Deep eutectic solvents, a versatile alternative to ionic liquids in ionogel-like electrolytes

Abstract

The automotive industry is investing heavily in the vehicle electrification. At the center of this revolution is the energy carrier, no longer combustible fuels but a battery capable of a high action radius. The Li-ion battery technology is at the forefront due to its high volumetric and gravimetric energy density. However, increasing the action radius, safety and reliability of electric vehicles requires further development of these chemistries. For instance, replacing the typical graphite anode with a lithium metal anode, increases the capacity of the battery significantly. Unfortunately, solid electrolytes; e.g. LISICON, garnets, perovskites; are needed to counteract the hazards of rechargeable lithium metal batteries. The aforementioned solid state electrolytes suffer from low ionic conductivity (<10-4 mS/cm), interfacial resistances, and difficulty of implementing these in conventional battery manufacturing, thereby impeding their economic viability. [1] Hybrid solid-state electrolytes address these issues and allow for an easier implementation into conventional battery technology. The hybrid solid-state electrolyte combines the desirable properties of both liquid and solid electrolytes by confining the liquid electrolyte within a (meso-)porous solid framework. [2,3] Prime examples are impregnated gel polymer electrolytes and ionogels/solid composite electrolytes/ionobrids. The latter consist of ionic liquid electrolytes (e.g. BMIMTFSI + LiTFSI) confined within a porous (hybrid) silica framework. [4] Unfortunately these ionic liquids are very costly, which impedes their implementation in commercial batteries. In this work, the costly ionic liquid is replaced by a deep eutectic solvent. The latter has proven to be a versatile and cheap alternative to the expensive ionic liquids for lithium ion batteries and capacitors. In this work, we demonstrate the potential of these deep eutectic solvents in hybrid solid-state electrolytes next to their use as liquid electrolytes. A series of hybrid electrolytes is synthesized in a facile one-pot sol-gel route at room temperature and investigated for their electrochemical properties. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) reveal a beneficial anodic stability limit of up to 4.8 V vs. Li + /Li while electrochemical impedance spectroscopy (EIS) reveals a high ionic conductivity of up to 1.15 mS/cm depending on composition. Demonstrator cells with LiFePO4 (LFP) display reversible capacities and remain stable for over 100 cycles. By using these deep eutectic solvents, the costs of these hybrid solid-state electrolytes could be lowered significantly.