Journal of Alloys and Compounds

Volume 978, 25 March 2024, 173507
Journal of Alloys and Compounds

Review 回顾
Opportunities and challenges of nano Si/C composites in lithium ion battery: A mini review

https://doi.org/10.1016/j.jallcom.2024.173507Get rights and content 获取权限和内容

Abstract 抽象

Under the requirements of reducing carbon emissions and developing a green and low-carbon economy, Li ion batteries (LIBs) play an important role in electric vehicles (EV), electric grid energy systems, and other energy storage power plants. R & D of higher energy density, safer and more stable LIBs has become an urgent task in these application fields. Unfortunately, the theoretical capacity of commercial graphite anode is 372 mAhg─1 which cannot meet the future demands for high-performance LIBs. By comparation, silicon (Si) as anode materials has shown good application prospects in the high-performance LIBs, and been intensively investigated worldwide in the past few decades owing to its high theoretical capacity of about 4200 mAhg-1, abundant reserve, low cost, environmentally friendly and so on. However, severe volume expansion, low electronic conductivity, low Coulombic efficiency in the first cycle, and continuous formation of SEI (solid electrolyte interphase) during lithiation/delithiation processes hinder its practical application in the LIBs. To address these shortcomings or alleviate problems, various technologies and strategies have been devoted to the Si anodes. In this review, the research progress of Si and various carbon composite materials including 1D, 2D and 3D structural features are summarized. Porous Si/C 3D architecture materials are considered excellent and promising composite materials due to their abundant pore channels for Li ions transportation, ability to release the significant stress caused by Si volume expansion, and a short distance for mass diffusion. This article also covers two important materials: new polymer binders and electrolyte additives. Their latest research progress and impact on electrochemical performances, as well as future development prospects are outlined.
在减少碳排放和发展绿色低碳经济的要求下,锂离子电池(LIB)在电动汽车(EV)、电网能源系统和其他储能发电厂中发挥着重要作用。研发更高能量密度、更安全、更稳定的锂离子电池已成为这些应用领域的当务之急。遗憾的是,商用石墨阳极的理论容量为372 mAhg ─1 ,无法满足未来对高性能锂离子电池的需求。相比之下,硅(Si)作为负极材料在高性能锂离子电池中显示出良好的应用前景,并因其理论容量约为4200 mAhg -1 、储量充足、成本低、环保等优点,在过去几十年中在全球范围内得到了深入研究。然而,严重的体积膨胀、低电子电导率、第一循环的低库仑效率以及锂化/脱锂过程中SEI(固体电解质界面)的连续形成阻碍了其在锂离子电池中的实际应用。为了解决这些缺点或缓解问题,各种技术和策略都致力于硅阳极。本文综述了Si和各种碳复合材料的研究进展,包括一维、二维和三维结构特征。多孔Si/C 3D结构材料因其丰富的锂离子传输孔隙通道、释放Si体积膨胀引起的显著应力以及质量扩散距离短等特点,被认为是优秀且有前途的复合材料。本文还介绍了两种重要材料:新型聚合物粘结剂和电解质添加剂。 概述了他们的最新研究进展和对电化学性能的影响,以及未来的发展前景。

Graphical Abstract 图形摘要

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The prospects and challenges of nanostructured (porous) Si/C composite anodes and their practical applications in LIBs.

Keywords 关键字

Li-ion battery
Nano Si/carbon composite
Porous structure
Additive of electrolyte


1. Introduction 1. 引言

Since the world first Lithium ion battery (LIBs) was commercialized by Sony and Asahi Group in 1991, it has been become a prime power source for portable electronic appliances such as mobile phone, laptops, digital cameras, current electric vehicles (EV) and electric grid energy systems and so on [1], [2], [3], [4], [5], [6]. Battery components, the electrode materials, play an important role on the performance of LIBs. Except for positive electrode materials, negative materials have significant impact on improving energy density and safety, especially in the context of the soaring international price of lithium carbonate two years ago, which hindered the development of ternary positive electrodes. The anode materials in the LIBs can be classified into two types: carbonaceous and no-carbonaceous materials. Carbonaceous materials contain graphite, hard carbon, soft carbon and graphene etc. Non-carbonaceous materials contain titanium-based, germanium-based and silicon-based materials [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. The graphite anode material has a maximum theoretical capacity of 372 mAhg─1, and the value has almost been achieved in the present LIBs. The limited capacity of graphite no longer meets the requirements of future high energy-consuming electronic products and long-distance endurance EVs. For the non-carbonaceous materials specifically alloy-based materials (for example Si, Ge, Sn, Sb, Al, etc.), their theoretical capacity exhibits much higher values compared to the carbon-based materials [18], [19], [20], [21], [22]. Among all the alloy-based anodes, Si has the highest theoretically specific and volumetric capacity of about 4200 mAhg─1 and 8322 mAh cm-3, respectively, which is almost 10 time higher than that of conventional graphite materials [23], [24], [25], [26]. It is because a single silicon atom can accommodate more than four lithium ions (Li4.4Si/Li22Si5) while six carbon atoms in graphite make only bonds with a single Li ion (LiC6) [27], [28], [29], [30]. Moreover, Si is the 2nd most abundant element on earth, low cost and environmentally friendly, as well as active at a low operating potential less than 0.5 V [31], [32], [33], [34]. Therefore, Si is the mainstream anode material in the next generation of LIBs with a high specific energy density. Unfortunately, Si has large volume expansion (300%) during repeated lithiation/delithiation, which causes particle pulverization and destabilizes the formation of solid electrolyte interphase (SEI), resulting in fast capacity decay [35], [36], [37]. Furthermore, the large volume expansion of Si anodes further causes following issues: 1) losing the contact of active materials with current collectors [38], [39]; 2) cracking and powder loss of Si anodes [40]; 3) consumption of Li+ and electrolyte due to continuous formation of SEI layers [41]; 4) performance failure [42] etc. Remarkable efforts have been devoted to tackle the aforementioned issues and to design architecture of Si/C composites in the past decades. One of strategies is to introduce conductive carbon network to increase the entire conductivity and buffer the volume expansion of Si-based anodes. The Si/C composite materials can be generally categorized into nanoparticles and porous Si materials based on the structure and property of Si. Dou et al. discussed the advantages and disadvantages of various structural Si/C composite materials through analyses of morphologies and electrochemical properties [43]. The preparing approaches of various carbon materials and Si/C composites, and performance of Si-based anodes related to nanoscale structure were discussed in details [44]. Chae et al. emphasized the necessity of co-utilization of graphite and Si for commercialization, and systematically discussed the criteria for the co-utilization of graphite and Si [45]. Recently, Zhang et al. elaborated challenges and progress on Si-based anode materials and reviewed various new polymer binders, improved electrolytes, different pre-lithiation approaches, and Si/graphite designs [46]. To improve the cycling stability, integrity of electrode and Coulombic efficiency (CE), these polymer binders have a specific structure and usually contain polar groups such as -OH, -COOH and -NH2 which can form strong hydrogen bonds on the surface of Si particles and ensure good bonding between the Si materials and current collectors through intermolecular forces. For practical application with high areal capacity, however, there are still challenges in the conductivity, SEI stability, initial Coulombic efficiency (ICE), and cost of Si-based anodes.
自1991年索尼和朝日集团将世界上第一个锂离子电池(LIB)商业化以来,它已成为便携式电子设备的主要电源,如手机、笔记本电脑、数码相机、当前的电动汽车(EV)和电网能源系统等[1]、[2]、[3]、[4]、[5]、[6]。电池组件,即电极材料,对锂离子电池的性能起着重要作用。除正极材料外,负极材料对提高能量密度和安全性有重大影响,尤其是在两年前国际碳酸锂价格飙升的背景下,阻碍了三元正极的发展。锂离子电池中的负极材料可分为两种类型:碳质和无碳质材料。碳质材料含有石墨、硬碳、软碳和石墨烯等。非碳质材料包含钛基、锗基和硅基材料[7]、[8]、[9]、[10]、[11]、[12]、[13]、[14]、[15]、[16]、[17]。石墨负极材料的最大理论容量为372 mAhg ─1 ,在目前的锂离子电池中几乎已经达到了这一值。石墨有限的产能已无法满足未来高耗能电子产品和长距离续航电动汽车的要求。对于非碳质材料,特别是合金基材料(如Si、Ge、Sn、Sb、Al等),其理论容量比碳基材料高得多[18]、[19]、[20]、[21]、[22]。 在所有合金基阳极中,Si的理论比容量和体积容量最高,分别约为4200 mAhg ─1 和8322 mAh cm -3 ,几乎是传统石墨材料的10倍[23],[24],[25],[26]。这是因为单个硅原子可以容纳四个以上的锂离子(Li 4.4 Si/Li 22 Si 5 ),而石墨中的六个碳原子仅与单个锂离子(LiC 6 )形成键合[27],[28],[29],[30]。此外,硅是地球上第二丰富的元素,成本低,环保,并且在低于0.5 V的低工作电位下具有活性[31],[32],[33],[34]。因此,Si是下一代锂离子电池中具有高比能量密度的主流负极材料。不幸的是,Si在反复锂化/脱锂过程中具有较大的体积膨胀(300%),这会导致颗粒粉碎并破坏固体电解质界面(SEI)的形成,从而导致快速容量衰减[35],[36],[37]。此外,硅阳极的大体积膨胀进一步导致以下问题:1)失去活性材料与集流体的接触[38],[39];2)硅阳极的开裂和粉末损失[40];3)由于SEI层的连续形成而消耗锂 + 和电解质[41];4)性能故障[42]等。在过去的几十年里,人们为解决上述问题和设计Si/C复合材料的结构做出了巨大的努力。其中一种策略是引入导电碳网络,以增加整个电导率并缓冲硅基阳极的体积膨胀。 根据Si的结构和性能,Si/C复合材料一般可分为纳米颗粒和多孔Si材料,Dou等通过对Si/C的形貌和电化学性能的分析,讨论了各种结构Si/C复合材料的优缺点[43]。详细讨论了各种碳材料和Si/C复合材料的制备方法,以及与纳米级结构相关的硅基阳极的性能[44]。Chae等强调了石墨和Si共同利用商业化的必要性,并系统地讨论了石墨和Si共同利用的标准[45]。最近,Zhang等人详细阐述了硅基负极材料的挑战和进展,并回顾了各种新的聚合物粘合剂、改进的电解质、不同的预锂化方法和硅/石墨设计[46]。为了提高循环稳定性、电极完整性和库仑效率(CE),这些聚合物粘结剂具有特定的结构,通常含有-OH、-COOH和-NH等极性基团 2 ,可以在硅颗粒表面形成强氢键,并通过分子间作用力保证硅材料与集流体之间的良好结合。然而,对于高面容量的实际应用,硅基阳极的导电性、SEI稳定性、初始库仑效率(ICE)和成本等方面仍存在挑战。

The design of Si/C composites using 1D, 2D and 3D carbon materials can significantly improve the conductivity and electronic contact between Si particles, and can better withstand the stress of Si volume expansion, greatly limiting the degree of Si particle fracture and improving the lifespan. An important advancement in Si-based materials is the development of porous nano Si architecture, which exhibits excellent structural characteristics, reduces the volume expansion, provides sufficient channels for ion transportation, and improves the cycling stability and rate capability. Due to irreversible chemical reactions (or parasitic reactions) during the first cycles, a large initial capacity loss usually occurs, which causes the excessive consumption of Li ions in electrolyte. The effective way to overcome this issue is to use prelithiation approaches. Recently, Bhujbal et al. comprehensively summarized and discussed various prelithiation techniques for the Si-based anodes by considering the operational simplicity, cost, stability, safety, compatibility with existing manufacturing processes [47]. In addition to these efforts of designing nano porous architectures and using prelithiation approaches to improve electrochemical properties, the development of new binders and electrolyte additives in the past decades also remarkedly enhanced the integrity and cycling performance of Si-based anodes, as well as improved the ICE and SEI stability.

