Negative electrode sheet, secondary battery, electric device, hard carbon material, and preparation method therefor

Abstract

A negative electrode sheet, a secondary battery, an electric device, a hard carbon material, and a preparation method therefor. The negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector; the negative electrode film layer comprises a hard carbon material; an X-ray diffraction pattern of the hard carbon material has a diffraction peak at the position where the diffraction angle 2θ ranges from 22° to 25°, wherein a line tangent to a first shoulder part on the left side of the diffraction peak and a first shoulder part on the right side of the diffraction peak is a straight line L, a line parallel to the straight line L and tangent to the diffraction peak is a tangent line M, and the intensity corresponding to the point of tangency of the tangent line M serves as B; if the intensity corresponding to an intersection point between a perpendicular line passing through the point of tangency and perpendicular to the horizontal axis serves as A, then 1≤B / A≤2.5; the saturated water vapor adsorption amount at 25° of the hard carbon material is ≥ 200 cm^3 / g; and the methylene blue adsorption value of the hard carbon material is ≤ 10 mg / g.

Description

Negative electrode sheet, secondary battery, electric device, hard carbon material and preparation method thereof

[0001] Cross-reference to related applications

[0002] The present disclosure is based on a Chinese patent application No. 202410543434.X, filed on April 30, 2024, entitled "Negative electrode sheet, secondary battery, electric device, hard carbon material and preparation method thereof", and claims priority to the Chinese patent application, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure relates to the technical field of batteries, and in particular to a negative electrode sheet, a secondary battery, an electric device, a hard carbon material and a preparation method thereof. BACKGROUND

[0004] In recent years, secondary batteries are widely used in energy storage power systems such as hydroelectric, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, etc. With the application and promotion of secondary batteries, people have increasingly high performance requirements for secondary batteries.

[0005] As an important component of secondary batteries, the performance of negative active material to some extent affects the performance of secondary batteries. Hard carbon as a kind of negative active material of secondary batteries has attracted much attention. In order to further improve the performance of the battery, people urgently need a hard carbon material with high capacity.

[0006] SUMMARY

[0007] The present disclosure is made in view of the above-mentioned problems, and aims to provide a negative electrode sheet, a secondary battery, an electric device, a hard carbon material and a preparation method thereof. The hard carbon material has high capacity.

[0008] To achieve the above-mentioned purpose, the first aspect of the present disclosure provides a negative electrode sheet, comprising a negative current collector and a negative film layer located on at least one surface of the negative current collector, the negative film layer comprising a hard carbon material, the X-ray diffraction pattern of the hard carbon material having a diffraction peak at a position of 2θ of 22°-25°, wherein: a straight line L tangent to the first shoulder on the left and the first shoulder on the right of the diffraction peak, a tangent line M parallel to the straight line L and tangent to the diffraction peak, the intensity corresponding to the tangent point of the tangent line M as B; the intensity corresponding to the intersection of the vertical line passing through the tangent point and perpendicular to the horizontal axis and the straight line L as A, then 1≤B / A≤2.5, the saturated water vapor adsorption amount of the hard carbon material at 25℃≥200cm 3 / g, and the methylene blue adsorption value of the hard carbon material≤10mg / g.

[0009] In the present disclosure, by making the value of B / A in the above range, the adsorption capacity of the hard carbon material is increased, in addition, by making the water vapor adsorption amount and the methylene blue adsorption value in the above range, the electrolyte is unable to enter the pore structure of the hard carbon material, the pore filling capacity is increased, thereby making the hard carbon material have high capacity.

[0010] In some embodiments, the saturated water vapor adsorption amount of the hard carbon material at 25℃ is 350cm 3 / g-390cm 3 / g.

[0011] In some embodiments, the methylene blue adsorption value of the hard carbon material is 0.1mg / g-4mg / g. Thereby, the pore filling capacity of the hard carbon material can be further increased.

[0012] In some embodiments, the compact density of the hard carbon material is measured twice continuously under a pressure of 5 tons, the first measured compact density is set as PD1, and the second measured compact density is set as PD2, PD1 and PD2 satisfy 0≤PD2-PD1≤0.03g / cm 3 By making the difference between PD2 and PD1 in the above range, the structural stability of the hard carbon material is improved, which is beneficial to the transmission of active ions in the charging and discharging process, and is more beneficial to improve the charging gram capacity and the first coulombic efficiency of the battery.

[0013] In some embodiments, the hard carbon material comprises a substrate and a carbon coating layer located on at least part of the surface of the substrate. Thereby, it is beneficial to improve the stacking of the sheets and the pore structure in the hard carbon material, so that the value of B / A, and the water vapor adsorption amount and the methylene blue adsorption value are in the above range.

[0014] In some embodiments, the substrate comprises a porous carbon skeleton and a phosphorus element located in the porous carbon skeleton, and the mass percentage content of the phosphorus element is 0.5wt%-2.5wt% relative to the hard carbon material. Thereby, it is more beneficial to improve the stacking of the sheets and the pore structure.

[0015] In some embodiments, the mass percentage content of the coating layer is 1.5wt%-7.5wt% relative to the hard carbon material. This is more beneficial to form a dense coating layer, so that the water vapor adsorption amount and the methylene blue adsorption value are in the above range.

[0016] In some embodiments, the charging gram capacity of the hard carbon material is 350mAh / g or more in the potential range of 0V to 2.5V of Na / Na +

[0017] ​In some embodiments, the hard carbon material has a surface oxygen content X1 measured under vacuum after sintering at 1000℃ for 2h in an inert atmosphere, and a surface oxygen content X2 measured after exposure to air with a humidity of ≤2% for 30 days, and X1 and X2 satisfy X2-X1≤5wt%. By having the value of X2-X1 in the above range, the surface activity of the hard carbon material is reduced, the ability of the hard carbon material to catalyze the decomposition of the electrolyte is reduced, and thus the amount of active Na consumed in the formation of the solid electrolyte interface film (SEI) is reduced, thereby facilitating further improvement in the initial coulombic efficiency.

[0018] The second aspect of the present disclosure provides a secondary battery. The secondary battery comprises the negative electrode sheet of the first aspect of the present disclosure.

[0019] In some embodiments, the secondary battery further comprises a positive electrode sheet. The positive electrode sheet comprises at least one selected from transition metal oxides, polyanionic compounds and Prussian blue compounds as a positive active material.

[0020] The third aspect of the present disclosure provides an electric device comprising the secondary battery of the second aspect of the present disclosure.

[0021] The electric device of the present disclosure comprises the secondary battery provided by the present disclosure, and thus has at least the same advantages as the secondary battery.

[0022] The fourth aspect of the present disclosure provides a hard carbon material. The hard carbon material has a diffraction peak at a position of diffraction angle 2θ of 22°-25° in an X-ray diffraction pattern of the hard carbon material, wherein: a straight line L tangent to a left first shoulder and a right first shoulder of the diffraction peak, a tangent line M parallel to the straight line L and tangent to the diffraction peak, the intensity corresponding to the tangent point of the tangent line M as B, and the intensity corresponding to the intersection of the vertical line passing through the tangent point and the horizontal axis and the straight line L as A, then 1≤B / A≤2.5, the saturated water vapor adsorption amount of the hard carbon material at 25℃ is ≥200cm 3 / g, and the methylene blue adsorption value of the hard carbon material is ≤10mg / g.

