Symposium ES01 : Organic Materials in Electrochemical Energy Storage

The ability to store large amounts of electrical energy is of increasing importance with the growing fraction of electricity generation from intermittent renewable sources such as wind and solar. Wide-scale utilization of flow batteries is limited by the cost of redox-active metals such as vanadium or precious metal electrocatalysts. We have developed high performance flow batteries based on the aqueous redox behavior of small organic and organometallic molecules, e.g. [1-8]. These redox active materials can be very inexpensive and exhibit rapid redox kinetics and high solubilities, potentially enabling massive electrical energy storage at greatly reduced cost. We have developed new protocols for measuring capacity fade rates and have discovered that the capacity fade rate is determined by the molecular calendar life, which can depend on state of charge, but is independent of the number of charge-discharge cycles imposed [7]. We will report the performance of the very few chemistries with long enough calendar life for practical application in stationary storage.
[1] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, "A metal-free organic-inorganic aqueous flow battery", Nature 505, 195 (2014), http://dx.doi.org/10.1038/nature12909

[2] K. Lin, Q. Chen, M.R. Gerhardt, L. Tong, S.B. Kim, L. Eisenach, A.W. Valle, D. Hardee, R.G. Gordon, M.J. Aziz and M.P. Marshak, "Alkaline Quinone Flow Battery", Science 349, 1529 (2015), http://dx.doi.org/10.1126/science.aab3033

[3] K. Lin, R. Gómez-Bombarelli, E.S. Beh, L. Tong, Q. Chen, A.W. Valle, A. Aspuru-Guzik, M.J. Aziz, and R.G. Gordon, "A redox flow battery with an alloxazine-based organic electrolyte", Nature Energy 1, 16102 (2016). http://dx.doi.org/10.1038/nenergy.2016.102

[4] E.S. Beh, D. De Porcellinis, R.L. Gracia, K.T. Xia, R.G. Gordon and M.J. Aziz, "A Neutral pH Aqueous Organic/Organometallic Redox Flow Battery with Extremely High Capacity Retention", ACS Energy Letters 2, 639 (2017). http://dx.doi.org/10.1021/acsenergylett.7b00019

[5] Z. Yang, L. Tong, D.P. Tabor, E.S. Beh, M.-A. Goulet, D. De Porcellinis, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, "Alkaline benzoquinone aqueous flow battery for large-scale storage of electrical energy” Advanced Energy Materials 2017, 1702056 (2017).
http://dx.doi.org/10.1002/aenm.201702056

[6] D.G. Kwabi, K. Lin, Y. Ji, E.F. Kerr, M.-A. Goulet, D. DePorcellinis, D.P. Tabor, D.A. Pollack, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, “Alkaline Quinone Flow Battery with Long Lifetime at pH 12” Joule 2, 1907 (2018).

[7] M.-A. Goulet & M.J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods”, J. Electrochem. Soc. 165, A1466 (2018). http://dx.doi.org/10.1149/2.0891807jes

[8] http://aziz.seas.harvard.edu/electrochemistry

Until very recently, the choice of electroactive materials in aqueous RFBs has been limited mostly to transition metal redox species, such as all-vanadium RFBs, which however are limited by the high cost of vanadium. Aqueous soluble organic (ASO) redox-active materials have recently shown promise as alternatives to transition metal ions to be employed in RFBs because of structural tunability, cost-effectiveness, availability, and safety features. To date, reported research on ASO species is rather limited and has been focused mostly on quinone, viologen, TEMPO, and ferrocene compounds.

In this presentation, we describe development of a new high energy density organic redox material as a promising ASO anolyte.¹In our research, we focused on investigating the primary ASO properties (i.e., solubility and redox potential) through a framework of combined nuclear magnetic resonance (NMR), density functional theory (DFT), organic synthesis, and electrochemical studies. Rational introduction of functional moieties in an asymmetrical configuration initiates preferential solvation²that significantly enhances the solubility from near-zero to up to 1.8 M in potassium based supporting electrolyte. The electrochemical performance of the new organic redox couple based anolytes was evaluated in a RFB using the well-established ferro/ferricyanide catholyte that leads to a high cell voltage of 1.4 V. Cycled at a high ASO concentration of 1.4 M (96 % of its maximum solubility of 1.45 M in 1 M NaOH ), the flow battery produced an exceptional reversible volumetric capacity of 67.4 Ah L^-1demonstrating for the first time of a ASO redox active material with reversible capacity equivalent to 2.8 M electron concentration.


1 Hollas, A.et al.A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries. Nat Energy3, 508-514, doi:10.1038/s41560-018-0167-3 (2018).
2 Han, K. S.et al.Preferential Solvation of an Asymmetric Redox Molecule. J Phys Chem C120, 27834-27839, doi:10.1021/acs.jpcc.6b09114 (2016).

Redox flow batteries (RFBs) are an exciting target for storing renewable energy at a large scale. Currently available commercial RFBs relying on vanadium as the redox material have suffered from high cost to be scale to a grid-level size. Such a challenge could be reduced if redox-active organic molecules could be used instead of vanadium. In addition, the design of redox molecules with various functionalities can tune the molecular properties, which eventually allows for improving the solubility and controlling the redox potentials to optimize their performance. However, there are very few candidates that could serve as organic redox materials and almost all of them act as a single-electron carrier. For increasing the energy density, the effective strategies are (1) increasing voltage gap as the redox materials reversibly respond more negative/positive potentials for negolyte/posolyte, respectively, (2) increasing solubility, and (3) designing redox materials capable of two (or more)-electron storage, which instantly doubles the energy density. Here we present some research results which have been done in my laboratory for the target of increasing energy density. We show a new class of organic material of naphthalene diimide (NDI) that can reversibly store two electrons at neutral pH in aqueous solution. By decorating with glycinate, the solubility of n-type organic semiconducting NDI was improved in aqueous solution. We studied the fundamental two-electron redox behavior from experimental and computational analyses and displayed a prototype RFB containing [K₂-BNDI] negolyte and 4-OH-TEMPO posolyte with reasonable cyclability, energy efficiency and excellent stability. To improve the stability of redox materials, we also attempted to use redox-active organometallic molecules such as pseudo-octahedral Co-polypyridyl complexes, and developed rational strategies for enhancing the robustness, namely, the spin-crossover between low and high-spin states and the chelation effect emerging from replacing three bidentate ligands with two tridentate analogues.

Redox flow batteries are particularly attractive for stationary energy storage with potential impact to a variety of grid applications. High chemical cost or long-term cyclability have limited the conventional inorganic-based redox materials. In this regard, organic redox materials have recently demonstrated encouraging property and performance characteristics, promising to address these challenges.¹ Solubility and chemical stability are among the most important properties of ROMs, the former determining the energy density and the latter of corresponding flow batteries. Currently, ROM candidates that satisfy these requirements simultaneously are rather limited.
Here, we will report our recent progress in the development of organic redox materials covering a diversity of structures and reaction mechanisms, such as diquats and phenazine in aqueous^1-2 as well as dialkoxybenzenes in nonaqueous³ flow batteries. Our studies have unraveled the molecular-level fundamental mechanisms of low solubility and chemical instability via combined spectroscopic and computational methods. Rational materials design principles have been suggested and demonstrated to be effective in enabling new highly soluble, stable redox materials.

References
1. Hollas, A.; Wei, X. L.; Murugesan, V.; Nie, Z. M.; Li, B.; Reed, D.; Liu, J.; Sprenkle, V.; Wang, W., A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries. Nature Energy 2018, 3 (6), 508-514.
2. Huang, J.; Yang, Z.; Murugesan, V.; Walter, E.; Hollas, A.; Pan, B.; Assary, R. S.; Shkrob, I. A.; Wei, X.; Zhang, Z., Spatially Constrained Organic Diquat Anolyte for Stable Aqueous Flow Batteries. Acs Energy Lett 2018, 3, 2533–2538.
3. Zhang, J.; Yang, Z.; Shkrob, I. A.; Assary, R. S.; Tung, S. o.; Silcox, B.; Duan, W.; Zhang, J.; Su, C. C.; Hu, B.; Pan, B.; Liao, C.; Zhang, Z.; Wang, W.; Curtiss, L. A.; Thompson, L. T.; Wei, X.; Zhang, L., Annulated Dialkoxybenzenes as Catholyte Materials for Nonaqueous Redox Flow Batteries: Achieving High Chemical Stability through Bicyclic Substitution. Adv Energy Mater 2017, 7, 1701272.

To efficiently utilize the intermittent renewable energy source such as wind and solar energy and achieve sustainable society, advanced large-scale energy storage technologies are highly demanded. Among various energy storage devices, aqueous organic redox flow batteries (AORFBs) are one of the most promising battery technologies for large scale storage of intermittent energy because of a number technological merits including decoupled energy and power, higher current and high power performance, safety features, and synthetic tunability of charge storage molecules. Herein, we developed a serial of low-cost, highly water soluble sulfonate functionalized viologens as anolyte for AORFB applications. The negative charged sulfonate pendant side chains and favorable molecular sizes of these viologens enable their compatibility with cation exchange membranes. The newly designed viologens were paired with low-cost I₃^-/I^-, Br₃^-/Br^-, and [Fe(CN)₆]^3-/[Fe(CN)₆]^4- catholytes in RFBs using cation exchange mechanism. The 1,1’-bis(3-sulfonatopropyl)-4,4’-bipyridinium, (SPr)₂V/I RFB delivered 1.0 V battery voltage and reliable battery performance under a pH neutral condition. When the (SPr)₂V was paired with a newly designed organometallic catholyte material (NH₄)₄[Fe(CN)₆], a 0.9 M pH neutral (SPr)₂V/[Fe(CN)₆]^4- system delivered unprecedented storage capacity and cycling stability, specifically, 24.1 Ah/L electrolyte capacity, 62.6% energy efficiency and 100% capacity retention in 1000 cycles (more than 45 days testing duration), which represents the best cycling stability among all reported organic RFBs. To further improve battery voltage and energy density, the (SPr)₂V was paired with Br₃^-/Br^- catholyte. The (SPr)₂V/Br RFB delivered a battery voltage of 1.51V, 78% energy efficiency, and up to a 30.4 Wh/L operated energy density in a 1.5 M battery. Other asymmetric sulfonate functionalized viologens, such as (SEt)(SPr)V and (SPr)(SBu)V, with even higher chemical stability and water solubility were also prepared for AORFB applications.

The urgent need for cleaner energy technologies calls for a radical change in the energy mix to favor renewable energy (+138.5 GW added in 2016 [1]) and environmentally responsible energy storage solutions. Within this background, the development of reliable, efficient, low-polluting and low-cost electrochemical storage systems can be considered as particularly important. Among the various possible technologies, Redox Flow Batteries (RFBs) is believed as suitable devices for large-scale energy storage [2]. Basically, both physico-chemical and electrochemical properties of the selected redox-active species are particularly crucial. Thus their solubility, chemical stability and the resulting output voltage (after assembly) define the energy density, the cyclability and the power density of the system, respectively. Interestingly, the use of organic electroactive species enable access to low cost and possibly greener compounds because composed of naturally abundant elements. Moreover, they offer high structural designability through the well-established principles of organic chemistry and notably access to both n- and p-type electrochemical storage mechanisms [3–5].
As part of our ongoing effort in developing novel Aqueous Organic Redox Flow Batteries (ORFBs), we will present our recent results dealing with the synthesis and characterizations of a novel highly soluble organic derivative used as catholyte and based on the stable tetramethylpiperidine N–oxyl moiety [6,7]. The electrochemical properties of the catholyte will be reported as well as preliminary data obtained in a full flow battery configuration.
References
[1] H. Chen, G. Cong, Y.-C. Lu, Journal of Energy Chemistry 27 (2018) 1304–1325.
[2] X. Wei, W. Pan, W. Duan, A. Hollas, Z. Yang, B. Li, Z. Nie, J. Liu, D. Reed, W. Wang, V. Sprenkle, ACS Energy Lett. 2 (2017) 2187–2204.
[3] P. Leung, A.A. Shah, L. Sanz, C. Flox, J.R. Morante, Q. Xu, M.R. Mohamed, C. Ponce de León, F.C. Walsh, J. Power Sources 360 (2017) 243–283.
[4] J. Winsberg, T. Hagemann, T. Janoschka, M.D. Hager, U.S. Schubert, Angew. Chem. Int. Ed. Engl. 56 (2017) 686–711.
[5] Q. Zhao, Z. Zhu, J. Chen, Adv. Mater. 29 (2017).
[6] K. Nakahara, K. Oyaizu, H. Nishide, Chem. Lett. 40 (2011) 222–227.
[7] J.E. Nutting, M. Rafiee, S.S. Stahl, Chem. Rev. 118 (2018) 4834–4885.

