Porous graphite, preparation method and application thereof and sodium ion battery

Abstract

The application provides a kind of porous graphite and its preparation method and application and sodium ion battery, wherein the preparation method comprises: after mixing natural graphite and grinding ball, it is carried out in inert atmosphere sufficient ball milling, and ball-milled graphite is obtained;Graphite and pore-forming agent are added to the dispersion liquid and uniformly dispersed, and then heated to obtain graphite / pore-forming agent composite material;Graphite / pore-forming agent composite material and protective agent are added to water and uniformly mixed to obtain a mixed solution;The mixed solution is carried out after spray granulation and then dried to obtain graphite / pore-forming agent / protective agent composite material;The graphite / pore-forming agent / protective agent composite material is subjected to segmented heat treatment under the protection of inert atmosphere, and then the segmented heat treatment product is washed to neutral and dried to obtain the porous graphite.The surface phase and bulk phase of the porous graphite have abundant pore structure, i.e., have more number of sodium storage sites, which helps to further improve its sodium storage capacity.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of porous graphite and its preparation method and application and sodium ion battery, belong to sodium ion battery technical field. BACKGROUND

[0002] At present, the chemical power technology mainly based on secondary battery is a key link in new energy industry. Traditional lithium ion battery faces problems such as high production cost and lack of lithium resources. The market has a more urgent demand for environmentally friendly and low-cost secondary batteries. Sodium ion battery has similar working principle as lithium ion battery, and sodium resource is widely distributed, with low production cost. Sodium ion battery is expected to replace lithium ion battery in large-scale energy storage field, and is getting more and more attention in the battery field.

[0003] At present, sodium ion battery usually uses hard carbon obtained by high-temperature carbonization of carbon source precursor as negative electrode material, which has high cost and produces a large amount of energy consumption and carbon emission. Graphite is the most successful negative electrode material in lithium ion battery, with a lithium storage capacity of ~ 360 mAh / g. However, when graphite is applied to sodium ion battery negative electrode material, it only exhibits a sodium storage capacity of ~ 30 mAh / g. Related studies have shown that there is an adverse interaction between sodium ions and graphite layers, which inhibits the formation of high-order intercalation compounds between sodium ions and graphite. In view of the difficulty that graphite cannot reversibly store sodium, researchers have prepared expanded graphite by modifying the Hummers method, which expands the graphite interlayer spacing and improves the sodium storage capacity of graphite. However, this method requires strong acid and strong oxidant, and has high preparation cost. Another study shows that graphite material can reversibly store sodium in the form of "solvated sodium ion co-intercalation" in ether-based electrolyte, showing a reversible capacity of nearly 100 mAh / g. However, the sodium storage capacity of this method is still low.

[0004] Therefore, it has become a technical problem to be solved in the field to provide a new type of porous graphite and its preparation method and application and sodium ion battery. SUMMARY

[0005] In order to solve the above-mentioned shortcomings and deficiencies, one object of the present application is to provide a preparation method of porous graphite.

[0006] Another object of the present application is also to provide a porous graphite prepared by the above-mentioned preparation method of porous graphite.

[0007] Still another object of the present application is also to provide the above-mentioned application of porous graphite as a negative electrode material of sodium ion battery.

[0008] Still another object of the present application is also to provide a sodium ion battery, wherein the negative electrode material is the above-mentioned porous graphite.

[0009] To achieve the above object, in one aspect, the present application provides a method for preparing porous graphite, wherein the method comprises:

[0010] Step (1): mixing natural graphite and grinding balls and then performing sufficient ball milling in an inert atmosphere to obtain ball-milled graphite;

[0011] Step (2): adding the ball-milled graphite and a pore-forming agent into a dispersion liquid and uniformly dispersing them, and then heating to obtain a graphite / pore-forming agent composite material;

[0012] Step (3): adding the graphite / pore-forming agent composite material and a protective agent into water and uniformly mixing them to obtain a mixed liquid; performing spray granulation on the mixed liquid and then drying to obtain a graphite / pore-forming agent / protective agent composite material;

[0013] Step (4): performing segmented heat treatment on the graphite / pore-forming agent / protective agent composite material under inert atmosphere protection, washing the segmented heat treatment product to neutral, and then drying to obtain the porous graphite.

