CN117059765B

CN117059765B - Silicon-carbon composite material and preparation method and application thereof

Info

Publication number: CN117059765B
Application number: CN202310985480.0A
Authority: CN
Inventors: 吴建华; 简正军; 吴易华; 冯荣标
Original assignee: Jiangmen Hechuang Energy Materials Co ltd
Current assignee: Jiangmen Hechuang Energy Materials Co ltd
Filing date: 2023-08-04
Publication date: 2024-06-07
Anticipated expiration: 2043-08-04

Abstract

The invention discloses a silicon-carbon composite material, a preparation method and application thereof, and belongs to the technical field of secondary battery materials. The silicon-carbon composite material has a core-shell structure, and the core comprises a porous carbon skeleton and silicon-based materials dispersed therein; the strongest peak position theta _20～30 of the porous carbon skeleton is 20-30 degrees, and the strongest peak position theta _40～60 is 40-60 degrees; the peak position of the strongest diffraction peak in the range of 20-30 degrees of the silicon-carbon composite material is theta '_20～30, and the peak position of the strongest diffraction peak in the range of 40-60 degrees is theta' _40～60; and, 2.7 DEG is less than or equal to B=θ' _20～30-θ_20～30≤11.4°;4.3°≤A＝θ`_40～60-θ_40～60 is less than or equal to 10.3 deg. The silicon-carbon composite material provided by the invention can achieve excellent performances of high capacity, small volume expansion ratio, high initial efficiency and the like. The invention also provides a preparation method and application of the silicon-carbon composite material.

Description

Silicon-carbon composite material and preparation method and application thereof

Technical Field

The invention relates to the technical field of secondary battery materials, in particular to a silicon-carbon composite material and a preparation method and application thereof.

Background

In the current power battery and consumer battery markets, lithium ion batteries become an absolute mainstream energy storage system because of great advantages in terms of voltage, service life, self-discharge, energy density and the like. Also, because of the endless increasing demands of power batteries and consumer batteries as different usage scenarios for endurance, the increasing of the Energy Density (ED) (volumetric energy density (VED), mass energy density (GED)) of lithium ion batteries is the final effort direction under the premise of meeting other basic performances.

In a lithium ion battery system, the capacity of the positive electrode material is basically exerted to the limit, and the lifting ratio is not high (150/175 mAh/g of lithium iron phosphate, 190/205mAh/g of nickel cobalt manganese ternary positive electrode and 142/148mAh/g of lithium manganate); graphite anodes have also reached a capacity and initial efficiency compromise (355/372 mAh/g). On this basis, silicon cathodes are increasingly being used by consumer batteries and begin to evolve towards power batteries, which have a major value of more than ten times the high gram capacity of graphite (4200 mAh/g vs372 mAh/g), but at the same time produce a volume expansion of >300% during delithiation of the battery cycle, because they can accommodate a large amount of lithium ions for alloying reactions.

For commercial application of silicon-based anode materials, the related art developed several paths: 1. route of silica: i.e., form oxygen deficient silica (also known as silica), but the first pass of this material is low (76%) and can cause significant loss of positive active lithium ions; 2. in order to compensate for the low first effect problem of the silicone route, pre-lithiated silicone materials were developed: namely, before the lithium source is used, partial lithiation is carried out, the lithium source used by the lithium source is expensive, and the initial efficiency is improved to be close to the limit (82 percent); 3. silicon-carbon material by traditional grinding method: the particle size of the silicon-based material is reduced by adopting a mechanical method, and the silicon-based material is combined with a carbon-based material, so that the size effect of the silicon-based particles is avoided to a certain extent, and the problem of poor conductivity of the silicon-based material is solved; however, the limit of the grinding method of silicon particles is reached (the silicon particles are in the shape of sheets with the thickness of 20nm and the length of 100 nm), but the structural impact on the battery core caused by the expansion of the silicon embedded lithium cannot be solved by using the silicon particles in a large amount, namely, the size of the silicon particles obtained by the grinding method, and the compatibility problem caused by the expansion of the silicon embedded lithium cannot be solved even if the silicon components are uniformly dispersed in a carbon medium.

In summary, the related art has verified that the above routes have all reached the design limit of capacity: 390. 420, 400mAh/g. Therefore, there is a need to develop new silicon-based materials that combine higher capacity, smaller volume expansion ratio, and higher initial efficiency.

Disclosure of Invention

The present invention aims to solve at least one of the technical problems existing in the prior art. Therefore, the invention provides the silicon-carbon composite material, which can be used as the anode material of the secondary battery and has the excellent performances of high capacity, small volume expansion ratio, high first efficiency and the like compared with the traditional silicon-carbon composite material.

The invention also provides a preparation method of the silicon-carbon composite material.

The invention also provides application of the silicon-carbon composite material.

According to an embodiment of the first aspect of the present invention, there is provided a silicon-carbon composite material having a core-shell structure, wherein:

the core comprises a porous carbon skeleton and a silicon-based material dispersed in the porous carbon skeleton;

The peak position of the strongest diffraction peak of the porous carbon skeleton is theta _20～30 within 20-30 degrees, and the peak position of the strongest diffraction peak is theta _40～60 within 40-60 degrees;

The peak position of the strongest diffraction peak of the silicon-carbon composite material is theta '_20～30 within 20-30 degrees, and the peak position of the strongest diffraction peak is theta' _40～60 within 40-60 degrees;

And the value range of B=θ' _20～30-θ_20～30 is 2.7-11.4 degrees; the value range of A=θ' _40～60-θ_40～60 is 4.3-10.3 degrees.

