A new technique to rival the infamous nail penetration test may soon be available to lithium-ion battery researchers, Dr Harry Doring told the AABC conferencing in Mainz this week.
Doering, of ZSW laboratory, has been using a 400W ND Yag solid-state laser to punch holes, layer by layer, through the jelly roll of lithium-ion cells with a beam diameter of 0.3m. Doring said the pulse energy of the laser is extremely controllable and can be compared to that of a dendritic short, based on the calculated energy levels published by Tiax, the Boston based company which did much to explain lithium-ion battery internal shorts some years ago.
The laser can be set up to make multi-pulse strikes, penetrating several layers of the battery windings. Doring said the laser pulse caused battery voltage drops and some heat evolution, indicative of real internal shorts. This did not vary with different states of charge of the cell so tested. Indeed, 100-pulse (2 Joule) in a 4Ah NMC cell caused the device to go into thermal runaway.
Currently, the nail penetration test is the most commonly used to simulate internal short circuits but researchers are all too aware of how a small variation in the procedure (such as speed of the penetration) can produced widely varying results.
With some standardisation of the laser procedure, the nail test could become obsolete, Doring believed.