Silicon-carbon composite material, secondary battery, and electronic device

Abstract

A silicon-carbon composite material, a secondary battery, and an electronic device are disclosed. The differential capacity-voltage curves of the silicon-carbon composite material during lithium delithiation exhibit characteristic peaks at 250mV to 300mV, 400mV to 500mV, and 600mV to 750mV, respectively. The peak heights A (250mV to 300mV), B (400mV to 500mV), and C (600mV to 750mV) satisfy the condition: 0.9 ≤ (B+C) / A ≤ 1. The silicon-carbon composite material with these characteristic peak heights exhibits high specific capacity, good initial coulombic efficiency, cycle performance, expansion performance, and rate performance, effectively improving the energy density, cycle performance, and rate performance of the secondary battery.

Description

Silicon-carbon composite materials, secondary batteries and electronic devices Technical Field

[0001] The present application relates to the field of battery technology, and more particularly, to a silicon-carbon composite material, a secondary battery, and an electronic device. Background Art

[0002] Secondary batteries represented by lithium-ion batteries are widely used in digital electronic products, energy storage, drones, power tools, electric vehicles and other products due to their high energy density, long cycle life, high safety, and fast charging capabilities. As the demand for thinner and lighter products becomes more urgent, batteries need to have higher and higher energy density. Graphite is the negative electrode material of traditional commercial secondary batteries, but its low gram capacity is not conducive to improving the energy density of batteries. Elemental silicon has an ultra-high theoretical gram capacity (Li 15 Si4, 3579mAh / g) and suitable operating voltage (<0.5V vs.Li / Li + ) and other characteristics, making it considered the most promising anode material to replace graphite. However, low conductivity and significant volume expansion during alloying / de-alloying processes severely restrict the large-scale application of elemental silicon in secondary batteries. Therefore, it is necessary to improve the initial coulombic efficiency, cycling performance, expansion performance, and rate capability of anode materials while maintaining the gram capacity.

[0003] Summary of the Invention

[0004] The present application provides a silicon-carbon composite material, a secondary battery, and an electronic device. The silicon-carbon composite material has high gram capacity while having good first coulombic efficiency, cycle performance, expansion performance, and rate performance.

[0005] In the first aspect, the present application provides a silicon-carbon composite material, wherein the differential capacity-voltage curve of the silicon-carbon composite material during delithiation has characteristic peaks at 250mV to 300mV, 400mV to 500mV and 600mV to 750mV, respectively, and the peak height A of the characteristic peak from 250mV to 300mV, the peak height B of the characteristic peak from 400mV to 500mV and the peak height C of the characteristic peak from 600mV to 750mV satisfy: 0.9≤(B+C) / A≤1.

[0006] According to the present application, the silicon-carbon composite material whose characteristic peak height meets the above conditions has a high gram capacity while having good first coulombic efficiency, cycle performance, expansion performance and rate performance, which can effectively improve the energy density, cycle performance and rate performance of secondary batteries.

[0007] In some embodiments, 0.92≤(B+C) / A≤0.94.

[0008] In some embodiments, the silicon-carbon composite material includes: a carbon matrix having a pore structure, a silicon-based material disposed in the pore structure, and a carbon coating layer disposed on a surface of the carbon matrix.

[0009] In some embodiments, the D peak intensity I in the Raman spectrum of the silicon-carbon composite material is D and G peak intensity I G Satisfy: 0.75≤I D / I G ≤0.85.

[0010] In some embodiments, the silicon-carbon composite material contains pores with a pore diameter of 2 nm to 50 nm.

[0011] In some embodiments, based on the total mass of silicon in the silicon-carbon composite material, the mass percentage of silicon on the outer surface of the silicon-carbon composite material is 0.5% to 2%.

[0012] In some embodiments, the silicon-carbon composite material includes silicon grains, and in the XRD spectrum of the silicon-carbon composite material, the characteristic peak of the Si(111) crystal plane is located between 27.5° and 28.5°, and the characteristic peak position of the Si(111) crystal plane of the silicon-carbon composite material is shifted to the left by 0.5° to 1° compared with the characteristic peak position of the Si(111) crystal plane in the XRD spectrum of pure silicon material, and the size of the silicon grains is not greater than 1 nm.

[0013] In some embodiments, based on the total mass of the silicon-carbon composite material, the mass percentage a of the carbon element in the silicon-carbon composite material is 40% to 70%, and the mass percentage b of the silicon element in the silicon-carbon composite material is 30% to 60%; optionally, a and b satisfy: 0.5≤a / b≤2.

[0014] In some embodiments, the silicon-carbon composite material satisfies at least one of the following conditions: 1) the sphericity of the silicon-carbon composite material is not less than 0.7; 2) the particle size D of the silicon-carbon composite material is V 50 is 5μm to 10μm, D V 99 is 15 μm to 25 μm; 3) the specific surface area of ​​the silicon-carbon composite material is 1 m 2 / g to 50m 2 / g; 4) based on the total mass of the silicon-carbon composite material, the mass percentage of the oxygen element in the silicon-carbon composite material is 1% to 5%; 5) the first delithiation capacity of the silicon-carbon composite material is 500mAh / g to 2500mAh / g.

[0015] In a second aspect, the present application provides a secondary battery comprising a negative electrode plate, wherein the negative electrode plate comprises a negative electrode film layer, wherein the negative electrode film layer comprises a negative electrode active material, and wherein the negative electrode active material comprises the silicon-carbon composite material according to any embodiment of the first aspect.

[0016] In some embodiments, the negative electrode active material further comprises graphite, and the mass percentage of the graphite is 35% to 95% based on the total mass of the negative electrode active material, and the negative electrode active material satisfies at least one of the following conditions: 1) the particle size D of the negative electrode active material V 50 is 5μm to 15μm, D V 99 is 15 μm to 40 μm; 2) the specific surface area of ​​the negative electrode active material is 1m 2 / g to 10m 2 / g; 3) the first delithiation capacity of the negative electrode active material is 400 mAh / g to 1000 mAh / g.

[0017] In a third aspect, the present application provides an electronic device, comprising: a secondary battery according to any embodiment of the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on the drawings without creative work.

[0019] FIG1 is a charge and discharge curve of a silicon-carbon composite material in one embodiment of the present application.

[0020] FIG2 is a differential capacity-voltage curve of the silicon-carbon composite material during delithiation in one embodiment of the present application.

[0021] FIG3 is a scanning transmission electron microscope image of the silicon-carbon composite material in one embodiment of the present application.

[0022] FIG4 is an XRD spectrum of the silicon-carbon composite material in one embodiment of the present application. DETAILED DESCRIPTION

[0023] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0024] Unless otherwise defined, all technical and scientific terms used in this application have the same meanings as commonly understood by those skilled in the art to which this application belongs. The terms used in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned drawings are intended to cover non-exclusive inclusions. The terms "first" and "second" in the specification and claims of this application or the above-mentioned drawings are used to distinguish different objects, rather than to describe a specific order or a primary-secondary relationship.

[0025] References to "embodiments" in this application mean that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments.

[0026] The term "and / or" in this application simply describes an association between related objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this application generally indicates that the related objects are in an "or" relationship.

