Recent rechargeable battery technologies have been dominated by lithium-ion chemistries, with use in a wide range of applications including medical devices, mobile phones, and electric vehicles. Despite its success, the future of lithium-ion batteries is challenged by concerns over energy capacity, longevity, scarcity of raw materials, and consumer safety.
The leading edge of energy storage has been guided by multi-electron battery systems. Instead of Li+ ions, divalent cations such as Mg2+ and Ca2+ are used as the mobile species for electrochemical charging and discharging. Magnesium battery technology has been the focus of considerable research while exploration into calcium battery technology is currently in its early stages. Calcium-based batteries have the potential to achieve 3-5 times the specific energy of lithium-based batteries, as well as higher energy densities. The natural abundance of calcium also provides an economical benefit. However, calcium’s relatively high reduction potential (similar to that of lithium) drives electrolyte decomposition leading to poor reversibility of deposited calcium. Although reversibility has been achieved using an electrolyte formed by dissolving Ca(BH4)2 salt in tetrahydrofuran (THF) solvent, decomposition of THF resulted in fouling of the calcium metal surface. Therefore, a need persists for a calcium salt that enables electrolytes to reversibly deposit calcium with minimal solvent decomposition.
Researchers at Arizona State University have developed a new calcium salt—Ca(HMDS)2, where HMDS is the hexamethyldisilazide anion—for use in creating electrolytes in rechargeable calcium batteries. Using this salt, extremely high concentrations of Ca2+ is achievable in various solvents including THF. As a result, enhanced current densities for calcium metal deposition and dissolution exceed 30 mA/cm2 (at 25 mV/s), which is more than twice that achieved with Ca(BH4)2. Additionally, at sufficiently high concentrations of Ca2+, solvent decomposition drops and causes a marked increase in coulombic efficiency for the deposition/dissolution calcium cycle.
• Rechargeable batteries
• Biomedical devices
• Electric vehicles
• Off-grid, remote power systems
Benefits and Advantages
• Novel – To the best of the inventors’ knowledge, this innovation represents the first use of Ca(HMDS)2 in an electrochemical setting
• High-Performing – Enables high electrolyte concentrations of Ca2+ and thus high current densities
• Efficient – Significant reduction in unwanted solvent decomposition leads to high coulombic efficiency