(Invited) Battery System Safety Design Based on Studying the Runaway in Battery Cell
Authors: Skarstind Martin Jens 
Published: February 2024

Lithium ion battery systems towards certain military application such as submarines and torpedoes are considered “fire critical applications”. Meaning a fire in the battery system on board a submarine is equal to a lost submarine with many crewmen dead. There have already been critical lithium ion battery fires on board submarines. In the united states a midget submarine were destroyed about fifteen years ago during the development phase of the submarine delaying the incorporation of lithium ion batteries in to marine platforms in the USA by about ten years even though no one were killed by the fire. Four years ago the Russian spy submarine Loshariks lithium ion battery system caught fire during the new battery systems maiden voyage. The fire started when the submarine were at a depth of about 200m below the surface of Barents ocean and 14 crewmen were killed almost all were high ranking officers. Too, be able to come up with a safe battery system design for the Swedish submarines we concluded that we need a strategy for the battery system design. We studied literature, conducted modelling work and carried out a lot of abuse test to understand what happens when a battery cell fails.
What happens when a 21700 battery cell is overcharged? First the cell is heated to about 110-115°C and then the membrane between the anode and the cathode melts. In the next step the CIP opens up and hot gas is released at about 125 - 135°C. The cell self-heat itself, the temperature rises fast to about 190-200 ° C, then the gas caught fire, and a jet flame is formed with a temperature above 1100°C. A huge amount of hot gases mixed with sluggish particles is released from the top of the battery cell at high speed. The sluggish particles are black and look like a lava flow and when analysed they are found to be electric conductive. What is also interesting is that the lava particles slowly releases its heat to the surroundings, thus the particles kept at a high temperature for a relatively long-time. The surface of the battery cell reaches a peak temperature of about 300°C. This information is enough to start the battery module design work.
Why is a round lithium battery cell the obvious choice for a safe high performing battery system? A runaway in round 21700 battery cell leads to axial expansion of 1-2mm the radial expiation is taken up by the design materials. A runaway in a prismatic or pouch cell battery the horizontal expansion can be several centimetres leading to an inefficient design to be able to take up the expansion of failing battery cell. To be able to do an efficient design to keep down the weight of the battery system the cells can be part of the mechanical design. When organizing the round battery cells symmetrically in a battery module each battery cell will work as springs in the right design material, thus be able to take up energy from an impact from an object. If the incoming object is a bullet at speed of 900m /s and a weight of 50 gram, the symmetrically organized battery cells will absorb the energy fast and only few of the battery cells in the module will be de destroyed and will start to burn.
Finally yet importantly, the round battery cells are in most cases smaller than the prismatic battery cells for the same application thus contains less energy per battery cell.
One battery cell can always burn, but a propagation in battery module or system is not an option for “Fire Critical Military Applications”.