Medium-entropy sodium iron phosphate pyrophosphate composite material and preparation method thereof

Abstract

The application relates to the technical field of chemical power sources, in particular to a medium-entropy sodium iron phosphate pyrophosphate composite material and a preparation method. 4‑a Fe 3‑3a M a Mg a Cu a (PO4)2P2O7, 0.01<=a<=0.1, and M is one of Al, Cr, Ce, Co, Sc and Ga. The sodium source, the phosphorus source, the iron source, the carbon source and the doping source are mixed and then ball-milled to obtain the medium-entropy sodium iron phosphate pyrophosphate composite material. The composite material prepared by the application has good rate performance and cycle performance as a polyanion positive electrode material. The preparation method is simple and easy to control, is suitable for large-scale production, and indicates a wide application prospect in the field of electrochemical energy storage.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode material technology, specifically to a medium-entropy sodium iron pyrophosphate composite material and its preparation method. Background Technology

[0002] With the continuous growth of the global economy and the increasing demand for energy, the pursuit of efficient and clean energy storage technologies is becoming increasingly urgent. Among various energy storage technologies, lithium-ion batteries have become one of the most mainstream battery technologies due to their widespread application in portable electronic devices, electric vehicles, and energy storage systems. However, the limited availability of lithium resources and the continuous rise in costs make it essential to find alternative solutions. Sodium-ion batteries, with their abundant resources, low cost, and environmental friendliness, have become a highly promising alternative. However, sodium ions have a larger ionic radius than lithium ions, making the intercalation and diffusion processes in electrode materials relatively more difficult, thus affecting battery performance. Therefore, developing high-performance electrode materials suitable for sodium-ion batteries has become a key research direction in this field.

[0003] Among numerous candidate materials, sodium iron pyrophosphate is considered an ideal choice for cathode materials in sodium-ion batteries due to its low cost, good structural stability, and excellent cycle performance. However, its inherent properties lead to low electronic conductivity and slow ion diffusion, affecting electrochemical performance. Therefore, researchers have developed a series of modification strategies to address this issue. Among them, the "medium-entropy concept" has been widely applied to the structural design system of electrochemical energy storage materials. Medium-entropy materials are a class of materials containing three or more elements that are solidly dissolved in equimolar or near-molar ratios to form a single phase. Their unique configuration and electrochemical tunability bring new opportunities to overcome the current performance bottlenecks of electrode materials. Summary of the Invention

[0004] This invention aims to provide a medium-entropy sodium iron pyrophosphate sodium-ion battery cathode material and its preparation method, using an equimolar ratio of Mg... 2+ Cu 2+ M 3+ (M 3+ For Al 3+ Cr 3+ Ce 3+ Co 3+ ,Sc 3+ Ga 3+ One of the three metal ions in Fe 2 + The formation of a combined crystal, with diverse local structures and high-valence M states. 3+ The resulting Na vacancies effectively delayed the formation of Na +The phase transition reaction and charge compensation in the insertion / extraction reaction effectively suppressed the phase transition reaction during the charge and discharge process of pure phase Na4Fe3(PO4)2P2O7, stabilizing the matrix structure; the obtained medium-entropy sodium-ion battery cathode material has a single phase and good crystallinity, and has excellent electrochemical performance in sodium-ion batteries.

[0005] Therefore, in a first aspect, the present invention provides a medium-entropy sodium ferric pyrophosphate composite material; wherein the chemical formula of the sodium ferric pyrophosphate is Na. 4-a Fe 3-3a M a Mg a Cu a (PO4)2P2O7, 0.01≤a≤0.1, M is one of Al, Cr, Ce, Co, Sc, Ga, and V.

[0006] Secondly, the present invention also provides a method for preparing the medium-entropy iron pyrophosphate sodium composite material described in the first aspect, comprising the following steps:

[0007] Step 1: Mix the sodium source, iron source, phosphorus source, carbon source and doping source evenly, wherein the doping source includes magnesium source, divalent copper source and trivalent metal M source;

[0008] Step 2: Place the above materials in a ball mill jar filled with inert gas and ball mill them;

[0009] Step 3: The product obtained in Step 2 is heated and calcined in an inert atmosphere, and then cooled to room temperature to obtain a medium-entropy sodium iron pyrophosphate composite material.

[0010] Preferably, in step one, the sodium source, iron source, phosphorus source and dopant source are added in a molar ratio of 4-a:3-3a:4:a:a:a, where the value of a ranges from 0.01 to a ≤ 0.1; the amount of carbon source added is based on the carbon content in the final prepared medium-entropy sodium pyrophosphate pyrophosphate composite material being between 0.1% and 9%.

