If recent media reports are to be believed, battery-powered electric vehicles (EVs) are death traps with the potential to burn their drivers to death in the event of a crash. The reality is that batteries in EVs are no more prone to catching fire than a conventional gas tank filled with unleaded fuel.1 In fact, completely “fire safe” chemistries have been developed.
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One of the most high-profile incidents occurred last October involving a Tesla Model S in Seattle (see the figure). Photos of the fire, caused by hitting road debris, are dramatic. But it’s important to note that the driver was unharmed, and while the battery itself did have cascading cell ruptures within the pack and burned to ash, it did not explode.
“The fire was caused by the direct impact of a large metallic object to one of the 16 modules within the Model S battery pack. Because each module within the battery pack is, by design, isolated by fire barriers to limit any potential (module-to-module cascading) damage, the fire in the battery pack was contained to a small section in the front of the vehicle,” Tesla said in a statement to the press (italics added).
This file type includes high resolution graphics and schematics when applicable.
The Tesla S battery cells are small cylindrical lithium-cobalt-nickel-oxide cells from Panasonic. While they did go into “thermal-runaway” in succession when exposed to high temperatures, most lithium-ion (Li-ion) chemistries will burn similarly when exposed to direct flames.2
Of course, battery safety is an issue when designing electrified transportation. But new materials coupled with advanced design techniques and rigorous safety testing have brought EVs to the point where they are as safe as houses—or more importantly, safer than conventional automobiles. In fact, one in every four fire department responses is to a conventional vehicle fire.3
Safety is the paramount concern, though environmental concerns linger as well. EVs don’t emit any pollutants, which is part of their appeal. But can batteries leak harmful materials if they’re defective or old? Will they explode upon impact, or penetration, or when enveloped in direct flames? Can they be disposed of safely when they have expired without poisoning the water supply?
All of these questions are valid. Yet it’s critical that consumers and industry understand that EVs and, more specifically, their batteries can be employed on a large scale without a detrimental impact on the planet and its inhabitants.
Balancing Density With Safety
Several different Li-ion battery formulations are available for electric vehicles. One of the key safety considerations is how much oxygen is released, when decomposing (in high temperatures), to catalyze combustion—the feared fires highlighted in media reports. This is what we call thermal stability or thermal-runaway thresholds.
A lithium-cobalt-oxide battery, for example, decomposes rapidly with highly flammable electrolyte (burning at temperatures well above the melting points of many metals such as aluminum) and begins releasing high volumes of oxygen at temperatures less than 180°C. Panasonic uses this battery formulation in Tesla electric vehicles. Meanwhile, the lithium-manganese formulation starts decomposition at yet higher temperatures, but still contains highly flammable electrolytes and releases high volumes of oxygen as well. These battery formulations can be found in the Nissan Leaf (Japanese cells from NEC) and GM Volt (Korean cells from LG Chemical) EV battery cells.
A third option is a lithium-iron-phosphate battery, or a variation depending on the dominant materials, an iron-phosphate battery. The lithium-iron-phosphate battery remains stable at temperatures as high as 600°C. Just as significantly, in some formulations, no oxygen is released during decomposition, which means there is no catalyst for additional combustion.
There are tradeoffs, however. The price for thermal stability can be energy density, but this is an acceptable compromise in vehicle applications where space is available. Cell chemistry is heavily influenced by the demand for extended battery life. This has resulted in higher energy and power densities that require more reactive chemical combinations, which in turn increase the risk of danger through thermal events and possible cell failures. This is where a compromise is required that balances maximum power delivered with fire safety.
Stability Thanks To Better Chemistry
Most commercially available Li-ion batteries comprise an inorganic lithium-intercalating compound as a positive electrode, a lithium-intercalating carbon negative electrode, and a lithium salt in an organic liquid, known as the electrolyte. Some sort of insulator such as a thermoplastic polymer separates both electrodes.
Most manufacturers choose polypropylene, which has a melting point of 160°C and is very resistant to many chemical solvents, bases, and acids. When the cell charges and discharges, lithium ions move between the cathode (the positive electrode) and the anode (the negative electrode). Upon discharge, the anode loses electrons, while the cathode gains them.
Li-ion cells have historically used lithium metal oxides as cathode materials due to their high capacity for lithium intercalation and their suitable chemical and physical properties required for Li-ion electrodes. Layered materials, such as lithium cobalt oxide and lithium nickelates, or a combination of these metals, have been the most extensively used and investigated for cathodes. These types of cathodes demonstrate excellent performance, but suffer from higher cost and significant toxicity. They also suffer heavily in thermal instability, which can lead to chemical thermal runaway.
