| Lithium-ion battery is the most mature and widely used chemical power source in new energy vehicles,but its safety problem is very outstanding,which seriously restricts its development.The safety issues of lithium-ion batteries are divided into intrinsic safety,failure safety,and abuse safety.Abuse is popular situation for users during using.Furt hermore,abuse includes thermal,mechanical,and electrical abuse.Three types of abuse: external heating,mechanical puncture,and overcharging are the most likely to cause fire or explosion,and the most difficult to overcome.To this end,national stand ards specify a number of safety testing items such as hot boxes,acupuncture,and overcharge.In this paper,thermal simulation and experimental research are performed for these three cases,and effective improvement methods are proposed.With reference to the experimental conditions stipulated by national standards,1 C 100% DOD charge and discharge cycle tests and 130 ?-160? hot box tests are performed on the individual cells of lithium ion batteries,needle and overcharge tests on cells and modules.Thermal generation and thermal diffusion processes such as electrochemical reaction heat and side reaction heat are analyzed,and an improvement is made on the basis of an electrochemical-thermal coupling model,and a thermal simulation model is established.Perform thermal simulation with finite element simulation software,compare the simulation and measured data,and veri fy the accuracy of the simulation model;analyze the results of simulation and testing and find out the possible cause of thermal runaway,and propose safety improvement methods in a targeted manner.The effectiveness of the improved method was verified by charging and discharging test,cycle test,safety test,and XRD characterization.It was found in the hot box test that the external high temperature accelerates the side reaction processes such as the decomposition of SEI and the interaction between the anode and the electrolyte.The reaction heat generation leads to the internal temperature of the core exceeding the control temperature of the hot box,which softens the diaphragm and causes the internal short circuit.The Joule heat of the internal short circuit is fed back to the above-mentioned side reaction process,which further accelerates thermal runaway.Based on this,the simulation model of the hot box is perfected,and the simulation results are in good agreement with the measured results,and th e temperature error is not more than 2.5?.The simulation results prove that when the temperature of the hot box reaches above 160?,the heat generation power of the internal short circuit exceeds 63% of the total heat generation power,which is the most important factor leading to thermal runaway.A high temperature resistant polyimide composite coating was proposed to prevent internal short circuits.The 5 ampere-hour core manufactured by the method has no thermal runaway in the heat box test at 160?.It has been found that the short-circuit form of the single-cell acupuncture of the lithium-ion battery is not continuous and single.At the beginning of the acupuncture,the short-circuit concentration appeared in the position of the needle,and rapidly evolved into the short circuit of the whole electric core and the thermal runaway occurred.The thermal runaway heat source of the cell comes from the release of short-circuit electrical energy at the acupuncture position and the occurrence of exothermic side reactions.Thereafter,a larger area of short circuit caused by the shrinkage of the diaphragm will also occur at other positions of the battery cell.The thermal diffusion rate of the module is correlated to the temperature gradient of the cell and the thermal conductivity of the case.The curve of cell simulation temperature and test temperature are basically the same.The simulation found that at the end of the acupuncture test,the evaporation and combustion of the electrolyte contributed the main heat generation power;Use sodium bicarbonate and other materials to isolate oxygen;Explosion-proof valve is used to drive flammable solvent vapor away from the heat source;Use flame-retardant and heat-resistant materials such as silicone to prevent heat spread.The test results show that the battery produced by this method does not ignite or explode during the needle puncture test.The overcharge thermal runaway diffusion mechanism of power lithium-ion battery cells and modules was analyzed and researched and found that: In the early stage of charging,the heat generated by the battery mainly comes from Joule heat and polarization heat during the normal charging and discharging process;When the voltage of the soft package core with Li Ni 1/3Co1/3Mn1/3O2 as cathode material is more than 5.1 V,the heat generation comes from a variety of side reactions and internal short circuit factors.The heat calculation is carried out based on the equivalent DC internal resistance method of the electric core and the simulation model is established,the error betwee n the simulation and the measured results is small.The overcharge test and simulation of the battery cell have confirmed that after the battery voltage exceeds 5.1 V,lithium dendrites generated by the negative electrode will cause multiple short circuits and contribute the main heat generating power.The use of hard carbon anodes increases the lithium storage space and suppresses the formation of lithium dendrites,thereby improving the overcharge resistance of the cells.Through the overcharging test and simulation of the module,it is found that if not suppressed,the core voltage and temperature after overcharging will soon reach the positive electrode oxygen evolution condition,and accelerate the process of thermal runaway.Li Ni1/3Co1/3Mn1/3O2 is combined with highly active tubular structure lithium manganate to increase the positive oxygen evolution overpotential and suppress thermal runaway of overcharge;Multi-level overcharge protection devices and other methods are used to improve the battery's re sistance to overcharge. |
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