Research Progress of Silicon Anode Materials for High Performance Lithium-ion Batteries
INTRODUCTION
Recent demand for electric and hybrid cars, coupled with lower prices, has led to lithium- ion batteries
(LIB) has become an increasingly popular rechargeable battery technology. The market for lithium-ion batteries is also growing. However, graphite (Product No:P196402), the traditional anode material for lithium-ion batteries, has a limited theoretical specific capacity of ~370 mAhg−1[2], cannot meet the high energy demand of the advanced electric and hybrid electric vehicle market. In the past decade, a large number of anode materials with enhanced storage capacity, high energy density and improved cycle characteristics have emerged in the direction of lithium- ion batteries[3-7]. Table 1 summarizes the properties of several different anode materials. Among these advanced anode materials, Si has attracted a lot of attention as an alternative to lithium- ion batteries, mainly for the following reasons:
1) Its specific capacity is 4200 mAhg−1, volume capacity is 9786 mAhcm−3, is the highest known LIB negative;
2) the operating potential is relatively low (0.5V vs. Li/Li+);
3) the natural abundance of Si element and its environmental goodness[8-10]
Table 1 Comparison of various anode materials
However, the practical application of silicon anode is still hampered by three major problems. First, the cycle life of silicon materials is poor. This is due to comminution during the huge volume fluctuations (>300%) that accompany lithium-ion intercalation and delamination. Secondly, mechanical fracture of the silicon negative during alloying/dealloying leads to dramatic irreversible capacity loss and low coulomb efficiency. Finally, the solid electrolyte interphase (SEI) breaks as the nanostructure shrinks during the removal process. This causes the fresh silicon surface to be exposed to the electrolyte, and the SEI reorganizes, causing the SEI to thicken with each charge-discharge cycle, as shown in Figure 1[11,12]
Nano Silicon Anode Material
To solve these problems, several strategies have been developed to accommodate large volume changes. One effective strategy is to reduce the active particle size to the nanoscale, as nanoscale particles can adapt to large stresses without cracking, while also reducing the distance between electrons and ions. In addition, the high density of grain boundaries in nanomaterials also provides rapid diffusion paths for lithium ions and acts as additional lithium storage sites [13-16]. Huang et al. demonstrated the effect of Si nanoparticle size on structural stress release using in situ transmission electron microscopy (TEM) and proposed that if particle diameter <150 nm, the stored strain energy in the electrochemical reaction is insufficient to drive crack growth in Si nanoparticle (FIG. 2) [17]. Recently, Kim et al reported that Si nanoparticles in 5, 10, and 20 nm sizes can be synthesized at 380°C under high pressure by using a variety of surface-active agent [18]. Cycling these materials between 0 and 1.5 V at a rate of 0.2 C achieves more than 40 charge-discharge cycles with capacity retention rates of 71,81 and 67 percent, respectively.
Figure 1 Schematic diagram of the stability of silicon during cycling as it is affected by diameter size
Kim et al. also reported a 3D silicon structure with a highly interconnected porous structure [19]. This Si structure, with a 40 nm thick pore wall, can withstand large stresses without deformation even after 100 cycles, and can be obtained with 1 C (2000 mAg-1) at a rate that preserves a charge capacity greater than 2800 mAhg-1. Despite advantages of nanostructured Si negative, nanoparticles also have disadvantages such as large surface area, high manufacturing cost, and difficult handling[22]. Even so, nanoscale silicon materials are considered one of the most promising ways to overcome the silicon negative challenges of next-generation lithium- ion batteries.
Silicon-Based Carbon Composite Anode Materials
Another way to overcome volume changes during cycling is to form a composite material[23]. If the matrix does not experience significant volume changes, this may buffer the expansion of the silicon, preserve the structural integrity of the electrode, and enhance stability by reducing silicon aggregation or electrochemical sintering.
Silicon-based carbon composites are a promising area of research, with the advantages of improving electrical conductivity and the expansion buffering effect of the carbon matrix. In addition, carbon additives have excellent ion conductivity and lithium-ion storage capacity [28,29]. However, the conformal carbon coating on the Si active material can break during the cycle, resulting in the Si being exposed to the electrolyte and additional deposition of SEI. Therefore, a form of carbon coating that can accommodate large volume fluctuations in Si is necessary.
Wang et al. successfully prepared a silicon-based composite negative catalyst by in-situ catalytic growth of graphene surface nano-silicon (Si@Graphene) [30]. As shown in Figure 2, the material exhibits excellent cyclic stability and rate capability, maintaining A reversible discharge capacity of up to 1909 mAhg-1 after 100 cycles at 0.2 Ag-1. Even a high current of 52 Ag-1 provides a discharge capacity of 975 mAhg-1.
