Silicon / graphite composite negative electrode material and preparation method thereof, lithium ion battery and electric equipment

Abstract

The invention provides a silicon / graphite composite negative electrode material and a preparation method thereof, a lithium ion battery and electric equipment, and relates to the field of lithium ion batteries. The preparation method of the silicon / graphite composite negative electrode material comprises the following steps: mixing a silicon source with melamine and water, and carrying out hydrothermal reaction to obtain a composite precursor; performing heat treatment on the composite precursor to obtain an intermediate; and mixing the intermediate with reductive iron powder, and carrying out catalytic graphitization reaction to obtain the silicon / graphite composite negative electrode material. According to the silicon / graphite composite negative electrode material, the silicon material can be uniformly embedded between graphite layers to form a composite structure on a micro-level, the composite structure is more stable than a conventional structure formed by physically mixing or combining the silicon material and a carbon material together by using an adhesive, and the prepared silicon / graphite composite negative electrode material has excellent cycle stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion batteries, in particular to a silicon / graphite composite negative electrode material, a preparation method thereof, a lithium ion battery and an electric device. BACKGROUND

[0002] With the rapid development of new energy industry, the market demand for energy density of lithium ion batteries is increasing, and thus higher demand for specific capacity of negative electrode materials is proposed. The currently widely used graphite negative electrode has gradually been difficult to meet the design requirements of high energy density batteries due to its limited theoretical specific capacity (about 375 mAh / g). Silicon-based negative electrode materials are considered as one of the most promising next-generation negative electrode materials due to their high theoretical specific capacity (4200 mAh / g), abundant resources and environmental friendliness. However, silicon material is a semiconductor with poor electrical conductivity, and there is significant volume expansion (>300%) during charging and discharging, which leads to active material pulverization, shedding, and continuous consumption of electrolyte to form an unstable solid electrolyte interface film (SEI), resulting in rapid capacity decay and cycle performance deterioration. Therefore, developing silicon-based negative electrode materials with high electrical conductivity, structural stability and long cycle life has become a key issue to improve battery performance and promote industrial technology upgrading.

[0003] To overcome the above problems, the prior art usually adopts the method of compounding silicon with carbon material which has good electrical conductivity and structural stability to improve the electrical conductivity and structural stability of silicon-based materials. At present, the mainstream solution is to load silicon particles on the surface or in the pores of finished graphite. However, this method essentially belongs to "secondary compounding" between two materials, and the interface bonding strength is often limited. During long-term cycling, the weak interface is prone to peeling, leading to structural damage and making it difficult to completely solve the problem of short cycle life.

[0004] Therefore, how to strengthen the silicon-graphite interface bonding to solve the problem of poor cycle stability of negative electrode materials has become a technical problem to be solved in the field of lithium ion battery negative electrode materials. SUMMARY

[0005] The purpose of the present application is to provide a silicon / graphite composite negative electrode material, a preparation method thereof, a lithium ion battery and an electric device to solve the above problems.

[0006] To achieve the above purpose, the following technical solutions are adopted in the present application: A preparation method of a silicon / graphite composite negative electrode material, comprising: mixing a silicon source, melamine and water, and performing a hydrothermal reaction, and after the reaction is completed, performing solid-liquid separation and drying to obtain a composite precursor; performing heat treatment on the composite precursor to obtain an intermediate; Mixing the intermediate with the reducing iron powder to perform a catalytic graphitization reaction to obtain the silicon / graphite composite negative electrode material.

[0007] According to the embodiments of the present application, the silicon source comprises at least one of silicon and silicon carbon; The mass ratio of the melamine to the silicon source is (3-25):1; In the step of preparing the composite precursor, the ratio of the total mass of the silicon source and the melamine to the mass of water is (10-50):100.

[0008] According to the embodiments of the present application, the temperature of the hydrothermal reaction is 120-250℃; The time of the hydrothermal reaction is 3 h-48 h.

[0009] According to the embodiments of the present application, the temperature of the heat treatment is 400-800℃; The heating rate of the heat treatment is 1-10℃ / min; The time of the heat treatment is 2h-10h.

