Preparation Of Single-ion Conductive Gel Polymer Electrolyte And Study Of Electrochemical Performance In Lithium-Sulfur Batteries

Sulfur,a rich element in the earth,has a theoretical specific capacity of 1675mAh g^-1,and an energy density of 2600 Wh kg^-1when used as cathode material of lithium batteries.However,during charge and discharge,the sulfur transforms to polysulfides,which is able to solvate in liquid electrolyte.As a result,polysulfides penetrate the separator to deposit on the surface of lithium anode,leading to a shuttle effect.The Li-S batteries reveal low coulombic efficiency,and thus bad cycling performance.On the other hand,the discharged products Li₂S₂/Li₂S and the charged products are insulative in nature,resulting in a poor rate capability.The commercial application of Li-S batteries is seriously limited by these problems.To suppress the shuttle effect,solid polymer electrolyte（SPE）is used to replace the liquid electrolyte.Nevertheless,low ionic conductivity at room temperature limits its wide use in Li-S batteries.Fortunately,gel polymer electrolyte（GPE）with high ionic conductivity shows a promising alternative,but it would cause serious polarization due to its low lithium ion transference number.In this work,single-ion conductive GPE based on P（VDF-HFP）was prepared using ionic liquid as plasticizer to improve electrochemical properties of GPE.The GPE was doped with BN nanoparticles to form a nanocomposite single-ion conductive GPE as well.The study contents included:（1）P（VDF-HFP）/PAALi based single-ion GPE was prepared by one-pot method.Lithiated MAA monomer are added to the solution of P（VDF-HFP）in ionic liquid,and polymerized in situ to occur a phase separation.The as-prepared single-ion GPE possesses highly thermal stability and low crystallinity.When the mass ratio of MAA to P（VDF-HFP）was 1 in the casting dope,the resultant GPE displays the maximum ionic conductivity of 0.733×10^-33 S cm^-11 at room temperature,a lithium transference number of 0.85,and a stable potential of 4.5 V（vs.Li⁺/Li）in anodic direction.The Li-S batteries assembled with this GPE membrane can deliver the capacities of 1288mAh g^-11 at 0.1 C rate in the first cycle,and 900 mAh g^-11 after 50 cycles.The initial capacities are 1094,773 and 474 mAh g^-11 at the rate of 0.2 C,0.5 C and 1 C,respectively.The results indicate that the electrochemical properties of the obtained GPE are unsatisfied,which needs improved.（2）BN nanosheets（BNNS）were prepared by a Hummers method,and the hydroxylated BNNS（BNNS-OH）were prepared by a thermal method.The BNNS-OH was characterized by FTIR and XRD technologies.The results suggest that the–OH groups are bonded on the surface of BNNS through covalent bonds,thus modifying the BNNS.SEM/TEM images show the exfoliated BNNS-OH particles are in a reduced thickness down to about 7 nm.The BNNS-OH particles can be dispersed uniformly in water or ethanol,displaying a Tyndall effect.The resultant dispersion is so stable in storage as to not precipitate over six months.（3）BNNS-OH particles were doped into the single-ion GPE based on P（VDF-HFP）/PAALi to improve its ionic conductivity,interfacial stability and mechanical performance.Effect of the amount of BNNS-OH on the morphology,mechanical properties,and electrochemical performance is studied.When the filling amount of BNNS-OH is 0.4%,the resultant composite GPE/BNNS-OH-2 sample has a room-temperature ionic conductivity of 1.29×10^-33 S cm^-1,and a lithium-ion transference number of 0.91.It shows a stable potential of 4.5 V（vs.Li⁺/Li）in anodic direction,accompanying with a good interfacial stability.The Li-S batteries assembled by this composite GPE can deliver an initial capacity of 1607 mAh g^-11 at0.1 C rate,and a capacity of 1272 mAh g^-11 after 50 cycles.It can deliver the capacities of 1402,1126 and 705 mAh g^-1,at the rates of 0.2C,0.5C and 1.0 C,respectively.The results indicate that the electrochemical and mechanical properties of the P（VDF-HFP）/PAALi-based single-ion GPE can be improved significantly by doping with the BN nanosheets modified by–OH groups on the surface（BNNS-OH）.The Li-S batteries assembled by this composite GPE membrane possess good electrochemical performance.