Layered K_0.54Mn_0.78Mg_0.22O₂ as a high-performance cathode material for potassium-ion batteries

Layered Mn-based oxides are one of the promising cathode materials for potassium-ion batteries (KIBs) owing to their high theoretical capacities, abundant material supply, and simple synthesis method. However, the structural deterioration resulting from the Jahn-Teller effect of Mn ions hinders their further development in KIBs. Herein, a novel Mn-based layered oxide, K_0.54Mn_0.78Mg_0.22O₂, is successfully designed and fabricated as KIBs cathode for the first time. It delivers smooth charging/discharging curves with high specific capacity of 132.4 mAh·g^‒1 at 20 mA·g^‒1 and good high-rate cycling stability with a capacity retention of 84% over 100 cycles at 200 mA·g^‒1. Combining in-situ X-ray diffraction (XRD) and ex-situ X-ray photoelectron spectroscopy (XPS) analysis, the storage of K-ions by K_0.54Mn_0.78Mg_0.22O₂ is revealed to be a solid-solution processes with reversible slip of the crystal lattice. The studies suggest that the rational doping of inactive Mg²⁺ can effectively suppress the Jahn-Teller effect and provide outstanding structure stability. This work deepens the understanding of the structural evolution of Mn-based layered materials doped with inactive materials during de/potassiation processes.