Mapping and Metastability of Heterogeneity in LiMn 2 O 4 Battery Electrodes with High Energy Density

Abstract

The hierarchical nature of the cathode in Li-ion batteries can result in phenomena determining electrochemical performance occurring at different length-scales, from individual atoms to the whole electrode. In architectures designed for high density of charge storage, transport limitations can emerge at the microscale that compromise effective utilization and accelerate degradation. These limitations manifest as chemical heterogeneity within the electrode. Micro-focused diffraction mapping using a laboratory X-ray source provides maps with sub-mm resolution of a whole electrode composed of commercial LiMn 2 O 4 . Evidence of disparate local utilization both laterally and along the depth of the electrode was obtained, especially at high rates and after multiple charge-discharge cycles. As a model to study the persistence of heterogeneity due to transport limitations, lateral gradients to lithium transport were introduced by cycling against Li anodes of small diameter. The resulting maps revealed the effects of anisotropic electric migration and diffusion to be separated, confirming that diffusion is the primary limitation for long-range kinetics. Tracking of subsequent relaxation revealed that the heterogeneity was metastable despite a strong thermodynamic driving force, maintained by poor lithium transport through the solid electrode matrix. This study enriches our understanding of transport across thick electrode architectures and the imposition of unique frustrated states, away from equilibrium.