Fluorination of (Mg,Cu,Al,Fe)-based LDHs template for conversion materials usable in all-solid state lithium metal batteries

Abstract

Iron and copper fluorides are of interest as conversion cathode materials in lithium batteries but they suffer from high hysteresis and low cyclability, respectively. To overcome these limitations and take advantage of the electrochemical properties of each of the fluorides, fluoride mixtures that combine the two redox centers Fe³⁺ and Cu²⁺ are synthesized using Layered Double Hydroxides (LDH) as a 2D multi-metallic template. Hydrotalcite-type phases substituted with Cu²⁺ and Fe³⁺ ions are prepared by coprecipitation then fluorinated with fluorine gas in static mode at temperatures chosen according to their thermal evolution monitored by mass spectrometry. After each fluorination treatment, the materials are characterized by X-ray diffraction and most LDH materials are found stable up to 200 °C, treatment above leads to the formation of fluorides of each of the cations. The initial dispersion of the cations in the LDH sheets nevertheless allows, after fluorination, a composite of fluorides making them totally accessible to the phenomenon of electrochemical conversion shown here in a metallic lithium battery assembly with a Solid Polymer Electrolyte (SPE). In such All-Solid-State Battery (ASSB) configuration often limiting in terms of columbic efficiency, 76% the first discharge capacity is recovered in charge for an optimized composition (up to 560 mAh.g⁻¹). This good reversibility positions these LDH templates very well for future investigations in the field of electrochemical storage.