[0027] The term "plurality" used in this application refers to two or more (including two).

[0028] As described in the background art, silicon-based materials have a high theoretical gram capacity, but their volume expansion rate during the alloying / de-alloying process is large, resulting in poor cycle performance and poor electrical conductivity, thus affecting their application as negative electrode active materials.

[0029] In response to the above problems, related technologies mostly use methods such as silicon nano-sizing, porous silicon, and the introduction of transition metal oxides. Although silicon nano-sizing and porous silicon can buffer the volume expansion of elemental silicon to a certain extent, their high specific surface area and low tap density limit their large-scale application; although the introduction of transition metal oxides can also reduce the volume expansion of silicon, the mechanical properties and chemical stability of metal oxides are not outstanding, and therefore they are not good buffer media. Although carbon-based materials have a small gram capacity as negative electrode materials, they can be used as good conductive media and buffer matrices for silicon-based materials due to their low price, good conductivity, and outstanding chemical and thermal stability. Therefore, combining silicon-based compounds and carbon-based materials to prepare silicon-carbon composite materials is an effective method to reduce the volume expansion rate. However, in the current related technologies, no silicon-carbon composite material has been found that has a high gram capacity while having good first coulombic efficiency, cycle performance, expansion performance, and rate performance.

[0030] Based on this, the present application provides a silicon-carbon composite material, a secondary battery, and an electronic device. The differential capacity-voltage curve of the silicon-carbon composite material during delithiation satisfies specific conditions. Thus, while having a high gram capacity, it also exhibits good initial coulombic efficiency, cycle performance, expansion performance, and rate performance, effectively improving the cycle performance and rate performance of the secondary battery. The following describes the embodiments of the present application in detail.

[0031] Silicon-carbon composite materials

[0032] In the first aspect, the present application provides a silicon-carbon composite material, wherein the differential capacity-voltage curve of the silicon-carbon composite material during delithiation has characteristic peaks at 250mV to 300mV, 400mV to 500mV, and 600mV to 750mV, respectively, and the peak height A of the characteristic peak from 250mV to 300mV, the peak height B of the characteristic peak from 400mV to 500mV, and the peak height C of the characteristic peak from 600mV to 750mV satisfy: 0.9≤(B+C) / A≤1.

[0033] According to the present application, the differential capacity-voltage curve of the silicon-carbon composite material when delithiated has a well-known meaning in the art. The charge-discharge gram capacity of the silicon-carbon composite material is used as the horizontal coordinate and the voltage is used as the vertical coordinate to obtain the charge-discharge curve of the silicon-carbon composite material. Then, the first-order derivative of the delithiation gram capacity of the silicon-carbon composite material with respect to the voltage is taken as the vertical coordinate and the voltage is used as the horizontal coordinate to obtain the differential capacity-voltage curve of the silicon-carbon composite material when delithiated. The differential capacity-voltage curve can reflect the capacity contained in the silicon-carbon composite material within a unit range. If the capacity at a certain voltage platform is higher, it means that a lot of capacity will be contributed within a very small voltage fluctuation range, and a characteristic peak will be shown on the curve. Each characteristic peak represents an electrochemical reaction, and the peak height of each characteristic peak also indicates the contribution of the corresponding electrochemical reaction to the capacity.

[0034] The differential capacity-voltage curve of the silicon-carbon composite material during lithium removal has characteristic peaks at 250mV to 300mV, 400mV to 500mV, and 600mV to 750mV, respectively, indicating that there are at least three different lithium removal reactions in the lithium removal process of the silicon-carbon composite material. It is generally believed that the characteristic peak at 250mV to 300mV indicates that the amorphous Li x The delithiation reaction of Si, the characteristic peak of 400mV to 500mV indicates the crystalline Li 15The delithiation reaction of Si4, the characteristic peak of 600mV to 750mV represents the delithiation reaction of lithium metal precipitated in the silicon-carbon composite material. At the same time, the peak height A of the characteristic peak of 250mV to 300mV, the peak height B of the characteristic peak of 400mV to 500mV and the peak height C of the characteristic peak of 600mV to 750mV of the silicon-carbon composite material satisfy: 0.9≤(B+C) / A≤1. It can be understood that the peak height of each characteristic peak represents the size of its contribution to the specific capacity of the silicon-carbon composite material, among which the amorphous Li x When the delithiation reaction of Si accounts for a high proportion, due to its small volume expansion rate, it is beneficial to improve the cycle performance and expansion performance of silicon-carbon composite materials, but compared with crystalline Li 15 The delithiation reaction of Si4 and the precipitation of lithium metal is not conducive to improving the specific capacity of the silicon-carbon composite material. At the same time, the delithiation reaction of lithium metal can also effectively improve the rate performance of the silicon-carbon composite material. However, the crystalline Li 15 The proportion of Si4's delithiation reaction is too high. Due to its large volume expansion rate, it is unstable during the cycle, which will deteriorate the cycle performance and expansion performance of the silicon-carbon composite material. In addition, the proportion of delithiation reaction of lithium metal precipitation is too high. Excessive lithium metal precipitation will increase the amount of dead lithium in the silicon-carbon composite material and reduce the first coulombic efficiency of the silicon-carbon composite material.

[0035] The inventors found that by adjusting the (B+C) / A of the silicon-carbon composite material to 0.9 to 1, the silicon-carbon composite material has good first coulombic efficiency, cycle performance, expansion performance and rate performance while having a higher gram capacity, and can effectively improve the cycle performance and rate performance of the secondary battery. As an example, (B+C) / A can be 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1 or any of the above values. Further preferably, (B+C) / A is 0.92 to 0.94, at this time, the silicon-carbon composite material can better take into account its first coulombic efficiency, cycle performance, expansion performance and rate performance while having a higher gram capacity.

[0036] It should also be noted that the differential capacity-voltage curve of the silicon-carbon composite material during delithiation includes but is not limited to the differential capacity-voltage curve of the silicon-carbon composite material during the first cycle of delithiation, and can also be the differential capacity-voltage curve of the silicon-carbon composite material during any number of cycles of delithiation. It can be obtained by methods and instruments known in the art. As an example, the differential capacity-voltage curve of the silicon-carbon composite material during delithiation can be obtained by the following method, comprising the following steps:

[0037] (1) The silicon-carbon composite material to be tested is used as the negative electrode active material, conductive carbon black is used as the conductive agent, and polyacrylic acid is used as the binder. The mass ratio of the negative electrode active material, conductive carbon black, and polyacrylic acid is 70:20:10. The negative electrode active material, conductive agent, and binder aqueous solution are fully mixed to obtain a mixture slurry, and the mixture slurry is evenly coated on a copper foil and dried to obtain a negative electrode sheet;

[0038] (2) In an argon-filled glove box (water <10 ppm, oxygen <1 ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1:1. LiPF6 and 5 vol% fluoroethylene carbonate (FEC) were slowly added to the mixed solution and stirred to obtain a non-aqueous electrolyte. Celgard 2400 membrane was used as the isolation membrane.

