Negative electrode active material and method for preparing same, secondary battery, and electronic device

Abstract

The present invention provides a negative electrode active material, a method for preparing the same, a secondary battery, and an electronic device. The negative electrode active material includes a hard carbon material, and the hard carbon material includes micropores and ultramicropores, the micropores having a pore diameter of less than 2 nm, the ultramicropores having a pore diameter of less than 0.7 nm, the pore volume of the micropores accounting for 95% to 100% of the total pore volume, and the pore volume of the ultramicropores being 0.01 cm. 3 / g~0.2cm 3 / g, and the proportion of the pore volume of the ultrafine pores to the total pore volume is 80% to 99%. The negative electrode active material provided by the present invention has a stable low potential platform, a high capacity per gram, and a high reversible capacity, and therefore, by applying the negative electrode active material of the present invention to a secondary battery, the energy density of the secondary battery can be improved and the cycle characteristics of the secondary battery can be improved.

Description

[Technical Field]

[0001] This invention claims priority to a Chinese patent application filed with the China Patent Office on April 7, 2023, bearing application number 202310369251.6 and entitled "Negative electrode active material and preparation method thereof, negative electrode strip, and secondary battery," the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to the field of electrochemistry, and in particular to a negative electrode active material and a preparation method thereof, a secondary battery, and an electronic device. [Background technology]

[0003] Secondary batteries have outstanding advantages such as high energy density, low self-discharge rate, long cycle life, and stable discharge characteristics, and are therefore widely used in industrial production and people's daily lives.

[0004] Currently, the capacity development of graphite, an anode active material, has reached its limit, approaching its theoretical capacity of 372 mAh / g, making it difficult to further improve the energy density of lithium-ion batteries using graphite as an anode active material. Furthermore, commercialized graphite cannot be directly applied to sodium-ion batteries. Among the many anode active materials being developed, hard carbon materials have gained attention due to their high theoretical capacity per gram, low volume expansion, and rapid charge / discharge characteristics. Furthermore, hard carbon materials have broad application prospects because they can be used simultaneously as anode active materials in both lithium-ion and sodium-ion batteries. However, existing hard carbon materials have relatively low actual capacity per gram, high irreversible capacity, and a lack of a stable charge / discharge platform, limiting the improvement of energy density in secondary batteries and making it difficult to meet practical needs. Summary of the Invention

[0005] The present invention aims to provide a negative electrode active material for improving the energy density of a secondary battery, a method for preparing the same, a secondary battery, and an electronic device. Specific technical solutions are as follows:

[0006] A first aspect of the present invention is a negative electrode active material, the negative electrode active material comprising a hard carbon material, the hard carbon material comprising micropores and ultramicropores, the micropores having a pore diameter of less than 2 nm, the ultramicropores having a pore diameter of less than 0.7 nm, the pore volume of the micropores accounting for 95% to 100% of the total pore volume, and the pore volume of the ultramicropores being 0.01 cm 3 / g~0.2cm 3 / g, and the proportion of the pore volume of the ultrafine pores to the total pore volume is 80% to 99%. By controlling the proportion of the pore volume of the fine pores to the total pore volume, the proportion of the pore volume of the ultrafine pores to the total pore volume, and the pore volume of the ultrafine pores within the above ranges, the negative electrode active material can be made to have a stable low-potential platform, a high capacity per gram, and a high reversible capacity, and therefore, by applying the negative electrode active material of the present invention to a secondary battery, the energy density of the secondary battery can be improved and the cycle characteristics of the secondary battery can be improved.

[0007] In some embodiments of the present invention, the hard carbon material contains carbon and oxygen elements, and the mass percentage of the oxygen element in the hard carbon material is 2% to 7%. A hard carbon material containing carbon and oxygen elements and having a mass percentage of the oxygen element within the above range contributes to the adsorption of active metal ions in the micropores and the release of active metal ions from the ultrafine pores, which is advantageous for improving the capacity per gram of the negative electrode active material.

[0008] In some embodiments of the present invention, the oxygen element in the hard carbon material exists in the form of a carbonyl group and a carboxy group, and the mass of the oxygen element in the carbonyl group and the carboxy group accounts for 60% to 99% of the total mass of the oxygen element in the hard carbon material. The oxygen element in the hard carbon material exists in the form described above, and by controlling the content thereof within the above range, it contributes more to the occurrence of the oxidation-reduction reaction of the active metal ions, thereby further improving the capacity per gram of the negative electrode active material.

[0009] In some embodiments of the present invention, the hard carbon material further comprises element A, which comprises at least one of N and S, and the mass percentage of element A is 0.05% to 2% based on the mass of the hard carbon material. By controlling the content of element A within this range, the hard carbon material can contribute to increasing the carbon layer spacing of the hard carbon material, promoting the release of active metal ions from the hard carbon material, and providing the negative electrode active material with a low potential platform and high reversible capacity.

[0010] In some embodiments of the present invention, the hard carbon material is D is the peak area of ​​the D peak in the Raman spectrum of the hard carbon material, and I G is the peak area of ​​the G peak in the Raman spectrum of a hard carbon material, 0.8≦I D / I G ≦1.5, which indicates that the hard carbon material has an appropriate degree of defects. Having an appropriate degree of defects in hard carbon materials can promote ion adsorption and binding and reduce irreversible capacity loss due to high defect levels, which is advantageous for improving the capacity per gram of the negative electrode active material.

[0011] In some embodiments of the present invention, Li / Li between 0 V and 2.5 V +Within this potential range, the lithium storage capacity per gram of the hard carbon material is 300mAh / g to 700mAh / g. When the lithium release energy of the hard carbon material is E1Wh, the lithium release capacity of the hard carbon material is C1Ah, and the average potential of the lithium release of the hard carbon material is E1 / C1V, the relationship between E1 / C1 and the average potential of the lithium release of the hard carbon material is 0.13≦E1 / C1≦0.28. The Li / Li ratio between 0V and 0.1V is 0.13≦E1 / C1≦0.28. + Within the potential range of 0V to 0.8V, the capacity per gram of hard carbon material accounts for 30% to 65% of the total capacity per gram of lithium storage. + In this potential range, the capacity per gram of the hard carbon material accounts for 70% to 96% of the total lithium storage capacity per gram. This indicates that the hard carbon material has a high lithium storage capacity per gram and a low average lithium release potential. Using a hard carbon material with the above characteristics as a negative electrode active material for a lithium ion battery is advantageous in improving the energy density of the lithium ion battery.

[0012] In some embodiments of the present invention, Na / Na + Within this potential range, the sodium storage capacity per gram of the hard carbon material is 250mAh / g to 400mAh / g. When the sodium release energy of the hard carbon material is E2Wh, the sodium release capacity of the hard carbon material is C2Ah, and the average potential of the sodium release of the hard carbon material is E2 / C2V, the relationship between E2 / C2 and the average potential of the sodium release of the hard carbon material is 0.2≦E2 / C2≦0.4. The Na / Na ratio between 0V and 0.5V is 0.2≦E2 / C2≦0.4. + Within the potential range of 0 V to 0.8 V, the capacity per gram of the hard carbon material accounts for 76% to 91% of the total capacity per gram of sodium storage. +Within this potential range, the capacity per gram of the hard carbon material accounts for 89% to 95% of the total capacity per gram of sodium storage. This indicates that the hard carbon material has a high capacity per gram of total sodium storage and a low average potential of sodium release. Using a hard carbon material with these characteristics as the negative electrode active material of a sodium-ion battery is advantageous in improving the energy density of the battery.

[0013] In some embodiments of the present invention, the electrical conductivity of the hard carbon material is 0.5 S / cm to 10 S / cm. Controlling the electrical conductivity of the hard carbon material within this range is advantageous for improving the capacity and rate characteristics of the secondary battery, as well as the cycle characteristics of the secondary battery.

[0014] A second aspect of the present invention is a method for preparing a negative electrode active material according to the first aspect of the present invention, which is a method for preparing a hard carbon material, comprising the steps of: (1) crushing and sieving a precursor, and then placing it in a sealed reactor; and replacing the gas in the sealed reactor with a first gas; the precursor includes at least one of lignin, cellulose, alkali lignin, asphalt, epoxy resin, and phenolic resin; and the first gas includes any one of oxygen gas, air, and carbon dioxide; (2) After sealing the sealed reactor, place it in a nitrogen atmosphere, perform primary firing, raise the temperature to 700-900°C at a rate of 0.5°C / min to 5°C / min, pre-carbonize for 1 to 4 hours, and then cool to obtain a pre-carbonized material. (3) placing the pre-carbonized material in a nitrogen atmosphere, performing secondary baking, heating the material to 1000°C to 1500°C at a rate of 0.5°C / min to 5°C / min, carbonizing for 1 hour to 8 hours, and then cooling to obtain a carbonized material, which is then classified according to particle size; (4) A method for preparing a negative electrode active material includes heating the carbonized material classified in step (3) to 700°C to 1200°C, passing a mixed gas of a reducing gas and argon gas through the carbonized material, maintaining the temperature for 0.1 to 12 hours, replacing the gas with nitrogen gas, and then cooling the carbonized material to obtain a hard carbon material, wherein the reducing gas contains at least one of acetylene and methane, and the mass percentage of the reducing gas is 5 wt% to 20 wt% based on the mass of the mixed gas.

