Evidence for slow droplet formation during cubic-to-tetragonal phase transition in LixMn2O4 spinel

We studied the electroanalytical response of LixMn2O4 electrodes around 3 V (vs. Li/Li+) using slow scan rate cyclic voltammetry (SSCV), potentiostati c intermittent titration technique, and electrochemical impedance spectrosc opy. 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 LixMn2O4 electrodes around 3 V (vs. Li/ Li+). The subsequent potentiostatic titrations performed at 10 mV steps rev ealed 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 a bove features of the cyclic voltammograms (at slow scan rate) and the curre nt vs, time response can be understood in terms of a phase transition contr olled by slow formation of droplets of a new phase in the bulk of the old o ne. A model describing such a process, based on the refined Frumkin-type so rption isotherm, which relates to both the electronic and the ionic species which participate in the intercalation reaction, is developed. The SSCV cu rves simulated according to this model agree well with the experimental one s. We also show a semiquantitative comparison of the electroanalytical char acteristics of LixMn2O4 electrodes around 3 and 4 V (vs. Li/Li+). (C) 2000 The Electrochemical Society. S0013-4651(99)05-081-8. All rights reserved.