Negative pole piece, secondary battery, electric device, hard carbon material and preparation method of hard carbon material

Abstract

The invention provides a negative pole piece, a secondary battery, an electric device, a hard carbon material and a preparation method thereof. The negative electrode piece 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, and in an X-ray diffraction pattern of the hard carbon material, a diffraction peak exists at the position where the diffraction angle 2 theta is 22-25 degrees, the straight line L is tangent to the first shoulder on the left side of the diffraction peak and the first shoulder on the right side of the diffraction peak, the tangent line M is parallel to the straight line L and is tangent to the diffraction peak, and the intensity corresponding to the tangent point of the tangent line M is taken as B; the strength corresponding to the intersection point of a vertical line which passes through the tangent point and is perpendicular to the horizontal axis and the straight line L is A, B / A is greater than or equal to 1 and less than or equal to 2.5, the 25 DEG C saturated vapor adsorption capacity of the hard carbon material is greater than or equal to 200cm < 3 > / g, and the methylene blue adsorption value of the hard carbon material is less than or equal to 10mg / g. The hard carbon material has high capacity.

Description

Technical Field

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

[0002] In recent years, rechargeable batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric cars, military equipment, aerospace, and many other fields. With the increasing application and promotion of rechargeable batteries, people are placing increasingly higher demands on their performance.

[0003] As a crucial component of rechargeable batteries, the performance of the negative electrode active material significantly impacts the overall battery performance. Hard carbon has garnered considerable attention as a negative electrode active material for rechargeable batteries. To further improve battery performance, there is an urgent need for a high-capacity hard carbon material. Summary of the Invention

[0004] This application is made in view of the above-mentioned issues, and its purpose is to provide a negative electrode sheet, a secondary battery, an electrical device, a hard carbon material, and a method for preparing the same. The hard carbon material has high capacity.

[0005] To achieve the above objectives, a first aspect of this application 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, characterized in that the X-ray diffraction pattern of the hard carbon material has a diffraction peak at a diffraction angle 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 M parallel to the straight line L and tangent to the diffraction peak, the intensity corresponding to the tangent point of the tangent M is taken as B; the intensity corresponding to the intersection point of the perpendicular line passing through the tangent point and perpendicular to the horizontal axis with the straight line L is taken as A, then 1≤B / A≤2.5, and the saturated water vapor adsorption capacity of the hard carbon material at 25°C is ≥200cm³. 3 / g, and the methylene blue adsorption value of the hard carbon material is ≤10mg / g.

[0006] In this application, by keeping the B / A value within the above range, the adsorption capacity of the hard carbon material is increased. In addition, by keeping the water vapor adsorption amount and methylene blue adsorption value within the above range, the electrolyte cannot enter the pore structure of the hard carbon material, thereby increasing the pore filling capacity and thus giving the hard carbon material a high capacity.

[0007] In some embodiments, the saturated water vapor adsorption capacity of the hard carbon material at 25°C is 350 cm⁻¹. 3 / g-390cm 3 / g.

[0008] In some embodiments, the methylene blue adsorption value of the hard carbon material is 0.1 mg / g to 4 mg / g. This further increases the pore-filling capacity of the hard carbon material.

[0009] In some embodiments, the compaction density of the hard carbon material is measured twice consecutively under a pressure of 5 tons. The compaction density measured in the first measurement is designated as PD1, and the compaction density measured in the second measurement is designated as PD2. PD1 and PD2 satisfy 0 ≤ PD2 - PD1 ≤ 0.03 g / cm³. 3 By keeping the difference between PD2 and PD1 within the aforementioned range, the structural stability of the hard carbon material is improved, which is beneficial for the transport of active ions during charging and discharging, and further conducive to improving the battery's specific capacity and initial coulombic efficiency.

[0010] In some embodiments, the hard carbon material includes a matrix and a carbon coating layer located on at least a portion of the surface of the matrix. This facilitates improved lamellar stacking and pore structure in the hard carbon material, ensuring that the B / A ratio, water vapor adsorption capacity, and methylene blue adsorption value are within the aforementioned ranges.

[0011] In some embodiments, the matrix comprises a porous carbon framework and phosphorus elements located within the porous carbon framework, wherein the phosphorus element has a mass percentage content of 0.5 wt% to 2.5 wt% relative to the hard carbon material. This is more conducive to improving the lamellar stacking and pore structure.

[0012] In some embodiments, the coating layer comprises 1.5 wt% to 7.5 wt% of the hard carbon material by mass. This is more conducive to forming a dense coating layer, resulting in water vapor adsorption and methylene blue adsorption values ​​within the aforementioned ranges.

[0013] In some implementations, Na / Na is at a voltage of 0V to 2.5V. + Within the potential range, the charging capacity of the hard carbon material is above 350 mAh / g.

[0014] In some embodiments, after the hard carbon material is sintered at 1000°C for 2 hours in an inert atmosphere, the surface oxygen content measured under vacuum conditions is set as X1, and the surface oxygen content measured after exposure to air with humidity ≤2% for 30 days is set as X2, wherein X1 and X2 satisfy X2-X1≤5wt%. By keeping the values ​​of X2-X1 within the above range, the surface activity of the hard carbon material is reduced, thereby decreasing the catalytic decomposition ability of the hard carbon material on the electrolyte. This reduces the amount of active Na required to form the solid electrolyte interphase (SEI) film, which is beneficial for further improving the first coulombic efficiency.

[0015] A second aspect of this application provides a secondary battery. The secondary battery includes the negative electrode sheet of the first aspect of this application.

[0016] In some embodiments, the secondary battery further includes a positive electrode. The positive electrode comprises at least one selected from transition metal oxides, polyanionic compounds, and Prussian blue compounds as the positive electrode active material.

[0017] A third aspect of this application provides an electrical device, including the secondary battery of the second aspect of this application.

[0018] The electrical device of this application includes the secondary battery provided in this application, and therefore has at least the same advantages as the secondary battery.

[0019] The fourth aspect of this application provides a hard carbon material. In the X-ray diffraction pattern of the hard carbon material, diffraction peaks are present at diffraction angles 2θ of 22°-25°. Specifically: 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 is taken as B; the intensity corresponding to the intersection point of the perpendicular line passing through the tangent point and perpendicular to the horizontal axis with the straight line L is taken as A. Then, 1 ≤ B / A ≤ 2.5, and the saturated water vapor adsorption capacity of the hard carbon material at 25°C is ≥200 cm³ / s. 3 / g, and the methylene blue adsorption value of the hard carbon material is ≤10mg / g.

