CN118039843A - Positive electrode active material of sodium ion battery, preparation method of positive electrode active material and sodium ion battery - Google Patents

Abstract

The present invention relates to a sodium ion battery, and in particular to a sodium ion battery positive electrode active material, a preparation method thereof and a sodium ion battery. The sodium ion battery positive electrode active material has a core-shell structure, and comprises an inner core layer, a protective layer and an amorphous carbon layer from the inside to the outside. The components of the inner core layer include Na _4-y Fe _3-x-y Mn _x Al _y (PO ₄ ) ₂ P ₂ O ₇ , wherein 0≤x＜3, 0＜y＜0.3, 0＜x+y≤3, the components of the protective layer include aluminum oxide, and the components of the amorphous carbon layer include elemental carbon. One or more of a sodium source, a phosphorus source, an aluminum source, an iron source and a manganese source are weighed, mixed and dispersed in a dispersant in a molar ratio of Na:(Fe+Mn):Al:P＝(4.0-y～5.0-1.2y):(2.4-y～3.6-y):(0.8y～1.2y):(3.2～4.8), wherein 0＜y＜0.3. After activation and drying, the product is pre-calcined, dispersed in a solvent containing aluminum raw materials and an organic carbon source, dried, and sintered to obtain a positive electrode active material for a sodium ion battery. The positive electrode active material for a sodium ion battery has a high specific capacity, electrical conductivity, and cycle life.

Description

A sodium ion battery positive electrode active material and preparation method thereof and a sodium ion battery

Technical Field

The invention relates to a sodium ion battery, and in particular to a sodium ion battery positive electrode active material and a preparation method thereof, and a sodium ion battery.

Background technique

Lithium-ion batteries are a relatively mature secondary battery, but the shortage and uneven distribution of lithium resources have greatly restricted their sustainable development. Sodium and lithium belong to the same main group and have similar chemical properties. At the same time, sodium is highly abundant in the earth's crust, and its resources are abundant and low-cost. Therefore, sodium-ion batteries are expected to be widely used in the fields of energy storage and low-speed electric vehicles. The positive electrode material is the key to the performance of sodium-ion batteries. The battery's energy density and power density and other characteristics are closely related to the working voltage, specific capacity, rate performance, cycle performance, compaction density, etc. of the positive electrode active material. Therefore, the positive electrode active material of sodium-ion batteries is the focus of research.

The sodium ion battery positive electrode active materials that are widely studied at present mainly include anionic polymers and layered oxides. Layered oxides have a multilayer structure, in which anions are loosely stacked. The deformation of the structure when sodium ions are embedded and de-embedded between layers will cause structural changes in the lattice. Anionic polymers have a framework structure. The embedding and de-embedding of sodium ions in different framework structures will have different effects on the structure. Therefore, the structural stability of anionic polymers with different framework structures varies greatly. Among the various anionic polymers with different structures, phosphate pyrophosphate positive electrode active materials have the advantages of low cost, simple preparation process, and stable structure. The strong P-O covalent bond can induce the ionization of metal ions to increase the redox potential, and help stabilize the oxygen atoms in the lattice, so that it has high structural stability and safety.

Phosphate pyrophosphate Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ is a positive electrode active material composed of NaFePO ₄ and Na ₂ FeP ₂ O ₇ in a molar ratio of 2:1. It has a three-dimensional sodium ion diffusion channel, low cost, environmentally friendly materials and is easy to obtain. However, its theoretical specific capacity is 129 mAh/g, and the average operating voltage is 2.9V-3.0V, that is, the specific capacity and operating voltage of the material are relatively low, so the energy density of the battery is relatively low, and it is only suitable for energy storage and other fields that do not require high battery energy density.

Summary of the invention

The object of the present invention is to provide a sodium ion battery positive electrode active material with excellent electrochemical performance, a preparation method thereof and a sodium ion battery.

In order to achieve the above object, the present invention provides the following technical solutions:

A sodium ion battery positive electrode active material, comprising:

An inner core layer, the components of the inner core layer include phosphate pyrophosphate, the molecular formula of the phosphate pyrophosphate is Na _4-y Fe _3-xy Mn _x Al _y (PO ₄ ) ₂ P ₂ O ₇ , wherein 0≤x＜3, 0＜y＜0.3, 0＜x+y≤3;

A protective layer, wrapped around the outer side of the inner core layer, wherein the protective layer comprises aluminum oxide;

The amorphous carbon layer is wrapped around the outer side of the protective layer, and the component of the amorphous carbon layer includes elemental carbon.

Optionally, the phospho-pyrophosphate comprises Mn ²⁺ and Fe ²⁺ .

Optionally, the mass ratio of the phosphoric acid pyrophosphate, the alumina and the elemental carbon is 100:

(0.1～5):(0.1～10).