This article summarizes the problems existing in the commercialization process of Si-based anodes and various solutions for developing nano Si/C composites with 1D, 2D, and 3D carbon materials, and discusses the challenges and opportunities faced by Si-based anodes. The latest research progresses in the areas of binder materials including conductive polymer binders and self-healing binders [48]Y, [49], [50] and electrolyte additives such as the FEC additive [51] that plays a new role on stabilizing SEI films, as well as lithium difluorophosphate (LiPOF2), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis (trifluoromethanesulfonyl)imide (LiTFSI) and lithium difluoro(oxalato)borate (LiDFOB) additives etc. are also introduced [52], [53]. Their impact on electrochemical performance of Si-based anodes is highlighted. Finally, the critical issues, future development trends, and prospects of Si/C composite materials used in LIBs are outlined.
本文总结了硅基负极商业化过程中存在的问题,以及开发一维、二维和三维碳材料纳米Si/C复合材料的各种解决方案,并探讨了硅基负极面临的挑战和机遇。在粘结剂材料领域的最新研究进展包括导电聚合物粘结剂和自修复粘结剂[48]Y,[49],[50]和电解质添加剂,如对稳定SEI薄膜起新作用的FEC添加剂[51],以及二氟磷酸锂(LiPOF 2 )、双(氟磺酰基)酰亚胺锂(LiFSI)、双(三氟甲磺酰基)亚胺锂(LiTFSI)和二氟(草酸)硼酸锂(LiDFOB)添加剂等[52], [53]. 强调了它们对硅基阳极电化学性能的影响。最后,概述了锂离子电池中使用Si/C复合材料的关键问题、未来发展趋势和前景。

2. A brief history of Si anodes for LIBs
2. 锂离子电池用硅阳极简史

In early 1971, Dey et al. reported an important discovery that some metallic elements such as (Si, Mg, Al, Zn, Pt, Sn etc) could react with Li+ at certain temperature to form alloys [54]. In 1976, Sharma and Seefurth investigated the formation of different phases of Li-Si alloys, specifically Li12Si7, Li14Si6, Li12Si4, and Li22Si5 phases in the temperature range of 400 to 500 oC. It is worth noting that the Li22Si5 phase exhibits the highest theoretical specific capacity of 4200 mAhg─1 among these phases [55]. In 1990 s, bulk silicon was focused as anode materials for LIBs, but the large volume expansion and contraction of Si during lithiation/delithiation processes cause severe mechanical stress, resulting into performance degradation and poor cycle stability of LIBs. Xing et al. firstly reported the preparation method of Si-based anode materials through pyrolysis of silicon-containing polymers such as polysiloxane and silicane epoxide [56]. In the late 1990 s, nanostructured and composited Si-based materials emerged, enhancing electrochemical performances and alleviating the volume changes in the lithiation/delithiation processes. In 1999, Zhou et al. investigated the insertion/extraction mechanism using single crystalline silicon [57]. Meanwhile, Kim utilized high energy mechanical milling to prepare Si/Sn composites where Si particles were uniformly distributed into Sn arrays, effectively mitigating the huge volume change of Si-based anodes during lithiation/delithiation processes [58].
1971年初,Dey等人报道了一项重要发现,即一些金属元素,如(Si、Mg、Al、Zn、Pt、Sn等) + 在一定温度下可以与Li反应形成合金[54]。1976年,Sharma和Seefurth研究了Li-Si合金不同相的形成,特别是在400至500°C的温度范围内, 12 LiSi 714 LiSi 612 LiSi 4 和Li 22 Si 5 相。 o 值得注意的是,在这些相中,Li 22 Si 5 相的理论比容量最高,为4200 mAhg ─1 [55]。在1990年代,块状硅被集中作为锂离子电池的负极材料,但硅在锂化/脱锂过程中的大量膨胀和收缩会引起严重的机械应力,导致锂离子电池的性能下降和循环稳定性差。Xing等首先报道了聚硅氧烷和硅烷环氧化物等含硅聚合物热解制备硅基负极材料的方法[56]。在1990年代后期,出现了纳米结构和复合硅基材料,增强了电化学性能并减轻了锂化/脱锂过程中的体积变化。1999年,周等人研究了使用单晶硅的插入/提取机制[57]。同时,Kim利用高能机械铣削制备了Si/Sn复合材料,其中Si颗粒均匀分布在Sn阵列中,有效地缓解了锂化/脱锂过程中Si基阳极的巨大体积变化[58]。

In the 21st century, Si-based anodes for LIBs have attracted considerable attention and conducted more comprehensive investigations. From 2000 to 2005, there was focus on utilizing low-dimensional Si nanomaterials such as nano particles and alloying. Films and composite structures with active/inactive matrices were reported [59], [60]. Between 2006 and 2010, the focus of research shifted towards one-dimensional Si nanomaterials, including nanowires and nanotubes, which has made significant contributions to improving the stability of Si-based anodes [61], [62]. Although improvement of Si-based anode materials has been made in the aspects of volume expansion, conductivity and cycle life, there are still significant challenges in terms of Coulombic efficiency, high cost, and rate performance of nano Si-based anodes.

3. Challenges and strategies of Si anodes
3. 硅阳极的挑战与策略

High cost of nano Si materials is one of the major obstacles because the processing method of nano Si particles from bulk Si has at least double their cost [63]. Poor initial Coulombic efficiency is another problem of Si-based anodes for LIBs. The easy oxidation of nano Si materials leads to irreversible reactions which further decrease the initial Coulombic efficiency [64], [65]. In addition, other factors such as the selection of binders, additives and electrolyte also affect the performance of Si-based anodes. Fig. 1 describes the failure mechanism of Si-based anodes, which is highly correlated with pulverization, changes in the morphology and volume of Si materials, as well as the growth of SEI layers [66]. The particle size of Si powder usually ranges from micrometers to nanometers, and is covered with a natural oxide layer. The oxide on the surface acts as a passivation layer, which further reduces the performance. Therefore, before using the silicon powder, the particles need to be treated in the environmental atmosphere.

Fig. 1
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Fig. 1. Si electrode failure mechanisms: (a) Material pulverization. (b) Morphology and volume change of the entire Si electrode. (c) Continuous SEI growth [66].
图 1.硅电极失效机理:(a)材料粉碎。(b) 整个硅电极的形貌和体积变化。(c)SEI持续增长[66]。

Nano Si not only has less expansion, but also provides a large surface area, short pathways for Li+ diffusion and fast electron transportation. By introducing a low-dimensional Si structure, the volume expansion can be further alleviated. Therefore, the nano Si has become the focus of attention and been classified according to dimensions such as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) nanostructures.
纳米硅不仅膨胀小,而且为Li + 扩散提供了较大的表面积、短的途径和快速的电子传输。通过引入低维Si结构,可以进一步缓解体积膨胀。因此,纳米硅成为人们关注的焦点,并按照零维(0D)、一维(1D)、二维(2D)和三维(3D)纳米结构等维度进行分类。

Zero-dimensional nano Si with a size less than 150 nm exhibits better tolerance for the volume expansion and morphology change during the lithiation process. The stability and cycle performance can be improved via further reducing the size of Si particles. But, large surface areas also cause agglomerates of nanoparticles, leading to the increase of irreversible capacity and reduction of Coulombic efficiency [67]. Jeong et al. reported silicon quantum dots (Si QDs) cluster with π-conjugated molecule bridges via Sonogashira C−C cross-coupling reactions between 4-Bs/Oct Si QDs and 1,4-diethynylbenzene, and highlighted Li+ properties as anode materials [68], as shown in Fig. 2(a). By comparison, Si QDs cluster exhibited excellent electrochemical performances while the 4-Bs/Oct Si QD electrode did not participate in delithiation and showed a low specific capacity close to zero. The Si QDs showed good cycling stability with 63% capacity retention after 100 cycles at 200 mAhg-1. The excellent electrochemical performance of the Si QDs cluster was attributed to the π-conjugated molecules between the Si QDs and Si QDs clusters on the surface, which acted as a buffer layer to alleviate the Si expansion during operation.
尺寸小于150 nm的零维纳米硅对锂化过程中的体积膨胀和形貌变化表现出更好的耐受性。通过进一步减小硅颗粒的尺寸,可以提高稳定性和循环性能。但是,大表面积也会导致纳米颗粒团聚,导致不可逆容量的增加和库仑效率的降低[67]。Jeong等人报道了硅量子点(Si QDs)通过4-Bs/Oct Si QDs和1,4-二炔基苯之间的Sonogashira C−C交叉偶联反应,具有π共轭分子桥的硅量子点(Si QDs),并强调了Li作为阳极材料 + 的性质[68],如图2(a)所示。相比之下,Si QDs团簇表现出优异的电化学性能,而4-Bs/Oct Si QD电极不参与脱锂,比容量较低,接近于零。Si QDs在200 mAhg -1 下循环100次后表现出良好的循环稳定性,容量保持率为63%。Si QDs团簇的优异电化学性能归因于表面Si QDs和Si QDs团簇之间的π共轭分子,作为缓冲层缓解了Si在运行过程中的膨胀。

Fig. 2
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Fig. 2. Cross-Coupling Reaction between 4-Bs/Oct Si QDs and 1,4-Diethynylbenzene and electrochemical performances [68]. (b) Si nanowires produced from the chloromethylsilane precursor via CVD method and the electrochemical performances for LIBs [75].
图 2.4-Bs/Oct Si QDs与1,4-二乙炔基苯的交叉偶联反应及电化学性能[68].(b)氯甲基硅烷前驱体通过CVD法制备的硅纳米线和锂离子电池的电化学性能[75]。

Compared to the zero-dimensional nano Si, one-dimensional (1D) Si such as nanowires [69], [70] can more effectively endure the extensive volume change due to the presence of space between nanowires perpendicular to their growth. Moreover, 1D nano Si materials can create more pathways for electrolyte penetration, and provide essential channels for charge transfer. Several technologies including chemical vapor deposition (CVD), electrochemical etching and vapor-liquid-solid (VLS) growth were reported to prepare the 1D Si nanowires [71], [72], [73], [74]. Liu et al. synthesized the carbon-coated Si nanowire on the surface of graphite nanosphere (GM) through the CVD method with chloromethylsilane as the Si precursor source [75], as shown in Fig. 2(b). The obtained structures of C/SiNW/GM were investigated by utilizing the basic characterization techniques and confirming the existence of Si nanowire with diameter 60 nm. The C/SiNW/GM composite showed a specific capacity of 580 mAhg-1 at a 0.2 C rate. The good electrochemical performance of the C/SiNW/GM was related to the carbon protection layer on the surface of the Si nanowire which allowed the volume changes during lithiation/delithiation.
与零维纳米硅相比,一维(1D)硅(如纳米线)[69],[70]可以更有效地承受由于纳米线之间存在垂直于其生长的空间而导致的广泛体积变化。此外,一维纳米硅材料可以为电解质渗透创造更多的途径,并为电荷转移提供必要的通道。据报道,化学气相沉积(CVD)、电化学蚀刻和气-液-固(VLS)生长等多种技术制备了一维硅纳米线[71]、[72]、[73]、[74]。Liu等人通过CVD法合成了以氯甲基硅烷为Si前驱体源的碳涂层Si纳米线[75],如图2(b)所示。利用基本表征技术研究了C/SiNW/GM的结构,确认了直径为60 nm的Si纳米线的存在。C/SiNW/GM复合材料在0.2 C速率下显示出580 mAhg -1 的比容量。C/SiNW/GM的良好电化学性能与Si纳米线表面的碳保护层有关,该保护层允许锂化/脱锂过程中的体积变化。

The 2D nanostructured Si materials also attracted considerable attention for the ion/electron migration and mitigated the volume expansion during lithiation/delithiation [76]. In addition to traditional synthesis methods of CVD and physical vapor deposition (PVD), 2D Si materials were prepared using topochemical exfoliation, DC thermal plasma and solvent-induced growth [77], [78], [79]. An excellent example of the 2D nanostructure was the calcium silicide (CaSi2) Zintl compound structure where both Ca and Si atomic layers were alternatingly stacked. It could be an ideal way to obtain 2D nano Si materials by introducing the low-cost and simple stripping of CaSi2 [80], [81]. In the past several years, numerous strategies have been introduced to disrupt the heteropolar bonds presented in the CaSi2. Liu et al. reported an innovative synthesis approach by oxidizing Si anion layers into neutral layers while preserving the reliability of the silicene structure [82], as illustrated in Fig. 3(a). The synthesized ultrathin silicene sheets are in micrometer, as shown in the SEM image in Fig. 3(b). The clarity of the size can be clearly seen in the inset of Fig. 3(b). The silicene sheets showed excellent dispersion in NMP solution, as observed in Fig. 3(c). The silicene nanosheets exhibited a discharge capacity of about 721 mAhg-1 at 0.1 Ag-1 along with superior stability of 1800 cycles at 1 Ag─1, as shown in Fig. 3(d).
二维纳米结构硅材料在离子/电子迁移方面也引起了相当大的关注,并减轻了锂化/脱锂过程中的体积膨胀[76]。除了传统的CVD和物理气相沉积(PVD)合成方法外,还利用拓扑化学剥离、直流热等离子体和溶剂诱导生长制备了二维硅材料[77]、[78]、[79]。二维纳米结构的一个很好的例子是硅化钙(CaSi 2 )Zintl化合物结构,其中Ca和Si原子层交替堆叠。通过引入CaSi的低成本和简单剥离 2 [80],[81],这可能是获得二维纳米硅材料的理想途径。在过去的几年中,已经引入了许多策略来破坏 CaSi 中呈现的异极键 2 。Liu等人报道了一种创新的合成方法,将Si阴离子层氧化成中性层,同时保持硅烯结构的可靠性[82],如图3(a)所示。合成的超薄硅烯片以微米为单位,如图3(b)中的SEM图像所示。在图3(b)的插图中可以清楚地看到尺寸的清晰度。硅烯片在NMP溶液中表现出优异的分散性,如图3(c)所示。硅烯纳米片在0.1 Ag下表现出约721 mAhg -1 的放电容量 -1 ,在1 Ag下具有1800次循环的优异稳定性 ─1 ,如图3(d)所示。

