When LiBs fail
LiBs can store a large amount of energy in very small space and can be cycled many times over their lifetime with very little loss of capacity. However, if the stored energy is released all at once it can be very destructive.
In normal operation charged lithium ions enter the carbon particle matrix on the anode by leaving the lithium paste at the cathode and pass through the solvent and separator. When discharging, or in use, this ion flow is reversed. When LiBs are made the fist charge is very slow and the batteries are left for 10 or more days to form an SEI (Solid Electrolyte Interface): a passive layer of solids around the carbon partials. Without this SEI layer, the charged lithium ions chemically attack the carbon partials. This creates an exothermic reaction that releases a huge amount of heat and gas and the cell explodes. This reaction can also be triggered if the SEI layer or polymer separator is damaged, leading to TR (thermal runaway) in a cell and then TP (Thermal Propagation) from cell to cell. This is the big baddie!
Image from BSLBATT Lithium
TR (Thermal Runaway)
Thermal runway is just heat, not a fire. It occurs when the limited chemical reactions within the cell become self-sustaining. Once this state is reached it can't be stopped. Degradation of the separator can start at as low as 60 C. Heat is produced in the volume of the object but only dissipated by its surface area. Thermal propagation heats up adjacent cells which then go into TR.
Lithium batteries don't commit suicide. They can be murdered by abuse. Physical damage, overcharging, over discharge, short circuit, overheating, or manufacturing defect.
Cylindrical cell It’s not smoke… and apply common sense
When LiBs go into TR a number of things can happen. The first is the battery may simply explode. Throwing hot shrapnel around. This has happened with light electric vehicles {LEVs), scooters, bikes, water hover boards, jet skis, etc.
If it doesn't explode, you may hear pressure caps popping and hissing of gas escaping with a chemical smell. You might briefly see a black cloud as the carbon particles are blasted out. The flammable gas escapes at high pressure and can ignite producing jet or rocket like flames at 1,000 C. If ignition is delayed the gases vent off creating an explosive atmosphere with an LEL of 11% to 16% with a risk of VCE (Vapour Cloud Explosion). This vapour contains hydrogen, hydrogen cyanide, hydrogen fluoride, hydrogen sulphide, carbon monoxide, CO2 and visible droplets of the solvent mistaken for smoke or steam. Hydrogen fluoride turns into hydrofluoric acid on contact with water or sweat on skin. The DTLH (Danger to Life and Health) level is well below the LEL.
The volume of gas boiled off is huge. In the region of 300 to 5000 litres per kWh of battery capacity. There are also two vapour clouds. One is lighter than air, and the other denser than air. At the moment it’s not known how to predict which will dominate.
It’s the VCE risk that is the real problem, especially for the marine industry. And current fire suppression systems cut out active fire burning time. But flip the risk to VCE. Because TR continues and cells continue to boil off vapour. As the cells are watertight “It’s like pouring water on the roof of your house when your kitchen is on fire.”
THE REPORT | MAR 2025 | ISSUE 111 | 75
Pouch cell
Prismatic cell
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