Unraveling the intercalation chemistry of multi-electron reaction for polyanionic cathode Li3V2(PO4)3

Intercalation compounds with a crystallographic framework form the basis of rechargeable lithium-ion batteries. However, the reversible electrochemical process accompanied by multi-electron reactions is proven challenging owing to complex or asymmetric reaction pathway. Herein, taking monoclinic Li3V2(PO4)3 as a model, we fundamentally reveal structural transformations and detailed charge compensation mechanism under the multi-electron transfer condition. The delithiation starts with extraction from Li(3) site via a two-step phase transition accompanied by electron transforming from V(1) 3d orbits. Subsequent Li(1) and Li(2) extractions primarily yield V(2) oxidation, presenting other two phase-transition reactions. In particular, the third Li+ ion extraction induces a slight loss of oxygen, which seriously reduces the stability of the delithiated phase. Furthermore, NASICON-like LixV2(PO4)3 phases undergo a knock-on type Li migration, which is responsible for the sluggish kinetics for Li-poor phases. For the re-embedding process, however, the electronic structure perturbation induced by oxygen loss along with interpenetrating ion migration results in a solid-solution behavior. In addition, our electroanalytical techniques and modeling parameters serve as evidence in identifying the reaction pathways. These findings provide a comprehensive understanding of intercalation thermodynamics and kinetics for multi-electron reaction in polyanionic cathodes, offering powerful guidance for the future design of high-performance electrode materials.