An optimized approach toward high energy density cathode material for K-ion batteries

Herein, we propose P′2-K_x[Ni_0.05Mn_0.95]O₂ as a promising cathode material for high-energy-density potassium-ion batteries (KIBs). The P′2-K_0.83[Ni_0.05Mn_0.95]O₂ delivers a high discharge capacity of 155 ​mAh g⁻¹ (52 ​mA ​g⁻¹) as well as a high energy density of 420 ​Wh kg⁻¹ in the voltage range of 1.5–4.3 ​V. Surprisingly, the fast K-ion migration in the P′2-K_0.83[Ni_0.05Mn_0.95]O₂ structure with a low activation barrier energy of ~271 ​meV enables achievement of high capacity at high currents, 78 ​mAh g⁻¹ ​at 2600 ​mA ​g⁻¹, as well as long-term cycling stability with capacity retention of ~77% after 500 cycles at 520 ​mA ​g⁻¹. Operando synchrotron X-ray diffraction analysis reveals that P′2-K_0.83[Ni_0.05Mn_0.95]O₂ retains the P′2-phase without P′2-OP4 phase transition during charge/discharge in the voltage range of 1.5–4.3 ​V, which is abnormal compared to the other P′2-based layered cathode materials for sodium-ion batteries, being responsible for the long-term cycle stability of P′2-K_0.83[Ni_0.05Mn_0.95]O₂. First-principles calculation results indicate that the excellent electrochemical performance results from the structural stability associated with a single -phase reaction upon K⁺ extraction/insertion out of/into the host structure. The remarkable potassium storage capability of P′2-K_0.83[Ni_0.05Mn_0.95]O₂ makes it a promising cathode candidate for KIBs.