Silicon-carbon negative electrode material, secondary battery and electric equipment

Abstract

The invention provides a silicon-carbon negative electrode material, a secondary battery and electric equipment. The silicon-carbon negative electrode material comprises a porous carbon substrate; the silicon crystal is filled in at least part of pores of the porous carbon substrate; the carbon coating layer coats at least part of the surface of the porous carbon substrate and at least part of the surface of the silicon crystal; the grain size D1 of the silicon crystal at the diffraction angle of 28 degrees in an X-ray diffraction pattern is 1-5nm, and the grain size D2 of the silicon crystal at the diffraction angle of 47.5 degrees in the X-ray diffraction pattern is 0.5-3nm. Therefore, the anti-expansion performance and the electrical performance of the negative electrode material can be improved.

Description

Technical Field

[0001] The exemplary embodiments disclosed herein generally relate to the field of batteries, and particularly to silicon-carbon anode materials, secondary batteries, and electrical devices. Background Technology

[0002] With the continuous development of industries such as electronics, new energy vehicles, and smart grids, higher demands are being placed on battery technology. Silicon-carbon anode materials, due to their high specific capacity, are considered one of the core materials for next-generation high-energy-density lithium-ion battery anodes. However, current silicon-carbon materials still have limitations in terms of volume expansion and conductivity. How to achieve lower volume expansion rates and higher conductivity in silicon-carbon materials to improve the cycle performance and rate capability of secondary batteries is a problem that urgently needs to be solved in the industry. Summary of the Invention

[0003] This application aims to provide a silicon-carbon anode material that improves the anti-expansion ability and conductivity of the silicon-carbon anode material by optimizing the carbon substrate structure and the silicon crystal structure deposited on the carbon substrate.

[0004] In a first aspect of this disclosure, a silicon-carbon anode material is provided. The silicon-carbon anode material comprises: a porous carbon substrate; silicon crystals filling at least a portion of the pores of the porous carbon substrate; and a carbon coating layer coating at least a portion of the surface of the porous carbon substrate and at least a portion of the surface of the silicon crystals; wherein the silicon crystals have a grain size D1 of 1–5 nm at a diffraction angle of 28° and a grain size D2 of 0.5–3 nm at a diffraction angle of 47.5° in an X-ray diffraction pattern.

[0005] According to the embodiments of this disclosure, the silicon-carbon anode material has a carbon coating layer that completely covers the silicon crystal and the porous carbon substrate. This coating layer isolates the silicon crystal, preventing direct contact between the silicon crystal and the electrolyte and reducing the disordered growth of the solid electrolyte interphase (SEI) film during the pre-cycling stage. Furthermore, by utilizing the high conductivity of amorphous carbon, a continuous conductive network is constructed on the porous carbon substrate, simultaneously enhancing the conductivity and interfacial stability of the material at both the interface and bulk phases, effectively suppressing material expansion.

[0006] By controlling the grain size of silicon crystals deposited in the porous structure of a porous carbon substrate, the grain size D1 at a diffraction angle of 28° in the X-ray diffraction pattern is 1–5 nm, and the grain size D2 at a diffraction angle of 47.5° is 0.5–3 nm. Small silicon grains help alleviate the mechanical stress caused by silicon volume expansion, thereby improving the cycle stability and mechanical stability of the silicon-carbon anode material. During high-rate charging, small silicon grains facilitate the reduction of active ions, reducing dendrite formation on the anode surface and improving the electron transport efficiency of the silicon-carbon anode material.

[0007] In some embodiments, the grain size D1 of silicon at a diffraction angle of 28° in the X-ray diffraction pattern can be in the range of 1 to 3 nm.

[0008] In some specific embodiments, the grain size D1 of the silicon crystal at a diffraction angle of 28° in the X-ray diffraction pattern can be 1.2nm, 1.26nm, 1.39nm, 1.42nm, 1.52nm, 1.61nm, 1.67nm, 1.74nm, 2.45nm, or 2.87nm.

[0009] In some embodiments, the grain size D2 of silicon at a diffraction angle of 47.5° in the X-ray diffraction pattern can be in the range of 0.5 to 2.5 nm.

[0010] In some specific embodiments, the grain size D2 of the silicon crystal at a diffraction angle of 47.5° in the X-ray diffraction pattern can be 0.69nm, 0.73nm, 0.75nm, 0.76nm, 0.77nm, 0.78nm, 0.84nm, 1.24nm, or 2.02nm.

[0011] In some embodiments, the powder resistivity R of the silicon-carbon anode material under a pressure of 20 kN is 0.1–15 Ω·cm.

[0012] By controlling the powder resistivity of the silicon-carbon anode material, the powder resistivity R of the silicon-carbon anode material under a pressure of 20 kN is made to be in the range of 0.1 to 15 Ω·cm. This is beneficial to the electron transport in the silicon-carbon anode material, accelerates the reaction between active ions and electrons, and improves the fast-charging performance of the battery.

[0013] In some embodiments, the powder resistivity R of the silicon-carbon anode material under a pressure of 20 kN is 0.3–15 Ω·cm.

[0014] In some embodiments, the powder resistivity R of the silicon-carbon anode material under a pressure of 20 kN is 0.3 to 3 Ω·cm.

