Crystallite Size Control of Prussian White Analogues for Nonaqueous Potassium-Ion Batteries

Nonaqueous potassium-ion batteries have emerged as possible low-cost alternatives to Li-ion batteries for large-scale energy storage, owing to their ability to use graphitic carbon as the negative electrode. Positive electrode materials remain a challenge. Here, we report control of the crystal dimensions of the Prussian white hexacyanoferrate (HCF), K1.7FeFe(CN)60.9, using solution chemistry to obtain either nano, submicron, or micron crystallites. We observe a very strong effect of crystallite size on electrochemical behavior. The optimal cathode material comprised of 20 nm crystallites delivers a close-to-theoretical reversible capacity of 140 mAh g–1 with two well-defined plateaus at 4.0 and 3.2 V vs K/K+ upon discharge. Slightly inferior electrochemical behavior is observed for crystallites up to ∼160–200 nm in diameter, but unlike the analogous Na HCFs, micron-sized crystals show very limited capacity. For the nanosized crystallites, however, the energy density of ∼500 Wh kg–1 is comparable to that of the best Na HCF cathode materials. At a relatively high current density of 100 mA g–1, half-cells cycled with ethylene carbonate/diethyl carbonate (EC/DEC) and 5% fluoroethylene carbonate (FEC) demonstrate an initial discharge capacity of 120 mAh g–1 with a capacity retention of 85% after 100 cycles and 65% after 300 cycles.