A sodium chromite core-shell structure cathode material coated by NFPP and a preparation method thereof

Abstract

The application discloses a sodium chromite core-shell structure positive electrode material coated with NFPP and a preparation method thereof, and belongs to the technical field of sodium ion battery positive electrode materials. First, a sodium chromite inner core material is synthesized by using a high-temperature solid phase method; then, a NFPP precursor sol containing an iron source, a phosphorus source and a sodium source is prepared; the NaCrO2 inner core is uniformly dispersed in the sol; by controlling the pH value and the concentration, controllable heterogeneous nucleation and growth of the NFPP precursor on the surface of the NaCrO2 particles are realized; after drying, heat treatment is carried out in an inert atmosphere; and finally, the core-shell structure positive electrode material with the NaCrO2 as the core and the NFPP as the shell is formed. The application effectively inhibits the side reaction of NaCrO2 with an electrolyte, transition metal dissolution and collapse of a layered structure in a cycle process by using the NFPP coating layer, greatly improves the cycle stability and safety of the material, and meanwhile, the high-voltage and high-capacity characteristics of the NaCrO2 are maintained.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and to an NFPP-coated sodium chromite core-shell structure cathode material and its preparation method. Specifically, it refers to a high-performance composite cathode material and its preparation method that uses an NFPP (Na4Fe3(PO4)2P2O7) coating layer to stabilize the sodium chromite (NaCrO2) core. Background Technology

[0002] Layered oxide cathode materials (such as NaCrO2) are promising cathode materials for sodium-ion batteries due to their high operating voltage, high theoretical specific capacity, and high tap density. However, NaCrO2 has serious problems in electrochemical cycling: ① During charge and discharge, especially at high voltages, it is prone to side reactions with the electrolyte, leading to increased interfacial impedance and capacity decay; ② Cr ions are easily dissolved during cycling, damaging the material structure; ③ Repeated insertion and extraction of sodium ions can easily cause irreversible phase transitions or collapse of the layered structure, resulting in poor cycle stability. Summary of the Invention

[0003] The first objective of this invention is to overcome the inherent defects of NaCrO2 layered oxide cathode materials in the prior art, such as poor cycle stability, severe interfacial side reactions, and dissolution of transition metal ions, and to provide an NFPP-coated sodium chromite core-shell structure cathode material with good cycle stability and interfacial compatibility.

[0004] The technical solution adopted in this invention is as follows: A core-shell structured cathode material coated with NFPP, the structural formula of which is: NaCrO2@NFPP, wherein: the cathode material is a core-shell structure, NaCrO2 is the core, and NFPP is the shell coated on NaCrO2.

[0005] The innovative mechanism of this invention is as follows: This invention employs a core-shell structure with NaCrO2 as the core and NFPP as the shell. Through the physical and chemical stabilizing effects of the NFPP coating, the electrochemical cycling stability and interfacial compatibility of NaCrO2 are significantly improved without sacrificing its high energy density advantage. The polyanionic compound NFPP possesses a robust three-dimensional framework structure, excellent thermal and chemical stability, and good ionic conductivity. However, its operating voltage (~2.8V) and theoretical capacity are lower than those of NaCrO2. Existing modification methods mostly focus on optimizing NFPP itself, while research on using NFPP as a "protective shell" to modify other high-energy-density materials is still lacking. This invention constructs a core-shell structure with NaCrO2 as the core and NFPP as the shell: the core NaCrO2 provides high energy density; the outer shell NFPP acts as a physical barrier, effectively isolating the electrolyte from direct contact with NaCrO2, inhibiting side reactions and Cr dissolution, while its three-dimensional sodium ion channels provide a stable interface for ion migration. This "strong-strong combination" structural design solves the bottleneck problem of NaCrO2.

[0006] The second aspect of this invention aims to provide a method for preparing an NFPP-coated sodium chromite core-shell structure cathode material, comprising the following steps: (1) Preparation of NaCrO2 core Sodium source and trivalent chromium source are mechanically mixed at a sodium to chromium molar ratio of (1.0-1.1):1. Then, the mixture is calcined at 750-900℃ for 6-15 hours under an inert or reducing atmosphere. After cooling, the mixture is crushed, ground, and sieved to obtain pure sodium chromite powder with uniform particle size distribution.

