Lithium ion battery safety challenges and UL's efforts to address them

Issue 45: Lithium ion battery safety challenges and UL's efforts to address them

By Laurie Florence

Lithium ion batteries are the energy source of choice for portable electronics from cell phones to notebook computers. They are also making inroads with products such as portable tools, toys, portable appliances and electric vehicle applications that had previously relied upon other chemistries. A key reason for this increased use of lithium ion batteries is that they can provide more energy in a smaller package.

For example, they can run all a notebook computer's energy-consuming applications for hours at a time before it is necessary to recharge the battery. Lithium ion batteries were first commercialized in the early 1990s by the Sony Corp. with lithium cobalt oxide as the most commonly used lithium ion chemistry. With the increased expansion of applications using lithium ion batteries and the search for improved materials for these new applications, other lithium ion chemistries such as lithium iron phosphate, lithium manganese oxide, etc., and more recently the implementation of nanomaterial to improve energy density are being used to manufacture commercially available lithium ion batteries.

The ability to provide a lot of energy in a small package makes lithium ion batteries an attractive energy source, but the batteries can become unstable, leading to an exothermic self-sustaining uncontrolled chemical reaction of the cell active materials, called "thermal runaway," which results in fire and typically rupture of the cell casing or even explosion of the cell. This instability can occur with external abuse, or it can happen due to internal defects such as contamination, resulting in a localized internal short circuit that leads to thermal runaway.

Although lithium ion batteries have had some bad press with large notebook computer and cell phone battery recalls in the recent past, the actual field failure rate compared to the number manufactured is extremely small, considering that over a billion lithium ion batteries have been sold in the past decade. A few incidents in the field in 2006 resulted in the recall of over four million notebook computer batteries. The fact that most people who use lithium ion battery products on a daily basis never experience these types of incidents supports this assertion. However, in the rare cases, when an event leads to a thermal runaway, the results can be severe.

 

Lithium ion batteries require protection from abuse and must be used within their operating rating parameters, which includes temperature, voltage and current. They should not be overcharged or over-discharged and they should be used within their temperature ratings. For example, a lithium cobalt oxide cell should not be charged beyond a maximum charging voltage limit of 4.25 Vdc within a charging temperature range from 10°C to 45°C. This means that protection circuitry should be built into the battery pack to monitor these values and shut down charging or discharging should the battery be reaching it limits to prevent an excess condition. It is important that the compatibility of the battery system (i.e. the battery pack, the charger and the host product energized by the battery) has been evaluated to ensure that the system maintains the batteries within their limits. Safety standards1,2 evaluate cell and battery packs and their inherent protection. End product standards3,4 include requirements that evaluate the battery system. Other documents address new applications that utilize lithium ion batteries.5,6,7,8

It is important that lithium ion batteries be provided with appropriate protection to prevent their use outside of their operating limits. Testing and evaluation of the lithium ion battery system to ensure compatibility is critical. However, some of the most publicized recalls were the result of inherent failures of the lithium ion cells, the building blocks of the lithium ion batteries, which external protection mechanisms could not stop.

These inherent failures were due to a localized internal short circuit that resulted in a thermal runaway of a cell. In multi-cell packs such as those for notebook computers, the heat generated from the failing cell resulted in the surrounding cells also going into thermal runaway.

The concern with this type of incident is that the inherent and external protection of the cell and battery pack and system could not stop this event. In addition, it was determined that the failure was due to manufacturing defects, a contaminant too small to pick up with cell screening processes, that did not lead to the field failure until after the battery had been in use for some time. What makes this issue more difficult to address is the fact that not all internal short circuit conditions will lead to a thermal runaway. More commonly, small defects within the cell do not lead to thermal runaway but simply add to the reduced performance over time as is typical of battery aging. The battery industry and others have been grappling with this issue for years.

 

Industry has made many improvements to lithium ion cell design to prevent an internal short circuit from turning into thermal runaway event. Stricter controls on manufacturing either as part of the certification program, as is the case with UL lithium ion cell certification, or as part of the standard,9,10 are methods that standards development and safety certification organizations have tried to address manufacturing controls. UL is also developing a manufacturing quality control proposal for inclusion in UL 1642.

There is a direct link to good quality lithium ion cell manufacturing and cell safety. However, even with an excellent manufacturing quality program in place, manufacturing defects will still occur. Thus, it is important that the lithium ion cell be designed to prevent internal short circuits from resulting in a thermal runaway condition. Developing a test to determine the cell's ability to withstand a localized internal short circuit event has been the goal of many researchers including UL's Corporate Research Department. Since 2007, UL researchers have been studying some classic test methods and making modifications, leading to the development of an indentation induced internal short circuit (IIISC) type test. The IIISC test consists of a localized indentation, without puncturing the cell case, until a 100 mVolt drop is recorded. Other industry groups and research labs have also been working on this issue.

The Battery Association of Japan (BAJ) has developed the forced internal short circuit (FISC) test which relies upon particle insertion, which is published in the JIS standards and being proposed in the IEC standards. Government labs such as NASA, Sandia, Argonne, NREL, and ORNL are involved in development of an internal short circuit test. At this time, there is no test that groups can agree is adequate to cover the various lithium ion cell designs (i.e. cylindrical, prismatic and pouch case types) and can address the issue of internal short circuit in a manner that will lead safer cells. UL has established a lithium test task group to assist UL research staff in this important work. The goal for the task group is to provide input to UL in order to develop a proposal for a meaningful test or if necessary a group of tests to determine a lithium ion cell's ability to withstand a localized internal short circuit condition safely.

UL has been active in making improvements to its lithium ion battery safety program through continuous improvements to its standards, stricter production surveillance criteria and researching new test methods to evaluate a lithium ion cell's ability to withstand a localized internal short circuit.


Laurie Florence is with Underwriters Laboratories Inc.

  1. UL 1642, Standard for Lithium Batteries, Underwriters Laboratories, Northbrook, IL, 2005.
  2. UL 2054, Standard for Household and Commercial Batteries, Underwriters Laboratories, Northbrook, IL, 2004.
  3. UL 60950-1, Information Technology Equipment - Safety - Part 1: General Requirements, Underwriters Laboratories, Northbrook, IL, 2007.
  4. UL 60065, Standard for Audio, Video and Similar Electronic Apparatus - Safety Requirements, Underwriters Laboratories, Northbrook, IL, 2003.
  5. UL 2575, Standard for Lithium Ion Battery Systems for Use in Electric Power Tool and Motor Operated, Heating and Lighting Appliances, Underwriters Laboratories, Northbrook, IL, 2011.
  6. UL Subject 2580, Batteries For Use In Electric Vehicles, Underwriters Laboratories, Northbrook, IL, 2009.
  7. UL Subject 2271, Batteries For Use In Light Electric Vehicle (LEV) Applications, Underwriters Laboratories, Northbrook, IL, 2010.
  8. UL Subject 1973, Batteries For Use In Light Electric Rail (LER) Applications and Stationary Applications, Underwriters Laboratories, Northbrook, IL, 2010.
  9. IEEE 1625, IEEE Standard for Rechargeable Batteries for Multi-Cell Mobile Computing Devices, IEEE, Piscataway, NJ, 2008.
  10. Draft Standard for Rechargeable Batteries for Cellular Telephones, IEEE, Piscataway, NJ, 2011.
For questions concerning delivery of this e-Newsletter, please contact our Customer Service Department at (216) 931-9934 or magazine.sfpe.org.