Operando Optical Tracking of Ion Dynamics and Degradation in Battery Electrodes
Key to advancing lithium-ion battery technology is our ability to follow and understand the dynamic processes occurring in functioning materials under realistic conditions, in real time, and on the nano- to meso-scale. Currently, operando imaging of lithium-ion dynamics requires sophisticated synchrotron X-ray or electron microscopy techniques, which are not suited for high-throughput material screening and often cannot access realistic cycling rates. This limits rapid and rational materials improvements, particularly when non-equilibrium kinetic effects play an important role. This work introduces and establishes a laboratory-based optical scattering microscopy methodology to resolve lithium-ion dynamics in individual active particles, under operating conditions.
First we apply this technique to follow cycling of the archetypical cathode material LiCoO₂ (LCO). We visualise the insulator-metal, solid solution and lithium ordering phase transitions directly, determine effective rates of lithium insertion and removal at the single-particle level, and identify different mechanisms on charge vs. discharge. We also capture the dynamic formation of domain boundaries between different crystal orientations associated with the monoclinic lattice distortion at Li₀.₅CoO₂.
Secondly, we employ optical scattering microscopy to examine state-of-the-art Ni-rich layered oxide cathodes. Here, a continuous change in scattering intensity with state-of-charge (SOC) enables direct observation of kinetically-induced lithium-heterogeneities within single-crystal LiNixMnyCo(1−x−y)O₂ (NMC). Upon delithiation, a rapid increase in lithium diffusivity at the beginning of charge results in particles with lithium-poor peripheries and lithium-rich cores. Finite-element modelling confirms that a SOC-dependent ion diffusivity is necessary to reproduce these phenomena. The slow diffusion at near-full lithiation states also seen to produce a more subtle lithium-heterogeneity at the end of discharge, with a lithium-rich surface preventing complete lithiation. These results demonstrate the kinetic origin of significant first-cycle capacity losses in Ni-rich cathodes.
Finally, we study rod-like particles of Nb₁₄W₃O₄₄ (NWO) anodes during high-rate cycling. We resolve elongation of the particles which, by comparison with ensemble X-ray diffraction, allows us to determine changes in the SOC of individual particles. Non-equilibrium phase fronts are observed propagating through the particles, both during early lithiation and high-rate delithiation. Phase field modelling (informed by nuclear magnetic resonance and electrochemical experiments) verifies the kinetic origin of this apparent phase separation, arising from the SOC dependence of the lithium-ion diffusivity. The lithium-heterogeneity leads to particle fracture at high rates of delithiation, particularly in longer particles, with some of the resulting fragments becoming electrically disconnected on subsequent cycling and contributing to capacity degradation.
These results demonstrate the power of optical scattering microscopy to track rapid and non-equilibrium processes in battery electrodes that would be inaccessible with established characterisation techniques.