Negative electrode material and preparation method thereof, negative electrode sheet and sodium ion battery

Abstract

The present application provides a negative electrode material and a preparation method thereof, a negative electrode sheet and a sodium ion battery, which relate to the field of new energy technology. The negative electrode material provided in the present application includes a core and a soft carbon coating layer coated on the surface of the core, the core includes hard carbon and carbon dots distributed inside and on the surface of the hard carbon, and closed pores are distributed inside the hard carbon. The soft carbon coating modifies the surface of the material, reduces the material specific surface area, and improves the first efficiency of the hard carbon material; the carbon dots are cross-linked with the coal-based precursor to inhibit the graphitization of the precursor material during high-temperature carbonization, increase the degree of disorder, help to form closed pores, and improve the sodium storage performance of the hard carbon material. The negative electrode material of the present application uses coal as a precursor material, mixes asphalt and carbon dots to form a composite precursor as an anode, an inert electrode as a cathode, and a mixed acid solution as an electrolyte. The composite precursor is electrolytically modified by an electrochemical method, and then separated, washed, and dried to obtain a modified composite precursor, which is finally obtained by sintering.

Description

Technical Field

[0001] The present application relates to the field of new energy technology, and in particular to a negative electrode material and a preparation method thereof, a negative electrode sheet and a sodium ion battery. Background Art

[0002] Hard carbon refers to carbon materials that are difficult to graphitize even at temperatures above 2500°C, deriving its name from its high mechanical hardness. Its morphology can be spherical, linear, or porous, and its geometric structure can be described as a finitely twisted stack of short, parallel graphene sheets. This exhibits a larger interlayer spacing (~0.38 nm) and numerous nanopores, providing more space for the insertion and diffusion of sodium ions, which promotes structural stability during cycling. This is manifested in a high sodium storage capacity, a low storage potential, and excellent cycling stability.

[0003] Hard carbon is mainly divided into three categories based on the source of the precursor: biomass-based, resin-based, and fossil fuel-based. Among them, biomass precursors (such as coconut shells, apricot shells, bamboo, straw, etc.) are widely available, inexpensive, and environmentally friendly, and they themselves have rich heteroatoms and unique microstructures. However, as the industry enters the growth and expansion stage, the stability, low cost, and consistency of its raw material supply chain are difficult to guarantee. The most common resin-based precursor is phenolic resin. The hard carbon product obtained from it has good uniformity and high purity, excellent cycle performance, higher reversible specific capacity and better rate performance, and the raw materials are controllable and the process is highly designable. However, its cost is the highest, and the cost disadvantage has become a pain point for resin-based hard carbon. Fossil fuel precursors include coal-based carbon materials, which refer to carbon materials prepared with coal and its derivatives as carbon-containing precursors, including coal tar, needle coke, anthracite-based carbon materials, etc. Among them, coal has a high carbon yield and rich by-products. Using coal or its by-products as precursors to develop sodium-ion battery negative electrode materials can not only reduce environmental pollution but also enhance its utilization value.

[0004] However, since the structure of coal-based or its by-product precursors after high-temperature pyrolysis is soft carbon and tends to graphitize, the ordered carbon microstructure of coal-derived hard carbon materials leads to poor sodium storage capacity and low initial Coulombic efficiency (ICE). Summary of the Invention

[0005] The purpose of this application is to provide a negative electrode material and a preparation method thereof, a negative electrode sheet and a sodium ion battery, aiming to solve the problems of poor sodium storage capacity and low initial Coulomb efficiency of existing coal-derived hard carbon materials.

[0006] To achieve the above objectives, the present application provides a negative electrode material, which includes a core and a soft carbon coating layer coated on the surface of the core, the core includes hard carbon and carbon dots distributed inside and on the surface of the hard carbon; closed pores are distributed inside the hard carbon.

[0007] In some embodiments, the pore size of the closed pores is 0.35-0.85 nm, and the volume density of the closed pores is 0.03-0.27 cm 3 / g.

[0008] In some embodiments, the ash content of the negative electrode material is less than 0.3%.

[0009] In some embodiments, at least one of the following conditions is met:

[0010] A. The average particle size of the negative electrode material is 3-20 μm;

[0011] B. The specific surface area of ​​the negative electrode material is less than 10 m 2 / g;

[0012] C. The 3T compaction density of the negative electrode material is 1.00-1.12 g / cm 3 ;

[0013] D. the hard carbon is spherical;

[0014] E. The average particle size of the carbon dots is 1-20 nm;

[0015] F. The oxygen content of the carbon dots is 30%-70%.

[0016] The present application also provides a method for preparing the above-mentioned negative electrode material, comprising:

[0017] Coal powder, asphalt and carbon dots are solid-phase mixed and hot-pressed to form a composite precursor sheet;

[0018] The composite precursor sheet is used as an anode, an inert electrode is used as a cathode, and a mixed acid solution is used as an electrolyte, and an electrochemical reaction is carried out at a constant voltage, and high-speed stirring is maintained during the reaction;

[0019] Separating, washing and drying the electrolysis product obtained by the electrochemical reaction from the electrolyte to obtain a modified composite precursor;

[0020] The modified composite precursor is sintered in stages under an inert atmosphere to obtain the negative electrode material.

[0021] In some embodiments, at least one of the following conditions is met:

[0022] A. The particle size of the pulverized coal is 3-20 μm;

[0023] B. the pulverized coal comprises at least one of anthracite, sub-bituminous coal, bituminous coal and lignite;

[0024] C. The particle size of the asphalt is 1-5 μm;

[0025] D. the asphalt comprises at least one of petroleum asphalt, coal tar asphalt and natural asphalt;

[0026] E. The softening point R0 of the asphalt satisfies 180°C ≤ R0 ≤ 320°C;

[0027] F. The carbon dots include at least one of carbon quantum dots, graphene quantum dots, and polymer quantum dots;

[0028] G. The average particle size of the carbon dots is 1-20 nm, and the oxygen content is 30%-70%;

[0029] H. The mass ratio of the coal powder, the asphalt and the carbon dots is (94.5-98.4): (1.5-5): (0.1-0.5);

[0030] I. The solid phase mixing includes any one of VC mixing, triple eccentric mixing, and ball milling mixing;

[0031] J. The thickness of the composite precursor sheet is 0.5 cm-1.0 cm.