[0011] Preferably, in step one, the sodium source is one or more of sodium acetate, sodium carbonate, sodium bicarbonate, sodium citrate, and sodium oxalate; the iron source is one or more of ferric acetate and its hydrate, ferrous oxalate and its hydrate, ferrous citrate, and ferrous ammonium sulfate and its hydrate; the phosphorus source is one or more of ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and triammonium phosphate; and the carbon source is one or more of sucrose, glucose, citric acid, ascorbic acid, and malic acid.

[0012] Preferably, in step one, the magnesium source is one or more of magnesium oxide, magnesium carbonate, magnesium acetate, magnesium nitrate, and their hydrates; the divalent copper source is one or more of copper oxide, copper carbonate, calcium acetate, calcium nitrate, and their hydrates; the trivalent metal M source has M in the form of Al, Cr, Ce, Co, Sc, Ga, and V, wherein the aluminum source is one or more of aluminum oxide, aluminum carbonate, aluminum oxalate, aluminum nitrate, and their hydrates; the chromium source is one or more of chromium oxide, chromium acetate, chromium nitrate, and their hydrates; the cerium source is one or more of cerium oxide, cerium sulfate, cerium acetate, cerium nitrate, and their hydrates; the cobalt source is one or more of cobalt oxide, cobalt carbonate, cobalt sulfate, cobalt acetate, cobalt nitrate, and their hydrates; the scandium source is one or more of scandium oxide, scandium oxalate, scandium nitrate, and their hydrates; the gallium source is one or more of gallium oxide, gallium acetate, gallium nitrate, and their hydrates; and the vanadium source is one or more of vanadium oxide, vanadium oxysulfate, vanadium oxyoxalate, vanadium nitrate, and their hydrates.

[0013] Preferably, in step two, the inert gas is one or more of argon and a hydrogen-argon mixture; the milling container is selected from one of agate milling jars, polyurethane milling jars, polytetrafluoroethylene milling jars, nylon milling jars, corundum milling jars, tungsten carbide milling jars, zirconia milling jars, and ceramic milling jars; the milling speed is 200-800 r / min; and the milling time is 4-24 h.

[0014] Preferably, in step three, the specific operation is as follows: the ball-milled product is transferred to a high-temperature furnace and heat-treated in an inert atmosphere, heated to 200-400°C at a rate of 3-9°C / min and calcined at this temperature for 2-5 hours; then the temperature is further increased to 500-600°C and calcined at this temperature for 5-11 hours, the inert atmosphere being one or more of argon and a hydrogen-argon mixture; the sample is cooled to room temperature in an inert atmosphere to obtain a medium-entropy sodium iron pyrophosphate composite material.

[0015] Thirdly, the present invention also provides the use of the medium-entropy sodium pyrophosphate composite material described in the first aspect as a cathode material for sodium-ion batteries.

[0016] Compared with the prior art, the advantages of the present invention are:

[0017] 1. This invention provides a Na 4-a Fe 3-3a M a Mg a Cu a (PO4)2P2O7 cathode material, which has a single phase, good crystallinity, and excellent electrochemical performance;

[0018] 2. This invention utilizes equimolar ratios of Mg 2+Cu 2+ M 3+ (M 3+ For Al 3+ Cr 3+ Ce 3+ Co 3+ ,Sc 3+ Ga 3+ One of the three metal ions in Fe 2+ The formation of a combined crystal, with diverse local structures and high-valence M states. 3+ The resulting Na vacancies effectively delayed the formation of Na + The phase transition reaction and charge compensation in the insertion / extraction reaction effectively suppressed the phase transition reaction during the charge and discharge process of pure phase Na4Fe3(PO4)2P2O7, stabilizing the matrix structure; after 1000 cycles of charge and discharge at 10C rate, the capacity retention rate of the material is 97.5%, indicating that the medium entropy effect greatly improves the cycle stability of the material.

[0019] 3. Preparation of Na by this invention 4-a Fe 3-3a M a Mg a Cu a (PO4)2P2O7 medium-entropy sodium-ion battery cathode material can be directly prepared by solid-state ball milling, which is easy to industrialize, environmentally friendly, low-cost, and easy to mass-produce for industrial applications. Attached Figure Description

[0020] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0021] Figure 1 The Na prepared in Example 1 of this invention 3.97 Fe 2.91 Al 0.03 Mg 0.03 Cu 0.03 X-ray diffraction pattern of (PO4)2P2O7 material;

[0022] Figure 2 The Na prepared in Example 1 of this invention 3.97 Fe 2.91 Al 0.03 Mg 0.03 Cu 0.03 Scanning electron microscope image of (PO4)2P2O7 material;

[0023] Figure 3 The Na prepared in Example 1 and Comparative Example 1 of this invention 3.97 Fe 2.91 Al 0.03Mg 0.03 Cu 0.03 0.1C rate charge-discharge curves of (PO4)2P2O7 and Na4Fe3(PO4)2P2O7 materials;