[0014] As a specific embodiment of the above-mentioned method for preparing porous graphite, in step (1), the mass ratio of the grinding balls to the natural graphite, i.e. the ball-to-material ratio, is 10:1-5:1.

[0015] As a specific embodiment of the above-mentioned method for preparing porous graphite, in step (1), the natural graphite and the grinding balls are first added into a ball milling tank, and then the ball milling tank is transferred to a planetary ball mill for sufficient ball milling to obtain the ball-milled graphite. In some embodiments of the present application, the grinding balls can be, for example, zirconia grinding balls, and the natural graphite can be, for example, flake graphite and / or spherical graphite.

[0016] As a specific embodiment of the above-mentioned method for preparing porous graphite, in step (1), the rotation speed of the ball milling is 100-500 rpm, and the time is 2-10 h.

[0017] As a specific embodiment of the above-mentioned method for preparing porous graphite, in step (1), the inert atmosphere includes argon or nitrogen, etc.

[0018] As a specific embodiment of the above-mentioned method for preparing porous graphite, in step (2), the mass ratio of the ball-milled graphite to the pore-forming agent is 10:1-1:1.

[0019] As a specific embodiment of the above-mentioned method for preparing porous graphite, in step (2), the uniform dispersion is achieved by ultrasonic, and the time of the ultrasonic is 10-60 min.

[0020] As a specific embodiment of the above preparation method of the present application, in step (2), the dispersion liquid comprises water or the like. The present application does not make specific requirements on the amount of water used in step (2) and step (3), and the amount of water can be reasonably adjusted according to the actual needs of the site operation.

[0021] As a specific embodiment of the above preparation method of the present application, in step (2), the heating temperature is 80-95℃. In some embodiments of the present application, the system after uniform dispersion is heated under stirring in step (2).

[0022] As a specific embodiment of the above preparation method of the present application, in step (2), the pore-forming agent comprises one or a combination of several of potassium hydroxide, zinc chloride, ammonium carbonate, and ammonium chloride, or the pore-forming agent comprises gluconate or the like.

[0023] As a specific embodiment of the above preparation method of the present application, in step (2), the gluconate comprises one or a combination of several of magnesium gluconate, potassium gluconate, and zinc gluconate or the like.

[0024] As a specific embodiment of the above preparation method of the present application, in step (3), the mass ratio of graphite / pore-forming agent composite material to protective agent is 100:1-20:1.

[0025] As a specific embodiment of the above preparation method of the present application, in step (3), the temperature of the spray granulation is 100-200℃. The present application does not have special restrictions on the specific operation of spray granulation, and the spray granulation operation well known to those skilled in the art can be used.

[0026] As a specific embodiment of the above preparation method of the present application, in step (3), the protective agent comprises one or a combination of several of sucrose, glucose, water-soluble starch, polyacrylonitrile, and polyvinylpyrrolidone or the like.

[0027] As a specific embodiment of the above preparation method of the present application, in step (3), the uniform mixing is achieved by ultrasonic stirring treatment, and the time of ultrasonic stirring treatment can be reasonably adjusted according to the actual needs of the site operation, as long as the purpose of uniform mixing can be achieved.

[0028] As a specific embodiment of the above preparation method of the present application, in step (4), the segmented heat treatment comprises first-stage heat treatment and second-stage heat treatment, wherein the temperature of the first-stage heat treatment is 100-500℃, and the time is 1-4h, and the temperature of the second-stage heat treatment is 700-1200℃, and the time is 1-7h.

[0029] The preparation method provided by the present application can improve the number of sodium storage active sites of the graphite and enhance the sodium storage performance of the graphite by adopting appropriate ball-to-material ratio, ball milling speed, ball milling time, pore-forming agent dosage, heat treatment temperature and time and other process parameters to construct rich microporous and mesoporous structures in the graphite phase and surface phase.

[0030] As a specific embodiment of the above preparation method of the present application, in steps (1) and (4), the inert atmosphere includes argon or nitrogen and the like.

[0031] As a specific embodiment of the above preparation method of the present application, in step (4), the segmented heat treatment can be performed in a tube furnace.

[0032] As a specific embodiment of the above preparation method of the present application, in step (4), the segmented heat treatment product is washed to neutral by using inorganic acid and deionized water respectively, or the segmented heat treatment product is washed to neutral by using deionized water.

