Understanding Reaction Pathways in High Dielectric Electrolytes Using β-Mo₂C as a Catalyst for Li–CO₂ Batteries

Abstract

The rechargeable Li–CO2 battery has attracted considerable attention in recent years because of its carbon dioxide (CO2) utilization and because it represents a practical Li–air battery. As with other battery systems such as the Li-ion, Li–O2, and Li–S battery systems, understanding the reaction pathway is the first step to achieving high battery performance because the performance is strongly affected by reaction intermediates. Despite intensive efforts in this area, the effect of material parameters (e.g., the electrolyte, the cathode, and the catalyst) on the reaction pathway in Li–CO2 batteries is not yet fully understood. Here, we show for the first time that the discharge reaction pathway of a Li–CO2 battery composed of graphene nanoplatelets/beta phase of molybdenum carbide (GNPs/β-Mo2C) is strongly influenced by the dielectric constant of its electrolyte. Calculations using the continuum solvents model show that the energy of adsorption of oxalate (C2O4 2–) onto Mo2C under the low-dielectric electrolyte tetraethylene glycol dimethyl ether is lower than that under the high-dielectric electrolyte N,N-dimethylacetamide (DMA), indicating that the electrolyte plays a critical role in determining the reaction pathway. The experimental results show that under the high-dielectric DMA electrolyte, the formation of lithium carbonate (Li2CO3) as a discharge product is favorable because of the instability of the oxalate species, confirming that the dielectric properties of the electrolyte play an important role in the formation of the discharge product. The resulting Li–CO2 battery exhibits improved battery performance, including a reduced overpotential and a remarkable discharge capacity as high as 14,000 mA h g–1 because of its lower internal resistance. We believe that this work provides insights for the design of Li–CO2 batteries with enhanced performance for practical Li–air battery applications.

Recent Advances in Rechargeable Li–CO2 Batteries

“…In addition, improved boiling points and low vapor pressure are imperative. Researchers have performed a train of studies about the effects of diverse solvents in aprotic electrolytes. − However, current liquid electrolytes still suffer from decomposition under high voltage together with leakage risks and volatilization . Some redox mediators (RMs), ,− additives, polymer electrolytes, ,, and solid-state electrolytes , have been proposed to modify or replace traditional electrolytes.…”

“…Copyright 2019 American Chemical Society. (b) Reprinted with permission from ref . Copyright 2020 American Chemical Society.…”

“…Recently, Jung et al reported that the discharge reaction mechanism of the Li–CO 2 battery with the graphene nanoplatelets/beta phase of molybdenum carbide (GNPs/β-Mo 2 C) cathode depends on the dielectric constant of the electrolyte. As presented in Figure b, the adsorption energy of C 2 O 4 2– was −0.028 eV in TEGDME and achieved +1.168 eV in high-dielectric N , N -dimethyl­acetamide (DMA), demonstrating that DMA destabilized C 2 O 4 2– intermediates on the Mo 2 C cathode, which was inconsistent with the previous report .…”

Ultrafine Co-Doped NiO Nanoparticles Decorated on Carbon Nanotubes Improving the Electrochemical Performance and Cycling Stability of Li–CO2 Batteries

“…Carbon dioxide (CO 2 ) as the dominating greenhouse gas is accelerating environmental warming. Nevertheless, CO 2 chemical fixation strategies transforming CO 2 into chemicals (i.e., methanol, ethylene, formic acid, and carbon monoxide) require a large consumption of additional energy to activate the highest oxidation state of the carbon element rested in CO 2 . − Li–CO 2 batteries significantly are green electrochemical energy storage systems powered by CO 2 , forwarding the realization of carbon neutrality and peak CO 2 emission in an environmentally friendly way. − Rechargeable Li–CO 2 batteries can offer a high theoretical energy density of up to 1876 W h kg –1 according to the electrochemical reaction 4Li + 3CO 2 ↔ 2Li 2 CO 3 +C. , The main discharge product, Li 2 CO 3 , is a wide-band gap insulator with highly thermodynamic stability. , For this reason, several scientific issues should be solved on the way to achieve good electrochemical performance for practical applications. ,, The most significant one is the large overpotential during the CO 2 reduction and evolution reactions (CO 2 RR and CO 2 ER), , resulting in the low energy efficiency and short cycling life owing to severe side reactions. , To this end, developing highly effective electrocatalysts capable of improving the electrochemical reaction kinetics, the voltage polarization, and the cycling performance toward the sluggish oxidation and reduction processes is greatly necessary.…”

“…4−6 Rechargeable Li−CO 2 batteries can offer a high theoretical energy density of up to 1876 W h kg −1 according to the electrochemical reaction 4Li + 3CO 2 ↔ 2Li 2 CO 3 +C. 7,8 The main discharge product, Li 2 CO 3 , is a wide-band gap insulator with highly thermodynamic stability. 9,10 For this reason, several scientific issues should be solved on the way to achieve good electrochemical performance for practical applications.…”