[0032] In some embodiments, at least one of the following conditions is met:

[0033] A. The inert electrode includes a mixed metal electrode formed by one or more of titanium, platinum, and iridium;

[0034] B. the mixed acid solution is a mixed solution of sulfuric acid and nitric acid;

[0035] C. When condition B is met, the mass percentage concentration of the sulfuric acid is 98%, the mass percentage concentration of the nitric acid is 30%-60%, and the volume ratio of the sulfuric acid to the nitric acid is 3:1;

[0036] D. the amount of the mixed acid solution added is greater than or equal to the mass of the coal powder;

[0037] E. The range of the constant voltage is 5-30V;

[0038] F. The linear velocity of the high-speed stirring is 10-20 m / s.

[0039] In some embodiments, at least one of the following conditions is met:

[0040] A. The separation method includes any one of centrifugal separation and membrane filtration;

[0041] B. The washing is repeated several times with deionized water until the pH of the washing solution is 6.5-7.5;

[0042] C. The drying method includes spray drying, fluidized bed drying, vacuum drying or forced air drying, or a combination thereof;

[0043] D. the inert atmosphere comprises at least one of nitrogen, argon and helium;

[0044] E. The staged sintering includes a first stage sintering and a second stage sintering. The heating rate of the first stage sintering is 0.5-2°C / min, the insulation temperature of the first stage sintering is the softening point of the asphalt R0+20°C, and the insulation time of the first stage sintering is 0.5-1h; the heating rate of the second stage sintering is 1-5°C / min, the insulation temperature of the second stage sintering is 1100-1400°C, and the insulation time of the second stage sintering is 2-5h.

[0045] The present application also provides a negative electrode sheet, comprising the above-mentioned negative electrode material.

[0046] The present application also provides a sodium ion battery, comprising the above-mentioned negative electrode sheet.

[0047] Compared with the prior art, the advantages of this application include:

[0048] The negative electrode material provided in this application includes a core and a soft carbon coating coated on the surface of the core. The core includes hard carbon and carbon dots distributed within and on the surface of the hard carbon, and the hard carbon has closed pores distributed within. Hard carbon is widely available and inexpensive, making it a suitable raw material for hard carbon negative electrodes in sodium-ion batteries. The soft carbon coating modifies the material surface, reducing the material specific surface area and improving the initial efficiency of the hard carbon material. The carbon dots cross-link with the hard carbon, inhibiting graphitization of the hard carbon material during high-temperature carbonization, increasing the degree of disorder, and promoting the formation of closed pores, thereby improving the sodium storage performance of the hard carbon material.

[0049] The preparation method of the negative electrode material provided in the present application uses coal as the precursor material, mixes asphalt and carbon dots to form a composite precursor as the anode, an inert electrode as the cathode, and a mixed acid solution as the electrolyte. The composite precursor is electrochemically modified. During the electrolysis process, the anode composite precursor sheets will be continuously consumed and dispersed into the electrolyte, and then separated, washed and dried to obtain the modified composite precursor, and finally the negative electrode material is obtained by high-temperature sintering.

[0050] The negative electrode sheet and sodium ion battery provided in this application use the negative electrode material of this application, which can improve the capacity and initial efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] 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. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope of the present application.

[0052] Figure 1 Schematic diagram of the structure of the negative electrode material of this application;

[0053] Figure 2 Schematic diagram of the process for preparing the negative electrode material of the present application;

[0054] Figure 3 A diagram of the electrochemical reaction device of this application;

[0055] Figure 4 is a SEM image of the negative electrode material of Example 1;

[0056] Figure 5 This is a closed pore size distribution diagram of the negative electrode material of Example 1;

[0057] Figure 6 This is a charge and discharge curve diagram of the negative electrode material of Example 1.

[0058] Reference numerals:

[0059] 1-soft carbon coating, 2-hard carbon, 3-carbon dots. DETAILED DESCRIPTION

[0060] As used herein:

[0061] "Prepared from" is synonymous with "comprising." As used herein, the terms "comprising," "including," "having," "containing," or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises the listed elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0062] When an amount, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper preferred values ​​and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pairing of any range upper limit or preferred value with any range lower limit or preferred value, regardless of whether the range is disclosed alone. For example, when a range of "1 to 5" is disclosed, the described range should be interpreted as including the range "1 to 4", "1 to 3", "1 to 2", "1 to 2 and 4 to 5", "1 to 3 and 5", etc. When a numerical range is described herein, unless otherwise stated, the range is intended to include its end values ​​and all integers and fractions within the range.

[0063] In these examples, parts and percentages are by mass unless otherwise indicated.

[0064] "Parts by mass" refers to the basic unit of measurement used to express the mass ratio of multiple components. One part can represent any unit of mass, such as 1g or 2.689g. For example, if we say that the parts by mass of component A are a parts and the parts by mass of component B are b parts, this means the ratio of the mass of component A to the mass of component B is a:b. Alternatively, we could say that the mass of component A is aK and the mass of component B is bK (K is an arbitrary number representing a multiplication factor). It's important to note that, unlike parts by mass, the sum of the parts by mass of all components is not limited to 100 parts.

[0065] "And / or" is used to indicate that one or both of the stated situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0066] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature identified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0067] This application provides a negative electrode material, please refer to Figure 1 The negative electrode material includes a core and a soft carbon coating layer 1 coated on the surface of the core, the core includes hard carbon 2 and carbon dots 3 distributed inside and / or on the surface of the hard carbon 2; closed pores are distributed inside the hard carbon 2.