[0024] Figure 4 The Na prepared in Example 1 and Comparative Example 1 of this invention 3.97 Fe 2.91 Al 0.03 Mg 0.03 Cu 0.03 Cycling curves of (PO4)2P2O7 and Na4Fe3(PO4)2P2O7 materials at different magnifications;

[0025] Figure 5 The Na prepared in Example 1 and Comparative Example 1 of this invention 3.97 Fe 2.91 Al 0.03 Mg 0.03 Cu 0.03 10C rate cycling curves of (PO4)2P2O7 and Na4Fe3(PO4)2P2O7 materials. Detailed Implementation

[0026] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0027] It should be noted that terms such as "upper", "lower", "left", "right", and "middle" used in this specification are only for clarity of description and are not intended to limit the scope of implementation. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered as within the scope of this application.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the term “and / or” as used herein includes any and all combinations of one or more of the associated listed items.

[0029] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0030] As used herein, the term “about” is used to provide for the flexibility and imprecision associated with a given term, measure, or value. Those skilled in the art can readily determine the degree of flexibility for a particular variable.

[0031] As used herein, the term “at least one of…” is intended to be synonymous with “one or more of…”. For example, “at least one of A, B, and C” explicitly includes only A, only B, only C, and combinations thereof.

[0032] Concentration, amount, and other numerical data may be presented in range format herein. It should be understood that such range format is used solely for convenience and brevity and should be flexibly interpreted to include not only the values ​​explicitly stated as the limits of the range, but also all individual values ​​or subranges encompassed within the range, as if each value and subrange were explicitly stated. For example, a range of values ​​from about 1 to about 4.5 should be interpreted to include not only the explicitly stated limits of 1 to 4.5, but also individual numbers (such as 2, 3, 4) and subranges (such as 1 to 3, 2 to 4, etc.). The same principle applies to ranges that describe only a single value, such as “less than about 4.5,” which should be interpreted to include all the aforementioned values ​​and ranges. Furthermore, this interpretation should apply regardless of the breadth of the range or characteristic described.

[0033] The purpose of this invention is to provide a method for preparing a medium-entropy sodium iron pyrophosphate composite material and its application. This invention provides a Na... 4-a Fe 3-3a M a Mg a Cu a (PO4)2P2O7 cathode material, with its single phase, high crystallinity, and excellent electrochemical performance. This material is produced by using equimolar ratios of Mg... 2+ Cu 2+ M 3+ (M 3+ For Al 3+ Cr 3+ Ce 3+ Co 3+ ,Sc 3+ Ga 3+ One of the three metal ions in Fe 2+ The formation of a combined crystal, with diverse local structures and high-valence M states. 3+ The resulting Na vacancies effectively delayed the formation of Na +The phase transition reaction and charge compensation in the insertion / extraction reaction effectively suppressed the phase transition reaction during the charge / discharge process of pure-phase Na4Fe3(PO4)2P2O7, stabilizing the matrix structure. After 1000 cycles of charge / discharge at 10C, the material retained 97.5% of its capacity, indicating that the medium entropy effect significantly improved the material's cycle stability. Furthermore, this preparation method, achieved directly through solid-state ball milling, is easy to industrialize, environmentally friendly, low-cost, and readily applicable for mass industrial applications.

[0034] The present invention employs the following specific operational steps to prepare a medium-entropy sodium pyrophosphate composite material, and then assembles it into a battery to test its performance.

[0035] Step 1: Mix the sodium source, iron source, phosphorus source, carbon source and doping source evenly, wherein the doping source includes magnesium source, divalent copper source and trivalent metal M source;

[0036] Preferably, the sodium source, iron source, phosphorus source and dopant source in step one are added in a molar ratio of 4-a:3-3a:4:a:a:a, where the value of a ranges from 0.01 to a ≤ 0.1; the amount of carbon source added is based on the carbon content in the final prepared medium-entropy sodium pyrophosphate pyrophosphate composite material being between 0.1% and 9%.

[0037] Preferably, the sodium source in step one is one or more of sodium acetate, sodium carbonate, sodium bicarbonate, sodium citrate, and sodium oxalate; the iron source is one or more of ferric acetate and its hydrate, ferrous oxalate and its hydrate, ferrous citrate, and ferrous ammonium sulfate and its hydrate; the phosphorus source is one or more of ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and triammonium phosphate; and the carbon source is one or more of sucrose, glucose, citric acid, ascorbic acid, and malic acid.

