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

Abstract

Provided are a negative electrode sheet, a secondary battery, an electric device, and a hard carbon material and a preparation method therefor. The negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a hard carbon material, and in the solid-state 23Na NMR spectrum of the hard carbon material, a characteristic peak is present when the δ displacement is within the range of 400 ppm to 1100 ppm.

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 and claims priority to Chinese Patent Application No. 202410538366.8, filed on April 30, 2024, entitled “Negative electrode sheet, secondary battery, electric device, hard carbon material and preparation method thereof”, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure relates to the technical field of sodium ion 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, with the continuous development and expansion of the new energy industry, the demand for lithium ion batteries has been increasing year by year. Under this background, the consumption of lithium resources leads to a rapid increase in the cost of lithium ion batteries, which is not conducive to the long-term development of the new energy industry. Sodium ion batteries can share part of the supply and demand pressure. Due to the advantages of sodium in resources and cost, sodium ion batteries have become an important development direction of energy storage batteries. Carbon negative electrode materials play a role in storing sodium during the charging process of sodium ion batteries and are an important part of sodium ion batteries. Hard carbon has ordered carbon microcrystals, rich micro-nano pore defects and surface defects, has a large interlayer spacing and rich pore structure, and has a relatively stable structure during sodium ion deintercalation. Hard carbon is one of the mainstream negative electrode active materials in this field due to its excellent overall performance, wide applicability and relatively low price.

[0005] Sodium ion batteries, like lithium ion batteries, have the same problem of consuming sodium ions due to the formation of SEI film during charging and discharging, resulting in a decrease in the first coulomb efficiency and a significant decrease in battery performance. Therefore, finding a solution to supplement sodium in the negative electrode of sodium ion batteries is of great significance in the field.

[0006] SUMMARY

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

[0008] To achieve the above-mentioned purpose, the first aspect of the present disclosure provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a hard carbon material. The hard carbon material has a self-sodium supplement effect and improved first coulomb efficiency. 23The Na solid-state NMR spectrum has a characteristic peak with a δ shift in the range of 400-1100 ppm.

[0009] The above characteristic peak of the hard carbon material provided by the present disclosure reflects that the hard carbon material has Na in a cluster-sized metalloid state, which is beneficial for realizing self-supply of Na when the hard carbon material is used as a negative electrode material, thereby improving the initial coulombic efficiency.

[0010] In some embodiments, the hard carbon material has a total Na element content of 0.5-10 wt% based on the weight of the hard carbon material. 23 The Na solid-state NMR spectrum has a characteristic peak with a δ shift in the range of 500-900 ppm. This is more beneficial for improving the activity of Na in a metalloid state in the hard carbon material, thereby improving the Na supply effect.

[0011] In some embodiments, the hard carbon material has a coating layer. This is beneficial for protecting Na in a metalloid state in the hard carbon material and reducing side reactions, thereby further improving the initial coulombic efficiency.

[0012] In some embodiments, the hard carbon material has a total Na element content of 0.5-10 wt% based on the weight of the hard carbon material.

[0013] In some embodiments, the hard carbon material has a total Na element content of 5-9.5 wt% based on the weight of the hard carbon material. This is more beneficial for forming Na in a cluster-sized metalloid state from sufficient Na elements in the hard carbon material, thereby facilitating Na supply.

[0014] In some embodiments, the hard carbon material has a surface Na element content of ≤0.1 wt% based on the weight of the hard carbon material. This is more beneficial for protecting Na in a metalloid state in the hard carbon material and reducing side reactions, thereby further improving the initial coulombic efficiency.

[0015] The second aspect of the present disclosure also provides a secondary battery comprising the negative electrode tab according to any of the above embodiments.

[0016] The negative electrode film layer in the secondary battery of the present disclosure comprises the negative electrode tab of the present disclosure, and the hard carbon material can realize self-supply of Na when used as a negative electrode material, thereby improving the initial coulombic efficiency.

[0017] In some embodiments, the secondary battery is a sodium-ion battery.

[0018] In some embodiments, the secondary battery further comprises a positive electrode tab, and the positive electrode tab comprises at least one selected from transition metal oxides, polyanion compounds, and Prussian blue compounds as a positive electrode active material.

[0019] The third aspect of the present disclosure provides a power consumption device comprising the secondary battery of the second aspect of the present disclosure.

[0020] The power consumption 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.

[0021] The fourth aspect of the present disclosure provides a hard carbon material. The hard carbon material has 23 The characteristic peak in the Na solid-state NMR spectrum is in the range of 400-1100 ppm.

[0022] The above characteristic peak of the hard carbon material provided by the present disclosure indicates that the hard carbon material has Na in the cluster size of the metalloid state, which is beneficial to realize self-supply of Na when the hard carbon material is used as a negative electrode material, thereby improving the first coulombic efficiency.

[0023] In some embodiments, the hard carbon material has 23 The characteristic peak in the Na solid-state NMR spectrum is in the range of 500-900 ppm. This is more beneficial to improve the activity of Na in the metalloid state in the hard carbon material, thereby improving the Na supply effect.

[0024] In some embodiments, the hard carbon material has a coating layer. This is beneficial to protect Na in the metalloid state in the hard carbon material and reduce side reactions, thereby further improving the first coulombic efficiency.

[0025] In some embodiments, the total Na element content of the hard carbon material is 0.5wt%-10wt% based on the weight of the hard carbon material. This is beneficial to sufficient Na supply of the hard carbon material, and the Na element content is not too high to form metal particles that cannot release Na.

