Preparation method and application of sodium ion battery negative electrode material with improved conductivity

Abstract

The present invention discloses a preparation method and application of a negative electrode material for a sodium-ion battery with improved conductivity, belonging to the technical field of battery material preparation. The present invention provides a preparation method of a negative electrode material for a sodium-ion battery, with the chemical formula MTiOPO4@C, comprising the following steps: dispersing a phosphorus source, an M source, and a titanium source in solvent A to obtain slurry A; dispersing a first carbon source and a second carbon source in solvent B to obtain slurry B; mixing slurry A and slurry B and then performing pressure spray drying to obtain a precursor; and performing heat treatment on the precursor at a set temperature to obtain the negative electrode material for the sodium-ion battery. Among them, the first carbon source is a carbon material containing dot-like structures, and the second carbon source is a carbon material containing two-dimensional sheet-like structures. The present invention sets different conductive structures between primary particles and secondary particles, and through the design of the preparation process, the powder does not need to be pulverized, ensuring a high electronic conductivity between primary particles.

Description

Technical Field

[0001] The invention relates to a preparation method and application of a sodium ion battery negative electrode material with improved electrical conductivity, belonging to the technical field of batteries. Background Art

[0002] As a key component in large-scale energy storage systems, the electrochemical performance of secondary ion batteries is particularly important. Secondary sodium ion batteries have attracted much attention due to their low cost and excellent low-temperature performance, and the electrode material directly determines the electrochemical performance of the battery.

[0003] Among the titanium-based phosphates used as negative electrode materials for sodium-ion batteries, KTiOPO4 (KTOP) has a larger cross-cavity, and the potential of KTiOPO4 is the lowest, at 1.23V, which has great potential in achieving faster ion dynamics and lower cycle strain in sodium-ion batteries. Graphene is widely used in battery materials. For example, the patent with publication number CN 119018885 A, entitled "Few-layer graphene conductive slurry, preparation method and application thereof, and conductivity evaluation method", provides a method for preparing a few-layer graphene conductive slurry, which can improve the rapid charge and discharge performance of the battery. However, at this time, the graphene exists between the secondary particles of the active material, and the electronic conductivity between the primary particles of the active material cannot be improved. In addition, the conventionally prepared graphene-coated / doped powder has a large particle size and needs to be crushed when used, which will destroy the carbon connection network, thereby reducing the conductivity. Summary of the invention

[0004] To solve the above problems, the present invention dopes point-like carbon materials and graphene, and controls the particle size of the obtained material, so that the subsequent powder does not need to be crushed, thereby ensuring that the primary particles have a high electronic conductivity, and only a small amount of carbon material needs to be added when preparing the slurry for coating.

[0005] The first object of the present invention is to provide a method for preparing a negative electrode material for a sodium ion battery, comprising the following steps:

[0006] S1. Dispersing a certain proportion of a phosphorus source, an M source and a titanium source in a solvent A according to the chemical formula MTiOPO4 to obtain a slurry A; wherein M is one or more of Na, Li and K;

[0007] Dispersing the first carbon source and the second carbon source in solvent B to obtain slurry B;

[0008] S2, mixing slurry A and slurry B and performing pressure spray drying to obtain a precursor;

[0009] S3, heat-treating the precursor at a set temperature to obtain the sodium ion battery negative electrode material;

[0010] Wherein, the first carbon source is a carbon material containing a dot structure, and the second carbon source is a carbon material containing a two-dimensional sheet structure.

[0011] Furthermore, in step S1, the solvent A includes but is not limited to an aqueous solvent or a mixed solvent (such as a water-oil mixed solvent), preferably water.

[0012] Furthermore, in step S1:

[0013] The M source includes one or more of a sodium source, a lithium source and a potassium source;

[0014] The sodium source includes one or more of sodium carbonate, sodium hydroxide, sodium sulfate and sodium dihydrogen phosphate;

[0015] The lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium sulfate and lithium dihydrogen phosphate;

[0016] The potassium source includes one or more of potassium carbonate, potassium hydroxide, potassium sulfate and potassium dihydrogen phosphate, preferably potassium dihydrogen phosphate;

[0017] The titanium source includes any one of titanium dioxide, metatitanic acid, titanic acid, and titanium tetrachloride, preferably titanium dioxide;

[0018] The phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate and phosphorus pentoxide.

[0019] Further, in step S1, the first carbon source includes one or more of activated carbon, carbon fiber with an aspect ratio of less than 5, graphite, soft carbon, acetylene black, and hard carbon, preferably acetylene black.

