Gel Electrolyte Comprising Solvate Ionic Liquid and Methyl Cellulose

Self-standing, high conductivity solid iongel electrolytes with good thermal stability (T_d ∼200 °C), mechanical properties, and an anodic stability up to 5 V were prepared using the solvate ionic liquid [G₄Li]⁺[TFSI]^- in methyl cellulose (MC) with 90 wt % liquid. The excellent combination of high room temperature conductivity (σ > 10^-4) and storage modulus (E′ ∼60 MPa) is due to the semicrystalline fibrillar network formed by the MC. The high anodic stability of 5 V for the iongel is attributed to the known 4.5 V stability of the [G₄Li]⁺[TFSI]^- and the -OH groups on MC, which hydrogen bond with any free G₄ not in the [G₄Li]⁺[TFSI]^- complex, so that they are unavailable for oxidation. Li⁰/iongel/Li⁰ plating/stripping at low current densities is stable with low polarization, but at high current densities the polarization and cell impedance increase, which is due to mosaic lithium plating. Coin cells of Li⁰/iongel/LiFePO₄ could be cycled at C/2 (1C = 170 mA h/g) for at least 100 cycles with 98-99% Coulombic efficiency and capacities of 125 mA h/g, comparable to that of the pure liquid [G₄Li]⁺[TFSI]^- cells.