Sodium-ion battery positive electrode active material, preparation method thereof and sodium-ion battery

Abstract

The invention relates to the field of new energy, in particular to a sodium-ion battery positive active material, a preparation method thereof and a sodium-ion battery. The sodium ion battery positive electrode active material is composed of active particles, and each active particle comprises a core layer, and a transition layer, a wrapping layer and a carbon layer which sequentially wrap the outer side of the core layer. The core layer, the transition layer and the wrapping layer all take sodium ferric phosphate pyrophosphate doped and modified salt as a main component, a first doping element and a manganese element of which the common valence state is higher than or equal to + 3 are doped in the core layer, the transition layer is doped with the manganese element, the wrapping layer is doped with an electrochemically inert second doping element, and the doping amounts of the three layers of structures are gradually reduced from inside to outside. The main component of the carbon layer is carbon element. The content of manganese in the active particles is in a gradient decreasing distribution rule, so that the long-range order of manganese in the active material is destroyed, the Jahn-Teller synergistic effect and manganese dissolution are inhibited, and the structural stability of the material is improved.

Description

Technical Field

[0001] The present invention relates to the field of new energy, and in particular to a sodium ion battery positive electrode active material and a preparation method thereof, and a sodium ion battery. Background Art

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

[0003] In order to improve the energy density of the sodium iron pyrophosphate positive electrode material, technicians in this field replace part of the iron element with manganese and other doping elements with higher valence, thereby increasing the Mn 2+ / Mn 3+ and Mn 3+ / Mn 4+ Two redox pairs can release more sodium ions during charging and discharging, and increase the working voltage, giving the material a higher energy density.

[0004] However, when the manganese content increases, its electronic and ionic conductivity will decrease significantly, resulting in a decrease in the material's charge and discharge capacity, initial efficiency and rate performance. At the same time, there is a Jahn-Teller effect in manganese-containing positive electrode materials, which causes serious lattice distortion in the material during the charge and discharge process, resulting in poor cycle performance. On the other hand, manganese also dissolves during the charge and discharge process. The dissolution of manganese will cause the reversible capacity of the material to gradually decrease and the cycle performance to further deteriorate. In addition, manganese-based materials generally have a sodium-manganese mixed arrangement phenomenon, which will lead to voltage hysteresis, thereby greatly reducing the energy density of the material. In summary, due to the addition of manganese, the sodium manganese pyrophosphate material has poor structural stability during the charge and discharge process, resulting in poor electrochemical performance of the sodium manganese pyrophosphate material. Summary of the invention

[0005] The object of the present invention is to provide a sodium phosphate manganese iron pyrophosphate modified sodium ion battery positive electrode active material with high structural stability during the charge and discharge process, and a preparation method and a sodium ion battery.

[0006] In order to achieve the above object, the present invention provides the following technical solutions: A sodium ion battery positive electrode active material, composed of active particles, the active particles comprising: The core layer, the main component of which is Na 4 Mn x-a Fe 3-x M a(PO 4 ) 2 P 2 O 7 , wherein M is a first doping element, 1≤x≤3, and 0≤a≤0.1, and a common valence state of the first doping element is greater than or equal to +3; The transition layer is wrapped around the outside of the core layer, and the chemical formula of the main component is Na 4 Mn y Fe 3-y (PO 4 ) 2 P 2 O 7 , where 0.5≤y≤2, and y <x; The wrapping layer is wrapped around the outside of the transition layer, and the chemical formula of the main component is Na 4 Fe 3-b N b (PO 4 ) 2 P 2 O 7 , wherein N is a second doping element, 0≤b≤0.1, and the second doping element is an electrochemically inert element used to stabilize the structure of the encapsulation layer; The carbon layer is wrapped around the outer side of the wrapping layer, and its main component is carbon element.

[0007] Optionally, the first doping element is one or more of Mo, Ti, V, Zr, W, Nb, Y and Ce.

[0008] Optionally, the potential of the second doping element relative to the sodium element is less than 1.8V or greater than 4.3V.

[0009] Optionally, the second doping element is one or more of Ni, Mg, Cu, Co, Al, Cr, Zn and La.

[0010] Optionally, the molar ratio of the core layer, the transition layer and the wrapping layer in the active particles is any value of 1:(0.05-2):(0.05-2).

