Negative electrode active material, preparation method thereof and electrochemical device comprising the same

Abstract

The present application provides a negative electrode active material and an electrochemical device comprising the negative electrode active material. The negative electrode active material provided by the present application includes a first material and a second material, wherein the d002 interplanar spacing of the first material measured in the X-ray diffraction pattern is d1, and the d002 interplanar spacing of the second material measured in the X-ray diffraction pattern is d2, and the d1 and d2 satisfy: (d2 / d1-1)≤0.01; the first material includes at least one of natural graphite, artificial graphite, hard carbon and soft carbon. The negative electrode active material of the present application can effectively improve the specific capacity of the negative electrode and the diffusion efficiency of lithium ions by reducing graphite oxide, thereby improving the energy density and rate performance of the electrochemical device. The present application also provides a method for preparing the negative electrode active material.

Description

Technical Field

[0001] The present application relates to the field of lithium-ion batteries, and in particular to a negative electrode active material, a preparation method thereof, and an electrochemical device comprising the negative electrode active material. Background Art

[0002] Electrochemical devices such as lithium-ion batteries, a new type of high-energy, green battery, are widely used in laptops, mobile phones, and new energy electric vehicles. The increasing energy density requirements in these applications have led to a bottleneck in increasing the capacity of anode materials such as graphite. Furthermore, while other anode materials, such as silicon-carbon and silicon-oxygen materials, offer higher specific capacities, these materials face challenges such as cycle life and expansion. Therefore, improving the capacity and performance of carbon materials, particularly graphite, is the most direct and effective approach.

[0003] In lithium-ion batteries, graphene can be used directly as a negative electrode material. Its theoretical specific capacity is higher than that of graphite, and its large interlamellar spacing allows for smooth diffusion of lithium ions within the graphene sheets, which improves the battery's rate performance. However, graphene itself has a high oxygen content and micropores, which hinder lithium ion intercalation and deintercalation, resulting in undesirable first coulombic efficiency and cycle performance, limiting its large-scale application. Therefore, there is a need in the art for a negative electrode active material that improves capacity while minimizing the negative impact on first coulombic efficiency and cycle performance. Summary of the Invention

[0004] In order to solve the above problems, the purpose of the present application is to provide a negative electrode active material, a preparation method thereof, and an electrochemical device comprising the negative electrode active material.

[0005] In a first aspect, the present application provides a negative electrode active material, which includes a first material and a second material, wherein the d002 crystal plane spacing of the first material measured in the X-ray diffraction pattern is d1, and the d002 crystal plane spacing of the second material measured in the X-ray diffraction pattern is d2, and the d1 and d2 satisfy: (d2 / d1-1)≤0.01; wherein the first material includes at least one of natural graphite, artificial graphite, hard carbon and soft carbon.

[0006] With the aid of the negative electrode active material according to the present application, the specific capacity of the negative electrode and the diffusion efficiency of lithium ions can be effectively improved by reducing graphite oxide, thereby improving the energy density and rate performance of the electrochemical device. In particular, the negative electrode active material according to the present application greatly utilizes the advantages of the high specific capacity of reduced graphite oxide, thereby reducing the negative impact on the first coulombic efficiency and cycle performance while improving the capacity. In some embodiments, 0.001≤(d2 / d1-1)≤0.008, which can enable the electrochemical device to have better rate performance and cycle performance.

[0007] According to some embodiments of the present application, the range of the d002 interplanar spacing d1 of the first material measured in the X-ray diffraction pattern is: to In some embodiments, the range of the d002 interplanar spacing d2 of the second material measured in the X-ray diffraction pattern is: to In some embodiments, d2 is to The second material retains the carbon layer structure of graphite, and its slightly enlarged interplanar spacing is conducive to the deintercalation and extraction of lithium ions. The specific capacity of the material is increased, further improving the rate performance of the electrochemical device.

[0008] According to some embodiments of the present application, the second material includes reduced graphite oxide.

[0009] According to some embodiments of the present application, the reduced graphite oxide comprises 1% to 10% by mass of the negative electrode active material. In some embodiments, the reduced graphite oxide comprises 2% to 5% by mass. When the reduced graphite oxide comprises within these ranges, the lithium ion diffusion efficiency and the gram capacity of the material can be more effectively improved, thereby enhancing the energy density, rate capability, and cycle performance of the electrochemical device.

[0010] According to some embodiments of the present application, the second material and the first material are coated together by a carbon layer. The carbon layer can bond the first and second materials together, while reducing the specific surface area and defect content of the negative electrode active material, thereby preventing a reduction in the initial charge and discharge efficiency of the electrochemical device.

[0011] According to some embodiments of the present application, the structure of the second material is a lamellar corrugated structure, which is more conducive to the diffusion of lithium ions and improves the rate performance of the electrochemical device.

[0012] According to some embodiments of the present application, the calculation formula for the difference between the d002 interplanar spacing of the second material measured in the X-ray diffraction pattern and the d002 interplanar spacing of the first material measured in the X-ray diffraction pattern is:

[0013] Difference = (d002 第二材料 / d002 第一材料 -1)×100%=(d2 / d1-1)×100%.

[0014] According to some embodiments of the present application, the second material contains boron, wherein the mass content of the boron is in the range of 0.1 mass % to 1 mass % based on the mass of the negative electrode active material. In some embodiments, the mass content of the boron is in the range of 0.2 mass % to 0.6 mass %. After boron doping, the interplanar spacing of the carbon layer is slightly enlarged, the lithium insertion sites on the carbon layer are increased, and the activation energy required for lithium ion diffusion conduction is reduced, which is beneficial to the deintercalation of lithium ions, and can increase the gram capacity of the material and enhance the rate performance of the electrochemical device.

[0015] According to some embodiments of the present application, the second material contains oxygen, wherein the mass content of oxygen is in the range of 1 mass % to 5 mass % based on the mass of the negative electrode active material. In some embodiments, the mass content of oxygen is in the range of 2 mass % to 4 mass %. An increase in the oxygen content will increase the surface defects and specific surface area of ​​the negative electrode active material. Excessive oxygen content will cause adsorption and intercalation of lithium, which is not conducive to the reversible intercalation and deintercalation of lithium ions. When the oxygen content is controlled within the above range, the negative electrode sheet compaction density and lithium ion diffusion coefficient can be increased, thereby improving the rate performance of the electrochemical device.

[0016] According to some embodiments of the present application, the X-ray diffraction pattern of the second material has a characteristic peak within the 2θ range of 26° to 27°. In some embodiments, the half-value width of the characteristic peak is 1° to 2°. The presence of the characteristic peak in the X-ray diffraction pattern of the second material indicates that the second material has a graphite-like carbon layer structure, which facilitates the intercalation and deintercalation of lithium ions and improves the rate performance of the electrochemical device.

