Symposium S.EN08—Multivalent-Based Electrochemical Energy Storage

Mg is an earth abundant and low-cost metal (ca. 24 times cheaper than Li), and as an anode material, Mg is safe to use (vs. Li, Li-ion, or Na-ion batteries). Its high gravimetric capacity (2,333 Ah/kg) and high reduction potential (-2.37 V vs SHE) allow the assembly of high-energy density batteries. However, despite rapid research progress, the lack of high performance Mg²⁺ conductive electrolytes still presents a primary technical hurdle for developing practical Mg batteries. The presentation will cover our research efforts in developing high performance Mg electrolytes for Mg rechargeable batteries. Specifically, we will present the synthesis and electrochemical performance of ternary Mg/MgCl₂/AlCl₃ (MMAC) electrolytes and a novel Cl-free perfluorinated pinacolatoborate Mg electrolyte (Mg-FPB). These MMAC and Mg-FPB electrolytes exhibited up to 100% Coulumbic efficiency, and 164 mV overpotenail for Mg deposition, and 4.5 V vs Mg anodic stability. Solution and interfacial chemistry of the presented Mg electrolytes will be discussed in detail.

The stagnation in the development of Li ion batteries based on liquid electrolytes and resource issues associated with the current battery configuration have fueled activities which try to solve or circumvent these issues. The aim is to both increase the energy content of battery cells and to make them ready for future application. This requires a set of materials which do not only have exceptional standalone properties but can mutually interact in a harmonic manner. Multivalent batteries promise to have superior properties in terms of volumetric and gravimetric capacities, however suffer from major roadblocks such as low ionic conductivity in the solid, reversible and efficient transport in the liquid and reversibility of insertion/conversion reactions.
One concept to tap the potential of multivalent systems is using conversion electrodes, e.g. with sulfur/carbon composites as cathode. The contribution will comment on the actual state of development of sulfur-compatible electrolytes with high efficiencies, high ionic conductivities, wide electrochemical stability window, easy synthesis and excellent compatibility with electrode materials and other cell components. Recent results in the development of Mg- and Ca-sulfur batteries will be presented and the remaining challenges will be discussed. Furthermore, a new cathode material with a high capacity for Mg and a mixed storage mechanism will be presented.

Magnesium is a promising element for future batteries due to high gravimetric (2206 mAh/g) and volumetric capacity (3834 mAh/cm3) of Mg anode. It is considered as geopolitically neutral, sustainable and abundant metal, which can be used in batteries due to non-dendritic deposition during battery operation. Besides discussed benefits, there are several challenges connected with the use of Mg metal, mainly incompatibility of the Mg metal with the electrolytes and difficult insertion of magnesium into oxide based solid state host matrix. Alternative solutions can be use of redox active polymers with different active redox groups.
In this presentation, overview of recent achievements in our group on the field of magnesium batteries will be discussed with a focus on proper selection of battery components in order to achieve electrochemical properties attractive for the commercialization.

Acknowledgement: This work is supported HONDA R&D Europe and by the Slovenian Research Agency (research core funding No. P2-0393 and research project J2-8167).

References:
1) J. Bitenc, K. Pirnat, T. Bancic, M. Gaberšček, B. Genorio, A. Randon-Vitanova and R. Dominko, Anthraquinone-Based Polymer as Cathode in Rechargeable Magnesium Batteries, ChemSusChem, 2015, 8, 4128–4132.
2) A. Vizintin, J. Bitenc, A. Kopač Lautar, K. Pirnat, J. Grdadolnik, J. Stare, A. Radon-Vitanova and R. Dominko, Probing electrochemical reactions in organic cathode materials via in operando infrared spectroscopy, Nat. Commun., 2018, 9, 661.
3) J. Bitenc, K. Pirnat, G. Mali, B. Novosel, A. Randon Vitanova and R. Dominko, Poly(hydroquinoyl-benzoquinonyl sulfide) as an active material in Mg and Li organic batteries, Electrochem. commun., 2016, 69, 1–5.
4) T. Bancic, J. Bitenc, K. Pirnat, A. Kopac Lautar, J. Grdadolnik, A. Randon Vitanova and R. Dominko, Electrochemical performance and redox mechanism of naphthalene-hydrazine diimide polymer as a cathode in magnesium battery, J. Power Sources, 2018, 395, 25–30.
5) Jan Bitenc, Alen Vizintin, Joze Grdadolnik, Robert Dominko, Tracking electrochemical reactions inside organic electrodes by operando IR spectroscopy Energy Storage Materials 21 (2019) 347–353
6) A. Robba, A. Vizintin, J. Bitenc, G. Mali, I. Arcon, M. Kavcic, M. Zitnik, K. Bucar, G. Aquilanti, C. Martineau-Corcos, A. Randon-Vitanova, R. Dominko, Chem. Mater.,2018, 29, 9555-9564.

The non-relenting urge for sustainable, safe, energy dense and low cost energy storage systems has been driving eager interests in beyond Li-ion batteries^[1,2,3] such as those based on Li and Mg metal anodes. In principle, Mg batteries are poised to meet these demands, however have been confronted with daunting challenges that so far have confined these technologies to fundamental studies in research laboratories. Nonetheless, recently these batteries have been undergoing rapid critical advancements that are altering the notion of what an ideal material needs to be like and have generated systems that overcome several existing challenges. ^[1,3,4] These tremendous transformations are reshaping the landscape of Mg batteries R&D and are permeating increased confidence in the practical potential of these batteries.
Herein, key developments will be highlighted ^[1,3,4,5] and the path forward to overcome critical remaining challenges outlined.
References
[1] Choi J. W. Aurbach D. Nature Reviews Materials 1, 2016, 16013, 1.
[2] Manthiram A., Yu X., Wang S. Nature Reviews Materials 2, 2017, 16103.
[3] Mohtadi R., Orimo S., Nature Reviews Materials, 2016, 2,16091, 1311.
[4] Mohtadi R., Mizuno F. Beilstein J. Nanotechnol. 2014, 5, 1291.
[5] Kar M., Tutusaus O., MacFarlane D.R., Mohtadi R., Energy & Environmental Science, 2019, 12, 566.

