Anchoring Li₃PO₄ on Li₅FeO₄ Surface as a Prelithiation Additive for Lithium-Ion Batteries

Antifluorite-type Li₅FeO₄ is regarded as an important cathode prelithiation material owing to its high specific capacity, yet its air stability needs to be further improved due to the high activity of Li₅FeO₄ with CO₂ and water in the air. In this article, amorphous Li₃PO₄ was anchored on the surface of Li₅FeO₄ through a straightforward method to enhance its air stability. First, Fe₂O₃ particles were dissolved in a certain concentration of NH₄H₂PO₄ solution followed by a drying process and calcination with Li₂O to obtain Li₃PO₄-coated Li₅FeO₄. The structure and ingredients were confirmed by Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray Diffraction, Fourier Transform Infrared Spectroscopy, and X-ray Photoelectron Spectroscopy. Furthermore, the electrochemical performances were examined by Galvanostatic charge-discharge, Cyclic Voltammetry, and Electrochemical Impedance Spectroscopy. The results indicate that the final product exhibits a core-shell structure, with Li₅FeO₄ as the core and Li₃PO₄ as the shell. This unique structure not only provides better environmental stability for Li₅FeO₄ but also improves its delithiation performance. Under optimal conditions, the product with 5% NH₄H₂PO₄ delivers the most remarkable performance. At a 0.05C current rate, it can deliver a specific capacity of 650.9 mAh^-1, which is higher than that of pure Li₅FeO₄ (612.3 mAh·g^-1). After 3 h of exposure to air, the optimized sample demonstrates a superior capacity of 498.2 mAh·g^-1 while the pure Li₅FeO₄ retains only 193.9 mAh·g^-1. Moreover, the diffusion coefficient of the optimized sample is also remarkably improved due to the advantage of the fast ion conductivity property of Li₃PO₄. Finally, the prelithiation performance was tested through a full battery system employing NCM₈₁₁ as the cathode and silicon carbon as the anode, and the results indicate that the as-prepared product is a promising prelithiation additive to enhance capacity retention as well as improve the cyclability of lithium-ion batteries.