[0026] In some embodiments, the total Na element content of the hard carbon material is 5wt%-9.5wt% based on the weight of the hard carbon material. This is more beneficial to form Na in the cluster size of the metalloid state in the hard carbon material from sufficient Na element, thereby facilitating Na supply.

[0027] In some embodiments, the surface Na element content is ≤0.1wt% based on the weight of the hard carbon material. This is more beneficial to protect Na in the metalloid state in the hard carbon material and reduce side reactions, thereby further improving the first coulombic efficiency.

[0028] The fifth aspect of the present disclosure also provides a preparation method of a hard carbon material. The method comprises: carbonizing a carbon source in an inert atmosphere, wherein the carbon source comprises a material with at least part of the side groups being Na ion-containing groups.

[0029] The present disclosure provides a method for preparing a hard carbon material. The method can form cluster-sized metalloid Na in the hard carbon material by carbonizing a carbon source in an inert gas. The hard carbon material can be used as an anode material to achieve self-supply of Na, thereby improving the initial coulombic efficiency.

[0030] In some embodiments, the Na ion content of the carbon source is 3wt%-15wt% based on the weight of the carbon source. The Na ion content of the carbon source in the above range can provide sufficient Na element for the final hard carbon material, and a suitable Na ion content in the carbon source is more conducive to the formation of cluster-sized metalloid Na.

[0031] In some embodiments, the Na ion content of the carbon source is 5wt%-13wt% based on the weight of the carbon source. This is more conducive to the formation of a suitable amount of metalloid Na in the hard carbon material, thereby more fully supplementing Na.

[0032] In some embodiments, the carbon source includes at least one of sodium lignosulfonate, cellulose sodium, and a composite material containing sodium lignosulfonate and / or cellulose sodium. The carbon source is more conducive to the formation of cluster-sized metalloid Na, thereby improving the activity of metalloid Na in the hard carbon material and improving the Na supplementing effect.

[0033] In some embodiments, after carbonization, the preparation method further includes coating by chemical vapor deposition (CVD) in a mixed gas of inert gas and organic compound gas. The formation of a coating layer on the surface of the hard carbon is conducive to reducing the exposure of sodium in the hard carbon material, improving the stability of the material and reducing the surface activity of the material. The CVD method is conducive to reducing side reactions during the coating process, and the formed coating layer more fully protects the metalloid Na in the hard carbon material, thereby further improving the initial coulombic efficiency.

[0034] In some embodiments, the coating is carried out at a temperature of 700°C-850°C. This temperature is conducive to forming a dense coating layer that protects the metalloid Na in the hard carbon material, and is also more conducive to reducing the escape of sodium formed by reduction.

[0035] In some embodiments, the volume fraction of the organic compound gas in the mixed gas is 2vol%-35vol%. The content of the organic compound gas used to form the coating layer in the mixed gas is conducive to a suitable amount of organic compound gas forming a carbon coating layer of a suitable thickness outside the hard carbon particles, thereby fully protecting the metalloid sodium in the hard carbon material.

[0036] In some embodiments, carbonization and coating are continuously carried out in the same reactor. This is more conducive to fully protecting the metalloid sodium in the hard carbon material during the preparation process and reducing the risk of material exposure to air during the preparation process.

[0037] In some embodiments, the temperature of carbonization is 600-850°C. The temperature of carbonization is within the above range, which is beneficial for both sufficient decomposition of carbon source to form suitable hard carbon structure and reduction of sodium vaporization escape formed by reduction.

[0038] In some embodiments, before carbonization, the preparation method further comprises a crushing treatment. Through the crushing treatment, the particle size can be reduced, which is more beneficial to the efficiency of subsequent carbonization. BRIEF DESCRIPTION OF DRAWINGS

[0039] FIG. 1 is a schematic view of a battery cell according to an embodiment of the present disclosure.

[0040] FIG. 2 is an exploded view of the battery cell shown in FIG. 1 according to an embodiment of the present disclosure.

[0041] FIG. 3 is a schematic view of a battery module according to an embodiment of the present disclosure.

[0042] FIG. 4 is a schematic view of a battery pack according to an embodiment of the present disclosure.

[0043] FIG. 5 is an exploded view of the battery pack shown in FIG. 4 according to an embodiment of the present disclosure.

[0044] FIG. 6 is a schematic view of an electric device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0045] FIG. 7 is a Na solid-state NMR spectrum of a hard carbon material prepared in Example 2 of the present disclosure. 23 Na solid-state NMR spectrum.

[0046] FIG. 8 is a Na solid-state NMR spectrum of a hard carbon material prepared in Example 6 of the present disclosure. 23 Na solid-state NMR spectrum.

[0047] BRIEF DESCRIPTION OF DRAWINGS 1: battery pack; 2: upper case; 3: lower case; 4: battery module; 5: battery cell; 51: case; 52: electrode assembly; 53: top cover assembly DETAILED DESCRIPTION

[0048] Hereinafter, embodiments of the negative electrode sheet, the secondary battery, the electric device, the hard carbon material, and the preparation method thereof according to the present disclosure are specifically disclosed in detail with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters that are well known, repeated description of actually 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. In addition, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present disclosure, and are not intended to limit the subject matter recited in the claims.

[0049] The ranges disclosed herein are defined by their lower and upper limit, and any given range is defined by selecting a lower limit and an upper limit, the selected lower limit and upper limit defining the boundaries of a particular range. Ranges defined by such limits are inclusive of the end values and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if a range of 60-120 and a range of 80-110 are listed, it is understood that a range of 60-110 and a range of 80-120 are also contemplated. Furthermore, if a minimum range value of 1 and 2 are listed, and if a maximum range value of 3, 4, and 5 are listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present disclosure, unless otherwise stated, a numerical range "a-b" indicates a shorthand way of describing each and every intervening real number, a and b, between the upper and lower boundary of that range. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, "0-5" is merely a shorthand way of describing these numerical combinations. Also, when a parameter is stated to be an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0050] Unless otherwise specified, all embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions.

