Just how safe are Lithium-ion Batteries?
Just how safe are Lithium-ion Batteries?
By Matthew Larkin, Product Safety Specialist, TÜV Product Service Ltd
Published: January 2007 - EMC Compliance Journal
In recent months consumers and consumer electronics manufacturers have shown increasing concern regarding Lithium-ion batteries. Currently consumers and manufacturers enjoy the benefits of Lithium-ion batteries, lightweight and higher capacity. But now this advantage is being questioned, in particular by lap-top computer and mobile handset manufacture’s, who in recent years have experienced safety related problems with their products resulting in much bad publicity.
History:
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 1970’s when non-rechargeable lithium-ion batteries became commercially available and another 20 years still until commercially available re-chargeable lithium-ion batteries became available. This was through the work of John Goodenough and his contemporaries. John Goodenough1) received a Japan Prize awarded by The Science & Technology Foundation of Japan in 1991 for his outstanding achievements in discoveries of material critical to the development of lightweight re-chargeable batteries.
Basic Construction 2) 3) 4)
The lithium-ion battery is available in three main types of package "cylindrical", "prismatic" and in the case of lithium polymer "pouch" designs; however the basic construction of each type is virtually identical.
Nowadays lithium-ion batteries do not actually contain lithium metal due to its inherent instability. Early re-chargeable batteries contained lithium based electrodes but it was discovered in the 1980’s that the cycling function of battery re-charging resulted in changes to the electrodes and in turn reduced thermal stability. This instability lead to thermal runaway with a rapid increase in temperature resulting in the cell reaching the melting point of lithium, and as a consequence violently venting and flaming. Today the electrodes are made from alternative materials such as lithium cobaltate (for the cathode) and graphite (for the anode).
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 due to the higher voltage (4V) of the battery. Lithium salt is used instead of an aqueous solution (for example lead acid used in Nickel-cadmium (Ni-Cd) batteries) where due to the higher voltage, electrolysis of the water would occur. Lithium salt benefits from inherent characteristics of high conductivity, electric chemical stability (at voltages over 4V), chemical & 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 & mechanical strength, are chemically & 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 so 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 lithium-ion polymer batteries where the pouch material is typically plastic or metal foil.
Difference between Lithium-ion (Li-ion) and Lithium-ion Polymer (Li-Ion Polymer) batteries 4)
The main difference is that lithium-ion polymer batteries 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 lithium-ion battery. However, due to poor conductivity at room temperatures, hybrid lithium-ion polymer batteries are often used in mobile handset applications which contain gel electrolyte so enhancing ion conductivity.
This results in a more robust, thinner, and safer battery. The enhanced safety being achieved as minimal liquid or gel electrolyte is used so reducing the flammable material in the battery.
Safety Features within the design 3)
Manufacturer’s of lithium-ion (Li-ion) and lithium-ion polymer (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:
1. 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.
2. Positive Temperature Coefficient (PTC)
PTC’s 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.
3. Separator
When the separator reaches its defined temperature (typically 130øC) 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.
4. Thermal Fuse
Some prismatic batteries have an additional feature, a thermal fuse which limits the current under fault conditions.
Protection within Battery Pack
A protection circuit is usually fitted within the battery pack consisting of a custom designed integrated circuit (IC) that monitors the cell and prevents overcharge (>4.3V), over discharge (< 2.3 V), and over current. This in combination with two Field Effect Transitior (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.
Advantages and Disadvantages of Lithium-ion Batteries 2) 3) 4)
Advantages
1. High energy density
Compared to other battery technologies such as nickel-cadmium (Ni-Cd) the energy density of the lithium-ion (Li-ion) battery is greater with the opportunity to increase capacity, for example by adding more nickel to the cathode.
