Electrolyte Solutions for Magnesium Rechageable Batteries

New electrolyte solutions for rechargeable magnesium metal batteries were studied. The electrolyte solution composed of triethylene glycol dimethyl ether (triglyme) and butyl methyl triglyme as solvents and magnesium(II) bis(trifluoromethanesulfonyl)amide as a magnesium salt. The reversible deposition and dissolution of magnesium metal was achieved in these electrolyte solutions at ambient temperature. The electrochemical stability of triglyme-based electrolyte solutions showed high stability toward oxidation.

Solar cells hold a function of photovoltaic conversion, while rechargeable metal batteries have an advantage of high energy storage. The conventional charge mode of batteries is made based on complete utilization of electric energy. The combination of solar cells and rechargeable metal batteries brings a new opportunity for the development of photo‐assisted rechargeable batteries, in which the solar energy can be utilized to partially achieve photo‐charging with or without external electrical bias. This review highlights the working mechanism and structure design of photo‐assisted rechargeable metal batteries according to the characteristics of rechargeable metal batteries and advantage of the photovoltaic technology. In particular, the recent advances are introduced for photo‐assisted rechargeable batteries based on light‐weight metal anodes, including metal lithium, metal sodium, and metal zinc. The working features of the integrated devices are also discussed for energy saving under photo‐assisted charging mode. Finally, a future outlook is provided for further improving the performance of photo‐assisted rechargeable metal batteries.

The field of advanced batteries faces today two main challenges: development of power sources for electro-mobility (i.e., electrochemical propulsion) and development of storage technologies for load-leveling applications (i.e., large scale batteries). Our starting point in this presentation is state-of-the-art Li-ion batteries. Using carbonaceous anodes and lithiated transition metal cathodes with selected combinations of transition metals may offer very suitable batteries for full EVs. Ni rich Li[NiCoMn]O2, Mn and Li rich Li1+x[NiMnCo]1-xO2 cathode materials operating in the right potential windows can provide Li-ion batteries high enough energy density as needed for full electrochemical propulsion. Also, by combining Li-Ti-O anodes with LiMPO4 olivine cathodes it is possible to demonstrate very suitable battery systems for stationary large energy storage applications. We will review briefly the updated frontier of Li-ion battery technology. Despite the high capability of this battery technology, there are so many applications for power sources in modern life, what promote very strongly a search for more new battery systems. The use of sulfur cathodes in Li batteries can increase considerably their gravimetric energy density, however, the volumetric energy density of Li-sulfur battery systems is not advantageous compared to advanced Li ion batteries. The good news are that in recent years, we see a nice progress in developing durable composite sulfur cathodes. It seems that there are 3 main approaches: encapsulating sulfur in activated carbon matrices, the use of substrates to which the LiSn species formed by sulfur reduction adsorb very strongly and the use of composite cathodes with high loading of sulfur and semi-permeable membranes that avoid transport of LiSn species to the negative electrodes. There is intensive work on Li-oxygen systems. We see progress in development of electro-catalysis for these systems. We will demonstrate electro-catalytic electrodes in Li-oxygen cells, which anodic reactions (Li-peroxide oxidation and molecular oxygen evolution) occur at very low over-potential, that does not endanger the stability of the electrolyte solutions. We demonstrate also red-ox mediators in solutions that facilitate both ORR and OER. Despite the frustration from the slow progress made towards practical directions, it is important to continue investing efforts in Li-oxygen systems. The community should understand that a long-term research is still required in order to see practical horizons to Li-air battery systems. A main problem is the reactivity of all kinds of polar-aprotic solvents towards superoxide and peroxide species formed by oxygen reduction, in the presence of the super-electrophilic Li ions. The intensive work on both Li-S and Li-oxygen systems is connected to a renaissance in efforts to develop practical Li metal anodes for rechargeable batteries. However, intensive studies that were carried out during more than 3 decades, concluded more than 15 years ago that there is no way to cycle Li metal anodes in practical rechargeable Li battery systems. Even if dendrites formation is avoided, continuous detrimental reactions between the active metal and any type of relevant electrolyte solution is inevitable. We will examine briefly what are the chances to overcome the intrinsic problems in using Li metal anodes in practical rechargeable batteries. We may be able to suggest practical alternative anodes for non-aqueous batteries based on sulfur or oxygen cathodes. Discussing other options for “beyond Li-ion batteries” usually includes Na-ion and magnesium batteries. In fact, the plethora of relevant Na-ion insertion cathodes available today is a good surprise, especially since many of them demonstrate excellent kinetics. However, there is no way that Na-ion systems can rival Li-ion battery systems in terms of energy density. Hence, Na-ion systems may be relevant for large energy storage applications, that can benefit from the high abundance of Na in earth crust. However for such uses, demonstrating high stability during prolonged cycling is mandatory. Consequently, a major challenge in R&D of Na-ion batteries is to exhibit the durability required for load-leveling applications. Finally, we will mention the status of R&D of rechargeable Mg battery systems: excellent work was done so far on the anode-solutions part. Several families of electrolyte solutions with wide electrochemical windows, in which Mg anodes are fully reversible were demonstrated. We have not seen yet cathodes materials that function well in the solutions in which Mg anodes are fully reversible, except the low voltage/low capacity Chevrel phase cathode materials. Hence, these systems still require long term research work, especially towards high voltage/high capacity cathode materials. A main incentive to develop rechargeable Mg batteries may relate to advantages in safety and high volumetric energy density.

