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Historically, the nickel-cadmium (NiCd) cell has been the best solution for handheld applications with usage profiles requiring large current pulses. But new environmental regulations may have a marked effect on cells containing heavy metals such as cadmium. Meanwhile, a new variety of lithiumion (Li-ion) cells can support the high discharge currents required for many applications.

Such batteries with high discharge rates represent a shift in Li-ion technologies from products driven by the desire for high capacity to cells developed to deliver high power for shorter periods of time. These new technologies prompted engineers of portable devices to ask about the cells during a recent webcast.

How are the new technologies different from “high-rate cells” based on traditional materials?
Designing a cell that can accommodate high discharge and charge rates is an effort to reduce the path length and resistance for the transport of ions and electrons. The resistance of the cells must be lowered by using thin materials, increasing the amount of current collectors, increasing the electrolyte concentration, and reducing its viscosity with solvents.

Traditional Li-ion cells are based on a lithium-cobalite (LiCoO₂) cathode compound. In this material, Li-ions can only be inserted through two-dimensional paths, so the rate capability is fundamentally limited. However, the rate capability for short pulses can be improved by making the aforementioned changes, and cells for high current pulses have been available for some time. The rate capability of cells based on traditional materials is only about 5C, whereas the cells based on new materials can support more than 30C.

What different types of high-rate cells are there?
The new cells have fundamental material changes in the cathode, moving to a three-dimensional insertion structure. Two 3D structures have been researched extensively: manganese spinel (LiMn₂O₄) and iron phosphate olivine (LiFePO₄). In addition, the problem can be addressed physically by decreasing the particle size of the materials to the nanoscale.

These materials offer great ionic conductivity and low resistance with a tradeoff in capacity. Most notably, E-One Moli Energy has commercialized the manganese material, and A123 Systems has commercialized a nanoscale phosphate.

Do I need high-rate cells in my battery to complete a fast charge?
Designers of portable devices, especially laptops, have made incredible efforts to reduce charge time. But they cannot simply increase the charge current. Li-ion batteries need to be charged with a constant current followed by a constant-voltage method. Increasing the current in the first portion only increases dwell time at constant voltage. Many modifications have been used, such as “express charge,” with moderate success. Yet a truly fast charge requires a cell designed to accept high current. These new cells boast charge times as low as 15 minutes.

What applications are best suited for the high-power technology?
Like other Li-ion cells, the high-power cells have operating voltages roughly three times that of the nickel chemistries. The manganese operating voltage is about 3.6 V, and the phosphate is about 3.3 V. Hence, any application that would benefit from the current capability of NiCd and the voltage of Li-ion is a good target. Handheld and motorized devices are likely candidates, and the performance improvement in the new power tools is a great proof point.

Are there special design considerations for using these cells?
Yes. Most battery packs aren’t designed to withstand the high currents associated with the charge and discharge of these cells. Hence, off-the-shelf protection circuits and fuel gauges aren’t yet available. Contacts and welds will need to be specially designed too.

Are the new cells less safe than a traditional technology?
There have been numerous news reports of safety problems associated with Li-ion batteries. Many of these incidents are due to faulty designs or bad manufacturing practices—usually associated with aftermarket supply. However, some characteristics of the Li-ion building blocks make the cells susceptible to dangerous failures.

The cathode material, to a large extent, determines the thermal runaway temperature. The new cells’ cathode material is actually less thermally volatile than traditional materials, making the high-power cells safer. High-capacity cells have also seen many safety improvements over the last couple of years, so Li-ion batteries from reputable suppliers with good design and manufacturing practices should be considered safe.

GOT QUESTIONS?
If you have any questions about these or other new power technologies, send them toquestions@micro-power.com, and they may be answered in a future column.