[0038] Preferably, the magnesium source in step one is one or more of magnesium oxide, magnesium carbonate, magnesium acetate, magnesium nitrate, and their hydrates; the divalent copper source is one or more of copper oxide, copper carbonate, calcium acetate, calcium nitrate, and their hydrates; the trivalent metal M source has M in the form of Al, Cr, Ce, Co, Sc, Ga, and V, wherein the aluminum source is one or more of aluminum oxide, aluminum carbonate, aluminum oxalate, aluminum nitrate, and their hydrates; the chromium source is one or more of chromium oxide, chromium acetate, chromium nitrate, and their hydrates; the cerium source is one or more of cerium oxide, cerium sulfate, cerium acetate, cerium nitrate, and their hydrates; the cobalt source is one or more of cobalt oxide, cobalt carbonate, cobalt sulfate, cobalt acetate, cobalt nitrate, and their hydrates; the scandium source is one or more of scandium oxide, scandium oxalate, scandium nitrate, and their hydrates; and the gallium source is one or more of gallium oxide, gallium acetate, gallium nitrate, and their hydrates.

[0039] Furthermore, the carbon content in step one is preferably 0.1-1%, 1-3%, 3-5%, 5-7%, or 7-9%.

[0040] Step 2: Place the above materials in a ball mill jar filled with inert gas and ball mill them.

[0041] Preferably, the inert gas in step two is one or more of argon, nitrogen, and a mixture of hydrogen and argon; the milling container is selected from one of agate milling jars, polyurethane milling jars, polytetrafluoroethylene milling jars, nylon milling jars, corundum milling jars, tungsten carbide milling jars, zirconia milling jars, and ceramic milling jars; the milling speed range is 200–800 r / min; and the milling time ranges from 4 to 24 h.

[0042] Furthermore, in the embodiments described, the preferred speed range is 200–400 r / min, 400–600 r / min, or 600–800 r / min.

[0043] Furthermore, in the embodiments described, the preferred ball milling time range is 4–8h, 8–12h, 12–16h, 16–20h, or 20–24h.

[0044] Step 3: The product obtained in Step 2 is heated and calcined in an inert atmosphere, and then cooled to room temperature to obtain a medium-entropy sodium iron pyrophosphate composite material.

[0045] Preferably, the sintering conditions in step three include sintering atmosphere, sintering temperature, and sintering time. The sintering atmosphere is an inert atmosphere, which is one or more of argon and a hydrogen-argon mixture; the sintering temperature is in the range of 200-600℃, and the sintering time is in the range of 3-24h. The specific operation is as follows: the ball-milled product is transferred to a high-temperature furnace for heat treatment, heated to 200-400℃ at a rate of 3-9℃ / min, and calcined at this temperature for 2-5 hours; then the temperature is further increased to 500-600℃ and calcined at this temperature for 5-11 hours; the sample is cooled to room temperature in an inert atmosphere to obtain a medium-entropy sodium iron pyrophosphate composite material.

[0046] Furthermore, in the embodiments described, the preferred constant temperature calcination temperature for the first stage is 200–300°C or 300–400°C.

[0047] Furthermore, in the embodiments described, the first stage of constant temperature calcination time is preferably 2-3.5h or 3.5-5h.

[0048] Furthermore, in the embodiments described, the preferred constant temperature calcination temperature for the second stage is 500–550°C or 550–600°C.

[0049] Furthermore, in the embodiments described, the second stage of constant temperature calcination time is preferably 5-7h, 7-9h, or 9-11h.

[0050] Electrode materials, conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were weighed at a mass ratio of 8:1:1, and mixed evenly with N-methylpyrrolidone as a dispersant. The mixture was then uniformly coated onto aluminum foil, dried, and cut into 12mm diameter positive electrode discs. These discs were then baked in a vacuum oven at 120℃ for 10 hours. Using the prepared positive electrode discs as the working electrode and a sodium metal sheet as the counter electrode, a 1 mol / L NaPF6 solution dissolved in a mixture of ethylene carbonate and dimethyl carbonate (wt% = 1:1) was used as the electrolyte. A 2032 type coin cell (sodium-ion battery) was assembled in a glove box. The selected voltage window for charge-discharge cycles was 1.8–4.2V.