[0015] The negative electrode active material prepared by the method provided by the second aspect of the present invention has a stable low potential platform, a high capacity per gram, and a high reversible capacity, and therefore, by using it in the negative electrode piece of a secondary battery, the energy density of the secondary battery can be increased and the cycle characteristics of the secondary battery can be improved.

[0016] A third aspect of the present invention provides a secondary battery, the secondary battery comprising a positive electrode piece, a negative electrode piece, and an electrolyte, the negative electrode piece comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising the negative electrode active material provided by the first aspect of the present invention or the negative electrode active material prepared according to the preparation method provided by the second aspect of the present invention. The negative electrode piece of the present invention has a high capacity, which can improve the energy density of the secondary battery and also improve the cycle characteristics of the secondary battery, thereby providing the secondary battery with a high energy density and good cycle characteristics.

[0017] In some embodiments of the present invention, the compressed density of the negative electrode active material layer is 0.8 g / cm 3 ~1.2g / cm 3 By controlling the compressed density of the negative electrode active material layer within the above range, the content of the negative electrode active material per unit area can be increased, thereby improving the energy density of the secondary battery.

[0018] A fourth aspect of the present invention provides an electronic device including the secondary battery provided by the third aspect of the present invention. The secondary battery provided by the third aspect of the present invention has a high energy density and good cycle characteristics, and therefore the electronic device provided by the fourth aspect of the present invention has a relatively long service life.

[0019] Beneficial Effects of the Invention The present invention provides a negative electrode active material, a method for preparing the same, a secondary battery, and an electronic device. The negative electrode active material includes a hard carbon material, and the hard carbon material includes micropores and ultramicropores, the micropores having a pore diameter of less than 2 nm, the ultramicropores having a pore diameter of less than 0.7 nm, the pore volume of the micropores accounting for 95% to 100% of the total pore volume, and the pore volume of the ultramicropores being 0.01 cm. 3 / g~0.2cm 3 / g, and the proportion of the pore volume of the ultrafine pores to the total pore volume is 80% to 99%. By controlling the proportion of the pore volume of the fine pores to the total pore volume, the proportion of the pore volume of the ultrafine pores to the total pore volume, and the pore volume of the ultrafine pores within the above ranges, the negative electrode active material can be made to have a stable low-potential platform, a high capacity per gram, and a high reversible capacity, and therefore, by applying the negative electrode active material of the present invention to a secondary battery, the energy density of the secondary battery can be improved and the cycle characteristics of the secondary battery can be improved.

[0020] Of course, any one product or method of the present invention need not simultaneously achieve all of the above advantages. [Brief explanation of the drawings]

[0021] In the following, in order to more clearly explain the embodiments of the present invention and the technical solutions of the prior art, the drawings necessary for explaining the embodiments and the prior art will be briefly described. Obviously, the drawings described below are only a part of the embodiments of the present invention, and those skilled in the art can obtain other embodiments based on these drawings. [Figure 1] FIG. 1 is a scanning electron microscope photograph of the hard carbon material prepared in Examples 1-5. [Figure 2] FIG. 2 is a pore size distribution diagram of the hard carbon material prepared in Example 1-5, determined by nitrogen gas adsorption / desorption. [Figure 3] FIG. 3 shows charge-discharge curves of the hard carbon materials of Examples 1-5 within the Li / Li+ potential range of 0 V to 2.5 V. [Figure 4] FIG. 4 shows charge-discharge curves of the hard carbon material of Comparative Example 2 within the Li / Li+ potential range of 0V to 2.5V. [Figure 5] FIG. 5 shows charge-discharge curves of the hard carbon material of Example 2-1 within the Na / Na+ potential range of 0 V to 2.5 V. [Figure 6] FIG. 6 shows the Raman spectra of the hard carbon materials prepared in Examples 1-5. [Figure 7] FIG. 7 shows the X-ray diffraction spectrum of the hard carbon material prepared in Example 1-5. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, the technical solutions in the embodiments of the present invention will be described in detail with reference to the drawings in the embodiments of the present invention. Of course, the described embodiments are only a part of the embodiments of the present invention, and are not all of the embodiments. All other embodiments that a person skilled in the art can obtain based on the embodiments of the present invention fall within the scope of protection of the present invention.

[0023] In specific embodiments of the present invention, the present invention will be described using lithium ion batteries and sodium ion batteries as examples of secondary batteries, but the secondary batteries of the present invention are not limited to lithium ion batteries and sodium ion batteries.

[0024] A first aspect of the present invention is a negative electrode active material, the negative electrode active material comprising a hard carbon material, the hard carbon material comprising micropores and ultramicropores, the micropores having a pore diameter of less than 2 nm, the ultramicropores having a pore diameter of less than 0.7 nm, the pore volume of the micropores accounting for 95% to 100% of the total pore volume, and the pore volume of the ultramicropores being 0.01 cm 3 / g~0.2cm 3 / g, and the pore volume of the ultrafine pores accounts for 80% to 99% of the total pore volume. For example, the pore volume of the ultrafine pores may be 95%, 96%, 97%, 98%, or 100%, or may be within a range consisting of any two of these values. The pore volume of the ultrafine pores is 0.01 cm 3 / g, 0.04cm 3 / g, 0.08cm 3 / g, 0.12cm 3 / g, 0.15cm 3 / g, 0.17cm 3 / g, 0.2cm 3 / g, or may be within a range consisting of any two of these values. The ratio of the pore volume of the ultrafine pores to the total pore volume may be 80%, 83%, 85%, 87%, 89%, 91%, 93%, 95%, 97%, or 99%, or may be within a range consisting of any two of these values. In the present invention, the total pore volume refers to the sum of the pore volumes of all pores in the hard carbon material, the pore volume of the fine pores refers to the sum of the pore volumes of all the fine pores in the hard carbon material, and the pore volume of the ultrafine pores refers to the sum of the pore volumes of all the ultrafine pores in the hard carbon material.

[0025] The anode active material provided by the present invention includes a hard carbon material, which has abundant micropores and ultramicropores within it, and active metal ions (e.g., lithium ions, sodium ions) can be stored in the micropores or ultramicropores, thereby increasing the capacity per gram and providing the hard carbon material with a stable low-potential platform. Furthermore, the proportion of the pore volume of the micropores and the pore volume of the ultramicropores within the hard carbon material to the total pore volume is high, and the amount of mesopores (pore diameter 2 nm to 50 nm) and macropores (pore diameter greater than 50 nm) generated by overactivation of the hard carbon material is extremely small, so the initial coulombic efficiency and irreversible capacity of the hard carbon material are not affected. By controlling the proportion of the pore volume of micropores to the total pore volume, the proportion of the pore volume of ultrafine pores to the total pore volume, and the pore volume of ultrafine pores within the above-mentioned ranges, the negative electrode active material can be made to have a stable low-potential platform, a high capacity per gram, and a high reversible capacity. Therefore, by applying the negative electrode active material of the present invention to a secondary battery, the energy density of the secondary battery can be improved and the cycle characteristics of the secondary battery can be improved.

[0026] In some embodiments of the present invention, the hard carbon material contains carbon and oxygen elements, and the mass percentage of the oxygen element in the hard carbon material is 2% to 7%. For example, the mass percentage of the oxygen element in the hard carbon material may be 2%, 3%, 4%, 5%, 6%, or 7%, or may be within a range consisting of any two of these values. A hard carbon material containing carbon and oxygen elements and having a mass percentage of the oxygen element within the above range contributes to the release of active metal ions in micropores and ultramicropores, which is advantageous for improving the capacity per gram of the negative electrode active material.

[0027] In some embodiments of the present invention, the oxygen atoms in the hard carbon material are present in the form of carbonyl groups and carboxy groups, and the mass of the oxygen atoms in the carbonyl groups and carboxy groups accounts for 60% to 99% of the total mass of the oxygen atoms in the hard carbon material. For example, the mass of the oxygen atoms in the carbonyl groups and carboxy groups may be 60%, 70%, 75%, 80%, 90%, or 99% of the total mass of the oxygen atoms in the hard carbon material, or may be within a range consisting of any two of these values. The oxygen atoms in the hard carbon material are present in the form of ether groups in addition to carbonyl groups and carboxy groups. Carbonyl groups and carboxy groups have higher electronegativity than ether groups, and are therefore more advantageous in bonding with ions (e.g., lithium ions and sodium ions). The oxygen element in the hard carbon material is present in the above-mentioned form, and the ratio of the mass of the oxygen element in the carbonyl group and the carboxy group to the total mass of the oxygen element in the hard carbon material is within the above-mentioned range, which further contributes to the occurrence of the oxidation-reduction reaction of the active metal ions and further improves the capacity per gram of the negative electrode active material.

[0028] In some embodiments of the present invention, the hard carbon material further includes element A, which includes at least one of N and S. The mass percentage of element A is 0.05% to 2% based on the mass of the hard carbon material. For example, the mass percentage of element A may be 0.05%, 0.1%, 0.2%, 0.5%, 0.7%, 1%, 1.3%, 1.5%, 1.7%, or 2%, or may be within a range consisting of any two of these values. By containing element A within the above range, the hard carbon material can contribute to expanding the layer spacing of the hard carbon material, promoting the release of active ions from the hard carbon material, and providing the negative electrode active material with a low potential platform and high reversible capacity.

