Modification Of Poly (Vinylidene Fluoride) Gel Electrolyte By Layered Double Hydroxide

Polymer electrolyte is one of the important parts of lithium battery and proton conducting fuel cell. The property of the polymer electrolyte is a key factor affecting the performance of cell. Poly(vinylidene fluoride)-based polymer electrolytes have good performance and have received much attentions. In order to improve the performance of PVDF electrolytes, layered double hydroxides (LDHs) were added into in the polymer electrolytes. The nano-layers of LDHs exhibit positive charges and can form electrostatic interaction with the negative ion of the electrolytes, thus, the ion conductivity can be enhanced. Meanwhile, the nano-layers of LDHs can improve the tensile modulus and the barrier property of the composite electrolyte films.LDHs intercalated with different anions were prepared and dispersed. It was found that LDHs intercalated with dodecyl sulfate anions (DS-LDH) can be easily exfoliated and dispersed in DMF, which is the solvent of PVDF and plasticizer of electrolytes. From the TEM image of PVDF membrane composited with DS-LDH and the particle size distribution of DS-LDH in DMF, it showed that size of most DS-LDH particles were lower than 100nm. The volume fraction of DS-LDH particles with size greater than 100nm was about 6.8%.As DS-LDH added, the crystallinity of PVDF was reduced and the porosity of the membranes was increased. When the weight percentage of added DS-LDH increased from 0 to 7.41%, the crystallinity of PVDF decreased from 70.4% to 61.2%, and the porosity of the membrane prepared by the dry method was increased from 2.0% to 8.31%. The porosity of the membranes was increased to 69.3% and to 66.7% when the wet method and the non-solvent method (the volume ratio of non-solvent was 33.3%) were applied to prepare the membranes, respectively. The PVDF membranes prepared by non-solvent method exhibited a loose and honeycomb-like morphology. The membranes prepared by the wet method exhibited fingerlike and sponge pores in the entire sub-layer and the dense top layer. Macroscopic phase separation was observed when these two methods were applied to prepare the membrances.The ion conductivity of both lithium conducting and proton conducting PVDF electrolytes increased with DS-LDH content initially and then decreased. For the proton conducting electrolytes, the maximum ion conductivity was achieved when the weight percentage of DS-LDH was 7.41wt%. The ion conductivity was about lO^S/cm for the composite membranes prepared by the dry method. However, the ion conductivity could reach 10"4 S/cm for the membranes prepared by the wet method and the non-solvent method. For the gel electrolyte, the ion conductivity could reach 3.8mS/cm at the room temperature. For the lithium conducting gel electrolytes, the maximum ion conductivity was observed in the sample having a content of 5.66wt% and reached 53mS/cm. The temperature dependence of conductivity of all gel electrolytes obeys Arrhenius equation.Based on the morphology of the membrane and the ion transport characteristics, the mechanism of ion conducting was presented. The action between the positive layers of the LDHs and the anions of the electrolyte had advantages for the transport of cations in the membranes of PVDF.