Positive electrode material, preparation method, pole piece and battery

Abstract

The invention relates to a positive electrode material, a preparation method, a pole piece and a battery. The positive electrode material comprises a wire wrapping structure, wherein the wire wrapping structure comprises a core material and a shell layer coating the core material in the axial direction; the core material comprises a sodium vanadium phosphate material, and the shell layer comprises a sodium ferric pyrophosphate material. The preparation method of the positive electrode material comprises the following steps that S1, coaxial electrostatic spinning is conducted on a sodium vanadium phosphate spinning solution and a sodium ferric phosphate pyrophosphate spinning solution, the sodium vanadium phosphate spinning solution is located on the inner layer, the sodium ferric phosphate pyrophosphate spinning solution is located on the outer layer, and a precursor film is obtained; and S2, carrying out heat treatment on the precursor film in a protective atmosphere. According to the invention, the sodium vanadium phosphate material is coated with the sodium ferric phosphate pyrophosphate material to form a one-dimensional nano envelope structure, and the one-dimensional nano envelope structure is used as the positive electrode material in the battery, so that the rate capability and the cycle performance of the battery are improved.

Description

Technical Field

[0001] The present invention relates to a cathode material, a preparation method, an electrode sheet and a battery. Background Art

[0002] Among the numerous energy storage technologies under research currently, electrochemical energy storage has its unique advantages, such as high energy density, high energy conversion efficiency, fast response speed, etc., and has broad application prospects in the energy field. Currently, there are four types of secondary batteries that already have energy storage demonstration projects: lead-acid batteries, sodium-sulfur batteries, vanadium redox flow batteries and lithium-ion batteries. However, each of the above batteries has its own limitations, awaiting improvement by relevant staff. Sodium-ion batteries have the same working principle as lithium-ion batteries, are rich in resources and have excellent low-temperature performance, thus attracting extensive attention from research teams around the world.

[0003] The sodium-ion cathode material is one of the key materials for sodium-ion batteries. The existing cathode materials are mainly divided into layered oxides, polyanion types and Prussian blue types. Sodium vanadium phosphate (NVP) is a polyanion cathode material with a nasicon structure, and its theoretical specific capacity is 117 mAh / g, having a relatively high working voltage (3.4 V vs Na / Na + ) and a crystal structure with high stability, and can maintain the structural stability during charge and discharge.

[0004] However, the electrical conductivity of this type of material is poor, and it is necessary to optimize its ionic and electronic conductivities by nano-scale sizing and surface carbon coating. Summary of the Invention

[0005] In order to solve the defects existing in the prior art, the present invention provides a cathode material, a preparation method, an electrode sheet and a battery. The present invention wraps the sodium vanadium pyrophosphate material around the sodium vanadium phosphate material to form a one-dimensional nano-core-shell structure, improving the rate performance and cycling performance of the material.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a cathode material, which includes a core-shell structure, and the core-shell structure includes a core material and a shell layer that axially coats the core material; the core material includes sodium vanadium phosphate, and the shell layer includes sodium iron pyrophosphate.

[0008] In the present invention, the ratio of the average equivalent diameter of the core material to the average thickness of the shell layer can be (100 - 160):(50 - 100), preferably (120 - 130):(70 - 90), such as 120:70, 125:72 or 130:90.

[0009] In the present invention, the average equivalent diameter of the core material can be 100 - 160 nm, preferably 120 - 130 nm, such as 120 nm, 125 nm or 130 nm.

[0010] In the present invention, the average thickness of the shell layer can be 50 - 100 nm, preferably 70 - 90 nm, such as 70 nm, 72 nm or 90 nm.

[0011] In the present invention, a first carbon layer may further be included between the core material and the shell layer. The thickness of the first carbon layer preferably does not exceed 10 nm.

[0012] In the present invention, a second carbon layer axially coating the shell layer may further be included outside the shell layer. The thickness of the second carbon layer preferably does not exceed 10 nm.

[0013] In the present invention, the sizes of the core material, the shell layer and the carbon layer can be obtained by conventional testing methods in the art. A typical method is as follows: Obtain the TEM image of the positive electrode material, combine with the structural features of the material presented in the TEM image, and cooperate with auxiliary means such as EDS to identify the core material, the shell layer and the carbon layer, and measure the corresponding sizes; Measure the relevant sizes of several sampling points in the TEM image and obtain their average value.

[0014] In the present invention, the shell layer may further include a conductive agent. The conductive agent is a conventional selection in the art, preferably graphene. The mass ratio of the conductive agent to the molar amount of Fe atoms in the sodium iron pyrophosphate phosphate is preferably (0.1 - 1) g: 12 mmol, such as 0.5 g: 12 mmol.

[0015] In a second aspect, the present invention provides a method for preparing a positive electrode material, which includes the following steps:

[0016] S1. Coaxial electrospinning of a sodium vanadium phosphate spinning solution and a sodium iron pyrophosphate phosphate spinning solution, with the sodium vanadium phosphate spinning solution located in the inner layer and the sodium iron pyrophosphate phosphate spinning solution located in the outer layer, to obtain a precursor film;

[0017] S2. Heat-treat the precursor film in a protective atmosphere.

