Negative electrode material, negative electrode plate, secondary battery and preparation method

Abstract

The invention discloses a negative electrode material, a negative electrode plate, a secondary battery and a preparation method. The negative electrode material comprises secondary particles and a coating layer coating at least part of the surfaces of the secondary particles, the secondary particle comprises a secondary particle inner core and graphene oxide coating at least part of the surface of the secondary particle inner core, and the secondary particle inner core comprises hard carbon primary particles and a one-dimensional carbon material dispersed around the hard carbon primary particles; the oxygen content of the graphene oxide is less than or equal to 25%, the particle size D50 is 3-20 [mu] m, and the number of layers is 1-10; the coating layer comprises an NASICON type solid electrolyte, the structural formula is Na < 1 + x > M < 2 > A < x > P < 3-x > O < 12 >, M is selected from any one of Ti, Zr, Ge and Sn, A is selected from any one of Si, V and Nb, and x is more than 0 and less than 3. The negative electrode material is used as a negative electrode plate of a secondary battery, and the capacity, the first efficiency, the rate capability and the cycling stability of the secondary battery can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electrochemical materials, in particular to a negative electrode material, a negative electrode sheet, a secondary battery and a preparation method. BACKGROUND

[0002] Hard carbon has high mechanical hardness. When applied to a negative electrode material of a sodium ion battery, the geometric structure of the hard carbon has a large interlayer spacing and a large number of nanopores, which provides more space for the intercalation and diffusion of active ions, thereby being beneficial to maintaining the stability of the structure during the cycle process.

[0003] However, the hard carbon negative electrode material has low capacity, poor electrical conductivity, low compaction density, low first coulomb efficiency, poor rate performance and cycle stability, and the like, and therefore, it is of important research significance and application value to develop a hard carbon negative electrode with high capacity, high compaction, high first coulomb efficiency and excellent electrochemical performance. SUMMARY

[0004] Therefore, the present application provides a negative electrode material, which comprises secondary particles and a coating layer coating at least part of the surface of the secondary particles; the secondary particles comprise a secondary particle core and graphene oxide coating at least part of the surface of the secondary particle core, and the secondary particle core comprises hard carbon primary particles and one-dimensional carbon materials dispersed around the hard carbon primary particles; the oxygen content of the graphene oxide is less than or equal to 25%, the particle size D50 of the graphene oxide is 3 μm to 20 μm, and the number of layers of the graphene oxide is 1 to 10; the coating layer comprises a NASICON-type solid electrolyte, and the structural formula of the NASICON-type solid electrolyte is Na 1+x M2A x P 3– x O 12 , M is selected from any one of Ti, Zr, Ge and Sn, A is selected from any one of Si, V and Nb, and 0 < x < 3.

[0005] In some possible implementations, the average particle size of the hard carbon primary particles is less than or equal to 3 μm.

[0006] In some possible implementations, the particle size D50 of the hard carbon primary particles is 1.2 μm to 1.8 μm.

[0007] In some possible implementations, the specific surface area of the hard carbon primary particles is greater than 10 m 2 / g.

[0008] In some possible implementations, the one-dimensional carbon material comprises at least one of carbon nanotubes and carbon fibers, and the average tube diameter of the carbon nanotubes is 1 nm to 30 nm.

[0009] In some possible implementation manners, the length of the one-dimensional carbon material is 1-10 microns.

[0010] In some possible implementation manners, the secondary particles further comprise carbon dot material, the carbon dot material is arranged on at least the surfaces of the one-dimensional carbon material and the graphene oxide, and the carbon dot material comprises at least one of carbon quantum dots, graphene quantum dots and polymer quantum dots.

[0011] In some possible implementation manners, the chemical formula of the cladding layer NASICON solid electrolyte is Na3Zr2Si2PO 12 .

[0012] In some possible implementation manners, the thickness of the cladding layer is 10-30 nanometers.

[0013] In some possible implementation manners, the mass ratio of the cladding layer in the negative electrode material is less than 10%.

[0014] In some possible implementation manners, the specific surface area of the negative electrode material is less than or equal to 10 m 2 / g.

[0015] In some possible implementation manners, the compaction density of the negative electrode material under a test tonnage of 3 tons is 1.05-1.12 g / cm 3 . 3 .

