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Will Lithium-Ion batteries power the new millennium?

Isidor Buchmann
Cadex Electronics Inc.
isidor.buchmann@cadex.com
www.buchmann.ca

April 2001 (Edited Sep.2008)

For many years, the Nickel Cadmium (NiCd) was the only suitable battery for portable applications such as wireless communications and mobile computing. In 1990, the Nickel Metal Hydride (NiMH) and lithium-ion (Li-ion) emerged, offering higher capacities. Both chemistries fought nose to nose, each claiming better performance and smaller sizes.

In the 21st century, lithium-ion is the undisputed winner. It is a low maintenance battery, an advantage that no other chemistry can claim. There is no memory and no scheduled cycling is required to prolong the battery's life. In addition to high energy density and lightweight, the self-discharge is less than half compared to the NiCd and NiMH, making the lithium-ion well suited for modern fuel gauge applications.

On the negative, the lithium-ion is fragile and requires a protection circuit to maintain safe operation, and like all battery chemistries, it is subject to aging, whether used or not.

History

Pioneering work for the lithium battery began in 1912 by G. N. Lewis but it was not until the early 1970's when the first non-rechargeable lithium batteries became commercially available. Attempts to develop rechargeable lithium batteries followed in the eighties, but failed due to safety concerns.
Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy content. Rechargeable batteries using lithium metal as the negative electrodes (anode) are capable of providing both high voltage and excellent capacity, resulting in an extraordinary high energy density.

After much research on rechargeable Lithium (metal) batteries during the eighties, it was found that cycling alters the lithium electrode, thereby reducing its thermal stability and causing potential thermal run-away. If this occurs, the cell temperature quickly approaches melting point of the lithium, which results in a violent reaction. A large quantity of rechargeable lithium batteries sent to Japan had to be recalled in 1991 after a battery in a cellular phone released hot gases and inflicted burns to a man's face.

Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density than lithium metal, the lithium-ion is safe, provided certain precautions are met when charging and discharging. In 1991, the Sony Corporation commercialized the first lithium-ion battery. Other manufacturers followed suit. Today, the lithium-ion is the fastest growing and most promising battery chemistry.

Sony's original version used coke as negative electrode (anode). Since 1997, most lithium-ion, including Sony's, has shifted to graphite. This electrode provides a flatter discharge voltage curve than coke and offers a sharp knee bend, followed by a rapid voltage drop before the discharge cut off (see Figure 1).

Figure 1:  Li‑ion discharge characteristics.
The graphite lithium-ion obtains its energy by a discharge to 3.0V/cel; the coke version needs discharging to 2.5V/cell to achieve similar performance.

For the positive electrode (cathode), two distinct chemistries have emerged. They are cobalt and manganese, also know as spinel. Whereas the cobalt has been in use longer, spinel is inherently safer and more forgiving if abused. Protection circuits can be simplified or even eliminated. Small prismatic spinel packs for mobile phones may only include a thermal fuse and temperature sensor. In addition to the added safety, the raw material cost for manganese is lower than cobalt.

As a trade-off, the spinel offers a slightly lower energy density, suffers capacity loss at temperature above 40ºC and ages quicker than cobalt. Figure 2 compares the advantages and disadvantages of the two chemistries.


  Cobalt Manganese (Spinel)
Energy density (Wh/kg) 140 1 120 1
Safety On overcharge, the cobalt electrode provides extra lithium, which can form into metallic lithium, causing a potential safety risk if not protected by a safety circuit. On overcharge, the manganese electrode runs out of lithium causing the cell only to get warm. Safety circuits can be eliminated for small 1 and 2 cell packs.
Temperature Wide temperature range. Best suited for operation at elevated temperature. Capacity loss above +40°C. Not as durable at higher temperatures.
Aging Short-term storage possible. Impedance increases with age. Newer versions offer longer storage. Slightly less than cobalt. Impedance changes little over the life of the cell. Due to continuous improvements, storage time is difficult to predict.
Life Expectancy 300 cycles, 50% capacity at 500 cycles. May be shorter than cobalt.
Cost Raw material relatively high; protection circuit adds to costs. Raw material 30% lower than cobalt. Cost advantage on simplified protection circuit.

