Silicon-carbon negative electrode material and preparation method, application and solid-state battery thereof

Abstract

The application provides a silicon-carbon negative electrode material and a preparation method, application and solid-state battery thereof, and relates to the technical field of lithium batteries.The silicon-carbon negative electrode material is prepared by mixing silicon oil, a molten salt and sodium hydride, and then sequentially performing low-temperature pre-reaction, medium-temperature reduction reaction and carbonization.The content of paraffin in the sodium hydride is 30%-50%, the temperature of the low-temperature pre-reaction is 150 DEG C-200 DEG C, and the temperature of the medium-temperature reduction reaction is 300 DEG C-400 DEG C.The silicon-carbon material prepared by the application has the advantages that the paraffin and the elemental silicon obtained by reduction are fully mixed, the preparation effect of the silicon-carbon material is improved, the C-Si interface bonding force is optimized, the electrochemical cycle stability is improved, and the volume expansion in the charging and discharging process is better limited.

Description

Technical Field

[0001] This invention relates to the technical field of lithium batteries, and in particular to a silicon-carbon anode material, its preparation method, applications, and solid-state batteries. Background Technology

[0002] Elemental silicon can be obtained through carbothermic reduction of silicon ore, and in the laboratory, it can also be obtained through magnesothermic reduction and hydrogen reduction, but all of these require high reduction temperatures. Furthermore, silane vapor deposition using industrial silicon sources poses significant safety risks. Simultaneously, the preparation of silicon-carbon anode materials requires the addition of a carbon source to elemental silicon to ensure homogeneous mixing, which is cumbersome and difficult to achieve thorough mixing. In addition, existing technologies lack environmental constraints during the raw material mixing and pretreatment stages. The raw material mixing process is susceptible to contact with air and moisture, leading to oxidation failure and silicone oil hydrolysis, thus affecting the performance of the final product. Existing molten salt systems are mostly single-component or simply compounded, and have not been optimized for the high-purity requirements of anode materials.

[0003] In view of this, the present invention is hereby proposed. Summary of the Invention

[0004] One of the objectives of this invention is to provide a method for preparing silicon-carbon anode materials, which not only fully mixes paraffin and reduced elemental silicon, thus improving the preparation effect of silicon-carbon materials, but also optimizes the C-Si interface bonding force, thus improving the electrochemical cycle stability, and better restricts elemental silicon, thus helping to cope with volume expansion during charge and discharge.

[0005] The second objective of this invention is to provide a silicon-carbon anode material with good electrochemical cycle stability and small volume expansion during charge and discharge.

[0006] The third objective of this invention is to provide an application of silicon-carbon anode material.

[0007] The fourth objective of this invention is to provide a solid-state battery.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: In a first aspect, a method for preparing a silicon-carbon anode material includes the following steps: Silicon oil, molten salt and sodium hydride are mixed and then subjected to low-temperature pre-reaction, medium-temperature reduction reaction and carbonization in sequence to obtain the silicon-carbon anode material. The sodium hydride contains 30%-50% paraffin. The low-temperature pre-reaction temperature is 150℃-200℃, and the time is 30min-60min; The intermediate-temperature reduction reaction is carried out at a temperature of 300℃-400℃ for 2-5 hours.

[0009] Furthermore, the carbonization temperature is 700℃-1500℃, and the time is 2.5h-5h.

[0010] Furthermore, the carbonization stages include a low-temperature carbonization stage, a medium-temperature transition stage, and a high-temperature stabilization stage. The temperature gradient between the low-temperature carbonization stage, the medium-temperature transition stage, and the high-temperature stabilization stage is 300℃-500℃.

[0011] Furthermore, the temperature of the low-temperature carbonization stage is 700℃-900℃, and the time is 1h-2h; Preferably, the temperature of the intermediate temperature transition stage is 1000℃-1200℃, and the time is 0.5h-1h; Preferably, the temperature of the high-temperature stabilization stage is 1300℃-1500℃, and the time is 1h-2h.

[0012] Furthermore, the molten salt includes at least one of aluminum trichloride and a lithium-containing molten salt compound.

