Lithium-ion (Li-ion) has become the dominant rechargeable battery chemistry for consumer electronics devices (e.g., smart phones and notebook computers) and is poised to become commonplace for industrial, transportation, and power-storage applications. Li-ion battery chemistry is different from previously popular rechargeable battery chemistries ( e.g., nickel metal hydride [NiMH], nickel cadmium [NiCad], and lead acid) in a number of ways. From a technological standpoint, because of high energy density, Li-ion technology has enabled entire families of portable devices, such as smart phones. From a safety and fire protection standpoint, a high energy density coupled with a flammable organic, rather than traditional aqueous electrolyte, has created a number of new fire protection challenges. Specific challenges include the design of batteries containing Li-ion cells, the storage and handling of these batteries, and challenges in determining the best response to suppress and control fires involving Li-ion batteries.

The Fire Protection Research Foundation (FPRF) completed an assessment of the hazards associated with Li-ion batteries related to storage of Li-ion batteries and fire protection; this article provides a brief overview of this work to-date.1 Before the global fire safety challenges associated with Li-ion batteries can be addressed, an understanding of Li-ion technology is useful and follows.

Li-ion Cells and Batteries

The term Li-ion battery refers to an entire family of battery chemistries. It is beyond the scope of this article to describe all of the chemistries used in commercial Li-ion batteries. Li-ion battery chemistry is an active area of research, and new materials are constantly being developed. Li-ion cells are distinct from lithium (orlithium primary cells). The term “lithium cell” most accurately refers to non-rechargeable battery chemistry, where lithium metal is used as one of the cell electrodes. Lithium metal is not used as an anode in Li-ion cells. However, the similarity in these two names has routinely led to confusion with regards to appropriate fire protection techniques. The following is an over view of rechargeable Li-ion technology and focuses on the characteristics of Li-ion batteries common to the majority of available batteries.

In the most basic sense, the term Li-ion battery refers to a battery where the negative electrode (anode) and positive electrode (cathode) materials serve as a host for the lithium ion (Li+). Lithium ions move from the anode to the cathode during discharge and are intercalated into (inserted into voids in the crystallographic structure of) the cathode. The ions reverse direction during charging (see Figure 1). Since lithium ions are intercalated into host materials during charge or discharge, there is no free lithium metal within a Li-ion cell; thus, if a cell ignites due to external flame impingement or an internal fault, metal fire suppression techniques are not appropriate for controlling a Li-ion battery fire. Under certain abuse conditions, lithium metal in very small quantities can plate onto anode surfaces. However, this should not have any appreciable effect on the fire behavior of the cell.

Cells can be constructed by stacking alternating layers of electrodes (such as in prismatic cells or by winding long strips of electrodes into a “jellyroll” configuration typical for cylindrical cells. Generally, cell form factors are classified as prismatic, cylindrical, and pouch cells (also known as polymer, soft-pack polymer, or lithium polymer).

In a Li-ion cell, alternating layers of anode and cathode are separated by a porous film (separator). An electrolyte composed of an organic solvent and dissolved lithium salt provides the media for lithium ion transport. A variety of safety mechanisms might also be included in a cell mechanical design, such as current interrupt devices (CID) and positive temperature coefficient switches.

An individual Li-ion cell will have a safe voltage range over which it can be cycled that will be determined by the specific cell chemistry. A safe voltage range will be a range in which the cell electrodes will not rapidly degrade due to lithium plating, copper dissolution, or other undesirable reactions. For most cells, charging significantly above 100% state of charge (SOC) can lead to rapid, exothermic degradation of the electrodes. Charging above the manufacturer’s high voltage specification is referred to as overcharge. Since overcharging can lead to violent thermal runaway reactions,2 a number of overcharge protection devices are either designed into the cells or included in the electronics protection packages for Li-ion battery packs.

A Li-ion battery (or battery pack) is made from one or more individual cells packaged together with their associated protection electronics. By connecting cells in parallel, designers increase pack capacity. By connecting cells in series, pack voltage is increased.

For large format battery packs, cells may be connected together (in series or in parallel) into modules. The modules may then be connected in series or in parallel to form full battery packs. Thus, large format battery pack architecture can be significantly more complex than small consumer electronics battery packs, which typically contain series connected elements consisting of two or more parallel-connected cells.