Rechargeable zinc-based battery systems hold great promise for future energy storage devices due to their low cost and high theoretical energy density.1 However, the development of portable or stationary rechargeable zinc batteries has been hindered by several critical issues. The key issue has been the evolution of dendritic zinc electrodeposit morphology during battery charging,2,3leading to battery capacity fade and cell shorting. In this work, we focus on the use of organic electrolyte additives to suppress zinc dendrite growth. We examine both acidic and alkaline electrolytes, which are relevant to acidic zinc-halogen flow batteries and alkaline zinc-metal batteries, respectively. To study zinc dendrite propagation, we employ in-situ microscopy during growth of the electrodeposit.4 To study the mechanism by which organic additives modulate the dendrite propagation rate, we utilize classical polarization measurements on a rotating disc electrode. For acidic zinc electrolytes, we studied numerous additives (Figure 1) and their dendrite suppression efficacy. We observed that strongly polarizing additives suppress zinc dendrites more than weak polarizers. For zinc electrodeposition from an alkaline electrolyte, we identified that polyethylenimine (PEI) enables near-complete dendrite suppression,5even at low concentrations (~50 ppm, see Figure 2). Figure 2 also shows electrochemical polarization measurements on a rotating disk electrode, which confirm the polarizing effect of the PEI. The aforementioned characterization techniques, together with electrochemical quartz crystal microgravimetry, ex-situscanning electron microscopy, and various ‘additive-injection’ tests, provide insights into the mechanism by which additives suppress dendritic growth of zinc. Additives are believed to adsorb onto the zinc surface and suppress zinc electrodeposition kinetics locally, thereby inhibiting activation-controlled dendrite propagation. In the talk, we will develop the mechanistic understanding further and provide guidelines for judicious selection of the additive chemistry for dendrite-free zinc electrodeposition in acidic and alkaline systems.

