Versatile battery suffers from unstable chemistry

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Demand for rechargeable lithium batteries has soared in recent years because of surging sales of portable PCs, mobile phones and wireless email devices such as the BlackBerry, PDAs (personal digital assistants), digital cameras and video camcorders and digital music players.

Portable PCs account for about 50 per cent of lithium-ion battery sales and Sony is the second-largest manufacturer of lithium-ion batteries for notebook computers after Sanyo Electric, with about a 25 per cent global market share. Matsushita is another big supplier.

Rechargeable batteries based on lithium-ion (Li-ion) technology, which were first commercialised by Sony in 1991, have largely replaced earlier technologies such as nickel metal hydride (NMh) and nickel cadmium because of their superior energy-to-weight ratios, ability to be formed into a wide range of shapes and sizes, and lack of so-called ‘memory effect’ which afflicts other battery types. They also have a low self-discharge rate of around 5 per cent per month, compared with 20 per cent per month or more for other rechargeables.

Unfortunately, however, the chemistry of lithium-ion power packs is inherently unstable. They are also difficult to manufacture, can be dangerous if mistreated and may have a shorter lifespan than other battery types unless they are managed carefully. As a result they have a number of mandatory safety mechanisms built into them – but very occasionally even these systems fail.

A unique and little publicised drawback of the Li-ion battery is that its lifespan is dependent on ageing from time of manufacturing, regardless of whether it was charged, and not just on the number of charge/discharge cycles.

More advanced lithium-ion polymer (Li-Poly) batteries have evolved from lithium-ion batteries and have a number of significant advantages over traditional lithium-ion design, including the fact that the solid polymer electrolyte is not flammable (unlike the organic solvent that the Li-Ion cell uses).

That means they are less hazardous if mistreated and are taking a growing chunk of the rechargeable battery market. Leading battery makers such as Panasonic, Sanyo, Sony and Toshiba have all been ramping up Li-polymer cell production.

Lithium-ion polymer batteries debuted in consumer electronics about 1996. They are used in an expanding range of devices including Apple Computer’s family of iPod digital music players.

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