Negative electrode material, preparation method thereof and secondary battery

Abstract

The invention relates to a negative electrode material and a preparation method thereof, and a secondary battery, the negative electrode material comprises silicon crystal grains, magnesium oxide crystal grains and a silicon oxide matrix, a part of the silicon crystal grains and a part of the magnesium oxide crystal grains are gathered to form a multiphase symbiotic cluster complex, the multiphase symbiotic cluster complex is regarded as a sphere-like shape, the size of the multiphase symbiotic cluster complex is 80-200 nm, and the particle size of the multiphase symbiotic cluster complex is 100-200 nm. A plurality of multiphase symbiotic cluster complexes are distributed in the silicon oxide matrix. And the silicon crystal grains and the magnesium oxide crystal grains have good chemical reactivity, so that the gram volume of the negative electrode material is favorably improved. The plurality of multiphase symbiotic cluster complexes are uniformly distributed in the silicon oxide matrix, so that the structural stability of the negative electrode material is improved, and the cycle performance of the negative electrode material is improved. And a multiphase symbiotic cluster complex structure is formed, so that silicon crystal grains are optimized, magnesium oxide crystal grains and a silicon oxide substrate have good distribution, and the negative electrode material can obtain higher capacity and higher coulombic efficiency.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage, specifically to a negative electrode material and its preparation method, and a secondary battery. Background Technology

[0002] With the continuous development of electrochemical new energy in energy storage, 3C (computers, communications, and consumer electronics), and power fields, the industry has put forward new requirements for the energy density, safety rate performance, and cycle performance of secondary batteries (such as lithium-ion batteries). As a crucial component of lithium-ion batteries, the performance of the anode material directly affects the overall battery performance. Traditional graphite anode materials have encountered bottlenecks in improving specific capacity and initial coulombic efficiency. Therefore, developing anode materials with high specific capacity and high initial coulombic efficiency has become a hot topic in lithium-ion battery research.

[0003] Novel lithium-ion battery anode materials, such as silicon-based anode materials, have superior theoretical specific capacity compared to traditional graphite anode materials, showing great development potential. However, the volume expansion phenomenon of silicon-based anode materials severely affects the cycle performance of lithium-ion batteries, limiting their application and development. Research has found that silicon-oxygen anode materials (SiOx) have a relatively small volume expansion rate, making them a more ideal anode material. However, during electrochemical cycling, SiOx anode materials form lithium oxide and lithium silicate, leading to a decrease in the initial coulombic efficiency, which also limits their application in the rechargeable battery field.

[0004] Therefore, the existing anode materials in the industry can no longer meet the needs of industry development in terms of performance. It is essential to develop anode materials and their preparation methods with higher capacity and higher coulombic efficiency than the existing technologies in the industry. Summary of the Invention

[0005] In view of this, this application provides a negative electrode material and a method for preparing the same, to solve at least one of the aforementioned technical problems. Furthermore, this application also provides a secondary battery.

[0006] In a first aspect, this application provides a negative electrode material comprising silicon grains, magnesium oxide grains and a silicon oxide matrix, wherein some silicon grains and some magnesium oxide grains aggregate to form a multiphase symbiotic cluster complex, the multiphase symbiotic cluster complex is regarded as a spherical shape, the size of the multiphase symbiotic cluster complex is 80 nm to 200 nm, and multiple multiphase symbiotic cluster complexes are distributed in the silicon oxide matrix.

[0007] Based on the first aspect, in some possible implementations, based on the anode material, the mass percentage of silicon grains is 10wt% to 50wt%, and the mass percentage of magnesium oxide grains is 3wt% to 25wt%.

[0008] Based on the first aspect, in some possible implementations, the size of silicon grains is 1 nm to 50 nm, and the size of magnesium oxide grains is 1 nm to 30 nm.

[0009] Based on the first aspect, in some possible implementations, the anode material further includes a coating layer located on at least a portion of the surface of the silicon oxide matrix, the coating layer comprising carbon.

[0010] Secondly, this application provides a method for preparing a negative electrode material, the method comprising: mixing silicon suboxide and a magnesium source to obtain a mixture; subjecting the mixture to a first heat treatment and a second heat treatment to obtain a first intermediate; wherein the first heat treatment is performed under a first atmosphere, the first atmosphere comprising an inert gas, the temperature of the first heat treatment being A Kelvin and the time being B hours; the second heat treatment is performed under a second atmosphere, the second atmosphere comprising a reducing gas, the temperature of the second heat treatment being C Kelvin and the time being D hours, wherein A, B, C, and D satisfy the following relationship: A is 0.6T. Mg Up to 0.9T Mg B = K1M2 / M1, C is from A+50 to A+130. The magnesium source has a melting point of T. Mg Kelvin, the mass of the magnesium source is M2 kg, the mass of silicon suboxide is M1 kg, K1 is 3.5 to 4.6, and K2 is 0.7 × 10⁻⁶. 4 Up to 1.3×10 4 The first intermediate is cooled to obtain the second intermediate; the second intermediate is ground, and the ground second intermediate is sintered in a third atmosphere, which includes one or more of the following: reducing gas, hydrocarbon gas, and a composite gas of reducing gas and carbon source gas. The sintering temperature is 873 Kelvin to 953 Kelvin, and the time is 0.75 h to 2 h to obtain the anode material.

