The introduction of electric vehicles (EVs) and plug-in hybrid vehicles (PHVs) via the use of such alternative technologies (e.g. batteries) as stand-alone or tandem energy sources is slowly revolutionizing the face of the automobile industry. However, while batteries are a cleaner alternative to fossil fuels, concerns over their long range performance in automobiles have hampered their widespread use. Hence, high performance battery systems which meet automobile energy use, and especially space requirements, remain paramount in establishing the next generation of EVs and PHVs.

In this regard, multivalent battery systems like rechargeable magnesium (Mg), calcium and aluminum-ion batteries are garnering more interest as candidate post-lithium (Li) systems. 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). Reports on the advantages of such high energy-density Mg batteries, and interest in the field of new cathodes and electrolytes for Mg batteries have steadily risen.

—Singh et al.

Because magnesium is divalent, it can thereby displace double the charge per ion (i.e., Mg²⁺ rather than Li⁺). As an element, magnesium is much more abundant than lithium, and more stable. Magnesium-ion batteries theoretically could offer good electrochemical performance, while being safer and less expensive than Li-ion batteries. However, Mg-ion batteries have suffered from a number of limitations, among them being anode/electrolyte incompatibility.

To date, various organohaloaluminates...have been utilized as alternative electrolytes for Mg systems, since conventional battery electrolytes (TFSI-, ClO4-, PF6-) form a Mg²⁺ blocking layer on the Mg metal anode surface. However, recent reports have shown that these organohaloaluminate electrolytes provide a limited operating voltage window when tested against typical battery current collectors. It is nonetheless possible to develop high voltage Mg systems via the use of conventional battery electrolytes, if the anode system is changed from a Mg metal anode to a Mg-ion insertion-type anode. This change would negate the challenge posed by the Mg²⁺ blocking layer on the Mg metal anode surface. The use of such insertion-type anodes has been abundant in Li- ion battery technology, where the use of a Li metal anode is deemed impractical, primarily due to dendrite formation.

—Singh et al.

The TRINA team had earlier used Bi as a Mg²⁺ insertion-type anode material, showing it to be compatible with conventional battery electrolytes. However, since high energy-density depends on cell voltage and capacity, they sought to develop next generation Mg-ion battery anode materials which remain compatible with high voltage conventional battery electrolytes, while displaying lower Mg²⁺ insertion/extraction voltages and higher capacities than Bi.

The TRINA team is now focusing on improving the rate capability of the Sn material and cyclability by exploring advancements in nanomaterials and anode architectures, while identifying and understanding the mechanisms responsible for the material’s poor coulombic efficiency.

Resources

• Nikhilendra Singh, Timothy S Arthur, Chen Ling, Masaki Matsui and Fuminori Mizuno (2012) A High Energy-Density Tin Anode for Rechargeable Magnesium-ion Batteries. Chem. Commun. doi: 10.1039/C2CC34673G

• A. Mitelman, M. D. Levi, E. Lancry, E. Levi and D. Aurbach (2007) New cathode materials for rechargeable Mg batteries: fast Mg ion transport and reversible copper extrusion in Cu_yMo₆S₈ compounds. Chem. Commun. 4212-4214 doi: 10.1039/B710743A