A phosphorus-doped hard carbon composite material, its preparation method and application

Abstract

The present application provides a phosphorus-doped hard carbon composite material, a preparation method thereof and an application. The method includes: mixing alginate and phosphate in water, and then adding a calcium salt solution to obtain a gel-like product; drying the gel-like product to obtain a precursor material; ball-milling the precursor material and coal to obtain a powder material; placing the powder material in an inert gas, and then introducing an oxygen-containing gas to perform low-temperature carbonization so that the precursor material and coal are co-gasified to obtain an intermediate product; performing high-temperature carbonization on the intermediate product to obtain a phosphorus-doped hard carbon composite material. The method provided by the present application first prepares a biomass precursor material rich in phosphorus, then dries it and ball-mills it with coal, then performs co-gasification in an oxygen-containing gas, and finally performs high-temperature carbonization, which can not only increase the doping amount of phosphorus element, but also simultaneously form a rich closed-pore structure, and the prepared phosphorus-doped hard carbon composite material has excellent reversible discharge specific capacity and first Coulomb efficiency.

Description

Technical Field

[0001] The present invention belongs to the field of sodium-ion batteries, and in particular, relates to a phosphorus-doped hard carbon composite material, a preparation method thereof, and an application thereof. Background Art

[0002] With the development of the new energy industry, lithium-ion batteries have been widely used in industries such as electric vehicles, but their further development is restricted by a series of key problems such as limited lithium resource reserves and high costs of lithium-ion batteries.

[0003] Sodium-ion batteries have the advantages of low cost, abundant and widely distributed sodium reserves. As the anode material of sodium-ion batteries, hard carbon has more defects and micropores in its disordered amorphous structure, which can provide more active sodium storage sites in the hard carbon composite material. At the same time, hard carbon has a large interlayer spacing, which is beneficial to the diffusion of sodium ions and high stability of the structure during the sodiation / delithiation process. However, the main problem of hard carbon composite materials is their inherently low initial Coulomb efficiency.

[0004] The content of the background art part is only the technology known to the inventor and does not of course represent the prior art in this field. Summary of the Invention

[0005] The purpose of the present invention is to provide a new phosphorus-doped hard carbon composite material to solve problems such as low initial Coulomb efficiency.

[0006] The first aspect of the present application provides a phosphorus-doped hard carbon composite material, wherein the doping amount of phosphorus element is 5.5-6.5%; the phosphorus-doped hard carbon composite material satisfies: 4.11 ≤ SSA + (P / (P + C)) * d 002 ≤ 4.53, where SSA is the specific surface area of the phosphorus-doped hard carbon composite material, P / (P + C) is the mass fraction of phosphorus element in the phosphorus-doped hard carbon composite material, and d 002 is the carbon layer spacing of the phosphorus-doped hard carbon composite material.

[0007] Phosphorus doping can inhibit the ordered stacking of graphene layers, increase the specific surface area and provide a large number of active sites, and can also increase the interlayer spacing of carbon materials, thereby improving their catalytic performance and significantly increasing the reversible capacity of hard carbon materials. The P-C bond has high electrochemical activity and can participate in redox reactions. The adsorption of alkali metal ions by the P-C bond and the reversible redox process of the P-C bond during cycling are both beneficial to the improvement of electrochemical performance.

[0008] The relatively high doping amount of phosphorus element can effectively increase the interlayer spacing, promote the efficient electron transfer, and effectively improve the initial Coulomb efficiency of the phosphorus-doped hard carbon composite material.

[0009] In some embodiments of the present application, the phosphorus-doped hard carbon composite material satisfies at least one of the following conditions:

[0010] a) The specific surface area of the phosphorus-doped hard carbon composite material is 4.1 - 4.5 cm 2 / g;

[0011] b) The carbon layer spacing of the phosphorus-doped hard carbon composite material is ≥ 0.39 nm;

[0012] c) The average pore diameter of the phosphorus-doped hard carbon composite material is 3.5 - 7.0 nm;

[0013] d) The total pore volume of the phosphorus-doped hard carbon composite material is 0.4 - 0.5 cm 3 / g, the total pore volume of micropores with a pore diameter of 0 - 2 nm is ≥ 0.12 cm 3 / g, the microporosity is ≥ 20%, and the phosphorus-doped hard carbon composite material includes closed pores;

[0014] e) The ratio of the intensity of the amorphous carbon peak to the graphitized carbon peak of the phosphorus-doped hard carbon composite material I D / I G is 2.1 - 2.5.

[0015] The performance of the hard carbon negative electrode material that satisfies any one of the above conditions is better.

[0016] The second aspect of the present application provides a method for preparing a phosphorus-doped hard carbon composite material, which includes the following steps:

[0017] S1: Mix alginate and phosphate in water, and then add a calcium salt solution to obtain a gel-like product;

[0018] S2: Dry the gel-like product to obtain a precursor material;

[0019] S3: Mix the precursor material and coal and then ball-mill to obtain a powder material;

[0020] S4: Place the powder material in an inert gas, then introduce an oxygen-containing gas, and perform low-temperature carbonization to co-gasify the precursor material and the coal to obtain an intermediate product;

[0021] S5: Perform high-temperature carbonization on the intermediate product to obtain the phosphorus-doped hard carbon composite material.