[0020] In this application, by keeping the B / A value within the above range, the adsorption capacity of the hard carbon material is increased. In addition, by keeping the water vapor adsorption amount and methylene blue adsorption value within the above range, the electrolyte cannot enter the pore structure of the hard carbon material, thereby increasing the pore filling capacity and thus giving the hard carbon material a high capacity.

[0021] In some embodiments, the saturated water vapor adsorption capacity of the hard carbon material at 25°C is 350 cm⁻¹. 3 / g-390cm 3 / g.

[0022] In some embodiments, the methylene blue adsorption value of the hard carbon material is 0.1 mg / g to 4 mg / g. This further increases the pore-filling capacity of the hard carbon material.

[0023] In some embodiments, the compaction density of the hard carbon material is measured twice consecutively under a pressure of 5 tons. The compaction density measured in the first measurement is designated as PD1, and the compaction density measured in the second measurement is designated as PD2. PD1 and PD2 satisfy 0 ≤ PD2 - PD1 ≤ 0.03 g / cm³. 3By keeping the difference between PD2 and PD1 within the aforementioned range, the structural stability of the hard carbon material is improved, which is beneficial for the transport of active ions during charging and discharging, and further conducive to improving the battery's specific capacity and initial coulombic efficiency.

[0024] In some embodiments, the hard carbon material includes a matrix and a carbon coating layer located on at least a portion of the surface of the matrix. This facilitates improved lamellar stacking and pore structure in the hard carbon material, ensuring that the B / A ratio, water vapor adsorption capacity, and methylene blue adsorption value are within the aforementioned ranges.

[0025] In some embodiments, the matrix comprises a porous carbon framework and phosphorus elements located within the porous carbon framework, wherein the phosphorus element has a mass percentage content of 0.5 wt% to 2.5 wt% relative to the hard carbon material. This is more conducive to improving the lamellar stacking and pore structure.

[0026] In some embodiments, the coating layer comprises 1.5 wt% to 7.5 wt% of the hard carbon material by mass. This is more conducive to forming a dense coating layer, resulting in water vapor adsorption and methylene blue adsorption values ​​within the aforementioned ranges.

[0027] In some implementations, Na / Na is at a voltage of 0V to 2.5V. + Within the potential range, the charging capacity of the hard carbon material is above 350 mAh / g.

[0028] In some embodiments, after the hard carbon material is sintered at 1000°C for 2 hours in an inert atmosphere, the surface oxygen content measured under vacuum conditions is set as X1, and the surface oxygen content measured after exposure to air with humidity ≤2% for 30 days is set as X2, wherein X1 and X2 satisfy X2-X1≤5wt%. By keeping the values ​​of X2-X1 within the above range, the surface activity of the hard carbon material is reduced, thereby decreasing the catalytic decomposition ability of the hard carbon material on the electrolyte. This reduces the amount of active Na required to form the solid electrolyte interphase (SEI) film, which is beneficial for further improving the first coulombic efficiency.

[0029] The fifth aspect of this application provides a method for preparing a hard carbon material, characterized by the following steps:

[0030] The impregnation step involves immersing the carbon source in a liquid containing a dopant for at least 2 hours. The dopant contains phosphorus or zinc as doping elements, and the mass percentage of phosphorus relative to the mass of the carbon source is 9.5wt%-32.4wt%, and the mass percentage of zinc is 14wt%-50wt%.

[0031] The low-temperature heat treatment step involves heating at 400℃-750℃ to obtain the matrix;

[0032] The kneading step involves kneading a mixture containing a resin-based polymer material and the matrix in a kneader, wherein the mass ratio of the resin-based polymer material to the matrix is ​​(0.5-2):10; and

[0033] Carbonization step.

[0034] The preparation method of this application enables the production of a high-capacity hard carbon material according to the fourth aspect of this application.

[0035] In some embodiments, the dopant includes at least one selected from phosphoric acid, phosphates, polyphosphoric acid, and zinc chloride. Using the above dopant can improve the layer stacking and pore structure.

[0036] In some embodiments, the resin-based polymer material includes at least one selected from 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.

[0037] In some embodiments, the kneading step lasts for 0.5 hours or more. This facilitates a more uniform distribution of the resin-based polymer material on the matrix surface, thereby promoting the formation of a dense and uniform coating layer.

[0038] In some embodiments, the solid content of the mixture during the kneading step is 55wt%-75wt%. This further facilitates a more uniform distribution of the resin-based polymer material on the matrix surface, thereby promoting the formation of a dense and uniform coating layer.

[0039] In some embodiments, during the low-temperature heat treatment step, the temperature is increased to 400°C-750°C at a heating rate of 1-20°C / min, and held for 1-12 hours. This is more conducive to improving the lamellar stacking and pore structure.

[0040] In some embodiments, during the carbonization step, the temperature is increased to 1000℃-1800℃ at a rate of 2-20℃ / min. This is more conducive to controlling the cracking process of the resin-based polymer material and reducing surface defects.

[0041] In some embodiments, during the carbonization step, the temperature is increased to 1100℃-1600℃ and held at a rate of 2-10℃ / min under a pressure of 10 MPa or higher. This reduces the release of carbon free radicals from the thermal decomposition of the polymer material during the carbonization step, allowing for the full repair of defects that make the material susceptible to oxidation, thereby improving the initial coulombic efficiency. Attached Figure Description

[0042] Figure 1This is a schematic diagram of a battery cell according to one embodiment of this application.

[0043] Figure 2 yes Figure 1 An exploded view of a battery cell according to one embodiment of this application is shown.

[0044] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application.

[0045] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0046] Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown.

[0047] Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0048] Figure 7 This is the XRD pattern of the hard carbon material of Example 1 of this application.

[0049] Explanation of reference numerals in the attached figures:

[0050] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly Detailed Implementation

[0051] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the negative electrode sheet, secondary battery, electrical device, hard carbon material, and preparation method thereof. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0052] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. Ranges defined in this way, unless otherwise stated, include endpoints and can be arbitrarily combined; that is, 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 specific parameter, it is understood that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0053] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0054] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0055] Currently, research on negative electrode sheets mainly focuses on improving the electrical performance of the hard carbon materials included in the negative electrode film. To further improve battery performance, there is an urgent need for a negative electrode sheet with high capacity.

