Reaching the initial coulombic efficiency and structural stability limit of P2/O3 biphasic layered cathode for sodium-ion batteries.

The P2/O3 biphasic layered oxide (NaMnMO, M: doping elements) is a cathode family with great promise for sodium-ion batteries (SIBs) because of their tunable electrochemical performance and low cost. However, the ultrahigh initial coulombic efficiency (ICE) and inferior cycling performance of P2/O3-NaMnMO need to be improved for practical application. Herein, Ni/Cu co-doped P2/O3-NaMnNiCuO materials are well-designed. The ultrahigh ICE can be restrained by altering the ratio of P2/O3 via adjusting Ni content, and the structural stability can be improved by Cu doping via enlarging parameter c of O3 phase and suppressing irreversible P2-O2 phase transformation. The optimal P2/O3-NaMnNiCuO delivers a capacity of 142.4 with ICE of 107.8%, superior capacity retention in the temperature range of -40 ∼ 30 °C, and rate performance of 95.9 mAh g at 1.2 A g. The overall storage mechanism of P2/O3-NaMnNiCuO is revealed by the combination of electrochemical profiles, in situ X-ray diffraction, and first-principles calculations. The Na-ion full battery based on P2/O3-NaMnNiCuO cathode can achieve a remarkable energy density of 306.9 Wh kg with a power density of 695.5 W kg at 200 mA g. This work may shed light on the rational design of high-performance P2/O3 biphasic layered cathode for SIBs.