Research On Electrochemical Lithium Storage Performance Of Cobalt-based Oxide Quantum Dots/MOFs-derived Carbon Composites

When graphite is used as lithium ion battery anode,the theoretical specific capacity is low,only 372 mAh g-1 which cannot meet the requirement of high energy density of batteries.Cobalt-based oxides(such as CoO,Co2O3 and Co3O4)stand out among a large number of electrode materials because of their high theoretical specific capacity.But the cobalt-based oxide anodes also face two serious problems.One is the large volume expansion on cycling,which is not conducive to the improvement of battery cycle performance.Second,the conductivity of cobalt-based oxides is poor,which will limit the rate performance of the battery.In this paper,we designed and fabricated composites of cobalt-based oxides and MOFs-derived carbon.Both have complementary advantages.MOFs-derived carbon can not only serve as matrices to improve the conductivity of the composites,but also buffer the volume expansion of cobalt-based oxides during the cycling.Cobalt-based oxides provide high specific capacity as active materials.Firstly,nanoporous CoOx quantum dots/ZIF-67-derived carbon composite materials were synthesized by high temperature pyrolysis method and chemical precipitation method.The composite anodes exhibit excellent cycle performance.0D/3D nanoporous structure has excellent structural stability.The in situ growth of CoOx quantum dots along ZIF-67-derived carbon can effectively inhibit the volume expansion and agglomeration of CoOx quantum dots during the cycling.The anodes have a reversible specific capacity of 1873 mAh g-1 at 200 mA g-1 after 200 cycles.When the current density increased to 1000 mA g-1,the reversible specific capacity is 1246 mAh g-1 after 900 cycles.Then,Zn-HMT precursor was pyrolyzed at high temperature to prepare nitrogen-doped porous carbon nanosheets(NPCNs).The composite anodes show good cycle and rate performance.0D/2D composite materials of Co3O4 quantum dots and NPCNs were designed and synthesized.NPCNs are rich in active sites and defects,which can increase the electrochemical reaction rate of composite anodes.NPCNs can also accommodate the volume expansion of Co3O4 during charging and discharging.At a current density of 200 mA g-1,a reversible specific capacity of 1099 mAh g-1 is shown after 200 cycles.At current densities of 100,200,500 and 1000 mA g-1,the reversible specific capacities are 1175,1080,1041,969 mAh g-1,respectively.Quantitative kinetic analysis shows that capacitor type lithium storage dominates the total lithium storage capacity.