New Materials Extend Li-Ion Performance

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With little fanfare, Li-ion batteries continue to make inroads into new applications, while slowly reaching higher levels of performance in terms of capacity, safety and power delivery. Though these trends in Li-ion development have been ongoing for years, they take on added significance now that some of the existing Li-ion technology is reaching a performance plateau.

This plateau concerns the most popular Li-ion cells, those used to power cell phones, laptops and other handheld applications. First brought to market in the early 1990s, Li-ion cells have made steady gains in energy density, which led to regular increases in capacity for cells produced in popular formats such as the 18650 cylindrical (see the figure).

Until recently, cell makers have been able to raise cell capacity using existing electrode systems. However, it appears that vendors are now reaching the capacity limits they can achieve with existing cathode and anode materials, and they are starting to introduce cells that incorporate new cathodes and, in some cases, new anodes. For equipment designers, the changeover in materials will mean not only longer runtimes, but also changes in the battery's charge and discharge voltages.

As cell capacity and energy density rise, so too does concern for battery safety. With most new cell developments, vendors document the safety tests that have been performed on their cells. Some of this information can be found in the references listed at the end of the article. In some cases, new electrode materials are viewed as a means to improving the inherent safety of the cell.

Although most Li-ion cells are being used in low-power applications, their use in power tools and other high-power applications is growing. Cell makers have developed special product families to address these high-discharge-rate applications. Some of these developments will be discussed in next month's Analog Feedback.

Last Hurrah for Lithium-Cobalt Oxide?

Cell makers are now producing Li-ion cylindricals in the popular 18650 format with capacities as high as 2.6 Ah of capacity. These cells use the same lithium-cobalt oxide cathode materials and graphite anode materials used in previous generations of Li-ion cells. And as was true in previous generations, these cells attain their high capacity by packing more active materials into the same size package.

Customers currently pay more for these high-capacity cells because they take longer to produce than cells with lower capacity. One vendor indicated that pricing for these 2.6-Ah cells is currently in the “upper $3 range,” while the more quickly manufactured 2.4-Ah cells are priced in the “low $3 range.” For now, many laptop manufacturers are said to be sticking with the latter cells, which are considered mainstream products.

The 2.6-Ah cells may be the last 18650 Li-ion cells to eke out more capacity from the existing electrode materials as suppliers seemed to have reached the limit on cell performance that can be attained by packing the cells more densely. Consequently, several cell manufacturers are now on the verge of making the leap to new cathode and anode materials (see the figure). In the next generation of Li-ion cells, new cathode materials will extend Li-ion capacity up to 2.9 Ah. Beyond this improvement, new anode materials are expected to push cell capacity even higher.

Matsushita Battery Industrial Co., known more commonly as Panasonic in the United States, currently offers a 2.6-Ah cylindrical cell (model number CGR18650E) that uses a lithium-cobalt oxide cathode. The company, which began sampling this cell in fall 2005, currently offers the cell through its authorized pack assemblers, which are listed on the Panasonic website (see www.panasonic.com/industrial/battery/oem/chem/lithion/lithdist.htm).

For its next-generation Li-ion cell, Matsushita will switch from a conventional lithium-cobalt oxide cathode (LiCoO₂) to a nickel-cobalt-aluminum oxide (LiNiCoAlO₂) referred to as NNP.^[2] The new cathode material enables higher capacity without affecting the current charge voltage and is said to have excellent storage characteristics. It also reduces the impact of fluctuations in the price of cobalt.

Using NNP, Matsushita achieves an energy density of 620 Wh/l. For the 18650 cell, this translates to a capacity of 2.9 Ah. The company expects to sample these 2.9-Ah cells to major laptop manufacturers in mid-2006. General availability of these cells will depend partly on the feedback received from these customers.

Although this cell still charges up to 4.2 V, the discharge voltage for cells built with NNP cathodes is 0.15 V lower than for cells using LiCoO₂ cathodes. But in return, the NNP cathodes offer 30% higher capacity.

Another cell manufacturer, Sanyo, has pushed the capacity of its 18650 cylindrical with the introduction of a 2600-mAh (typ.) cell (model number UR18650F), which went into production last year.^[3] This cell employs the same electrode materials as in the previous 2.2-Ah and 2.4-Ah cells produced by Sanyo, so the discharge characteristics of the new cell are similiar to that of previous Li-ion cells.

In addition, Sanyo is developing a next-generation battery technology based on what it describes as a “neo-hybrid cathode material.”^[3] The new cathode mixes a nickel, manganese and cobalt oxide (Li(Ni-Mg-Co) O₂) with cobalt-oxide material (LiCoO₂). Because of the similarity of these materials to the existing cathode materials, the new cathode material can be manufactured using existing processes.