[0056] Negative electrode sheet

[0057] Based on this, the first aspect of this application proposes a negative electrode sheet, including 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 and the first shoulder on the right of the diffraction peak, a tangent M parallel to the straight line L and tangent to the diffraction peak, the intensity corresponding to the tangent point of the tangent M is taken as B; the intensity corresponding to the intersection point of the perpendicular line passing through the tangent point and perpendicular to the horizontal axis with the straight line L is taken as A, then 1≤B / A≤2.5, and the saturated water vapor adsorption capacity of the hard carbon material at 25℃ is ≥200cm³. 3 / g, and the methylene blue adsorption value of the hard carbon material is ≤10mg / g.

[0058] The capacity of hard carbon materials includes adsorption capacity (corresponding to the adsorption of active ions mainly occurring on the outer surface of the stacked layers of hard carbon materials) and pore filling capacity (corresponding to the filling of active ions mainly occurring in the pore structure that the electrolyte cannot enter). The B / A ratio represents the average number of stacked layers of hard carbon (refer 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). The smaller the value, the fewer the number of stacked layers, the more outer surface area is available, providing adsorption sites and increasing the adsorption capacity. In addition, the greater the water vapor adsorption and the smaller the methylene blue adsorption value, the more pore structures that the electrolyte cannot enter, that is, the more active ions can be stored in the internal pore regions, increasing the pore filling capacity. The hard carbon material of this application achieves high capacity by keeping the B / A ratio, the saturated water vapor adsorption at 25°C, and the methylene blue adsorption value within the above ranges.

[0059] In this application, 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 a value between 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 any two of these values.

[0060] In this application, the saturated water vapor adsorption capacity at 25°C reflects the pore volume within the hard carbon material that can store Na. This value can be determined using the dynamic water vapor adsorption (DVS) method. Specifically, after drying the hard carbon material sample at 200°C for 2 hours, the sample is purged at 25°C with nitrogen gas carrying a constant water vapor pressure of 3.14 kPa at a certain flow rate, allowing adsorption at this water vapor partial pressure until equilibrium is reached. The final adsorption value is the saturated water vapor adsorption capacity of the sample at 25°C.

[0061] For example, the saturated water vapor adsorption capacity at 25°C is 200 cm⁻¹. 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、400cm3 / g or a value within a range of any two of these values. Preferably, the saturated water vapor adsorption capacity at 25°C is 350-390 cm³ / g. 3 / g.

[0062] Furthermore, the methylene blue adsorption value in this application reflects the surface coating density of the material. This value can be determined with reference to GB / T 12496.10-1999. Specifically, a hard carbon material sample is mixed with a certain amount (in milliliters) of methylene blue solution, allowed to stand for 30 minutes, then filtered, and the absorbance of the filtrate is measured using a spectrophotometer. If this absorbance is lower than that of a standard solution at a specified concentration, then the number of milliliters of methylene blue consumed × the concentration of methylene blue in the methylene blue solution = the mass of methylene blue adsorbed, thus yielding the methylene blue adsorption value per unit mass of hard carbon.

[0063] For example, 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 within a range of any two of these values. Preferably, the methylene blue adsorption value is 0.1-4 mg / g.

[0064] In some embodiments, the compaction density of the hard carbon material is measured twice consecutively under a pressure of 5 tons. The compaction density measured in the first measurement is designated as PD1, and the compaction density measured in the second measurement is designated as PD2. PD1 and PD2 satisfy 0 ≤ PD2 - PD1 ≤ 0.03 g / cm³. 3 The difference between PD2 and PD1 within the aforementioned range indicates that the hard carbon material has a stable structure, reducing the possibility of structural collapse due to the release of active ions, which could block ion transport pathways. This facilitates the smooth transport of active ions during charging and discharging, and further improves the battery's specific capacity and initial coulombic efficiency. The value of PD2-PD1 can be, for example, 0 or 0.01 g / cm³. 3 0.02g / cm 3 0.03g / cm 3 The smaller the PD2-PD1 value, the more stable the sample structure, enabling it to maintain a stable structure during subsequent electrochemical processes and achieve reversible insertion / extraction of active ions.

[0065] In this application, the compaction density of hard carbon materials has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be determined by an electronic pressure testing machine (e.g., a UTM7305 type electronic pressure testing machine) in accordance with GB / T 24533-2009.

[0066] In some embodiments, the hard carbon material includes a matrix and a carbon coating layer located on at least a portion of the surface of the matrix. This facilitates improved lamellar stacking and pore structure in the hard carbon material, ensuring that the B / A ratio, water vapor adsorption capacity, and methylene blue adsorption value are within the aforementioned ranges.

[0067] In some embodiments, the matrix further comprises a porous carbon framework and phosphorus elements located within the porous carbon framework, wherein the mass percentage of phosphorus elements is 0.5 wt% to 2.5 wt% relative to the hard carbon material. This is more conducive to improving the lamellar stacking and pore structure. Exemplarily, the mass percentage of phosphorus elements is 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, 1.5 wt%, 1.75 wt%, 2.0 wt%, 2.25 wt%, 2.5 wt%, or a value between any two of these values.

[0068] In some embodiments, the coating layer comprises 1.5 wt% to 7.5 wt% of the hard carbon material by mass. This is more conducive to forming a dense coating layer, resulting in water vapor adsorption and methylene blue adsorption values ​​within the aforementioned ranges.

[0069] In some implementations, Na / Na is at a voltage of 0V to 2.5V. + Within a certain potential range, the specific charging capacity of the hard carbon material is 350 mAh / g or higher. For example, the specific charging capacity of the hard carbon material is 350 mAh / g, 370 mAh / g, 400 mAh / g, 420 mAh / g, 450 mAh / g, 470 mAh / g, 490 mAh / g, or a value within a range of any two of these values.

[0070] In some embodiments, after the hard carbon material is sintered at 1000°C for 2 hours in an inert atmosphere, the surface oxygen content measured under vacuum conditions is set as X1, and the surface oxygen content measured after exposure to air with humidity ≤2% for 30 days is set as X2, wherein X1 and X2 satisfy X2-X1≤5wt%.

[0071] Due to the high surface activity of hard carbon materials, they are easily oxidized in air, introducing a large number of oxygen-containing groups onto the surface. In this application, the surface oxygen content X1 measured under vacuum conditions is used as a benchmark, and the surface oxygen content X2 measured after 30 days of exposure to air with humidity ≤2% is compared with it. The increase in X2 compared to X1 (i.e., the increase in O due to oxidation of the hard carbon material due to its high surface activity) characterizes the level of surface activity of the hard carbon material. A higher X2-X1 value indicates a higher surface activity of the hard carbon material.

