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 (μXANES) and micro-X-ray diffraction (μXRD) were used to investigate the electrochemical lithiation of the layered oxysulfide Sr₂MnO₂Cu_3.5S₃. Microfocused X-ray fluorescence (XRF) imaging was used to image the elemental components within the battery electrode while μXANES and μXRD provided information about the Cu oxidation state and phase distribution, respectively. Sr₂MnO₂Cu_3.5S₃ operates by a combined insertion/displacement mechanism. After 1 mol of Li intercalation, Cu metal extrusion is observed by μXRD, which also reveals the formation of the Sr₂MnO₂Cu_{3.5–<i>x</i>}Li_<i>x</i>S₃ phase. Ex situ μXRF images of the electrode after 3.75 mol of Li intercalation show segregated Cu metal and Sr₂MnO₂Cu_{3.5–<i>x</i>}Li_<i>x</i>S₃ particles, while in situ μXRF 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 μXRF imaging data suggest that the microstructure of the electrode, on a microscale level, is not affected by the Cu extrusion process

Investigating Li Microstructure Formation on Li Anodes for Lithium Batteries by in Situ ⁶Li/⁷Li NMR and SEM

The growth of lithium microstructures during battery cycling has, to date, prohibited the use of Li metal anodes and raises serious safety concerns even in conventional lithium-ion rechargeable batteries, particularly if they are charged at high rates. The electrochemical conditions under which these Li microstructures grow have, therefore, been investigated by in situ nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and susceptibility calculations. Lithium metal symmetric bag cells containing LiPF₆ in EC/DMC electrolytes were used. Distinct ⁷Li NMR resonances were observed due to the Li metal bulk electrodes and microstructures, the changes in peak positions and intensities being monitored in situ during Li deposition. The changes in the NMR spectra, observed as a function of separator thickness and porosity (using Celgard and Whatmann glass microfiber membranes) and different applied pressures, were correlated with changes in the type of microstructure, by using SEM. Isotopically enriched ⁶Li metal electrodes were used against natural abundance predominantly ⁷Li metal counter electrodes to investigate radiofrequency (rf) field penetration into the Li anode and to confirm the assignment of the higher frequency peak to Li dendrites. The conclusions were supported by calculations performed to explore the effect of the different microstructures on peak position/broadening, the study showing that Li NMR spectroscopy can be used as a sensitive probe of both the amount and type of microstructure formation

Composition-Structure Relationships in the Li-Ion Battery Electrode Material LiNi_0.5Mn_1.5O₄

A study of the correlations between the stoichiometry, secondary phases, and transition metal ordering of LiNi_0.5Mn_1.5O₄ was undertaken by characterizing samples synthesized at different temperatures. Insight into the composition of the samples was obtained by electron microscopy, neutron diffraction, and X-ray absorption spectroscopy. In turn, analysis of cationic ordering was performed by combining neutron diffraction with Li MAS NMR spectroscopy. Under the conditions chosen for the synthesis, all samples systematically showed an excess of Mn, which was compensated by the formation of a secondary rock-salt phase and not via the creation of oxygen vacancies. Local deviations from the ideal 3:1 Mn:Ni ordering were found, even for samples that show the superlattice ordering by diffraction, with different disordered schemes also being possible. The magnetic behavior of the samples was correlated with the deviations from this ideal ordering arrangement. The in-depth crystal-chemical knowledge generated was employed to evaluate the influence of these parameters on the electrochemical behavior of the materials