The alkaline Zn/MnO2 battery was introduced in the early 1960s. Its advantages over the Zn-C system can be summarized as:
If the cost-per hour of service is considered, especially at high drains and continuous discharge, the alkaline battery becomes cheaper than Zn-C. Its higher capacity and energy vs the standard Zn-C battery is due to the use of high-grade anode and cathode materials, and to the more conductive alkaline electrolyte.
The anode is essentially high-purity Zn powder. Its higher surface area vs that of a Zn can afford higher discharge rates, while the electrolyte is more uniformly distributed. Furthermore, the combination of a porous anode and a conductive electrolyte reduces the extent of accumulation of reaction products near the electrode, resulting in a higher rate capability. The low impurity level of the zinc powder (especially Fe) has facilitated elimination of Hg, Pb or other heavy metals as gassing suppressors. A gelling agent is instead necessary for immobilizing the electrolyte and improving electrode processibility. To this end, starch, cellulose derivatives or polyacrylates are often used. The anode also contains the electrolyte, that is an aqueous KOH solution (35-52%).
The cathode is based on electrolytic MnO2, as only this form can grant high power and long shelf life. The electronic conductor is carbon in the form of graphite, although some acetylene black may also be added to enhance the surface area.
The separator, which has to be chemically stable in the concentrated alkaline solution, is normally a non-woven fabric, such as cellulose, vinyl polymer, polyolefin or a combination thereof.
The high porosity of cathode, anode and separator allows their saturation with the electrolyte. The homogeneous electrolyte distribution and its high conductivity afford high discharge rates also on continuous drains and at low temperatures.
Zn powder is obviously quite reactive and can decompose H2O with the production of hydrogen, which can cause MnO2 self-discharge and generate an overpressure. As mentioned above, reducing the impurity level in the Zn powder greatly limits gassing. Otherwise, additives for the anode are necessary, such as ZnO (or other oxides) or organic inhibitors (polyethylene oxide compounds). Alkaline batteries can be built with cylindrical, button or prismatic configurations.
In a cylindrical alkaline cell, the can is not an active material, as it is made of steel or nickel-plated steel and acts as the cathode current collector. The cathode is pressed against the steel can either applying a high pressure to the powder when in contact with the can or forming annular pellets, which are then inserted into the can. The Zn powder is allocated in the central cavity, around a brass current collector welded to the cell bottom (negative cap). A plastic grommet, sealed to the cell can, ensures that the cell is leakproof. The grommet incorporates a membrane vent for relieving overpressure in case of short circuits or cell abuse.
In a button cell, the Zn powder is in the upper part and contacts the negative cell top, a steel foil usually having an external layer of nickel and an internal layer of Cu or Sn. The can, acting as a container and cathode collector, is made of Ni-plated steel. It is insulated from the cell top by a plastic grommet over which is crimped to seal the cell. The MnO2 pellet, at the bottom of the cell, is covered by a separator and by an absorber for the electrolyte.
A standard prismatic battery is multicell and constructed as described for the Zn-C battery.
In 1999, premium alkaline cells were commercialized. They have a better performance at high discharge rates than the standard models. This was made possible by a further reduction of the cell resistance through (1) coating both the negative and positive current collector, (2) using a finer graphite grade and (3) packing more MnO2 into the space available for the cathode. Coating reduces the build up of corrosion products on the current collectors, while a finer graphite powder improves the electronic conductivity.
Button cells have capacities of 25-60mAh, cylindrical cells of 0.5-22Ah and prismatic batteries of 0.16-44 Ah. This wide capacity range makes alkaline batteries suitable for several applications, from consumer to industrial/military devices. The former are more numerous and include remote controls, photographic equipment, flashlights, radios, watches, calculators, home healthcare devices, etc., while the latter include portable medical and industrial instrumentation, emergency lighting, communication equipment, electrical measurement devices, etc.
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