Crystallite Size Control of Prussian White Analogues for Nonaqueous Potassium-Ion Batteries Guang He† and Linda F. Nazar* Department of Chemistry and the Waterloo Institute of Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada S Supporting Information *
ABSTRACT: Nonaqueous potassium-ion batteries have emerged as possible lowcost 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 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.
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Inspired by the above, a few studies on cathodes for nonaqueous KIBs have rapidly emerged. Ji et al. reported reversible capacities of 130−200 mAh g−1 with both 3,4,9,10perylene-tetracarboxylicacid−dianhydride (PTCDA) and poly(anthraquinonyl sulfide) (PAQS) between 3.5 and 1.5 V vs K/ K+.15,16 The overall energy density is
