With the increasing demand for batteries with high energy density, lithium metal anode-based batteries are suitable candidates. Although lithium metal is an attractive material with excellent theoretical specific capacity (3860 mAh/g) and very redox potential (-3.04 V vs. strandard hydrogen electrode), it causes electrolyte leakage, combustion, and explosion problems due to dendrite growth. In the case of conventional liquid electrolytes, dendrites grow more freely and flammable organic solvents are used, making them unsuitable for use with lithium metal anodes. On the contrary, solid polymer electrolyte-based batteries have the advantage of being able to achieve higher energy density as well as better stability to lithium metal. However, the organic component of the solid polymer electrolyte is still flammable, and the polymer/ceramic composite material is also generally flammable, so the safety issue cannot be completely avoided. Here, we show that the addition of a flame retardant effectively reduces the flammability of solid polymer electrolytes. The solid electrolyte consisted of a combination of PEO and flame retardant DMMP, and the composed LFP/solid polymer electrolyte/Li cell exhibited improved cycle stability and ionic conductivity.