[0022] The method provided by the present application first prepares a biomass precursor material rich in phosphorus, then dries it and ball-mills it with coal, then performs co-gasification in an oxygen-containing gas, and finally performs high-temperature carbonization. It can not only increase the doping amount of phosphorus element, but also simultaneously form a rich closed pore structure, and the prepared phosphorus-doped hard carbon composite material has excellent reversible discharge specific capacity and first Coulomb efficiency.

[0023] In some embodiments of the present application, in step S1, the dosage ratio of the alginate to the phosphate is 1:(1-2);

[0024] Optionally, the phosphate is selected from one or more of sodium hydrogen phosphate, sodium dihydrogen phosphate, and dodecyl phosphate;

[0025] Optionally, the molar concentration of the calcium salt solution is 0.1-0.5 mol / L;

[0026] Optionally, the calcium salt is selected from one or several of calcium chloride, calcium gluconate, calcium nitrate, calcium bicarbonate, calcium acetate, calcium hypochlorite, calcium perchlorate, calcium permanganate, and calcium lactate;

[0027] Optionally, the addition amount of the calcium salt solution is 10-20% of the total weight of the alginate, the phosphate, and water;

[0028] Optionally, the alginate and the phosphate are stirred and mixed in water for 10-14 h.

[0029] In this step, after the alginate and the phosphate are mixed evenly in water, a calcium salt solution is added to generate a gel-like calcium alginate-based phosphate complex.

[0030] In some embodiments of the present application, in step S3, the coal is selected from one or more of bituminous coal, anthracite, and lignite;

[0031] Optionally, the dosage ratio of the precursor material to the coal is 1:(2-2.5);

[0032] Optionally, the rotation speed of the ball milling is 150-250 r / min, and the time is 2-4 h.

[0033] In this step, the precursor material and the coal are ball milled and then mixed evenly to form a powder material, which is beneficial to the subsequent gasification.

[0034] In some embodiments of the present application, in step S4, the temperature of the low-temperature carbonization is 300-500 °C, and the time is 3-6 h;

[0035] Optionally, the heating rate of the low-temperature carbonization is 1-3 °C / min;

[0036] Optionally, the oxygen-containing gas is selected from air and / or oxygen;

[0037] Optionally, the flow rate of the oxygen-containing gas is 2-10 L / min;

[0038] Optionally, the inert gas is selected from one or more of nitrogen, argon, helium, xenon, and radon.

[0039] In this step, the precursor material and coal are subjected to low-temperature carbonization in an oxygen-containing gas, enabling the co-gasification of the precursor material and coal. The phosphorus compounds in the precursor material decompose and volatilize at a relatively low temperature, and phosphorus forms Al(PO3)3 and Ca3(PO4)2 with Ca and Al in the coal ash mineral components, improving the stability of the phosphides and effectively reducing phosphorus loss during this process.

[0040] In some embodiments of the present application, in step S5, the temperature of the high-temperature carbonization is 1100 - 1300 °C, and the time is 4 - 10 h;

[0041] Optionally, the heating rate during the high-temperature carbonization is 3 - 5 °C / min.

[0042] In this step, the gasified intermediate product is further subjected to high-temperature carbonization to obtain a phosphorus-doped hard carbon composite material. Under high-temperature conditions, calcium phosphate reacts with silicon in coal using carbon or a reducing gas as a reducing agent at high temperature as follows: 2Ca3(PO4)2 + 6SiO2 + 10C → 6CaSiO3 + P4 + 10CO, etching the surface of the carbon material, and simultaneously volatilizing phosphorus in the form of elemental phosphorus. In an atmosphere rich in phosphorus, the doping of phosphorus elements and the formation of a rich closed-pore structure are synchronously completed.

[0043] The third aspect of the present application provides a negative electrode sheet, which includes the phosphorus-doped hard carbon composite material described in any one of the above, or the phosphorus-doped hard carbon composite material prepared by the preparation method described in any one of the above.

[0044] The fourth aspect of the present application provides a sodium-ion battery, which is characterized by including the above negative electrode sheet.

[0045] In some embodiments of the present application, the reversible discharge specific capacity of the sodium-ion battery is ≥295 mAh / g, and the initial Coulombic efficiency is ≥90%.

[0046] The sodium-ion battery provided by the present application has very good reversible discharge specific capacity and initial Coulombic efficiency.

[0047] Additional aspects and advantages of the present invention will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present invention. Description of the Drawings

[0048] The drawings constituting a part of this disclosure are used to provide a further understanding of this disclosure. The schematic embodiments and descriptions thereof of this disclosure are used to explain this disclosure and do not constitute an improper limitation of this disclosure.

[0049] Figure 1 It is a process flow diagram for preparing a phosphorus-doped hard carbon composite material provided by an embodiment of the present application.

[0050] Figure 2 This is the SEM image of the phosphorus-doped hard carbon composite material obtained in an embodiment of the present application, with a magnification of 10,000×.

[0051] Figure 3 This is Figure 2 a partial enlarged view thereof, with a magnification of 30,000×. Detailed Description of the Invention

[0052] In the following, only some exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the present invention. Therefore, the drawings and the description are to be regarded as illustrative in nature and not restrictive.

