Hard carbon negative electrode material and preparation method thereof and lithium ion battery

Abstract

This application discloses a hard carbon anode material, a preparation method thereof, and a lithium-ion battery. The preparation method comprises the following steps: mixing starch with a crosslinking agent to obtain a carbonized precursor; carbonizing the carbonized precursor in a protective atmosphere using a stepwise heating method; and cooling the carbonized precursor to obtain the hard carbon anode material. Specifically, the technical solution of this application promotes the crosslinking reaction during the starch dehydration condensation process, increasing the gaps between carbon layers, thereby improving the capacity and rate performance of the hard carbon anode material.

Description

Technical Field

[0001] The present application relates to the technical field of negative electrode materials, and in particular to a hard carbon negative electrode material, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] Hard carbon anode materials are ideal for power battery anodes due to their high power output, long cycle life, and excellent low-temperature safety. Currently, with the continuous development of the lithium-ion power battery market, the requirements for the rate performance and capacity of anode materials are becoming increasingly stringent. However, due to the shortcomings of hard carbon anode materials such as low capacity and compaction density, their energy density cannot meet the requirements of lithium-ion power batteries, thus limiting their application in lithium-ion power batteries. Therefore, the development of hard carbon anode materials with high capacity, high compaction density, and excellent rate performance is of great research significance and application value.

[0003] For example, there is a technical solution that uses an oxidizing gas as a stabilizing atmosphere, stabilizes the product, and then carbonizes it to produce starch-based carbon microspheres. Although this method maintains the original spherical morphology of the starch, the long-term stabilization treatment in an oxidizing atmosphere causes the prepared hard carbon microspheres to contain more oxygen-containing functional groups. Oxygen-containing functional groups can react with lithium ions, thereby increasing the irreversible capacity of the lithium-ion battery and reducing the initial efficiency. In addition, this method requires pretreatment in a stabilized atmosphere, which increases the preparation time, reduces the preparation efficiency, and has high energy consumption, which is not conducive to industrial production.

[0004] Another example is a technical solution that uses ammonium chloride or ammonium sulfate as a cross-linking agent, prepares a precursor solution with the cross-linking agent and starch, pre-treats it at a temperature between 80°C and 170°C, and then carbonizes it at high temperature to prepare hard carbon anode materials. This technology requires a long drying process for the precursor solution, which not only increases the preparation time but also destroys the spherical morphology of the starch raw material. In addition, the acidic atmosphere generated by the high-temperature decomposition of ammonium chloride or ammonium sulfate severely corrodes the carbonization equipment, accelerating its aging.

[0005] Another example is a two-step stabilization-carbonization method for preparing spherical hard carbon materials. While this method is simple, it requires 8 to 100 hours of stabilization, which increases the preparation time and reduces the efficiency, making it unsuitable for industrial production.

[0006] The hard carbon materials prepared by the above method cannot maintain high capacity and long cycle performance while maintaining a spherical structure. Summary of the Invention

[0007] The main purpose of this application is to provide a method for preparing a hard carbon negative electrode material, aiming to improve the capacity and rate performance of the hard carbon negative electrode material.

[0008] To achieve the above objectives, the present application proposes a hard carbon negative electrode material, which is spherical and has micropores distributed inside. The size of the micropores is 0.3 nm to 0.7 nm.

[0009] Optionally, the hard carbon negative electrode material satisfies at least one of the following characteristics a to g:

[0010] a. The micropore volume is 0.02cm 3 / g~0.20cm 3 / g;

[0011] b. The micropore porosity is 5% to 27%;

[0012] c. The total specific surface area of ​​the micropores is 90m 2 / g~500m 2 / g;

[0013] d. The true density of the hard carbon negative electrode material is 1.3g / cm 3 ~1.8g / cm 3 ;

[0014] e. The particle size of the hard carbon negative electrode material is 3um to 50um;

[0015] f. The compaction density of the hard carbon negative electrode material is 0.96g / cm 3 ~1.20g / cm 3 ;

[0016] g. The specific surface area of ​​the hard carbon negative electrode material is 0.5m 2 / g~3.0m 2 / g.

[0017] This application also proposes a method for preparing a hard carbon negative electrode material, comprising the following steps:

[0018] mixing starch with a cross-linking agent to obtain a carbonized precursor;

[0019] The carbonized precursor is carbonized in a protective atmosphere by adopting a staged temperature increase method, and after cooling, a hard carbon negative electrode material is obtained.

[0020] Optionally, in the method for preparing the hard carbon negative electrode material, the step of “carbonizing the carbonized precursor by increasing the temperature in stages” includes:

[0021] The carbonized precursor is heated to 150° C. to 170° C. for the first time and kept at this temperature for 0.5 to 3 hours, then heated to 200° C. to 350° C. for the second time and kept at this temperature for 1 to 4 hours, then heated to 1000° C. to 1500° C. for the third time and kept at this temperature for 1 to 4 hours.

[0022] Optionally, in the method for preparing the hard carbon negative electrode material, in the step of “raising the temperature of the carbonized precursor to 150° C. to 170° C. for the first time”, the first heating rate is 0.5° C. / min to 5° C. / min.

