Preparation, Structure And Properties Of Acrylonitrile Copolymer Nanocomposite Electrolyte

Secondary lithium battery and proton-conducting membrane fuel cell (PCMFC) are typical representitives of the secondary battery and fuel cell, respectively, which exhibit envionment-friendly, renewable and high efficient characteristics. Lithium ion and proton-conducting polymer electrolytes are the main components of lithium ion battery and PCMFC, respectively, which significantly influence the performances of cells. In view of the low ion conductivity of solid lithium polymer electrolytes and high methanol crossover of common proton-conducting membrane, such as perfluorosulfonate membrane, the modification of polymer electrolytes by copolymerization and compositing with nanometer material were carried out in this thesis. Acrylonitrile-N-[4-(aminosulfonyl) phenyl] acrylamide (AN-ASPAA) copolymers were synthesized through the solution copolymerization and composited with lithium salt to obtain lithium ion polymer electrolyte. AN-sodium 4-sulphonate styrene (AN-SSNa) were synthesized through precipitation copolymerization and treated with acid to obtain proton-conducting polymer membranes. A novel layered nanometer material-layered double hydroxide (LDH) was further composited with AN copolymers to improve the properties of polymer electrolyte membranes. The influences of copolymer composition and LDH content on the properties of polymer electrolyte were investigated, and the mechanism of ion transportation in the modified polymer electrolytes was discussed.In order to obtain a new host for lithium ion polymer electrolyte, the synthesis of AN-ASPAA copolymer with homogeneous composition was carried out at first. It was found that the reactivity ratios of AN-ASPAA copolymerization in DMF were r_AN=0.60 and r_ASPAA=1.76, which indicated the ideal copolymerization behavior of two monomers and the formation of random copolymer.Lithium ion polymer electrolyte membrane was obtained by combining of AN-ASPAA copolymer with 20wt% (based on polymer) LiClO₄. An equivalent circuit model was proposed based on AC impendance spectra and practical circuit analysis. The element parameters of the equivalent circuit model, including resistance of polymer electrolyte, were calculated, and the simulated AC impendance spectra were fitted well with experimental data. It was considered that the addition of ASPAA unit was a favor to the association between Li⁺ and polymer chains, but reduced the local mobility of polymer chains. As a result, the conductivity, the dielectric constant and dielectric loss of copolymer electrolyte were increased with the increase of ASPAA content initially, and decreased when the molar fraction of ASPAA was greater than 32.6%. The membrane exhibited a maximum conductivity of 1.54×10^-2S/m (30℃) when ASPAA molar fraction was 32.6%. The ion transportation mechanism of AN-ASPAA copolymer/LiClO₄ composite was proposed as follows: the introduction of ASPAA unit into copolymer would provided more nucleophilic groups to complex with Li⁺, and lithium ions transported mainly through association-disassociation with the nucleophilic groups under the electric field force and concentration gradient.In order to improve the dispersion and exfoliation of LDH in AN polymer matrix, the dispersion behavior and in-situ solution polymerization of LDH intercalated with different organic anions in organic mediums were studied. According to the dispersion behavior of 4-styrene sulfonate intercalated LDH, 10-undecylenate intercalated LDH and polyoxyethyleneα-propenyl alkylphenyl ether sulfate intercalated LDH (MgAl-HS10 LDH) in different solvents, Hansen's solubility parameters of the intercalated hydrophobic groups and that of the solvents were adopted to judge the dispersing ability and exfoliation of LDH. It was found that MgAl-HS10 LDH could be stably dispersed and partially exfoliated in DMSO. Thus, in-situ polymerization of AN and copolymerization of AN-ASPAA were carried out in the presence of MgAl-HS10 LDH to obtain PAN/LDH and AN-ASPAA copolymer/LDH nanocomposite with well dispersed and exfoliated LDH layers.The properties of AN-ASPAA copolymer/LDH nanocomposite and AN-ASPAA copolymer/LDH/LiClO4 nanocomposite electrolyte were investigated. It was found that the thermal stability of AN-ASPAA copolymer/LDH nanocomposite was increased as LDH content increased. The ion conductivity of nanocomposite electrolyte was also increased when the weight fraction of added LDH was lower, and reached a maximum (5℃) when the weight fraction of LDH was 1%. Further increase of LDH content would lead the decrease of ion conductivity of nanocomposite electrolyte. A new equivalent circuit model for AN-ASPAA copolymer /LDH/UClO₄ nanocomposite electrolyte was proposed, and the role of LDH played in the nanocomposite electrolyte was analyzed. It was considered that LDH layers with positive charges acted as capacitances in the nanocomposite electrolyte, which would associated with ClO₄^- and enhanced the transportation of Li⁺. However, LDH layers also hindered the transportation of Li⁺.AN-(sodium 4-styrene sulfonated) (AN-SSNa) copolymers with different composition were prepared through semi-continuous precipitation polymerization, and protonated to obtain AN-4-styrene sulfonic acid (AN-SSA) copolymer proton-conducting membrane with different sulfonation degree. It was found that the water uptake, ion exchange capacity, proton conductivity and methanol permeation were all increased as the molar fraction of SSA in copolymer increased. The water uptake, proton conductivity and methanol permeation coefficient of AN-SSA copolymer proton-conducting membrane were increased slowly when the molar fraction of SSA ranged from 0 to 3.12%, and increased rapidly when the molar fraction of SSA was greater than 3.12%. The selectivity of membrane defined as the ratio of the proton conductivity to methanol permeation coefficient reached a maximum value for the membrane prepared from AN-SSA copolymer with 3.12mol% of SSA. The proton diffusion coefficient in the proton-conducting membrane obtained through simulation of equivalent circuit model and fitting of AC impedance spectra also reached a maximum (5℃) when the molar fraction of SSA in copolymer was 3.12%.AN-SSNa copolymer/LDH nanocomposites were prepared by in-situ aqueous precipitation copolymerization of AN and SSNa in the presence of sodium 4-styrene sulfonated intercalated LDH(MgAl-SS LDH) and transferred to AN-SSA copolymer/LDH nanocomposites as a proton-conducting polymer electrolyte. It was found that the methanol permeation coefficient of the proton-conducting nanocomposite membrane was decreased as the weight fraction of LDH in nanocomposite increased, while the ion exchange capacity and water uptake were increased as the weight fraction of LDH increased. The ion conductivity, dielectric content and dielectric loss of proton-conducting nanocomposite membrane showed an increasing trend at the lower content of LDH, and a decreasing trend when the weight fraction of LDH was greater than 4%. It was considered that the incorporated LDH layers would lead the formation of more proton conducting channels in the membrane due to its hydrophilicity, while they would also hinder the transportation of protons due to their barrier property. As a result, the proton conductivity of nanocomposite membrane showed a maximum of 1.04×10^<sup>-3S/m (30℃) when the LDH content was 4wt%.