Negative electrode sheet, secondary battery, electric device, hard carbon material, and method for producing hard carbon material

Abstract

The application provides a negative electrode sheet, a secondary battery, a power utilization device, a hard carbon material and a preparation method thereof. After the hard carbon material is sintered and heated at 1000 DEG C for 2h under an inert atmosphere, the surface oxygen content measured under vacuum is set as X1, the surface oxygen content measured after exposure in air with humidity less than or equal to 2% for 30 days is set as X2, and the X1 and the X2 satisfy X2-X1 less than or equal to 5wt%. Thus, the first coulomb efficiency of the secondary battery can be improved.

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 negative electrode active material significantly impacts their performance and cost. One such material is hard carbon; however, hard carbon materials suffer from high surface activity, leading to low initial coulombic efficiency. Summary of the Invention

[0004] This application was made in view of the above-mentioned problems, and its purpose is to provide a negative electrode sheet, a method for preparing the same, a secondary battery, an electrical device, a hard carbon material, and a method for preparing the same. This application can improve the initial coulombic efficiency of a secondary battery.

[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 comprises a hard carbon material. After the hard carbon material is sintered and heated 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%.

[0006] In this application, by keeping the values ​​of X2-X1 within the above range, the surface activity of the hard carbon material is reduced, thereby decreasing its ability to catalyze the decomposition of the electrolyte. This reduces the amount of active Na required to form the solid electrolyte interphase (SEI) film, thereby improving the initial coulombic efficiency of the secondary battery.

[0007] In some embodiments, the X2-X1 of the hard carbon material is ≤2.5wt%. This further reduces the surface activity of the hard carbon material, which is more conducive to improving the initial coulombic efficiency of the secondary battery.

[0008] In some embodiments, the hard carbon material includes a matrix and a carbon-based coating layer located on the surface of the matrix. By forming the coating layer, surface activity can be further reduced, thereby improving the initial coulombic efficiency of the battery.

[0009] In some embodiments, the coating layer comprises 1.5 wt% to 7 wt% of the hard carbon material by mass. Maintaining the coating layer within this range by mass is more conducive to reducing surface activity.

[0010] In some embodiments, the compaction density of the hard carbon material under 5 tons of pressure is 0.70 g / cm³. 3 -1.05g / cm 3 .

[0011] A second aspect of this application provides a secondary battery, including a negative electrode sheet comprising the same as described in the first aspect of this application. Consequently, the initial coulombic efficiency of the secondary battery of this application is improved.

[0012] In some embodiments, the secondary battery includes a positive electrode sheet, which comprises at least one positive electrode active material selected from sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, etc.

[0013] A third aspect of this application provides an electrical device, including the secondary battery of the second aspect of this application.

[0014] 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.

[0015] The fourth aspect of this application provides a hard carbon material, wherein 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%.

[0016] In this application, by keeping the values ​​of X2-X1 within the above range, the surface activity of the hard carbon material is reduced, thereby decreasing its ability to catalyze the decomposition of the electrolyte. This reduces the amount of active Na required to form the solid electrolyte interphase (SEI) film, thereby improving the initial coulombic efficiency of the secondary battery.

[0017] In some embodiments, the X2-X1 of the hard carbon material is ≤2.5wt%. This further reduces the surface activity of the hard carbon material, which is more conducive to improving the initial coulombic efficiency of the secondary battery.

[0018] In some embodiments, the hard carbon material includes a matrix and a carbon-based coating layer located on the surface of the matrix. By forming the coating layer, surface activity can be further reduced, thereby improving the initial coulombic efficiency of the battery.

[0019] In some embodiments, the coating layer comprises 1.5 wt% to 7 wt% of the hard carbon material by mass. Maintaining the coating layer within this range by mass is more conducive to reducing surface activity.

[0020] In some embodiments, the compaction density of the hard carbon material under 5 tons of pressure is 0.70 g / cm³. 3 -1.05g / cm 3 .