[0068] Among them, hard carbon is widely available and inexpensive, and is used as the raw material for hard carbon negative electrodes of sodium ion batteries; the soft carbon coating modifies the surface of the material, reduces the material specific surface area, and improves the primary efficiency of the hard carbon material; carbon dots are cross-linked with hard carbon to inhibit the graphitization of the hard carbon material during high-temperature carbonization, increase the degree of disorder, help form closed pores, and improve the sodium storage performance of the hard carbon material.

[0069] Among them, closed pores are closed cavities inside hard carbon that are not connected to the outside world. They are formed by stacking twisted graphene sheets, and the pore walls are composed of graphite-like crystallites.

[0070] In some embodiments, the pore size of the closed pores is 0.35-0.85 nm, for example, 0.35, 0.4 nm, 0.45 nm, 0.50 nm, 0.55 nm, 0.60 nm, 0.65 nm, 0.70 nm, 0.75 nm, 0.80 nm, 0.85 nm or any value between 0.35-0.85 nm, preferably 0.45-0.65 nm; the volume density of the closed pores is 0.03-0.27 cm 3 / g, for example, 0.03 cm3 / g, 0.05cm 3 / g, 0.07cm 3 / g, 0.09cm 3 / g, 0.10cm 3 / g, 0.12cm 3 / g, 0.14cm 3 / g, 0.15cm 3 / g, 0.17cm 3 / g, 0.18cm 3 / g, 0.19cm 3 / g, 0.20cm 3 / g, 0.22cm 3 / g, 0.23cm 3 / g, 0.24cm 3 / g, 0.25cm 3 / g, 0.27cm 3 / g or 0.03-0.27 cm 3 Any value between 0.18-0.24 cm / g, preferably 0.18-0.24 cm 3 / g.

[0071] In some embodiments, the ash content of the negative electrode material is less than 0.3%, for example, it can be any value of 0.05%, 0.1%, 0.15%, 0.2%, 0.25% or less than 0.3%, preferably less than 0.2%.

[0072] In some embodiments, the average particle size of the negative electrode material is 3-20 μm, for example, 3 μm, 5 μm, 7 μm, 9 μm, 10 μm, 12 μm, 14 μm, 15 μm, 17 μm, 19 μm, 20 μm or any value between 3-20 μm.

[0073] In some embodiments, the specific surface area of ​​the negative electrode material is less than 10 m 2 / g, for example, 1m 2 / g, 2m 2 / g、3m 2 / g、4m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g or <10 m 2 / g, preferably 3m 2 / g-7m 2 / g.

[0074] In some embodiments, the 3T compaction density of the negative electrode material is 1.00-1.12 g / cm 3 , for example, it can be 1.00 g / cm 3 , 1.01g / cm 3 , 1.02g / cm 3 , 1.03g / cm 3 , 1.04g / cm 3 , 1.05g / cm 3 , 1.06g / cm 3 , 1.07g / cm 3 、1.08g / cm 3 , 1.09g / cm 3 , 1.10g / cm 3 , 1.11g / cm 3 , 1.12g / cm 3 or 1.00-1.12g / cm 3 Any value in between.

[0075] In some embodiments, the hard carbon is spherical.

[0076] In some embodiments, the average particle size of the carbon dots is 1-20 nm, for example, 1 nm, 2 nm, 4 nm, 5 nm, 7 nm, 9 nm, 10 nm, 12 nm, 14 nm, 15 nm, 17 nm, 19 nm, 20 nm, or any value between 1-20 nm.

[0077] In some embodiments, the oxygen content of the carbon dots is 30%-70%, for example, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or any value between 30% and 70%.

[0078] This application also provides a method for preparing the above-mentioned negative electrode material, please refer to Figure 2 ,include:

[0079] S100: solid-phase mixing of coal powder, asphalt and carbon dots, and hot pressing to form a composite precursor sheet;

[0080] S200: The composite precursor sheet is used as the anode, the inert electrode is used as the cathode, and the mixed acid solution is used as the electrolyte. The electrochemical reaction is carried out at a constant voltage and high-speed stirring is maintained during the reaction.

[0081] S300: separating, washing, and drying the electrolysis product obtained by the electrochemical reaction from the electrolyte to obtain a modified composite precursor;

[0082] S400: Sintering the modified composite precursor in sections under an inert atmosphere to obtain a negative electrode material.

[0083] The preparation method of the negative electrode material provided in the present application uses coal as the precursor material, mixes asphalt and carbon dots to form a composite precursor as the anode, an inert electrode as the cathode, and a mixed acid solution as the electrolyte. The composite precursor is electrochemically modified. During the electrolysis process, the anode composite precursor sheets will be continuously consumed and dispersed into the electrolyte, and then separated, washed and dried to obtain the modified composite precursor, and finally the negative electrode material is obtained by high-temperature sintering.

[0084] In some embodiments, the particle size of the coal powder in step S100 is 3-20 μm, for example, it can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm or any value between 3-20 μm.

[0085] In some embodiments, the pulverized coal of step S100 includes at least one of anthracite, sub-bituminous coal, bituminous coal, and lignite.

[0086] Coal-based precursors are widely available and inexpensive, and are used as raw materials for hard carbon negative electrodes of sodium ion batteries in this application.

[0087] In some embodiments, the particle size of the asphalt in step S100 is 1-5 μm, for example, it can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or any value between 1-5 μm.

[0088] In some embodiments, the asphalt in step S100 includes at least one of petroleum asphalt, coal tar asphalt, and natural asphalt.

[0089] In some embodiments, the softening point R0 of the asphalt in step S100 satisfies 180°C ≤ R0 ≤ 320°C. R0 can be, for example, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C, 300°C, 310°C, 320°C, or any value between 180°C and 320°C. Asphalt with a low softening point contains a large amount of light components, which is detrimental to the coating effect.