[0029] In some embodiments of the present invention, the hard carbon material has a viscosity of 0.8≦I D / I G ≦1.5. D is the peak area of ​​the D peak in the Raman spectrum of the hard carbon material, and I G is the peak area of ​​the G peak in the Raman spectrum of the hard carbon material. For example, I D / I G The value of may be 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, or 1.5, or may be in a range consisting of any two of these values. This indicates that the hard carbon material has an appropriate degree of defects. Having an appropriate degree of defects in the hard carbon material can promote ion adsorption and binding and reduce irreversible capacity loss due to high defect levels, which is advantageous for improving the capacity per gram of the negative electrode active material.

[0030] In some embodiments of the present invention, Li / Li between 0 V and 2.5 V + Within this potential range, the lithium storage capacity per gram of the hard carbon material is 300mAh / g to 700mAh / g, and when the lithium release energy of the hard carbon material is E1Wh, the lithium release capacity of the hard carbon material is C1Ah, and the average potential of the lithium release of the hard carbon material is E1 / C1V, the relationship between E1 / C1 and the average potential of the lithium release of the hard carbon material is 0.13≦E1 / C1≦0.28. For example, in the Li / Li range from 0V to 2.5V, +Within this potential range, the lithium storage capacity per total gram of the hard carbon material may be 300 mAh / g, 400 mAh / g, 500 mAh / g, 600 mAh / g, or 700 mAh / g, or may be within a range consisting of any two of these values, and the E1 / C1 value may be 0.13, 0.15, 0.18, 0.2, 0.23, 0.25, or 0.28, or may be within a range consisting of any two of these values. This indicates that the hard carbon material has a high lithium storage capacity per total gram and a low average lithium release potential. Using a hard carbon material having the above characteristics as a negative electrode active material for a lithium-ion battery is advantageous in improving the energy density of the lithium-ion battery. In the present invention, the lithium storage capacity per total gram refers to the Li / Li ratio between 0 V and 2.5 V. + This refers to the lithium storage capacity per whole gram of hard carbon material within the potential range of 0.1V.

[0031] In some embodiments of the present invention, Li / Li between 0 V and 0.1 V + Within this potential range, the capacity per gram of hard carbon material accounts for 30% to 65% of the total capacity per gram of lithium storage. For example, in the Li / Li range from 0 V to 0.1 V, + Within this potential range, the ratio of the capacity per gram of the hard carbon material to the total capacity per gram of lithium storage may be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or may be within a range consisting of any two of these values. This indicates that the hard carbon material has a low average potential for lithium release. Using a hard carbon material having the above characteristics as a negative electrode active material for a lithium ion battery is advantageous in improving the energy density of the lithium ion battery.

[0032] In some embodiments of the present invention, Li / Li between 0 V and 0.8 V +In the potential range of 0 V to 0.8 V, the capacity per gram of hard carbon material accounts for 70% to 96% of the total capacity per gram of lithium storage. + In this potential range, the ratio of the capacity per gram of the hard carbon material to the total capacity per gram of lithium storage may be 70%, 75%, 80%, 85%, 90%, 95%, 96%, or may be within a range consisting of any two of these values. This indicates that the hard carbon material has a low average potential for lithium release. Using a hard carbon material having the above characteristics as a negative electrode active material for a lithium ion battery is advantageous in improving the energy density of the lithium ion battery.

[0033] In some embodiments of the present invention, Na / Na + In the potential range of 0V to 2.5V, the sodium storage capacity per gram of the hard carbon material is 250mAh / g to 400mAh / g, and when the sodium release energy of the hard carbon material is E2Wh, the sodium release capacity of the hard carbon material is C2Ah, and the average potential of the sodium release of the hard carbon material is E2 / C2V, the ratio of E2 / C2 is 0.2≦E2 / C2≦0.4. For example, in the potential range of 0V to 2.5V, the sodium storage capacity per gram of the hard carbon material is 250mAh / g to 400mAh / g. + Within this potential range, the sodium storage capacity per whole gram of the hard carbon material may be 250 mAh / g, 300 mAh / g, 325 mAh / g, 350 mAh / g, or 400 mAh / g, or may be within a range consisting of any two of these values, and the E2 / C2 value may be 0.2, 0.25, 0.3, 0.35, or 0.4, or may be within a range consisting of any two of these values. This indicates that the hard carbon material has a high sodium storage capacity per whole gram and a low average potential for sodium release. Using a hard carbon material having the above characteristics as a negative electrode active material for a sodium-ion battery is advantageous in improving the energy density of the sodium-ion battery. In the present invention, the sodium storage capacity per whole gram refers to the Na / Na ratio between 0 V and 2.5 V.+ This refers to the sodium storage capacity per whole gram of hard carbon material within the potential range of 0.1 V.

[0034] In some embodiments of the present invention, Na / Na + In the potential range of 0 V to 0.5 V, the capacity per gram of the hard carbon material accounts for 76% to 91% of the total capacity per gram of sodium storage. + Within this potential range, the ratio of the capacity per gram of the hard carbon material to the total capacity per gram of sodium storage may be 76%, 80%, 83%, 85%, 88%, 91%, or may be within a range consisting of any two of these values. This indicates that the hard carbon material has a low average potential for sodium release. Using a hard carbon material having the above characteristics as a negative electrode active material for a sodium-ion battery is advantageous in improving the energy density of the sodium-ion battery.

[0035] In some embodiments of the present invention, Na / Na + Within this potential range, the capacity per gram of the hard carbon material accounts for 89% to 95% of the total capacity per gram of sodium storage. + Within this potential range, the ratio of the capacity per gram of the hard carbon material to the total capacity per gram of sodium storage may be 89%, 90%, 91%, 92%, 93%, 94%, 95%, or may be within a range consisting of any two of these values. This indicates that the hard carbon material has a low average potential for sodium release. Using a hard carbon material having the above characteristics as a negative electrode active material for a sodium-ion battery is advantageous in improving the energy density of the sodium-ion battery.

[0036] In some embodiments of the present invention, the electrical conductivity of the hard carbon material is 0.5 S / cm to 10 S / cm. For example, the electrical conductivity of the hard carbon material may be 0.5 S / cm, 1 S / cm, 3 S / cm, 5 S / cm, 7 S / cm, 9 S / cm, or 10 S / cm, or may be within a range consisting of any two of these values. Controlling the electrical conductivity of the hard carbon material within this range can reduce the internal resistance of the secondary battery, which is advantageous for improving the capacity and rate characteristics of the secondary battery as well as the cycle characteristics of the secondary battery.

[0037] A second aspect of the present invention is a method for preparing a negative electrode active material according to the first aspect of the present invention, which is a method for preparing a hard carbon material, comprising the steps of: (1) crushing and sieving a precursor, and then placing it in a sealed reactor; and replacing the gas in the sealed reactor with a first gas; the precursor includes at least one of lignin, cellulose, alkali lignin, asphalt, epoxy resin, and phenolic resin; and the first gas includes any one of oxygen gas, air, and carbon dioxide; (2) After sealing the sealed reactor, place it in a nitrogen atmosphere, perform primary firing, raise the temperature to 700-900°C at a rate of 0.5°C / min to 5°C / min, pre-carbonize for 1 to 4 hours, and then cool to obtain a pre-carbonized material. (3) placing the pre-carbonized material in a nitrogen atmosphere, performing secondary baking, heating the material to 1000°C to 1500°C at a rate of 0.5°C / min to 5°C / min, carbonizing for 1 hour to 8 hours, and then cooling to obtain a carbonized material, which is then classified according to particle size; (4) A method for preparing a negative electrode active material includes heating the carbonized material classified in step (3) to 700°C to 1200°C, passing a mixed gas of a reducing gas and argon gas through the carbonized material, maintaining the temperature for 0.1 to 12 hours, replacing the gas with nitrogen gas, and then cooling the carbonized material to obtain a hard carbon material, wherein the reducing gas contains at least one of acetylene and methane, and the mass percentage of the reducing gas is 5 wt% to 20 wt% based on the mass of the mixed gas.

[0038] The negative electrode active material prepared by the method provided by the second aspect of the present invention has a stable low potential platform, a high capacity per gram, and a high reversible capacity, and therefore, by using it in the negative electrode piece of a secondary battery, the energy density of the secondary battery can be increased and the cycle characteristics of the secondary battery can be improved.

[0039] The present invention is not particularly limited to the classification method in step (3), as long as the objectives of the present invention can be achieved. For example, the calcined negative electrode active material precursor is crushed and classified using a classifier crusher. The present invention is not particularly limited to the sealed reactor, and those skilled in the art can select any reactor according to actual needs as long as the objectives of the present invention can be achieved. For example, the sealed reactor may include, but is not limited to, a sealed graphite reactor, a sealed corundum reactor, a sealed nickel crucible, a sealed iron crucible, etc. In the present invention, the particle diameter Dv99 of the carbonized material particles classified in step (3) may be 30 μm to 50 μm. In the present invention, the particle diameter Dv50 of the hard carbon material particles is 3 μm to 20 μm. The particle diameter Dv99 of the hard carbon material particles may be in the range of 30 μm to 50 μm. Dv50 refers to the particle diameter at which the cumulative volume reaches 50% from the small particle diameter side in the volume-based particle size distribution of the material. Dv99 refers to the particle size at which the cumulative volume reaches 99% from the small particle size side in the volume-based particle size distribution of a material.