Fig. 3
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Fig. 3. (a) Schematic illustration for the synthesis of silicene from CaSi2. (b) SEM images of silicene nanosheets and a single silicene sheet (inset), respectively. c) TEM image of silicene sheets with the inset photograph of a stable dispersion of silicene in NMP solution. (d) Cycling performance of silicene up to 1800 cycles at 1.0 A g─1 [82].
图 3.(a) CaSi合成硅烯的示意图 2 。(b)硅烯纳米片和单个硅烯片(插图)的SEM图像。c) 硅烯片的 TEM 图像,以及硅烯在 NMP 溶液中稳定分散的插图。(d)硅烯在1.0 A g ─1 下循环1800次的循环性能[82]。

The 3D Si materials along with the conductive network can provide more porous channels [83], [84], [85], [86]. The 3D nano Si materials are synthesized usually using a "top-down" method, and their porosity is mainly achieved through electrochemical etching [87]. A “bottom-up” approach was also used to prepare the 3D nano Si materials [88]. Yu et al. synthesized 3D stacked silicon nanosheets (s-SiNS) by following the electro-deoxidizing natural attapulgite in molten salts [89]. The morphology of s-SiNS is closely related with intermediate Ca3Si3O9 by following the selective etching process, as shown in Fig. 4(a-b). The obtained s-SiNS had excellent cycle stability and delivered a high specific capacity of 1205 mAhg─1 at 0.5Ag─1 with 60.2% capacity retention after 200 cycles. The s-SiNS also showed excellent rate performance even at high current densities, as illustrated in Fig. 4(c-d). The excellent electrochemical performance of s-SiNS was attributed to consisting nanosheets by forming three-dimensional structure which can effectively buffer the Si expansion and provide a wide pathway for Li+ and electrons transportation.
3D Si材料与导电网络一起可以提供更多的多孔通道[83],[84],[85],[86]。3D纳米硅材料通常采用“自上而下”的方法合成,其孔隙率主要通过电化学蚀刻实现[87]。还采用了“自下而上”的方法制备了3D纳米硅材料[88]。Yu等人通过在熔盐中电脱氧天然凹凸棒土合成了3D堆叠硅纳米片(s-SiNS)[89]。s-SiNS的形貌与中间体Ca 3 Si 3 O 9 密切相关,如图4(a-b)所示。所得的s-SiNS具有优异的循环稳定性,在0.5Ag下提供1205 mAhg ─1 的高比容量 ─1 ,200次循环后容量保持率为60.2%。即使在高电流密度下,s-SiNS也表现出出色的速率性能,如图4(c-d)所示。s-SiNS优异的电化学性能归因于纳米片的形成三维结构,可以有效缓冲Si的膨胀,为Li + 和电子的传输提供宽阔的途径。

Fig. 4
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Fig. 4. (a,b) SEM images of s-SINS surface. (c) Long-term cycling stability of the s-SINSC electrode. (d) Rate performance of the s-SINS at different current densities [89].
图 4.(a,b)s-SINS表面的SEM图像。(c) s-SINSC电极的长期循环稳定性。(d)s-SINS在不同电流密度下的速率性能[89]。

Compared to nano-scale Si, micro structural Si materials can provide a high tape density and higher volumetric capacity under the same mass load, as well as less active surface areas which can avoid or reduce side reactions. However, microstructural Si anodes have several challenges in term of the long cycle stability and insufficient pathways for Li+ transport. Therefore, it is a high challenge to balance between the tap density and particle size in nano-scale to achieve the high volumetric capacity density.
与纳米级硅相比,微结构硅材料在相同质量载荷下可以提供较高的胶带密度和更高的容积容量,以及更少的活性表面积,可以避免或减少副反应。然而,微观结构硅阳极在长循环稳定性和锂 + 传输途径不足方面存在一些挑战。因此,在纳米尺度上平衡振实密度和粒径以实现高体积容量密度是一项很高的挑战。

4. Si/C composites 4. Si/C复合材料

4.1. using 1D carbon materials such as nanotube and nanofiber
4.1. 使用一维碳材料,如纳米管和纳米纤维

In order to improve the electrochemical performance of Si/C anodes, the synthesis of 1D Si/C composite materials using nano Si and nano C materials has attracted great interest. Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are the most common 1D nanomaterials, which can be used to prepare 1D Si/C composites combined with nano Si. 1D Si/C composites offer several advantages as an anode of LIBs. The elongated structure of 1D composites provides short pathways for Li+ diffusion and reduces internal resistance. This enables the enhancement of rate capability and improvement of the power density [90], [91]. CNTs can buffer the volume expansion during lithiation/delithiation processes and prevent the pulverization of the Si particles, and also maintain the overall integrity of the Si anode.
为了提高Si/C负极的电化学性能,利用纳米Si和纳米C材料合成一维Si/C复合材料引起了人们的极大兴趣。碳纳米管(CNTs)和碳纳米纤维(CNFs)是最常见的一维纳米材料,可用于制备与纳米硅结合的一维Si/C复合材料。一维复合材料的细长结构为锂 + 扩散提供了短路径,并降低了内阻。这样可以增强速率能力并改善功率密度[90],[91]。碳纳米管可以缓冲锂化/脱锂过程中的体积膨胀,防止硅颗粒粉碎,并保持硅阳极的整体完整性。

Xu et al. reported Si composites with graphene and CNTs (G-Si-CNTs) by the CVD method, where Si nanoparticles, MgCl2 and Na2CO3 were used as precursor [92]. Iron oxide Fe2O3 acted as a catalytic template during calcination process, responsible for the CNTs growth. The schematic of the synthesis of G-Si-CNTs is shown in Fig. 5(a). The obtained G-Si-CNTs displayed spherical morphology, as shown in Fig. 5(b-c). Fig. 5(d) shows the cycle performance of G-Si-CNTs composites and maintained the reversible capacity of 907 mAhg─1 at 2Ag─1 after 700 cycles. The electrochemical performances of G-Si-CNTs were superior to that of the bare graphene Si composite, mainly ascribed to the interpenetrating of the CNTs in the composite.
Xu等报道了采用CVD法制备的Si复合材料与石墨烯和碳纳米管(G-Si-CNTs),其中Si纳米颗粒、MgCl 2 和Na 2 CO 3 为前驱体[92]。氧化铁Fe 2 O 3 在煅烧过程中起到催化模板的作用,负责碳纳米管的生长。G-Si-CNTs的合成示意图如图5(a)所示。得到的G-Si-CNTs呈现球形形貌,如图5(b-c)所示。图5(d)显示了G-Si-CNTs复合材料 ─1 在700次循环后在2Ag下保持907 mAhg ─1 的可逆容量的循环性能。G-Si-CNTs的电化学性能优于裸石墨烯Si复合材料,这主要归因于复合材料中碳纳米管的互穿性。

Fig. 5
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Fig. 5. (a) Schematic illustration G-Si-CNTs. (b,c) Morphology of G-Si-CNTs. (d) Cycling capacity of G-Si-CNTs at 2 A g−1 after the initial five cycles at 0.2 A g−1 [92].
图 5.(a) G-Si-CNT示意图。(二、三)G-Si-CNTs的形态。(d)在0.2 A g −1 的初始5次循环后,G-Si-CNTs在2 A g −1 下的循环容量[92]。

Recently, Yan et al. reported Si/CNTs/C composites by following one-step chemical reactions of calcium carbonate (CaCO3) with magnesium silicide (Mg2Si) in the presence of ferrocene (FeC10H10) [93]. The schematic illustration of synthesis is shown in Fig. 6(a). The CNTs were generated in the presence of iron atoms serving as a catalyst derived from the decomposition of ferrocene. The product morphology is shown in Fig. 6(b). The obtained tubes were intertwined each other and provided a platform for the encapsulation of Si particles. By comparison, Si/CNTs/C composites exhibited a higher specific capacity of 2052 mAhg─1 after 150 cycles, compared to the Si/C composite anode. The Si/CNTs/C anode also showed excellent cycle stability with the specific capacity of 137 mAhg─1 after 300 cycles at 0.5 Ag─1, depicted in Fig. 6(c-d). The outstanding electrochemical performances was mainly attributed to the encapsulation of Si particles by CTNs cages. The CNTs network can effectively buffer volume expansion, increase the conductivity of the composite and provide channels for Li+ transportation. CNFs not only have chemical stability and significant conductivity, but also excellent stress resistance, making them ideal electrode materials for LIBs and hydrogen storage materials [94]. CNFs can be synthesized using different technologies such as CVD, solid-phase synthesis and electrospinning [95], [96].
最近,Yan等人报道了碳酸钙(CaCO 3 )与硅化镁(Mg 2 Si)在二茂铁(FeC 10 H 10 )存在下一步化学反应[93]。合成示意图如图6(a)所示。碳纳米管是在铁原子存在下产生的,铁原子作为二茂铁分解产生的催化剂。产物形貌如图6(b)所示。所得到的管子相互缠绕,为Si颗粒的封装提供了平台。相比之下,与Si/C复合阳极相比,Si/CNTs/C复合材料在150次循环后表现出更高的比容量,为2052 mAhg ─1 。Si/CNTs/C阳极在0.5 Ag下循环300次后,比容量为137 mAhg ─1 ─1 ,显示出优异的循环稳定性,如图6(c-d)所示。出色的电化学性能主要归功于CTNs笼对Si颗粒的封装。碳纳米管网络可以有效缓冲体积膨胀,提高复合材料的电导率,为锂 + 输送提供通道。CNF不仅具有化学稳定性和显著的导电性,而且具有优异的抗应力性,使其成为锂离子电池和储氢材料的理想电极材料[94]。CNF可以使用CVD、固相合成和静电纺丝等不同技术进行合成[95],[96]。

Fig. 6
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Fig. 6. (a) Schematic of the Si/CNT/C composite. (b) SEM image of Si/CNT/C composite. (c) cycling performance of Si/C and Si/CNTs/C at 0.2 Ag−1. (d) Long cycle performance of Si/CNTs/C composite at 1 C rate [93].
图 6.(a) Si/CNT/C复合材料示意图。(b) Si/CNT/C复合材料的SEM图像。(c) Si/C和Si/CNTs/C在0.2 Ag下的循环性能 −1 。(d) Si/CNTs/C复合材料在1 C速率下的长循环性能[93]。

