Double site doped sodium iron phosphate / carbon composite positive electrode material and preparation method thereof

Abstract

Dual-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material and preparation method thereof, the positive electrode material of the present invention is introduced into tungstate during the raw material mixing process, and W is used in the synthesis process. 6+ and WO4 2‑ The double-site doped sodium iron phosphate pyrophosphate / carbon composite cathode material is Na4Fe 3‑x W x (PO4) 2‑y (WO4) y (P2O7) / C, 0≤x≤0.3, 0≤y≤0.3. The present invention also includes a method for preparing the dual-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material. The dual-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the present invention has greatly improved phase purity, does not contain inactive marmorite-type sodium iron phosphate and low-capacity sodium iron pyrophosphate impurity phases, and significantly reduces sodium iron antisite defects in the structure. The positive electrode material has good electronic conductivity and ionic conductivity, high specific capacity, good rate performance, long cycle life, and significantly improved high and low temperature performance.

Description

TECHNICAL FIELD

[0001] The application relates to a sodium ion battery positive electrode material, in particular to a double-site doped sodium iron phosphate / carbon composite positive electrode material of a sodium ion battery and a preparation method thereof. BACKGROUND

[0002] As a representative product in new energy batteries, lithium ion batteries play a key role in the technical "cornerstone" of digital 3C, electric vehicles and large-scale energy storage power stations. However, the limited lithium resources in geochemistry will not be able to fully meet the growing energy needs of human society, and sodium ion batteries can be used as an ideal alternative to lithium ion batteries, especially in the context of large-scale energy storage applications.

[0003] The performance and cost of sodium ion batteries are largely determined by the positive electrode material, and a low-cost, long-life positive electrode material is the key to promoting the development of high-performance sodium ion batteries. Polyanion-type positive electrode materials are important candidates for the industrialization of sodium ion battery positive electrode materials due to their low cost and long cycle life. Among them, iron-based mixed phosphate sodium iron pyrophosphate Na4Fe3(PO4)2(P2O7) is the most promising sodium battery positive electrode material for industrialization and application. It is composed of high-abundance, low-cost and non-toxic Fe and P elements, has high structural stability (volume strain < 4% during sodiumization / desodiation), moderate theoretical capacity (128.9 mAh g -1 ) and working voltage (3.0 V vs. Na + / Na). However, the problems of non-active marumeite-type sodium iron phosphate (NaFePO4) and low-capacity sodium iron pyrophosphate (Na2Fe(P2O7)) impurities, low electronic conductivity and slow ion transport rate formed during synthesis limit its further development.

[0004] CN2022108476538 and CN2022109282351 respectively disclose a sodium ion battery positive electrode material sodium iron pyrophosphate / carbon and a synthesis method thereof, both of which use phosphorus-containing organic compounds as phosphorus and carbon sources. The organic phosphorus source is expensive, and the problems of low electronic conductivity of NFPP and impurity formation have not been solved.

[0005] CN2023103030205 discloses a synthesis method of a sodium ion battery positive electrode material sodium iron pyrophosphate / carbon, which uses Mn and V for doping to improve the working voltage. The defect is that the Jahn-Teller effect of Mn will affect the structural stability of NFPP, sacrificing the cycle stability, and V is a highly toxic element, which is not environmentally friendly and expensive.

[0006] CN2023107601573 discloses a porous sodium iron phosphate pyrophosphate / carbon composite material and its preparation method and application, and the porous morphology is obtained by adjusting the spray drying process, but the low conductivity and impurity phase problems are not solved, and the porous morphology leads to low compaction density.

[0007] CN2023110621642 discloses a vanadium-doped sodium iron phosphate pyrophosphate / carbon composite material and its preparation method and application, and the working voltage is improved by V doping, but V is toxic and expensive.

[0008] CN2023115878412 discloses a sodium iron phosphate pyrophosphate / carbon composite positive electrode material and its preparation method and application, which mainly directly uses solid-phase mixing to dry-mix the raw materials, and then directly sinter to avoid the ball milling process, but this leads to uneven mixing of raw materials and is more prone to impurity phases, resulting in poor material performance.

