Modified composite sodium ferric phosphate positive electrode material and preparation method and application thereof

Abstract

The invention belongs to the field of battery manufacturing, and relates to a preparation method of a sodium ion battery, in particular to a modified composite sodium ferric phosphate positive electrode material as well as a preparation method and application thereof. The preparation method comprises the following steps: 1) crushing and sieving dried citric acid; (2) uniformly mixing the treated citric acid serving as a carbon-coated material with a composite sodium ferric phosphate material in manners of ball milling and the like; (3) sintering the uniformly mixed material in a high-temperature furnace and preserving heat; and 4) sieving the modified material cooled to room temperature to obtain the modified composite sodium ferric phosphate positive electrode material. According to the sodium ion battery composite sodium ferric phosphate positive electrode material, citric acid is adopted as a modifier, the pH value of the material is reduced, the tap density of the material is improved, and the prepared sodium ion battery has excellent cycle performance while having high capacity exerting and rate performance.

Description

Technical Field

[0001] The present invention belongs to the field of battery manufacturing and relates to a method for preparing a sodium ion battery, and in particular to a modified composite sodium iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] As a highly promising electrochemical energy storage medium, sodium-ion batteries (Na-ion batteries) offer broad application prospects in large-scale energy storage systems and smart grid construction, leveraging their significant advantages of abundant sodium resources and low raw material costs. Among the three types of cathode material systems currently receiving significant industry attention, polyanionic composite sodium iron phosphate (NFPP, chemical formula Na4Fe3(PO4)2P2O7) exhibits significant application value in energy storage devices due to its unique olivine crystal structure, resulting in excellent thermodynamic stability, and rapid charge transfer properties through three-dimensional ion channels. This material not only possesses the cost advantages of iron-based materials, but also possesses core characteristics such as long cycle life, outstanding thermal safety, and significant environmental friendliness. However, due to inherent defects such as low intrinsic electronic conductivity and sluggish sodium ion diffusion kinetics, practical applications often face technical bottlenecks such as limited mass-to-capacity, insufficient adaptability to low-temperature environments, and significant performance degradation under high-rate charge and discharge conditions.

[0003] In terms of material chemical properties, compared with layered oxide positive electrode materials, although the pH value of NFPP is in the sub-strong alkaline range of 10-12, this alkaline environment will still trigger a series of interfacial side reactions during the long-term service of the battery. Specifically, it manifests as continuous decomposition of the electrolyte leading to gas expansion, increased impedance of the electrode / electrolyte interface film causing capacity decay, and increased dendrite growth on the surface of the sodium metal negative electrode, which are safety hazards. An in-depth analysis of the source of its alkalinity shows that it is mainly attributed to free alkaline substances such as sodium hydroxide and sodium carbonate remaining in the synthesis process, hydrolysis and alkalinization reactions on the surface of the material under the action of environmental humidity, and sodium-containing alkaline compounds introduced during the doping and modification process. When the pH value is too high during the preparation of the electrode slurry, the polyvinylidene fluoride binder is prone to β-phase dehydrofluorination reaction, forming conjugated double bonds between the molecular chains, resulting in gelation of the slurry. At the same time, the alkaline catalytic hydrogen evolution reaction further exacerbates the risk of battery gas production.

[0004] To address the above technical difficulties, the industry currently adopts four main modification strategies: 1) Optimizing the NFPP chemical environment, such as introducing low-alkalinity or acidic cathode materials (NFS, pH ~ 3.6) or substances to neutralize the residual alkali on the surface; 2) Thoroughly washing the synthesized materials to remove residual alkali and other impurities; 3) Selecting electrolytes with better compatibility with high pH cathode materials to reduce interfacial reactions; 4) Precisely controlling the reaction conditions during the synthesis process to avoid excessive use of alkali.

[0005] It is worth noting that surface coating modification technology has attracted widespread attention due to its strong process compatibility and good cost controllability. By constructing a carbon-based or metal oxide protective layer, it can effectively isolate the material surface from direct contact with the electrolyte and improve the electronic conduction network structure. However, existing modification methods generally suffer from a contradiction between process complexity and the extent of performance improvement. How to achieve long-term control of surface alkalinity while maintaining the intrinsic electrochemical properties of the material remains a key technical obstacle restricting the industrial application of this material.