[0016] In some possible implementation manners, the average particle size of the negative electrode material is 4.5-6.5 microns.

[0017] In some possible implementation manners, the powder conductivity of the negative electrode material is greater than or equal to 48 S / cm.

[0018] The application further provides a preparation method of the negative electrode material, comprising: mixing, grinding a hard carbon, a carbon dot material, a one-dimensional carbon material and a graphene oxide solution, the oxygen content of the graphene oxide being less than or equal to 25%, the particle size D50 of the graphene oxide being 3-20 microns, the number of layers of the graphene oxide being 1-10, adsorbing the carbon dot material on the surfaces of the hard carbon, the one-dimensional carbon material and the graphene oxide, to obtain a mixed solution, the solid content of the mixed solution being 30-50 wt%. The mixed solution is subjected to spray drying to form secondary particles. The secondary particles are mixed with a NASICON solid electrolyte solution and sintered, so that the NASICON solid electrolyte clads at least part of the surfaces of the secondary particles, and the structural formula of the NASICON solid electrolyte is Na 1+x M2A x P 3–x O 12M is selected from any one of Ti, Zr, Ge and Sn, and A is selected from any one of Si, V and Nb, wherein 0 < x < 3, to obtain a negative electrode material.

[0019] In some possible implementation manners, the average tube diameter of the carbon nanotubes in the preparation method is 1 nm to 30 nm.

[0020] In some possible implementation manners, the mass ratio of the hard carbon, the carbon dot material, the one-dimensional carbon material and the graphene oxide in the preparation method is (40-50):1:(1-5):(1-5).

[0021] In some possible implementation manners, the thickness of the coating layer in the preparation method is 10 nm to 30 nm.

[0022] The application further provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer arranged on the negative electrode current collector, the negative electrode active material layer comprising the negative electrode material or the negative electrode material prepared by the preparation method.

[0023] The application further provides a secondary battery comprising the negative electrode sheet.

[0024] In the negative electrode material of the application, the hard carbon primary particles and the one-dimensional carbon material are secondarily granulated by using graphene oxide, and the coating of the NASICON solid electrolyte is performed on the basis of the secondary particles, which is conducive to reducing the risk of aggravation of electrolyte consumption in the cycle process, thereby improving the cycle performance of the secondary battery. The one-dimensional carbon material is distributed between or on the surface of the hard carbon particles to form a conductive network structure, which improves the electronic conductivity of the hard carbon material, reduces the charge transfer resistance and accelerates the electron migration. The graphene oxide helps to reduce the specific surface area of the negative electrode material and improve the particle size uniformity and the compaction density of the negative electrode material. The coating layer formed by the NASICON solid electrolyte also helps to improve the contact interface between the material and the electrolyte, reduce the interface side reaction, improve the ionic conductivity of the hard carbon material and accelerate the diffusion of sodium ions. In the preparation method of the application, the dispersibility of the one-dimensional carbon material and the graphene oxide is improved by the nanocarbon dot material. Therefore, when the negative electrode material of the application is used as the negative electrode sheet of the sodium ion battery, it has the advantages of high capacity, high initial coulomb efficiency, high rate performance and good cycle stability. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 A structure schematic diagram of the negative electrode material provided by an embodiment of the application.

[0026] Figure 2 A scanning electron microscope spectrum of the hard carbon crushing tailings provided by another embodiment of the application.

[0027] Figure 3A scanning electron microscope image of the secondary particles provided for Example 1 of the present application.

[0028] Figure 4 A scanning electron microscope image of the negative electrode material provided for Example 1 of the present application.

[0029] Figure 5 The first cycle charge-discharge test results of the button cell provided for Example 1 of the present application.

[0030] Figure 6 The rate performance test results of the button cell provided for Example 1 of the present application.

[0031] Explanation of main element symbols

[0032] Negative electrode material 100

[0033] Hard carbon primary particles 111

[0034] One-dimensional carbon material 112

[0035] Secondary particles 10

[0036] Secondary particle core 11

[0037] Graphene oxide 12

[0038] Coating layer 20 DETAILED DESCRIPTION

[0039] Embodiments of the present application are described in detail below. The embodiments described below with reference to the drawings are exemplary and are used only to explain the present application, and are not to be construed as limiting the present application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meanings as understood by those skilled in the art to which the present application belongs; the embodiments of the present application and the features in the embodiments can be combined with each other without conflict; in the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the present application, and the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments.

