Lithium Ion Battery Thermal Runaway And Fire Risk Analysis And The Development On The Safer Battery System

Due to its high voltage, high specific energy, long cycle life, no pollution to the environment and other excellent performance, Li-ion battery has become the dominant power in the field of consumer electronics. Meanwhile, Li-ion battery has been contemplated by energy storage and power supply industry such as photovoltaic energy, nuclear energy, wind energy,(hybrid) electric vehicles and aircraft. However, fire incidents caused by thermal runaway of Li-ion battery have been reported recent years. Safety has become one of the most intractable barriers to the wide application of Li-ion battery in the above industry. Therefore, study on thermal runaway and fire hazard of Li-ion battery and high-safety battery system can improve our understanding on the origins of thermal runaway and fire of the battery, clarify the main unsafe reason, determine the behavior and hazard of thermal runaway and fire quantitatively, and propose the safe battery system to reduce the possibility of the incidents, to provide data base and technical support for the battery safety and fire protection design.The work of this thesis are summarized as following:Thermal hazards of some electrolytes based on different lithium salts are studied and compared based on thermodynamic analysis. It is shown that the lithium salt influences the thermal hazard of electrolyte markedly. However, the thermal stabilities of different electrolytes are not coincident with that of the corresponding lithium salts. Furthermore, the LiBOB-based electrolyte shows the higher thermal stability than other tested electrolytes.Deconnvolution method is proposed to investigate the thermal runaway hazard of the full cell system, based on the thermal analysis results of the electrolyte-electrode coexisting system. The detail process of the thermal runaway behavior for the full cell system is clarified. It is shown that the content of electrolyte is related with the thermal reactivity of the system; the decomposition of solid electrolyte interface (SEI) film initiates the thermal reaction of the system; the reactions involved with positive material release much heat, which is the largest part of the overall heat; the Joule heat caused by the short circuit following with the melting of separator increases the hazard of thermal runaway for the system.Full-scale burning test system for the large and high-energy Li-ion battery is established. The fire behavior and hazard of50Ah LiFePO4/graphite battery is studied systematically. During the test, the Li-ion battery shows a special fire behavior, the repeated jetting fire. Based on the oxygen mass released from the electrode and the Joule heat caused by the short circuit of the battery, the overall heat generation in the burning test is calculated as18195.07,10368.98,4639.65kJ for100.50、0%states of charge (SOC) batteries, respectively. The peak heat release rate (HRR) is49.35,30.05,12.85kW and the mass loss is405.78g,342.12g,294.69g for100,50,0%SOC batteries, respectively. It can be concluded that the heat generation, peak HRR and mass loss is directly related with the SOCs. Considering the thermal analysis on the full cell, it is shown that the Joule heat generated by the short circuit following with the melting of separator triggers the jetting fire directly. It makes miner difference for the temperature of the flame whether the SOC is large or small. Even though the battery is fully discharged, it still shows very high flame temperature. The highest temperature of the flame is1500,1020,1091℃for100,50,0%SOC batteries, respectively.Safety and electrochemical characteristics of LiBOB/GBL electrolyte are discussed. The LiBOB/GBL+DMS electrolyte is proposed based on the addition of sulfite. The reason why the LiBOB/GBL electrolyte decomposed and causes irreversible capacity loss is probed. The effeteness of using sulfite to overcome the above problem is tested. The good electrochemical stability and high safety of the LiBOB/GBL+DMS electrolyte is characterized.Compatibility between the electrolyte with high-content TPP and the positive or negative electrode is examined. The highly safe battery system composed of Li(Ni0.8Co0.15Al0.05)O2,high-content TPP electrolyte and MCMB is proposed. The electrochemical analysis show that the dissolvability of lithium ions would be effected by the higher content of TPP, then the batteries show higher impedance and polarization. However, the storage tests show that TPP decrease the lithium consumed during the tests at higher temperature. It implies that TPP forms more stable SEI, but the SEI shows higher impedance. After the addition of2%VC, the electrochemical performance of the battery is improved. Compared with the control battery system, the coexisting system of electrolyte with TPP and the electrode shows much higher safety.