The silicon-carbon composite material provided by the embodiment of the invention has at least the following beneficial effects:

(1) The invention limits the relation between the silicon-carbon composite material and the peak position in the specific area of the porous carbon skeleton, so that compared with the traditional silicon-carbon composite material, the silicon-carbon composite material provided by the invention has the characteristic peak position red shift; the red shift phenomenon actually reflects the crystallization state of the silicon-based material, the interaction among porous carbon frameworks, surface defects and other properties:

1. the pure porous carbon skeleton and the silicon-based material have diffraction peaks in the ranges of 40-60 degrees and 20-30 degrees; wherein the diffraction peak of the silicon-based material is more right; according to the crystallization performance of the silicon-based material and the porous carbon material, in the silicon-carbon composite material, the strongest peak in a certain area can be the peak of the silicon-based material or the peak of a porous carbon skeleton; from this point of view, the value A and the value B reflect the crystallization performance of the silicon-based material, and the better the crystallization performance, the faster the lithium is extracted from the silicon-based material, so the first effect and the multiplying power are better. I.e. the larger the A value and the B value, the better the rate capability of the silicon-carbon composite material.

2. In the invention, the silicon-based material not only forms particles, but also modifies the exposed crystal face of the self carbon of the carbon skeleton to form the silicon-carbon composite material with chemical combination; the method is equivalent to optimizing the combination of silicon and carbon two phases, so that the volume change brought by lithium intercalation of the silicon-carbon composite material can be further relieved, the shuttle difficulty of lithium ions at the combination position of the two phases is reduced, and finally the cycle performance and the first effect can be obviously improved. However, a silicon-based material having a modified porous carbon skeleton cannot form a crystalline state, and thus a and B are degraded. From this point of view, the increase in the a and B values may reduce the cycle performance to some extent and reduce the first effect.

3. In the invention, the carbon component and the silicon component in the silicon-based material have a certain amount of defects, including surface defects, lattice defects and the like (because in theory, the transmission of ions between interfaces has the maximum transmission rate), and the defects can accelerate the reaction with active lithium ions on one hand, thereby improving the rate capability; on the other hand, since the radius of lithium ions is small, the above defects can provide a lattice space for accommodating lithium ions in advance, thereby further reducing the volume change ratio during deintercalation of lithium. Defects may further red shift diffraction peaks. From this point of view, the improvement of the A value and the B value improves the cycle performance.

4. The peak position of the characteristic diffraction peak also implies the crystallization state of the material, and if the crystallization state of the material is poor, the half-width of the diffraction peak is widened, even the characteristic peak does not appear, and the deviation of the characteristic peak also appears. In the diffraction peak position range defined by the invention, the silicon component in the silicon-carbon composite material has good crystallization performance, can more effectively exert electrochemical performance, and particularly has the first coulomb efficiency, which is caused by the following reasons: the lithium intercalation capacity of the material is provided by the lithium intercalation of the silicon-based material, the lithium intercalation of the silicon-based material is an alloying behavior of lithium intercalation into silicon lattices, the behavior is an entropy increasing process, if the initial silicon-based material has higher crystallinity, the lithium will destroy more original Si-Si bonds in the process of intercalating into the silicon-based material lattices, but the reaction rate of alloying is higher because of the reduction of lattice energy and the trend of entropy increasing in the process, namely, the lithium intercalation dynamics is higher, the lithium ion is guaranteed to have higher transmission rate, and therefore, the higher first coulombic efficiency is shown in a more macroscopic level. However, when the particle size of the silicon-based material is in the nanometer level, the crystallization performance of the silicon-based material on the XRD spectrum is poor, and meanwhile, the volume change of the silicon-carbon composite material is reduced by the nanometer level silicon-based material. Therefore, the relationship among the peak positions of the XRD patterns effectively balances the relationship among the silicon-based material in the silicon-carbon composite material in terms of particle size, crystallinity, distribution uniformity and the like.

In conclusion, the dispersion distribution uniformity of the silicon-based material, the crystallization performance of the granular silicon-based material and the defects in the silicon-based material are practically limited by limiting the XRD peak position, and the first effect of the material can be regulated and controlled on the premise of obviously inhibiting the lithium intercalation expansion of the silicon-based material by balancing all the formulas, so that the electrochemical performance in the aspects of circulation and the like is finally improved.

(2) Compared with the traditional silicon-carbon (Si abrasive particles produced by Samsung and other enterprises are degraded, and are 20nm thin, 100nm long and sheet-shaped), in the silicon-carbon composite material provided by the invention, the silicon-based material is uniformly dispersed in the porous carbon skeleton, so that the dispersion degree of the silicon-based material is improved to be maximized, the stress distribution of the silicon-carbon composite material is maximized by the lithium intercalation-deintercalation expansion of the silicon-based material, and the impact of the expansion of the silicon-based material in a lithium ion battery on a cell structure is minimized. In addition, the dispersion distribution mode can obviously promote the synergistic effect between the silicon-based material and the porous carbon skeleton and promote the electronic conductivity and the ionic conductivity of the silicon-carbon composite material. And finally, the cycle performance and the multiplying power performance of the obtained silicon-carbon composite material are improved. Due to the structural design, the silicon-carbon composite material provided by the invention can be used in lithium ion batteries, and the gram specific capacity of the silicon-carbon composite material is obviously higher than that of graphite, so that the energy density of the lithium ion batteries can be obviously improved. Specific:

Taking the volume expansion noninductivity of the battery cell as a requirement, in the prior art, the highest gram specific capacity of the silicon-based material and the graphite composite material is about 400mAh/g; after the silicon-carbon composite material provided by the invention is compounded with graphite, the capacity can be improved to be more than 500mAh/g, even 800mAh/g; compared with the traditional graphite cathode, the energy density of the battery core made of the silicon-carbon composite material can be improved by 20%, and the endurance mileage of the power battery is obviously improved.

(3) The silicon-carbon composite material provided by the invention is provided with the shell, so that other gases such as hydrogen and the like which are unfavorable for the production of the cell process can be protected after the silicon-carbon composite material is stirred in the water-based binder and eroded and oxidized in the pulping process in the cell preparation process.

According to some embodiments of the invention, the porous carbon skeleton has a pore size in the range of 0.33-25 nm.