Due to the intermittent nature of sunlight, practical solar energy utilization systems demand both efficient solar energy conversion and inexpensive large scale energy storage. We have developed novel hybrid solar-charged storage devices that integrate organic redox flow batteries (RFBs) and regenerative semiconductor solar cells that share the same pair of redox couples. In these integrated solar flow batteries (SFBs), solar energy is absorbed by semiconductor electrodes and photoexcited caries are collected at the semiconductor-liquid electrolyte interface and used to convert the redox couples in the RFB to fully charge up the battery. When electricity is needed, the charged up redox couples are discharged to generate the electricity. We have demonstrated that solar energy harvest, conversion, storage, and redelivery can be completed by such a single integrated SFB without any external electrical energy input. After developing high performance III-V solar cells that are carefully matched with various high voltage organic couples and optimizing several generations of SFB device designs, we have achieved integrated SFB device with an overall direct solar-to-output electricity efficiency (SOEE) of 14%. We have further improved the cycling performance of the SFBs by integrating robust organic redox couples. To enable SFBs in practical distributed and standalone solar energy conversion and storage systems in remote locations, we aim to keep a low overall cost for SFB devices while maintaining its high performance, thus lowering the chemical cost of redox active materials could be one of the effective ways to achieve such goal. Therefore, we are particularly interested in new, inexpensive, and robust redox couples with diverse redox potentials.

During the last decades, the demand for environmental friendly renewable power source has grown rapidly to achieve the sustainable society. To efficiently utilize the widespread but intermittent renewable energy source such as solar and wind energy, low-cost and reliable large-scale energy storage technologies were required. Aqueous organic redox flow batteries (AORFBs) with the merits of decoupled energy and power, higher current and high power performance, safety features, synthetic tunability of charge storage molecules, and potentially low-cost, are ideal solution for the intermittent energy storage and electricity grids balancing. Herein, we designed and synthesized sulfonate functionalized viologen compound [1,1’-bis(3-sulfonatopropyl)-4,4’-bipyridinium, (SPr)₂V] as a low-cost, highly soluble anode material for pH neutral AORFB application. DFT computational studies suggested charge repulsion and size exclusion enable the compatibility of (SPr)₂V with a cation exchange membrane. The superior chemical stability of the (SPr)₂V anolyte was confirmed by a 0.5 M half-cell (SPr)₂V/(SPr)₂V^.- battery test. Paired with the inexpensive I₃^-/I^- catholyte, the (SPr)₂V/I neutral AORFB delivered 1.0 V battery voltage and up to 71% energy efficiency with 99.99% capacity retention per cycle. When combined with (NH₄)₄[Fe(CN)₆], a 0.9 M or 24.1 Wh/L symmetric (SPr)₂V/(NH₄)₄[Fe(CN)₆] delivered ultra-stable cycling performance, nearly no capacity decay for 1000 cycles. To further boost the battery voltage and energy density of the (SPr)₂V-based AORFBs, Br₃^-/Br^- catholyte was chosen to combine with (SPr)₂V for AORFB demonstration. The (SPr)₂V/Br AQRFB delivered 1.51 V battery voltage and 30.4 Wh/L operated energy density due to the high potential of Br₃^-/Br^- redox couple and high solubility of (SPr)₂V anode material. Furthermore, the (SPr)₂V/Br AORFB maintained excellent energy and power performance under 1.5 M concentration, respectively, up to 78% energy efficiency and 228 mW/cm² power density, which is the highest power density ever reported in neutral AORFBs.

Ref:
1. DeBruler C.; Hu, B.; Moss, J.; Luo, J.; Liu, T. L. A Sulfonate Functionalized Viologen Enabling Neutral Cation Exchange Aqueous Organic Redox Flow Batteries towards Renewable Energy Storage ACS Energy Letters 2018, 3, 663-668.
2. Luo, J. (co-1^st author); Hu, B. (co-1^st author); DeBruler C. Zhao, Y., Yuan B. Hu, M. Wu, W. Liu, T. L.* "Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries", Joule, 2019, 3, 1-15.

In conventional batteries which store energy in solid materials, the active materials are coated on current collector in order for good electrical contact with the electrode (current collector). Based on the “redox targeting” reactions of solid energy storage materials with redox mediators, the active material can be reversibly oxidized and reduced without being attached to the current collector. The transport of electrons between the material and the current collector is mediated by the diffusion of redox molecules dissolved in the electrolyte. The application of redox targeting reactions to both the anode and cathode intuitively leads to a novel energy storage device — redox targeting-based flow battery. Insuch a new battery configuration, the active materials are stored statically in two separate tanks and power is produced in the cell stack by the redox reactions of redox mediators, disruptively changing the operation mode of the conventional batteries. Redox targeting-based flow batteryis poised to have advantages over other types of electrochemical energy storage devices in terms of energy density, safety, and operation flexibility for large-scale stationary energy storage. Various redox targeting-based battery systems have been demonstrated since the first report in 2013.¹ The battery chemistry has been extended with charge balancing ions from lithium-ion to sodium-ion and proton, electrolytes from non-aqueous to aqueous², and redox mediators from dual to single redox molecule³.

In this talk, I will report the latest progress on the development of redox targeting-based flow battery, with special focus on the visualization and kinetics of the redox targeting reactions between organic redox species with the energy storage materials.

Reference:
1. Q. Huang, H. Li, M. Grätzel, and Q. Wang, Reversible Chemical Delithiation/Lithiation of LiFePO4: Towards A Redox Flow Lithium-ion Battery. Phys. Chem. Chem. Phys., 15 (6), 1793-1797 (2013).
2. J. Yu, L. Fan, R. Yan, M. Zhou, and Q. Wang, A Redox Targeting-based Aqueous Redox Flow Lithium Battery. ACS Energy Lett., 3, 2314-2320 (2018).
3. M. Zhou, Q. Huang. T. N. P. Truong, J. Ghilane, Y. G. Zhu, C. Jia, R. Yan, L. Fan, H. Randriamahazaka, Q. Wang, Nernstian Potential-driven Redox Targeting Reactions of Battery Materials. Chem, 3 (6), 1036-1049 (2017).

Highly-soluble redox-active polymers (RAPs) and colloids (RACs) [1] are a new class of materials in the form of fluid dispersions that support a new concept in size-exclusion flow batteries. Because RAPs and RACs rely on intra-particle charge transfer to yield quantitative charge accessibility and high rate, understanding the intrinsic properties of these particles, as opposed to them in the bulk fluids, is of great interest to understand their limitations and to identify design opportunities. In this talk, I will present on the of RAPs and RACs through a spectrum of powerful electrochemical techniques, ranging from spectroelectrochemical approaches to single-particle analysis.

In a first application, I will describe how nano-resolved scanning electrochemical microscopy (SECM) and its combination with Raman spectroscopy has helped us understand the mechanisms of individual electrochemical entities.[2] These experiments provide us with unprecedented versatility to identify kinetic bottlenecks, such as charge trapping, and to determine the maximum current densities attainable in flow devices. Our data indicate that RACs undergo some conditioning upon electrolysis, and that their charge transport is sensitive to state-of-charge (SoC).

In a second application, I will describe how new redox mediation electrocatalysis using each redox-active pendant in RAPs as an electron transfer agent can be exploited to solve pervasive problems with passivating interfaces in complex chemistries, such as those involved in Li-air batteries. Here, investigations of charge transfer and of transient titration of reaction intermediates using SECM is paving the way to understanding how to leverage the properties of polymers to solve rate-limiting interfacial processes.

The insightful use of analytical approaches allows us to understand charge transfer, and the conditions that lead to effective storage or to failure. We expect that our methods will be extendable to other energy storage systems of interest to the community.

[1] Burgess, M.; Moore, J.S.; Rodríguez-López, J. Redox Active Polymers as Soluble Nanomaterials for Energy Storage. Acc. Chem. Res. 2016, 49, 2649-2657.
[2] Gossage, Z.T.; Hernandez-Burgos, K.; Moore, J.S.; Rodríguez-López, J. Impact of Charge Transport Dynamics and Conditioning on Cycling Efficiency within Single Redox-Active Colloids. ChemElectroChem, 2018, 5, 3006-3013.

Ethyl viologen-based nonconjugated redox active colloids (RAC) demonstrate efficient and reversible charge transport in aqueous and non-aqueous environments. They can be used as electrode materials in flow-battery setup to prevent crossover issues between anolytes and catholytes. RAC is also intriguing for the colloid science community as a novel type of colloid bearing intrinsic function (charge transport through electron hopping) at the molecular level. There is tremendous scientific value in direct visualization of contact-mediated electron and or energy transfer between these colloids, which will provide deep understanding of the unique dynamics as well as facilitate redox-state mapping in this system. Electrofluorochromism of RAC was discovered and exploited to serve the purpose. Via coupling a distinct fluorescent contrast with the respective redox states we successfully conducted in-situ imaging of intra- and inter-colloid electron hopping processes during electrochemical cycling. We captured an over 20μm electron diffusion in a RAC monolayer. Due to the percolation nature of RAC monolayer, inter-colloid charge transport diffusion coefficient D_CT was facilely extracted and first-time reported. This system also displayed a sensitivity to fluorescence quenching, the photophysics of which was preliminarily investigated and ascribed to fast electron hopping among neighboring redox groups. By correlating charge input and fluorescence emission over time we developed a working curve which renders RAC a redox state meter in and of itself under optical microscopes.

Redox-active organic materials (ROMs) have shown great promise for redox flow battery applications but generally encounter limited cycling efficiency and stability at relevant redox material concentrations in nonaqueous systems. In this talk, a new family of heterocyclic organic anolyte molecules, 2,1,3-benzothiadiazole (BZNSN) and derivatives,^1-2 will be discussed, which demonstrated high solubility, low redox potential, and fast electrochemical kinetics. By coupling BZNSN with a benchmark catholyte ROM (DBMMB),³ the nonaqueous organic flow battery delivered significant improvement in cyclable redox material concentrations and cell efficiencies compared to the state-of-the-art nonaqueous systems. In order to obtain insightful understanding of this family of molecules, a series of derivatives have been developed by varying the 5-position substitutions with various electron accepting/withdrawing ability. The substituent effects on their properties of interest have been examed, including redox potentials, calendar lives of charged ROMs in electrolyte, and the flow cell cycling performance. While the calendar life of energized fluids can be tuned in a predictable fashion over a wide range, the improvements in the calendar life do not directly translate into the enhanced cycling performance, indicating that in addition to the slow reactions of charges species in the solvent bulk, there are other parasitic reactions that occur only during electrochemical cycling of the cell and can dramatically affect the cycling lifetime.
1. Duan, W.; Huang, J.; Kowalski, J. A.; Shkrob, I. A.; Vijayakumar, M.; Walter, E.; Pan, B.; Yang, Z.; Milshtein, J. D.; Li, B.; Liao, C.; Zhang, Z.; Wang, W.; Liu, J.; Moore, J. S.; Brushett, F. R.; Zhang, L.; Wei, X., “Wine-Dark Sea” in an Organic Flow Battery: Storing Negative Charge in 2,1,3-Benzothiadiazole Radicals Leads to Improved Cyclability. ACS Energy Letters 2017, 2 (5), 1156-1161.
2. Huang, J.; Duan, W.; Zhang, J.; Shkrob, I. A.; Assary, R. S.; Pan, B.; Liao, C.; Zhang, Z.; Wei, X.; Zhang, L., Substituted thiadiazoles as energy-rich anolytes for nonaqueous redox flow cells. Journal of Materials Chemistry A 2018, 6 (15), 6251-6254.
3. Huang, J.; Cheng, L.; Assary, R. S.; Wang, P.; Xue, Z.; Burrell, A. K.; Curtiss, L. A.; Zhang, L., Liquid Catholyte Molecules for Nonaqueous Redox Flow Batteries. Advanced Energy Materials 2015, 5 (6), 1401782.

