Novel Synthesis of Anhydrous and Hydroxylated CuF 2 Nanoparticles and Their Potential for Lithium Ion Batteries.

Anhydrous nanoscopic CuF ₂ is synthesized from alkoxides Cu(OR) ₂ (R=Me, tBu) by their reaction either in pure liquid HF at -70 °C, or under solvothermal conditions at 150 °C using excess HF and THF as solvent. Depending on the synthesis method, nanoparticles of sizes between 10 and 100 nm are obtained. The compound is highly hygroscopic and forms different hydrolysis products under moist air, namely CuF ₂ ⋅2 H ₂ O, Cu ₂ (OH)F ₃ , and Cu(OH)F, of which only the latter is stable at room temperature. CuF ₂ exhibits an electrochemical plateau at a potential of ≈2.7 V when cycled versus Li in half cell Li-ion batteries, which is attributed to a non-reversible conversion mechanism. The cell capacity in the first cycle depends on the particle size, being 468 mAh g ^-1 for ≈8 nm crystallite diameter, and 353 mAh g ^-1 for ≈12 nm crystallite diameter, referred to CuF ₂ . However, such a high capacity cannot be sustained for several cycles and the capacity rapidly fades out. The cell voltage decreases to ≈2.0 V for CuF ₂ ⋅2 H ₂ O, Cu ₂ (OH)F ₃ , and Cu(OH)F. As all the compounds studied in this work show irreversible conversion reactions, it can be concluded that copper-based fluorides are unsuitable for Li-ion battery applications.
(© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)