[0023] Optionally, in the method for preparing the hard carbon negative electrode material, in the step of "raising the temperature to 200° C. to 350° C. for the second time", the second heating rate is 0.5° C. / min to 5° C. / min.

[0024] Optionally, in the method for preparing the hard carbon negative electrode material, in the step of "raising the temperature to 1000° C. to 1500° C. for the third time", the third heating rate is 1° C. / min to 10° C. / min.

[0025] Optionally, in the method for preparing the hard carbon negative electrode material, in the step of “mixing starch with a cross-linking agent”, the cross-linking agent includes at least one of phenylhydrazine, phenylpropenol, 2-aminobiphenyl, 2-aminoethanol and 4-aminopyridine;

[0026] And / or, the mass ratio of the starch to the cross-linking agent is (9-1):1.

[0027] Optionally, in the method for preparing the hard carbon negative electrode material, in the step of "mixing starch with a cross-linking agent", the starch includes at least one of corn starch, wheat starch, sorghum starch, rice starch, potato starch, cassava starch and sweet potato starch.

[0028] The present application also proposes a lithium-ion battery, wherein the negative electrode material of the lithium-ion battery is the above-mentioned hard carbon negative electrode material or the hard carbon negative electrode material prepared by the above-mentioned preparation method.

[0029] The technical solution of the present application adopts a staged heating method to carbonize the carbonized precursor under a protective atmosphere, so that the starch can be carbonized under the action of a cross-linking agent to obtain a hard carbon negative electrode material. Among them, the starch is carbonized by staged heating under the action of a cross-linking agent, which promotes the cross-linking reaction in the process of starch dehydration condensation and increases the gap between the carbon layer structures. In this way, the prepared hard carbon negative electrode material can provide more lithium cluster storage space, thereby improving the capacity of the hard carbon negative electrode material. In addition, the increase in cross-linked structures is conducive to the rapid deintercalation of lithium, thereby improving the rate performance of the hard carbon negative electrode material. That is, the technical solution of the present application can promote the cross-linking reaction in the process of starch dehydration condensation and increase the gap between the carbon layer structures, thereby improving the capacity and rate performance of the hard carbon negative electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a flow chart of an embodiment of a method for preparing a hard carbon negative electrode material of the present application;

[0031] Figure 2 for Figure 1 Detailed flow chart of step S20;

[0032] Figure 3 This is a process flow chart of the preparation method of the hard carbon negative electrode material of the present application;

[0033] Figure 4 This is a small-angle X-ray scattering diagram of the hard carbon negative electrode material of Example 1 of the present application;

[0034] Figure 5 This is a small-angle X-ray scattering diagram of the hard carbon negative electrode material of Comparative Example 2 of the present application;

[0035] Figure 6 This is a small-angle X-ray scattering diagram of the hard carbon negative electrode material of Comparative Example 3 of the present application;

[0036] Figure 7 This is a scanning electron microscope image of the hard carbon negative electrode material of Example 1 of the present application;

[0037] Figure 8 This is a scanning electron microscope image of the hard carbon negative electrode material of Example 2 of the present application;

[0038] Figure 9 This is a scanning electron microscope image of the hard carbon negative electrode material of Comparative Example 1 of the present application;

[0039] Figure 10 This is a charge-discharge curve of the hard carbon negative electrode material of Example 1 of the present application tested in a button cell at a current density of 400 mA / g;

[0040] Figure 11 This is a graph showing the cycling performance data of the hard carbon negative electrode material of Example 1 of the present application tested in a cylindrical battery at a current density of 3C. DETAILED DESCRIPTION

[0041] To make the purpose, technical solutions and advantages of the examples of the present application clearer, the technical solutions in the examples of the present application will be described clearly and completely below. Where specific conditions are not specified in the examples, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased commercially.

[0042] The present application proposes a hard carbon negative electrode material. The hard carbon negative electrode material is spherical and has micropores distributed inside. The size of the micropores is 0.3 nm to 0.7 nm.

[0043] It should be noted that the micropores described in this application refer to the internal gaps of the hard carbon negative electrode material. The hard carbon negative electrode material is formed by stacking hard carbon particles, and there will be gaps between adjacent hard carbon particles to form the internal gaps of the hard carbon negative electrode material. This application adjusts the micropore size to 0.3nm~0.7nm, so that the hard carbon negative electrode material provides more lithium cluster storage space, thereby increasing the capacity of the hard carbon negative electrode material, and is conducive to rapid lithium deintercalation, thereby improving the rate performance of the hard carbon negative electrode material. At the same time, the hard carbon negative electrode material of this application is spherical. By sphericalizing the hard carbon particles, the packing density between the hard carbon particles is reduced, the compaction density of the hard carbon negative electrode material is increased, and the hard carbon negative electrode material has a high energy density. That is, the technical solution of this application can improve the capacity and rate performance of the hard carbon negative electrode material.

