Preparation And Characterization Of Polyurethane-based Gel Polymer Electrolyte For Lithium Ion Batteries

Among all kinds of energy storage equipment,lithium-ion battery（LIBs）is widely used in various products due to its advantages of high energy density,high efficiency and low cost,and has a broad prospect.From the point of view of material science,LIBs is mainly composed of anode,cathode,electrolyte and membrane.The main function of the membranes is to separate the anode from the cathode.Although it is not an active component during the charge and discharge process of the battery,it can influence the property of the battery.Thermoplastic polyurethane（TPU）has a two-phase structure of soft segment and hard segment.The soft segment has ether bond,which can support the ionic liquid and facilitate the dissociation of alkali metal salts,thus improving the ionic conductivity.The hard segments can form hydrogen bonds,which can guarantee the spatial stability of the polymer and make the membrane have good film-forming and mechanical properties.Therefore,in this paper,I choose TPU as the matrix material for the preparation of electrolytes with stable electrochemical properties and good mechanical properties.In this paper,cellulose acetate/thermoplastic polyurethane（CA/TPU）gel polymer electrolytes（GPEs）are prepared by non-solvent induced phase separation（NIPS）method.The influence of mass ratio of CA to TPU on the membrane performance is investigated.It is found that the overall performance of the membrane is the best when the mass ratio of CA to TPU is 3:7.Compared with pure TPU electrolytes,C3 electrolytes have a very uniform porous structure,low crystallinity,high porosity（78%）and high liquid uptake rate（300%）,as well as excellent thermal stability and good mechanical properties,which greatly improve the safety performance of the battery.In addition,the ionic conductivity of C3 is also significantly increased to 1.04 m S·cm^-1.The batteries assembled by C3 have stable charge-discharge performance,good rate performance and cycle performance.At 0.5 C,The first specific discharge capacity of Li Fe PO₄/C3 GPEs/Li is 139.5 m Ah·g^-1.After 100charge-discharge processes,the capacity still retains 95.7%,with stable electrochemical performance.A series of CA/TPU/Al₂O₃ composite gel polymer electrolytes（CGPEs）are successfully prepared by modifying CA/TPU membranes with Al₂O₃ nanoparticles in order to further improve the mechanical strength and ionic conductivity of the electrolytes and improve the feasibility and safety of its practical application.The modified membranes still have uniform porous structure and excellent thermal stability.Moreover,the crystallinity is further reduced,and the porosity（85%）and liquid uptake rate（402%）are further increased.In addition,the mechanical strength and electrochemical stability are significantly enhanced,the tensile strength reaches 8.8 MPa,and the electrochemical window reaches more than 7 V（vs.Li/Li⁺）.The ionic conductivity of A9 electrolytes is 1.61 m S·cm^-1.The batteries assembled by A9have a first discharge specific capacity of 147.1 m Ah·g^-1 at 0.5 C.The battery performance of the modified electrolytes has also been improved.On the basis of the above experiments,Poly（vinylidene fluoride）（PVDF）is continued to be added for blending.CA/TPU/PVDF/Al₂O₃ CGPEs are prepared by NIPS method with CA,TPU,PVDF and Al₂O₃ as raw materials,in order to further enhance the ionic conductivity of the electrolytes and the battery performance.The results show that the membranes with PVDF still have low crystallinity,high porosity（88%）,high liquid uptake rate（445%）,excellent thermal stability,good mechanical strength and excellent electrochemical stability.In addition,the ionic conductivity of the membranes is also greatly improved,and the ionic conductivity of P1 is 3.55 m S·cm^-1.The assembled batteries have a discharge specific capacity of 152.4m Ah·g^-1 at 0.5 C and a capacity retention rate of 98.5%after 100 cycles.The battery performance is also improved effectively.Therefore,CA/TPU/PVDF/Al₂O₃ is a kind of polymer electrolytes with good comprehensive performance,and the preparation method is simple and feasible,which is an effective method to improve the performance of LIBs electrolytes.