[0051] Unless otherwise specified, all technical features and optional technical features of the present disclosure can be combined with each other to form new technical solutions.

[0052] Unless otherwise specified, all steps of the present disclosure can be performed in sequence or randomly, and preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, it is mentioned that the method can further comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0053] At present, the surface of the hard carbon material in the negative pole piece has high activity, is easy to react with the electrolyte, forms SEI, consumes more active ions, and thus reduces the first coulomb efficiency of the hard carbon material. The preparation process of the existing negative pole sodium supplement material is complex, and the activity of Na therein is low, and the sodium supplement effect is poor.

[0054] Negative pole piece

[0055] Based on this, the negative pole piece provided by the present disclosure comprises a negative pole current collector and a negative pole film layer on at least one surface of the negative pole current collector, and the negative pole film layer comprises a hard carbon material. 23 The Na solid-state NMR spectrum has a characteristic peak in the range of 400 ppm-1100 ppm.

[0056] The present disclosure refers to 23 The δ shift in the Na solid-state NMR spectrum can reflect the existing state of Na element. The hard carbon material provided by the present disclosure has 23 The δ shift in the Na solid-state NMR spectrum in the above range reflects the quasi-metallic state of Na with cluster size, which corresponds to the existence of Na in the form of Na2C6 in the hard carbon material. 23 The Na solid-state NMR spectrum has a characteristic peak in the range of 400 ppm-1100 ppm. Na in the hard carbon material exists in the form of quasi-metallic state with cluster size, has higher electrochemical activity, and has good Na supplement effect. When the hard carbon material is used as a negative pole material, it can achieve self-supplement of Na, thereby improving the initial coulombic efficiency.

[0057] Exemplarily, the hard carbon material has 23 The δ shift in the Na solid-state NMR spectrum is 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, or a value between any two of the above values.

[0058] In some embodiments, the hard carbon material has 23 The Na solid-state NMR spectrum has a characteristic peak in the range of 500 ppm-900 ppm. Na in the hard carbon material has more suitable activity in this range, thereby having better Na supplement effect.

[0059] The present disclosure refers to 23 The δ shift in the Na solid-state NMR spectrum can be measured by a nuclear magnetic resonance instrument (such as Bruker Avance Ⅲ 400M).

[0060] In some embodiments, the hard carbon material has a coating layer. The coating layer is beneficial to protect the Na in the hard carbon material from reacting with the outside environment, thus improving the stability of the hard carbon material. For example, the coating layer can reduce the side reaction of Na in the hard carbon material with the electrolyte, thus further improving the first coulombic efficiency. For another example, during the storage and transportation of the hard carbon material and the preparation of the battery, the coating layer can reduce the contact of Na in the hard carbon material with the outside environment, thus improving the stability and safety of the material. In the present disclosure, the coating layer is a continuous coating layer or a discontinuous coating layer. Optionally, the coating layer is a continuous coating layer. If the hard carbon material does not have a coating layer, the handling of the material will be more difficult, for example, the transfer and use of the material are carried out in an inert atmosphere as protection to avoid the violent oxidation reaction of Na in the material.

[0061] In some embodiments, the total Na element content of the hard carbon material is 0.5wt%-10wt% based on the weight of the hard carbon material. The total Na element content of the hard carbon material in the above range is beneficial to the sufficient Na supplement of the hard carbon material, and the Na element content is not too high to form metal particles that cannot release Na.

[0062] For example, the total Na element content of the hard carbon material is 0.5wt%, 1wt%, 1.5wt%, 2wt%, 3wt%, 5wt%, 7wt%, 9wt%, 10wt% or a value between any two of the above values based on the weight of the hard carbon material.

[0063] In some embodiments, the total Na element content of the hard carbon material is 5wt%-9.5wt% based on the weight of the hard carbon material. The total Na element content of the hard carbon material in the above range is more beneficial to the formation of cluster-sized metalloid state Na by the sufficient Na element of the hard carbon material, thus more beneficial to the Na supplement.

[0064] The total Na element content of the hard carbon material mentioned in the present disclosure can be determined by instruments and methods known in the art. In the present disclosure, referring to the People's Republic of China National Environmental Protection Standard HJ 781-2016 Determination of 22 Metal Elements in Solid Waste, the test instrument can be, for example, ICP-OES, Thermo ICAP7400.

[0065] In some embodiments, the surface Na element content is ≤ 0.1 wt% based on the weight of the hard carbon material. The surface Na element content of the hard carbon material within the above range is beneficial to protect the metalloid state Na in the hard carbon material and reduce side reactions, thereby further improving the initial coulombic efficiency. Illustratively, the surface Na element content is 0.002 wt%, 0.005 wt%, 0.01 wt%, 0.03 wt%, 0.05 wt%, 0.07 wt%, 0.1 wt% or a value between any two of the above values based on the weight of the hard carbon material. Preferably, the surface Na element content is less than or equal to 0.05 wt% or even less than or equal to 0.01 wt% based on the weight of the hard carbon material. The smaller the surface Na element content of the hard carbon material, the better the stability of the material.

