Preparation method of sodium-rich sodium iron sulfate composite material and its application in sodium storage

Abstract

The present invention discloses a preparation method of a sodium-rich sodium iron sulfate composite material and its application in sodium storage. Iron-based sulfate and CNTs after dispersion treatment are ball-milled at low speed, heat-treated after vacuum drying, and naturally cooled to room temperature to obtain FeSO4 / CNTs; according to the molar ratio of Na to Fe being 7:5.5, FeSO4 / CNTs and anhydrous Na2SO4 are mixed and ball-milled wet at low speed, vacuum dried, and ground to obtain Na2SO4 / FeSO4 / CNTs; Na2SO4 / FeSO4 / CNTs is compacted, placed in a tube furnace, kept at 350-400 °C for 10-24 h, cooled to room temperature, and ground to obtain the Na7Fe 5.5 (SO4)9 / CNTs composite material. The Na7Fe 5.5 (SO4)9 cathode material of the present invention has good specific sodium storage capacity, excellent rate performance, and stable long cycle life.

Description

Technical Field

[0001] The invention belongs to the field of sodium ion batteries and relates to a preparation method of a sodium-enriched sodium iron sulfate composite material and a sodium storage application thereof. Background Art

[0002] Sodium-ion batteries are potential candidates for replacing lithium-ion batteries for large-scale energy storage applications. Polyanionic compounds are one of the four important materials (polyanionic compounds, layered oxides, Prussian blue analogs and organic materials) for electrochemical energy storage and conversion in sodium-ion batteries. Sodium ferric sulfate (NFS) among polyanionic compounds has significant low-cost advantages, a wide source of production raw materials and is green, and is a type of economically efficient and environmentally friendly material. When applied to the positive electrode of sodium-ion batteries, NFS has an operating voltage of up to ~3.8V and a theoretical capacity of 120mAh / g, thus having a significant advantage in high energy density. In addition, NFS has a stable crystal structure and strong resistance to thermal failure during the charge and discharge process, so it has long-cycle application potential and high safety. At the same time, compared with lithium-ion batteries, sodium-ion batteries have strong low-temperature fast charging capabilities and a small capacity decay. Therefore, sodium-ion batteries with NFS as the positive electrode material are more suitable to replace lithium-ion batteries and be used in large-scale energy storage scenarios with low energy density requirements and low cost requirements; they are expected to completely replace lead-acid batteries and be used in two-wheeled and three-wheeled electric vehicles; they can also partially replace lithium-ion batteries and be used in energy storage applications such as four-wheeled low-speed vehicles.

[0003] At present, spray drying, freeze drying, sol-gel and one-step high-energy ball milling technology are the main methods for preparing sodium iron sulfate composite materials. However, spray drying will cause certain dust pollution, freeze drying and sol-gel methods are inefficient; at the same time, the above three methods require a lot of water resources. In order to avoid Fe in NFS precursor, 2+ Oxidation requires the use of a large amount of chelating agents, and the heat treatment process should be additionally kept at about 200°C to remove excess crystal water. The above challenges are not conducive to the large-scale industrial preparation and application of NFS materials in the field of electrochemical energy storage. In contrast, the ball milling process has the advantages of simplicity, high efficiency and high yield. However, one-step high-energy ball milling has strict requirements on equipment, high energy consumption, certain safety hazards and difficulty in synthesizing uniform materials. In addition, the Na-rich composition can supplement a certain amount of Na to the full battery to improve the coulombic efficiency without significantly sacrificing the material's cycle stability, but the NFS material family still lacks Na-rich materials. Therefore, it is still a difficult problem to develop an efficient, inexpensive, high-yield and uniform synthetic strategy to prepare sodium-rich sodium iron sulfate composite materials. Summary of the invention

[0004] In view of the shortcomings of the prior art and the blank of material synthesis, the problem to be solved by the present invention is to provide a method for preparing a sodium iron sulfate composite material, synthesizing sodium-enriched sodium iron sulfate Na7 Fe 5.5 (SO 4 ) 9 composite with carbon nanotubes (CNTs). This method has the advantages of simple process, large output, high efficiency, strong controllability and good repeatability; the prepared Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite not only has good uniformity, but also has excellent electrochemical performance for sodium ion storage, and is suitable for large-scale production and industrial application.