[0033] In steps (3) and (4) of the above preparation method of the present application, drying treatment and baking and the like are all conventional operations, and appropriate modes and appropriate process parameters (including temperature and time and the like) can be selected for drying and baking according to the actual operation needs on site, as long as the purpose of drying and baking can be achieved.

[0034] On the other hand, the present application also provides a porous graphite, wherein the porous graphite is prepared by the above preparation method of the porous graphite, and the surface phase and the body phase both have rich pores.

[0035] In another aspect, the present application also provides the application of the above porous graphite as a negative electrode material of a sodium ion battery.

[0036] In still another aspect, the present application also provides a sodium ion battery, wherein the negative electrode material of the sodium ion battery is the above porous graphite.

[0037] In the preparation of the porous graphite, the natural graphite and the grinding ball are mixed and then ball milled in an inert atmosphere to obtain the ball milled graphite. The ball milling in this step can reduce the particle size of the graphite, increase the specific surface area of the graphite, and expand the graphite interlayer spacing, which is beneficial to the subsequent diffusion of the pore-forming agent in the melt state to the bulk phase to manufacture the bulk phase pores. Then, the ball milled graphite and the pore-forming agent are added to a dispersion liquid and uniformly dispersed, and then heated to obtain a graphite / pore-forming agent composite material. Then, the graphite / pore-forming agent composite material and a protective agent are added to water and uniformly mixed, and then the obtained mixed liquid is spray granulated. Through the spray granulation in this step, the pore-forming agent can be located on the surface and inside of the particles as a template agent or an etching agent. Specifically, when the pore-forming agent is one of potassium hydroxide, zinc chloride, ammonium carbonate and ammonium chloride, the pore-forming agent is an etching agent, and after the spray granulation, the etching agent is distributed on the surface and inside of the particles. At this time, the etching agent and the ball milled graphite composite particle structure are formed. In the preheating treatment step, i.e. the first stage heat treatment step, the etching agent is in a molten state, and the molten state etching agent can diffuse to the graphite bulk phase. At the same time, part of the molten state etching agent still exists on the surface of the particles. In the second stage heat treatment process, the etching agent existing on the surface and in the bulk phase can etch the surface carbon layer and the bulk phase carbon layer of the ball milled graphite to construct a rich surface and bulk phase pore structure. After acid washing, the residual etching agent inside is removed to further construct the bulk phase pore structure.

[0038] When the pore-forming agent is a gluconate salt, the pore-forming agent is a template agent, and after the spray granulation, the template agent is distributed on the surface and inside of the particles. At this time, the template agent and the ball milled graphite composite particle structure are formed. In the heat treatment process, the gluconate salt decomposes to generate gas and metal oxides. The gas escapes to enrich the internal pore structure of the particles. The removal of the metal oxides by acid washing can further construct pores in the bulk phase. At the same time, the decomposition / pyrolysis of the template agent existing on the surface of the particles in the heat treatment process can also construct a rich surface and bulk phase pore structure on the surface of the particles.

[0039] The porous graphite is prepared only through the three main steps of ball milling, spray granulation and heat treatment pore forming, and has the advantages of simple process, safety and low cost.

[0040] The porous graphite provided by the application has rich pore structures (micropores and mesopores) in the surface and bulk phases, i.e. has more number of sodium storage sites, which helps to further improve the sodium storage capacity. When the porous graphite is used as a negative electrode material of a sodium ion battery with an ester-based or ether-based electrolyte, it can exhibit higher sodium storage capacity compared with the natural graphite bulk, and has a good application prospect in long cycle and low cost sodium ion battery negative electrode materials. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to make the technical solutions in the embodiments of the present application or the prior art clearer, the accompanying drawings needed in the embodiments will be briefly introduced. Obviously, the accompanying drawings in the following description are some embodiments of the present application, and based on these drawings, those skilled in the art can obtain other drawings without any creative effort.

[0042] Figures 1-3 SEM image of sample 1 in test example 1 of the present application.

[0043] Figure 4 SEM image of sample 2 in test example 1 of the present application.

[0044] Figure 5 SEM image of comparative sample 1 in test example 1 of the present application.

[0045] Figure 6 Charge-discharge curve of battery 1-1 and battery 1-2 in test example 2 of the present application at 0.1C (1C=200 mA / g) current density in the voltage range of 0.005-2.5V.

