Symposium EE5 : Next-Generation Electrical Energy Storage Chemistries

Electrodeposition is used in various manufacturing processes for creating metal, colloid, and polymer coatings on conductive electrode substrates. The process also plays an important role in electrochemical storage technologies based on batteries, where it must be carefully managed to facilitate stable and safe operations at low operating temperatures, high rates and over many cycles of charge and discharge. A successful electrodeposition processes requires fast transport of charged species (e.g. ions, particles, polymers) in an electrolyte and stable redox reactions and transport at the electrolyte/electrode interface at which the deposition occurs. This talk considers the stability of electrodeposition of metals on planar electrodes with an emphasis on its role in enabling next-generation secondary batteries based on lithium and sodium metal anodes. Such batteries promise substantial improvements in electrochemical energy storage over todays’s state-of-the art lithium ion technology and are under active investigation worldwide.

Development of a practical rechargeable lithium metal battery (LMB) remains a challenge due to uneven lithium electrodeposition and formation of ramified denderitic electrodeposits during repeated cycles of charge and discharge. Known consequences of unstable electrodeposition in LMBs include accumulation of electrically disconnected regions of the anode or “dead lithium”, thermal runaway of the cell, and internal short circuits, which limit cell lifetime and may pose serious hazards if a flammable, liquid electrolyte is used in a LMB. Lithium-ion batteries (LIBs) are designed to eliminate the most serious of these problems by hosting the lithium in a graphitic carbon substrate, but this configuration is not entirely immune from uneven lithium plating and dendrite formation. Specifically, the small potential difference separating lithium intercalation into versus lithium plating onto graphite, means that a too quickly charged or overcharged LIB may fail by similar mechanisms as a LMB.

Using a continuum transport analysis for electrodeposition in a structured electrolyte in which a fraction of the anions are fixed in space, the talk will show that electrodeposition at the lithium anode can be stabilized through rational design of the electrolyte and salt. Building upon these ideas, the talk will explore structure and transport in novel nanoporous hybrid electrolyte configurations designed to stabilize metal anodes against dendritic electrodeposition and premature failure. Finally, the talk will explore an application of these electrolyte designs for LMBs to evaluate stability conditions deduced from theory.

Successful strategies of silicon (Si)-based anode materials for lithium ion batteries (LIBs) have numerously reported during past decade, such as nano-designed Si structure, stronger new binder system, Si composite with other materials, and so on. However most of those strategies provided only specific energy or power density with low volumetric energy density or area capacity. The superior stability of specific energy or power density couldn’t represent the higher electrochemical performance in practical application for LIBs. To investigate the practical use of anode materials, initial area capacity of anode should be higher than 3.7 mAh/cm² which is commercial level of graphite. It appears in high loading level that the more critical problems due to volume expansion which doesn’t appear in low loading level. Moreover, high loading level causes fast degrading of lithium metal in half-cell test. Thus electrochemical test of high loaded electrode should be conducted in a full-cell test. Therefore, here we conducted full-cell electrochemical test of Si oxide-based anode with high loading (also using only 3wt% of CMC/SBR binder) while physical mixing of LiCoO₂ and Ni-rich cathode as cathode material, also known as commercialized cathode material for LIBs, and investigated its detailed fading mechanisms out to 1000 cycles.
Our investigation scope of the fading mechanism is from electrode level to atomic level. To verify the effect of electrode volume change, the thickness of electrode and solid-electrolyte-interphase (SEI) layer was measured after cycle test and chemical compositions of SEI layer were analyzed by X-ray photoelectron spectroscopy (XPS). To observe the atomic structure of electrode materials, high resolution-transmission electron microscopy (HR-TEM) were operated after long-term cycling. On the evidence of ex-situ analysis and electrochemical result, we created the algorithm for the possible fading mechanism of Si-based anode. We also separated the reasons of fast and gradual degrading respectively and suggested the behaviors of idealized Si-based anode. We believe that our findings provide a foundation to clearly verify fading mechanism of Si-based anode for LIBs and envision the considerations of future Si-based anode for practical use.

Silicon clathrates contain cage-like structures that can encapsulate various guest atoms or molecules. Here we present an electrochemical evaluation of type I silicon clathrates based on M₈Y_xSi_46-x (M = Ba, Sr; Y = Al, Cu, Ni) as the anode material for lithium-ion batteries. For the Ba-Al-Si system, post-cycling characterization with NMR and XRD show no discernible structural or volume changes even after electrochemical insertion of 44 Li (~1 Li/Si) into the clathrate structure. The observed properties are in stark contrast with lithiation of other silicon anodes, which become amorphous and suffer from large volume changes. The electrochemical reactions are proposed to occur as single phase reactions at approximately 0.2 and 0.4 V vs. Li/Li⁺ during lithiation and delithiation, respectively, distinct from diamond cubic or amorphous silicon anodes. Reversible capacities as high as 499 mAh g^-1 at a 5 mA g^-1 rate were observed for silicon clathrate with composition Ba₈Al_8.54Si_37.46, corresponding to ~1.18 Li/Si. These results show that silicon clathrates could be promising durable anodes for lithium-ion batteries. Changing the composition of the clathrate, namely replacing the Ba guest atom and Al framework substitution with other metals, was found to have a strong effect on the number of Li reversibly inserted into the structure and the shape of the voltage profile.

Si has been considered as a promising alternative anode for next-generation Li-ion batteries (LIBs) because of its high theoretical energy density, relatively low working potential, and abundance in nature. However, Si anodes exhibit a rapid capacity decay and increase in the internal resistance, which are caused by the large volume changes upon Li insertion and extraction. This unfortunately obstructs their practical applications. Therefore, managing the total volume change remains a critical challenge for effectively alleviating the mechanical fractures and instability of solid-electrolyte-interphase products. In this regard, in spite of many new ideas being published, all of them are still from practical implantation in the Li-ion batteries. Accordingly, it is inevitable to composite with the graphite to minimize the volume change and to balance with the cathode material. In this talk, I am going to present new advanced results of the Si and graphite composites with reversible capacity of < 600 mAh/g, which can be immediately implanted in the full cell.

Silicon is considered as a potential next-generation anode material for lithium ion battery (LIB). Experimental reports of up to 40% increase in energy density of silicon anode based LIBs have been reported in literature. However, such increase in energy density is achieved when silicon anode based LIB is allowed to swell more than graphite based LIB and beyond permissible limits. For practical applications such as in automotive or mobile devices, one cannot have any volume expansion. We determine the theoretical bounds of silicon composition in a silicon – carbon composite (SCC) based anode to maximize the volumetric energy density of LIB by assuming no increase in the external dimensions of the anode during charging. The porosity of SCC anode is adjusted to accommodate the volume expansion during lithiation. The determined threshold value of silicon was then used to calculate the volumetric energy densities of SCC anode based LIBs and improvement over graphite anode based LIBs for three types of cathodes - lithium cobalt oxide (LCO), lithium manganese cobalt oxide (NMC) and lithium nickel cobalt aluminum oxide (NCA), and at a constant cathode thickness of 70 μm. The maximum improvement in the volumetric capacity of SCC anode based LIB over graphite anode based LIB for LCO, NMC and NCA cathodes was determined to be ~20%, ~22%, and ~24%, respectively. Theoretical maximum in the volumetric capacity and energy density is obtained when it is assumed that there is zero porosity in the lithiated anode and that the displaced electrolyte does not need additional volume. The level of practically achievable improvements in capacity and energy density of silicon anode based LIBs is expected to be between 5-15% for lithiated anode porosities of 10-30% to ensure the battery has similar life and power characteristics of conventional LIB.