In a second aspect, the present invention further provides a method for preparing the above-mentioned positive electrode active material for a sodium ion battery, comprising:

Prepare a mixed raw material, the mixed raw material includes a sodium source, a phosphorus source and an aluminum source, and also includes one or more of an iron source and a manganese source, the molar ratio of elements in the mixed raw material is Na:(Fe+Mn):Al:P=(4.0-y～5.0-1.2y):(2.4-y～3.6-y):(0.8y～1.2y):(3.2～4.8), wherein 0＜y＜0.3;

Dispersing the mixed raw material in a dispersant and activating it, drying it and heating it at a first preset temperature for a first preset time in an inert atmosphere to obtain a precursor, wherein the dispersant is one or more of ethanol, water, and acetone;

Dissolving an aluminum-containing raw material and an organic carbon source in a solvent to obtain a reaction solution, dispersing the precursor in the reaction solution and drying the precursor so that the aluminum-containing raw material and the organic carbon source are wrapped around the outside of the precursor to obtain an intermediate;

The intermediate is heated to a second preset temperature in an inert atmosphere and maintained for a second preset time, so that the precursor is converted into the phosphate pyrophosphate, the aluminum-containing raw material is converted into the aluminum oxide, and the organic carbon source is converted into the elemental carbon, thereby obtaining the sodium ion battery positive electrode active material.

Optionally, the solvent is water, ethanol or isopropanol, the aluminum-containing raw material is one or more of aluminum nitrate, aluminum chloride, aluminum sulfate, aluminum isopropoxide, aluminum acetate and aluminum chloride; and the organic carbon source is one or more of glucose, sucrose, oxalic acid, ascorbic acid, tartaric acid, citric acid and oleic acid.

Optionally, the iron source may be selected as materials in different shapes, such as flakes, needles, particles or blocks.

Optionally, the manganese source and the iron source have the same shape.

Optionally, the first preset temperature is any value between 300°C and 350°C, and the first preset time is any value between 1h and 5h; the second preset temperature is any value between 450°C and 600°C, and the second preset time is any value between 1h and 24h.

Optionally, the sodium source can be one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium dihydrogen citrate, sodium oxalate, sodium alginate, and sodium pyrophosphate; the phosphorus source can be one or more of phosphoric acid, triammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, ferrous phosphate, and ferrous phosphate; the iron source can be one or more of iron powder, ferrous oxalate, ferric nitrate, ferric phosphate, ferrous phosphate, ferric citrate, ferric oxide, and ferric oxide; the manganese source can be one or more of manganese phosphate, manganese dihydrogen phosphate, manganese carbonate, manganese monoxide, manganese dioxide, manganese trioxide, and manganese tetraoxide; the aluminum source can be one or more of aluminum oxide, aluminum hydroxide, aluminum nitrate, aluminum sulfate, aluminum phosphate, aluminum dihydrogen phosphate, and aluminum dihydrogen phosphate.

In a second aspect, the present invention also provides a sodium ion battery comprising the above-mentioned sodium ion battery positive electrode active material.

According to the first aspect of the present invention, the main component of the inner core layer is Na _4-y Fe _3-xy Mn _x Al _y (PO ₄ ) ₂ P ₂ O ₇ , wherein 0≤x＜3, 0＜y＜0.3, 0＜x+y≤3. Since the Mn element has two redox pairs, namely Mn ²⁺ /Mn ³⁺ and Mn ³⁺ /Mn ⁴⁺ , doping Mn ²⁺ or replacing Fe ²⁺ with Mn ²⁺ can help the material to release more sodium ions during the charge and discharge process, thereby improving its working voltage and theoretical specific capacity. The Mn element can cause a serious Jahn-Teller effect. When Mn ²⁺ is converted into Mn ³⁺ , the structural stability of the inner core layer deteriorates. In order to improve the structural stability, Al ³⁺ is used to replace some metal ions. Since Al ³⁺ has a small radius, when it replaces other metal ions, the lattice of phosphate pyrophosphate will shrink. This shrinkage helps to improve the structural stability of the material, so that it can better maintain its original structure during the charge and discharge process. In addition, Al ³⁺ has excellent conductivity and is suitable for battery positive electrode materials, which helps to promote electron transfer. It is electrochemically inert, does not participate in redox reactions, does not undergo structural and property changes due to charging and discharging, and helps to maintain the stability of the lattice. Doping Al ³⁺ in phosphate pyrophosphate can shrink the material lattice, make the structure more stable, prevent the material from decreasing in crystallinity after multiple discharge cycles, and improve the material's cycle performance. The protective layer composed of aluminum oxide can avoid direct contact between the positive electrode material and the electrolyte, inhibit the side reaction between the electrolyte and the positive electrode material, and thus significantly improve the cycle life of the material. The amorphous carbon layer helps to improve conductivity. Modification of Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ by manganese doping or substitution, aluminum doping, aluminum oxide coating, and elemental carbon coating helps to increase its operating voltage and theoretical specific capacity, and improve the cycle life of the sodium battery.

