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February 7, 2023

Leclanché Battery Chemistry FAQ

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Leclanché Cells are the carbon-zinc primary batteries which have been largely replaced by alkaline cells.

Leclanché Cells (zinc carbon or dry cell )

Anode: Zinc

Cathode: Manganese Dioxide (MnO2)

Electrolyte: Ammonium chloride or zinc chloride dissolved in water

Applications: Flashlights, toys, moderate drain use

The basic design of the Leclanché cell has been around since the 1860s, and until World War II, was the only one in wide use. It is still the most commonly used of all primary battery designs because of its low cost, availability, and applicability in various situations. However, because the Leclanché cell must be discharged intermittently for best capacity, much of battery research in the last three decades has focused on zinc-chloride cell systems, which have been found to perform better than the Leclanché under heavier drain.

Discharge curve for Leclanche batteries

This figure shows typical discharge curves for general-purpose Leclanché zinc chloride D-size cells discharge 2 h/day at 20º C. Solid line—zinc chloride; broken line—Leclanché (Linden 8.18). The zinc-chloride cell has a higher service life and voltage than the Leclanché (at both higher and lower discharge rates).

In an ordinary Leclanché cell the electrolyte consists (in percent of atomic weight) of 26% NH4Cl (ammonium chloride), 8.8% ZnCl2 (zinc chloride), and 65.2% water. The overall cell reaction can be expressed:

Zn + 2MnO2 +2NH4Cl —> 2MnOOH + Zn(NH3)2Cl2 E=1.26

The electrolyte in a typical zinc chloride cell consists of 15-40% ZnCl2 and 60-85% water, sometimes with a small amount of NH4Cl for optimal performance. The overall cell reaction of the zinc chloride as the electrolyte can be expressed:

Zn + 2MnO2 + 2H2O + ZnCl2 —> 2MnOOH + 2Zn(OH)Cl

MnO2, is only slightly conductive, so graphite is added to improve conductivity. The cell voltage increases by using synthetically produced manganese dioxide instead of that found naturally (called pyrolusite). This does drive the cost up a bit, but it is still inexpensive and environmentally friendly, making it a popular cathode.

These cells are the cheapest ones in wide use, but they also have the lowest energy density and perform poorly under high-current applications. In fact some pyrolusite MnO2 deposits are pure enought to be used directly after milling. The zinc carbon design is reliable and more than adequate for many everyday applications.

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