[0015] In some specific embodiments, the powder resistivity of the silicon-carbon anode material under a pressure of 20 kN can be 0.392 Ω·cm, 0.492 Ω·cm, 0.580 Ω·cm, 0.983 Ω·cm, 1.225 Ω·cm, 1.427 Ω·cm, 1.632 Ω·cm, 2.745 Ω·cm, 6.818 Ω·cm, or 14.062 Ω·cm.

[0016] In some embodiments, the polarization voltage dV of the silicon-carbon anode material is 50–150 mV.

[0017] In some embodiments, the polarization voltage dV of a silicon-carbon anode material refers to the polarization voltage after the silicon-carbon anode material is fabricated into a negative electrode sheet and assembled into a coin cell. Polarization voltage is an important indicator for evaluating internal resistance and interfacial effects during electrochemical reactions. Polarization voltage reflects the redistribution of active ions within the bulk phase of the material. Controlling the polarization voltage dV within the range of 50–150 mV is beneficial for increasing the electron exchange rate between active ions and the material, thereby resulting in more uniform lithium intercalation. A low polarization voltage of the silicon-carbon anode material also means that the prepared battery can have lower energy loss and better electrochemical kinetic characteristics.

[0018] In some embodiments, the polarization voltage dV of the silicon-carbon anode material can be 60–100 mV.

[0019] In some specific embodiments, the polarization voltage dV of the silicon-carbon material can be 65.1mV, 80.1mV, 80.9mV, 91.1mV, 102.3mV, 118.7mV, 118.75mV, 122.7mV, 131.55mV, or 135.1mV.

[0020] In some embodiments, the porous carbon substrate exhibits a Raman spectrum at 1350 cm⁻¹. -1 Raman Peak (Id) at 1580cm -1 The ratio of the Raman peak (Ig) at that location, Id / Ig, is 0.8 to 1.

[0021] The degree of defects in the carbon substrate of the material affects the fast charging capability of the battery. Raman spectroscopy measurements show the Id peak (approximately 1350 cm⁻¹). -1 The peak (Ig) represents the disordered structure in the carbon structure. (Ig peak approximately 1580 cm⁻¹) -1 The Id / Ig ratio represents the degree of order in the carbon structure. By constraining the Id / Ig ratio to be within the range of 0.8–1, it is possible to improve the conductivity of the porous carbon substrate as a carbon framework and the electron transport capability of the silicon-carbon anode material, thereby enhancing the overall rate performance and cycle stability of the battery. Furthermore, it allows the carbon framework to maintain appropriate buffering properties to cope with the expansion stress of silicon crystals, reducing material pulverization failure caused by localized stress concentration. In addition, an Id / Ig ratio within the range of 0.8–1 can also reduce the impact on lithium-ion insertion / extraction channels, improving the kinetic performance of the silicon-carbon anode material.

[0022] In some embodiments, the porous carbon substrate is nitrogen-doped porous carbon, with nitrogen atoms accounting for 0.5 wt% to 0.8 wt% by weight.

[0023] The nitrogen doping content of porous carbon substrates can affect the conductivity of silicon-carbon anode materials. By limiting the nitrogen doping amount in porous carbon substrates to between 0.5 wt% and 0.8 wt%, sufficient anchoring sites can be provided for silicon atom deposition, reducing silicon crystal migration and agglomeration, and improving the conductivity of porous carbon substrates. Furthermore, by limiting the upper limit of nitrogen doping in porous carbon substrates, the reduction of the mechanical strength of the carbon framework due to excessive nitrogen doping can be avoided.

[0024] In some embodiments, the thickness of the carbon coating layer is 140–200 nm.

[0025] The thickness of the carbon coating layer in silicon-carbon anode materials affects the ability of active ions to insert and extract. An appropriate carbon coating layer thickness can buffer the volume expansion of silicon and form a more complete conductive network, improving electron transport efficiency and thus enhancing the material's kinetic properties. Controlling the carbon coating layer thickness within the range of 140–200 nm allows the silicon-carbon anode material to form a continuous and stable conductive network, ensuring its conductivity while reducing the diffusion resistance of lithium-ion insertion / extraction and improving the fast-charging performance of the secondary battery.

[0026] In some embodiments, the silicon crystal content in the silicon-carbon anode material is 40 wt% to 60 wt% by weight.

[0027] Silicon content directly affects the energy density and volume expansion characteristics of anode materials. As a high-capacity anode material, silicon theoretically has a much higher specific capacity than traditional graphite. However, it undergoes significant volume changes during charge and discharge, leading to structural damage and decreased cycle performance. By controlling the silicon content between 40wt% and 60wt% and combining it with a suitable carbon substrate pore structure, it is possible to mitigate the volume expansion problem of silicon while ensuring high energy density and improving the cycle stability of the battery.

[0028] In some embodiments, the specific surface area of ​​the silicon-carbon anode material is 0.5–3 m². 2 / g.

[0029] The specific surface area of ​​silicon-carbon anode materials affects the fast-charging capability of batteries. Controlling the specific surface area of ​​silicon-carbon anode materials within the range of 0.5–3 m² is crucial. 2 Within the range of / g, side reactions can be reduced, the active sites for silicon-active ion reactions can be increased, and the kinetic performance of the battery can be improved.

