Aluminum-doped modified mixed sodium iron phosphate positive electrode material and preparation method thereof

Abstract

The application provides an aluminum-doped modified mixed sodium iron phosphate positive material and a preparation method thereof, and belongs to the technical field of sodium ion batteries. 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C, Al 3+ is doped into the NFPP through a sol-gel method, and is obtained after high-temperature calcination. When the aluminum-doped modified mixed sodium iron phosphate positive material is used as a positive electrode of a sodium ion battery, the initial capacity is high, and the cycle stability and high-rate performance are very excellent, the reversible capacity is 133 mAh.g ‑1 -1 at a current density of 0.2 C, the capacity retention rate is 99% after 500 cycles at a current density of 20 C, and the capacity retention rate is 88% after 2500 cycles at a large current density of 50 C.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to an aluminum-doped modified mixed sodium iron phosphate positive electrode material and a preparation method thereof. BACKGROUND

[0002] Sodium ion batteries (SIBs) have abundant Na + reserves and SIBs and lithium ion batteries (LIBs) have similar electrochemical properties, thus causing great research interest in the field of grid-scale energy storage systems (ESS). However, it is still a great challenge to obtain a sodium ion battery positive electrode material with excellent electrochemical performance and good cost-effectiveness, which meets the requirements of ESS.

[0003] In view of the commercial success of the LiFePO4 cathode material in lithium ion batteries, the benign and inexpensive iron-based phosphate with similar properties is considered to be one of the best choices for polyanion cathode materials of sodium ion batteries. However, due to the special thermodynamic properties of the olivine phase NaFePO4 (NFP), the phosphoferrite phase NaFePO4 is often obtained in the traditional synthesis process, so the expected olivine phase NaFePO4 cannot be obtained by conventional methods. Due to the lack of effective Na + ion diffusion channels, it is generally believed that the phosphoferrite phase NaFePO4 has no electrochemical activity. Fortunately, Na2FeP2O7 derived from NaFePO4 can be directly synthesized by simple methods such as mechanical synthesis, sol-gel method, etc., and has low cost. And because there is no distortion in the crystal lattice during the charging and discharging process, it has strong cycle performance and good structural stability. Although Na2FeP2O7 shows open three-dimensional Na + channels and allows a higher working voltage (~3.0 V), this cathode material can only pass one sodium ion during the charging and discharging cycle, resulting in a low theoretical capacity of 97.1 mAh·g -1 , which hinders its large-scale commercial application. In this regard, polyanion phosphates composed of NaFePO4 and Na2FeP2O7 in a 1:1 molar ratio have attracted widespread research interest in terms of structure, and this mixed phosphate compound combines the advantages of phosphate and pyrophosphate cathodes, thus having good electrochemical performance. Na4Fe3(PO4)2(P2O7) (NFPP) (orthorhombic Pn21a) can obtain a higher theoretical capacity (~129 mAh·g -1). Doping is an effective method to improve the electronic conductivity of materials. It can introduce impurity energy levels and improve structural stability by introducing inert ions. Therefore, it is of great significance to develop a doped NaFePO4 with high specific capacity, excellent cycle stability and rate performance as a positive electrode material for sodium ion batteries. SUMMARY

[0004] In view of the above problems of the prior art, the present application provides an aluminum-doped modified mixed sodium iron phosphate positive electrode material and a preparation method thereof. The aluminum-doped modified mixed sodium iron phosphate positive electrode material provided by the present application has high specific capacity, excellent cycle stability and rate performance, and good cycle stability even at a large current density.

[0005] To achieve the above-mentioned purpose, the specific technical solutions of the present application are as follows:

[0006] In a first aspect, the present application provides an aluminum-doped modified mixed sodium iron phosphate positive electrode material, which has a chemical formula of Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C.

[0007] Preferably, the aluminum-doped modified mixed sodium iron phosphate positive electrode material is prepared by doping Al 3+ into NFPP through a sol-gel method and then high-temperature calcination.

