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.