Electrical contacts, zinc carbon cells

In systems with solid cathodes, the active positive electrode material is usually mixed with a conductive carbon that conveys electrons firom the current collector to the electrode active material. In this case (and similar to the zinc-air cell discussed below), liquid SOCI2 is the active material and needs to come into contact with an electrode site (cathode) where it can take up electrons as the cell discharges. The cathode in Li/SOCl2 cells is a porous carbon, such as acetylene black, and a PTEE binder. An aluminum screen can be used as a current collector to electrically connect the cathode and the positive cell terminal. 

The flat cell is illustrated in Fig. 8.5. In this construction, a duplex electrode is formed by coating a zinc plate with either a carbon-filled conductive paint or laminating it to a carbon-filled conductive plastic film. Either coating provides electrical contact to the zinc anode, isolates the zinc from the cathode of the next cell, and performs the function of cathode collector. The collector function is the same as that performed by the carbon rod in cylindrical cells. When the conductive paint method is used, an adhesive must be placed onto the painted side of the zinc prior to assembly to effectively seal the painted surface directly to the vinyl band to encapsulate the cell. No expansion chamber or carbon rod is used as in the cylindrical cell. The use of conductive polyisobutylene film laminated to the zinc instead of the conductive paint and adhesive usually results in improved sealing to the vinyl however, the film typically occupies more volume than the paint and adhesive design. These methods of construction readily lend themselves to the assembly of multi-cell batteries. 

The top and bottom of most batteries are capped with shiny, tin-plated steel (or brass) terminals to aid conductivity, prevent exposure of any zinc and in many designs enhance the appearance of the cell. Some of the bottom covers are swaged onto the zinc can, others are locked into paper jackets or captured under the jacket crimp. Top covers are almost always fitted onto the carbon electrode with interference. All of the designs try to minimize the electrical contact resistance. 

The "classical" Leclanche cell uses zinc sheet formed into a cylindrical can serving simultaneously as the anode and as the cell container (AB1C1). The cathode is a mixture of Mn02 and graphite wrapped into a piece of separator and contacted by a central carbon rod. The can dissolves slowly when the cell is not in use and faster when the cell delivers electrical energy. The reaction following the primary electrochemical zinc dissolution [Eq. (19)] leads, in the case of an ammonium chloride electrolyte, to a zinc diammine cation ... 

Dry-Cell Battery This is a common galvanic cell which contains a moist ammonium chloride electrolyte. An schematic battery is shown in Figure 1.7. The zinc casing and the solid carbon in contact with the electrolyte (electric conductor) develop a potential difference, which in turn, produces an electron flow when the zinc and carbon are electrically connected. Consequently, the zinc eventually conodes galvanically since it provides the electrons to the electrolyte for generating reduction reactions. The electrolyte (moist paste) carries the current from the zinc anode to the carbon cathode. 

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