[0018] In the present invention, in step S1, the sodium vanadium phosphate spinning solution is a conventional selection in the art. Generally, it is considered that sodium vanadium phosphate can be generated after heat treatment in a protective atmosphere.

[0019] In certain specific embodiments of the present invention, in step S1, the sodium vanadium phosphate spinning solution may include a sodium vanadium phosphate precursor, a first polymer and a first solvent.

[0020] In certain specific embodiments of the present invention, in step S1, the sodium vanadium phosphate spinning solution may include sodium vanadium phosphate, a first polymer, and a first solvent.

[0021] In the present invention, in step S1, the sodium iron pyrophosphate phosphate spinning solution is a conventional selection in the art. Generally, it is considered that after heat treatment in a protective atmosphere, sodium iron pyrophosphate phosphate can be generated.

[0022] In certain specific embodiments of the present invention, in step S1, the sodium iron pyrophosphate phosphate spinning solution may include sodium iron pyrophosphate phosphate, a second polymer, and a second solvent.

[0023] In certain specific embodiments of the present invention, in step S1, the sodium iron pyrophosphate phosphate spinning solution may include a sodium iron pyrophosphate phosphate precursor, a second polymer, and a second solvent.

[0024] In the present invention, in step S1, the raw materials of the sodium vanadium phosphate precursor may include a sodium source, a vanadium source, a phosphorus source, and a reducing agent.

[0025] Among them, the sodium source is a conventional selection in the art and may be selected from one or more of sodium carbonate, sodium oxalate, sodium acetate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0026] Among them, the phosphorus source is a conventional selection in the art and may be selected from one or more of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, ammonium phosphate, and diammonium hydrogen phosphate.

[0027] Among them, the reducing agent is a conventional selection in the art and may be selected from one or more of citric acid, oxalic acid, tartaric acid, oxalic acid, adipic acid, malonic acid, ascorbic acid, sucrose, mandelic acid, malic acid, formaldehyde, acetaldehyde, n-butanal, isobutanal, tetraethylene glycol, isopropanol, hydrazine hydrate, and urea.

[0028] Among them, the molar ratio of Na atoms, V atoms, and P atoms in the sodium source, the vanadium source, and the phosphorus source is preferably 3:2:3.

[0029] Among them, the molar ratio of the reducing agent to the V atoms in the vanadium source is preferably 1:1.

[0030] Among them, the vanadium source is selected from one or more of vanadium pentoxide, vanadium dioxide, and ammonium metavanadate as a vanadium salt.

[0031] Among them, the molar amount of V atoms in the vanadium source and the mass of the first polymer are preferably 10 mmol:(2.5 - 4.5) g, for example, 10 mmol:3.3 g.

[0032] Among them, the molar amount of V atoms in the vanadium source and the mass of the first solvent are preferably 10 mmol:(2.5 - 4.5) g, such as 10 mmol:3.3 g.

[0033] In the present invention, in step S1, the first polymer is a conventional choice in the art and can be selected from one or more of polyacrylonitrile, polyvinyl alcohol, polylactic acid, polyurethane, and polyvinylpyrrolidone.

[0034] In the present invention, in step S1, the first solvent can be 1-butyl-3-methylimidazolium dihydrogen phosphate.

[0035] In the present invention, in step S1, in the sodium iron pyrophosphate phosphate spinning solution, the mass ratio of the sodium iron pyrophosphate precursor to the second polymer can be 1:(2.5 - 4), such as 1:3.

[0036] In the present invention, in step S1, the mass ratio of the second solvent to the sodium iron pyrophosphate precursor can be (2 - 3):1, such as 3:1.

[0037] In the present invention, in step S1, the raw materials of the sodium iron pyrophosphate precursor can include a sodium source, an iron source, a phosphorus source, and a reducing agent.

[0038] Among them, the sodium source is a conventional choice in the art and can be selected from one or more of sodium carbonate, sodium oxalate, sodium acetate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0039] Among them, the phosphorus source is a conventional choice in the art and can be selected from one or more of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, ammonium phosphate, and diammonium hydrogen phosphate.

[0040] Among them, the reducing agent is a conventional choice in the art and can be selected from one or more of citric acid, oxalic acid, tartaric acid, oxalic acid, adipic acid, malonic acid, ascorbic acid, sucrose, mandelic acid, malic acid, formaldehyde, acetaldehyde, n-butanal, isobutanal, tetraethylene glycol, isopropanol, hydrazine hydrate, and urea.

[0041] Among them, the molar ratio of Na atoms, Fe atoms, and P atoms in the sodium source, the iron source, and the phosphorus source is preferably 4:3:4.

[0042] Among them, the molar ratio of the reducing agent to the Fe atoms in the iron source is preferably 1:1.

[0043] Among them, the iron source is a conventional choice in the art, such as iron(III) nitrate nonahydrate.

[0044] Among them, the raw materials of the sodium iron pyrophosphate phosphate precursor preferably further include a conductive agent. The conductive agent is preferably graphene. The mass ratio of the conductive agent to the molar amount of Fe atoms in the iron source is preferably (0.1-1) g: 12 mmol, such as 0.5 g: 12 mmol.