[0051] Example 1

[0052] Sodium bicarbonate, ferrous oxalate, ammonium hydrogen phosphate, alumina, copper oxide, and magnesium oxide were placed in a ceramic-lined ball mill jar containing zirconium balls of different sizes in a molar ratio of 3.97:2.91:4:0.03:0.03. Glucose was added and mixed thoroughly, with the glucose content accounting for 5% of the total mass of the added materials. Argon gas was introduced into the ball mill jar, and the mixture was ball-milled at 500 r / min for 8 hours. The ball-milled product was placed in a high-temperature furnace under argon atmosphere control and heated to 300℃ at a heating rate of 3℃ / min, and kept at a constant temperature for 4 hours. Then, it was heated to 550℃ at the same heating rate and kept at a constant temperature for 8 hours. Finally, it was cooled to room temperature with the furnace to obtain Na. 3.97 Fe 2.91 Al 0.03 Mg 0.03 Cu 0.03 (PO4)2P2O7 material. Figure 1 The X-ray diffraction characterization results showed that the sample contained Na 3.97 Fe 2.91 Al 0.03 Mg 0.03 Cu 0.03 (PO4)2P2O7 is mainly a crystalline compound. Figure 2 The electron microscope image shows the material after heat treatment. It can be seen that the final product has a porous structure. This structure can effectively increase the specific surface area of ​​the material, which is conducive to the full reaction.

[0053] Within a voltage range of 1.8–4.2V, the Na prepared in Example 1 was subjected to... 3.97 Fe 2.91 Al 0.03 Mg 0.03 Cu 0.03 A sodium-ion battery with (PO4)2P2O7 as the positive electrode was subjected to charge-discharge tests, and the relevant parameters are shown in Table 1. Figure 3 and Figure 5The charge-discharge characteristic curves at 0.1C and 10C rates show that the material exhibits a reversible discharge specific capacity of 114.9 mAh / g at 0.1C and 87.9 mAh / g at 10C, with excellent cycle stability; the capacity retention is 97.5% after 1000 cycles. Furthermore, as... Figure 4 As shown, the Na prepared in Example 1 3.97 Fe 2.91 Al 0.03 Mg 0.03 Cu 0.03 The (PO4)2P2O7 material exhibits significant advantages at different rates from 0.5C to 50C, indicating that the diverse local structures caused by the entropy effect and the Na vacancies brought by high-valence ions effectively delay the phase transition reaction and charge compensation in the Na+ insertion / extraction reaction, stabilize the matrix structure, and play an important role in improving the material's electrical conductivity.

[0054] Comparative Example 1

[0055] The difference between this comparative example and Example 1 is that no doping element was added:

[0056] Sodium bicarbonate, ferrous oxalate, and ammonium hydrogen phosphate were placed in a ceramic-lined ball mill jar containing zirconium balls of different sizes in a molar ratio of 4:3:4. Glucose was added and mixed evenly, with the glucose content accounting for 5% of the total mass of the added materials. Argon gas was introduced into the ball mill jar, and the mixture was ball-milled at a speed of 500 r / min for 8 hours. The product obtained after ball milling was placed in a high-temperature furnace under argon atmosphere control and heated to 300℃ at a heating rate of 3℃ / min, and kept at a constant temperature for 4 hours. Then, it was heated to 550℃ at the same heating rate and kept at a constant temperature for 8 hours. Finally, it was cooled to room temperature with the furnace to obtain Na4Fe3(PO4)2P2O7 material.

[0057] Charge-discharge tests were conducted on a sodium-ion battery using the Na4Fe3(PO4)2P2O7 material prepared in Comparative Example 1 as the positive electrode within a voltage range of 1.8–4.2 V. Figure 3 and Figure 5 The charge-discharge characteristic curves at 0.1C and 10C rates show that the material has a reversible discharge specific capacity of 94.3 mAh / g at 0.1C and a capacity of 67 mAh / g at 10C, which is lower than that of the material prepared in Example 1. Figure 4 The poor rate performance also reflects the inherent low electronic conductivity of Na4Fe3(PO4)2P2O7 material.

[0058] Comparative Example 2

[0059] The difference between this comparative example and Example 1 is that the proportion of sodium bicarbonate added remains unchanged:

[0060] Sodium bicarbonate, ferrous oxalate, ammonium hydrogen phosphate, alumina, copper oxide, and magnesium oxide were placed in a ceramic-lined ball mill jar containing zirconium balls of different sizes in a molar ratio of 4:2.91:4:0.03:0.03:0.03. Glucose was added and mixed thoroughly, with the glucose content accounting for 5% of the total mass of the added materials. Argon gas was introduced into the ball mill jar, and the mixture was ball-milled at 500 r / min for 8 hours. The ball-milled product was placed in a high-temperature furnace under argon atmosphere control and heated to 300℃ at a heating rate of 3℃ / min, and kept at a constant temperature for 4 hours. Then, it was heated to 550℃ at the same heating rate and kept at a constant temperature for 8 hours. Finally, it was cooled to room temperature with the furnace to obtain Na4Fe. 2.91 Al 0.03 Mg 0.03 Cu 0.03 (PO4)2P2O7 material.

[0061] Within a voltage range of 1.8–4.2 V, the Na₄Fe₄ prepared in Comparative Example 2 was subjected to... 2.91 Al 0.03 Mg 0.03 Cu 0.03 A sodium-ion battery with (PO4)2P2O7 as the positive electrode was subjected to charge-discharge tests, and the relevant parameters are shown in Table 1.

