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

Abstract

The invention provides a negative pole piece, a secondary battery, an electric device, a hard carbon material and a preparation method thereof. The negative electrode piece comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer comprises a hard carbon material, and the hard carbon material has a characteristic peak in a 23Na solid-state NMR spectrum within the delta displacement range of 400 ppm to 1100 ppm. When the hard carbon material is used as a negative electrode material, self-supplementation of Na can be realized, so that the first coulombic efficiency is improved.

Description

Technical Field

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

[0002] In recent years, with the continuous development and expansion of the new energy industry, the demand for lithium-ion batteries has been increasing year by year. Against this backdrop, the depletion of lithium resources has led to a rapid increase in the cost of lithium-ion batteries, which is detrimental to the long-term development of the new energy industry. Sodium-ion batteries can alleviate some of the supply and demand pressure. Due to sodium's advantages in resources and cost, sodium-ion batteries have become an important development direction for energy storage batteries. Carbon anode materials play a role in storing sodium during the charging process of sodium-ion batteries and are an important component of sodium-ion batteries. Because hard carbon has ordered carbon microcrystals, abundant micro- and nano-pore defects and surface defects, it has a large interlayer spacing and abundant pore structure, resulting in a relatively stable structure during sodium ion insertion / extraction. Due to its excellent overall performance, wide applicability, and relatively low price, hard carbon is one of the mainstream anode active materials in this field.

[0003] Like lithium-ion batteries, sodium-ion batteries suffer from the same problem during charging and discharging: the formation of an SEI film consumes sodium ions, leading to a decrease in initial coulombic efficiency and a significant reduction in battery performance. Therefore, finding a solution for sodium replenishment at the negative electrode of sodium-ion batteries is of great significance in this field. Summary of the Invention

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

[0005] To achieve the above objectives, a first aspect of this application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a hard carbon material. The 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.

[0006] The characteristic peaks of the hard carbon material provided in this application indicate that the hard carbon material has quasi-metallic Na with cluster size, which is beneficial for the hard carbon material to achieve self-replenishment of Na when used as a negative electrode material, thereby improving the first coulombic efficiency.

[0007] In some embodiments, the hard carbon material is in 23The Na solid-state NMR spectrum exhibits a characteristic peak in the δ shift range of 500 ppm–900 ppm. This is more conducive to improving the activity of quasi-metallic Na in the hard carbon material, thereby improving the Na replenishment effect.

[0008] In some embodiments, the hard carbon material has a coating layer. This helps protect the quasi-metallic Na in the hard carbon material, reduces side reactions, and thus further improves the first coulombic efficiency.

[0009] In some embodiments, the total Na content of the hard carbon material is 0.5wt%-10wt% based on its weight. This facilitates sufficient Na replenishment in the hard carbon material and prevents the formation of metal particles that cannot release Na due to excessive Na content.

[0010] In some embodiments, the total Na content of the hard carbon material is 5 wt% to 9.5 wt% based on its weight. This is more conducive to the formation of sufficient Na in the hard carbon material to form quasi-metallic Na clusters, thereby facilitating sodium replenishment.

[0011] In some embodiments, the surface Na content is ≤0.1wt% based on the weight of the hard carbon material. This is more conducive to protecting the quasi-metallic Na in the hard carbon material, reducing side reactions, and thus further improving the first coulombic efficiency.

[0012] A second aspect of this application also provides a secondary battery, including a negative electrode sheet according to any of the above embodiments.

[0013] The negative electrode film layer in the secondary battery of this application includes the negative electrode sheet of this application. When the hard carbon material is used as the negative electrode material, it can achieve self-replenishment of Na, thereby improving the first coulombic efficiency.

[0014] In some embodiments, the secondary battery is a sodium-ion battery.

[0015] In some embodiments, the secondary battery further includes a positive electrode, which comprises at least one selected from transition metal oxides, polyanionic compounds, and Prussian blue compounds as the positive electrode active material.

[0016] A third aspect of this application provides an electrical device, including the secondary battery of the second aspect of this application.

[0017] The electrical device of this application includes the secondary battery provided in this application, and therefore has at least the same advantages as the secondary battery.

[0018] A fourth aspect of this application provides a hard carbon material. The hard carbon material in... 23In Na solid-state NMR spectra, there are characteristic peaks in the δ shift range of 400 ppm to 1100 ppm.

