High-purity na5feo4, and preparation method and application thereof

Abstract

The application discloses high-purity Na5FeO4 as well as a preparation method and application thereof, belongs to the technical field of electrochemistry, and overcomes the problems in the preparation and application process of the sodium supplement material Na5FeO4 of high-sodium transition metal oxide; in the synthesis method, the mixed powder of Na2O and Fe2O3 is strictly sealed in a sealed crucible in a sintering process; in the heating and heat preservation process, due to the entropy increase effect, the air pressure in the crucible is greater than the atmospheric pressure outside, a certain pressure is applied, the stability of Na5FeO4 at high temperature is maintained, the window for the stable existence of Na5FeO4 is expanded in thermodynamics, and the transformation of Na5FeO4 into other phase structures with higher stability under normal pressure during high-temperature sintering is inhibited; and the crystallinity and purity of Na5FeO4 are improved.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical technology, specifically relating to a high-purity Na5FeO4, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, with their high energy density, long cycle life, and relatively mature technology, have been widely used in numerous fields such as consumer electronics, electric vehicles, and large-scale energy storage. However, with the acceleration of the global energy transition and the rapid development of the electric vehicle industry, the demand for lithium resources is surging. Sodium and lithium belong to the same main group of elements, have similar physicochemical properties, and sodium is extremely abundant and widely distributed in the Earth's crust and oceans, making it a low-cost raw material. Against this backdrop, sodium-ion batteries are considered one of the most promising next-generation electrochemical energy storage technologies.

[0003] In practical applications of sodium-ion batteries, during the initial charge and discharge cycle, the electrolyte undergoes reduction and decomposition on the negative electrode surface, forming a solid electrolyte interphase (SEI) film. This process irreversibly consumes active sodium ions from the positive electrode, leading to a significant reduction in the reversible capacity and energy density of the entire battery. To overcome this bottleneck, pre-sodiuming technology has emerged and become one of the key strategies. The core idea of ​​this technology is to pre-introduce an additional sodium source into the positive or negative electrode using physical or chemical methods before battery assembly.

[0004] Currently, various potential sodium supplements have been explored, mainly including Na2NiO2, Na2O2, NaN3 (sodium azide), Na3P, NaNO2, sodium oxalate (Na2C2O4), sodium succinate (Na2C4O4), sodium acetate (NaAc), and Na5FeO4. These materials vary in terms of sodium content, decomposition voltage, and byproduct properties. For example, although NaN3 has a high sodium content and a suitable decomposition potential, the nitrogen gas produced during its decomposition may pose a safety hazard, and the azides themselves are toxic and explosive; Na2O2 is extremely sensitive to air and difficult to handle. Na5FeO4 has a very high sodium content, but studies have found that Na5FeO4 is a pressure-sensitive material (…). Figure 2 Currently, the preparation of Na5FeO4 involves high-temperature sintering or crystal growth using alumina crucibles, typically employing a "lid-on but open" design. Na5FeO4 is a metastable phase at ambient pressure, therefore existing methods cannot synthesize high-purity Na5FeO4. This approach is primarily based on long-standing technical biases. Technicians generally believe that the high-temperature sintering process relies on sufficient contact between the sample and a specific atmosphere (such as argon or oxygen), while the internal pressure of a sealed crucible increases dramatically at high temperatures, easily causing the crucible to crack, leading to sample contamination and safety hazards.

[0005] Overall, existing sodium replenishment material Na5FeO4 struggles to simultaneously achieve an optimal balance among key performance indicators such as high capacity compensation, low and safe decomposition voltage, inert byproducts, and good environmental stability. Therefore, developing a next-generation sodium replenishment material with superior overall performance through material design is an urgent and crucial scientific and technological challenge for advancing sodium-ion battery technology towards maturity and large-scale application. Summary of the Invention

[0006] This invention provides a high-purity Na5FeO4, its preparation method, and its application, overcoming the problem of low purity in the preparation of high-sodium transition metal oxide Na5FeO4 in the prior art.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for preparing high-purity Na5FeO4, which involves synthesis via a high-temperature solid-state reaction, includes the following steps:

[0009] S1: Mix the sodium source and the transition metal source uniformly according to the preset stoichiometric ratio;

[0010] S2: The uniformly mixed powder is placed in a high-temperature resistant crucible and heated and kept warm in a sealed environment to allow sufficient diffusion and chemical reaction between solid substances to form a target sodium-rich oxide phase with good crystallinity. During the heating and holding process, due to the entropy increase effect, the internal gas pressure is greater than the external atmospheric pressure. By applying a certain pressure, the stability of Na5FeO4 at high temperature is maintained, thereby thermodynamically expanding its stable existence window.