[0005] The technical solution to solve the problems of the present invention is as follows:

[0006] The first aspect of the present invention is to provide a preparation method of a sodium-rich sodium iron sulfate composite, which uses ferrous sulfate salt, sodium sulfate salt and carbon nanotubes to prepare Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite through a two-step low-energy solid-phase ball milling strategy combined with pyrolysis. The specific steps are as follows:

[0007] Step 1: Load ferrous sulfate-based salt and the dispersed CNTs into the ball milling tank according to a mass ratio of 6-78:0.4-6, inject ethanol or acetone solution, and perform low-speed ball milling at 350-450 r / min for 12-24 h under the protection of an inert atmosphere. After vacuum drying, perform heat preservation heat treatment at 350-400 °C for 8-24 h in a air-free environment, and naturally cool to room temperature to obtain FeSO 4 / CNTs;

[0008] Step 2: According to the molar ratio of Na to Fe of 7:5.5, mix FeSO 4 / CNTs and anhydrous Na 2 SO 4 and put them into the ball milling tank, perform low-speed wet ball milling at 350-450 r / min for 12-24 h under the protection of an inert atmosphere, vacuum dry in an oven at 40-80 °C for 6-12 h, and grind to obtain a black precursor powder Na 2 SO 4 / FeSO 4 / CNTs;

[0009] Step 3: Put the prepared precursor powder Na 2 SO 4 / FeSO 4The CNTs are tightly compacted under a pressure of 8 - 12 MPa, placed in a tube furnace, and high-purity argon or nitrogen is introduced. The temperature is raised at a rate of 1 - 2 °C / min to 350 - 400 °C and held for 10 - 24 h, then cooled to room temperature. After grinding, a black powder product Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material is obtained.

[0010] Among them, the iron-based sulfate and the dispersed CNTs are in a mass ratio of 6 - 78:0.4 - 6. A lower CNT content will cause a decrease in the conductivity of Na 7 Fe 5.5 (SO 4 ) 9 / CNTs and a deterioration in electrochemical performance; a higher CNT content will increase the cost and cause uneven ball milling, resulting in a decrease in product consistency.

[0011] Among them, the molar ratio of Na to Fe is 7:5.5. The material with this molar ratio has excellent sodium storage electrochemical performance, can provide a relatively high capacity; has high product consistency; this Na-Fe ratio composition has not been reported in the literature and patents and is a new composition material.

[0012] Among them, in step three, it is held at 350 - 400 °C for 10 - 24 h. Samples synthesized below 350 °C have poor crystallinity, resulting in a decrease in conductivity and electrochemical performance; above 400 °C, some sulfates will decompose to produce impurity phases; less than 10 h leads to insufficient material reaction and poor crystallinity; more than 24 h results in an increase in synthesis cost and a decrease in synthesis efficiency.

[0013] Furthermore, the dispersion treatment is to heat-treat multi-walled carbon nanotubes in a concentrated nitric acid solution at 60 - 80 °C for 12 - 15 h, wash with water until the pH value is 6, and then vacuum-dry at 60 - 80 °C for 12 - 15 h.

[0014] Furthermore, the iron-based sulfate is ferrous sulfate heptahydrate, ferrous sulfate monohydrate or anhydrous ferrous sulfate.

[0015] Furthermore, zirconia balls are used as the ball milling medium in both step one and step two, and the ball-to-material ratio is set to 10:1 - 20:1.

