Composite polyanionic sodium battery positive electrode material as well as preparation method and application thereof

Abstract

The invention provides a composite polyanionic sodium battery positive electrode material as well as a preparation method and application thereof. The preparation method comprises the following steps: (1) mixing a ferrous ion solution, a phosphate ion solution, a pyrophosphate ion solution, an oxidant solution and a precipitator solution, and carrying out a co-precipitation reaction to obtain a ferric sodium pyrophosphate precursor; (2) mixing the ferric pyrophosphate precursor, a sodium source, a biomass carbon source and a pore forming agent, and performing spray drying to obtain a ferric pyrophosphate sodium precursor coated with the biomass carbon source; and (3) sintering the biomass carbon source coated ferric pyrophosphate sodium precursor to obtain the composite polyanionic sodium battery positive electrode material. The NFPP precursor is prepared by a coprecipitation method, and the problems of poor conductivity, low capacity, low multiplying power, high cost and the like of the NFPP positive electrode material can be improved by synergistically matching the effects of the biomass carbon and the pore-forming agent.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries and relates to a composite polyanion sodium battery positive electrode material and a preparation method and application thereof. Background Art

[0002] Sodium-ion batteries have certain advantages in terms of resource abundance and cost, and are the ballast stone for promoting the development of the new energy industry. Among the key materials of sodium-ion batteries, the positive electrode material is one of the main factors determining battery performance and cost. Based on the differences in positive electrode materials, sodium-ion batteries have developed into three major technical routes: layered oxides, polyanion compounds, and Prussian blue compounds. Among them, polyanion compounds have the characteristics of good electrochemical stability, high safety, long cycle life, low cost, and a wide temperature range, making them more suitable for energy storage. Polyanions include phosphates, pyrophosphates, mixed phosphates, and sulfates. Among them, the mixed phosphate sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2(P2O7), abbreviated as NFPP) has a higher voltage platform and has a lower volume change when used as the positive electrode of sodium-ion batteries. This material also has the advantages of being green, low-cost, and having high cycle performance, and is regarded as an energy storage material with great application potential.

[0003] However, this type of material has defects such as poor electronic conductivity, low specific capacity, and poor rate performance, which limits its practical application.

[0004] Therefore, improving capacity, electronic conductivity and rate performance becomes the key to improving the performance of polyanion cathode materials. Summary of the Invention

[0005] To address the shortcomings of the existing technology, the present invention aims to provide a composite polyanionic sodium cathode material, its preparation method, and its application. This invention utilizes a coprecipitation method to prepare a NFPP precursor, which, in synergistically with biomass carbon and a pore-forming agent, can address the poor conductivity, low capacity, low rate capability, and high cost of NFPP cathode materials.

[0006] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a method for preparing a composite polyanionic sodium cathode material, the preparation method comprising the following steps:

[0008] (1) mixing a ferrous ion solution, a phosphate ion solution, a pyrophosphate ion solution, an oxidant solution, and a precipitant solution to perform a coprecipitation reaction to obtain a sodium ferric pyrophosphate precursor;

[0009] (2) mixing a phosphate iron pyrophosphate precursor, a sodium source, a biomass carbon source, and a pore-forming agent, and spray drying the mixture to obtain a biomass carbon source-coated sodium phosphate iron pyrophosphate precursor;

[0010] (3) Sintering the biomass carbon source-coated sodium iron pyrophosphate precursor to obtain the composite polyanion sodium cathode material.

[0011] The present invention first uses an aqueous solution co-precipitation method to synthesize a sodium iron phosphate pyrophosphate (NFPP) precursor. Compared with the widely used pure solid phase, the element distribution can be more uniform, the impurity phase can be effectively suppressed, and the performance of the positive electrode material, such as capacity, is optimized. Further, the NFPP precursor is synergistically combined with a biomass carbon source and a pore-forming agent to improve the problems of poor conductivity, low capacity and high cost of NFPP, without affecting the electrochemical properties of the polyanionic sodium cathode material itself. The preparation method, preparation sequence and preparation raw materials cooperate with each other to suppress the impurity phase and improve the conductivity, and optimize the uniformity and stability of the material. At the same time, it can also effectively reduce the internal resistance of the battery and increase the transmission rate of electrons and sodium ions, thereby significantly improving the rate performance of the polyanionic sodium cathode material. At the same time, the biomass carbon source also has the advantages of abundant resources, environmental friendliness and low price, which reduces production costs and improves overall performance.

