Solid polymer electrolytes (SPEs) have been playing a crucial role in the development of a high-performance solid-state lithium metal battery. The safety and the easy tailoring of the polymers designate these materials as promising candidates to be implemented as electrolytes. Poly(ethylene oxide) (PEO) has been widely employed during the past four decades, but its inferior electrochemical stability against high-voltage cathode active materials strongly urges the search for alternative polymers. In recent years, several carbonyl-containing polymers have arisen as possible replacements for PEO, with poly(ε-caprolactone) (PCL) being one of the most representative. In this work, we combine molecular dynamics simulations and a range of experimental measurements to gain in-depth insights into the ionic transport in polyester-based SPEs. Specifically, the physicochemical properties and morphological behaviors of the blend SPEs comprising PEO and PCL including the two end members are comprehensively investigated. The results reveal that the preferential coordination between Li + cation and ethylene oxide units and partial phase separation between PEO and PCL control the ionic transport in PEO and PCL blends. The present study is believed to inspire novel design strategies for improving the properties of SPEs and batteries made from them.

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Polymer electrolytes (PEs) with excellent flexibility, processability, and good contact with lithium metal (Li°) anodes have attracted substantial attention in both academic and industrial settings. However, conventional poly­(ethylene oxide) (PEO)-based PEs suffer from a low lithium-ion transference number (T Li +), leading to a notorious concentration gradient and internal cell polarization. Here, we report two kinds of highly lithium-ion conductive and solvent-free PEs using the benzene-based lithium salts, lithium (benzenesulfonyl)­(trifluoromethanesulfonyl)­imide (LiBTFSI) and lithium (2,4,6-triisopropyl­benzenesulfonyl)­(trifluoromethanesulfonyl)­imide (LiTPBTFSI), which show significantly improved T Li + and selective lithium-ion conductivity. Using molecular dynamics simulations, we pinpoint the strong π–π stacking interaction between pairs of benzene-based anions as the cause of this improvement. In addition, we show that Li°∥Li° and Li°∥LiFePO4 cells with the LiBTFSI/PEO electrolytes present enhanced cycling performance. By considering π–π stacking interactions as a new molecular-level design route of salts for electrolyte, this work provides an efficient and facile novel strategy for attaining highly selective lithium-ion conductive PEs.