[0008] The aluminum-doped modified mixed sodium iron phosphate positive electrode material Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C provided by the present application has excellent electronic conductivity and structural stability, and the diffusion kinetics and conductivity of Na + are significantly improved; in addition, the aluminum-doped modified mixed sodium iron phosphate positive electrode material has high initial capacity and exhibits very excellent cycle stability and high rate performance. The present application prepares the carbon-coated aluminum-doped modified mixed sodium iron phosphate positive electrode material Na 3+ Fe 4.4 Al 3.4 (PO4) 0.1 (P2O7) / C by doping Al 2.5 into NFPP through a traditional and simple sol-gel method and then high-temperature calcination; wherein, the aluminum-doped transition metal site replaces part of the iron atoms, and since the radius of Al 3+ is small, aluminum doping produces Na + and Fe 2+ vacancies, thereby improving the electronic conductivity; in addition, the inactive Al 3+Not involved in redox reaction, can reduce the change of crystal volume, prevent the collapse of crystal structure during charge and discharge cycle, thereby improving the cycle stability.

[0009] In a second aspect, the application provides a preparation method of an aluminum-doped modified mixed sodium iron phosphate positive electrode material, comprising the following steps:

[0010] S1. Dissolve an iron source and a sodium source in water according to a molar ratio, mix uniformly, and obtain a solution M1;

[0011] S2. Add a phosphorus source and an aluminum source to the solution M1 according to a molar ratio, mix uniformly, and obtain a solution M2;

[0012] S3. Add a carbon source to the solution M2, mix uniformly, and obtain a solution M3;

[0013] S4. Heat the solution M3 to form a wet gel;

[0014] S5. Vacuum dry the wet gel to obtain a dry gel;

[0015] S6. Grind the dry gel to obtain a powdery precursor, and after pre-burning and calcination of the powdery precursor under an inert atmosphere, cool to room temperature to obtain the aluminum-doped modified mixed sodium iron phosphate positive electrode material.

[0016] The aluminum-doped modified mixed sodium iron phosphate positive electrode material is prepared by a sol-gel method and subsequent high-temperature calcination, and in the steps, first, an iron source and a sodium source are added to be dissolved uniformly under stirring; then a phosphorus source and an aluminum source are added, and stirring is maintained at room temperature to form Na 4.5-x Fe 3.5-x Al x (PO4) 2.5 (P2O7); then a carbon source is added, and after uniform mixing, water is evaporated by heating and stirring to obtain a wet gel; the water in the wet gel is further evaporated by heating to obtain a dry gel; the dry gel is ground into powder, and high-temperature calcination is performed under the protection of an inert gas to obtain Na 4.5-x Fe 3.5-x Al x (PO4) 2.5 (P2O7) / C.

[0017] Preferably, in step S1, the molar ratio of the iron source to the sodium source is 3.4: 4.4.

[0018] Preferably, in step S1, the mixing time is 30-60 min, and the mixing temperature is 20-25 ℃.

[0019] Preferably, in step S1, the iron source comprises a mixture of one or more of ferric nitrate nonahydrate, ferrous oxalate, ferrous sulfate; and the sodium source comprises a mixture of one or more of sodium fluoride, sodium acetate, sodium carbonate, sodium nitrate.

[0020] Preferably, in step S2, the molar ratio of the phosphorus source to the aluminum source is 4.5:0.1.

[0021] Preferably, in step S2, the phosphorus source comprises a mixture of one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate; and the aluminum source comprises a mixture of one or more of aluminum nitrate nonahydrate, aluminum sulfate, aluminum chloride.

[0022] Preferably, in step S3, the mass of the carbon source is 20% of the total mass of the iron source, the sodium source, the phosphorus source, and the aluminum source.

[0023] Preferably, in step S3, the carbon source comprises a mixture of one or more of citric acid, oxalic acid, and glucose.

[0024] Preferably, in step S4, the heating temperature is 70-90℃.

[0025] Preferably, in step S5, the vacuum drying temperature is 100-120℃, and the time is 10-12h.

[0026] Preferably, in step S6, the inert atmosphere is argon or nitrogen.

[0027] Preferably, in step S6, the pre-burning temperature is 300-400℃, and the time is 2-5h.

