Dynamics: Research progress in silicon-based anode materials

Silicon-based anode materials are regarded as one of the alternative products of the existing commercial carbon anode materials. However, due to the large volume effect during charging and discharging, it is not commercialized. For this reason, researchers have carried out a large number of modification studies. . Based on theoretical research and experimental research, the research progress of silicon-based anode materials is summarized, and it is hoped that the research on new alloy anode materials can be promoted.

In recent years, the rapid development of new energy power generation has put forward new requirements for the matching energy storage system. In the upgrading of energy storage batteries, lithium-ion batteries have become a key research field due to their various advantages, and have achieved practical application in a large number of energy storage projects, and achieved certain results.

The capacity of the lithium-ion battery is determined by the active lithium ion of the positive electrode material and the lithium-encapsulating ability of the negative electrode material. The stability of the positive and negative electrodes in various environments determines the performance of the battery and even seriously affects the safety of the battery. The performance of the electrode determines the overall performance of the lithium ion battery to some extent.

However, the commercial anode material of lithium ion battery is mainly graphite carbon anode material, and its theoretical specific capacity is only 372 mAh/g (LiC6), which seriously limits the further development of lithium ion battery, the silicon-based material is the research system with the highest theoretical specific capacity in the anode material. The alloy formed is LixSi (x=0~4.4), and the theoretical specific capacity is up to 4200mAh/g due to its low embedded lithium potential and low atomic mass. High energy density and high Li mole fraction in Li-Si alloys are considered as alternatives to carbon anode materials. However, the silicon negative electrode has a severe volume expansion and contraction during the lithium-intercalation cycle, causing damage to the material structure and mechanical pulverization, resulting in poor cycle performance of the electrode.

In recent years, researchers have made a lot of research on the modification of silicon-based anode materials, and made some progress. Based on theoretical research and experimental research, this paper summarizes the research methods and research methods of silicon-based anode materials at home and abroad, and hopes to promote the research of new alloy anode materials.

1. Theoretical study

At present, when researchers choose a research system, they mainly try to select some systems based on existing relevant experience, which takes a long time, wastes resources and is inefficient. Due to the number of candidate systems and the uncertainty of the synthesis process, the new alloy-based anode materials based on experimental research are progressing slowly.

In recent years, through the theoretical simulation method, the structure and performance of the material are predicted, so as to optimize the research object and develop new materials in a targeted manner. This research method combining theoretical research with experimental research has been more and more the attention of researchers.

At present, the theoretical research on silicon-based anode materials is mainly based on the simulation study of density functional theory. The software used is mainly MaterialsStudio developed by Accelrys, USA. It contains a large number of program modules, which are used for simulation of alloy materials. CASTEP module, CASTEP module is an advanced program module based on solid state physics and quantum physics development in MaterialsStudio. Its theoretical basis is charge density functional theory (DFT), and local charge density approximation (LDA) or generalized gradient can be selected approximation (GGA).

According to ICSD2009 (ICSD#29287), the lattice structure of silicon belongs to cubic crystal system, the space group type is Fd-3ms, and the space group number is 227. The lattice structure diagram is shown in Figure 1, where the lattice constant a=b= c = 0.543071 nm, the angle of the edge a = b = g = 90 °. During the charging process, lithium ions from the positive electrode material are embedded in the interstitial position of the host material under the action of the electrolyte, and the reaction equation is as shown in equation (1):

Hou et al. studied the mechanism of Li-Si alloy as the anode material for lithium ion battery based on the first-principles plane wave pseudopotential method. Studies have shown that the first irreversible capacity loss of Si is derived from the formation of SEI film and the lithium-depleted phase Li12Si7 which is difficult to de-alloy.

Chou et al. conducted a first-principles study of silicon intercalation and lithium intercalation behavior. The results show that when the concentration of lithium ions is low, the interface of the slightly lithium-rich Li-Si alloy is in a relatively stable state. With the increase of the concentration of lithium ions, in addition to the outermost Si-Si bond, the structure of the near surface and the composition becomes similar to the host material; the influence of the interface on the material is mainly the first two atomic layers. The transport of lithium ions is related to the composition of the alloy, and the diffusion coefficient of lithium ions is enhanced by geometric orders of magnitude in the advanced lithium insertion stage.