The first cell being developed with the neo-hybrid material is a 33.85-mm × 35.8-mm × 5.4-mm prismatic cell offering 820 mAh. With its 463 Wh/l energy density, this cell (model number UF553436T) increases cell capacity by 100 mAh when compared with the company's existing cell in this form factor. However, in exchange for that performance, the cell's charge voltage increases from the usual 4.2 V to 4.38 V. As a result, the limit for overcharge protection rises from 4.28 V to 4.47 V. The UF553436T is currently in production. For more information, see www.sanyobatteries.com.

Meanwhile, Sony is charging ahead with changes to both cathode and anode materials. In February of last year, the company introduced its Nexelion family of hybrid Li-ion batteries. The Nexelion battery replaces the graphite-based anode with a tin-based (Co-Sn-C) amorphous anode to achieve a 50% increase in storage capacity per volume when compared with conventional Li-ion cells. In terms of overall battery capacity, the improvement is 30% versus existing Li-ion cells.

The Nexelion battery also incorporates a new cathode material comprised of Li(Ni,Co,Mn)O₂ and LiCoO₂.^[4] In addition, Nexelion incorporates a new electrolyte and improvements in battery structure. Although Sony is noted for its development of Li-polymer cells, which use a polymer-type electrolyte, the electrolyte employed by Nexelion is the liquid type.

In developing the Nexelion battery, Sony overcame a problem that was previously encountered when using compounds that contain tin and silicon. Although use of these elements enables high capacity in Li-ion batteries, the particles in these compounds tend to change shape during battery charging and discharging, resulting in poor cycling characteristics for the battery. But, by adding several elements to the tin-based compound, Sony has been able to minimize the change in particle shape during charge and discharge.

In addition to offering higher capacity, Nexelion batteries are said to offer higher durability than other Li-ion batteries, as well as improved charge and discharge at low temperatures and quicker recharge times.

The first battery to exploit Nexelion technology is the 14430W1, a 14-mm (diameter) × 43-mm (height) cylindrical cell specifying 900 mAh of capacity or 3.1 Wh. The vendor compares this performance with that of a similarly sized conventional Li-ion cell offering 700 mAh or 2.6 Wh. But unlike the existing Li-ion cell, which discharges from 4.2 V to 3 V, the Nexelion cell discharges from 4.2 V to 2.5 V.

Faster charging is another area of distinction. A conventional cell charging at a 1 C rate would require 60 minutes to reach 90% capacity at room temperature. However, the Nexelion cell can be charged at a 2 C rate, enabling it to reach the same capacity in just 30 minutes.

Since its introduction last year, the Nexelion battery has been sold as part of an optional battery pack for the Sony Handycam camcorders. According to the company, the price of the Nexelion cells will vary depending on the specification of the battery pack, which combines multiple cells with circuitry and packaging.

Although chemistry offers an important path to higher capacity, vendors have not given up on packaging as a means to better performance. For example, Sony has developed a new casing technology called S-pack.^[4] The concept here is to increase cell capacity by reducing the thickness of battery-pack material.

In most conventional Li-ion batteries, whether they be Li-ion or Li-polymer types, the cells are housed in a hard battery case, which employs a thick plastic material and also introduces space between the cell and the plastic case.

The S-pack minimizes the ratio of pack to cells in Li-polymer batteries by replacing the hard case with a two-layer laminate film. The film consists of a hard outer laminated film that provides durability and a soft inner laminated film that allows easy embossing. When compared with the hard battery case approach, the S-pack provides a 10% to 20% increase in energy density at the battery-pack level.

The S-pack is being offered for three 34-mm × 53-mm Li-polymer models. The S3C is a 4.05-mm thick version with 690-mAh capacity; the S4C is a 4.7-mm version with 830 mAh; and the S5C is a 5.7-mm version with 1050 mAh capacity. For more information, see www.sony.com/energy.

References

1. Onnerud, Per, and Barnett, Brian, both of TIAX LLC. “Li-ion Battery Tutorial at Portable Power 2005,” Portable Power 2005 Conference, presented on Sept. 18, 2005.

2. Nakura, K.; Taniguchi, A.; Hashimoto, T.; Nagasaki, A.; and Kinoshita, K. “New High-Capacity Nickel-based Cathode Material for Li-ion Battery,” Portable Power 2005 Conference, presented on Sept. 20, 2005. The NNP cathode material is referred to in this presentation by its previous name, NCA.

3. Takahashi, Masatoshi, Sanyo Electric Co., Power Solutions Group. “New Concept Lithium Ion Batteries in Sanyo,” Portable Power 2005 Conference, presented on Sept. 20, 2005.

4. Ogisu, Kenji, R&D Division, Energy Group, Sony. “R&D Activities & Results for Sony Batteries,” Portable Power 2005 Conference, presented on Sept. 20, 2005.