[0072] The hard carbon material of this application has X2-X1≤5wt%, indicating that it has low surface activity and low catalytic decomposition ability of electrolyte, thereby reducing the amount of active Na required to form a solid electrolyte interphase (SEI) film and improving the initial coulombic efficiency.

[0073] The surface oxygen content mentioned in this application refers to the oxygen content on the surface of a solid material. This value can be determined using conventional methods in the art. For example, referring to GB / T 33502-2017, the testing instrument can be an X-ray photoelectron spectrometer (Thermo Fisher K-Alpha). The excitation source is Al K-Alpha, the energy step is 1.0 eV, and the carbon peak is calibrated using the standard C1s (284.6 eV).

[0074] The values ​​for X2-X1 can be, for example, 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, or a range between any two of these values. Preferably, the X2-X1 of the hard carbon material is ≤2.5wt%. This indicates that the hard carbon material has lower surface activity, which is more conducive to improving the initial coulombic efficiency of the secondary battery.

[0075] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0076] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil or aluminum foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0077] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may 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).

[0078] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0079] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0080] 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, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0081] The negative electrode sheet prepared using the above-mentioned hard carbon material has high capacity.

[0082] Secondary batteries

[0083] A second aspect of this application provides a secondary battery, which will be described below with appropriate reference to the accompanying drawings. The term "secondary battery" as used herein refers to a single battery cell, a battery module, or a battery pack. These will be described separately below.

[0084] Typically, a single secondary battery cell includes a positive electrode, a negative electrode as described in the above embodiment, an electrolyte, and a separator. During battery charging and discharging, active ions, such as sodium ions, repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0085] [Positive electrode plate]

[0086] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0087] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0088] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0089] In some embodiments, the battery cell is a sodium-ion battery, and the positive electrode active material can be any positive electrode active material known in the art for use in sodium-ion batteries. As examples, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, etc., but this application is not limited to these materials; other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries can also be used. For example, as an optional technical solution in this application, the transition metal in the sodium transition metal oxide 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, 0 <x≤1。

[0090] As an optional technical approach in this application, the polyanionic compound can be a compound containing sodium ions, transition metal ions, or a tetrahedral (YO4) structure. n- A class of compounds with anionic 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; n represents (YO4). n- The valence state. Polyanionic compounds can also have sodium ions, transition metal ions, or tetrahedral (YO4) ions. n- A class of compounds containing anionic units and halide 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, and n represents (YO4). n- The valence state; the halogen can be at least one of F, Cl, and Br. Polyanionic compounds can also have sodium ions, tetrahedral (YO4) valence states. n- Anionic unit, polyhedral unit (ZO) y ) m+ And a class of compounds with optional halide anions. Y can be at least one of P, S, and Si, and n represents (YO4). n- Valence state: Z represents a transition metal, which can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; m represents (ZO) y ) m+ The valence state; the halogen can be at least one of F, Cl, and Br. Examples of polyanionic compounds include NaFePO4, Na3V2(PO4)3, NaM'PO4F (M' is one or more of V, Fe, Mn, and Ni), and Na3(VO4)2(PO4)3. y )2(PO4)2F 3-2yAt least one of the following (0≤y≤1). Prussian blue compounds can be a class of compounds containing 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. Examples of Prussian blue compounds include 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<1,0<c<1。

[0091] In some embodiments, when the battery cell is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries.

[0092] During the charging and discharging process, batteries experience the insertion / extraction and consumption of active ions (Na or K), resulting in varying molar contents of Li or Na at different discharge states. In the examples of positive electrode active materials listed in this application, the molar contents of Li or Na represent the initial state of the material, i.e., the state before material addition. As the positive electrode active material is applied to the battery system, the molar contents of Li or Na change after charge-discharge cycles. Similarly, the molar contents of oxygen listed in the examples of positive electrode active materials in this application are only theoretical values. Lattice oxygen release leads to changes in the molar contents of oxygen, resulting in fluctuations in the actual molar contents of oxygen.

[0093] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0094] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0095] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0096] [Electrolytes]

[0097] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0098] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0099] In some embodiments, when the battery cell is a sodium-ion battery, the electrolyte salt may be selected from at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium difluorosulfonamide, sodium ditrifluoromethanesulfonamide, sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluorooxalate borate, sodium dioxalate borate, sodium difluorodioxalate phosphate, and sodium tetrafluorooxalate phosphate.

[0100] In some embodiments, when the battery cell is a lithium-ion battery, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0101] In some embodiments, the solvent may 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, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0102] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0103] [Isolation membrane]

[0104] In some embodiments, the battery cell also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0105] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0106] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0107] 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.

[0108] 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.

[0109] This application 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 The example shown is a square-structured battery cell 5.

[0110] In some implementations, refer to Figure 2 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. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 through 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 a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0111] 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.

[0112] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0113] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0114] 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.

[0115] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0116] Electrical appliances

[0117] A third aspect of the embodiments of this application also provides an electrical device, and the secondary battery of this application will be described below with appropriate reference to the accompanying drawings.

[0118] The electrical device mentioned in the embodiments of this application includes the secondary battery provided in this application. The secondary battery can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0119] As the electrical device, a single battery cell, a battery module, or a battery pack can be selected according to its usage requirements.

[0120] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0121] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0122] Hard carbon materials

[0123] The fourth aspect of this application provides a hard carbon material. In the X-ray diffraction pattern of the hard carbon material, diffraction peaks are present at diffraction angles 2θ of 22°-25°. Specifically: 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 is taken as B; the intensity corresponding to the intersection point of the perpendicular line passing through the tangent point and perpendicular to the horizontal axis with the straight line L is taken as A. Then, 1 ≤ B / A ≤ 2.5, and the saturated water vapor adsorption capacity of the hard carbon material at 25°C is ≥200 cm³ / s. 3 / g, and the methylene blue adsorption value of the hard carbon material is ≤10mg / g.

[0124] In this application, by keeping the B / A value within the above range, the adsorption capacity of the hard carbon material is increased. In addition, by keeping the water vapor adsorption amount and methylene blue adsorption value within the above range, the electrolyte cannot enter the pore structure of the hard carbon material, thereby increasing the pore filling capacity and thus giving the hard carbon material a high capacity.