Redox flow batteries (RFBs) are some of the most promising energy storage systems because of their scalability and design flexibility; however, their low energy density is a major drawback limiting their widespread application. Most conventional approaches to increase the energy density have involved exploiting high-concentration electrolytes. However, this approach results in many technical issues such as increased viscosity and sluggish kinetics. In this work, we propose a strategy of boosting the energy density by exploiting an active material based on phenazine molecule (5,10-dihydro-5,10-dimethyl phenazine or DMPZ), which is capable of multi-redox reaction at −0.15 and 0.61 V vs.Ag/Ag⁺. This novel positive electrode material exhibits reversible double-redox activity with fast kinetics and remarkable chemical stability. Coupled with 9-fluorenone (FL), the DMPZ/FL flow cell can provide the highest energy density per mole (≈85 W h mol⁻¹) ever reported for RFBs. Furthermore, the marked color change of DMPZ at different charged states enables the state of charge to be precisely visualized. This novel strategy on multi-electron redox material can provide a potential pathway toward high-energy-density RFBs.

Intermittent sunlight necessitates the storage of electricity, ideally in controlled chemical reactions analogous to photosynthesis. Many efforts have focused on the oxidation of water at metal oxide semiconductor surfaces, although light-driven electrochemical reactions at photoelectrodes continue to suffer from sluggish half reactions. One major challenge continues to be the control of multi-electron transfer events with well-defined rate constants. An ideal materials design approach would enable predictive rate constants prior to photo-electrode fabrication.

Electrodes comprised of organic semiconductor films offer the possibility to control redox properties independent of opto-electronic behaviors, making these electrodes idea for next-generation photoelectrodes. Examples of proof-of-concept include photo-capacitors and photo-driven water splitting. Yet to date, electron-transfer rates between conductive polymers and redox species remain slower than inorganic materials including metals and oxides, making it difficult to incorporate these inexpensive, printable systems in redox flow systems.
Most recently, we showed that the rate of electron transfer is predictable using a Marcus-Gerischer relationship, whereby the dominant factor controlling the rate of electron-transfer is the overlap in the density of states of the electrode with the density of states of the redox probe. With this new insight, we demonstrated that the microstructure of the polymer electrode becomes a critical component to controlling the symmetry of charge transfer reactions. Control of the microstructure enables moving the charge transfer events into the mass transport limit necessary for energy storage systems.

In this work, we focus on nanoscale manipulation of electron transfer events, relative to mass transfer of the redox probe, to further control electrode performance. Specifically, we investigate the role of heterogeneity of blended polymer electrodes. Both macroscale and nanoscale electrochemical phenomena will be discussed using a combination of spectroelectrochemcial and scanning electrochemical microscopy. We demonstrate that the macroscale rate of electron transfer can be further controlled beyond the density of states framework, offering a new paradigm to control multi-electron transfer events not available in inorganic systems.

There is a pressing need for new battery technologies in two key areas: high-energy power batteries for electric vehicles, and large-scale storage batteries to buffer the output from the renewable-but-intermittent solar and wind power generation. For storage batteries, the goal is to minimize the levelized energy cost over the devices’ lifetime. This means that electrode materials containing rare and expensive elements should be avoided. Secondly, the cycle life of storage batteries must be excellent — ideally over 10,000 cycles. Thirdly, these batteries should be quickly rechargeable to store, for example, energy from wind farms. Most importantly, storage batteries should be intrinsically safe, i.e., nonflammable. To meet this list of requirements, batteries with aqueous electrolytes relish several distinct advantages. Aqueous electrolytes are cheaper and safer. The simplicity of an aqueous chemistry environment may facilitate long battery longevity, and its high conductivity brings an innate power advantage. In battery chemistry design, one of the most important considerations is the choice of working charge carriers. To date, a majority of battery technologies rely on metal-ion charge carriers. Surprisingly, non-metal cations, particularly proton-containing cations, i.e., H⁺, H₃O⁺,¹ and NH₄⁺,^1,2, have received exceedingly little attention. The simplest form of hydrogen cation, a single proton, is nearly “invisible” with a measured radius of ~0.89 fm or ~2.1 fm, using muon or e^- spectroscopy, respectively. Due to the negligible strain of hosting protons, the rate capability and cycle life of proton batteries have the potential to be far superior to those of existing batteries. In this talk, I will introduce some of our new results, experimental and computational, on the storage of new charge carriers in battery chemistry for grid-storage purposes, particularly related to the Grotthuss mechanism³ and the ion/electrode interactions. I will compare different charge carriers in terms of their correlations to electrochemical properties, such as capacity fading, polarization, and operation potentials.

References
Wang, X., Bommier, C., Jian, Z., Li, Z., Chandrabose, R. S., Rodríguez Pérez, I., Ji*, X.“Hydronium-Ion Batteries with Perylenetetracarboxylic Dianhydride Crystals as an Electrode”Angew. Chem. Int. Ed. 56, (2017): 2909-2913 doi:10.1002/ange.201700148
Wu, X., Qi, Y., Hong, J. J., Hernandez, A. S., Ji*, X. “Rocking-Chair NH₄-Ion Battery: A Highly Reversible Aqueous Energy Storage System” Angewandte Chemie International Edition 56, (2017) 13026-13030 doi: 10.1002/anie.201707473
[3] Wu, X., Hong, J. J., Shin, W., Ma, L., Liu, T., Bi, X., Yuan, Y., Qi, Y. (undergraduate), Surta, T., Huang, W., Neuefeind, J., Wu, T., Greaney*, P. A., Lu*, J., and Ji* X., “Diffusion-Free Grotthuss Topochemistry for High-rate and Long-Life Proton Batteries" Nature Energy, (2019) DOI: 10.1038/s41560-018-0309-7

Nowadays electrodes of lithium batteries are mainly constituted by inorganic compounds based on transition metals such as cobalt, nickel or manganese. Although their performances are satisfying, these materials present several important drawbacks. Indeed these compounds are expensive because they are prepared due to energy-consuming techniques from rare mineral precursors. Moreover, some metals are toxic and often hard to recycle. Eventually their reactivity leads to safety issues in abusive conditions.
Organic electroactive compounds¹ such as nitroxide based polymers² or carboxylate salts³ offer a cost-effective and environmental friendly alternative to conventional electrode materials for electrochemical storage. Interestingly these products can be prepared from low cost precursors using classical organic and polymer chemistry techniques. Moreover these compounds are easy to recycle or reuse at their end of life. Eventually organic electrode materials can followed n-doped but also p-doped redox mechanisms which enable to imagine new battery configurations (cation-ion, dual-ion or anion-ion). But until now, their use is still challenging due to low cycle life usually related to their high solubility in organic solvents of electrolytes.
Following the pioneer work of Yao et al.⁴ with its concept of molecular-ion based “rocking chair” battery, this work is focused on the development of optimized full organic battery using anion as a shuttle during charge/discharge in order to study their electrochemical performances for in particular high power applications.
First various polymers based on viologen redox unit were studied as negative electrode materials and the introduction of crosslinker⁵ but also the influence of the nature of counter-anion have been investigated. A particular strategy was identified in order to stabilize the specific capacity of polyviologen (PV) along cycling. In parallel, an original and insoluble structure based on lithium dianilinoterephthalate (Li₂DAnT) has been developed and even if some moderate electrochemical performances have been obtained, this development demonstrates for the first time the interest of terephthalate backbone to suppress the dissolution of p-doped organic materials.⁶
Finally the formulation of organic electrodes using PV, Li₂DAnt but also using polynitroxydes (PTMA)² based composites were optimized in particular for screen printing process and several full organic batteries have been assembled and tested in anion-ion configuration. These results pave the way for the development of metal-free battery with high electrochemical performances.

¹ P. Poizot et al, Energy Environ. Sci., 2011, 4, 2003-2019 ; Y. Liang et al., Adv. Energy Mater., 2012, 2 742-769
² Nakahara al., Chem. Phys., Lett., 2002, 359, 351-354
³ A. Iordache et al. Adv. Sustain. Syst. 1600032
⁴ M. Yao et al., Sci. Reports, 2015, 5, 10962-10969:10692
⁵ Sano et al., Appl. Materials & Interfaces, 2013, 5, 1355-1361
⁶ E. Deunf et al, J. Mater. Chem., 2016, 4, 6131-6139 ; E. Deunf et al, CrystEngComm, 2016, 18, 6076-6082 ; E. Quarez et al., CrystEngComm, 2017, 19, 6787-6796

Forming stable cathode-solid electrolyte interface is one of the greatest challenges in sulfide-based all-solid-state sodium batteries (ASSSBs). So far these ASSSBs suffer from low specific energy and poor cycling performances due to the interfacial incompatibility between cathode materials and sulfide electrolytes. Resistive layer forms at the interface due to the electrolyte decomposition at high potentials. Most previous cathodes are also too rigid to accomodate the volume change upon cycling, resulting in the inter-particle contact loss. Herein we show an organic cathde material, pyrene-4,5,9,10-tetraone (PTO), that can form an (electro)chemically and mechanically compatible interface with Na₃PS₄. PTO has a moderate redox potential (2.2 V vs. Na⁺/Na) that aligns with the electrochemical stability window of Na₃PS₄. PTO also has a low Young’s modulus (4.2 ± 0.2 GPa) that is similar to that of Na₃PS₄ (10.7 ± 0.6 GPa), hence ensuring intimate PTO-Na₃PS₄ contact during cycling. The PTO-based ASSSB exhibits high specific energy of 587 Wh kg^-1 at material-level and 89% capacity retention over 500 cycles. This work reveals an effective cathode design strategy towards compatibility with solid electrolytes and thus high-performance ASSSBs.

In face of the climate change there is a strong and growing demand for the storage of renewable energies. Organic electrode materials have attracted great interest, as they can be prepared from renewable, sustainable or less-limited resources, they are easy to recycle as well as potentially safer and cheaper to produce, leading to a low carbon footprint. A promising class of organic electrode materials are redox polymers – polymers containing groups that can be reversibly reduced or oxidized.
In this talk organic redox polymers will be presented containing heteroaromatics as redox-active functionalities. The design, synthesis and electrochemical properties of these polymers will be discussed as well as their application as electrode-active materials in batteries. Mechanistic studies will be presented that show how the mobility of the redox polymer within the composite electrode as well as interactions between redox-active groups influence the specific capacity of the electrode as well as its cyclability.

The structure tunability and built-in porosity of metal organic frameworks (MOFs) materials makes them promising electrode materials to host metal ions. Herein, Novel MOFs materials consisting of redox active ligands and earth abundant transition metals were rationally designed and synthesized for ion intercalation energy storage applications. Structural characterization by X-ray diffraction, IR absorption, and elemental analysis confirmed target structures were assembled successfully. Electrochemical studies including cyclic voltammetry, electrochemical impedance spectroscopy, and full cell battery tests were conducted to verify these organic/inorganic materials have improved electrochemical reversibility, conductivity, and stability over purely organic or inorganic materials. For example, a Ni redox MOFs delivered outstanding battery performance for 1000 cycles with capacity retention up to 82%. Further studies on ionic conductivity, valence state analysis, gas adsorption, and charge storage mechanism were conducted to better understand the electrochemistry, structure/function relationship of these novel MOFs materials.