Preventing A Thermal Event
A thermal event occurs when rapidly increased temperatures cascade out of control, causing rapid disassembly and possible explosion. Such an explosive event may result in throwing shrapnel and projecting its flaming contents, a phenomenon known as “venting.” To avoid thermal instability, other lithium-metal-oxide materials with a “spinel” structure, such as lithium-manganese spinels, have been proposed to substitute the layered materials. This oxide is inexpensive and environmentally friendly, but it has significant disadvantages related to capacity degradation issues (i.e., the Leaf battery cells will degrade faster than the Tesla battery cells, diminishing vehicle range faster over time), especially at elevated operating temperatures (reducing cycle life significantly).
Both lithium-iron-phosphate and iron-phosphate batteries have excellent thermal stability. The iron phosphate releases oxygen in temperatures around 410°C and at a rate of 210 J/g. In contrast, lithium cobalt oxide begins to decompose oxygen at only 240°C and at a rate of more than 1000 J/g. This dramatic release of oxygen is the main reason why Li-ion batteries can rapidly flame up and explode (or vent) during thermal events. A user can easily see this phenomenon by placing lithium-cobalt-oxide batteries (the ones commonly used in cell phones and laptops) in direct flames. There is an overall industry agreement on the superior thermal stability of lithium iron phosphate and the recognition that it is a safer cathode material than the commonly used lithium-metal-oxide cathodes.
Designing To Prevent Problems
In addition to chemistry and the design of the cell, the battery and the battery compartment are just as critical to ensure optimum, reliable, and safe operation. Many problems that are normally attributed to the battery can be prevented through proper precautions taken during the design of the cell and battery packs, including cell chemistry, electrode design, pack capacity design, mechanical design, cell construction, and venting design.
Electrode design—including the testing and verification of electrode structures using current distribution models, thermal modes, electrochemical modes, and mechanical models—makes it possible to reduce the resistance and the optimize the current and thermal distribution in the cell. Stable current and thermal distribution ensures the long-term stability and safety of the cell.
Generally speaking, the higher the capacity of the cell is, the greater the risk of instability. This is where pack capacity design plays a role. By optimizing cell capacity and safety using failure modes and effects analysis (FMEA) calculations and real-world safety testing, mechanical design can address safety issues such as seal integrity and anti-eroding levels.
For higher-power cells, the thermal design can be a source of weakness, so eliminating the excess heat produced while charging and discharging cells is the focus. This is where cell construction plays a role, as poor product design often results in localized hotspots within the cell, which may lead to premature cell failures. Optimum thermal performance for high-power cells requires substantial thermal conduction paths.
If other safety devices fail or a cell is exposed to higher than normal operational temperatures, chemical reactions may result in a process known as out-gassing, in which the active materials expand. This can build up pressure inside the sealed cell, which may rupture the cell casing and cause a corresponding pop or loud bang.
Safety vents are needed as a final precaution to release this potential increase in pressure before it reaches a critical level. Automatic release guard vents prevent the absorption of external air into the cell, but allow controlled release of excess internal pressure to avoid leakage and prevent uncontrolled rupture of the cell casing.
Proper design needs to be followed up by high-quality manufacturing to realize safety and performance goals. This is accomplished through strict adherence to quality control throughout the entire manufacturing process. A single defect such as burrs on the electrodes, misaligned or out of tolerance components, or contaminated electrode coatings or electrolytes can cause short circuits or separator penetration, resulting in latent thermal events later in cell cycling.
A lithium-iron-phosphate battery cathode looks to be the safest cathode material available because there is no thermal runaway mechanism and no oxygen generated during decomposition. It is also the most robust when cycled, due to the fact that there is no net-net volume gain (oxidation) that can result in premature cell swelling, impedance growth, and electrolyte starvation due to excessive internal pressures.
Test, Test, And Test Again
Testing battery modules and pack designs obviously is a key part of the quality control process and ultimately guaranteeing overall safety. Several different types of testing should be applied:
• Vibration/shaker-table testing: This testing simulates roadway vibrations during extended periods that equal how long a battery might be on the road in an active vehicle for a normal period of operation.
• Thermal shock test: The reliability of a battery when the vehicle would be operated at extreme temperature ranges also needs to be tested, with a recommended temperature range between 85°C, ±2°C, to –40°C, ±2°C.
• Salt spray test: Exposure to salt can come from environments that are near the ocean and from environments that use road salt in the winter. Testing over the course of months is recommended to ensure that salt exposure does not lead to conditions that might cause the battery module to catch fire, explode, rupture the enclosure, or leak electrolyte outside the enclosure.
