Solid-state 23Na NMR study of Ga-doped Na3V2(PO4)2F3 cathodes for sodium-ion batteries

Sodium-ion batteries have gained attention as a suitable alternative for lithium-ion batteries due to their analogous electrochemistry in addition to the availability and affordability of sodium. Among cathode materials for SIBs, Na3V2(PO4)2F3 (NVPF) is a promising candidate due to its rapid ion transport capability and high operating potential, however, NVPF is burdened by low energy density and slow ion diffusion rates. Herein, NVPF is doped with Ga to enhance structural stability and promote alternative Na+ diffusion pathways. A sol-gel method is used to synthesize three mixed V/Ga phases and a fully substituted Ga phase: Na3V2-xGax(PO4)F3, (x = 0.5, 0.7, 1, and 2). Powder XRD analysis reveals the mixed phase materials experience lattice contraction owing to increased Ga-content, while solid-state 23Na MAS NMR of pristine phases indicates oxidation of vanadium in surrounding Na local sites upon Ga addition. The electrochemical performance of the Na3V1.5Ga0.5(PO4)F3, (x = 0.5) cathode exhibited a specific capacity of 89 mAh g-1, 95% of the phase theoretical capacity. Ex-situ 23Na MAS NMR on cycled cathodes suggests the lower Ga-substituted phases exhibit a high level of oxidation and reversibility based on the degree of 23Na shift upon charge and discharge. However, higher Ga-substituted phases display a marginal 23Na shift difference during cycling, indicating minimal oxidation processes, which translates to reduced electrochemical performance. This report intends to observe the structural and electrochemical effects of Ga-doped NVPF, while reinforcing the advantages of NMR spectroscopy when investigating alkali-ion battery systems.