Tunnel/P2 Interlocking Structure Coupled with TiO2 Interface Modification for Synergistically Enhanced Structural Stability of Sodium-Ion Battery Cathodes

Abstract Layered Mn-based oxides are promising cathode materials for sodium-ion batteries, which are prone to an irreversible P2-O2 phase transition during cycling and thus exhibit poor cyclic stability. Although tunnel-type Mn-based oxides feature a stable structure, they deliver a low sodium storage capacity. In this work, tunnel/P2 interlocked cathode materials are synthesized by adjusting sodium content and further modified via Ti/Cu co-doping and 1 wt % TiO2 surface coating for synergistic optimization. The results show that Ti/Cu co-doping lowers the proportion of Mn3+ to 42.83% and efficiently restrains Jahn–Teller distortion. The optimized Na0.66Mn0.8Ti0.1Cu0.1O2 achieves an initial discharge specific capacity of 153.7 mAh·g–1 and an average sodium-ion diffusion coefficient of 3.9 × 10–11 cm2·s–1, showing outstanding kinetic performance. The TiO2-coated sample still retains a stable dual-phase structure consisting of a 64.3% P2 phase and a 35.7% tunnel phase, with the average sodium-ion diffusion coefficient increasing to 5.06 × 10–11 cm2·s–1. It maintains a high capacity retention of 91.85% after 50 cycles at 20 mA·g–1 and 67.5% after 200 cycles at 200 mA·g–1. In situ XRD tests confirm that the (002) diffraction peak of the P2 phase shifts reversibly by 0.12° during charge and discharge. The diffraction peaks of the tunnel phase weaken temporarily at high voltage and recover completely after discharge, demonstrating excellent structural reversibility. This study greatly boosts the electrochemical properties of Mn-based cathodes through combined modification strategies, offering solid experimental data and practical references for the development of high-performance cathode materials for sodium-ion batteries.