[0016] Furthermore, the preparation method of anhydrous Na 2 SO 4 is: heat-treat the sodium-based sulfate with or without crystal water in a vacuum oven at 180 - 220 °C for 5 - 8 h to remove surface adsorbed water and / or crystal water, and obtain anhydrous Na 2 SO 4 .

[0017] Further, the inert atmosphere described in Step 1 and Step 2 is argon or nitrogen.

[0018] The second aspect of the present invention is to provide the application of the Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material as a cathode material for sodium-ion batteries. A button cell is assembled with the Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material as the positive electrode active material and a sodium metal sheet as the negative electrode. Among them, the positive electrode part is composed of Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material, Ketjen black, and polyvinylidene fluoride (PVDF) added with an appropriate amount of N-methylpyrrolidone (NMP) in a mass ratio of 8:1:1 to form a homogeneous slurry, which is uniformly coated on an aluminum current collector in a dry environment and vacuum dried at 100 °C for 12 h. The electrolyte is 1 M sodium perchlorate (NaClO 4 ) dissolved in a volume ratio of ethylene carbonate (EC) to diethyl carbonate (DEC) of 1:1 with 5% volume of fluoroethylene carbonate (FEC). The test temperature is room temperature.

[0019] Advantages and beneficial effects of the present invention:

[0020] (1) The Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material prepared by the method of the present invention has high crystal phase homogeneity and uniform morphology, and good product consistency. The synthesis process has strong repeatability. By changing the iron source, sodium source, and using CNTs with different aspect ratios, the Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material has high Na 7 Fe 5.5 (SO 4 ) 9 purity, large yield, and good controllability, and is easy to be prepared and applied on a large scale in the future.

[0021] (2) The Na 7 Fe 5.5 (SO 4 ) 9The / CNTs composite material has a sodium-rich composition. This design can ensure the overall charge-discharge cycle stability of the battery by not destroying the iron-based sulfate structure even when providing sodium to the hard carbon negative electrode in the full battery causes sodium loss in the positive electrode.

[0022] (3) The Na 7 Fe 5.5 (SO 4 ) 9 positive electrode material has good specific sodium storage capacity, excellent rate performance, and stable long cycle life. In addition, compared with layered oxide and vanadium-based polyanion compound positive electrode materials, the Na 7 Fe 5.5 (SO 4 ) 9 positive electrode material has a higher working voltage (~3.7V) and lower production raw material cost, and has significant advantages in terms of energy density, long cycle performance, and production cost.

[0023] (4) The Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material prepared by the method of the present invention has a nano-sized structure, which is conducive to rapid ion transport, electron diffusion, and sufficient contact between the electrode and the electrolyte. Description of the Drawings

[0024] Figure 1 X-ray powder diffraction (XRD) pattern of the Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material prepared in Example 1;

[0025] Figure 2 XRD pattern of the Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material prepared in Comparative Example 1, which contains Na 3 Fe(SO 4 ) 3 and FeSO 4 impurities;

[0026] Figure 3 Fourier transform infrared spectroscopy (FTIR) pattern of the Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material prepared in Example 1;

[0027] Figure 4Na prepared for Example 1 7 Fe 5.5 (SO 4 ) 9 X-ray photoelectron spectroscopy (XPS) spectra of the Na

[0028] Figure 5 Fe 7 Fe 5.5 (SO 4 ) 9 / CNTs composite materials, where a is for Example 1 and b is for Example 5; Scanning electron microscope (SEM) images

[0029] Figure 6 Na prepared for Example 1 7 Fe 5.5 (SO 4 ) 9 / CNTs composite materials; Transmission electron microscope (TEM) images

[0030] Figure 7 Na prepared for Example 1 7 Fe 5.5 (SO 4 ) 9 / CNTs composite materials as the positive electrode of a sodium-ion battery; Galvanostatic charge-discharge curves (GCD) for the first 5 cycles at a current density of 0.1C

[0031] Figure 8 Na prepared for Example 1 7 Fe 5.5 (SO 4 ) 9 / CNTs composite materials as the positive electrode of a sodium-ion battery; Rate performance at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0C

