An Optimized Approach Toward High Energy Density Cathode Material for K-Ion Batteries

Herein, we propose P′2-Kx[Ni0.05Mn0.95]O2 as a promising cathode material for high-energy-density potassium-ion batteries (KIBs). The P′2-K0.83[Ni0.05Mn0.95]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-K0.83[Ni0.05Mn0.95]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-K0.83[Ni0.05Mn0.95]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-K0.83[Ni0.05Mn0.95]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-K0.83[Ni0.05Mn0.95]O2 makes it a promising cathode candidate for KIBs.