| Lithium ion batteries demonstrated many advantages,so it has been used more and more widely,especially in the field of digital electronic products and electric vehicles.However,the safety of lithium-ion batteries has been plagued by the further development of lithium-ion batteries.Numerous studies have shown that a large amount of heat cannot be effectively dissipated,which can easily lead to unsafe behaviors such as rupture,burning and explosion.In response to this problem,this paper explores a self-blocking polypyrrole(PPy)-based new temperature-sensitive electrode for lithium-ion batteries,which can effectively prevent the battery from thermal runaway.This improves the safety performance of the battery and lays a theoretical foundation for building a safer lithium-ion battery.The specific research work of this paper is as follows:(1)After doping the intrinsic conductive polymer(ICP),the room temperature conductivity is greatly improved,and some of them will drop sharply after thermal dedoping,showing a good positive temperature coefficient resistance(PTC-R)effect.In this paper,PPy doped with dodecylbenzenesulfonate ion(DBS-),p-toluenesulfonate ion(TOS-)and chloride ion(Cl-)was prepared by chemical methods.And studied the structure,morphology,thermal stability and PTC-R characteristics of PPy/DBS’,PPy/TOS’ and PPy/Cl-.SEM results show that PPy/DBS-has a bulk structure and a large size,while PPy/TOS-and PPy/Cl-have a spherical structure and a relatively small size.TG test results show that there is obvious weightlessness around 90℃.On the one hand,it may be related to the loss of water absorbed in PPy at 90℃.On the other hand,it may be related to the dedoping of doped ions,while PPy/Cl-has the most weight loss and the degree of dedoping may be the largest.The resistance-temperature(R-T)test results show that the resistance of PPy/DBS-,PPy/TOS-and PPy/Cl-all rise sharply to about 104Ω due to dedoping and have a significant PTC effect.It shows that PPy has the possibility of being a temperature-sensitive electrode.(2)PPy powder doped with three different ions(PPy/DBS-,PPy/TOS-,PPy/Cl-)were mixed with polyvinylidene fluoride(PVDF)at a mass ratio of 9:1 and an appropriate amount of N-methyl-2-pyrrolidone(NMP)was added dropwise to mix well.A PPy coating with a thickness of 5 μm was coated on the aluminum foil.After drying,LiNi0.6Co0.2Mn0.2O2(622)active material was coated on it to prepare a composite electrode with Al/PPy/622(622-PPy)sandwich structure.The electrochemical performances of the 622-PPy/DBS-,622-PPy/TOS-and 622-PPy/Cl-composite electrodes at room temperature and high temperature were tested respectively.The test results at room temperature show that the three composite electrodes 622-PPy/DBS-,622-PPy/TOS-and 622-PPy/Cl-are very similar to the pure 622 electrode without PPy coating.The lower composite electrode and the pure 622 electrode showed similar charge and discharge performance.At a current density of 50 mA/g,the difference in discharge specific capacity of the three composite electrodes and the pure 622 electrode was less than 4%.However,when the battery was heated to about 90℃,the three composite electrodes all exhibited obvious PTC behavior.The rapid rise of the battery’s resistance caused a large ohmic instant and the specific capacity of the battery was greatly reduced.In particular,the voltage of the 622-PPy/Cl-composite electrode immediately fell to the cut-off voltage at 90℃,that is,the electrode reaction was turned off.The specific capacity of the 622-PPy/Cl-composite electrode dropped sharply to 0.04 mAh/g,accounting for only 0.2%of the initial specific capacity at room temperature.The pure 622 electrode can still exert a specific discharge capacity of 161.8 mAh/g,which is only 1.7%lower than that at room temperature.This shows that the three composite electrodes have excellent high-temperature safety performance,which is expected to protect the battery from thermal runaway and also provides a great possibility for building a safer lithium-ion battery. |
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