包覆结构Si/C复合负极材料研究进展

以包覆结构Si/C复合材料作为负极的锂离子电池(LIBs)具有能量密度高、自放电效率低、循环寿命长等特点。然而,锂在硅中插入/脱出过程的体积膨胀和固体电解质界面膜(SEI)的不稳定性,阻碍了硅的商业化应用。本文通过对近年来新型包覆结构Si/C复合负极材料的构筑方法、电化学性能、比容量和循环性能进行分析和研究,发现包覆结构Si/C复合负极材料不仅可以缓解硅在锂化过程中的体积膨胀和炭层破裂,而且可以有效提高LIBs循环稳定性。因此,Si/C复合材料有望取代石墨成为高容量LIBs的主要负极材料。     

三维针刺C/SiC复合材料显微结构演变分析

以三维针刺碳毡作为预制体,采用树脂浸渍-热解工艺制备C/C多孔体,然后采用反应熔体浸渍法(Reactive melt infiltration,RMI)对C/C多孔体分别浸渗Si和Si-Mo合金制备C/SiC复合材料。首先研究了C/C多孔体制备过程中的显微结构演变。结果表明,浸渍过程中树脂主要填充在纤维束内小孔隙中,热解后裂纹增多,生成网格状C/C亚结构单元;高温热处理使C/C复合材料裂纹进一步扩展,石墨化度提高,束内闭气孔打开,从而为RMI渗Si提供通道。然后对C/C多孔体分别渗Si和Si-Mo合金所得材料的物相组成和显微结构进行对比分析。发现纯Si浸渗得到的复合材料残余Si较多,束内纤维受损严重;而浸渗Si-Mo合金可以减少残余Si含量,束内纤维受损轻微,仍保持着完整的C/C亚结构单元。     

短纤维增强C/C-SiC复合材料的微观结构与力学性能EI北大核心CSCD

综合原料的热物理性能分析和配比设计,实现了C/C复合材料载体孔隙体积的精细控制,采用热压-熔渗两步法在低温条件下制备了具有高致密、低残余Si含量特征的短碳纤维增强C/C-SiC复合材料。系统解析了C/C-SiC复合材料成型过程中的结构演变行为,研究了短纤维增强C/C-SiC复合材料的力学性能和失效机制。结果表明:多孔C/C复合材料载体孔隙的孔径呈双极分布特征,添加芳纶纤维可提高网络孔隙结构的连通性,具有显著的孔隙结构调控作用。SiC基体以网络骨架形态分布于C/C-SiC复合材料内部,与纤维束形成了强界面结合钉扎结构,高含量纤维协同作用下使C/C-SiC复合材料具有优异的综合力学性能,添加芳纶纤维可明显增加复合材料内部裂纹扩展路径,提高C/C-SiC复合材料的断裂韧性。碳纤维的面内各向同性分布及陶瓷相层间均匀分布对C/C-SiC复合材料承载、摩擦稳定性提升均具有积极作用。     

预制体对C/C复合材料热膨胀系数的影响

以天然气为前驱气体,整体碳毡和2D针刺碳毡为预制体,采用热梯度化学气相渗积技术制备了两种C/C复合材料,其表观密度均为1.74 g/cm3。借助光学显微镜和扫描电子显微镜观察了热解碳基体的生长特征和微观形貌,采用热膨胀仪测量了两种材料的热膨胀系数(CTE),研究了由不同预制体增强C/C复合材料的CTE,解释了造成材料不同方向CTE差异的主要原因。结果表明,随着温度升高,材料A和B的CTE是逐渐升高的,且Z向CTE值均大于XY向。当两种材料在Z向的纤维体积分数接近时,随着XY方向纤维体积分数增大,材料在Z向的CTE增大,在XY向的CTE降低,两种材料在XY和Z向的CTE呈如下分布:αB-Z>αA-Z>αA-XY>αB-XY。C/C复合材料的CTE主要取决于纤维体积分数和排布、碳基体及材料中的孔隙分布情况,前者起决定作用。     

