The Impact of Selectively Plasticized Poly (ethylene oxide) (PEO) Block in Nanostructured Polystyrene−PEO−Polystyrene Triblock Copolymer Electrolytes

Abstract

All polymer electrolytes, including block polymer (BP) electrolytes exhibit low ionic conductivity compared to liquid electrolytes. For homopolymer electrolytes, the ionic conductivity can be increased by incorporating plasticizer into the polymer network. However, the subsequent weakened mechanical structure of the network precludes its use in many practical applications. In this work, we selectively plasticize the PEO block in a polystyrene PS-PEO-PS triblock copolymer electrolyte using a PEO-like plasticizer: tetraethylene glycol dimethyl ether (TEGDME). Unlike its homopolymer (PEO) electrolyte counterpart, the mechanical properties of the PS-PEO-PS triblock copolymer electrolyte are not drastically degraded by this plasticizer because it is immiscible in the nonpolar PS block domains. Polymer electrolyte membranes were cast from lithium trifluoromethanesulfonate (LiTFS) solutions with the corresponding polymers. The molar ratio of ethylene oxide to Li+ is 16 for all electrolytes (nEO:nLi = 16:1). The ionic conductivity of the PS-PEO-PS electrolyte at 80 °C is about one order of magnitude lower than that of the homopolymer (PEO) counterpart at the same temperature. However, the shear modulus (G’) of the PS-PEO-PS electrolyte is about three orders of magnitude higher than the homopolymer, indicating the mechanical properties of the copolymer electrolyte is primarily dominated by the PS phase. The addition of TEGDME plasticizer to the PEO homopolymer electrolyte resulted in a ~10% increase in conductivity. The addition of same amount of TEGDME to the PS-PEO-PS electrolyte resulted in ~100% increase in conductivity. Compared to the dry electrolyte counterpart, the low isothermal frequency (ω = 0.1 rad/s) elastic modulus, G’ of the TEGDME plastized PS-PEO-PS was reduced to half at 80 °C (on the order of 106 Pa). Surprisingly, G’ of the plastized membrane surpassed its dry PS-PEO-PS counterpart at higher frequency values (17 < ω < 100 rad/s). This counter intuitive result is further explored using: 1) Fourier-transform infrared spectroscopy (FTIR) to elucidate the salt solvation and coordination to ether oxygens, and 2) small angle X-ray scattering (SAXS) to decipher the domain spacing of the adjacent copolymer blocks. Acknowledgment This work is supported by Dr. Imre Gyuk, Manager, Energy Storage Program, Office of Electricity Delivery and Reliability, Department of Energy. CC acknowldge partial financial support by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.