Wang et al. reported carbon nanofibers with uniformly embedded Si nanoparticles as self-standing anodes for high-performance LIBs [97]. To prepare the CNFs electrospinning precursor, a solution was adopted where Si nanoparticles and a triblock copolymer of polyethylene oxide-polypropylene oxide-polyethylene oxide (P123) were dissolved in N,N-dimethylformamide by stirring and ultrasonic vibration. Polyacrylonitrile (PAN) was added into the mixed solution and then electrospinning was used on the desired fixed parameter, and then finally conducted carbonization process to obtain (Si/PSNF-15) electrodes for LIBs, as shown in Fig. 7(a). They believed that PAN as a carbon precursor was beneficial for the formation of a highly desirable CNFs structure. The homogeneous morphology was obtained with no Si nanoparticles aggregation on the surfaces of Si/PCNF-15, depicted in Fig. 7(b). The Si/CNF, as a free-standing electrode, demonstrated excellent electrochemical performance and also could replace the Cu metal as a current collector. By comparison, the Si/PCNF-15 composites exhibited significant cycle stability and maintained the reversible capacity of 1063.2 mAhg─1 after 250 cycles with impressive coulombic efficiency, shown in Fig. 7(c). Moreover, the free-standing electrode could offer several advantages such as light weight, excellent conductivity, and good mechanical properties. The superior electrochemical performance was attributed to the unique design of Si/PCNF-15 containing abundant mesopores which allowed Li+ to diffuse faster and the electrolyte to penetrate more easily.
Wang等人报道了具有均匀嵌入Si纳米颗粒的碳纳米纤维作为高性能锂离子电池的自立阳极[97]。为了制备CNFs静电纺丝前驱体,采用Si纳米颗粒和聚环氧乙烷-聚环氧丙烷-聚环氧乙烷(P123)的三嵌段共聚物通过搅拌和超声振动溶解在N,N-二甲基甲酰胺中的溶液。在混合溶液中加入聚丙烯腈(PAN),然后对所需的固定参数进行静电纺丝,最后进行碳化过程,得到用于锂离子电池的(Si/PSNF-15)电极,如图7(a)所示。他们认为,PAN作为碳前体有利于形成非常理想的CNFs结构。Si/PCNF-15表面没有Si纳米颗粒聚集,获得均匀的形貌,如图7(b)所示。Si/CNF作为独立式电极,表现出优异的电化学性能,也可以替代Cu金属作为集流体。相比之下,Si/PCNF-15复合材料表现出显著的循环稳定性,并在250次循环后保持了1063.2 mAhg ─1 的可逆容量,库仑效率令人印象深刻,如图7(c)所示。此外,独立式电极具有重量轻、导电性好、机械性能好等优点。优异的电化学性能归因于Si/PCNF-15的独特设计,它含有丰富的介孔,使Li + 扩散得更快,电解质更容易渗透。

Fig. 7
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Fig. 7. (a) Schematic drawing of the producing processes for Si//PCNF-15 and Si//CNF composites. (b) SEM image of Si/PCNF-15. (c) CV results of different cycles of Si/C carbon fiber. (d) Cycling performance and coulombic efficiencies of the three electrodes at a current density of 0.3 Ag─1 [97].
图 7.(a) Si//PCNF-15和Si//CNF复合材料的生产工艺示意图。(b) Si/PCNF-15的SEM图像。(c) Si/C碳纤维不同循环的CV结果。(d)电流密度为0.3 Ag时三个电极的循环性能和库仑效率 ─1 [97]。

Carbon nanofibers doped with heteroatoms were designed and prepared through electrospinning and pyrolysis process [98]. Research found that the carbon nanofibers shortened the Li⁺ diffusion distance and facilitated ion transport while porous Si efficiently relieved the volume expansion. The freestanding porous Si@heteroatom-doped carbon fiber anodes exhibited an excellent reversible capacity of 1112.7 mAhg-1 after 1000 cycles at 2.0 Ag-1 [99]. Other approaches such as anchoring of Si particles, the combination of branched CNFs with Si particles, the incorporation of branched CNFs on Si nanowires, and Si-core carbon shell fibers were also explored to prepare Si@CNFs composites [100], [101], [102], [103]. Jin et al. reported a scalable production of self-supporting Si/C carbon nanofibers composites by utilizing the electrospinning and hydrothermal methods [104]. The Si nanoparticles were dissolved in the DMF solution and then PAN was added subsequently into the prepared solution. Electrospinning and hydrothermal followed by pyrolysis along with carbonization were employed to obtain the final product of the carbon-coated Si/carbon nanofiber membrane (C-Si-CNF). The morphology observation confirmed the existence of Si NPs on the surface of CNF and displayed rough surface with increase in size compared to the PAN-based CNF. After hydrothermal treatments and pyrolysis, the C-Si-CNF displayed the fine structure, as shown in Fig. 8(a-c). Compared to other fiber membranes, the C-Si-CNF showed excellent cycling stability. Fig. 8(d) presents the cycling performance of the synthesized C-Si-CNF, which has the reversible capacity of 694.7 mAhg─1 at 0.2 Ag─1 and capacity retention of more than 100% after 300 cycles. The C-Si-CNF electrode also exhibited excellent rate capability at different current densities, and presented 210 mAhg-1 at 5 Ag-1, depicted in Fig. 8(e). The outstanding electrochemical performances was mainly related to the ionic and electronic conductivity of the fiber membrane itself, and increased with the increase of carbon coating, which further provided more channels for the rapid diffusion of Li+.
通过静电纺丝和热解工艺设计制备了掺杂杂原子的碳纳米纤维[98]。研究发现,碳纳米纤维缩短了Li⁺的扩散距离,促进了离子的传输,而多孔Si有效地缓解了体积膨胀。独立式多孔Si@heteroatom掺杂碳纤维负极在2.0 Ag下循环1000次后表现出1112.7 mAhg -1 的优异可逆容量 -1 [99]。还探索了其他方法,如Si颗粒的锚定、支链CNFs与Si颗粒的结合、在Si纳米线上掺入支链CNFs以及Si芯碳壳纤维等Si@CNFs复合材料[100]、[101]、[102]、[103]。Jin等人报道了利用静电纺丝和水热法可扩大生产自支撑Si/C碳纳米纤维复合材料的方法[104]。将Si纳米颗粒溶解在DMF溶液中,然后将PAN加入到制备的溶液中。采用静电纺丝和水热法,然后热解和碳化,得到碳包覆硅/碳纳米纤维膜(C-Si-CNF)的最终产品。形貌观察证实了CNF表面存在Si NPs,与PAN基CNF相比,其表面粗糙且尺寸增大。经水热处理和热解后,C-Si-CNF呈现出精细结构,如图8(a-c)所示。与其他纤维膜相比,C-Si-CNF表现出优异的循环稳定性。图8(d)显示了合成的C-Si-CNF的循环性能,其在0.2 Ag下的可逆容量为694.7 mAhg ─1 ─1 ,循环300次后容量保持率超过100%。 C-Si-CNF电极在不同电流密度下也表现出优异的倍率能力,在5 Ag时呈现210 mAhg -1 -1 ,如图8(e)所示。突出的电化学性能主要与纤维膜本身的离子和电子电导率有关,并随着碳涂层的增加而增加,进一步为Li的快速扩散提供了更多的通道 +

Fig. 8
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Fig. 8. (a-c) SEM and optical images of corresponding CNF, Si-CNF, C-Si-CNF, and schematic illustration of surface and cross-sectional sections. (d) Cycling performance of the Si, CNF, Si-CNF, and C-Si-CNF electrodes, at 0.2 Ag─1. (e) Rate performance of the CNF, Si-CNF, and C-Si-CNF electrodes [104].
图 8.(a-c)相应CNF、Si-CNF、C-Si-CNF的SEM和光学图像,以及表面和横截面的示意图。(d) Si、CNF、Si-CNF 和 C-Si-CNF 电极在 0.2 Ag 下的循环性能 ─1 。(e)CNF、Si-CNF和C-Si-CNF电极的倍率性能[104]。

4.2. using 2D materials such as graphene and MXenes
4.2. 使用石墨烯和MXenes等2D材料

Graphene is theoretically a single-layer two-dimensional carbon structure, where carbon atoms are arranged in a hexagonal lattice. As is well known, graphene has unique and special properties such as excellent electrical and thermal conductivity, mechanical strength, and high surface areas, and has received considerable attention in many fields of energy storage, electronics, electrochemical capacitors and biomedical applications [105], [106], [107]. Chemically doped graphene has been considered a hot research topic because it generates band gaps through doping, thus bringing more advantages [108]. Therefore, doped graphene has been widely used as composite electrodes in the LIBs.

The graphene in Si/2D composite anodes is synthesized using the economical and scalable Hummer’s method. Li et al. reported the carbon/ graphene double-layer coated nano Si/C/G composite anodes via two step chemical reactions [109]. The Si particles were first coated with an amorphous carbon layer to obtain a stable core-shell structure. The obtained Si/C core-shell was then surrounded with graphene coatings to maintain the integrity of Si particles. The schematic illustration of the Si/C/G is shown in Fig. 9(a). The graphene oxide (GO) sheets were synthesized via the Hummer’s method from natural graphite flakes and subsequently followed by hydrothermal methods. Four different samples were fabricated to investigate electrochemical properties of the Si/C/G composites anodes. CV results of the Si/C/G anodes are displayed in Fig. 9(b). The first cathodic peak presents the SEI layer associated with the interaction of Li+ and electrolyte. The anodic peak fits well with the decomposition of Li-Si phases. Fig. 9(c) presents the EIS results of the four samples. Their results indicated that the Si/C/G composite anode had less charge transfer resistance compared to other electrodes. Furthermore, the Si/C/G anode exhibited excellent cycle performance within 300 cycles and displayed the discharge capacity of 2000 mAhg─1 and 1600 mAhg─1 under two different current densities of 1 Ag-1 and 2 Ag-1, respectively, as shown in Fig. 9(d). The excellent performance of Si/C/G composites was attributed to the amorphous carbon and the graphene coating on the surface of the Si/C composite. Moreover, the presence of graphene sheets in the composites improved the ionic conductivity and reduced the internal resistance of the Si electrode.
Si/2D复合阳极中的石墨烯采用经济且可扩展的悍马方法合成。Li等人报道了碳/石墨烯双层涂层纳米Si/C/G复合阳极的两步化学反应[109]。首先在Si颗粒上涂覆无定形碳层,以获得稳定的核壳结构。然后用石墨烯涂层包围所得的Si/C核壳,以保持Si颗粒的完整性。Si/C/G的原理图如图9(a)所示。氧化石墨烯(GO)片是通过悍马法从天然石墨片中合成的,随后采用水热法。制备了四种不同的样品,以研究Si/C/G复合材料阳极的电化学性能。Si/C/G阳极的CV结果如图9(b)所示。第一个阴极峰呈现与锂 + 和电解质相互作用相关的 SEI 层。阳极峰与Li-Si相的分解非常吻合。图9(c)显示了四个样品的EIS结果。结果表明,与其他电极相比,Si/C/G复合阳极的电荷转移电阻较小。此外,Si/C/G阳极在300次循环内表现出优异的循环性能,在1 Ag -1 和2 Ag -1 两种不同电流密度下分别表现出2000 mAhg ─1 和1600 mAhg ─1 的放电容量,如图9(d)所示。Si/C/G复合材料的优异性能归功于Si/C复合材料表面的非晶碳和石墨烯涂层。此外,复合材料中石墨烯片的存在提高了离子电导率并降低了硅电极的内阻。

Fig. 9
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Fig. 9. (a) Schematic illustration of Si/C/G composite. (b) Cyclic voltammogram of Si/C/G. (c) EIS results of Si, Si/C, Si/G and Si/C/G. (d) Long cycle performance of Si/C/G composite electrode [109].
图 9.(a) Si/C/G复合材料示意图。(b)Si/C/G的循环伏安图。 (c)Si、Si/C、Si/G和Si/C/G的EIS结果。 (d)Si/C/G复合电极的长循环性能[109]。

