【作者】 王锭笙；

【导师】 潘洪革； 高明霞；

【作者基本信息】 浙江大学 ， 材料学， 2014， 博士

【摘要】 硅具有储锂能量密度高(理论容量高达3580mA h/g),储量丰富等优点,是极具应用前景的锂离子电池负极材料。然而硅负极材料存在导电性差、脱嵌锂过程中体积变化大,从而导致其循环稳定性差等问题。因而常采用纳米材料、多孔结构和引入高电导率的碳以缓解体积变化、增加硅材料的电接触、提高其循环稳定性,但纳米化和多孔结构导致了材料体积比能量密度降低,且其制备方法普遍复杂,成本高,限制了其商业化应用。本文在全面综述国内外Si基负极材料研究现状的基础上,提出用微米／亚微米尺度的硅为原材料,采用高效的喷雾干燥—高温裂解法和球磨法等制备方法,通过球磨非晶化、原位引入低含量的碳导电网络和高电导率的硅合金导电相,并在硅表面原位引入硅氧化物缓冲相,以获得兼具高容量和良好循环性能、高振实密度和低制备成本的复合材料。研究影响微米／亚微米硅电极材料电化学性能的关键因素及其影响机理。本文以微米硅粉和柠檬酸为原料,采用喷雾干燥—高温裂解法制备微米Si/C原位复合材料。研究了不同裂解温度下微米硅的晶体结构转变和不同含量裂解碳的引入对Si/C复合材料电化学性能的影响。研究表明,复合材料中形成了直片状和弯曲片状碳,其有效缓解了Si颗粒充放电过程中体积变化,并增加了硅颗粒间的电接触。在复合材料中碳含量仅为5.9wt%的条件下,经过60次循环后可逆容量高达1860mA h/g,容量保持率为69%。采用氩气作为球磨保护气氛制备非晶硅粉体材料,球磨后获得了颗粒尺寸基本小于200nm的非晶态硅(a-Si),球磨后Si颗粒的高表面活性特性,诱发其表面生成较高含量的SiOx,形成a-Si＠SiOx复合结构。a-Si@SiOx经100次循环后容量达1060mA h/g,容量保持率由初始Si的23%提高到38%。该a-Si@SiOx材料进一步结合柠檬酸,经高温裂解获得了a-Si@SiOx/C复合材料。系统研究了柠檬酸引入量对复合材料中碳的含量及其形貌结构的影响,研究了碳和SiOx的引入对复合材料电化学性能的影响。研究结果表明,当C含量为8.4wt%时,复合材料在100mA/g电流下,经过100次循环后容量达1450mAh/g,保持率为73%,在500mA/g较大电流下循环100次后,容量为1230mAh/g。复合材料中形成了絮状碳导电网络,其有效的提高了a-Si颗粒间的电接触。SiOx的引入进一步提高了复合材料循环性能。本文系统研究了在较低球磨能量下,球磨气氛(NH3、H2、N2、N2/H2混合气体和Ar气)对球磨Fe和Si混合粉末的产物结构的影响。NH3、N2/H2混合气体及H2和N2气氛均有利于FeSi和FeSi2合金相的生成。结合采用HCl去除球磨产物中残余Fe,得到了FeSiy/SiOx复合层包覆无定形和纳米晶Si结构的微米／亚微米尺寸的Si@FeSiy/SiOx复合材料。采用NH3辅助球磨制备Si@Fe-Si/SiOx复合材料的过程中,NH3经球磨后分解为N2/H2混合气体。当NH3未完全分解时(球磨20-40小时),复合材料中生成了少量的FeSi相(5wt%)和较高含量的SiOx(41-59wt%);当NH3完全分解为N2/H2时(球磨60-100小时),材料中含较高量的FeSi2及少量的FeSi相(37-40wt%)和较低的SiOx(14-18wt%)。非晶态的SiOx起到了部分抑制体积膨胀的作用,较高含量高电导率Fe-Si相的引入,大幅提高了复合材料的电导率,并对Si颗粒在循环过程中的体积变化起到有效缓冲作用,非晶态及纳米晶硅在充放电过程中的体积变化小,因而复合材料可逆容量高、循环稳定性和倍率性能好。其中球磨80小时的材料表现出良好的综合性能,其首次可逆容量为1150mAh/g,150次循环后容量为880mAh/g,容量保持率为77%。球磨60小时的材料因其略低的惰性FeSi2和SiOx包覆层具有优良的倍率性能,在4000mA/g电流密度下可逆容量达560mAh/g。本论文的制备方法工艺简单,成本低,适合规模化生产。获得的硅基复合材料只含少量的碳或不含碳,颗粒尺寸为亚微米级,振实密度高。本论文在缓解微米、亚微米低碳和无碳硅基负极材料的体积膨胀和提高导电性的研究结果为获得高容量和高振实密度的硅基负极材料提供了新的思路。更多还原

【Abstract】 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.更多还原