Furthermore, since the increase of Mn content in phosphoric acid pyrophosphate increases its working voltage and specific capacity, it also leads to a decrease in the electronic conductivity of the material and causes a serious Jahn-Teller effect, which leads to a decrease in the electrochemical performance of phosphoric acid pyrophosphate due to excessive Mn content. Through iron-manganese composite, the working voltage and specific capacity of the modified phosphoric acid pyrophosphate are higher than those of Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ , and the rate performance and structural stability are higher than those of Na ₄ Mn ₃ (PO ₄ ) ₂ P ₂ O ₇ , and its electrochemical performance changes with the change of Mn/Fe ratio.

Furthermore, if the content of aluminum oxide is too low, it is difficult to wrap the internal Na _4-y Fe _3-xy Mn _x Al _y (PO ₄ ) ₂ P ₂ O ₇ , which will make it difficult to protect the internal materials. If the content of elemental carbon is too low, the conductivity of the material will be too low. Neither aluminum oxide nor elemental carbon participates in the redox reaction, so if the content is too high, the overall electrochemical performance of the positive electrode material will be reduced.

According to a second aspect of the present invention, a mixed raw material containing Na and P elements, as well as one or more of Fe, Mn and Al elements is dispersed in a dispersant and activated, so that the components in the mixed raw material are evenly mixed and ionized to be reacted. After drying and removing the dispersant, the first preset time is heated at a first preset temperature so that the ions are preliminarily combined to obtain a precursor that is preliminarily combined but the structure is not yet stable. After dissolving the aluminum-containing raw material and the organic carbon source in a solvent, the precursor is dispersed in the system, and the aluminum-containing raw material, the organic carbon source and the precursor particles are evenly mixed. After drying and removing the solvent, an intermediate is obtained in which the aluminum-containing raw material and the organic carbon source are evenly wrapped on the outside of the precursor particles. The intermediate is sintered to further react the precursor particles to generate Na _4-y Fe _3-xy Mn _x Al _y (PO ₄ ) ₂ P ₂ O ₇ (0≤x＜3, 0＜y＜0.3, 0＜x+y≤3) with a stable structure to form an inner core layer; the aluminum-containing raw material is converted into aluminum oxide at high temperature to form a protective layer; the organic carbon source undergoes a reduction reaction to form amorphous elemental carbon, most of which is precipitated on the outer surface of the protective layer to form an amorphous carbon layer, and a small part of the elemental carbon is doped and embedded in the protective layer. At this time, a sodium ion battery positive electrode active material suitable for the positive electrode of a sodium ion battery is obtained. By controlling the element ratio in the raw materials and reducing the generation of impurities, it is helpful to improve the overall electrochemical performance.

Furthermore, the aluminum-containing raw material and the organic carbon source are both soluble in the solvent, and the precursor is dispersed in the solvent. After drying, the aluminum-containing raw material and the organic carbon source are evenly attached to the outside of the precursor to form a basic structure of a core-shell structure, which is conducive to evenly mixing the aluminum-containing raw material, the organic carbon source and the precursor.

Furthermore, by using raw materials of special shapes, the product has a special morphology, which can adjust the specific surface area of the material and the diffusion rate of sodium ions, thereby affecting the electrochemical properties of the material.

Furthermore, the use of a treatment method of pre-sintering before sintering can prevent the rapid heating of raw materials from causing excessive reaction and oxidation of the materials, as well as deformation of the materials, and improve the sintering quality. It can also reduce the waste of raw materials and energy and the wear of equipment, thereby reducing production costs.

According to the third aspect of the present invention, by using the above-mentioned sodium ion battery positive electrode active material as raw material to prepare the sodium ion battery positive electrode and assemble the sodium ion battery, the sodium ion battery can have a higher operating voltage and specific capacity, as well as good rate performance and structural stability, thereby obtaining a wider range of application scenarios and excellent cycle life.

The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of a method for preparing a positive electrode active material for a sodium ion battery according to the present invention;

FIG2 is a SEM image of the positive electrode active material of the sodium ion battery prepared in Example 1;

FIG3 is a SEM image of the positive electrode active material of the sodium ion battery prepared in Example 2;

FIG4 is an XRD pattern of the positive electrode active material of the sodium ion battery prepared in Example 1;

FIG5 is an XRD pattern of the positive electrode active material of the sodium ion battery prepared in Example 1 after 2000 cycles at 5C;

FIG6 is an XRD pattern of the positive electrode active material of the sodium ion battery prepared in Example 5;

FIG7 is an XRD pattern of the positive electrode active material of the sodium ion battery prepared in Example 6;

FIG8 is an XRD pattern of the positive electrode active material of the sodium ion battery prepared in Comparative Example 3;

FIG9 is an XRD pattern of the positive electrode active material of the sodium ion battery prepared in Comparative Example 3 after 2000 cycles at 5C;

FIG10 is a charge and discharge curve of the positive electrode active material of the sodium ion battery prepared in Example 1 at a rate of 0.1C;

FIG11 is a charge and discharge curve of the positive electrode active material of the sodium ion battery prepared in Comparative Example 1 at a rate of 0.1C.