[0030] The pore structure of carbon materials in silicon-carbon materials affects the distribution of silicon atoms, thereby influencing the conductivity of the silicon-carbon material. In some embodiments, the porous carbon substrate includes microporous and mesoporous structures. Based on the total specific surface area of ​​the porous carbon substrate, the pore volume of the microporous structure accounts for 80%–95%, and the pore volume of the mesoporous structure accounts for 1%–15%. By controlling the pore volume ratios of the microporous and mesoporous structures, the structural collapse of the porous carbon substrate during silicon expansion can be reduced, and a high electrolyte migration rate can be ensured during chemical reactions, thus guaranteeing the conductivity and electrochemical reaction rate of the silicon-carbon anode material. Microporous structures refer to pore structures with an average pore diameter of less than 2 nm, while mesoporous structures refer to pore structures with an average pore diameter between 2 and 50 nm.

[0031] In some embodiments, the average particle size D50 of the silicon-carbon anode material is 5–15 μm.

[0032] The rate performance of a battery is related to the particle size of the silicon-carbon anode material. When the average particle size (D50) of the silicon-carbon anode material is within the range of 5–15 μm, the diffusion path of active ions within the silicon-carbon anode material is smaller, which is beneficial for high-rate charging of the secondary battery. Furthermore, controlling the average particle size of the silicon-carbon anode material also helps to reduce the coating thickness of the anode slurry, which is conducive to the diffusion of active ions, avoids polarization and lithium plating, and improves cycle performance.

[0033] In some embodiments, the silicon-carbon anode material satisfies the following relationship:

[0034] 100≤(0.3*D1+0.7*D2)*R+2*dV)≤300 (I) In formula (I), D1 is the grain size of silicon at a diffraction angle of 28° in the X-ray diffraction pattern, and D2 is the grain size of silicon at a diffraction angle of 47.5° in the X-ray diffraction pattern, in nm; R is the powder resistivity of silicon-carbon anode material under a pressure of 20 kN, in Ω*cm; dV is the polarization voltage.

[0035] In some embodiments, the conductivity of silicon-carbon anode materials satisfies the following relationship:

[0036] 100≤(0.3*D1+0.7*D2)*R+2*dV≤200 (II).

[0037] In some embodiments, the tap density of the negative electrode active material is between 0.5 and 2 g / cm³. 3 Tap density refers to the mass of particles packed per unit volume, reflecting the compactness of the material packing. The optimal tap density is controlled between 0.5 and 2 g / cm³. 3 Within a certain range, the energy density and kinetic performance of the electrode can be balanced.

[0038] In some embodiments, the true density of the silicon-carbon anode active material is 1.6–2.5 g / cm³. 3 True density reflects the degree of silicon deposition and the richness of the pore structure within the material. The true density of silicon-carbon anode materials is controlled to be between 1.6 and 2.5 g / cm³. 3 Within this range, it helps improve the adhesion between silicon-carbon anode active material particles, thereby increasing the electrode's energy density. Simultaneously, it ensures that the porous carbon substrate contains sufficient pore structure, thus guaranteeing complete silicon crystal deposition within the porous carbon substrate. In this way, the overall energy density of the silicon-carbon anode material can be improved, fully utilizing the high specific capacity advantage of silicon crystals.

[0039] In some embodiments, coin cells using silicon-carbon anode materials exhibit an initial coulombic efficiency of 78–86% and a specific capacity of 1500–2200 mAh / g at a charging cutoff voltage of 0.8V. Initial charge specific capacity is the amount of charge stored per unit mass of active material (silicon-carbon anode material) in a coin cell. A higher initial charge specific capacity indicates a higher energy density in the battery made from this silicon-carbon anode material. Initial coulombic efficiency, the ratio of initial charge capacity to initial discharge capacity, is an important indicator of irreversible capacity loss during the initial charge and discharge process. Higher initial coulombic efficiency means less irreversible capacity loss, which is crucial for extending battery life and improving energy utilization.

[0040] In a second aspect of this disclosure, a method for preparing a silicon-carbon anode material is provided. The preparation method includes the following steps:

[0041] S1. Preparation of a porous carbon substrate, comprising: placing a carbon source in a protective atmosphere and heating and holding it at a gradient temperature to carbonize the carbon source to obtain an initial carbon substrate; and activating the initial carbon substrate in an activator environment to obtain a porous carbon substrate including a pore structure.

[0042] S2. Deposition of silicon crystals, including: reacting a porous carbon substrate and a silicon source in an environment of 400-800°C for 6-24 hours, so that silicon crystals are deposited in the pore structure of the porous carbon substrate in the form of nanoparticles, to obtain the initial silicon-carbon anode material.

[0043] S 3, the coating of the carbon coating layer, including: reacting the initial silicon-carbon composite material with the carbon source in an environment of 300-900°C for 0.5-4 hours to coat the surface of the initial silicon-carbon composite material with a carbon layer to obtain the silicon-carbon anode material.

[0044] By treating the substrate carbon source with a gradient temperature rise, the molecular chains of the substrate carbon source can be orderedly reorganized with different hybrid structures under the drive of high and low temperature differences, thus forming the initial microporous / mesoporous framework of the carbon substrate. After the gradient temperature rise treatment, a high concentration of SP forms on the outer layer of the porous carbon substrate. 3 Hybrid structure and high concentration of SP formed inside 2 Hybrid structure. Due to the sp hybrid structure of carbon. 3 Hybridization provides strong mechanical strength, with the outer SP layer... 3 Increasing the hybridization concentration effectively improves the compressive strength of the carbon substrate, enabling it to provide effective support for the volume expansion of silicon grains during charge and discharge, thereby improving the cycle stability and lifespan of silicon-carbon materials. The inner layer has a higher concentration of SP... 2 Hybridization can improve the conductivity of silicon-carbon anode materials, reduce their internal resistance, and thus improve the rate performance and overall energy efficiency of the battery.