[0021] The fifth aspect of this application provides a method for preparing a hard carbon material, the method comprising the following steps: a pretreatment step, wherein a carbon source is pre-carbonized to obtain a matrix; a kneading step, wherein a mixture containing the matrix and a resin-based polymer material is kneaded in a kneader for a kneading time of 0.5 hours or more, wherein the solid content of the mixture is 55wt%-75wt%; and a hot-pressing carbonization step, wherein the temperature is increased to 1100℃-1600℃ at a heating rate of less than 10℃ / min, and hot-pressing carbonization is performed under a pressure of more than 10MPa to obtain the hard carbon material.

[0022] The preparation method of this application can obtain the hard carbon material of the first aspect of this application, thereby improving the first coulombic efficiency of the secondary battery.

[0023] In some embodiments, during the kneading step, the mass ratio of the resin-based polymer to the matrix is ​​(0.5-2):10. This facilitates the formation of a dense and uniform coating layer, reduces the number of surface defects, and improves surface activity.

[0024] In some embodiments, during the kneading step, the resin-based polymer includes at least one of epoxy resin, phenolic resin, or furan resin. This is more conducive to the subsequent formation of a dense and uniform coating layer.

[0025] In some embodiments, the pressure during the hot-pressing carbonization step is 30 MPa or higher. This reduces the escape of carbon free radicals formed by the decomposition of the coating organic matter, allowing them to remain at the surface defect site for a longer period, thus improving the efficiency of surface defect repair.

[0026] In some embodiments, the holding time in the hot-pressing carbonization step is 1 to 12 hours. This is more conducive to repairing defects that are easily oxidized during the previous heat treatment process and to balancing the carbonization time.

[0027] In some embodiments, the pretreatment step involves pre-carbonization at 600°C-900°C. This helps reduce the amount of gas generated inside the carbon source during subsequent hot-pressing carbonization, thereby reducing the possibility of surface repair being damaged again. Simultaneously, controlling the H and O content in the matrix within an appropriate range facilitates the firm and uniform adhesion of the resin-based polymer material to the matrix surface, thus reducing the number of surface defects and lowering surface activity.

[0028] In some embodiments, the pretreatment step, following the pre-carbonization treatment, further includes a crushing process and a deashing and drying process. The crushing process reduces particle size, which is beneficial for improving the efficiency of the surface repair reaction. The deashing and drying process removes impurities, thereby reducing the initial coulombic efficiency drop caused by impurities. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0030] Figure 2 yes Figure 1 An exploded view of a battery cell according to one embodiment of this application is shown.

[0031] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application.

[0032] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0033] Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown.

[0034] 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.

[0035] Explanation of reference numerals in the attached figures:

[0036] 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

[0037] 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.

[0038] 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 a particular range. Ranges defined in this way 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 also expected that ranges of 60-110 and 80-120 are also included. Furthermore, if the minimum range values ​​are 1 and 2, and the maximum range values ​​are 3, 4, and 5, 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" means that all real numbers between "0-5" have been listed herein; "0-5" is merely a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer greater than or equal to 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.

[0039] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0040] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0041] Currently, hard carbon materials are mostly used as negative electrode materials for secondary batteries. However, due to the high surface activity of hard carbon materials and their strong ability to catalyze electrolyte decomposition, a large amount of active Na is required to form SEI, which adversely affects the first coulombic efficiency of secondary batteries.

[0042] Negative electrode sheet

[0043] Based on this, the first aspect of this application proposes 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. The hard carbon material is sintered at 1000°C for 2 hours under an inert atmosphere, and the surface oxygen content measured under vacuum conditions is defined as X1, and the surface oxygen content measured after exposure to air with humidity ≤2% for 30 days is defined as X2, wherein X1 and X2 satisfy X2-X1≤5wt%.

[0044] Due to the high surface activity of hard carbon materials, they are easily oxidized in air, thereby 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.

[0045] 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 of the secondary battery.

[0046] 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).