Detailed ways

The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

The sodium ion battery positive electrode active material protected by the present invention includes an inner core layer, a protective layer wrapped around the outer side of the inner core layer, and an amorphous carbon layer wrapped around the outer side of the protective layer. The main component of the inner core layer is Na _4-y Fe _3-xy Mn _x Al _y (PO ₄ ) ₂ P ₂ O ₇ , where 0≤x＜3, 0＜y＜0.3, 0＜x+y≤3. The main component of the protective layer is aluminum oxide, which is used to protect the inner core layer and prevent the positive electrode material from directly contacting the electrolyte, thereby preventing the occurrence of side reactions. The main component of the amorphous carbon layer is elemental carbon, which is used to improve the overall conductivity. In some embodiments, the elemental carbon is partially analyzed outside the protective layer to form an amorphous carbon layer, and is partially distributed in the protective layer.

Phospho-pyrophosphate Na _4-y Fe _3-xy Mn _x Al _y (PO ₄ ) ₂ P ₂ O ₇ is obtained by modifying Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O _7. Since the element manganese has a high redox potential and has two redox pairs, namely Mn ²⁺ /Mn ³⁺ and Mn ³⁺ /Mn ⁴⁺ , doping Mn ²⁺ into phospho-pyrophosphate is beneficial to improving the specific capacity and working voltage of the material. Aluminum ions have a small radius, excellent conductivity, and are electrochemically inert. They do not participate in redox reactions, which can shrink the material lattice and make the structure more stable, thereby improving the cycle life of sodium ion batteries.

In some embodiments, the phosphate pyrophosphate includes both Mn ²⁺ and Fe ²⁺ , which retains the advantages of Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ in terms of good rate performance and structural stability, and can also improve the working voltage and specific capacity of the material.

In some embodiments, the mass ratio of phosphoric acid pyrophosphate, aluminum oxide and elemental carbon is 100:(0.1-5):(0.1-10), so that the content of the protective layer and the amorphous carbon layer is sufficient to protect and improve the conductivity.

Referring to FIG. 1 , the method for preparing the positive electrode active material of the sodium ion battery claimed in the present invention comprises:

S1. Prepare a mixed raw material, the mixed raw material includes a sodium source, a phosphorus source and an aluminum source, and also includes one or more of an iron source and a manganese source, and the molar ratio of each element in the mixed raw material is Na:(Fe+Mn):Al:P=(4.0-y～5.0-1.2y):(2.4-y～3.6-y):(0.8y～1.2y):(3.2～4.8), wherein 0＜y＜0.3;

S2, dispersing the mixed raw material in a dispersant and activating it, drying it and heating it at a first preset temperature for a first preset time in an inert atmosphere to obtain a precursor;

S3, dissolving the aluminum-containing raw material and the organic carbon source in a solvent to obtain a reaction solution, dispersing the precursor in the reaction solution and drying it, so that the aluminum-containing raw material and the organic carbon source are wrapped around the outside of the precursor, to obtain an intermediate;

S4. Heating the intermediate at a second preset temperature for a second preset time in an inert atmosphere to convert the aluminum-containing raw material into aluminum oxide and the organic carbon source into elemental carbon to obtain the positive electrode active material for the sodium ion battery.

The mixed raw material is dispersed in a dispersant and activated to increase the activity of each element in the mixed raw material. After drying and removing the dispersant, the first preset temperature is heated for a first preset time to preliminarily combine the elements to obtain a precursor with an unstable structure. After dissolving the aluminum-containing raw material and the organic carbon source in a solvent, the precursor is dispersed in the system, and the aluminum-containing raw material, the organic carbon source and the precursor particles are uniformly mixed. After drying and removing the solvent, an intermediate is obtained in which the aluminum-containing raw material and the organic carbon source are uniformly wrapped on the outside of the precursor particles. The intermediate is sintered to further react the precursor particles to generate Na _4-y Fe _3-xy Mn _x Al _y (PO ₄ ) ₂ P ₂ O ₇ (0≤x＜3, 0＜y＜0.3, 0＜x+y≤3) with a stable structure. The aluminum-containing raw material is converted into aluminum oxide at high temperature, and the organic carbon source is reduced to elemental carbon. The aluminum oxide is wrapped around the outer side of Na _4-y Fe _3-xy Mn _x Al _y (PO ₄ ) ₂ P ₂ O ₇ to form a shell, and an amorphous carbon layer composed of elemental carbon is precipitated and wrapped on the outer side. At this time, the positive active material of the sodium ion battery is obtained. By controlling the content of each element, the composition in the inner core layer is mainly the pure phase of Na _4-y Fe _3-xy Mn _x Al _y (PO ₄ ) ₂ P ₂ O _7. In order to reduce the influence of segregation and the like, more sodium source is usually added to reduce the generation of impurities.