[0045] By controlling the temperature of the porous carbon substrate treated with gradient temperature rise and the silicon deposition temperature, the grain size of the deposited silicon can be controlled. This allows the grain size of the silicon crystals at a diffraction angle of 28° in the X-ray diffraction pattern to be controlled within the range of 1–5 nm, and the grain size at a diffraction angle of 47.5° to be controlled within the range of 0.5–3 nm. This method can alleviate the mechanical stress caused by the volume expansion of silicon, thereby improving the cycle stability and mechanical stability of the silicon-carbon anode material.

[0046] In addition, the carbon coating layer covers the silicon crystal and the carbon substrate, which can isolate the direct contact between silicon and electrolyte and reduce the disordered growth of SEI film during the pre-cycle stage. On the other hand, by utilizing the high conductivity of amorphous carbon, a continuous conductive network is constructed on the basis of the mesoporous framework of the carbon substrate, thereby simultaneously enhancing the conductivity and interfacial stability of the material from the interface and the bulk phase.

[0047] In some embodiments, the base carbon source is selected from one or more of phenolic resin, mesophase pitch, coconut shell biochar, and wood biochar.

[0048] In some embodiments, heating and holding at a gradient temperature includes: a first stage of heating to 700-1200°C at a rate of 5-20°C / min and holding for 2-8 hours; and a second stage of heating to 900-1500°C at a rate of 5-20°C / min and holding for 2-8 hours; wherein the temperature of the second stage is higher than that of the first stage.

[0049] In some embodiments, the endpoint of the first-stage heating can be 700°C, 800°C, 900°C, or 950°C.

[0050] In some embodiments, the endpoint of the two-stage heating process can be 900°C, 1000°C, 1100°C, 1150°C, or 1200°C.

[0051] In the gradient temperature heating and holding process, the endpoints of the two stages should have a certain temperature difference, meaning the temperature of the second stage should be higher than that of the first stage. This allows the porous carbon substrate to undergo ordered recombination of its hybrid structure under gradient high and low temperatures.

[0052] In some embodiments, the porous carbon substrate reacts with a silicon source in an environment of 450–650°C.

[0053] In some embodiments, the heat preservation time for the first stage is 2-4 hours, and the heat preservation time for the second stage is 5-8 hours. In some embodiments, the heat preservation time for the first stage can be 3 hours, and the heat preservation time for the second stage can be 6 hours.

[0054] In some embodiments, the activator is selected from one or more of CO2, H2O, and alkaline activators.

[0055] In some embodiments, the activation temperature is 800–1000°C. In some embodiments, the activation temperature may be 800–900°C.

[0056] In some embodiments, the activation time is 5 to 20 hours. In some embodiments, the activation time may be 8 to 10 hours.

[0057] In some embodiments, the silicon source is selected from one or more of SiH4 and Si2H6.

[0058] In some embodiments, the coating carbon source is selected from one or more of CH4, C2H4, and C3H8.

[0059] In some embodiments, the reaction time between the porous carbon substrate and the silicon source can be 12 to 15 hours.

[0060] In a third aspect of this disclosure, a negative electrode sheet is provided. The negative electrode sheet includes a negative current collector; and a negative electrode material layer coated on the negative current collector, the negative electrode material layer including the silicon-carbon negative electrode material provided in the first aspect of this disclosure.

[0061] In a fourth aspect of this disclosure, a secondary battery is provided. The secondary battery includes: a negative electrode sheet, the negative electrode sheet including a negative electrode active material layer, the negative electrode active material layer comprising a silicon-carbon negative electrode material provided according to a first aspect of this disclosure. In some embodiments, the secondary battery further includes a positive electrode sheet; and an electrolyte filled between the positive electrode sheet and the negative electrode sheet.

[0062] In a fifth aspect of this disclosure, an electrical device is provided, including a secondary battery provided according to a third aspect of this disclosure, the secondary battery serving as a power source for the electrical device. In some embodiments, the electrical device may be a vehicle, mobile phone, portable electronic device, wearable electronic device, laptop computer, ship, spacecraft, electric toy, or power tool, etc. In some specific embodiments, the vehicle may be a new energy vehicle, which may be a pure electric vehicle, a hybrid electric vehicle, or a range-extended electric vehicle, etc.

[0063] This disclosure utilizes a specific porous carbon structure as a substrate, deposits silicon crystals within the porous structure of the substrate, and coats both the carbon substrate and the silicon crystals with a carbon coating layer. This reduces the direct contact between the silicon crystals and the electrolyte, minimizing the disordered growth of the solid electrolyte interphase (SEI) film during the pre-cycling stage. By controlling the size of the silicon crystal grains deposited within the porous carbon substrate—resulting in grains of 1–5 nm at a diffraction angle of 28° and 0.5–3 nm at a diffraction angle of 47.5°—the mechanical stress caused by silicon volume expansion can be reduced, improving the cycle stability and mechanical stability of the silicon-carbon anode material. Furthermore, during high-rate charging, the smaller silicon particles facilitate the reduction of active ions, thereby reducing dendrite formation on the anode surface and improving the electron transport efficiency of the silicon-carbon anode material.