[0039] (3) Assembling the above-mentioned negative electrode sheet with the lithium sheet as the counter electrode in a glove box in the order of negative electrode sheet, separator, and lithium sheet, with the lithium sheet as the counter electrode into a button half-cell;

[0040] (4) The half-cell assembled above was subjected to charge and discharge tests on a LAND CT2001A battery test system. The half-cell test used an operating voltage range of 0.01V to 2.0V, discharged at a constant current of 0.1C to 0.01V, allowed to stand for 5 minutes, then discharged at a constant current of 50μA to 0.01V, allowed to stand for 5 minutes, and charged at a constant current of 0.1C to 2.0V, allowed to stand for 5 minutes. The charge and discharge gram capacity of the half-cell was recorded, and the charge and discharge gram capacity of the silicon-carbon composite material was used as the horizontal axis and the voltage was used as the vertical axis to obtain the charge and discharge curve of the silicon-carbon composite material. Then, the first-order derivative of the delithiation gram capacity of the silicon-carbon composite material with respect to the voltage was taken as the vertical axis and the voltage was used as the horizontal axis to obtain the differential capacity-voltage curve of the silicon-carbon composite material when delithiation occurred.

[0041] As an example, Figure 1 shows the charge and discharge curves of the silicon-carbon composite material in an embodiment of the present application, and Figure 2 shows the differential capacity-voltage curve of the silicon-carbon composite material during delithiation in an embodiment of the present application. It can be seen that there are obvious characteristic peaks at 250mV to 300mV, 400mV to 450mV, and 600mV to 750mV, indicating that there are three different delithiation reactions in the silicon-carbon composite material during the delithiation process, and their corresponding peak heights are A, B, and C, respectively.

[0042] In some embodiments, the silicon-carbon composite material includes: a carbon matrix having a pore structure, a silicon-based material disposed in the pore structure, and a carbon coating layer disposed on a surface of the carbon matrix.

[0043] In some of the above embodiments, the structure of the silicon-carbon composite material is specifically defined. By using a carbon matrix with a pore structure as the skeleton of the silicon-carbon composite material, the silicon-carbon composite material can have good electrical conductivity, which is beneficial to the performance of the specific capacity and can further improve the specific capacity of the silicon-carbon composite material. In addition, the silicon-based material is arranged in the pore structure of the carbon matrix. On the one hand, it can limit the size of the silicon-based material, thereby reducing the expansion stress. On the other hand, the pore structure can disperse the expansion stress of the silicon-based material, thereby improving the stability of the silicon-based material and reducing the volume expansion rate of the silicon-carbon composite material, further improving the cycle performance and expansion performance of the silicon-carbon composite material. The carbon coating layer arranged on the surface of the carbon matrix can, on the one hand, reduce the specific surface area of ​​the silicon-carbon composite material, thereby reducing the side reaction of the silicon-carbon composite material with the electrolyte and improving its first coulombic efficiency. On the other hand, it can further improve the electrical conductivity of the silicon-carbon composite material, which is beneficial to further improve its rate performance.

[0044] In some embodiments, the D peak intensity I in the Raman spectrum of the silicon-carbon composite material is D and G peak intensity I G Satisfy: 0.75≤I D / I G ≤0.85.

[0045] In some of the above embodiments, the D peak intensity I in the Raman spectrum of the silicon-carbon composite material is D and G peak intensity I G have meanings well known in the art, wherein I D The Raman spectrum of the silicon-carbon composite material is 1360±5cm -1 The intensity of the peak at I G The Raman spectrum of the silicon-carbon composite material is 1580±5cm -1 The intensity of the peak at , the ratio of the two can represent the defect degree of carbon-based materials in silicon-carbon composite materials, I D / I G The larger the value, the higher the defect degree, which will deteriorate the over-discharge resistance of the silicon-carbon composite material. D / I G The smaller the value, the higher the degree of graphitization, which will affect the ion transmission rate to a certain extent, thereby affecting the rate performance of the silicon-carbon composite material. D / I G When the ratio is 0.75 to 0.85, the cycle performance and over-discharge resistance of the silicon-carbon composite material can be further improved.

[0046] The D peak intensity I in the Raman spectrum of silicon-carbon composites D and G peak intensity I GThe detection can be performed according to methods and instruments known in the art. As an example, a laser confocal Raman spectrometer (Raman, HR Evolution, HORIBA Scientific Instruments Division) is used to scan the sample particles to obtain the D peak and G peak of all particles within the area. The data is processed using LabSpec software to obtain the peak intensities of the D peak and G peak of each particle, which are I and I, respectively. D and I G , I D / I G Statistics I with a step size of 0.02 D / I G The frequency of the normal distribution diagram is obtained, and I is calculated. D / I G The average value of the D peak and G peak intensity ratio of the active material I D / I G The laser wavelength of the Raman spectrometer can be in the range of 532nm to 785nm. Among them, the D peak is at 1360cm -1 Peak intensity at 1580cm -1 Peak intensity.

[0047] In some embodiments, the silicon-carbon composite material contains pores with a pore size of 2 nm to 50 nm.

[0048] In some of the above embodiments, the pores with a pore size of 2nm to 50nm in the silicon-carbon composite material are, on the one hand, beneficial as a place for lithium precipitation, reducing the amount of dead lithium, thereby appropriately increasing the content of silicon-based materials in the silicon-carbon composite material, further increasing the gram capacity of the silicon-carbon composite material, and at the same time increasing the content of precipitated lithium, which is also beneficial to further improve the cycle performance of the silicon-carbon composite material; on the other hand, these pores can reserve expansion space for the silicon-based material, thereby improving the stability of the silicon-based material, while reducing the overall volume expansion rate of the silicon-carbon composite material, and further improving the cycle performance and expansion performance of the silicon-carbon composite material. For example, the pores containing pores in the silicon-carbon composite material can be 2nm, 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm or within the range of any of the above values. In addition, the silicon-carbon composite material can contain one or more pores with a pore size of 2nm to 50nm.

[0049] It should be noted that whether the silicon-carbon composite material contains pores with a pore size of 2nm to 50nm can be detected by methods and instruments known in the art. As an example, the following can be done: Use FIB (focused ion beam) to slice the silicon-carbon composite material to be tested, and then use a field FEI Talos F200S high-resolution transmission electron microscope to test the silicon-carbon composite material at a voltage of 200kV and a current of 100nA.

[0050] As an example, FIG3 shows a scanning transmission electron microscope image of a silicon-carbon composite material in an embodiment of the present application. As can be seen from the image, the silicon-carbon composite material contains a plurality of pores with a pore size of 2 nm to 50 nm.

[0051] In some embodiments, based on the total mass of silicon in the silicon-carbon composite material, the mass percentage of silicon on the outer surface of the silicon-carbon composite material is 0.5% to 2%.

[0052] In some of the above embodiments, the silicon element on the outer surface of the silicon-carbon composite material accounts for 0.5% to 2% of the total mass of the silicon element. In this case, there is a certain silicon-rich phenomenon on the outer surface of the silicon-carbon composite material. A certain amount of silicon element on the outer surface is more conducive to improving the gram capacity and first coulombic efficiency of the silicon-carbon composite material. At the same time, the silicon element on the outer surface will also affect the cyclic performance and expansion performance of the silicon-carbon composite material. For example, based on the total mass of the silicon element in the silicon-carbon composite material, the mass percentage of silicon element on the outer surface of the silicon-carbon composite material can be 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, or within the range composed of any of the above values.

