New Polyurethane-based Electrolyet For Lithium Batteries

Lithium-ion batteries have been widely used in all aspects of social life due to the high working voltage,long cycle life and environmentally friendly.However,the traditional lithium battery contains a large amount of organic liquid electrolyte,which has potential safety hazards such as easy leakage,flammability,and explosion.It is urgent to further improve the safety and electrochemical performance of lithium batteries.Because of the advantages of easy processing into membranes and high energy density,the polymer electrolytes have become one of the most preferred electrolytes for lithium-ion batteries in the future.And polyurethane（PU）can have rigidity and flexibility owing to its unique two-phase structure which provide both good electrochemical performance and mechanical strength.As a polymer electrolyte for lithium-ion batteries,PU has been favored by many researchers.In this paper,starting from the design and development of a new type of polyurethane-based electrolyte,PU with different soft and hard segment contents are synthesized to form the composite gel electrolyte and solid electrolyte by using cellulose membrane as a framework material.The main works are as follows:1.Using polycarbonate diol（PCDL）as the soft segment and isophorone diisocyanate（IPDI）as the hard segment,five polyurethanes with different soft and hard segment contents were synthesized.By FTIR,stretching and DSC,it is shown that PU has high mechanical strength（>10 MPa）and low glass transition temperature（<-30℃）,which is beneficial to its application in polymer electrolytes of lithium-ion batteries.Then cellulose membrane as the framework material,polyurethane as a matrix material and Li PF₆ commercial electrolyte were used to form a composite gel electrolyte（C-PU）.Studies have shown that the C-PU20 electrolyte with hard segment content of 20%exhibits a room temperature ionic conductivity up to 2.37 m S cm^-1,electrochemical stability window of 4.8 V,excellent long cycle and rate performance.2.Select the composite film（containing 20%hard segment content）and add different mass fractions of Li TFSI to form a solid polymer electrolyte（T-PU）,which is used in lithium batteries.Studies have shown that the T-PU20%electrolyte（containing20 wt%Li TFSI）has an ionic conductivity of 9.97×10^-5 S cm^-1 at 30℃and 2.72×10^-4 S cm^-1 at 60℃.The electrochemical stability window reaches 4.5 V and the lithium-ion migration number is 0.51.It also has good interface contact with lithium metal electrode and better cycle and rate performance at 60℃.3.For improving the electrochemical performance of the solid polymer electrolyte,different mass fractions of Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）were added to the T-PU20%electrolyte to form a new solid polymer electrolyte（L-PU）,which a trace amount of fluoroethylene carbonate（FEC）is used as an additive.Studies have shown that L-PU4%electrolyte（4 wt%LLZTO）has an ionic conductivity of 2.22×10^-4 S cm^-1 at 30℃,electrochemical stability window of 4.8 V,lithium-ion migration number of 0.55 and excellent lithium metal interface stability.Assembling cells with LiFePO₄ as the positive electrode have discharge capacity of 162 mAh g^-1 which is close to the theoretical specific capacity of LiFePO₄ positive electrode(170 mAh g^-1)at 60℃and 0.5 C.More importantly,there is no significant decrease in capacity after 100 cycles.Even at rate of1 C,the battery based on L-PU4%electrolyte still has a specific discharge capacity of150 mAh g^-1 and the capacity retention rate is 92%after 150 cycles.In addition,cell with LiCoO₂ as the positive electrode was assembled,which provided discharge specific capacity of 109.5 mAh g^-1 at 60℃and 0.5 C.