[0020] Further, in step S1, the second carbon source includes one or more of graphene, graphene oxide, and reduced graphene oxide, preferably with a solid content of 0.1-20%, a BET specific surface area (m 2 / g)≥500 graphene slurry.

[0021] Further, in step S1, solvent B is an aqueous solution of water or ethanol, and of course, solvent B may also contain a surfactant. The main purpose of this step is to achieve uniform mixing of the carbon source, because the surface energy of the carbon material is relatively high, and adding a certain surfactant promotes the dispersion of the carbon material. Similarly, since the carbon material is easy to agglomerate, solvent B is preferably used for dispersion.

[0022] Further, in step S1, the primary particle size of the water-insoluble raw materials in the phosphorus source, M source, titanium source, first carbon source and second carbon source is D90<0.2μm. The size of the primary particle size in the present invention is closely related to the high conductivity of potassium titanium oxyphosphate: since the powder material obtained after sintering is grown on the basis of all raw materials, the small primary particle size of the raw material also ensures the small particle size of the powder after sintering. At the same time, the powder with a small particle size is easier to react uniformly during the sintering process, reducing the generation of impurities caused by local reaction differences. In addition to providing more active sites, the smaller primary particle size of the powder after sintering is conducive to the embedding and extraction of sodium ions, and shortening the diffusion path of ions inside the active material to increase the battery capacity, more importantly, the appropriate primary particle size can not only avoid the crushing of the active material when preparing the negative electrode slurry, thereby avoiding the destruction of the surface network structure of the active material, which is the main reason for achieving a significant increase in conductivity.

[0023] Preferably, the D90 is less than 0.2 μm, such as 0.19 μm, 0.18 μm, 0.17 μm, 0.16 μm, 0.15 μm, 0.14 μm, 0.13 μm, 0.12 μm, 0.11 μm, 0.1 μm, 0.08 μm, 0.06 μm, 0.05 μm, 0.04 μm, 0.03 μm, 0.02 μm, 0.01 μm, including but not limited to the values ​​listed above.

[0024] Furthermore, in step S1, the dispersing method includes wet ball milling.

[0025] Furthermore, in step S2, the mass ratio of slurry A to slurry B is (10-20):1.

[0026] The present invention innovatively adopts a two-step pulping and mixing preparation process to optimize and improve material performance. Specifically, the main raw materials are first mixed, and various parameters in the mixing process, such as stirring speed, time, and temperature, are precisely controlled to promote the fusion of the main raw materials to form a uniform and stable slurry A. This step is crucial to ensure the particle size of the main raw materials, and the appropriate particle size distribution lays the foundation for subsequent material performance.

[0027] At the same time, when preparing slurry B, by carefully selecting and adding specific types and appropriate amounts of surfactants, and utilizing the unique amphiphilic structure of the surfactant, its hydrophilic group interacts with water molecules, while the lipophilic group combines with the surface of the carbon material, thereby effectively reducing the surface tension between the carbon material particles, hindering their agglomeration tendency, and forming a uniformly dispersed carbon slurry.

[0028] Subsequently, slurry A is mixed with slurry B. This method of slurrying separately and then mixing has significant advantages. It can not only maintain the ideal particle size formed by the previous treatment of the main raw material, but also ensure the good dispersion uniformity of the carbon material in the whole system. The uniform dispersion of carbon materials is particularly critical for the carbon coating process, which enables the carbon to be evenly coated on the surface of the main raw material particles to form a complete and uniform carbon coating layer. This uniform carbon coating layer can not only improve the conductivity of the material, but also play a key role in ensuring the uniformity of the primary particle size of the obtained powder during subsequent processing. Uniform primary particle size distribution helps to improve the consistency and stability of the material, thereby improving the overall performance of the material in practical applications.

[0029] Preferably, the mass ratio of slurry A to slurry B is (10-20):1, such as 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, etc., including but not limited to the values ​​listed above.

[0030] Further, in step S2, the temperature of the pressure spray drying is 120-210°C, and the spray pressure is 10-20MPa. Pressure spray drying is to pressurize the slurry to a higher pressure using a high-pressure pump, and then fully contact it with hot air to achieve rapid drying of the slurry. In the present invention, under the high pressure of 10-20MPa, the liquid substance is atomized into tiny droplets, the surface area of ​​these small droplets is greatly increased, and the contact area with hot air is greatly increased, thereby quickly evaporating the water in the droplets and quickly drying the droplets into fine particles. Therefore, the present invention must adopt pressure spray drying to maintain a smaller particle size to avoid subsequent crushing.

[0031] Preferably, the temperature of the pressure spray drying is 120-210°C, such as 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, etc., including but not limited to the values ​​listed above.

