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

Abstract

The present disclosure provides a negative electrode sheet, a secondary battery, an electric device, and a hard carbon material and a preparation method therefor. After the hard carbon material is sintered and heated for 2 h at 1000°C in an inert atmosphere, the surface oxygen content measured under a vacuum condition is set as X1, and the surface oxygen content measured after 30 days of exposure in air with a humidity less than or equal to 2% is set as X2, wherein X1 and X2 satisfy: X2-X1≤5 wt%.

Description

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

[0001] Cross-reference to related applications

[0002] The present disclosure is based on a Chinese patent application No. 202410543432.0, filed on April 30, 2024, entitled "Negative electrode sheet, secondary battery, electric device, hard carbon material and preparation method thereof", and claims priority to the Chinese patent application, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

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

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

[0005] The negative active material, as an important component of the secondary battery, affects the performance and cost of the secondary battery. The negative active material includes a hard carbon material, but the hard carbon material has a high surface activity, which leads to a low first coulomb efficiency of the battery.

[0006] SUMMARY

[0007] The present disclosure is made in view of the above-mentioned problems, and aims to provide a negative electrode sheet, a preparation method thereof, a secondary battery, an electric device, a hard carbon material and a preparation method thereof. The first coulomb efficiency of the secondary battery can be improved by the present disclosure.

[0008] To achieve the above-mentioned purpose, the first aspect of the present disclosure provides a negative electrode sheet, comprising a negative current collector and a negative film layer on at least one surface of the negative current collector, the negative film layer comprising a hard carbon material, the surface oxygen content measured under vacuum conditions after sintering and heating the hard carbon material at 1000°C for 2h in an inert atmosphere is set as X1, and the surface oxygen content measured after exposing the hard carbon material in air with a humidity of ≤2% for 30 days is set as X2, X1 and X2 satisfy X2-X1≤5wt%.

[0009] In the present disclosure, by making the value of X2-X1 in the above range, the surface activity of the hard carbon material is reduced, the catalytic decomposition ability of the hard carbon material to the electrolyte is reduced, and the amount of active Na consumed in the formation of the solid electrolyte interface film (SEI) is reduced, thereby improving the first coulomb efficiency of the secondary battery.

[0010] In some embodiments, the hard carbon material has X2-X1≤2.5wt%. Thereby, the surface activity of the hard carbon material is further reduced, which is more conducive to improving the initial coulombic efficiency of the secondary battery.

[0011] In some embodiments, the hard carbon material comprises a substrate and a carbon-based coating layer on the surface of the substrate. By forming the coating layer, the surface activity can be further reduced, thereby improving the initial coulombic efficiency of the battery.

[0012] In some embodiments, the mass percentage of the coating layer relative to the hard carbon material is 1.5wt%-7wt%. By making the mass percentage of the coating layer within the above range, the surface activity is more conducive to being reduced.

[0013] In some embodiments, the hard carbon material has a compaction density of 0.70g / cm 3 -1.05g / cm 3 .

[0014] The second aspect of the present disclosure provides a secondary battery comprising the negative electrode sheet of the first aspect of the present disclosure. Thereby, the initial coulombic efficiency of the secondary battery of the present disclosure is improved.

[0015] In some embodiments, the secondary battery comprises a positive electrode sheet comprising at least one positive electrode active material selected from sodium transition metal oxides, polyanionic compounds, prussian blue compounds, and the like.

[0016] The third aspect of the present disclosure provides an electric device comprising the secondary battery of the second aspect of the present disclosure.

[0017] The electric device of the present disclosure comprises the secondary battery provided by the present disclosure, and thus has at least the same advantages as the secondary battery.

[0018] The fourth aspect of the present disclosure provides a hard carbon material, wherein after sintering heating at 1000℃ for 2h under an inert atmosphere, the surface oxygen content measured under vacuum conditions is set as X1, the surface oxygen content measured after exposure to air with a humidity of ≤2% for 30 days is set as X2, and X1 and X2 satisfy X2-X1≤5wt%.

[0019] In the present disclosure, by making the value of X2-X1 within the above range, the surface activity of the hard carbon material is reduced, the catalytic decomposition ability of the electrolyte is reduced, and the amount of active Na consumed in the formation of the solid electrolyte interface film (SEI) is further reduced, thereby improving the initial coulombic efficiency of the secondary battery.

[0020] In some embodiments, the hard carbon material has X2-X1≤ 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.

[0021] In some embodiments, the hard carbon material includes a substrate and a carbon-based coating layer on the surface of the substrate. By forming the coating layer, the surface activity can be further reduced, thereby improving the initial coulombic efficiency of the battery.

[0022] In some embodiments, the mass percentage of the coating layer with respect to the hard carbon material is 1.5wt%-7wt%. By making the mass percentage of the coating layer within the above range, it is more conducive to reducing the surface activity.