[0044] In one embodiment of the present application, the hard carbon negative electrode material satisfies at least one of the following characteristics a to g: a. The micropore volume is 0.02 cm 3 / g~0.20cm 3 / g; b. The micropore porosity is 5% to 27%; c. The total specific surface area of ​​the micropores is 90m 2 / g~500m 2 / g; d. The true density of the hard carbon negative electrode material is 1.3g / cm 3 ~1.8g / cm 3 ; e. The particle size of the hard carbon negative electrode material is 3um to 50um; f. The compaction density of the hard carbon negative electrode material is 0.96g / cm 3 ~1.20g / cm 3 ; g. The specific surface area of ​​the hard carbon negative electrode material is 0.5m 2 / g~3.0m 2 / g. This application regulates the size of the hard carbon negative electrode material to provide more lithium cluster storage space, thereby ensuring the capacity and rate performance of the hard carbon negative electrode material, thereby meeting the market development requirements for lithium-ion batteries and broadening the application prospects of lithium-ion batteries.

[0045] See also Figure 1 As shown, in one embodiment of the present application, a method for preparing a hard carbon negative electrode material includes the following steps:

[0046] Step S10, mixing starch with a cross-linking agent to obtain a carbonized precursor;

[0047] Step S20: Carbonizing the carbonized precursor in a protective atmosphere by stepwise heating, and obtaining a hard carbon negative electrode material after cooling.

[0048] In the technical solution of the present application, under a protective atmosphere, the precursor is carbonized by a staged heating method, so that the starch can be carbonized under the action of a cross-linking agent to obtain a hard carbon negative electrode material. Among them, the starch is carbonized by a staged heating method under the action of a cross-linking agent, which promotes the cross-linking reaction in the process of starch dehydration condensation and increases the gap between the carbon layer structures. In this way, the prepared hard carbon negative electrode material can provide more lithium cluster storage space, thereby improving the capacity of the hard carbon negative electrode material, and the increase in cross-linked structures is conducive to rapid lithium deintercalation, thereby improving the rate performance of the hard carbon negative electrode material. That is, the technical solution of the present application can promote the cross-linking reaction in the process of starch dehydration condensation and increase the gap between the carbon layer structures, thereby improving the capacity and rate performance of the hard carbon negative electrode material. Furthermore, the starch is carbonized in a staged heating manner under the action of a cross-linking agent, ensuring the orderly progress of the starch carbonization and dehydration process, avoiding the destruction of the spherical morphology of the starch raw material itself caused by direct high-temperature carbonization, and ensuring that the particles of the hard carbon negative electrode material formed after carbonization have the spherical morphology of the starch raw material, thereby reducing the stacking gaps between the particles through particle sphericity, improving the compaction density of the prepared hard carbon negative electrode material, and thus improving the energy density of the hard carbon negative electrode material. The embodiment of the present application will sphericize the particles that form the hard carbon negative electrode material, reduce the specific surface area of ​​the hard carbon negative electrode material, thereby reducing the lithium consumed in forming the solid electrolyte interface film (SEI film), which is conducive to improving the first coulombic efficiency of the hard carbon negative electrode material.

[0049] It should be noted that compared with inorganic cross-linking agents, for example, when ammonium chloride and ammonium sulfate are used as inorganic cross-linking agents, during the high-temperature carbonization process of starch, ammonium chloride decomposes into hydrochloric acid and ammonia, and ammonium sulfate decomposes into sulfuric acid and ammonia. Since the decomposition products, hydrochloric acid and sulfuric acid, are highly acidic, they will corrode the carbonization equipment, causing aging and damage to the carbonization equipment. Preferably, the cross-linking agent is an organic cross-linking agent, thereby avoiding the corrosion of the carbonization equipment by strong acidity. In addition, the embodiment of the present application is carbonized under a protective atmosphere, thereby avoiding the irreversible capacity increase caused by the reaction of oxygen functional groups with lithium ions caused by the generation of oxygen-containing functional groups, ensuring the first efficiency of the lithium-ion battery, and improving the first coulomb efficiency of the hard carbon negative electrode material. The hard carbon negative electrode material prepared in the present application has a uniform particle size, a smooth surface, a high compaction density, and reduces the specific surface area of ​​the hard carbon negative electrode material, thereby showing high capacity and long cycle life in lithium-ion batteries.

[0050] See also Figure 2As shown, in one embodiment of the present application, the step of "carbonizing the carbonization precursor by means of staged heating" includes: step S21, heating the carbonization precursor to 150°C~170°C for the first time, and keeping it warm for 0.5 hour to 3 hours; step S22, heating it to 200°C~350°C for the second time, and keeping it warm for 1 hour to 4 hours; step S23, heating it to 1000°C~1500°C for the third time, and keeping it warm for 1 hour to 4 hours. It should be noted that the present application controls the temperature to 150°C to 170°C by programmed temperature rise, so that the starch and the cross-linking agent undergo dehydration condensation at this low temperature stage, promotes dehydration between molecules, and obtains the product of dehydration condensation of starch and the cross-linking agent; in this way, the temperature is then controlled to 200°C to 350°C, and the product of dehydration condensation of starch and the cross-linking agent undergoes polycondensation and cross-linking reaction at this medium temperature stage; in this way, the spherical morphology of the starch raw material is maintained through two orderly dehydration condensation reactions, and the spherical morphology of the starch raw material is destroyed due to the melting and expansion of starch particles caused by the thermal breakage of the glycosidic bonds between the pyranose glucose units of the starch. In addition, the introduction of the cross-linking agent increases the cross-linking reaction between the carbon chains, thereby adjusting the number of pores between the carbon layers, providing more lithium cluster storage space, and bringing higher lithium storage capacity. At the same time, the increase in the cross-linking structure promotes the rapid insertion and extraction of lithium ions, thereby improving the rate performance of the hard carbon negative electrode material. In addition, the present application performs carbonization at 1000° C. to 1500° C. for 1 to 4 hours, thereby ensuring sufficient carbonization of the carbonization precursor and ensuring the recycling performance of the prepared hard carbon negative electrode material.