SnO_2/C复合材料的制备及其电化学性能研究

以纤维素为碳源,采用水热法制备了碳微球,通过添加金属Sn,制得SnO2/C复合材料。通过扫描电子显微镜(SEM)、红外光谱仪(FT-IR)及X-射线衍射仪(XRD)对其形貌及结构进行表征,采用循环伏安法(CV)及电化学阻抗(EIS)测试对材料的电化学性能进行表征。结果表明:制得的SnO2/C复合材料表面存在羟基、羰基等多种含氧基团,同时复合材料中存在Sn-O键,锡在复合材料中以SnO2的形式存在;当碳化温度为280℃、反应时间为8h时,SnO2/C复合材料呈球状,粒径分布在4~6μm之间;与水热碳化后的产物对比,SnO2/C复合材料作为电极材料具有更优良的循环性能和电容特性,电子传递能力也有显著提高。     

软硬混编预制体增强沥青基4D-C/C材料的拉伸破坏行为

以X-Y平面依次铺设炭纤维束、Z向穿插炭棒的4D软硬混编为预制体,采用沥青液相常压、高压浸渍/炭化-石墨化循环致密工艺制备4D-C/C复合材料。通过该材料Z向(炭棒方向)的拉伸实验,测定其拉伸性能和力学行为,并采用SEM分析试样表面及断口形貌。结果表明:宏观上拉伸试样以炭棒整体拔出的形式破坏;细观尺度上,试样表面形成了与载荷方向垂直的贯穿性裂纹,裂纹以2 mm左右的距离呈等间距分布;材料进一步的破坏过程中,基体裂纹在X-Y向纤维束中呈线性扩展,快速分割了基体材料,使4D-C/C复合材料的拉伸破坏演变为1D-C/C复合材料的破坏模式,由于炭棒与基体炭界面结合弱,炭棒以拔出方式失效和破坏。     

硅化镁还原CO2一步原位合成Si/C复合负极

硅碳(Si/C)负极被认为是高能量密度锂离子电池的首选负极材料,本文提出了一种利用Mg₂Si一步还原CO₂原位制备硅碳复合材料的新方法,研究了Ar∶CO₂混合气体积比和反应温度等关键工艺对Si/C负极材料微结构和电化学性能的影响。研究发现,该方法原位合成的Si/C颗粒尺寸为几百纳米,晶态硅和无定形碳相互交织、分布均匀。当反应温度为700℃、Ar∶CO₂=7∶1时合成的Si/C复合材料作为锂离子电池负极材料时,在0.2 A/g的电流密度下,500个循环后仍有1134 mA·h/g可逆容量。本文利用温室气体CO₂来制备储能用Si/C复合负极材料,既能实现变废为宝,同时该方法合成工艺简便,容易工业化实施,具有商业化开发的潜力。      

不同纤维体积分数CVI 炭/ 炭复合材料的石墨化度

为确定不同纤维体积分数的化学气相浸渗(CVI) C/ C 复合材料的最佳热处理工艺, 以40 %、30 %、25 %三种不同纤维体积分数的针刺整体毡为坯体, 经三次CVI 后制得C/ C 复合材料, 采用X射线衍射和拉曼光谱微区分析测试了三种不同纤维体积分数的CVI C/ C 复合材料试样未经热处理及经2200 ℃、2400 ℃热处理下宏观和微区石墨化度。结果表明: 三次CVI 热解炭均为光滑层结构, 且纤维体积分数越高, C/ C 复合材料的石墨化度也越高;纤维与光滑层热解炭界面及两种不同热解炭界面在高温热处理时会发生应力石墨化, 应力石墨化程度前者大于后者, 这是纤维体积分数高的C/ C 复合材料石墨化度高的原因; 热处理温度越高, 应力石墨化程度越大。      

