Electrochemistry

A nickel–cadmium battery uses cadmium for the anode (negative terminal), nickel oxyhydroxide for the cathode (positive terminal) and aqueous potassium hydroxide as the electrolyte.

A lithium-ion battery uses graphite as the anode, lithium oxide for the cathode and a lithium salt as the electrolyte. Lithium ions move from the negative electrode to the positive electrode during discharge, and back when charging. Lithium-ion electrochemical cells use an intercalated lithium compound as the electrode material instead of metallic lithium, unlike the disposable lithium primary batteries.

Environmental Impact

NiCad batteries contain between 6% (industrial batteries) and 18% (consumer batteries) cadmium, which is a toxic heavy metal and therefore requires special care during battery disposal. The federal government classifies it as hazardous waste. In the United States, part of the battery price is a fee for its proper disposal at the end of its service lifetime.

The components of lithium-ion batteries are environmentally safe as lithium is nonhazardous waste.

Cost

A lithium-ion battery costs about 40 percent more to manufacture because of the extra protection circuit to monitor the voltage and current.

Operation and Performance

The biggest drawback of nickel-cadmium batteries is they suffer from a "memory effect" if they are discharged and recharged to the same state of charge several times. The battery "remembers" the point in its charge cycle where recharging began and during subsequent use the voltage suddenly drops at that point, as if the battery had been discharged. However, the capacity of the battery does not reduce substantially. Some electronics are especially designed to withstand this reduced voltage long enough for the voltage to return to normal. However some devices are unable to operate through this period of decreased voltage, and the battery appears "dead" earlier than normal.

A similar effect called voltage depression or lazy battery effect, results from repeated overcharging. In this instance the battery appears to be fully charged but discharges quickly after only a brief period of operation. If treated well, a nickel-cadmium battery can last for 1,000 cycles or more before its capacity drops below half its original capacity.

Another problem is reverse charging, which occurs due to an error by the user, or when a battery of several cells is fully discharged. Reverse charging can reducing battery life. The by-product of reverse charging is hydrogen gas, which can be dangerous.

When not used regularly, dendrites tend to develop in NiCad batteries. Dendrites are thin, conductive crystals that may penetrate the separator membrane between electrodes. This leads to internal short circuits and premature failure.

Lithium-ion batteries are low maintenance. They can be recharged before they are fully discharged without creating a “memory effect” and operate in a wider temperature range. When compared to Ni-Cd, the self-discharge in lithium-ion is less than half, making it well suited for modern fuel gauge applications. The only drawback is lithium-ion battery is fragile and requires a protection circuit to maintain safe operation. The protection circuit is built into each pack, which limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. To prevent temperature extremes the cell temperature is also monitored.

Sizes and Types

Ni-Cd cells are available from AAA through D, the same sizes as alkaline batteries, as well as several multi-cell sizes. In addition to single cells they are available in packs of up to 300 cells, commonly used in automotive and heavy-duty industrial applications. For portable applications, the number of cells is below 18 cells. There are 2 types of NiCd batteries: sealed and vented.

Li-ion batteries are smaller, lighter and provide more energy than nickel-cadmium batteries. They are also available in a wide variety of shapes and sizes in 4 types of formats:

• Small cylindrical (solid body without terminals, such as those used in laptop batteries)
• Large cylindrical (solid body with large threaded terminals)
• Pouch (soft, flat body, such as those used in cell phones)
• Prismatic (semi-hard plastic case with large threaded terminals, often used in vehicles' traction packs)

The pouch cells have the highest energy density due to the absence of case. However it requires some external form of containment to prevent expansion when its state-of-charge (SOC) level is high.

Applications

NiCad batteries may be assembled into battery packs or used individually. Small and miniature cells can be used in flashlights, portable electronics, cameras, and toys. They can supply high surge currents with a relatively low internal resistance, making them a favorable choice for remote-controlled electric model airplanes, boats, cars, cordless power tools and camera flash units. Larger flooded cells are used for aircraft starting batteries, electric vehicles, and standby power.

With qualities like high energy densities, no memory effect, and a slow loss of charge when not in use, lithium-ion batteries are the most popular choice for consumer electronics. They are also growing in popularity for military, electric vehicle, and aerospace applications.

References

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