[0010] According to the embodiments of the present application, the mass ratio of the intermediate to the reducing iron powder is (1-5):1.

[0011] According to the embodiments of the present application, the temperature of the catalytic graphitization reaction is 500-1200℃; The heating rate of the catalytic graphitization reaction is 1-10℃ / min.

[0012] According to the embodiments of the present application, the time of the catalytic graphitization reaction is 1-10h.

[0013] The present application also provides a silicon / graphite composite negative electrode material prepared by the preparation method of the silicon / graphite composite negative electrode material.

[0014] The present application also provides a lithium ion battery comprising the silicon / graphite composite negative electrode material prepared by the preparation method of the silicon / graphite composite negative electrode material or comprising the silicon / graphite composite negative electrode material.

[0015] The present application also provides an electrical equipment comprising the lithium ion battery.

[0016] Compared with the prior art, the present application has the following beneficial effects: This application enables the uniform composite of silicon and graphitic carbon nitride through chemical self-assembly. During the process of graphitic carbon nitride forming graphite, silicon can be embedded in situ into the interlayer microstructure of graphite. The composite structure is more stable than the structure of conventional physical mixing or using binders to combine silicon and carbon materials. The silicon / graphite composite anode material prepared in this application has high initial coulombic efficiency and cycle stability.

[0017] Specifically, this application utilizes a chemical self-assembly method to embed a silicon source into a graphite precursor, and then uses high-temperature catalysis to convert the precursor into graphite, forming a tightly bonded silicon-graphite composite material. This material does not use additional binders and forms a composite product before graphitization. Through the graphitization process, the silicon material and graphite precursor are more tightly bonded to the material structure during high-temperature thermodynamic action, forming a silicon / graphite anode material with a microstructure composite. This silicon / graphite composite anode material has excellent electrochemical performance. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation on the scope of this application.

[0019] Figure 1 The XRD pattern of the silicon / graphite composite anode material in Example 1 is shown below. Figure 2 The graph shows the capacity retention rate of Example 1 and Comparative Examples 1-2 as a function of the number of cycles. Detailed Implementation

[0020] As used in this article: "Prepared from" is synonymous with "comprising". The terms "comprising", "including", "having", "containing", or any other variations thereof as used herein are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0021] The conjunction "composed of..." excludes any unspecified elements, steps, or components. If used in a claim, this phrase makes the claim closed, excluding materials other than those described, except for associated conventional impurities. When the phrase "composed of..." appears in a clause of the body of a claim rather than immediately following it, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0022] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1–5” is disclosed, the described range should be interpreted as including ranges “1–4”, “1–3”, “1–2”, “1–2 and 4–5”, “1–3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0023] In these embodiments, unless otherwise specified, the portions and percentages are all by weight.

[0024] "Parts by mass" refers to the basic unit of measurement that expresses the mass ratio of multiple components. One part can represent any unit mass, such as 1g or 2.689g. If we say that component A has "a" parts by mass and component B has "b" parts by mass, it means the ratio of the mass of component A to the mass of component B is a:b. Alternatively, it can mean that the mass of component A is aK and the mass of component B is bK (where K is any number representing a multiplier). It is important to understand that, unlike parts by mass, the sum of the mass parts of all components is not limited to 100 parts.

[0025] "And / or" is used to indicate that one or both of the described situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0026] A method for preparing a silicon / graphite composite anode material includes: A composite precursor was obtained by mixing a silicon source with melamine and water and carrying out a hydrothermal reaction. The composite precursor is subjected to heat treatment to obtain an intermediate; The intermediate was mixed with reducing iron powder and subjected to a catalytic graphitization reaction to obtain a silicon / graphite composite anode material.

[0027] In the hydrothermal reaction step, melamine and water undergo a hydrolysis reaction under heating conditions to produce cyanuric acid, NH3, and CO2. After the cyanuric acid and unreacted melamine are thoroughly mixed, they self-assemble to form a composite precursor. The generated gas creates a large number of porous structures in the composite precursor. The precursor, which gradually forms during the hydrothermal reaction, grows with silicon material as its core, uniformly coating the silicon material to form the composite precursor.

