NASICON型Na_1+xV_xTi_2-x(PO₄)₃カソードにおける多電子反応の解明：DFTと実験的アプローチの相乗効果

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+xV_xTi_2-x(PO₄)₃ is considered a promising cathode for sodium-ion batteries (SIBs) due to its high capacity and structural stability. However, the high reactivity of V³⁺ during redox reactions results in rapid capacity fading, while a higher Ti³⁺ proportion reduces the achievable capacity. In this work, we systematically tune the V/Ti ratio of NASICON-type Na_1+xV_xTi_2-x(PO₄)₃ to obtain cathode materials with optimized electrochemical performance and study the crystal structure and battery performance of Na₂VTi(PO₄)₃. Density functional theory (DFT) calculations were used to predict the voltage plateau and phase stability of Na₂VTi(PO₄)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_1.6V_0.6Ti_1.4(PO₄)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.