[0040] In the embodiments of the present application, in order to facilitate the description without limiting the present application, the term "connected" used in the patent application specification and claims of the present application is not limited to physical or mechanical connection, whether direct or indirect. "Up", "down", "above", "below", "left", "right", and the like are only used to indicate relative positional relationships, and when the absolute position of the described object changes, the relative positional relationship also changes accordingly.

[0041] An embodiment of the present application provides a secondary battery, which includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located in the housing.

[0042] The outer casing can be a packaging bag sealed with an encapsulating film (such as aluminum-plastic film), for example, a pouch battery for a secondary battery. In other embodiments, the secondary battery can also be a steel-cased battery, an aluminum-cased battery, etc.

[0043] The electrode assembly includes a positive electrode, a negative electrode, and a separator, with the separator disposed between the positive and negative electrode. The electrode assembly can be a stacked structure, formed by alternating layers of the positive electrode, separator, and negative electrode. In other embodiments, the electrode assembly can also be a wound structure, formed by winding the positive electrode, separator, and negative electrode after they have been stacked sequentially.

[0044] Positive electrode sheet

[0045] The positive electrode includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector. The positive current collector can be a metal foil or a composite current collector. In some embodiments, the metal foil may be aluminum foil. In some embodiments, the positive current collector may include a polymeric material base layer and a metal layer formed on at least one surface of the polymeric material base layer. In some embodiments, non-limiting examples of the metal layer material may include one or more of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys; in some embodiments, non-limiting examples of the polymeric material base layer material may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), and other substrates. However, this application is not limited to these materials, and any material disclosed in the prior art or conventionally known that can be used as a positive current collector for sodium-ion batteries may also be used.

[0046] The positive electrode active layer includes a positive electrode active material, which comprises a compound capable of reversibly inserting and deintercalating sodium ions. In some embodiments, the positive electrode active material may include one or more of sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds. In some embodiments, the transition metal in the sodium transition metal oxide may include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, and a non-limiting example of the sodium transition metal oxide may be Na. x MO2, wherein M may include one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < x ≤ 1. In some embodiments, the sodium transition metal oxide may include sodium-iron composite oxide (NaFeO2), sodium-cobalt composite oxide (NaCoO2), sodium-chromium composite oxide (NaCrO2), sodium-manganese composite oxide (NaMnO2), sodium-nickel composite oxide (NaNiO2), and sodium-nickel-titanium composite oxide (NaNi). 1 / 2 Ti 1 / 2O2), sodium nickel manganese composite oxide (NaNi 1 / 2 Mn 1 / 2 O2), sodium iron manganese composite oxide (Na 2 / 3 Fe 1 / 3 Mn 2 / 3 O2), sodium nickel cobalt manganese composite oxide (NaNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), sodium iron phosphate compound (NaFePO4), sodium manganese phosphate compound (NaMnPO4), sodium cobalt phosphate compound (NaCoPO4). In some embodiments, the polyanionic compound can be a compound having sodium ions, transition metal ions, and tetrahedral (YO4) n- anion units, the transition metal can include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, Y can be one or more of P, S, and Si, and n represents the valence state of (YO4) n- . Optionally, the polyanionic compound can further have halide anions, and the halide can be one or more of F, Cl, and Br. In other embodiments, the polyanionic compound can also be a compound having sodium ions, tetrahedral (YO4) n- anion units, polyhedral units (ZO y ) m+ , and optional halide anions, Y can be one or more of P, S, and Si, n represents the valence state of (YO4) n- , Z represents a transition metal, which can include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, and m represents the valence state of (ZO y ) m+ . In some embodiments, the polyanionic compound can include one or more of NaFePO4, Na3V2(PO4)3, Na4Fe3(PO4)2(P2O7), NaMPO4F, Na3V2(PO4)2F3, and Na3(VO y )2(PO4)2F 3-2y (0≤y≤1). In NaMPO4F, M can include one or more of V, Fe, Mn, and Ni. In some embodiments, the Prussian blue type compound can be a compound having sodium ions, transition metal ions, and cyanide ions (CN - ). The transition metal can include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. Non-limiting examples of the Prussian blue type compound can be Na a Meb Me’ c (CN)6, where Me and Me’ can each independently be one or more of Ni, Cu, Fe, Mn, Co, and Zn, 0 < a < 2, 0 < b < 1, 0 < c < 1. However, the present application is not limited to these materials, and any material known in the art, or conventionally known, that can be used as a sodium-ion battery cathode active material can be used.

