Synthesis And Electrochemical Performance Of Si/SiO_x/C And Si@Fe-Si/SiO_x Composite Anode Materials For Lithium-ion Batteries

As one of the most promising anode materials for Lithium-ion batteries, Si-based materils have been attracting much attention in recent years due to its high energy density and its high abundance. However, Si suffers from a large volume change (>300%) during lithiation/delithiation processes, which leads to its mechanical disintegration and thus the low cyclic stability. To bufferi the huge volume expansion, increase the electrical conductivity and improve the electrochemical performance of Si anodes, various nano-structured, porous structured Si and Si-C composite anodes were studied. The commercial applications of nano-or porous structured Si anodes are hindered by low tap density, complicated syntheses and high cost. On the basis of an overall review on the research advances of the Si anode materials, the present work puts forward a simple way for obtaining high capacity, long cycling performance, high energy density and low cost of Si anodes by means of spray drying-carbonization method and ball milling process. In this thesis, micro-Si and submicron Si particles with carbon conductive network were synthesized by spray drying-carbonization method, while Si particles with high electrical conductivity FeSiy nanocrystallite and inactive SiOx layer were acquired by ball milling process.Micro-Si and citric acid were used as raw materials to fabricate Si/C composites by spray drying and subsequent carbonization. The effect of citric acid addtions on the carbon content and structre in the composites was studied systematically. Micro-Si/C composites with high capacity and low carbon content were successfully synthesized. The composite with5.9wt%carbon has well dispersed flake-like layers and crooked carbon, providing a reversible capacity of2710mA h/g with capacity retention of69%after60cycles. It is found that, the carbon flakes are beneficial to accommodate the volume expansion of silicon, which improves the electrical conductivity of Si particles.Amorphous Si powders were synthesized by a ball milling process. Amorphous Si@SiOx powders were obtained after being milled for100h with a main particle size distribution smaller than200nm with a SiOx layer on its surfure. The reversible capacity of a-Si@SiOx after100cycles is1060mA h/g,38%capacity retention, whereas that of the pristine electrodes is only23%. Moreover, the Li+diffusion coefficient of the a-Si@SiOx is3.5times higher than that of the pristine Si. a-Si@SiOx and citric acid were used as raw materials to fabricate a-Si@SiOx/C composites. The effect of citric acid addtions on the carbon content and structre in the composites was studied systematically. The effects of introducing carbon and SiOx on the electrochemical performance of the composites were also studied. Composites with high capacity and stable cycling performance were successfully synthesized. By wrapping of silicon particle with floc-like carbon network, a markedly improved cycling performance is achieved with reversible discharge capacity over1450mAh/g and capacity retention of73%after100cycles at the current density of100mA/g. Furthermore, this sample has excellent rate performance with reversble capacity of1230mA h/g after100cycles at500mA/g.Si@FeSiy/SiOx composites were obtained by a low energy ball milling Fe and Si powders at different gases, such as Ar, H2, N2, NH3as well as gas mixture of N2and H2. The microstructure and composition of the composites depend strongly on milling atmosphere. The surface activity of the Fe and Si powders are enhanced and FeSi and FeSi2phase are formed when milling in H2, N2, NH3as well as gas mixture of N2and H2. The composites show nanocrystallite and amorphous Si core coated with an uneven thick SiOx (41-59wt%) layer and embedded with a few FeSi (5wt%) nanocrystallites after20-40h of milling, during which NH3is partially decomposed to H2and N2. Whereas high content of FeSiy (37-40wt%) and lower content of SiOx (14-18wt%) are formed for60-100h of milling, during which NH3is fully decomposed to H2and N2. The initial charge capacity of the sample with80h of milling at a current density of100mA/g is1150mA h/g, and still maintains a high charge capacity of880mA h/g after150cycles. Rate capacity of the sample with60h of milling is560mA h/g at4000mA/g due to the appropriate content of FeSi2and SiOx. It is found that SiOx layer uppresses the valume changes, FeSiy, inactive phase improves the conductivity, while the amorphous Si decreases the valume changes as well as optimizes the kinetic properities.The synthetic method developed in this work possesses several advantages of facile, low-cost and larg-scale. Micro-or submicron Si based composites without or with low carbon content and with high tap density were obtained. This present work provides novel ways to synthesize Si anodes with high capacity, and high tap density.