31st March 2022
Battery Safety for Lithium-Ion Batteries
The Importance of battery safety
Lithium-ion batteries were developed in the 1970s and first commercialized by Sony in 1991 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 2008, Tesla unveiled the Roadster making it the first car company to commercialize a battery-powered electric vehicle. By 2025, 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.
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, 2019, 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.
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.
Now that we know why lithium-ion batteries catch fire, let’s look at the 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.
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.
What to do when a battery catches fire?
If you notice the lithium-ion battery overheat, try moving the device away from flammable materials and cutting of 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.
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.
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