The properties of polymer hosts and their interactions with lithium salt are critically important for the design of alternative polymer electrolytes for safe lithium battery applications. Herein, we report a poly(ionic liquid) host as a solid electrolyte platform and propose a different coordination mechanism manipulating lithiumion diffusion compared with traditional polymer systems. Our finding provides a new strategy to develop high-performance polymer electrolytes for nextgeneration high-energy-density lithium-metal batteries.

Electrolytes of a room temperature ionic liquid (RTIL), trimethyl(isobutyl)phosphonium (P111i4) bis(fluorosulfonyl)imide (FSI) with a wide range of lithium bis(fluorosulfonyl)imide (LiFSI) salt concentrations (up to 3.8 mol kg(-1) of salt in the RTIL) were characterised using a combination of techniques including viscosity, conductivity, differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), nuclear magnetic resonance (NMR) and cyclic voltammetry (CV). We show that the FSI-based electrolyte containing a high salt concentration (e.g. 1 : 1 salt to IL molar ratio, equivalent to 3.2 mol kg(-1) of LiFSI) displays unusual transport behavior with respect to lithium ion mobility and promising electrochemical behavior, despite an increase in viscosity. These electrolytes could compete with the more traditionally studied nitrogen-based ionic liquids (ILs) in lithium battery applications.

Poly(ionic liquids)-based gel polymer electrolytes containing high lithium salt concentration ionic liquids are demonstrated.

The chemical composition of the solid electrolyte interphase (SEI) layer formed on the surface of lithium metal electrodes cycled in phosphonium bis(fluorosulfonyl)imide ionic liquid (IL) electrolytes are characterized by magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray photoelectron spectroscopy (XPS), fourier transformed infrared spectroscopy, and electrochemical impedance spectroscopy. A multiphase layered structure is revealed, which is shown to remain relatively unchanged during extended cycling (up to 250 cycles at 1.5 mA·cm, 3 mA h·cm, 50 °C). The main components detected by MAS NMR and XPS after several hundreds of cycles are LiF and breakdown products from the bis(fluorosulfonyl)imide anion including LiS. Similarities in chemical composition are observed in the case of the dilute (0.5 mol·kg of Li salt in IL) and the highly concentrated (3.8 mol·kg of Li salt in IL) electrolyte during cycling. The concentrated system is found to promote the formation of a thicker and more uniform SEI with larger amounts of reduced species from the anion. These SEI features are thought to facilitate more stable and efficient Li cycling and a reduced tendency for dendrite formation.

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In this study the performance of the lithium (Li) anode is characterized in two alternative ionic liquid electrolytes: (i) a solution of 0.5 mol·kg–1 of lithium bis­(fluorosulfonyl)­imide (LiFSI) in trimethyl­(isobutyl)­phosphonium FSI (P111i4FSI) and (ii) an equimolar mixture of these two salts, effectively an inorganic–organic mixture IL. We have investigated the formation of the solid electrolyte interphase (SEI) at the lithium electrode and its influence on the polarization potential, the electrode surface impedance and deposition morphologies. Lithium metal cycling is revealed to be significantly more stable in the electrolyte with high lithium salt concentration due to the creation of a more uniform SEI. Stable and effective cycling was demonstrated at high applied currents (up to 12 mA·cm–2) with large areal capacities being transferred with each polarization cycle (up to 6 mAh·cm–2 at 50 °C). An average Coulombic efficiency of not less than 99.2% was demonstrated under these conditions and SEM observations of the cycled electrode surfaces show a uniform and compact deposit. Combined with spectroscopic characterization of the electrolyte and electrode surface, these observations indicate a role for the speciation and transport properties of these high concentration ionic liquid electrolytes in modifiying the physicochemical properties of the SEI which result in enhanced cycling performance of the Li metal electrode.

Solid-state electrolytes with mechanical integrity and high ionic conductivity are important components in high performance all-solid-state lithium (Li) batteries. Relative to these electrolytes, ionic liquid-based composite polymer electrolytes exhibit high ionic conductivity and improved safety. However, the incorporation of large concentration of nonactive ions and the presence of a liquid phase lead to relatively low Li + transference number and poor mechanical properties. In this study, poly(ionic liquid)s or polymerized ionic liquids (polyILs) are combined with an electrospun fibrous support to afford electrolytes with high ionic liquid content and greatly increased Li + transference number. The incorporation of electrospun PVDF nanofibers effectively improves the mechanical strength of the composite polymer electrolytes, and consequently, flexible electrolytes with superior mechanical properties are described. Finally, we demonstrate the performance of high energy density Li metal batteries under high-voltage operation (up to 4.5 V) using LiNiMnCoO 2 (NMC) and LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) cathodes with an areal capacity up to 1.1 mAh cm −2 .

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