[0011] Optionally, the mass ratio of the total mass of the core layer, the transition layer and the wrapping layer to the mass of the carbon layer is 100: any value in the range of (0.1-10).

[0012] In a second aspect, the present invention also provides a method for preparing the above active material, comprising: 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 the manganese ions, ferrous ions, first doping element ions and hydrogen phosphate ions react in a molar ratio of (xa): (3-x): a: 3 to generate hydrogen phosphate precipitate, thereby obtaining a first precursor; Under the protection of an inert gas, the first precursor is dispersed in deionized water, and ferrous ions and manganese ions are dissolved therein, and after the ammonium phosphate is added, the manganese ions, ferrous ions and hydrogen phosphate ions react at a molar ratio of y:(3-y):3 to generate hydrogen phosphate wrapped in the outer layer of the first precursor, and a solid second precursor is obtained by separation; 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, and after adding ammonium phosphate, the ferrous ions, second doping element ions and hydrogen phosphate ions react in a molar ratio of (3-b):b:3 to generate hydrogen phosphate wrapped in the outer layer of the second precursor, and a solid third precursor is obtained by separation; The third precursor, phosphorus source, sodium source and organic carbon source are dispersed in a dispersant, and mechanically activated by a sand grinder to obtain a slurry. The powder obtained by spray drying the slurry is pre-fired 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 the sodium source to be converted into phosphate pyrophosphate to obtain the active material.

[0013] Optionally, the ferrous ions are derived from one or more of ferrous chloride, ferrous sulfate, and ferrous nitrate; The manganese ions are derived from one or more of manganese chloride, manganese dihydrogen phosphate, manganese sulfate, manganese nitrate, and manganese acetate; The first doping element ion is derived 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; The second doping element ion is derived 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; The ammonium phosphate salt is one or more of triammonium phosphate, diammonium phosphate, and diammonium hydrogen phosphate; The phosphorus source is one or more of phosphoric acid, triammonium phosphate, diammonium phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and 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, and sodium pyrophosphate; The organic carbon source is one or more of glucose, sucrose, oxalic acid, ascorbic acid, tartaric acid, citric acid, and oleic acid; The dispersant is one or more of ethanol, water, acetone and isopropanol.

[0014] Optionally, the mechanical activation time is any value between 1h and 12h, the first temperature is any value between 300℃ and 350℃, the pre-burning time is any value between 1h and 5h, the second temperature is any value between 450℃ and 600℃, and the sintering time is any value between 1h and 24h.

[0015] In a third aspect, the present invention further provides a sodium ion battery, comprising a positive electrode, wherein the active material of the positive electrode is the above-mentioned active material.

[0016] According to the first aspect of the present invention, a manganese-rich core layer and an iron-rich wrapping layer are formed, and a transition layer with a manganese content between the core layer and the wrapping layer is formed, so that the manganese content in the active particles is distributed in a gradient decreasing pattern, thereby destroying the long-range order of manganese in the active material, thereby inhibiting the Jahn-Teller synergistic effect and manganese dissolution, and improving the structural stability of the material. In addition, the manganese-rich region has an "unconventional" Jahn-Teller effect, which helps to alleviate the closure of sodium ion channels caused by pyrophosphate distortion and improve the diffusion capacity of sodium ions. Doping high-valent elements in the manganese-rich core layer can inhibit sodium-manganese mixing and solve the voltage hysteresis problem. Doping electrochemically inert elements in the iron-rich wrapping layer can improve the conductivity of the material by adjusting the internal charge distribution. In addition, the pinning effect of the doped element can also be used to inhibit pyrophosphate distortion and improve the structural stability and ionic conductivity of the material. Moreover, through lattice reconstruction and stress regulation, the internal strain of the material can be suppressed, the structural stability of the material can be improved, and the electronic and ionic conductivity of the material can be improved at the same time.

[0017] According to the second aspect of the present invention, a multi-step co-precipitation method is first used to synthesize a hydrogen manganese ferrophosphate precursor with a gradient distribution of iron and manganese elements inside the particles and dual doping inside and outside. The precursor is then mixed with a sodium source, additives, etc. through a sand milling-spraying-calcination process to prepare a composite sodium manganese ferrophosphate positive electrode material, which helps to form a gradient and helps to inhibit side reactions.