Using a redox active organic material as an electrode material of rechargeable lithium batteries can reduce the amount of minor metal-based materials from the current system. Among many organic candidates, we have focused on a series of low-molecular-weight quinone derivatives since they show high capacities based on their multi-electron transfer type redox reaction. However, many of them often show poor cycle-stabilities; therefore, improving the cycle-stability is an important concern. One of the reasons for the capacity fade is believed to be the dissolution of redox active low-molecular-weight molecules into the electrolyte solution.
To suppress the solubility of the quinone-based molecules, we synthesized some oligomers in which the quinone sites are connected by some covalent bond or fused, and proved that these oligomers have longer cycle-lives than the monomers. For example, anthraquinone (AQ), a quinone containing the anthracene skeleton, shows a high initial discharge capacity of about 200 mAh/g with the average potential of 2.3 V vs. Li⁺/Li; however, its capacity notably decreases upon cycling. On the other hand, a fused larger polycyclic quinone having the pentacene skeleton (pentacenetetrone), which has a lower solubility, exhibits a longer cycle-life than AQ. In addition, the solubility of the AQ derivatives can be lowered by oligomerization. We synthesized an AQ dimer and trimer, in which the AQ units are connected by the acetylene unit, and found that they are insoluble in ordinary solvents and show a longer cycle-performance. In particular, the trimer retained an almost constant capacity during the one hundred cycles.
A similar result was also observed for the naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) derivatives which undergo a four-electron transfer redox reaction. While the lithium salt of the naphthazarin itself (monomer) showed a high capacity of about 400 mAh/g with the average potential of 2.7 V vs. Li⁺/Li during the first cycle, its cycle stability was also poor. On the other hand, a dimer fused by the dithiin ring showed an improved battery performance; a high initial capacity of 416 mAh/g with a relatively stable cycle-performance was obtained.
We consider that the above-mentioned oligomers have a stronger attractive intermolecular force than the monomers which should contribute to a less solubility than the monomers. To obtain a theoretical insight into the intermolecular interactions, a quantum chemistry calculation was performed. Our calculation gave π-stacked structures for a series of the AQ oligomers, the charged naphthazarin and its fused dimer. The estimated binding energy for the monomers are about 20 kJ/mol, which are typical values for such stacked small molecules. As for the oligomers, a few times higher values (70-100 kJ/mol) were obtained. In general, intermolecular forces represented by the Van der Waals forces are considered to be very weak; however, the obtained stacking force values are much higher than the binding energy of the hydrogen bonding (10-40 kJ/mol) which is a relatively strong intermolecular force and comparable to the level of the covalent bonding (100-300 kJ/mol). This calculation indicates that oligomerization will be very effective to enhance the intermolecular attractive interaction which should contribute to suppressing the dissolution.
In summary, the oligomerization and/or ring-fusion of a redox active molecule showing multi-electron transfer reaction will be a guide for designing a new organic active material that can satisfy both the long cycle-life and high energy density requirements.

Organic redox-active molecules composed of only earth-abundant elements such as C, H, N, O, and S are very promising for the next-generation secondary batteries with low cost and sustainability. A lot of organic cathode materials have been reported, so far; however, their redox potentials are still mostly below 3.0 V vs. Li/Li⁺, which is considerably inferior to the conventional ones. Recently, a few organic molecules showing high redox potential above 3.5 V vs. Li/Li⁺ are reported including triphenylamine (TPA), carbazole (Cbz), and phenothiazine (PTZ), whose charge/discharge mechanisms rely on oxidation reactions of N-containing heterocyclic redox centers. Although they show excellent cycle stability and high rate performance, most of them can undergo only a one-electron oxidation reaction, which limits their practical capacity below 100 mAh/g. Here, we report molecular design strategies to achieve a large specific capacity of the N-containing heterocyclic organic molecules by inducing multi-electron redox reactions.
First, a series of novel organic molecules bearing phenoxazine (PXZ) as a new p-type redox center is reported. In the molecules, multiple PXZ units are covalently connected together by a phenyl ring core as a minimal linker. Through the combination of experimental and theoretical studies, it is revealed that the three-dimensional molecular geometry and the negligible re-organization during the redox reactions allow them to undergo reversible multi-electron redox reactions with minimal redox peak splitting. In the coin cells, they can deliver large specific capacity of more than 120 mAh/g with a discharge plateau at 3.7 V vs. Li/Li⁺. It is noteworthy that they show excellent cycle stability over 500 cycles and good rate capability.
Second, we present a series of fused pyrroles recently developed in my group as a new cathode active materials. They exhibit multi-electron oxidation reactions at above 3.5 V vs. Li/Li⁺ to provide large specific capacity. Their electrochemical properties and electrode performances are discussed in this presentation.

Redox flow batteries (RFBs) are a viable technology to store renewable energy in the form of electricity that can be supplied to electricity grids. However, widespread implementation of traditional aqueous inorganic redox flow batteries (AIRFBs), such as vanadium and Zn-Br₂ RFBs, is limited due to a number of challenges related to materials, including low abundance and high costs of redox-active metals, expensive separators, and corrosive and hazardous electrolytes. To address these challenges, our group has demonstrated a series of pH neutral aqueous organic redox flow batteries (AORFBs) based on tunable and sustainable viologen anolytes (e.g. methyl viologen (MV), 1,1′-bis[3-(trimethylammonio)propyl]-4,4′-bipyridinium ((NPr)2V), and 1,1′-bis(3-sulfonatopropyl)-4,4′-bipyridinium ((SPr)2V) with a variety of organic and inorganic catholytes. Viologen compounds display excellent chemical and electrochemical stability, desirable redox potential, and high solubility in water. The MV/FcNCl (FcNCl abbreviated for (ferrocenylmethyl)trimethylammonium chloride ) AORFB was reported by us with outsantding cycling stability and power density, representing as a benchmark organic flow battery. By means of molecular engineering, we have not only tuned the molecular structures of viologen molecules and also achieved even better stability compared to the benchmark molecule, methyl viologen. These efforts have leading to several outstanding pH neutral AORFBs: (1) the (NPr)₂V/FcNCl and (NPr)₂V/TEMPO AORFBs were reported as the first examples employing two electron storage viologen anolytes in AORFBs, achieving a cell voltage of 1.72 V and a power density of 130 mW/cm²; (2) The 1.38 V (NPr)₂V/TEMPO AORFB represents the most stable total organic RFB (99.993% capacity retention per cycle for overall 500 cycles), and (3) the (SPr)2V/(NH4)2Fe(CN)6 AORFB represents the most stable AORFB (nearly 100% capacity retention after 1000 cycles) with a demonstrated high capacity (24.1 Ah/L). The presented high performance viologen AORFBs underline the great promise of soluble redox-active organic molecules for green energy storage. Particularly, this poster presentation emphasizes that fundamental understandings of redox active electrolytes at molecular level are crucial to develop new generations of redox flow batteries for large scale and dispatchable renewable energy storage.
References:
(1) Luo, J. (co-1^st author); Hu, B. (co-1^st author); Bi, Y.; Hu, M.; Wu, W.; Liu, T. L.*, Joule. 2018, DOI:10.1016/j.joule.2018.10.010.
(2) DeBruler C.; Hu, B.; Moss, J.; Luo, J.; Liu, T. L.*, ACS Energy Letters 2018, 3, 663.
(3) Luo, J.; Hu, B.; DeBruler C.; Liu, T. L.*, Angew. Chem. Int. Ed. 2018, 57, 231.
(4) Hu, B.; Tang, Y.; Luo, J.; Grove, G.; Guo, Y.; Liu, T. L.*, Chem. Commun. 2018, 54, 6871.
(5) Hu, B.; Liu, T. L.*, J. Energy Chem. 2018, 27, 1326.
(6) DeBruler C. (co-1^st author); Hu, B. (co-1^st author); Moss, J.; Liu, X. Luo, J.; Sun, Y; Liu, T. L.*, Chem, 2017, 3, 961.
(7) Luo, J.; Sam, A.; Hu, B.; DeBruler C. Liu, T. L.* Nano Energy, 2017, 42, 215.
(8) Hu, B.; Seefeldt, C., DeBruler C.; Liu, T. L.*, J. Mater. Chem. A 2017, 5, 22137.
(9) Hu, B.; DeBruler C., Rhodes, Z.; Liu, T. L.*, J. Am. Chem. Soc., 2017, 139, 1207.
(10) Filed US Patents covering applications of water soluble viologen, ferrocene derivatives, and TEMPO for RFBs.

Novel organosilicon nitrile (OSN) solvents have low flammability, broad electrochemical windows and excellent thermal stability, which make them promising materials for lithium-ion battery (LIB) electrolytes.¹ This presentation will explore the ion transport of a variety of electrolytes composed of the organosilyl nitrile (OSN) solvents which vary in the degree of fluorination of the solvent. Electrolytes of the organosilyl nitrile (OSN) solvents were prepared with the salts LiPF₆ or LiTFSI at concentrations ranging from 0.3 to 1.6 M salt and their VT ionic conductivities were measured by electrochemical impedance spectroscopy (EIS). We have also measured VT ⁷Li, ¹⁹F (anion) and ¹H (solvent) diffusion in PFG-STE NMR experiments which yield lithium and anion transference numbers. By comparing the molar conductivity from the impedance measurements with the molar conductivities based on NMR diffusion measurements, salt dissociation values for the electrolytes over the same temperature range were calculated. All electrolytes show VTF temperature dependencies in their ionic conductivities and reached the commercial viability benchmark of 1 mScm^-1 at 278 K. Increasing fluorination of the OSN solvents improves ionic conductivity, resulting from higher activation energies and higher values of salt dissociation. Electrolytes containing LiPF₆ demonstrated higher ionic conductivities and higher salt dissociation than the same solvents with the LiTFSI salt. Ionic conductivities of electrolytes at 25°C range from 1.5 to 3.2 mScm^-1 for LiPF₆ salt concentrations at 0.6 M or 0.7 M. Based on these promising ionic conductivities, we have prepared a variety of solvent blend electrolytes in which the OSN solvent is mixed with the conventional battery solvents, a cyclic carbonate ethylene carbonate (EC) and the linear carbonate diethyl carbonate (DEC) with the LiPF₆ salt concentration at 1.2 M. In general, ionic conductivities of the organosilyl nitrile (OSN) solvent/carbonate blend electrolytes increase in comparison to the analogous electrolyte of the pure organosilyl nitrile (OSN) solvent with the blends achieving ionic conductivities at 25°C of 5.5 to 6.3 mScm^-1 and salt dissociation values ranging from 0.42 to 0.45. In our NMR experiments, among the pure OSN electrolytes we found the mono-fluorinated solvent electrolytes have the highest ⁷Li and ¹⁹F diffusion coefficients. Lithium transference numbers in OSN electrolytes are higher than those in analogous cyclic carbonate electrolytes. In the organosilyl nitrile/carbonate blend electrolytes, lithium diffusion is fastest in the blend electrolytes followed by lithium diffusion in the all carbonate electrolyte which is again faster than the lithium diffusion in the pure silyl nitrile electrolyte. In the proton NMR studies of the blend electrolytes, the solvent DEC diffuses fastest while the EC and organosilyl solvents diffuse at the same rates suggesting that both of these solvents participate cooperatively in solvating the lithium cation. The lithium transference numbers for all OSN electrolytes with LiPF₆ and LiTFSI salts were in between 0.35 to 0.50, and the mono-fluorinated samples have the highest Li⁺ transference numbers, above 0.40 for all temperature ranges. From both the ionic conductivity and NMR diffusion measurements, we have calculated the extent of salt dissociation in these electrolytes. A general trend is that the degree of LiPF₆ and LiTFSI salt dissociation decreases with increasing temperature. Calculations of the effective lithium transference numbers to exclude the contributions of neutral ion pairs result in less ion pair formation occurring in the organosilyl nitrile/carbonate blend electrolytes than either the all carbonate electrolyte or the pure organosilyl nitrile electrolyte.
¹Guillot, S. L.; Peña-Hueso, A.; Usrey, M. L.; Hamers, R. J. Journal of The Electrochemical Society 2017, 164(9), A1907–A1917.

