Research On Degradation Of Cycle Performance And Accelerated Life Test Of LiFePO₄ Battery

Due to the high energy density and power density and the advancement of battery technology,the applications of lithium-ion batteries have gradually extended from consumer electronic products to hybrid vehicles and pure electric vehicles as well as the energy storage in power grids.Contrasting with the service life of consumer batteries,for the energy system,the cost of power batteries or energy storage batteries is high,and the equipment with batteries often requires the life span of more than 10 years.This puts higher requirements on the lithium-ion battery.At the same time,the battery will experience complicated working conditions and external environmental conditions.The test for the life of lithium-ion battery based on actual use and working conditions usually requires a long time,and it is unable to meet the needs of product development.Therefore,research on the accelerated life test of lithium ion batteries has become very impo rtant and urgent.In this paper,the accelerated life tests of LiFePO₄/graphite full battery are carried out with ambient temperature,cycle rate and high discharge rate as accelerated stress factors.The accelerated effect of different factors on the decay performance of the cycle-aged battery was studied.The full battery was disassembled,and the cathode and anode materials and the electrolyte were tested and characterized.The aging factors affecting the decay of the battery capaci ty and the influencing modes were quantitatively analyzed.The battery performance attenuation behavior under accelerated conditions and non-accelerated conditions were compared,and then the range of accelerated stress conditions which can reasonably accelerate the aging of the lithium ion battery was obtained.The acceleration effects of ambient temperatures?25°C,35°C,45°C and55°C?on the degradation of cycle performance of LiFePO₄/graphite full cells?cycled at 1/3C and 100%DOD?are studied,and the causes of aging factors at different ambient temperatures are analyzed.Studies have shown that ambient temperature can significantly accelerate the decay rate of battery capacity,and the battery life at 55°C is about one-seventh of the battery life at 25°C.The exponent term z of cycle time at different temperatures is obtained by Arrhenius analysis.The values of z at 45°C and 55°C increase with the temperature.Calculation of the irreversible consumption of active lithium and the performance decrease of the cathode material shows that the performance of the cathode is not significantly reduced at 25°C.The capacity decay of the battery is mainly due to the irreversible consumption of active lithium which is caused by the repair of the SEI film on the anode surface.However,as the ambient temperature increases,the structure decay and the thickening of the surface layer result in a decrease in the performance of the cathode material.When the ambient temperature is higher than 35°C,the decrease in cathode performance becomes an important factor in the battery capacity fading.In addition,the elevated ambient temperature significantly promotes the electrolyte decomposition,accelerates the consumption of active lithium,and changes the consumption mode.Therefore,when the ambient temperatures are 45°C and 55°C,although the decay rate of the battery capacity increases,the attenuation mechanism is also changed.At 35°C,the decay rate of battery capacity increases slightly.In short,for LiFePO₄/graphite full cell,raising the ambient temperature is not an effective method for accelerated life test.The acceleration effects of cycle rates?1/3C,1.0C,1.5C,1.8C,2.0C,and 2.5C?on the performance degradation of LiFePO₄/graphite full cells?cycled at 25°C and100%DOD?are studied,and the causes of aging factors at different cycle rates are analyzed.Studies have shown that increasing the cycle rate can significantly accelerate the capacity decay of the battery.The cycle life at 2.5C is only 1/34 of the cycle life at 1/3C.Disassembling analysis shows that the lithium iron phosphate cathode material does not significantly deteriorate after aging at different cycle rates.For the graphite anode material,the SEI film and the formation of lithium deposits are the main cause of the performance degradation of the anode electrode,and the irreversible consumption of active lithium in this process is the most important factor leading to the decay of the battery capacity.However,when the cycle rate is greater than 1.8C,lithium precipitation occurs on the anode surface,resulting in a large and rapid loss of active lithium.Therefore,the cycle rate of 1.8C is the maximum current value that can be used in the accelerated life test of LiFePO₄/graphite full cell without changing the battery attenuation behavior.When the cycle rate is 1.8C,the time for the full cell being decayed to the end of life is about 60 days,which is less than one tenth of the time at 1/3C,and this can significantly accelerate the capacity decay of the battery.When the consumption mode of active lithium changes,the linear relationship between the acceleration parameter A_rate and the cycle rate will change accordingly,which can help make a quick judgment on whether the attenuation mechanism of lithium ion battery capacity changes.At the same time,a reasonable range of A_rate can be used to estimate the cycle life of the full cell.The accelerated effects of high discharge rates?0.5C,1.0C,2.0C,3.0C,4.0C and 5.0C?on the cycle performance degradation of LiFePO₄/graphite full cells tested at 25°C and 100%DOD are investigated,and the causes of aging factors at different discharge rates are analyzed.Studies have shown that under smaller discharge rates,the main reason for the full cell capacity attenuation is the irreversible consumption of active lithium caused by the formation and recovery of SEI film on the anode surface.Under the conditions of 4.0C and 5.0C discharge rates,lithium ions are rapidly released in a large amount,so that the SEI film on the surface of the negative electrode is not stable enough and can be easily destroyed,thereby making a relatively high decay rate of the battery capacity.The high discharge rates have an important effect on the structural decay of the graphite anode material and the surface composition,which leads to the increased internal resistance in the full cell.In addition,the performance of the lithium iron phosphate cathode material will degrade,which increases the decay rate of battery capacity in the later period of cycling.When the discharge rate is less than 3.0C,the cycle time is reduced mainly by the shortened discharge time whereas the acceleration effect is limited.For the 2.0C discharge rate,the multiplication of capacity decay rate is less than 3 times.When the discharge rate is greater than 3.0C,the large discharge current adversely affects the performance of the cathode and anode materials and the surface film on the anode,and causes rising internal temperature of the battery,resulting in a change in the battery capacity attenuation mechanism.Here,the high discharge rates are not suitable for the accelerated life test of LiFePO ₄/graphite full cells.