[0032] Preferably, the spray pressure of the pressure spray drying is 10-20 MPa, such as 11 MPa, 12 MPa, 13 MPa, 14 MPa, 15 MPa, 16 MPa, 17 MPa, 18 MPa, 19 MPa, etc., including but not limited to the values ​​listed above.

[0033] Furthermore, in step S3, at least one of the following is included:

[0034] (1) performing heat treatment in an inert gas atmosphere; preferably, the inert gas comprises nitrogen or argon;

[0035] (2) The set temperature is 600-800°C;

[0036] (3) The heat treatment time is 1-24 hours.

[0037] Furthermore, in the sodium ion battery negative electrode material, the content of the first carbon source is 1-5wt%, and the content of the second carbon source is 0.01-1.0wt%.

[0038] Preferably, the content of the first carbon source is 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, etc., including but not limited to the values ​​listed above.

[0039] Preferably, the content of the second carbon source is 0.05wt%, 0.07wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt% and the like, including but not limited to the values ​​listed above.

[0040] Furthermore, the particle size D90 of the precursor is less than 5 μm, such as 4.99 μm, 4.95 μm, 4.9 μm, 4.7 μm, 4.5 μm, 4.2 μm, 4 μm, 3.5 μm, 3 μm, 2.5 μm, 2 μm, 1.5 μm, 1 μm, etc., including but not limited to the values ​​listed above.

[0041] The second object of the present invention is to provide a sodium ion battery negative electrode material prepared by the above preparation method.

[0042] The third object of the present invention is to provide a negative electrode plate, the negative electrode plate comprising a current collector and a negative electrode active layer disposed on at least one side of the current collector along the thickness direction, the negative electrode active layer containing the negative electrode material of the sodium ion battery. When preparing the active coating of the negative electrode plate, the active material does not need to be crushed and can be used directly, which is also the innovation of the present invention.

[0043] Furthermore, the negative electrode active layer also contains a conductive agent, which is a carbon material with a linear structure, such as carbon nanotubes, more preferably, carbon nanotubes with an aspect ratio of not less than 15. Most preferably, carbon nanotubes with an aspect ratio of 20-100.

[0044] Furthermore, the negative electrode active layer comprises the following components in mass percentage: 90-99% of negative electrode material, 0.5%-2% of conductive agent and 0.5-5% of binder. In a general active layer, at least 5-15% of conductive agent is required, but in the present invention, due to the use of high conductivity active materials and the combination of conductive agent types, a conductive path is established between the primary particles and the secondary particles, which greatly reduces the amount of conductive agent used and improves the electrical performance of the negative electrode sheet.

[0045] Preferably, the content of the conductive agent is 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt%, 1.99wt% and the like, including but not limited to the values ​​listed above.

[0046] Furthermore, the current collector is aluminum foil, or other materials that can be used as current collectors.

[0047] Furthermore, the binder can be any binder known to those skilled in the art without any particular limitation, such as chitosan, xanthan gum, gellan gum, gum arabic, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, polymethacryloyl, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, polyacrylamide, styrene-butadiene rubber, sodium alginate, polyethylene glycol, guar gum, guar gum polymer and guar gum copolymer. In the embodiment of the present invention, PVDF (polyvinylidene fluoride) is selected.

[0048] A fourth object of the present invention is to provide a method for preparing a negative electrode sheet, comprising the following steps:

[0049] S01, preparing a negative electrode active slurry by mixing 90-99% of a negative electrode material of a sodium ion battery (without being crushed), 0.5%-2% of a conductive agent and 0.5-5% of a binder;

[0050] S02, coating the negative electrode active slurry on at least one side of the current collector along the thickness direction, drying to obtain a negative electrode active layer, and pressing to obtain the negative electrode sheet.

[0051] A fifth object of the present invention is to provide a sodium ion battery comprising the negative electrode plate.

[0052] Furthermore, the sodium ion battery also contains an electrolyte, and the electrolyte contains a sodium salt electrolyte.

[0053] Furthermore, the electrolyte can be any electrolyte well known to those skilled in the art without any particular limitation, the solvent is at least one of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate and diethylene glycol dimethanol ether; the electrolyte salt is at least one of sodium perchlorate, sodium bis(oxalate)borate, and sodium hexafluorophosphate.