[0019] The characteristic peaks of the hard carbon material provided in this application indicate that the hard carbon material has quasi-metallic Na with cluster size, which is beneficial for the hard carbon material to achieve self-replenishment of Na when used as a negative electrode material, thereby improving the first coulombic efficiency.

[0020] In some embodiments, the hard carbon material is in 23 The Na solid-state NMR spectrum exhibits a characteristic peak in the δ shift range of 500 ppm–900 ppm. This is more conducive to improving the activity of quasi-metallic Na in the hard carbon material, thereby improving the Na replenishment effect.

[0021] In some embodiments, the hard carbon material has a coating layer. This helps protect the quasi-metallic Na in the hard carbon material, reduces side reactions, and thus further improves the first coulombic efficiency.

[0022] In some embodiments, the total Na content of the hard carbon material is 0.5wt%-10wt% based on its weight. This facilitates sufficient Na replenishment in the hard carbon material and prevents the formation of metal particles that cannot release Na due to excessive Na content.

[0023] In some embodiments, the total Na content of the hard carbon material is 5 wt% to 9.5 wt% based on its weight. This is more conducive to the formation of sufficient Na in the hard carbon material to form quasi-metallic Na clusters, thereby facilitating sodium replenishment.

[0024] In some embodiments, the surface Na content is ≤0.1wt% based on the weight of the hard carbon material. This is more conducive to protecting the quasi-metallic Na in the hard carbon material, reducing side reactions, and thus further improving the first coulombic efficiency.

[0025] A fifth aspect of this application also provides a method for preparing a hard carbon material. The method includes: carbonizing a carbon source under an inert atmosphere, wherein the carbon source comprises a material with at least a portion of its side groups containing Na ions.

[0026] The method for preparing hard carbon material provided in this application enables the formation of quasi-metallic Na clusters in the hard carbon material by carbonizing the carbon source in an inert gas. When the hard carbon material is used as a negative electrode material, it can achieve self-replenishment of Na, thereby improving the first coulombic efficiency.

[0027] In some embodiments, the Na ion content of the carbon source is 3wt%-15wt% based on its weight. A Na ion content within this range provides sufficient Na for the final hard carbon material, and a suitable Na ion content is more conducive to the formation of quasi-metallic Na clusters.

[0028] In some embodiments, the Na ion content of the carbon source is 5wt%-13wt% based on its weight. This is more conducive to forming an appropriate amount of quasi-metallic Na in the hard carbon material, thereby more fully replenishing Na.

[0029] In some embodiments, the carbon source includes at least one of sodium lignosulfonate, sodium cellulose, and composite materials containing sodium lignosulfonate and / or sodium cellulose. This carbon source is more conducive to the formation of quasi-metallic Na at cluster sizes, thereby improving the activity of quasi-metallic Na in the hard carbon material and enhancing the Na replenishment effect.

[0030] In some embodiments, after carbonization, the preparation method further includes coating via chemical vapor deposition (CVD) in a mixture of inert and organic compound gases. Forming a coating layer on the hard carbon surface helps reduce sodium exposure in the hard carbon material, improving material stability and reducing surface activity. CVD helps reduce side reactions during the coating process, and the resulting coating layer more effectively protects the quasi-metallic Na in the hard carbon material, thereby further improving the initial coulombic efficiency.

[0031] In some embodiments, the coating is performed at a temperature of 700°C to 850°C. This temperature is beneficial for forming a dense coating layer that protects the quasi-metallic Na in the hard carbon material, and also helps to reduce the escape of sodium vapor formed during reduction.

[0032] In some embodiments, the volume fraction of the organic compound gas in the mixed gas is 2 vol% to 35 vol%. The content of the organic compound gas in the mixed gas is conducive to the formation of a carbon coating of suitable thickness on the outside of the hard carbon particles by an appropriate amount of organic compound gas, thereby fully protecting the quasi-metallic sodium in the hard carbon material.

[0033] In some embodiments, the carbonization and coating are carried out continuously in the same reactor. This is more conducive to fully protecting the quasi-metallic sodium in the hard carbon material during the preparation process and reducing the risk of the material being exposed to air during preparation.