[0017] According to some embodiments of the present application, Raman testing is performed on particles of the negative electrode active material within a range of 100 μm×100 μm, and the test results satisfy the following conditions: 0.5≤Id / Ig≤1.5, where Id is the particle size of the negative electrode active material at 1350 cm -1 Ig is the peak intensity of the negative electrode active material particles at 1580cm -1 By using the negative electrode active material according to the present application, the defect content of the negative electrode active material is set within a suitable range, which not only facilitates the entry of lithium ions into the graphite layer through the defects, but also controls the thickness of the SEI film generated by the reaction between the defects of the negative electrode active material and the electrolyte within a certain range, thereby controlling the first coulombic efficiency within a suitable range and improving the rate performance and cycle performance of the electrochemical device.

[0018] According to some embodiments of the present application, the specific surface area of ​​the negative electrode active material is 1m 2 / g to 3m 2 / g range.

[0019] According to some embodiments of the present application, the particle size D of the negative electrode active material is 50 Satisfy: 10μm≤D 50 ≤20μm.

[0020] According to some embodiments of the present application, the gram capacity of the negative electrode active material is greater than or equal to 360 mAh / g.

[0021] According to some embodiments of the present application, the powder compaction density of the negative electrode active material is greater than or equal to 1.80 g / cm 3 and less than or equal to 2.00g / cm 3 .

[0022] In a second aspect, the present application provides a method for preparing a negative electrode active material, the method comprising the following steps:

[0023] S1, mixing natural flake graphite and an oxidant in a mass ratio of 2:1 to 1:1, and reacting at 20 to 30° C. to obtain graphite oxide;

[0024] S2, mixing the graphite oxide obtained in step S1 with a boron-containing compound in a mass ratio of 1:5 to 1:10, heating the mixture to 800° C. to 1200° C. at 2° C. / min to 5° C. / min in an inert gas and keeping the mixture at that temperature for 2 to 4 hours, then cooling the mixture to room temperature at 5° C. / min to 10° C. / min, and washing the mixture with water at 80° C. to 95° C., such as distilled water, to obtain a second material;

[0025] S3, mixing the first material and the second material obtained in step S2 at a mass ratio of 100:1 to 100:10, and then coating and carbonizing at 800°C to 1200°C to obtain a negative electrode active material. By means of the method according to the present application, the graphite oxide obtained in step S1 is a particle that has not been completely exfoliated into a single-layer graphene structure. The graphite oxide is then reduced through the catalytic action of a boron-containing compound to reduce the oxygen content and defects, while also improving the diffusion rate of lithium ions. The structure of the reduced graphite oxide particles is closer to that of graphite itself, thereby increasing the specific capacity without affecting the graphite potential, bringing positive technical effects in terms of electrochemical device kinetics.

[0026] In some embodiments, the method for preparing the negative electrode active material comprises the following steps:

[0027] Step A: reacting graphite with an oxidant to obtain graphite oxide;

[0028] Step B: In an inert gas, the graphite oxide prepared in step A is mixed with a boron-containing compound and subjected to a reduction reaction to obtain a second material;

[0029] Step C: After mixing the first material and the second material of step B, coating and carbonizing are performed to obtain a negative electrode active material.

[0030] In some embodiments, the oxidant in step S1 or step A comprises at least one of concentrated nitric acid, concentrated sulfuric acid, concentrated phosphoric acid, potassium chromate, potassium perchlorate, potassium permanganate, and persulfate. In some embodiments, the oxidant in step S1 or step A is concentrated sulfuric acid, potassium permanganate, or a combination thereof.

[0031] According to some embodiments of the present application, the graphite oxide obtained in step S1 or step A is graphite oxide particles with weak oxidative intercalation. In some embodiments, the oxidative intercalation comprises at least one oxidant intercalation selected from concentrated nitric acid, concentrated sulfuric acid, concentrated phosphoric acid, potassium chromate, potassium perchlorate, potassium permanganate, and persulfate. According to some preferred embodiments of the present application, the oxidative intercalation is an oxidant intercalation of concentrated sulfuric acid or potassium permanganate, or a combination thereof.

[0032] According to some embodiments of the present application, in step S1, the reaction time is 0.5 h to 6 h.

[0033] In some embodiments, in step A, the reaction temperature is 20° C. to 30° C. In some embodiments, in step A, the reaction time is 0.5 h to 6 h. In some embodiments, in step A, the mass ratio of the graphite to the oxidant is 2:1 to 1:1. In some embodiments, the graphite comprises natural flake graphite.

[0034] According to some embodiments of the present application, in step S2 or step B, the inert gas includes at least one of argon, nitrogen, helium and an argon-hydrogen mixture.

[0035] In some embodiments, in step B, the mass ratio of the graphite oxide to the boron-containing compound is 1:5 to 1:10. In some embodiments, in step B, the reduction reaction temperature is 800°C to 1200°C. In some embodiments, the reduction reaction temperature is achieved by temperature programming, for example, heating to 800°C to 1200°C at a rate of 2°C / min to 5°C / min. In some embodiments, in step B, the reduction reaction time is 2 hours to 4 hours. In some embodiments, after the reduction reaction is completed, the temperature is cooled to room temperature at a rate of 5°C / min to 10°C / min.

[0036] According to some embodiments of the present application, in step (ii) or step B, the boron-containing compound includes at least one of boric acid, boron oxide, tetraphenylboric acid, sodium tetraphenylborate, sodium borate, sodium metaborate, calcium borate, and sodium tetraborate. Preferably, the boron-containing compound is boric acid.

[0037] According to some embodiments of the present application, a reduction reaction occurs in step (ii), and in particular, the graphite oxide particles obtained from step (i) are reduced to a second material, particularly, the second material is reduced graphite oxide particles.

[0038] By means of the method according to the present application, graphite oxide is reduced by in-situ catalytic reduction, so that the reduced graphite oxide material is closer to the structure of the graphite body, thereby improving the capacity while reducing the negative impact on the first coulombic efficiency and cycle performance.

[0039] According to some embodiments of the present application, in step (iii), asphalt is used for coating carbonization, wherein the mass content of the asphalt is in the range of 1 mass % to 10 mass % based on the total mass of the negative electrode active material.

[0040] According to some embodiments of the present application, in step C, the coating carbonization is performed at a temperature of 800° C. to 1200° C. In some embodiments, in step C, the mass ratio of the first material to the second material is 100:1 to 100:10.

[0041] In the present application, "graphite oxide" means oxidized graphite, and "reduced graphite oxide" means reduced graphite oxide.

[0042] In a third aspect of the present application, the present application provides an electrochemical device, which includes a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode includes a negative electrode collector and a negative electrode active material layer arranged on the negative electrode collector, and the negative electrode active material layer includes the negative electrode active material according to the present application.

[0043] According to some embodiments of the present application, in the electrochemical device according to the present application, the compaction density of the negative electrode is greater than or equal to 1.50 g / cm 3 and less than or equal to 1.70g / cm 3 The compaction density of the negative electrode within this range can further reduce the transmission path of lithium ions, increase the insertion and extraction rate of lithium ions, and improve the rate performance of the electrochemical device.

[0044] The negative electrode active material according to the present application has a high gram capacity, and the electrochemical device prepared according to the negative electrode active material of the present application has a high lithium ion diffusion coefficient, capacity retention rate and energy density, as well as improved rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 Schematically shows the XRD pattern of graphite oxide according to the present application;

[0046] Figure 2 The XRD pattern of the reduced graphite oxide according to the present application is schematically shown. DETAILED DESCRIPTION

[0047] 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.

