Classification of Cells

Classification of Cells Points : Classification of Cells, Primary cells, Secondary cells, Dry Cell, Leclanche Cell, Voltaic Cell, Daniel Cell, Zinc-chloride Cell, Magnesium Dry-Cell Batteries, Alakline-Mn02 Cell (Zn-MnO₂), Lithium-Sulfur Dioxide Cell The battery cells may be classified into two general classes:
(i) Primary cells
(ii) Secondary cells (i) Primary Cells Primary cells are those cells in which chemical substance is used to produce e.m.f. by chemical action. Primary cells are those cells that are not rechargeable. That is the chemical reaction that occurs during discharge is not easily reversed. When the chemicals used in the reaction are all used up as electric energy is drawn from them the cells is fully discharged. A new cell must then replace it. The most common used primary cells are as given below:
1. Dry Cell
2. Leclanche Cell
3. Voltaic Cell
4. Daniel Cell
5. Zinc-chloride Cell
6. Magnesium Dry-Cell Batteries
7. Alakline-Mn02 Cell (Zn-MnO₂)
8. Lithium-Sulfur Dioxide Cell 1. Dry cell This cell is the modification of Laclanche cell. This is really not ‘dry’; if it were, it would not work. It consists of a zinc cylinder, Zn in Fig next to which is a paste, W, composed of plaster of paris flour, zinc chloride, sal.ammoniac and water. Adjoining this is a second paste, B is carbon, manganese dioxide, zinc chloride, ammoniac and water, C in Fig is a rod carbon forming the high potential element of the cell (i.e., +ve). This whole is covered with a case of millboard, is sealed with pitch, and provided with a vent for the escape of gas. The E.M.F., is about 1.5 volts.

2. Voltaic Cell It consist of a glass vessel containing dilute sulphuric acid with copper and zinc rods as electrodes. In case copper becomes positive relative to zinc. If the circuit is complete by joining the electrodes with a conductor, the current flows. Sulphuric acid decomposes into hydrogen and sulphate ions. The hydrogen travels in the direction of current and adheres to the copper electrode in the form of small bubbles. Sulphate ions attack the zinc; forming zinc sulphate(ZnSO₄). Thus, it is zinc plate with which the chemical action of the acid takes place.

Zn+ H₂S0₄ = ZnSO₄ + H₂

The current flows from zinc to copper inside the cell and from copper to zinc out side the cell. This is why the copper plate forms anode and zinc plate cathode of the cell. The cell suffers from certain defects such as local action and polarization. In the following cells these defects have been over come.

Local Action When the zinc rod is placed within the cell, it keeps on dissolving, even when it is not connected to the external circuit. This is due to the impurities like iron and lead, etc. being present within the commercial zinc.. These impurities from small tiny cells which are short-circuited by the main body of the zinc rod. This action of the tiny cells cannot be controlled, so there is a wastage of zinc. This process is known as the local action. This defect can be removed by amalgamating the zinc rod, or by rubbing the mercury over the zinc rod. Polarization When the external circuit of the cell is completed, we find that the value of the current goes on decreasing. The decreasing of current in the external circuit is due to, the collection of hydrogen bubbles on the surface of the copper plate. The effect of the hydrogen bubbles. (1) It acts as an insulator and hence increases the internal resistance of the cell. (2) Sticking hydrogen ions on the +ve plate exert a repulsive force on the other hydrogen ions coming towards the copper plate. Hence, the current is reduced. The action of the cell is known as the polarizatIon. The method used to minimize the action is to surround the cathode by a solid or liquid depolarizer which oxidizes the hydrogen bubbles as soon as these are produced. 3. Daniel Cell It consists of a copper plate (+) dipped in a saturated solution of copper sulphate which acts as a depolarizer and is contained in a glass or earthenware vessel. An amalgamated zinc rod (-ye) is placed in a porous pot containing diluted sulphuric acid. If the copper plate and zinc rod are connected by a wire, an electric current flows from copper through the wire to zinc and the following reaction takes place:

4. Laclanche Cell In the Laclanche Cell carbon and (+ve) is, placed in porous pot and is packed round with a mixture of powdered manganese dioxide and graphite which acts as a polarizer. The zinc rod (-ve) is placed in the saturated solution of ammonium chloride contained in an enamel-coated jar. The NH₄CI solution reacts with zinc rod when the circuit is connected to an external load.

Zn + 2NH₄Cl = ZnCI₂ + 2NH₃ + H₂
2H + 2MnO₂ = Mn₂O₃ + H₂0

Mn₂O₃ is further oxidized to MnO₂ by atmospheric oxygen. The E.M.F. is 1.46 volts The action of depolarizer is very slow and the internal resistance is very high. The cell is suitable for small and intermittent currents such as for telephone exchanges.
In this cell polarization is minimized by using manganese dioxide (MnO₂) as a depolarizer which is kept in the porous pot:

5. Zinc Chloride Cell A recent modification of the Leclanche cell is the zinc chloride electrolyte cell. The construction is similar to the conventional carbon-zinc cell but the electrolyte contains only zinc chloride, without the saturated solution of ammonium chloride. The zinc chloride cell is a high-performance cell with improved high-rate and low temperature performance and a reduced incidence of leakage. A comparison of the performance of the zinc chloride cell with the conventional cell is presented in figure. 6. Magnesium Dry-Cell Batteries The magnesium battery was developed for military use and has two principal advantages over the zinc dry cell; (i) it has twice the capacity or service life of an equivalently sized zinc cell, and (2) it can retain this capacity during storage, even at elevated temperatures. The construction of the magnesium dry cell is similar to that of the cylindrical zinc cell, except that a magnesium can is used instead of the zinc container.