Figure 2:  Comparison of Cobalt and Manganese as positive electrodes.
Manganese is inherently safer and more forgiving if abused but offers a slightly lower energy density. Manganese suffers capacity loss at temperature above 40°C and ages quicker than cobalt.

1 Based on present generation 18650 cells. The energy density tend to be lower for prismatic cells,

Chemicals and additives help to balance the critical trade-off between high energy density, long storage time, extended cycle life and safety. High energy densities can be achieved with relative ease. For example, adding more nickel in lieu of cobalt increases the ampere/hours rating and lowers the manufacturing cost but makes the cell less safe. While a start-up company may focus on high energy density to gain quick market acceptance, safety, cycle life and storage may be compromised. Reputable manufacturers, such as Sony, Panasonic, Sanyo and E-One Moli place high importance on safety.

Lithium-ion cells cause less harm when disposed than lead or cadmium based batteries. Among the lithium-ion battery family, the spinel is the friendliest in terms of disposal.


Charging the Li-ion battery

The lithium-ion charger is a voltage-limiting device that is similar to the lead acid battery charger with the differences of a higher voltage per cell, tighter voltage tolerance and the absence of trickle or float charge at full charge.

Whereas the Valve Regulated Lead Acid (VRLA) offers some flexibility in terms of voltage cut off, the manufacturers of lithium-ion cells are very strict about the voltage choice. When first introduced, the charge voltage limit of the graphite system was 4.10 volts per cell. Although higher voltages deliver increased energy density, cell oxidation severely limited the service life in the early graphite cells if charged above the 4.10V/cell threshold. This effect has now been solved with chemical additives, and most new lithium-ion cells are now set to 4.20V. The tolerance on all lithium-ion batteries is a tight +/- 0.05 volts per cell.

The charge time of all lithium-ion batteries is about 3 hours at a 1C initial charge current, and the battery remains cool during charge. Full charge is attained after the voltage reaches the upper voltage threshold, and the current drops and levels off at about 3% of its nominal rating, or about 0.03C.

Increasing the charge current on a lithium-ion charger does not shorten the charge time by much. Although the voltage peak is reached quicker with higher current, the topping charge will take longer. Figure 3 shows the voltage and current signature of a charger as the lithium-ion cell passes through stage one and two.

Figure 3:  Charge stages of a Li-ion Battery.
Increasing the charge current on a lithium-ion charger does not shorten the charge time by much. Although the voltage peak is reached quicker with higher current, the topping charge will take longer
.

Claims of fast charging a Li-ion battery in one hour or less usually results in lower charge levels. Such a charger simply eliminates stage two and goes directly into ‘ready’ once the voltage threshold is reached at the end of stage one. The charge level at this point is about 70%. The topping charge typically takes twice as long as the initial charge.

No trickle charge is applied because the Li-ion is unable to absorb overcharge. Trickle charge could cause plating of metallic lithium, a condition that makes the cell unstable. Instead, a brief topping charge is applied to compensate for the small amount of self-discharge the battery and its protective circuit consume.

Depending on the charger and the self-discharge of the battery, a topping charge may be implemented once every 500 hours or 20 days. Typically, the charge kicks in when the open terminal voltage drops to 4.05 volts per cell and turns off when it reaches 4.20V/cell.

Protection circuit

Commercial lithium-ion battery packs contain redundant protection devices to assure safety under all circumstances. Typically, a solid-state switch opens if the charge voltage of any cell reaches 4.30V, and a fuse activates if the cell temperature approaches 90°C (194°F). In addition, a pressure switch in each cell permanently interrupts the charge current if a safe pressure threshold is exceeded, and internal voltage control circuits cut off the battery at low and high voltage points. Exceptions are made to prismatic and cylindrical spinel packs containing one or two cells only.

The lithium-ion is typically discharged to 3 volts per cell. The lowest 'low-voltage' power cut-off is 2.5V/cell. During prolonged storage, however, a discharge below this voltage level is possible. Manufacturers recommend a 'trickle' charge to raise such a battery gradually back up into the 'acceptable' voltage window. Not all chargers are designed to apply a charge once a lithium-ion battery has dipped below 2.5V/cell.