[0023] In the present disclosure, by having the value of B / A in the above range, the adsorption capacity of the hard carbon material is increased, and in addition, by having the water vapor adsorption amount and the methylene blue adsorption value in the above range, the electrolyte is unable to enter the pore structure of the hard carbon material, the pore filling capacity is increased, and thus the hard carbon material has a high capacity.

[0024] In some embodiments, the saturated water vapor adsorption amount of the hard carbon material at 25℃ is 350cm 3 / g-390cm 3 / g.

[0025] In some embodiments, the methylene blue adsorption value of the hard carbon material is 0.1 mg / g-4 mg / g. In this way, the pore filling capacity of the hard carbon material can be further increased.

[0026] In some embodiments, the hard carbon material is measured for compactness density twice continuously under a pressure of 5 tons, the first measured compactness density is denoted as PD1, and the second measured compactness density is denoted as PD2, PD1 and PD2 satisfy 0≤PD2-PD1≤0.03 g / cm 3 By making the difference between PD2 and PD1 within the above range, the structural stability of the hard carbon material is improved, which is beneficial to the transmission of active ions during charging and discharging, and is more beneficial to increasing the charging gram capacity and the first coulombic efficiency of the battery.

[0027] In some embodiments, the hard carbon material comprises a substrate and a carbon coating layer located on at least part of the surface of the substrate. In this way, it is beneficial to improve the stacking of the layers and the pore structure in the hard carbon material, so that the value of B / A, and the water vapor adsorption amount and the methylene blue adsorption value are within the above range.

[0028] In some embodiments, the substrate comprises a porous carbon skeleton and a phosphorus element located in the porous carbon skeleton, and the mass percentage of the phosphorus element is 0.5wt%-2.5wt% relative to the hard carbon material. In this way, it is more beneficial to improve the stacking of the layers and the pore structure.

[0029] In some embodiments, the mass percentage of the coating layer is 1.5wt%-7.5wt% relative to the hard carbon material. This is more beneficial to forming a dense coating layer, so that the water vapor adsorption amount and the methylene blue adsorption value are within the above range.

[0030] In some embodiments, the charging gram capacity of the hard carbon material is 350 mAh / g or more within the potential range of 0V-2.5V of Na / Na + .

[0031] In some embodiments, after the hard carbon material is sintered and heated at 1000°C for 2h under an inert atmosphere, the surface oxygen content measured under vacuum conditions is denoted as X1, and the surface oxygen content measured after being exposed to air with a humidity of ≤2% for 30 days is denoted as X2, X1 and X2 satisfy X2-X1≤5wt%. By making the value of X2-X1 within the above range, the surface activity of the hard carbon material is reduced, the catalytic decomposition ability of the hard carbon material to the electrolyte is reduced, and the amount of active Na consumed when forming a solid electrolyte interface film (SEI) can be reduced, thereby it is beneficial to further improve the first coulombic efficiency.

[0032] The fifth aspect of the present disclosure provides a preparation method of a hard carbon material, comprising the following steps:

[0033] The impregnation step impregnates the carbon source in a liquid containing a dopant, the impregnation time is more than 2 hours, the dopant contains phosphorus element or zinc element as a doping element, and the mass percentage of the phosphorus element is 9.5wt%-32.4wt% and the mass percentage of the zinc element is 14wt%-50wt% relative to the mass of the carbon source;

[0034] The low-temperature heat treatment step heats at 400°C-750°C to obtain the substrate;

[0035] The kneading step kneads the mixed solution in which the resin-based polymer material and the substrate are dispersed in a kneader, and the mass ratio of the resin-based polymer material to the substrate is (0.5-2):10; and

[0036] The carbonization step.

[0037] By the preparation method of the present disclosure, the hard carbon material with high capacity of the fourth aspect of the present disclosure can be obtained.

[0038] In some embodiments, the dopant includes at least one of phosphoric acid, phosphate, polyphosphoric acid, and zinc chloride. By using the above-mentioned dopant, the stacking of the sheets and the pore structure can be improved.

[0039] In some embodiments, the resin-based polymer material includes at least one of epoxy resin, phenolic resin, unsaturated polyester resin, and furan resin. The resin-based polymer material is more conducive to forming a dense and uniform coating layer.

[0040] In some embodiments, in the kneading step, the kneading time is more than 0.5 hours. Thus, the resin-based polymer material is more evenly distributed on the surface of the substrate, thereby being more conducive to forming a dense and uniform coating layer.

[0041] In some embodiments, in the kneading step, the solid content of the mixed solution is 55wt%-75wt%. Thus, the resin-based polymer material is more evenly distributed on the surface of the substrate, thereby being conducive to forming a dense and uniform coating layer.

[0042] In some embodiments, in the low-temperature heat treatment step, the temperature is raised to 400°C-750°C at a temperature raising rate of 1-20°C / min, and the holding time is 1 hour-12 hours. Thus, the stacking of the sheets and the pore structure can be improved.

[0043] In some embodiments, in the carbonization step, the temperature is raised to 1000°C-1800°C at a temperature raising rate of 2-20°C / min. Thus, the cracking process of the resin-based polymer material can be controlled, and the surface defects can be reduced.

[0044] In some embodiments, in the carbonization step, the temperature is raised to 1100-1600°C at a temperature raising rate of 2-10°C / min under a pressure of 10 MPa or more. Thus, the escape of carbon radicals from the thermal cracking of the high molecular material in the carbonization step can be reduced, and the defects that are easily oxidized can be sufficiently repaired, thereby facilitating the improvement of the initial coulombic efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0045] FIG. 1 is a schematic view of a battery cell according to an embodiment of the present disclosure.

[0046] FIG. 2 is an exploded view of the battery cell according to an embodiment of the present disclosure shown in FIG. 1.

[0047] FIG. 3 is a schematic view of a battery module according to an embodiment of the present disclosure.

[0048] FIG. 4 is a schematic view of a battery pack according to an embodiment of the present disclosure.

[0049] FIG. 5 is an exploded view of the battery pack according to an embodiment of the present disclosure shown in FIG. 4.

[0050] FIG. 6 is a schematic view of an electric device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0051] FIG. 7 is an XRD spectrum of a hard carbon material according to Example 1 of the present disclosure.

[0052] REFERENCE NUMERALS

[0053] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 battery cell; 51 case; 52 electrode assembly; 53 top cap assembly DETAILED DESCRIPTION

[0054] Hereinafter, embodiments of the negative electrode sheet, the secondary battery, the electric device, the hard carbon material, and the method for manufacturing the same according to the present disclosure are specifically disclosed in detail with appropriate reference to the accompanying drawings. However, there can be cases where unnecessary detailed descriptions are omitted. For example, there can be cases where detailed descriptions of matters that are well known, repeated descriptions of actually identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present disclosure, and are not intended to limit the subject matter recited in the claims.

[0055] The ranges disclosed herein are defined by their lower and upper limits. Ranges that are given by selecting a lower limit and a upper limit define the range by the endpoints, and the ranges are inclusive of the endpoints and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. In addition, if a minimum range value of 1 and 2 is listed, and if a maximum range value of 3, 4, and 5 is listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present disclosure, unless otherwise stated, a numerical range "a-b" indicates a shorthand way of describing all of the real combinations of values that have been listed herein between "a" and "b", where "a" and "b" are both real numbers. For example, the numerical range "0-5" indicates that all of the real numbers between "0" and "5" have been listed herein, and "0-5" is merely a shorthand way of describing these combinations of values. In addition, when a parameter is stated to be an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0056] Unless otherwise specified, all embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions.