[0053] The following disclosure provides many different embodiments or examples for implementing the present invention. To simplify the disclosure of the present invention, the components and settings of specific examples are described below. Of course, they are merely examples and are not intended to limit the present invention. In addition, the present invention may repeat reference numerals and / or reference letters in different examples. Such repetition is for the purpose of simplification and clarity and does not in itself indicate the relationship between the various embodiments and / or settings discussed. In addition, the present invention provides examples of various specific processes and materials, but those of ordinary skill in the art can recognize the application of other processes and / or the use of other materials.

[0054] Furthermore, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will also be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning consistent with their meaning in the context of the relevant art and the present invention, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0055] Taking into account the measurements discussed and the errors associated with the measurements of specific quantities (i.e., the limitations of the measurement system), as used herein, "about" or "approximately" includes the stated value and means within an acceptable range of deviation from the specific value as determined by one of ordinary skill in the art. For example, "about" can mean within one or more standard deviations, or within ±30%, ±20%, ±10%, or ±5% of the stated value.

[0056] The following further describes the specific embodiments of the present invention in more detail in conjunction with the drawings and embodiments, so as to better understand the solution of the present invention and the advantages of its various aspects. However, the specific embodiments and examples described below are for illustrative purposes only and are not limitations on the present invention.

[0057] The present application provides a phosphorus-doped hard carbon composite material, which is doped with phosphorus elements. Phosphorus forms a bond with the carbon matrix in the form of P-C. Phosphorus doping can improve defects and oxygen functional groups, increase the sodium storage capacity, and thus improve the specific capacity of the phosphorus-doped hard carbon composite material. In the present application, the doping amount of phosphorus elements in the phosphorus-doped hard carbon composite material is 5.5-6.5%. The doping amount of phosphorus elements is relatively high, which can effectively increase the layer spacing, promote the efficient electron transfer, and effectively improve the initial Coulomb efficiency of the phosphorus-doped hard carbon composite material. In some specific embodiments, the doping amount of phosphorus elements can be 5.5%, 5.7%, 5.9%, 6.1%, 6.3% or 6.5%.

[0058] In the present application, the phosphorus-doped hard carbon composite material satisfies: 4.11 ≤ SSA + (P / (P + C)) * d 002 | ≤ 4.53, where SSA is the specific surface area of the phosphorus-doped hard carbon composite material, P / (P + C) is the mass fraction of phosphorus elements in the phosphorus-doped hard carbon composite material (i.e., the doping amount of phosphorus elements), and d 002 is the carbon layer spacing of the phosphorus-doped hard carbon composite material. When the specific surface area, carbon layer spacing, and doping amount of phosphorus elements meet the above requirements, the performance of the phosphorus-doped hard carbon composite material is better.

[0059] Optionally, the specific surface area of the phosphorus-doped hard carbon composite material is 4.1-4.5 cm 2 / g. The smaller the specific surface area and the fewer the surface defects, the better the performance of the phosphorus-doped hard carbon composite material. In some specific embodiments, the specific surface area of the phosphorus-doped hard carbon composite material can be 4.1 cm 2 / g, 4.2 cm 2 / g, 4.3 cm 2 / g, 4.4 cm 2 / g or 4.5 cm 2 / g.

[0060] Optionally, the carbon layer spacing d 002 of the phosphorus-doped hard carbon composite material ≥ 0.039 nm. A larger carbon layer spacing is beneficial to providing more sodium storage sites. Further optionally, the carbon layer spacing d 002 is 0.039-0.040 nm. In some specific embodiments, the carbon layer spacing d 002 can be 0.039 nm, 0.0392 nm, 0.0394 nm, 0.0396 nm, 0.0398 nm or 0.040 nm.

[0061] Optionally, the average pore diameter of the phosphorus-doped hard carbon composite material is 3.5 to 7.0 nm. Within this pore diameter range, the phosphorus-doped hard carbon composite material can have more phosphorus elements, and thus, the prepared sodium-ion battery has better performance. In some specific embodiments, the average pore diameter of the phosphorus-doped hard carbon composite material can be 3.5 nm, 4.0 nm, 4.5 nm, 5.0 nm, 5.5 nm, 6.0 nm, 6.5 nm, or 7.0 nm.

[0062] Optionally, the phosphorus-doped hard carbon composite material has a rich pore structure, and its total pore volume is 0.4 to 0.5 cm 3 / g. Optionally, the total pore volume of micropores (pores with a pore diameter of 0 to 2 nm) ≥ 0.12 cm 3 / g, and the microporosity ≥ 20%. In some specific embodiments, the total pore volume can be 0.40 cm 3 / g, 0.41 cm 3 / g, 0.42 cm 3 / g, 0.44 cm 3 / g, 0.46 cm 3 / g, 0.48 cm 3 / g, or 0.50 cm 3 / g. The phosphorus-doped hard carbon composite material includes closed pores. The closed pore structure can reduce the formation of SEI, and the closed pore structure can provide more sodium-ion insertion sites, improving the specific capacity of the phosphorus-doped hard carbon composite material. Optionally, the intensity ratio I D / I G of the amorphous carbon peak and the graphitized carbon peak of the phosphorus-doped hard carbon composite material is 2.1 to 2.5. The low degree of graphitization shows a higher capacity. In some specific embodiments, the I D / I G value can be 2.1, 2.2, 2.3, 2.4, or 2.5.