Some batteries feature an ultra-low voltage cutoff that permanently disconnects the pack if a cell dips below 1.5 volts. This precaution is done to prohibit recharge if a battery has dwelled in an illegal voltage state. A deep discharge causes copper plating, which can lead to short circuit in the cell.
Most manufactures do not sell the lithium-ion cells by themselves but make them available in a battery pack, complete with protection circuit. This precaution is understandable when considering the danger of explosion and fire if the battery is charged and discharged beyond safe limits.

A major concern arises if static electricity or a faulty charger has managed to destroy the battery's protection circuit. Such damage often causes the solid-state switches to fuse to a permanent ON position without the user's knowledge. A battery with a faulty protection circuit may function normally but will not provide the required safely. If charged beyond safe voltage limits with a poorly designed accessory charger, the battery may heat up, then bulge and in some cases vent with flame. Shorting such a battery can also be hazardous.

Analyzers for the Lithium Ion batteries

In the past, battery analyzers were used to restore batteries affected by 'memory'. With today's nickel-free batteries, memory is no longer a problem and the emphasis of an analyzer is shifting to battery performance verification, quality control and quick-test.

Conventional wisdom says that a new battery always performs flawlessly. Yet many users have learned that a battery fresh from the shrink-wrap does not always meet the manufacturer's specifications. With a battery analyzer, all incoming batteries can be checked as part of a quality control procedure. In addition, warranty claims can be made if the capacity drops below the specified level at the end of the warranty period.

A typical life of a lithium-ion is 300-500 discharge/charge cycles or about three years from time of manufacturing. The loss of battery capacity occurs gradually and often without the knowledge of the user. Although fully charged, the battery eventually regresses to a point where it may hold less than half of its original capacity. The function of the battery analyzer is to identify these weak batteries and "weed' them out.

A battery analyzer can also be used to troubleshoot the cause of short runtimes, some can simulate the load signature of a digital device and verify the runtime based on the available battery capacity.
An important feature of modern battery analyzers is its ability to read the internal battery resistance, a test that only takes a few seconds to complete. As part of natural aging, the internal resistance of the cobalt lithium-ion gradually increases due to cell oxidation. The higher the resistance, the less energy the battery can deliver. On the manganese lithium-ion, the resistance stays low but the capacity drops.

A more reliable method of measuring the state-of-health of a battery is through quick-test methods. Cadex has developed a system that uses an inference algorithm to test battery capacities. The QuickTest™ algorithm is made battery-specific by using a trend-learning algorithm that resembles the thinking process of the human brain. QuickSort™ uses a generic matrix and services the vast pool of single cell lithium-ion batteries for cell phones.

Summary

Lithium-ion has found a strong market niche for portable devices demanding small form-factor and light weight. The high energy cobalt version is popular for cell phones, cameras and laptop computers; the rugged but lower energy-dense manganese and phosphate variants are employed for high current applications such as power tools, medical instruments, and soon also the hybrid car and electric vehicle.

Lithium-ion has proven less favorable for a laptop that is mostly powered by AC. High heat on a fully charge battery cause premature aging. Most battery chemistries suffer from this phenomenon.

                                   

This article contains excerpts from the second edition book entitled Batteries in a Portable World — A Handbook on Rechargeable Batteries for Non-Engineers. In the book, Mr. Buchmann evaluates the battery in everyday use and explains their strengths and weaknesses in laymen’s terms. The 300-page book is available from Cadex Electronics Inc. through book@cadex.com, tel. 604-231-7777 or most bookstores. For additional information on battery technology visit www.buchmann.ca.


About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Richmond (Vancouver) British Columbia, Canada. Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. The author of many articles and books on battery maintenance technology, Mr. Buchmann is a well-known speaker who has delivered technical papers and presentations at seminars and conferences around the world.


About the Company

Cadex Electronics Inc. is a world leader in the design and manufacture of advanced battery analyzers and chargers. Their award-winning products are used to prolong battery life in wireless communications, emergency services, mobile computing, avionics, biomedical, broadcasting and defense. Cadex products are sold in over 100 countries.


Acknowledgements

The author would like to thank Mr. Ulrich Von-Sacken, Ph.D., Mr. Mark Reid and Mr. Paul Craig of NEC Moli Energy (Canada) Ltd. for their comments and suggestions.

 

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