Research On Thermal-electrical Characteristics Of 18650 Power Cells And The Thermal Management Systems

The thermal safety of power battery module,as the heart of the electric vehicles(EVs),has been restricting the EVs development.The large quantity of heat generation during the high rates and/or long-term discharge process will inevitably cause the sharply rise of the internal temperature.Meanwhile the difference of heat dissipation boundary conditions will cause obvious uneven temperature distribution,which will furtherly lead to the severe temperature inconsistency and result in the severely fading of power batteries service performance.On the other hand,rapidly accumulation of heat generation will form the vicious cycle,eventually leading to the thermal runaway and cause serious hazards,such as burning and explosion accidents.Therefore solving the thermal safety issues in the practical application process will become the key for EVs popularization.Building an efficient heat dissipation system matching with the power batteries modules can achieve peak temperature and difference decrease quickly,and realise the fundamentally improvement of thermal safety issues.The cells determine the fundermental performance of the battery modules.18650-type power batteries,as the earliest,the most mature and stable batteries,owe highly mature production technology,which have been incorporated into the preferred technical route of the EVs enterprises represented by Tesla.The heat generation reactions and its correlation with electro-chemical performance of the power batteries under different working conditions were analysed,also the matched mechanical and electronic structure,battery thermal management composites and schemes were designed in the manuscript.Above research contents will provide systematic basic data and theoretical guidance for the 18650-type power batteries and thermal management system design.Relative research contents and main conclusions of this manuscript are as follows:1.Selecting the mainstream lithium iron phosphate and ternary 18650 power batteries for heat generation behaviors research on the present market.Two kinds of representative 18650-type power batteries product from the imported and domestic famous manufacturers were conducted for heat production reactions analysis under different work:ing conditions.Also the impact of the experiment environment and discharge current on thermal-electrical performances were researched.Furtherly,the correlation of electro-chemical properties fading and heat production increase in extreme high/ultra-low environment after overcharge operations were analysed.Finally the essential reason about the performance recession were explained through the electrode material from the micro perspective,also the relationship between micro thermoelectric chemistry properties of electrode materials and the macroscopic heat generation rules of battery module was revealed.The main conclusions indicate:(1)A commom phenomenon describes that high temperature environment has the most severe impact on the electrochemical reactions,resulting in the sharply rise of maximum temperature and heat generation rate for LiFePO4 and ternary batteries under the same discharge current.In addition,the temperature rise rate of domestic cells is higher than that of imported cells under room and high temperature condition,especially at the high discharge rate.For example,peak temperature of O-type and A-type 18650 power LiFePO4 batteries came to 69.8℃and 65.5℃ at 10℃ discharge rate under high temperature(55℃)condition,which reaches 49.2℃and 44.0℃at room temperature condition.The temperature rise gradient are 0.074℃/s and 0.056℃/s respectively,revealing higher temperature rising speed of domestic batteries.(2)The electrochemical performances of imported and domestic power batteries under ultra-low temperature(-20 ℃)condition have severely recession,compared to that of room temperature environment.Meanwhile,obvious performance difference exists between the imported and domestic batteries,especially the decrease of discharge time and capacity,furtherly indicating more serious electrochemical performance deterioration about domestic power battery.It is worth noting that the domestic batteries nearly lossed the normal discharge voltage platform and the electrochemical energy storage capacity at 3C discharge rate under ultra-low temperature condition.For example,the discharged caparity ratio of imported A-type LiFePO4 battery came to 55.17%.46.95%and 28.25%respectively when the discharge rate was 0.5C、1.0C and 3.0C.However the discharged capacity ratio of domestic O-type LiFePO4 power batteries was 7.4%、6.16%and 2.65%.(3)Overcharge phenomenon may inevitably occur if the battery management system(BMS)fails.In order to simulate the phenomenon,artificial overcharge experiments were carried out.Testing results indicated the overcharge behavior will lead to the increase of capacity recession rate,internal resistance,AC impedance and heat generation rate,furtherly causing the heating effect intensifies and internal heat accumulation.Meanwhile,the charge voltage platform increased and discharge voltage platform of power batteries decreased after overcharge.