In step S1, the material shapes of the iron source and the manganese source will affect the crystal properties of the positive electrode active material of the sodium ion battery.

In some embodiments, the iron source and the manganese source can be selected as materials with different morphologies, such as flakes, needles, particles or blocks, so that the product has a special shape, thereby regulating the specific surface area of the material and the diffusion rate of sodium ions, affecting the electrochemical properties of the material. When the iron source and the manganese source are selected to have the same crystal shape, the crystal shape of the phosphoric acid pyrophosphate is the same; when the iron source and the manganese source are selected to have different crystal shapes, the phosphoric acid pyrophosphate includes a variety of different crystal shapes.

In some embodiments, the sodium source can be one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium dihydrogen citrate, sodium oxalate, sodium alginate, and sodium pyrophosphate;

The phosphorus source may be one or more of phosphoric acid, triammonium phosphate, diammonium phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, ferric phosphate, and ferrous phosphate;

The iron source may be one or more of iron powder, ferrous oxalate, ferric nitrate, ferric phosphate, ferrous phosphate, ferric citrate, ferric oxide, and ferric oxide;

The manganese source may be one or more of manganese phosphate, manganese dihydrogen phosphate, manganese carbonate, manganese monoxide, manganese dioxide, manganese trioxide, and manganese tetraoxide;

The aluminum source may be one of aluminum oxide, aluminum hydroxide, aluminum nitrate, aluminum sulfate, aluminum phosphate, dialuminum hydrogen phosphate, and aluminum dihydrogen phosphate.

In step S2, the inert atmosphere is usually selected to be nitrogen or argon.

In some embodiments, dispersing the mixed raw material in a dispersant and activating the mixed raw material comprises:

The mixed raw materials are added with a dispersant, and the mixture is ground and mechanically activated for 1 to 12 hours to form a slurry, wherein the dispersant is one or more of ethanol, water, and acetone. Mechanical activation by grinding helps to uniformly mix the mixed raw materials and makes it easier for each element to react.

In some embodiments, the first preset temperature is any value between 300°C and 350°C, for example, it can be 300°C, 310°C, 320°C, 330°C, 340°C or 350°C; the first preset time is any value between 1h and 5h, for example, it can be 1h, 2h, 3h, 4h or 5h.

In step S3, the mass of the required aluminum oxide and elemental carbon is estimated by the amount of the obtained precursor, and the amount of the required aluminum-containing raw material and the organic carbon source is calculated.

In some embodiments, the mass ratio of the precursor, alumina and elemental carbon is 100:(0.1-5):(0.1-10), for example, it can be (100:0.1:0.1), (100:0.1:1), (100:0.1:3), (100:0.1:5), (100:0.1:8), (100:0.1:10), (100:1:0.1), (100:3:0.1), (100:5:0.1), (100:1:1), (100:1.5:2.5), (100:3.5:7) or (100:5:10).

In some embodiments, the solvent is water, ethanol or isopropanol. When the solvent is water, the aluminum-containing raw material can be selected as aluminum nitrate, aluminum chloride, aluminum sulfate, aluminum isopropoxide or aluminum acetate; when the solvent is ethanol, the aluminum-containing raw material can be selected as aluminum isopropoxide, aluminum nitrate, aluminum acetate or aluminum chloride; when the solvent is isopropanol, the aluminum-containing raw material can be selected as aluminum isopropoxide or aluminum chloride.

In some embodiments, the organic carbon source is one or more of glucose, sucrose, oxalic acid, ascorbic acid, tartaric acid, citric acid, and oleic acid.

In step S4, the inert atmosphere is usually selected to be nitrogen or argon.

In some embodiments, the second preset temperature is any value between 450°C and 600°C, for example, 450°C, 500°C, 550°C or 600°C; the second preset time is any value between 1h and 24h, for example, 1h, 8h, 16h, 20h or 24h.

See the following embodiments for details.

Embodiment 1:

In this embodiment, a sodium ion battery was prepared and its electrochemical effect was tested.

Referring to FIG. 1 , a method for preparing a positive electrode active material for a sodium ion battery shown in a preferred embodiment of the present application includes:

S1. Sodium acetate, flaky iron phosphate, flaky manganese carbonate, aluminum sulfate and diammonium phosphate are weighed and mixed according to the element molar ratio of Na:Fe:Mn:Al:P=4:1.5:1.41:0.09:2.5 to obtain a mixed raw material.

S2. Select ethanol as a dispersant, add the mixed raw materials into the dispersant, and mechanically activate them in a ball mill for 8 hours. The obtained slurry is spray dried. The dried powder is heat treated at 350° C. in nitrogen for 5 hours to obtain a pre-calcined precursor.

S3. Weigh the mass of the precursor, and weigh appropriate amounts of aluminum nitrate and oxalic acid, and calculate the amount of aluminum nitrate and oxalic acid required for conversion according to the amount of aluminum oxide and elemental carbon required, wherein the mass ratio of precursor: aluminum oxide: elemental carbon is 100:0.5:3. Dissolve aluminum nitrate and oxalic acid in water to form a reaction solution, and disperse the precursor in the reaction solution, and stir and evaporate the above suspension to obtain an intermediate.