[0051] In one embodiment of the present application, in the step of "raising the temperature of the carbonized precursor to 150°C to 170°C for the first time", the first heating rate is 0.5°C / min to 5°C / min. The heating rate in the low-temperature stage of the present application should not be too slow or too fast, and the heating rate is controlled to be 0.5°C / min to 5°C / min. This not only increases the carbonization rate, but also avoids excessive carbonization, which causes the starch granules to melt and expand, thereby preventing the spherical morphology of the starch raw material from being destroyed, thereby ensuring the formation of a spherical hard carbon negative electrode material.

[0052] In one embodiment of the present application, in the step of "raising the temperature for the second time to 200°C to 350°C", the second heating rate is 0.5°C / min to 5°C / min. Similarly, the heating rate in the medium temperature stage of the embodiment of the present application should not be too slow or too fast. The heating rate of the carbonization furnace is controlled to be 0.5°C / min to 5°C / min. In this way, while increasing the carbonization rate, it also avoids the destruction of the spherical morphology of the starch raw material due to the melting and expansion of the starch particles caused by excessive carbonization, thereby ensuring the spherical morphology of the particles forming the hard carbon negative electrode material.

[0053] In one embodiment of the present application, in the step of "raising the temperature to 1000°C to 1500°C for the third time", the third heating rate is 1°C / min to 10°C / min. Similarly, the heating rate in the high-temperature stage of the embodiment of the present application should not be too slow or too fast. When the heating rate is too fast, the dehydrogenation and carbonization of the hard carbon precursor are too rapid, resulting in incomplete dehydrogenation inside the material, and the prepared hard carbon material has a low first efficiency; when the heating rate is too slow, the pore structure between the carbon layer structure will slowly shrink, resulting in a slight decrease in the lithium storage capacity. The embodiment of the present application controls the heating rate of the carbonization furnace to 1°C / min to 10°C / min, thereby improving the first efficiency while avoiding the reduction of capacity, thereby ensuring the formation of a hard carbon negative electrode material with high capacity and high first efficiency.

[0054] In one embodiment of the present application, in the step of “mixing starch with a cross-linking agent”, the cross-linking agent includes at least one of phenylhydrazine, phenylpropenol, 2-aminobiphenyl, 2-aminoethanol and 4-aminopyridine. In this embodiment of the present application, a cross-linking agent is added to the starch for mixing. The addition of the cross-linking agent allows the dehydration condensation process of the starch to proceed in a controllable and orderly manner, promotes the cross-linking reaction during the dehydration condensation of the starch, further increases the gap between the carbon layer structures, and enables the prepared hard carbon negative electrode material to provide more lithium cluster storage space, thereby improving the capacity and rate performance of the hard carbon negative electrode material. In addition, the controllable and orderly dehydration condensation process also ensures the spherical morphology of the starch raw material itself. The rapid dehydration condensation reaction between the starch and the cross-linking agent shortens the carbonization time of the starch, thereby shortening the preparation time of the hard carbon negative electrode material. Preferably, the cross-linking agent is phenylpropenol, which further dehydrates and condenses with starch through enol, thereby making the cross-linking between carbon chains more complex, thereby making the internal pores of the hard carbon negative electrode material richer, and improving the capacity and rate performance of the hard carbon negative electrode material.

[0055] In one embodiment of the present application, in the step of "mixing starch with a cross-linking agent", the mass ratio of the starch to the cross-linking agent is (9-1):1. It should be noted that the embodiment of the present application adjusts the amount of starch and the cross-linking agent so that the starch is fully carbonized under the action of the cross-linking agent, thereby ensuring the charge and discharge performance of the prepared hard carbon negative electrode material. Of course, the embodiment of the present application can appropriately adjust the amount ratio of starch and cross-linking agent according to the different types of starch and cross-linking agent used. The embodiment of the present application is not limited to this, and the above are all within the protection scope of the embodiment of the present application.

[0056] In one embodiment of the present application, in the step of "mixing starch with a cross-linking agent," the starch includes at least one of corn starch, wheat starch, sorghum starch, rice starch, potato starch, tapioca starch, and sweet potato starch. It should be noted that in this embodiment of the present application, by selecting different types of starch, hard carbon anode material particles of varying sizes or morphologies can be prepared.