Metal fluoride conversion electrodes have been of fundamental interest as high energy density electrodes for lithium batteries for over 40 years, however, the theoretical electrochemical activity of such materials remained elusive as a result of their high bandgap and poor ionic and electronic charge transport characteristics. Well over a decade ago, electronically and mixed conducting matrices to form metal fluoride nanocomposites resulted in the revelation of the theoretical voltages, high energy densities, and minimal reversibility of some of the most promising fluorides and oxyfluorides which operate over 2V. Since this time many in our community have investigated these materials and advanced the state of the science significantly. This paper will discuss a sampling of the scientific, technological, and practical questions that still stand today as supported by examples of research from the community and our laboratories.

Achieving solid state batteries that operate at room temperature is an elusive, but compelling goal, one that researchers have been working on decades. Solid state batteries hold the promise of much safer and robust energy storage, with potentially higher energy density as well. But the challenges for a thin, stable solid electrolyte with adequate transport and mechanical properties, plus a practical route for large scale manufacturing is daunting. Perhaps more worrisome is the challenge of a stable interface with electrodes being single phase or solid composites. How can the electrodes cycle many times while still maintaining good physical contact and low resistance to ion transport with the solid electrolyte? Should we perhaps compromise with an “almost all” solid state battery? What energy densities can we reasonably expect? Can advanced manufacturing methods provide solutions?
This presentation highlights recent research at ORNL and with our collaborators, as well as reports from other groups, that clearly point out the challenges facing solid state battery development, where we lack fundamental understanding of materials and interfaces, and pragmatic approaches that might move us toward a near term success.
Acknowledgements: The presenter thanks co-editors, William E. West and Jagjit Nanda, and the contributing authors of the Handbook of Solid State Batteries, 2015, for their insights. Research conducted at ORNL was supported by the U.S. Department of Energy through the Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (for inorganic solid electrolytes) and through the Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office, Advanced Battery Materials Research program (for polymer and composite electrolytes).

Achieving significant gains in energy density and specific energy beyond lithium ion battery technology will require the use of alkali (lithium) and alkaline earth (magnesium) metals as anodes. Power requirements over a practical temperature range necessitate the use of these reactive metals in direct contact with a liquid electrolyte resulting in parasitic reactions yielding solid electrolyte interphases that control the accommodation and removal of metal during energy release (discharge) and energy storage (charging cycles). Where Li dendrite formation is the most readily recognized form of loss of dimensional control leading to safety concerns, structural changes that lead to Coulombic efficiency loss are far more common for Li anodes in oxygen and sulfur cells and for Mg anodes coupled with insertion and sulfur cathodes. We explore the origins of loss of dimensional control due to film formation within the Mg system starting with ether-based Mg chloro complex forming electrolytes. Well faceted Mg deposits form in these electrolytes as step-flow growth dominates deposition. Using a combination of chronopotentiometric trace analysis and operando imaging and spectroscopic analysis of the interface during metal addition and removal, we show that interfacial films are responsible for guiding localized dissolution phenomenon that result in cumulative morphology evolution leading to Mg loss with repeated cycling. Interfacial films also play an important role when weakly coordinating anions are used in the place of chloride to deliver the cation to the Mg surface. In these systems, re-nucleation of Mg onto itself plays a dominant role in defining structure and in dictating subsequent efficiency loss. We compare interfacial film composition for several weakly coordinating anions, including bis(trifluoromethylsulfonyl)imide, and contrast film identity and role when chloride anion is present. Lastly, we focus on recent work that explores the role of lithium fluoride – lithium imide salt combinations in ethers at concentrations that yield a solvate electrolyte. Within this system of electrolyte, we probe the role the fluoride film plays in directing Li accommodation and removal from the Li substrate. Loss of dimensional control is probed as a function of local ionic transport within and mechanical properties of the film.

This work is supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science. Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. DOE’s NNSA under contract DE-AC04-94AL85000.

Interphase has been the central component that enables battery chemistries of high voltage to reversibly operate, the prominent example of which is the very successful Li-ion ^{1, 2}. The possibility of such a protective interphase has been confined to non-aqueous electrolytes thus far, where the carbonate solvents serve as the main contributor of chemical building blocks of interphase. Recently, we found that by manipulating the inner solvation sphere of Li-ion, one could form such interphase in aqueous electrolytes ³. The expanded electrochemical stability window of such new electrolytes opens new possibilities of aqueous electrochemical devices. In this talk we will examine the criteria for electrolyte components that enables the formation of aqueous SEI as well as the formation mechanism involved.

In recent years, lithium sulfur (Li-S) batteries have garnered drastic research interest for both transportation and large-scale (grid) energy storage applications mainly because of this electrochemical couple's high theoretical gravimetric energy density which is projected to be twice that of the state-of-art lithium-ion (Li-ion) batteries and the potential for a greatly reduced battery cost. For practical applications of Li-S batteries, many challenges exist, including poor cycling stability, low round-trip efficiency and severe self-discharge, all of which are rooted in the dissolution of long-chain polysulfide species. The soluble intermediate polysulfides diffuse out of the cathode gradually and “contaminate” almost all of the exposed surfaces in the cell, leading to a series of side reactions and thus poor electrochemical performance.

To overcome the aforementioned hurdles in Li-S batteries, many different approaches have been proposed to modulate the cathode structure, decorate the separator, and alter the interfacial reactions on the anode side of the cell by using new electrolyte recipes or additives. Although the number of publications on Li-S battery research has increased exponentially, still the progress towards the market penetration of Li-S battery is limited. In this talk, an overview will be given on the recent advances in Li-S battery research but focus will be put on the scientific gap between lab R&D and industry need. All the related components in Li-S cells including cathode, electrolyte, separator, anode electrode/electrolyte interfaces and current collector will be reviewed in this talk and future research directions will also be proposed.

Lithium sulfur (Li-S) batteries may catch up with lithium ion batteries (LIBs) due to its high specific energy and reduced cost from the use of sulfur. Li-S has the highest theoretical specific and volumetric energy densities of any rechargeable battery chemistry (2550 Wh/kg and 2862 Wh/L). The best announced Li-S batteries can offer around 350 Wh/kg and 320 Wh/L currently, which is still significantly better than LIBs (150-200 Wh/kg). The high capacity is based on the conversion reaction of sulfur to form lithium sulfide (Li₂S) by reversibly incorporating two electrons per sulfur atom compared to one or less than one per transition-metal ion in insertion oxide cathodes. Sulfur cathode suffers from poor cyclability, which is mainly due to shuttling effect of polysulfides and volume change (ca. 80%). To unlock this potential battery, efforts have been made such as carbon accommodation/coating, electrolyte additives, membrane and lithium anode modification, however, none of them were commercially available until now. Here we propose a total solution based on polymer binders, carrageenan, which is a natural polysaccharide obtained from red seaweeds. Based on the polymer, we build a high areal capacity electrode (ca. 7 mg/cm²) that could buffer the polysulfides shuttle effect and help to maintain the mechanical integrity of the sulfur electrode. A simple, cheap, scalable and versatile way was chosen to obtain sulfur electrodes with high binding capability, the property gain in terms of electrochemical performance (high areal capacity with high current density) is demonstrated, which proves our proposal that an effective binder is one of the key factors to improve the loading of sulfur electrode. This discovery will arouse battery community’s interest in developing advanced green electrode fabrication process for Li-S cells based on binders.

