Low flammability and nonflammable localized superconcentrated electrolytes (LSEs) for stable operation of electrochemical devices, such as rechargeable batteries, supercapacitors, and sensors, are disclosed. Electrochemical devices, such as rechargeable batteries, supercapacitors, and sensors, including the low flammability and nonflammable LSEs are also disclosed. The low flammability and nonflammable LSEs include an active salt, a solvent comprising a flame retardant compound, wherein the active salt is soluble in the solvent, and a diluent in which the active salt is insoluble or poorly soluble. In certain embodiments, such as when the solvent and diluent are immiscible, the LSE further includes a bridge solvent.