Novel Method for Monitoring the Electrochemical Capacitance by in Situ Impedance Spectroscopy as Indicator for Particle Cracking of Nickel-Rich NCMs: Part I. Theory and Validation

Nickel-rich NCM (LiMO₂, with M = Ni, Co, and Mn) cathode active materials for lithium-ion batteries are being increasingly commercialized due to their high specific capacity. However, their capacity retention upon cycling is impaired by crack formation of NCM secondary agglomerates induced by the volume change upon repeated (de)lithiation that depends on the nickel content and the cutoff potential. Particle cracking leads to loss of electrical contact and enhanced side reactions caused by an increased surface area. Here, we introduce a novel method based on electrochemical impedance spectroscopy (EIS) in blocking conditions to quantify the increase in the active material's surface area upon cycling, utilizing the correlation between the surface area of the electrode and the electrochemical double-layer capacitance that is validated experimentally by comparing the capacitance and BET surface area increase of NCM electrodes upon mechanical compression. To quantify the cracking of the particles upon 200 charge/discharge cycles, we perform in situ EIS measurements utilizing a micro-reference electrode and monitor the cathode's impedance response. In addition, the crack formation of cycled NCM particles is validated visually by post mortem FIB-SEM. The effect of volume change on cracking is illuminated through the analysis of LFP and LTO as model materials.