Introduction Rechargeable battery systems with different transporting ions are an alternative choice to Li batteries that can expand and diversify the existing energy storage market. Magnesium batteries are the most promising technology to achieve substantially greater energy density than Li-ion batteries, due to their divalent nature (two electron redox couple). Furthermore, because of its “green” character Mg is environmentally friendly, with high natural abundance in the Earth’s crust (13.9% as compared to 0.0007 % for Li), and atmospheric stability.The current quest for new cathode materials for rechargeable Mg batteries is focused on intercalation compounds exhibiting high working potential and capacity1. When transporting ions have much larger diameter or charge than the host metal ion, segregation of transporting and host metal ions in the crystalline structure is forseable. In this case deintercalation will lead to the collapse of the crystalline structure and breaking of electric conductance. Therefore, our approach to achieving magnesium intercalation is to use nanoscale material that have adjustable d spacing and two-dimensional layered structure that can to control polarizability of Mg2+ions and accommodate large charge/volume changes.Nanostructured electrode materials represent one of the most attractive strategies to dramatically enhance battery performance. Short diffusion length associated with the nanosized dimensions effectively reduces the distance that ions and electrons must travel during cycling. Their large surface area increases the contact area between electrode and electrolyte and hence the number of active sites for electrode reactions, which in turn reduces electrode polarization loss and improves power density, energy efficiency, and usable energy.Nanostructured bilayered V2O5 is highly efficient 3 V cathode material for ambient temperature sodium ion batteries2. It shows superb performance: theoretical reversible capacity for Na2V2O5 stochiometry of 250 mAh/g, excellent rate capability and cycle life, as well as high energy and power densities. Preliminary results show that it is possible to extend the reversible ion-insertion chemistry of nanostructured layered V2O5 beyond lithium and larger in size Na+ ions to those with higher charge: Mg2+ions. Results and Discussion As–prepared nanostructured bilayered V2O5 cathode is synthesized in the charged state. To provide source enrichment, prior to assembling into full Mg-ion cell, it was necessary to pre-condition V2O5 cathode (intercalate with Mg2+ ions). Half-cells were discharged galvanostatically at 20mA down to and 0.01 V versus Mg/ Mg2+, respectively, using an automated Maccor battery tester at ambient temperature. As shown in Figure 1., upon electrochemical pre-condition (Mg-ion half cell) the X-ray edge energy for vanadium is shifted toward the lower energy as a consequence of a vanadium reduction due to Mg intercalation. Pre-edge peak is proportional to the octahedral symmetry of the vanadium sites and its decrease upon transporting ion intercalation is due to modification of vanadium local environment into more ordered, symmetric structure. Based on the main-edge peak position and comparison with Na-ion half cell we estimate 1 Mg is incorporated in the V2O5 structure (V5+ is reduced to V4+). It was previously shown that the V5+ was reduced to V4+ when discharged to 1.5V versus Na/ Na+ (Na-ion half cell)2. As a result of this electrochemical pre-condition we created Mg-rich V2O5 cathodes that were able to provide source of Mg-ions in full-cell. Furthermore, we employed HRTEM, X-Ray fluorescence (XRF) imaging and classical molecular dynamics simulations to understand the role of environment in the intercalation processes of Mg-ions into V2O5cathode. Conclusions We have developed high-voltage nanostructured V2O5 cathode for Mg-ion rechargeable batteries that can operate in commercial electrolytes. Our results emphasize the importance of tailoring nanoarchitecture of electrode materials and open up new opportunities for rechargeable Mg batteries. In the near future, we plan using the combined experimental and theoretical approach to guide our efforts to increase the Mg enrichment to reach the ultimate goal of inserting 2 Mg-ions/V2O5. Acknowledgement. This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by DOE, Office of Science, Basic Energy Science (BES). Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. References (1) On the Way to Rechargeable Mg Batteries: The Challenge of New Cathode Materials, E. Levi, Y. Gofer, and D. Aurbach, Chem. Mater. 2010, 22, 860–868.(2) Vanadium Oxide Electrodes for Rechargeable Sodium-Ion Batteries, Sanja Tepavcevic, Hui Xiong, Vojislav R. Stamenkovic, Xiaobing Zuo, Mahalingam Balasubramanian, Vitali B. Prakapenka, Christopher S. Johnson, and Tijana Rajh, Nanostructured Bilayered ACS Nano, 2012, 6 (1), 530–538.

Magnesium (Mg) metal anode is a highly desirable candidate among various high energy density metal anodes, possessing higher volumetric capacity and better safety characteristic compared to lithium metal. However, most Mg salts in conventional Mg electrolytes easily react with Mg metal to form blocking layers, leading to inferior reversibility of Mg plating/stripping. Here, a stable Mg2+ -conducting solid electrolyte interphase (SEI) is successfully constructed on Mg metal anode by regulating the molecular-orbital-energy-level toward an aluminum(III)-centered anion Mg salt through anion-solvent coordination. Of which, the LUMO energy level of perfluorinated pinacolatoaluminate (Al(O2 C2 (CF3 )4 )2 - , abbreviated as FPA) anion has been adjusted by coordinating with solvent molecule (tetrahydrofuran) for facilitating the formation of advantageous SEI. The existence of SEI formed by FPA anion greatly improves the reversibility and long-term stability of Mg plating/stripping process. More importantly, based on this aluminum(III)-centered Mg electrolyte, the Mo6 S8 /Mg batteries can achieve a fantastic cycle performance of 9000 cycles, proving the beneficial effect of SEI on the cycling stability of Mg battery system. These findings open up a promising avenue to construct stable and compatible SEI on Mg metal anode, and lay significant foundations for the successful development of rechargeable Mg metal batteries.