[0007] Preferably, in step (1): The sodium source is selected from one or more of sodium carbonate and sodium acetate.

[0008] The trivalent chromium source is selected from one or more of chromium trioxide and chromium chloride (CrCl3).

[0009] The inert atmosphere is selected from one or more of nitrogen and argon.

[0010] The high-temperature calcination is carried out at a heating rate of 2-5℃ / min to ensure uniform grain size.

[0011] Preferably, appropriate amounts of doping elements, such as Ca, K, Sr, Bi, Ba and La, can be added during the calcination process. The doping elements can improve the electrochemical performance of the material. The compounds of the doping elements are generally oxides or hydroxides, and the amount added is 0.1-5 mol of the total components (mass).

[0012] The sodium chromite powder prepared in step (1) has an average particle size (D50) controlled at 1-5 micrometers.

[0013] (2) Preparation of NFPP precursor sol Soluble iron, phosphorus, and sodium sources were weighed according to a Na:Fe:P molar ratio of (4.0-4.5):3:4. Then, a complexing agent was added with a molar ratio of (1.0-2.0):1 to the metal ions. The mixture was stirred continuously at 40-80℃ to form a homogeneous, stable, and transparent NFPP precursor sol.

[0014] Preferably, in step (2): The soluble iron source is selected from one or more of ferric nitrate and ferric chloride, the phosphorus source is selected from one or more of ammonium dihydrogen phosphate and diammonium hydrogen phosphate, and the sodium source is selected from one or more of sodium acetate and sodium nitrate.

[0015] The complexing agent is selected from one or more of citric acid, ascorbic acid, and ethylenediaminetetraacetic acid (EDTA) to improve the stability of metal ions and the uniformity of the sol.

[0016] The solid content of the NFPP precursor sol is preferably controlled at 10-30 wt% to ensure uniform coating in subsequent processes.

[0017] (3) Preparation of NFPP precursor coated with NaCrO2 The NaCrO2 core powder obtained in step (1) is uniformly dispersed in the NFPP precursor sol prepared in step (2), and reacted at a certain temperature and pH value to obtain NFPP precursor coated with NaCrO2.

[0018] Preferably, in step (3): The pH value of the reaction is precisely controlled within the range of 4.0-7.0 by slowly adding a pH adjuster. The pH adjuster can be one or more of ammonia, ethanolamine, and isopropylamine.

[0019] The reaction temperature is controlled between 40-70℃. Under these conditions, the NFPP precursor tends to undergo heterogeneous nucleation and gradual growth on the surface of NaCrO2 particles, rather than homogeneous nucleation in solution to generate independent impurity phases. The coating thickness can be controlled by adjusting parameters such as reaction pH, reaction temperature, reaction time, and sol concentration.

[0020] After the reaction is completed, a centrifuge is used for solid-liquid separation and washing to remove residual ions, and then the precursor powder is dried to obtain coated precursor powder. The drying process preferably adopts vacuum drying, freeze drying or spray drying process to ensure the uniformity and stability of the powder.

[0021] (4) Preparation of NFPP-coated sodium chromite core-shell structure cathode material The NFPP precursor prepared in step (3) was coated with NaCrO2 and heat-treated under an inert protective atmosphere at a temperature of 500-650℃ for 2-10 hours.

[0022] Preferably, in step (4): The inert protective atmosphere is argon. Preferably, the argon flow rate during the heat preservation process is 50-200 mL / min to ensure the stability and uniformity of the atmosphere.

[0023] The preferred heat treatment temperature is 550-620℃. This heat treatment process aims to transform the amorphous NFPP precursor into well-crystallized pure-phase NFPP, forming a complete and dense coating shell. The selected heat treatment temperature is much lower than the phase transition temperature of NaCrO2, thereby ensuring that the core crystal structure is not damaged during the coating process. Preferably, the heating rate is 2-5℃ / min during the heating process, and the holding temperature is kept constant or the temperature rise does not exceed ±5℃ during the holding process to ensure uniform heat treatment.

[0024] The third objective of this invention is to provide an application of the NFPP-coated sodium chromite core-shell structure cathode material in the preparation of sodium-ion batteries, which has been experimentally proven to have excellent cycle stability.

