Thermal runaway features of large-format LiFePO4 cells under internal short-circuit scenarios: Impact of penetration parameter, cell property, and short-circuit form

Considering that large-format LiFePO 4 (LFP) cells may experience internal short-circuits caused by mechanical abuse or separator failure, this study comprehensively investigates the thermal runaway behavior of large-format LFP cells under internal short-circuit conditions induced by penetration and partial separator failure. Thermal runaway triggered by penetration evolves much more rapidly than that caused by overheating or overcharging, with the internal temperature exceeding 800 ℃ within only a few tens of seconds. Penetration parameters including nail diameter, penetration rate, penetration depth, and penetration location, exert significant influence on the resulting thermal runaway characteristics. In particular, heat generation inside the cell intensifies markedly as the nail diameter and penetration depth increase. Penetration through the safety valve, however, results in only minor cell damage and fails to trigger thermal runaway due to the absence of a severe internal short-circuit. In addition, reducing the cell capacity substantially mitigates the severity of thermal runaway, whereas replacing the LFP cathode with LiNi 0.5 Mn 0.3 Co 0.2 O 2 greatly decreases the cell’s sensitivity to penetration, accompanied by violent gas generation, ejection, and combustion. Compared with penetration-induced short-circuits, thermal runaway develops more slowly when triggered by partial internal short-circuits caused by localized single-layer separator failure. Under such conditions, full internal short-circuiting and subsequent thermal runaway evolve progressively as the internal temperature rises. Distinct from the behaviors observed under overheating and overcharging, no reliable quantitative relationship is found between the internal and external parameters of the cell during internal short-circuit scenarios, as the parameter evolution depends strongly on the specific short-circuit conditions. • Internal short-circuit induced by penetration and partial separator failure is investigated. • Thermal runaway intensifies markedly as the nail diameter and penetration depth increase. • NMC cells are more sensitive to penetration than LFP cells. • Thermal runaway develops more slowly when triggered by partial separator failure. • No reliable quantitative relation between internal and external parameters under internal short-circuit.