[0057] Unless otherwise specified, all technical features and optional technical features of the present disclosure can be combined with each other to form new technical solutions.

[0058] At present, the research on the negative electrode sheet is mostly focused on improving the electrical performance of the hard carbon material included in the negative electrode film layer. In order to further improve the performance of the battery, there is an urgent need for a negative electrode sheet with high capacity.

[0059] Negative electrode sheet

[0060] Based on this, the first aspect of the present disclosure provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a hard carbon material. In the X-ray diffraction pattern of the hard carbon material, there is a diffraction peak at a diffraction angle 2θ of 22°-25°, wherein: a straight line L tangent to the first shoulder on the left side and the first shoulder on the right side of the diffraction peak, a tangent line M parallel to the straight line L and tangent to the diffraction peak, the intensity corresponding to the tangent point of the tangent line M is B; the intensity corresponding to the intersection of the vertical line passing through the tangent point and the horizontal axis and the straight line L is A, then 1≤B / A≤2.5, the 25℃ saturated water vapor adsorption amount of the hard carbon material is ≥200cm 3 / g, and the methylene blue adsorption value of the hard carbon material is ≤10mg / g.

[0061] The capacity of hard carbon material includes adsorption capacity (corresponding to the adsorption of active ions mainly occurring on the outer surface of the stacked layers of hard carbon material) and pore filling capacity (corresponding to the filling of active ions mainly occurring in the pore structure inaccessible to electrolyte). The ratio of B / A represents the average number of stacked layers of hard carbon (referring to YINGHU Liu, et al. "MECHANISM OF LITHIUM INSERTION IN HARD CARBONS PREPARED BY PYROLYSIS OF EPOXY RESINS." Carbon Vol. 34, No. 2, pp. 193-200, 1996), and the smaller the value, the fewer the number of layers of stacked layers, the increased outer surface, which can provide adsorption sites, and the increased adsorption capacity. In addition, the greater the water vapor adsorption and the smaller the methylene blue adsorption value, the more the pore structure inaccessible to electrolyte, i.e., the more active ions that can be stored in the internal pore region, and the increased pore filling capacity. The hard carbon material of the present disclosure has high capacity by having B / A, 25°C saturated water vapor adsorption, and methylene blue adsorption value within the above ranges.

[0062] In the present disclosure, the X-ray diffraction pattern of the hard carbon material can be tested using an X-ray diffractometer according to JIS K 0131-1996. Exemplarily, B / A is 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.2, 2.4, 2.5, or a value between any two of the above values.

[0063] The 25°C saturated water vapor adsorption in the present disclosure reflects the pore volume inside the hard carbon material that can store Na. This value can be determined by dynamic water vapor adsorption DVS method, specifically, after drying the sample of hard carbon material at 200°C for 2h, using a dynamic water vapor adsorption instrument to sweep the sample with a constant water vapor of 3.14kPa partial pressure carried by nitrogen gas at a certain flow rate, so that it adsorbs at this water vapor partial pressure until equilibrium, and the final adsorption value is the 25°C saturated water vapor adsorption of the sample.

[0064] Exemplarily, the 25°C saturated water vapor adsorption is 200cm 3 / g, 210cm 3 / g, 220cm 3 / g, 250cm 3 / g, 280cm 3 / g, 300cm 3 / g, 320cm 3 / g, 350cm 3 / g, 380cm 3 / g, 400cm 3 / g or a value between any two of the numerical values. Preferably, the 25°C saturated water vapor adsorption amount is 350-390 cm 3 / g.

[0065] In addition, the methylene blue adsorption value in the present disclosure reflects the surface coating compactness of the material. The value can be determined by referring to GB / T 12496.10-1999, specifically, the hard carbon material sample is mixed with a certain amount (in milliliters) of methylene blue solution, and then filtered, and the absorbance of the filtrate is determined by spectrophotometer. If the absorbance is lower than that of the standard solution under the specified concentration, then the methylene blue adsorption mass is equal to the methylene blue adsorption value of the hard carbon per unit mass.

[0066] Illustratively, the methylene blue adsorption value can be 10 mg / g, 9 mg / g, 8 mg / g, 7 mg / g, 6 mg / g, 5 mg / g, 4 mg / g, 3 mg / g, 2 mg / g, 1 mg / g, 0.5 mg / g, 0.1 mg / g or a value between any two of the numerical values. Preferably, the methylene blue adsorption value is 0.1-4 mg / g.

[0067] In some embodiments, the hard carbon material is measured for compactness density twice continuously under a pressure of 5 tons, the first measured compactness density is set as PD1, and the second measured compactness density is set as PD2, PD1 and PD2 satisfy 0≤PD2-PD1≤0.03 g / cm 3 . The difference between PD2 and PD1 within the above range indicates that the structure of the hard carbon material is stable, which can reduce the possibility of structure collapse caused by the escape of active ions and block the ion transmission path, which is conducive to the smooth transmission of active ions during the charging and discharging process, and is more conducive to improving the charge capacity and the first coulombic efficiency of the battery. As the value of PD2-PD1, for example, it can be 0, 0.01 g / cm 3 , 0.02 g / cm 3 , 0.03 g / cm 3 , etc. The smaller the value of PD2-PD1, the more stable the structure of the sample, which can maintain a stable structure during subsequent electrochemical processes to realize the reversible deintercalation of active ions.

[0068] In the present disclosure, the compactness density of the hard carbon material is the meaning known in the art, which can be determined by instruments and methods known in the art. For example, it can be determined by referring to GB / T 24533-2009, by using an electronic pressure testing machine (for example, it can be a UTM7305 type electronic pressure testing machine).

[0069] In some embodiments, the hard carbon material comprises a substrate and a carbon coating layer on at least part of the surface of the substrate. This is advantageous for improving the stacking of the layers and the pore structure of the hard carbon material, so that the value of B / A, and the water vapor adsorption capacity and the methylene blue adsorption value are within the above ranges.

[0070] In some embodiments, the substrate further comprises a porous carbon skeleton and a phosphorus element within the porous carbon skeleton, and the mass percentage of the phosphorus element is 0.5wt%-2.5wt% relative to the hard carbon material. This is more advantageous for improving the stacking of the layers and the pore structure. For example, the mass percentage of the phosphorus element is 0.5wt%, 0.75wt%, 1.0wt%, 1.25wt%, 1.5wt%, 1.75wt%, 2.0wt%, 2.25wt%, 2.5wt%, or a value between any two of the above values.

[0071] In some embodiments, the mass percentage of the coating layer is 1.5wt%-7.5wt% relative to the hard carbon material. This is more advantageous for forming a dense coating layer, so that the water vapor adsorption capacity and the methylene blue adsorption value are within the above ranges.

[0072] In some embodiments, the hard carbon material has a charge gram capacity of 350mAh / g or more in the potential range of 0V to 2.5V of Na / Na + For example, the hard carbon material has a charge gram capacity of 350mAh / g, 370mAh / g, 400mAh / g, 420mAh / g, 450mAh / g, 470mAh / g, 490mAh / g, or a value between any two of the above values.

[0073] In some embodiments, the hard carbon material is sintered at 1000℃ for 2h under an inert atmosphere, the surface oxygen content measured under vacuum is set as X1, the surface oxygen content measured after exposure to air with a humidity of ≤2% for 30 days is set as X2, and X1 and X2 satisfy X2-X1≤5wt%.

