Negative electrode composite materials and their preparation methods, negative electrode sheets and sodium-ion batteries

Abstract

This invention relates to the field of sodium-ion battery technology, and in particular to a negative electrode composite material and its preparation method, a negative electrode sheet, and a sodium-ion battery. The negative electrode composite material comprises a cotton fiber-based carbon material with FeP and Fe7S8 on its surface. The negative electrode composite material provided by this invention has a high pore volume, which can serve as a channel for sodium ion insertion / extraction, and exhibits high specific capacity and high initial coulombic efficiency as a negative electrode material for sodium-ion batteries.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a negative electrode composite material and its preparation method, a negative electrode sheet, and a sodium-ion battery. Background Technology

[0002] Due to their high energy density, power output, excellent safety, and long cycle life, lithium-ion batteries (LIBs) are widely used in the new energy field. The global grid battery energy storage market is projected to grow at an annual rate of 23% or higher by 2030. However, lithium (Li) is a very limited resource in the Earth's crust, comprising only 0.0017 wt%. With the vigorous development of the new energy sector, the demand for lithium-ion batteries is increasing, leading to a supply shortage and potentially causing a major crisis for lithium-ion batteries in the future.

[0003] Therefore, there is a need to focus on developing energy storage devices using abundant and sustainable elements on Earth, such as sodium-ion batteries (SIBs), which are considered the most suitable battery technology to replace lithium-ion batteries. Sodium (Na) is cheaper and more abundant than lithium, estimated to account for 2.8 wt% of the total Na concentration in the Earth's crust. Furthermore, sodium and lithium share many similarities in their physical and chemical properties, making sodium-ion batteries a highly advantageous alternative.

[0004] However, because the atomic radius of sodium (0.102 nm) is much larger than that of lithium (0.076 nm), this significantly restricts the transport of sodium ions and compromises the structural stability of the material during charge-discharge cycles. Therefore, sodium-ion battery anode materials require larger interlayer spacing to accommodate the diffusion and transport of sodium ions. Currently, hard carbon is the mainstream anode material for sodium-ion batteries. However, conventional hard carbon exhibits poor specific capacity and initial coulombic efficiency. Therefore, there is an urgent need to develop high-performance sodium-ion battery anode materials with high specific capacity and high initial coulombic efficiency. Summary of the Invention

[0005] To address the aforementioned shortcomings in the existing technology, the present invention aims to provide a negative electrode composite material and its preparation method, a negative electrode sheet, and a sodium-ion battery. The negative electrode composite material provided by the present invention has a high pore volume, which can serve as a channel for sodium ion insertion / extraction, and exhibits high specific capacity and high initial coulombic efficiency as a negative electrode material for sodium-ion batteries.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a negative electrode composite material, the negative electrode composite material comprising a cotton fiber-based carbon material having FeP and Fe7S8 on its surface.

[0008] The negative electrode composite material provided by this invention has a high pore volume, which can serve as a channel for sodium ion insertion and extraction, and exhibits high specific capacity and high initial coulombic efficiency as a negative electrode material for sodium-ion batteries.

[0009] Furthermore, the negative electrode composite material includes cotton fiber-based carbon material and FeP and Fe7S8 loaded on the cotton fiber-based carbon material;

[0010] And / or, the negative electrode composite material is a sodium-ion battery negative electrode composite material;

[0011] And / or, in the negative electrode composite material, the mass fraction of FeP is 5% to 20%, and the mass fraction of Fe7S8 is 5% to 30%;

[0012] And / or, the FeP is obtained by reacting cotton fiber-based carbon / Fe3C composite material with a phosphorus source, and the Fe7S8 is obtained by reacting cotton fiber-based carbon / Fe3C composite material with a sulfur source. The reaction temperature is 500-700℃, and the mass ratio of the cotton fiber-based carbon / Fe3C composite material, the phosphorus source, and the sulfur source is 1:(1-20):(0.5-2). The phosphorus source includes NaH2PO2. . At least one of H2O and red phosphorus, wherein the sulfur source includes sulfur powder;

[0013] And / or, the cotton fibers include waste cotton fabric.

[0014] In a second aspect, the present invention provides a method for preparing the negative electrode composite material as described in the first aspect, the method comprising the following steps:

[0015] S1. Soak the cotton fibers in an alkaline solution for treatment. After taking them out, wash the cotton fibers with water until the pH value of the filtrate is in the range of 6-8.

[0016] S2. Soak the cotton fibers treated in step S1 in an iron-containing compound solution, shake them, remove them and dry them for later use.

[0017] S3. Under a protective gas atmosphere, the sample treated in step S2 is calcined and ground to obtain cotton fiber-based carbon / Fe3C composite material.

[0018] S4. Place the cotton fiber-based carbon / Fe3C composite material into a small crucible, and simultaneously place the small crucible into a large crucible containing a phosphorus source and a sulfur source. Calcinate the crucible under a protective gas atmosphere, and then grind it to obtain the negative electrode composite material.

[0019] Furthermore, in step S1, the cotton fibers are cut into small pieces with a length and width of 4cm*4cm-6cm*6cm and soaked in the alkaline solution for 30min-60min.