[0032] Figure 9 Na prepared for Example 1 7 Fe 5.5 (SO 4 ) 9 / CNTs composite materials as the positive electrode of a sodium-ion battery; Performance graph of charge-discharge cycles for 700 cycles at a current density of 1C

[0033] Figure 10 Na prepared for Examples 1-5 and Comparative Example 1 7 Fe 5.5 (SO 4 )9 Performance graph of the / CNTs composite material as the positive electrode of a sodium-ion battery during charge and discharge cycling 100 times at a current density of 1C. Detailed implementation manners

[0034] The present invention will be further described in detail below through specific embodiments. The following embodiments are only descriptive and not restrictive, and the protection scope of the present invention cannot be limited thereby.

[0035] Embodiment 1

[0036] Disperse multi-walled carbon nanotubes CNTs: Heat 0.6 g of multi-walled carbon nanotubes in 30 mL of concentrated nitric acid solution at 70 °C for 12 h, wash with water until the pH is 6, and after centrifugation or filtration, dry in vacuum at 80 °C for 12 h to obtain uniformly dispersed carbon nanotubes CNTs. The first ball milling: Mix 6.501 g of FeSO 4 ·H 2 O and 0.488 g of CNTs and put them into a ball milling jar with a volume of 70 mL. Use ZrO 2 balls as the ball milling medium, set the ball mass to 20 times the total material, inject 15 mL of acetone, use argon as the protective gas, ball mill at 400 r / min for 12 h, and dry in vacuum at 60 °C for 6 h. Collect the obtained black powder FeSO 4 ·H 2 O / CNTs. Subsequently, place the black powder FeSO 4 ·H 2 O / CNTs in a tube furnace, use argon as the carrier gas, heat it to 360 °C at a rate of 2 °C / min, hold for 18 h, and cool to room temperature to obtain the black powder FeSO 4 / CNTs. The second ball milling: Take 3.874 g of the black powder FeSO 4 / CNTs and 2.138 g of anhydrous Na 2 SO 4 and mix them in a 70 mL ball milling jar. The ball-to-material ratio is 20:1. Inject 15 mL of acetone, ball mill at 400 r / min for 12 h under argon protection, and dry in vacuum at 60 °C for 8 h. Collect the obtained black powder Na 2 SO 4 / FeSO 4 / CNTs. Tablet pressing: Manually grind the Na 2 SO 4 / FeSO 4 / CNTs powder and compact it under a pressure of 10 MPa. One-step pyrolysis: Compress the Na 2 SO 4 / FeSO 4The / CNTs were placed in a tubular furnace with argon as the carrier gas and heated to 400 °C at a rate of 2 °C / min for 12 h, then cooled to room temperature. After grinding, the final black powder product Na 7 Fe 5.5 (SO 4 ) 9 / CNTs was obtained.

[0037] It can be seen from Figure 1 that the Na 7 Fe 5.5 (SO 4 ) 9 / CNTs synthesized by this method is a pure phase.

[0038] It can be seen from Figure 4 that the main elemental composition of the sample includes Na, Fe, S, O, and C, which is consistent with the Na 7 Fe 5.5 (SO 4 ) 9 / CNT phase; In Na 7 Fe 5.5 (SO 4 ) 9 / CNTs, a small part of the surface Fe is oxidized, but it still mainly exists in the form of Fe 2+ and SO 4 2- ; The C signal exists in large amounts in the form of C-C chemical bonds, indicating that CNTs are present in large amounts on the material surface.

[0039] It can be seen from Figure 5 that in the Na 7 Fe 5.5 (SO 4 ) 9 / CNT product, the CNTs are evenly dispersed, and the Na 7 Fe 5.5 (SO 4 ) 9 exists in the form of nanoscale particles.

[0040] It can be seen from Figure 6 that the Na 7 Fe 5.5 (SO 4 ) 9 / CNT synthesized by this method has good crystallinity and obvious lattice fringes.

