Sodium ion battery positive electrode material with air stability and preparation method thereof

Abstract

The present invention belongs to the technical field of battery materials and discloses a positive electrode material for a sodium ion battery with air stability and a preparation method thereof. The positive electrode material comprises a matrix material Na 0.67 Mn x Fe y Cu z O2 and TPA coated on the surface of the substrate material. The substrate material and TPA solution are evenly mixed, stirred, and evaporated to dryness to obtain an air-stable sodium-ion battery cathode material. The present invention attaches TPA to the cathode material interface, hindering its reaction with moisture and carbon dioxide in the air, improving its structural stability in air; effectively mitigating the production of inactive byproducts, and enhancing rate and cycle performance. The method of the present invention is simple, highly reproducible, and easily adaptable for large-scale application.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and in particular relates to a sodium ion battery positive electrode material and a preparation method thereof. Background Art

[0002] Since their commercialization, lithium-ion batteries have been widely used in a variety of everyday applications, such as digital 3C devices and electric vehicles. However, this has also led to significant lithium resource depletion, resulting in low lithium content and uneven distribution in the Earth's crust. The massive annual consumption of lithium resources and the fluctuating supply and demand relationship between lithium mining and production have led to a surge in the price of raw materials for lithium-ion batteries. The price of battery-grade lithium carbonate has skyrocketed from 40,000 to 50,000 yuan per ton in the second half of 2020 to a record high of nearly 500,000 yuan per ton in the first half of 2022. However, the global energy transition requires a large number of electrochemical energy storage systems for clean energy storage. Limited by resources, lithium-ion batteries are unable to simultaneously support the massive demand for polymer batteries for digital 3C devices, power batteries, and large-scale energy storage systems. This has created an urgent need to find new, low-cost energy storage batteries to alleviate lithium resource depletion. At the same time, major breakthroughs have been made in the research of sodium-ion batteries. Relevant studies have shown that graphite negative electrode materials have extremely poor electrochemical properties, but another type of hard carbon material has a larger interlayer spacing compared to graphite, which is conducive to the intercalation and deintercalation of sodium ions. When used as the negative electrode of sodium-ion batteries, it exhibits excellent sodium storage performance. This enables sodium-ion positive electrode materials to maximize their own performance, thereby driving changes in the entire industry.

[0003] However, the air instability of layered sodium-ion battery cathode materials has hindered their further application. To address this issue, researchers have successfully prepared sodium-ion battery cathode materials by isolating them from air and moisture during the preparation process. However, altering external conditions cannot address the inherent structural defects of the material. Therefore, how to adapt the material to complex air environments (primarily moisture) remains a major challenge facing those skilled in the art. Summary of the Invention

[0004] In view of the problems existing in the prior art, one of the objectives of the present invention is to provide a sodium ion battery positive electrode material with air stability.

[0005] A second object of the present invention is to provide a method for preparing an air-stable sodium ion battery cathode material.

[0006] To achieve the above objectives, the present invention provides the following specific technical solutions.

[0007] First, the present invention provides a sodium ion battery cathode material with air stability, which has surface hydrophobicity, including a matrix material Na 0.67 Mn x Fey Cu z O2 and TPA (tetradecylphosphonic acid) coated on the surface of the base material, wherein 0.92≤x≤1; 0≤y+z≤0.08.

[0008] Furthermore, in some preferred embodiments of the present invention, in the sodium ion battery positive electrode material, the matrix material is a flaky particle with a thickness of 200-240 nm, and the thickness of the TPA coated on the surface of the flaky particle is 2-5 nm.

[0009] Furthermore, in some preferred embodiments of the present invention, the mass ratio of the coating amount of TPA to the base material is 1-5:500, preferably 1-3:500.

[0010] Too low TPA content will lead to uneven coating on the surface of the positive electrode material, while too high TPA content will lead to too thick coating layer, which will hinder the transmission of sodium ions and reduce the electrochemical performance.

[0011] Secondly, the present invention provides a method for preparing the above-mentioned air-stable sodium ion battery positive electrode material, comprising the following steps:

[0012] Step S1, preparing TPA solution;

[0013] Step S2: Evenly mix the matrix material and the TPA solution, stir and evaporate to dryness to obtain an air-stable sodium ion battery positive electrode material.

[0014] Furthermore, in some preferred embodiments of the present invention, the TPA solution is prepared by dissolving TPA in water and then adding anhydrous ethanol solution to obtain the TPA solution.