反应熔体渗透法制备C/C-SiC复合材料的微观结构及抗氧化性能

采用反应熔渗法(RMI)制备了C/C-Si C复合材料,对比研究了不同密度C/C预制体所制备C/C-Si C复合材料在1 500℃静态空气环境中抗氧化性能和1 500℃室温抗热震性能,借助X射线衍射分析仪(XRD)与扫描电子显微镜(SEM)对C/C-Si C复合材料的相组成及微观形貌进行了分析。结果表明,密度为1.0 g/cm~3的C/C复合材料孔径分布在10~100μm范围,有利于液相Si的渗入,进而可获得高致密度的C/C-Si C复合材料;而密度为1.7 g/cm~3的C/C复合材料因孔径太小(10μm),不利于Si熔体的渗入,仅能在C/C复合材料表面形成Si C涂层。由于C/C复合材料与Si C的热膨胀系数不同,在氧化和热震试验过程易造成Si C涂层开裂,致使用密度为1.7 g/cm~3的C/C预制体制备的C/C-Si C复合材料抗氧化与抗热震性能下降。而密度为1.0 g/cm~3的C/C预制体制备的复合材料内部致密的Si C基体与低密度C/C复合材料形成镶嵌界面,有效缓解热膨胀系数不匹配而造成的缺陷,从而具有优异的抗氧化和抗热震性能。     

Si粉对SiO_2凝胶/铝硅纤维复合材料性能的影响

采用浸渍法制备SiO_2凝胶/铝硅纤维复合材料。研究Si粉质量分数为0%,0.2%,0.4%,0.6%和0.8%时,SiO_2凝胶/铝硅纤维复合材料的微观形貌、体积密度、回弹率和物相组成。结果表明:Si粉质量分数低于0.6%时,其氧化产生的体积膨胀填补了纤维与凝胶间的裂纹,体积密度由0.468g/cm~3增大到0.723g/cm~3,回弹率由43.1%升高为59.6%,并且Si粉高温下的结晶化抑制了纤维析晶和促进了辉石的生成,这是材料的力学性能提高及高温损坏程度降低的重要原因;Si粉质量分数为0.8%时,高温下的Si粉产生了过分的体积膨胀,导致复合材料内部出现较大裂纹,压缩回弹率降低为44.5%。     

A new type of nanostructure in Si/C composite electrodes for lithium-ion batteries

The composition and structure of thin-film Si/C composite anodes produced by alternately depositing controlled amounts of silicon and carbon using magnetron plasma sputtering have been determined by atomic force microscopy, x-ray diffraction, and optical spectroscopy (Raman and UV through IR specular reflectance spectra). The silicon-to-carbon volume ratio in the films was varied from 39.5: 60.5 to 87: 13, and their thickness ranged from 100 to 480 nm. The surface of the films was found to have a nanogranular structure, which had not been reported previously for Si/C composites. This morphology is atypical of structureless silicon layers deposited under the same conditions but is similar to the nanostructure of a thin carbon film consisting of grains uniform in shape and size (D _av = 20–25 nm). Reducing the carbon content of the composites from 60 to 36% increases the grain size from 25 to 45–50 nm. At high silicon contents (near 80%), the nanostructure of the composites is less homogeneous: in addition to nanograins, there are structureless silicon zones. The homogeneity of the nanostructure depends on the Si: C ratio and the sequence and thicknesses of the deposited Si and C layers. Thin (104–173 nm) films containing more than 30% carbon (they have isolated silicon clusters) reveal the highest activity for the lithium intercalation-deintercalation process. Their Raman spectra show strong luminescence characteristic of silicon nanoparticles less than 5–6 nm in size. This effect is missing in the thicker films, in which the silicon forms an infinite cluster and which have a stronger tendency to degrade.     

Synthesis of anode active material particles for lithium-ion batteries by surface modification via chemical vapor deposition and their electrochemical characteristics

The high capacity and optimal cycle characteristics of silicon render it essential in lithium-ion batteries. We have attempted to realize a composite material by coating individual silicon (Si) particles of μm-order diameter with a silicon oxide film to serve as an active material in the anode of a lithium-ion battery and thus improve its charge-discharge characteristics. The particles were coated using an inductively coupled plasma-chemical vapor deposition (ICP-CVD) process that realized a homogeneously coated silicon oxide film on each Si particle. The film was synthesized using tetraethyl orthosilicate (TEOS) with hydrogen (H₂) gas used as a reducing agent to deoxidize the silicon dioxide. This enabled the control of the silicon oxidation number in the layers produced by adjusting the H₂ flow during the silicon oxide deposition by ICP-CVD. The silicon oxide covering the Si particles included both silicon monoxide and suboxide, which served to improve the charge-discharge characteristics. We succeeded in realizing an active material using Si, which is abundant in nature, for the anode of a lithium-ion battery with highly charged, improved cycle properties.     

