Grain Size Control Toward Room‐Temperature Operable Solid Polymer Electrolytes

The practical application of solid polymer electrolytes (SPE) is limited due to notorious high crystallinity and low ionic conductivity. Existing research concentrated on reducing crystallinity and increasing Li salt concentration have made certain process. However, the segmentation and isolation effects of large and numerous grains on amorphous region have always been overlooked and the effect of grain size remains largely unexplored. Herein, take polyethylene oxide (PEO) as an example, “grain size refinement” strategy is adopted to improve the related room‐temperature ionic conductivity by simply placing PEO based SPE on Li sheets coated with ester monomers and conducting in‐situ polymerization. During these processes, in addition to reducing the interaction force between polymer chains and decreasing the driving force for crystallization, ester monomers are conducive to form interface with polymer clusters, which serves as additional nucleation sites and promotes the formation of refined grains. Then instantaneous high‐temperature provided by muffle furnace triggers rapid solidification of monomers, leading to the locking of refined grain structure and the formation of more interconnected amorphous regions. Time‐of‐flight secondary ion mass spectrometry and polarization microscope confirm these processes, while small‐angle X‐ray scattering results indicate that the grain size reduces to one‐third of its original size. Then the room‐temperature conductivity increased by at least two orders of magnitude for PEO‐based SPE.