Due to arising environmental problems caused by exhaust gases and the concerns of driving bans for fossil fuel vehicles in major cities, the popularity of electric vehicles has been increasing rapidly. This fast growing market has prompted the automotive industry to invest in research and development of lithium-ion batteries as a suitable power source. In particular, the topic of safety has become of paramount interest. Because of their high energy and power densities, modern batteries consist mainly of cathodes made of lithium metal oxides such as nickel, cobalt and aluminum in combination with graphite as the anode. Difficulties arise in the case of mechanical or electrical abuse, which may result in thermal runaway. This can, in the worst cases, result in a battery fire. In order to prevent such fire hazards, extensive research is undertaken to determine the causes of thermal runaway, as well as the factors influencing it. The primary purpose of this work is to determine the correlation between battery aging and the thermal runaway. For this purpose, commercial Sony VTC 6 18650 batteries are brought to 80 percent of their initial capacity. Both the cyclic and the calendric aging of the batteries was evaluated and they are subsequently brought to thermal runaway, using a battery calorimeter (Accelerating Rate Calorimetry, ARC). The dominant aging process presents itself as the loss of lithium cyclability, which is caused by either the growth of a passivation layer (Solid Electrolyt Interface, SEI) or by the deposition of metallic lithium (Plating) on the anode. The cathode shows changes in the charge transfer resistance. This could be caused by decreased conductivity through reduced amount of carbon black or electrolyt. The calorimetric analyses of cyclically aged batteries show an increase of 0.6 degree Celsius per minute in temperature rates in the temperature range of 80 degree Celsius to 130 degree Celsius, compared to reference cells. This is an indication for a reaction between the electrolyte solution and the metallic lithium. When compared to reference cells, the temperature rates are reduced during further process, which can be correlated with decline in battery capacity. Therefore, it can be concluded that no significant reduction in safety is caused by the aging of these batteries.
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