Over-heating triggered thermal runaway characteristic of lithium ion hybrid supercapacitor based lithium nickel cobalt manganate oxide/activated carbon cathode and hard carbon anode

The energy density increase and the application of battery-type electrode materials and organic electrolyte for lithium ion hybrid supercapacitors (Li-HSCs) inevitably accompany the rising of thermal runaway. Herein, we fabricated a large-format pouch cell Li-HSC with activated carbon (AC) and lithium nickel cobalt manganate oxide (NCM) composite cathode, hard carbon (HC) anode, and ceramic polyethylene terephthalate (PET) nonwoven separator. The as-prepared Li-HSC delivers a capacitance of 34,476 F and a high energy density of 126 Wh kg−1 (based on the mass of device). The thermal runaway characteristic and gas release of this large-format Li-HSC are analyzed. Thermal runaway occurs for fully charged NCM/AC//HC Li-HSC at 249 °C, but not for fully charged AC//HC Li-HSC. The O2 releasing by NCM component accelerates the self-heating reactions, and the introduction of NCM into cathode is proved to contribute on triggering thermal runaway of NCM/AC//HC Li-HSC. The Li-HSC displays the highest runaway temperature (Tpeak) of 490.9 °C with smoke spurting out and does not catch fire or explode during thermal runaway. The main gas components at thermal runaway are C2H4, O2 C3H6 and C4H8, produced by the reaction between anode and electrolyte, and decomposition of NCM and electrolyte. The ratio of hydrocarbon gases and O2 decreases, while that of CO and CO2 increases at the Tpeak.