[0023] In some embodiments, the hard carbon material has a compaction density of 0.70g / cm 3 -1.05 g / cm 3 .

[0024] The fifth aspect of the present disclosure provides a method for preparing a hard carbon material, the method comprising the following steps: a pretreatment step, pre-carbonizing a carbon source to obtain a substrate; a kneading step, kneading a mixed solution in which the substrate and a resin-based polymer material are dispersed in a kneader, the kneading time being 0.5 hours or more, and the solid content of the mixed solution being 55wt%-75wt%; and a hot-pressing carbonization step, heating to 1100°C-1600°C at a heating rate of 10°C / min or less, and performing hot-pressing carbonization under a pressure of 10MPa or more, to obtain the hard carbon material.

[0025] By the preparation method of the present disclosure, the hard carbon material of the first aspect of the present disclosure can be obtained, thereby improving the initial coulombic efficiency of the secondary battery.

[0026] In some embodiments, in the kneading step, the mass ratio of the resin-based polymer material to the substrate is (0.5-2):10. This is conducive to forming a dense and uniform coating layer, reducing the number of surface defects, and improving the surface activity.

[0027] In some embodiments, in the kneading step, the resin-based polymer includes at least one of epoxy resin, phenolic resin, or furan resin. This is more conducive to forming a dense and uniform coating layer.

[0028] In some embodiments, in the hot-pressing carbonization step, the pressure is 30MPa or more. This can reduce the escape of carbon radicals formed by the decomposition of the coating layer organic matter, allowing them to stay at the surface defect position for a longer period of time, thereby improving the efficiency of surface defect repair.

[0029] In some embodiments, in the hot-pressing carbonization step, the holding time is 1 hour to 12 hours. Thus, it is more advantageous to repair defects that are easily oxidized in the previous heat treatment process and to balance the carbonization time.

[0030] In some embodiments, in the pretreatment step, a pre-carbonization treatment is performed at 600°C to 900°C. This is advantageous to reduce the amount of gas generated inside the carbon source in the subsequent hot-pressing carbonization process, thereby reducing the possibility of the surface being damaged again after the surface repair, while controlling the content of H and O elements in the matrix within an appropriate range, thereby facilitating the firm and uniform adhesion of the resin-based high molecular material to the surface of the matrix, thereby facilitating the reduction in the number of surface defects, resulting in a reduction in the surface activity.

[0031] In some embodiments, in the pretreatment step, after the pre-carbonization treatment, a crushing treatment and a deashing drying treatment are further included. The crushing treatment can reduce the particle size, thereby facilitating the improvement in the efficiency of the surface repair reaction. The deashing drying treatment can remove impurities, thereby reducing the first coulombic efficiency drop caused by the impurities. BRIEF DESCRIPTION OF DRAWINGS

[0032] FIG. 1 is a schematic view of a battery cell according to an embodiment of the disclosure.

[0033] FIG. 2 is an exploded view of the battery cell according to an embodiment of the disclosure shown in FIG. 1.

[0034] FIG. 3 is a schematic view of a battery module according to an embodiment of the disclosure.

[0035] FIG. 4 is a schematic view of a battery pack according to an embodiment of the disclosure.

[0036] FIG. 5 is an exploded view of the battery pack according to an embodiment of the disclosure shown in FIG. 4.

[0037] FIG. 6 is a schematic view of a power-using device using a secondary battery as a power source according to an embodiment of the disclosure.

[0038] REFERENCE NUMERALS:

[0039] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 battery cell; 51 case; 52 electrode assembly; 53 top cap assembly DETAILED DESCRIPTION

[0040] Hereinafter, specific embodiments of the negative electrode sheet, the secondary battery, the power-using device, the hard carbon material, and the method for manufacturing the same according to the present disclosure are described in detail with appropriate reference to the accompanying drawings. However, there are cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of matters known well, repetitive descriptions of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present disclosure, and are not intended to limit the subject matter recited in the claims.

[0041] The "range" disclosed in the present disclosure is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting one lower limit and one upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The range defined in this way is inclusive of the end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if the ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that the ranges of 60-110 and 80-120 are also contemplated. Furthermore, if the minimum range values of 1 and 2 are listed, and if the maximum range values of 3, 4, and 5 are listed, the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present disclosure, unless otherwise specified, a numerical range "a-b" represents a shorthand manner of describing the arbitrary real number combinations between a and b, where a and b are both real numbers. For example, the numerical range "0-5" indicates that all the real numbers between "0-5" have been listed herein, and "0-5" is just a shorthand notation for these numerical combinations. In addition, when it is stated that a parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and the like.

[0042] If not specifically stated, all the embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions.

[0043] If not specifically stated, all the technical features and optional technical features of the present disclosure can be combined with each other to form new technical solutions.