[0028] Preferably, in step S6, the calcination temperature is 500-600℃, and the time is 5-10h.

[0029] In a third aspect, the application provides the use of the aluminum-doped modified mixed sodium iron phosphate positive electrode material or the aluminum-doped modified mixed sodium iron phosphate positive electrode material prepared by the method in the preparation of a sodium ion battery positive electrode.

[0030] Compared with the prior art, the application has the following advantages:

[0031] (1) The aluminum-doped modified mixed sodium iron phosphate positive electrode material Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C provided by the application has excellent electronic conductivity and structural stability, and Na +The diffusion kinetics and conductivity of the mixed sodium iron phosphate positive electrode material are significantly improved; in addition, the aluminum-doped modified mixed sodium iron phosphate positive electrode material has a high initial capacity and exhibits very excellent cycle stability and high-rate performance.

[0032] (2) The aluminum-doped modified mixed sodium iron phosphate positive electrode material Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C can reach 133 mAh·g -1 The reversible capacity at a current density of 0.2C is 133 mAh·g BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The XRD pattern of Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C prepared in Example 1 is shown in Figure 1.

[0034] Figure 2 The SEM pattern of Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C prepared in Example 1 is shown in Figure 2.

[0035] Figure 3 The TEM pattern of Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C prepared in Example 1 is shown in Figure 3.

[0036] Figure 4 The XRD pattern of Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C prepared in Example 1, Na 4.5 Fe 3.5 (PO4) 2.5 (P2O7) / C prepared in Comparative Example 1, Na 4.45 Fe 3.45 Al 0.05 (PO4) 2.5 (P2O7) / C prepared in Comparative Example 2, and Na 4.35 Fe 3.35 Al 0.15 (PO4)2.5 First cycle charge-discharge curves of Na

[0037] Figure 5 Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 Na 4.5 Fe 3.5 (PO4) 2.5 Na 4.45 Fe 3.45 Al 0.05 (PO4) 2.5 Na 4.35 Fe 3.35 Al 0.15 (PO4) 2.5 Rate plot of Na

[0038] Figure 6 Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 Na 4.5 Fe 3.5 (PO4) 2.5 Na 4.45 Fe 3.45 Al 0.05 (PO4) 2.5 Na 4.35 Fe 3.35 Al 0.15 (PO4) 2.5 Cycle performance plot of Na

[0039] Figure 7 Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 Cycle performance plot of Na DETAILED DESCRIPTION

[0040] The technical solutions of the present application will be described clearly and completely below. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts fall within the scope of the present application.

[0041] The present application provides an aluminum-doped modified mixed sodium iron phosphate positive material, with a chemical formula of Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C.

[0042] The aluminum-doped modified mixed sodium iron phosphate positive material is obtained by doping Al 3+ into NFPP through a sol-gel method and then high-temperature calcination.

[0043] The preparation method of the aluminum-doped modified mixed sodium iron phosphate positive material comprises the following steps:

[0044] S1. Dissolve an iron source and a sodium source in water according to a molar ratio, mix uniformly, and obtain a solution M1;

[0045] S2. Add a phosphorus source and an aluminum source to the solution M1 according to a molar ratio, mix uniformly, and obtain a solution M2;

[0046] S3. Add a carbon source to the solution M2, mix uniformly, and obtain a solution M3;

[0047] S4. Heat the solution M3 to form a wet gel;

[0048] S5. Vacuum dry the wet gel to obtain a dry gel;

[0049] S6. Grind the dry gel to obtain a powdery precursor, and then pre-burn and calcine the powdery precursor under an inert atmosphere, and cool to room temperature to obtain the aluminum-doped modified mixed sodium iron phosphate positive material.

