The reaction of lithium with CuCr2S4—lithium intercalation and copper displacement/extrusion

The electrochemical reduction of CuCr2S4 by Li was shown to occur in two steps, involving first a combined Cu extrusion/Li insertion leading to a Li1.75Cu0.25Cr2S4 phase that, upon further lithiation, undergoes a conversion reaction. Analytical transmission electron microscopy, high resolution electron energy loss spectroscopy and neutron diffraction experiments have been used to elucidate the copper, sulphur and chromium oxidation states during the first electrochemical step. HRTEM shows the growth of copper dendrites during the discharge of the compound and their disappearance upon subsequent re-oxidation, while EDS analysis indicates that all of the copper cannot be extruded from the pristine phase upon discharge, nor be fully reinserted on charging. HR-EELS confirms the following oxidation state Cu+Cr23+(S2-)3S- for the pristine material and demonstrates that the insertion of lithium in the sample only corresponds to a change of the sulphur oxidation state whereas the valence of chromium was not modified. This observation was directly confirmed by neutron data that show an increase in the S-S bond length upon lithium insertion.