[0047] 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. In some embodiments, preferably, 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.

[0048] In some embodiments, the hard carbon material includes a matrix and a carbon-based coating layer located on the surface of the matrix. The high surface activity of hard carbon materials is due to numerous surface defects (numerous active sites). By providing a dense coating layer, these defects can be repaired, reducing surface activity and thereby improving the initial coulombic efficiency of the battery.

[0049] In some embodiments, the coating layer has a mass percentage content of 1.5 wt% to 7 wt% relative to the hard carbon material, preferably 3.0 wt% to 4.5 wt%. Maintaining the mass percentage content of the coating layer within the above range is more conducive to reducing surface activity.

[0050] In some embodiments, the compaction density of the hard carbon material under 5 tons of pressure is 0.70 g / cm³. 3 -1.05g / cm 3 Preferably, it is 0.80 g / cm³. 3 -1.0g / cm 3 .

[0051] In some embodiments, the hard carbon material of this application may further satisfy at least one of the following to further improve at least one aspect of the performance of the hard carbon material.

[0052] (1) The hard carbon material I D / I G Preferably, within the range of 0.80-1.35, the I of the hard carbon material D / I G The value is 0.8-1.32. For example, the I of the hard carbon material... D / I G It can be any value within the range of 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.35, or any two values. Where I... D This indicates that the Raman spectrum is at 1350±50 cm⁻¹ -1 D peak intensity at I G This indicates that the Raman spectrum is at 1580±50 cm⁻¹. -1 The intensity of the G peak at that location. By making the I of the hard carbon material... D / I G Within the aforementioned range, maintaining a suitable proportion of disordered carbon on the hard carbon surface and a certain amount of ordered carbon layered structure can help improve the compaction density of the hard carbon material through interlayer slippage of carbon layers, thereby increasing the negative electrode energy storage density.

[0053] (2) The particle size of the hard carbon material satisfies the following conditions: the volume distribution particle size Dv50 is 3.0-15μm; the volume distribution particle size Dv90 is 8-30μm; and Dv50 < Dv90. The particle size of the hard carbon material satisfies the above combination, which is more conducive to the compaction density of the negative electrode sheet.

[0054] (3) The specific surface area of ​​the hard carbon material is 1.5 m². 2 / g-15m 2 / g, preferably, the specific surface area is 3m². 2 / g-8m 2 / g. By ensuring that the specific surface area of ​​the hard carbon material is within the above-mentioned range, it is beneficial to obtain a suitable pore structure and to balance the specific capacity.

[0055] (4) The tap density of the hard carbon material is 0.4 g / cm³. 3 -0.68g / cm 3 .

[0056] 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.

[0057] 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.).

[0058] 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).

[0059] 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.

[0060] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0061] 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.

[0062] Secondary batteries

[0063] The second aspect of this application provides a secondary battery, which will be described below with appropriate reference to the accompanying drawings.

[0064] The term "secondary battery" used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0065] 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.

[0066] [Positive electrode plate]

[0067] 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.

[0068] 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.

[0069] 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.).

[0070] 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。

[0071] 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 3-2y At 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。

[0072] In some embodiments, the battery cell may also be a lithium-ion battery, and the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries.

[0073] During the charging and discharging process, the active ions (Na) undergo insertion / extraction and consumption, resulting in different molar contents of Li or Na at different discharge states. In the examples of positive electrode active materials in this application, the molar contents of Li or Na refer to the initial state of the material, i.e., the state before feeding. When the positive electrode active material is applied to the battery system, the molar contents of Li or Na will change after charge-discharge cycles.

[0074] In the examples of positive electrode active materials in this application, the molar content of oxygen is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of oxygen will fluctuate.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] [Electrolytes]

[0079] 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.

[0080] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] [Isolation membrane]

[0086] 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.

[0087] 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.

[0088] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0096] 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.

[0097] 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.

[0098] Electrical appliances

[0099] The third aspect of this application provides an electrical device, and the secondary battery of this application will be described below with appropriate reference to the accompanying drawings.

