CN115911380A - Positive electrode material, preparation method of positive electrode material, positive electrode piece and sodium-ion battery - Google Patents
Positive electrode material, preparation method of positive electrode material, positive electrode piece and sodium-ion battery Download PDFInfo
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Abstract
The application discloses a positive electrode material, a preparation method of the positive electrode material, a positive electrode piece and a sodium-ion battery. In the present application, the positive electrode material includes a prussian blue material and a carbon layer coated on a surface of the prussian blue material. According to the method, the damage of high-temperature sintering to the Prussian blue material substrate is reduced by using an inorganic carbon source coating process; the carbon source is uniformly coated on the surface of Prussian blue in the firing process by using an inorganic conductive coating agent, so that the surface smoothness of the particles is improved while the surface conductivity of the material is improved, and the circulation stability of the obtained sodium-ion battery is further improved.
Description
Technical Field
The invention relates to the field of secondary batteries, in particular to a positive electrode material, a preparation method of the positive electrode material, a positive electrode piece and a sodium ion battery.
Background
Sodium ion batteries have the advantages of wide raw material sources, low cost, long service life and the like, and are more and more concerned by people. Among them, prussian blue, which is a positive electrode material of a sodium ion battery, is widely spotlighted due to its high theoretical specific capacity (200 mAh/g), excellent cycle life, simple preparation process, and low cost.
Prussian blue is excellent in various properties, but is inherently poor in conductivity. A large amount of conductive agent is often mixed in the pulping process to improve the conductivity of the electrode, so that the use of active substances is greatly reduced, and the energy density of the battery cell is reduced. Therefore, how to effectively improve the conductivity of the prussian blue material and increase the proportion of active substances is a challenging problem in the field.
In the prior art, the conductivity of the prussian blue material is improved by coating carbon outside the prussian blue material, and a large amount of conductive agents are avoided from being mixed, so that the proportion of active substances is increased, but carbon sources used in actual operation are organic glycogen such as glucose, sucrose, fructose and the like, the cracking temperature of the organic glycogen is higher, the cracking is started only when the temperature is higher than 600 ℃ (700-800 ℃), the decomposition temperature of the prussian blue material is low at high temperature, the cracking is started when the temperature is 350 ℃, active materials are seriously lost, even coating products cannot be obtained, and only the mixture of the prussian blue material and the carbon materials is used. Therefore, there is still a need in the art to develop a method for preparing a carbon-coated prussian blue cathode material that avoids loss of active material.
Disclosure of Invention
The invention aims to provide a positive electrode material.
Another object of the present invention is to provide a method for preparing the above-mentioned positive electrode material.
The invention also provides a positive pole piece containing the positive pole material.
The invention also provides a sodium-ion battery comprising the positive pole piece.
In order to solve the technical problems, the invention provides a positive electrode material in a first aspect, where the positive electrode material includes a prussian blue material and a carbon layer coated on the surface of the prussian blue material.
In some preferred embodiments, the carbon layer is composed of an inorganic carbon source selected from at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, ketjen black, and acetylene black.
In some preferred embodiments, the carbon layer has a thickness of 50 to 300nm.
In some preferred embodiments, the prussian blue material is Na x M[Fe(CN) 6 ]·nH 2 O, wherein x is more than or equal to 1.5 and less than or equal to 2.0, M is Fe or Mn, and n is more than or equal to 0 and less than or equal to 6.
In some preferred embodiments, the method for preparing the cathode material includes the steps of:
mixing a prussian blue material and an inorganic carbon source, and sintering at a temperature lower than 350 ℃.
In some preferred embodiments, the inorganic carbon source is at least one selected from the group consisting of single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, ketjen black, and acetylene black.
In some preferred embodiments, the mass ratio of the prussian blue material to the inorganic carbon source is 1 (0.02-0.25), more preferably 1 (0.02-0.22), more preferably 1 (0.05-0.2), more preferably 1: (0.06-0.18).
In some preferred embodiments, the method for preparing the cathode material includes the steps of:
mixing the prussian blue material, the inorganic carbon source and a liquid additive; and (3) crushing, and then sintering, wherein the sintering temperature is lower than 350 ℃.