[0050] Embodiment 1

[0051] An aluminum-doped modified mixed sodium iron phosphate positive material is prepared by the following method:

[0052] S1. Dissolve 0.0034 mol of iron nitrate nonahydrate in 100 mL of deionized water, stir to dissolve into a clear solution, add 0.0044 mol of sodium acetate to the above solution, and stir uniformly under room temperature conditions for 1 h to obtain a clear solution M1;

[0053] S2. 0.0045 mol of ammonium dihydrogen phosphate and 0.0001 mol of aluminum nitrate nonahydrate were sequentially added to the solution M1, and stirring was maintained to obtain a yellow-green solution M2;

[0054] S3. The total mass of the iron source, the sodium source, the phosphorus source, and 20% of glucose (0.886 g) was added to the solution M2, and the stirring condition was maintained without changing the color of the solution to obtain a solution M3;

[0055] S4. The solution M3 was placed in an 80 ℃ oil bath and heated and stirred until a white gel was formed;

[0056] S5. The white gel was transferred to a vacuum drying oven and dried at 120 ℃ for 12 h to obtain a white dry gel;

[0057] S6. The obtained white dry gel was finely ground with a pestle to obtain a powdery precursor, which was placed in a tube furnace and pre-fired at 350 ℃ for 3 h under the protection of an argon atmosphere, and then calcined at a rate of 5 ℃ / min to 550 ℃ for 5 h, and finally cooled to room temperature with the tube furnace to obtain an aluminum-doped modified mixed sodium iron phosphate positive electrode material Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C. The XRD, SEM, and TEM images of the Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C are shown in Figure 1 、 Figure 2 、 Figure 3 , respectively.

[0058] Comparative Example 1

[0059] A preparation method of a mixed sodium iron phosphate positive electrode material, and the preparation method is as follows:

[0060] S1. 0.0035 mol of iron nitrate nonahydrate was dissolved in 100 mL of deionized water, and the solution was stirred to be clear. 0.0044 mol of sodium acetate was added to the solution, and the solution was stirred uniformly at room temperature for 1 h to obtain a clear solution M1;

[0061] S2. 0.0045 mol of ammonium dihydrogen phosphate was added to the solution M1, and stirring was maintained to obtain a yellow-green solution M2;

[0062] S3. The total mass of the iron source, the sodium source, and the phosphorus source and 20% of glucose (0.821 g) was added to the solution M2, and the stirring condition was maintained without changing the color of the solution to obtain a solution M3;

[0063] S4. The solution M3 was placed in an 80 ℃ oil bath to heat and stir until a white gel was formed;

[0064] S5. The white gel was transferred to a vacuum drying oven and dried at 120 ℃ for 12 h to obtain a white dry gel;

[0065] S6. The obtained white dry gel was ground with a pestle to obtain a powdery precursor, which was placed in a tube furnace and pre-fired at 350 ℃ for 3 h under an argon atmosphere, then calcined at a rate of 5 ℃ / min to 550 ℃ for 5 h, and finally cooled to room temperature with the tube furnace to obtain a mixed sodium iron phosphate positive electrode material Na 4.5 Fe 3.5 (PO4) 2.5 (P2O7) / C.

[0066] Comparative Example 2

[0067] A kind of aluminum doped modified mixed sodium iron phosphate positive electrode material, the preparation method is as follows:

[0068] S1. 0.00345 mol of iron nitrate nonahydrate was dissolved in 100 mL of deionized water, and the dissolved solution was stirred to a clear solution. 0.00445 mol of sodium acetate was added to the above solution, and stirred uniformly at room temperature for 1 h to obtain a clear solution M1.

[0069] S2. 0.0045 mol of ammonium dihydrogen phosphate and 0.00005 mol of aluminum nitrate nonahydrate were sequentially added to solution M1, and the stirring condition was maintained to obtain a yellow-green solution M2.

[0070] S3. 20% of glucose (0.832 g) of the total mass of iron source, sodium source, phosphorus source and aluminum source was added to solution M2, and the stirring condition was maintained without changing the color of the solution to obtain solution M3.

[0071] S4. The solution M3 was placed in an 80 ℃ oil bath to heat and stir until a white gel was formed;

[0072] S5. The white gel was transferred to a vacuum drying oven and dried at 120 ℃ for 12 h to obtain a white dry gel;

[0073] S6. The obtained white dry gel was ground with a pestle to obtain a powdery precursor, which was placed in a tube furnace and pre-fired at 350 ℃ for 3 h under an argon atmosphere, then calcined at a rate of 5 ℃ / min to 550 ℃ for 5 h, and finally cooled to room temperature with the tube furnace to obtain a mixed sodium iron phosphate positive electrode material Na4.45 Fe 3.45 Al 0.05 (PO4) 2.5 (P2O7) / C.

