Insights into the Development and Performances of Ionogel-Based Electrolytes for Solid-State Lithium-Based Batteries

Solid-state electrolytes (SSEs) aim to overcome safety issues, decrease package volume, and increase the energy density of next-generation energy storage devices, such as Li-ion and post-lithium batteries. Among different types of SSEs, the ionogel (or ion gel) systems, in which polymer electrolytes are doped with room-temperature ionic liquids (RTILs), are amongst the most attractive choices to reach ionic conductivity values comparable to those of liquid electrolytes, ensuring better safety and electrode/electrolyte interface stability [1]. In our previous studies, several ionogel samples were prepared by in-situ UV-curing [2]. Here, we present the results of newly optimized methacrylate-based solid-state electrolyte systems conceived for ambient temperature cycling with high-energy cathodes. We characterized the electrolyte formulations using various physico-chemical and electrochemical methods, including gel content, FTIR, rheology, DMTA, TGA, SEM, voltammetry, impedance spectroscopy, and galvanostatic cycling. The focus is particularly on the influence of using two different RTILs as reaction media on the properties of the resulting materials and their electrochemical behaviours. The achieved results indicate that viscosity affects the polymerization kinetics of the ionogels, which in turn affects their thermal stability and galvanostatic cycling behavior. The obtained ready-to-use and self-standing crosslinked ionogel electrolyte membranes (~100 µm thickness) showed relatively high ionic conductivity (0.8 to 2 mS/cm at 20 °C) and anodic stability (up to 4.5-5 V vs. Li). Lithium lab-scale cells were assembled with ionogel-polymerized-on-NMC-cathodes for galvanostatic cycling measurements, and they demonstrated initial discharge capacities of around 180 mAh/g at low C-rate in the voltage range of 3.0-4.3 V at room temperature. Furthermore, motivated by the demand of increasing the overall performance of the SSEs, different types of composite solid electrolytes are now under our investigation and further development, including the utilization of active and inert inorganic fillers. For example, we introduced LLZO ceramic microparticles into the aforementioned ionogel systems for fabricating composite SSEs via in-situ photopolymerization. So far, the best achieved preliminary cycling results show enhanced and more stable cycling behaviour at higher C-rates up to 0.2C at room temperature, suggesting the promising prospects of our composite SSEs and their potential contribution to the fabrication of solid-state, high-energy lithium-based batteries.