General Motors expanded the recall of its Chevrolet Bolt electric vehicles due to the risk of fire from bag-type lithium-ion battery cells manufactured by LG of South Korea.
The recall, the second-largest involving batteries made by LG Chem’s battery pack, LG Energy Solution (LGES), highlights the challenges battery companies face in making a stable product to power electric cars.
How Does a Lithium Ion Battery Work?
Cells come in different shapes and sizes, but most have three main elements: electrodes, electrolyte and separator.
Electrodes store lithium. The electrolyte carries lithium ions between the electrodes. The separator prevents the positive electrode from contacting the negative electrode.
Energy, in the form of electricity, is discharged from the battery cell when lithium ions flow from the negative electrode, or anode, to the positive electrode or cathode. When the cell is charging, these ions flow in the opposite direction, from the cathode to the anode.
Why are Li-ion batteries a fire hazard?
Li-ion batteries, whether used in cars or electronic devices, can catch fire if they are improperly manufactured or damaged, or if the software that operates the battery is not designed correctly.
The main weakness of lithium-ion batteries in electric cars is the use of organic liquid electrolytes, which are volatile and flammable when operating at high temperatures. An external force, such as a collision, can also cause chemical leakage.
“For EV fires, it’s always been very difficult to identify the exact root cause of the fire, because it’s extremely difficult to ‘re-enact’ the fire incident with the same conditions,” said Kim Pil-soo, professor of automotive engineering at Daelim University.
In addition, authorities, car manufacturers and battery manufacturers often do not disclose what the exact security risk is.
What caused the fires in screws and konas?
In February, the South Korean transport ministry said defects had been found in some battery cells manufactured at LGS’s China plant and used in Hyundai Motor’s electric cars, including the Kona EV. Hyundai’s recall cost about KRW 1 trillion (about Rs. 6,350 crores).
GM said LG-supplied batteries for the Bolt EV and Bolt EUV may have two manufacturing defects – a torn anode flap and a bent separator – present in the same battery cell, which increases the risk of fire.
For a box of facts about major battery fires:
Are bag-type batteries more vulnerable?
All three types of lithium-ion batteries currently used in electric cars – cylindrical, prismatic and bag-type – are fundamentally the same in functionality, but each has its pros and cons.
Cylindrical and prismatic batteries are lined with rigid materials. Bag types use sealed flexible sheets and are protected by thin metal bags.
The technology used in cylindrical batteries is old and produces consistent results. These cells can withstand high internal pressure without deforming. They are also cheaper, making them ideal for mass production. But they are heavier – and their shape prevents the cells from being compacted with the same density as other forms of battery. Tesla mainly uses cylindrical batteries, some supplied by LGS.
Prismatic batteries are considered safer and lighter than cylindrical cells and, being rectangular, can be more densely compacted. They optimize space better than cylindrical cells, but they are typically more expensive and have a shorter life cycle. They can also swell.
Compared to cylindrical and prismatic cells, bag-type battery cells allow for the fabrication of lighter and thinner cells and design flexibility for different capacities and space requirements for different vehicle models. However, they are vulnerable to swelling and are more vulnerable to collisions, presenting a greater risk of fire.
GM and Hyundai Motor use bagged battery cells from LG Energy Solution (formerly LG Chem). Volkswagen said earlier this year that it would switch bag-type cells made by LG and SK Innovation to prismatic technology.
Are there other solutions?
Companies like China’s BYD Co produce EV battery cells that use lithium iron phosphate cathodes, which are less likely to catch fire but are not able to store as much energy as standard cells that use nickel cobalt-manganese cathodes .
Others, including GM, are testing different chemicals, such as nickel-cobalt-manganese-aluminium (NCMA) technology, which uses less cobalt, making cells more stable and cheaper.
Chinese battery maker CATL last month unveiled a sodium ion battery that does not contain lithium, cobalt or nickel.
Several companies, including Toyota Motor, are also developing battery cells with solid-state electrolytes, which could minimize overheating and fire hazards, but could take another three to five years to market.
© Thomson Reuters 2021