[0020] And / or, in step S1, the mass of the cotton fiber is 2.5g-3.5g;

[0021] And / or, the base includes at least one of sodium hydroxide and potassium hydroxide;

[0022] And / or, the solid content of the alkaline solution is 10%-25%;

[0023] And / or, in step S1, after the washing, the cotton fibers are vacuum dried at a temperature of 90-120°C for 10-15 hours.

[0024] Furthermore, the iron-containing compound includes Fe(NO3)3;

[0025] And / or, the concentration of iron ions in the iron-containing compound solution is 0.5 mol / L to 2 mol / L;

[0026] And / or, in step S2, an isothermal oscillator is used for oscillation, the oscillation temperature is 20℃-30℃, and the oscillation time is 8h-24h;

[0027] And / or, in step S2, a vacuum drying oven is used for drying, the drying temperature is 60℃-80℃, and the drying time is 12h-24h.

[0028] Furthermore, in step S3, the protective gas includes nitrogen;

[0029] And / or, in step S3, the calcination includes heating to 600-900°C at a heating rate of 2-5°C / min and holding at that temperature for 2-4 hours.

[0030] Furthermore, in step S4, the phosphorus source and the sulfur source are located at different positions in the large crucible, and the stacking height of the phosphorus source and the sulfur source is not higher than the height of the small crucible;

[0031] And / or, in step S4, the volume ratio of the small crucible to the large crucible is 1:3-1:4;

[0032] And / or, the small crucible is left uncovered, while the large crucible is covered.

[0033] Furthermore, in step S4, the protective gas includes nitrogen, and the calcination includes heating to 500-700°C at a heating rate of 2-5°C / min and holding at that temperature for 4-8 hours.

[0034] And / or, in step S4, the mass ratio of the cotton fiber-based carbon / Fe3C composite material, the phosphorus source, and the sulfur source is 1:(1-20):(0.5-2).

[0035] Thirdly, the present invention provides a negative electrode sheet, the negative electrode sheet comprising:

[0036] Negative electrode current collector layer; and

[0037] A negative electrode material layer is disposed on the surface of the negative electrode current collector layer, and the negative electrode material layer includes the negative electrode composite material described in the first aspect or the negative electrode composite material prepared by the preparation method described in the second aspect.

[0038] Fourthly, the present invention provides a sodium-ion battery, the sodium-ion battery comprising the negative electrode sheet described in the third aspect.

[0039] Compared with the prior art, the beneficial effects of the present invention include at least one of the following:

[0040] (1) The negative electrode composite material provided by the present invention has a high pore volume, which can serve as a channel for sodium ion insertion and extraction, and exhibits high specific capacity and high initial coulombic efficiency as a negative electrode material for sodium ion batteries.

[0041] (2) The negative electrode composite material provided by the present invention exhibits good rate performance and cycle stability as a negative electrode material for sodium-ion batteries.

[0042] (3) This invention uses common waste cotton as raw material, which greatly reduces the manufacturing cost.

[0043] (4) The negative electrode carbon material of the present invention is calcined at low temperature after adsorbing an iron source to obtain a cotton fiber-based carbon@Fe3C material with a high degree of graphitization. After the composite material is calcined at low temperature for a second time, a composite material with iron phosphide and iron sulfide active substances on the surface is obtained, which can improve the sodium storage capacity of the carbon material, thereby improving the specific capacity of the carbon material.

[0044] (5) Carbon materials serve as the substrate to provide the carbon source, while iron phosphide and iron sulfide serve as active materials to coat the surface of the carbon materials, providing high porosity and improving the ICE (first coulomb efficiency) and rate performance of the carbon materials.

[0045] (6) Compared with other processes, the entire process adopts a low-temperature calcination method, which greatly reduces the synthesis cost and is conducive to industrialization. Attached Figure Description

[0046] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0047] Figure 1 This is a flowchart of the preparation method of the negative electrode composite material provided in Embodiment 1 of the present invention;

[0048] Figure 2 This is a schematic diagram showing the positions of raw materials in the preparation method of the negative electrode composite material provided in Example 1 of the present invention;

[0049] Figure 3 The XRD pattern of the negative electrode composite material prepared in Embodiment 1 of the present invention;

[0050] Figure 4 The cycling spectrum of the negative electrode composite material provided in Example 1 of this invention at 2 A / g;

[0051] Figure 5 The images show the SEM images of the negative electrode composite material prepared in Example 1 of this invention at 10,000x (a) and 5,000x (b) magnification. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0053] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0054] In a first aspect, the present invention provides a negative electrode composite material, the negative electrode composite material comprising a cotton fiber-based carbon material having FeP and Fe7S8 on its surface.

[0055] The negative electrode composite material provided by this invention has a high pore volume, which can serve as a channel for sodium ion insertion and extraction, and exhibits high specific capacity and high initial coulombic efficiency as a negative electrode material for sodium-ion batteries.

[0056] In one optional embodiment of the above-mentioned negative electrode composite material, the negative electrode composite material includes cotton fiber-based carbon material and FeP and Fe7S8 coated on the surface of the cotton fiber-based carbon material.

[0057] In one optional embodiment of the above-mentioned negative electrode composite material, the negative electrode composite material includes cotton fiber-based carbon material and FeP and Fe7S8 loaded on the cotton fiber-based carbon material.

[0058] In one optional embodiment of the above-mentioned negative electrode composite material, the negative electrode composite material is a sodium-ion battery negative electrode composite material.