[0025] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention provides an NFPP-coated sodium chromite core-shell structure cathode material NaCrO2@NFPP, with NaCrO2 as the core and NFPP as the shell coating on NaCrO2, the shell thickness being 50-300 nanometers. By forming a dense NFPP coating layer around the NaCrO2 core, direct contact between the electrolyte and NaCrO2 is effectively isolated, significantly suppressing interfacial side reactions during charging and discharging, reducing the increase in electrode-electrolyte interfacial impedance, and effectively alleviating the Cr 3+ The problem of ion dissolution and loss is solved, thereby greatly improving the cycle life and safety of the material.

[0026] (2) The present invention utilizes the three-dimensional sodium ion channel of NFPP material to provide a stable and low-resistance channel for the migration of sodium ions at the core-shell material interface, improves the kinetic process of electrode reaction, and may buffer the lattice stress generated during sodium ion insertion and extraction, thereby reducing mechanical damage to the material structure.

[0027] (3) Through optimized core-shell structure design, combined with the high energy density of NaCrO2 and the good cycle performance of NFPP, the effective unity of high energy density and long cycle life is achieved, which meets the stringent requirements of sodium-ion batteries for high-performance cathode materials.

[0028] (4) The preparation method of the present invention adopts the sol-gel combined controllable heterogeneous nucleation technology, which realizes the uniform and dense composite of two materials with different crystal structures (layered oxide and polyanionic compound) at the nanoscale, and overcomes the shortcomings of the simple physical mixing in the prior art which makes it difficult to achieve uniform and dense coating.

[0029] (5) By precisely controlling the preparation process parameters, such as precursor concentration, pH value, reaction temperature and time, the thickness of the coating layer is precisely controlled, ensuring the uniformity and stability of the coating layer, while ensuring the structural integrity of the NaCrO2 core. Attached Figure Description

[0030] Figure 1 This is a SEM image of the cathode material prepared in Example 1.

[0031] Figure 2 This is a SEM image of the cathode material prepared in Example 2.

[0032] Figure 3 This is a SEM image of the cathode material prepared in Example 3.

[0033] Figure 4 This is a SEM image of the cathode material prepared in Example 4.

[0034] Figure 5 The image shows the SEM image of the NaCrO2 cathode prepared in Comparative Example 1.

[0035] Figure 6 SEM image of the mechanically hybrid cathode prepared for Comparative Example 2. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. However, the scope of protection of this invention is not limited to the listed embodiments. Unless otherwise specified, the raw materials and reagents used in the embodiments are existing technologies in the art or commercially available products.

[0037] Example 1

[0038] (1) Preparation of NaCrO2 core Sodium carbonate and chromium trioxide were accurately weighed and prepared according to a Na:Cr molar ratio of 1.025:1. The mixture was placed in a planetary ball mill and milled for 6 hours with anhydrous ethanol as the medium to ensure thorough mixing. The homogeneous powder was placed in an alumina crucible and heated to 900°C at a heating rate of 5°C / min under a flowing high-purity nitrogen atmosphere. The mixture was then calcined at this temperature for 12 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. The product was then ground and passed through a 400-mesh sieve to obtain pure-phase NaCrO2 powder with an average particle size D50 of approximately 2 μm.

[0039] NFPP precursor sol preparation Based on the stoichiometric ratio of Na:Fe:P = 4.2:3:4, 3.92 g of sodium acetate (CH3COONa), 12.12 g of ferric nitrate nonahydrate (Fe(NO3)3·9H2O), and 4.60 g of ammonium dihydrogen phosphate (NH4H2PO4) were weighed and dissolved in 129.36 g of deionized water. Citric acid was then added to the solution as a complexing agent, controlling the molar ratio of total metal ions to citric acid to be 1:1.5. The mixed solution was continuously magnetically stirred in a 60°C constant temperature water bath for 4 hours to form a uniform and stable deep red sol. The resulting NFPP precursor sol had a solid content of 10 wt% and a total mass of 150 g.