Rechargeable batteries are playing an increasingly prominent role in our lives due to the ongoing transition from fossil energy sources to green energy. The purpose of this study was to investigate variables that impact the effectiveness of rechargeable batteries. Alkaline (non-rechargeable) and rechargeable batteries share common features that are critical for the operation of a battery. The positive and negative electrodes, also known as the cathode and anode, are where the energy of the battery is stored. The electrolyte is what facilitates the transfer of cations and anions in a battery to generate electricity. Due to the importance of these components, we felt that a systematic investigation examining the surface area of the cathode and anode as well the impact of electrolytes with different properties on battery performance was justified. Utilizing a copper cathode and aluminum anode coupled with a water in salt electrolyte, a model rechargeable battery system was developed to test two hypotheses: a) increasing the contact area between the electrodes and electrolyte would improve battery capacity, and b) more soluble salt-based electrolytes would improve battery capacity. After soaking in an electrolyte solution, the battery was charged and the capacity, starting voltage, and ending voltage of each battery were measured. The results of this study supported our hypothesis that larger anode/cathodes surface areas and more ionic electrolytes such as sodium chloride, potassium chloride and potassium sulfate resulted in superior battery capacity. Incorporating these findings can help maximize the efficiency of commercial rechargeable batteries.

A new ether-based electrolyte to match lithium metal electrode is prepared by introducing 1, 4-dioxane as co-solvent into lithium bis(fluorosulfonyl)imide/1,2-dimethoxyethane solution. Under the synergetic effect of solvents and salt, this simple liquid electrolyte presents stable Li cycling with dendrite-free Li deposition even at relatively high current rate, high coulombic efficiency of ca. 98%, and good anodic stability up to ~4.87 V vs Li RE. Its excellent performance will open up a new possibility for high energy-density rechargeable Li metal battery system.

The newly emerging rechargeable batteries beyond lithium‐ion, including aqueous and nonaqueous Na‐/K‐/Zn‐/Mg‐/Ca‐/Al‐ion batteries, are rapidly developing toward large‐scale energy storage application. The properties of electrode materials are determinant for electrochemical performance of the batteries. By virtue of the prominent features of low cost, non‐toxicity, high voltage, and rich valence states, Mn‐based electrode materials have attracted increasing attention. The big family of Mn‐based materials with rich composition and polymorphs, provides great possibilities for exploring and designing advanced electrode materials for these emerging rechargeable batteries. In this review, three main categories of Mn‐based materials, including oxides, Prussian blue analogous, and polyanion type materials, are systematically introduced to offer a comprehensive overview about the development and applications of Mn‐based materials in various emerging rechargeable battery systems. Their crystal structure, electrochemical performance, and reaction mechanism are highlighted. In addition, the key issues encountered by many Mn‐based materials, including Jahn–Teller distortion, Mn dissolution, crystal water, impact of electrolyte, etc., are also discussed. Finally, challenges and perspectives on the future development of manganese‐based materials are provided as well. It is believed this review is timely and important to further promote exploration and applications of Mn‐based materials in both aqueous and nonaqueous rechargeable battery systems beyond lithium‐ion.