[0063] Figure 1 FIG. shows the preparation method of the phosphorus-doped hard carbon composite material provided by an embodiment of the present application, including the following steps S1 to S5.

[0064] S1: Mix alginate and phosphate in water, and then add a calcium salt solution to obtain a gel-like product.

[0065] In this step, after mixing alginate and phosphate evenly in water, a calcium salt solution is added to generate a gel-like calcium alginate-based phosphate complex. Optionally, alginate and phosphate are stirred and mixed in water for 10 to 14 h to be fully stirred and evenly mixed. In some specific embodiments, the stirring time can be 10 h, 11 h, 12 h, 13 h, or 14 h.

[0066] In step S1, the dosage ratio of alginate to phosphate is 1:(1-2). In the presence of phosphate, the reaction rate of alginate and calcium salt will not be too fast, so that an ideal gel-like product can be obtained. In some specific embodiments, the dosage ratio of alginate to phosphate can be 1:1, 1:1.2, 1:1.4, 1:1.6, 1:1.8 or 1:2.

[0067] The alginate can be selected from one or more of sodium alginate, potassium alginate, magnesium alginate, ammonium alginate, etc. The phosphate can be selected from one or more of sodium hydrogen phosphate, sodium dihydrogen phosphate, dodecyl phosphate ester salt, etc. The calcium salt can be selected from one or more of calcium chloride, calcium gluconate, calcium nitrate, calcium bicarbonate, calcium acetate, calcium hypochlorite, calcium perchlorate, calcium permanganate, calcium lactate, etc.

[0068] Optionally, the molar concentration of the calcium salt solution is 0.1-0.5 mol / L. If the calcium salt concentration is too high, the reaction rate of alginate and calcium salt is very fast. At the above concentration, a relatively ideal gel-like product can be obtained. In some specific embodiments, the molar concentration of the calcium salt solution can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L or 0.5 mol / L.

[0069] Optionally, the addition amount of the calcium salt solution is 10-20% of the total weight of alginate, phosphate and water. In some specific embodiments, the addition amount can be 10%, 12%, 14%, 16%, 18% or 20%.

[0070] S2: Dry the gel-like product to obtain a precursor material.

[0071] This step removes the moisture in the gel-like product, so that it can be ball-milled with coal in the subsequent steps. Drying can be carried out by drying.

[0072] S3: Mix the precursor material and coal and then ball-mill to obtain a powder material.

[0073] In this step, the precursor material and coal are evenly mixed after ball-milling and made into a powder material, which is beneficial to the subsequent gasification. Coal is inexpensive and contains a large number of elements, and has great potential for the preparation of hard carbon. Optionally, the coal is selected from one or more of bituminous coal, anthracite and lignite.

[0074] Optionally, the dosage ratio of the precursor material to coal is 1:(2-2.5). In some specific embodiments, the dosage ratio of the precursor material to coal can be 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4 or 1:2.5.

[0075] Optionally, the rotation speed of ball milling is 150 - 250 r / min, and the time is 2 - 4 h. In some specific embodiments, the rotation speed of ball milling is 150 r / min, 170 r / min, 190 r / min, 210 r / min, 230 r / min or 250 r / min. In some specific embodiments, the time of ball milling can be 2 h, 2.5 h, 3 h, 3.5 h or 4 h.

[0076] S4: Place the powder material in an inert gas, then introduce an oxygen-containing gas, and carry out low-temperature carbonization to co-gasify the precursor material and coal to obtain an intermediate product.

[0077] In this step, the precursor material and coal are subjected to low-temperature carbonization in an oxygen-containing gas to co-gasify the precursor material and coal. The phosphorus compound of the precursor material decomposes and volatilizes at a relatively low temperature, and phosphorus forms Al(PO3)3 and Ca3(PO4)2 with Ca and Al in the coal ash mineral components, improving the stability of the phosphide and effectively reducing the loss of phosphorus during this process.

[0078] Optionally, the inert gas is selected from one or more of nitrogen, argon, helium, xenon and radon.

[0079] Optionally, the temperature of low-temperature carbonization is 300 - 500 °C, and the time is 3 - 6 h. In some specific embodiments, the temperature of low-temperature carbonization can be 300 °C, 320 °C, 340 °C, 360 °C, 380 °C, 400 °C, 420 °C, 440 °C, 460 °C, 480 °C or 500 °C. In some specific embodiments, the time of low-temperature carbonization can be 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h or 6 h.

[0080] Optionally, the heating rate of low-temperature carbonization is 1 - 3 °C / min. In some specific embodiments, the heating rate of low-temperature carbonization can be 1 °C / min, 1.5 °C / min, 2 °C / min, 2.5 °C / min or 3 °C / min.

[0081] Optionally, the oxygen-containing gas is selected from air and / or oxygen. Optionally, the flow rate of the oxygen-containing gas is 2 - 10 L / min. In some specific embodiments, the flow rate of the oxygen-containing gas can be 2 L / min, 3 L / min, 4 L / min, 5 L / min, 6 L / min, 7 L / min, 8 L / min, 9 L / min or 10 L / min.

[0082] S5: Carry out high-temperature carbonization on the intermediate product to obtain a phosphorus-doped hard carbon composite material.

