Towards solid-state batteries operating at ambient temperature: (composite) polymer electrolytes in a cross-linked poly(ethylene oxide) matrix

Among the oxide ceramic super lithium ion conductors, garnet-type Li7La3Zr2O12 (LLZO) has recently attracted much attention because of its relatively high ionic conductivity at room temperature (>10-4 S cm–1), negligible electronic conductivity and absence of harmful decomposition products upon contact with atmospheric moisture. Recent efforts have been dedicated to the formulation of composite hybrid polymer electrolytes (CPEs), where the ceramic material is embedded in a polymeric matrix. CPEs are stiff while preserving flexibility, are easily processed, and can be conceived to attain improved ionic conductivity and interfacial contact with the electrodes. Here, a polymer based matrix containing poly(ethylene oxide) (PEO), lithium bis (trifluoromethylsulphonyl)imide (LiTFSI), tetra(ethylene glycol dimethyl ether) (G4) and a photoinitiator was added with LLZO particles, thoroughly mixed, formed into a film and crosslinked under UV radiation to obtain a composite hybrid electrolyte. This easy procedure allows obtaining self-standing CPEs with desirable properties of flexibility, shape retention upon thermal stress, improved interfacial contact with the electrodes and ionic conductivity suitable for practical application. Lab-scale lithium metal cells assembled with the CPEs and LiFePO4 cathodes demonstrated specific capacities up to 125 mAh g-1 at 1C rate and could work for hundreds of cycles at ambient temperature.