[0028] After heat treatment, the composite precursor forms a composite structure (Si / g-C3N4) with silicon embedded in g-C3N4 (graphitic carbon nitride), i.e., an intermediate. g-C3N4 has a two-dimensional layered structure similar to graphite, exhibiting good mechanical properties, thermal stability, and chemical stability, but its electrical conductivity is relatively weak. Therefore, reducing iron powder is mixed with the intermediate (Si / g-C3N4 material) as a catalyst. Under high temperature and the catalysis of reducing iron powder, g-C3N4 is converted into graphite, yielding a silicon / graphite composite anode material.

[0029] This application mainly utilizes the self-assembly process of melamine to introduce silicon-based materials into it, forming a tightly bonded Si / g-C3N4 material. The material is then subjected to catalytic graphitization treatment to form a silicon / graphite composite anode material. This process allows silicon materials to be uniformly embedded between graphite layers to form a composite structure at the microscopic level. Compared with conventional liquid-phase mixing and solid-phase mixing methods that only involve mixing at the two-phase level, the chemical composite process of this application can combine silicon and carbon from the internal structure, which can stabilize the structure of silicon-based materials and improve the conductivity and cycle performance of the materials.

[0030] According to embodiments of this application, the silicon source includes at least one of silicon and silicon-carbon; In some embodiments, the silicon source D v50 The range is 100-600 nm.

[0031] The mass ratio of melamine to silicon source is (3~25):1; For example, the mass ratio of melamine to silicon source can be any value between 3:1, 5:1, 7:1, 9:1, 10:1, 11:1, 13:1, 15:1, 17:1, 19:1, 20:1, 21:1, 23:1, 25:1 or (3~25):1.

[0032] The mass ratio of melamine to silicon source determines the ratio of graphite to silicon-based material in the final silicon / graphite material. When the melamine ratio is too low, the graphite content is low, and the silicon-based material will be exposed, making it difficult to form an effective embedded structure. This will result in poor structural stability of the anode material and consequently poor cycle performance. On the other hand, if there is too much melamine, the specific capacity of the final material will be too low, losing the high capacity advantage of silicon-based materials.

[0033] In the step of preparing the composite precursor, the ratio of the total mass of silicon source and melamine to the mass of water is (10~50):100.

[0034] For example, the ratio of the total mass of silicon source and melamine to the mass of water can be any value between 10:100, 20:100, 30:100, 40:100, 50:100, or (10~50):100.

[0035] Specifically, the hydrothermal reaction is carried out in a high-pressure reactor.

[0036] According to an embodiment of this application, the temperature of the hydrothermal reaction is 120~250°C; For example, the temperature of the hydrothermal reaction can be 120℃, 140℃, 160℃, 180℃, 200℃, 220℃, 240℃, 250℃ or any value between 120℃ and 250℃.

[0037] When the hydrothermal temperature is too high, melamine will undergo excessive hydrolysis and will not form a precursor. Furthermore, the pressure in the autoclave will increase dramatically, posing a safety risk. Conversely, when the hydrothermal temperature is too low, the hydrolysis reaction will not occur.

[0038] The hydrothermal reaction time is 3 h to 48 h.

[0039] For example, the hydrothermal reaction time can be any value between 3 h, 4 h, 5 h, 7 h, 9 h, 11 h, 13 h, 15 h, 17 h, 19 h, 21 h, 23 h, 25 h, 27 h, 29 h, 31 h, 33 h, 35 h, 37 h, 39 h, 41 h, 43 h, 45 h, 48 h, or 3 h to 48 h.

[0040] The hydrothermal reaction time gradually increases with the proportion of melamine. If the hydrothermal reaction time is too short, the reaction will be incomplete, thus affecting the graphite content in the final silicon / graphite material. If the hydrothermal reaction time is too long, that is, continuing to heat after the reaction is complete, it will result in energy waste. According to an embodiment of this application, the temperature of the heat treatment is 400~800℃; For example, the heat treatment temperature is 400℃, 500℃, 600℃, 700℃, 800℃ or any value between 400 and 800℃.