[0009] CN2024107558084 discloses a sodium iron phosphate pyrophosphate / carbon composite positive electrode material and its preparation method and application, which mainly pre-reduces the trivalent iron of the raw materials by a reducing agent to avoid the formation of impurity phases, but the iron source itself can use divalent iron raw materials, and the pre-reduction of trivalent iron raw materials by a reducing agent is a common operation, and this does not solve the problem of low conductivity of NFPP.

[0010] CN2024109093221 discloses a sodium iron phosphate pyrophosphate / carbon composite positive electrode material and its preparation method, which mainly prepares a starch-based hydrogel to obtain a NFPP / C aerogel material with a porous structure. This method is complex, and the porous structure of the aerogel reduces the compaction density of the material. SUMMARY

[0011] The technical problem to be solved by the present application is to overcome the above-mentioned defects in the prior art, and to provide a sodium iron phosphate pyrophosphate / carbon composite positive electrode material with high electronic conductivity, fast ion transmission rate and double-site doping, and a preparation method thereof.

[0012] The further technical problem to be solved by the present application is to overcome the above-mentioned defects in the prior art, and to provide a preparation method of a double-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material with low cost, simple operation, environmental friendliness and no impurity phase in the prepared composite positive electrode material.

[0013] The technical solution adopted by the present application to solve the technical problem is a double-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material, whose chemical formula is Na4Fe 3-x W x (PO4) 2-y (WO4) y(P2O7) / C, wherein 0≤x≤0.3, 0≤y≤0.3.

[0014] The technical scheme adopted by the present application to solve the technical problem is further a preparation method of a double-site doped sodium iron pyrophosphate / carbon composite positive electrode material, comprising the following steps:

[0015] (1) uniformly mixing raw materials of a sodium source, an iron source, a phosphorus source, a tungsten source, a carbon source and a solvent, drying to obtain a mixed precursor;

[0016] (2) pre-burning, grinding and sintering the precursor obtained in step (1), and then cooling to obtain the double-site doped sodium iron pyrophosphate / carbon composite positive electrode material Na4Fe 3-x W x (PO4) 2-y (WO4) y (P2O7) / C.

[0017] Further, in step (1), the molar ratio of the elements sodium, iron, phosphorus and tungsten in the raw materials is: sodium: iron: phosphorus: tungsten = 4.0: 2.7-3.0: 3.7-4.0: 0.01-0.6; the mass of the carbon source is 1-40% of the mass of the theoretical sodium iron pyrophosphate positive electrode material, preferably 5-35%, more preferably 10-30%.

[0018] Further, in step (1), the sodium source is one or more of sodium dihydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium pyrophosphate, disodium pyrophosphate, sodium acetate, sodium oxalate, and sodium citrate; the iron source is one or more of iron phosphate, iron nitrate, iron chloride, ferrous oxalate, diiron trioxide, triiron tetroxide, ferrous acetate, ferrous sulfate, and ammonium ferrous phosphate; the phosphorus source is one or more of phosphoric acid, pyrophosphoric acid, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, iron phosphate, trisodium phosphate, and sodium pyrophosphate; and the carbon source is one or more of citric acid, glucose, sucrose, fructose, starch, ascorbic acid, oxalic acid, polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, polydopamine, resorcinol-formaldehyde resin, cellulose, ketjen black, carbon nanotubes, graphene oxide, and reduced graphene oxide.

[0019] Further, in step (1), the tungsten source is one or more of ammonium tungstate, sodium tungstate, and ferrous tungstate.

[0020] Further, in step (1), the mixing method is one of stirring and dissolving, ball milling, and sand milling; and the drying method is any one of evaporation drying, spray drying, and freeze drying.

[0021] Further, in step (2), the pre-burning temperature is 290-400°C, and the pre-burning time is 2-5 hours.

[0022] Further, in step (3), the sintering temperature is 500-600°C, preferably 540-560°C, and the sintering time is 5-20 hours, preferably 8-12 hours.

[0023] Further, in step (4), the pre-sintering and sintering atmosphere is an inert gas such as nitrogen, argon, argon-hydrogen mixture, or / and a reducing gas.