[0015] Furthermore, in the TPA solution, the content of TPA is 0.1-0.5 wt%, the water content is 10-20 wt%, and the remainder is anhydrous ethanol; preferably, the content of TPA is 0.1-0.2 wt%.

[0016] Furthermore, in some preferred embodiments of the present invention, the evaporation temperature is 60-100° C. and the evaporation time is 6-9 hours. Too high an evaporation temperature will cause the solution to evaporate too quickly, resulting in uneven coating.

[0017] The present invention introduces TPA on the surface of the sodium ion battery cathode material to achieve hydrophobic properties of the material interface, thereby ensuring air stability, which is beneficial to improving the electrochemical performance of the cathode material after long-term storage and lays the foundation for large-scale application.

[0018] In addition, the introduction of the TPA coating layer can effectively avoid the reaction between the positive electrode material interface and CO2 and H2O, thereby avoiding the generation of NaOH, Na2CO3, Na2CO3∙H2O and NiO impurities, and optimizing the electrochemical performance of the material.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The interfacially hydrophobic sodium ion cathode material of the present invention successfully attaches TPA to the cathode material interface, hindering its reaction with moisture and carbon dioxide in the air and improving its structural stability in the air;

[0021] (2) The interfacial hydrophobic sodium ion cathode material of the present invention effectively alleviates the generation of inactive by-products and improves rate performance and cycle performance;

[0022] (3) The method of the present invention has simple process, high repeatability and is easy to promote on a large scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0024] Figure 1 This is the SEM image of the NMFCO positive electrode material obtained in Example 1.

[0025] Figure 2 This is the SEM image of the NMFCO@TPA positive electrode material obtained in Example 1.

[0026] Figure 3 This is the TEM image of the NMFCO positive electrode material obtained in Example 1.

[0027] Figure 4 This is the TEM image of the NMFCO@TPA positive electrode material obtained in Example 1.

[0028] Figure 5 Schematic diagrams of the interfacial hydrophobicity of the NMFCO positive electrode material and the NMFCO@TPA positive electrode material obtained in Example 1, wherein Figures (a) to (c) are schematic diagrams of the interfacial hydrophobicity of the NMFCO positive electrode material after being placed for 0s, 15s, and 30s, respectively, and Figures (d) to (f) are schematic diagrams of the interfacial hydrophobicity of the NMFCO@TPA positive electrode material after being placed for 0s, 15s, and 30s, respectively.

[0029] Figure 6Figure 2 shows the first constant current charge-discharge graph of the battery assembled after the NMFCO positive electrode material and the NMFCO@TPA positive electrode material were exposed to air for 0 days, 14 days, and 28 days, respectively. Figure (a) shows the first constant current charge-discharge graph of the battery assembled after the NMFCO positive electrode material was exposed to air for 0 days, 14 days, and 28 days, respectively. Figure (b) shows the first constant current charge-discharge graph of the battery assembled after the NMFCO@TPA positive electrode material was exposed to air for 0 days, 14 days, and 28 days, respectively.

[0030] Figure 7 Figure 2 is the cycle performance diagram of the battery assembled after NMFCO positive electrode material and NMFCO@TPA positive electrode material were exposed to air for 0 days, 14 days and 28 days, respectively. Figure (a) is the cycle performance diagram of the battery assembled after NMFCO positive electrode material was exposed to air for 0 days, 14 days and 28 days, respectively. Figure (b) is the cycle performance diagram of the battery assembled after NMFCO@TPA positive electrode material was exposed to air for 0 days, 14 days and 28 days, respectively.

[0031] Figure 8 These are the rate performance diagrams of the batteries assembled after the NMFCO positive electrode material and the NMFCO@TPA positive electrode material were exposed to the air for 0 days, 14 days, and 28 days, respectively. Figures (a), (b), and (c) are the rate performance diagrams of the batteries assembled after the NMFCO positive electrode material was exposed to the air for 0 days, 14 days, and 28 days, respectively. Figures (b), (d), and (f) are the rate performance diagrams of the batteries assembled after the NMFCO@TPA positive electrode material was exposed to the air for 0 days, 14 days, and 28 days, respectively. DETAILED DESCRIPTION

[0032] To facilitate understanding of the present invention, the present invention will be described in more comprehensive and detailed form below in conjunction with the accompanying drawings and preferred embodiments. However, the protection scope of the present invention is not limited to the following specific embodiments.

