Top 5 Reasons Why Lithium-Ion Batteries Catch Fire

The importance of battery safety

Lithium-ion batteries were developed in the s and first commercialized by Sony in for the company&#;s handheld video recorder. Today everything you see is powered by batteries from smartphones to electric cars to even the International Space Station, which makes increased battery safety all the more crucial.
In , Tesla unveiled the Roadster making it the first car company to commercialize a battery-powered electric vehicle. By , the global lithium-ion (Li-ion) battery market is expected to reach USD 100.4 billion, over 50% of which will be used for the automotive market.

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Why such a craze for lithium-ion?

Lithium-ion batteries are popular because of how much power they can put out at a given size and weight. A typical lithium-ion battery stores 150 watt-hours of electricity in 1 kilogram of battery, compared to NiMH Battery pack (100 watt-hours per kg) or Lead Acid Battery (25 watt-hours per kg). It takes 6 kilograms to store the same amount of energy in a lead-acid battery that a 1-kilogram lithium-ion battery can handle.

However, lithium-ion batteries are extremely sensitive to high temperatures and inherently flammable. These battery packs tend to degrade much faster than they normally would, due to heat. If a lithium-ion battery pack fails, it will burst into flames and can cause widespread damage. This calls for immediate measures and guidelines for battery safety.

Recently, there have been a few incidents of fires caused by Lithium-Ion batteries. On January 8, , spontaneous combustion of a lithium-ion battery caused the fire to break out on the COSCO Pacific, a vessel in the Arabian Sea, caused by the. In April last year, a 2MW battery at an APS facility in Arizona exploded, injuring four firefighters.

Hans-Otto Schjerven, head of the Vestfold Fire Department, said that rechargeable lithium batteries can cause &#;fires that are difficult to extinguish and the batteries emit fire that quickly spreads.&#; As the adoption of electric vehicles grows, these incidents are set to increase.

Before analyzing why lithium-ion batteries catch fire, let&#;s understand how they work.

A lithium-ion battery pack consists of lithium-ion cells stacked together in modules, temperature sensors, voltage tap and an onboard computer (Battery Management System) to manage the individual cells. Like any other cell, the lithium-ion cell has a positive electrode (cathode), a negative electrode (anode) and a chemical called an electrolyte in between them. While the anode is generally made from graphite (carbon), different lithium materials are used for the cathode &#; Lithium Cobalt Oxide (LCO), Lithium Nickel Manganese Cobalt (or NMC), etc.

When a charging current is provided to the cell, lithium ions move from the cathode to the anode through the electrolyte. Electrons also flow but take the longer path outside the circuit. The opposite movement takes place during discharge with the result that the electrons power up the application that the cell has been connected to.

When all the ions have moved back to the cathode, the cell has been completely discharged and will need charging.

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The lithium-ion cells have been designed with battery safety measures like:

A. Pressure-sensitive vent holes

Batteries are pressurized and so they need an outer wall made of metal, which has a pressure-sensitive vent hole. If there&#;s a risk of the battery becoming very hot and exploding from over-pressure (pressure buildup at 3,000 kPa), this vent will release the extra pressure and prevent other cells in the battery pack from catching fire.

B. Separator serves as a fuse

Most lithium-ion cells use a separator made of a material known as polyolefin, which boasts of good chemical stability, excellent mechanical properties and is affordable. It serves as a fuse when the cell heats up. On excessive heat, when the core reaches 130°C (266°F), the separator melts which stops the transport of ions. This action immediately shuts down the cell.

Had this provision not been provided, there would have been a possibility of the heat in the failing cell to give rise to the thermal runaway threshold and vent with flame.

C. Positive Temperature Coefficient (PTC)

This a switch that prevents the battery from overheating by protecting it against current surges

Lithium-ion cells like all chemistries undergo self-discharge. Self-discharge means the batteries lose their stored charge without connecting the electrodes or the external circuit. This takes place due to chemical reactions inside the cell. Self-discharge of cells increases with age, cycling, and elevated temperature.

Elevated self-discharge can cause temperatures to rise which if uncontrolled can lead to a Thermal Runaway also known as &#;venting with flame&#;. A mild short won&#;t cause thermal runway because the discharging energy is very low and little heat is generated.

If however due to some damage to the cell, impurities penetrate into the cell, a major electrical short can develop and a sizable current will flow between the positive and negative plates. There is a sudden rise in temperature and the energy stored in the battery is released within milliseconds. Battery packs consist of thousands of cells packed together.

During a thermal runaway, the heat generated by a failed cell can move to the next cell, causing it to become thermally unstable as well. This chain reaction can cause the entire pack to be destroyed within a few short seconds.

