Improved Performance Of Silicon-based Anode Materials For Li-ion Batteries By Buffer Structure Design

Silicon is most likely to replace graphite as the next-generation anode material for lithium-ion batteries due to its 10 times specific capacity higher than graphite and its natural abundance.However,the large volume change during cycling leads to pulverization of Si,fracture of the electrode,and an unstable solid electrolyte interphase(SEI),seriously affecting its cycle life.Besides,the poor conductivity of silicon affects its rate performance.In this dissertation,in order to solve the problem of large volume expansion in the lithiation process of Si,different buffer structures were designed to improve the structure and interface stability of silicon-based anode materials,and the controllable preparation of silicon-based anode materials was achieved through the optimization of synthesis methods.The specific research contents are as follows:1.Design and fabrication of graphene wrapped silicon composite fibers(nSi@rGS).Volume expansion of Si is buffered by the characteristic of the graphene scrolls in synchrony with the repeated expansion/contraction of Si,maintaining structure and interface stability.The silicon nanoparticles were combined with the graphene oxide by electrostatic attraction,then the graphene oxide was curled by means of isolation freeze-drying to wrap Si nanoparticles in the interlayers of graphene oxide scrolls.nSi@rGS was finally fabricated after high-temperature thermal reduction.nSi@rGS shows a high cycling stability of 1200 mA h g-1 at 4 A g-1 with 99.2%capacity retention after 200 cycles and an excellent rate performance of 1000 mA h g-1 even at 8 A g-1.The excellent electrochemical performance of nSi@rGS is closely related to its structure:The Si nanoparticles are firmly wrapped in the reduced graphene oxide scrolls,which prevents them peeling from graphene sheets,leading to a high specific capacity;The reduced graphene oxide scrolls have a self-adaptive ability in synchrony with the repeated expansion/contraction of nSi to maintain the structural integrity,leading to a good cycling stability;The high conductivity and the open structures at the ends and sides of the reduced graphene oxide scrolls increase the electron and lithium ion transport speeds,leading to an outstanding rate performance.2.Design and fabrication of TiO2-x coated mesoporous hollow Si nanospheres(MHSi@TiO2-x).The outward expansion of Si is suppressed,and the inward expansion is buffered,maintaining structure and interface stability.MHSi@TiO2-x was fabricated through the magnesiothermic reduction of hollow silica nanospheres to generate mesoporous hollow silicon nanospheres,tetrabutyl titanate hydrolysis on the surface of mesoporous hollow silicon nanospheres,and sequent calcination in an inert atmosphere.MHSi@TiO2-x shows an excellent cycling stability of 1303.1 mA h g-1 at 2 A g-1 with 84.5%capacity retention after 500 cycles,and the coulomb efficiency is higher than 99%after 10 cycles.The excellent electrochemical performance of MHSi@TiO2-X is closely related to its structure:The TiO2-X shell suppresses the outward expansion of the mesoporous hollow Si nanospheres,and the mesoporous hollow structure provides enough inner void space for the inward expansion of Si,leading to an excellent cycling stability;The TiO2-X shell not only prevents the direct contact between the electrolyte and Si,but also contributes a slight volume vibration upon lithiation to maintain the interface stability,leading to a high coulombic efficiency during cycling.3.The preparation process of yolk-shell structure precursor is simplified,and the 'controllable preparation of yolk-shell structure composite(Si@Void@C)is achieved;Constructing another yolk-shell structure with carbon-encapsulated Si core(Si@C@Void@C)to reserve buffer space for the outward expansion of Si and increase the electrical contact between Si core and the carbon shell.One-step coating of SiO2 and phenolic resin(RF)on the surface of Si nanoparticles to obtain the precursor Si@SiO2@RF was realized because the speed of ethyl orthosilicate hydrolysis to generate SiO2 is faster than that of the condensation of resorcinol and formaldehyde to form RF.Then the precursor was calcined and etched by hydrofluoric acid to generate Si@Void@C.This method reduces the washing and drying process when SiO2 and RF are stepwise coated.Si@C@Void@C was also fabricated through the calcination and etch of the precursor Si@RF@SiO2@RF,which was synthesized by the one-step coating of Si02 and RF on the surface of Si@RF nanoparticles.Both yolk-shell structure composites have excellent cycle stability,and the capacity retention rates exceed 88%after 100 cycles.Compared with Si@Void@C,Si@C@Void@C exhibits a higher initial coulombic efficiency and a better rate performance,which are attributed to the fact that the carbon coating on the inner Si core surface reduces the erosion of the Si core by the electrolyte and increases the electrical contact between the Si core and the outer carbon shell.4.Design and fabrication of flake-graphite-based Si/C composite microspheres(Si-SiO/C-CNTs@pC).Si,SiO and flake graphite are uniformly distributed in the microsphere to increase the silicon content in the Si/C composite which is mainly composed of graphite.SiO and flake graphite were first broken and mixed with Si nanoparticles by ball-milling,and further reduction of the sizes of SiO and flake graphite are realized by sand-milling.Si-SiO/C were prepared through spray drying,in which Si,SiO and flake graphite were uniformly dispersed.Then Si-SiO/C was modified by adding carbon nanotubes and asphalt coating to generate Si-SiO/C-CNTs@pC.The composite microsphere has a tap density of 0.69 g cm-3,and the total content of Si and SiO was 25.2%.Si-SiO/C-CNTs@pC shows a high reversible specific capacity of is 587.6 mAh g-1 at 0.25 A g-1,and the capacity retention rate after 100 cycles is 97.7%.The good electrochemical performance of Si-SiO/C-CNTs@pC is closely related to its structure:Si and SiO are uniformly dispersed in flake graphite,increasing the content of Si and SiO,thereby increasing the specific capacity;The use of Si and SiO as dual silicon sources to utilize Li2O and Li4SiO4 formed during lithiation of SiO,which act as a buffer layer to buffer the expansion of Si,reducing the effect of volume change when using single Si source;while the expansion of Si is further buffered by the microvoids formed by the stacking of flake graphite;and the carbon coating on the surface of the microsphere maintains structural stability,improving the cycle stability.The addition of CNTs effectively connects the silicon source with the flake graphite and forms a strong conductive network,which facilitates the rapid transmission of electrons,leading to a good rate performance.In this dissertation,buffer structure design is used to solve the problem of large volume expansion during the lithiation of silicon and improve the cycle stability of Si-based anode materials.Through the optimization of the synthesis method,the preparation process is simplified,and the controllable preparation of the Si-based anode materials is realized.The buffer structure proposed in this dissertation can be extended to other electrode materials with large volume changes.