[0011] Based on the second aspect, in some possible implementations, the mass ratio of magnesium source to silicon suboxide is 0.1 to 0.5.

[0012] Based on the second aspect, in some possible implementations, the second atmosphere also includes an inert gas, wherein the partial pressure of the reducing gas in the second atmosphere is 3% to 15%.

[0013] Based on the second aspect, in some possible implementations, the cooling rate is greater than or equal to 10. 5 Kelvin / second.

[0014] Based on the second aspect, in some possible implementations, before obtaining the negative electrode material, the above preparation method further includes: drying the sintered second intermediate at a temperature of 353 Kelvin to 393 Kelvin for a time of 4 h to 6 h.

[0015] Thirdly, this application provides a secondary battery comprising the aforementioned negative electrode material.

[0016] In the anode material of this application, the silicon and magnesium oxide grains exhibit good chemical reactivity, which helps to improve the specific capacity of the anode material compared to the inert magnesium silicate formed during conventional pre-magnesification treatment. Simultaneously, the uniform distribution of multiple multiphase symbiotic clusters within the silicon oxide matrix helps to improve the structural stability of the anode material, thereby enhancing its cycle performance. Furthermore, the formation of the multiphase symbiotic cluster structure also helps to optimize the distribution of silicon grains, magnesium oxide grains, and the silicon oxide matrix, which is beneficial for achieving higher capacity and higher coulombic efficiency in the anode material. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the negative electrode material provided in one embodiment of this application.

[0018] Figure 2 The XRD diffraction pattern of the negative electrode material provided in Example 1 of this application.

[0019] Figure 3 This is a high-resolution electron microscope image of the negative electrode material provided in Embodiment 1 of this application.

[0020] Figure 4 Transmission electron microscope image of the negative electrode material provided in Embodiment 1 of this application.

[0021] Figure 5 Transmission electron microscope image of the negative electrode material provided in Embodiment 5 of this application.

[0022] Figure 6 Transmission electron microscope image of the negative electrode material provided in Embodiment 9 of this application.

[0023] Figure 7 The electrochemical performance test results are for Examples 1, 3, 5, 9 and Comparative Examples 1-3 of this application.

[0024] Explanation of main component symbols Anode material 100 Silicon grain 101 Magnesium oxide grains 102 nucleation basement 103 Multiphase symbiotic cluster complex 104 Silicon oxide matrix 105 Coating layer 106 The following detailed description, in conjunction with the accompanying drawings, will further illustrate this application. Detailed Implementation

[0025] The embodiments of this application are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; where there is no conflict, the implementation methods and features of the implementation methods of this application can be combined with each other; many specific details are set forth in the following description to provide a full understanding of this application, and the described implementation methods are only a part of the implementation methods of this application, and not all of the implementation methods.

[0026] In related technologies, pre-lithiation or pre-magnesiation processes are typically used to improve the initial coulombic efficiency of SiOx anode materials. Pre-lithiation usually involves modifying SiOx materials with lithium hydride, which can improve the initial coulombic efficiency. However, lithium hydride is expensive, has demanding process conditions and equipment requirements, and can reduce the specific capacity of SiOx anode materials. Pre-magnesiation is more cost-effective and has more controllable requirements, but it also reduces the specific capacity of SiOx anode materials, and the initial coulombic efficiency improvement obtained through pre-magnesiation is not sufficient. More importantly, after pre-magnesiation, a large portion of the Si within the SiOx anode material is converted into electrochemically less active lithium silicates or magnesium silicates, leading to a decrease in the specific capacity of the silicon-oxygen anode material. Furthermore, the large amount of heat released during the pre-magnesiation reaction can cause rapid localized growth of nano-silicon grains within the material, forming larger silicon crystals that severely affect the material's cycle performance.

[0027] In summary, existing pre-magnesification modification technologies for silicon-oxygen anode materials can improve the initial coulombic efficiency to some extent, but the improved initial coulombic efficiency is still significantly lower than that of similar materials, and it also greatly reduces the specific capacity of silicon-oxygen anode materials. The publicly disclosed pre-magnesification technologies for silicon-oxygen anode materials in the industry cannot overcome the two aforementioned drawbacks: the reaction of magnesium with silicon to form silicates, leading to a decrease in specific capacity, and the excessive heat generated during the reaction process causing excessively rapid growth of silicon grains, resulting in a decline in cycle performance.

[0028] Based on this, the embodiments of this application improve the pre-magnesification process of silicon-oxygen anode materials, thereby mitigating the problems of reduced specific capacity caused by the reaction of magnesium and silicon-oxygen to form silicates, and reduced cycle performance caused by excessively rapid growth of silicon grains due to heat generation. This achieves the goal of developing anode materials with higher capacity and higher coulombic efficiency and their preparation methods.