We are in the midst of an electrochemical energy storage boom where rapid development of next-generation battery chemistries is mandatory to keep pace with demand for lower cost, lighter, and smaller batteries that are capable of powering a range of technologies from mobile devices to on-demand delivery of renewable energy to the electric grid. Lithium-sulfur (Li-S) batteries are one promising solution that offer 3-5 times more energy per unit weight, improved safety, and dramatic cost reductions relative to conventional Li-ion batteries. A key component is the sulfur cathode because of its high theoretical capacity (1672 mAh/g), abundance (0.03%), non-toxicity, and low cost 8 and Li₂S. Upon discharge a conformal coating of insulating Li₂S is deposited on conductive carbon current collectors resulting in large overpotentials and incomplete sulfur utilization. The current paradigm to improve utilization involves higher loadings of high-surface area conductive carbon in cathode formulations where the enhanced utilization is correlated to the engineered micro/mesopores that accommodate additional Li₂S electrodeposition.

We have developed a set of organic redox mediators that are electrochemically active in either the soluble regime or precipitation plateau of the polysulfide cathode that mitigate surface area restrictions on current collector design.^[1,2] Improved charge transport in the presence of redox mediators results in higher sulfur utilization and in the case of Li₂S, an entirely new porous 3-D morphology is observed upon electrodeposition. Our theory guided selection criteria, the mode of operation for the redox mediators, and their effects on Li-S cell performance will be presented.

[1] Frischmann, P. D.; Gerber, L. C. H.; Doris, S. E.; Tsai, E. Y.; Fan, F. Y.; Qu, X.; Jain, A.; Persson, K. A.; Chiang, Y.-M.; Helms, B. A. “Supramolecular Perylene Bisimide-Polysulfide Gel Networks as Nanostructured Redox Mediators in Lithium-Sulfur Batteries” Chem. Mater. 2015, 27, 6765.

Provisional Patent Application “Electronically Percolating Networks for Redox Flow Batteries” US Patent Application Ser. No: 62/036,056.

[2] Gerber, L. C. H.; Frischmann, P. D.; Fan, F. Y.; Doris, S. E.; Qu, X.; Scheuermann, A. M.; Persson, K.; Chiang, Y.-M.; Helms, B. A. “3-Dimensional Growth of Li₂S in Lithium–Sulfur Batteries Promoted by a Redox Mediator” submitted.

The demand for rechargeable batteries with high gravimetric and volumetric energy densities will continue to grow, due to the rapidly increasing integration of renewable energy into the global energy scheme. In terms of energy density, modern high-end rechargeable-battery technologies are reaching their fundamental limits and major development leaps are not to be expected in this field. Promising material combinations for near-future non-aqueous rechargeable batteries suitable for portable electronics and automotive applications can be identified using the energy-cost model, which has been developed for the comparative evaluation of battery-cell chemistries in a commercial-type pouch-cell configuration and helps to establish the relationship between cost and energy densities.
Among the wide variety of positive-electrode materials, only few have a sufficiently high potential for commercialization and undoubtedly the immediate future will still be dominated by Li-ion technology, with Li-rich and Ni-rich materials as definite winners, but Li–S and Na-ion chemistries as leading contenders, owing to the low cost and abundance of their key components. Further significant improvements in energy density — be it gravimetric or volumetric — and cost cannot be achieved through the use of new battery chemistries alone. Instead, engineering aspects, targeting cost reduction and safety assurance, will likely be the main driving force of future rechargeable-battery development.
Due to the low cost of raw sulphur and its high theoretical specific charge upon lithiation, the Li–S system, despite its quite narrow potential window, offers the most attractive energy–cost ratio. Also, Li–S batteries can deliver higher gravimetric energy densities than any of their Li-ion counterparts. However, the volumetric energy densities for Li–S systems will at best be on par with state-of-the-art Li-ion batteries. To optimize the performance of the Li–S cell, further engineering efforts are essential, as the Li–S chemistry is exquisitely sensitive to changes in its constituents and in their relative or absolute amounts. We will present an overview of recent advances in the field and will in particular discuss the advantages of introducing standard electrodes and of standardizing testing protocols.

The rechargeable lithium-sulfur battery is a promising option for energy storage applications because of its low cost and high energy density. The electrochemical performance of the sulfur cathode, however, is substantially compromised due to fast capacity decay caused by polysulfide dissolution/shuttling and low specific capacity caused by the poor electrical conductivities of the active materials.
Recently we have proposed a strategy of utilizing ternay hybrid material structures in which each component takes a different function and synergistically cooperates to reach high electrochemical performance. A hybrid material composed of multi-wall carbon nanotubes (MWCNTs), NiFe₂O₄ nanosheets and S nanoparticles has demonstrated superior cycling stability together with good specific capacity and rate capability.
We further advance the strategy with a ternary hybrid structured free-standing and flexible cathode composed of single-wall carbon nanotubes (SWCNTs), S nanoparticles and polypyrrole (PPy). The SWCNT-S-PPy flexible cathode can deliever a specific capacity of more than 1200 mAh g^-1 at 2 A g^-1 with a low decay rate of 0.032% per cycle over 500 recharging cycles, representing the highest electrochemical performance for flexible Li-S battery electrodes.

