| Lithium ion batteries (LIBs) have been considered as the potential candidates in various energy storage fields due to its high energy density and long cycle life. Graphite is currently commercial anode materials, however, its low theoretical specific capacity (372 mAh g"1) and serious safety problems resulting from the lithium dendrites limit its wide applications. Therefore, to develop a new alternative anode material with high energy capability is a new challenge for researchers. Among all of the current anode materials, transitional metal sulfides have attracted great attention owing to their unique properties, such as low-cost, abundant in earthcrust and high specific capacity. Still it have some drawbacks to limit their practical applications for energy storage. For instance, the volume changes during electrochemical reaction can lead to the pulverization and fracture of active materials, which can peel off easily from the conductive substrate. Meanwhile, the dissolution of sulfur ion into the electrolyte is another problem. The above difficulties can be addressed by the structure modification and carbon coating. In this paper, two kinds of transition metal sulfides including Co9S8 and VS4 are discussed. The nanomaterials with various specific morphologies and carbon coated composites have been synthesized via a facile solvothermal method. As anode materials for LIBs, the electrochemical performances of all of the prepared materials have been investigated.The main content in this research work can be described as follows:(1) The amorphous CoS2 hollow nanospheres have been prepared by a facile solvothermal approach, to increase the crystallinity of the above product, a subsequent annealing process was conducted, while the final product is cubic mesoporous Co9S8 hollow nanospheres. As anode materials, the electrochemical performance of mesoporous Co9S8 hollow nanospheres is better than that of CoS2, and the capacity for Co9S8 hollow nanospheres increases to some degree as the cycle number increases, finally, the reversible capacity is as high as 1414 mAh g-1 after 100 cycles. The continuous capacity recovery is associated with enhanced capacitance caused by repeated electrochemical milling. Unfortunately, the cycle stability of mesoporous Co9S8 hollow nanospheres is not ideal when cycled at high current densities. Thus, the carbon coated mesoporous Co9S8 hollow nanospheres (Co9S8@C) were further fabricated via the chemical vapor deposition (CVD) method. When cycled at 2 A g-1, the reversible capacity can retain at 896 mAh g’1 after 800 cycles. The carbon layer can not only increase the electroconductivity, but can effectively buffer the volume change during cycling, and also can depress the dissolution of sulfur ion into the electrolyte. The excellent lithium-storage performances shed light on the promising potential of sulfides as a high capacity, high rate and long life anodes.(2) Based on the above work, the one-dimensional MWCNT@a-C@Co9S8 nanocomposite has been prepared via a facile solvothermal reaction followed by a calcination process, in which MWCNTs@a-C were used as a growth template. If using MWCNTs as growth template, the obtained product is not good, and some agglomeration of Co9S8 nanoparticles can be observed except for the MWCNT@Co9S8 nanocomposite. The results show that the amorphous carbon between MWCNTs and Co9S8 as a linker can increase the loading of Co9S8 on MWCNTs. When evaluated as anode materials for LIBs, the MWCNT@a-C@Co9S8 nanocomposite shows the high capacity and long cycle life, superior to Co9S8 nanoparticles and MWCNT@Co9S8 nanocomposites. The reversible capacity could be retained at 662 mAh g-1 after 120 cycles at 1 A g-1. The good cycle stability for MWCNT@Co9S8 nanocomposites is attributed to its unique hierarchical structure, first, the amorphous carbon can inhibit the Co9S8 peel off from MWCNTs. Besides this, the nanosized Co9S8 can improve the diffusion kinetics and buffer the volume changes during cycling. Moreover, the MWCNTs@a-C can increase the electroconductivity of this nanocomposite, which can improve the electron transfer efficiency.(3) Well dispersed VS4 sub-microspheres with a particle size of 500-800 run, have been successfully synthesized via a simple solvothermal reaction. In this case, we first prepared the pure VS4 without graphitic layer. When these particles were examined as an electrode material, they present the poor cycling stability and rate capability. Thus, three conductive polymers, PEDOT, PPY and PANI are deposited on the surface of the VS4 particles to improve the electron conductivity, suppress the diffusion of polysulfides and modify the interface between electrode/electrolyte. As a result, all the composites exhibit enhanced electrochemical performance, compared with naked VS4 sub-microparticles. The coulombic efficiency for the first cycle is～86% for PANI-coated particles, which is higher than that of reported GO@VS4 composites. They also exhibit the best cycling stability and rate capability in the polymer-coated VS4. The reversible capacity have been maintained upto 755 mAh g-1 after 50 cycles. The insights are discussed on the basis of the interaction between the conductive polymers and VS4, and it has been confirmed by related theoretical simulation.(4) Based on the above work, the hierarchical MWCNTs@a-C@VS4 composites have been fabricated via the simple solvothermal method. Meanwhile, the different amount of MWCNTs@a-C are discussed, and finally three kinds of MWCNTs@a-C@VS4 composites named MWCNTs60@a-C@VS4, MWCNTs100@a-C@VS4 and MWCNTs150@a-C@VS4 composites have been obtained. Then, all the above obtained composites were used as anode materials to discuss their electrochemical performance. It can be observed that the cycle stability of MWCNTs60@a-C@VS4 is poor, and the reversible, capacity decreases very quickly as the cycle number increases. The main reason is that the amount of MWCNTs wrapping VS4 sub-microspheres is little, which cannot buffer the volume change during cycling and suppress the dissolution of the polysulfide into the electrolyte. While for the MWCNTs100@a-C@VS4 and MWCNTs150@a-C@VS4 composites, they exhibit high reversible capacity and good cycle stability at a current density of 0.1 A g-1, the reversible capacities for both of the two composites are 932 and 1244 mAh g-1 after 100 cycles, respectively. But, when cycled at high current densities such as 1 A g-1, the specific capacity of MWCNTs100@a-C@VS4 composites decreases dramatically, while for the MWCNTs150@a-C@VS4 composites, it present better cycle stability. Besides, two other kinds of composites (MWCNTs150@VS4 and MWCNTs 150+VS4) are also prepared, when used as electrode materials, their electrochemical performance is inferior to that of MWCNTs150@a-C@VS4 composites. Meanwhile, the composites were also tried to be used as anode materials for sodium ion batteries (SIBs) due to its large layer distance and specific chain structure of VS4. The reversible capacity can reach 722 mAh g-1 at 0.1 A g-1, meanwhile, after 30 cycles, the good cycle stability of the MWCNTs150@a-C@VS4 composites have been obtained. |
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