Lithium Local Coordination Structure in Polyether-Based Electrolyte with Cyanoethoxy Sidechain: An Experimental and Theoretical Approach
Authors: Matsuoka Riho , Shibata Masayuki , Matsuo Kousuke , Sai Ryansu , Tsutsumi Hiromori , Fujii Kenta , Katayama Yu 
Published: December 2020

Solid polymer electrolytes (SPEs) have been studied as the alternative for the current liquid electrolytes. Although poly(ethylene oxide) (PEO) with ether structure and poly(acrylonitrile) (PAN) with cyano group have studied for more than decades, these electrolytes have issues such as low ionic conductivity and small transference number of lithium-ion (t
Li
+). We have reported improved lithium transference number in PEO-based SPE having both ether and cyano groups (PCEO), which showed t
Li
+ of 0.5, compared to that of ca. 0.2–0.3 for PEO counterparts. However, the molecular-level understanding of its ion transport mechanism is still lacking. The main obstacle is the difficulty in experimentally clarify the complex interaction between the ions and the polymer matrix in the PCEO electrolyte.

In this study, we combine infrared (IR) spectroscopy with molecular dynamics (MD) simulation to understand the complex interaction within the electrolyte, in order to clarify the Li+ coordination structure as well as its transport mechanism in PCEO electrolytes. We selected lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) as a salt and prepared three PCEO electrolytes with different salt concentrations (a = 10, 3, and 1). High-energy X-ray diffraction patterns and corresponding MD simulated curves were in good agreement, confirming that MD simulation could apply to this system. The local coordination structures around lithium-ion were thus estimated by the distribution function obtained from MD simulation. The simulated average coordination number for PCEO3LiTFSI, with the highest ionic conductivity among the 3 electrolytes, were Li+–CN = 2.2, Li+–O (ether) = 0.5, and Li+–TFSI- = 1.3. The result suggests that the cyano group preferably coordinate with lithium-ion than the other functional group, including ether group. Further supports comes from IR spectroscopy, where the estimated coordination number obtained from the IR peak area of the deconvoluted CN stretching vibration (n = ca. 2250–2280 cm-1) agreed well with the MD result. Based on the experimental and theoretical evidence, we conclude that cyano groups mainly responsible for transporting lithium-ion in PCEO electrolytes. Arrhenius plots of the ionic conductivity showed a curved shape, suggesting that the segmental motion of polymer main-chain transported the ions.

In summary, we conclude that the high t
Li
+ for PCEO electrolyte is mainly due to its characteristic lithium coordination structure; lithium-ions are mostly surrounded by cyano groups and transported via the segmental motion of polymer main-chain in PCEO electrolytes. The results emphasize the importance of the coordination structure on the characteristics of the electrolyte, which can provide additional knobs to improve the ionic conductivity as well as lithium transference number, leading further improvements in the performance of polymer electrolytes.