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 (Na_xMn_1-yM_yO₂, 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-Na_xMn_1-yM_yO₂ need to be improved for practical application. Herein, Ni/Cu co-doped P2/O3-Na_0.75Mn_1-yNi_y-zCu_zO₂ 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-Na_0.75Mn_0.6Ni_0.3Cu_0.1O₂ 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^-1 at 1.2 A g^-1. The overall storage mechanism of P2/O3-Na_0.75Mn_0.6Ni_0.3Cu_0.1O₂ 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-Na_0.75Mn_0.6Ni_0.3Cu_0.1O₂ cathode can achieve a remarkable energy density of 306.9 Wh kg^-1 with a power density of 695.5 W kg^-1 at 200 mA g^-1. This work may shed light on the rational design of high-performance P2/O3 biphasic layered cathode for SIBs.