Reduced graphene oxide modified sodium vanadium fluorophosphate positive electrode material, preparation method and application thereof

Abstract

This invention provides a reduced graphene oxide-modified sodium vanadium fluorophosphate cathode material, its preparation method, and its applications. The preparation method includes: ball milling a sodium vanadium fluorophosphate precursor and graphene oxide, followed by sintering to obtain the reduced graphene oxide-modified sodium vanadium fluorophosphate cathode material. This invention uses reduced graphene oxide to modify sodium vanadium fluorophosphate, which can improve the conductivity of the sodium vanadium fluorophosphate material and enhance its sodium ion conductivity, enabling rapid insertion and extraction of sodium ions, thereby effectively improving the rate performance and cycle performance of the cathode material.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to reduced graphene oxide modified sodium vanadium fluorophosphate cathode material, its preparation method and application. Background Technology

[0002] Lithium-ion batteries boast high energy density and are used in various fields, including portable electronic devices, electric vehicles, and smart grids. The rapid development of portable electronic devices and electric vehicles has significantly increased the demand for lithium batteries, leading to a sharp rise in the price of scarce lithium ore. Consequently, many researchers have begun searching for new alternative energy sources. Sodium is the fourth most abundant element in the world, relatively inexpensive, and readily available for mass production. Furthermore, sodium's electrode potential relative to the standard hydrogen electrode is -2.71V, only 0.3V higher than that of lithium, making sodium-ion batteries a potential next-generation energy storage device.

[0003] Sodium-ion battery cathode systems can be broadly classified into layered oxides, Prussian blue compounds, and polyanionic compounds based on their crystal structure. For example, CN115954463B discloses a layered oxide composite material for sodium-ion batteries with the molecular formula M. x B y / NaNi a Fe b Mn c O2, where M is an alkali metal element, 1≤x, y≤9; 0≤a, b, c≤1, and a+b+c=1. For example, CN115611296B discloses a method for preparing and applying a Prussian blue sodium-ion battery cathode material. The preparation method is as follows: under a protective atmosphere, a soluble sodium hexacyano salt of transition metal N is dissolved in deionized water to obtain solution A; a soluble salt of transition metal M, a sodium salt of complexing agent, and an antioxidant are dissolved in water to obtain solution B; solution B is slowly added dropwise to solution A, stirred and reacted, aged after reaction, and then separated by solid-liquid separation, washing, and drying to obtain the Prussian blue sodium-ion battery cathode material to be treated; the Prussian blue sodium-ion battery cathode material to be treated and a dehydrating agent are added to an anhydrous organic solvent, then ball-milled, and separated by solid-liquid separation to obtain undried Prussian blue sodium-ion battery cathode material.

[0004] Sodium vanadium fluoride phosphate (Na3V2(PO4)2O2F, abbreviated as NVPOF), a polyanionic cathode material, has attracted widespread attention due to its advantages of good structural stability and high operating platform voltage. However, its low electronic conductivity and slow sodium ion diffusion kinetics lead to poor rate performance and cycle stability.

[0005] Therefore, there is an urgent need for a method to improve the electronic conductivity and sodium ion diffusion capacity of NVPOF, thereby enhancing its rate performance and cycling stability. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a reduced graphene oxide-modified sodium vanadium fluorophosphate cathode material, its preparation method, and its applications. This invention utilizes reduced graphene oxide (rGO) to modify sodium vanadium fluorophosphate (NVPOF), which enhances the conductivity of the sodium vanadium fluorophosphate material and improves its sodium ion conductivity, enabling rapid insertion and extraction of sodium ions. This effectively improves the rate performance and cycle performance of the NVPOF cathode material.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a reduced graphene oxide-modified sodium vanadium fluorophosphate cathode material, the preparation method comprising:

[0009] The sodium vanadium fluorophosphate precursor and graphene oxide were ball-milled and sintered to obtain the reduced graphene oxide modified sodium vanadium fluorophosphate cathode material.

