Synthesis And Catalytic Active Sites Regulation Of Cathode Materials For Lithium-Gas Batteries Based On Organic Electrolytes

The metal lithium-gas battery possesses ultra-high theoretical energy density(11430wh kg^-1)rivalling to that of fossil fuels,and is expected to replace the lithium-ion battery as a power source for driving electric vehicles.Due to the wide electrochemical window and high ionic conductivity of organic electrolytes,lithium-gas batteries based on organic electrolyte systems have promising future.However,the sluggish electrode reaction kinetic of the lithium-gas battery has caused serious polarization problems,resulting in a low energy conversion rate and poor cycle stability.In order to solve the above problems,a large number of efforts are devoted to designing highly efficient catalysts to promote the further development of lithium-gas batteries.So far,people have systematically studied catalysts with different chemical compositions.Among various types of catalysts,transition metal oxide is expected to work as suitable positive electrode catalysts for lithium-gas batteries due to their low cost and high stability in hush redox reactions.In this paper,starting from the construction of a free-standing effective transition metal oxide cathode,methods such as doping,magnetron sputtering,chemical reduction are employed to introduce oxygen vacancies on the surface of the transition metal oxide then adjusting the catalytic active sites on the surface of the material in order to build up a high energy conversion efficiency lithium-gas battery with good reversible cycle performance.The research results are described as follows:?1?Using hydrothermal method and calcination,Co-doped Ni Fe₂O₄ composite foam nickel positive electrode?Co Ni Fe LDO@Ni?was successfully prepared.Battery based on Co Ni Fe LDO@Ni shows a small overpotential?0.98 V?and excellent cycle stability?110 cycles with no obvious attenuation?.The improvement of the bifunctional catalytic activity of the Co Ni Fe LDO@Ni electrode should be attributed to the following two points:?1?the independent self-supporting electrode structure is beneficial to accelerate the of oxygen and electrons;?2?Co regulates the electrons at the Fe site density and induce the formation of oxygen vacancies.Oxygen vacancies acting as active sites can promote the adsorption of oxygen and enhance the transport of electrons at the interface.?2?We introduce small amounts of Ce in Co₃O₄ to fabricate oxygen vacancies.Compared with pure Co₃O₄ free-standing electrodes,batteries based on vacant Ce-Co₃O₄self-supporting electrodes showed a low overpotential of 0.9 V,a discharge capacity of up to 8250 m Ah g^-1 and extended cycle life.The excellent catalytic activity of this defective Ce-Co₃O₄ confirms the positive effect of surface oxygen vacancies on the regulation of catalyst activity.Oxygen vacancies can induce the formation of unsaturated coordination sites and cause electron delocalization near the oxide surface,thereby enhancing charge/Li⁺transfer.In addition,oxygen vacancies can also act as active sites for bonding O₂/Li₂O₂,thereby promoting reaction kinetics involving oxygen participation.Therefore,it is these factors work together to promote the bi-functional catalytic activity of Ce-Co₃O₄ in Li-O₂batteries.?3?The magnetron sputtering method is used to adjust the concentration of oxygen vacancies in Ni Co₂O₄ by controlling the sputtering time.Comparing the electrochemical performance of batteries based on NCO-0 min,NCO-5 min,NCO-10min and NCO-15 min,only the battery with NCO-10 min showed the smallest overpotential?0.49 V?and operated for 100 cycles without no attenuation.Combined with DFT calculations,it is confirmed that the positive effect of oxygen vacancies on the regulation of catalytic activity comes from the vacancies up-shifting the metal E_dorbitals relative to the Fermi level,thereby enhancing oxygen adsorption and accelerating electron transfer at the defect site.However,excessive oxygen vacancies will have a certain negative effect on the improvement of the catalytic activity of the material:with the excess of oxygen vacancies,according to the principle of electro-neutrality,most metal ions will be reduced,which will cause unstable crystal structure.?4?The etch method was used to introduce oxygen vacancies into Ni O and oxygen vacancy itself can optimize CO₂reduction and adsorption capacity of Ni O.Electrochemical performance test results prove that compared with ordinary Ni O self-supporting electrodes,lithium-carbon dioxide batteries catalyzed by self-supporting Ni O electrodes with rich oxygen vacancies have the lowest overpotential?1.2 V?and the longest cycle life?58 cycles?.Through material characterization,we concluded that the reason why oxygen vacancies can improve the catalytic activity of Ni O is that the extra electrons generated at oxygen vacancies sites creating electron donor?Lewis base?surface where electrons can be easily transferred to electron-deficient carbon atom in CO₂,which is difficult to obtain electrons by virtue of its chemically inert,therefore promoting the activation and the reduction of CO₂ adsorbed on the surface of defective Ni O.Additionally,Ni ions in asymmetric coordination environment induced by oxygen vacancies can also work as active sites for preferential adsorption of CO₂ then enhancing the capture and adsorption of CO₂,which is an essential step to initialize the following electrochemical reaction.