Redox flow batteries (RFBs) are one of the promising energy storages with a grid scale owing to their decoupled energy and power density, which lead to scalability and flexibility in design. To aim at higher energy density more than that of the current technology of vanadium RFBs, the redox-active molecules should have higher solubility and larger voltage gap than vanadium ions. The nonaqueous electrolyte media are also essential to offer wide electrochemical windows (> 2 V). Many redox-active organic materials have been studied for the role of posolyte in nonaqueous medium ^{[1],[2].[3],[4]} while only a few negolytes have been designed in this system^[5],[6],[7]. Herein, we present a new category of fused heterocyclic rings that can be employed as the negolyte. Our attempts to introduce various substituents to the core cyclic ring altered the electrochemical and chemical reversibility. The degree of molecular planarity and Lewis acidity/basicity of redox-active heterocyclic molecules critically determine the reversibility in cyclic voltammetry profile. The decorations of functionality also shift the redox potential and the number of electrons transferred. In addition, the asymmetric structure of heterocyclic rings may increase the solubility in the nonaqueous media. Through the fundamental studies on the design of heterocyclic rings and the corresponding electrochemical/chemical analyses, we demonstrated the optimum negolyte that can potentially provide high energy density and cyclic stability in nonaqueous RFBs. This study gives the guideline to develop new redox-active organic molecules for the application of RFBs.

References
[1] Hu, B.; DeBruler, C.; Rhodes, Z.; Liu, T. L. Long-Cycling Aqueous Organic Redox Flow Battery (AORFB) toward Sustainable and Safe Energy Storage. J. Am. Chem. Soc. 2017, 139 (3), 1207−1214.
[2] Huskinson, B.; Marshak, M. P.; Suh, C.; Er, S.; Gerhardt, M. R.; Galvin, C. J.; Chen, X.; Aspuru-Guzik, A.; Gordon, R. G.; Aziz, M. J. A metal-free organic-inorganic aqueous flow battery. Nature, 2014, 505 (7482), 195−198.
[3] Zhang, J.; Yang, Z.; Shkrob, I. A.; Assary, R. S.; Tung, S. o.; Silcox, B.; Duan, W.; Zhang, J.; Su, C. C.; Hu, B.; Pan, B.; Liao, C.; Zhang, Z.; Wang, W.; Curtiss, L. A.; Thompson, L. T.; Wei, X.; Zhang, L. Annulated Dialkoxybenzenes as Catholyte Materials for Nonaqueous Redox Flow Batteries: Achieving High Chemical Stability through Bicyclic Substitution. Adv. Energy Mater. 2017, 7, 1701272
[4] Liu, T.; Wei, X.; Nie, Z.; Sprenkle, V.; Wang, W. A Total Organic Aqueous Redox Flow Battery Employing a Low Cost and Sustainable Methyl Viologen Anolyte and 4-HO-TEMPO Catholyte. Adv. Energy Mater. 2016, 6 (3), 1501449
[5] Sevov, C. S.; Hickey, D. P.; Cook, M. E.; Robinson, S. G.; Barnett, S.; Minteer, S. D.; Sigman, M. S.; Sanford, M. S. Physical Organic Approach to Persistent, Cyclable, Low-Potential Electrolytes for Flow Battery Applications. J. Am. Chem. Soc. 2017, 139 (8), 2924−2927.
[6] Wei, X.; Xu, W.; Huang, J.; Zhang, L.; Walter, E.; Lawrence, C.; Vijayakumar, M.; Henderson, W. A.; Liu, T.; Cosimbescu, L.; Li, B.; Sprenkle, V.; Wang, W. Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All-Organic Redox Flow Battery. Angew. Chem., Int. Ed. 2015, 54 (30), 8684−8687.
[7] Duan, W.; Huang, J.; Kowalski, J. A.; Shkrob, I. A.; Vijayakumar, M.; Walter, E.; Pan, B.; Yang, Z.; Milshtein, J. D.; Li, B.; et al. Wine-Dark Sea” in an Organic Flow Battery: Storing Negative Charge in
2,1,3-Benzothiadiazole Radicals Leads to Improved Cyclability. ACS Energy Lett. 2017, 2, 1156−1161

The use of redox-active organic molecules (ROMs) as cathode materials holds a great potential in the sustainable energy storage. In particular, ROMs with high redox potential (> 3.5 V vs. Li/Li⁺) are one of the most promising alternatives to the metal oxide cathodes in the conventional secondary batteries. Recently, phenothiazine (PTZ)-based polymers showed excellent cycle stability and fast charge/discharge kinetics as cathode materials; however, their significantly low specific capacity due to low utilization of the redox-active centers is a critical drawback for practical use.
Here, we present phenoxazine (PXZ) as a new organic redox-active center for high voltage cathode materials in the secondary batteries. In this study, a series of organic molecules bearing multiple PXZ moieties are synthesized, and their electrochemical properties are thoroughly investigated by both experimental and computational methods. Among them, 1,3,5-tri(10H-phenoxazin-10-yl)benzene (3PXZ), in which three PXZs are covalently linked to a phenyl core, shows three reversible one-electron redox reactions to deliver a specific capacity of 121 mAh/g (94% of the theoretical specific capacity) with average discharge voltage of 3.7 V vs Li/Li⁺ in the coin cell. It should be noted that the 3PXZ electrode shows excellent cycle stability with a capacity retention of 81% after 500 cycles at a 1C rate, and even shows a high rate capability with a specific capacity of 128 mAh/g (99% of the theoretical specific capacity) with 72% retention after 500 cycles at a 5C rate.

Since the development of the transition metal oxide-based electrodes has been reaching its theoretical limits, organic electrode materials have attracted much attention as alternatives due to light weight, abundance, low cost, sustainability, and chemical diversity. Among others, cyclic imides, which consist of two acyl groups bound to a nitrogen atom, are one of the most promising redox centers for organic electrode materials due to their stable and reversible one-electron reduction. Although a lot of organic electrode materials bearing two cyclic aromatic imides such as perylene diimides, naphthalene diimides, and pyromellitic diimides showed superior cycle and rate performance, they can deliver rather low specific capacities. In fact, regarding their reversible two-electron reduction, the theoretical capacities of the diimide cores possessing no substituents at the two N-positions are up to 137, 201, 248 mAh/g for perylene, naphthalene, and pyromellitic diimide, respectively. Practically, small substituents are necessarily required to utilize them for electrode materials, which typically limits their practical capacity far below 200 mAh/g.
Here, we present a series of mellitic triimides which bear three cyclic imide groups fused to one phenyl ring with C₃-symmetry. Theoretically, the non-substituted mellitic triimide core is able to accept three electrons to deliver a specific capacity up to 282 mAh/g. As a model compound, ethyl-substituted mellitic triimide (ETI, theoretical specific capacity = 218 mAh/g) shows well distinguished and reversible three one-electron redox reactions at -0.97, -1.62, and -2.36 V vs. Ag/Ag+, respectively, in the cyclic voltammetry experiments. In the galvanostatic charge/discharge test, ETI electrode delivers a specific capacity of 225 mAh/g, one of the highest capacity among the imides electrodes, with three distinct plateaus at 2.82, 2.30, and 2.09 V vs. Li/Li⁺, respectively, in the coin cell.

Controlling interfacial electron-transfer rates is fundamental to maximizing device efficiencies in electrochemical technologies including redox-flow batteries, chemical sensors, bioelectronics, and photo-electrochemical devices. In many of these technologies, control of multi-electron transfer events with well-defined rates constants is required, but is often limited by materials choice. Electrodes comprised of organic semiconductor films offer the possibility to control redox properties independent of opto-electronic behaviors, making these electrodes idea for next-generation photoelectrodes. Yet to date, electron-transfer rates between conductive polymers and redox species remain slower than inorganic materials including metals and oxides. Most recently, we showed that the rate of electron transfer is predictable using a Marcus-Gerischer relationship, whereby the dominant factor controlling the rate of electron-transfer is the overlap in the density of states of the electrode with the density of states of the redox probe. With this new insight, we demonstrated that the microstructure of the polymer electrode becomes a critical component to controlling the symmetry of charge transfer reactions. In this work, we focus on nanoscale manipulation of electron transfer events, relative to mass transfer of the redox probe, to further control electrode performance. Specifically, we investigate the role of heterogeneity of conductive polymer electrodes, using a model system comprised of varying sized ultra-microelectrodes formed from blends of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA). We demonstrate that the macroscale rate of electron transfer can be further controlled beyond the density of states framework, offering a new paradigm to control multi-electron transfer events.

Redox flow batteries (RFBs) are a viable technology to store renewable energy in the form of electricity that can be supplied to electricity grids. Methyl viologen is a highly attractive anode material due to it high solubility in water, low cost, fast electrochemical kinetics, and excellent chemical stability under pH neutral conditions. Our group reported a 1.2 V methyl viologen (MV)/4-hydroxyl-TEMPO (4-HO-TEMPO) neutral aqueous organic redox flow battery (AORFB) and a high theoretical energy density FcNCl/MV AORFB. In this presentation, we introduce a highly stable 1.38 V viologen/TEMPO AORFB using 1,1’-bis[3-(trimethylammonio)-propyl]-4,4’-bipyridinium tetrachloride ((NPr)₂V) as an anolyte and N^Me-TEMPO as a catholyte. The (NPr)₂V/N^Me-TEMPO AORFB displayed outstanding cycling stability, 97.48% total capacity retention or 99.995% capacity retention per cycle for 500 cycles at 60mA/cm2 and a high power density of 128.2 mW/cm2 under pH neutral conditions, representing the most stable total AORFBs known to date. Compared to the (NPr)₂V/N^Me-TEMPO AORFB (97.48% total capacity or 99.995% capacity retention per cycle), the long-term cycling stability of the MV/N^Me-TEMPO AORFB (91.21% total capacity, which is equivalent to 99.982% capacity retention per cycle) is apparently inferior to that of the (NPr)₂V/N^Me-TEMPO. We believe that the more stable charged state, [(NPr)₂V]^+., than [MV]^+. contributes to the observed improved cycling stability of the (NPr)₂V/N^Me-TEMPO AORFB. To confirm this hypothesis, we using UV-vis absorption spectroscopy test the radical stability of [(NPr)₂V]^+. and [MV]^+. (0.1 mM in 2.0 M NaCl). Over 48 hour continuous measurements, the absorption of [MV]^+. showed a slow decay. For [(NPr)₂V]^+. , almost no absorption decrease was observed. This studies confirmed that the outstanding cycling stability of the (NPr)₂V/N^Me-TEMPO AORFB is attributed to the exceptional radical stability of [(NPr)₂V]^+.. The present study not only stresses the importance of the molecular engineering strategy to improve active materials’ stability but also advances the state of the art AORFBs for sustainable and green energy storage of renewable energy.
Reference
1. DeBruler, C.; Hu, B.; Moss, J.; Liu, X; Luo, J; Sun, Y.; Liu, T. L., Chem., 2017, 3, 1-978
2. Hu, B.; DeBruler, C.; Rhodes, Z.; Liu, T. L., J. Am. Chem. Soc., 2017, 139, 1207-1214.
3. Hu, B.; Tang, Y.; Luo, J.; Grove, G.; Guo, Y.; Liu, T. L., Chem. Commun., 2018, 54, 6871-6874.

