A hard carbon material, a negative electrode sheet, a secondary battery, and an electrical device.

Abstract

This application provides a hard carbon material, a negative electrode sheet, a secondary battery, and an electrical device. In the X-ray diffraction pattern of the hard carbon material, a first characteristic peak exists between 15° and 35°, and a second characteristic peak exists between 25° and 28°. The starting position of the first characteristic peak is A°, and the ending position is B°, where B - A ≥ 8°. The peak intensity of the first characteristic peak is I1, and the peak intensity of the second characteristic peak is I2, where 0.1 ≤ I2 / I1 ≤ 3.0. The hard carbon material is used as the negative electrode active material to prepare the negative electrode sheet. Charge-discharge tests are performed on the negative electrode sheet using metallic lithium as the counter electrode to obtain a differential capacity curve. The negative electrode sheet exhibits four reversible lithium insertion peaks and three to four reversible lithium extraction peaks within the voltage range of 0V to 0.4V. With the above configuration, the resulting secondary battery possesses high energy density and good cycle performance.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a hard carbon material, a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0002] Rechargeable batteries, represented by lithium-ion batteries, possess outstanding characteristics such as high energy density, long cycle life, low pollution, and no memory effect. As a clean energy source, the application of rechargeable batteries has gradually expanded from electronic products to large-scale devices such as electric vehicles, in order to adapt to the sustainable development strategy of environment and energy. This has also placed higher demands on the energy density of rechargeable batteries.

[0003] Currently, graphite remains the primary anode active material for commercially available rechargeable batteries. Graphite possesses advantages such as high conductivity and high stability. However, the theoretical capacity of graphite is approximately 372 mAh / g, and in recent years, its theoretical capacity has been almost fully realized, making it difficult to further increase the energy density of lithium-ion batteries using graphite as the anode active material. Summary of the Invention

[0004] The purpose of this application is to provide a hard carbon material, a negative electrode sheet, a secondary battery, and an electrical device to improve the energy density and cycle performance of the secondary battery. The specific technical solution is as follows:

[0005] It should be noted that the invention description in this application uses lithium-ion batteries and sodium-ion batteries as examples of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries and sodium-ion batteries.

[0006] The first aspect of this application provides a hard carbon material. In the X-ray diffraction pattern of the hard carbon material, there is a first characteristic peak between 15° and 35°, and a second characteristic peak between 25° and 28°. The starting position of the first characteristic peak is A°, and the ending position of the first characteristic peak is B°, where BA ≥ 8°. The peak intensity of the first characteristic peak is I1, and the peak intensity of the second characteristic peak is I2, where 0.1 ≤ I2 / I1 ≤ 3.0. A negative electrode sheet is prepared by using the hard carbon material as the negative electrode active material. The negative electrode sheet is subjected to charge-discharge tests using metallic lithium as the counter electrode to obtain a differential capacity curve. The negative electrode sheet exhibits four reversible lithium insertion peaks and three to four reversible lithium extraction peaks within the voltage range of 0V to 0.4V. The hard carbon material of this application exhibits a first characteristic peak and a second characteristic peak in its X-ray diffraction pattern. Furthermore, its dQ / dV curve shows four reversible lithium insertion peaks and three to four reversible lithium extraction peaks within the voltage range of 0V to 0.4V. By further adjusting the values ​​of BA and I2 / I1 within this range, it is demonstrated that the hard carbon material of this application possesses a locally ordered graphitized structure. By introducing a locally ordered graphitized structure into the hard carbon material, compared to completely disordered hard carbon materials, it achieves higher reversible capacity while improving its true density. Applying hard carbon materials to secondary batteries can increase the energy density of the batteries. Simultaneously, the locally ordered graphitized structure can shorten the lithium-ion transport path within the hard carbon material, which is beneficial for improving the kinetic performance of the secondary battery, thereby enhancing its cycle performance.

[0007] In one embodiment of this application, the hard carbon material has an internal porous structure, wherein the maximum inscribed circle diameter of the pores in the internal porous structure is 0.40 nm to 10 nm, preferably 0.40 nm to 2 nm, and more preferably 0.40 nm to 0.80 nm. When the hard carbon material has an internal porous structure, it exhibits a high reversible capacity. By controlling the maximum inscribed circle diameter of the pores in the internal porous structure of the hard carbon material within the above-mentioned range, it is beneficial to improve the reversible capacity of the hard carbon material, especially its low-voltage plateau capacity. Applying hard carbon materials to secondary batteries is beneficial to improving the energy density and initial coulombic efficiency of the secondary battery, and thus improving its cycle performance.

[0008] In one embodiment of this application, the surface of the hard carbon material has an external pore structure, and the pore volume of the external pore structure obtained by adsorption testing is ≤0.05cc / g. The fact that the pore volume of the external pore structure of the hard carbon material obtained by adsorption testing is within the above range indicates that the external pore volume of the hard carbon material in this application is small, which is beneficial to improving the lithium storage performance of the hard carbon material. Applying the hard carbon material to secondary batteries is beneficial to improving the energy density and initial coulombic efficiency of the secondary battery, and improving the cycle performance of the secondary battery.

[0009] In one embodiment of this application, the true density of the hard carbon material is between 0.9 g / cc and 2.0 g / cc. By controlling the true density of the hard carbon material within the above range, the hard carbon material has a large number of internal pore structures, which is beneficial for storing lithium ions or sodium ions. Applying hard carbon materials to secondary batteries can improve the energy density and first coulombic efficiency of the secondary battery, and improve the cycle performance of the secondary battery.

[0010] In one embodiment of this application, the specific surface area of ​​the hard carbon material is 0.5 m². 2 / g to 50m 2 / g, preferably 0.5m 2 / g to 5m 2 / g. By controlling the specific surface area of ​​hard carbon materials within the above range, when hard carbon materials are applied to secondary batteries, it is beneficial to form an SEI film of appropriate area during the first charge process, reduce the content of negative electrode binder in the negative electrode active material layer, thereby reducing the loss of active ions, lowering the internal resistance of the negative electrode active material layer, thereby improving the energy density and first coulombic efficiency of the secondary battery, and improving the cycle performance and safety performance of the secondary battery.

[0011] In one embodiment of this application, the Dv50 of the hard carbon material is 3 μm to 12 μm, preferably 5 μm to 9 μm. By controlling the Dv50 of the hard carbon material within the above range, the hard carbon material particles have good electrolyte wetting properties while having a small specific surface area, thereby reducing the active ions consumed during the formation of an SEI film on the surface of the hard carbon material particles during the first charge. Applying hard carbon materials to secondary batteries is beneficial for improving the energy density and first coulombic efficiency of secondary batteries, and improving the cycle performance of secondary batteries.

[0012] In one embodiment of this application, the hard carbon material includes heterogeneous elements, including at least one selected from Li, Na, K, Cs, Mg, Al, Ca, Rb, Zn, Fe, Ni, Co, N, O, H, P, S, B, or Se. Based on the mass of the hard carbon material, the mass percentage of the heterogeneous elements is greater than 0 and less than or equal to 3%. By controlling the mass percentage of the heterogeneous elements within the above range, it is beneficial to locally graphitize the hard carbon material while improving its stability during electrochemical reactions, resulting in higher specific capacity and initial coulombic efficiency. Applying hard carbon materials to secondary batteries is beneficial to improving the energy density and initial coulombic efficiency of the secondary batteries, and improving their cycle performance.

[0013] A second aspect of this application provides a method for preparing a hard carbon material, comprising the following steps:

[0014] (1) The reactants, catalyst and solvent are mixed evenly to obtain a reaction solution, and the reaction solution is dried to obtain a precursor; wherein, the mass ratio of catalyst to reactants is 0.01 to 1, the reactants include at least one of methyl phenolic resin, furfural resin, epoxy resin, asphalt, starch, glucose, sucrose or fructose, the solvent includes at least one of deionized water, ethanol, benzene, toluene or N-methylpyrrolidone, and the catalyst includes at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, magnesium hydroxide, cesium hydroxide, rubidium hydroxide, calcium hydroxide, aluminum hydroxide, aluminum oxide, aluminum trioxide, zinc oxide, zinc chloride, iron, iron tetroxide, ferric chloride, iron oxide, nickel oxide, nickel sulfate, nickel hydroxide, nickel chloride, cobalt nitrate, cobalt oxide, urea, phosphoric acid, lignin, boric acid, sodium selenate, sodium benzoate, potassium benzoate or sodium cinnamate.

[0015] (2) The precursor is subjected to a first carbonization treatment in an inert atmosphere and then pickled, followed by a second carbonization treatment in an inert atmosphere to obtain the carbonized product; wherein, the temperature T1 of the first carbonization treatment is 500℃ to 900℃, the time t1 of the first carbonization treatment is 0.5h to 5h, and the heating rate v1 of the first carbonization treatment is 0.5℃ / min to 10℃ / min; the temperature T2 of the second carbonization treatment is 1000℃ to 1600℃, the time t2 of the second carbonization treatment is 0.5h to 15h, and the heating rate v2 of the second carbonization treatment is 0.5℃ / min to 10℃ / min.

[0016] (3) The carbonized product is heated to 500°C to 1100°C in an inert atmosphere at a heating rate of 0.5°C / min to 20°C / min, and then kept in a reducing atmosphere to obtain hard carbon material; wherein the inert atmosphere includes at least one of nitrogen, argon or helium; the holding time t3 is 0.1h to 8h, and the reducing atmosphere includes at least one of argon and acetylene or methane.

[0017] Hard carbon materials are prepared using the method provided in the second aspect of this application, and the parameters are adjusted within the above range. The resulting hard carbon materials have high specific capacity and first coulombic efficiency. When used as negative electrode sheets in secondary batteries, they can improve the energy density of secondary batteries and enhance their cycle performance.