[0018] 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.

[0019] 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

[0020] Figure 1 It is a schematic diagram of the structure of active particles shown in Example 1 of the present invention; Figure 2 This is a flow chart of a method for preparing a sodium ion positive electrode active material as shown in Example 1 of the present invention; Figure 3 This is an electron microscope image of the active particles shown in Example 1 of the present invention; Figure 4 This is an X-ray diffraction analysis spectrum of the positive electrode active material of the sodium ion battery shown in Example 1 of the present invention; Figure 5 This is a charge and discharge curve diagram of the sodium ion battery shown in Example 1 of the present invention at a rate of 0.1C; Figure 6 This is a charge and discharge curve diagram of the sodium ion battery shown in Comparative Example 1 of the present invention at a rate of 0.1C.

[0021] Legend: 1-core layer, 2-transition layer, 3-wrapping layer, 4-carbon layer. DETAILED DESCRIPTION

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] Please refer to Figure 1 , the positive electrode active material of the sodium ion battery protected by the present invention application is composed of active particles, and the active particles include 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. The chemical formula of the main component of the core layer 1 is Na 4 Mn x-a Fe 3-x M a (PO 4 ) 2 P 2 O 7 , 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. The chemical formula of the main component of the transition layer 2 is Na 4 Mn y Fe 3-y (PO 4 ) 2 P 2 O 7 , where 0.5 ≤ y ≤ 2, and y < x. The chemical formula of the main component of the coating layer 3 is Na 4 Fe 3-b N b (PO 4 ) 2 P 2 O 7 , 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 coating layer 3. The main component of the carbon layer 4 is carbon element. In the present invention, x can be, for example, any value among 1, 1.5, 2, 2.5, and 3, a can be, for example, any value among 0.03, 0.05, 0.07, 0.09, and 0.1, y can be, for example, 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, for example, any value among 0.03, 0.05, 0.07, 0.09, and 0.1.

[0027] A manganese-rich core layer 1 and an iron-rich wrapping layer 3 are formed in the active particles, and a transition layer 2 with a manganese content between the core layer 1 and the wrapping layer 3 is formed, so that the manganese content in the active particles is distributed in a gradient decreasing pattern, thereby destroying the long-range order of manganese in the active material, thereby inhibiting the Jahn-Teller synergistic effect and manganese dissolution, and improving the structural stability of the material. In addition, the manganese-rich region has an "unconventional" Jahn-Teller effect, which helps to alleviate the closure of sodium ion channels caused by pyrophosphate distortion and improve the diffusion capacity of sodium ions. Doping high-valent elements in the manganese-rich core layer 1 can inhibit sodium-manganese mixing and solve the voltage hysteresis problem. The electrochemically inert elements doped in the iron-rich wrapping layer 3 can improve the conductivity of the material by adjusting the internal charge distribution. In addition, the pinning effect of the doped elements can also be used to inhibit pyrophosphate distortion and improve the structural stability and ionic conductivity of the material. Moreover, through lattice reconstruction and stress regulation, the internal strain of the material can be suppressed, the structural stability of the material can be improved, and the electronic and ionic conductivity of the material can be improved.

[0028] In some embodiments, the first doping element is one or more of Mo, Ti, V, Zr, W, Nb, Y, and Ce.

[0029] In some embodiments, the potential of the second doping element relative to the sodium element should be less than 1.8V or greater than 4.3V, for example, any value among 1.2V, 1.5V, 1.7V, 4.4V, 4.5V, and 4.8V. 2+ / Fe 3+ The voltage platform is around 2.9V, Mn 2+ / Mn 3+ The voltage platform is around 3.8V. According to the material voltage platform and combined with the charge and discharge efficiency, 1.8V to 4.3V is selected as the voltage window. An element whose potential relative to the sodium element is not in the voltage window is selected as the second doping element. The pinning effect that does not participate in the redox reaction is utilized, which helps to improve the stability of the material structure.

[0030] In some embodiments, the second doping element is one or more of Ni, Mg, Cu, Co, Al, Cr, Zn, and La.

