| To meet the demands for miniaturization of electronic devices and effectively solve the energy storage issues of hybrid power systems,currently,all kinds of the energy storage devices have been studied and widely used in daily life and industrial production.Among them,rechargeable sodium ion batteries?SIBs?are a potential candidate for the new-style energy storage devices that can be expected to replace lithium ion batteries?LIBs?to achieve large-scale and low-cost production based on its abundant resources,low price and widely distributed.Although sodium has a similar chemical properties with lithium,however,sodium ion has a larger ionic radius than lithium one,making its kinetics and reversible insertion/extraction in rigid materials is hindered and resulting in an unsatisfied sodium storage performance.Therefore,selecting the host electrode materials and designing a new electrode structure to construct the electrode of SIBs can effectively promote its rapid development.With the rapid development of nanotechnology,a series of new nanomaterials such as metal oxides,sulfides,selenides and binary metal oxides have received enough attention because of their high theoretical capacity,but their poor electronic conductivity and serious structure collapse greatly restrict their electrochemical performance upon cycling.Therefore,it is an important topic that how to further optimize and construct the unique nanocomposite to achieve a high capacity and long life in the SIBs fileds.In this research project,four different nanocomposites were prepared by means of carbon-coating and two dimensional graphene combined with antimony-based nanomaterials,which can improve the electronic conductivity and suppress the volume change of antimony-based nanomaterials upon the discharge-charge process,achieving superior sodium storage performance with the large capacity,long life and high rate capability.On this basis,a new conceptual electrode architecture was constructed and used as a binder-free anode for SIBs,and systematically studied the effect of electrochemical performance of electrode structure to nanomaterials.The main contents and results are as follows.1.Metallic antimony has been extensively studied due to its large theoretical capacity,but its huge volume expansion result in structure collapse and shows a rapid capacity loss.The carbon-coated antimony nanofibers were prepared by electrostatic spinning techniques followed by the high temperature sintering.Meanwhile,the morphology of antimony nanofibers are further investigated by regulating the amount of Sb?CH3COO?3 precursor.Electrochemical tests are used to discuss the influence of amount of precursor on the electrochemical performance.The results indicate that the antimony nanoparticles are uniformly dispersed inside the C@Sb nanofiber and have a certain gap between the nanoparticles when Sb?CH3COO?3 precursor is 1 mmol,thus realizing the effective protection of the antimony nanostructures upon cycling.In addition,a mesh structured nanofibers can improve the fast transport of electrons and enhance the diffusion kinetics of sodium ion.The results show that the obtained C@Sb nanofiber has a good cycling stability,showing a high discharge capacity of 386.3 mAh g-1 with 73.8% retention of the capacity at 2nd cycle after 700 cycles at 1 A g-1,as well as273 mAh g-1 with capacity retention of 60% after 4000 cycles at 5 A g-1,respectively.Meanwhile,C@Sb nanofiber has a superior rate capacity that the discharge capacity is about 272.1 mAh g-1 at a current density of 20 A g-1.2.Metal oxides have been widely studied due to their low cost and easy availability,but their defects such as low electronic conductivity and large volume expansion limit their practical application.Herein,the prepared polymethacrylic acid brushes with a pyrenyl derivative can functionally modify the surface of graphene oxide to construct a three-dimensional frame structure.And then,the polymer brush modified graphene@Sb2O3?PMGS?hybrids were prepared via hydrothermal process.The prepared PMGS electrode shows a high specific capacity and good cycling stability,delivering a discharge capacity of 442 mAh g-1after 120 cycles at 100 mA g-1,whichare about 2.3 and 8.2 times larger than that of rGO@Sb2O3 and plain Sb2O3 electrode,respectively,followed by a large revisbile capacity of 220 mAh g-1 after 200 cycles at400 mA g-1.In addition,it also shows a good rate capability,delivering a larger capacity of 160 mAh g-1at a high current density of 800 mA g-1,which are 1.4 and 5.7 times larger than that of rGO@Sb2O3 and plain Sb2O3,respectively.Meanwhile,the PMGS electrode shows the most rapid ion diffusion kinetics in three electrodes by AC impedance