[0064] Furthermore, this disclosure also allows for the control of the powder resistivity of the silicon-carbon anode material, achieving a powder resistivity in the range of 0.1–15 Ω·cm under a pressure of 20 kN. This facilitates electron transport within the silicon-carbon anode material, accelerates the reaction between active ions and electrons, and improves the fast-charging performance of the battery.

[0065] When silicon-carbon anode materials are fabricated into anode sheets and assembled into coin cells, their polarization voltage can be controlled within the range of 50–150 mV. This increases the exchange rate of active ions and electrons within the material. The low polarization voltage of silicon-carbon anode materials also means that the prepared secondary batteries can have lower energy loss and better electrochemical kinetic characteristics.

[0066] It should be understood that the content described in this content section is not intended to limit the key or essential features of the embodiments of this disclosure, nor is it intended to restrict the scope of this disclosure. Other features of this disclosure will become readily apparent from the following description. Detailed Implementation

[0067] It should be noted that the headings of any section / subsection provided herein are not limiting. Various embodiments are described throughout this document, and embodiments of any type may be included under any section / subsection. Furthermore, embodiments described in any section / subsection may be combined in any way with any other embodiments described in the same section / subsection and / or different sections / subsections.

[0068] In the description of embodiments of this disclosure, the term "comprising" and similar terms should be understood as open-ended inclusion, i.e., "including but not limited to". The term "based on" should be understood as "at least partially based on". The term "one embodiment" or "the embodiment" should be understood as "at least one embodiment". The term "some embodiments" should be understood as "at least some embodiments". Other explicit and implicit definitions may also be included below. The terms "first", "second", etc., may refer to different or the same objects. Other explicit and implicit definitions may also be included below.

[0069] The present disclosure will be further described below with reference to specific embodiments. It should be noted that the following implementation methods are further explanations of the present disclosure and should not be construed as limiting the present disclosure.

[0070] Preparation of silicon-carbon anode materials

[0071] The silicon-carbon anode material in Example 1 was prepared using the following method:

[0072] First, the phenolic resin, the carbon source of the substrate, is placed in a nitrogen protective atmosphere. In the first stage, the temperature is increased to 800℃ at a rate of 10℃ / min and held for 3 hours. In the second stage, the temperature is increased to 900℃ at a rate of 5℃ / min and held for 6 hours to obtain the initial carbon substrate. The initial carbon substrate is then activated with water as an activator at 800-1000℃ for 10 hours to obtain a porous carbon substrate with a porous structure.

[0073] Subsequently, the porous carbon substrate was reacted with silane gas (SiH4) at 500°C for 13 hours to obtain a silicon-carbon composite material. In the silicon-carbon composite material, silicon was deposited in the porous structure of the porous carbon substrate in the form of nanoparticles. Finally, the silicon-carbon composite material was reacted with a carbon-coated acetylene source at 500°C for 2 hours to coat the surface of the silicon-carbon composite material with a carbon coating layer, thus obtaining a silicon-carbon anode material.

[0074] The silicon-carbon anode materials of Examples 2-10 and Comparative Examples 1 and 2 differ from those of Example 1 in that the reaction temperatures of the first stage, the second stage, and the silicon source deposition stage are different, as are the reaction times of the silicon source deposition stage, as shown in Table 1.

[0075] Table 1

[0076]

[0077]

[0078] Preparation of negative electrode sheet:

[0079] Some embodiments of this disclosure also provide negative electrode sheets prepared based on the above-described silicon-carbon negative electrode material. The negative electrode sheets can be prepared by the following method: Silicon-carbon negative electrode material, conductive agent carbon black, binder styrene-butadiene rubber, and additive sodium carboxymethyl cellulose are mixed in a mass ratio of 8:1:0.5:0.5, and deionized water is added. The mixture is stirred under vacuum to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated onto a negative electrode current collector copper foil, air-dried at room temperature, and then transferred to an oven for drying. Finally, the negative electrode sheets are obtained through rolling and cutting.

[0080] The silicon-carbon anode material can be any of the silicon-carbon anode materials prepared in Examples 1-13 above.

[0081] Preparation of the positive electrode sheet:

[0082] The positive electrode sheet can be prepared by the following method: LiNi 0.8 Co 0.1 Mn 0.1 O2 cathode material, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 96:2:2. N-methylpyrrolidone (NMP) solvent is added, and the mixture is stirred under vacuum to obtain cathode slurry. The cathode slurry is uniformly coated on cathode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. After rolling and cutting, cathode electrode sheets are obtained.

[0083] Electrolyte preparation:

[0084] The electrolyte is prepared as follows: ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1. Then, 1 mol / L LiPF6 and 5 wt% fluoroethylene carbonate are dissolved in the mixed organic solvent and mixed evenly to prepare the electrolyte.

[0085] Some embodiments of this disclosure also provide a secondary battery, which includes a positive electrode, a negative electrode, and an electrolyte.