[0031] In some embodiments, the molar ratio of the core layer 1, the transition layer 2 and the encapsulating layer 3 in the active particles is any value of 1:(0.05-2):(0.05-2), for example, it can be any value of 1:0.05:0.05, 1:0.05:1, 1:1:0.5, 1:1:1 and 1:1.6:2, which helps to form a uniform concentration gradient.

[0032] 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 carbon layer 4 is any value in the range of 100:(0.1-10), for example, it can be any value in the range of 100:0.1, 100:0.5, 100:1, 100:5 and 100:10. The carbon layer 4 helps to improve the conductivity of the active material and protect the internal structure with strong chemical activity.

[0033] In a second aspect, the present invention also provides a method for preparing the above active material, comprising: 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 the manganese ions, ferrous ions, first doping element ions and hydrogen phosphate ions react in a molar ratio of (xa):(3-x):a:3 to generate hydrogen phosphate precipitate, thereby obtaining a first precursor.

[0034] 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, the manganese ions, ferrous ions and hydrogen phosphate ions react in a molar ratio of y: (3-y): 3 to generate hydrogen phosphate wrapped in the outer layer of the first precursor, and a solid second precursor is obtained by separation.

[0035] S3. 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, the ferrous ions, second doping element ions and hydrogen phosphate ions react in a molar ratio of (3-b):b:3 to generate hydrogen phosphate wrapped in the outer layer of the second precursor, and a solid third precursor is obtained by separation.

[0036] S4. Dispersing a third precursor, a phosphorus source, a sodium source and an organic carbon source in a dispersant, mechanically activating the mixture by a sand grinder to obtain a slurry, spray-drying the slurry to obtain a powder, pre-calcining the slurry at a first temperature under the protection of an inert gas, and then heating the mixture to a second temperature for sintering, so that the third precursor reacts with the phosphorus source and the sodium source to be converted into phosphoric acid pyrophosphate to obtain an active material.

[0037] Through a multi-step co-precipitation method, a hydrogen manganese ferrophosphate precursor with a gradient distribution of iron and manganese elements inside the particles and dual doping inside and outside is first synthesized, and then the precursor is mixed with a sodium source, additives, etc. through a sand milling-spraying-calcination process to prepare a composite sodium manganese ferrophosphate positive electrode material, which helps to form a gradient and helps to inhibit side reactions.

[0038] In some embodiments, the ferrous ions are derived from one or more of ferrous chloride, ferrous sulfate, and ferrous nitrate; The manganese ion is derived from one or more of manganese chloride, manganese dihydrogen phosphate, manganese sulfate, manganese nitrate, and manganese acetate; The first doping element ion is derived 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; The second doping element ion is derived 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; The ammonium phosphate salt is one or more of triammonium phosphate, diammonium phosphate, and diammonium hydrogen phosphate; the phosphorus source is one or more of phosphoric acid, triammonium phosphate, diammonium hydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and 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, and sodium pyrophosphate; The organic carbon source is one or more of glucose, sucrose, oxalic acid, ascorbic acid, tartaric acid, citric acid, and oleic acid; The dispersant is one or more of ethanol, water, acetone and isopropanol.

[0039] The phosphorus source can be a sodium salt, i.e. sodium dihydrogen phosphate, disodium hydrogen phosphate and sodium phosphate, and the sodium source can also be sodium dihydrogen phosphate, disodium hydrogen phosphate and sodium phosphate containing phosphorus element, with the sodium ions ionized from the phosphorus source and the sodium source as the additional sodium ions, and the phosphate, hydrogen phosphate and dihydrogen phosphate ionized from the phosphorus source and the sodium source as the additional phosphorus-containing ions. 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 to 3):1, the sodium content is low, so it is necessary to supplement the additional sodium ions with other sodium sources, and the system may not contain other phosphorus sources.

[0040] In some embodiments, the time of mechanical activation is any value in the range of 1h to 12h, for example, it can be any value in the range of 1h, 3h, 5h, 8h, 10h and 12h. The first temperature is any value in the range of 300°C to 350°C, for example, it can be any value in the range of 300°C, 310°C, 320°C, 330°C, 340°C and 350°C. The time of pre-burning is any value in the range of 1h to 5h, for example, it can be any value in the range of 1h, 2h, 3h, 4h and 5h. The second temperature is any value in the range of 450°C to 600°C, for example, it can be any value in the range of 450°C, 500°C, 550°C and 600°C. The time of sintering is any value in the range of 1h to 24h, for example, it can be any value in the range of 1h, 3h, 5h, 8h, 10h and 12h.

