Insight into relationship between Crystal/Interface structure and properties of capacity, stability and rate capability are important for developing advanced Li-ion batteries. Using theoretical calculations combined with experimental in-situ tests, we did extensive studies on the kinetic of Li-ion diffusion for two representative cathode materials: layered Li(NixMnyCoz)O2 (NMC) (x + y + z = 1) and LiFePO4. We not only focus on the bulk kinetics, but also the kinetics across electrode/electrolyte solid-liquid interface and in the electrolytes. For example, we first proposed that "Janus" solid-liquid interface would facilitate the Li-ion transport in battery and introducing some disordering in non-active cathode materials would activate them for Li-ion storage. Finally, we also developed some in-situ technologies for battery studies. For example, using electrochemical quartz crystal microbalance (EQCM), we achieve an in situ experimental investigation of the LiFePO4 (LFP) and NaFePO4 (NFP)/electrolyte interfacial kinetics for Li(Na)-batteries. For high energy and power density applications (e.g., EVs), the safety becomes especially important. Using ab initio calculations combined with experiments, we clarified how the thermal stability of NMC materials can be tuned by the most unstable oxygen, which is determined by the local coordination structure unit (LCSU) of oxygen (TM(Ni, Mn, Co)3-O-Li3-x’): each O atom bonds with three of transition metal (TM) from the TM-layer and three to zero of Li from fully discharged to charged states from the Li-layer. Under this model, how the lithium content, valence states of Ni, contents of Ni, Mn, and Co, and Ni/Li disorder to tune the thermal stability of NMC materials by affecting the sites, content, and the release temperature of the most unstable oxygen is proposed.

Crystal/interface structure vs. properties of Li-ion batteries

255th National Meeting (2018)

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