[0054] Beneficial effects of the present invention:

[0055] The present invention provides a novel method for preparing negative electrode materials for sodium ion batteries. The prepared material has the chemical formula of MTiOPO4@C. Specifically, in the present invention, a point-like carbon source and a sheet-like carbon source are added before preparing secondary particles, and a conductive network is formed between primary particles. At the same time, by controlling the primary particle size and the particle size after spraying, the primary particle size is ensured when preparing the slurry, and no crushing step is required, thereby ensuring the integrity of the conductive network in the active material, thereby improving the conductivity of the powder material. On this basis, only a small amount of conductive agent needs to be added during the subsequent preparation of the pole piece to greatly increase the proportion of active materials and improve the specific energy of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 The present invention is a process flow chart of the preparation process.

[0057] Figure 2 The middle left picture is a schematic diagram of the powder microstructure, and the right picture is a schematic diagram of the microstructure of the coating on the electrode; among them, 1 represents KTiOPO4, 2 represents point carbon source, 3 represents graphene, and 4 represents carbon nanotubes.

[0058] Figure 3 This is the charge and discharge curve of the powder in Example 1. DETAILED DESCRIPTION

[0059] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0060] The preparation method involved in the present invention is as follows:

[0061] On the one hand, the present invention provides a method for preparing a high-conductivity sodium-ion battery negative electrode material (chemical formula: MTiOPO4@C, where M is one or more of Na, Li and K) as follows:

[0062] 1) preparing slurry A with a stoichiometric ratio of phosphorus source, potassium source and titanium source, and grinding the slurry to a particle size D90 of less than 0.5 μm; the solvent is deionized water;

[0063] 2) wet-ball-milling a carbon source, a solvent B and a surfactant to obtain a slurry B; the carbon source includes a point-shaped carbon source and a graphene slurry, the solvent B is water, or a mixed solvent of water and ethanol, the content of ethanol in the solvent B is 0-10 wt%, and the surfactant includes at least one of sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), gum arabic (GA), TritonX-100 and cetyltrimethylammonium bromide (CTAB);

[0064] 3) Slurry A and slurry B are mixed to obtain a mixed slurry; the mass ratio of slurry A to slurry B is (10-20):1;

[0065] 4) The mixed slurry is spray-dried to obtain a spherical carbon-coated potassium titanyl phosphate compound precursor mixed powder; the spray drying parameters are: temperature 120-210° C., spray pressure 10-20 MPa, the content of the point-like carbon source in the synthetic powder is 1-5wt%, and the content of graphene is 0.01-1.0wt%;

[0066] 5) Sintering the precursor mixed powder to obtain carbon-coated potassium titanyl phosphate compound powder; the sintering is carried out at a constant temperature of 600-800° C. for 1-24 hours under a protective atmosphere.

[0067] In another aspect, the present invention provides a negative electrode sheet containing the above-mentioned high-conductivity sodium ion battery negative electrode material, and the preparation method is as follows:

[0068] 1) The above-mentioned high conductivity sodium ion battery negative electrode material is used as the active material, mixed with carbon nanotubes and a binder in a certain proportion, and added to deionized water to prepare an aqueous slurry. The solid content of the aqueous electrode slurry is 40-65%, and the viscosity is 2000-15000 mPa.S. The solid material of the aqueous electrode slurry includes 90-99% of the active material, 0.5%-2% of the carbon nanotube conductive agent, and 0.5-5% of the binder. The conductive agent can be the carbon nanotube aqueous slurry; after ball milling and dispersion, it is coated and compacted to obtain the negative electrode sheet. Example 1

[0069] 1. Preparation of negative electrode materials for sodium ion batteries

[0070] 1) According to 60 kg of KTiOPO4, slurry A is prepared with titanium dioxide and potassium dihydrogen phosphate in a stoichiometric ratio, and finely ground to a particle size D90 of 0.2 μm; the solvent is deionized water; the mass of slurry A is 150 kg.

[0071] 2) Sodium dodecyl sulfate (SDS) was dissolved in deionized water containing 5 wt% ethanol to obtain solvent B, and then 2 kg of acetylene black (2 / 62.05=3.2%, 62.5 is the total mass of the powder) and 0.5 kg of graphene slurry (solid content of 10%) (0.05 / 62.05=0.08%) were added to solvent B to obtain slurry B; the mass of slurry B was 10 kg.

[0072] 3) Slurry A and slurry B are mixed to obtain a mixed slurry.

[0073] 4) A spherical carbon-coated potassium titanyl phosphate compound precursor mixed powder was obtained by pressure spraying, the spray drying air inlet temperature was 150° C., the spray pressure was 10 MPa, and a powder with a D90 of 4.5 μm was obtained;

[0074] 5) The precursor mixed powder is heated to 700° C. at a heating rate of 3° C. / min, and sintered at 700° C. for 12 h to obtain a carbon-coated potassium titanyl phosphate compound powder.

