Watching lithium move in battery materials

The type and amount of active material in a lithium ion battery determines the theoretical capacity and voltage provided by the battery. Since the commercialization of lithium ion batteries, the active materials on the positive side of rechargeable lithium ion batteries have been layered transition metal oxides, such as lithium cobalt oxide LiCoO₂. Batteries in electric vehicles contain higher performance and lower cost variations of LiCoO₂, such as lithium nickel manganese cobalt oxide Li(Ni,Mn,Co)O₂ or mixtures of lithium manganese LiMn₂O₄ and lithium nickel manganese cobalt oxide Li(Ni,Co,Al)O₂.

During charge and discharge of the battery, the transition metals – Ni, Co, and Mn – in these active materials change their oxidation states. When their oxidiation states change, the X-ray absorption spectra of these metals change. Spatially-resolved X-ray absorption spectroscopy (XAS) can therefore be used to quantify where lithium is in the battery.

However, typical spatially resolved XAS measurements are too slow to resolve the dynamic processes going on during battery charge and discharge. Furthermore, prolonged X-ray exposure can cause thermal degradation of the materials being studied.

Researchers from ETH Zurich and PSI have collaborated to demonstrate how measurement time and X-ray exposure can be reduced so that XAS can be used to track lithiation dynamics during battery operation.

In the Nature subjournal Scientific Reports, the researchers explain the clever trick that makes this possible. They apply a highly discretized energy resolution – in other words, fewer measurement points – coupled with advanced post-processing to enable quick yet reliable identification of the oxidation state of the transition metals during battery operation. They apply this technique to monitor the lithium dynamics in the commercial NCA particles. They watch the material’s behavior during discharge, charge, and how it degrades during overcharge, and affect that can occur locally due to structural inhomogeneities or more globally due to poor battery management.

The work was carried out the microXAS beamline of the Swiss Light Source by Lea Nowack, PhD student in the group of ETH Professor Vanessa Wood. microXAS beamline group leader, Dr. Daniel Grolimund, an author on the study, said This works shows how spatially resolved x-ray absorption spectroscopy can be a powerful tool for energy related research.

In collaboration with Dr. Federica Marone from the TOMCAT beamline at PSI, the authors implemented a full-field microscopy setup, providing sub-particle resolution over a large area of battery electrode, enabling the lithium dynamics to be tracked in many transition metal oxide particles simultaneously. Professor Wood explains This is important in order to study the effect of inhomogeneities in the lithiation and delithiation process that impact battery performance.