[0045] Among them, the raw materials of the sodium iron pyrophosphate phosphate precursor preferably further include a dispersant. The dispersant is preferably azobisisobutyramidine hydrochloride. The mass ratio of the dispersant to the molar amount of Fe atoms in the iron source is preferably (0.05-0.5) g: 12 mmol, such as 0.1 g: 12 mmol.

[0046] In the present invention, in step S1, the second polymer is a conventional selection in the art and can be selected from one or more of polyacrylonitrile, polyvinyl alcohol, polylactic acid, polyurethane, and polyvinylpyrrolidone.

[0047] In the present invention, in step S1, the second solvent can be 1-butyl-3-methylimidazolium dihydrogen phosphate.

[0048] In the present invention, in step S1, the volume flow rate ratio of the sodium vanadate phosphate spinning solution to the sodium iron pyrophosphate phosphate spinning solution can be (1-3): 1, such as 2: 1.

[0049] In the present invention, in step S1, the linear flow rates of the sodium vanadate phosphate spinning solution and the sodium iron pyrophosphate phosphate spinning solution can be the same.

[0050] In the present invention, in step S1, the voltage of the coaxial electrospinning is 10-30 kV, such as 15 kV.

[0051] In the present invention, in step S1, the feeding speed of the coaxial electrospinning is 1-20 mL / h, such as 10 mL / h.

[0052] In the present invention, in step S1, the rotation speed of the receiver of the coaxial electrospinning is 500-5000 rpm, such as 2500 rpm.

[0053] In the present invention, in step S2, the protective atmosphere can be a non-oxidizing atmosphere. The non-oxidizing atmosphere is preferably an argon atmosphere or a mixed atmosphere of hydrogen and argon. In the mixed atmosphere of hydrogen and argon, the volume fraction of hydrogen is preferably 5 vol%.

[0054] In the present invention, in step S2, the temperature of the heat treatment can be 300-550 °C.

[0055] In the present invention, in step S2, the time of the heat treatment is 2-16 h.

[0056] In the present invention, in step S2, the heating rate of the heat treatment is 2-3 °C / min.

[0057] In some specific embodiments of the present invention, in step S2, the process of the heat treatment is as follows: heating to 300 °C at a rate of 3 °C / min, heating for 2-6 h, then heating to 550 °C at a rate of 3 °C / min, and heating for 4-10 h, for example, 8.5 h.

[0058] In a third aspect, the present invention provides a positive electrode material prepared by the method for preparing a positive electrode material as described above.

[0059] In a fourth aspect, the present invention provides a pole piece, which includes a current collector and a positive electrode material layer located on at least one surface of the current collector, and the positive electrode material layer includes the positive electrode material as described above.

[0060] In a fifth aspect, the present invention provides a battery, which includes the pole piece as described above.

[0061] On the basis of not violating the common knowledge in the art, the above preferred conditions can be combined arbitrarily to obtain various preferred examples of the present invention.

[0062] The reagents and raw materials used in the present invention are all commercially available.

[0063] The positive progress effects of the present invention are as follows:

[0064] (1) For the positive electrode material of the present invention, sodium iron pyrophosphate phosphate Na4Fe3(PO4)2P2O7 is wrapped around sodium vanadium phosphate Na3V2(PO4)3 to form a one-dimensional nano-wire structure, which improves the rate performance and cycle performance of the material;

[0065] (2) For the method for preparing the positive electrode material of the present invention, through the double-layer coaxial needle electrospinning technology, the raw materials involved are relatively cheap and easy to obtain, and it is easy to industrialize;

[0066] (3) The battery of the present invention has low resistance, high energy density, good rate performance and cycle performance, and has good electrochemical performance. Specific Embodiments

[0067] The present invention will be further illustrated by the following examples, but the present invention is not limited to the scope of the examples. The experimental methods without specific conditions in the following examples are carried out according to conventional methods and conditions, or selected according to the product instructions.

[0068] In the following examples and comparative examples, the raw material conditions are as follows:

[0069] Polyacrylonitrile (PAN), purchased from BASF in Germany, model BSF1005K.

[0070] PVDF, purchased from Arkema, model Kynar Flex 2801.

[0071] The dispersant, azodiisobutyramidine hydrochloride, was purchased from Shanghai Songyue New Energy Technology Co., Ltd., model V-50.

[0072] Whatman glass fiber membrane, model GF / D, 1823-090, pore size 2.7 μm.

[0073] Example 1

[0074] A cathode material with a one-dimensional nano-wire structure suitable for sodium-ion batteries and its preparation method, including the following steps:

[0075] 1. Take 4.84 g (12 mmol) of ferric nitrate nonahydrate, 1.08 g of anhydrous oxalic acid, 1.7 g of sodium carbonate, and 1.92 g of sodium dihydrogen phosphate, dissolve them in 100 mL of deionized water in turn, put them into a beaker, and finally add 0.5 g of graphene and 0.1 g of dispersant V-50. Place the mixed solution on a magnetic stirrer and stir for 30 min until it is evenly mixed.

