A cathode active material for a sodium-ion battery, a preparation method thereof, and a sodium-ion battery

Abstract

This application relates to the field of new energy, and particularly to a positive electrode active material for a sodium-ion battery, a preparation method thereof, and a sodium-ion battery. The positive electrode active material for the sodium-ion battery is composed of active particles, and the active particles include a core layer, a transition layer, a coating layer, and a carbon layer that are sequentially coated on the outer side of the core layer. The core layer, the transition layer, and the coating layer are all mainly composed of salts doped and modified with sodium iron pyrophosphate phosphate. The core layer is doped with a first doping element and manganese element with a common valence state higher than or equal to +3. The transition layer is doped with manganese element, and the coating layer is doped with a second doping element that is electrochemically inert. The doping amounts of these three-layer structures decrease from the inside to the outside. The main component of the carbon layer is carbon element. By making the manganese content in the active particles show a gradient decreasing distribution law, the long-range order of manganese in the active material is destroyed, thereby suppressing the Jahn-Teller cooperative effect and manganese dissolution, and improving the structural stability of the material.

Description

Technical Field

[0001] The present invention relates to the field of new energy, and particularly relates to a cathode active material for a sodium-ion battery, a preparation method thereof, and a sodium-ion battery. Background Art

[0002] With the development of the research on sodium-ion batteries, sodium iron pyrophosphate cathode materials have attracted wide attention due to their wide raw material sources, low cost, and good cycle stability. However, the specific capacity and working voltage of this material are both relatively low, resulting in a relatively low energy density, making it difficult to be applied to fields with high energy density requirements such as electric vehicles and 3C electronic products.

[0003] In order to improve the energy density of sodium iron pyrophosphate cathode materials, those skilled in the art replace part of the iron element with manganese elements and other doping elements with higher valence states, thereby increasing the Mn 2+ / Mn 3+ and Mn 3+ / Mn 4+ two redox pairs, so that more sodium ions can be released during charge and discharge, and the working voltage can be increased, making the material have a higher energy density.

[0004] However, when the manganese content increases, its electronic and ionic conductivities will decrease significantly, resulting in a reduction in the charge-discharge capacity, first efficiency, and rate performance of the material. At the same time, the manganese-containing cathode material has the Jahn-Teller effect, resulting in relatively serious lattice distortion of the material during the charge-discharge process, thus leading to poor cycle performance. On the other hand, there is also a dissolution phenomenon of manganese during the charge-discharge process. The dissolution of manganese will cause the reversible capacity of the material to gradually decrease and the cycle performance to deteriorate further. In addition, sodium-manganese mixing generally exists in manganese-based materials, which will cause voltage hysteresis, thus greatly reducing the energy density of the material. To sum up, due to the addition of manganese elements, the sodium iron manganese pyrophosphate material has poor structural stability during the charge-discharge process, resulting in poor electrochemical performance of the sodium iron manganese pyrophosphate material. Summary of the Invention

[0005] The purpose of the present invention is to provide a modified sodium-ion battery cathode active material of sodium iron manganese pyrophosphate with high structural stability during charge and discharge, a preparation method thereof, and a sodium-ion battery.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] A sodium-ion battery cathode active material, which is composed of active particles, and the active particles include:

[0008] A core layer, the main component of which has a chemical formula of Na4Mn x-a Fe 3-x Ma (PO4)2P2O7, where M is the first doping element, 1 ≤ x ≤ 3, and 0 ≤ a ≤ 0.1, and the common valence state of the first doping element is higher than or equal to +3;

[0009] The transition layer, which is wrapped around the outside of the core layer, and the chemical formula of the main component is Na4Mn y Fe 3-y (PO4)2P2O7, where 0.5 ≤ y ≤ 2, and y < x;

[0010] The encapsulating layer, which is wrapped around the outside of the transition layer, and the chemical formula of the main component is Na4Fe 3-b N b (PO4)2P2O7, where N is the second doping element, 0 ≤ b ≤ 0.1, and the second doping element is an electrochemically inert element for stabilizing the structure of the encapsulating layer;

[0011] The carbon layer, which is wrapped around the outside of the encapsulating layer, and the main component is carbon element.

[0012] Optionally, the first doping element is one or more of Mo, Ti, V, Zr, W, Nb, Y, and Ce.

[0013] Optionally, the potential of the second doping element relative to the sodium element is any value less than 1.8 V or greater than 4.3 V.

[0014] Optionally, the second doping element is one or more of Ni, Mg, Cu, Co, Al, Cr, Zn, and La.

[0015] Optionally, the molar ratio of the core layer, the transition layer, and the encapsulating layer in the active particles is any value in 1:(0.05 - 2):(0.05 - 2).

[0016] Optionally, the mass ratio of the total mass of the core layer, the transition layer, and the encapsulating layer to the mass of the carbon layer is any value in 100:(0.1 - 10).

