Design,Synthesis And Lithium Storage Performance Of Silicon Oxide Composites

At present,lithium-ion batteries(LIBs)are one of the most important energy storage devices due to their high energy density,no memory effect,and good safety.The choice of electrode materials plays a crucial role in the electrochemical performance of LIBs.Among various materials,silicon oxide(SiOx)has been regarded as a promising anode material due to their high theoretical specific capacity,abundant natural resources,easy preparation and nontoxicity.However,SiOx regarded as LIBs anodes still suffers from inherently low electrical conductivity and large volume expansion during the intercalation of Li+,which hinders its practical application.To solve above two main problems,in this paper,we design an electrospray-carbonization approach to modify SiOx materials by nanocrystallization,compositing with conductive materials,and constructing adequate buffer space,which is generally applicable.When used modified SiOx as LIBs anodes,the lithium storage performance including discharge specific capacity,cycle life,and rate capability can be effectively improved.The connections between structure,composition and electrochemical performance of various designed SiOx hybrids have been revealled by a series of typical characterization technique and electrochemical tests.The conditions of the surface and cross-sectional thickness of SiOx-based electrodes before or after cycling can be mearsured by typical SEM images to evaluate the volume effect during the charge/discharge process.The difference of lithium adsorption energy as well as electrical conductivity of the SiOx composites can be provided by density functional theory(DFT)calculations.Moreover,in situ XRD and ex situ XPS tests have been utilized to further analyze the electrochemical mechanism of the SiOx anodes.The five chapters of the paper are given as follows:1.Synthesis and lithium storage performance of silicon oxide/nitrogen doped carbon microspheresBased on biomass rice husks(RHs)as Si source,SiO2 nanoparticles(NPs)with a small size and good dispersion can be directly obtained through pretreatment,calcination,ultrasonic,and centrifugation process.Herein,nitrogen doped carbon(N-C)microspheres with SiOx NPs encapsulated inside are prepared by a facile electrospray-carbonization method,using polyacrylonitrile(PAN)as a source of N-C.The effect of the amount of SiO2 NPs on the morphology and structure of a series of SiOx/N-C microspheres is systematically investigated,which is closely related with the lithium storage performance.The results showed that the introduction of amounts of SiO2 NPs affects the coating of the N-C matrix and dispersity of SiOx NPs.When the content of SiOx in the composites is～64.3%,SiOx/N-C composite(SNC2)exhibits the best performance including discharge specific capacity,cycle performance and rate capability.At a current density of 0.1 A g-1,the discharge specific capacity of 622.8 mA h g-1 can be maintained after 100 cycles.The suitable carbon coating and good dispersion of SiOx NPs promote the electrochemical activity and conductivity of SiOx,and effectively buffer the volume expansion during the charging/discharging process.2.Synthesis and lithium storage performance of silicon oxide/ultrafine metal nanoparticles/nitrogen-doped carbon compositesBased on the previous chapter,transition-metal(Fe,Co,Ni)acetates have been introduced into the precursor solution and a carbonization process under H2-Ar atmosphere is adopted for obtaining a series of SiOx/M/N-C composites.Here,M refers Fe,Co or Ni.The effects of ultrafine metal NPs and N-C matrix on the structure and electrochemical properties of SiOx composites have been studied.It is shown that the calcination temperature has a significant effect on the size of the metal NPs.Taking metallic Ni as an example,increasing calcination temperature leads to the increase of Ni NP size.When the calcination temperature is determined to be 650℃,Fe NPs(～5 nm),Co NPs(～10 nm)and Ni NPs(～5 nm)are all well dispersed in the N-C matrix.Ultrafine Ni NPs can significantly enhance the conductivity of SiOx,effectively buffer the change of volume,and can improve the electrochemical activity and display a certain catalytic effect.The SiOx/Ni/N-C microspheres show the best electrochemical performance.The SiOx/Ni/N-C anode exhibits a high specific capacity of～757.5 mA h g-1 after 200 cycles at a current density of 100 mA g-1,and maintains a discharge specific capacity of 665.0 mA h g-1 after 500 cycles at 0.5 A g-1.In addition,it also shows superb long cycle performance at high current densities of 5 A g-1 and 10 A g-1.Additionally,the ultrafine metal and N-C matrix can inhibit the volume expansion of SiOx,which has been mearsured by observing the surface and cross-section conditions of the electrode before and after cycling.3.Synthesis and lithium storage performance of silicon oxide/iron-nitrogen co-doped carbon compositesIn the previous chapter,it has been found that the samples encapsulated with metallic Fe NPs and Ni NPs