[0033] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0034] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0035] The present invention provides a sodium ion battery cathode material with air stability, which has surface hydrophobicity and comprises a matrix material Na 0.67 Mn x Fe y Cu zO2 and TPA (tetradecylphosphonic acid) coated on the surface of the base material, wherein 0.92≤x≤1; 0≤y+z≤0.08.

[0036] The matrix material can be Na 0.67 MnO2、Na 0.67 Mn 0.92 Cu 0.08 O2、Na 0.67 Mn 0.92 Fe 0.08 O2、Na 0.67 Mn 0.92 Fe 0.04 Cu 0.04 O2, etc.

[0037] Furthermore, the substrate material is a flaky particle with a thickness of 200-240 nm, and the TPA coating on the surface of the flaky particle is 2-5 nm thick. Too little coating is insufficient to optimize the substrate interface, while too much coating will hinder the transport of sodium ions, thereby reducing the electrochemical performance of the material.

[0038] Furthermore, the mass ratio of the TPA coating amount to the base material is 1 to 5:500, preferably 1 to 3:500. Too low a TPA content will result in uneven coating on the surface of the positive electrode material, while too high a TPA content will result in an overly thick coating layer, hindering sodium ion transport and reducing electrochemical performance.

[0039] The present invention prepares a sodium ion battery cathode material with air stability by the following method:

[0040] Step S1, preparing TPA solution;

[0041] Step S2: Evenly mix the matrix material and the TPA solution, stir and evaporate to dryness to obtain an air-stable sodium ion battery positive electrode material.

[0042] Furthermore, a TPA solution is prepared by dissolving TPA in water, and then adding anhydrous ethanol solution to obtain a TPA solution.

[0043] The TPA solution contains 0.1-0.5 wt% TPA, 10-20 wt% water, and the remainder is anhydrous ethanol. Preferably, the TPA content is 0.1-0.2 wt%.

[0044] Furthermore, the evaporation temperature is 60-100°C, which can be selected from 60°C, 70°C, 80°C, 90°C, 100°C, etc.; the evaporation time is 6-9h, generally 6h, 7h, 8h, 9h, etc. according to actual conditions.

[0045] In the actual research process, the electrochemical performance of the positive electrode material after exposure to air for 0 to 28 days can be compared and analyzed. The exposure time can be 0 days, or 1 day, 2 days, 5 days, 10 days, 14 days, 20 days, 22 days, 25 days, 28 days, etc.

[0046] Furthermore, those skilled in the art can synthesize the matrix material in the following manner:

[0047] (1) According to the metal element composition of the matrix material, a manganese source, an iron source, a copper source, and a sodium source are mixed in an ultrapure aqueous solution and continuously stirred to form a solution A; citric acid monohydrate is placed in a certain amount of ultrapure water and continuously stirred until it is completely dissolved to form a solution B; solution A is slowly added dropwise to solution B and stirred to obtain a uniformly mixed solution, and ammonia water is added to adjust the pH to 6 to obtain a slurry;

[0048] (2) evaporating and aging the slurry obtained in step (1) to obtain a precursor powder;

[0049] (3) The precursor powder obtained in step (2) is crushed, calcined, and cooled to obtain a matrix material.

[0050] During the synthesis of the matrix material, those skilled in the art:

[0051] Furthermore, the manganese source is one of manganese acetate, manganese nitrate and hydrates thereof; the iron source is one of ferric acetate, ferric nitrate and hydrates thereof; the copper source is one of copper acetate, copper nitrate and hydrates thereof; the sodium source is at least one of sodium nitrate, sodium acetate and hydrates thereof;

[0052] Furthermore, in step (1), in the solution A, the total molar ratio of Na to Mn, Fe, and Cu is 0.67:1.

[0053] Furthermore, in step (1), the molar ratio of the amount of citric acid monohydrate to the total metal ions in solution A is 1-1.2:1, preferably 1-1.1:1.

[0054] Furthermore, in step (1), when solution A is slowly added dropwise to solution B, the stirring rate is 100-150 r / min, and the stirring time is 0.5-3 h. A stirring rate that is too low is not conducive to mixing the two solutions, while a rate that is too high will cause splashing of the liquid and lead to loss of the solution.

[0055] Furthermore, in step (2), the evaporation temperature is 80°C-110°C, and the evaporation time is 6-10 hours; the aging temperature is 100°C-130°C, and the evaporation time is 8-12 hours. Evaporation temperatures that are too low or evaporation times that are too short are not conducive to the volatilization of the solvent.

[0056] Furthermore, in step (3), the pulverization time is 0.2-1 h, preferably 0.2-0.4 h; too short a time will result in incomplete pulverization, while too long a time will increase costs.