(4)Microstructure characterization results showed overcharge bahaviors caused the obvious reunion of active substance,which will heavily obstacle the normal migration of the Li+ during charge and discharge process,eventually leading to the electrochemical performance severly fading.2.AlN/paraffin/Expanded graphite(EG)/epoxy resin composite phase change materials were prepared.The impact of different propotions of AIN on the mechanical and thermal properties was analysed.Research results indicated the thermal conductivity coefficient,mechanical strength and electrical insulation performance of composites kepting rising with the increase of AIN about the AIN mass percentage range referred in this paper.When the AIN is 20%wt,the coefficient of thermal conductivity of the composite reached its maxmum 4.331 W·m-1·K-1 with 116.61J/g latent heat,also the volume resistivity comes to the highest value 26.62 X 1011 Ω cm.The tensile,bending and impact strength increases by 164.2%、67.6%and 38.1%compared to that of 0%AIN composites.The precipitation rate of composite materials with 20%AIN was the lowest,reaching 4.59%after 800 hours cycles testing under high temperature condition.Meanwhile,the thermal stability was greatly improved owing to thte participation of AIN.3.In order to build the module thermal management system,a thermal insulation polymer matrix/solid-liquid phase change composite module was developed.The scheme can greatly improve the safety coefficient of the phase change material conductivity module,such as anti-leakage,anti-deformation and anti-inflation..Injection moulding method was selected for processing A-type and B-type high thermal conductive insulation polymer matrix.Meanwhile,the physical properties,the thermal conductivity,mechanical properties and insulating properties were tested and evaluated,aiming to choose the more excellent one for the follow-up heat dissipation performance experiment and battery modue assemble.Research results showed that the B-type matrix exhibits much higher overall performance owing to the the addition of special function additives.Through the surface resistivity and volume resistivity calculation with 500and 1000V,insulation performance of A-type and B-type matrix can meet the insulation material performance requirements.4.Thermal management systems of LiFePO4 and ternary batteries modules consisting of 30 18650 cells in parallel were researched.In order to analyse the heat dissipation performance,furtherly evaluate controlling and balancing temperature ability of different thermal management methods,composite PCM were applied to the 18650 power battery modules for the constant discharge current and cycle experiments under 45 ℃ constant temperature condition.(1)Experiment results indicate that PCM cooling system perform excellent controlling and balancing temperature ability,which can make the maximum temperature decrease quickly and maintain the excellent uniform temperature distribution,especially at the higher discharge rate and charge-discharge cycle experiments.For LiFePO4 battery modules,the maximum temperature is controlled within 50℃ at 3C discharge rate with less than 2℃ temperature difference.Even in the high current(66A)charge-discharge cycle experiments,peak temperature difference of PCM cooling module can still be maintained within 3 ℃.For imported S-type ternary battery modules,the highest temperature of the module with air-cooled method was 88.18 ℃ and the PCM module conmes to 72.74℃which was reduced by 15.44℃ with less than 5℃ peak temperature differenceat the end of cycles process.(2)Temperature rise performance and heat generation of ternary power battery modules is much higher than that of LiFePO4 modules under the same discharge current,which shows that ternary battery modules will have much higher heat dissipation demand standard about the battery thermal management system design.Innovation points of the manuscript are as follows:1.Heat generation behaviors and discharge rates performance about imported and domestic 18650 power batteries were systematically studied.This paper reveals a number of electrochemical and thermotics rules,which will provide fundermental data for the subsequent cell assembly and the design of thermal management system.2.AlN/epoxy resin composite material,which originally have mature application technology in electronic packaging materials area,were firstly applied in power battery cooling field through combining with phase change materials.AlN/paraffin/graphite/epoxy resin composite phase change materials were successfully prepared.3.A kind of thermal insulation polymer skeleton/composite phase change material heat dissipation solutions was designed for solving the leakage,low mechanical strength,low protection degree problems of composite phase change materials.Excellent controlling and balancing temperature ablity were researched.