Lithium-ion capacitor safety assessment under electrical abuse tests based on ultrasound characterization and cell opening

Safety issues related to lithium-ion batteries are a driving force in the search for new energy storage systems. Lithium-ion capacitors are becoming recognised as promising devices to address the question of safety. These products are a combination of lithium-ion batteries and electric double-layer capacitors in terms of energy and power density. The aim of this work is to assess lithium-ion capacitor safety under over-charge and under-discharge processes for pouch and prismatic cells. In the course of performing abuse tests, no evidence was found of severe hazard (explosion, fire or flame, rupture or major leakage). Quantitative external parameters (thickness, resistance and mass) and electro-thermal measurements showed an increase of swelling and internal resistance, which caused a decrease of capacity and energy efficiency in all cases. The ultrasound characterization technique confirmed that there were irreversible physical modifications of the materials under abuse conditions on prismatic cells, which could not be seen with the commonly used magnitudes (U, I, T). This technique was also used for the identification of the start of cell degradation. In post-mortem analysis, were observed different degradation phenomena such as melting of the separator and delamination in the electrodes.