Electronic Design

Li-Polymer: Practical, Or Just Promising?

Almost five years have passed since Sony began large-scale commercialization of Li-polymer technology with the introduction of its 3.2- by 53- by 85-mm cell. Since then, a number of other cell makers have joined them in producing Li-polymer. But so far, all of these cells have captured only a small fraction of the market for rechargeable batteries. The slow growth in the adoption of Li-polymer batteries has tempered initial enthusiasm that surrounded its introduction. Some question its current value, even as they look forward to its future development.

Using a polymer or gelled form of electrolyte, rather than the liquid electrolyte found in standard Li-ion cells, has many potential advantages. A gelled electrolyte eliminates the need for the metal that a liquid electrolyte requires for generating the stack pressure within the cell. So, Li-polymer cells could be encased in the very thin foil pouches that help to reduce cell thickness, and are simpler and less expensive to make than aluminum or steel cans.

But stack pressure wasn't the only concern in changing cell packaging from a metal can to a foil pouch. Leakage was another issue addressed via the gelled electrolyte. By eliminating liquid electrolyte within the cell, the Li-polymer technology would prevent leakage in the event of a punctured package. So, Li-polymer cells could be housed in foil pouches, while Li-ion cells would generally require the more durable cans.

Eliminating electrolyte leakage—ideally via fully solid polymer electrolyte—also raises the possibility that cell protection devices, either the PTC or IC, can be eliminated. This reduces cost and frees up space for active battery materials.

However, it's thin cell design and light weight are key benefits of Li-polymer and the polymer approach to thinning the cell's internal construction and packaging. Within the cell, the polymer electrolyte permits layering of electrode, electrolyte, and separators in a flat sandwich-like stack, as opposed to the wound stack or "jellyroll" normally employed in Li-ion cells. The stacked approach allows fabrication of 1-mm or thinner cells.

These characteristics endow Li-ion polymer cells with the potential for great design flexibility. In theory, cell makers could produce Li-polymer cells in varying voltages and capacities, with varying thicknesses, different form factors, and custom shapes. So, cell manufacturers are pursuing development of Li-polymer cells with the hope that their flexibility will open doors to new applications.

But the promise of Li-polymer has yet to be realized. Michael Mummert, senior product manager of GS-Melcotec, has noted, "Most manufacturers are relying on gel chemistry approaching a solid. Therefore, protection circuitry remains a requirement due to outgassing at high temperatures."

"Custom sizes and configurations haven't been achieved either. Even when the advent of solid polymer arrives, the costs of these custom formed and configured cells will be far too expensive to meet the market cost requirements. Mass production of the cells keeps price points and competition keen to allow wireless manufacturers competitive pricing."

At the moment, Li-polymer technology has lost some of its luster because thinness—one of Li-polymer's main claims to fame—doesn't seem like such a great advantage now that Li-ion cells are available in 3- to 4-mm thicknesses. Although Li-polymer cells can be manufactured in even thinner dimensions, no great demand for such cells exists yet.

Despite the current obstacles to Li-polymer deployment, GS-Melcotec and other vendors continue to commercialize the technology, confident that the ideal of the Li-polymer cell can ultimately be achieved. Probably its biggest proponent is Sony, the company that first ramped up production for Li-polymer. It now offers at least 13 models in a range of sizes and capacities.

Sony's Sage Nishimura cites some reasons why Sony still views Li-polymer as the better technology. Although, Li-ion and Li-polymer now offer almost the same energy density, Nishimura claims that the capacity of Li-polymer cells could easily rise another 50% in the next few years, pushing its performance ahead of Li-ion.

Safety is another concern. While vendors strive to increase cell capacity, they also are trying to make the cell safer, particularly because the newer cell contains more energy. According to Nishimura, use of polymer electrolyte enables Sony to offer the safest battery.

Some recent consumer product introductions serve as proof of the industry's interest in Li-polymer. Two major laptop makers have already introduced models powered by Li-polymer batteries, and Apple's iPod MP3 player contains one as well. In addition, Li-polymer also is being considered for more futuristic applications, such as wearable devices.

Some companies also see hope for Li-polymer in niche applications that some of the big cell vendors may not have addressed. One company, NewTurn Energ of Soowon Kyonggi-Do, Korea, plans to develop custom Li-polymer batteries in low volumes. These batteries are expected to have odd sizes and shapes with larger than usual capacities above 2500 mAh.

Another vendor, Electrovaya (formerly Electrofuel) of Mississauga, Ontario, is pursuing niche battery markets by developing its Li-polymer technology into standard and custom products with very high energy density—better than 200 Wh/kg (gravimetric) and up to 500 Wh/l (volumetric) (see Table 2 of "Li-Ion Batteries Reach For Higher Performance," p. 62).

Despite the high performance numbers, the materials employed don't appear to be anything exotic. Electrovaya's Li-polymer cells contain graphite anodes and lithium-cobalt-oxide cathodes, as do most other cells. But as President Sankar Das Gupta notes, "We're pushing the technology closer to its theoretical limits."

As standard product, the company offers external batteries that extend the runtimes of laptops by adding 160 or 120 Wh of capacity (PowerPad 160/120) with packs that weigh less than 2.5 and 2 lbs, respectively. Beyond these products, the company wants to be a flexible custom cell manufacturer. According to Das Gupta, the company designs its own production equipment. Using this flexibility, Li-polymer batteries can be built with capacities in the tens of ampere-hours for a range of applications.

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