A study of poly(vinyl alcohol) as a solid polymer electrolyte for lithium-ion batteries

Abstract: The use of solid polymer electrolytes in lithium-ion batteries has the advantage in terms of safety and processability, however they often lack in terms of performance. This is of major concern in applications where high current densities or rapidly changing currents are important. Such applications include electrical vehicles and energy storage of the electrical grid to accommodate fluctuations when using renewable energy sources such as wind and solar. In this study, the use of commercial poly(vinyl alcohol) (PVA) as a solid polymer electrolyte for use in lithium-ion batteries has been evaluated. Films were prepared using various lithium salts such as lithium bis(trifluoromethane)sulfonimide (LiTFSI) and casting techniques. Solvent free films were produced by substituting the solvent Dimethyl sulfoxide (DMSO) with water and rigouros drying or by employing a hot-pressing technique. The best performing system studied was PVA-LiTFSI-DMSO, which reached ionic conductivities of 4.5E-5 S/cm at room temperature and 0.45 mS/cm at 60 °C. The solvent free films showed a drop of ionic conductivity by roughly one order of magnitude compared to films with residual DMSO present. High ionic conductivities in PVA-LiTFSI-DMSO electrolytes are thus ascribed to fast lithium ion transport through the liquid domain of DMSO, or by plasticizing effects of salt and solvent on the polymer. Thermal analysis of the films showed a clear plasticizing effect of DMSO by a decrease in the glass transition temperature. FTIR analysis showed complexation of all the lithium salts investigated with the OH-groups of the polymer by a shift in the characteristic frequencies of both salts and polymer. For the first time, prototype battery cells containing PVA electrolytes were manufactured and evaluated by galvanostatic cycling. PVA-LiTFSI-DMSO showed stable cycling performance for 15 cycles. Solvent free electrolytes were also investigated but did not result in any stable cycling performance.