[0057] Furthermore, in step (3), the sintering temperature is 500°C-850°C and the time is 8-12 hours. A sintering temperature lower than 500°C is not conducive to the volatilization of nitrate and organic matter.

[0058] The following is further described with reference to specific examples.

[0059] Example 1

[0060] This embodiment includes the following steps:

[0061] (1) According to the molar ratio of Na:Mn:Fe:Cu = 0.67:0.92:0.02:0.06, anhydrous sodium acetate, manganese acetate tetrahydrate, copper acetate monohydrate, and ferric nitrate nonahydrate in the corresponding stoichiometric ratio were dissolved in 50 mL of ultrapure water and stirred continuously to prepare a mixed solution A, wherein anhydrous sodium acetate was in excess of 5% based on the corresponding stoichiometric ratio. 2.1014 g of citric acid monohydrate was dissolved in 50 mL of ultrapure water and stirred continuously to prepare a mixed solution B, wherein the molar amount of citric acid monohydrate and the total molar amount of (Na+Mn+Fe+Cu) in mixed solution A were 1:1. Solution A was slowly added dropwise to solution B and stirred for 1 hour to obtain a uniformly mixed solution. 2 mL of ammonia water was then added to adjust the pH of the system to 6.

[0062] (2) The mixed solution in step (1) was placed in a constant temperature water bath at 90°C to evaporate the solvent to obtain a wet gel, and the wet gel was placed in a 120°C forced air drying oven for 10 h to obtain a dry gel, which was then ground in a mortar.

[0063] (3) The crushed sample in step (2) was placed in a muffle furnace and sintered at 500 °C for 10 h to remove nitrate and organic matter, and then heated to 850 °C and sintered for 10 h to obtain Na 0.67 Mn 0.92 Fe 0.02 Cu 0.06 O2 positive electrode material (NMFCO positive electrode material);

[0064] (4) Preparation of TPA solution: Dissolve an appropriate amount of tetradecylphosphonic acid (TPA) in a small amount of water and add it to anhydrous ethanol solution, stirring and dispersing it to prepare a 0.1 wt% mixed solution.

[0065] (5) Take 1 g of NMFCO cathode material and an appropriate amount of TPA solution in a mass ratio (NMFCO cathode material: TPA solution = 1:2), and stir continuously in a constant temperature water bath at 80 °C until dry. Then, transfer the solid powder to a vacuum oven at 60 °C and dry it for 10 h to obtain a surface-modified sample, which is recorded as NMFCO@TPA.

[0066] The prepared NMFCO positive electrode material and NMFCO@TPA positive electrode material were exposed to air for 0 days respectively.

[0067] Figure 1 This is the SEM image of the NMFCO positive electrode material prepared in step (3) of Example 1. It can be seen from the figure that the NMFCO positive electrode material has a sheet structure with a thickness of about 200 nm.

[0068] Figure 2 This is the SEM image of the NMFCO@TPA positive electrode material prepared in step (5) of Example 1. It can be seen from the figure that the material has a sheet structure with a thickness of about 200 nm. The introduction of TPA does not destroy the intrinsic structure of the material.

[0069] Figure 3 This is the TEM image of the NMFCO positive electrode material prepared in step (3) of Example 1. Figure 4 This is a TEM image of the NMFCO@TPA cathode material prepared in step (5) of Example 1. By comparison, it can be found that the present invention can effectively coat TPA on the surface of the NMFCO material.

[0070] Figure 5 The figure shows the interface hydrophobicity of the NMFCO cathode material prepared in step (3) of Example 1 and the NMFCO@TPA cathode material prepared in step (5). The surface of the NMFCO cathode material is easily wetted by water. The initial static contact angle is only 12.8°. After only 15 seconds, the static contact angle drops to 6.6°. After less than 30 seconds, the water droplet completely penetrates into the interior of the material. This shows that the NMFCO cathode material is easily wetted by water, which may be because soluble residual alkaline substances are produced on the surface and it has a high degree of hydrophilicity. In contrast, the surface of the NMFCO@TPA cathode material is difficult to be wetted by water. The static contact angle reaches 76.2° at the beginning. As the placement time increases, the static contact angle slowly decreases, but after 30 seconds, it still has a static contact angle of 68.8°. This shows that compared with the NMFCO cathode material, the surface of the NMFCO@TPA cathode material has a certain degree of hydrophobicity and is difficult to be wetted by water.

