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5.5.2.1 Batteries

A battery is a device that converts chemical energy directly to electrical energy. It consists of one or more voltaic cells. Each voltaic cell consists of two half cells connected in series by a conductive electrolyte. One half-cell is the negative electrode (the cathode) and the other is the positive electrode (the anode). In the redox reaction that powers the battery, reduction occurs in the cathode, while oxidation occurs in the anode. The electrode needs to be from different materials. The electrodes do not touch each other but are electrically connected by the electrolyte, which can be either solid or liquid. In many cells, the materials are enclosed in a container, and a separator, which is porous to the electrolyte, prevents the electrodes from coming into contact.

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Figure: Li-ion battery diagram

There has been constant materials progress from the compact rechargeable NiCd (Nickel-Cadmium) batteries to NiMH (Nickel-Metal-Hydride) and then Li-ion (Lithium-Ion) and Lithium-polymer. This evolution corresponds to a progression of the power output and of the speed of the discharge/recharge cycles.

Li-ion battery presents the best characteristics as Li is the most electropositive and the lightest metal. Lithium is intercalated in another metal oxide (manganese, nickel, cobalt oxides) and constitutes the cathode. During charge phase, Li migrates to the anode (usually graphite) and when the battery is put under a load, Li ions are released from the anode to migrate back to the cathode. The electrolyte used is organic and therefore creates safety problems, which can be solved in Li-polymer batteries. [i]

[i] Road Maps for Nanotechnology in Energy; The Institute of Nanotechnology (2006) - http://www.nanoroadmap.it/