S4. The intermediate is heat treated at 600° C. for 24 h under the protection of nitrogen, and then cooled in the furnace to obtain a positive electrode active material for a sodium ion battery.

The obtained sodium ion battery positive electrode active material was used as the positive electrode active material, and it was weighed and mixed with acetylene black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and then ground evenly in a mortar, and N-methylpyrrolidone (NMP) was added and continued to grind, and the uniform black viscous slurry was coated on aluminum foil to form a film of uniform thickness. A metal sodium sheet was used as the counter electrode, a glass fiber membrane was used as the diaphragm, and 1 mol/L NaClO ₄ /propylene carbonate (PC) was used as the electrolyte, and a CR2032 button cell was assembled in an anhydrous and oxygen-free argon atmosphere glove box. The discharge specific capacity, average working voltage, cycle performance, specific surface area and compaction density of the positive electrode material were tested.

Embodiment 2:

The difference between this embodiment and the first embodiment is that in step S1, sodium carbonate, granular ferrous oxalate, granular manganese carbonate, aluminum hydroxide and phosphoric acid are weighed in a molar ratio of Na:Fe:Mn:Al:P of 4:1.5:1.47:0.03:4; in step S2, acetone is selected as a dispersant, mechanical activation is performed for 8 hours, and the first preset time is 2 hours; in step S3, the solvent is selected as isopropanol, the aluminum-containing raw material is selected as aluminum isopropoxide, the organic carbon source is selected as ascorbic acid, and the mass ratio of precursor: alumina: elemental carbon is 100:2:5; in step S4, the second preset temperature is 550°C, and the second preset time is 12 hours.

Embodiment three:

The only difference between this embodiment and the first embodiment is that in step S1, sodium oxalate, blocky iron phosphate, blocky manganese oxide, aluminum nitrate and triammonium phosphate are weighed at a molar ratio of Na:Fe:Mn:Al:P=3.8:1.5:1.25:0.25:4; in step S2, mechanical activation is performed for 10 hours, the first preset temperature is 300°C, and the first preset time is 1 hour; in step S3, the aluminum-containing raw material is selected as aluminum chloride, the organic carbon source is selected as citric acid, and the mass ratio of precursor: aluminum oxide: elemental carbon is 100:5:1; in step S4, the second preset temperature is 550°C, and the second preset time is 12 hours.

Embodiment 4:

The difference between this embodiment and the first embodiment is that in step S1, sodium alginate, granular ferric nitrate, granular manganese dihydrogen phosphate, aluminum phosphate and ammonium dihydrogen phosphate are weighed in a molar ratio of Na:Fe:Mn:Al:P=3.9:2:0.85:0.15:4; in step S2, ethanol is selected as a dispersant, mechanical activation is performed for 6 hours, the first preset temperature is 300°C, and the first preset time is 5 hours; in step S3, the solvent is selected as anhydrous ethanol, the aluminum-containing raw material is selected as aluminum nitrate, the organic carbon source is selected as citric acid, and the mass ratio of precursor: alumina: elemental carbon is 100:1:10; in step S4, the second preset temperature is 530°C, and the second preset time is 12 hours.

Embodiment five:

The only difference between this embodiment and the first embodiment is that in step S1, sodium dihydrogen phosphate, ferric nitrate and aluminum oxide are weighed at a molar ratio of Na:Fe:Al:P=3.8:2.75:0.25:4; in step S2, mechanical activation is performed for 4 hours, the first preset temperature is 300°C, and the first preset time is 2 hours; in step S3, the aluminum-containing raw material is selected as aluminum isopropoxide, the organic carbon source is selected as sucrose, and the mass ratio of precursor: aluminum oxide: elemental carbon is 100:0.5:4; in step S4, the second preset temperature is 520°C, and the second preset time is 5 hours.

Embodiment six:

The difference between this embodiment and the first embodiment is that in step S1, sodium carbonate, granular ferrous oxalate, granular manganese carbonate, aluminum hydroxide and phosphoric acid are weighed in a molar ratio of Na:Mn:Al:P=3.8:2.73:0.27:4; in step S2, acetone is selected as a dispersant, mechanical activation is performed for 4 hours, the first preset temperature is 300°C, and the first preset time is 5 hours; in step S3, the aluminum-containing raw material is selected as aluminum sulfate, the organic carbon source is selected as sucrose, and the mass ratio of precursor: alumina: elemental carbon is 100:0.3:7; in step S4, the second preset temperature is 570°C, and the second preset time is 6 hours.