[0074] Due to the high activity of the surface of the hard carbon material, it is easily oxidized in air to introduce a large amount of oxygen-containing groups on the surface. In the present disclosure, the surface oxygen content X2 measured after exposure to air with a humidity of ≤2% for 30 days is compared with the surface oxygen content X1 measured under vacuum, and the increase of X2 relative to X1 (i.e., the amount of O increased due to the high activity of the surface of the hard carbon material) is used to characterize the activity of the surface of the hard carbon material. The higher the value of X2-X1, the higher the surface activity of the hard carbon material.

[0075] The X2-X1 of the hard carbon material of the present disclosure is less than or equal to 5 wt%, indicating that the surface activity of the hard carbon material is low, and the ability to catalyze the decomposition of the electrolyte is small, thereby reducing the amount of active Na consumed when forming the solid electrolyte interface film (SEI) and improving the initial coulombic efficiency.

[0076] The surface oxygen content mentioned in the present disclosure refers to the oxygen element content of the surface of the solid material. The value can be determined by conventional determination methods in the art. For example, GB / T 33502-2017 can be referred to, and the test instrument can use an X-ray photoelectron spectrometer (Thermo Fisher K-Alpha). Among them, the excitation light source uses Al K-Alpha, the energy step is 1.0 eV, and the carbon peak is calibrated with standard C1s (284.6 eV).

[0077] For the value of X2-X1, for example, it can be 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, or a value between any two of the above values. Preferably, the X2-X1 of the hard carbon material is less than or equal to 2.5 wt%. Thus, it is indicated that the surface activity of the hard carbon material is lower, which is more conducive to improving the initial coulombic efficiency of the secondary battery.

[0078] As an example, the negative electrode current collector has two surfaces opposite in the thickness direction of the negative electrode current collector, and the negative electrode film layer is arranged on any one or both of the two opposite surfaces of the negative electrode current collector.

[0079] In some embodiments, the negative electrode current collector can use a metal foil or a composite current collector. For example, as a metal foil, a copper foil or an aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base material. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0080] In some embodiments, the negative electrode film layer can also optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0081] In some embodiments, the negative electrode film layer can also optionally include a conductive agent. The conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0082] In some embodiments, the negative electrode film layer can also optionally include other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)), etc.

[0083] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material including the hard carbon material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector, and after processes such as drying, cold pressing, etc., a negative electrode sheet can be obtained.

[0084] The negative electrode sheet prepared using the above-mentioned hard carbon material has high capacity.

[0085] Secondary battery

[0086] A second aspect of the present disclosure provides a secondary battery, which will be described below with appropriate reference to the accompanying drawings. The term “secondary battery” referred to herein means a battery cell, a battery module, or a battery pack. Each will be described below.

[0087] Generally, a secondary battery cell includes a positive electrode sheet, a negative electrode sheet of the above-mentioned embodiments, an electrolyte, and a separator. During charging and discharging of the battery, active ions, such as sodium ions, are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet, and mainly functions to prevent short circuiting of the positive and negative electrodes, while allowing ions to pass through.

[0088] [Positive electrode sheet]

[0089] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, the positive electrode film layer including a positive electrode active material.

[0090] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0091] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0092] In some embodiments, the battery cell is a sodium-ion battery, and the positive active material can employ positive active materials known in the art for sodium-ion batteries. As an example, the positive active material can include sodium transition metal oxides, polyanionic compounds, Prussian blue type compounds, etc., but the present disclosure is not limited to these materials, and other conventionally known materials that can be used as positive active materials for sodium-ion batteries can also be used. For example, as an alternative technical solution of the present disclosure, in the sodium transition metal oxides, the transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide is, for example, Na x MO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < x < 1.

[0093] As an alternative technical solution of the present disclosure, the polyanionic compound can be a compound having sodium ions, transition metal ions, and tetrahedral (YO4) n- anion units. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si; and n represents the valence of (YO4) n- . The polyanionic compound can also be a compound having sodium ions, tetrahedral (YO4) n- anion units, and halogen anions. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si; n represents the valence of (YO4) n- ; and the halogen can be at least one of F, Cl, and Br. The polyanionic compound can also be a compound having sodium ions, tetrahedral (YO4) n- anion units, and polyhedral (ZO y ) m+ anion units. Y can be at least one of P, S, and Si; n represents the valence of (YO4) n- ; Z represents a transition metal, which can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; and m represents the valence of (ZO y ) m+ . The polyanionic compound is, for example, NaFePO4, Na3V2(PO4)3, NaM’PO4F (M’ is one or more of V, Fe, Mn, and Ni), and Na3(VO y )2(PO4)2F 3-2yat least one of (0 < y < 1). The Prussian blue type compound can be a compound having sodium ions, transition metal ions, and cyanide ions (CN ). The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The Prussian blue type compound is, for example, Na a Me b Me’ c (CN)6, wherein Me and Me’ are each independently at least one of Ni, Cu, Fe, Mn, Co, and Zn, 0 < a < 2, 0 < b < 1, and 0 < c < 1.

[0094] In some embodiments, when the battery cell is a lithium ion battery, the positive active material can employ a positive active material for a lithium ion battery known in the art.

[0095] The battery is accompanied by the deintercalation and consumption of active ions (Na or K) during the charging and discharging process, and the molar content of Li or Na is different when the battery is discharged to different states. In the enumeration of the positive active material in the present disclosure, the molar content of Li or Na is the initial state of the material, i.e., the state before feeding, and the positive active material is applied to the battery system. After charging and discharging cycles, the molar content of Li or Na will change. In the enumeration of the positive active material in the present disclosure, the molar content of oxygen is only the theoretical state value, and the release of oxygen from the lattice will cause the molar content of oxygen to change, and the actual molar content of oxygen will fluctuate.

[0096] In some embodiments, the positive electrode film layer can further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0097] In some embodiments, the positive electrode film layer can further optionally include a conductive agent. As an example, the conductive agent can include at least one of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0098] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as the positive active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and after processes such as drying and cold pressing, the positive electrode sheet can be obtained.

[0099] [Electrolyte]

[0100] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The kind of electrolyte is not particularly limited in the present disclosure, and can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

[0101] In some embodiments, the electrolyte employs an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0102] In some embodiments, when the battery cell is a sodium ion battery, the electrolyte salt can be selected from at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium bisfluorosulfonylimide, sodium bis-trifluoromethylsulfonylimide, sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluoroboric oxalate, sodium bisoxalate borate, sodium difluorobisoxalate phosphate, and sodium tetrafluorobisoxalate phosphate.

[0103] In some embodiments, when the battery cell is a lithium ion battery, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethylsulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoroboric oxalate, lithium bisoxalate borate, lithium difluorobisoxalate phosphate, and lithium tetrafluorobisoxalate phosphate.

[0104] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0105] In some embodiments, the electrolyte solution can also optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can also include an additive capable of improving certain properties of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0106] [Separator]

[0107] In some embodiments, the battery cell further includes a separator. The kind of separator is not particularly limited in the present disclosure, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0108] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0109] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0110] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0111] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0112] This disclosure does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square battery cell 5 as an example.

[0113] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0114] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0115] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple battery cells 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple battery cells 5 can be fixed in place using fasteners.