[0059] In the above-mentioned negative electrode composite material, as an optional embodiment, the mass fraction of FeP in the negative electrode composite material is 5% to 20% (for example, it can be 5%, 10%, 15% or 20%), and the mass fraction of Fe7S8 is 5% to 30% (for example, it can be 5%, 10%, 15%, 20%, 25% or 30%).

[0060] In the above-mentioned negative electrode composite material, as an optional embodiment, the FeP is obtained by reacting cotton fiber-based carbon / Fe3C composite material with a phosphorus source, and the Fe7S8 is obtained by reacting cotton fiber-based carbon / Fe3C composite material with a sulfur source. The reaction temperature is 500-700℃ (for example, it can be 500℃, 550℃, 600℃, 650℃, or 700℃). The mass ratio of the cotton fiber-based carbon / Fe3C composite material, the phosphorus source, and the sulfur source is 1:(1-20):(0.5-2) (for example, it can be 1:5:1.5, 1:10:1, or 1:15:0.5).

[0061] In the aforementioned negative electrode composite material, as an optional embodiment, the cotton fiber includes waste cotton cloth. The role of waste cotton in the negative electrode composite material provided by this invention is twofold: firstly, it serves as a carbon source; secondly, it has strong adsorption capacity and can uniformly adsorb Fe. 3+ From a technological perspective, choosing waste cotton as a carbon source only requires alkali treatment of the raw materials before calcination to generate the carbon source. The process is simple, and waste cotton has strong adsorption properties, which can effectively adsorb active substances. From a production perspective, it is chosen because of its low cost and its common and readily available nature in daily life.

[0062] In the above-mentioned negative electrode composite material, as an optional embodiment, the phosphorus source includes NaH2PO2. . At least one of H2O and red phosphorus, preferably NaH2PO2 for improved safety. . H2O.

[0063] In the above-mentioned negative electrode composite material, as an optional embodiment, the sulfur source includes sulfur powder.

[0064] In a second aspect, the present invention provides a method for preparing the negative electrode composite material as described in the first aspect, the method comprising the following steps:

[0065] S1. Soak the cotton fibers in an alkaline solution for treatment. After taking them out, wash the cotton fibers with water until the pH value of the filtrate is in the range of 6-8.

[0066] S2. Soak the cotton fibers treated in step S1 in an iron-containing compound solution, shake them, remove them and dry them for later use.

[0067] S3. Under a protective gas atmosphere, the sample treated in step S2 is calcined and ground to obtain cotton fiber-based carbon / Fe3C composite material (Fe3C particles are coated inside the cotton fiber-based carbon).

[0068] S4. Place the cotton fiber-based carbon / Fe3C composite material into a small crucible, and simultaneously place the small crucible into a large crucible containing a phosphorus source and a sulfur source. Calcinate the crucible under a protective gas atmosphere, and then grind it to obtain the negative electrode composite material.

[0069] This invention removes natural impurities from cotton fibers, as well as sizing agents, oils, and contaminants that may have been added to the cotton fibers during processing, by soaking them in an alkaline solution.

[0070] In step S4, the present invention employs a gas-phase synthesis method, in which the sulfur and phosphorus sources in the large crucible are sublimated by calcination to form a gas that reacts with the composite material in the small crucible to synthesize a cotton fiber-based carbon material with a surface rich in FeP and Fe7S8 coating.

[0071] In the above-mentioned method for preparing negative electrode composite material, as an optional embodiment, in step S1, the cotton fiber is cut into small pieces with a length and width of 4cm*4cm-6cm*6cm and soaked in the alkaline solution for 30min-60min, for example, 30min, 40min, 50min or 60min.

[0072] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, in step S1, the mass of the cotton fiber is 2.5g-3.5g, for example, it can be 2.5g, 3g or 3.5g.

[0073] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, the alkali includes at least one of sodium hydroxide and potassium hydroxide.

[0074] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, the solid content of the alkaline solution is 10%-25%, for example, it can be 10%, 15%, 20% or 25%.

[0075] In the above-mentioned method for preparing negative electrode composite material, as an optional embodiment, in step S1, after the water washing, the cotton fiber is vacuum dried at a temperature of 90-120℃ (for example, 90℃, 100℃, 110℃ or 120℃) for 10-15 hours (for example, 10 hours, 11 hours, 12 hours, 14 hours or 15 hours).

[0076] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, the iron-containing compound includes Fe(NO3)3.

[0077] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, the concentration of iron ions in the iron-containing compound solution is 0.5 mol / L-2 mol / L, for example, it can be 0.5 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L.

[0078] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, in step S2, a constant temperature oscillator is used for oscillation. The oscillation temperature is 20℃-30℃, for example, 20℃, 22℃, 24℃, 26℃, 28℃ or 30℃, and the oscillation time is 8h-24h, for example, 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h or 24h.

[0079] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, in step S2, a vacuum drying oven is used for drying. The drying temperature is 60℃-80℃, for example, 60℃, 65℃, 70℃, 75℃ or 80℃, and the drying time is 12h-24h, for example, 12h, 14h, 16h, 18h, 20h, 22h or 24h.

[0080] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, in step S3, the protective gas includes nitrogen.

[0081] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, in step S3, the calcination includes heating to 600-900℃ (e.g., 600℃, 700℃, 800℃ or 900℃) at a heating rate of 2-5℃ / min (e.g., 2℃ / min, 3℃ / min, 4℃ / min or 5℃ / min), and holding at that temperature for 2-4 hours.

