WO2024193654A1 - Negative electrode material, secondary battery, and electronic apparatus - Google Patents

Abstract

Provided in the present application are a negative electrode material, a secondary battery, and an electronic apparatus. The negative electrode material in the present application comprises silicon-carbon composite particles, the silicon-carbon composite particles comprising a porous silicon substrate, wherein lithium is used as a negative electrode, and the negative electrode material is used as a button-type battery of a positive electrode to perform charging and discharging tests, the charging curve of the button-type battery has a first voltage platform and a second voltage platform, the range of the first voltage platform is between 0 V and 0.02 V, and the range of the second voltage platform is between 0.40 V and 0.45 V. By means of a two-stage lithium storage behavior of the negative electrode material in the present application, the cycle performance and storage performance of a secondary battery which comprises the negative electrode material can be effectively improved.

Description

Negative electrode material, secondary battery and electronic device

This application claims priority to a Chinese patent application filed with the Chinese Patent Office on March 23, 2023, with application number 202310292772.6 and invention name “Negative Electrode Materials, Secondary Batteries and Electronic Devices”, the entire contents of which are incorporated by reference into this application.

Technical Field

The present application relates to the field of energy storage, and in particular to a negative electrode material, a secondary battery and an electronic device.

Background Art

The negative electrode material is the key factor affecting the volume energy density of lithium-ion batteries. The theoretical gram capacity of the current mature graphite negative electrode material is only 372mAh/g, which greatly limits the improvement of the volume energy density of lithium-ion batteries. Pure silicon material has become the most promising new negative electrode material for building high volume energy density lithium-ion batteries due to its high capacity (3578mAh/g, measured in the form of Li ₁₅ Si ₄ ) and low lithium desorption potential (0.4V vs. Li ⁺ /Li). However, for pure silicon materials, during the lithium insertion and extraction process, the volume expansion of nearly 280% will lead to rapid attenuation of battery capacity, which cannot meet the actual electrical appliance requirements for battery performance.

In response to the problems encountered by pure silicon materials, researchers introduced the elements O and C that constitute buffers into pure silicon materials to synthesize silicon-oxygen materials and silicon-carbon materials respectively. Although the "introduction of buffers" strategy alleviates the attenuation of battery capacity to a certain extent, the low first coulombic efficiency and gram capacity still cannot achieve the construction of high volume energy density lithium-ion batteries.

In order to achieve the effective application of silicon-oxygen materials and silicon-carbon materials, researchers have proposed that silicon-oxygen materials or silicon-carbon materials can be physically mixed with graphite to construct hybrid negative electrode materials, which is expected to achieve an increase in volume energy density. However, the energy density that can be improved by physically mixing silicon-oxygen materials or silicon-carbon materials with graphite to construct hybrid negative electrode materials is theoretically very limited, and the actual usability of the battery and the volume energy density gain are mutually balanced.

Summary of the invention

In view of the above problems existing in the prior art, the present application provides a negative electrode material and a secondary battery including the negative electrode material to improve the lithium storage behavior of the negative electrode material, thereby improving the cycle performance and storage performance of the secondary battery including the negative electrode material.

The first aspect of the present application provides a negative electrode material, which includes silicon-carbon composite particles, and the silicon-carbon composite particles include a porous silicon matrix, wherein a button battery with lithium as the negative electrode and the negative electrode material as the positive electrode is used for charge and discharge testing, and the charging curve of the button battery has a first voltage platform and a second voltage platform, the first voltage platform ranges from 0V to 0.02V, and the second voltage platform ranges from 0.40V to 0.45V. In the present application, in the charging curve of the button battery of the negative electrode material, the first voltage platform corresponds to the capacity contributed by the lithium metal dissolution reaction, and the second voltage platform corresponds to the capacity contributed by the lithium silicon alloy dissolution reaction. The alloying reaction contributes to the capacity. Compared with conventional silicon-carbon materials that only show the second voltage platform, that is, the alloying reaction and dealloying reaction of silicon and lithium contribute to the capacity, the porous silicon-carbon composite particles of the present application can make lithium ions selectively deposited in the pores of the porous silicon matrix and can be reversibly dissolved. During the charge and discharge process, the deposited lithium metal is used as the main active material, making it difficult for the alloying reaction of silicon and lithium to proceed, thereby reducing the participation of silicon in the electrochemical reaction, improving the structural stability of the silicon-carbon composite particles, and making the secondary battery containing the silicon-carbon composite particles have excellent cycle performance and storage performance.

In some embodiments, in the charging curve, the charging gram capacity of the button battery is Q mAh/g, and the charging gram capacity corresponding to the first voltage platform is Q1 mAh/g, wherein 30% ≤ Q1/Q × 100% ≤ 90%. The higher the value of Q1/Q, the higher the proportion of reversible capacity contributed by lithium metal, and the better the long cycle stability of the secondary battery. However, when the value of Q1/Q is too high, the overall gram capacity of the silicon-carbon composite particles will decrease, which is not conducive to the improvement of the energy density of the secondary battery. In some embodiments, 60% ≤ Q1/Q × 100% ≤ 80%.

In some embodiments, the lowest potential in the discharge curve of the button cell is 0 to -0.10 V. The lowest potential of the discharge curve is within the above range, indicating that the overpotential for lithium metal deposition on the porous micron silicon surface is not large, and under the action of extremely small SiC, the negative electrode potential drops to close to -0.1 V, so that the charging voltage of the secondary battery is increased, which is conducive to improving the discharge voltage platform of the secondary battery.

In some embodiments, the silicon-carbon composite particles include a carbon coating layer located on the surface of the porous silicon substrate, and the carbon coating layer includes silicon carbide (SiC) particles. The silicon carbide particles in the coating layer can passivate the surface of the silicon skeleton, inhibit the reaction of lithium ions with the silicon skeleton, promote the selective deposition of lithium ions in the pores of the porous silicon substrate, reduce the participation of silicon in the electrochemical reaction, and improve the structural stability of the silicon-carbon composite particles.

In some embodiments, the thickness of the carbon coating layer is 3nm to 25nm. When the thickness of the carbon coating layer is too high, it is not conducive to improving the energy density of the battery. When it is too low, it cannot completely cover the pore surface of the porous micron silicon, which cannot achieve spatial isolation between the porous micron silicon particles and the electrolyte, and is not conducive to achieving long-cycle stability. In some embodiments, the thickness of the carbon coating layer is 5nm to 15nm.