[0006] In addition, on the one hand, there are differences in understanding among those skilled in the art; on the other hand, the inventor studied a large number of documents and patents when making the present invention, but due to space limitations, not all details and contents are listed in detail. However, this does not mean that the present invention does not have the characteristics of these prior arts. On the contrary, the present invention already has all the characteristics of the prior art, and the applicant reserves the right to add relevant prior art to the background art. Summary of the Invention

[0007] Based on the above technical problems, one of the objectives of the present invention is to provide a modified composite sodium iron phosphate positive electrode material and its preparation method and application to overcome the shortcomings of the existing technology.

[0008] To achieve the above object, the present invention adopts the following technical solutions: A method for preparing a modified composite sodium iron phosphate positive electrode material comprises the following steps: The citric acid and the composite sodium iron phosphate material are uniformly mixed, and the mixed material is subjected to a heating treatment under an inert atmosphere and then an annealing treatment; The material cooled to room temperature was sieved to obtain a modified composite sodium iron phosphate positive electrode material.

[0009] Preferably, the preparation method comprises the following steps: S1: citric acid pretreatment; S2: The amount of citric acid added is 3wt%~10wt% of the composite sodium iron phosphate material. The citric acid and composite sodium iron phosphate material are mixed according to the above addition amount, and the temperature is increased to 100~200℃ at 2~5℃ / min under nitrogen atmosphere for 2~6 hours; S3: Constant temperature treatment at 250-500℃ for 2-6 hours; S4: The material is sieved after cooling to room temperature.

[0010] According to a preferred embodiment, the citric acid may be anhydrous citric acid or citric acid monohydrate.

[0011] According to a preferred embodiment, the pretreatment step of citric acid comprises: Anhydrous citric acid and citric acid monohydrate are mixed, dried, crushed, and sieved, wherein the particle size D50 of the citric acid after sieving is ≤0.5 μm.

[0012] Preferably, the citric acid pretreatment step comprises: Anhydrous citric acid and monohydrate citric acid are mixed and dried at a temperature of 80-120°C for 2-6 hours; Crush and sieve.

[0013] According to a preferred embodiment, the general chemical formula of the composite sodium iron phosphate material is NaFePO 2+x (2 <x<4)。

[0014] According to a preferred embodiment, the inert atmosphere is selected from one or more of nitrogen, argon and hydrogen.

[0015] According to a preferred embodiment, the step of heating treatment comprises: The temperature was raised to 0-200°C at a heating rate of 2-5°C / min and continued for 2-6 h.

[0016] According to a preferred embodiment, the annealing temperature is 250-500°C.

[0017] One of the purposes of the present invention is to provide a modified composite sodium iron phosphate positive electrode material, which is prepared based on the above-mentioned modified composite sodium iron phosphate positive electrode material preparation method.

[0018] One of the purposes of the present invention is also to provide a sodium ion battery positive electrode, which uses a modified composite sodium iron phosphate positive electrode material prepared based on the above-mentioned modified composite sodium iron phosphate positive electrode material preparation method.

[0019] Another object of the present invention is to provide a sodium ion battery using a modified composite sodium iron phosphate cathode material prepared using the above-mentioned method for preparing the modified composite sodium iron phosphate cathode material. The sodium ion battery comprises a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode uses the modified composite sodium iron phosphate cathode material prepared using the above-mentioned method for preparing the modified composite sodium iron phosphate cathode material.

[0020] This patented technical solution achieves a triple synergistic effect through citric acid modification, as described below: (1) Based on the acid-base neutralization mechanism, the carboxyl functional groups in the citric acid molecules react quantitatively with the sodium hydroxide remaining on the surface of the material (C6H8O7+3NaOH→Na3C6H5O7+3H2O), generating water-soluble organic sodium salts accompanied by the release of water. This process can effectively eliminate the free strong alkaline components, significantly reduce the pH value of the material surface, and thus inhibit the gelation tendency in the electrode slurry preparation stage ( Figures 1 and 2) and the risk of gas production due to electrolyte decomposition during battery cycling ( Figure 4 ), improve the safety of the battery system from the source.

