Preparation method and application of cobalt tetroxide-coated carbon self-supporting negative electrode material

Abstract

The present invention belongs to the field of batteries, and specifically relates to a method for preparing a cobalt tetroxide-coated carbon self-supporting negative electrode material and its application; the method for preparing the cobalt tetroxide-coated carbon self-supporting negative electrode material uses cobalt acetate as a precursor, uses a cellulose sheet as a template and a carbon source, utilizes a surface sol-gel method, assembles a cobalt hydroxide gel layer layer by layer on the surface of a cellulose fiber, and then combines a heat treatment process of carbonization in an argon atmosphere and oxidation in air to obtain a cobalt tetroxide-coated carbon self-supporting negative electrode material; wherein the number of layer-by-layer assembly cycles is 10-50 times; the cobalt tetroxide-coated carbon self-supporting negative electrode material is directly used as a negative electrode of a lithium ion battery or a sodium ion battery after slicing. The cobalt tetroxide-coated carbon self-supporting negative electrode prepared by the present invention has the advantages of large specific capacity, high coulombic efficiency, and good cycle stability.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries, and in particular relates to a preparation method and application of a cobalt tetroxide-coated carbon self-supporting negative electrode material. Background Art

[0002] Among secondary energy storage batteries, lithium-ion batteries have become the dominant power source for portable electronic products and electric vehicles due to their high energy density, strong power performance, no memory effect, and environmental friendliness. Complementary to limited lithium resources, sodium-ion batteries have the advantages of abundant sodium reserves, high safety performance, and good low-temperature performance. Although their energy density is lower than that of lithium-ion batteries, they are expected to be improved through modification. The physical and chemical properties of electrode materials are one of the key factors affecting battery performance. Compared with positive electrode materials, negative electrode materials provide higher capacity for batteries and affect the performance of lithium / sodium-ion batteries to a greater extent. Currently commercial carbon negative electrode materials are limited by their relatively low theoretical specific capacity (372 mAh g -1 ). In order to meet the needs of multiple fields for high energy and power density, long cycle life lithium / sodium ion batteries, the research and development of new negative electrode materials has important practical significance and application value.

[0003] Among the many candidate negative electrode materials, conversion-type negative electrode materials represented by transition metal oxides have the outstanding advantage of high theoretical specific capacity. Among them, the theoretical specific capacity of cobalt tetroxide (Co3O4) negative electrode material is as high as 890 mAh g -1 , which is about 2.4 times that of commercial carbon negative electrode materials. However, the inherent structural instability, serious volume effect and poor conductivity of Co3O4 during charging and discharging have seriously limited its practical application and development. In recent years, the modification strategies for Co3O4-based negative electrode materials have mainly focused on two aspects: one is to construct micro-nano structures, using microscopic gaps to increase the capacity to accommodate volume changes in materials during charging and discharging and shorten the diffusion path of lithium ions or sodium ions; the other is to regulate the chemical composition and exert a synergistic effect through multi-component hybridization, such as using carbon composites to construct C / Co3O4 negative electrode materials to increase conductivity and alleviate volume effects.

[0004] Most of the current Co3O4-based negative electrodes still use the traditional pulping and coating method for electrode preparation, that is, the Co3O4-based active material and the conductive agent and the binder are mixed in a certain proportion, and an appropriate amount of solvent is added to make a slurry and coated on the current collector. The steps are complicated, the cost is high, the energy consumption is high, and there is a "dead volume / mass" that reduces the overall energy density of the battery. In contrast, the Co3O4-based self-supporting negative electrode has the huge advantages of integration and integration. However, the current Co3O4-based self-supporting negative electrode materials mostly use carbon materials such as graphene and carbon cloth as self-supporting substrates, and are prepared by hydrothermal methods, electrospinning and other methods. The cost is high and it is not convenient to finely control the microstructure of Co3O4 and its combination with the substrate.

[0005] Therefore, it is necessary to provide an improved technical solution to address the above-mentioned deficiencies in the prior art. Summary of the invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a method for preparing a cobalt tetroxide-coated carbon self-supporting negative electrode material and its application.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] A method for preparing a cobalt tetroxide-coated carbon self-supporting negative electrode material, using cobalt acetate as a precursor, using a cellulose sheet as a template and a carbon source, using a surface sol-gel method, assembling a cobalt hydroxide gel layer on the surface of the cellulose fiber layer by layer, and then performing a two-step heat treatment to obtain a cobalt tetroxide-coated carbon self-supporting negative electrode material;

[0009] The layer-by-layer assembly process is as follows: placing a cellulose sheet in a vacuum filtration device, adding a 90-290 mmol / L cobalt hydroxide solution, vacuum filtering until the liquid level is higher than the surface of the cellulose sheet, letting it stand and then filtering out the remaining solution, then adding deionized water and letting it stand to fully hydrolyze the cobalt hydroxide and cross-link with the cellulose, then adding anhydrous ethanol and filtering and drying, thus completing a layer-by-layer assembly cycle; the above steps are repeated 10-50 times to obtain a cellulose sheet with a cobalt hydroxide gel layer deposited layer by layer on the surface, which is recorded as material A;

[0010] The two-step heat treatment process is: material A is clamped on both sides of a silicon wafer, first annealed and carbonized in an argon atmosphere, the annealing temperature is 570-630°C, the heating rate is 0.8-1.2°C / min, and the calcination time is 1.5-2.5 h; then, an oxidative heat treatment is performed in an air atmosphere, the heat treatment temperature is 280-320°C, the heating rate is 1.5-2.5°C / min, and the calcination time is 1.5-2.5 h, so as to obtain a black paper-like cobalt tetroxide-coated carbon self-supporting negative electrode material.