both have ultrafine size and show high initial discharge capacity,which can be mainly attributed to the introduction of ultrafine metal NPs as well as the catalytic effect.Based on above results,metal-doped carbon matrix can also be designed to modify SiOx,which can combine the advantages of the size and catalytic effect of metal,so as to further improve the conductivity and electrochemical activity of the SiOx system.In this chapter,ironnitrogen co-doped carbon matrix encapsulated SiOx NPs(SiOx/Fe-N-C)was obtained by dissolving Fe(Ⅲ)acetylacetonate in a PAN/SiO2 mixed solution based on the electrospray and carbonization method.It is noted that Fe element has been successfully doped and the main form of Fe single atoms is verified by HAADF-STEM and XANES tests.When SiOx/FeN-C materials were used as LIBs anodes,it exhibits a high discharge capacity of 799.1 mA h g-1 after 100 cycles at 100 mA g-1 and a discharge capacity of 173.7 mA h g-1 after 5000 cycles at a high current density of 5 A g-1.Compared with pure SiO2 and SiOx/N-C,the discharge capacity and rate capability of SiOx/Fe-N-C are significantly improved.Lithium adsorption energy and the density of states based on DFT calculations reveal that the introduction of FeN-C indeed significantly improves the Li adsorption capability and electrical conductivity of the SiOx hybrids.In addition,the electrochemical tests and in-situ XRD characterization of SiOx/Fe-N-C reveal the oxidation of LixSi phase as well as the storage mechasim and thus imprve the electrochemical reversibility,which can be ascribed to the catalytic effect of Fe single atoms.4.Synthesis and lithium storage performance of silicon oxide/nitrogen doped carbon@reduced graphene oxide compositesAs a typical carbon material,graphene with superb electron transport ability can significantly improve the electronic conductivity of SiOx and the specific capacity.Therefore,multi-layered reduced graphene oxide(rGO)nanosheets coating SiOx/N-C microspheres(SiOx/N-C@rGO)are designed by a hydrothermal method based on the principle of electrostatic adsorption.The effect of rGO coating on the structure and performance of SiOx composites is discussed.The results indicate that suitable rGO coating is benificial for the generation of crimped rGO nanosheets coated on the surface of the SiOx/N-C microspheres.Excess rGO nanosheets lead to poor dispersity and difficulty in generating a complete coating layer.The microspheres are seriously coated within the stacking rGO interlayers(SiOx/NC/rGO).When used as LIBs anodes,the introduction of rGO can both improve the performance of the two kinds of SiOx composites.and SiOx/N-C/rGO.However,SiOx/NC@rGO shows better cyclability than SiOx/N-C/rGO.At the current density of 100 mA g-1,the specific discharge capacities of the SiOx/N-C@rGO and SiOx/N-C/rGO anodes can maintain 709.6 mA h g-1 and 531.4 mA h g-1 after 100 cycles,respectively.This is mainly due to rGO as a kind of carbon materials also suffers from volume expansion and multilayered rGO coating with flexible buffer space can reduce the side reactions between SiOx and electrolyte.Excessive and agglomerated rGO hinders electron transport and cannot effectively accommodate the volume change during cycling.5.Synthesis and lithium storage performance of of silicon oxide/nitrogen doped carbon/reduced graphene oxide compositesUltrafine metal NPs have been proved to be conductive and stable.The effect of metal doping on the improvement of conductivity and catalytic effect is more desirable.Nevertheless,it is difficult and unreasonable to combine the advantages.Moreover,it is no doubt that the advantages brought by the distribution and size of ultrasmall metal NPs should be preserved,for further improving the electrochemical performance.Thus,this charpter focuses on further optimization based on Fe element doping in the fourth charpter.Fe atoms can be fixed by PAN derived N-C matrix,which can prevent the changes in the existing form of metal elements caused by secondary calcination.Meanwhile,graphene oxide(GO)obtained through solvent exchange can be dispersed in the precursor solution.Combined with the electrospraycarbonization strategy,SiOx/Fe-N-C/rGO composites can be obtained,which simplifies its synthetic method of the SiOx composites in the previous chapter.This strategy can possess the advantages of metal and non-metal co-doping,relatively improve the dispersion of SiOx and rGO.Moreover,rGO nanosheets dispersed inside and outside and Fe-N-C matrix can construct a continueous 3D conductive network,promoting the electron transport and ion diffusion.In comparison with SiOx/Fe-N-C and SiOx/N-C/rGO anodes,a series of electorchemical tests have proved that SiOx/Fe-N-C/rGO composites exhibit the optimized lithium storage performance,and the effect of the Fe-N-C matrix and rGO nanosheets has been investigated.As a result,the SiOx/Fe-N-C/rGO anode delivers specific discharge capacities of 701.2 mA h g-1 after 300 cycles at 500 mA g-1 and 203.2 mA h g-1 after 5000 cycles even at a high current density of 5 A g-1.