[0041] If the heat treatment temperature is too high, the morphology of the composite precursor is prone to collapse, which in turn deteriorates the electrochemical performance of the anode material; if the heat treatment temperature is too low, the conversion reaction from the composite precursor to graphitic carbon nitride cannot occur.

[0042] The heating rate of the heat treatment is 1~10℃ / min; For example, the heating rate of the heat treatment is 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min or any value between 1 and 10℃ / min.

[0043] The heat treatment time is 2h to 10h.

[0044] For example, the heat treatment time can be 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h or any value between 2h and 10h.

[0045] If the heat treatment time is too short, the conversion of the composite precursor to the intermediate will be incomplete; if the heat treatment time is too long, energy will be wasted after the composite precursor has completely reacted.

[0046] According to an embodiment of this application, the mass ratio of the intermediate to the reduced iron powder is (1~5):1.

[0047] For example, the mass ratio of the intermediate to the reducing iron powder is any value between 1:1, 2:1, 3:1, 4:1, 5:1 or (1~5):1.

[0048] When the proportion of reduced iron powder is too low, the catalytic reaction will be incomplete, resulting in poor graphitization. As the proportion of reduced iron powder increases, the degree of graphitization gradually increases to a stable level. When the proportion of reduced iron powder is too high, the pickling process will be complicated, residues will be easily left, and the electrical properties of the material will be affected.

[0049] In some embodiments, the intermediate and the reduced iron powder are mixed by ball milling, the ball milling speed is 50~200 rpm, the ball milling time is 1~8 h, and the ball-to-material ratio is (2~15):1.

[0050] According to an embodiment of this application, the temperature of the catalytic graphitization reaction is 500~1200℃; For example, the temperature for the catalytic graphitization reaction is 500℃, 600℃, 700℃, 800℃, 900℃, 1000℃, 1100℃, 1200℃ or any value between 500℃ and 1200℃.

[0051] If the temperature for catalytic graphitization is too low, the catalytic graphitization reaction cannot occur; as the reaction temperature increases, the degree of graphitization increases, but excessively high temperatures can easily cause the material structure to collapse, thereby reducing the material's electrical properties.

[0052] The heating rate of the catalytic graphitization reaction is 1~10℃ / min.

[0053] For example, the heating rate of the catalytic graphitization reaction is 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min or any value between 1 and 10℃ / min.

[0054] According to an embodiment of this application, the catalytic graphitization reaction takes 1 to 10 hours.

[0055] For example, the time for catalytic graphitization reaction can be 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h or any value between 1 and 10h.

[0056] The time required for catalytic graphitization affects the completeness of the graphitization of intermediates; too short a time will result in an incomplete reaction.

[0057] Under the catalysis of reducing iron powder, the intermediate is transformed into graphite.

[0058] In some embodiments, after the catalytic graphitization reaction is completed, the method further includes: The product of the catalytic graphitization reaction was washed with dilute hydrochloric acid to remove excess reduced iron powder, and after drying, a silicon / graphite composite anode material was obtained.

[0059] After the catalytic graphitization reaction is completed, excess Fe is washed away with dilute hydrochloric acid, and then the material is washed with pure water until neutral. After drying, silicon / graphite composite material is obtained.

[0060] In some embodiments, the specific surface area of ​​the silicon / graphite composite anode material is 3.55~5.88 m². 2 / g.

[0061] This application also provides a silicon / graphite composite anode material, which is prepared by the method described above for preparing silicon / graphite composite anode materials.

[0062] This application also provides a lithium-ion battery, comprising a silicon / graphite composite anode material prepared by the method described above, or comprising the silicon / graphite composite anode material described above.

[0063] This application also provides an electrical device, including the lithium-ion battery described above.

[0064] The implementation schemes of this application will be described in detail below with reference to specific embodiments. However, those skilled in the art will understand that the following embodiments are only for illustrating this application and should not be regarded as limiting the scope of this application. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used without specified manufacturers are all conventional products that can be purchased commercially.

[0065] Example 13, D of the nano-silicon-carbon material used in Comparative Example 3 v50 =100-600nm, purchased from Sichuan Huayiqing Innovative Materials Technology Co., Ltd.