Comparative Example 1:

Sodium acetate, flaky iron phosphate, flaky manganese carbonate and ammonium dihydrogen phosphate were weighed and mixed to obtain a mixed raw material according to a molar ratio of Na:Fe:Mn:P=4:1.5:1.5:4, citric acid was added in a ratio of 100:3 of the mass ratio of the mixed raw material to the elemental carbon, ethanol was used as a dispersant, and mechanical activation was performed in a ball mill for 8 hours. The obtained slurry was spray dried, and the dried powder was pre-calcined at 350°C for 5 hours in an inert gas, and then heat-treated at 600°C for 24 hours. The material obtained after cooling in the furnace was used as the positive electrode active material, and a button cell was prepared in the same manner as in Example 1, and the discharge specific capacity, average working voltage, cycle performance, specific surface area and compacted density of the positive electrode material were tested.

Comparative Example 2:

The only difference between this comparative example and comparative example 1 is that sodium acetate, flaky iron phosphate, flaky manganese carbonate, aluminum sulfate and diammonium phosphate are weighed and mixed according to a molar ratio of Na:Fe:Mn:Al:P of 4:1.5:1.41:0.09:4 to obtain a mixed raw material.

Comparative Example 3:

The difference between this comparative example and Example 1 is that sodium acetate, flaky iron phosphate, flaky manganese carbonate and diammonium phosphate are weighed in a molar ratio of Na:Fe:Mn:P=4:1.5:1.5:4 in step S1, and ethanol is selected as the dispersant in step S2.

Comparative Example 4:

The difference between this comparative example and comparative example 1 is that sodium dihydrogen phosphate and ferric nitrate are weighed according to a molar ratio of Na:Fe:P of 4:3:4, and sucrose is added according to a mass ratio of mixed raw materials to elemental carbon of 100:4. The first preset temperature is 300°C, the first preset time is 2h, the second preset temperature is 520°C, and the second preset time is 5h.

Comparative Example 5:

The only difference between this comparative example and comparative example 1 is that sodium acetate, manganese carbonate and triammonium phosphate are weighed according to a molar ratio of Na:Mn:P of 4:3:4, no citric acid is added, deionized water is used as a dispersant, the first preset temperature is 300°C, the first preset time is 5h, the second preset temperature is 570°C, and the second preset time is 6h.

The data detected in Examples 1 to 6 and Comparative Examples 1 to 5 were sorted to obtain the following table.

Table 1: Comparison of specific capacities of different samples.

Table 2: Comparison of median voltage, cycling performance, specific surface area and compaction density of different samples.

Combined with the raw materials and experimental methods in Examples 1 to 6 and Comparative Examples 1 to 5, the data in Tables 1 and 2 were analyzed.

Please refer to Figures 2, 3, 4 and Table 2. The microstructure surface of the positive active material of the sodium ion battery generated in Example 1 has needle-like protrusions, while the microstructure of the positive active material of the sodium ion battery generated in Example 2 is granular with a smooth surface, and the material obtained in Example 1 has a larger specific surface area and compacted density than the material obtained in Example 2. This is because both ferrous phosphate and manganese carbonate in Example 1 use flaky raw materials, and both ferrous oxalate and manganese carbonate in Example 2 use granular raw materials, and the molecular structure of the raw materials affects the molecular structure and electrochemical properties of the generated positive active material of the sodium ion battery. It can be seen from the XRD spectrum that the positive active material of the sodium ion battery is a core-shell structure at the microscopic level, with the outermost layer being an amorphous layer, which is wrapped with a protective layer with a higher degree of crystallinity, and the protective layer is wrapped with a core inside.

Please refer to Table 1 and Table 2. The raw materials of Examples 1 to 4 and Comparative Example 2 all include Na, Fe, Mn, Al and P elements, that is, the generated phosphate pyrophosphates are all sodium manganese iron phosphate pyrophosphate doped with aluminum ions. No aluminum-containing raw materials are added in Comparative Example 2, so that the material produced in Comparative Example 2 does not include a protective layer, which makes the cycle life of the material generated in Comparative Example 2 significantly lower.

The raw materials of Example 5 do not contain the Mn element, that is, the generated phosphate pyrophosphates are all sodium iron phosphate pyrophosphate doped with aluminum ions, and its XRD spectrum is shown in Figure 6. The raw materials of Example 6 do not contain the Fe element, that is, the generated phosphate pyrophosphates are all sodium manganese phosphate pyrophosphate doped with aluminum ions, and its XRD spectrum is shown in Figure 7. Comparing the test data of Examples 5 and 6 in Tables 1 and 2 with the test data of Examples 1 to 4, it can be found that the absence of manganese elements will lead to lower operating voltage and specific capacity of the sodium ion battery, but better capacity retention rate; the absence of iron elements will lead to a lower capacity retention rate of the sodium ion battery, but a higher operating voltage, and a specific capacity close to the theoretical value.

The other raw materials, proportions and preparation methods used in Example 1 and Comparative Example 3 are similar, the only difference is that aluminum ions are not doped in the comparative example. It can be seen from Tables 1 and 2 that the capacity retention rate in Comparative Example 3 decreases significantly. Please refer to Figures 4, 5, 8 and 9. After 2000 cycles, the crystallinity of the material obtained in Example 1 has no obvious change, while the crystallinity of the material obtained in Comparative Example 1 decreases significantly after 2000 cycles.

