A Comparison of as-synthesized P2-K0.70[Cr0.85Sb0.15]O2 and Ion-Exchanged P2-K0.62Na0.08[Cr0.85Sb0.15]O2 Demonstrates the Superiority of the Latter as a Potassium-Ion Battery Cathode

Abstract

In this study, P2-type K0.7[Cr0.85Sb0.15]O2 (KCSO) is prepared via a solid-state route and its electrochemical properties are compared with those of P2-K0.62Na0.08[Cr0.85Sb0.15]O2 (IE-KCSO), which is prepared via an electrochemical ion-exchange of P2-Na0.7[Cr0.85Sb0.15]O2. Despite the near-identical chemical compositions and crystallographic structures, the IE-KCSO demonstrates rate capability and cyclic stability that is distinctively better than those of KCSO when used as a cathode in potassium-ion batteries (KIBs). For example, while KCSO shows a significant reduction in discharge capacities at high rates (48 mAh·g−1 at 2C vs 70 mAh·g−1 at 0.1C), IE-KCSO retains substantial capacities at high rates (67 mAh·g−1 at 2C vs 78 mAh·g−1 at 0.1C). The cyclic stability of IE-KCSO is also superior, as it delivers 96% of initial capacity after 100 charge/discharge (C/D) cycles, in contrast to 76% retention for KCSO. This study confirms that the Na+ ions remaining in IE-KCSO contribute to the fast K+ diffusion and, thereby, to its superior rate capability. Smaller dimensional changes in IE-KCSO during C/D are also apparent and these result in better cyclic stability. This work suggests that preparing KIB cathode materials via an ion-exchange protocol is more efficient than direct synthesis, particularly when the latter creates difficulties in obtaining phase-pure target compounds.