(396e) Study of Concentration Dependent Diffusion Coefficient of Lithium Salt in Block Copolymer

Recent interests in energy storage technology comprise safety, energy density, efficiency, and cost. Due to its high energy density, lithium metal batteries are attractive alternatives to lithium ion batteries. The two biggest technical issues to be solved to replace the lithium ion batteries are low ionic conductivity of solid polymer electrolytes, which serve as electrolyte and separator, in the lithium metal batteries, and dendrite growth from the lithium electrode surface. Polystyrene-poly(ethylene oxide) block copolymer (SEO) is a candidate polymer electrolyte because of its high shear storage modulus to suppress dendrite growth. Ionic conductivity can be improved by adding lithium bis-trifluoromethylsulfonimide (LiTFSI) that coordinates to the ether group in the poly(ethylene oxide) phase. Diffusion of LiTFSI is a key property that directly relates to battery rate capability. The diffusion coefficient of LiTFSI in SEO electrolytes was investigated using time-resolved Fourier Transform infrared - attenuated total reflectance spectroscopy (FTIR-ATR). One advantageous feature of this method is that it is a measurement without electric current, i.e., concentration gradient is the only driving force of the transport of the salts. This makes the diffusion coefficient to be a function of the concentration of salts only and the results to be straightforward. Another merit of the FTIR-ATR measurement is its relatively simpler and faster analysis as compared to conventional electrochemical methods that have been used in studies of diffusion coefficient of salts in block copolymers. Our findings imply concentration-dependent diffusion coefficients of lithium salts in the block copolymer membrane, which corresponds to our previous research of concentration dependence of diffusion coefficient of LiTFSI using ⁷Li MRI.