The ever-increasing energy storage demands of laptops, mobile phones, cameras, electric tools, unmanned aerial vehicles and cars have motivated research on safer, lighter and lower cost rechargeable batteries. Commercial Li-ion batteries utilizing intercalation-type Co- and Ni- containing cathode materials suffer from natural toxicity, relatively high cost and limited specific capacities ^[1]. Since 2009, the lithium-sulfur chemistry with the high theoretical energy of 2600 Whkg^-1 has attracted great attention in the battery community ^[2-4]. Sulfur with the theoretical capacity of 1675mAh g^-1 is abundant in the earth crust, cheap and environmentally friendly. However, the large expansion (~79 vol. %) of S during its lithiation to Li₂S may induce mechanical damages within electrodes or S-comprising composite electrode particles, while polysulfide dissolution during cycling induces active material loss and increases cell resistance. Compared with S, the Li₂S cathode can pair with the anode materials that do not contain Li, such as C, Si and Sn, and thus offer greatly improved safety.
Here, we reported on the development of a hierarchical particle-shell architecture aimed to enhance long-term stability of the Li₂S-based cathodes. Such architecture provides multi-level protection against failure in the protective shelling material. Numerical modeling demonstrated an opportunity to significantly reduce stresses within the outer shell, which should further enhance mechanical stability of the particles and thereby enable effective operation of the electrochemical cells. As an example of the practical hierarchical composite, we synthesized C-Li₂S-C particles by using a simple solution-based method followed by carbon outer shell deposition through atmospheric pressure CVD ^[5-6]. Electrochemical tests showed excellent capacity utilization and rate performance, enabled by embedding Li₂S (5-10 nm in size) into conductive material matrix. More importantly, by employing the hierarchical core-shell nanocomposite design, our Li₂S cathodes effectively mitigated polysulfide dissolution and achieved remarkable stability within 600 cycles. The hierarchical particle-shell architecture could be applied in many other high-energy volume-changing active materials that could be damaged or dissolved during undesirable interactions with electrolytes.
References
[1] D. Andre, S.-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, B. Stiaszny, Journal of Materials Chemistry A 2015, 3, 6709.
[2] H. D. Yoo, E. Markevich, G. Salitra, D. Sharon, D. Aurbach, Materials Today 2014, 17, 110.
[3] F. Wu, H. Kim, A. Magasinski, J. T. Lee, H.-T. Lin, G. Yushin, Advanced Energy Materials 2014, 4, 1400196.
[4] X. Ji, L. F. Nazar, Journal of Materials Chemistry 2010, 20, 9821.
[5] F. Wu, H. Kim, A. Magasinski, J. T. Lee, H.-T. Lin, G. Yushin, Advanced Energy Materials 2014, 4, 1400196.
[6] F. Wu, J. T. Lee, F. Fan, N. Nitta, H. Kim, T. Zhu, G. Yushin, Advanced materials 2015, 27, 5579.

To keep up with the increasing energy demands, lithium-sulfur battery (Li-S) poses itself as the next-generation battery technology due to its high theoretical capacity (1672 mA h g^-1). Li-S batteries are promising due to their low cost, high abundance, and benign environmental impact. However, sulfur and the end discharge product (Li₂S) are insulating in nature. This, combined with the problem of polysulfide (PS) diffusion, leads to low active-material utilization and limited rate capability. To address these issues, a huge focus has been centered on increasing the conductive performance and localizing PS within the cathode region.
There are many ways scientists have approached this problem, including the use of S-carbon composite materials. Most composites, however, lack sufficient malleability to effectively trap PSs without any substantial impact on the mass energy density of the cell. This presentation introduces light-weight heteroatom boron-doped (B-rGO) and nitrogen-doped (N-rGO) reduced graphene oxide into the cathode architecture as a comprehensive examination into (i) the conductivity that rGO networks provide, (ii) the conductive contribution from the heteroatom dopant, and (iii) the filtering ability that pseudo-stacked heteroatom-doped rGO offers. The graphene network increases conductivity by shortening ion/electron transport pathways while heteroatom doping improves the contact between nonpolar graphene network and the polar PSs. The stacking of the heteroatom-doped rGOs provides enough room for the PS filter to minimize diffusion.
We first present the B/N-doped rGO properties as a binder-free coated separator on a polypropylene membrane (Celgard) and then as a self-supported binder-free PS filter under higher S loadings. As a binder-free filter coated separator, N-rGO was able to attain a peak discharge capacity of 1191 mA h g^-1 and a reversible capacity of 849 mA h g^-1 over 50 cycles. The similarly prepared B-rGO-coated separator was able to attain a peak discharge capacity of 1027 mA h g^-1 and a reversible capacity of 608 mA h g^-1 over 50 cycles. Due to the promising performance of binder-free filter-coated separators, S loading was increased in both cell configurations. A B-rGO binder-free self-supported PS filter was able to accommodate a S loading of 4.2 mg cm^-2 and attain a peak discharge capacity of 1257 mA h g^-1 while maintaining a reversible capacity of 876 mA h g^-1 over 50 cycles. As a binder-free self-supported PS filter, N-rGO was able to accommodate a S loading of 3.9 mg cm^-2 with a high peak discharge capacity of over 1670 mA h g^-1- a value very close to the full electrochemical utilization of S. It is additionally able to maintain a reversible capacity of 1110 mA h g^-1 after 50 cycles. Thus, B/N-doped rGOs prove themselves to be a viable material for improving the capacity, cycle stability, and rate capability while accommodating higher S loadings in Li-S cathodes.

There has been enormous interest lately in lithium sulfur batteries, since they have 5 times the theoretical capacity of lithium ion batteries¹ and already satisfy the energy requirements of the DOE 2020 goals for transportation. Large-scale adoption of this technology has been hampered by numerous shortcomings, chiefly the poor utilization of the active cathode material and rapid capacity fading during cycling. This in turn requires methods capable of identifying and quantifying the products of the poorly understood electrochemical reactions.
One recent advance has been the use of X-ray absorption spectroscopy (XAS), an element-specific probe of the unoccupied energy levels around an excited atom upon absorption of an X-ray photon, to identify the reaction products and intermediates². In this talk, we’ll present first principles molecular dynamics and spectral simulations of dissolved lithium polysulfide species, showing how finite temperature dynamics, molecular geometry, molecular charge state and solvent environment conspire to determine the peak positions and intensity of the XAS^3-5. We’ll present a spectral analysis of the radical (-1e charge) species, reveal a unique low energy feature that can be used to identify these species from their more common dianion (-2e charge) counterparts. The impact of our findings on the proposed LiS discharge electrochemical pathways will be discussed⁶.

References
(1) Peramunage, D.; Licht, S. Science 1993, 261, 1029.
(2) Gao, J.; Lowe, M. A.; Kiya, Y.; Abruña, H. D. The Journal of Physical Chemistry C 2011, 115, 25132.
(3) Pascal, T. A.; Pemmaraju, C. D.; Prendergast, D. Physical Chemistry Chemical Physics 2015, 17, 7743.
(4) Pascal, T. A.; Boesenberg, U.; Kostecki, R.; Richardson, T. J.; Weng, T.-C.; Sokaras, D.; Nordlund, D.; McDermott, E.; Moewes, A.; Cabana, J.; Prendergast, D. The Journal of Chemical Physics 2014, 140, 034107.
(5) Pascal, T. A.; Wujcik, K. H.; Velasco-Velez, J.; Wu, C.; Teran, A. A.; Kapilashrami, M.; Cabana, J.; Guo, J.; Salmeron, M.; Balsara, N.; Prendergast, D. The Journal of Physical Chemistry Letters 2014, 5, 1547.
(6) Wujcik, K. H.; Pascal, T. A.; Pemmaraju, C.; Devaux, D.; Stolte, W. C.; Balsara, N. P.; Prendergast, D. Advanced Energy Materials 2015.

