Effects of cation and anion substitution in KVPO4F for K-ion batteries

Abstract

The effects of cation (Ti4+) and anion (O2-) substitution on the electrochemical properties of KVPO4F cathodes are investigated in this work. Both forms of substitution lead to smoothing of the associated voltage profile as well as reduced charging time at high voltage (>4.8 V vs. K/K+). These changes effectively suppress electrolyte decomposition, thereby enhancing the capacity retention in the ion-substituted KV(1-x)TixPO4+yF1-y materials. Ionic substitution also enables the intercalation of excess K ions at a reasonably high voltage (similar to 1.8 V vs. K/K+) relative to that required in pure KVPO4F (<1.0 V vs. K/K+). Finally, this study reveals that detours in the synthesis reaction pathways can be created by using a pre-reacted precursor, VPO4, rather than the conventional precursors V2O3 and NH4H2PO4 to form stoichiometric (or non-substituted) KVPO4F. These results highlight several design criteria that can be used to optimize KVPO4F and related compositions to prepare K-ion batteries with high capacity, good cyclability, and fast rate capability.