| Compared with traditional fuel vehicles,electric vehicles can effectively reduce carbon emissions in the field of transportation.With the advantages of high energy density,extended cycle life,and low self-discharge rate,Li-ion battery is the preferred power source for electric cars.Nonetheless,Li-ion battery safety issues persist,which has become one of the primary obstacles restricting its adoption and development.The main cause of fire of electric cars is the thermal runaway of batteries,and the inducements of thermal runaway are divided into external short circuit,internal short circuit,overcharge and over-discharge.In this study,a number of experimental investigations on the thermal runaway behavior of Li-ion batteries during external short circuits have been conducted with the goal of investigation the failure mechanism of the battery during external short circuit from two aspects of macro features and micro ion transport.The results of the work performed are as follows:(1)The impacts of various discharge C-rates and operating temperatures on the electrochemical and thermal characteristics of the battery were investigated,and the performance variations between LFP and NCM battery with various electrode thickness were also contrasted and studied.While NCM batteries witnessed a significant voltage decrease at high discharge rates(>2C),LFP batteries experienced a minor voltage drop at low discharge rates(0.5-1C).Additionally,compared to the NCM batteries,the LFP batteries generated more heat as a result of the high temperature environment.The internal resistance of the battery increases as the thickness of the battery electrode grows,which raises the battery’s temperature.(2)The characteristics of LFP battery under different external short-circuit currents were studied experimentally.At the same time,the cycle performance of the battery without damage due to short circuits and the potential risk of thermal runaway were evaluated.The results showed that the temperature rise of the battery were significantly related to the short-circuit current and the initial SOC.According to the SEM results of the electrodes,it is found that the short circuit will lead to electrolyte consumption,metal deposition,electrode particle breakage and diaphragm closure.The capacity of the undamaged battery after short circuit will recover again due to the reduction of ohmic resistance and the weakening of polarization effect.Nevertheless,when the undamaged battery undergoes the second short circuit during the first short circuit,the battery shows greater voltage drop and faster temperature rise rate than the fresh battery.(3)The thermal runaway behavior of external short circuit of batteries with different electrode thickness after aging was studied experimentally.The cyclic aging characteristics of batteries with different electrode thicknesses and the external short-circuit thermal runaway characteristics of batteries after aging were analyzed.The ohmic internal resistance and polarization internal resistance increase after the battery with thicker electrode is cycled.The reason is that the cracks of the positive electrode material and the formation of the electrolyte film block the pores of the ion diffusion.During external short circuit after aging of the battery,due to the consumption of electrolyte in the circulation process,no electrolyte leakage occurred in the battery with thicker electrode.The pore closure after separator contraction cut off the reaction between positive and negative electrodes,and the temperature rise of the battery stopped. |
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