Figure 2 Preparation process diagram and performance test diagram of Si@Graphene
Liu et al. reported a model with excellent capacity (2833 mAhg-1 at C/10), cycle life (1000 cycles, capacity maintained 74%), and Coulombic efficiency (99.84%) Si@C (FIG. 3A)[31]. Si nanoparticles are SiO2 first The layers are then coated with polydopamine layers, which are subsequently carbonized to form a nitrogen-doped carbon coating that is treated with hydrofluoric acid (HF) to selectively remove SiO2After the layer, the Si@Void@C "yolk-shell" structure is obtained. Recently, Lie al. reported hollow core-shell porous Si-C nanocomposites with a reversible capacity of 650 mAhg-1 after 100 cycles (The current density is 1Ag-1), corresponding to 86% capacity retention [32]. The advantages of these unique structures boil down to two things:
1) The interstitial space between the Si core and the carbon shell allows the Si nanoparticles to expand without damaging the shell when lithium is applied;
2) The electrical and ionic conductivity of the carbon shell prevents the electrolyte from reaching the Si surface.
Another strategy is to produce porous Si@C composites. Magasinski et al reported high capacity (reversible capacity: 1950 mAhg-1Si@C porous composite with long cycle life[33]. The porous Si@C structure was prepared using a layered bottom-up assembly method, in which the irregular channels ensure the rapid entry of lithium-ions into the particle body, while the porosity inside the particle can adapt to the large changes in Si volume during the cycle.
Graphene (Product No:G139798,G139805,G139804) has a high surface area (2600m) due to its superior electrical conductivity 2g-1), excellent chemical stability and strong mechanical strength, is also used in Si negative electrodes to buffer volume changes and improve electronic conductivity[34-38].Recently, Wen et al reported that treating Si (project number:S108980) with aminopropyl trime thoxylsilane (APS) and replacing carboxymethyl cellulose (CMC) with sodium alginate improved the electro chemic a l performance of graphene-encapsulated Si anodes (Product No:C294622,C104978). Both methods can improve the interaction between graphene-bonded and encapsulated Si groups and the collector. These graphene-encapsulated functionalized silicon nanoparticles have a capacity of 2250 mAhg-1 at 0.1°C, with a capacity of 1,000 mAhg-1 at 10℃, maintains 85% of its initial capacity even after 120 cycles.
Zhao et al. reported silicon nanoparticles embedded in a 3D graphene scaffold (Figure 3B) and exhibited about 3200 mAg-1(Current density :1Ag-1) of a reversible capacity that retains 83% of its theoretical capacity after 150 cycles [39]. In this case, the 3D conductive graphene scaffold was constructed using a simple wet chemical method from stripped graphene oxide. The ability to maintain a high capacity in this negative material is attributed to the excellent cross-plane ion diffusion rate, which shortens the diffusion path of lithium ions throughout the electrode, allowing full access to the interior and rapid lithium in the Si nanoparticles. Xin et al. also reported the synthesis of Si/ graphene nanocomposites with 3D porous structures through a series of chemical processes [40]. This architecture offers 900 mAhg-1 Reversible capacity, even at 1Ag-1At the charging rate of 1 a g, there is almost no fading after 30 cycles. Because the 3D graphene network enhances the electrical conductivity of the electrodes, 3D graphene - based composites show superior cyclic stability and high magnification performance, showing better magnification characteristics than 2D nanostructures.
Figure. 3 A) Schematic of a single Si@Void@C particle (top) and in situ TEM images of the synthetic Si@Void@C before and after lithium (bottom); B) Cross-sectional diagram of composite electrode material constructed by in-plane carbon-vacancy defect graphene scaffold (above); (Si: large particles; Li ions: small spheres) and an SEM image of a cross-section of a Si-3D graphene scaffold (below), inset showing Si nanoparticles uniformly embedded between graphene sheets [39].
The Direction of the Future
Recently, Wu et al. reported the ideal 3D porous silicon/conductive polymer hydrogel composite electrode with a relatively stable reversible capacity (1,600 mAhg-1 after 1000 deep cycles) and very stable performance (no significant capacity attenuation after 5000 cycles) [2]. Porous layered hydrogel frames have significant advantages: conductive polymer 3D networks provide fast electron and ion transfer channels, in addition to providing porous Spaces for Si particle volume expansion. This preparation method of in-situ polymerization shows promise for scalability and industrial commercialization, as shown in Figure 4.
Figure 4 Schematic diagram of A 3D porous silicon nanoparticle/conductive polymer hydrogel composite electrode: (A) where each silicon nanoparticle is encapsulated in a conductive polymer surface coating and further attached to a highly porous hydrogel frame, photo (B-D) shows the key steps in the electrode manufacturing process.
Summarization and Challenges
Silicon is one of the most promising anode materials for lithium-ion batteries, with advantages including the highest known capacity and relatively low operating potential. However, the problem of volume expansion must be overcome before silico n anode can be applied to actual lithium batteries. This paper illustrates various silico n anodes and silicon-based composite negative electrodes that improve electro chemical performance, demonstrating two feasible solutions to bypass the silicon negative electrode. Further research is needed to address the practical requirements of Si anode, including aspects such as high power density, long life, simple manufacturing and low cost.
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