[0041] In a third aspect, the present invention further provides a sodium ion battery, comprising a positive electrode, wherein the active material of the positive electrode is the above-mentioned active material. By improving the stability of the positive electrode active material during charging and discharging, the electrochemical performance of the battery is improved.

[0042] Please refer to the following examples for details.

[0043] Embodiment 1: See also Figure 1 The sodium ion battery positive electrode active material 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 wrapping layer 3 and a carbon layer 4 that are sequentially wrapped around the outer side of the core layer 1. After forming a three-layer hydrogen phosphate structure with a gradient content of manganese and iron elements through multi-step co-precipitation, it is synchronously converted into phosphoric acid pyrophosphate, and the carbon layer 4 is wrapped around the outer layer to enhance conductivity.

[0044] See also Figure 2 In this embodiment, the preparation method of the positive electrode active material of the sodium ion battery includes: 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 the manganese ions, ferrous ions, first doping element ions and hydrogen phosphate ions react in a molar ratio of (xa):(3-x):a:3 to generate hydrogen phosphate precipitate, thereby obtaining a first precursor.

[0045] 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, the manganese ions, ferrous ions and hydrogen phosphate ions react in a molar ratio of y: (3-y): 3 to generate hydrogen phosphate wrapped in the outer layer of the first precursor, and a solid second precursor is obtained by separation.

[0046] S3. 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, the ferrous ions, second doping element ions and hydrogen phosphate ions react in a molar ratio of (3-b):b:3 to generate hydrogen phosphate wrapped in the outer layer of the second precursor, and a solid third precursor is obtained by separation.

[0047] S4. Dispersing a third precursor, a phosphorus source, a sodium source and an organic carbon source in a dispersant, mechanically activating the mixture by a sand grinder to obtain a slurry, spray-drying the slurry to obtain a powder, pre-calcining the slurry at a first temperature under the protection of an inert gas, and then heating the mixture to a second temperature for sintering, so that the third precursor reacts with the phosphorus source and the sodium source to be converted into phosphoric acid pyrophosphate to obtain an active material.

[0048] In each step of each embodiment of the present invention, when weighing and adding each raw material, the molar ratio of ions corresponding to each element in the raw material is controlled to be equal to the preset molar ratio of each ion participating in the reaction or the element in each reactant.

[0049] In the prior art for preparing sodium manganese iron phosphate pyrophosphate, the molar ratio of each ion in the deionized water is made the same or close to the molar ratio required for the reaction by limiting the amount added, so that the reaction can be carried out according to the preset molar ratio. In actual operation, those skilled in the art may weigh some raw materials in moderate excess, so that the corresponding ions in the deionized water are in excess by an appropriate amplitude in order to promote the reaction. Since the excess amplitude is small, there is no obvious effect on the composition and structure of the generated hydrogen phosphate precursor. This excess limit is common knowledge in the art, so it will not be repeated. 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 solution is also within the scope of protection of the present application.

[0050] 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. The molar ratio of each element in each raw material weighed in the present invention 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 diammonium phosphate are weighed respectively at an element molar ratio of Mn:Fe:Mo:P of 2.85:0.1:0.05:3. Under the protection of argon, manganese sulfate, ferrous nitrate and ammonium molybdate are dissolved in deionized water, and diammonium phosphate is added to react to obtain water-insoluble hydrogen phosphate, so that the liquid is converted into a suspension. After the reaction is completed, the first precursor is obtained by suction filtration.

[0051] In step S2, y is taken as 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 and dispersing, ferrous chloride and manganese nitrate are added in turn and stirred and dissolved, and finally ammonium dihydrogen phosphate is added for stirring reaction precipitation. At this time, the hydrogen iron manganese phosphate obtained by the reaction is wrapped in 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.

[0052] In step S3, 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 and dispersing, ferrous sulfate and nickel acetate are added in turn and stirred to dissolve, and finally ammonium dihydrogen phosphate is added for stirring reaction precipitation. At this time, the water-insoluble hydrogen phosphate obtained by the reaction is wrapped in 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.

