High Energy Polyanion Cathode for K-Ion Batteries: KVPO4F

Abstract

The development of high-energy cathode materials is crucial for the commercialization of emerging K-ion batteries (KIBs). While layered potassium-transition-metal-oxides (K x MO2, M=transition metals) have been investigated as potential cathodes, they have two intrinsic challenges. First, most of the K-layered oxides have K-poor composition (x<1.0 in K x MO2) and it leads practical difficulty of realizing KIBs because all the K ions should come from the cathode in a rocking-chair KIB.[1-6] Second, K-layered oxides have too high a voltage slope, resulting in low specific capacity and average voltage. Both the problems are attributable to much stronger K+-K+ interaction than Na+-Na+ and Li+-Li+ in the layered oxide structure.[2, 4] In contrast, polyanion cathodes can be better alternatives because 3-dimensional arrangement of K ions in their frameworks can significantly reduce the strength of effective interaction between K ions. As a result, polyanion cathodes can have K-rich (or K-stoichiometric) composition and high working voltage.In this work, we demonstrate that KVPO4F, a polyanion compound, is a high-energy cathode of KIBs. The KVPO4F cathode delivers a reversible capacity of ~105 mAh g−1 with an average voltage of ~4.3 V (vs. K/K+) (Figure 1), which is the highest among the K-cathodes developed up to date.[7] The specific energy (a gravimetric energy) of KVPO4F reaches ~450 Wh kg−1. Ex-situ X-ray diffraction characterization and ab-initio calculations demonstrate reversible multiple phase transitions of K x VPO4F during electrochemical cycling. K x VPO4F goes through various intermediate phases at x = 0.75, 0.625, and 0.5 upon K extraction and reinsertion. We further explain the role of oxygen substitution in KVPO4+xF1−x: the oxygenation of KVPO4F leads to an anion-disordered structure which prevents the formation of K+/vacancy orderings without electrochemical plateaus and hence to a smoother voltage profile.