The mechanism of capacity fade of rechargeable alkaline manganese dioxide zinc cells

Recent experiments showed, that in contrast to traditional opinion, if the cathode was protected by anode limitation the capacity fade of Rechargeable Alkaline Manganese Dioxide Zinc (RAM(TM)) cells was not caused by the EMD cathode, but by the gelled zinc anode. The key is the electrolyte. The cathode competition for the electrolyte and the increasing requirement of chemically formed ZnO for more electrolyte caused an electrolyte deficiency at the front face of the anode and finally caused precipitation of zincate and passivation of zinc. The crust is a mixed material of precipitation and passivation products. The low solubility ZnO is formed by decomposition of electrochemically generated zincate ions [Zn(OH)(4)](2-) and also by recombination of zinc with oxygen during overcharge. The progressively thickened "crust" at the front face of the anode increases the resistance, then finally causes the cell to fade. The crusting is a redistribution of active material and electrolyte between the front and rear of the cylindrical gelled zinc anode. More electrolyte and proper charging can delay such a "crusting" phenomenon. (C) 2000 Elsevier Science S.A. All rights reserved.