In situ, real-time Raman microscopy of embedded single particle graphite electrodes

Dynamic aspects of Li⁺ intercalation-deintercalation into single KS-44 carbon particles (8-50 μm in diameter) embedded in thermally annealed Ni foils were examined in 1 M LiClO₄, ethylene carbonate (EC), diethyl carbonate (DEC) (1:1 v/v) solutions by in situ, time-resolved Raman microscopy. A direct correlation was found between the position of the prominent G band, in the range 1581-1590 cm^-1, and the amount of Li⁺ in KS-44 within the so-called dilute stage 1 phase. This information was used to determine spectroscopically and in real time the average concentration of Li⁺ within the volume of the particle probed by the laser beam following application of a potential step between 0.05 V vs. Li/Li⁺, i.e., nominally full Li⁺ intercalation and 0.7 V vs. Li/Li⁺, i.e., full Li⁺ deintercalation. Quantitative analysis of these transient data based on a spherical diffusion model yielded a time constant for Li⁺ deintercalation for dilute stage 1 phase consistent with reported values of Li⁺ diffusion coefficients within graphitic materials and the size of the particles probed. Single KS-44 particles cycled repeatedly into the deep Li⁺-intercalation region recorded at high potentials E > 0.5 V vs. Li/Li⁺, displayed a new Raman band attributed to bounding graphite layers not observed for pristine KS-44 carbon under otherwise identical conditions. This new spectral feature has been tentatively associated with chemical modifications of the carbon itself which may be at least partially responsible for irreversible capacity losses.