It has demonstrated that some organic functional groups such as R–CH2-OH and R–COOH facilitated the formation of a stable SEI layer during the first lithiation process, showing high reversible capacity and good cycle life [110]. The GO on the Si particles can provide hydroxyl (-OH) and carboxyl (-COOH) groups. These specious create chemical bonds with Si particles. The interaction forms a strong bond between rGO and Si particles, effectively preventing loss of Si particles and enhancing the electrochemical performance. The preparation of composites can be completed by the self-assembly process of spray drying or freeze drying of Si and GO solutions [111], [112], [113]. Xu et al. reported the nitrogen-rich Si/graphene composite anode (NR/Si/G) through self-assembly of electrostatic attractions between amino and carboxyl groups [114]. Si particles were well encapsulated in N-doped graphene because the electrostatic attraction of the amino and carboxyl groups inhibited the agglomeration of Si particles and the accumulation of graphene sheets. As results, the volume expansion and collapse of Si particles were suppressed during charging-discharging processes, preventing Si particles from being directly exposed to the electrolyte. The N-doped graphene improved the conductivity and performance of the Si-based anodes. In comparison with Si electrodes, the NR/Si/G electrodes showed good electrochemical performance with the initial Coulombic efficiency of 75.2%. The electrode maintained the discharge capacity of 937 mAhg-1 at 1 Ag-1 with a capacity retention of 62.8% after 500 cycles. Cong et al. reported N-doped graphene/carbon encapsulated Si nanoparticles and carbon nanofibers (NG/C@Si/CNF) electrodes by following the surface modification, electrostatic self-assembly, cross-linking with heat treatment and further carbonization processes [115]. The presence of CNF and NG/C layers allows the volume changes of Si and also enhances the overall conductivity of the electrode. The as synthesized NG/C@Si/CNF electrode exhibited better rate capability and cycling performance, compared to the other electrode materials. After 100 cycles, the electrode maintained a high reversible specific capacity of 1371.4 mAhg-1. Lin et al. obtained graphene nanowalls (GNWs) on nickel foam by RF plasma horizontal tube furnace deposition [116]. Afterward, Si particles were deposited on the synthesized GNWs to form Si@GNWs composites. The Si@GNWs composites exhibited relatively low graphitization in Raman spectroscopy since intergranular defects created short paths for charge transfer into the conductive network formed by 2D GNWs. The composites could be achieved without any additives and complex fabrication processes.
研究表明,一些有机官能团,如R-CH 2 -OH和R-COOH,在第一次锂化过程中促进了稳定SEI层的形成,显示出高可逆能力和良好的循环寿命[110]。硅颗粒上的GO可以提供羟基(-OH)和羧基(-COOH)基团。这些似是而非的与硅粒子形成化学键。这种相互作用在rGO和Si颗粒之间形成强键,有效防止了Si颗粒的损失,增强了电化学性能。复合材料的制备可以通过Si和GO溶液的喷雾干燥或冷冻干燥的自组装过程来完成[111],[112],[113]。Xu等报道了富氮Si/石墨烯复合阳极(NR/Si/G)通过氨基和羧基之间的静电吸引力自组装[114]。由于氨基和羧基的静电吸引力抑制了Si颗粒的团聚和石墨烯片的积累,Si颗粒很好地包裹在N掺杂石墨烯中。结果,在充放电过程中抑制了Si颗粒的体积膨胀和塌陷,防止了Si颗粒直接暴露于电解液中。N掺杂石墨烯提高了硅基阳极的导电性和性能。与硅电极相比,NR/Si/G电极表现出良好的电化学性能,初始库仑效率为75.2%。电极在1 Ag下保持937 mAhg -1 的放电容量 -1 ,500次循环后容量保持率为62.8%。Cong 等人。 报道了N掺杂石墨烯/碳包封的Si纳米颗粒和碳纳米纤维(NG/C@Si/CNF)电极,通过表面改性、静电自组装、交联热处理和进一步的碳化过程[115]。CNF 和 NG/C 层的存在允许 Si 的体积变化,并增强电极的整体电导率。与其他电极材料相比,合成的NG/C@Si/CNF电极表现出更好的倍率能力和循环性能。循环100次后,电极保持了1371.4 mAhg -1 的高可逆比容量。Lin等人通过RF等离子体水平管炉沉积在泡沫镍上获得了石墨烯纳米壁(GNWs)[116]。然后,Si颗粒沉积在合成的GNW上,形成Si@GNWs复合材料。Si@GNWs复合材料在拉曼光谱中表现出相对较低的石墨化,因为晶间缺陷为电荷转移到由二维GNW形成的导电网络中创造了短路径。这些复合材料可以在没有任何添加剂和复杂的制造工艺的情况下实现。

MXenes as another category of 2D materials for LIBs recently aroused great interest [117]. Similar to the 2D graphene structure, MXenes not only provide a large number of active sites on the surface, but also have high conductivity, making them more promising electrode materials. In addition, MXenes can be used as current collectors, free-standing electrodes, multifunctional binders, and composite materials [118], [119], [120], [121], [122], [123], [124]. Zhou et al. synthesized 2D V4C3Tx @MXenes by utilizing selective etching V4AlC3 in hydrofluoric acid at temperature of 55 °C [125]. The obtained V4C3Tx-HF and V4C3Tx-BM-HF (BM:ball milling) exhibited similar layered morphological characteristics. Both V4C3Tx-HF and V4C3Tx-BM-HF exhibited a high capacity with good stability under different current densities. Tian et al. revealed the free-standing Si@MXene composite electrodes via vacuum filtration [119]. Fig. 10 (a) depicts the schematic diagram of the preparation of Si@MXene composite papers. The TEM and SEM images of the Si@MXene composite electrodes are shown in Fig. 10(b-c). The CV results of first three cycles of the Si@MXene composites revealed the formation process of SEI layers, as shown in Fig. 10 (d). The Si@MXene composite electrodes displayed the superior electrochemical performance with a capacity of 2118 mAhg─1 at 200 mAg─1 and 1672 mAhg─1 at 1000 mAg─1, respectively, as depicted in Fig. 10 (e). The superior electrochemical performance was related to the unique configuration of the Si@MXene composite, which effectively enhanced the conductivity of the Si@MXene electrode. In addition, Nam et al. utilized the liquid phase epitaxial growth and sonochemical methods to prepare functionalized titanium carbide nanorods (FTCNs) on Ti3C2 nanosheets [126]. The ion intercalation expanded the interlayer spacing of Ti3C2, effectively boosting the reaction sites. The presence of grown fluoride anions (F-) on Ti3C2 increased the electronegativity and further enhanced the stability of FTCNs. The specific capacity of FTCN-MXene was recoded as 1043 mAh g-1 and the Coulombic efficiency remained 98.78% over 250 cycles.
MXenes作为LIB的另一类二维材料,最近引起了人们的极大兴趣[117]。与二维石墨烯结构类似,MXenes不仅在表面提供大量活性位点,而且具有高导电性,使其成为更有前途的电极材料。此外,MXenes还可用作集流体、独立电极、多功能粘合剂和复合材料[118]、[119]、[120]、[121]、[122]、[123]、[124]。周等人在55°C温度下利用氢氟酸中的选择性蚀刻V AlC 3 合成了2D V 4 C 3 T x @MXenes[125]。 4 得到的V 4 C 3 T x -HF和V 4 C 3 T x -BM-HF(BM:球磨)表现出相似的层状形貌特征。V 4 C 3 T x -HF和V 4 C 3 T x -BM-HF在不同电流密度下均表现出较高的容量和良好的稳定性。Tian等人通过真空过滤揭示了独立式Si@MXene复合电极[119]。图10(a)描绘了Si@MXene复合纸的制备示意图。Si@MXene复合电极的TEM和SEM图像如图10(b-c)所示。Si@MXene复合材料前三个循环的CV结果揭示了SEI层的形成过程,如图10(d)所示。如图10(e)所示,Si@MXene复合电极在200 mAg和1000 mAg ─1 时的容量分别为2118 mAhg ─1 ─1 和1672 mAhg ─1 ,表现出优异的电化学性能。 优异的电化学性能与Si@MXene复合材料的独特构型有关,有效增强了Si@MXene电极的导电性。此外,Nam等人利用液相外延生长和声化学方法在Ti 3 C 2 纳米片上制备了功能化碳化钛纳米棒(FTCN)[126]。离子插层扩大了Ti 3 C 2 的层间距,有效地促进了反应位点。Ti 3 C 2 上生长的氟化物阴离子(F - )的存在增加了FTCNs的电负性,并进一步增强了FTCNs的稳定性。FTCN-MXene的比容量被重新编码为1043 mAh g -1 ,库仑效率在250次循环中保持在98.78%。

Fig. 10
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Fig. 10. (a) Schematic diagram for the preparation of Si@MXene composite paper. (b) Cross-sectional SEM image. (c) TEM image. (d) CV plots of first three cycles for Si@MXene anodes. (e) Long-term cycling capability of Si@MXene anodes at 1000 mAhg─1 [119].
图 10.(a) Si@MXene复合纸的制备示意图。(b) 横截面SEM图像。(c) 透射电镜图像。(d) Si@MXene阳极前三个循环的CV图。(e)Si@MXene阳极在1000 mAhg ─1 下的长期循环能力[119]。

4.3. Porous Si/C composites using 3D architecture materials
4.3. 使用3D建筑材料的多孔Si/C复合材料

3D porous Si/C composites can be synthesized by various technologies such as the CVD, electrodeposition, and solution-based methods etc. Recently, Zhang et al. reported 3D honeycomb Si/carbon/reduced GO (Si/C/rGO) via electrostatic reactions as anodes for LIBs [127]. In the honeycomb 3D nanostructure, acetylene black and the Si/C composite were formed via a ball milling method and then modified with NH3·H2O. Afterward, the Si/C composites were dispersed homogeneously in dilute suspension of GO solution. Then the reduction process was followed by adding hydrazine hydrate (N2H4·H2O) to convert the Si/C/GO into Si/C/rGO composites, as shown in Fig. 11(a). The presence of acetylene black provided electron transport while the rGO buffered the volume expansion of Si during lithiation/delithiation reactions. The mesoporous channels allowed Li+ diffusion. The TEM and SEM images are shown in Fig. 11(b-c), respectively. For the comparison, three types of electrodes including the pure Si, Si/C and Si/C/rGO composites were tested. The Si/C/rGO anode showed excellent rate capability from 1 C to 5 C rates, as shown in Fig. 11(d). The CV curves of the 1st to 4th circle of the Si/C/rGO half-cell are shown in Fig. 11(e). After the first cycle, the activity of the electrode was enhanced, and the peak current also increased gradually. The 3D honeycomb Si/C/rGO composite exhibited excellent cycling performance compared to other electrodes and displayed the capacity of about 1004 mAhg─1 at 1 Ag─1 after 270 cycles, as shown in Fig. 11(f). The excellent performance of the electrode was attributed to the unique nanostructure of Si/C/rGO composites that could provide channels for Li+ fast diffusion. Moreover, the combination of acetylene black and rGO made the composites more favorable for electrode reaction kinetics and toleration of Si volume expansion.
三维多孔Si/C复合材料可以通过CVD、电沉积、溶液等多种技术合成,最近,Zhang等人报道了通过静电反应将3D蜂窝Si/碳/还原GO(Si/C/rGO)作为锂离子电池的阳极[127]。在蜂窝状三维纳米结构中,采用球磨法形成乙炔黑和Si/C复合材料,然后用NH 3 · 2 随后,将Si/C复合材料均匀分散在GO溶液的稀悬液中。然后进行还原过程,加入水合肼(N 2 H 4 ·H 2 O)将Si/C/GO转化为Si/C/rGO复合材料,如图11(a)所示。乙炔黑的存在提供了电子传递,而rGO在锂化/脱锂反应中缓冲了Si的体积膨胀。介孔通道允许Li + 扩散。TEM和SEM图像分别如图11(b-c)所示。为了进行比较,测试了纯Si、Si/C和Si/C/rGO复合材料三种类型的电极。Si/C/rGO阳极在1 C至5 C速率下表现出优异的倍率能力,如图11(d)所示。Si/C/rGO半电池第1圈至第4圈的CV曲线如图11(e)所示。第一次循环后,电极的活性增强,峰值电流也逐渐增加。与其他电极相比,3D蜂窝Si/C/rGO复合材料表现出优异的循环性能, ─1 在270次循环后,在1 Ag下显示出约1004 mAhg ─1 的容量,如图11(f)所示。该电极的优异性能归因于Si/C/rGO复合材料独特的纳米结构,可以为Li + 的快速扩散提供通道。 此外,乙炔黑和rGO的结合使复合材料更有利于电极反应动力学和Si体积膨胀的耐受性。

Fig. 11
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Fig. 11. (a) Schematic of the process for fabricating 3D honeycomb nanostructure Si/C/rGO composite. (b) TEM image of Si/C/rGO composite. (c) SEM images of Si/C/rGO composite. (d) Rate performance of Si/C/rGO (red), Si/C (black), and pure Si (blue). (e) CV curves of the Si/C/rGO composite at a scan rate of 0.1 mVs─1 within potential range of 0.01–1.5 V. (f) Specific capacity and CE of Si/C/rGO, Si/C at 1 Ag-1 [127].
图 11.(a) 三维蜂窝纳米结构Si/C/rGO复合材料的制备工艺示意图。(b) Si/C/rGO复合材料的TEM图。(c) Si/C/rGO复合材料的SEM图像。(d) Si/C/rGO(红色)、Si/C(黑色)和纯Si(蓝色)的倍率性能。(e)在0.01–1.5 V电位范围内,扫描速率为0.1 mVs ─1 的Si/C/rGO复合材料的CV曲线。 (f)Si/C/rGO、Si/C在1 Ag时的比容量和CE -1 [127]。