According to some embodiments of the invention, the porous carbon skeleton has a pore volume of 0.45 to 0.85cc/g. For example, the ratio may be specifically 0.65 to 0.75cc/g.

According to some embodiments of the invention, the porous carbon skeleton has a specific surface area of 1000-2300 cm ²/g.

The porous carbon skeleton with the limited range can effectively contain the silicon-based material, and the parameters of the porous carbon skeleton have a limiting effect on the particle size of the silicon-based material because the silicon-based material is mainly deposited in the pores of the porous carbon skeleton, so that the dispersion uniformity degree of the silicon-based material is improved, and the volume change of the silicon-carbon composite material in the lithium removal process can be effectively inhibited.

According to some embodiments of the invention, the material of the shell comprises at least one of a carbon-based material, a metal oxide, a fast ion conductor, or an organic polymer. Therefore, the silicon-carbon composite material can be protected from being corroded in the pulping process and the working process; and the gas production phenomenon of the battery cell is reduced.

According to some embodiments of the invention, a=θ' _40～60-θ_40～60 ranges from 6.1 ° to 8.2 °.

According to some embodiments of the invention, b=θ' _20～30-θ_20～30 has a value in the range of 5.6 to 9.9 °.

According to some embodiments of the invention, I _D/I_G = M of the porous carbon skeleton, I _D/I_G = N, C = M-N-1 of the silicon carbon composite, and 3% < C <43%. I _D/I_G reflects the graphitization degree of the carbon material, and the silicon-based material is deposited in the porous carbon skeleton to impact the porous carbon skeleton and form Si-C bonds in the porous carbon skeleton; thereby reducing the graphitization degree of the porous carbon skeleton. However, because of the shorter bond length of the Si-C bond, the graphitization degree of the end face of the carbon component is integrated, resulting in a decrease in the C value; the effective bonding of the carbon component and the silicon component boundary can reduce the transmission resistance of ions in two phases, so that the lithium ions are transmitted between two-dimensional layered lattices of the carbon component and are more efficiently intercalated between lattices of the silicon component, the lithium intercalation dynamics of the material is improved, and the first effect is improved. Accordingly, the present invention limits the C value to the above range, and in fact, limits the reduction in graphitization degree of the porous carbon skeleton and the proportion of interaction with the silicon-based material. Within this ratio range, a good first effect can be obtained.

According to some embodiments of the invention, the value of c=m-N-1 ranges from 7% to 30%.

According to some embodiments of the invention, the silicon element in the silicon-carbon composite material is 35% -65% by mass.

According to some embodiments of the invention, the silicon-carbon composite material comprises 35% -65% of carbon element by mass.

According to some embodiments of the invention, the sum of the mass percentages of the silicon element and the carbon element in the silicon-carbon composite material is more than or equal to 95%.

Since elemental silicon is the primary source of lithium intercalation capacity, if the proportion of elemental silicon is low, the gram specific capacity of the silicon-carbon composite material is significantly reduced; in addition, in the silicon-carbon composite material, if the proportion of the silicon-based material is small, the residual pore structure is large (the specific surface area is large) in the porous carbon skeleton, so that the proportion of active lithium consumed for forming the SEI film is high, and the initial effect is possibly reduced, but at the same time, the space capable of accommodating the volume change of the silicon-based material in the porous carbon skeleton is large, so that the whole volume change proportion of the silicon-carbon composite material is small, and the cycle performance is excellent.

Therefore, the mass percent of the silicon element and the carbon element is limited in the range, and the capacity, the first effect, the cycle performance and the like of the obtained silicon-carbon anode material can be effectively balanced.

According to some embodiments of the invention, 6.3 μm < D _V50 <11 μm of the silicon-carbon composite.

In the silicon carbon composite material, 0.75 μm < D _n10 <2.3 μm.

In the silicon carbon composite material, 2.5 μm < D _V10 <5.9 μm.

In the silicon carbon composite material, 11.4 μm < D _V90 <20 μm.

In the silicon carbon composite material, 21.3 μm < D _V99 <35 μm.

In the silicon carbon composite material, 35 μm < D _VMAX <100 μm.

Regarding the above symbols, the following are exemplified: d _V50 is a particle size value below which 50% by volume of the small particles have a particle size. D _n50 is a particle size value, and 50% of the small particles have a particle size smaller than the particle size value.

The particle size of the silicon-carbon composite material can influence the compaction density and the like of the silicon-carbon composite material to a certain extent, and in the range, higher compaction density can be obtained, and finally, the energy density and the rate capability of the negative electrode comprising the silicon-carbon negative electrode material are obviously improved. And because the thickness of the shell is relatively thin, the particle size of the silicon-carbon composite material is similar (slightly larger) to the particle size of the porous carbon skeleton.

According to some embodiments of the invention, the first-effect FCE _2.0V has a value ranging from 86.5 to 94.5% when the cut-off voltage is 2.0V in the buckling process.

According to some embodiments of the invention, the first-effect FCE _0.8V has a value ranging from 78.5 to 84.5% when the cut-off voltage is 0.8V in the buckling process.

According to an embodiment of the second aspect of the present invention, there is provided a method for preparing the silicon-carbon composite material, the method comprising the steps of:

s1, cracking a carbon raw material to obtain a porous carbon skeleton;

s2, depositing the silicon-based material in the porous carbon skeleton by adopting a chemical vapor deposition method to obtain a core;

S3, wrapping a shell on the surface of the core.

The preparation method provided by the invention has at least the following beneficial effects:

(1) The invention adopts the cracking carbon as the porous carbon skeleton, and has the advantages of wide sources of raw materials, low cost, good isotropy, controllable performance, easy modification of pore structure and better distribution uniformity and dispersivity of silicon-based materials.

