A method for preparing Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology

Abstract

The present invention discloses a method for preparing a Na2FePO4F@TiO2 heterostructure material using atomic layer deposition technology. The method combines atomic layer deposition technology with annealing technology. By adjusting the number of atomic layer deposition cycles and the annealing temperature, a Ti-substituted Na2FePO4F@TiO2 heterostructure material with controllable physical and chemical properties is prepared. A TiO2 coating layer with adjustable thickness and tight bonding is formed on the outer layer of the material. The microscale heterostructure formed by annealing effectively enhances the mechanical properties of the material and reduces direct contact between the material and the electrolyte, thereby suppressing the occurrence of possible side reactions. At the same time, the trace Ti substitution achieved in one step acts as an anchoring agent without almost changing the original crystal structure, effectively preventing structural deformation and reducing internal stress changes, effectively improving the mechanical properties and cyclic stability of the Na2FePO4F material.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion battery positive electrode materials and relates to a method for preparing a Na2FePO4F@TiO2 heterogeneous structure material, and specifically relates to a method for preparing the Na2FePO4F@TiO2 heterogeneous structure material by atomic layer deposition technology. Background Art

[0002] As an emerging rechargeable battery, sodium-ion batteries have broad development prospects due to their abundant raw material resources, excellent safety performance, and production lines that can draw on existing lithium-ion batteries. As a type of phosphate polyanion material, Na2FePO4F has a relatively simple synthesis process. Due to the stronger inductive effect of fluoride ions, it effectively improves the redox couple, has a higher operating voltage and discharge specific capacity, but the specific capacity retention rate also deteriorates slightly. Traditional modification methods mainly improve the intrinsic conductivity of Na2FePO4F materials by coating with materials such as carbon. This method has low controllability over the thickness of the coating layer, making it difficult to achieve uniform and dense coverage of carbon. In addition, the coating layer has poor bonding with the material and may fall off during subsequent processing such as paste mixing and coating, seriously affecting the performance of the battery. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the present invention provides a method for preparing Na2FePO4F@TiO2 heterostructure materials via atomic layer deposition (ALD). This method combines ALD with annealing techniques. By varying the number of deposition cycles and annealing temperature, a Na2FePO4F@TiO2 heterostructure with a phase interface whose physical and chemical properties can be freely controlled is constructed. This method also achieves Ti substitution in the Na2FePO4F. The unique heterostructure and the anchoring effect of trace amounts of Ti enhance the material's mechanical properties and cyclic stability.

[0004] The purpose of the present invention is achieved through the following technical solutions:

[0005] A method for preparing Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology comprises the following steps:

[0006] Step 1: Preparation of sodium fluoroferric phosphate:

[0007] Step 11, ball milling: The phosphorus source, fluorine source, iron source, and sodium source are mixed and ball milled in a molar ratio of 1.0-1.5:1.0-1.3:1.0-1.5:2.0-2.5, wherein:

[0008] The phosphorus source is one or more of ferric phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium monohydrogen phosphate, sodium monohydrogen phosphate, phosphoric acid, sodium hypophosphite, sodium phosphite, ferric hypophosphite, ferric phosphite, ammonium hypophosphite, sodium phosphate compounds, and ammonium phosphate compounds;

[0009] The fluorine source is one or more of sodium fluoride, ferric fluoride, tetrafluoroethylene, vinylidene fluoride, polytetrafluoroethylene, polyvinylidene fluoride, fluorobenzene, fluoroacetic acid, and perfluoroalkyl carboxylic acid;

[0010] The iron source is one or more of iron powder, iron oxide, ferrous oxalate, ferrous acetate, ferric hydroxide, ferrous hydroxide, ferrous lactate, ferric citrate, and ferrocene;

[0011] The sodium source is one or more of sodium bicarbonate, sodium carbonate, sodium hydroxide, sodium citrate, sodium gluconate, sodium acetate, sodium phenolate, sodium ethoxide, and sodium acetylide;

[0012] The ball milling time is 18 to 24 hours, and the ball milling medium is one of deionized water and anhydrous ethanol;