[0046] Figure 7 Cycle curve of battery 1-2 in test example 2 of the present application at 1C (1C=200 mA / g) current density in the voltage range of 0.005-2.5V.

[0047] Figure 8 Cycle curve of battery 2-1 in test example 2 of the present application at 1C (1C=200 mA / g) current density in the voltage range of 0.005-2.5V.

[0048] Figure 9 Charge-discharge curve of comparative battery 1-1 and comparative battery 1-2 in test example 2 of the present application at 0.1C (1C=200 mA / g) current density in the voltage range of 0.005-2.5V.

[0049] Figure 10 Cycle data of comparative battery 1-2 in test example 2 of the present application at 1C current density in the voltage range of 0.005-2.5V.

[0050] Figure 11 Pore size distribution curve of sample 1 in test example 1 of the present application obtained by BET test.

[0051] Figure 12 Pore size distribution curve of sample 2 in test example 1 of the present application obtained by BET test. DETAILED DESCRIPTION

[0052] It has to be understood that the terms "comprising", "including", "containing", "characterized by" and any other variation thereof in the specification and in the claims are not to be construed as excluding any feature, step or element, but are meant to encompass the possibility of non-exclusive inclusion. Thus, the methods and compositions described herein can include, consist essentially of, or consist of, any element or combination of elements described herein, in any order or arrangement.

[0053] The ranges disclosed herein are given in their absolute form. They can each be one or more lower limits, and one or more upper limits. A given range is defined by selecting one lower limit and one upper limit. The selected lower and upper limits define the boundaries of a particular range. All ranges defined in this way are combinable, i.e. any lower limit can be combined with any upper limit to form a range. For example, where ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. Furthermore, if a minimum range value of 1 and 2 is listed, and a maximum range value of 3, 4 and 5 is listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.

[0054] In the present application, unless otherwise stated, the numerical range "a-b" indicates a shorthand way of describing all the individual real combinations between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all the real numbers between "0-5" have been listed in the present application, "0-5" is just a shorthand way of describing these numerical combinations.

[0055] In the present application, unless otherwise stated, all the embodiments and preferred embodiments mentioned in the present application can be combined with each other to form new technical solutions.

[0056] In the present application, unless otherwise stated, all the technical features and preferred features mentioned in the present application can be combined with each other to form new technical solutions.

[0057] In the present application, unless otherwise stated, the term "two" used in the present application means "at least two".

[0058] In the present application, if not otherwise specified, all the steps mentioned herein can be carried out in sequence or randomly, but preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method mentioned herein can further comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0059] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below in combination with the drawings and examples. The examples described below are part of the examples of the present application, but not all the examples, which are only used to illustrate the present application, and should not be regarded as limiting the scope of the present application. Based on the examples in the present application, all the other examples obtained by those skilled in the art without making creative efforts fall within the scope of protection of the present application. If the specific conditions are not specified in the examples, the conventional conditions or the conditions recommended by the manufacturer are used. If the reagents or instruments used are not specified by the manufacturer, they are all conventional products that can be purchased on the market.

[0060] Porous graphite examples and comparative examples

[0061] Example 1

[0062] The present example provides a porous graphite, which is prepared by a preparation method comprising the following specific steps:

[0063] Step (1): 2 g of natural graphite (flake graphite) is weighed and added to a ball mill tank with zirconium oxide grinding balls according to a ball-to-material ratio of 10:1; the ball mill tank is in an argon atmosphere; the ball mill tank is transferred to a planetary ball mill and ball milled at a speed of 400 rpm for 3 h, and the ball milled graphite is collected.

[0064] Step (2): 3 g of the ball milled graphite and KOH are added to deionized water according to a mass ratio of 2:1, and after ultrasonic treatment for 30 min, the solvent is heated and stirred to dryness at a temperature of 80°C, and a graphite / KOH composite material is obtained.

[0065] Step (3): the graphite / pore-forming agent composite material and sucrose are mixed in deionized water according to a mass ratio of 100:1, and stirred at 400 rpm for 2 h, followed by ultrasonic treatment for 1 h; the obtained mixture is then spray granulated, the outlet temperature is 120°C, and the granulated material is collected and dried in a vacuum oven at 60°C for 12 h, to obtain a graphite / KOH / sucrose composite material.