[0044] At present, hard carbon materials are mostly used as negative electrode materials of secondary batteries, but due to the relatively high surface activity of hard carbon materials, the catalytic decomposition ability of electrolyte is relatively strong, and more active Na needs to be consumed when SEI is formed, thereby adversely affecting the initial coulombic efficiency of the secondary battery.

[0045] Negative electrode sheet

[0046] Based on this, the first aspect of the present disclosure provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a hard carbon material. After the hard carbon material is sintered at 1000°C for 2h in an inert atmosphere, the surface oxygen content measured under vacuum conditions is denoted as X1, and the surface oxygen content measured after exposure to air with a humidity of ≤2% for 30 days is denoted as X2, and X1 and X2 satisfy X2-X1≤5wt%.

[0047] Due to the high activity of the surface of the hard carbon material, it is prone to be oxidized in air, thereby introducing a large amount of oxygen-containing groups on the surface. In the present disclosure, the surface oxygen content X2 measured after exposure to air with a humidity of ≤2% for 30 days is compared with the surface oxygen content X1 measured under vacuum conditions, and the increase of X2 from X1 (i.e., the amount of O increased due to the oxidation of the hard carbon material due to the high activity of the surface) characterizes the activity of the surface of the hard carbon material. The higher the value of X2-X1, the higher the surface activity of the hard carbon material.

[0048] The X2-X1 of the hard carbon material of the present disclosure is ≤5wt%, which indicates that the surface activity of the hard carbon material is low, and the catalytic decomposition ability of the electrolyte is small, thereby reducing the amount of active Na consumed when forming the solid electrolyte interface film (SEI), and improving the initial coulombic efficiency of the secondary battery.

[0049] The surface oxygen content mentioned in the present disclosure refers to the oxygen element content of the surface of a solid material. This value can be measured by conventional methods in the art. For example, GB / T 33502-2017 can be referred to, and the testing instrument can use an X-ray photoelectron spectrometer (Thermo Fisher K-Alpha).

[0050] For the value of X2-X1, it can be 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, or a value between any two values in the range. In some embodiments, preferably, the X2-X1 of the hard carbon material is ≤2.5wt%. Thus, it is indicated that the surface activity of the hard carbon material is lower, which is more conducive to improving the initial coulombic efficiency of the secondary battery.

[0051] In some embodiments, the hard carbon material comprises a substrate and a carbon-based coating layer on the surface of the substrate. The high surface activity of the hard carbon material is caused by the high surface defects (high active sites), and the defects can be repaired by setting a dense coating layer to reduce the surface activity, thereby improving the initial coulombic efficiency of the battery.

[0052] In some embodiments, the mass percentage content of the coating layer is 1.5wt%-7wt% relative to the hard carbon material, preferably 3.0wt%-4.5wt%. By having the mass percentage content of the coating layer in the above range, it is more conducive to reducing the surface activity.

[0053] In some embodiments, the hard carbon material has a compaction density of 0.70g / cm 3 -1.05g / cm 3 , preferably 0.80g / cm 3 -1.0g / cm 3 .

[0054] In some embodiments, the hard carbon material of the present disclosure can further satisfy at least one of the following to further improve the performance of at least one aspect of the hard carbon material.

[0055] (1) The I D / I G of the hard carbon material is in the range of 0.80-1.35, preferably the I D / I G of the hard carbon material is 0.8-1.32. Exemplarily, the I D / I G of the hard carbon material is 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.35 or any value within the range of any two of the values. Wherein I D represents the D peak intensity of the Raman spectrum at 1350±50cm -1 , and I G represents the G peak intensity of the Raman spectrum at 1580±50cm -1 . By having the I D / I G of the hard carbon material in the above range, a suitable proportion of the hard carbon surface disordered carbon can be maintained, and a certain amount of ordered carbon layer structure is conducive to improving the compaction density of the hard carbon material by carbon layer interlayer slip, and improving the energy storage density of the negative electrode.

[0056] (2) The particle size of the hard carbon material satisfies: the volume distribution particle size Dv50 is 3.0-15μm; the volume distribution particle size Dv90 is 8-30μm; and Dv50

[0057] (3) The specific surface area of the hard carbon material is 1.5m 2 / g-15m 2 / g, preferably the specific surface area is 3m 2 / g-8m 2g / cm3. By making the tap density of the hard carbon material in the above range, it is possible to obtain a suitable pore structure, and to balance the capacity per volume.

[0058] (4) The tap density of the hard carbon material is 0.4 g / cm3 3 -0.68 g / cm3 3 .

[0059] As an example, the negative electrode current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode film layer is provided on either one or both of the two opposite surfaces of the negative electrode current collector.

[0060] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, a copper foil or an aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer such as a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0061] In some embodiments, the negative electrode film layer can further optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0062] In some embodiments, the negative electrode film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0063] In some embodiments, the negative electrode film layer can further optionally include other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0064] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material including the hard carbon material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on the negative electrode current collector, and then performing processes such as drying, cold pressing, etc., to obtain the negative electrode sheet.