Guan et al. proposed a granadilla-like porous Si/C composite structure by mixing Si nanoparticles, tetraethoxysilane and poly (vinylpyrrolidone) (PVP) as a carbon source. Fig. 12(a) depicts the preparing processes of the granadilla-like P/Si/C composite [128]. The porous P/Si/C composite was fabricated by spray drying. During this process, the PVP particles incorporated on the surface of every single SiO2/Si NP to produce the core shell on the surface of the composite (PVP SiO2/Si NPs). Meanwhile, the pre-carbonization of PVP turned into micro-size spherical morphology interconnected with the obtained composite. The SiO2/Si NPs were uniformly distributed inside PVP framework. By following the etching process to carbonize and remove SiO2, the pomegranate-like porous Si/C composite was obtained. After high temperature treatments, PVP network generated numerous micro-sized pores on outer surface of the entire and within the carbon matrix, creating a unique porous structure. Fig. 12(b-d) shows the SEM images of the P/Si/C-400 and HRTEM illustration of PSi/C after etching. The carbon core shell allowed Si to expand in volume, and micropores created pathways for fast transfer of Li+. The granadilla-like porous P/Si/C composite exhibited excellent electrochemical performances. The P/Si/C-400 composite delivered the discharge capacity of 610.38 mAhg─1 at 1000 mAg─1 after 3000 cycles, as shown in Fig. 12(e). Fig. 12(f) presents the EIS plots of the P/Si/C-400, P/Si/C-800 and Si/C composites, demonstrating that the P/Si/C-400 sample had the smallest charge transfer resistance. Fig. 12(g) depicts the CV results of the first three cycles of P/Si/C-400 composites.
Guan等人通过混合Si纳米颗粒、四乙氧基硅烷和聚乙烯基吡咯烷酮(PVP)作为碳源,提出了一种类似颗粒的多孔Si/C复合结构。图12(a)描绘了粒状P/Si/C复合材料的制备过程[128]。多孔P/Si/C复合材料采用喷雾干燥法制备。在此过程中,PVP颗粒掺入每个SiO 2 /Si NP的表面,在复合材料表面产生核壳(PVP SiO 2 /Si NPs)。同时,PVP的预碳化转变为与所获得的复合材料相互连接的微粒球形貌。SiO 2 /Si NPs均匀分布在PVP框架内。通过蚀刻工艺碳化除去SiO 2 ,得到了石榴状多孔Si/C复合材料。经过高温处理后,PVP网络在整体外表面和碳基体内产生了许多微小的孔隙,形成了独特的多孔结构。图12(b-d)显示了P/Si/C-400的SEM图像和PSi/C的HRTEM图。碳核壳允许Si在体积上膨胀,微孔为Li的快速转移创造了途径 + 。粒状多孔P/Si/C复合材料表现出优异的电化学性能。P/Si/C-400复合材料 ─1 在3000次循环后在1000 mAg下提供610.38 mAhg ─1 的放电容量,如图12(e)所示。图12(f)显示了P/Si/C-400、P/Si/C-800和Si/C复合材料的EIS图,表明P/Si/C-400样品具有最小的电荷转移电阻。图12(g)描绘了P/Si/C-400复合材料前三个循环的CV结果。

Fig. 12
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Fig. 12. (a) Schematic illustration of the preparation of the granadilla-like PSi/C composite. (b-c) SEM images of PSi/C-400. (d) HRTEM illustration of PSi/C after etching. (e) Long cycle performance of PSi/C-400 at 1000 mAg─1 current density. (f) EIS plots of PSi/C-400, PSi/C-800 and Si/C-400. (g) Cyclic voltammograms for the first three cycles of PSi/C-400 [128].
图 12.(a) 类似花岗岩的PSi/C复合材料的制备示意图。(乙-丙)PSi/C-400 的 SEM 图像。(d) 蚀刻后PSi/C的HRTEM图示。(e) PSi/C-400在1000 mAg ─1 电流密度下的长循环性能。(f) PSi/C-400、PSi/C-800和Si/C-400的EIS图。(g)PSi/C-400前三个周期的循环伏安图[128]。

Shi et al. developed a Si core-graphene shell structure from vertical graphene (vG) by using the CVD method on commercial available SiO microparticles (d-SiO@vG), where methane was used as the carbon source [129]. During graphene synthesis, the SiO microparticles underwent disproportionation reactions, resulting in conversion to silicon and silica. Meanwhile, the reaction was heated initially and the SiO2 phase was formed, which further provided catalytic nucleation sites for graphene synthesis. The obtained d-SiO@vG sample delivered a capacity of about 450 mAhg─1 at 1600 mAg─1, and showed small capacity fade after 100 cycles. The outstanding electrochemical performance was attributed to the unique structure of the d-SiO@vG composite. Additionally, the vertical graphene coating on the surface of SiO led to high electrical conductivity.
Shi等人通过在市售SiO微粒(d-SiO@vG)上使用CVD方法,从垂直石墨烯(vG)开发了Si核-石墨烯壳结构,其中甲烷用作碳源[129]。在石墨烯合成过程中,SiO微粒发生歧化反应,转化为硅和二氧化硅。同时,对反应进行初步加热,形成SiO 2 相,进一步为石墨烯的合成提供了催化成核位点。在1600 mAg下,d-SiO@vG样品的容量约为450 mAhg ─1 ─1 ,并且在100次循环后显示出较小的容量衰减。出色的电化学性能归功于d-SiO@vG复合材料的独特结构。此外,SiO表面的垂直石墨烯涂层导致了高导电性。

Ma et al. reported 3D cross-linked graphene-wrapped porous nano-Si composites as anodes for LIBs [130]. The synthesis process is shown in Fig. 13(a). The SiO2 was prepared by following the Stöber sol-gel method, while GO was synthesized with the Hummers. Subsequently, the GO, SiO2 and Mg were mixed in 30 mL DI water and then sonicated for 15 min to obtain a homogenous solution. The solution was first freeze-dried, and then put it in furnace for different time of 5,7.5 and 10 h at 660 °C at a heat rate of 10 °C min─1, respectively. The obtained samples were named accordingly to the reaction time as P-Si@rGO-5, P-Si@rGO-7.5 and P-Si@rGO-10, respectively. Then the products were treated with 0.1 M HCl and DI water to remove extra products formed during reactions, and then finally the three P-Si@rGO composites were obtained by freeze-drying again. Fig. 13(b-c) shows the SEM images of the obtained P-Si@rGO-7.5. The P-Si@rGO-7.5 product exhibited excellent rate capability and cycling performance compared to other two products of P-Si@rGO-5 and P-Si@rGO-10, as shown in Fig. 13(d-e). The P-Si@rGO-7.5 showed a capacity of about 1123 mAhg─1 at 1 Ag─1 over 500 cycles. The electrode of P-Si@rGO-7.5 contained a high content of Si and porous structure, while the P-Si@rGO-5 contained SiOx products and the P-Si@rGO-10 contained large quantity of Si/C. By comparison, 3D carbon materials revealed good prospects in protecting Si particles from electrolyte erosion and improved battery performance. However, the preparation cost of 3D carbon materials was high, and the preparation process was complex.
马等人报道了3D交联石墨烯包裹的多孔纳米硅复合材料作为锂离子电池的阳极[130]。合成过程如图13(a)所示。SiO 2 采用Stöber溶胶-凝胶法制备,GO采用悍马合成。随后,将GO、SiO 2 和Mg混合在30 mL去离子水中,然后超声处理15 min,得到均匀的溶液。首先将溶液冷冻干燥,然后分别以10 ° ° ─1 C的升温在660 ° °C下放入炉中5,7.5小时和10小时。根据反应时间分别将得到的样品命名为P-Si@rGO-5、P-Si@rGO-7.5和P-Si@rGO-10。然后用0.1 M HCl和去离子水处理产物,除去反应过程中形成的多余产物,最后再次冷冻干燥得到3种P-Si@rGO复合材料。图13(b-c)显示了获得的P-Si@rGO-7.5的SEM图像。P-Si@rGO-7.5产品与P-Si@rGO-5和P-Si@rGO-10的其他两种产品相比,表现出优异的倍率能力和循环性能,如图13(d-e)所示。P-Si@rGO-7.5在500次循环中在1 Ag ─1 下的容量约为1123 mAhg ─1 。P-Si@rGO-7.5电极含有高含量的Si和多孔结构,而P-Si@rGO-5含有SiOx产物,P-Si@rGO-10含有大量的Si/C。相比之下,3D碳材料在保护硅颗粒免受电解质侵蚀和提高电池性能方面显示出良好的前景。然而,三维碳材料的制备成本高,制备工艺复杂。

Fig. 13
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Fig. 13. (a) Synthesis of P-Si@rGO composite. (b, c) SEM images. (d) Rate capability of P-Si@rGO-5, P-Si@rGO-7.5 and P-Si@rGO-10 electrodes. (e) Cycling performance of P-Si@rGO-5, P-Si@rGO-7.5 and P-Si@rGO-10 electrodes [130].
图 13.(a) P-Si@rGO复合材料的合成。(二、三)SEM 图像。(d) P-Si@rGO-5、P-Si@rGO-7.5和P-Si@rGO-10电极的倍率能力。(e)P-Si@rGO-5、P-Si@rGO-7.5和P-Si@rGO-10电极的循环性能[130]。

5. Binders and their derivatives
5. 粘结剂及其衍生物

Binder materials provide adhesion between the active materials and current collector, ensuring good contact and efficient electron transfer. In addition, they can assist in forming a uniform and continuous film on the electrode surface, which helps improve electrode stability and performance, particularly for Si-based anodes [131], [132]. Appropriate binders can enhance the mechanical integrity of electrodes by holding active materials along with conductive additives, which further prevents electrode disintegration during lithiation/delithiation processes [133]. Different binders significantly impact the electrode’s performance such as initial Coulombic efficiency, capacity, cycling stability and rate capability etc. [134].

Polyvinylidene difluoride (PVDF) is commonly used as electrode binders in the LIBs [135]. Unfortunately, the PVDF has a weak van der Waals force which is unable to cope with the huge volume changes of Si particles, and unable to maintain the integrity of the electrode structure [136], [137]. In addition, the PVDF binder only dissolves in the N-methylpyrrolidone (NMP) solvent, which is not only expensive, but also harmful to the environment. Therefore, water-soluble binders are gradually becoming the environmentally friendly mainstream binders. For example, the polyacrylic acid (PAA), carboxymethyl cellulose (CMC), 1-pyrenemethyl methacrylate, polyethylene glycol, sodium alginate, β-cyclodextrin, polyimide, polyimine polyaniline and crosslinked polymer were widely studied [138], [139], [140], [141], [142], [143], [144]. These binders contain abundant polar functional groups (-OH, -COOH, -NH2), which have strong interaction forces with the surface of Si-based materials, forming a protective layer that helps the Si electrode form a stable SEI film during cycling. Gao et al. reported the polyimine polymer binder for Si-based anodes via one-step condensation reactions [145]. The fabrication process was as follows: the 4, 4- biphenyldicarboxaldehyde (BCA) and 1, 5-naphthalenediamine (NDA) were firstly mixed. Then condensation process was conducted under Ar atmosphere at 95 °C for 3 h, as depicted in Fig. 14(a). The weight ratio of Si nanoparticles, supper P carbon and binder kept 70: 15: 15 in the electrode preparation. The Si-based electrode using the polyimine polymer as binders exhibited good cycles and rate performance, as shown in Fig. 14(b-c). The Si anode displayed the first discharge capacity of 3280.2 mAhg─1 at C/10 with capacity retention of 85.6% after 100 cycles. In contrast, the Si electrodes using PAA, CMC and PVDF binders delivered capacity retention of only 45.2%, 52.1% and 8.0% after 100 cycles, respectively. Moreover, the Si electrode using the polyimine binder also showed excellent long-term stability after 1000 cycles and presented 800 mAhg-1 at 1 C and capacity retention of 82.4%, as shown in Fig. 14(d). The ultra-efficiency of polyimine binder was thought to be related to the abundant imine groups in the repeating units which can provide efficient interaction with Si surface.
聚偏二氟乙烯(PVDF)通常用作锂离子电池中的电极粘合剂[135]。不幸的是,PVDF的范德华力较弱,无法应对Si颗粒的巨大体积变化,也无法保持电极结构的完整性[136],[137]。此外,PVDF粘结剂仅溶于N-甲基吡咯烷酮(NMP)溶剂,不仅价格昂贵,而且对环境有害。因此,水溶性粘结剂正逐渐成为环保的主流粘结剂。例如,聚丙烯酸(PAA)、羧甲基纤维素(CMC)、甲基丙烯酸1-芘甲基酯、聚乙二醇、海藻酸钠、β-环糊精、聚酰亚胺、聚亚胺聚苯胺和交联聚合物被广泛研究[138]、[139]、[140]、[141]、[142]、[143]、[144]。这些粘结剂含有丰富的极性官能团(-OH、-COOH、-NH 2 ),它们与硅基材料表面具有很强的相互作用力,形成保护层,帮助硅电极在循环过程中形成稳定的SEI膜。Gao等人报道了通过一步缩合反应将聚酰亚胺聚合物粘合剂用于硅基阳极[145]。制备工艺如下:首先将4,4-联苯二甲醛(BCA)和1,5-萘二胺(NDA)混合。然后在95 ° °C的氩气气氛下进行冷凝过程3 h,如图14(a)所示。在电极制备中,Si纳米颗粒、硫磷碳和粘结剂的重量比保持在70:15:15。使用聚亚胺聚合物作为粘结剂的硅基电极表现出良好的循环和倍率性能,如图14(b-c)所示。硅阳极的首次放电容量为3280。C/10 浓度为 2 mAhg ─1 ,循环 100 次后容量保持率为 85.6%。相比之下,使用PAA、CMC和PVDF粘结剂的硅电极在100次循环后分别仅提供45.2%、52.1%和8.0%的容量保持率。此外,使用聚亚胺粘合剂的硅电极在1000次循环后也表现出优异的长期稳定性,在1 C时呈现出800 mAhg -1 和82.4%的容量保持率,如图14(d)所示。聚亚胺结合剂的超高效被认为与重复单元中丰富的亚胺基团有关,这些亚胺基团可以与Si表面提供有效的相互作用。