[0048] In addition, amounts, ratios, and other numerical values ​​are sometimes presented herein in a range format. It should be understood that such a range format is used for convenience and brevity and should be interpreted flexibly to include not only the values ​​explicitly specified as limits of the range, but also all individual values ​​or sub-ranges encompassed within the range, as if each value and sub-range were explicitly specified.

[0049] In the description herein, unless otherwise specified, “above” and “below” include the number itself.

[0050] 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 contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0051] 1. Negative electrode active materials

[0052] The negative electrode active material provided by the present application includes a first material and a second material, wherein the d002 interplanar spacing of the first material measured in the X-ray diffraction pattern is d1, and the d002 interplanar spacing of the second material measured in the X-ray diffraction pattern is d2, and the d1 and d2 satisfy: (d2 / d1-1)≤0.01; wherein the first material includes at least one of natural graphite, artificial graphite, hard carbon and soft carbon. With the aid of the negative electrode active material according to the present application, the specific capacity of the negative electrode and the diffusion efficiency of lithium ions can be effectively improved by reducing graphite oxide, thereby improving the energy density and rate performance of the electrochemical device. In particular, the negative electrode active material according to the present application maximizes the advantage of the high specific capacity of reduced graphite oxide, so that while improving the capacity, the negative impact on the first coulombic efficiency and cycle performance is reduced. In some embodiments, d2 / d1-1 is 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, or a range consisting of any two of these values. In some embodiments, 0.001≤(d2 / d1-1)≤0.008.

[0053] In some embodiments, the range of the d002 interplanar spacing d1 of the first material measured in the X-ray diffraction pattern is: to In some embodiments, d1 is or a range consisting of any two of these values.

[0054] In some embodiments, the range of the d002 interplanar spacing d2 of the second material measured in the X-ray diffraction pattern is: to In some embodiments, d2 is Or a range consisting of any two of these values. In some embodiments, the range of d2 is to The second material retains the carbon layer structure of graphite, and its slightly enlarged interplanar spacing is conducive to the deintercalation and extraction of lithium ions. The specific capacity of the material is increased, further improving the rate performance of the electrochemical device.

[0055] In some embodiments, the second material includes reduced graphite oxide. In some embodiments, based on the mass of the negative electrode active material, the mass proportion of the reduced graphite oxide is 1% to 10%, and in some embodiments, the mass proportion of the reduced graphite oxide is 2% to 5%. When the mass proportion of reduced graphite oxide is within the above range, the diffusion efficiency of lithium ions and the gram capacity of the material can be more effectively improved, thereby improving the energy density, rate and cycle performance of the electrochemical device. In some embodiments, based on the mass of the negative electrode active material, the mass proportion of the reduced graphite oxide is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range consisting of any two of these values.

[0056] In some embodiments, the second material contains boron element, wherein the mass content of boron element is in the range of 0.1 mass % to 1 mass % based on the mass of the negative electrode active material. In some embodiments, the mass content of boron element is in the range of 0.2 mass % to 0.6 mass %. After boron doping, the interplanar spacing of the carbon layer is slightly enlarged, the lithium insertion positions on the carbon layer are increased, the activation energy required for lithium ion diffusion conduction is reduced, which is beneficial to the deintercalation of lithium ions, and can increase the gram capacity of the material and enhance the rate performance of the electrochemical device. In some embodiments, based on the mass of the negative electrode active material, the mass content of boron element is 0.2 mass %, 0.3 mass %, 0.4 mass %, 0.5 mass %, 0.6 mass % or a range consisting of any two of these values.

[0057] In some embodiments, the second material contains oxygen, wherein the mass content of oxygen is in the range of 1% to 5% by mass based on the mass of the negative electrode active material. In some embodiments, the mass content of oxygen is in the range of 2% to 4% by mass. An increase in the oxygen content will increase the surface defects and specific surface area of ​​the negative electrode active material. Excessive oxygen content will cause adsorption and intercalation of lithium, which is not conducive to the reversible intercalation and deintercalation of lithium ions. When the oxygen content is controlled within the above range, the negative electrode sheet compaction density and lithium ion diffusion coefficient can be increased, thereby improving the rate performance of the electrochemical device. In some embodiments, based on the mass of the negative electrode active material, the mass content of oxygen element is 2.0 mass%, 2.1 mass%, 2.2 mass%, 2.3 mass%, 2.4 mass%, 2.5 mass%, 2.6 mass%, 2.7 mass%, 2.8 mass%, 2.9 mass%, 3.0 mass%, 3.1 mass%, 3.2 mass%, 3.3 mass%, 3.4 mass%, 3.5 mass%, 3.6 mass%, 3.7 mass%, 3.8 mass%, 3.9 mass%, 4.0 mass% or a range consisting of any two of these values.

[0058] In some embodiments, the X-ray diffraction pattern of the second material has a characteristic peak within a 2θ range of 26° to 27°. In some embodiments, the half-value width of the characteristic peak is 1° to 2°. The presence of the characteristic peak in the X-ray diffraction pattern of the second material indicates that the second material has a graphite-like carbon layer structure, which facilitates lithium ion intercalation and deintercalation, thereby improving the rate performance of the electrochemical device.

[0059] In some embodiments, Raman testing is performed on particles of the negative electrode active material within a range of 100 μm×100 μm, and the test result satisfies the following conditions: 0.5≤Id / Ig≤1.5, where Id is the particle size of the negative electrode active material at 1350 cm -1 Ig is the peak intensity of the negative electrode active material particles at 1580cm -1 The peak intensity at . The Id / Ig ratio can characterize the crystal defect degree of the negative electrode active material. The larger the value, the higher the defect degree. A high defect degree can increase the deintercalation channel of the active ions and increase the deintercalation speed of the active ions, thereby improving the kinetic performance of the negative electrode active material. However, too many defects will lead to a decrease in the first coulombic efficiency, cycle, storage and other performance of the electrochemical device. When the Id / Ig ratio is within the above range, it is beneficial for lithium ions to enter the graphite layer through the defects, and the thickness of the SEI film generated by the reaction of the defects of the negative electrode active material with the electrolyte can be controlled within a certain range, thereby controlling the first coulombic efficiency within a suitable range and improving the rate performance and cycle performance of the electrochemical device. In some embodiments, Id / Ig is 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5 or a range consisting of any two of these values.

[0060] In some embodiments, the particle size D of the negative electrode active material is 50 Satisfy: 10μm≤D 50 ≤20 μm. In some embodiments, D 50 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm or a range consisting of any two of these values. 50 The particle size distribution of the negative electrode material on a volume basis, from the small particle size side, reaches 50% of the volume accumulation is the D of the negative electrode material. 50 .

[0061] In some embodiments, the gram capacity of the negative electrode active material is greater than or equal to 360 mAh / g. In some embodiments, the gram capacity of the negative electrode active material is 360 mAh / g, 365 mAh / g, 370 mAh / g, 375 mAh / g, 380 mAh / g, 385 mAh / g, 390 mAh / g, 395 mAh / g, 400 mAh / g, or a range consisting of any two of these values.