[0012] Biomass carbon sources have the advantages of good conductivity, porosity, abundant resources, environmental friendliness and low price. Under the action of pore-forming agents, they act on the NFPP precursor prepared by co-precipitation method. The pore-forming agents increase the porosity of the carbon layer and improve the conductivity. After subsequent sintering, the obtained composite polyanion sodium cathode material has good conductivity and porosity.

[0013] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved and realized.

[0014] Preferably, the mixing in step (1) comprises: first mixing a ferrous ion solution and a phosphate ion solution to obtain a first mixed solution, then adding the first mixed solution, a pyrophosphate ion solution and an oxidant solution in parallel, and using a precipitant solution to adjust the pH value during the co-precipitation reaction.

[0015] The present invention does not impose any special restrictions on the specific types of materials used in step (1). Any conventional material that can provide the corresponding required ions to be reacted and can prepare the NFPP precursor is applicable to the present invention.

[0016] For example, the ferrous ion solution includes but is not limited to ferrous sulfate heptahydrate solution; the phosphate ion solution includes but is not limited to at least one of phosphoric acid solution, ammonium dihydrogen phosphate solution or diammonium hydrogen phosphate solution; the pyrophosphate ion solution includes but is not limited to sodium pyrophosphate, etc.; the oxidant solution includes but is not limited to hydrogen peroxide solution, etc.; the precipitant solution includes but is not limited to sodium hydroxide solution and / or potassium hydroxide solution, etc.

[0017] During the coprecipitation reaction process, the present invention first performs a first mixing of a ferrous ion solution and a phosphate ion solution, then adds other raw materials in parallel, and controls the pH value of the desired reaction by using a precipitant solution. The above preparation sequence is more conducive to preventing the oxidation of ferrous ions.

[0018] Preferably, the feed flow rate of the first mixed solution is 3 to 35 L / h, for example, 3 L / h, 5 L / h, 10 L / h, 13 L / h, 15 L / h, 18 L / h, 20 L / h, 23 L / h, 25 L / h, 28 L / h, 30 L / h, 33 L / h or 35 L / h, etc., but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0019] Preferably, the feed flow rate of the pyrophosphate ion solution is 5 to 50 L / h, for example, 5 L / h, 8 L / h, 10 L / h, 13 L / h, 15 L / h, 18 L / h, 20 L / h, 23 L / h, 25 L / h, 28 L / h, 30 L / h, 33 L / h, 35 L / h, 38 L / h, 40 L / h, 43 L / h, 45 L / h, 48 L / h or 50 L / h, etc., but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0020] Preferably, the feed flow rate of the oxidant solution is 1 to 10 L / h, for example, 1 L / h, 2 L / h, 3 L / h, 4 L / h, 5 L / h, 6 L / h, 7 L / h, 8 L / h, 9 L / h or 10 L / h, etc., but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0021] Preferably, the pH value during the coprecipitation reaction in step (1) is 1.6 to 3.5, for example, 1.6, 1.8, 2, 2.3, 2.5, 2.8, 3, 3.3 or 3.5, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0022] Preferably, the reaction temperature during the coprecipitation reaction in step (1) is 40 to 90°C, for example, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C, etc., but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0023] Preferably, the stirring rate during the coprecipitation reaction in step (1) is 180 to 300 r / min, for example, 180 r / min, 200 r / min, 230 r / min, 250 r / min, 280 r / min or 300 r / min, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0024] Preferably, after the coprecipitation reaction in step (1) is completed, the reaction slurry is sequentially subjected to solid-liquid separation and washing to obtain a sodium iron pyrophosphate precursor.

