Surface Modification of the LiFePO₄ Cathode for the Aqueous Rechargeable Lithium Ion Battery

The LiFePO₄ surface is coated with AlF₃ via a simple chemical precipitation for aqueous rechargeable lithium ion batteries (ARLBs). During electrochemical cycling, the unfavorable side reactions between LiFePO₄ and the aqueous electrolyte (1 M Li₂SO₄ in water) leave a highly resistant passivation film, which causes a deterioration in the electrochemical performance. The coated LiFePO₄ by 1 wt % AlF₃ has a high discharge capacity of 132 mAh g⁻¹ and a highly improved cycle life, which shows 93% capacity retention even after 100 cycles, whereas the pristine LiFePO₄ has a specific capacity of 123 mAh g⁻¹ and a poor capacity retention of 82%. The surface analysis results, which include X-ray photoelectron spectroscopy and transmission electron microscopy results, show that the AlF₃ coating material is highly effective for reducing the detrimental surface passivation by relieving the electrochemical side reactions of the fragile aqueous electrolyte. The AlF₃ coating material has good compatibility with the LiFePO₄ cathode material, which mitigates the surface diffusion obstacles, reduces the charge-transfer resistances and improves the electrochemical performance and surface stability of the LiFePO₄ material in aqueous electrolyte solutions.