Research On Safety And Protection Of Cylindrical Power Lithium-ion Battery Under Mechanical Abuse

New energy vehicles have been promoted because of the advantages of no exhaust emission and green during driving.However,in recent years,many accidents of electric vehicles caused by extruded collision have aroused people’s worries.The safety of electric vehicles hinders the development of electric vehicles.As the central component of electric vehicles,it is very important to study the safety of power battery.Due to the uncertainty of the collision accidents of new energy vehicles,the form of the collision,the speed of the collision,and the State of Charge(SOC)of the battery cells are all factors that affect battery safety.In this paper,18650 cylindrical lithium-ion battery was used to explore the influences of different loading conditions,SOC and loading speed on the safety performance of batteries.And drilling foam aluminum protection structure was used to protect batteries.The failure process of lithium-ion battery is complex,including the changes of load,voltage and temperature.During the loading process,the voltage and temperature of the battery was recorded respectively by digital oscilloscope and infrared thermal camera,and the failure process of the battery under mechanical abuse was analyzed.The main research contents are as follows:(1)The influence of SOC and loading speed on the safety performance of battery cell was explored in radial two-plate compression tests.In the range of quasi-static loading speed selected in the tests,the battery with SOC = 100% had occurred severe thermal runaway,and the maximum load and temperature rise almost occurred at the same time.In the range of SOC selected in the tests,taking the battery with SOC = 60% as the limit,the battery with SOC ≤ 60% failed without thermal runaway,while the battery with SOC > 60% failed with severe thermal runaway.Moreover,batteries with high SOC have more obvious temperature transfer process than batteries with low SOC.The peak force does not increase monotonously with the increase of SOC and the peak force is also related to the thermal runaway of the battery.Because the time of the thermal runaway trigger of high SOC battery in radial two-plate compression is very short,the turning point of nominal modulus is regarded as the prediction point of battery failure.The trend of nominal modulus-displacement curves of different SOC batteries is similar,and the nominal modulus peak appears before the maximum load.The compressed battery was mechanically cut,and it was found that the battery failed in shear failure and local yield bending under radial compression.(2)The safety performance of the battery under different loading speed was explored in radial indentation tests.In the selected range of quasi-static speed,the load-displacement curve of the battery had the similar trend.There is the turning point of force-displacement curve near the displacement of 4.3 mm,and changing of the later is faster.And the turning point of load-displacement curve is taken as the prediction point of battery failure.In the radial indentation tests,the maximum load and the beginning of temperature rise occurred almost simultaneously.In the failure process of the battery under the indentation tests,it was found that the battery under the radial indentation failed due to the shear failure of the battery.Compared with the radial two-plate compression,the radial indentation test is to load the battery locally,and the radial two-plate compression is to load the whole cell.Under these two conditions,the battery both had severe thermal runaway,but the maximum load of the battery under the radial indentation is much smaller than that under the radial two-plate compression.(3)The effects of loading speed and SOC on the safety performance of the battery were discussed in axial compression tests,and the failure process of the battery was analyzed.The results show that load-displacement curve of the battery is similar in the selected loading range in the test;in the SOC range used in the test,the voltage during the axial compression process showed the unique stepped drop.The peak load increases slightly with the increase of SOC.The side of the battery first appears annular fold and cracks in the failure of the battery under axial compression,and the location of the cracks is close to the positive.The local groove structure of the positive induced the battery to rupture near the positive.Comparing with the radial two-plate compression,the axial compression has no severe thermal runaway,and the maximum load of battery under the axial compression is much lower than that under the radial two-plate compression,and the voltage of the axial compression battery has the unique stepped drop phenomenon.The axial loading tests of the battery is along in-plane direction of pole pieces,which is prone to wrinkle buckling.The radial two-plate tests of the battery is along the direction of perpendicular to pole pieces winding,and the pole pieces will appear compaction during the loading process.(4)There are two kinds of tests in the battery protection test,including the quasi-static two-plate compression of aluminum foam with drilling hole(using foam aluminum as the protective material for the battery,take the rectangular foam aluminum to drill a circular through hole in a certain section of the foam aluminum,and the through hole can fit the battery)and aluminum foam with drilling hole embedded in the lithium battery.The protection effect of aluminum foam with drilling hole was discussed from the heat diffusion and deformation to absorpt energy,the failure of batteries with and without protective structures was compared.In radial quasi-static compression test of the drilling foam aluminum structure with embedded batteries,the batteries with SOC = 80% occurred thermal runaway.The aluminum foam protection structure can obstruct the high temperature jet and flame,and largely obstruct the propagation of heat to the neighboring battery.There are two kinds failure modes of batteries with and without protective structure,interlayer fractures and local buckling.The number of layers of interlayer fractures of batteries with protective structures is less.The local buckling and bending of batteries with protective structures are mostly in the outer shell and its adjacent pole pieces.The partial buckling of the unprotected battery is more inside the battery.