[0100] 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.

[0101] As the electrical device, a single battery cell, a battery module, or a battery pack can be selected according to its usage requirements.

[0102] 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.

[0103] 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.

[0104] Hard carbon materials

[0105] The fourth aspect of this application provides a hard carbon material. After sintering the hard carbon material at 1000°C for 2 hours in an inert atmosphere, the surface oxygen content measured under vacuum conditions is designated as X1, and the surface oxygen content measured after exposure to air with humidity ≤2% for 30 days is designated as X2, wherein X1 and X2 satisfy X2-X1≤5wt%.

[0106] In this application, by keeping the values ​​of X2-X1 within the above range, the surface activity of the hard carbon material is reduced, thereby decreasing its ability to catalyze the decomposition of the electrolyte. This reduces the amount of active Na required to form the solid electrolyte interphase (SEI) film, thereby improving the initial coulombic efficiency of the secondary battery.

[0107] In some embodiments, the X2-X1 of the hard carbon material is ≤2.5wt%. This further reduces the surface activity of the hard carbon material, which is more conducive to improving the initial coulombic efficiency of the secondary battery.

[0108] In some embodiments, the hard carbon material includes a matrix and a carbon-based coating layer located on the surface of the matrix. By forming the coating layer, surface activity can be further reduced, thereby improving the initial coulombic efficiency of the battery.

[0109] In some embodiments, the coating layer comprises 1.5 wt% to 7 wt% of the hard carbon material by mass. Maintaining the coating layer within this range by mass is more conducive to reducing surface activity.

[0110] In some embodiments, the compaction density of the hard carbon material under 5 tons of pressure is 0.70 g / cm³. 3 -1.05g / cm 3 .

[0111] Preparation method of hard carbon materials

[0112] The fifth aspect of this application provides a method for preparing a hard carbon material, the method comprising the following steps:

[0113] The pretreatment step involves pre-carbonizing the carbon source to obtain the matrix;

[0114] The kneading step involves kneading the mixture containing the matrix and resin-based polymer material in a kneader for at least 0.5 hours, wherein the solid content of the mixture is 55wt%-75wt%; and

[0115] The hot-press carbonization step involves heating the material to 1100℃-1600℃ at a rate of less than 10℃ / min and then performing hot-press carbonization under a pressure of more than 10MPa to obtain hard carbon material.

[0116] The preparation method of this application enables the acquisition of the hard carbon material of the first aspect of this application, thereby improving the initial coulombic efficiency of secondary batteries. The preparation method described above will be explained in detail below.

[0117] (1) Pretreatment steps

[0118] In some embodiments, the carbon source may include bitumen / coal, biomass materials, polymer materials, or composite materials. Biomass materials are widely available, such as coconut shells, rice husks, bamboo, wheat husks, and straw. Polymer materials include, for example, phenolic resins, epoxy resins, and furan resins. Optionally, different carbon sources can be used in combination. This invention does not impose any particular limitations on this.

[0119] In some embodiments, during the pretreatment step, the carbon source is pre-carbonized at a heating rate of 1°C / min-20°C / min to 600°C-900°C, held for 1-12 hours in an atmosphere of N2 or Ar. This pre-carbonization process removes a large amount of H and O elements from the carbon source, transforming it into coke and forming a basic framework structure (matrix). This ensures that only trace amounts of small-molecule gas are generated inside the particles during subsequent hot-pressing carbonization, reducing the likelihood of large amounts of gas escaping from inside the particles to the outside, thus preventing re-cracking of the repaired surface.

[0120] For example, the pre-carbonization temperature is 600℃, 650℃, 700℃, 750℃, 800℃, 850℃, 900℃, or any range between two of these. This helps reduce the amount of gas generated inside the particles during subsequent hot-pressing carbonization, thereby reducing the risk of surface repair being damaged again. Simultaneously, controlling the H and O content in the matrix within an appropriate range facilitates the firm and uniform adhesion of the resin-based polymer material to the matrix surface, thus reducing the number of surface defects and consequently lowering surface activity. Preferably, the pre-carbonization temperature is 750℃-850℃.