[0018] A third aspect of this application provides a negative electrode sheet, comprising a negative current collector and a negative active material layer located on at least one surface of the negative current collector. The negative active material layer comprises the hard carbon material provided in the first aspect of this application or a hard carbon material prepared according to the preparation method provided in the second aspect of this application. The negative electrode sheet provided in the third aspect of this application has high reversible capacity and good kinetic performance. When applied to a secondary battery, it can improve the energy density and initial coulombic efficiency of the secondary battery, and improve the cycle performance of the secondary battery.

[0019] In one embodiment of this application, the compaction density of the negative electrode active material layer is 0.7 g / cm³. 3 Up to 1.7 g / cm 3 The low compaction density of the negative electrode active material layer indicates the presence of numerous internal pores, which facilitates electrolyte wetting of the negative electrode active material particles and thus improves the cycle kinetics performance of the secondary battery. Controlling the compaction density of the negative electrode active material layer within the aforementioned range is beneficial for increasing the volumetric energy density of the secondary battery. Therefore, the secondary battery exhibits high energy density and good cycle performance.

[0020] In one embodiment of this application, the porosity of the negative electrode active material layer is 10% to 50%. By controlling the porosity of the negative electrode active material layer within the above range, it is beneficial for the electrolyte to wet the negative electrode active material particles, thereby improving the cycle dynamics performance of the secondary battery. This ensures that the contact points between the negative electrode active material particles are within a suitable range, balancing the internal resistance of the secondary battery while increasing the volumetric energy density of the secondary battery.

[0021] A fourth aspect of this application provides a secondary battery comprising the negative electrode sheet of any of the foregoing embodiments. The secondary battery provided by the fourth aspect of this application has high energy density, initial coulombic efficiency, and good cycle performance.

[0022] A fifth aspect of this application provides an electrical device that includes a secondary battery as described in any of the foregoing embodiments. The electrical device of this application has a long service life.

[0023] The beneficial effects of this application are:

[0024] This application provides a hard carbon material. In the X-ray diffraction pattern of the hard carbon material, there is a first characteristic peak between 15° and 35°, and a second characteristic peak between 25° and 28°. The starting position of the first characteristic peak is A°, and the ending position of the first characteristic peak is B°, where BA ≥ 8°. The peak intensity of the first characteristic peak is I1, and the peak intensity of the second characteristic peak is I2, where 0.1 ≤ I2 / I1 ≤ 3.0. A negative electrode sheet is prepared by using the hard carbon material as the negative electrode active material. The negative electrode sheet is subjected to charge-discharge tests using metallic lithium as the counter electrode to obtain a differential capacity curve. The negative electrode sheet exhibits four reversible lithium insertion peaks and three to four reversible lithium extraction peaks within the voltage range of 0V to 0.4V. The hard carbon material of this application exhibits a first characteristic peak and a second characteristic peak in its X-ray diffraction pattern. Furthermore, its dQ / dV curve shows four reversible lithium insertion peaks and three to four reversible lithium extraction peaks within the voltage range of 0V to 0.4V. By further adjusting the values ​​of BA and I2 / I1 within this range, it is demonstrated that the hard carbon material of this application possesses a locally ordered graphitized structure. By introducing a locally ordered graphitized structure into the hard carbon material, compared to completely disordered hard carbon materials, it achieves higher reversible capacity while increasing its true density, thereby improving the energy density of the secondary battery. Simultaneously, the locally ordered graphitized structure shortens the diffusion path of lithium ions within the hard carbon material, thus improving the kinetic performance of the secondary battery. Therefore, the secondary battery exhibits high energy density and good cycle performance.

[0025] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0027] Figure 1 This is the X-ray diffraction pattern of Example 4 of this application;

[0028] Figure 2 for Figure 1 X-ray diffraction peak pattern after substrate removal;

[0029] Figure 3 This is the differential capacity curve of Embodiment 4 of this application. Detailed Implementation

[0030] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0031] It should be noted that the invention description in this application uses lithium-ion batteries and sodium-ion batteries as examples of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0032] Among the many undeveloped anode active materials, hard carbon materials have attracted considerable attention due to their high specific capacity, excellent rate performance, low-temperature performance, and cycle performance. Furthermore, hard carbon materials can be used not only as anode active materials in lithium-ion batteries but also in sodium-ion batteries, showing great application potential. However, existing hard carbon materials suffer from problems such as high irreversible capacity, high average delithiation potential, and low density. Their application in secondary batteries offers limited improvement in energy density, failing to meet practical application needs. Therefore, this application provides a hard carbon material, a secondary battery, and a power supply device. The hard carbon material exhibits high initial coulombic efficiency and high reversible capacity. Applying hard carbon materials to secondary batteries can improve their energy density and cycle performance.

[0033] The first aspect of this application provides a hard carbon material. In the X-ray diffraction pattern of the hard carbon material, a first characteristic peak exists between 15° and 35°, and a second characteristic peak exists between 25° and 28°. The starting position of the first characteristic peak is A°, and the ending position of the first characteristic peak is B°, where BA ≥ 8°. For example, BA can be 8°, 9°, 10°, 11°, 12°, 13°, 14°, 15°, 16°, 17°, 18°, 19°, 20°, or any range of two values ​​therein. The peak intensity of the first characteristic peak is I1, and the peak intensity of the second characteristic peak is I2, where 0.1 ≤ I2 / I1 ≤ 3.0. For example, the value of I2 / I1 can be 0.1, 0.3, 0.5, 0.7, 1.0, 1.3, 1.5, 1.7, 2.0, 2.2, 2.5, 2.7, 3.0, or any range of two values ​​therein. A negative electrode sheet was prepared by using hard carbon material as the negative electrode active material. The negative electrode sheet was charged and discharged with lithium metal as the counter electrode to obtain the differential capacity (dQ / dV) curve. The negative electrode sheet has 4 reversible lithium insertion peaks and 3 to 4 reversible lithium extraction peaks in the voltage range of 0V to 0.4V.

[0034] The inventors discovered that the precursor of hard carbon materials has a high content of heteroatoms such as H, O, and N during the preparation process. During low-temperature pyrolysis, these heteroatoms form numerous cross-linked structures, hindering the formation of crystalline regions. This makes it difficult for hard carbon materials to graphitize even at temperatures above 2500℃. After high-temperature carbonization, the hard carbon material exhibits a short-range ordered arrangement of curved graphite microcrystals, forming 2 to 6 layers without stacking, resulting in a highly distorted structure that significantly reduces the true density of the hard carbon material. In the X-ray diffraction pattern of the completely disordered hard carbon material, only the first characteristic peak exists, with no second characteristic peak. Using the completely disordered hard carbon material as the negative electrode active material, a negative electrode sheet was prepared. Charge-discharge tests were conducted on the negative electrode sheet using metallic lithium as the counter electrode to obtain the dQ / dV curve. The negative electrode sheet exhibited one reversible lithium insertion peak and one reversible lithium extraction peak within the voltage range of 0V to 0.4V. In the X-ray diffraction pattern of graphite, only the second characteristic peak exists, and the first characteristic peak is absent. A negative electrode sheet was prepared using graphite as the negative electrode active material. Charge-discharge tests were conducted on the negative electrode sheet using lithium metal as the counter electrode to obtain the dQ / dV curve. The negative electrode sheet exhibited three reversible lithium insertion peaks and three reversible lithium extraction peaks within the voltage range of 0V to 0.4V. A mixture of completely disordered hard carbon material and graphite was used as the negative electrode active material. Its X-ray diffraction pattern showed both the first and second characteristic peaks, and the dQ / dV curve exhibited four reversible lithium insertion peaks and four reversible lithium extraction peaks within the voltage range of 0V to 0.4V. However, the I2 / I1 value was higher than the upper limit of the scope of this application, and the potential ranges used by the two active materials differed, which may result in some active materials not fully utilizing their capacity, reducing the energy density of the secondary battery. The hard carbon material of this application exhibits a first characteristic peak and a second characteristic peak in its X-ray diffraction pattern. Furthermore, its dQ / dV curve shows four reversible lithium insertion peaks and three to four reversible lithium extraction peaks within the voltage range of 0V to 0.4V. Further adjustment of the values ​​of BA and I2 / I1 within this range indicates that the hard carbon material of this application possesses a locally ordered graphitized structure. By introducing a locally ordered graphitized structure into the hard carbon material, compared to completely disordered hard carbon materials, it achieves higher reversible capacity while improving its true density. Applying the hard carbon material of this application to secondary batteries can increase the energy density of the secondary battery. Simultaneously, the locally ordered graphitized structure can shorten the lithium-ion transport path within the hard carbon material, which is beneficial for improving the kinetic performance of the secondary battery, thereby enhancing its cycle performance. In this application, the peak intensity of the highest point within the range of the first characteristic peak in the X-ray diffraction peak pattern after substrate removal is the peak intensity I1 of the first characteristic peak, and the peak intensity of the highest point within the range of the second characteristic peak in the X-ray diffraction peak pattern after substrate removal is the peak intensity I2 of the second characteristic peak.

[0035] In one embodiment of this application, the hard carbon material has an internal porous structure, wherein the maximum inscribed circle diameter D of the pores in the internal porous structure is 0.40 nm to 10 nm, preferably 0.40 nm to 2 nm, and more preferably 0.40 nm to 0.80 nm. For example, the maximum inscribed circle diameter D of the pores in the internal porous structure can be 0.40 nm, 0.50 nm, 0.60 nm, 0.80 nm, 1 nm, 1.3 nm, 1.5 nm, 1.7 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or a range consisting of any two of these values. When the hard carbon material has an internal porous structure, it is beneficial for the storage of lithium ions during lithium storage, thus the hard carbon material has a high reversible capacity. By controlling the maximum inscribed circle diameter of the pores in the internal pore structure of hard carbon materials within the aforementioned range, the average delithiation potential of the negative electrode sheet using hard carbon materials as the negative electrode active material and metallic lithium as the counter electrode can be kept in a lower range. This is beneficial to improving the reversible capacity of hard carbon materials, especially the low voltage plateau capacity. Applying hard carbon materials to secondary batteries is beneficial to improving the energy density and initial coulombic efficiency of secondary batteries, and improving the cycle performance of secondary batteries.

