Physicochemical Screen Effect of Li Ions in Oxygen Redox Cathodes for Advanced Sodium-Ion Batteries

Unlike in lithium-ion batteries (LIBs), in sodium-ion batteries (SIBs), nonhysteretic oxygen redox (OR) reactions are observed in Li-excess Na-layered oxides. This necessitates an understanding of the reaction mechanism of an O3-type Li-excess Mn oxide, Na[Li1/3Mn2/3]O2, a novel OR material designed for advanced SIBs. It could establish the role of Li in triggering nonhysteretic oxygen capacities during (de)sodiation. Three biphasic mechanisms were compared using first-principles calculations under the desodiation modes: (i) Na/vacancy ordering, (ii) Li migration in the NaO2 layer, and (iii) in-plane Mn migration. The migrated Li ions generated a "physicochemical screen"effect upon electrochemical OR reactions in the oxide cathode. Thermodynamic formation energies showed different biphasic pathways upon charging in Na1-x[Li2/6Mn4/6]O2 (NLMO) under the three modes. O-O bond population indicated that biphasic-reaction paths -i and -iii were derived from generating inter/intralayer O-O dimers, and path-iii was triggered by the formation of a Mn-O2-Mn moiety. However, Li migration exhibited an ideal OR process (O2-/On-) without forming anionic dimers. The electronic structures of Mn(3d) and O(2p) revealed that Li migration pushed lattice-based O(2p)-hole states to a high energy level, resulting in the chemical suppression of O2 molecule formation. Selectively decoupled oxygen ordering indicated that the oxygen species coordinated with two Mn (OMn2) derived from Li migration played an important role in nonhysteretic oxygen capacities during cycling. From these findings, we propose the "physicochemical screen"concept that physically suppresses interlayer O-O dimers and chemically hinders discretized O(2p)-O(2p) states formed by molecular O2. This could significantly impact the role of Li ions in Li-excess OR-layered oxides for SIBs.