Study On Failure Mechanism And Safety Of Lithium Batteries In Portable Devices

With the development of battery science and technology,lithium ion batteries have been widely used in various fields and play an indispensable role in our life.With the wide application of lithium ion batteries,the transport volume of lithium ion batteries is increasing,which occupies an important position in civil aviation transportation.However,with the increase of transportation volume,safety problems caused by lithium-ion batteries in portable devices continue to emerge,which has aroused extensive concern of civil aviation organizations at home and abroad.Lithium-ion batteries in portable devices will be used in a variety of environments and conditions.Battery in the recycling process,thermal stability and electrochemical properties of the internal batteries can change and attenuation,the battery performance degradation,seriously affecting the life of the battery and safety,resulting in a decline in the performance of the electric equipment directly or malfunction,so the lithium ion battery failure mechanism and safety become the key to ensure the safety of lithium ion battery air transport.In this paper,the structure,charge and discharge process and basic parameters of lithium cobalt oxide battery which is widely used in portable equipment of air transportation are introduced.Through the battery test platform,the electrochemical performance of the batteries was tested to detect the stability of this batch of batteries,and the overcharge cycle aging experiment of lithium ion batteries was carried out.The results show that the SOC of the battery in the overcharge cycle is about 118.6%SOC compared with that in the normal cycle.The discharge voltage platform of LIB_S is significantly higher than that of normal charging,and the overcharging cycle accelerates the capacity attenuation of LIB_Sgreatly.The coulomb efficiency of the battery decreases rapidly with the overcharge cycle,indicating that the heat of the battery increases rapidly during operation.At the same time,the DC internal resistance of the battery also increases rapidly.Secondly,the principle and application of electrochemical impedance spectroscopy were introduced,and the effect of temperature on the impedance spectrum of lithium-ion battery was studied.The correlation between the battery temperature and the single-frequency phase angle was studied in the constant temperature and single-side heated state.The results show that in the range of 30～60℃,the phase angle in the range of low frequency（5 Hz）to high frequency（500 Hz）has a second-order linear relationship with the internal temperature,and monotonically increases with the increase of the internal temperature of the battery.The internal temperature of the battery at the phase angle of 5 Hz,at constant temperature and under unilateral heating state has a second-order linear relationship,and has a good goodness of fit.Then,the construction of the electrochemical equivalent circuit of lithium ion battery and the internal failure mechanism during the aging process were introduced and studied.The decay process and aging mechanism of the battery under the overcharge cycle were qualitatively and quantitatively studied by using ZVIEW software fitting.The results showed that in the overcharging cycle,R_ohm,R_SEI and R_W increased linearly with the number of cycles,while R_CTincreased exponentially.With the increase of the number of cycles,LLI increases exponentially,while CL and LAM increase linearly.LLI is the main factor affecting battery aging,LAM is at a medium level,and CL has a small effect on capacity decay.Finally,the failure mechanism and heat generation of the Li-ion battery during overcharging were analyzed.The behavior characteristics of fresh batteries under different overcharge current and cut-off voltage conditions were studied.The results show that the peak temperature increases with the increase of charging rate at different cut-off voltages.With the increase of cut-off voltage,the first peak temperature T₁ is not affected by the cut-off voltage at 2C or below,but increases obviously at 3C.At the same time,at the condition of 5.0V overcharge,a side reaction will occur inside the lithium cobalt oxide battery,resulting in a second peak of the battery surface temperature.It should be noted that the second peak temperature T₂ is higher than T₁ in the case of 1C,but less than T₁ in other cases,indicating that the cut-off voltage has a greater impact on the safety of the battery when the low-speed current is overcharged.Then,the overcharge test was carried out on the lithium ion batteries with different aging degrees after the overcharge cycle.Combined with the failure mechanism of the battery in the overcharge cycle in Chapter 3,the influence of the failure state of the battery on the safety was analyzed.The results showed that the overcharge cycle would lead to the deterioration of the stability and safety of the lithium ion battery.With the deepening of aging,side reactions occurred in the overcharge test of the battery after 92%SOH,and the energy released by the battery increased significantly.LLI and LAM decline are the most related factors to cause side effects during overcharge.The maximum temperature of 79.6%SOH aged battery increased by 68%compared with the fresh battery,and the battery could not be used again after the experiment.Considering the safety of aging LIB_S caused by overcharging,it may not be suitable for secondary use when approaching 80%SOH.This study can provide suggestions for the revision of the relevant standards for air transport of lithium ion batteries,and also provide references for battery design and battery management systems,so as to improve the tolerance and safety of batteries against overcharge.