Design,Synthesis And Electrochemical Performances Of The Silicon And Germanium Based Anode Materials In Lithium-Ion Batteries

With the development and popularisation of portable electronic devices and electric vehicles,electrochemical energy storage devices such as lithium-ion batteries（LIBs）with high energy density have attracted increasing attention.The commercial graphite anode material in LIBs has low specific capacity of 372 mAh g^-1,which could not meet the increasing requirement of energy storge devices needing high energy density.Compared to graphite,silicon（Si）and germanium（Ge）have higher specific capacities and higher lithium insertion potentials.Also,Si is the second most abundant element in the earth’s crust.As a result,Si and Ge are the most promising electrode materials for next-generation lithium-ion batteries.However,the Si and Ge based anodes suffer from substantial volume changes during the charge-discharge process,resulting in problems such as fragmentation and pulverzation of the active material,destruction of the electrode structure,and instability of the solid-electrolyte-interface,hindering the practical applications.In this thesis,we carry our studies of high-performance Si-and Ge-based anodes from three different levels:material structure design,low-cost and large-scale fabrication as well as electrode structure optimization.The goal is to produce Si and Ge-based anode materials to achieve large capacity,long life,and high rate capability.The main contents and innovations can be concluded as follows.（1）The low-cost and large scale preparation and design of nano Si/C composite with high cycle stability are essential for the application of Si based anode materials.We proposed a large-scale preparation of ultrafine Si nanoparticles using agricultural waste bamboo leaves as a raw material through molten salt modified magnesiothermic reduction and then integrated Si with graphene（RGO）and carbon（C）to design a double layer protected Si@C/RGO nanostructures.The prepared ultrafine Si nanoparticles have a diameter about 6-8 nm,and the overall yield is about 10%.Under a current density of 840mA g^-1,the Si nanoparticles deliver a reversible capacity of 1800 mAh g^-1 after 100 cycles.Si@C/RGO exhibits a reversible capacity of 1200 mAh g^-1 at a current density of 4 C(1 C=4.2 A g^-1).This result not only provides new ideas for the large-scale high value-added use of agricultural waste but also offers a good guide for the design of highly electrochemically active Si-C structures.（2）Compared to the nano-silicon based anode material,the york-shell carbon-encapsulated micro-silicon can further enhance the lithium storage performance because of higher structure stability and tap density.Through the method of carbon-coated catalytic growth and wet de-alloying,we developed a route to synthesise a micron-sized yolk-like graphene-coated porous silicon（pSi@NG）using the overproduced ferrosilicon as a precursor in metallurgy.This graphene-covered hollow structure can accommodate the volumetric expansion of the Si.Due to the high electrical conductivity,large Si loading and a small specific surface,pSi@NG can exhibit exceptional capacity,high Initial coulombic efficiency（85%）and long cycle life.The reversible capacity is still higher than1600 mAh g^-1 after 500 cycles.When current density increased to 1.5 C,the capacity remains at 59%of initial value.This research not only provides a sound research basis for further improving the electrochemical properties of the hollow Si-C structure but also offers new ideas for the high value-added use of ferrosilicon.（3）Based on the previous study of active materials,we further designed a new type of conductive glue and optimized the electrode structure of lithium ion batteries.The binder in the lithium-ion battery plays an essential role in the structural integrity of the electrode.The current electrode preparation process is to mix the active material,the binder and the conductive agent,and then paste it onto the current collector.The silicon anode suffers from problems caused by large volume expansion during the cycle,leading to issues such as separation and aggregation of conductive additives,massive interface resistance,and destruction of the electrode structure.We have developed conductive glue（CG）that combines stretchability with high electrical conductivity,and further applied it in Si anode as both binder and conductive additives.The conductivity of the CG does not change even at 250%stretching,and 400%volume expansion,and therefore the CG can efficiently accommodate the enormous volume expansion of Si during the lithiation process,and at the same time can efficiently prevent the electrolyte and silicon from direct contact.This integrated silicon negative electrode（Si-CG）has good electrochemical performances.Under 90%silicon particle loading,the stable capacity is 1500 mAh g^-1after 700 cycles at 0.2 C,and the initial Coulomb efficiency is 80%.The areal capacity of Si-CG anode at an areal loading of 2 mg cm^-2 can reach 5.13 mA h cm^-2.The invented CG and integrated Si-CG anode provide thinking for the development of high-performance silicon-based anodes.（4）Based on the research of silicon anode materials,we successfully extend our research to the Ge in the same group.Compared with Si,Ge possesses higher conductivity for both electrons and Li ions,leading to better lithium storage performances.Ge nanoparticles are entirely capsulated in crumpled CN_x nanotubes.The CN_x can accommodate the volume expansion of the Ge nanoparticles during lithiation efficiently.The material shows excellent stability.The specific capacity decayed only 3.5%after 1200cycles.Moreover,hollow structure facilitates ion diffusion,and one-dimensional shells could provide continuous electron transport channels.The Ge/CN_x also exhibits exceptional rate performance.As the current density increases from 0.5 C to 8 C,the capacity is maintained at 62.3%of the initial value.This structure provides a practical reference for the design of high-performance alloy anode nanomaterials.