[0116] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0117] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0118] FIGS. 4 and 5 are battery packs 1 as an example. Referring to FIGS. 4 and 5, a battery case and a plurality of battery modules 4 provided in the battery case can be included in the battery pack 1. The battery case includes an upper case 2 and a lower case 3, and the upper case 2 is capable of being provided on the lower case 3 and forms a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery case in any manner.

[0119] Electric device

[0120] A third aspect of the embodiments of the present disclosure also provides an electric device. The secondary battery of the present disclosure is described below with reference to the accompanying drawings as appropriate.

[0121] The electric device mentioned in the embodiments of the present disclosure includes the secondary battery provided by the present disclosure. The secondary battery can be used as a power source of the electric device or as an energy storage unit of the electric device. The electric device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0122] As the electric device, a battery monomer, a battery module or a battery pack can be selected according to the use requirement thereof.

[0123] FIG. 6 is an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of high power and high energy density of the secondary battery for the electric device, a battery pack or a battery module can be used.

[0124] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thin and light, and a battery monomer can be used as a power source.

[0125] Hard carbon material

[0126] A fourth aspect of the present disclosure provides a hard carbon material. In the X-ray diffraction pattern of the hard carbon material, a diffraction peak is present at a position with a diffraction angle 2θ of 22°-25°, wherein: a straight line L tangent to the first shoulder on the left side and the first shoulder on the right side of the diffraction peak, a tangent line M parallel to the straight line L and tangent to the diffraction peak, and the intensity corresponding to the intersection point of the tangent line M is taken as B; the intensity corresponding to the intersection point of the vertical line passing through the intersection point and perpendicular to the horizontal axis and the straight line L is taken as A, then 1≤B / A≤2.5, the saturated water vapor adsorption amount of the hard carbon material at 25°C≥200 cm 3 / g, and the methylene blue adsorption value of the hard carbon material≤10 mg / g.

[0127] In the present disclosure, by making the value of B / A in the above range, the adsorption capacity of the hard carbon material is increased, in addition, by making the water vapor adsorption amount and the methylene blue adsorption value in the above range, the electrolyte is unable to enter the pore structure of the hard carbon material, the pore filling capacity is increased, thereby making the hard carbon material have high capacity.

[0128] In some embodiments, the saturated water vapor adsorption amount of the hard carbon material at 25℃ is 350cm 3 / g-390cm 3 / g.

[0129] In some embodiments, the methylene blue adsorption value of the hard carbon material is 0.1mg / g-4mg / g. Thereby, the pore filling capacity of the hard carbon material can be further increased.

[0130] In some embodiments, the compact density of the hard carbon material is measured twice continuously under a pressure of 5 tons, the first measured compact density is set as PD1, and the second measured compact density is set as PD2, PD1 and PD2 satisfy 0≤PD2-PD1≤0.03g / cm 3 By making the difference between PD2 and PD1 in the above range, the structural stability of the hard carbon material is improved, which is beneficial to the transmission of active ions in the charging and discharging process, and is more beneficial to improve the charging gram capacity and the first coulombic efficiency of the battery.

[0131] In some embodiments, the hard carbon material comprises a matrix and a carbon coating layer located on at least part of the surface of the matrix. Thereby, it is beneficial to improve the stacking of the sheets and the pore structure in the hard carbon material, so that the value of B / A, and the water vapor adsorption amount and the methylene blue adsorption value are in the above range.

[0132] In some embodiments, the matrix comprises a porous carbon skeleton and a phosphorus element located in the porous carbon skeleton, and the mass percentage content of the phosphorus element is 0.5wt%-2.5wt% relative to the hard carbon material. Thereby, it is more beneficial to improve the stacking of the sheets and the pore structure.

[0133] In some embodiments, the mass percentage content of the coating layer is 1.5wt%-7.5wt% relative to the hard carbon material. This is more beneficial to form a dense coating layer, so that the water vapor adsorption amount and the methylene blue adsorption value are in the above range.

[0134] In some embodiments, the charging gram capacity of the hard carbon material is 350mAh / g or more in the potential range of 0V to 2.5V of Na / Na + .

[0135] In some embodiments, the hard carbon material is sintered at 1000°C for 2 hours under an inert atmosphere, the surface oxygen content measured under vacuum conditions is set as X1, the surface oxygen content measured after exposure to air with a humidity of ≤2% for 30 days is set as X2, and X1 and X2 satisfy X2-X1≤5wt%. By setting the value of X2-X1 within the above range, the surface activity of the hard carbon material is reduced, the catalytic decomposition ability of the hard carbon material to the electrolyte is reduced, and thus the amount of active Na consumed in the formation of the solid electrolyte interface film (SEI) can be reduced, thereby facilitating further improvement in the initial coulombic efficiency.

[0136] Method for preparing hard carbon material

[0137] The fifth aspect of the present disclosure provides a method for preparing the hard carbon material of the fourth aspect of the present disclosure. The method comprises the following steps: an impregnation step, in which a carbon source is impregnated in a liquid containing a dopant, the impregnation time is 2 hours or more, the dopant contains phosphorus or zinc as a doping element, and the mass percentage of phosphorus is 9.5wt%-32.4wt% and the mass percentage of zinc is 14wt%-50% relative to the mass of the carbon source; a low-temperature heat treatment step, in which heating is performed at 400°C-750°C to obtain a matrix; a kneading step, in which a mixed solution in which a resin-based polymer material and the matrix are dispersed is kneaded in a kneader, and the mass ratio of the resin-based polymer material to the matrix is (0.5-2):10; and a carbonization step.

[0138] Hereinafter, the above steps are described respectively.

[0139] (1) Impregnation step

[0140] By impregnating the carbon source in the liquid containing the dopant sufficiently, the activation reaction in the subsequent low-temperature heat treatment step is uniformly performed.

[0141] In the impregnation step, as the carbon source, a carbon-hydrogen-oxygen polymer, for example, any one or several of phenolic resin, epoxy resin, unsaturated polyester resin, and furan resin, can be used. The carbon source has structural designability and low impurity content, which is more conducive to improving the stacking of the lamellas and the pore structure.

[0142] As a dopant, at least one of the following is included: phosphorus element or zinc element, such as phosphoric acid, phosphate, polyphosphoric acid, zinc chloride. In some embodiments, the mass percentage of the phosphorus element is, for example, 9.5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 32.4 wt%, etc., and any range between any two of them, relative to the carbon source. The mass percentage of the zinc element is 14 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, etc., and any range between any two of them, relative to the carbon source. In the present disclosure, the above-mentioned doping elements are incorporated into the carbon skeleton structure by forming P-C, P-O, Zn-C, Zn-O, etc. covalent bonds during subsequent low-temperature heat treatment, and thus as dopants, they can inhibit the stacking of the layers and the increase of B / A, so that B / A is in the range of 1-2.5. In addition, by introducing the above-mentioned doping elements, an appropriate amount of pore structure can be introduced, so that the 25°C saturated water vapor adsorption capacity is ≥200 cm 3 / g and the methylene blue adsorption value of the hard carbon material is ≤10 mg / g.

[0143] In some embodiments, the impregnation time is more than 2 hours, for example, it can be 2 hours-24 hours. By making the impregnation time in the above range, the dopant can be uniformly distributed in the carbon source, the stacking of the layers can be improved, the B / A value can be controlled, and the carbon source can also be uniformly activated to introduce an appropriate amount of pore structure so that the above-mentioned adsorption capacity and adsorption value are in the above-mentioned specific range.