High-performance lithium-ion batteries based on organic active materials currently constitute one of the most investigated energy sources due to their inherent advantages such as renewability, lightness, fast charge and discharge, and more environmentally friendly features. Among different active organic materials investigated so far, poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA), a polymer bearing persistent nitroxide radicals as repeating units, has become one of the most promising candidate as cathode polymeric material for lithium-ion batteries, which exhibits fast reversible redox reactions and allows high energy storage, high potentials and a long cycling life.^1,2 However, PTMA is easily dissolved into organic electrolytes and displays limited electrical conductivity, greatly hindering the performances of lithium ion batteries based on this polymer.³ Moreover a high mass loading of PTMA leads to the aggregation of active materials which cannot be fully utilized and hence in a decrease of the performance of the battery.⁴
Here, we report a facile and novel method for the fabrication of high performance free-standing cathodes using PTMA@CTAB nanospheres as active units and rGO@MWCNT as conductive framework. The introduction of PTMA nanostructures and their uniform distribution in the composite system shortens the ions pathways in the active material and allows almost 100% active substance utilization. At the same time, the rGO coating on each of PTMA nanosphere not only effectively increases its stability in the electrolyte (0.03% capacity loss per cycle), but also avoids the aggregation of PTMA and leads to a high effective mass loading of active material. Most importantly, the composite electrode shows as high as 105 mAh/g specific capacity based on the mass of the whole electrode in the voltage range from 2V to 4V, which is almost the highest reported value for PTMA cathode materials. The presented work successfully improves conductivity and stability of PTMA as cathode which can be also extended to various organic materials to obtain high performance free-standing electrodes.
1 B. Ernould, M. Devos, J.-P. Bourgeois, J. Rolland, A. Vlad and J.-F. Gohy, J. Mater. Chem. A, 2015, 3, 8832–8839.
2 F. Boujioui, O. Bertrand, B. Ernould, J. Brassinne, T. Janoschka, U. S. Schubert, A. Vlad and J.-F. Gohy, Polym. Chem., 2017, 8, 441-450.
3 H. Nishide and T. Suga, Electrochem. Soc. ‘Interface’, 2005, 14, 32–36.
4 A. Vlad, J. Rolland, G. Hauffman, B. Ernould and J. F. Gohy, ChemSusChem, 2015, 8, 1692–1696.

Electroactive polymers exhibit desirable properties for electrochemical storage, but their broader implementation is limited by the conventional methods by which they are prepared. Electropolymerization routes produce high-quality films, yet with thickness typically <1 µm. Solution-phase synthesis yields larger quantities of material, but in precipitate forms that can be difficult to process into high-performance electrodes. The Reynolds group at Georgia Tech has recently introduced a family of solution-processable electroactive polymers, where solubility characteristics are controlled by derivatizing ProDOT and EDOT with particular side chains [1,2]. The ability to dissolve/disperse such polymers at high concentrations (e.g., in polar organic solvents) facilitates their incorporation into macroscopically thick carbon nanotube/nanofiber papers that serve as a base electrode architecture for macroscale devices. By distributing the polymer as a thin coating on a 3D carbon-based current collector, the fast-switching characteristics inherent to these ProDOT and EDOT polymers is maintained, while the capacity per geometric footprint is amplified by factors of ten or greater compared to analogous planar thin films. The chemical structures of these polymers can be further tuned for reactivity in either aqueous or nonaqueous electrolytes, which expands the range of electrochemical device configurations that will benefit from these charge-storing materials. We perform electrochemical tests on polymer-modified carbon paper electrodes in three-electrode half-cells and two-terminal devices, assessing such critical performance metrics as rate-dependent capacitance/capacity, self-discharge, and cycle life.

1. J. F. Ponder Jr., A. M. Österholm, J. R. Reynolds, “Designing a Soluble PEDOT Analogue without Surfactants or Dispersants,” Macromolecules, 49 (2016) 2106.
2. A. M. Österholm,; J. F. Ponder Jr., J. A. Kerszulis, J. R. Reynolds, “Solution Processed PEDOT Analogues in Electrochemical Supercapacitors,” ACS Appl. Mater. Interfaces, 8 (2016) 13492.

In the recent decades, we are seeing the booming of renewable energy markets, and it consequently leads to the rise of different sorts of energy storage. The constant interest on emerging energy particularly promotes the development of electrochemical energy storage. Amongst numerous battery innovations, flexible and stretchable power sources have becoming a new hotspot because of the increasing requirements from soft and portable electronic equipment. However, in previous work, the flexible battery usually used sophisticated composite with carbon substrate or carbon conductive network embedded with active material, and the flexibility was mainly supplied by carbon and thus big amount of inactive carbon component was required to obtain the acceptable mechanical property and softness. Herein, the free-standing, really flexible and applicable electrode was fulfilled by utilizing the soft nature of polymer electrode material, and some commercialized plastics was first proposed as the membrane electrode. After the well-designed processing, plastic-carbon composite could be obtained, the mechanical property test indicated that its tensile strength was comparable to some nature or commercial textile, and in addition, the binder-free feature and plastic nature made it a real self-standing and highly flexible electrode with decent practical capacity. We further fabricated the counter membrane polymer electrode with similar method. All-organic, membrane battery was thus constructed. The battery well resolves the issues bothering the organic electrode material, such as, the rely on Li/Na metal electrode. In addition, the low-density shortcoming of organic material could be well avoided due to the special application in membrane battery. Generally, the soft nature of polymer material was fully utilized and plastic-based battery concept was realized in this work, the research gives some clues on the potential application of organic electrode material.

Magnesium batteries could offer high energy density without compromising safety due to the use of non-dendritic Mg metal anode. However, the charge-dense divalent Mg²⁺ also makes cation ingress into and diffusion within cathode materials kinetically sluggish. It is therefore intriguing that recently organic cathodes were reported to deliver high energy and power even at room temperature. Herein we reveal that previous organic cathodes likely all operate on an MgCl-storage chemistry sustained by a large amount of electrolyte that significantly reduces cell-level energy. We go on to demonstrate Mg batteries featuring an Mg²⁺-storage chemistry using quinone polymer cathode, Cl-free electrolytes, and Mg metal anode. Under lean electrolyte conditions, the organic cathode in cells on Mg-storage chemistry deliver the same energy while using ca. one tenth of the amount of electrolyte needed for the MgCl-based counterparts. With the right combination of organic cathodes and chloride-free electrolytes, the observed specific energy (up to 243 Wh kg^-1), power (up to 3.4 kW kg^-1), and cycling stability (up to 87%@2500 cycles) of Mg-storage cells consolidate organic polymers as promising cathode candidates for high-energy Mg batteries.

In recent years, development of a high performance secondary battery is demanded. In particular, it is desired to develop a battery using a material that is free from bias in resources and that can be stably supplied. Lithium, which is a charge carrier, is also a minor element, so replacing Li-ion with another metal ion such as Na-ion or K-ion is being studied. However, it is difficult to utilize these elements as an electrode active material because of the larger ion size than Li⁺. The ion radius of Na⁺ and K⁺ are about 1.3 times and 1.8 times the ion radius of Li⁺ respectively. Since organic molecules form crystals by weak interaction such as van der Waals force, it is more flexible than general inorganic materials with strong atomic bonding.
The theoretical capacity of benzoquinone (BQ) derivatives is high. However, the redox potential of BQ derivatives is lower than those of common inorganic cathode materials. The BQ derivatives are an electron acceptor whose redox potential is in the range of around 2-3 V vs. Li⁺/Li. On the other hand, the tetrathiafulvalene (TTF), which is an electron donor, exhibits a high redox potential (the range of around 3-4 V vs. Li⁺/Li), but its capacity is lower than that of the electron acceptor. Therefore, in order to achieve both high capacity and high discharge potential, we focused on molecules combining BQ units with TTF units.
A fused triad molecule incorporating N-type benzoquinone (BQ) and P-type tetrathiofulvalene (TTF), Q-TTF-Q, which is known to accept both cations and anions during the redox process in the solution state and has higher electrical conductivity than common organic compounds. The Q-TTF-Q is a 6 electrons redox system (Theoretical capacity: 441 mAh/g). We present Q-TTF-Q as positive electrode active material. In addition, those performances of a battery using lithium-ion, sodium-ion or potassium-ion as the charge carriers were evaluated. In the K-system, it is difficult to reversibly preserve K⁺ with the oxide-based inorganic compounds in conventional organic solvent systems, but the organic compound Q-TTF-Q reversibly preserves K⁺.

Organic electrode materials (OEMs) for lithium-ion batteries (LIBs) constitute a very promising alternative to standard electrodes materials that are prepared from finite and non-renewable minerals resources, since they instead are being potentially environmental friendly, cheap, and abundant if derived from biomass via ecofriendly processes. However, their commercial use is currently held back, primarily due to their poor energy density. High specific capacity OEMs are therefore of uttermost interest. During the last 5 years, OEMs with the ability of an unexpected reversible reduction of carbon-carbon double bonds have sporadically been reported [1]. As a consequence of this redox process – coined ‘superlithiation’ – specific energies several times higher than commercial standards (graphite) and Li/C ratios of 1/1 have been reported.
In a previous work on the ‘superlithiation’ of dilithium benzenedipropiolate [2], we reported that this material can reversibly reduce its unsaturated carbon-carbon bonds in addition to the expected reduction of its carbonyls, leading to a Li/C ratio of 1/1 and specific capacity as high as 1363 mAh g^-1: the highest ever reported for a lithium carboxylate. However, the stability of this redox behavior was poor and obvious capacity fading was observed after a few cycles. We here show that better capacity and stability can be achieved with appropriate electrode formulation and optimization of parameters such as calendaring or temperature/electrolyte match (liquid or industrial quality polymer electrolytes).
[1] a) X. Han, G. Qing, J. Sun, T. Sun, Angew. Chem. Int. Ed., 2012, 51 5147. b) W. Luo, M. Allen, V. Raju, X. Ji, Adv. Energy Mater., 2014, 4 1400554.
[2] S. Renault, V.A. Oltean, C.M. Araujo, A. Grigoriev, K. Edström, D. Brandell, Chem. Mater., 2016, 28, 1920.

Discussion of application of redox active polymers will be focused polyanthraquinones. The polymer was successfully implemented in two forms of batteries: multivalent ion batteries and lithium sulfur batteries. In typical multivalent ion batteries, for example, magnesium ion batteries, the mobile carriers are multivalent ions (Mg²⁺) and the anode is magnesium metal. By utilization of a high voltage magnesium ion electrolyte, the idea of multivalent ion batteries based on a prototype consisting of a polyanthraquinone cathode and a magnesium anode was realized. Further discussion will be focused on the functionality of the electrolyte, the performance and mechanistic insight of the conversion reaction.
In the lithium sulfur batteries, the polymer was functioning as a redox active binder to “electrocatalyze” the process of polysulfide to Li₂S and reduce the potential dissolution, diffusion, decomposition of the polysulfide. The electrocatalytic effect was provided with evidence for the first time: there is incontrovertible evidence that the polymer can be reduced by presence of polysulfide. Further performance improvements in the Li-S batteries will be discussed.

Polymeric salen-type metal complexes, often denoted as poly[M(Schiff)] (M = transition metals), is one of promising electronically conducting polymers. Among the poly[M(Schiff)] family, poly[Ni(CH3-Salen)] has been most intensively investigated due to its promising electrochemical properties for energy storage devices such as super-capacitor and lithium-ion (Li-ion) battery. The poly[Ni(CH3-Salen)]have been proven to have similar performance to other conducting polymers in terms of conductivity, while often surpassing them in terms of stability, capacity, and charge transfer parameters. However, due to its low practical specific capacity (c.a., ~ 50 mAh/g), the poly[Ni(CH3-Salen)] would not be considered as an alternative to conventional cathode materials for Li-ion batteries.

In this presentation, we first report unique positive multifunction of poly[Ni(CH3-Salen)] as an additive for conventional cathodes in lithium-ion (Li-ion) batteries. Our systematic physical and electrochemical characterizations revealed that the poly[Ni(CH3-Salen)] can eliminate conductive carbon and replace significant portion of polyvinylidene fluoride (PVdF) binder for cathodes. For example, by replacing such electrochemically inactive components (i.e., carbon and PVdF), LiFePO₄ cathodes with poly[Ni(CH3-Salen)] deliver improved energy density compared with the conventional LiFePO₄ cathode. Also, the redox-active poly[Ni(CH3-Salen)] itself provides extra capacity in electrode-level.

In addition, the poly[Ni(CH3-Salen)] forms three-dimensional (3D) web-like electron-network structure surrounding the LiFePO₄ cathode particles, which can significantly accelerate charge-transfer reaction as evidenced by electrochemical impedance spectroscopy (EIS) and rate-capability test of the cathodes. Moreover, unlike PVdF, poly[Ni(CH3-Salen)] retains steady Young’s modulus values after immersing in an electrolyte solvent, measured by in-situ atomic force microscopy (AFM). This result suggests a superior mechanical stability of poly[Ni(CH3-Salen)] to PVdF that would affects long-term cycleability of battery cells. The unique multifunction of the poly[Ni(CH3-Salen)] will be of broad interest for its application in next-generation energy storage devices.

