Study On Thermal-Degradation Characteristics And Thermal Management System Of Prismatic Lithium-ion Battery

Lithium-ion battery（LIB）has a wide application in electronic equipment,electric vehicles,and renewable energy utilization due to its light weight,long life,and higher energy density and power density,which can meet the requirements of fast charge and high power discharge conditions.However,with the continuous improvement of requirements on energy density and power density,security problems such as high temperature,degradation performance,and difficulty in regulation become increasingly prominent,which limits the wide range and deep application of LIB in various fields.Given the above problems,it is necessary to reveal the thermal-degradation characteristics of LIB and construct the thermal management system.Hence,the dissertation develops appropriate mathematical models and numerical methods to accurately simulate the characteristics of LIB and the thermal management system.At the same time,it is necessary to conduct theoretical and experimental analyses to verify these simulations.Based on the above background and research ideas,taking prismatic lithium iron phosphate（LFP）battery as the research object,numerical simulations,theoretical analyses,and experimental measurements are carried out as follows:（1）To reveal the thermal-degradation characteristics of prismatic LIB more comprehensively,a one-dimensional electrochemical model,a three-dimensional thermal model,and an accurate degradation model are coupled for a prismatic LFP battery.The degradation mechanism mainly considers the formation of solid electrolyte interface（SEI）,lithium plating,and loss of active material in electrodes.Multiple charge and discharge cycles under different conditions are simulated,and the results are verified by the experiments.Finally,the thermal-degradation characteristics and their relation are analyzed and summarized.The lithium plating,loss of active material in negative electrode and SEI formation,and loss of active material in negative electrode dominate the capacity fading at low temperature,normal temperature,and high temperature,respectively.（2）To make up for the lack of research on the prismatic LIB thermal management system and ensure the battery safety,a parallel multi-channel cold plate is proposed.The thermal management system of 6 in series and 4 in parallel（6s4p）prismatic LFP battery pack is modeled and analyzed in three-dimensional condition,and the results are verified by experiments.The thermal characteristics and influencing factors at 5 C discharge rate and external short circuit are discussed.In addition,the parameters of glycol aqueous solution are optimized by the temperature uniformity index of the battery pack.At the flow rate of 0.5m·s^-1,glycol aqueous solution with 70%volume concentration is the best cooling concentration.（3）To optimize and control the prismatic LIB in different cycle conditions efficiently,a one-dimensional thermal management system is established for 6s4p LFP battery pack with multiphysics model coupled,including electric,thermal,and liquid.And the optimized Single particle（SP）battery model is verified by the experiments.The thermal-degradation characteristics of the battery pack are analyzed and evaluated in two liquid cooling modes,series and parallel.Finally,the triple-step nonlinear method and extended Proportion-Integral-Differential（PID）control strategy are used to optimize the battery pack under different cycle conditions.The results indicate that the parallel mode has more advantages.The optimization effect of the battery pack is evaluated in 2 C-rate,NEDC and US06 cycle condition,the effectiveness of the two control strategies is verified,and the triple-step nonlinear control strategy is more advantageous.The research covers the thermal-degradation characteristics,thermal management system,and optimization control strategy of LIB,which can provide useful guidance for the design and optimization of large-scale lithium-ion battery thermal management system.