[0057] The present application also proposes a hard carbon negative electrode material, which is prepared by a hard carbon negative electrode material preparation method, comprising the following steps: mixing starch with a cross-linking agent to obtain a carbonized precursor; carbonizing the carbonized precursor in a stepwise heating manner under a protective atmosphere, and obtaining a hard carbon negative electrode material after cooling. It should be noted that the micropore volume inside the material was tested by a Bruker small-angle X-ray scattering instrument, and the micropore volume of the hard carbon negative electrode material was 0.02 cm 3 / g~0.20cm 3 / g; the porosity is calculated according to Porod's law, and the porosity is the ratio of the micropore volume to the real volume. The real volume is obtained by testing with a true density meter. The porosity of the hard carbon negative electrode material is 5 to 27%. The starch of the present application is carbonized by a segmented heating method under the action of a cross-linking agent, which promotes the cross-linking reaction in the starch dehydration condensation process and increases the gap between the carbon layer structures. In this way, the prepared hard carbon negative electrode material can provide more lithium cluster storage space, thereby improving the capacity of the hard carbon negative electrode material, and the increase in cross-linked structures is conducive to rapid lithium deintercalation, thereby improving the rate performance of the hard carbon negative electrode material. In addition, the starch is carbonized by a segmented heating method under the action of a cross-linking agent, which ensures the orderly progress of the starch carbonization and dehydration process, avoids the destruction of the spherical morphology of the starch raw material itself caused by direct high-temperature carbonization, and ensures that the particles of the hard carbon negative electrode material formed after carbonization have the spherical morphology of the starch raw material, thereby reducing the stacking gap between the particles through particle sphericity, improving the compaction density of the prepared hard carbon negative electrode material, and then improving the energy density of the hard carbon negative electrode material. The embodiment of the present application spheroidizes the particles of the hard carbon negative electrode material to reduce the specific surface area of ​​the hard carbon negative electrode material. In this way, the lithium consumed in forming the solid electrolyte interface film (SEI film) is reduced, which is beneficial to improving the first coulombic efficiency of the hard carbon negative electrode material.

[0058] This application also proposes a lithium-ion battery whose negative electrode material is prepared using a method for preparing a hard carbon negative electrode material. By promoting the cross-linking reaction during the dehydration condensation of starch, this hard carbon negative electrode material provides more lithium cluster storage space, thereby improving the capacity and rate performance of the hard carbon negative electrode material. This meets the market requirements for lithium-ion batteries and broadens the application prospects of lithium-ion batteries.

[0059] Specifically, see Figure 3 As shown, a preparation method of a hard carbon negative electrode material comprises the following steps: mixing starch with a cross-linking agent to obtain a carbonized precursor; placing the mixed carbonized precursor into a crucible, and placing the crucible containing the carbonized precursor in a heating furnace, introducing an inert gas, and carbonizing by a staged heating method, wherein the heating program is: heating from room temperature to 150°C to 70°C at a heating rate of 0.5°C / min to 5°C / min, and keeping warm for 0.5 hour to 3 hours to allow the low-temperature dehydration condensation process to proceed fully; heating to 200°C to 350°C at a heating rate of 0.5°C / min to 5°C / min, and keeping warm for 1 hour to 4 hours to allow the medium-temperature condensation cross-linking process to proceed fully; heating to 1000-1500°C at a heating rate of 1°C to 10°C / min, and keeping warm for 1 hour to 4 hours to allow the high-temperature carbonization process to proceed fully. After the process is fully cooled, the material is discharged, and the carbonized product is broken up using a mixing device. After screening by a screening machine, a hard carbon negative electrode material is obtained. It should be noted that the crucible is one of a graphite crucible, a corundum crucible and a mullite crucible; the heating furnace is one of a box furnace, a push plate kiln, a roller kiln, a rotary kiln and a tube furnace; the inert gas is any one of nitrogen, helium, neon, argon, krypton or xenon, or a combination of at least two; the mixing equipment is one of a VC mixer, a three-dimensional mixer and a ball mill; the screening machine is an ultrasonic vibrating screen. The embodiments of the present application are not limited to this, and the above are all within the protection scope of the embodiments of the present application.

[0060] The internal gap of the hard carbon negative electrode material was detected by a Bruker small angle X-ray scattering instrument, and the specific surface area was calculated according to the Porod law. The micropore size of the hard carbon negative electrode material in the embodiment of the present application is 0.3nm~0.7nm, and the total specific surface area of ​​the micropores is 90m 2 / g~500m 2 / g. The micropore volume inside the material was tested by Bruker small-angle X-ray scattering, and the porosity was calculated according to Porod's law. The true volume was obtained by true density measurement. The micropore volume of the hard carbon negative electrode material of this application is 0.02cm 3 / g~0.20cm 3 / g, porosity 5% to 27%, true density 1.3g / cm 3 ~1.8g / cm 3 The particle size of the hard carbon negative electrode material was measured using a Malvern laser particle size analyzer 2000. The particle size of the hard carbon negative electrode material was 3μm to 50μm, with a preferred particle size of 5μm to 30μm. Under the condition that the pressure test instrument applied a pressure of 1 ton, the compaction density of the hard carbon negative electrode material was 0.96g / cm 3 ~1.20g / cm 3; The specific surface area of ​​the hard carbon negative electrode material calculated by BET method is 0.5m 2 / g~3.0m 2 / g; in the button half-cell system test of the spherical hard carbon negative electrode material, with metallic lithium as the counter electrode, the mass specific capacity of the hard carbon negative electrode material is 460mA hour / g~680mA hour / g; with metallic lithium as the counter electrode, the first coulombic efficiency obtained from the test is 76%~88%.

