Electrochemical-thermal Coupling Characteristics And Combined Thermal Management System Of Lithium Batteries

Lithium-ion battery plays an important role in many fields such as electric vehicle,electrochemical energy storage,and electronic information.With the necessity of achieving national carbon peaking and carbon neutrality goals as well as the diversification application of Lithium-ion battery,its working performance,cycle life and safety performance have received extensive attention.The working process of lithium-ion battery is actually a complex interaction process including electrochemical reaction kinetics as well as heat and mass transport,which is also closely affected by the external environment.Based on the electrochemical-thermal coupling characteristics of lithium-ion battery,this paper focuses on the battery life attenuation,thermal management and thermal runaway propagation in battery module.First,the voltage plateau and temperature change of NCM battery at different charge/discharge rate and ambient temperatures were measured experimentally.On this basis,an electrochemical-thermal coupling model was established,and the electrochemical as well as thermal distribution inside the battery under constant current discharge conditions were deeply explored.In order to be closer to the actual vehicle operating,the dynamic electrothermal characteristics of the battery under NEDC conditions were analyzed.It can be found that the fluctuation range of the working voltage increased with the increase of depth of discharge.According to the analysis and comparison of the dynamic voltage at di fferent ambient temperatures,the working voltage range with higher battery capacity during cycles can be obtained,and the temperature optimization in this range is more helpful to improve the working performance of battery.Finally,according to the dynamic change of the battery temperature during cyclic process,the cooling strategies such as multi-frequency physical cooling and increasing the heat dissipation coefficient in the idling interval of electric vehicle is proposed to control the battery temperature within an appropriate range.Considering the battery aging mechanisms of the positive and negative active materials loss,SEI(solid electrolyte interface),electrolyte oxidation and lithium plating,an electrochemical-mechanical-thermal coupled aging model of NCM battery was established.Based on the model,the effects of charge/discharge rate,upper and lower voltages and ambient temperatures on the capacity fading of the battery were analyzed.Among the attenuation losses of the battery at normal temperature,the loss of cathode active material accounts for the largest proportion,followed by the growth of SEI film,and the loss of lithium plating is the minimum.With the aging of the battery,the voltage loss and the temperature rises obviously;the electrode particles break unevenly in the thickness direction of electrode,the volume fraction of positive and negative solid phase decreases faster near the separator than that near the current collector,and the thickness of SEI film and lithium plating was larger near the separator.In addition,with the increase of charge-discharge rate,the increase of the upper charge voltage and the decrease of the lower discharge voltage,the battery aging rate increases,but the increasing trend gradually slows down.For the influence of ambient temperature,low temperature and high temperature both accelerate the aging of the battery.The increase of lithium plating loss at low ambient temperature is the main factor for the rapid attenuation of battery capacity,the losses of positive,negative active materials and electrolyte oxidation also increase with the decrease of temperature.The decrease of battery capacity at high ambient temperature is mainly due to the increase of SEI film growth rate.Finally,the influence of different electrical connection modes on the capacity fading of battery module was analyzed.The results shows that the aging rate of battery rises after being used in module,and aging performance differs among batteries;Due to the large current fluctuation during operation,the aging characteristics of module with parallel electrical connection is worse than that of module in series.Based on the temperature uniformity characteristics of PCM and the excellent cooling performance of liquid cooling,a liquid cooling/PCM combined thermal management system was proposed.For the battery module,to establish a multi-layer electrochemical conjugate heat transfer coupling model fully considering the differences of discharge and heat generation of each single cell.Firstly,with the thermal characteristics of the battery module as the optimization goal,the cell spacing and coolant flow rates were optimized.Secondly,the various polarization differences of battery module are calculated and analyzed by mathematical method,and the discharge inconsistency of battery modules with different cooling systems was quantitatively compared;Based on the above methods,the effects of the initial coolant temperature on the discharge characteristics and temperature variation of the module were analyzed and compared.Finally,the dynamic thermal characteristics of hybrid cooling module and the latent heat recovery of PCM under continuous chargedischarge cycles were calculated and analyzed.According to the research results,it is found that the hybrid cooling can still control the maximum temperature and temperature difference of the battery module within an appropriate range when discharging at a high rate(3 C).With the increase of the cell spacing,the maximum temperature and temperature difference of the battery module both decrease,but the reduction rate decreases when it exceeded a certain spacing.The increase of coolant flow rate reduces the maximum temperature of the battery module,but also worsens its temperature uniformity.In addition,it can be concluded that the diffusion polarization of electrolyte is the main cause of the unbalanced discharge in battery module by comparing the maximum differences of each polarization.When the initial coolant temperature was low,the maximum differences of polarization increased sharply,and the discharge unbalance increased.The discharge capacity of battery module decreased with the reduction of the coolant temperature,the available capacity decreased by about 1.3-3.6%for every 5℃ decreases in the initial coolant temperature when the discharge rate was 3 C.During a continuous charge-discharge cycle,the temperature and temperature difference of the hybrid cooling module can be maintained within an appropriate range,and the latent heat of PCM can be recovered in time before the start of the next cycle.Regarding the 3×3 battery module composed of cylindrical battery,a thermal runaway model of lumped reaction kinetics coupled with heat transfer was established,the thermal runaway propagation characteristics of runaway monomer in battery module under different battery spacing,convective heat transfer coefficient and ambient temperature was explored.In addition,the inhibitory effect of liquid/PCM hybrid cooling system on the thermal runaway of battery module was analyzed.By increasing the cell spacing,the convective heat transfer coefficient(water cooling,oil cooling as well as heat pipe cooling)and reducing the ambient temperature,the propagation spread of thermal runaway in battery module can be delayed or suppressed.For the hybrid cooling structure,the low coolant flow rate will accelerate the thermal runaway propagation due to the high thermal conductivity of cooling structure.However,with the increase of the flow rate,the suppression of the thermal runaway propagation in battery module can be realized.