[0062] Comparative Example 3

[0063] The difference between this comparative example and Example 1 is that no aluminum doping source was added:

[0064] Sodium bicarbonate, ferrous oxalate, ammonium hydrogen phosphate, copper oxide, and magnesium oxide were placed in a ceramic-lined ball mill jar containing zirconium balls of different sizes in a molar ratio of 4:2.91:4:0.045:0.045. Glucose was added and mixed thoroughly, with the glucose content accounting for 5% of the total mass of the added materials. Argon gas was introduced into the ball mill jar, and the mixture was ball-milled at 500 r / min for 8 hours. The product obtained after ball milling was placed in a high-temperature furnace under argon atmosphere control and heated to 300℃ at a heating rate of 3℃ / min, and kept at a constant temperature for 4 hours. Then, it was heated to 550℃ at the same heating rate and kept at a constant temperature for 8 hours. Finally, it was cooled to room temperature with the furnace to obtain Na4Fe. 2.91 Mg 0.045 Cu 0.045 (PO4)2P2O7 material.

[0065] Within a voltage range of 1.8–4.2 V, the Na₄Fe₄ prepared in Comparative Example 2 was subjected to... 2.91 Mg 0.045 Cu 0.045 A sodium-ion battery with (PO4)2P2O7 as the positive electrode was subjected to charge-discharge tests, and the relevant parameters are shown in Table 1.

[0066] Example 2

[0067] The difference between this embodiment and Embodiment 1 is that the doping element is different (Al is replaced with Cr):

[0068] Sodium bicarbonate, ferrous oxalate, ammonium hydrogen phosphate, chromium oxide, copper oxide, and magnesium oxide were placed in a ceramic-lined ball mill jar containing zirconium balls of different sizes in a molar ratio of 3.97:2.91:4:0.03:0.03. Glucose was added and mixed thoroughly, with the glucose content accounting for 5% of the total mass of the added materials. Argon gas was introduced into the ball mill jar, and the mixture was ball-milled at 500 r / min for 8 hours. The product obtained after ball milling was placed in a high-temperature furnace under argon atmosphere control and heated to 300℃ at a heating rate of 3℃ / min, and kept at a constant temperature for 4 hours. Then, it was heated to 550℃ at the same heating rate and kept at a constant temperature for 8 hours. Finally, it was cooled to room temperature with the furnace to obtain Na. 3.97 Fe 2.91 Cr 0.03 Mg 0.03 Cu 0.03 (PO4)2P2O7 material.

[0069] Within a voltage range of 1.8–4.2V, the Na prepared in Example 2 of this paper... 3.97 Fe 2.91 Cr 0.03 Mg 0.03 Cu 0.03 A sodium-ion battery with (PO4)2P2O7 as the positive electrode was subjected to charge-discharge tests, and the relevant parameters are shown in Table 1.

[0070] Example 3

[0071] The difference between this embodiment and Embodiment 1 is that the doping element is different (Al is replaced by Ce):

[0072] Sodium bicarbonate, ferrous oxalate, ammonium monohydrogen phosphate, cerium nitrate, copper oxide, and magnesium oxide were placed in a ceramic-lined ball mill jar containing zirconium balls of different sizes in a molar ratio of 3.97:2.91:4:0.03:0.03:0.03. Glucose was added and mixed thoroughly, with the glucose content accounting for 5% of the total mass of the added materials. Argon gas was introduced into the ball mill jar, and the mixture was ball-milled at 500 r / min for 8 hours. The product obtained after ball milling was placed in a high-temperature furnace under argon atmosphere control and heated to 300℃ at a heating rate of 3℃ / min, and kept at a constant temperature for 4 hours. Then, it was heated to 550℃ at the same heating rate and kept at a constant temperature for 8 hours. Finally, it was cooled to room temperature with the furnace to obtain Na. 3.97 Fe 2.91 Ce 0.03 Mg0.03 Cu 0.03 (PO4)2P2O7 material.

[0073] Within a voltage range of 1.8 to 4.2 V, the Na prepared in Example 3 of this study... 3.97 Fe 2.91 Ce 0.03 Mg 0.03 Cu 0.03 A sodium-ion battery with (PO4)2P2O7 as the positive electrode was subjected to charge-discharge tests, and the relevant parameters are shown in Table 1.

