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Silicon based materials, as negative electrodes for lithium-ion batteries, have advantages such as high capacity, wide availability, and environmental friendliness. They are expected to replace the widely used graphite negative electrode as the main negative electrode material for the next generation of lithium-ion batteries. This article briefly introduces the latest research progress of silicon/carbon composite materials from the aspects of material selection, structural design, and electrode optimization, and looks forward to future development directions.

Silicon can alloy with lithium at room temperature to form Li15Si4 phase, with a theoretical specific capacity of up to 3572 mA · h/g, far higher than the theoretical specific capacity of commercial graphite (372 mA · h/g). It is abundant in crustal elements (26.4%, second place), low in cost, and environmentally friendly. Therefore, silicon negative electrode materials have always been of great concern to researchers and are one of the most promising next-generation negative electrode materials for lithium-ion batteries.

However, silicon undergoes severe volume expansion (~300%) during the charging and discharging process, and the huge volume effect and low conductivity limit the commercial application of silicon anode technology. To overcome these shortcomings, researchers have made numerous attempts using composite technology and compensating for material expansion with a "buffer skeleton".
Carbonaceous negative electrode materials have small volume changes during charge and discharge processes, and have good cycling stability. Moreover, carbonaceous negative electrode materials themselves are mixed conductors of ions and electrons; In addition, silicon and carbon have similar chemical properties and can be tightly combined, so carbon is often used as the * * matrix for silicon composites.

In the Si/C composite system, Si particles serve as the active material, providing lithium storage capacity; C can buffer the volume change of silicon negative electrode during charge and discharge processes, improve the conductivity of Si based materials, and prevent Si particles from aggregating during charge and discharge cycles. Therefore, Si/C composite materials combine the advantages of both, exhibiting high specific capacity and longer cycle life, and are expected to replace graphite as the new generation of negative electrode materials for lithium-ion batteries.In recent years, the technology related to silicon carbon negative electrode materials has developed rapidly, and a small number of products have been put into practical use. Maxell, a subsidiary of Hitachi Group in Japan, has developed a new type of lithium battery using "SiO-C" material as the negative electrode, which has been successfully applied to commercial products such as smart ones. However, there are still many scientific issues that urgently need to be addressed before the large-scale commercial application of silicon carbon negative lithium-ion batteries can be achieved.

Structural Design of Silicon Carbon Composite Materials
Starting from the structure of silicon carbon composite materials, the currently studied silicon carbon composite materials can be divided into coating structures and embedding structures.
1.1 Coating Structure
The encapsulation structure is to cover a carbon layer on the surface of the active material silicon to alleviate the volume effect of silicon and enhance its conductivity. According to the coating structure and morphology of silicon particles, the coating structure can be divided into core-shell type, yolk shell type, and porous type.
1.1.1 Core shell type
Core shell silicon/carbon composite materials are composed of silicon particles as the core, uniformly coated with a layer of carbon on the outer surface of the core. The presence of a carbon layer is not only beneficial for increasing the conductivity of silicon and buffering the partial volume effect of silicon during lithium extraction, but also can greatly reduce the direct contact between the silicon surface and the electrolyte, thereby alleviating electrolyte decomposition and improving the cycling performance of the entire electrode.
Zhang et al. used lotion polymerization to coat polyacrylonitrile (PAN) on the surface of silicon nanoparticles, and heat treated at 800 ℃ to obtain silicon carbon core-shell structure composites（ Si@C ）The amorphous carbon layer inhibits the aggregation of silicon particles during charge and discharge processes, Si@C After 20 cycles, the capacity remains at around 50% of the initial capacity. In contrast, silicon nanoparticles experience significant capacity degradation after 20 cycles.
Hwa et al. used polyvinyl alcohol (PVA) as a carbon source and employed high-temperature pyrolysis under inert atmosphere to encapsulate silicon nanoparticles, resulting in silicon carbon composite materials with a carbon shell thickness of 5-10 nm. The use of silicon nanoparticles can reduce the volume effect of silicon and weaken the internal stress of the material. Carbon coating further buffers the expansion of the silicon core. After 50 cycles at a current of 100 mA/g, the specific capacity of the composite material can still reach 1800 mA · h/g, demonstrating excellent cycling stability. However, the capacity of pure nano Si and carbon coated micro silicon (4 μ m) decreases by less than 200 mA · h/g.
Xu et al. obtained core-shell silicon carbon composite materials by high-temperature pyrolysis of polyvinylidene fluoride (PVDF), with a carbon layer thickness of 20-30 nm; The reversible specific capacity of the silicon carbon composite electrode under 50 mA/g current conditions in the voltage range of 0.02~1.5V is 1328.8 mA · h/g. After 30 cycles, the capacity remains at 1290 mA · h/g, with a capacity retention rate of 97%. In core-shell silicon/carbon composite materials, the selection of different pyrolytic carbon source materials has varying effects on the interface between silicon and carbon intercalated lithium matrix in the composite system.