[0010] This invention provides a method for preparing sodium vanadium fluorophosphate (NVPOF) cathode material modified with reduced graphene oxide (rGO). The ball milling process ensures uniform mixing of the NVPOF precursor and graphene oxide (GO), forming a GO coating layer. This structure effectively improves the structural stability of the NVPOF material. Through sintering, the GO coating layer transforms into a reduced graphene oxide (rGO) coating layer. The reduced graphene oxide enhances the conductivity of the NVPOF material and improves its sodium ion conductivity, enabling rapid sodium ion insertion and extraction. Therefore, the preparation method of this invention effectively improves the rate performance and cycle performance of the NVPOF cathode material. Furthermore, the process steps of this invention are simple, the total process time is short, and it is suitable for large-scale mass production.

[0011] Preferably, the oxygen content of the graphene oxide is 0-10%, and not 0, for example, it can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, etc.

[0012] In this invention, the structural modification and function of the graphene oxide material are superior within the aforementioned oxygen content range. Excessive oxygen content will affect the structural stability of the coating layer.

[0013] Preferably, the particle size D50 of the graphene oxide is 10-50 μm, for example, it can be 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm or 45 μm, etc.

[0014] Preferably, the graphene oxide has 1 to 5 layers, for example, 1, 2, 3, 4 or 5 layers.

[0015] Preferably, the particle size D50 of the sodium vanadium fluorophosphate precursor is 6-10 μm, for example, it can be 6 μm, 7 μm, 8 μm, 9 μm or 10 μm.

[0016] Preferably, the mass ratio of the sodium vanadium fluorophosphate precursor to graphene oxide is 1:(0.02-0.1), for example, it can be 1:0.02, 1:0.04, 1:0.06, 1:0.08 or 1:0.1, etc.

[0017] In this invention, when the mass ratio of NVPOF precursor to graphene oxide is too small, the amount of graphene oxide added is too large, which will lead to an increase in side reactions and poor rate and cycle performance; when the mass ratio of NVPOF precursor to graphene oxide is too large, the amount of graphene oxide added is insufficient, which will lead to poor rate and cycle performance.

[0018] Preferably, the rotational speed of the ball mill is 600-1000 rpm, for example, it can be 600 rpm, 650 rpm, 700 rpm, 750 rpm, 800 rpm, 850 rpm, 900 rpm, 950 rpm or 1000 rpm.

[0019] In this invention, when the ball mill speed is too low or too high, it will lead to uneven mixing, poor mixing ratio, and poor circulation performance.

[0020] Preferably, the ball milling time is 4-6 hours, for example, 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours.

[0021] Preferably, the sintering temperature is 500-550℃, for example, it can be 500℃, 510℃, 520℃, 530℃, 540℃ or 550℃.

[0022] In this invention, when the sintering temperature is too low or too high, it will lead to uneven sintering of the coating layer and poor rate and cycle performance.

[0023] Preferably, the heating rate of the sintering is 3-5℃ / min, for example, it can be 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min or 5℃ / min, etc.

[0024] Preferably, the sintering time is 0.8-1h, for example, it can be 0.8h, 0.85h, 0.9h, 0.95h or 1h.

[0025] Preferably, the sintering atmosphere is an inert atmosphere, and the gas in the inert atmosphere includes, but is not limited to, nitrogen.

[0026] As a preferred technical solution of the present invention, the preparation method includes the following steps:

[0027] (1) Dissolve sodium source, fluorine source, vanadium source and phosphorus source in solvent, and then hydrothermally react at 160-180℃ (e.g., 165℃, 170℃ or 175℃, etc.) for 20-24h (e.g., 21h, 22h or 23h, etc.) to obtain sodium vanadium fluoride phosphate precursor, wherein the molar ratio of sodium source, fluorine source, vanadium source and phosphorus source is 1:1:2:2;

[0028] (2) The sodium vanadium fluorophosphate precursor and graphene oxide are ball-milled at 600-1000 rpm for 4-6 hours, and then sintered at 500-550℃ for 0.8-1 hours to obtain the reduced graphene oxide modified sodium vanadium fluorophosphate cathode material.