[0029] Based on this, please refer to Figure 1One embodiment of this application provides a negative electrode material 100, comprising silicon grains 101, magnesium oxide grains 102, and a silicon oxide matrix 105. A portion of the silicon grains 101 and a portion of the magnesium oxide grains 102 aggregate to form a multiphase symbiotic cluster composite 104. The multiphase symbiotic cluster composite 104 is considered to be spherical, with a size ranging from 80 nm to 200 nm. Multiple multiphase symbiotic cluster composites 104 are distributed within the silicon oxide matrix 105. Understandably, after the aggregation of some silicon grains 101 and some magnesium oxide grains 102, a reaction first occurs to form magnesium silicate, constituting a nucleation substrate 103. The nucleation substrate 103 further grows to form the multiphase symbiotic cluster composite 104. The size of the multiphase symbiotic cluster complex 104 can be 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, or any value within the range of any two of the above values. The multiple multiphase symbiotic cluster complexes 104 distributed in the silicon oxide matrix 105 remain relatively independent or only partially adhered, forming a relatively uniform and diffuse distribution overall.

[0030] In this application, silicon grains 101 and magnesium oxide grains 102 exhibit good chemical reactivity, which helps to improve the specific capacity of the anode material 100 compared to the inert magnesium silicate formed during conventional pre-magnesification treatment. Simultaneously, the uniform distribution of multiple multiphase symbiotic clusters 104 within the silicon oxide matrix 105 helps to improve the structural stability of the anode material 100, thereby improving its cycle performance. Furthermore, the formation of the multiphase symbiotic clusters 104 structure also helps to optimize the distribution of silicon grains 101, magnesium oxide grains 102, and the silicon oxide matrix 105, which is beneficial for the anode material 100 to achieve higher capacity and higher coulombic efficiency.

[0031] In some embodiments, based on the negative electrode material 100, the mass percentage of silicon grains 101 is 10wt% to 50wt%, and the mass percentage of magnesium oxide grains 102 is 3wt% to 25wt%. For example, the mass percentage of silicon grains 101 can be 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, or any value within the range of any two of the above values. The mass percentage of magnesium oxide grains 102 can be 3wt%, 5wt%, 8wt%, 10wt%, 12wt%, 15wt%, 17wt%, 20wt%, 23wt%, 25wt%, or any value within the range of any two of the above values. Controlling the mass percentages of silicon grains 101 and magnesium oxide grains 102 within the above ranges is beneficial for further improving the specific capacity of the negative electrode material 100, and also helps to reduce the mutual encroachment or local segregation of excessive silicon grains 101 or magnesium oxide grains 102 to form large-sized grains, which would affect the capacity utilization of the negative electrode material 100.

[0032] In some embodiments, the size of silicon grain 101 is from 1 nm to 50 nm, and the size of magnesium oxide grain 102 is from 1 nm to 30 nm. For example, the size of silicon grain 101 can be 1 nm, 3 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, or any value within the range of any two of the above values. The size of magnesium oxide grain 102 can be 1 nm, 3 nm, 5 nm, 8 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, or any value within the range of any two of the above values. In related technologies, larger silicon or magnesium oxide grains can affect the capacity performance of the anode. Therefore, controlling the size of silicon grain 101 and magnesium oxide grain 102 within the above ranges is beneficial for maintaining a higher capacity and coulombic efficiency of the anode material 100. Meanwhile, controlling the above-mentioned size range is also conducive to the aggregation of silicon grains 101 and magnesium oxide grains 102 to form multiphase symbiotic cluster composites 104, reducing the risk of grains engulfing each other and forming large-sized grains due to excessively large grain size. With multiple multiphase symbiotic cluster composites 104 uniformly distributed in the silicon oxide matrix 105, the silicon grains 101 and magnesium oxide grains 102 can have a good distribution, which is more conducive to the capacity utilization of the negative electrode material 100.

[0033] In some embodiments, the negative electrode material 100 further includes a coating layer 106 located on at least a portion of the surface of the silicon oxide matrix 105, the coating layer 106 comprising carbon. The carbon coating layer helps to further improve the cycle stability of the negative electrode material 100.

[0034] An embodiment of this application also provides a method for preparing a negative electrode material, the method comprising: Step 1: Mix silicon suboxide and magnesium source to obtain a mixture.

[0035] In some embodiments, the mass ratio of magnesium source to silicon suboxide is 0.1 to 0.5. For example, the mass ratio of magnesium source to silicon suboxide can be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or any value within the range of any two of the above values. This application has found that the content of magnesium source in the mixture simultaneously affects the growth rate of subsequently formed silicon grains and the distribution of magnesium oxide grains. Controlling the mass ratio of magnesium source to silicon suboxide in the mixture within the above range, while providing sufficient magnesium source modification, allows for the generation of controllable heat of reaction with an appropriate amount of magnesium source, which is beneficial for maintaining silicon grain size and also helps reduce the mutual encroachment of magnesium oxide grains, reducing local segregation of silicon and magnesium oxide grains, thereby optimizing the distribution of silicon and magnesium oxide grains in the anode material.

