Yushin and coworkers previously showed that using an elastic separator (a polyethylene oxide (PEO)-based solid polymer electrolyte (SPE)) in Li-FeF2 batteries alleviated several of the issues above. In particular, separator flexibility during volume expansion stabilized the cathode electrolyte interface, which in turn decreased voltage hysteresis and the maladies of mechanical fatigue (e.g., ion leaching and shortened cycle life). Subsequent reports of solid state iron fluoride batteries improved on this design (achieving ~150-200 mAh g -1 after 1200 cycles at 1 C) by adding various fillers to PEO-based SPEs (e.g., graphitic carbon nitride and a Ce metal organic framework). Despite these gains, previous solid state iron fluoride batteries suffered from two major limitations: (1) low ionic conductivities (maximum of 0.03 mS cm -1 at 30°C and 0.31 mS cm -1 at 60°C) and (2) low mass loadings (maximum of 0.6-0.8 mg cm -2 ). These properties prevented practical room temperature operation.