[0029] In this invention, a hydrothermal method is used to synthesize sodium vanadium fluorophosphate precursor, followed by ball milling and sintering. This process is simple, has a short total processing time, and is suitable for large-scale mass production. However, if the hydrothermal reaction temperature is too low, it will affect the synthesis of the NVPOF precursor, resulting in poor rate performance and cycle life of the rGO-modified NVPOF cathode material.

[0030] Preferably, the sodium source includes at least one of sodium carbonate, sodium chloride, sodium nitrate, sodium sulfate, and sodium acetate.

[0031] Preferably, the fluorine source includes sodium fluoride and / or ammonium fluoride.

[0032] Preferably, the vanadium source includes at least one selected from sodium vanadate, ammonium vanadate, sodium metavanadate, and ammonium metavanadate.

[0033] Preferably, the phosphorus source includes ammonium dihydrogen phosphate and / or ammonium hydrogen phosphate.

[0034] In a second aspect, the present invention provides a reduced graphene oxide modified sodium vanadium fluorophosphate cathode material, wherein the reduced graphene oxide modified sodium vanadium fluorophosphate cathode material is prepared by the preparation method described in the first aspect.

[0035] Preferably, the reduced graphene oxide modified sodium vanadium fluorophosphate cathode material comprises a sodium vanadium fluorophosphate matrix and a reduced graphene oxide coating layer covering the surface of the matrix.

[0036] Thirdly, the present invention provides a sodium-ion battery, wherein the positive electrode of the sodium-ion battery comprises the reduced graphene oxide modified sodium vanadium fluorophosphate positive electrode material described in the second aspect.

[0037] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0039] This invention provides a method for preparing sodium vanadium fluorophosphate (NVPOF) cathode material modified with reduced graphene oxide (rGO). The ball milling process ensures uniform mixing of the NVPOF precursor and graphene oxide (GO), forming a GO coating layer. This structure effectively improves the structural stability of the NVPOF material. Through sintering, the GO coating layer transforms into a reduced graphene oxide (rGO) coating layer. The reduced graphene oxide enhances the conductivity of the NVPOF material and improves its sodium ion conductivity, enabling rapid sodium ion insertion and extraction. Therefore, the preparation method of this invention effectively improves the rate performance and cycle performance of the NVPOF cathode material. Furthermore, the process steps of this invention are simple, the total process time is short, and it is suitable for large-scale mass production.

[0040] Compared to the unmodified form, the specific capacity of the rGO-modified sodium vanadium fluorophosphate cathode material can be increased by 22%. Under the condition of 100 cycles at a current of 50 mA / g, the discharge capacity and capacity retention of the modified material can be increased by approximately 23 mAh and 27%, respectively; under the condition of 500 cycles at a current of 200 mA / g, the post-cycle capacity and capacity retention of the modified material can be increased by approximately 30 mAh and 42%, respectively. The rate capability and cycle performance of the rGO-modified sodium vanadium fluorophosphate cathode material are both improved. Detailed Implementation

[0041] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0042] Example 1

[0043] This embodiment provides a method for preparing rGO-modified NVPOF cathode material, including the following steps:

[0044] (1) NaF, Na2CO3, NH4VO3 and NH4H2PO4 were dissolved in deionized water / ethylene glycol in a molar ratio of 1:1:2:2 and stirred until homogeneous to form a mixed solution. The mixed solution was then transferred to a polytetrafluoroethylene (PTFE) hydrothermal reactor and kept at 180°C for 24 hours. After natural cooling, the precipitate was washed with deionized water and alcohol to obtain the NVPOF precursor.