[0066] Step (4): The graphite / KOH / cane sugar composite material was transferred to a tube furnace for staged heat treatment under an argon atmosphere. Specifically, the tube furnace was first raised to 400°C at a temperature increase rate of 5°C / min, and held for 4 h. Then, the tube furnace was raised to 800°C at a temperature increase rate of 5°C / min, and held for 4 h, to obtain a staged heat treatment product.

[0067] Step (5): After the staged heat treatment product was washed to neutral with deionized water, it was dried at 80°C for 12 h to obtain a porous graphite material, which was recorded as sample 1.

[0068] Example 2

[0069] This example provides a porous graphite prepared by a preparation method comprising the following specific steps:

[0070] Step (1): 3 g of natural graphite (flake graphite) was weighed, and zirconium oxide grinding balls were added to a ball mill tank at a ball-to-material ratio of 10:1. The ball mill tank was filled with nitrogen. The ball mill tank was then transferred to a planetary ball mill and ball milled at a speed of 400 rpm for 3 h to obtain ball milled graphite.

[0071] Step (2): 3 g of the ball milled graphite and magnesium gluconate were added to deionized water at a mass ratio of 2:1. After ultrasonic treatment for 30 min, the solvent was evaporated by heating and stirring at a temperature of 80°C to obtain a graphite / magnesium gluconate composite material.

[0072] Step (3): The graphite / magnesium gluconate composite material and cane sugar were mixed in deionized water at a mass ratio of 100:1 to obtain a mixture. The mixture was stirred at 400 rpm for 2 h and then ultrasonically treated for 1 h. The mixture was then spray granulated, and the outlet temperature was 120°C. The granulated material was collected and dried in an oven at 60°C under vacuum for 12 h to obtain a graphite / magnesium gluconate / cane sugar composite material.

[0073] Step (4): The graphite / magnesium gluconate / cane sugar composite material was transferred to a tube furnace for staged heat treatment under an argon atmosphere. Specifically, the tube furnace was first raised to 500°C at a temperature increase rate of 5°C / min, and held for 4 h. Then, the tube furnace was raised to 1000°C at a temperature increase rate of 5°C / min, and held for 4 h, to obtain a staged heat treatment product.

[0074] Step (5): The staged heat treatment product was first washed with hydrochloric acid to remove magnesium oxide, and then washed with deionized water until the product was neutral. The product was dried at 80°C for 12 h to obtain a porous graphite material, which was recorded as sample 2.

[0075] Comparative Example 1

[0076] The comparative example provides a ball-milled and step-heat-treated graphite, which is prepared by a preparation method comprising the following specific steps:

[0077] Step (1): 2 g of natural graphite (flake graphite) was weighed, and was added into a ball mill tank with zirconium oxide grinding balls at a ball-to-material ratio of 10:1. The ball mill tank was then transferred to a planetary ball mill and ball-milled at a rotation speed of 400 rpm for 3 h. The ball-milled graphite was collected.

[0078] Step (2): The ball-milled graphite was transferred to a tube furnace and was subjected to step-heat treatment under an argon atmosphere. Specifically, the tube furnace was first raised to 400 ℃ at a temperature raising rate of 5 ℃ / min, and was kept at 400 ℃ for 4 h. Then, the tube furnace was raised to 800 ℃ at a temperature raising rate of 5 ℃ / min, and was kept at 800 ℃ for 4 h. The ball-milled and step-heat-treated graphite was obtained, and was recorded as comparative sample 1.

[0079] Sodium-ion battery examples and comparative examples

[0080] Example 1-1

[0081] The example provides a sodium-ion battery, which is assembled by an assembling method comprising the following specific steps:

[0082] Sample 1, acetylene black and polyacrylic acid were mixed at a mass ratio of 8:1:1 to obtain a slurry.

[0083] The slurry was coated on an aluminum foil to prepare a negative electrode sheet. A button cell was assembled in an argon glove box with the negative electrode sheet as a working electrode, a piece of metallic sodium as a counter electrode, Whatman glass fiber as a separator, and a 1M NaPF6 solution of diethylene glycol dimethyl ether as an electrolyte, and was recorded as battery 1-1.

[0084] Example 1-2

[0085] The example provides a sodium-ion battery, which is assembled by an assembling method comprising the following specific steps:

[0086] Sample 1, acetylene black and polyacrylic acid were mixed at a mass ratio of 8:1:1 to obtain a slurry.