In some preferred aspects, the positive electrode material has a stacking angle of 40 to 70 °; more preferably 45-70 °, more preferably 45 to 65 °, more preferably 50 to 65 °, more preferably 55 to 65 °.
A second aspect of the present invention provides a method of preparing a positive electrode material, the method comprising the steps of:
mixing a Prussian blue material and an inorganic carbon source, and sintering, wherein the sintering temperature is lower than 350 ℃.
In some preferred embodiments, the method for preparing the cathode material includes the steps of:
mixing the prussian blue material, the inorganic carbon source and a liquid auxiliary agent; and (3) crushing, and then sintering, wherein the sintering temperature is lower than 350 ℃.
In some preferred embodiments, the pulverization treatment is a ball milling treatment.
In some preferred embodiments, the comminution process is carried out in a ball mill.
In some preferred embodiments, the liquid adjuvant is acetone, ethanol, or water.
In some preferred schemes, the method further comprises the following steps between the crushing treatment and the sintering treatment:
and removing the liquid auxiliary agent.
In some preferred embodiments, the step of removing the liquid adjuvant is performed in an oven.
In some preferred embodiments, the ball milling speed of the ball milling process is from 10 to 200rpm/min, such as 100rpm/min.
In some preferred embodiments, the time of the ball milling treatment is not less than 10 hours, more preferably not less than 15 hours, such as 24 hours.
In some preferred embodiments, the ball milling treatment time is not greater than 30 hours, more preferably not greater than 28 hours.
In some preferred embodiments, the sintering process is performed in a muffle furnace or a tube furnace.
In some preferred embodiments, the sintering treatment temperature is 200 to 300 ℃, more preferably 210 to 290 ℃, more preferably 220 to 280 ℃, and more preferably 230 to 270 ℃.
In some preferred embodiments, the sintering treatment time is 6 to 30 hours, more preferably 7 to 28 hours, and still more preferably 8 to 26 hours.
A third aspect of the present invention provides a positive electrode plate, including a current collector and a positive electrode slurry layer applied to the current collector, where the positive electrode slurry layer includes the positive electrode material according to the first aspect of the present invention.
In some preferred embodiments, the positive electrode slurry layer further includes a conductive agent and a binder.
In some preferred embodiments, the conductive agent is selected from Super P, carbon Nanotubes (CNT).
In some preferred embodiments, the binder is selected from polyvinylidene fluoride (PVDF).
The invention provides a sodium-ion battery, which comprises a positive pole piece, a negative pole piece, a diaphragm and electrolyte.
The fifth aspect of the present invention provides a method for increasing the compaction density of a positive electrode sheet in which the positive electrode material according to the first aspect of the present invention is used as a positive electrode active material, wherein the stacking angle of the positive electrode material is 45 to 70 °.
Compared with the prior art, the invention has at least the following advantages:
(1) According to the carbon-coated Prussian blue cathode material provided by the invention, the damage of high-temperature sintering to a Prussian blue material substrate is reduced by using an inorganic carbon source coating process;
(2) According to the carbon-coated Prussian blue cathode material provided by the embodiment of the invention, the carbon source is uniformly coated on the surface of Prussian blue by using the inorganic conductive coating agent in the firing process, so that the surface smoothness of particles is improved while the surface conductivity of the material is improved, and the cycle stability of the obtained sodium-ion battery is further improved;
(3) According to the carbon-coated Prussian blue cathode material provided by the embodiment of the invention, the compacted density of the prepared cathode pole piece is higher by controlling the particle stacking angle of the material to be 40-70 degrees;
(4) According to the carbon-coated Prussian blue cathode material provided by the embodiment of the invention, the conductivity of the battery is further improved by controlling the thickness and volume distribution of the carbon coating layer on the surface of the material;
(5) The sodium ion battery provided by the invention has the advantages of lower resistivity, and better discharge capacity and cycle performance.
It is to be understood that within the scope of the present invention, the above-described features of the present invention and those specifically described below (e.g., in the examples) may be combined with each other to form new or preferred embodiments. Not to be reiterated herein, but to the extent of space.
Drawings
One or more embodiments are illustrated by the figures in the accompanying drawings, which correspond to and are not intended to limit the embodiments.