Liu et al. conducted a comparative analysis of silicon-based core-shell negative electrode materials using polyethylene oxide (PEO), polyvinyl chloride (PVC), polyethylene (PE), chlorinated polyethylene (CPE), and PVDF as pyrolytic carbon sources. They found that due to the etching effect of fluorine-containing materials on silicon, some F can be embedded into Si Si bonds, effectively enhancing the interface compatibility between pyrolytic carbon and silicon core. The corresponding Si PVDF based active materials also exhibited superior cycling stability.
Therefore, when the organic precursor of the carbon source contains F or Cl elements, it is beneficial to obtain a more stable silicon carbon interface, which makes the electrochemical performance of the material more excellent.
In summary, by carbon coating silicon materials and constructing core-shell structures, it helps to improve the cyclic stability of the materials. However, when the pyrolytic carbon in the silicon carbon core-shell structure is seamlessly coated on the surface of silicon particles, the volume effect of the lithiation process of the silicon core is too large, which can cause the entire core-shell particles to expand, and even lead to the rupture of the surface carbon layer, collapse of the composite material structure, and rapid decrease in cyclic stability. To address this issue, researchers have designed a double shell structure by enhancing the mechanical properties of the shell.
Tao et al. prepared composite materials with a double shell structure by coating SiO2 and pyrolytic carbon on the surface of silicon nanoparticles（ Si@SiO2 @C) Refer to Figure 1. Compared to a single shell Si@C comparison, Si@SiO2 @C has a higher capacity retention rate, with a reversible capacity of 785 mA · h/g even after 100 cycles within the voltage range of 0.01~5 V.
Research has shown that the intermediate layer SiO2, as a buffer phase, can further reduce the expansion stress generated during the cycling process; At the same time, the SiO2 layer can undergo irreversible reactions with diffused Li+to form Si and Li4SiO4 alloys, further ensuring the reversible capacity of the material.
1.1.2 Egg yolk shell type
Egg yolk shell structure is a new type of nano multiphase composite material formed by introducing gaps between the core and shell through certain technical means based on the core-shell structure. Egg yolk shell type silicon/carbon composite material presents a special Si@void @The configuration of the C-shell not only has the advantages of the ordinary core-shell structure, but its cavity also accommodates the volume expansion of silicon, allowing for more free expansion and contraction of the silicon core, thereby ensuring the stability of the overall structure of the material during charging and discharging, and facilitating the production of a stable solid electrolyte (SEI) film.
Zhou et al. used the sol gel method to coat a layer of SiO2 shell on the surface of silicon nanoparticles, and used sucrose as the carbon source for pyrolytic carbon coating. After etching SiO2 with HF, egg yolk shell structure composites were obtained（ Si@void @C) The mass fraction of active substance silicon is 28.54%. Compared to silicon nanoparticles and hollow carbon, Si@void @C has better cycling stability, with a specific capacity of 813.9 mA · h/g, and the capacity remains at 500 mA · h/g after 40 cycles.
Tao et al. also prepared stable samples using a similar method Si@void @The specific capacity of C composite material after 100 cycles is 780 mA · h/g. The optimization of carbon loading revealed that the specific capacity (780 mA · h/g) of the composite material with a carbon loading of 63% was higher than that with a carbon loading of 72% (690 mA · h/g). This indicates the need to achieve Si@void @The high capacity of C composite materials requires in-depth optimization design of the yolk shell structure.
Liu et al. synthesized egg yolk shell composite material using polydopamine as a carbon source（ Si@void @C) . In this structure, sufficient space is reserved between the silicon core and the thin carbon layer, allowing silicon to expand during lithiation without damaging the carbon shell layer, thereby enabling the formation of a stable SEI film on the surface of the composite material.
such Si@void @At a current density of 0.1C, the reversible capacity of C can reach up to 2800 mA · h/g, with a capacity retention rate of 74% and a Coulomb efficiency of 99.84% after 1000 cycles.
Recently, researchers have introduced the concept of multiple shell layers into the design of silicon carbon yolk shell structures to enhance the mechanical properties of the carbon layer and improve the material's ability to resist silicon volume expansion stress.
Sun et al. prepared it using the vesicle template method Si@void @SiO2 material, coated with polysaccharides on the inner and outer sides of porous SiO2 shell, and thermally decomposed at high temperature under inert atmosphere to obtain Si@void @C@ SiO2@C After removing SiO2 through HF etching, a double shell structure was obtained（ Si@void @C@ void@C ）Egg yolk shell composite material（ Si@DC ）See Figure 2.