[0036] In one embodiment of this application, the surface of the hard carbon material has an external pore structure, and the pore volume V of the external pore structure obtained by adsorption testing is ≤0.05cc / g. For example, the pore volume V of the external pore structure obtained by adsorption testing can be 0.01cc / g, 0.02cc / g, 0.03cc / g, 0.04cc / g, 0.05cc / g, or a range of any two of these values. The fact that the pore volume of the external pore structure of the hard carbon material obtained by adsorption testing is within the above range indicates that the external pore volume of the hard carbon material of this application is small, which is beneficial to improving the lithium storage performance of the hard carbon material. It also helps to reduce the irreversible capacity caused by the formation of the SEI (solid electrolyte interface) film in the first cycle of the hard carbon material. Applying hard carbon materials to secondary batteries is beneficial to improving the energy density and first coulombic efficiency of the secondary battery, and improving the cycle performance of the secondary battery.

[0037] In one embodiment of this application, the true density ρ of the hard carbon material is from 0.9 g / cc to 2.0 g / cc. For example, the true density ρ of the hard carbon material can be 0.9 g / cc, 1.0 g / cc, 1.3 g / cc, 1.5 g / cc, 1.7 g / cc, 2.0 g / cc, or a range consisting of any two of these values. By controlling the true density of the hard carbon material within the above range, the hard carbon material has a greater number of internal pores, which is beneficial for storing lithium ions or sodium ions. Therefore, the hard carbon material has a higher reversible capacity. Applying hard carbon materials to secondary batteries can improve the energy density and initial coulombic efficiency of the secondary battery, and improve its cycle performance.

[0038] In one embodiment of this application, the specific surface area S of the hard carbon material is 0.5 m². 2 / g to 50m 2 / g, S is preferably 0.5m 2 / g to 5m 2 / g. For example, the specific surface area S of hard carbon materials can be 0.5m². 2 / g、1m 2 / g、3m 2 / g、5m 2 / g、7m 2 / g, 10m 2 / g、13m 2 / g, 15m 2 / g、17m 2 / g、20m 2 / g、23m 2 / g、25m 2 / g、27m 2 / g、30m 2 / g、33m 2 / g、35m 2 / g、37m 2 / g、40m 2 / g、43m 2 / g、45m 2 / g、47m 2 / g, 50m 2 / g or a range consisting of any two of these values. By controlling the specific surface area of ​​the hard carbon material within the above range, when the hard carbon material is applied to a secondary battery, it is beneficial to form an SEI film of appropriate area during the first charge, reducing the content of negative electrode binder in the negative electrode active material layer, thereby reducing the loss of active ions such as lithium ions or sodium ions, lowering the internal resistance of the negative electrode active material layer, thereby improving the energy density and first coulombic efficiency of the secondary battery, and improving the cycle performance and safety performance of the secondary battery.

[0039] In one embodiment of this application, the Dv50 of the hard carbon material is from 3 μm to 12 μm, preferably from 5 μm to 9 μm. For example, the Dv50 of the hard carbon material can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, or a range of any two of these values. In this application, Dv50 represents the particle size that, measured from the smallest particle size, reaches 50% of the total volumetric particle size in the particle size distribution on a volume basis. By controlling the Dv50 of the hard carbon material within the above range through crushing and grading, the hard carbon material particles have good electrolyte wetting properties while having a small specific surface area, thereby reducing the consumption of active ions such as lithium ions or sodium ions during the formation of an SEI film on the surface of the hard carbon material particles during the first charge. Applying hard carbon materials to secondary batteries is beneficial for improving the energy density and initial coulombic efficiency of secondary batteries, and improving the cycle performance of secondary batteries.

[0040] In one embodiment of this application, the hard carbon material includes heterogeneous elements, which include at least one selected from Li, Na, K, Cs, Mg, Al, Ca, Rb, Zn, Fe, Ni, Co, N, O, H, P, S, B, or Se. Based on the mass of the hard carbon material, the mass percentage X of the heterogeneous element is greater than 0 and less than or equal to 3%. For example, the mass percentage X of the heterogeneous element can be 0.01%, 0.03%, 0.05%, 0.07%, 0.1%, 0.3%, 0.5%, 0.7%, 1%, 1.3%, 1.5%, 1.7%, 2%, 2.3%, 2.5%, 2.7%, 3%, or a range consisting of any two of these values. One or more catalysts are generally used when preparing the hard carbon material precursor. The catalyst can promote the local graphitization of the hard carbon material at a relatively low pyrolysis temperature, which may result in some heterogeneous elements remaining inside the hard carbon material. By controlling the mass percentage content of impurity elements within the aforementioned range, local graphitization of hard carbon materials is facilitated, while simultaneously improving the stability of hard carbon materials during electrochemical reactions. This results in hard carbon materials exhibiting higher specific capacity and initial coulombic efficiency. Applying hard carbon materials to secondary batteries is beneficial for improving the energy density and initial coulombic efficiency of secondary batteries, thereby enhancing their cycle performance.

[0041] A second aspect of this application provides a method for preparing a hard carbon material, comprising the following steps:

[0042] (1) The reactants, catalyst and solvent are mixed evenly to obtain a reaction solution, and the reaction solution is dried to obtain a precursor; wherein the mass ratio of catalyst to reactants is 0.01 to 1, for example, the mass ratio of catalyst to reactants can be 0.01, 0.03, 0.05, 0.07, 0.1, 0.3, 0.5, 0.7, 1 or any two of these values. The reactants include at least one of methyl phenolic resin, furfural resin, epoxy resin, asphalt, starch, glucose, sucrose, or fructose; the solvent includes at least one of deionized water, ethanol, benzene, toluene, or N-methylpyrrolidone; and the catalyst includes at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, magnesium hydroxide, cesium hydroxide, rubidium hydroxide, calcium hydroxide, aluminum hydroxide, aluminum oxide, aluminum trioxide, zinc oxide, zinc chloride, iron, iron tetroxide, ferric chloride, iron oxide, nickel oxide, nickel sulfate, nickel hydroxide, nickel chloride, cobalt nitrate, cobalt oxide, urea, phosphoric acid, lignin, boric acid, sodium selenate, sodium benzoate, potassium benzoate, or sodium cinnamate.

[0043] (2) The precursor is subjected to a first carbonization treatment in an inert atmosphere and then pickled, followed by a second carbonization treatment in an inert atmosphere to obtain the carbonized product. The temperature T1 of the first carbonization treatment is 500℃ to 900℃. For example, the temperature T1 of the first carbonization treatment can be 500℃, 530℃, 550℃, 570℃, 600℃, 630℃, 650℃, 670℃, 700℃, 730℃, 750℃, 770℃, 800℃, 830℃, 850℃, 870℃, 900℃, or a range of any two of these values. The time t1 of the first carbonization treatment is from 0.5h to 5h. For example, the time t1 of the first carbonization treatment can be 0.5h, 0.7h, 1h, 1.3h, 1.5h, 1.7h, 2h, 2.3h, 2.5h, 2.7h, 3h, 3.3h, 3.5h, 3.7h, 4h, 4.3h, 4.5h, 4.7h, 5h, or a range of any two of these values. The heating rate v1 of the first carbonization treatment is from 0.5℃ / min to 10℃ / min. For example, the heating rate v1 of the first carbonization treatment can be 0.5℃ / min, 0.7℃ / min, 1℃ / min, 1.3℃ / min, 1.5℃ / min, 1.7℃ / min, 2℃ / min, 2.3℃ / min, 2.5℃ / min, 2.7℃ / min, 3℃ / min, 3.3℃ / min, 3.5℃ / min, 3.7℃ / min, 4℃ / min, 4.3℃ / min, 4.5℃ / min, 4.7℃ / min, etc. The temperature T2 for the second carbonization treatment is 1000℃ to 1600℃. For example, the temperature T2 for the second carbonization treatment can be 1000℃, 1100℃, 1200℃, 1300℃, 1400℃, 1500℃, 1600℃, or a range of any two of these values. The second carbonization treatment time t2 is from 0.5h to 15h. For example, the second carbonization treatment time t2 can be 0.5h, 0.7h, 1h, 1.3h, 1.5h, 1.7h, 2h, 2.3h, 2.5h, 2.7h, 3h, 3.3h, 3.5h, 3.7h, 4h, 4.3h, 4.5h, 4.7h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, or a range of any two of these values.The heating rate v2 for the second carbonization treatment is from 0.5℃ / min to 10℃ / min. For example, the heating rate v2 for the second carbonization treatment can be 0.5℃ / min, 0.7℃ / min, 1℃ / min, 1.3℃ / min, 1.5℃ / min, 1.7℃ / min, 2℃ / min, 2.3℃ / min, 2.5℃ / min, 2.7℃ / min, 3℃ / min, 3.3℃ / min, 3.5℃ / min, 3.7℃ / min, 4℃ / min, 4.3℃ / min, 4.5℃ / min, 4.7℃ / min. ℃ / min, 5℃ / min, 5.3℃ / min, 5.5℃ / min, 5.7℃ / min, 6℃ / min, 6.3℃ / min, 6.5℃ / min, 6.7℃ / min, 7℃ / min, 7.3℃ / min, 7.5℃ / min, 7.7℃ / min, 8℃ / min, 8.3℃ / min, 8.5℃ / min, 8.7℃ / min, 9℃ / min, 9.3℃ / min, 9.5℃ / min, 9.7℃ / min, 10℃ / min, or a range of any two of these values.