[0040] Typically, the mass of the precursor, the time of the primary firing, the temperature of the primary firing, the time of vapor deposition, and the time of the secondary firing all affect the ratio of the pore volume of micropores to the total pore volume and the ratio of the pore volume of ultrafine pores to the total pore volume in a hard carbon material. As the mass of the precursor increases, the ratio of the pore volume of micropores to the total pore volume and the ratio of the pore volume of ultrafine pores to the total pore volume increase and then decrease. As the time of the primary firing extends, the ratio of the pore volume of micropores to the total pore volume and the ratio of the pore volume of ultrafine pores to the total pore volume decrease. On the other hand, as the time of the primary firing shortens, the ratio of the pore volume of micropores to the total pore volume and the ratio of the pore volume of ultrafine pores to the total pore volume increase. As the temperature of the primary firing increases, the ratio of the pore volume of micropores to the total pore volume and the ratio of the pore volume of ultrafine pores to the total pore volume increase. On the other hand, as the temperature of the primary firing decreases, the proportion of the pore volume of micropores to the total pore volume and the proportion of the pore volume of ultrafine pores to the total pore volume decrease. As the vapor growth time increases, the proportion of the pore volume of micropores to the total pore volume and the proportion of the pore volume of ultrafine pores to the total pore volume increase. On the other hand, as the vapor growth time decreases, the proportion of the pore volume of micropores to the total pore volume and the proportion of the pore volume of ultrafine pores to the total pore volume decrease. As the temperature of the secondary firing increases, the proportion of the pore volume of micropores to the total pore volume and the proportion of the pore volume of ultrafine pores to the total pore volume decrease. On the other hand, as the temperature of the secondary firing decreases, the proportion of the pore volume of micropores to the total pore volume and the proportion of the pore volume of ultrafine pores to the total pore volume increase. In the present invention, carbonized materials having different particle sizes can be selected by classifying the carbonized materials according to particle size.

[0041] A third aspect of the present invention provides a secondary battery, the secondary battery comprising a positive electrode piece, a negative electrode piece, and an electrolyte, the negative electrode piece comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising the negative electrode active material provided by the first aspect of the present invention or the negative electrode active material prepared according to the preparation method provided by the second aspect of the present invention. The negative electrode piece of the present invention has high capacity, which can improve the energy density of the secondary battery and also improve the cycle characteristics of the secondary battery, thereby providing the secondary battery with high energy density and good cycle characteristics.

[0042] In some embodiments of the present invention, the compressed density of the negative electrode active material layer is 0.8 g / cm 3 ~1.2g / cm 3 For example, the compressed density of the negative electrode active material layer is 0.8 g / cm 3 , 0.9g / cm 3 , 1g / cm 3 , 1.1g / cm 3 , 1.2g / cm 3 or may be within a range consisting of any two of these values. By controlling the compressed density of the negative electrode active material layer within the above range, the content of the negative electrode active material per unit area can be increased, thereby improving the energy density of the secondary battery.

[0043] The negative electrode current collector of the present invention is not particularly limited as long as the object of the present invention can be achieved. For example, the negative electrode current collector may include aluminum foil, copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a polymer substrate coated with a conductive metal. The conductive metal may include, but is not limited to, copper, nickel, or titanium. The polymer substrate material may include, but is not limited to, at least one of polyethylene, polypropylene, ethylene-propylene copolymer, polyethylene terephthalate, polyethylene naphthalate, and polyparaphenylene terephthalamide. In the present invention, the thickness of the negative electrode current collector and the thickness of the negative electrode active material layer are not particularly limited as long as the object of the present invention can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm, and the thickness of the negative electrode active material layer on one side is 30 μm to 130 μm. In the present invention, the negative electrode active material layer may be formed on one surface in the thickness direction of the negative electrode current collector, or on two surfaces in the thickness direction of the negative electrode current collector. Here, the "surface" may refer to the entire area of ​​the negative electrode current collector or a partial area of ​​the negative electrode current collector, and the present invention is not particularly limited as long as the object of the present invention can be achieved. Optionally, the negative electrode active material may contain a thickener, and the thickener may include, but is not limited to, sodium carboxymethyl cellulose. The negative electrode active material layer of the present invention may further contain a conductive agent and a binder.

[0044] The conductive agent and binder of the present invention are not particularly limited as long as the object of the present invention can be achieved. For example, the conductive agent may include, but is not limited to, a carbon material, a metal, or a conductive polymer. The carbon material may include at least one of conductive carbon black (SuperP), carbon nanotubes (CNTs), carbon nanofibers, natural graphite, artificial graphite, flake graphite, carbon dots, and graphene. The metal may include metal powder or metal fiber such as copper, iron, or aluminum. The conductive polymer may include at least one of polythiophene, polypyrrole, polybenzeneamine, polyphenylene, and polyphenylene vinylene. The binder may include at least one of, but is not limited to, polypropylene alcohol, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyamideimide, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinylpyrrolidone, polyethylene, polypropylene, epoxy resin, nylon, styrene butadiene rubber (SBR), polyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral, water-based acrylic resin, carboxymethyl cellulose (CMC), and sodium carboxymethyl cellulose (CMC-Na), and the like.

[0045] The positive electrode piece of the present invention is not particularly limited as long as the objectives of the present invention can be achieved. For example, the positive electrode piece includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode current collector of the present invention is not particularly limited as long as the objectives of the present invention can be achieved. For example, the positive electrode current collector may include a metal foil sheet or a composite current collector. For example, the metal foil sheet may be aluminum foil. The composite current collector may include a polymer base layer and a metal material layer disposed on at least one surface of the polymer base layer. For example, the material of the metal material layer may include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. The polymer base layer may include at least one of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, and polyethylene. The positive electrode active material layer of the present invention includes a positive electrode active material. The type of positive electrode active material of the present invention is not particularly limited as long as the objectives of the present invention can be achieved.

[0046] In some embodiments, the secondary battery is a lithium-ion battery, and the positive electrode active material may include a lithium transition metal oxide, which may include at least one of lithium nickel cobalt manganese oxide (NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO), lithium manganese oxide, lithium iron manganese phosphate, and lithium titanate, etc., but is not limited to these. In other embodiments, the secondary battery is a sodium-ion battery, and the positive electrode active material may include at least one of sodium transition metal oxide, a polyanion-type compound, and a Prussian blue-type compound. The sodium transition metal oxide may include at least one of Na 1-x Cu h FE k Mn l M 1 m O 2-y , Na 0.67 Mn 0.7 NI z M2 0.3-z O2 and Na a Li b NI c Mn d FE e may contain O2, where M 1 is at least one of Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, and Ba, 0 < x ≦ 0.33, 0 < h ≦ 0.24, 0 ≦ k ≦ 0.32, 0 < l ≦ 0.68, 0 ≦ m < 0.1, h + k + l + m = 1, 0 ≦ y < 0.2, and M 2 is at least one of Li, Mg, Al, Ca, Ti, Fe, Cu, Zn, and Ba, 0 < z ≦ 0.1, 0.67 < a ≦ 1, 0 < b < 0.2, 0 < c < 0.3, 0.67 < d + e < 0.8, and b + c + d + e = 1. The polyanion-type compound is A 1 f M 3 g (PO4) i O j X 1 3-j , Na n M 4 PO4X 2 , Na p M 5 q (SO4)3, Na s Mn t FE 3-t may contain (PO4)2(P2O7), but is not limited thereto, where A 1 is at least one of H, Li, Na, K, and NH4, and M 3 is at least one of Ti, Cr, Mn, Fe, Co, Ni, V, Cu, and Zn, X 1 is at least one of F, Cl, and Br, 0 < f ≦ 4, 0 < g ≦ 2, 1 ≦ i ≦ 3, 0 ≦ j ≦ 2, and M 4 is at least one of Mn, Fe, Co, Ni, Cu, and Zn, X 2 is at least one of F, Cl, and Br, 0 < n ≦ 2, and M 5is at least one of Mn, Fe, Co, Ni, Cu, and Zn, where 0 < p ≤ 2, 0 < q ≤ 2, 0 < s ≤ 4, and 0 ≤ t ≤ 3. The Prussian blue compound may include, but is not limited to, at least one of Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], and Na2Mn[Mn(CN)6].

[0047] The thicknesses of the positive electrode current collector and the positive electrode active material layer of the present invention are not particularly limited as long as the object of the present invention can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, preferably 6 μm to 18 μm. The thickness of the positive electrode active material layer on one side is 30 μm to 120 μm. In the present invention, the positive electrode active material layer may be provided on one surface in the thickness direction of the positive electrode current collector or on two surfaces in the thickness direction of the positive electrode current collector. Here, the "surface" may be the entire region of the positive electrode current collector or a partial region of the positive electrode current collector, and the present invention is not particularly limited as long as the object of the present invention can be achieved. The positive electrode active material layer of the present invention may further contain the conductive agent and the binder.