[0053] In step S4, the phosphorus source is sodium dihydrogen phosphate, the sodium source is sodium carbonate, and the organic carbon source is citric acid. The total molar amount of hydrogen phosphate in the third precursor is used as the molar amount of the third precursor, the sodium ions contained in the phosphorus source and the sodium source are used as additional sodium ions, and the dihydrogen phosphate contained in the phosphorus source is used as additional phosphorus ions. The molar ratio of the third precursor, the additional sodium ions and the additional phosphorus ions participating in the reaction is 3:4:1. According to the reaction molar ratio, the third precursor, the sodium source and the phosphorus source are weighed, dispersed in ethanol together with the organic carbon source, and mechanically activated in a sand mill for 5 hours. The obtained slurry is spray dried to form a powder. The powder is heat treated at 320°C for 2 hours under the protection of argon, and then the temperature is raised to 550°C for heat treatment for 14 hours. At this time, part of the reaction formed by the first precursor in the third precursor is converted into Na 4 Mn 2.85 Fe 0.1 Mo 0.05 (PO 4 ) 2 P 2 O 7 , forming the core layer 1, and the hydrogen iron manganese phosphate formed in step S2 is converted into Na 4 Mn 1.8 Fe 1.2 (PO 4 ) 2 P 2 O 7 , forming transition layer 2, the hydrogen phosphate formed in step S3 is converted into Na 4 Fe 2.9 Ni 0.1 (PO 4 ) 2 P 2 O 7 , forming a wrapping layer 3. The citric acid is wrapped around the third precursor, and after high temperature treatment, a carbon layer 4 is formed which is wrapped outside the wrapping layer 3 and has carbon as the main component. After cooling in the furnace, the positive active material of the sodium ion battery of the present invention is obtained.

[0054] By adjusting the molar amount of the raw materials in each step, the molar ratio of the core layer 1, transition layer 2 and wrapping 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, transition layer 2 and wrapping layer 3 to the mass ratio of the carbon layer 4 is 100:5.

[0055] See also Figure 3 The positive electrode active material of the sodium ion battery in this embodiment is observed under an electron microscope to obtain an electron microscope image, thereby revealing the internal microstructure of the active material.

[0056] See also Figure 4 , the X-ray diffraction analysis spectrum of the positive electrode active material of the sodium ion battery was detected and drawn, and it can be seen that the active material is a pure phase.

[0057] The prepared sodium ion battery positive electrode active material was mixed with the conductive agent acetylene black and the adhesive polyvinylidene fluoride PVDF in a mass ratio of 8:1:1, ground in a mortar until uniform, and then N-methylpyrrolidone NMP was added. The grinding continued for a period of time to finally obtain a uniform black viscous mixed slurry. The ground mixed slurry was placed on aluminum foil and evenly coated with a scraper to form a film of uniform thickness to form a positive electrode. The metal sodium sheet was used as the counter electrode, the glass fiber membrane was used as the diaphragm, and 1 mol / L NaClO 4 / PC propylene carbonate was used as the electrolyte to assemble CR2032 button cells 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.

[0058] See also Figure 5 , the charge and discharge curve of the sodium ion battery at a rate of 0.1C is detected and plotted. It can be seen that the sodium ion battery prepared in this embodiment has a higher specific capacity and a stable voltage platform during the charge and discharge process.

[0059] Embodiment 2: The difference between this embodiment and the first embodiment is that: In step S1 of this embodiment, the first doping element is zirconium, x is 2.5, a is 0.04, ferrous ions are derived from ferrous sulfate, manganese ions are derived from manganese acetate, hydrogen phosphate is derived from diammonium hydrogen phosphate, and the first doping element ions are derived from zirconium nitrate; In step S2 of this embodiment, y is 1.5, ferrous ions are derived from ferrous chloride, manganese ions are derived from manganese sulfate, and hydrogen phosphate is derived from diammonium hydrogen phosphate; In step S3 of this embodiment, the second doping element is lanthanum, the value of b is 0.06, the ferrous ions are derived from ferrous nitrate, the hydrogen phosphate is derived from diammonium hydrogen phosphate, and the second doping element ions are derived from lanthanum nitrate; 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 mechanical activation time is 8 hours, the first temperature is 300° C., the pre-burning time is 5 hours, the second temperature is 500° C., and the sintering time is 20 hours; In this embodiment, the chemical formula of the main component of the core layer 1 is Na 4 Mn 2.46 Fe 0.5 Zr 0.04 (PO 4 ) 2 P 2 O 7 , the chemical formula of the main component of transition layer 2 is Na 4 Mn 1.5 Fe 1.5 (PO 4 ) 2 P 2 O 7 The chemical formula of the main component of the coating layer 3 is Na 4 Fe 2.94 La 0.06 (PO 4 ) 2 P 2 O 7 The molar ratio of the core layer 1, the transition layer 2 and the encapsulating 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 encapsulating layer 3 to the mass ratio of the carbon layer 4 is 100:6.

