A New Partial Atomic Layer Deposition Coating Technique to Assess Battery Cycle-Performance Effects of Different Surface Sites of LiNi0.5Mn0.3Co0.2O2

Abstract

In order to better understand electrolyte degradation at cathode-electrolyte interfaces and mechanisms by which coatings can protect against such degradation, it is important to determine to what extent capacity fade occurs predominantly at highly reactive sites vs. uniformly over the cathode surface. To explore this question we have developed an atomic layer deposition (ALD) based thin film coating technique to study the varied effects of LiNi0.5Mn0.3Co0.2O2 (NMC) surface sites on Li-ion battery charge capacity fade. Small organic molecules selected for their chemical functionality are grafted to the oxide surface to block ALD coating of NMC surface sites of complimentary chemical reactivity to the template molecules. The ALD films are deposited using the well-established Al2O3 process, which has been studied extensively in the coating of NMC and similar cathode materials. After ALD, the organic template molecules are removed using ozone treatment, leaving the underlying reactive surface sites of NMC exposed. By exposing different surface sites, it is possible to study their separate effects on battery cycle performance. The protocol for partial ALD coating is established, with screening performed on some candidate template molecules. Both the reproducibility of the grafting, the partial ALD coating, and the template removal are established. The benzylbromide (BB) template molecule will react with hydroxyl groups at the surface, while benzoic acid (BA) will react with basic metal oxide sides. In order to insure the stability of the grafted template molecules, deposition was performed at 100°C, a temperature lower than that employed in a typical ALD process. X-ray photoelectron spectroscopy shows that the BA-NMC surfaces have a higher proportion of Ni exposed on the surface compared to a control sample, while the BB-NMC surface does not have any disproportionate elemental composition. Preliminary results show that despite similar amounts of film deposited, the BA-NMC surface performs better than BB NMC in terms of discharge capacity fade during cycle-performance testing. This preliminary result implies that Ni sites on the NMC surface do not negatively affect cycle-performance. This result is contrary to other recent reports in the field reporting a connection between NiO and solid electrolyte interphase formation, showing further insight into the complex electrochemical phenomena occurring at the cathode surface.