Studt On Self-assembly Modification Of Polyethylene Lithium Ion Battery Separator

Separator is one of the key components of lithium ion battery,which has a great influence on the safety and discharge performance of the lithium ion battery.The polyolefin separator with stable chemical properties,high mechanical strength and low cost is the most widely used in lithium ion battery.However,due to the weak polarity and low surface energy of the polyolefin materials,the wetting effect between the separator and the polar organic electrolyte is poor,which leads to the large mass transfer resistance of lithium ions.On the other hand,due to the low melting point of polyolefin materials,it is easy to cause safety accidents such as short connection of positive and negative electrodes for shrinkage of separator at higher temperatures.It’s has become the focus of research to improve the wettability,electrochemical performance and thermal stability of polyolefin separator.In this paper,an organic-inorganic composite coating was constructed on the surface of polyethylene separator by self-assembly modification to improve the wettability,electrochemical performance and thermal stability of the polyethylene separator.In this paper,layer by layer self-assembly and in-situ synthesis method was used to construct the（CTS/PAA-CSH）₂ modification layer on the surface of polyethylene separator by using CTS-Ca Ac₂ polycation electrolyte solution and PAA–Na₂Si O₃polyanionic electrolyte solution.Characterize the structure and properties of separator before and after the modification.Compared with the pristane separator,the roughness,the surface free energy and the electrolyte uptake of modified separator which is assembled by the polycation electrolyte solution containing 0.04mol/L Ca Ac₂ and polyanionic electrolyte solution containing 0.04mol/L Na₂Si O₃ increased from 57nm,42.6m J·m^-2,87%to 70nm,54.6m J·m^-2 and 133%,respectively.The wettability of the separator has improved significantly.Moreover,the thermal stability of separator has been remarkably improved with the decreased thermal shrinkage rate（39%）when it is placed at 140℃for 0.5h.The electrochemical performance of the PE（CTS/PAA-CSH）₂separator are also tested.The results show that the bulk impedance of the PE（CTS/PAA-CSH）₂ separator decrease from 3.41Ωto 1.70Ωand the interfacial impedance of the PE（CTS/PAA-CSH）₂ separator decrease from 334.60Ωto 120.30Ω,respectively.At the same time,the lithium ion transference number of modification separator increases from 0.45 to 0.57.In the cycle stability test,the capacity retention of celles assembled with modified separator（88%）is higher than the cells assembled with pristine PE separator（58%）after 200 cycles.The C-rate performance of celles assembled with modified separator are also tested,showing that cell assembled with modificated separator has better C-rate performance with capacity retention（37.05m Ah/g）than the cell assembled with PE separator（82.82m Ah/g）at 7C.Based on the modification of PE separator by layer by layer self-assembly and in-situ synthesis method,a one-step deposition self-assembly method was used to construct a（CTS/PAA-CSH）modification layer on the surface of polyethylene separator.The characterization results of the separator modified by the polycation electrolyte solution containing 0.05mol/L Ca Ac₂ and polyanionic electrolyte solution containing 0.05mol/L Na₂Si O₃ for 25 miniute shows that the modified separator achieved the superphilic property to the electrolyte with a contact angle of 5°.As a result,the wettability of the separator is significantly improved.Moreover,after modification,the bulk impedance of separtor reduce from 3.41Ωto 1.18Ω,the interfacial impedance decrease from 334.60Ωto 90.56Ω,lithium ion transference number increased from 0.45 to 0.78.As a consequence,the capacity retention of cell assembled with modified separator is 99%which is higher than the cells assembled with pristine PE separator（58%）after 200 cycels in the cycle stability test.The C-rate performance of celles assembled with modified separator are also tested,showing that cell assembled with modificated separator has better C-rate performance with capacity retention（37.05m Ah/g）than the cell assembled with PE separator（94.33m Ah/g）at 7C.