[0047] The cathode active layer can also include a binder to bind the cathode active material particles to facilitate formation of a film layer, and to improve the adhesion between the cathode active layer and the cathode current collector. In some embodiments, the binder can include one or more 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 acrylate resin. However, the present application is not limited to these materials, and any material known in the art, or conventionally known, that can be used as a sodium-ion battery cathode binder can be used.

[0048] The cathode active layer can also include a conductive material. In some embodiments, the conductive material can include, but is not limited to, one or more of super-P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. However, the present application is not limited to these materials, and any material known in the art, or conventionally known, that can be used as a sodium-ion battery cathode conductive material can be used.

[0049] Negative electrode sheet

[0050] The anode sheet includes an anode current collector and an anode active layer disposed on at least one surface of the anode current collector. The anode current collector can be at least one of a copper foil, a nickel foil, a stainless steel foil, a titanium foil, or a carbon-based current collector, and can be any composite current collector known in the art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foils and a polymer substrate. The anode active layer includes an anode material 100.

[0051] Referring to Figure 1 , the anode material 100 includes secondary particles 10 and a coating layer 20 coating at least a portion of the surface of the secondary particles 10. The secondary particles 10 include a secondary particle core 11 and a graphene oxide 12 coating at least a portion of the surface of the secondary particle core 11. The secondary particle core 11 includes hard carbon primary particles 111 and one-dimensional carbon material 112 dispersed around the hard carbon primary particles 111.

[0052] In some embodiments, the average particle size of the hard carbon primary particles 111 is less than or equal to 3 pm. For example, the average particle size of the hard carbon primary particles 111 can be 0.5 pm, 1 pm, 1.5 pm, 2 pm, 2.5 pm, 3 pm, or any value within a range defined by any two of the aforementioned values. In some embodiments, the particle size D50 of the hard carbon primary particles 111 is 1.2 pm to 1.8 pm. For example, the particle size D50 of the hard carbon primary particles 111 can be 1.2 pm, 1.4 pm, 1.5 pm, 1.6 pm, 1.8 pm, or any value within a range defined by any two of the aforementioned values. The D50, also referred to as the "median particle size", represents the particle size of the hard carbon primary particles 111 at which 50% of the volume is accounted for from the small particle size side in the particle size distribution on a volume basis, i.e., the volume of the hard carbon primary particles 111 smaller than this particle size accounts for 50% of the total volume of the hard carbon primary particles 111. The average particle size or the particle size D50 of the hard carbon primary particles 111 can be determined by a laser particle size analyzer.

[0053] In some embodiments, the specific surface area of the hard carbon primary particles 111 is greater than 10 m 2 / g. The specific surface area of the hard carbon primary particles 111 can be measured by a specific surface and pore size analyzer. Selecting the hard carbon primary particles 111 within the particle size range or the specific surface area range is conducive to forming the secondary particles 10, thereby improving the compaction density and rate capability of the negative electrode material 100. When the average particle size of the hard carbon primary particles 111 is too large, it is not conducive to secondary granulation, and the ion migration path is too long, which is not conducive to the rate capability of the negative electrode material 100.

[0054] It should be noted that the hard carbon primary particles 111 selected in the present application can be hard carbon crushing tailings obtained by crushing and grading of biomass-based, resin-based, coal-based and the like raw materials after pre-carbonization. Such hard carbon primary particles 111 have a smaller average particle size or a larger specific surface area, and their own contact area with the electrolyte is increased, which is easy to consume more Na + +, promote the formation of irreversible SEI film, and is not conducive to good initial coulombic efficiency and capacity. However, after the above hard carbon primary particles 111 are made into the negative electrode material 100 of the present application, the negative electrode material 100 can have a relatively higher average particle size (4.5 pm to 6.5 pm) and a relatively smaller specific surface area (less than or equal to 10 m 2 / g), thereby reducing the area of contact with the electrolyte and reducing the risk of continuous generation of irreversible SEI film, which is conducive to good initial coulombic efficiency and capacity. Moreover, by fully utilizing the tailings to obtain the negative electrode material 100, the hard carbon crushing tailings can be reused, reducing the direct disposal of hard carbon crushing tailings and reducing production costs.

