Silicon-carbon composite negative electrode material with index attenuation gradient silicon layer and preparation method and application of silicon-carbon composite negative electrode material

Abstract

The invention relates to the technical field of lithium battery negative electrode materials, in particular to a silicon-carbon composite negative electrode material with an exponential decay gradient silicon layer and a preparation method and application thereof.The silicon-carbon composite negative electrode material is formed by sequentially and alternately depositing multiple silicon layers and carbon layers on the inner wall of a porous carbon matrix pore channel from inside to outside, the thickness of the silicon layers meets the condition that i represents the number of layers, c represents the thickness of the first silicon layer, d represents an attenuation constant, i is larger than or equal to 2 and smaller than or equal to 10, c is 1-15 nm, d is 0.1-0.5, the thickness of the outermost silicon layer is smaller than 2.5 nm, and the thicknesses of the carbon layers are the same; the aperture of the silicon-carbon composite negative electrode material is satisfied; d0 is the initial aperture of the porous carbon matrix, N is the number of silicon layers, and TC is the thickness of the carbon layers. The pore diameter of the obtained product is gradually changed from inside to outside to realize a small orifice, so that the interface stability is favorably improved, the side reaction of electrolyte is reduced, and the first effect and cycle stability are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery anode material technology, and in particular to a silicon-carbon composite anode material with an exponentially decaying silicon layer, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries are widely used in portable electronic devices and electric vehicles due to their long lifespan, high energy conversion and storage efficiency, and environmental friendliness. However, with the pursuit of higher energy density in lithium-ion batteries, the development of next-generation high-performance anode materials has become a research hotspot. Silicon is considered one of the most promising next-generation anode materials due to its extremely high theoretical specific capacity. However, silicon undergoes significant volume changes during charge and discharge, leading to material pulverization, continuous growth and shedding of the solid electrolyte interfacial film, ultimately resulting in rapid capacity decay and poor cycle life, severely hindering its commercial application.

[0003] To overcome the above-mentioned shortcomings, existing technologies have nanoscaled silicon and combined it with porous carbon materials. Porous carbon materials are ideal substrates for loading nanoscale silicon due to their abundant pore structure, high specific surface area and good conductivity. The carbon framework can not only buffer the volume effect of silicon, but also provide a continuous electronic conduction path.

[0004] However, existing silicon-carbon composite materials still have some shortcomings. For example, simply filling porous carbon with silicon nanoparticles results in poor bonding between silicon and carbon. During cycling, the volume expansion of silicon particles can still cause stress concentration on the carbon framework, potentially leading to structural damage under long-term cycling. Furthermore, traditional uniform coating or filling methods fail to adequately consider the gradient changes in stress distribution during lithium ion diffusion and intercalation within the pores, resulting in uneven stress distribution within the material and affecting the long-term stability of the structure.

[0005] Therefore, developing a novel anode material that can actively regulate the internal microstructure of silicon-carbon composite materials and achieve optimized stress distribution is crucial for obtaining lithium-ion batteries with high specific capacity, high initial coulombic efficiency, and ultra-long cycle life. Summary of the Invention

[0006] Based on the background technology, this invention provides a silicon-carbon composite anode material with an exponentially decaying gradient silicon layer, its preparation method, and its application.

[0007] To achieve the above objectives, the main technical solutions adopted by the present invention are as follows.

[0008] Firstly, this invention proposes a silicon-carbon composite anode material with an exponentially decaying gradient silicon layer, comprising a porous carbon matrix with a mesoporous structure and large pore volume. Multiple silicon layers and multiple carbon layers are alternately deposited from the inside to the outside on the inner wall of the pores of the porous carbon matrix, and the thickness of each silicon layer decreases exponentially from the inside to the outside, while the thickness of each carbon layer is the same. The pore size of the silicon-carbon composite anode material gradually changes from the inside to the outside. The silicon layer thickness T Si,i Satisfying relation 1: Relation 1, i represents the layer number, which is numbered 1, 2, 3... from the center of the channel in order of distance. c represents the thickness of the first silicon layer, and d represents the attenuation constant. 2≤i≤10, c is 1-15nm, d is 0.1-0.5, and the thickness of the outermost silicon layer is less than 2.5nm. The pore size of the silicon-carbon composite anode material satisfies Equation 2: Relation 2, c and d have the same meaning as above, D0 represents the initial pore size of the porous carbon matrix, N represents the number of silicon layers, and T C The carbon layer thickness is represented by D0 and T. C All units are in nm.

[0009] This invention designs an alternating deposition structure of silicon and carbon layers on the inner wall of a porous carbon matrix. The thickness of the silicon layer decreases exponentially from the inside out, as shown in Equation 1. All carbon layers have the same thickness, which matches the diffusion gradient of lithium ions within the material, optimizes stress distribution, and achieves more uniform lithium ion insertion / extraction behavior. The gradient composite structure of silicon and carbon layers creates a buffer space at the nanoscale, effectively alleviating the volume change stress of silicon. Furthermore, the pore size satisfies the constraint of Equation 2, and its gradual change from the inside out results in small pores, which helps to exert a solvent sieving effect, effectively preventing solvent from entering the material and causing side reactions. This further improves interface stability, reduces electrolyte side reactions, increases first-time efficiency, and forms a more stable SEI. Compared with existing silicon-carbon anode materials, this invention, through a radially exponentially decreasing silicon layer structure with a thick inner layer and a thin outer layer, and a silicon layer thickness of less than 2.5 nm, achieves stress relief within the pores and interface stability, ultimately ensuring high first-time coulombic efficiency and cycle stability.