[0066] The graphite used in Comparative Example 1 was purchased from Hunan Zhongke Xingcheng Graphite Co., Ltd.

[0067] Example 1 Example 1 provides a silicon / graphite composite anode material, the preparation method of which includes: (1) Melamine was mixed with nano-silicon and ultrapure water, wherein the mass ratio of melamine to nano-silicon was 5:1, and the mass ratio of the total mass of silicon source and melamine to the mass of water was 25:100. The mixture of melamine, nano-silicon and ultrapure water was placed in a high-pressure reactor and subjected to a hydrothermal reaction at 200°C for 4 hours. After the reaction was completed, a composite precursor was obtained. (2) Take out the composite precursor obtained in step (1), wash and dry it multiple times with ultrapure water, and heat treat the dried composite precursor: heat it to 600°C at a heating rate of 5°C / min, and keep it at 600°C for 3 hours to obtain the intermediate.

[0068] (3) The intermediate and reduced iron powder were ball-milled at a mass ratio of 3:1 at a ball milling rate of 100 rpm for 3 h. After ball milling, a catalytic graphitization reaction was carried out: the temperature was heated to 800℃ at a heating rate of 5℃ / min and kept at 800℃ for 4 h. After the reaction was completed, the material was mixed with 10% hydrochloric acid and stirred for 2 h. The material was separated by filtration to wash away excess iron powder. This step was repeated 3 times or more until there were no Fe ions in the filtrate. The material was then washed with pure water until neutral and then solid-liquid separation was carried out. The separated material was placed in an 80℃ oven to dry and then passed through a 300-mesh standard sieve to obtain silicon / graphite composite anode material.

[0069] Figure 1 The XRD pattern of the silicon / graphite composite anode material in Example 1 is shown below. Figure 1 It can be seen that the silicon / graphite composite material prepared by self-assembly contains obvious graphite, Si, and SiC phases.

[0070] Example 2 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 2 and Example 1 is that in step (1), the mass ratio of melamine to nano-silicon is 10:1, and the rest is the same as in Example 1.

[0071] Example 3 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 3 and Example 1 is that in step (1), the mass ratio of melamine to nano-silicon is 15:1, and the rest is the same as in Example 1.

[0072] Example 4 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 4 and Example 1 is that in step (1), the mass ratio of melamine to nano-silicon is 20:1, and the rest is the same as in Example 1.

[0073] Example 5 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 5 and Example 1 is that in step (1), the temperature of the hydrothermal reaction is 150°C, and the rest is the same as in Example 1.

[0074] Example 6 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 6 and Example 1 is that in step (1), the temperature of the hydrothermal reaction is 250°C, and the rest is the same as in Example 1.

[0075] Example 7 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 7 and Example 1 is that in step (1), the hydrothermal reaction time is 8 h, and the rest is the same as in Example 1.

[0076] Example 8 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 8 and Example 1 is that in step (1), the hydrothermal reaction time is 24 h, and the rest is the same as in Example 1.

[0077] Example 9 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 8 and Example 1 is that in step (1), the hydrothermal reaction time is 48 h, and the rest is the same as in Example 1.

[0078] Example 10 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 10 and Example 1 is that in step (3), the temperature of the catalytic graphitization reaction is 600℃, and the rest is the same as in Example 1.

[0079] Example 11 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 11 and Example 1 is that in step (3), the temperature of the catalytic graphitization reaction is 1000℃, and the rest is the same as in Example 1.

[0080] Example 12 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 12 and Example 1 is that in step (3), the temperature of the catalytic graphitization reaction is 1200℃, and the rest is the same as in Example 1.

[0081] Example 13 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Example 13 and Example 1 is that in step (1), nano-silicon carbon material was used to replace the nano-silicon in Example 1, and the rest was the same as in Example 1.

[0082] Comparative Example 1 Graphite and nano-silicon were ball-milled at a mass ratio of 1:2 and then ball-milled at a rate of 300 rpm for 4 hours to obtain a silicon-graphite hybrid material.