[0040] (3) NFPP precursor coated with NaCrO2 Weigh 10.0 g of the prepared NaCrO2 powder and slowly add it to the NFPP precursor sol prepared above. First, disperse the powder ultrasonically for 15 minutes, then mechanically stir at 300 rpm for 30 minutes to ensure uniform dispersion of the NaCrO2 particles. While continuously stirring, slowly add dilute ammonia solution using a micro-injection pump to precisely adjust the pH of the system to 5.5. Then, continue stirring at 60°C for 4 hours to allow the NFPP precursor to fully hydrolyze, condense, and coat the NaCrO2 surface. After the reaction, perform solid-liquid separation using a centrifuge. Wash the obtained solid product three times each with deionized water and anhydrous ethanol, and finally dry it in a vacuum drying oven at 80°C for 12 hours to obtain NFPP precursor-coated NaCrO2 powder.

[0041] (4) Preparation of NFPP-coated sodium chromite core-shell structure cathode material The dried NFPP precursor-coated NaCrO2 powder was transferred to a crucible and placed in a tube furnace. High-purity argon was introduced as a protective gas at a flow rate of 100 mL / min, and the furnace temperature was raised to 580℃ at a heating rate of 3℃ / min, and held at this temperature for 6 hours for crystallization. After heat treatment, the sample was cooled to room temperature with the furnace to obtain the final product—NFPP-coated sodium chromite core-shell structure cathode material (denoted as NaCrO2@NFPP-10wt%).

[0042] Example 2

[0043] The preparation method is the same as in Example 1, except that the solid content of the NFPP precursor sol in step (2) is adjusted to 15wt%, and the product is denoted as NaCrO2@NFPP-15wt%.

[0044] Example 3

[0045] The preparation method is the same as in Example 1, except that the pH value in step (3) is adjusted to 6 and its effect on the performance of the cathode material is tested.

[0046] Example 4

[0047] The preparation method is the same as in Example 1, except that the reaction time in step (3) is adjusted to 6 hours, and its effect on the performance of the cathode material is tested.

[0048] Comparative Example 1 This embodiment describes the preparation of a pure-phase NaCrO2 cathode material without any NFPP coating treatment.

[0049] Sodium carbonate and chromium trioxide were accurately weighed and prepared according to a Na:Cr molar ratio of 1.025:1. The mixture was placed in a planetary ball mill and milled for 6 hours with anhydrous ethanol as the medium to ensure thorough mixing. The homogeneous powder was placed in an alumina crucible and heated to 900°C at a heating rate of 5°C / min under a flowing high-purity nitrogen atmosphere. The mixture was then calcined at this temperature for 12 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. The product was then ground and passed through a 400-mesh sieve to obtain pure-phase NaCrO2 powder with an average particle size D50 of approximately 2 μm.

[0050] Comparative Example 2 In this embodiment, a NaCrO2 and NFPP blended cathode material is prepared by mechanical mixing.

[0051] The preparation method is as follows: The pure phase NaCrO2 cathode material prepared in Comparative Example 1 is mechanically mixed with NFPP powder synthesized by conventional solid-phase method at a mass ratio of 9:1 (ball milling for 30 minutes) to obtain NaCrO2 and NFPP blended cathode material.

[0052] Product characteristics: First, combined Figure 1 , Figure 5 The comparison is shown below: Figure 1 The NaCrO2@NFPP cathode material prepared in this invention has a continuous and dense NFPP coating layer on the surface of NaCrO2 particles, which confirms the successful construction of a core-shell structure. Figure 5 The prepared pure-phase NaCrO2 cathode material exhibits a smooth, exposed surface morphology. The NaCrO2@NFPP cathode material prepared in this invention shows a uniform coating characteristic on the NFPP-coated sample surface. Locally, due to the larger coating thickness, some areas display a bright white contrast in electron microscopy, further confirming the presence and integrity of the coating layer. In this core-shell structure, the outer NFPP coating layer effectively blocks direct contact between the electrolyte and the NaCrO2 core, thereby significantly suppressing interfacial side reactions, reducing the rate of increase in electrode-electrolyte interfacial impedance, and alleviating Cr during cycling. 3+ The dissolution of Na. Meanwhile, NFPP, as a framework material with three-dimensional sodium ion transport channels, provides a suitable environment for Na... + The migration at the core-shell interface provides an efficient and stable diffusion pathway, effectively improving the kinetics of the electrode reaction. Furthermore, this coating layer may also buffer Na to some extent. + The changes in lattice stress caused during the insertion / extraction process reduce structural damage to the material, thereby jointly promoting the electrode material to maintain higher structural integrity and chemical stability during long-term cycling, ultimately achieving a significant improvement in cycle life and safety performance.