In some embodiments, the X-ray diffraction spectrum of the silicon-carbon composite particles is tested by X-ray diffraction method, and has a first diffraction peak in the range of 2θ of 28.0° to 29.0°. In some embodiments, the half-peak width of the first diffraction peak is 0.2° to 0.9°. In the present application, the first diffraction peak is the strongest diffraction peak of the silicon particles. The half-peak width of the first diffraction peak can characterize the grain size of the silicon particles.

In some embodiments, the X-ray diffraction spectrum of the silicon-carbon composite particles has a second diffraction peak in the range of 2θ of 38.0° to 39.0° as tested by X-ray diffraction. In the present application, the second diffraction peak confirms the presence of SiC particles.

In some embodiments, the particle size of the silicon carbide particles is less than or equal to 5 nm. When the particle size of the silicon carbide particles is too large, the SiC particles tend to cover the porous silicon matrix too much, thereby hindering the transmission of electrons, and in severe cases, the silicon-carbon composite particles lose their lithium storage activity. In some embodiments, the particle size of the silicon carbide particles is less than 2 nm.

In some embodiments, through X-ray photoelectron spectroscopy testing, the silicon-carbon composite particles satisfy: 0.1≤S1/S≤0.8, wherein, in the X-ray photoelectron spectroscopy diagram, the area of the peak corresponding to the silicon-carbon bond is S1, and the area of the peak corresponding to the silicon element is S. The value of S1/S can reflect the content of silicon carbide particles in the silicon-carbon composite particles. The higher the value of S1/S, the higher the content of silicon carbide particles. As the content of silicon carbide increases, the proportion of capacity contributed by the lithium metal dissolution reaction gradually increases, which is beneficial to improving the long-cycle stability of the secondary battery. However, since SiC contributes almost no reversible capacity, too much SiC will affect the gram capacity of the material, which is not conducive to the improvement of the energy density of the secondary battery. In some embodiments, 0.2≤S1/S≤0.6.

In some embodiments, through Raman testing, the silicon-carbon composite particles satisfy: 0 <I _2D /I _G ≤0.4, 0.9 ≤I _D /I _G ≤1.2, wherein I _2D is the intensity of the 2600cm ^-1 peak in the Raman spectrum, I _G is the intensity of the 1600cm ^-1 peak in the Raman spectrum, and _ID is the intensity of the 1300cm ^-1 peak in the Raman spectrum. In the present application, the values of I _2D /I _G and _ID /I _G can characterize the degree of graphitization of the silicon-carbon composite particles. The larger the ratio of I _2D /I _G and the smaller the ratio of _ID /I _G , the higher the degree of graphitization. A high degree of graphitization can provide high electronic conductivity, which is conducive to lithium ions quickly obtaining electrons on the surface of porous micron silicon and depositing, so that the structure has the potential to achieve high rate charge and discharge, and provides high mechanical strength, which is conducive to building a stable coating layer. In some embodiments, 0.1 ≤I _2D /I _G ≤0.3.

In some embodiments, the pore volume of the silicon-carbon composite particles is 1.0 cm ³ /g to 4.0 cm ³ /g. In some embodiments, the area of the pores per unit cross section of the silicon-carbon composite particles is 0.35 μm ² /μm ² to 0.85 μm ² /μm ^2. The function of the pores is to accommodate the deposited lithium metal. The larger the volume provided by the pores, the more lithium metal can be deposited in the pores, and the more lithium metal can be reversibly dissolved, that is, the higher the reversible capacity. However, the lithium metal deposited in the pores will undergo a certain degree of alloying reaction on the silicon located on the pore walls. When the pore diameter or the pore volume is too large, the silicon skeleton is at risk of collapse due to alloying and dealloying of the pore walls, which in turn leads to poor cycle stability of the secondary battery. In some embodiments, the pore volume of the silicon-carbon composite particles is 1.5 cm ³ /g to 3.0 cm ³ /g. In some embodiments, the area of pores per unit cross section of the silicon-carbon composite particles is 0.35 μm ² /μm ² to 0.7 μm ² /μm ² .

In some embodiments, the specific surface area of the silicon-carbon composite particles is 6 m ² /g to 30 m ² /g. In some embodiments, the specific surface area of the silicon-carbon composite particles is 6 m ² /g to 15 m ² /g.

In some embodiments, based on the mass of the silicon-carbon composite particles, the mass content of silicon is 75% to 95%, and the mass content of carbon is 4% to 25%. In some embodiments, based on the mass of the silicon-carbon composite particles, the mass content of silicon is 84% to 92%, and the mass content of carbon is 6% to 20%.

In some embodiments, the preparation method of silicon-carbon composite particles includes: using porous silicon material as a substrate, depositing a carbon source on the surface of the porous silicon material by magnetron sputtering. In the present application, magnetron sputtering is used to deposit the carbon source on the porous silicon material, so that SiC particles with extremely small particle size can be obtained.

In some embodiments, the magnetron sputtering is performed in an inert atmosphere, the power of the magnetron sputtering is 200 W to 500 W, the pulse frequency of the magnetron sputtering is 1 time/s to 5 times/s, and the time of the magnetron sputtering is 3s to 25s.

In some embodiments, the carbon source is selected from at least one of soft carbon and hard carbon.

A second aspect of the present application provides a secondary battery comprising a negative electrode, the negative electrode comprising a negative electrode collector and a negative electrode active material layer disposed on at least one surface of the negative electrode collector, wherein the negative electrode active material layer comprises the negative electrode material of the first aspect.

In some embodiments, when the negative electrode charge capacity is ≤2000mAh/g, lithium metal is only deposited in the pores of the composite silicon-carbon particles; when the negative electrode charge capacity is >2000mAh/g, lithium metal is deposited in the pores of the silicon-carbon composite particles and on the particle surface.

In some embodiments, when the SOC is between 30% and 100%, an optical microscope is used to observe the cross section of the negative electrode, and discretely distributed bright spots can be observed in the cross section, which are deposited lithium metals.

A third aspect of the present application provides an electronic device, which includes the secondary battery of the second aspect.

The present application performs surface modification on a porous silicon-based structure to make the alloying reaction between silicon and lithium difficult to proceed, and lithium ions are selectively deposited in the pores of silicon-carbon composite particles and can be reversibly dissolved. During the charge and discharge process, the deposited lithium metal is used as the main active material, thereby improving the structural stability of the silicon-carbon composite particles, and making the secondary battery containing the silicon-carbon composite particles have excellent cycle performance and storage performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 shows the first cycle charge and discharge curve of the button cell of Example 2-2.

