Failure Analysis And New Structure Study Of Flexible Lithium-ion Batteries

In recent years,with the advancement of technology and the continuous development of artificial intelligence technology,flexible electronic devices due to its thin,light,portable,wearable and many other advantages become an important development direction of future electronic products.The development of new flexible energy storage devices with high energy density,long life and high mechanical stability is crucial to the rapid development of wearable devices and flexible consumer electronics as well as new energy industries.In order to explore high-performance flexible lithium-ion batteries,from the research theory of"battery generation-battery use-battery optimization"and the characteristics of flexible batteries,it is clear that the failure analysis of flexible lithium-ion batteries and the study of highly mechanically stable structures are essential.Therefore,in this thesis,a failure analysis of flexible lithium-ion batteries based on lithium-ion battery system of lithium Li Co O₂/graphite is carried out,and a new structure with high mechanical stability using corrugated cardboard as a prototype is proposed.The main work of the thesis research is as follows:Firstly,the fabrication process of flexible lithium-ion batteries is briefly described,and the process of key steps such as stacking,top and side sealing,liquid injection,chemical formation,and volume separation are analyzed in detail.Secondly,the failure analysis of the thin-plate flexible battery under high magnification,overcharge and over-discharge and external load conditions were conducted to study the effect on the battery’s internal resistance and cycling performance.After disassembling the failed battery to analyze the causes of its failure,it is used to provide meaningful guidance on the possible failure of the battery.It was found that the battery has a certain degree of flexibility（capacity does not decay after500 times of bending）,but in the case of more severe external forces,the battery is prone to failure due to the concentration of forces.Therefore,after fully considering the usage environment of the flexible battery,the research on the new structured flexible battery was subsequently carried out.Then,inspired by the common bending-resistant structures in life,a new high-performance flexible lithium-ion battery based on corrugated cardboard as a prototype is proposed.and the advantages of its structure are verified by finite element simulation.Next,new flexible battery fabrication methods（shaping）with different structural depths have been investigated,and five new flexible lithium-ion batteries with structural depths of 1 mm,4 mm,7 mm,10 mm,and 13 mm have been successfully fabricated.Finally,the electrochemical performance of the new flexible cells was tested to exclude cells that failed due to excessive shaping,（structural depths of 10 mm and 13mm）and analyzed the reasons for their failure.Then the mechanical stability of the battery was evaluated by dynamic mechanical tests such as bending,twisting and coiling.The studies on mechanical abuse,long cycle,discharge performance under stress and power supply stability under different forms were also carried out.The results show that the new flexible battery achieves the goals of high performance and long life.The mass-energy density of the battery is about 149 Wh/kg.Its capacity retention rate is about 80%and 95%after 150000 bending or 100000 twisting,respectively,and it has more than 90%capacity after 50 days of continuous charging and discharging cycles.Furthermore,it can maintain a stable power supply under various conditions such as bending,stretching,twisting,winding and folding.At last,the battery was installed on wearable electronic devices such as smartwatches and electronic collars to demonstrate its practical application.