[0036] In some embodiments, silica and magnesium sources are mixed by ball milling. Ball milling helps improve mixing uniformity. In some embodiments, the ball-to-material ratio is 8:1 to 12:1, for example, 8:1, 9:1, 10:1, 11:1, or 12:1; the ball milling speed is 200 rpm to 400 rpm, for example, 200 rpm, 250 rpm, 300 rpm, 350 rpm, or 400 rpm; and the ball milling time is 2 h to 6 h, for example, 2 h, 3 h, 4 h, 5 h, or 6 h. Controlling the above ball milling parameters helps to further improve mixing uniformity.

[0037] Step 2: The mixture undergoes a first heat treatment and a second heat treatment to obtain the first intermediate. The first heat treatment is performed under a first atmosphere containing an inert gas at a temperature of A Kelvin for a duration of B hours. The second heat treatment is performed under a second atmosphere containing a reducing gas at a temperature of C Kelvin for a duration of D hours. A, B, C, and D satisfy the following relationship: A is 0.6T. Mg Up to 0.9T Mg B = K1M2 / M1, C is from A+50 to A+130. The magnesium source has a melting point of T. Mg Kelvin, the mass of the magnesium source is M2 kg, the mass of silicon suboxide is M1 kg, K1 is 3.5 to 4.6, and K2 is 0.7 × 10⁻⁶. 4 Up to 1.3×10 4For example, K1 can be 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, or any value within the range of any two of the above values. K2 can be 0.7 × 10⁻⁶. 4 0.8×10 4 0.9×10 4 1.0×10 4 1.1×10 4 1.2×10 4 1.3×10 4 Or any value within the range formed by any two of the above values.

[0038] During the first heat treatment, the mixture is fully heated, increasing the internal energy of the materials and the kinetic energy of molecules and atoms. This enhances the reactivity of the mixture, promotes rapid reaction, and makes the reaction process more controllable. In the second heat treatment, a reducing gas atmosphere promotes reduction, causing the magnesium source to react rapidly with some silicon suboxide to form silicon grains and highly active magnesium silicates. Therefore, the first heat treatment places the mixture in a high-internal-energy, high-activity state, creating favorable conditions for the reduction process in the second heat treatment and facilitating the synthesis of highly active magnesium silicates. This application employs a combination of the first and second heat treatments, which better controls the large amount of heat released during the magnesian reaction, yields highly active magnesium silicates, reduces the growth rate of silicon grains during the magnesian reaction, and maintains suitable grain size. This approach is beneficial for obtaining anode material precursors with highly active magnesium silicates and small-sized silicon grains.

[0039] In some embodiments, the second atmosphere further includes an inert gas, and the partial pressure of the reducing gas in the second atmosphere is 3% to 15%. For example, the partial pressure of the reducing gas in the second atmosphere can be 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or any value within the range of any two of the above values. Controlling the partial pressure of the reducing gas in the second atmosphere within the above range is beneficial in two ways: firstly, it helps to reduce the oxidation of small-sized silicon grains and promote the continuous progress of the reaction; secondly, it helps to maintain the activity of highly reactive magnesium silicate while reducing partial decomposition.

[0040] Step 3: Cool the first intermediate to obtain the second intermediate. For example, cooling methods can include water cooling, air cooling, or metal thermal conductivity cooling.

[0041] Cooling helps to slow the growth of silicon grains and highly reactive magnesium silicates in the first intermediate, thereby helping to maintain the size of the silicon grains and the reactivity of the highly reactive magnesium silicates.

[0042] In some embodiments, the cooling rate is greater than or equal to 10. 5 Kelvin / second. Controlling the above cooling rate to rapidly cool the first intermediate helps to better slow down the growth of silicon grains and highly reactive magnesium silicate in the first intermediate, thereby helping to maintain the size of the silicon grains and the reactivity of the highly reactive magnesium silicate.

[0043] Step 4: Grind the second intermediate and sinter it under a third atmosphere, which includes one or more of the following: reducing gas, hydrocarbon gas, and a composite gas of reducing gas and carbon source gas. The sintering temperature is between 873 Kelvin and 953 Kelvin, and the sintering time is between 0.75 h and 2 h, to obtain the negative electrode material. For example, the sintering temperature can be 873 Kelvin, 883 Kelvin, 893 Kelvin, 903 Kelvin, 913 Kelvin, 923 Kelvin, 933 Kelvin, 943 Kelvin, 953 Kelvin, or any value within the range of any two of the above values. The sintering time can be 0.75 h, 1 h, 1.25 h, 1.5 h, 1.75 h, 2 h, or any value within the range of any two of the above values. It should be noted that the reducing carbon source gas in this application refers to a gas that can be decomposed at high temperatures to form carbon and hydrogen. For example, reducing carbon source gases include one or more of acetylene, methane, ethylene, and propylene.