• Crush testing: The safety of the battery is tested under conditions where it is impacted directly during a crash and crushed, much like Tesla’s real-world under carriage impacts. Packs should experience more than 100 kilonewtons of force when fully charged. The impact may render the module non-functional, but it should not catch fire or explode, as Tesla’s did.
• Short-circuit testing: The safety of the battery is tested when all printed-circuit board (PCB) assembly protection circuit devices fail to work, and the battery is “hard” short-circuited. While the module may be rendered non-functional, the module must not catch fire or explode.
• Pack level tests: These tests include simulated collisions with different objects at a variety of speeds inflicted directly on the pack.
• Fire and gas flaming testing: Packs also should be subjected to fire, including a one-hour fire simulation test in which the vehicle has caught fire from some external combustion source. This generally results in a non-functioning battery, but the goal is to make sure cells do not catch fire and explode, potentially hurting rescue workers and emergency personnel. The same outcome is desired for a longer-term, gas flaming test, where the battery is entirely destroyed in fire to make sure it does not explode even when placed in direct flames over longer periods of time. The individual cells, modules, and pack-casings may be consumed, the separators may melt, and the plastic components of the battery and organics may be consumed in the flames. But the goal is to eliminate the risk of an explosion that results in flying debris or shrapnel, which is common in EV batteries subject to thermal events.
A gas flaming test is a complete consumption test that evaluates the ultimate safety and stability of the battery and chemistry in the most extreme of conditions, as the pack is continually bombarded with flames from an external source. While the pack might catch fire and burn to ash, success is achieved if the pack does not explode. This is a test presently missing in U.S. Department of Transportation, Federal Motor Vehicle Safety Standard, and Society of Automotive Engineers standards requirements, but it is included in many of China’s vehicle safety testing programs. It should be added to the U.S. standards if we are to ensure public safety.
Safety In The Long Term
The safety of batteries while in use clearly is the most critical concern of drivers of electric vehicles, but the disposal of batteries when they finally achieve their end of life also is a concern. There are strict recycling requirements for lead-acid, nickel-cadmium, and nickel-metal-hydride batteries. There are even more strict requirements and environmental concerns for most Li-ion batteries.
As discussed before, all Li-ion cobalt and manganese batteries contain a toxic and flammable electrolyte. If these battery cells are thrown in a landfill improperly and decompose, they will leak. Unfortunately, the Li-ion battery electrolyte has a propensity to mobilize other heavy metals easily into the water stream and is extremely toxic. This is not so for BYD’s iron-phosphate chemistry.
Ultimately, iron-phosphate batteries are the most appealing from an environmental standpoint. They contain no toxic electrolytes, nor do they contain any heavy metals in either the cathode or the anode. Also, they are not manufactured with any caustic or harmful materials. They are completely environmentally friendly, while also boasting a very good energy density.
With proper adherence to quality control throughout the manufacturing process, combined with state-of-the-art technology and rigorous safety testing, the thermal stability of iron-phosphate batteries exceeds fire safety standards. In general, the fear about commonly used EV batteries is unfounded, and these vehicles are safer than conventional ones. There are also chemistries that are more advanced and “fire safe” that can be used to increase the trust that is due to the electrified transportation community.
References
1. “Electric Cars Pose Different Risks, U.S. Regulator Says,” Angela Greiling Keane and Bernard Kohn, www.bloomberg.com/news/2014-01-14/electric-cars-pose-different-risks-u-s-regulator-says.html
2. “Panasonic Enters Into Supply Agreement With Tesla Motors To Supply Automotive-Grade Battery Cells,” www.teslamotors.com/about/press/releases/panasonic-enters-supply-agreement-tesla-motors-supply-automotivegrade-battery-c
3. U.S. Fire Administration’s Topical Fire Research Series, July 2001.
Micheal Austin is a founding board member of the IEEE Transportation Electrification Initiative. Entering his eighth year as vice president, he has played an integral role in bringing BYD to the Americas. Originally from Colorado, he attended Brigham Young University (BYU) earning a bachelor’s degree in design engineering as well as a master’s in mechanical engineering. Prior to joining BYD, he was with Motorola for 15 years serving as a senior director and in engineering, honored with Motorola’s Distinguished Innovator award for receiving 23 patents. Though he spends much of his time serving as BYD’s spokesman leading the overseas PR and marketing departments, he remains actively involved in the Institute of Electrical and Electronics Engineers (IEEE) and is editor-in-chief of the IEEE’s TEI monthly publication.