FIG. 2 shows the first cycle charge and discharge curve of the button cell of Comparative Example 1-1.

FIG3 is an XRD diagram of the silicon-carbon composite particles of Examples 1-4.

FIG. 4 is an XRD diagram of the silicon-carbon composite particles of Comparative Examples 1-3.

FIG5 is a focused resonance Raman image of the silicon-carbon composite particles of Examples 1-4.

DETAILED DESCRIPTION

The embodiments of the present application will be described in detail below. The embodiments of the present application should not be interpreted as limiting the present application.

In addition, sometimes amounts, ratios and other numerical values are presented in a range format in this application. It should be understood that such a range format is for convenience and brevity, and should be flexibly understood to include not only the numerical values explicitly specified as range limits, but also all individual numerical values or sub-ranges included in the range, as if each numerical value and sub-range were explicitly specified.

In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single element or multiple elements. Item B may include a single element or multiple elements. Item C may include a single element or multiple elements.

1. Anode Materials

The negative electrode material provided in the present application includes silicon-carbon composite particles, which include a porous silicon matrix, wherein a button-type battery using lithium as the negative electrode and the negative electrode material as the positive electrode is used for charge and discharge testing, and the charging curve of the button-type battery has a first voltage platform and a second voltage platform, the first voltage platform ranges from 0V to 0.02V, and the second voltage platform ranges from 0.40V to 0.45V. In the present application, in the charging curve of the button-type battery of the negative electrode material, the first voltage platform corresponds to the capacity contributed by the lithium metal dissolution reaction, and the second voltage platform corresponds to the capacity contributed by the lithium silicon alloy dealloying reaction. Compared to conventional silicon-carbon materials that only exhibit a second voltage platform, i.e., contribute capacity to the alloying reaction and dealloying reaction of silicon and lithium, the porous silicon-carbon composite particles of the present application enable lithium ions to be selectively deposited in the pores of the porous silicon matrix and to be reversibly dissolved. During the charge and discharge process, the deposited lithium metal is used as the main active substance, making it difficult for the alloying reaction of silicon and lithium to proceed, thereby reducing the participation of silicon in the electrochemical reaction, improving the structural stability of the silicon-carbon composite particles, and making the secondary battery containing the silicon-carbon composite particles have excellent cycle performance and storage performance.

In some embodiments, in the charging curve, the charging capacity of the button battery is Q mAh/g, and the charging capacity corresponding to the first voltage platform is Q1 mAh/g, wherein 30%≤Q1/Q×100%≤90%. In some embodiments, Q1/Q×100% is 35%, 40%, 45%, 50%, 55%, 60%, 65%, 67%, 70%, 73%, 75%, 77%, 80%, 83%, 85%, 87% or a range consisting of any two of these values. The higher the value of Q1/Q, the higher the charge capacity of the button battery. The higher the proportion of the contributed reversible capacity, the better the long cycle stability of the secondary battery. However, when the value of Q1/Q is too high, the overall gram capacity of the silicon-carbon composite particles will decrease, which is not conducive to improving the energy density of the secondary battery. In some embodiments, 60%≤Q1/Q×100%≤80%.

In some embodiments, the lowest potential in the discharge curve of the button cell is 0 to -0.10 V. The lowest potential of the discharge curve is within the above range, indicating that the overpotential for lithium metal deposition on the porous micron silicon surface is not large, and under the action of the extremely small size SiC, the negative electrode potential drops to close to -0.1 V, so that the charging voltage of the secondary battery is increased, which is conducive to improving the discharge voltage platform of the secondary battery.

In some embodiments, the silicon-carbon composite particles include a carbon coating layer located on the surface of the porous silicon substrate, and the carbon coating layer includes silicon carbide (SiC) particles. The silicon carbide particles in the coating layer can passivate the surface of the silicon skeleton, inhibit the reaction of lithium ions with the silicon skeleton, promote the selective deposition of lithium ions in the pores of the porous silicon substrate, reduce the participation of silicon in the electrochemical reaction, and improve the structural stability of the silicon-carbon composite particles.

In some embodiments, the thickness of the carbon coating layer is 3nm to 25nm. In some embodiments, the thickness of the carbon coating layer is 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, 16nm, 17nm, 18nm, 19nm, 20nm, 21nm, 22nm, 23nm, 24nm or a range consisting of any two of these values. When the thickness of the carbon coating layer is too high, it is not conducive to the improvement of the battery energy density. When it is too low, it is impossible to completely cover the pore surface of the porous micron silicon, which cannot achieve spatial isolation of the porous micron silicon particles from the electrolyte, which is not conducive to achieving long cycle stability. In some embodiments, the thickness of the carbon coating layer is 5nm to 15nm.

In some embodiments, the X-ray diffraction spectrum of the silicon-carbon composite particles is tested by X-ray diffraction method, and has a first diffraction peak in the range of 2θ of 28.0° to 29.0°. In some embodiments, the half-peak width of the first diffraction peak is 0.2° to 0.9°, for example, 0.3°, 0.4°, 0.5°, 0.6°, 0.7° or 0.9°. In the present application, the first diffraction peak is the strongest diffraction peak of the silicon particles. The half-peak width of the first diffraction peak can characterize the grain size of the silicon particles.

In some embodiments, the X-ray diffraction spectrum of the silicon-carbon composite particles has a second diffraction peak in the range of 2θ of 38.0° to 39.0° as tested by X-ray diffraction. In the present application, the second diffraction peak confirms the presence of SiC particles.

In the present application, the particle size of silicon carbide particles is calculated by the Scherrer formula based on the second diffraction peak. In some embodiments, the particle size of silicon carbide particles is less than or equal to 5nm. In some embodiments, the particle size of silicon carbide particles is 2.5nm, 3nm, 4nm or 4.5nm. When the particle size of silicon carbide particles is too large, SiC particles tend to cover the porous silicon matrix too much, thereby hindering the transmission of electrons, and in severe cases, the silicon-carbon composite particles may lose their lithium storage activity. In some embodiments, the particle size of silicon carbide particles is less than 2nm. In the present application, the X-ray diffraction spectrum of silicon-carbon composite particles is When there is no obvious second diffraction peak within the range of 2θ of 38.0° to 39.0°, it indicates that the particle size of the silicon carbide particles of the present application is less than 2 nm.