[0061] The technical solution of the present application will be further described below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not constitute a limitation on the scope of protection of the present application.

[0062] Example 1

[0063] Take 2kg of corn starch and 222g of 10% phenylpropenol, mix them evenly with a VC mixer, put them in a box furnace, and heat them from room temperature to 190℃ at a heating rate of 2℃ / min under a nitrogen atmosphere, and keep them warm for 0.5 hours; then heat them to 270℃ at a heating rate of 2℃ / min and keep them warm for 4 hours; then heat them to 1100℃ at a rate of 8℃ / min, keep them warm for 2 hours, and then cool them to room temperature to obtain a corn starch-based hard carbon negative electrode material. The micropore specific surface area of ​​the obtained corn starch-based hard carbon negative electrode material is 372m 2 / g, micropore volume is 0.14cm 3 / g, true density is 1.47g / cm 3 , porosity is 20.58%, particle size D50 is 10.5μm, compacted density is 1.05g / cm 3 , with a specific surface area of ​​1.52m 2 / g.

[0064] Example 2

[0065] Take 4kg of potato starch and 1.71kg of 30% phenylpropenol, mix them evenly in a three-dimensional mixer, put them in a rotary kiln, and heat them from room temperature to 170℃ at a heating rate of 0.5℃ / min under argon atmosphere, and keep them warm for 3 hours; then heat them to 220℃ at a heating rate of 1℃ / min, and keep them warm for 2 hours; then heat them to 1200℃ at a rate of 5℃ / min, keep them warm for 3 hours, and then cool them to room temperature to obtain a potato starch-based hard carbon negative electrode material. The micropore specific surface area of ​​the obtained potato starch-based hard carbon negative electrode material is 269m 2 / g, micropore volume is 0.12cm 3 / g, true density is 1.52g / cm 3 , porosity is 18.24%, particle size D50 is 17.6 μm, and compacted density is 1.15 g / cm 3 , with a specific surface area of ​​0.79m2 / g.

[0066] Comparative Example 1

[0067] 2 kg of corn starch and 222 g of 10% phenylpropenol were mixed evenly in a VC mixer, placed in a box furnace, and heated from room temperature to 1100 ° C at a heating rate of 0.5 ° C / min under a nitrogen atmosphere to obtain a corn starch-based hard carbon negative electrode material. The micropore specific surface area of ​​the obtained corn starch-based hard carbon negative electrode material is 81 m 2 / g, micropore volume is 0.11cm 3 / g, true density is 1.63g / cm 3 , porosity is 17.92%, particle size D50 is 14.1 μm, and compacted density is 0.9 g / cm 3 , with a specific surface area of ​​5.76m 2 / g.

[0068] Comparative Example 2:

[0069] Take 2kg of corn starch, mix it evenly with a VC mixer, put it into a box furnace, and heat it from room temperature to 190℃ at a heating rate of 0.5℃ / min under a nitrogen atmosphere, and keep it warm for 0.5 hours; then heat it to 270℃ at a heating rate of 2℃ / min, and keep it warm for 4 hours; then heat it to 1100℃ at a rate of 8℃ / min, keep it warm for 2 hours, and then cool it to room temperature to obtain a corn starch-based hard carbon negative electrode material. The micropore specific surface area of ​​the obtained corn starch-based hard carbon negative electrode material is 21m 2 / g, micropore volume is 0.04cm 3 / g, true density is 1.76g / cm 3 , porosity is 7.04%, particle size D50 is 12.1 μm, and compacted density is 0.95 g / cm 3 , with a specific surface area of ​​2.58m 2 / g.

[0070] Comparative Example 3

[0071] Take 2kg of corn starch and 222g of urotropine (10%), mix them evenly with a VC mixer, put them in a box furnace, and heat them from room temperature to 190℃ at a heating rate of 2℃ / min under a nitrogen atmosphere, and keep them warm for 0.5 hours; then heat them to 270℃ at a heating rate of 2℃ / min and keep them warm for 4 hours; then heat them to 1100℃ at a rate of 8℃ / min, keep them warm for 2 hours, and then cool them to room temperature to obtain a corn starch-based hard carbon negative electrode material. The micropore specific surface area of ​​the obtained corn starch-based hard carbon negative electrode material is 57m 2 / g, micropore volume is 0.07cm 3 / g, true density is 1.68g / cm 3, porosity is 11.76%, particle size D50 is 11.4 μm, and compacted density is 0.92 g / cm 3 , with a specific surface area of ​​4.39m 2 / g.

