[Technology Report]
Li-Ion Batteries Reach For Higher Performance
Better chemistry and packaging reduce cell thickness to below 3 mm while boosting the capacity of already popular sizes.
Over the last 10 years, lithium-ion (Li-ion) batteries have emerged as the high-performance choice for powering cell phones and notebook computers. Their high energy density has enabled them to supplant NiMH and NiCd in many new designs, despite their higher cost and special requirements for cell protection and packaging. First introduced in cylindrical sizes like the popular 18650, Li-ion cells subsequently appeared in flat, rectangular cases, called prismatics, as a way to optimize the package for cell phones and notebooks.
As Li-ion cell manufacturers have put these batteries into production, they have steadily improved their performance. Enhancements in cell chemistry—better cathode and anode electrode materials, electrolytes, and separators—have boosted energy capacity, improved cell safety, and lowered cost. For example, when the 18650 cylindrical was introduced in 1992, it provided 1000 mAh of capacity and cost about $10. Today, the same size cell offers capacities of 2000 mAh (or higher) for $2.50.1
Nevertheless, in recent years, much of the focus has been on prismatics. With improvements in cell chemistry and packaging, these prismatic Li-ion cells have migrated from thicknesses of 8 and 10 mm to under 3 mm. During the last few years, the quest for thin cells has fostered Li-polymer cell development. They employ a gelled or polymer electrolyte, rather than the liquid electrolyte used in standard Li-ion cells.
Using a gelled electrolyte in Li-polymer cells eliminates the need for a case that provides stack pressure within the cell, while reducing worries of leakage if the foil is punctured. As a result, Li-polymer cells can be encased in aluminum foil laminate pouches that are just 0.1 mm thick, rather than the 0.25- to 0.4-mm thick aluminum or steel cans traditionally used with Li-ion cells.
Another benefit of Li-polymer cells is the ability to construct them by stacking electrode and electrolyte materials in a flat sandwich, rather than winding them in a jellyroll fashion as is done with Li-ion cells.2 The stacked cell structure, in combination with the foil pouch packaging, makes it possible to build Li-polymer cells thinner than 1 mm, although many recently developed thin cells are in the 3- to 4-mm range.
At those thicknesses, Li-polymer faces competition with Li-ion cells, which currently offer equal or better performance less expensively. (See the sidebar, "Li-Polymer: Practical, Or Just Promising?" in the online edition of this article at www.elecdesign.com.) But despite these limitations, development of Li-polymer cells continues as vendors seek to exploit its potential for building thin cells in custom shapes and sizes.
Recent Developments: Li-ion and Li-polymer cell makers are continually striving to develop batteries with higher levels of energy density to meet the growing power demands of cell phones and notebook computers. In particular, 3G phones are expected to increase cell capacity requirements and accelerate the usage of Li-ion cells in cellular handsets. In addition, Li-ion cells are being developed for PDAs and Bluetooth devices.
Naturally, different applications demand different form factors and capacities. For example, cell phones may require around 500 to 700 mAh; PDAs, perhaps 600 to 1800 mAh; and notebook computers, which often rely on cylindrical cells, might need a total of 6000 mAh.
On the other hand, a Bluetooth headset may only call for between 150 and 200 mAh. Designers working in other areas could benefit from the popularity of the cells created for these mainstream products. So, while much of the cell and battery pack development is custom work, many projects may be able to adapt readily available cells, like the 18650 or the 6-mm prismatics, to suit their purposes. (See the online sidebar, "Li-Ion Availability.")
If the first decade of Li-ion commercialization is any guide, these popular applications will probably keep driving Li-ion performance higher. The latest generation of 3- to 4-mm thick devices includes cells that approach or exceed 400 Wh/l in volumetric energy density and 200 Wh/kg in gravimetric energy density. (See Tables 1 and 2 for lists of recently introduced cells.)3
Among cylindrical cells, such as the 18650, energy density is even better. For example, Gold Peak Industries offers a 2100-mAh 18650 that boasts 492 Wh/l, while Toshiba offers a 2200-mAh model with 470 Wh/l and 190 Wh/kg. (Small differences in cell dimensions may account for the higher energy density of the lower-capacity 18650.) Meanwhile, Sony plans to release a 2150-mAh 18650 this year or next year, with a 2500-mAh version to follow by 2004.
In general, the industry should continue making modest, steady gains in the future. Toshiba has charted the growth in energy density since Li-ion's introduction. The company projects a steady 9% to 10% increase in capacity over the coming decade.
Within a few years, that progress should yield cylindrical and prismatic cells with about 500 Wh/l (Fig. 1). Meanwhile, increasing production and growing competition among vendors should drive down the cost of Li-ion and Li-polymer cells to the point where cylindrical style Li-ion cells will be comparable to NiMH in terms of dollars per watt-hour (Fig. 2).
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