In-Situ Computed Tomography of Particle Microcracking and Electrode Damage in Cycled NMC622/Graphite Pouch Cell Batteries

Abstract

Mechanical degradation of electrode materials is an important potential failure mode in lithium-ion batteries. High-energy-density cathode materials like nickel-rich NMC (LiNixMnyCozO2) undergo high anisotropic volume expansion during cycling that applies significant mechanical stress to the material. Computed tomography (CT) of cells can be used to image cell-level and electrode-level changes that result from long-term cycling, without the need for cell disassembly or destructive sampling. Previous work by our group has used synchrotron CT to show cathode thickness growth and depletion of liquid electrolyte after long-term (> 2 years) cycling of polycrystalline NMC622/graphite cells. These phenomena were attributed to cathode microcracking, but direct evidence of this was not available at the time. In this study, we present in-situ, sub-micron CT of these unmodified pouch cells, providing new insights into the morphological changes occurring at the particle level. These results confirm that extensive microcracking and dramatic morphological changes are occurring in the cathode that were not previously observed. Combined with the cell-level and electrode-level scans presented previously, this study provides a complete, multi-scale picture of cathode microcracking and how its effects propagate throughout the cell.

Understanding Interfaces at the Positive and Negative Electrodes on Sulfide-Based Solid-State Batteries

“…In addition, Figure e displays the ion-milled cross-section SEM micrograph of the composite cathode after cycling, where cracking of the NMC active material and the contact loss with the solid electrolyte are clearly noticeable. It is known that periodic delithiation and lithiation of NMC yield significant volume changes (4–5 vol % for NMC622), buildup of mechanical stresses in the crystal lattice of the cathode material, accompanied by its cracking, contact loss with the SE phase, resulting in performance drop. ,,, …”

“…It is known that periodic delithiation and lithiation of NMC yield significant volume changes (4−5 vol % for NMC622), buildup of mechanical stresses in the crystal lattice of the cathode material, accompanied by its cracking, contact loss with the SE phase, resulting in performance drop. 23,27,28,58 Comparison of the EIS at different cell potentials and after repeated cycling allows us to estimate the contribution of individual rate-determining steps to the overall degradation. 59,60 The evolution of Nyquist plots and corresponding DRT spectra for the full cell during the charge/discharge steps of the first cycle is summarized in Figure 5a−d, while the first charge and discharge curve obtained during the GEIS experiment is shown in Figure S4, and Table S3 lists the possible processes assigned to each component.…”

“…Sulfide-based materials have certain drawbacks related to their poor chemical stability toward ambient atmosphere and narrow electrochemical stability window. ,, For instance, the Li 6 PS 5 Cl (LPSCl) argyrodite phase exhibits thermodynamic stability in the potential range of 1.7–2.3 V vs Li/Li + , resulting in incompatibility between the electrolyte and Li metal anode or high-voltage Li­(Ni,Mn,Co)­O 2 (NMC) cathodes. Although in practice the stability window is generally larger for kinetic reasons or due to the formation of passivating layers, − there are numerous studies highlighting the reactivity at the sulfide/Li − or sulfide/NMC interface. − Additionally, mechanisms of degradation of sulfide-containing SSBs are ascribed to microstructural changes arising from the redox processes during cycling, − especially in the case of large volume changes of the cathode active material upon delithiation, ,, poor percolation within the electron-conductive phase, contact loss between the electrolyte and cathode or anode layers, ,− as well as dendrite propagation. , …”

“…Although in practice the stability window is generally larger for kinetic reasons or due to the formation of passivating layers, 16−18 there are numerous studies highlighting the reactivity at the sulfide/Li 19−21 or sulfide/NMC interface. 22−25 Additionally, mechanisms of degradation of sulfide-containing SSBs are ascribed to microstructural changes arising from the redox processes during cycling, 25−27 large volume changes of the cathode active material upon delithiation, 23,27,28 poor percolation within the electronconductive phase, 26 contact loss between the electrolyte and cathode or anode layers, 23,29−31 as well as dendrite propagation. 21,30 Aging and degradation phenomena in SSBs can be comprehensively studied by electrochemical impedance spectroscopy (EIS), especially in the case of changes in the interfacial kinetics or conductivity mechanism of cell components.…”

Differential voltage analysis for battery manufacturing process control

“…These measurements can still be made on formation equipment, and would only require a small, dedicated number of formation cyclers, to be withheld for this purpose. This partitioning method can also encourage the adoption of more advanced characterization methods such as computed tomography (CT) (Wu et al, 2018;Bond et al, 2022;Gauthier et al, 2022), ultrasound imaging (Bommier et al, 2020;Deng et al, 2020), and strain measurements (Mohtat et al, 2022). Measuring all of these parameters on the same cell could enable a more complete perspective on the cell electrochemical and mechanical state, enabling deeper insights.…”