[0021] (2) Citric acid acts as an auxiliary carbon precursor during the high-temperature carbonization process, and constructs a continuous and dense carbon-based protective layer on the surface of NFPP particles through in-situ pyrolysis. This carbon layer forms a topological interpenetrating network structure with the inherent carbon source of the material, while maintaining the integrity of the crystal structure ( Figure 3 ), which not only optimizes the spatial distribution of the conductive path, but also significantly improves the volumetric capacity characteristics of the electrode by regulating the particle packing density ( Figure 4 In addition, the nano-scale carbon coating has both physical confinement effect and stress buffering function, which can not only inhibit the agglomeration and coarsening of active particles during the cycle to maintain the high specific surface area characteristics, but also alleviate the lattice volume deformation caused by sodium ion deintercalation through mechanical constraint (Table 3 tap density), thereby synergistically improving the material's rate performance and cycle durability ( Figure 4 ).

[0022] (3) The lyophilic modification of the surface carbon layer makes the modified NFPP particles have better interfacial wettability, forming a uniform and stable colloidal dispersion system during the electrode slurry mixing stage, effectively avoiding coating defects caused by abnormal increase in local viscosity ( Figures 1 and 2 ). BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The SEM images of the materials prepared in Examples 1, 4, 5 and Comparative Example 1 of the present invention are shown; Figure 2 SEM images of the materials prepared in Examples 1 and 3 of the present invention; Figure 3 The XRD patterns of the materials prepared in Examples 1, 3, 4, 5, 7 and Comparative Example 1 of the present invention are shown below: Figure 4 The charge and discharge curves of button-type half-cells prepared using the composite sodium iron phosphate materials prepared in Examples 1, 3, 5, and 9 of the present invention and Comparative Example 1 at a current of 0.1 C are shown. DETAILED DESCRIPTION

[0024] In the description of the present invention, terms are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly indicate the number of technical features indicated. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0025] The raw materials used in the following examples were all commercially available or homemade; unless otherwise specified, their mass percentages are indicated.

[0026] Preferably, the amount of citric acid added is 3wt% of the composite sodium iron phosphate material. More preferably, the amount of citric acid added is 4wt% of the composite sodium iron phosphate material. More preferably, the amount of citric acid added is 5wt% of the composite sodium iron phosphate material. More preferably, the amount of citric acid added is 6wt% of the composite sodium iron phosphate material. More preferably, the amount of citric acid added is 7wt% of the composite sodium iron phosphate material. More preferably, the amount of citric acid added is 8wt% of the composite sodium iron phosphate material. More preferably, the amount of citric acid added is 9wt% of the composite sodium iron phosphate material. More preferably, the amount of citric acid added is 10wt% of the composite sodium iron phosphate material.

[0027] Preferably, the "temperature raising treatment of S2" comprises raising the temperature to 180°C at a rate of 2°C / min under a nitrogen atmosphere for 4 hours.

[0028] Preferably, the "annealing treatment in S3" includes a constant temperature treatment at 400°C for 4 hours.

[0029] The “inert atmosphere conditions” in the following examples refer to nitrogen conditions.

[0030] Example 1 This embodiment provides a method for modifying the pH value of a composite sodium iron phosphate cathode material and improving its reversible capacity, and its application. The specific preparation method includes the following steps: 1) Weigh 50 g of anhydrous citric acid and dry it in a vacuum drying oven at 80°C for 4 h. 2) crushing the dried anhydrous citric acid granules and sieving them through a 300 mesh sieve to obtain citric acid powder with a particle size D50 ≤ 0.5 μm; 3) Weigh 1.5 g of citric acid powder and 30 g of composite sodium iron phosphate NFPP and mix them, that is, the mass of citric acid is 5wt% of NFPP. The mixed material is kept warm in an inert atmosphere in two stages: heating to 180°C at a rate of 2°C / min and keeping warm for 2 h, then heating to 300°C and keeping warm for 4 h, and then naturally cooling to obtain a modified composite sodium iron phosphate positive electrode material with a reduced pH value and high reversible capacity.