[0013] Furthermore, the lithium-containing molten salt complex includes an AlCl3-LiCl-KCl ternary complex; Preferably, the molar ratio of AlCl3, LiCl and KCl in the ternary compound is (50-70):(15-25):(15-25).

[0014] Furthermore, the silicone oil, molten salt, and sodium hydride are mixed in an inert gas. Preferably, the water and oxygen content in the mixture is controlled to be ≤1ppm; Preferably, the mixing process further includes a pretreatment step; The pretreatment includes: vacuum drying the silicone oil at 100℃-120℃ for 2h-4h; and / or grinding and dispersing the sodium hydride under an inert atmosphere.

[0015] Furthermore, the mass ratio of the silicone oil, molten salt, and sodium hydride is 1:(5-30):(1-10).

[0016] Secondly, a silicon-carbon anode material prepared by any of the above-described preparation methods.

[0017] Thirdly, the application of the aforementioned silicon-carbon anode material in lithium batteries.

[0018] Fourthly, a solid-state battery, wherein the negative electrode of the solid-state battery comprises the silicon-carbon negative electrode material described above.

[0019] Compared with the prior art, the present invention has at least the following beneficial effects: The method for preparing silicon-carbon anode materials provided by this invention utilizes the strong reducing effect of sodium hydride to efficiently reduce elemental silicon in silicone oil. During the reaction, paraffin and the reduced elemental silicon can be fully mixed, which is beneficial to improving the preparation effect of silicon-carbon materials. By using a segmented temperature-controlled reaction process, the location and content of SiC formation can be precisely controlled, and the C-Si interface bonding force can be optimized, which is beneficial to improving the electrochemical cycle stability. The silicon-carbon anode material obtained by carbonization forms a core-shell structure of carbon-coated silicon or a Si@C / SiC composite structure, which better confines elemental silicon and is beneficial to cope with the volume expansion during charge and discharge. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 A flowchart illustrating the preparation process of a silicon-carbon anode material according to one embodiment of the present invention; Figure 2 The image shows an XRD pattern of a sample obtained by sodium hydride reduction according to one embodiment of the present invention. Detailed Implementation

[0022] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] According to a first aspect of the present invention, a method for preparing a silicon-carbon anode material is provided, comprising the following steps: Silicon oil, molten salt and sodium hydride are mixed and then subjected to low-temperature pre-reaction, medium-temperature reduction reaction and carbonization in sequence to obtain silicon-carbon anode material; Molten salts can lower the activation energy of a reaction; Sodium hydride contains 30%-50% paraffin. Because sodium hydride contains solid paraffin as a protective agent, the paraffin and the reduced elemental silicon can be fully mixed during the reaction. The low-temperature pre-reaction temperature is 150℃-200℃ and the time is 30min-60min, which can initially activate the contact interface between sodium hydride and silicone oil. The temperature of the medium-temperature reduction reaction is 300℃-400℃ and the time is 2h-5h. Sodium hydride reduces silicone oil to produce elemental silicon, and at the same time, paraffin and silicon can be initially combined. Carbonization forms a carbon coating layer and a controllable SiC interface layer, which can improve the structural stability of the material.

[0024] In this invention, a simple and low-energy-consumption method for reducing elemental silicon is employed. A safer organosilicon source is used as the precursor, and the reducing agent (sodium hydride) contains a carbon source. During the reduction process, silicon and carbon are thoroughly and uniformly mixed, which is more conducive to the subsequent preparation of silicon-carbon materials. Through a segmented temperature-controlled reaction process, a low-temperature pre-reaction can initially activate the interface between sodium hydride and silicone oil. The medium-temperature reduction reaction involves sodium hydride reducing silicone oil to generate elemental silicon, while paraffin wax and silicon undergo initial composite formation. Carbonization then forms a carbon coating layer and a controllable SiC interface layer, improving the material's structural stability. The synergistic effect of the segmented temperature-controlled reaction effectively improves the material's structural uniformity and electrochemical performance.