FIG. 1 is another electron micrograph of a carbon-coated Prussian blue positive electrode material used in accordance with the present invention;
FIG. 2 is another electron micrograph of a carbon-coated Prussian blue positive electrode material used in accordance with the present invention;
FIG. 3 is an electron microscope scan of a glucose-coated Prussian blue positive electrode material according to an embodiment of the present invention.
Detailed Description
When the carbon-coated Prussian blue cathode material is prepared, the Prussian blue material is unstable at high temperature, so the material loss is serious. In the invention, the inventor tries to use an inorganic carbon source to replace organic glycogen as a coating agent to develop a process for coating the Prussian blue material by the inorganic carbon source, so that the sintering temperature is reduced, and the material loss is reduced.
Positive electrode material
The invention relates to a positive electrode material, which comprises a Prussian blue material and a carbon layer coated on the surface of the Prussian blue material, wherein the positive electrode material is preferably in a shell-core structure, the shell is the carbon layer, and the core is Prussian blue core particles.
In the present invention, the term "Prussian blue" refers to Na 1.5～2 M[Fe(CN) 6 ]·nH 2 O, wherein M is Fe,Mn, co or Ni, n is more than or equal to 0 and less than or equal to 6. In a preferred embodiment, "prussian blue" is Na 1.5～2 M[Fe(CN) 6 ]·nH 2 O, wherein M is Fe or Mn, and n is 0.5-3).
In the invention, the coating carbon is a carbon layer formed by pyrolyzing an inorganic carbon source, and the carbon layer is used as the inorganic carbon source and can be at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, ketjen black and acetylene black.
Different carbon sources are used for sintering treatment, and the obtained carbon layer has different microstructures to obtain different conductivity. When an organic carbon source is used, the obtained carbon layer is irregular amorphous carbon, even the carbon layer cannot be successfully coated to form a shell-core structure, and the mixture of lamellar particles and carbon is observed under an electron microscope; when the inorganic carbon source is used, the microscopic molecular arrangement of the obtained carbon layer is more regular. Different inorganic carbon sources are used, the carbon layers formed after sintering are different, for example, ketjen black is preferably used as the inorganic carbon source in the invention, and the electrical property of the obtained cathode material is superior to other inorganic carbon sources such as single-walled carbon nanotubes, multi-walled carbon nanotubes, acetylene black, graphene and the like.
In the present invention, the positive electrode material stacking angle is 40 to 70 °, more preferably 45 to 65 °. When the stacking angle is less than 40 degrees, the adhesion capability of the anode material, the conductive agent and the binder is poor, so that the slurry is not uniform easily, and the battery capacity exertion is influenced; when the stacking angle is greater than 70 °, the positive electrode material is poor in conductivity, deteriorating the battery capacity and rate performance. In a more preferred embodiment, the stack angle is 55 to 65 °.
In the invention, the carbon layer of the anode material has the thickness of 50-300nm. When the thickness of the carbon layer of the anode material is less than 50nm, the surface conductivity of the material is poor; when the thickness of the carbon layer of the anode material is less than 300nm, sodium ions are difficult to pass through, so that the battery cell capacity is low.
In the present invention, the D50 of the positive electrode material is 2.0 to 8.0. Mu.m. When the D50 is less than 2.0 μm, the anode material is easy to form gel in the homogenization process, and the high-temperature performance of the obtained battery is poor; when the D50 is more than 8.0 μm, the positive electrode material particles have poor conductivity, deteriorating battery capacity and rate performance.
Preparation method of cathode material
The invention also relates to a preparation method of the cathode material, which comprises the following steps:
mixing a prussian blue material and an inorganic carbon source, and sintering at a temperature lower than 350 ℃.
Preferably, mixing the prussian blue material, the inorganic carbon source and a liquid additive; the pulverization treatment is carried out, and then the sintering treatment is carried out, wherein the temperature of the sintering treatment is lower than 350 ℃, and more preferably lower than 300 ℃.
Preferably, the method further comprises the following steps between the crushing treatment and the sintering treatment:
and removing the liquid auxiliary agent.
