DFT+U insights into the structural, surface and hydrogen fluoride adsorptions on Li2MnO3 for Li-ion batteries

Abstract

The formation of acidic hydrogen fluoride (HF) upon liquid electrolyte decomposition results in Li-Mn-O cathode material degradations and capacity fading during cycling. Even with the extensive research on electrolyte decomposition, degradation mechanisms, and capacity retention improvement strategies, further research is required to comprehend the surface interactions of HF with the layered Li-rich Mn-based Li₂MnO₃ cathode material. Hence, this work discusses the surface properties and effect of HF adsorption on the major Li₂MnO₃ surface using the density functional theory (DFT) method. During single HF adsorption, the molecule spontaneously dissociates to form Li-F and H-O species on the surface with an average adsorption energy (E_ads) of −1.90 eV. However, increasing the HF surface coverage ($\theta$) generally resulted in a decrease in E_ads, whereas for the full monolayer, we observed a slight drop with reference to the $\theta$ = 0.33. It can be noted that the incorporation of HF enhances the stability of the (001) surface, which improves with an increase in surface coverage. The calculated work function increases with an increase in surface coverage, with a slight drop upon full coverage, suggesting a decrease in reactivity compared to the pristine (001) surface. Upon attaining a monolayer, the newly formed Li-F species on the surface greatly relaxed outwards with the highest average charge accumulation of −0.191 e^-. The electronic density of states shows no effect of HF surface adsorptions on the bandgap, however, the emergence of Li-F and H-O peaks on the valence band. Our results provide a closer look into the surface properties and HF interactions with the major Li₂MnO₃ surface for aqueous lithium-ion battery.