Mechanism-of-Action Elucidation of Reversible Li–CO2 Batteries Using the Water-in-Salt Electrolyte

Abstract

Li–CO₂ batteries have attracted worldwide attention because of their dual characteristics of high energy density and effective CO₂ capture. However, the basic electrochemistry mechanism involved has been unclear, which is mainly confused by the complicated decomposition of organic electrolytes. Herein, water-in-salt (WIS, LiTFSI/H₂O 21.0 mol/1 kg) has been explored as a suitable electrolyte for the first time to investigate the reaction mechanism of Li–CO₂ batteries with different cathodes (carbon nanotube (CNT) and Mo₂C/CNT, respectively). An Mo₂C-based Li–CO₂ battery with WIS delivers a higher energy efficiency of 83% and a superior cyclability, compared to those of the CNT-based counterpart cell. Through various ex/in situ qualitative/quantitative characterizations, the Mo₂C-based Li–CO₂ battery with WIS can operate on the reversible conversion of CO₂-to-Li₂C₂O₄ ((e^–/CO₂)_ideal = 1) at lower discharge/charge overpotentials, while the CNT-based counterpart cell is based on the formation/decomposition of Li₂CO₃ ((e^–/CO₂)_ideal ≈ 1.33) at high overpotentials. Such a difference in CO₂ reduction products stems from the stronger interaction between Mo₂C(101) and Li₂C₂O₄ than that of the CNT and Li₂C₂O₄ based on the density functional theory calculations, resulting in the selective stabilization of the intermediate product Li₂C₂O₄ on the Mo₂C surface.