According to the invention, the adhesion of the inorganic carbon source on the surface of the material is low, and the stack angle, the volume distribution and the coating thickness of the obtained positive electrode material can be in a preferred range by adjusting the proportion of the Prussian blue material and the inorganic carbon source and the process conditions of crushing treatment and sintering treatment, so that higher pole piece compaction and better battery performance are obtained.
In the invention, the crushing treatment is only to fully crush the materials, and preferably, the crushing treatment is ball milling treatment. The ball milling treatment in the invention is carried out by a ball mill. For example, the prussian blue material and the inorganic carbon source are mixed in a mixing stage and then subjected to ball milling treatment using a ball mill. The ball milling treatment is preferably wet ball milling, and during the wet ball milling, a certain amount of liquid auxiliary agent is added to assist the ball milling. Such liquid auxiliaries do not react with the material, preferably acetone, ethanol or water, for example acetone.
In order to obtain a more suitable material for the inorganic carbon coating of the present invention, in a preferred embodiment of the present invention, the ball milling speed of the ball milling process is 10 to 200rpm/min, such as 100rpm/min. The time of the ball milling treatment is not less than 15 hours, more preferably not less than 18 hours, and still more preferably not less than 20 hours. The time of the ball milling treatment is not more than 30 hours, more preferably not more than 28 hours, and more preferably not more than 26 hours. For example 24 hours
The process of the sintering treatment significantly affects the adhesion capability of the resulting carbon layer of the coating layer of the positive electrode material. At smaller particle sizes, the difficulty of using inorganic carbon coatings is greater. In a preferred embodiment of the invention, the sintering process is carried out in a muffle or tube furnace. The sintering treatment is carried out in an inert atmosphere.
The temperature of the sintering treatment is 200 to 300 ℃, more preferably 230 to 270 ℃.
The time of the sintering treatment is 6 to 24 hours, more preferably 9 to 21 hours.
Even for the same inorganic carbon source, the microstructure of the carbon layer formed by sintering is significantly affected by the difference in sintering temperature. For example, carbon nanotubes are used as the inorganic carbon source, the carbon layer formed at 220 ℃ has a longer particle distribution distance observed by an electron microscope, the carbon layer formed at 260 ℃ has a dense particle distribution observed by an electron microscope, and the carbon layer formed at 270 ℃ has a dense interweaving of the pipes observed by an electron microscope.
Positive pole piece
In the invention, the positive pole piece comprises a current collector and a positive slurry layer applied to the current collector, wherein the positive slurry layer comprises the positive pole material.
The positive pole piece is prepared by adopting a conventional method in the field. As the positive electrode, it can be prepared by a method conventional in the art. In some embodiments of the invention, the positive electrode material and additives such as binders (e.g., polyvinylidene fluoride (PVDF) or Styrene Butadiene Rubber (SBR)), conductive agents (e.g., carbon black or carbon nanotubes), fillers (e.g., graphitic carbon for coating viscosity modification, adhesion promoters, thickeners, such as carboxymethyl cellulose), and other additives known to those skilled in the art may be mixed into an appropriate coating solvent (such as water or N-methyl pyrrolidone (NMP)) to form a coating dispersion or coating mixture. The coating dispersion or coating mixture can be thoroughly mixed and then applied to the current collector by any suitable coating technique, such as reverse-tone coating, knife coating, dip coating, spray coating, electrospray coating, or gravure coating. The current collector may be a conductive metal can, such as, for example, copper, aluminum, stainless steel, or nickel foil. The slurry may be coated onto a current collector box and then allowed to dry in air, followed by drying, typically in a heated oven, typically at about 80 ℃ to about 300 ℃ for about 1 hour to remove the solvent. The resulting negative pole piece is then compressed to increase its density. For example, the positive electrode material, the conductive agent, and the binder are mixed, sufficiently stirred to form a positive electrode slurry, the slurry is coated on a positive electrode current collector (e.g., aluminum foil), and dried and compacted to obtain a positive electrode sheet.
As the conductive agent, there are included, but not limited to, carbon black, graphene, ketjen black, carbon nanotube, carbon fiber, acetylene black, furnace black, lamp black, or a mixture thereof, and the like.