[0011] S3: After sintering, the high-purity Na5FeO4 is obtained by naturally cooling to room temperature.

[0012] In the steps described above, the molar ratio of sodium source to transition metal source is greater than 2:1 and less than 8:1; the sodium source includes, but is not limited to, sodium peroxide (Na2O2), sodium nitrite (NaNO2), sodium oxide (Na2O), sodium acetate (CH3COONa), or sodium carbonate (Na2CO3); the transition metal source is the corresponding metal oxide.

[0013] S2 is carried out in a protective atmosphere to prevent oxygen or moisture from the air from penetrating into the sealed crucible. The protective atmosphere is argon (Ar). The heating rate is 5-10℃ / min, the heating temperature is 350℃-850℃, and the holding time is 12h-48h. Na5FeO4 needs to be sintered under sealed conditions because Na5FeO4 is a metastable phase under normal pressure. If it is sintered at high temperature under open normal pressure conditions, it tends to transform into other more stable phase structures. The sealed crucible provides a positive pressure environment, under which Na5FeO4 is in a thermodynamically stable state, which can maintain a good crystal structure and high purity.

[0014] The Na5FeO4 prepared by the above method has high crystallinity and high purity.

[0015] The aforementioned high-purity Na5FeO4 can be used as a sodium supplement in the dry process for preparing the positive electrode of sodium-ion batteries.

[0016] Beneficial effects: This invention provides a high-purity Na5FeO4, its preparation method, and its applications, which have the following advantages compared with the prior art:

[0017] In the synthesis of Na5FeO4, the reactant Na2O is highly hygroscopic and reacts with CO2 to form sodium carbonate. Simultaneously, Na5FeO4 is prone to reverse reactions or desodium decomposition at high temperature and normal pressure, often resulting in low purity of the target phase and easy decomposition of the product. This invention employs a sealed reaction system constructed with a sealed crucible to strictly seal the Na2O+Fe2O3 mixed powder. Sintering at an appropriate temperature prevents crucible cracking. During sintering, the increased entropy at high temperature increases the internal pressure of the crucible, along with the sodium saturated vapor pressure generated by the precursor itself, establishing a chemically stable environment that inhibits high-temperature decomposition of Na5FeO4 and promotes intact lattice growth. This thermodynamically suppresses the intrinsic decomposition tendency of Na5FeO4 in the >350℃ high-temperature region; inhibits the transformation into more stable phase structures under normal pressure during high-temperature sintering; and improves the crystallinity and purity of Na5FeO4. Attached Figure Description

[0018] Figure 1 This is a photograph of PVDF gelled under alkaline conditions.

[0019] Figure 2 This is a comparison of XRD patterns of Na5FeO4 sintered overnight in a sealed crucible and an open crucible in an embodiment of the present invention.

[0020] Figure 3 shows the X-ray diffraction (XRD) pattern of Na5FeO4 in an embodiment of the present invention;

[0021] Figure 4 shows the scanning electron microscope (SEM) image and EDS image of Na5FeO4 in the embodiment of the present invention;

[0022] Figure 5 shows the first charge-discharge curve of the Na|| Na5FeO4 half-cell at 0.1C in the embodiment of the present invention;

[0023] Figure 6 shows the first charge-discharge curves of Na||MFN half-cells with different proportions of Na5FeO4 addition in the embodiments of the present invention.

[0024] Figure 7 shows the cycle stability curves of the HC||MFN and HC||MFN+10% Na5FeO4 full cells in the embodiments of the present invention;

[0025] Figure 8 This is a comparison of the electrochemical performance test results of Na5FeO4 sintered in sealed crucibles and open crucibles after assembling half-cells in this embodiment of the invention. Detailed Implementation

[0026] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments:

[0027] Example 1

[0028] This embodiment prepares high-purity sodium-rich transition metal oxide Na5FeO4 via a high-temperature solid-state method, specifically including the following steps: using Na2O as the sodium source and α-Fe2O3 as the iron source, Na2O and α-Fe2O3 are mixed at a molar ratio of 5:1, and ball-milled at 450 rpm for 2 hours using a planetary ball mill. The powder is then placed in a sealed crucible, heated to 420°C at a heating rate of 5°C / min under an argon atmosphere, and held at that temperature for 24 hours. After natural cooling to room temperature, Na5FeO4 sodium-supplementing material is obtained.