[0053] Combination Figures 2-4 As shown, different coating processes result in variations in coating quality, specifically in the thickness and uniformity of the NFPP coating layer.

[0054] according to Figure 2 It can be clearly seen that the sample prepared in Example 2 has a continuous and dense amorphous NFPP coating layer on the surface of its NaCrO2 core particles, which confirms the successful construction of the core-shell structure. The key feature is that the thickness and surface morphology of the NFPP coating layer can be controlled by the precursor concentration: as the amount of NFPP coating increases, the coating layer thickness increases accordingly, and the surface roughness improves, ultimately resulting in a moderate increase in the overall size of the composite particles.

[0055] according to Figure 3It is clearly evident that, although the sample prepared in Example 3 exhibits a continuous NFPP coating layer on the surface of the NaCrO2 particles, confirming the formation of a core-shell structure, its morphology and structure differ significantly from those of Example 1. When the pH of the coating system was increased to 6, the hydrolysis process of the NFPP precursor was accelerated, resulting in an excessively rapid rate of release of active monomers. This, in turn, shifted the reaction pathway from the ideal heterogeneous deposition (film formation on the NaCrO2 surface) to the unfavorable homogeneous nucleation (self-forming particles in solution), generating a large number of fine NFPP particles. These particles subsequently accumulated randomly on the core surface, ultimately forming a porous coating layer composed of secondary particles, with high porosity and poor density.

[0056] according to Figure 4 It can be clearly seen that the sample prepared in Example 4 successfully constructed a core-shell structure with NaCrO2 as the core and NFPP as the shell. However, when the coating reaction time was extended to 6 hours, a significant change in the morphology of the coating layer was observed: on the one hand, the longer reaction time provided conditions for the homogeneous nucleated NFPP fine particles in the solution to be adsorbed and deposited again on the already formed coating layer, resulting in an increase in the overall thickness of the coating layer; on the other hand, the continuous condensation reaction intensified the homogeneous nucleation tendency, causing some areas of the coating layer to exhibit the characteristics of being composed of fine particle accumulation. This is directly manifested in the electronic image as a bright white contrast in the local coating layer due to excessive thickness and material accumulation.

[0057] Performance testing: The positive electrode materials prepared in Examples 1-4 and Comparative Examples 1-2 were used as positive electrode active materials. Positive electrode slurries were prepared according to a mass ratio of active material: conductive agent (acetylene black): binder (polyvinylidene fluoride, PVDF) of 8:1:1. These slurries were coated onto aluminum foil current collectors and then dried, rolled, and stamped to form positive electrode sheets. Using a sodium metal sheet as the counter electrode, a glass fiber membrane as the separator, and a 1M NaClO4 EC / PC (volume ratio 1:1) solution as the electrolyte, CR2032 coin cells were assembled in an argon-protected glove box. Electrochemical performance tests were conducted, and the results are shown in Table 1.

[0058] Table 1 .

[0059] As shown in Table 1: Although the NaCrO2@NFPP core-shell structured materials prepared in Examples 1 and 2 of this invention showed a slight decrease in initial discharge specific capacity compared to pure NaCrO2 (Comparative Example 1), their cycle stability was significantly improved. After 200 cycles at 1C rate, the capacity retention rates of Examples 1 and 2 reached 89% and 92%, respectively, far exceeding the 68% of Comparative Example 1 and the 75% of the physically mixed sample in Comparative Example 2. This fully demonstrates that the NFPP coating layer plays a crucial role in suppressing the capacity decay of NaCrO2 and improving its cycle life, and that the core-shell structure design is superior to simple physical mixing. Example 2 shows that a higher coating amount leads to better cycle stability, but with a slight sacrifice in capacity, reflecting the performance trade-off.

[0060] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the present invention. Without conflict, those skilled in the art can make various modifications or improvements based on the technical solutions of the present invention, and such modifications or improvements should also be considered to fall within the protection scope of the present invention.