In some embodiments, through the X-ray photoelectron spectroscopy test, the silicon-carbon composite particles meet: 0.1≤S1/S≤0.8, wherein, in the X-ray photoelectron spectroscopy, the area of the peak corresponding to the silicon-carbon bond is S1, and the area of the peak corresponding to the silicon element is S. In some embodiments, S1/S is 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75 or a range consisting of any two of these values. The value of S1/S can reflect the content of silicon carbide particles in the silicon-carbon composite particles. The higher the value of S1/S, the higher the content of silicon carbide particles. As the content of silicon carbide increases, the capacity ratio contributed by the lithium metal dissolution reaction gradually increases, which is beneficial to improve the long cycle stability of the secondary battery. However, since SiC hardly contributes to reversible capacity, too much SiC will affect the gram capacity of the material, which is not conducive to the improvement of the energy density of the secondary battery. In some embodiments, 0.2≤S1/S≤0.6.

In some embodiments, through Raman testing, the silicon-carbon composite particles satisfy: 0 <I _2D /I _G ≤ 0.4, 0.9 ≤I _D /I _G ≤ 1.2, wherein I _2D is the intensity of the 2600 cm ^-1 peak in the Raman spectrum, I _G is the intensity of the 1600 cm ^-1 peak in the Raman spectrum, and _ID is the intensity of the 1300 cm ^-1 peak in the Raman spectrum. In the present application, the values of I _2D /I _G and _ID /I _G can characterize the degree of graphitization of the silicon-carbon composite particles. The larger the ratio of I _2D /I _G and the smaller the ratio of _ID /IG, the higher the degree of graphitization. A high degree of graphitization can provide high electronic conductivity, which is conducive to the rapid acquisition of electrons by lithium ions on the surface of porous micron silicon and deposition, so that the structure has the potential to achieve high rate charge and discharge, and provides high mechanical strength, which is conducive to the construction of a stable coating layer. In some embodiments, I _2D / _IG is 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.3, or a range consisting of any two of these values. In some embodiments, 0.1≤I _{2D /IG≤0.3} . In some embodiments, _ID / _IG is 0.95, 1.0, 1.05, 1.1, 1.15, or a range consisting of any two of these values.

In some embodiments, the pore volume of the silicon-carbon composite particles is 1.0 ^{cm 3} /g to 4.0 cm ³ /g, for example, 1.2 cm 3 /g, 1.4 cm ³ /g, 1.6 cm 3 /g, 1.8 cm 3 /g, 2.0 cm ³ /g, 2.2 cm ³ /g, 2.4 cm ³ /g, 2.6 cm ³ / ^g , 2.8 cm ³ /g, 3.0 cm ³ /g ^, 3.2 cm ³ /g ^, 3.4 cm 3 /g, 3.6 cm ³ /g, 3.8 cm ³ /g, or a range consisting of any two of these values. In some embodiments, the area of pores per unit cross section of the silicon-carbon composite particles is 0.35 μm ² /μm ² to 0.85 μm ² /μm ² , for example, 0.4 μm ² /μm ² , 0.45 μm ² /μm ² , 0.5 μm ² /μm ² , 0.55 μm ² /μm ² , 0.6 μm ² /μm ² , 0.65 μm ² /μm ² , 0.7 μm ² /μm ² , 0.75 μm ² /μm ² , 0.8 μm ² /μm ² , or a range consisting of any two of these values. The function of the pores is to accommodate the deposited lithium metal. The larger the volume provided by the pores, the more lithium metal can be deposited in the pores, the more lithium metal can be reversibly dissolved, and the higher the reversible capacity. However, the lithium metal deposited in the pores will undergo a certain degree of alloying reaction with the silicon located on the pore walls. When the pore diameter or pore volume is too large, the silicon skeleton is at risk of collapse due to alloying and dealloying of the pore walls, which in turn leads to poor cycle stability of the secondary battery. In some embodiments, the pore volume of the silicon-carbon composite particles is 1.5 cm ³ /g In some ^embodiments , the area of pores per unit cross section of the silicon-carbon composite particles is 0.35 μm ² /μm ² to 0.7 μm ² /μm ² .

In some embodiments, the specific surface area of the silicon-carbon composite particles is 6 m ² /g to 30 m ² /g. In some embodiments, the specific surface area of the silicon-carbon composite particles is 6.5 m ² /g, 7 m ² /g, 8 m ² /g, 9 m ² /g, 10 m ² /g, 11 m ² /g, 12 m ² /g, 13 m ² /g, 14 m ² /g, 15 m ² /g, 17 m ² /g, 19 m ² /g, 20 m ² /g, 23 m ^{2 /g, 25 m 2 /} g, 27 m ² /g, or a range consisting of any two of these values. In some embodiments, the specific surface area of the silicon-carbon composite particles is 6 m ² /g to 15 m ² /g.

In some embodiments, the Dv10 of the silicon-carbon composite particles is 1 μm to 10 μm, such as 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm or 9 μm. In some embodiments, the Dv90 of the silicon-carbon composite particles is 15 μm to 30 μm, such as 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm or 29 μm.

In some embodiments, based on the mass of the silicon-carbon composite particles, the mass content of silicon is 75% to 95%, and the mass content of carbon is 4% to 25%. In some embodiments, based on the mass of the silicon-carbon composite particles, the mass content of silicon is 77%, 79%, 80%, 83%, 85%, 87%, 89%, 90%, 93% or a range consisting of any two of these values. In some embodiments, based on the mass of the silicon-carbon composite particles, the mass content of carbon is 5%, 7%, 9%, 10%, 13%, 15%, 17%, 19%, 20%, 23% or a range consisting of any two of these values. In some embodiments, based on the mass of the silicon-carbon composite particles, the mass content of silicon is 84% to 92%, and the mass content of carbon is 6% to 20%.

In some embodiments, the preparation method of silicon-carbon composite particles includes: using porous silicon material as a substrate, depositing a carbon source on the surface of the porous silicon material by magnetron sputtering. In the present application, magnetron sputtering is used to deposit the carbon source on the porous silicon material, and SiC particles with extremely small particle size can be obtained.

In some embodiments, magnetron sputtering is performed in an inert atmosphere, and the power of magnetron sputtering is 200W to 500W. For example, 250W, 300W, 350W, 400W or 450W. In some embodiments, the pulse frequency of magnetron sputtering is 1 time/s to 5 times/s, for example, 2 times/s, 3 times/s or 4 times/s. In some embodiments, the time of magnetron sputtering is 3s to 25s, for example, 4s, 5s, 6s, 7s, 8s, 9s, 10s, 11s, 12s, 13s, 14s, 15s, 16s, 17s, 18s, 19s, 20s, 21s, 22s, 23s or 24s.