[0072] Supplementary explanation: The following instruments were used to perform performance tests on the hard carbon negative electrode material prepared in this application: a Bruker small-angle X-ray scattering instrument to characterize the gaps, specific surface area and gap volume inside the material, a Beijing Best 3H-2000TD true density tester to measure the true density of the material, a Tristar 3000 fully automatic specific surface area and porosity analyzer from Micrometer Instruments of the United States to test the specific surface area of ​​the material, a 4350.22 tablet press from Carver of the United States to test the compaction density of the material at a test pressure of 1 ton, a Malvern laser particle size tester MS 2000 to test the particle size range of the material and the average particle size of the raw material particles, and a Hitachi S4800 scanning electron microscope to observe the surface morphology and particle size of the material.

[0073] Small angle X-ray scattering was performed on Example 1, Comparative Example 2 and Comparative Example 3 to analyze the gap inside the material. The results are shown in Figures 4 to 6 .from Figure 4 It can be seen that the internal gap is mainly distributed in the Q value of 1.3-21nm -1 In this range, the hard carbon negative electrode material has a broad peak, indicating that there are a large number of gaps in the nanoscale inside the material. According to the Polord law, the micropore specific surface area is 372m 2 / g. Comparative Example 2 was calculated to have a micropore specific surface area of ​​21m 2 / g. Through the comparative analysis of Example 1 and Comparative Example 2, it can be seen that the addition of the crosslinking agent increases the crosslinking degree of the carbon chain, thereby enriching the gaps inside the material and increasing the micropore specific surface area. Compared with Comparative Example 3, it can be seen that Comparative Example 3 adds urotropine as a crosslinking agent, which produces a certain amount of carbon chain crosslinking. -1 ~4.3nm -1 There are certain gaps. Due to insufficient cross-linking, the micropore specific surface area is only 57m 2 / g. As can be seen from the above, the cross-linking agent used in this application promotes the mutual cross-linking between carbon chains during the polycondensation cross-linking process, generating abundant gaps. These gaps form more lithium cluster storage spaces, further improving the capacity and rate performance of the hard carbon negative electrode material.

[0074] The hard carbon negative electrode materials prepared in Example 1, Example 2 and Comparative Example 1 were tested by scanning electron microscope (SEM), and the results are shown in FIG. Figures 7 to 9. As can be seen from the figure, the hard carbon negative electrode materials prepared in Example 1 and Example 2 are both spherical, among which the particle size prepared in Example 1 is more uniform and the overall sphericity is very high. The surface of the material prepared in Example 2 is smoother, there is no adhesion between the particles, and some of the particles are elliptical, which is mainly due to the morphology of the potato starch itself. The hard carbon negative electrode material prepared in Comparative Example 1 is in the shape of an irregular block and contains many fine particles. It can be seen from the above that direct high-temperature carbonization will lead to the thermal breakage of the glycosidic bonds between the pyranose glucose units of starch, thereby causing the starch particles to melt and expand, making it impossible to maintain the spherical morphology of the starch raw material. In this way, a large number of irregular blocks and fine powders are produced when the crushed particles are broken. In contrast, the staged heating method ensures the orderly progress of the starch carbonization and dehydration process, avoids the destruction of the spherical morphology of the starch raw material itself caused by direct high-temperature carbonization, and ensures that the particles of the hard carbon negative electrode material formed after carbonization have the spherical morphology of the starch raw material.

[0075] The hard carbon negative electrode materials obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were prepared into batteries and then subjected to electrochemical performance testing.

[0076] The specific button cell preparation process is as follows: a hard carbon negative electrode material, a conductive agent, and a binder are mixed in a mass ratio of 91:3:6, and the mixture is adjusted to a solid content of 50% with distilled water. After uniform mixing, the mixture is coated on a copper foil current collector and vacuum dried to produce a negative electrode. A lithium sheet is used as the positive electrode, a 1 mol / L LiPF6 / EC+DMC+EMC (v / v = 1:1:1) electrolyte is used, Celgrad 2400 is used as the separator, and the outer shell is a 2016 button cell shell. The first reversible capacity and first coulombic efficiency of the hard carbon negative electrode materials prepared in each embodiment and comparative example are tested using a button cell. The specific test conditions are as follows: the test is carried out on the LAND battery test system of Wuhan Jinnuo Electronics Co., Ltd., first 0.1C constant current discharge with a cutoff voltage of 0mV, then constant voltage discharge with a cutoff current of 2.5uA, and then 0.1C charging with a cutoff voltage of 1.5V.

[0077] The specific cylindrical battery preparation method is as follows: a hard carbon negative electrode material, a conductive agent, and a binder are mixed in a ratio of 94:1:5 by mass, dispersed in a solvent, and the solid content is controlled to be 50%. After uniform mixing, the mixture is coated on a copper foil current collector and vacuum dried to produce a negative electrode plate. A lithium cobalt oxide positive electrode plate, a 1 mol / L LiPF6 / EC+DMC+EMC (v / v = 1:1:1) electrolyte, a Celgard 2400 separator, and an 18650 cylindrical single cell housing are used. The cylindrical batteries are used to test the rate discharge performance of the hard carbon negative electrode materials of each embodiment and comparative example. The specific test conditions are as follows: testing is carried out on the LAND battery testing system of Wuhan Jinnuo Electronics Co., Ltd., with activation at charge and discharge rates of 0.1C, 0.2C, and 0.5C for 2 weeks, then the charge rate is increased to 1C, and the discharge rates are tested at 1C, 5C, and 30C, respectively. The 30C / 1C rate discharge retention rate is calculated.