[0011] Furthermore, the preparation steps of the cobalt hydroxide solution are: preparing a cobalt acetate precursor solution at a concentration of 25-75 g / L, adding ammonia water with a concentration of 25%-28% to adjust the pH of the cobalt acetate precursor solution to 8-9, and stirring the obtained purple solution for 10-30 min to allow the cobalt acetate and ammonia water to fully react, so as to obtain a cobalt hydroxide solution with a molar concentration of 90-290 mmol / L.

[0012] Furthermore, by adding ammonia water dropwise into the cobalt acetate precursor solution, the pH value of the solution is adjusted to 8.4-8.6.

[0013] Furthermore, the pH value of the cobalt acetate precursor solution is adjusted to 8.5 by dripping ammonia water; and the number of cycles in the layer-by-layer assembly process is 30.

[0014] Furthermore, after the cellulose sheet is placed in a vacuum filtration device, it is first washed with anhydrous ethanol for 2 to 3 times and then vacuum dried.

[0015] Furthermore, in the vacuum filtration device, the ratio of the cellulose sheet to the added cobalt hydroxide solution is 0.005-0.017 g / mL.

[0016] Furthermore, the number of cycles in the layer-by-layer assembly process is 20-40.

[0017] Furthermore, in the two-step heat treatment process, the argon annealing carbonization temperature is 600°C, the heating rate is 1°C / min, and the calcination time is 2 h; the oxidation heat treatment temperature in air is 300°C, the heating rate is 2°C / min, and the calcination time is 2 h.

[0018] Furthermore, the cellulose sheet material has a multi-level porous network structure with fibers interwoven with each other at a microscopic level, and the cellulose sheet material is a quantitative filter paper, a pure cotton non-woven fabric or a water-wettable pure wood pulp paper towel.

[0019] The present invention also proposes the application of the aforementioned cobalt tetroxide-coated carbon self-supporting negative electrode material. After the cobalt tetroxide-coated carbon self-supporting negative electrode material is cut into small black discs of a specified diameter, it is directly used as the negative electrode of a lithium ion battery or a sodium ion battery.

[0020] The beneficial effects of the present invention are:

[0021] In terms of process, the present invention can uniformly and tightly coat the cobalt hydroxide gel on a single cellulose fiber through layer-by-layer assembly, and can accurately replicate the micron fiber interwoven three-dimensional porous network and nanofiber structure of the cellulose sheet. At the same time, by adjusting the number of assemblies, the thickness of the Co3O4 coating layer generated after the cobalt hydroxide gel is calcined can be controllably adjusted, thereby improving the electrochemical performance through fine regulation of the microstructure; the present invention obtains a Co3O4-coated carbon self-supporting material that retains the three-dimensional carbon fiber network derived from the cellulose sheet through a two-step heat treatment of carbonization followed by oxidation; the final state of the cellulose sheet not only retains the macroscopic sheet shape as a self-supporting carbon substrate, but the microscopic three-dimensional hierarchical porous network also provides a buffer space for the volume expansion of the Co3O4 particles, and the carbon fibers also prevent the coated Co3O4 particles from agglomerating, thereby enhancing the structural stability of the material, and providing a conductive network pathway to promote charge transfer, thereby improving the cycle and rate performance of the negative electrode.

[0022] In terms of performance, the Co3O4-coated carbon self-supporting negative electrode prepared by the present invention has the advantages of large specific capacity, high coulombic efficiency, good cycle stability, etc.

[0023] In terms of application, the finished product prepared by the present invention is in the shape of a black paper sheet on a macro scale, has certain flexibility and strong compressive resistance, can be directly sliced ​​and processed as a negative electrode for assembling lithium-ion batteries or sodium-ion batteries, without the need to add conductive agents, adhesives and current collector coatings, is more economical and environmentally friendly, and can reduce the weight of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. Among them:

[0025] Figure 1 The cyclic voltammetry curve of the cobalt tetroxide-coated carbon self-supporting negative electrode material prepared in Example 1 of the present invention applied to a lithium-ion battery.

[0026] Figure 2 The cyclic voltammetry curve of the cobalt tetroxide-coated carbon self-supporting negative electrode material prepared in Example 1 of the present invention applied to a sodium ion battery.

[0027] Figure 3 This is a graph showing the cycling performance of the cobalt tetroxide-coated carbon self-supporting negative electrode material prepared in Example 1 of the present invention when applied to a lithium-ion battery.

[0028] Figure 4 This is a cycle performance diagram of the cobalt tetroxide-coated carbon self-supporting negative electrode material prepared in Example 1 of the present invention when applied to a sodium ion battery.

[0029] Figure 5This is a rate performance diagram of the cobalt tetroxide-coated carbon self-supporting negative electrode material prepared in Example 1 of the present invention when applied to a lithium-ion battery.

[0030] Figure 6 This is a rate performance diagram of the cobalt tetroxide-coated carbon self-supporting negative electrode material prepared in Example 1 of the present invention and applied to sodium ion batteries.

[0031] Figure 7 The scanning electron microscope images of the cobalt tetroxide-coated carbon self-supporting negative electrode materials prepared in Example 2, Example 1, and Example 3 of the present invention at different magnifications; wherein,

[0032] (a)-(d) are scanning electron micrographs of C@Co3O4-Lb prepared in Example 2;

[0033] (e)-(h) are scanning electron micrographs of C@Co3O4-Mb prepared in Example 1;

[0034] (i)-(l) are scanning electron microscope images of C@Co3O4-Hb prepared in Example 3.

[0035] Figure 8 The transmission electron microscope images of the cobalt tetroxide-coated carbon self-supporting negative electrode materials prepared in Example 2, Example 1, and Example 3 of the present invention at different magnifications; wherein,

[0036] (a)-(c) are transmission electron microscopy images of C@Co3O4-Lb prepared in Example 2;

[0037] (d)-(f) are transmission electron micrographs of C@Co3O4-Mb prepared in Example 1;

[0038] (g)-(i) are transmission electron micrographs of C@Co3O4-Hb prepared in Example 3.