[0044] By controlling the sintering temperature and time, hydrocarbon gases can be decomposed to form carbon and hydrogen. Carbon can deposit to form a carbon coating layer, while hydrogen, with its high reactivity, can promote the decomposition of highly active magnesium silicate in the milled second intermediate to form magnesium oxide and silicon grains. These decomposition products aggregate and grow together with the silicon grains generated by the magnesian thermal reduction reaction, thus forming a multiphase symbiotic cluster complex. Multiple multiphase symbiotic cluster complexes are distributed within the silicon oxide matrix. The uniform distribution of these symbiotic clusters helps improve the structural stability of the anode material, thereby improving its cycle performance. The formation of this multiphase symbiotic cluster complex structure also helps optimize the distribution of silicon grains, magnesium oxide grains, and the silicon oxide matrix, which is beneficial for achieving higher capacity and higher coulombic efficiency in the anode material.

[0045] Understandably, when the third atmosphere includes hydrocarbon gases, under the aforementioned sintering temperature and time conditions, the hydrocarbon gases can simultaneously act as a reducing gas to promote the reaction and as a carbon source gas to deposit and coat the material. When the third atmosphere includes a reducing gas, the reducing gas promotes the decomposition of highly reactive magnesium silicate in the milled second intermediate to form magnesium oxide grains and silicon grains. It can also further composite the carbon source gas, for example, by using a composite gas of reducing gas and carbon source gas, to achieve the formation of a carbon coating layer during the sintering process.

[0046] In some embodiments, before obtaining the negative electrode material, the above preparation method further includes: drying the sintered second intermediate at a temperature of 353 Kelvin to 393 Kelvin for a time of 4 h to 6 h. For example, the drying temperature can be 353 Kelvin, 363 Kelvin, 373 Kelvin, 383 Kelvin, 393 Kelvin, or any value within the range of any two of the above values. The drying time can be 4 h, 4.1 h, 4.2 h, 4.3 h, 4.4 h, 4.5 h, 4.6 h, 4.7 h, 4.8 h, 4.9 h, 5 h, 5.1 h, 5.2 h, 5.3 h, 5.4 h, 5.5 h, 5.6 h, 5.7 h, 5.8 h, 5.9 h, 6 h, or any value within the range of any two of the above values. The above drying of the sintered second intermediate helps to remove residual moisture formed by the decomposition of highly active magnesium silicate under reducing atmosphere conditions.

[0047] One embodiment of this application also provides a secondary battery comprising the aforementioned negative electrode material. The secondary battery of this application exhibits good discharge capacity, initial coulombic efficiency, and cycle stability.

[0048] In some embodiments, the secondary battery includes a casing, an electrode assembly, and an electrolyte. Both the electrode assembly and the electrolyte are located within the casing. The casing can be a packaging bag encapsulated with a film (such as an aluminum-plastic film), for example, a pouch battery. In other embodiments, it can also be a steel-cased battery, an aluminum-cased battery, etc. The electrode assembly includes electrode sheets and a separator. The electrode sheets include a positive electrode sheet and a negative electrode sheet, and the separator is used to separate the positive and negative electrode sheets and can be disposed between the positive and negative electrode sheets. In some embodiments, the electrode assembly can be a stacked structure, for example, it is formed by alternately stacking a positive electrode sheet, a separator, and a negative electrode sheet. In other embodiments, the electrode assembly can also be a wound structure, for example, it is formed by sequentially stacking and then winding a positive electrode sheet, a separator, and a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode material active layer disposed on at least one surface of the negative current collector. The negative electrode material active layer includes the aforementioned negative electrode material.

[0049] The present application's solution will be explained below with reference to embodiments. Those skilled in the art will understand that the following examples are for illustrative purposes only and should not be construed as limiting the present application. Unless otherwise stated, reagents, software, and instruments involved in the following embodiments that are not specifically mentioned are all conventional commercially available products or open-source materials.

[0050] Example 1: A negative electrode material, the preparation method of which includes: S1: Weigh 10 kg of silica powder (SiOx) and 1.5 kg of magnesium powder and mix them thoroughly in an argon atmosphere using an atmosphere mixer. Then add the mixture to a mechanical ball mill jar and grind it at 350 r / min for 6 h using agate balls and agate balls with a ball-to-material ratio of 10:1 to obtain a uniform SiOx / Mg mixture.

[0051] S2: The SiOx / Mg mixture was subjected to a first heat treatment in a resistance furnace under vacuum conditions, with a reaction temperature A = 0.794 × T. Mg =0.794×923.15=733 K, holding time B=K1M2 / M1=3.6×1.5 / 10=0.54 h, then argon-hydrogen mixed gas is introduced into the furnace cavity at a flow rate of 0.5 L / min for a second heat treatment, heating to the reaction temperature C=A+90=733+90=823 K, holding time D= =1.09×10 4 The first intermediate is obtained by multiplying the mass by 2 × 1.5 / 10 / 823 by 4 h. The melting point of the magnesium metal is 923.15 K.

