High Rate Capability of All-Solid-State Lithium Batteries Using Quasi-Solid-State Electrolytes Containing Ionic Liquids

Here we demonstrate a high rate capability of all-solid-state lithium batteries using quasi-solid-state electrolytes containing an ionic liquid. We fabricated solid-state electrolyte using an ionic liquid: 1 mol l^-1 lithium bis(fluorosulfonyl) imide dissolved 1-Ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (LiFSI/EMI-FSI) and fumed silica nanoparticles with a variety of volume fractions. The fabricated freestanding film with 85% volume fraction of LiFSI/EMI-FSI exhibited an ionic conductivity and self-diffusion coefficient of lithium-containing species; 10.2 mS cm^-1 and 3.3 × 10^-11 m² s^-1 at 35 °C. We revealed that the increase in the volume fraction of the LiFSI/EMI-FSI led to the decrease in concentration polarization resistance, leading to an enhanced rate capability in Li|LiFePO₄ batteries. The fabricated Li|LiFePO₄ batteries using freestanding electrolyte films with 85 vol% LiFSI/EMI-FSI exhibited a high capacity (>150 mAh g^-1) at 1 C (0.6 mA cm^-2) based on that at 0.1 C. Further, we fabricated bipolar-type all-solid-state lithium batteries assembled by stacking of Li|LiFePO₄ cell components in a single package. The bipolar-type lithium batteries exhibited the increased packing energy density, depending on the number of stacked cells. These results open opportunities of designing all-solid-state lithium batteries for high energy and power density using quasi-solid-state electrolytes.