[0034] In some embodiments, the carbonization temperature is 600°C-850°C. A carbonization temperature within this range is beneficial for both the complete decomposition of the carbon source to form a suitable hard carbon structure and for reducing the escape of sodium vapor formed during reduction.

[0035] In some embodiments, the preparation method further includes a crushing process prior to carbonization. The crushing process reduces particle size, which is more beneficial for the efficiency of subsequent carbonization. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0037] Figure 2 yes Figure 1 An exploded view of a battery cell according to one embodiment of this application is shown.

[0038] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application.

[0039] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0040] Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown.

[0041] Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0042] Figure 7 The hard carbon material prepared in Example 2 of this application 23 Na solid-state NMR spectrum.

[0043] Figure 8 The hard carbon material prepared in Example 6 of this application 23 Na solid-state NMR spectrum.

[0044] Explanation of reference numerals in the attached figures:

[0045] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly Detailed Implementation

[0046] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the negative electrode sheet, secondary battery, electrical device, hard carbon material, and preparation method thereof. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0047] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. Ranges defined in this way, unless otherwise stated, include endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is understood that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0048] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0049] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0050] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0051] Currently, the hard carbon material in the negative electrode has high surface activity, making it prone to reacting with the electrolyte to form an electrolyte interphase (SEI), which consumes a significant amount of active ions and thus reduces the initial coulombic efficiency of the hard carbon material. Existing sodium-replenishing materials for negative electrodes have complex preparation processes, and the low activity of Na in them results in poor sodium replenishment effects.

[0052] Negative electrode sheet

[0053] Based on this, this application proposes a negative electrode sheet, including a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a hard carbon material. The 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.

[0054] This application mentions 23 The δ shift in a Na solid-state NMR spectrum reflects the state of Na. The hard carbon material provided in this application... 23 In the Na solid-state NMR spectrum, the δ shift within the above range reflects Na in a quasi-metallic state with cluster size, corresponding to... 23 In the solid-state NMR spectrum of Na, characteristic peaks with δ shifts exist in the range of 400 ppm–1100 ppm. Na in the hard carbon material exists in a quasi-metallic state with the aforementioned cluster size, exhibiting high electrochemical activity and good Na replenishment effect. When used as an anode material, the hard carbon material can achieve self-replenishment of Na, thereby improving the initial coulombic efficiency.

[0055] For example, the hard carbon material in 23 In Na solid-state NMR spectra, the δ shift is 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, or a value between any two of these values.

[0056] In some embodiments, the hard carbon material is in 23In Na solid-state NMR spectra, characteristic peaks with delta shifts in the range of 500 ppm to 900 ppm are observed. Within this delta shift range, Na exhibits more suitable activity in the hard carbon material, resulting in better Na replenishment.

[0057] This application mentions 23 The δ shift in Na solid NMR spectra can be measured using a nuclear magnetic resonance spectrometer (e.g., Bruker Avance III 400M).

[0058] In some embodiments, the hard carbon material has a coating layer. Since sodium in its quasi-metallic state is highly reactive, the coating layer helps protect the Na in the hard carbon material, reducing its contact with the external environment and thus improving the stability of the hard carbon material. For example, the coating layer can reduce side reactions between Na in the hard carbon material and the electrolyte, thereby further improving the initial coulombic efficiency. As another example, during the storage, transportation, and battery fabrication of the hard carbon material, the coating layer can reduce the contact between Na in the hard carbon material and the external environment, thereby improving the stability and safety of the material.

[0059] In some embodiments, the total Na content of the hard carbon material is 0.5 wt% to 10 wt% based on its weight. A total Na content within this range is beneficial for the hard carbon material to adequately replenish Na without forming metal particles that cannot release Na due to excessive Na content.

[0060] For example, based on the weight of the hard carbon material, the total Na 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 these values.

[0061] In some embodiments, the total Na content of the hard carbon material is 5 wt% to 9.5 wt% based on its weight. A total Na content within this range is more conducive to the formation of quasi-metallic Na clusters through sufficient Na, thus facilitating sodium replenishment.

[0062] The total Na content of the hard carbon material mentioned in this application can be determined using instruments and methods known in the art. In this application, the determination of 22 metallic elements in solid waste is referenced to the National Environmental Protection Standard of the People's Republic of China HJ 781-2016. The testing instruments that can be used include, for example, ICP-OES and Thermo ICAP7400.