[0144] (2) Low-temperature heat treatment step

[0145] In the present disclosure, by low-temperature heat treatment, on the one hand, the dopant etches the raw material by chemical activation, introducing pore structure into the carbon skeleton structure; on the other hand, the dopant and the raw material molecules undergo chemical reaction to form P-C, P-O, Zn-C, Zn-O, etc. covalent bonds, and P atoms / Zn atoms are incorporated into the carbon skeleton structure. Since the size of P atoms / Zn atoms is larger than that of carbon atoms, they can start to play a role in inhibiting the stacking of the layers at the initial stage of layer formation.

[0146] The temperature of the low-temperature heat treatment is 400°C-750°C. Exemplarily, it can be 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, and any range between any two of them. By making the temperature in the above range, it is more conducive to obtain sufficient activation effect, introduce an appropriate amount of pore structure, control the stacking of the layers, and reduce the B / A value.

[0147] In some embodiments, the temperature is increased to 400°C-750°C at a rate of 1-20°C / min under N2or Ar atmosphere, and held for 1-12h. In this way, the improvement of the stacking of the layers and the pore structure is more facilitated. Illustratively, the temperature increasing rate of the low-temperature heat treatment step can be 1°C / min, 2°C / min, 4°C / min, 6°C / min, 8°C / min, 10°C / min, 12°C / min, 15°C / min, 18°C / min, 20°C / min, and a range between any two of them.

[0148] In some embodiments, after the above low-temperature heat treatment, a crushing treatment is performed to reduce the particle size, and then a washing is performed to remove the P-containing impurities and the like after the reaction.

[0149] (3) kneading step

[0150] By the kneading step, the kneading raw material (resin-based polymer material) is uniformly coated on the surface of the substrate to form a dense organic coating layer.

[0151] In some embodiments, in the kneading step, the kneading time is 0.5h or more, and in this way, the resin-based polymer material is more uniformly distributed on the surface of the substrate. Illustratively, the kneading time can be 0.5h, 1h, 2h, 3h, 4h, 5h, 6h, 8h, 9h, 10h, and a range between any two of them. Preferably, the kneading time is 3-9h.

[0152] In some embodiments, the resin-based polymer material includes at least one of epoxy resin, phenol resin, unsaturated polyester resin, and furan resin. The resin-based polymer material is more advantageous to form a dense and uniform coating layer.

[0153] In addition, in some embodiments, in this step, the mixture liquid in which the substrate and the resin-based polymer material are dispersed is kneaded, and the solid content of the mixture liquid is 55wt%-75wt%. Illustratively, the solid content can be 55wt%, 60wt%, 65wt%, 70wt%, 75wt%, and a range between any two of them. The solid content of the mixture liquid refers to the content of the substrate and the resin-based polymer material in the mixture liquid. By making the solid content in the above range, the resin-based polymer material is more uniformly distributed on the surface of the hard carbon, and it is more advantageous to form a dense coating layer, so as to control the methylene blue adsorption value to be 10mg / g or less.

[0154] In addition, the mass ratio of the resin-based polymer material to the substrate is (0.5-2):10, which is more conducive to forming a dense and uniform coating layer with a suitable thickness and forming an appropriate pore structure, thereby controlling the methylene blue adsorption value. Illustratively, the mass ratio of the resin-based polymer material to the substrate of the present disclosure is 0.5:10, 0.7:10, 1:10, 1.2:10, 1.5:10, 1.7:10, 2:10, and a range between any two of them.

[0155] In some embodiments, the kneading is performed by a double screw kneader at a rotation speed of 10-50 rpm.

[0156] (4) Carbonization step

[0157] In some embodiments, in the carbonization step, the temperature is raised to 1000-1800°C at a temperature raising speed of 2-20°C / min. This is more conducive to controlling the cracking process of the resin-based polymer material and reducing surface defects. Illustratively, the temperature raising speed of the carbonization step can be 2°C / min, 4°C / min, 6°C / min, 8°C / min, 10°C / min, 12°C / min, 15°C / min, 18°C / min, 20°C / min, and a range between any two of them. Illustratively, the carbonization temperature is 1000°C, 1200°C, 1400°C, 1600°C, 1800°C, and a range between any two of them. In the carbonization step, when a dopant containing phosphorus elements is used, at least part of the phosphorus elements combined into the substrate are retained; when a dopant containing zinc elements is used, substantially all the zinc elements combined into the substrate are volatilized.

[0158] In some embodiments, in the carbonization step, the temperature is raised to 1100-1600°C at a temperature raising speed of 2-10°C / min under a pressure of 10 MPa or more.

[0159] Through the above-mentioned hot-pressing carbonization, the excess H and O in the carbon structure of the substrate can be removed to form a suitable Na storage skeleton structure; at the same time, the organic matter in the coating layer is thermally cracked into carbon radicals to repair surface defects. Through the pressurized mode, the escape of the carbon radicals formed by the cracking of the organic matter in the coating layer can be reduced, so that they can stay in the surface defect position for a longer time to repair the surface, the completion rate of the repair is improved, thereby reducing the content of surface defects and further reducing the activity of the surface.

[0160] In this step, the hot-pressing carbonization is performed under a pressure of 10 MPa or more, for example, it can be 10 MPa, 20 MPa, 30 MPa, 35 MPa, 40 MPa, 50 MPa, 60 MPa, etc., and preferably, the pressure is 30 MPa or more. In this way, the escape of the carbon radicals formed by the cracking of the organic matter in the coating layer can be reduced, so that they can stay in the surface defect position for a longer time to improve the efficiency of surface defect repair.

[0161] Examples

[0162] Hereinafter, examples of the present disclosure will be described. The examples described below are exemplary and are for the purpose of explanation of the present disclosure and cannot be understood as a limitation of the present disclosure. In the examples, specific techniques or conditions not noted are performed in accordance with techniques or conditions described in the literature in the field or in accordance with product manuals. The reagents or instruments not noted for the manufacturer are all conventional products that can be obtained commercially.

[0163] Example 1

[0164] Preparation of hard carbon material:

[0165] 1) Immersion

[0166] In a reaction kettle, 1000 g of phenol-formaldehyde resin (CAS No. 9003-35-4, purchased from Shanghai Aladdin Biochem Technology Co., Ltd.) as a carbon source was added to a dopant solution (807 g of phosphoric acid, 85 wt%) and immersed for 6 hours. The mass percentage of P element with respect to the carbon source was 21.7 wt%.

[0167] 2) Low-temperature heat treatment

[0168] The liquid in step 1) above was heated to 600°C at a temperature increase rate of 5°C / min in a tube furnace under N2atmosphere, and held for 8 hours. After that, crushing was performed, followed by washing (to remove P-containing impurities after reaction) to obtain a matrix.

[0169] 3) Kneading

[0170] Phenol-formaldehyde resin (CAS No. 9003-35-4, purchased from Shanghai Aladdin Biochem Technology Co., Ltd.) as a kneading raw material and the above matrix were mixed at a mass ratio of 1:10, dispersed into an ethanol solution to obtain a mixed solution having a solid content of 65 wt%, and then the mixed solution was added to a double-screw kneader, and kneading was performed at a rotation speed of 40 rpm for 6 hours. The product was filtered out, and then vacuum dried at 80°C for 12 h to obtain a matrix having a coating layer.

