Copper-iron ternary metal fluorides from multi-metallic template fluorination and their first use as cathode in solid state Li-batteries

International audienceConversion cathodes such as metal fluorides are particularly interesting for their theoretical capacities up to 711 mAh/g when used in Li-ion batteries. However, they generally suffer from high hysteresis and/or poor cyclability. To overcome these problems, ternary metal fluorides are being explored to improve electrochemical properties compared to the usual binary fluorides: a polymer electrolyte membrane is also used to avoid undesirable cathode-electrolyte reactions and to assess the compatibility of such cathode materials with All Solid State Batteries (ASSB). Among the multi-metallic models, copper hexacyanoferrate Cu3[Fe(CN)6]2.nH2O from the Prussian blue analogue family (PBA), is fluorinated under pure fluorine gas at selected temperatures according to preliminary thermogravimetric mass spectrometry analyses. Using X-ray absorption spectroscopy and diffraction, an inverse perovskite structure [Cu(H2O)4]3·(FeF6)2 is identified at a fluorination temperature of 140 ​°C and an intimate mixture of CuF2 and FeF3 is formed at 350 ​°C. The cyclic voltammogram of the latter compared to that of a ball-milled sample of similar composition CuF2/FeF3 highlights the advantages of MMTF multi-metallic template fluorination for electrochemical properties, interpreted by the high level of homogeneous dispersion of the redox centers obtained with this method compared to the mechanical milling reference evaluated by SEM. The fluorides are also found to be compatible with the commercial solid polymer electrolyte membrane PEO-LiTFSI (Poly Ethylene Oxide impregnated with Lithium bis(trifluoromethanesulfonyl)imide), underlining the suitability of the MMTF-prepared cathodes for use in the ASSB