[0075] 2. Preparation of negative electrode sheet

[0076] The carbon-coated potassium titanyl phosphate compound powder prepared above is used as an active material, mixed with a conductive agent and a binder in a certain proportion, and added to deionized water to prepare an aqueous slurry. The solid material of the aqueous electrode slurry includes 97wt% of active material, 1wt% of conductive agent, and 2wt% of binder. The conductive agent is a carbon nanotube aqueous slurry with an aspect ratio of 35, and the binder is carboxymethyl cellulose (CMC). The materials are dispersed and then coated and compacted to obtain a negative electrode sheet. The compaction density of the electrode sheet is 1.9g / cm 3 . Example 2

[0077] The mass percentage of acetylene black in step 2) was adjusted from 3.2% to 1%, and the rest was the same as in Example 1. Example 3

[0078] The mass percentage of acetylene black in step 2) was adjusted from 3.2% to 5%, and the rest was the same as in Example 1. Example 4

[0079] The mass percentage of acetylene black in step 2) was adjusted from 3.2% to 8%, and the rest was the same as in Example 1. Example 5

[0080] (1) The mass percentage of graphene solid in step 2) was adjusted from 0.08% to 0.2%, and the rest was the same as in Example 1.

[0081] (2) The mass percentage of graphene solid in step 2) was adjusted from 0.08% to 0.4%, and the rest was the same as in Example 1.

[0082] (3) The mass percentage of graphene solid in step 2) was adjusted from 0.08% to 0.6%, and the rest was the same as in Example 1. Example 6

[0083] The spray drying parameters in step 4) were adjusted as follows: the spray temperature was 200° C., the spray pressure was 15 MPa, and the rest was the same as in Example 1. Example 7

[0084] (1) The precursor mixed powder in step 5) is calcined at 800° C. for 2 h to obtain a carbon-coated potassium titanyl phosphate compound powder, and the rest is the same as in Example 1.

[0085] (2) The precursor mixed powder in step 5) was calcined at 600° C. for 24 h to obtain a carbon-coated potassium titanyl phosphate compound powder, and the rest was the same as in Example 1. Example 8

[0086] (1) The carbon nanotubes with an aspect ratio of 35 used in the preparation of the negative electrode plate were replaced with carbon nanotubes with an aspect ratio of 15, and the rest were the same as in Example 1.

[0087] (2) The carbon nanotubes with an aspect ratio of 35 used in the preparation of the negative electrode plate were replaced with carbon nanotubes with an aspect ratio of 20, and the rest were the same as in Example 1.

[0088] (3) The carbon nanotubes with an aspect ratio of 35 used in the preparation of the negative electrode plate were replaced with carbon nanotubes with an aspect ratio of 100, and the rest were the same as in Example 1.

[0089] (4) The carbon nanotubes with an aspect ratio of 35 used in the preparation of the negative electrode plate were replaced with carbon nanotubes with an aspect ratio of 120, and the rest were the same as in Example 1. Example 9

[0090] (1) When preparing the negative electrode sheet, the proportions of the components were adjusted to 97 wt % of active material and 3 wt % of binder, and the rest were the same as in Example 1.

[0091] (2) When preparing the negative electrode sheet, the proportions of the components are adjusted to 97 wt% of active material, 0.5 wt% of conductive agent, and 2.5 wt% of binder. The rest is the same as in Example 1.

[0092] (3) When preparing the negative electrode sheet, the proportions of the components are adjusted to 97 wt% of active material, 2 wt% of conductive agent, and 1 wt% of binder. The rest are the same as in Example 1.

[0093] (4) When preparing the negative electrode sheet, the proportions of the components are adjusted to 97 wt% of active material, 2.5 wt% of conductive agent, and 0.5 wt% of binder. The rest is the same as in Example 1. Example 10

[0094] According to the preparation method of the negative electrode material of the sodium ion battery in Example 1, K 0.8 Na 0.2 TiOPO4@C material, the sodium source used is Na2CO3. Comparative Example 1

[0095] 1. Preparation of negative electrode materials for sodium ion batteries

[0096] 1) Prepare slurry A according to Example 1.

[0097] 2) Sodium dodecyl sulfate (SDS) was dissolved in deionized water containing 5 wt % ethanol to obtain solvent B, and 2 kg of acetylene black was added to solvent B to obtain slurry B'.

[0098] 3) Slurry A and slurry B' are mixed to obtain a mixed slurry.