Referring to FIG. 10 and FIG. 11 , it can be seen that the positive electrode active material for the sodium ion battery prepared in Example 1 has an obvious cycle performance advantage over that of Comparative Example 1.

Therefore, by synergistically modifying the phosphate pyrophosphate positive electrode material through shape regulation, iron-manganese composite, aluminum doping and alumina coating, the electronic/ionic conductivity of the material can be improved, its diffusion kinetics and structural stability can be enhanced, thereby significantly improving its electrochemical performance.

The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims (10)

1. A positive electrode active material for a sodium ion battery, comprising:

The inner core layer comprises a phosphoric acid pyrophosphate, and the molecular formula of the phosphoric acid pyrophosphate is Na _{4- y}Fe_3-x-yMn_xAl_y(PO₄)₂P₂O₇, wherein x is more than or equal to 0 and less than 3, y is more than or equal to 0 and less than or equal to 0.3, and x+y is more than or equal to 0 and less than or equal to 3;

The protective layer is wrapped outside the inner core layer, and the components of the protective layer comprise aluminum oxide;

and the amorphous carbon layer is wrapped outside the protective layer, and the components of the amorphous carbon layer comprise elemental carbon.

2. The positive electrode active material of a sodium ion battery according to claim 1, wherein the phosphoric pyrophosphate comprises Mn ²⁺ and Fe ²⁺.

3. The positive electrode active material of sodium ion battery according to claim 1, wherein the mass ratio of the phosphoric pyrophosphate, the alumina and the elemental carbon is 100 (0.1 to 5): 0.1 to 10.

4. A method for producing the positive electrode active material for sodium-ion batteries according to any one of claims 1 to 3, comprising:

Preparing a mixed raw material, wherein the mixed raw material comprises a sodium source, a phosphorus source and an aluminum source, and further comprises one or more of an iron source and a manganese source, and the molar ratio of elements in the mixed raw material is Na (Fe+Mn): al: P= (4.0-y-5.0-1.2 y): (2.4-y to 3.6-y): (0.8 y-1.2 y): (3.2-4.8), wherein y is more than 0 and less than 0.3;

Dispersing and activating the mixed raw materials in a dispersing agent, drying and heating for a first preset time at a first preset temperature in an inert atmosphere to obtain a precursor, wherein the dispersing agent is one or more of ethanol, water and acetone;

dissolving an aluminum-containing raw material and an organic carbon source in a solvent to obtain a reaction solution, dispersing the precursor in the reaction solution, and drying to wrap the aluminum-containing raw material and the organic carbon source outside the precursor to obtain an intermediate;

And heating the intermediate to a second preset temperature in an inert atmosphere for a second preset time to convert the precursor into the phosphoric acid pyrophosphate, converting the aluminum-containing raw material into the aluminum oxide, and converting the organic carbon source into the elemental carbon to obtain the sodium ion battery anode active material.

5. The method according to claim 4, wherein the solvent is water, ethanol or isopropanol, and the aluminum-containing raw material is one or more of aluminum nitrate, aluminum chloride, aluminum sulfate, aluminum isopropoxide, aluminum acetate and aluminum chloride; the organic carbon source is one or more of glucose, sucrose, oxalic acid, ascorbic acid, tartaric acid, citric acid and oleic acid.

6. The method of claim 4, wherein the iron source is selected from materials of different shapes, such as flakes, needles, granules or blocks.

7. The method of claim 6, wherein the manganese source is the same shape as the iron source.

8. The method according to claim 4, wherein the first preset temperature is any one of 300 to 350 ℃, and the first preset time is any one of 1 to 5 hours; the second preset temperature is any value of 450-600 ℃, and the second preset time is any value of 1-24 h.

9. The method of claim 4, wherein the sodium source is one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium dihydrogen citrate, sodium oxalate, sodium alginate, and sodium pyrophosphate; the phosphorus source can be one or more of phosphoric acid, triammonium phosphate, monoammonium phosphate, diammonium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, ferric phosphate and ferrous phosphate; the iron source can be one or more of iron powder, ferrous oxalate, ferric nitrate, ferric phosphate, ferrous phosphate, ferric citrate, ferric oxide and ferric oxide; the manganese source can be one or more of manganese phosphate, manganese dihydrogen phosphate, manganese carbonate, manganese monoxide, manganese dioxide, manganese sesquioxide and manganous oxide; the aluminum source may be one or more of aluminum oxide, aluminum hydroxide, aluminum nitrate, aluminum sulfate, aluminum phosphate, aluminum dihydrogen phosphate, and aluminum dihydrogen phosphate.

10. A sodium ion battery comprising the positive electrode active material of the sodium ion battery according to any one of claims 1 to 3.