So on the ageing aspects causing failure of lithium batteries I did some research and found this document with the following extract:
https://www.mdpi.com/2076-3417/9/12/2483/pdf3.4. Manufacturing Defect and Aging
Apart from the external factors, battery internal defect due to poor manufacturing, such as a low-quality separator, material contaminant, and improperly arranged constituents, can also cause battery failure and result in thermally hazardous conditions [66]. As stated in Section 2, the main cause of incidents in the Samsung Note 7 was a battery fault, in other words, Samsung adopted an excessively thin separator to increase the energy density of battery which raised the possibility of battery short circuit [67]. Mohanty et al. [68] undertook a systematic investigation on several plausible cathode defects such as agglomeration, pinholes, metal particle contamination and non-uniformity. The findings showed that cathode agglomeration aggravated the cycle efficiency and resulted in faster capacity fading. Electrode pinholes showed substantially lower discharge capacities compared to the baseline of electrodes. Metal particle contaminants had an extremely negative effect on performance, and the electrodes with severe non-uniform coatings showed poor cycle life. In summary, cathode material defects decrease battery performance and increase the risk of thermal hazard. In addition, a low-quality separator will reduce the efficiency of Li+ passing through the separator and result in serious Li plating, which will further penetrate the separator and induce the internal short circuit [69]. The improper arrangement of constituents is also harmful to the operation of battery, deteriorates the heat generation and heat release, therefore decreases battery safety.
Finally, due to deterioration associated with battery aging, its thermal hazards will increase correspondingly. The aged battery will lose quantities of lithium and active materials [70,71]. Partially inactivated Li+ will transfer to Li metal and deposit onto the surface of electrodes and even the separator to form Li plating. Upon the progression of battery aging, the degree of Li plating will increase gradually and subsequently, dendrite is generated. The dendrite may penetrate the separator and form a bridge between the electrodes, which will cause micro-shorting inside the battery and eventually lead to failure of the battery. At the same time, the thickness of the solid electrolyte interface (SEI) layer will gradually increase with the aging of the battery, which will lead to increases in the battery resistance, heat generation and thermal hazards.
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