[0125] In some embodiments, the saturated water vapor adsorption capacity of the hard carbon material at 25°C is 350 cm⁻¹. 3 / g-390cm 3 / g.

[0126] In some embodiments, the methylene blue adsorption value of the hard carbon material is 0.1 mg / g to 4 mg / g. This further increases the pore-filling capacity of the hard carbon material.

[0127] In some embodiments, the compaction density of the hard carbon material is measured twice consecutively under a pressure of 5 tons. The compaction density measured in the first measurement is designated as PD1, and the compaction density measured in the second measurement is designated as PD2. PD1 and PD2 satisfy 0 ≤ PD2 - PD1 ≤ 0.03 g / cm³. 3 By keeping the difference between PD2 and PD1 within the aforementioned range, the structural stability of the hard carbon material is improved, which is beneficial for the transport of active ions during charging and discharging, and further conducive to improving the battery's specific capacity and initial coulombic efficiency.

[0128] In some embodiments, the hard carbon material includes a matrix and a carbon coating layer located on at least a portion of the surface of the matrix. This facilitates improved lamellar stacking and pore structure in the hard carbon material, ensuring that the B / A ratio, water vapor adsorption capacity, and methylene blue adsorption value are within the aforementioned ranges.

[0129] In some embodiments, the matrix comprises a porous carbon framework and phosphorus elements located within the porous carbon framework, wherein the phosphorus element has a mass percentage content of 0.5 wt% to 2.5 wt% relative to the hard carbon material. This is more conducive to improving the lamellar stacking and pore structure.

[0130] In some embodiments, the coating layer comprises 1.5 wt% to 7.5 wt% of the hard carbon material by mass. This is more conducive to forming a dense coating layer, resulting in water vapor adsorption and methylene blue adsorption values ​​within the aforementioned ranges.

[0131] In some implementations, Na / Na is at a voltage of 0V to 2.5V. + Within the potential range, the charging capacity of the hard carbon material is above 350 mAh / g.

[0132] In some embodiments, after the hard carbon material is sintered at 1000°C for 2 hours in an inert atmosphere, the surface oxygen content measured under vacuum conditions is set as X1, and the surface oxygen content measured after exposure to air with humidity ≤2% for 30 days is set as X2, wherein X1 and X2 satisfy X2-X1≤5wt%. By keeping the values ​​of X2-X1 within the above range, the surface activity of the hard carbon material is reduced, thereby decreasing the catalytic decomposition ability of the hard carbon material on the electrolyte. This reduces the amount of active Na required to form the solid electrolyte interphase (SEI) film, which is beneficial for further improving the first coulombic efficiency.

[0133] Preparation method of hard carbon materials

[0134] The fifth aspect of this application provides a method for preparing the hard carbon material of the fourth aspect of this application. The method comprises the following steps: an impregnation step, in which a carbon source is impregnated in a liquid containing a dopant for at least 2 hours, wherein the dopant contains phosphorus or zinc as a dopant element, and the mass percentage of phosphorus relative to the mass of the carbon source is 9.5 wt%-32.4 wt%, and the mass percentage of zinc is 14 wt%-50 wt%; a low-temperature heat treatment step, in which the material is heated at 400℃-750℃ to obtain a matrix; a kneading step, in which a mixture of a resin-based polymer material and the matrix is ​​kneaded in a kneader, wherein the mass ratio of the resin-based polymer material to the matrix is ​​(0.5-2):10; and a carbonization step.

[0135] The following sections will explain each of the above steps.

[0136] (1) Impregnation step

[0137] By thoroughly immersing the carbon source in a liquid containing dopant, the activation reaction is carried out uniformly in the subsequent low-temperature heat treatment step.

[0138] In the impregnation step, the carbon source includes hydrocarbon polymers, such as any one or more of phenolic resins, epoxy resins, unsaturated polyester resins, and furan resins. The carbon source has structural designability and low impurity content, which is more conducive to improving layer stacking and pore structure.

[0139] As a dopant, it contains phosphorus or zinc, for example, at least one selected from phosphoric acid, phosphates, polyphosphoric acid, and zinc chloride. In some embodiments, the mass percentage of phosphorus relative to the carbon source is, for example, 9.5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 32.4 wt%, etc., and any range between the two. The mass percentage of zinc relative to the carbon source is 14 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, etc., and any range between the two. In this application, the above-mentioned dopant elements are incorporated into the carbon framework structure by forming covalent bonds such as PC, PO, Zn-C, and Zn-O during subsequent low-temperature heat treatment. Since P and Zn atoms have relatively large atomic sizes, they can suppress the stacking of layers and the increase of B / A as dopants, thereby keeping the B / A ratio in the range of 1-2.5. Furthermore, by introducing the aforementioned doping elements, an appropriate amount of porous structure can be introduced, resulting in an adsorption capacity of saturated water vapor at 25℃ ≥ 200 cm⁻¹. 3 / g and the methylene blue adsorption value of the hard carbon material is ≤10mg / g.

[0140] In some embodiments, the impregnation time is 2 hours or more, for example, 2 hours to 24 hours. By keeping the impregnation time within 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 be uniformly activated to introduce an appropriate amount of porous structure so that the adsorption amount and adsorption value are within the above-mentioned specific range.

[0141] (2) Low-temperature heat treatment steps

[0142] This application achieves two goals through low-temperature heat treatment: firstly, the dopant is chemically activated to etch the raw material, introducing a porous structure into the carbon framework; secondly, the dopant and the raw material molecules undergo a chemical reaction to form covalent bonds such as PC, PO, Zn-C, and Zn-O, and P atoms / Zn atoms are incorporated into the carbon framework. Since the size of P atoms / Zn atoms is larger than that of carbon atoms, they can play a role in inhibiting the stacking of layers from the initial stage of layer formation.

[0143] The temperature for low-temperature heat treatment is between 400℃ and 750℃. For example, it can be 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, or any range between two of these. By keeping the temperature within the above range, it is more beneficial to obtain a sufficient activation effect, introduce an appropriate amount of porous structure, control the layer stacking, and reduce the B / A value.

[0144] In some embodiments, the temperature is increased to 400°C-750°C at a rate of 1-20°C / min in an N2 or Ar atmosphere and held for 1-12 hours. This is more conducive to improving the lamellar stacking and pore structure. Exemplarily, the heating 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 any range between the two.