[0017] In a second aspect, the present invention also provides a preparation method of the above active material, including:

[0018] Under the protection of an inert gas, ammonium phosphate salt is added to deionized water containing ferrous ions, manganese ions, and first doping element ions, so that the manganese ions, ferrous ions, first doping element ions, and hydrogen phosphate ions react in a molar ratio of (x - a):(3 - x):a:3 to form a hydrogen phosphate precipitate, obtaining a first precursor;

[0019] Under the protection of an inert gas, the first precursor is dispersed in deionized water, and ferrous ions and manganese ions are dissolved therein. After adding ammonium phosphate salt, the manganese ions, ferrous ions and hydrogen phosphate ions react in a molar ratio of y:(3 - y):3 to form hydrogen phosphate salt coating the outer layer of the first precursor, and the solid second precursor is separated;

[0020] Under the protection of an inert gas, the second precursor is dispersed in deionized water, and ferrous ions and second doping element ions are dissolved therein. After adding ammonium phosphate salt, the ferrous ions, second doping element ions and hydrogen phosphate ions react in a molar ratio of (3 - b):b:3 to form hydrogen phosphate salt coating the outer layer of the second precursor, and the solid third precursor is separated;

[0021] The third precursor, phosphorus source, sodium source and organic carbon source are dispersed in a dispersant, and mechanical activation is carried out by a grinding machine to obtain a slurry. The powder obtained after spray-drying the slurry is pre-burned at a first temperature under the protection of an inert gas and then sintered by raising the temperature to a second temperature, so that the third precursor reacts with the phosphorus source and the sodium source to be converted into pyrophosphate, and the active material is obtained.

[0022] Optionally, the ferrous ions are from one or more of ferrous chloride, ferrous sulfate, ferrous nitrate;

[0023] The manganese ions are from one or more of manganese chloride, manganese dihydrogen phosphate, manganese sulfate, manganese nitrate, manganese acetate;

[0024] The first doping element ions are from one or more of ammonium molybdate, sodium molybdate, tetrabutyl titanate, metatitanic acid, zirconium nitrate, zirconium acetate, ammonium metavanadate, ammonium tungstate, niobium oxide, niobium nitrate, yttrium nitrate, cerium nitrate;

[0025] The second doping element ions are from one or more of nickel acetate, nickel sulfate, magnesium acetate, magnesium nitrate, cobalt acetate, cobalt nitrate, copper sulfate, copper nitrate, lanthanum nitrate, chromium nitrate, aluminum nitrate, zinc acetate;

[0026] The ammonium phosphate salt is one or more of ammonium triphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate;

[0027] The phosphorus source is one or more of phosphoric acid, ammonium triphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate;

[0028] The sodium source is 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, sodium pyrophosphate;

[0029] The organic carbon source is one or more of glucose, sucrose, oxalic acid, ascorbic acid, tartaric acid, citric acid, and oleic acid;

[0030] The dispersant is one or more of ethanol, water, acetone, and isopropanol.

[0031] Optionally, the time of mechanical activation is any value in 1h to 12h, the first temperature is any value in 300°C to 350°C, the pre-burning time is any value in 1h to 5h, the second temperature is any value in 450°C to 600°C, and the sintering time is any value in 1h to 24h.

[0032] In a third aspect, the present invention also provides a sodium ion battery, including a positive electrode, and the active material of the positive electrode is the above-mentioned active material.

[0033] According to the first aspect of the present invention, a manganese-rich core layer and an iron-rich coating layer are formed, and a transition layer with a manganese element content between the two is formed between the core layer and the coating layer, so that the manganese content in the active particles shows a gradient decreasing distribution law, thereby destroying the long-range order of manganese in the active material, further inhibiting the Jahn-Teller cooperative effect and manganese dissolution, and improving the structural stability of the material. In addition, there is an "unconventional" Jahn-Teller effect in the manganese-rich region, which helps to relieve the closing of sodium ion channels caused by pyrophosphate distortion and improve the sodium ion diffusion ability. Doping with high-valence elements in the manganese-rich core layer can inhibit sodium-manganese mixing and solve the voltage hysteresis problem. Doping with electrochemically inert elements in the iron-rich coating layer can improve the conductivity of the material by adjusting the internal charge distribution. In addition, the pinning effect of doping elements can be used to inhibit pyrophosphate distortion and improve the structural stability and ionic conductivity of the material. And through lattice reconstruction and stress regulation, the strain in the material can be inhibited, the structural stability of the material can be improved, and at the same time, the electronic and ionic conductivities of the material can be improved.

[0034] According to the second aspect of the present invention, through the multi-step coprecipitation method, first synthesize a manganese iron hydrogen phosphate precursor with a gradient distribution of iron and manganese elements inside the particles and double doping inside and outside, and then prepare a composite sodium manganese iron pyrophosphate positive electrode material from the precursor, sodium source, additive, etc. through processes such as sanding - spraying - calcining, which helps the formation of the gradient and helps to inhibit side reactions.

[0035] According to the third aspect of the present invention, by improving the stability of the positive electrode active material during charge and discharge, it helps to improve the electrochemical performance of the battery.