[0052] S3. After the heat preservation is completed, stop the supply of argon-hydrogen mixed gas, and replace it with argon gas at a rate of 1 L / min to supply gas to the furnace cavity. Use a circulating water cooling system to rapidly cool the furnace cavity at a cooling rate of 10. 5 K / S, after the furnace is cooled to room temperature, the material is discharged to obtain the second intermediate; S4. The second intermediate is broken up and the particle size is adjusted to 10 μm (median particle size) and then transferred to a rotary kiln. Under an argon atmosphere (gas flow rate 1 L / min), the temperature is raised to 913 K and then the gas flow is stopped. Argon-hydrogen mixed gas (gas flow rate 0.5 L / min) and acetylene gas (gas flow rate 0.5 L / min) are added to the furnace cavity and the temperature is maintained for 1 h. After the temperature maintenance is completed, the gas flow is stopped and argon gas is replaced to circulate gas into the furnace cavity (gas flow rate 1 L / min) until it is cooled to room temperature. Then the material is discharged to obtain bulk anode material. S5. The bulk anode material is baked under vacuum at 373 K for 4 h and then crushed and ground to adjust the particle size to 8 μm (median particle size) to obtain the anode material.

[0053] Examples 2-9: The difference from Example 1 is the preparation conditions shown in Table 1. The rest of the preparation of the negative electrode material is carried out in the same way as in Example 1.

[0054] Comparative Example 1: A negative electrode material, the preparation method of which includes: S1: Weigh 10 kg of silica fume (SiOx) and 1.5 kg of magnesium powder and mix them thoroughly in an argon atmosphere using an atmosphere mixer to obtain a uniform SiOx / Mg mixture.

[0055] S2: The SiOx / Mg mixture was heated to the reaction temperature of 823 K and held in an argon atmosphere using a resistance furnace.

[0056] S3: After the heat preservation is completed, the material is cooled to room temperature in the furnace to obtain a block, and then crushed and ground to adjust the particle size to 5 μm (median particle size) to obtain crushed material.

[0057] S4: The crushed material from S3 is heated in a rotary kiln under an argon atmosphere. After heating to 1173 K, methane gas (flow rate 0.5 L / min) is introduced into the furnace cavity and held at that temperature for 120 min. The material is then discharged after cooling in the furnace.

[0058] S5. The particle size of the material after S4 treatment is adjusted to 5 μm (median particle size) to obtain the negative electrode material.

[0059] Comparative Example 2: A negative electrode material, the preparation method of which includes: S1: Weigh 10 kg of silica fume (SiOx), 1.5 kg of magnesium powder, and 1.5 kg of sodium chloride, and mix them thoroughly in an argon atmosphere using an atmosphere mixer to obtain a homogeneous mixture of SiOx / Mg / NaCl.

[0060] S2: The SiOx / Mg / NaCl mixture was heated to the reaction temperature of 973 K and held at that temperature for 6 h in an argon atmosphere using a resistance furnace.

[0061] S3: After the heat preservation is completed, the material is cooled to room temperature in the furnace to obtain a block, which is then crushed and washed with water. The particle size is then adjusted to 5μm (median particle size) to obtain crushed material.

[0062] S4: The crushed material from S3 is heated in a rotary kiln under an argon atmosphere until it reaches 1173 K. Then, methane gas (flow rate 0.5 L / min) is introduced into the furnace cavity and kept at that temperature for 2 hours. The material is then discharged after cooling in the furnace.

[0063] S5. The particle size of the material after S4 treatment is adjusted to 5 μm (median particle size) to obtain the negative electrode material.

[0064] Comparative Example 3: A negative electrode material, the preparation method of which includes: S1: Weigh 10 kg of silica fume (SiOx) and 1.5 kg of magnesium powder and mix them thoroughly in an argon atmosphere using an atmosphere mixer to obtain a uniform SiOx / Mg mixture.

[0065] S2: The SiOx / Mg mixture was heated to the reaction temperature of 823 K and held in an argon atmosphere using a resistance furnace.

[0066] S3: After the heat preservation is completed, the material is cooled to room temperature in the furnace to obtain a block. Then, it is crushed and ground to adjust the particle size to 5 μm (median particle size) to obtain fragments, thus obtaining the negative electrode material.

[0067] Comparative Example 4: A negative electrode material, the preparation method of which includes: S1: Weigh 10 kg of silicon suboxide powder (SiOx) and heat it in a rotary kiln under an argon atmosphere. After heating to 1173 K, introduce methane gas (flow rate 0.5 L / min) into the furnace cavity and keep it at that temperature for 2 h. After cooling in the furnace, discharge the material.

[0068] S2: The particle size of the material after S1 treatment is adjusted to 5 μm (median particle size) to obtain the negative electrode material.

[0069] Table 1. Preparation conditions of Examples 1-9 of this application .

