Damage Detection And Flame Retardant For Lithium Battery Packaging Materials

Recently,lithium ion baterries（LIBs）with high energy density and long cycle lifetime,have been widely used in portable products,standby power supplies,electrical vehicles and so on.However,in the long-term use and storage process,LIBs are inevitably damaged by extrusion,puncture,temperature shock,overcharge,short corcuit and so on,causing burn or even explode.Hence,based on the typical mechanical damage and thermal damage of LIBs,we develop two kinds of real-time sensors to monitoring and positioning damages that happened on the LIBs,and propose a polycarbonate（PC）thin film with flame retardant function to improve the flame retardant performance of LIBs.The specific contents of this works are as follows:（1）For the quasi-static damage of LIBs,we design a capacitance sensor through parameters optimization of PVDF-TrFE piezoelectric thin film and capacitance sensor,and systematically characterize real-time detecting and positioning performance of LIBs udner various quasi-static damage conditions.A PVDF-TrFE piezoelectric thin film with high surface flatness is prepared via the combianation of doctor blade coating technology and vacuum drying technology.And then,the in-situ polarization technology is used to enhance the piezoelectric performance of the thin film.The piezoelectric constant d₃₃value can be improved to 27.7±0.7 pC/N at a polarization voltage of 7 kV.As for the fabrication of capacitance sensor,combined a PVDF-TrFE piezoelectric thin film with a thin film transistor（TFT）,a capacitance sensor that can graphically display various damage conditions has been prepared.In specific,the fabricated capacitance sensor can not only picture the damage morphology of the damage LIB with extrusion damage,hole damage,and micron-scale scratch damage,but also detect the shape and position of the bulge damage,helping to vesualizing the bulging behavior of LIBs.（2）For the dynamic damage and thermal damage of LIBs,we design a PVDF-TrFE based piezoelectric sensor through the structure design and parameter optimazation,and study thier response performance of LIBs under stab damage,drop damage and temperature shock damage.Firstly,through piezoelectric simulation,it was found that the arrangement interval of the circular electrodes should be greater than the radius length to avoid mutual interference between the electrodes.Secondly,combination of the arrayed copper electrode as the bottom electrode and magnetron sputtered Ag layer as the top electrode,a PVDF-TrFE dynamic damage detection piezoelectric sensor based on the piezoelectric and pyroelectric effects of the thin film is designed and manufactured.The device response cycle is 8.4 ms.The force-response response fits a linear relationship with a slope of 0.011 V/kPa.The sensor has a sensitive response to both dynamic damage and thermal damage,and can detect small changes such as collisions at a frequency of 10cpm,drops at a height of 5 cm,and a temperature rise of 10℃.（3）For the flame retardant of LIBs,we prepare a PC thin film with good flame retardant performance.First,through heat transfer simulation,it is found that when the thickness of the polycarbonate film layer is less than 0.2 mm,it will not significantly affect the heat dissipation capacity of the battery.Then,a polycarbonate thin film with good mechanical properties is prepared by optimization of parameters of the solution knife coating process.After then,the flame retardant performance of PC thin film is optimized by controling the proportion of components between inorganic flame retardant and organophosphorus flame retardant,and the limiting oxygen index of PC thin film reaches 29.1%,which can reach VTM-0 flammability rating.Therefore,the fabricated PC thin film with good flame retardant performance can enhance the safety of LIBs.