[0074] Comparative Example 3

[0075] An aluminum-doped modified mixed sodium iron phosphate positive electrode material is prepared by the following method:

[0076] S1. Dissolve 0.00335 mol of iron nitrate nonahydrate in 100 mL of deionized water, stir to dissolve to a clear solution, and add 0.00435 mol of sodium acetate to the above solution. Stir uniformly at room temperature for 1 h to obtain a clear solution M1.

[0077] S2. Add 0.0045 mol of ammonium dihydrogen phosphate and 0.00015 mol of aluminum nitrate nonahydrate to solution M1 in sequence, and keep stirring to obtain a yellow-green solution M2.

[0078] S3. Add 20% of glucose (0.853 g) based on the total mass of the iron source, sodium source, phosphorus source, and aluminum source to solution M2, and keep the stirring conditions unchanged. The solution color does not change to obtain solution M3.

[0079] S4. Place solution M3 in an 80 ℃ oil bath and heat and stir until a white gel is formed.

[0080] S5. Transfer the above white gel to a vacuum drying oven, and keep drying at 120 ℃ for 12 h to obtain a white dry gel.

[0081] S6. Grind the obtained white dry gel with a pestle to obtain a powdery precursor. Place the powdery precursor in a tube furnace, pre-burn at 350 ℃ under the protection of an argon atmosphere, heat at a rate of 5 ℃ / min to 550 ℃ for calcination, and keep the temperature for 5 h. Finally, cool the tube furnace to room temperature to obtain an aluminum-doped modified mixed sodium iron phosphate positive electrode material Na 4.35 Fe 3.35 Al 0.15 (PO4) 2.5 (P2O7) / C.

[0082] Application Example

[0083] The positive electrode material, conductive agent (Super P), and polyvinylidene fluoride (PVDF) prepared in Example 1 and Comparative Examples 1-3 were mixed in a mass ratio of 7:2:1, uniformly dispersed in N-methyl-2-pyrrolidone (NMP) solvent to form a uniform slurry, and coated on an aluminum foil using a doctor blade or a four-way applicator to prepare a positive electrode sheet. The coated aluminum foil was dried in a drying oven at 120°C for 12 h, and then the aluminum foil with the coated material was cut into a circular electrode sheet with a diameter of 12 mm, and placed in a glove box to assemble a button cell (CR2032). The prepared positive electrode material was used as a working electrode, and a sodium block was used as a counter electrode to test the electrochemical performance, wherein the separator was Whatman GF / A, and the electrolyte system was 1M NaClO4solute and ethylene carbonate (EC)+dimethyl carbonate (DMC)+5% FEC solvent, and the volume ratio of EC and DMC was 1:1.

[0084] The assembled button cell was subjected to electrochemical performance test, and the test instrument used for electrochemical performance test was as follows: a LANHE-CT2001A type multi-channel battery test system produced by Wuhan Lanhe Electronics Co., Ltd., the static time was 8 h, the voltage window range was 1.8-4.3V, the current density range was 20-5000 mA·g -1 , and the cycle number range was 100-3000 times.

[0085] Figure 4 The first cycle charge-discharge curve of Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C prepared in Example 1, Na 4.5 Fe 3.5 (PO4) 2.5 (P2O7) / C prepared in Comparative Example 1, Na 4.45 Fe 3.45 Al 0.05 (PO4) 2.5 (P2O7) / C prepared in Comparative Example 2, and Na 4.35 Fe 3.35 Al 0.15 (PO4) 2.5 (P2O7) / C prepared in Comparative Example 3 was shown in FIG. 1. Figure 4 It can be seen that the first cycle specific capacity of Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C prepared in Example 1 was the highest, which was 133 mA·g -1 .