Sodium ion battery
The invention also relates to a sodium ion battery, a positive pole piece, a negative pole piece, a diaphragm and electrolyte, wherein the positive pole piece is the positive pole piece in other aspects of the invention, and the negative pole piece, the diaphragm and the electrolyte can refer to the negative pole piece, the diaphragm and the electrolyte commonly used in the field of sodium ion batteries.
As the electrolytic solution, a nonaqueous solvent and an electrolyte are included. Examples of the nonaqueous solvent include organic carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, butylene carbonate, vinylene carbonate, and fluoroethylene carbonate. The electrolyte may be prepared by adding a sodium salt electrolyte to a non-aqueous solvent. Exemplary sodium salts include NaPF 6 、NaCIO 4 、NaN(CF 3 SO 2 ) 2 、NaN(C 2 F 5 SO 2 ) 2 、NaAsF 6 And combinations thereof. In some embodiments, a sodium ion battery including the provided positive electrode slurry layer can be prepared by taking one positive electrode and one negative electrode as described above and placing them in an electrolyte. Generally, a separator is used to prevent the anode from directly contacting the cathode. As the separator, a commercially available polyethylene separator or other separator may be used, and preferably, the thickness is 9 to 18 μm.
In order to make the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the present invention is further described below with reference to specific embodiments. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. The experimental procedures, in which specific conditions are not noted in the following examples, are generally carried out under conventional conditions or conditions recommended by the manufacturers. Unless otherwise indicated, percentages and parts are percentages and parts by weight. The test materials and reagents used in the following examples are commercially available without specific reference.
Unless otherwise defined, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs, and it is to be noted that the terms used herein are merely for describing particular embodiments and are not intended to limit example embodiments of the present application.
Fig. 1 and 2 show scanning electron micrographs of prussian blue materials prepared by the present application. The preparation conditions of the prussian blue material in fig. 1 are as follows: mixing Prussian blue (Na) 1.8 Fe[Fe(CN) 6 ]·1.6H 2 O), keqin black and acetone in a mass ratio of 1:0.11:1 into a ball milling tank. Adding zirconium bead balls for ball milling at the speed of 100rpm/min for 24h. And then placing the mixed materials in an oven at 80 ℃ for baking for 6 hours until the acetone solvent is completely removed. And placing the mixed material in a muffle furnace, and sintering for 9 hours at a sintering temperature of 270 ℃ in an inert atmosphere. And taking out the material after sintering, and physically dispersing in a mechanical mill to obtain the carbon-coated Prussian blue cathode material of the embodiment 1. The preparation conditions of the prussian blue material in fig. 2 are as follows: mixing Prussian blue (Na) 1.8 Fe[Fe(CN) 6 ]·1.6H 2 O), carbon nano tubes and acetone in a mass ratio of 1:0.02: the ratio of 1 is added into a ball milling tank. Adding zirconium bead balls for ball milling at the speed of 100rpm/min for 24h. Then the mixed materials are placed in an oven at 80 ℃ for baking for 6 hours until the acetone solvent is completely removed. And placing the mixed materials in a muffle furnace, and sintering for 12 hours at the sintering temperature of 250 ℃. And taking out the material after sintering is finished, and physically dispersing the material in a mechanical mill to obtain the target material. As can be seen from the scanning electron micrographs of fig. 1 and fig. 2, the prussian blue material expected to be obtained in the present application can be prepared under both of the above conditions, that is, the surface of the product presents a non-smooth outer layer structure, which can be regarded as being successfully coated with a carbon layer.
The test properties of the prussian blue material of the present application are further described below.
Example 1
Mixing Prussian blue (Na) 1.8 Fe[Fe(CN) 6 ]·1.6H 2 O), keqin black and acetone in a mass ratio of 1:0.11: the ratio of 1 is added into a ball milling tank. Adding zirconium bead balls for ball milling at the speed of 100rpm/min for 24h. And then placing the mixed materials in an oven at 80 ℃ for baking for 6 hours until the acetone solvent is completely removed. And placing the mixed material in a muffle furnace, and sintering for 9 hours at a sintering temperature of 270 ℃ in an inert atmosphere. And taking out the material after sintering, and physically dispersing the material in a mechanical mill to obtain the carbon-coated Prussian blue cathode material (shown in figure 1) of the embodiment 1.
