Rechargeable Ca-Ion Batteries: A New Energy Storage System

We present that as new uses for larger scale energy storage systems are realized, new chemistries that are less expensive or have higher energy density are needed. While lithium-ion systems have been well studied, the availability of new energy storage chemistries opens up the possibilities for more diverse strategies and uses. Electrochemical energy storage systems utilizing multivalent ions, such as Ca²⁺ or Mg²⁺, provide a path towards achieving this goal and dramatic increases in volumetric energy storage densities relative to lithium-ion systems. Herein, we first demonstrate this concept for a Ca-ion system using manganese hexacyanoferrate (MFCN) as a Ca-intercalation cathode with a nonaqueous electrolyte and then establish the first reported rechargeable Ca-ion battery utilizing MFCN vs. elemental tin as the anode. Lastly, through characterization via X-ray absorption near-edge spectroscopy, it is determined that only the manganese changes oxidation state during cycling with Ca. X-ray diffraction indicates the MFCN maintains its crystallinity during cycling, with only minor structural changes associated with expansion and contraction.