[0048] In one embodiment, the electrolyte includes a lithium salt and a non-aqueous solvent. The lithium salt may include at least one of LiPF, LiBF, LiAsF, LiClO, LiB(CH), LiCHSO, LiCFSO, LiN(SOCF), LiC(SOCF), LiSiF, lithium bis(oxalato)borate (LiBOB), and lithium difluoroborate. The present invention does not particularly limit the concentration of the lithium salt in the electrolyte, as long as the objective of the present invention is achieved. In another embodiment, the electrolyte includes a sodium salt and a non-aqueous solvent. The sodium salt may include at least one of NaPF, NaClO, NaBCl, NaSOCF, and Na(CH)CHSO. The present invention does not particularly limit the concentration of the sodium salt in the electrolyte, as long as the objective of the present invention is achieved. The non-aqueous solvent is not particularly limited, as long as the objective of the present invention is achieved. For example, the non-aqueous solvent may include, but is not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, and other organic solvents. The carbonate compound may include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, and a fluorocarbonate compound. The chain carbonate compound may include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and methyl ethyl carbonate (MEC). The cyclic carbonate may include, but is not limited to, at least one of vinyl carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinyl ethylene carbonate (VEC).The fluorocarbonate compound may include, but is not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, and trifluoromethylethylene carbonate. The carboxylate compound may include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decalactone, valerolactone, and caprolactone. The ether compound may include, but is not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, and tetrahydrofuran. The other organic solvent may include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, and trioctyl phosphate.

[0049] The secondary battery of the present invention further includes a separator that isolates the positive and negative electrode pieces, prevents short circuits within the secondary battery, allows electrolyte ions to pass freely, and does not affect the electrochemical charge and discharge process. The separator of the present invention is not particularly limited as long as it achieves the objectives of the present invention. For example, the separator material may include, but is not limited to, at least one of polyethylene (PE), polypropylene (PP), polytetrafluoroethylene-based polyolefin (PO) separators, polyester film (e.g., polyethylene terephthalate (PET) film), cellulose film, polyimide film (PI), polyamide film (PA), spandex film, and aramid film. The separator type may include, but is not limited to, at least one of woven film, nonwoven film, microporous film, composite film, pressed film, and spun film. The separator of the present invention may have a porous structure, and a porous layer is disposed on at least one surface of the separator. The porous layer includes inorganic particles and a binder. The inorganic particles may include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder may include at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethylcellulose, polyvinylpyrrolidone, polyethylene ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The present invention does not particularly limit the pore size of the porous structure, as long as the objective of the present invention can be achieved. For example, the pore size of the porous structure may be 0.01 μm to 1 μm. In the present invention, the thickness of the separator is not particularly limited as long as the object of the present invention can be achieved.For example, the thickness of the separator may be 5 μm to 500 μm.

[0050] The secondary battery of the present invention further includes a packaging bag for containing the positive electrode pieces, separator, negative electrode pieces, and electrolyte, as well as other components known in the art for secondary batteries. The present invention does not limit the other components. The packaging bag of the present invention is not particularly limited and may be any packaging bag known in the art as long as the object of the present invention is achieved. For example, the packaging bag may be an aluminum-plastic film packaging bag.

[0051] The secondary battery of the present invention is not particularly limited and may include any device in which an electrochemical reaction occurs. In one embodiment of the present invention, the secondary battery may include, but is not limited to, a lithium ion battery, a sodium ion battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0052] The process for preparing the secondary battery of the present invention is well known to those skilled in the art, and is not particularly limited to the steps of, for example, stacking a positive electrode piece, a separator, and a negative electrode piece in this order, and if necessary, winding or folding the stack to obtain a wound electrode assembly, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag, and sealing the packaging bag to obtain a secondary battery; or, stacking a positive electrode piece, a separator, and a negative electrode piece in this order, and then fixing the four corners of the entire stack with adhesive tape to obtain a stacked electrode assembly, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag, and sealing the packaging bag to obtain a secondary battery. Furthermore, to prevent internal pressure buildup and overcharging and discharging of the secondary battery, an overcurrent protection element, lead plates, etc. may be placed in the packaging bag as needed.

[0053] A fourth aspect of the present invention provides an electronic device including the secondary battery provided by the third aspect of the present invention. The secondary battery provided by the third aspect of the present invention has a high energy density and good cycle characteristics, and therefore the electronic device provided by the fourth aspect of the present invention has a relatively long service life.

[0054] The electronic device of the present invention is not particularly limited and may be used in any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, a notebook computer, a pen-based computer, a mobile computer, an electronic book player, a mobile phone, a portable facsimile machine, a portable copier, a portable printer, a stereo headset, a video recorder, an LCD television, a portable vacuum cleaner, a portable CD player, a minidisc, a walkie-talkie, an electronic notebook, a calculator, a memory card, a portable tape recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, an electric bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household storage battery, or a lithium ion capacitor. Example

[0055] Hereinafter, the embodiments of the present invention will be described in more detail with reference to examples and comparative examples. Various tests and evaluations were carried out as follows. Unless otherwise specified, "parts" and "%" are by mass.

[0056] Measurement methods and equipment Raman measurements The Raman spectrum of the hard carbon material was measured using a Raman spectrometer. A powder of the hard carbon material was taken and measured. A 200 μm × 500 μm area was selected for measurement, and measurements were taken at 200 or more equally spaced points within this area. The measurement range for each point was 1000 cm. -1 ~2000cm -1 Between 1320cm -1 ~1370cm -1 The peak that appeared between 1570 cm and 1570 cm was designated as the D peak. -1 ~1620cm -1 When the peak that appears between is the G peak, I D / I G The ratio of the values ​​was then calculated as the final I D / I G It was calculated as the ratio of I Dis the peak area of ​​the D peak in the Raman spectrum of the hard carbon material, and I G was defined as the peak area of ​​the G peak in the Raman spectrum of the hard carbon material.

[0057] X-ray diffraction (XRD) measurements The X-ray diffraction spectrum of the hard carbon material was measured and analyzed using an X-ray powder diffractometer (XRD, device model: Bruker D8 ADVANCE), and the position of the XRD 002 peak of the hard carbon material was obtained.

[0058] Scanning electron microscope (SEM) measurements The particles of the hard carbon material were observed using a scanning electron microscope (device model: ZEISS SEM), and SEM photographs were taken.

[0059] Powder particle size measurement The particle size distribution of the hard carbon material was measured using a Malvern particle size analyzer (instrument model: Master Sizer 2000). In the volume-based particle size distribution of the material, the particle size at which the cumulative volume reaches 50% from the small particle size side was defined as Dv50, and the particle size at which the cumulative volume reaches 99% was defined as Dv99.

[0060] Pore ​​size distribution measurement Hard carbon powder was taken and pressurized with a 5-ton weight for 30 seconds, then placed in a sample tube. Vacuum was applied to 100°C for degassing and maintained for 12 hours. The carbon dioxide adsorption capacity of the hard carbon material at various gas pressures was measured using an ASAP2460 physical adsorption analyzer, and adsorption and desorption isotherms were plotted. The pore shape was determined based on the shape of the hysteresis loop, and the pore size distribution curve of the micropores was fitted using a DFT model to calculate the pore volume of the ultrafine pores, the pore volume of the micropores, and the total pore volume of the hard carbon material. The ratio of the pore volume of the micropores to the total pore volume was calculated as follows: micropore volume / total pore volume; and the ratio of the ultrafine pore volume to the total pore volume was calculated as ultrafine pore volume / total pore volume.

[0061] Elemental analysis measurement The proportions and contents of the elements C, O, N, and S in the hard carbon material were measured using an X-ray photoelectron spectrometer (instrument model: ThermoESCALAB250XI). Specifically, three different parts of the same material were selected, and the contents of the elements were measured in each part, and their mass percentages were calculated.

[0062] The mass ratio of oxygen atoms in carbonyl and carboxyl groups to the total mass of oxygen atoms in the hard carbon material was measured by X-ray photoelectron spectroscopy. The O 1s peak in the X-ray photoelectron spectrum was fitted, and the peaks around 530.8 eV and 533.9 eV corresponded to carbonyl oxygen and carboxyl oxygen, respectively. The ratio of the mass of oxygen atoms in carbonyl and carboxyl groups to the total mass of oxygen atoms in the hard carbon material was obtained by dividing the corresponding peak area by the total O 1s peak area.

[0063] Electrical conductivity measurement The electrical conductivity of the hard carbon materials was measured using a four-probe powder electrical conductivity meter (model: Suzhou Jingge Co., Ltd., ST2742B). The measurement pressure was 5 MPa, the temperature was 25°C, and the humidity was 65%. Each sample was measured three times and the average value was calculated.

[0064] Measurement of total gram capacity and initial efficiency of lithium storage in hard carbon materials The initial reversible capacity per gram of the hard carbon material at 0V to 2.5V was measured using the following method. The hard carbon material, styrene butadiene rubber (a binder), and sodium carboxymethyl cellulose (a thickener) were mixed in a mass ratio of 97:1.5:1.5, and then deionized water was added as a solvent to prepare a slurry with a solids content of 40 wt%. The slurry was then coated on one side of a copper foil negative electrode current collector to obtain a negative electrode piece. The negative electrode piece was then cut into 14 mm diameter wafers to form the working electrode. A lithium sheet was then used as the counter electrode, and a porous polyethylene film (provided by Celgard) was used as the separator. After injecting the electrolyte, a button battery was assembled. The button battery was discharged to 0V at three small currents: 0.05C, 0.01C, and 0.005C. The initial discharge capacity of the button battery was recorded. It was then charged at a constant current of 0.1C to 2.5V, and the initial charge capacity of the button battery was recorded. The mass of the hard carbon material in the negative electrode piece was calculated from the coating weight and area of ​​the slurry in the preparation process of the negative electrode piece. Initial efficiency = initial charge capacity / initial discharge capacity × 100%, and the initial reversible capacity per gram of the hard carbon material at 0V to 2.5V, i.e., the total lithium storage capacity per gram, Q = initial charge capacity / mass of the hard carbon material, in mAh / g. Li / Li at 0V to 0.1V + Within the potential range, the capacity per gram of hard carbon material is Q1, and the Li / Li + The capacity per gram of the hard carbon material within this potential range was defined as Q2.