[0055] The one-dimensional carbon material 112 can be distributed between or on the surface of the hard carbon primary particles 111 to form a conductive network structure. The one-dimensional carbon material 112 has good electrical conductivity and unique electron conduction channels along its length direction, which is conducive to improving the electronic conductivity of the hard carbon primary particles 111, reducing the charge transfer resistance, and accelerating the migration of electrons.

[0056] In some embodiments, the one-dimensional carbon material 112 includes carbon nanotubes, which can be selected from at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, or vapor-grown carbon fibers.

[0057] In some embodiments, the average tube diameter of the carbon nanotubes can be 1 nm to 30 nm, and the length can be 1 μm to 10 μm. For example, the tube diameter of the carbon nanotubes can be 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, or any value within the range between any two of the above values. Controlling the average tube diameter of the carbon nanotubes within the above range is conducive to improving the electrical conductivity of the composite material. The length of the carbon nanotubes can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or any value within the range between any two of the above values. When the carbon nanotubes have the above tube diameter or length, it is conducive to the mutual connection of the carbon nanotubes to form a more stable network structure. The average tube diameter and length of the carbon nanotubes can be characterized by transmission electron microscopy.

[0058] The oxygen content of the graphene oxide 12 is less than or equal to 25%. For example, the oxygen content of the graphene oxide 12 can be 5%, 10%, 13%, 15%, 17%, 19%, 20%, 22%, 23%, 25%, or any value within the range between any two of the above values.

[0059] Generally, the graphene oxide 12 contains oxygen-containing functional groups such as hydroxyl and carboxyl groups. The defect regions formed by oxygen atoms in graphene can play a role in supporting and connecting its own structure or combining with other materials in multiple directions. Compared with graphene, the graphene oxide 12 has better hydrophilicity and flexibility, which is conducive to bending to form a wrapped morphology outside the inner core.

[0060] In this application, the oxygen content of the graphene oxide 12 is less than or equal to 25%, which belongs to weakly oxidized graphene. Due to the low degree of oxidation, the graphene oxide 12 has relatively few oxygen-containing functional groups and good hydrophilicity, so it can retain the excellent electrical conductivity of graphene as much as possible, thereby improving the electronic conductivity of the negative electrode material 100. Too high oxygen content can lead to too many defect regions in the material, resulting in a decrease in the electrical conductivity of the negative electrode material 100. The degree of oxidation of the graphene oxide 12 can be measured by an oxygen content analyzer.

[0061] In some embodiments, the specific surface area of the graphene oxide 12 is greater than 10 m 2The specific surface area of the negative electrode material 100 of the hard carbon primary particle 111 is less than or equal to 10 m2 / g 2 / g, indicating that the secondary granulation by the graphene oxide 12 can be beneficial to reduce the specific surface area of the negative electrode material 100. The secondary particle 10 formed by the secondary granulation of the graphene oxide 12 has a core-shell structure, which is also beneficial to improve the particle uniformity of the negative electrode material 100, thereby improving the compaction density and reducing the specific surface area of the negative electrode material 100. Therefore, the risk of electrolyte consumption aggravation during the cycle process can be reduced, thereby improving the cycle performance of the secondary battery. Moreover, since the risk of increased by-products and impedance caused by electrolyte consumption is reduced, it is also beneficial to improve the rate performance of the secondary battery.