[0063] In some embodiments, the surface Na content is ≤0.1 wt% based on the weight of the hard carbon material. A surface Na content within this range is beneficial for protecting the quasi-metallic Na in the hard carbon material, reducing side reactions, and thus further improving the first coulombic efficiency. Exemplarily, the surface Na content, based on the weight of the hard carbon material, 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 within a range of any two of these values. Preferably, the surface Na content, based on the weight of the hard carbon material, is less than or equal to 0.05 wt%, or even less than or equal to 0.01 wt%. The lower the surface Na content of the hard carbon material, the better the material stability.

[0064] The surface Na content of the hard carbon material mentioned in this application can be obtained by measuring the hard carbon material using conventional methods in the relevant field, such as scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and X-ray fluorescence spectroscopy. In this application, referring to the specifications for data recording and reporting of X-ray photoelectron spectroscopy (XPS) for surface chemical analysis in GB / T 33502-2017, the testing instrument can be an AxisSupra+ X-ray photoelectron spectroscopy instrument.

[0065] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0066] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil or aluminum foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0067] 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 prepared by the preparation method described in any of the above embodiments. The negative electrode film layer in the secondary battery of this application includes the hard carbon material of this application, which, when used as a negative electrode material, can achieve self-replenishment of Na, thereby improving the initial coulombic efficiency.

[0068] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0069] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0070] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0071] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material including the hard carbon material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0072] Secondary batteries

[0073] A second aspect of the embodiments of this application provides a secondary battery, which will be described below with appropriate reference to the accompanying drawings.

[0074] The term "secondary battery" used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0075] Typically, a single secondary battery cell includes a positive electrode, a negative electrode as described in the above embodiment, an electrolyte, and a separator. During battery charging and discharging, active ions, such as sodium ions, repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0076] [Positive electrode plate]

[0077] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0078] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

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

[0080] In some embodiments, the battery cell is a sodium-ion battery, and the positive electrode active material can be any positive electrode active material known in the art for use in sodium-ion batteries. As examples, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, etc., but this application is not limited to these materials; other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries can also be used. For example, as an optional technical solution in this application, the transition metal in the sodium transition metal oxide can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide is, for example, Na. x MO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, 0 <x≤1。

[0081] As an optional technical approach in this application, the polyanionic compound can be a compound containing sodium ions, transition metal ions, or a tetrahedral (YO4) structure. n- A class of compounds with anionic units. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si; n represents (YO4). n- The valence state. Polyanionic compounds can also have sodium ions, transition metal ions, or tetrahedral (YO4) ions. n- A class of compounds containing anionic units and halide anions. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si, and n represents (YO4). n- The valence state; the halogen can be at least one of F, Cl, and Br. Polyanionic compounds can also have sodium ions, tetrahedral (YO4) valence states. n- Anionic unit, polyhedral unit (ZO) y ) m+ And a class of compounds with optional halide anions. Y can be at least one of P, S, and Si, and n represents (YO4).n- Valence state: Z represents a transition metal, which can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; m represents (ZO) y ) m+ The valence state; the halogen can be at least one of F, Cl, and Br. Examples of polyanionic compounds include NaFePO4, Na3V2(PO4)3, NaM'PO4F (M' is one or more of V, Fe, Mn, and Ni), and Na3(VO4)2(PO4)3. y )2(PO4)2F 3-2y At least one of the following (0≤y≤1). Prussian blue compounds can be a class of compounds containing sodium ions, transition metal ions, and cyanide ions (CN-). The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. Examples of Prussian blue compounds include Na. a Me b Me' c (CN)6, wherein Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co, and Zn, 0 <a≤2,0<b<1,0<c<1。

[0082] During the charging and discharging process, batteries experience the insertion and extraction of active ions (such as Na), and the molar content of Na varies depending on the battery's discharge state. In the examples of positive electrode active materials in this application, the molar content of Na refers to the initial state of the material, i.e., the state before feeding. When the positive electrode active material is applied to the battery system, the molar content of Na changes after charge-discharge cycles.

[0083] In the examples of positive electrode active materials in this application, the molar content of oxygen is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of oxygen will fluctuate.

