Biphase-to-monophase structure evolution of Na_0.766+xLi_xNi_0.33−xMn_0.5Fe_0.1Ti_0.07O₂ toward ultradurable Na-ion batteries

Layered composite oxide materials with O3/P2 biphasic crystallographic structure typically demonstrate a combination of high capacities of the O3 phase and high operation voltages of the P2 phase. However, their practical applications are seriously obstructed by difficulties in thermodynamic phase regulation, complicated electrochemical phase transition, and unsatisfactory cycling life. Herein, we propose an efficient structural evolution strategy from biphase to monophase of Na_0.766+xLi_xNi_0.33−xMn_0.5Fe_0.1Ti_0.07O₂ through Li⁺ substitution. The role of Li⁺substitution not only simplifies the unfavorable phase transition by altering the local coordination of transition metal (TM) cations but also stabilizes the cathode–electrolyte interphase to prevent the degradation of TM cations during battery cycling. As a result, the thermodynamically robust O3-Na_0.826Li_0.06Ni_0.27Mn_0.5Fe_0.1Ti_0.07O₂ cathode delivers a high capacity of 139.4 mAh g⁻¹ at 0.1 C and shows prolonged cycling life at high rates, with capacity retention of 81.6% at 5 C over 500 cycles. This work establishes a solid relationship between the thermodynamic structure evolution and electrochemistry of layered cathode materials, contributing to the development of long-life sodium-ion batteries.