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

Abstract

Unlike in lithium-ion batteries (LIBs), in sodiumion 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[Li 1/3 Mn 2/3 ]O 2 , 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 NaO 2 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 Na 1−x [Li 2/6 Mn 4/6 ]O 2 (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−O 2 −Mn moiety. However, Li migration exhibited an ideal OR process (O 2− /O n− ) 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 O 2 molecule formation. Selectively decoupled oxygen ordering indicated that the oxygen species coordinated with two Mn (O Mn2 ) 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 O 2 . This could significantly impact the role of Li ions in Li-excess OR-layered oxides for SIBs.

Recent Advances in Mn‐Rich Layered Materials for Sodium‐Ion Batteries

“…The substitution of Li in transition metal layers is the major approach to utilizing ARR in Mn‐rich layered cathode materials. [ 107 ] Similar to the ARR mechanism in typical Li 2 MnO 3 , the coordination configuration of O with four Li ions and two Mn ions drives the generation of the non‐bonding O 2p states. [ 108 ] The approach of Li substitution in the TM layer generates non‐bonding O 2p states in layered Mn‐rich Na‐ion oxides through the weak electron overlap between alkali metal ions (Na and Li) and oxygen, such as P2‐Na 5/6 [Li 1/4 Mn 3/4 ]O 2 , [ 100 ] P3‐Na 0.6 [Li 0.2 Mn 0.8 ]O 2 [ 47 ] and P2‐Na 0.6 [Li 0.2 Mn 0.8 ]O 2 [ 109 ] with different contents of Li substitution in the TM layer.…”

Data‐Driven Decoupling Structural Feature Correlation for Harnessing Anionic Capacity in Na Layered Oxide

“…In this regard, our group highlighted that a phase transition of O3−O1 accompanying with the Li rearrangement under the Ir immobility was observed, and it was determined to be a structural design concept mimicking the Li-excess Ir cathode for a Na[Li 1/3 Mn 2/3 ]O 2 -layered cathode. [29,30] From the perspective of a local structure, forming O−O dimers has been acknowledged as a critical factor in triggering hysteretic oxygen capacities under its decrease during the first cycle for OR-exhibiting Na cathodes. Similarly, the anionic by-products were observed in the O3-type Mn oxide depending on the desodiation pathways.…”

“…[33,[35][36][37] Based on this understanding of the local structures, the decoordination of oxygen ions from the octahedron is regarded as an unavoidable mechanism upon anionic oxidation, leading to formation of the O 2 -contained moiety in the superstructure of OR-active Mn oxides. [11,17,30,38,39] These considerations in the macroscopic and local structure perspectives motivated a systematic decoupling of the complex factors resulting in oxygen capacity degradation with a severe voltage drop during (de)sodiation to provide a systematic direction toward practical OR reactions. [29,30] DFT calculations have been employed to understand the fundamental reaction mechanisms upon the (dis)charging of various layered oxides in alkali(A)-ion batteries and to diagnose the specific atomistic origins leading to performance degradation.…”

“…[11,17,30,38,39] These considerations in the macroscopic and local structure perspectives motivated a systematic decoupling of the complex factors resulting in oxygen capacity degradation with a severe voltage drop during (de)sodiation to provide a systematic direction toward practical OR reactions. [29,30] DFT calculations have been employed to understand the fundamental reaction mechanisms upon the (dis)charging of various layered oxides in alkali(A)-ion batteries and to diagnose the specific atomistic origins leading to performance degradation. [40][41][42] Recently, House et al reported on the influence of superstructure on the first cycle voltage hysteresis of oxygen-redox cathodes, attributed to the formation of vacancy clusters and the trapping of molecular O 2 .…”

“…[ 11,17,30,38,39 ] These considerations in the macroscopic and local structure perspectives motivated a systematic decoupling of the complex factors resulting in oxygen capacity degradation with a severe voltage drop during (de)sodiation to provide a systematic direction toward practical OR reactions. [ 29,30 ]…”