Research On The Preparation And Application Of Inorganic-Organic Solid Polymer Electrolyte

With the use of solid polymer electrolytes (SPEs) instead of liquid electrolyte in lithium batteries, high specific energy and specific power, safe operation, flexibility in packaging and variety geometries can be expected. However SPEs still have some drawbacks such as low conductivity at ambient temperature and poor mechanical properties at high temperature for dry SPEs, and weak mechanical strength and poor interfacial properties for gel and porous SPEs.Considering that the inorganic-organic polymer has amorphous network which is benefit for the ion transport as well as the well dimensional stability, two kinds of inorganic-organic SPEs were prepared based on two novel monomers designed and synthesized in my lab. One was organically modified polysiloxane as SPE based on organically modified polysiloxane macromonomer, and the other was nanocomposite SPE based on the nano-SiO₂-contained poly(ethylene glycol) diacrylate (PEGDA).Organically modified polysiloxane macromonomer was synthesized by hydrolytic condensation of tetramethoxysilane (TMOS) and in combination with demethanol-ation reaction using 2-hydroxyethyl acrylate (2-HEA). The molecular weight and the number of surface functional groups were controlled well. The rational formula was SiO_a(OH)_b(OCH₃)_c(OCH₂CH₂OCOCH=CH₂)_d, which was determined by GPC, ¹H-NMR, FT-IR and silica analysis.Nano-Si02-contained PEGDA was prepared by solvent exchange process using aqueous colloidal silica as one of starting materials. The partial silanol surface groups of SiO₂ were modified into functional acrylic groups by employing methacryloxy-propyltrimethoxysilane (MAPTMS), which made the dispersion of nanosize SiO₂ in PEGDA uniform and stable and the viscosity very low. This was the first time for the aqueous colloidal silica, in which nano-sized SiO₂ with silanol groups dispersing uniformly, to be used into the preparation of SPE monomer. It was the modification and elimination of silanol groups make it possible.Dry and gel SPEs were prepared by thermal or UV polymerization with these two monomers described above respectively. In contrast to hardly reproducible filling of polymer electrolytes with nanoparticles, the polymerization of organically modifiedpolysiloxane or the nano-SiC^-contained PEGDA enables an extremely homogenized and reproducible distribution of the nano-sized oxidic units.Two types of dry inorganic-organic SPEs were prepared. One was semi interpenetrating networks which were prepared by thermal polymerization with the mixture of monomers, PEO and LiClC>4. The conductivities were high with 10* S.cm'1 at room temperature and 10"3S.cm"' at 70°C. The other was plasticized SPEs which were prepared by UV polymerization with the mixture of monomers, L1CIO4 and low molecular weight poly(ethylene glycol) dimethylether (PEGDME) as the plasticizer. The conductivities were up to 10"3 S.cm"1 at room temperature when the PEGDME content was about 75wt.%. The conductivity of both types of dry SPE obeyed the VTF equation. In the system of dry inorganic-organic SPEs, the inorganic components play fourfold role: I. improving the mechanical properties; II. decreasing the crystallinity and stabilizing the conductive amorphous phase; III. forming a chemically crosslinked networks for the functional acrylic groups in both polysiloxane and nano-SiCh; IV. increasing the fraction of free Li+ and the conducting pathways, and decreasing the activation energy.The gel-type inorganic-organic SPEs were also prepared by UV polymerization and thermal polymerization respectively in the presence of liquid electrolyte. The conductivities exceeded 10*3 S.cm"1 at room temperature when the liquid electrolyte content was larger than 40wt.%. The gel SPEs exhibited excellent electrochemical stability and interfacial stability both by UV polymerization and thermal polymerization except that a little lower conductivity by latter means.The electronic properties of the interphase film on Li/inorganic-organic SPEs interface were studied by AC impedance spectroscopy. A model of the interphase film with 4 layers structure was put forward and the impedance spectra were modeled by equivalent circuit analog. The calculated parameters values enabled the thickness and resistivity of the various layers and their changes upon storage time. The results showed that the layer a (closest to the Li electrode surface) in the interphase film on Li/inorganic-organic SPEs interface was thinner and more compact than that on Li/liquid electrolyte interface. This indicated that the interfacial stability of inorganic-organic SPEs with Li electrode were better. The mechanism could be explained from two aspects: i. the inorganic-organic matrix stably trapped the liquid electrolyte and prohibited it to react with Li electrode; ii. The inorganic components could trap the impurities, mainly the water in liquid electrolyte, and protect the Li electrode from the corrosion.