The four primary functional components of a practical Li-ion cell are the anode, cathode, separator, and electrolyte. Additional components of Li-ion cells, such as the current collectors, case or pouch, internal insulators, headers, and vent ports also affect cell reliability, safety, and behavior in a fire.


Figure 1. Li-ion cell operation: during charging, Lithium ions intercalate into the anode; the reverse occurs during discharge

The chemistry and design of these components can vary widely across multiple parameters. Cell components, chemistry, electrode materials, particle sizes, particle size distributions, coatings on individual particles, binder materials, cell construction styles, etc., generally will be selected by a cell designer to optimize a family of cell properties and performance criteria. As a result, no “standard” Li-ion cell exists, and even cells that nominally appear to be the same (e.g., lithium cobalt oxide/graphite electrodes) can exhibit significantly different performance and safety behavior. In addition, since Li-ion cell chemistry is an area of active research, one can expect cell manufacturers to continue to change cell designs for the foreseeable future.

Li-ion Technology Applications

Li-ion cells have gained a dominant position in the rechargeable battery market for consumer electronic devices. The primary reason for Li-ion battery dominance is the chemistry’s high specific energy (Wh/kg) and volumetric energy density (Wh/L), or more simply, the fact that a Li-ion cell of a specific size and weight will provide substantially more energy than competing technologies of the same size or weight. Li-ion cells have enabled smaller, more slender, and more feature-rich portable electronics. The smallest Li-ion cells are found in devices such as hearing aids and Bluetooth headsets. Larger, single cell applications include batteries for digital cameras, MP3 players, and e-readers.The most common single-cell Li-ion battery applications are cell phones and smartphones.

For larger electronic devices, such as notebook computers, power tools, portable DVD players, and portable test instruments, multi-cell battery packs are used. Multi-cell devices, such as notebook computer battery packs, utilize complex protection electronics.

Notebook computers represent the largest population of relatively complex Li-ion batteries in the commercial market. Many of these packs contain between six and 12 cylindrical, 18, 650-size cells connected in series and parallel, though smaller cylindrical cells and flat soft pouch Li-ion polymer cells are becoming more common (see Figure 2).

The demand for Hybrid Electric Vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and purely electric vehicles (EVs) is expected to increase. At present, many hybrid vehicles implement NiMH batteries. A few vehicles that implement Li-ion battery technology have recently entered the U.S. market.


Figure 2. Examples of 18, 650 cylindrical cells (these are the most common consumer electronics Li-ion cell form factor).

With penetration of electric vehicles comes the addition of charging stations in public areas, as well as in private residences. Automotive battery packs will also be serviced and thus, stored at service and battery switching locations. This new type of infrastructure will pose high voltage and fire safety challenges in addition to those associated with Li-ion batteries themselves.

Considerable interest has been generated in the last two to three years for applying Li-ion batteries for a variety of energy storage and grid stabilization (stationary) applications.3 Prototype systems have been installed.4 Megawatt-scale systems typically include thousands of cells housed in shipping container sized structures that can be situated on power utility locations. These systems usually include integrated fire suppression in their installations. Smaller systems have also been planned and are being delivered for evaluation purposes, particularly for use with renewable energy sources.

Li-ion Battery Failures

The fact that batteries can fail in an uncontrolled manner on rare occasions has brought an increased public awareness for battery safety, in particular as a result of some very large product recalls of portable notebook computer and cell phone batteries. Both energetic and non-energetic failures of Li-ion cells and batteries can occur for a number of reasons, including poor cell design (electrochemical or mechanical), cell manufacturing flaws, external abuse of cells (thermal, mechanical, or electrical), poor battery pack design or manufacture, poor protection electronics design or manufacture, and poor charger or system design or manufacture. Thus, Li-ion battery reliability and safety is generally considered a function of the entirety of the cell, pack, system design, and manufacture.5,6

Performance standards are designed to test cell and battery pack designs. Failures that occur in the field are seldom related to cell design; rather, they are predominantly the result of manufacturing defects or subtle abuse scenarios that result in the development of latent cell internal faults.