[0071] Example 2

[0072] The difference between Example 2 and Example 1 is that:

[0073] The prepared NMFCO cathode material and NMFCO@TPA cathode material were exposed to air for 14 days respectively.

[0074] The only difference between Example 3 and Example 1 is that:

[0075] The prepared NMFCO cathode material and NMFCO@TPA cathode material were exposed to air for 28 days respectively.

[0076] Battery assembly: Weigh 0.08 g of the NMFCO positive electrode material and NMFCO@TPA positive electrode material exposed to air for 0 days, 14 days, and 28 days in Examples 1-3, respectively, add 0.01 g of acetylene black as a conductive agent and 0.01 g of polyvinylidene fluoride as a binder, and N-methylpyrrolidone as a dispersant. After mixing evenly, apply the mixture on aluminum foil to make a positive electrode sheet. In a vacuum glove box, a metal lithium sheet is used as the negative electrode, a composite film of PE and PP is used as the separator, and 1 mol / LLiPF6 / DMC:EC (volume ratio 1:1) is used as the electrolyte to assemble a CR2032 button battery.

[0077] Test the electrochemical performance of the button battery, the results are as follows Figure 6-8 shown.

[0078] like Figure 6 As shown in the figure, as the exposure time of NMFCO cathode material increases, the initial discharge capacity of the battery decreases. However, the extension of the exposure time of NMFCO@TPA cathode material does not affect the initial discharge capacity of the battery.

[0079] like Figure 7 As shown in the figure, as the exposure time of the NMFCO cathode material increases, the battery's first discharge specific capacity and first efficiency continue to decrease. After 28 days of exposure, the battery's first efficiency is only 77.98%. However, the extension of the exposure time of the NMFCO@TPA cathode material does not have a significant impact on the battery's first discharge specific capacity and first efficiency. After 28 days of exposure, the battery's first efficiency is still 92.22%.

[0080] like Figure 8 As shown in the figure, at the rate performance of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 5 C, the NMFCO@TPA positive electrode material with different exposure time did not cause a significant decline in the rate performance of the battery; but with the NMFCO positive electrode material, the rate performance of the battery containing it declined significantly.

[0081] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of the present invention.

Claims

1. A sodium ion battery cathode material with air stability, characterized in that The positive electrode material includes a base material Na 0.67 Mn 0.92 Fe 0.02 Cu 0.06 O2 and tetradecylphosphonic acid coated on the surface of the base material; the base material is a flaky particle with a thickness of 200-240 nm; The matrix material is prepared by the following method: (1) According to the metal element composition of the matrix material, a manganese source, an iron source, a copper source, and a sodium source are mixed in an ultrapure aqueous solution and continuously stirred to form solution A; Place citric acid monohydrate in a certain amount of ultrapure water and stir continuously until it is completely dissolved to form solution B; slowly add solution A dropwise to solution B and stir to obtain a uniformly mixed solution; add ammonia water to adjust the pH to 6 to obtain a slurry; (2) evaporating and aging the slurry obtained in step (1) to obtain a precursor powder; (3) crushing the precursor powder obtained in step (2), calcining, and cooling to obtain a matrix material; In step (1), the molar ratio of the amount of citric acid monohydrate to the total metal ions in solution A is 1-1.2:

1.

2. The air-stable sodium ion battery cathode material according to claim 1, wherein The thickness of the tetradecylphosphonic acid coated on the surface of the flaky particles is 2-5 nm.

3. The air-stable sodium ion battery cathode material according to claim 1 or 2, wherein: The mass ratio of the coating amount of tetradecylphosphonic acid to the base material is 1-5:

500.

4. The method for preparing an air-stable sodium ion battery cathode material according to any one of claims 1 to 3, wherein: The following steps are involved: Step S1, preparing a tetradecylphosphonic acid solution; Step S2: Evenly mix the matrix material and the tetradecylphosphonic acid solution, stir and evaporate to dryness to obtain an air-stable sodium ion battery positive electrode material.

5. The preparation method according to claim 4, wherein The tetradecylphosphonic acid solution is prepared in the following manner: tetradecylphosphonic acid is dissolved in water, and then anhydrous ethanol is added to obtain the tetradecylphosphonic acid solution.

6. The preparation method according to claim 5, wherein The tetradecylphosphonic acid solution contains 0.1-0.5 wt % of tetradecylphosphonic acid, 10-20 wt % of water, and the remainder is anhydrous ethanol.

7. The preparation method according to claim 4, wherein The evaporation temperature is 60-100° C., and the evaporation time is 6-9 hours.

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