(Invited, Digital Presentation) Phase Interactions and Degradation in Battery Composite Electrodes

The composite electrodes of lithium-ion and sodium-ion batteries must balance complex interactions between the many physical phenomena that underpin battery operation. Active materials host electroactive species. Porous regions support the movement of ions, and secondary phases containing binders and conductive additives provide a supporting electron conducting matrix. Cycling and aggressive operating conditions drive changes in these phases and the balance between them. The alteration of these electrode phases manifests in declining performance and ultimately battery failure. Here, we present an overview of research on degradation processes observed in a diverse set of battery materials using X-ray imaging methods—nanotomography, microtomography, and X-ray absorption near edge structure (XANES) imaging—complemented by electrochemical experiments and other materials characterization methods. Metallic anodes offer a high-capacity alternative to carbon-based anodes for lithium-ion and sodium-ion batteries. Among these metallic anodes tin-based active materials present high-capacity and electrochemical activity in both lithium and sodium chemistries. However, high capacity comes at the price of excessive volume expansion and attendant performance degradation. Studies of Cu6Sn5 alloy anodes in lithium-ion batteries performed using X-ray tomography and nanoscale 2D XANES reveal material volume expansion, fracture, and dissolution as key failure mechanisms within the active material. However, the expansion and contraction of the metallic active material during cycling also leads to significant changes in the supporting carbon-binder domains of the electrodes. Such changes are also observed in the cycling of tin-based composite anodes in both lithium-ion and sodium-ion applications. Active material pulverization of tin due to lithiation and sodiation processes is evident in phase size distributions observed with X-ray microtomography. These changes are accompanied by alteration of the supporting carbon-binder regions. Low cobalt layered oxide cathode materials for lithium-ion batteries present an opportunity for maintaining desirable theoretical capacities while removing cobalt, a metal that presents significant humanitarian issues within its supply chain. However, removal of cobalt from these layered oxide systems removes its stabilizing influence on the electrode structure, especially when cycling at voltages above 4.0 V vs. Li/Li+. Degradation of the low cobalt cathode material Li(Ni0.8Mn0.1Co0.1)O2 has been assessed under different cut-off voltage and operating temperatures using a suite of electrochemical and materials characterization techniques. Higher cycling temperature was found to accelerate degradation mechanisms within the active material, specifically rock-salt phase formation and active material dissolution. Elevated temperature was found to cause significant changes in the supporting carbon-binder domains as well. These changes in both the active material and supporting phases manifest in degradation of electrochemical performance with respect to lost capacity and altered electrode impedances. Taken together, the above studies provide a picture of battery degradation that is driven by anticipated changes in the active material and more nuanced variations that occur in the supporting phases. These observations motivate holistic approaches for the analysis and design of battery electrodes that fit their status as heterogeneous functional materials.