[0036] The above description is only an overview of the technical solution of the present invention. In order to be able to understand the technical means of the present invention more clearly and implement it according to the content of the specification, the following takes the preferred embodiments of the present invention and combines with the drawings to describe in detail as follows. Brief Description of the Drawings

[0037] Figure 1 Schematic diagram of the structure of the active particles shown in the first embodiment of the present invention;

[0038] Figure 2 Flow chart of the preparation method of the sodium ion positive electrode active material shown in the first embodiment of the present invention;

[0039] Figure 3 Electron microscope image of the active particles shown in the first embodiment of the present invention;

[0040] Figure 4 X-ray diffraction analysis pattern of the positive electrode active material of the sodium ion battery shown in the first embodiment of the present invention;

[0041] Figure 5 Charge and discharge curve of the sodium ion battery shown in the first embodiment of the present invention at a rate of 0.1C;

[0042] Figure 6 Charge and discharge curve of the sodium ion battery shown in Comparative Example 1 of the present invention at a rate of 0.1C.

[0043] Legend: 1 - core layer, 2 - transition layer, 3 - coating layer, 4 - carbon layer. Detailed Description of the Invention

[0044] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0045] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. It is 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 thus should not be construed as a limitation of the present invention. In addition, the terms "first", "second", "third" are only used for descriptive purposes and should not be construed as indicating or implying relative importance.

[0046] In the description of the present invention, it should be noted that unless otherwise clearly specified and defined, the terms "installation", "connection", and "coupling" 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 directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0047] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0048] Please refer to Figure 1 , the positive electrode active material protected by the present invention application is composed of active particles, and the active particles include a core layer 1, a transition layer 2 successively coated on the outer side of the core layer 1, a coating layer 3, and a carbon layer 4. The chemical formula of the main component of the core layer 1 is Na4Mn x-a Fe 3-x M a (PO4)2P2O7, where M is a first doping element, 1 ≤ x ≤ 3, and 0 ≤ a ≤ 0.1, and the common valence state of the first doping element is higher than or equal to +3. The chemical formula of the main component of the transition layer 2 is Na4Mn y Fe 3-y (PO4)2P2O7, where 0.5 ≤ y ≤ 2, and y < x. The chemical formula of the main component of the coating layer 3 is Na4Fe 3-b N b (PO4)2P2O7, where N is a second doping element, 0 ≤ b ≤ 0.1, and the second doping element is an electrochemically inert element for stabilizing the structure of the coating layer 3. The main component of the carbon layer 4 is carbon. In the present invention, x can be any value among 1, 1.5, 2, 2.5, and 3, a can be any value among 0.03, 0.05, 0.07, 0.09, and 0.1, y can be any value among 0.5, 0.9, 1.4, 1.9, and 2, and the selected value of y should be less than x, and b can be any value among 0.03, 0.05, 0.07, 0.09, and 0.1.

[0049] A manganese-rich core layer 1 and an iron-rich coating layer 3 are formed within the active particles, and a transition layer 2 with a manganese element content between the two is formed between the core layer 1 and the coating layer 3, such that the manganese content within the active particles shows a gradient decreasing distribution pattern, thereby disrupting the long-range order of manganese in the active material, further inhibiting the Jahn-Teller cooperative effect and manganese dissolution, and improving the structural stability of the material. Additionally, there is an "unconventional" Jahn-Teller effect in the manganese-rich region, which helps to alleviate the closing of sodium ion channels caused by pyrophosphate distortion and enhance the sodium ion diffusion ability. Doping with high-valence elements in the manganese-rich core layer 1 can inhibit sodium-manganese mixing and solve the voltage hysteresis problem. Electrochemically inert elements doped in the iron-rich coating layer 3 can enhance the conductivity of the material by adjusting the internal charge distribution. In addition, the pinning effect of the doped elements can be utilized to inhibit pyrophosphate distortion and improve the structural stability and ionic conductivity of the material. Moreover, through lattice reconstruction and stress regulation, the strain within the material can be inhibited, the structural stability of the material can be improved, and at the same time, the electronic and ionic conductivities of the material can be enhanced.

[0050] In some embodiments, the first doping element is one or more of Mo, Ti, V, Zr, W, Nb, Y, and Ce.

[0051] In some embodiments, the electric potential of the second doping element relative to the sodium element should be less than 1.8 V or greater than 4.3 V. For example, it can be any value among 1.2 V, 1.5 V, 1.7 V, 4.4 V, 4.5 V, and 4.8 V. The voltage plateau of Fe in sodium manganese pyrophosphate 2+ / Fe 3+ is around 2.9 V, and the voltage plateau of Mn 2+ / Mn 3+ is around 3.8 V. According to the voltage plateau of the material and in combination with the charge-discharge efficiency, 1.8 V to 4.3 V is proximally selected as the voltage window, and an element with an electric potential relative to the sodium element not within the voltage window is selected as the second doping element. Utilizing its pinning effect that does not participate in the redox reaction helps to improve the structural stability of the material.

[0052] In some embodiments, the second doping element is one or more of Ni, Mg, Cu, Co, Al, Cr, Zn, and La.

[0053] In some embodiments, the molar ratio of the core layer 1, the transition layer 2, and the coating layer 3 in the active particles is any value within 1:(0.05 - 2):(0.05 - 2), such as 1:0.05:0.05, 1:0.05:1, 1:1:0.5, 1:1:1, and 1:1.6:2, which helps to uniformly form the concentration gradient.

