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.

Polymeric materials are considered as promising electrolytes for all-solid-state secondary lithium batteries with superior energy and power densities, long cycle lives, and safety. To further improve the ionic conductivity of...

Although solid-state polymer electrolytes (SPE) have attracted much attention as a safe electrolyte material for lithium-ion batteries (LIB), one major limitation of SPE resides in the low ionic conductivity[1]. Previous study reported the improvement of ionic conductivity by adding inorganic particles into polymer electrolytes[2]. However, a molecular-level understanding of its effect on the Li+ conduction mechanism is still lacking. In this study, we select polyether having cyanoethoxy sidechain as a polymer matrix, and clarify the effect of TiO2 nanoparticles on the Li+ conduction mechanism. Differential scanning calorimetry and thermogravimetric analysis confirmed that glass transition temperature (T g) and the thermal stability of electrolytes were not affected by TiO2. On the other hand, electrolyte with TiO2 showed higher ionic conductivity than the electrolyte without TiO2. Various (electro)chemical and spectroscopic analyses were performed to clarify the key descriptor for the ionic conductivity. Infrared spectroscopy confirmed that the Li+–CN interaction was not affected by the existence of TiO2. However, it also suggested two types of the dissociated anion (free anion and anion interacting with the surface hydroxyl group on TiO2) in the electrolyte with TiO2. Based on these results, observed improvement of Li+ conduction for TiO2-contained electrolytes can be due to the trapping of bulky anion by surface hydroxyl groups on TiO2. In conclusion, we clarify that the anion trapping by the surface hydroxyl group on TiO2 nanoparticles could facilitate Li+ conduction, probably due to the absence of bulky anion, which sterically inhibits Li+ transportation. The surface hydroxyl groups on inorganic fillers thus play a vital role in facilitating Li+ conduction. [1] X. Zhigang et al., J. Matter. Chem. A, 2015, 3, 19218–19253. [2] F. Croce et al., Nature, 1998, 394, 456–458.

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.