Nickel cross-linked sodium alginate induced sodium vanadium phosphate composite positive electrode material, preparation method and application thereof

Abstract

This invention belongs to the field of sodium-ion battery cathode material technology. To address the problems of poor intrinsic conductivity and low energy density of sodium vanadium phosphate, it provides a nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material, its preparation method, and its applications. Using nickel acetate tetrahydrate, vanadium pentoxide, and sodium carbonate as raw materials, and sodium alginate as a partial sodium source and reducing agent, a solid-state method is employed. Nickel-doped vanadium sites are introduced into redox couples, generating tetravalent and pentavalent vanadium redox couples, constructing three high-discharge plateaus. Nickel-crosslinked sodium alginate allows nickel ions to exchange with and integrate with the G-blocks in the sodium alginate structure to form an "egg-shell" structure. After sintering, a nickel-doped carbon coating layer with numerous defects and a thin, fibrous carbon interconnect network are formed. The resulting nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material, composed of a nickel-doped carbon coating layer of sodium vanadium phosphate, exhibits stable multi-segment high-voltage plateaus, high energy density, and significant practical value.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery cathode material technology, specifically relating to a nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, as the most advanced rechargeable energy storage batteries, have been widely used. However, the upstream battery resources required are insufficient to meet the large-scale demand in downstream applications, exposing the pain points of lithium resource shortages and susceptibility to geopolitical influences and oligopolistic monopolies. Sodium and lithium are elements in the same group and have similar chemical properties. Furthermore, the architecture and packaging process of sodium-ion batteries are highly similar to those of lithium batteries, allowing lithium battery factories to directly produce sodium batteries without significant modifications. Therefore, sodium-ion batteries are considered the next generation of mainstream batteries and the optimal choice to replace lithium batteries.

[0003] Sodium vanadium phosphate (PVP), with its hexagonal crystal system, possesses a high voltage plateau (3.4 V vs. Na) and a high theoretical specific capacity (117.6 mAh / g), making it a promising polyanionic cathode material for sodium-ion batteries. It exhibits a sodium superionic conductor (NASICON) structure, good chemical stability, long lifespan, and high natural abundance. However, compared to lithium ions, sodium ions have a larger atomic radius and mass, resulting in low energy density in practical applications, which hinders the application of sodium-ion batteries in the power battery field. Therefore, improving the energy density of PPVP cathode materials is crucial for their practical application. Simultaneously, the strong bonding and interaction of the VO bonds lead to low electronic conductivity in PPVP materials, limiting their further development.

[0004] Since the discharge capacity of sodium vanadium phosphate is determined by the content of active sodium ions, its energy density can only be improved by increasing the discharge plateau. The discharge plateau corresponds to the redox couple, but common redox couples are difficult to achieve a high discharge plateau, thus limiting the improvement of energy density for sodium vanadium phosphate electrode materials. Furthermore, improving the electronic conductivity of the battery is usually achieved through carbon coating. However, the orderliness of the carbon layer hinders the diffusion process of sodium ions, negatively impacting the discharge process. Summary of the Invention

[0005] This invention addresses the issues of poor intrinsic conductivity and low energy density inherent in sodium vanadium phosphate (CNP), providing a nickel-crosslinked sodium alginate-induced CNP composite cathode material, its preparation method, and its applications. By doping vanadium sites with nickel, new redox couples are introduced, promoting the formation of tetravalent and pentavalent vanadium redox couples, thus achieving the construction of three high discharge plateaus and significantly improving the material's energy density. Simultaneously, during sintering, the nickel-crosslinked CNP forms a nickel-doped carbon coating layer with numerous defects and a thin, fibrous carbon interconnect network, enhancing the material's ionic and electronic conductivity, resulting in excellent electrochemical performance. When loaded into a 2025 coin cell, the electrode material exhibits excellent cycle stability and high-rate, long-cycle performance, making it a promising cathode material for sodium-ion batteries.