[0083] Comparative Example 2 Comparative Example 2 uses nano-silicon as the negative electrode material, and the nano-silicon is not modified in any way.

[0084] Comparative Example 3 Comparative Example 3 uses nano-silicon carbon as the negative electrode material, and the nano-silicon carbon is not modified in any way.

[0085] Comparative Example 4 Silicon / graphite composite anode material was prepared according to the method of Example 1. The difference between Comparative Example 4 and Example 1 is that step (3) is omitted in Comparative Example 4, and the intermediate obtained in step (2) is directly used as the anode material. The rest is the same as in Example 1.

[0086] The specific surface area of ​​the negative electrode materials in Examples 1-13 and Comparative Examples 1-4 was tested using the nitrogen adsorption-desorption isotherm method.

[0087] The specific surface areas of the negative electrode materials in Examples 1-13 and Comparative Examples 1-4 are shown in Table 1.

[0088] Table 1. Comparison of specific surface area of ​​negative electrode materials in Examples 1-13 and Comparative Examples 1-4

[0089] As shown in Table 1, the specific surface area of ​​the negative electrode materials in Examples 1-13 is smaller than that of the negative electrode materials in Comparative Examples 1-4. A larger specific surface area of ​​the negative electrode material results in more lithium ions being consumed during charge-discharge to form the SEI film, thus leading to a corresponding decrease in the first-cycle coulombic efficiency under the same conditions. The specific surface area of ​​the negative electrode materials in Examples 1-13 ranges from 3.55 to 5.88 m². 2 / g, Examples 1-13 have a smaller specific surface area of ​​anode materials, which reduces irreversible lithium loss caused by the formation of SEI film and is conducive to obtaining anode materials with higher first-cycle coulombic efficiency.

[0090] The negative electrode materials of Examples 1-13 and Comparative Examples 1-4 were assembled into coin cells. Specifically, the negative electrode materials of Examples 1-13 or Comparative Examples 1-4, conductive agents, and binders were mixed in a mass ratio of 8:1:1, and an appropriate amount of NMP solvent was added. The mixture was stirred evenly using a homogenizer, and the slurry coating thickness was 150 μm. After drying, the electrode sheets were cut into 12 mm diameter discs, and the coin cells were assembled in the following order: positive electrode shell, electrode sheet, separator, lithium sheet, spacer, spring sheet, and negative electrode shell.

[0091] The button cells were subjected to charge / discharge tests and cycle performance tests. The conditions for charge / discharge tests and cycle performance tests included: charge / discharge rate: 0.1C, voltage range: 0.05V~1.5V.

[0092] The electrochemical performance of Examples 1-13 and Comparative Examples 1-4 is shown in Table 2.

[0093] Table 2. Comparison of electrochemical performance in Examples 1-13 and Comparative Examples 1-4

[0094] As shown in Table 2, the initial coulombic efficiency of Examples 1-13 is greater than that of Comparative Examples 1-4. Examples 1-4 demonstrate that the initial efficiency gradually increases with increasing melamine content. This is because the graphite content in the material gradually increases, making the conductive network more complete and thus increasing the initial efficiency.

[0095] As can be seen from Examples 1 and 10-12, as the temperature of the catalytic graphitization reaction increases, the first-cycle coulombic efficiency first increases and then decreases. This is because the increase in temperature of the catalytic graphitization reaction will increase the degree of graphitization accordingly, but excessively high temperature will cause the material morphology to collapse.

[0096] As can be seen from Example 1 and Comparative Example 1, Comparative Example 1 uses a physical mixing method to prepare silicon-graphite composite material. The first-cycle coulombic efficiency of Example 1 is higher than that of Comparative Example 1. This may be because the mixing uniformity and composite strength of silicon-graphite composite material prepared by the physical mixing method are lower than those of silicon / graphite composite material formed by self-assembly. Therefore, the first-cycle coulombic efficiency of Comparative Example 1 is relatively low.

[0097] As can be seen from Example 1 and Comparative Example 2, the first-cycle coulombic efficiency of Example 1 is higher than that of Comparative Example 2. The first-cycle coulombic efficiency of unmodified nano-silicon is much lower than that of Example 1, because the intrinsic conductivity of nano-silicon is poor.