[0041] Electrochemical performance test:

[0042] The Na 7 Fe 5.5 (SO 4 ) 9The button cell assembled with the / CNTs composite as the positive active material and the sodium metal sheet as the negative electrode is completed. Among them, the positive electrode part consists of Na 7 Fe 5.5 (SO 4 ) 9 / CNTs composite material, Ketjen black and polyvinylidene fluoride (PVDF) are added with an appropriate amount of N-methylpyrrolidone (NMP) according to a mass ratio of 8:1:1 and mixed into a homogeneous slurry, which is uniformly coated on an aluminum current collector under a dry environment and vacuum dried at 100 °C for 12 h. The electrolyte is 1 M sodium perchlorate (NaClO 4 ) dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1:1 and added with 5% by volume of fluoroethylene carbonate (FEC). The test temperature is room temperature.

[0043] It can be seen from Figure 7 that the average working voltage of the Na 7 Fe 5.5 (SO 4 ) 9 / CNT product is as high as 3.7 V, and it has good cycle stability.

[0044] It can be seen from Figure 8 that the rate performance of the Na 7 Fe 5.5 (SO 4 ) 9 / CNT product is excellent, and it still maintains a reversible specific capacity of more than 80 mAh g -1 at a high current of 5C.

[0045] It can be seen from Figure 9 that the cycle performance of the Na 7 Fe 5.5 (SO 4 ) 9 / CNT sample is excellent. After 700 cycles, it has a capacity retention rate of 88%.

[0046] Example 2

[0047] Disperse multi-walled carbon nanotubes CNTs: 1.2 g of multi-walled carbon nanotubes are heated in 50 mL of concentrated nitric acid solution at 70 °C for 12 h, washed with water until the pH is 6, and vacuum dried at 80 °C for 12 h after centrifugation or filtration to obtain uniformly dispersed carbon nanotubes CNTs. First step of ball milling: 13 g of FeSO 4 ·H 2 O and 0.976 g of CNTs are mixed and placed in a ball milling jar with a volume of 100 mL, and ZrO 2The ball is used as the ball milling medium, the mass of the ball is set to 12 times the total material, 21 mL of acetone is injected, and ball milling is carried out at 400 r / min for 18 h under argon as the protective gas, and then vacuum drying is carried out at 80 °C for 6 h to collect the black powder FeSO 4 ·H 2 O / CNTs. Subsequently, the black powder FeSO 4 ·H 2 O / CNTs is placed in a tubular furnace, the carrier gas is argon, and the temperature is raised to 380 °C at a rate of 2 °C / min and held for 15 h, and then cooled to room temperature to obtain the black powder FeSO 4 / CNTs. Second step of ball milling: Take 7.748 g of the black powder FeSO 4 / CNTs and 4.276 g of anhydrous Na 2 SO 4 Mix and place in a 100 mL ball milling tank, the ball-to-material ratio is 14:1, inject 21 mL of acetone, carry out ball milling at 400 r / min for 18 h under argon protection, and then vacuum dry at 80 °C for 8 h to collect the black powder Na 2 SO 4 / FeSO 4 / CNTs. Tablet pressing: Grind the Na 2 SO 4 / FeSO 4 / CNTs powder and compact it under a pressure of 10 MPa. One-step pyrolysis: Place the compacted Na 2 SO 4 / FeSO 4 / CNTs in a tubular furnace with argon as the carrier gas, raise the temperature to 400 °C at a rate of 1 °C / min and hold for 12 h, cool to room temperature, and grind to obtain the final black powder product Na 7 Fe 5.5 (SO 4 ) 9 / CNTs.