[0086] Figure 5 Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C, Na 4.5 Fe 3.5 (PO4) 2.5 (P2O7) / C, Na 4.45 Fe 3.45 Al 0.05 (PO4) 2.5 (P2O7) / C, Na 4.35 Fe 3.35 Al 0.15 (PO4) 2.5 (P2O7) / C at various current densities. Na Figure 5 Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C maintained higher specific capacity at various current densities.

[0087] Figure 6 Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 Na 4.5 Fe 3.5 (PO4) 2.5 Na 4.45 Fe 3.45 Al 0.05 (PO4) 2.5 Na 4.35 Fe 3.35 Al 0.15 (PO4) 2.5 (P2O7) / C at 20C current density. Na Figure 6 Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C still had ultra-high capacity retention rate and cycle stability at 20C current density.

[0088] Figure 7 Na 4.4 Fe 3.4 Al0.1 (PO4) 2.5 (P2O7) / C at 50C current density. The capacity retention of Na Figure 7 It can be seen that the Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C prepared in Example 1 still has a high capacity retention of 88% after 2500 cycles at a high current density of 50C.

[0089] In summary, the aluminum-doped modified mixed sodium iron phosphate positive electrode material Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C has excellent electronic conductivity and structural stability, and the diffusion kinetics and conductivity of Na + are significantly improved; in addition, the aluminum-doped modified mixed sodium iron phosphate positive electrode material has a high initial capacity and exhibits very excellent cycle stability and high-rate performance.

[0090] The above specific embodiments describe the implementation of the present application, but the present application is not limited to the specific details in the above embodiments. Within the scope of the claims and technical concepts of the present application, various simple modifications and changes can be made to the technical solutions of the present application, and these simple modifications all belong to the protection scope of the present application.

Claims

1. An aluminum-doped modified mixed sodium iron phosphate cathode material, characterized in that, The chemical formula is Na 4.4 Fe 3.4 Al 0.1 (PO4) 2.5 (P2O7) / C; The preparation method of the aluminum-doped modified mixed sodium iron phosphate cathode material includes the following steps: S1. Dissolve the iron source and sodium source in water according to the molar ratio, mix them evenly, and obtain solution M1; S2. Add the phosphorus source and aluminum source to solution M1 according to the molar ratio, mix them evenly, and obtain solution M2; S3. Add the carbon source to solution M2 and mix thoroughly to obtain solution M3; S4. Heating solution M3 until a wet gel is formed; the heating temperature is 70~90 ℃; S5. Vacuum dry the wet gel to obtain a dry gel; S6. Grind the dry gel to obtain a powdered precursor. Pre-calcine and bake the powdered precursor under an inert atmosphere, and then cool it to room temperature to obtain an aluminum-doped modified mixed sodium iron phosphate cathode material. The pre-calcine temperature is 300~400 ℃ and the time is 2~5 h. The baking temperature is 500~600 ℃ and the time is 5~10 h.

2. The aluminum-doped modified mixed sodium iron phosphate cathode material according to claim 1, characterized in that, The molar ratio of the iron source to the sodium source is 3.4:4.

4.

3. The aluminum-doped modified mixed sodium iron phosphate cathode material according to claim 1, characterized in that, The molar ratio of the phosphorus source to the aluminum source is 4.5:0.

1.

4. The aluminum-doped modified mixed sodium iron phosphate cathode material according to claim 1, characterized in that, The mass of the carbon source is 20% of the total mass of the iron source, sodium source, phosphorus source, and aluminum source.

5. The aluminum-doped modified mixed sodium iron phosphate cathode material according to claim 1, characterized in that, The iron source includes one or more of ferric nitrate nonahydrate, ferrous oxalate, and ferrous sulfate, or a mixture thereof. The sodium source includes one or more of sodium fluoride, sodium acetate, sodium carbonate, and sodium nitrate; The phosphorus source includes one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and sodium dihydrogen phosphate; The aluminum source includes one or a mixture of aluminum nitrate nonahydrate, aluminum sulfate, and aluminum chloride. The carbon source includes one or more of citric acid, oxalic acid, and glucose, or a mixture thereof.

6. The application of the aluminum-doped modified mixed sodium iron phosphate cathode material as described in any one of claims 1-5 in the preparation of sodium-ion battery cathodes.

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