Polymeric solid electrolytes have been subject of intensive research for potential applications in electrochemical devices, especially lithium-ion batteries. The most widely studied system is based on poly(ethylene glycol) (PEG), which can effectively solvate a variety of alkali and alkaline cations.¹ Although mixtures of polymers with salts show reasonable conductivity at room temperature,² they suffer from the undesirable electrode polarization due to anion aggregation at the cathode surface.³ Single-ion conductors with anions fixed to polymer backbone are able to resolve this problem and achieve a cation transference number of 1.¹ Unfortunately, the single-ion conductors studied to date show far lower conductivity than the bi-ion conductor. Understanding the cation transport mechanism and structure in single-ion conductors is critical for designing future cation transport materials with high conductivity.

In this study, the structure and ionic association behavior of PEG-based ionomers are investigated as a function of PEG segment length, cation type, and temperature. A series of sulfonated polyester ionomers with well-defined PEG spacer lengths are synthesized by melt polycondensation of PEG oligomers and dimethyl 5-sulfoisophthalate sodium salt. Multiple-angle X-ray scattering data from scattering vector q = 0.1 to 23 nm^-1 reveal the morphology of the ionomers at different length scales. When the counterion is sodium, there is no “ionomer peak” in the expected angular range. In contrast, when the cation is exchanged from sodium to a higher atomic number species such as cesium, a broad peak appears at ~7 nm^-1, reminiscent of conventional ionomers. Ionomers with longer PEG segment length are semicrystalline, and show multiple small-angle scattering peaks. The peak positions relative to the first peak position are 1:2:3, which might indicate a layered structure. The effects of temperature on the nanoscale structures of these ionomers are investigated by in situ X-ray scattering over a wide temperature range. Electron microscopy is performed to study the distribution of ionic groups. These results will provide new insights into the development of advanced single-ion conductors for lithium-ion batteries.

References

(1) Xu, K. Chem. Rev. 2004, 104, 4303.

(2) Zhang, S.; Runt J.; J. Phys. Chem. B 2004, 108, 6295

(3) Dou, S.; Zhang, S.; Klein R. J.; Runt J.; Colby, R. H. Chem. Mater. 2006, 18, 4288-4295