Media Reports Fuel Unfounded Fears about Electric Vehicle Batteries

Republished with permission by electronic design  Media Reports Fuel Unfounded Fears About EV Batteries

New chemistries coupled with advanced design techniques have birthed battery technologies that provide high energy density while meeting stringent safety requirements exceeding US standards.

By Micheal Austin

If recent media reports are to be believed, battery-powered electric vehicles are death traps with the potential to burn their drivers to death in the event of a crash.  The reality is that batteries in electric vehicles are no more prone to catching fire than a conventional gas tank filled with unleaded fuel[1], and chemistries have been developed that are now completely “fire safe”. 

Probably one of the most high profile incidents is from this past October involving a Tesla Model S in Seattle.  Photos of the fire, caused by hitting road debris, are dramatic to be sure, 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.  Tesla’s statement to press included, “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.” (Italics added)  The Tesla S battery cells are small cylindrical Lithium-Cobalt-Nickel-Oxide from Panasonic and they did go into “thermal-runaway” in succession when exposed to high temperatures, most Lithium-Ion chemistries will burn similarly when exposed to direct flames. [2]

Is battery safety an issue when designing electrified transportation?  Without a doubt.  But new materials coupled with advanced design techniques and rigorous safety testing have brought electric vehicles to the point where they are as safe as houses.  Or more importantly, safer than conventional automobiles [3 NOTE: “one in every four fire department responses is to a conventional vehicle fire” (Italics added U.S. Fire Administration’s TOPICAL FIRE RESEARCH SERIES, July 2001)].

While safety is the paramount concern, environmental concerns linger.  And while one of the primary appeals of electric vehicles are their near-zero impact on the environment, battery recycling and impact remain a cause for concern.  Can they leak harmful materials if defective or old?  Will they explode upon on 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 these questions are valid, but it’s critical that consumers and industry understand that electric vehicles, and more specifically, their batteries, can be employed at a large scale without a detrimental impact on the planet and its inhabitants.

Balancing Density with Safety

There are several different lithium-ion battery formulations 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 that are 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.  This formulation of battery is used by Panasonic 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 Japanese Leaf (cells from NEC) and Korean Volt (cells from LG Chemical) electric vehicle battery cells.

A third option is a lithium-iron-phosphate battery, or a variation depending on the dominate 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 trade-offs, however.  The price for thermal stability can be sacrificing 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 lithium-ion batteries are made up of 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.  Both electrodes must be separated by some sort of insulator such a thermoplastic polymer.  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 (which is the positive electrode), and the anode (negative electrode).  Upon discharge, the anode undergoes loss of electrons, while the cathode sees a gain.

Lithium ion cells have historically used lithium-metal-oxides as cathode materials due to their high capacity for lithium intercalation, and have 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, significant toxicity and they 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, thus 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”.  In order 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 has significant disadvantages related to capacity degradation issues (ie. 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 lithium 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 lithium-iron-phosphate is a safer cathode material than the commonly used lithium metal oxide cathodes.

Designing to Prevent Problems

In addition to chemistry, design of the cell, battery and the battery compartment is just as critical to assure 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 includes testing and verification of electrode structures using current distribution models, thermal modes, electrochemical modes and mechanics 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 capacity cell is, the greater risk of instability.  This is where pack capacity design plays a role.  By optimizing cell capacity and safety using 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 cause a build-up of pressures inside the sealed cell, which may result in a rupture of the cell casing, along with a corresponding pop or loud bang.  Safety vents are needed as a final safety 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 the 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, contaminated electrode coatings or electrolytes can all cause short circuits or penetration of the separator 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) which 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 are obviously a key part of quality control process and ultimately, guaranteeing overall safety.  There are a number of different types of testing that should be applied:

Vibration/shaker-table testing: This simulates roadway vibrations during extending 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: Reliability of a battery when the vehicle would be operated at extreme temperature ranges also needs to be tested, with recommended temperature range between 85 ±2°C to -40 ±2°C.

Salt spray test: Exposure to salt can come from environments that use road salt in the winter and those in near ocean environments.  Testing over the course of months is recommended to assure that salt exposure does note lead to conditions that might cause the battery module to catch fire or explode, or cause a rupture of enclosure or leakage of electrolyte outside of enclosure, and that it remains operational.

Crush testing: This tests the safety of the battery under conditions where it is crushed and the battery was impacted directly during a crash (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: This involves testing the safety of the battery when all printed circuit board assembly protection circuit devices fail to work, and the battery was “hard” short circuited. While the module may be rendered non-functional, it is critical that the module does not catch fire or explode.

Pack level tests: These include collision tests, such as simulating a vehicle colliding with different objects at a variety of speeds, and with a collision inflicted directly on the pack.

Fire and gas flaming testing: Packs should also 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, 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, which 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 US DOT, FMVSS and SAE standards requirements, but is included in many of China’s vehicle safety testing programs.  This should be added to the U.S. standards if we are to assure the public safety.

Looking at Safety in the Long Term

Safety of batteries while in use are clearly the most critical concern drivers of electric vehicles, but disposal of batteries when they finally achieve their end of life are also 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 Lithium-Ion batteries.  As discussed before all Lithium-Ion Cobalt and Manganese batteries contain a toxic and flammable electrolyte.   If these battery cells are thrown in a landfill improperly and they decompose, they will leak.  Unfortunately, the Lithium-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 the BYD iron-phosphate chemistry.

Ultimately, iron-phosphate batteries are the most appealing from an environmental standpoint, as they contain no toxic electrolytes, contain no heavy metals in either the cathode or the anode, and are not manufactured with any caustic or harmful materials.  It is 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, it’s been demonstrated that the thermal stability of iron-phosphate batteries exceeds fire safety standards.  In general, the fear about EV batteries commonly used in electrical vehicles 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.

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[1] http://www.bloomberg.com/news/2014-01-14/electric-cars-pose-different-risks-u-s-regulator-says.html
[2] http://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 — Vice President – BYD America

Micheal Austin received his degree in Design Engineering as well as a Masters Degree in Mechanical Engineering from BYU. He worked for Motorola 15 years in functions including ODM Director for the Mobile Devices Business responsible for over $3B in purchases annually and serving as Motorola’s Global Energy Commodity Manager, purchasing Motorola’s battery products. He was selected as Motorola’s Distinguished Innovator (with 22 US patents) in 1999. He has considerable Asian International business experience which proves invaluable in his current role as Vice President for BYD America. BYD is a $40B Chinese company listed on the HKE and has over 200,000 employees.