[0087] The slurry was coated on an aluminum foil to prepare a negative electrode sheet. A button cell was assembled in an argon glove box with the negative electrode sheet as a working electrode, a piece of metallic sodium as a counter electrode, Whatman glass fiber as a separator, and a 1M NaPF6 solution of EC / EMC (the volume ratio of ethylene carbonate to methyl ethyl carbonate was 1:1) as an electrolyte, and was recorded as battery 1-2.

[0088] Example 2-1

[0089] The embodiment provides a sodium ion battery which is assembled by using an assembling method comprising the following specific steps.

[0090] Sample 2, acetylene black and polyacrylic acid are mixed according to a mass ratio of 8:1:1 to obtain a slurry;

[0091] The slurry is coated on an aluminum foil to prepare a negative electrode sheet, the negative electrode sheet is used as a working electrode, a metal sodium sheet is used as a counter electrode, Whatman glass fiber is used as a diaphragm, and a 1M NaPF6 diethylene glycol dimethyl ether solution is used as an electrolyte, and a button cell is assembled in an argon glove box, and the button cell is recorded as battery 2-1.

[0092] Embodiment 2-2

[0093] The embodiment provides a sodium ion battery which is assembled by using an assembling method comprising the following specific steps.

[0094] Sample 2, acetylene black and polyacrylic acid are mixed according to a mass ratio of 8:1:1 to obtain a slurry;

[0095] The slurry is coated on an aluminum foil to prepare a negative electrode sheet, the negative electrode sheet is used as a working electrode, a metal sodium sheet is used as a counter electrode, Whatman glass fiber is used as a diaphragm, and a 1M NaPF6 diethylene glycol dimethyl ether solution is used as an electrolyte, and a button cell is assembled in an argon glove box, and the button cell is recorded as battery 2-2.

[0096] Comparative example 1-1

[0097] The comparative example provides a sodium ion battery which is assembled by using an assembling method comprising the following specific steps.

[0098] Comparative sample 1, acetylene black and polyacrylic acid are mixed according to a mass ratio of 8:1:1 to obtain a slurry;

[0099] The slurry is coated on an aluminum foil to prepare a negative electrode sheet, the negative electrode sheet is used as a working electrode, a metal sodium sheet is used as a counter electrode, Whatman glass fiber is used as a diaphragm, and a 1M NaPF6 diethylene glycol dimethyl ether solution is used as an electrolyte, and a button cell is assembled in an argon glove box, and the button cell is recorded as comparative battery 1-1.

[0100] Comparative example 1-2

[0101] The comparative example provides a sodium ion battery which is assembled by using an assembling method comprising the following specific steps.

[0102] Comparative sample 1, acetylene black and polyacrylic acid are mixed according to a mass ratio of 8:1:1 to obtain a slurry;

[0103] The slurry was coated on aluminum foil to make a negative electrode sheet, a sodium metal sheet was used as a counter electrode, Whatman glass fiber was used as a separator, and a 1M NaPF6 EC / EMC solution (volume ratio of ethylene carbonate to methyl ethyl carbonate was 1:1) was used as an electrolyte to assemble a coin cell in an argon glove box, which was recorded as a comparative cell 1-2.

[0104] Test Example 1

[0105] The sample 1, the sample 2 and the comparative sample 1 were respectively subjected to SEM testing, and the sample 1 and the sample 2 were subjected to BET testing, wherein the SEM image of the sample 1 is as shown in Figures 1-3 , the pore size distribution curve of the sample 1 obtained by BET testing is as shown in Figure 11 , the SEM image of the sample 2 is as shown in Figure 4 , and the pore size distribution curve of the sample 2 obtained by BET testing is as shown in Figure 12 , and the SEM image of the comparative sample 1 is as shown in Figure 5 .

[0106] As can be seen from Figure 1 and Figure 2 , the particle size of the sample 1 is uneven, and the surface of the graphite material is rough. As can be clearly seen from the high magnification SEM image as shown in Figure 3 , the porous structure on the surface of the graphite and the holes inside the graphite can be clearly seen. The SEM data show that after the pore-forming agent is activated and pores are formed in the embodiment 1 of the present application, there are rich pore structures in the surface phase and the bulk phase of the graphite. The pore size distribution curve as shown in Figure 11 proves that the sample 1 has a large amount of microporous structure and mesoporous structure.