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Now that we know why lithium-ion batteries catch fire, let&#;s look at some of the ways this can happen:

A. Manufacturing Defects

Flaws in production can cause metallic particles (impurities) to seep into the lithium-ion cell during the manufacturing process. Battery manufacturers need to ensure stringently controlled cleanrooms for manufacturing batteries.

Another defect could be the thinning of separators which could prove detrimental in actual use. Cells should undergo strict quality-control tests and validation before being sold.

B. Design Flaws

Car companies want to design their cars as sleek and slim while giving the maximum range and performance. These requirements push battery pack manufacturers to come up with compact designs by packing high-capacity cells into a smaller body, messing with an otherwise well-built battery.

Compromising on the design can cause damage to the electrodes or the separator. Either of which could result in a short circuit. Further, the absence of a proper cooling system or vent can cause battery temperatures to rise as the flammable electrolyte heats up.

If uncontrolled, it could result in a chain reaction of cell failures, causing the battery to heat up even more and spiral out of control.

C. Abnormal or Improper Usage

External factors like keeping the battery very close to a heat source or near a fire can cause it to explode. Penetrating the battery pack either deliberately or through an accident is bound to cause a short circuit and the battery to catch fire. That&#;s why unauthorized disassembly of the battery pack in electric vehicles leads to the lapse of warranty.

Users are advised to only get the batteries checked and repaired from the car maker&#;s authorized service centers. Even high-voltage charging or excessive discharging of the battery could damage it.

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D. Charger Issues

Using poorly insulated chargers can damage the battery. If the charger shorts or generates heat near the battery, it can do enough damage to cause failure.

While lithium-ion batteries have built-in protections to stop them from overcharging, using unofficial chargers can damage the battery in the long term.

E. Low-quality components

In addition to manufacturing defects, using low-quality components is one of the highest causes of battery failures. Increasing competition is driving the prices of batteries down, causing battery manufacturers to cut corners where they shouldn&#;t. By skimping on poor-quality electronics like the battery management system, the risk of battery failure increases.

The battery management system is critical to battery safety and performance. It protects the battery pack from operating outside of its safe operating area. As batteries form a high-value component of an electric vehicle or energy storage system, it&#;s essential to invest in a smart battery management system that can detect cell failures immediately and prevent the battery from exploding.

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What to do when a battery catches fire?

If you notice the lithium-ion battery overheating, try moving the device away from flammable materials and cutting off the current supply. If you&#;re in an electric vehicle, you should immediately evacuate and never attempt to extinguish lithium battery fires yourself. Your health and safety are far more important, call the emergency services instead.

In case of fire,  a standard ABC or BC dry chemical fire extinguisher must be used since these are considered Class B fire. A common misconception is that lithium-ion batteries contain any actual lithium metal. They don&#;t and that&#;s why you shouldn&#;t use a Class D Fire Extinguisher.

There are new and improved methods to douse lithium fires as well. The Aqueous Vermiculite Dispersion (AVD) is a fire extinguishing agent that disperses chemically exfoliated vermiculite in the form of a mist. However, larger lithium-ion fires as that of EVs or ESS may need to burn out. Using water with copper material is effective but is costly.

Battery Safety experts advise using water even for large lithium-ion fires. Fires like these may burn for days and it&#;s important to isolate them from flammable materials and prevent them from expanding.

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Ensuring battery safety

Battery pack makers should adopt a no-compromise approach to battery safety. Lithium-Ion batteries can be made safer by making them &#;smart&#;. By building a layer of intelligence into the batteries, we can not just diagnose but also predict abnormal usage or performance of the battery. This will help us take timely action, prevent damage to the system and ensure battery safety.

To know more about lithium-ion battery safety, write to us on

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Maxwell is an advanced battery management and intelligence platform. We&#;re focused on building technologies that improve the life and performance of lithium-ion batteries that power electric vehicles and energy storage systems.

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All batteries are affected by self-discharge. Self-discharge is not a manufacturing defect but a battery characteristic; although poor fabrication practices and improper handling can increase the problem. Self-discharge is permanent and cannot be reversed. Figure 1 illustrates self-discharge in the form of leaking fluid.

The amount of electrical self-discharge varies with battery type and chemistry. Primary cells such as lithium-metal and alkaline retain the stored energy best, and can be kept in storage for several years. Among rechargeable batteries, lead acid has one of the lowest self-discharge rates and loses only about 5 percent per month. With usage and age, however, the flooded lead acid builds up sludge in the sediment trap, which causes a soft short when this semi-conductive substance reaches the plates(See BU-804a: Corrosion, shedding and Internal Short)

The energy loss is asymptotical, meaning that the self-discharge is highest right after charge and then tapers off. Nickel-based batteries lose 10&#;15 percent of their capacity in the first 24 hours after charge, then 10&#;15 percent per month. Figure 2 shows the typical loss of a nickel-based battery while in storage.