[0076] 2. Vacuum-dry the above solution at 60 °C, grind the obtained solid to obtain a powdery precursor.

[0077] 3. Dissolve 1-butyl-3-methylimidazolium dihydrogen phosphate, polyacrylonitrile (PAN), and the above precursor powder in 1-butyl-3-methylimidazolium dihydrogen phosphate ([BMIm]H2PO4) at a mass ratio of 3:3:1 to obtain a sodium iron pyrophosphate NaFeP2O7 spinning solution, abbreviated as NFPP spinning solution.

[0078] 4. Weigh 2.01 g of sodium oxalate, 1.82 g of vanadium pentoxide V2O5, 3.45 g of ammonium dihydrogen phosphate (30 mmol), 3.84 g of citric acid, 3.3 g of polyacrylonitrile (PAN), and 3.3 g of [BMIm]H2PO4, mix and stir evenly to prepare a sodium vanadium phosphate Na3V2(PO4)3 spinning solution, abbreviated as NVP spinning solution.

[0079] 5. Draw the NFPP spinning solution into the outer injection pump of a double-layer coaxial needle, draw the NVP spinning solution into the inner injection pump of the double-layer coaxial needle, and perform electrospinning to obtain an electrospinning precursor; the specific parameters are: voltage 15 kV, propulsion speed 10 mL / h, receiver rotation speed 2500 rpm. The volume flow rate ratio of the NFPP spinning solution to the NVP spinning solution is 1:2, and the linear flow rate is the same.

[0080] 6. Place the above precursor film in a vacuum tube furnace with an argon or 5 vol% H2 / Ar mixed atmosphere. First, introduce gas at room temperature for 30 min to fill the quartz tube with an inert atmosphere, then heat it to 300 °C at a rate of 3 °C / min and heat for 2 hours, then heat it to 550 °C at the same rate and heat for 8.5 hours. Finally, naturally cool it to room temperature under an argon or 5 vol% H2 / Ar mixed atmosphere, and a one-dimensional nano-encapsulated wire structure product with NVP as the core material and NFPP as the shell material is obtained. In the core material, the average equivalent diameter of NVP is 120 nm. In the shell material, the average thickness of NFPP is 70 nm. Between the core material and the shell material, and outside the shell material, a carbon layer with a thickness of less than 10 nm is coated. The sizes of the core material, the shell layer, and the carbon layer are respectively obtained by acquiring the TEM images of the cathode material, combining the structural characteristics of the materials presented in the TEM images, and cooperating with auxiliary means such as EDS to identify the core material, the shell layer, and the carbon layer, and measuring the corresponding sizes; the relevant sizes of several sampling points are measured in the TEM images, and their average values are obtained.

[0081] Example 2

[0082] A cathode material with a one-dimensional nano-encapsulated wire structure suitable for sodium-ion batteries and its preparation method, including the following steps:

[0083] 1. Take 4.84 g (12 mmol) of ferric nitrate nonahydrate, 2.12 g of ascorbic acid, 2.14 g of sodium oxalate, and 1.92 g of sodium dihydrogen phosphate, dissolve them in 100 mL of deionized water in turn, put them into a beaker, and finally add 0.5 g of graphene and 0.1 g of dispersant V-50. Place the mixed solution on a magnetic stirrer and stir for 30 min until it is evenly mixed.

[0084] 2. Vacuum-dry the above solution at 60 °C, and grind the obtained solid to obtain a powdery precursor.

[0085] 3. Dissolve 1-butyl-3-methylimidazolium dihydrogen phosphate, polyacrylonitrile (PAN), and the above precursor powder in 1-butyl-3-methylimidazolium dihydrogen phosphate ([BMIm]H2PO4) at a mass ratio of 3:3:1 to obtain a sodium iron pyrophosphate NaFeP2O7 spinning solution, abbreviated as NFPP spinning solution.

[0086] 4. Weigh 1.59 g of sodium carbonate, 1.82 g of vanadium pentoxide V2O5 (10 mmol), 1.47 g of phosphoric acid, 3.84 g of citric acid, 3.3 g of polyacrylonitrile (PAN), and 3.3 g of [BMIm]H2PO4, mix and stir evenly to prepare a sodium vanadium phosphate Na3V2(PO4)3 spinning solution, abbreviated as NVP spinning solution.

[0087] 5. Pump the NFPP spinning solution into the outer injection pump of the double-layer coaxial needle, and pump the NVP spinning solution into the inner injection pump of the double-layer coaxial needle, and perform electrospinning to obtain an electrospinning precursor; the specific parameters are: voltage 15 kV, propulsion speed 10 mL / h, and receiver rotation speed 2500 rpm. The volume flow rate ratio of the NFPP spinning solution to the NVP spinning solution is 1:2, and the linear flow rates are the same.