[0090] Asphalt precursor has two functions. On the one hand, in the production of anode sheets, asphalt acts as a binder to bond the composite precursor powder together for extrusion molding; on the other hand, during the high-temperature sintering process of the modified composite precursor, asphalt acts as a coating agent to modify the surface of the material, reduce the material specific surface area, and improve the initial efficiency of the hard carbon material.

[0091] In some embodiments, the carbon dots in step S100 include at least one of carbon quantum dots, graphene quantum dots, and polymer quantum dots.

[0092] In some embodiments, the average particle size of the carbon dots in step S100 is 1-20 nm, for example, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, or any value between 1 and 20 μm, and the oxygen content is 30%-70%, for example, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or any value between 30% and 70%.

[0093] In some embodiments, the mass ratio of coal powder, asphalt and carbon dots in step S100 is (94.5-98.4): (1.5-5): (0.1-0.5), for example, it can be 94.5:5:0.5, or 95:2.5:0.3, or 96:3.5:0.1, or 98:1.5:0.1, or 97:2.0:0.4, or any ratio between (94.5-98.4): (1.5-5): (0.1-0.5).

[0094] In some embodiments, the solid phase mixing in step S100 includes any one of VC mixing, triple eccentric mixing, and ball milling mixing.

[0095] In some embodiments, the thickness of the composite precursor sheet in step S100 is 0.5 cm-1.0 cm, for example, it can be 0.5 cm, 0.6 cm, 0.7 cm, 0.8 cm, 0.9 cm, 1.0 cm or any value between 0.5 cm and 1.0 cm. The thickness of the composite precursor sheet has little effect on the electrolysis effect, as long as the composite precursor sheet is designed to a thickness suitable for electrolysis.

[0096] In some embodiments, the inert electrode in step S200 includes a mixed metal electrode formed by one or more of titanium, platinum, and iridium.

[0097] The main purpose of the inert electrode is to conduct electricity. At the same time, since the electrolyte is an acidic solution, it must also prevent itself from corrosion.

[0098] In some embodiments, the mixed acid solution of step S200 is a mixed solution of sulfuric acid and nitric acid; the mass percentage concentration of the sulfuric acid is 98%, and the mass percentage concentration of the nitric acid is 30%-60%, for example, it can be 30%, 40%, 50%, 60% or any value between 30%-60%, and the volume ratio of the sulfuric acid and the nitric acid is 3:1; the amount of the mixed acid solution added is greater than or equal to the mass of the coal powder.

[0099] Among them, sulfuric acid has high conductivity and stability, and can act as an electrolyte, providing an ionic environment for the electrolysis reaction and ensuring the smooth progress of the electrolysis process. Nitric acid has strong oxidizing properties and can oxidatively modify the composite precursor under electrolysis.

[0100] In some embodiments, the constant voltage in step S200 is in a range of 5-30V, for example, 5V, 10V, 15V, 20V, 25V, 30V or any value between 5-30V.

[0101] In some embodiments, the linear velocity of the high-speed stirring in step S200 is 10-20 m / s, for example, it can be 10 m / s, 12 m / s, 14 m / s, 16 m / s, 17 m / s, 18 m / s, 20 m / s or any value between 10-20 m / s.

[0102] In some embodiments, the device for implementing the electrochemical reaction of step S200 is shown in FIG. Figure 3 shown.

[0103] High-speed stirring during the electrochemical reaction is beneficial to a more uniform and sufficient reaction between the coal-based precursor and the mixed acid, avoiding an overly violent reaction on the anode side; at the same time, high-speed stirring can also prevent carbon dots from agglomerating, facilitating uniform penetration into the interior of the coal-based precursor.

[0104] The role of electrochemical reaction: (1) Promote oxidation: The electrochemical reaction provides an external electric field, which causes the ions in the system to move in a directional manner. During the electrochemical reaction, anions will accumulate near the anode. Nitrate has strong oxidizing properties under acidic conditions. Under the action of electrolysis, its oxidizing properties are enhanced, thereby more effectively oxidizing the composite precursor; (2) Remove impurities: After being oxidized, some elements in coal enter the solution in the form of ions, or generate gaseous products to escape, thereby realizing the conversion and separation of certain components in coal.

[0105] In some embodiments, the separation method in step S300 includes any one of centrifugal separation and membrane filtration.

[0106] In some embodiments, the washing in step S300 is repeated multiple times using deionized water until the pH of the washing solution is 6.5-7.5.

[0107] In some embodiments, the drying method of step S300 includes one or a combination of spray drying, fluidized bed drying, vacuum drying or forced air drying.

[0108] In some embodiments, the inert atmosphere of step S400 includes at least one of nitrogen, argon, and helium.

[0109] In some embodiments, the segmented sintering of step S400 includes a first stage sintering and a second stage sintering, the heating rate of the first stage sintering is 0.5-2°C / min, the insulation temperature of the first stage sintering is the softening point of asphalt R0+20°C, and the insulation time of the first stage sintering is 0.5-1h; the heating rate of the second stage sintering is 1-5°C / min, the insulation temperature of the second stage sintering is 1100-1400°C, for example, it can be 1100°C, 1150°C, 1200°C, 1250°C, 1300°C, 1350°C, 1400°C or any value between 1100-1400°C, and the insulation time of the second stage sintering is 2-5h.

[0110] The first stage of sintering is kept warm near the softening point of the asphalt, and the asphalt will soften and have good fluidity. At this time, the softened asphalt can better spread on the surface of the hard carbon, achieve uniform coating of the hard carbon particles, and form a uniform asphalt film. This uniform coating layer is crucial for the subsequent improvement of the performance of the hard carbon. In the second stage of high-temperature sintering, the amorphous carbon matrix of the hard carbon will gradually form a short-range ordered graphite microcrystalline structure. This microcrystalline structure is arranged in a graphite-like layer, providing rich embedding sites and diffusion channels for sodium ions. In addition, the pore structure inside the hard carbon changes, and the open pores gradually close to form closed nanopores, which enhances the sodium ion "filling" mechanism and improves the platform area capacity.