[0086] Assembly of secondary batteries:

[0087] The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in sequence, then wound into a core and placed into a soft-pack shell. After top-side sealing, liquid injection, formation, sorting and other processes, a lithium-ion battery is obtained.

[0088] The separator membrane used is Celgard 2300 membrane from Celgard.

[0089] Performance testing

[0090] Assembly of button batteries

[0091] This disclosure also provides a coin cell based on silicon-carbon anode material.

[0092] Button batteries can be prepared using the following methods:

[0093] Using the above-described method for preparing negative electrode sheets, the silicon-carbon negative electrode materials of Examples 1-13 and Comparative Examples 1 and 2 were respectively prepared into corresponding negative electrode sheets.

[0094] The positive lithium electrode, separator, and negative electrode are sequentially stacked and encapsulated in a CR2032 coin cell. The separator used is a Celgard 2300 separator. The electrolyte is the electrolyte prepared above.

[0095] Test content

[0096] 1. The grain size of silicon crystals in silicon-carbon anode materials at diffraction angles of 28° and 47.5° was detected using an X-ray diffractometer.

[0097] 2. After pressing the silicon-carbon anode material into a sheet under a pressure of 20 kN, the powder resistivity of the silicon-carbon anode material under a pressure of 20 kN is directly tested using a four-probe tester.

[0098] 3. Raman spectroscopy was used to detect the properties of porous carbon substrates at an excitation wavelength of 532 nm and a wavelength of 1350 cm⁻¹. -1 Raman Peak (Id) at 1580cm -1 The Raman peak (Ig) at the location is determined, and the ratio Id / Ig is calculated.

[0099] 4. The nitrogen atom content (wt%) in the porous carbon substrate was determined using an elemental analyzer.

[0100] 5. The thickness of the carbon coating layer in the silicon-carbon anode material was detected using transmission electron microscopy (TEM).

[0101] 6. The silicon crystal content in silicon-carbon anode materials was detected using a thermogravimetric analyzer.

[0102] 7. Calculate the specific surface area of ​​silicon-carbon anode materials using an adsorption analyzer based on the BET method.

[0103] 8. Calculate the pore volume ratio of mesopores in porous carbon substrates using an adsorption analyzer based on the DFT method.

[0104] 9. Test the polarization voltage of silicon-carbon anode materials after they are assembled into coin cells.

[0105] The coin cell was left to stand at 25°C for 30 minutes, then discharged at 0.1C to a voltage of 0.005V, followed by charging at 0.1C to a voltage of 2V, and left to stand for 1 hour. Next, it was discharged at a current density of 0.1C for 300 minutes and left to stand for 1 hour. This state represents the battery's 50% SOC. The voltage difference between the end of the discharge and the end of the resting period is the polarization voltage.

[0106] 10. Measure the constant current charge ratio of coin cells assembled based on silicon-carbon anode materials.

[0107] The coin cell battery was left to stand at 25°C for 30 minutes, then discharged at 0.5C to a voltage of 0.005V, and the 0.5C discharge capacity was measured. Afterwards, it was left to stand for 5 minutes, then discharged at 0.1C to 0.005V, and the 0.1C discharge capacity was measured. The constant current charge ratio was calculated using the following formula:

[0108] 0.5C discharge specific capacity = 0.5C discharge capacity / active material mass * 100%

[0109] 0.1C discharge specific capacity = 0.1C discharge capacity / active material mass * 100%

[0110] Constant current charge ratio = 0.5C discharge specific capacity / (0.5C discharge specific capacity + 0.1C discharge specific capacity),

[0111] The mass of the active material is the mass of the silicon-carbon anode material in the negative electrode of the coin cell.

[0112] The constant current charge ratio reflects the material's kinetics; the higher the constant current charge ratio, the better the material's kinetics.

[0113] 11. Measure the initial coulombic efficiency (also known as 0.8V first efficiency) of a coin cell assembled based on silicon-carbon anode material at a charging cutoff voltage of 0.8V.

[0114] 12. Measure the specific capacity of a coin cell assembled based on silicon-carbon anode material during its first charge at a charging cutoff voltage of 0.8V (also known as the 0.8V specific capacity).

[0115] The initial coulombic efficiency and initial charge specific capacity were tested using the following method: First, the initial discharge capacity was measured under conditions of a discharge current density of 200 mA / g and an initial discharge cutoff voltage of 0.01 V. Then, the initial charge capacity was measured under conditions of a charging current density of 200 mA / g and an initial charge cutoff voltage of 0.8 V. The initial coulombic efficiency and initial charge specific capacity of the coin cell were determined based on the following formula:

[0116] Initial coulombic efficiency = (Initial charge capacity / Initial discharge capacity) * 100%;

[0117] First charge specific capacity = first charge capacity / active material mass * 100%.

[0118] The mass of the active material is the mass of the silicon-carbon anode material in the negative electrode of the coin cell.

[0119] 13. Measure the expansion rate of the negative electrode sheet prepared based on silicon-carbon negative electrode material.

[0120] Anode sheets were assembled using silicon-carbon anode material, and the thickness of the coated anode sheets was measured.

[0121] Assemble a coin cell using the negative electrode sheet. Let the coin cell stand at 25°C for 30 minutes, then discharge it at 0.5C until the voltage reaches 0.005V, at which point the negative electrode sheet is fully lithium-intercalated. Remove the coin cell in a glove box or dry environment and measure the thickness of the negative electrode sheet. Determine the expansion rate of the negative electrode sheet based on the following formula.

