An Appraisal of Tetraethylsulfamide as Plasticizer for Poly(ethylene oxide)−LiN(CF3SO2)2 Rubbery Electrolytes

Abstract

Tetraethylsulfamide (TESA), (CH₃CH₂)₂NSO₂N(CH₂CH₃)₂, is a nonvolatile liquid of moderate permittivity (ε = 29) that exhibits a good stability toward reducing agents. This glass-forming liquid (T_g = −111 °C) is compared to propylene carbonate (PC, ε = 65, T_g = −110 °C) as diluent for PEO−LiN(CF₃SO₂)₂ electrolytes based on high molecular weight (M = 4 × 10⁶) poly(ethylene oxide) (PEO). This study concerns amorphous mixtures with salt weight fractions fixed near the eutectic composition (W_salt = 0.37) of the PEO−salt binary system. It shows that, at 0 °C and up to 30% diluent (with respect to PEO), the conductivity elevation produced by TESA is about half that produced by PC. This difference is mainly due to the small T_g depression produced by TESA. This latter feature suggests that TESA is not as competitive as PC for lithium cation solvation. This situation is more severe at higher TESA contents where less and less PEO is available for cation solvation. Over this range, a neutral or a charged complex of very low mobility forms with TESA. Also examined is the effect of PC and TESA on the crystallization behavior of these salt-rich mixtures.