The introduction of dual carbon layers endows the material with superior electrical conductivity. At a current density of 50mA/g, Si@DC After 80 cycles, the discharge specific capacity remained at 943.8 mA · h/g, while the silicon/single shell layer（ Si@SC ）After 80 cycles, the capacity of pure silicon particles decreased by 719.8 and 115.3 mA · h/g, respectively.
Yang et al. used St ö ber method and pyrolysis method to sequentially coat SiO2 layer and carbon layer on the surface of silicon nanoparticles. After HF selective etching, a double shell structure composite material was obtained（ Si@void @ SiO2@void @C) .
The material exhibits excellent cycling stability, maintaining a capacity of 956mA · h/g after 430 cycles at a current density of 460 mA/g, with a capacity retention rate of up to 83% Si@C Under the same testing conditions, the capacity of the core-shell material deteriorates significantly in the first 10 cycles, and after 430 cycles, the capacity is less than 200 mA · h/g.
In this composite structure, the carbon layer can improve conductivity, the SiO2 layer increases material stability, and the cavity provides a buffer space for the expansion of the silicon core. At the same time, SiO2 and carbon double shell layers block the electrolyte and silicon nanoparticles, preventing irreversible reactions between silicon nanoparticles and the electrolyte, playing a dual layer protective role.
1.1.3 Multi hole type
Porous silicon is commonly prepared by template method, and the internal voids of silicon can reserve buffer space for volume expansion during lithium silicon alloying process, alleviating internal mechanical stress of the material. Silicon carbon composite materials formed from porous silicon have a more stable structure during cycling.
Research has shown that in porous silicon/carbon composites, pore structures uniformly distributed around silicon particles can provide fast ion transport channels, and a larger specific surface area increases material reactivity, thereby exhibiting excellent rate performance and significant advantages in battery fast charging performance.
Li et al. synthesized 3D connected porous silicon carbon composite by controllable reduction of silica aerogel. The capacity of the material was kept at 1552 mA · h/g when it was cycled 200 times under 200 mA/g current density, and the specific capacity was still kept at 1057 mA · h/g after it was cycled 50 times under 2000 mA/g high current charge and discharge.
Bang et al. deposited Ag particles on the surface of silicon powder (particle size 10 μ m) through galvanic displacement reaction, removed Ag by etching, and obtained block shaped silicon with 3D pore structure. Then, carbon coating was carried out by acetylene pyrolysis to prepare porous silicon carbon composite materials, which had an initial capacity of 2390 mA · h/g and a * * * Coulomb efficiency of 94.4% at a rate of 0.1C; The capacity at 5C magnification can still reach 92% of the capacity at 0.1C magnification, demonstrating excellent magnification performance. In addition, after 50 cycles, the thickness of the electrode changed from 18 μ m to 25 μ m, and the volume expansion was only 39%; At the same time, the volumetric capacity of the material is close to 2830 mA · h/cm3, which is five times that of commercial graphite electrodes (600 mA · h/cm3).
Yi et al. treated micrometer sized SiO2 powder at 950 ℃ for 5 hours to obtain a Si/SiO2 mixture. After removing SiO2 by HF acid etching, porous silicon composed of primary silicon particles with a particle size of 10 nm was obtained. Then, using acetylene as the carbon source, the porous silicon was pyrolyzed at 620 ℃ for 20 minutes to coat it with carbon, resulting in the preparation of porous silicon carbon composite materials. The material maintains a capacity of 1459 mA · h/g after 200 cycles at a current density of 1 A/g, which is much higher than pure silicon; At a high current density of 12.8 A/g, the specific capacity can still reach 700mA · h/g, demonstrating excellent rate performance. In addition, the material has a high tap density (0.78g/cm3) and a high volumetric capacity. After 50 charge discharge cycles at a current density of 400mA/g, the capacity remains at 1326mA · h/cm3.
Further research has found that by adjusting the reaction temperature to optimize the particle size of silicon primary particles, the performance of porous silicon carbon composite materials is superior when the primary particle size is 15 nm. After 100 cycles at a current density of 400 mA/g, the capacity can reach 1800 mA · h/cm3, which is much higher than that of composite materials with primary particle sizes of 30 nm and 80 nm. This is mainly because the smaller the particle size of silicon primary particles, the smaller the volume change during lithium deintercalation, thus forming a more stable SEI film.
Furthermore, further optimization of carbonization temperature and time revealed that the porous silicon/carbon composite material exhibited excellent performance at a carbonization temperature of 800 ℃ and a carbon loading mass fraction of 20%. After 600 cycles at a current density of 1.2 A/g, the capacity remained at 1200mA · h/g with almost no capacity loss, and the Coulomb efficiency reached 99.5%.