Li-S batteries are intensively investigated nowadays aiming to replace the commercial lithium intercalation-based Li-ion batteries, due to their higher energy density and lower raw materials price. Nonetheless, several problems have to be well addressed before they are viable for commercialization. First, because of the insulating nature of elemental sulfur and discharge products (e.g. Li₂S/Li₂S₂), large amount of inactive carbonaceous materials or conductive polymers are necessary to lower the ohmic polarization. Second, irreversible sulfur and electrolyte losses during cycling are known to account for the rapid capacity fading and low columbic efficiency. Meanwhile, the deposition or aggregation of solid sulfur and/or Li₂S in cathode often makes the active materials inaccessible, deteriorating the cycling and rate performances.
To solve these problems, various approaches have been proposed to trap solid sulfur in micro-/nano-porous structures. However, none of them have been proven to completely prevent the dissolution and shuttling of polysulfides. Contrary to the strategy of avoiding the dissolution of polysulfides, liquid-type polysulfides-containing catholyte has been recently used as active materials. Compared to the sluggish reaction associated with insulating solid-phase sulfur, dissolved polysulfides can alleviate the irreversible deposition of S or Li₂S at “dead” sites, and therefore offer a higher utilization of active materials.
Herein, we designed binder-free Li-S batteries with free-standing 3-dimensional networks of carbon nanotube (CNT) sponges, which were used as bifunctional catholyte reservoir and conductive framework in the cathode. The unique microporous structure of the CNT sponge can accommodate a large amount of catholyte as well as form intimate contacts with dissolved redox species in the cathode. Two different catholyte concentrations and loading volumes were used and compared to investigate their effects on battery performances. The electrochemical impedance spectra (EIS) were measured at seven different stages in the first discharge/charge cycle to understand the reaction mechanism of our sulfur batteries. By optimizing the catholyte concentrations and loading volumes, we have achieved a highly stable cycling performance with capacity retention higher than 90% at 0.5-C rate for 300 cycles. According to the impedance spectra during the first discharge/charge cycle along with electron microscopy images, sulfur was uniformly distributed/deposited on the surface of CNT sponge at the fully charged state, and the deposition of solid Li₂S₂/Li₂S at fully discharged state was found to be the rate-determining step. The facile scalable one-step synthesis process of the CNT sponge with inexpensive raw materials used in our sulfur batteries will be a big step toward developing practically viable Li-S batteries in the near future.

Transition metal oxides (TMO) have been reported to have a catalytic effect on the oxygen evolution reaction in aprotic Li-oxygen batteries, resulting in lowered charging overpotentials and increased cycling lives. Here, we were able to unravel the role of three TMO’s (MnO_x, Co₃O₄ and RuO₂) in the oxidation of Li₂O₂ using Differential Electrochemical Mass Spectrometry (DEMS). MnO_x, Co₃O₄ and RuO₂ particles, synthesized without using any templates, surfactants or ligands, were incorporated in cathodes of Li-oxygen batteries operated with 1 M LiTFSI DME electrolyte to investigate their electrochemical functionalities. First, using our DEMS instrument, we could evaluate the oxygen recovery-to-consumption ratio (OER/ORR efficiency), and relate it to the galvanostatic profile obtained over an electrochemical cycle. In addition, linear sweep voltammetry coupled with mass spectrometry clearly showed two distinguished oxygen evolution stages and how these stages varied with TMO cathodes in charging Li-oxygen batteries. Second, the origin of decomposition of DME in the presence of these TMO’s was studied by discharging cells with ¹³C based TMO cathodes in an ¹⁸O₂ environment. The combination of electrochemical and mass spectrometric data allowed us to determine the exact contribution of the TMO cathodes to the ORR and OER reactions. We believe that these results provide a useful guidance for selecting high performance cathode materials by considering all necessary aspects for next generation, high energy density, Li-oxygen batteries.

A rechargeable battery based on an Mg/O₂ couple presents an attractive chemistry due to its high theoretical energy density and potential for low cost. Nevertheless, many fundamental aspects of this system remain poorly understood, such as the reaction mechanisms associated with discharge and charge. The present study employs first-principles calculations to characterize electrochemical processes on the surfaces of likely Mg/O₂ battery discharge products, MgO and MgO₂. Thermodynamic limiting potentials for charge and discharge are calculated for several scenarios, including variations in surface stoichiometry and the presence/absence of intermediate species in the reaction pathway. The calculations suggest that pathways involving oxygen intermediates are preferred, as they generally result in higher discharge and lower charging voltages. In agreement with recent experiments, cells that discharge to MgO are predicted to exhibit low round-trip efficiencies, which are rationalized by the presence of large thermodynamic overvoltages. In contrast, MgO₂-based cells are much more efficient: high discharge and low charging voltages on oxygen-rich facets result in a round-trip efficiency of approximately 90%. We conclude that the performance of Mg/O₂ batteries could be dramatically improved by biasing discharge towards the formation of MgO₂ rather than MgO.

The primary discharge product in sodium−air batteries has been reported in some experiments to be sodium peroxide, Na₂O₂, while in others sodium superoxide, NaO₂, is observed. Importantly, cells that discharge to NaO₂ exhibit low charging overpotentials, while those that discharge to Na₂O₂ do not. These differences could arise from a higher conductivity within the superoxide; however, this explanation remains speculative given that charge transport in superoxides is relatively unexplored. Here, density functional and quasi-particle GW methods are used to comparatively assess the conductivities of Na−O₂ discharge phases by calculating the concentrations and mobilities of intrinsic charge carriers in Na₂O₂ and NaO₂. Both compounds are predicted to be electrical insulators, with bandgaps in excess of 5 eV. In the case of sodium peroxide, the transport properties are similar to those reported previously for lithium peroxide, suggesting low bulk conductivity. Transport in the superoxide has some features in common with the peroxide but also differs in important ways. Similar to Na₂O₂, NaO₂ is predicted to be a poor electrical conductor, wherein transport is limited by sluggish charge hopping between O₂ dimers. Different from Na₂O₂, in NaO₂ this transport is mediated by a combination of electron and hole polarons. An additional distinguishing feature of the superoxide is its ionic conductivity, which is 10 orders of magnitude larger than the electronic component. The ionic component is comprised primarily of p-type contributions from (surprisingly mobile) oxygen dimer vacancies, and from n-type contributions from negative sodium vacancies. In the context of sodium−air batteries, the low electronic conductivity afforded by NaO₂ suggests that enhanced bulk transport within this phase is unlikely to account for the low overpotentials associated with its decomposition. Rather, the enhanced efficiency of NaO₂-based cells should be attributed to other factors, such as a reduced tendency for electrolyte decomposition.

The Li–O₂ battery exhibits high theoretical specific energy, making it a promising candidate for energy storage in electric vehicles (EV). One of the main issues hindering the application of these batteries is the decomposition of the organic electrolyte during cycling. This process results in high charging overpotentials, capacity fading, and limited cycle life. Hence, the understanding of these reactions is a crucial step in developing practical Li-O₂ batteries. Our prior studies [Kumar et al., J. Phys. Chem. C, 2015, 119(17), 9050-9060] used Van der Waals-augmented DFT calculations to study the initial decomposition reactions of the commonly used electrolyte solvent, DME, on peroxide and superoxide terminated surfaces of the Li₂O₂ discharge product. We find that a possible degradation reaction route is via hydrogen abstraction from the DME molecule. In addition, DME was found to be more susceptible to undergo chemical decomposition on superoxide terminated surfaces than on peroxide terminations. To improve the stability of the electrolyte we investigate the influence of varying degrees of fluorination of DME (DME-F) on degradation kinetics. A combination of classical Monte Carlo and DFT calculations are used to identify low energy geometries for DME-F adsorption. Activation energies for plausible decomposition pathways are subsequently evaluated. We find that the fluorination reduces the likelihood of electrolyte degradation by increasing the activation energy of hydrogen abstraction.

