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 sodium-ion batteries, 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.