We previously demonstrated that reformulating zinc into a monolithic spongy form factor moved aqueous zinc-based batteries onto a new performance curve—one in which the zinc electrode can be rechargeably cycled at high rate, to deep utilization of the metal, all without generating separator-piercing dendrites. This breakthrough enables development of a broad class of rechargeable zinc-based aqueous batteries. Ideally a next-generation of batteries in which the accompanying cathode can match the two-electron redox of zinc. We will describe the improvement of system metrics when pairing the two-electron zinc sponge versus the two-electron silver oxide cathode, even without optimizing the silver electrode, including electrochemical capacitor-worthy power. We then will describe the nanostructuring of α-Ni(OH)₂ in an attempt to stabilize the 1.67 electron redox characteristic of this precursor to the NiOOH cathode over the one-electron redox limit characteristic of β-Ni(OH)₂.

The fossil fuel energy crisis causes significant geopolitical stress, and renewable energy such and wind, tide and solar is a green solution to this problem. Energy storage is required for any renewable energy utilization as they are of intermittent nature with usually high production during off-peak hours and low production in peak hours.
Lithium ion battery is the main workhorse for portable electronics and portable tools and start adopting the market of electric vehicles, as well as electrochemical energy storage. However, they are reaching their maximum performance and a new breakthrough in electrochemical energy storage is necessary for a wide adaption of renewable energy. Multivalent ion batteries, particularly Mg and Ca, are of high interest because of their very electropositive electrochemical potentials: Mg has a reduction potential of -2.4 V vs NHE, while Ca has a reduction potential of -2.9 V vs NHE. Other favorable properties include high volumetric energy density and high crustal abundance.
The development of Mg has been mostly impeded by both the cathode and electrolyte developments, unlike its counterpart Li ion, Mg is notoriously famous for formation of a strong bond with oxygen and does not move easily in the lattice of MgO. A premise for a working Mg ion battery is that the surface of the Mg metal cannot be passivated in order to facilitate reversible Mg dissolution/deposition. A classical way of conditioning or preparing the Mg metal surface is either to provide a Cl- in a Mg solution.(1,2) The limited anodic stability of the Cl- would be problematic for Mg ion batteries because of corrosion issue with current collector. Using high-purity Mg salts with weakly associated anion (WCA) such as trifluorosulfonylimide (TFSI) (3) and carborane (4) has been a standard practice in the field since 2015. In 2019, our group introduced a new WCA group for the MV salts, the t-butylperfluoroaluminate ([TPFA]− = [Al{OC(CF3)3}4]−) anion for Mg salts,(5) and demonstrated the highly reversibility of deposition/dissolution, unprecedented anodic stability, and lack of passivation layer formation during the high potentiostatic hold. The favorable properties of the salt make it a friendly anion for the quest of cathode development, especially the transition metal oxide spinels for Mg ion batteries.
We will cover the brief history of the development of Cl- containing electrolyte in our group since 2014 and will present the detailed electrochemical performance, and collaborated work on the decomposition mechanism understanding, solvation behavior in O-donor solvents, and expansion of these research

(1) Liao, C.; Sa, N.; Key, B.; Burrell, A. K.; Cheng, L.; Curtiss, L. A.; Vaughey, J. T.; Woo, J.-J.; Hu, L.; Pan, B. The unexpected discovery of the Mg (HMDS) 2/MgCl 2 complex as a magnesium electrolyte for rechargeable magnesium batteries. Journal of Materials Chemistry A 2015, 3 (11), 6082.
(2) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Prototype systems for rechargeable magnesium batteries. Nature 2000, 407 (6805), 724.
(3) Ha, S.-Y.; Lee, Y.-W.; Woo, S. W.; Koo, B.; Kim, J.-S.; Cho, J.; Lee, K. T.; Choi, N.-S. Magnesium(II) Bis(trifluoromethane sulfonyl) Imide-Based Electrolytes with Wide Electrochemical Windows for Rechargeable Magnesium Batteries. ACS Applied Materials & Interfaces 2014, 6 (6), 4063.
(4) Carter, T. J.; Mohtadi, R.; Arthur, T. S.; Mizuno, F.; Zhang, R.; Shirai, S.; Kampf, J. W. Boron Clusters as Highly Stable Magnesium-Battery Electrolytes. 2014, 53 (12), 3173.
(5) Lau, K.-C.; Seguin, T. J.; Carino, E. V.; Hahn, N. T.; Connell, J. G.; Ingram, B. J.; Persson, K. A.; Zavadil, K. R.; Liao, C. Widening Electrochemical Window of Mg Salt by Weakly Coordinating Perfluoroalkoxyaluminate Anion for Mg Battery Electrolyte. J. Electrochem. Soc. 2019, 166 (8), A1510.

As compared to Li-ion and Na-ion battery electrolytes the electrolytes for Mg, Ca and Al metal batteries are quite different.^1,2
This is also reflected in the nature of the charge carriers e.g. the size and number of ligands coordinating to the central multivalent atom.
Furthermore the charge carriers are also affected by the salt concentration in a different way than the electrolytes based on monovalent central atoms.
We have studied several multivalent electrolyte designs³ and elucidated the preferrable local structures both by modelling (MD + DFT calculations) and by experimental methods (IR, Raman and NMR spectroscopy) in terms of stability of the first solvation shell, while
also adressing other properties such as the electrochemical stabilities and the coordination of active sites at the electrode.⁴
These results can be used for a more general understanding of the central atom - ligand relationship for these basic electrolytes.

References
1. A. Ponrouch, J. Bitenc, R. Dominko, N. Lindahl, P. Johansson and M. R. Palacín, Energy Storage Materials, 2019.
2. M. Arroyo-de Dompablo, A. Ponrouch, P. Johansson and M. R. Palacín, Chem. Rev., 2019.
3. T. Mandai, H. Masu and P. Johansson, Dalton Transactions, 2015.
4. J. Bitenc, N. Lindahl, A. Vizintin, M. E. Abdelhamid, R. Dominko and P. Johansson, Energy Storage Materials, 2019.

