Solid-State Electrochemistry of Copper(I) Coordination Polymers Containing Tetrafluoroborate Anions

Host-guest materials based on coordination polymers (CPs) are currently emerging as potential candidates for battery applications. In this context, we describe the preparation of three-dimensional network structures containing BF₄ anions and water molecules in the one-dimensional (1D) channels via hydrothermal reactions between Cu(BF₄)₂ and 4,4'-bipyridine or 1,2-di-4-pyridylethylene. A systematic characterization of the obtained CPs using single-crystal X-ray diffraction, X-ray absorption fine structure, and an electrochemical test was performed. The results showed that the BF₄ anions were electrochemically reduced to BF₃ in the cavities of the CPs, with concomitant elimination of a leaving fluoride at room temperature. Using this electrochemical property, a prototype battery, in which the CPs act as the anode and graphite as the cathode, was demonstrated. The cell exhibited a practical discharge potential of ∼1.5 V. This constitutes the first demonstration of CPs showing electrochemical B-F bond activation in the 1D channels and rocking-chair-type fluoride insertion and extraction by changes in the electric potential.