[0048] Example 3

[0049] Disperse multi-walled carbon nanotubes CNTs: Heat 2.4 g of multi-walled carbon nanotubes in 50 mL of concentrated nitric acid solution at 70 °C for 12 h, wash with water until the pH is 6, and carry out centrifugation or filtration and then vacuum dry at 80 °C for 12 h to obtain uniformly dispersed carbon nanotubes CNTs. First step of ball milling: Mix 50.79 g of FeSO 4 ·10H 2 O and 1.952 g of CNTs and put them into a ball milling tank with a volume of 200 mL, and use ZrO 2Using balls as ball-milling media, the mass of the balls was set to 10 times that of the total material, 45 mL of acetone was injected, and ball-milling was carried out at 400 r / min for 24 h under argon as the protective gas, followed by vacuum drying at 40 °C for 12 h to collect the black powder FeSO 4 ·H 2 O / CNTs. Subsequently, the black powder FeSO 4 ·H 2 O / CNTs was placed in a tubular furnace with argon as the carrier gas, heated to 400 °C at a rate of 2 °C / min and held for 8 h, and then cooled to room temperature to obtain the black powder FeSO 4 / CNTs. Second step of ball-milling: Take 15.496 g of the black powder FeSO 4 / CNTs and 8.552 g of anhydrous Na 2 SO 4 Mix and place in a 200 mL ball-milling jar with a ball-to-material ratio of 15:1, inject 45 mL of acetone, ball-mill at 350 r / min for 24 h under argon protection, and vacuum dry at 60 °C for 12 h to collect the black powder Na 2 SO 4 / FeSO 4 / CNTs. Tabletting: Grind the Na 2 SO 4 / FeSO 4 / CNTs powder and compact it under a pressure of 10 MPa. One-step pyrolysis: Place the compacted Na 2 SO 4 / FeSO 4 / CNTs in a tubular furnace with argon as the carrier gas, heat it to 400 °C at a rate of 2 °C / min and hold for 24 h, cool it to room temperature, and grind it to obtain the final black powder product Na 7 Fe 5.5 (SO 4 ) 9 / CNTs.

[0050] Example 4

[0051] Disperse multi-walled carbon nanotubes CNTs: Heat 4.29 g of multi-walled carbon nanotubes in 100 mL of concentrated nitric acid solution at 70 °C for 15 h, wash with water until the pH is 6, and vacuum dry at 80 °C for 12 h after centrifugation or filtration to obtain uniformly dispersed carbon nanotubes CNTs. First step of ball-milling: Mix 46.48 g of FeSO 4 ·H 2 O and 3.49 g of CNTs and put them into a ball-milling jar with a volume of 500 mL, and use ZrO 2The ball is used as the ball milling medium, the mass of the ball is set to 20 times that of the total material, 100 mL of acetone is injected, and ball milling is carried out at 350 r / min for 24 h using argon as the protective gas, and then vacuum drying is carried out at 80 °C for 10 h to collect the black powder FeSO 4 ·H 2 O / CNTs. Subsequently, the black powder FeSO 4 ·H 2 O / CNTs is placed in a tube furnace, the carrier gas is argon, heated to 400 °C at a rate of 2 °C / min and held for 12 h, and then cooled to room temperature to obtain the black powder FeSO 4 / CNTs. Second step of ball milling: Take 27.7 g of the black powder FeSO 4 / CNTs and 15.29 g of anhydrous Na 2 SO 4 Mix and place in a 500 mL ball milling tank, the ball-to-material ratio is 20:1, inject 100 mL of acetone, ball mill at 400 r / min for 24 h under argon protection, and vacuum dry at 80 °C for 10 h to collect the black powder Na 2 SO 4 / FeSO 4 / CNTs. Tablet pressing: Grind the Na 2 SO 4 / FeSO 4 / CNTs powder and compact it tightly under a pressure of 10 MPa. One-step pyrolysis: Place the compacted Na 2 SO 4 / FeSO 4 / CNTs in a tube furnace with argon as the carrier gas, heat it to 400 °C at a rate of 1.5 °C / min and hold for 24 h, cool to room temperature, and grind to obtain the final black powder product Na 7 Fe 5.5 (SO 4 ) 9 / CNTs.

