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.