[0082] In the above-described method for preparing the negative electrode composite material, as an optional embodiment, in step S4, the phosphorus source and the sulfur source are located at different positions within the large crucible, and the stacking height of the phosphorus source and the sulfur source is not higher than the height of the small crucible. This embodiment, by limiting the different positions of the phosphorus source and the sulfur source within the large crucible, can prevent the phosphorus source and sulfur source from reacting in the molten state to generate byproducts after mixing.

[0083] The volume ratio of the small crucible to the large crucible can be determined based on the size of the muffle furnace space and the suitability of the two crucibles. In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, in step S4, the volume ratio of the small crucible to the large crucible is 1:3-1:4, for example, the volume of the small crucible is 30ml and the volume of the large crucible is 100ml.

[0084] In the above-mentioned method for preparing negative electrode composite material, as an optional embodiment, the small crucible is not covered, while the large crucible is covered.

[0085] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, in step S4, the protective gas includes nitrogen, and the calcination includes heating to 500-700℃ (e.g., 500℃, 550℃, 600℃, 650℃, 700℃) at a heating rate of 2-5℃ / min (e.g., 2℃ / min, 3℃ / min, 4℃ / min, or 5℃ / min), and holding at that temperature for 4-8 hours (e.g., 4 hours, 5 hours, 6 hours, or 8 hours). This invention involves sublimating phosphorus and sulfur sources into a gaseous phase under high-temperature conditions and then combining them with a cotton fiber-based carbon material containing Fe3C. The heating rate affects the sublimation state of the phosphorus and sulfur sources. Different products are obtained from calcination at excessively high or low temperatures, such as obtaining FeP2 or FeS phases.

[0086] In the above-mentioned method for preparing the negative electrode composite material, as an optional embodiment, in step S4, the mass ratio of the cotton fiber-based carbon / Fe3C composite material, the phosphorus source, and the sulfur source is 1:(1-20):(0.5-2) (for example, it can be 1:5:1.5, 1:10:1, or 1:15:0.5).

[0087] Thirdly, the present invention provides a negative electrode sheet, the negative electrode sheet comprising:

[0088] Negative electrode current collector layer; and

[0089] A negative electrode material layer is disposed on the surface of the negative electrode current collector layer, and the negative electrode material layer includes the negative electrode composite material described in the first aspect or the negative electrode composite material prepared by the preparation method described in the second aspect.

[0090] Fourthly, the present invention provides a sodium-ion battery, the sodium-ion battery comprising the negative electrode sheet described in the third aspect.

[0091] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0092] Example 1

[0093] This embodiment provides a negative electrode composite material, which includes waste cotton cloth-based carbon material containing FeP and Fe7S8 on its surface. The mass fraction of FeP in the negative electrode composite material is 10.14%, and the mass fraction of Fe7S8 is 25.33%.

[0094] The method for preparing the negative electrode composite material provided in this embodiment includes the following steps:

[0095] S1. Cut 3g of waste cotton cloth into small pieces of 4cm*4cm and soak them in 18% solid content NaOH solution for 30min. At the same time, prepare 1mol / L Fe(NO3)3 solution.

[0096] Waste cotton cloth soaked in NaOH solution was treated with deionized water, washed until the pH of the filtrate was neutral (pH value 7), and then dried under vacuum (vacuum degree -0.1Mpa) at 100℃ for 12 hours.

[0097] S2. Soak the dried waste cotton cloth in a 1 mol / L Fe(NO3)3 solution and maintain it at 25℃ for 8 hours using a constant temperature shaker. After taking it out, put it in a vacuum drying oven and dry it at 60℃ under vacuum (vacuum degree of -0.1Mpa) for 12 hours for later use.

[0098] S3. The dried waste cotton cloth from step S2 is calcined in a muffle furnace under a nitrogen atmosphere. During the calcination process, the temperature is increased to 700℃ at a heating rate of 2℃ / min and held for 2 hours. After that, it is cooled to ambient temperature and ground in an agate mortar to obtain a carbon material (waste cotton cloth-based carbon / Fe3C composite material) with Fe3C coated inside the waste cotton cloth.

[0099] S4. Place the composite material prepared in step S3 into a small crucible with a volume of 30 ml, and place the small crucible on one side with NaH2PO2. . In a large crucible containing H2O and sulfur powder (100 ml on the other side), NaH2PO2 . The height of the H2O and sulfur powder piled up is less than the height of the small crucible. The small crucible is not covered, while the large crucible is covered (e.g., ...). Figure 2 As shown), the composite material is NaH2PO2. .The mass ratio of H2O to sulfur powder was 1:5:1.5. The mixture was calcined in a muffle furnace under nitrogen protection. During calcination, the temperature was increased to 600℃ at a rate of 2℃ / min and held for 6 hours. After cooling to ambient temperature, the material was ground in an agate mortar to obtain a waste cotton cloth-based carbon material (negative electrode composite material) with a surface rich in FeP and Fe7S8. The specific preparation flow chart is shown below. Figure 1 As shown.