[0066] The surface Na element content of the hard carbon material mentioned in the present disclosure can be obtained by measuring the hard carbon material by a conventional determination method in the art, for example, scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray photoelectron spectrometer (XPS), Auger electron spectrometer (AES), X-ray fluorescence spectrometry, etc. In the present disclosure, the test instrument can use Axis Supra+ X-ray photoelectron spectrometer according to the specification requirements of GB / T 33502-2017 Surface Chemical Analysis X-ray Photoelectron Spectroscopy (XPS) Data Recording and Reporting.

[0067] 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.

[0068] In some embodiments, the negative electrode current collector can use 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.).

[0069] In some embodiments, the negative electrode active material includes the hard carbon material according to any of the above embodiments or the hard carbon material obtained by the preparation method according to any of the above embodiments. The negative electrode film layer in the secondary battery of the present disclosure includes the hard carbon material of the present disclosure, which can achieve self-supply of Na when used as a negative electrode material, thereby improving the initial coulombic efficiency.

[0070] In some embodiments, the negative electrode film layer further optionally comprises 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).

[0071] In some embodiments, the negative electrode film layer further optionally comprises a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0072] In some embodiments, the negative electrode film layer further optionally comprises other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)), and the like.

[0073] 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 comprising the hard carbon material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector, and after processes such as drying, cold pressing, and the like, the negative electrode sheet can be obtained.

[0074] Secondary battery

[0075] In a second aspect of the embodiments of the present disclosure, a secondary battery is provided, and the secondary battery of the present disclosure is described below with appropriate reference to the accompanying drawings.

[0076] The term “secondary battery” referred to herein means a battery cell, a battery module, or a battery pack. Each is described below.

[0077] Generally, a secondary battery cell comprises a positive electrode sheet, the negative electrode sheet of the above-mentioned embodiments, an electrolyte, and a separator film. During charging and discharging of the battery, active ions, such as sodium ions, are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The separator film is disposed between the positive electrode sheet and the negative electrode sheet, and mainly functions to prevent short circuiting of the positive and negative electrodes, while allowing ions to pass through.

[0078] [Positive electrode sheet]

[0079] The positive electrode sheet comprises 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 comprising a positive electrode active material.

[0080] By way of example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0081] In some embodiments, the positive current collector can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be employed. 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.).

[0082] In some embodiments, the battery cell is a sodium ion battery, and the positive active material can employ a positive active material for a sodium ion battery known in the art. As an example, the positive 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 a positive active material for a sodium ion battery 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.

[0083] As an alternative technical solution of the present disclosure, the polyanion 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 polyanion 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 polyanion compound can also be a compound having a sodium ion, a tetrahedral (YO4) n- anion unit, a polyhedral (ZO y ) m+ nion unit, and an optional halogen anion. Y can be at least one of P, S, and Si, and n represents the valence of (YO4) n-valence: 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+ valence; halogen can be at least one of F, CI, and Br. The polyanionic compound is, for example, NaFeP04, Na3V2(P04)3, NaM’P04F (M’ is one or more of V, Fe, Mn, and Ni), and Na3(VO y )2(P04)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 is, for example, Na a Me b Me’ c (CN)6, wherein Me and Me’ are each independently at least one of Ni, Cu, Fe, Mn, Co, and Zn, 0 < a < 2, 0 < b < 1, and 0 < c < 1.

[0084] The battery will be accompanied by the de-intercalation and consumption of active ions (such as Na) during the charging and discharging process, and the molar content of 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 Na is the initial state of the material, i.e., the state before feeding, and the positive electrode active material is applied to the battery system. After charging and discharging cycles, the molar content of Na will change.

[0085] In the enumeration of the positive electrode active material in the present disclosure, the molar content of oxygen is only the theoretical state value, and 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.

[0086] 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), a vinylidene-fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene-fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0087] 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 super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0088] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as the positive 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 subjecting the positive electrode current collector to drying, cold pressing, and the like to obtain the positive electrode sheet.

[0089] [Electrolyte]

[0090] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of electrolyte is not particularly limited in the present disclosure and can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

[0091] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0092] 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-trifluoromethanesulfonylimide, sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluoroboric oxalate, sodium difluorophosphoric oxalate, and sodium tetrafluorophosphoric oxalate.

[0093] 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.

[0094] In some embodiments, the electrolyte solution can optionally further 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, and the like.

[0095] [Separator]

[0096] In some embodiments, the battery cell further includes a separator. The type of separator is not particularly limited in the present disclosure and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0097] In some embodiments, the material of the separator film can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator film can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator film is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0098] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator film can be used to form an electrode assembly through a winding process or a stacking process.

[0099] In some embodiments, the battery cell can include an outer package. The outer package can be used to encapsulate the electrode assembly and the electrolyte described above.

[0100] 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, or the like. 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, or the like can be listed.

[0101] The present disclosure does not particularly limit the shape of the battery cell, which can be cylindrical, square, or any other shape. For example, FIG. 1 is a battery cell 5 of a square structure as an example.

[0102] In some embodiments, referring to FIG. 2, the outer package can include a shell 51 and a top cover assembly 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can be provided on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab, and the separator film can be used to form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated 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 a person skilled in the art according to specific actual needs.

[0103] 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 a person skilled in the art according to the application and capacity of the battery module.

[0104] 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 arrangements can also be used. Further, the plurality of battery cells 5 can be fixed by fasteners.

[0105] Optionally, the battery module 4 can further include a housing having a receiving space, and the plurality of battery cells 5 are received in the receiving space.

[0106] In some embodiments, the above 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, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0107] 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 an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0108] Electric device

[0109] A third aspect of the embodiments of the present disclosure also provides an electric device, and the secondary battery of the present disclosure is described below with reference to the accompanying drawings as appropriate.