[0053] Based on the total mass of silicon in the silicon-carbon composite material, the mass percentage of silicon on the outer surface of the silicon-carbon composite material can be measured according to methods and instruments known in the art. For example, the mass percentage of silicon on the outer surface of the silicon-carbon composite material can be calculated by thermogravimetric analysis (TG). The specific principle is that the crystallinity of silicon on the outer surface of the silicon-carbon composite material is stronger than that of silicon inside the outer surface of the silicon-carbon composite material. Therefore, the oxidation resistance of silicon on the outer surface is higher than that of silicon inside. The specific method is to heat the silicon-carbon composite material from 25°C to 1100°C in air. The increase in mass of the sample at about 300°C to 500°C represents that the low-crystalline internal silicon is oxidized to SiO2. Subsequently, due to the burning of the carbon material, the sample shows a mass loss. When the temperature continues to rise to close to 1100°C, the mass of the sample increases again, which means that the high-crystalline outer surface silicon is completely oxidized. By comparing the degree of mass increase at high temperature, the silicon content on the outer surface of the particle can be calculated.

[0054] In some embodiments, the silicon-carbon composite material includes silicon grains, and in the XRD spectrum of the silicon-carbon composite material, the characteristic peak of the Si(111) crystal plane is located between 27.5° and 28.5°, the characteristic peak position of the Si(111) crystal plane of the silicon-carbon composite material is shifted to the left by 0.5° to 1° compared with the characteristic peak position of the Si(111) crystal plane in the XRD spectrum of pure silicon material, and the size of the silicon grains is not greater than 1 nm.

[0055] In some of the above embodiments, the silicon-carbon composite material includes silicon grains, which can further improve the specific capacity of the silicon-carbon composite material. In addition, in the XRD spectrum of the silicon-carbon composite material, the characteristic peak of the Si(111) crystal plane is located between 27.5° and 28.5°. The characteristic peak position of the Si(111) crystal plane of the silicon-carbon composite material is shifted to the left by 0.5° to 1° compared with the characteristic peak position of the Si(111) crystal plane of the pure silicon material. This may be due to the presence of nanoscale pores near the silicon grains in the silicon-carbon composite material, which causes a certain degree of shift in the characteristic peak. These nanoscale pores can reserve space for the expansion of the silicon grains, improve the stability of the silicon grains, and limit the volume expansion of the silicon grains. At the same time, the size of the silicon grains is not greater than 1 nm. The smaller grain size is conducive to reducing its own expansion stress, so that the silicon grains have a smaller volume expansion rate and higher stability, thereby further improving the cycle performance and expansion performance of the silicon-carbon composite material. For example, in the XRD test pattern of the silicon-carbon composite material, the characteristic peak of the Si(111) crystal plane can be located at 27.5°, 27.7°, 27.9°, 28.1°, 28.3°, or 28.5°. The position of the characteristic peak of the Si(111) crystal plane in the XRD pattern of the silicon-carbon composite material can be shifted to the left by 0.5°, 0.6°, 0.7°, 0.8°, 0.9°, 1°, or within the range composed of any of the above values ​​compared to the position of the characteristic peak of the Si(111) crystal plane in the XRD pattern of the pure silicon material. The size of the silicon grains can be 0.1nm, 0.2nm, 0.3nm, 0.4nm, 0.5nm, 0.6nm, 0.7nm, 0.8nm, 0.9nm, 1nm, or within the range composed of any of the above values.

[0056] The XRD pattern of the silicon-carbon composite material and the size of the silicon grains have well-known meanings in the art and can be detected by methods and instruments known in the art. For example, using a D8Advance device with a Cu target Using a target material, the test was conducted at a voltage of 60 kV over an angle range of 2θ from 10° to 80°. After obtaining the XRD pattern of the silicon-carbon composite, the size of the silicon crystallites in the silicon-carbon composite was calculated using the Debye-Scherrer formula: D = Kλ / βcosθ at 2θ = 28.4°. Here, D is the size of the silicon crystallites, K is the Scherrer constant, λ is the X-ray wavelength, β is the half-width at half-maximum of the measured sample diffraction peak, and θ is the Bragg diffraction angle.

[0057] As an example, FIG4 shows an XRD spectrum of the silicon-carbon composite material in an embodiment of the present application. At the same time, the characteristic peak of the Si(111) crystal plane and the characteristic peak position of the Si(111) crystal plane of the pure silicon material in the XRD spectrum are marked in the figure. It can be seen that the characteristic peak position of the Si(111) crystal plane of the silicon-carbon composite material is significantly shifted to the left compared to the characteristic peak position of the Si(111) crystal plane of the pure silicon material.

[0058] In some embodiments, based on the total mass of the silicon-carbon composite material, the mass percentage a of the carbon element in the silicon-carbon composite material is 40% to 70%, and the mass percentage b of the silicon element in the silicon-carbon composite material is 30% to 60%.

[0059] In some of the above embodiments, the mass percentages of silicon and carbon in the silicon-carbon composite material are further limited, wherein increasing the carbon content can improve the conductivity of the silicon-carbon composite material, which is beneficial to the performance of the gram capacity, while too high a carbon content will reduce the gram capacity of the silicon-carbon composite material; in addition, increasing the silicon content can increase the gram capacity of the silicon-carbon composite material, but too high a silicon content will affect the cycle performance and expansion performance of the silicon-carbon composite material. Therefore, controlling the carbon and silicon elements within the above ranges can further take into account the battery energy density, cycle performance and expansion performance. For example, a can be 40%, 45%, 50%, 55%, 60%, 65%, 70% or within the range composed of any of the above values, and b can be 30%, 35%, 40%, 45%, 50%, 55%, 60% or within the range composed of any of the above values.

[0060] Furthermore, a and b satisfy the following: 0.5 ≤ a / b ≤ 2. When the carbon and silicon contents meet the aforementioned ranges, the silicon-carbon composite material can achieve both specific capacity and cycling performance, expansion performance, and rate performance. For example, a / b can be 0.5, 1, 1.5, 2, or any of the aforementioned values.

[0061] The mass percentages of carbon and silicon in the silicon-carbon composite material have well-known meanings in the art and can be detected by methods and instruments known in the art. For example, the contents of carbon and silicon in the silicon-carbon composite material can be characterized by using ICP (inductively coupled plasma spectrometer) technology.

[0062] In some embodiments, the sphericity of the silicon-carbon composite material is not less than 0.7. The higher the sphericity of the silicon-carbon composite material, the more conducive it is to increasing the compaction density of the silicon-carbon composite material, thereby further increasing the energy density of the secondary battery.