[0088] 6. Place the above precursor film in a vacuum tube furnace with an argon or 5 vol% H2 / Ar mixed atmosphere. First, introduce gas at room temperature for 30 min to fill the quartz tube with an inert atmosphere, then heat it to 300 °C at a rate of 3 °C / min, heat for 2 hours, then heat it to 550 °C at the same rate, and heat for 8.5 hours. Finally, cool it naturally to room temperature under an argon or 5 vol% H2 / Ar mixed atmosphere, and a one-dimensional nano-encapsulated wire structure product with NVP as the core material and NFPP as the shell material is obtained. In the core material, the average equivalent diameter of NVP is 125 nm. In the shell material, the average thickness of NFPP is 72 nm. Between the core material and the shell material, and outside the shell material, there is a carbon layer with a thickness of less than 10 nm. The sizes of the core material, the shell layer, and the carbon layer are respectively obtained by acquiring the TEM images of the cathode material, combining the structural characteristics of the materials presented in the TEM images, and cooperating with auxiliary means such as EDS to identify the core material, the shell layer, and the carbon layer, and measuring the corresponding sizes; relevant sizes of several sampling points are measured in the TEM images, and their average values are obtained.

[0089] Example 3

[0090] A cathode material with a one-dimensional nano-encapsulated wire structure suitable for sodium ion batteries and its preparation method, including the following steps:

[0091] 1. Take 4.84 g of ferric nitrate nonahydrate, 1.08 g of anhydrous oxalic acid, 1.7 g of sodium carbonate, and 1.92 g of sodium dihydrogen phosphate, dissolve them in 100 mL of deionized water in sequence, put them into a beaker, and finally add 0.5 g of graphene and 0.1 g of dispersant V-50. Place the mixed solution on a magnetic stirrer and stir for 30 min until it is evenly mixed.

[0092] 2. Vacuum dry the above solution at 60 °C, and grind the obtained solid to obtain a powdery precursor.

[0093] 3. Dissolve 1-butyl-3-methylimidazolium dihydrogen phosphate, polyacrylonitrile (PAN), and the above precursor powder in 1-butyl-3-methylimidazolium dihydrogen phosphate ([BMIm]H2PO4) at a mass ratio of 3:3:1 to obtain a sodium iron pyrophosphate NaFeP2O7 spinning solution, abbreviated as NFPP spinning solution.

[0094] 4. Weigh 2.01 g of sodium oxalate, 1.82 g of vanadium pentoxide V2O5, 3.45 g of ammonium dihydrogen phosphate, 3.84 g of citric acid, 3.3 g of polyacrylonitrile (PAN) and 3.3 g of [BMIm]H2PO4, and mix them evenly by stirring to prepare a sodium vanadium phosphate Na3V2(PO4)3 spinning solution, abbreviated as NVP spinning solution.

[0095] 5. Extract the NFPP spinning solution into the outer injection pump of the double-layer coaxial needle, and extract the NVP spinning solution into the inner injection pump of the double-layer coaxial needle, and perform electrospinning to obtain an electrospinning precursor; the specific parameters are: voltage 15 kV, propulsion speed 10 mL / h, receiver rotation speed 2500 rpm. The volume flow rate ratio of the NFPP spinning solution to the NVP spinning solution is 1:2, and the linear flow rates are the same.

[0096] 6. Place the above precursor film in a vacuum tube furnace with an argon or 5 vol% H2 / Ar mixed atmosphere. First, ventilate for 30 min at room temperature to fill the quartz tube with an inert atmosphere, then heat it to 300 °C at a rate of 3 °C / min and heat for 2 hours, then heat it to 550 °C at the same rate and heat for 8.5 hours, and finally cool it naturally to room temperature in an argon or 5 vol% H2 / Ar mixed atmosphere, that is, a one-dimensional nano-encapsulated wire structure product with NVP as the core material and NFPP as the shell material is obtained. In the core material, the average equivalent diameter of NVP is 130 nm, in the shell material, the average thickness of NFPP is 90 nm. Between the core material and the shell material, and outside the shell material, a carbon layer with a thickness of less than 10 nm is coated. The sizes of the core material, the shell layer and the carbon layer are respectively obtained by obtaining the TEM images of the cathode material, combining the structural characteristics of the materials presented in the TEM images, and cooperating with auxiliary means such as EDS to identify the core material, the shell layer and the carbon layer, and measuring the corresponding sizes; the relevant sizes of several sampling points are measured in the TEM images, and their average values are obtained.

[0097] Comparative Example 1: Preparation of Na3V2(PO4)3 by solid-phase method

[0098] Weigh 0.182 g of V2O5 and 0.36 g of NaH2PO4, fully grind and mix them evenly, then transfer them to a tube furnace with an argon atmosphere, and calcine them at a high temperature of 800 °C for 8 h to obtain Na3V2(PO4)3 powder.

[0099] Comparative Example 2: Preparation of surface carbon-coated Na3V2(PO4)3 / C by sol-gel method

[0100] Weigh 0.182 g of V2O5 and 0.378 g of H2C2O4·2H2O, add them to 25 ml of deionized water, heat in a water bath at 80 °C with magnetic stirring until the solution turns blue, add 0.36 g of NaH2PO4 and continue stirring for 15 min. Weigh a certain amount of glucose according to the molar ratio of 0.18 g of glucose 1:1 and add it to the above solution, and continue to stir vigorously, then evaporate to dryness to form a gel.