[0145] In some embodiments, after the above-mentioned low-temperature heat treatment, crushing is performed to reduce the particle size, followed by washing to remove P-containing impurities and the like after the reaction.

[0146] (3) Kneading steps

[0147] Through the kneading step, the kneading raw material (resin-based polymer material) is uniformly coated on the surface of the matrix to form a dense organic coating layer.

[0148] In some embodiments, the kneading step involves a kneading time of 0.5 hours or more, which is more conducive to a more uniform distribution of the resin-based polymer material on the matrix surface. Exemplarily, the kneading time is 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 8 hours, 9 hours, 10 hours, and any range between these. Preferably, the kneading time is 3-9 hours.

[0149] In some embodiments, the resin-based polymer material includes at least one selected from 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.

[0150] In some embodiments, this step involves kneading a mixture containing a matrix and a resin-based polymer material, the solid content of which is between 55 wt% and 75 wt%. Exemplarily, the solid content is 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, or any range between these values. The solid content of the mixture refers to the content of the matrix and the resin-based polymer material in the mixture. By keeping the solid content within the aforementioned range, the resin-based polymer material can be more uniformly distributed on the hard carbon surface, which is more conducive to the formation of a dense coating layer, thereby controlling the methylene blue adsorption value to below 10 mg / g.

[0151] Furthermore, a resin-based polymer material to matrix mass ratio of (0.5-2):10 is more conducive to forming a dense and uniform coating layer of suitable thickness and an appropriate amount of porous structure, thereby controlling the methylene blue adsorption value. For example, the resin-based polymer material to matrix mass ratio of this application is 0.5:10, 0.7:10, 1:10, 1.2:10, 1.5:10, 1.7:10, 2:10, and any range between these two.

[0152] In some implementations, kneading is performed using a twin-screw kneader at a speed of 10-50 rpm.

[0153] (4) Carbonization step

[0154] In some embodiments, the carbonization step involves heating to 1000℃-1800℃ at a rate of 2-20℃ / min. This is more conducive to controlling the cracking process of the resin-based polymer material and reducing surface defects. Exemplarily, the heating rate of the carbonization step can be 2℃ / min, 4℃ / min, 6℃ / min, 8℃ / min, 10℃ / min, 12℃ / min, 15℃ / min, 18℃ / min, 20℃ / min, or any range between these two. Exemplarily, the carbonization temperature is 1000℃, 1200℃, 1400℃, 1600℃, 1800℃, or any range between these two. In the carbonization step, when using a phosphorus-containing dopant, at least some of the phosphorus bound to the matrix is ​​retained; when using a zinc-containing dopant, the zinc bound to the matrix is ​​almost entirely volatilized.

[0155] In some embodiments, during the carbonization step, the temperature is increased to 1100℃-1600℃ and held at a rate of 2-10℃ / min under a pressure of 10MPa or higher.

[0156] Through the aforementioned hot-press carbonization, excess H and O in the matrix carbon structure can be removed, forming a suitable Na-storage framework structure. Simultaneously, the organic matter in the coating layer undergoes thermal decomposition into carbon free radicals to repair surface defects. By applying pressure, the escape of carbon free radicals formed from the decomposition of the coating layer's organic matter can be reduced, allowing them to remain at the surface defect sites for a longer period to repair the surface. This increases the repair completion rate, thereby reducing the content of surface defects and ultimately lowering surface activity.

[0157] In this step, hot-press carbonization is performed under a pressure of 10 MPa or higher, for example, 10 MPa, 20 MPa, 30 MPa, 35 MPa, 40 MPa, 50 MPa, 60 MPa, etc., preferably 30 MPa or higher. This reduces the escape of carbon free radicals formed by the decomposition of the organic matter in the coating layer, allowing them to remain at the surface defect site for a longer period, thus improving the efficiency of surface defect repair.

[0158] Example

[0159] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0160] Example 1

[0161] Preparation of hard carbon materials:

[0162] 1) Impregnation

[0163] In a reaction vessel, 1000 g of phenolic resin (CAS No. 9003-35-4, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) as the carbon source was added to a dopant solution (807 g phosphoric acid, 85 wt%) and impregnated for 6 hours. The mass percentage of phosphorus relative to the carbon source was 21.7 wt%.

[0164] 2) Low-temperature heat treatment

[0165] The liquid from step 1) above was heated to 600°C in a tube furnace under N2 atmosphere at a heating rate of 5°C / min and held at that temperature for 8 hours. Afterwards, it was crushed and washed (to remove P-containing impurities from the reaction) to obtain the matrix.

[0166] 3) Kneading

[0167] The phenolic resin (CAS No. 9003-35-4, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) used as kneading raw material was mixed with the above matrix at a mass ratio of 1:10 and dispersed in an ethanol solution to obtain a mixture with a solid content of 65 wt%. The mixture was then added to a twin-screw kneader and kneaded at a speed of 40 rpm for 6 hours. The mixture was filtered and then vacuum dried at 80°C for 12 hours to obtain a matrix with a coating layer.

[0168] 4) Carbonization

[0169] The substrate with the coating layer was heated to 1250°C at a heating rate of 2°C / min under N2 atmosphere and held at that temperature for 4 hours to obtain hard carbon material.

[0170] Preparation of negative electrode sheet:

[0171] The above-mentioned hard carbon material, styrene-butadiene rubber (SBR) binder, sodium carboxymethyl cellulose (CMC-Na) thickener, and carbon black conductive agent are mixed in a mass ratio of 96.2:1.8:1.2:0.8 with an appropriate amount of deionized water to form a uniform negative electrode slurry.

[0172] The uniformly stirred negative electrode slurry is coated onto one side of Cu foil using a double-sided coating machine. After coating, the negative electrode sheet is prepared by drying, cold pressing, and stamping.

[0173] Fabrication of button cells:

[0174] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Sodium hexafluorophosphate (NaPF6) was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L. Then, using a metallic sodium sheet as the counter electrode and a glass fiber separator, a CR2430 coin cell was assembled with the prepared negative electrode sheet in an argon-protected glove box.

[0175] Examples 2-7

[0176] The battery was prepared in the same manner as in Example 1, except that the preparation conditions were changed according to Table 1.

[0177] Comparative Examples 1-3

[0178] The battery was prepared in the same manner as in Example 1, except that the preparation conditions were changed according to Table 1.