In the journey of the ions from the bulk to the interface the changes in their solvation structures and in their relative populations determine the essential characteristics of all the most important electrochemical processes. The ion solvation energy and the stiffness of its solvation sphere determine both thermodynamics and kinetics of the charge transfer at the interfaces. However, the atomistic details and thermodynamic characteristics of ion (de-)solvation process in the confined environment (e.g., near an electrode) remain largely unknown. For example, we still lack clarity on the locus and particular ion-solvent configurations at which the charge transfer occurs. On the other hand, the current paradigm of the solvation science provides well developed concepts and tools to study the ion solvation in the bulk liquid electrolytes (molecular dynamics and quantum chemistry hybrid protocols). These rely on the fundamental assumption of the uniqueness of the ion solvation structure. However, we have recently shown [1] that some even monoatomic cations in aprotic solvents exhibit multiple minima in the free energy landscapes with respect to ion-solvent coordination. As we will demonstrate, it has profound implications for the methodology to evaluate thermodynamic characteristics of electrolytes. Here we extend our approach of using advanced molecular dynamics free energy sampling techniques – both classical and ab initio – and explore the dynamics of the solvation structures of mono- and multivalent (Li+, Mg2+, Ca2+, and Zn2+) ions as they approach electrodes from the bulk in a range of aprotic solvents.

[1] A. Baskin and D. Prendergast, J. Phys. Chem. Lett. 10, 4920−4928 (2019)

Reversible electrochemical reactions of aluminum (Al) that can be potentially used for energy storage technologies is an intriguing topic from both scientific and technological perspectives. The centerpiece of reversible Al electrochemistry is the electrolyte. To date, the overwhelming majority of Al battery research activities utilize Lewis acidic chloroaluminate ionic liquids as the electrolytes. However, the complex nature of these electrolytes, such as corrosiveness, proneness to oxidation, and rich coordination, can lead to unintended ambiguity. In this presentation, we demonstrate that cautions must be used to interpret results of Al electrochemical reactions using chloroaluminate ionic liquid electrolytes. We use aluminum chloride/1-ethyl-3-methylimidazolium chloride (AlCl₃-[EMIm]Cl) ionic liquid as the example. We find Lewis acidic AlCl₃-[EMIm]Cl is corrosive to many commonly used metal current collectors, and the resulting corrosion current could be mistaken as the current from the electrochemical reactions of Al. The proneness of electrochemical oxidation of AlCl₄^- anion (ultimately leading to chlorine evolution) causes complicated process in the previously reported reversible AlCl₄^- intercalation in graphite. As the results, fast self-discharge and large irreversible capacity is observed when graphite (either natural or synthetic) is used as the cathode martials. We further investigated the chemical compatibility of AlCl₃-[EMIm]Cl with V₂O₅, which is a common cathode material reported in the literature of Al battery. Our study indicates V₂O₅ chemically reacts to both Lewis neutral and Lewis acidic AlCl₃-[EMIm]Cl. By elucidating the reaction mechanisms, we conclude V₂O₅ is not a feasible Al storage material in the chloroaluminate ionic liquid electrolytes. These findings could provide guideline for design more reliable electrolytes for rechargeable Al batteries.

Multivalent cation-based electrochemical energy storage concepts provide significant promise and challenges as beyond Li-ion technologies. In particular, chemistries based on Mg²⁺ or Ca²⁺ cations and their associated reactive metal anodes stretch the current limits of understanding regarding solvation and resultant electrochemistry in organic electrolytes. Furthermore, the recent discovery of semi-reversible calcium metal plating at room temperature in a very limited number of organic electrolytes provides a new and exciting opportunity to study the key aspects of solvation interactions that apply to transport and electrochemical stability across this set of alkaline earth metals. To this end, we present comparative studies of solvation phenomena and related electrochemical behavior using analogous sets of Mg²⁺ and Ca²⁺ salts in ethereal solvents. Differences in dication size and attendant solvation shell structures drive unique ion pairing tendencies and coordinating solvent structures, as determined through combined experimental and computational investigation. These coordination differences are further linked to both ion transport and electrochemical stability as they pertain to reversible metal plating. Based on these results, we will discuss strategies for designing electrolytes capable of maintaining efficient delivery of alkaline earth cations to and from their respective metal anodes or cathode hosts within a battery architecture.

The increasing demand of large-scale energy storage with improved performance, safety and sustainability for renewable variable energy sources and automotive applications drives the search for post-Li-ion batteries based on multivalent cation chemistries (e.g., Mg, Zn and Ca). However, poor electrolyte stability against metal anodes and high voltage cathodes has been so far one of many barriers to the development of multivalent cation rechargeable batteries. Unserstanding and controlling (electro)chemistry at the electrolyte-electrode interface is critical on improving battery performance. Multivalent cations are prone to complex formation with solvent molecules and anions, leading to different interfacial species with characteristic (electro)chemical properties. Although multivalent cation solvation/complexation structures have been investigated in the bulk, fundamental understanding of speciation at the interface is lacking.

Recently, these buried solid-liquid interfaces have been accessed experimentally via total electron yield (TEY) soft X-ray absorption spectroscopy (XAS), sensitive to just a few nanometers above the solid electrode surface, and total fluorescence yield (TFY), more sensitive to the liquid bulk electrolyte. Interfacial species identification has been realized only in a handful of systems, formed by chemically stable electrodes (Au, Pt) and water or sulfuric acid solutions, although efforts toward exploring more complex systems are ongoing.

This work explores the structure, dynamics and spectroscopic signatures of a matrix of multivalent cations (Zn²⁺ , Mg²⁺ , Ca²⁺ ), nonaqueous electrolytes, and the bis(trifluoromethylsulfonyl)imide anion (TFSI^- ) through TEY/TFY soft X-ray absorption spectroscopy and atomistic simulations. While the metal K-edge may provide insight into cation coordination, the previously unexplored F K-edge contains unique information about the complexation in bulk and at the interface. The limited studies in the literature show the rich behavior of the F K-edge in materials. However, the origin of the high sensitivity of the F K-edge XAS spectral profile to the chemical environment still lacks theoretical interpretation.