[0100] Figure 3 The XRD pattern of the negative electrode composite material prepared in this embodiment (XRD pattern of cotton fiber-based carbon material doped with FeP-Fe7S8) is shown below. Figure 3 It can be seen that the negative electrode composite material includes FeP and Fe7S8 phases, proving that a composite material containing FeP and Fe7S8 has been synthesized.

[0101] Figure 4 The cycling spectrum of the negative electrode composite material provided in this embodiment at 2 A / g is obtained from... Figure 4 It can be seen that the capacity retention rate of the negative electrode composite material is 90.45% after 480 cycles. The charge and discharge at a current density of 0.1A / g for the first 10 cycles is to activate the battery.

[0102] Figure 5 The SEM images of the negative electrode composite material prepared in this embodiment under 10000x (a) and 5000x (b) magnification are provided by [the image source]. Figure 5 It can be seen that FeP and Fe7S8 particles are evenly distributed on the surface of waste cotton cloth.

[0103] Example 2

[0104] This embodiment provides a negative electrode composite material, including waste cotton cloth-based carbon material with FeP and Fe7S8 on its surface. The mass fraction of FeP in the negative electrode composite material is 13.76%, and the mass fraction of Fe7S8 is 21.46%.

[0105] The preparation method of the negative electrode composite material provided in this embodiment is basically the same as that in Example 1, except that in step S1, 3g of waste cotton is cut into small pieces of 6cm*6cm, and the solid content of the NaOH solution is 10%; in step S2, it is kept at 25℃ for 12h and then vacuum dried for 20h; in step S4, the composite material is NaH2PO2. . The mass ratio of H2O to sulfur powder is 1:10:1.

[0106] Example 3

[0107] This embodiment provides a negative electrode composite material, including waste cotton cloth-based carbon material with FeP and Fe7S8 on its surface. The mass fraction of FeP in the negative electrode composite material is 19.38%, and the mass fraction of Fe7S8 is 17.41%.

[0108] The preparation method of the negative electrode composite material provided in this embodiment is basically the same as that in Example 1, except that in step S4, the composite material is NaH2PO2. . The mass ratio of H2O to sulfur powder is 1:15:0.5.

[0109] Example 4

[0110] This embodiment provides a negative electrode composite material, including waste cotton cloth-based carbon material with FeP and Fe7S8 on its surface. The mass fraction of FeP in the negative electrode composite material is 7.45%, and the mass fraction of Fe7S8 is 21.21%.

[0111] The preparation method of the negative electrode composite material provided in this embodiment is basically the same as that in Example 1, except that the heating rate in step S4 is 5℃ / min.

[0112] Comparative Example 1

[0113] This comparative example provides a method for preparing a negative electrode composite material, which includes the following steps:

[0114] S1. Cut 3g of waste cotton cloth into small pieces of 4cm*4cm and soak them in 18% solid content NaOH solution for 30min. At the same time, prepare 1mol / L Fe(NO3)3 solution.

[0115] Waste cotton cloth soaked in NaOH solution was treated with deionized water, washed until the pH of the filtrate was neutral (pH value 7), and then dried under vacuum (vacuum degree -0.1Mpa) at 100℃ for 12 hours.

[0116] S2. Soak the dried waste cotton cloth in a 1 mol / L Fe(NO3)3 solution and maintain it at 25℃ for 8 hours using a constant temperature shaker. After taking it out, put it in a vacuum drying oven and dry it at 60℃ under vacuum (vacuum degree of -0.1Mpa) for 12 hours for later use.

[0117] S3. The dried waste cotton cloth from step S2 is calcined in a muffle furnace under a nitrogen atmosphere. During the calcination process, the temperature is increased to 700℃ at a heating rate of 2℃ / min and held for 2 hours. After that, it is cooled to ambient temperature and ground in an agate mortar to obtain a carbon material (waste cotton cloth-based carbon / Fe3C composite material) with Fe3C coated inside the waste cotton cloth.

[0118] S4. Place the composite material prepared in step S3 into a small crucible with a volume of 30 ml, and place NaH2PO2 on both sides of the crucible. . In a large crucible with a volume of 100 ml for H2O, NaH2PO2 . The H2O buildup height is less than the height of the small crucible. The small crucible is uncovered, while the large crucible is covered. The composite material is NaH2PO2. . The mass ratio of H2O was 1:20 (at this ratio, the composite material can fully react with the phosphorus source), and the material was calcined in a muffle furnace under nitrogen protection. During the calcination process, the temperature was increased to 600℃ at a heating rate of 2℃ / min and held for 6 hours. After that, it was cooled to ambient temperature and ground in an agate mortar to obtain a waste cotton cloth-based carbon material with FeP-rich surface.

[0119] Comparative Example 2

[0120] This comparative example provides a method for preparing a negative electrode composite material, which includes the following steps:

[0121] S1-S3 are exactly the same as in Example 1.

[0122] S4. Place the composite material prepared in step S3 into a small crucible with a volume of 30 ml, and place the small crucible into a large crucible with a volume of 100 ml on both sides, where sulfur powder is placed. The height of the sulfur powder pile is less than the height of the small crucible. The small crucible is not covered, while the large crucible is covered. The mass ratio of composite material to sulfur powder is 1:2 (at this ratio, the composite material can fully react with the sulfur source). Calcination is carried out in a muffle furnace under nitrogen protection. During calcination, the temperature is increased to 600℃ at a heating rate of 2℃ / min and held for 6 hours. After that, it is cooled to ambient temperature and ground in an agate mortar to obtain a waste cotton cloth-based carbon material with a surface rich in Fe7S8.

