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The credit for developing thin film batteries goes to a team of scientists lead by Dr. John Bates. They conducted, for over a decade, research at the Oak Ridge National Laboratory for the development of a thin film battery. Conventional batteries are bulky and non-flexible, making them unsuitable for use where space is a constraint. Another factor is the energy to weight ratio, which is quite low for conventional batteries.
Features which are specific to the thin film batteries are the all solid state construction. They can be formed in any shape or size and are completely safe under any operating conditions. These specific batteries can also be used under a wider operating temperature range. Because of their all solid state construction, thin film batteries can stand temperatures as high as 280 degrees centigrade or 586 degrees Fahrenheit without failure.
This makes thin film batteries amenable to be soldered along with other electronic components in a solder re-flow process for assembly of electronic circuits. In this process, all the components are heated to a temperature at which solder typically melts and flows to bond each component to the printed circuit board. As this temperature is about 250-280 degrees Celsius, 482-586 degrees Fahrenheit, conventional batteries containing organic liquid compounds are unable to survive, and therefore have to be added manually, after the assembly has had time to cool down. This unique feature of thin film batteries has earned them the name electronics battery.
The construction of a thin film battery is very simple. Different layers are deposited by evaporation or sputtering, a method commonly used in the semiconductor manufacturing industry. The cathode is usually a large surface and is covered on the top with a layer of electrolyte over which the anode is deposited. The electrolytic layer isolates the entire cathode from the anode. A base or substrate at the bottom, and a packaging on the top, protects the battery from damage. Depending on the substrate and packaging method, the total thickness of the battery could be as thin as 0.35 mm to 0.62 mm. On account of the battery being able to be manufactured in any shape and size, any specific space, energy and power capabilities can be targeted.
An electronics battery is capable of delivering electricity with high current densities because of the good cathode utilization. The current density, and hence, the discharge capacity, are dependent on the area of the cathode. With a good cathode size, the thin film battery can boast a high energy output at a specified discharge rate.
A practical example of a thin film battery is a lithium battery. The anode is metallic lithium, with a lithium cobalt oxide cathode. This arrangement makes for rechargeable batteries, on which can be charged up to 4.2 volts, and can be discharged down to 3.0 volts, repeatedly. The capacity of lithium ion batteries is expressed as the amount of current that the battery can deliver in a specified time in hours, and denoted by AH or mAH. The energy of thin film batteries is given as the product of the voltage and the charge supplied by it, expressed in WH or mWH.