The high flammability of state-of-the-art liquid electrolytes is a major hazard for the safe operation of lithium (Li) ion batteries (LIBs) and Li metal batteries (LMBs). In this invention, flame retardants (FRs) with distinct solvation abilities were employed as the solvent and diluent of highly safe electrolytes. Because of the difference in the solvating abilities of the two selected flame retardants, the unique solvation structure of localized high concentration electrolytes (LHCEs) is created, which facilitates the formation of effective solid electrolyte interphases (SEIs) on Li and graphite (Gr) electrodes. Consequently, the FR-based LHCEs allow long-term cycling of LIBs and LMBs. Since both the solvating solvent and the diluent are FRs, the FR-based LHCEs exhibit superior flame resistivity to the conventional LiPF6-organocarbonate electrolytes and the previous LHCEs.