Operando space-resolved inhomogeneity in lithium diffusion across NMC and graphite electrodes in cylinder-type Li-ion batteries

Homogeneity in Li-ion battery electrodes is a crucial element in obtaining high performance and long cycle life. In this work inhomogeneities in lithium distribution in NMC cathodes and graphite anodes in cylindrical cells have been tracked in real time during cycling, using operando and ex situ high energy X-ray diffraction. A set of 18650 cells having undergone different cycling protocols and at different stages of degradation have been studied during operation at two different C-rates using synchrotron radiation, scanned along the height of the cells. Significant lithium gradients along the height of the cells have been found, with large changes in inhomogeneity over the course of up to 11 charge/discharge cycles performed operando. Lithium inhomogeneity has been found to be different in cathodes and anodes, and depend heavily on degradation state, state of charge, C-rate, and relaxation steps. Phase relaxation during OCV was quantified, such that after charge LiC₆+LiC₁₈+C→LiC₁₂, and after discharge LiC₁₂+C→LiC₁₈.