Highly stable P′3-K0.8CrO2 cathode with limited dimensional changes for potassium ion batteries

O3–KCrO2 is the only known stoichiometric layered transition metal oxide with a layered structure, which enables efficient coupling with a graphite anode for K+-shuttling batteries. O3–KCrO2, however, shows low Coulombic efficiency during 1st charge/discharge cycle and continuous capacity fading during subsequent cycles. Here, we present a slightly K+-deficient compound P′3-K0.8CrO2, synthesized from a commercially available K2CrO4, as a stable cathode in potassium ion batteries. During the 1st charge, K0·8CrO2 mostly retains its P′3-phase with a short P3 region and finally returns to a P′3-phase (K0·48CrO2). Subsequent discharge shows reversible phase transitions, but ultimately yielded a new O′3-K0.9CrO2 phase. Absence of an O3-phase and persistence of a P′3-phase during charge/discharge, result in negligible volume changes (1.08%) and facile K+-diffusion through the spacious-prismatic sites (D = 10−11 - 10−10 cm2 s−1). This eventually contributes to a significant improvement in cyclic stability (99% retention after 300 cycles at 1 C) and rate capability (52 mAh·g−1 at 2 C), respectively. We believe that the use of slightly K+-deficient chromium oxides synthesized via a simple protocol should enhance the commercial viability of KxCrO2 in potassium ion batteries.