No abstract

Na3V2(PO4)3 has been considered as the most promising cathode material for sodium-ion batteries because of its suitable sodium insertion/extraction plateau and good cycling stability. However, the inferior rate-capability resulting from the lower electronic conductivity restricts its commercial application. To address this issue, niobium ions (Nb5+) are introduced to partially substitute V3+ for generating V-mixed valence, ensuring the material with improved intrinsic electronic conductivity. Herein, nano-sized Na3V2−xNbx(PO4)3/C (x = 0, 0.05, 0.1, 0.2) materials with a core–shell structure are prepared via a simple sol–gel method. The optimized Na3V1.9Nb0.1(PO4)3/C material shows a high specific capacity (∼114 mA h g−1 at 0.5C), a superior rate-capability (∼100 mA h g−1 at 20C), and an excellent long-term cycling stability (a specific capacity of 74.7 mA h g−1 over 1000 cycles at 50C, corresponding to a capacity decay rate of 0.0258%/cycle). The improved cycling performance is mainly attributed to the enhanced intrinsic electronic conductivity and Na+ mobility, and the reduced charge-transfer resistance, as confirmed by the first-principles calculations, cyclic voltammetry, and electrochemical impedance spectra analysis. The constructed fast electron network, from the surface to the bulk, combining with the nano-sized feature, contributes to the facilitated electrode reaction kinetics and, thus, ensures improved rate-capability.