[0044] (3) The carbonized product is heated to 500℃ to 1100℃ in an inert atmosphere at a heating rate of 0.5℃ / min to 20℃ / min. For example, the heating rate of the carbonized product in the inert atmosphere can be 0.5℃ / min, 0.7℃ / min, 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, 11℃ / min, 12℃ / min, 13℃ / min, 14℃ / min, 15℃ / min, or 16℃ / min. The temperature range is 17℃ / min, 18℃ / min, 19℃ / min, 20℃ / min, or any two of these values. The carbonized products can be heated in an inert atmosphere to 500℃, 530℃, 550℃, 570℃, 600℃, 630℃, 650℃, 670℃, 700℃, 730℃, 750℃, 770℃, 800℃, 830℃, 850℃, 870℃, 900℃, 930℃, 950℃, 970℃, 1000℃, 1030℃, 1050℃, 1070℃, 1100℃, or any two of these values. Hard carbon material is obtained by heat treatment in a reducing atmosphere; wherein the inert atmosphere includes at least one of nitrogen, argon, or helium; the heat treatment time t3 is from 0.1h to 8h, for example, the heat treatment time t3 can be 0.1h, 0.3h, 0.5h, 0.7h, 1h, 1.3h, 1.5h, 1.7h, 2h, 2.3h, 2.5h, 2.7h, 3h, 3.3h, 3.5h, 3.7h, 4h, 4.3h, 4.5h, 4.7h, 5h, 5.3h, 5.5h, 5.7h, 6h, 6.3h, 6.5h, 6.7h, 7h, 7.3h, 7.5h, 7.7h, 8h, or a range of any two of these values. The reducing atmosphere includes argon and at least one of acetylene or methane.

[0045] The inventors discovered that the catalyst component can form a melt with the reactants, promoting the rearrangement of atoms within the reactants and thus precipitating graphite microcrystals. It can also form carbides with carbon materials, decomposing into graphite and catalyst elements at high temperatures. By using the catalyst and reactants within the scope of this application, local graphitization of the reactants can be promoted at lower pyrolysis temperatures, and the formation of internal porous structures in hard carbon materials can also be promoted. Simultaneously, the interaction between the catalyst and reactants also carries away some impurities such as oxygen or hydrogen, reducing the impurity content in the hard carbon material. This helps reduce irreversible reactions between impurities and lithium ions during lithium storage in hard carbon materials, thereby improving the reversible capacity of the hard carbon material. The first and second carbonization treatments facilitate the formation of locally graphitized hard carbon materials with internal porous structures and low impurity content. Acid washing and heat treatment in a reducing atmosphere further reduce the impurity content in the hard carbon material, improving its stability during electrochemical reactions and facilitating the formation of hard carbon materials within the scope of this application. Hard carbon materials are prepared using the method provided in the second aspect of this application, and the parameters are adjusted within the above range. The resulting hard carbon materials have high specific capacity and first coulombic efficiency. When used as negative electrode sheets in secondary batteries, they can improve the energy density of secondary batteries and enhance their cycle performance.

[0046] This application does not impose any particular limitation on the mass ratio of solvent to reactants in step (1) above, as long as the purpose of this application can be achieved. For example, the mass ratio of solvent to reactants can be 0.1 to 8. This application does not impose any particular limitation on the drying method in step (1) above, as long as the purpose of this application can be achieved. For example, the drying method can be spray drying, with the inlet air temperature set to 130°C to 300°C and the outlet air temperature set to 50°C to 150°C. It should be noted that if the mixing effect between some catalyst and reactants is poor under the action of solvent, the catalyst and reactants of the same mass ratio can be ball-milled or mechanically mixed and then placed in the reaction vessel before adding solvent to react, in order to obtain the precursor. This application does not impose any particular limitation on the gas flow rate of the inert atmosphere in steps (2) and (3), as long as the purpose of this application can be achieved. For example, the gas flow rate of the inert atmosphere can be 0.1 L / min to 30 L / min. This application does not impose any particular restrictions on the gas flow rate of the reducing atmosphere in step (3), as long as the purpose of this application can be achieved. For example, the gas flow rate of the reducing atmosphere can be from 0.1 L / min to 3 L / min.

[0047] In one embodiment of this application, after the above-mentioned heat preservation treatment in step (3), the reducing atmosphere can be replaced with an inert atmosphere for a second heat preservation treatment. This application does not have a particular limitation on the temperature of the second heat preservation, as long as the purpose of this application can be achieved. For example, the temperature of the second heat preservation can be 900℃ to 1300℃, and the heat preservation time can be 1h to 5h. After the second heat preservation treatment, the content of impurity elements inside the hard carbon material is less, and the stability of the hard carbon material in the electrochemical reaction process is higher, so that the hard carbon material has a higher specific capacity and first coulombic efficiency. Applying hard carbon material to secondary batteries is beneficial to improving the energy density and first coulombic efficiency of secondary batteries and improving the cycle performance of secondary batteries.

[0048] A third aspect of this application provides a negative electrode sheet, comprising a negative current collector and a negative active material layer located on at least one surface of the negative current collector. The negative active material layer comprises the hard carbon material provided in the first aspect of this application or a hard carbon material prepared according to the preparation method provided in the second aspect of this application. The negative electrode sheet provided in the third aspect of this application has high reversible capacity and good kinetic performance. When applied to a secondary battery, it can improve the energy density and initial coulombic efficiency of the secondary battery, and improve the cycle performance of the secondary battery.

[0049] In one embodiment of this application, the compaction density (PD) of the negative electrode active material layer is 0.7 g / cm³. 3 Up to 1.7 g / cm 3 For example, the compaction density (PD) of the negative electrode active material layer can be 0.7 g / cm³. 3 0.9g / cm 3 1g / cm 3 1.2g / cm 3 1.4g / cm 3 1.5g / cm 3 1.6g / cm 3 1.7g / cm 3 This can be a range consisting of any two of these values. A lower compaction density of the negative electrode active material layer indicates the presence of numerous internal pores, which facilitates the wetting of the negative electrode active material particles by the electrolyte, thereby improving the cycle kinetics performance of the secondary battery. Furthermore, the hard carbon material of this application exhibits high lithium storage capacity and a low average lithium delithiation potential. Controlling the compaction density of the negative electrode active material layer within the aforementioned range is beneficial for improving the volumetric energy density of the secondary battery. Therefore, the secondary battery possesses high energy density and excellent cycle performance.

[0050] In one embodiment of this application, the porosity P of the negative electrode active material layer is 10% to 50%. For example, the porosity P of the negative electrode active material layer can be 10%, 12%, 15%, 17%, 20%, 22%, 25%, 27%, 30%, 32%, 35%, 37%, 40%, 42%, 45%, 47%, 50%, or a range of any two of these values. By controlling the porosity of the negative electrode active material layer within the above range, it is beneficial for the electrolyte to wet the negative electrode active material particles, thereby improving the cycle kinetics performance of the secondary battery. This ensures that the contact points between the negative electrode active material particles are within a suitable range, balancing the internal resistance of the secondary battery while increasing its volumetric energy density.

[0051] In one embodiment of this application, the negative electrode active material layer may include other negative electrode active materials besides hard carbon materials, such as artificial graphite. By mixing hard carbon materials with other negative electrode active materials, it is beneficial to increase the compaction density of the negative electrode active material layer, thereby increasing the energy density of the secondary battery.

[0052] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, the negative electrode current collector may comprise a metal foil, a porous metal plate, or a composite current collector. Metal foils include copper foil, copper alloy foil, nickel foil, stainless steel foil, or titanium foil; porous metal plates include foamed nickel or foamed copper, etc.; and composite current collectors may comprise a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. Composite current collectors can be formed by forming metal materials (copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). In this application, there are no particular limitations on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm, and the thickness of the single-sided negative electrode active material layer is 30 μm to 200 μm. In this application, the negative electrode active material layer can be disposed on one surface or on two surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector or a part of the negative electrode current collector; this application has no particular limitation, as long as the purpose of this application can be achieved. The negative electrode active material layer of this application may also contain a conductive agent and a negative electrode binder. This application has no particular limitation on the conductive agent and negative electrode binder in the negative electrode active material layer, as long as the purpose of this application can be achieved. For example, the conductive agent may include, but is not limited to, carbon materials, metals, or conductive polymers. Carbon materials may include at least one of natural graphite, artificial graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. Metals may include metal powders or metal fibers such as copper, iron, and aluminum. Conductive polymers may include polyphenylene derivatives. The negative electrode binder may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon. Optionally, the negative electrode active material layer may also include a thickener, which may include, but is not limited to, sodium carboxymethyl cellulose.

[0053] The negative electrode sheet in this application can be prepared according to conventional methods in the art. For example, hard carbon material or hard carbon material with other optional negative electrode active materials, conductive agents, negative electrode binders and thickeners are dispersed in a solvent, which can be N-methylpyrrolidone (NMP) or deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is then coated onto a negative electrode current collector, and the negative electrode sheet is obtained through processes such as drying and cold pressing.

[0054] The negative electrode sheet in this application does not exclude additional functional layers besides the negative electrode active material layer. For example, in one embodiment, the negative electrode sheet of this application further includes a conductive undercoat (e.g., composed of a conductive agent and a negative electrode binder) sandwiched between the negative electrode current collector and the negative electrode active material layer and disposed on the surface of the negative electrode current collector. In another embodiment, the negative electrode sheet of this application further includes a protective layer covering the surface of the negative electrode active material layer.

[0055] A fourth aspect of this application provides a secondary battery comprising the negative electrode sheet of any of the foregoing embodiments. The secondary battery provided by the fourth aspect of this application has high energy density, initial coulombic efficiency, and good cycle performance.