[0025] Preferably, in the sodium iron phosphate pyrophosphate precursor of step (1), the molar ratio of Fe to P is (2.7 to 3.1):4, for example, 2.7:4, 2.8:4, 2.9:4, 3:4 or 3.1:4, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0026] Preferably, the sodium iron pyrophosphate precursor obtained in step (1) of the present invention is nano-scale precursor particles.

[0027] It should be noted that the nanoscale in the present invention means that the average particle size of the particles reaches the nanoscale.

[0028] Preferably, the amount of the biomass carbon source added in step (2) is 10% to 25% of the mass of the ferric phosphate pyrophosphate precursor, for example, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24% or 25%, etc., but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0029] Preferably, the amount of the pore-forming agent added in step (2) is 3% to 12% of the mass of the biomass carbon source, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11% or 12%, etc., but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0030] In the present invention, the addition amount of a suitable biomass carbon source and the addition amount of a suitable pore-forming agent can effectively improve the conductivity and rate performance; after exploration, it is preferred that the addition amount of the biomass carbon source is 10% to 25% of the mass of the ferric phosphate pyrophosphate precursor and / or the addition amount of the pore-forming agent is 3% to 12% of the mass of the biomass carbon source, which can better improve the problems of poor conductivity, low capacity, low rate and high cost.

[0031] In addition, in step (2) of the present invention, a sodium source is added as a supplement. After the sodium source is added, the molar ratio of Na:Fe:P in the final product is 4:(2.7-3.14):4, for example, 4:2.7:4, 4:2.8:4, 4:2.9:4, 4:3:4, 4:3.1:4 or 4:3.14:4, etc.

[0032] The sodium source includes but is not limited to at least one of sodium carbonate, sodium pyrophosphate, sodium hydroxide, sodium oxide, sodium peroxide or sodium oxalate.

[0033] Preferably, the mixing method in step (2) comprises wet slurry ball milling.

[0034] Furthermore, the wet slurry ball milling method includes: in the mixing process of step (2), in addition to the ferric phosphate pyrophosphate precursor, the sodium source, the biomass carbon source and the pore-forming agent, a conventional solvent is additionally added to perform wet slurry ball milling to obtain a ball-milled slurry.

[0035] The conventional solvents include but are not limited to water and / or anhydrous ethanol.

[0036] Preferably, during the wet slurry ball milling process, the ball milling speed can be 200-400 rpm, for example, 200 rpm, 250 rpm, 300 rpm, 350 rpm or 400 rpm, and the ball-to-material ratio can be 10:1-20:1, for example, 10:1, 13:1, 15:1, 18:1 or 20:1, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0037] Preferably, the spray drying temperature is 350-550°C, such as 350°C, 400°C, 450°C, 500°C or 550°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0038] In the present invention, after spray drying, the biomass carbon source is not carbonized, but is attached and coated on the surface of the single crystal particles of the ferric phosphate pyrophosphate precursor together with the sodium source and the pore-forming agent.

[0039] Preferably, the heating rate of the sintering in step (3) is 1 to 15°C / min, for example, 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, 11°C / min, 12°C / min, 13°C / min, 14°C / min or 15°C / min, etc., but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0040] Preferably, the holding temperature of the sintering in step (3) is 480-600°C, for example, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C or 600°C, etc., but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0041] Preferably, the holding time for sintering in step (3) is 5 to 15 hours, for example, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours or 15 hours, etc., but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0042] It should be noted that:

[0043] The present invention does not specifically limit the specific source of the biomass carbon source, and the present invention is applicable to conventional types of biomass carbon sources, such as at least one of corn stalks, peanut shells, bamboo powder or rice husks.

[0044] The pore-forming agent is also a conventional technical solution, such as potassium hydroxide and / or sodium carbonate.

[0045] In the second aspect, the present invention provides a composite polyanionic sodium cathode material, which is prepared by the preparation method described in the first aspect; the composite polyanionic sodium cathode material includes secondary particles formed by the accumulation of primary particles, and the primary particles include a sodium ferric phosphate pyrophosphate base material and an amorphous carbon layer coated on the surface of the sodium ferric phosphate pyrophosphate base material.

