Unraveling the State of Charge-Dependent Electronic and Ionic Structure-Property Relationships in NCM622 Cells by Multiscale Characterization

LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) undergoes crystallographic and electronic changes when charging and discharging, which drive the cathode material close to or even beyond its stability window. To unravel the charge compensation mechanism of NCM622, spatially resolved atomic force microscopy (AFM) measurements in electrochemical strain microscopy (ESM) and conductive AFM (C-AFM) modes are obtained, and the spectroscopic information and crystallographic information are compared. All experiments are performed with two sets of samples: state-of-the-art samples that are composed of a binder, a conductive additive, and an active material and polished samples for single-particle analysis. Near-edge X-ray absorption fine structure spectroscopy shows that ionic Ni²⁺ reacts to give Ni³⁺ when charging and forms covalent bonds with its oxygen neighbors. A Ni²⁺/Ni³⁺ gradient across the particles balances out with the increasing state of charge, as verified by ESM. Therefore, the results also provide an important view that improves the mechanistic understanding of ESM in electrode materials. Finally, the interplay between the electronic and ionic conductivities and the crystallinities of NCM622 cathodes is elaborated and discussed.