Design And Fabrication Of Carbon Nanostructures And Their Applications In Lithium/Sodium Ion Batteries

In order to meet the rapid development of electric vehicles,lithium ion batteries?LIBs?need to have higher energy density and power density.Transition metal oxides have been recognized as potential candidates for LIBs anode material due to their high theoretical capacity.However,their large expansion during cycling process and low electrical conductivity limit their commercial application.In addition,the development of clean energy requires large-scale energy storage devices.Sodium ion batteries?SIBs?have cost advantages over LIBs in this field.As a series of promising anode materials for SIBs,alloy-based materials can store Na⁺by alloying reaction with high capacity and low reaction potential.Both of two kinds of materials are faced with the rapid capacity decay in the electrochemical reaction.At present,numerous efforts have been devoted to focus on this issue,such as reducing the particle size to the nanometer scale,compositing with conductive elastic matrix,and preparing hollow nanostructures etc.It has been demonstrated that the ingenious nanostructures could greatly enhance the electrochemical performance of the electrode materials in the batteries.Based on this idea,the main research contents in this paper are divided into the following two aspects:1.By using a hard templating method,we synthesized the hybrid composite of NiO nanoparticles and carbon nanohelmets?NiO/CNHs?,which reserve all advantages of hollow structures but also fully utilize the inner walls of carbon.The hybrid material of NiO/CNHs provides a large amount of extra active sites for Li⁺insertion/extraction from inner walls of carbon;buffers the large volume change of NiO during charging/discharging cycles effectively contributed by the open structure;reduces the internal resistance significantly attributed by the contact between NiO and carbon;and supplies efficient electrolyte pathways from the porous and open carbon skeleton.As expected,the NiO/CNHs hybrid anode shows much better electrochemical performances than the bowl-like structure in LIBs:high specific capacity(1740 mAh g^-1 at 0.3C),superior cycling stability(424 mAh g^-1 after1500 cycles at 7.5C),and excellent rate performances(450 mAh g^-1 at 15C).At the same time,this open helmet-like structure can also be used for other materials.For example,Co₃O₄/CNHs hybrid material was prepared by similar methods,providing a new direction for the development of high performance anode materials for lithium ion batteries2.A hybrid material of ultra-small Sb nanoparticles encapsulated in unique N-doped carbon nanonecklace?Sb/N-CNN?has been synthesized for the first time via electrospinning and in-situ substitution.As an anode material in sodium-ion batteries,the as-fabricated Sb/N-CNN exhibits superior rate property(314 mAh g^-1 at 20 A g^-1)and cycling stability(401 mAh g^-1 after 6000 cycles at 1 A g^-1),outperforming other Sb-based materials.Such excellent performances originate from the ingenious necklace-like structure of Sb/N-CNN,which supplies efficient electrolyte diffusion paths and more active sites for Na⁺insertion/extraction;disperses the stress caused by the large volume change of Sb nanoparticles during the cycling process;and also reduces the electrode resistance.This study provides a facile strategy to construct well-designed structure for advanced energy storage technologies.Moreover,we assembled the full cell of Sb/N-CNN//Na₃V₂?PO₄?₃/C.The initial charge and discharge capacities of the full cell are 812 and 590 mAh g^-1,respectively,corresponding to an initial coulombic efficiency of 72.7%at 100 mA g^-1.Furthermore,the discharge capacity retains as high as 445 mAh g^-1 after 100 cycles at 100mA g^-1,indicating excellent cyclability of the Sb/N-CNN//Na₃V₂?PO₄?₃/C full cell.