Cell and Battery Failure Modes

Li-ion batteries can fail in both non-energetic and energetic modes. Typical non-energetic failure modes (usually considered benign failures) include loss of capacity, internal impedance increase (loss of rate capability), activation of a permanent disabling mechanism such as a CID, shut down separator, fuse, or battery pack permanent disable, electrolyte leakage with subsequent cell dry-out, and cell swelling.

Often, energetic failures lead to thermal runaway. Cell thermal runaway refers to rapid self-heating of a cell derived from the exothermic chemical reaction of the highly oxidizing positive electrode and the highly reducing negative electrode; it can occur with batteries of almost any chemistry.

In a thermal runaway reaction, a cell rapidly releases its stored energy. The more energy a cell has stored, the more energetic a thermal runaway reaction will be. One of the reasons Li-ion cell thermal runaway reactions can be very energetic is these cells have very high-energy densities compared to other cell chemistries. The other reason that Li-ion cell thermal runaway reactions can be very energetic is because these cells contain flammable electrolyte. As a result, not only do they store electrical energy in the form of electrochemical potential energy, they store appreciable chemical energy (especially compared to cells with water-based electrolytes) in the form of combustible materials.

The severity of a cell thermal runaway event will depend upon a number of factors, including the SOC of a cell (how much electrical energy is stored in the form of chemical potential energy), the ambient environmental temperature, the electrochemical design of the cell (cell chemistry), and the mechanical design of the cell (cell size, electrolyte volume, etc.).

For any given cell, the most severe thermal runaway reaction will be achieved when that cell is at 100% (or greater, if overcharged) SOC, because the cell will contain maximum electrical energy. If a typical fully charged (or overcharged) Li-ion cell undergoes a thermal runaway reaction, a number of things occur, including:

  • Cell internal temperature increases;
  • Cell internal pressure increases;
  • Cell undergoes venting;
  • Cell vent gases may ignite;
  • Cell contents may be ejected; and
  • Cell thermal runaway may propagate to adjacent cells.

Root Causes of Energetic Cell and Battery Failures

There are a number of ways to exceed the thermal stability limits of a Li-ion cell and cause an energetic failure. Energetic Li-ion battery failures may be induced by external forces, such as exposure to fire or mechanical damage, or they may be the result of problems involving charge, discharge, and/or battery protection circuitry design and implementation, or they may be caused by internal cell faults that result from rare or subtle manufacturing problems. Generally, the root causes of energetic cell and battery failures can be classified as:

  • Thermal abuse (e.g., external heating);
  • Mechanical abuse (e.g., denting, dropping);
  • Electrical abuse (e.g., overcharge, external short circuit, over discharge);
  • Poor cell electrochemical design (e.g., imbalance between positive and negative electrodes); and
  • Internal cell faults associated with cell manufacturing defects (e.g., foreign metallic particles, poor electrode alignment).

Factors That Influence the Effect of Failure

The severity of a Li-ion cell failure will be strongly affected by the total energy stored in that cell: a combination of chemical energy and electrical energy. Thus, the severity of a potential thermal runaway event can be mitigated by reducing stored chemical energy (i.e., by reducing the volume of electrolyte within a cell), or by changing the electrolyte to a noncombustible material (i.e., the cell chemistry).

The most flammable component of a Li-ion cell is the hydrocarbon-based electrolyte. The hydrocarbon-based electrolyte in Li-ion cells means that under fire conditions, these cells will behave in a fundamentally different way than lead acid, NiMH or NiCad cells, which contain water-based electrolytes.

Although all charged cells contain stored electrical energy, even fully discharged Li-ion cells contain appreciable chemical energy that can be released through combustion of the electrolyte. Water-based battery chemistries, under some charging conditions, can produce hydrogen gas through electrolysis of the water; however, this hazard is seldom a concern during storage where no charging occurs.

If cells with water-based electrolyte are punctured or damaged, leakage of the electrolyte can pose a corrosive hazard; however, it does not pose a flammability hazard. In comparison, leakage or venting of Li-ion cells will release flammable vapors. Fire impingement on Li-ion cells will cause release of flammable electrolyte, increasing the total heat release of the fire (assuming there are well-ventilated conditions).

Other combustible components in a Li-ion cell include a polymeric separator, various binders used in the electrodes, and the graphite of the anode.