[0060] Embodiment three: The difference between this embodiment and the first embodiment is that: In step S1 of this embodiment, the first doping element is titanium, x is 1.5, a is 0.08, ferrous ions are derived from ferrous chloride, manganese ions are derived from manganese acetate, and the first doping element ions are derived from tetrabutyl titanate; In step S2 of this embodiment, the value of y is 1.2, the ferrous ions are derived from ferrous sulfate, and the manganese ions are derived from manganese nitrate; In step S3 of this embodiment, the second doping element is aluminum, the value of b is 0.09, the ferrous ions are derived from ferrous nitrate, and the second doping element ions are derived from aluminum nitrate; 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 mechanical activation time is 2 hours, the first temperature is 350° C., the pre-burning time is 5 hours, the second temperature is 600° C., and the sintering time is 24 hours; In this embodiment, the chemical formula of the main component of the core layer 1 is Na 4 Mn 1.42 Fe 1.5 Ti 0.08(PO 4 ) 2 P 2 O 7 , the chemical formula of the main component of transition layer 2 is Na 4 Mn 1.2 Fe 1.8 (PO 4 ) 2 P 2 O 7 The chemical formula of the main component of the coating layer 3 is Na 4 Fe 2.91 Al 0.09 (PO 4 ) 2 P 2 O 7 The molar ratio of the core layer 1, the transition layer 2 and the encapsulating 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 encapsulating layer 3 to the mass ratio of the carbon layer 4 is 100:2.

[0061] Comparative Example 1: The difference between this comparative example and the first embodiment is that steps S2 and S3 are not performed, and step S4 is performed by directly replacing the third precursor with the first precursor, so that the active particles only include the core layer 1 and the carbon layer 4 wrapped around the outside of the core layer 1.

[0062] See also Figure 6 , detect and draw the charge and discharge curve of the sodium ion battery prepared in this comparative example at a rate of 0.1C, and Figure 5 It can be seen that the sodium ion battery prepared in this comparative example has a low specific capacity, poor redox activity during the charge and discharge process, and a short voltage platform, resulting in a low energy density.

[0063] Comparative Example 2: The difference between this comparative example and Example 1 is that S3 is not performed, and the third precursor is directly replaced by the second precursor to perform step S4, so that the active particles only include the core layer 1, the transition layer 2 wrapped around the outside of the core layer 1, and the carbon layer 4 wrapped around the outside of the transition layer 2.

[0064] Comparative Example 3: The difference between this comparative example and Example 1 is that S2 is not performed, and step S3 is performed by directly replacing the second precursor with the first precursor, so that the active particles only include the core layer 1, the wrapping layer 3 wrapped around the outside of the core layer 1, and the carbon layer 4 wrapped around the outside of the wrapping layer 3.

[0065] The electrochemical properties of the positive electrode materials are shown in Table 1 below.

[0066]

[0067] Combining Example 1 and the comparative examples with the data in Table 1, it can be seen that by constructing the positive electrode active material into a multilayer structure with a gradient distribution of iron and manganese content, the discharge specific capacity and cycle efficiency can be effectively improved. In addition, the 0.1C average operating voltage in Example 1 is only slightly lower than that in Comparative Example 1, proving that even if the manganese doping amount in the outer layer material is reduced by the gradient, the operating voltage can still be maintained at a high level, that is, the positive electrode active material obtained in the present invention has the advantages of good cyclicity of doped sodium iron pyrophosphate and high energy density of doped sodium iron manganese pyrophosphate, and has high practicality.