[0045] (2) Place 1g of NVPOF precursor and 0.05g of graphene oxide (GO) in a high-energy ball mill at 1000rpm for 6h to obtain the ball milled product. The particle size D50 of the NVPOF precursor is 8μm, the particle size D50 of the graphene oxide is 30μm, the number of graphene oxide layers is 1 to 5, and the oxygen content of the graphene oxide is 0-10%, and not 0.

[0046] (3) The ball-milled product was placed in an N2 environment and heated to 550°C at a heating rate of 5°C / min. The temperature was maintained at this temperature for 1 hour, and then allowed to cool naturally to obtain the NVPOF-rGO cathode material. The NVPOF-rGO cathode material includes an NVPOF matrix and an rGO coating layer on the surface of the matrix.

[0047] Example 2

[0048] This embodiment provides a method for preparing rGO-modified NVPOF cathode material, including the following steps:

[0049] (1) NaF, Na2CO3, NH4VO3 and NH4H2PO4 were dissolved in deionized water / ethylene glycol in a molar ratio of 1:1:2:2 and stirred until homogeneous to form a mixed solution. The mixed solution was then transferred to a polytetrafluoroethylene (PTFE) hydrothermal reactor and kept at 180°C for 24 hours. After natural cooling, the precipitate was washed with deionized water and alcohol to obtain the NVPOF precursor.

[0050] (2) Place 1g of NVPOF precursor and 0.05g of graphene oxide (GO) in a high-energy ball mill at 800rpm for 1h to obtain the ball milled product. The particle size D50 of the NVPOF precursor is 8μm, the particle size D50 of the graphene oxide is 30μm, the number of layers of the graphene oxide is 1 to 5, and the oxygen content of the graphene oxide is 0-10%, and not 0.

[0051] (3) The ball-milled product was placed in an N2 environment and heated to 550°C at a heating rate of 5°C / min. The temperature was maintained at this temperature for 1 hour, and then allowed to cool naturally to obtain the NVPOF-rGO cathode material. The NVPOF-rGO cathode material includes an NVPOF matrix and an rGO coating layer on the surface of the matrix.

[0052] Example 3

[0053] This embodiment provides a method for preparing rGO-modified NVPOF cathode material, including the following steps:

[0054] (1) NaF, Na2CO3, NH4VO3 and NH4H2PO4 were dissolved in deionized water / ethylene glycol in a molar ratio of 1:1:2:2 and stirred until homogeneous to form a mixed solution. The mixed solution was then transferred to a polytetrafluoroethylene (PTFE) hydrothermal reactor and kept at 180°C for 24 hours. After natural cooling, the precipitate was washed with deionized water and alcohol to obtain the NVPOF precursor.

[0055] (2) Place 1g of NVPOF precursor and 0.05g of graphene oxide (GO) in a high-energy ball mill at 600rpm for 1h to obtain the ball milled product. The particle size D50 of the NVPOF precursor is 8μm, the particle size D50 of the graphene oxide is 30μm, the number of graphene oxide layers is 1 to 5, and the oxygen content of the graphene oxide is 0-10%, and not 0.

[0056] (3) The ball-milled product was placed in an N2 environment and heated to 550°C at a heating rate of 5°C / min. The temperature was maintained at this temperature for 1 hour, and then allowed to cool naturally to obtain the NVPOF-rGO cathode material. The NVPOF-rGO cathode material includes an NVPOF matrix and an rGO coating layer on the surface of the matrix.

[0057] Example 4

[0058] The difference between this embodiment and embodiment 1 is that the hydrothermal temperature in step (1) is adjusted to 160°C, while the other parameters are exactly the same as in embodiment 1.

[0059] Example 5

[0060] The difference between this embodiment and embodiment 1 is that the hydrothermal temperature in step (1) is adjusted to 170°C, while the other parameters are exactly the same as in embodiment 1.