[0046] Preferably, the chemical formula of the composite polyanion sodium cathode material is Na4Fe x (PO4)2(P2O7)@C, in fact, 2.5≤x≤3, for example, 2.5, 2.6, 2.7, 2.8, 2.9 or 3, etc., but it is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0047] In a third aspect, the present invention further provides a sodium ion battery, which comprises the composite polyanion sodium cathode material as described in the second aspect.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] The present invention first uses an aqueous solution co-precipitation method to synthesize a sodium iron phosphate pyrophosphate (NFPP) precursor. Compared with the widely used pure solid phase, the element distribution can be more uniform, the impurity phase can be effectively suppressed, and the performance of the positive electrode material, such as capacity, is optimized. Further, the NFPP precursor is synergistically combined with a biomass carbon source and a pore-forming agent to improve the problems of poor conductivity, low capacity and high cost of NFPP, without affecting the electrochemical properties of the polyanionic sodium cathode material itself. The preparation method, preparation sequence and preparation raw materials cooperate with each other to suppress the impurity phase and improve the conductivity, and optimize the uniformity and stability of the material. At the same time, it can also effectively reduce the internal resistance of the battery and increase the transmission rate of electrons and sodium ions, thereby significantly improving the rate performance of the polyanionic sodium cathode material. At the same time, the biomass carbon source also has the advantages of abundant resources, environmental friendliness and low price, which reduces production costs and improves overall performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 and Figure 2 These are SEM images of the NFPP precursor prepared in step S1 of Example 1 at different magnifications. DETAILED DESCRIPTION

[0051] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0052] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in this application are intended to cover non-exclusive inclusions.

[0053] In the description of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly indicate the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0054] Example 1

[0055] This embodiment provides a method for preparing a composite polyanion sodium cathode material, the preparation method being as follows:

[0056] S1: Solution preparation: Dissolve 14 mol of ferrous sulfate heptahydrate in 13 L of deionized water to obtain ferrous ion precursor solution A; dissolve 9 mol of phosphoric acid solution in 5 L of deionized water to obtain phosphate ion precursor solution B; dissolve 6 mol of sodium pyrophosphate in 20 L of deionized water to obtain pyrophosphate precursor solution C; dilute 9 mol of hydrogen peroxide solution to 5 L to obtain an oxidant solution to obtain an oxidant precursor solution D; dissolve 5 mol of sodium hydroxide in 9 L of deionized water to obtain a pH adjuster solution to obtain a precipitant precursor solution E;

[0057] First, the precursor solution A and the precursor solution B were mixed evenly, which was defined as the precursor solution M. 5 L of M was taken as the reaction base liquid. Then, the precursor solution M, the precursor solution C, and the precursor solution D were fed into the reactor at flow rates of 8 L / h, 5 L / h, and 3 L / h, respectively, and the coprecipitation reaction was carried out at a stirring speed of 300 r / min and a temperature of 40° C. The pH in the reactor was monitored. When the pH was less than 1.6, the precursor solution E was added dropwise to adjust the pH in the reactor to be maintained between 3.0. When a suspension was formed in the reactor, a sodium iron pyrophosphate precursor (NFPP precursor) with a nanometer particle size was prepared.

[0058] The suspension in the reactor was washed and dehydrated in a centrifuge to obtain a precipitate, i.e., NFPP precursor, with a Fe:P molar ratio of 2.9:4;

[0059] S2: 300 g of the NFPP precursor obtained by the above coprecipitation was transferred to a ball mill, and 3 mol of a sodium source was added according to a molar ratio of Na:Fe:P of 4:2.9:4. 15 L of deionized water was added, and then 60 g of bamboo powder and 5 g of potassium hydroxide were added for ball milling and dispersion. The ball-milled mixed slurry was transferred to a spray dryer for spray granulation and drying to obtain nanometer-sized NFPP-1 single crystal particles.