[0099] 4) Preparing a potassium titanyl phosphate compound precursor mixed powder according to the spray drying method of Example 1;

[0100] 5) Prepare potassium titanyl phosphate compound powder according to the sintering method of Example 1.

[0101] 2. Preparation of negative electrode sheet

[0102] The potassium titanium phosphate compound powder prepared above was used as an active material, mixed with a conductive agent and a binder in a certain proportion, and added to deionized water to prepare an aqueous slurry. The solid material of the aqueous electrode slurry included 97wt% of active material, 1wt% of carbon nanotubes with an aspect ratio of 35, and 2wt% of binder CMC. 0.5kg of graphene slurry (solid content of 10%) was additionally added. The materials were dispersed and then coated and compacted to obtain a negative electrode sheet. The compaction density of the electrode sheet was 1.9g / cm 3 . Comparative Example 2

[0103] In step 2), acetylene black is not added, and the mass percentage of graphene is adjusted from 0.08% to 3.28%, and the rest is the same as in Example 1. Comparative Example 3

[0104] In step 2), no graphene is added, and the mass percentage of acetylene black is adjusted from 3.2% to 3.28%, and the rest is the same as in Example 1. Comparative Example 4

[0105] The pressure spray drying in step 4) was replaced by centrifugal spray drying, and the rest was the same as in Example 1. Comparative Example 5

[0106] The conductive carbon nanotubes used in the preparation of the negative electrode plate were replaced with conductive carbon black Super P, and the rest was the same as in Example 1. Comparative Example 6

[0107] The slurry was finely ground to a particle size D90 of 0.5 μm, and the rest was the same as in Example 1.

[0108] Test Case

[0109] The test method is as follows:

[0110] (1) Powder charge and discharge test: The obtained KTiOPO4 carbon composite material was used as the working electrode, and a slurry was prepared with a mass ratio of active material: acetylene black: PVDF of 8:1:1. The slurry was coated on aluminum foil, and a Na sheet was used as the counter electrode. The electrolyte used PC as the solvent and sodium hexafluorophosphate as the electrolyte salt. The 1C first charge and discharge curve is shown in the figure. Figure 3 As shown, the charge and discharge curves of the test material in the charge and discharge voltage range of 0-3V.

[0111] (2) Powder resistivity R (Ω·cm) test: The powder resistivity of the sodium titanium phosphate composite material was measured using an ST2742C automatic powder resistivity tester. Under the same carbon content, the electronic conductivity of the sodium titanium phosphate composite material can be directly reflected. The curve between pressure (MPa) and resistivity (Ω·cm) is recorded, and the resistivity at a pressure of 60 MPa is used as a comparison between different samples.

[0112] (3) Electrode resistivity test: The two-probe electrode overall resistivity direct measurement method is to place two electrodes on the upper and lower sides of the sample, apply an excitation current and detect the voltage on the upper and lower sides of the sample, thereby calculating the overall resistivity of the sample.

[0113] (4) Powder particle size test: The D90 of the obtained powder was tested by Malvern Mastersizer 300 particle size tester.

[0114] The results are as follows:

[0115] Figure 3 This is the powder charge and discharge curve (SOC: the state of charge parameter of the battery, which is the ratio of the remaining capacity to the battery capacity) tested in Example 1. It can be seen from the curve that the material exhibits excellent electrochemical properties. The charging medium voltage and the discharging medium voltage of the material are 1.25V and 1.24V, respectively, and the difference between the two is small, indicating that its reversibility is very good. This is attributed to the good conductive structure mentioned above, which effectively reduces the ohmic polarization of the electrode and makes the charging and discharging process smoother. By calculation, the specific capacity of the material is 125mAh / g, which is greater than 95% of the theoretical specific capacity, which further proves the effectiveness of the preparation process adopted by the present invention in improving the performance of the material. The good process creates a material with good conductivity and high specific capacity, laying the foundation for its high efficiency performance in practical applications.

[0116] Table 1 Test results

[0117]

[0118]

[0119] In Table 1:

[0120] From the comparison between Comparative Example 1 and Example 1, it can be seen that since only point-like carbon sources are added to the primary particles of the active material without adding graphene, the resistivity of the powder material has increased by nearly 7 times. Although the same proportion of graphene is added when preparing the pole piece, the pole piece resistivity is still high. Therefore, compared with the presence of point-like carbon sources and graphene between the secondary particles of the active material, when they are present between the primary particles of the active material, even with the same dosage, the electronic conductivity of the powder material and the pole piece is also significantly improved. This may be because the conductive agent material added during the preparation of the powder will be tightly attached to the surface of the primary particles during the sintering process of the material to form a good conductive network, while the conductive agent material added during the preparation of the pole piece slurry has a far different contact with the active material, and cannot effectively build an efficient conductive network, resulting in an increase in resistivity.