[0062] The particle size D50 of the graphene oxide 12 is 3 μm to 20 μm. For example, the particle size D50 of the graphene oxide 12 can be 3 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, or any value within the range composed of any two of the above values. By controlling the particle size D50 of the graphene oxide 12 within a suitable range, the flexible graphene oxide 12 can be beneficial to coat the hard carbon primary particle 111 and the one-dimensional carbon material 112 to form the secondary particle 10 during the spray drying process. If the particle size D50 of the graphene oxide 12 is too small, it will increase the difficulty of forming the secondary particle 10, and if the particle size D50 of the graphene oxide 12 is too large, it will cause the graphene oxide 12 to agglomerate more and be more difficult to disperse, also increasing the difficulty of forming the secondary particle 10. Too small or too large particle size D50 of the graphene oxide 12 is not conducive to forming the negative electrode material 100 with small specific surface area, high powder conductivity, and large compaction density. Among them, D50 is also called "median particle size", which represents the particle size of the graphene oxide 12 reaching 50% of the total volume of the graphene oxide 12 in the volume-based particle size distribution from the small particle size side, that is, the volume of the graphene oxide 12 smaller than this particle size accounts for 50% of the total volume of the graphene oxide 12. The particle size D50 of the graphene oxide 12 can be measured by a laser particle size analyzer.

[0063] The number of layers of the graphene oxide 12 is 1 to 10 layers. For example, the number of layers of the graphene oxide 12 can be 1 layer, 2 layers, 3 layers, 4 layers, 5 layers, 6 layers, 7 layers, 8 layers, 9 layers, or 10 layers. The thickness of the graphene oxide 12 with the number of layers of 1 to 10 layers corresponds to 0.335 nm to 3.35 nm. By controlling the number of layers of the graphene oxide 12 within a suitable range, the dispersibility and flexibility of the graphene oxide 12 are both good, which is beneficial to coat the hard carbon primary particle 111 and the one-dimensional carbon material 112 to form the secondary particle 10, and reduces the risk of not being able to form the secondary particle 10 due to too high number of layers and too thick material. Among them, the number of layers of the graphene oxide 12 can be measured by an atomic force microscope or a transmission electron microscope.

[0064] The coating layer 20 comprises a NASICON-type solid-state electrolyte, and the structural formula of the NASICON-type solid-state electrolyte is Na 1+ x M2A x P 3–x O 12 , M is selected from any one of Ti, Zr, Ge and Sn, A is selected from any one of Si, V and Nb, and 0 < x < 3.

[0065] The NASICON-type solid-state electrolyte used in the present application belongs to a sodium superionic conductor type solid-state electrolyte. Such a sodium superionic conductor type solid-state electrolyte has high ionic conductivity, high mechanical strength, high air, chemical and electrochemical stability, is conducive to improving the contact interface between the negative electrode material 100 and the electrolyte, reducing the interface side reaction, and can also improve the ionic conductivity of the hard carbon material and accelerate the diffusion of sodium ions, thereby further improving the cycle performance and rate performance of the secondary battery.

[0066] In some embodiments, the chemical formula of the NASICON-type solid-state electrolyte is Na3Zr2Si2PO 12 In the NASICON-type solid-state electrolyte, the partial substitution of P by Si in the monoclinic crystal structure leads to a decrease in the symmetry of the crystal structure, splitting of the Na2 site (generating Na2 and Na3 sites), and formation of two Na + transport channels, and the additional Na + occupied sites can serve as exchange sites for ion transport, thereby helping to improve the ionic conductivity and further improve the rate performance of the secondary battery.

[0067] In some embodiments, the thickness of the coating layer 20 is 10 nm to 30 nm. For example, the thickness of the coating layer 20 can be 10 nm, 13 nm, 15 nm, 20 nm, 25 nm, 30 nm or any value within the range between any two of the above values. This is conducive to improving the interface contact between the hard carbon material and the electrolyte, reducing the interface side reaction, improving the ionic conductivity of the hard carbon material, and accelerating the diffusion of sodium ions.

[0068] In some embodiments, the mass percentage of the coating layer 20 in the negative electrode material 100 is less than 10%. For example, the mass percentage of the coating layer 20 in the negative electrode material 100 can be 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, or any value within a range defined by any two of the above values. The mass percentage of the coating layer 20 in the negative electrode material 100 within the above range can allow the electrode active ions to have sufficient intercalation capacity, thereby allowing the battery to have a higher charge and discharge capacity. A too high mass percentage of the coating layer 20 in the negative electrode material 100 can cause the coating layer 20 to thicken, and the thicker coating layer 20 can reduce the electronic conductivity of the negative electrode material 100.