One of the most ambitious challenges for the planet for the next future years is to reverse the worldwide CO2-related global warming and decrease the pollution of large modern cities induced by the burning of fossil fuels. Accordingly, the full or partial replacement of inner combustion engines by electric motors and the use of alternative green energy sources, such as solar or wind or geothermal for powering the stationary applications appear as necessary to reach these objectives, in conditions that energy storage systems could ensure a/o regulate the energy production.¹
Henceforth, batteries and more especially rechargeable lithium batteries really represent a viable energy storage technology for supplying low-emission plug-in hybrid electric vehicles (HEVs), even for emission-free pure electric vehicles (EVs) or for the integration of renewable intermittent energy sources. Indeed, lithium based batteries exhibit high specific energy (high voltage joined with high specific capacity), long cyclelife, high efficiency, high charge/discharge rate capability and low self-discharge. In addition, it is a versatile technology easily adaptable to the application by playing with the electrode materials and electrolyte composition.^2-3
However, most commercial batteries widely use liquid electrolyte composed of organic solvents containing salt and impregnating a separator. This implies to define inevitably safety measures to prevent any risks of electrolyte leakage and/or violent reactions as fire a/o explosion in case of inner short-circuit.⁴ It is the reason why CEA-Liten and Solvay has engaged a large R&D program to propose a new safer and durable lithium battery system constituted of a hybrid polymer membrane and two gelled electrodes.
The gelled character of the electrodes, i.e. the retention of the electrolyte in the electrode microstructure is favored by the use of a Solvay proprietary poly(vinylidene fluoride) based polymer as binder, capable to trap a lithium ion conducting organic solution. Specifically designed it ensures high adhesion, it allows obtaining adhesive microporous electrodes to metallic (Al or Cu based) current collector with porosity fully filled by carbonate based electrolyte.
The method for preparing such gelled electrodes was developed in acetone media, according to a three-step procedure similar to classical electrodes of Li-ion cell. Thus, after dispersion of the powdered compounds in acetone in presence of the electrolyte, the electrode ink is coated, dried and densified by calendering in air under appropriate conditions. Prepared with ethylene carbonate / Propylene Carbonate (EC:PC 1:1) solution containing bis(trifluoromethanesulfonyl)imide lithium salt, LiTFSI (1M) or lithium hexafluorophosphate, LiPF₆ (1M) at the labscale but also in continuous mode, the gelled graphite (Cgr) based or LiNi_0.33Mn_0.33Co_0.330₂ (NMC) based electrodes exhibit regular surface, homogeneous thickness with low roughness and good adhesion. In addition, its properties of flexibility and cohesion allow its implementation in wound roll of several meters.
Moreover, assembled with hybrid polymer membrane without any addition of complementary electrolyte, the gelled electrodes work successfully at various discharge rates, even at high rates and exhibit a very stable behavior with very long cyclelife.



References
¹ N.L. Panwar, S.C. Kaushik, Surendra Kothari, ”Role of renewable energy sources in environmental protection: A review”, Renewable and Sustainable Energy Reviews, Vol. 15, Issue 3, (2011), 1513-1524
² M. Armand, J-M. Tarascon, Building better batteries, Nature 451 (2008) 652-657
³ S. Fletcher, Bottle Lightning: superbatteries, electric cars and the new lithium economy, Hill and Wang, New York, 2011, B. Scrosati, Nature 473 (2011) 448-449
⁴ A. Manthiram, X. Yu, S. Wang, “Lithium battery chemistries enabled by solid-state electrolytes”, Nature Reviews Materials vol 2, (2017) Article number: 16103

Low-Cost and ion-selective membranes are required to meet the growing demands for peak performance by next-generation batteries for EVs, aviation, and the grid. To that end, I will showcase a highly disruptive membrane platform based on polymers of intrinsic microporosity, whose pores are on the length scale of solvated ions and small molecules that are sometimes used as the battery’s active materials (e.g., as in flow batteries). The design space for these polymer membranes is advanced using a variety of computational tools, including computational materials genomics. The foundational knowledge built using these tools provides a roadmap for the diversity-oriented synthetic development of polymer membranes with specific pore architectures and chemistry, specifically tailored for the battery’s chemistry. I will also outline foundations on which to build adaptive membranes, where judiciously placed molecular switches allow for the membrane’s transport properties to be modulated in-situ in response to excursions that are otherwise detrimental to the battery’s cycle-life. There remains much to be learned about the origins of their adaptive and dynamic properties, and how these feed back across multiple length and timescales in the electrochemical cell.

Block copolyelectrolytes are solid-state single-ion conductors which phase separate into ubiquitous microdomains to enable both high ion transference number and structural integrity. Ion transport in these charged block copolymers highly depends on the nanoscale microdomain morphology; however, the influence of electrostatic interactions on morphology and ion diffusion pathways in block copolyelectrolytes remains an obscure feature. In this paper, we systematically predict the phase diagram and morphology of diblock copolyelectrolytes using a modified dissipative particle dynamics simulation framework, considering both explicit electrostatic interactions and ion diffusion dynamics. Various experimentally controllable conditions are considered here, including block volume fraction, Flory–Huggins parameter, block charge fraction or ion concentration, and dielectric constant. Boundaries for microphase transitions are identified based on the computed structure factors, mimicking small-angle X-ray scattering patterns. Furthermore, we develop a novel “diffusivity tensor” approach to predict the degree of anisotropy in ion diffusivity along the principal microdomain orientations, which leads to high-throughput mapping of phase-dependent ion transport properties. Inclusion of ions leads to a significant leftward and upward shift of the phase diagram due to ion-induced excluded volume, increased entropy of mixing, and reduced interfacial tension between dissimilar blocks. Interestingly, we discover that the inverse topology gyroid and cylindrical phases are ideal candidates for solid-state electrolytes in metal-ion batteries. These inverse phases exhibit an optimal combination of high ion conductivity, well-percolated diffusion pathways, and mechanical robustness. Finally, we find that higher dielectric constants can lead to higher ion diffusivity by reducing electrostatic cohesions between the charged block and counterions to facilitate ion diffusion across block microdomain interfaces. This work significantly expands the design space for emerging block copolyelectrolytes and motivates future efforts to explore inverse phases to avoid engineering hurdles of aligning microdomains or removing grain boundaries.

Redox flow batteries (RFBs) offer a scalable solution to grid scale energy storage. The polymer membrane in the RFB that separates the anolyte and catholyte is often underappreciated, preventing mixing of anolyte and catholyte while enabling counter-ion transport. During typical RFB operation, many membranes allow not only inert counter-ions, but also redox-active species to be transported through them. This unwanted transport of redox-active species, called crossover, effectively robs the battery of capacity and decreases performance. Understanding the fundamental mechanisms by which redox-active species are transported through the membrane is essential to designing next generation membrane materials. Here we discuss our recent progress evaluating the performance of several commercial and custom-made membranes using model aqueous and non-aqueous RFB chemistries. We demonstrate how fundamental membrane materials properties can be related to overall RFB performance. Trade-offs in membrane design (e.g. high conductivity vs. high diffusion) are shown to influence crossover mechanisms and overall RFB performance, offering insight to improving the performance of RFBs for grid scale energy storage.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Hydrogels have become an appealing material platform for energy storage technologies. Developing hydrogels with enhanced physicochemical properties, such as mechanical strength, flexibility, dimensional stability, and fast charge transport, offers new opportunities for significantly improving the performance of energy storage devices. In this talk, I will discuss recent innovations in rational design and synthesis of hydrogels with diverse features, including stimuli-responsive, self-healing, and highly stretchable for next-generation electrochemical devices. I will then focus on how hydrogels have been integrated into energy storage systems, highlighting exciting examples that demonstrate the versatility of hydrogel materials in terms of tailorable architectures, conductive nanostructures, 3D frameworks and multi-functionalities.

The key to enabling high-energy-density lithium (Li) metal batteries is the development of electrolyte for resolving Li metal challenges, in particular, the dendritic growth and low coulombic efficiency. The low-temperature operation is a large challenge for lithium-ion batteries, resulting from the sluggish diffusion, including the reduced electrolytic conductivity by electrolyte freezing, largely increased SEI resistances and the limited Li-ion diffusivity in the electrodes. However, there has been little change of electrolyte chemistry since their commercialization. These liquid electrolytes often limit the energy density and low-temperature operation of these devices, which hinder many potential applications. Our work uses electrolytes based on solvent systems which are typically gaseous under standard conditions and show excellent performance in electrochemical energy storage devices. Systems using fluoromethane -based liquefied gas electrolytes demonstrate remarkable rate capability (up to 10 mA cm^-2) and excellent dendrite-free long-term (>500 cycles) cycling of Li metal anodes with a high coulombic efficiency of up to 99.9%. Both the rate and cycling performance are well maintained at a low temperature down to -60 °C. The superior performance is ascribed to the formation of a compacted Li metal deposition microstructure and a conductive, stable solid electrolyte interphase (SEI). This study opens up a promising avenue toward the applications of high-energy-density Li metal batteries.

A series of polybenzimidazole (PBI) membranes were synthesized using sol-gel process and modified for all vanadium redox flow batteries (VRFB). These membranes demonstrated not only a high in-situ and ex-situ stability in concentrated sulfuric acid and in oxidative vanadium (V) solutions but also exhibited high conductivities and low performance degradation during in-cell testing comparing to the ‘conventionally imbibed’ meta-polybenzimidazole (m-PBI) membranes cast from N,N’-dimethylacetamide (DMAc) solutions. The PBI membrane doped with 2.6 M H₂SO₄ shows a proton conductivity of 390 mS/cm at room temperature and VO²⁺ permeability as 2.30E-08 cm²/s. Different strategies were anticipated to block the porous structure of PBI membranes and further decrease vanadium ion permeability while maintaining the high proton conductivity. A VRFB operated with the modified PBI membrane shows over 98% coulombic efficiency under a large range of applied cell cycling current densities from 100 to 450 mA/cm².

The state-of-the-art lithium (Li)-ion batteries are approaching their energy density limit and cannot meet the fast increasing demand for higher energy density requirements. Due to the ultrahigh theoretical specific capacity (3,860 mAh g^-1) and the extremely low standard electrochemical redox potential (-3.040 V) of Li metal, it has long been regarded as an ideal anode material. The research and development of rechargeable Li metal batteries has therefore been revived in recent years. However, the big challenges related to Li metal anode, such as the safety issues due to the formation of metallic Li dendrites during repeated charge/discharge cycles and the short cycle life because of low Li Coulombic efficiency (CE), have not been completely solved and there is still a long way to go for the commercialization of rechargeable Li metal batteries. Many approaches have been reported to address the above challenges to Li metal anode and Li metal batteries, and many achievements have been reported recently. Among the reported strategies, the electrolyte design is one of the most effective and facile approaches to improve Li CE and cycling stability. Organic ether solvents have been found to be more stable with Li metal than other organic solvents, but they suffer low oxidation stability so the conventional ether-based electrolytes cannot be charged above 4 V vs. Li/Li⁺ so they are normally used in low voltage battery systems. In this work, we have re-investigated the ether-based electrolytes and achieved excellent stability and performance of ether electrolytes in Li metal protection and on high voltage (up to 4.3 V) Li metal batteries. The details will be reported at the presentation.

Electrodeposition plays an important role in electrochemical storage technologies based on batteries. It should be carefully managed to facilitate safe and stable operations in a broad range of conditions. In liquid electrolyte, deposition is subject to a variety of hydrodynamic and morphological instabilities, leading to a strong convective process near the interfaces. In many situations, the instabilities are problematic as the produced complex flows cause dendrite formation at planar interfaces, which is linked to the failure of microcircuits and overlimiting conductance.