[0061] Example 6

[0062] The difference between this embodiment and Embodiment 1 is that the mass ratio of NVPOF precursor to graphene oxide is adjusted to 1:0.03, while the other parameters are exactly the same as in Embodiment 1.

[0063] Example 7

[0064] The difference between this embodiment and Embodiment 1 is that the mass ratio of NVPOF precursor to graphene oxide is adjusted to 1:0.07, while the other parameters are exactly the same as in Embodiment 1.

[0065] Example 8

[0066] The difference between this embodiment and embodiment 1 is that the sintering temperature in step (3) is adjusted to 500°C, while the other parameters are exactly the same as in embodiment 1.

[0067] Example 9

[0068] The difference between this embodiment and embodiment 1 is that the sintering temperature in step (3) is adjusted to 600°C, while the other parameters are exactly the same as in embodiment 1.

[0069] Comparative Example 1

[0070] The difference between this comparative example and Example 1 is that step (2) is omitted, that is, graphene oxide is not added and ball milling is not performed, while the other parameters are exactly the same as those in Example 1.

[0071] Comparative Example 2

[0072] This comparative example provides a method for preparing NVPOF-rGO cathode material, including:

[0073] (1) NaF, Na2CO3, NH4VO3, and NH4H2PO4 were dissolved in deionized water / ethylene glycol in a molar ratio of 1:1:2:2 and stirred until homogeneous to form a mixed solution. Then, the graphene oxide dispersion was added to the mixed solution and mixed until homogeneous. The final solution was then transferred to a polytetrafluoroethylene (PTFE) hydrothermal reactor and kept at 180°C for 24 hours. After natural cooling, the precipitate was washed with deionized water and alcohol to obtain the NVPOF precursor.

[0074] (2) The NVPOF precursor was placed in an N2 environment and heated to 550°C at a heating rate of 5°C / min. It was then kept at this temperature for 1 hour and then cooled naturally to obtain the NVPOF-rGO cathode material.

[0075] Comparative Example 3

[0076] The difference between this comparative example and Example 1 is that step (1) is omitted, and the NVPOF precursor in step (2) is replaced with NVPF (sodium vanadium fluorophosphate, Na3V2(PO4)2F3) precursor. The remaining parameters are exactly the same as those in Example 1.

[0077] Performance testing

[0078] The positive electrode materials, Super P, and polyvinylidene fluoride (PVDF) provided in Examples 1-9 and Comparative Examples 1-3 were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 96.5:2:1.5 to obtain a slurry. After coating and drying, a positive electrode was obtained. The positive electrode, negative electrode, and separator were assembled into a sodium-ion battery, wherein the negative electrode was graphite, the separator was a polyethylene (PE) membrane, and NaPF6 electrolyte was used.

[0079] Test A: The battery is subjected to 100 charge-discharge cycles at a current density of 50 mA / g. The initial discharge capacity and the discharge capacity after 100 cycles are recorded, and the capacity retention rate is calculated as: discharge capacity after 100 cycles / initial discharge capacity * 100%.

[0080] Test B: The battery was subjected to 500 charge-discharge cycles at a current density of 200 mA / g. The initial discharge capacity and the discharge capacity after 500 cycles were recorded, and the capacity retention rate was calculated as: discharge capacity after 500 cycles / initial discharge capacity * 100%.

[0081] The test results are shown in Table 1.

[0082] Table 1

[0083]

[0084] analyze:

[0085] As can be seen from the results of Examples 1-3, the NVPOF-rGO cathode material prepared by the method of the present invention exhibits excellent cycle performance and high capacity retention in sodium-ion batteries at current densities of 50 mA / g and 200 mA / g. Furthermore, the ratio of the initial capacity of the same sodium-ion battery at a current density of 200 mA / g to that at a current density of 50 mA / g is relatively large, indicating that the battery has excellent rate performance.

