The present work is concerned with the stress changes generated during lithium transport through the $Li_xMn_2O_4$ film electrode quantitatively measured using a laser beam deflection method combined with galvanotatic intermittent titration technique, potentiostatic current transient technique, galvanostatic discharge experiment and cyclic voltammetry. The $Li_xMn_2O_4$ film electrodes were prepared by spin-coating a viscous gel on the Pt/Ti/thin glass substrate and then annealed at 773 and 873 K in air. The annealed electrodes were characterised by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy.
The stress change Δσ generated during a real potential step between initial electrode potential and final applied potential was uniquely specified by the lithium stoichiometry x. The value of Δσ coincided well with that value derived from Δσ versus x curve measured simultaneously along with the galvanostatic intermittent titration discharge curve. The $Li_xMn_2O_4$ film electrode annealed at 773 K (LT- $Li_xMn_2O_4$) showed larger capacity than the $Li_xMn_2O_4$ film electrode annealed at 873 K (HT- $Li_xMn_2O_4$) and this result is ascribed to the fact that the smaller the grain size is, the more increases the electrochemically active area of the film electrode.
From the analysis of the stress change transients measured along with the galvanostatic discharge curves and the cyclic voltammograms, it is found that the value of Δσ generated in the LT- $Li_xMn_2O_4$ was smaller than that in the HT- $Li_xMn_2O_4$ during the lithium intercalation/deintercalation around 3 V$_{Li/Li+}$ region although the amount of intercalated/deintercalated lithium in the LT- $Li_xMn_2O_4$ was larger than that in the HT- $Li_xMn_2O_4$. This anormalous behaviour of Δσ was discussed in terms of the suppression of the Jahn-Teller distortion. From the analysis of the ternary Li-Mn-O phase diagram, it is suggested that the oxidation state of manganese in $Li_xMn_2O_4$ is strongly dependent on the annealing temperature. From these results, it is concluded that the Jahn-Teller distortion is suppressed by the increase in the average oxidation state of manganese. Furthermore, the beneficial effect of the suppression of the Jahn-Teller distortion on the cyclability justified by improvement of cyclability obtained from cyclic voltammetry around 3 $V_{Li/Li+}$ region and by the suppressed surface cracking observed by scanning electron microscopy.