[0062] In some embodiments, the powder compaction density of the negative electrode active material is greater than or equal to 1.80 g / cm 3 and less than or equal to 2.00g / cm 3 In some embodiments, the powder compaction density of the negative electrode active material is 1.80 g / cm 3 、1.81g / cm 3 , 1.82g / cm 3 , 1.83g / cm 3 , 1.84g / cm 3 , 1.85g / cm 3 , 1.86g / cm 3 , 1.87g / cm 3 , 1.88g / cm 3 , 1.89g / cm 3 、1.90g / cm 3 、1.91g / cm 3 、1.92g / cm 3 、1.93g / cm 3 , 1.94g / cm 3 , 1.95g / cm 3 , 1.96g / cm 3 , 1.97g / cm 3 、1.98g / cm 3 , 1.99g / cm 3 , 2.0g / cm 3 or a range consisting of any two of these values.

[0063] In some embodiments, the method for preparing a negative electrode active material comprises the following steps:

[0064] (i) mixing natural flake graphite and an oxidant in a mass ratio of 2:1 to 1:1 and reacting at 20 to 30° C. to obtain graphite oxide;

[0065] (ii) mixing the graphite oxide obtained in step (i) with a boron-containing compound in a mass ratio of 1:5 to 1:10, heating the mixture to 800° C. to 1200° C. at a rate of 2° C. / min to 5° C. / min in an inert gas and maintaining the temperature for 2 to 4 hours, then cooling the mixture to room temperature at a rate of 5° C. / min to 10° C. / min, and washing the mixture with water, such as distilled water, at a temperature of 80° C. to 95° C. to obtain a second material;

[0066] (iii) After mixing the first material and the second material obtained in step (ii) at a mass ratio of 100:1 to 100:10, coating and carbonizing the mixture at 800° C. to 1200° C. to obtain a negative electrode active material.

[0067] In some embodiments, the method for preparing the negative electrode active material comprises the following steps:

[0068] Step A: reacting graphite with an oxidant to obtain graphite oxide;

[0069] Step B: In an inert gas, the graphite oxide prepared in step A is mixed with a boron-containing compound and subjected to a reduction reaction to obtain a second material;

[0070] Step C: After mixing the first material and the second material of step B, coating and carbonizing are performed to obtain a negative electrode active material.

[0071] In some embodiments, in step S1, the mass ratio of natural flake graphite to oxidant is 2:1, 1.9:1, 1.8:1, 1.7:1, 1.6:1, 1.5:1, 1.4:1, 1.3:1, 1.2:1, 1.1:1, 1.0:1, or a range consisting of any two of these values. In some embodiments, in step S1, the reaction time is 1 hour, 2 hours, 3 hours, 4 hours, or 5 hours.

[0072] In some embodiments, in step A, the mass ratio of graphite to oxidant is 2:1, 1.9:1, 1.8:1, 1.7:1, 1.6:1, 1.5:1, 1.4:1, 1.3:1, 1.2:1, 1.1:1, 1.0:1, or a range consisting of any two of these values. In some embodiments, the graphite comprises natural flake graphite.

[0073] In some embodiments, in step A, the reaction temperature is 20° C. to 30° C., for example, 22° C., 24° C., 26° C., or 28° C. In some embodiments, in step A, the reaction time is 0.5 h to 6 h, for example, 1 h, 2 h, 3 h, 4 h, or 5 h.

[0074] In some embodiments, in step (ii) or step B, the mass ratio of graphite oxide to the boron-containing compound is 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10.

[0075] In some embodiments, in step (ii), heating is performed in an inert gas at a heating rate of 2° C. / min, 3° C. / min, 4° C. / min, or 5° C. / min.

[0076] In some embodiments, in step (ii), heating is performed in an inert gas atmosphere to 800°C, 900°C, 1000°C, 1100°C, or 1200°C.

[0077] In some embodiments, in step (ii), the temperature is lowered in an inert gas at a rate of 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, or 10°C / min.

[0078] In some embodiments, in step B, the temperature of the reduction reaction is 800°C to 1200°C, for example, 800°C, 900°C, 1000°C, 1100°C or 1200°C. In some embodiments, the temperature of the reduction reaction is achieved by temperature programming, for example, heating to 800°C to 1200°C at 2°C / min to 5°C / min (for example, 2°C / min, 3°C / min, 4°C / min or 5°C / min). In some embodiments, in step B, the reduction reaction time is 2h to 4h, for example, 2.5h, 3h or 3.5h. In some embodiments, after the reduction reaction is completed, the temperature is cooled to room temperature at 5°C / min to 10°C / min (for example, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min or 10°C / min).

[0079] In some embodiments, in step (iii) or step C, the first material and the second material are in a mass ratio of 100:1, 100:2, 100:3, 100:4, 100:5, 100:6, 100:7, 100:8, 100:9 or 100:10.

[0080] In some embodiments, in step (iii) or step C, the coating carbonization is performed at 800°C, 900°C, 1000°C, 1100°C, or 1200°C. According to some embodiments of the present application, the second material and the first material are coated together by a carbon layer. The carbon layer can bond the first and second materials together, while reducing the specific surface area and defect content of the negative electrode active material, thereby preventing a decrease in the initial charge and discharge efficiency of the electrochemical device.

[0081] In some embodiments, the graphite oxide obtained in step S1 is a particle that has not been completely exfoliated into a single-layer graphene structure. The graphite oxide is then reduced and the oxygen content and defects are reduced through the catalytic action of a boron-containing compound, while at the same time improving the diffusion rate of lithium ions. The structure of the reduced graphite oxide particles is closer to that of the graphite body, thereby being able to increase the specific capacity without affecting the graphite potential, bringing positive technical effects in terms of the kinetics of the electrochemical device.

[0082] In some embodiments, the graphite oxide obtained in step (i) or step A is graphite oxide particles with weak oxidative intercalation. In some embodiments, the oxidative intercalation comprises at least one oxidant intercalation selected from concentrated nitric acid, concentrated sulfuric acid, concentrated phosphoric acid, potassium chromate, potassium perchlorate, potassium permanganate, and persulfate. According to some preferred embodiments of the present application, the oxidative intercalation is an oxidant intercalation of concentrated sulfuric acid or potassium permanganate, or a combination thereof.

[0083] In some embodiments, in step S1, the reaction of natural flake graphite and the oxidant is carried out at room temperature, and the reaction time is 0.5 h to 6 h, for example, 0.5 h, 1 h, 1.5 h, 2.0 h, 2.5 h, 3.0 h, 3.5 h, 4.0 h, 4.5 h, 5.0 h, 5.5 h, 6.0 h.

[0084] In some embodiments, the oxidant in step S1 or step A is, for example, concentrated sulfuric acid, potassium permanganate, or a combination thereof.

[0085] In some embodiments, in step S2 or step B, the inert gas includes at least one of argon, nitrogen, helium, and an argon-hydrogen mixture.