[0171] 4) Carbonization

[0172] The above matrix having a coating layer was heated to 1250°C at a temperature increase rate of 2°C / min under N2atmosphere, and held for 4 hours to obtain a hard carbon material.

[0173] Preparation of negative electrode sheet:

[0174] The hard carbon material, a binder styrene-butadiene rubber (SBR), a thickening agent sodium carboxymethyl cellulose (CMC-Na), and a conductive agent carbon black are mixed in a mass ratio of 96.2:1.8:1.2:0.8 in a proper amount of solvent deionized water, and a uniform negative electrode slurry is formed.

[0175] The uniformly stirred negative electrode slurry is coated on one side of a Cu foil by a double-sided coating machine, and after coating, drying, cold pressing, and punching are sequentially performed to prepare a negative electrode sheet.

[0176] Preparation of button cell:

[0177] Vinyl carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and sodium hexafluorophosphate (NaPF6) is dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L. Then, a metal Na sheet is used as a counter electrode, a glass fiber is used as a separator, and a CR2430 button cell is assembled with the prepared negative electrode sheet in an argon glove box.

[0178] Examples 2-7

[0179] The battery is prepared according to Table 1 by changing the preparation conditions, except that the preparation is the same as in Example 1.

[0180] Comparative Examples 1-3

[0181] The battery is prepared according to Table 1 by changing the preparation conditions, except that the preparation is the same as in Example 1.

[0182] Hard carbon material related tests:

[0183] Test of B / A

[0184] The hard carbon materials prepared in the above examples and comparative examples are tested using a Bruker D8 Discover X-ray diffractometer. The hard carbon material is prepared by a flat plate method, CuKα ray is used as the radiation source, a copper target is used as the anode target, the voltage is 40 KV, the current is 40 mA, the anti-scattering slit is 1 mm, the scanning 2θ angle range is 20°-80°, the step size is 0.01671°, the time for each step is 0.24 s, and the scanning rate is 4° / min. Specifically, in the obtained X-ray diffraction pattern, a diffraction peak is present at a diffraction angle 2θ of 22°-25°, a straight line L is drawn tangent to the left first shoulder and the right first shoulder of the diffraction peak, a tangent line M is drawn parallel to the straight line L and tangent to the diffraction peak, the intensity corresponding to the tangent point is taken as B, and a vertical line is drawn through the tangent point and perpendicular to the horizontal axis, and the intensity corresponding to the intersection of the vertical line and the straight line L is taken as A.

[0185] The above determination was performed for the hard carbon material of Example 1, and its X-ray diffraction pattern is shown in Figure 7. In Figure 7, B / A = 1.42.

[0186] 25℃ saturated water vapor adsorption amount

[0187] After drying the sample of the hard carbon material prepared in the above examples and comparative examples at 200℃ for 2h, the sample was subjected to a constant water vapor sweep at 25℃ at a flow rate of 100mL / min of nitrogen carrying a water vapor partial pressure of 3.14kPa, so that it adsorbed at this water vapor partial pressure until equilibrium, and the final adsorption value was the 25℃ saturated water vapor adsorption amount of the sample, in cm 3 / g.

[0188] Methylene blue adsorption value

[0189] After drying the sample of the hard carbon material prepared in the above examples and comparative examples at 200℃ for 2h, 100mg was placed in a 100mL conical flask, 10mL of methylene blue solution (methylene blue concentration: 1.5g / L) was added with a burette, after the hard carbon sample was completely wetted, it was immediately placed in an electric shaker for 20min, and then filtered with a neutral qualitative filter paper. The filtrate was placed in a cuvette with an optical path of 1cm, and the absorbance was measured at a wavelength of 665nm with a spectrophotometer, and compared with the absorbance of a copper sulfate standard filter solution (4.000g of copper sulfate pentahydrate was dissolved in 1000mL of deionized water), the number of milliliters of methylene blue test solution consumed multiplied by the concentration of methylene blue divided by 0.1g of hard carbon mass, i.e. the methylene blue adsorption value of the hard carbon, in mg / g (refer to GB / T 12496.10-1999).

[0190] PD2-PD1

[0191] For the hard carbon material prepared in the above examples and comparative examples, the determination was performed by a UTM7305 electronic pressure testing machine according to GB / T 24533-2009. Specifically, 1g of sample was weighed and added to a mold with a bottom area of 1.327cm 2 , pressurized to 5 tons for 30s, then released, and kept for 10s, and the powder compaction density PD1 of the sample at 5 tons was recorded and calculated. Next, the sample was pressurized to 5 tons again for the same operation as above, and the powder compaction density PD2 of the sample at 5 tons was obtained.

[0192] Determination of the content of P element in the hard carbon material

[0193] The hard carbon materials prepared in the above examples and comparative examples were digested according to the national standard method (LY / T 1232-1999 Determination of total phosphorus in forest soil) to oxidize all phosphorus compounds in the hard carbon materials into orthophosphate, and then transferred to a 50 mL volumetric flask for constant volume. After mixing uniformly, the solution was used as the test solution. The test solution was diluted 10 times with deionized water (0.5 mL of the test solution was added to 4.5 mL of deionized water), introduced into an atomizer by an automatic sampler, and carried into a plasma torch with a temperature of 6000 K-10000 K by a carrier gas. The components in the sample were atomized, ionized and excited. When these excited particles returned to the stable state, they emitted certain energy (in the form of light spectrum of a certain wavelength). The intensity of the spectrum line specific to P element was measured and compared with that of a P standard solution to quantitatively analyze the P element in the sample. The P standard solution was prepared by adding 0 mL, 0.1 mL, 0.2 mL, 0.4 mL, 0.8 mL, 1.6 mL and 10 mL of PO4 3- The concentration was 50 mg / L. A control solution with the same background ion strength as the sample was added, and then deionized water was added to constant volume. The selected test wavelengths were 214.914 nm and 178.222 nm.

[0194] Battery performance test

[0195] Gravimetric capacity and first coulombic efficiency

[0196] At 25°C, the coin cells prepared in the above examples and comparative examples were first discharged at a current density of 10 mA / g to 0 V, and the initial discharge capacity of the coin cells was recorded. Then the coin cells were charged at a current density of 10 mA / g to 2.5 V, and the initial charge capacity of the coin cells was recorded. The mass of the hard carbon material in the negative electrode tab was calculated according to the coating weight and area of the slurry in the above tab preparation process. The initial charge gravimetric capacity = initial charge capacity / mass of hard carbon material, the initial discharge gravimetric capacity = initial discharge capacity / mass of hard carbon material, and the first coulombic efficiency of the coin cell (%) = initial charge gravimetric capacity of the coin cell / initial discharge gravimetric capacity of the coin cell x 100%.

[0197] The B / A, methylene blue adsorption value, water vapor adsorption amount, PD2-PD1, and gravimetric capacity test results of the hard carbon materials prepared in the above examples 1-7 and comparative examples 1-3 are shown in Table 2.

[0198] Table 1:

[0199] Table 2

[0200] As can be seen from Table 2, by making 1≤B / A≤2.5 and 25°C saturated water vapor adsorption amount≥200 cm3 / g, and a methylene blue adsorption value of 10 mg / g or less, the charge g capacity can be significantly improved. In addition, by making the value of PD2-PD1 within the range of 1 g / cm 3 -0.03 g / cm 3 , an excellent initial coulombic efficiency can be further obtained.