When a cell vents, the released gases mix with the surrounding atmosphere. Depending upon a number of factors, including fuel concentration, oxygen concentration, and temperature, the resulting mixture may or may not be flammable.

Fire Behavior of Cells and Battery Packs

Currently, there is no publicly available data from large-scale Li-ion cell or battery pack fire tests. There are a number of reasons for the lack of large-scale test data. The Li-ion cell industry has been evolving rapidly, so there has been an inherent difficulty in defining an “average” cell, battery pack, or device. Thus, if testing were to be conducted and considered reasonably comprehensive, it would require testing of multiple models of cells, packs, or devices from multiple suppliers, and even so might quickly become obsolete, as cell chemistries and mechanical designs evolve.

In 2010, testing was conducted involving a consumer electronic device package that contained a Li-ion battery pack, and a cell within a consumer electronics device package undergoing a thermal runaway reaction (no external heating).7

The observations from this testing may have significant implications on firefighting procedures, specifically fire protection and fighting strategies, fire scene overhaul procedures, and fire scene monitoring for rekindles. Specifically, if a fire occurs adjacent to stored Li-ion cells and battery packs, those cells and battery packs must be protected from relatively modest (compared to flashover) overheating, or cells may begin to vent and ignite, spreading the fire more rapidly than would be expected for normal combustibles.

On fire scenes where large quantities of Li-ion cells have been involved, decisions regarding overhaul procedures must be made with an understanding that as cells are uncovered, moved, or damaged by overhaul procedures, they may undergo thermal runaway reactions and vent, they may ignite, and they may generate (or may themselves become) hot projectiles. Similarly, the potential for rekindles will be high at such fire scenes, and these scenes will require extended monitoring.

As these products saturate the marketplace, distribution, storage, warehousing and retail locations will store Li-ion batteries, as well as the products that contain them. With new battery technologies come new hazards and new challenges for determining the best way to suppress and control fires, including determination of the most effective suppression agents.

Fire Protection Standards

Li-ion batteries and battery packs have a higher energy density than other, more common battery types, which is appealing to the end user, but provides distinct fire protection challenges given the current body of knowledge available regarding Li-ion battery fires.

At present, the authors are not aware of any fire protection standards specific to Li-ion cells. None of the widely accepted standards applicable to Li-ion battery packs includes water application tests. The publicly available information from testing conducted to date does not allow a comprehensive assessment of whether traditional water-based automatic sprinkler systems, water mist systems, or some other water-based suppression system would be most effective in the protection of stored Li-ion cells or batteries.

Water-based automatic sprinklers are the most widely used fire suppression system and have proven their efficiency and reliability over the years. Many locations are currently provided with the infrastructure necessary to facilitate suppression strategies using water-based suppression systems. Therefore, based on current knowledge and infrastructure, a water-based fire suppression system is the strongest candidate for the protection of stored Li-ion cells and batteries. As warehouse and retail spaces see an increase in the volume of these products, the current codes and standards do not provide adequate guidance on how to best protect Li-ion batteries or classification of their commodity type.

Commodity classifications for water-based suppression strategies are described in NFPA 13,8 which addresses sprinkler system applications and proposes requirements for storage protection. Commodity classifications relate directly to the fire protection system design requirements.

Classification of actual commodities is primarily based on comparing the commodity to be protected to the definitions for the various commodity classes. NFPA 13 provides a list of commodity classes for various commodities.8 Once the commodity classification is known along with the geometry and configuration of the stored product, sprinkler design densities can be selected. Different types of batteries and the recommended commodity classification for those batteries are mentioned:

  • Dry cells (non-lithium or similar exotic metals) packaged in cartons: Class I (i.e., alkaline cells);
  • Dry cells (non-lithium or similar exotic metals) blister packed in cartons: Class II (i.e., alkaline cells);
  • Automobile batteries - filled:Class I (i.e., lead acid batteries with water-based electrolyte); and
  • Truck or larger batteries, empty or filled Group A Plastics (i.e., lead acid batteries with water-based electrolyte).

