Power Electronics

Batteries Seek Higher Capacity, Power and Safety

At the recent Portable Power Conference in San Francisco, battery vendors and others connected with the battery industry presented some of the latest developments relating to Li-ion/Li-polymer batteries.

Although there is continued use of NiCd and NiMH batteries in high-volume applications such as cordless phones and power tools, Li-ion is the dominant rechargeable chemistry thanks to its use in cell phones, notebook computers and PDAs.[1] Li-ion batteries also are being adopted in many other applications such as DVD players, digital still cameras, MP3 players and medical instruments.

For these types of products with modest power requirements, falling battery costs, widespread availability of charging circuits and high energy density are making Li-ion batteries the power source of choice. However, in their search for new markets, cell manufactures continue to develop Li-ion batteries capable of supplying the higher discharge rates and the ruggedness demanded by power tools. As these batteries emerge from the lab into production, they will seek to supplant NiCd and NiMH batteries, with Li-ion cutting perhaps half the weight of the battery pack.

Meanwhile, cell makers continue to optimize the existing Li-ion chemistries to achieve greater capacity with the expectation that changes in the electrode composition will soon be needed to push battery performance higher. At the same time, efforts continue to exploit the benefits of Li-polymer cells and to expand production of these cells. While many of these efforts involve incremental improvements of existing batteries, some companies are pursuing new chemistries to achieve a major step up in performance.

One example is Sion Power's efforts to commercialize lithium-sulfur (Li-S) cells, which promise higher energy densities than Li-ion cells, albeit with a lower cell voltage of 2.1 V. The company plans to begin sampling Li-S cells next July.

Incremental Gains for Li-ion

The 18650 cylindrical cells serve as a marker of Li-ion performance advances over time. According to data from the conference, most 18650 cells shipped in 2004 had a capacity of 2200 mAh.[1] However, several cell makers began producing 2400-mAh cells this year. According to data presented for a Sony cell, that capacity corresponds to a gravimetric energy density of 205 Wh/kg and a volumetric energy density of 530 Wh/l.

Because the material composition of the highest capacity cells is slightly different from those of the lower capacity cells, OEMs pay a premium for their performance. One vendor, Expower (part of HYB battery Co.), has indicated that 2400 mAH cells are currently priced at $3 to $4 per cell in volume. However, 18650 cells are still being offered with capacities ranging down below 2000 mAh, and these cells are priced closer to $2 per cell.[1]

The recent increases in performance for Li-ion batteries are attributed to optimization of a familiar Li-ion cell chemistry using lithium cobalt oxide as the cathode material and graphite as the anode material. Continuing with this approach, cell vendors expect to introduce cells with up to 2600 mAh in the coming year. However, some think that value is the limit for cells built with lithium cobalt oxide.

For cell makers to achieve higher capacities up to about 3000 mAh in the 18650 format, change in cathode materials, possibly a combination of nickel and cobalt, will be required. Such an electrode system will require further development to ensure the safety and stability of the cell.

Safety isn't only a concern in the development of future Li-ion cells with higher performance, it's an issue with existing cells, which generally require multiple layers of protection. Consequently, the pursuit of inherently safer Li-ion cells continues using more thermally stable cathode materials such as lithium manganese oxide and lithium metal phosphate. Valence Technology is developing batteries based on the latter material.

One variation on safer cell design is Sanyo's composite cathode, which combines lithium manganese oxide with lithium cobalt oxide, blending the safety of the former material with the performance of the latter.[2] Cells using the composite cathode are safe enough to remove the protection circuit and reduce costs, given certain restraints on charging conditions, leakage current and continued use of a PTC or thermal fuse. At the same time, Li-ion cells built with the composite have discharge characteristics close to that achieved using lithium cobalt oxide alone.

Progress for Li-polymer

Work continues to boost the performance of Lithium polymer batteries, which are essentially Li-ion batteries but with two distinctions. The Li-polymer cells use a gelled electrolyte rather a liquid electrolyte and an aluminum laminate casing instead of the aluminum or steel can associated with Li-ion prismatics. Since their introduction by Sony in the 1990s, Li-polymer cells have been touted because of their ability to be manufactured in the thinnest form factors (below 4 mm).

But Li-polymer batteries have trailed Li-ion batteries in a few performance areas. Because of the lower ionic conductivity of the electrolyte, they have exhibited degraded performance at lower temperatures and over varying load conditions. In the past, leakage and swelling were concerns with some Li-polymer cells. The development of new types of completely gelled electrolyte has addressed these concerns. One example is a 3.8-mm × 35-mm × 62-mm, 900-mAh Li-polymer cell from Sony.[3] This cell, which specifies energy density at 410 Wh/l (volumetric) and 210 Wh/kg (gravimetric), operates from -20°C to +60°C.

While cell phones and notebooks may require batteries to supply 2C and 3C discharge rates, power tools may drain the battery at rates of 10C. To meet that requirement, vendors are developing Li-ion cells with lower impedance but making tradeoffs in energy density. For example, BYD's LC1865PR cylindrical cell features less than 20 mΩ of impedance and is characterized at discharge rates up to 10C.[4] The capacity of this cell at 10C is 93% of that at 1C; at the 10C rate, the cell retains 80% of its capacity after 500 cycles.

This high-rate Li-ion cell achieves an energy density of 108 Wh/kg (gravimetric) or 260 Wh/l (volumetric). Although those numbers are below that of the benchmark cylindrical cells cited above, they compare favorably with NiMH, which offers an energy density of 50 Wh/kg (gravimetic) and 150 Wh/l (volumetric) in similar power tool applications.

BYD's high-rate discharge battery is still in development. The focus is improving cell safety by adopting lithium manganese oxide in the cathode. The company is working to enhance performance with this cathode. Specifically, BYD seeks to increase initial capacity at the 10C rate, improve high temperature performance and continue life-cycle testing.

References

  1. Takeshita, Hideo. “Worldwide Battery Market Status and Forecast.”

  2. Nagaya, Hideyuki. “Alternative Cathode Material For Cobalt.”

  3. Nishi, Yoshio. “Can DFMC Replace Lithium Ion Secondary Batteries As the Leading Position in Portable Power Sources?”

  4. Wang, Robert. “Next-Generation Li-ion Batteries for Power Tool Application.”



*These papers were presented at the Portable Power Conference & Expo 2004.

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