[0063] The sphericity of the silicon-carbon composite material has a well-known meaning in the art and can be detected by instruments and methods known in the art, for example: first, the silicon-carbon composite material to be tested is polished by an IB-09010CP / ion polisher (voltage is 6kV), and then the silicon-carbon composite material is tested using a JEOL-JSM-6700F scanning electron microscope at a voltage of 5kV and a current of 0.8nA in backscattered mode to obtain a scanning electron microscope image of the cross section of the silicon-carbon composite material in backscattered mode. The sphericity of the silicon-carbon composite material is measured by the British lattice code SHAPE industrial image analysis and processing software. Sphericity is the ratio of the surface area of ​​a sphere of the same volume as the object to the surface area of ​​the object. It is a parameter that characterizes the morphology of the particle. The closer the particle is to a sphere in morphology, the closer its sphericity is to 1. The calculation formula for the sphericity of any particle is:

[0064] Among them, V p is the particle volume, S p is the particle surface area.

[0065] In some embodiments, the particle size D of the silicon-carbon composite material is V 50 is 5μm to 10μm, D V 99 is 15μm to 25μm. This can avoid the defects of excessive specific surface area, increased electrolyte consumption and low material compaction density caused by too small particle size, and can also prevent the problems of low ion conductivity and poor rate performance caused by too large particle size, thereby further improving the first coulombic efficiency, cycle performance and rate performance of the silicon-carbon composite material. For example, the particle size D of the silicon-carbon composite material V 50 can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or any range thereof, D V 99 can be 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm or a range consisting of any of the above values.

[0066] D of silicon carbon composites V50. D V 99 has a well-known meaning in the art, i.e., the particle size corresponding to the 50% and 90% cumulative particle size distribution on a volume basis, which can be measured by known methods. For example, using a laser diffraction particle size distribution measuring instrument (Malvern Mastersizer 3000), according to the particle size distribution laser diffraction method GB / T19077-2016, the particle size distribution is measured to obtain D V 50. D V 99.

[0067] In some embodiments, the specific surface area of ​​the silicon-carbon composite material is 1 m 2 / g to 50m 2 / g. This is conducive to reducing the side reactions between the silicon-carbon composite material and the electrolyte, thereby further improving the initial coulombic efficiency and cycle performance of the silicon-carbon composite material. For example, the specific surface area of ​​the silicon-carbon composite material can be 1m 2 / g、5m 2 / g、10m 2 / g、15m 2 / g, 20m 2 / g, 25m 2 / g、30m 2 / g、35m 2 / g, 40m 2 / g、45m 2 / g, 50m 2 / g or a range consisting of any two of the above values.

[0068] The specific surface area of ​​a silicon-carbon composite material has a well-known meaning in the art and can be measured by known methods. For example, the specific surface area can be tested using a gas adsorption method in accordance with the GB / T19587-2017 test standard, specifically as follows: a silicon-carbon composite material is taken as a sample, the sample tube is immersed in liquid nitrogen at -196°C, and the amount of nitrogen adsorbed on the solid surface at different pressures is measured at a relative pressure of 0.05 to 0.30. The amount of sample monolayer adsorption is calculated based on the BET multilayer adsorption theory and its formula, thereby calculating the specific surface area of ​​the solid.

[0069] In some embodiments, the mass percentage of oxygen in the silicon-carbon composite material is 1% to 5% based on the total mass of the silicon-carbon composite material. In this case, the silicon-carbon composite material can generate Li2O during the lithium insertion process, which can serve as a buffer substance, thereby further improving the cycle performance, expansion performance, and rate performance of the silicon-carbon composite material. For example, the mass percentage of oxygen in the silicon-carbon composite material can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range consisting of any of the above values.

[0070] The mass percentage of oxygen in the silicon-carbon composite material has a well-known meaning in the art and can be detected by methods and instruments known in the art. For example, the oxygen content of the silicon-carbon composite material is tested using a German Elementar elemental analyzer.

[0071] In some embodiments, the first delithiation gram capacity of the silicon-carbon composite material is 500mAh / g to 2500mAh / g. Now the gram capacity of the silicon-carbon composite material can be better taken into account as well as the cycle performance, expansion performance and rate performance. For example, the first delithiation gram capacity of the silicon-carbon composite material can be 500mAh / g, 600mAh / g, 700mAh / g, 800mAh / g, 900mAh / g, 1000mAh / g, 1100mAh / g, 1200mAh / g, 1300mAh / g, 1400mAh / g, 1500mAh / g, 1600mAh / g, 1700mAh / g, 1800mAh / g, 1900mAh / g, 2000mAh / g, 2100mAh / g, 2200mAh / g, 2300mAh / g, 2400mAh / g, 2500mAh / g or the scope of any of the above numerical values.

[0072] The first delithiation gram capacity of the silicon-carbon composite material has a meaning well known in the art and can be measured according to methods known in the art, for example, by the following method:

[0073] (1) The silicon-carbon composite material to be tested is used as the negative electrode active material, conductive carbon black is used as the conductive agent, and polyacrylic acid is used as the binder. The mass ratio of the negative electrode active material, conductive carbon black, and polyacrylic acid is 70:20:10. The negative electrode active material, conductive agent, and binder aqueous solution are fully mixed to obtain a mixture slurry, and the mixture slurry is evenly coated on a copper foil and dried to obtain a negative electrode sheet;

[0074] (2) In an argon-filled glove box (water <10 ppm, oxygen <1 ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1:1. LiPF6 and 5 vol% fluoroethylene carbonate (FEC) were slowly added to the mixed solution and stirred to obtain a non-aqueous electrolyte. Celgard 2400 membrane was used as the isolation membrane.

[0075] (3) Assembling the above-mentioned negative electrode sheet with the lithium sheet as the counter electrode in a glove box in the order of negative electrode sheet, separator, and lithium sheet, with the lithium sheet as the counter electrode into a button half-cell;

[0076] (4) The half-cell assembled above was subjected to charge and discharge tests on a LAND CT2001A battery test system. The half-cell test used an operating voltage range of 0.01V to 2.0V, discharged at a constant current of 0.1C to 0.01V, allowed to stand for 5 minutes, then discharged at a constant current of 50μA to 0.01V, allowed to stand for 5 minutes, and charged at a constant current of 0.1C to 2.0V, allowed to stand for 5 minutes. The first charge and discharge capacity in grams of the half-cell was recorded, where the first charge capacity was the first delithiation capacity in grams of the silicon-carbon composite material.

[0077] The present application does not particularly limit the preparation method of the silicon-carbon composite material. For example, the preparation method of the silicon-carbon composite material may include but is not limited to the following steps: placing a carbon matrix having a porous structure in a reaction apparatus, introducing a silicon source gas under negative pressure conditions, causing the silicon source gas to pyrolyze and deposit into a silicon-based material in the porous structure of the carbon matrix, and then introducing a carbon source gas under negative pressure conditions, causing the carbon source gas to pyrolyze and deposit into a carbon coating layer, thereby obtaining a silicon-carbon composite material. The silicon source gas may include but is not limited to at least one of monosilane, disilane, trisilane, tetrasilane, chlorosilane, dichlorosilane, trichlorosilane, or tetrachlorosilane; the carbon source gas may include but is not limited to at least one of methane, acetylene, ethylene, ethane, propyne, propylene, propane, butyne, butene, or butane.