[0065] Secondary battery

[0066] A second aspect of the present disclosure provides a secondary battery, which will be described below with appropriate reference to the accompanying drawings.

[0067] The term "secondary battery" referred to herein means a battery cell, a battery module, or a battery pack. Each will be described below.

[0068] Generally, a secondary battery cell includes a positive electrode tab, the negative electrode tab of the above-described embodiments, an electrolyte, and a separator. During charging and discharging of the battery, active ions, such as sodium ions, are intercalated and deintercalated between the positive electrode tab and the negative electrode tab. The electrolyte functions to conduct ions between the positive electrode tab and the negative electrode tab. The separator is disposed between the positive electrode tab and the negative electrode tab and functions to prevent short circuiting between the positive and negative electrodes while allowing ions to pass through.

[0069] [Positive electrode tab]

[0070] The positive electrode tab includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, the positive electrode film layer including a positive electrode active material.

[0071] As an example, the positive electrode current collector has two opposing surfaces in the thickness direction thereof, and the positive electrode film layer is disposed on either one or both of the two opposing surfaces of the positive electrode current collector.

[0072] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0073] In some embodiments, the battery cell is a sodium ion battery, and the positive electrode active material can be any of the positive electrode active materials conventionally known in the art for use in sodium ion batteries. As an example, the positive electrode active material can include a sodium transition metal oxide, a polyanion compound, a Prussian blue compound, etc., but the present disclosure is not limited to these materials and 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 alternative technical solution of the present disclosure, in the sodium transition metal oxide, the transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide is, for example, Na x MO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < x ≤ 1.

[0074] As an alternative technical approach of the present disclosure, the polyanionic compound can be a compound having a sodium ion, a transition metal ion, and a tetrahedral (YO4) n- anion unit. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si; and n represents the valence of (YO4) n- The polyanionic compound can also be a compound having a sodium ion, a transition metal ion, a tetrahedral (YO4) n- anion unit, and a halogen anion. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si; n represents the valence of (YO4) n- ; and the halogen can be at least one of F, Cl, and Br. The polyanionic compound can also be a compound having a sodium ion, a tetrahedral (YO4) n- anion unit, a polyhedral unit (ZO y ) m+ , and an optional halogen anion. Y can be at least one of P, S, and Si; n represents the valence of (YO4) n- ; Z represents a transition metal, which can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; and m represents the valence of (ZO y ) m+ ; and the halogen can be at least one of F, Cl, and Br. The polyanionic compound can be, for example, at least one of NaFePO4, Na3V2(PO4)3, NaM’PO4F (M’ being one or more of V, Fe, Mn, and Ni), and Na3(VO y )2(PO4)2F 3-2y (0≤y≤1). The Prussian blue compound can be a compound having a sodium ion, a transition metal ion, and a cyanide ion (CN ). The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The Prussian blue compound can be, for example, Na a Me b Me’ c (CN)6, where Me and Me’ are each independently at least one of Ni, Cu, Fe, Mn, Co, and Zn, 0

[0075] In some embodiments, the battery cell can also be a lithium ion battery, and the positive active material can employ a positive active material known in the art for lithium ion batteries.

[0076] The battery will be accompanied by the deintercalation and consumption of active ions (Na) during the charging and discharging process. The molar content of Li or Na is different when the battery is discharged to different states. In the enumeration of the positive electrode active material in the present disclosure, the molar content of Li or Na is the initial state of the material, i.e. the state before feeding. When the positive electrode active material is applied to the battery system, the molar content of Li or Na will change after charging and discharging cycles.

[0077] In the enumeration of the positive electrode active material in the present disclosure, the molar content of oxygen is only the theoretical state value. The release of oxygen from the crystal lattice will cause the molar content of oxygen to change, and the actual molar content of oxygen will fluctuate.

[0078] In some embodiments, the positive electrode film layer can also optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylic ester resin.

[0079] In some embodiments, the positive electrode film layer can also optionally include a conductive agent. As an example, the conductive agent can include at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0080] In some embodiments, the positive electrode tab can be prepared by dispersing the above-mentioned components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and after processes such as drying, cold pressing, etc., the positive electrode tab can be obtained.

[0081] [Electrolyte]

[0082] The electrolyte plays a role in conducting ions between the positive electrode tab and the negative electrode tab. The present disclosure does not have specific limitations on the type of electrolyte, which can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid-state.

[0083] In some embodiments, the electrolyte uses an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0084] In some embodiments, when the battery cell is a sodium-ion battery, the electrolyte salt can be selected from at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium bisfluorosulfonylimide, sodium bis-trifluoromethylsulfonylimide, sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluoro oxalate borate, sodium difluoro oxalate borate, sodium difluoro di-oxalate phosphate, and sodium tetrafluoro oxalate phosphate.