[0123] Comparative Example 3

[0124] This comparative example provides a method for preparing a negative electrode composite material, which includes the following steps:

[0125] S1. Cut 3g of waste cotton cloth into small pieces of 4cm*4cm and soak them in 18% solid content NaOH solution for 30min. At the same time, prepare 1mol / L Fe(NO3)3 solution.

[0126] Waste cotton cloth soaked in NaOH solution was treated with deionized water, washed until the pH of the filtrate was neutral (pH value 7), and then dried under vacuum (vacuum degree -0.1Mpa) at 100℃ for 12 hours.

[0127] S2. Soak the dried waste cotton cloth in a 1 mol / L Fe(NO3)3 solution and maintain it at 25℃ for 8 hours using a constant temperature shaker. After taking it out, put it in a vacuum drying oven and dry it at 60℃ under vacuum (vacuum degree of -0.1Mpa) for 12 hours for later use.

[0128] S3. The dried waste cotton cloth from step S2 is calcined in a muffle furnace under a nitrogen atmosphere. During the calcination process, the temperature is increased to 700℃ at a heating rate of 2℃ / min and held for 2 hours. After that, it is cooled to ambient temperature and ground in an agate mortar to obtain a carbon material (waste cotton cloth-based carbon / Fe3C composite material) with Fe3C coated inside the waste cotton cloth.

[0129] Comparative Example 4

[0130] This comparative example provides a method for preparing a negative electrode composite material, which includes the following steps:

[0131] S1. Cut 3g of waste cotton cloth into small pieces of 4cm*4cm and soak them in 18% solid content NaOH solution for 30min. At the same time, prepare 1mol / L Fe(NO3)3 solution.

[0132] Waste cotton cloth soaked in NaOH solution was treated with deionized water, washed until the pH of the filtrate was neutral (pH value 7), and then dried under vacuum (vacuum degree -0.1Mpa) at 100℃ for 12 hours.

[0133] S2. The dried waste cotton cloth from step S1 is calcined in a muffle furnace under a nitrogen atmosphere. During the calcination process, the temperature is increased to 700°C at a rate of 2°C / min and held for 2 hours. After that, it is cooled to ambient temperature and ground in an agate mortar to obtain waste cotton cloth-based carbon material (hard carbon material).

[0134] Comparative Example 5

[0135] This comparative example provides a method for preparing a negative electrode composite material, which is basically the same as that in Example 1, except that step S3 in Example 1 is not included. Specifically, it includes:

[0136] S1. Cut 3g of waste cotton cloth into small pieces of 4cm*4cm and soak them in 18% solid content NaOH solution for 30min. At the same time, prepare 1mol / L Fe(NO3)3 solution.

[0137] Waste cotton cloth soaked in NaOH solution was treated with deionized water, washed until the pH of the filtrate was neutral (pH value 7), and then dried under vacuum (vacuum degree -0.1Mpa) at 100℃ for 12 hours.

[0138] S2. Soak the dried waste cotton cloth in a 1 mol / L Fe(NO3)3 solution and maintain it at 25℃ for 8 hours using a constant temperature shaker. After taking it out, put it in a vacuum drying oven and dry it at 60℃ under vacuum (vacuum degree of -0.1Mpa) for 12 hours for later use.

[0139] S3. Place the sample obtained in step S2 into a small crucible with a volume of 30 ml, and place the small crucible on one side with NaH2PO2. . In a large crucible containing H2O and sulfur powder (100 ml on the other side), NaH2PO2 . The height of the H2O and sulfur powder piled up is less than the height of the small crucible. The small crucible is not covered, while the large crucible is covered (e.g., ...). Figure 2 As shown), the composite material is NaH2PO2. . The mass ratio of H2O to sulfur powder is 1:5:1.5. The mixture is calcined in a muffle furnace under nitrogen protection. During the calcination process, the temperature is increased to 600℃ at a heating rate of 2℃ / min and held for 6 hours. After cooling to ambient temperature, the mixture is ground in an agate mortar to obtain a waste cotton cloth-based carbon material (negative electrode composite material) with a surface rich in FeP and Fe7S8.

[0140] Comparative Example 6

[0141] This comparative example provides a method for preparing a negative electrode composite material, which is basically the same as that in Example 1, except that in step S4, the composite material prepared in step S3 and NaH2PO2 are mixed. . H2O and sulfur powder were placed in three ceramic boats in a mass ratio of 1:5:1.5. Then, from upstream to downstream of the airflow, a magnetic boat containing sulfur powder and a boat containing NaH2PO2 were placed in sequence. . H2O magnetic boats and magnetic boats containing composite materials are calcined under nitrogen protection. During the calcination process, the temperature is raised to 600℃ at a heating rate of 2℃ / min and held for 6 hours. After that, the temperature is cooled to ambient temperature and ground in an agate mortar to obtain waste cotton cloth-based carbon material (negative electrode composite material) with FeP and Fe7S8 on the surface.

[0142] Performance testing

[0143] The pore volume of the negative electrode composite materials prepared in the examples and comparative examples was tested using the physical adsorption-desorption test method (BET test method), and the results are shown in Table 1.

