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

Abstract

LiNi 0.6 Co 0.2 Mn 0.2 O 2 (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 Ni 2+ reacts to give Ni 3+ when charging and forms covalent bonds with its oxygen neighbors. A Ni 2+ /Ni 3+ 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.

Stacking Fault Formation in LiNi0.6Co0.2Mn0.2O2 during Cycling: Fundamental Insights into the Direct Recycling of Spent Lithium-Ion Batteries

“…Figures S2 and S3 show the Rietveld analysis results of other delithiated NCM622 samples, and Figures S2 and S3 list the structural parameters of all the samples. The delithiated samples can be assigned to a single phase with the R 3̅ m space group until 244 C ( x = 0.12), although previous in situ XRD studies on NCM622 limited the charge voltage to 4.6 V or 4.5 V, x = 0.35, or Q cha = 150 mA h g –1 . NCM622 differs from Li x CoO 2 , and Li x NiO 2 ; for example, at least four different layered phases were observed in Li 0.1 NiO 2 .…”

“…Changes in the crystal structure and lattice parameters for the a h - and c h -axes from those in the initial state are essential for determining the properties of degraded electrodes, but previous results on NCM622 remain controversial. − For example, Wang et al and Jetybayeva et al claimed that it could be modeled as a single-phase material up to a cell voltage ( E ) of 4.6 V vs Li + /Li, whereas Zhu et al proposed a two-phase model at E ≥ 3.78 V. In addition, the Q dependences of a h and c h apparently differ, although these values were previously examined with respect to changes in E , , x in Li x Ni 0.6 Co 0.2 Mn 0.2 O 2 , and the state of charge . Previous studies on the structure of NCM622 utilized laboratory or synchrotron in situ X-ray diffraction (XRD), − which has limitations in determining the exact structural parameters and detecting minor phases due to narrow 2θ range and weak peak intensity of the XRD patterns. We thus conducted whole-powder pattern fitting analyses, i.e.…”

Correlating Nanoscale Structures with Electrochemical Properties of Solid Electrolyte Interphases in Solid-State Battery Electrodes

“…ESM images show the spatial distribution of Li-ions on the surface of Li-ion battery materials. ,, Therefore, the average ESM amplitude is expected to linearly scale with DOL . As DOL increases, Li distribution (bright region in the amplitude map) is observed from the boundary of NG particles (DOL20) to all areas on the electrode (DOL100).…”

“…18,36,37 Therefore, the average ESM amplitude is expected to linearly scale with DOL. 38 As DOL increases, Li distribution (bright region in the amplitude map) is observed from the boundary of NG particles (DOL20) to all areas on the electrode (DOL100). However, the amplitude increased in a nonlinear fashion in Figure 1c.…”

Spatially Uniform Lithiation Enabled by Single-Walled Carbon Nanotubes

“…Furthermore, conductive-AFM (C-AFM) is widely used to image the distribution of electron-conduction paths in the electrode. , Park et al used C-AFM imaging to examine cycled Si-based anodes and discovered that the average current over a microscale region was considerably higher in the anode with SWCNTs than in the anode without SWCNTs . However, it was unclear how the electrochemical property deterioration of each component in the anode was related to the presence or absence of SWCNTs, which affects the structure of the electron-conduction channel after cycling.…”