[0122] Expansion rate = (thickness of negative electrode sheet in full lithium insertion state - thickness of negative electrode sheet after coating) / thickness of negative electrode sheet after coating.

[0123] 14. Test the cycle performance of coin cells assembled based on silicon-carbon anode materials.

[0124] The voltage cycling range was set to 0.01–2V, the charge / discharge current density for the first three cycles was 200 mA / g, and then the charge / discharge current density was set to 500 mA / g for cyclic testing.

[0125] Test results

[0126] The detection results of Examples 1-13 and Comparative Examples 1 and 2 are shown in Tables 2 and 3.

[0127] Table 2

[0128]

[0129]

[0130] Table 3

[0131]

[0132]

[0133] Referring to Tables 2 and 3 and combining them with Table 1, the reaction temperatures during the first stage, the second stage, and the silicon source deposition stage of silicon-carbon anode material preparation affect factors such as the size of silicon crystals (at diffraction angles of 28° and 47.5°), powder resistance, and polarization voltage in silicon-carbon anode materials.

[0134] By controlling the reaction temperature of the first stage at 700–950℃ and the reaction temperature of the second stage at 900–1300℃, and by creating a temperature gradient between the two stages (i.e., the reaction temperature of the second stage is higher than that of the first stage), carbon-based materials can be heat-treated at different temperatures. This utilizes the high and low temperature difference to drive the carbon source molecular chains to undergo SP (split reaction). 2 and SP 3 The structure undergoes orderly reorganization, simultaneously forming the initial microporous / mesoporous framework of the carbon substrate. Subsequently, an activator is introduced, and the mesoporous content of the carbon substrate is controlled through a "carbon etching" process. This mesoporous-dominated hierarchical porous structure provides highly dispersed loading sites for subsequent silicon crystallization and also serves as an elastic buffer space for silicon volume expansion, enhancing the mechanical stability of the carbon substrate. This, in turn, regulates the hybridization of carbon atoms in the carbon substrate, resulting in a high concentration of spline atoms (SP) on the outer layer. 3 Hybrid structure and high internal SP concentration 2 The hybrid structure balances high conductivity and structural stability, effectively reducing the internal resistance of silicon-carbon materials and improving the rate performance and overall energy efficiency of the battery. The SP structure of carbon elements... 3 Hybridization has strong mechanical strength, SP 3 Increasing the hybridization concentration can effectively improve the compressive strength of the carbon substrate, enabling it to provide effective support for the volume expansion of silicon crystals during charging and discharging, thereby improving the cycle stability and lifespan of silicon-carbon materials.

[0135] Compared to Examples 1-13, Comparative Examples 1 and 2 did not use gradient temperature heat treatment of the carbon substrate during preparation. Furthermore, the reaction temperature in Comparative Example 1 was 800°C in the second stage (not within the 900-1500°C range), and the silicon deposition temperatures in Comparative Examples 1 and 2 were 700°C and 750°C, respectively, higher than the 500-600°C in Examples 1-13. This resulted in larger silicon grains at diffraction angles of 28° and 47.5° in Comparative Examples 1 and 2. Although there was no significant deterioration in physical properties, the lithium-ion diffusion rate was slower during the coin electrode polarization capability test, leading to reduced kinetic performance and a higher polarization capability in the negative electrode. In contrast, Examples 1-13, by controlling the reaction temperature during the preparation stage, controlled the silicon grain size within a defined range (D1 = 1-5 nm, D2 = 0.5-3 nm), resulting in higher capacity and better kinetic performance compared to Comparative Examples 1 and 2.

[0136] In the silicon-carbon anode materials prepared in Examples 1-13, the ratio Id / Ig of the porous carbon substrate is in the range of 0.8 to 1, the nitrogen atom content of the porous carbon substrate is 0.5 wt% to 0.8 wt%, the thickness of the carbon coating layer is 140 to 200 nm, the silicon content in the silicon-carbon anode material is 40 wt% to 60 wt%, and the specific surface area of ​​the silicon-carbon anode material is 0.5 to 3 m². 2 / g, the pore volume ratio of the mesoporous structure of the porous carbon substrate is 1% to 15%. By controlling the above-mentioned characteristic parameters of the silicon-carbon anode material within their respective predetermined ranges, the mechanical strength of the silicon-carbon anode material can be improved while ensuring its conductivity, thereby improving its resistance to expansion.

[0137] After assembling the coin cells, the constant current charge ratio of the coin cells assembled based on the silicon-carbon anode materials of Examples 1-13 can exceed 80%, which is significantly higher than the constant current charge ratio of the coin cells prepared using Comparative Examples 1 and 2. This indicates that the silicon-carbon anode materials of Examples 1-13 have stable kinetic performance, and the secondary batteries assembled based on the silicon-carbon anode materials of the embodiments of this disclosure have good rate performance.

[0138] The coin cells assembled using the silicon-carbon anode materials of Examples 1-13 have 107-206 cycles at 80% capacity, exceeding the coin cells assembled using the silicon-carbon anode materials of Comparative Examples 1 and 2. This indicates that the secondary batteries prepared using the silicon-carbon anode materials provided in this disclosure have better cycle performance and lifespan. The coin cells assembled using the silicon-carbon anode material of Example 4 can reach 206 cycles at 80% capacity.