[0013] Step 1 and 2, sintering: After vacuum drying the product obtained by ball milling, sintering it in a tube furnace under an inert atmosphere, and cooling it to obtain a sodium iron fluoride phosphate material, wherein:

[0014] The vacuum drying time is 20 to 30 hours;

[0015] The inert atmosphere is one or more of argon, hydrogen, nitrogen and helium;

[0016] The sintering temperature is 600-800°C and the sintering time is 10-14h;

[0017] Step 2: Deposit TiO2 thin film using atomic layer deposition technology:

[0018] Step 21. Depositing TiO2 thin film: Using ALD atomic deposition technology, titanium source and oxygen source are selected as precursors, inert gas is introduced, and redox reaction occurs on the surface of sodium ferric phosphate fluoride material to deposit a dense TiO2 thin film, wherein:

[0019] The titanium source is one or more of titanium tetrachloride, titanium tetraiodide, titanium tetraisopropoxide, tetramethyl titanium, titanium tetraethoxide, titanium tetrabutoxide, and titanium pentacarbonyl;

[0020] The oxygen source is one or more of H2O, O2, and O3;

[0021] The inert gas is one or more of argon, hydrogen, nitrogen and helium, the flow rate of the inert gas is 50-100 ml / min, and the duration is 60-120 s;

[0022] Step 22: Cycle: The thickness and physical and chemical properties of the TiO2 film deposited on the surface of the sodium ferric phosphate fluoride material are controlled by varying the number of atomic deposition cycles, wherein:

[0023] The number of cycles of the atomic deposition is 4 to 10;

[0024] Step 3: Annealing:

[0025] The sodium ferric phosphate fluoride material with a TiO2 film deposited on the surface is annealed, and the Ti in the partially deposited TiO2 on the surface enters the sodium ferric phosphate fluoride lattice and replaces the Fe ions, forming a partially Ti-doped Na2FePO4F@TiO2 heterostructure material, in which:

[0026] The annealing temperature is 300-600° C.

[0027] The Na2FePO4F@TiO2 heterostructure material can be used as a positive electrode material for sodium ion batteries.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] This invention utilizes a combination of atomic layer deposition (ALD) and annealing techniques to prepare a Ti-substituted Na2FePO4F@TiO2 heterostructure material with controllable physical and chemical properties by adjusting the number of atomic layer deposition cycles and the annealing temperature. This creates a tightly bound TiO2 coating with adjustable thickness on the outer layer of the material. The microscale heterostructure formed by annealing effectively enhances the material's mechanical properties and reduces direct contact between the material and the electrolyte, thereby suppressing potential side reactions. Furthermore, the trace Ti substitution achieved in one step, while barely altering the original crystal structure, acts as an anchor, effectively preventing structural deformation and reducing internal stress changes, thereby significantly improving the mechanical properties and cyclic stability of the Na2FePO4F material. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the process of depositing TiO2 thin film using the atomic layer deposition technology of the present invention;

[0031] Figure 2 Schematic diagram of the structure of the Na2FePO4F@TiO2 heterostructure material prepared in Example 2;

[0032] Figure 3 This is the SEM image of the Na2FePO4F@TiO2 heterostructure material prepared in Example 2;

[0033] Figure 4 This is a charge-discharge curve diagram of the Na2FePO4F@TiO2 heterostructure material prepared in Example 2;

[0034] Figure 5 This is a comparison chart of the cycling performance of the Na2FePO4F@TiO2 heterostructure material prepared in Example 2 and the unmodified Na2FePO4F material. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is further described below with reference to the embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0036] Example 1:

[0037] This embodiment provides a method for preparing a Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology, the method comprising the following steps:

[0038] Step 1: Preparation of sodium fluoroferric phosphate:

[0039] Step 11, ball milling: ferric phosphate, sodium fluoride, ferrous oxalate, and sodium bicarbonate were mixed at a molar ratio of 1.0:1.0:1.0:2.1 and ball milled for 18 h using deionized water as the solvent;

[0040] Step 1 and 2, sintering: vacuum drying the ball milled product for 24 hours, sintering it in a tubular furnace at 600°C for 14 hours under an inert atmosphere, and cooling it to obtain the sodium ferric phosphate fluoride material, wherein the inert atmosphere is hydrogen and argon;

[0041] Step 2: Deposit TiO2 thin film using atomic layer deposition technology:

[0042] Step 21: Depositing TiO2 thin film: Using ALD atomic deposition technology, titanium tetrachloride and H2O are selected as precursors, and an inert gas is introduced at 100 ml / min for 60 seconds to cause a redox reaction on the surface of the sodium ferric phosphate fluoride material to deposit a dense TiO2 thin film. The inert gas is hydrogen and argon.

