In-situ probing the near-surface structural thermal stability of high-nickel layered cathode materials

The thermal stability of cathode materials is very important to the safety of lithium-ion batteries (LIBs), especially the promising high-nickel LiNixCoyMn1-x-yO2 (NCM, 0.6 ≤ x < 1) materials. Generally, the thermal decomposition is believed to begin at the electrode/electrolyte interface. However, due to the lack of suitable diagnostic tools, current recognition of their near-surface structural thermal stability still remains limited. Raman spectroscopy can not only sensitively reflect changes in the local metal-oxygen coordination structure, but also conveniently detect the near-surface structural information with the suitable spatial resolution and penetration depth. Here, through developing the in-situ heating Raman spectroscopy method, the thermal decomposition process of the near-surface structure of the fully charged high-nickel NCM material is confirmed, which is much lower than existing recognition. Interestingly, the thermal decomposition of the secondary particle bulk is evidenced to be obviously lagging behind the particle surface, exhibiting a centripetally diffused thermal decomposition within the secondary particle. In addition, the near-surface structural thermal stability is revealed to be significantly modulated by the electrolyte components by means of the dehydrogenation, adsorption, oxidation of carbonate solvents and the decomposition of lithium salt. Meanwhile, it weakens with the increased surface oxidation state of high-nickel NCM materials. Consequently, this work can remind us to rethink the true thermal stability of high-nickel NCM materials and guide targeted improvement of their interfacial thermal stability.