This study focuses on the effects of aqueous viscoelastic electrolyte and the buoyancy force on the electrodeposition instabilities. By introducing a small amount of the polymers, the viscosity of the electrolyte largely increases without compromising ionic mobility. As a result, the voltage window of limiting current regime increases in the viscoelastic aqueous electrolyte. In the gravitationally unstable configuration, the variation of salt density leads to a Rayleigh-Bernard flow that increases the current. In this case, the influence of individual factors of the buoyancy effect is studied in the viscoelastic electrolyte system. And the qualitative and quantitative comparison with theoretical and simulation studies are performed to help clarify the mechanisms.

Organic electrolyte is the key part in battery, but for various types of battery the requirements are different. An intercalation battery may prefer the formation of protective but ionic conducting solid-electrolyte interphase that is usually decomposed from the organic electrolytes, while the sulfur-based batteries need the electrolytes to suppress the formation of polysulfides. How to develop suitable electrolytes still remain technically challenging. In my group, we take the in situ operando view to check the performance of electrolytes and electrodes using X-ray absorption spectroscopy at both hard and soft X-ray regions. These element-specific methods enable us to exam the light elements in electrolytes and heavy elements in electrodes for comprehensive understanding. I will illustrate examples in lithium-ion batteries and lithium/magnesium-sulfur batteries to demonstrate how electrolyte can promote/inhibit the reactions.

Next generation electrochemical energy storage systems based on divalent metal cations occupy a new frontier of research and development efforts related to materials chemistry and design. High energy density batteries based on Mg or Ca metal anodes are yet to be realized due to problems associated with the stability and ion transport properties of organic electrolytes and insertion cathodes. In this presentation we will discuss results related to controlling Mg²⁺ electrolyte properties via tuning of the organic solvent structure. While the role of competitive solvent and anion interactions in defining the properties of univalent organic electrolyte systems has received considerable investigation, such knowledge is still emergent in the divalent electrolyte field. This presentation will serve as a discussion of our recent findings on how the ethereal solvent or co-solvent (e.g. glymes, cyclic ethers) structure defines stable Mg²⁺:anion solvation populations, which in turn determine critical electrolyte performance attributes including ionic conductivity and Mg metal plating efficiency. Specifically, we will present electrochemical characterization of several inorganic and organic Mg-salts including MgCl₂, MgTFSI₂, Mg(CB₁₁H₁₂)₂, and Mg(B(OR^F)₄)₂ across a systematic variation of ethereal solvents along with characterization of the resulting solvation environments through solution spectroscopies including Raman, NMR, and X-ray absorption spectroscopy (XAS). These experimental results are further augmented with the aid of density functional theory and molecular dynamics simulations, providing insight into how the solution and interfacial chemistry regulates the behavior of Mg²⁺ electrolyte systems.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Lithium-ion batteries (LIBs) have been predominant in electric vehicles and electronic devices applications because of their unparalleled combination of energy density and reasonable cost. Stabilizing the cathode/electrolyte interface at high voltage is necessary to achieve higher capacities while still maintaining capacity retention. However, the traditional organic carbonate-based electrolytes are not stable at these high voltages, new strategies must be adopted to stabilize this interface. One such strategy is through the use of additives in the electrolyte: components in low concentration (<10%) that have a lower anodic stability than the baseline electrolyte, so that during the initial cycles, the additive will decompose on the charged cathode surface preferentially over the baseline electrolyte. This reaction will then yield a layer which will inhibit further reaction between the electrolyte and the cathode surface, i.e. cathode passivation. While many of these additives tested in the literature do show improvements in cycling performance, the mechanism of improvement remains unclear. In the present work, Density Functional Theory is used to gain insights and understanding on experimental results using a potentiostatic hold technique to evaluate cathode/electrolyte reactivity for two families of additives: phosphites and phosphates. Simulations indicate the susceptibility of the various additives to electrochemical and chemical oxidation, showing chemical oxidation to be much more likely with the phosphite moiety. The identity of the ligands on the phosphorus-containing additive can dramatically affect both the decomposition current and the cathode surface film.

We report on a novel approach thruogh which metal-organic framework constructed on graphene platelets is converted, in situ, into highly active and durable water splitting electrocatalyst. This approach to construct highly active and durable Ni(OH)₂ nanoparticle/graphene hybrid electrocatalyst utilized the Ni-loaded, graphene-supported metal-organic framework (UiO-66-NH₂-Ni@G) as a sacrificial pre-catalyst to generate the true catalyst, in situ, under the electrolysis conditions. The resulted nanocatalyst was shown to enclose Ni(OH)2 nanoparticles imbeded within hydrous zirconia and deposited on top of G platelets, demonstrating a high electrochemical activity towards oxygen-evolution reaction (η₁₀ = 0.38 V vs. RHE) and highly durable catalyst. This strategy can potentially be extended to several other systems as a less energy-demanding alternative to the commonly utilized pyrolysis pathway to generate electrocatalysts from MOFs.

The development of new rechargeable battery systems employing novel chemistries is imperative to meet the increasing energy storage demands of emerging technologies. Metal-sulfur batteries are a promising set of chemistries for moving beyond lithium-ion, owing to the widespread abundance of sulfur and the high theoretical energy density of many metal-sulfur couples. However, it is well known that metal-sulfur systems suffer poor performance as a result of the polysulfide shuttle effect. Building on previous work demonstrating the delicate balance of ion transport in a functional interlayer¹, i.e. allowing cation transport while minimizing polysulfude transport, thin single-ion conducting polymer coatings are investigated for their ability to mitigate the polysulfide shuttle effect in lithium- and magnesium-sulfur cells. Relative to a bulk polymer layer, a thin coating contributes significantly less to the total cation conduction resistance observed in the cell. The decreased resistance stemming from a thin layer design enables exploration of a polymer composition space that favors polysulfide restriction at the expense of cationic conduction, such as the use of highly crosslinked ionomer networks in which the primary mechanism of cation conduction is solid state. Differences in solid state Li⁺ and Mg²⁺ conduction present the need for polymer compositions tailored specifically to the cation identity. Relationships between cell performance and a given ionomer chemical composition, structure, and transport mechanisms are determined with a combination of ex-situ characterization techniques and full cell cycling.

Hunter O. Ford, Laura C. Merrill, Peng He, Sunil P. Upadhyay, and Jennifer L. Schaefer
Cross-Linked Ionomer Gel Separators for Polysulfide Shuttle Mitigation in Magnesium–Sulfur Batteries: Elucidation of Structure–Property Relationships
Macromolecules Article ASAP

Nanomaterials can offer large surface area, facile strain relaxation upon cycling and efficient electron transport pathway to achieve high electrochemical performance. Organic compounds have achieved immense priority over inorganic materials for their manageable structures, suitable operating voltage and abundance as well as renewability. We have synthesized graphene oxide-wrapped organic dipotassium terephthalate (K₂TP@GO) hollow microrods using an abundant and renewable organic resource. During the potassiation process, the crystalline phase of K₂TP gradually transformed into amorphous K₄TP. They demonstrate enhanced potassium storage performance compared to bulk K₂TP, which mainly ascribed to the fast K⁺ ion transfer kinetics, high electronic conductivity and short diffusion distance. In addition, we have also developed a facile and high-yield strategy for the oriented formation of CNTs from metal-organic frameworks (MOFs). The appropriate graphitic N doping and the confined metal nanoparticles in CNTs both increase the densities of states near the Fermi level and reduce the work function, hence efficiently enhancing its oxygen reduction activity. Moreover, we also obtained thiocyanato-functionalized SiO₂ (SCN-SiO₂) microspheres through the hydrolysis and condensation of organic silicon source (3-thiocyanatopropyltriethoxysilane). A vinyl-functionalized SiO₂ (CH₂CH-SiO₂) layer and a thiol-functionalized SiO₂ (SH-SiO₂) layer was also coated onto the surface of SCN-SiO₂ in sequence. The rationally designed yolk@shell structured SiO_x/C composite anode materials manifest high specific capacity, excellent rate capability and superior cycling stability in lithium storage. Our work presented here can inspire new thought in constructing novel organic compound structures and accelerate the development of energy storage applications.

High-energy-density lithium metal batteries have aroused wide attention due to the high theoretical capacity. However, uncontrolled dendrite formation and infinite volume expansion hinder the practical application of lithium metal anode. Herein, we develop a new strategy to make adsorption induction and ionic intervention well-combined to modulate the Li deposition behavior. A K⁺-doped Nafion nanofiber separator, having abundant lithophilic groups and porous framework is electrospun. It can provide high affinity with electrolyte and large electrolyte uptake. It also provides uniform pathway for lithium ions transportation, and a parclose for the stress of volumetric change during lithium plating/stripping. Moreover, it can release K⁺ as an inhibitor to prevent aggregation of Li ions by forming an electrostatic shielding on the surface of electrode. As a result, the functional separator can guide uniform lithium deposition, leading to high Coulombic efficiency (98.5%) and low overpotential (~15 mV) in symmetric cells with long lifespan. This work shed the dawn of the research on the multi-functional electrospun separators for dendrite-free lithium metal batteries.

In order to realize a perfect solid polymer electrolyte (SPE) concept for lithium batteries, there is a need for lithium new salts. The new salts must be cheap and stable and able to provide a large number of charge carriers to meet the demands on ion conductivity. In polymer electrolytes, the salt concentration is usually high, thus an important factor affecting the ion conductivity is the equilibrium between “free” anions, ion pairs, and higher aggregates, which tend to form in concentrated electrolytes. Weakly cation coordinating anions, such as the triflate, perchlorate or the “imide”/TFSI anions are therefore good candidates for SPE lithium salts. The chemistry of new salts employed in SPEs, today, is in many cases based on the electron withdrawing power of fluorine atoms. Lithium salts with organic anions, heterocyclic aromatic groups are considered as candidates of choice for this application because they are expected to combine good ionic conductivity and wide electrochemical stability. Especially, those organic salts have electrochemical stability toward Li metal anode and have a good efficiency. A high value of the lithium ion transference number is also desirable. Much less is published on the corresponding triazolate anion; Pi stacking and association of the flat aromatic molecules are probably responsible for the depressed mobility of the anion, thus accounting for the enhancement in the cation transference number.

Here, we demonstrated the structure-related energy storage performance of covalent organic nanosheets (CONs) synthesized by Stille cross-coupling under conventional reflux and solvothermal conditions, displaying that the specific surface area and self-assembled morphology of the nanosheets can be effective regulated by a deliberate choice of the synthetic method and monomer combination. The Sodium-ion storage capacity in the mentioned above CONs could be increased by elevating their conductivity of charge carriers via enforcement of a network polymer backbone planarity or by enhancing their specific surface area while maintaining polymer structure constitution. Comparing the anodes manufactured by combining each synthesized CONs, the electrodes based on CON-16 showed the highest cycling performance and rate capability, maintainig a reversible discharge capacity of up to 250mAh/g after 30 cycles at a current density of 100 mA/g.

Dissolution of active material is one of the primary reasons for capacity fade in lithium-ion batteries, particularly at elevated temperatures. Ultra-thin coated cathode particles via Atomic Layer Deposition (ALD) exhibit superior battery performance over bare particles. However, we have observed a coating layer can decrease or accelerate the metal dissolution depending on the coating materials. For instance, an ultra-thin CeO₂ coating intensifies Mn dissolution of lithium manganese oxide cathode material during cycling of battery, whereas ultra-thin Al2O3 coating tends to inhibit Mn dissolution. A detailed study of Density Functional Theory (DFT) has been carried out to illustrate the experimental observations. First, the manganese vacancy formation energy is calculated, along with the bonding strengths of Mn-O of uncoated, Al₂O₃ coated, and CeO₂ coated particles via Crystal Orbital Overlap Population (COOP) calculations. Further, the projected density of state calculation of Mn is used to confirm the electronic occupancy of Mn atom for each case. Surprisingly, the atomic analysis is consistent with the experimental observations. This finding can provide new insights into ALD coatings and their impact on metal dissolution in cathode materials.