Characterization of FeF3 cathodes. Figure shows that the range of FeF3 particle sizes is ~100-400 nm. Raman spectroscopy (Figure ) indicates the presence of just two species: FeF3 (170, 303, and 437 cm -1 ) and carbon (1350 and 1600 cm -1 ; D and G bands, 6 respectively). The lack of additional peaks suggests that the FeF3 is phase pure. XRD (Figure ) supports this claim, with the exception of a minor FeF2 impurity. Cross-sectional SEM (Figure ) of a representative FeF3 cathode indicates that that the average layer thickness of FeF3 (mixed with C) is 20 ± 5 μm (Figure ; N = 200 thickness measurements of randomly selected cathode regions). Additional PVDF-HFP gel characterization was accomplished using electrochemical impedance spectroscopy (EIS) and Raman spectroscopy. EIS, in particular, was used to analyze the impact of gel-1 and gel-2 on ionic conductivity (σ), activation energy (Ea), and the charge transfer resistance (RCT) of assembled Li-FeF3 full cells.