[0052] Example 5

[0053] Disperse multi-walled carbon nanotubes CNTs: Heat 7.2 g of multi-walled carbon nanotubes in 150 mL of concentrated nitric acid solution at 70 °C for 15 h, wash with water until the pH is 6, and vacuum dry at 80 °C for 15 h after centrifugation or filtration to obtain uniformly dispersed carbon nanotubes CNTs. First step of ball milling: Mix 78 g of FeSO 4 ·H 2 O and 5.86 g of CNTs and put them into a ball milling tank with a volume of 500 mL, and use ZrO 2The ball was used as the ball milling medium, the mass of the ball was set to 10 times the total material, 100 mL of acetone was injected, nitrogen was used as the protective gas, and ball milling was carried out at 450 r / min for 24 h, followed by vacuum drying at 80 °C for 12 h to collect the black powder FeSO 4 ·H 2 O / CNTs. Subsequently, the black powder FeSO 4 ·H 2 O / CNTs was placed in a tube furnace, the carrier gas was nitrogen, the temperature was raised to 400 °C at a rate of 2 °C / min and held for 24 h, and then cooled to room temperature to obtain the black powder FeSO 4 / CNTs. Second step of ball milling: Take 46.49 g of the black powder FeSO 4 / CNTs and 25.66 g of anhydrous Na 2 SO 4 Mix and place in a 500 mL ball milling tank, the ball-to-material ratio is 10:1, inject 100 mL of acetone, carry out ball milling at 450 r / min for 24 h under nitrogen protection, and vacuum dry at 80 °C for 12 h to collect the black powder Na 2 SO 4 / FeSO 4 / CNTs. Tablet pressing: After thoroughly grinding the Na 2 SO 4 / FeSO 4 / CNTs powder, compact it tightly under a pressure of 10 MPa. One-step pyrolysis: Place the compacted Na 2 SO 4 / FeSO 4 / CNTs in a tube furnace with nitrogen as the carrier gas, raise the temperature to 400 °C at a rate of 1 °C / min and hold for 24 h, cool to room temperature, and grind to obtain the final black powder product Na 7 Fe 5.5 (SO 4 ) 9 / CNTs.

[0054] Comparative Example 1

[0055] Disperse multi-walled carbon nanotubes CNTs: Heat 0.6 g of multi-walled carbon nanotubes in 30 mL of concentrated nitric acid solution at 70 °C for 12 h, wash with water until the pH is 6, and vacuum dry at 80 °C for 12 h after centrifugation or filtration to obtain uniformly dispersed carbon nanotubes CNTs. Heat 5.81 g of FeSO 4 ·H 2 O in a vacuum drying oven at 200 °C for 12 h to remove the crystal water. First step of ball milling: Take 3.574 g of FeSO 4 and 2.138 g of anhydrous Na 2 SO 4Mix with 0.3 g of CNTs and place them in a 70 mL ball milling jar. The ball-to-material ratio is 20:1. Inject 15 mL of acetone and ball mill at 400 r / min for 12 h under argon protection, then vacuum dry at 60 °C for 8 h to collect the black powder Na 2 SO 4 / FeSO 4 / CNTs. Tablet pressing: Manually grind the Na 2 SO 4 / FeSO 4 / CNTs powder and compact it under a pressure of 10 MPa. One-step pyrolysis: Place the compacted Na 2 SO 4 / FeSO 4 / CNTs in a tube furnace with argon as the carrier gas, heat it to 400 °C at a rate of 2 °C / min and hold for 12 h, then cool to room temperature and grind to obtain the final black powder product Na 7 Fe 5.5 (SO 4 ) 9 / CNTs. Note that the final sample prepared by this one-step ball milling process is not a pure phase and contains Na 3 Fe(SO 4 ) 3 and FeSO 4 impurities.

[0056] It can be seen from Figure 2 that the final sample synthesized by one-step ball milling is not a pure phase and shows many impurity peaks.