In some embodiments, the carbon source is selected from at least one of soft carbon and hard carbon.

In some embodiments, the pore volume of the porous silicon material is 1.0 cm ³ /g to 4.0 cm ³ /g, such as 1.5 cm ³ /g, 2 ^cm 3 /g, 2.5 cm ³ /g, 3 cm ³ /g, or 3.5 cm ³ /g. In some embodiments, the specific surface area of the porous silicon material is 6 m ² /g to 30 m ² /g, such as 6 m ² /g, 10 m ² /g, 13 m 2 /g, 15 m ² /g, 17 m ² /g, 20 m ² /g, 23 m ^{2 /g, 25 m 2 /} g, or 30 m ² /g. In some embodiments, the area of pores per unit cross section of the porous silicon material particles is 0.35 μm ² /μm ² to 0.85 ^{μm 2} /μm ² , for example 0.4 μm ² /μm ² , 0.45 μm ² /μm ² , 0.5 μm ² /μm ² , 0.55 μm ² /μm ² , 0.6 μm ² /μm ² , 0.65 μm ² /μm ² , 0.7 μm ² /μm ² , 0.75 μm ² /μm ² or 0.8 μm ² /μm ² .

In some embodiments, the porous silicon material can be prepared by the following method: photovoltaic silicon scraps and metal chromium blocks are melted and then cooled through a mold to form a silicon-chromium alloy strip, and then the silicon-chromium alloy strip is crushed to obtain a silicon-chromium alloy powder, and finally the metal chromium is washed away with dilute hydrochloric acid or dilute sulfuric acid. During the cooling process, the molten metal chromium will precipitate from the silicon-chromium alloy melt and form a phase. The precipitated chromium element tends to form a columnar structure, and these columnar structures are arranged parallel to each other.

2. Secondary battery

The secondary battery provided in the present application includes a negative electrode, wherein the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes the negative electrode material of the first aspect.

In some embodiments, when the negative electrode charge capacity is ≤2000mAh/g, lithium metal is only deposited in the pores of the composite silicon-carbon particles; when the negative electrode charge capacity is >2000mAh/g, lithium metal is deposited in the pores of the composite silicon-carbon particles and on the surface of the particles.

In some embodiments, when the SOC is between 30% and 100%, an optical microscope is used to observe the cross section of the negative electrode, and discretely distributed bright spots can be observed in the cross section, which are deposited lithium metals.

In some embodiments, the negative electrode has a porosity of 25% to 45%, such as 27%, 30%, 33%, 35%, 37%, 40%, or 43%.

In some embodiments, the conductivity of the negative electrode is 0.1 S/cm to 0.5 S/cm, for example, 0.2 S/cm, 0.3 S/cm or 0.4 S/cm.

In some embodiments, the negative electrode current collector includes: copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

In some embodiments, the negative electrode active material layer further includes a binder and a conductive agent. In some embodiments, the binder includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

In some embodiments, the conductive agent includes, but is not limited to: carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, Acetylene black, Ketjen black, carbon fiber or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

In some embodiments, the positive electrode further includes a conductive agent, which includes a carbon-based material, such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black or carbon fiber; a metal-based material, such as metal powder or metal fiber of copper, nickel, aluminum, silver, etc.; a conductive polymer, such as a polyphenylene derivative; or a mixture thereof.

In some embodiments, the positive electrode further includes a positive electrode current collector, which may be a metal foil or a composite current collector. For example, aluminum foil may be used. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer substrate.

In some embodiments, the negative electrode further comprises a binder and a conductive agent. In some embodiments, the binder comprises, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

In some embodiments, the conductive agent includes, but is not limited to: carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

In some embodiments, the negative electrode further includes a negative electrode current collector, and the negative electrode current collector includes: copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

In some embodiments, the electrolyte includes an organic solvent, a lithium salt, and an optional additive. The electrolyte used in the electrolyte according to the present application is not limited, and it can be any electrolyte known in the prior art. The additive of the electrolyte according to the present application can be any additive known in the prior art that can be used as an electrolyte additive.

In some embodiments, the organic solvent further includes, but is not limited to, ethylene carbonate (EC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate or ethyl propionate. In some embodiments, the organic solvent includes an ether solvent, for example, at least one of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME).

In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to, lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium difluorophosphate (LiPO ₂ F ₂ ), lithium bis(trifluoromethanesulfonyl)imide LiN(CF ₃ SO ₂ ) ₂ (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO ₂ F) ₂ )(LiFSI), lithium bis(oxalatoborate) LiB(C ₂ O ₄ ) ₂ (LiBOB) or lithium di(oxalatoborate) LiBF ₂ (C ₂ O ₄ )(LiDFOB). In an embodiment, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

The secondary battery of the present application also includes a separator. The material and shape of the separator used in the secondary battery of the present application are not particularly limited, and it can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic substance formed of a material that is stable to the electrolyte of the present application.

For example, the isolation film may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film may be selected.

A surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. The inorganic layer includes inorganic particles and a binder, and the inorganic particles are selected from at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyethylene alkoxy, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene. The polymer layer contains a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyethylene alkoxy, polyvinylidene fluoride and poly (vinylidene fluoride-hexafluoropropylene).

In some embodiments, the secondary battery of the present application includes, but is not limited to: a lithium ion battery or a sodium ion battery. In some embodiments, the secondary battery includes a lithium ion battery.

3. Electronic Devices

The present application further provides an electronic device, which includes the secondary battery according to the second aspect of the present application.

The electronic device or device of the present application is not particularly limited. In some embodiments, the electronic device of the present application includes, but is not limited to, a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD TV, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery and a lithium-ion capacitor, etc.

In the following examples and comparative examples, the reagents, materials and instruments used are all commercially available unless otherwise specified.

Examples and Comparative Examples

Example 1-1

Negative electrode material preparation:

The negative electrode material of the present application uses micron silicon particles with a porous structure as a substrate, and pulse magnetron sputtering is performed on the carbon source. The sputtered carbon atoms or carbon atom clusters are deposited on the surface of the porous silicon to form a carbon coating layer containing SiC particles.

