| Due to their superior theoretical energy density and low to zero emissions etc, rechargeable Li-O2 batteries have attracted significant attention as promising next-generation power sources for vehicle applications. Some exciting progresses have been achieved in recent years. However, certain problems must be resolved before these novel energy systems will be feasible for practical applications. One problem is the cathode catalyst, large discharge-charge overpotential due to the slow kinetics of the oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) at the cathode is one of the biggest obstacles. The problem also resulted in the high charge voltage, which may lead the decomposition of the electrolyte, the instability of the cathode and other issues. Currently, research work focused on the electrocatalyst, efficient electrocatalyst can reduce the discharge-charge overpotential, improve the specific energy efficiency, and also improve the cycling stability.In this thesis, we prepared and investigated a type of cathode catalysts by supporting metals/metal oxides nanoparticles on the undoped/doped reduced graphene oxide(r GO), or graphene-like materials. Using undoped/doped carbon-based material as the substrate, through the impregnation–reduction method and hydrothermal method to prepare the composite catalyst with noble metal nanoparticle mounted on doped carbon materials. Combined with the morphology, structure, chemical composition, catalytic activity of ORR/OER and other factors of the catalyst, to analysis its performance as the cathode catalyst of the Li-O2 battery. The purpose is to prepare an efficient cathode catalyst for the battery.Firstly, using the r GO as the substrate, through the impregnation–reduction method to prepare different noble metal mounted on r GO. The results showed that the four catalysts were successfully prepared and all of them have maintained the unique morphology of graphene. In these four catalysts, Ru/r GO catalyst showed the best performance in the Li-O2 battery. The energy efficiency decreased from 74.9% to 57% after 38 cycles. Ir/r GO catalyst also showed the good performance in the Li-O2 battery, the energy efficiency decreased from 70.3% to 53% after 30 cycles. Ag/r GO and Au/r GO catalyst have similar performance in the Li-O2 battery, both of them have an activation process, and compared with r GO catalyst, they cycling stability has improved, but the discharge-charge overpotential does not improved, which may be due to the low catalytic activity of Ag and Au. We believe that Ru and Ir were very promising cathode catalyst of Li-O2 battery, while the low ORR activity and low stability of the original r GO also have an impact on the overall performance of Li-O2 battery.Secondly, we have designed a new bifunctional catalyst by mounting Ru nanoparticles on multielements co-doped r GO; in our novel material, reduced graphene oxide co-doped with N, Fe, and Co may play as the ORR catalyst, and ruthenium nanoparticles play as the OER catalyst. The catalyst exhibited significantly higher ORR and OER activity than a commercial Pt/C catalyst in both aqueous and non-aqueous electrolytes. With this novel catalyst as cathode, the battery exhibited an ultra-high reversible capacity of 23,905 m Ah/g at a current density of 200 m A/g. Further, the battery also exhibits an excellent cycling stability—after 300 cycles of limited capacity, the discharge plateau potential decreased only slightly, and the energy efficiency was still above 60%. The battery also demonstrated good rate performance; with discharge current densities of up to 1000 and 2000 m A/g, the capacities still reached 14,560 and 6,420 m Ah/g, respectively. We suggest that the excellent performance of our catalyst can be ascribed to the excellent ORR performance of the multielements co-doped graphene and the excellent OER performance of the mounted Ru nanoparticles. In addition, the nanosheet structure with high surface area of the multielements co-doped graphene may result in the formation of uniform Li2O2 nanocrystals, which make the formation(discharge) and decomposition(charge) processes much more reversible.Thirdly, in contrast to the discharge process, the charge process plays an important role in relieving the cathode passivation to realize the high cyclability. Therefore, we also studied the performance of the Li-O2 battery with the Ir O2 catalyst, which is an excellent OER catalyst in aqueous electrolyte. The results showed that the total amount of active N species(graphitic, pyrrolic, pyridinic and Co-Nx) reaches 84.4 at.% of the total nitrogen content, which is an important factor to improve the activity of the catalyst. According to the high resolution XPS spectra of Ir 4f, Ir4+ was the main species on the catalyst surface, indicating that the majority of the iridium probably existed in the catalyst as iridium oxide. Compared with r GO catalyst, Ir O2-Co N/r GO cathode shows lower discharge-charge overpotential, better capacity retention, excellent reversibility and cycle stability. With this novel catalyst as cathode, the battery exhibited an reversible capacity of 9,237 m Ah/g at a current density of 200 m A/g. More interestingly, the battery with Ir O2-Co N/r GO exhibits relatively good discharge-charge behavior, delivering a high reversible capacity of 11,731 m Ah/g at the 5th cycles. Further, the battery also exhibits an excellent cycling stability—after 200 cycles of limited capacity, the energy efficiency decreased from 75% to 65%, and a lower charge plateau about 3.8 V. After 200 cycles, the charge voltage region of the Ir O2-Co N/r GO electrode remained below 4.2 V—far behind the onset voltage for electrolyte decomposition.Finally, we prepared a novel biomass-derived N-doped carbon catalyst(NORI) through a heat-treatment method and hydrothermal carbonization method. The results show that the catalyst has a graphene-like structure and that its ORR/OER activity was very good. Notably, the Li–O2 cell with the NORI cathode exhibited high discharge capacity: the initial value was up to 4,222 m Ah/g at a discharge current density of 200 m A g which was 2.4 times higher than that with the XC-72 R cathode(1,743 m Ah/g). They also exhibits an excellent cycling stability—after 100 cycles of limited capacity, the energy efficiency was still above 56.5%. On the basis of this unique NORI catalyst, we designed a solid-liquid two-phase catalyst for the Li-O2 battery. Doped carbon catalyst on the basis of this unique topography, we designed a solid-liquid two-phase catalyst in lithium-air batteries, choose NORI as the solid phase catalyst and Li I as the liquid phase catalyst. With this two-phase catalyst, the battery exhibited an ultra-high performance with the charge voltage down to 3.4 V, the reversible capacity up to 10,430 m Ah/g at a current density of 200 m A/g. Further, the battery also exhibits an excellent cycling stability and good rate performance. After 200 cycles of limited capacity, the energy efficiency was still above 90%. |
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