Electronic Design

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).

Changes in material systems are responsible for some of the capacity enhancements, including one by Toshiba. For its negative (anode) electrode material, the company is moving from existing graphite materials to higher-capacity graphite materials, and eventually to "nanocomposite" graphite, to achieve greater packing density for the anode. At the same time, the lithium-cobalt oxide material used for the positive (cathode) electrode material is being enhanced with greater boron doping.

But Toshiba anticipates even greater capacity improvements when it re-places its lithium-cobalt oxide-based electrodes with nickel-cobalt hybrids. In the 18650 cell, the result will be a 2300-mAh cell that achieves 480 Wh/l and 195 Wh/kg. That cell is expected in the fourth quarter of this year.

Further improvements should be obtained by migrating to a positive electrode based on nickel-manganese oxide and, ultimately, an all-nickel positive electrode. The all-nickel cathode should boost cell capacity and lower cost because it will replace more expensive cobalt with an inherently safer metal.

Now the company is developing a nickel-based cell that could provide 540 Wh/l. But the operating voltage will be only 2.8 V, instead of the 3.7 V normally associated with Li-ions.

Toshiba's Li-ion technology actually contains a polymer-style electrolyte. But the company shies away from calling its cells Li-polymers because they contain trace amounts of liquid electrolyte. This reduces the cell's electrical resistance. It also improves low temperature performance, and reduces swelling.

The application of small amounts of liquid electrolyte in Li-polymer cells is considered a common practice. That may change, though, as vendors attempt to build cells with greater resistance to leakage.

Typically with Li-polymer cells, manufacturers convert the polymer from liquid into gel form, then apply it along with the separator between the positive and negative electrodes as the cell layers are wound. This approach requires that a small amount of liquid electrolyte be inserted into the electrodes to achieve the necessary electrical conductivity.

However, Sanyo has created a technique for gelling the electrolyte within the battery case. After the cell electrodes and separators are wound, a solution of Li-salt, solvent, and prepolymer material is poured into the battery case. This is then heated to form a completely gelled electrolyte. In addition to promising better leakage protection, the Sanyo polymer electrolyte is said to maintain good discharge characteristics.

Vendors also are making strides into the area of cell packaging. Although thin foil packages have been adopted in both Li-polymer and Li-ion cell designs, Hitachi Maxell has released a cell that features foil-like thinness and can-like ruggedness. The vendor has exploited an aluminum alloy that contains greater than 4.5% magnesium to produce a rigid material with just 0.15-mm thickness. That's almost as thin as a foil pouch, but unlike the pouch, Hitachi Maxell's metal can resists bending, cracking, and piercing.

Using this metal can, the company has produced a 2.8- by 34- by 65-mm Li-ion prismatic that provides just over 600 mAh of capacity (Table 1, again). That's deemed just enough for cell phones with fewer features. Nevertheless, the cell's capacity can be modified by changing length and width dimensions. (For a description of how this cell is fabricated, read the online sidebar, "Canning Thin Li-Ion Cells.")

The availability of Li-ion cells with thicknesses below 3 mm creates competition with Li-polymer cells whose main claim to fame is their thinness. Moreover, others are developing Li-ion cells almost that thin. Panasonic plans to offer Li-ion cells housed in aluminum cans with dimensions as low as 4.3 mm this year.

Meanwhile, NEC Electronics continues to exploit manganese-based cathodes (less common than the cobalt-based cells) to build Li-ion cells in foil packages. Among the vendor's recent introductions are two Bluetooth oriented cells—a 3.9-mm thick Li-polymer cell with 150-mAh capacity, and a 5.6-mm thick version with 300 mAh. The company plans to develop this Li-ion technology further and introduce cells with 4- to 8-mm thicknesses.

Another packaging development comes from GS-Melcotec. Originally offered in the foil pouch, its LY series of Li-polymer cells is now being produced in a Buttercup-shaped aluminum-laminated film case. The squared-off shape fits better than the pouch in some designs.

The availability of coin-type cells is another packaging twist seen in recent product introductions. Panasonic, for one, has developed a Li-ion coin cell that offers 130 mAh in a 30-mm (diameter) by 3.2-mm (thick) cell, the CGL3032. Another vendor, Korea Power Cell (www.powercellkorea.com), has announced plans to offer a Li-polymer of the same dimensions that will deliver 180 mAh (PD3032 or PowerDisc).

References
1. "Worldwide Battery Market Status and Forecast," presented at the Power 2001 Conference by Hideo Takeshita, vice president of the Institute of Information Technology Ltd.; [email protected]

2. For a description of Li-ion and Li-polymer charge/discharge methods and cell structures, see Sony's Li-ion Rechargeable Battery Catalog, p. 3, at www.sony.co.jp/en/Products/BAT/ION/index.html.

3. For some perspective on existing performance levels, read "Thinner Li-Ion Batteries Power Next-Generation Portable Devices," Electronic Design, Feb. 7, 2000, p. 95-106; available online at www.elecdesign.com.


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