| Nowadays,there are still fire accidents occurring from time to time caused by the thermal failure of lithium-ion battery(LIB)and thermal safety issue has become a key challenge in the further application of LIBs.Since the performance of LIB is quite sensitive to the operating temperature,developing a reliable battery thermal management system(BTMs)to improve the LIB safety has become an urgent need.Phase Change Material(PCM)and PCM-based BTMs are considered efficient and promising which attract numerous research interests.PCM,a heat storage material,enables efficient temperature control and temperature equalization of battery modules without additional energy,which indicates a wide application prospect in the next generation of high energy BTMs.However,the current mainstream organic PCM still has some deficiencies such as poor secondary heat dissipation and flammability,which limit the PCM heat dissipation behavior and even lead to fire after heat accumulation.However,there is still a large research gap in understanding the degradation of PCM cooling performance,PCM burning behavior and mechanism in fire scenarios.In this study,a type of solid-solid PCM is synthesized,and a series of battery thermal management tests as well as battery fire tests are conducted on the self-built experimental platform.This thesis comprehensively considered its thermal safety issues and fire risk characteristics following the logic of(ⅰ)PCM cooling performance in normal working state;(ⅱ)degradation behavior of PCM cooling performance under high temperature environment;(ⅲ)fire hazards of PCM-based BTMs and influence of PCM on battery thermal runaway under abnormal situation of heat generation and heat dissipation.Following this logic,a series of experimental studies are carried out,and the temperature control mechanism of PCM and secondary heat dissipation enhancement are discussed.Based on this,an optimized BTMs coupling PCM and thermal insulation material is proposed,which can cooperatively optimize the system heat dissipation and inhibit thermal runaway diffusion,enhancing the fire safety of the PCM-based BTMs.The main research work is described as follows:(1)Heat dissipation mechanism and influencing factors of PCM-based BTMs.In this study,the thermoelectric behavior of LIBs with and without PCM in a single cycle is studied and compared.It is found that the temperature-control performance of PCM for LIBs is not only reflected on the peak temperature of battery surface,but also reflected in the amplitude variation of battery surface temperature during charging and discharging.The influence of rest stage,charging-discharging current mode,ambient temperature and other factors on the PCM cooling behavior in multiple chargedischarge cycles are explored.The influence characteristics of rest stage on the PCM cooling performance is obtained,and the heat production rate in the discharge stage of constant current mode during charging-discharging process is higher,which deserves more attention to its cooling performance degradation.(2)In high temperature environment,the PCM shows poor heat dissipation performance due to limited secondary heat dissipation.In this study,the synthesized macromolecule cross-linked solid-solid PCM is used,and the self-built variable temperature experiment chamber is used to simulate the medium-high temperature working environment,where PCM cooling behavior under high temperature is studied.The study found that although the solid-solid PCM had good thermal stability,its latent heat value is not sufficient.The failure of PCM cooling can be accelerated under high ambient temperature and even leads to the problem of localized heat accumulation.This increases its own temperature,and thus further weakening the secondary heat dissipation between the battery and the environment,aggravating the temperature difference between the battery and the battery.The author further explores the influence of the addition amount of thermal conductivity additives namely expanded graphite(EG).It is found that the effect of 6wt%EG can improve thermal conductivity.However,due to the limited thickness,the cooling performance of PCM decays significantly after several cycles.(3)Fire hazards in PCM-based BTMs and influence mechanism of PCM on battery thermal runaway.Considering the above mentioned PCM in high temperature environment,the cooling technology and BTMs may go failure and abnormal heat balance can occur.The inconsistency between batteries will further aggravate the"voltage grab" phenomenon,resulting in localized overheating of battery modules and causing extreme conditions such as thermal runaway.The flammable organic PCM will further intensify the thermal runaway propagation,and lead to strong heat feedback to the unburnt battery,which can accelerate LIB thermal runaway propagation and battery fire.Finally,the fire spread model and the fire safety factor(SFV)are combined to quantify and compare the effects of different additives and different group structure on the PCM-based battery modules.It is concluded that the 15 wt%ATH additive only increases SFV from 4.11%to 18.75%,while the optimization of group structure increases SFV to 32.95%.(4)Optimized PCM-based BTMs using thermal insulation and its mitigation effects on battery thermal runaway.In order to further improve the fire safety of the PCM-based BTMs,an optimized BTMs is constructed by coupling PCM and thermal insulation,which can cooperatively satisfy heat dissipation requirement and mitigate battery thermal runaway propagation.On one hand,the heat generated during the normal charging-discharging process can be absorbed through the PCM of the inner structure.On the other hand,the thermal insulation can delay thermal runaway and fire propagation to some degree.Such a composite PCM thermal management module design can not only realize the safe operation of battery packs,but also reduce the risk of disaster caused by thermal runaway propagation of LIB packs and thus improve the fire safety of battery system.This research is majorly targeted on PCM-based BTMs,investigating on its normal working mechanism;thermal safety problems under the condition of limited heat dissipation;and its influence mechanism on thermal runaway in abnormal heat dissipation.Finally,an optimized PCM-based BTMs is proposed to optimize heat dissipation and inhibit thermal runaway propagation,enhancing the fire safety of the PCM-based BTMs. |
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