| A new class of highly conductive and processable composite polymer electrolytes for use in rechargeable lithium batteries have been synthesized. The principal characteristic of these systems is the use of surface functionalized fumed silica fillers to control the mechanical properties of the composites. When a low molecular weight polyether solution is immobilized in fumed silica, an open three-dimensional network forms due to van der Waals attractions or hydrogen bond linkages between silica particles. The open network structure is conducive to high ionic mobility and responsible for the reinforcement of the composite.; The most promising electrolytes are cross-linkable composite polymer electrolytes that incorporate a silica that bears both hydrophobic and cross-linkable functional groups. Hydrophobic groups contribute to the formation of open networks, while cross-linking groups provide mechanical strength and dimensional stability through the formation of covalent bonds between silica particles.; Cross-linkable composite electrolytes were prepared from PEGDM-500, LiClO{dollar}sb4,{dollar} fumed silica, and 10 wt% of methyl, butyl, or octyl methacrylate. Before cross-linking, the electrolytes were thixotropic and had ionic conductivities {dollar}{lcub}>{rcub}10sp{lcub}-4{rcub}{dollar} S/cm. After UV-induced cross-linking, the electrolytes were rubbery and dimensionally stable. DSC results showed that the addition of silica and monomer to the polyether-Li salt matrix did not affect the glass transition of polyether. In addition, the conductivities were unchanged according to impedance spectroscopy. After solvent extraction of the cross-linked composites to remove the polyether, lithium salt, and homopolymer, the solid phase was analyzed. The FTIR spectrum showed qualitatively the grafting of the guest polymer, and TGA analyses demonstrated that nearly all the polymer had been chemically bound to the silica surface. These data all support a model where the added methacrylate monomer and initiator phase separate from polyether-Li salt matrix and form a thin layer surrounding the silica network. Upon photoinitiation, the added monomer copolymerizes with cross-linkable groups on silica surface to yield a silica/polymer phase and little or no polymer in the PEGDM-500 phase. Thus, the mechanical properties of the composite electrolyte and its ionic conductivity are de-coupled, and can be optimized independently. |