[0085] In some embodiments, when the battery cell is a lithium ion battery, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoroboric oxalate, lithium difluoroboric bisoxalate, lithium difluorophosphoric bisoxalate, and lithium tetrafluorophosphoric oxalate.

[0086] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0087] In some embodiments, the electrolyte solution can further optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain properties of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0088] [Separator]

[0089] In some embodiments, the battery cell further includes a separator. The type of the separator is not particularly limited in the present disclosure, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0090] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0091] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator can be made into an electrode assembly through a winding process or a stacking process.

[0092] In some embodiments, the battery cell can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0093] In some embodiments, the outer package of the battery cell can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the battery cell can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be listed.

[0094] The shape of the battery cell is not particularly limited in the present disclosure, and can be cylindrical, square, or any other shape. For example, FIG. 1 is a battery cell 5 of a square structure as an example.

[0095] In some embodiments, referring to FIG. 2, the outer package can include a housing 51 and a top cover assembly 53. The housing 51 can include a bottom plate and side plates connected to the bottom plate, which enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can be arranged on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab, and the separator film can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific actual needs.

[0096] In some embodiments, the battery cell can be assembled into a battery module, and the number of battery cells contained in the battery module can be one or more, which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0097] FIG. 3 is a battery module 4 as an example. Referring to FIG. 3, in the battery module 4, a plurality of battery cells 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arbitrary arrangements can also be used. Further, the plurality of battery cells 5 can be fixed by fasteners.

[0098] Optionally, the battery module 4 can also include a housing having a receiving space, and the plurality of battery cells 5 are received in the receiving space.

[0099] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0100] FIGS. 4 and 5 are a battery pack 1 as an example. Referring to FIGS. 4 and 5, the battery pack 1 can include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be arranged on the lower box body 3 to form a closed space for receiving the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0101] Electric device

[0102] The third aspect of the present disclosure provides an electric device, and the secondary battery of the present disclosure is described below with appropriate reference to the accompanying drawings.

[0103] The power consuming device mentioned in the embodiments of the present disclosure includes the secondary battery provided by the present disclosure. The secondary battery can be used as a power source of the power consuming device, or can be used as an energy storage unit of the power consuming device. The power consuming device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0104] As the power consuming device, the battery monomer, the battery module or the battery pack can be selected according to the use requirement thereof.

[0105] FIG. 6 is a power consuming device as an example. The power consuming device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of high power and high energy density of the secondary battery for the power consuming device, the battery pack or the battery module can be used.

[0106] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and the battery monomer can be used as a power source.

[0107] Hard carbon material

[0108] The fourth aspect of the present disclosure provides a hard carbon material. After the hard carbon material is sintered and heated at 1000°C for 2h under an inert atmosphere, the surface oxygen content measured under vacuum conditions is set as X1, the surface oxygen content measured after exposure to air with humidity ≤2% for 30 days is set as X2, and X1 and X2 satisfy X2-X1≤5wt%.

[0109] In the present disclosure, by making the value of X2-X1 in the above range, the surface activity of the hard carbon material is reduced, the catalytic decomposition ability of the hard carbon material to the electrolyte is reduced, and thus the amount of active Na consumed in the formation of the solid electrolyte interface film (SEI) can be reduced, thereby improving the initial coulombic efficiency of the secondary battery.

[0110] In some embodiments, X2-X1 of the hard carbon material is ≤2.5wt%. Thus, the surface activity of the hard carbon material is further reduced, which is more conducive to improving the initial coulombic efficiency of the secondary battery.

[0111] In some embodiments, the hard carbon material includes a substrate and a carbon-based coating layer on the surface of the substrate. By forming the coating layer, the surface activity can be further reduced, thereby improving the initial coulombic efficiency of the battery.

[0112] In some embodiments, the mass percentage content of the coating layer is 1.5wt%-7wt% relative to the hard carbon material. By making the mass percentage content of the coating layer in the above range, the surface activity is more conducive to being reduced.

[0113] In some embodiments, the hard carbon material has a compaction density of 0.70 g / cm3 under a pressure of 5 tons. 3 - 1.05 g / cm3 3 .

[0114] Method for preparing hard carbon material

[0115] A fifth aspect of the present disclosure provides a method for preparing a hard carbon material, the method comprising the following steps:

[0116] A pre-treatment step, in which a carbon source is pre-carbonized to obtain a matrix;

[0117] A kneading step, in which a mixed solution in which the matrix and a resin-based polymer material are dispersed is kneaded in a kneader for more than 0.5 hours, and the solid content of the mixed solution is 55 wt% to 75 wt%; and

[0118] A hot-pressing carbonization step, in which the temperature is raised to 1100°C to 1600°C at a temperature raising rate of 10°C / min or less, and the hot-pressing carbonization is performed under a pressure of 10 MPa or more, to obtain the hard carbon material.

