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 ? and at 55 ? for comparison of its performance at 25 ?.

Automated summary

In this study, the electrochemical performance of Na3V2(PO4)(3) cathode material was improved through dual-doping with Cr and Mg. The optimal doping concentration of 0.1 for both Cr and Mg showed positive effects on the capacity, rate performance, and cycling stability of the cathode material.