[0025] In summary, this invention utilizes the strong reducing effect of sodium hydride to efficiently reduce elemental silicon in silicone oil. During the reaction, paraffin and the reduced elemental silicon can be fully mixed, which is beneficial to improving the preparation effect of silicon-carbon materials. Through a segmented temperature-controlled reaction process, the location and content of SiC formation can be precisely controlled, and the C-Si interface bonding force can be optimized, which is beneficial to improving the electrochemical cycle stability. The silicon-carbon anode material obtained by carbonization forms a core-shell structure of carbon-coated silicon or a Si@C / SiC composite structure, which better confines elemental silicon and is beneficial to cope with the volume expansion during charge and discharge.

[0026] In a preferred embodiment, the carbonization temperature can be 700℃-1500℃ and the time can be 2.5h-5h, which is more conducive to forming a carbon-coated silicon core-shell structure or a Si@C / SiC composite structure.

[0027] In a preferred embodiment, the carbonization stages include, but are not limited to, a low-temperature carbonization stage, a medium-temperature transition stage, and a high-temperature stabilization stage; the temperature gradient between the low-temperature carbonization stage, the medium-temperature transition stage, and the high-temperature stabilization stage can be 300℃-500℃.

[0028] In a preferred embodiment, the temperature of the low-temperature carbonization stage can be 700℃-900℃ and the time can be 1h-2h, which is more conducive to the full formation of the carbon coating layer.

[0029] In a preferred embodiment, the temperature of the intermediate temperature transition stage can be 1000℃-1200℃ and the time can be 0.5h-1h, which is more conducive to the controllable generation of the SiC interface layer.

[0030] In a preferred embodiment, the temperature of the high-temperature stabilization stage can be 1300℃-1500℃, and the time can be 1h-2h, which is more conducive to fully improving the structural stability of the material.

[0031] In a preferred embodiment, the molten salt includes, but is not limited to, at least one of aluminum trichloride and lithium-containing molten salt complexes, which is more conducive to reducing the reaction activation energy and greatly reducing the reaction temperature.

[0032] In a preferred embodiment, the lithium-containing molten salt complex can be an AlCl3-LiCl-KCl ternary complex, which is more conducive to improving the first coulombic efficiency and rate performance of the material.

[0033] In a preferred embodiment, the molar ratio of AlCl3, LiCl and KCl in the ternary compound can be (50-70):(15-25):(15-25), which can reduce the melting temperature to about 150°C, improve ionic conductivity and reaction medium fluidity, introduce lithium ions which is more conducive to the subsequent lithium battery anode interface compatibility, and improve the first coulombic efficiency.

[0034] In a preferred embodiment, the silicone oil, molten salt, and sodium hydride can be mixed in an inert gas; the water and oxygen content in the mixture can be controlled to be ≤1ppm.

[0035] In a preferred embodiment, a pretreatment step is further included before mixing.

[0036] In a preferred embodiment, the pretreatment includes: vacuum drying the silicone oil at 100°C-120°C for 2-4 hours; and / or grinding and dispersing the sodium hydride under an inert atmosphere.

[0037] Pretreatment can prevent sodium hydride from oxidizing and failing, and silicone oil from hydrolyzing and deteriorating.

[0038] In a preferred embodiment, the mass ratio of silicone oil, molten salt, and sodium hydride can be 1:5-30:1-10.