[0098] As can be seen from Example 13 and Comparative Example 3, the first-cycle coulombic efficiency of Example 13 is higher than that of Comparative Example 3, while the first-cycle coulombic efficiency of unmodified nano-silicon carbon is much lower than that of Example 13.

[0099] As can be seen from Example 1 and Comparative Example 4, the initial coulombic efficiency of Example 1 is significantly greater than that of Comparative Example 4. This may be because Comparative Example 4 omitted step (3) and did not carry out the catalytic graphitization reaction.

[0100] From Table 2 and Figure 1It can be seen that the capacity retention of the negative electrode materials in Examples 1-13 after 100 cycles is all above 90%, and their cycle stability is significantly better than that of Comparative Examples 1-4. Among them, the cycle performance of the nano-silicon in Comparative Example 2 is poor, with the capacity decreasing rapidly in the first 10 cycles. Comparative Example 2 failed to complete the 100-cycle test due to failure during the cycle test. The cycle performance of the silicon-graphite composite material of Comparative Example 1 after physical mixing is somewhat improved compared with nano-silicon, but the cycle stability is still poor. The capacity retention after 100 cycles is only 55.44%, indicating that the physical composite structure is not stable and cannot effectively improve the stability of silicon materials and improve cycle performance. The silicon-graphite composite material formed by chemical self-assembly in Example 1 has excellent cycle performance. After 100 cycles, the capacity retention is 92.28%, indicating that the chemically self-assembled composite material has high strength and can better maintain the integrity of silicon materials during cycling, thus exhibiting excellent cycle performance.

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application 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 therein. Such 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 this application.

[0102] Furthermore, those skilled in the art will understand that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and form different embodiments. For example, in the foregoing claims, any of the claimed embodiments can be used in any combination. The information disclosed in this background section is intended only to enhance the understanding of the general background of this application and should not be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

Claims

1. A method for preparing a silicon / graphite composite anode material, characterized in that, include: A composite precursor was obtained by mixing a silicon source with melamine and water and carrying out a hydrothermal reaction. The composite precursor is subjected to heat treatment to obtain an intermediate; The intermediate was mixed with reducing iron powder and subjected to a catalytic graphitization reaction to obtain a silicon / graphite composite anode material.

2. The method for preparing the silicon / graphite composite anode material according to claim 1, characterized in that, The silicon source includes at least one of silicon and silicon-carbon. The mass ratio of melamine to silicon source is (3~25):1; In the step of preparing the composite precursor, the ratio of the total mass of silicon source and melamine to the mass of water is (10~50):

100.

3. The method for preparing the silicon / graphite composite anode material according to claim 2, characterized in that, The temperature of the hydrothermal reaction is 120~250℃; The hydrothermal reaction time is 3 h to 48 h.

4. The method for preparing the silicon / graphite composite anode material according to claim 1, characterized in that, The heat treatment temperature is 400~800℃; The heating rate of the heat treatment is 1~10℃ / min; The heat treatment time is 2h to 10h.

5. The method for preparing the silicon / graphite composite anode material according to claim 1, characterized in that, The mass ratio of the intermediate to the reduced iron powder is (1~5):

1.

6. The method for preparing the silicon / graphite composite anode material according to claim 5, characterized in that, The temperature for the catalytic graphitization reaction is 500~1200℃; The heating rate of the catalytic graphitization reaction is 1~10℃ / min.

7. The method for preparing the silicon / graphite composite anode material according to any one of claims 1-6, characterized in that, The catalytic graphitization reaction takes 1 to 10 hours.

8. A silicon / graphite composite anode material, characterized in that, The silicon / graphite composite anode material is prepared by the method for preparing silicon / graphite composite anode material according to any one of claims 1-7.

9. A lithium-ion battery, characterized in that, The silicon / graphite composite anode material prepared by the method described in any one of claims 1-7, or the silicon / graphite composite anode material described in claim 8.

10. An electrical appliance, characterized in that, Including the lithium-ion battery as described in claim 9.

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