The influence of the precursors, namely potassium ferrocyanide and potassium ferricyanide on the particles sizes of Prussian Blue (PB) and Prussian Green (PG), under identical reaction conditions have been investigated. It was found that the particle sizes influence the gravimetric capacity utilization of these materials as cathodes for aqueous potassium (K +) ion batteries. The PG particle sizes were on the order of 50-75 nm, whereas PB particles size were on the order of 2-10 microns. The PG cathodes demonstrated a reversible capacity of 121.4 mAhr/g, with a coulombic efficiency of 98.7% compared to PB cathodes which demonstrated 53.8 mAhr/g, with a coulombic efficiency of 100%. We interpret the increased capacity of PG batteries relative to PB batteries as being a result of the smaller particle size of PG, which results in greater accessibility of the cathode to K+ ions.

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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 0.55 for Mg2+, 94.8% for Ca2+, and 95.53% forBa2+. 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.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 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 0.55 for Mg2+, 94.8% for Ca2+, and 95.53% forBa2+. 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.

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