[0065] The electrolyte solution contained a base solvent and a lithium salt. The base solvent was obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a mass ratio of 1:1. The lithium salt was LiPF6, and the concentration of the lithium salt was 1 mol / L.

[0066] The methods for measuring the capacity and initial efficiency of lithium storage per total gram of the graphite of Comparative Example 1 were the same as the methods for measuring the capacity and initial efficiency of lithium storage per total gram of the hard carbon material.

[0067] Measurement of capacity and initial efficiency per whole gram of sodium storage for hard carbon materials The method for measuring the capacity and initial efficiency of sodium storage per total gram of the hard carbon material was the same as that for measuring the capacity and initial efficiency of lithium storage per total gram of the hard carbon material, except that a sodium sheet was used instead of a lithium sheet as the counter electrode, aluminum foil was used as the negative electrode current collector, and NaPF6, a sodium salt, was used instead of a lithium salt. + The capacity of the hard carbon material per gram of sodium storage in the potential range of 0V to 0.5V is R, and the Na / Na + In the potential range of 0V to 0.8V, the capacity per gram of hard carbon material is R1, and the Na / Na + The capacity per gram of the carbon material within this potential range was defined as R2.

[0068] Measurement of compressed density of negative electrode active material layer A lithium-ion battery discharged to 2V at 1C was taken, disassembled, and the negative electrode piece was removed. It was washed and dried, and the negative electrode piece with an area of ​​S was weighed using an electronic balance. The weight was designated W1, and the thickness of the negative electrode piece, T1, was measured using a micrometer. The negative electrode active material layer was washed off using DMC as a solvent and dried. The weight of the negative electrode current collector was measured. The weight was designated W2, and the thickness of the negative electrode current collector, T2, was measured using a micrometer. The weight W0, thickness T0, and compressed density of the negative electrode active material layer installed on the negative electrode current collector side were calculated using W0 = W1 - W2, T0 = T1 - T2, and compressed density = W0 / (T0 × S), respectively.

[0069] Energy Density (ED) Measurement In a 25°C environment, a lithium-ion battery was charged at a constant current of 0.2 C to a voltage of 4.48 V (or a sodium-ion battery was charged at a constant current of 0.2 C to a voltage of 3.95 V), then charged at a constant voltage of 0.02 C, and then discharged at a constant current of 0.2 C to a voltage of 2 V. This constituted one cycle. The discharge capacity C and discharge energy E during this cycle were recorded. E was the fully discharged energy. The length, width, and height of the battery at 50% charge depth were then measured, and the volume V of the battery was calculated. m The average discharge voltage U = E / C and the energy density ED = E / V m It was.

[0070] Measurement of cycle characteristics Five samples were taken from each of the lithium-ion batteries and sodium-ion batteries prepared in each example and comparative example. The lithium-ion batteries and sodium-ion batteries were repeatedly charged and discharged using the following steps, and the capacity retention rate was calculated by averaging the five battery samples.

[0071] The lithium-ion battery was first charged and discharged at 25°C. It was charged at a constant current of 0.7 C, and after the voltage reached 4.48 V, it was charged at a constant voltage of 0.02 C. It was then discharged at a constant current of 1 C until the voltage reached 2 V, and the discharge capacity of the first cycle was recorded. Next, the battery was subjected to 400 charge and discharge cycles according to the same charge and discharge process, and the discharge capacity of the 400th cycle was recorded. The capacity retention rate (%) was calculated as follows: (discharge capacity at the 400th cycle / discharge capacity at the first cycle) × 100%.

[0072] The sodium-ion battery was first charged and discharged at 25°C. It was charged at a constant current of 0.5 C, and after the voltage reached 3.95 V, it was charged at a constant voltage of 0.02 C. It was then discharged at a constant current of 1 C until the voltage reached 2 V, and the discharge capacity of the first cycle was recorded. Next, 400 charge and discharge cycles were performed according to the same charge and discharge process, and the discharge capacity of the 400th cycle was recorded. The capacity retention rate (%) was calculated as follows: (discharge capacity at the 400th cycle / discharge capacity at the first cycle) × 100%.

[0073] Example 1-1 <Preparation of negative electrode active material> A mass (m) of 20 g of precursor alkaline lignin was taken and placed in a 500 mL reactor with a lidded graphite crucible. The gas in the reactor was replaced with air as the first gas. After sealing the reactor, it was transferred to a box furnace and subjected to primary firing in a nitrogen atmosphere. The primary firing temperature T1 was increased to 700 ° C at a heating rate of 5 ° C / min, and the primary firing time t1 was set to 2 hours, after which the temperature was lowered. The powder of the material after pre-carbonization in the reactor was then transferred to a nitrogen atmosphere protective furnace and subjected to secondary firing. The secondary firing temperature T2 was increased to 1100 ° C at a heating rate of 2 ° C / min, and carbonized for 2 hours. The temperature was lowered, and the carbonized material was obtained after cooling. The powder was crushed to obtain a powder, which was then classified according to particle size. The particle diameter Dv99 of the classified powder particles was controlled to 40 μm and Dv50 to 10 μm. The classified powder was then transferred to a nitrogen-atmosphere furnace and heated to 900°C (T3) at a rate of 5°C / min. The atmosphere was then replaced with a reducing gas mixture of methane and argon gas. The vapor growth time (t3) was set to 2 hours. After vapor growth was completed, the mixture was switched off and replaced with nitrogen gas. The mixture was then cooled to room temperature to obtain a hard carbon material with a Dv50 of 18μm (i.e., anode active material). The mass percentage of the reducing gas (methane) was 20% based on the mass of the mixture.

[0074] <Preparation of negative electrode pieces> The above-prepared hard carbon material (anode active material), styrene butadiene rubber (binder), and sodium carboxymethyl cellulose (thickener) were mixed in a mass ratio of 97:1.5:1.5, followed by the addition of deionized water as a solvent to prepare a 40 wt% solids anode slurry. The slurry was then uniformly coated onto one surface of a 6 μm-thick copper foil anode current collector. The copper foil was then dried at 85°C for 4 hours to obtain a 50 μm-thick anode piece coated on one side with a 50 μm-thick anode active material layer. After cold pressing, cutting, and slitting, the anode piece was dried under vacuum at 120°C for 12 hours to obtain a 76.6 mm x 875 mm anode piece.

[0075] <Preparation of positive electrode piece> Lithium cobalt oxide (positive electrode active material), conductive carbon black (SuperP), and PVDF (binder) were mixed in a mass ratio of 97:1.4:1.6, and N-methylpyrrolidine ketone (NMP) was added as a solvent. The mixture was stirred uniformly to prepare a positive electrode slurry with a solids content of 72 wt%. The positive electrode slurry was uniformly coated onto one surface of a 13 μm-thick aluminum foil positive electrode current collector and dried at 85°C to obtain a positive electrode piece coated with positive electrode active material on one side with a positive electrode active material layer thickness of 80 μm. After cold pressing, cutting, and slitting, the piece was dried in a vacuum at 85°C for 4 hours to obtain a positive electrode piece with dimensions of 74 mm x 867 mm.

[0076] <Preparation of electrolyte> In a glove box with a dry argon atmosphere, the base solvents ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:DEC = 1:1:1, then fluoroethylene carbonate was added and dissolved, stirred thoroughly, and the lithium salt LiPF6 was added and mixed uniformly to obtain an electrolyte. Based on the mass of the electrolyte, the mass percentage of the lithium salt was 12.5%, the mass percentage of the fluoroethylene carbonate was 3.5%, and the remainder was the base solvent.

[0077] <Preparation of separator> A 7 μm thick polyethylene film (provided by Celgard) was used as the separator.

[0078] <Preparation of lithium-ion batteries> The positive electrode, separator, and negative electrode pieces prepared above were stacked in order with the separator interposed between the positive and negative electrode pieces to serve as insulators, and then wound to obtain an electrode assembly. After tab welding, the electrode assembly was placed in an aluminum plastic film packaging bag and placed in a vacuum oven at 80°C to dry for 12 hours to remove moisture. The above-prepared electrolyte solution was then injected, and the assembly was vacuum packaged, left to stand, formed, degassed, trimmed, and other processes to obtain a lithium-ion battery.

[0079] Examples 1-2 to 1-14 The same as Example 1-1 was carried out except that the preparation parameters were adjusted according to Table 1.

[0080] [Table 1]

[0081] Example 2-1 <Preparation of negative electrode active material> The procedure was the same as in Example 1-5, except that the secondary firing temperature T2 was adjusted to 1300°C.

[0082] <Preparation of negative electrode pieces> The negative electrode active material prepared above, styrene butadiene rubber as a binder, and sodium carboxymethyl cellulose as a thickener were mixed in a mass ratio of 97:2:1, and aluminum foil was used as the negative electrode current collector instead of copper foil, except that the same procedure was followed as in Example 1-5.

[0083] <Preparation of positive electrode piece> Instead of lithium cobalt oxide, which is the positive electrode active material, copper nickel iron manganese oxide (NaCu 1 / 9 NI 2 / 9 FE 1 / 3 Mn 1 / 3The procedure was the same as in Example 1-5, except that O2) was used.