[0086] In some embodiments, in step S2, the graphite oxide particles obtained from step S1 and the boron-containing compound are heated to a temperature in the range of 800°C to 1200°C, and in particular, the temperature is maintained for 2 hours to 4 hours after heating, for example, 2.0 hours, 2.5 hours, 3.0 hours, 3.5 hours, or 4.0 hours.

[0087] In some embodiments, in step S2 or step B, the boron-containing compound includes at least one of boric acid, boron oxide, tetraphenylboric acid, sodium tetraphenylborate, sodium borate, sodium metaborate, calcium borate, and sodium tetraborate. Preferably, the boron-containing compound is boric acid.

[0088] In some embodiments, a reduction reaction occurs in step S2 or step B, particularly reducing the graphite oxide particles obtained in step S1 or step A to a second material. In some embodiments, the second material is reduced graphite oxide particles. By in-situ catalytic reduction of graphite oxide, the reduced graphite oxide material more closely resembles the structure of bulk graphite, thereby improving capacity while minimizing negative impacts on initial coulombic efficiency and cycling performance.

[0089] In some embodiments, in step S2 or step C, asphalt is used for coating carbonization, wherein the mass content of the asphalt is in the range of 1 mass % to 10 mass %, for example, 1 mass %, 2 mass %, 3 mass %, 4 mass %, 5 mass %, 6 mass %, 7 mass %, 8 mass %, 9 mass %, 10 mass %, based on the total mass of the negative electrode active material.

[0090] 2. Electrochemical Device

[0091] The electrochemical device of the present application includes a negative electrode, which includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes the negative electrode active material of the first aspect.

[0092] In some embodiments, the specific surface area of ​​the negative electrode active material layer is 1 m 2 / g to 3m 2 In some embodiments, the specific surface area of ​​the negative electrode active material layer is 1.0 m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.3m 2 / g, 1.4m 2 / g, 1.5m 2 / g, 1.6m 2 / g, 1.7m 2 / g, 1.8m 2 / g, 1.9m 2 / g, 2.0m 2 / g, 2.1m 2 / g, 2.2m 2 / g, 2.3m 2 / g, 2.4m 2 / g, 2.5m 2 / g, 2.6m 2 / g, 2.7m 2 / g, 2.8m 2 / g, 2.9m 2 / g、3m 2In the present application, the specific surface area of ​​the negative electrode active material layer is the specific surface area measured by disassembling the negative electrode after the electrochemical device is heated to 3V and scraping off the powder on the negative electrode sheet.

[0093] In some embodiments, the compaction density of the negative electrode is greater than or equal to 1.50 g / cm 3 and less than or equal to 1.70g / cm 3 In some embodiments, the negative electrode has a compacted density of 1.50 g / cm 3 , 1.51g / cm 3 , 1.52g / cm 3 , 1.53g / cm 3 , 1.54g / cm 3 , 1.55g / cm 3 , 1.56g / cm 3 、1.57g / cm 3 、1.58g / cm 3 , 1.59g / cm 3 , 1.60g / cm 3 , 1.61g / cm 3 , 1.62g / cm 3 , 1.63g / cm 3 , 1.64g / cm 3 , 1.65g / cm 3 , 1.66g / cm 3 , 1.67g / cm 3 , 1.68g / cm 3 , 1.69g / cm 3 , 1.70g / cm 3 or a range consisting of any two of these values.

[0094] In some embodiments, the negative electrode active material layer further comprises a binder. In some embodiments, the binder includes, but is not limited to, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0095] In some embodiments, the negative electrode active material layer includes a conductive material. In some embodiments, the conductive material includes, but is not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, metal powder, metal fiber, copper, nickel, aluminum, silver, or polyphenylene derivatives.

[0096] In some embodiments, the negative electrode current collector includes, but is not limited to, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a polymer substrate coated with a conductive metal.

[0097] The electrochemical device of the present application further includes a positive electrode. The materials, compositions and manufacturing methods of the positive electrode that can be used in the embodiments of the present application include any technology disclosed in the prior art.

[0098] In some embodiments, the positive electrode includes a current collector and a positive electrode active material layer on the current collector.

[0099] In some embodiments, the positive electrode active material includes, but is not limited to, lithium cobalt oxide (LiCoO2), lithium nickel cobalt manganese (NCM) ternary material, lithium iron phosphate (LiFePO4), or lithium manganese oxide (LiMn2O4).

[0100] In some embodiments, the positive electrode active material layer further includes a binder, and optionally a conductive material. The binder improves the bonding between the positive electrode active material particles and also improves the bonding between the positive electrode active material and the current collector.

[0101] In some embodiments, the binder includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0102] In some embodiments, the conductive material 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.

[0103] In some embodiments, the current collector may include, but is not limited to, aluminum.

[0104] The positive electrode can be prepared using methods known in the art. For example, the positive electrode can be obtained by mixing an active material, a conductive material, and a binder in a solvent to prepare an active material composition, and then coating the active material composition on a current collector. In some embodiments, the solvent may include, but is not limited to, N-methylpyrrolidone.

[0105] The electrochemical device of the present application also includes an electrolyte. The electrolyte that can be used in the embodiments of the present application can be an electrolyte known in the prior art.

[0106] In some embodiments, the electrolyte includes an organic solvent, a lithium salt, and an additive. The organic solvent of the electrolyte according to the present application may be any organic solvent known in the prior art that can be used as a solvent for the electrolyte. The electrolyte used in the electrolyte according to the present application is not limited and can be any electrolyte known in the prior art. The additive of the electrolyte according to the present application may be any additive known in the prior art that can be used as an electrolyte additive.

[0107] In some embodiments, the organic solvent includes, but is not limited to, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate.

[0108] In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt.

[0109] In some embodiments, the lithium salt includes, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalatoborate) LiB(C2O4)2 (LiBOB), or lithium difluorooxalatoborate LiBF2(C2O4) (LiDFOB).

[0110] In some embodiments, the concentration of the lithium salt in the electrolyte is 0.5-3 mol / L, 0.5-2 mol / L, or 0.8-1.5 mol / L.

[0111] The electrochemical device of the present application includes a separator between the positive electrode and the negative electrode to prevent short circuits. The material and shape of the separator used in the embodiments of the present application are not particularly limited and may be any known prior art. In some embodiments, the separator comprises a polymer or inorganic material, for example, formed from a material that is stable to the electrolyte of the present application.

[0112] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, film, or 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, polypropylene porous film, polyethylene porous film, polypropylene non-woven fabric, polyethylene non-woven fabric, or polypropylene-polyethylene-polypropylene porous composite film can be used.

[0113] A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic layer.

[0114] The inorganic layer includes inorganic particles and a binder, wherein the inorganic particles are selected from one or a combination 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 one or a combination of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0115] 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, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0116] In some embodiments, the electrochemical device of the present application includes, but is not limited to: a primary battery or a secondary battery.

[0117] In some embodiments, the electrochemical device is a lithium secondary battery.

[0118] In some embodiments, the lithium secondary battery includes, but is not limited to, a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0119] 3. Electronic Devices

[0120] The electronic device of the present application may be any device using the electrochemical device according to the embodiment of the present application.

