BU-304a: Safety Concerns with Li-ion

Safety of lithium-based batteries has attracted much media and legal attention. Any energy storage device carries a risk, as demonstrated in the 1800s when steam engines exploded and people got hurt. Carrying highly flammable gasoline in cars was a hot topic in the early 1900s. All batteries carry a safety risk, and battery makers are obligated to meet safety requirements; less reputable firms are known to make shortcuts and it’s “buyer beware!”

Lithium-ion is safe but with millions of consumers using batteries, failures are bound to happen. In 2006, a one-in-200,000 breakdown triggered a recall of almost six million lithium-ion packs. Sony, the maker of the lithium-ion cells in question, points out that on rare occasion microscopic metal particles may come into contact with other parts of the battery cell, leading to a short circuit within the cell.

Battery manufacturers strive to minimize the presence of metallic particles. The semiconductor industry has spent billions of dollars to find ways in reducing particles that reduce the yield in wafers. Advanced cleanrooms are Class 10 in which 10,000 particles larger than 0.1µm per cubic meter are present (ISO 4 under ISO 14644 and ISO 14698). In spite of this high cleanliness, particle defects still occur in semiconductor wafers. Class 10 reduces the particles count but does not fully eliminate them.

Battery manufacturers may use less stringently controlled cleanrooms than the semiconductor industry. While a non-functioning semiconductor simply ends up in the garbage bin, a compromised Li-ion can make its way into the workforce undetected and deteriorate without knowing. Resulting failures are especially critical with the thinning of the separators to increase the specific energy.

Cells with ultra-thin separators of 24µm or less (24-thousandth of an mm) are more susceptible to impurities than the older designs with lower Ah ratings. Whereas the 1,350 mAh cell in the 18650 package could tolerate a nail penetration test, the high-density 3,400 mAh can ignite when performing the same test. (See BU-306: What is the Function of the Separator?) New safety standards direct how batteries are used, and the UL1642 Underwriters Laboratories (UL) test no longer mandates nail penetration for safety acceptance of lithium-based batteries.

To verify the safety of a new cell, a manufacturer may release 1 million samples into a workforce on observation. The cell is approved for the use of critical missions, such as medical, if no failure occurs in one year that could compromise safety. Similar field-testing is also common with pharmaceutical products.

Li-ion using conventional metal oxides is nearing its theoretical limit on specific energy. Rather than optimizing capacity, battery makers are improving manufacturing methods to enhance safety and increase calendar life. The real problem lies when on rare occasions an electrical short develops inside the cell. The external protection peripherals are ineffective to stop a thermal runaway once in progress. The batteries recalled in 2006 had passed the UL safety requirements — yet they failed under normal use with appropriate protection circuits.

There are two basic types of battery failures. One occurs at a predictable interval-per-million and is connected with a design flaw involving the electrode, separator, electrolyte or processes. These defects often involve a recall to correct a discovered flaw. The more difficult failures are random events that do not point to a design flaw. It may be a stress event like charging at sub-freezing temperature, vibration, or a fluke incident that is comparable to being hit by a meteor.

Let’s examine the inner workings of the cell more closely. A mild short will only cause elevated self-discharge and the heat buildup is minimal because the discharging power is very low. If enough microscopic metallic particles converge on one spot, a sizable current begins to flow between the electrodes of the cell, and the spot heats up and weakens. As a small water leak in a faulty hydro dam can develop into a torrent and take a structure down, so too can heat buildup damage the insulation layer in a cell and cause an electrical short. The temperature can quickly reach 500°C (932°F), at which point the cell catches fire or it explodes. This thermal runaway that occurs is known as “venting with flame.” “Rapid disassembly” is the preferred term by the battery industry.

Uneven separators can also trigger cell failure. Poor conductivity due to dry areas increases the resistance, which can generate local heat spots that weaken the integrity of the separator. Heat is always an enemy of the battery.

Most major Li-ion cell manufacturer x-ray every single cell as part of automated quality control. Software examines anomalies such as bent tabs or crushed jelly rolls. This is the reason why Li-ion batteries are so safe today, but such careful manufacturing practices may only be offered with recognized brands.

Why Batteries Fail

Quality lithium-ion batteries are safe if used as intended. However, a high number of heat and fire failures had been reported in consumer products that use non-certified batteries, and the hoverboard is an example. This may have been solved with the use of certified Li-ion on most current models. A UL official at a meeting in the Washington, D.C. area said that no new incident of overheating or fire had been reported since Li-ion in hoverboards was certified. Fires originating in the Samsung Galaxy Note 7 were due to a manufacturing defect that had been solved. The main-ship battery in the Boeing 787 Dreamliner also had defects that were resolved.

