Lithium-ion battery (LIB) safety is a major concern for designers. LIBs are generally safe to use if they have been properly manufactured and integrated into systems. However, using sub-standard materials and the possibility of manufacturing or design defects can result in hazardous conditions.
As a result, there are several global safety standards related to LIBs. This article reviews the:
- Potential of and consequences of thermal runaway in LIBs
- Commonly cited LIB safety standards
- Non-standard nature of “nail penetration” testing
- Hazard levels associated with LIBs in electric vehicles (EVs)
Thermal runaway (TR) in LIBs can be a catastrophic event. It can have several causes, such as dendrite formation, manufacturing defects, physical damage, and more. It typially doesn’t happen at once but in a series of stages.
The first stage is the start of overheating (Figure 1). If not extinguished, the solid electrolyte interface (SEI) decomposes, and heat builds up, resulting in more side reactions, possibly melting the separator. As the heating continues and the pace of temperature rise increases, the TR phase is entered.
TR is a self-heating rate of at least 10° C /min. As it accelerates, TR leads to uncontrollable temperatures, and the cell may burst or explode. Temperatures of 200° C or higher will be experienced, and the cell will catch fire.
Small LIB standards
The three most-commonly cited LIB safety standards are:
- UN/DOT 38.3 5th Edition, Amendment 1 — recommendations on the Transport of Dangerous Goods
- IEC 62133-2:2017 — safety requirements for portable sealed secondary lithium cells, and batteries made from them, for use in portable applications — part 2: lithium systems
- UL 2054 2nd Edition — household and Commercial Batteries
LIBs that do not satisfy UN/DOT 38.3 are limited to shipment via ground transport as Class 9 Hazardous Goods. While it’s possible to self-certify to this standard, most companies use third-party testing labs to limit liability. UN 38.3 includes a combination of environmental, mechanical, and electrical stresses in sequence (T1-T5):
- T1 — altitude simulation (primary and secondary cells and batteries)
- T2 — thermal test (primary and secondary cells and batteries)
- T3 — vibration (primary and secondary cells and batteries)
- T4 — shock (primary and secondary cells and batteries)
- T5 — external short circuit (primary and secondary cells and batteries)
- T6 — impact (primary and secondary cells)
- T7 — overcharge (secondary batteries)
- T8 — forced discharge (primary and secondary cells)
The vibration test (T3) is the most challenging because it includes survival of intense vibration for three hours in each of the three cardinal planes. The sequence from T1 to T5 typically has a negative cumulative impact, making it more challenging to pass T6, T7, and/or T8 — depending on the requirements.
UN/DOT 38.3 is an integral requirement of IEC 62133-2:2017. IEC 62133-2 adds four additional tests:
- Molded case stress
- External short circuit
- Free fall
- Overcharging of the battery
These tests are not nearly as challenging as the tests required to meet UN/DOT 38.3.
The future of UL 2054 is unclear. Typically, compliance with US end-device standards is mandated as it requires a single fault to be applied to the battery pack. The first edition of UL 62133 has been released and aligns with IEC 62133. The requirements of UL 2054 and UL 62113 are fairly different.
For example, IEC 62133 is primarily concerned with fire or explosion hazards with the battery, while UL 2054 looks at exposure concerns. As a result, the IEC test specifies the use of fully charged batteries, and the UL test specifies the use of fully discharged batteries.
UL 2054 is the more complex of these standards and includes about twice the number of tests compared with UN/DOT 38.3 or IEC/UL 62113:
- 7 Electrical tests
- 4 Battery enclosure tests
- 4 Mechanical tests
- 1 Fire exposure test
- 2 Environmental tests
Cylindrical, pouch, prismatic, or coin?
The four most common LIB cell formats are cylindrical, prismatic, coin, and pouch. Cylindrical cells are available in high- and low-capacity formats. Low-capacity cells have lower impedances and are used in more applications (for a discussion of the various cylindrical LIBs, see “18650, 21700, 30700, 4680, and other Li-ions – what’s the difference?”).
Prismatic cells typically have larger capacities and can simplify system design, but they’re also more costly than cylindrical cells. Pouch cells are gaining popularity since they offer packaging flexibility to match the cell to a specific device shape. Coin cells are inherently low-capacity devices and are used in small portable devices.
Internal short-circuiting is the most critical abuse test applied to most LIBs. In real-world applications, internal short-circuiting can arise from various causes, such as mechanical damage or production flaws.
The most impressive form of internal short-circuit testing can be the “nail penetration” test. Generally, nail penetration testing is reserved for scientific investigations into thermal runaway and internal short-circuiting of LIBs. It’s not a standardized test and is subject to several variables, including the type and size of the nail used, the rate at which the nail penetrates the cell, the state of charge of the cell, and others. (Figure 2).
A cell’s design and size affect its ability to keep it cool. Larger cell sizes are a more significant safety concern and are at risk of faster degradation. Cylindrical cells are the safest, prismatic cells are the least safe, and pouch cells are somewhere in the middle.
Although prismatic cells can have higher heat dissipation due to their larger surface area, the gaps between cylindrical cells in battery packs tend to simplify thermal management. Cylindrical and prismatic cells have metallic casings that can withstand high pressures. However, if there’s an internal short circuit, this is a concern since pressures can build up to potentially explosive levels.
Pouch cells tend to burst under lower pressures, can catch fire, and rarely explode. Determining the ideal cell format for a vehicle is a complex process, and most EV manufacturers use each of the three packaging formats.
Various regional and global EV battery safety standards have differing requirements, adding to the complexity. (Table 1).
Hazard levels
EVs combine hundreds to thousands of cells in the battery pack, increasing the challenges for battery safety. Under normal operating conditions, EV battery thermal management is difficult. The challenges can become unmanageable when abnormal conditions occur, such as an accident. That’s where the concept of “hazard levels” is applied.
The EUCAR and SAE-J Hazard Levels, along with their associated criteria, are widely used to assess the safety of EV battery packs (Table 2).
Summary
A carefully developed LIB in a well-designed system that’s properly used poses extremely low safety hazards. The LIB standards recognize that perfection does not exist, even with the most advanced manufacturing and system integration technologies. So, these standards aim to maximize safety under real-world conditions.
As the standards are under constant review and are evolving, designers must keep up with the latest developments.
References
A review of lithium-ion battery safety concerns: The issues, strategies, and testing standards, Journal of Energy Chemistry
Button and coin batteries safety requirements specification, British Standards Institute
Development of a New Procedure for Nail Penetration of Lithium-Ion Cells to Obtain Meaningful and Reproducible Results, Journal of the Electrochemical Society
IEC 62133-2:2017, International Electrotechnical Commission
Products Incorporating Button or Coin Cell Batteries of Lithium Technologies, UL 4200A, Underwriters Laboratories
Safety Issues in Lithium Ion Batteries: Materials and Cell Design, Frontiers in Energy Research
Tracking Internal Temperature and Structural Dynamics during Nail Penetration of Lithium-Ion Cells, Journal of the Electrochemical Society
UN DOT 38.3 Testing, TÜV SÜD
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