[0060] S3: NFPP-1 was sintered at a heating rate of 5℃ / min, a holding temperature of 550℃, and a holding time of 10h, so that sodium was embedded in the crystal nucleus and the carbon source was also carbonized to form a conductive carbon layer, forming a high conductivity and high rate Na4Fe 2.9 (PO4)2(P2O7)@C composite polyanion sodium cathode material;

[0061] The composite polyanionic sodium cathode material includes secondary particles formed by the accumulation of primary particles, and the primary particles include a sodium ferric phosphate pyrophosphate base material and an amorphous carbon layer coated on the surface of the sodium ferric phosphate pyrophosphate base material.

[0062] Figure 1 and Figure 2 The SEM images of the NFPP precursor prepared in step S1 of Example 1 at different magnifications are shown. Figure 1 and Figure 2 It can be seen that the precursor particles provided in this embodiment have a smaller particle size, which is more conducive to subsequent carbon layer coating.

[0063] Example 2

[0064] This embodiment provides a method for preparing a composite polyanion sodium cathode material, the preparation method being as follows:

[0065] S1: Solution preparation: Dissolve 14 mol of ferrous sulfate heptahydrate in 13 L of deionized water to obtain ferrous ion precursor solution A; dissolve 9 mol of phosphoric acid solution in 5 L of deionized water to obtain phosphate ion precursor solution B; dissolve 6 mol of sodium pyrophosphate in 20 L of deionized water to obtain pyrophosphate precursor solution C; dilute 9 mol of hydrogen peroxide solution to 5 L to obtain an oxidant solution to obtain an oxidant precursor solution D; dissolve 5 mol of sodium hydroxide in 9 L of deionized water to obtain a pH adjuster solution to obtain a precipitant precursor solution E;

[0066] First, the precursor solution A and the precursor solution B were mixed evenly, which was defined as the precursor solution M. 5 L of M was taken as the reaction base liquid. Then, the precursor solution M, the precursor solution C, and the precursor solution D were fed into the reactor at flow rates of 3 L / h, 8 L / h, and 1 L / h, respectively, and a co-precipitation reaction was carried out at a stirring speed of 180 r / min and a temperature of 60° C. The pH in the reactor was monitored. When the pH was less than 1.6, the precursor solution E was added dropwise to adjust the pH in the reactor to be maintained between 2.4. A suspension was formed in the reactor to prepare a sodium iron pyrophosphate precursor (NFPP precursor) with a nanometer particle size.

[0067] The suspension in the reactor was washed and dehydrated in a centrifuge to obtain a precipitate, i.e., NFPP precursor, with a Fe:P molar ratio of 2.9:4;

[0068] S2: 300 g of the NFPP precursor obtained by the above coprecipitation was transferred to a ball mill, and 3 mol of a sodium source was added according to a molar ratio of Na:Fe:P of 4:2.9:4, and 15 L of deionized water was added. Then, 30 g of bamboo powder and 0.9 g of potassium hydroxide were added for ball milling dispersion; the mixed slurry uniformly dispersed by ball milling was transferred to a spray dryer for spray granulation and drying to obtain NFPP-1 single crystal particles with a particle size of nanometers. Scanning electron microscopy showed Figure 1 ;

[0069] S3: NFPP-1 was sintered at a heating rate of 1°C / min, a holding temperature of 480°C, and a holding time of 150h, so that sodium was embedded in the crystal nucleus and the carbon source was also carbonized to form a conductive carbon layer, forming a high conductivity and high rate Na4Fe 2.9 (PO4)2(P2O7)@C composite polyanion sodium cathode material;

[0070] The composite polyanionic sodium cathode material includes secondary particles formed by the accumulation of primary particles, and the primary particles include a sodium ferric phosphate pyrophosphate base material and an amorphous carbon layer coated on the surface of the sodium ferric phosphate pyrophosphate base material.