[0039] A typical preparation method for a silicon-carbon anode material is described in [reference needed]. Figure 1 This includes the following steps: (a) Pretreatment and mixing process: (1) Pre-drying of raw materials: Vacuum dry the silicone oil at 100℃-120℃ for 2h-4h; Sodium hydride (paraffin content 30%-50%) is ground and dispersed under an inert atmosphere at room temperature to avoid local overheating; (2) Environmental control: Silicon oil (liquid phase silicon source), molten salt (AlCl3 or AlCl3-LiCl-KCl ternary compound) and sodium hydride (NaH) are mixed in an inert gas glove box, and the water and oxygen content in the glove box is controlled to be ≤1ppm; The mortar, reaction vessel and other contact parts should be dried at 120℃ for 2 hours and cooled to room temperature before use; (3) Feeding and mixing: Weigh silicone oil, molten salt and sodium hydride according to the mass ratio of 1:(5-30):(1-10). The feeding order is to first add molten salt as a dispersion medium, then add silicone oil and sodium hydride, and mix thoroughly with a mortar and pestle in an inert gas glove box. (b) Segmented temperature-controlled gradient reaction process: The mixture was transferred to a stainless steel reactor and subjected to low-temperature pre-reaction, medium-temperature reduction reaction and carbonization in an inert gas atmosphere to obtain silicon-carbon anode material. The low-temperature pre-reaction involves heating the temperature at 2℃ / min-5℃ / min to 150℃-200℃ and holding it at that temperature for 30min-60min to initially activate the interface between sodium hydride and silicone oil. The intermediate-temperature reduction reaction involves further heating to 300℃-400℃ and holding at that temperature for 2-5 hours. Sodium hydride reduces the silicone oil to elemental silicon (Si), while paraffin and silicon undergo initial composite formation. The XRD image of the sample obtained from sodium hydride reduction is shown below. Figure 2 Paraffin was present in the sample but could not be detected by XRD. Carbonization involves taking out the reactants, washing, drying, grinding them, and placing them in a tube furnace. Under an inert gas atmosphere, the process proceeds sequentially through a low-temperature carbonization stage, a medium-temperature transition stage, and a high-temperature stabilization stage. The low-temperature carbonization stage involves holding the temperature at 700℃-900℃ for 1-2 hours to form a carbon coating layer. During the intermediate temperature transition stage, the SiC interface layer can be generated by holding the temperature at 1000℃-1200℃ for 0.5h-1h. During the high-temperature stabilization phase, the material is kept at 1300℃-1500℃ for 1-2 hours to improve its structural stability.

[0040] According to a second aspect of the present invention, a silicon-carbon anode material prepared by any of the above-described preparation methods is provided.

[0041] According to a third aspect of the present invention, an application of the aforementioned silicon-carbon anode material in a lithium battery is provided.

[0042] According to a fourth aspect of the present invention, a solid-state battery is provided, wherein the negative electrode of the solid-state battery comprises the silicon-carbon negative electrode material described above.

[0043] The present invention will be further illustrated by the following examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0044] Example 1 A method for preparing a silicon-carbon anode material includes the following steps: (a) Pretreatment and mixing process: (1) Pre-drying of raw materials: The silicone oil was vacuum dried at 110°C for 3 hours; Sodium hydride was ground and dispersed at room temperature under an inert atmosphere, and the paraffin content in the sodium hydride was 40%. (2) Environmental control: Silicon oil (liquid phase silicon source), molten salt AlCl3 and sodium hydride (NaH) are mixed in an inert gas glove box, and the water and oxygen content in the glove box is controlled to be ≤1ppm; The mortar, reaction vessel and other contact parts should be dried at 120℃ for 2 hours and cooled to room temperature before use; (3) Feeding and mixing: Weigh silicone oil, molten salt and sodium hydride according to the mass ratio of 1:20:5. The feeding order is to first add molten salt as a dispersion medium, then add silicone oil and sodium hydride, and mix thoroughly with a mortar and pestle in an inert gas glove box. (b) Segmented temperature-controlled gradient reaction process: The mixture was transferred to a stainless steel reactor and subjected to low-temperature pre-reaction, medium-temperature reduction reaction and carbonization in an inert gas atmosphere to obtain silicon-carbon anode material. The low-temperature pre-reaction involves heating to 180℃ at a rate of 3℃ / min and holding for 45min to initially activate the interface between sodium hydride and silicone oil. The intermediate-temperature reduction reaction involves further heating to 350℃ and holding at that temperature for 2.5 hours. Sodium hydride reduces the silicone oil to generate elemental silicon (Si), while paraffin wax and silicon undergo preliminary composite formation. Carbonization involves taking out the reactants, washing, drying, grinding them, and placing them in a tube furnace. Under an inert gas atmosphere, the process proceeds sequentially through a low-temperature carbonization stage, a medium-temperature transition stage, and a high-temperature stabilization stage. The low-temperature carbonization stage involves holding the temperature at 800℃ for 1.5 hours to form a carbon coating layer. During the intermediate temperature transition stage, the SiC interface layer is generated controllably by holding the temperature at 1100℃ for 0.8h. The high-temperature stabilization stage involves holding the material at 1400℃ for 1.5 hours to improve its structural stability.

