Electrochemical and microstructural studies of AlPO₄-nanoparticle coated LiCoO₂ for lithium-ion batteries

Abstract

AlPO₄-nanoparticle coated LiCoO₂ is studied as a positive electrode for lithium rechargeable batteries for a high-voltage charge limit of 4.7V. To understand the role of the coating in transport phenomena and in deintercalation of Li+, the electrochemical behavior is studied alongside the microstructure of the coating. The galvanostatic charge profile is studied with electrochemical impedance tests conducted at various lithium contents during charging. The coating is found to increase the flatness of the two-phase region plateau and to cause the appearance of the monoclinic distortion at x=0.5 as well as distinct plateaus for 03 to H1-3 and H1-3 to 01 transitions, indicating that application of the coating transformed Li-rich LiCoO₂ into stoichiometric LiCoO₂. According to electronic resistance (Re) values from impedance spectra, the coated LiCoO₂ has a more rapid decrease in Re with delithiation, and both materials show a rise in Re above 4.5V. Structurally, SEM images show pitted surface morphology only on the edges of the layers of coated LiCoO₂.

(cont.) Furthermore, cross-sectional TEM images reveal a continuous coating layer with compositional variations seen through EDX mapping and point spectroscopy, indicating that a thin layer of aluminum covers the particle uniformly while the phosphorus is more concentrated in thicker, isolated clusters, which is further supported y XPS surface analysis. This coating composition may increase the ionic conductivity of the LiCoO₂ surface while protecting it from harmful side reactions and stabilizing the surface structure to inhibit Co dissolution.