  Discontinuous solid-solid phase transformations play a pivotal role in
determining properties of rechargeable battery electrodes. By leveraging
operando Bragg Coherent Diffractive Imaging (BCDI), we investigate the
discontinuous phase transformation in LixNi0.5Mn1.5O4 within a fully
operational battery. Throughout Li-intercalation, we directly observe the
nucleation and growth of the Li-rich phase within the initially charged Li-poor
phase in a 500 nm particle. Supported by the microelasticity model, the
operando imaging unveils an evolution from a curved coherent to planar
semi-coherent interface driven by dislocation dynamics. We hypothesize these
dislocations exhibit a glissile motion that facilitates interface migration
without diffusion of host ions, leaving the particle defect-free
post-transformation. Our data indicates negligible kinetic limitations
impacting the transformation kinetics, even at discharge rates as fast as C/2.
This study underscores BCDI's capability to provide operando insights into
nanoscale phase transformations, offering valuable guidance for electrochemical
materials design and optimization.
