Investigation of the Conversion Reaction Mechanisms for Binary Copper(II) Compounds by Solid-State NMR Spectroscopy and X-ray Diffraction

The conversion reaction mechanisms of CuS, CuF2, and CuO during the electrochemical reaction with Li are studied by solid-state Cu-63, F-19, and Li-7 nuclear magnetic resonance (NMR) and X-ray diffraction (XRD). For CuS, a two-step reaction is observed that is associated with an insertion reaction involving first limited incorporation of Li into CuS and then a two-phase reaction to form a material with the approximate composition LiCuS. This is followed by a conversion reaction to form Li2S and Cu, Cu1.96S being formed as a side product of the decomposition of LiCuS. Evidence for the insertion phases is found from both NMR and XRD. A direct conversion reaction to form LiF and Cu is seen for CuF2, whereas the Li-7 NMR results indicate that CuO can tolerate a small amount of Li substitution before reacting to form Li2O and Cu. Both the diffract ion and NM R results indicate that the size of the Cu particles formed on discharge are much larger in the CuS system. which is thought to result from the higher Cu1+ mobilities in the intermediate intercalation compounds Li,CuS. The factors that control the possible mechanisms for these conversion reactions are discussed.