Si/C particles on graphene sheet as stable anode for lithium-ion batteries

Silicon is considered as one of the most promising anodes for Li-ion batteries (LIBs),but it is limited for commercial applications by the critical issue of large volume expansion during the lithiation.In this work,the structure of silicon/carbon (Si/C) particles on graphene sheets (Si/C-G) was obtained to solve the issue by using the void space of Si/C particles and graphene.Si/C-G material was from Si/PDA-GO that silicon particles was coated by polydopamine (PDA) and reacted with oxide graphene (GO).The Si/C-G material have good cycling performance as the stability of the structure during the lithiation/dislithiation.The Si/C-G anode materials exhibited high reversible capacity of 1910.5 mA h g-1 and 1196.1 mA h g-1 after 700 cycles at 357.9 mA g-1,and have good rate property of 507.2 mA h g-1 at high current density,showing significantly improved commercial viability of silicon electrodes in high-energy-density LIBs.     

Fullerene C60 coated silicon nanowires as anode materials for lithium secondary batteries

A Fullerene C60 film was introduced as a coating layer for silicon nanowires (Si NWs) by a plasma assisted thermal evaporation technique. The morphology and structural characteristics of the materials were studied by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). SEM observations showed that the shape of the nanowire structure was maintained after the C60 coating and the XPS analysis confirmed the presence of the carbon coating layer. The electrochemical characteristics of C60 coated Si NWs as anode materials were examined by charge-discharge tests and electrochemical impedance measurements. With the C60 film coating, Si NW electrodes exhibited a higher initial coulombic efficiency of 77% and a higher specific capacity of 2020 mA h g(-1) after the 30th cycle at a current density of 100 microA cm(-2) with cut-off voltage between 0-1.5 V. These improved electrochemical characteristics are attributed to the presence of the C60 coating layer which suppresses side reaction with the electrolyte and maintains the structural integrity of the Si NW electrodes during cycle tests.     

锂离子电池硅/碳复合负极材料的研究进展

负极理论容量最大的硅在充放电过程中,体积过度膨胀粉化导致容量衰减快,成为其作为商用负极材料的最大障碍.碳材料不仅具有一定的电化学活性,结构也较稳定,可以作为硅电极的"缓冲基体";具有高容量和优良循环性能的硅-碳复合负极材料已经成为该领域的研究热点.按照碳材料的分类,评述了Si/C复合电极材料,并初步展望了该领域的研究方向.     

多孔硅/碳复合负极材料的制备及电化学性能

以介孔二氧化硅SBA-15为硅源, 采用镁热还原法和化学气相沉积(CVD)法合成了具有莲藕状结构的多孔硅/碳复合材料。利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和氮气吸脱附法研究了原料配比, 混压方式以及反应温度对多孔硅物相组成和结构形貌的影响。多孔硅的最佳合成条件为: 镁粉过量20wt%, 原料经球磨混合均匀后直接在750℃下高温反应4 h。对多孔硅及多孔硅/碳材料进行了电化学性能测试, 在0.5 A/g电流密度下循环70次后仍保持1633.1 mAh/g的可逆容量, 并且在8 A/g下容量达到580.1 mAh/g, 表现出优异的循环稳定性以及倍率性能。     

Carbon Nanotube-Reinforced Dual Carbon Stress-Buffering for Highly Stable Silicon Anode Material in Lithium-Ion Battery

