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290                Polymer-based Nanocomposites for Energy and Environmental Applications

         (Al-BTC) MOF and a proper lithium salt could enhance ionic conductivity by two
         orders of magnitude at ambient temperature and also stabilize the lithium-metal/
         PCE interphase for a prolonged period of storage. MOFs have also been proposed
         as a solid ion conductor for lithium batteries. These are hybrid nanoporous solids
         where the ion transport could occur through 1-D pores. These large nanocrystalline
         pores are expected to benefit high ionic conductivity of >3 10  4 Scm  1  [57] as
         the ionic mobility is not sterically hindered through their channels.


         10.2.1.4 Layered clays
         Layered silicates such as montmorillonite (MMT) consist of two fused silica tetrahe-
         dral sheets that sandwich an edge-shared octahedral sheet of either magnesium or
         aluminum hydroxide [58,59]. Large interfacial area can be created when polymer
         chains are appropriately intercalated in between the silicate layers, which can promote
         the dissolution of a lithium salt [60]. The addition of MMT to PCEs further enhances
         the ionic conductivity, flame-retardant properties, thermal stability, and mechanical
         strength [59,61]. The mixing of carbon nanotubes and clays in PCEs was found to
         be beneficial to the lithium-ion conductivity and mechanical properties [62] because
         the isolation of the CNTs by the insulating clay layers eliminates the electrical
         shorting.


         10.2.2 Polymers

         Selection of a suitable polymer matrix is essential for high-performance electrolytes.
         Tremendous efforts have been put in investigating PEO-based electrolytes, but
         its semicrystalline nature results in low ionic conductivity at room temperature
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         (10 –10  8 Scm ) [56]. Poly(acrylonitrile) (PAN)-based electrolytes offer homo-
         genous films that exhibit good mechanical strength, heat resistance, good flame
         retardancy, and chemical stability. However, the C^N groups of PAN can interact
                      +
         strongly with Li ions, resulting in poor interfacial stability, which limits its applica-
         tion in rechargeable lithium polymer batteries. By combining with ceramic materials,
         for example, PAN/LiClO 4 composites containing 15 wt% of Li 0.33 La 0.557 TiO 3
         (LLTO) delivered ionic conductivity over 10  4 Scm  1  at room temperature [40].
         Poly(methyl methacrylate) (PMMA) was firstly used as polymer electrolyte matrix
         in 1985. It has been used in gel form or blends with PEO, PAN, or PVDF. PMMA
         has the advantage of easy gelation and rigidity due to its high T g . PMMA in PCE
         configuration delivered an ionic conductivity of >10  4 Scm  1  with the addition of
         5 wt% of Al 2 O 3 [63]. In any case, most of the results were achieved by using plasti-
         cizers such as ethylene carbonate or other organic carbonate species. Poly(vinyl chlo-
         ride) (PVC) has been studied as a polymer host for PCEs, where plasticizers are
         necessary. Some of the major problems with PVC are the incompatibility with lithium
         metal and narrow electrochemical stability window. PVC/ZrO 2 electrolyte delivered
         an ionic conductivity of >10  7 Scm  1  with a 10 wt% of ZrO 2 and plasticizer dibutyl
         phthalate. Poly(vinylidene fluoride) (PVDF)-based polymer electrolytes exhibit high
         anodic stability thanks to the dCdF group (strong electron-withdrawing functional
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