A New Strategy for High-Voltage Cathodes for K-Ion Batteries: Stoichiometric KVPO 4 F
The exploration of high-energy-density cathode materials is crucial to the practical use of K-ion batteries. Layered K-metal oxides have too high a voltage slope due to their large K + –K + interaction, resulting in low specific capacity and average voltage. In contrast, the 3D arrangement of K + , with polyanions separating them, reduces the strength of the effective K + -K + repulsion, which in turn increases specific capacity and voltage. Here, stoichiometric KVPO 4 F for use as a high-energy-density K-ion cathode is developed. The KVPO 4 F cathode delivers a reversible capacity of ≈105 mAh g -1 with an average voltage of ≈4.3 V (vs K/K + ), resulting in a gravimetric energy density of ≈450 Wh kg -1 . During electrochemical cycling, the K x VPO 4 F cathode goes through various intermediate phases at x = 0.75, 0.625, and 0.5 upon K extraction and reinsertion, as determined by ex situ X-ray diffraction characterization and ab initio calculations. Moreover, our research explains the role of oxygen substitution in KVPO 4+ x F 1- x : the oxygenation of KVPO 4 F leads to an anion-disordered structure which prevents the formation of K + /vacancy orderings without electrochemical plateaus and hence to a smoother voltage profile.