[0029] The phase composition of Na5FeO4 was analyzed using X-ray diffraction (XRD). Diffraction patterns were recorded by stepwise scanning 2θ from 10° to 80° at room temperature. The XRD pattern of the synthesized Na5FeO4 is shown below. Figure 3 Na5FeO4 and PDF#97-000-2485 have the same characteristic peaks, indicating that Na5FeO4 was obtained through sintering.

[0030] The microstructure of Na5FeO4 was determined using scanning electron microscopy. The scanning electron microscope image of synthesized Na5FeO4 is shown below. Figure 4 As shown, the microstructure of Na5FeO4 after sintering consists of columnar particles at the micrometer level, with a particle size of approximately 15 μm. EDS images show that Na, Fe, and O elements are uniformly distributed within the particles, indicating that there is no elemental segregation within the particles.

[0031] Example 2

[0032] In this embodiment, both a sealed crucible and an open crucible were used as sintering containers. Na5FeO4 was sintered at 420℃ for 24 hours under an argon atmosphere. Figure 2 and Figure 3 The XRD results show that Na5FeO4 maintains good crystallinity in a sealed crucible, and its XRD pattern matches PDF card 97-000-2485, proving its high purity. In contrast, Na5FeO4 sintered in an open crucible not only has poor crystallinity, but its main peak also does not correspond to the main peak of the Na5FeO4 PDF card. Therefore, it is impossible to sinter Na5FeO4 with high crystallinity and high purity under these conditions. These results indicate that the positive pressure atmosphere in the sealed crucible provides a thermodynamically stable condition for Na5FeO4 at high temperatures. In contrast, the poor crystallinity of Na5FeO4 in an open crucible suggests that this sodium supplement tends to decompose under normal pressure.

[0033] Half-cells were assembled from materials sintered in sealed and open crucibles, respectively, and electrochemical tests were performed within a voltage window of 2–4.5 V. Figure 8 As shown, Na5FeO4 sintered in a sealed crucible exhibits a capacity of 500 mAh g⁻¹ during the first charge cycle to 4.5 V. -1 The material exhibits a high specific capacity; in contrast, at the same sintering temperature and time, the material sintered in an open crucible has a capacity of only 350 mAh g⁻¹. -1 Furthermore, the polarization voltage is large, and the charging curve exhibits significant fluctuations. This demonstrates that Na5FeO4 sintered in a sealed crucible possesses higher purity and electrochemical performance, making it more suitable for use as a sodium supplement.

[0034] Example 3

[0035] This invention uses high-purity Na5FeO4 prepared in Example 1 as a sodium-supplementing material to prepare electrodes: Na5FeO4 (80wt%) and conductive carbon black (15wt%) are mixed in a mortar in a glove box and ground thoroughly for 30 minutes to ensure uniform mixing. Then, PTFE (5wt%) is added and mechanically mixed until the solid powder is fully bonded into sheets. These sheets are then transferred to a roller press and repeatedly rolled until the thick electrode sheet becomes a uniform thin electrode sheet with a thickness of approximately 25μm. The sheets are then cut using a punch into circular positive electrode sheets with a diameter of 12mm, obtaining a sodium-ion battery composite positive electrode sheet. Simultaneously, a wet process is used to prepare Na5FeO4 electrodes: Na5FeO4 sodium supplement (80wt%), conductive carbon black (15wt%), 5wt% PVDF, and an appropriate amount of NMP are added to a glass jar and stirred with a magnetic stirrer for 4-6 hours. The resulting slurry exhibits obvious gelation, indicating that the wet process is not suitable for Na5FeO4 materials.

[0036] Half-cell assembly: The above-mentioned positive electrode sheet is used as the positive electrode, and a sodium metal disc with a diameter of 14 mm is used as the negative electrode sheet. A 19 mm diameter aluminum foil is placed in the positive electrode shell as the positive electrode current collector. The circular positive electrode sheet is placed in the center of the positive electrode shell, and 75 μL of 1 mol NaPF6 in PC with 2% FEC electrolyte is added dropwise. Then, a glass fiber separator is placed and 75 μL of electrolyte is added dropwise to fully wet the separator. Finally, the hard carbon negative electrode, gasket, spring contact, and negative electrode shell are placed in sequence. The assembled coin cell sodium-ion battery is then sealed using a packaging machine and subjected to electrochemical testing after standing for 12 hours.