[0054] In some embodiments, the mass ratio of the total mass of the core layer 1, the transition layer 2, and the wrapping layer 3 to the mass of the carbon layer 4 is any value in the range of 100:(0.1 - 10), for example, it can be any value among 100:0.1, 100:0.5, 100:1, 100:5, and 100:10. The carbon layer 4 helps improve the conductivity of the active material and protects the internally structured with strong chemical activity.

[0055] In a second aspect, the present invention also provides a method for preparing the above-mentioned active material, including:

[0056] S1. Under the protection of an inert gas, ammonium phosphate salt is added to deionized water containing ferrous ions, manganese ions, and first doping element ions, and the manganese ions, ferrous ions, first doping element ions, and hydrogen phosphate ions react in a molar ratio of (x - a):(3 - x):a:3 to form a hydrogen phosphate precipitate, obtaining a first precursor.

[0057] S2. Under the protection of an inert gas, the first precursor is dispersed in deionized water, ferrous ions and manganese ions are dissolved, and after adding ammonium phosphate salt, the manganese ions, ferrous ions, and hydrogen phosphate ions react in a molar ratio of y:(3 - y):3 to form a hydrogen phosphate coating on the outer layer of the first precursor, and the solid second precursor is separated.

[0058] S3. Under the protection of an inert gas, the second precursor is dispersed in deionized water, ferrous ions and second doping element ions are dissolved, and after adding ammonium phosphate salt, the ferrous ions, second doping element ions, and hydrogen phosphate ions react in a molar ratio of (3 - b):b:3 to form a hydrogen phosphate coating on the outer layer of the second precursor, and the solid third precursor is separated.

[0059] S4. The third precursor, phosphorus source, sodium source, and organic carbon source are dispersed in a dispersant, and mechanical activation is performed by a sand mill to obtain a slurry. The powder obtained after spray drying the slurry is pre-burned at a first temperature under the protection of an inert gas and then sintered by raising the temperature to a second temperature, causing the third precursor to react with the phosphorus source and sodium source to be converted into pyrophosphate, obtaining the active material.

[0060] Through a multi-step coprecipitation method, first, a hydrogen phosphate manganese iron precursor with a gradient distribution of iron and manganese elements inside the particles and double doping inside and outside is synthesized, and then the precursor, sodium source, additives, etc. are prepared into a composite sodium manganese iron pyrophosphate cathode material through processes such as sand milling - spraying - calcining, which helps the formation of the gradient and helps suppress side reactions.

[0061] In some embodiments, the ferrous ions are sourced from one or more of ferrous chloride, ferrous sulfate, and ferrous nitrate;

[0062] The manganese ions are sourced from one or more of manganese chloride, manganese dihydrogen phosphate, manganese sulfate, manganese nitrate, and manganese acetate;

[0063] The first doping element ions are from one or more of ammonium molybdate, sodium molybdate, tetrabutyl titanate, metatitanic acid, zirconium nitrate, zirconium acetate, ammonium metavanadate, ammonium tungstate, niobium oxide, niobium nitrate, yttrium nitrate, and cerium nitrate;

[0064] The second doping element ions are from one or more of nickel acetate, nickel sulfate, magnesium acetate, magnesium nitrate, cobalt acetate, cobalt nitrate, copper sulfate, copper nitrate, lanthanum nitrate, chromium nitrate, aluminum nitrate, and zinc acetate;

[0065] The ammonium phosphate salt is one or more of ammonium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the phosphorus source is one or more of phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate;

[0066] The sodium source is 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;

[0067] The organic carbon source is one or more of glucose, sucrose, oxalic acid, ascorbic acid, tartaric acid, citric acid, and oleic acid;

[0068] The dispersant is one or more of ethanol, water, acetone, and isopropyl alcohol.

[0069] The phosphorus source can be sodium salts, namely sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate, while the sodium source can also be sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate containing phosphorus elements. The additional sodium ions are the sodium ions ionized from the phosphorus source and the sodium source, and the additional phosphorus-containing ions are the phosphate, hydrogen phosphate, and dihydrogen phosphate ions ionized from the phosphorus source and the sodium source. In step S4 of the present invention, the molar ratio of the elements to be supplemented by the sodium source and the phosphorus source is 4:1, that is, the molar ratio of the additional sodium ions to the additional phosphorus-containing ions is 4:1. When the phosphorus source and the sodium source include one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium phosphate, since the ratio of the sodium element to the phosphorus element contained therein is (1-3):1 and the sodium content is low, other sodium sources are needed to supplement the additional sodium ions, and there may be no other phosphorus sources in the system.

