Investigation of structural fatigue in spinel electrodes using in situ laser probe beam deflection technique

The presence of the tetragonal phase at the surface of a LiMn₂O₄ particle due to a Jahn-Teller effect was previously reported to be one of the reasons for capacity fade observed during cycling of Li//Li_xMn₂O₄ in the 4 V range. This study reports on the evidence of the onset of a Jahn-Teller effect above 3 V at the surface of LiMn₂O₄ particle of thin-film electrodes using in situ laser probe beam deflection technique. The thin film electrode was prepared by electrostatic spray deposition technique. The cyclic deflectograms and cyclic voltammograms were obtained simultaneously, and the potential range responsible for the Jahn-Teller distortion in the 4 V range was determined by differentiating the deflectograms against time. We observed three sets of peaks in both the cathodic and anodic scan of the differential cyclic deflectograms and the magnitudes of these are changed with the scan rate. In the absence of Jahn-Teller distortion, the shape of the differential deflectograms would be identical to that of cyclic voltammograms. Analysis of the differential strain curves obtained from the cyclic deflectograms and the cyclic voltammograms indicates that one of the cathodic peaks of the differential cyclic deflectograms is due to the onset of the Jahn-Teller effect and one of the anodic peaks is due to the relaxation of the tetragonal phase, formed in the previous cathodic scan due to the Jahn-Teller effect, to cubic phase. The Jahn-Teller effect during the cathodic scan is observed to occur around 3.95 ∼ 4.0 V depending on the scan rates, i.e., at slower scan rates the cathodic peak of the differential strain curves became smaller. This could be ascribed to the suppression of the Jahn-Teller effect at near equilibrium conditions.