Fast Charge Becomes a Reality For Li-Ion Batteries

Oct 1, 2007 12:00 PM
By Robin Sarah Tichy, Technical Marketing Manager, and Jeff Van Zwol, Marketing Manager, Micro Power

High-drain-rate Li-ion batteries developed for power tools accept high-charge currents, but alter the demands on the battery charger.

Consumers of frequent-use products, such as mobile phones, bar-code scanners and military radios, have been clamoring for fast battery charging since the introduction of rechargeable batteries. The adoption of lithium-ion (Li-ion) batteries in portable systems has decreased charge time significantly compared to nickel-metal-hydride (NiMH)-based systems, but traditional Li-ion cells still can only accept a 0.7-C charge rate.

However, the power-tool industry's demand for high-discharge-rate batteries with lighter weight and smaller and better cycle life than nickel-cadmium (NiCd) batteries has driven a few cell manufacturers to invest in the development of high-drain Li-ion cells. The impending elimination of NiCd — due to the hazardous-waste restriction imposed by the RoHS mandate — has greatly improved the chance of success for high-power Li-ions in the market.

With the emergence of this new variety of cells, there are now two types of Li-ion cells. One type is shaped by the demands for high capacity, while the other is developed to deliver high power for shorter periods of time. The latter variety of Li-ion cells can support the high-discharge currents required for many applications, and as a result, they will also support high-charge currents for fast charging.

There is no standard definition for high-drain-rate cells, but basic design guidelines dictate that standard cobalt-oxide-based cells can support a 2-C or maybe a 3-C rate, continuous current. High-drain cells based on cobalt-oxide support roughly double those currents, but only for seconds. The new high-drain cells support 20 C continuous.

Given that a high-discharge-rate cell can support high-current discharges over a very short period, in theory, a battery charger could fully charge that cell in an equally short amount of time. But to take advantage of this possibility, the conventional battery-charger design must be modified. For the sake of simplicity, these changes can be illustrated with the example of a single-bay charger supporting a single-cell battery pack.

Cell Characteristics

On the surface, fast-charging Li-ion cells seem straightforward. It seems that one could simply increase the current delivered during the constant-current phase of the charge cycle. However, as shown in the table, the overall charge time is not significantly decreased when the current is increased from 1 C to higher rates.

The difference in charge time with a 2-C rate versus a 3-C rate is only about one minute, regardless of the cell vendor. Essentially, the cells will just reach the upper-voltage cutoff faster, but the time in the constant-voltage charge mode will be much longer. Obviously, this increases the potential for damage to the battery due to overvoltage. The resistance of traditional Li-ion cells will cause them to heat up more during faster charges, so the cells will begin to break down. Fast charging significantly reduces the battery life cycle.

Designing a cell that can accommodate high-discharge and high-charge rates is an effort to reduce the path length and resistance for the transport of ions and electrons. Fig. 1 shows a cross section of a typical Li-ion cylindrical cell. Changes start with the battery's active materials. Traditional Li-ion cells are based on a lithium-cobalt-oxide (LiCoO₂) cathode compound. In this material, Li-ions, which diffuse in and out of the cathode, can only be inserted through 2-D paths in the crystal structure.

The path length can be shortened by changing the physical morphology of the battery's active material or changing the material's chemical structure, or by doing both. One approach to addressing the problem physically is to decrease the particle size of the materials to as small as nano-scale. New chemistries such as manganese spinel (LiMn₂O₄) offer 3-D pathways for ion insertion.

In addition to these changes, the resistance of the cells must be lowered by using thin materials, increasing the amount of current collectors, and increasing the electrolyte concentration and reducing its viscosity with solvents. Many of these changes suggest that Li-polymer cells, which can be very thin, lend themselves for use in designing for high rates.

Li-ion cell manufacturers have been experimenting with their formulations in order to implement designs specific to high-rate applications. A few manufacturers have come up with solutions. E-One Moli Energy introduced a high-discharge-rate cell based on a manganese-spinel cathode material for cordless power tools.

As Fig. 2 shows, this cylindrical 26700 (26-mm in diameter × 70-mm in length) cell can support 80-A pulses for more than 10 seconds. This cell has been highly successful in power-tool applications, so Moli very recently introduced a spinel-chemistry cell in the traditional 18650 size.

The drawback of a high-rate cell is the lower capacity. The 26700 and 18650 have capacities of only about 2.9 Ah and 1.4 Ah, respectively. Another cell supplier, A123 has also introduced a cell that supports very-high-drain rates for the power-tool market.

Offered in 26650 and 18650 sizes, the A123 cell exploits nano-scale particles to achieve a performance that is very similar to the Moli cell, as seen in Fig. 3. The fundamental cathode chemistry is also different from the Moli technology, so the voltage is somewhat lower. The A123 has an operating voltage of 3.2 V instead of 3.6 V, because lithium-iron-phosphate (LiFePO₄) material is used for the cathode.

Yet another cell vendor, Kokam has brought a polymer option to the market. This Li-polymer battery — which uses a polymer rather than liquid electrolyte — is able to draw up to a 20-C rate discharge continuously, with a peak discharge rate of 40 C. All of these new cells offer similar advantages and disadvantages. The cell impedance is low and the discharge and charge rates can be high, so fast charge, high power and fewer cells per battery pack may be possibilities.

Unfortunately, the energy densities for such cells are relatively low. Because the cells are new and somewhat uncommon, they are more expensive. Moreover, off-the-shelf safety circuits and fuel gauges are not yet available.