The process is as follows:

Take 10g of micron silicon particles with a porous structure and place them in a substrate box, and 1g of soft carbon film as a carbon source is placed on the sputtering target. When the vacuum degree of the entire furnace body drops below 0.001Pa, introduce argon gas to raise the vacuum degree to 1.0Pa. The power is set to 300W, the pulse frequency is set to 3 times/s, the magnetron sputtering time is 5s, and the vacuum degree is stabilized at 1.0Pa during the sputtering process. After the sputtering is completed, let it stand for 1h, let the vacuum degree of the furnace body drop below 0.001Pa, then introduce argon gas to make the vacuum degree 0, open the furnace and take the material, and obtain the silicon-carbon composite material of this embodiment.

Negative electrode preparation:

Step 1) In a MSK-SFM-10 vacuum mixer, 500 g of the silicon-carbon composite material prepared above and 35 g of conductive carbon nanotubes were added to the mixer and stirred for 40 min at a revolution speed of 20 rpm.

Step 2) Add 95g of binder polyacrylic acid to the mixture stirred in step 1), stir for 60min to disperse evenly, then add deionized water and stir for 120min to disperse evenly to obtain slurry. The revolution speed is 20rpm, and the rotation speed is 1200rpm. The viscosity of the slurry is controlled at 6000mPa·s, and the solid content is controlled at 30%.

Step 3) The slurry obtained in step 2) is filtered through a 170-mesh double-layer sieve to obtain a negative electrode slurry.

Step 4) The slurry obtained in step 3) is coated on both sides of the copper foil current collector with a coating thickness of 50 μm. The electrode piece is dried and then cold pressed (the cold pressing density of the negative electrode piece is 1.0-1.4 g/cm ³ ) to obtain a negative electrode piece, also called a negative electrode.

Cathode preparation:

The positive electrode active material LiCoO ₂ , conductive carbon black and polyvinylidene fluoride (PVDF) were fully stirred and mixed in N-methylpyrrolidone at a weight ratio of 96.7:1.7:1.6, and then coated on an aluminum foil current collector, dried and cold pressed to obtain a positive electrode sheet, also called a positive electrode.

Electrolyte: mass ratio EC:DMC:DEC=1:1:1, 1M LiPF ₆ , 10 wt % fluoroethylene carbonate.

Preparation of button cell: Assemble the above-prepared negative electrode sheet, lithium sheet, separator (PE porous polymer film), gasket, spring, button cell shell, etc. into a button cell, and inject the above-prepared electrolyte.

Preparation of lithium-ion full battery: stack the prepared positive electrode sheet, separator (PE porous polymer film), and the prepared negative electrode sheet in order, with the separator in the middle of the positive and negative electrode sheets, and wind them to obtain an electrode assembly. Place it in an outer package, inject the prepared electrolyte and seal it, and go through the formation, degassing, trimming and other process steps to obtain a full lithium-ion battery.

Examples 1-2 to 1-9, Comparative Example 1-1

Anode material preparation

The preparation process of the negative electrode material is similar to that of Example 1-1, except that the corresponding negative electrode material is prepared by adjusting the magnetron sputtering time during the preparation process. The specific preparation parameters are shown in Table a:

Table a

The preparation of the negative electrode, positive electrode, electrolyte, button cell and lithium-ion full battery is the same as that in Example 1-1.

Comparative Example 1-2

Anode material preparation

20 g of micron silicon particles with a porous structure were placed in a rotary deposition furnace, argon gas was introduced, and the furnace temperature was raised from room temperature to the deposition temperature of 500°C at a heating rate of 10°C/min. A mixed gas of acetylene and argon (the mass fraction of acetylene was 20%) was introduced for 360 minutes, and then changed to argon gas. The temperature was cooled to room temperature and then sampled.

The preparation of the negative electrode, positive electrode, electrolyte, button cell and lithium-ion full battery is the same as that in Example 1-1.

Comparative Examples 1-3

Anode material preparation

20 g of micron silicon particles with a porous structure were placed in a rotary deposition furnace, argon gas was introduced, and the furnace temperature was raised from room temperature to the deposition temperature of 1000°C at a heating rate of 10°C/min. A mixed gas of acetylene and argon (the mass fraction of acetylene was 20%) was introduced for 360 minutes, and then changed to argon gas. The temperature was cooled to room temperature and then sampled.

The preparation of the negative electrode, positive electrode, electrolyte, button cell and lithium-ion full battery is the same as that in Example 1-1.

Examples 2-1 to 2-5, Comparative Example 2-1

Anode material preparation

The preparation process of the negative electrode material is similar to that of Example 1-4, except that porous micron silicon particles with different pore structures are used to prepare the corresponding negative electrode material. The specific preparation parameters and the parameters of the adjusted silicon-carbon particles are shown in Table b:

Table b

The preparation of the negative electrode, positive electrode, electrolyte, button cell, and lithium-ion full cell is the same as in Examples 1-4.

Test Method

1. Button battery charge and discharge test

After placing the button battery on the Blue Electric test system channel, start setting the test process to obtain the charge and discharge curve of the button battery. The test process is as follows:

1) Let stand for 4 hours;

2) Discharge at a constant rate of 0.05C until the capacity reaches 1C. (1C is the designed rated capacity value, for example, 1C = 2000mAh/g × electrode mass (g), 0.05C means it takes 20h to discharge to 1C capacity)

3) Charge at a constant rate of 0.05C until the battery reaches 2.0V.

Test steps for the gram capacity contributed by lithium metal dissolution and its proportion of the total charge gram capacity:

The intersection of the 0.1-0.3V line segment on the "withdrawal charge curve" and the X-axis (gram capacity) is the gram capacity Q1 contributed by lithium metal dissolution. The gram capacity value corresponding to 2.0V on the "withdrawal charge curve" is Q, and the gram capacity ratio contributed by lithium metal dissolution is Q1/Q.

2. Scanning electron microscopy analysis test

The SEM images were recorded by a Philips XL-30 field emission scanning electron microscope at 10 kV and 10 mA.

3. Specific surface area test

ASAP2020 was used as the test equipment, software version V3.04, and _N2 isothermal adsorption and desorption tests were performed. The vacuum degassing pretreatment temperature was 200°C, the pretreatment time was 2 hours, and the mass of the silicon-carbon composite material sample was 0.5 g. The specific surface area and pore volume were calculated by analyzing the recorded isothermal adsorption and desorption curves.

