Study On The Thermal Runaway Risk And Deflagration Of Nickel Cobalt Lithium Manganate Batteries

As a new type of energy storage medium,the lithium-ion batteries have been widely used in consumer electronics,transportation,aerospace and energy storage fields.However,there are certain potential safety hazards for the lithium-ion battery.The electrochemistry performance of the battery is improved by the development and application of the new manufacturing techniques and new materials,but at the same time,the thermal runaway(TR)hazard is increased.Besides,in the fields of new energy vehicles and energy storage,the lithium-ion batteries are mostly placed in confined spaces,gas and heat released from runaway batteries accumulate easily due to the poor heat dissipation and ventilation conditions,which could lead to serious fires and even explosions.The risk of thermal runaway and deflagration of typical nickel cobalt lithium manganate(NCM)batteries are carried out in this thesis.The TR characteristics of typical NCM batteries were studied,the influences of the state of charge(SOC),the number of cycles,the heating methods and the overcharge on the risk of TR were analyzed.The TR hazards increase with the increase of the SOC and the number of cycles.For fresh batteries,the batteries with higher SOC exhibited lower onset temperature of TR and higher peak temperature and pressure.For aged batteries,the time from start to be heated to TR was lower than that of fresh batteries,and the onset temperature of TR was also lower than fresh batteries.In addition,more attention should be paid to the TR hazards of aged batteries,the thermal stability of these aged batteries had been decreasing although the electrochemical performance was still excellent.The influence of slightly overcharged on TR behaviors was mainly concentrated in the early stage of the whole process.The voltage drop temperature of overcharged batteries dropped sharply during TR,the separator of 110%or 120%SOC battery was coverd by some films,while no significant changes were found in the electrode materials.Thus indicating that the films with weak thermal stability appeared on the surface of the separator was the main reason causing the decrease of the thermal stability of slightly overcharged batteries.The heat distribution law of TR batteries was revealed,the property of gas released from runaway batteries was analyzed.The batteries were induced to TR by overheating.The experiment results showed that the total heat produced by a fully charged lithium-ion battery with a rated capacity of 2 Ah was 61.72 kJ,which could be equivalent to 5.57 g TNT.The heat generated by a runaway battery could be divided into two parts,one part was used to raise the battery body temperature and another part was used to heat the ambient(canister)temperature.Besides,the heat absorbed by the battery body accounted for 31-44%of the total heat.Moreover,the peak pressure in confined space showed a much more similar tendency to the total energy released from the batteries at different SOCs,so it could be used as one of the judging criteria of the TR hazards.The main components of gases produced from NCM batteries under thermal abuse conditions were not affected by SOC.The main gas components could reach more than 9 kinds,mainly including CO2,H2,CO,CH4,C2H2 C2H4,C2H6,C3H6,C3H8.With the increase of SOC,the amount of the gas increased,the content of CO2 decreased,and the content of combustible gases increased,such as H2 and CO.The flammability limit range of the gas released from the battery with higher SOC was wider than that of batteries with lower SOC.The gas quantity and lower flammability limit of a fully charged battery were 1.84 L/Ah and 6.14%,respectively.The gas deflagration hazard in the lithium-ion battery energy storage container was explored,the effects of the position of the ignition point initial temperature and pressure relief plate setting on gas explosion were analyzed.One 1:1 geometric model based on the actual energy storage container was established,the gas deflagration hazard in a single container was analyzed by the FLACS software.It had been found that when the fire point was in the middle of the battery rack or closing to the small pressure relief plate,the pressure and velocity in the container would be higher due to the longer flame propagation time under restraint conditions.Besides,the higher the initial ambient temperature,the higher the fire or explosion risk.The setting of the pressure relief device significantly affected the explosion intensity.The pressure relief device with lower burst pressure should be set as many as possible,and the pressure relief device should avoid being blocked by obstacles.Injecting inert gas before the fire starts could reduce the explosion risk.When the short side of the container was 5 m apart,the effect of a single container explosion on the shock wave pressure of the surrounding containers limited.The influence of thermal radiation and flying debris should be mainly considered in this condition.