Jean-Louis Evans, Managing Director, TÜV Product Service
Lithium-ion (Li-ion) batteries have helped to revolutionize technology development. Lightweight and long lasting, they have proven invaluable in the evolution of consumer technologies such as mobile phones and notebook PCs. However, they also have a reputation for volatility and until the CTIA certification scheme was introduced, bad press had meant that both consumers and consumer electronics manufacturers were increasingly questioning their long-term viability.
Fighting Back
Consumer electronic devices are becoming smaller with many power hungry features, resulting in the battery pack also becoming smaller, but at the same time requiring greater capacity. Such devices are often carried in people’s pockets, increasing the potential safety hazard and risk of personal injury.
Education of the consumer plays a significant part by promoting the safe use of batteries and discouraging the purchase of non-genuine or counterfeit batteries and chargers, where they are tempted by the lower prices without realizing the potential safety implications.
This is especially important for so called smart batteries, which use the SMBus protocol to communicate between the battery pack, charger and end-use product. A scenario could exist whereby the branded charger supplied with the phone meets Level 2 of the SMBus protocol and is therefore dependent on the branded battery pack for direction on charging algorithm requirements. If a non-genuine battery is fitted, it may not have this protection that could result in an overcharge situation. The same applies for non-genuine chargers.
It is true that the increase in capacity of today’s batteries, and end-user misuse, is part of the reason for the Li-ion battery’s bad publicity, but the manufacturing process has also been questioned. Many battery safety incidents involving notebook PCs have been linked to inadequate procedures relating to the avoidance of contaminates in production. In such cases, it is suspected that metal particles penetrated the battery separator and caused a short circuit between the cathode and the anode resulting in thermal runaway.
As a result of the number of incidents involving Li-ion batteries, there was an international move to improve the testing and quality control of the cell and battery packs. This is now addressed by the application of standards including UL 1642 “Lithium Batteries”, IEEE 1725 “IEEE Standard for Rechargeable Batteries for Cellular Telephones” and “UN Recommendations on the Transport of Dangerous Goods Manual of Tests and Criteria” (ST/SG/AC.10/11).
Another key catalyst in the improvement of Li-ion battery safety was the introduction of the CTIA (Cellular Telecommunications and Internet Association) certification scheme. This is an industry led initiative in the US for mobile handsets and is based on the IEEE 1725 standard.
The CTIA Battery Certification Program has had a positive effect worldwide as manufacturers are unlikely to exclude themselves from the massive potential of the US market by not developing products to this standard.
The CTIA program was devised in partnership with leading cellular network operators such as AT&T and Verizon, as well as battery industry experts. It requires all products to undergo mandatory third-party testing and auditing, with certification categories covering cells, battery packs and power adapters, as well as complete cellular product systems. The CTIA requirements now also include another standard, IEEE 1625, which relates to notebook computers with GSM functionality that use Li-ion battery packs.
The Ugly – The Birth of Lithium-Ion
The lithium battery can be traced back as far as 1912 due to the work of American physical chemist Gilbert Newton, but it was not until the 1970s when non-rechargeable Li-ion batteries became commercially available. A further 20 years on and commercially available re-chargeable Li-ion batteries finally hit the market.
The Li-ion battery is available in three main types of package: cylindrical, prismatic and lithium polymer pouch designs. However, the basic construction of each type is virtually identical.
The main difference between Li-ion polymer batteries and other types of Li-ion batteries is that they use a dry solid polymer electrolyte. The electrolyte has the appearance of a plastic-like film that does not conduct electricity but allows an exchange of ions. The polymer replaces the porous separator as used in the Li-ion battery. However, due to poor conductivity at room temperatures, hybrid Li-ion polymer batteries are often used in mobile handset applications that contain gel electrolyte, thus enhancing ion conductivity. This results in a more robust, thinner and safer battery. Enhanced safety is achieved through a minimal amount of liquid or gel electrolyte being used to reduce the flammable material in the battery.
Early re-chargeable batteries contained lithium based electrodes, but in the 1980s it was discovered that re-charging resulted in changes to the electrodes that reduced thermal stability. Thermal runaway led to a rapid increase in temperature, with the cell reaching the melting point of lithium, resulting in violent venting and flaming.
As a result, today’s Li-ion batteries do not actually contain lithium metal and the electrodes are made from alternative materials such as lithium cobaltate (for the cathode) and graphite (for the anode).
Today, the electrolyte (which has the function of carrying lithium-ions and so producing current flow) is lithium salt, a non aqueous organic solvent which is required because of the higher voltage (4 V) of the battery. Lithium salt is used instead of an aqueous solution (for example lead acid used in NiCD batteries), as the high voltage would cause electrolysis of the water. Lithium salt benefits from inherent characteristics of high conductivity, electric chemical stability (at voltages over 4 V), chemical and thermal stability, and has a wide temperature range.