Furthermore, the porous carbon skeleton adopted by the invention has good matching performance with the grain diameter and crystalline structure which can be achieved by the silicon-based material, the regulation and control are carried out on the space provided by the porous carbon skeleton for the silicon-based material and the mechanical strength of the porous carbon skeleton, the volume impact of the silicon-carbon composite material caused by the volume change brought by the lithium intercalation of the silicon-based material in the silicon-carbon composite material is maximally inhibited, and the initial coulomb efficiency of the silicon-carbon composite material is continuously improved at the same time: finally, a large amount of application and introduction of the silicon-based material in the cell cathode are realized.

(2) The invention adopts a deposition method to produce the silicon-based material, is a bottom-up micro-nano particle construction method, has smaller particle size and is limited by the pore diameter of the porous carbon skeleton compared with the traditional top-down construction method such as grinding and the like, and can realize the maximum dispersion distribution of the silicon-based material in the porous carbon skeleton. Therefore, the stress impact of the volume change of the silicon-based material on the battery in the charging and discharging processes is further relieved. Specifically, compared with a top-down method, the silicon-carbon composite material prepared by the method can reduce the volume change ratio by 50%.

Furthermore, in the process of adopting a chemical vapor deposition method, part of the silicon-based material can also react with the porous carbon skeleton to generate Si-C bonds, so that the graphitization degree of the porous carbon skeleton is reduced, and the first effect of the obtained silicon-carbon composite material is improved.

According to some embodiments of the invention, in step S1, the carbon feedstock comprises at least one of a bio-based carbon feedstock and other organics.

According to some embodiments of the invention, the bio-based carbon feedstock comprises at least one of corn stover, wheat straw, rice straw, peanut straw, soybean straw, sorghum straw, cotton straw, reed straw, common sweetgum fruit, common flowery herb, sesame straw, wood flour, peanut shells, husks, rice husks, walnut shells, rubber shells, bagasse, red-surgical-residue, soybean residue, pine needles, pine cones, apple branches, pear branches, peach branches, eucalyptus branches, pig manure, cow manure, sheep manure, chicken manure, kitchen waste, apples, and bananas.

According to some embodiments of the invention, the other organic matter includes at least one of benzene ring, c=c, nitrogen atom, sulfur atom, heterocycle, and bridged ring.

According to some embodiments of the invention, the other organics include polymers of diphenic acid and melamine. In this way, in the cracking process, the diphenic acid and the melamine can undergo polymerization reaction first and then undergo carbon decomposition, so as to generate a porous carbon skeleton with a specific pore structure.

The polymerization is carried out in the presence of a free radical initiator. The free radical initiator includes AIBN (CAS: 78-67-1).

The mass ratio between the diphenic acid and the free radical initiator is 10-20:1. For example, it may be about 15:1.

The mass ratio between the melamine and the free radical initiator is 3-8:1. For example, it may be about 5:1. According to some embodiments of the invention, in step S1, the cleavage is performed in a protective atmosphere. The protective atmosphere includes at least one of nitrogen and argon.

According to some embodiments of the invention, in step S1, the lysing comprises a first stage constant temperature and a second stage constant temperature performed sequentially.

According to some embodiments of the invention, the first stage constant temperature is 250-350 ℃. For example, it may be about 300 ℃.

According to some embodiments of the invention, the first stage is thermostated for a period of time ranging from 1.5 to 2.5 hours. For example, it may be about 2 hours.

When the carbon raw material comprises other organic matters, the carbon raw material undergoes polymerization, crosslinking and other reactions in the constant temperature process of the first stage, so that the molecular weight is increased. Provides a foundation for the subsequent generation of a porous carbon skeleton with stable structure and uniform material.

According to some embodiments of the invention, the second stage constant temperature is 700-900 ℃. For example, the temperature may be specifically 800 to 850 ℃.

According to some embodiments of the invention, the second stage is thermostated for a period of 3 to 8 hours. For example, the time period may be about 4 to 5 hours.

According to some embodiments of the invention, in step S1, pore-forming is further included after the pyrolysis.

According to some embodiments of the invention, the method of pore-forming comprises at least one of a physical method and a chemical method.

The pore-forming is carried out in a physical activation device. The physical activation apparatus comprises a horizontal rotary kiln.

The pore-forming is carried out by adopting carbon dioxide and water vapor. The volume ratio of the carbon dioxide to the water vapor is 1:1.

The temperature of pore-forming is 700-1000 ℃. For example, it may be about 850 ℃.

Since the carbon-based material is deposited substantially in the pores inside the porous carbon skeleton without substantially affecting the particle size, the particle size of the silicon-carbon composite material is affected by the particle size of the porous carbon skeleton and the thickness of the shell. Therefore, in view of particle size, it is necessary to perform pulverization before and/or after the pore-forming to obtain a porous carbon skeleton having a suitable particle size.

According to some embodiments of the invention, in step S2, the temperature of the chemical vapor deposition method is 450-800 ℃. In the temperature range, the obtained silicon-based material and porous carbon material have sufficient physical and chemical effects, and the defect density and crystallization performance meet the requirements.

According to some embodiments of the invention, in step S2, the temperature of the chemical vapor deposition method is 475-700 ℃. For example, the temperature may be specifically 500 to 625 ℃. And more particularly may be about 600 c.

According to some embodiments of the invention, in step S2, the chemical vapor deposition process is performed at a constant temperature.

According to some embodiments of the invention, in step S2, the chemical vapor deposition method includes an a constant temperature section and a B constant temperature section, which are sequentially performed. The temperature difference between the constant temperature section A and the constant temperature section B is less than or equal to 50 ℃. For example, it may be about 25 ℃.

The change of the temperature in the constant temperature stage can obviously influence the existence state of the silicon-based material, and when the temperature difference between the two stages is large, a discontinuous interface is easy to generate, so that the electrochemical performance of the silicon-based material is influenced.

According to some embodiments of the invention, in step S2, the duration of the chemical vapor deposition method is 3h to 12h. The duration affects the mass ratio of silicon element in the silicon-carbon composite material to a certain extent.

