AC impedance analysis of NCM523 composite electrodes in all-solid-state three electrode cells and their degradation behavior

Electrochemical analysis offers useful information to understand the behavior of electrochemical systems including lithium-ion batteries and to take measures for their improvement. In this study, we report AC impedance analysis of LiNi0.5Co0.2Mn0.3O2 (NCM523) composite electrodes in all-solid-state three-electrode cells using glass-ceramic Li2S-P2S5-LiI (LPSI) or crystalline Li6PS5Cl (LPSCl) electrolytes to evaluate the rate-limiting processes in NCM523 electrodes and their variations after the charge-discharge cycles. Besides the lithium-ion transfer resistance in the electrolyte layer (i), electron transfer resistance between NCM523 composite electrode and current collector (ii), charge transfer resistance between NCM523 and solid electrolyte (iii), component that has been unidentified (iv), and lithium-ion diffusion resistance in NCM523 particles as Warburg impedance (v) are observed. The activation energies of (iii) are relatively low compared to those in liquid electrolyte systems. After the charge-discharge cycle tests, the impedance of the NCM523 electrode using both LPSI and LPSCl electrolytes shows a significant increase in (iii). On the other hand, the LPSI system exhibits larger increase in (iv) and fine voids in NCM523 secondary particles more than those in the LPSCl system, showing that (iv) can be attributed to the charge transfer resistance between NCM523 primary particles.