[0068] Combining the various embodiments with the data in Table 1, it can be seen that the higher the manganese doping amount, the greater the gradient, the higher the energy density of the material, and the worse the cyclability.

[0069] 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.

[0070] 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

1. A sodium ion battery positive electrode active material, characterized in that: Composed of active particles, the active particles include: The core layer (1) has a main chemical formula of Na4Mn x-a Fe 3-x M a (PO4)2P2O7, wherein M is a first doping element, 1≤x≤3, and 0≤a≤0.1, and a common valence state of the first doping element is greater than or equal to +3; The transition layer (2) is wrapped around the outside of the core layer (1), and the chemical formula of the main component is Na4Mn y Fe 3-y (PO4)2P2O7, where 0.5≤y≤2, and y <x; The wrapping 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, wherein N is a second doping element, 0≤b≤0.1, and the second doping element is an electrochemically inert element used to stabilize the structure of the encapsulation layer (3); The carbon layer (4) is wrapped around the outer side of the wrapping layer (3), and its main component is carbon element.

2. The active material according to claim 1, characterized in that The first doping element is one or more of Mo, Ti, V, Zr, W, Nb, Y and Ce.

3. The active material according to claim 1, characterized in that The potential of the second doping element relative to the sodium element is less than 1.8V or greater than 4.3V.

4. The active material according to claim 3, characterized in that The second doping element is one or more of Ni, Mg, Cu, Co, Al, Cr, Zn and La.

5. The active material according to claim 1, characterized in that The molar ratio of the core layer (1), the transition layer (2) and the wrapping layer (3) in the active particles is any value in the range of 1:(0.05-2):(0.05-2).

6. 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 wrapping layer (3) to the mass ratio of the carbon layer (4) is 100: any value in the range of (0.1 to 10).

7. The method for preparing the active material according to any one of claims 1 to 6, characterized in that: include: 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 the manganese ions, ferrous ions, first doping element ions and hydrogen phosphate ions react in a molar ratio of (xa): (3-x): a: 3 to generate hydrogen phosphate precipitate, thereby obtaining a first precursor; Under the protection of an inert gas, the first precursor is dispersed in deionized water, and ferrous ions and manganese ions are dissolved therein, and after the ammonium phosphate is added, the manganese ions, ferrous ions and hydrogen phosphate ions react at a molar ratio of y:(3-y):3 to generate hydrogen phosphate wrapped in the outer layer of the first precursor, and a solid second precursor is obtained by separation; 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, and after adding ammonium phosphate, the ferrous ions, second doping element ions and hydrogen phosphate ions react in a molar ratio of (3-b):b:3 to generate hydrogen phosphate wrapped in the outer layer of the second precursor, and a solid third precursor is obtained by separation; The third precursor, phosphorus source, sodium source and organic carbon source are dispersed in a dispersant, and mechanically activated by a sand grinder to obtain a slurry. The powder obtained by spray drying the slurry is pre-fired 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 the sodium source to be converted into phosphate pyrophosphate to obtain the active material.

8. The preparation method according to claim 7, characterized in that: The ferrous ions are derived from one or more of ferrous chloride, ferrous sulfate, and ferrous nitrate; The manganese ions are derived from one or more of manganese chloride, manganese dihydrogen phosphate, manganese sulfate, manganese nitrate, and manganese acetate; The first doping element ion is derived 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; The second doping element ion is derived 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; The ammonium phosphate salt is one or more of triammonium phosphate, diammonium phosphate, and diammonium hydrogen phosphate; The phosphorus source is one or more of phosphoric acid, triammonium phosphate, diammonium phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and 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, and sodium pyrophosphate; The organic carbon source is one or more of glucose, sucrose, oxalic acid, ascorbic acid, tartaric acid, citric acid, and oleic acid; The dispersant is one or more of ethanol, water, acetone and isopropanol.

9. The preparation method according to claim 7, characterized in that: The mechanical activation time is any value between 1h and 12h, the first temperature is any value between 300°C and 350°C, the pre-burning time is any value between 1h and 5h, the second temperature is any value between 450°C and 600°C, and the sintering time is any value between 1h and 24h.

10. A sodium ion battery, characterized in that: A positive electrode is included, and the active material of the positive electrode is the active material according to any one of claims 1 to 6.

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