Boosting Structural Reversibility of Na3V2(PO4)3 Cathode via Ti-Cr-Al Synergistic Substitution: From Two-Phase to Quasi-Solid-Solution Reaction in Aqueous Sodium-Ion Storage

Aqueous sodium-ion batteries (ASIBs) are considered promising candidates for large-scale energy storage due to the natural abundance and low cost of sodium. However, the practical application of NASICON-type Na3V2(PO4)3 (NVP) cathodes in aqueous electrolytes is severely hindered by structural degradation and material dissolution, leading to rapid capacity fading. Herein, we propose a multi-metal substitution strategy to address these challenges by successfully synthesizing a Ti, Cr, and Al co-substituted Na3V1Ti1/3Cr1/3Al1/3(PO4)3 (NVTCAP) cathode material via a sol-gel method. The synergistic effect of the incorporated metal ions optimally tunes the crystal lattice, enhancing structural stability and mitigating volume expansion during cycling. This multi-metal substitution effectively suppresses active material dissolution, as revealed by in-situ X-ray diffraction (XRD), and enables the structural evolution of crystals during charging and discharging to exhibit the characteristics of quasi-solid-solution reactions. Quantitatively, the NVTCAP electrode undergoes a continuous structural evolution during Na+ extraction/insertion, with the unit cell volume recovering to within just 0.31% of its initial value after a full cycle. Consequently, the optimized NVTCAP electrode exhibits significantly improved electrochemical kinetics, delivering a high reversible capacity, outstanding rate capability (retaining 86.7% of its initial capacity from 0.1 to 5 A g-1), and exceptional long-term cycling stability (88.6% capacity retention after 3000 cycles at 5 A g-1). When coupled with an activated carbon anode, the NVTCAP cathode maintains superior performance, with 62.1% capacity retention after 5000 cycles. This work demonstrates that multi-metal synergistic substitution is a highly effective strategy for developing high-performance NVP-based cathodes, paving the way for advanced ASIBs with enhanced durability and rate capability.