Described herein are semi-solid polymer electrolytes (SSPEs) based on a polymer backbone incorporating a flame-retardant crosslinker and fluorinated counterions that are useful in the production of high energy rechargeable lithium metal batteries. The described SSPEs are not liquid electrolytes, are not solid state electrolytes (SSEs), and are differentiated from standard state-of-the-art gel polymer electrolytes (GPEs). The described SSPEs are formed from a first solvent, an optional second solvent, a crosslinker, a lithium salt, and an initiator. The unique coordination structure of the described SSPEs yields non-flammable, low-volatility, non-vaporizable, high Coulombic efficiency (CE), stable solid-electrolyte-interphase (SEI)-forming electrochemical devices, such as lithium metal rechargeable batteries, that are easily adaptable to existing mass-production lines.