[0144] The negative electrode composite materials prepared in the examples and comparative examples were assembled into button batteries, and the assembly process is as follows:

[0145] Negative electrode preparation: The negative electrode composite materials prepared in the examples and comparative examples were used as negative electrode carbon. 0.8g of negative electrode carbon and 0.1g of Super P were manually ground and mixed in an agate mortar. The mixture was then poured into a small centrifuge tube, and 0.05g of CMC powder and 0.125g of SBR (40% solid content) adhesive were added. The mixture was then ball-milled in a small ball mill to prepare a slurry. The slurry was evenly coated onto copper foil and vacuum dried. The dried negative electrode sheet was sliced ​​to obtain 12mm negative electrode discs for later use, with a loading of 1.5mg / cm³. 2 ;

[0146] Electrolyte preparation: 1.0M sodium hexafluorophosphate was used in a mixed solution of DME = 100 vol%.

[0147] Separator preparation: Glass fiber film is punched into 16mm diameter sheets to serve as separators;

[0148] Positive electrode preparation: A commercially available 12mm sodium sheet was used as the positive electrode;

[0149] Assembly: In an argon-filled glove box, the negative electrode shell, sodium sheet, electrolyte, separator, electrolyte, negative electrode plate, gasket, spring contact, and positive electrode shell are stacked sequentially and sealed using a sealing machine. Before sealing, approximately 8 μL of electrolyte is added, and then the battery is encapsulated in a battery case to complete the battery assembly. The assembled button battery is then subjected to electrochemical performance testing using the following method:

[0150] Cyclic performance test: The constant current charge-discharge test method was adopted, the voltage test range was 0-3V, and after 10 cycles at a current density of 0.1A / g, a long-term cycle performance test was conducted at a current density of 2A / g. After 480 cycles, the capacity retention rate was calculated as: Capacity retention rate = Discharge capacity at 480 cycles / Discharge capacity at 11 cycles * 100%.

[0151] 0.1A / g discharge specific capacity test: The constant current charge-discharge test method was adopted, the voltage test range was 0-3V, the test current density was 0.1A / g, and the average discharge specific capacity of the negative electrode material in the first 3 tests was calculated.

[0152] 2A / g discharge specific capacity test: The constant current charge-discharge test method was adopted, the voltage test range was 0-3V, the test current density was 2A / g, and the average discharge specific capacity of the negative electrode material in the first 3 tests was calculated.

[0153] ICE test (first coulombic efficiency): The constant current charge-discharge test method is adopted. Under the voltage range of 0-3V and the current density of 0.1A / g, the first coulombic efficiency is calculated. First coulombic efficiency = first charge capacity / first discharge capacity.

[0154] The test results are shown in Table 1.

[0155] Table 1

[0156]

[0157]

[0158] From Table 1, it can be seen at least that:

[0159] (1) Comparing Examples 1-4 with Comparative Example 4, it can be seen that the negative electrode composite material provided by the present invention, by loading FeP and Fe7S8, has a high pore volume, which can serve as a channel for sodium ion insertion and extraction. As a negative electrode material for sodium-ion batteries, it exhibits high specific capacity, high initial coulombic efficiency and good rate performance, and excellent long-cycle stability.

[0160] (2) Comparing Example 1 and Comparative Example 1, it can be seen that when only FeP is loaded on the waste cotton cloth-based carbon material, although the initial coulombic efficiency is better, the discharge specific capacity is poor, and it is impossible to simultaneously achieve both the initial coulombic efficiency and the specific capacity.

[0161] (3) Comparing Example 1 and Comparative Example 2, it can be seen that when only Fe7S8 is loaded on the waste cotton cloth-based carbon material, the long-term cycle stability is poor.

[0162] (4) Comparing Example 1 and Comparative Example 3, it can be seen that when the negative electrode material is Fe3C carbon material coated in waste cotton cloth, the initial coulombic efficiency, specific capacity and long cycle stability all decrease significantly.

[0163] (5) Comparing Example 1 and Comparative Example 5, it can be seen that when only one calcination is performed, the initial coulombic efficiency decreases significantly, especially the long-term cycle stability decreases more significantly. The inventors found that the one-step calcination method results in uneven distribution of FeP and Fe7S8 doped on the surface of waste cotton cloth, which will affect the cycle performance of the battery in the later stage. However, the two-step calcination method first synthesizes the active material in the carbon-coated state of waste cotton cloth, and then the two-step calcination makes the coated active material more uniformly converted into FeP and Fe7S8 particles in situ. At the same time, it is not easy to control the temperature when directly synthesizing composite materials by one-step calcination.

[0164] (6) Comparing Example 1 and Comparative Example 6, it can be seen that when sulfur powder is placed sequentially in a magnetic boat containing NaH2PO2 from upstream to downstream of the airflow, .When preparing anode materials using H2O magnetic boats and magnetic boats containing composite materials, although the specific capacity and initial coulombic efficiency are superior, the long-term cycling stability is significantly reduced. The inventors found that the preparation method provided in Comparative Example 6 leads to a low FeP content and an excessive Fe7S8 content in the anode composite material. FeP plays a role in maintaining the stability of the composite material and is beneficial to its cycling stability. If the Fe7S8 content is too high, it will lead to poor overall cycling performance of the material. Moreover, the long-term cycling stability of the anode material obtained by this preparation method is worse than that of the anode material with only Fe7S8 loaded in Comparative Example 2, indicating that the preparation method itself will also reduce the long-term cycling stability of the anode material. The preparation method provided in Example 1 of this invention can ensure the uniformity of phosphorus and sulfur doping by adjusting the ratio of phosphorus source and sulfur source, thereby simultaneously taking into account specific capacity and long-term cycling stability.