[0045] Example 2 The only difference between this embodiment and Embodiment 1 is that, in step (1), the paraffin content in the sodium hydride is 30%; The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0046] Example 3 The only difference between this embodiment and Embodiment 1 is that, in step (1), the paraffin content in the sodium hydride is 50%; The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0047] Example 4 The only difference between this embodiment and Example 1 is that in step (2), the molten salt AlCl3 is replaced with an AlCl3-LiCl-KCl ternary complex, and the molar ratio of AlCl3, LiCl and KCl in the ternary complex is 60:20:20. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0048] Example 5 The only difference between this embodiment and embodiment 1 is that in step (2), the molten salt AlCl3 is replaced with an AlCl3-LiCl-KCl ternary complex, and the molar ratio of AlCl3, LiCl and KCl in the ternary complex is 50:25:25; The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0049] Example 6 The only difference between this embodiment and embodiment 1 is that in step (2), the molten salt AlCl3 is replaced with an AlCl3-LiCl-KCl ternary complex, and the molar ratio of AlCl3, LiCl and KCl in the ternary complex is 70:15:15. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0050] Example 7 The only difference between this embodiment and embodiment 1 is that in step (3), the mass ratio of silicone oil, molten salt and sodium hydride is 1:5:1; The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0051] Example 8 The only difference between this embodiment and embodiment 1 is that in step (3), the mass ratio of silicone oil, molten salt and sodium hydride is 1:10:1; The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0052] Example 9 The only difference between this embodiment and embodiment 1 is that in step (3), the mass ratio of silicone oil, molten salt and sodium hydride is 1:20:1; The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0053] Example 10 The only difference between this embodiment and embodiment 1 is that in step (3), the mass ratio of silicone oil, molten salt and sodium hydride is 1:30:1; The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0054] Example 11 The only difference between this embodiment and embodiment 1 is that in step (3), the mass ratio of silicone oil, molten salt and sodium hydride is 1:30:5; The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0055] Example 12 The only difference between this embodiment and embodiment 1 is that in step (3), the mass ratio of silicone oil, molten salt and sodium hydride is 1:30:10; The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0056] Example 13 The only difference between this embodiment and Embodiment 1 is that the temperature of the low-temperature pre-reaction in the segmented temperature-controlled gradient reaction process is 150°C. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0057] Example 14 The only difference between this embodiment and Embodiment 1 is that the temperature of the low-temperature pre-reaction in the segmented temperature-controlled gradient reaction process is 200°C. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0058] Example 15 The only difference between this embodiment and Embodiment 1 is that the temperature of the medium-temperature reduction reaction in the segmented temperature-controlled gradient reaction process is 300°C. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0059] Example 16 The only difference between this embodiment and Embodiment 1 is that the temperature of the medium-temperature reduction reaction in the segmented temperature-controlled gradient reaction process is 400°C. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0060] Example 17 The only difference between this embodiment and Embodiment 1 is that the carbonization in the segmented temperature-controlled gradient reaction process does not include a low-temperature carbonization stage, but only a medium-temperature transition stage and a high-temperature stabilization stage. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0061] Example 18 The only difference between this embodiment and Embodiment 1 is that the carbonization in the segmented temperature-controlled gradient reaction process does not include a medium-temperature transition stage; that is, the high-temperature stabilization stage is directly carried out after the low-temperature carbonization stage. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0062] Example 19 The only difference between this embodiment and Embodiment 1 is that the carbonization in the segmented temperature-controlled gradient reaction process does not include a high-temperature stabilization stage, but only a low-temperature carbonization stage and a medium-temperature transition stage. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0063] Comparative Example 1 The only difference between this comparative example and Example 1 is that, in step (2), molten salt was not added for mixing; The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0064] Compared to Example 1, the drawback of this comparative example is that the absence of molten salt prevents silicon from being reduced, thus failing to obtain elemental silicon.

