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A large number of applications such as mobile electronics and electric vehicles requires rechargeable batteries with high energy density and enhanced safety. To achieve these goals, lithium metal batteries employing solid-state electrolytes have become common despite the safety concerns associated with lithium metal. Polymer electrolytes have been studied as a solution for enhancing the safety of lithium metal batteries because they are non-volatile, non-flammable, and suppress the growth of lithium dendrites. In this study, highly elastic polyurethane (PU)-based polymer electrolytes were prepared in the form of thin flexible films, and their electrochemical characteristics were investigated. To improve the ionic conductivity, non-volatile and non-flammable 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide was added as a plasticizing additive to the polymer electrolyte. The cell assembled using a Li anode, PU-based elastomeric polymer electrolyte and composite LiNi0.6Co0.2Mn0.2O2 cathode exhibited stable cycling performance by suppressing the growth of lithium dendrites as well as maintaining good interfacial contacts between electrolyte and electrodes during repeated cycling.

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Since the use of lithium metal ensures a high specific capacity of 3,862 mAh g-1,the lithium metal batteries can be one of the attractive candidates for achieving high energy density. In order to obtain good cycling stability and enhanced safety, it is crucial to suppress the lithium dendrite formation on the lithium metal surface and reduce the interfacial resistance between the electrode and electrolyte. According to previous studies, there are several approaches to address the above issues; formation of protective layer on the lithium surface, morphology control of the lithium metal and use of additives. As the protective layer, the polymer-based thin layer has a critical drawback of poor mechanical strength, and thus addition of inorganic particles can be a solution to enhance its mechanical strength. In this work, we applied a composite protective layer composed of ion-conductive polymer and inorganic particles to all-solid-state lithium batteries, which effectively suppressed the lithium dendrite and relieved the interfacial resistances during cycling, resulted in improvement of the cycling stability of lithium metal batteries.