Rahaman et al. conducted a first-principles study of the effects of oxygen ratios in silicon oxide on structural and electronic properties. The results show that the oxygen atom reacts violently with the embedded lithium ions, resulting in decomposition of the host material. High concentration of oxygen atoms can inhibit the volume expansion of silicon during charge and discharge, and help to suppress material powder failure caused by volume effect. The increase in oxygen content can increase the lithium intercalation capacity of the silicon oxide negative electrode, but it can lead to the formation of lithium silicate which is difficult to re-allocate, thereby introducing irreversible capacity loss.

2. Experimental research

2.1 Silicon modification

For the modification of elemental silicon, the Si-M alloy is formed by incorporation of the second component, the volume expansion coefficient of the silicon alloy is reduced, or the silicon is made porous and nanosized by various engineering techniques, and the volume expansion of silicon is reserved, space, reducing the effect of silicon volume effect on material cycle stability.

2.1.1 Alloying of silicon

The biggest obstacle to the commercialization of silicon anode materials is the failure of material powdering caused by the large volume effect of silicon during charge and discharge. Experiments have shown that the introduction of the second component to form a "Si-M" active-active or active-inactive system can effectively reduce the volume expansion coefficient of silicon, utilizing some properties of the active or inactive elements themselves, such as metal ductility, The bond characteristics, etc., alleviate the volume effect of silicon during the process of inserting and deintercalating lithium.

Lee et al. placed the silicon powder on the surface of the copper substrate and heated it to 2000 ° C under vacuum to form a negative electrode material of Si-Cu alloy film which gradually transitioned from the bottom to the top to the Si-rich state from the bottom to the top with Cu as the matrix. The half-cell test showed that after 100 cycles, the film sample had a mass specific capacity of 1250 mAh/g and an area specific capacity of 1956 mAh/cm3. However, excessive Cu causes the presence of partially crystalline silicon, making the cycle stability of the sample relatively poor.

Yang Juan et al. used a combination of mechanical ball milling and annealing to prepare Si-Fe composite anode materials, and improved the cycle performance of Si by using Si-Fe alloy with good conductivity and ductility. The results show that the material after the experimental treatment has reached alloying, and different forms of Si-Fe alloy phase are formed, but the degree of alloying is not complete. The formation of Si-Fe alloy improves the cycle performance of Si as a negative electrode material for lithium ion batteries, and the higher the degree of alloying, the better the electrochemical performance of the alloy material.

Zhang et al. used a combination of chemical etching, electrochemical reduction and magnetron sputtering to prepare a three-dimensional nanostructured multilayer Si/Al thin film anode material. The sample exhibited good electrochemical performance at 4.2 A/g discharge. At current density, the reversible specific capacity was 1015 mAh/g after 120 cycles, and the reversible specific capacity reached 919 mAh/g even though the discharge current increased to 10 A/g. The improvement in electrochemical performance is mainly attributed to the effective distribution of three-dimensional nanostructures.

2.1.2 Porosity of silicon

On the one hand, the porosity of silicon can increase the specific surface area of ??the silicon host material in contact with the electrolyte, increase the transport efficiency of lithium ions into the material, enhance the conductivity of the material, and on the other hand, it may exist for the charge and discharge process of silicon. Volume expansion reserves space to reduce the effect of silicon volume effect on the pole piece. The porosity of silicon is now widely recognized as an effective means of solving the volumetric effect of silicon. Figure 2 is a SEM topographical view of porous silicon.

Tang et al. used a PVA carbon source coating, HF acid etching and secondary asphalt coating to prepare a porous Si/C composite anode material. The results show that when the content of the secondary coated asphalt is 40% (mass fraction), the discharge specific capacity of the second week charge and discharge cycle of the sample reaches 773 mAh/g at a current density of 100 mA/g, after 60 cycles, the specific capacity was still maintained at 669 mAh/g, and the capacity loss rate was only 0.23%/week, and the material exhibited good cycle stability.

Han et al. Electrochemical etching and high-energy ball milling combined with P-type Si as the bottom plate and HF solution as the etching solution to obtain a porous silicon film material with a porosity of 70%, which was then ball-milled and heat-treated in PAN. Carbon coated porous silicon anode material. The sample has a reversible specific capacity of 1179 mAh/g after 120 cycles at 0.1 C, and has good electrochemical performance. The method is low in cost and is suitable for large-scale preparation of porous silicon materials.