[0070] In some embodiments, the time of mechanical activation is any value from 1 h to 12 h, for example, it can be any value from 1 h, 3 h, 5 h, 8 h, 10 h, and 12 h. The first temperature is any value from 300 °C to 350 °C, for example, it can be any value from 300 °C, 310 °C, 320 °C, 330 °C, 340 °C, and 350 °C. The pre-burning time is any value from 1 h to 5 h, for example, it can be any value from 1 h, 2 h, 3 h, 4 h, and 5 h. The second temperature is any value from 450 °C to 600 °C, for example, it can be any value from 450 °C, 500 °C, 550 °C, and 600 °C. The sintering time is any value from 1 h to 24 h, for example, it can be any value from 1 h, 3 h, 5 h, 8 h, 10 h, and 12 h.

[0071] In a third aspect, the present invention also provides a sodium-ion battery, including a positive electrode, and the active material of the positive electrode is the above-mentioned active material. By improving the stability of the positive electrode active substance during charge and discharge, it helps to improve the electrochemical performance of the battery.

[0072] For details, please refer to the following embodiments.

[0073] Example 1:

[0074] Please refer to Figure 1 , the positive electrode active material of the sodium-ion battery shown in a preferred embodiment of the present application is composed of active particles. The active particles have a multi-layer structure, including a core layer 1 and a transition layer 2, a coating layer 3, and a carbon layer 4 sequentially wrapped outside the core layer 1. After forming a phosphate hydrogen salt structure with a gradient change in the content of manganese and iron elements in three layers by multi-step coprecipitation, it is synchronously converted into pyrophosphate phosphate, and a carbon layer 4 is wrapped on the outer layer to enhance conductivity.

[0075] Please refer to Figure 2 , in this embodiment, the preparation method of the positive electrode active material of the sodium-ion battery includes:

[0076] S1. Under the protection of an inert gas, ammonium phosphate is added to deionized water containing ferrous ions, manganese ions, and first doping element ions, so that manganese ions, ferrous ions, first doping element ions, and hydrogen phosphate ions react in a molar ratio of (x - a): (3 - x): a: 3 to form a phosphate hydrogen salt precipitate, obtaining a first precursor.

[0077] S2. Under the protection of an inert gas, the first precursor is dispersed in deionized water, and ferrous ions and manganese ions are dissolved. After adding ammonium phosphate, manganese ions, ferrous ions, and hydrogen phosphate ions react in a molar ratio of y: (3 - y): 3 to form a phosphate hydrogen salt wrapped on the outer layer of the first precursor, and a solid second precursor is separated.

[0078] S3. Under the protection of an inert gas, disperse the second precursor in deionized water, dissolve ferrous ions and second doping element ions, and after adding ammonium phosphate salt, the ferrous ions, second doping element ions and hydrogen phosphate ions react in a molar ratio of (3 - b):b:3 to form a hydrogen phosphate salt wrapped around the outer layer of the second precursor, and then separate to obtain a solid third precursor.

[0079] S4. Disperse the third precursor, phosphorus source, sodium source and organic carbon source in a dispersant, and perform mechanical activation through a sand mill to obtain a slurry. The powder obtained after spray drying the slurry is pre - sintered at a first temperature under the protection of an inert gas, and then heated to a second temperature for sintering, so that the third precursor reacts with the phosphorus source and sodium source to be converted into pyrophosphate phosphate, obtaining an active material.

[0080] In each step of the embodiments of the present invention, when weighing and adding each raw material, control the molar ratio of the ions corresponding to each element in the raw material to be equal to the molar ratio of the elements in each ion or each reactant participating in the reaction preset.

[0081] In the prior art for preparing sodium manganese - iron pyrophosphate phosphate, by limiting the addition amount, the molar ratio of each ion in deionized water is made the same as or close to the molar ratio required for the reaction, so that the reaction can proceed according to the preset molar ratio. In actual operation, those skilled in the art may appropriately over - weigh some raw materials during weighing, so that the corresponding ions in deionized water are appropriately in excess, in order to promote the reaction. Since the excess amplitude is small, it has no obvious influence on the composition and structure of the generated hydrogen phosphate precursor. This limit of excess is common knowledge in the art, so it will not be elaborated here. It should be noted that when such an excess that does not affect the reaction molar ratio occurs during the weighing of raw materials, the scheme is also within the protection scope of this application.

[0082] In this embodiment, the inert gas is argon. In step S1, the value of x is 2.9, the value of a is 0.05, and the first doping element is molybdenum, that is, manganese ions, ferrous ions, first doping element ions and hydrogen phosphate ions react in a molar ratio of 2.85:0.1:0.05:3. In the present invention, the molar ratio of each element in each weighed raw material is consistent with the molar ratio of the elements in the ions required for the reaction. In step S1 of this embodiment, manganese sulfate, ferrous nitrate, ammonium molybdate and ammonium dihydrogen phosphate are weighed respectively in a ratio of element molar ratio Mn:Fe:Mo:P of 2.85:0.1:0.05:3. Under the protection of argon, after dissolving manganese sulfate, ferrous nitrate and ammonium molybdate in deionized water, add ammonium dihydrogen phosphate to react to obtain a water - insoluble hydrogen phosphate salt, converting the liquid into a suspension, and after the reaction is completed, obtain the first precursor by suction filtration.