NiMH and NiCd belong to rechargeable batteries that have the highest self-discharge; they need recharging before use when placed on a shelf for a few weeks. High-performance NiCd has a higher self-discharge than the standard versions. Furthermore, the self-discharge increases with use and age, of which crystalline formation (memory) is a contributing factor. Regular full discharge cycles keeps memory under control(See BU-807: How to restore Nickel-based Batteries)

Li-ion self-discharges about 5 percent in the first 24 hours and then loses 1&#;2 percent per month; the protection circuit adds another 3 percent per month. A faulty separator can lead to elevated self-discharge that could develop into a current path, generating heat and, in an extreme case, initiate a thermal breakdown. In terms of self-discharge, lead acid is similar to Li-ion. Table 3 summarizes the expected self-discharge of different battery systems.

Battery System Estimated Self-Discharge Primary lithium-metal 10% in 5 years Alkaline 2&#;3% per year (7-10 years shelf life) Lead-acid 10&#;15% in 24h, then 10-15% per month Nickel-based Li-ion, NiCd, NiMH Lithium-ion 5% in 24h, then 1&#;2% per month (plus 3% for safety circuit)

The self-discharge of all battery chemistries increases at higher temperature, and the rate typically doubles with every 10°C (18°F). A noticeable energy loss occurs if a battery is left in a hot vehicle. High cycle count and aging also increase self-discharge of all systems. Nickel-metal-hydride is good for 300&#;400 cycles, whereas the standard nickel-cadmium lasts for over 1,000 cycles before elevated self-discharge starts interfering with performance. The self-discharge on an older nickel-based battery can get so high that the pack goes flat from leakage rather than normal use(See BU-208: Cycling Performance demonstrating the relationship of capacity, internal resistance and self-discharge)

Under normal circumstances the self-discharge of Li-ion is reasonably steady throughout its service life; however, full state-of-charge and elevated temperature cause an increase. These same factors also affect longevity. Furthermore, a fully charged Li-ion is more prone to failure than one that is partially charged. Table 4 shows the self-discharge per month of Li-ion at various temperatures and state-of-charge. The high self-discharge at full state-of-charge and high temperatures comes as a surprise(See BU-808: How to Prolong Lithium-based Batteries)

Type 0°C (32°F) 25°C (77°F) 60°C (140°F) Full Charge 6% 20% 35% 40&#;60% Charge 2% 4% 15%

Lithium-ion should not be discharged below 2.50V/cell. The protection circuit turns off and most chargers will not charge the battery in that state. A &#;boost&#; program applying a gentle charge current to wake up the protection circuit often restores the battery to full capacity(See BU-803a: How to Awaken Sleeping Li-ion)

There are reasons why Li-ion is put to sleep when discharging below 2.50V/cell. Copper dendrites grow if the cell is allowed to dwell in a low-voltage state for longer than a week. This results in elevated self-discharge, which could compromise safety.

Self-discharge mechanisms must also be observed in manufacturing. They vary from corrosion to impurities in the electrodes that reflect in self-discharge variations not only from batch to batch but also form cell to cell. A quality manufacturer checks the self-discharge of each cell and rejects those that fall outside tolerances.

Regular charge and discharge causes an unwanted deposit of lithium metal on the anode (negative electrode) of Li-ion, resulting in capacity loss through a depletion of the lithium inventory and the possibility of creating an internal short circuit. An internal short is often preceded with elevated self-discharge, a field that needs further research to learn what levels of self-discharge would pose a hazard that can lead to a thermal runaway. Unwanted lithium deposition also increases the internal resistance that reduces loading capability.

Figure 5 compares the self-discharge of a new Li-ion cell with a cell that underwent forced deep discharges and one that was fully discharged, shorted for 14 days and then recharged. The cell that was exposed to deep discharges beyond 2.50V/cell shows a slightly higher self-discharge than a new cell. The largest self-discharge is visible with the cell that was stored at zero volts.

Figure 6 illustrates the self-discharge of a lead acid battery at different ambient temperatures At a room temperature of 20°C (68°F), the self-discharge is roughly 3% per month and the battery can theoretically be stored of 12 months without recharge. With a warm temperature of 30°C (86°F), the self-discharge increases and a recharge will be needed after 6 months. Letting the battery drop below 60 percent SoC for some time causes sulfation(See also BU-702: How to Store Batteries)

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