[0039] Fig. 9 The particle size distribution diagram of cobalt tetroxide in the cobalt tetroxide-coated carbon self-supporting negative electrode material prepared in Example 2, Example 1 and Example 3 of the present invention; wherein,

[0040] (a) is the particle size distribution diagram of C@Co3O4-Lb prepared in Example 2;

[0041] (b) is the particle size distribution diagram of C@Co3O4-Mb prepared in Example 1;

[0042] (c) is the particle size distribution diagram of C@Co3O4-Hb prepared in Example 3.

[0043] Fig.10The scanning electron microscope images of the cobalt tetroxide-coated carbon self-supporting negative electrode materials prepared in Example 4, Example 1, and Example 5 of the present invention at different magnifications; wherein,

[0044] (a)-(d) are scanning electron micrographs of C@Co3O4-Ma prepared in Example 4;

[0045] (e)-(h) are scanning electron micrographs of C@Co3O4-Mb prepared in Example 1;

[0046] (i)-(l) are scanning electron microscope images of C@Co3O4-Mc prepared in Example 5.

[0047] Fig.11 Transmission electron microscope images of the cobalt tetroxide-coated carbon self-supporting negative electrode materials prepared in Examples 4, 1 and 5 of the present invention at different magnifications; wherein,

[0048] (a)-(c) are transmission electron micrographs of C@Co3O4-Ma prepared in Example 4;

[0049] (d)-(f) are transmission electron micrographs of C@Co3O4-Mb prepared in Example 1;

[0050] (g)-(i) are transmission electron micrographs of C@Co3O4-Mc prepared in Example 5.

[0051] Fig.12 This is a photo of C@Co3O4-Mb prepared in Example 1 of the present invention.

[0052] Fig.13 This is a photo of the button battery negative electrode cut from C@Co3O4-Mb prepared in Example 1 of the present invention.

[0053] Fig.14 This is a photograph showing that the negative electrode of a button cell cut from C@Co3O4-Mb prepared in Example 1 of the present invention has flexibility. DETAILED DESCRIPTION

[0054] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field belong to the scope of protection of the present invention.

[0055] A method for preparing a cobalt tetroxide-coated carbon self-supporting negative electrode material, using cobalt acetate as a precursor, using a cellulose sheet as a template and a carbon source, and using a surface sol-gel method to assemble a cobalt hydroxide gel layer on the surface of cellulose micrometer and nanometer fibers layer by layer, and then combining a heat treatment process of carbonization in an argon atmosphere and oxidation in air to obtain a cobalt tetroxide-coated carbon self-supporting negative electrode material; wherein:

[0056] ① The layer-by-layer assembly process is as follows: a cobalt acetate precursor solution is prepared at a concentration of 25-75 g / L (for example, 25 g / L, 30 g / L, 35 g / L, 40 g / L, 45 g / L, 50 g / L, 55 g / L, 60 g / L, 65 g / L, 70 g / L or 75 g / L), and a 25%-28% ammonia solution is added dropwise to adjust the pH of the cobalt acetate precursor solution to 8-9 (for example, pH 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0), and the obtained purple solution is stirred for 10-30 min (for example, 10 min, 20 min, 25 min, 30 min) to allow cobalt acetate and ammonia to react fully to obtain a substance concentration of 90-290 mmol / L (for example, 90 1. Prepare a cobalt hydroxide solution of 100 mmol / L, 200 mmol / L, 100 mmol / L, 120 mmol / L, 150 mmol / L, 170 mmol / L, 190 mmol / L, 220 mmol / L, 260 mmol / L, and 290 mmol / L; place a cellulose sheet in a filtration device, wash it with ethanol 2 to 3 times, and dry it under vacuum; add cobalt hydroxide solution, the ratio of cellulose sheet to the added solution is 0.005-0.017 g / mL (for example, 0.005 g / mL, 0.006 g / mL, 0.008 g / mL, 0.010 g / mL, 0.012 g / mL, 0.014 g / mL, 0.015 g / mL, and 0.017 g / mL), vacuum filter until the liquid level is higher than the surface of the cellulose sheet, so that the cellulose sheet is always immersed in the solution, and let it stand for 3 hours. min, so that the cobalt hydroxide colloidal particles in the solution are fully adsorbed; then the remaining solution is filtered and dried, and deionized water is added and allowed to stand for 3 min to fully hydrolyze the cobalt hydroxide and cross-link with the cellulose in the cellulose sheet; finally, anhydrous ethanol is added and filtered for 10 min, and after the surface of the cellulose sheet is completely dried, a layer-by-layer assembly cycle is completed; the above steps are repeated 10-50 times to obtain a cellulose sheet with a layer-by-layer cobalt hydroxide gel layer deposited on the surface, which is recorded as material A;

[0057] ② The two-step heat treatment process is: clamp material A on both sides with a silicon wafer, place it in a tubular furnace and carbonize it under an argon atmosphere, the annealing temperature is 570-630 ° C, the heating rate is 0.8-1.2 ° C / min, and the calcination time is 1.5-2.5 h; then perform an oxidative heat treatment in an air atmosphere in a muffle furnace, the heat treatment temperature is 280-320 ° C, the heating rate is 1.5-2.5 ° C / min, and the calcination time is 1.5-2.5 h, so as to obtain a black paper-like cobalt tetroxide-coated carbon self-supporting negative electrode material.

[0058] The main components of the cobalt tetroxide-coated carbon self-supporting negative electrode material of the present invention are cobalt tetroxide (Co3O4) and cellulose sheet-derived carbon, and the cobalt tetroxide-coated cellulose sheet-derived carbon is used as a self-supporting negative electrode for lithium / sodium ion batteries; wherein the mass fraction of cobalt tetroxide is 60%-80%, and the carbon content is 20%-40%. Cobalt tetroxide, as a conversion negative electrode material, provides the main redox reaction and high capacity for the negative electrode. By adjusting its particle size and coating thickness, the charge and discharge volume effect can be alleviated and the charge transfer rate can be improved; cellulose-derived carbon fibers play the role of a self-supporting substrate, while allowing the self-supporting negative electrode to maintain the hierarchical porous microstructure of the cellulose material, providing a buffer space for the charge and discharge volume change of cobalt tetroxide, and as a highly conductive network, it is conducive to electron transfer and ion diffusion.

