Conductivity increase effect in nanocomposite polymer gel electrolytes: manifestation in the IR spectra

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Conductivity increase effect in nanocomposite polymer gel electrolytes: manifestation in the IR spectra A. V. Yudina,a G. R. Baymuratova,a G. Z. Tulibaeva,a A. L. Litvinov,a A. F. Shestakov,a,b and O. V. Yarmolenkoa aInstitute

of Problems of Chemical Physics, Russian Academy of Sciences, 1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation. E-mail: [email protected] bFaculty of Fundamental Physical and Chemical Engineering, M. V. Lomonosov Moscow State University, Build. 51, 1 Leninskie Gory, 119991 Moscow, Russian Federation Nanocomposite polymer electrolytes based on poly(ethylene glycol) diacrylate and 1 M LiBF4 solution in γ-butyrolactone containing SiO2 nanoparticles were studied by FT IR spectroscopy. Quantum chemical modeling of five different solvate complexes of ions composed of LiBF4 with solvent molecules was carried out and their theoretical IR spectra were calculated. The compositions of the solvate complexes of LiBF4 in the nanocomposite gel electrolyte were studied by comparing the experimental and theoretical IR spectra. It was concluded that the conductivity peak observed upon adding 2 wt.% SiO2 is due to the appearance of mobile ions as a consequence of ionic dissociation on the surface of nanoparticles at their optimal configuration. Key words: nanocomposite electrolytes, poly(ethylene glycol) diacrylate, LiBF4, SiO2 nanoparticles, density functional theory, IR spectroscopy.

Nanocomposite polymer electrolytes (NPE) are a promising class of electrolytes for solid-state electrochemical devices that require safe operation and flexibility.1,2 Most NPE are prepared from a copolymer of polyvinylidene fluoride with hexafluoropropylene, which contains nanoparticles as an additive. The copolymer is then used for membrane fabrication by solution cast3,4 or electrospinning.5,6 One more synthetic route to NPE based on polymer networks is to carry out a radical polymerization7—9 of diacrylates with bridging polyether chain. Threedimensional polymer networks are formed upon crosslinking by the radical mechanism in a liquid organic electrolyte (or without the electrolyte) in the presence of inorganic nanoparticles. Earlier, we developed a new nanocomposite electrolyte based on poly(ethylene glycol) diacrylate (PEG-DA) and 1 М LiBF4 solution in γ-butyrolactone (GBL), and SiO2 nanopowder. The dependence of the conductivity of this system on the amount of SiO2 nanopowder added exhibits two maxima9—12 unlike the dependence of the conductivity of a similar system containing TiO2 and Li2TiO3 nanoparticles8,13 where only one maximum is observed. The ionic conductivity in nanocomposites can increase due to various effects including an increase in structural disordering of a system,14,15 the interaction between the nanofiller and the salt or polymer in a Lewis acid—Lewis base manner,16 enhancement of ion mobility,17,18 an increase in the transference numbers,10,19 enhanced seg-

mental mobility of the polymer chain,20 electrostatic interaction between ions and nanofiller particles,21 formation of