Controllable Synthesis And Lithium Storage Properties Of MnCo₂O₄ Nanowire Arrays And Their Composite Structures

Lithium-ion battery is a kind of rechargeable battery as opposed to lead acid battery,nickel-cadmium battery and nickel-hydrogen battery.As a green energy storage device,lithium-ion battery has become the first choice of portable electronic equipment battery,and has been widely used in power battery field.However,the present lithium ion battery system is unable to meet the requirements of high capacity,high power and rapid charge and discharge rate.Therefore,it has become a hot and challengeable issue to explore novel materials that can meet the requirements of high capacity and fast charge and discharge rate.Manganese cobalt oxide has high theoretical specific capacity,but its low cycling stability and high redox potential restrict its practical applications.In this thesis,potential of manganese cobalt oxide as an anode for lithium ion batteries has been investigated and explored.In the third chapter,urchin-like nanowire arrays with the diameter of 20-50nm were prepared on the porous copper foam by hydrothermal method.The effect of growth temperature on the morphology,structure as well as lithium storage properties of the nanostructures were systematically investigated,and the evolution mechanism of the morphology was proposed.Under the optimized growth parameters,and the first discharge capacity of the manganese cobalt oxide nanowires is as high as 3600m Ah g^-1,under the growth temperature of 120℃.The nanostructured manganese cobalt oxide provides a large number of active sites for the conversion reaction,and shortens the diffusion path of lithium ions,thus ensuring its high capacity.In the fourth chapter,Mn Co₂O₄@C composites were prepared by chemical vapor deposition（CVD）and high-temperature solid-state sintering（HTSintering）.The Mn Co₂O₄@C obtained by high-temperature solid-state sintering showed excellent cycle stability in the first 50 cycles,and the capacity remained above 1200m Ah g^-1.The capacity started to decrease after 50 turns,and the capacity after 100 cycles decreased below900m Ah g^-1.The capacity of Mn Co₂O₄@C composites obtained by chemical vapor deposition was initially lower than that obtained by high temperature solid-state sintering at first several cycles and increased with the increase of the number of cycles,and finally resumed to the capacity of 100%after 100 cycles.Carbon coating process under the chemical vapor deposition was optimized.The optimum parameters were obtained as follows:carbonation time 2 h,methane flow rate 50 sccm.At the current density of 100m A g^-1,the high specific capacity of 1239m Ah g^-1was obtained after 100 cycles of cyclic charge^-discharge test.Under the condition of 1000 m A g^-1,the specific capacity can be maintained at 600 m Ah g^-1with good stability,and the reduction potential of Mn Co₂O₄@C can be reduced to about 0.5V at the same time.In the fifth chapter,the Mn Co₂O₄@C composite material and the cathode materials Li Fe PO₄,Li Co O₂,NMC811 and NMC111 were assembled into the full battery,respectively,and their lithium storage performance and cycle stability were compared.Among the batteries,the assembled Li Co O₂/Mn Co₂O₄@C battery kept 92%capacity after 100 cycles,suggesting the best match between Mn Co₂O₄@C and Li Co O₂among the assembled full batteries.