Claims

1. An NFPP-coated sodium chromite core-shell structure cathode material, characterized in that, The cathode material has the following structural formula: NaCrO2@NFPP, where: the cathode material has a core-shell structure, with NaCrO2 as the core and NFPP as the shell covering NaCrO2; The method for preparing the NFPP-coated sodium chromite core-shell structure cathode material is characterized by comprising the following steps: Sodium chromite (NaCrO2) core powder was prepared by a high-temperature solid-state method. Provides NFPP precursor sol containing iron, phosphorus and sodium sources; The NaCrO2 core powder is dispersed in the NFPP precursor sol, so that the NFPP precursor preferentially nucleates and coats the NaCrO2 particles, resulting in NFPP precursor coated NaCrO2. The NFPP precursor was coated with NaCrO2 and then heat-treated under a protective atmosphere to obtain NFPP-coated sodium chromite core-shell structure cathode material. The structural formula of the NFPP is: Na4Fe3(PO4)2P2O7.

2. A method for preparing the NFPP-coated sodium chromite core-shell structure cathode material according to claim 1, characterized in that: (1) Preparation of NaCrO2 core Sodium source and trivalent chromium source are mechanically mixed at a sodium to chromium molar ratio of 1.0-1.1:1, and then calcined at 750-900℃ for 6-15 hours under an inert or reducing atmosphere. After cooling, the mixture is crushed, ground, and sieved to obtain sodium chromite powder. (2) Preparation of NFPP precursor sol Soluble iron, phosphorus and sodium sources were weighed according to the molar ratio of Na:Fe:P of 4.0-4.5:3:

4. Then a complexing agent was added, with the molar ratio of the complexing agent to the metal ions being 1.0-2.0:

1. The mixture was stirred continuously at 40-80℃ to form an NFPP precursor sol. (3) Preparation of NFPP precursor coated with NaCrO2 The NaCrO2 core powder obtained in step (1) is uniformly dispersed in the NFPP precursor sol prepared in step (2), and reacted at a certain temperature and pH value to obtain NFPP precursor coated with NaCrO2. (4) Preparation of NFPP-coated sodium chromite core-shell structure cathode material The NFPP precursor prepared in step (3) was coated with NaCrO2 and heat-treated under an inert protective atmosphere at a temperature of 500-650℃ for 2-10 hours.

3. The method for preparing an NFPP-coated sodium chromite core-shell structure cathode material according to claim 2, characterized in that: In step (1): the sodium source is selected from one or more of sodium carbonate and sodium acetate; the trivalent chromium source is selected from one or more of chromium trioxide and chromium chloride.

4. The method for preparing an NFPP-coated sodium chromite core-shell structure cathode material according to claim 2, characterized in that: In step (1): the high-temperature calcination is carried out at a heating rate of 2-5℃ / min, and the inert atmosphere is selected from one or more of nitrogen and argon.

5. The method for preparing an NFPP-coated sodium chromite core-shell structure cathode material according to claim 2, characterized in that: In step (2): the soluble iron source is selected from one or more of ferric nitrate and ferric chloride, the phosphorus source is selected from one or more of ammonium dihydrogen phosphate and diammonium hydrogen phosphate, and the sodium source is selected from one or more of sodium acetate and sodium nitrate.

6. The method for preparing an NFPP-coated sodium chromite core-shell structure cathode material according to claim 2, characterized in that: In step (2): the complexing agent is selected from one or more of citric acid, ascorbic acid, and ethylenediaminetetraacetic acid.

7. The method for preparing an NFPP-coated sodium chromite core-shell structure cathode material according to claim 2, characterized in that: In step (3): control the pH value of the reaction in the range of 4.0-7.0 and the reaction temperature in the range of 40-70℃.

8. The method for preparing an NFPP-coated sodium chromite core-shell structure cathode material according to claim 2, characterized in that: In step (4): the inert protective atmosphere is argon, the argon flow rate is 50-200 mL / min, the heat treatment temperature is 550-620℃, and the heating rate is 2-5℃ / min.

9. The application of the NFPP-coated sodium chromite core-shell structure cathode material of claim 1 in the preparation of sodium-ion batteries.

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