[0084] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0085] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0086] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0087] [Electrolytes]

[0088] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0089] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0090] In some embodiments, when the battery cell is a sodium-ion battery, the electrolyte salt may be selected from at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium difluorosulfonamide, sodium ditrifluoromethanesulfonamide, sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluorooxalate borate, sodium dioxalate borate, sodium difluorodioxalate phosphate, and sodium tetrafluorooxalate phosphate.

[0091] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0092] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0093] [Isolation membrane]

[0094] In some embodiments, the battery cell also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0095] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0096] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0097] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0098] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0099] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 The example shown is a square-structured battery cell 5.

[0100] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0101] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0102] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0103] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0104] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0105] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0106] Electrical appliances

[0107] A third aspect of the embodiments of this application also provides an electrical device, and the secondary battery of this application will be described below with appropriate reference to the accompanying drawings.

[0108] The electrical device mentioned in the embodiments of this application includes the secondary battery provided in this application. The secondary battery can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0109] As the electrical device, a single battery cell, a battery module, or a battery pack can be selected according to its usage requirements.

[0110] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0111] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0112] Hard carbon materials

[0113] Based on this, a fourth aspect of this application proposes 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.

[0114] The characteristic peaks of the hard carbon material provided in this application indicate that the hard carbon material has quasi-metallic Na with cluster size, which is beneficial for the hard carbon material to achieve self-replenishment of Na when used as a negative electrode material, thereby improving the first coulombic efficiency.

[0115] In some embodiments, the hard carbon material is in 23 The Na solid-state NMR spectrum exhibits a characteristic peak in the δ shift range of 500 ppm–900 ppm. This is more conducive to improving the activity of quasi-metallic Na in the hard carbon material, thereby improving the Na replenishment effect.

[0116] In some embodiments, the hard carbon material has a coating layer. This helps protect the quasi-metallic Na in the hard carbon material, reduces side reactions, and thus further improves the first coulombic efficiency.

[0117] In some embodiments, the total Na content of the hard carbon material is 0.5wt%-10wt% based on its weight. This facilitates sufficient Na replenishment in the hard carbon material and prevents the formation of metal particles that cannot release Na due to excessive Na content.

[0118] In some embodiments, the total Na content of the hard carbon material is 5 wt% to 9.5 wt% based on its weight. This is more conducive to the formation of sufficient Na in the hard carbon material to form quasi-metallic Na clusters, thereby facilitating sodium replenishment.

[0119] In some embodiments, the surface Na content is ≤0.1wt% based on the weight of the hard carbon material. This is more conducive to protecting the quasi-metallic Na in the hard carbon material, reducing side reactions, and thus further improving the first coulombic efficiency.

[0120] Preparation method of hard carbon materials

[0121] A fifth aspect of this application also provides a method for preparing a hard carbon material. The method includes: carbonizing a carbon source under an inert atmosphere, wherein the carbon source comprises a material with at least a portion of its side groups containing Na ions.

[0122] The method for preparing hard carbon materials provided in this application involves carbonizing the carbon source in an inert gas. Because the side groups are uniformly distributed at the microscale within the bulk structure of the carbon source, sodium does not agglomerate during subsequent high-temperature carbonization due to the steric hindrance of the nearby main structure. This allows the formation of quasi-metallic Na clusters in the hard carbon material, corresponding to... 23 The Na solid-state NMR spectrum has a characteristic peak in the δ shift range of 400ppm-1100ppm. When the hard carbon material is used as a negative electrode material, it can achieve self-replenishment of Na, thereby improving the first coulombic efficiency.

[0123] The inert atmosphere refers to a non-oxidizing atmosphere. For example, the inert atmosphere may be a nitrogen atmosphere, an argon atmosphere, or a mixture of nitrogen and helium.

[0124] 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 some sodium is lost as gaseous sodium during carbonization, hard carbon materials formed using Na-containing carbon sources with Na ion content within the above range can provide sufficient sodium in secondary batteries. On the other hand, it is beneficial to form quasi-metallic sodium with cluster size, thereby achieving its high electrochemical activity.

[0125] For example, the Na ion content in the carbon source is 3wt%, 4wt%, 5wt%, 7wt%, 9wt%, 11wt%, 13wt%, 14wt%, 15wt%, or a value between any two of these values.