[0078] Typically, the (B+C) / A value in the differential capacity-voltage curve of a silicon-carbon composite material during delithiation can be controlled by changing the pyrolysis temperature, gas flow rate, and the time of introducing the silicon source gas. For example, increasing the pyrolysis temperature increases the B value, increases the C value, and increases the (B+C) / A value; decreasing the pyrolysis temperature decreases the B value, decreases the C value, and decreases the (B+C) / A value; increasing the gas flow rate increases the B value and decreases the C value; decreasing the gas flow rate decreases the B value and increases the C value; extending the time of introducing the silicon source gas increases the B value and decreases the C value); shortening the time of introducing the silicon source gas decreases the B value and increases the C value. Those skilled in the art can adjust the pyrolysis temperature of the silicon source gas or the carbon source gas, the gas flow rate of the silicon source gas or the carbon source gas, and the introduction time of the silicon source gas or the carbon source gas as needed. For example, the pyrolysis temperature of the silicon source gas or the carbon source gas is 400°C to 800°C, the gas flow rate of the silicon source gas or the carbon source gas is 50sccm to 500sccm, and the time for introducing the silicon source gas is 1h to 20h.

[0079] secondary batteries

[0080] In a second aspect, the present application provides a secondary battery comprising a negative electrode plate, the negative electrode plate comprising a negative electrode film layer, the negative electrode film layer comprising a negative electrode active material, and the negative electrode active material comprising a silicon-carbon composite material according to any embodiment of the first aspect.

[0081] According to the present application, since the negative electrode plate of the secondary battery includes a silicon-carbon composite material, it has the beneficial effects of any embodiment of the first aspect, that is, it has good energy density, cycle performance and rate performance.

[0082] Typically, a secondary battery includes a negative electrode sheet, a positive electrode sheet, an electrolyte, and a separator.

[0083]

Negative electrode

[0084] In some embodiments, the negative electrode active material further comprises graphite, and the mass percentage of graphite is 35% to 95% based on the total mass of the negative electrode active material, and the negative electrode active material satisfies at least one of the following conditions: 1) the particle size D of the negative electrode active material is V 50 is 5μm to 15μm, D V 99 is 15μm to 40μm; 2) the specific surface area of ​​the negative electrode active material is 1m 2 / g to 10m 2 / g; 3) the first delithiation capacity of the negative electrode active material is 400mAh / g to 1000mAh / g.

[0085] In some of the above embodiments, the negative electrode active material may also include graphite. Although graphite has a low gram capacity, it has good cycle stability and a low volume expansion rate. Therefore, controlling the mass percentage of graphite in the negative electrode active material to 35% to 95% can further improve the cycle performance of the secondary battery.

[0086] In addition, by controlling the overall particle size, specific surface area and first delithiation capacity of the negative electrode active material, the energy density, first coulombic efficiency, cycle performance and rate performance of the secondary battery can be further improved.

[0087] It should also be noted that the overall particle size, specific surface area and first delithiation capacity of the negative electrode active material can be tested by referring to the test methods for the particle size, specific surface area and first delithiation capacity of the silicon-carbon composite material mentioned above.

[0088] In some embodiments, the negative electrode sheet includes a negative electrode current collector, and the negative electrode film layer is disposed on at least one side of the negative electrode current collector. For example, the negative electrode current collector has two opposing surfaces in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposing surfaces of the negative electrode current collector.

[0089] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0090] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0091] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0092] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0093] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0094]

Positive electrode

[0095] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0096] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0097] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0098] In some embodiments, the positive electrode active material may adopt the positive electrode active material for secondary batteries that is well known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0099] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0100] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0101] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0102]

Isolation film

[0103] The separator is placed between the positive and negative electrodes to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through. This application does not impose any particular restrictions on the type of separator; any known porous separator with good chemical and mechanical stability can be used.

[0104] In some embodiments, the material of the isolation membrane can be selected from one or more of, but not limited to, fiberglass, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. Alternatively, the isolation membrane can include polyethylene and / or polypropylene. The isolation membrane can be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different. In some embodiments, the isolation membrane can also be provided with a ceramic coating or a metal oxide coating.

[0105]

Electrolyte

[0106] The electrolyte plays a role in conducting active ions between the positive electrode and the negative electrode. The electrolyte that can be used in the secondary electrolyte of this application can be an electrolyte known in the prior art.

[0107] In some embodiments, the electrolyte may include an organic solvent, an electrolyte salt, and optional additives. The types of the organic solvent, the lithium salt, and the additives are not particularly limited and may be selected according to needs.

[0108] In some embodiments, the secondary battery is a lithium-ion battery, and the electrolyte salt may include a lithium salt. As an example, the lithium salt includes, but is not limited to, at least one of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiFSI (lithium bis(trifluoromethanesulfonyl imide), LiTFSI (lithium bis(trifluoromethanesulfonyl imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalatoborate), LiBOB (lithium dioxalatoborate), LiPO2F2 (lithium difluorophosphate), LiDODFP (lithium difluorooxalatophosphate), and LiOTFP (lithium tetrafluorooxalatophosphate). The above lithium salts can be used alone or in combination.

[0109] In some embodiments, the secondary battery is a sodium ion battery, and the electrolyte salt may include a sodium salt. As an example, the sodium salt may be selected from at least one of NaPF6, NaClO4, NaBCl4, NaSO3CF3, and Na(CH3)C6H4SO3.

[0110] In some embodiments, as an example, the organic solvent includes but is not limited to ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), cyclopentane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS) and diethyl sulfone (ESE). The above organic solvents can be used alone or in combination. Alternatively, two or more organic solvents are used in combination.

[0111] In some embodiments, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

[0112] As an example, the additive includes but is not limited to at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), diethylene sulfate (DTD), propylene sulfate, vinyl sulfite (ES), 1,3-propane sultone (PS), 1,3-propene sultone (PST), sulfonate cyclic quaternary ammonium salt, succinic anhydride, succinonitrile (SN), adiponitrile (AND), tris(trimethylsilyl) phosphate (TMSP), and tris(trimethylsilyl) borate (TMSB).

[0113] The electrolyte solution can be prepared according to conventional methods in the art. For example, an organic solvent, an electrolyte salt, and optional additives can be uniformly mixed to obtain the electrolyte solution. The order in which the materials are added is not particularly limited. For example, the electrolyte salt and optional additives can be added to the organic solvent and mixed uniformly to obtain the electrolyte solution; alternatively, the electrolyte salt can be first added to the organic solvent, and then the optional additives can be added to the organic solvent and mixed uniformly to obtain the electrolyte solution.

[0114] electronic devices

[0115] In a third aspect, the present application provides an electronic device, comprising: a secondary battery according to any embodiment of the second aspect.

[0116] Since the electronic device includes the secondary battery of any embodiment of the second aspect, it has the beneficial effects of the second aspect.