[0121] In some embodiments, the pre-carbonization treatment is followed by a crushing process and a deashing and drying process. The crushing process reduces particle size, which is beneficial for improving the efficiency of the surface repair reaction. The deashing and drying process removes impurities, thereby reducing the initial decrease in coulombic efficiency caused by impurities.

[0122] The crushing process can be carried out using conventional methods in the art, such as crushing the pre-carbonized product into particles with a Dv50 of 4μm-8μm by air jet milling or mechanical milling.

[0123] The deashing and drying process can be performed using conventional methods in the art, such as washing with an acidic aqueous solution followed by drying. Exemplarily, the acidic aqueous solution contains, for example, hydrochloric acid, nitric acid, sulfuric acid, hypochlorous acid, hydrofluoric acid, or perchloric acid. The washing temperature is between room temperature and 95°C, the washing time is 1-12 hours, and the process is repeated 1-5 times. The deashing and drying process also includes filtration, taking the filter cake, repeatedly washing it with deionized water and anhydrous ethanol until the pH of the filtrate is >6, and then drying the washed filter cake in a vacuum oven at 80°C for 24 hours.

[0124] (2) Kneading steps

[0125] Through the above kneading steps, the resin-based polymer (raw material for surface defect repair) is uniformly coated onto the substrate surface, forming a dense organic coating layer, providing raw materials for the subsequent repair process. The aforementioned resin-based polymer includes at least one of epoxy resin, phenolic resin, or furan resin.

[0126] In the kneading step, the solid content of the mixture refers to the content of both the resin polymer and the matrix as solid components in the mixture. The solid content of the mixture is 55wt%-75wt%, exemplarily 55wt%, 60wt%, 65wt%, 70wt%, 75wt%, or any range between the two. By keeping the solid content within the above range, the resin polymer material can be more uniformly distributed on the matrix surface, which is beneficial for forming a dense coating layer, thereby repairing surface defects. Preferably, the solid content of the mixture is 60wt%-70wt%.

[0127] In the kneading step described above, the mass ratio of the resin-based polymer material to the matrix is ​​(0.5-2):10. Exemplarily, the mass ratio of the resin-based polymer material to the matrix is ​​0.5:10, 0.7:10, 1:10, 1.3:10, 1.5:10, 1.7:10, 2:10, or any range between these values. Maintaining the mass ratio within the above ranges is more conducive to forming a dense and uniform coating layer, reducing the number of surface defects, and lowering surface activity. Preferably, the mass ratio of the resin-based polymer material to the matrix is ​​(1-1.5):10.

[0128] Furthermore, the kneading time in the kneading step is 0.5 hours or more. For example, the kneading time can be 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or any range between these. This allows the resin-based polymer material to be more evenly distributed on the hard carbon surface, which is more conducive to the repair of surface defects.

[0129] (3) Hot pressing carbonization

[0130] Hot-press carbonization removes excess H and O from the matrix carbon structure, 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. Applying pressure reduces the escape of carbon free radicals generated from the decomposition of the coating layer's organic matter, 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.

[0131] In this step, hot-press carbonization is performed by heating to 1100℃-1600℃ at a heating rate of less than 10℃ / min. Exemplarily, the heating rate is 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, or any range between these. By keeping the heating rate within the above range, it is beneficial to control the cracking rate of the resin-based polymer material, improve the repair efficiency of surface defects, and reduce surface activity.

[0132] 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. 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. Preferably, the pressure is 30 MPa or higher.

[0133] In the hot-press carbonization step, for example, the holding time can be 1-12 hours in a N2 or Ar atmosphere.

[0134] Example

[0135] 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.

