Accelerating the Gelation of in-Situ Gel Polymer Electrolyte by Lithium Salts-Anion-like Initiator for Lithium-ion Batteries

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The demand for high-energy-density batteries is expanding with the rise of EV markets. Moreover, the benefits of solid-state electrolytes are emerging to achieve high energy density. However, the technology of solid-state electrolytes needs to mature further. In the present state, a gel polymer electrolytes (GPEs) could act as a bridge between liquid (LEs) and solid-state electrolytes (SSEs). GPEs under certain conditions can offer the advantages of both LEs and SSEs. Firstly, GPEs should be feasible to create a gel structure with minimal polymer content to ensure high ionic conductivity. Ideally, they should also exhibit a high lithium ion transference number. Secondly, GPEs should ensure safety from the risk of fire. It is essential not only to prioritize the non-flammability of polymer matrix but also to suppress the ignition of the LEs contained within GPEs. Lastly, in the current manufacturing process, the injection of liquid-state GPEs without additional steps should be possible. Also, in-situ gelation is necessary to mitigate interfacial problems. The non-flammable GPEs developed in the previous study satisfied above three conditions but had the disadvantage of long gelation time about one day at 60 oC. The gelation reaction was hindered by the bulky size of the functional groups substituted to enhance the non-flammability. Therefore, the amount of substituted functional groups was inevitably limited.In this study, fully substituted polymers were used to form the GPEs rapidly at room temperature or low temperatures (45 oC) with the addition of a lithium salts-anion-like initiator. The strong CN bonds were allowed to form carbocation due to easily ionized initiators. As a result, generated highly conjugated C=N-C bond great help to the formation of non-flammable polymer matrix compared with conjugated C=N-C bond from LiPF6. Furthermore, thermal degradation of the batteries was minimized because of rapid gelation at room temperature or lukewarm temperature. As the GPEs can undergo gelation without LiPF6, which was employed as an initiator in the previous method, stable operation at 60 oC or higher became possible. In the previous method, the decomposition of LipF6 into hydrogen fluoride at 60 oC resulted in a side reaction at the interphase between the electrolyte and the electrode. However, with the use of an initiator consisting of two anions instead of LiPF6, a stable lithium salt could be employed at high temperatures, ensuring stable operation. Moreover, the absence of hydroxyl groups enables the operation of a lithium metal battery that was previously inoperable when using PVA-CN. Due to the highly potent oxidizing power of the initiator, the gelation could rapidly progress even within a bulky substitute structure. Therefore, bulky flame-retardant structures could be fully incorporated into the polymer instead of the hydroxyl group (OH). Consequently, the side reaction of OH was eliminated. Specifically, the maximized presence of the fluorine functional group with radical scavenging ability ensured enhanced non-flammability of the entire electrolyte compared to previous studies.In summary, the addition of the initiator enabled rapid gelation with minimal energy consumption, facilitating the development of a lithium metal battery suitable for high-temperature operation. Furthermore, complete substitution of the functional group enhanced various functions and the flame retardancy of the electrolyte.