[0078] The capacity, first coulombic efficiency and rate performance measured in Example 1, Example 2 and Comparative Examples 1 to 3 are shown in Table 1:

[0079] Table 1

[0080]

[0081]

[0082] As shown in Table 1, compared with Comparative Examples 1 to 3, the hard carbon negative electrode materials prepared in Example 1 and Example 2 have higher capacity and first coulombic efficiency, and at the same time greatly improve the 30C / 1C rate discharge retention rate. This is because under controllable condensation conditions, the increase in cross-linking reaction makes it easy for the carbon chain to generate more internal gaps, thereby increasing the lithium cluster storage capacity. The increase in cross-linking structure can promote the deintercalation and extraction of lithium ions at a high rate, so that the embodiments of the present application have higher rate performance. At the same time, due to the segmented heating, the volatile components are sequentially removed, reducing the production of substances such as tar, so that the volatile components on the surface of the particles of Example 1 and Example 2 are less relative to Comparative Examples 1 to 3, and less lithium ions are consumed in forming the SEI film in the battery test, thereby improving the first coulombic efficiency of the hard carbon negative electrode material.

[0083] in addition, Figure 10 This is a charge-discharge curve of the hard carbon negative electrode material of Example 1 of the present application tested in a button cell at a current density of 400 mA / g. Figure 11 This figure shows the cycling performance data of the hard carbon anode material of Example 1 of this application tested in a cylindrical battery at a current density of 3C. As can be seen from the figure, the hard carbon anode material prepared in this example not only has good charge and discharge performance, but also has good cycling performance, and can be widely used in lithium-ion batteries.

[0084] The above description is only a preferred embodiment of the present application and does not limit the patent scope of the present application. All equivalent structural transformations made using the contents of the present application specification under the inventive concept of the present application, or direct / indirect application in other related technical fields are included in the patent protection scope of the present application.

Claims

1. A hard carbon negative electrode material, characterized in that: The hard carbon negative electrode material is spherical, and micropores are distributed inside the hard carbon negative electrode material. The size of the micropores is 0.3nm to 0.7nm, and the volume of the micropores is 0.02cm 3 / g~0.20cm 3 / g, the micropore porosity is 5% to 27%, and the total specific surface area of ​​the micropores is 90m 2 / g~500m 2 / g; The compaction density of the hard carbon negative electrode material is 0.96 g / cm 3 ~1.20g / cm 3 ; The specific surface area of ​​the hard carbon negative electrode material is 0.5m 2 / g~3.0m 2 / g.

2. The hard carbon negative electrode material according to claim 1, wherein It satisfies at least one of the following characteristics a-b: a. The true density of the hard carbon negative electrode material is 1.3g / cm 3 ~1.8g / cm 3 ; b. The particle size of the hard carbon negative electrode material is 3um to 50um.

3. A method for preparing a hard carbon negative electrode material according to claim 1 or 2, characterized in that: The following steps are involved: Mixing starch with a cross-linking agent to obtain a carbonized precursor; the cross-linking agent includes at least one of phenylhydrazine, phenylpropenol, 2-aminobiphenyl, 2-aminoethanol and 4-aminopyridine; Under a protective atmosphere, the carbonized precursor is heated to 150°C to 170°C for the first time and kept warm for 0.5 to 3 hours, then heated to 200°C to 350°C for the second time and kept warm for 1 to 4 hours, then heated to 1000°C to 1500°C for the third time and kept warm for 1 to 4 hours. After cooling, a hard carbon negative electrode material is obtained.

4. The method for preparing a hard carbon negative electrode material according to claim 3, wherein: In the step of "raising the temperature of the carbonized precursor to 150° C. to 170° C. for the first time", the first heating rate is 0.5° C. / min to 5° C. / min.

5. The method for preparing a hard carbon negative electrode material according to claim 3, wherein: In the step of "raising the temperature for the second time to 200°C to 350°C", the second heating rate is 0.5°C / min to 5°C / min.

6. The method for preparing a hard carbon negative electrode material according to claim 3, wherein: In the step of "raising the temperature for the third time to 1000° C. to 1500° C.", the third heating rate is 1° C. / min to 10° C. / min.

7. The method for preparing a hard carbon negative electrode material according to claim 3, wherein: In the step of "mixing starch with a cross-linking agent": The mass ratio of the starch to the cross-linking agent is (9-1):

1.

8. The method for preparing a hard carbon negative electrode material according to claim 3, wherein: In the step of "mixing starch with a cross-linking agent", the starch includes at least one of corn starch, wheat starch, sorghum starch, rice starch, potato starch, tapioca starch and sweet potato starch.

9. A lithium-ion battery, characterized in that: The negative electrode material of the lithium-ion battery is the hard carbon negative electrode material according to any one of claims 1 to 2 or the hard carbon negative electrode material prepared by the preparation method according to any one of claims 3 to 8.

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