[0010] Furthermore, the thickness of the carbon layer is 1-10nm, for example, the specific thickness of each carbon layer can be 1nm, 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, or 10nm.

[0011] Furthermore, the number of silicon and carbon layers is 5, with the first silicon layer having a thickness of 3.5 nm, an attenuation constant d of 0.238, and a carbon layer having a thickness of 1 nm.

[0012] Secondly, this invention proposes a method for preparing a silicon-carbon composite anode material with an exponentially decaying silicon layer, comprising the following steps: S1. Provides a porous carbon matrix with mesoporous structure and macroporous volume; S2. Multiple silicon layers and multiple carbon layers are alternately deposited from the inside to the outside on the inner wall of the porous carbon matrix by chemical vapor deposition. S3, Annealing treatment.

[0013] It should be noted that the annealing treatment in step S3 is mainly a high-temperature heat treatment, that is, after the final carbon layer is deposited, it is kept at the carbon layer deposition temperature for a period of time, such as 60-180 minutes, to enhance the crystallinity of the deposited layer.

[0014] Furthermore, in step S1, the pore volume of the porous carbon matrix is ​​0.7-1.8 cm³. 3 / g, specific surface area of ​​900-3500m² 2 / g, with an average pore size of 0.7-15nm; preferably, the porous carbon matrix is ​​pretreated before use to ensure that the pores of the porous carbon matrix are unobstructed.

[0015] In this technical solution, the pretreatment can specifically be high-temperature heat treatment, which removes the pores of the porous carbon matrix to facilitate subsequent silicon and carbon layer deposition. High-temperature heat treatment is a conventional technique in this field and is not required by this invention. For example, the specific operation of high-temperature heat treatment can be: heating the porous carbon matrix to 800-1000℃ in an inert atmosphere and holding for 100-600 minutes to remove residual organic matter in the pores of the porous carbon matrix, thereby unblocking the pores.

[0016] In this invention, the silicon and carbon layers are deposited alternately, each through the control of the concentration, pressure, temperature and time of the reaction system. The dynamic and coordinated control of these four technical parameters helps to achieve the orderly construction of the silicon and carbon layers inside the pores, thereby forming a silicon-carbon composite anode material with a clear structural gradient and stable interface.

[0017] Furthermore, this invention uses silicon source gas as a silicon-containing precursor. A silicon layer is obtained by introducing silicon source gas and controlling its concentration, deposition pressure, deposition temperature, and deposition time. The specific method for controlling the silicon source gas concentration is as follows: the silicon source gas concentration required for depositing a silicon layer with a thickness of 1-15 nm is generally 2.85 vol%-42.7 vol%. The required silicon source gas concentration for deposition is determined empirically based on the thickness of the first silicon layer. Combined with a defined attenuation constant d, the silicon source gas concentration for depositing the i-th silicon layer is calculated using Equation 3: Equation 3. A1 represents the silicon source gas concentration during the deposition of the first silicon layer; the specific method for controlling the deposition pressure is as follows: the deposition pressure of the inner silicon layer is 1-120 kPa, the deposition pressure of the intermediate silicon layer decreases from the inside to the outside compared to the deposition pressure of the inner silicon layer, and the deposition pressure of the outer silicon layer returns to the deposition pressure of the inner silicon layer; the specific method for controlling the deposition temperature is as follows: the deposition temperature of the inner silicon layer is 350-600℃, and the deposition temperature of the remaining silicon layers increases from the inside to the outside compared to the deposition temperature of the inner silicon layer; the specific method for controlling the deposition time is as follows: the deposition time for each silicon layer is 50-250 min. Preferably, the silicon source gas includes one or more of silane, disilane, monochlorosilane, dichlorosilane, trichlorosilane, and methylsilane.

[0018] In this technical solution, the thickness of the silicon layer is essentially determined by the concentration of silicon source gas in the silicon precursor. The silicon source gas concentration corresponding to the deposition of each silicon layer is obtained through Equation 3, and this calculated value guides and controls the amount of silicon source gas introduced.

[0019] In this technical solution, the silicon layer achieves gradient growth with exponentially decreasing thickness from the inside of the channel to the orifice by dynamically and synergistically controlling the silicon source gas concentration, deposition pressure, deposition temperature, and deposition time. The deposition pressure is specifically controlled by employing a pulsed decompression method to regulate the total pressure of the reaction system during silicon layer deposition, and the deposition temperature can be controlled to regulate the deposition rate of the silicon layer. In the initial stage of deposition, due to the unobstructed pores and small deposition volume, the kinetics of silicon source gas molecules diffusing into the pores are excellent. At this time, the concentration of silicon source gas is mainly controlled to enhance the diffusion driving force, prompting it to migrate and deposit deeper into the pores. As deposition progresses, the pore size gradually shrinks, and the diffusion resistance of silicon source gas molecules increases. At this time, pressure needs to be introduced for control to provide a stronger mass transfer driving force, ensuring that silicon source gas molecules continue to penetrate into the pores and deposit uniformly. Towards the end of the deposition process, the deposition space on the inner wall of the pores is almost filled. In order to further precisely control the surface structure of the silicon layer, temperature-assisted control is used to induce silicon source gas molecules to quickly complete the final deposition at the pore opening and near the surface. Under the dynamic synergistic control of silicon source gas concentration, deposition pressure, deposition temperature, and deposition time, a gradient structure with exponentially decreasing thickness of the silicon layer from the inside of the pores to the pore opening is constructed.

