Observations of Gas Evolution at End-of-Life in Commercial Li-Ion Cells – Analyses and Diagnostics

It is well-known that Lithium-ion cells can have safety issues. A large number of battery fire incidents have been reported the last years within e.g. battery energy storage systems [1], electric and hybrid-electric ships [2], electric vehicles and mobile phones [3]. The causes for these incidents are numerous and can include internal shortage of the Li-ion cell, external shortage of the battery system, over-heating and over-charging. All these causes involve overheating of the battery that can lead to an eventual thermal runaway which again could lead to a full battery fire. We have earlier reported changes in thermal stability of Li-ion cells aged by cycling and shown that specific cycling regimes can decrease the thermal stability of Li-ion cells to a large extent [4]. Another safety hazard is the gas evolution in Li-ion pouch cells during and after cycling at normal operating temperatures. The gas evolution can be so high that the pouch cell will expand significantly and eventually burst due to the increased overpressure building up inside the cell. While several studies focus on gas evolution under extreme thermal runaway conditions, reports on gas evolution under “normal” operating conditions and even storage are rare. This gas evolution is not well described in literature and the gas composition might be different compared to gases commonly released during a thermal event in the Li-ion battery. This study is based on results from a cycle life study performed on a large renowned Li-ion pouch cell with an initial capacity of more than 50 Ah. A total of 39 cells were cycled following a selected test matrix including various temperatures, current rates and State-of-Charge limits. All cells were cycled until they reached a remaining capacity of between 60 to 70 % State-of-Health. Already during cycling, the thickness of the cells increased, for some cells even by up to 50% by the time they reached end of life. In addition to the swelling during cycling, one third of the tested cells started to evolve significant amounts of gas during storage at 5 °C, i.e. after the cycling was finished and the cells had reached the lower remaining capacity. This gas evolution was further analyzed to scrutinize the causes of this gassing. To determine the composition of the gas evolved in the cells, the gas was retrieved by carefully inserting a syringe with a sharp needle through the top of the cell into the gas-filled void. The gas composition was then analyzed by gas chromatography (GC), coupled with three different detectors: mass spectrometry (MS), thermal conductivity (TCD) and flame ionization. The analysis revealed that more than 95 % of the gas within the cell comprised of hydrogen, methane and ethane, with hydrogen being the largest contributor. A small amount of electrolyte solvents (EMC and DMC) was found, which was expected due to the low vapor pressure at low temperatures. CO and CO2 levels were found to be below 1 %. A post-mortem study of selected gassed cells was performed to investigate the causes of the gassing. During cycling a large set of diagnostic data was compiled to investigate links between the cycling conditions and the specific findings from the post-mortem study. References [1] McKinnon MB, DeCrance S, Kerber S. Four Firefigheters Injured In Lithium-Ion Battery Energy Storage System Explosion - Arizona. In: Laboratories U, editor. Columbia, MD: Underwriters Laboratories; 2020. p. 66. [2] Norwegian Maritime Authority. Supporting preliminary findings after battery incident. Norwegian Maritime Authority2019. [3] BBC. Samsung confirms battery faults as cause of Note 7 fires. BBC: BBC; 2017. [4] Lian T, Vie PJS, Gilljam M, Forseth S. (Invited) Changes in Thermal Stability of Cyclic Aged Commercial Lithium-Ion Cells. ECS Transactions. 2019;89(1):73-81.