[0110] The electric device mentioned in the embodiments of the present disclosure includes the secondary battery provided by the present disclosure. The secondary battery can be used as a power supply of the electric device or as an energy storage unit of the electric device. The electric device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0111] As the electric device, the battery monomer, the battery module or the battery pack can be selected according to the use requirement thereof.

[0112] FIG. 6 is an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of high power and high energy density of the secondary battery for the electric device, the battery pack or the battery module can be used.

[0113] As another example of the device, it 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 supply.

[0114] Hard carbon material

[0115] Based on this, a fourth aspect of the present disclosure provides a hard carbon material, and the hard carbon material is used in the secondary battery provided by the present disclosure. 23 The Na solid-state NMR spectrum has a characteristic peak in the range of 400 ppm-1100 ppm of δ shift.

[0116] The above characteristic peak of the hard carbon material provided by the present disclosure reflects that the hard carbon material has sodium in a cluster-sized metalloid state, which is beneficial for realizing self-supply of sodium when the hard carbon material is used as a negative electrode material, thereby improving the first coulombic efficiency.

[0117] In some embodiments, the hard carbon material has a total sodium element content of 0.5wt%-10wt% based on the weight of the hard carbon material. 23 The δ shift has a characteristic peak in the range of 500ppm-900ppm in the Na solid-state NMR spectrum. This is more beneficial for improving the activity of sodium in a metalloid state in the hard carbon material, thereby further improving the sodium supply effect.

[0118] In some embodiments, the hard carbon material has a coating layer. This is beneficial for protecting sodium in a metalloid state in the hard carbon material and reducing side reactions, thereby further improving the first coulombic efficiency.

[0119] In some embodiments, the total sodium element content of the hard carbon material is 0.5wt%-10wt% based on the weight of the hard carbon material. This is beneficial for sufficient sodium supply of the hard carbon material, and the sodium element content is not too high to form metal particles that cannot release sodium.

[0120] In some embodiments, the total sodium element content of the hard carbon material is 5wt%-9.5wt% based on the weight of the hard carbon material. This is more beneficial for sufficient sodium element in the hard carbon material to form sodium in a cluster-sized metalloid state, thereby facilitating sodium supply.

[0121] In some embodiments, the surface sodium element content is ≤0.1wt% based on the weight of the hard carbon material. This is more beneficial for protecting sodium in a metalloid state in the hard carbon material and reducing side reactions, thereby further improving the first coulombic efficiency.

[0122] A method for preparing a hard carbon material

[0123] The fifth aspect of the present disclosure also provides a method for preparing a hard carbon material. The method comprises: carbonizing a carbon source in an inert atmosphere, wherein the carbon source comprises a material with at least part of the side groups being groups containing Na ions.

[0124] The method for preparing a hard carbon material provided by the present disclosure carbonizes the carbon source in an inert gas, and since the side groups are uniformly distributed in the carbon source body phase structure at a microscale, sodium does not agglomerate due to the steric hindance of the nearby main structure in the subsequent high-temperature carbonization process, which enables sodium in a cluster-sized metalloid state to be formed in the hard carbon material, corresponding to a characteristic peak of the δ shift in the Na solid-state NMR spectrum being in the range of 400ppm-1100ppm, and the hard carbon material being able to realize self-supply of sodium when used as a negative electrode material, thereby improving the first coulombic efficiency. 23 The δ shift has a characteristic peak in the range of 500ppm-900ppm in the Na solid-state NMR spectrum. This is more beneficial for improving the activity of sodium in a metalloid state in the hard carbon material, thereby further improving the sodium supply effect.

[0125] Inert atmosphere refers to a non-oxidizing atmosphere. Exemplarily, the inert atmosphere is, for example, a nitrogen atmosphere, an argon atmosphere, or a mixed atmosphere of nitrogen and helium, etc.

[0126] In some embodiments, the Na ion content of the carbon source is 3wt%-15wt% based on the weight of the carbon source. On the one hand, since part of the sodium will form gaseous sodium and be lost during the carbonization process, the hard carbon material formed by the Na ion content in the Na-containing carbon source within the above range can provide a sufficient amount of sodium in the secondary battery, on the other hand, it is beneficial to form sodium in the cluster size of the metalloid state, and thus achieve high electrochemical activity.

[0127] Exemplarily, the Na ion content of the carbon source is 3wt%, 4wt%, 5wt%, 7wt%, 9wt%, 11wt%, 13wt%, 14wt%, 15wt%, or a value between any two of the above values.

[0128] In some embodiments, the Na ion content of the carbon source is 5wt%-13wt% based on the weight of the carbon source. This is more conducive to forming a more suitable amount of sodium in the metalloid state in the hard carbon material, thereby more fully supplementing Na.

[0129] In some embodiments, the carbon source includes at least one of sodium lignosulfonate, cellulose sodium, and a composite material containing sodium lignosulfonate and / or cellulose sodium. Due to the presence of a large number of cyclic molecular structural units in sodium lignosulfonate and cellulose sodium, the carbon skeleton is stable during carbonization, and the sodium connected to the side groups of the carbon chain forms sodium in the cluster size of the metalloid state. The composite material containing sodium lignosulfonate and / or cellulose sodium refers to a material that, in addition to the side groups of the Na ion-containing groups of sodium lignosulfonate and / or cellulose sodium, can further include other suitable carbon sources. Other carbon sources forming the composite material can not contain sodium elements, such as phenolic resin, epoxy resin, monosaccharides and polysaccharides, and other carbon hydroxide compounds. Exemplarily, the carbon source can be sodium lignosulfonate, cellulose sodium, sodium lignosulfonate-phenolic resin composite material, cellulose sodium-phenolic resin composite material, etc.