[0083] In this step, the gasified intermediate product is further subjected to high-temperature carbonization to obtain a phosphorus-doped hard carbon composite material. Under high-temperature conditions, calcium phosphate reacts with silicon in coal using carbon or a reducing gas as a reducing agent at high temperature as follows: 2Ca3(PO4)2 + 6SiO2 + 10C → 6CaSiO3 + P4 + 10CO. This etches the surface of the carbon material and simultaneously causes phosphorus to volatilize in the form of elemental phosphorus. In an atmosphere rich in phosphorus, the doping of phosphorus elements and the formation of a rich closed-pore structure are completed synchronously.

[0084] Optionally, the temperature of high-temperature carbonization is 1100 - 1300 °C, and the time is 4 - 10 h. In some specific embodiments, the temperature of high-temperature carbonization can be 1100 °C, 1120 °C, 1140 °C, 1160 °C, 1180 °C, 1200 °C, 1220 °C, 1240 °C, 1260 °C, 1280 °C, or 1300 °C. In some specific embodiments, the time of high-temperature carbonization can be 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, or 10 h.

[0085] Optionally, the heating rate during high-temperature carbonization is 3 - 5 °C / min. Low-temperature carbonization and high-temperature carbonization can be carried out in the same equipment. For example, first place the powder material in an inert gas, then introduce an oxygen-containing gas for low-temperature carbonization. After completion, stop introducing the oxygen-containing gas and continue to heat up to the temperature of high-temperature carbonization for high-temperature carbonization. In some specific embodiments, the heating rate during high-temperature carbonization can be 3 °C / min, 3.5 °C / min, 4 °C / min, 4.5 °C / min, or 5 °C / min.

[0086] The method provided in this application first prepares a biomass precursor material rich in phosphorus, then dries it and ball-mills it with coal, then performs co-gasification in an oxygen-containing gas, and finally performs high-temperature carbonization. This can not only increase the doping amount of phosphorus elements but also synchronously form a rich closed-pore structure.

[0087] This application further provides a negative electrode sheet, which includes the above-mentioned phosphorus-doped hard carbon composite material or a phosphorus-doped hard carbon composite material prepared by the above-mentioned preparation method.

[0088] The negative electrode sheet generally includes a negative electrode current collector and a negative electrode film layer provided on the negative electrode current collector. Among them, the negative electrode film layer can include the above-mentioned phosphorus-doped hard carbon composite material.

[0089] The current collector can be a metal foil, such as an aluminum foil, a copper foil, etc., preferably a copper foil. The negative electrode film layer can also include a binder, a conductive agent, etc. The binder can be, for example, styrene-butadiene rubber (SBR), polyvinylidene chloride (PVDF), polyacrylic acid (PAA), sodium carboxymethyl cellulose (CMC-Na), sodium alginate, etc. The conductive agent can be, for example, graphene, carbon nanotubes, Ketjen black, conductive carbon black (SP), etc. Optionally, the negative electrode film layer can also include other additives, such as a dispersant (such as carboxymethyl cellulose (CMC)), etc.

[0090] The present application also provides a sodium-ion battery, which includes the above-mentioned negative electrode sheet. The sodium-ion battery also includes a positive electrode sheet, a separator, and an electrolyte. The sodium-ion battery provided by the present application has very good reversible discharge specific capacity and initial Coulomb efficiency. Optionally, the reversible discharge specific capacity of the sodium-ion battery ≥ 295 mAh / g, and can even be higher than 300 mAh / g. Optionally, the initial Coulomb efficiency of the sodium-ion battery ≥ 90%.

[0091] The sodium-ion battery of the present application is a secondary battery, which means a battery that can be activated by charging after battery discharge and continue to be used. Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. During the charge and discharge process of the battery, active ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, playing a role of isolation. The electrolyte plays a role of conducting ions between the positive electrode sheet and the negative electrode sheet.

[0092] The present invention will be described below with reference to specific embodiments. The numerical values of the process conditions taken in the following embodiments and comparative examples are all exemplary, and the range of their available numerical values is as shown in the foregoing invention content. For process parameters not specifically noted, conventional techniques can be referred to. Unless otherwise specified, the reagents and instruments used in the technical solutions provided by the present invention can be obtained from conventional channels or the market. It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other.

[0093] Example 1

[0094] In this embodiment, a phosphorus-doped hard carbon composite material is prepared, and the specific steps are as follows:

[0095] 1) Take 10 g of sodium alginate and 15 g of sodium dodecyl phosphate, add 100 mL of deionized water, stir at room temperature for 30 min, add 5 mL of 0.5 mol / L CaCl2 solution, and stir to form a gel-like product; then dry the gel-like product to obtain a precursor material.

[0096] 2) Mix the precursor material and lignite according to a mass ratio of 1:2.5, and ball-mill at 200 r / min for 3 h to obtain a powder material.

[0097] 3) Place the powder material in argon gas, then introduce oxygen with an oxygen flow rate of 8 L / min and a heating rate of 2 °C / min. Heat it to 400 °C for low-temperature carbonization and hold for 6 h. Close the oxygen and continue heating with a heating rate of 5 °C / min. Heat it to 1300 °C for high-temperature carbonization and hold for 4 h. After cooling, a phosphorus-doped hard carbon composite material is obtained.

