  Liquid electrolytes in batteries are typically treated as macroscopically
homogeneous ionic transport media despite having complex chemical composition
and atomistic solvation structures, leaving a knowledge gap of microstructural
characteristics. Here, we reveal a unique micelle-like structure in a localized
high-concentration electrolyte (LHCE), in which the solvent acts as a
surfactant between an insoluble salt in diluent. The miscibility of the solvent
with the diluent and simultaneous solubility of the salt results in a
micelle-like structure with a smeared interface and an increased salt
concentration at the centre of the salt-solvent clusters that extends the salt
solubility. These intermingling miscibility effects have temperature
dependencies, wherein an exemplified LHCE peaks in localized cluster salt
concentration near room temperature and is utilized to form a stable
solid-electrolyte interphase (SEI) on Li-metal anode. These findings serve as a
guide to predicting a stable ternary phase diagram and connecting the
electrolyte microstructure with electrolyte formulation and formation protocols
to form stable SEI for enhanced battery cyclability.
