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.

A Moisture‐Assisted Rechargeable Mg−CO2 Battery

“…By contrast, MgCO 3 ⋅3 H 2 O has a closer contact with the CNT substrate of −0.67 eV, which enhances electron conduction during the charge process. Besides, C=O in MgCO 3 ⋅3 H 2 O is stretched to 1.46 Å under the influence of the CNT substrate and the H 2 O molecule in the structure, which would make MgCO 3 ⋅3 H 2 O easier to decompose (Figure 5c) [25] . As demonstrated by experimental observations and theoretical calculations, the Mg−CO 2 battery shows superior reversibility with H 2 O assistance.…”

Carbon Tube-Based Cathode for Li-CO2 Batteries: A Review

“…The electron transfer between the intermediate product and the Mo 2 C catalyst plays a crucial role in the stability of the discharge product and the electrochemical mechanism of the battery. Xia's group designed a water-in-salt electrolyte (LiTFSI/H 2 O 21.0 mol/1 kg) and studied the reaction mechanism of a Li-CO 2 battery composed of CNT and Mo 2 C/CNT cathodes [45]. Through a variety of in situ/non-in situ and qualitative/quantitative characterization analyses, the electrode based on Mo 2 C/CNT can realize the reversible conversion between CO 2 and Li 2 C 2 O 4 at a low charge-discharge overpotential.…”

Recent Advances in Rechargeable Li–CO2 Batteries

“…Collectively, all these data suggested that Li + concentration, Li + availability, and its coordination to neutral CO 2 are crucial to the discharge process, thereby inspiring a comprehensive design criteria for electrolyte ahead. Shortly before this, Xia’s group explored a water-in-salt (WIS, LiTFSI/H 2 O 21.0 mol/1 kg) electrolyte in the Li–CO 2 battery for the first time. On account of the wide potential window (2–5 V), high transference number of Li + (0.73), great CO 2 solubility, and rapid diffusion, the Li–CO 2 battery with CNT and Mo 2 C/CNT cathode showed better electrochemical performance.…”

“…Collectively, all these data suggested that Li + concentration, Li + availability, and its coordination to neutral CO 2 are crucial to the discharge process, thereby inspiring a comprehensive design criteria for electrolyte ahead. Shortly before this, Xia's group 147 8c,d). Such improved electrochemical performance at low temperatures was attributed to morphology differences under different temperatures, where small granular particles tended to loosely deposit on the cathode after discharge at −60 °C.…”