[0074] Example 4

[0075] The difference between this embodiment and Embodiment 1 is that the doped element is different (Al is replaced with Co):

[0076] Sodium bicarbonate, ferrous oxalate, ammonium hydrogen phosphate, cobalt nitrate, copper oxide, and magnesium oxide were placed in a ceramic-lined ball mill jar containing zirconium balls of different sizes in a molar ratio of 3.97:2.91:4:0.03:0.03:0.03. Glucose was added and mixed thoroughly, with the glucose content accounting for 5% of the total mass of the added materials. Argon gas was introduced into the ball mill jar, and the mixture was ball-milled at 500 r / min for 8 hours. The product obtained after ball milling was placed in a high-temperature furnace under argon atmosphere control and heated to 300℃ at a heating rate of 3℃ / min, and kept at a constant temperature for 4 hours. Then, it was heated to 550℃ at the same heating rate and kept at a constant temperature for 8 hours. Finally, it was cooled to room temperature with the furnace to obtain Na. 3.97 Fe 2.91 Co 0.03 Mg 0.03 Cu 0.03 (PO4)2P2O7 material.

[0077] Within a voltage range of 1.8 to 4.2 V, the Na prepared in Example 4 of this paper... 3.97 Fe 2.91 Co 0.03 Mg 0.03 Cu 0.03 A sodium-ion battery with (PO4)2P2O7 as the positive electrode was subjected to charge-discharge tests, and the relevant parameters are shown in Table 1.

[0078] Example 5

[0079] The difference between this embodiment and Embodiment 1 is that the doping element is different (Al is replaced with Sc):

[0080] Sodium bicarbonate, ferrous oxalate, ammonium hydrogen phosphate, scandium oxalate, copper oxide, and magnesium oxide were placed in a ceramic-lined ball mill jar containing zirconium balls of different sizes in a molar ratio of 3.97:2.91:4:0.03:0.03. Glucose was added and mixed thoroughly, with the glucose content accounting for 5% of the total mass of the added materials. Argon gas was introduced into the ball mill jar, and the mixture was ball-milled at 500 r / min for 8 hours. The ball-milled product was placed in a high-temperature furnace under argon atmosphere control and heated to 300℃ at a heating rate of 3℃ / min, and kept at a constant temperature for 4 hours. Then, it was heated to 550℃ at the same heating rate and kept at a constant temperature for 8 hours. Finally, it was cooled to room temperature with the furnace to obtain Na. 3.97 Fe 2.91 Sc 0.03 Mg 0.03 Cu 0.03 (PO4)2P2O7 material.

[0081] Within a voltage range of 1.8 to 4.2 V, the Na prepared in Example 5 of this paper... 3.97 Fe 2.91 Sc 0.03 Mg 0.03 Cu 0.03 A sodium-ion battery with (PO4)2P2O7 as the positive electrode was subjected to charge-discharge tests, and the relevant parameters are shown in Table 1.

[0082] Example 6

[0083] The difference between this embodiment and Embodiment 1 is that the doping element is different (Al is replaced with Ga):

[0084] Sodium bicarbonate, ferrous oxalate, ammonium monohydrogen phosphate, gallium acetate, copper oxide, and magnesium oxide were placed in a ceramic-lined ball mill jar containing zirconium balls of different sizes in a molar ratio of 3.97:2.91:4:0.03:0.03:0.03. Glucose was added and mixed thoroughly, with the glucose content accounting for 5% of the total mass of the added materials. Argon gas was introduced into the ball mill jar, and the mixture was ball-milled at 500 r / min for 8 hours. The ball-milled product was placed in a high-temperature furnace under argon atmosphere control and heated to 300℃ at a heating rate of 3℃ / min, and kept at a constant temperature for 4 hours. Then, it was heated to 550℃ at the same heating rate and kept at a constant temperature for 8 hours. Finally, it was cooled to room temperature with the furnace to obtain Na. 3.97 Fe 2.91 Ga 0.03 Mg 0.03 Cu 0.03 (PO4)2P2O7 material.

[0085] Within a voltage range of 1.8–4.2V, the Na prepared in Example 6 of this paper... 3.97 Fe 2.91Ga 0.03 Mg 0.03 Cu 0.03 A sodium-ion battery with (PO4)2P2O7 as the positive electrode was subjected to charge-discharge tests, and the relevant parameters are shown in Table 1.

[0086] The electrode materials provided in Examples 1 to 6 and the electrode materials provided in Comparative Examples 1 to 3 were weighed with conductive agent carbon black (SuperP) and binder polyvinylidene fluoride (PVDF) at a mass ratio of 8:1:1. N-methylpyrrolidone was used as a dispersant, and the mixture was stirred until homogeneous. The mixture was then uniformly coated onto aluminum foil, dried, and cut into positive electrode discs with a diameter of 12 mm. These discs were then baked in a vacuum oven at 120°C for 10 hours. The prepared positive electrode discs were used as the working electrode, and a sodium metal sheet as the counter electrode. A 1 mol / L solution of NaPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (wt% = 1:1) was used as the electrolyte. A 2032 coin cell was assembled in a glove box. The voltage window selected for charge-discharge cycles was 1.8–4.2 V.