[0121] In some embodiments, the electronic device includes, 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 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 or a lithium-ion capacitor, etc.

[0122] 4. Test Method

[0123] 1. X-ray diffraction (XRD)

[0124] The negative electrode active material was tested using an X-ray powder diffractometer (Bruker D8 ADVANCE) with a Cu Kα target; the voltage and current were 40 kV / 40 mA, the scanning angle range was 5° to 80°, the scanning step was 0.00836°, and the time per step was 0.3 s.

[0125] 2. 002 crystal plane spacing test of negative electrode active material

[0126] High-purity silicon powder (purity ≥99.99%) was used as an internal standard for calibration. The materials were mixed at a weight ratio of 5:1 for the anode active material and ground uniformly, then pressed into tablets. The interplanar spacing (d002) of the anode active material was measured using an X-ray diffractometer. The difference between the d002 interplanar spacing of the second material and the d002 interplanar spacing of the first material was calculated using the following formula:

[0127] Difference = (d002 第二材料 / d002 第一材料 -1)×100%, wherein the first material may be artificial graphite, and the d002 of the artificial graphite is

[0128] 3. Element content test in negative electrode active materials

[0129] The boron and oxygen content can be obtained by testing the negative electrode active material using the German Elementar element analyzer.

[0130] 4. Surface defectivity of negative electrode active material (Id / Ig)

[0131] The surface defects of the negative electrode active material were tested by laser microconfocal Raman spectroscopy. -1 The peak intensity Id at 1580 cm -1 The ratio of the peak intensity Id at each potential, Id / Ig, represents the surface defectivity of the sample. Each sample was tested at multiple potentials, and the standard deviation of these values ​​indicated the uniformity of surface defectivity across different regions. The average and standard deviation of the Id / Ig values ​​for the negative electrode active material were determined by measuring 100 points of the unscreened negative electrode active material, obtaining the corresponding Id / Ig values, and calculating the average and standard deviation of these 100 values.

[0132] 5. Specific surface area of ​​negative electrode active material

[0133] The specific surface area of ​​the negative electrode active material was measured by nitrogen adsorption / desorption method using a specific surface area analyzer (Tristar II 3020M), wherein the negative electrode active material sample was dried in a vacuum drying oven and then placed in a sample tube for measurement in the analyzer.

[0134] 6. Powder compaction density of negative electrode active material

[0135] The powder compaction density test standard refers to GB / T 24533-2009, "Graphite-Based Anode Materials for Lithium-Ion Batteries." The specific test method involves weighing 1.0000±0.0500g of sample and placing it in a test mold (CARVER#3619 (13mm)). The sample is then placed in the Sansi Zongheng UTM7305 test equipment, with a test capacity of 0.3t to 5.0t. The powder compaction density reported herein is measured at 5t. The formula for calculating powder compaction density is: Powder compaction density = Mass of negative electrode active material / Force-bearing area of ​​negative electrode active material / Sample thickness.

[0136] 7. Gram capacity of negative electrode active material

[0137] A negative button cell was prepared using the negative active material, with a lithium sheet used as the positive electrode. The negative button cell was discharged at 0.05C to 5.0 mV, then at 50 μA to 5.0 mV, and then at 10 μA to 5.0 mV. The cell was then charged at 0.1C to 2.0 V. The capacity of the cell at this point was recorded (gram capacity).

[0138] 8. Compaction density of negative electrode

[0139] Use an electronic balance to weigh a negative electrode sheet of a certain area S (wherein a negative electrode active material layer is arranged on both sides of the negative electrode current collector), and record the weight as W1. Use a ten-thousandth ruler to measure the thickness T1 of the negative electrode sheet. Use a solvent to wash off the negative electrode active material layer, dry it, measure the weight of the negative electrode current collector, record it as W2, and use a ten-thousandth ruler to measure the thickness T2 of the negative electrode current collector. Calculate the weight W0 and thickness T0 of the negative electrode active material layer provided on one side of the negative electrode current collector and the compaction density of the negative electrode active material layer by the following formula:

[0140] W0 = (W1 - W2) / 2; T0 = (T1 - T2) / 2; compacted density = W0 / (T0×S).

[0141] 9. Lithium-ion battery energy density

[0142] At a temperature of 25°C, the lithium-ion battery was charged and discharged for the first time, and then charged at a constant current and constant voltage at a charging current of 0.5C until the upper limit voltage reached 4.48V, and then discharged at a constant current at a discharge current of 0.2C, with a discharge cut-off voltage of 3V. Five lithium-ion batteries were provided for each embodiment and comparative example for energy density testing, and the average value was taken.

[0143] 10. Lithium-ion battery cycle retention rate

[0144] Five lithium-ion batteries prepared in all comparative examples and examples were taken. The lithium-ion batteries were repeatedly charged and discharged using the following steps, and the cycle capacity retention rates of the lithium-ion batteries were calculated and averaged.

[0145] First, in an environment of 25°C, the lithium-ion battery was charged and discharged for the first time. Constant current charging was performed at a charging current of 1C until the upper limit voltage of 4.48V was reached, and then constant voltage charging was switched. Then, constant current discharge was performed at a discharge current of 1C until the final voltage was 3V, and the discharge capacity of the first cycle was recorded. Then, 400 charge and discharge cycles were performed, and the discharge capacity of the 400th cycle was recorded.

[0146] Cycle capacity retention rate = (discharge capacity at the 400th cycle / discharge capacity at the first cycle) × 100%;

[0147] 11. Lithium-ion battery charge rate performance

[0148] Place the lithium-ion battery in a 25°C environment for 1 hour. Perform constant current charging (CC) on the battery at a charging rate of 1C. After charging to 4.48V, switch to constant voltage charging (CV). Stop charging when the charging current is lower than 0.05C and let it sit for 5 minutes. Then discharge the battery to 3V at a constant current of 0.2C and let it sit for 5 minutes to ensure the integrity of the subsequent charging and discharging process. Then use a current of 2C to fully charge the battery according to the previous CC+CV charging mode, and calculate the proportion of CC segment capacity at the 2C charging rate. Calculation formula:

[0149] CC segment capacity ratio (2C) = [CC segment charging capacity / (CC + CV) total charging capacity] × 100%.

[0150] The above charging and discharging process was repeated, and the capacity (average value) of each charging stage was counted to calculate the CC segment capacity ratio. Five lithium-ion batteries were provided for each embodiment and comparative example for charge rate performance testing and the average value was taken.

[0151] 12. Rct (charge transfer resistance) and lithium ion diffusion coefficient

[0152] a) Preparation and lithium plating of three-electrode batteries: During the preparation of lithium-ion batteries, a copper wire was connected to the battery as a reference electrode. The copper wire contacted the blank area of ​​the negative electrode current collector without an active material layer. The contact position was close to the tab position but not in contact with the tab. Lithium was then plated at the contact position of the copper wire and the negative electrode current collector at a current of 20 μA for 6 h. EIS was measured after lithium plating.