[0101] Transfer the obtained gel to an incubator at 120 °C for sufficient drying and then grind it. Then, keep it at a constant temperature of 400 °C for 4 h in a tubular furnace with Ar / H2 mixed gas, and then raise the temperature to 800 °C and keep it at a constant temperature for 8 h to obtain the Na3V2(PO4)3 / C nanoparticle material.

[0102] Comparative Example 3: Preparation of Na4Fe3(PO4)2(P2O7) by solid-phase method

[0103] Weigh 2.42 g of ferric nitrate nonahydrate, 0.54 g of anhydrous oxalic acid, 0.85 g of sodium carbonate, and 0.96 g of sodium dihydrogen phosphate. After fully grinding and mixing evenly, transfer them to a tubular furnace under argon atmosphere and calcine at a high temperature of 550 °C for 8 h to obtain Na4Fe3(PO4)2(P2O7) powder.

[0104] Comparative Example 4: Preparation of Na4Fe3(PO4)2(P2O7) / C by sol-gel method

[0105] First, weigh 2.42 g of ferric nitrate nonahydrate and 1.06 g of ascorbic acid, add them to 25 ml of deionized water, heat in a water bath at 80 °C with magnetic stirring until the solution turns dark green, slowly add 0.92 g of ammonium dihydrogen phosphate and 1.07 g of sodium oxalate, and continue stirring for 15 min. Weigh a certain amount of glucose according to the molar ratio of 0.18 g of glucose 1:1 and add it to the above solution, and continue to stir vigorously, then evaporate to dryness to form a gel.

[0106] Transfer the obtained gel to an incubator at 120 °C for sufficient drying and then grind it. Then, keep it at a constant temperature of 300 °C for 4 h in a tubular furnace with Ar / H2 mixed gas, and then raise the temperature to 550 °C and keep it at a constant temperature for 8 h to obtain the Na4Fe3(PO4)2(P2O7) / C nanoparticle material.

[0107] Comparative Example 5: Preparation of Na3V2(PO4)3 / C by electrospinning method

[0108] Weigh 1.59 g of sodium carbonate, 1.82 g of vanadium pentoxide V2O5, 1.47 g of phosphoric acid, 3.84 g of citric acid, 3.3 g of polyacrylonitrile (PAN) and 3.3 g of [BMIm]H2PO4, mix and stir evenly to prepare the NVP spinning solution. Draw the NVP spinning solution into the inner layer injection pump of a single-layer needle and carry out electrospinning to obtain an electrospinning precursor; the specific parameters are: voltage 15 kV, propulsion speed 10 mL / h, and receiver rotation speed 2500 rpm.

[0109] Place the above precursor film in a vacuum tube furnace with an argon or 5 vol% H2 / Ar mixed atmosphere. First, let the inert atmosphere fill the quartz tube by introducing gas at room temperature for 30 min, then heat it to 300 °C at a rate of 3 °C / min and heat for 2 hours, then heat it to 550 °C at the same rate and heat for 8.5 hours. Finally, naturally cool it to room temperature under an argon or 5 vol% H2 / Ar mixed atmosphere to obtain a one-dimensional structure product of NVP. The average equivalent diameter of NVP is 200 nm, coated with a carbon layer with a thickness of less than 20 nm. The sizes of the core material, shell layer, and carbon layer are respectively identified by obtaining the TEM images of the cathode material, combining the structural characteristics of the material presented in the TEM images, and cooperating with auxiliary means such as EDS, and measuring the corresponding sizes; relevant sizes of several sampling points are measured in the TEM image, and their average values are obtained.

[0110] Comparative Example 6: Preparation of Na4Fe3(PO4)2(P2O7) / C by electrospinning method

[0111] Take 4.84 g of ferric nitrate nonahydrate, 2.12 g of ascorbic acid, 2.14 g of sodium oxalate, and 1.92 g of sodium dihydrogen phosphate, dissolve them in 100 mL of deionized water in sequence, put them into a beaker, and finally add 0.5 g of graphene and 0.1 g of dispersant V-50. Place the mixed solution on a magnetic stirrer and stir for 30 min until it is evenly mixed. Vacuum dry the above solution at 60 °C, grind the obtained solid to obtain a powdery precursor. Dissolve 1-butyl-3-methylimidazolium dihydrogen phosphate, polyacrylonitrile (PAN), and the above precursor powder in 1-butyl-3-methylimidazolium dihydrogen phosphate ([BMIm]H2PO4) at a mass ratio of 3:3:1 to obtain the NFPP spinning solution. Draw the NFPP spinning solution into the injection pump of a single-layer needle and carry out electrospinning to obtain an electrospinning precursor; the specific parameters are: voltage 15 kV, propulsion speed 10 mL / h, and receiver rotation speed 2500 rpm.