[0121] From the comparison between Comparative Examples 2-3 and Example 1, it can be seen that when there is only graphene, the resistivity of the powder and the electrode both increase greatly due to the lack of the connecting effect of the point-like carbon source; and when there is only a point-like carbon source, the rising trend of the resistance of the powder and the electrode is more serious. This may be mainly because the two-dimensional sheet structure of graphene enables it to achieve "surface-point" contact with the active material. Compared with the traditional zero-dimensional carbon black particles and one-dimensional carbon nanotubes' "point-point" or "line-point" contact mode, graphene has a lower conductivity threshold and can construct a conductive network in the electrode from a larger spatial span, realizing a long-range conductive single-layer carbon structure on the entire electrode. The present invention coats a mixture of point-to-point carbon sources and surface-to-point graphene on the surface of the active material, constructing a complete conductive network structure on the surface of the primary particles. Subsequently, carbon nanotubes in a line-to-point contact mode are added between the secondary particles to achieve conductive connection between the secondary particles, that is, to connect the conductive networks on the surfaces of different secondary particles, thereby forming a larger and interconnected conductive structure in the electrode coating.

[0122] From the comparison between Comparative Example 4 and Example 1, it can be seen that when centrifugal spray is used to prepare powder, the powder particle size is generally tens of microns. When preparing slurry, the particles need to be crushed and sieved until D90 is less than 5 microns. Compared with powders of the same particle size prepared by pressure spraying, the conductivity of the powder after crushing increases by 50%. This is mainly because the friction and collision between the particles during the crushing process cause the internal conductive structure to be destroyed. Although carbon nanotubes are added later, the resistivity of the electrode is still greatly improved.

[0123] From the comparison between Comparative Example 5 and Example 1, it can be seen that when switching to the universal conductive agent Super P, the electrode resistivity increases by nearly an order of magnitude. This is mainly because Super P is a point-like conductive agent, which exists in the form of a point-like coating on the surface of 4.5-micron particles and cannot form long-range conduction, so the electrode resistivity increases.

[0124] From the comparison between Comparative Example 6 and Example 1, it can be seen that when the initial primary particle size of the raw material is greater than 0.2 microns, even if pressure spraying is adopted, by adjusting the spray control parameters, the particle size of the obtained spray powder is less than 5 microns. However, since the particle size of the raw material is too large, the primary particle size of the obtained powder is too large, which means that the conductive layer on its surface cannot form a good coating, and the resistivity of the powder and the electrode is increased.

[0125] From the comparison between Examples 2-4 and Example 1, it can be seen that there is no obvious difference in resistivity when the acetylene black content is in the range of 1wt%-5wt%, and it decreases slightly with the increase of the point-like carbon source content; when the amount of acetylene black added as the point-like conductive agent is too high, although the resistivity of the powder decreases slightly, the bulk tap density of the powder is very low due to the large specific surface area of ​​the point-like conductive agent. When the powder is used to prepare the electrode, the compaction density of the electrode reaches 1.9g / cm 3 When the battery is used, the electrode has cracked and cannot be used normally.

[0126] From the comparison between Example 5 and Example 1, it can be seen that with the increase of graphene content, the powder resistivity and the electrode resistivity show a downward trend, but the rate of decline is slowing down, which shows that both can meet the requirements of low-resistance powder preparation.

[0127] From the comparison between Example 6 and Example 1, it can be seen that the parameter setting of the pressure spray has no significant effect on the resistivity.

[0128] From the comparison between Example 7 and Example 1, it can be seen that the sintering temperature and time may change the microstructure of the active material, which will also have a corresponding impact on the resistivity (when the sintering temperature is high, the energy obtained by the system increases. This makes the material grow larger in particle size. As the material particle size increases, the larger particle size may lead to increased electron scattering, and the resistivity of the powder and the electrode will increase slightly). As long as the calcination process is carried out within a suitable range, the resistivity can be reduced.

[0129] From the comparison between Example 8 and Example 1, it can be seen that the aspect ratio of the added carbon nanotubes is very important when preparing the pole piece. When the aspect ratio of the carbon nanotubes is within 20-100, the resistivity of the pole piece is not much different, but when the aspect ratio is less than 20, the resistivity of the pole piece is increased by nearly 4 times. This is mainly because the short-range carbon nanotubes cannot tightly wrap the powder particles to form a smooth electron channel; when the aspect ratio is greater than 100, there will be great challenges in the process of preparing the pole piece slurry. Due to the large aspect ratio, the shape of the particles becomes slender, and in the slurry system, these particles are more likely to entangle and aggregate with each other. Even under conventional dispersion means such as stirring, it is difficult to break this aggregation trend, making it difficult to form a uniformly dispersed slurry state. This uneven slurry cannot be evenly coated when coating the pole piece, and it is difficult to obtain a qualified pole piece.