The carbon-coated prussian blue positive electrode material prepared in example 1, super P, carbon Nanotubes (CNT), polyvinylidene fluoride (PVDF), N-methylpyrrolidone (NMP) were added to a stirring tank at a mass ratio of (80 to 90) to 4. The slurry was mixed for 6h at a stirring speed of 1000 rpm/min. Thereafter, the slurry was coated on an aluminum foil substrate by extrusion coating. And baking the coated pole piece for 15min at the baking temperature of 110 ℃, and removing the NMP solvent to obtain the positive pole piece.
Example 2
Mixing Prussian blue (Na) 1.8 Fe[Fe(CN) 6 ]·1.6H 2 O), keqin black and acetone in a mass ratio of 1:0.18: the ratio of 1 is added into a ball milling tank. Adding zirconium bead balls for ball milling at the speed of 100rpm/min for 24h. And then placing the mixed materials in an oven at 80 ℃ for baking for 6 hours until the acetone solvent is completely removed. And placing the mixed material into a muffle furnace or a tubular furnace, and sintering for 24 hours at a sintering temperature of 270 ℃. And taking out the material after sintering, and physically dispersing in a mechanical mill to obtain the target material.
A positive electrode sheet was prepared in the same manner as in example 1. Example 3
A carbon-coated prussian blue positive electrode material was prepared in the same manner as in example 1, except that the addition ratio of prussian blue, ketjen black, and acetone in this example was 1:0.06:1. and a positive electrode sheet was prepared in the same manner as in example 1.
Example 4
A carbon-coated prussian blue positive electrode material was prepared in the same manner as in example 1, except that the addition ratio of prussian blue, ketjen black, and acetone in this example was 1:0.25:1. and a positive electrode sheet was prepared in the same manner as in example 1.
Example 5
A carbon-coated prussian blue positive electrode material was prepared in the same manner as in example 1, except that the addition ratio of prussian blue, ketjen black, and acetone in this example was 1:0.01:1. and a positive electrode sheet was prepared in the same manner as in example 1.
Example 6
In the same manner as in example 1 and in the same material ratio as in example 4, the ratio of the prussian blue, ketjen black and acetone added was 1:0.06: the carbon-coated prussian blue cathode material was prepared in the manner of 1, except that in this example, the sintering temperature and the sintering time were different in the sintering step. In the present example, the sintering temperature was 230 ℃, the sintering time was 21 hours, and the other steps were the same. And a positive electrode sheet was prepared in the same manner as in example 1.
Example 7
A carbon-coated prussian blue positive electrode material was prepared in the same manner and in the same material ratio as in example 3, except that in this example, the sintering temperature and the sintering time in the sintering step were different. In the embodiment, the sintering temperature is 250 ℃, the sintering time is 15h, and the rest steps are the same. And a positive electrode sheet was prepared in the same manner as in example 1.
Example 8
A carbon-coated prussian blue positive electrode material was prepared in the same manner and in the same material ratio as in example 3, except that in this example, the sintering temperature and the sintering time in the sintering step were different. In the embodiment, the sintering temperature is 270 ℃, the sintering time is 9h, and the rest steps are the same. And a positive electrode sheet was prepared in the same manner as in example 1.
Example 9
A carbon-coated prussian blue positive electrode material was prepared in the same manner and in the same material ratio as in example 3, except that in this example, the sintering temperature and the sintering time in the sintering step were different. In the embodiment, the sintering temperature is 300 ℃, the sintering time is 6h, and the rest steps are the same. And a positive electrode sheet was prepared in the same manner as in example 1.
Example 10
A carbon-coated prussian blue positive electrode material was prepared in the same manner as in example 8, except that the ball milling time was different in this example. In this example, the ball milling time was 36 hours, and the rest of the steps were the same. And a positive electrode sheet was prepared in the same manner as in example 1.
Example 11
A carbon-coated prussian blue positive electrode material was prepared in the same manner as in example 8, except that in this example, the ball milling time was different. In this example, the ball milling time was 12 hours, and the rest of the steps were the same. And a positive electrode sheet was prepared in the same manner as in example 1.