The synthesis process of this porous silicon carbon composite material is low-cost and easy to scale up production.
Recently, Lu et al. designed and synthesized a special structure of carbon coated porous silicon material (nC pSiMPs), in which porous microsilica (pSiMPs) is composed of a stack of primary silicon nanoparticles, with no carbon coating layer on the surface of the silicon nanoparticles inside, and the carbon layer only coating the outer surface of the microsilica.
This material is made from commercial SiO particles as raw materials, using resorcinol formaldehyde resin as a carbon source, and high-temperature carbonization treatment under Ar atmosphere to obtain a carbon coating layer. At the same time, the core SiO undergoes high-temperature disproportionation reaction to generate Si and SiO2, and after HF etching, porous silicon with a volume ratio of silicon to cavity of 3:7 is obtained. In this structure, the cavity size can well accommodate the volume change of silicon during lithium extraction without causing the carbon shell to break, ensuring the stability of the material structure; At the same time, the carbon shell layer covering the outer surface of porous silicon can prevent electrolyte from penetrating into the interior of porous silicon, reduce the contact area between silicon and electrolyte, and only form a stable SEI film on the carbon coating layer on the outer surface of micron sized silicon.
Correspondingly, for materials with internal silicon nanoparticles also coated with a carbon layer (iC pHiMP), the contact area between the electrolyte and the active substance is larger. At the same time, the expansion of silicon volume can easily cause the carbon layer to break, exposing the internal silicon nanoparticles and coming into contact with the electrolyte, resulting in the formation of a thicker SEI film during charge discharge cycles.
Therefore, nC pHiMPs electrodes (with an active material loading of 0.5 mg/cm2) have superior cycling stability compared to iC pHiMP and pSiMP, with a reversible capacity of up to 1500 mA · h/g after 1000 cycles at 1/4C (1C=4.2 A/g active material).
In addition, after 100 cycles, the thickness of the electrode material increased from 16.2 μ m to 17.3 μ m, with an expansion rate of only 7%. Its volumetric capacity (1003 mA · h/cm3) is also much higher than that of commercial graphite (600mA · h/cm3).
1.2 Embedded type
Embedded silicon carbon composite materials refer to the dispersion of silicon particles into a carbon carrier through physical or chemical means. The silicon particles are tightly bound to the carbon matrix, forming a stable and uniform two-phase or multi-phase system. The carbon carrier provides transport channels and support skeleton for electrons and ions, providing stability to the material structure.
In embedded silicon carbon composite materials, the silicon content is generally low, and the capacity that can be contributed is relatively small, so its reversible specific capacity is usually low. However, there are a large number of carbon materials in the composite material, so its cyclic stability is generally good.
1.2.1 Graphite
Graphite is currently the most widely used negative electrode material for lithium-ion batteries, divided into two types: natural graphite and artificial graphite. The raw materials are widely sourced and inexpensive. Graphite has a layered structure, with small volume changes during charging and discharging, good cycling stability, and can buffer the volume expansion caused by the reconstruction of silicon structure during charging and discharging, avoiding the collapse of negative electrode material structure. It is suitable as a buffer matrix; Meanwhile, the excellent electronic conductivity of graphite effectively solves the problem of poor electronic conductivity in silicon. However, graphite has stable chemical properties at room temperature and is difficult to generate strong forces with silicon. Therefore, currently, silicon/graphite composites are mainly prepared through two methods: high-energy ball milling and chemical vapor deposition.
Pengjian et al. used high-energy ball milling to mix graphite and silicon powder to produce silicon/graphite composite materials. Research has shown that no alloy phase is generated in the composite material, and its reversible specific capacity is 595 mA · h/g, Coulomb efficiency is 66%; After 40 cycles, the specific capacity is 469 mA · h/g, and the capacity loss rate per cycle is about 0.6%.
Holzapfel et al. used chemical deposition (CVD) to deposit silicon nanoparticles in graphite. When the silicon mass fraction was 7.1%, the reversible capacity of the electrode was 520mA · h/g, with the specific capacity contributed by silicon exceeding 2500 mA · h/g. After 100 cycles, the specific capacity contributed by silicon was still as high as 1900 mA · h/g.
The interaction force between graphite and silicon is weak, making it difficult to form a stable composite structure. Therefore, graphite is generally used as a conductive skeleton or medium to construct structurally stable ternary composite systems together with other silicon/carbon materials. For lithium-ion battery negative electrode materials, silicon/amorphous carbon/graphite (Si-C-G) is a popular and early researched ternary composite system. Its preparation methods mainly include mechanical mixing high-temperature pyrolysis, solvothermal high-temperature pyrolysis, and chemical vapor deposition.