[0043] Step 22, Cycle: By changing the number of atomic deposition cycles, the thickness and physical and chemical properties of the TiO2 film deposited on the surface of the sodium ferric fluoride phosphate material are controlled, wherein the number of atomic deposition cycles is 4 times;

[0044] Step 3: Annealing:

[0045] The sodium ferric phosphate fluoride material with TiO2 film deposited on the surface was annealed at 480℃. Part of the Ti in the surface-deposited TiO2 entered the sodium ferric phosphate fluoride lattice and replaced Fe ions, forming a partially Ti-doped Na2FePO4F@TiO2 heterostructure material.

[0046] Example 2:

[0047] This embodiment provides a method for preparing a Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology, the method comprising the following steps:

[0048] Step 1: Preparation of sodium fluoroferric phosphate:

[0049] Step 11, ball milling: ammonium dihydrogen phosphate, sodium fluoride, ferrous acetate, and sodium hydroxide are mixed in a molar ratio of 1.1:1.0:1.2:2.2 and ball milled for 20 hours, using deionized water as the solvent;

[0050] Step 1 and 2, sintering: vacuum drying the ball milled product for 24 hours, sintering it in a tube furnace at 700°C for 17 hours under an inert atmosphere, and cooling it to obtain the sodium iron phosphate fluoride material, wherein the inert atmosphere is argon;

[0051] Step 2: Deposit TiO2 thin film using atomic layer deposition technology:

[0052] Step 21: Depositing a TiO2 thin film: Using ALD atomic deposition technology, titanium tetrachloride, H2O, and O2 are selected as precursors, and an inert gas is introduced at 80 ml / min for 100 seconds to cause a redox reaction on the surface of the sodium ferric phosphate fluoride material to deposit a dense TiO2 thin film. The inert gas is argon.

[0053] Step 22, Cycle: By changing the number of atomic deposition cycles, the thickness and physical and chemical properties of the TiO2 film deposited on the surface of the sodium ferric fluoride phosphate material are controlled, wherein the number of atomic deposition cycles is 5 times;

[0054] Step 3: Annealing:

[0055] The sodium ferric phosphate fluoride material with TiO2 film deposited on the surface was annealed at 550℃. Part of the Ti in the surface deposited TiO2 entered the sodium ferric phosphate fluoride lattice and replaced the Fe ions, forming a partially Ti-doped Na2FePO4F@TiO2 heterostructure material. The structural diagram is shown in the figure. Figure 2 shown.

[0056] Example 3:

[0057] This embodiment provides a method for preparing a Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology, the method comprising the following steps:

[0058] Step 1: Preparation of sodium fluoroferric phosphate:

[0059] Step 11, ball milling: Sodium dihydrogen phosphate, ferric fluoride, ferric hydroxide, and sodium carbonate are mixed at a molar ratio of 1.1:1.1:1.1:2.1 and ball milled for 24 hours using anhydrous ethanol as the solvent;

[0060] Step 1 and 2, sintering: vacuum drying the ball milled product for 24 hours, sintering it in a tube furnace at 800°C for 10 hours under an inert atmosphere, and cooling it to obtain the sodium iron phosphate fluoride material. The inert atmosphere is a mixture of argon and nitrogen.