According to some embodiments of the invention, in step S2, the duration of the chemical vapor deposition method is 4-10 hours. For example, about 4.5h, 5h, 6h, 7.5h or 8.5h may be mentioned.

According to some embodiments of the invention, in step S2, the mixed gas introduced in the chemical vapor deposition method includes a carrier gas and a silicon source.

According to some embodiments of the invention, the silicon source comprises 10-45% by volume of the mixed gas.

According to some embodiments of the invention, the silicon source comprises 20-40% by volume of the mixed gas. For example, the content may be specifically 25 to 35%.

According to some embodiments of the invention, the carrier gas comprises at least one of nitrogen and argon.

According to some embodiments of the invention, the silicon source is in a gas phase at 500 ℃. Thus, during the deposition process, the silicon source may be cleaved and then enter the porous carbon skeleton, or may be diffused into the carbon skeleton and then be cleaved.

According to some embodiments of the invention, the silicon source comprises at least one of silane and substituted silane.

According to some embodiments of the invention, the silane comprises at least one of SiH₄、Si₂H₆、Si₂H₄、Si₃H₈、Si₃H₆、Si₃H₄、Si₄H₁₀、Si₄H₈、Si₄H₆、Si₅H₁₂、Si₅H₁₀、Si₅H₈、Si₅H₆、Si₆H₁₄、Si₆H₁₂、Si₆H₁₀、Si₆H₈、Si₆H₆、Si₇H₁₆、Si₇H₁₄、Si₇H₁₂、Si₇H₁₀、Si₇H₈、Si₇H₆、Si₈H₁₈、Si₈H₁₆、Si₈H₁₄、Si₈H₁₂、Si₈H₁₀、Si₈H₈、Si₉H₂₀、Si₉H₁₈、Si₉H₁₆、Si₉H₁₄、Si₉H₁₂、Si₉H₁₀、Si₉H₈、Si₁₀H₂₂、Si₁₀H₂₀、Si₁₀H₁₈、Si₁₀H₁₆、Si₁₀H₁₄ and Si ₁₀H₁₀.

According to some embodiments of the invention, the substituted silane is obtained by substituting H of the silane with at least one of B, N, F, P, S and Cl.

Compared with the traditional silicon-oxygen material, the pre-lithiated silicon-oxygen material and the silicon-carbon material prepared by grinding, the silicon raw material adopted by the silicon-carbon anode material provided by the invention is a relatively upstream product in a silicon industry chain, and has obvious cost advantage in a calculation mode of unit capacity. Specifically, referring to the recent selling price of the silicon carbon of about 160w/ton, the silicon carbon negative electrode material provided by the invention can realize the advantage of 50% reduction of the cost of the silicon carbon negative electrode material on the premise of ensuring production profits.

According to some embodiments of the invention, in step S3, the method of wrapping is chemical vapor deposition.

According to some embodiments of the invention, the wrapped chemical vapor deposition employs a gas flow comprising a precursor of the shell.

According to some embodiments of the invention, the precursor of the shell comprises at least one of a carbon source, a metal oxide precursor, a fast ion conductor precursor, and an organic polymer precursor.

The carbon source includes a hydrocarbon. For example, at least one of an alkane, alkene, alkyne and aromatic hydrocarbon may be specifically mentioned. Further specifically acetylene is possible.

The flow rate of the carbon source is 1.5-7.5L/min. For example, it may be about 4L/min.

The temperature of the wrapped chemical vapor deposition is 350-750 ℃. For example, the temperature may be 550 to 600 ℃. This temperature can affect the crystallization properties of the resulting shell; within this range, the majority of the shell is amorphous carbon, thereby enhancing the wetting effect on the electrolyte.

And the chemical vapor deposition time of the package is 0.5-6 h. For example, it may be about 2 hours. The duration and the flow rate of the carbon source jointly influence the thickness of the shell, and the shell obtained in the range can not only provide a protection effect, but also avoid reducing the gram specific capacity of the silicon-carbon composite material, and further avoid reducing other performances such as first effect and the like.

According to an embodiment of the third aspect of the present invention, there is provided an application of the silicon-carbon composite material in a lithium ion battery anode active material.

The application adopts all the technical schemes of the silicon-carbon composite material of the embodiment, so that the silicon-carbon composite material has at least all the beneficial effects brought by the technical schemes of the embodiment. Namely, in the application process, the lithium ion battery anode active material has smaller volume change proportion, higher initial effect and better cycle performance.

According to an embodiment of the fourth aspect of the present invention, there is provided a lithium ion battery, wherein the preparation raw material of the lithium ion battery includes the silicon-carbon composite material.

The lithium ion battery adopts all the technical schemes of the silicon-carbon composite material of the embodiment, so that the lithium ion battery has at least all the beneficial effects brought by the technical schemes of the embodiment. Namely, the lithium ion battery has higher energy density, cycle performance, first effect and the like.

According to some embodiments of the invention, the lithium ion battery comprises at least one of a lithium ion half-cell and a lithium ion full-cell.

According to some embodiments of the invention, the lithium ion battery comprises a negative electrode.

The negative electrode comprises a negative electrode current collector and a negative electrode coating layer arranged on the surface of the negative electrode current collector, wherein the negative electrode coating layer comprises a negative electrode active material, and the negative electrode active material comprises the silicon-carbon composite material.

Unless otherwise specified, in the present invention:

The actual meaning that the peak positions are all the peak positions of 2θ, for example, "the peak position of the strongest diffraction peak within 20 ° to 30 ° is θ _20～30" means that when 2θ=20 ° to 30 °, the 2θ value of the diffraction peak with the highest intensity is θ _20～30.

I _D represents the peak intensity of the D peak with a peak position of about 1350cm ^-1 in the Raman spectrum; i _G represents the peak intensity of the G peak with a peak position of about 1600cm ^-1.