[0107] As can be seen from Figure 4 , the porous graphite particles obtained after spray granulation have uneven particle size, the surface of the particle material is rough, and the particle material has a spherical morphology and obvious holes. In the embodiment 2 of the present application, magnesium gluconate is used as a pore-forming agent, and is located inside the granulated material after spray granulation. During the subsequent heat treatment process, the magnesium gluconate is dehydrogenated and deoxidized, which on the one hand produces gas to enrich the pores inside the particles, and on the other hand forms magnesium oxide particles during the heat treatment process. The subsequent acid washing to remove the magnesium oxide particles will further enrich the pore structure inside the particles. The pore size distribution curve as shown in Figure 12 proves that the sample 2 also has a large amount of microporous structure and mesoporous structure.

[0108] As can be seen from Figure 5 , after simple ball milling and segmented heat treatment, the material, i.e. the comparative sample 1, has a broken graphite particle morphology, and the surface is relatively smooth without obvious pores.

[0109] Test Example 2

[0110] The test example respectively carries out electrochemical test on the assembled battery 1-1, battery 1-2, battery 2-1, battery 2-2, comparative battery 1-1, comparative battery 1-2 on the LAND battery test system, and specifically includes: after the above-mentioned battery is placed for 8h, the charge-discharge cycle is carried out at a rate of 0.05-5C, the test voltage range is 0.005-2.5V, and the 1C current density is defined as 200mA / g.

[0111] The first cycle charge-discharge curve of the battery 1-1, the battery 1-2 (i.e. the battery assembled by the sample 1 under the ether-based electrolyte and the ester-based electrolyte respectively) under the current density of 0.1C (1C = 200mA / g) in the voltage interval of 0.005-2.5V is as shown in Figure 6 It can be seen from Figure 6 that the sample 1 presents a reversible capacity of about 160mAh / g in the ether-based electrolyte, has a high first cycle coulombic efficiency of 80%; and presents a reversible capacity of about 170mAh / g in the ester-based electrolyte, has a first cycle coulombic efficiency of 41%.

[0112] The cycle curve of the battery 1-2 (i.e. the battery assembled by the sample 1 under the ester-based electrolyte) under the current density of 1C (1C = 200mA / g) in the voltage interval of 0.005-2.5V is as shown in Figure 7 It can be seen from Figure 7 that the sample 1 under the ester-based electrolyte can still maintain a high specific capacity of 155mAh / g after 100 cycles.

[0113] The cycle curve of the battery 2-1 (i.e. the battery assembled by the sample 2 under the ether-based electrolyte) under the current density of 1C (1C = 200mA / g) in the voltage interval of 0.005-2.5V is as shown in Figure 8 It can be seen from Figure 8 that the sample 2 under the ether-based electrolyte can still maintain a high specific capacity of 175mAh / g after 100 cycles.

[0114] The first cycle charge-discharge curve of the comparative battery 1-1, the comparative battery 1-2 (i.e. the battery assembled by the comparative sample 1 under the ether-based electrolyte and the ester-based electrolyte respectively) under the current density of 0.1C (1C = 200mA / g) in the voltage interval of 0.005-2.5V is as shown in Figure 9 It can be seen from Figure 9 that the comparative sample 1 only presents a reversible capacity of about 10mAh / g in the ester-based electrolyte, has a first cycle coulombic efficiency of 15%, and has a poor sodium storage capacity; and presents a reversible capacity of about 110mAh / g in the ether-based electrolyte, has a first cycle coulombic efficiency of 86%.

[0115] The cycling data of comparison batteries 1-2 (i.e., the batteries assembled from comparison sample 1 using ester electrolyte) at 1C current density within a voltage window of 0.005-2.5V are shown in the figure below. Figure 10 As shown. From Figure 10 As can be seen, compared to sample 1, which has almost no sodium storage capacity at a current density of 1C in the ester electrolyte.

