Construction And Research Of Electrocatalyst For High Performance Rechargeable Lithium-air Battery

As a promising alternative to fossil fuels,rechargeable lithium-air(Li-air)battery has achieved wide attention due to its highest theoretical energy density(11,400 Wh kg-1).Nowadays,these research work towards Li-air battery mainly focus on the development of electrolyte,the synthesis of cathode materials and the preparation of the electrode catalysts.The latter two have great influence on the performance of the battery.The cathode materials should have good electrical conductivity(favorable for electron transportation)、large specific surface area(favorable for the retention of the discharge products)、suitable pore structure(favorable for the diffusion of the electrolyte),and good stability.Moreover,a stable and efficient electrode catalyst should be applied in the battery.Recently,we have developed a three dimensional porous graphene-like(3DG)material with large surface area,high conductivity and interconnected pore structure.It meets the abovementioned requirements of the cathode material.As a result,the Li-air battery shows an ultrahigh complete discharge capacity with 3DG,but its cycling stability and rate performance are quite limited.These performances are further improved by supporting Fe2O3 on the 3DG and adding an efficient soluble redox media in the electrolyte as the electrode catalyst.In this thesis,the Fe2O3/3DG was immobilized on 3DG and the soluble redox mediator Fe(acac)2 was added into the electrolyte to study the performance of Li-air batteries.The main progresses are demonstrated.1.Surface-modified 3DG as cathode material of Li-air battery:The 3DG material with different C/O atomic ratio was obtained by controlling the air atmosphere during annealing treatment.It was found that the 3DG with higher oxygen content had higher discharge capacity and cyclic stability.It was inferred that the carbon oxygen group on the surface was the nucleation center of the discharge product of Li2O2.As a result,3DG cathode materials with high oxygen content show high complete discharge capacity(23,000 mAh g-1)and cycle stability(69 cycles),far better than that of XC-72.2,Fe2O3/3DG with good Li-air battery performance:Fe2O3/3DG was first synthesized by impregnation method.And the catalytic performance of Fe2O3 nanoparticles was enhanced under the support of 3DG.At the current density of 0.5 A g-1,the overpotential decreased by 0.5 V and the battery cycle 95 times.This good performance can be attributable to the high specific surface area and excellent conductivity of 3DG,as well as the increment of ORR/OER catalytic activity of Fe2O3 nanoparticles.3.Excellent Li-air battery performance with Fe(acac)2 as the soluble redox mediator:The soluble Fe(acac)2 with the Fe(II)center was added to the electrolyte as a redox mediator.On the one hand,it acts as the mediator of oxygen species and electron transportation to form Li2O2 in the solution phase,and then deposit.the Li2O2 on the surface of 3DG with typical morphology.On the other hand,Fe(acac)2 can interact with Li2O2 and effectively catalyzes the decomposition of Li2O2 during charging process.Additionally,comparing with FePc,the Fe(acac)2 molecule is absent of large πconjugated rings with nonplanar structure.Therefore,the interaction between Fe(acac)2 and the carbon material is appropriate.At high current density,Fe(acac)2 can rapidly adsorb and desorb on 3DG and catalyze ORR/OER processes efficiently.At the current density of 0.1 A g-1,the complete discharge capacity raises up to 24800 mAh g-1 with Fe(acac)2,and the cycle stability increase to 193 cycles at the 0.5 A g-1.When the current density is raised to 1,2 A g-1,the cyclic stability still remains 130 and 122 cycles respectively.The excellent performance can be attributed to:(1)Fe(acac)2 can shuttle the oxygen species,leading to the solution phase formation of Li2O2;(2)The Fe(Ⅱ)center of Fe(acac)2 can effectively catalyze the ORR/OER processes,reducing the overpotential and improve the cyclic stability;(3)Fe(acac)2 is very soluble in DMSO,producing more nucleation center for Li2O2.These result provide a promising strategy to explore the high-performance Li-air batteries with lower overpotentials,high-rate capability,and long-term cyclibilty:(i)developing cathode catalysts with ORR/OER bifunctional catalysis,high specific surface area,hierarchical pore structure,large pore volume,and high conductivity;(ii)applying highly soluble redox mediator with the bifunction of shuttling oxygen species and catalyzing ORR/OER,proper interaction with cathode materials.