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Battery Types & Chemistries

A battery can be crudely described as a vessel containing various chemicals which interact to produce electricity. The reactions are called electrochemical reactions and are explained by Faraday’s Law of Electrolysis. The earliest recorded version of the battery was created by Alessandro Volta between 1780 to 1800. He created the battery using alternating layers of zinc, sliver and blotting paper that had been soaked in salt water. This is known as a voltaic pile.

There are several types of battery used today. The most common is a dry zinc-carbon battery (discussed below). To demonstrate the electrolytic reaction that takes place in the cell, a simple wet zinc-carbon battery (Leclanché cell) can be made in the laboratory using dilute sulphuric acid as an electrolyte solution, a carbon anode (+ve electrode) and a zinc cathode (-ve electrode). The electrodes (normally rods) are immersed into the sulphuric acid without touching. At this point the battery is an open circuit and will not produce an electro motive force (emf). Whilst is open circuit a small reaction can be seen at the cathode, this is caused by the sulphuric acid oxidising the zinc anode producing the visible hydrogen gas bubbles. By placing a wire between the two electrodes a closed circuit is made and emf will be created. The emf can be measured in volts by placing a voltmeter across the two electrodes. As mentioned briefly above, the emf is created by an electrolytic reaction between the negative cathode and positive anode. The zinc cathode is oxidised by the sulphuric acid releasing two positive electrons that travel through the wire to the anode making it positively charged (this is the opposite direction to previously thought) and two negative hydrogen ions that travel towards the anode in solution as zinc sulphate. When the negative hydrogen ions reach the now positive anode they combine with two electrons to produce hydrogen gas and deposit zinc metal onto the surface of the anode. The production of hydrogen and the deposition of zinc eventually reduces the emf potential of the cell. The hydrogen causes polarisation and the zinc deposition eventually causes the anode to effectively choke. These two side effects of electrolysis can be reduced by: a) coating the anode in another chemical that prevents the production of hydrogen using the redox reaction and b) using a chemically pure zinc anode or by adding other chemicals to the electrolyte to prevent the zinc deposition. The Leclanché cell has an open circuit shelf life because the zinc cathode is constantly oxidized by the sulphuric acid. Wet cells can be created using other electrolytic solutions and different electrodes that avoid this problem (eg. Daniel Cell).

The wet cell is impractical as a mobile power source because the electrolyte is prone to leakage and doesn’t function well when used in anything but the vertical plane. Modern dry cells are not truly dry but they do overcome the problems of a wet cell by using an acidic paste as the electrolyte. There are several different designs of dry cell, however they can all be classified as either a primary or secondary cell. Primary cells have a fixed emf potential and can only be used once, where as secondary cells can be recharged. A typical secondary cell is designed to be recharged several hundred times.

Primary

Zinc/Carbon Battery: Used in all inexpensive AA, C and D dry cell batteries. It has Zinc and Carbon as electrodes and an acidic paste as the electrolyte.
Alkaline Battery: Used in Duracell and Energizer batteries, it uses Zinc and Manganese dioxide in powdered form as the electrodes and an Alkaline, like Potassium hydroxide, as the electrolyte.
Lithium Cell: Used in cameras, calculators and pacemakers. Different Lithium cells exist because of its stability and low reactivity with a number of cathodes and nonaqueous electrolytes. The most common electrolytes are organic liquids.
Zinc/Air Cells: Used in pagers, hearing aids. Amalgamated Zinc powder and Oxygen (Air) are the electrodes and Potassium Hydroxide is used as the electrolyte. This cell is lightweight, environmentally friendly and has a relatively low cost.

Secondary

Lead Acid Battery: Used in cars. Lead and Lead oxide are used as electrodes with a very strong acidic electrolyte. This is rechargeable. The modern variation is known as Gel Cell, where the electrolyte is in a gelatin form.
Nickel/Cadmium Cells (NiCd): Used in Digital cameras, laptops, calculators. Cadmium and Nickel are used as electrodes and aqueous Potassium Hydroxide is used as the electrolyte. It is a rechargeable battery, but it is prone to the memory effect, where the cell retains the characteristics of the previous cycle. The image on the right shows the inside of a NiCd battery, showing the actual cells that make up the battery.
Nickel Metal Hydride (NiMh): Used in laptops, camcorders, mobile phones, power tools. Rare-earth or nickel alloys with many metals are used as the electrodes and Potassium Hydroxide is used as the electrolyte. This is rechargeable and is also prone to the memory effect, but less so than the Ni Cad cells. The image on the right is of a NiMH laptop battery showing the actual cells inside.
Lithium Ion Cells: Used in laptops and mobile phones. Carbon compound and Lithium Oxide are used as electrodes and organic solvents are used as the electrolyte. Has a very good power to weight ratio and is rechargeable. Check Apple's page on Lithium Ion batteries. The image on the right shows the cells and the microprocessor inside a Lithium Ion battery.
Lithium Ion Polymer Cells: Used in PDAs, handhelds, micro-models, MP3 players. The electrolyte used is a polymer. It resembles a plastic like film which does not conduct electricity, but allows the exchange of ions. The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile. There is no danger of flammability because no liquid or gelled electrolyte is used.

Specialist

Silver/Zinc Cells: Used in aeronautical and defence applications. Has a very good power to weight ratio.
Sodium/Sulphur Cells: Used in electric vehicles and aerospace applications such as satellites. Uses molten Sodium and Sulphur as electrodes and ceramic beta alumina as the electrolyte. This has been studied extensively for electric vehicles because of its inexpensive materials, high cycle life, and high specific energy and power. The problems with this cell are that the temperature has to be kept at 350C to keep the Sulphur in liquid form. This is achieved through insulation or heating through the cells own power. This lowers the energy density. The electrolyte is brittle and develops microfissures. Thus the liquid sodium and sulfur come in contact—with explosively violent results.