Comparative example 1
A positive electrode material was prepared in the same manner as in example 1, except that in the sintering step, the sintering temperature and sintering time were different. In the comparative example, the sintering temperature was 360 ℃ and the sintering time was 6 hours. And a positive electrode sheet was prepared in the same manner as in example 1.
Comparative example 2
A positive electrode sheet was produced in the same manner as in example 1, except that glucose was used as a carbon source. The material obtained after sintering is shown in figure 3. As can be seen from fig. 3, the carbon-coated prussian blue cathode material cannot be obtained under the same conditions by using glucose as a carbon source, and the obtained product has a smooth surface and is not coated with a carbon layer successfully. Then, a positive electrode sheet was prepared in the same manner as in example 1.
The preparation conditions are summarized in Table 1 below.
TABLE 1
The inorganic carbon-coated Prussian blue anode material is easy to have insufficient adhesive force, so that a coating layer falls off and the performance of a pole piece is poor. In order to solve the problem, the inventor researches the influence of the carbon source proportion of the positive electrode material on the stacking angle of particles, the thickness of a coating layer, the compacted density of a pole piece and the resistivity of the pole piece, and measures the stacking angle of the carbon-coated prussian blue positive electrode material prepared in the embodiment, the compacted density of the pole piece and the volume resistivity of the pole piece according to the following methods:
[ method of measuring Stacking Angle ]
Dropping powder of more than 1kg onto a plane from the height of 0.2m at the speed of less than 1g/s, continuously accumulating until the powder is completely accumulated, and testing the acute included angle between the powder accumulation slope and the plane, namely the accumulation angle of the powder.
[ method for measuring compacted density of pole piece ]
And rolling the finished electrode plate for 2min under the pressure of 20MPa, and measuring the thickness and the quality of the electrode plate to calculate the compacted density of the electrode plate.
[ method for measuring volume resistivity of pole piece ]
And (3) rolling the finished electrode plate for 2min under the pressure of 20MPa, and testing the diaphragm resistance of the coated electrode plate by using a four-probe diaphragm resistance instrument.
The results obtained are referred to in tables 2 and 3 below.
TABLE 2
Table 2 shows the effect of changes in the amount of ketjen black added on the bank angle and the compacted density. As can be seen from the data in table 2, by adjusting the input mass ratio of the coated carbon material to the prussian blue, a positive electrode sheet with a higher compacted density can be obtained. In a preferred embodiment, when the mass ratio of prussian blue to ketjen is 1: (0.06-0.18), the pole piece compaction density can reach more than 1.9, and most preferably, when the mass ratio of the Prussian blue to the Keqin is 1: and when the pressure is 0.11, the pole piece is compacted to the highest degree, and the resistivity is lower.
The compaction density of the comparative example is low, the battery rate of the pole piece is high, and the compaction density of the positive pole piece prepared by adopting the Prussian blue material coated by the inorganic carbon source (the material stacking angle is 40-70 degrees, preferably 45-65 degrees) as the positive active material is high and can reach more than 1.5.
Table 3 shows the effect of the ball milling and sintering process on the performance of the resulting carbon-coated prussian blue positive electrode material and positive electrode sheet.
TABLE 3
As can be seen from the data in Table 3, when the sintering temperature is 230-270 ℃ and the sintering time is 9-21 hours, the stacking angle of the obtained pole piece is 40-70 degrees, and the pole piece compaction density can reach more than 1.8.
When the ball milling time is too high (more than 30 hours) or too low (less than 15 hours), the compaction density of the obtained pole piece is lower, about 1.4-1.6.
When the sintering temperature exceeds 270 ℃ and is close to 300 ℃, the obtained pole piece has low compaction density which is about 1.5.
In addition, when the sintering temperature exceeded 350 ℃, significant loss of active material was observed in the pole pieces, and the angle of repose and the compacted density could not be measured.
Sodium ion battery preparation
The electrode sheets prepared in examples and comparative examples were used as positive electrode sheets.
Preparing a negative pole piece: hard carbon, SP, CMC and SBR were mixed in a mass ratio of (90 to 95): 2. The negative electrode slurry was mixed at a stirring speed of 1000rpm/min for 6h. And then, coating the negative electrode slurry on a copper foil substrate in an extrusion coating mode. And baking the coated pole piece for 15min at the baking temperature of 110 ℃ to obtain the negative pole piece.