[0006] This invention is achieved by the following technical solution: a nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material, wherein the composite cathode material is prepared by solid-state method using nickel acetate tetrahydrate, vanadium pentoxide, and sodium carbonate as raw materials, and sodium alginate as part of the sodium source and reducing agent. Nickel is doped at vanadium sites, introducing redox couples and promoting the generation of vanadium tetravalent and vanadium pentavalent redox couples, thus constructing three high discharge platforms. Simultaneously, nickel crosslinks sodium alginate, and nickel ions undergo ion exchange with the G-blocks in the sodium alginate structure and integrate to form an "egg-shell" structure. After sintering, a nickel-doped carbon coating layer with numerous defects and a thin, veil-like carbon interconnect network are formed; thus, a nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material is obtained, consisting of a sodium vanadium phosphate nickel-doped carbon coating layer.

[0007] The specific steps for preparing the nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material are as follows: (1) Sodium alginate, vanadium pentoxide, sodium carbonate, ammonium dihydrogen phosphate and nickel acetate tetrahydrate were placed in a planetary ball mill jar; the mass ratio of sodium alginate, vanadium pentoxide, sodium carbonate, ammonium dihydrogen phosphate and nickel acetate tetrahydrate was 0.5:2.3935:1.0461:2.2707:0.0143~0.0257. (2) Add 25 mL of anhydrous ethanol to the planetary ball mill jar and put in the ball milling beads; the mass ratio of raw material to beads is 1:60, the ball milling frequency is 40 Hz, the ball milling time is 8 h, and the forward and reverse rotation is 4 h each to obtain the precursor; (3) Place the precursor in a forced-air drying oven and dry it at 80°C for 12 hours; (4) The precursor obtained is pre-calcined and final-calcined under a nitrogen atmosphere to obtain the final product; wherein, the pre-calcination process is to heat the temperature from room temperature to 450°C at a heating rate of 2°C / min, hold the temperature for 4 hours and then cool naturally; the final-calcination process is to heat the temperature from room temperature to 700°C at a heating rate of 2°C / min, hold the temperature for 6 hours and then cool naturally.

[0008] The present invention also provides the application of the nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material as a cathode material in sodium-ion batteries.

[0009] The specific method is as follows: (1) Preparation of positive electrode: 0.21 g of nickel crosslinked sodium alginate-induced sodium vanadium phosphate composite positive electrode material, 0.06 g of acetylene black conductive filler and 0.03 g of polyvinylidene fluoride binder were mixed in 1.6 mL of N-methylpyrrolidone organic solvent in a ratio of 7:2:1; the mixture was placed in a ball mill jar and ball milled unidirectionally for 4 h to obtain a slurry, which was then coated on a single-sided carbon-coated aluminum foil; the aluminum foil coated with the slurry was then dried at 40 °C for 4 h, followed by vacuum drying at 120 °C for 6 h, and cut into 16 mm circular electrode sheets; (2) Preparation of button cell: Using the circular electrode obtained in step (1) as the positive electrode, metallic sodium as the negative electrode, and ceramic Celgard membrane as the separator, the 2025 type button cell is assembled in a vacuum glove box; wherein, the electrolyte is 1M sodium perchlorate dissolved in a ethylene carbonate / diethyl carbonate system with a volume ratio of 1:1, and 5wt% fluoroethylene carbonate is added based on sodium perchlorate.

[0010] This invention utilizes sodium alginate as a carbon source, adds nickel acetate tetrahydrate as a carbon layer, and incorporates heteroatoms and additional redox couples as dopants for sodium vanadium phosphate crystals. Through a one-step solid-state method, sodium vanadium phosphate with multiple discharge platforms is synthesized, significantly improving the material's energy density. Nickel ions can undergo ion exchange with the G-blocks in the sodium alginate structure and integrate to form an "egg-shell" structure. After sintering, a nickel-doped highly conductive carbon coating layer and a carbon fiber network are formed. The nickel-doped highly conductive carbon coating layer has numerous defects, which allows for rapid electron and sodium ion transport. The carbon fiber conductive network promotes electron transfer between sodium vanadium phosphate particles, thereby simultaneously improving both intrinsic and ionic conductivity.

[0011] Compared with the prior art, the present invention has the following advantages and beneficial effects: This invention utilizes sodium alginate as both a sodium source and a carbon source, and introduces nickel heteroatoms into the carbon layer and sodium vanadium phosphate crystal to prepare a high-energy-density cathode material with a triple discharge platform. The preparation steps are simple and the raw materials are inexpensive.