[0130] In some embodiments, after carbonization, the preparation method further includes coating by chemical vapor deposition (CVD) in a mixed gas of inert gas and organic compound gas. The coating layer is formed on the surface of the hard carbon, which is beneficial to reduce the exposure of sodium on the surface of the hard carbon material, improve the stability of the material, and reduce the surface activity of the material. The CVD coating method is beneficial to reduce the side reactions in the coating process, and the coating layer formed can more fully protect the sodium in the metalloid state in the hard carbon material, thereby further improving the first coulombic efficiency.

[0131] According to the specific embodiments, the coating layer is a carbon coating layer.

[0132] In some embodiments, the temperature of the coating is 700-850°C. This is more conducive to forming a dense coating layer to protect the metalloid Na in the hard carbon material, and also more conducive to reducing the escape of sodium vapor formed by reduction during carbonization. Illustratively, the temperature of the coating is 700°C, 750°C, 800°C, 850°C, or a value between any two of the values.

[0133] In some embodiments, the volume fraction of the organic compound gas in the mixed gas is 2-35 vol%. This is more conducive to the formation of a carbon coating layer of a suitable thickness on the outside of the hard carbon particles by a suitable amount of the organic compound gas to sufficiently protect the metalloid Na in the hard carbon material. Illustratively, the volume fraction of the organic compound gas in the mixed gas is 2 vol%, 5 vol%, 10 vol%, 15 vol%, 20 vol%, 25 vol%, 30 vol%, 35 vol%, or a value between any two of the values. Alternatively, the volume fraction of the organic compound gas in the mixed gas is 2-30 vol%.

[0134] Illustratively, the organic compound can be at least one of an alkane, an alkene, an alkyne, an alcohol, and a benzene series.

[0135] Illustratively, the inert gas can be at least one of nitrogen and argon.

[0136] In some embodiments, carbonization and CVD coating are continuously performed in the same reactor. This is more conducive to sufficiently protecting the metalloid Na in the hard carbon material during preparation and reducing the risk of oxidation by air. Illustratively, carbonization and CVD coating can be performed in a tube furnace, and the gas pipeline of the inlet is connected through a three-way connector. When performing CVD coating, the three-way connector is switched to switch the source of the former inert atmosphere gas to the source of the mixed gas containing the organic compound gas.

[0137] In some embodiments, the temperature of carbonization is 600-850°C. The temperature of carbonization in the above range is conducive to both the sufficient splitting of the carbon source to form a suitable hard carbon structure and the reduction of sodium vapor escape. Illustratively, the temperature of carbonization is 600°C, 650°C, 700°C, 800°C, 850°C, or a value between any two of the values.

[0138] In some embodiments, before carbonization, the preparation method further comprises a crushing treatment. The crushing treatment can reduce the particle size and be more conducive to the efficiency of subsequent carbonization. The crushing treatment can adopt conventional methods in the art, for example, illustratively, crushing into particles with a Dv50 of 4-8 μm by an air jet mill or mechanical or ball milling.

[0139] Embodiments

[0140] Hereinafter, the embodiments of the present disclosure will be described. The embodiments described below are exemplary and are for the purpose of explanation of the present disclosure and cannot be understood as a limitation of the present disclosure. In the embodiments, the specific techniques or conditions not mentioned are performed in accordance with the techniques or conditions described in the literature in the field 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.

[0141] Example 1

[0142] Preparation of hard carbon material:

[0143] 1) Breaking

[0144] A sodium lignosulfonate raw material (CAS No. 8061-51-6, purchased from Shanghai Aldrin Biotech Co., Ltd., with a total Na ion content of 9.0 wt%) was prepared into a 10 wt% aqueous solution. To 1000 g of the above sodium lignosulfonate aqueous solution, 21.4 g of Ca(OH)2 powder was added in batches, the pH value of the solution was controlled between 7.5 and 9.0, and the solution was fully stirred for 6 h, filtered, and dried to obtain treated sodium lignosulfonate, and the Na ion content thereof was measured by ICP to be 8.4 wt%. The treated sodium lignosulfonate was ball milled, using zirconia milling beads and a milling jar, the mass ratio of the product to the milling beads was 1:3, the rotation speed was 800 rpm, and the time was 8 hours. The particle size distribution Dv50 of the product obtained by breaking was measured by a Mastersizer 3000 laser particle size analyzer to be 5 μm.

[0145] 2) Carbonization

[0146] The product obtained in step 1) above was heated to 800℃ at a heating rate of 5℃ / min in a tube furnace (Hefei Kejing, GSL-1400X) under a normal pressure nitrogen atmosphere for 3 hours to obtain a substrate.

[0147] 3) Coating

[0148] The substrate obtained in step 2) above was continuously heated to 800℃ at a heating rate of 5℃ / min in a tube furnace under an atmosphere of a mixture of nitrogen and acetylene (the volume fraction of acetylene in the mixture was 25 vol%) for 8 hours to obtain a hard carbon material.

[0149] Preparation of negative electrode slurry:

[0150] The above hard carbon material, binder styrene-butadiene rubber (SBR), thickening agent sodium carboxymethyl cellulose (CMC-Na), and conductive agent carbon black were mixed in a mass ratio of 96.2:1.8:1.2:0.8 in an appropriate amount of solvent deionized water, and a uniform negative electrode slurry was formed.