For Si-C-G composite materials, silicon has a higher specific capacity (about 3579 mA · h/g), which is 10 times that of graphite and pyrolytic carbon. It is the key active substance that determines the capacity of the composite material, and the capacity can be designed by adjusting the content of silicon in the composite system; Graphite as a supporting material can improve the dispersion effect and conductivity of silicon; Amorphous carbon serves as a binder and coating carbon, effectively combining silicon powder with graphite and forming a conductive carbon mesh structure together with graphite. At the same time, amorphous carbon can also improve the interfacial properties between silicon and electrolyte.
Therefore, the organic combination of silicon amorphous carbon graphite materials can effectively improve the electrochemical performance of silicon negative electrodes.
Kim et al. used a combination of mechanochemical ball milling and granulation processes to mix silicon nanoparticles with larger flake graphite particles for granulation, allowing smaller silicon nanoparticles to be embedded in the gaps between flake graphite particles, thereby preparing silicon graphite/amorphous carbon composite materials. This composite material effectively solves the problems of poor conductivity and volume expansion of silicon, and the resulting composite material has a reversible specific capacity of 568 mA · h/g*** The Coulomb efficiency can reach 86.4%.
Lee et al. added silicon nanoparticles (100nm) and natural flake graphite (~5 μ m) to asphalt solution, and obtained Si-G-C ternary composite material through ball milling granulation high-temperature pyrolysis carbonization. Its reversible specific capacity is 700 mA · h/g, and the * * efficiency is as high as 86%. After 50 cycles, the specific capacity hardly deteriorates.
Ma et al. dissolved silicon nanoparticles, polyvinyl chloride (PVC), and expanded graphite in THF, evaporated the solvent, and carbonized to obtain silicon carbon expanded graphite composite materials. The reversible capacity of the material is 902.8 mA · h/g at 200mA/g, and the capacity retention rate is 98.4% after 40 cycles.
Research has found that silicon nanoparticles broken due to expansion during the cycling process can still be well dispersed on expanded graphite, mainly due to the porous nature and good flexibility of expanded graphite.
In summary, the capacity of silicon/graphite or silicon/graphite/carbon systems is generally not high, below 1000mA · h/g, and the silicon content is generally low. The purpose of reducing silicon usage is to improve the capacity of composite materials while ensuring that their various properties are as consistent as graphite, especially * * Coulomb efficiency and cycle life, in order to improve the quality and volumetric energy density of existing battery systems. The current design capacity is 450-600 mA · h/g, but considering the explosive demand for mileage and lifespan in the new energy vehicle market, the development of 300-350 W · h/kg power lithium batteries is an inevitable trend. Therefore, the development of high-capacity silicon-based materials is also imperative.
1.2.2 Carbon nanotubes/nanofibers
Compared to graphite particles, carbon nanotubes/nanofibers (CNT/CNFs) benefit from their high aspect ratio. When combined with silicon, their conductivity and network structure can be utilized to construct a continuous electron transfer network, alleviate the volume change of silicon during cycling, suppress particle aggregation, and thus improve the electrochemical performance of silicon-based negative electrode materials.
Camer et al. used chemical synthesis to obtain phenolic polymer silicon composite materials, and then carbonized them in an inert atmosphere to obtain Si/SiOx/carbon fiber composite materials. The presence of carbon fiber enhances the conductivity of the electrode while limiting the expansion and contraction during the process of silicon deintercalation and lithium insertion. The composite material has a specific capacity of 2500 mA · h/g at a current density of 500 mA/g and exhibits good cycling stability.
Mangolini et al. coated quantum dot Si solution, CNTs, and polyvinylpyrrolidone (PVP) onto copper foil and subjected it to heat treatment in an inert atmosphere to obtain Si/CNTs composite material, in which Si particles were uniformly dispersed in CNTs and a heterojunction layer was formed between the two. After 200 cycles, the charging specific capacity of the material can still reach 1000 mA ·/g, and its Coulomb efficiency is 99.8%.
Additionally, introducing CNT and CNF into Si@C In composite materials, the synergistic effect between the three materials also helps to further enhance the electrochemical performance of the material.
Zhang et al. combined CNT and CNF Si@C Mixing to prepare composite materials with high capacity and excellent cycling performance（ Si@C /CNT&CNF). Among them, CNT and CNF, along with the carbon coating layer on the silicon surface, construct an efficient electron transfer network within the composite material, which transfers most of the electrons Si@C Connecting particles together to enhance the conductivity of composite materials; Simultaneously CNT and CNF Si@C The interwoven and mixed pores formed within the composite material can withstand the expansion of silicon during lithium insertion, suppress the rupture of the transmission network during cycling, and thereby enhance the cycling stability of the material.