[0084] <Preparation of electrolyte> In a glove box with a dry argon atmosphere, the base solvents ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a mass ratio of 50:50, then fluoroethylene carbonate was added and dissolved, and after thorough stirring, the sodium salt NaPF6 was added and mixed uniformly to obtain an electrolyte. Based on the mass of the electrolyte, the mass percentage of the sodium salt was 12.5%, the mass percentage of the fluoroethylene carbonate was 3.5%, and the remainder was the base solvent.

[0085] <Preparation of separator> PVDF and aluminum oxide ceramic were mixed in a mass ratio of 9:1, and deionized water was added as a solvent to prepare a slurry with a solid content of 12 wt%. The mixture was stirred uniformly and the slurry was uniformly coated onto one surface of a 9 μm-thick polyethylene (provided by Celgard) porous polymer film. After drying, a separator with a 2 μm-thick aluminum oxide ceramic layer coated on one side was obtained.

[0086] <Preparation of sodium ion battery> The cathode, separator, and anode pieces prepared above were stacked in order, with the aluminum oxide ceramic layer-coated side of the separator facing the cathode piece, and the separator interposed between the cathode and anode pieces to serve as an insulator, and then wound to obtain an electrode assembly. After the tabs were welded, the electrode assembly was placed in an aluminum plastic film packaging bag and placed in a vacuum oven at 80°C for 12 hours to remove moisture. The prepared electrolyte was then injected, and the assembly was vacuum packaged, left to stand, formed, degassed, trimmed, and other processes to obtain a sodium-ion battery.

[0087] Example 2-2 <Preparation of negative electrode active material> The procedure was the same as in Example 1-7, except that the secondary firing temperature T2 was adjusted to 1300°C. <Preparation of negative electrode pieces>, <Preparation of positive electrode pieces>, <Preparation of electrolyte>, <Preparation of separator> and <Preparation of sodium ion battery> were the same as in Example 2-1.

[0088] Example 2-3 <Preparation of negative electrode active material> The same procedure as in Example 1-8 was carried out except that the temperature T2 of the secondary firing was adjusted to 1300°C. <Preparation of negative electrode pieces>, <Preparation of positive electrode pieces>, <Preparation of electrolyte>, <Preparation of separator> and <Preparation of sodium ion battery> were the same as in Example 2-1.

[0089] Examples 2-4 The procedure was the same as in Example 1-10, except that the secondary firing temperature T2 was adjusted to 1300°C. <Preparation of negative electrode pieces>, <Preparation of positive electrode pieces>, <Preparation of electrolyte>, <Preparation of separator> and <Preparation of sodium ion battery> were the same as in Example 2-1.

[0090] Comparative Example 1 The procedure was the same as in Example 1-1, except that artificial graphite was used instead of the negative electrode active material prepared in <Preparation of negative electrode active material>.

[0091] Comparative Example 2 The procedure was the same as in Example 1-1, except that the negative electrode active material prepared in the following <Preparation of negative electrode active material> was used.

[0092] <Preparation of negative electrode active material> 20 g of the precursor alkaline lignin was taken and placed in a box furnace, and subjected to secondary firing in a nitrogen atmosphere. The secondary firing temperature T2 was raised to 1100 ° C. at a heating rate of 2 ° C. / min, and carbonized for 2 hours. The temperature was lowered, and after cooling, the carbonized material was obtained. The powder was crushed to obtain a powder, and the powder was classified according to particle size. The particle diameter Dv99 of the powder particles was controlled to 40 μm and Dv50 to 10 μm. The classified powder was then transferred to a nitrogen atmosphere protective furnace, and the vapor growth temperature T3 was raised to 900 ° C. at a heating rate of 5 ° C. / min. The atmosphere was then replaced with a mixture of reducing gases, methane and argon, and the vapor growth time t3 was set to 2 hours. After the vapor growth was completed, the mixed gas was turned off, replaced with nitrogen gas, and cooled to room temperature. A hard carbon material with a Dv50 of 18 μm, i.e., a negative electrode active material, was obtained. Based on the mass of the mixed gas, the mass percentage of methane, which is a reducing gas, was 20%.

[0093] Comparative Example 3 The procedure was the same as in Example 1-1, except that the negative electrode active material prepared in the following <Preparation of negative electrode active material> was used.

[0094] <Preparation of negative electrode active material> 20 g of the precursor alkaline lignin was placed in a 500 mL graphite reactor with a lid, and the gas in the reactor was replaced with air. After sealing the reactor, it was transferred to a box furnace and subjected to primary firing in a nitrogen atmosphere. The primary firing temperature T1 was increased to 700°C at a heating rate of 5°C / min, and the primary firing time t1 was set to 2 hours, after which the temperature was lowered. The powder after primary firing was crushed and classified to control the particle diameter Dv99 of the powder particles to 40 μm and Dv50 to 10 μm. The classified powder was then transferred to a nitrogen-atmosphere furnace and heated to 900°C (T3) at a rate of 5°C / min. The atmosphere was then replaced with a reducing gas mixture of methane and argon gas. The vapor growth time (t3) was set to 2 hours. After vapor growth was completed, the gas mixture was switched off and replaced with nitrogen gas. The mixture was then cooled to room temperature to obtain a hard carbon material with a Dv50 of 18μm (i.e., anode active material). The mass percentage of the reducing gas, methane, was 20% based on the mass of the gas mixture.

[0095] Comparative Example 4 <Preparation of negative electrode active material> The procedure was the same as in Comparative Example 2, except that the secondary firing temperature T2 was adjusted to 1300°C. <Preparation of negative electrode pieces>, <Preparation of positive electrode pieces>, <Preparation of electrolyte>, <Preparation of separator> and <Preparation of sodium ion battery> were the same as in Example 2-1. The relevant parameters and measurements of the properties of each example and each comparative example are shown in Tables 2 to 4. In Tables 2 to 4 below, the ratio of the pore volume of micropores to the total pore volume is abbreviated as the ratio of micropores, the ratio of the pore volume of ultrafine pores to the total pore volume is abbreviated as the ratio of ultrafine pores, and the ratio of the mass of oxygen elements in carbonyl groups and carboxy groups to the total mass of oxygen elements in the hard carbon material is abbreviated as X.

[0096] [Table 2]

[0097] [Table 3]

[0098] [Table 4]

[0099] As can be seen from Examples 1-1 to 1-14 and Comparative Examples 1 to 3, when the negative electrode active material contains a hard carbon material, and the micropore ratio, ultramicropore ratio, and pore volume of the ultramicropores of the hard carbon material are within the ranges of the present invention, the values ​​of Q1 and Q2 become larger, and Q1 / Q and Q2 / Q can reach levels comparable to those of graphite materials. The hard carbon material has a relatively low average lithium release potential, a higher lithium storage capacity per gram Q between 0 V and 2.5 V, and a higher initial efficiency. This indicates that the negative electrode active material provided by the present invention has a stable low-potential platform, higher capacity per gram, and higher reversible capacity. Lithium-ion batteries containing the negative electrode active material of the present invention have higher energy density and capacity retention. This indicates that the lithium-ion batteries provided by the present invention have higher energy density and better cycle characteristics.

[0100] Mass percentage of oxygen element in hard carbon material, mass percentage of element A (N + S), value of X, I D / I G The value of and the electrical conductivity of the hard carbon material typically affect the properties of the negative electrode active material, such as the average lithium release potential, capacity per gram, and initial efficiency, thereby affecting the cycle characteristics and energy density of the lithium-ion battery. As can be seen from Examples 1-1 to 1-14, when the above parameters are within the ranges of the present invention, the negative electrode active material has a stable low-potential platform, a relatively high capacity per gram, and a reversible capacity, and the resulting lithium-ion battery has a relatively high energy density and capacity retention. This indicates that the lithium-ion battery provided by the present invention has high energy density and good cycle characteristics.

[0101] The compressed density of the negative electrode active material layer generally affects the cycle characteristics and energy density of a lithium ion battery. As can be seen from Examples 1-1 to 1-14, when the compressed density of the negative electrode active material layer is within the range of the present invention, the resulting lithium ion battery has a relatively high energy density and capacity retention rate. This indicates that the lithium ion battery provided by the present invention has a high energy density and good cycle characteristics.

[0102] As can be seen from Examples 2-1 to 2-4 and Comparative Example 4, when the negative electrode active material contains a hard carbon material and the micropore ratio, ultrafinepore ratio, and pore volume of the ultrafinepores of the hard carbon material are within the ranges of the present invention, the values ​​of R1 and R2 become larger, the values ​​of R1 / R and R2 / R become relatively large, the hard carbon material has a relatively low average potential for sodium release, and the capacity R per total gram of sodium storage at 0 V to 2.5 V becomes higher, and the initial efficiency becomes higher. This indicates that the negative electrode active material provided by the present invention has a stable low-potential platform, higher capacity per gram, and reversible capacity. Sodium-ion batteries containing the negative electrode active material of the present invention have higher energy density and capacity retention. This indicates that the sodium-ion batteries provided by the present invention have higher energy density and better cycle characteristics.