TEY/TFY experiments carried out at the ALS reveal unique interfacial speciation as a function of the cation and the coordinative strength of the solvent (2- methyltetrahydrofuran, 2MeTHF; 1,2-dimethoxyethane, G1; 1,2-dimethoxyethane, G2). A recently developed many-body formalism based on Density Functional Theory (MBXAS) was used to calculate the F K-edge X-ray absorption spectroscopic signature of the counterion TFSI^- in different coordination environments (contact and solvent-separated ion pairs) and in proximity to the electrode. Furthermore, the energy lanscapes of the different interfacial species were probed using classical and first-principles Molecular Dynamics simulations so as to understand subsequent temperature effects on the spectra.

With this work, we intend to highlight the promise of F K-edge X-ray absorption spectroscopy in combination with computational modelling as a unique channel to detect interfacial populations of anions (and associated solvent and cations).

Although lead-acid batteries managed to dominate market share of the low-cost power source, their high toxicity and negative environmental impact urge the development of low-cost batteries.^[1] The divalent cation (e.g. Zn²⁺ and Mg²⁺ etc.) chemistries are principally advantageous for their adequate resources and improved safety. However, rechargeable divalent metal-based batteries is currently limited by the unavailability of electrolytes which must simultaneously satisfy wide enough electrochemical stability window, good compatibility with electrodes and current collector corrosion resistance.^[2] Based on the precise design of electrolytes with reasonable composition and structures, we developed a series of boron-centered-anion-based Mg-ion electrolytes characterized by high ionic conductivity, non-nucleophilicity, and wide electrochemical window (Adv. Energy Mater. 2017, 1602055; Electrochem. Commun. 2017, 83, 72). Based on an in situ crosslinking reaction of magnesium borohydride and hydroxyl-terminated polytetrahydrofuran, a gel polymer electrolyte exhibits reversible Mg plating/stripping performance, high Mg-ion conductivity, and remarkable Mg-ion transfer number (Adv. Mater. 2019, 1805930; Energy Environ. Sci. 2017, 10, 2616; Small 2017, 1702277). Meanwhile, we demonstrated the solid-electrolyte interface formation on Zn anodes based on eutectic electrolytes that can optimize the stripping/deposition processes (Nano Energy 2019, 57, 625; Nat. Commun. 2019, accepted; Electrochem. Commun. 2016, 69, 6.). We further creatively proposed a thermoreverisble polymer-based electrolyte with the ability of cooling-recovery to repair the interfacial failure during cycling (Angew. Chem. Int. Ed. 2017, 56, 7871) and a brightener-inspired interface for suppressing Zn dentrite and side reactions (Energy Environ. Sci. 2019, 12, 1938; NPG. Asia Mater. 2019, accepted). These prospective researches on the rechargeable Zn and Mg batteries are highly in compliance with the lead-free trend in the EVs and corresponding low-cost applications.

Various metals have been used as battery anodes in electrochemical cells ever since the birth of the batteries with Volta’s pile and in the first commercialized primary (Zn/MnO₂, Leclanché 1866) and secondary (Pb/acid, Planté 1859) batteries. Li-MoS₂ cells, employing Li metal anodes, with specific energies two to three times higher than both Ni/Cd and Pb/acid cells, were withdrawn from the market due to safety issues related to dendrites growth. Instead, Ca is currently being considered as safer metal anode alternative.[1] Pioneering work by Aurbach et al. in the early 1990’s showed a surface-film controlled electrochemical behavior of Ca metal anodes in electrolytes with conventional organic solvents.[2,3] The lack of metal plating was attributed to the poor divalent cation migration through the passivation layer.

Nevertheless, recent demonstration of Ca plating and stripping in the presence of a passivation layer [4,5] has paved the way for assessment of new electrolyte formulations with high resilience towards oxidation. However, several challenges remain to be tackled for the development of Ca based batteries.[1,6] Among these, the need for reliable electrochemical test protocols, mass transport limitations and high desolvation energies (due to strong cation-solvent and cation–anion interactions) are implied.[8] Here, the reliability of electrochemical set-ups involving multivalent chemistries is discussed,[9] and a systematic investigation of the electrolyte formulation impact on the cation solvation structure and transport is presented.[10] Finally, a systematic characterization of the SEI formed on the Ca metal anode in various electrolyte formulations using complementary techniques allowed for the identification of the most suitable SEI components in terms of divalent cation mobility.

References
1) M. E. Arroyo-de Dompablo, A. Ponrouch, P. Johansson, M. R. Palacin, Chem. Rev. doi.org/10.1021/acs.chemrev.9b00339.
2) D. Aurbach, R. Skaletsky, Y. Gofer, J. Electrochem. Soc.138 (1991) 3536
3) Z. Lu, A. Schechter, M. Moshkovich, D. Aurbach, J. Electroanal. Chem. 466 (1999) 203
4) A. Ponrouch, C. Frontera, F. Bardé, M.R. Palacin, Nat. Mater., 15 (2016) 169
5) D. Wang, X. Gao, Y. Chen, L. Jin, C. Kuss, P. G. Bruce, Nat. Mater. 17 (2018) 16
6) A. Ponrouch, M.R. Palacin, Current Opinion in Electrochemistry 9 (2018) 1
7) A. Ponrouch, J. Bitenc, R. Dominko, N. Lindahl, P. Johansson, M.R. Palacin, Energy Storage Materials 20 (2019) 253
8) D. S. Tchitchekova, D. Monti, P. Johansson, F. Bardé, A. Randon-Vitanova, M. R. Palacin, A. Ponrouch, J. Electrochem. Soc., 164 (2017) A1384
9) R. Dugas, J. D. Forero-Saboya, A. Ponrouch, Chem. Mater. doi.org/10.1021/acs.chemmater.9b02776
10) J. D. Forero-Saboya, E. Marchante, R. B. Araujo, D. Monti, P. Johansson, A. Ponrouch, Submitted.

