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Recent years have seen a rapidly escalating demand for battery technologiescapable of storing more energy, charging more quickly and having longer usablelifetimes, driven largely by increased electrification of transport and bygrid-scale energy storage systems. This has led to the development of manypromising new electrode materials for high-rate lithium ion batteries. In orderto rationalise and improve upon material performance, it is crucial tounderstand the fundamental ion-intercalation and degradation mechanismsoccurring during realistic battery operation, on the nano- to meso-scale. Herewe apply a straightforward laboratory-based operando optical scatteringmicroscopy method to study micron-sized rods of the high-rate anode materialNb$_{14}$W$_3$O$_{44}$ during cycling at rates of up to 30C. We directlyvisualise an elongation of the rods, which, by comparison with ensemble X-raydiffraction, allows us to determine the state of charge (SOC) of the individualparticle. A continuous change in scattering intensity with SOC is also seen,enabling observation of a non-equilibrium kinetic phase separation withinindividual particles. Phase field modelling (informed by pulsed-field-gradientnuclear magnetic resonance and electrochemical experiments) is used to verifythe kinetic origin of this separation, which arises from a dependence of theLi-ion diffusion coefficient upon SOC. Finally, we witness how suchintra-particle SOC heterogeneity can lead to particle cracking; we follow thecycling behaviour of the resultant fragments, and show that they may becomeelectrically disconnected from the electrode. These results demonstrate thepower of optical scattering microscopy to track rapid non-equilibriumprocesses, often occurring over less than 1 minute, which would be inaccessiblewith established characterisation techniques.

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