Currently, NFPA 13 does not provide a specific recommendation of a commodity classification for Li-ion cells or complete batteries containing several cells. A number of features specific to Li-ion batteries could make any of the existing battery classifications inaccurate and the recommended fire suppression strategy may not be appropriate:

  • Flammable versus aqueous electrolyte;
  • The potential to eject electrodes/case material (projectiles) upon thermal runaway;
  • Latency of thermal runaway reactions (cell venting can occur sequentially and after a significant delay resulting in re-ignition of materials);
  • Large format battery packs may exhibit voltages much higher than typical truck batteries; and
  • Individual cells generally have metal versus plastic outer shells.

The venting and projectile potential of Li-ion cells has some similarities with aerosol products, which typically utilize a flammable propellant, such as propane, butane, dimethyl ether, and methyl ethyl ether. However, these products generally have no associated electrical energy and are not as susceptible to re-ignition events. As they contain flammable electrolyte, Li-ion cells might also be compared to commodities such as ammunition or butane lighters in blister packed cartons (high-energy density).

For commodities not specifically covered by NFPA 13, full-scale fire suppression tests are typically used to determine the commodity classification. Most current sprinkler system design criteria are based on classifications of occupancies or commodities that have been developed from the results of full-scale fire suppression test data and the application of experimental results that have been shown to provide a minimum level of protection. According to the Automatic Sprinkler System Handbook:9

Where commodities are not currently defined, commodity classification testing can provide an accurate comparison between the proposed commodity and known commodity classifications. This testing is essential when determining acceptable sprinkler design criteria for new or unknown commodities where a meaningful comparison cannot be made between the given commodity and other known commodity classifications. Bench-scale testing is not useful for making precise commodity classifications.

One of the main reasons that specific test data are required when determining the commodity classification of a new or unknown commodity is that the current ability of an engineering analysis is insufficient to define sprinkler suppression characteristics.9

At present, there is no publicly available large-scale fire test data for Li-ion cells that can be used to fully assess the storage hazards of Li-ion cells or batteries or to determine an appropriate commodity classification that could be used to provide an overall fire protection suppression strategy.

The FPRF is currently preparing for Phase II of the Li-ion Hazard and Use Assessment, which will likely involve fire tests aimed at determining the fire behavior characteristics of bulk packaged Li-ion batteries with the end goal of determining commodity classifications, as appropriate, for various Li-ion battery products. This data will likely prove useful to the NFPA 13 technical committees responsible for the development of provisions related to the suppression of Li-ion battery fires in various occupancies.

R. Thomas Long, P.E., and Michael Kahn, Ph.D., are with Exponent Failure Analysis Associates; Celina Mikolajczak, P.E., is a battery consultant.

ACKNOWLEDGEMENTS

This work was made possible by the Fire Protection Research Foundation. The authors are indebted to the project steering committee members, task group members, and industry representatives for their valuable suggestions.

References:

  1. Adapted from Mikolajczak C., Kahn M., White K., Long R.T., Lithium-ion batteries hazard and use assessment, Fire Protection Research Foundation, July 2011.
  2. Reddy T.B. (ed), Linden’s Handbook of Batteries, 4th Edition, McGraw Hill, NY: 2011.
  3. Kamath H., “Integrating Batteries with the Grid,” 28th International Battery Seminar and Exhibit 2011, Curran Associates, Inc., Red Hook, NY, 2011, pp. 921-940.
  4. Gengo T., Kobayashi Y., Hashimoto T., Minami M., Shigemizu T., Kobayashi K., “Development of Grid-Stabilization Power-Storage System with Lithium-Ion Secondary Battery,” Mitsubishi Heavy Industries Technical Review 46(2) June 2009.
  5. IEEE 1625, “IEEE Standard for Rechargeable Batteries for Multi-Cell Mobile Computing Devices,” IEEE, New York, NY, 2008.
  6. IEEE 1725, “IEEE Standard for Rechargeable Batteries for Cellular Telephones,” IEEE, New York, NY, 2011.
  7. Harmon J., Gopalakrishnan P., Mikolajczak C., “US FAA-style flammability assessment of lithium-ion batteries packed with and contained in equipment (UN3481),” Exponent Report 1000025.000 A0F0 0310 FREP, The Rechargeable Battery Association, Washington, DC, March 2010.
  8. NFPA 13, Standard for the Installation of Sprinkler Systems, National Fire Protection Association, Quincy, MA, 2010.
  9. Dubay C. (ed), Automatic Sprinkler System Handbook; National Fire Protection Association, Quincy, MA, 2010.