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), K1.7Fe[Fe(CN)6]0.9, 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 tha...