[0083] In step S2, the value of y is 1.8. Manganese nitrate, ferrous chloride, and ammonium dihydrogen phosphate are weighed respectively at a molar ratio of Mn:Fe:P of 1.8:1.2:3. After adding the first precursor to deionized water and stirring to disperse it, ferrous chloride and manganese nitrate are added successively and stirred to dissolve, and finally ammonium dihydrogen phosphate is added for stirring reaction precipitation. At this time, the manganese iron phosphate formed by the reaction is wrapped around the outer layer of the first precursor to form a second precursor, and the solid second precursor is separated from the suspension by suction filtration.

[0084] In step S3, the value of b is 0.1, and the second doping element is nickel. Ferrous sulfate, nickel acetate, and ammonium dihydrogen phosphate are weighed respectively at a molar ratio of Fe:Ni:P of 2.9:0.1:3. After adding the second precursor to deionized water and stirring to disperse it, ferrous sulfate and nickel acetate are added successively and stirred to dissolve, and finally ammonium dihydrogen phosphate is added for stirring reaction precipitation. At this time, the water-insoluble hydrogen phosphate formed by the reaction is wrapped around the outer layer of the second precursor to form a third precursor, and the solid third precursor is separated from the suspension by suction filtration.

[0085] In step S4, the phosphorus source is sodium dihydrogen phosphate, the sodium source is sodium carbonate, and the organic carbon source is citric acid. Taking the total molar amount of hydrogen phosphate in the third precursor as the molar amount of the third precursor, taking the sodium ions contained in the phosphorus source and the sodium source as the additional sodium ions, and taking the dihydrogen phosphate contained in the phosphorus source as the additional phosphorus-containing ions. The molar ratio of the third precursor, additional sodium ions, and additional phosphorus-containing ions participating in the reaction is 3:4:1. Weigh the third precursor, sodium source, and phosphorus source according to the reaction molar ratio, disperse them together with the organic carbon source in ethanol, and mechanically activate them in a sand mill for 5 h. The obtained slurry is spray-dried to form a powder. The powder is heat-treated at 320 °C for 2 h under the protection of argon, and then heated to 550 °C for heat treatment for 14 h. At this time, part of the first precursor formed in the third precursor reacts and is converted into Na4Mn 2.85 Fe 0.1 Mo 0.05 (PO4)2P2O7 to form the core layer 1, and the manganese iron phosphate formed in step S2 is converted into Na4Mn 1.8 Fe 1.2 (PO4)2P2O7 to form the transition layer 2, and the hydrogen phosphate formed in step S3 is converted into Na4Fe 2.9 Ni 0.1 (PO4)2P2O7 to form the coating layer 3. Citric acid is wrapped around the third precursor and after high-temperature treatment, it forms a carbon layer 4 mainly composed of carbon elements and wrapped outside the coating layer 3. After cooling with the furnace, the positive electrode active material for the sodium-ion battery in the present invention is obtained.

[0086] By adjusting the molar amounts of the raw materials in each step, the molar ratio of the core layer 1, the transition layer 2, and the coating layer 3 of the active particles in this embodiment is 1:0.5:0.5, and the mass ratio of the total mass of the core layer 1, the transition layer 2, and the coating layer 3 to the mass of the carbon layer 4 is 100:5.

[0087] Please refer to Figure 3 , the positive electrode active material of the sodium-ion battery in this embodiment was observed under an electron microscope to obtain an electron micrograph, thereby revealing the internal microstructure of the active material.

[0088] Please refer to Figure 4 , the X-ray diffraction analysis pattern of the positive electrode active material of the sodium-ion battery was detected and plotted, and it can be seen that the active material is a pure phase.

[0089] The prepared positive electrode active material of the sodium-ion battery was mixed with the conductive agent acetylene black and the binder polyvinylidene fluoride PVDF in a mass ratio of 8:1:1. After grinding evenly in a mortar, N-methylpyrrolidone NMP was added and grinding continued for a period of time, and finally a uniform black viscous mixed slurry was obtained. The mixed slurry obtained by grinding was placed on an aluminum foil, and a scraper was used to evenly coat it into a film with a uniform thickness to form a positive electrode. Using a sodium metal sheet as the counter electrode and a glass fiber membrane as the separator, 1 mol / L of NaClO4 / PC (propylene carbonate) was used as the electrolyte, and a CR2032 type coin cell was assembled in an anhydrous and oxygen-free argon atmosphere glove box to obtain a sodium-ion battery. This assembly process ensures the performance stability and safety of the battery.

[0090] Please refer to Figure 5 , the charge-discharge curve of the sodium-ion battery at a 0.1C rate was detected and plotted, and it can be seen that the sodium-ion battery prepared in this embodiment has a high specific capacity and a stable voltage platform during the charge-discharge process.