[0179] Tests related to hard carbon materials:

[0180] B / A Test

[0181] The hard carbon materials prepared in the above examples and comparative examples were tested using a Bruker D8 Discover X-ray diffractometer. The hard carbon materials were prepared using the flat plate method, with CuKα rays as the radiation source and a copper target as the anode target. The voltage was 40 kV, the current was 40 mA, the anti-scattering slit was 1 mm, the scanning 2θ angle range was 20°–80°, the step size was 0.01671°, the step duration was 0.24 s, and the scanning rate was 4° / min. Specifically, in the obtained X-ray diffraction pattern, a diffraction peak was found at a diffraction angle of 2θ between 22° and 25°. A straight line L was drawn that was tangent to both the first shoulder on the left and the first shoulder on the right of the diffraction peak. A tangent line M, parallel to line L and tangent to the diffraction peak, was drawn, and the intensity corresponding to the tangent point was taken as B. A perpendicular line was drawn through the tangent point and perpendicular to the horizontal axis, and the intensity corresponding to the intersection of the perpendicular line and line L was taken as A.

[0182] The above measurements were performed on the hard carbon material of Example 1, and its X-ray diffraction pattern is shown below. Figure 7 As shown. In Figure 7 In the equation, B / A = 1.42.

[0183] 25℃ saturated water vapor adsorption capacity

[0184] After drying the hard carbon material samples prepared in the above examples and comparative examples at 200℃ for 2 hours, the samples were purged with nitrogen gas carrying a constant water vapor pressure of 3.14 kPa at a flow rate of 100 mL / min using a dynamic water vapor adsorption apparatus at 25℃. The adsorption was allowed to proceed until equilibrium was reached. The final adsorption value is the saturated water vapor adsorption capacity of the sample at 25℃, expressed in cm⁻¹. 3 / g is the unit.

[0185] methylene blue adsorption value

[0186] After drying the hard carbon material samples prepared in the above examples and comparative examples at 200℃ for 2 hours, 100 mg of each sample was placed in a 100 mL Erlenmeyer flask. 10 mL of methylene blue solution (methylene blue concentration: 1.5 g / L) was added using a burette. Once the hard carbon sample was completely wetted, it was immediately placed on an electric shaker and shaken for 20 minutes. The sample was then filtered using neutral qualitative filter paper. The filtrate was placed in a cuvette with a 1 cm optical path and the absorbance was measured at 665 nm using a spectrophotometer. The absorbance was compared with that of a copper sulfate standard filter solution (4.000 g of copper sulfate pentahydrate dissolved in 1000 mL of deionized water). The volume of methylene blue test solution consumed multiplied by the concentration of methylene blue and divided by the mass of 0.1 g of hard carbon gave the methylene blue adsorption value of the hard carbon, expressed in mg / g (refer to GB / T 12496.10-1999).

[0187] PD2-PD1

[0188] The hard carbon materials prepared in the above embodiments and comparative examples were tested using a UTM7305 electronic pressure testing machine, referring to GB / T 24533-2009. Specifically, 1g of sample was weighed and added to a container with a bottom area of ​​1.327cm². 2 In the mold, the pressure was increased to 5 tons and held for 30 seconds, then the pressure was released and held for 10 seconds. The compaction density PD1 of the powder under 5 tons of pressure was recorded and calculated. Next, the sample was pressurized to 5 tons again and the same operation was performed to obtain the compaction density PD2 of the powder under 5 tons of pressure.

[0189] Determination of phosphorus (P) content in hard carbon materials

[0190] The hard carbon materials prepared in the above embodiments and comparative examples were digested according to the national standard method (LY / T 1232-1999 Determination of Total Phosphorus in Forest Soils) to oxidize all phosphorus compounds into orthophosphate. The solution was transferred to a 50 mL volumetric flask and diluted to volume, then mixed thoroughly to obtain the test solution. The test solution was diluted 10 times with deionized water (0.5 mL test solution plus 4.5 mL deionized water), and introduced into the nebulizer via an autosampler. The nebulizer was then carried by the carrier gas into a plasma flame at a temperature of 6000 K-10000 K. The components in the sample were atomized, ionized, and excited. When these excited-state particles returned to a steady state, they released a certain amount of energy (manifested as a spectrum of a certain wavelength). The specific spectral lines and intensities of phosphorus (P) were measured, and compared with a standard P solution, allowing for quantitative analysis of P 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 to a 50 mL volumetric flask, respectively. 3- A 50 mg / L solution was prepared, and a control solution with the same background ionic strength as the sample was added. The solution was then brought to volume with deionized water. The selected test wavelengths were 214.914 nm and 178.222 nm.

[0191] Battery performance test

[0192] Capacity and first coulomb efficiency

[0193] At 25°C, the coin cells prepared in the above examples and comparative examples were first discharged to 0V at a constant current density of 10mA / g, and the initial discharge capacity of the coin cells was recorded. Then, they were charged to 2.5V at a constant current density of 10mA / g, and the initial charge capacity of the coin cells was recorded. The mass of the hard carbon material in the negative electrode was calculated based on the coating weight and area of ​​the slurry during the electrode preparation process. Initial charge specific capacity = initial charge capacity / mass of hard carbon material; initial discharge specific capacity = initial discharge capacity / mass of hard carbon material; initial coulombic efficiency (%) of the coin cell = initial charge specific capacity of the coin cell / initial discharge specific capacity of the coin cell × 100%.

[0194] The results of B / A ratio, methylene blue adsorption value, water vapor adsorption capacity, PD2-PD1, and specific capacity tests of the hard carbon materials prepared in Examples 1-7 and Comparative Examples 1-3 are shown in Table 2.

[0195] Table 1:

[0196]

[0197] Table 2

[0198]

[0199] As can be seen from Table 2, by ensuring that 1 ≤ B / A ≤ 2.5 for the hard carbon material, the saturated water vapor adsorption capacity at 25℃ is ≥ 200 cm⁻¹. 3 A concentration of 1 g / g and a methylene blue adsorption value ≤10 mg / g can significantly improve the charging capacity. Furthermore, by maintaining the PD2-PD1 value within 1 g / cm³, the charging capacity can be significantly improved. 3 -0.03g / cm 3 Within this range, it is possible to further achieve excellent first-round coulomb efficiency.

