| Owing to the increasing demands for energy,the transformation of energy structure has been pushed by global energy crisis and environment pollution problems caused by the excessive consumption of fossil energy.Development and utilization of clean energy resources has become an effective way to solve the above challenges.Lithium-ion batteries(LIBs)have been used as energy storage device in cell phones,laptop computer and portable electronic devices due to their high energy density and long-life.However,with the rapid development of large-scale energy storage devices and electric cars,the current LIBs have not yet been able to meet their requirements.Meanwhile,the limited lithium resources are bound to retrict the large-scale applications of LIBs.Sodium-ion batteries(SIBs)are considered to be the most promising ideal energy storage devices because of abundance and low cost of sodium resources,especially for large-scale energy storage.It is well known that the electrode material is the key factor for LIBs/SIBs.Thus,it is highly important to study and develop suitable electrode material with higher capacity,better rate performance and long-life cycling for LIBs/SIBs.Recently,a class of electrode materials derived from metal-organic frameworks(MOFs)has been successfully used in the field of energy storage,and gained excellent electrochemical performance.In this thesis,we prepared a series of shuttle-like nanostructure composites using the environmentally friendly MIL-88(Fe)as precursors by changing the subsequent heating treatment process,and studied the electrochemical performances of their lithium/sodium ion storage.The results show the composite electrode materials exhibit high-rate performance and long-life cycling,which can be attributed to their hierarchically porous structure and carbon-coating.Nanoscale particles and porous structure can increase the contact area between electrode materials and electrolyte,and carbon layer is beneficial for fast charge transfer,thus improving the electrochemical reaction kinetics and rate performance.In addition,the analysis and discussion on the performances of different electrode materials have also been carried out.The main contents of this thesis are as follows1.Using MIL-88(Fe)as templates,spindle structure amorphous porous carbon nanoplates(PC)were prepared through carbonization and subsequent pickling treatment.The results show that as-prepared PC obtained at 600? exhibit high capacity,rate performance and long-life cycling.For LIBs,a maximum reversible capacity of 504.7 mAh g-1 can be kept at a current density of 0.1 A g-1 after 100 cycles,and even at a very high current density of 5.0 A g-1,the capacity can be reached up to 172 mAh g-1.And for SIBs,maximum reversible capacities of 192 mAh g-1 and 138.8 mAh g-1 can be obtained at 0.05 A g-1 and 2.5 A g-1,respectively,exhibiting excellent rate capability.2.The Fe2O3@C composite electrode material was synthesized via two-step heat treatment process,and they exhibit a hierarchially porous structure.The Fe2O3@C composite electrode shows excellent electrochemical performance owing to the improved electron transfer and the buffering of volume changes from carbon layer and porous structure.The results show that:(1)a maximum reversible capacity of 1000 mAh g-1 is achieved at 0.1 A g-1 after 100 cycles for LIBs with good cycling stability;(2)a maximum reversible capacity of 237 mAh g-1 is obtained at a current density of 0.05 A g-1 for SIBs.3.Shuttle-like porous carbon-coated FeP was prepared from Fe-based MOFs(MIL-88)as precursors and used as anodes for LIBs/SIBs.Benefiting from the advantages of untrafine FeP nanoparticles distributed in porous carbon,the FeP@C-600 delivers high capacity for lithium storage with excellent cycling stability(902.4 mAh g-1 at 0.1 A g-1 for 100 cycles),superior rate performance(reversible capacity of 416 mAh g-1 at 5.0 A g-1)and long-life cycling(3000 cycles at 5.0 A g-1),and a reversible capacity of 230 mAh g-1 as well as excellent rate performance and long-life cycling are also achieved for SIBs.The outstanding electrochemical properties should be ascribed to unique structure and pseudocapacitive behaviors during charge/discharge process,especially at high current density.4.The heterojunction structure Fe2O3/Fe1-xS@C composite was obtained via one-pot heat treatment of MIL-88(Fe)and S powder mixture.The composite displays a hollow and porous structure,with a pore size distribution in micro and mesoporous range.When used for LIBs,it shows excellent electrochemical performance for lithium storage with a maximum reversible capacity of 1200 mAh g-1 at 0.1 A g-1 after 100 cycles.A capacity of 345 mAh g-1 can be maintained even at a high current density of 5.0 A g-1,with a long-life cycling.Though it shows high capacity for sodium storage,the cycling stability and rate performance are still unsatisfactory owing to abundant micropore unsuitable for large sodium ions and low pseudocapacitive contribution. |
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