Study On Mechanical Properties Of Lithium Iron Phosphate Energy Storage Battery Under Overcharge Fault Condition

Lithium iron phosphate battery(LFP)is widely used in the field of electric vehicles and power grid energy storage due to its long service life and low explosion risk.However,LFP batteries still have safety problems under abuse conditions(especially overcharge),resulting in heating,gas production and deformation.Under overcharge fault,the deformation of a single battery will have a force impact on adj acent batteries.Most of the existing studies focus on the impact extrusion deformation under external mechanical load,and there is little research on the mechanical characteristics of the battery under overcharge fault.Therefore,taking LFP battery as the research object,this paper proposes a model to realize the pressure swelling of single battery.For the pressure swelling deformation,the swelling characteristics of pouch and prismatic battery and the contact characteristics between batteries were studied;For prismatic batteries,the relationship between the internal pressure of the battery and the displacement of the hard shell and the fracture of the safety valve was studied.It is conducive to better understand and make use of the deformation characteristics of lithium iron phosphate battery to improve the operation safety of the battery.Firstly,the overcharge experimental platform of LFP battery is built,and the swelling characteristics experiments of pouch,prismatic single battery and module are carried out respectively.The overcharge characteristics of LFP battery are analyzed,including visible light deformation characteristics,temperature and gas changes,and contact extrusion between swelling batteries.The monomer experiment shows that the swelling deformation degree of pouch battery is greater than that of prismatic battery;The shell displacement of pouch and prismatic batteries can reach more than 10 mm from overcharge to battery rupture or safety valve opening.The module experiment shows that the swelling of single cell will squeeze the surrounding cells and destroy the overall structure of the module,and the deformation characteristics of the battery are often earlier than the large temperature rise and the release of side reaction gas,so it can be used as an important information to characterize the safety state of the battery.The above experimental results provide a basis for setting the parameters of heat source and gas source in the subsequent simulation analysis.Then,the overcharge heat and gas generation mechanism of LFP battery is analyzed,and the heat calculation method and gas type of overcharge battery pressure swelling are determined.The thermal gas solid coupling swelling model of single battery is established based on finite element software,and the quantitative relationship between the internal pressure of battery and external mechanical parameters such as displacement,stress and contact pressure is established.For the pouch battery,the displacement and stress of aluminum composite film under different internal pressure are simulated and analyzed,and the volume measurement of single battery is realized.The simulation results show that the maximum displacement of pouch battery is 13.5 mm,the maximum lateral stress is 138 N/mm2,and the volume change offset can reach 277.6%under the internal pressure of 32.43 kPa;The contact simulation results show that compared with the swelling of a single battery,when two batteries swelled at the same time,it is easy to lead to battery rupture.For the prismatic battery,the swelling coupling model suitable for the prismatic battery is obtained by changing the shell material.The displacement and stress of A13003-H14 aluminum alloy shell under different internal pressures are simulated.The single cell simulation results show that the displacement of prismatic battery can reach 14 mm under 1740.7 kPa;The contact simulation results show that when both sides are limited,the displacement direction of the front and rear sides of the shell changes opposite before and after contact,and when the internal pressure is 2498.60 kPa,the maximum contact pressure peak can reach 41.01 N/mm2,which will cause great influence to the surrounding batteries.Based on this,in order to slow down the extrusion effect of overcharge deformation on adjacent batteries,the fracture failure of safety valves of different materials and structures in prismatic batteries is simulated and analyzed.The results show that the damage distribution of the safety valve with a semicircular impression is concentrated in the embossing center,and it is easier to reach the fracture condition so that the battery can be deflated in time and can effectively reduce the shape variable of the battery.