Evidence for slow droplet formation during cubic-to-tetragonal phase transition in Li_xMn₂O₄ spinel

We studied the electroanalytical response of Li_xMn₂O₄ electrodes around 3 V (vs. Li/Li⁺) using slow scan rate cyclic voltammetry (SSCV), potentiostatic intermittent titration technique, and electrochemical impedance spectroscopy. Broad anodic and cathodic peaks of typical 50 to 60 mV half-peak width, and a surprisingly large peak potential separation (ca. 150 mV and higher) appear on the SSCV curves of thin Li_xMn₂O₄ electrodes around 3 V (vs. Li/Li⁺). The subsequent potentiostatic titrations performed at 10 mV steps revealed quite an unusual response; an increase in the current with time at 2.94 and 3.00 V (vs. Li/Li⁺) during discharge and charge, respectively. The above features of the cyclic voltammograms (at slow scan rate) and the current vs. time response can be understood in terms of a phase transition controlled by slow formation of droplets of a new phase in the bulk of the old one. A model describing such a process, based on the refined Frumkin-type sorption isotherm, which relates to both the electronic and the ionic species which participate in the intercalation reaction, is developed. The SSCV curves simulated according to this model agree well with the experimental ones. We also show a semiquantitative comparison of the electroanalytical characteristics of Li_xMn₂O₄ electrodes around 3 and 4 V (vs. Li/Li⁺).