[0111] The present application also provides a negative electrode sheet, comprising the above-mentioned negative electrode material.

[0112] The present application also provides a sodium ion battery, comprising the above-mentioned negative electrode sheet.

[0113] The negative electrode sheet and sodium ion battery provided in this application use the negative electrode material of this application, which can improve the capacity and initial efficiency.

[0114] The embodiments of the present application will be described in detail below in conjunction with specific examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present application and should not be considered as limiting the scope of the present application. In the examples, if specific conditions are not specified, the conditions are carried out according to conventional conditions or manufacturer recommendations. If the manufacturer is not specified for reagents or instruments used, they are all conventional products that can be purchased commercially.

[0115] Example 1

[0116] Example 1 provides a negative electrode material, and the preparation method thereof comprises the following steps:

[0117] (1) The coal and asphalt raw materials were crushed by a gas pulverizer, and the particle size of the coal powder was controlled to be 5 μm, the particle size of the asphalt was 1 μm, the softening point of the asphalt was 320 °C, the average particle size of the carbon dots was 3 nm, and the oxygen content was 50%. 94.5 g of coal powder, 5 g of asphalt and 0.5 g of carbon dots were weighed, mixed evenly by VC equipment, and further hot pressed by a grinding tool to form a composite precursor sheet.

[0118] (2) The mass percentage concentration of sulfuric acid is 98%, and the mass percentage concentration of nitric acid is 50%. Sulfuric acid and nitric acid are mixed in a volume ratio of 3:1. 94.5 g of the mixed acid is taken as the electrolyte. The above-mentioned composite precursor sheet is used as the anode and the inert titanium electrode is used as the cathode. The stirring linear speed is 10 m / s. A constant voltage DC power supply is used and the voltage is adjusted to 30 V for the electrochemical reaction.

[0119] (3) After the electrolysis reaction is completed, the electrolysis product is centrifuged and separated from the electrolyte, and washed with deionized water several times until the pH of the solution is 6.5. The electrolysis product is placed in a forced air drying oven at 80°C for 6 hours to dry out the moisture, thereby obtaining a modified composite precursor material.

[0120] (4) The modified composite precursor material was placed in a high-temperature carbonization furnace and sintered in stages under a nitrogen atmosphere. The first stage was sintered from room temperature to 340°C at a heating rate of 1°C / min and kept warm for 1 hour. The second stage was sintered from 340°C to 1200°C at a heating rate of 3°C / min and kept warm for 3 hours to obtain the negative electrode material of Example 1. Its SEM image is shown in FIG. Figure 4 shown.

[0121] Example 2

[0122] Example 2 provides a negative electrode material, the preparation method of which includes the following steps:

[0123] (1) The coal and asphalt raw materials were crushed by a gas pulverizer, and the particle size of the coal powder was controlled to be 10 μm, the particle size of the asphalt was 3 μm, the softening point was 250 °C, the average particle size of the carbon dots was 5 nm, and the oxygen content was 30%. 95 g of coal powder, 4.7 g of asphalt and 0.3 g of carbon dots were weighed, mixed evenly by VC equipment, and further hot pressed by a grinding tool to form a composite precursor sheet.

[0124] (2) The mass percentage concentration of sulfuric acid is 98%, and the mass percentage concentration of nitric acid is 50%. Sulfuric acid and nitric acid are mixed in a volume ratio of 3:1. 95 g of the mixed acid is taken as the electrolyte. The above-mentioned composite precursor sheet is used as the anode and the inert titanium electrode is used as the cathode. The stirring linear speed is 12 m / s. A constant voltage DC power supply is used and the voltage is adjusted to 25 V for the electrochemical reaction.

[0125] (3) After the electrolysis reaction is completed, the electrolysis product is centrifuged and separated from the electrolyte, and washed with deionized water several times until the pH of the solution is 6.5. The electrolysis product is placed in a forced air drying oven at 90°C for 4 hours to dry out the moisture to obtain a modified composite precursor material.

[0126] (4) The modified composite precursor material is placed in a high-temperature carbonization furnace and sintered in stages under a nitrogen protective atmosphere. The first stage is sintered from room temperature to 270°C, with a heating rate of 2°C / min and a heat preservation time of 0.5h. The second stage is sintered from 270°C to 1230°C, with a heating rate of 5°C / min and a heat preservation time of 4h to obtain the negative electrode material of Example 2.

[0127] Example 3

[0128] Example 3 provides a negative electrode material, the preparation method of which includes the following steps:

[0129] (1) The coal and asphalt raw materials were crushed by a gas pulverizer. The particle size of the coal powder was controlled to 7 μm, the particle size of the asphalt was controlled to 2 μm, the softening point was controlled to 300 °C, the average particle size of the carbon dots was controlled to 10 nm, and the oxygen content was controlled to 70%. 96 g of coal powder, 3.8 g of asphalt and 0.2 g of carbon dots were weighed, mixed evenly by VC equipment, and further hot pressed by a grinding tool to form a composite precursor sheet.

[0130] (2) The mass percentage concentration of sulfuric acid is 98%, and the mass percentage concentration of nitric acid is 50%. Sulfuric acid and nitric acid are mixed in a volume ratio of 3:1. 100 g of the mixed acid is taken as the electrolyte. The above-mentioned composite precursor sheet is used as the anode and the inert titanium electrode is used as the cathode. The stirring linear speed is 15 m / s. A constant voltage DC power supply is used and the voltage is adjusted to 15 V for the electrochemical reaction.