Preparing an electrolyte: adding sodium salt NaPF into non-aqueous solvent with mass ratio of EC, EMC, DMC and PC of (2-4): (3-5): (2-4): 0-1) 6 An electrolyte having an electrolyte content of 4 to 24wt%,
the separator is a conventional polyethylene film.
And stacking the positive pole piece, the diaphragm and the negative pole piece in sequence, and encapsulating and injecting electrolyte to obtain the sodium-ion battery core. According to the same method, the positive pole pieces obtained in the examples are used for preparing the sodium-ion battery.
Battery performance testing
In this example, the influence of different examples on the direct current internal resistance, cycle life and first coulombic efficiency of the obtained sodium ion battery was studied.
[ measurement of DC internal resistance ]
The SoC of the cell was adjusted to 50% at 25 ℃ at a current density of 0.063A/g (calculated as mass m0 of the positive electrode material), and after leaving for 2 hours, the voltage at that time V1 was recorded, and after discharging at a current density of 0.76A/g for 30s, the voltage at that time was recorded as V2, and the initial DCR value was (V1-V2)/(0.76 × m 0).
[ measurement of cycle Life ]
The cycle is carried out under the condition of 25 ℃ in a charge-discharge mode of 0.19A/g (calculated by the mass m0 of the anode material, the voltage range is 2.0-3.8V). And after the battery is circulated to the capacity of 80%, the obtained cycle number is the cycle life of the battery.
[ gram Capacity and first coulombic efficiency assay ]
At 25 deg.C, charging and discharging for one week in the 0.063A/g charging and discharging mode, and the obtained charging and discharging capacity is divided by the using amount of positive electrode, namely the first charging/discharging gram capacity. The first coulombic efficiency is obtained by dividing the first discharge capacity by the first charge capacity.
The results are shown in Table 4.
TABLE 4
As can be seen from the data in table 4, when the pole piece compaction density is low as shown in example 5, the dc resistance, cycle life and first coulombic efficiency are all low. On the other hand, when the compaction density is higher than 1.8 as shown in examples 1 to 3, 6 and the like, the dc resistance, the cycle life and the first coulombic efficiency all showed good performances.
It will be understood by those of ordinary skill in the art that the foregoing embodiments are specific examples of practicing the invention, and that various changes in form and detail may be made therein without departing from the spirit and scope of the invention in practice.
Claims (10)
1. A positive electrode material, characterized in that the positive electrode material comprises a prussian blue material and a carbon layer coating the prussian blue material.
2. The positive electrode material according to claim 1, wherein the carbon layer is composed of an inorganic carbon source selected from at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, ketjen black, and acetylene black.
3. The positive electrode material according to claim 1, wherein the Prussian blue material is Na x M[Fe(CN) 6 ]·nH 2 O, wherein x is more than or equal to 1.5 and less than or equal to 2.0, M is Fe or Mn, and n is more than or equal to 0 and less than or equal to 6.
4. The positive electrode material according to claim 2, wherein the mass ratio of the Prussian blue material to the inorganic carbon source is 1 (0.02-0.25).
5. The positive electrode material according to any one of claims 1 to 4, wherein a stacking angle of the positive electrode material is 40 to 70 °.
6. The positive electrode material according to any one of claims 1 to 4, wherein the carbon layer has a thickness of 50 to 300nm.
7. A method of preparing a positive electrode material, comprising the steps of:
mixing a Prussian blue material and an inorganic carbon source, and sintering, wherein the sintering temperature is lower than 350 ℃.
8. The method according to claim 7, wherein the temperature of the sintering process is 200-300 ℃;
and/or the time of the sintering treatment is 6 to 30 hours.
9. A positive electrode sheet comprising a current collector and a positive electrode slurry layer applied to the current collector, the positive electrode slurry layer comprising the positive electrode material according to any one of claims 1 to 6.
10. A sodium-ion battery comprising the positive electrode sheet, the negative electrode sheet, the separator and the electrolyte according to claim 9.
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