Polymer-in-salt electrolytes based on acrylonitrile/butyl acrylate copolymers and lithium salts

Solid polymeric electrolytes for battery purposes in the form of composites of lithium salts [LiI, LiN(CF₃SO₂)₂, LiClCO₄, LiAlCl₄, LiCF₃SO₃, and LiBF₄] and acrylic polymeric matrixes [poly(acrylonitrile-co-butyl acrylate), poly(methyl methacrylate), and poly(butyl acrylate)] have been obtained by film casting from acetonitrile. The ionic conductivity (σ) as a function of temperature was studied by the impedance spectroscopy method. These systems show the highest σ values, on the order of 10 ^-4-10^-7 S·cm^-1, at high salt concentrations (above 50 wt %), characteristic of polymer-in-salt electrolytes. The ionic conductivity and mechanical properties of composites depend on the chemical structure of the polymer matrix, the anion, and the salt concentration. The glass transition temperature (T_g) was determined from DSC studies. The introduction of a salt causes, in a majority of the composites studied, a considerable decrease in the T_g values, indicating a strong plasticizing effect. DSC studies show a multiphase character of the composites, in which, with the exception of the amorphous system with LiN(CF₃SO₂)₂, phases of the plasticized matrix, complexes of the salt with the matrix of varying stoichiometry, and often the separating salt are observed. The logarithm of the decoupling index (log Rτ) on the order of 3-5 as well as the shift in the IR spectrum of the groups present in the polymer (C≅N and C≃O) by about 20-30 cm^-1 indicate a weak interaction of the salt with the matrix. The ion transference numbers (0.5-0.8) determined by the electrochemical method indicate an increased participation of cations in the electrical charge conduction and a different conduction mechanism compared to that of classical electrolytes based on complexes with polyethers.