[0131] (3) After the electrolysis reaction is completed, the electrolysis product is centrifuged from the electrolyte and washed with deionized water several times until the pH of the solution is 7.0. The electrolysis product is placed in a vacuum drying oven at 60°C for 10 hours to dry out the moisture to obtain a modified composite precursor material.

[0132] (4) The modified composite precursor material was placed in a high-temperature carbonization furnace and sintered in stages under a nitrogen atmosphere. The first stage was sintered from room temperature to 320°C at a heating rate of 1.5°C / min and kept warm for 1 hour. The second stage was sintered from 320°C to 1150°C at a heating rate of 4°C / min and kept warm for 5 hours to obtain the negative electrode material of Example 3.

[0133] Example 4

[0134] Example 4 provides a negative electrode material, the preparation method of which includes the following steps:

[0135] (1) The coal and asphalt raw materials were crushed by gas-powder-based crushing, and the particle size of the coal powder was controlled to be 20 μm, the particle size of the asphalt was 5 μm, the softening point was 180 °C, the average particle size of the carbon dots was 17 nm, and the oxygen content was 55%. 95.6 g of coal powder, 4 g of asphalt and 0.4 g of carbon dots were weighed, mixed evenly through VC equipment, and further hot-pressed through a grinding tool to form a composite precursor sheet.

[0136] (2) The mass percentage concentration of sulfuric acid is 98%, and the mass percentage concentration of nitric acid is 50%. Sulfuric acid and nitric acid are mixed in a volume ratio of 3:1. 100 g of the mixed acid is taken as the electrolyte. The above-mentioned composite precursor sheet is used as the anode and the inert titanium electrode is used as the cathode. The stirring linear speed is 10 m / s. A constant voltage DC power supply is used and the voltage is adjusted to 5 V for the electrochemical reaction.

[0137] (3) After the electrolysis reaction is completed, the electrolysis product is centrifuged and separated from the electrolyte, and washed with deionized water several times until the pH of the solution is 6.8. The electrolysis product is placed in a forced air drying oven at 80°C for 6 hours to dry out the moisture to obtain a modified composite precursor material.

[0138] (4) The modified composite precursor material is placed in a high-temperature carbonization furnace and sintered in stages under a nitrogen protective atmosphere. The first stage is sintered from room temperature to 200°C, with a heating rate of 1°C / min and a heat preservation period of 1 hour. The second stage is sintered from 200°C to 1200°C, with a heating rate of 3°C / min and a heat preservation period of 3 hours to obtain the negative electrode material of Example 4.

[0139] Comparative Example 1

[0140] Comparative Example 1 provides a negative electrode material, the preparation method of which is different from that of Example 1 in that no carbon dots are added in step (1), and the other steps are the same as those of Example 1.

[0141] Comparative Example 2

[0142] Comparative Example 2 provides a negative electrode material, the preparation method of which is different from that of Example 1 in that no asphalt is added in step (1), and the other steps are the same as those of Example 1.

[0143] Comparative Example 3

[0144] Comparative Example 3 provides a negative electrode material, the preparation method of which differs from that of Example 1 in that: in step (1), 70.5 g of coal powder, 29 g of asphalt, and 0.5 g of carbon dots are taken, i.e., 70.5:29:0.5 (asphalt in excess), and the other steps are the same as those of Example 1.

[0145] Comparative Example 4

[0146] Comparative Example 4 provides a negative electrode material, the preparation method of which is different from that of Example 1 in that: in step (1), 94 g of coal powder, 5 g of asphalt, and 1 g of carbon dots are taken, i.e., 94:5:1 (carbon dots are in excess), and the other steps are the same as those of Example 1.

[0147] Comparative Example 5

[0148] Comparative Example 5 provides a negative electrode material, the preparation method of which differs from that of Example 1 in that: in step (2), no electrolytic reaction is performed, and the composite precursor sheet is directly placed in the mixed acid electrolyte and stirred for a natural reaction.

[0149] Comparative Example 6

[0150] Comparative Example 6 provides a negative electrode material, the preparation method of which is different from that of Example 1 in that the mass of the mixed acid in step (2) is 50 g, and the other steps are the same as those in Example 1.

[0151] Comparative Example 7

[0152] Comparative Example 7 provides a negative electrode material, the preparation method of which differs from that of Example 1 in that the voltage of the electrochemical reaction in step (2) is 1 V, and the other steps are the same as those in Example 1.

[0153] Comparative Example 8

[0154] Comparative Example 8 provides a negative electrode material, the preparation method of which differs from that of Example 1 in that the voltage of the electrochemical reaction in step (2) is 50 V, and the other steps are the same as those in Example 1.

[0155] Comparative Example 9

[0156] Comparative Example 9 provides a negative electrode material, the preparation method of which differs from that of Example 1 in that: in step (4), no segmented sintering is performed, and the temperature is directly raised from room temperature to 1200°C at a heating rate of 3°C / min and kept warm for 3 hours. The other steps are the same as those of Example 1.

[0157] Comparative Example 10

[0158] Comparative Example 10 provides a negative electrode material, the preparation method of which is different from that of Example 1 in that the holding temperature of the first sintering stage in step (4) is 280°C, and the other steps are the same as those of Example 1.

[0159] Performance Testing

[0160] Button Cell Fabrication: The negative electrode materials of Examples 1 to 4 and Comparative Examples 1 to 10 were respectively mixed with conductive carbon black and LA133 binder in a mass ratio of 91:3:6 in pure water. The mixture was then uniformly coated on an aluminum foil current collector and dried under vacuum at 80°C to obtain electrode sheets. Button cells were then assembled in a glove box for testing. The counter electrode was sodium metal sheet, the separator was Whateman GF / D glass fiber, and the electrolyte was a 1.4 M NaPF6in (EC:DMC:EMC = 1:2:2) solution.

[0161] To strongly support the beneficial effects of the technical solutions of the embodiments of the present invention, the following performance tests are provided:

[0162] Particle size test:

[0163] The particle size of the material was tested using a Zhuhai OMEC laser particle size tester.