[0070] This application analyzes the negative electrode material using XRD. For example, in Example 1, please refer to [link to Example 1]. Figure 2 The anode material of this application comprises silicon grains and magnesium oxide grains. The XRD pattern shows characteristic diffraction peaks of silicon (JCPDS 27-1402) near 2θ = 28.4°, 47.3°, and 56.1°, with a full width at half maximum (FWHM) ranging from 0.15° to 0.35°. Based on the XRD analysis results of each embodiment and comparative example, the sizes of the silicon grains and magnesium oxide grains in each embodiment and comparative example were calculated, and the results are shown in Table 2. The formula for calculating the size of the nano-silicon grains is as follows: The Scherrer constant K is 0.89, the X-ray wavelength γ is 1.54056 Å, θ is the Bragg diffraction angle at 2θ = 28.4°, E is the radian corresponding to the half-width at half-maximum (FWHM) of the diffraction peak at 2θ = 28.4°, and the equipment correction factor K3 ranges from 0.95 to 1.05. The magnesium oxide grain size is calculated from the grain size in the high-resolution transmission electron microscope (TEM) image using the formula: grain size = (L + N) / 2, where L is the major axis of the grain and N is the minor axis. Taking Example 1 as an example, the calculation process for the silicon grain size in the silicon-oxygen anode material synthesized in Example 1 is as follows: The calculation process of magnesium oxide grain size in the silicon-oxygen anode material synthesized in Example 1: Grain size = (L+N) / 2 = (12.365+18.171) / 2 = 15.268 nm.

[0071] This application uses a high-resolution electron microscope to test the negative electrode material of the embodiments of this application. Taking Embodiment 1 as an example, please refer to [link to Embodiment 1]. Figure 3 (The scale bars for both images are 200 nm). The internal structure of the negative electrode material particles is based on silicon oxide, which contains chain-like regions formed by nanocomposite structures with high contrast. The surface of the negative electrode material is covered with many thin-film carbon layers. Highly active magnesium silicate decomposes under reducing atmosphere conditions to form nanoscale magnesium oxide and silicon grains. After the carbon source gas decomposes, carbon is deposited on the surface of silicon grains and SiOx to form a sheet-like coating layer. Finally, nanoscale silicon, magnesium oxide, C, and SiOx form a silicon-oxygen composite material.

[0072] This application also tests the negative electrode material of the embodiments of this application using transmission electron microscopy. Please refer to [link to relevant documentation]. Figure 4 (Scale bar is 5 nm) Figure 5 (Scale bar is 5 nm) and Figure 6 (Scale bar is 10 nm) In the negative electrode material of this application, magnesium oxide and silicon grains are regularly combined in clusters to form clusters of a specific size. The combination of magnesium oxide and nano-silicon is derived from the decomposition of highly active magnesium silicate, indicating that magnesium oxide and silicon grains are combined to form symbiotic clusters. Therefore, in the negative electrode material of this application, some silicon grains and some magnesium oxide grains aggregate to form a multiphase symbiotic cluster complex. The multiphase symbiotic cluster complex is regarded as a spherical shape, and the size of the multiphase symbiotic cluster complex is 80 nm to 200 nm. Multiple multiphase symbiotic cluster complexes are distributed in the silicon oxide matrix.

[0073] This application also conducted electrochemical performance tests on the negative electrode materials of the embodiments and comparative examples, specifically including: the negative electrode slurry ratio was: active material: conductive carbon: binder slurry: water = 9.6:0.02:0.2:10, wherein the binder slurry was: styrene-butadiene rubber slurry: sodium carboxymethyl cellulose: water = 1:1:100. The counter electrode of the half-cell was a lithium metal sheet, and charge-discharge cycle tests were conducted at 25°C and 0.1C / 0.1C rate using the Blue Electric testing system. Please refer to the electrochemical performance test results. Figure 7 And Table 3.

[0074] Table 2. XRD test results and grain size calculation results of Examples 1-9 and Comparative Examples 1-4 of this application. .

[0075] Table 3. Electrochemical performance test results of the negative electrode materials of Examples 1-9 and Comparative Examples 1-4 of this application. .

[0076] Referring to Tables 2 and 3, Examples 1-9 of this application employ a specific process to pre-magnesize silicon suboxide: the first heat treatment places the mixture in a high-internal-energy, high-activity state, creating favorable preconditions for the reduction process in the second heat treatment, which is beneficial for promoting the synthesis of highly active magnesium silicates; the combination of the first and second heat treatments allows for better control of the large amount of heat released by the magnesothermic reaction, resulting in highly active magnesium silicates. It also reduces the growth rate of silicon grains during the magnesothermic reaction, maintaining suitable grain size, which is beneficial for obtaining a cathode material precursor with highly active magnesium silicates and small-sized silicon grains. Simultaneously, by controlling the sintering temperature and time under a third atmosphere, the highly active magnesium silicates decompose to form magnesium oxide and silicon grains. These decomposition products aggregate and grow together with the silicon grains generated by the magnesothermic reduction reaction, forming a multiphase symbiotic cluster complex. Multiple multiphase symbiotic cluster complexes are distributed within the silicon oxide matrix. The uniformly distributed symbiotic clusters help improve the structural stability of the cathode material, thereby improving its cycle performance. The formation of multiphase symbiotic cluster complex structure also helps to optimize the distribution of silicon grains, magnesium oxide grains and silicon oxide matrix, which is beneficial for anode materials to obtain higher capacity and higher coulombic efficiency.