[0119] By the method for preparing the hard carbon material of the present disclosure, the hard carbon material of the first aspect of the present disclosure can be obtained, thereby improving the initial coulombic efficiency of the secondary battery. Hereinafter, the method for preparing the hard carbon material of the present disclosure will be described in detail.

[0120] (1) Pre-treatment step

[0121] In some embodiments, the carbon source can include pitch / coal, biomass material, polymer material, or composite material, etc. The biomass material is widely available, such as coconut shell, rice husk, bamboo, wheat chaff, straw, etc. The polymer material includes, for example, phenolic resin, epoxy resin, furan resin, etc. Alternatively, different carbon sources can also be used in combination. The present disclosure does not have a particular limitation in this regard.

[0122] In some embodiments, in the pre-treatment step, the carbon source is pre-carbonized by raising the temperature to 600°C to 900°C at a temperature raising rate of 1°C / min to 20°C / min, and the holding time is 1 hour to 12 hours, and the atmosphere is N2 or Ar. By the pre-carbonization, the carbon source is deprived of a large amount of H and O elements, and is converted into coke to form a basic skeleton structure (matrix). Thus, only a small amount of gas is generated inside the particles during the subsequent hot-pressing carbonization, thereby reducing the possibility that the location of the surface repair caused by the large amount of gas escaping from the inside of the particles to the outside of the particles is broken again.

[0123] Exemplarily, the temperature of the pre-carbonization is 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, and a range between any two of them. This is advantageous to reduce the amount of gas generated inside the particles during the subsequent hot-pressing carbonization process, thereby reducing the risk of the surface being repaired again being damaged, while controlling the content of H and O elements in the matrix within a proper range, thereby facilitating the resin-based high molecular material to be firmly and uniformly attached to the surface of the matrix, thereby reducing the number of surface defects, thereby reducing the surface activity. Preferably, the temperature of the pre-carbonization is 750°C-850°C.

[0124] In some embodiments, after the pre-carbonization treatment, a crushing treatment and a deashing drying treatment are further included. The particle size can be reduced by the crushing treatment, which is advantageous to improve the efficiency of the surface repair reaction. The impurities can be removed by the deashing drying treatment, thereby reducing the first coulomb efficiency decline caused by impurities.

[0125] The crushing treatment can adopt conventional methods in the art, for example, the pre-carbonized product is crushed into particles with a Dv50 of 4-8 μm by an air flow mill or a mechanical mill.

[0126] The deashing drying treatment can adopt conventional methods in the art, for example, washing with an acidic aqueous solution, and then drying. Exemplarily, the acidic aqueous solution contains, for example, hydrochloric acid, nitric acid, sulfuric acid, hypochlorous acid, hydrofluoric acid, or perchloric acid. The temperature of the washing is room temperature-95°C, the time is 1-12 h, and the number of repetitions is 1-5 times. The deashing drying further includes filtering, taking the filter cake, repeatedly washing with deionized water and anhydrous ethanol until the filtrate pH>6, and drying the washed filter cake in a vacuum oven at 80°C for 24 hours.

[0127] (2) kneading step

[0128] By the above-mentioned kneading step, the resin-based high molecular (raw material for surface defect repair) is uniformly coated on the surface of the matrix to form a dense organic coating layer, which provides raw material for the subsequent repair process. The above-mentioned resin-based high molecular includes at least one of epoxy resin, phenolic resin, or furan resin.

[0129] In the kneading step, the solid content of the mixed solution refers to the content of both the resin-based high molecular and the matrix as solid components in the mixed solution. The solid content of the mixed solution is 55wt%-75wt%, exemplarily, the solid content of the mixed solution is 55wt%, 60wt%, 65wt%, 70wt%, 75wt%, and a range between any two of them. By making the solid content within the above-mentioned range, the resin-based high molecular material can be more uniformly distributed on the surface of the matrix, which is advantageous to form a dense coating layer, thereby repairing the surface defects. Preferably, the solid content of the mixed solution is 60wt%-70wt%.

[0130] In the kneading step, the mass ratio of the resin-based polymer material to the substrate is (0.5-2): 10. Illustratively, the mass ratio of the resin-based polymer material to the substrate is 0.5: 10, 0.7: 10, 1: 10, 1.3: 10, 1.5: 10, 1.7: 10, 2: 10, and a range between any two of them. By having the mass ratio in the above range, it is more conducive to forming a dense and uniform coating layer, reducing the number of surface defects, and reducing the surface activity. Preferably, the mass ratio of the resin-based polymer material to the substrate is (1-1.5): 10.

[0131] In addition, the kneading time of the kneading step is 0.5 hours or more. Illustratively, the kneading time is 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, and a range between any two of them. Thus, the resin-based polymer material can be more uniformly distributed on the hard carbon surface, and it is more conducive to repairing surface defects.