[0056] The secondary battery of this application also includes a positive electrode sheet. This application does not impose any particular limitation on the positive electrode sheet, as long as it achieves the purpose of this application. For example, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. This application does not impose any particular limitation on the positive current collector, as long as it achieves the purpose of this application. For example, the positive current collector may include a metal foil or a composite current collector. For example, the metal foil may be aluminum foil. The composite current collector may include a polymer material base layer and a metal material layer disposed on at least one surface of the polymer material 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, or silver alloy. The polymer material base layer may include at least one of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, or polyethylene. The positive active material layer of this application includes a positive active material. This application does not impose any particular limitation on the type of positive active material, as long as it achieves the purpose of this application, and can be selected according to actual needs.

[0057] In some embodiments, the secondary battery is a lithium-ion battery, and the positive electrode active material may include lithium transition metal oxide, which may include, but is not limited to, lithium nickel cobalt manganese oxide (LiNi). 0.90 Co 0.05 Mn 0.05 At least one of the following: O2 (NCM955), NCM811, NCM622, NCM523, NCM111, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate. 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 oxides, polyanionic compounds, and Prussian blue compounds. Sodium transition metal oxides may include Na... 1-x Cu h Fe k Mn l M1 m O 2-y Na 0.67 Mn 0.7 Ni z M 2 0.3-z O2, Na a Li b Ni c Mn d Fe e O2, where M 1 It is at least one of Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, or 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; M 2 It is at least one of Li, Mg, Al, Ca, Ti, Fe, Cu, Zn, or Ba, where 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. Polyanionic compounds may include, but are not limited to: 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 (PO4)2(P2O7), where A 1 M is at least one of H, Li, Na, K or NH4. 3 X is at least one of Ti, Cr, Mn, Fe, Co, Ni, V, Cu, or Zn. 1 It is at least one of F, Cl or Br, 0 < f ≤ 4, 0 < g ≤ 2, 1 ≤ i ≤ 3, 0 ≤ j ≤ 2; M 4 X is at least one of Mn, Fe, Co, Ni, Cu, or Zn. 2 It is at least one of F, Cl, or Br, 0 < n ≤ 2; M 5It is at least one of Mn, Fe, Co, Ni, Cu, or Zn, 0 < p ≤ 2, 0 < q ≤ 2; 0 < s ≤ 4, 0 ≤ t ≤ 3. Prussian blue compounds may include, but are not limited to, A. 2 u M 6 v [M 7 (CN)6] w ·xH2O, where A 2 For H + NH4 + M is one or more of alkali metal cations and alkaline earth metal cations. 6 and M 7 Each is independently at least one of the transition metal cations, 0 < u ≤ 2, 0 < v ≤ 1, 0 < w ≤ 1, 0 < x < 6. For example, A 2 For H + Li + Na + K + NH4 + 、Rb + Cs + 、Fr + Be 2+ Mg 2+ Ca 2+ 、Sr 2+ Ba 2+ and Ra 2+ At least one of them, M 6 and M 7 Each is an independently selected cation of at least one transition metal element chosen from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, and W. Preferably, A 2 For Li + Na + and K + At least one of them, M 6 It is a cation of at least one transition metal element selected from Mn, Fe, Co, Ni, and Cu, M 7 It is a cation of at least one transition metal element selected from Mn, Fe, Co, Ni and Cu.

[0058] This application does not impose any particular limitation on the thickness of the positive electrode current collector and the positive electrode active material layer, as long as the purpose of this application 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 single-sided positive electrode active material layer is 30 μm to 400 μm. In this application, the positive electrode active material layer can be disposed on one surface or two surfaces in the thickness direction of the positive electrode current collector. It should be noted that the "surface" here can be the entire area of ​​the positive electrode current collector or a part of the positive electrode current collector; this application does not impose any particular limitation, as long as the purpose of this application can be achieved. The positive electrode active material layer of this application may also contain a conductive agent and a positive electrode binder. This application does not impose any particular limitation on the type of conductive agent in the positive electrode active material layer, as long as the purpose of this application can be achieved. For example, the conductive agent can be the same type as the conductive agent in the negative electrode active material layer described above. This application does not impose any particular limitation on the type of positive electrode binder in the positive electrode active material layer, as long as the purpose of this application can be achieved. For example, the positive electrode binder can be the same type as the negative electrode binder in the aforementioned negative electrode active material layer. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, conductive agent, and positive electrode binder in the positive electrode active material layer; those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved.

[0059] The positive electrode sheet in this application can be prepared according to conventional methods in the art. For example, the positive electrode active material layer is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode active material, optional conductive agent, optional positive electrode binder, and any other components in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP), but is not limited to it.

[0060] The positive electrode sheet of this application does not exclude other additional functional layers besides the positive electrode active material layer. For example, in one embodiment, the positive electrode sheet of this application further includes a conductive undercoat (e.g., composed of a conductive agent and a binder) sandwiched between the positive electrode current collector and the positive electrode active material layer and disposed on the surface of the positive electrode current collector. In another embodiment, the positive electrode sheet of this application further includes a protective layer covering the surface of the positive electrode active material layer.

[0061] The secondary battery of this application also includes an electrolyte. In one embodiment, the electrolyte may include an organic solvent, an electrolyte salt, and optional additives. The types of organic solvent, lithium salt, and additives are not specifically limited and can be selected according to needs. In one embodiment, the secondary battery is a lithium-ion battery, and the electrolyte salt may include a lithium salt, which may include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), or lithium tetrafluorooxalate phosphate (LiTFOP). This application does not have a particular limitation on the concentration of lithium salt in the electrolyte, as long as it achieves the purpose of this application. In another embodiment, the secondary battery is a sodium-ion battery, and the electrolyte salt may include a sodium salt, which may include, but is not limited to, at least one selected from NaPF6, NaClO4, NaBCl4, NaSO3CF3, or Na(CH3)C6H4SO3. This application does not impose any particular limitation on the concentration of the sodium salt in the electrolyte, as long as it achieves the purpose of this application.

[0062] This application does not impose any particular limitation on organic solvents, as long as they can achieve the purpose of this application. For example, they may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other non-aqueous organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorinated carbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorocarbonate compounds may include, but are 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, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), n-propyl acetate, tert-butyl acetate, methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), ethyl butyrate (EB), 1,4-butyrolactone (GBL), decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are 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, or tetrahydrofuran. Other non-aqueous organic solvents may include, but are not limited to, at least one of dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), diethyl sulfone (ESE), sulfolane (SF), 1,2-dioxolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0063] In one embodiment, the additive may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives capable of improving certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc. The additive may include, but is not limited to, at least one of the following: fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), vinyl sulfate (DTD), propylene sulfate, vinyl sulfite (ES), 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), sulfonate cyclic quaternary ammonium salts, succinic anhydride, succinic anhydride (SN), adiponitrile (AND), tris(trimethylsilane) phosphate (TMSP), and tris(trimethylsilane) borate (TMSB).

[0064] The electrolyte can be prepared according to conventional methods in the art. For example, an organic solvent, an electrolyte salt, and optional additives can be mixed evenly to obtain an electrolyte. There are no particular restrictions on the order of addition of the materials; for example, the electrolyte salt and optional additives can be added to the organic solvent and mixed evenly to obtain an electrolyte; or, the electrolyte salt can be added to the organic solvent first, and then the optional additives can be added to the organic solvent and mixed evenly to obtain an electrolyte.

[0065] The secondary battery of this application also includes a separator for separating the positive and negative electrode plates, preventing internal short circuits, allowing electrolyte ions to pass freely, and not affecting the electrochemical charging and discharging process. This application does not impose any particular limitation on the separator, as long as it achieves the purpose of this application. For example, the separator material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The separator type may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. The separator of this application may have a porous structure, and this application does not impose any particular limitation on the pore size of the porous structure of the separator, as long as it achieves the purpose of this application. For example, the pore size may be from 0.01 μm to 1 μm. This application does not impose any particular limitation on the thickness of the separator, as long as it achieves the purpose of this application. For example, the separator thickness may be from 3 μm to 20 μm.

[0066] The secondary battery of this application also includes an outer packaging for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the art for secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular restrictions on the outer packaging, as long as it achieves the purpose of this application. In one embodiment, the outer packaging can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. In another embodiment, the outer packaging can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic; for example, the plastic can include, but is not limited to, at least one of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0067] The secondary battery described in this application is not particularly limited and may include any device in which an electrochemical reaction occurs. In one embodiment of this application, the secondary battery may include, but is not limited to, lithium-ion batteries, sodium-ion batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries. This application does not impose any particular limitation on the shape of the secondary battery; it may be cylindrical, square, or any other arbitrary shape.

[0068] The preparation process of the secondary battery described in this application is well known to those skilled in the art, and this application has no particular limitations. For example, it may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, overcurrent protection components, conductive plates, etc., may be placed in the packaging bag as needed to prevent the internal pressure of the secondary battery from rising and overcharging / discharging.

[0069] A fifth aspect of this application provides an electrical device comprising the secondary battery of any of the foregoing embodiments. The secondary battery provided by this application has high energy density, initial coulombic efficiency, and good cycle performance; therefore, the electrical device of this application has a long service life.

[0070] The electrical device used in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electrical device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0071] Example

[0072] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0073] Test methods and equipment:

[0074] Sampling methods for negative electrode sheets and negative electrode active materials:

[0075] The lithium-ion battery discharged at 0.2C to 2.0V was disassembled, and the negative electrode sheet was removed. It was then soaked in dimethyl carbonate (DMC) for 20 minutes, followed by rinsing with DMC and acetone sequentially. The negative electrode sheet was then placed in an oven and baked at 80℃ for 12 hours to obtain the treated negative electrode sheet sample. The negative electrode sheet samples used in the following tests of the compaction density (PD) and porosity (P) of the negative electrode active material layer were all sampled using the above method.