[0065] Comparative Example 2 The only difference between this comparative example and Example 1 is that the paraffin content in the sodium hydride is 20%. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0066] Compared with Example 1, the drawback of this comparative example is that the low paraffin content will result in incomplete or too thin coating of the reduced elemental silicon, affecting the expansion inhibition effect of the carbon layer on the elemental silicon in the later stage. At the same time, the discontinuous carbon layer can form a conductive network, increasing the resistance of the material.

[0067] Comparative Example 3 The only difference between this comparative example and Example 1 is that the paraffin content in the sodium hydride is 60%. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0068] Compared with Example 1, the drawback of this comparative example is that the silicon content in the material is reduced, which reduces the specific capacity of the material and limits the improvement of energy density. Excessive carbon source will also lead to the generation of excessive silicon carbide in the later stage, consuming more elemental silicon.

[0069] Comparative Example 4 The only difference between this comparative example and Example 1 is that the low-temperature pre-reaction temperature in the segmented temperature-controlled gradient reaction process is 100°C. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0070] Compared with Example 1, the drawback of this comparative example is that the silicone oil and sodium hydride cannot be fully contacted, and AlCl3 cannot be in a molten state, which hinders the reduction reaction.

[0071] Comparative Example 5 The only difference between this comparative example and Example 1 is that the low-temperature pre-reaction temperature in the segmented temperature-controlled gradient reaction process is 250°C. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0072] Compared with Example 1, the drawback of this comparative example is that the excessively high temperature may cause the paraffin to decompose or carbonize prematurely at high temperatures, losing its dispersion and coating effect on the newly formed silicon, resulting in silicon particle agglomeration; the silicone oil may volatilize or undergo non-target pyrolysis at high temperatures, affecting the final silicon yield and purity.

[0073] Comparative Example 6 The only difference between this comparative example and Example 1 is that the temperature of the medium-temperature reduction reaction in the segmented temperature-controlled gradient reaction process is 250°C. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0074] Compared with Example 1, the drawback of this comparative example is that the reduction kinetics of sodium hydride to silicone oil is insufficient, a large amount of silicone oil is not reduced, and the product contains unreacted silicone oil or its pyrolysis fragments.

[0075] Comparative Example 7 The only difference between this comparative example and Example 1 is that the temperature of the intermediate-temperature reduction reaction in the segmented temperature-controlled gradient reaction process is 450°C. The remaining steps are the same as in Example 1, and silicon-carbon anode material is obtained.

[0076] Compared with Example 1, the drawback of this comparative example is that the high temperature causes the generated silicon grains to coarsen, forming micron-sized silicon, which exacerbates the volume expansion problem and deteriorates the cycle stability.

[0077] Test case The silicon-carbon anode materials of Examples 1-19 and the silicon-carbon anode materials of Comparative Examples 1-7 were subjected to electrochemical performance tests and charge-discharge volume expansion tests. The results are shown in Table 1.

[0078] Test method: 1. Battery fabrication Electrode preparation: The silicon-carbon anode material, conductive agent (Super P), and binder (SBR+CMC) are mixed at a mass ratio of 90:5:5, and an appropriate amount of deionized water is added to form a slurry. The slurry is coated onto copper foil, dried in a vacuum oven at 120°C for 12 hours, and then cut into Φ14mm circular electrodes.

[0079] Battery Assembly: CR2032 coin cells were assembled in an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm). The counter electrode was a lithium metal sheet, the electrolyte was 1M LiPF6 in EC / DEC (1:1 vol%) containing 10% FEC, and the separator was Celgard 2400.

[0080] 2. Electrochemical performance testing (room temperature, 25℃) First-cycle discharge specific capacity & first-cycle coulombic efficiency: A constant current charge-discharge tester was used, with a voltage range of 0.005V - 1.5V.

[0081] The first cycle involves discharging (lithium insertion) at a rate of 0.1C to 0.005V, followed by charging (lithium removal) at a rate of 0.1C to 1.5V.

[0082] First-cycle discharge specific capacity (mAh / g) = First-cycle discharge capacity / Mass of negative electrode active material.

[0083] Initial Coulomb efficiency (%) = (First charge capacity / First discharge capacity) × 100%.