[0098] Example 2

[0099] The specific steps for preparing a phosphorus-doped hard carbon composite material in this example are as follows:

[0100] 1) Take 10 g of sodium alginate, 10 g of disodium hydrogen phosphate, and 10 g of sodium dihydrogen phosphate, add 100 mL of deionized water, stir at room temperature for 30 min, add 5 mL of 0.5 mol / L CaCl2 solution, and stir to form a gel-like product. Then dry the gel-like product to obtain a precursor material.

[0101] 2) Mix the precursor material with bituminous coal at a mass ratio of 1:2 and ball mill for 3 h at 200 r / min to obtain a powder material.

[0102] 3) Place the powder material in argon gas, then introduce oxygen with an oxygen flow rate of 8 L / min and a heating rate of 2 °C / min. Heat it to 400 °C for low-temperature carbonization and hold for 4 h. Close the oxygen and continue heating with a heating rate of 5 °C / min. Heat it to 1300 °C for high-temperature carbonization and hold for 4 h. After cooling, a phosphorus-doped hard carbon composite material is obtained.

[0103] Example 3

[0104] The specific steps for preparing a phosphorus-doped hard carbon composite material in this example are as follows:

[0105] 1) Take 10 g of potassium alginate, 10 g of disodium hydrogen phosphate, and 10 g of sodium dihydrogen phosphate, add 100 mL of deionized water, stir at room temperature for 30 min, add 5 mL of 0.5 mol / L Ca(NO3)2 solution, and stir to form a gel-like product. Then dry the gel-like product to obtain a precursor material.

[0106] 2) Mix the precursor material with anthracite at a mass ratio of 1:2 and ball mill for 3 h at 200 r / min to obtain a powder material.

[0107] 3) Place the powder material in argon gas, then introduce oxygen with an oxygen flow rate of 8 L / min and a heating rate of 2 °C / min. Heat it to 400 °C for low-temperature carbonization and keep it warm for 4 h. Close the oxygen and continue heating with a heating rate of 5 °C / min. Heat it to 1300 °C for high-temperature carbonization and keep it warm for 4 h. After cooling, a phosphorus-doped hard carbon composite material is obtained.

[0108] Example 4

[0109] In this example, a phosphorus-doped hard carbon composite material is prepared, and the specific steps are as follows:

[0110] 1) Take 10 g of sodium alginate and 15 g of sodium dodecyl phosphate, add 100 mL of deionized water, stir at room temperature for 30 min, add 5 mL of 0.5 mol / L CaCl2 solution, and stir to form a gel-like product; then dry the gel-like product to obtain a precursor material.

[0111] 2) Mix the precursor material and lignite in a mass ratio of 1:2.5, and ball-mill for 3 h at 200 r / min to obtain a powder material.

[0112] 3) Place the powder material in argon gas, then introduce oxygen with an oxygen flow rate of 4 L / min and a heating rate of 2 °C / min. Heat it to 300 °C for low-temperature carbonization and keep it warm for 6 h. Close the oxygen and continue heating with a heating rate of 5 °C / min. Heat it to 1300 °C for high-temperature carbonization and keep it warm for 4 h. After cooling, a phosphorus-doped hard carbon composite material is obtained.

[0113] Example 5

[0114] In this example, a phosphorus-doped hard carbon composite material is prepared, and the specific steps are as follows:

[0115] 1) Take 10 g of sodium alginate and 20 g of disodium hydrogen phosphate, add 100 mL of deionized water, stir at room temperature for 30 min, add 5 mL of 0.5 mol / L CaCl2 solution, and stir to form a gel-like product; then dry the gel-like product to obtain a precursor material.

[0116] 2) Mix the precursor material and lignite in a mass ratio of 1:2.5, and ball-mill for 3 h at 400 r / min to obtain a powder material.

[0117] 3) Place the powder material in argon gas, then introduce oxygen with an oxygen flow rate of 4 L / min and a heating rate of 2 °C / min. Heat it to 400 °C for low-temperature carbonization and keep it warm for 6 h. Close the oxygen and continue heating with a heating rate of 5 °C / min. Heat it to 1200 °C for high-temperature carbonization and keep it warm for 6 h. After cooling, a phosphorus-doped hard carbon composite material is obtained.

[0118] Example 6

[0119] In this example, a phosphorus-doped hard carbon composite material was prepared, and the specific steps are as follows:

[0120] 1) Take 10 g of sodium alginate and 15 g of sodium dodecyl phosphate, add 100 mL of deionized water, stir at room temperature for 30 min, add 5 mL of 0.5 mol / L CaCl2 solution, and stir to form a gel-like product; then dry the gel-like product to obtain a precursor material.

[0121] 2) Mix the precursor material and lignite in a mass ratio of 1:2.5, and ball mill for 3 h at 400 r / min to obtain a powder material.

[0122] 3) Place the powder material in argon gas, then introduce oxygen with an oxygen flow rate of 8 L / min and a heating rate of 1 °C / min, heat up to 400 °C for low-temperature carbonization, and keep the temperature for 6 h; close the oxygen and continue heating with a heating rate of 3 °C / min, heat up to 1200 °C for high-temperature carbonization, keep the temperature for 6 h, and cool to obtain the phosphorus-doped hard carbon composite material.

[0123] Comparative Example 1

[0124] The difference between this comparative example and Example 1 is that the lignite in step 2) is replaced with coconut shell.

[0125] Comparative Example 2

[0126] The difference between this comparative example and Example 1 is that sodium dodecyl phosphate is not added in step 1).

