Volume-complementary bipolar layered oxide enables stable symmetric sodium-ion batteries

The commercialization of sodium-ion batteries is based on developing low-cost, highly stable, and safe cathode and anode electrodes. However, the promising hard carbon anode and layered oxide cathode suffer from low sodium-embedded potential near 0.1 V and severe phase transitions, which cause safe problem and short lifespan, respectively. Herein, we design a low-strain bipolar P2-Na_0.7Ni_0.25Fe_0.2Ti_0.55O₂ to solve the mentioned obstacles, whereas (Ni, Fe) and Ti provide charge compensation when it is used as cathode and anode, respectively. It is revealed that the bipolar layered oxide undergoes solid–solution reaction when used as cathode or anode, and exhibits volume-complementary feature in a sodium-ion full-cell, as identified by in-situ X-ray diffraction. Remarkably, the safe symmetric sodium-ion full-cell exhibits excellent cyclic stability with 91.7% capacity retention after 200 cycles. This work will provide a new horizon for designing safe and stable sodium-ion batteries.