[0037] The first charge-discharge test of the Na||Na5FeO4 half-cell was conducted at a temperature of 25℃, a voltage range of 2-4.5V, and a rate of 0.1C. Figure 5 As shown, the specific capacity of the Na||Na5FeO4 half-cell during the first charge cycle is 500 mAh g. -1 The discharge specific capacity is 10 mAh g. -1 The average decomposition voltage is 3.1 V, which demonstrates the dual advantages of Na5FeO4: high irreversible capacity and low desodiuming voltage.

[0038] Example 4

[0039] Assembly of the composite positive electrode half-cell: In a glove box, 80 wt% MFN sodium-ion positive electrode material, 15 wt% conductive carbon black Ketjen black, and 5 wt% and 10 wt% high-purity Na5FeO4 prepared in Example 1, respectively, were mixed in a mortar and ground thoroughly for 30 min to ensure uniform mixing. Then, 5 wt% PTFE was added and grinding continued until the solid powder was fully bonded into sheets. These sheets were then transferred to a roller press and repeatedly rolled until the thick electrode sheet became a uniform thin electrode sheet with a thickness of approximately 25 μm. The sheets were then cut with a punch into circular positive electrode sheets with a diameter of 12 mm to obtain the sodium-ion battery positive electrode sheet.

[0040] Half-cell assembly: The above-mentioned positive electrode sheet is used as the positive electrode, and a sodium metal disc with a diameter of 14 mm is used as the negative electrode sheet. A 19 mm diameter aluminum foil is placed in the positive electrode shell as the positive electrode current collector. The circular positive electrode sheet is placed in the center of the positive electrode shell, and 75 μL of 1 mol NaPF6 in PC with 2% FEC electrolyte is added. Then, a glass fiber separator is placed and 75 μL of electrolyte is added to fully wet the separator. Finally, the hard carbon negative electrode, gasket, spring contact, and negative electrode shell are placed in sequence. The assembled coin cell sodium-ion battery is then sealed using a packaging machine and subjected to electrochemical testing after standing for 12 hours.

[0041] First-cycle charge-discharge tests were conducted on Na||MFN+5wt% Na5FeO4 (NFO) and Na||MFN+10wt% Na5FeO4 (NFO) half-cells. The test temperature was 25℃, the voltage range was 2-4.5V, and the rate was 0.1C. Figure 6 As shown, the specific capacity of the Na||MFN+5wt%NFO half-cell during the first charge cycle is 182mAh g. -1 It is higher than the 152mAh g of the MFN half-cell without sodium supplementation. -1 The first charge specific capacity of the Na||MFN+10wt%NFO half-cell is 212mAh g. -1 It is higher than the 152mAh g of the MFN half-cell without sodium supplementation. -1 The difference in the first charge capacity of the half-cell demonstrates the excellent sodium replenishment performance of Na5FeO4 prepared in Example 1.

[0042] Example 5

[0043] Preparation of the positive electrode: Since Na5FeO4 is a strong alkaline sodium replenishing agent, if the electrode is prepared using the traditional wet process, the residual alkali on its surface will lead to PVDF defluorination and gelation. Figure 1 This process leads to slurry agglomeration and failure. This embodiment abandons the conventional wet process and adopts a dry electrode process, using PTFE as a binder to completely avoid the gelation process of PVDF in an alkaline environment; the specific preparation process is as follows:

[0044] In a glove box, 80 wt% MFN sodium-ion battery positive electrode material, 10 wt% conductive carbon black Ketjen black, and 10 wt% Na5FeO4 sodium-supplementing material are mixed in a mortar and ground thoroughly for 30 minutes to ensure uniform mixing. Then, 5 wt% PTFE is added and grinding continues until the solid powder is fully bonded into sheets. These sheets are then transferred to a roller press and repeatedly rolled until the thick electrode sheet becomes a uniform thin electrode sheet with a thickness of approximately 100 μm. The sheets are then cut with a punch into circular positive electrode sheets with a diameter of 12 mm to obtain the sodium-ion battery positive electrode sheet.

[0045] Assembly of the full battery: Using the above-mentioned positive electrode sheet as the positive electrode, a mixture of hard carbon, binder (polyvinylidene fluoride PVDF), and Super P (85:8:7) was uniformly coated onto a single-sided carbon-coated aluminum foil. After vacuum drying at 110℃, it was cut into 14mm diameter circular pieces as negative electrode sheets. A 19mm diameter aluminum foil was placed in the positive electrode shell, and the circular positive electrode sheet was placed in the center of the positive electrode shell. 75μL of 1 mol NaPF6 in PC with 2% FEC electrolyte was added, followed by the placement of a glass fiber separator and the addition of 75μL of electrolyte to fully wet the separator. Finally, the hard carbon negative electrode, gasket, spring contact, and negative electrode shell were placed in sequence. The assembled coin cell sodium-ion battery was then sealed using a packaging machine and subjected to electrochemical testing after standing for 12 hours.