[0020] Furthermore, the specific control methods for deposition pressure and deposition temperature when depositing 5 silicon layers are as follows: The deposition pressure of the first and second silicon layers is 80-120 kPa; the deposition pressure of the third and fourth silicon layers is reduced by 40-60 kPa compared to the first and second silicon layers; and the deposition pressure of the fifth silicon layer is restored to 80-120 kPa. The deposition temperature of the first silicon layer is 400-600℃; the deposition temperature of the second silicon layer is 20-40℃ higher than that of the first silicon layer; the deposition temperature of the third silicon layer is 40-60℃ higher than that of the second silicon layer; the deposition temperature of the fourth silicon layer is 60-80℃ higher than that of the third silicon layer; and the deposition temperature of the fifth silicon layer is 80-100℃ higher than that of the fourth silicon layer.

[0021] Furthermore, this invention uses a carbon source gas as a carbon-containing precursor, and obtains the carbon layer by introducing the carbon source gas and controlling its concentration, deposition pressure, deposition temperature, and deposition time. During carbon layer deposition, the amount of carbon source gas introduced is controlled according to the designed thickness of the carbon layer, and the deposition pressure is controlled at 1-120 kPa, the deposition temperature at 600-700℃, and the deposition time at 30-60 min. Preferably, the carbon source gas includes one or more of methane, acetylene, ethylene, ethane, propane, propylene, isobutylene, benzene, toluene, and xylene.

[0022] In this technical solution, the carbon layer thickness is also controlled through dynamic and coordinated regulation of reaction system concentration, deposition pressure, deposition temperature, and deposition time. Since the thickness of each carbon layer is consistent, the deposition pressure, deposition temperature, and deposition time are kept consistent for each carbon layer during deposition.

[0023] Furthermore, each deposition of one silicon layer and one carbon layer is called a deposition cycle, and the number of deposition cycles is 2-10.

[0024] Based on the design of a multi-level gradient structure for silicon-carbon composite anode materials, this invention achieves the controllable preparation of a multi-level gradient structure by dynamically and synergistically controlling the concentration of the reaction system, deposition pressure, deposition temperature, and deposition time, thereby realizing the alternating deposition of silicon layers with exponential decay gradients and carbon layers of uniform thickness in the pores from deep to shallow.

[0025] The preparation method works synergistically with the silicon-carbon composite anode material, resulting in a silicon-carbon composite anode material with an exponentially decaying silicon layer exhibiting excellent comprehensive electrochemical performance. Its advantages in energy density, cycle life, and safety are significantly superior to those of traditional silicon-carbon anode materials.

[0026] Thirdly, the present invention proposes a lithium-ion battery in which the negative electrode active material is either the silicon-carbon composite negative electrode material with an exponentially decaying silicon layer as described above, or the silicon-carbon composite negative electrode material with an exponentially decaying silicon layer prepared according to the above preparation method. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 Nitrogen (77K) desorption curves of the porous carbon matrix and the silicon-carbon composite anode material with an exponentially decaying silicon layer in Example 1 of this application; Figure 2 This is a cross-sectional transmission electron microscope (TEM) image of the silicon-carbon composite anode material with an exponentially decaying silicon layer in Example 1 of this application; Figure 3 This is a charge-discharge diagram of a coin cell composed of a silicon-carbon composite anode material with an exponentially decaying silicon layer, as shown in Example 1 of this application. Figure 4 This is a cycle diagram of a 2.5Ah pouch cell composed of a silicon-carbon composite anode material with an exponentially decaying silicon layer, as described in Example 1 of this application. Detailed Implementation

[0029] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0030] All chemical raw materials used in the following examples and comparative examples are commercially available, and all apparatus and operations involved are conventional in the art.

[0031] In the following examples and comparative examples, silane (SiH4) was used as the silicon source gas and propylene (C2H4) was used as the carbon source gas.

[0032] Example 1

[0033] Silicon-carbon composite materials with exponentially decaying silicon layers were prepared according to the following procedure: S1, Providing a porous carbon matrix with mesoporous structure and macropore volume The porous carbon matrix has a pore volume of 1.0 cm³.3 / g, specific surface area is 1200m² 2 / g, with an average pore size of 4nm, and has undergone high-temperature heat treatment, resulting in unobstructed pores; S2. Five silicon layers and five carbon layers are alternately deposited from the inside to the outside on the inner wall of the porous carbon matrix by chemical vapor deposition. The first silicon layer is designed to have a thickness of 3.5 nm and an attenuation constant d = 0.238. The carbon layer thickness T is also designed. C The density is 1 nm; a porous carbon matrix is ​​placed in a CVD reactor, and the system concentration, deposition pressure, deposition temperature and deposition time are dynamically controlled to first deposit a silicon layer and then deposit a carbon layer, thus completing the alternating deposition of silicon and carbon layers; Specifically, by substituting the thickness of the first silicon layer (3.5 nm) and the attenuation constant (d = 0.238) into Equation 1, the thicknesses of the second, third, fourth, and fifth silicon layers are calculated as shown in Table 1 below.

