Ti⁴⁺ doping-induced regulation of P2/O3 biphasic structure enhances the sodium storage performance of NaFe_0.45Mn_0.55O₂ cathode material

Layered sodium-ion battery cathode materials are considered ideal candidates for achieving high energy density and low-cost energy storage owing to their high specific capacity, strong structural tunability, and rich transition metal chemistry. In this study, a series of Ti-doped layered oxide cathodes NaFe_0.45Mn_0.55-xTi_xO₂ (x = 0.00, 0.05, 0.09, 0.13) were successfully synthesized via a sol-gel method, and the influence of Ti substitution for Mn on the structural evolution and electrochemical performance was systematically investigated. The results reveal that the sample with x = 0.09 (denoted as NFMT9) exhibits the best overall performance. Structural analysis indicates that the introduction of Ti⁴⁺ induces a phase transformation from a single O3 phase to a more stable P2/O3 composite phase, significantly enhancing structural stability during cycling. Electrochemical measurements demonstrate that NFMT9 delivers a high discharge capacity of 110.3 mAh g^-1 at 1 C, (1 C = 200 mA g^-1) with an initial capacity of 97.2 mAh g^-1 and an excellent capacity retention of 95.9 % after 100 cycles. Moreover, it exhibits outstanding rate capability under various current densities. GITT analysis further confirms that Ti doping facilitates Na⁺ diffusion kinetics, thereby improving the charge/discharge rate and energy efficiency. The performance enhancement is attributed to the synergistic effect of the P2/O3 dual-phase structure and the optimization of the charge compensation mechanism. Specifically, the introduction of Ti⁴⁺ reduces the content of unstable Mn³⁺and suppresses the Jahn-Teller distortion. Furthermore, to assess the practical applicability of NFMT9, a full cell is assembled using NFMT9 as the cathode and hard carbon (HC) as the anode. After 100 cycles at 0.5 C, the full cell retained a discharge capacity of 87.7 mAh g⁻¹, corresponding to a capacity retention of 91.9 %. The work provides new insights into phase structure engineering and valence-state modulation for the development of high-performance sodium-ion battery cathode materials.