[0127] Comparative Example 3

[0128] The difference between this comparative example and Example 1 is that sodium alginate is not added in step 1).

[0129] Comparative Example 4

[0130] The difference between this comparative example and Example 1 is that in step 2), the precursor material and lignite are not ball milled, and step 3) is directly carried out after mixing.

[0131] Comparative Example 5

[0132] The difference between this comparative example and Example 1 is that in step 3), oxygen is not introduced, and low-temperature carbonization is carried out in argon gas.

[0133] Comparative Example 6

[0134] The difference between this comparative example and Example 1 is that in step 3), after introducing oxygen, low-temperature carbonization is not carried out, but high-temperature carbonization is directly carried out.

[0135] Test Example 1

[0136] The N2 adsorption-desorption specific surface areas of the phosphorus-doped hard carbon composites of Examples 1-6 and Comparative Examples 1-6 were detected. The specific surface area and pore structure were determined by gas adsorption method, and the carbon layer spacing d was calculated by XRD 002 with an incident light wavelength of and the intensity ratio I of the typical D peak and G peak of the amorphous carbon peak and graphitized carbon peak was measured by Raman spectroscopy D / I G . The content of phosphorus (P) element in the material was determined by using an organic elemental analyzer (EA). The results are shown in Table 1.

[0137] The SEM images of the phosphorus-doped hard carbon composite prepared in Example 1 can be seen in Figure 2 and Figure 3 where Figure 2 has a magnification of 10,000×, Figure 3 is Figure 2 a partial enlarged view with a magnification of 30,000×.

[0138] Table 1

[0139]

[0140] As can be seen from Examples 1-6 and Comparative Examples 1-6 in Table 1, the composite of alginate and phosphate can effectively increase the doping amount of phosphorus in the subsequently prepared phosphorus-doped hard carbon composite. Using other biomass that is difficult to composite with phosphorus for doping, and not adding phosphate and alginate for mixing, the phosphorus doping amount of the subsequently prepared phosphorus-doped hard carbon composite is significantly reduced, which is not conducive to the increase of the layer spacing after carbonization. For the precursor not mixed with coal, in the low-temperature stage, phosphorus elements escape, and the subsequent phosphorus doping amount decreases. Without ball milling, the material mixing is uneven, which is not conducive to the release of phosphorus during the subsequent carbonization process. The specific surface area of the prepared phosphorus-doped hard carbon composite is too large and the phosphorus doping amount is low. For the subsequent two-stage heating, the first-stage carbonization is carried out in an oxygen-containing atmosphere. On the one hand, it can pre-oxidize and crosslink the coal. On the other hand, at low temperature, the escaped phosphorus elements can be effectively trapped by Ca and Al in the coal to form calcium phosphate and aluminum phosphate, which can effectively increase the phosphorus doping amount of the subsequently prepared hard carbon.

[0141] Test Example 2

[0142] The phosphorus-doped hard carbon composites obtained in Examples 1-6 and Comparative Examples 1-6 were used as anode materials, and sodium-ion batteries were prepared respectively according to the following methods. The preparation method of a button-type sodium-ion battery includes the following steps:

[0143] According to the ratio: active material: SP: CMC: SBR = 94:2:1.5:2.5, weigh the anode material, SP, CMC, and SBR respectively and uniformly mix them in deionized water to prepare a slurry; coat the uniformly mixed slurry on an aluminum foil current collector, dry it in an oven at 80 °C for 1 h, take it out and cool it to room temperature;

[0144] Adjust the roller pressing distance to roll the electrode sheet. Cut the rolled electrode sheet to make small round pieces with a diameter of 14 mm and weigh them as m1. Similarly, cut the aluminum foil current collector to make aluminum foil round pieces with a diameter of 14 mm and weigh them as m2. Among them, 0.94*(m1 - m2) is the mass of the active material, denoted as m3. Put the weighed small round pieces into an oven at 80 °C and vacuum dry them for 12 h;

[0145] Transfer the vacuum-dried small round pieces to a glove box. Using sodium pieces as the counter electrode and auxiliary electrode, with the electrolyte 1M NaPF6 / EC:DMC:DEC = 2:2:1 and a glass fiber separator as the separator, assemble a sodium-ion button cell in a glove box with the oxygen and water content both less than 0.01 ppm; let the assembled button-type sodium-ion battery stand for 12 h.

[0146] Test the electrochemical performance of the stationary button-type sodium-ion battery on a Wuhan Blue Electric battery test system. The test results are shown in Table 2.

[0147] Table 2

[0148]

[0149] As can be seen from Table 2, after mixing a phosphorus-rich precursor material with coal and performing carbonization in an oxygen-containing gas, the electrochemical performance of the subsequently prepared hard carbon can be effectively improved. By adding a phosphorus-containing biomass precursor and performing low-temperature carbonization, the phosphorus compound decomposes and volatilizes at a relatively low temperature, and reacts with Ca and Al in the coal ash mineral components to form calcium phosphate and aluminum phosphate and is trapped. After stopping the supply of the oxygen-containing gas, continue to heat up for high-temperature carbonization. Under high-temperature conditions, calcium phosphate reacts with silicon using carbon or a reducing gas as a reducing agent to etch the surface of the carbon material, and at the same time, the inorganic phosphorus volatilizes in the form of elemental phosphorus. In a phosphorus-rich atmosphere, the doping of phosphorus elements and the formation of a rich closed-pore structure are completed synchronously. For the precursor without mixing with coal, in the low-temperature stage, the phosphorus element escapes, reducing the subsequent phosphorus doping amount. Ball milling can effectively increase the uniformity after mixing coal and the precursor, and is also beneficial to the subsequent reaction. The phosphorus-doped hard carbon composite material prepared by this method has a small specific surface area, excellent reversible discharge specific capacity and first Coulomb efficiency.

