Cycling Stability of Layered Potassium Manganese Oxide in Nonaqueous Potassium Cells

Potassium-ion batteries have emerged as an alternative to lithium-ion batteries as energy storage systems. In particular, K_xMnO₂ has attracted considerable attention as a cathode material because of its high theoretical capacity and low cost. In this study, partial substitution of Mn in P3-type K_0.5MnO₂ with divalent Ni is performed, resulting in a first discharge capacity of approximately 121 mAh (g-oxide)^-1 with 82% retention for 100 cycles. Operando synchrotron X-ray diffraction analysis reveals the occurrence of phase transition from P3 to O3 on charge and O3-P3-P'3 transition on discharge at the first cycle, where P'3 is a new distorted form of the P3 phase, accompanied by reversible Mn^4+/3+ and Ni^3+/2+ redox pairs, as evidenced by X-ray absorption spectroscopy. The reduced variation in the lattice parameters during de/potassiation for P3-K_0.5[Ni_0.1Mn_0.9]O₂ relative to P3-K_0.5MnO₂ is suggested as a possible reason for the enhanced electrochemical performance of K_0.5[Ni_0.1Mn_0.9]O₂. These results open the possibility of using inexpensive and high-capacity Mn-based cathode active materials for potassium-ion batteries.