Secondary batteries based on divalent calcium ions are attractive due to the high theoretical capacities of calcium metal anodes, low reduction potential nearing that of lithium, and earth abundance of calcium. In a significant milestone towards calcium-based batteries, the feasibility of reversibly plating and stripping calcium at room temperature from ether-based electrolytes was recently demonstrated. However, secondary phases such as calcium hydride [1] and calcium fluoride [2] form concurrently with deposited Ca metal and accumulate over time, contributing to coulombic efficiencies far below the threshold required for a practical rechargeable battery. Determining the formation mechanisms and distribution of these secondary phases is a critical first step in understanding and harnessing potential benefits of their presence to improve reversible battery operation. In this talk, we present the results of experiments utilizing a suite of electron microscopy techniques to map the distribution of calcium metal and secondary phases throughout electrochemically deposited and cycled calcium anodes. Calcium borohydride [Ca(BH₄)₂] in tetrahydrofuran (THF) was used as the primary model electrolyte for this study. Focused ion beam-generated cross sections were extracted from deposited and cycled anodes and characterized using Scanning Transmission Electron Microscopy (STEM) and Transmission Kikuchi Diffraction (TKD), revealing the preferred locations for calcium nucleation and growth, as well as the evolving morphology and accumulation of secondary phases over time. The deposits were also characterized using XRD in an inert environment, allowing for quantification of secondary phases and revealing crystallographic texturing of the calcium metal. We also present preliminary studies of cycled calcium anodes using STEM-EELS to chemically map calcium hydride. The work presented here provides critical insights regarding the morphology of deposited calcium metal and the extent and distribution of secondary phase formation and is an important step towards increasing the coulombic efficiency of calcium plating and stripping for next-generation rechargeable batteries.

References:

[1] D. Wang, X. Gao, Y. Chen, L. Jin, C. Kuss, and P. G. Bruce, "Plating and stripping calcium in an organic electrolyte," Nat Mater, vol. 17, no. 1, pp. 16-20, Jan 2018.
[2] A. Shyamsunder, L. E. Blanc, A. Assoud, and L. F. Nazar, "Reversible Calcium Plating and Stripping at Room Temperature Using a Borate Salt," ACS Energy Letters, vol. 4, no. 9, pp. 2271-2276, 2019.

Rechargeable batteries are critical components of portable electronics, electric vehicles and grid storage. Li-ion is the dominant technology because of its low weight, large voltage and high capacity, but the demand for raw materials is putting strain on the supply chain, and there are several safety, ethical and environmental concerns. Therefore, developing adequate alternatives to Li-ion batteries is an active research area around the world. Multivalent ion batteries (MVIBs), utilizing Mg, Ca, Zn and Al, are promising due to being based on earth-abundant materials, and their higher weight can be compensated by their multivalent nature, which allows their ions to carry multiple charges. We are particularly interested in Ca ion batteries, which have received relatively little attention compared to Mg and Al, yet they are predicted to provide the largest voltage for a multivalent ion. However, their development has been limited by challenges in identifying adequate electrolytes to reversibly plate and strip metal Ca anodes. While there have been some encouraging recent developments, a systematic understanding of the interactions between electrolytes and Ca is needed. For example, it is not clear how the electrolyte (e.g., ethylene carbonate) decomposes at a Ca metal surface, nor are the structure and composition known of the solid-electrolyte interphase (SEI) that forms between the metal anode and the electrolyte. Furthermore, the diffusion of Ca²⁺ ions through the SEI is not well understood.

We aim to address some of these points with computational modeling that combines density functional theory (DFT) with ab initio molecular dynamics (AIMD). We investigate the electrolyte decomposition of ethylene carbonate, with and without salt, at the Ca metal interface. We then compare this process to those occurring at Li and Al metal surfaces. The DFT calculations reveal the mechanism and energy barriers associated with the possible electrolyte decomposition pathways, while the AIMD trajectories reveal how quickly molecules decompose and their location. Next, we investigate the electron conductivity of the anode-electrolyte interface region at various stages of SEI layer formation. We accomplish this with the non-equilibrium Green’s function technique combined with DFT (NEGF-DFT), which not only tells us the conductivity but also the electron pathway through the interface. Finally, we investigate the possibility of protecting the electrolyte from reacting with the anode by separating them with an artificial SEI that still allows Ca²⁺ ions to diffuse through it. We propose that this strategy can aid in the development of rechargeable Ca-ion batteries by bypassing the formation of an ionically insulating SEI due to electrolyte decomposition.

Calcium is the 5^th most abundant element in the Earth’s crust and a candidate material for next-generation non-Lithium rechargeable batteries. Its standard electrode potential of -2.87 V is close to that of Lithium and double the energy density compared to that of Lithium-ion batteries would be possible due to Calcium’s bivalent nature. Additionally, dendrite formation is alleviated with Calcium due to its high density and high thermal stability. In comparison to other multivalent ions such as Magnesium, Calcium possesses a smaller charge density and higher standard electrode potential, affording faster ion diffusion through cathode materials. However, to practically realize these advantages, Calcium metal anodes are necessary, which makes reversible plating and stripping Calcium on metallic substrates an important area of research. Recent work has successfully demonstrated electrochemical deposition of Calcium in EC/PC at elevated temperatures¹, THF², and DME³, whereas here, we report reversible plating and stripping of Calcium in an EC/PC based electrolyte at room temperature. Stable redox reactions were observed with cyclic voltammetry, and reversible plating and stripping was achieved galvanostatically at room temperature. Crystalline deposits in the form of thin films were successfully confirmed using XRD and FTIR measurements to be consisting of Calcium metal with traces of side products. Deposition on other substrates in other non-aqueous electrolytes is currently being investigated. This work provides an avenue for reliably developing Ca-metal electrodes for next-generation batteries.

References
[1] Ponrouch, Alexandre, et al. "Towards a calcium-based rechargeable battery." Nature materials 15.2 (2016): 169.
[2] Wang, Da, et al. "Plating and stripping calcium in an organic electrolyte." Nature materials 17.1 (2018): 16.
[3] Li, Zhenyou, et al. "Towards stable and efficient electrolytes for room-temperature rechargeable calcium batteries." Energy & Environmental Science (2019).