[0084] 500-cycle discharge capacity: After the first activation cycle, subsequent cycles are performed at a charge / discharge rate of 0.5C.

[0085] Record the discharge specific capacity on the 500th cycle.

[0086] 3. Volume expansion rate test Test subject: specially prepared "silicon-carbon negative electrode sheet" rather than a complete battery (or using an in-situ expansion analyzer).

[0087] method: Initial measurement: After coating and drying, measure the average thickness (d0) of the electrode sheet after stamping.

[0088] Simulate full lithium insertion state: Assemble the electrode sheets into a half cell, discharge it to 0.005V at 0.1C, then disassemble and remove the electrode sheets in a glove box, carefully clean off the electrolyte and dry them.

[0089] Thickness after expansion: The thickness (d1) of the electrode after full lithium insertion was measured using a high-precision thickness gauge (accuracy ±1μm).

[0090] Volume expansion rate (%) = [(d1- d0) / d0] × 100%. (Note: This method approximates the assumption that thickness variation dominates volume variation).

[0091] Table 1

[0092] The data in Table 1 shows the following: 1. Comparative analysis between implementation examples Effect of paraffin content (Examples 1 vs 2 vs 3): Data: Example 1 (40% paraffin) showed the best performance. Example 2 (30%) had slightly lower capacity and greater expansion (48%); Example 3 (50%) showed a significant decrease in capacity (1680).

[0093] Analysis: Paraffin wax serves as both a carbon source and a dispersant. Incomplete coating with 30% paraffin wax leads to poor cycling stability and significant expansion. While 50% paraffin wax improves cycling slightly, the excessively thick carbon layer reduces the proportion of active silicon, sacrificing specific capacity. 40% represents the optimal balance, validating the preference for the 30%-50% range in this invention.

[0094] Effect of molten salt type (Examples 1 vs 4): Data: The initial coulombic efficiency (91.5%) and 500-cycle capacity (1580) of Example 4 (lithium-containing ternary complex) are significantly better than those of Example 1 (AlCl3, 88.5% / 1425).

[0095] Analysis: The lithium-containing molten salt (AlCl3-LiCl-KCl) not only lowers the reaction activation energy but also introduces lithium ions, improving interfacial compatibility with the negative electrode, thereby significantly enhancing the initial coulombic efficiency and long-term cycling stability.

[0096] Effect of mass ratio (Examples 7-12): Data: As the proportion of molten salt increases (1:5→1:30) and the amount of sodium hydride increases moderately (1→5), the capacity and cycle life gradually improve; however, excessive sodium hydride (1:30:10) leads to a decrease in capacity.

[0097] Analysis: Sufficient molten salt provides a good reaction medium; an appropriate amount of sodium hydride ensures reduction efficiency. Too little molten salt results in incomplete reaction (the capacity of Example 7 was only 1650), while too much sodium hydride may lead to violent local reactions and structural defects.

[0098] Integrity of the carbonization stage (Examples 1 vs 17, 18, 19): Data shows that the absence of any carbonization stage leads to performance degradation, especially the absence of low-temperature carbonization (Example 17, without carbon coating), where the capacity after 500 cycles plummets to 900, and the expansion rate is as high as 68%.

[0099] Analysis: The three-stage gradient carbonization process (low-temperature carbon coating formation → mid-temperature SiC interface layer formation → high-temperature structural stabilization) is indispensable. In particular, the carbon shell formed by low-temperature carbonization is the first line of defense against volume expansion.

[0100] 2. Comparative Analysis of Examples and Comparative Cases Comparative Example 1 (without molten salt): Data: Discharge specific capacity is only 350 mAh / g, coulombic efficiency is 55%, and there is almost no cycling performance.

[0101] Analysis: Molten salt is the key medium for lowering the activation energy of the reaction. Without molten salt, silicone oil cannot be effectively reduced by sodium hydride, and the products are mainly inactive substances, verifying the necessity of the method of this invention.

[0102] Comparative Examples 2-3 (outside the paraffin content boundary): Data: Comparative Example 2 (20% paraffin) had extremely poor cycling performance (950 cycles out of 500); Comparative Example 3 (60% paraffin) had very low capacity (1200 cycles).