[0071] Example 3

[0072] This embodiment provides a method for preparing a composite polyanion sodium cathode material, the preparation method being as follows:

[0073] S1: Solution preparation: Dissolve 14 mol of ferrous sulfate heptahydrate in 13 L of deionized water to obtain ferrous ion precursor solution A; dissolve 9 mol of phosphoric acid solution in 5 L of deionized water to obtain phosphate ion precursor solution B; dissolve 6 mol of sodium pyrophosphate in 20 L of deionized water to obtain pyrophosphate precursor solution C; dilute 9 mol of hydrogen peroxide solution to 5 L to obtain an oxidant solution to obtain an oxidant precursor solution D; dissolve 5 mol of sodium hydroxide in 9 L of deionized water to obtain a pH adjuster solution to obtain a precipitant precursor solution E;

[0074] First, the precursor solution A and the precursor solution B were mixed evenly, which was defined as the precursor solution M. 5 L of M was taken as the reaction base liquid. Then, the precursor solution M, the precursor solution C, and the precursor solution D were fed into the reactor at flow rates of 35 L / h, 50 L / h, and 10 L / h, respectively, and the co-precipitation reaction was carried out at a stirring speed of 250 r / min and a temperature of 90° C. The pH in the reactor was monitored. When the pH was less than 1.6, the precursor solution E was added dropwise to adjust the pH in the reactor to be maintained between 1.6. A suspension was formed in the reactor to prepare a sodium iron pyrophosphate precursor (NFPP precursor) with a nanometer particle size.

[0075] The suspension in the reactor was washed and dehydrated in a centrifuge to obtain a precipitate, i.e., NFPP precursor, with a Fe:P molar ratio of 2.9:4;

[0076] S2: 300 g of the NFPP precursor obtained by the above coprecipitation was transferred to a ball mill, and 3 mol of a sodium source was added according to a molar ratio of Na:Fe:P of 4:2.9:4, and 15 L of deionized water was added. Then, 75 g of bamboo powder and 9 g of potassium hydroxide were added for ball milling dispersion; the ball-milled mixed slurry was transferred to a spray dryer for spray granulation and drying to obtain nano-sized NFPP-1 single crystal particles. Scanning electron microscopy showed that Figure 1 ;

[0077] S3: NFPP-1 was sintered at a heating rate of 5℃ / min, a holding temperature of 550℃, and a holding time of 10h, so that sodium was embedded in the crystal nucleus and the carbon source was also carbonized to form a conductive carbon layer, forming a high conductivity and high rate Na4Fe 2.9 (PO4)2(P2O7)@C composite polyanion sodium cathode material;

[0078] The composite polyanionic sodium cathode material includes secondary particles formed by the accumulation of primary particles, and the primary particles include a sodium ferric phosphate pyrophosphate base material and an amorphous carbon layer coated on the surface of the sodium ferric phosphate pyrophosphate base material.

[0079] Example 4

[0080] The difference between this embodiment and embodiment 1 is that in step S1 of this embodiment, the molar ratio of Fe:P in the NFPP precursor is 2.7:4;

[0081] The carbon source in step S2 is corn stalks.

[0082] The chemical formula of the prepared composite polyanion sodium cathode material is Na4Fe 2.7 (PO4)2(P2O7)@C.

[0083] The rest of the preparation methods and parameters were the same as those in Example 1.

[0084] Example 5

[0085] The difference between this embodiment and embodiment 1 is that in step S1 of this embodiment, the pH value of the coprecipitation reaction is regulated to 4.

[0086] The rest of the preparation methods and parameters were the same as those in Example 1.

[0087] Example 6

[0088] The difference between this embodiment and embodiment 1 is that in step S2 of this embodiment, the amount of bamboo powder added is 90 g, which is 30% of the mass of the precursor.

[0089] The rest of the preparation methods and parameters were the same as those in Example 1.

[0090] Example 7

[0091] The difference between this embodiment and embodiment 1 is that in step S2 of this embodiment, the amount of bamboo powder added is 15 g, which is 5% of the mass of the precursor.

[0092] The rest of the preparation methods and parameters were the same as those in Example 1.

[0093] Example 8

[0094] The difference between this embodiment and embodiment 1 is that in step S2 of this embodiment, the amount of potassium hydroxide added as a pore-forming agent is 9 g, which is 15% of the mass of the bamboo powder.