[0031] Example 2 The specific steps of this embodiment are similar to those of embodiment 1, except that: in step 3), the mass of citric acid is 3 wt% of NFPP, i.e., 0.9 g citric acid and 30 g composite sodium iron phosphate NFPP.

[0032] Example 3 The specific preparation method steps of this embodiment are similar to those of Example 1, except that: in step 3), the mass of citric acid is 10 wt% of NFPP, that is, 3 g citric acid and 30 g composite sodium iron phosphate NFPP.

[0033] Example 4 The specific preparation method steps of this embodiment are similar to those of Example 1, except that: Step 3) sintering procedure: heating to 180°C at a rate of 2°C / min and holding for 2 h, then heating to 400°C and holding for 4 h, and then cooling in the furnace.

[0034] Example 5 The specific preparation method steps of this embodiment are similar to those of Example 1, except that: Step 3) sintering procedure: heating to 180°C at a rate of 2°C / min and holding for 2 h, then heating to 500°C and holding for 4 h, and then cooling in the furnace.

[0035] Example 6 The specific preparation method steps of this embodiment are similar to those of Example 1, except that: Step 3) sintering procedure: heating to 150°C at a rate of 2°C / min and holding for 2 h, then heating to 400°C and holding for 4 h before cooling in the furnace.

[0036] Example 7 The specific preparation method steps of this embodiment are similar to those of embodiment 1, except that: Step 3) sintering procedure: heating to 300°C at a rate of 2°C / min, holding for 4 hours, and then cooling in the furnace.

[0037] Example 8 The specific preparation steps of this embodiment are similar to those of embodiment 1, except that: Step 3) sintering procedure: heating to 300°C at a rate of 2°C / min, holding for 3 h, and then cooling in the furnace.

[0038] Example 9 The specific preparation method steps of this embodiment are similar to those of embodiment 1, except that: Step 3) sintering procedure: heating to 300°C at a rate of 2°C / min, holding for 5 h, and then cooling in the furnace.

[0039] Comparative Example 1 The composite sodium iron phosphate positive electrode material provided in this comparative example was not subjected to any modification treatment, that is, original NFPP.

[0040] Comparative Example 2 The specific preparation steps of the composite sodium iron phosphate positive electrode material provided in this comparative example are as follows: 30 g of composite sodium iron phosphate NFPP was weighed and kept warm in two stages under inert atmosphere. The temperature was raised to 180°C at a rate of 2°C / min and kept warm for 2 h, then raised to 300°C and kept warm for 4 h, and then naturally cooled to obtain a composite sodium iron phosphate positive electrode material.

[0041] Comparative Example 3 In this comparative example, oxalic acid was used as an additive to prepare modified composite sodium iron phosphate. The specific preparation steps are as follows: 30 g of composite sodium iron phosphate NFPP and 1.5 g of oxalic acid were weighed and uniformly mixed to obtain an oxalic acid-modified composite sodium iron phosphate positive electrode material.

[0042] Detailed information on ingredients and sintering procedures is shown in Table 1.

[0043] Table 1. Sodium-ion battery performance data

[0044] In order to verify the use effect of the positive electrode material of the present invention, the materials prepared in each of Examples 1-9 and Comparative Examples 1-2 were prepared as positive electrode sheets. The preparation method was as follows: the positive electrode material, sp, and PVDF were mixed into a slurry in a mass ratio of 94:3:3, and the slurry was evenly coated on aluminum foil using a 200 μm four-sided preparation device. The film was then placed in a 100°C blast drying oven and dried for 8 hours. The electrode film was punched into a disc with a diameter of 14 mm using a sheet puncher. The cut electrode discs were placed in a vacuum drying oven at 100°C and dried for 4 hours and then transferred to a glove box. Using a metal sodium disc as the counter electrode, NaPF6 as the electrolyte, and a Whatman GF / D glass fiber diaphragm as the diaphragm, a CR2016 button cell was assembled in the glove box and a constant current charge and discharge test was performed at 0.1C. The specific results and the pH value of the material are shown in Table 2.