[0117] The electronic device of the present application is not particularly limited and can be any electronic device known in the prior art. In some embodiments, the electronic device can include, but is not limited to, a laptop computer, a pen-type 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 television, 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, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0118] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0119] The first lithium-free gram capacity and first coulombic efficiency test of silicon-carbon composite materials:

[0120] Half-cell charge and discharge tests were performed on a LAND CT2001A battery test system. The half-cell test employed an operating voltage range of 0.01V to 2.0V. The test involved discharging the battery at a constant current of 0.1C to 0.01V, allowing it to rest for 5 minutes. The battery then discharged at a constant current of 50μA to 0.01V, allowing it to rest for 5 minutes, and then charging the battery at a constant current of 0.1C to 2.0V, allowing it to rest for 5 minutes. The initial discharge and charge capacities of the half-cell were recorded as the initial lithium insertion and removal capacities, respectively.

[0121] First coulombic efficiency (%) = (first lithium removal capacity / first lithium insertion capacity) × 100%.

[0122] Cyclic performance test of silicon-carbon composite materials:

[0123] Discharge the half-cell at 25°C at a constant current of 0.5C to 0.01V, let it rest for 5 minutes, then discharge it at a constant current of 50μA to 0.01V, let it rest for 5 minutes, and charge it at a constant current of 0.5C to 2.0V, let it rest for 5 minutes. Record the discharge capacity of the first cycle. Then, repeat the same charge and discharge steps for 50 cycles, and record the discharge capacity at the 50th cycle.

[0124] Capacity retention rate of half-cell after 50 cycles (%) = (discharge capacity at the 50th cycle / discharge capacity at the first cycle) × 100%.

[0125] The full battery was charged at 25°C at a constant current of 0.5C to 3.8V, then charged at a constant voltage of 3.8V to a cutoff of 50μA, allowed to rest for 5 minutes, and discharged at a constant current of 0.5C to 2.4V, allowed to rest for 5 minutes. The discharge capacity of the first cycle was recorded. The same charge and discharge steps were then repeated for 100 cycles, and the discharge capacity at the 100th cycle was recorded.

[0126] Full battery 100-cycle capacity retention rate (%) = (discharge capacity at the 100th cycle / discharge capacity at the first cycle) × 100%.

[0127] Rate performance test of silicon-carbon composite materials:

[0128] Half-cells were rate-tested on a LAND CT2001A battery test system. The test employed an operating voltage range of 0.01V to 2V, discharging at a constant current of 0.5C to 0.01V, allowing the battery to rest for 5 minutes. The battery then charged at a constant current of 0.5C to 2.0V, allowing the battery to rest for 5 minutes. The charge capacity of the half-cell was recorded as the delithiation capacity at 0.5C. Similarly, the test was repeated at 5.0C using the same process to determine the delithiation capacity of the half-cell at 5.0C.

[0129] Expansion performance test of silicon-carbon composite materials:

[0130] The button-type half-cells were disassembled before and after 50 cycles, and the negative electrode sheets were obtained. The thickness of the sheet was measured 12 times with a vernier caliper and the average value was taken. If the copper foil thickness is l, the sheet thickness before 50 cycles is m, and the sheet thickness after 50 cycles is n, then the thickness expansion rate k of the negative electrode sheet after 50 cycles is: k = (nm) / (ml) × 100%.

[0131] Over-discharge resistance test of silicon-carbon composite materials:

[0132] The full battery was charged at 25°C at a constant current of 0.5C to 3.8V, then charged at a constant voltage of 3.8V to a cutoff of 50μA, allowed to rest for 5 minutes, and discharged at a constant current of 0.5C to 1.5V, allowed to rest for 5 minutes. The discharge capacity of the first cycle was recorded. The same charge and discharge steps were then repeated for 100 cycles, and the discharge capacity at the 100th cycle was recorded.

[0133] Full battery 100-cycle deep discharge capacity retention rate (%) = (discharge capacity of the 100th cycle / discharge capacity of the first cycle) × 100%.

[0134] Example 1

[0135] Preparation of silicon-carbon composite materials:

[0136] (1) The porous carbon material was sieved and pre-treated for demagnetization. 100 g of the pre-treated porous carbon was placed in a reaction chamber of a rotary kiln, vacuumed to maintain a pressure of -95 kPa, and heated to 200 °C for 1 h to remove the trace water remaining in the porous carbon.

[0137] (2) Slowly introduce nitrogen gas with a purity of 99.999%, and continue to heat the rotary kiln to 500°C. In order to improve the utilization rate of the porous carbon pores and monosilane gas, the rotary kiln is continuously evacuated to maintain the pressure inside the furnace below -90kPa. A 5% concentration of monosilane / nitrogen mixture is introduced at a flow rate of 1L / min, and the reaction is continued for 5h, so that monosilane is adsorbed and deposited in the pore structure of the porous carbon to form elemental silicon grains;

[0138] (3) The temperature of the rotary kiln was raised to 600°C at a heating rate of 10°C / min, and an acetylene / propylene / nitrogen mixture (40% acetylene, 10% propylene, 50% nitrogen) was used as the carbon source. In order to improve the utilization rate of the carbon source gas, the rotary kiln was continuously evacuated to maintain the pressure inside the furnace below -90kPa. The acetylene / propylene / nitrogen mixture was introduced at a flow rate of 1L / min and the reaction was continued for 7h, so that the gaseous carbon source was deposited on the outer surface of the material and in some pores to form amorphous carbon with a high degree of graphitization;

[0139] (4) The material obtained in step (3) is screened and demagnetized to obtain a silicon-carbon composite material. The sphericity of the silicon-carbon composite material, the number of the first silicon-carbon composite particles, the size of the silicon grains, the mass percentage of the silicon element, the mass percentage of the carbon element, D V 50. D V 99. The specific surface area and the mass percentage of oxygen element were tested, and the results are shown in Table 2.

[0140] Preparation of half-cell and full-cell:

[0141] (1) Preparation of the negative electrode sheet: The silicon-carbon composite material of the present invention is used as the negative electrode active material, conductive carbon black is used as the conductive agent, and polyacrylic acid is used as the binder. The mass ratio of the negative electrode active material, conductive carbon black, and polyacrylic acid is 70:20:10. The negative electrode active material, conductive agent, and binder aqueous solution are thoroughly mixed to obtain a mixture slurry. The mixture slurry is evenly coated on copper foil and dried to obtain the negative electrode sheet.

[0142] (2) Preparation of the positive electrode sheet: Super P is used as a conductive agent and PVDF is used as a binder. The mass ratio of the positive electrode active material (LiFePO4), Super P, and PVDF is 70:20:10. The positive electrode active material, Super P, and 10 wt% PVDF solution are thoroughly mixed to obtain a mixture slurry. The mixture slurry is evenly coated on aluminum foil and dried to obtain a positive electrode sheet.

[0143] (3) Electrolyte and isolation membrane: In an argon-filled glove box (water <10 ppm, oxygen <1 ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1:1. LiPF6 and 5 vol% fluoroethylene carbonate (FEC) were slowly added to the mixed solution and stirred to obtain a non-aqueous electrolyte. Celgard 2400 membrane was used as the isolation membrane.

[0144] (4) Assembly of half-cells and full-cells: The above-mentioned negative electrode sheets are respectively used as counter electrodes with lithium sheets or positive electrode sheets, and are assembled into button-type half-cells in a glove box with lithium sheets as counter electrodes in the order of negative electrode sheets, separators, lithium sheets or positive electrode sheets, and button-type full-cells are assembled with the above-prepared positive electrode sheets as counter electrodes.