[0130] From the comparison between Example 9 and Example 1, it can be seen that when the carbon nanotube content is 0, the resistance of the electrode increases by an order of magnitude, because there is no channel for electron conduction between the particles; with the increase of the carbon nanotube content, the resistivity of the electrode decreases slightly. When the amount of carbon nanotubes added is too high (greater than 2%), the prepared electrode slurry is difficult to form a uniform dispersion and the electrode cannot be evenly coated.

[0131] Example 10 The active material is K 0.8 Na 0.2 The resistivity of TiOPO4, powder material and electrode has almost no change and is not affected by the active material.

[0132] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A negative electrode plate, characterized in that: The negative electrode sheet comprises a current collector and a negative electrode active layer disposed on at least one side of the current collector along the thickness direction, wherein the negative electrode active layer contains a negative electrode material for a sodium ion battery and a conductive agent; The conductive agent is a carbon nanotube with an aspect ratio of not less than 15, The method for preparing the negative electrode material of a sodium ion battery comprises the following steps: S1. Dispersing a certain proportion of a phosphorus source, an M source and a titanium source in a solvent A according to the chemical formula MTiOPO4 to obtain a slurry A; wherein M is one or more of Na, Li and K; Dispersing the first carbon source and the second carbon source in solvent B to obtain slurry B; S2, mixing slurry A and slurry B and performing pressure spray drying to obtain a precursor; S3, heat-treating the precursor at a set temperature to obtain the sodium ion battery negative electrode material; Wherein, the first carbon source is selected from one or more of activated carbon, carbon fiber or carbon nanotube with an aspect ratio of less than 5, graphite, soft carbon, and hard carbon; the second carbon source is selected from one or more of graphene, graphene oxide, and reduced graphene oxide. The primary particle size of the water-insoluble raw material among the phosphorus source, M source, titanium source, first carbon source and second carbon source is D90 not more than 0.2 μm, The pressure spray drying temperature is 120-210° C., and the spray pressure is 10-20 MPa.

2. The negative electrode sheet according to claim 1, characterized in that: Include at least one of the following characteristics: (1) In the sodium ion battery negative electrode material, the content of the first carbon source is 1-5wt%; (2) In the sodium ion battery negative electrode material, the content of the second carbon source is 0.01-1.0wt%.

3. The negative electrode sheet according to claim 1, characterized in that: Include at least one of the following characteristics: (1) The M source includes one or more of a sodium source, a lithium source and a potassium source; The sodium source includes one or more of sodium carbonate, sodium hydroxide, sodium sulfate and sodium dihydrogen phosphate; The lithium source includes one or more of lithium carbonate, lithium hydroxide, lithium sulfate and lithium dihydrogen phosphate; The potassium source includes one or more of potassium carbonate, potassium hydroxide, potassium sulfate and potassium dihydrogen phosphate; (2) The titanium source includes one or more of titanium dioxide, metatitanic acid, titanic acid, and titanium tetrachloride; (3) The phosphorus source includes one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate and phosphorus pentoxide; (4) The particle size D90 of the precursor is less than 5 μm.

4. The negative electrode sheet according to claim 1, characterized in that: In step S1, at least one of the following features is included: (1) The solvent A comprises an aqueous solvent and / or a mixed solvent; (2) The solvent B contains a surfactant; (3) The solvent B contains an aqueous solution of water or ethanol; (4) The dispersion method includes wet ball milling.

5. The negative electrode sheet according to claim 1, characterized in that: Include at least one of the following characteristics: (1) In step S2, the mass ratio of slurry A to slurry B is (10-20):1; (2) In step S3, heat treatment is performed under an inert gas atmosphere; (3) In step S3, the set temperature is 600-800°C; (4) In step S3, the heat treatment duration is 1-24 hours.

6. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active layer comprises the following components in mass percentage: 90-99% of negative electrode material, 0.5%-2% of conductive agent and 0.5-5% of binder.

7. The negative electrode sheet according to claim 1, characterized in that: The conductive agent includes carbon nanotubes with an aspect ratio of 20-100.

8. A sodium ion battery, characterized in that: The sodium ion battery contains the negative electrode sheet according to any one of claims 1 to 7.

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