Another major component of the battery is the separator. The main function of the separator is to insulate the positive and negative electrodes, to retain the electrolyte and to transmit ions. Typical materials used for this component are polyethlene and polypropylene porous thin films. These materials provide good insulation and mechanical strength, are chemically and thermally stable (against the electrolyte), have the ability of holding electrolyte, and are porous, allowing the movement of the lithium-ions. The separator plays an important part in the safety of the battery due to the fact that the pores of the material melt at temperature thus blocking the movement of the lithium-ions. The outer enclosure often known as the can is typically made of nickel-plated iron or aluminium alloy except for Li-ion polymer batteries where the pouch material is typically plastic or metal foil.
Safety Features within the Design
Manufacturers of Li-ion and Li-ion polymer batteries include internal protection devices in addition to the protection circuits within the overall battery pack to guard against excessive heat and pressure. Typical protection devices are:
Vent Plate/Vent Tear Away Tab: Excessive build up of pressure within battery cells is caused primarily from excessive abnormal heat generation or over-charging. The vent allows the safe release of gas.
Positive Temperature Coefficient (PTC): PTCs act as both a current fuse and a thermal fuse so that, when excessive current is drawn, the resistance of the PTC increases resulting in increased heat generation. The resistance of the PTC is selected so that it trips at the pre-determined current.
Separator: When the separator reaches its defined temperature (typically 130ºC), the pores are blocked by the melting of the material, preventing electrical current to flow between the electrodes. The separator is also sometimes known as a shut-down separator.
Thermal Fuse: Some prismatic batteries have an additional feature, a thermal fuse which limits the current under fault conditions.
A protection circuit is also usually fitted within the battery pack consisting of a custom designed integrated circuit that monitors the cell and prevents over-charge, over-discharge and over-current. This in combination with two field effect transistor (FET) devices control the charge and discharging. Also present is a temperature sensing device (thermistor) designed to invoke protective action via the control IC in the event of an over-temperature scenario.
The Good and the Bad
Advantages
High Energy Density: Compared to other battery technologies such as NiCd the energy density of the Li-ion battery is greater with the opportunity to increase capacity, for example by adding more nickel to the cathode.
Small Package Size and Weight: The Li-ion battery is ideal for portable consumer products. Designers have the option of using the prismatic package, which is typically thinner than 19 mm, or the Li-ion polymer pouch, which is typically thinner than 5 mm. In addition to the size advantage, there is also a reduction in weight due to the chemistry (e.g. solid /gel electrolytes rather than liquid electrolytes) and the packaging used (e.g. foil).
Memory Effect: Unlike NiCd Li-ion batteries do not suffer from memory effect. Memory effect occurs where over time a battery has been consistently partly used and then fully recharged which results in the appearance of rapid discharge. In modern batteries this is more likely to be caused by voltage depression as a result of repeated overcharging leading to clogged plates which increases internal resistance thus lowering the voltage of the battery.
Low Discharge Rate: Compared with other rechargeable batteries Li-ion have a low self-discharge rate which means they can be left unused for longer.
Disadvantages
Protection: Li-ion batteries are sensitive to temperature and the chemistry is complex, therefore circuitry is required to protect the battery against over-charge, over-discharge and over-temperature.
Premature Aging: Li-ion batteries are susceptible to capacity deterioration over time; however, storage of the battery in a cool environment can reduce the effects. Once the battery is shipped by the manufacturer it is important that it is used it as soon as practical in order to provide the end-user with the longest possible battery life.
Chemistry: Due to the nature of lithium, severe temperature or mechanical impact can result in venting and possible thermal runaway. This requires more extensive testing than other forms of battery technology to demonstrate stability in the final battery product and safeguard against potential foreseeable misuse.
Production Costs: Compared to other types of rechargeable battery production costs can be high.
The Future
The development of Li-ion technology has played a significant role in the pace of technology evolution, never more so than in the consumer electronics sector. Today’s consumer demands that mobile devices and other technologies give them increased functionality with portability. Li-ion has helped manufacturers deliver on that.
While Li-ion batteries still have some disadvantages, their progressive development over the last few years has meant that these are far outweighed by the advantages. An improvement of manufacturing processes through the introduction of more robust standards, as well as increasing consumer understanding of how to respect these batteries, means that the safety of Li-ion has dramatically improved. It is a battery technology that has moved through the bad and ugly phase, resulting now in a good battery option that enhances all of our daily lives. Bad publicity and safety scares should be a thing of the past, at least for the non-counterfeit product.
TÜV Product Service is one of the world’s leading experts on product testing with 170,000 product certifications in circulation globally. It is also the leader in the test and certification of medical devices, with more than 1,000 customers globally. With 13,000 employees worldwide, TÜV covers regulatory and voluntary aspects associated with satisfying legal and cultural requirements for products. It also helps retailers across the world to ensure a consistent supply chain by helping them to ensure safe, compliant and reliable products are put on the shelves.
Jean-Louis Evans, managing director at TÜV Product Service, a global product testing and certification organization, and at its sister company, British Approvals Board of Telecommunications (BABT), the world’s leading radio and telecommunications certification body.
For more information, please contact TÜV Product Service at www.tuvps.co.uk.
This article appeared in the March/April 201o issue of Battery Power magazine.
