Operando monitoring of single-particle kinetic state-of-charge heterogeneities and cracking in high-rate Li-ion anodes

Recent years have seen a rapidly escalating demand for battery technologies capable of storing more energy, charging more quickly and having longer usable lifetimes, driven largely by increased electrification of transport and by grid-scale energy storage systems. This has led to the development of many promising new electrode materials for high-rate lithium ion batteries. In order to rationalise and improve upon material performance, it is crucial to understand the fundamental ion-intercalation and degradation mechanisms occurring during realistic battery operation, on the nano- to meso-scale. Here we apply a straightforward laboratory-based operando optical scattering microscopy method to study micron-sized rods of the high-rate anode material Nb$_{14}$W$_3$O$_{44}$ during cycling at rates of up to 30C. We directly visualise an elongation of the rods, which, by comparison with ensemble X-ray diffraction, allows us to determine the state of charge (SOC) of the individual particle. A continuous change in scattering intensity with SOC is also seen, enabling observation of a non-equilibrium kinetic phase separation within individual particles. Phase field modelling (informed by pulsed-field-gradient nuclear magnetic resonance and electrochemical experiments) is used to verify the kinetic origin of this separation, which arises from a dependence of the Li-ion diffusion coefficient upon SOC. Finally, we witness how such intra-particle SOC heterogeneity can lead to particle cracking; we follow the cycling behaviour of the resultant fragments, and show that they may become electrically disconnected from the electrode. These results demonstrate the power of optical scattering microscopy to track rapid non-equilibrium processes, often occurring over less than 1 minute, which would be inaccessible with established characterisation techniques.