| With the rapid increase of electric energy consumption,the development of energy storage technology and devices becomes more and more important.Currently widely used lithium-ion batteries(LIBs)possess high energy density and high output voltage.However,it is challenging to significantly reduce the cost due to the limited reserves and nonuniform distribution of lithium resources.Therefore,it is urgent to develop alternative low-cost energy storage devices to meet the large-scale and low-cost electricity demand.Because the sodium and potassium elements with similar properties to lithium are more abundant in the earth’s crust,sodium-ion batteries(SIBs)and potassium-ion batteries(PIBs)with a similar charge-discharge mechanism to LIBs are considered as promising energy storage devices.However,due to the large ion radius Na+ and K+,it is necessary to design electrode materials rationally to overcome the sluggish reaction kinetics in the research of SIBs and PIBs.In this thesis,the alternating intercalation structure of layered MoS2(VS2)/carbon was used to improve the conductivity of nanostructures and expand interlayer spacing.Moreover,the structure was combined with conductive substrates to construct a three-dimensional(3D)conductive network.This structure can significantly enhance the electrical conductivity of composite,reduce the ions diffusion barrier,and effectively improve the structural stability of the electrode materials.As a result,electrode materials with greatly improved electrochemical performance were prepared,and high-capacity&high-stability sodium-/potassium-ion batteries were fabricated.The major results include:(1)Cetylpyridine chloride(CPC)was intercalated into the adjacent monolayers of ultrathin MoS2 nanosheets,and the alternate intercalation structure of MoS2/N-C assembled by N-doped porous carbon(N-C)and monolayer MoS2 was obtained through following high temperature treatment.In this composite,the N-doped porous carbon with high electrical conductivity is tightly bonded with MoS2 layers,which can greatly enhance the overall electrical conductivity of the composite.The significantly expanded interlayer spacing of MoS2 can greatly decrease the ions diffusion barrier,and effectively buffer the dramatic volume change during the ions intercalation-deintercalation processes.Due to these structural advantages,MoS2/N-C composite shows excellent electrochemical performance and structural stability.When used as the electrode of SIBs,the wool spherical-like MoS2/N-C composite delivers the specific capacity of 393 mAh g-1 after 650 cycles at 1.0 A g-1.When used as the electrode of PIBs,it delivers the specific capacity of 313 mAh g-1 after 250 cycles at 1.0 A g-1.(2)By anchoring the ultrathin nanosheets of monolayer carbon(m-C)and MoS2 with alternate intercalation structure on both sides of the conductive Ti3C2 substrate,hierarchical MoS2/m-C@a-C@Ti3C2 composite with high electrical conductivity,abundant active sites,low ions diffusion barrier,and stable structure were successfully prepared.In this composite,the 3D interconnected conductive networks constructed by vertically anchored m-C and lateral amorphous carbon(a-C)coated Ti3C2 substrate can facilitate the rapid charge transfer.Few-layered MoS2 nanosheets can not only expose more active sites for Na+ storage,but also greatly shorten the diffusion distance of ions.On one hand,the nanostructure with significantly expanded interlayer spacing can greatly reduce the ions diffusion barrier.On the other hand,it can effectively buffer the dramatic volume change during charge-discharge processes and improve the stability of electrode materials.In addition,the abundant functional groups on the surface of Ti3C2 networks can enhance the interfacial bonding strength,which is beneficial to the structural stability of the electrode.The synergistic effect of these advantages makes MoS2/m-C@a-C@Ti3C2 composite with excellent electrochemical properties.When used as the electrode of SIBs,the hierarchical MoS2/m-C@a-C@Ti3C2 composite delivers the specific capacity of 212 and 135 mAh g-1 after 2000 cycles at large current densities of 2.0 and 5.0 A g-1,respectively.(3)The stabilized 1T-rich MoS2/m-C nanoflowers with rich 1T-MoS2 were successfully prepared via a synergetic strategy of electron-injunction and atomic interface engineering.In this composite,the 1T-rich MoS2 with high electrical conductivity and carbon monolayers are alternately intercalated and tightly bonded by strong interfacial interaction.This special structure can effectively stabilize metastable 1T-pahse MoS2,and improve the electrical conductivity and structural stability of this composite.Moreover,the intercalation of monolayer carbon also can greatly expand the interlayer spacing of MoS2 and strikingly promote the intercalation-deintercalation processes of the ions.As expected,the 1T-rich MoS2/m-C composite exhibits superior electrochemical performance.When used as the electrode of SIBs,the IT-rich MoS2/m-C composite delivers high specific capacities of 557 mAh g-1 after 80 cycles at 0.1 A g-1 and 364 mAh g-1 after 1000 cycles even at a large current density of 2.0 A g-1,respectively.In 1T-rich MoS2/m-C//NaV2(PO4)3/C sodium-ion full cell,the reversible specific capacity still reaches 215 mAh g-1 after 400 cycles at 0.5 A g-1.When used as the electrode of PIBs,it delivers the specific capacity of 415 mAhg-1 after 100 cycles at 0.1 A g-1.(4)On the basis of structural design,VS2/N-C@rGO composite was prepared by our unique high-pressure solvothermal method,achieving the extension of alternate intercalation structure.In this composite,an alternate intercalation structure of metallic VS2 monolayer N-doped porous carbon can effectively improve the conductivity of the composite,and significantly expand the interlayer spacing of VS2,which could dramatically decrease the ions intercalation-deintercalation barrier.The ultrathin structure of VS2 nanosheets can greatly reduce the diffusion distance of ions in the electrode.Moreover,the conductive rGO framework can notably improve the overall electrical conductivity of the composite and effectively inhibit the aggregation of VS2 nanosheets.Considering the synergistic effect of these structural characteristics,the VS2/N-C@rGO composite exhibits excellent electrochemical properties.When used as the electrode of SIBs,the VS2/N-C@rGO composite delivers high specific capacity of 316 and 220 mAh g-1 after 1400 cycles at large current densities of 2.0 and 5.0 A g-1,respectively.When used as the electrode of PIBs,it delivers the specific capacity of 216 mAh g-1 after 500 cycles at 1.0 A g-1. |