[0112] The above precursor film was placed in a vacuum tube furnace with an argon or 5 vol% H2 / Ar mixed atmosphere. First, the inert atmosphere was filled with quartz tube by aeration at room temperature for 30 min, then the temperature was raised to 300 °C at a rate of 3 °C / min and heated for 2 hours, and then the temperature was raised to 550 °C at the same rate and heated for 8.5 hours. Finally, it was naturally cooled to room temperature under an argon or 5 vol% H2 / Ar mixed atmosphere to obtain a one-dimensional structure product of NFPP. The average equivalent diameter of NFPP was 190 nm, coated with a carbon layer less than 20 nm thick. The sizes of the core material, shell layer and carbon layer were respectively identified by obtaining the TEM images of the cathode material, combining the structural characteristics of the material presented in the TEM images, and cooperating with auxiliary means such as EDS, and measuring the corresponding sizes; the relevant sizes of several sampling points were measured in the TEM images and the average value was obtained.

[0113] Effect Example 1: Electrochemical performance

[0114] 1. Test objects: The final products of Examples 1-3 and Comparative Examples 1-6 were used as active electrode materials respectively.

[0115] 2. Test methods:

[0116] (1) Half-cell assembly:

[0117] The electrode slurry was prepared with the active electrode material, conductive carbon black and binder PVDF in a mass ratio of 8:1:1, and the solvent was NMP organic solvent. Then it was coated on the aluminum foil by a doctor blade coating method with a coating thickness of 250 μm. The aluminum foil was dried in an oven at 80 °C for 2 h, then transferred to a vacuum drying oven and dried at 105 °C for 12 hours. After being compacted by a rolling press, it was cut into electrode discs with a diameter of 10 mm, weighed and transferred into a glove box as the positive electrode sheet.

[0118] Using a sodium metal sheet as the negative electrode sheet and a Whatman glass fiber membrane as the separator, in the glove box, it was assembled in the order of positive electrode sheet, separator, and negative electrode sheet, and 80 μL of electrolyte was filled. The electrolyte used a solvent with a volume ratio of EC:DEC = 1:1, and the solute was 1 M NaPF6 to obtain a CR 2032 type button cell to be tested.

[0119] (2) Electrochemical performance test:

[0120] Using a battery test device with a rated voltage and current of 5 V and 10 mA respectively, the assembled button cell was connected to the battery test system for charge and discharge tests.

[0121] 3. Test results: As shown in the following table.

[0122]

[0123]

[0124] The positive electrode material of the present invention has a one-dimensional nano-encapsulated wire structure with NVP as the core material and NFPP as the shell material. Compared with single NVP or NFPP materials, it has significantly better rate performance and capacity retention rate.

Claims

1. A cathode material, characterized in that, It includes a wire-wrapping structure, and the wire-wrapping structure includes a core material and a shell layer axially coating the core material; the core material includes sodium vanadium phosphate, and the shell layer includes sodium iron pyrophosphate phosphate.

2. The cathode material according to claim 1, characterized in that, It satisfies at least one of the following conditions: (1) The ratio of the average equivalent diameter of the core material to the average thickness of the shell layer is (100 - 160):(50 - 100), preferably (120 - 130):(70 - 90), such as 120:70, 125:72 or 130:90; (2) The average equivalent diameter of the core material is 100 - 160 nm, preferably 120 - 130 nm, such as 120 nm, 125 nm or 130 nm; (3) The average thickness of the shell layer is 50 - 100 nm, preferably 70 - 90 nm, such as 70 nm, 72 nm or 90 nm; (4) A first carbon layer is further included between the core material and the shell layer; the thickness of the first carbon layer preferably does not exceed 10 nm; (5) A second carbon layer axially coating the shell layer is further included outside the shell layer; the thickness of the second carbon layer preferably does not exceed 10 nm; (6) The shell layer further includes a conductive agent; the conductive agent is preferably graphene; the mass ratio of the conductive agent to the molar amount of Fe atoms in the sodium iron pyrophosphate phosphate is preferably (0.1 - 1) g:12 mmol, such as 0.5 g:12 mmol.

3. A method for preparing a cathode material, characterized in that, It includes the following steps: S1. Coaxial electrospinning is performed on the sodium vanadium phosphate spinning solution and the sodium iron pyrophosphate phosphate spinning solution, with the sodium vanadium phosphate spinning solution located in the inner layer and the sodium iron pyrophosphate phosphate spinning solution located in the outer layer, to obtain a precursor film; S2. The precursor film is heat-treated in a protective atmosphere.

4. The method for preparing the cathode material according to claim 3, wherein, It satisfies at least one of the following conditions: (1) In step S1, the sodium vanadium phosphate spinning solution includes a sodium vanadium phosphate precursor, a first polymer, and a first solvent; (2) In step S1, the sodium vanadium phosphate spinning solution includes sodium vanadium phosphate, a first polymer, and a first solvent; (3) In step S1, the sodium iron pyrophosphate phosphate spinning solution includes sodium iron pyrophosphate phosphate, a second polymer, and a second solvent; (4) In step S1, the sodium iron pyrophosphate phosphate spinning solution includes a sodium iron pyrophosphate phosphate precursor, a second polymer, and a second solvent.