[0132] (3) Hot-pressing carbonization

[0133] Through hot-pressing carbonization, excess H and O in the carbon structure of the substrate can be removed to form a suitable Na storage skeleton structure; at the same time, the organic matter of the coating layer is thermally cracked into carbon radicals to repair surface defects. Through pressurization, the escape of carbon radicals formed by cracking of the organic matter of the coating layer can be reduced, allowing them to stay at the surface defect position for a longer time to repair the surface, improving the completion rate of repair, thereby reducing the content of surface defects and further reducing the surface activity.

[0134] In this step, the temperature is raised to 1100-1600°C at a temperature rise rate of 10°C / min or less for hot-pressing carbonization. Illustratively, the temperature rise rate is 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, and a range between any two of them. By having the temperature rise rate in the above range, it is conducive to controlling the cracking rate of the resin-based polymer material, improving the repair efficiency of surface defects, and reducing the surface activity.

[0135] In this step, hot-pressing carbonization is carried out under a pressure of 10 MPa or more, for example, 10 MPa, 20 MPa, 30 MPa, 35 MPa, 40 MPa, 50 MPa, 60 MPa, etc. Thus, the escape of carbon radicals formed by cracking of the organic matter of the coating layer can be reduced, allowing them to stay at the surface defect position for a longer time to improve the efficiency of surface defect repair. Preferably, the pressure is 30 MPa or more.

[0136] In the hot-pressing carbonization step, for example, the holding time can be 1-12 hours in a N2 or Ar atmosphere.

[0137] Examples

[0138] Hereinafter, the embodiments of the present disclosure will be described. The embodiments described below are exemplary and are for the purpose of explaining the present disclosure only and are not to be understood as limiting the present disclosure. In the embodiments, the specific techniques or conditions not mentioned are performed in accordance with the techniques or conditions described in the literature in the art or in accordance with the product manual. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be obtained commercially.

[0139] Example 1

[0140] Preparation of hard carbon material:

[0141] 1) Pre-carbonization

[0142] Phenolic resin (CAS No. 9003-35-4, purchased from Shanghai Aladdin Biochem Technology Co., Ltd.) was heated to 850℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, and held for 8 hours. After that, the material was broken, dehydrated and dried to obtain a substrate.

[0143] 2) Kneading

[0144] Phenolic resin (CAS No. 9003-35-4, purchased from Shanghai Aladdin Biochem Technology Co., Ltd.) and the substrate obtained in step 1) above were dispersed into a mixed solution of water and ethanol (volume ratio of ethanol to water was 1:2) at a mass ratio of 1.2:1 to obtain a mixture, and the solid content of the mixture was 65wt%. Then, the mixture was added into a double screw kneader, and kneaded at a rotation speed of 30 rpm for 6 hours. The material was taken out and vacuum dried at 80℃ for 12h to obtain a substrate coated with resin.

[0145] 3) Hot-pressing carbonization

[0146] The substrate coated with resin obtained in step 2) above was heated to 1450℃ at a heating rate of 2℃ / min under a nitrogen atmosphere and at a pressure of 30 MPa, and held for 6 hours to obtain a hard carbon material.

[0147] Preparation of negative electrode sheet:

[0148] The hard carbon material and the binder styrene-butadiene rubber (SBR), thickening agent sodium carboxymethyl cellulose (CMC-Na), and conductive agent carbon black were mixed in a proper amount of solvent deionized water at a mass ratio of 96.2:1.8:1.2:0.8 to form a uniform negative electrode slurry.

[0149] The uniformly stirred negative electrode slurry was coated on one side of a Cu foil by a double-sided coating machine. After coating, the material was sequentially subjected to drying, cold pressing and punching to prepare a negative electrode sheet.

[0150] Fabrication of button cells:

[0151] 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.

[0152] Example 2

[0153] 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.

[0154] Example 3

[0155] 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.

[0156] Comparative Example 1

[0157] 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.

[0158] Comparative Example 2

[0159] 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.

[0160] Comparative Example 3

[0161] 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.

[0162] Comparative Example 4

[0163] 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.

[0164] Hard carbon material testing:

[0165] Tests of surface oxygen content X1 and X2

[0166] After sintering and heating the hard carbon material prepared in the above examples and comparative examples at 1000℃ for 2h under a normal pressure of N2 atmosphere, and cooling to room temperature, the sample was transferred under inert atmosphere protection, and the surface oxygen content X1 was determined by X-ray photoelectron spectroscopy (instrument model: Thermo Fisher K-Alpha) under vacuum conditions. Then, after exposing the sample to dry air (humidity ≤ 2%) at room temperature for 30 days, the surface oxygen content X2 was determined, and X2-X1 was calculated.

