Electrolyte Selection Criteria for Electrochemical Conversion of CO2 in Aprotic, Li-CO2 Batteries

Abstract

The search for practical end-uses for post-combustion CO2 has generated significant research interest in CO2 utilization in non-aqueous storage and conversion devices, such as electrochemical reactors, as well as alkali metal-based O2/CO2 and -CO2 batteries. In nonaqueous systems, CO2 is typically converted to environmentally benign solids such as mineralized carbonates and/or carbon. Employing aprotic solvents for realizing room temperature electrochemical CO2-to-solids conversion reactions is particularly advantageous because it enables the use of highly reducing alkali/alkaline earth metals (typically Li) that yield large thermodynamic driving forces for the activation and conversion of CO2. Furthermore, the lack of protons suppresses parasitic side reactions such as hydrogen evolution, and their wide stability window enables access to a wide energy range over which to probe activity, which may be useful for designing new reaction pathways. Due to severe kinetic limitations, however, direct CO2 reduction, which entails electron-coupled formation of the CO2 anion radical intermediate,1 has been reported to have little to no activity in most common nonaqueous solvents.2 For example, past literature has shown that nearly all prior reports of Li-CO2 batteries have almost exclusively relied upon tetraethylene glycol dimethyl ether (TEGDME)-based electrolytes containing either 1 M lithium trifluoromethanesulfonate (LiCF3SO3)3-4 or 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)5-6 salts. While the choice of such electrolytes in Li-CO2 batteries appears to be critical, fundamental knowledge regarding why TEGDME-based electrolyte chemistries appear to be most promising for affecting direct electrochemical CO2 transformations largely remains lacking. In this work, we study the effect of varying electrolyte compositions on the discharge performance of conventional Li-CO2 batteries. This entails systematic investigations highlighting the role of key electrolyte parameters such as intrinsic solvent properties, and the concentration and composition of the electrolyte co-salt for facilitating CO2 discharge. To probe solvent-dependent CO2 activity, we perform cyclic voltammetry (CVs) and discharge measurements in a diverse set of standard battery-grade solvents (e.g.: TEDGME, DME, PC and DMSO) to investigate the role of physical solvent properties such as donor number (DN), dielectric constant, ionic conductivity, and viscosity for enabling CO2 reduction. CVs are also conducted in CO2-rich electrolytes with varying alkali cations to explore the cation-dependence of onset CO2 reduction potentials and elucidate the discharge reaction mechanism. Furthermore, we also study CO2 solubility as a function of the identity and concentration of the electrolyte co-salt anion in several TEGDME-based electrolytes to determine how electrolyte compositions modulate transport properties and overall reaction rates. Collectively, these data provide a mechanistic explanation for why certain electrolyte chemistries impart activity for electrochemical CO2 conversion while others do not, and help establish comprehensive design criteria to guide future electrolyte development for nonaqueous CO2 conversion.