Incorrect uses of all batteries are excessive vibration, elevated heat and charging Li-ion below freezing. (See BU-410: Charging at High and Low Temperature) Li-ion and lead acid batteries cannot be fully discharged and must be stored with a remaining charge. While nickel-based batteries can be stored in a fully discharged state with no apparent side effect, Li-ion must not dip below 2V/cell for any length of time. Copper shunts form inside the cells that can lead to elevated self-discharge or a partial electrical short. If recharged, the cells might become unstable, causing excessive heat or showing other anomalies.

Heat combined with a full charge is said to induce more stress to Li-ion than regular cycling. Keep the battery and a device away from sun exposure and store in a cool place at a partial charge. Exceeding the recommended charge current by ultra-fast changing also harms Li-ion. Nickel-cadmium is the only chemistry that accepts ultra-fast charging with minimal stress. (See BU-401a: Fast and Ultra-fast Chargers)

Li-ion batteries that have been exposed to stresses may function normally but they become more sensitive to mechanical abuse. The liability for a failed battery goes to the manufacturer even if the fault may have been caused by improper use and handling. This worries the battery manufacturers and they go the extra mile to make their products safe. Treat the battery as if it were a living organism by preventing excess stress.

With more than a billion mobile phones and computers used in the world every day, the number of accidents is small. By comparison, the National Oceanic and Atmospheric Administration say that your chance of being struck by lightning in the course of a lifetime is about 1 in 13,000. Lithium-ion batteries have a failure rate that is less than one in a million. The failure rate of a quality Li-ion cell is better than 1 in 10 million.

Industrial batteries, such as those used for power tools, are generally more rugged than those in consumer products. Besides solid construction, power tool batteries are maximized for power delivery and less on energy for long runtimes. Power Cells have a lower Ah rating than Energy Cells and are in general more tolerant and safer if abused.

Battery Safety in Public addresses concerns with consumer batteries. One of the most accident-prone batteries is Li-ion in an 18650 cell with an unfamiliar brand name. These batteries made available for vaping do not have the same quality and safety as a recognized brand name. Li-ion is safe if made by a reputable manufacturer, but there have been a number fires and injuries with cells that developed defects and caught fire while carrying in clothing and while traveling. An onboard fire forced a WestJet plane to return to the airport in 2018 soon after takeoff. The burning e-cigarette battery was illegally placed in baggage as spare and checked in. The plane’s cargo bay is not accessible when in flight and a burning battery requires an unscheduled landing. The U.S. Federal Aviation Administration (FAA) recorded 206 incidents involving Li-ion batteries between 1991 and 2018.

What to Do When a Battery Overheats or Catches Fire

If a Li-ion battery overheats, hisses or bulges, immediately move the device away from flammable materials and place it on a non-combustible surface. If at all possible, remove the battery and put it outdoors to burn out. Simply disconnecting the battery from charge may not stop its destructive path.

A small Li-ion fire can be handled like any other combustible fire. For best result use a foam extinguisher, CO2, ABC dry chemical, powdered graphite, copper powder or soda (sodium carbonate). If the fire occurs in an airplane cabin, the FAA instructs flight attendants to use water or soda pop. Water-based products are most readily available and are appropriate since Li-ion contains very little lithium metal that reacts with water. Water also cools the adjacent area and prevents the fire from spreading. Research laboratories and factories also use water to extinguish Li-ion battery fires.

Crew can’t access the cargo areas of a passenger aircraft during flight. To assure safety in case of a fire, planes rely on fire suppression systems. Halon is a common fire suppressant, but this agent may not be sufficient to extinguish a Li-ion fire in the cargo bay. FAA tests found that the anti-fire halon gas installed in airline cargo areas can’t extinguish a battery fire that combines with other highly flammable material, such as the gas in an aerosol can or cosmetics commonly carried by travelers. However, the system prevents the blaze from spreading to adjacent flammable material such as cardboard or clothing.

With the increased use of Li-ion batteries, improved methods to extinguish lithium fires have been developed. The Aqueous Vermiculite Dispersion (AVD) fire extinguishing agent disperses chemically exfoliated vermiculite in the form of a mist that provides advantages over existing products. AVD fire extinguishers are available in a 400ml aerosol can for a small fire; AVD canister for warehouses and factories; a 50 liter AVD trolley system for larger fires, and a modular system that can be carried on a pickup truck.

Extover® is another fire extinguishing medium that effectively suppresses and smothers burning Li-ion batteries to minimize damage to the surrounding area through isolation of the fire source. The lightweight and free-flowing glass aggregate can be applied to a burning battery by hand, bucket, or shovel. Grain sizes vary from 0.04mm to 2mm to suit different applications. Extover® allows the battery to safely burn out with a homogenous covering because a flaming Li-ion battery cannot be easily extinguished. Extover® does not contain chemical agents, is made from 100% post-consumer recycled content, and is suitable for small and large batteries. The material can be reused if clean.