[0167] Battery performance determination

[0168] First coulombic efficiency of secondary battery

[0169] At 25℃, the coin cell prepared in the above examples and comparative examples was first discharged at a current density of 10mA / g to 0V, and the initial discharge capacity of the coin cell was recorded; then the coin cell was charged at a current density of 10mA / g to 2.5V, and the initial charge capacity of the coin cell was recorded. The mass of the hard carbon material in the negative electrode tab was calculated according to the coating weight and area of the slurry in the above tab preparation process. The initial charge gram capacity = initial charge capacity / mass of hard carbon material, the initial discharge gram capacity = initial discharge capacity / mass of hard carbon material, and the first coulombic efficiency of the coin cell (%) = initial charge gram capacity of the coin cell / initial discharge gram capacity of the coin cell × 100%.

[0170] The above performance determination was performed on the above examples and comparative examples, and the results are shown in Table 1.

[0171] Table 1:

[0172] As can be seen from Table 1, by making X2-X1 of the hard carbon material less than 5wt%, the first coulombic efficiency of the secondary battery can be improved. By making X2-X1 of the hard carbon material less than 2.5wt%, the first coulombic efficiency can be further improved.

[0173] It should be noted that the present disclosure is not limited to the above embodiments. The above embodiments are only examples, and embodiments having substantially the same configuration and playing the same role and effect within the scope of the technical solutions of the present disclosure are all included in the technical scope of the present disclosure. In addition, within the scope of the main idea of the present disclosure, various modifications that can be thought of by those skilled in the art, and other modes constructed by combining part of the components in the embodiments are also included in the scope of the present disclosure.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a hard carbon material, a surface oxygen content of the hard carbon material measured under vacuum after sintering at 1000℃ for 2 hours in an inert atmosphere is set as X1, and a surface oxygen content of the hard carbon material measured after exposure to air with a humidity of 2% or less for 30 days is set as X2, and the X1 and the X2 satisfy the following relationship: X2-X1≤5wt%.

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

3. The negative electrode sheet according to claim 1 or 2, wherein The hard carbon material comprises a substrate and a carbon-based coating layer on a surface of the substrate.

4. The negative electrode sheet according to claim 3, wherein The mass percentage of the coating layer with respect to the hard carbon material is 1.5wt%-7wt%.

5. The negative electrode sheet according to any one of claims 1 to 4, wherein The hard carbon material has a compaction density of 0.70 g / cm 3 - 1.05 g / cm 3 . 6.A secondary battery comprising the negative electrode sheet according to any one of claims 1-5.

7. The secondary battery according to claim 6, wherein The secondary battery comprises a positive electrode sheet comprising at least one positive electrode active material selected from sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, and the like. 8.An electric device comprising the secondary battery according to claim 6 or 7. 9.A hard carbon material, wherein a surface oxygen content of the hard carbon material measured under vacuum after sintering at 1000℃ for 2 hours in an inert atmosphere is set as X1, and a surface oxygen content of the hard carbon material measured after exposure to air with a humidity of 2% or less for 30 days is set as X2, and the X1 and the X2 satisfy the following relationship: X2-X1≤5wt%.

10. The hard carbon material of claim 9, wherein, X2-X1≤2.5wt%.

11. The hard carbon material of claim 9 or 10, wherein, The hard carbon material comprises a substrate and a carbon-based coating layer on a surface of the substrate.

12. The hard carbon material of claim 11, wherein, The mass percentage of the coating layer with respect to the hard carbon material is 1.5wt%-7wt%.

13. The hard carbon material of any one of claims 9-12, wherein, The hard carbon material has a compaction density of 0.70 g / cm 3 - 1.05 g / cm 3 . 14.A method for preparing a hard carbon material, the method comprising the following steps: a pretreatment step of performing a pre-carbonization treatment on a carbon source to obtain a substrate; a kneading step of kneading a mixed solution in which the substrate and a resin-based polymer material are dispersed in a kneader for 0.5 hours or more, the mixed solution having a solid content of 55wt%-75wt%; and a hot-press carbonization step of performing hot-press carbonization at a temperature of 1100℃-1600℃ at a temperature increasing rate of 10℃ / min or less under a pressure of 10MPa or more to obtain the hard carbon material.

15. The method of manufacturing according to claim 14, wherein, In the kneading step, the mass ratio of the resin-based polymer material to the substrate is (0.5-2):

10.

16. The hard carbon material of claim 14 or 15, wherein, The resin-based polymer comprises at least one of an epoxy resin, a phenol resin, or a furan resin.

17. The method of manufacturing according to any one of claims 14-16, wherein, In the hot-press carbonization step, the pressure is 30MPa or more.

18. The method of manufacturing according to any one of claims 14-17, wherein, In the hot-press carbonization step, the holding time is 1 hour-12 hours.

19. The method of manufacturing according to any one of claims 14-18, wherein, In the pretreatment step, the pre-carbonization treatment is performed at 600℃-900℃.

20. The method of manufacturing according to any one of claims 14-19, wherein, In the pretreatment step, after the pre-carbonization treatment, a crushing treatment and a deashing drying treatment are further included.

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