[0151] Preparation of negative electrode sheet:

[0152] The uniformly stirred negative electrode slurry is coated onto one side of a copper foil using a double-sided coating machine. After the single-sided coating is completed, the foil is dried, cold-pressed, and punched in sequence to prepare the negative electrode sheet.

[0153] Preparation of coin cell half-cells:

[0154] The prepared negative electrode sheet was used for battery assembly in a glove box. A sodium metal sheet was used as the counter electrode. A 1 mol / L electrolyte was prepared by adding NaPF6 to a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a 1:1:1 volume ratio. Glass fiber was used as the separator. CR2430 coin cells were then assembled with the prepared negative electrode sheet in the glove box.

[0155] Example 2

[0156] Hard carbon materials were prepared using a method similar to that in Example 1, except that the carbon source was cross-linked sodium carboxymethyl cellulose (CAS No. 74811-65-7, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., with a total Na ion content of 10 wt%), and 26.5 g of Ca(OH)2 powder was added. The total Na ion content of the treated cross-linked sodium carboxymethyl cellulose was 6.1 wt%.

[0157] Example 3

[0158] Hard carbon material was prepared using a method similar to that in Example 1, except that 25.5 g of Ca(OH)2 powder was added, and the total Na ion content of the treated sodium lignosulfonate was 4.7 wt%.

[0159] Example 4

[0160] Hard carbon material was prepared using a method similar to that in Example 1, except that 27.4 g of Ca(OH)2 powder was added, and the total Na ion content of the treated sodium lignin sulfonate was 3 wt%.

[0161] Example 5

[0162] Hard carbon material was prepared using a method similar to that in Example 1, except that 6.3 g of NaOH powder was added, and the total Na ion content of the treated sodium lignosulfonate was 12.6 wt%.

[0163] Comparative Example 1

[0164] Hard carbon material was prepared using a method similar to that in Example 1, except that 11.8 g of NaOH powder was added, and the total Na ion content of the treated sodium lignosulfonate was 15.8 wt%.

[0165] Tests related to hard carbon materials:

[0166] 23 Na solid-state NMR test

[0167] The hard carbon materials of the above embodiments and comparative examples were vacuum dried overnight at 80°C, then transferred to a 2.5 mm rotor and sealed with a Vespel cap. Testing was performed using a Bruker Avance III (400 MHz). 23 The Na single-pulse experiment was conducted with the following parameters: a 2.5mm probe rotation rate of 25kHz and a π / 4 pulse length of 2.0μs. A saturation recovery experiment was used. 23 Measurement of Na spin-lattice relaxation time. 1 H- 23 Na CPMAS NMR was performed with a contact time of 0.6 ms and SPINA-1H decoupling at 70.0 kHz. 23 Na chemical shift external reference: 1 mol / L NaCl aqueous solution (0 ppm). Samples from Examples 2 and 6. 23 The solid NMR spectra of Na are shown in Figures 7 and 8.

[0168] Total Na content test

[0169] For the hard carbon materials of the above embodiments and comparative examples, the determination of 22 metallic elements in solid waste was carried out in accordance with the People's Republic of China National Environmental Protection Standard HJ 781-2016. The testing instrument used was ICP-OES, Thermo ICAP7400.

[0170] Surface Na content test

[0171] For the hard carbon materials in the above embodiments and comparative examples, referring to GB / T 33502-2017, three different parts of the same material were selected, and the Na content on the surface of the hard carbon material was tested by X-ray photoelectron spectroscopy (instrument model: Axis Supra / Supra+).

[0172] Specific capacity and first coulombic efficiency test of hard carbon materials

[0173] For the coin cell half-cells of the above embodiments and comparative examples: The capacity obtained by inserting sodium at a rate of 0.05C to 0V is the initial charge capacity; the capacity obtained by desodiuming at a rate of 0.1C to 2.5V is the initial discharge capacity. The mass of the hard carbon material in the negative electrode is calculated based on the coating weight and area of ​​the slurry during the electrode preparation process. Charge capacity = Initial charge capacity / Mass of hard carbon material. Discharge capacity = Initial discharge capacity / Mass of hard carbon material. Initial coulombic efficiency (%) = Initial charge capacity / Initial discharge capacity × 100%.

[0174] The hard carbon materials prepared in Examples 1-5 and Comparative Example 1 above 23 The characteristic peak shifts of Na solid-state NMR spectra, total Na content, surface Na content, discharge specific capacity, charge specific capacity, and initial coulombic efficiency test results are shown in Table 1.

[0175] Table 1:

[0176] As can be seen from Table 1 above, the Na ion content of the carbon source in Examples 1-5 is in the range of 3wt%-15wt%, which is suitable for hard carbon materials. 23 In solid-state NMR spectra of Na, the δ shift ranges from 400 ppm to 1100 ppm. Hard carbon materials contain quasi-metallic Na with cluster-sized aggregates, which can significantly improve the first coulomb efficiency.

[0177] Example 6

[0178] Hard carbon materials were prepared using a method similar to that in Example 1, except that the carbonization temperature was 600°C.

[0179] Example 7

[0180] Hard carbon materials were prepared using a method similar to that in Example 1, except that the carbonization temperature was 700°C.

[0181] Example 8

[0182] Hard carbon materials were prepared using a method similar to that in Example 1, except that the carbonization temperature was 850°C.

[0183] The hard carbon materials prepared in Examples 6-8 above 23 The characteristic peak shifts of the Na solid-state NMR spectrum, total Na content, surface Na content, discharge specific capacity, charge specific capacity, and the results of the first coulombic efficiency test are shown in Table 2.