[0153] b) EIS (lithium ion impedance spectroscopy) test steps: The above three-electrode battery was connected to the Bio-Logic VMP3B electrochemical workstation produced by Bio-LogiC (France) for testing. The test frequency range was 30mHz to 50kHz and the amplitude was 5mV. After data collection, the data was analyzed using the impedance complex plane plot to obtain the Rct data.

[0154] c) Since the impedance of the battery is mainly determined by the mass transfer step under the action of low-frequency AC signal, the corresponding Warburg coefficient σ can be calculated according to the AC impedance spectrum curve, and then the differential value of the negative electrode potential and the battery state of charge can be determined by the coulometric titration method. The lithium ion diffusion coefficient D of the corresponding battery state of charge is calculated by the following formula Li+ .

[0155]

[0156] Where V M is the average molar volume of the negative electrode active material; S is the effective area of ​​the negative electrode; F is the Faraday constant; the Warburg coefficient σ is calculated from the Z'-ω of the corresponding EIS curve -1 / 2 The slope value is determined.

[0157] The differential value of the negative electrode potential and the battery state of charge The method for determining is: charge the battery at 0.1C, charge for 1 hour, let it stand for 2 hours, measure the negative electrode open circuit voltage after the voltage stabilizes, and obtain the charging coulomb titration curve.

[0158] The lithium ion diffusion coefficient test was conducted when the state of charge of the lithium ion battery was 90%.

[0159] Example 1

[0160] Preparation of negative electrode active materials

[0161] In Example 1, the negative electrode active material is prepared by the following steps, wherein the first material is artificial graphite and the second material is reduced graphite oxide:

[0162] (i) 5 g of natural flake graphite was weighed, mixed with 100 mL of concentrated sulfuric acid and 10 g of potassium permanganate, and the mixture was stirred thoroughly and allowed to react at room temperature for 3 hours. The resulting product was washed with a 5 wt% hydrochloric acid solution and then with distilled water until the pH was 7. The product was vacuum filtered and dried in a 70°C oven to obtain weakly oxidized intercalated graphite particles.

[0163] (ii) mixing the graphite oxide particles with boric acid in a mass ratio of 1:10, heating the mixture to 900°C at 5°C / min in a nitrogen atmosphere, maintaining the temperature for 3 hours, and then cooling the mixture to room temperature at 5°C / min. The mixture was then washed three times with 90°C distilled water, vacuum filtered, and dried in a 70°C oven to obtain a second material.

[0164] (iii) The second material and the first material (artificial graphite) were mixed at a mass ratio of 5:95, and pitch (2 wt %) was added for coating and carbonization at a carbonization temperature of 900° C. to obtain a negative electrode active material.

[0165] Preparation of lithium-ion batteries

[0166] In Example 1, a lithium ion battery was prepared by the following steps:

[0167] (a) providing a positive electrode sheet, wherein a positive electrode active material, lithium cobalt oxide (molecular formula: LiCoO2), a conductive agent, acetylene black, and a binder, polyvinylidene fluoride (abbreviated as PVDF), are mixed in a weight ratio of 96:2:2, and then thoroughly stirred and mixed in an appropriate amount of N-methylpyrrolidone (abbreviated as NMP) solvent to form a uniform positive electrode slurry; the slurry is coated on a current collector aluminum foil, and dried to obtain a positive electrode sheet;

[0168] (b) providing a negative electrode sheet, wherein the negative electrode active material obtained in this example, a thickener sodium carboxymethyl cellulose (CMC), and a binder styrene-butadiene rubber (SBR) are mixed in a weight ratio of 95:2:3, and thoroughly stirred in an appropriate amount of deionized water solvent to form a uniform negative electrode slurry; the negative electrode slurry is coated on a current collector copper foil pre-coated with a primer layer, wherein the conductive carbon material of the primer layer is carbon black, and dried to obtain a negative electrode sheet;

[0169] (c) providing an electrolyte solution, wherein EC, DMC, and DEC are mixed in a weight ratio of 1:1:1, and then LiPF6 is added and mixed thoroughly, wherein the mass percentage of LiPF6 is 12.5%, to obtain an electrolyte solution;

[0170] (d) providing a separator, wherein the separator is a polyethylene porous polymer film having a thickness of 7 μm;

[0171] (e) Assembling a lithium-ion battery, wherein the positive electrode sheet, the negative electrode sheet and the separator are wound to form a bare cell, and then the lithium-ion battery is formed by injecting an electrolyte and other steps.

[0172] Example 2

[0173] Example 2 was carried out according to the method of Example 1, except that, in step (ii), heating was performed to 1000° C. in a nitrogen atmosphere.

[0174] Example 3

[0175] Example 3 was carried out according to the method of Example 1, except that, in step (ii), heating was performed to 800° C. in a nitrogen atmosphere.

[0176] Example 4

[0177] Example 4 was carried out according to the method of Example 1, except that, in step (iii), the second material and the first material (artificial graphite) were mixed in a mass ratio of 10:90.

[0178] Example 5

[0179] Example 5 was carried out according to the method of Example 1, except that, in step (iii), the second material and the first material (artificial graphite) were mixed in a mass ratio of 2:98.

[0180] Example 6

[0181] Example 6 was carried out according to the method of Example 1, except that, in step (iii), the second material and the first material (artificial graphite) were mixed in a mass ratio of 15:85.

[0182] Example 7

[0183] Example 7 was carried out according to the method of Example 1, except that in step (ii), heating was performed to 950° C. in a nitrogen atmosphere.

[0184] Example 8

[0185] Example 8 was carried out according to the method of Example 1, except that in step (ii), heating was performed to 1100° C. in a nitrogen atmosphere.

[0186] Example 9

[0187] Example 9 was carried out according to the method of Example 1, except that, in step (ii), heating was performed to 1000° C. in a nitrogen atmosphere; and before step (iii) and after step (ii), the second material obtained from step (ii) was immersed in a 3M lithium hydroxide solution and reacted at room temperature for 3 hours, and then washed with distilled water until the solution pH = 7 to obtain a treated second material.

[0188] Example 10

[0189] Example 10 was carried out according to the method of Example 1, except that in step (ii), heating was performed to 750° C. in a nitrogen atmosphere.

[0190] Example 11

[0191] Example 11 was carried out according to the method of Example 1, except that, in step (ii), the graphite oxide particles and boric acid were mixed at a mass ratio of 1:20.

[0192] Comparative Example 1

[0193] Comparative Example 1 was carried out according to the method of Example 1, except that, in step (iii), the second material and the first material (artificial graphite) were mixed at a mass ratio of 0:100.

[0194] Comparative Example 2

[0195] Comparative Example 2 was carried out according to the method of Example 1, except that, in step (ii), heating was performed to 700° C. in a nitrogen atmosphere.

[0196] XRD tests were performed on the graphite oxide and the second material, i.e., reduced graphite oxide, obtained from Example 1. Figure 1 shows the X-ray diffraction pattern of graphite oxide, and Figure 2 The X-ray diffraction pattern of the reduced graphite oxide is shown in FIG. Figure 2 It can be seen that there is an obvious characteristic peak in the range of 26°-27° in 2θ, and the half-peak width of the characteristic peak is 1° to 2°, which indicates that the second material, ie, reduced graphite oxide, retains the carbon layer structure of graphite.