[0145] Examples 2 to 9, Comparative Examples 1 to 6

[0146] It is substantially the same as Example 1, except that different silicon-carbon composite materials are prepared according to the relevant preparation parameters in Table 1, wherein the relevant parameters of the silicon-carbon composite materials are shown in Table 2.

[0147] Comparative Example 7

[0148] The method is substantially the same as Example 1, except that the silicon-carbon composite material is prepared according to the following method: micron silicon powder that has been sieved through 400 mesh and pre-treated for demagnetization is mixed with graphite and ground under an inert atmosphere to obtain a traditional silicon-carbon composite material.

[0149] Table 1

[0150] Table 2

[0151] Performance Testing

[0152] The first delithiation capacity, first coulombic efficiency, cycle performance, rate performance, expansion performance and over-discharge resistance of the silicon-carbon composite materials obtained in Examples 1 to 7 and Comparative Examples 1 to 7 were tested, and the results are shown in Table 3.

[0153] Table 3

[0154] According to Table 3, the silicon-carbon composite materials obtained in each embodiment can take into account good first coulombic efficiency, cycle performance, rate performance and expansion performance while having a higher gram capacity than the comparative examples, thereby helping to improve the energy density, cycle performance and rate performance of the secondary battery. When (B+C) / A is 0.92 to 0.94, better first coulombic efficiency, cycle performance, rate performance and expansion performance can be taken into account. In addition, it can be seen that the I of the silicon-carbon composite material D / I G The value has a certain influence on its cycle performance and over-discharge resistance. When other conditions are constant, I D / I G When the value is between 0.75 and 0.85, the silicon-carbon composite material has better cycle performance and over-discharge resistance.

[0155] According to Comparative Examples 1 to 6, when (B+C) / A is too low (i.e., less than 0.9), the gram capacity and the first coulombic efficiency of the silicon-carbon composite material are low. The possible reason is that the content of silicon in the silicon-carbon negative electrode material is low at this time, which will cause the delithiation reaction of the crystalline silicon-lithium alloy to contribute less to the gram capacity. Although the precipitation of lithium can increase the gram capacity to a certain extent, the improvement effect is limited due to the formation of dead lithium, which will affect the first coulombic efficiency of the material. When (B+C) / A is too high (i.e., greater than 1), the cycle performance, expansion performance and rate performance of the silicon-carbon composite material are poor. The possible reason is that increasing the content of silicon in the silicon-carbon negative electrode material will increase the contribution of the delithiation reaction of the crystalline silicon-lithium alloy to the gram capacity, but will reduce the content of precipitated lithium. At the same time, the volume change rate of the delithiation reaction of the crystalline silicon-lithium alloy is large, thereby affecting the expansion performance and rate performance of the silicon-carbon composite material.

[0156] According to Comparative Example 7, it can be seen that its (B+C) / A value is as high as 25.2. This is because the delithiation curve of the traditional silicon-carbon material has almost no characteristic peaks at 600-750mV and 250-300mV, and only a strong characteristic peak at 400-500mV. At this time, its cycle performance, expansion performance and rate performance are poor. The possible reason is that the volume change rate of the delithiation reaction of the crystalline silicon-lithium alloy is large, and there are almost no nanopores that can stably accommodate volume expansion, which affects the cycle performance, rate performance and expansion performance of the silicon-carbon composite material.

[0157] It should be noted that, unless there is any conflict, the embodiments and features in the embodiments of this application can be combined with each other.

[0158] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A silicon-carbon composite material, wherein the differential capacity-voltage curve of the silicon-carbon composite material during delithiation has characteristic peaks at 250mV to 300mV, 400mV to 500mV, and 600 to 750mV, respectively, and the peak height A of the characteristic peak at 250mV to 300mV, the peak height B of the characteristic peak at 400mV to 500mV, and the peak height C of the characteristic peak at 600mV to 750mV satisfy: 0.9≤(B+C) / A≤1.

2. The silicon-carbon composite material according to claim 1, wherein 0.92≤(B+C) / A≤0.

94.

3. The silicon-carbon composite material according to claim 1 or 2, wherein The silicon-carbon composite material comprises: a carbon matrix with a pore structure, a silicon-based material arranged in the pore structure, and a carbon coating layer arranged on the surface of the carbon matrix.

4. The silicon-carbon composite material according to any one of claims 1 to 3, wherein The D peak intensity I in the Raman spectrum of the silicon-carbon composite material D and G peak intensity I G Satisfy: 0.75≤I D / I G ≤0.

85.

5. The silicon-carbon composite material according to any one of claims 1 to 4, wherein The silicon-carbon composite material contains pores with a pore diameter of 2 nm to 50 nm.

6. The silicon-carbon composite material according to any one of claims 1 to 5, wherein Based on the total mass of silicon in the silicon-carbon composite material, the mass percentage of silicon on the outer surface of the silicon-carbon composite material is 0.5% to 2%.

7. The silicon-carbon composite material according to any one of claims 1 to 6, wherein The silicon-carbon composite material includes silicon grains. The characteristic peak of the Si(111) crystal plane in the XRD spectrum of the silicon-carbon composite material is located between 27.5° and 28.5°. The size of the silicon grains is no greater than 1 nm.

8. The silicon-carbon composite material according to any one of claims 1 to 7, wherein Based on the total mass of the silicon-carbon composite material, the mass percentage a of the carbon element in the silicon-carbon composite material is 40% to 70%, and the mass percentage b of the silicon element in the silicon-carbon composite material is 30% to 60%; Optionally, a and b satisfy: 0.5≤a / b≤2.

9. The silicon-carbon composite material according to any one of claims 1 to 8, wherein: The silicon-carbon composite material satisfies at least one of the following conditions: 1) The sphericity of the silicon-carbon composite material is not less than 0.7; 2) Particle size D of the silicon-carbon composite material V 50 is 5μm to 10μm, D V 99 is 15μm to 25μm; 3) The specific surface area of ​​the silicon-carbon composite material is 1m 2 / g to 50m 2 / g; 4) The mass percentage of oxygen in the silicon-carbon composite material is 1% to 5% based on the total mass of the silicon-carbon composite material; 5) The first delithiation capacity of the silicon-carbon composite material is 500 mAh / g to 2500 mAh / g.

10. A secondary battery comprising a negative electrode plate, wherein the negative electrode plate comprises a negative electrode film layer, the negative electrode film layer comprises a negative electrode active material, and the negative electrode active material comprises the silicon-carbon composite material according to any one of claims 1 to 9.

11. The secondary battery according to claim 10, wherein The negative electrode active material further includes graphite, wherein the mass percentage of the graphite is 35% to 95% based on the total mass of the negative electrode active material, and the negative electrode active material satisfies at least one of the following conditions: 1) Particle size D of the negative electrode active material V 50 is 5μm to 15μm, D V 99 is 15μm to 40μm; 2) The specific surface area of ​​the negative electrode active material is 1m 2 / g to 10m 2 / g; 3) The first delithiation capacity of the negative electrode active material is 400 mAh / g to 1000 mAh / g.

12. An electronic device comprising: The secondary battery according to claim 10 or 11.

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