Visualizing Unstable Lithium Electrodeposition within Polymer-ceramic Composite Electrolytes

Solid polymer and perovskite-type ceramic electrolytes have both shown promise in advancing solid-state lithium metal batteries. Despite their favorable interfacial stability against lithium metal, polymer electrolytes face issues with dendrite formation due to their low ionic conductivity and poor mechanical strength. Highly conductive and mechanically robust ceramics, on the other hand, can be unstable against pure lithium metal. To overcome the disadvantages of each material, we incorporate Li_0.33La_0.56TiO₃ (LLTO) nanoparticles into a block copolymer, polystyrene-b-polyethylene oxide (SEO), matrix to develop a polymer-composite electrolyte (SEO-LLTO). We use synchrotron hard x-ray microtomography to noninvasively study the cell failure and interfacial stability of SEO-LLTO in a lithium-lithium symmetric cell. We observe no clear trend in ionic conductivity or current fraction with increased nanoparticle loading. While the SEO-LLTO electrolyte demonstrates a higher storage modulus than SEO, it is unable to sustain large current densities due to dendrite formation. Three-dimensional tomograms reveal the formation of large globular lithium structures preferentially around regions of high LLTO concentrations in contact with lithium metal. However, upon repeated cycling at lower current densities, the formation of similar globular dendritic structures is not observed. Moreover, by encasing the SEO-LLTO between layers of SEO, we prevent direct contact of LLTO with lithium metal, allowing for the passage of seven-fold higher current densities without signatures of lithium deposition around LLTO. We posit that in addition to direct contact of LLTO and lithium metal, there exists a relatively low critical current density that induces interfacial instabilities in these polymer-composite electrolytes.