[0024] Compared with the prior art, the present application has the following beneficial effects: 1) The double-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the present application has the advantages of good electrical conductivity, high specific capacity, good cycle stability and high rate capacity; for example, the 0.1C specific capacity in the voltage range of 2.0-4.0V can reach 113.8mAh g -1 ; the stability after 300 cycles at 1C rate can reach 99.6%; the 10C high-rate capacity is 91.6mAh g -1 ; 2) The method for preparing the double-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the present application uses the same element to simultaneously perform double doping at the Fe site and the PO4 site, which improves the configurational entropy of the material (~1.5R, medium-high entropy material), increases the formation energy of harmful impurity phases, effectively inhibits the formation of impurity phases, and produces a material without impurity phases; the strong W-O bond can stabilize the crystal structure, and the substitution of W 6+ and WO4 2- in the crystal structure can widen the migration channel of sodium ions, reduce the band gap, and greatly improve the electronic conductivity and sodium ion diffusion rate of the positive electrode material; 3) The method for preparing the double-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the present application has a short process flow, simple process, is green and environmentally friendly, and has good repeatability. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 XRD patterns of the modified sodium iron phosphate pyrophosphate materials prepared in Example 1 and Comparative Example 1;

[0026] Figure 2 SEM images of the modified sodium iron phosphate pyrophosphate materials prepared in Example 1 and Comparative Example 1;

[0027] Figure 3 First cycle charge-discharge curves of the modified sodium iron phosphate pyrophosphate materials prepared in Example 1 and Comparative Example 1;

[0028] Figure 4 Rate performance graphs of the sodium iron phosphate pyrophosphate materials prepared in Example 1 and Comparative Example 1;

[0029] Figure 5 Cycle performance graphs of the sodium iron phosphate pyrophosphate materials prepared in Example 1 and Comparative Example 1; DETAILED DESCRIPTION

[0030] The application will be further described in conjunction with specific embodiments and the accompanying drawings, but the application is not limited to the following embodiments. Unless otherwise defined, all the professional terms used below have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the application.

[0031] In the following examples, the experimental methods are conventional methods unless otherwise specified; the reagents and materials are commercially available unless otherwise specified.

[0032] Example 1

[0033] The chemical formula of the two-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the present embodiment is: Na4Fe 2.9 W 0.1 (PO4) 1.9 (WO4) 0.1 (P2O7) / C.

[0034] The preparation method comprises the following steps:

[0035] (1) 0.5 mmol of sodium carbonate, 39 mmol of sodium dihydrogen phosphate, 29 mmol of iron nitrate nonahydrate, 0.17 mmol of ammonium tungstate, and 20 mmol of citric acid monohydrate are added to 50 mL of deionized water and mixed, and then spray dried at 220°C and a flow rate of 300 mL / min using a spray dryer to obtain a mixed precursor;

[0036] (2) The mixed precursor obtained in step (1) is pre-sintered at 300°C for 3h in 95% / 5% argon-hydrogen mixed gas, and the pre-sintered product is taken out and ground again, and then sintered at 550°C for 10h, and then cooled to room temperature in the furnace to obtain the two-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material Na4Fe 2.9 W 0.1 (PO4) 1.9 (WO4) 0.1 (P2O7) / C.

[0037] The XRD pattern of the obtained modified positive electrode material is shown in Figure 1 From Figure 1 it can be seen that the synthesized sodium iron phosphate pyrophosphate has an orthorhombic structure with a space group of Pn21a, showing high phase purity without impurity phases. The SEM image of the positive electrode material is shown in Figure 2 The material particles have good sphericity, and the surface is wrapped with a uniform carbon layer.

[0038] The obtained positive electrode material was mixed with a conductive agent and PVDF, and then coated on an aluminum foil after slurry preparation. A CR2032 button half-cell was assembled with a sodium sheet as the negative electrode, a 1M NaClO4 solution of EC:PC (1:1 Vol%, containing 5% FEC) as the electrolyte, and the electrochemical performance of the positive electrode material was tested.

[0039] The first cycle charge-discharge curve of the assembled battery is shown in Figure 3 , and the specific capacity at 0.1C discharge is 113.8mAh g -1 . The rate performance graph is shown in Figure 4 , and the high-rate capacity at 10C is 91.6mAh g -1 . The cycle performance graph is shown in Figure 5 , and the capacity retention rate after 300 cycles at 1C is as high as 99.6%.