[0197] Performance tests were performed on the second material, negative electrode active material, negative electrode sheet and lithium-ion battery obtained in Examples 1 to 11 and Comparative Examples 1 to 2, including 002 crystal plane spacing, surface defectivity of the negative electrode active material (Id / Ig), specific surface area of ​​the negative electrode active material, gram capacity of the negative electrode active material, compaction density of the negative electrode active material layer, energy density of the lithium-ion battery, charge rate performance of the lithium-ion battery and lithium ion diffusion coefficient.

[0198] The test results are summarized in Table 1 and Table 2, wherein energy density ratio = energy density of the lithium ion battery obtained in each embodiment or comparative example / energy density of the lithium ion battery obtained in Comparative Example 1.

[0199] Table 1

[0200]

[0201]

[0202] Note: The characteristic peak of the second material is the characteristic peak of the X-ray diffraction pattern of the second material in the range of 26° to 27°

[0203] As shown in Examples 1-5 and Comparative Examples 1-2, the difference between the d002 interplanar spacing of the second material measured in the X-ray diffraction pattern and the d002 interplanar spacing of the first material measured in the X-ray diffraction pattern is less than 1%, and the d002 interplanar spacing of the second material is less than 1%. The half-maximum width of the characteristic peak of the second material is within 1-2°, indicating that the second material retains the carbon layer structure of graphite, has a slightly enlarged interplanar spacing, and the boron-doped carbon layer increases the lithium insertion position, which is beneficial to the deintercalation of lithium ions. Compared with the comparative example, the 1 gram capacity and rate performance are improved; when the difference in interplanar spacing is greater than 1%, it means that the interplanar spacing of the carbon layer in the second material is larger, and the diffusion and embedding distance of lithium ions between the carbon layers is increased, affecting the rate performance of the battery.

[0204] As can be seen from Examples 4-6, the addition of the second material can increase the gram capacity of the negative electrode active material, and the energy density ratio of the battery is significantly improved. However, excessive addition of the second material will affect the cycle retention rate of the lithium-ion battery. This is because the microcrystalline structure of the second material is not as regular as that of graphite, and more electrolyte may be consumed during the cycle of the lithium-ion battery. Therefore, the appropriate addition of the second material and carbon coating can improve the cycle problem.

[0205] Table 2

[0206]

[0207] Comparing the results of Examples 2, 7-11 with Comparative Example 1 shows that the presence of a certain amount of boron in the carbon layer of the second material allows it to deposit between the carbon layers, reducing the activation energy required for lithium ion diffusion and conduction, thereby facilitating lithium ion diffusion. Furthermore, by controlling the oxygen content of the second material within a certain range, surface defects in the negative electrode active material can be mitigated, the specific surface area of ​​the second material can be controlled, the negative electrode sheet compaction density and lithium ion diffusion coefficient can be increased, and the reversible intercalation and deintercalation of lithium ions can be enhanced.

[0208] 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 certain 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, and such modifications and changes also fall within the scope of protection of the present invention.

Claims

1. A negative electrode active material comprising a first material and a second material, wherein the first material has a d002 interplanar spacing of d1 as measured in an X-ray diffraction pattern, and the second material has a d002 interplanar spacing of d2 as measured in an X-ray diffraction pattern, wherein d1 and d2 satisfy the following relationship: (d2 / d1-1)≤0.01; The first material includes at least one of natural graphite, artificial graphite, hard carbon and soft carbon; The second material includes reduced graphite oxide and boron element.

2. The negative electrode active material according to claim 1, wherein The negative electrode active material satisfies at least one of the following conditions: (i) The range of d1 is: to (ii) The range of d2 is: to (iii)0.001≤(d2 / d1-1)≤0.

008.

3. The negative electrode active material according to claim 1, wherein The negative electrode active material satisfies at least one of the following conditions: (iv) the mass content of the boron element is in the range of 0.1 mass % to 1 mass % based on the mass of the negative electrode active material, (v) the second material contains oxygen, wherein the mass content of the oxygen is in the range of 1 mass % to 5 mass % based on the mass of the negative electrode active material, (vi) The X-ray diffraction pattern of the second material has a characteristic peak in the range of 2θ from 26° to 27°, and the half-value width of the characteristic peak is from 1° to 2°. 4 . The negative electrode active material according to claim 3 , wherein the reduced graphite oxide accounts for 1% to 10% by mass based on the mass of the negative electrode active material.

5. The negative electrode active material according to claim 1, wherein the negative electrode active material satisfies at least one of the following conditions: (1) The specific surface area of ​​the negative electrode active material is 1 m 2 / g to 3m 2 / g; (2) The gram capacity of the negative electrode active material is greater than or equal to 360 mAh / g; (3) The powder compaction density of the negative electrode active material is greater than or equal to 1.80 g / cm 3 and less than or equal to 2.00g / cm 3 .

6. The negative electrode active material according to claim 1, wherein Raman spectroscopy is performed on particles of the negative electrode active material within a range of 100 μm×100 μm, and the test result satisfies the following conditions: 0.5≤Id / Ig≤1.5, wherein Id is the particle size of the negative electrode active material at 1350 cm -1 Ig is the peak intensity of the negative electrode active material particles at 1580cm -1 The peak intensity at .

7. The negative electrode active material according to claim 1, wherein the negative electrode active material satisfies at least one of the following conditions: (4) The range of d2 is: to (5) The second material includes reduced graphite oxide, and the mass of the reduced graphite oxide accounts for 2% to 5% based on the mass of the negative electrode active material; (6) The second material contains boron, and the mass content of the boron is in the range of 0.2 mass % to 0.6 mass % based on the mass of the negative electrode active material; (7) The second material contains oxygen element, and the mass content of the oxygen element is in the range of 2 mass % to 4 mass % based on the mass of the negative electrode active material.

8. A method for preparing the negative electrode active material according to any one of claims 1 to 7, the method comprising the following steps: S1, mixing natural flake graphite and an oxidant in a mass ratio of 2:1 to 1:1, and reacting at 20° C. to 30° C. to obtain graphite oxide; S2, mixing the graphite oxide obtained in step S1 with a boron-containing compound in a mass ratio of 1:5 to 1:10, heating the mixture to 800° C. to 1200° C. at 2° C. / min to 5° C. / min in an inert gas and keeping the mixture at that temperature for 2 to 4 hours, then cooling the mixture to room temperature at 5° C. / min to 10° C. / min, and washing the mixture with water at 80° C. to 95° C. to obtain a second material; S3, mixing the first material and the second material obtained from step S2 in a mass ratio of 100:1 to 100:10, and then coating and carbonizing at a temperature of 800°C to 1200°C to obtain a negative electrode active material, wherein the first material includes at least one of natural graphite, artificial graphite, hard carbon and soft carbon.

9. An electrochemical device comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode active material according to any one of claims 1 to 7 or the negative electrode active material prepared by the method according to claim 8.

10. The electrochemical device according to claim 9, wherein the compaction density of the negative electrode is greater than or equal to 1.50 g / cm 3 and less than or equal to 1.70g / cm 3 .

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