In situ Raman scattering measurements of a LiMn2O4 single crystal microelectrode

Raman spectra of a LiMn₂O₄ single crystal microelectrode have been recorded in situ as a function of potential in 1 M LiPF₆ ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1) solutions at room temperature. Measurements were acquired during a linear voltammetric scan at 0.1 mV/s over the potential (E) range in which the material undergoes redox transitions, i.e., 3.6 < E < 4.4 V vs Li/Li ⁺. Spectral data were analyzed using both classical least squares (CLS) and multivariate curve resolution (MCR) techniques. A plot of the state of charge of the LiMn₂O₄ single crystal microelectrode derived from the optical data vs the applied potential obtained from data collected during the reduction of the fully oxidized material displayed two well-defined steps. A similar behavior was found for corresponding plots extracted from a coulometric analysis of the voltammetric scan, except that the curves in this latter case were shifted along the potential axis by ca. 40 mV toward more negative values. This effect was attributed to the slow rates of Li⁺ diffusion within the lattice, allowing the surface region probed by the laser beam to reach equilibrium with the applied potential much faster than the particle as a whole.