[0069] In some embodiments, the negative electrode material 100 further includes a carbon dot material, which is disposed at least on the surfaces of the one-dimensional carbon material 112 and the graphene oxide 12. The carbon dot material can include at least one of a carbon quantum dot, a graphene quantum dot, and a polymer quantum dot. The carbon dot material used in the present application is a zero-dimensional surface-oxidized nanocarbon material, which can have an average particle size of 1 nm to 20 nm, a carbon content of about 30% to 70%, and an oxygen content of greater than 30%. The carbon dot material has a π bond on the surface and an oxygen functional group on the edge, and thus can be adsorbed on the surfaces of the hard carbon primary particles 111, the one-dimensional carbon material 112, and the graphene oxide 12 through the π bond and can modify the one-dimensional carbon material 112 and the graphene oxide 12 through non-covalent bonding. The carbon dot material can also act as an active agent and a dispersant, reducing the risk of aggregation of the one-dimensional carbon material 112 and folding of the graphene oxide 12, and filling the space gaps, thereby improving the poor dispersibility of the one-dimensional carbon material 112 and the graphene oxide 12. The carbon dot material of the present application can be characterized by a transmission electron microscope.

[0070] In some embodiments, the specific surface area of the negative electrode material 100 is less than or equal to 10 m 2 / g. For example, the specific surface area of the negative electrode material 100 can be 0.5 m 2 / g, 1 m 2 / g, 2 m 2 / g, 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 7 m 2 / g, 8 m 2 / g, 9 m 2 / g, 10 m 2 / g, or any value within a range defined by any two of the above values. This can reduce the risk of increased consumption of electrolyte during the cycle, thereby improving the cycle performance of the secondary battery. Moreover, since the risk of increased by-products and impedance due to the consumption of electrolyte is reduced, the rate performance of the secondary battery can also be improved.

[0071] In some embodiments, the compacted density of the negative electrode material 100 is 1.05 g / cm3to 1.12 g / cm3. 3 3 For example, the compacted density of the negative electrode material 100 can be 1.05 g / cm3, 1.07 g / cm3, 1.09 g / cm3, 1.10 g / cm3, 1.11 g / cm3, 1.12 g / cm3, or any value within a range defined by any two of the above values. This can result in good contact between the negative electrode particles, reduce impedance, and thus facilitate the transport of active ions in the negative electrode material 100, thereby further improving the rate performance of the secondary battery. 3 3 3 3 3 3

[0072] In some embodiments, the average particle size of the negative electrode material 100 is 4.5 pm to 6.5 pm. For example, the average particle size of the negative electrode material 100 can be 4.5 pm, 4.8 pm, 5.0 pm, 5.2 pm, 5.5 pm, 5.8 pm, 6.0 pm, 6.2 pm, 6.5 pm, or any value within a range defined by any two of the above values. In some embodiments, the powder conductivity of the negative electrode material 100 is greater than or equal to 48 S / cm. For example, the powder conductivity of the negative electrode material 100 can be 48 S / cm, 51 S / cm, 55 S / cm, 60 S / cm, 65 S / cm, 70 S / cm, 75 S / cm, or 80 S / cm.

[0073] The negative electrode material 100 of the present application has the advantages of average particle size, low specific surface area, high conductivity, high capacity, high compacted density, and high first coulombic efficiency, and thus is helpful to solve the problems of low capacity, low first coulombic efficiency, low compacted density, poor rate performance, and poor long cycle stability of hard carbon negative electrodes.

[0074] The negative electrode active layer also includes a binder to bind the negative electrode active material particles to facilitate the formation of a film layer, and also to improve the bonding force between the negative electrode active layer and the negative electrode current collector. In some embodiments, the binder can include one or more 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). However, the present application is not limited to these materials, and any material disclosed in the prior art or conventionally known as a binder for a sodium ion battery negative electrode can also be used.

[0075] ​​​​​​​The negative active layer can also include a conductive material. In some embodiments, the conductive agent can include one or more of super-P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. However, the present application is not limited to these materials, and any material known in the art, or conventionally known, that can be used as a sodium-ion battery negative electrode conductive material can also be used.

