Preparation And Energy Storage Application Of Bi/Bi-Based Compounds@CNTs Composites With Encapsulated Structure

The bismuth（Bi）/bismuth-based compounds have attracted great attentions as electrodes of lithium-ion batteries（LIBs）due to their high theoretical capacities(Bi:386m A h g^-1,Bi₂O₃:690 m A h g^-1,Bi₂S₃:625 m A h g^-1),unique layer structure,large layer spacing and semiconductor properties.In addition,they also have been considered as the ideal sulfur host materials for lithium-sulfur（LSBs）owing to their strong chemisorption and good electrocatalytic performances.However,there are some issues such as the nanoparticles agglomeration/pulverization,poor conductivity,low sulfur loading and serious volume expansion,which seriously limit the application of these materials both in LIBs and LSBs.To solve the above problems,an innovative method is to construct one-dimensional nanocomposites with encapsulation structure,in which the Bi/Bi-based compounds are fixed in the cavities of carbon nanotubes（CNTs）.Sush a structure can not only increase the conductivity of the electrode and avoid the agglomeration/pulverization of nanoparticles,but also enhance the structural stability of the electrode,resulting in high electrochemical performance for LIBs.Meanwhile,as the host of LSBs,under the synergy of physical detention of abundant pores and chemical adsorption of Bi/Bi-based compounds,the diffusion of polysulfides（LIPs）can be firmly held and the shuttle effect can be effectively curbed,resulting in the improved electrochemical performance of LSBs.Specific research contents are listed as follows:1.Preparation and electrochemical performances of Bi/Bi-based compounds@CNTs for LIBsFirstly,1D Bi@CNTs nanocomposites with encapsulated structure were prepared by the sol-gel and calcination method.The effects of temperatures on the morphologies and electrochemical performances of the composites were discussed.It turns out that the Bi nanorods in the nanocomposites（BNT3）prepared at 750 ^oC are fixed into the cavities of CNTs.Benefit from it,the contact area between the electrolyte and the active materials can be effectively enlarged,which can promote the electrochemical reaction and improve the utilization rate of the active materials.As a result,BNT3 exhibits excellent cycling stability and rate performance.Meanwhile,BNT3 was further oxidized to prepare the 1D Bi₂O₃@CNTs nanocomposites with encapsulated structure,and the effects of temperatures on the morphologies and electrochemical performances of the nanocomposites were investigated.The experimental results show that the Bi₂O₃@CNTs nanocomposites（BONT2）prepared at 200℃have an intact encapsulated structure.Bi₂O₃ in nanocomposites is very pure.Therefore,the electrode exhibits excellent cycle stability in LIBs(the discharge capacity can reach 469 m A h g^-1 after 400 cycles at 0.4A g^-1).Furthermore,BNT3 was vulcanized to prepare the 1D Bi₂S₃@CNTs nanocomposites with encapsulated structure.The effects of S mass on the structural characteristics and electrochemical performance of composites were discussed.The results show that Bi₂S₃exhibits the nanorod structure when the mass ratio（Bi@CNTs nanocomposites vs sulfur）is 1:1,which can enlarge the contact area between the active material and the electrolyte,leading to a high capability of 581.9 m A h g^-1 after 600 cycles at 1 A g^-1.Three Bi/Bi-based compounds@CNTs nanocomposites in LIBs showed excellent electrochemical performance due to their encapsulated structures,in which the Bi,Bi₂O₃and Bi₂S₃ nanoparticles/nanorods are completely fixed in the cavities of CNTs,successfully resolving the problems of the aggregation/pulverization.Meanwhile,the 3D conductive network formed by CNTs can enhance the conductivity of the electrode,and accelerate the ion/electron transmission rate,resulting high reaction kinetics.In addition,the unique encapsulated structure can provide a large space to accommodate the volume expansion in the alloying process,ensuring the integrity of the electrode structure and improving the cycling stability of the electrode in the cycling process.2.Preparation and electrochemical performances of Bi/Bi-based compounds@CNTs/S for lithium-sulfur batteriesBased on the unique encapsulated structure and excellent electrochemical performance of Bi@CNTs,Bi₂O₃@CNTs and Bi₂S₃@CNTs in LIBs,they can be further used as sulfur host.The Bi@CNTs/S,Bi₂O₃@CNTs/S and Bi₂S₃@CNTs/composites were prepared by loading sulfur in-situ and melting infiltration,where the sulfur permeates through the walls of CNTs and is dispersed on the surface of Bi,Bi₂O₃ and Bi₂S₃ nanoprods in the melting state.Such a structure can avoid the agglomeration of sulfur and effectively improve the conductivity of the electrode.Meanwhile,the encapsulated structure can provide enough space to alleviate the volume expansion in the cycling process.More importantly,under the synergistic effect of the retention of CNTs and the chemisorption of bismuth-based compounds,the dissolution of LIPs can be effectively prevented to alleviate the"shuttle effect".As a result,the capacities of the Bi₂O₃@CNTs/S and Bi₂S₃@CNTs/S electrodes can achieve 639.4 and 640.7 m A h g^-1after 150 cycles at 0.5 C.By contrast,the Bi@CNTs/S can not only have a stong absorption for LIPs,but also accelerate the conversion rate of LIPs and improve the utilization rate of the active materials due to the excellent catalytic performance of Bi.Therefore,the discharge capacity of 708 m A h g^-1 for the Bi@CNTs/S electrode can be obtained after 200 cycles at 1C,which is higher than the Bi₂O₃@CNTs/S and Bi₂S₃@CNTs/S electrodes.