Stabilizing Anionic Redox Reactions By Regulating Frontier Orbitals in Cation-Disordered Rock-Salt Oxides

Abstract

Cation-disordered rock-salt oxides (DRXs) with both cationic and O2-/O2 n- redox reactions, such as Li1.2Mn0.4Ti0.4O2, Li1.25Nb0.25Fe0.5O2, and Li1.3Ta0.3Mn0.4O2 are promising materials for designing a new generation of lithium-ion batteries with high energy density. [1, 2] However, the irreversible loss of lattice oxygen during charge/discharge cycling, fast capacity fading, and severe voltage decay are three major challenges, which impede the practical application of those materials. [3, 4] Therefore, developing strategies to stabilize the anionic redox processes in DRXs cathodes is currently a major focus of the battery research community.Here, we combine experiment and theory to demonstrate that regulating frontier orbitals via the redox-active transition-metal ratio is an effective strategy to enhance the oxygen reaction stability to enhance the cycling stability of DRXs [5]. We investigated a series of xLi2TiO3-(1 - x)LiMnO2 (0 ≤ x ≤1) materials and discovered that only Li1.2Mn0.4Ti0.4O2 (x=0.4) and Li1.1Mn0.7Ti0.2O2 (x=0.2) can form phase-pure DRXs. The newly discovered Li1.1Mn0.7Ti0.2O2 DRX exhibits a remarkable capacity retention of 84.4% after 20 cycles compared to only 60.8% for Li1.2Mn0.4Ti0.4O2. Based on the dQ/dV curves and X-ray absorption near edge structure (XANES) results, we show that the reversibility of the lattice oxygen redox of the materials improves while the cationic Mn redox reaction dominates. Therefore, adjusting the TM and Li content in the oxides is an important strategy to balance the high capacity enabled by the oxygen redox and the cyclic stability of the DRXs. Density functional theory (DFT) calculations reveal that the Mn-to-Ti ratio, which also determines the lithium content by charge balance, intrinsically regulates redox-active frontier orbitals close to the Fermi level, i.e., (Mn-O)* antibonding orbitals in Li1.1Mn0.7Ti0.2O2 and Li-O-Li unhybridized orbitals in Li1.2Mn0.4Ti0.4O2. The former results in low charge transfer to anionic O and a high aggregation barrier, promoting high reversibility of the oxygen redox reaction. Whereas, the latter generates high charge transfer to anionic O and a low O···O aggregation barrier, resulting in low cycling stability and O2 release.