[0076] The negative electrode active material layer on the negative electrode sheet was scraped off with a scraper, and the scraped powder was heat-treated in a tube furnace at 400°C for 4 hours under argon protection to obtain a negative electrode active material powder sample. The following X-ray diffraction (XRD) tests; specific capacity and initial efficiency tests of the negative electrode active material; maximum inscribed circle diameter D of the pores in the internal pore structure; pore volume V of the external pore structure; true density ρ of the negative electrode active material; Dv50 test of the negative electrode active material; specific surface area S of the negative electrode active material; and elemental analysis tests were all performed using the above methods.

[0077] X-ray diffraction (XRD) test:

[0078] The negative electrode active material was tested using an X-ray powder diffractometer (XRD, Bruker D8 ADVANCE). Test parameters: target material: Cu Kα; voltage: 30 kV; current: 10 mA.

[0079] Specific capacity and initial efficiency tests of negative electrode active materials:

[0080] The negative electrode active material, styrene-butadiene rubber (SBR) binder, and sodium carboxymethyl cellulose (CMC-Na) were mixed at a mass ratio of 97:1.5:1.5. Deionized water was then added as a solvent and stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 40 wt%. This slurry was uniformly coated onto one surface of a 10 μm thick copper foil current collector and dried at 85 °C to obtain the negative electrode sheet. The negative electrode sheet was cut into 14 mm diameter discs and used as the working electrode. A lithium metal sheet was used as the counter electrode, and a 7 μm thick polyethylene (PE) film was used as the separator. After injecting test electrolyte, the discs were assembled into a coin cell. The coin cells were subjected to charge-discharge cycles. The coin cells were first discharged at 0.05 C to 0 mV, then discharged at a constant voltage of 0 mV to 20 μA, and the initial discharge specific capacity was recorded. Finally, the coin cells were charged at a constant current of 0.05 C to 2.5 V, and the initial charge specific capacity was recorded. The mass of the negative electrode active material in the negative electrode sheet was calculated based on the coating weight and area of ​​the negative electrode slurry during the electrode preparation process described above. Initial efficiency = initial charge specific capacity / initial discharge specific capacity × 100%; specific capacity of the negative electrode active material Q = initial charge specific capacity / mass of the negative electrode active material, in mAh / g. The specific capacity-voltage curve was differentiated to obtain the dQ / dV curve, and the number of reversible lithium insertion peaks and reversible lithium extraction peaks in the voltage range of 0V to 0.4V for the tested negative electrode sheet was recorded. The specific parameters and steps for preparing the test electrolyte were the same as those in Example 1.

[0081] Test of the maximum inscribed circle diameter D of the pore in the internal hole structure:

[0082] The scattering vector of the tested negative electrode active material sample was obtained by small-angle X-ray scattering (SAXS) at 0.01 nm. -1 Up to 7nm -1 The internal scattering intensity variation was used to calculate the internal pore structure information of the negative electrode active material. Further, high-magnification transmission electron microscopy (HRTEM) was employed to visually observe the pore structure of the hard carbon material. Specifically, the negative electrode active material was embedded and cured with epoxy resin. Then, using an ultrathin sectioning method, the tested negative electrode active material was cut into samples ranging from 20 nm to 150 nm. HRTEM was used to observe the cross-sections of the particle samples. Carbon microcrystals and pores could be distinguished by the difference in brightness contrast under HRTEM imaging. The criterion for determining pores in the internal pore structure was: within a defined area, there were no microcrystalline layers; this area was blank, surrounded by microcrystalline layers. Five clearly defined areas of 10 nm × 10 nm were randomly selected for analysis. The pore size was calculated using image processing software, obtaining the range of the diameter D of the largest inscribed circle of the pores in the internal pore structure.

[0083] Test of pore volume V of external hole structure:

[0084] The pore volume of the external pore structure of the negative electrode active material was measured using an ASAP2460 physical adsorption analyzer. Specifically, after drying and degassing pretreatment, the adsorption capacity of the negative electrode active material for nitrogen gas at different pressures was measured using the 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 obtain the pore volume V of the external pore structure of the negative electrode active material.

[0085] True density ρ test of negative electrode active materials:

[0086] According to the standard GB / T24586-2009 "Determination of apparent density, true density and porosity of iron ore", the true density ρ of the negative electrode active material under test is tested by a true density meter.

[0087] Dv50 test of negative electrode active material:

[0088] The particle size distribution of the negative electrode active material was determined using a laser particle size analyzer (Malvern, UK, model: Mastersizer2000E) according to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0089] Test of the specific surface area S of the negative electrode active material:

[0090] After drying the negative electrode active material in a vacuum drying oven, it was placed into a sample tube. The specific surface area S of the negative electrode active material was measured using a specific surface area analyzer (TristarⅡ3020M) via nitrogen adsorption / desorption. The specific testing was conducted according to GB / T 19587-2017, "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method".

[0091] Elemental analysis test:

[0092] The mass ratio 'a' of the elements with larger relative atomic masses to carbon in hard carbon materials was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). 0.5 g of the tested negative electrode active material was weighed and mixed with 10 mL of concentrated HNO3. Microwave digestion was performed, and the digested solution was introduced into an inductively coupled plasma optical emission (ICP) light source. The ICP-OES equipment detects the content of different substances based on the characteristic radiation energy emitted when the outer electrons of gaseous atoms in the sample return from the excited state to the ground state after excitation. Elements with smaller relative atomic masses in hard carbon materials can be measured using an elemental analyzer (company: Elementar, model: Unicube). The hard carbon material samples obtained in the above steps were tested using carbon, hydrogen, and nitrogen (CHN) mode and oxygen (O) mode, respectively, to determine the mass content of C, H, O, and N elements in the hard carbon material, thus obtaining the mass ratio 'b' of the elements with smaller relative atomic masses in the hard carbon material. Therefore, the impurity element content X of hard carbon particles is X = a + b. When X < 0.01%, the content of impurities in hard carbon materials can be ignored, and X is counted as < 0.01%.

[0093] Compact density (PD) test of the negative electrode active material layer:

[0094] The negative electrode sheet with area S is weighed using an electronic balance and recorded as W1. The thickness T1 of the negative electrode sheet is measured using a micrometer. The negative electrode active material layer is washed away with solvent DMC, dried, and the weight of the negative electrode current collector is measured and recorded as W2. The thickness T2 of the negative electrode current collector is measured using a micrometer. The compaction density PD of the negative electrode active material layer on the side of the negative electrode current collector is then calculated as (W1-W2) / [(T1-T2)S].

[0095] Porosity P of the negative electrode active material layer:

[0096] The negative electrode active material layer sample to be tested was prepared into a complete circular disc. Thirty samples were tested for each embodiment or comparative example, with each sample having a volume of 0.35 cm³. 3 According to the standard GB / T24586-2009 "Determination of Apparent Density, True Density and Porosity of Iron Ore", the true density ρ' of the negative electrode active material layer in each embodiment or comparative example was obtained by testing with a true density meter, and the apparent density ρ” of the negative electrode active material layer in each embodiment or comparative example was obtained by testing. The porosity of the negative electrode active material layer P = (ρ'-ρ”) / ρ'.

[0097] Energy density test:

[0098] Five lithium-ion batteries from each group were taken and subjected to their first charge and discharge at 25°C. Constant current and constant voltage charging was performed at a charging current of 0.5C until the upper voltage limit was reached, followed by constant current discharge at a discharge current of 0.2C to 3.0V. The discharge capacity and average discharge voltage of the lithium-ion batteries were obtained. The length, width, and thickness of each lithium-ion battery at 50% SOC were measured to calculate the volume. The volumetric energy density of the lithium-ion battery was calculated as: discharge capacity × average discharge voltage / volume. The percentage of energy density of each embodiment and comparative example relative to Comparative Example 1 was further calculated.

[0099] Among them, the upper limit voltage for charging of the lithium-ion battery containing graphite as the negative electrode active material is 4.48V, and the discharge cutoff voltage is 3.0V; the upper limit voltage for charging of the lithium-ion battery of the embodiment with only hard carbon material as the negative electrode active material is 4.53V, and the discharge cutoff voltage is 2.0V.

[0100] Cyclic performance test:

[0101] Take 5 lithium-ion batteries from each group of tested batteries, and repeatedly charge and discharge the lithium-ion batteries through the following steps, and calculate the discharge capacity retention rate of the lithium-ion batteries.

[0102] The first charge and discharge cycle was performed at 25℃. Constant current charging was initiated at 1C until the upper limit voltage was reached, then constant voltage charging was switched to constant voltage charging. Constant current discharging was then performed at 1C until the discharge cutoff voltage was reached, and the discharge capacity of the first cycle was recorded. Subsequently, 800 charge and discharge cycles were performed, and the discharge capacity of the 800th cycle was recorded. The upper limit voltage for charging of lithium-ion batteries containing graphite as the negative electrode active material was 4.48V, and the discharge cutoff voltage was 3.0V. The upper limit voltage for charging of lithium-ion batteries containing only hard carbon as the negative electrode active material was 4.53V, and the discharge cutoff voltage was 2.0V.

[0103] Cycle capacity retention (%) = (Discharge capacity of the 800th cycle / Discharge capacity of the first cycle) × 100%.

[0104] High and low temperature discharge performance testing of lithium-ion batteries:

[0105] Five lithium-ion batteries were taken from each group of tested batteries and charged at 25°C. Constant current and constant voltage charging was performed at a charging current of 0.5C until the upper limit voltage was reached. The fully charged lithium-ion batteries were then placed in environments of 25°C, -20°C, and 45°C for 1 hour each, and then discharged at a constant current of 0.2C until the cutoff voltage was reached. The capacity value D of each lithium-ion battery was obtained. R D L D HLow-temperature capacity retention rate (%) of lithium batteries = D L / D R ×100%, Low-temperature capacity retention rate of lithium batteries (%) = D H / D R ×100%.

