Study On Evolution Of Electro-thermal Characteristics Of Power Lithium-ion Battery During Its Entire Life

As the increasing demand for protection of energy and environment,the research and commercialization of electric vehicles have been developed rapidly.However,the further application of electric vehicles has been restricted by the recession of performance due to the problems of aging and thermal safety in its whole life with the improvement of energy density for power lithium-ion batteries.Therefore,it is an essential basis for accurate fault diagnosis,state assessment and warning of thermal runaway by exploring the evolution of electro-thermal characteristics in the whole life of power lithium-ion batteries.In this paper,the NCA 21700 power lithium-ion battery with high specific energy is selected as the research object,which has been used to study the aging characteristics,aging paths and evolution of thermal safety during its whole life on the basis of aging and performance test system,adiabatic thermal test system and external short-circuit test system.Firstly,the variation of characteristics of power lithium-ion battery in the aging process has been studied and analyzed under multiple charge/discharge and environmental conditions on the basis of accelerated aging tests,capacity increment tests,rate characteristic tests and HPPC tests.It is found that 21700 battery has extremely poor cycle performance due to the nickel-lithium mixed phenomena in the continuous aging process though it has high available capacity and energy.During the 250 cycles,the capacity,energy and energy efficiency decreased obviously.Meanwhile,there is a significant behavior of gas generation internally during the aging process of the battery,which results in the expansion of the negative electrode terminal dramatically.Through the analysis of increment capacity increase curve,it is investigated that the height,position and area of the four peaks varies with the aging simultaneously.The capacity loss corresponding to the voltage range between 3.50 V to 3.80 V accounts for the biggest of overall loss for this battery.In addition,both of the rate characteristics and environmental adaptability of the battery suffers from a worse change.The open-circuit voltage becomes more stable.And the temperature sensitivity of internal resistance and internal resistance increases obviously.Secondly,the effects of charge/discharge rates,charge/discharge depth and ambient temperature on the aging rate of power lithium ion batteries are investigated by designing a variety of aging paths.Besides,the characteristic parameters of the discharge voltage curves in the aging process are identified by combining with the principle of battery voltage variation and the method of genetic algorithm to solve the optimal solution,which could help for studying the influence of aging path on its performance degradation quantitatively.The result of the research shows that the aging rate could be accelerated by the higher charge rate,the bigger charge/discharge depth,the higher voltage range and the lower environmental temperature.Both of active lithium loss and active material loss occurs inside the battery when it is aged with a higher charge rate.However,the lower charge and discharge depth could contribute to slowing down the loss of active material,but it could not effectively inhibit the loss of active lithium-ion.Moreover,the lithium deposition in a low-temperature environment could accelerate the loss of anode material and active lithium terribly.Finally,considering the actual scenario of electric vehicle safety incidents,the effect of aging on the thermal safety of battery is investigated from three aspects,respectively regular use,thermal abuse and electrical abuse.Both of temperature variation rate and maximum temperature of power lithium-ion batteries increase significantly with the aging degree.The phenomenon of temperature rise caused by aging is more evident under high charge/discharge rates or low temperature.In detail,the increase of temperature results from the enlargement of irreversible heat generation primarily.The critical temperature corresponding to self-heating behavior of battery increases firstly and then decreases with the aging in the brewing stage of thermal runaway.The temperature corresponding to voltage failure continues to decrease.In the process of thermal runaway,the reactions between various materials inside battery are more sufficient,which leads to the higher temperature and temperature gradient.And the ejection of active materials is more serious.For the abuse of external short circuit,the short circuit current of battery lasts longer with the aging though its maximum gradually decreases,which results in the slightly higher surface temperature.