  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.