The rapid worldwide energy consumption is necessitating more energy generation from environmentally benign sources like solar, wind, or tide. However, the lack of efficient energy storage devices is the major limitation for the practical harvest of these clean energies. Rechargeable batteries are the promising solution for the efficient energy storage. We have witnessed the great success of secondary lithium-ion batteries (LIBs) in portable electronic devices in the last two decades. However, the cost and safety barriers make traditional LIBs not desirable to meet the requirements for large power applications, such as the electric or plug-in hybrid vehicles. In contrast, the multivalent ion rechargeable batteries such as Mg1-3 and Ca4 , 5 ion batteries are more promising as larger and safer energy storage devices due to their advantages in safety, cost and capacity. For instance, compared to lithium, magnesium metal is non-toxic, more air resistant and free from the dangerous dendrite formation. The higher natural abundance of magnesium makes it more than 20 times cheaper than lithium in cost. In addition, the divalent characteristic of magnesium ion (Mg2+) enables magnesium to have a nearly doubled volumetric capacity as an anode than the lithium (3832 mA cm-3 vs 2062 mA cm-3). All these merits have brought significant attention on magnesium-ion batteries as the new generation rechargeable energy storage device. Inspired by the pioneer work by Aurbach group for the development of the famous dichloro-complex magnesium electrolyte (2EtAlCl2-MgBu2),6 a lot of efforts have been devoted to the development of high voltage electrolytes for rechargeable magnesium-ion batteries. In contrast to the rapid development of effective magnesium electrolytes capable of reversibly plating/stripping magnesium with wide electrochemical windows, the design of high potential cathode material for magnesium ion batteries is far behind. To date, the Chevrel phase Mo6S8 is still the only cathode material with excellent cycling reversibility and reasonable rate capability in rechargeable magnesium-ion batteries. However, Mo6S8 could only operate at a relatively low voltage in magnesium-ion batteries (< 1.3 V) with a typical below 100 mAh g-1 capacity. Thus, development of high operating voltage cathode material deserves great attention in nowadays. Herein, we would like to present our recent progress on the development of high voltage and high performance rechargeable magnesium-ion batteries using redox active organic molecules as cathode material. As shown in Figure 1(a), when the 0.3M Mg(HMDS)2-4MgCl2 in THF solution was used as the electrolyte,7 excellent reversible redox processes have been observed in the cyclic voltammetry measurement when the redox active organic molecule was used as the cathode material. The redox potential was detected at around 1.7 V (vs Mg/Mg2+), which is much higher than the well-known Chevral phase Mo6S8. The compatibility and reversibility of the organic cathode material with the electrolyte and magnesium metal anode was further demonstrated in the batteries upon cycling. As shown in Figure 1(b), excellent cycling performance has been observed with around 120 mAh g-1 initial specific discharge capacity at the current rate of 0.5C, and still about 102 mAh g-1specific discharge capacity was obtained in 100 cycles (around 84% capacity retention). The results further suggest the great success of using redox active organic molecules as cathode material for secondary multivalent-ion batteries. (1) Muldoon, J.; Bucur, C. B.; Gregory, T. Chem Rev 2014, 114, 11683. (2) Saha, P.; Datta, M. K.; Velikokhatnyi, O. I.; Manivannan, A.; Alman, D.; Kumta, P. N. Prog Mater Sci 2014, 66, 1. (3) Yoo, H. D.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.; Aurbach, D. Energ Environ Sci 2013, 6, 2265. (4) Padigi, P.; Goncher, G.; Evans, D.; Solanki, R. J Power Sources 2015, 273, 460. (5) Lipson, A. L.; Pan, B.; Lapidus, S. H.; Liao, C.; Vaughey, J. T.; Ingram, B. J. Chem Mater 2015, ASAP. (6) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Nature 2000, 407, 724. (7) Liao, C.; Sa, N.; Key, B.; Burrell, A. K.; Cheng, L.; Curtiss, L. A.; Vaughey, J. T.; Woo, J. J.; Hu, L. B.; Pan, B. F.; Zhang, Z. C. Journal of Materials Chemistry A 2015, 3, 6082. Figure 1

Rechargeable magnesium metal batteries (RMBs) have shown promising prospects in sustainable energy storage due to the high crustal abundance, safety, and potentially large specific capacity of magnesium. However, their development is constrained by the lack of effective cathode materials that can achieve high capacity and stable magnesium storage at a practically reasonable rate. Herein, we construct a three-dimensional (3D) iron(III)-dihydroxy-benzoquinone (Fe2(DHBQ)3) metal-organic framework (MOF) material with dual redox centers of Fe3+ cations and DHBQ2- anions for reversible storage of Mg2+ in RMBs. Spectroscopic analysis and density functional theory (DFT) calculations reveal the redox chemistry of both Fe3+ ions and carbonyls from DHBQ ligands during electrochemical processes. Benefiting from the rational structure, the Fe2(DHBQ)3∥Mg cells exhibit a high reversible capacity of 395.3 mAh/g, large energy density of 463.5 Wh/kg, and high power density of 2456.0 W/kg. Moreover, the high electronic conductivity (8.35 × 10-5 S/cm) and favorable diffusion path of Mg2+ in Fe2(DHBQ)3 endow the cells with exceptional cycling stability and rate capability with a long life of 5000 cycles at 2000 mA/g. The dual redox-active MOF demonstrates a category of advanced cathode materials for high-performance RMBs.

