Improved Lithium-Ion Transport Property in Concentrated Polyether Electrolytes with Cyano Groups

Lithium-ion batteries (LIBs) are currently the most comprehensive secondary battery technologies due to their high energy density and long cycle life ^[1]. However, LIBs still have safety issues such as flammability or leakage due to the use of liquid electrolytes. Polymer-based solid electrolytes thus have attracted much attention owing to its flame retardancy and structural stability. However, its low ionic conductivity limits its practical use ^[2]. The typical strategy for improving ionic conductivity in polyether (PEO)-based electrolyte is increasing the number of ion carriers by increasing salt concentration. Ionic conductivity for the polyether-based electrolytes heavily depends on the segmental mobility of the polymer, which is known to decrease under high salt concentration due to the strong interaction between Li⁺ and an ether oxygen atom ^[3]. On the other hand, it is also known that the segmental mobility increases with the increase in salt concentration for polymers with cyano groups (e.g., PAN ^[4]).However, PAN showed inferior ion transport property than PEO, mainly because of the low segmental mobility (T_g = 125ºC) of PAN than PEO (T_g = –64ºC). Therefore, it is crucial to develop polymer electrolytes having both the salt concentration dependence of PAN and the low T_g of PEO, to achieve high ionic conductivity.

In this study, we focused on the cyanoethoxy group, having both ether oxygen and cyano (CN) group, and clarify the effect of the cyanoethoxy groups on the electrolyte properties, in order to establish the design strategy for improving ionic conductivity. A series of polymers having different numbers (0, 1, 2) of cyanoethoxy groups per unit were synthesized. The glass transition temperature (T_g), which was often used as a descriptor of the segmental mobility, was obtained by the differential scanning calorimetry (DSC). It was confirmed that the segmental mobility was improved with the increase in the number of introduced cyano groups. One of the possible explanations for this phenomenon is the difference in the free volume of the coordination structure; a free volume of the Li⁺–CN coordination structure can be larger than that of Li⁺-ether counterparts, considering the characteristic linear coordination structure of Li⁺–CN. Infrared (IR) spectroscopy was performed to further understand the intermolecular interaction. The result revealed that the Li⁺–CN interaction was promoted by introducing cyanoethoxy groups. Electrochemical impedance spectroscopy confirmed that electrolyte having the largest number of cyanoethoxy groups in the repeating unit showed the highest ionic conductivity under high salt concentration (CN/Li⁺ = 0.4), as well as the most significant transference number of Li⁺ (t_Li = 0.90). Therefore, we conclude that the introduction of the cyanoethoxy groups into the polymer sidechain improves the ion transport property, mainly due to the improved segmental mobility and the optimized Li⁺–CN interaction. The introduction of multiple cyanoethoxy groups is thus one of the promising strategies to improve the ion transport property, by enabling both the salt concentration dependence of PAN and the segmental mobility of PEO.

[1] Julien, C. et al., Lithium batteries, Springer: New York, 2015; pp 431–440.

[2] Xue, Z. et al., J. Mater. Chem. A, 2015, 3, 19218–19253.

[3] Perrier, M. et al., Electrochim. Acta, 1995, 40, 2123–2129.

[4] Florjanczyk, Z. et al., J. Phys. Chem., 2004, 108, 14907–14914.