After 50 cycles at a current density of 300mA/g, the capacity of the material can still reach 1195 mA · h/g, while undoped CNTs&CNFs Si@C The material has poor cycling stability, with a capacity of only 601 mA · h/g after 50 cycles. The pure silicon nanoparticles without carbon coating have almost zero capacity decay after 15 cycles.
1.2.3 Graphene
In addition to graphite and carbon nanotubes/nanofibers, graphene has also become one of the hot materials for modified silicon-based negative electrodes due to its excellent conductivity, high specific surface area, and good flexibility. Researchers have developed several methods for preparing silicon/graphene composite negative electrode materials for lithium-ion batteries.
Chou et al. obtained a material with a reversible specific capacity of 2158 mA · h/g by simply mechanically mixing silicon nanoparticles with graphene, which remained at 1168 mA · h/g after 30 cycles.
Chabot et al. prepared silicon/graphene composites by freeze-drying a mixture of silicon nanoparticles and graphene oxide, followed by thermal reduction in an Ar atmosphere containing 10% (volume fraction) H2. The initial discharge capacity of the material is 2312 mA · h/g, and the capacity retention rate after 100 cycles is 78.7%.
Luo et al. designed an aerosol assisted capillary driven self-assembly method, in which graphene oxide is mixed with silicon by ultrasound, heated to form droplets, and then brought into a carbonization furnace by gas to reduce carbonization, resulting in a wrinkled graphene coated silicon composite material. After 250 cycles at a current of 1A/g, the capacity of the material can still reach 940mA · h/g, with an average capacity loss of only 0.05% per cycle.
Research has shown that combining graphene (G) with silicon can improve the conductivity and cycling stability of silicon negative electrodes, but introducing graphene alone cannot greatly improve the electrochemical performance of silicon negative electrode materials. By combining silicon, graphene, and amorphous pyrolytic carbon together and utilizing the synergistic effect between the three, it is expected to obtain silicon-based negative electrode materials with better electrochemical performance.
Zhou et al. designed graphene/ Si@C Composite materials, by coating a layer of pyrolytic carbon protective layer on the surface of silicon nanoparticles, not only promote the structural stability of silicon, but also enhance the bonding ability between silicon particles and graphene interface, promoting electron transfer between interfaces. The reversible capacity of this composite material with a double-layer protective structure can reach 902 mA · h/g after 100 cycles at a current density of 300 mA/g.
Li et al. first grafted polyaniline onto the surface of silicon nanoparticles, and then utilized the π - π interaction and electrostatic attraction between polyaniline and graphene to self assemble and encapsulate graphene on the surface of the particles, followed by high-temperature carbonization to obtain Si@C /G composite material. The reversible capacity of the composite material at a current density of 50mA/g is 1500mA · h/g, and the capacity at a high current density of 2000mA/g exceeds 900 mA · h/g. The capacity retention rate after 300 cycles can reach 70% of the initial capacity.
Zhou et al. coated negatively charged silicon nanoparticles with positively charged polydialkylamine chloride (PDDA), and then self-assembled them with negatively charged graphene oxide under electrostatic action, carbonizing them to obtain a coated structure Si@C /G composite material. The material still has a reversible capacity of 1205 mA · h/g after 150 cycles at a current density of 100mA/g.
Yi et al. used a similar method to encapsulate PDDA onto a mixture of SiO and graphene oxide (GO), followed by high-temperature carbonization and HF acid etching to obtain microporous silicon/graphene composite material (G/Si). Subsequently, acetylene was used as the carbon source and subjected to high-temperature pyrolysis carbonization for carbon coating to obtain G/ Si@C Three element composite materials. The material has a specific capacity of up to 1150 mA · h/g, and the capacity remains basically unchanged after 100 cycles.
Research has found that under the synergistic effect of graphene support skeleton and carbon coating, the composite material still exhibits high area specific capacity under high loading conditions of negative electrode active material. After 100 cycles, the area specific capacity is about 3.2 mA · h/cm2.
Carbon coated microporous silicon without graphene support skeleton under the same load（ Si@C ）The area specific capacity of composite materials deteriorates significantly, with an area specific capacity of approximately 1.8 mA · h/cm2 after 100 cycles.
This is mainly due to the introduction of graphene support skeleton and carbon coating layer, which constructs an efficient electronic conduction network in the composite material, effectively connecting all silicon particles together and enhancing the electrochemical performance of the negative electrode highly active material. Unlike graphite and carbon nanotubes/nanofibers, graphene has a special single-layer 2D planar structure that can be combined with silicon to construct silicon/graphene composite materials with a "sandwich" structure.
In this "sandwich" structure, graphene sheets are stacked on top of each other, sandwiching silicon nanoparticles like a "sandwich" in its stacked elastic layer, effectively suppressing the contact between silicon and electrolyte and particle agglomeration. At the same time, the hole defects in the stacking layer (holes between layers and holes in graphene sheets) can buffer the volume expansion of silicon particles and reduce the deformation stress generated during lithium extraction and insertion.
