Li-ion diffusion characteristics of surface modified Ni-rich NCM cathode material

Abstract

The article considers the mechanism of the particles’ surface treatment influence on the electrochemical characteristics of the lithium-ion battery cathode active material based on Ni-Rich Layered Oxides with following composition LiNi0.85Co0.10Mn0.05O2 (NCM). To study the characteristics of the electrode material, a complex of structural, morphological, and electrochemical research methods was applied. To characterize the structural and morphological state of the material, X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods were used. From the XRD data, the unit cell parameters depending on the material’s particles surface treatment were determined. SEM allowed determining the morphology of the studied material. It is presented by globular agglomerates ∼ 20 μm in size formed by clusters of crystallites ∼ 0.3 – 0.5 μm in size. Each crystallite has distinct boundaries. The article presents the evaluation data of the practical electrochemical characteristics of the studied NCM samples: the capacity values and their change during repeated charge–discharge cycles were determined. The capabilities of the electrochemical methods such as galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) in their combined application allowed to analyze several charge transfer processes in the material under study, differing in characteristic times. The dependences of the diffusion coefficient of lithium ions in each process on the electrode potential or concentration of lithium ions were found. The range of D values for the process with the longest characteristic times was 10-15 – 10-12 cm2·s−1. Comparison of the dependences D(E), obtained using the GITT and EIS methods allowed to determine the degree of diffusion inhomogeneity, which differs for the three samples under consideration and is a criterion for the level of practical characteristics of the electrode material.