Without a doubt the Holy Grail of battery research is the development of a post lithium ion technology. This may require a shift towards batteries containing a pure metal anode. Li metal is an attractive metal anode in part due to its high volumetric capacity (2062 mAh cm^-3), a high reductive potential of -3.0 V vs. NHE and the wide availability of lithium electrolytes. However, its deposition occurs unevenly with formation of dendrites which leads to safety concerns during cycling. In contrast to lithium metal, magnesium metal deposition is not plagued by dendritic formation. Additionally, magnesium is more stable than lithium when exposed to air, more abundant in the earth crust and provides a higher volumetric capacity (3832 mAh cm^-3). However, magnesium has a reductive potential of -2.36 V vs. NHE and has a unique electrochemistry which prohibited the use of magnesium analogues of lithium electrolytes. Since the oxidative stability of electrolytes governs the choice of cathodes it is of paramount importance to develop non-corrosive magnesium electrolyte with wide electrochemical windows which will permit discovery of high voltage cathodes. In this talk we will present the latest developments and future challenges which must be overcome.^1,2,3,4,5,6

1. Aurbach, D., Lu, Z., Schechter, A., Gofer, Y., Gizbar, H., Turgeman, R., Cohen, Y., Moshkovich, M. and Levi, E., Nature, 2000, 407, 724-727.
2. Kim, H.S., Arthur, T.S., Allred, G.D., Zajicek, J., Newman, J.G., Rodnyansky, A.E., Oliver, A.G., Boggess, W.C. and Muldoon, J., Nat. Commun., 2011, 2, 427.
3. Muldoon, J., Bucur, C.B., Oliver, A.G., Sugimoto, T., Matsui, M., Kim, H.S., Allred, G.D., Zajicek, J. and Kotani, Y., Energy Environ. Sci., 2012, 5, 5941-5950.
4. Muldoon, J., Bucur, C.B., Oliver, A.G., Zajicek, J., Allred, G.D and Boggess, W.C. Energy Environ. Sci, 2013, 6, 482-487.
5. Muldoon, J., Bucur, C.B. and Gregory. T. Chem. Rev., 2014, 114, 11683-11720
6. Muldoon, J., Bucur, C.B. and Gregory. T. Phys. Chem. Lett., 2015, 6, 3578–3591

Nonaqueous magnesium-ion battery research has been growing due to the attractive characteristics of such systems, including a high theoretical energy density and, compared to lithium-ion systems, a relatively low cost of materials. Increasing the electronegativity of precursor ligands is an important trend in enhancing the oxidative stability of electrolytes, and thus can enable the use of higher voltage cathode materials.^1,2
We have performed electrochemical analysis on a series of solutions made from various magnesium alkoxide and aryloxide compounds in combination with AlCl₃ in ethereal solvents. Displaying comparable electrochemical windows, deposition efficiencies, and conductivities compared to previously researched systems, these compounds show a high degree of promise. Additionally, these free flowing alkoxide powder precursors are significantly easier to handle compared to traditional Grignard-based and amido-based magnesium electrolyte precursors. Our work seeks to more fully understand these trends and materials. The synthesis and electrochemical behavior of a series of magnesium alkoxide and aryloxide salts were prepared as precursors for electrolytes for magnesium-ion batteries, and analyzed using cyclic voltammetry and coin-cell measurements.

1. Yoo, H. D.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.; Aurbach, D.Energy Environ. Sci. 2013, 6, 2265.
2. Nelson, E. G.; Kampf, J. W.; Bartlett, B. M. Chem. Commun. (Camb). 2014, 50, 5193–5195.

Lithium-ion batteries (LIBs) are nowadays the predominant battery technology for portable electronics and are of growing importance in the area of electrical mobility. However, the limited abundance and uneven global distribution of lithium salts are raising concerns regarding future price development and supply security. In this regard, Sodium-ion batteries (SIBs) are gaining increasing attention as more economical alternative, due to the ubiquitous nature of sodium salts. However, especially due to the ~50% larger ionic radius of the Na-ion the electrochemistry of SIBs and LIBs is generally very different and the development of new materials is essential in this field. In fact, many alloying and conversion materials are considered promising candidates as electrode materials for SIBs based on their high specific and volumetric capacities. However, these materials undergo drastic volume changes leading to rapid capacity fading after only a few cycles. In this regard, nanocrystals (NCs) are advantageous electrode materials, since they can improve the cycling stability by mitigating the impact of volume changes. Moreover, as a result of the reduced dimensions and higher surface area, NCs offer faster kinetics compared to their bulk counterparts.
Due to its non-toxicity, low raw material cost and high storage capacity FeS₂ so far has been only considered as promising cathode material for LIBs and SIBs. We demonstrate, for the first time, that FeS₂ NCs can in fact serve as excellent Na-ion anode material with good cycling stability and rate capability.^[1] Namely, FeS₂ NCs deliver capacities ≥500 mAhg^-1 for 400 cycles at a current rate of 1000 mAg^-1 clearly exceeding the performance of both bulk FeS₂ as well as various other nanostructured metal sulfides.
Further, we present a hybrid intercalation battery based on a FeS₂ NC cathode, metallic magnesium anode and sodium/magnesium dual salt electrolyte. ^[2] Unlike lithium or sodium, metallic magnesium can be safely used due to dendrite-free electroplating and offers high volumetric (3833 mAhcm^-3) and gravimetric capacities (2205 mAhg^-1). Na-ion cathodes – in particular FeS₂ NCs – may serve as attractive alternatives to Mg-ion cathodes due fast, highly reversible insertion of Na-ions. In the presented proof-of-concept study, electrochemical cycling of the Na/Mg hybrid battery is characterized by high rate capability, high coulombic efficiency of 99.8% and high energy density. In particular, with an average discharge voltage of ~1.0 V and an average cathodic capacity of 189 mAhg^-1 at a current of 200 mAg^-1, the presented Mg/FeS₂ hybrid battery delivers energy densities of up to 215 Whkg^-1. Such a hybrid Na-Mg battery, fully based on Earth-abundant elements, is highly promising for future large-scale energy storage solutions.

[1] M. Walter, T. Zünd and M. V. Kovalenko Nanoscale, 2015, 7, 9158-9163.
[2] M. Walter, K. V. Kravchyk, M. Ibanez and M. V. Kovalenko, submitted.