&lt;div class="htmlview paragraph"&gt;At the request of the FAA, a committee was formed by the RTCA organization to prepare a Minimum Operating Performance Standard (MOPS) for Rechargeable Aircraft Lithium Battery Systems. This committee has being working for the last 2 years and completed the final draft during their 6&lt;sup&gt;th&lt;/sup&gt; meeting in February 2008.&lt;/div&gt; &lt;div class="htmlview paragraph"&gt;Lithium Battery approval will be different than Nickel-Cadmium and Lead-Acid in that they will incorporate additional safety and documentation requirements.&lt;/div&gt; &lt;div class="htmlview paragraph"&gt;The MOPS, to be used by the FAA as the basis for a Technical Standard Order (TSO), includes the testing and evaluation criteria required for rechargeable lithium battery systems airworthiness demonstration.&lt;/div&gt;

With high energy density and improved safety, rechargeable battery chemistries with a zinc (Zn) metal anode offer promising and sustainable alternatives to those based on lithium metal or lithium‐ion intercalation/alloying anode materials; however, the poor electrochemical reversibility of Zn plating/stripping, induced by parasitic reactions with both aqueous and non‐aqueous electrolytes, presently limits the practical appeal of these systems. Although recent efforts in rechargeable Zn metal batteries (RZMBs) have achieved certain advancements in Zn metal reversibility, as quantified by the Coulombic efficiency (CE), a standard protocol for CE has not been established, and results across chemistries and systems are often conflicting. More importantly, there is still an insufficient understanding regarding the critical factors dictating Zn reversibility. In this work, a rigorous, established protocol for determining CE of lithium metal anodes is transplanted to the Zn chemistry and is used for systematically examining how a series of factors including current collector chemistry, current density, temperature, and the upper voltage limit during stripping affect the measured reversibility of different Zn electrolytes. With support from density functional theory calculations, this standardized Zn CE protocol is then leveraged to identify an important correlation between electrolyte solvation strength toward Zn2+ and the measured Zn CE in the corresponding electrolyte, providing new guidance for future development and evaluation of Zn electrolytes.

Electrolyte solutions of magnesium organo-halo-aluminates in ethers are suitable for rechargeable magnesium batteries as they enable highly reversible electrodeposition for magnesium while they possess a wide electrochemical window (>2.2 V). Adding LiCI or tetrabutylammonium chloride to these solutions considerably improves their ionic conductivity, the kinetics of the Mg deposition-dissolution processes, and the intercalation behavior of Mg x MO 6 S 8 Chevrel cathodes. The dissolution of both salts in the electrolytic solutions involves acid-base reactions with complex species. Multinuclei nuclear magnetic resonance and Raman spectroscopy were used in conjunction with electrochemical techniques to study these systems. The nature of these reactions, their products, and the way they influence the various properties of these solutions, are discussed herein.

The development of new rechargeable battery systems could fuel various energy applications, from personal electronics to grid storage. Rechargeable aluminium-based batteries offer the possibilities of low cost and low flammability, together with three-electron-redox properties leading to high capacity. However, research efforts over the past 30years have encountered numerous problems, such as cathode material disintegration, low cell discharge voltage (about 0.55volts; ref. 5), capacitive behaviour without discharge voltage plateaus (1.1-0.2volts or 1.8-0.8volts) and insufficient cycle life (less than 100 cycles) with rapid capacity decay (by 26-85 per cent over 100 cycles). Here we present a rechargeable aluminium battery with high-rate capability that uses an aluminium metal anode and a three-dimensional graphitic-foam cathode. The battery operates through the electrochemical deposition and dissolution of aluminium at the anode, and intercalation/de-intercalation of chloroaluminate anions in the graphite, using a non-flammable ionic liquid electrolyte. The cell exhibits well-defined discharge voltage plateaus near 2volts, a specific capacity of about 70mAhg(-1) and a Coulombic efficiency of approximately 98 per cent. The cathode was found to enable fast anion diffusion and intercalation, affording charging times of around one minute with a current density of ~4,000mAg(-1) (equivalent to ~3,000Wkg(-1)), and to withstand more than 7,500 cycles without capacity decay.