Leclanchd cells

Figure 7.20 depicts the Leclanchd cell in schematic form. The zinc can is generally coated with plastic for encapsulation (i.e. to prevent it from splitting) and to stop the intrusion of moisture. Plastic is an insulator, and so we place a conductive cap of stainless steel at the base of the cell to conduct away the electrons originating from the dissolution of the zinc from the inside of the can. A carbon rod then acts as an inert electrode to conduct electrons away from the reduction of Mn02 at the cathode. The reaction at the cathode is given by... 

In Scotland, the cost of domestic mains electricity is 0.0713/kWh (in 1997). A D-size Leclanchd cell, delivering say 5 Wh, currently retails at 0.50. Thus, energy from the primary battery costs I00/kWh - a factor of over 1000 more expensive. For a 150 mWh zinc-silver oxide button cell, retailing at 1.50, the cost of energy is over 10 000/kWh ... 

The practical capacity of a Leclanchd cell does not have a fixed value since it varies according to the pattern and conditions of discharge - to a much greater extent than for most other cells. Different forms of this type of cell are designed for particular purposes, and as pointed out in Chapter 2, test procedures imitate as far as possible the discharge pattern of the applica-... 

The service schedule is of critical importance in relation to the recuperation reaction. Unless the current drain is ver low, Leclanchd cells give a much better performance when used on an intermittent basis. The effect of rest periods was shown earlier in Fig. 3.5. In Fig. 3.15 the effect on the service life of D-size cells of two different operating schedules is illustrated. A schematic three-dimensional representation of capacity as a function of current drain and operating programme based on Union Carbide technical data is given in Fig. 3.16. 

D-size cells on 2.25 O continuous test are reported. Cell (a) is a standard Leclanchd cell using a natural ore cell (b) is a HD Leclanche with electrolyte Mn02 cell (c) is a zinc chloride cell and cell (d) is an alkaline manganese primary unit. The differences at this current drain are striking the discharge capacities with a 0.9 V cut-off are in the ratio 0.12 0.24 0.55 1.00 for the four types. However, when less severe tests are considered, the disparities are less pronounced. Thus for the light industrial flashlight (LIF) test, the ratios are 0.40 0.61 0.96 1.00. 

A number of cylindrical and flat magnesium-based cells have been developed on a commercial scale, mainly for military applications where high discharge currents and low unit weight are important. However, for most of these applications, magnesium batteries have now been replaced by various lithium/organic systems. There are no commercial aluminium-based Leclanchd cells. Magnesium and aluminium are both exploited as anodes in metal-air cells which are considered below. 

The most obvious advantages of the oxygen cathode are that it has low weight and infinite capacity. Consequently, prototype D-size cells based on the zinc-air system have been shown to have twice the overall practical capacity of zinc-mercuric oxide cells (and 10 times that of a standard Leclanchd cell) when subjected to a continuous current drain of 250 mA. In the larger industrial cells, energy densities of up to 200 Wh/kg and specific capacities of 150 Ah/dm3 may be obtained. On the other hand, a catalytic surface must be provided for efficient charge transfer at the oxygen cathode, and by its nature the electrode is susceptible to concentration polarization. 

Despite the long history of manufacture, the dry Leclanchd cell dating from the 1880s, its detailed electrochemistry is again far from understood. Indeed the open-circuit potential measured for the completed cell is frequently higher than that estimated on the basis of the equations in Table 10.1 or the more complex equation... 

A number of batteries are based on modifications of the technology or the substitution of electrode reactions into either the Ni/Cd or Leclanchd cells the changes usually improve one characteristic of importance to a particular duty but usually only at an increased cost. In an alkaline manganese battery, the cathode is manganese dioxide and graphite in the form of compressed tablets, and the electrolyte is strongly alkaline. The cell reaction is... 

The ordinary Leclanchd cell uses a mixture of ammonium chloride and zinc chloride, with the former predominating. Zinc-chloride cells typically use only Z11CI2, but can contain a small amount of NH4CI to ensure high rate performance. Eixamples of typical electrolyte formulation for the zinc-carbon battery systems are listed in Table 8.3. 

Fig. 1.5 Cross-sectional view of a round Leclanchd cell (http //sciencescollege3eme.blogspot.fr/ 2008/12/histoire-de-pile.htm)...

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