Effects of capacity on the thermal runaway and gas venting behaviors of large-format lithium iron phosphate batteries induced by overcharge

Large-capacity lithium iron phosphate (LFP) batteries are widely used in electric bicycles. However, while crucial, thermal runaway (TR) behaviors under overcharge conditions have rarely been studied, leading to frequent fire accidents. This paper investigates the overcharge behavior and TR characteristics of four LFP batteries with the same components and materials but varying capacities (86, 100, 120, and 140 Ah). Overcharge tests were conducted in a customized apparatus, and the surface temperature, mass loss, and gas venting phenomenon were recorded. The results revealed that under the same overcharge conditions, the TR early warning temperature can differ considerably with respect to battery capacity, highlighting the importance of selecting the appropriate early warning temperature in real-time temperature monitoring based on the capacity of the LFP battery. Three characteristic times were used to describe the thermal behavior of the batteries, namely, the time to reach the voltage plateau (tPV), the time when the safety valve opens (tSV), and the start time of TR (tTR). Analysis of critical heat accumulation indicated that the 86 Ah LFP battery has the lowest tolerance to overcharge and, thus, the highest TR risk. However, after TR, the 86 Ah LFP battery exhibited the smallest maximum temperature (Tmax), maximum temperature increase rate (Rmax), maximum total mass loss, and maximum mass loss rate (Mmax) among all the batteries, suggesting the least TR hazard. Consequently, a safety evaluation and scoring system using these six parameters was established considering the overcharge behavior and TR characteristics of LFP batteries. The results indicated that the TR risk ranking of LFP batteries with different capacities was 86 Ah LFP > 100 Ah LFP > 140 Ah LFP > 120 Ah LFP, while the TR hazard ranking was 140 Ah LFP > 120 Ah LFP > 100 Ah LFP > 86 Ah LFP. This comparative study of LFP batteries with varying capacities enhances our understanding of their overcharge behavior and TR characteristics, and a safety evaluation and scoring system can also be used for risk and hazard assessments of other types of batteries.