Rechargeable batteries are having to keep up with the pace being set by the mobile comms revolution. “No chemical reaction is 100 per cent reversible,” says Tony Gozdz, chief scientist at US firm Telcordia Technologies. This may be true but rechargable lithium ion (Liion) batteries – as used in portable computers, mobile phones & electronic cigarettes – make a pretty good stab at it, although there can be complications.
The accepted performance metric for rechargable batteries is that after 500 recharging cycles, the overall dip in performance is less than 20 per cent. “For each cycle this equates to a reversibility of 99.96 per cent – an incredibly small degree of degradation,” says Gozdz.
As head of battery R&D at Telcordia, formeriy Bellcore, improving battery technology is a topic close to Gozdz’s heart. What occupies him, and many other scientists at companies like Sony, Sanyo, Panasonic, Toshiba and Philips, are ways to squeeze Liion technology into ever slimmer packages. Liion is the predominant technology used in laptops. Being packed in cylindrical metal cases provides the necessary uniform pressure to hold the battery’s electrochemical materials together.
The drawback of the cylindrical shape is the space it takes up, especially when several are placed in series to achieve the 11 to 15V needed by notebooks. In turn, more than one bank is required to achieve an acceptable operating life. Overall a quarter of the overall volume is wasted. The development of fiat battery technology means there is more battery per unit volume. Refine the battery thickness and further possibilities arise.
Thomas & Betts, a licensee of Telcordia’s plastic Liion technology (Plion), is working on a large fiat battery to be placed behind a notebook’s screen. Placing it there not only results in a greater overall capacity, it also separates it from local hot-spots such as the PC’s microprocessor.
Mobile handsets, in contrast, embrace three battery types, each at a different stage in their evolution: nickel metal hydride (NiMH), Liion and Liion solid-polymer (lithium-polymer). Telcordia’s Plion is an example of the latter.
NiMH is the most mature and cheapest of the three. It also has the advantage of not needing any protection circuitry. For this reason NiMH AAA cells still claim the largest share of the European mobile phone market.
In Japan, the situation is different, says Daiichiro Eguchi, senior manager at Toshiba’s battery division. With Japan’s appetite for petite handsets – some 8mm thick and weighing as little as 55g – slimmer, lighter batteries are required. This is why battery makers are embracing Liion battery systems.
What Liion offers is a battery thickness down to 4mm. “It is very difficult to make Liion batteries below 4mm,” said Eguchi.
Typically a metal can is used to bind the internal electrodes and separator material. Although the can’s 0.6mm wall thickness sounds insignificant, when the battery is only 4mm, both walls – each 0.6mm – account for a significant proportion of the battery’s volume. Enter lithium-polymer. It is based on the same electrochemical active components as Liion, where it differs is in the battery’s construction.
It promises lightweight batteries with a thickness down to 1mm, enclosed in a thin aluminium bag. “Like a coffee bag,” says Gozdz. The aluminium sheath prevents chemical solvent leaking out and water vapour getting in, which degrades the battery.
Panasonic manufactures lithium-polymer technologies for mobile phone and portable computers. Stephen Evangelou, Panasonic’s battery sales group’s technical manager, says the company has already produced lithium polymer in limited volumes for mobile handsets. Volume production is expected by April next year.
Toshiba is another company developing lithium polymer technology. According to Eguchi, Toshiba is not close to releasing product. “We will wait one or two years,” says Eguchi, who admits that Toshiba is still having problems achieving the performance required at low temperatures. Meanwhile Toshiba is launching an advanced Liion battery which while using a gel, has a different chemical composition to lithium-polymer.
Philips, while not a battery manufacturer, is also investigating lithium-polymer battery technology at its Research Laboratories in Eindhoven. “It’s something we need to know about,” explained Dr. Hans Feil, general manager of Philips Lithylene. What Philips Research has done is develop a lithium-polymer manufacturing process which it claims extends the battery’s capacity.
Dubbed lithylene, the process uses rivets instead of a can to give the structure rigidity. Micro-holes are introduced into the electrodes and separator. These are then filled with polymer, which, when set, binds together the battery’s materials.
A prototype battery has already been demonstrated by Philips Research. Feil claims the lithylene battery will deliver 800mAh, while competitor batteries with the same volume offer only 600mAh. The battery form factor for the latest mobile phones is 62 x 35 x 3.6mm. Toshiba’s advanced Liion battery, to be launched next February, will deliver 610mA while Panasonic’s lithium-polymer battery of the same dimension will offer 550mAh.
Telcordia’s Plion technology uses a “totally different” manufacturing process to that of Philips’. It binds the layers together without the need for rivets. Gozdz claims that this aids the manufacturing process. The company has licensed its Plion battery technology to several companies, four of which have already been announced.
Philips is also making its lithylene technology available for licensing. “We haven’t licensed The technology yet but we already have some very serious candidates,” said Feil. The challenge facing all the companies pursuing lithium polymer is to make the manufacturing process reliable. It is not a technology or electrochemical issue, but rather the optimisation of equipment for volume manufacturing and its debugging, says Gozdz.
As he points out: “It took seven to eight years and hundreds of millions of cells to get lithium ion to the point where it was right.”