The first Zn/MnO2 battery was introduced in the middle of the nineteenth century; its electrolyte is immobilized in an inert support, which justifies the name of “dry battery”. This cheap battery is still largely used in moderate and light drain applications. However, it cannot compete with alkaline Zn/MnO2 in terms of performance, and its use is declining except for emerging countries Dry batteries can use either the Leclanche´ or the ZnCl2 system. The former uses an aqueous electrolyte containing NH4Cl (26%) and ZnCl2 (8.8%), while the latter contains ZnCl2 (15–40%). In both electrolytes, inhibitors of Zn corrosion are added.
The electrodes are basically the same in both systems. The Zn can of the cell is also the anode, while the cathode is a mix of electrochemically active MnO2 and carbon. In principle, the electrochemistry of the Zn-C cell is quite simple with Zn oxidation to Zn2þ and Mn4þ reduction to Mn3þ (MnOOH or Mn2O3). In practice, the reactions are rather complicated and depend on several factors, such as electrolyte concentration, temperature, rate and depth of discharge.
These batteries can have a cylindrical or a flat configuration. In the former, a bobbin containing a mixture of MnO2, carbon black and electrolyte surrounds the carbon rod, serving as a current collector for the cathode (hence the name Zn-C). The separator between the Zn can and the bobbin is usually paper thinly coated with a paste of gelled flour and starch absorbing the electrolyte. To prevent electrolyte leakage due to perforation of the Zn can, the latter is jacketed with a polymer film or polymer-coated steel.
In the flat configuration, rectangular cells are stacked to give prismatic batteries, for example the popular 9-V size. The construction in this case is quite different from that of cylindrical cells. The Zn anode is coated with a carbon layer, to act as an electron conductor for the cathode side of the adjacent cell. In the 9V battery, six flat single cells are stacked in series. In these cells, the flat design provides more space to the cathode mixture, so the energy density is higher. The prismatic battery shape, in turn, is more favourable to space saving: the volumetric energy density is twice that of a cylindrical battery. The flat configuration is available in multicell batteries only (from four to several hundred cells in a stack or set of stacks).
Modern cells mostly use either chemical MnO2 or electrolytic MnO2, whose percentage of active material is 90–95% (the remainder is mostly H2O plus several impurities). As a carbon, the highly porous acetylene black is preferred as it remarkably increases the poor conductivity of MnO2. The Zn anode is ultrapure and is used to alloy with Cd (0.3%) and Pb (0.6%) to improve its metallurgical properties and reduce corrosion. The legislation of several countries now prohibits the use of these toxic metals beyond a given (very low) limit, so that their content in modern cells is practically zero. Similarly, the use of Hg as the main corrosion inhibitor has been eliminated. Other materials now considered as inhibitors include Ga, Sn, Bi, glycols or silicates. Zn corrosion is primarily due to the acidic character of both the NH4Cl and the ZnCl2 solution (the latter is more acidic).
The solution containing ZnCl2 is preferred. Indeed, formation of sparingly soluble Zn salts, which tend to accumulate near the electrode, greatly limits ion diffusion in the Leclanche´ cell. With the ZnCl2 solution, this phenomenon is reduced, so that faster diffusion and enhanced rates of discharge are allowed. The better performance of the ZnCl2 cell, especially at high currents and moderately low temperature (down to 10C), is counterbalanced by a higher cost. In terms of performance and cost, this cell lies between the common Leclanche´ and the alkaline cell. Another advantage of the ZnCl2 cell is given by its lower self-discharge rate.
Our website is not responsible for the information contained by this article. Webworldarticles.com is a free articles resource thus practically any visitor can submit an article. However if you notice any copyrighted material, please contact us and we will remove the article(s) in discussion right away.
This article was sent to us by:
Giani Rimeollo at
06272010
1. Alkaline Batteries
All articles in this directory are property of their respective authors. Additionally, read our Privacy Policy
© 2010 WebWorldarticles.com - All Rights Reserved. Partners: Gunblade Saga