[0059] The invention relies on the layer-by-layer assembly technology of cellulose sheets, and adjusts the pH of the cobalt acetate precursor solution and the number of assemblies to control the Co3O4 particle size and coating thickness, thereby achieving the improvement of the electrochemical performance of the Co3O4-based self-supporting negative electrode. The Co3O4-based self-supporting negative electrode material obtained by the present invention has a unique hierarchical structure of natural cellulose material and a porous network structure with interwoven fibers at the microscopic level. It is in the shape of a black paper sheet at the macroscopic level, has a certain flexibility and strong compressive resistance, and (taking quantitative filter paper as a cellulose sheet as an example) the compressive resistance can reach 2255 kPa.

[0060] The two-step heat treatment process of carbonization followed by oxidation in the present invention has unique advantages: in the first step of carbonization, argon provides an inert environment, which can prevent the oxidation and decomposition of the three-dimensional fiber network of the cellulose sheet, and is conducive to maintaining the structural integrity of the cross-linked carbon fibers converted from cellulose, thereby providing good support and stabilization for the electrode structure; however, after calcination under an inert atmosphere, cobalt hydroxide cannot be completely converted into Co3O4 with a high theoretical specific capacity, but CoO (the theoretical specific capacity of CoO is low, only 715 mAh / g), forming a Co3O4 / CoO complex, so further treatment is required. The second step is oxidized at a lower temperature, which can completely oxidize the cobalt hydroxide gel layer assembled on the cellulose surface into Co3O4 with a high theoretical specific capacity and good crystallinity under the condition of retaining the self-supporting skeleton of cellulose-derived carbon fibers. Note that the order of the two-step heat treatment cannot be reversed. If oxidation is performed first, the cellulose sheet after direct oxidation will be partially decomposed, which is not conducive to the material maintaining its hierarchical fiber porous network-like microstructure. The self-supporting negative electrode material obtained after carbonization is easily crushed and cannot maintain good flexibility.

[0061] Furthermore, the cellulose sheet has a multi-level porous mesh structure with interwoven fibers at the microscopic level. The cellulose sheet is made of pure natural cellulose. Natural cellulose is the most widely distributed and largest natural material in nature. It comes from plants and has good environmental protection. The cellulose sheet made of pure natural cellulose ensures that there are no other impurities after calcination and carbonization; the multi-level structure ensures that the cellulose sheet after two-step heat treatment can still maintain a certain mechanical strength and is not easy to break. The present invention utilizes the abundant active hydroxyl groups on its surface, and uses the intermolecular interaction force to controllably assemble the transition metal oxide material, and then converts the filter paper fiber into a skeleton carbon fiber through carbonization to obtain a carbon paper-type self-supporting material with high compressive strength and good flexibility; it can also improve the overall electronic conductivity and structural stability of the negative electrode material, thereby improving the charge and discharge performance of the battery.

[0062] Specifically, the cellulose sheet is quantitative filter paper, pure cotton non-woven fabric or wettable pure wood pulp paper towel. In order to ensure that the cellulose sheet can withstand the negative pressure of suction filtration and the prepared self-supporting material has a strong compressive strength, the cellulose sheet has a pressure resistance of not less than 5240 kPa, a thickness of not less than 100 μm, and a minimum size of not less than the diameter of the suction funnel. The thickness of the quantitative filter paper commonly used in the laboratory is 150-400 μm, and the sizes are 9 cm, 12.5 cm, 15 cm, 18 cm, etc.; the present invention preferably uses quantitative filter paper as the cellulose sheet.

[0063] The present invention also proposes the application of the cobalt tetroxide-coated carbon self-supporting negative electrode material. After the cobalt tetroxide-coated carbon self-supporting negative electrode material is cut into small black discs of a specified diameter, it is directly used as the negative electrode of a lithium-ion battery or a sodium-ion battery. It is suitable for a variety of application scenarios that require high energy density and long cycle life. For example, after being cut into slices by a slicer, it can be used in portable electronic devices, electric vehicles, and energy storage systems.

[0064] The following will be described in detail with reference to the embodiments and comparative examples.

[0065] In the following examples and comparative examples, the cellulose sheet material is ash-free rapid quantitative filter paper with a diameter of 4 cm. Conventional 9 cm ash-free rapid quantitative filter paper is purchased and cut into a diameter of 4 cm to fit the 4 cm diameter suction funnel in the suction filtration device used in the test of this example.

[0066] Take 4 g of cobalt acetate powder and dissolve it in 80 mL of deionized water, add 25%-28% ammonia water, adjust the pH of the solution to 8.4-8.6, and stir the obtained purple solution for 20 min to allow the cobalt acetate and ammonia water to react fully to obtain a cobalt hydroxide solution with a molar concentration of 190 mmol / L. The quantitative filter paper is placed in a suction filtration device, first washed with ethanol 2 to 3 times, and vacuum-dried; a cobalt hydroxide solution is added, and the ratio of the filter paper to the added solution is 0.010 g / mL (0.08 g of a filter paper corresponds to about 8 mL of cobalt hydroxide solution). In this embodiment, 3 quantitative filter papers with a diameter of 4 cm are placed, and vacuum filtration is performed until the liquid level is higher than the surface of the quantitative filter paper fiber, so that the quantitative filter paper is always immersed in the solution, and the solution is allowed to stand for 3 minutes to fully adsorb the cobalt hydroxide colloidal particles in the solution; then the remaining solution is filtered and dried, and deionized water is added and allowed to stand for 3 minutes to fully hydrolyze the cobalt hydroxide and cross-link with the filter paper cellulose to form a uniform gel film coating; finally, anhydrous ethanol is added and filtered for 10 minutes. After the surface of the filter paper is completely dried, a layer-by-layer assembly cycle is completed (a layer of cobalt hydroxide film is deposited on the surface of the quantitative filter paper); the number of assembly cycles is 20-40 times, and different thicknesses of cobalt hydroxide films can be obtained with different numbers of assembly cycles, which are recorded as material A;

