Preparation And Properties Of Separators In Lithium Ion Batteries For Electric Vehicles

The electric vehicle industry is currently in the early stages of industrial development,and has a very broad development prospect in the long run.According to the 13 th Five-Year Plan,the annual output of electric vehicles in China will reach 2 million units in 2020.Based on this calculation,the demand for lithium batteries for electric vehicles will exceed 110 GWh.The lithium battery industry for electric vehicles has a compound growth rate of 40% in the past four years,and lithium battery separators for electric vehicles have also benefited from the rapid development of the electric vehicle industry.Lithium batteries for electric vehicles generally require high safety performance,electrical performance and long cycle life,the performance requirements for the separators are also relatively high.Polyolefin lithium battery separators generally have a lower melting point.When the temperature is too high,the heat shrinkage is likely to cause dimensional deformation.In severe cases,the battery short-circuiting causes thermal runaway.Additionally,polyolefin separators have a poor ability to absorb and retain the electrolyte,which is not conducive to the transport of lithium ions at the interface and the electrochemical reaction during the charge and discharge of lithium-ion batteries.In order to make up for the shortcomings of polyolefin-based separators,the coating and modification measures for polyolefin separators were generally adopted using inorganic ceramics to improve the high-temperature dimensional stability and the ability to wet the electrolyte.However,the ceramic-modified separators have no adhesion between the separator and the electrodes for the larger-sized polymer batteries,and the overall hardness of the battery is poor,affecting the yield due to the difficulty to insert the case.The permeability of the inorganic ceramic coating is mainly based on the void formed between the inorganic particles.Thickening the coating will increase the heat-resistant dimensional stability of the separator,but the plugging of the separator will become serious and the permeability will decrease.When the coating is thinned,the air permeability of the separator is increased,but the heat-resistant dimensional stability is deteriorated.Therefore,the thickness,air permeability,and heat stability of the coating are mutually restricted and have certain limitations.The inorganic coating itself does not improve the mechanical properties such as tensile strength and puncture strength of the separator.When it is subject to impact and extrusion of heavy objects,it is prone to fracture and cause safety accidents such as short circuit,combustion,and even explosion.In brief,the ceramic-modified separators cannot fully meet the requirements of lithium-ion batteries for electric vehicles.In this thesis,theoretical research is combined with production practice.PVDF(polyvinylidene fluoride)is used to coat and modify polyolefin lithium battery separators.By designing the formulation of the PVDF coating solution and optimizing the preparation process of the PV coating solution,PVDF coated modified separator with excellent comprehensive performance were prepared for lithium battery separator for electric vehicles.The prepared separator was tested for AC impedance,air permeability,heat-resistant dimensional stability,mechanical strength,and so on.At the same time,the prepared coated modified separator was made into a soft-packed lithium-ion battery.With reference to some relevant national standards,the cycle life,safety performance,and electrical performance tests were performed.The test results show that,compared with the conventional PE separator,the ion conductivity of the modified PVDF coating is increased by about 1.1 to 1.2 times,the ability to absorb and retain liquid is increased by about 2 times,and the heat-resistant dimensional stability is increased from 130 °C to 150 °C.The assembled lithium-ion battery has a 20 C rate discharge capacity retention rate as high as 47%;The capacity retention rate in the 500-cycle cycle life test has been increased from 23.57% to 56.09%,and the safety under needle punching and heavy object impact has been significantly improved.In conclusion,the prepared PVDF-coated separator has excellent comprehensive performance and can meet the application requirements of lithium batteries for electric vehicles.