Design And Construction Of Tin-based Composites And Their Application In Anodes For Sodium-ion Battery

As a promising substitution for lithium-ion batteries,sodium-ion batteries?SIBs?possess the merits of low-cost and nature abundance.In this regard,SIBs become a potential energy storage device for grid-scale applications.However,due to the larger ionic radius and atomic mass of sodium respect to lithium,the energy density,power density,as well as cycling stability of SIBs are still unsatisfied for practical implementations.Given the fact that the electrochemical performance of cathodes is intrinsically restricted to a certain value,the performance of full cells are highly dependent on the properties of anodes.Therefore,there is an urgent need to explore anode materials with enhanced electrochemical performances and simplify the synthesis approach.Tin-based materials with relatively high theoretical specific capacities are widespread and ease-synthesis,which become a promising candidate for SIBs anodes.During the repetitive sodiation/desodiation process,tin-based materials suffer from huge volume variation,which result in the pulverization of active materials and lost contact with the collector.As a result,poor specific sodium storage capacity and cycling stability are obtained.In order to solve the above mention issues as well as enhance the electrical performance of tin-based anodes,various approachs were used to constructed composites with different carbonaceous matrixes.The structural and component information were systematically investigated and sodium storage mechanisms were demonstrated.The brief works of this paper are as below.?1?SnO₂/CNTs composite was firstly synthesized through a facile hydrothermal approach.For further improve the electrochemical performance,the obtained composite was subsequently sulfurized via a fixed-bed assisted time-dependent process.The relative structural and component information reveal that with the sulfurization time increase,the content of SnS₂ with sheet structure is increase,which can effectively prevent the aggregation of SnO₂particles.The product obtained under 4 h sulfurization treatment was consisted with SnO₂ particles and SnS₂ nanoplates.Further characterizations show that the particles were both attach to the surface of CNTs and SnS₂ nanoplates.As a result,heterostructures were constructed,which favors the transportation of electrons between interfaces.When the material wass tested as anode for sodium-ion batteries,an initial discharge specific capacity of 543 mAh g^-1 was maintained at a current density of 50 mA g^-1.After 100 cycles,the capacity degraded to 355 mAh g^-1,corresponding to a capacity retention of 65.3%.In contrast,the pristine SnO₂/CNTs composite only delivered 215 mAh g^-1 after100 cycles under the same conditions due to the severe aggregation of active particles.?2?Besides,CNTs modified carbon nafibers?CNFs?confined SnO₂hybrid was obtained by electrospinning and the following thermal treatment?SnO₂@CNFs/CNTs?s.On one hand,the incorporated CNTs in CNFs could improve the electrical condivity of the material.On the other hand,CNTs favored the graphitization behavior of CNFs,and thus the harsh graphitizing process could be simplified.Thanks to the flexible structure of the fibers,the resultant materials can be directly applied as anodes for SIBs.Take advantage of the finely encapsulation effect of CNFs,the cyclic life was signally prolonged.When tested as freestanding andoe for SIBs,a reversible capacity of 460.3 mAh g^-1 was obtained after 200 cycles at 100 mA g^-1.Impressively,a specific capacity of¹60 mAh g^-1 was retained after 1000 cycles at 2 A g^-1.?3?Nanostructure of active materials has great effect on the sodium storage performance.The effect of dimensionality or morphology on electrochemical performances of SnS₂/CNTs was demonstrated.These elaborately engineered composites were obtained by an easily time-dependent solvothermal approach.Structural and morphological characterizations depict that 2D composites were constituted by 2D SnS₂ sheets with CNTs crossed through the void space.While,the 3D composite displays a tight self-assembled structure and the cores present a visual solid structure.Therefore,2D composite possesses higher specific surface area than that of 3D counterpart.Galvanostatic discharge–charge measurements demonstrated that2D electrode maintained an enhanced electrochemical performance than 3D electrode.After 100 cycles,2D and 3D electrode could gain specific capacities of 476 mAh g^-11 and 362 mAh g^-1,respectively.In order to elucidate the reasons associated with this performance difference,the sodium storage mechanism was investigated.The calculated results reveal that pseudocapacitance contribution of 2D composite is higher than 3D composite,which favors for rate and cycling performances.?4?Take advantage of merits of the CNTs,we designed and prepared CNTs?length:15³0?m?wrapped flower-like SnS composite.The adjacent self-assembled flowers were bridged together by CNTs,which enhanced the electronic conductivity throughout the whole electrode.Furthermore,thanks to the flexible structure of CNTs,the volumetric stress derived from sodiation/desodiation cycles could be easily accommodated.Such a porous structure with high specific surface area(86.8 m² g^-1)provided sufficient reaction sites between active materials and electrolyte.As a result,the composite obtained a highly improved cycling stability and rate capability when employed as anode for SIBs.Specifically,a reversible capacity of 464mAh g^-1 was gained at 50 mA g^-1.Evan at a current density of 3200 mA g^-1,the electrode still could deliver a capacity of 244 mAh g^-1(126 mAh g^-1 for bare SnS).?5?Considering that 2D composites have higher apparent diffusion coefficient than their 3D counterparts and SnS experience less volume change than SnS₂.We obtained CNFs confined 2D SnS through electrospinning and thermal treatment with 2D SnS₂/CNTs and PAN as precursors.During the heat treatment,2D SnS₂ transformed to 2D SnS through a phase transition process.Besides,S atoms generated form SnS₂ dissociation in-situ doped to CNFs,which also favored the sodium storage performance.The final product with optimized composition could obtain a reversible capacity of 499.9 mAh g^-1 at100 mA g^-1.After 600 depth cycles at current density of 0.8 A g^-1,the cell still could maintain a capacity of 296.6 mAh g^-1,corresponding to a capacity retention of 78.9%.