NSRRC Activity Report 2022

Chemical Science 023 T he development of portable devices has led to the pursuit of high-energy-density storage systems. To date, Li-ion batteries (LIB) have predominantly been used in energy storage systems as they provide considerable energy density with a high energy efficiency of approximately 90%. To replace LIBs, it has been proposed that the graphite anode could be replaced with Li metal. It is notable that Li metal can provide a capacity that is 10 times larger than that of the graphite anode, thus boosting the battery’s overall energy density. On the cathode side, CO 2 has been proposed to be a suitable candidate as its redox potential (2.8 V vs. Li/Li + ) is comparable to that of the LiFePO 4 cathode (3.4 V vs. Li/Li + ), with the former also weighing less than the LiFePO 4 cathode. When a cell discharges, CO 2 is reduced to C according to the following cell equation: 4Li + 3CO 2 → 2Li 2 CO 3 + C. Moreover, the use of CO 2 in the battery system indicates the possibility of CO 2 utilization. Combining these two novel electrodes, a Li-CO 2 battery with an energy density of 1876 Wh/kg can be developed. Figure 1 shows a typical Li-CO 2 battery in which coin cells with holes that permit CO 2 flow are sealed into a glass bottle filled with CO 2 for electrochemical tests. However, Li-CO 2 batteries faces problems such as a high discharge/charge overpotential due to their sluggish kinetics. The discharge product, Li 2 CO 3 , is an electric and ionic insulator, which results in a high charge overpotential. When discharging, the intrinsically stable CO 2 must be reduced. In the commonly applied aprotic environment, CO 2 can hardly be reduced, which results in a high discharge overpotential. As a result, an effective electrocatalyst is required to not only reduce the overpotential but also increase the energy efficiency. The teams of Ru-Shi Liu (National Taiwan University) and Jin-Ming Chen (NSRRC) recently studied the reaction mechanism of Li-CO 2 batteries using soft X-ray absorption spectroscopy (sXAS) at TLS 20A1 . 1 Considerable information can be obtained as changes in the reactants can be probed using C K-edge XAS. The team found that the major discharge product is Li 2 CO 3 with no C signals observed. This observation held irrespective of Fig. 1 : Configuration of a typical Li-CO 2 battery. [Reproduced from Ref. 1] Fig. 2 : Soft X-ray spectra of (a,c) Pt and (b,d) carbon nanotube cathodes at different discharge states with different treatments. The H 2 O-washed discharged cathodes (red lines) resemble their pristine (brown lines) counterparts, indicating the absence of C. [Reproduced from Ref. 1] Utilizing CO 2 in an Energy Storage System Li-CO 2 batteries with high energy efficiency are key to CO 2 utilization. Sluggish kinetics limit their practical application, but electrocatalysts may be a solution to this. whether the cells were discharged to 2 V or 1 V, indicating that C was not formed throughout the discharge process, which was contrary to most reports on Li-CO 2 batteries. The team claimed that the reaction should instead be 2Li + 2CO 2 → Li 2 CO 3 + CO, where CO could be detected