The development of new rechargeable battery systems employing novel chemistries is imperative to meet the increasing energy storage demands of emerging technologies such as electric vehicles and grid load leveling. Of particular interest is the development of battery systems employing multivalent metal anodes. Using a multivalent metal anode increases the energy density of the cell by providing an electrode consisting of entirely active material. Furthermore, many such elements such as magnesium are far more naturally abundant than lithium, ensuring they are economically viable and sustainable. Due to these benefits, multivalent-ion battery systems have recently received significant research attention. One of the outstanding challenges is dealing with the multivalency of the active cation, in regards to how the high charge density of the ion generally exacerbates traditional transport problems. For instance, conduction of multivalent cations Mg²⁺ and Ca²⁺ within single-ion conducting polymers is 1-3 orders of magnitude slower than their monovalent cation equivalents. This general problem of charge density presents itself in all components of the battery, from promoting decomposition on the anode to limiting the selection of electrolyte salts to the few that can adequately dissociate in solution. In regards to magnesium-sulfur batteries, poor cation diffusivity in solid discharge products makes oxidation to higher order polysulfides and sulfur difficult, a problem made worse by the low solubility of magnesium polysulfides and consequent lack of intrinsic redox mediators. Herein, we explore recent attempts within our group to provide solutions to these challenges, focusing primarily on electrolyte and conversion cathode development. We aim to link material properties to observed transport behavior, specifically in how to enhance divalent cation transport and minimize anion transport under an applied potential while maintaining electrochemical activity.

Multivalent batteries are promising candidates for next generation battery technologies in terms of capacity and resource abundance. However, the theoretically high capacity of multivalent metals attributes to the multiple charge of the cations, which usually leads to a high charge density, thus causing sluggish insertion and diffusion kinetics at the corresponding cathode side. Taking rechargeable Mg batteries (RMBs) as an example, there is no report about insertion-type oxide cathode, which could provide reversible Mg storage with fair capacity at an acceptable current rate.
The kinetic issue can be alleviated to some extent by using layered transition metal sulfide, due to the lower charge density of S than O. However, both diffusion kinetics and achievable capacity are not satisfying even if cointercalation of a second species is employed, which is supposed to screen the high charge density of Mg²⁺ . [1] In this regard, further “softening” the host structure is essential to realize fast Mg²⁺ mobility. On the other hand, the realization of the high energy density requires enabling a reversible multi-electron redox reaction, which is seldom reported in insertion-type cathode for multivalent batteries. In fact, classical cationic redox with stepwise single electron reactions have not been able to fulfil the requirement for multi-electron redox reactions so far. Here, anionic redox chemistry with multi-electron transfer may be a possible solution. [2]
In this talk, we will present a new insertion-type cathode for multivalent batteries, which not only offers much more structural flexibility but also exhibits high energy density than the layered materials. Mg²⁺ storage mechanism and possible diffusion pathways will be discussed in detail.

References
[1]. Z. Li, X. Mu, Z. Zhao-Karger, T. Diemant, R. J. Behm, C. Kübel, M. Fichtner, Fast kinetics of multivalent intercalation chemistry enabled by solvated magnesium-ions into self-established metallic layered materials. Nat Commun 9, 5115 (2018)
[2]. E. D. Grayfer, E. M. Pazhetnov, M. N. Kozlova, S. B. Artemkina, V. E. Fedorov, Anionic Redox Chemistry in Polysulfide Electrode Materials for Rechargeable Batteries. ChemSusChem 2017, 10, 4805.

We study the electrochemical and structural responses of nanoparticles of different sizes as cathode materials in magnesium ion batteries. By systematically varying the electrolytes, we observe a consistent solid-solution phase transition in small nanoparticles during discharge, in contrast to the mixed-phase evolution or surface reactions in big nanoparticles. Further examination by scanning electron nanobeam diffraction (SEND) in scanning transmission electron microscope (STEM) revealed the strain and phase distribution in the small nanoparticles at nanometer resolution. We observe a non-uniform, scattered phase domain distribution in small nanoparticles in the non-aqueous electrolyte, in contrast to the uniform one in the aqueous electrolyte. The varied phase behaviors are associated with the co-intercalation of solvent molecules in the discharge processes. Our work shows that nanoparticle size and solvent molecules have a direct influence on Mg ion insertion processes, which rationally guide the design of Mg ion batteries with high capacity and high stability.

Li-ion batteries are currently approaching its fundamental limits in terms of energy density, which in addition to the reduced supply of Li instigate the race for the development of alternative technologies exhibiting advantages in terms of energy density, safety and cost. Although rechargeable batteries based on a Ca-metal anode are attractive, the development of such batteries is intricate due -among other factors- to the lack of competitive cathode materials. To the date, restricted reversible electrochemical Ca intercalation has been reported for a handful of compounds (see [1] and references therein), all of them showing poor cycling capability, if any. Parallel experimental-computational investigations establish a link between the lack of reversible electrochemical activity (experimentally observed) and a hampered Ca diffusion (computationally estimated). Hence Ca diffusion is a key parameter, for which the migration energy barriers provide an approximate indication. In this communication we analyse the calculated DFT-energy barriers for Ca migration found for a large set of Ca-transition metal compounds including oxides, sulphides, phosphates, nitrides, carbonates and phosphates. The possible application of Ca intercalation materials as cathode in rechargeable batteries is further discussed considering other basic electrode characteristics.


Acknowledgments: Authors are grateful for financial support from European Union H2020-FETOPEN funded project CARBAT-766617.

[1] M.E. Arroyo-de Dompablo, A. Ponrouch, P. Johansson and M.R. Palacín Chemical Reviews 10.1021/acs.chemrev.9b00339

Developing batteries beyond those based on Lithium is critical to the advancement of next-generation energy storage technology with lower cost and higher energy density. To this end, Calcium has been identified as a candidate material due to its Earth-abundancy, reduction potential similar to that of Lithium, and divalency, which can enable the development of high-voltage and high-energy density batteries. While various Calcium based compounds have been suggested as potential cathode materials for Ca-ion batteries using DFT calculations¹, very few have been synthesized and evaluated experimentally. Here, we report the facile synthesis and evaluation of some Calcium containing transition metal oxides as potential cathode materials for Ca-based batteries. X-ray diffractograms of the as-synthesized powders are in agreement with powder patterns obtained theoretically. Electrochemical de-insertion/insertion from and into the structure of the host materials was conducted in 0.5 M Ca(BF₄)₂ in Acetonitrile electrolyte and evaluated using X-ray diffraction. Further electrochemical evaluation of these cathode materials by galvanostatic charge/discharge measurements is currently being investigated. This work provides opportunities to identify, synthesize and evaluate low-cost cathodes for next-generation Calcium batteries.