[0045] Table 2. Comparison of electrochemical performance of Examples 1-9 and Comparative Examples 1-2

[0046] Comparison of the data in Table 2 shows that citric acid significantly suppresses the pH value of the NFPP cathode material. Compared to the original sample (Comparative Example 1), the pH value of the modified sample with 3% citric acid addition (Example 2) decreased from 10.65 to 9.88. The pH value decreased significantly with increasing addition levels, decreasing to 9.57 with 5% addition (Example 1) and 9.07 with 10% addition (Example 3). Furthermore, when 5% citric acid was added (Example 1), the sample exhibited a discharge capacity of 101.80 mAh / g at 0.1C, compared to the discharge capacity of 99.58 mAh / g for the original sample (Comparative Example 1), indicating an improvement in the reversible capacity of the battery. However, when the addition level was 10% (Example 3), the discharge capacity of the sample decreased to only 97.52 mAh / g.

[0047] For the modified samples of Examples 1, 4, and 5, the first holding temperature during heat treatment was 180°C, and the second holding temperatures were 300, 400, and 500°C, respectively. The results show that a sintering temperature of 300°C produces a low material pH value and a high reversible capacity. Further increasing the sintering temperature to 400°C further increases the discharge specific capacity to 103.25 mAh / g. However, the pH reduction effect is not as good as at 300°C, reaching only 9.94. At 500°C, the pH reduction effect is even worse, and the capacity enhancement effect is also inferior to that at 300°C.

[0048] Examples 1, 6, and 7 compare the effects of different holding temperatures during the first stage on the modification effect. The results show that Example 1, with an intermediate sintering temperature of 180°C, achieved the best pH reduction compared to the other conditions, while also significantly improving the discharge specific capacity. When the second holding time was extended to 5 hours (Example 9), the pH of the sample slightly increased compared to 4 hours (Example 1), and the capacity began to decrease. In this invention, increasing the intermediate sintering temperature allows the citric acid to melt at the pre-calcination temperature, reacting with the residual alkali on the surface of the original NFPP to form sodium citrate (C6H5O7Na3), thereby reducing the residual alkali. This allows the molten CA to infiltrate the surface of the NFPP particles and react more fully with the residual alkali. The carbon content data in Table 3 suggest that another reason for the citric acid modification effect may be that the carbonized citric acid coats the surface of the YN3 particles, protecting the NFPP grains from solution etching.

[0049] Table 3. Comparison of some physical and chemical properties of Examples 1, 3 and Comparative Example 1

[0050] Table 3 compares some of the physical and chemical properties of Examples 1 and 3 with Comparative Example 1. The results show that the D50 of the modified NFPP material is slightly increased, proving the growth of its particles. Figure 1 and Figure 2 SEM images further support this observation. With increasing final sintering temperature and citric acid addition, particle growth becomes more pronounced, and particle agglomeration is significantly reduced, leading to a smoother particle surface. This reduces the forces acting on the particles during positive electrode slurry preparation, thus mitigating slurry gelation. Furthermore, the tap density of sintered samples increased with the addition of citric acid, which could contribute to the development of high-energy-density batteries.

[0051] Figure 3 The XRD patterns of the materials obtained in Examples 1, 3, 4, 5, 7 and Comparative Example 1 are shown in FIG. Figure 3It can be seen that, compared with the PDF card of Na4Fe3(PO4)2(P2O7), the diffraction peaks of different embodiments and comparative samples correspond one to one with the PDF card, and there are no obvious impurity peaks, indicating that citric acid modification has no obvious effect on the purity of the original sample.

[0052] Figure 4 The charge and discharge curves of button half-cells prepared using the composite sodium iron phosphate materials prepared in Examples 1, 3, 5, 9 and Comparative Example 1 at a current of 0.1 C are shown.

