TOKYO — With their thin, liquidless structure, polymer batteries have been gaining attention as the next-generation line of rechargeables. But as Japan's major battery vendors ready production of low-profile, liquidless polymer batteries next year, they're groping for a clear advantage beyond just thinness to differentiate them from existing lithium-ion batteries.

Mitsubishi Electric Corp., working through a battery-supply problem last autumn, has already learned, for example, that it can maintain a low-profile product without a polymer battery. The company introduced a notebook PC, the Pedion, using polymer batteries in September 1997. It features the world's thinnest profile, 18 mm. But its hopes to use that low profile to move to the top of the market were shattered by problems with the polymer-battery supply. Sales leveled off at thousands of units, not the tens of thousands that had been envisioned, because of large back orders.

Last February, Mitsubishi replaced the 6.5-mm-thick polymer battery pack with lithium-ion batteries and started offering the notebook in volume.

"The battery pack was 6.5-mm thick from the beginning. So the thickness of the PC has not changed even if it uses a lithium-ion battery pack as long as it is 6.5-mm thick. We used the polymer battery because only the battery could achieve the 6.5-mm thinness three years ago when the PC was developed," said a Mitsubishi spokesman. Lithium-polymer batteries employ the same materials for electrodes as their lithium-ion cousins but replace the electrolytic solution with a gel.

That means that a polymer battery is not mandatory to realize the 18-mm profile.

"A lithium-polymer battery enables us to make the battery thin-3 mm or less-while maintaining energy capacity comparable to that of a lithium-ion battery," Satoshi Narukawa, senior manager at Sanyo's soft-energy development center, said. "But what is the most suitable application? A battery needs to have a certain capacity regardless of whether it is lithium-ion or polymer. To have high capacity, the battery becomes thick. Furthermore, the volumetric energy density should be disadvantageous for polymer because of the difference in encapsulation methods."

Sanyo has already begun production of lithium-ion batteries that are 5-mm thick, which "satisfies current demand for thinness," said Narukawa.

Narukawa listed cellular phones and simple-function PDAs as possible applications for polymer batteries. But, he said, "unless we can add some new value to polymer batteries, the battery market won't take off." He declined to discuss Sanyo's production plans, though he said Sanyo is ready to supply samples anytime.

Sony, Sanyo and Matsushita Battery now dominate the lithium-ion battery market with more than an 80 percent share.

Sony recently exhibited prototypes of lithium-polymer batteries-one is a magazine-size sheet and another is credit-card sized-and conducted a survey asking visitors about the best applications. Sony plans to begin marketing the battery next year. A Sony engineer at the exhibit said, "Sony is going to offer lithium-ion and polymer batteries to different market segments depending on the shape that the segment requires." But he did not disclose what application Sony intends to target.

The B5 (magazine-size) prototype is a 2-mm thick sheet form battery with a 7-ampere-hour (Ah) capacity. "The capacity is sufficiently competitive with lithium-ion batteries used for notebook PCs," he said. The credit-card-size prototype has 470-mAh capacity, which is close to 600 mAh, an average capacity of batteries for cellular phones, he said.

The sheet-type battery, however, raises a question about its suitability for notebook applications. Thin notebook PCs now have a battery pack attached outside. Heat dissipation is one of most important considerations in notebook design. Yet, designers point out, if a sheet-type battery covers one side of a notebook PC, it would make efficient heat radiation difficult to design.

Matsushita Battery plans to begin shipping polymer batteries next April, but announced no dates for volume production. "Nothing has been decided yet. Generally speaking, it takes a half year to one year from the supply of samples to volume production. Unless we get about 100 customers, we don't begin volume production," a spokesman said.

Lithium-polymer batteries will give manufacturers a chance to enter the rechargeable market now dominated by Hitachi Maxell Ltd., Toshiba Corp. and Yuasa Corp.

Hitachi Maxell developed a credit-card-sized prototype in April 1997 and has just begun sampling larger-sized batteries targeting at PDA and note PC applications. The company is preparing a line with a capacity of 30,000 units a month slated to begin operation next spring.

Toshiba plans to offer the battery next fall. The startup application will focus on mini PCs like Libretto, said a spokesman.

Yuasa provided a credit-card-sized prototype for a "solid-state audio" player that was proposed by NTT and Kobe Steel (see July 27, page 4). The battery, 1.35 mm thick, helped to realize the 8-mm thick flash audio player. The polymer battery has a volume of 5 cc and weighs 8 grams. It has an energy capacity of 175 mAh and a 3.6-V output.

The solid-audio project, however, will be launched next year or later. Yuasa is now working with potential customers to develop practical applications for polymer batteries. "Polymer batteries will not have standard sizes, at least at the beginning. So we are working with customers to satisfy their needs," said a Yuasa spokesman.

Yuasa is also promoting a thin, non-rechargeable lithium polymer battery. The sample battery is postage-stamp-sized and 0.3 mm thick.

"We've got a lot of inquiries about the possibility of applications that we had not imagined," said a spokesman. For example, a battery could be pasted to a concert ticket with an audio IC to provide a pre-concert. For the rechargeable version, however, "Just to pursue thinness will limit the applications. Rather we want to stress the safety-the polymer battery does not use a liquid that causes troubles."