[0095] The rest of the preparation methods and parameters were the same as those in Example 1.

[0096] Example 9

[0097] The difference between this embodiment and embodiment 1 is that in step S2 of this embodiment, the amount of potassium hydroxide added as a pore-forming agent is 0.6 g, which is 1% of the mass of the bamboo powder.

[0098] The rest of the preparation methods and parameters were the same as those in Example 1.

[0099] Comparative Example 1

[0100] This comparative example provides a method for preparing a composite polyanion sodium cathode material, and the preparation method is as follows:

[0101] 6.5 mol of phosphoric acid, 6 mol of ferric acetate, 8 mol of NaH2PO4·2H2O, 60 g of bamboo powder, 5 g of potassium hydroxide and deionized water were ball-milled for 8 h, and then spray-dried at 150 °C. The dried yellow powder was calcined at 550 °C for 10 h at a heating rate of 5 °C / min, and then cooled and crushed to obtain Na4Fe 2.9 (PO4)2(P2O7)@C composite polyanion sodium battery positive electrode material.

[0102] Comparative Example 2

[0103] The difference between this comparative example and Example 1 is that the carbon source in step S2 of this comparative example is glucose.

[0104] The rest of the preparation methods and parameters were the same as those in Example 1.

[0105] Comparative Example 3

[0106] The difference between this comparative example and Example 1 is that in step S2 of this comparative example, potassium hydroxide as a pore-forming agent is not added.

[0107] The rest of the preparation methods and parameters were the same as those in Example 1.

[0108] Comparative Example 4

[0109] The difference between this comparative example and Example 1 is that in step S2 of this comparative example, neither bamboo powder nor pore-forming agent potassium hydroxide is added.

[0110] The rest of the preparation methods and parameters were the same as those in Example 1.

[0111] [Battery preparation and performance testing]

[0112] I. Battery Preparation

[0113] Preparation of the positive electrode sheet: The positive electrode materials, conductive agent additive Super P and binder polyvinylidene fluoride (PVDF) provided in Examples 1-9 and Comparative Examples 1-3 were weighed in a mass ratio of 8:1:1, added to a mortar and ground and mixed, the resulting material was transferred to a glass bottle and N-methylpyrrolidone (NMP) was added as a solvent, and a magnetic stirring rotor was added and stirred for 20 hours to obtain a positive electrode slurry; the positive electrode slurry was coated on an aluminum foil using a coater with a coating thickness of 150 μm, and the aluminum foil coated with the positive electrode slurry was transferred to a vacuum oven at 120°C and dried for 8 hours. It was cut into electrode discs with a diameter of 12 mm using a pole piece punch, and the amount of active material of each pole piece was weighed using a high-precision balance, and then transferred to an argon glove box to obtain a positive electrode sheet;

[0114] 2) Button-type half-cell assembly: According to the assembly order of positive electrode shell - positive electrode disc - electrolyte - glass fiber separator - electrolyte - sodium sheet - circular nickel mesh - negative electrode shell, stack them neatly with the center of the circle as the axis, and finally complete the sealing work with an electric button-type battery sealing machine.

[0115] II Performance Test

[0116] The batteries provided in Examples 1-9 and Comparative Examples 1-4 were placed in a test box at 25° C., and their electrochemical properties were tested on a LAND battery testing station at a test voltage range of 1.5-4.0V.

[0117] Capacity and cycle: Charge and discharge tests were conducted at a rate of 0.1C. The discharge capacity of the first cycle was taken as the capacity value, and the capacity retention rate after 300 cycles was taken as the cycle performance.

[0118] The test results of the above tests are shown in Table 1.