[0067] The prepared material A was then clamped on both sides with a silicon wafer with a diameter larger than the quantitative filter paper (a silicon wafer with a diameter of 5 cm is sufficient here), placed in a tubular furnace and carbonized in an argon atmosphere at a temperature of 600 °C, a heating rate of 1 °C / min, and a calcination time of 2 h; subsequently, it was subjected to an oxidative heat treatment in an air atmosphere in a muffle furnace at a temperature of 300 °C, a heating rate of 2 °C / min, and a calcination time of 2 h.

[0068] The present invention provides 5 embodiments, in each embodiment, only the pH of the cobalt acetate precursor solution and the number of layer-by-layer assembly are different, and the other steps are the same as the above steps; specifically:

[0069] Example 1: The pH of the cobalt acetate precursor solution is 8.5, and the number of layer-by-layer assembly is 30, which is recorded as C@Co3O4-Mb; Example 1 is the best example; Fig.12 This is a photo of C@Co3O4-Mb prepared in Example 1. In this photo, the shape of C@Co3O4-Mb is a circle with a diameter of 4 cm; Fig.13 This is a photo of a button cell negative electrode cut from C@Co3O4-Mb prepared in Example 1 of the present invention. In this photo, the diameter of the button cell negative electrode is 11 mm; Fig.14 This is a photograph showing the flexibility of the button battery negative electrode cut from C@Co3O4-Mb prepared in Example 1. The figure shows the button battery negative electrode bent by tweezers ( Figure 12-14 The tweezers in the figure are the same tweezers); It can be seen from these figures that the cobalt tetroxide-coated carbon self-supporting negative electrode material prepared using the quantitative filter paper as a template maintains the original shape of the quantitative filter paper and has good flexibility;

[0070] Example 2: The pH of the cobalt acetate precursor solution is 8.4, and the number of layer-by-layer assembly is 30, recorded as C@Co3O4-Lb;

[0071] Example 3: The pH of the cobalt acetate precursor solution is 8.6, and the number of layer-by-layer assembly is 30, recorded as C@Co3O4-Hb;

[0072] Example 4: The pH of the cobalt acetate precursor solution is 8.5, and the number of layer-by-layer assembly is 20, recorded as C@Co3O4-Ma;

[0073] Example 5: The pH of the cobalt acetate precursor solution is 8.5, and the number of layer-by-layer assembly is 40, recorded as C@Co3O4-Mc.

[0074] Compare the above examples 1-5 with the following comparative examples:

[0075] Comparative Example 1

[0076] In Comparative Example 1, the layer-by-layer assembly process is not used, and the material A is prepared by the immersion method; the other processes are the same as those in Example 1.

[0077] 4 g of cobalt acetate was dissolved in 80 mL of deionized water, and then 70 μL of ammonia solution with a concentration of 25%-28% was added, and the pH of the solution was adjusted to 8.5. The obtained purple solution was ultrasonicated for 20 min to allow cobalt acetate and ammonia to react fully to obtain a cobalt hydroxide solution with a molar concentration of 190 mmol / L, and then 10 quantitative filter papers were immersed in 80 mL of the above purple cobalt hydroxide solution for 24 h, and the ratio of filter paper to solution was 0.010 g / mL; the prepared material was clamped on both sides of a silicon wafer with a diameter of 5 cm, and carbonized in an argon atmosphere at a temperature of 600 °C, a heating rate of 1 °C / min, and a calcination time of 2 h; the oxidation heat treatment process was carried out in a muffle furnace at an oxidation temperature of 300 °C, a heating rate of 2 °C / min, and a calcination time of 2 h to obtain a cobalt tetraoxide-coated carbon self-supporting negative electrode material, recorded as C@Co3O4-SG.

[0078] Comparative Example 2

[0079] In Comparative Example 2, a one-step oxidation heat treatment is used to replace the two-step heat treatment process in Example 1. Other processes are the same as in Example 1.

[0080] The one-step oxidation heat treatment process is as follows: Material A prepared in Example 1 is directly placed in a muffle furnace in air for oxidation at a temperature of 300 °C, a heating rate of 2 °C / min, and a calcination time of 2 h. A Co3O4-coated filter paper fiber material is obtained, which is recorded as cellulose@Co3O4-OS.

[0081] Comparative Example 3

[0082] In Comparative Example 3, a one-step carbonization heat treatment is used to replace the two-step heat treatment process in Example 1. Other processes are the same as in Example 1.

[0083] The one-step carbonization heat treatment process is as follows: Material A prepared in Example 1 is directly placed in a tube furnace, carbonized in an argon atmosphere, the temperature is 600 °C, the heating rate is 1 °C / min, and the calcination time is 2 h. In this comparative example, cobalt hydroxide cannot be completely converted into Co3O4 with a high theoretical specific capacity (890 mAh / g), but CoO (the theoretical specific capacity of CoO is 715 mAh / g) exists, forming a Co3O4 / CoO complex, recorded as C@Co3O4 / CoO-CS.

[0084] Comparative Example 4

[0085] Comparative Example 4 adopts the sol-gel method without using a quantitative filter paper template, and no quantitative filter paper is used to provide a carbon source. The final product prepared by this method is a pure Co3O4 powder material.