[0040] Comparative Example 1

[0041] (1) 40mmol of sodium dihydrogen phosphate, 30mmol of iron nitrate nonahydrate, and 20mmol of citric acid monohydrate were added to 50mL of deionized water and mixed, and then spray dried at 220℃ and a flow rate of 300mL / min using a spray dryer to obtain a mixed precursor;

[0042] (2) The above was pre-sintered at 300℃ for 3h in 95% / 5% argon-hydrogen mixed gas, the pre-sintered product was taken out and ground again, and then sintered at 550℃ for 10h, and cooled to room temperature in the furnace to obtain a double-site doped sodium iron pyrophosphate / carbon composite positive electrode material Na4Fe3(PO4)2(P2O7) / C.

[0043] The XRD pattern of the prepared positive electrode material is shown in Figure 1 , and the SEM image is shown in Figure 2 . It can be seen from Figure 1 and Figure 2 that the NFPP positive electrode material prepared in Comparative Example 1 has obvious impurities compared with the NFPP positive electrode material prepared in Example 1.

[0044] A CR2032 button half-cell was assembled in the same way as in Example 1, and the electrochemical performance of the positive electrode material was tested. The first cycle charge-discharge curve of the assembled battery is shown in Figure 3 , and the specific capacity at 0.1C discharge is 105.2mAh g -1 . The rate performance graph is shown in Figure 4 , and the high-rate capacity at 10C is 75.4mAh g -1 . The cycle performance is shown in Figure 5 , and the capacity retention rate after 300 cycles at 1C is 93.3%, and the performance indicators are all lower than those of Example 1.

[0045] Example 2

[0046] The chemical formula of the double-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the embodiment is: Na4Fe 2.95 W 0.05 (PO4) 1.95 (WO4) 0.05 (P2O7) / C.

[0047] The preparation method comprises the following steps:

[0048] (1) 0.25 mmol of sodium carbonate, 39.5 mmol of sodium dihydrogen phosphate, 29.5 mmol of iron nitrate nonahydrate, 0.09 mmol of ammonium tungstate, and 20 mmol of citric acid monohydrate are added to 50 mL of deionized water for mixing, and spray drying is performed at 220°C and a flow rate of 300 mL / min using a spray dryer to obtain a mixed precursor;

[0049] (2) The mixed precursor obtained in step (1) is pre-sintered at 350°C for 2.5 hours in an argon atmosphere, and the pre-sintered product is taken out and ground again, and then sintered at 520°C for 12 hours, and cooled to room temperature in the furnace, to obtain the double-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material Na4Fe 2.95 W 0.05 (PO4) 1.95 (WO4) 0.05 (P2O7) / C.

[0050] Example 3

[0051] The chemical formula of the double-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the embodiment is: Na4Fe 2.85 W 0.15 (PO4) 1.85 (WO4) 0.15 (P2O7) / C.

[0052] The preparation method comprises the following steps:

[0053] (1) 0.75 mmol of sodium carbonate, 38.5 mmol of sodium dihydrogen phosphate, 28.5 mmol of iron nitrate nonahydrate, 0.26 mmol of ammonium tungstate, and 20 mmol of citric acid monohydrate are added to 50 mL of deionized water for mixing, and spray drying is performed at 220°C and a flow rate of 300 mL / min using a spray dryer to obtain a mixed precursor;

[0054] (2) The mixed precursor obtained in step (1) is pre-sintered at 300°C for 3h under nitrogen, and the pre-sintered product is taken out and ground again, and then sintered at 570°C for 8h, and cooled to room temperature in the furnace, to obtain a two-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material Na4Fe 2.85 W 0.15 (PO4) 1.85 (WO4) 0.15 (P2O7) / C.

[0055] Example 4

[0056] The chemical formula of the two-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the present example is:

[0057] Na4Fe 2.80 W 0.20 (PO4) 1.80 (WO4) 0.20 (P2O7) / C.

