Study On The Preparation Of Fe₃O₄/RGO Composites And Their Properties As Cathode Materials For Lithium Air Batteries

The metal air battery takes the oxygen in the air as the positive electrode active material,the air electrode as the energy conversion tool,the metal as the negative electrode active material,and the metal anode to store energy.The lithium air battery due to its high theoretical energy density?close to oil and other fossil fuels?,which are considered as the next generation energy source for many large equipment.However,for the actual commercial applications,there are still many technical difficulties,mainly reflected in the battery cathode.As the central of oxygen reduction and oxygen precipitation reaction,discharge products accumulates on the electrode will not only reduce the actual capacity density,at the same time will seriously affect the cycle life of the battery.In addition,the resulting serious polarization is also the important factors that affect battery performance.Therefore,it is very important to find a suitable anode catalyst to effectively improve the charge discharge performance and reversibility of lithium air battery,and reduce the polarization effect.Firstly,graphite oxide was prepared with Hummer method,by comparing the products with different reagents,results show that the oxidation degree of graphite oxide is mainly controlled by the amount of Potassium Permanganate.By comparing the products of graphite oxide reduced by hydrazine hydrate and vitamin C,It is found that hydrazine hydrate is more effective in the reduction of graphite oxide,mainly reflected in the reduction of carboxyl.Single reduced graphene as anode catalysis for lithium air battery,the total discharge capacity of the battery after the first and ten cycles were 2102 mAh/g and 832 mAh/g respectively,and the capacity retention rate was about 39.24 %.Secondly,in the early exploration of solvothermal synthesis of Fe₃O₄/RGO,by changing the dispersant,it is found that citric acid is helpful to the dispersion of nano particles and the formation of small particle size,and the reduction of Fe3+ by ethylene glycol in the solvothermal process.Fe₃O₄/RGO composite prepared by solvothermal method as positive electrode catalysis,the total discharge capacity of batteries for the first time and after ten cycles was 3534 mAh/g and 1062 mAh/g,which are higher than that of a single graphene material,but the capacity retention rate is slightly lower.The results of thermogravimetric analysis and SEM indicate that this may be related to the excessive loading of nano Fe₃O₄,and the charging and discharging process is easy to cause the damage of the structure.Then,graphite oxide were used as raw materials,through the self-assembly in the solution surface,as well as the reduction and gelation of oxalic acid and Na I,three dimensional graphene gel was obtained.At the same time,the process of coprecipitation of Fe₃O₄ was investigated,it indicated that when the adding rate of ammonia is too fast,it is easily lead to the deposition process of colloid is greater than the crystallization process,what eventually led to the formation of the cluster of nano Fe₃O₄.Just because of the interlayer spacing and the specific surface area of the cluster structure of Fe₃O₄,It also bring about the increase of the total discharge capacity for the first time compared with the spherical nano Fe₃O₄ material,which is up to 1708 mAh/g.At the same time,the two-dimensional Fe₃O₄/RGO and three-dimensional Fe₃O₄/RGO were prepared by the co-precipitation method on the basis of the reduction of graphene and three-dimensional graphene gel.The first discharge capacity of the two materials were 3285 mAh/g and 3157 mAh/g respectively,and the difference was not significant.However the total discharge capacity were 975 mAh/g and 1359 mAh/g after ten discharge,the capacity retention rates were 29.7 % and 43 %,respectively.It also shows the advantages of the three-dimensional structure of the Fe₃O₄/RGO in terms of control attenuationFinally,the preparation of CoFe₂O₄ composite was prepared by sol-gel method,and the preparation of CoFe₂O₄/RGO composites and the construction of the CoFe₂O₄/RGO-Ni electrode were studied.Battery assembled with CoFe₂O₄/RGO-Ni electrode,whose total discharge capacity for the first time and after ten cycles were 3700 m Ah/g and 1451 mAh/g,which is higher than that of CoFe₂O₄/RGO material?3152 mAh/g and 957 mAh/g?,also higher than that of the previous three-dimensional Fe₃O₄/RGO material.The capacity retention rate of battery assembled with CoFe₂O₄/RGO-Ni electrode was 39.21 %,which is higher than that of CoFe₂O₄/RGO?30.36 %?.With the charge and discharge process continued,Kulun efficiency is also rising,which indicates the system was becoming more stable.Because foam nickel provides more solid three-dimensional support,it also can alleviate the graphene volume changes caused by the continuous cycle of metal oxide nanoparticles,All these make the material not easily damaged by the reaction process.