[0086] Take 4 g of cobalt acetate and dissolve it in 80 mL of deionized water, then add 70 μL of ammonia solution with a concentration of 25%-28%, adjust the pH of the solution to 8.5, stir the purple solution for 20 min, make the cobalt acetate and ammonia water react fully, and obtain a cobalt hydroxide solution with a molar concentration of 190 mmol / L, vacuum filter, collect the colloidal product, and then dry it in a vacuum drying oven at 60°C for 12 h. The obtained powder is placed in a tubular furnace under argon atmosphere for carbonization, the temperature is 600°C, the heating rate is 1°C / min, and the calcination time is 2 h. The post-oxidation heat treatment process is carried out in a muffle furnace, the oxidation temperature is 300°C, the heating rate is 2°C / min, and the calcination time is 2 h, and a Co3O4 powder material is obtained.

[0087] In Comparative Examples 5 and 6, only the pH of the cobalt acetate precursor solution is different from that in Example 1, and the other steps are the same as those in Example 1; in Comparative Examples 7 and 8, only the number of times the cobalt acetate precursor solution is assembled layer by layer is different, and the other steps are the same as those in Example 1.

[0088] Comparative Example 5: The pH value of the cobalt acetate precursor solution is 8, and the number of layer-by-layer assembly is 30.

[0089] Comparative Example 6: The pH value of the cobalt acetate precursor solution is 9, and the number of layer-by-layer assembly is 30.

[0090] Comparative Example 7: The pH value of the cobalt acetate precursor solution is 8.5, and the number of layer-by-layer assembly is 10.

[0091] Comparative Example 8: The pH value of the cobalt acetate precursor solution is 8.5, and the number of layer-by-layer assembly is 50.

[0092] In order to detect the electrochemical performance of the prepared cobalt tetroxide-coated carbon self-supporting negative electrode, the final materials obtained in Examples 1-5 and Comparative Examples 1-8 were assembled into lithium / sodium ion button batteries for electrochemical performance testing.

[0093] ① Assembly of lithium-ion battery: Take a piece of composite material and cut it into small black discs with a diameter of 11 mm using a manual slicer. Use it as the negative electrode of the lithium-ion battery and assemble it into a button battery with metal lithium. The battery assembled by this method is a CR2025 button battery, the electrolyte is 1 mol / L LiPF6 (solvent EC:EDC=1:1 vol%), and the two electrodes are separated by a Celgard 2300 diaphragm. In order to prevent poor contact inside the battery, a piece of nickel foam is added to one end of the metal lithium sheet as a gasket; the battery assembly process is carried out in a glove box filled with argon gas, and the water and oxygen content in the glove box is not higher than 0.1ppm.

[0094] ②Assembly of sodium ion battery: Take a piece of composite material, cut it into small black discs with a diameter of 11 mm using a manual slicer, and directly use it as the negative electrode of the sodium ion battery to assemble it into a button battery with metallic sodium. The sodium ion battery of the present invention is assembled as a CR2032 button battery, the diaphragm is a glass fiber diaphragm (diameter 16 mm), the electrolyte is 1 mol / L NaClO4 (solvent EC:PC=1:1 vol%), in order to prevent poor contact inside the battery, a gasket and a shrapnel are added to one end of the metallic sodium sheet; the assembly is carried out in a glove box filled with argon gas, and the water and oxygen content in the glove box is not higher than 0.1 ppm.

[0095] ③ Preparation of electrode sheets by pulp coating method and assembly of batteries: The Co3O4 powder material prepared in Comparative Example 4 was mixed with the conductive agent acetylene black and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1 and ground under an infrared lamp for 1 h to ensure uniform mixing. Then, N-methylpyrrolidone (NMP) and acetone were added to the mixed powder, and stirring continued under infrared lamp irradiation until a honey-like slurry was formed; the prepared slurry was evenly coated on a circular nickel foam with a diameter of 11 mm, and placed in a vacuum drying oven at 80 ° C for 12 h; the dried nickel foam was placed on a roller press and pressed into a thickness of 0.25 mm, that is, the required electrode sheet was prepared. The assembly of lithium / sodium ion button batteries is the same as the assembly methods of ① and ②.

[0096] Electrochemical performance tests include cyclic voltammetry (CV) curves, cycle performance tests and rate performance tests.

[0097] ①CV curve: The instrument used for the cyclic voltammetry test in the present invention is Shanghai Chenhua CHI660E electrochemical workstation. The voltage range is set to 0.01-3.0 V, the scan rate is 0.1 mV / s, and 3 cycles of charge and discharge operation are performed to obtain the CV curve.

[0098] ② Cycle performance: The instrument used for the cycle performance test in the present invention is the CT-4008 Xinwei battery test system. The battery is tested for cycle performance in a constant current charge and discharge mode, and the test environment temperature is 25°C. The test voltage range in the present invention is 0.01-3.0 V, and the cycle performance test is performed at a current density of 0.1 A / g.

[0099] ③ Rate performance: The instrument used for the rate performance test in the present invention is the CT-4008 Xinwei battery test system, and the battery is tested for rate performance in a constant current charge and discharge mode, and the test environment temperature is 25 ° C. The test voltage range of the present invention is 0.01-3.0 V, and the rate performance test is performed at current densities of 0.1 A / g, 0.2 A / g, 0.5 A / g, 1 A / g, and 0.1 A / g, respectively.

[0100] The test results are shown in Table 1, Table 2, Figures 1 to 11 shown.

