Room-Temperature Pseudo-Solid-State Iron Fluoride Conversion Battery with High Ionic Conductivity

Abstract

Li-metal batteries (LMBs) employing conversion cathode materials (e.g., FeF3) are a promising way to prepare inexpensive, environmentally friendly batteries with high energy density. Pseudo-solid-state ionogel separators harness the energy density and safety advantages of solid-state LMBs, while alleviating key drawbacks (e.g., poor ionic conductivity and high interfacial resistance). In this work, a pseudo-solid-state conversion battery (Li-FeF3) is presented that achieves stable, high rate (1.0 mA cm–2) cycling at room temperature. The batteries described herein contain gel-infiltrated FeF3 cathodes prepared by exchanging the ionic liquid in a polymer ionogel with a localized high-concentration electrolyte (LHCE). The LHCE gel merges the benefits of a flexible separator (e.g., adaptation to conversion-related volume changes) with the excellent chemical stability and high ionic conductivity (∼2 mS cm–1 at 25 °C) of an LHCE. The latter property is in contrast to previous solid-state iron fluoride batteries, where poor ionic conductivities necessitated elevated temperatures to realize practical power levels. The stable, room-temperature Li-FeF3 cycling performance obtained with the LHCE gel at high current densities paves the way for exploring a range of architectures including flexible, three-dimensional, and custom shape batteries.

Composite Ionogel Electrodes for Polymeric Solid-State Li-Ion Batteries

“…This was confirmed by taking energy dispersive spectra (EDS) and overlaying each on the corresponding scanning electron microscope (SEM) image of a composite LTO electrode with an IG SSE (Figure 2). The lack of an N and S signal and the presence of Cl correspond to the removal of LiFSI and PYR14TFSI and successful substitution with LiClO 4 , as has been previously demonstrated [8]. The Al signal is likely contamination from the cross-sectioning process.…”

“…A 1 M lithium perchlorate (LiClO 4 ) in propylene carbonate (PC) solution was prepared and used for a solvent exchange process. Further details of this procedure are found in previous publications [8,9,17], which found improved capacity retention during room temperature cycling after performing a solvent exchange. Briefly, in this process, the disks with the composite electrodes and ionogel layer were immersed in small volumes of stock solvent exchange solution.…”

“…IGs consist of an ionic liquid electrolyte (ILE) contained within a solid matrix, where the coexistence of solid and liquid phases combines the mechanical properties of a solid (e.g., structural support and elasticity) with the physical properties of a liquid electrolyte. ILEs are attractive because they have high ionic conductivities, are stable over a broad range of temperatures and chemical environments, are non-flammable, and typically have high voltage windows [8][9][10][11][12]. Furthermore, once solidified, we recently demonstrated that the IG functions as a separator without additional separator membranes [9].…”