On the Development of Intercalation-Based Cathode Materials for All-Solid-State Fluoride Ion Batteries

Abstract

Recently reversible batteries based on fluoride ions as a charge carriers have attracted some attentions as an alternative electrochemical energy storage system to conventional lithium ion batteries (LIBs). Fluoride is the most stable anion with a high mobility and therefore, fluoride ion batteries (FIBs) can theoretically provide a wide electrochemical potential window. Moreover, FIBs are capable of being built in an all solid-state modification. Previously, electrochemical fluoride ion cells based on conversion-based electrode materials have been built. However, the state of the art of the FIBs suffer from poor cycling performance in lack of well-developed cell components including the electrode materials. In the current study, intercalation-based cathode materials have been investigated as an alternative approach to make electrode materials for FIBs. In this respect, various compounds with mainly Ruddlesden-Popper-type structure including LaSrMO4 (M = Mn, Co, Fe) and La2MO4+d (M = Co, Ni) as well as Schafarzikite-type compounds of Fe0.5M0.5Sb2O4 (M = Mg, Co) have been subjected to electrochemical measurements including galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy and the structural changes upon electrochemical fluorination/de-fluorination were analyzed by X-ray Diffraction (XRD). LaSrMnO4 has been fluorinated/de-fluorinated via electrochemical method confirming successful intercalation/de-intercalation of the fluoride ions, but showed problems for long-term operation. In contrast, La2NiO4+d showed to be the most promising intercalation-based cathode material (for FIB) in terms of cycling stability (>220 cycles and 60 cycles for cutoff capacities of 30 and 50 mAh/g, respectivly) with a nearly 100% Coulombic efficiency (average Coulombic efficiency of 97.68% and 95.44% for cutoff capacities of 30 and 50 mAh/g, respectively). This is the highest cycle life that has been reported so far for a FIB. One of the major challenges of the proposed FIB systems was found in avoiding oxidation of the conductive carbon which has been mixed with the electrodes to improve the electronic conductivity. This decomposition of the carbon matrix results in a remarkable increase in the impedance of the cell and can significantly impair the cycle life and discharge capacity. However, the critical charging conditions which could be determined by cyclic voltammetry and electrochemical impedance spectroscopy have a major impact on preserving the conductivity of the cell. In addition, the effect of volume change in the conversion-based anode materials has been studied showing that the overpotentials arising from the volume change can significantly influence the cycling behavior of the battery system (due to absence of well-developed intercalation-based anode materials for FIBs, conversion-based counter electrodes have been used as anode materials).