Dual surface-bulk aluminum modification in a-Al2O3@Na3VAl(PO4)3 sodium-ion batteries cathode to boost high voltage utilization and electrolyte protection

Amorphous alumina (a-Al₂O₃)-coated Na₃VAl(PO₄)₃ samples are prepared by a low-cost, easily scalable, and environmentally friendly sol-gel process. X-ray diffraction reveals no significant structural changes after deposition. The presence of the amorphous carbon conductive and a-Al₂O₃ phases will be respectively confirmed in the light of Raman and NMR spectroscopies. Electron microscopy evidences the presence of alumina particles deposited on the substrate. Ex-situ XRD shows the reversible structural changes while sodium is inserted. Ex-situ XPS reveals the effective participation of V⁵⁺/V⁴⁺/V³⁺ species during the electrochemical reaction, while the formation of aluminum oxyfluorides justifies the efficient HF removal that prevents electrode degradation.

Electrochemical tests will validate this proposal. Thus, rate capability essays indicate that 1–3 % a-Al₂O₃ coating enhances capacity at high rates, with coated samples exhibiting a fast sodium migration and lower cell resistance at both the beginning and conclusion of cycling tests. It is supported by evaluating the kinetic response showing their high capacitive contributions and diffusion coefficients, especially in the 2 % a-Al₂O₃ coated sample. Eventually, these findings are corroborated by the good capacity retention of the 2 % coated sample during prolonged cycling at both room and low temperatures.