Effect of molecular weight and end groups on ionic conductivity of polyester‑based solid‐state electrolytes

Abstract

Aliphatic polyesters have emerged as a promising host polymer in lithium‐ion solid‐state electrolytes due to their appropriate coordination strength between carbonyl oxygen active sites and Li⁺, and high ion transference numbers. However, the effects of polyester molecular weight and end groups on ionic conductivity of polymer solid‐state electrolytes have been less studied. In this work, poly (hexamethylene succinate) (PHS) with various molecular weights and PHS‐based electrolytes with 30 wt% bis(trifluoromethane)‐sulfonimide lithium salt (LiTFSI) were prepared. From Raman spectra, PHS/LiTFSI with more end carboxyl groups of 5.3 × 10^-5 mol/g had the highest LiTFSI dissociation rate up to 68.8 % at room temperature, due to the strong binding between end carboxyl groups and Li⁺. Both the glass transition temperature and crystallinity of PHS/LiTFSI exhibited a characteristic convex profile with minimum values occurring at − 48.1°C and 8.32 %. It is suggested that more amorphous regions dilute the Li⁺ concentration which weakens the interactions of ions and polymer segments and restricts chain mobility. The ionic conductivity from electrochemical impedance spectroscopy confirmed that the relationship between molecular weight and ionic conductivity followed the fractional linear function, indicating the importance of tailoring solvation capacity and the dominant ion transport mechanism.