[0091] Example Two:

[0092] The difference between this embodiment and Example One is:

[0093] In step S1 of this embodiment, the first doping element is zirconium, x takes a value of 2.5, a takes a value of 0.04, the ferrous ions are from ferrous sulfate, the manganese ions are from manganese acetate, the hydrogen phosphate group is from diammonium hydrogen phosphate, and the first doping element ions are from zirconium nitrate;

[0094] In step S2 of this embodiment, y takes a value of 1.5, the ferrous ions are from ferrous chloride, the manganese ions are from manganese sulfate, and the hydrogen phosphate group is from diammonium hydrogen phosphate;

[0095] In step S3 of this embodiment, the second doping element is lanthanum, b is 0.06, the ferrous ions are from ferrous nitrate, the hydrogen phosphate ions are from diammonium hydrogen phosphate, and the second doping element ions are from lanthanum nitrate;

[0096] In step S4 of this embodiment, the phosphorus source is sodium dihydrogen phosphate, the sodium source is sodium bicarbonate, the organic carbon source is glucose, the time of mechanical activation is 8 h, the first temperature is 300 °C, the time of pre-sintering is 5 h, the second temperature is 500 °C, and the time of sintering is 20 h;

[0097] In this embodiment, the chemical formula of the main component of the core layer 1 is Na4Mn 2.46 Fe 0.5 Zr 0.04 (PO4)2P2O7, the chemical formula of the main component of the transition layer 2 is Na4Mn 1.5 Fe 1.5 (PO4)2P2O7, the chemical formula of the main component of the coating layer 3 is Na4Fe 2.94 La 0.06 (PO4)2P2O7, the molar ratio of the core layer 1, the transition layer 2 and the coating layer 3 of the active particles is 1:0.5:1, and the mass ratio of the total mass of the core layer 1, the transition layer 2 and the coating layer 3 to the mass of the carbon layer 4 is 100:6.

[0098] Example 3:

[0099] The difference between this embodiment and Example 1 is that:

[0100] In step S1 of this embodiment, the first doping element is titanium, x is 1.5, a is 0.08, the ferrous ions are from ferrous chloride, the manganese ions are from manganese acetate, and the first doping element ions are from tetrabutyl titanate;

[0101] In step S2 of this embodiment, y is 1.2, the ferrous ions are from ferrous sulfate, and the manganese ions are from manganese nitrate;

[0102] In step S3 of this embodiment, the second doping element is aluminum, b is 0.09, the ferrous ions are from ferrous nitrate, and the second doping element ions are from aluminum nitrate;

[0103] In step S4 of this embodiment, the phosphorus source is sodium dihydrogen phosphate, the sodium source is sodium acetate, the organic carbon source is ascorbic acid, the time of mechanical activation is 2 h, the first temperature is 350 °C, the time of pre-sintering is 5 h, the second temperature is 600 °C, and the time of sintering is 24 h;

[0104] In this embodiment, the chemical formula of the main component of the core layer 1 is Na4Mn 1.42 Fe 1.5 Ti 0.08(PO4)2P2O7, the chemical formula of the main component of the transition layer 2 is Na4Mn 1.2 Fe 1.8 (PO4)2P2O7, the chemical formula of the main component of the coating layer 3 is Na4Fe 2.91 Al 0.09 (PO4)2P2O7, the molar ratio of the core layer 1, the transition layer 2 and the coating layer 3 of the active particles is 1:1:0.5, and the mass ratio of the total mass of the core layer 1, the transition layer 2 and the coating layer 3 to the mass of the carbon layer 4 is 100:2.

[0105] Comparative Example 1:

[0106] The difference between this comparative example and Example 1 is that steps S2 and S3 are not carried out, and the third precursor is directly replaced by the first precursor to carry out step S4, so that the active particles only include the core layer 1 and the carbon layer 4 wrapped outside the core layer 1.

[0107] Please refer to Figure 6 , detect and plot the charge-discharge curve of the sodium-ion battery prepared in this comparative example at a rate of 0.1C, and compare it with Figure 5 It can be seen that the specific capacity of the sodium-ion battery prepared in this comparative example is relatively low, and the redox activity is poor during the charge-discharge process, and the voltage platform is short, resulting in a low energy density.

[0108] Comparative Example 2:

[0109] The difference between this comparative example and Example 1 is that S3 is not carried out, and the third precursor is directly replaced by the second precursor to carry out step S4, so that the active particles only include the core layer 1, the transition layer 2 wrapped outside the core layer 1 and the carbon layer 4 wrapped outside the transition layer 2.

[0110] Comparative Example 3:

[0111] The difference between this comparative example and Example 1 is that S2 is not carried out, and the second precursor is directly replaced by the first precursor to carry out step S3, so that the active particles only include the core layer 1, the coating layer 3 wrapped outside the core layer 1 and the carbon layer 4 wrapped outside the coating layer 3.

[0112] The electrochemical performance of the positive electrode material is shown in Table 1 below.

[0113]

[0114] From the data in Example 1 and each comparative example combined with those in Table 1, it can be seen that by constructing the positive electrode active material into a multi-layer structure with a gradient distribution of the contents of iron and manganese elements, the discharge specific capacity and cycle efficiency can be effectively improved. Moreover, the average working voltage at 0.1C in Example 1 is only slightly lower than that in Comparative Example 1, which proves that even though the doping amount of manganese in the outer layer material is reduced by gradient, the working voltage can still be maintained at a relatively high level. That is to say, the positive electrode active material obtained in the present invention has the advantages of good cycle performance of doped sodium iron pyrophosphate phosphate and high energy density of doped sodium iron manganese pyrophosphate phosphate, and has high practicability.