[0012] Nickel ions undergo ion exchange with the G-block in the sodium alginate structure and integrate to form an "egg-shell" structure. After sintering, a carbon coating layer with numerous defects is formed, enabling rapid electron and ion transport.

[0013] Nickel ions replace vanadium sites and dope into sodium vanadium phosphate crystals, introducing additional redox pairs and promoting a corresponding high discharge plateau, namely V0. 3+ / 4+ -3.4 V, Ni 2+ / 3+ -3.7 V, V 4+ / 5+ -3.9 V. The material obtained by this invention has a stable multi-segment high voltage plateau, high energy density, and significant practical value. Attached Figure Description

[0014] Figure 1 A photograph of the nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material prepared in Example 1; Figure 2 The image shows the Raman spectrum of the nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material prepared in Example 1. The nickel-doped carbon coating layer formed after the nickel-crosslinked sodium alginate carbonization has a high degree of disorder. Figure 3 The fine XPS spectrum of nickel in the nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material prepared in Example 1 shows that nickel ions were successfully doped into the sodium vanadium phosphate crystal. Figure 4 The constant current charge-discharge curve of the nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material prepared in Example 1 was measured when it was assembled into a 2025 button battery, with a current density of 0.1 C. Figure 5 The constant current charge-discharge curves measured at multiple current densities when the nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material prepared in Example 1 was assembled into a 2025 button battery. Figure 6 The constant current intermittent titration curve was measured when the nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material prepared in Example 1 was assembled into a 2025 button battery. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and all materials publicly cited herein and cited by them are incorporated herein by reference.

[0017] Equivalent technologies of the specific embodiments described herein that are apparent to those skilled in the art through routine experimentation are included in this application.

[0018] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the instruments and equipment used in the following examples are all standard laboratory instruments and equipment; unless otherwise specified, the experimental materials used in the following examples were all purchased from regular biochemical reagent stores.

[0019] Example 1: Preparation of nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material: 0.5 g of sodium alginate, 2.3935 g of vanadium pentoxide, 1.0461 g of sodium carbonate, 2.2707 g of ammonium dihydrogen phosphate, and 0.0257 g of nickel acetate tetrahydrate were placed in a planetary ball mill jar, and 25 mL of anhydrous ethanol was added. Then, ball milling beads were added at a raw material to milling bead mass ratio of 1:60. The mixture was milled in both directions at 40 Hz for 4 hours each. After milling, it was placed in a forced-air oven and dried at 80 °C for 12 hours. The resulting precursor was pre-calcined at 450 °C for 4 hours under a nitrogen atmosphere, then compressed into tablets, and finally calcined at 700 °C for 6 hours, followed by milling for 1 hour to obtain the final product.

[0020] The prepared nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material is shown in the image below. Figure 1 As shown in the figure, the material has a low particle size and is evenly dispersed. After in-situ carbonization of sodium alginate, a nickel-doped carbon coating layer and a thin veil-like conductive network are formed, which are interconnected with the particles, which helps to improve the electronic conduction between the particles.

[0021] Raman spectrum as follows Figure 2 As shown, from Figure 2 It can be seen that the nickel-doped carbon coating layer formed after nickel cross-linking and sodium alginate carbonization has a high degree of disorder.

[0022] Fine XPS spectra of nickel in nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode materials are shown below. Figure 3 As shown, Figure 3 This indicates that nickel ions have been successfully doped into the sodium vanadium phosphate crystal.

[0023] The positive electrode material prepared in this embodiment was used as the active material to prepare a slurry. 0.21 g of the positive electrode material, 0.06 g of acetylene black (conductive filler), and 0.03 g of polyvinylidene fluoride (binder) were mixed in 1.6 mL of N-methylpyrrolidone (organic solvent) at a ratio of 7:2:1. The mixture was placed in a ball mill jar and ball-milled unidirectionally for four hours to obtain a slurry, which was then coated onto a single-sided carbon-coated aluminum foil. The aluminum foil coated with the slurry was then dried at 40°C for four hours, followed by vacuum drying at 120°C for six hours, and cut into 16 mm circular electrode sheets. Using these as the positive electrode, metallic sodium as the negative electrode, and a ceramic Celgard separator as the separator, a 2025 type button cell was assembled in a vacuum glove box. The electrolyte was prepared by dissolving 1 M sodium perchlorate in a 1:1 volume ratio of ethylene carbonate / diethyl carbonate, with the addition of 5 wt% fluoroethylene carbonate.