spectroscopy?EIS?.The superior sodium storage performance is mainly due to the presence of polymer brush,which can reduce the nanoparticles size and increase the contact area between the nanoparticles and electrolyte,as well as improve the mechanical properties of the materials and prevent the aggregation of Sb2O3 nanoparticles upon cycling,resulting in a good electrochemical performance.At the same time,the phase transition mechanism of PMGS at different charged and discharged state was systematically studied by means of XRD,XPS and TEM,showing the presence of conversion and alloy reaction upon the initial cycling.3.It is well known that the combination of nanomaterials with graphene can effectively improve the electronic conductivity,but graphene directly used as a conductive component of an electrode in a sodium-ion battery cannot maximize capacity,and its low tapping density and volumetric capacity limited the sodium storage performance of nanomaterials.In this work,the urea is used as the nitrogen source,followed by a mesh-structured nitrogen-doped graphene@Sb2Se3 hybrid?N-doped graphene@Sb2Se3,NGS?was one-pot hydrothermal prepared.Meanwhile,the graphene@Sb2Se3?GS?and plain Sb2Se3 were also fabricated using same methods without adding urea or graphene.As compared to GS and Sb2Se3,NGS has a three dimensional mesh structure,resulting in a large contact area between NGS and electrolyte as well as the fast electron transport.Electrochemical tests show that the type of charge storage of NGS is the ion diffusion behavior during the entire electrochemical process and it has a good sodium storage capability,delivering a high discharge capacity of 548.6 mAh g-1after 50 cycles at 100 m A g-1,which are larger than that of GS and Sb2Se3 by 1.6 and 2.8 times,respectively.Even if a current density of 1.5 A g-1,the specific capacity of NGS electrode reach to about 337 mAh g-1,which are 3 and 12.5times larger than that of GS and Sb2Se3,respectively.The diffusion coefficient of NGS is larger than that of GS and plain Sb2Se3 by 3 and 8.8 times,respectively.This strongly indicates that the mesh-like NGS structure can facilitate a fast sodium ion transport.As a basis,the better performance is ascribed to the unique intertwined porous mesh-likestructure associated with a strong synergistic effect of N-doped graphene for dramatic improvement of electronic and ionic conductivity by the unique porous structure,the specific capacity of graphene from N doping and fast interfacial electron transfer rate by N-doping induced surface effect and the structure-shortening insertion/desertion pathway of Na+.In addition,a flexible mesh-like graphene covering the Sb2Se3 nanorods provides an elastic buffer spacing to accommodate the volume expansion/reduction,thus efficiently retaining the structural integrity during the sodiation and desodiation process for good cycle life stability.4.In order to solve the thermodynamics and kinetics of sodium ion diffusion during the charge and discharge process.Herein,we synthesized Sb2S3/CNT composites by a facile hydrothermal method.And then,a multilayered?Sb2S3/CNT@RGO?n electrode was constructed by means of the dip coating and electrochemical deposition methods.Different layered electrode?2,3,4,5 and 6 layers?were constructed and investigated to further explore the relation of electrochemical performance versus number of layers of Sb2S3/CNT,revealing the highest reversible capacity and specific energy occurs when the layer number equals to 3.The results show that the?Sb2S3/CNT@RGO?3 electrode has a much higher discharge capacity of 604 mAh g-1after 100 cycles at 400 mA g-1 and more outstanding rate capability of 400 mAh g-1 at 3A g-1than that of a bulk Sb2S3/CNT electrode?Sb2S3/CNT: carbon black: binder = 7:2:1?by 7.5 and 6.1 times,respectively.In addition,the diffusion coefficient of sodium ion in?Sb2S3/CNT@RGO?3 is 5.3 times higher than that of the non-layered Sb2S3/CNT one,showing that the former structrure can also boost fast Na+transport.The superior electrochemical performance is attributed to the multi-layered structure for significantly improvement of the thermodynamic and kinetic limits with faster charger transfer,higher sodium ion accessibility and higher mass transport.This work provides a fundamental new method to construct a practical porous electrode with high efficient.In summary,effectivly design the electrode materials or reasonably construct the electrode structure can greatly improve electronic conductively as well as prevent structure collapse relating to sodium insertion/extraction caused of electrode materials,and further enhancing the sodium storage capability. |
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