Syntheses And Lithium-storage Properties Of 3D Graphene Confined MOF-derived Metal Oxides

The rapid development of electric vehicles has put forward higher requirements on the rate performance and energy density of lithium-ion batteries.Metal oxides are expected to replace traditional graphite anode materials due to their good safety and high theoretical specific capacity.However,the poor conductivity and serious volume expansion during cycling greatly affect the specific capacitiy and cycle stability of metal oxide.This paper focused on the improvement of the conductivity of metal oxides and relieving the volume expansion.A series of composites containing metal oxides and three-dimentional（3D）graphene were constructed by using 3D graphene（graphene foam,GF;graphene aerogel,GA）as substrates to confine the growth or encapsulation of metal-organic framework（MOF）under the high-temperature calcination.The obtained materials were used directly as self-standing electrodes for lithium-ion batteries.Combined with the characterizations and electrochemical test,the effects of metal oxide type,microstructure and mass loading on lithium-storage performances of composites were investigated.The research contents are as follows:（1）Ni-MOF hollow microspheres were grown on the surface of GF by a solvothermal method.The NiO/GF composite was obtained after the calcination process,which could be used as self-standing electrode for lithium-ion batteries.Compared with pure NiO and GF,the NiO/GF composites exhibited excellent specific capacities and cycle stabilities.The optimal composite showed a specific capacity of640 mAh g^-1 at a current density of 100 mA g^-1 after 50 cycles.The specific capacity at a high current density of 1 A g^-1 could reach 330 mAh g^-1.The excellent lithium-storage properties of the composite electrode can be attributed to the synergistic effect between hollow NiO microsphere and conductive GF substrate,in which the hollow structure could alleviate the volume expansion of metal oxide particles during the cycle,while the GF substrate could increase the overall conductivity of electrode as well as improve the cycle stability of metal oxide.（2）Co-MOF particles with different morphologies were grown on the surface of GF by a solution impregnation method,and the Co₃O₄/GF composites were obtained by the calcination process.The effect of the morphology of metal oxide on lithium-storage properties of composites was investigated.The lithium-storage capacity of the leaf-like Co₃O₄/GF composite was better than those of the other ones.This was attributed to the increase of specific surface area for the quasi-two-dimensional structure of leaf-like Co3O4,where the migration of lithium ions at the electrode/electrolyte interface was promoted.The research showed that the specific capacity of the leaf-like Co₃O₄/GF electrode was 986 mAh g^-1 at a current density of 100 mA g^-1 after 250 cycles,and the specific capacity could still maintain at340 mAh g^-1 even at a high current density of 2 A g^-1.（3）CoCo-MOF nanocubes were grown in the network of graphene aerogel（GA）by a self-assembly method,and the Co3O4@GA composite with confined structure was obtained after the calcination process.The obtained composites were used as self-standing anodes of lithium-ion battery.The result showed that the synergistic effect between metal oxide and GA resulted in a better cycle stability of the Co3O4@GA composite electrode than pure components.The specific capacity was624 mAh g^-1 at a current density of 100 mA g^-1 after 100 cycle,and the composite still had specific capacities of 466 and 253 mAh g^-1,respectively,when the current densities were increased to 500 and 1000 mA g^-1.This can be attributed to the confinement effect of GA enhanced the conductivity of the electrode and greatly alleviated the aggregation and volume expansion of metal oxide particles during the charge-discharge process.（4）The bimetallic FeCo-MOF nanocubes was wrapped into the framework of GA by a self-assembly method,and the FeCo-oxide@GA composites were obtained by the calcination as self-standing anodes of lithium-ion batteries.The intrinsic properties of bimetal oxides,and the synergistic interaction between metal oxides with GA allowed that the composite electrodes exhibited better specific capacities and cycle stabilities.The composite delivered a specific capacity of 947 mAh g^-1 after 130cycles at the current density of 100 mA g^-1,and the specific capacity could still maintain at 286 mAh g^-1 even at a high current density of 2 A g^-1.