[0077] In Examples 1-5 and 8-9, as the initial amount of magnesium source added increased, the particle size of silicon grains in the resulting anode material gradually increased. This indicates that the content of magnesium source in the mixture affects both the growth rate of the subsequently formed silicon grains and the distribution of magnesium oxide grains. By controlling the mass ratio of magnesium source to silicon suboxide in the mixture within the above range, and on the basis of providing sufficient magnesium source modification, the appropriate amount of magnesium source generates controllable heat of reaction, which is beneficial to maintaining the silicon grain size. It is also beneficial to reduce the mutual phagocytosis of magnesium oxide grains and reduce the local segregation of silicon grains and magnesium oxide grains, thereby optimizing the distribution of silicon grains and magnesium oxide grains in the anode material.

[0078] Compared to Examples 1-9, Comparative Examples 1-3 did not employ specific processes during preparation (no initial heat treatment, no rapid cooling). The uncontrolled release of a large amount of heat from the magnesothermic reaction accelerated silicon nucleation and growth, resulting in larger silicon grains. Furthermore, the magnesium oxide formed during the magnesothermic reaction continued to react with silicon suboxide to form non-electrochemically active magnesium silicate. This non-active magnesium silicate significantly reduces the specific capacity of the anode material, while the large silicon grains affect the cycle performance of the anode material and also form a thicker SEI film, impacting the initial coulombic efficiency.

[0079] Comparative Example 4 uses a conventional carbon coating process for silicon suboxide. It can be seen that the silicon grain size in the material synthesized by this process is smaller, which is closer to the embodiment of this application. This shows that the pre-magnesification treatment using the preparation method of the embodiment of this application will not lead to excessive growth of silicon grains and can maintain a suitable silicon grain size.

[0080] The above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application should not depart from the spirit and scope of the technical solutions of this application.

Claims

1. A negative electrode material, characterized in that, The negative electrode material includes silicon grains, magnesium oxide grains and a silicon oxide matrix. Some of the silicon grains and some of the magnesium oxide grains aggregate to form a multiphase symbiotic cluster complex. The multiphase symbiotic cluster complex is considered to be spherical. The size of the multiphase symbiotic cluster complex is 80 nm to 200 nm. Multiple multiphase symbiotic cluster complexes are distributed in the silicon oxide matrix.

2. The negative electrode material as described in claim 1, characterized in that, Based on the negative electrode material, the mass percentage of silicon grains is 10wt% to 50wt%, and the mass percentage of magnesium oxide grains is 3wt% to 25wt%.

3. The negative electrode material as described in claim 1, characterized in that, The silicon grains have a size of 1 nm to 50 nm, and the magnesium oxide grains have a size of 1 nm to 30 nm.

4. The negative electrode material as described in claim 1, characterized in that, The negative electrode material further includes a coating layer located on at least a portion of the surface of the silicon oxide matrix, the coating layer comprising carbon.

5. A method for preparing the negative electrode material as described in claim 1, characterized in that, The preparation method includes: A mixture is obtained by mixing silicon suboxide and a magnesium source; The mixture is subjected to a first heat treatment and a second heat treatment to obtain a first intermediate. The first heat treatment is performed under a first atmosphere comprising an inert gas, at a temperature of A Kelvin and a time of B hours. The second heat treatment is performed under a second atmosphere comprising a reducing gas, at a temperature of C Kelvin and a time of D hours. A, B, C, and D satisfy the following relationship: A is 0.6T. Mg Up to 0.9T Mg B = K1M2 / M1, C is from A+50 to A+130. Wherein, the melting point of the magnesium source is T Mg Kelvin, the mass of the magnesium source is M2 kg, the mass of the silicon suboxide is M1 kg, K1 is 3.5 to 4.6, and K2 is 0.7 × 10⁻⁶. 4 Up to 1.3×10 4 ; The first intermediate is cooled to obtain the second intermediate; The second intermediate is ground, and the ground second intermediate is sintered in a third atmosphere, the third atmosphere including one or more of reducing gas, hydrocarbon gas and composite gas of reducing gas and carbon source gas, the sintering temperature is 873 Kelvin to 953 Kelvin, and the time is 0.75 h to 2 h, to obtain the negative electrode material.

6. The preparation method according to claim 5, characterized in that, The mass ratio of the magnesium source to the silicon suboxide is 0.1 to 0.

5.

7. The preparation method according to claim 5, characterized in that, The second atmosphere also includes an inert gas, wherein the partial pressure of the reducing gas in the second atmosphere is 3% to 15%.

8. The preparation method according to claim 5, characterized in that, The cooling rate is greater than or equal to 10. 5 Kelvin / second.

9. The preparation method according to claim 5, characterized in that, Before obtaining the negative electrode material, the preparation method further includes: drying the sintered second intermediate at a temperature of 353 Kelvin to 393 Kelvin for a time of 4 h to 6 h.

10. A secondary battery, characterized in that, The secondary battery includes the negative electrode material as described in any one of claims 1-4.

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