Scanning X-ray Fluorescence Imaging Study of Lithium Insertion into Copper Based Oxysulfides for Li-Ion Batteries

Ex situ and in situ Synchrotron X-ray fluorescence imaging coupled with selective micro-X-ray absorption near-edge spectroscopy (mu XANES) and micro-X-ray diffraction (mu XRD) were used to investigate the electrochemical lithiation of the layered oxysulfide Sr2MnO2Cu3.5S3. Microfocused X-ray fluorescence (XRF) imaging was used to image the elemental components within the battery electrode while mu XANES and mu XRD provided information about the Cu oxidation state and phase distribution, respectively. Sr2MnO2Cu3.5S3 operates by a combined insertion/displacement mechanism. After 1 mol of Li intercalation, Cu metal extrusion is observed by mu XRD, which also reveals the formation of the Sr2MnO2Cu3.5-xLixS3 phase. Ex situ mu XRF images of the electrode after 3.75 mol of Li intercalation show segregated Cu metal and Sr2MnO2Cu3.5-xLixS3 particles, while in situ mu XRF imaging experiments reveal that the Cu and Mn elemental distribution maps are highly correlated to the particle orientation giving different results when the particle is oriented either perpendicular or parallel to the incident beam. In situ electrochemical synchrotron XRF imaging has the advantage over the ex situ mode in that it allows the reaction mechanism of a single particle to be followed vs time. In situ mu XRF imaging data suggest that the microstructure of the electrode, on a microscale level, is not affected by the Cu extrusion process.