Aluminum is a very attractive material for next-generation energy storage, since it is the most abundant metal in the earth’s crust. Its relatively low atomic weight of 26.98 along with its trivalence give a corresponding electrochemical equivalent of 2.98 Ah/g and a extremely high volume capacity(8.04 Ah/cm³). It is noted that the Al³⁺ cation has a smaller radius (53.5 pm), which may act as a guest species in intercalation chemistry. Fabrication of rechargeable aluminum battery working at room temperature did not succeed until haloaluminate containing ionic liquids were used as electrolytes. Anions of ionic liquids are expected to have a great effect on performance of rechargeable aluminum batteries. Concentration of Al₂Cl₇^- is considered as a key factor in chloroaluminate ionic liquids when used as electrolytes. Vanadium pentoxide (V₂O₅) is a favorable candidate as a Na-ion and Al-ion intercalation electrode because of its layered structure, which is open to reversible metal-ion insertion/extraction. Considering acidic AlCl₃ contained ionic liquids are not compatible with some binders, a binder-free cathode was synthesized by in-situ hydrothermal deposition of V₂O₅ on Ni foam current collector, which delivered an initial discharge capacity of 239 mAh/g. An obvious discharge voltage plateau appeared at 0.6 V in the discharge curves of the Ni-V₂O₅ cathode, which is slightly higher than that of the V₂O₅ nanowire cathodes with common binders attributing to reduced electrochemical polarization. In comparison with crystalline V₂O₅, the hydrated form of vanadium pentoxide (V₂O₅●nH₂O) has good chemical stability. Due to the expended interlayer distance, large atomic and molecular species and even polymers can be reversibly intercalated between the layers of V₂O₅●nH₂O. Therefore, it is a very promising strategy to prepare V₂O₅●nH₂O with a particular structure on a conductive substrate, to achieve aluminum battery with good performance by enhancing charge-transfer conductivity and preserving the interface morphology integrity. A simple and clean hydrothermal approach was used for synthesizing a cathode material, consisting of V₂O₅●0.5H₂O nanometer-thick lamella grown on stainless steel mesh. It could effectively maintain the electrode integrity during charge/discharge processes and facilitate the electronic and ionic transportation, with the help of the mechanical strength and the high porosity of the stainless steel mesh. A stabilized capacity of 110 mAh/g under a high current density of 100 mA/g was achieved at 180 cycles.

Magnesium-ion batteries represent one possible path to affordable grid level energy storage, due to the high volumetric energy density afforded by metallic magnesium anodes. One of the primary goals of the work being done in this field is the development of electrolytes that are capable of reversible magnesium electrodeposition. Stable electrolytes that possess a large electrochemical window, high cycling efficiency, and compatibility with intercalation-type cathodes are uncommon. Understanding surface phenomena in these systems, particularly with respect to the magnesium anode, is crucial to rational development of future electrolytes. Using a combination of x-ray photoelectron spectroscopy and energy-dispersive x-ray spectroscopy, we have systematically analyzed how electrolyte composition affects surface structures and elemental makeup of both the magnesium anode and deposits on the counter electrode. By performing long-term tests on sequential generations of electrolytes developed during the past ten years, trends relating electrochemical performance to interfacial phenomena begin to emerge. Factors such as long metallic growths and formation of passivating surface films have a significant impact on the long term performance of cells that use magnesium anodes, and dictate whether a given electrolyte is commercially viable.

Multivalent battery systems like rechargeable magnesium (Mg) batteries have recently gained more interest as candidate post-lithium (Li) battery systems, for possible applications in electric vehicles (EVs) and plug-in hybrid vehicles (PHVs). This is primarily due to concerns over the range performance of current Li battery systems, and the space requirements for future EVs and PHVs. Mg, being divalent and denser, is theoretically capable of delivering a higher volumetric energy-density (3833 mAh cm^-3) than Li (2061 mAh cm^-3), making it a viable battery system for addressing current range and space concerns.^1-4 To date, various organohaloaluminate electrolytes and electrolytes containing the B-H family have been utilized in Mg batteries, due to the incompatibility of conventional battery electrolytes (TFSI^-, ClO₄^-, PF₆^-) with Mg metal anodes.^3,5 However, as recently reported, it is also possible to use conventional battery electrolytes for Mg-ion batteries, by changing the type of anode, from a Mg metal anode to a Mg-ion insertion-type anode. This change enables Mg-ion transport through the anode/electrolyte interface during the use of conventional battery electrolytes.^2-4,6
Here, we report recent advancements in alternate architectures, as well as new materials for insertion-type anodes for rechargeable Mg-ion batteries. Further, we address specific studies related to the observation of the anode/electrolyte interface for Mg batteries, which have recently been studied in some detail.^7,8 Results from the utilization of alternate architectures and recent fundamental analytical analyses, focused on studying and understanding the nature of the anode/electrolyte interface, will be presented and discussed.

References:

1 Mizuno F, Singh N, Arthur TS, Fanson PT, Ramanathan M, Benmayza A, Prakash J, Liu Y-S, Glans P-A, Guo J, Frontiers in Energy Research, 2014, 2, 1.
2 N. Singh, T. S. Arthur, C. Ling, M. Matsui and F. Mizuno, Chem. Commun., 2013, 49, 149.
3 Muldoon J, Bucur CB, Gregory T, Chemical Reviews 2014, 114, 11683.
4 T. S. Arthur, N. Singh and M. Matsui, Electrochem. Commun., 2012, 16, 103.
5 Mohtadi R, Matsui M, Arthur TS, Hwang S-J, Angewandte Chemie International Edition 2012, 51, 1.
6 Shao Y, Gu M, Li X, Nie Z, Zuo P, Li G, Liu T, Xiao J, Cheng Y, Wang C, Zhang J-G, Liu J, Nano Letters, 2014, 14, 255.
7 T. S. Arthur, P-A. Glans, M. Matsui, R. Zhang, B. Ma and J. Guo, Electrochem. Commun., 2012, 24, 43.
8 Benmayza A, Ramanathan M, Arthur T, Matsui M, Mizuno F, Guo J, Glans P-A, Prakash J, Journal of Physical Chemistry C, 2013, 117, 26881.

With an aim to increase the energy density and lower the cost, there has been immense interest recently in developing rechargeable batteries based on multivalent working cations, such as Mg²⁺ and Zn²⁺. However, it is often tedious to design and develop appropriate host structures for multivalent-ion transport. We present here a new microwave-assisted chemical insertion technique, which has been used successfully to intercalate Mg²⁺ and Zn²⁺ ions into microporous Mo_2.5+yVO_9+z host framework with large open channels. Mo_2.5+yVO_9+z with tunnels constructed by three-, six-, and seven-membered ring units of MO₆ octahedra (M = Mo^5+/6+ or V^4+/5+) belongs to the family of isostructural MoVNbTeO compounds, which are very active oxidation catalysts for light alkanes due to the redox activities of Mo and V. Mo_2.5+yVO_9+z has been shown previously to intercalate Li⁺ ions up to 6 Li per formula unit. With a novel microwave-assisted technique using metal acetates as a metal-ion source and diethylene glycol (DEG) as a reducing agent, divalent-ion-inserted compounds A_xMo_2.5+yVO_9+z (A = Mg and Zn, 0 < x < 3) could be prepared in as little as 30 min at 160 - 200 degrees Celcius. This method provides fast chemical insertion unlike the traditional technique that takes days by using moisture-sensitive organometallic reagents in heptane or hexane, such as di-n-butylmagnesium for Mg²⁺ and dimethylzinc or diethylzinc for Zn²⁺. The Mg-inserted compounds Mg_xMo_2.5+yVO_9+z (0 < x < 3) thus obtained have been investigated as cathode materials in Mg-ion batteries. Our preliminary results suggest that the inserted Mg²⁺ ions can be removed electrochemically from the framework and reinserted for many cycles, despite the fact that oxides have higher ionicity compared to sulfides (e.g., Chevrel phases). The crystal structure is found to be well maintained after cycling. This new chemical intercalation process offers a fast, inexpensive, easily scalable method to insert multivalent metal ions using relatively much safer chemicals under ambient atmosphere.