[0053] In all cases, although the pH-lowering effects and capacity increases in Examples 1-9 vary slightly depending on the modification conditions, overall, they all exhibit lower pH values ​​and higher reversible capacities than the original material (Comparative Example 1) and the sample sintered without the addition of citric acid (Comparative Example 2). Sintering in Comparative Example 2 according to the process described in the examples has little impact on performance, so the primary effect is the addition of CA. Comparative Example 3, while using oxalic acid as an additive, effectively lowers the pH value of the material, it has a detrimental effect on charge-discharge performance. Clearly, the modified composite sodium iron phosphate material produced using the technical approach provided by the present invention can both lower the material pH and reduce slurry gelation and cell gassing, thereby increasing reversible capacity and boosting the energy density of sodium-ion batteries.

[0054] In the existing technology, citric acid is cracked into carbon at 300°C. The carbon thermal reduction environment provided by the carbon enables trivalent iron ions to be reduced to ferrous ions. At this stage, the excess carbon source will be coated on the surface of the NFPP particles, and citric acid cannot increase its participation in material modification based on its own properties.

[0055] Adding excessive citric acid in the initial process without secondary treatment can lead to delamination of the modified base sample and modifier, and also fails to achieve the desired neutralization alkalinity and increased coating efficiency. The citric acid of the present invention reacts with residual alkali (such as NaOH and Na2CO3) to produce sodium citrate, which can be used as a sodium supplement in the positive electrode material. During the calcination process, the different decomposition temperatures of citric acid and sodium citrate are cleverly utilized to carbonize excess citric acid without affecting the sodium citrate. This avoids the problem of foreign matter introduced into the NFPP material, which reduces its capacity, while still achieving the purpose of material modification.

[0056] The aforementioned basic examples and their respective alternatives can be freely combined to form a plurality of embodiments, all of which are applicable and claimed embodiments of the present invention. In the scheme of the present invention, each alternative can be arbitrarily combined with any other basic examples and alternatives. Those skilled in the art will appreciate the numerous possible combinations.

[0057] It should be noted that the above-described specific embodiments are illustrative only. Those skilled in the art may devise various solutions based on the disclosure of the present invention, and such solutions fall within the scope of the present invention and are intended to be protected by the present invention. Those skilled in the art should understand that the present description and its accompanying drawings are intended to be illustrative only and are not intended to limit the scope of the claims. The scope of protection of the present invention is defined by the claims and their equivalents.

Claims

1. A method for preparing a modified composite sodium iron phosphate positive electrode material, characterized in that: The preparation method comprises the following steps: The citric acid and the composite sodium iron phosphate material are uniformly mixed, and the mixed material is subjected to a heating treatment under an inert atmosphere and then an annealing treatment; The material cooled to room temperature was sieved to obtain a modified composite sodium iron phosphate positive electrode material.

2. The preparation method according to claim 1, characterized in that The citric acid includes anhydrous citric acid and monohydrate citric acid.

3. The preparation method according to claim 2, characterized in that The citric acid pretreatment step comprises: Anhydrous citric acid and citric acid monohydrate are mixed, dried, crushed, and sieved, wherein the particle size D50 of the citric acid after sieving is ≤0.5 μm.

4. The preparation method according to claim 1 or 2, characterized in that The general chemical formula of the composite sodium iron phosphate material is NaFePO 2+x (2 <x<4)。 5. The preparation method according to claim 1 or 2, characterized in that The inert atmosphere is selected from one or more of nitrogen, argon and hydrogen.

6. The preparation method according to claim 1 or 2, characterized in that The step of heating treatment comprises: The temperature was raised to 0-200°C at a heating rate of 2-5°C / min and continued for 2-6 h.

7. The preparation method according to claim 1 or 2, characterized in that The annealing temperature is 250-500°C.

8. A modified composite sodium iron phosphate positive electrode material, characterized in that: The modified composite sodium iron phosphate positive electrode material is prepared based on the modified composite sodium iron phosphate positive electrode material preparation method according to any one of claims 1 to 7.

9. A sodium ion battery positive electrode, characterized in that A modified composite sodium iron phosphate positive electrode material prepared using the modified composite sodium iron phosphate positive electrode material preparation method according to any one of claims 1 to 7.

10. A sodium ion battery, characterized in that: A modified composite sodium iron phosphate positive electrode material prepared using the modified composite sodium iron phosphate positive electrode material preparation method according to any one of claims 1 to 7.

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