[0184] Table 2:

[0185] As can be seen from Table 2, the carbonization temperatures of Examples 6-8 are all in the range of 600℃-850℃. In particular, the carbonization temperature of Example 8 reaches 850℃, and the total Na content decreases significantly, but is still above 0.5wt%, which can significantly improve the first coulombic efficiency.

[0186] Example 9

[0187] Hard carbon materials were prepared using a method similar to that in Example 1, except that the volume fraction of organic compound gas was 5 vol% during CVD coating.

[0188] Example 10

[0189] Hard carbon materials were prepared using a method similar to that in Example 1, except that the CVD coating temperature was 700°C.

[0190] Example 11

[0191] Hard carbon materials were prepared using a method similar to that in Example 1, except that the CVD coating temperature was 850°C.

[0192] The hard carbon materials prepared in Examples 9-11 above 23 The characteristic peak shifts of the Na solid-state NMR spectrum, total Na content, surface Na content, discharge specific capacity, charge specific capacity, and the results of the first coulombic efficiency test are shown in Table 3.

[0193] Table 3:

[0194] As can be seen from Table 3 above, the volume fraction of organic compound gas in Example 9 is above 2 vol%, and the CVD coating temperature in Examples 10-11 is in the range of 700-850℃. The coating layers formed can fully protect the hard carbon material. The hard carbon material has Na in a quasi-metallic state with cluster size, which can significantly improve the first coulombic efficiency.

[0195] It should be noted that this disclosure is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same essential structure and achieving the same effect as the technical concept within the scope of this disclosure are included in the technical scope of this disclosure. 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, are also included in the scope of this disclosure without departing from the spirit of this disclosure.

Claims

1. A negative electrode sheet, comprising a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, the negative electrode film layer comprising a hard carbon material, the hard carbon material being in... 23 In Na solid-state NMR spectra, there are characteristic peaks in the δ shift range of 400 ppm to 1100 ppm.

2. The negative electrode sheet according to claim 1, wherein, The hard carbon material in 23 In Na solid-state NMR spectra, characteristic peaks of δ shift are observed in the range of 500 ppm to 900 ppm.

3. The negative electrode sheet according to claim 1 or 2, wherein, The hard carbon material has a coating layer.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein, Based on the weight of the hard carbon material, the total Na content of the hard carbon material is 0.5wt%-10wt%.

5. The negative electrode sheet according to claim 4, wherein, Based on the weight of the hard carbon material, the total Na content of the hard carbon material is 5wt%-9.5wt%.

6. The negative electrode sheet according to any one of claims 1 to 5, wherein, Based on the weight of the hard carbon material, the surface Na content of the hard carbon material is ≤0.1wt%.

7. A secondary battery, the secondary battery comprising a negative electrode sheet according to any one of claims 1 to 6.

8. The secondary battery according to claim 7, wherein, The secondary battery is a sodium-ion battery.

9. The secondary battery according to claim 7 or 8, wherein, The secondary battery also includes a positive electrode sheet, wherein the positive electrode sheet comprises at least one selected from transition metal oxides, polyanionic compounds and Prussian blue compounds as the positive electrode active material.

10. An electrical device comprising a secondary battery according to any one of claims 7-9.

11. A hard carbon material, said hard carbon material in 23 In Na solid-state NMR spectra, there are characteristic peaks in the δ shift range of 400 ppm to 1100 ppm.

12. The hard carbon material according to claim 11, wherein, The hard carbon material in 23 In Na solid-state NMR spectra, characteristic peaks of δ shift are observed in the range of 500 ppm to 900 ppm.

13. The hard carbon material according to claim 11 or 12, wherein, The hard carbon material has a coating layer.

14. The hard carbon material according to any one of claims 11 to 13, wherein, Based on the weight of the hard carbon material, the total Na content of the hard carbon material is 0.5wt%-10wt%.

15. The hard carbon material according to claim 14, wherein, Based on the weight of the hard carbon material, the total Na content of the hard carbon material is 5wt%-9.5wt%.

16. The hard carbon material according to any one of claims 11 to 15, wherein, Based on the weight of the hard carbon material, the surface Na content of the hard carbon material is ≤0.1wt%.

17. A method for preparing a hard carbon material, the method comprising: Carbon source is carbonized under an inert atmosphere. The carbon source includes materials whose side groups are at least partially Na-containing groups.

18. The preparation method according to claim 17, wherein, Based on the weight of the carbon source, the Na ion content of the carbon source is 3wt%-15wt%.

19. The preparation method according to claim 18, wherein, Based on the weight of the carbon source, the Na ion content of the carbon source is 5wt%-13wt%.

20. The preparation method according to any one of claims 17-19, wherein, The carbon source includes at least one of sodium lignin sulfonate, sodium cellulose, and composite materials containing sodium lignin sulfonate and / or sodium cellulose.

21. The preparation method according to any one of claims 17-20, wherein, Following the carbonization, the preparation method further includes coating the mixture of inert gas and organic compound gas by chemical vapor deposition (CVD).

22. The preparation method according to claim 21, wherein, The coating is carried out at a temperature of 700℃-850℃.

23. The preparation method according to claim 21 or 22, wherein, The volume fraction of the organic compound gas in the mixed gas is 2 vol% to 35 vol%.

24. The preparation method according to any one of claims 21-23, wherein, The carbonization and coating are carried out continuously in the same reactor.

25. The preparation method according to any one of claims 17-24, wherein, The carbonization temperature is 600℃-850℃.

26. The preparation method according to any one of claims 17-25, wherein, Prior to carbonization, the preparation method further includes a crushing process.

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