The Effect of Cation Disorder on the Average Li Intercalation Voltage of Transition-Metal Oxides

Cation disorder is a phenomenon that is becoming increasingly important for the design of high-energy lithium transition-metal oxide positive electrodes (LiMO₂) for Li-ion batteries. Disordered Li-excess rocksalts have recently been shown to achieve high reversible capacity, and <i>in operando</i> cation disorder (i.e., disorder induced by electrochemical cycling) has been observed in a large class of ordered materials. Despite the growing importance of cation disorder in the Li-ion battery field, very little is known about the effect of cation disorder on the average voltage (i.e., energy density) of lithium transition metal oxides. In this study, we use first-principles methods to demonstrate that, depending on the transition metal species, cation disorder can lead to an increase or a decrease of the average voltage of lithium transition metal oxides. We further demonstrate that the Ni^3+/4+ redox can be high in disordered compounds, so that it may be preceded by oxygen activity. Finally, we establish rules for the voltage evolution of compounds that experience <i>in operando</i> disorder

Understanding the Effect of Cation Disorder on the Voltage Profile of Lithium Transition-Metal Oxides

Cation disorder is a phenomenon that is becoming increasingly important for the design of high-energy lithium transition metal oxide cathodes (LiMO₂) for Li-ion batteries. Disordered Li-excess rocksalts have recently been shown to achieve high reversible capacity, while <i>in operando</i> cation disorder has been observed in a large class of ordered compounds. The voltage slope (dVdxLi) is a critical quantity for the design of cation-disordered rocksalts, as it controls the Li capacity accessible at voltages below the stability limit of the electrolyte (∼4.5–4.7 V). In this study, we develop a lattice model based on first principles to understand and quantify the voltage slope of cation-disordered LiMO₂. We show that cation disorder increases the voltage slope of Li transition metal oxides by creating a statistical distribution of transition metal environments around Li sites, as well as by allowing Li occupation of high-voltage tetrahedral sites. We further demonstrate that the voltage slope increase upon disorder is generally smaller for high-voltage transition metals than for low-voltage transition metals due to a more effective screening of Li–M interactions by oxygen electrons. Short-range order in practical disordered compounds is found to further mitigate the voltage slope increase upon disorder. Finally, our analysis shows that the additional high-voltage tetrahedral capacity induced by disorder is smaller in Li-excess compounds than in stoichiometric LiMO₂ compounds