[0165] In summary, the cotton fiber-based carbon composite material prepared according to the preparation method of the present invention has a lower specific surface area and the powder has a higher pore volume, which can serve as a channel for sodium ion insertion and extraction. Compared with the carbon material obtained by conventional cotton textile calcination, it has better ICE, higher specific capacity and excellent rate performance.

[0166] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for producing a negative electrode composite material, characterized by, The preparation method comprises the following steps: S1, treating cotton fibers by immersing them in an alkali solution, and after taking out, washing the cotton fibers with water until the pH value of the filtrate is in the range of 6-8; S2, immersing the cotton fibers treated in step S1 in a solution containing iron compounds, oscillating, and after taking out, drying for standby; S3, calcining the sample treated in step S2 under a protective gas atmosphere, and after grinding, obtaining a cotton fiber-based carbon / Fe3C composite material; S4, placing the cotton fiber-based carbon / Fe3C composite material into a small crucible, at the same time, placing the small crucible in a large crucible containing a phosphorus source and a sulfur source, and under a protective gas atmosphere, calcining, and after grinding, obtaining the negative electrode composite material; The negative electrode composite material comprises a cotton fiber-based carbon material containing FeP and Fe7S8 on the surface; In the negative electrode composite material, the mass fraction of FeP is 7.45%-10.14%, and the mass fraction of Fe7S8 is 21.21%-25.33%; In step S4, the phosphorus source and the sulfur source are located at different positions in the large crucible, and the accumulation height of the phosphorus source and the sulfur source is not higher than the height of the small crucible; The small crucible is not covered with a cover, and the large crucible is covered with a cover.

2. The method of claim 1, wherein the negative electrode composite is prepared by mixing the active material, the binder, and the conductive agent. In step S1, the cotton fibers are cut into small pieces with a length and width of 4cm*4cm-6cm*6cm, and are immersed in the alkali solution for 30min-60min; And / or, in step S1, the mass of the cotton fibers is 2.5g-3.5g; And / or, the alkali comprises at least one of sodium hydroxide and potassium hydroxide; And / or, the solid content of the alkali solution is 10%-25%; And / or, in step S1, after the water washing, the cotton fibers are vacuum dried, the temperature is 90-120℃, and the time is 10-15 hours; And / or, the negative electrode composite material comprises a cotton fiber-based carbon material and FeP and Fe7S8 loaded on the cotton fiber-based carbon material; And / or, the negative electrode composite material is a sodium ion battery negative electrode composite material; And / or, the FeP is obtained by reacting the cotton fiber-based carbon / Fe3C composite material with a phosphorus source, and the Fe7S8 is obtained by reacting the cotton fiber-based carbon / Fe3C composite material with a sulfur source, the reaction temperature is 500-700℃, the mass ratio of the cotton fiber-based carbon / Fe3C composite material, the phosphorus source and the sulfur source is 1:(1-20):(0.5-2), the phosphorus source comprises at least one of NaH2PO2.H2O and red phosphorus, and the sulfur source comprises sulfur powder; And / or, the cotton fibers comprise waste cotton cloth.

3. The method of claim 1, wherein the negative electrode composite is prepared by mixing the carbon material, the binder, and the lithium metal oxide. The iron-containing compound comprises Fe(NO3)3; And / or, the concentration of iron ions in the iron-containing compound solution is 0.5mol / L-2mol / L; And / or, in step S2, a constant temperature oscillator is used for oscillation, the oscillation temperature is 20℃-30℃, and the oscillation time is 8h-24h; And / or, in step S2, a vacuum drying oven is used for drying, the drying temperature is 60℃-80℃, and the drying time is 12h-24h.

4. The method of claim 1, wherein the negative electrode composite is prepared by mixing the active material, the binder, and the conductive material. In step S3, the protective gas comprises nitrogen. In step S3, the calcination comprises heating at a temperature increasing rate of 2-5℃ / min to 600-900℃, and holding for 2-4h.

5. The method of claim 1, wherein the negative electrode composite is prepared by mixing the carbon material, the binder, and the electrolyte solution. In step S4, the volume ratio of the small crucible to the large crucible is 1:3-1:

4.

6. The method of claim 1, wherein the negative electrode composite is prepared by mixing the carbon material, the binder, and the electrolyte solution. In step S4, the protective gas comprises nitrogen, and the calcination comprises heating at a temperature increasing rate of 2-5℃ / min to 500-700℃, and holding for 4-8h. In step S4, the mass ratio of the cotton fiber-based carbon / Fe3C composite material, the phosphorus source, and the sulfur source is 1:(1-20):(0.5-2).

7. A negative electrode sheet characterized by comprising: The negative electrode sheet comprises: a negative electrode current collector layer; and a negative electrode material layer disposed on the surface of the negative electrode current collector layer, the negative electrode material layer comprising a negative electrode composite material prepared by the preparation method of any one of claims 1-6.

8. A sodium-ion battery, characterized in that, The sodium ion battery comprises the negative electrode sheet of claim 7.

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