The magnesium cell has a mechanical vent for the escape of hydrogen gas, which forms as a result of a parasitic reaction during the discharge of the battery. Magnesium batteries have not been fabricated successfully in flat-cell designs.

The good shelf life of the magnesium battery results from a film which forms on the inside of the magnesium can, preventing corrosion. This film, however, is responsible for a delay in the battery’s ability to deliver full output voltage after it is placed under load. The delay is usually less than 0.3 s but can be longer at low temperatures and high current drains.

7. AlaklIne-MnO₂ Cell (Zn-MnO₂) The zinc-alkaline-MnO₂ cell uses the same electrochemically active materials, zinc and manganese dioxide, as the Leclanche cell but differs in construction and in the use of highly conductive potassium hydroxide electrolyte which result in a lower internal resistance. The advantage on low-rate or intermittent discharge is marginal, but on high and continuous drain conditions, the alkaline cell can deliver from 2 to 10 times the ampere-hour capacity of the Leclanche cell. Its performance at low temperatures is superior to other commercially available dry batteries, operating at temperatures as low as —25°C. The electrolyte undergoes no change during the discharge, maintaining its high conductivity throughout the cell’s life. It thus differs from the Leclanche cell, whose resistance increases during the discharge. Typical discharge curves are given in figure, The shelf life of the alkaline-Mn02 cell is moderately superior to that of the Leclanche cell. Capacity retention is about 90% after 1 year of storage. 8. Lithium-Sulfur Dioxide Cell This lithium primary ccli uses sulfur dioxide for the cathode material and an electrolyte consisting of acetonitrile and lithium bromide, The cell is typically fabricated In a cylindrical hermetically sealed structure, A Jelly-roll construction is used, made by spirally winding strips of lithium ribbon, a polypropylene separator, and the cathode electrode (a Tenon-carbon mix pressed on an aluminum screen). This design provides the high surface area and low cell resistance which are necessary to obtain high current and low-temperature performance.

The good shelf life of the lithium-SO₂ cell is attributed to, the protective film formed by the initial reaction of lithium and SO₂, which prevents further reaction or loss of capacity on stand. During discharge, the SO₂ is depleted and the cell pressure reduced.

(ii) Secondary Cells Secondary cells are those cells that are first charged from external source storing up electrical energy in the form of chemical energy. Hence secondary cells may be discharged and recharged many times. The number of discharge, charge cycles a cell can withstand depends upon the type of size of the cell and on the operating conditions. The number of cycles will vary from fewer than 100 to many thousands. When the cell becomes discharged forcing current back into it from an external source recharges it.
The most common used secondary cells are as given below:
(i) Rechargeable alkaline
(a) Nickel cadmium
(b) Nickel-iron or. Edison Cell 1. Rechargeable Alkaline In alkaline cell the electrolyte solution is of potassium hydroxide. The positive and negative plates are immersed in electrolyte. The two kinds of alkaline batteries which hare in general uses are:
(a) Nickel Iron Cell also know n as the Edison Cell.
(b) Nickel Cadmium Cell also known as Junger Cell.

As far as construction is concerned, constructional there is no difference between the two cells except that the negative plate of nickel iron cell is made up of iron and in the nickel cadmium cell, it is of cadmium. Alkaline cells produce about 1.5 volt. The cell voltage decreases gradually as the cell is discharged. (a) Nickel Iron Battery (Edison Cell) The nickel-iron battery is a storage battery having a nickel (III) oxide-hydroxide cathode and an iron anode, with an electrolyte of potassium hydroxide. The active materials are held in nickel-plated steel tubes or perforated pockets. The nominal cell voltage is 1 .2V. It is a very robust battery which is tolerant of abuse, (overcharge, over discharge, short-circuiting and thermal shock) and can have very long life even if so treated. It is often used in backup situations where it can be continuously charged and can last for more than 20 years. Its limitations, namely, low specific energy, poor charge retention, and poor low-temperature performance, and its high cost of manufacture compared with the lead-acid battery led to a decline in usage along with it having the lowest energy-to-weight ratio.

The ability of these batteries to survive frequent cycling is due to the low solubility of the reactants in the electrolyte. The formation of metallic iron during charge is slow because of the low solubility of the Fe304, which is good and bad. It is good because the slow formation of iron crystals preserves the electrodes, bad because it limits the high rate performance: these cells charge slowly, and are only able to discharge slowly.

Nickel-iron batteries have long been used in European mining operations because of their ability to withstand vibrations, high temperatures and other physical stress. They are being examined again for use in wind and solar power systems and for modem electric vehicle applications.

In many respects the Nickel/Iron battery was almost “too good’. A battery that lasts for decades in many cases can outlast the equipment that is was originally designed to power. So from an economic standpoint lead acid, NiCd and other technologies have been deemed “good enough” and are the predominant technologies in use today even though they do not last as long as a Nickel/Iron counterpart.