The Electrochemical And Thermal Performance Of Lithium Batteries And An Ultra-thin Heat Pipe-based Thermal Management System

Lithium ion batteries have a wide application in the renewable energy,electric vehicles,electronic equipments,etc.Therefore,the performance and safety of lithium ion batteries has been increasingly weighted.The thermally driven issues,such as degration and inconsistency,are becoming prominent,especially at the background of battery energy density increasing.The research about these issues plays a key role in battery designs,operation conditions,equalization of modules,battery thermal management system?BTMS?,etc.Furthermore,the development of an efficient BTMS helps to ensure the performance and safety of batteries.The present study focuses on the degration of batteries,the inconsistency of modules and the BTMS.The main contents and conclusions are as following,1. An electrochemical-thermal-mehchanical coupled aging model is developed for a LiFePO₄ battery,including the aging mechanisms of SEI?Solid electrolyte interface?,Li plating and loss of anode material.As the initial temperature of batteries increases,the capacity fading experiences three stages,where the corresponding dominated aging mechanism varies,i.e.the loss of anode material,the loss of anode material and the SEI,the SEI.As the C-rate increases,the temperature range corresponding to the second stage moves toward higher temperature.The high charging C-rate does not definitely leads to a significant increase of capacity fading,because the charging rapidly enters into the stage of constant voltage.The Li plating mainly occurs in the region near to the separator,and the loss of anode material is significant in the region.The Li plating can reduce the SEI and loss of anode material.Among the active particle diameter,thickness of electrodes,solid phase volume fraction and electric conductivity,the active particle diameter has the most significant effect on the fading.The electric conductivity affects the fading via the distribution of current density and the dynamic working current.2. A multilayer electrochemical-thermal coupled model is built for a battery module and each parallel connected cell inside each battery is included in the modeling.The electrochemical performance,temperature non-uniformity and the unbalanced charging under a two-stage fast charging are studied.When the charing turns from high C-rate into low one,there is a large change or increase in the electrochemical papramter spatial distribution,solid phase Li⁺concentration gradient and the cell inconsistency.When the charing turns from low C-rate into high one,the unbalanced charging at the terminal stage of charging is aggravated.Otherwise,a fluctuation in the unbalanced charging occurs at the monement when changing the C-rate.Based on the multilayer electrochemical-thermal coupled model,the unbalanced discharing is investigated including the cooling process.The unbalanced discharging increases significantly at the terminal stage of discharging.As the convective heat transfer coeeficient increases,the unbalanced discharging firstly increases,and then decreases after the improvement of cooling performance becomes limited.The average temperature of modules is an important parameter in the unbalanced discharging.Reducing the average temperature increases the sensitivity of unbalanced discharging on the temperature non-uniformity,especially when the average temperature is lower than 20°C.Reducing the initial temperature of modules or increasing the discharing C-rate aggravates the unbalanced discharging.When the C-rate exceeds 4 C,the C-rate shows little effect on on the aggravation.When the coolant temperature is lower than the initial temperature,the unbalanced discharing is aggravated.The local temperature difference of a battery increases the unbalanced discharging,which will be more significant if the local temperature difference is various in different batteries.Compared with that in the module with more cell number,the unbalanced discharing in the module with less cell number is significant when the convective heat transfer coeeficient,but will decreases rapidly after the improvement of cooling performance becomes limited.3. The model development,the analysis of electrochemical characteristics and the cooling performance are conducted.The evaporation section of the heat pipe is flattened?2 mm in thickness?.The wick of the heat pipe is sinter copper powder.The thermal-hydraulic model is developed for the heat pipe.When the flattened length increases or the flattened height reduces,the temperature difference of the heat pipe linearly and exponentially increases,respectively.Reducing the flattened height and the operating temperature aggravates the non-uniform distributon of evaporation rate inside the flattened evaporation section.The evaporation rate is higher when approaching the outlet of evaporation sections.When the flattened height is 2 mm and the height of vapor region is 0.5 mm,the vapor pressure drop cannot theorectically calculated via the laminar flow pressure drop equation of circular pipe.The laminar flow pressure drop equation for the two infinite flat parallel plates should be adopted,and then a segemented-layered conduction-based model is provided for the heat pipe.Furthermore,an electrochemical-thermal coupled model is developed for a battery module using heat pipe cooling.The electrochemical and thermal characateriestic of each battery is included.When the cooling begins,the coolant temperature should be not 10°C lower than the initial temperature of the module.Under different coolant temperatures,both the local current density and Li⁺concentration change at a similar speed at the initial stage of discharging.Subsequently,those under lower coolant temperatures changes more violently,forming a larger spatial gradient within a cell.The cooling increases the gradient of solid phase Li⁺concentration in cathode,which dominates the loss of available capacity in the cooling process.With reducing coolant temperature by 10°C,the the available capacity decreases by about 1.14%?1.17%and the unbalanced discharging at the end ofdischarging increases by about 100%?150%.Finally,the cooling performance of the heat pipe coupled with liquid cooling is experimentally investigated in the adjustable parameters of the cooling process.Under different ambient temperatures,when the coolant flow rate increases to the same certain value,the improvement of cooling performance is limited.When the ambient temperature is lower than 35°C,the allowable maximum heat load under different ambient temperatures can be kept nearly unchanged via reducing inlet coolant temperature.When the ambient temperature is lower than 25°C,it is unnecessary to reduce the inlet coolant temperature.A delay cooling strategy is provided and the thermal equilibrium temperature can be used as an index to initiate the cooling system.An intermittent cooling strategy is provided and the cooling performance is close to the constant cooling.The heat pipe coupled with flow boiling is proposed.The coolant flow rate can be reduced by 50% and the temperature is more uniform,compared with the heat pipe coupled with liquid cooling.As the coolant flow rate increases,the cooling performance firstly improves and then decreases due to the transition of heat transfer mechanism.