The expressions of constant temperature, etc. mean that the temperature floats within the range of ±5 ℃ of the set temperature, and this float is mainly derived from an error of the temperature controlling member of the heating apparatus, and is difficult to avoid.

The term "about" as used herein, unless otherwise specified, means that the tolerance is within + -2%, for example, about 100 is actually 100 + -2%. Times.100.

Unless otherwise specified, the term "between … …" in the present invention includes the present number, e.g. "between 2 and 3" includes the end values 2 and 3.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

Drawings

The foregoing and/or additional aspects and advantages of the invention will become apparent and may be better understood from the following description of embodiments taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic cross-sectional view of a silicon-carbon composite material according to example 1 of the present invention.

Fig. 2 is an SEM image of the silicon carbon composite material obtained in example 1 of the present invention.

Figure 3 is an XRD pattern of the silicon carbon composite obtained in example 1 of the present invention.

FIG. 4 is a Raman spectrum of the silicon carbon composite obtained in example 1 of the present invention.

Detailed Description

The conception and the technical effects produced by the present invention will be clearly and completely described in conjunction with the embodiments below to fully understand the objects, features and effects of the present invention. It is apparent that the described embodiments are only some embodiments of the present invention, but not all embodiments, and that other embodiments obtained by those skilled in the art without inventive effort are within the scope of the present invention based on the embodiments of the present invention.

In the description of the present invention, the descriptions of the terms "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiments or examples. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

Example 1

The silicon-carbon composite material is prepared by the method, which comprises the following specific steps:

D1. Carbon raw material: diphenic acid, melamine and AIBN free radical initiator according to the mass ratio of 3:1:0.2 and placed in a suitable heating vessel, mixed and polymerized at 300℃for 2h. Then inert gas (nitrogen) is introduced, and heating and cracking are continued for 5 hours at 850 ℃ to obtain the porous carbon skeleton precursor.

D2. and D1, primarily crushing the porous carbon skeleton precursor obtained in the step D1 into millimeter-grade particles (jaw crushing), and activating and pore-forming the particles at 850 ℃ in a physical method activation device (a horizontal rotary furnace) by using mixed gas (volume ratio of water vapor to carbon dioxide is 1:1). After pore formation, the total pore volume was 0.7cc/g.

D3. and D2, carrying out jet milling and classification on the material obtained in the step D2 to obtain the porous carbon skeleton. The particle size distribution of the porous carbon skeleton is as follows dv10=2.7 μm; dv50=7.9 μm; dv99 = 28 μm.

D4. And D3, placing the porous carbon skeleton obtained in the step D3 in a chemical vapor deposition chamber, taking SiH ₄ as a silicon source, taking N ₂ as a carrier gas, and carrying out silicon deposition for 6 hours at 600 ℃ by using 35% of SiH ₄ in the mixed gas to obtain the core.

D5. continuously placing the core obtained in the step D4 in a chemical vapor deposition chamber, introducing acetylene, and performing CVD coating, wherein the flow rate of the acetylene is 4L/min; the temperature of the CVD was 600 c and the deposition time was 2h.

The schematic structure of the silicon-carbon composite material obtained in this example is shown in fig. 1.

In order to verify the influence of the synthesis process conditions of the silicon-carbon composite material on the final physicochemical properties and electrochemical properties, examples 2-12 and comparative examples 1-4 are also designed. The specific differences from example 1 are:

example 2: in step D4, the silicon deposition time was 4.5h.

Example 3: in step D4, the silicon deposition time was 7.5h.

Example 4: in step D4, the temperature of the last 0.5h period of silicon deposition is lower than 25 ℃ for the main period.

Example 5: in step D4, the temperature of the last 0.5h period of silicon deposition is higher than 25 ℃ for the main period.

Example 6: in step D4, the silicon deposition temperature is 500 ℃.

Example 7: in step D4, the silicon deposition temperature is 625 ℃.

Example 8: in step D4, the SiH ₄ volume ratio was 25% and the deposition time was 8.4h.

Example 9: in step D4, during silicon deposition, the SiH ₄ volume ratio is 45%, and the deposition time is 4.7h.

Example 10: in step D4, the silicon deposition time is 3h.

Example 11: in step D4, the SiH ₄ volume ratio is 12% and the deposition time is 16h.

Example 12: in step D3, the jet milling time is doubled.

Comparative example 1: in step D4, the temperature of the last 0.5h period of silicon deposition is lower than 75 ℃ for the main period.

Comparative example 2: in step D4, the silicon deposition temperature is 850 ℃.

Comparative example 3: there is no step D5.

Comparative example 4: in step D2, the activation temperature was 450 ℃.

Test case

This example first uses a differential method to test the mass percent of silicon in the silicon-carbon composites obtained in the examples and comparative examples.

The XRD patterns and raman spectra of the silicon-carbon composites obtained in the examples and comparative examples, the porous carbon skeleton used (step D3 product) were also tested in this example, and it was confirmed that the value of A, B, C was calculated from the test results. Wherein XRD test uses Bruke D of instrument, slit width is 1.25nm, scanning angle scope: 5-90 degrees; scanning speed: 5 DEG/min. The model of the Raman spectrum is WItec brand-Alpha 300R; the scanning range is 200-2000 cm ^-1; the wavelength of the light source is 532nm; the sampling time of a single test point is 3s;10EA*10EA mapping taking the point 100EA to obtain 100 Raman spectral lines to be superimposed, deleting 10EA of lower and higher abnormal data respectively, and outputting average peak intensities of a D peak and a G peak.

