Solid Fluoride Electrolytes and Their Composite with Carbon: Issues and Challenges for Rechargeable Solid State Fluoride-Ion Batteries

Solid-state batteries relying on fluoride-ion shuttle are still at their early stage of development. Assessing the fluoride solid electrolyte’s electrochemical stability and its conduction properties in a mixture with carbon, as well as the possible interaction of fluoride-ion with carbon both during the electrode preparation and upon electrochemical reactions, are mandatory to enable future practical applications. Here, we discuss these points using LaF₃ doped with BaF₂ (La_0.95Ba_0.05F_2.95, LBF) as a benchmark solid fluoride electrolyte. We establish that lithium may be used as a pseudoreference electrode to assess the electrochemical stability window of LBF and support the experiment with thermodynamic calculations. We demonstrate the chemical compatibility of LBF with carbon upon ball-milling and investigate the electrical conductivity of the formed LBF-C composite. We use a LBF|LBF-C|LBF cell (in this configuration, LBF serves as electronically blocking electrode) to assess the ionic conductivity of the LBF-C composite. The results confirm that both electronic and ionic percolations are ensured within the LBF-C composite despite a noticeable decrease of the ionic conductivity. Finally, we use a Li|LBF|LBF-C cell to evaluate the electrochemical fluorination of the carbon in the LBF-C composite. Our results suggest an electrochemical activity of carbon with fluoride ions. The possible interactions of carbon with fluorides to form insulating carbon fluoride (CF_<i>x</i>) must be considered when determining the operating voltage of FIBs

“Two-Story” Calix[6]arene-Based Zinc and Copper Complexes: Structure, Properties, and O₂ Binding

A new “two-story” calix[6]­arene-based ligand was synthesized, and its coordination chemistry was explored. It presents a tren cap connected to the calixarene small rim through three amido spacers. X-ray diffraction studies of its metal complexes revealed a six-coordinate Zn^II complex with all of the carbonyl groups of the amido arms bound and a five-coordinate Cu^II complex with only one amido arm bound. These dicationic complexes were poorly responsive toward exogenous neutral donors, but the amido arms were readily displaced by small anions or deprotonated with a base to give the corresponding monocationic complexes. Cyclic voltammetry in various solvents showed a reversible wave for the Cu^II/Cu^I couple at very negative potentials, denoting an electron-rich environment. The reversibility of the system was attributed to the amido arms, which can coordinate the metal center in both its +II and +I redox states. The reversibility was lost upon anion binding to Cu. Upon exposure of the Cu^I complex to O₂ at low temperature, a green species was obtained with a UV–vis signature typical of an <i>end-on</i> superoxide Cu^II complex. Such a species was proposed to be responsible for oxygen insertion reactions onto the ligand according to the unusual and selective four-electron oxidative pathway previously described with a “one-story” calix[6]­tren ligand