  Key to advancing lithium-ion battery technology, and in particular fast
charging capabilities, is our ability to follow and understand the dynamic
processes occurring in operating 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 do not lend themselves to high-throughput material
screening. This limits rapid and rational materials improvements. Here we
introduce a simple lab-based, optical interferometric scattering microscope to
resolve nanoscopic lithium-ion dynamics in battery materials and apply it to
follow the repeated cycling of the archetypical cathode material
Li$_\textit{x}$CoO$_2$. The method allows us to visualise directly the
insulator-metal, solid solution and lithium ordering phase transitions in this
material. We determine rates of lithium insertion and removal at the
single-particle level and identify different mechanisms that occur on charge
vs. discharge. Finally, we capture the dynamic formation of domain boundaries
between different crystal orientations associated with the monoclinic lattice
distortion at around Li$_{0.5}$CoO$_2$. The high throughput nature of our
methodology allows many particles to be sampled across the entire electrode
and, moving forward, will enable exploration of the role of dislocations,
morphologies and cycling rate on battery degradation. The generality of our
imaging concept means that it can be applied to study any battery electrode,
and more broadly, systems where the transport of ions is associated with
electronic or structural changes, including nanoionic films, ionic conducting
polymers, photocatalytic materials and memristors.
