Reversible K+-Insertion/Deinsertion and Concomitant Na+-Redistribution in P ' 3-Na0.52CrO2 for High-Performance Potassium-Ion Battery Cathodes

P'3-type Na052CrO2 is proposed as a viable cathode material for potassium-ion batteries (KIBs). The in situ-generated title compound during the first charge of O3-NaCrO2 in K+-containing electrolytes can reversibly accommodate 0.35 K+-ions with no interference with Na+. In addition to the sequential interlayer slippage that occurs with Na+ insertion, K+-insertion into Na052CrO2 induces a sudden phase separation, which ultimately results in a biphasic structure when fully discharged (K+-free O3-NaCrO2 and K+-rich P3-K0.6Na0.17CrO2). A reversible transition between monophasic (Na0.52CrO2) and biphasic states during repeated Ktinsertion/deinsertion is also maintained, which contributes to superior electrochemical properties of the title compound when used as a KIB cathode. Na0.52CrO2 delivers a specific capacity of 88 mA h g(-1) with an average discharge potential of 2.95 V versus K/K+ This high level of energy density (260 W h kg(-1) at 0.05C) is not substantially decreased at fast C-rates (195 W h kg(-1) at 5C). When cycled at 2C, the first reversible capacity of 77 mA h gradually decreases to 52 mA h g(-1) during initial 20 cycles, but no further capacity fading is observed for subsequent cycles (51 mA h g(-1) after 200 cycles). Density-functional-theory computation reveals that the rearrangement of Na+ is an energetically favored process rather than a homogeneous distribution of Na+/K+.