[0058] The preparation method comprises the following steps:

[0059] (1) 1mmol of sodium carbonate, 38.0mmol of sodium dihydrogen phosphate, 28.0mmol of iron nitrate nonahydrate, 0.34mmol of ammonium tungstate, and 20mmol of citric acid monohydrate are added to 50mL of deionized water for mixing, and a spray dryer is used to spray dry at a flow rate of 300mL / min at 220°C, to obtain a mixed precursor;

[0060] (2) The above is pre-sintered at 330°C for 2.8h under 95% / 5% argon-hydrogen mixed gas, and the pre-sintered product is taken out and ground again, and then sintered at 550°C for 10h, and cooled to room temperature in the furnace, to obtain a two-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material Na4Fe 2.80 W 0.20 (PO4) 1.80 (WO4) 0.20 (P2O7) / C.

[0061] The 0.1C initial discharge specific capacity, 10C discharge specific capacity, and 1C cycle capacity retention rate of 300 times of the two-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode materials of Examples 1-4 and the sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the control example are listed in the following table:

[0062]

[0063] As can be seen from the table, the 10C discharge specific capacity and the capacity retention rate after 300 cycles at 1C of the two-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material of embodiments 1-4 of the present application and the sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the comparative examples are obviously higher than the 10C discharge specific capacity and the capacity retention rate after 300 cycles at 1C of the sodium iron phosphate pyrophosphate / carbon composite positive electrode material of the comparative examples.

[0064] The above only describes the preferred embodiments of the present application, and it should be noted that, for those skilled in the art, several improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A double site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material, characterized in that, Na4Fe 3-x W x (PO4) 2-y (WO4) y (P2O7) / C, wherein 0 2. A method for preparing the double site doped sodium iron phosphate pyrophosphate / carbon composite cathode material according to claim 1, characterized in that, The method comprises the following steps: (1) mixing raw materials of sodium source, iron source, phosphorus source, tungsten source, carbon source and solvent uniformly, drying to obtain mixed precursors; (2) The mixed precursor obtained in step (1) is pre-burned, ground, sintered, and cooled to obtain a two-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material Na4Fe 3-x W x (PO4) 2-y (WO4) y (P2O7) / C.

3. The method for preparing a double site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material according to claim 2, characterized in that: In step (1), the molar ratio of sodium, iron, phosphorus and tungsten in the raw materials is: sodium: iron: phosphorus: tungsten = 4.0: 2.7~3: 3: 3.7~4.0: 0.01~0.6; the mass of the carbon source is 1~40% of the mass of the theoretical sodium iron phosphate pyrophosphate positive electrode material.

4. The preparation method of the double-site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material according to claim 2 or 3, characterized in that: In step (1), the sodium source is one or more of sodium dihydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium pyrophosphate, disodium pyrophosphate, sodium acetate, sodium oxalate, and sodium citrate; the iron source is one or more of iron phosphate, iron nitrate, iron chloride, ferrous oxalate, diiron trioxide, triiron tetroxide, ferrous acetate, ferrous sulfate, and ammonium ferrous phosphate; the phosphorus source is one or more of phosphoric acid, pyrophosphoric acid, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, iron phosphate, trisodium phosphate, and sodium pyrophosphate; the carbon source is one or more of citric acid, glucose, sucrose, fructose, starch, ascorbic acid, oxalic acid, polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, polydopamine, resorcinol-formaldehyde resin, cellulose, ketjen black, carbon nanotubes, graphene oxide, and reduced graphene oxide.

5. The method for preparing a double site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material according to claim 2 or 3, characterized in that: In step (1), the tungsten source is one or more of ammonium tungstate, sodium tungstate, and ferrous tungstate.

6. The method for preparing a double site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material according to claim 2 or 3, characterized in that: In step (1), the mixing method is one or more of stirring and dissolving, ball milling, and sand milling; the drying method is any one of evaporation drying, spray drying, and freeze drying.

7. The method for preparing a double site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material according to claim 2 or 3, characterized in that: In step (2), the pre-burning temperature is 290~400℃, and the pre-burning time is 2~5 hours.

8. The method for preparing a double site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material according to claim 2 or 3, characterized in that: In step (2), the sintering temperature is 500~600℃, and the sintering time is 5~20 hours.

9. The method for preparing a double site doped sodium iron phosphate pyrophosphate / carbon composite positive electrode material according to claim 2 or 3, characterized in that: In step (2), the pre-burning and sintering atmospheres are one of nitrogen, argon, and argon-hydrogen mixed gas.

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