The electrochemical properties of the silicon carbon composites obtained in the examples and comparative examples were also tested in this example. Specifically, a button cell is prepared, and a working electrode (negative electrode) of the button cell comprises a copper foil and a negative electrode coating coated on the copper foil; the preparation of the negative electrode coating comprises the steps of coating a copper foil with negative electrode slurry and drying, wherein the composition of the negative electrode slurry is as follows: 84% + CNTs slurry +10% PAA-Li (lithiated PAA binder) +5% sp; in the CNTs slurry, the solids are CNTs and CMC mixed according to the mass ratio of 4:6, wherein the mass of the solids accounts for 1% of the mass of all solids in the test electrode formula. The opposite electrode of the button cell is a lithium metal sheet.

The structure of the buckling electricity is as follows: 2430, and a spring sheet, foam nickel or a gasket is arranged between the test electrode (negative electrode) and the battery case to avoid breaking, so that the test result can be accurately reflected.

Electrolyte solution: the carbonate solution of 1M lithium hexafluorophosphate contains 15% FEC and 1-2% VC.

Test electrode coat weight: 7-8 mg/cm ²;

the testing process of the button cell comprises the following steps: after the assembly is completed, standing for 8-12 h;0.05C lithium is intercalated to 5mV and kept stand for 5min;50uA intercalates lithium to 5mV and stands for 5min;10 mu A lithium is intercalated to 5mV and kept stand for 5min; and (3) carrying out lithium removal at 0.05C to 2V (or 0.8V) standing for 5min. After that, the cycle was performed at a rate of 0.33C between 2V and 5 mV.

The test results in a percentage increase in thickness of the resulting silicon carbon composite material at a first effect of 2.0V or 0.8V and full lithium intercalation.

At the same time, the cycle number (compared with the first week of 0.33C rate) at which the gram specific capacity decays to 90% was tested at 0.33C charge-discharge rate (1C current of 1700 mAh/g).

The results of the above tests are shown in Table 1 and FIGS. 2 to 4.

Table 1 properties of the silicon carbon composite materials obtained in examples and comparative examples

In table 1, the pore volume is the pore volume of the porous carbon skeleton.

The above results illustrate:

the silicon-carbon composite material with the core-shell structure in the XRD parameter range required by the invention has excellent cycle performance, small volume expansion coefficient and high initial effect when being used as a cathode active material.

In the process of depositing silicon by a vapor deposition method, the deposition time firstly influences the deposition amount of the silicon-based material, and the silicon-based material is more prone to form silicon in a crystalline state along with the improvement of the deposition amount of the silicon-based material; i.e. the values of a and B become large; along with the improvement of crystallinity, the lithium intercalation process of the silicon-carbon composite material is smoother, the initial effect is higher, and meanwhile, the volume change proportion is improved and the cycle performance is reduced in the lithium intercalation process. Comparison of the results of comparative examples 1 to 3 and example 10 shows that: the A, B values of examples 1-3 and 10 are within the scope of the claimed invention, demonstrating that the degree of crystallinity of the silicon-based materials balances the relationship between the first effect and the cycling performance. However, in example 10, the values a and B are smaller than those in example 1, which means that the silicon-based material has poor crystallization property in the silicon-carbon composite material, so that the first effect is reduced to some extent; but cycle performance is significantly improved.

In the process of depositing silicon by a vapor deposition method, one section of constant temperature or two sections of constant temperature are adopted, and the constant temperature can influence the following parameters: the crystallization degree, deposition amount, uniformity degree and interaction with the porous carbon skeleton of silicon, specifically, the higher the temperature, the higher the crystallization degree and the interaction with the carbon skeleton, the uniformity is lowered to a certain extent, and the deposition amount of silicon tends to be raised first and then lowered (the more deposition amount, the easier the silicon in a crystalline state is generated). Further, in the case of using the same carbon skeleton, because the silicon-based material has a high lithium intercalation capacity, more silicon-based material content (more silicon deposition amount from the deposition section) inevitably results in greater expansion of the material for lithium intercalation, which in turn causes higher expansion of the pole pieces, which in turn will lead to continuous loss of electron and ion contact during the cycle, resulting in a decrease in cycle performance. But more silicon-based materials are deposited, holes of the carbon skeleton are more fully utilized, and the first effect of the materials is improved; meanwhile, the better the crystallization state, the higher the first effect of the material. In XRD, the increase of the A value and the B value indicates that the deposition amount of the silicon-based material is increased, or the deposition uniformity of the silicon-based material is improved, or the crystallization degree of the silicon-based material is improved; or a decrease in the number of Si-C bonds (decrease in the interaction with the porous carbon skeleton). Comparison of the results of examples 1, 4,5 and comparative example 1 shows that: the last 0.5h of deposition at a temperature lower than the temperature of the main period will result in lower crystallinity of the silicon component on the surface of the material, lower diffraction peaks of the silicon component in XRD, and less masking of the XRD diffraction peaks of the original carbon skeleton. The lower deposition temperature represents lower conversion rate of SiH ₄ pyrolysis deposition, which leads to that partial carbon skeleton on the surface of the material is not filled by silicon component and is fully crystallized, so that the first effect of the material is lower, the expansion is smaller, and the cycle performance is slightly excellent. The last 0.5h of deposition is at a temperature higher than the temperature of the main period, and vice versa.

Comparison of the results of examples 1, 6, 7 and comparative example 2 shows that: the lower overall temperature of the deposition section will result in lower crystallinity of all silicon components in the material, less pronounced peak intensity at the main peak around 28 deg., and less inclusion-like peaks that mask the graphite crystalline inclusion of the carbon skeleton as the matrix. While lower silicon crystallinity will result in lower overall first efficiency of the silicon component and the silicon carbon material. However, the low-crystallization silicon component has lower true density, and the lithium intercalation expansion is lower and the cycle performance is better under the condition of the same lithium intercalation amount. And the opposite performance difference is caused by the improvement of the crystallization degree of the silicon caused by the overall temperature increase of the deposition section. And the silane gas is easily decomposed violently due to the excessively high temperature, and is self-agglomerated into silicon particles, so that the single silicon particles can slightly improve the initial effect, but the existence of the single silicon particles inevitably causes the aggravation of electrolyte consumption and expansion, and the cycle life of the material is deteriorated. .

