  Understanding (de)lithiation heterogeneities in battery materials is key to
ensuring optimal electrochemical performance and developing better energy
storage devices. However, this remains challenging due to the complex three
dimensional morphology of microscopic electrode particles, the involvement of
both solid and liquid phase reactants, and range of relevant timescales
(seconds to hours). Here, we overcome this problem and demonstrate the use of
bench-top laser scanning confocal microscopy for simultaneous three-dimensional
operando measurement of lithium ion dynamics in single particles, and the
electrolyte, in batteries. We examine two technologically important cathode
materials that are known to suffer from intercalation heterogeneities: LixCoO2
and LixNi0.8Mn0.1Co0.1O2. The single-particle surface-to-core transport
velocity of Li-phase fronts, and volume changes - as well as their
inter-particle heterogeneity - are captured as a function of C-rate, and
benchmarked to previous ensemble measurements. Additionally, we visualise
heterogeneities in the bulk and at the surface of particles during cycling, and
image the formation of spatially non-uniform concentration gradients within the
liquid electrolyte. Importantly, the conditions under which optical imaging can
be performed inside absorbing and multiply scattering materials such as battery
intercalation compounds are outlined.
