context: The charging process of the Li–CO2 battery was also studied systematically (Fig. 5). The Li–CO2 battery with no illumination exhibits a high charge voltage of about 4.17 V, whereas the charge voltage sharply drops to 3.28 V after illumination (Fig. 5a). Impressively, the photo-assisted Li–CO2 battery with the SiC/RGO cathode outperforms the majority of Li–CO2 batteries (Table S1†). Furthermore, the fast response of the charge voltage to the on–off irradiation (Fig. S10†) demonstrates the superior kinetics under illumination, which is partially responsible for the decrease of charge overpotential. As presented in Fig. 5b, LSV curves of the batteries obtained under illumination also show a lower onset voltage and higher current density compared with that in the absence of light, in accordance with the galvanostatic charging profiles. Such a light response phenomenon presented in the charging process further supports the proposed working mechanism for the photo-assisted Li–CO2 batteries (Fig. 1c): the photoexcited holes play a key role in the decomposition of the discharge products due to their high oxidation capability. Thus, reduction of the charge voltage could be achieved by providing extra energy from light irradiation to compensate the required input energy from the external circuit. Also, the kinetics of the battery could be promoted under illumination in virtue of the quick response of the photo-excitation process compared to that of the electric field, which is also beneficial in reducing the charging overpotential. The morphology evolution of the discharge products upon charging was further investigated via SEM analysis. After the recharge process under illumination, the nanosheet-like discharge product disappears completely (Fig. 5d), whereas some membranous discharge product remains on the surface of the cathode charged in the absence of light (Fig. 5e). Analogously, XRD patterns of recharged cathodes show that the typical diffraction peaks of Li2CO3 completely disappear after photo-assisted charging, while these peaks still exist after being recharged without illumination (Fig. 5c and S7†), consistent with the results obtained from FTIR spectra (Fig. 5f and S8†). Overall, the light-assisted Li–CO2 battery possesses better reversibility in comparison with the conventional Li–CO2 battery, owing to the facile CO2ER process promoted by the photogenerated holes.
answer: SiC/RGO