References
[1] Arroyo-de Dompablo, M. Elena, et al. "In quest of cathode materials for Ca ion batteries: the CaMO 3 perovskites (M= Mo, Cr, Mn, Fe, Co, and Ni)." Physical Chemistry Chemical Physics 18.29 (2016): 19966-19972

The market of electric vehicles (EVs) is growing fast and is expected to take over the gas-powered vehicles in the upcoming years. However, the bottleneck for the development of EVs remains to be the battery technology, where batteries with higher energy and power densities are required. Also, battery chemistries with higher safety and lower cost must be developed to enable EVs to compete with gasoline-powered cars in the price range of under $30K. Despite the significant developments made in lithium-ion (Li-ion) batteries, their flammability and high cost, as well as concerns about the availability of economically viable lithium resources have considerably increased the need for alternative battery chemistries. Rechargeable aluminum batteries with aluminum metal anode are considered as one of the most promising alternative energy storage systems to current Li-ion batteries. Aluminum is the most abundant metal in Earth’s crust, and it can potentially offer three-electron redox reactions resulting in the highest theoretical volumetric capacity of 8040 mAh cm^-3 among all metals and a reasonably high theoretical gravimetric capacity of 2980 mAh g^-1. Recently, we reported on the performance of two-dimensional (2D) V₂CT_x MXene as an intercalation-type cathode for rechargeable aluminum batteries delivering exceptional capacities and rate-capabilities.¹ MXenes are a family of 2D transition metal carbides and nitrides that are produced by selective etching of the A layer atoms (i.e., Al) from MAX phases (i.e., V₂AlC), a large group of layered ternary carbides and nitrides. Despite the promising performances in our early work, the V₂CT_x MXene cathode showed severe capacity decay in over hundreds of cycles. In this presentation, through a combination of various structural characterizations techniques (XRD, SEM, HRTEM, and XPS) and electroanalytical analyses such as Galvanostatic Intermittent Titration Technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) we provide valuable insights about the underlying reasons for the capacity loss and further shed light on the thermodynamics and kinetics of Al³⁺ intercalation into V₂CT_x MXene cathode. We further demonstrate that by designing new electrode architectures and modifying electrolyte composition, different 2D MXenes can deliver stable cyclic performance in the aluminum battery system. The 2D MXene cathodes can also be fabricated as freestanding and binder-free electrodes with exceptional volumetric capacities and long cycle life. Considering that the family of the MXenes now includes ~30 different compositions, our research guides the preparation of an entire group of cathode materials for rechargeable aluminum batteries based on these 2D materials.
References
1. Vahidmohammadi, A., Hadjikhani, A., Shahbazmohamadi, S. & Beidaghi, M. Two-Dimensional Vanadium Carbide (MXene) as a High-Capacity Cathode Material for Rechargeable Aluminum Batteries. ACS Nano 11, 11135–11144 (2017).

We previously demonstrated that reformulating zinc into a monolithic spongy form factor moved aqueous zinc-based batteries onto a new performance curve—one in which the zinc electrode can be rechargeably cycled at high rate, to deep utilization of the metal, all without generating separator-piercing dendrites. This breakthrough enables development of a broad class of rechargeable zinc-based aqueous batteries. Ideally a next-generation of batteries in which the accompanying cathode can match the two-electron redox of zinc. We will describe the improvement of system metrics when pairing the two-electron zinc sponge versus the two-electron silver oxide cathode, even without optimizing the silver electrode, including electrochemical capacitor-worthy power. We then will describe the nanostructuring of α-Ni(OH)₂ in an attempt to stabilize the 1.67 electron redox characteristic of this precursor to the NiOOH cathode over the one-electron redox limit characteristic of β-Ni(OH)₂.

The fossil fuel energy crisis causes significant geopolitical stress, and renewable energy such and wind, tide and solar is a green solution to this problem. Energy storage is required for any renewable energy utilization as they are of intermittent nature with usually high production during off-peak hours and low production in peak hours.
Lithium ion battery is the main workhorse for portable electronics and portable tools and start adopting the market of electric vehicles, as well as electrochemical energy storage. However, they are reaching their maximum performance and a new breakthrough in electrochemical energy storage is necessary for a wide adaption of renewable energy. Multivalent ion batteries, particularly Mg and Ca, are of high interest because of their very electropositive electrochemical potentials: Mg has a reduction potential of -2.4 V vs NHE, while Ca has a reduction potential of -2.9 V vs NHE. Other favorable properties include high volumetric energy density and high crustal abundance.
The development of Mg has been mostly impeded by both the cathode and electrolyte developments, unlike its counterpart Li ion, Mg is notoriously famous for formation of a strong bond with oxygen and does not move easily in the lattice of MgO. A premise for a working Mg ion battery is that the surface of the Mg metal cannot be passivated in order to facilitate reversible Mg dissolution/deposition. A classical way of conditioning or preparing the Mg metal surface is either to provide a Cl- in a Mg solution.(1,2) The limited anodic stability of the Cl- would be problematic for Mg ion batteries because of corrosion issue with current collector. Using high-purity Mg salts with weakly associated anion (WCA) such as trifluorosulfonylimide (TFSI) (3) and carborane (4) has been a standard practice in the field since 2015. In 2019, our group introduced a new WCA group for the MV salts, the t-butylperfluoroaluminate ([TPFA]− = [Al{OC(CF3)3}4]−) anion for Mg salts,(5) and demonstrated the highly reversibility of deposition/dissolution, unprecedented anodic stability, and lack of passivation layer formation during the high potentiostatic hold. The favorable properties of the salt make it a friendly anion for the quest of cathode development, especially the transition metal oxide spinels for Mg ion batteries.
We will cover the brief history of the development of Cl- containing electrolyte in our group since 2014 and will present the detailed electrochemical performance, and collaborated work on the decomposition mechanism understanding, solvation behavior in O-donor solvents, and expansion of these research