[0086] The results of Examples 1 and 4-5 show that if the hydrothermal reaction temperature is too low when preparing the NVPOF precursor, it will affect the synthesis of the NVPOF precursor, and thus make the rate performance and cycle performance of the rGO modified NVPOF cathode material slightly worse.

[0087] The results of Examples 1 and 6-7 show that when the mass ratio of NVPOF precursor to graphene oxide is too high, the amount of GO added is insufficient, which will result in slightly poor rate and cycle performance; when the mass ratio of NVPOF precursor to graphene oxide is too low, the amount of GO added is too high, which will result in increased side reactions and slightly poor rate and cycle performance.

[0088] The results of Examples 1 and 8-9 show that when the sintering temperature is too low, the coating layer will be unevenly sintered, resulting in slightly poor rate of change and cycle performance; when the sintering temperature is too high, the coating layer will be unevenly sintered, resulting in poor rate of change and cycle performance.

[0089] The results of Example 1 and Comparative Example 1 show that without the addition of graphene oxide and without ball milling, a uniform coating layer cannot be formed on the surface of the NVPOF raw material, resulting in poor rate and cycle performance and low capacity.

[0090] The results of Example 1 and Comparative Example 2 show that if a graphene oxide dispersion is added during the hydrothermal synthesis of the NVPOF precursor, a uniform coating layer cannot be formed, but a complex is formed instead, resulting in poor rate and cycle performance and low capacity.

[0091] As can be seen from the results of Example 1 and Comparative Example 3, modifying NVPOF using the method of the present invention can effectively improve its rate capability and cycle performance, and its capacity is higher, all of which are superior to modified NVPF materials.

[0092] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a reduced graphene oxide-modified sodium vanadium fluorophosphate cathode material, characterized in that, The preparation method includes: The sodium vanadium fluorophosphate precursor and graphene oxide were ball-milled and sintered to obtain the reduced graphene oxide modified sodium vanadium fluorophosphate cathode material. The mass ratio of the sodium vanadium fluorophosphate precursor to graphene oxide is 1:(0.05-0.06); the particle size D50 of the sodium vanadium fluorophosphate precursor is 6-10. m; The sintering temperature is 500-550℃; the sintering heating rate is 3-5℃ / min; the sintering time is 0.8-1h; and the sintering atmosphere is an inert atmosphere. The oxygen content of the graphene oxide is 0-10%, and not 0; the particle size D50 of the graphene oxide is 10-50. m; the number of layers of the graphene oxide is 1-5; The ball mill rotates at 600-1000 rpm; the ball milling time is 4-6 hours.

2. The preparation method according to claim 1, characterized in that, The preparation method includes the following steps: (1) Dissolve sodium source, fluorine source, vanadium source and phosphorus source in solvent, and then hydrothermally react at 160-180℃ for 20-24h to obtain sodium vanadium fluoride phosphate precursor, wherein the molar ratio of sodium source, fluorine source, vanadium source and phosphorus source is 1:1:2:2; (2) The sodium vanadium fluorophosphate precursor and graphene oxide are ball-milled at 600-1000 rpm for 4-6 h, and then sintered at 500-550℃ for 0.8-1 h to obtain the reduced graphene oxide modified sodium vanadium fluorophosphate cathode material.

3. A reduced graphene oxide-modified sodium vanadium fluorophosphate cathode material, characterized in that, The reduced graphene oxide modified sodium vanadium fluorophosphate cathode material is prepared by the preparation method described in claim 1 or 2.

4. The reduced graphene oxide modified sodium vanadium fluorophosphate cathode material according to claim 3, characterized in that, The reduced graphene oxide modified sodium vanadium fluorophosphate cathode material includes a sodium vanadium fluorophosphate matrix and a reduced graphene oxide coating layer covering the surface of the matrix.

5. A sodium-ion battery, characterized in that, The positive electrode of the sodium-ion battery includes the reduced graphene oxide modified sodium vanadium fluorophosphate positive electrode material as described in claim 4.

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