[0201] Note that the present disclosure is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration as the technical idea and exerting the same effects within the scope of the technical solution of the present disclosure are included in the technical scope of the present disclosure. Furthermore, within the scope of the gist of the present disclosure, other modes obtained by applying various modifications that can be thought of by those skilled in the art to the embodiments, or by combining part of the constituent elements of the embodiments, are also included in the scope of the present disclosure.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a hard carbon material, The hard carbon material has a diffraction peak at a position of diffraction angle 2θ of 22°-25° in an X-ray diffraction pattern, wherein: a tangent line L tangent to the first left shoulder and the first right shoulder of the diffraction peak, a tangent line M parallel to the tangent line L and tangent to the diffraction peak, the intensity corresponding to the tangent point of the tangent line M as B, and the intensity corresponding to the intersection of the vertical line passing through the tangent point and the tangent line L as A, wherein 1≤B / A≤2.5, The hard carbon material has a saturated water vapor adsorption amount of ≥ 200 cm 3 / g at 25°C, and a methylene blue adsorption value of ≤ 10 mg / g.

2. The negative electrode sheet according to claim 1, wherein The hard carbon material has a saturated water vapor adsorption amount of 350 cm 3 / g-390 cm 3 / g at 25°C.

3. The negative electrode sheet according to claim 1 or 2, wherein the methylene blue adsorption value of the hard carbon material is 0.1 mg / g-4 mg / g.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein The hard carbon material is measured for compacted density twice continuously under a pressure of 5 tons, the first measured compacted density is denoted as PD1, and the second measured compacted density is denoted as PD2, the PD1 and the PD2 satisfy the following relationship: 0≤PD2-PD1≤0.03 g / cm3 3 .

5. The negative electrode sheet according to any one of claims 1 to 4, wherein The hard carbon material comprises a substrate and a carbon coating layer on at least part of the surface of the substrate.

6. The negative electrode sheet according to any one of claims 1 to 5, wherein The substrate comprises a porous carbon skeleton and phosphorus elements in the porous carbon skeleton, and the mass percentage of the phosphorus elements is 0.5 wt%-2.5 wt% relative to the hard carbon material.

7. The negative electrode sheet according to any one of claims 1 to 6, wherein The mass percentage of the coating layer is 1.5 wt%-7.5 wt% relative to the hard carbon material.

8. The negative electrode sheet according to any one of claims 1 to 7, wherein The hard carbon material has a charge specific capacity of 350 mAh / g or more in a potential range of 0 V to 2.5 V vs. Na / Na + + 9. The negative electrode sheet according to any one of claims 1 to 8, wherein After the hard carbon material is sintered at 1000℃ for 2h in an inert atmosphere, the surface oxygen content measured under vacuum is set as X1, and the surface oxygen content measured after exposure to air with a humidity of ≤2% for 30 days is set as X2, wherein X2-X1≤5 wt%. 10.A secondary battery comprising the negative electrode sheet according to any one of claims 1-9.

11. The secondary battery according to claim 10, wherein The secondary battery further comprises a positive electrode sheet comprising at least one selected from transition metal oxides, polyanion compounds and Prussian blue compounds as a positive electrode active material. 12.An electric device comprising the secondary battery according to claim 10 or 11.

13. A hard carbon material having a diffraction peak in an X-ray diffraction pattern at a position of diffraction angle 2Θ of 22° to 25°, wherein: a tangent line L tangent to the first left shoulder and the first right shoulder of the diffraction peak, a tangent line M parallel to the tangent line L and tangent to the diffraction peak, the intensity corresponding to the tangent point of the tangent line M as B, and the intensity corresponding to the intersection of the vertical line passing through the tangent point and the tangent line L as A, wherein 1≤B / A≤2.5, The hard carbon material has a saturated water vapor adsorption amount of ≥ 200 cm 3 / g at 25°C, and a methylene blue adsorption value of ≤ 10 mg / g.

14. The hard carbon material of claim 13, wherein, The hard carbon material has a saturated water vapor adsorption amount of 350 cm 3 / g-390 cm 3 / g at 25°C.

15. The hard carbon material of claim 13 or 14, wherein, the methylene blue adsorption value of the hard carbon material is 0.1 mg / g-4 mg / g.

16. The hard carbon material of any one of claims 13-15, wherein, The hard carbon material is measured for compacted density twice continuously under a pressure of 5 tons, the first measured compacted density is denoted as PD1, and the second measured compacted density is denoted as PD2, the PD1 and the PD2 satisfy the following relationship: 0≤PD2-PD1≤0.03 g / cm3. 3 .

17. The hard carbon material of any one of claims 13-16, wherein, The hard carbon material comprises a substrate and a carbon coating layer on at least part of the surface of the substrate.

18. The hard carbon material of any one of claims 13-17, wherein, The substrate comprises a porous carbon skeleton and phosphorus elements in the porous carbon skeleton, and the mass percentage of the phosphorus elements is 0.5 wt%-2.5 wt% relative to the hard carbon material.

19. The hard carbon material of any one of claims 13-18, wherein, The mass percentage of the coating layer is 1.5 wt%-7.5 wt% relative to the hard carbon material.

20. The hard carbon material of any one of claims 13-19, wherein, The hard carbon material has a charge specific capacity of 350 mAh / g or more in a potential range of 0 V to 2.5 V vs. Na / Na + + 21. The hard carbon material of any one of claims 13-20, wherein, After the hard carbon material is sintered at 1000℃ for 2h in an inert atmosphere, the surface oxygen content measured under vacuum is set as X1, and the surface oxygen content measured after exposure to air with a humidity of ≤2% for 30 days is set as X2, wherein X2-X1≤5 wt%. 22.A method for preparing a hard carbon material, comprising the following steps: The impregnation step impregnates the carbon source in a liquid containing a dopant containing phosphorus or zinc as a dopant element, and the mass percentage of phosphorus is 9.5wt%-32.4wt% and the mass percentage of zinc is 14wt%-50wt% relative to the mass of the carbon source, for more than 2 hours; The low-temperature heat treatment step heats at 400-750℃ to obtain a matrix; The kneading step kneads a mixed solution in which a resin-based polymer material and the matrix are dispersed in a kneader, and the mass ratio of the resin-based polymer material to the matrix is (0.5-2):10; and The carbonization step.

23. The method of making according to claim 22, wherein, The dopant includes at least one of phosphoric acid, phosphate, polyphosphoric acid, and zinc chloride.

24. The method of manufacturing according to claim 22 or 23, wherein, The resin-based polymer material includes at least one of epoxy resin, phenol resin, unsaturated polyester resin, and furan resin.

25. The method of making according to any one of claims 22-24, wherein, In the kneading step, the kneading time is more than 0.5 hours.

26. The method of making according to any one of claims 22-25, wherein, In the kneading step, the solid content of the mixed solution is 55wt%-75wt%.

27. The method of making according to any one of claims 22-26, wherein, In the low-temperature heat treatment step, the temperature is raised to 400-750℃ at a temperature raising rate of 1-20℃ / min, and the holding time is 1-12 hours.

28. The method of making according to any one of claims 22-27, wherein, In the carbonization step, the temperature is raised to 1000-1800℃ at a temperature raising rate of 2-20℃ / min.

29. The method of making according to claim 28, wherein, In the carbonization step, the temperature is raised to 1100-1600℃ at a temperature raising rate of 2-10℃ / min under a pressure of more than 10MPa.