[0034]

[0035] The empirical concentration of silicon source gas for depositing a 3.5 nm thick silicon layer is 10 vol%. Substituting this concentration with the attenuation constant d = 0.238 into Equation 3, the silicon source gas concentrations for depositing the 2nd, 3rd, 4th, and 5th silicon layers are calculated to be 7.88 vol%, 6.21 vol%, 4.89 vol%, and 3.86 vol%, respectively. These calculated values ​​are used to control the silicon source gas concentration during the deposition of the corresponding silicon layers. For carbon layer deposition, the carbon source gas concentration is kept constant at 5 vol%. Note: All volume concentrations mentioned above are diluted with argon gas. During silicon deposition, the pressure control operation is as follows: pulse decompression technology is used to maintain the reaction system pressure at 100 kPa during the deposition of the first and second silicon layers, the pressure is reduced by 50 kPa during the deposition of the third and fourth silicon layers, and the pressure is restored to 100 kPa during the deposition of the fifth silicon layer; during carbon deposition, the reaction system pressure is maintained at 100 kPa. The temperature control during silicon deposition is as follows: 500℃ for the first silicon layer, 530℃ for the second silicon layer, 580℃ for the third silicon layer, 650℃ for the fourth silicon layer, and 740℃ for the fifth silicon layer; the temperature is maintained at 650℃ during carbon deposition. The deposition time for each silicon layer was controlled at 200 min, and the deposition time for each carbon layer was controlled at 45 min. S3, Annealing treatment Continue to maintain at 650°C for 120 minutes in argon atmosphere.

[0036] like Figure 1As shown: The pore volume of the porous carbon matrix is ​​1.0 cm³. 3 / g, specific surface area is 1200 m² 2 / g, with an average pore size of 4nm; after alternating deposition of silicon and carbon layers, the pore volume of the final product decreased to 0.0084cm³. 3 / g, the specific surface area decreased to 3.3m². 2 / g indicates that the channels have been fully filled. Furthermore, as... Figure 2 As shown: The cross-section of the final product obtained by FIB cutting was obtained. TEM images show that a gradient multilayer structure has been successfully constructed within the channels. Further measurements show that the thickness of the innermost layer, i.e., the first silicon layer, is approximately 3.5 nm, and the thickness of the outermost layer, i.e., the fifth silicon layer, is approximately 1.4 nm, meeting the design requirements.

[0037] like Figure 3 As shown: A negative electrode sheet was prepared using this final product as the negative electrode active material, with lithium metal as the counter electrode. The electrolyte was 1 M lithium hexafluorophosphate (the solvent system was a mixture of ethylene carbonate and diethyl carbonate in a 1:1 volume ratio, with 10 vol% fluoroethylene carbonate and 1 vol% vinylene carbonate added as additives). A coin cell was assembled using the above configuration, and the initial coulombic efficiency of the coin cell was 94.5%. Figure 4 As shown: Using this final product as the negative electrode active material to make a negative electrode sheet, and using NCM811 as the positive electrode, a 2.5Ah pouch cell was assembled. After being charged to 4.2V with a current of 6A and then discharged to 2.5V with a current of 20A, it still retained 80% of its capacity after 1000 cycles. The final product obtained in Example 1 exhibits excellent initial coulombic efficiency and capacity retention.

[0038] Structural verification: Using c=3.5nm, d=0.238, D0=4nm, T... C Substituting =1nm and N=5 into Equation 2, the result of the parameter part is -24.1, which satisfies Equation 2.

[0039] Example 2

[0040] Silicon-carbon composite materials with exponentially decaying silicon layers were prepared according to the following procedure: S1, Providing a porous carbon matrix with mesoporous structure and macropore volume The porous carbon matrix has a pore volume of 1.8 cm³. 3 / g, specific surface area of ​​2500m² 2 / g, with an average pore size of 15nm, and has undergone high-temperature heat treatment, resulting in unobstructed pores; S2. Eight silicon layers and eight carbon layers are alternately deposited from the inside to the outside on the inner wall of the porous carbon matrix by chemical vapor deposition. The first silicon layer is designed with a thickness c of 12 nm and an attenuation coefficient d of 0.3. Using Equation 1, the thickness of the eighth silicon layer is calculated to be 1.45 nm. The carbon layer thickness T is also designed. C The thickness is 8nm; a porous carbon matrix is ​​placed in a CVD reactor, and the system concentration, deposition pressure, deposition temperature and deposition time are dynamically controlled to first deposit a silicon layer and then deposit a carbon layer, thus completing the alternating deposition of silicon and carbon layers; During silicon layer deposition, the concentration control was as follows: the silicon source gas concentration decreased exponentially layer by layer; the silicon source gas concentration was 34.2 vol% during the deposition of the first layer, and then the silicon source gas concentration during the deposition of the remaining silicon layers was controlled according to Equation 3; during carbon layer deposition, the carbon source gas concentration remained constant at 40 vol%. During silicon deposition, the pressure control was as follows: a stepped pressure reduction technique was used, with the reaction system pressure maintained at 100 kPa during the deposition of silicon layers 1-3, at 30 kPa during the deposition of silicon layers 4-6, and the pressure restored to 100 kPa during the deposition of silicon layers 7-8; during carbon deposition, the reaction system pressure was maintained at 100 kPa. The temperature control during silicon deposition is as follows: 500℃ for the first silicon layer, 503℃ for the second, 508℃ for the third, 516℃ for the fourth, 541℃ for the fifth, 561℃ for the sixth, 586℃ for the seventh, and 616℃ for the eighth. The temperature is maintained at 650℃ during carbon deposition. The deposition time for each silicon layer was controlled at 200 min, and the deposition time for each carbon layer was controlled at 45 min. S3, Annealing treatment Continue to maintain at 650°C for 120 minutes in argon atmosphere.