In addition, the "sandwich" structural units are interconnected in a three-dimensional graphite network structure, which can reconstruct a silicon/graphene three-dimensional network composite material. Li+can freely move on graphene layers and can also be transferred between layers through planar hole defects, thereby enhancing Li+conduction and electrochemical reactions in the composite material.
Mori et al. prepared multi-layer "sandwich" structured silicon/graphene composites using electron beam deposition technology under isolated air conditions.
Research has shown that the number of layers and thickness of the "sandwich" structure have a direct impact on the discharge capacity, Coulomb efficiency, and reversible capacity of composite materials. When the number of layers is 7 and the thickness is 100nm, the electrochemical performance of the composite material is optimal, with a discharge capacity exceeding 1600 mA · h/g after 30 cycles at a current density of 100 mA/g. The flexible lithium-ion soft pack battery assembled with LiCoO2 as the positive electrode and silicon/graphene as the negative electrode can be used in commercial LED light power supply devices and has potential advantages in the field of flexible thin-film electronic devices.
Liu et al. designed and prepared a self curled silicon/reduced graphene oxide (rGO) nano "sandwich" structure thin film material based on the principle of stress release. The internal holes in the structure and the mechanical stability of the nano film effectively alleviate the expansion stress generated during the process of silicon deintercalation and lithium insertion. In addition, the uniformly distributed rGO layer in the nanofilm can not only enhance conductivity, alleviate the volume expansion and agglomeration of silicon nanoparticles, but also effectively suppress the formation of thicker SEI films during charge and discharge processes. The sandwich structured composite film exhibited excellent cycling stability after 2000 cycles under 3 A/g conditions, with a lifespan decay of only 3.3% per 100 cycles.
Electrode optimization
In addition to structure, surface, and interface, other factors such as electrolyte additives and binders also have important effects on the capacity and cycling performance of silicon carbon composite materials.
2.1 Electrolyte additives
Severe volume expansion (~300%) of silicon during lithium insertion can cause pulverization of active material particles, making it difficult to form a stable SEI film on their surface; Moreover, during the process of lithium extraction and insertion, the volume change of silicon can easily damage the film layer. The SEI film is damaged, exposing the surface of new silicon particles. The electrolyte will continue to decompose on its surface, forming a new SEI film, causing the SEI film to become thicker and the internal resistance of the electrode to continuously increase, exacerbating the degradation of electrode capacity.
The composition of electrolyte affects the formation of SEI film, which in turn affects the electrochemical performance of negative electrode materials. To form a uniform and stable SEI thin layer, researchers improved the electrochemical performance of silicon negative electrodes by adding electrolyte additives. The currently used additives include ethylene carbonate, trihydroxymethyl aminomethane, succinic anhydride, fluorinated ethylene carbonate (FEC), etc. Among them, the most effective additive is FEC. Silicon nanoparticles (~50nm) experience only a 5% capacity loss after 80 cycles in an electrolyte with a FEC mass fraction of 10%, The Coulomb efficiency is close to 99%, but after 80 cycles in the electrolyte without FEC, the capacity retention rate is only 70%, and the Coulomb efficiency also decreases by * * 97%.
Research has shown that the main products of FEC reduction are - CHF-OCO2 type compounds and LiF. During the charging and discharging process, the compounds generated by FEC reduction form the initial SEI film coating on the silicon surface. This layer of SEI film has good mechanical properties and is not easily broken. It can effectively block the contact between silicon and electrolyte, slow down the decomposition of electrolyte, and suppress the continuous generation of uneven SEI film. Meanwhile, the production of another product, LiF, also facilitates the conduction of Li+within the SEI film.
2.2 Adhesive
In the preparation process of lithium-ion battery electrodes, polymer binders are usually used to bond the active material and conductive agent to the current collector. Therefore, the characteristics of the binder are also crucial to the battery performance, especially the initial Coulomb efficiency and cycling performance. Polyvinylidene fluoride (PVDF) is currently a widely used commercial adhesive, but when combined with silicon negative electrode materials, it exhibits Van der Waals force and weak adhesion, making it difficult to adapt to the huge volume effect during silicon deintercalation and lithium insertion, and insufficient to maintain the integrity of the electrode structure. Recently, researchers have made significant progress in the research of silicon-based material adhesives, such as carboxyl containing compounds and their derivatives, including polyacrylic acid (PAA), carboxymethyl cellulose (CMC) - based polymers, alginate (Alg) - based polymers, etc.
Compared to PVDF and styrene butadiene rubber (SER), these polymers can form hydrogen bonds and/or covalent bonds with silicon, exhibiting better adhesion ability.