[0139] The expansion rate of the negative electrode sheet of the coin cells assembled based on the silicon-carbon negative electrode materials of Examples 1-13 is in the range of 65.6% to 81.4%, which is lower than that of the negative electrode sheet of the coin cells assembled based on the silicon-carbon negative electrode materials of Comparative Examples 1 and 2. The negative electrode sheet assembled based on the silicon-carbon negative electrode materials of this disclosure has a low expansion rate, which helps to reduce the mechanical stress on the negative electrode material during the charging and discharging of the secondary battery and extend the cycle life of the secondary battery. The expansion rate of the negative electrode sheet prepared based on the silicon-carbon negative electrode material of Example 4 can be controlled at 65.6%.

[0140] The coin cells assembled using the silicon-carbon anode materials of Examples 1-13 can achieve an initial efficiency of over 81% at 0.8V and a specific capacity of over 1790 mAh / g at 0.8V. This indicates that the secondary batteries prepared using the silicon-carbon anode materials of Examples 1-13 have high energy density and high energy utilization, while also contributing to extended cycle life. The coin cells prepared using the silicon-carbon anode material of Example 9 can achieve an initial efficiency of 84.77% at 0.8V and a specific capacity of 2206.1 mAh / g at 0.8V.

[0141] Furthermore, by comprehensively adjusting the silicon crystal size, powder resistance, and polarization capability of the silicon anode material in Examples 1-13, the (0.3D1+0.7D2)*R+2*dV was kept between 100 and 300, resulting in batteries with high capacity and high kinetic performance. If (0.3D1+0.7D2)*R+2*dV is between 100 and 200, the battery achieves even better high capacity, high kinetic performance, and cycle performance. For example, the calculation results of (0.3D1+0.7D2)*R+2*dV for Examples 1-13 are all within the range of 100 to 300, while the calculation results for Comparative Examples 1 and 2 exceed 300. Therefore, it can be considered that the kinetic and cycle performance of Examples 1-13 are superior to those of Comparative Examples 1 and 2. The formula values ​​for Examples 3, 4, 9, and 10 are in the range of 1 to 200, indicating that Examples 3, 4, 9, and 10 exhibit better kinetic and cycle performance. The constant current charge ratio in Examples 3, 4, 9, and 10 can reach over 81%, the expansion of the electrode is between 60% and 80%, and the coin cell cycle life is over 170 cycles.

[0142] Various implementations of this disclosure have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed implementations. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described implementations. The terminology used herein is chosen to best explain the principles, practical applications, or improvements to technology in the market, or to enable others skilled in the art to understand the various implementations disclosed herein.

Claims

1. A silicon-carbon anode material, characterized in that, include: Porous carbon substrate; Silicon crystals fill at least a portion of the pores in the porous carbon substrate; as well as A carbon coating layer covers at least a portion of the surface of the porous carbon substrate and at least a portion of the surface of the silicon crystal; The silicon crystal has a grain size D1 of 1–5 nm at a diffraction angle of 28° in the X-ray diffraction pattern, and a grain size D2 of 0.5–3 nm at a diffraction angle of 47.5° in the X-ray diffraction pattern.

2. The silicon-carbon anode material according to claim 1, characterized in that, The resistivity R of the silicon-carbon anode material under a pressure of 20 kN is 0.1–15 Ω·cm.

3. The silicon-carbon anode material according to claim 2, characterized in that, The polarization voltage dV of the silicon-carbon anode material is 50–150 mV.

4. The silicon-carbon anode material according to claim 1, characterized in that, The porous carbon substrate exhibits a Raman spectrum at 1350 cm⁻¹ -1 Raman Peak (Id) at 1580cm -1 The ratio of the Raman peak (Ig) at that location, Id / Ig, is 0.8 to 1.

5. The silicon-carbon anode material according to claim 1, characterized in that, The porous carbon substrate contains nitrogen atoms, and the nitrogen atoms account for 0.5 wt% to 0.8 wt% of the porous carbon substrate by weight.

6. The silicon-carbon anode material according to claim 1, characterized in that, The thickness of the carbon coating layer is 140–200 nm.

7. The silicon-carbon anode material according to claim 1, characterized in that, The silicon content in the silicon-carbon anode material is 40 wt% to 60 wt% based on the weight of the silicon-carbon anode material.

8. The silicon-carbon anode material according to claim 1, characterized in that, The specific surface area of ​​the silicon-carbon anode material is 0.5–3 m². 2 / g.

9. The silicon-carbon anode material according to claim 1, characterized in that, The porous carbon substrate includes microporous structures and mesoporous structures. Based on the total specific surface area of ​​the porous carbon substrate, the pore volume of the microporous structures accounts for 80% to 95%, and the pore volume of the mesoporous structures accounts for 1% to 15%.

10. The silicon-carbon anode material according to claim 3, characterized in that, The following relationship must be satisfied: 100≤(0.3*D1+0.7*D2)*R+2*dV)≤300.

11. A secondary battery, characterized in that, include: A negative electrode sheet, wherein the negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer comprising the silicon-carbon negative electrode material according to any one of claims 1-10.

12. An electrical appliance, characterized in that, include: The secondary battery of claim 11, wherein the secondary battery serves as the power supply for the electrical device.

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