A readily-available and effective fire retardant is sand kept in a fire-proof bucked. In case of fire, the flaming battery is moved into the bucked and covered with sand to allow a controlled burn-out. The sand can also be heaved over the hot battery to prevent the fire from spreading. Sand is about three-times heavier than Extover® made from post-consumer glass.

A large Li-ion fire, such as in an EV, may need to burn out. Water with copper material can be used, but this may not be available and is costly for fire halls. Increasingly, experts advise using water even with large Li-ion fires. Water lowers combustion temperature but is not recommended for battery fires containing lithium-metal.

Research by IdTechEx reveals that 17% of EV fires occur during regular driving; 25% when charging; 20% in a crash; and 4% when unduly exposes the battery to air or water. The report says EV fires are more severe than with conventional ICE vehicles. A Tesla Model S fire required nearly 30,000 gallons (over 100,000 liters) of water to extinguish the fire due to reigniting and continuously burning for four hours. By comparison, a typical car fire with ICE engine can be extinguished with about 300 gallons (>1,000 liters) of water. Remedies to EV fires are often software changes to lower the battery capacity to 90%. There have also been rare manufacturing defects in cells. IDTechEx expects tighter regulations in design with the use of more fire-retardant materials.

When encountering a fire with a lithium-metal battery, only use a Class D fire extinguisher. Lithium-metal contains plenty of lithium that reacts with water and makes the fire worse. As the number of EVs grows, so must the methods to extinguish such fires.

During a thermal runaway, the high heat of the failing cell inside a battery pack may propagate to the next cells, causing them to become thermally unstable also. A chain reaction can occur in which each cell disintegrates on its own timetable. A pack can thus be destroyed in a few seconds or over several hours as each cell is being consumed. To increase safety, packs should include dividers to protect the failing cell from spreading to the neighboring one. Figure 1 shows a laptop that was damaged by a faulty Li-ion battery.

The gas released by a venting Li-ion cell is mainly carbon dioxide (CO2). Other gases that form through heating are vaporized electrolyte consisting of hydrogen fluoride (HF) from 20–200mg/Wh, and phosphoryl fluoride (POF3) from 15–22mg/Wh. Burning gases also include combustion products and organic solvents.

The knowledge on the toxicity of burning electrolyte is limited and toxicity can be higher than with regular combustibles. Ventilate the room and vacate area if smoke and gases are present. Gas and smoke in a confined area such as an aircraft, submarine and mine shaft will present a potential health risk.

While lithium-based batteries are heavily studied for safety, nickel- and lead-based batteries also cause fires and are being recalled. The reasons are faulty separators resulting from aging, rough handling, excessive vibration and high-temperature. Lithium-ion batteries have become very safe and heat-related failures occur rarely when used correctly.

Definition

• Hydrogen fluoride (HF): a colorless gas or liquid substance. It is the principal source of fluorine, a feedstock for pharmaceuticals, polymers (Teflon) and assisting petrochemical industries. Hydrogen fluoride is a highly dangerous gas, forming corrosive and penetrating hydrofluoric acid with moisture. In large quantities, gas can cause blindness by destruction of the corneas.

• Phosphoryl fluoride (POF3): a colorless gas that hydrolyzes rapidly.

• Lithium hexafluorophosphate (LiPF6): an inorganic compound in the form of white crystalline powder serving as electrolyte in Li-ion batteries.

Simple Guidelines for Using Lithium-ion Batteries

• A failing Li-ion begins to hiss, bulge and leak electrolyte.

• The electrolyte consists of lithium salt in an organic solvent (lithium hexafluorophosphate) and is highly flammable. Burning electrolyte can ignite combustible material in close proximity.

• Dowse Li-ion fire with water or use a regular fire extinguisher. Only use a Class D fire extinguisher for lithium-metal fires because of the reaction of water with lithium. (Li-ion contains little lithium metal reacting with water.)

• If a Class D extinguisher is not available, douse a lithium-metal fire with water to prevent the fire from spreading.

• For best results dowsing a Li-ion fire, use a foam extinguisher, CO2, ABC dry chemical, powdered graphite, copper powder or soda (sodium carbonate) as you would extinguish other combustible fires. Reserve the Class D extinguishers for lithium-metal fires only.

• If the fire of a burning lithium-ion battery cannot be extinguished, allow the pack to burn in a controlled and safe way.

• Be aware of cell propagation as each cell might be consumed on its own time table when hot. Place a seemingly burned-out pack outside for a time.

References

[1] Source: Shmuel De-Leon