Controlled Preparation And Lithium Storage Properties Of Si/Ge-based Anode Materials

In recent years,due to the rapid rise of portable electronics and electric vehicle markets,a huge demand for advanced lithium-ion batteries（LIBs）has arisen,prompting many scholars to develop a variety of new LIBs materials.In order to better meet the market demand,the energy density and power density of Li-ion batteries should be further improved,of which the anode specific capacity is one of the important factors in determining the high energy density of batteries.The graphite negative electrode currently commercially available has a specific capacity of only 372 m Ah g^-1,which is no longer sufficient to meet the needs of electric vehicles.Si and Ge materials from theⅣA group of elements are considered ideal candidates for the next generation of LIBs because of their high theoretical specific capacity and low embedded lithium potential.However,they are alloyed reactive mechanism materials,and their large volume expansion during the alloying process of de/embedded lithium can lead to electrode particle fragmentation,while the repeated growth of the solid electrolyte interface film（SEI）subsequently destroys the battery cycle stability performance and multiplier performance,both of severely limiting the development and application.Based on an overview of the research progress of two anode materials,this paper proposes a method to prepare the porous Si and Ge anode materials in order to improve the volume expansion problem of two materials and further enhance the cycle stability and multiplier performance of Li-ion batteries.The method of alloying Si and Ge is also explored,using the high ion mobility and electron conductivity of Ge to increase the overall conductivity of the electrode and cover the shortage of Si in limiting electron transfer and diffusion due to its low conductivity.The details of the research and the results are as follows.（1）Modification studies focusing on the bulk expansion and low electrical conductivity of Si.Mg₂Si with different pore size precursors was designed,Mg was removed by CO₂oxidation in one step,nanoscale pores were introduced into the micron-scale porous Si system,and a homogeneous composite of carbon layers on the Si backbone was achieved to prepare micro-nano-graded three-dimensional porous Si/C composites（MN-p-Si/C）for high-performance LIBs.The structure not only reduces the volume expansion of silicon during lithium de-embedding,but also provides a three-dimensional efficient channel for ions and electrons transport.The carbon layer in the structure also greatly improves the conductivity of the material.Owing to these advantages,the MN-p-Si/C anode exhibits excellent electrochemical performance in LIBs,The first charge/discharge specific capacities of the electrode were 2869.2 and 2364.5 m Ah g^-1,respectively,with an initial Coulomb efficiency of 82.41%,and a reversible capacity of 1270.1 m Ah g^-1 after 100 cycles at high current densities of 1 A g^-1,with a capacity retention rates of 70.01%.（2）A simple synthetic route with high utilisation rate is designed to modify germanium-based anode materials in this chapter.Using Ge and Mg as raw materials,a micron-scale Mg₂Ge is synthesized,and a honeycomb porous germanium（HB-p Ge）is prepared by nitriding Mg₂Ge as a soft template,taking advantage of its decomposition at high temperatures.The continuous and homogeneous pores of this structure not only provide an effective channel for ion transport,but also provide space for the volume expansion caused by Ge in the process of lithium de-embedding,maintain the structural integrity of the electrode,prevent the crushing of germanium particles,and improve the cycling stability and multiplicity performance of the HB-p Ge electrode.The first charge/discharge specific capacities of the electrode were 1690.7 and 1429.8 m Ah g^-1,respectively,with an initial coulombic efficiency of 84.57%,and the reversible capacity of HB-p Ge remained at 1235.1m Ah g^-1 after 350 cycles at a current density of 1 A g^-1,with the capacity retention rate of97.06%.（3）In this chapter,Si O is used as the silicon source,and the similar crystal structure and high capacity density of Si and Ge are exploited to synthesize lamellar stacked Si_1.5Ge₁ alloys by one-step electroreduction of Si O and Ge blends in chloride molten salts with adjustable electrolytic voltages and ratios of Si O and Ge.The three-dimensional lamellar structure with a large number of voids also provides sufficient space for volume expansion between the layers,and the large interlayer spacing and weak interactions facilitate free Li⁺intercalation and rapid diffusion.An initial Coulomb efficiency up to 82.59%of the Si_1.5Ge₁ electrode with first charge/discharge specific capacity of 2062.3/1703.3 m Ah g^-1.The reversible specific capacity of 1090.4 m Ah g^-1 after 200 cycles at a current density of 1 A g^-1,with a capacity retention rate of 84.74%,...