[0101] Table 1 Comparison of electrochemical performance of various materials prepared in Examples and Comparative Examples for application in lithium-ion batteries

[0102]

[0103] From the above results, it can be seen that the data of the lithium-ion batteries assembled in Examples 1 to 5 all have good results, with a 100-cycle discharge capacity retention rate of 45.61%-60.16%, and a capacity retention rate and a reversible discharge specific capacity that are higher than all the comparative examples. -1 At a current density of 1.5 %, the first discharge capacity can reach 11.27 mAh cm -2 (1788 mAh g -1 ), the first coulombic efficiency is 60.0%, and the specific capacity remains at 7.10 mAhcm after 50 cycles. -2 (1126 mAh g -1 ), the specific capacity remains at 6.78 mAh cm after 100 cycles. -2 (1076 mAh g -1 ). Compared with other literature (Performance and morphology of centrifugally spun Co3O4 / C composite fibers for anode materials in lithium-ion batteries, DOI: 10.1007 / s10853-021-06285-3), at a current density of 0.1 A / g, the specific capacity is maintained at 632 mAh / g after 100 cycles, and the present invention exhibits better lithium storage performance. Compared with the test results disclosed in patent CN110683589A (at a current density of 0.1 A / g, the first discharge specific capacity is greater than 1332 mAh / g, and after 50 cycles, the charge and discharge specific capacity is retained at 998 mAh / g), the present invention exhibits better lithium storage electrochemical performance.

[0104] Table 2 Comparison of electrochemical performance of various materials prepared in Examples and Comparative Examples for sodium ion batteries

[0105]

[0106] As can be seen from Table 2, the data of the sodium ion batteries assembled in Examples 1 to 5 all have good results, and the 100-cycle discharge capacity retention rate is 32.23%-49.57%, which is better than all the comparative examples. Among them, the sodium ion battery assembled with the cobalt tetroxide-coated carbon self-supporting negative electrode material of the best example 1 can achieve a first discharge specific capacity of 4.45 mAh cm -2 (706mAh g -1 ), the first coulombic efficiency is 53.97%, and the specific capacity remains at 2.24 mAh cm after 50 cycles. -2 (356 mAh g -1 ), and the specific capacity is stable at 2.21 mAh cm after 100 cycles. -2 (350 mAh g -1 ), the cobalt tetroxide-coated carbon self-supporting negative electrode prepared by the present invention exhibits excellent cycle stability performance in sodium ion batteries.

[0107] In addition, when comparing Example 1 to Example 3, the number of assemblies is the same and the pH value of the cobalt acetate precursor solution changes, the Figures 7 and 8 As can be seen from the figure, with the increase of pH of the cobalt acetate precursor solution ( Figure 7 , Figure 8 The pH values ​​of the cobalt acetate precursor solutions from top to bottom are 8.4, 8.5, and 8.6, respectively. The particle size of the Co3O4 particles coated on the fiber surface increases; Fig. 9 It can be seen that when the pH of the cobalt acetate precursor solution is 8.4, 8.5, and 8.6, the average particle size of the Co3O4 particles coated on the carbon fiber is 5.87 nm, 8.42 nm, and 9.82 nm, respectively. Therefore, by adjusting the pH, the fine regulation of the particle size of the Co3O4 particles can be achieved. From the performance test results, the pH of the cobalt acetate precursor solution in Example 1 is 8.5, which can achieve the best effect. In addition, the pH of the cobalt acetate precursor solution in Comparative Examples 5 and 6 is 8 and 9, respectively, but the test results are all at a relatively low level, which shows that controlling the particle size of the Co3O4 particles within a suitable range is very important for increasing the lithium / sodium storage sites and improving the electronic and ionic conductivity.

[0108] Comparing Example 1, Example 4 and Example 5, the pH of their cobalt acetate precursor solutions is 8.5, and the number of layer-by-layer assembly is 30, 20, and 40, respectively. Fig.10 and Fig.11 It can be seen that the fiber surface becomes rougher after more layers of coating. Fig.11The average Co3O4 coating thickness of the carbon fiber after 20, 30, and 40 assembly times was measured to be 128.4 nm, 208.7 nm, and 297.7 nm, respectively, indicating that the fine regulation of the nano-coating layer can be achieved by controlling the number of layer-by-layer assembly cycles. Comparing Example 1 (30 assembly times), Examples 4-5 (20 and 40 assembly times), and Comparative Examples 7-8 (10 and 50 assembly times), under the premise that the pH of the cobalt acetate precursor solution is 8.5, with the increase of the number of layer-by-layer assembly times, the various properties show a pattern of first increasing and then decreasing, and the best performance can be obtained when the number of layer-by-layer assembly times is 30.

[0109] Comparing Example 1, Comparative Example 1 and Comparative Example 4, in Comparative Example 1, quantitative filter paper was soaked in cobalt hydroxide solution before the two-step heat treatment, while in Comparative Example 4, quantitative filter paper was not used, and only cobalt hydroxide solution was used before the two-step heat treatment, and finally pure Co3O4 powder was obtained. The first coulomb efficiency, specific capacity after 50 and 100 cycles, and capacity retention rate in Tables 1 and 2 are all better in Example 1 than in Comparative Example 1, and Comparative Example 1 is better than Comparative Example 4. This shows that if the layer-by-layer assembly process is not adopted but the immersion method is used, the microscopic morphology of the coated Co3O4 cannot be accurately controlled, and its bonding with carbon fiber is poor, then the test results are significantly reduced, indicating that the present invention is better than the conventional immersion method; and if cellulose sheets are not used, the electrode sheets made by coating Co3O4 powder obviously do not have a porous network structure at the microscopic level, which will lead to a reduction in lithium / sodium storage active sites, and the Co3O4 particles lack carbon fiber support. During the charge and discharge process, the volume changes dramatically and is easy to agglomerate, resulting in the collapse of the electrode structure and even cracks. Therefore, the electrochemical performance of the lithium / sodium ion battery assembled therein is the worst among the three.