2. Small package size and weight
The lithium-ion (Li-ion) battery is ideal for portable consumer products. Designers have the option of utilising the prismatic package which is typically thinner than 19mm or the Li-ion polymer pouch which is typically thinner than 5mm. In addition to the size advantage is the reduction in weight due to the chemistry (e.g. solid /gel electrolytes rather than liquid electrolytes) and the packaging used (e.g. foil).
3. Memory effect
Unlike nickel-cadmium batteries (Ni-Cd) lithium-ion (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.
4. Low discharge rate
Compared with other rechargeable batteries lithium-ion (Li-ion) have a low self-discharge rate which means they can be left unused for longer.
Disadvantages
1. Protection
Lithium-ion batteries are sensitive to temperature and the chemistry is complex therefore circuitry is required to protect the battery against overcharge, overdischarge, and over temperature.
2. Premature ageing
Lithium-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 to utilise it as soon as practical in order to provide the end user with the longest possible use.
3. 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.
4. Production costs
Compared to other types of rechargeable battery production costs can be high.
Reasons for recent bad publicity 2) 5) 6)
Consumer electronic devices are becoming smaller with many power hungry features resulting in the battery pack itself becoming smaller but requiring greater capacity. Today’s mobile handsets are often carried in people’s pockets meaning these batteries are closer than ever to the user increasing the potential safety hazard and risk of personal injury. In recent years incidents involving batteries have dramatically increased; 2002 statistics show that more than 1,100 children under 14 in the UK were injured in incidents involving batteries, usually sustaining chemical or heat burns. This has culminated in recent headline news stories featuring some of the worlds leading consumer electronics companies.
So what are the underlying reasons behind these incidents?
It is true that the increase in capacity of today’s batteries is part of the reason, placing greater pressure on design tolerances; but the manufacturing process has also been questioned. Many of the recent incidents involving lap-top computers have been linked to potentially inadequate procedures relating to the avoidance of contaminates in production; it is suspected that metal particles penetrated the separator and caused a short circuit between the cathode and the anode resulting in thermal runaway. Another area of concern is non-genuine batteries and chargers intended for use with mobile handsets; consumers being tempted by the lower prices without realising the potential safety implications. This is especially important for so called "smart batteries" using 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 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 which could result in an overcharge situation. The same applies for non-genuine chargers.
What can be done? - Comprehensive battery testing
Although education of the consumer plays a significant part in increasing battery safety through discouraging the purchase of non-genuine batteries & chargers and in the safe use of batteries, it is accepted that more needs to be done. Primarily the testing and quality control of the cell and battery packs must be improved. This is being addressed by the application of standards such as 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).
1. UL 1642
This standard addresses safety requirements for the lithium batteries and can broadly be split into three main sections.
a) Electrical tests such as short circuit, abnormal charge and forced discharge;
b) Mechanical tests including crush (13kN applied via a hydraulic ram), impact (9.1kg from a distance of 610mm), shock, & vibration;
c) Environmental tests including dry heat, temperature cycling, altitude simulation (1.68psi applied for 6 hours) and the projectile test (the battery is enclosed in a metal mesh test box and a burner is placed underneath, the test concluding when the battery explodes or ignites and burns out; compliance being demonstrated as long as parts of the battery or cell do not penetrate the mesh).
2. IEEE 1725
This standard differs in many respects as it requires a system level approach to be considered; i.e. each sub-assembly within the cellular system needs to meet specific requirements including the interaction of the complete system and covers the cell, battery pack, host (mobile handset) and AC or DC chargers. In addition it introduces system integration, reliability, security considerations and validation.
The standard also focuses on design approaches to ensure reliable and safe operation of the mobile handset and its power sources. Application of design analysis tools such as FMEA (Failure Mode and Effects Analysis) are required to demonstrate compliance.
In addition each sub-assembly must meet any applicable specific standards such as UL 1642 and EN / IEC / UL 60950-1.
Cell considerations: include auditing processes such as manufacturing traceability (so that the manufacturer can demonstrate they have a "Cell Traceability Plan") as well as constructional requirements such as burr control, positioning of insulating plates and tabs plus electrode alignment.
