Synergetic Effect of Isovalent- and Aliovalent-Ion Dual-Doping in the Vanadium Site of Na₃V₂(PO₄)₃ for Wide-Temperature Operating Sodium-Ion Batteries

NASICON-type vanadium-based cathodes have received significant attention due to the higher stability and theoretical capacity associated with polyanionic groups and vanadium redox-chemistry. However, improvements in structural stability and electronic conductivity to enhance the electrochemical performance are still a hot topic of research. In the present work, intrinsic modification in the vanadium site of Na3V2(PO4)3 (NVP) by dual-doping was investigated. The effect of dual-doping by isovalent Cr3+ and aliovalent Mg2+ ions in the vanadium site as Na3V2–x–yCrxMgy(PO4)3 (varying x and y = 0, 0.05, 0.1, and 0.15) was investigated. Among all the concentrations of doping, a concentration of 0.1 for both Cr and Mg showed a positive impact in capacity and rate performance as well as cycling compared to undoped and other doping conditions. The dual-doping significantly influences the particle size reduction, local lattice distortion, and populates higher valent V ions on the surface. These aspects synergistically help in reducing diffusion length, faster sodiation, and improved structural stability. Compared to the undoped and single-ion-doped NVP samples, the dual-doped NVP performed well. Dual-doped NVP exhibited 100 mA h/g at 25 mA/g and at 100 mA/g, 88% retention after 100 cycles. At the same time, at 250 mA/g, a specific capacity of 81.6 mA h/g for dual-doped NVP and a lower capacity of only 62.3 mA h/g for undoped NVP were recorded. Galvanostatic intermittent titration studies were also conducted to evaluate the kinetics. To establish the electrochemical performance of dual-doped NVP at different temperatures, electrochemical investigations were carried out at −10 °C and at 55 °C for comparison of its performance at 25 °C.