4. Raman test

Raman spectroscopy analysis was completed with an excitation light source with a wavelength of 514nm. Before testing the silicon-carbon composite material sample, the instrument accuracy was corrected using the 520.7cm ^-1 peak of the silicon standard. When testing the sample, the grating parameters were set to 300gr/mm, the wavenumber range was 100cm ^-1 to 3000cm ^-1 , the laser power was set to 10%, the spectrum acquisition time was 10s for each sample point, and the surface scanning spectrum was performed on any focused sample surface, recording a total of 10×10 spectral lines.

On each spectrum line, I _2D is the intensity of the peak at ~2600 cm ^-1 in the Raman spectrum, _IG is the intensity of the peak at ~1600 cm ^-1 in the Raman spectrum, and _ID is the intensity of the peak at ~1300 cm ^-1 in the Raman spectrum. After deducting the horizontal base of each spectrum line, the ratios of I _2D / _IG and _ID / _IG on each spectrum line were calculated, and the average values were calculated as the I _2D / _IG and _ID / _IG of the sample.

5. X-ray powder diffraction test

Powder diffraction analysis of silicon-carbon composites was performed using Cu K_αX-rays with a wavelength of 1.5406 angstroms.

6. Carbon coating thickness test

The silicon-carbon composite material test sample was treated with a focused ion beam in an inert atmosphere and placed in a TEM sample holder for testing. Particles with a particle size close to Dv50 were selected, and the coating thickness of 10 sample positions in the field of view was randomly measured with a measuring tool at a magnification of 50nm (the interface between the carbon coating layer and Si was clearly discernible), and the average value was taken as the carbon coating thickness of the sample.

7. X-ray photoelectron spectroscopy test

2 mg of silicon-carbon composite powder was glued to the XPS test sample stage with ordinary double-sided tape (not double-sided carbon conductive tape), and XPS-Si 2p fine spectrum was collected using Thermo-Avantage, with the energy range set to 110.00 eV to 96.00 eV, the energy step set to 0.05 eV, and the spectrum acquisition time set to 6 min. The spectrum was analyzed by peak separation, and the signal peaks with binding energies of 99.6 eV, 100.6 eV, and 103.6 eV were marked as Si-Si peak, Si-C peak, and Si-O peak, respectively. The area obtained by integrating the entire spectrum was S, and the area obtained by integrating the Si-C peak was S1. The area ratio of the Si-C bond signal peak to all Si 2p signal peaks was x = S1/S.

8. Test of the mass content of silicon and carbon in silicon-carbon composite particles

The mass content of silicon was tested by inductively coupled plasma spectrometry (ICP). Specifically, the silicon-carbon composite material sample was dissolved in high-temperature alkali, and then diluted to prepare the test solution. The peak intensity of the test solution at 251.6 nm was recorded and brought into the peak intensity-concentration conversion relationship to obtain the mass fraction of silicon in the sample.

The mass content of carbon is tested by a high-frequency infrared carbon-sulfur analyzer. The test principle is: the silicon-carbon composite material sample is heated and burned at high temperature in a high-frequency furnace under oxygen-rich conditions to oxidize carbon into carbon dioxide. The gas enters the corresponding absorption cell after treatment, absorbs the corresponding infrared radiation and is then converted into a corresponding signal by the detector. This signal is sampled and processed by a computer and converted into the mass content of carbon in the silicon-carbon composite material sample.

9. Test of the area of pores per unit cross section of silicon-carbon composite particles

The negative electrode sheet containing silicon-carbon composite material particles was polished by ion beam to obtain the cross section. The negative electrode sheet cross section was recorded by SEM, 10 particle cross sections were randomly selected, and a 2μm×2μm rectangular selection area was selected inside each particle cross section. The area S contributed by the hole in the selection area (contributed by the darker pixel point) was calculated, and then

The pore area per unit cross section of the i-th particle

The pore area per unit cross section of the powder particles in the pole piece

10. Lithium-ion full battery cycle performance test

The test temperature is 45°C. The lithium-ion full battery is charged to 4.45V at 0.5C constant current, and then discharged to 3.0V at 0.5C constant current after standing for 5 minutes. The first discharge capacity is taken as the initial capacity C, and a 0.5C charge and 0.5C discharge cycle test is performed. The discharge capacity (Cn) of each subsequent cycle is compared with the initial capacity to obtain the discharge capacity decay curve. Among them, at the nth cycle, the cycle capacity retention rate of the lithium-ion full battery = Cn/C×100%.

After the lithium-ion full battery completes a charge and discharge cycle, it is charged to 3.85V and the thickness Ti of the lithium-ion full battery is recorded. Each time it is charged to 4.45V, the thickness Tf of the battery is recorded again. The thickness expansion rate of the lithium-ion full battery = (Tf/Ti-1) × 100%.

Test Results

Table 1 shows the effects of the thickness of the surface coating layer, the particle size of silicon carbide particles, and the silicon carbide content in the silicon-carbon composite particles on the performance of the lithium-ion full battery. Among them, the silicon-carbon composite particles of Examples 1-1 to 1-9 and Comparative Examples 1-1 to 1-3 have the same particle size and similar pore structure characteristics.

Specifically, the Dv10 of the silicon-carbon composite particles of Examples 1-1 to 1-9 and Comparative Examples 1-1 to 1-3 were all 5.2 μm, and the Dv90 were all 24.5 μm.

The pore volume of the silicon-carbon composite particles of Examples 1-1 to 1-9 and Comparative Examples 1-1 to 1-3 is 1.5 cm ³ /g, and the pore area per unit cross section of the particles is 0.38 μm ² /μm ² . The specific surface area of the silicon-carbon composite particles of Examples 1-1 to 1-9 and Comparative Examples 1-2 to 1-3 is 6.5 m ² /g, and the specific surface area of the silicon-carbon composite particles of Comparative Example 1-1 is 0.45 m ² /g. The electrical conductivity of the silicon-carbon composite particles of Examples 1-1 to 1-9 and Comparative Examples 1-2 to 1-3 is greater than 1000 μS/cm, and the electrical conductivity of the silicon-carbon composite particles of Comparative Example 1-1 is 1.75 μS/cm.

Table 1

It can be seen from the data in Table 1 that compared with silicon-carbon composite particles with only lithium alloying reaction and dealloying lithium storage behavior, the silicon-carbon composite particles lithium metal with two-stage lithium storage behavior (deposited lithium metal dissolution + lithium alloying reaction and dealloying) of the present application show better long-cycle stability of lithium-ion batteries (higher capacity retention rate at the 400th week and smaller thickness expansion).

