Cathode degradation is one of the most important factors that limit the lifetime of lithium (Li)-ion batteries. Major intrinsic causes of this degradation include instability against irreversible phase transformations, e.g., layered to spinel transformation in LixMO2 type cathodes, and dissolution of the redox-active transition metal (TM) ions into the electrolyte. Corrosive species are known to attack the cathode particles and accelerate TM dissolution, which often leads to a significant capacity loss upon cycling. Hydrofluoric acid (HF), for example, forms in the presence of only trace amount of water in the common LiPF6 based electrolytes. A strong correlation has been observed between HF content in the electrolyte and TM loss for common battery cathode materials including the layered LiCoO2, spinel LiMn2O4 and similar cathodes. For LiMn2O4, in particular, disproportionation of surface Mn3+ to Mn2+ and Mn4+, and subsequent dissolution of Mn2+ into the electrolyte is triggered by the H+ ion (i.e., acidic environments), and is a primary reason for capacity fade in this material. This dissolved Mn deposits at the anode surface and further contributes to degradation.
While alternative strategies such as doping, tailoring the particle morphology or core-shell structures have been suggested, a common approach to suppressing cathode degradation has been applying protective coatings on cathode particles. Stable binary oxides, such as Al2O3, MgO, ZnO, ZrO2, SiO2 and TiO2 may reduce the HF-content in the electrolyte, but they do not perform equally well in suppressing the TM-loss from the cathode or the capacity fade. However, the complex nature of reactions between the cathode, coating and electrolyte prohibited the design of generic guidelines to find effective coatings beyond such simple binary oxides. A density functional theory (DFT) based materials design approach considering the thermodynamic aspects of binary metal oxide cathode coatings has been introduced. (See, Aykol, M.; Kirklin, S.; Wolverton, C. Advanced Energy Materials 2014, 4, 1400690.) This reproduced the known effective coatings, such as Al2O3, and predicted trivalent transition metal oxides as a promising class of under-explored cathode coatings. This framework was limited to only binary metal oxides, because the description of HF-reactivity and electrochemical stability of coatings were described by hypothesized reactions based on “chemical intuition” (i.e., reactions that had predefined forms, such as MxO1/2+HF→MxF½H2O for HF-reactivity of a metal oxide MxO1/2) and could not be extended to other more complex materials.