Calcium Cobalt Hexacyanoferrate Cathodes for Rechargeable Divalent Ion Batteries

Calcium cobalt hexacyanoferrate (CaCoHCF) was synthesized and tested as a cathode material for rechargeable batteries, using divalent cations (Mg2+, Ca2+, Ba2+). CaCoHCF demonstrated reversible specific capacity and coulombic efficiency (in parentheses) of 45.49 mAh/g (99.18%) for Mg2+, '55.04 mAh/g (99.2%) for Ca2+, and 44.09 mAh/g (99.42%) for Ba2+, at a current density of 25 mA/g. Of the three ions, Ca2+ resulted in the highest absolute specific capacity as well as high specific capacity utilization. The cathodes were also subjected to rate capability measurements using current densities of 50 mA/g (30 cycles) and 0.1 A/g (100 cycles). Upon addition of 2 mL water to the non-aqueous electrolyte, the fraction of theoretical specific capacity increased to 55% for Mg2+, 94.8% for Ca2+, and 95.53% forBa(2+). This increase has been interpreted as the ability of the cathode material to intercalate and de-intercalate more ions due to the electrostatic shielding provided by water molecules between the host lattice and the guest cations. An empirical relationship between the cation size and specific capacity utilization is presented.