[0116] contrast Figure 6 and Figure 9 The data shows that, compared to Comparative Sample 1, Sample 1 provided in this embodiment of the invention exhibits significantly improved sodium storage capacity under both ether-based and ester-based electrolyte conditions. Specifically, under ester-based electrolyte conditions, the sodium storage capacity increased from 10 mAh / g to a specific capacity of 170 mAh / g, and under ether-based electrolyte conditions, the sodium storage capacity increased from 110 mAh / g to a specific capacity of 160 mAh / g. This indicates that the formation of a large number of pore structures contributes to the improvement of sodium storage capacity of graphite materials.

[0117] contrast Figures 7-8 and Figure 10 The data shows that, compared to control sample 1, the porous graphite materials, namely samples 1 and 2, exhibit significantly improved sodium storage stability under ester-based and ether-based electrolyte conditions. Specifically, sample 2 maintained a high specific capacity of 175 mAh / g after 100 cycles under ether-based electrolyte conditions, while sample 1 maintained a high specific capacity of 155 mAh / g after 100 cycles under ester-based electrolyte conditions. This indicates that the beneficial effect of the porous structure on the graphite material during cycling is stable.

[0118] The above description is merely a specific embodiment of the present invention and should not be construed as limiting the scope of the invention. Therefore, any substitution of equivalent components or equivalent changes and modifications made within the scope of protection of this patent should still fall within the scope of this patent. Furthermore, the technical features, technical features and technical inventions, and technical inventions in this invention can be freely combined and used.

Claims

1. A method for preparing porous graphite, characterized in that, The preparation method includes: Step (1): Mix natural graphite and grinding balls and then ball mill them thoroughly in an inert atmosphere to obtain ball-milled graphite; Step (2): Add the ball-milled graphite and the pore-forming agent to the dispersion and disperse them evenly. Then heat the mixture to obtain the graphite / pore-forming agent composite material. The pore-forming agent is one or a combination of ammonium carbonate and ammonium chloride, or the pore-forming agent is gluconate. Step (3): Add the graphite / pore-forming agent composite material and the protective agent to water and mix them evenly to obtain a mixture; spray granulate the mixture and then dry it to obtain the graphite / pore-forming agent / protective agent composite material; in step (3), the protective agent includes one or a combination of several of sucrose, glucose, water-soluble starch, polyacrylonitrile and polyvinylpyrrolidone. Step (4): The graphite / pore-forming agent / protectant composite material is subjected to segmented heat treatment under an inert atmosphere, and the segmented heat treatment product is washed until neutral and then dried to obtain the porous graphite. In step (4), the segmented heat treatment includes a first stage heat treatment and a second stage heat treatment, wherein the temperature of the first stage heat treatment is 100-500°C. o C, the time is 1-4 hours, and the temperature of the second stage of heat treatment is 700-1200℃. o C, the time is 1-7 hours.

2. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of grinding balls to natural graphite is 10:1-5:

1.

3. The preparation method according to claim 1 or 2, characterized in that, In step (1), the ball mill rotates at 100-500 rpm for 2-10 hours.

4. The preparation method according to claim 1 or 2, characterized in that, In step (1), the inert atmosphere includes argon or nitrogen.

5. The preparation method according to claim 1, characterized in that, In step (2), the mass ratio of ball milled graphite to pore-forming agent is 10:1-1:

1.

6. The preparation method according to claim 1 or 5, characterized in that, In step (2), the uniform dispersion is achieved by ultrasound, and the ultrasound time is 10-60 min.

7. The preparation method according to claim 1 or 5, characterized in that, In step (2), the heating temperature is 80-95°C. o C.

8. The preparation method according to claim 1 or 5, characterized in that, In step (2), the gluconate includes one or a combination of magnesium gluconate, potassium gluconate and zinc gluconate.

9. The preparation method according to claim 1, characterized in that, In step (3), the mass ratio of graphite / pore-forming agent composite material to protective agent is 100:1-20:

1.

10. The preparation method according to claim 1 or 9, characterized in that, In step (3), the temperature of the spray granulation is 100-200°C. o C.

11. The preparation method according to claim 1, characterized in that, In step (4), the inert atmosphere includes argon or nitrogen.

12. A porous graphite, characterized in that, The porous graphite is prepared by the method of any one of claims 1-11, and both its surface phase and bulk phase have abundant pores.

13. The application of the porous graphite of claim 12 as a negative electrode material for sodium-ion batteries.

14. A sodium-ion battery, characterized in that, The negative electrode material of the sodium-ion battery is the porous graphite as described in claim 12.

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