[0061] Step 2: Deposit TiO2 thin film using atomic layer deposition technology:

[0062] 3) Deposition of TiO2 thin film: Using ALD atomic deposition technology, titanium source and oxygen source are selected as precursors, and an inert gas is introduced at a certain flow rate and for a certain period of time, so that a redox reaction occurs on the surface of the sodium ferric phosphate fluoride material to deposit a dense TiO2 thin film;

[0063] 4) Cycling: By varying the number of atomic deposition cycles, the thickness and physical and chemical properties of the TiO2 film deposited on the surface of the sodium ferric phosphate fluoride material can be controlled;

[0064] Step 21: Depositing a TiO2 thin film: Using ALD atomic deposition technology, titanium tetraisopropoxide and O3 are selected as precursors, and an inert gas is introduced at 60 ml / min for 120 seconds to cause a redox reaction on the surface of the sodium ferric phosphate fluoride material to deposit a dense TiO2 thin film. The inert gas is a mixture of argon and nitrogen.

[0065] Step 22, Cycle: By changing the number of atomic deposition cycles, the thickness and physical and chemical properties of the TiO2 film deposited on the surface of the sodium ferric fluoride phosphate material are controlled, wherein the number of atomic deposition cycles is 8 times;

[0066] Step 3: Annealing:

[0067] The sodium ferric phosphate fluoride material with TiO2 film deposited on the surface was annealed at 600℃. Part of the Ti in the surface-deposited TiO2 entered the sodium ferric phosphate fluoride lattice and replaced Fe ions, forming a partially Ti-doped Na2FePO4F@TiO2 heterostructure material.

[0068] Test and analyze product morphology and battery cycle performance:

[0069] (1) SEM images of Na2FePO4F@TiO2 heterostructure materials

[0070] Scanning electron microscopy (SEM) is a fundamental analytical technique for exploring the microstructure of materials. This study utilized a GEMINI 500 SEM from the German company ZEISS to characterize the surface morphology of the sample. To ensure a clearer visualization of the material's morphology, the material was pre-treated with gold spraying to enhance its electrical conductivity. Figure 3 This is the SEM image of the Na2FePO4F@TiO2 heterostructure material prepared in Example 2. Figure 3 It can be seen that the Na2FePO4F@TiO2 heterostructure material prepared in Example 2 is composed of a series of particles with a diameter ranging from 6 to 8 μm.

[0071] (2) Electrochemical performance test of Na2FePO4F@TiO2 heterostructure materials

[0072] In order to evaluate the electrochemical performance of the prepared materials, a half-cell was designed and assembled for testing. First, 200 mg of the material prepared in Example 2 and 25 mg of the conductive additive were weighed and ground in a mortar for 20 minutes to ensure uniform mixing. The mixed powder was transferred to a paste bottle, 500 mg of a pre-made NMP solution containing 5 wt% PVDF was added thereto, and an appropriate amount of NMP was added to adjust the viscosity. After magnetic stirring for about 18 hours, a uniform paste was formed. Subsequently, a 120 μm scraper was used to evenly coat the paste on aluminum foil, placed under vacuum at 120°C for 12 hours to dry, and then cut into positive electrode sheets with a diameter of 12 mm. Finally, the electrode sheet, glass fiber separator, and sodium sheet were assembled into a CR2032 button cell in an inert argon atmosphere glove box. The electrolyte was 1.0 M NaClO4 / EC-PC (volume ratio 1:1, 5.0 Vol% FEC). The button battery is charged and discharged at a constant current of 0.2C (1C = 120mAh / g) in the voltage range of 2.0 to 4.0V. The charge and discharge curve is as follows: Figure 4 As shown in the figure, the cycle performance comparison of the unmodified Na2FePO4F material is shown in Figure 5 The charge and discharge test results are shown in Table 1.

[0073] Table 1 Charge and discharge test results

[0074]

[0075] As shown in Table 1, within a certain range of conditions, changing the raw material ratio, the number of atomic deposition cycles, the annealing temperature and other conditions will have a slight effect on the electrochemical properties of the obtained Na2FePO4F@TiO2 heterostructure material, but the overall performance consistency is still high. Figure 4 、 5It can be seen that the prepared Na2FePO4F@TiO2 heterostructure material has a smooth charge and discharge curve, and the discharge platform area is relatively concentrated. Compared with the unmodified Na2FePO4F material, it has obvious improvements in discharge specific capacity and cycle stability, and has good application prospects.

