Fabrication And Properties Of PAMPSLi-based Electrospun Fibrous Polymer Electrolytes

Polymer electrolytes as key materials for lithium ionic batteries are required topossess not only a good electrochemical performance, but also excellent thermalstability and dimensional stability. Aiming at the problems, such as the low ionicconductivity of single-ion conducting polymer electrolytes, the poor dimensionalstability of fiber-based polymer electrolytes and the leakage of electrolyte solutionfor PVDF-based gel polymer electrolytes,2-acrylamido-2-methylpropane-sulfonicacid （AMPS） was chosen as a main monomer for the modification of polymerelectrolytes utilizing ion-exchange processes and polymerization reaction. Themethods like blending, copolymerization and complex with organic-inorganichybrid particles were adopted to optimize the physical and electrochemicalproperties of polymer electrolytes. The preparation, modification, and therelationships between structures and properties were investigated.The nonvolatile and chemically inert plasticizer poly（siloxane-g-ethyleneoxide）（PSi-PE） was introduced into a PAMPSLi-based all-solid-state single-ionconducting polymer electrolyte. The addition of PSi-PE shortened the relaxationtime of the polymer chain segments and improved the mobility of the segments,which was beneficial to the migration of lithium ions. The test results of theconductivity showed that the conductivity first increased and then decreased withthe increase of PSi-PE. When35mass%PSi-PE was added, the ionic conductivity atroom temperature got the maximum value (6.9×10^-8S/cm), which was about20times larger than that of the membrane without PSi-PE.To further enhance the ionic conductivity, the method adopting an organicsolvent（EC/DMC,1:1, mass ratio） as a plasticizer to promote both the ionicdissociation of the polymeric lithium salt and the ionic migration was considered.Meanwhile, the large specific surface area of the electrospinning membrane wasused to improve the contact area between the polymeric lithium salt and theplasticizer, further to enhance the ionic conductivity of the single-ion conductor.PAMPSLi used for the electrospinning procedure was synthesized by free-radicalpolymerization of2-acrylamido-2-methyl propanesulfonic acid, followed byion-exchange of H⁺with Li⁺. TG analysis showed that its decomposition did notoccur until304℃. PAMPSLi electrospun membranes with different specific surfaceareas were fabricated by varying the solvent composition of polymer solutions.FTIR tests showed PAMPSLi dissociated effectively in the plasticizer. Theroom-temperature ionic conductivity was up to2.12×10-5S/cm. In addition, this kind of polymer electrolytes had excellent dimensional stability and electrochemicalstability.To solve the problem that it was difficult to obtain PMMA-based polymerelectroytes with good dimensional stability under high content of electrolyte,P（MMA-co-AMPSLi） fiber-based polymer electrolytes were prepared through thecopolymerization of AMPS and MMA. The introduction of AMPSLi units whichhave better solvent resistance can adjust the solvent affinity ofP（MMA-co-AMPSLi）. The test results manifested that the spinnability ofP（MMA-co-AMPSLi） was improved with the increase of the content of AMPSLiunits. Meanwhile, the solvent resistance of P（MMA-co-AMPSLi） was improved,and the degree of swelling of the fibers in the electrolyte solution decreased. Theholding ability of pore structure and the uptake of electrolyte solution wereenhanced. The dielectric constants of P（MMA-co-AMPSLi） electrospun membranesalso increased with the increase of the AMPSLi unit content. When the feed ratio ofMMA:AMPS was2:8, the ionic conductivity of the resultant fibrous polymerelectrolyte at room temperature was4.12×10^-3S/cm, and the electrochemicalwindow was up to5.0V. The synthesis of P（MMA-co-AMPSLi） successfullyovercame the requirements of the preparation conditions for PMMA gel polymerelectrolyte, and improved effectively the performance of the polymer electrolyte.To overcome the disadvantage of poor dimensional stability of PVDF-basedelectrospun fibrous polymer electrolytes and facilitate the assemblage of batteries, anew type of bicomponent fibrous membranes based on PVDF/PAMPSLi blendsystems with different blend ratios were fabricated via electrospinning method. Theinteraction between PAMPSLi and PVDF improved the miscibility of two polymers,suppressed the orientation of the PVDF molecular chain, and reduced thecrystallinity of PVDF. Meanwhile, the introduction of PAMPSLi improved thedimensional stability of the polymer electrolytes. The average diameters of thebicomponent fibrous membranes were far lesser than those of the pure PVDFfibrous membranes. The average fibrous diameter of the composite fibrousmembrane with the blend ratio of5:1（PVDF:PAMPSLi） was145nm, and theproperties of the corresponding polymer electrolyte, such as the uptake ofelectrolyte, ionic conductivity, dimensional stability, and electrochemical stability,reached optimal value. Incorporation of PAMPSLi has been verified to be aneffective method in improving the properties of PVDF based fibrous polymerelectrolytes.The high crystallinity of PVDF limits the increase of ionic conductivity, andoften causes the problem of the electrolyte leakage. To solve these problems, organic-inorganic hybrid nanoparticles PMMA-g-TiO₂was synthesized throughatom transfer radical polymerization. The PVDF/PMMA-g-TiO₂composite fiberpolymer electrolytes were fabricated using electrospinning technology. Themolecular weight of the grafted polymer distributed uniformly and the averagemolecular weight was relatively low, which was beneficial to the penetration of theelectrolyte solution. The introduction of PMMA-g-TiO₂reduced the crystallinity ofPVDF, and improved the electrolyte uptake due to the excellent affinity of PMMAtoward the liquid electrolyte. The capability of electrolyte retention of fibrouspolymer electrolytes was enhanced by the incorporation of PMMA-g-TiO₂. Theionic conductivity at room temperature was up to2.95×10^-3S/cm, and theelectrochemical window reached5.3V.