Research On Performance And Accelerated Life Of Lithium Manhanese Iron Phosphate Composite Battery For Vehicles

As an"upgraded version"of lithium iron phosphate,lithium manganese iron phosphate(LiMn_xFe_1-xPO₄)cathode material has broad application prospects and is considered to be one of the main development directions of power battery cathode materials in the future.Research on materials and battery performance is of great significance.Not only can LiMn_xFe_1-xPO₄ cathode materials with excellent electrochemical comprehensive performance be obtained to meet the requirements of their commercial applications,but also provide data support for the application of this system of batteries in the field of energy storage and electric vehicles.Among them,battery life is one of the important indicators to measure battery performance,and the correct evaluation of battery life plays an important guiding role in battery production and development and battery health management systems.This topic takes lithium manganese iron phosphate(LiMn_0.6Fe_0.4PO₄/C)cathode material and 18650 type lithium manganese iron phosphate composite ternary system battery（referred to as lithium manganese iron phosphate composite battery）as research objects,studies the material and battery performance,and proposes a degradation model for lithium manganese iron phosphate composite battery through accelerated life testing.The main work and conclusions are as follows:（1）LiMn_0.6Fe_0.4PO₄/C cathode material was successfully prepared by adjusting the proportion of ferromanganese to lithium and superposed carbon coating technology,and its material characterization and electrochemical performance tests were conducted.The results show that the material has a single-phase olivine structure,with a particle size of100-200 nm and a carbon layer thickness of 2-3 nm.The LiMn_0.6Fe_0.4PO₄/C sample prepared has good magnification performance,with a discharge capacity of 137.7 m Ah g^-1 at the rate of 1 C,approximately 83.6%of the discharge capacity at the rate of 0.1 C.After 100 cycles at 0.2 C rate,its capacity retention rate is 98.2%,with excellent cycle stability.At 45℃,after 80 cycles at 0.5 C rate,its discharge capacity is 148.7 m Ah g^-1,with excellent high-temperature performance.（2）In order to further realize the commercial application of LiMn_0.6Fe_0.4PO₄/C cathode material,a 18650 type lithium manganese iron phosphate composite battery was prepared by combining it with a ternary material(LiNi_0.5Co_0.2Mn_0.3O₂).Its electrochemical performance was studied,including magnification,temperature,internal resistance,and open circuit voltage characteristics.The results show that the composite battery has excellent high rate discharge performance,and its discharge capacity at 5 C rate is about 98.03%of that at 0.5 C rate.With the increase of ambient temperature,the battery capacity increases exponentially,but at low temperatures,the battery capacity declines rapidly.In the lower state of charge（SOC）range,the internal resistance increases significantly,and when the SOC is 40%to 50%,the internal resistance changes slightly.Storage in this state is most beneficial for the battery.The SOC-OCV curve decreases as the ambient temperature decreases,and the shift rate at low temperatures is greater.（3）According to the effect of different accelerated stresses on the life of lithium manganese iron phosphate composite batteries,the environmental temperature,charging cut-off voltage,and discharge rate are selected as the types of accelerated stresses,and the accelerated life test of lithium manganese iron phosphate composite batteries is conducted within their reasonable use range.The results showed that compared to ambient temperature（35℃、45℃）and discharge rate（1 C、2 C）,the effect of charging cutoff voltage on battery capacity degradation was more significant.When the cycle reached 250 cycles,the battery capacity retention rates at 4.3 V and 4.4 V charging cutoff voltages were and 91.6%and 84.8%,respectively.As the number of cycles increases,the battery capacity retention rate under different accelerated stresses exhibits a characteristic of approximately linear attenuation,and when the types of stresses are consistent,the battery capacity decreases more rapidly under high intensity stresses.By fitting and analyzing the accelerated life test data,a degradation model of lithium manganese iron phosphate composite battery under different accelerated stresses was established.By comparing and analyzing the predicted data of the model with the measured data,it can be concluded that the predicted error of the model is within 2%,which proves that the established model can well achieve the life prediction of lithium manganese iron phosphate composite batteries under the above experimental conditions.