Innovations in Post-Mortem Battery Material Characterization for Diagnosing Failure Mechanisms

Abstract

An increasing demand for clean and renewable energy provides motivation for additional innovations to be sought in energy storage. Lithium-ion batteries (LIBs) have seen many advancements in terms of their cathode chemistries, including electrode coatings that are used to increase durability and capacity of cathode materials. These coatings are effective, but create additional interfaces in the battery system, that adds to complexity in characterization. To address many key issues of LIB failures, further insights are necessary that provide details on currently unknown and partially known intermediate states, side reactions, and/or mechanisms of degradation at electrode interfaces. In this work, a method is developed to enable a detailed surface analysis and additional characterization of coated cathode particles harvested from post-mortem LIBs. Procedures are outlined for the safe handling and preparation of post-mortem materials for high-quality single particle electron microscopy analyses, paired with ultramicrotome techniques for high-throughput cross-sectional imaging with elemental and structural analyses of the coatings themselves.Commonly seen in post-mortem results, the materials are kept intact as whole electrodes or large collections of particles while still held within binders and other cathode slurry components from cell fabrication. These samples are often briefly rinsed with electrolyte solvents [e.g., most commonly dimethyl carbonate (DMC)] to rinse away residual lithium salts.1 While important insights have been gained from such studies, details of surface defects are difficult to discern amidst the debris of additives and binders. To address this, we developed a non-destructive procedure for the separation of cathodes particles from their supports, followed by washing to remove binders, additives, and residual electrolyte with relatively safe and accessible solvents. This method enables high-quality single particle imaging by scanning and transmission electron microscopy (SEM and TEM) techniques. Nano-scale features on the particle surfaces can be observed with high-resolution on the post-mortem cathode particles. With this procedure, insights can be gained about the retention of coatings after battery cycling, in addition to observing visible surface defects that result from charge cycling.Typical cross-sectional analyses of interfacial systems involve expensive, time-consuming methods like focused ion beam (FIB) milling that require extensive training, and are low-throughput when FIB lift-out samples are prepared for TEM. An alternative high-throughput technique for the cross-sectional analysis of coated cathode particles by ultramicrotome was previously developed in our group.2 This technique involves embedding coated cathode particles in epoxy, from which ultra-thin slices (<100 nm) that can contain many [approx. 50-200] particles at a time are cut with a diamond knife. The resultant sheets of epoxy are deposited onto TEM grids so the cross-sections can be imaged by TEM. These sections also enable the particle coatings to be characterized separately from the bulk cathode by electron dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). This technique was demonstrated on as-synthesized (pre-electrochemistry) samples but was adapted here for the prepared post-mortem samples to assess degradation of the particles and their coatings.Combining the newly developed techniques for obtaining high-quality images of single whole-particles and particle cross-sections this work provides a safe, relatively inexpensive, and high-throughput methodology for the post-mortem characterization of LIB materials. This data can be correlated to as-synthesized materials and electrochemical cycling data to create a detailed profile of cathode aging and degradation as it relates to LIB failures. We are expanding this method to a variety of standard and coated cathode systems to provide detailed information needed for the next steps towards developing new innovations in electrode designs aimed at achieving more durable LIBs.