[0057] Figure 10 Performance graphs of the Na 7 Fe 5.5 (SO 4 ) 9 / CNT composite materials prepared in Examples 1-5 and Comparative Example 1 as the positive electrode of a sodium-ion battery during charge and discharge cycling 100 times at a current density of 1C. It can be seen from Figure 10 that after continuously expanding the production of Na 7 Fe 5.5 (SO 4 ) 9 / CNT, the product performance basically remains the same, and the material obtained by the two-step ball milling process is a pure phase, with better performance than the impure phase prepared in Comparative Example 1.

[0058] The Na 7 Fe 5.5 (SO 4 ) 9The cathode material has good sodium storage specific capacity. At a rate of 1C, it is 89.7 mAh / g for Example 1, 89.2 mAh / g for Example 2, 88.1 mAh / g for Example 3, 87 mAh / g for Example 4, 86.8 mAh / g for Example 5, and 71.6 mAh / g for Comparative Example 1.

[0059] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the inventive concept, several modifications and improvements can be made, and these all fall within the protection scope of the present invention.

Claims

1. A method for preparing a sodium-enriched sodium iron sulfate composite material, characterized in that: Here are the steps: Step 1, iron-based sulfate and dispersed CNTs are loaded into a ball mill according to a mass ratio of 6-78:0.4-6, and ethanol or acetone solution is injected, and the ball mill is carried out at a low speed of 350-450r / min for 12-24h under the protection of an inert atmosphere, and after vacuum drying, the ball mill is heat-treated at 350-400°C in an airless environment for 8-24h, and the ball mill is naturally cooled to room temperature to obtain FeSO4 / CNTs; Step 2: According to the molar ratio of Na to Fe being 7:5.5, FeSO4 / CNTs and anhydrous Na2SO4 are mixed and put into a ball mill, and the mixture is wet-milled at a low speed of 350-450 r / min for 12-24 h under the protection of an inert atmosphere, and vacuum-dried in an oven at 40-80° C. for 6-12 h to obtain a black precursor powder Na2SO4 / FeSO4 / CNTs; Step 3: The obtained precursor powder Na2SO4 / FeSO4 / CNTs is compacted under a pressure of 8-12 MPa, placed in a tube furnace, and introduced with high-purity argon or nitrogen. The temperature is raised to 350-400°C at a rate of 1-2°C / min and kept at this temperature for 10-24 hours. The powder is then cooled to room temperature and ground to obtain a black powder product Na7Fe 5.5 (SO4)9 / CNTs composite material, the material is pure phase, CNTs are evenly dispersed, and Na7Fe 5.5 (SO4)9 exists in the form of nano-sized particles; The dispersion treatment is to heat treat the multi-walled carbon nanotubes in a concentrated nitric acid solution at 60-80°C for 12-15h, wash with water until the pH value is 6, and then vacuum dry at 60-80°C for 12-15h; The iron-based sulfate is ferrous sulfate heptahydrate, ferrous sulfate monohydrate or anhydrous ferrous sulfate.

2. The preparation method according to claim 1, characterized in that: In both step 1 and step 2, zirconia balls are used as ball milling media, and the ball-to-material ratio is set to 10:1 to 20:

1.

3. Na7Fe prepared by the preparation method according to any one of claims 1 to 2 5.5 Application of (SO4)9 / CNTs composites as positive electrode materials for sodium ion batteries.

4. The use according to claim 3, characterized in that: Na7Fe 5.5 (SO4)9 / CNTs composite material is used as positive electrode active material, sodium metal sheet is used as negative electrode, and the electrolyte is 1M sodium perchlorate dissolved in ethylene carbonate and diethyl carbonate with a volume ratio of 1:1 and 5% volume of fluoroethylene carbonate added.

5. Na7Fe prepared by the preparation method according to any one of claims 1 to 2 5.5 Application of (SO4)9 / CNTs composites as sodium storage materials.

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