[0106] Among them, the upper limit of charging voltage for lithium-ion batteries with graphite as the negative electrode active material is 4.48V, and the discharge cutoff voltage is 3.0V; the upper limit of charging voltage for lithium-ion batteries with only hard carbon as the negative electrode active material is 4.53V, and the discharge cutoff voltage is 2.0V.

[0107] Example 1

[0108] <Preparation of Hard Carbon Materials>

[0109] (1) 200g of reactant starch and 28.05g of catalyst potassium hydroxide were added to 300g of solvent deionized water and stirred until dissolved to obtain a reaction solution. The reaction solution was spray-dried to obtain a precursor. The mass ratio of catalyst to reactant was 0.14.

[0110] (2) The precursor was subjected to a first carbonization treatment in an inert nitrogen atmosphere to obtain a first carbonization product. The heating rate v1 of the first carbonization treatment was 3℃ / min, the temperature T1 of the first carbonization treatment was 900℃, and the time t1 of the first carbonization treatment was 4h. The first carbonization product was added to 500mL of 1mol / L hydrochloric acid solution, heated to 80℃, refluxed and stirred for 12h for acid washing to remove impurities, collected by vacuum filtration, and then washed three times with water. After vacuum filtration and drying, the acid-washed product was obtained. Then, the acid-washed product was subjected to a second carbonization treatment in an inert nitrogen atmosphere to obtain a carbonized product. The heating rate v2 of the second carbonization treatment was 5℃ / min, the temperature T2 of the second carbonization treatment was 1200℃, and the time t2 of the second carbonization treatment was 4h.

[0111] (3) After heating the carbonized product to 900°C in an inert atmosphere of nitrogen at a rate of 1°C / min, the inert atmosphere was replaced with a mixture of 5wt% methane and argon at a flow rate of 0.5L / min. The holding time was t3 for 2 hours to obtain hard carbon material.

[0112] <Preparation of Negative Electrode Sheets>

[0113] Hard carbon material (negative electrode active material), styrene-butadiene rubber (SBR) binder, and sodium carboxymethyl cellulose (CMC-Na) were mixed in a mass ratio of 97:1.5:1.5. Deionized water was then added as a solvent and stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 40 wt%. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector and dried at 85°C to obtain a single-sided coated negative electrode sheet with an 80 μm thick negative electrode active material layer. The above steps were repeated on the other surface of the copper foil to obtain a double-sided coated negative electrode sheet. After cold pressing, cutting, and slitting, the negative electrode sheets were obtained.

[0114] <Preparation of the positive electrode>

[0115] Lithium cobalt oxide, conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1.4:1.6. N-methylpyrrolidone (NMP) was added as a solvent and stirred until homogeneous, forming a positive electrode slurry with a solid content of 72 wt%. The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil used as a positive electrode current collector and dried at 85 °C to obtain a positive electrode sheet with a single-sided coating of positive electrode active material. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode active material. The positive electrode sheets were then obtained by cold pressing, cutting, and slitting. The N / P ratio of the positive and negative electrodes was set to 1.0, that is, the ratio of the negative electrode capacity per unit area to the positive electrode capacity per unit area was 1.0.

[0116] <Preparation of Electrolyte>

[0117] In a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Lithium salt LiPF6 was dissolved in the organic solvent, and then fluoroethylene carbonate (FEC) was added and mixed thoroughly to obtain the electrolyte. Based on the total mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, the mass percentage of FEC was 5%, and the remainder was the organic solvent.

[0118] <Septum>

[0119] A polyethylene (PE) membrane with a thickness of 7 μm was used as the separator.

[0120] <Preparation of Lithium-ion Batteries>

[0121] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrodes are then wound to form the electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag and dried in an 80°C vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected, and the battery undergoes vacuum sealing, settling, formation, degassing, and edge trimming to obtain the lithium-ion battery. The designed potential range for lithium-ion batteries is 2.0V to 4.53V.

[0122] Examples 2 to 7

[0123] Except for the amount of potassium hydroxide catalyst used in the <Preparation of Hard Carbon Materials> being 14.03g, 10.10g, 8.98g, 7.86g, 6.73g, and 5.61g respectively, the rest is the same as in Example 1.

[0124] Examples 8 to 12

[0125] Except for the second carbonization treatment time t2 in the <Preparation of Hard Carbon Materials> being 12h, 8h, 3h, 2h, and 1h respectively, the rest is the same as in Example 4.

[0126] Example 13

[0127] Except for the preparation of hard carbon materials according to the steps described below, the rest is the same as in Example 4.

[0128] <Preparation of Hard Carbon Materials>

[0129] (1) 200g of reactant starch and 8.98g of catalyst potassium hydroxide were added to 300g of solvent deionized water and stirred until dissolved to obtain a reaction solution. The reaction solution was spray-dried to obtain a precursor. The mass ratio of catalyst to reactant was 0.0449.

[0130] (2) The precursor was subjected to a first carbonization treatment in an inert nitrogen atmosphere to obtain a first carbonization product. The heating rate v1 of the first carbonization treatment was 3℃ / min, the temperature T1 of the first carbonization treatment was 900℃, and the time t1 of the first carbonization treatment was 4h. The first carbonization product was added to 500mL of a mixed solution of 2mol / L hydrochloric acid and 1mol / L hydrofluoric acid, heated to 80℃, refluxed and stirred for 12h for acid washing to remove impurities, collected by vacuum filtration, and then washed three times with water. After vacuum filtration and drying, the acid-washed product was obtained. Then, the acid-washed product was subjected to a second carbonization treatment in an inert nitrogen atmosphere to obtain a carbonized product. The heating rate v2 of the second carbonization treatment was 5℃ / min, the temperature T2 of the second carbonization treatment was 1200℃, and the time t2 of the second carbonization treatment was 12h.

[0131] (3) After heating the carbonized product to 900°C in an inert atmosphere of nitrogen at 1°C / min, the inert atmosphere was replaced with a mixture of 5wt% methane and argon at a flow rate of 0.5L / min. The holding time was t3 for 2h. Then, the product was heated to 1200°C in an inert atmosphere of nitrogen at 1°C / min. The holding time was t4 for 2h to obtain hard carbon material.

[0132] Examples 14 to 16

[0133] Except for the fact that the Dv50 of the hard carbon active materials in Examples 14 to 16 were set to 12.0 μm, 4.7 μm and 1.3 μm respectively by crushing and grading in the <Preparation of Hard Carbon Materials>, the rest is the same as in Example 4.

[0134] Comparative Example 1

[0135] Except for replacing the hard carbon material with artificial graphite as the negative electrode active material in the <Preparation of Negative Electrode Sheet> section, setting the N / P ratio of the positive and negative electrodes to 1.04, and adjusting the design potential range of the lithium-ion battery to 3.0V to 4.48V in the <Preparation of Lithium-ion Battery> section, the rest is the same as in Example 1.

[0136] Comparative Example 2

[0137] Except for the absence of potassium hydroxide catalyst in the <Preparation of Hard Carbon Materials>, the rest is the same as in Example 1.

[0138] Comparative Example 3

[0139] Except for the preparation of hard carbon materials according to the steps described below, the rest is the same as in Example 1.

[0140] <Preparation of Hard Carbon Materials>

[0141] Microporous activated carbon containing 0.35 cc / g pore volume was heated to 1200℃ in an inert nitrogen atmosphere at a rate of 1℃ / min. The inert atmosphere was then replaced with a mixture of 5wt% methane and argon at a flow rate of 0.5L / min. The holding time was t3 for 4 hours to obtain hard carbon material.

[0142] The relevant parameters and performance tests of each embodiment and comparative example are shown in Tables 1 and 2.

[0143]

[0144] Table 2

[0145]

[0146] Note: " / " in Table 2 indicates that there are no relevant parameters.

[0147] As can be seen from Examples 1 to 16 and Comparative Examples 1 to 3, the hard carbon material exhibits a first characteristic peak and a second characteristic peak in its X-ray diffraction pattern. Furthermore, its dQ / dV curve shows four reversible lithium insertion peaks and three to four reversible lithium extraction peaks within the voltage range of 0V to 4V. The values ​​of BA and I2 / I1 are within the range of this application. The hard carbon material has a larger specific capacity and its initial efficiency is improved compared to the hard carbon material in the comparative examples. The energy density ratio of the lithium-ion battery containing the hard carbon material of this application to the lithium-ion battery of Comparative Example 1 is larger, indicating that the lithium-ion battery containing the hard carbon material of this application has a higher energy density, which is improved compared to the hard carbon material in the comparative examples. Moreover, the capacity retention rate after 800 cycles, and the capacity retention rates at low and high temperatures, are all improved in the lithium-ion battery containing the hard carbon material of this application. The negative electrode active material in Comparative Example 1 is artificial graphite. Although artificial graphite itself has a higher initial efficiency, its specific capacity is lower, and lithium-ion batteries containing artificial graphite cannot simultaneously achieve high energy density and good cycle performance. This demonstrates that the hard carbon material of this application exhibits higher initial efficiency, achieving both high energy density and good cycle performance. The hard carbon material in Comparative Example 2 has a lower specific capacity and lower initial efficiency, resulting in poor performance; therefore, it is not very meaningful to use it as a negative electrode active material for further testing in lithium-ion battery fabrication. Although the hard carbon material in Comparative Example 3 has a higher specific capacity, its initial efficiency is low, and the lithium-ion battery containing artificial graphite exhibits low capacity retention after 800 cycles, failing to achieve a balance between high energy density and good cycle performance.