[0110] Comparing Example 1 with Comparative Examples 2-3, the performance results are from high to low in the order of Example 1, Comparative Example 3, and Comparative Example 2, indicating that although the cellulose sheet can achieve better results than the oxidation heat treatment alone by carbonization heat treatment, they are not as good as the treatment effect of the two-step heat treatment process of the present invention. This is because in Comparative Example 2, since only oxidation was performed, the final product obtained is the negative electrode material of Co3O4-coated filter paper cellulose. The partial oxidation and decomposition of the filter paper fibers causes the Co3O4-coated cellulose material to lose its good three-dimensional hierarchical porous network structure, resulting in a reduction in lithium / sodium storage active sites, and the cellulose itself has poor conductivity, and because the oxidation calcination temperature is low, the Co3O4 crystallinity is poor, the material specific capacity is low, and the decay is fast, so Comparative Example 2 has the worst effect. In Comparative Example 3, the inert atmosphere prevents the oxidation and decomposition of natural cellulose filter paper fibers, and the Co3O4 / CoO-coated carbon material replicates the cross-linked three-dimensional fiber network microstructure of the quantitative filter paper.

[0111] The present invention adopts advanced surface and interface control technology to give full play to the microstructure advantages of the material, so as to achieve a balance between performance indicators such as specific capacity and cycle stability, electron transfer and ion diffusion rate, and realize a controllable construction method of Co3O4-based self-supporting negative electrode materials;

[0112] The present invention uses cellulose sheets made of natural cellulose as structural templates and self-supporting carbon skeleton sources, and adopts a surface sol-gel method to prepare cobalt tetroxide-coated carbon (C@Co3O4) self-supporting negative electrode materials. The engineering design of the layer-by-layer assembly process based on natural cellulose materials is realized, and the three-dimensional fiber network microstructure in the filter paper fibers is utilized. By adjusting the pH of the Co3O4 precursor cobalt acetate and the number of layer-by-layer assembly cycles, two process parameters are adjusted to achieve fine regulation from nanoparticles to nano-coating layers, replicate the hierarchical microstructure of cellulose materials, reduce the damage to the electrode structure caused by the volume change of Co3O4 during charge and discharge, effectively increase the lithium / sodium storage sites and improve the electronic and ionic conductivity. Moreover, as a flexible self-supporting raw material, cellulose sheets can achieve lightweight batteries without sacrificing mechanical strength, which helps to improve the energy density and safety performance of batteries.

[0113] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are within the scope of protection of the pending claims of the present invention.

Claims

1. A method for preparing a cobalt tetroxide-coated carbon self-supporting negative electrode material, characterized in that: Cobalt acetate is used as a precursor, cellulose sheets are used as templates and carbon sources, and a surface sol-gel method is used to assemble cobalt hydroxide gel layers on the surface of cellulose fibers layer by layer, and then a two-step heat treatment is performed to obtain a cobalt tetraoxide-coated carbon self-supporting negative electrode material; the cellulose sheets have a multi-level porous network structure with fibers interwoven with each other at a microscopic level, and the cellulose sheets are quantitative filter paper, pure cotton non-woven fabrics or water-wettable pure wood pulp paper towels; The layer-by-layer assembly process is as follows: placing a cellulose sheet in a vacuum filtration device, adding a 90-290 mmol / L cobalt hydroxide solution, vacuum filtering until the liquid level is higher than the surface of the cellulose sheet, letting it stand and then filtering out the remaining solution, then adding deionized water and letting it stand to allow the cobalt hydroxide to be fully hydrolyzed and cross-linked with the cellulose, then adding anhydrous ethanol and filtering and drying, thus completing a layer-by-layer assembly cycle; the above steps are repeated 20-40 times to obtain a cellulose sheet with a cobalt hydroxide gel layer deposited layer by layer on the surface, which is recorded as material A; The two-step heat treatment process is: material A is clamped on both sides of a silicon wafer, and first annealed and carbonized in an argon atmosphere, the annealing temperature is 570-630 ° C, the heating rate is 0.8-1.2 ° C / min, and the calcination time is 1.5-2.5 h; then an oxidation heat treatment is performed in an air atmosphere, the heat treatment temperature is 280-320 ° C, the heating rate is 1.5-2.5 ° C / min, and the calcination time is 1.5-2.5 h, so as to obtain a black paper-like cobalt oxide-coated carbon self-supporting negative electrode material; The preparation steps of the cobalt hydroxide solution are: preparing a cobalt acetate precursor solution according to a concentration of 25-75 g / L, adding ammonia water to adjust the pH of the cobalt acetate precursor solution to 8.4-8.6, stirring the obtained purple solution for 10-30 min to allow the cobalt acetate and ammonia water to fully react, and obtaining a cobalt hydroxide solution with a molar concentration of 90-290 mmol / L.

2. The method for preparing the cobalt tetroxide-coated carbon self-supporting negative electrode material according to claim 1, characterized in that: The pH of the cobalt acetate precursor solution was adjusted to 8.5 by dropping ammonia water; the number of cycles in the layer-by-layer assembly process was 30.

3. The method for preparing the cobalt tetroxide-coated carbon self-supporting negative electrode material according to claim 1, characterized in that: After the cellulose sheet is placed in the vacuum filtration device, it is first washed with anhydrous ethanol 2 to 3 times and then vacuum dried.

4. The method for preparing the cobalt tetroxide-coated carbon self-supporting negative electrode material according to claim 1, characterized in that: In the vacuum filtration apparatus, the ratio of the cellulose sheet to the added cobalt hydroxide solution is 0.005-0.017 g / mL.

5. The method for preparing the cobalt tetroxide-coated carbon self-supporting negative electrode material according to claim 1, characterized in that: In the two-step heat treatment process, the argon annealing carbonization temperature is 600°C, the heating rate is 1°C / min, and the calcination time is 2h; the oxidation heat treatment temperature in air is 300°C, the heating rate is 2°C / min, and the calcination time is 2h.

6. An application of a cobalt tetraoxide-coated carbon self-supporting negative electrode material, characterized in that: The cobalt oxide-coated carbon self-supporting negative electrode material prepared according to any one of claims 1 to 5 is cut into small black discs of a specified diameter and directly used as the negative electrode of a lithium ion battery or a sodium ion battery.

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