[0200] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. 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, characterized in that, In the X-ray diffraction pattern of the hard carbon material, there are diffraction peaks at diffraction angles 2θ of 22°-25°. The intensity of the point of intersection of the first shoulder on the left and the first shoulder on the right of the diffraction peak (line L) and the tangent line M parallel to line L and tangent to the diffraction peak is defined as B. The intensity of the intersection of the perpendicular line passing through the tangent point and perpendicular to the horizontal axis with line L is defined as A. Therefore, 1 ≤ B / A ≤ 2.

5. The hard carbon material has a saturated water vapor adsorption capacity of ≥200 cm⁻² at 25°C. 3 / g, and the methylene blue adsorption value of the hard carbon material is ≤10mg / g.

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

3. The negative electrode sheet according to claim 1 or 2, characterized in that, 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 1-3, characterized in that, The compaction density of the hard carbon material was measured twice consecutively under a pressure of 5 tons. The compaction density measured in the first measurement is denoted as PD1, and the compaction density measured in the second measurement is denoted as PD2. The following relationship is satisfied between PD1 and PD2: 0≤PD2-PD1≤0.03g / cm 3 。 5. The negative electrode sheet according to any one of claims 1-4, characterized in that, The hard carbon material includes a matrix and a carbon coating layer located on at least a portion of the surface of the matrix.

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

7. The negative electrode sheet according to any one of claims 1-6, characterized in that, The coating layer has a mass percentage content of 1.5 wt% to 7.5 wt% relative to the hard carbon material.

8. The negative electrode sheet according to any one of claims 1-7, characterized in that, Na / Na from 0V to 2.5V + Within the potential range, the charging capacity of the hard carbon material is above 350 mAh / g.

9. The negative electrode sheet according to any one of claims 1-8, characterized in that, After the hard carbon material is sintered at 1000°C for 2 hours in an inert atmosphere, the surface oxygen content measured under vacuum conditions is set as X1, and the surface oxygen content measured after exposure to air with humidity ≤2% for 30 days is set as X2. X1 and X2 satisfy X2-X1≤5wt%.

10. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet as described in any one of claims 1-9.

11. The secondary battery according to claim 10, characterized in that, The secondary battery also includes a positive electrode sheet, wherein the positive electrode sheet comprises at least one selected from transition metal oxides, polyanionic compounds and Prussian blue compounds as the positive electrode active material.

12. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 10 or 11.

13. A hard carbon material, characterized in that, In the X-ray diffraction pattern of the hard carbon material, there are diffraction peaks at diffraction angles 2θ of 22°-25°. The intensity of the point of intersection of the first shoulder on the left and the first shoulder on the right of the diffraction peak (line L) and the tangent line M parallel to line L and tangent to the diffraction peak is defined as B. The intensity of the intersection of the perpendicular line passing through the tangent point and perpendicular to the horizontal axis with line L is defined as A. Therefore, 1 ≤ B / A ≤ 2.

5. The hard carbon material has a saturated water vapor adsorption capacity of ≥200 cm⁻² at 25°C. 3 / g, and the methylene blue adsorption value of the hard carbon material is ≤10mg / g.

14. The hard carbon material according to claim 13, characterized in that, The hard carbon material has a saturated water vapor adsorption capacity of 350 cm⁻¹ at 25°C. 3 / g-390cm 3 / g.

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

16. The hard carbon material according to any one of claims 13-15, characterized in that, The compaction density of the hard carbon material was measured twice consecutively under a pressure of 5 tons. The compaction density measured in the first measurement is denoted as PD1, and the compaction density measured in the second measurement is denoted as PD2. The following relationship is satisfied between PD1 and PD2: 0≤PD2-PD1≤0.03g / cm 3 。 17. The hard carbon material according to any one of claims 13-16, characterized in that, The hard carbon material includes a matrix and a carbon coating layer located on at least a portion of the surface of the matrix.

18. The hard carbon material according to any one of claims 13-17, characterized in that, The matrix comprises a porous carbon framework and phosphorus elements located within the porous carbon framework, wherein the mass percentage of phosphorus elements is 0.5wt%-2.5wt% relative to the hard carbon material.

19. The hard carbon material according to any one of claims 13-18, characterized in that, The coating layer has a mass percentage content of 1.5 wt% to 7.5 wt% relative to the hard carbon material.

20. The hard carbon material according to any one of claims 13-19, characterized in that, Na / Na from 0V to 2.5V + Within the potential range, the charging capacity of the hard carbon material is above 350 mAh / g.

21. The hard carbon material according to any one of claims 13-20, characterized in that, After the hard carbon material is sintered at 1000°C for 2 hours in an inert atmosphere, the surface oxygen content measured under vacuum conditions is set as X1, and the surface oxygen content measured after exposure to air with humidity ≤2% for 30 days is set as X2. X1 and X2 satisfy X2-X1≤5wt%.

22. A method for preparing a hard carbon material, characterized in that, Includes the following steps: The impregnation step involves immersing the carbon source in a liquid containing a dopant for at least 2 hours. The dopant contains phosphorus or zinc as doping elements, and the mass percentage of phosphorus relative to the mass of the carbon source is 9.5wt%-32.4wt%, and the mass percentage of zinc is 14wt%-50wt%. The low-temperature heat treatment step involves heating at 400℃-750℃ to obtain the matrix; The kneading step involves kneading a mixture containing a resin-based polymer material and the matrix in a kneader, wherein the mass ratio of the resin-based polymer material to the matrix is ​​(0.5-2):10; and Carbonization step.

23. The preparation method according to claim 22, characterized in that, The dopant includes at least one of phosphoric acid, phosphate ester, polyphosphoric acid, and zinc chloride.

24. The preparation method according to claim 22 or 23, characterized in that, The resin-based polymer material includes at least one of epoxy resin, phenolic resin, unsaturated polyester resin, and furan resin.

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

26. The preparation method according to any one of claims 22-25, characterized in that, In the kneading step, the solid content of the mixture is 55wt%-75wt%.

27. The preparation method according to any one of claims 22-26, characterized in that, In the low-temperature heat treatment step, the temperature is increased to 400℃-750℃ at a heating rate of 1-20℃ / min, and the holding time is 1 hour-12 hours.

28. The preparation method according to any one of claims 22-27, characterized in that, In the carbonization step, the temperature is increased to 1000℃-1800℃ at a heating rate of 2-20℃ / min.

29. The preparation method according to claim 28, characterized in that, In the carbonization step, the temperature is increased to 1100℃-1600℃ and held at a rate of 2-10℃ / min under a pressure of 10MPa or higher.

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