5. The preparation method of the cathode material according to claim 4, characterized in that, It satisfies at least one of the following conditions: (1) In step S1, the raw materials of the sodium vanadium phosphate precursor include a sodium source, a vanadium source, a phosphorus source, and a reducing agent; the molar ratio of Na atoms, V atoms, and P atoms in the sodium source, the vanadium source, and the phosphorus source is preferably 3:2:3; the molar ratio of the reducing agent to the V atoms in the vanadium source is preferably 1:1; the vanadium source is selected from one or more of vanadium pentoxide, vanadium dioxide, and ammonium metavanadate; the molar amount of V atoms in the vanadium source and the mass ratio of the first polymer is preferably 10 mmol:(2.5 - 4.5) g, such as 10 mmol:3.3 g; the molar amount of V atoms in the vanadium source and the mass ratio of the first solvent is preferably 10 mmol:(2.5 - 4.5) g, such as 10 mmol:3.3 g; (2) In step S1, the first polymer is selected from one or more of polyacrylonitrile, polyvinyl alcohol, polylactic acid, polyurethane, and polyvinylpyrrolidone; (3) In step S1, the first solvent is 1-butyl-3-methylimidazolium dihydrogen phosphate; (4) In step S1, in the sodium iron pyrophosphate phosphate spinning solution, the mass ratio of the sodium iron pyrophosphate precursor to the second polymer is 1:(2.5 - 4), such as 1:3; (5) In step S1, the mass ratio of the second solvent to the sodium iron pyrophosphate precursor is (2 - 3):1, such as 3:1; (6) In step S1, the raw materials of the sodium iron pyrophosphate precursor include a sodium source, an iron source, a phosphorus source, and a reducing agent; the molar ratio of Na atoms, Fe atoms, and P atoms in the sodium source, the iron source, and the phosphorus source is preferably 4:3:4; the molar ratio of the reducing agent to the Fe atoms in the iron source is preferably 1:1; the iron source is ferric nitrate nonahydrate; the raw materials of the sodium iron pyrophosphate precursor preferably further include a conductive agent; the conductive agent is preferably graphene; the mass ratio of the conductive agent to the molar amount of Fe atoms in the iron source is preferably (0.1 - 1) g:12 mmol, such as 0.5 g:12 mmol; the raw materials of the sodium iron pyrophosphate precursor preferably further include a dispersant; the dispersant is preferably azobisisobutyramidine hydrochloride; the mass ratio of the dispersant to the molar amount of Fe atoms in the iron source is preferably (0.05 - 0.5) g:12 mmol; (7) In step S1, the second polymer is selected from one or more of polyacrylonitrile, polyvinyl alcohol, polylactic acid, polyurethane, and polyvinylpyrrolidone; (8) In step S1, the second solvent is 1-butyl-3-methylimidazolium dihydrogen phosphate.

6. The preparation method of the cathode material according to claim 5, characterized in that, It satisfies at least one of the following conditions: (1) The sodium source is selected from one or more of sodium carbonate, sodium oxalate, sodium acetate, sodium dihydrogen phosphate, and disodium hydrogen phosphate; (2) The phosphorus source is selected from one or more of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, ammonium phosphate, and diammonium hydrogen phosphate; (3) The reducing agent is selected from one or more of citric acid, oxalic acid, tartaric acid, oxalic acid, adipic acid, malonic acid, ascorbic acid, sucrose, mandelic acid, malic acid, formaldehyde, acetaldehyde, n-butanal, isobutanal, tetraethyleneglycol, isopropanol, hydrazine hydrate, and urea.

7. The method for preparing the cathode material according to claim 3, wherein, It satisfies at least one of the following conditions: (1) In step S1, the volume flow rate ratio of the sodium vanadate phosphate spinning solution to the sodium iron pyrophosphate phosphate spinning solution is (1 - 3):1, such as 2:1; (2) In step S1, the linear flow rates of the sodium vanadate phosphate spinning solution and the sodium iron pyrophosphate phosphate spinning solution are the same; (3) In step S1, the voltage of the coaxial electrospinning is 10 - 30 kV, such as 15 kV; (4) In step S1, the advancing speed of the coaxial electrospinning is 1 - 20 mL / h, such as 10 mL / h; (5) In step S1, the rotational speed of the receiver for coaxial electrospinning is 500 - 5000 rpm, such as 2500 rpm; (6) In step S2, the protective atmosphere is a non-oxidizing atmosphere; preferably, the non-oxidizing atmosphere is an argon atmosphere or a mixed atmosphere of hydrogen and argon; in the mixed atmosphere of hydrogen and argon, the volume fraction of hydrogen is preferably 5 vol%; (7) In step S2, the temperature of the heat treatment is 300 - 550 °C; (8) In step S2, the time of the heat treatment is 2 - 16 h; (9) In step S2, the heating rate of the heat treatment is 2 - 3 °C / min; Preferably, in step S2, the process of the heat treatment is: heating to 300 °C at a rate of 3 °C / min, heating for 2 - 6 h, then heating to 550 °C at a rate of 3 °C / min, and heating for 4 - 10 h, such as 8.5 h.

8. A positive electrode material prepared by the method for preparing a positive electrode material according to any one of claims 3 - 7.

9. A pole piece, characterized in that, It includes a current collector and a positive electrode material layer located on at least one surface of the current collector, and the positive electrode material layer includes the positive electrode material according to any one of claims 1, 2, and 8.

10. A battery, characterized in that, It includes a pole piece according to claim 9.

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