[0164] Specific surface area and pore size distribution test:

[0165] The specific surface area and micropore size distribution of the material were tested using a TriStar II 3030 analyzer (Micromeritics Instruments, Inc., USA) with CO2 as the atmosphere.

[0166] Compaction density test:

[0167] The compaction density measured by the automatic compaction density meter UTM7305 at a pressure of 3T is the compaction density measured in this application.

[0168] Material micromorphology test:

[0169] A Hitachi S-4800 high-magnification scanning electron microscope was used to test the micromorphology of the materials.

[0170] Closed cell bulk density test:

[0171] The closed-cell bulk density can be calculated as follows: closed-cell bulk density = 1 / true density - 1 / 2.26. The true density of the material was measured using an American Micromeritics AccuPyc II 1340 analyzer.

[0172] Ash test:

[0173] Use a box furnace or muffle furnace to test the ash content of the material and keep it at 900℃ for 2h.

[0174] Electrochemical performance test:

[0175] The battery was tested for the first cycle of charge and discharge using the Wuhan Blue Electric battery testing system, with a voltage range of 0.001 to 2.0 V.

[0176] The physical and electrochemical performance test data of the negative electrode materials of each embodiment and comparative example are shown in Table 1. The closed pore size distribution diagram of the negative electrode material of Example 1 is shown in Table 1. Figure 5 As shown, the charge and discharge curve of the negative electrode material of Example 1 is as shown in FIG. Figure 6 shown.

[0177] Table 1 Physical and electrochemical properties of negative electrode materials of various examples and comparative examples

[0178]

[0179] It can be seen from Table 1 that the negative electrode materials prepared by the methods of each embodiment have a specific surface area of ​​<10m 2 / g, ash content <0.3%, 3T compacted density 1.04-1.10 g / cm 3 The average pore size is 0.54-0.58 nm, and the closed pore volume density is 0.20-0.24 cm 3 / g, as a negative electrode material for sodium ion batteries, the charge capacity is >323mAh / g, the initial efficiency is >90%, and the 200-week capacity retention rate is >99%, showing excellent electrochemical performance. The coal-derived precursor is oxidatively modified through electrolysis reaction to increase surface and edge functional groups. Nano-scale carbon dots are further used to cross-link with the modified precursor to inhibit the graphitization of the hard carbon material during high-temperature carbonization, increase the degree of disorder, form abundant closed pores, and improve the sodium storage performance of the hard carbon material. The soft carbon coating layer modifies the surface of the material, reduces the material specific surface area, and improves the initial efficiency of the hard carbon material, resulting in excellent performance of the negative electrode material.

[0180] Compared with Example 1, Comparative Example 1 does not add carbon dots, lacks cross-linking of carbon dots with coal-based precursors, and the material has poor disorder, resulting in fewer closed pores and larger pore sizes, which affects the sodium storage performance of the hard carbon composite material.

[0181] Compared with Example 1, comparative example 2 does not add asphalt. The lack of asphalt will make the hard carbon specific surface area larger, reduce the first effect of the material, and affect the cycle performance.

[0182] Compared with Example 1, in Comparative Example 3, asphalt is added in excess. Asphalt is fluid at high temperatures, and a small amount of asphalt will form a coating layer on the surface of the hard carbon. If the asphalt is excessive, it will act as a binder between the hard carbon particle layers, bonding small particles together to form large particles, which increases the average particle size. After high-temperature carbonization, the asphalt has a structure of soft carbon with a small interlayer spacing and poor sodium storage performance. Excessive asphalt will reduce the capacity of the hard carbon composite material.

[0183] Compared with Example 1, in Comparative Example 4, excessive carbon dots are added. The carbon dots can adjust the closed-pore structure, but the carbon dot concentration is too high, that is, excessive carbon dots, which will cause the coal-based hard carbon to have more unclosed pores, more material defects, and an increased specific surface area, affecting the capacity and initial effect of the hard carbon composite material.

[0184] Compared to Example 1, Comparative Example 5 did not perform an electrolytic reaction. Instead, the composite precursor sheet was placed in a mixed acid, stirred for reaction, and the reaction product was separated, dried, and sintered in stages to obtain the negative electrode material. Without the electrolytic reaction, the composite precursor was not sufficiently oxidatively modified, resulting in a negative electrode material structure that tended toward a soft carbon structure, poor impurity removal, and a high ash content, which affected the negative electrode material's capacity, initial efficiency, and cycle performance.

[0185] Compared to Example 1, Comparative Example 6 uses a smaller amount of mixed acid. The mixed acid solution serves as the electrolyte. Sulfuric acid, with its high conductivity and stability, can act as an electrolyte, providing an ionic environment for the electrolysis reaction and ensuring the smooth progress of the electrolysis process. Nitric acid has strong oxidizing properties and can oxidatively modify the composite precursor during electrolysis. Insufficient acid usage can result in an insufficient electrochemical reaction, affecting the electrochemical properties of the final material.

[0186] Compared with Example 1, the voltage of the electrochemical reaction in Comparative Example 7 is 1 V. The lower voltage will cause the electrolysis reaction process to proceed more slowly and the reaction degree to be insufficient, which is similar to Comparative Example 6 and affects the electrochemical properties of the final material.

[0187] Compared with Example 1, the voltage of the electrochemical reaction in Comparative Example 8 is 50V. The higher voltage will make the electrolysis reaction process occur faster. The coal-based precursor reacts violently with the electrolyte in a short period of time, causing the coal-based precursor to be over-oxidized and produce a large number of defects. More SEI films are produced during the charge and discharge process, affecting the capacity, first efficiency and cycle performance of the final material.

[0188] Compared with Example 1, Comparative Example 9 does not perform segmented sintering, and the temperature is raised from room temperature to 1200°C in one step. The asphalt softens faster during the heating process and has poor fluidity. It cannot spread better on the hard carbon surface to achieve uniform coating of the hard carbon particles. The hard carbon material has a larger specific surface area, which affects the primary effect of the material.

