The Application Of Polyoxometalates(POMs)as Cathode Catalysts For Lithium Oxygen Battery

The development of lithium-oxygen battery with extremely high theoretical energy density is sluggish due to the complexity of reaction system.Among many existing problems,the selection and design of cathode catalysts is undoubtedly a key challenge.Although carbon,precious metals and their alloys,metal oxides and sulfides and others have been proposed,a satisfactory catalytic system has not yet been formed.Thus,exploring some possible new catalysts have attracted significant attention.The polyoxometalates known as "electron sponge" exhibits a potential possibility in the design of cathode catalysts in lithium-oxygen battery because of its reversible multi-electron redox process,multiple catalytic sites and structural stability.Based on the above considerations,this research is devoted to employ suitable polyoxometalates as cathode catalysts in Li-O₂ battery.The specific research ideas and contents are as follows:1.Sandwich-type Ru-based POM with OER activity in water oxidation and higher solubility was used as cathode liquid phase catalyst in L1-O₂ battery.The discharge specific capacity was increased by around 2 times from the original 4934 mAh/g to 14436 mAh/g at a current density of 100 mA/g.Moreover,the overpotential during charging was reduced by 0.6 V from 3.8 V to about 3.2 V when the specific capacity was fixed at 500 mAh/g.The battery life was prolonged to 90 cycles while the coulombic efficiency could still maintain at 93.15%.The discharge products were analyzed by XRD,Raman,SEM and titration experiments of Li2O₂ and LiOH.The results showed that the discharge products were all crystalline LiOH instead of the traditional Li2O₂.DEMS further confirmed that this electrochemically generated LiOH was capable of undergoing 4e^-/O₂ decomposition.2.Fe-based POM framework?NH4-Mo-Fe?with O₂ adsorption capacity at room temperature was used as cathode solid phase catalyst in Li-O₂ battery.Compared with commercial Fe₃O₄ nanoparticles,the discharge specific capacity was doubled to 1740 mAh/g at a current density of 100 mA/g.In addition,the overpotential during charging was reduced by 0.58 V.The discharge products and side products were analyzed by XRD,SEM,Raman and XPS.The results showed that the discharge products were amorphous Li2O₂ shaped like broken particles,which became the reason for lower overpotential.And the presence of a part of by-products Li₂CO₃ reveals an important reason for battery failure after a long cycle.