Mechanical Behavior And Structural Evolution Of Lithium-ion Battery Separators In Multi-physics Fields

The widespread adoption of cell phones and electric vehicles has imposed higher requirements on the performance and safety of lithium-ion batteries(LIB).Although the separator,one of the key components of the battery,is not involved in the electrochemical reaction,it suffers from the coupling effects of temperature,stress,and electrolyte during cycling,which results in significant mechanical behavior and the battery performance fade.Most of the previous studies merely considered the influences of a single physic field on the separator,the coupling effect induced by the mutli-physics fields is usually ignored.In order to study the structural evolution of the separator during cycling,this paper investigates the separator mechanical behavior influenced by the coupling fields by means of a combination of experiment and simulation.The main research contents and conclusions are listed as follows:(1)A finite element model of the separator under the coupling effect of mutli-physics fields was established.The surface morphology and mechanical properties of the polyolefin separator were characterized by atomic force microscopy(AFM),and the microstructure characteristics of the separator were analyzed based on the characterization results,and then a finite element geometric model containing microstructure units of the separator that is close to the real separator structure was established.A finite element model was built in COMSOL based on the geometric model and the mechanical parameters of the separator.The validity of the model was verified by comparing the simulated results of the single physic field effects with the experimental data.(2)The micro scale structural evolution of the separator under the coupling of tensile and compressive stresses and the coupling of compressive stress-temperature was analyzed through simulation,and the deformation law of the separator microstructure under the coupling of tensile and compressive stresses is clarified,that is,the tensile stress promotes the deformation of the separator under compressive stress,and the trend of slow decline of porosity in the small stress phase of the separator under single compressive stress gradually disappears when tensile stress was applied.The compressive effect of stress is enhanced when temperature increases.And when the separator is under the compressive stress,the higher the temperature of the environment in which the separator is working,the faster the porosity of separator declines.(3)The deformation laws of the separator under the coupling effects of electrolyte-stress fields,electrolyte-temperature fields and electrolyte-stress-temperature fields were simulated and analyzed.It is found that the organic solvent in the electrolyte strengthen the effect of compressive stress and accelerates the reduction of porosity in the initial stage of compression.The swelling effect of the electrolyte on the separator is not sufficient to compensate for the compressive deformation caused by the stress,yet when the separator is affected by the electrolyte combined with temperature field,the swelling of the separator filaments increases,but the electrolyte has a certain inhibitory effect on the swelling of the separator caused by the temperature field.The reduction of separator porosity is further aggravated by the coupling of the three physics fields,indicating that the multi-physical field coupling effects is still dominated by the stress.