[0136] Example 1

[0137] Preparation of hard carbon materials:

[0138] 1) Pre-carbonization

[0139] Phenolic resin (CAS No. 9003-35-4, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) was heated to 850℃ at a heating rate of 5℃ / min under a nitrogen atmosphere and held at that temperature for 8 hours. After that, it was crushed, deashed and dried to obtain the matrix.

[0140] 2) Kneading

[0141] Phenolic resin (CAS No. 9003-35-4, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) and the matrix obtained in step 1) above were dispersed in a mixed solution of water and ethanol (volume ratio of ethanol to water is 1:2) at a mass ratio of 1.2:1 to obtain a mixture with a solid content of 65wt%. The mixture was then added to a twin-screw kneader and kneaded at a speed of 30 rpm for 6 hours. The material was then removed and vacuum dried at 80℃ for 12 hours to obtain the resin-coated matrix.

[0142] 3) Hot pressing carbonization

[0143] The resin-coated matrix obtained by kneading in step 2) above is heated to 1450°C at a heating rate of 2°C / min under N2 atmosphere and pressure of 30MPa, and held at that temperature for 6 hours to obtain hard carbon material.

[0144] Preparation of negative electrode sheet:

[0145] The above-mentioned hard carbon material, styrene-butadiene rubber (SBR) binder, sodium carboxymethyl cellulose (CMC-Na) thickener, and carbon black conductive agent are mixed thoroughly in an appropriate amount of deionized water at a mass ratio of 96.2:1.8:1.2:0.8 to form a uniform negative electrode slurry.

[0146] 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.

[0147] Fabrication of button cells:

[0148] 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.

[0149] Example 2

[0150] The button cell was prepared using a method similar to that in Example 1, except that the kneading time for the hard carbon material was 4 hours and the hot-pressing carbonization pressure was 20 MPa.

[0151] Example 3

[0152] The button cell was prepared using a method similar to that in Example 1, except that the kneading time for the hard carbon material was 0.5 hours and the hot-pressing carbonization pressure was 10 MPa.

[0153] Comparative Example 1

[0154] The button cell was prepared using a method similar to that in Example 1, except that the kneading time in the preparation of the hard carbon material was 0.1 hours.

[0155] Comparative Example 2

[0156] The coin cells were prepared using a method similar to that in Example 1, except that the solid content of the mixture was 30 wt% in the preparation of the hard carbon material.

[0157] Comparative Example 3

[0158] The coin cell was prepared using a method similar to that in Example 1, except that hot-press carbonization was performed at a heating rate of 25°C / min during the preparation of the hard carbon material.

[0159] Comparative Example 4

[0160] The coin cell was prepared using a method similar to that in Example 1, except that hot-press carbonization was performed at a pressure of 1 MPa during the preparation of the hard carbon material.

[0161] Hard carbon material testing:

[0162] Tests of surface oxygen content X1 and X2

[0163] The hard carbon materials prepared in the above examples and comparative examples were sintered at 1000°C for 2 hours under a normal pressure N2 atmosphere, then cooled to room temperature and transferred under an inert atmosphere. The surface oxygen content X1 was measured using X-ray photoelectron spectroscopy (Thermo Fisher K-Alpha instrument) under vacuum conditions. The samples were then exposed to dry air (humidity ≤2%) at room temperature for 30 days, and the surface oxygen content X2 was measured. X2-X1 was then calculated.

[0164] Battery performance testing

[0165] First coulombic efficiency of secondary battery

[0166] 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%.

[0167] The above performance measurements were performed on the above embodiments and comparative examples, and the results are shown in Table 1.

[0168] Table 1:

[0169]

[0170] As shown in Table 1, the initial coulombic efficiency of the secondary battery can be improved by keeping the X2-X1 content of the hard carbon material below 5 wt%. The initial coulombic efficiency can be further improved by keeping the X2-X1 content of the hard carbon material below 2.5 wt%.

[0171] 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, After the hard carbon material is sintered at 1000℃ 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 the following relationship: X2-X1≤5 wt%.

