Preparation And Electrochemical Study Of Silicon-and Germanium-based Composite Anodes For Lithium-ion Batteries

The increasingly serious energy crisis has brought attention to urgent transformation of the energy structure,resulting in a huge demand gap for high-performance energy storage devices.Lithium-ion batteries（LIBs）are widely favored due to the high energy/power density and eco-friendly features.The development of high-performance anode materials is of great significance for the mature development of energy storage devices.Alloy-based anode materials with high specific capacity and low working potential have received much attention from researchers and practitioners.However,Alloy-based anodes face fast capacity decay during cycling,which is caused by the excessive deformation of material within the lithium-ion insertion/extraction process.Therefore,to reduce the particular negative effects through structural design and composite design has become the top priority of current research.Silicon and germanium as members of the group IVA elements shown great electrochemical properties.The thesis takes silicon（Si）-and germanium（Ge）-based materials as the research objects,and takes the composite design with carbon materials as the starting point.Through the idea of??structural design and the introduction of other active components,the study of Si-and Ge-based anodes are carried out respectively to get low-cost,high-performance Si/C and Ge/C anode materials.The thesis systematically studys the prepared Si/C and Ge/C composites,with analysis of the relationship between structure and energy storage performance.Achieved innovative research results are listed below:（1）Reduced graphene oxide（RGO）caging silicon nanoparticles（Si NPs）of the yolk-shell structure（Si@RGOC）is prepared.Through the construction of the sacrificial layer,space is reserved for the volume change of Si NPs during the lithium-ion insertion/extraction process.Following surface amination,combination with graphene oxide（GO）,anneal and etching processes,the Si@RGOC is finally obtained.The effects of cage size and GO precursor content on the structure and electrochemical performance are systematically studied.The reasons for the low coating efficiency of yolk-shell structure are further discussed.To improve the coating efficiency,we demonstrate a novel method for the preparation of defect-repaired reduced graphene oxide caging silicon（Si@DRGOC）with the assistance of glucose.Si@DRGOC delivers enhanced lithium storage performance(a reversible specific capacity of 1911 m A h g^-1at 0.1 A g^-1after 100 cycles,and 1523 m A h g-1 at 2 A g^-1),which is due to good structural stability and dispersion provided by the DRGOC.（2）Considering practical application,a novel method for preparation of yolk-shell structural Si/C composite is proposed.ZIF-67 is selected as the carbon cage template.Through adjusting the carbonization temperature,a layer of extended carbon nanotubes（CNTs）outside the ZIF-67 framework is in situ formed under the catalysis of metallic cobalt nanoparticles（Co NPs）.The simple and controllable method to provide yolk-shell structural carbon nanotube caging silicon（Si@CNTC）can greatly improve the coating efficiency.With the introduction of cobalt oxide component with energy storage activity,the Si@CNTC anode exhibits high specific capacity:1252m A h g^-1at 0.1 A g^-1after 200 cycles with 0.09%capacity decay per cycle.In addition,the Co NPs-doped CNTCs possess a large number of ion/electron fast transport paths,providing sufficient reactive sites for the Si NPs,to improve the rate performance(a reversible specific capacity of 689 m A h g^-1at 5 A g^-1).The long-term cycle performance is also excellent,the reversible specific capacity reaches 1688 m A h g^-1at 1 A g^-1after 500 cycles,which benefits from the stable structure of the native framework derived from ZIF-67.（3）Germanium-based ternary oxides derived from low-cost Ge O₂as Ge source are studied instead of high-cost metallic Ge.The Calliandra Benth-like hierarchical Cu Ge O₃/RGO composite material is prepared for the first time.The micro-scale hierarchical structure is obtained based on the p H value control.The Cu Ge O₃/RGO anode material exhibits high reversible capacity(884 m A h g^-1@0.1 A g^-1@150 cycles),good rate performance(385 m A h g^-1@5 A g^-1),and excellent cycling stability(892 m A h g^-1@1 A g^-1@500 cycles).We systematically characterize the micro/nano hierarchical structure,study the bonding and interface states,as well as analyze the electrode reaction mechanism.The improved intrinsic/non-intrinsic conductivities profit from the hetero-coupling between Cu Ge O₃and the 2D RGO sheets,as well as the oxygen vacancy defects created during annealing process,to give the hierarchical Cu Ge O₃/RGO composite anode advanced reaction kinetics.Additionally,the unique micro/nano hierarchical structure and native cross-linked framework effectively improve the structural stability and provide extra ion/electron transport capacities.（4）Nickel（Ni）element is further selected to construct ternary oxide with Ge O₂,to give a Ni₃Ge@Ni₂Ge O₄/RGO composite material for the first time.The morphology of Ni₃Ge@Ni₂Ge O₄/RGO is engineered via controlling the content of GO precursor.RGO plays important role in dispersing the Ni precursor,which helps to build a uniform and continuous layered skeleton.This engineered two-dimensional structure can effectively improve the utilization of active materials and maintain the stability of the electrode structure.With an additional lithium-ion storage mechanism provided by nickel oxide,the first synthesized Ni₃Ge@Ni₂Ge O₄/RGO composite anode material exhibits excellent electrochemical performance.752 m A h g^-1of the reversible specific capacity is retained at 5 A g^-1in the rate test.The specific capacity increases to 1680 m A h g^-1after 230 cycles at 1 A g^-1.The interface composition and electrochemical behavior are analyzed,showing that the improved electrochemical performance is due to the interface enhancement effect brought by the heterostructure.The heterostructure greatly improves the interfacial bonding,as well as boosts the rapid migration of ions/electrons between the interfaces.This work conclusively ups the upper-limit of the lithium-ion storage capacity of existing germanium-based ternary oxides,and provides new solution for the practical application of germanium-based anode materials.