Electrolyte Effects on the Electrochemical Behavior of Electrolytic Manganese Dioxide in Alkaline Aqueous Media

Electrolytic manganese dioxide (EMD) is the cathode material typically used in primary alkaline cells, owing to its low cost and relatively high gravimetric capacity.   These qualities also make it attractive as a cathode for secondary cells, though unfortunately its cycle-life is relatively short. The cycle-life can be improved under certain conditions, e.g. at limited depth of discharge [1]; there also reports of improved cycleability in LiOH solutions or against non-zinc electrodes [2].  We have investigated the effects of electrolyte composition on the electrochemical properties of EMD with cyclic voltammetry and galvanostatic cycling.  More particularly, we focused on LiOH and KOH electrolytes, with ZnO present or absent.  The structural evolution of the material during electrochemical cycling was studied using in situ synchrotron x-ray diffraction.  While the structure of the EMD appears to remain largely intact after a single cycle in KOH solution, it undergoes an irreversible transformation at low depths of discharge when reduced in lithium hydroxide solutions.  Both electrochemical and diffraction measurements indicate this transformation was complete within the first reduction, though the material undergoes further structural evolution with extended cycling.  The electrochemical behavior of ramsdellite MnO₂, the prototypical structure for EMD, was also examined in a LiOH electrolyte as a point of comparison.  Ramsdellite underwent an irreversible transformation at low depths of discharge in a manner similar to that observed for EMD.  The evolution of the diffraction pattern of the ramsdellite upon electrochemical reduction was very similar to that reported for chemically reduced ramsdellite in organic media [3].  In situ measurements with zincate-containing electrolyte showed ZnMn₂O₄ (hetaerolyte) to form sooner in LiOH solutions than KOH solutions.  Curiously, hetaerolyte underwent some structural changes upon re-oxidation and we will provide some possible interpretations of this observation.  Short-term (<30 cycles) capacity and cycleability trends observed as a function of the various electrolyte compositions and potential limits will also be presented.

[1] K. Kordesch, J. Gsellmann, M. Peri, K. Tomantschger, R. Chemelli Electrochim. Acta 1981, 26, 1495-1504.

[2] M. Minakshi Electrochem. Solid-State Lett. 2010, 13, A125-A127.

[3] M.M. Thackeray, M.H. Rossouw, R.J. Gummow, D.C. Liles, K. Pearce, A. De Kock, W.I.F. David, S. Hull Electrochim. Acta 1993, 38, 1259-1267.