[0041] Example 3

[0042] Silicon-carbon composite materials with exponentially decaying silicon layers were prepared according to the following procedure: S1, Providing a porous carbon matrix with mesoporous structure and macropore volume The porous carbon matrix has a pore volume of 1.0 cm³. 3 / g, specific surface area is 1200m² 2 / g, with an average pore size of 8nm, and has undergone high-temperature heat treatment, resulting in unobstructed pores; S2. Four silicon layers and four carbon layers are alternately deposited from the inside to the outside on the inner wall of the porous carbon matrix by chemical vapor deposition. The first silicon layer is designed to have a thickness of 8 nm and an attenuation coefficient d of 0.5. Using Equation 1, the thickness of the eighth silicon layer is calculated to be 1.79 nm. The carbon layer thickness T is designed. C The thickness is 4nm; the porous carbon matrix is ​​placed in a CVD reactor, and the system concentration, deposition pressure, deposition temperature and deposition time are dynamically controlled to first deposit a silicon layer and then deposit a carbon layer, thus completing the alternating deposition of silicon and carbon layers; During silicon deposition, the concentration control was as follows: the silicon source gas concentration was increased exponentially from 22.8 vol% for the first layer, and then controlled according to Equation 3 for the deposition of the remaining silicon layers; during carbon deposition, the carbon source gas concentration was kept constant at 20 vol%. The pressure control during silicon deposition is as follows: the reaction system pressure is 80 kPa when depositing the first silicon layer, 65 kPa when depositing the second silicon layer, 40 kPa when depositing the third silicon layer, and the pressure returns to 80 kPa when depositing the fourth silicon layer; the reaction system pressure is maintained at 80 kPa when depositing the carbon layer. The temperature control during silicon deposition is as follows: 500℃ for the first silicon layer, 515℃ for the second silicon layer, 540℃ for the third silicon layer, and 570℃ for the fourth silicon layer; the temperature is maintained at 650℃ during carbon deposition. The deposition time for each silicon layer was controlled at 200 min, and the deposition time for each carbon layer was controlled at 45 min. S3, Annealing treatment Continue to maintain at 650°C for 120 minutes in argon atmosphere.

[0043] Example 4

[0044] Compared with Example 1, in step S2, the attenuation constant d was adjusted from 0.238 to 0.1, the thickness of the first silicon layer remained 3.5 nm, and the silicon source gas concentration during the deposition of the first silicon layer remained 10 vol%. The thickness of the fifth silicon layer was calculated to be 2.35 nm using Equation 1, and the silicon source gas concentration during the deposition of each silicon layer was controlled using Equation 3. All other aspects remained the same as in Example 1.

[0045] Example 5

[0046] Using the same porous carbon matrix as in Example 1, eight silicon layers and eight carbon layers were alternately deposited from the inside out via chemical vapor deposition. The thickness of the first silicon layer was designed to be 3.5 nm, with an attenuation constant d of 0.136. The thickness of the eighth silicon layer was calculated to be 1.35 nm, and the carbon layer thickness T was designed. CThe silicon source gas concentration was 10 vol% during the deposition of the first silicon layer, and the concentrations involved in the remaining layers were controlled according to Equation 3. The reaction system pressure was maintained at 100 kPa during the deposition of the first and second silicon layers, at 80 kPa during the deposition of the third and fourth silicon layers, at 60 kPa during the deposition of the fifth and sixth silicon layers, at 40 kPa during the deposition of the seventh silicon layer, and at 100 kPa during the deposition of the eighth silicon layer. The temperature was 400°C during the deposition of the first silicon layer, and then increased by 30°C for each subsequent layer. The carbon layer thickness, carbon layer deposition operation, and other parameters not mentioned were consistent with those in Example 1.

[0047] Example 6

[0048] Compared with Example 1, the thickness of the first silicon layer was adjusted to 15nm, the attenuation constant d was 0.5, and the thickness of the fifth silicon layer was calculated to be 2.03nm; the silicon source gas concentration during the deposition of the first silicon layer was 42.7 vol%, and the silicon source gas concentration of the remaining layers was controlled according to the law shown in Equation 3; the rest were consistent with Example 1.

[0049] Example 7

[0050] Using the same porous carbon matrix as in Example 1, two silicon layers and two carbon layers were alternately deposited from the inside out by chemical vapor deposition. The thickness of the first silicon layer was designed to be 1 nm and the attenuation constant d was 0.2. The thickness of the second silicon layer was calculated to be 0.82 nm. The silicon source gas concentration was 2.85 vol% when the first silicon layer was deposited and 2.33 vol% when the second silicon layer was deposited. The reaction system pressure was maintained at 120 kPa when the first silicon layer was deposited and at 90 kPa when the second silicon layer was deposited. The temperature was 450 °C when the first silicon layer was deposited and at 500 °C when the second silicon layer was deposited. The carbon layer thickness, carbon layer deposition operation, and other unmentioned parameters were consistent with those in Example 1.