[0126] In some embodiments, the Na ion content of the carbon source is 5wt%-13wt% based on its weight. This is more conducive to forming a more suitable amount of quasi-metallic sodium in hard carbon materials, thereby more fully replenishing Na.

[0127] In some embodiments, the carbon source includes at least one of sodium lignin sulfonate, sodium cellulose, and composite materials containing sodium lignin sulfonate and / or sodium cellulose. Due to the presence of numerous cyclic molecular structural units in sodium lignin sulfonate and sodium cellulose, the carbon skeleton is stable during carbonization, which facilitates the formation of cluster-sized quasi-metallic Na from sodium attached to the side groups of the carbon chain. The composite material containing sodium lignin sulfonate and / or sodium cellulose refers to materials that, in addition to sodium lignin sulfonate and / or sodium cellulose with Na-containing side groups, may further contain other suitable carbon sources. Other carbon sources forming the composite material may not contain sodium, such as phenolic resins, epoxy resins, monosaccharides, and polysaccharides, etc. Exemplarily, the carbon source may be sodium lignin sulfonate, sodium cellulose, sodium lignin sulfonate-phenolic resin composite materials, sodium cellulose-phenolic resin composite materials, etc.

[0128] In some embodiments, after carbonization, the preparation method further includes coating via chemical vapor deposition (CVD) in a mixture of inert and organic compound gases. Forming a coating layer on the hard carbon surface helps reduce sodium exposure, improves material stability, and reduces surface activity. CVD coating helps reduce side reactions during the coating process, and the resulting coating layer more effectively protects the quasi-metallic Na in the hard carbon material, thereby further improving the initial coulombic efficiency.

[0129] According to a specific implementation, the coating layer is a carbon coating layer.

[0130] In some embodiments, the coating temperature is 700°C-850°C. This is more conducive to forming a dense coating layer to protect the quasi-metallic Na in the hard carbon material, and also more conducive to reducing the vaporization escape of sodium formed during reduction during carbonization. Exemplarily, the coating temperature is 700°C, 750°C, 800°C, 850°C, or a value between any two of these values.

[0131] In some embodiments, the volume fraction of the organic compound gas in the mixed gas is 2 vol% to 35 vol%. This is more conducive to the formation of a carbon coating layer of suitable thickness on the exterior of the hard carbon particles by an appropriate amount of organic compound gas, so as to adequately protect the quasi-metallic Na in the hard carbon material. Exemplarily, 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 these values. Optionally, the volume fraction of the organic compound gas in the mixed gas is 2 vol% to 30 vol%.

[0132] For example, the organic compound may be at least one of alkanes, alkenes, alkynes, alcohols, and benzene compounds.

[0133] For example, the inert gas may be at least one of nitrogen and argon.

[0134] In some embodiments, the carbonization and CVD coating are carried out continuously in the same reactor. This is more beneficial for fully protecting the quasi-metallic Na in the hard carbon material during the preparation process and reducing the risk of oxidation by air. Exemplarily, the carbonization and CVD coating can be carried out in a tube furnace, with the inlet gas line connected by a T-junction. During CVD coating, the T-junction is used to switch the inert atmosphere gas source of the former to a mixed gas source containing organic compound gases.

[0135] In some embodiments, the carbonization temperature is 600°C-850°C. A carbonization temperature within this range is beneficial for both the complete decomposition of the carbon source to form a suitable hard carbon structure and for reducing sodium vaporization escape. Exemplarily, the carbonization temperature is 600°C, 650°C, 700°C, 800°C, 850°C, or a value between any two of these values.

[0136] In some embodiments, the preparation method further includes a crushing process prior to carbonization. This crushing process can reduce particle size, which is more beneficial for the efficiency of subsequent carbonization. The crushing process can employ conventional methods in the art, for example, by air jet milling, mechanical milling, or ball milling to produce particles with a Dv50 of 4-8 μm.