[0046] First-cycle charge-discharge tests were conducted on HC||MFN and HC||MFN+10% Na5FeO4 full cells at 25℃ and a voltage range of 1.2-4.0V. After three cycles of activation at 0.1C, a long-term electrochemical cycle at 0.33C was performed. Figure 5 As shown, the first-cycle discharge specific capacity of the HC||MFN full cell is 115 mAh g-1, while that of the HC||MFN+10% Na5FeO4 is 136 mAh g-1. This indicates that the Na5FeO4 cathode sodium sacrificial agent played a role in compensating for the active sodium ions consumed in the formation of SEI in the full cell, thereby improving the first-cycle discharge capacity.

[0047] Long-cycle performance tests were conducted on HC||MFN and HC||MFN+10% Na5FeO4 full cells. The test temperature was 25℃, the voltage range was 1.2-4.0V, the rate of change was 0.1C for the first three cycles, and 0.33C thereafter. Figure 7 As shown, the HC||MFN+10%Na5FeO4 full cell did not exhibit significant capacity decay compared to the control group after multiple charge-discharge cycles. This indicates that the HC||MFN+10%Na5FeO4 full cell possesses good cycle stability, and that Na5FeO4, as a sodium supplement, is well-suited to the MFN system.

[0048] The above description is merely a preferred embodiment of the present invention, and the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention, such as equivalent substitutions for precursor types, conventional adjustments to the cycle ratio, or reasonable optimization of the deposition temperature, should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing high-purity Na5FeO4, characterized in that, Includes the following steps: The sodium source and the transition metal oxide are uniformly mixed according to a preset stoichiometric ratio; the molar ratio of the sodium source to the transition metal oxide is greater than 2:1 and less than 8:

1. The uniformly mixed powder is placed in a high-temperature resistant sealed crucible and strictly sealed. It is heated and kept at a temperature under a protective atmosphere to allow sufficient diffusion and chemical reaction between the solid substances. During the heating and holding process, the crucible is kept in a sealed state. Due to the entropy increase effect, the gas pressure inside the crucible is greater than the external atmospheric pressure, thus forming a positive pressure environment inside the crucible. By applying this pressure, the stability of Na5FeO4 at high temperature is maintained, and the thermodynamic window for the stable existence of Na5FeO4 is expanded, inhibiting its transformation to other phase structures. After sintering, the sample is naturally cooled to room temperature to obtain the high-purity Na5FeO4.

2. The method for preparing high-purity Na5FeO4 according to claim 1, characterized in that, The sodium source includes sodium peroxide, sodium nitrite, sodium oxide, sodium acetate, or sodium carbonate.

3. The method for preparing high-purity Na5FeO4 according to claim 1, characterized in that, The transition metal oxide is an oxide of iron.

4. The method for preparing high-purity Na5FeO4 according to claim 1, characterized in that, The heating rate is 5-10℃ / min, the temperature is 350℃-850℃, and the holding time is 12h-48h.

5. The method for preparing high-purity Na5FeO4 according to claim 1, characterized in that, The protective atmosphere is argon.

6. A high-purity Na5FeO4, characterized in that, Prepared using the method described in any one of claims 1-5, the X-ray diffraction pattern of the high-purity Na5FeO4 is consistent with the characteristic peaks of the standard card PDF#97-000-2485, and the first charge specific capacity is 500mAh / g under a voltage window of 2-4.5V and a 0.1C rate.

7. An application of the high-purity Na5FeO4 as described in claim 6, characterized in that, The high-purity Na5FeO4 is used as a sodium supplement agent in the dry process for preparing the positive electrode of sodium-ion batteries.

8. The application of the high-purity Na5FeO4 according to claim 7, characterized in that, The dry process specifically includes the following steps: In an anhydrous and oxygen-free environment, the positive electrode material, conductive carbon black, and Na5FeO4 are mixed and ground thoroughly to ensure uniform mixing. Then, PTFE is added and grinding continues until the solid powder is fully bonded into a sheet. The sheet is then repeatedly rolled until it becomes a uniform sheet with a thickness of 100μm, thus obtaining the positive electrode for a sodium-ion battery.

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