The energy density of many lithium metal oxide battery cathodes is limited by redox reactions associated with the transition metal alone. Anionic redox reactions are affording new opportunities to increase this energy density. Here, we present results from first-principles density functional theory (DFT) calculations aimed at understanding the combined cation/anion redox of Li-rich materials, and designing new materials that will enable high-capacity, reversible cycling with minimal oxygen evolution. We illustrate this approach on three Li-rich compounds: Li₅FeO₄, Li₄Mn₂O₅, and Li₂MnO₃.

Li₅FeO₄: Initial removal of Li is accompanied by Fe migration to form a disordered rocksalt structure. A local Li₆-O coordination of oxygen, identified by DFT calculations, raises the O 2p band and enables reversible O-/O^2- redox behavior, previously unknown in this material. This insight leads to the prediction and subsequent experimental demonstration of anionic and cationic redox reactions with good reversibility and without any obvious O₂ gas release.

Li₄Mn₂O₅: We study the recently-reported, high-capacity, disordered rocksalt-type Li₄Mn₂O₅ compound and also determine the ground state ordered structure of Li4Mn2O5 via a DFT-based enumeration method. DFT calculations show that the delithiation process occurs via a three-step reaction pathway involving the complex interplay of cation and anion redox reactions: i) Mn3+ ➝ Mn4+ (Li_xMn₂O₅, 4 > x > 2), ii) O²⁻ ➝ O¹⁻ (2 > x > 1), and iii) Mn⁴⁺ ➝ Mn⁵⁺ (1 > x > 0) concomitant with Mn migration from the original octahedral site to the adjacent tetrahedral site. Finally, we predict that alloying with M = V and Cr in Li₄(Mn,M)₂O₅ would produce new stable compounds with substantially improved electrochemical properties.

Li₂MO₃: We catalog the family of Li₂MO₃ compounds as active cathodes or inactive stabilizing agents using high-throughput density functional theory (HT-DFT). With an exhaustive search based on design rules that include phase stability, cell potential, resistance to oxygen evolution, and metal migration, we predict a number of new Li₂M^IO₃–Li₂M^IIO₃ active/inactive electrode pairs, in which M^I and M^II are transition- or post-transition metal ions, that can be tested experimentally for high-energy-density lithium-ion batteries.