Synergistic activation of anionic redox through substitution strategy to design low-cost Co/Ni-free layered oxide cathode materials for high-performance Na-ion batteries

P2-type layered oxides with low Na+ diffusion barriers have emerged as promising cathode materials for sodium-ion batteries (SIBs) due to excellent cycle stability and rate performance. Among them, Co/Ni-free Fe-Mn-Cu based cathode materials have gained significant attention owing to their abundance, low cost, and environmentally friendless. However, their practical application has been hindered by irreversible phase changes during the charging/discharging process, leading to rapid capacity decay. To address this issue, the inactive element (Titanium) is introduced in Na0.70Fe0.20Cu0.20Mn0.60O2 oxides to mitigate the detrimental phase transitions and enhance electrochemical performance by facilitating anionic redox reactions. The resulting Na0.70Fe0.20Cu0.20Mn0.55Ti0.05O2 (NFCMT-0.05) cathode material exhibits a high initial discharge specific capacity of 186 mAh/g at 0.1C and superior cycling stability, with a capacity retention of 83.5% after 400 cycles at 5C between 2.0 and 4.3 V. Furthermore, the NFCMT-0.05 maintains P2 phase structure throughout the entire sodiation/desodiation process, and the reversible oxygen-related redox reaction provides additional discharge capacity for the Fe-Mn-based cathode above 4.1 V. The NFCMT-0.05 also demonstrates fast Na+ ion transfer kinetics and excellent rate performance, delivering a discharge capacity at 5C that is 60% of that at 0.1C. The DFT calculation confirms that the introduction of titanium effectively reduces the volume change and suppresses the relative slip of adjacent TMO6 layers. As a result, Ti-doping is instrumental in improving the cycle stability of NFCMT. This work offers a low-cost strategy for constructing high-performance cathode materials for SIBs. offering promising prospects for the development of efficient and affordable energy storage systems.