Insights Towards Mitigated Mn Dissolution through Structure-Sensitive Al2O3 Film Nucleation on LiMn2O4 By Atomic Layer Deposition

Abstract

Dissolution of transition metal (TM) cations into organic electrolytes is a significant impediment to capacity retention in various lithium ion battery electrode materials. Interfaces engineered via substitutional doping and protective coatings, however, have been demonstrated to be effective techniques to reduce TM dissolution, preventing counterelectrode crossover1–3. In this work, we present an integrated theoretical and experimental approach to gain a molecular level understanding towards the functionality of protective thin films. As a model system, we consider Al2O3 film growth by atomic layer deposition (ALD) on spinel LiMn2O4 (LMO), a cathode material exhibiting the well-documented capacity fade mechanism of Mn dissolution. Using first principles DFT+U calculations in concert with in situ experimental characterization, the mechanisms for Al2O3 ALD film growth by trimethylaluminum (TMA) and H2O half-reactions are evaluated. Using thermodynamically stable low-index surface truncations, as well as stepped defect features4, we identify facile pathways for full demethylation of the TMA precursor on the LMO surface. Trends from microbalance experiments indicate sub-monolayer film coverage for ~10 ALD cycles, wherein film growth is initially slow, but gradually increases until the limit of a fully formed Al2O3 film is reached. DFT calculations suggest that sluggish film growth at the onset is predicated by site blocking from decomposed CH3 groups. An electronic structure descriptor is presented to demonstrate evidence that, in early ALD pulses, Al2O3 preferentially nucleates at sites on LMO that are most susceptible to Mn dissolution. The chemical nature of these relevant defect sites will be discussed further in the context of Mn dissolution through charge disproportionation, with enhanced stability by selective nucleation of the protective films. The unique mechanisms for ALD on LMO lead to modest enhancements in electrochemical performance for only ~1-2 ALD cycles, whereas cycling capabilities reduce with further TMA/H2O pulses in the limit of a fully formed coating on the substrate. This research was supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.