(1) Liao, C.; Sa, N.; Key, B.; Burrell, A. K.; Cheng, L.; Curtiss, L. A.; Vaughey, J. T.; Woo, J.-J.; Hu, L.; Pan, B. The unexpected discovery of the Mg (HMDS) 2/MgCl 2 complex as a magnesium electrolyte for rechargeable magnesium batteries. Journal of Materials Chemistry A 2015, 3 (11), 6082.
(2) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Prototype systems for rechargeable magnesium batteries. Nature 2000, 407 (6805), 724.
(3) Ha, S.-Y.; Lee, Y.-W.; Woo, S. W.; Koo, B.; Kim, J.-S.; Cho, J.; Lee, K. T.; Choi, N.-S. Magnesium(II) Bis(trifluoromethane sulfonyl) Imide-Based Electrolytes with Wide Electrochemical Windows for Rechargeable Magnesium Batteries. ACS Applied Materials & Interfaces 2014, 6 (6), 4063.
(4) Carter, T. J.; Mohtadi, R.; Arthur, T. S.; Mizuno, F.; Zhang, R.; Shirai, S.; Kampf, J. W. Boron Clusters as Highly Stable Magnesium-Battery Electrolytes. 2014, 53 (12), 3173.
(5) Lau, K.-C.; Seguin, T. J.; Carino, E. V.; Hahn, N. T.; Connell, J. G.; Ingram, B. J.; Persson, K. A.; Zavadil, K. R.; Liao, C. Widening Electrochemical Window of Mg Salt by Weakly Coordinating Perfluoroalkoxyaluminate Anion for Mg Battery Electrolyte. J. Electrochem. Soc. 2019, 166 (8), A1510.

Magnesium is a promising element for future batteries due to high gravimetric (2206 mAh/g) and volumetric capacity (3834 mAh/cm3) of Mg anode. It is considered as geopolitically neutral, sustainable and abundant metal, which can be used in batteries due to non-dendritic deposition during battery operation. Besides discussed benefits, there are several challenges connected with the use of Mg metal, mainly incompatibility of the Mg metal with the electrolytes and difficult insertion of magnesium into oxide based solid state host matrix. Alternative solutions can be use of redox active polymers with different active redox groups.
In this presentation, overview of recent achievements in our group on the field of magnesium batteries will be discussed with a focus on proper selection of battery components in order to achieve electrochemical properties attractive for the commercialization.

Acknowledgement: This work is supported HONDA R&D Europe and by the Slovenian Research Agency (research core funding No. P2-0393 and research project J2-8167).

References:
1) J. Bitenc, K. Pirnat, T. Bancic, M. Gaberšček, B. Genorio, A. Randon-Vitanova and R. Dominko, Anthraquinone-Based Polymer as Cathode in Rechargeable Magnesium Batteries, ChemSusChem, 2015, 8, 4128–4132.
2) A. Vizintin, J. Bitenc, A. Kopač Lautar, K. Pirnat, J. Grdadolnik, J. Stare, A. Radon-Vitanova and R. Dominko, Probing electrochemical reactions in organic cathode materials via in operando infrared spectroscopy, Nat. Commun., 2018, 9, 661.
3) J. Bitenc, K. Pirnat, G. Mali, B. Novosel, A. Randon Vitanova and R. Dominko, Poly(hydroquinoyl-benzoquinonyl sulfide) as an active material in Mg and Li organic batteries, Electrochem. commun., 2016, 69, 1–5.
4) T. Bancic, J. Bitenc, K. Pirnat, A. Kopac Lautar, J. Grdadolnik, A. Randon Vitanova and R. Dominko, Electrochemical performance and redox mechanism of naphthalene-hydrazine diimide polymer as a cathode in magnesium battery, J. Power Sources, 2018, 395, 25–30.
5) Jan Bitenc, Alen Vizintin, Joze Grdadolnik, Robert Dominko, Tracking electrochemical reactions inside organic electrodes by operando IR spectroscopy Energy Storage Materials 21 (2019) 347–353
6) A. Robba, A. Vizintin, J. Bitenc, G. Mali, I. Arcon, M. Kavcic, M. Zitnik, K. Bucar, G. Aquilanti, C. Martineau-Corcos, A. Randon-Vitanova, R. Dominko, Chem. Mater.,2018, 29, 9555-9564.

Reversible electrochemical reactions of aluminum (Al) that can be potentially used for energy storage technologies is an intriguing topic from both scientific and technological perspectives. The centerpiece of reversible Al electrochemistry is the electrolyte. To date, the overwhelming majority of Al battery research activities utilize Lewis acidic chloroaluminate ionic liquids as the electrolytes. However, the complex nature of these electrolytes, such as corrosiveness, proneness to oxidation, and rich coordination, can lead to unintended ambiguity. In this presentation, we demonstrate that cautions must be used to interpret results of Al electrochemical reactions using chloroaluminate ionic liquid electrolytes. We use aluminum chloride/1-ethyl-3-methylimidazolium chloride (AlCl₃-[EMIm]Cl) ionic liquid as the example. We find Lewis acidic AlCl₃-[EMIm]Cl is corrosive to many commonly used metal current collectors, and the resulting corrosion current could be mistaken as the current from the electrochemical reactions of Al. The proneness of electrochemical oxidation of AlCl₄^- anion (ultimately leading to chlorine evolution) causes complicated process in the previously reported reversible AlCl₄^- intercalation in graphite. As the results, fast self-discharge and large irreversible capacity is observed when graphite (either natural or synthetic) is used as the cathode martials. We further investigated the chemical compatibility of AlCl₃-[EMIm]Cl with V₂O₅, which is a common cathode material reported in the literature of Al battery. Our study indicates V₂O₅ chemically reacts to both Lewis neutral and Lewis acidic AlCl₃-[EMIm]Cl. By elucidating the reaction mechanisms, we conclude V₂O₅ is not a feasible Al storage material in the chloroaluminate ionic liquid electrolytes. These findings could provide guideline for design more reliable electrolytes for rechargeable Al batteries.