[0051] This embodiment constructs the smallest discrete unit of an exponentially decaying gradient silicon layer.

[0052] Example 8

[0053] Compared to Example 1, the number of deposition layers in step S2 was adjusted to 10 layers, i.e., 10 silicon layers and 10 carbon layers were deposited. The thickness of the first silicon layer was designed to be 5 nm, and the attenuation constant d was 0.145. According to Equation 1, the thickness of the tenth silicon layer was calculated to be 1.35 nm. During silicon layer deposition, the silicon source gas concentration decreased exponentially from 15.1 vol% in the first layer according to the law shown in Equation 3. The reaction system pressure was maintained at 110 kPa during the deposition of the first and second silicon layers, at 90 kPa during the deposition of the third and fourth silicon layers, at 60 kPa during the deposition of the fifth and sixth silicon layers, at 30 kPa during the deposition of the seventh silicon layer, and the reaction system pressure returned to 110 kPa during the deposition of the eighth silicon layer. The temperature was 480°C during the deposition of the first silicon layer, and then increased by 35°C for each subsequent layer. The carbon layer thickness, carbon layer deposition operation, and other unmentioned parameters were consistent with those in Example 1.

[0054] This embodiment constructs the largest discrete unit of the exponentially decaying gradient silicon layer.

[0055] Example 9

[0056] Compared with Example 2, the thickness of the carbon layer was adjusted to 10 nm, the carbon source gas concentration was 50 vol% during carbon layer deposition, and the rest remained the same as in Example 2.

[0057] Example 10

[0058] Compared with Example 1, the technical parameters of deposition pressure and deposition temperature of silicon layer, deposition pressure of carbon layer, deposition temperature and deposition time have been adjusted. Specifically: (1) When depositing silicon layer, the pressure control operation is as follows: pulse decompression technology is used. When depositing the first and second silicon layers, the pressure of the reaction system is maintained at 80 kPa. When depositing the third and fourth silicon layers, the pressure is 40 kPa. When depositing the fifth silicon layer, the pressure is restored to 80 kPa. The temperature control operation is as follows: when depositing the first silicon layer, the temperature is 400℃. When depositing the second silicon layer, the temperature is 440℃. When depositing the third silicon layer, the temperature is 500℃. When depositing the fourth silicon layer, the temperature is 580℃. When depositing the fifth silicon layer, the temperature is 680℃. (2) When depositing carbon layer, the pressure of the reaction system is maintained at 80 kPa. The temperature is maintained at 600℃ and the time is 60 min. The rest are consistent with Example 1.

[0059] Example 11

[0060] Compared with Example 1, the technical parameters of deposition pressure and temperature of silicon layer, deposition pressure of carbon layer, deposition temperature and deposition time have been adjusted, specifically: (1) When depositing silicon layer, the pressure control operation is as follows: pulse decompression technology is used, the reaction system pressure is maintained at 120 kPa when depositing the first-second silicon layer, the pressure is 60 kPa when depositing the third-fourth silicon layer, and the pressure is restored to 120 kPa when depositing the fifth silicon layer; the temperature control operation is as follows: the temperature is 600℃ when depositing the first silicon layer, the temperature is 620℃ when depositing the second silicon layer, the temperature is 660℃ when depositing the third silicon layer, the temperature is 720℃ when depositing the fourth silicon layer, and the temperature is 800℃ when depositing the fifth silicon layer; (2) When depositing carbon layer, the reaction system pressure is maintained at 120 kPa, the temperature is maintained at 700℃, and the time is 30 min. The rest are consistent with Example 1.

[0061] Comparative Example 1 Compared with Example 1, the number of silicon and carbon layers remains the same, but the thickness of each silicon and carbon layer is 5 nm, and the deposition process is adjusted accordingly.

[0062] Comparative Example 2 Compared with Example 1, the silicon layer thickness in step S2 was changed from an exponential decrease to an exponential increase. That is, the thickness of the first silicon layer was designed to be 1.35 nm and the thickness of the fifth silicon layer was designed to be 3.5 nm. The deposition conditions were adjusted in the opposite direction accordingly.

[0063] Comparative Example 3 Compared to Example 1, the deposition of each silicon layer in step S2 was carried out under the same conditions: silicon source gas concentration of 5 vol%, deposition pressure of 101.3 kPa, and deposition temperature of 540°C. The deposition conditions for the carbon layer remained unchanged. All other conditions were consistent with those in Example 1.

[0064] Comparative Example 4 Compared to Example 1, the design and deposition conditions of the silicon layer in step S2 remain unchanged, and the deposition process of the carbon layer is adjusted so that the thickness of each carbon layer is 25 nm. Everything else is the same as in Example 1.

[0065] Comparative Example 5 Compared with Example 1, the porous carbon matrix in step S1 has been adjusted. The specific technical parameters of the porous carbon matrix used are as follows: pore volume 0.6 cm³. 3 / g, specific surface area 800m² 2 / g, average pore size 6nm; in step S2, the deposition process is adjusted so that the number of silicon layers is 3, the thickness of the first silicon layer is 5nm, the attenuation coefficient is 0.5, and the thickness of the carbon layer is adjusted from 1nm to 3nm. The rest are consistent with Example 1.