[0024] The assembled coin cells were subjected to constant current charge-discharge tests at room temperature within a voltage range of 2.3-4.1 V. Specifically, the first charge-discharge curves are shown below. Figure 4 Charge-discharge curves at different rates are as follows: Figure 5 .

[0025] Electrochemical tests show that the material achieves a discharge specific capacity of 111.3 mAh g⁻¹ at 0.1 C. -1 Furthermore, it exhibited three stable charge / discharge plateaus at 3.4 V, 3.7 V, and 3.9 V, respectively. All three plateaus existed and remained stable at different current densities.

[0026] The assembled coin cells were subjected to constant-current intermittent titration tests at room temperature within a voltage range of 2.3–4.1 V. The results are as follows: Figure 6 As shown, the results indicate that the sodium ion diffusion coefficient decreased significantly at the three voltage plateaus of 3.4 V, 3.7 V, and 3.9 V, which suggests that phase transition occurred at all three voltage plateaus.

[0027] The above embodiments illustrate that this invention uses a simple solid-state method to rapidly synthesize a nickel-crosslinked sodium alginate-induced multi-discharge-platform sodium vanadium phosphate cathode material via a planetary ball mill. This cathode material exhibits three stable high-voltage plateaus at 3.4 V, 3.7 V, and 3.9 V. This can be attributed to the Ni produced after nickel doping into the sodium vanadium phosphate crystal. 2+ / 3+ and V 4+5+The redox couple is formed. Furthermore, nickel ions can undergo ion exchange with the G-block in the sodium alginate structure, further forming an "egg-shell" structure. After sintering, a nickel-doped, highly conductive carbon coating layer and a carbon fiber network are formed, possessing numerous defects that allow for rapid ion and electron movement, thereby improving the intrinsic and ionic conductivity of the material. Characterization tests show that the electrode material of this invention exhibits three stable high-voltage plateaus and excellent electrochemical performance, especially high energy density. Simultaneously, the material preparation process is simple and inexpensive, making it promising for industrial application.

[0028] Example 2: Nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material, wherein the mass ratio of sodium alginate, vanadium pentoxide, sodium carbonate, ammonium dihydrogen phosphate and nickel acetate tetrahydrate is 0.5:2.3935:1.0461:2.2707:0.0143; the remaining methods are the same as those described in Example 1.

[0029] Example 3: Nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material, wherein the mass ratio of sodium alginate, vanadium pentoxide, sodium carbonate, ammonium dihydrogen phosphate and nickel acetate tetrahydrate is 0.5:2.3935:1.0461:2.2707:0.0200; the remaining methods are the same as those described in Example 1.

[0030] Comparative Example 1: Preparation of ordinary sodium vanadium phosphate cathode material: 2.3935 g of vanadium pentoxide, 1.0461 g of sodium carbonate, and 2.2707 g of ammonium dihydrogen phosphate were placed in a planetary ball mill jar, and 25 mL of anhydrous ethanol was added. Then, ball milling beads were added at a raw material to milling bead mass ratio of 1:60. The mixture was milled in both directions at 40 Hz for 4 hours each. After removal, it was placed in a forced-air oven and dried at 80 °C for 12 hours. The resulting precursor was pre-calcined at 450 °C for 4 hours under a nitrogen atmosphere, then compressed into tablets, and finally calcined at 700 °C for 6 hours, followed by milling for 1 hour to obtain the final product.

[0031] The positive electrode material prepared in this embodiment was used as the active material to prepare the slurry, following the same method as described in Example 1. The assembled coin cells were subjected to constant current charge-discharge tests within a voltage range of 2.3-4.1 V at room temperature. Specifically, the first charge-discharge curves are shown below. Figure 4 The material exhibits a discharge specific capacity of only 74.3 mAh g at 0.1 C. -1 And there is only one charge / discharge plateau at 3.4V, which corresponds to V 3+ / 4+ The redox couple.

