Imaging of Lithium Ion Release from Individual Cathode Particles Shows Evidence for Diversity of Intraparticle Contacts.

The ability to observe nanoscale transport of lithium ions in battery functional components with high spatial and temporal resolution can offer a powerful source of mechanistic insight. While existing methods can provide dynamics for single nanocrystals of cathode materials, the behavior of macroscopic battery systems cannot be extrapolated from this single-crystal limit. Key elements of these systems' complexity, such as those arising from particle-particle contacts, only emerge after the transition to the mesoscopic length scale. In this work we demonstrate how fluorescence imaging can be used to watch the electrochemically initiated release of lithium ions from mesoscopic lithium cobalt oxide cathode particles and to quantify the quantity of lithium released as a function of time. The particle-to-particle diversity of release kinetics observed with this method is remarkable. Additionally, correlative scanning electron microscopy of the particles highlights links between certain structural features and lithium storage capacity, while also revealing unexpected weaker correlations between structural motifs and the time scales of the dynamics. Importantly, the complexity of these dynamics provides evidence for a wide distribution of intraparticle contact resistances between nanocrystalline domains, a distribution that was previously hypothesized to be present but not yet directly observed. Quantification of the intraparticle lithium diffusion constant shows the robustness of the approach. This ability to quantify lithium ion release behavior from mesoscopic particles therefore offers a powerful path for obtaining chemical insight on this important length scale in between single nanocrystals and macroscopic devices.