[0119] Table 1

[0120]

[0121]

[0122] In summary, the present invention first uses an aqueous solution co-precipitation method to synthesize a sodium iron phosphate pyrophosphate NFPP precursor, which can make the element distribution more uniform compared with the widely used pure solid phase, the impurity phase can be effectively suppressed, and the performance of the positive electrode material such as capacity is optimized; further, the NFPP precursor is combined with a biomass carbon source and a pore-forming agent to improve the problems of poor conductivity, low capacity and high cost of NFPP, without affecting the electrochemical properties of the polyanionic sodium cathode material itself; the preparation method, preparation sequence and preparation raw materials work together to suppress impurities and improve conductivity, and optimize the uniformity and stability of the material, while also effectively reducing the internal resistance of the battery and increasing the transmission rate of electrons and sodium ions, thereby significantly improving the rate performance of the polyanionic sodium cathode material; at the same time, the biomass carbon source also has the advantages of abundant resources, environmental friendliness and low price, which reduces production costs and improves overall performance.

[0123] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a composite polyanion sodium cathode material, characterized in that: The preparation method comprises the following steps: (1) mixing a ferrous ion solution, a phosphate ion solution, a pyrophosphate ion solution, an oxidant solution, and a precipitant solution to perform a coprecipitation reaction to obtain a sodium ferric pyrophosphate precursor; (2) mixing a phosphate iron pyrophosphate precursor, a sodium source, a biomass carbon source, and a pore-forming agent, and spray drying the mixture to obtain a biomass carbon source-coated sodium phosphate iron pyrophosphate precursor; (3) Sintering the biomass carbon source-coated sodium iron pyrophosphate precursor to obtain the composite polyanion sodium cathode material.

2. The preparation method according to claim 1, characterized in that The mixing in step (1) comprises: first mixing a ferrous ion solution and a phosphate ion solution to obtain a first mixed solution, then adding the first mixed solution, a pyrophosphate ion solution and an oxidant solution in parallel, and using a precipitant solution to adjust the pH value during the coprecipitation reaction.

3. The preparation method according to claim 2, characterized in that The feed flow rate of the first mixed solution is 3 to 35 L / h; Preferably, the feed flow rate of the pyrophosphate ion solution is 5 to 50 L / h; Preferably, the feed flow rate of the oxidant solution is 1 to 10 L / h.

4. The preparation method according to claim 1 or 2, characterized in that The pH value during the coprecipitation reaction in step (1) is 1.6 to 3.5; Preferably, the reaction temperature during the coprecipitation reaction in step (1) is 40-90°C; Preferably, the stirring rate during the coprecipitation reaction in step (1) is 180 to 300 r / min; Preferably, after the coprecipitation reaction in step (1) is completed, the reaction slurry is sequentially subjected to solid-liquid separation and washing to obtain a sodium iron pyrophosphate precursor.

5. The preparation method according to claim 1, characterized in that In the sodium iron phosphate pyrophosphate precursor of step (1), the molar ratio of Fe to P is (2.7-3.1):

4.

6. The preparation method according to claim 1, characterized in that The amount of the biomass carbon source added in step (2) is 10% to 25% of the mass of the ferric phosphate pyrophosphate precursor; Preferably, the amount of the pore-forming agent added in step (2) is 3% to 12% of the mass of the biomass carbon source; Preferably, the mixing method in step (2) comprises wet slurry ball milling.

7. The preparation method according to claim 1, characterized in that The heating rate of the sintering in step (3) is 1 to 15°C / min; Preferably, the sintering holding temperature in step (3) is 480-600°C; Preferably, the sintering holding time in step (3) is 5 to 15 hours.

8. A composite polyanion sodium cathode material, characterized in that: The composite polyanionic sodium cathode material is prepared by the preparation method described in any one of claims 1 to 7; the composite polyanionic sodium cathode material includes secondary particles formed by the accumulation of primary particles, and the primary particles include a sodium ferric phosphate pyrophosphate base material and an amorphous carbon layer coated on the surface of the sodium ferric phosphate pyrophosphate base material.

9. The composite polyanionic sodium cathode material according to claim 8, characterized in that The chemical formula of the composite polyanion sodium cathode material is Na4Fe x (PO4)2(P2O7)@C, in fact, 2.5≤x≤3.

10. A sodium ion battery, characterized in that: The sodium ion battery comprises the composite polyanion sodium cathode material as claimed in claim 8 or 9.

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