Comparison of the results of examples 1, 8, 9, 11 shows that: at equal silicon deposition levels, the volume fraction of the silicon source in the mixed atmosphere is mainly due to the thermal effect released by the decomposition of SiH ₄ during the deposition of silicon, while the lower volume fraction of SiH ₄ is mainly due to the formation of more Si-C bonds at the end faces of the carbon components caused by the uniform distribution and decomposition of SiH ₄ during the deposition of silicon: the thermal shock of the former results in a decrease in graphitization degree of the carbon component, while the latter results in a decrease in C value due to the shorter bond length of si—c bonds, integrating graphitization degree of the end face of the carbon component. More Si-C bonds indicate closer interface contact between the silicon component and the carbon component, so that the lithium intercalation dynamics of the material can be improved, and the first effect is improved. On the other hand, the higher volume fraction of SiH ₄ inevitably results in insufficient deposition (but still nano-sized), leaving partially unused regions in the carbon skeleton that can be used to mitigate the volumetric expansion of the lithium intercalation material, enhancing cycle performance.

Comparison of the results of example 1 and comparative example 3 shows that: if the comparative example of the D4 process is not performed, i.e., if the protective coating layer of the outer core layer is not performed, the silicon component of the outer core layer is easily contacted with the electrolyte to form a large amount of SEI films, the active lithium ion utilization rate is reduced, the initial efficiency is reduced, and the cycle performance is reduced. At the same time, because of the SEI, additional volume expansion is formed, which also leads to the above results.

Comparison of the results of example 1 and comparative example 4 shows that: if the pore is not fully activated and formed, under the condition of insufficient pore volume, depositing a silicon-based material with the same mass ratio into the carbon skeleton, so that a considerable part of silicon is deposited outside the carbon skeleton, wherein the material is effective at the moment, but the expansion of the outer layer pure silicon is not inhibited by the carbon skeleton, and the possibility of cracking of the coating layer and the protecting outer layer pure silicon and pure silicon components is caused; and the high activity of the lithium silicon alloy can lead to the generation of a large amount of SEI films, so that the silicon-carbon composite material is greatly expanded, and the cycle life of the silicon-carbon composite material is obviously deteriorated.

Comparison of the results of example 1 and example 12 shows that: the particle size of the carbon skeleton determines the particle size of the silicon-carbon composite material, if the carbon skeleton with too small particle size is used, more material surfaces are necessarily introduced to be exposed to electrolyte under the condition of the same use amount, the high specific surface area slightly reduces the initial effect of the material, and the expansion and cycle performance of the material are slightly deteriorated because a little more SEI is formed on the surfaces; but the performance still meets the use requirements.

In conclusion, the silicon-carbon composite material provided by the invention can obviously improve the electrochemical performance when being used as a cathode active material due to the structural design, the material composition and the crystallization state limited by XRD. And thus is expected to find wide application in energy storage batteries, power batteries and 3C batteries.

The embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge of one of ordinary skill in the art without departing from the spirit of the present invention. Furthermore, embodiments of the invention and features of the embodiments may be combined with each other without conflict.

Claims

1. A silicon-carbon composite material, characterized in that the silicon-carbon composite material has a core-shell structure in which:

The peak position of the XRD strongest diffraction peak of the porous carbon skeleton is theta _20~30 within 20-30 degrees, and the peak position of the XRD strongest diffraction peak is theta _40~60 within 40-60 degrees;

The peak position of the XRD strongest diffraction peak of the silicon-carbon composite material is theta '_20~30 within 20-30 degrees, and the peak position of the XRD strongest diffraction peak is theta' _40~60 within 40-60 degrees;

And the value range of B=θ' _20~30-θ_20~30 is 2.7-9.9 degrees; the value range of A=θ' _40~60-θ_40~60 is 4.3-10.3 degrees;

I _D/I_G = M of the porous carbon skeleton, I _D/I_G = N, C = M-N-1 of the silicon carbon composite, and 3% < C <43%;

the porous carbon skeleton is obtained by cracking a carbon raw material, and the carbon raw material is a polymer of diphenic acid and melamine.

2. The silicon-carbon composite material according to claim 1, wherein the mass percentage of silicon element in the silicon-carbon composite material is 35% -65%.

3. The silicon-carbon composite material according to claim 1, wherein the pore diameter of the porous carbon skeleton is in the range of 0.33-25 nm; and/or the pore volume of the porous carbon skeleton is 0.45-0.85 cc/g.

4. The silicon-carbon composite of claim 1, wherein 6.3 μιη < D _V50 <11 μιη.

5. The silicon-carbon composite according to any one of claims 1 to 4, wherein the shell of the silicon-carbon composite is made of at least one of a carbon-based material, a metal oxide, a fast ion conductor, or an organic polymer.

6. A method for preparing a silicon-carbon composite material as defined in any one of claims 1 to 5, comprising the steps of:

s1, cracking a carbon raw material to obtain a porous carbon skeleton;

S3, wrapping a shell on the surface of the core;

In step S2, in the chemical vapor deposition method, the introduced mixed gas includes a carrier gas and a silicon source; the silicon source accounts for 25-35% of the volume of the mixed gas.

7. The method according to claim 6, wherein in the step S2, the temperature of the chemical vapor deposition method is 450-800 ℃.

8. The method according to claim 7, wherein in step S2, the duration of the chemical vapor deposition method is 3-12 hours.

9. The method of preparing according to claim 6, wherein the precursor of the shell comprises at least one of a carbon source, a metal oxide precursor, a fast ion conductor precursor, and an organic polymer precursor.

10. Use of the silicon-carbon composite material according to any one of claims 1-5 in a negative electrode active material of a lithium ion battery.

11. A lithium ion battery, characterized in that the preparation raw material of the lithium ion battery comprises the silicon-carbon composite material according to any one of claims 1 to 5.