Synthesis Of Carbon-Based Catalytic Cathodes For Lithium-Oxygen Batteries

With the development of electric vehicles and power storage stations,the demand for energy storage devices with high energy density is increasingly urgent.As for Li-ion batteries,the lithium storage capacity restricts its further application in energy storage fields.Although electrode materials such as Pb and Si have a high lithium-storage capacity,they suffer from lagre volume changes during lithiation/de-lithiation process,which leads to rapid capacity fading.Li-air battery(Li-O2 battery)has an extremely high theoretical energy density,and is considered as one of the promising next-generation lithium batteries.However,the insulating feature of the discharge product Li2O2 and accompanied sluggish oxygen reduction/evolution reactions(ORR/OER)kinetics result in high overpotential,poor rate performance and limited cycling performance.Carbon-based materials are widely used in energy storage devices due to their low density,high specific surface area,excellent electrical conductivity and superior electrochemical stability.Herein,based on problems faced by cathode materials for Li batteries,we propose several new carbon-based materials to improve electrochemical property.The results are as follows.(1)Graphene/PbSe nanohybrid(rGO/PbSe)was synthesized by a solvothermal method.The hybrid exhibited a sandwith structure with PbSe nanoparticles dispersed between graphene nanosheets.Graphene can buffer volume changes of the active materials,confine pulverized particles,and offer Li-ion and electron conducting channels.In situ TEM characterization reveals the lithiation/de-lithiation mechanism of rGO/PbSe,and confirms the buffering and confining effects of graphene.The lithium-storage property of rGO/PbSe indicates that graphene can improve cycling stability to some extent.However,graphene cannot prevent pulverization and volume change of active materials.The capacity and cycling performance are not as ideal as expected.When acting as catalyst for Li-O2 battery,rGO/PbSe will not suffer from volume changes and will exhibit a high capacity.Electrochemical tests show that rGO/PbSe-catalyzed Li-O2 battery exhibits a higher capacity than the corresponding Li battery,but with high Overpotential and poor cycling performance,probably owing to the low catalytic activity of rGO/PbSe towards ORR/OER.(2)A catalytic cathode for Li-O2 battery was designed,which is composed of mushroom-like Au/NiCo2O4 nanohybrid on three-dimensional graphene(3D-G)grown directly on Ni foam.NiCo2O4 arrays and Ni foam constitute a hierarchical porous structure,which avoids the use of binder,ensures penetration of electrolyte,and promotes rapid diffusion of reactants.Nanosheets on the top of NiCo2O4 enlarge surface area and provid more catalytic active sites.The introduction of Au and 3D-G increases the conductivity of the electrode.Besides,synergistic catalytic effect of Au and NiCo2O4 can improve ORR and OER kinetics.Ex situ SEM observation verifies that Au can induce Li2O2 to grow into thin-film form,which decomposes at lower potential compared with large particles during charge.Au/NiCo2O4/3D-G catalyzed Li-O2 battery delivers a high discharge capacity of 1275 mAh g-1 at 42.5 mA g-1 and a relatively low overpotential(1.01 V).The battery can sustain a stable cycling up to 40 cycles under a limited capacity of 510 mAh g-1.(3)A templating method was used to construct carbon submicron tube(CST)arrays grown directly on Ni foam,where Au nanoparticles were decorated on the inner tube walls of CST.Ex situ SEM observation confirms that Au can direct the growth of Li2O2 into thin-film form along the inner wall of CST.This growth mode facilitates decomposition of Li2O2,alleviates electrode passivation,and keeps the voids between CST arrays,Density functional theory(DFT)caculation further clarifies that the presence of Au will promote the adsorption of Li2O2 on carbon tube.Au@CST-catalyzed Li-O2 battery exhibits high specific capacity(5488 mAh g-1 at 400 mA g-1),excellent rate performance(1208 mAh g-1 at 1000 mA g-1)and long cycle life(112 cycles at 400 mA g-1 with a limited capacity of 500 mAh g-1).(4)A templating route was used to build a unique MnO2/CST array-type cathode which avoids the use of noble metals and minimizes the exposure of carbon to Li2O2.The a-MnO2 nanosheets grown on CST have large surface area,provide numerous Li2O2 nucleation sites and thus increase the loading of Li2O2.Besides,high catalytic activity of a-MnO2 nanosheets enables conformal growth of thin-layered Li2O2 on the a-MnO2 nanosheets,rendering easy decomposition of Li2O2 upon charge.In this design,MnO2 coating minimizes the contact between carbon and Li2O2,along with the absence of polymer binder,which largely reduces or inhibit parasitic reactions.As a result,MnO2/CST-catalyzed Li-O2 battery delivers a high discharge capacity(4675 mAh g-1)at 400 mA g-1.At a high current density of 800 mA g-1,the battery can sustain a stable cycling of over 300 cycles under a limited capacity of 1000 mAh g-1.In situ TEM characterization confirms the formation of Li2O2,and observes the phase transition and morphology changes upon charge.