[0032] As can be clearly seen from the comparative examples, the nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material prepared in this invention can achieve a discharge specific capacity of 111.3 mAh g at 0.1 C. -1The intrinsic and ionic conductivity of the material are significantly higher than those of the comparative example, indicating a significant improvement. Characterization tests show that the electrode material of this invention exhibits three stable charge-discharge plateaus at 3.4 V, 3.7 V, and 3.9 V. These three plateaus exist and remain stable at different current densities, demonstrating excellent electrochemical performance, especially at high energy densities. Furthermore, the material preparation process is simple and inexpensive, making it a promising candidate for industrial application.

[0033] 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 nickel cross-linked sodium alginate induced sodium vanadium phosphate composite cathode material, characterized in that: The composite cathode material is prepared using nickel acetate tetrahydrate, vanadium pentoxide, and sodium carbonate as raw materials, with sodium alginate as a partial sodium source and reducing agent, via a solid-state method. Nickel is doped at vanadium sites, introducing redox couples and promoting the formation of vanadium tetravalent and vanadium pentavalent redox couples, thus constructing three high-discharge platforms. Simultaneously, nickel crosslinks sodium alginate, allowing nickel ions to exchange with and integrate with the G-blocks in the sodium alginate structure to form an "egg-shell" structure. After sintering, a nickel-doped carbon coating layer with numerous defects and a thin, veil-like carbon interconnect network are formed, resulting in a nickel-crosslinked sodium alginate-induced vanadium phosphate composite cathode material composed of a nickel-doped carbon coating layer. The specific steps for preparing the nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material are as follows: (1) Sodium alginate, vanadium pentoxide, sodium carbonate, ammonium dihydrogen phosphate and nickel acetate tetrahydrate were placed in a planetary ball mill jar; the mass ratio of sodium alginate, vanadium pentoxide, sodium carbonate, ammonium dihydrogen phosphate and nickel acetate tetrahydrate was 0.5:2.3935:1.0461:2.2707:0.0143~0.0257. (2) Add 25 mL of anhydrous ethanol to the planetary ball mill jar and put in the ball milling beads; the mass ratio of raw material to beads is 1:60, the ball milling frequency is 40 Hz, the ball milling time is 8 h, and the forward and reverse rotation is 4 h each to obtain the precursor; (3) Place the precursor in a forced-air drying oven and dry it at 80°C for 12 hours; (4) The precursor obtained is pre-calcined and final-calcined under a nitrogen atmosphere to obtain the final product; wherein, the pre-calcination process is to heat the temperature from room temperature to 450°C at a heating rate of 2°C / min, hold the temperature for 4 hours and then cool naturally; the final-calcination process is to heat the temperature from room temperature to 700°C at a heating rate of 2°C / min, hold the temperature for 6 hours and then cool naturally.

2. Use of the nickel cross-linked sodium alginate induced sodium vanadium phosphate composite cathode material according to claim 1 in sodium-ion batteries, characterized in that: The nickel-crosslinked sodium alginate-induced sodium vanadium phosphate composite cathode material is used as a cathode material in sodium-ion batteries.

3. Use according to claim 2, characterized in that: The specific method is as follows: (1) Preparation of positive electrode sheet: 0.21 g of nickel crosslinked sodium alginate-induced sodium vanadium phosphate composite positive electrode material, 0.06 g of acetylene black conductive filler and 0.03 g of polyvinylidene fluoride binder were mixed in 1.6 mL of N-methylpyrrolidone organic solvent at a ratio of 7:2:1; the above mixture was placed in a ball mill jar and ball milled unidirectionally for 4 h to obtain a slurry, which was then coated on a single-sided carbon-coated aluminum foil; the aluminum foil coated with slurry was then dried at 40 °C for 4 h, followed by vacuum drying at 120 °C for 6 h, and cut into 16 mm circular electrode sheets; (2) Preparation of button cell: Using the circular electrode obtained in step (1) as the positive electrode, metallic sodium as the negative electrode, and ceramic Celgard membrane as the separator, the 2025 type button cell is assembled in a vacuum glove box; wherein, the electrolyte is 1M sodium perchlorate dissolved in a ethylene carbonate / diethyl carbonate system with a volume ratio of 1:1, and 5wt% fluoroethylene carbonate is added based on sodium perchlorate.

Images