[0115] From the data in each example combined with those in Table 1, it can be seen that the higher the doping amount of manganese and the greater the gradient, the higher the energy density of the material and the worse the cycle performance.

[0116] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of concise description, 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, it should be considered as the scope described in this specification.

[0117] The above-described embodiments only represent several implementation manners of the present invention, and the description thereof is relatively specific and detailed, but it should not be understood as a limitation to the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the invention patent should be subject to the appended claims.

Claims

1. A cathode active material for a sodium-ion battery, characterized in that Composed of active particles, the active particles include: Core layer (1), the chemical formula of the main component is Na4Mn x-a Fe 3-x M a (PO4)2P2O7, where M is the first doping element, 1 ≤ x ≤ 3, and 0 ≤ a ≤ 0.1, and the first doping element is one or more of Mo, Ti, V, Zr, W, Nb, Y, and Ce; The transition layer (2), which is wrapped around the outer side of the core layer (1), has a chemical formula of the main component as Na4Mn y Fe 3-y (PO4)2P2O7, where 0.5 ≤ y ≤ 2 and y < x; The coating layer (3) is wrapped around the outside of the transition layer (2), and the chemical formula of the main component is Na4Fe 3-b N b (PO4)2P2O7, where N is the second doping element, 0 ≤ b ≤ 0.1, and the second doping element is one or more of Ni, Mg, Cu, Co, Al, Cr, Zn, and La, and is used to stabilize the structure of the coating layer (3); A carbon layer (4), wrapped outside the encapsulation layer (3), with the main component being carbon element.

2. The active material according to claim 1, characterized in that, The mass ratio of the total mass of the core layer (1), the transition layer (2) and the encapsulation layer (3) to the mass of the carbon layer (4) is any value in 100:(0.1 - 10).

3. The method for preparing the active material according to claim 1 or 2, characterized in that, Including: Under the protection of inert gas, ammonium phosphate is added to deionized water containing ferrous ions, manganese ions and first doping element ions, so that manganese ions, ferrous ions, first doping element ions and hydrogen phosphate ions react in a molar ratio of (x - a):(3 - x):a:3 to form a hydrogen phosphate precipitate, obtaining a first precursor; Under the protection of inert gas, the first precursor is dispersed in deionized water, ferrous ions and manganese ions are dissolved, after adding ammonium phosphate, manganese ions, ferrous ions and hydrogen phosphate ions react in a molar ratio of y:(3 - y):3 to form hydrogen phosphate wrapped around the outer layer of the first precursor, and a solid second precursor is separated; Under the protection of inert gas, the second precursor is dispersed in deionized water, ferrous ions and second doping element ions are dissolved, after adding ammonium phosphate, ferrous ions, second doping element ions and hydrogen phosphate ions react in a molar ratio of (3 - b):b:3 to form hydrogen phosphate wrapped around the outer layer of the second precursor, and a solid third precursor is separated; The third precursor, phosphorus source, sodium source and organic carbon source are dispersed in a dispersant, and mechanical activation is carried out by a sand mill to obtain a slurry. The powder obtained after spray drying the slurry is pre - burned at a first temperature under the protection of inert gas, and then heated to a second temperature for sintering, so that the third precursor reacts with the phosphorus source and the sodium source to be converted into pyrophosphate, obtaining the active material.

4. The preparation method according to claim 3, wherein The ferrous ions are from one or more of ferrous chloride, ferrous sulfate, ferrous nitrate; The manganese ions are from one or more of manganese chloride, manganese dihydrogen phosphate, manganese sulfate, manganese nitrate, manganese acetate; The first doping element ions are from one or more of ammonium molybdate, sodium molybdate, tetrabutyl titanate, metatitanic acid, zirconium nitrate, zirconium acetate, ammonium metavanadate, ammonium tungstate, niobium oxide, niobium nitrate, yttrium nitrate, cerium nitrate; The second doping element ions are from one or more of nickel acetate, nickel sulfate, magnesium acetate, magnesium nitrate, cobalt acetate, cobalt nitrate, copper sulfate, copper nitrate, lanthanum nitrate, chromium nitrate, aluminum nitrate, zinc acetate; The ammonium phosphate is one or more of ammonium triphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate; The phosphorus source is one or more of phosphoric acid, ammonium triphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate; The sodium source is 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, sodium pyrophosphate; The organic carbon source is one or more of glucose, sucrose, oxalic acid, ascorbic acid, tartaric acid, citric acid, oleic acid; The dispersant is one or more of ethanol, water, acetone, and isopropyl alcohol.

5. The preparation method according to claim 3, wherein, The time of mechanical activation is any value in the range of 1 h to 12 h, the first temperature is any value in the range of 300 °C to 350 °C, the time of pre-sintering is any value in the range of 1 h to 5 h, the second temperature is any value in the range of 450 °C to 600 °C, and the time of sintering is any value in the range of 1 h to 24 h.

6. A sodium-ion battery, characterized in that, It includes a positive electrode, and the active material of the positive electrode is the active material as described in Claim 1 or 2.

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