Table 2 further studies the effect of the pore structure of silicon-carbon composite particles on the performance of lithium-ion batteries based on Examples 1-4. Among them, Examples 2-1 to 2-5 and Comparative Example 2-1 have the same particle size, pore inner surface coating thickness, Raman characteristics, XPS signal peak area ratio x, SiC particle size and similar powder conductivity as Example 1-4.

Table 2

From the data in Table 2, it can be seen that the silicon-carbon composite particles with a porous structure show better capacity retention and lower thickness expansion than the non-porous silicon-carbon composite particles. It is speculated that this is mainly because the silicon-carbon composite particles without a porous structure can only deposit lithium on the surface of solid silicon particles, which can easily generate lithium dendrites and cause short circuits, and rapid capacity decay due to self-discharge. For silicon-carbon composite particles with a certain porous structure, lithium can be selectively deposited in the pores, making full use of the space provided by the pores, and the deposited lithium, driven by chemical potential, will react with the outermost silicon in contact with the deposited lithium. Lithiation, regardless of whether the deposited lithium undergoes a lithiation reaction with silicon, can be reversibly dissolved or undergo a delithiation reaction of silicon-lithium alloy to provide capacity.

Although some exemplary embodiments of the present application have been illustrated and described, the present application is not limited to the disclosed embodiments. On the contrary, those skilled in the art will recognize that some modifications and changes may be made to the described embodiments without departing from the spirit and scope of the present application as described in the appended claims.

Claims (11)

A negative electrode material comprises silicon-carbon composite particles, wherein the silicon-carbon composite particles comprise a porous silicon matrix, wherein a button battery using lithium as a negative electrode and the negative electrode material as a positive electrode is used for charge and discharge testing, and a charging curve of the button battery has a first voltage platform and a second voltage platform, wherein the first voltage platform ranges from 0V to 0.02V, and the second voltage platform ranges from 0.40V to 0.45V. The negative electrode material according to claim 1, wherein in the charging curve, the charging capacity in grams of the button battery is Q mAh/g, and the charging capacity in grams corresponding to the first voltage platform is Q1 mAh/g, wherein 30%≤Q1/Q×100%≤90%. The negative electrode material according to claim 2, wherein 60%≤Q1/Q×100%≤80%; and/or The lowest potential in the discharge curve of the button battery is 0 to -0.10V. The negative electrode material according to any one of claims 1 to 3, wherein the silicon-carbon composite particles satisfy at least one of the following conditions (i) to (iv): (i) the silicon-carbon composite particles include a carbon coating layer located on the surface of the porous silicon substrate, and the carbon coating layer includes silicon carbide particles; (ii) The silicon-carbon composite particles satisfy the following conditions as determined by an X-ray photoelectron spectroscopy test: 0.1≤S1/S≤0.8, wherein in the X-ray photoelectron spectroscopy, the area of the peak corresponding to the silicon-carbon bond is S1, and the area of the peak corresponding to the silicon element is S; (iii) as tested by an X-ray diffraction method, the X-ray diffraction spectrum of the silicon-carbon composite particles has a first diffraction peak in the range of 2θ of 28.0° to 29.0°, and a second diffraction peak in the range of 2θ of 38.0° to 39.0°, wherein the half-peak width of the first diffraction peak is 0.2° to 0.9°; (iv) Through Raman testing, the silicon-carbon composite particles satisfy: 0<I _2D /I _G ≤0.4, 0.9≤I _D /I _G ≤1.2, wherein I _2D is the intensity of the 2600 cm ^-1 peak in the Raman spectrum, I _G is the intensity of the 1600 cm ^-1 peak in the Raman spectrum, and _ID is the intensity of the 1300 cm ^-1 peak in the Raman spectrum. The negative electrode material according to claim 4, wherein the silicon-carbon composite particles satisfy at least one of the following conditions (v) to (viii): (v) the thickness of the carbon coating layer is 3 nm to 25 nm; (vi) the particle size of the silicon carbide particles is less than or equal to 5 nm; (vii) 0.2 ≤ S1/S ≤ 0.6; (viii)0.1≤I _2D /I _G ≤0.3. The negative electrode material according to claim 4 or 5, wherein the thickness of the carbon coating layer is 5 nm to 15 nm; and/or The particle size of the silicon carbide particles is less than 2 nm. The negative electrode material according to any one of claims 1 to 6, wherein the silicon-carbon composite particles satisfy at least one of the following conditions (ix) to (xii): (ix) the pore volume of the silicon-carbon composite particles is 1.0 cm ³ /g to 4.0 cm ³ /g; (x) the specific surface area of the silicon-carbon composite particles is 6 m ² /g to 30 m ² /g; (xi) the area of pores per unit cross section of the silicon-carbon composite particles is 0.35 μm ² /μm ² to 0.85 μm ² /μm ² ; (xii) Based on the mass of the silicon-carbon composite particles, the mass content of silicon is 75% to 95%, and the mass content of carbon is 4% to 25%. The negative electrode material according to claim 7, wherein the silicon-carbon composite particles satisfy at least one of the following conditions (xiii) to (xvi): (xiii) the pore volume of the silicon-carbon composite particles is 1.5 cm ³ /g to 3.0 cm ³ /g; (xiv) the specific surface area of the silicon-carbon composite particles is 6 m ² /g to 15 m ² /g; (xv) the area of pores per unit cross section of the silicon-carbon composite particles is 0.35 μm ² /μm ² to 0.7 μm ² /μm ² ; (xvi) Based on the mass of the silicon-carbon composite particles, the mass content of the silicon element is 84% to 92%, and the mass content of the carbon element is 6% to 20%. The negative electrode material according to any one of claims 1 to 8, wherein the preparation method of the silicon-carbon composite particles comprises: taking a porous silicon material as a substrate, and depositing a carbon source on the surface of the porous silicon material by magnetron sputtering; the magnetron sputtering is carried out in an inert atmosphere, the power of the magnetron sputtering is 200W to 500W, the pulse frequency of the magnetron sputtering is 1 time/s to 5 times/s, and the time of the magnetron sputtering is 3s to 25s; the carbon source is selected from at least one of soft carbon and hard carbon. A secondary battery comprises a negative electrode, the negative electrode comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode material according to any one of claims 1 to 9. An electronic device comprising the secondary battery according to claim 10.