On-line gas detection during the thermal runaway of Li-ion cells by coupling ARC with a mass spectrometer and external sensors

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In addition to the costs, high energy density, and fast-charging capability, the safety of Li-ion batteries is most important – even after long-term usage or abusive conditions. A new combination of Accelerating Rate Calorimetry (ARC) coupled with a mass spectrometer (MS), as well as cell resistance and audio recording [1] was applied to study commercial 18650 and 21700-type Li-ion cells. This novel ARC-MS setup allows following the electrochemical and thermal behavior simultaneously to the evolved gases during cell venting and thermal runaway. The tested cells were (i) un-aged, (ii) aged by low-temperature cycling (main mechanism: Li deposition), as well as (iii) overcharged. The aged and the overcharged cells show an early onset-of-self-heating at 36°C whereas the self-heating started at 96°C for the un-aged cells. The rank of the time until the explosion of the cells is in the order of overcharged cell < aged cell with Li deposition < un-aged cell. The main volatile products of the studied cells are evaporated organic solvents during the venting of the cell, especially for unaged cells. Different types of gaseous products such as ethylene, CO2, and POF3 were found during the explosions and result from thermal and electrochemical side reactions of electrolyte decomposition. Initial decomposition of the metastable SEI components occurs early to produce CO2 and ethylene gases. CO2 might be also formed by the reaction of Li2CO3, which is produced by the reduction of ethylene carbonate at the anode, with traces of HF. Oxygen-containing species such as H2O or Li2CO3 initiate the decomposition of LiPF6 to produce POF3 gas. The amount of formed gases are depending on the history of the cell, i.e. aging mechanism or overcharging and SOC.

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