Claims

1. A method for preparing Na2FePO4F@TiO2 heterostructure materials by atomic layer deposition technology, characterized in that The method comprises the following steps: Step 1: Preparation of sodium fluoroferric phosphate: Step 11, ball milling: mixing a phosphorus source, a fluorine source, an iron source, and a sodium source in a molar ratio of 1.0-1.5:1.0-1.3:1.0-1.5:2.0-2.5 and ball milling; Step 1 and 2, sintering: vacuum drying the product obtained by ball milling, sintering it in a tube furnace under an inert atmosphere, and cooling it to obtain the sodium iron phosphate fluoride material; Step 2: Deposit TiO2 thin film using atomic layer deposition technology: Step 21. Depositing TiO2 thin film: Using ALD atomic deposition technology, titanium source and oxygen source are selected as precursors, an inert atmosphere is introduced, and a redox reaction occurs on the surface of the sodium ferric phosphate fluoride material to deposit a dense TiO2 thin film; Step 22: Cycle: The thickness and physical and chemical properties of the TiO2 film deposited on the surface of the sodium ferric fluoride phosphate material are controlled by changing the number of atomic deposition cycles; Step 3: Annealing: The sodium ferric phosphate fluoride material with TiO2 film deposited on the surface is annealed, and the Ti in part of the surface-deposited TiO2 enters the sodium ferric phosphate fluoride lattice and replaces Fe ions, forming a partially Ti-doped Na2FePO4F@TiO2 heterostructure material.

2. The method for preparing Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology according to claim 1, characterized in that The phosphorus source is one or more of ferric phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium monohydrogen phosphate, sodium monohydrogen phosphate, phosphoric acid, sodium hypophosphite, sodium phosphite, ferric hypophosphite, ferric phosphite, ammonium hypophosphite, sodium phosphate compounds, and ammonium phosphate compounds; the fluorine source is one or more of sodium fluoride, ferric fluoride, tetrafluoroethylene, vinylidene fluoride, polytetrafluoroethylene, polyvinylidene fluoride, fluorobenzene, fluoroacetic acid, and perfluoroalkyl carboxylic acid; the iron source is one or more of iron powder, iron oxide, ferrous oxalate, ferrous acetate, ferric hydroxide, ferrous hydroxide, ferrous lactate, ferric citrate, and ferrocene; and the sodium source is one or more of sodium bicarbonate, sodium carbonate, sodium hydroxide, sodium citrate, sodium gluconate, sodium acetate, sodium phenolate, sodium ethoxide, and sodium acetylide.

3. The method for preparing Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology according to claim 1, characterized in that The ball milling time is 18 to 24 hours, and the ball milling medium is one of deionized water and anhydrous ethanol.

4. The method for preparing Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology according to claim 1, characterized in that In steps one and two, the vacuum drying time is 20 to 30 hours; the inert atmosphere is one or more of argon, hydrogen, nitrogen and helium; the sintering temperature is 600 to 800°C and the time is 10 to 14 hours.

5. The method for preparing Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology according to claim 1, characterized in that The titanium source is one or more of titanium tetrachloride, titanium tetraiodide, titanium tetraisopropoxide, tetramethyl titanium, titanium tetraethoxide, titanium tetrabutoxide, and titanium pentacarbonyl; the oxygen source is one or more of H2O, O2, and O3.

6. The method for preparing Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology according to claim 1, characterized in that In step 21, the inert atmosphere is one or more of argon, hydrogen, nitrogen and helium, the flow rate of the inert atmosphere is 50-100 ml / min, and the duration is 60-120 s.

7. The method for preparing Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology according to claim 1, characterized in that The number of cycles of the atomic deposition is 4 to 10 times.

8. The method for preparing Na2FePO4F@TiO2 heterostructure material by atomic layer deposition technology according to claim 1, characterized in that The annealing temperature is 300-600°C.

9. A Na2FePO4F@TiO2 heterostructure material prepared by the method according to any one of claims 1 to 8.

10. Use of the Na2FePO4F@TiO2 heterostructure material prepared by the method according to any one of claims 1 to 8 in a positive electrode material for a sodium ion battery.

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