The spatiotemporal studies of the salt-hardening effect of the coacervates of nano-ions for aqueous super-ionic electrolytes with enhanced electrochemical stability

The application of aqueous electrolytes is generally limited by their narrow electrochemical window (ECW), which sets an intrinsic limit on the practical voltage and energy density of fabricated energy storage devices. Herein, the coacervates of nano-ions are proposed as aqueous lithium-ion electrolytes with both high ion conductivity and broadened ECW. The hybrid complex coacervates of 1 nm nano-anions, metatungstate ([W₁₂O₄₀]⁸⁻), and polyethylene glycol (PEG) are studied for their unique salt-hardening effect. The framework of random percolated nano-ions interweaved by PEG chains is demonstrated from small angle X-ray/neutron scattering (SAXS/SANS) and micro-rheology measurements. The introduced extra Li⁺ can complex with the PEG backbone for strengthened PEG/nano-ion interaction. The relaxation dynamics follows the model of sticky reptation by treating the binding of Li⁺ as effective stickers and this leads to the observed salt hardening effect, e.g., enhanced viscosity with the increased LiCl concentrations. This hybrid coacervate exhibits excellent ionic conductivity (∼0.027 S/cm), and the super-ionic nature of ion conduction is demonstrated employing fractional Walden rule. Suggested from diffusion ordered spectroscopy studies, the water molecules confined in the framework of the coacervates show hindered diffusive dynamics and this contributes to the extended ECW (∼2 V), demonstrating its potential application as aqueous lithium electrolytes.