Local State-of-Charge Inhomogeneities in Charged and Cycled High Capacity Lithium-Manganese-Rich Composite Cathodes

Materials for electrochemical energy storage often exhibit significant chemical and structural inhomogeneity at the particle and even sub-particle level. Understanding how these mesoscale variations in the state of charge and local phase transformations impact electrochemical performance is critical to developing improved electrode materials. Raman microscopy is an excellent technique for studying inhomogeneity and interfacial phenomena in batteries, because it can measure subtle changes in chemical composition and structure at sub-micron resolution. By monitoring changes in the spectroscopic signatures, we can infer information about ion and electron transport across interfaces and through particles. In this study we use ex situ Raman mapping to study layered-layered lithium-rich manganese-rich cathode material Li_1.2Mn_0.525Ni_0.175Co_0.1O₂ (LMR-NMC) as a function of the average charge. We show that variations in the microstructure and local state of charge occur at the level of individual particles within electrodes prepared from a commercial sample. Micro-Raman mapping provides both high-resolution and large field of view, but the full potential of this technique to analyze complex electrochemical systems is rarely realized, in part due to the challenge of processing the inherently large data sets. Here we take advantage of cluster analysis and other statistical methods to directly visualize interfaces and charge inhomogeneity in cathode particles.

Acknowledgment This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, is sponsored by the Vehicle Technologies Program for the Office of Energy Efficiency and Renewable Energy.

References:

1. Nanda, J.; Remillard, J.; O'Neill, A.; Bernardi, D.; Ro, T.; Nietering, K. E.; Go, J. Y.; Miller, T. J., Local State-of-Charge Mapping of Lithium-Ion Battery Electrodes. Adv. Funct. Mater. 2011, 21 (17), 3282-3290.
2. Ruther, R. E., et al., Raman Mapping Study of Inhomogeneity in Lithium-Manganese-Rich Cathode Particles, to be submitted 2014