Unlocking Multi-Electron Reactions in NASICON-Na _{1+ x} V _x Ti _{2- x} (PO ₄ ) ₃ Cathodes through a Synergistic DFT and Experimental Approach

Sodium-ion batteries (SIBs) are considered a promising alternative to lithium-ion batteries due to the abundance and low cost of sodium resources, wherein cathode materials play a critical role in determining their performance. The vanadium-based phosphate Na 1+ x V x Ti 2- x (PO 4 ) 3 is considered a promising cathode for sodium-ion batteries (SIBs) due to its high capacity and structural stability. However, the high reactivity of V 3+ during redox reactions results in rapid capacity fading, while a higher Ti 3+ proportion reduces the achievable capacity. In this work, we systematically tune the V/Ti ratio of NASICON-type Na 1+ x V x Ti 2- x (PO 4 ) 3 to obtain cathode materials with optimized electrochemical performance and study the crystal structure and battery performance of Na 2 VTi(PO 4 ) 3 . Density functional theory (DFT) calculations were used to predict the voltage plateau and phase stability of Na 2 VTi(PO 4 ) 3, indicating a low volume change (∼6.89%). Experiments indicated that elevated vanadium content promotes the activity of V 2+/3+ and V 3+/4+ redox couples, which facilitates multielectron reactions and improves specific capacity. Meanwhile, the Ti 3+/4+ redox couple contributes to structural integrity and supplementary charge compensation. Leveraging the synergy between V and Ti, the Na 1.6 V 0.6 Ti 1.4 (PO 4 ) 3 composition was determined to be optimal. This material exhibited an initial discharge capacity of 144.83 mAh g – 1 at 0.2C, with a capacity retention of 92.42% over 500 cycles, as well as outstanding rate performance and cycling reversibility. This study provides both theoretical and experimental guidance for designing high-performance, long-life cathode materials for SIBs.