[0137] Example

[0138] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0139] Example 1

[0140] Preparation of hard carbon materials:

[0141] 1) Broken

[0142] Sodium lignosulfonate raw material (CAS No. 8061-51-6, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., with a total Na ion content of 9.0 wt%) was prepared into a 10 wt% aqueous solution. 21.4 g of Ca(OH)₂ powder was added in batches to 1000 g of the above sodium lignosulfonate aqueous solution. The pH of the solution was controlled between 7.5 and 9.0, and the mixture was stirred thoroughly for 6 hours. After filtration and drying, the treated sodium lignosulfonate was obtained, and the Na ion content was determined to be 8.4 wt% by ICP. The treated sodium lignosulfonate was then ball-milled using zirconia grinding beads and a grinding jar. The product-to-grinding-bead mass ratio was 1:3, the milling speed was 800 rpm, and the time was 8 hours. The particle size distribution Dv50 of the crushed product was measured to be 5 μm using a Mastersizer 3000 laser particle size analyzer.

[0143] 2) Carbonization

[0144] The product obtained in step 1) above was heated to 800°C for 3 hours in a tube furnace (Hefei Kejing, GSL-1400X) under a nitrogen atmosphere at atmospheric pressure at a heating rate of 5°C / min to obtain the matrix.

[0145] 3) Covering

[0146] The matrix obtained in step 2) above is further heated to 800°C for 8 hours in a tube furnace in an atmosphere of nitrogen and acetylene (the volume fraction of acetylene in the mixed gas is 25 vol%) at a heating rate of 5°C / min to obtain hard carbon material.

[0147] Preparation of negative electrode slurry:

[0148] The above-mentioned hard carbon material, styrene-butadiene rubber (SBR) binder, sodium carboxymethyl cellulose (CMC-Na) thickener, and carbon black conductive agent are mixed thoroughly in an appropriate amount of deionized water at a mass ratio of 96.2:1.8:1.2:0.8 to form a uniform negative electrode slurry.

[0149] Preparation of negative electrode sheet:

[0150] 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.

[0151] Preparation of coin cell half-cells:

[0152] 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.

[0153] Example 2

[0154] 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%.

[0155] Example 3

[0156] 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%.

[0157] Example 4

[0158] 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%.

[0159] Example 5

[0160] 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%.

[0161] Comparative Example 1

[0162] 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%.

[0163] Tests related to hard carbon materials:

[0164] 23 Na solid-state NMR test

[0165] 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 Na solid NMR spectrum as follows Figure 7 and Figure 8 As shown.

[0166] Total Na content test

[0167] 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 HJ781-2016. The testing instrument used was ICP-OES, Thermo ICAP7400.

[0168] Surface Na content test

[0169] 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+).

[0170] Specific capacity and first coulombic efficiency test of hard carbon materials

[0171] 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%.

[0172] 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.

[0173] Table 1:

[0174]

[0175] 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.

[0176] Example 6

[0177] Hard carbon materials were prepared using a method similar to that in Example 1, except that the carbonization temperature was 600°C.

[0178] Example 7

[0179] Hard carbon materials were prepared using a method similar to that in Example 1, except that the carbonization temperature was 700°C.

[0180] Example 8

[0181] Hard carbon materials were prepared using a method similar to that in Example 1, except that the carbonization temperature was 850°C.

[0182] 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.

[0183] Table 2:

[0184]

[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]

[0195] 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.

[0196] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a hard carbon material, characterized in that, The 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.

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

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

4. The negative electrode sheet according to any one of claims 1 to 3, characterized in that, 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, characterized in that, 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, characterized in that, 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, characterized in that, The secondary battery includes a negative electrode sheet according to any one of claims 1 to 6.

8. The secondary battery according to claim 7, characterized in that, The secondary battery is a sodium-ion battery.

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

10. An electrical device, characterized in that, Includes the secondary battery according to any one of claims 7-9.

11. A hard carbon material, characterized in that, The 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, characterized in that, The hard carbon material in 23 In Na solid-state NMR spectra, characteristic peaks of δ shifts are observed in the range of 500 ppm to 900 ppm.

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

14. The hard carbon material according to any one of claims 11 to 13, characterized in that, 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, characterized in that, 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, characterized in that, 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, characterized in that, The method includes: 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, characterized in that, 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, characterized in that, 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, characterized in that, 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, characterized in that, 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, characterized in that, The coating is carried out at a temperature of 700℃-850℃.

23. The preparation method according to claim 21 or 22, characterized in that, 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, characterized in that, The carbonization and coating are carried out continuously in the same reactor.

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

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

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