Preparation,Properties And Reaction Mechanism Of Noble Metal-based Cathode Catalysts Of Li-CO₂/O₂ Batteries

The lithium-ion batteries(LIBs)have been widely used in mobile electronics,electric vehicles and scale energy storage.However,the lower specific energy density(250?300Wh kg-1)of LIBs could not satisfy theever-increasing demand of energy storage.Specifically,Li-O2 and Li-CO2 batteries exhibitthe specific energy densityof 3500 and 1876 Wh kg-1,respectively,which are potential to be the new generation of the energy storage systems.For the Li-O2 battery,the existence of CO2 contamination in the air is an inevitable issue which would result in the formation of the side product,Li2CO3,further leading to the inactivation of the catalysts and the degradation of the batteries.For the Li-CO2 battery,the discharge process is to convert the CO2 gas into Li2CO3and C.Thus,investigating the properties of Li2CO3 based on the Li-CO2/O2 batteries is beneficial for the improvment of Li-CO2 and Li-O2 batteries.The insoluble,dielectric and sluggish kinetic features of Li2CO3 lead to the higher decomposition potential,more than 4.0V.Thus,the design of effective cathode catalysts is necessary to improve the decomposition rate of Li2CO3 in Li-CO2/O2 battery.Herein,three different catalysts,Ru/NC,Ru/NiO@Ni/and Pt/CNT were prepared and used as the cathode catalysts for the Li-CO2/O2 batteries.With such catalysts,all the batteries showed preferable performance.Also,the reaction mechanism during discharge and charge process was investigated with the assistanceof TEM,SEM,XRD,in-situ FTIRs and DFT calculation.The main research works are as follows:(1)Ru/N-doped carbon nanotube(Ru/NC)catalyst was prepared by solvothermal method and Ru nanoparticles are uniformly distributed on the surface of NC.Due to the superior catalytic activity of Ru nanopartices and N-doped feature,the decomposition kinetics of the Li2CO3 could be accelerated.Therefore,the overpotential of Li-CO2/O2(VCO2:VO2=4:1)battery is decreased from 1.45 V to 1.06 V and thusassembledbattery could operate smoothly for 90 cycles with a capacity of 1000 mAh g1 or 184 cycles with 500 mAh g-1.XRD and SEM characterizations after cycles further revealed that the as-generated Li2CO3could be effectively decomposed by the Ru/NC.Besides,with Ru/NC catalyst,Li-CO2/O2(VCO2:VO2=0:1,2:1 and 1:0)batteries also show enhanced cycling stability of 200,190and 150 cycles,respectively,with 500 mAh g-1.Interestingly,the discharge potential increases from 2.35 V to 2.90 V when introducing 20%O2 in Li-CO2 battery.Last but not least,our investigation on the failure mechanismputs an emphasis on the consumption of electrolyte and the degradationof Li anode as the major causes for the decay of the Li-CO2/O2batteries.(2)A carbon nanotube supported Ru/NiO@Ni catalyst(Ru/NiO@Ni/CNT)was synthesized with Ru nanoparticles(?2.5 nm)anchored on the surface of core-shell structure NiO@Ni nanoparticles(?17 nm).We found a strong interfacial interaction between Ru nanoparticlesand NiO.XRD and XPS analysis revealed that the presence of Ru could protect the Ni species from being deep oxidized while the NiO species could modify the local electronic structure of Ru inducing higher oxidation state.When such Ru/NiO@Ni/CNT catalyst was used as cathode in Li-CO2/O2(VCO2:VO2=4:1)batteries,long cycling life of 105 cycles at a cut-off capacityof 1000 mAh g-1 with overpotential as low as 1.01 V was achieved,which is significantly betterthan 75 and 44 cycles with Ru/CNT and NiO@Ni/CNT catalysts,respectively,and confirmed the strong synergetic effect between the Ru and NiO species in electrocatalytic decomposition of Li2CO3.Density functional theory(DFT)calculations of electrochemical decompositionof Li2CO3withtheassistanceof RuO2 indicatedthattheformation O2isthe rate determining step.And the formation and decomposition process of Li2CO3 was illuminated at a molecule level by in-situ FTIRs spectroscopy with Ru/NiO@Ni/CNT catalyst.(3)The Pt/CNT catalyst was prepared byformaldehyde reduction method which the Pt nanoparticles with an average size of 2.5 nm were uniform distributed on the CNT.When introducing such Pt/CNT catalyst into the Li-CO2/O2 battery with 2%O2 content,the low overpotential of 0.3 V and a superior stability of 128 cycles with 1000 mAh g-1 could be obtained and this superior performance could be further confirmed with the assistance of electrochemical in-situ FTIR spectroscopy at molecular level which could timely detect the formation at 2.5 V and decomposition at 3.2 V of Li2CO3.Interestingly,when CO2was solely used the discharge products Li2CO3was uniformly distributed on the gas side and electrolyte and while the low O2contents,2%or 6%,were used,the Li2CO3,was prone to grow on the gas side of Pt/CNT catalyst.It is inferred that the prior reduction of O2 combining with the lower O2 content during the discharge process result in such growth mechanism.Three different catalysts,Ru/NC,Ru/NiO@Ni/CNT and Pt/CNT,were prepared based on the consideration of uniform distribution of active sites,synergetic effect between different active sites and investigation of new active sites.Insightful investigation of the influence of such catalysts for the improvement of Li-CO2/O2 battery was performed.These researches would be benefical for the development of highly-efficient cathode catalysts.