Thermotropic Ionic Liquid Crystalline Polymers for Lithium-Ion Battery Electrolytes

Due to their great success for powering portable electronic devices, lithium-ion batteries are presently also becoming increasingly important for (hybrid) electric vehicles and stationary energy storage. One major issue towards their implementation in such large-scale applications, however, concerns their safety. Commonly employed liquid organic electrolytes comprising, e.g., LiPF₆ as conducting salt entail the risks of high flammability, leakage, thermal instability, sensitivity towards hydrolysis, and high toxicity of the resulting decomposition products.^1,2 Thus, solid electrolytes are at present considered as the more promising approach.^3–5 Nonetheless, the state-of-the-art polyethylene oxide-based electrolytes suffer from insufficient ionic conductivities (10^-4-10^-5 S cm^-1) and polymer gel electrolytes (inactive polymer host comprising a conducting liquid electrolyte) do not really solve the aforementioned safety issues.^4,6 Recently, alternative strategies have emerged to enhance both chemical and electrochemical properties: block copolymer ionomers, for instance, provide the great advantage of being single-ion conductors (transference number = 1).^7,8 Also, another class of electrolytes, thermotropic ionic liquid crystals comprising a lithium salt, is currently attracting an increasing attention; basically for two reasons: tunable self-assembly and very high ionic conductivities of up to about 10^‑3 S cm^-1.^9–12

In an attempt to combine the properties of thermotropic ionic liquid crystals and polymers, we have developed a new class of single ion-conducting thermotropic ionic liquid crystalline polymers (TILCPs) (2 patents in progress, Figure 1), which potentially allow superior Li⁺ conductivities, thus, enabling the realization of high performance and safer lithium-ion batteries. Herein, we will present their morphological, structural, and electrochemical characterization, highlighting the interplay of nanostructure and ionic conductivity. Additionally, we will present our first results regarding their utilization as electrolyte in lithium-ion batteries.

Figure 1. General chemical formula of TILCPs.

References

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