[0066] Comparative Example 6 Compared with Example 1, the porous carbon matrix in step S1 was adjusted to microporous activated carbon. The specific technical parameters of the microporous activated carbon are as follows: pore volume 1.5 cm³. 3 / g, specific surface area 3000m² 2 / g, average pore size 1.5nm, microporous activated carbon with a micropore content >90%. All other aspects are consistent with Example 1.

[0067] The products obtained from Examples 2-11 and Comparative Examples 1-6 were assembled into coin cells and 2.5Ah pouch cells using the same procedures as in Example 1. The initial coulombic efficiency of the coin cells and the capacity retention of the 2.5Ah pouch cells after 1000 cycles were tested under the same conditions as in Example 1. The test results are shown in Table 2.

[0068]

[0069] From Table 1, Table 2, Figures 1-4 As shown in the following: A comparison of the test results of Example 1 with Comparative Examples 1 and 2 shows that, under the same conditions of depositing 5 silicon layers and 5 carbon layers, the silicon-carbon composite anode material with a specific exponential decay gradient silicon layer prepared according to the method of this invention exhibits superior electrochemical performance compared to the two anode materials with equal thickness of silicon layers and an exponential increase in the thickness of each silicon layer from the inside out. When fabricated as an anode sheet, a coin cell assembled with lithium metal as the counter electrode and 1 M lithium hexafluorophosphate (a solvent system consisting of a 1:1 volume ratio of ethylene carbonate and diethyl carbonate, with 10 vol% fluoroethylene carbonate and 1 vol% vinylene carbonate as additives) as the electrolyte achieves an initial coulombic efficiency of up to 94.5%. A 2.5Ah soft-pack battery assembled with NCM811 as the positive electrode, charged to 4.2V at 6A and then discharged to 2.5V at 20A, still retains 80% of its capacity after 1000 cycles. The reason for this is that during the lithiation of the silicon-carbon materials obtained in Comparative Examples 1 and 2, the outer thick silicon layer first undergoes a huge volume expansion, rapidly blocking the pores and preventing the internal silicon from being utilized. At the same time, the huge stress generated by the volume expansion easily leads to structural cracking. In particular, Comparative Example 2 also causes the electrode material to pulverize and lose contact with the current collector, thereby causing rapid capacity decay and battery failure. In contrast, the "gradient silicon layer with exponentially decreasing thickness from the inside to the outside" of the present invention can achieve stress gradient release and ensure the structural integrity of the negative electrode material.

[0070] A comparison of the test results of Examples 1, 10, and 11 with Comparative Example 3 shows that obtaining the exponential decay gradient silicon layer depends on the dynamic synergistic process of silicon source gas concentration, deposition pressure, deposition temperature, and deposition time proposed in this invention. Otherwise, the ideal design product cannot be obtained, and the performance improvement of the final product is extremely limited, with unsatisfactory first-efficiency and capacity retention rates.

[0071] A comparison of the test results from Example 1 and Comparative Example 4 shows that, based on the exponentially decaying gradient design of the silicon layer, the thickness of the carbon layer should not be too thick. If the carbon layer is too thick, the pores will be completely blocked by the excessively thick carbon layer after the second or third deposition cycle, preventing further deposition and causing the gradient structure construction to fail. The material essentially becomes a thick carbon shell structure with only a small amount of silicon filling some of the pores. The resulting material exhibits degraded energy density and hindered lithium-ion transport. Controlling the carbon layer thickness to 1-10 nm is crucial for balancing the structural stability and electrochemical performance of the anode material.

[0072] A comparison of the test results of Example 1 with those of Comparative Examples 5 and 6 shows that the exponentially decaying gradient silicon layer structure proposed in this invention places certain requirements on the porous carbon substrate, limiting the pore volume of the porous carbon substrate to 0.7-1.8 cm³. 3 / g, specific surface area of ​​900-3500m² 2 / g, with an average pore size of 0.7-15nm. If the structural characteristics of the porous carbon volume do not meet the above requirements, the resulting final product will essentially become silicon-carbon core-shell particles or a simple mixture with the carbon matrix, or silicon deposition will mainly occur on the outer surface of the carbon matrix and a small number of large pore entrances to form surface-coated or agglomerated silicon particles, resulting in the inability to successfully construct the gradient structure within the pores.

[0073] In summary, the test results of Examples 1-11 show that the silicon-carbon composite anode materials with specific exponential decay gradient silicon layers prepared according to the preparation method proposed in this invention all have high initial coulombic efficiency and high capacity retention. This indicates that the design of the radial exponential decay structure of the silicon layer with a thick inner layer and a thin outer layer, and the outermost silicon layer thickness of less than 2.5 nm, as proposed in this invention, has indeed achieved stress relief and interface stability within the channels, ultimately ensuring high initial coulombic efficiency and cycle stability.

[0074] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make modifications, alterations, substitutions, and variations to the above embodiments within the scope of the present invention. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of the different embodiments or examples.