Fig. 14
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Fig. 14. (a) Schematic illustration of the electrode fabrication process. (b) Cycling performance of the 70Si/ (Binder + SP) (Binder ¼ PAA, CMC and PVDF. With polyimine as binder, there is 30 wt% of polyimine in the electrode without any SP) electrodes. (c) Rate capability of the 70Si/ (Binder + SP) electrodes under different C rates. (d) Long-term cycling performance of the 70Si/Polyimine electrode at the current density of 1 C [145].
图 14.(a) 电极制造过程示意图。(b) 70Si/(粘结剂+SP)(粘结剂1/4 PAA、CMC和PVDF.以聚亚胺为粘合剂,电极中含有30wt%的聚亚胺,没有任何SP)电极。(c) 70Si/(粘结剂+SP)电极在不同C倍率下的倍率能力。(d)70Si/聚亚胺电极在1 C电流密度下的长期循环性能[145]。

Composite binders composed of two or more selected binders show better adhesion force and higher stretchability such as the composite binders of CMC/PDA (Polydopamine) [146], PAA/EVA(Ethylene vinyl acetate) [147] and PAA/PVA [148] etc. It was reported that the CMC/PDA composite binder exhibited the excellent bonding strength through hydrogen bonding, having the adhesion force of 10.8 N and high stretchability of 128.7% [146]. The high stretchability is beneficial for buffering changes in the volume of Si particles, while the better adhesion force improves the adhesion between the active Si material and the metal collector. Excellent mechanical performance helps maintain the integrity and porous structure of the electrode, thereby stabilizing the battery capacity during cycling.
由两种或两种以上选定的粘结剂组成的复合粘结剂具有更好的粘接力和更高的拉伸性,例如CMC/PDA(聚多巴胺)[146]、PAA/EVA(乙烯醋酸乙烯酯)[147]和PAA/PVA[148]等复合粘结剂。据报道,CMC/PDA复合粘结剂通过氢键表现出优异的粘接强度,粘附力为10.8 N,拉伸性为128.7%[146]。高拉伸性有利于缓冲Si颗粒体积的变化,而更好的粘附力提高了活性Si材料与金属集电极之间的粘附力。优异的机械性能有助于保持电极的完整性和多孔结构,从而在循环过程中稳定电池容量。

Choi et al. demonstrated that incorporating 5 wt% polyrotaxane (PR) into the polyacrylic acid (PAA) binder significantly enhanced the elasticity of the polymer network [149]. The improvement of elasticity was related to the ring sliding motion of polyrotaxane. Besides these, modified binder had a unique structure, which enabled Si-based anodes to have more stable cycling performance. Recently, an innovative concept of an “adaptive binder” tackled the challenge of the significant volume expansion of Si anodes. The covalent gallol-to-gallol (1,2,3-trihydroxybenzene) is the kind of self-crosslinking gallol-binder and provides stable microenvironment around Si particles, improving the capacity retention and structural stability of the Si-based electrode [150]. Another important kind of binders is the electronic conductive polymer binders, which aims to improve the conductivity of Si materials and reduce the use of conductive agents. The conductive binders typically contain a highly π-conjugated structures and exhibit the characteristics of tunable metal-like electronic conductivity [151], [152]. The conductive binders reduce the resistance, improve the initial Coulombic efficiency (ICE) and stabilize the Si-anode interface [48]. It was reported that the ion and electron conductive binder could enhance the Li+ diffusivity and electron conductivity, which were 14 and 90 times higher than the CMC binder, respectively [153]. Ye et al. reported a novel n-type conductive binder and used it in the Si anodes of LIBs [154]. By using condensation and reversible addition-fragmentation chain transfer polymerization methods, polybutadiene (PB) rubber was incorporated into the designed multi block polymers of polymethyl methacrylate and polybutadiene (PPy-b-PB), as shown in Fig. 15 (a-b). This method allowed for the condensation reaction between two carboxyl groups and the two terminal hydroxyl groups of HTPB. By adding PB into the binder, the deformation ability, flexibility, and adhesion performance of PPy-b-PB/Si anode were improved. The PPy-b-PB/Si anode exhibited excellent reversible capacity of 2274 mAhg-1 and capacity retention with 87.1% at 0.2 C after 200 cycles, as shown in Fig. 15 (c). The Coulombic efficiency of the two electrodes displayed in Fig. 15 (d).
Choi等人证明,在聚丙烯酸(PAA)粘合剂中加入5重量%的聚轮烷(PR)可显著增强聚合物网络的弹性[149]。弹性的提高与聚轮烷的环滑动运动有关。除此之外,改性粘结剂具有独特的结构,使硅基阳极具有更稳定的循环性能。最近,一种“自适应粘结剂”的创新概念解决了硅阳极体积显著膨胀的挑战。共价没食子-没食子(1,2,3-三羟基苯)是一种自交联的没食子粘合剂,在硅颗粒周围提供稳定的微环境,提高了硅基电极的容量保持和结构稳定性[150]。另一种重要的粘结剂是电子导电聚合物粘结剂,旨在提高Si材料的导电性,减少导电剂的使用。导电结合剂通常含有高度π共轭的结构,并表现出可调谐的类金属电子电导率的特性[151],[152]。导电粘结剂降低了电阻,提高了初始库仑效率(ICE)并稳定了硅阳极界面[48]。据报道,离子和电子导电结合剂可以提高Li + 扩散率和电子电导率,分别比CMC结合剂高14倍和90倍[153]。Ye等人报道了一种新型n型导电粘合剂,并将其用于锂离子电池的硅阳极[154]。采用缩合和可逆加成-碎裂链转移聚合方法,将聚丁二烯(PB)橡胶掺入聚甲基丙烯酸甲酯和聚丁二烯(PPy-b-PB)设计的多嵌段聚合物中,如图1所示。 第15段(a-b)。该方法允许HTPB的两个羧基和两个末端羟基之间的缩合反应。通过在粘结剂中加入PB,提高了PPy-b-PB/Si阳极的变形能力、柔韧性和粘附性能。PPy-b-PB/Si阳极在200次循环后表现出优异的可逆容量(2274 mAhg -1 )和87.1%的容量保持率(0.2 C),如图15(c)所示。图15(d)所示的两个电极的库仑效率。

Fig. 15
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Fig. 15. (a) Schematic illustration of the Si electrodes employing PPy (A) or PPy-b-PB. (b) as conductive binders during delithiation/lithiation process. (c) The discharge specific capacity. (d) Coulombic efficiency of the corresponding cycles [154].
图 15.(a) 采用PPy(A)或PPy-b-PB的硅电极示意图。(b) 在脱锂/锂化过程中作为导电粘合剂。(c) 排放比容量。(d)相应循环的库仑效率[154]。

Self-healing polymer (SHP) binders have rich hydrogen bonds, appropriate viscoelasticity, and stretchability, making them a hot topic in the recent years to improve the electrochemical performance of Si-based anodes and reduce the adverse effects of repeated volume changes [155], [156]. It was reported that the ureidopyrimidinone functionalized polyethylene glycol (UPy-PEG-UPy) self-healing binder had excellent self-healing function [157], [158]. The results showed that the first Coulombic efficiency of the Si-based electrode using the UPy-PEG-UPy self-healing binder was 81%, and the specific capacity maintained 1454 mAhg─1 after 400 cycles with a single cycle capacity decay of 0.04%. Compared to PAA binders, the UPy-PEG-Upy self-healing binder can maintain the structural integrity of the Si-based electrode and significantly improve electrochemical performance. Due to its ability to effectively solve material cracks and structural damage caused by volume changes in Si electrodes, self-healing binders significantly improve the cycling stability and rate performance.
自修复聚合物(SHP)粘结剂具有丰富的氢键、适当的粘弹性和拉伸性,使其成为近年来提高硅基阳极电化学性能和减少反复体积变化的不利影响的热点[155],[156]。据报道,脲嘧啶酮官能化聚乙二醇(UPy-PEG-UPy)自愈结合剂具有优异的自愈功能[157],[158]。结果表明,采用UPy-PEG-UPy自愈粘结剂的Si基电极的首次库仑效率为81%,循环400次后比容量保持为1454 mAhg ─1 ,单循环容量衰减为0.04%。与PAA粘结剂相比,UPy-PEG-Upy自修复粘结剂可以保持硅基电极的结构完整性,并显著提高电化学性能。自修复粘结剂能够有效解决硅电极体积变化引起的材料裂纹和结构损伤,显著提高了循环稳定性和倍率性能。

6. Electrolytes and additives
6. 电解质和添加剂

The electrolyte used in the LIBs consists of lithium salt, solvents and additives, and plays a significant role on overall electrochemical performance. Due to the large volume expansion of Si-based anodes, the SEI film formed on surface undergoes repeated cracking and re-construction during the lithiation/delithiation processes, leading to increase of its thickness. The thick SEI layer raises the interface impedance and also has negative impact on performance. A compact and stable SEI film is beneficial for maintaining the cycling stability of Si-based electrodes. Due to the significant impact of electrolyte composition and additives on the structure and performance of SEI films, extensive research has been conducted on various electrolyte solvents and functional additives to stabilize and enhance the passivation effect of SEI [159], [160], [161]. Among these, carbonate electrolytes containing flouroethylene carbonate (FEC) and vinyl carbonate (VC) additives have been shown to have a positive impact on cycling performance for Si-based anodes [156], [162]. Li et al. recently found a new role of the FEC additive to stabilize the SEI film [51]. In their work, they believed that the FEC played a more important role in suppressing Li-trapping within Si particles than stabilizing SEI films, which is different from traditional understanding. In electrolytes without FEC, the growth and rearrangement of SEI accounted for 47.8% of the capacity loss, while the remaining 52.2% was related to Li capture in Si particles. However, in the presence of 10% FEC, the capacity loss associated with SEI growth was reduced by 52.9%, while Li capture within Si particles was reduced by 82.3%. The schematic representation of the FEC role for suppressing Li-trapping in Si particles is presented in Fig. 16(a). The cycle performance of the Si electrode with and without FEC at constant charge/discharge current (CC) and constant voltage (CV) are shown in Fig. 16(b-c). Microscopic studies revealed that the formed LiF was not only distributed on the surface of Si particles, but also formed a large number of LiF-doped phases inside. The LiF-doping Si was beneficial for the formation of amorphous phase Li5Si4, and prevented the formation of crystalline phase Li5Si4. The amorphous phase Li5Si4 had good electrochemical activity and small volume expansion, which was a key mechanism to improve the performance of Si-based electrodes. Fig. 16(d-e) represents the TEM images of the SEI layer formation on Si particles. This new insight disclosed a hidden effect of FEC additive on the Si anode in the LIBs.
锂离子电池中使用的电解质由锂盐、溶剂和添加剂组成,对整体电化学性能起着重要作用。由于硅基阳极的体积膨胀较大,在锂化/脱锂过程中,在表面形成的SEI膜会经历反复开裂和重建,导致其厚度增加。厚的SEI层提高了接口阻抗,也对性能产生了负面影响。致密稳定的SEI薄膜有利于保持硅基电极的循环稳定性。由于电解质组成和添加剂对SEI薄膜的结构和性能有显著影响,因此对各种电解质溶剂和功能添加剂进行了广泛的研究,以稳定和增强SEI的钝化效果[159],[160],[161]。其中,含有碳酸氟乙烯酯(FEC)和碳酸乙烯酯(VC)添加剂的碳酸酯电解质已被证明对硅基阳极的循环性能有积极影响[156],[162]。Li等人最近发现了FEC添加剂在稳定SEI薄膜方面的新作用[51]。在他们的工作中,他们认为FEC在抑制Si颗粒内的Li捕获方面比稳定SEI薄膜起着更重要的作用,这与传统的理解不同。在不含FEC的电解液中,SEI的生长和重排占容量损失的47.8%,其余52.2%与硅颗粒中的Li捕获有关。然而,在10%FEC存在下,与SEI增长相关的容量损失减少了52.9%,而Si颗粒中的Li捕获减少了82.3%。 FEC在抑制硅颗粒中Li捕获的作用示意图如图16(a)所示。带和不带FEC的硅电极在恒定充放电电流(CC)和恒定电压(CV)下的循环性能如图16(b-c)所示。显微研究表明,形成的LiF不仅分布在Si颗粒表面,而且在内部形成了大量的LiF掺杂相。LiF掺杂Si有利于非晶相Li 5 Si的形成 4 ,阻止了结晶相Li 5 Si 4 的形成。非晶相Li 5 Si 4 具有良好的电化学活性和较小的体积膨胀,这是提高Si基电极性能的关键机制。图16(d-e)表示Si颗粒上SEI层形成的TEM图像。这一新发现揭示了FEC添加剂对锂离子电池中硅阳极的隐藏影响。