Mg rechargeable batteries (MgRBs) stand out as a promising candidate beyond lithium ion battery technologies due to high volumetric energy density, resource abundance, and the dendrite-free deposition behavior of Mg, which ensures safe operation. Many of the advantages of MgRBs originate from the divalent nature and small ionic size of Mg ions; however, these properties also render the cation too polarizing to diffuse easily in most ion-intercalation materials. We recently investigated interlayer expansion as a general atomic-level lattice engineering approach to transform inactive layered intercalation hosts into efficient Mg storage materials. The expansion boosts Mg diffusivity by three orders of magnitude, effectively enabling the otherwise barely active host to approach its theoretical capacity as well as to achieve one of the highest rate capabilities among Mg-intercalation materials. An electrochemical activation method to achieve ~200 mAh/g capacity of Mg intercalation will also be discussed.

In order to make available the increasing amount of energy produced by clean and renewable sources at any moment and conditions, cheap, efficient and high-capacity rechargeable systems are needed. Although lithium-ion batteries (LIB) meet these requirements for many applications, the possibility that LIB fulfil the growing demand of more energy-consuming uses is nowadays called into question.[1] Moreover, the future demand of Li seems difficult to satisfy since Li is relatively rare and not evenly distributed on Earth.[2] A strong interest is thus given to alternative solutions and, thanks to its abundance, low price, safety features and high volumetric energy density (3837 mAh/cm³), Mg is today considered as a promising candidate for next-generation energy storage applications. However, only highly air-sensitive electrolytes are compatible with Mg metal that undergoes irreversible passivation with conventional formulations.[3] One of the strategies to overcome this hurdle consists in replacing Mg metal with other materials able to reversibly alloy with Mg, building a veritable Mg-ion battery (MIBs), and allowing the use of safer electrolyte formulations. A few p-block elements (Sn, Sb and Bi) were identified as promising candidates for negative electrodes on MIBs. Following these leading studies, we showed that micron-sized Bi is able to reversibly alloy to Mg (372 mAh/g) even at fast current rates, undergoing a biphasic reaction with the direct formation of well-cristallized Mg₃Bi₂.[4] Cell capacities recorded during c-rate tests are in line with more elaborate Bi nanotubes.[5] Moreover, ball-milling made Mg₃Bi₂ was successfully tested in a complete cell and can be set as a promising candidate for next-generation of MiBs. Similar results were obtained for In, which reversibly alloys with Mg (425 mAh/g) at low c-rates exhibiting the lowest alloying voltage ever reported vs. Mg.[6] Fast rate cycling affects the performance of In that suffers of poor kinetics. Operando XRD shows the reversible formation of crystalline MgIn.
To improve the overall performance of the negative electrodes, we recently explored the possible combination of two p-group elements, in order to profit of a synergistic effect between good performance (Bi) and high capacity (In, Sn). The study of these mixed-metal systems and of their electrochemical mechanisms vs. Mg metal, which allows us to explain the observed improvement of their performance compared to pure metals, will be highlighted in this communication.

[1] J.B. Goodenough, K.S. Park, J. Am. Chem. Soc. 135 (2013) 1167.
[2] J.-M. Tarascon, Nat. Chem. 2 (2010) 510.
[3] P. Novák, R. Imhof, O. Haas, Electrochim. Acta 45 (1999) 351.
[4] F. Murgia, L. Stievano, L. Monconduit, R. Berthelot, J. Mater. Chem. A 3 (2015) 16478.
[5] Y. Shao, M. Gu, X. Li, Z. Nie, P. Zuo, G. Li, T. Liu, J. Xiao, Nano Lett. 20 (2014) 255.
[6] F. Murgia, E.T. Weldekidan, L. Stievano, L. Monconduit, R. Berthelot, Electrochem. Commun. 60 (2015) 56.

As progress on Li-ion batteries plateaus, safety and material availability continue to be the concerns, particularly as the technology is more widely deployed. Among possible alternatives to Li as a positive charge carrier in batteries, Mg is perhaps most promising. Early-stage cell-level modeling predicts that such systems can provide significantly higher energy density with distinct advantages in safety and material availability. However, one of the main bottlenecks to design better Mg-ion battery system and to achieve comparable performance as Li-ion batteries is to find or develop new cathode materials that can reversibly intercalate Mg-ion while maintaining relatively high intercalation voltage. Earlier pioneering work by Aurbach et al. have shown the applicability of MoO₃ thin film as the cathode material to intercalate Mg-ion. In this work, we use first-principles method to show how Mg-ion mobility can be improved in nanostructured MoO₃ cathode. Furthermore, creating such nanostructure will inevitably increase the surface/bulk ratio and therefore the behavior of Mg intercalation through the cathode surface is potentially important and may even dominate the kinetics. Here, the Mg desolvation and intercalation mechanisms at different MoO₃ surfaces are also studied when the cathode is in contact with active components from the electrolyte.

This work is supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

A promising strategy for increasing the energy density of batteries is to substitute a multivalent (MV) metal for the commonly-used lithiated carbon anode. Magnesium (Mg) is one of the prime candidates for a MV battery due to its high volumetric capacity, abundance, and low tendency to form dendrites. However, one of the key issues in the successful implementation of a Mg-based battery is the development of efficient and stable electrolytes. A starting point for the identification of a suitable electrolyte is screening for solvents having a wide enough electrochemical window. This window can be strongly impacted by interfacial interactions with electrodes, resulting in renormalization of the solvent’s HOMO and LUMO levels. This study examines the stability of several common electrolyte solvents (THF, DME, DMSO and ACN) on model electrodes of relevance for Mg batteries: Mg(0001) and MgO(100). Many-body perturbation theory calculations based on the G₀W₀ method were used to predict shifts in the solvent’s electronic levels during adsorption; trends across the different solvents are discussed. Compared to calculations on isolated molecules, characterization of HOMO/LUMO levels at electrode interfaces presents a more realistic depiction of the stability window of the electrolyte, and can be useful in screening for optimal electrolyte compositions.

Research on sodium layered oxides Na_xMO₂ (M: 3d or 4d transition metal, x: sodium content) as positive electrode in sodium ion batteries have regained interest for stationary energy storage applications. Furthermore, due to their structure, in which sodium occupy interstitial sites (octahedral or prismatic) between [MO₂] slabs constituted of MoO₆ edge-sharing octahedra, sodium layered oxides can exhibit original physical properties depending on their chemical composition (nature of the transition metal and sodium content).
This work focus on Na_xMoO₂ layered oxides, which have been the subject of only a few publications in the 1980’s concerning Na_2/3MoO₂ [1-4], and Na_0.5MoO₂ [5]. The only electrochemical investigation [4], shows a reversible sodium intercalation in Na_xMoO₂ for 0.28