Claims

1. A silicon-carbon composite anode material with an exponentially decaying silicon layer, characterized in that: The material includes a porous carbon matrix with mesoporous structure and macroporous volume. Multiple silicon layers and multiple carbon layers are alternately deposited on the inner wall of the pores of the porous carbon matrix from the inside to the outside. The thickness of each silicon layer decreases exponentially from the inside to the outside, and the thickness of each carbon layer is the same. The pore size of the silicon-carbon composite anode material gradually changes from the inside to the outside. The silicon layer thickness T Si,i Satisfying relation 1: Relation 1, i represents the layer number, which is numbered 1, 2, 3... from the center of the channel in order of distance. c represents the thickness of the first silicon layer, and d represents the attenuation constant. 2≤i≤10, c is 1-15nm, d is 0.1-0.5, and the thickness of the outermost silicon layer is less than 2.5nm. The pore size of the silicon-carbon composite anode material satisfies Equation 2: Relation 2, c and d have the same meaning as above, D0 represents the initial pore size of the porous carbon matrix, N represents the number of silicon layers, and T C The carbon layer thickness is represented by D0 and T. C All units are in nm.

2. The silicon-carbon composite anode material with an exponentially decaying silicon layer according to claim 1, characterized in that: The thickness of the carbon layer is 1-10 nm.

3. The silicon-carbon composite anode material with an exponentially decaying silicon layer according to claim 1, characterized in that: The number of silicon and carbon layers is 5, and the thickness of the first silicon layer is 3.5 nm with an attenuation constant d of 0.238, and the thickness of the carbon layer is 1 nm.

4. A method for preparing a silicon-carbon composite anode material with an exponentially decaying silicon layer as described in claim 1, characterized in that: The following steps are included: S1. Provides a porous carbon matrix with mesoporous structure and macroporous volume; S2. Multiple silicon layers and multiple carbon layers are alternately deposited from the inside to the outside on the inner wall of the porous carbon matrix by chemical vapor deposition. S3, Annealing treatment.

5. The method for preparing the silicon-carbon composite anode material with an exponentially decaying silicon layer according to claim 4, characterized in that: In step S1, the pore volume of the porous carbon matrix is ​​0.7-1.8 cm³. 3 / g, specific surface area of ​​900-3500m² 2 / g, with an average pore size of 0.7-15nm; Preferably, the porous carbon matrix is ​​pretreated before use to ensure that the pores of the porous carbon matrix are unobstructed.

6. The method for preparing the silicon-carbon composite anode material with an exponentially decaying silicon layer according to claim 4, characterized in that: The silicon layer is deposited by introducing silicon source gas, and the silicon layer is obtained by controlling the silicon source gas concentration, deposition pressure, deposition temperature and deposition time. The specific method for controlling the silicon source gas concentration is as follows: The required silicon source gas concentration for deposition is determined based on the thickness of the first silicon layer. Combined with a defined attenuation constant d, the silicon source gas concentration for deposition of the i-th silicon layer is calculated using Equation 3: Equation 3. A1 represents the silicon source gas concentration during the deposition of the first silicon layer; The specific method for controlling the deposition pressure is as follows: the deposition pressure of the inner silicon layer is 1-120 kPa, the deposition pressure of the middle silicon layer decreases from the inside to the outside compared with the deposition pressure of the inner silicon layer, and the deposition pressure of the outer silicon layer is restored to the deposition pressure of the inner silicon layer. The specific method for controlling the deposition temperature is as follows: the deposition temperature of the inner silicon layer is 350-600℃, and the deposition temperature of the remaining silicon layers increases from the inside to the outside compared with the deposition temperature of the inner silicon layer. The specific method for controlling the deposition time is as follows: the deposition time for each silicon layer is 50-250 min; Preferably, the silicon source gas includes one or more of silane, disilane, monochlorosilane, dichlorosilane, trichlorosilane, and methylsilane.

7. The method for preparing the silicon-carbon composite anode material with an exponentially decaying silicon layer according to claim 6, characterized in that: The specific methods for controlling the deposition pressure and deposition temperature when depositing 5 silicon layers are as follows: The deposition pressure of the first and second silicon layers is 80-120 kPa. The deposition pressure of the third and fourth silicon layers is 40-60 kPa lower than that of the first and second silicon layers. The deposition pressure of the fifth silicon layer returns to 80-120 kPa. The deposition temperature of the first silicon layer is 400-600℃. The deposition temperature of the second silicon layer is 20-40℃ higher than that of the first silicon layer. The deposition temperature of the third silicon layer is 40-60℃ higher than that of the second silicon layer. The deposition temperature of the fourth silicon layer is 60-80℃ higher than that of the third silicon layer. The deposition temperature of the fifth silicon layer is 80-100℃ higher than that of the fourth silicon layer.

8. The method for preparing the silicon-carbon composite anode material with an exponentially decaying silicon layer according to claim 4, characterized in that: Carbon source gas is introduced to deposit the carbon layer. During the deposition of each carbon layer, the amount of carbon source gas introduced is controlled according to the designed thickness of each carbon layer, and the deposition pressure is controlled to be 1-120 kPa, the deposition temperature to be 600-700℃, and the deposition time to be 30-60 min. Preferably, the carbon source gas includes one or more of methane, acetylene, ethylene, ethane, propane, propylene, isobutylene, benzene, toluene, and xylene.

9. The method for preparing the silicon-carbon composite anode material with an exponentially decaying silicon layer according to claim 4, characterized in that: Each deposition of one silicon layer and one carbon layer is called a deposition cycle, and the number of deposition cycles is 2-10.

10. A lithium-ion battery, characterized in that, The negative electrode active material is a silicon-carbon composite negative electrode material with an exponentially decaying silicon layer as described in any one of claims 1-3, or a silicon-carbon composite negative electrode material with an exponentially decaying silicon layer prepared according to the preparation method described in any one of claims 4-9.

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