Revealing the surface modification effect on Li-ion insertion into Ni-rich NCM cathode material by cyclic voltammetry

This article presents a new approach to the determination of electrochemical parameters characterizing the degradation behavior of electrodes based on LiNi0.85Co0.10Mn0.05O2 (Ni-rich NCM) using the method of cyclic voltammetry with a linear scan of the electrode potential (CV). To establish the relationship between electrochemical, structural, and morphological characteristics, XRD, synchrotron XRD (SXRD) in the operando mode, and SEM were used. The results of the galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) were used to verify the CV data. The features of the irreversible phase transition in NCM during the first cycle of extraction-insertion of lithium ions and its influence on the further behavior of the material during cycling were studied. It was found that the nature of the slope variation of capacity and coulomb efficiency dependences on the cycle number with an increase in the electrode potential scan rate is a criterion for the ratio of reversible and irreversible components of the electrode capacity degradation. Reversible electrode capacity degradation is due to the difference between the diffusion coefficients of lithium ions in the anodic and cathodic processes, irreversible degradation is due to the structural degradation of the material. For NCM samples with different particle surface conditions (with different composition of Cathode Electrolyte Interphase (CEI)) the values of Li+ ions diffusion coefficient (D) for each stage of the intercalation process were found in the range 4.5·10−14 – 1.2·10−13 cm2·s−1 for the anodic process, 1.7·10−14 – 5.6·10−14 cm2·s−1 for the cathodic process. It has been established that the reversible and irreversible degradation components of electrodes based on Ni-rich NCM in the potential range corresponding to the H2-H3/H3-H2 interphase transition correlate with each other. Therefore, the difference in D values in the anodic and cathodic processes, which correspond to the current peaks of the cyclic voltammogram in this region, is a criterion for the irreversible degradation of the material’s capacity: the greater the difference in the D values for these peaks, the higher the irreversible degradation of the material’s capacity.