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

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.