Atomic Layer Deposition of Alumina-Coated Thin-Film Cathodes for Lithium Microbatteries

This work shows the electrochemical performance of sputter-deposited, binder-free lithium cobalt oxide thin films with an alumina coating deposited via atomic layer deposition for use in lithium-metal-based microbatteries. The Al₂O₃ coating can improve the charge-discharge kinetics and suppress the phase transition that occurs at higher potential limits where the crystalline structure of the lithium cobalt oxide is damaged due to the formation of Co⁴⁺, causing irreversible capacity loss. The electrochemical performance of the thin film is analysed by imposing 4.2, 4.4 and 4.5 V upper potential limits, which deliver improved performances for 3 nm of Al₂O₃, while also highlighting evidence of Al doping. Al₂O₃-coated lithium cobalt oxide of 3 nm is cycled at 147 µA cm^-2 (~2.7 C) to an upper potential limit of 4.4 V with an initial capacity of 132 mAh g^-1 (65.7 µAh cm^-2 µm^-1) and a capacity retention of 87% and 70% at cycle 100 and 400, respectively. This shows the high-rate capability and cycling benefits of a 3 nm Al₂O₃ coating.