Research On Phase Change Heat Storage Thermal Management Of Lithium-ion Batteries In Cold Regions

Under the background of carbon neutrality,the global energy pattern is being reshaped,and the energy structure in our country has entered an important window period.It is of great strategic significance to accelerate the development of new energy technologies,especially new energy storage technologies.Electrochemical energy storage,as an important form of new energy storage,is one of the most potential energy storage solutions.Lithium-ion batteries have the advantages of high energy storage density,high discharge power,and no memory effect and thus are widely used in new energy vehicles and distributed energy storage.However,the performance and lifespan of lithium-ion batteries are highly dependent on the operating environment.Unsuitable ambient temperature will affect the normal operation of the battery and even bring about safety problems.Our country has a wide area of cold land,and the low temperature lasts for a long time in winter,which has a great impact on the stability of lithium battery performance.How to coordinate the contradiction between preheating and heat preservation of batteries in a lowtemperature environment of cold regions,and heat dissipation of batteries in a hightemperature environment is one of the key issues in the thermal management system of batteries in cold regions.It is intended to provide a theoretical basis for the promotion and application of lithium-ion batteries in cold climates,and to help achieve the “carbon peaking,carbon neutrality” goal.The paraffin/olefin block copolymer/expanded graphite(PW/OBC/EG)ternary composite phase change material was prepared by vacuum adsorption and directional compression.The physical properties such as microscopic morphology,physicochemical composition,and mechanical properties of the composite phase change material were characterized,and the relationship between the thermal conductivity and electrical conductivity of the composites and the additive amount of expanded graphite was analyzed.The thermal flexibility and shape stability of composite phase change materials were discussed.Furthermore,the heat storage,heat transfer and Joule heat characteristics of the composite phase change materials were studied experimentally.The research shows that the directional 3D thermal conduction network can be obtained by vacuum adsorption and directional compression method without destroying the expanded graphite sheet.Moreover,the3 D thermal conduction network can increase the thermal conductivity enhancement efficiency of the composite phase change material.The thermal conductivity of the composite phase change material can be increased by 74.4 times with only 30% loss of the latent heat.As the thermal conductivity increases,the heat storage power and thermal conductivity per unit area of the composite phase change material gradually increase,which is conducive to rapid absorption and conduction of heat.Olefin block copolymers can enhance the mechanical properties of composite phase change materials and make composite phase change materials thermally flexible for easy integration with batteries.A passive lithium-ion battery thermal management structure with phase change heat storage technology was constructed,and the relationship between the contact thermal resistance of battery and composite phase change materials and the phase change process was studied.A double-layer thermal management structure based on composite phase change materials with different thermal conductivity was designed.The inner layer adopts high thermal conductivity material to quickly absorb the heat of the battery,and the outer layer adopts low thermal conductivity material to keep the battery warm.The competition and coordination relationship between thermal conductivity and latent heat of composite phase change materials on battery heat dissipation and thermal insulation was explored.The results show that compared with the single-layer structure,the double-layer structure can better take into account the heat dissipation and heat preservation of the battery,and improve the performance of the battery in the cold environment.On this basis,the finite element model was established to optimize the thermal management structure of double-layer composite phase change material.The thermal insulation characteristics of different thermal management structures at different initial temperatures were analyzed.The results show that the phase change materials can rely on latent heat to keep the batteries warm in low temperature environments.The double-layer thermal management structure can further improve the thermal insulation capability by virtue of the large latent heat and low thermal conductivity of the outer composite phase change material.In a severely cold environment,the holding time of the battery system when the operating temperature drops to 0 ℃ is extended by 20.1%.Given the problem that the battery is polarized and cannot work normally due to the extremely low temperature in severe cold and extremely cold environments,a thermal management system with active preheating and heat preservation functions was constructed.The low-temperature DC preheating performance of the battery was tested to study the DC preheating effect of the battery under different ambient temperatures and different power levels.Experiments show that by reducing the output voltage of the battery,the preheating current can be increased,the battery can be preheated quickly,and the impact on the battery life is small.Furthermore,the composite phase change material was used as an external resistor to build a synergistic preheating structure with internal and external sources,and the effect and efficiency of synergistic preheating of internal and external sources under different ambient temperatures and battery power were studied.After the battery is preheated,the battery is connected in series with the composite phase change material and the load,and the Joule heating characteristics of the composite phase change material are used to continuously heat the battery to achieve the active heat preservation.Experiments show that active preheating can improve the battery’s discharge capacity in severe cold environments,and active heat preservation can further improve the performance of batteries in cold regions based on preheating.Compared with the single preheating mode,the discharge energy is increased by as much as 58.1% at a rate of 1C at-30℃.Aiming at the significant thermal effect after battery aging,a neural network model for health state evaluation and remaining life prediction of batteries was constructed to improve the battery thermal management system.The long-term and short-term memory neural network with a many-to-one structure was used to predict the discharge capacity of the battery by learning the data of the current and voltage variation with time during the battery charging process,and the capacity evaluation effect of the neural network was verified using the public battery database and the self-tested cold region battery data.The root mean square error between the estimated capacity value and the measured value is basically lower than 0.05,indicating that the neural network is versatile and robust in evaluating battery capacity.Then,the capacity data was decomposed by the method of empirical mode decomposition,and the remaining life of the battery was predicted by using the processed capacity data through a long-term and short-term memory neural network with a one-to-one structure.The results show that the neural network can learn the capacity data of the battery and predict the subsequent capacity to calculate the remaining life of the battery.The accuracy of the model prediction is highly correlated with the starting point of the prediction.