Investigation on the discharge and charge behaviors of Li-CO₂ batteries with carbon nanotube electrodes

Abstract

Li–CO2 batteries are regarded as promising electrochemical devices to simultaneously capture CO2 and deliver electric energy. Although efforts are made to exploring reaction routes and developing effective catalysts, the discharge and charge behaviors at different current densities and the intrinsic mechanisms are not reported. Herein, a Li–CO2 battery with a carbon nanotube electrode is investigated. It is found that with an increase of the current density, the discharge voltage plateau gradually decreases. After the initial charge polarization, the following charge process shows a two-stage charge voltage profile where the first stage is sensitive to the applied current density, whereas the second one is not. In addition, the electrode discharged at a lower current density has a higher voltage plateau of the first stage. The characterization results demonstrate that the discharge product is a combination of Li2CO3 and carbon in which crystalline Li2CO3 nanoparticles with the size of ∼5 nm are distributed. Upon charging, rich nanopores with the sizes of 5–10 nm are observed, which may come from the shrinkage of both crystalline and amorphous Li2CO3. Even at the end of charge, Li2CO3 and carbon remain on the electrode, resulting in the irreversible process. Thus, the first charge stage is proposed to be the decomposition of crystal and amorphous Li2CO3, whereas the second charge stage with a high voltage is attributed to the blockage of transport channels and the accumulation of side products. Furthermore, for the first charge stage, a low discharge current density leads to small sizes of crystalline Li2CO3 combining with amorphous carbon in the products, increasing the transport resistance and causing a high charge voltage. On the contrary, a high discharge current density results in large sizes of Li2CO3 crystals, improving the overall conductivity and leading to a low charge voltage.