[0103] Table 3 shows the positions of the 002 peaks in the XRD diffraction patterns of the hard carbon materials of Examples 1-5, 1-11, and 1-12. The positions of the 002 peaks of the hard carbon materials of the above examples were between 21.2° and 21.8°. This indicates that the lattice spacing of the hard carbon materials is large, which is advantageous for lithium ion absorption between the hard carbon material layers. Table 3 also shows the oxygen content and electrical conductivity of the hard carbon materials of the above examples. As can be seen, the oxygen content is significantly affected by the type of first gas. Specifically, when the first gas is CO2, air, and O2 in this order, the oxygen content increases sequentially and the electrical conductivity of the hard carbon materials decreases sequentially.

[0104] Table 4 shows the positions of the 002 peak in the XRD diffraction patterns of the hard carbon materials of Examples 2-1 to 2-4. The positions of the 002 peak of the hard carbon materials of the above examples were between 23.3° and 24.9°. This indicates that the lattice spacing of the hard carbon materials is large, which is advantageous for the absorption of sodium ions between the layers of the hard carbon material.

[0105] Figure 1 shows a scanning electron microscope image of the hard carbon material prepared in Examples 1-5, in which the hard carbon material is an irregular block with no obvious pore structure on the particle surface. This indicates that no macropores appear on the surface of the hard carbon material. Figure 2 shows the pore size distribution of the hard carbon material prepared in Examples 1-5 measured by nitrogen gas adsorption / desorption. This shows that the hard carbon material mainly consists of micropores with a pore diameter of less than 2 nm, with very fine pores with a pore diameter of less than 7 nm clearly observed, indicating that no mesopores or macropores appear in the hard carbon material.

[0106] As can be seen from Figure 3, the Li / Li +Within this potential range, the specific capacities of the hard carbon materials of Examples 1-5 were 285 mAh / g, 401 mAh / g, and 465 mAh / g, respectively. This indicates that the hard carbon materials of Examples 1-5 have relatively high specific capacities and relatively low lithium release potentials. As can be seen from Figure 4, the Li / Li + Within this potential range, the specific capacities of the hard carbon material of Comparative Example 2 were 144 mAh / g, 276 mAh / g, and 300 mAh / g, respectively. This indicates that the hard carbon material of Comparative Example 2 has a relatively low specific capacity and a relatively high lithium desorption potential. It can be seen from FIGS. 3 and 4 that the hard carbon materials of Examples 1-5 have higher specific capacities and lower lithium desorption potentials than Comparative Example 2.

[0107] As can be seen from Figure 5, there are clear platforms in the charge-discharge curves of the hard carbon materials, and the Na / Na + Within this potential range, the specific capacities of the hard carbon material of Example 2-1 were 274 mAh / g, 293 mAh / g, and 315 mAh / g, respectively, indicating that the hard carbon material of Example 2-1 had a relatively high specific capacity and a relatively low sodium release potential.

[0108] Figure 6 shows the Raman spectra of the hard carbon materials prepared in Examples 1-5. -1 ~1370cm -1 The peak area of ​​the D peak located between 1570 cm and 58213 cm is -1 ~1620cm -1 The peak area of ​​the G peak located between D / I G = 1.12, which indicates that having an appropriate degree of defects in hard carbon materials is advantageous for improving the capacity per gram of the hard carbon materials.

[0109] 7, the position of the 002 peak of the hard carbon material of Example 1-5 was at 21.8°, which indicates that the lattice spacing of the hard carbon material is large, which is advantageous for the absorption of lithium ions between the layers of the hard carbon material.

[0110] It should be noted that, in this specification, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily imply any actual relationship or order between those entities or operations. Furthermore, terms such as "have," "comprise," "include," "contain," or other variations thereof are intended to be non-exclusive inclusive, and a process, method, or article that includes a set of elements may include not only those elements but also other elements not expressly listed or may include inherent elements in such process, method, article, or device.

[0111] Since the embodiments in this specification are described in relation to each other, identical or similar parts between the embodiments can be referred to. In each embodiment, differences from other embodiments will be mainly described.

[0112] The above are only preferred embodiments of the present application, and do not limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A negative electrode active material, the negative electrode active material includes a hard carbon material, the hard carbon material contains micropores and ultramicropores; The pore diameter of the micropores is less than 2 nm, The pore diameter of the ultrafine pores is less than 0.7 nm, the ratio of the pore volume of the micropores to the total pore volume is 95% to 100%; The pore volume of the ultrafine pores is 0.01 cm ３ / g to 0.2 cm ３ / g, The ratio of the pore volume of the ultrafine pores to the total pore volume is 80% to 99%. Negative electrode active material.

2. the hard carbon material contains carbon and oxygen elements, 2. The negative electrode active material according to claim 1, wherein the mass percentage of oxygen element in the hard carbon material is 2% to 7%.

3. the oxygen element in the hard carbon material exists in the form of a carbonyl group and a carboxyl group; 3. The negative electrode active material according to claim 2, wherein the mass of oxygen elements in the carbonyl groups and carboxy groups accounts for 60% to 99% of the total mass of oxygen elements in the hard carbon material.

4. The hard carbon material further contains an element A, The element A includes at least one of N and S, 4. The negative electrode active material according to claim 2, wherein the mass percentage of the element A is 0.05% to 2% based on the mass of the hard carbon material.

5. The hard carbon material is I Ｄ is the peak area of ​​the D peak in the Raman spectrum of the hard carbon material, and I Ｇ is the peak area of ​​the G peak in the Raman spectrum of the hard carbon material, 0.8≦I Ｄ / I Ｇ 5. The negative electrode active material according to claim 1, wherein the negative electrode active material satisfies the following condition: ≦1.

5.

6. 0V~2.5V Li / Li ＋ In the potential range of 300 mAh / g to 700 mAh / g, the lithium storage capacity per total gram of the hard carbon material is 300 mAh / g to 700 mAh / g, and the lithium release energy of the hard carbon material is E １ Wh is the lithium desorption capacity of the hard carbon material, and C １ Ah, and the average potential of lithium desorption from the hard carbon material is E １ / C １ When V is set, 0.13≦E １ / C １ ≦0.28, 0V~0.1V Li / Li ＋ In the potential range of 0 V to 0.8 V, the capacity per gram of the hard carbon material accounts for 30% to 65% of the total capacity per gram of the lithium storage. ＋ 6. The negative electrode active material according to claim 1, wherein the capacity per gram of the hard carbon material accounts for 70% to 96% of the total capacity per gram of the lithium storage in a potential range of 0.1 to 0.

5.

7. 0V to 2.5V Na / Na ＋ In the potential range of 250 mAh / g to 400 mAh / g, the sodium storage capacity per gram of the hard carbon material is 250 mAh / g to 400 mAh / g, and the sodium release energy of the hard carbon material is E ２ Wh is the sodium release capacity of the hard carbon material, and C ２ Ah, and the average potential of sodium release of the hard carbon material is E ２ / C ２ When V is set, 0.2≦E ２ / C ２ ≦0.4, 0V to 0.5V Na / Na ＋ In the potential range of 0 V to 0.8 V, the capacity per gram of the hard carbon material accounts for 76% to 91% of the total capacity per gram of the sodium storage. ＋ 7. The negative electrode active material according to claim 1, wherein the capacity per gram of the hard carbon material accounts for 89% to 95% of the total capacity per gram of the sodium storage within a potential range of 0.1 to 0.5 V.

8. 8. The negative electrode active material according to claim 1, wherein the hard carbon material has an electrical conductivity of 0.5 S / cm to 10 S / cm.

9. A method for preparing the negative electrode active material according to any one of claims 1 to 8, comprising: The method for preparing hard carbon materials includes: (1) A precursor is crushed and sieved, and then placed in a sealed reactor. The gas in the sealed reactor is replaced with a first gas, wherein the precursor includes at least one of lignin, cellulose, alkaline lignin, asphalt, epoxy resin, and phenolic resin, and the first gas includes any one of oxygen gas, air, and carbon dioxide; (2) After sealing the closed reactor, place it in a nitrogen atmosphere, perform primary firing, raise the temperature to 700°C to 900°C at a rate of 0.5°C / min to 5°C / min, pre-carbonize for 1 hour to 4 hours, and cool to obtain a pre-carbonized material; (3) placing the pre-carbonized material in a nitrogen atmosphere, performing secondary baking, raising the temperature to 1000°C to 1500°C at a rate of 0.5°C / min to 5°C / min, carbonizing for 1 hour to 8 hours, cooling, obtaining a carbonized material, and then classifying the carbonized material according to particle size; (4) A method for preparing a negative electrode active material, comprising: heating the carbonized material classified in step (3) to 700°C to 1200°C; passing a mixed gas of a reducing gas and argon gas through the carbonized material; maintaining the mixed gas for 0.1 to 12 hours; replacing the mixed gas with nitrogen gas; and then cooling the carbonized material to obtain the hard carbon material, wherein the reducing gas includes at least one of acetylene and methane, and the mass percentage of the reducing gas is 5 wt% to 20 wt% based on the mass of the mixed gas.

10. A secondary battery, The secondary battery includes a positive electrode piece, a negative electrode piece, and an electrolyte solution; The negative electrode piece includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, The negative electrode active material layer contains the negative electrode active material according to any one of claims 1 to 8 or the negative electrode active material prepared by the method for preparing the negative electrode active material according to claim 9. Secondary battery.

11. The compressed density of the negative electrode active material layer is 0.8 g / cm ３ ~1.2 g / cm ３ The secondary battery according to claim 10 ,

12. An electronic device comprising the secondary battery according to claim 10 or 11.

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