Na₃MnTi(PO₄)₃ (NMTP) shows significant potential as a cathode for sodium-ion batteries (SIBs) owing to its multi-electron transfer capability and high theoretical capacity. Nevertheless, its practical application is significantly limited by sluggish ion diffusion and rapid capacity decay, which stem from structural evolution during the sodiation/desodiation process. Herein, an Fe-doping strategy is proposed to reinforce the structural framework and enhance the electrochemical performance of NMTP. Trace Fe doping is found to shorten the M–O (M = Ti and Mn) bond while extending the Na–O bond, effectively minimizing structural fluctuations in NMTP during charge/discharge cycles and enhancing sodium-ion diffusion kinetics. Consequently, the Na₃Mn_0.99Fe_0.02Ti_0.99(PO₄)₃ (NMTP-Fe0.02) cathode demonstrates exceptional rate capability and long-term stability, delivering a high reversible capacity of 153.2 mAh·g⁻¹ at 0.1 C and retaining 99.3 mAh·g⁻¹ after 800 cycles at 5 C, exhibiting a capacity preservation rate of 81.5%. Moreover, its outstanding performance in full-cell configurations highlights the significant potential of NMTP-Fe0.02 for practical applications.

Sodium-ion batteries (SIBs) are required to possess long cycle life when used for large-scale energy storage. The polyanionic Na₄MnV(PO₄)₃ (NMVP) reveals good cyclic stability due to its unique three-dimensional (3D) frame structure, but it still faces the challenge of interfacial degradation in practical applications. In this work, NASICON-type Na_1.3Al_0.7Ti_1.3(PO₄)₃ (NATP) was deposited on the surface of NMVP to promote interface stability by surface modification and gradient doping. As a result, the optimized NMVP@2%NATP released a capacity retention of 44.8% after 1000 cycles at 5 C, much higher than that of the initial NMVP (28.9%). The enhanced electrochemical performance was mainly attributed to NATP coating acting as a fast ion transport carrier and physical barrier, significantly facilitating the Na⁺ diffusion and isolating side reaction at the electrode/electrolyte interface. On the other hand, Ti⁴⁺ and Al³⁺ cations from the NATP were partially doped inside the NMVP surface to boost the transport of Na⁺, and the perfect lattice matching of NVMP and NATP improved the interface and structural stability accompanying long cycling. This work demonstrated the effectiveness of surface modification with high lattice match material and provided new perspectives for high energy density solid-state SIBs.