Fig. 16
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Fig. 16. (a) Schematic representation of FEC additive for suppressing Li-trapping with in Si particles. (b) Cycle performance of Si electrode at constant current CC. (c) Cycle performance at constant current and at constant voltage CCCV. (d) TEM image of Si electrode without FEC additive electrolyte. (e) TEM image of the Si electrode with FEC additive electrolyte [51].
图 16.(a) 用于抑制硅颗粒中锂俘获的FEC添加剂示意图。(b) 硅电极在恒流CC下的循环性能。 (c)恒流和恒压CCCV下的循环性能。(d)不含FEC添加剂电解液的硅电极的TEM图。(e)含FEC添加剂电解质的硅电极的TEM图[51]。

The FEC is an indispensable additive to improve the cycle stability of Si-based electrodes. But its incompatibility with LiPF6 electrolyte seriously damages the high temperature performance of LIBs. In order to increase the compatibility between LiPF6 and FEC at elevated temperatures, mono-component additives of lithium difluorophosphate (LiPO2F2), and bicomponent additives of LiPO2F2 with N, N-dimethyltrifluoroacetamide (DMTFA) were investigated [163], [164]. The introduction of LiPO2F2 additive effectively improved the stability of LiPF6 electrolyte, inhibited the side reactions and decreased the self-discharge of the LIBs at high temperatures. The discharge capacity of LIBs with the LiPO2F2 additive at temperature of 45 oC maintained 90.3% for 200 cycles and the swelling ratio was only 1.96% after storage at 55 oC for 7 days [163]. It was also reported that the additive of mesylethyl-methyl-pyrrolidinium (MEMP-DFOB) was beneficial for generating a film on the surface of SiO-graphite anodes to improve performance of LIBs at low and high temperatures [165]. Other additives such as dimethylacrylamide (DMAA), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB) were also investigated [166], [167], [168], and results indicated that the LiPF6-LiFSI-LiTFSI ternary composite lithium salt with a mole ratio of 7:1:2 exhibited the best cyclic stability and rate capability.
FEC是提高硅基电极循环稳定性不可或缺的添加剂。但其与 6 LiPF电解质的不相容性严重损害了锂离子电池的高温性能。为了提高LiPF 6 和FEC在高温下的相容性,研究了二氟磷酸锂(LiPO 2 F 2 )的单组分添加剂和LiPO 2 F 2 与N,N-二甲基三氟乙酰胺(DMTFA)的双组分添加剂[163],[164]。LiPO 2 F 2 添加剂的引入有效提高了 6 LiPF电解液的稳定性,抑制了锂离子电池在高温下的副反应,降低了锂离子电池的自放电。在45 o C温度下,LiPO 2 F 2 添加剂的锂离子电池在200次循环中保持90.3%的放电容量,在55 o C下储存7 d后溶胀率仅为1.96%[163]。另据报道,甲基乙基甲基吡咯烷(MEMP-DFOB)的添加剂有利于在SiO-石墨阳极表面生成薄膜,以提高锂离子电池在低温和高温下的性能[165]。还研究了二甲基丙烯酰胺(DMAA)、双(氟磺酰基)亚胺锂(LiFSI)、双(三氟甲磺酰基)亚胺锂(LiTFSI)、双(草酸)硼酸锂(LiBOB)、二氟(草酸)硼酸锂(LiDFOB)[166]、[167]、[168],结果表明,摩尔比为7:1:2的 6 LiPF-LiFSI-LiTFSI三元复合锂盐表现出最佳的循环稳定性和倍率能力。

Research found that a specific electrolyte composition, containing a 2.0 M LiPF6 solution in a 1:1 vol ratio mixture of tetrahydrofuran (THF) and 2-methyltetrahydrofuran (MTHF) called mixture THF, facilitated to form thin, uniform and flexible LiF-based SEI [169]. This electrolyte enabled the Si micro-particles to deliver a high specific capacity of 2800 mAhg─1 over 200 cycles. Moreover, it exhibited a high initial Coulombic efficiency (ICE) of > 90% and a CE > 99.9% during cycling tests. In order to improve the safety precautions, there were also other investigations using concentrated electrolytes based on lithium bis (fluorosulfonyl) imide (LiFSI), which are composed of nonflammable solvents such as di-2,2,2-trifluoroethyl carbonate and fluoroethylene carbonate. The concentrated LiFSI-based electrolyte demonstrated the improvement of cycling stability, delivering a high initial reversible capacity of 2644 mAhg─1 and exhibiting a slow capacity fading trend compared to a Si nanoparticle anode.
研究发现,一种特定的电解质组合物,含有2.0 M LiPF 6 溶液,溶液比例为1:1,为四氢呋喃(THF)和2-甲基四氢呋喃(MTHF)的混合物,称为THF,有助于形成薄、均匀和柔韧的LiF基SEI[169]。这种电解质使硅微粒能够在 200 次循环中提供 2800 mAhg ─1 的高比容量。此外,在循环测试中,它表现出> 90% 的高初始库仑效率 (ICE) 和 99.9% 的 CE >。为了提高安全防范措施,还研究了使用基于双(氟磺酰基)酰亚胺锂(LiFSI)的浓缩电解质,该电解质由不易燃溶剂(如二-2,2,2-三氟乙基碳酸酯和氟碳酸乙烯酯)组成。与硅纳米颗粒阳极相比,基于LiFSI的浓缩电解质表现出循环稳定性的改善,提供了2644 mAhg ─1 的高初始可逆容量,并表现出缓慢的容量衰减趋势。

In the recent years, ionic liquid electrolyte also attracts considerable attention due to its favorable properties such as excellent chemical and thermal stability, low vapor pressure, wide electrochemical window, and nonflammability. A few achievements on ionic liquids have been reported for applications in the Si-based anodes. The fluorine-substitute ionic liquid containing 0.1 M M(TFSI)x (where M = Al, Mg, Zn, or Ca) as a secondary salt in the electrolyte could significantly enhance the stability of anodes. During the charging process, the formation of Li-M-Si ternary phases altered the Li-Si binary behavior, and had less negative impact on the electrochemical characteristics and theoretical capacity of the Si anodes [170]. The performance of mixed ionic liquid electrolytes was superior to those based on carbonates because it helped to form a uniform SEI film, thereby improving the cycling stability. In addition, the organic gel electrolyte was not only used as an ionic conductor, but also provided cohesion between Si particles, thus maintaining the integrity of the Si electrode. This characteristics helped to suppress crack propagation and SEI growth. Therefore, when oleoresin organic alcohol electrolytes were used, the Si anode exhibited the significantly enhanced capacity retention during cycling processes compared to the common liquid electrolytes.
近年来,离子液体电解质因其优异的化学和热稳定性、低蒸气压、宽电化学窗口和不燃性等良好性能而备受关注。据报道,在硅基阳极中应用离子液体方面取得了一些成就。电解质中含有0.1 M M(TFSI) x (其中M = Al、Mg、Zn或Ca)作为仲盐的氟替代离子液体可以显著提高阳极的稳定性。在充电过程中,Li-M-Si三元相的形成改变了Li-Si二元行为,对Si阳极的电化学特性和理论容量的负面影响较小[170]。混合离子液体电解质的性能优于碳酸盐,因为它有助于形成均匀的SEI膜,从而提高循环稳定性。此外,有机凝胶电解质不仅用作离子导体,而且还提供了Si颗粒之间的内聚力,从而保持了Si电极的完整性。这一特性有助于抑制裂纹扩展和SEI扩展。因此,当使用油树脂有机醇电解质时,与普通液体电解质相比,Si阳极在循环过程中表现出显着增强的容量保持性。

7. Conclusion and future prospective
7. 结论和未来展望

With the rapid development of EVs and electronic portable devices, people have put forward higher requirements for LIBs, promoting the in-depth research and development of Si/C electrodes towards a high energy density and stability. In order to improve the performance of Si-based electrodes, many efforts have been made through development of various technologies, and progress has been achieved in alleviating problems related to Si materials, such as poor electronic conductivity, large volume expansion, slow Li+ diffusion kinetics, electrolyte consumption, reconstruction of the SEI layer and so on.
随着电动汽车和电子便携式设备的快速发展,人们对锂离子电池提出了更高的要求,推动了Si/C电极向高能量密度和稳定性的深入研究和开发。为了提高硅基电极的性能,通过各种技术的开发做出了许多努力,在缓解与硅材料相关的问题方面取得了进展,如电子导电性差、体积膨胀大、锂 + 扩散动力学慢、电解液消耗、SEI层重建等。

Due to having sufficient space and unique characteristics, 3D porous Si composites allow for volume expansion and maintain electrode integrity during cycling. The 3D porous configuration is beneficial for electrolyte storage and creates pathways for charge transfer. The volume expansion of Si-based materials can also be alleviated by combining two or more types of binders to form a composite binder. The synergistic effect of each component of composite binders can effectively inhibit the movement of nano Si particles, prevent the huge volume changes of Si materials and avoid damage of conductive network during charging-discharging processes, thereby stabilizing the electrode and improving the cycling stability. In addition, conductive binders and self-healable polymer binders can reduce the content of conductive agents and repair electrode cracks caused by volume changes of Si. The review discussed various electrolyte additives, and their impact on electrochemical performance of Si-based electrodes. It has been proven that the FEC, VC, LiFSI, LiTFSI and LiDFOB etc. additives have a significant positive effect on the cycling performance of Si-based electrodes.

Although significant efforts have been made to tackle the major challenges of Si-based anodes in the past decades, there are still challenges to achieve commercialization. The prospects of future R & D on Si materials are: 1) Design and prepare porous nano Si/C/G multi-component composites to alleviate the volume expansion of Si; 2) Optimize more compatible electrolyte additives and binders for Si-based electrodes; 3) Improve the active material loading and rate capability for the large-scale application of Si-based electrode materials in the LIBs.

CRediT authorship contribution statement
CRediT 作者贡献声明

Tong Luyou: Funding acquisition, Resources. Jiang Zhan-Guo: Project administration, Supervision. Saddique Jaffer: Writing – original draft, Conceptualization. Zheng Hao: Funding acquisition, Resources. Wu Mengjing: Investigation, Data curation. Wajid Ali: Methodology, Formal analysis. Xu Xiaoxue: Visualization, Validation. Hu Weikang: Writing – review & editing, Supervision.
佟鲁友:资金收购,资源。江占国:项目管理、监督。萨迪克·贾弗(Saddique Jaffer):写作 - 原始草稿,概念化。郑浩:资金获取,资源。吴梦晶:调查,数据整理。瓦吉德·阿里:方法论,形式分析。徐晓雪:可视化,验证。胡 Weikang:写作 - 审查和编辑,监督。

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments 确认

The key project on R & D of silicon carbon negative electrode materials for high specific energy Li ion batteries (2021-1-064) by the science and technology plan of Jinhua, Zhejiang, China and the authors gratefully acknowledge financial supports from the post-doctoral research grant (ZC304023914) of Zhejiang Normal University, Jinhua, Chian.

Data availability 数据可用性

No data was used for the research described in the article.

References 引用


Authors contributed equally to this work.







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