Comprehensive Characterization Tool for Modeling and Understanding the Behaviors of Li-ion Cells after Heavy Cycles

Title: The Complex and Spatially Heterogeneous Nature of Degradation in Heavily Cycled Li-ion Cells
Authors: Bond et al.,
Journal: Journal of The Electrochemical Society Link

Synchrotron X-ray diffraction (XRD) has become a widely adopted structural characterization technique in the battery research community. However, studies utilizing this technique are often conducted in overly simplified settings with model cells (e.g., single-layer cells subjected to hundreds of cycles), raising questions about the applicability of their conclusions to commercial battery cells.

In this study, the authors characterized commercially manufactured polycrystalline NMC622 and graphite cells (prismatic wound pouch-type) cycled over more than two years using a combination of non-destructive electrochemical and imaging methods. Spatially resolved, in-situ, and operando synchrotron XRD was employed to investigate spatially heterogeneous changes across three differently cycled cells: heavily cycled, lightly cycled, and only with formation cycling.

Static and time-resolved data were obtained under near-equilibrium conditions (CC-CV), non-equilibrium conditions (CC only), and after open-circuit relaxation. The diffraction peaks corresponding to NMC (113) were analyzed using a three-component fitting method to quantify the temporal and spatial lithiation state distribution (i.e., active, inactive, and semi-active regions). The heavily cycled cells exhibited complex, multi-modal, and heterogeneous kinetics during charge, discharge, and after open-circuit relaxation, in contrast to the more uniform behavior of the lightly cycled and formation-only cells. The authors also demonstrated that these behaviors correlated with microcracking and surface reconstruction in the heavily cycled cells could be mitigated by employing single-crystal electrodes, which suppress microcracking, reduce pore growth, and significantly decrease the fraction of inactive material.

This study presents a robust spatial and temporal methodology to characterize the complex behavior of degraded cells through a multi-faceted approach, accounting for the extent of cell degradation, equilibrium states under varied test protocols, and rigorous quantitative analysis. By demonstrating the depth and comprehensiveness of such characterization, the authors highlight the potential for advancing the knowledge of cycle-intensive systems for commercial application.