A quantitative discrimination between reversible Li+-insertion and irreversible solvent oxidation at a lithium/manganese-spinel electrode

Oxidic Li+-insertion compounds of the donor type in aprotic Li+-electrolytes are of high practical importance as positives in rechargeable Li+-ion batteries. The manganese oxides MnOx are of special interest as host lattices due to their high reversibility and to the relatively positive potentials. However, these materials may induce irreversible anodic decomposition of the organic solvent molecules, especially towards the end of the charging process. Protons are injected into the electrolyte in this case. It is possible to evaluate these side reactions quantitatively through a measurement at the rotating ring/disk-electrode (RRDE). The disk was made of a 1 mu m thick layer of MnO2 (anodic deposit) on Pt (I) or a ceramic layer of the spinel LiMn2O4 on Ti, which was fabricated after Beers's method at a firing temperature of 400 (II) and 600 degrees C (III). The ring was made of Pt. At the ring, protons were reduced to H-2 at a half wave potential of - 0.25 V versus saturated sodium chloride electrode (SSCE). The electrolytes were 0.1 M LIClO4, in propylenecarbonate (PC) and acetonitrile (MeCN). These measurements showed that the novel MnOx-layers behave similar to the conventional porous battery electrodes which are much thicker and are made of powdery materials. Charge stoichiometry gamma in Li gamma Mn2O4 is found to increase from I to III from 0.2 to 2.0. No solvent oxidation could be detected at the potentials of the reversible Li+-intercalation peaks in cyclic voltammogram. The irreversible side reaction can be excluded therefore with a margin of error of 1-2%. This is in contrast to the former results obtained from the graphite intercalation compounds of the acceptor type (A-GIC), albeit, at potentials more positive by 1 V. However, just behind the reversible Li+-deintercalation peak, i.e. charge direction, the positive end of the electrochemical window is attained. The current rises steeply. Abundant protons are injected into the electrolyte. The potentials are by 0.5-1 V more negative than at Pt due to the electrocatalytic activity of MnOx with respect to the irreversible anodic oxidation of the solvent molecules PC and MeCN. (C) 2000 Elsevier Science Ltd. All rights reserved.