2.1.3 Nanocrystallization of silicon

Researchers of silicon-based anode materials generally believe that when the scale of silicon is small to a certain extent, the effect of silicon volume effect can be relatively reduced, and the silicon of small particles is matched with the corresponding dispersion technology, and it is easy to reserve enough for the silicon particles. The expansion space, so the nanocrystallization of silicon is considered to be an important way to solve the commercialization of silicon-based anode materials. Figure 3 is a SEM topographical view of a carbon coated silicon nanotube array.

Wang et al. used ZnO nanowire template method to grow silicon nanotube arrays on carbon substrates, and compared the effects of carbon coating on silicon nanotube arrays. The results show that the carbon nanotube-coated silicon nanotube array samples show good cycle stability, and the specific discharge capacity still reaches 3654 mAh/g after 100 cycles.

Sun et al. used a plasma-assisted discharge method to prepare Si/graphite nanosheets from nano-silicon and expanded graphite, and used them as anode materials for lithium-ion batteries. The results show that the synthesized Si/C composite sample has good cycle stability, and the specific capacity of lithium intercalation is 1000 mAh/g. There is no capacity loss until the cycle of 350 weeks, and the coulombic efficiency is above 99%.

2.2 Structural design

The modification of the silicon monomer can reduce the volume expansion coefficient of silicon to a certain extent, but since the volume effect still exists, and the conductivity of silicon itself is insufficient to support the rapid transport of lithium ions, the silicon-based anode material is commercially available. Before the transformation, a lot of structural design is still needed to meet the requirements of commercial applications.

2.2.1 Core-shell structure

The purpose of the core-shell structure is to provide a buffer layer for the volume expansion of silicon or silicon alloy through the basic properties of the outer shell, and to control the volume effect of the silicon or silicon alloy within the core-shell structure. The researchers conducted a lot of research on the core-shell structure. 4 is a schematic structural view and a cycle curve of a Si/NiSi2/Ni/C core-shell structure sample.

Deng et al. used nano-Si as the inner core, NiSi2/Ni as the shell layer to coat the nano-Si, and coated the carbon layer to prepare a silicon-based anode material with a core-shell structure. The experimental sample had a reversible specific capacity of 1194 mAh/g, and the 105-cycle cycle capacity retention rate was 98%. The preparation method has the characteristics of simple process and low cost.

Wu et al. prepared a core/shell Si/C anode material by electrospinning nano-silicon particles into hollow carbon fibers. At a current density of 0.2 A/g, the reversible specific capacity of the sample was 903 mAh/g, and the capacity retention rate of the 100-week cycle was 89%. When the current density was increased to 2 A/g, the reversible specific capacity of the sample still reached 743 mAh/g has better rate performance. Hollow carbon fiber not only inhibits the volume expansion of nano-silicon, but also improves the electrical conductivity of the material.

2.2.2 Sandwich structure

Sun et al. used industrial silicon powder, graphite and sucrose as raw materials, reduced the scale of industrial silicon powder by mechanical high-energy ball milling, and then ball milled the industrial silicon powder with graphite. Finally, the sucrose pyrolysis carbon coating was used to form a sandwich structure MS. -G@C composite anode material. The sample has a reversible specific capacity of 830 mAh/g at 0.5 C and the 100-week cycle content is only attenuated by 0.02% per week, which has good cycle stability. The advanced structural design on the one hand provides a higher conductive network and on the other hand hinders the chalking failure of Si during charge and discharge.

3 Summary and outlook

In this paper, the research progress of silicon-based materials as anode materials for lithium ion batteries is summarized. In order to solve the volume effect that silicon may have during charging and discharging,

On the one hand, the researchers have carried out a large number of modification treatments on silicon, including the incorporation of the second component to form the Si-M alloy system, as well as the porous and nano-treatment of silicon.

On the other hand, the researchers constructed various structures to further modify the silicon-based anode to obtain a commercial silicon-based anode material.

Experiments prove that it is difficult for a single modification method to meet the commercial requirements of silicon-based anodes. In order to realize the commercial application of silicon-based materials, it is necessary to carry out compound modification by various means, and it is necessary to develop new engineering technologies to achieve scale. Control preparation.

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