[0103] Analysis: This study validated the boundary effect of paraffin content between 30% and 50%. Content below 30% leads to silicon agglomeration / exposedness; content above 50% results in excessively low silicon content and the formation of too much inactive SiC.

[0104] Comparative Examples 4-5 (outside the low-temperature pre-reaction temperature boundary): Data: The overall performance (capacity, efficiency, cycle life) of Comparative Example 4 (100°C, too low temperature) and Comparative Example 5 (250°C, too high temperature) was worse than that of the Example.

[0105] Analysis: 150-200℃ is the critical window. Too low a temperature (100℃) will not activate the interface; too high a temperature (250℃) will cause the paraffin to decompose and become ineffective prematurely, and the silicon particles will coarsen.

[0106] Comparative Examples 6-7 (outside the boundary of the mesotemperature reduction temperature): Data: Comparative Example 6 (250℃) showed insufficient reduction kinetics, extremely low capacity (800) and poor efficiency (70%). Comparative Example 7 (450℃) had acceptable capacity, but its cycling decay was extremely rapid (only 900 after 500 cycles).

[0107] Analysis: 300℃-400℃ is the optimal range for sodium hydride reduction of silicone oil. Too low a temperature leads to incomplete reduction; too high a temperature causes excessive growth of silicon grains into micron-sized silicon (exacerbating volume expansion) and premature graphitization of the carbon source, resulting in the loss of its flexible coating effect.

[0108] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a silicon-carbon anode material, characterized in that, Includes the following steps: Silicon oil, molten salt and sodium hydride are mixed and then subjected to low-temperature pre-reaction, medium-temperature reduction reaction and carbonization in sequence to obtain the silicon-carbon anode material. The sodium hydride contains 30%-50% paraffin. The temperature of the low-temperature pre-reaction is 150℃-200℃; The temperature of the intermediate-temperature reduction reaction is 300℃-400℃.

2. The preparation method according to claim 1, characterized in that, The carbonization temperature is 700℃-1500℃, and the time is 2.5h-5h.

3. The preparation method according to claim 2, characterized in that, The carbonization stages include a low-temperature carbonization stage, a medium-temperature transition stage, and a high-temperature stabilization stage. The temperature gradient between the low-temperature carbonization stage, the medium-temperature transition stage, and the high-temperature stabilization stage is 300℃-500℃.

4. The preparation method according to claim 3, characterized in that, The temperature of the low-temperature carbonization stage is 700℃-900℃, and the time is 1h-2h. Preferably, the temperature of the intermediate temperature transition stage is 1000℃-1200℃, and the time is 0.5h-1h; Preferably, the temperature of the high-temperature stabilization stage is 1300℃-1500℃, and the time is 1h-2h.

5. The preparation method according to claim 1, characterized in that, The molten salt includes at least one of aluminum trichloride and a lithium-containing molten salt compound.

6. The preparation method according to claim 5, characterized in that, The lithium-containing molten salt complex includes an AlCl3-LiCl-KCl ternary complex; Preferably, the molar ratio of AlCl3, LiCl and KCl in the ternary compound is (50-70):(15-25):(15-25).

7. The preparation method according to any one of claims 1-6, characterized in that, The silicone oil, molten salt, and sodium hydride are mixed in an inert gas. Preferably, the water and oxygen content in the mixture is controlled to be ≤1ppm; Preferably, the mixing process further includes a pretreatment step; The pretreatment includes: vacuum drying the silicone oil at 100℃-120℃ for 2h-4h; And / or, the sodium hydride is ground and dispersed under an inert atmosphere; Preferably, the mass ratio of the silicone oil, molten salt and sodium hydride is 1:(5-30):(1-10).

8. A silicon-carbon anode material prepared by the preparation method according to any one of claims 1-7.

9. The application of the silicon-carbon anode material according to claim 8 in a lithium battery.

10. A solid-state battery, characterized in that, The negative electrode of the solid-state battery comprises the silicon-carbon negative electrode material as described in claim 8.

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