Mechanism for Al₂O₃ Atomic Layer Deposition on LiMn₂O₄ from In Situ Measurements and Ab Initio Calculations

Here, we elucidate the mechanism for Al₂O₃ atomic layer deposition (ALD) on LiMn₂O₄ (LMO) cathodes for lithium-ion batteries by using in situ and ex situ experimental characterization coupled with density functional theory (DFT) calculations. We demonstrate that not only does Al₂O₃ coat the LMO, but the Al heteroatom of the trimethylaluminum (TMA) precursor also dopes to interstitial sites on the LMO surface, thereby reducing the oxidation state of near-surface Mn ions. DFT calculations further suggest facile transfer of methyl groups from the TMA precursor to oxygen atoms on the LMO surface, which blocks adsorption sites for subsequent TMA adsorption. These predictions are supported by quartz crystal microbalance experiments demonstrating inhibited growth below ten ALD Al₂O₃ cycles, suggesting that sub-monolayer coverages of alumina are present on the LMO surface in the early stages of film growth. In comparison with fully conformal films, these sub-monolayer coatings show enhanced electrochemical capacity when cycled in coin cells. There is great demand for rechargeable battery electrodes with improved energy density and cycle life. Although protective coatings deposited on lithium-ion electrodes show enhanced performance, many of the mechanistic details at the electrode-coating interface remain elusive. In this work, Al₂O₃ was grown on spinel LiMn₂O₄, a model cathode for studying the transition-metal-loss problem that plagues many promising cathode materials. We employed a suite of in situ and ex situ techniques, along with theoretical calculations, to elucidate mechanisms of Al₂O₃ growth by atomic layer deposition on LiMn₂O₄. The ALD Al₂O₃ reaction is multi-faceted in that it involves precursor decomposition, Al doping, Mn redox, and non-uniform film growth, each of which contributes to observed trends in electrochemical performance. The fundamental understanding demonstrated in this work provides insights toward rational tuning of electrode interfaces for enhanced electrochemical performance. Deposition of protective coatings on lithium-ion battery electrode materials has been shown to enhance electrochemical capacity retention. Despite this, little is understood regarding the nature of the interface formed between the protective coating and the electrode. Here, we report a detailed mechanism for Al₂O₃ atomic layer deposition on LiMn₂O₄. We find that the initial stages of Al₂O₃ ALD on LiMn₂O₄ lead to sub-monolayer deposits that enhance electrochemical performance.