[0150] Obviously, the above embodiments are merely examples given to clearly illustrate the present invention, rather than limitations on the implementation manners. For those of ordinary skill 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 enumerate all the implementation manners here. And the obvious changes or modifications derived therefrom still fall within the protection scope of the present invention.

Claims

1. A phosphorus-doped hard carbon composite material, characterized in that: The phosphorus doping amount of the phosphorus doped hard carbon composite material is 5.5-6.5%; the phosphorus doped hard carbon composite material satisfies: 4.11≤SSA+(P / (P+C))*d 002 ≤4.53, where SSA is the specific surface area of ​​the phosphorus-doped hard carbon composite material, P / (P+C) is the mass ratio of phosphorus in the phosphorus-doped hard carbon composite material, that is, the doping amount of phosphorus, and d 002 is the carbon layer spacing of the phosphorus-doped hard carbon composite material; The specific surface area SSA of the phosphorus-doped hard carbon composite material is 4.1-4.5 cm 2 / g; the carbon layer spacing d of the phosphorus-doped hard carbon composite material 002 ≥0.39nm.

2. The phosphorus-doped hard carbon composite material according to claim 1, characterized in that: The phosphorus-doped hard carbon composite material satisfies at least one of the following conditions: a) the average pore size of the phosphorus-doped hard carbon composite material is 3.5-7.0 nm; b) The total pore volume of the phosphorus-doped hard carbon composite material is 0.4-0.5 cm 3 / g, the total pore volume of micropores with a pore size of 0~2nm is ≥0.12cm 3 / g, microporosity ≥ 20%, the phosphorus-doped hard carbon composite material includes closed pores; c) The intensity ratio of the amorphous carbon peak and the graphitized carbon peak of the phosphorus-doped hard carbon composite material I D / I G It is 2.1~2.

5.

3. A method for preparing the phosphorus-doped hard carbon composite material according to claim 1 or 2, characterized in that: The steps include: S1: mixing alginate and phosphate in water, and then adding calcium salt solution to obtain a gel-like product; S2: drying the gel-like product to obtain a precursor material; S3: mixing the precursor material and coal and then ball milling them to obtain a powder material; S4: placing the powder material in an inert gas, and then introducing an oxygen-containing gas to perform low-temperature carbonization, so that the precursor material and the coal are co-gasified to obtain an intermediate product; S5: Carbonizing the intermediate product at high temperature to obtain the phosphorus-doped hard carbon composite material.

4. The preparation method according to claim 3, characterized in that: Step S1 satisfies at least one of the following conditions: 1) The weight ratio of the alginate to the phosphate is 1:(1-2); 2) The phosphate is selected from one or more of sodium monohydrogen phosphate, sodium dihydrogen phosphate, and dodecyl phosphate; 3) The molar concentration of the calcium salt solution is 0.1-0.5 mol / L; 4) The amount of the calcium salt solution added is 10-20% of the total weight of the alginate, the phosphate and the water; 5) The calcium salt is selected from one or more of calcium chloride, calcium gluconate, calcium nitrate, calcium bicarbonate, calcium acetate, calcium hypochlorite, calcium perchlorate, calcium permanganate, and calcium lactate; 6) The alginate and the phosphate are stirred and mixed in water for 10 to 14 hours.

5. The preparation method according to claim 3, characterized in that: Step S3 satisfies at least one of the following conditions: 1) The coal is selected from one or more of bituminous coal, anthracite and lignite; 2) The weight ratio of the precursor material to the coal is 1:(2-2.5); 3) The ball mill has a rotation speed of 150-250 r / min and a milling time of 2-4 h.

6. The preparation method according to claim 3, characterized in that: Step S4 satisfies at least one of the following conditions: 1) The temperature of the low-temperature carbonization is 300-500°C and the time is 3-6 hours; 2) The heating rate of the low temperature carbonization is 1-3°C / min; 3) The oxygen-containing gas is selected from air and / or oxygen; 4) The flow rate of the oxygen-containing gas is 2-10 L / min; 5) The inert gas is selected from one or more of nitrogen, argon, helium, xenon and radon.

7. The preparation method according to claim 3, characterized in that: In step S5, the high temperature carbonization temperature is 1100-1300°C and the time is 4-10 hours; and / or The heating rate during the high temperature carbonization is 3-5°C / min.

8. A negative electrode sheet, characterized in that: The invention comprises the phosphorus-doped hard carbon composite material as claimed in claim 1 or 2.

9. A sodium ion battery, characterized in that: Includes the negative electrode sheet as described in claim 8.

10. The sodium ion battery according to claim 9, characterized in that: The reversible discharge specific capacity of the sodium ion battery is ≥295 mAh / g, and the first coulombic efficiency is ≥90%.

Images