Study on Thermal Runaway Triggering Mechanisms and Propagation Characteristics of Lithium Iron Phosphate Batteries with Different Connections

Abstract

As an efficient and clean energy storage carrier, lithium-ion batteries (LIBs) play an essential role in the development of the new energy industry due to their high energy density and excellent cycle performance. However, owing to the flammability of electrolyte and the reactivity of the materials, LIBs are prone to thermal runaway (TR) under abuse conditions, which seriously threaten the lives and properties. Thus, it is necessary to study TR of LIBs. At present, the researchers mainly focus on TR behaviors and propagation characteristics of open-circuit LIBs. The TR triggering mechanisms and TR propagation behaviors of batteries with different connection modes are still unclear. With consideration of the universal use of electric connection modes in LIBs, the objectives of this thesis are to reveal TR triggering mechanisms of the batteries with different electric connections and clarify the dominant heat transfer paths in inducing TR propagation. By adopting the approaches of experiment and theoretical analysis, this thesis investigates TR characteristics and triggering mechanisms of the open-circuit and parallel-connected lithium iron phosphate batteries from the cell level. After revealing the TR triggering characteristics, the TR propagation behaviors and heat transfer laws between batteries with different electric connections are revealed from the battery module level, and the blocking mechanisms of thermal insulation material on TR propagation are revealed. The main research contents are as follows:

(1) The TR experiments of open-circuit lithium iron phosphate batteries at different heating temperatures were carried out. The TR behavior, internal heat generation, and TR propagation within the battery body of 50 Ah and 105 Ah batteries are compared. Only the opening of the safety valve can be observed in the battery exposed to 175°C heating temperature; once the heating temperature exceeds 200°C, the battery TR can be successfully triggered. Based on the temperature variation of the battery, the self-generated heat of the battery before TR and its contribution ratio to TR triggering at different heating temperatures are obtained. By analyzing the TR propagation within the battery body, it is found that the key temperature of TR in the lithium iron phosphate battery is about 200°C.

(2) The TR triggering experiments of open-circuit lithium iron phosphate batteries under typical thermal abuse conditions are conducted to investigate the effects of different thermal abuse conditions on the internal TR triggering mechanism of open-circuit battery. Compared with the heating quantity and heating power, the heating position plays a more significant influence on the battery TR behaviors, and the bottom heating can cause severe TR. The TR triggering inside the battery is mainly affected by its own temperature, temperature gradient, heating power, external heating quantity and heating position, which first occurs in the heating region and then propagates to the whole battery. Based on the Arrhenius formula and energy conservation law, a theoretical model describing the local TR trigger of the open-circuit battery is proposed, and the TR triggering mechanism of the open-circuit battery is revealed.

(3) By eliminating the interference of heat transfer between LIBs in parallel, the TR experiments of parallel-connected lithium iron phosphate batteries under bottom heating are conducted to identify the separate influence of the parallel connection on the battery TR behaviors and trigger characteristics. The parallel connection can reduce the TR trigger temperature from 221.7°C to less than 170°C and can advance the TR trigger to the moment of the safety valve opening. The TR triggering of the parallel battery is mainly affected by the electricity transfer between batteries, and the transferred electricity exceeding 4.6% of the nominal capacity of the battery is sufficient to trigger the thermal runaway in advance. By analyzing the temperature response of different positions in the battery, it is found that the TR of the parallel battery first occurs in the upper region near the electrode tabs.

(4) Considering the diversity of battery TR triggering scenarios, a series of TR experiments of lithium iron phosphate batteries exposed to the front heating modes are performed to investigate the effects of the parallel battery number on TR triggering characteristics and mechanisms. The results show that the onset time and triggering temperature of TR decrease with the increase of the parallel battery number. Once the number of parallel batteries exceeds 2, the continuous electricity transfer may occur in the parallel batteries after the TR. Based on the electricity transfer characteristics and temperature responses of different positions in the battery, combined with the analysis of the battery debris, the TR triggering mechanism of the parallel battery under front heating and the cause of the continuous electricity transfer phenomenon are revealed, and three methods to reduce the excessive TR risk of the parallel battery are proposed.

(5) Based on the investigations about TR triggering of the individual battery, the TR propagation experiments of open-circuit and parallel batteries are conducted. The TR propagation behaviors and heat transfer law are analyzed at the battery module level, and the effectiveness of the thermal insulation material to block the TR propagation of battery modules with different connection modes is verified. The results show that the parallel battery module exhibits a higher combustion risk during TR propagation in comparison with the open-circuit battery module. The thermal conduction between the battery shells is the main heat transfer path to drive the TR propagation, and the heat transfer through the parallel connection only accounts for 19% of the thermal conduction between the shells. Aiming at the main heat transfer path between batteries, the insertion of thermal insulation material between batteries with different connection modes can effectively prevent the propagation of TR, and the mechanism behind insulating thermal insulation materials blocking TR propagation is revealed.

Date of Award	25 Jun 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Lizhong Yang (External Supervisor) & Kim Meow LIEW (Supervisor)