*Recently, Kovalenko et al. found that silicon negative electrodes using alginate as a binder have better electrochemical performance than those using CMC as a binder. After 100 cycles at a high current density of 4200mA/g, the reversible capacity exceeds 1700mA · h/g, while the CMC based Si negative electrode has a capacity of less than 1000mA · h/g after 40 cycles. The reason is that the carboxyl groups of alginate are uniformly distributed in the polymer chain, while the carboxyl groups in CMC are randomly distributed.
In addition, multifunctional polymer adhesives have also received certain research attention. For example, Ryou et al. utilized the chelating effect of catechol groups with adhesive dopamine hydrochloride to graft it onto PAA and Alg frameworks, and used it as a binder to prepare Si-Alg-C and Si-PAA-C electrodes. Compared to Si Alg and Si PAA electrodes with PAA and Alg as binders, Si Alg-C and Si PAA-C electrodes have better cycling stability. Although introducing functional groups to improve polymer adhesion can enhance the electrochemical performance of silicon negative electrodes, this type of multifunctional polymer binder belongs to linear polymers. Once the silicon undergoes continuous volume changes during cycling, the binder is easily peeled off from the surface of silicon particles.
To solve this problem, researchers have prepared a silicon negative electrode binder with a three-dimensional (3D) network structure by crosslinking and fixing polymer segments. Koo et al. prepared a 3D cross-linked polymer binder c-PAA-CMC through the condensation reaction of PAA and CMC. Compared to CMC, PAA, and PVDF, c-PAA-CMC has better cycling stability when used as a silicon nanoparticle negative electrode* Recently, polymers containing a large number of hydrogen bonds and possessing self-healing functions (SHPs) have also been used as binders to stabilize silicon negative electrode materials. SHPs have self-healing ability in both mechanical and electrical properties, and can repeatedly heal broken or damaged silicon during battery cycling.
When the Si SHP/carbon black (Si SHP/CB) electrode is charged and discharged at a current of 0.1 mA/cm2 under high load conditions (1.13 mA/cm2), the initial unit area capacity is close to 3.22 mA/cm2. Even after 120 cycles at a current of 0.3 mA/cm2, the unit area capacity can still reach 2.72 mA/cm2. In contrast, using CMC or PVDF as binders, under the same silicon negative electrode material and load conditions, the capacity rapidly decreases after multiple cycles.
Conclusion
Silicon carbon composite materials combine the advantages of high conductivity and stability of carbon materials with high capacity of silicon materials. Through carbon material selection, preparation process optimization, and composite structure design, the * * * and subsequent Coulomb efficiency and cycling performance of silicon carbon negative electrode materials have been significantly improved. The preparation of egg yolk shell type or the introduction of graphene can obtain silicon carbon composite materials with special morphological structures and excellent properties, but it is difficult to achieve large-scale commercialization.
At present, the composite of silicon and graphite materials, using encapsulation and embedding to construct high-performance silicon carbon negative electrode materials, has been recognized by the industry and is regarded as a negative electrode product that is close to industrialization.
The future research directions of silicon carbon materials will focus on the following aspects:
1) Improve the dispersion of silicon nanoparticles and form effective composite structures with carbon materials;
2) Study the composite mechanism of silicon carbon materials and the mechanism of lithium insertion and extraction with different microstructures, and explore the influence of different microstructures on electrochemical performance, such as the effect of specific surface area on SEI film formation, and the influence of carbon content and structure on irreversible capacity in different composite systems;
3) Simplify and optimize the material preparation process, using cost-effective and short cycle raw materials and preparation methods;
4) Explore binders and electrolytes that better match the properties of silicon carbon materials;
5) Improving the high current charge and discharge performance of silicon carbon materials while maintaining cycling stability is of great significance for power and dynamic batteries;
6) The selection of materials and processes must consider safety and environmental protection, and develop towards a green, environmentally friendly, and recyclable direction.
Grinding and dispersing machine is a high-tech product composed of colloid grinding and dispersing machines.

The first level consists of three-level sawtooth protrusions and grooves with increasing precision. The stator can be infinitely adjusted to the desired distance between the rotors. Under enhanced fluid turbulence. The groove can change direction at each level.
The second stage is composed of a stator. The design of the dispersing head also effectively meets the needs of substances with different viscosities and particle sizes. The difference in the design of the stator and rotor (emulsifying head) between online and batch machines is mainly due to the requirements for conveying performance. It is particularly important to note that the difference between coarse precision, medium precision, fine precision, and other types of working heads is not only the arrangement of specified rotor teeth, but also an important difference in the geometric characteristics of different working heads. The width of the slot and other geometric features can alter the different functions of the stator and rotor working heads.

The following is a model table for reference:

Carbon silicon material high shear grinding and dispersing machine