High-voltage and long-life aqueous lithium-ion batteries based on polyanionic framework structured Na₃V₂(PO₄)₃ and NbOPO₄ in novel deep eutectic solvents

Due to the intrinsic structural stability, materials with polyanionic framework have attracted worldwide attention to build-up aqueous metal-ion batteries for large-scale energy storage. Anion-dependent electrochemical behaviors of graphene-modified Na₃V₂(PO₄)₃ (rGO/NVP/C) with rhombohedral structure have been explored. Compared to common saturated solutions, remarkable improvements on overall the electrochemical performances have been achieved in novel deep eutectic solvents (DES) based on methylsulfonylmethane, perchloride salts and water. A high-voltage aqueous lithium-ion battery consisted of rGO/NVP/C nanomaterials as cathode materials and carbon-coated NbOPO₄ as anode materials (rGO/NVP/C||DES-Li||NbOPO₄) exhibits a specific energy of 74 Wh kg⁻¹ with a voltage output up to 1.8 V and excellent capacity retention that is above 80.1% after 600 cycles at 2 C current density. Kinetic studies have clearly shown that Li⁺ (de)intercalation in rGO/NVP/C in DES is dominated by a diffusion-controlled process. During the repetitive Li⁺ (de)intercalation, a phase transformation from Na₃V₂(PO₄)₃ phase into Li₂NaV₂(PO₄)₃ is identified. The negligible changes of lattice volume within Na₃V₂(PO₄)₃ during Li⁺ (de)intercalation, and the wide electrochemical window of DES together with the low activity of water in DES enable the long cycle life of rGO/NVP/C||DES-Li||NbOPO₄.