2. The negative electrode sheet according to claim 1, characterized in that, X2-X1≤2.5 wt%.

3. The negative electrode sheet according to claim 1 or 2, characterized in that, The hard carbon material includes a matrix and a carbon-based coating layer located on the surface of the matrix.

4. The negative electrode sheet according to claim 3, characterized in that, The coating layer has a mass percentage of 1.5 wt% to 7 wt% relative to the hard carbon material.

5. The negative electrode sheet according to any one of claims 1, 2, and 4, characterized in that, The compaction density of the hard carbon material under 5 tons of pressure is 0.70 g / cm³. 3 -1.05 g / cm 3 .

6. The negative electrode sheet according to claim 3, characterized in that, The compaction density of the hard carbon material under 5 tons of pressure is 0.70 g / cm³. 3 -1.05 g / cm 3 .

7. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet as described in any one of claims 1-6.

8. The secondary battery according to claim 7, characterized in that, The secondary battery includes a positive electrode sheet, which comprises at least one positive electrode active material selected from sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds.

9. An electrical device, characterized in that, Includes the secondary battery as described in claim 7 or 8.

10. A hard carbon material, characterized in that, After the hard carbon material is sintered at 1000℃ 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 the following relationship: X2-X1≤5 wt%.

11. The hard carbon material according to claim 10, characterized in that, X2-X1≤2.5 wt%.

12. The hard carbon material according to claim 10 or 11, characterized in that, The hard carbon material includes a matrix and a carbon-based coating layer located on the surface of the matrix.

13. The hard carbon material according to claim 12, characterized in that, The coating layer has a mass percentage of 1.5 wt% to 7 wt% relative to the hard carbon material.

14. The hard carbon material according to any one of claims 10, 11, and 13, characterized in that, The compaction density of the hard carbon material under 5 tons of pressure is 0.70 g / cm³. 3 -1.05 g / cm 3 .

15. The hard carbon material according to claim 12, characterized in that, The compaction density of the hard carbon material under 5 tons of pressure is 0.70 g / cm³. 3 -1.05 g / cm 3 .

16. A method for preparing a hard carbon material, characterized in that, The method includes the following steps: The pretreatment step involves pre-carbonizing the carbon source to obtain the matrix; The kneading step involves kneading the mixture containing the matrix and resin-based polymer material in a kneader for at least 0.5 hours. The solid content of the mixture is 55 wt%-75 wt%. The hot-press carbonization step involves heating the material to 1100℃-1600℃ at a rate of less than 10℃ / min and then performing hot-press carbonization under a pressure of more than 10 MPa to obtain hard carbon material.

17. The preparation method according to claim 16, characterized in that, In the kneading step, the mass ratio of the resin-based polymer material to the matrix is ​​(0.5-2):

10.

18. The preparation method according to claim 16 or 17, characterized in that, The resin-based polymer includes at least one of epoxy resin, phenolic resin, or furan resin.

19. The preparation method according to claim 16 or 17, characterized in that, In the hot-press carbonization step, the pressure is above 30 MPa.

20. The preparation method according to claim 18, characterized in that, In the hot-press carbonization step, the pressure is above 30 MPa.

21. The preparation method according to claim 16, 17 or 20, characterized in that, In the hot-pressing carbonization step, the holding time is 1 hour to 12 hours.

22. The preparation method according to claim 18, characterized in that, In the hot-pressing carbonization step, the holding time is 1 hour to 12 hours.

23. The preparation method according to claim 16, 17, 20 or 22, characterized in that, In the pretreatment step, precarbonization is performed at 600℃-900℃.

24. The preparation method according to claim 18, characterized in that, In the pretreatment step, precarbonization is performed at 600℃-900℃.

25. The preparation method according to claim 16, 17, 20, 22 or 24, characterized in that, The pretreatment step, after the precarbonization treatment, also includes crushing and deashing drying.

26. The preparation method according to claim 18, characterized in that, The pretreatment step, after the precarbonization treatment, also includes crushing and deashing drying.

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