[0189] Compared with Example 1, the first stage sintering temperature of Comparative Example 10 is lower than R0. This situation is similar to Comparative Example 9. When the temperature is kept below R0, the asphalt basically does not soften. Then the temperature is raised to 1200°C. The asphalt softens faster during the heating process and has poor fluidity, which affects the coating effect. The hard carbon material has a larger specific surface area, which affects the initial effect of the material.

[0190] 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 or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

[0191] Furthermore, those skilled in the art will appreciate that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and to form distinct embodiments. For example, in the claims above, any of the claimed embodiments may be used in any combination. The information disclosed in this background section is intended solely to enhance understanding of the overall background of this application and should not be construed as an admission or any implication that such information constitutes prior art known to those skilled in the art.

Claims

1. A negative electrode material, characterized in that The negative electrode material comprises a core and a soft carbon coating layer coated on the surface of the core, wherein the core comprises hard carbon and carbon dots distributed inside and / or on the surface of the hard carbon; closed pores are distributed inside the hard carbon; The preparation method of the negative electrode material comprises: Coal powder, asphalt and carbon dots are solid-phase mixed and hot-pressed to form a composite precursor sheet; The composite precursor sheet is used as an anode, the inert electrode is used as a cathode, and the mixed acid solution is used as an electrolyte to perform an electrochemical reaction under a constant voltage; Separating, washing and drying the electrolysis product obtained by the electrochemical reaction from the electrolyte to obtain a modified composite precursor; The modified composite precursor is sintered in stages under an inert atmosphere to obtain the negative electrode material.

2. The negative electrode material according to claim 1, characterized in that The pore diameter of the closed pores is 0.35-0.85 nm, and the volume density of the closed pores is 0.03-0.27 cm 3 / g.

3. The negative electrode material according to claim 1, characterized in that The ash content of the negative electrode material is less than 0.3%.

4. The negative electrode material according to any one of claims 1 to 3, characterized in that At least one of the following conditions is met: A. The average particle size of the negative electrode material is 3-20 μm; B. The specific surface area of ​​the negative electrode material is less than 10 m 2 / g; C. The 3T compaction density of the negative electrode material is 1.00-1.12 g / cm 3 ; D. the hard carbon is spherical; E. The average particle size of the carbon dots is 1-20 nm; F. The oxygen content of the carbon dots is 30%-70%.

5. A method for preparing a negative electrode material according to any one of claims 1 to 4, characterized in that: include: Coal powder, asphalt and carbon dots are solid-phase mixed and hot-pressed to form a composite precursor sheet; The composite precursor sheet is used as an anode, the inert electrode is used as a cathode, and the mixed acid solution is used as an electrolyte to perform an electrochemical reaction under a constant voltage; Separating, washing and drying the electrolysis product obtained by the electrochemical reaction from the electrolyte to obtain a modified composite precursor; The modified composite precursor is sintered in stages under an inert atmosphere to obtain the negative electrode material.

6. The preparation method according to claim 5, characterized in that At least one of the following conditions is met: A. The particle size of the pulverized coal is 3-20 μm; B. the pulverized coal comprises at least one of anthracite, sub-bituminous coal, bituminous coal and lignite; C. The particle size of the asphalt is 1-5 μm; D. the asphalt comprises at least one of petroleum asphalt, coal tar asphalt and natural asphalt; E. The softening point R0 of the asphalt satisfies 180°C ≤ R0 ≤ 320°C; F. The carbon dots include at least one of carbon quantum dots, graphene quantum dots, and polymer quantum dots; G. The average particle size of the carbon dots is 1-20 nm, and the oxygen content is 30%-70%; H. The mass ratio of the coal powder, the asphalt and the carbon dots is (94.5-98.4): (1.5-5): (0.1-0.5); I. The solid phase mixing includes any one of VC mixing, triple eccentric mixing, and ball milling mixing; J. The thickness of the composite precursor sheet is 0.5 cm-1.0 cm.

7. The preparation method according to claim 5, characterized in that At least one of the following conditions is met: A. The inert electrode includes a mixed metal electrode formed by one or more of titanium, platinum, and iridium; B. the mixed acid solution is a mixed solution of sulfuric acid and nitric acid; C. When condition B is met, the mass percentage concentration of the sulfuric acid is 98%, the mass percentage concentration of the nitric acid is 30%-60%, and the volume ratio of the sulfuric acid to the nitric acid is 3:1; D. the amount of the mixed acid solution added is greater than or equal to the mass of the coal powder; E. The range of the constant voltage is 5-30V; F. Maintain high-speed stirring during the reaction, and the linear speed of the high-speed stirring is 10-20m / s.

8. The preparation method according to claim 5, characterized in that At least one of the following conditions is met: A. The separation method includes any one of centrifugal separation and membrane filtration; B. The washing is repeated several times with deionized water until the pH of the washing solution is 6.5-7.5; C. The drying method includes spray drying, fluidized bed drying, vacuum drying or forced air drying, or a combination thereof; D. the inert atmosphere comprises at least one of nitrogen, argon and helium; E. The staged sintering includes a first stage sintering and a second stage sintering. The heating rate of the first stage sintering is 0.5-2°C / min, the insulation temperature of the first stage sintering is the softening point of the asphalt R0+20°C, and the insulation time of the first stage sintering is 0.5-1h; the heating rate of the second stage sintering is 1-5°C / min, the insulation temperature of the second stage sintering is 1100-1400°C, and the insulation time of the second stage sintering is 2-5h.

9. A negative electrode sheet, characterized in that: The negative electrode material comprises the negative electrode material according to any one of claims 1 to 4.

10. A sodium ion battery, characterized in that: Including the negative electrode sheet according to claim 9.

Images