Importance of Lithium Coordination Structure to Lithium-Ion Transport in Polyether Electrolytes with Cyanoethoxy Side Chains: An Experimental and Theoretical Approach

Polymer electrolytes (PEs) have been studied as an alternative to the current liquid electrolytes in lithium-ion batteries. Although polyether electrolytes have been developed for more than decades, these electrolytes have limitations such as low ionic conductivity and a small lithium-ion (t_Li⁺) transference number. In this work, we combine spectro­(electro)­chemical analyses with molecular dynamics (MD) simulations to understand the complex interaction within the electrolyte, consisting of a polyether having both ether and cyano groups (poly­(3-(2-cyanoethoxymethyl)-3-ethyloxetane), PCEO) mixed with various concentrations of lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI), to clarify the Li⁺ coordination structure as well as its relevance to Li⁺ conductivity. Applicability of MD simulations was validated by high-energy X-ray total scattering measurements. The local coordination structures around Li⁺ were successfully estimated by the distribution function obtained from MD simulations, which suggested the preferable coordination of the cyano group with Li⁺ over the other elements, including ether oxygen. Further support came from infrared (IR) spectroscopy, where the estimated coordination number (N) obtained from the IR peak area of the deconvoluted CN stretching vibration (ca. 2250–2280 cm^–1) agreed well with the MD result. Arrhenius plots of the ionic conductivity showed a curved shape, indicating that the segmental motion of the polymer main chain was responsible for Li⁺ transportation in PCEO electrolytes. The Li⁺ conductivity varied with the salt concentration and was sensitive to the Li⁺ coordination structure. The highest Li⁺ conductivity was achieved at an intermediate salt concentration, where Li⁺ coordinated mostly by the cyano group (N = 2.2), followed by the TFSI anion (N = 1.3), and only a small contribution was from ether oxygen (N = 0.5). The characteristic cocontribution of a cyano group and ether oxygen to the Li⁺ coordination structure can be responsible for the improved Li⁺ conduction, by accelerating the interchain Li⁺ transfer, involving a decoordination process, (short-range Li⁺ conduction) while maintaining good segmental mobility of the polymer (long-range Li⁺ conduction). The results emphasize the importance of the coordination structure to the electrolyte property, which can provide additional knobs to improve the ionic conductivity as well as the Li⁺ transference number, leading to further improvements in the performance of polymer electrolytes.