[0087] The relevant electrochemical test results of the sodium-ion batteries prepared in each embodiment and comparative example are shown in Table 1:

[0088]

[0089]

[0090] Table 1. Electrochemical test results of Examples 1-6 and Comparative Examples 1-3

[0091] Table 1 shows that: through equimolar ratios of Mg 2+ Cu 2+ M 3+ (M 3+ For Al 3+ Cr 3+ Ce 3+ Co 3+ ,Sc 3+ Ga 3+ One of the three metal ions in Fe 2+ The formation of a combined crystal at specific sites, with its diverse local structures effectively delaying the formation of Na+ crystals. + The phase transition reaction and charge compensation in the insertion / extraction reaction effectively suppressed the phase transition reaction during the charge / discharge process and stabilized the matrix structure; among which Al 3+ Doping yields the best results. Furthermore, the Na vacancies created by the high-valence M ions dilute the Na. + The transfer of charge on and inside the tunnel improves the cyclic stability of the material at high rates.

[0092] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A medium-entropy iron pyrophosphate sodium composite material, characterized in that, The chemical formula of the sodium ferric pyrophosphate is Na. 4-a Fe 3-3a M a Mg a Cu a (PO4)2P2O7, 0.01≤a≤0.1, M is one of Al, Cr, Ce, Co, Sc, Ga, and V; The composite material has a porous structure; Prepared by the following steps: Step 1: Mix the sodium source, iron source, phosphorus source, carbon source and doping source evenly, wherein the doping source includes magnesium source, divalent copper source and trivalent metal M source; Step 2: Place the above materials in a ball mill jar filled with inert gas and ball mill them; Step 3: The product obtained in Step 2 is heated and calcined in an inert atmosphere, and then cooled to room temperature to obtain a medium-entropy sodium iron pyrophosphate composite material.

2. A method for preparing the medium-entropy sodium ferric pyrophosphate composite material as described in claim 1, characterized in that, Includes the following steps: Step 1: Mix the sodium source, iron source, phosphorus source, carbon source and doping source evenly, wherein the doping source includes magnesium source, divalent copper source and trivalent metal M source; Step 2: Place the above materials in a ball mill jar filled with inert gas and ball mill them; Step 3: The product obtained in Step 2 is heated and calcined in an inert atmosphere, and then cooled to room temperature to obtain a medium-entropy sodium iron pyrophosphate composite material.

3. The method as described in claim 2, characterized in that, The amount of carbon source added is determined based on the carbon content in the resulting composite material being 0.1% to 9%.

4. The method as described in claim 2, characterized in that, The sodium source is one or more of sodium acetate, sodium carbonate, sodium bicarbonate, sodium citrate, and sodium oxalate; the iron source is one or more of ferric acetate and its hydrate, ferrous oxalate and its hydrate, ferrous citrate, and ferrous ammonium sulfate and its hydrate; the phosphorus source is one or more of ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and triammonium phosphate; and the carbon source is one or more of sucrose, glucose, citric acid, ascorbic acid, and malic acid.

5. The method as described in claim 2, characterized in that, The magnesium source is one or more of magnesium oxide, magnesium carbonate, magnesium acetate, magnesium nitrate and their hydrates; the divalent copper source is one or more of copper oxide, copper carbonate, calcium acetate, calcium nitrate and their hydrates; the M in the trivalent metal M source is one of Al, Cr, Ce, Co, Sc, Ga and V.

6. The method as described in claim 2, characterized in that, In step two, the grinding container is selected from one of the following: agate grinding jar, polyurethane grinding jar, polytetrafluoroethylene grinding jar, nylon grinding jar, corundum grinding jar, tungsten carbide grinding jar, zirconia grinding jar, or ceramic grinding jar; the grinding speed of the grinding mill is 200–800 r / min; and the grinding time is 4–24 h.

7. The method as described in claim 2, characterized in that, In step three, the specific operations are as follows: the ball-milled product is transferred to a high-temperature furnace and heat-treated in an inert atmosphere, heated to 200-400℃ at a rate of 3-9℃ / min and calcined at this temperature for 2-5 hours; then the temperature is further increased to 500-600℃ and calcined at this temperature for 5-11 hours; the sample is cooled to room temperature in an inert atmosphere to obtain a medium-entropy sodium iron pyrophosphate composite material.

8. The method as described in claim 2, wherein in steps two and three, the inert gas is one or more of argon and a mixture of hydrogen and argon.

9. The use of the medium-entropy sodium pyrophosphate composite material as described in claim 1 as a cathode material for sodium-ion batteries.

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