Silicon (Si) anode suffers from huge volume expansion which causes poor structural stability in terms of electrode material, solid electrolyte interface, and electrode, limiting its practical application in high-energy-density lithium-ion batteries. Rationally designing architectures to optimize the stress distribution of Si/carbon (Si/C) composites has been proven to be effective in enhancing their structural stability and cycling stability, but this remains a big challenge. Here, metal-organic frameworks (ZIF-67)-derived carbon nanotube-reinforced carbon framework is employed as an outer protective layer to encapsulate the inner carbon-coated Si nanoparticles (Si@C@CNTs), which features dual carbon stress-buffering to enhance the structural stability of Si/C composite and prolong their cycling lifetime. Finite element simulation proves the structural advantage of dual carbon stress-buffering through significantly relieving stress concentration when Si lithiation. The outer carbon framework also accelerates the charge transfer efficiency during charging/discharging by the improvement of lithium-ion diffusion and electron transport. As a result, the Si@C@CNTs electrode exhibits excellent long-term lifetime and good rate capability, showing a specific capacity of 680 mAh g⁻¹ even at a high rate of 1 A g⁻¹ after 1000 cycles. This work provides insight into the design of robust architectures for Si/C composites by stress optimization.     

A high-strength SiCw/SiC–Si composite derived from pyrolyzed rice husks by liquid silicon infiltration

A high-strength SiC composite with SiC whiskers (SiC_w) as reinforcement has been fabricated by liquid silicon infiltration (LSI) using pyrolyzed rice husks (RHs) as raw material. RHs were coked and pyrolyzed subsequently at high temperature to obtain a mixture containing SiC whiskers, particles, and amorphous carbon. The pyrolyzed RHs were then milled and modeled to preforms, which were then used to fabricate biomorphic SiC_w/SiC–Si composites by liquid silicon infiltration at 1,450, 1,550, and 1,600 °C, respectively. Dense composite with a density of 3.0 g cm⁻³ was obtained at the infiltration temperature of 1,550 °C, which possesses superior mechanical properties compared with commercial reaction-sintered SiC (RS-SiC). The Vickers hardness, flexure strength, elastic modulus, and fracture toughness of the biomorphic SiC_w/SiC–Si composite were 18.8 ± 0.6 GPa, 354 ± 2 GPa, 450 ± 40 MPa, and 3.5 ± 0.3 MPa m^1/2, respectively. Whereas the composites obtained at the other two infiltration temperatures contain unreacted carbon and show lower mechanical properties. The high flexure strength of the biomorphic composite infiltrated at 1,550 °C is attributed to the dense structure and the reinforcement of the SiC whiskers. In addition, the fracture mechanism of the composite is also discussed.     

石墨烯桥联碳-聚乙二醇包覆的Si纳米颗粒Li离子电池复合负极材料

Si是一种很有前途的Li离子电池负极材料。为解决其巨大体积形变导致的容量衰退快、循环寿命短等问题,采用简单的搅拌和热还原,利用聚乙二醇衍生的薄碳修饰Si纳米颗粒(C-PEG@Si NPs),并通过石墨烯的桥联来制备具有多级包覆结构的石墨烯桥联C-PEG包覆的Si纳米颗粒(graphene@C-PEG@Si NPs)复合材料。利用SEM、 TEM、 X射线衍射、恒流充放电测试等一系列表征测试方法对材料结构、物相和电化学性能进行分析。C-PEG与石墨烯涂层可有效地减小Li离子储存过程中Si对电解质的暴露面积并缓解其体积膨胀。研究结果表明,相比纯Si, graphene@C-PEG@Si NPs复合材料表现出优异的电化学性能,在210 mA/g的电流密度下,经过100次循环可逆比容量仍高达1 032 mA·h/g,电极在4 200 mA/g的大电流密度下循环100次,其比容量仍保持在430 mA·h/g以上。     

紫外光辐照下钛硅共掺杂类金刚石薄膜微结构的演化

采用中频磁控溅射Ti80Si20合金靶在单晶硅表面制备了钛硅共掺杂的类金刚石薄膜。利用紫外-可见光多波长Raman光谱表征薄膜微结构, 并结合FTIR光谱研究了紫外光辐照对类金刚石薄膜微结构的影响, 进一步讨论了紫外光辐照下薄膜微结构的演化机理。结果表明: 非晶结构的类金刚石薄膜出现反式聚乙炔和聚对苯乙炔类聚合物结构以及sp杂化的线型卡宾碳结构。紫外光辐照诱导薄膜微结构驰豫和重构, 薄膜Si-O和C-O键含量增加, C=C和C-H键含量减少; 同时薄膜sp²团簇尺寸减小而无序度增大。