Design of Poly(ethylene oxide)-Based Polymer-in-High Concentrated Ionic Liquid Electrolyte for All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes (SPEs) have emerged as promising candidates for solid-state lithium metal batteries (LMBs) due to their inherent safety advantages and potential to facilitate high energy density devices [1]. Poly(ethylene oxide) (PEO) has been the most prominent representative polymer host in SPEs since 1970s because of their excellent ability to solvate Li ions and support ion transport [2,3]. However, it suffers from limited oxidation stability (<4 V vs. Li) and low ambient temperature conductivity (up to 10−6 S cm−1), thus hindering its practical application in high voltage (>4V) high energy density LMBs. In this study, a new type of polymer-in-“high concentrated ionic liquid” solid electrolyte is designed with high molecular weight PEO, N-propyl-N-methylpyrrolidinium bis(fuorosulfonyl) imide (C3mpyrFSI) ionic liquid and lithium bis(fluorosulfonyl)imide (LiFSI). The EO: [LiFSI/C3mpyrFSI] ratio has been widely varied and the resulting physicochemical and electrochemical properties have been explored. The optimal electrolyte provides promisingly high oxidative stability exceeding 5V and ambient temperature conductivity of 5.6 x10-4 S cm-1. It induces stable solid electrolyte interface with Li metal and demonstrates prolonged reversible Li plating-stripping in Li symmetrical cells even at high temperatures (up to 70°C). All solid-state Li metal cells assembled with LFP cathode show excellent rate capability and cycling performance (see Figure 1). The discharge capacity can be maintained at 47.3% when the current density increases 20 times from 0.1 C to 2 C (areal capacity ~0.4 mAh/cm2), and long-term cycling of Li||LFP cells showed 97% capacity retention after 100 cycles at 0.2 C. Based on the findings, an ion conduction network consisting of interconnected anion-rich clusters and pores is highlighted to help understand the structural change, ion dynamics and good battery performance of the designed SPE. Therefore, the novel approach of polymer-in-“high concentrated ionic liquid” solid electrolyte enables a new pathway to design high-performing SPEs for high energy density all-solid-state LMBs. Figure 1