[0148] Figure 1 The image shown is the XRD pattern of Example 4, which contains a first characteristic peak and a second characteristic peak. To more clearly identify the peak positions of the first and second characteristic peaks, Figure 2 It shows Figure 1 XRD peak pattern after substrate removal, from Figure 2 It can be seen that the first characteristic peak is located between 15° and 35°, and the second characteristic peak is located between 25° and 28°. Figure 3 The differential capacity curve for Example 4 is shown below. Further, for ease of observation, Figure 3 The top right corner shows a magnified view of the original differential capacity curve. Figure 3 It can be seen that the negative electrode in Example 4 exhibits four reversible lithium insertion peaks and four reversible lithium extraction peaks within the voltage range of 0V to 0.4V. This indicates that the hard carbon material in Example 4 possesses a locally ordered graphitized structure.

[0149] The maximum inscribed circle diameter D of the pores in the internal pore structure of hard carbon materials typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 7, when the maximum inscribed circle diameter D of the pores in the internal pore structure of the hard carbon material is within the range of this application, the specific capacity and initial efficiency of the hard carbon material are relatively high. The resulting lithium-ion battery achieves both high energy density and high capacity retention after 800 cycles, as well as high capacity retention at low and high temperatures. This demonstrates that the lithium-ion battery provided in this application has high energy density and good cycle performance.

[0150] The pore volume V in the external pore structure of hard carbon materials typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 16, when the pore volume V in the external pore structure of the hard carbon material is within the range of this application, the specific capacity and initial efficiency of the hard carbon material are high. The resulting lithium-ion battery achieves both high energy density and high capacity retention after 800 cycles, as well as high capacity retention at low and high temperatures. This demonstrates that the lithium-ion battery provided in this application has high energy density and good cycle performance.

[0151] The porosity P of hard carbon materials typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 16, when the porosity P of the hard carbon material is within the range specified in this application, the specific capacity and initial efficiency of the hard carbon material are relatively high. The resulting lithium-ion battery achieves both high energy density and high capacity retention after 800 cycles, as well as high capacity retention at low and high temperatures. This demonstrates that the lithium-ion battery provided in this application has high energy density and good cycle performance.

[0152] The true density ρ of hard carbon materials typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 16, when the true density ρ of the hard carbon material is within the range of this application, the specific capacity and initial efficiency of the hard carbon material are relatively high. The resulting lithium-ion battery achieves both high energy density and high capacity retention after 800 cycles, as well as high capacity retention at low and high temperatures. This demonstrates that the lithium-ion battery provided in this application has high energy density and good cycle performance.

[0153] The specific surface area S of hard carbon materials typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 16, when the specific surface area S of the hard carbon material is within the range specified in this application, the specific capacity and initial efficiency of the hard carbon material are relatively high. The resulting lithium-ion battery achieves both high energy density and high capacity retention after 800 cycles, as well as high capacity retention at low and high temperatures. This demonstrates that the lithium-ion battery provided in this application has high energy density and good cycle performance.

[0154] The Dv50 of hard carbon materials typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 16, when the Dv50 of the hard carbon material falls within the scope of this application, the specific capacity and initial efficiency of the hard carbon material are relatively high. The resulting lithium-ion battery achieves both high energy density and high capacity retention after 800 cycles, as well as high capacity retention at low and high temperatures. This demonstrates that the lithium-ion battery provided in this application has high energy density and good cycle performance.

[0155] The mass percentage X of impurity elements in hard carbon materials typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 4, 8 to 13, when the mass percentage X of impurity elements in the hard carbon material is within the range of this application, the specific capacity and initial efficiency of the hard carbon material are relatively high. The resulting lithium-ion battery achieves both high energy density and high capacity retention after 800 cycles, as well as high capacity retention at low and high temperatures. This demonstrates that the lithium-ion battery provided in this application has high energy density and good cycle performance.

[0156] The compaction density (PD) of the negative electrode active material layer typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 16, when the PD of the negative electrode active material layer is within the scope of this application, the hard carbon material exhibits higher specific capacity and initial efficiency. The resulting lithium-ion battery achieves both high energy density and high capacity retention after 800 cycles, as well as high capacity retention at low and high temperatures. This demonstrates that the lithium-ion battery provided in this application possesses high energy density and excellent cycle performance.

[0157] The porosity P of the negative electrode active material layer typically affects the energy density and cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 16, when the porosity P of the negative electrode active material layer is within the range of this application, the hard carbon material exhibits higher specific capacity and initial efficiency. The resulting lithium-ion battery achieves both high energy density and high capacity retention after 800 cycles, as well as high capacity retention at low and high temperatures. This demonstrates that the lithium-ion battery provided in this application possesses high energy density and excellent cycle performance.

[0158] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0159] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0160] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A hard carbon material, wherein in the X-ray diffraction pattern of the hard carbon material, there is a first characteristic peak between 15° and 35°, and a second characteristic peak between 25° and 28°, wherein the starting position of the first characteristic peak is A°, the ending position of the first characteristic peak is B°, BA≥8°, the peak intensity of the first characteristic peak is I1, the peak intensity of the second characteristic peak is I2, and 0.1≤I2 / I1≤3.0; The hard carbon material is used as the negative electrode active material to prepare a negative electrode sheet. The negative electrode sheet is subjected to charge-discharge test with metallic lithium as the counter electrode to obtain a differential capacity curve. The negative electrode sheet has 4 reversible lithium insertion peaks and 3 to 4 reversible lithium extraction peaks in the voltage range of 0V to 0.4V.

2. The hard carbon material according to claim 1, wherein, The hard carbon material has an internal pore structure, wherein the maximum inscribed circle diameter of the pores in the internal pore structure is 0.40 nm to 10 nm.

3. The hard carbon material according to claim 2, wherein the hard carbon material satisfies one of the following characteristics: (1) The maximum inscribed circle diameter of the pores in the internal pore structure is 0.40 nm to 2 nm; (2) The maximum inscribed circle diameter of the pores in the internal hole structure is 0.40 nm to 0.80 nm.

4. The hard carbon material according to any one of claims 1 to 3, wherein, The surface of the hard carbon material has an external pore structure, and the pore volume of the external pore structure obtained by adsorption testing is ≤0.05cc / g.

5. The hard carbon material according to any one of claims 1 to 3, wherein, The hard carbon material satisfies at least one of the following characteristics: (1) The true density of the hard carbon material is 0.9 g / cc to 2.0 g / cc; (2) The specific surface area of ​​the hard carbon material is 0.5 m². 2 / g to 50m 2 / g; (3) The Dv50 of the hard carbon material is 3 μm to 12 μm; (4) The hard carbon material includes heterogeneous elements, which include at least one of Li, Na, K, Cs, Mg, Al, Ca, Rb, Zn, Fe, Ni, Co, N, O, H, P, S, B or Se. Based on the mass of the hard carbon material, the mass percentage of the heterogeneous elements is greater than 0 and less than or equal to 3%.

6. The hard carbon material according to any one of claims 1 to 3, wherein, The hard carbon material satisfies at least one of the following characteristics: (1) The specific surface area of ​​the hard carbon material is 0.5 m². 2 / g to 5m 2 / g; (2) The Dv50 of the hard carbon material is 5μm to 9μm.

7. A method for preparing the hard carbon material according to any one of claims 1 to 6, comprising the following steps: (1) The reactants, catalyst and solvent are mixed evenly to obtain a reaction solution, and the reaction solution is dried to obtain a precursor; The mass ratio of the catalyst to the reactants is 0.01 to 1. The reactants include at least one of methyl phenolic resin, furfural resin, epoxy resin, asphalt, starch, glucose, sucrose, or fructose. The solvent includes at least one of deionized water, ethanol, benzene, toluene, or N-methylpyrrolidone. The catalyst includes at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, magnesium hydroxide, cesium hydroxide, rubidium hydroxide, calcium hydroxide, aluminum hydroxide, aluminum oxide, zinc oxide, zinc chloride, iron, iron(II,III) oxide, ferric chloride, iron oxide, nickel oxide, nickel sulfate, nickel hydroxide, nickel chloride, cobalt nitrate, cobalt oxide, urea, phosphoric acid, lignin, boric acid, sodium selenate, sodium benzoate, potassium benzoate, or sodium cinnamate. (2) The precursor is subjected to a first carbonization treatment in an inert atmosphere and then pickled, and then subjected to a second carbonization treatment in the inert atmosphere to obtain the carbonized product. Wherein, the temperature T1 of the first carbonization treatment is 500℃ to 900℃, the time t1 of the first carbonization treatment is 0.5h to 5h, and the heating rate v1 of the first carbonization treatment is 0.5℃ / min to 10℃ / min; the temperature T2 of the second carbonization treatment is 1000℃ to 1600℃, the time t2 of the second carbonization treatment is 0.5h to 15h, and the heating rate v2 of the second carbonization treatment is 0.5℃ / min to 10℃ / min; (3) The carbonized product is heated to 500°C to 1100°C in the inert atmosphere at a heating rate of 0.5°C / min to 20°C / min, and then kept in a reducing atmosphere to obtain the hard carbon material. The inert atmosphere includes at least one of nitrogen, argon, or helium; the heat preservation time t3 is 0.1 h to 8 h; and the reducing atmosphere includes at least one of argon and acetylene or methane.

8. A negative electrode sheet, comprising a negative current collector and a negative active material layer located on at least one surface of the negative current collector, wherein the negative active material layer comprises the hard carbon material according to any one of claims 1 to 6 or the hard carbon material prepared by the preparation method of claim 7.

9. The negative electrode sheet according to claim 8, wherein, The compaction density of the negative electrode active material layer is 0.7 g / cm³. 3 Up to 1.7 g / cm 3 And / or, the porosity of the negative electrode active material layer is 10% to 50%.

10. A secondary battery comprising the negative electrode sheet as described in claim 8 or 9.

11. An electrical device comprising the secondary battery of claim 10.

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