Battery pack considerations: include leakage protection, circuit layout, thermal protection, overcharge, overdischarge and overcurrent protection. Issues such as cell dimensional allowance, whereby the design of the pack should take into consideration the shrinking / expanding of both the pack and the cell over the lifetime of the product must also be considered. Integration is further examined by clauses relating to the cell vent mechanism to check that the pack design does not interfere with the correct operation of the vent and therefore escape of cell gasses.
Host considerations: include algorithm verification to ensure the host can identify the battery pack fitted and execute the correct charging algorithm and timer faults to demonstrate that there is no safety hazard if the host exceeds the specified charge time. Other issues addressed in this section of the standard include robustness, connector strength, electro-static discharge, shock and vibration.
System reliability consideration: includes required user warnings and instructions to ensure safe use.
System Security considerations: includes supply chain security whereby through the use of procedures and audits the manufacturer shall ensure products, sub-systems and components that enter the supply chain are appropriate so they will not compromise the safety or functionality of the final product.
The final section relates to validation and covers issues such as record keeping, quality systems, process capability, process monitoring and reaction to non-conforming products.
3. UN Recommendations on the Transport of Dangerous Goods Manual of Tests and Criteria (ST/SG/AC.10/11)
Details a suite of tests covering altitude simulation, thermal, vibration, shock, external short circuit, impact, overcharge, and forced discharge.
Industry led certification schemes
As part of an industry led initiative the CTIA (Cellular Telecommunications and Internet Association) have introduced a certification scheme for mobile handsets based on the IEEE 1725 standard. It is expected that this certification scheme will become mandatory during 2007 in order to obtain PTCRB approval which is required for anyone wishing to sell a mobile handset into North America. In addition there is another standard IEEE 1625 which relates to lap-top computer batteries which we expect will be used as the basis for a similar certification scheme in the near future.
The "CTIA Battery Certification Programme" has been devised in partnership with leading cellular network operators such as Cingular and Verizon and battery industry experts. The programme will be introduced in two phases, with the network operators giving preference to cellular products that are CTIA registered to IEEE 1725.
The 1st phase of the certification programme known as the "Registration Stage" relates to the complete cellular product system (cell, battery pack, cellular product (host), power adapter and / or charging cradle) with submissions being made from system vendors (handset manufacturers). The aim of the programme is to verify compliance of cellular products in accordance with the standard IEEE 1725 and associated CTIA documents CRD (Certification Requirements for Battery System Compliance) and CRSL (Certification Requirements Status List).
The process involves the system vendor evaluating compliance of the cellular product system to the requirements and compiling a ’compliance folder’ containing supporting documentation. This "compliance folder" is then submitted to a BCRO (Battery Compliance Review Organisation) for review. Currently there are four appointed BCRO’s; TšV Product Service / BABT being the only one based outside of North America.
The "compliance folder" will typically consist of:
- Declarations
- Design / manufacturing / test data
- Test reports
- Audit reports
- BCRO Worksheets
It is the responsibility of the BCRO to determine that all relevant requirements are satisfied and sufficiently documented. The BCRO will then provide an evaluation report and completed BCRO Worksheets to the CTIA, recommending that the cellular product system be registered on the CTIA certification database. Under the registration phase, compliance is established by independent evaluation of system vendor supplied data and results; no compulsory 3rd party testing and auditing are required.
The 2nd stage of the programme will include elements of mandatory 3rd party testing and auditing and will introduce certification categories covering cells, battery packs & power adapters (Recognised) and complete cellular product systems (Listed).
References
1) www.uspto.gov
2) Gold Peak Industries Ltd Lithium Ion Technical Handbook
3) www.sanyo.com
4) www.buchmann.ca
5) Scientific American Magazine December 2006
6) www.gloucestershire.gov.uk