[0076] Separator film

[0077] The separator film includes a film layer having a porous structure, and the material thereof includes, but is not limited to, at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the separator film can be a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film, etc. The separator film can also be a single-layer film, or can be a multi-layer composite film, and when the separator film is a multi-layer composite film, the materials of the respective layers can be the same or different. However, the present application is not limited to these materials, and any material known in the art, or conventionally known, that can be used as a sodium-ion battery separator film can also be used.

[0078] Electrolyte

[0079] The electrolyte has a function of conducting ions between the positive electrode sheet and the negative electrode sheet. The state of the electrolyte can be one or more of a gel state, a solid state, and a liquid state. In some embodiments, the electrolyte employs an electrolytic solution. The electrolytic solution has a function of conducting active ions between the positive electrode sheet and the negative electrode sheet. In some embodiments, the electrolytic solution includes an organic solvent, a sodium salt, and optional additives disclosed in the prior art. In some embodiments, the sodium salt can include at least one of NaPF6, NaClO4, NaBCl4, NaSO3CF3, and Na(CH3)C6H4SO3. The above-mentioned sodium salt can be used alone or simultaneously with two or more. In some embodiments, the organic solvent can include at least one of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE). The above-mentioned organic solvent can be used alone or simultaneously with two or more. In some embodiments, the additives can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include additives capable of improving certain properties of the battery, such as additives for improving overcharge performance of the battery, additives for improving high-temperature or low-temperature performance of the battery, etc. In some embodiments, the additives can include at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), ethylene sulfate (DTD), propylene sulfate, ethylene sulfite (ES), 1,3-propane sulfonate lactone (PS), 1,3-propylene sulfonate lactone (PST), sulfonate cyclic quaternary ammonium salt, succinic anhydride, succinonitrile (SN), adiponitrile (AND), tris(trimethylsilyl)phosphate (TMSP), tris(trimethylsilyl)borate (TMSB). The electrolytic solution can be prepared according to conventional methods in the art. In some embodiments, the organic solvent, the sodium salt, and the optional additives can be mixed uniformly to obtain the electrolytic solution. The order of addition of the materials is not particularly limited, and in some embodiments, the sodium salt and the optional additives can be added to the organic solvent and mixed uniformly to obtain the electrolytic solution, or the sodium salt can be added to the organic solvent first, and then the optional additives can be added to the organic solvent and mixed uniformly to obtain the electrolytic solution. However, the present application is not limited to these materials, and any material disclosed in the prior art or conventionally known as a material that can be used as an electrolyte for a sodium ion battery can be used.

[0080] Another embodiment of the present application also provides a preparation method of the negative material 100, comprising:

[0081] In the first step, the hard carbon primary particles 111, the carbon dot material, the one-dimensional carbon material 112 and the solution of the graphene oxide 12 are mixed and ground, the oxygen content of the graphene oxide 12 is less than or equal to 25%, the particle size D50 is 3-20 μm, and the number of layers is 1-10, so that the carbon dot material is adsorbed on the surface of the hard carbon primary particles 111, the one-dimensional carbon material 112 and the graphene oxide 12, and a mixed solution is obtained, and the solid content of the mixed solution is 30-50 wt%.

[0082] In this process, the one-dimensional carbon material 112 is dispersed around the hard carbon primary particles 111. The hard carbon primary particles 111 exist in the form of fine powder in the mixed solution, and the hard carbon crushing tailings can also be directly selected as the raw material on the basis of selection, such as biomass-based, resin-based and coal-based materials after pre-carbonization and crushing and grading. The specific surface area of the hard carbon crushing tailings is generally greater than 10 m 2 / g, and the average particle size is generally less than 3 μm. Please refer to Figure 2 The scanning electron microscope (SEM) spectrum characterization result shows that the hard carbon crushing tailings are irregular polygons, the particle size D50 is 1.2 μm, and the surface has protruding parts with a certain roughness. The surface of the hard carbon crushing tailings can be ground by the spherical zirconium oxide medium during the grinding process, so as to reduce the roughness of the surface. The relative motion between the medium and the hard carbon crushing tailings is generated by the high-speed rotation of the medium, so as to rub off the protruding parts on the surface and make them smoother.

[0083] The appropriate grinding is also beneficial to further improve the dispersibility and uniformity of the mixed solution as a whole, and is beneficial to the subsequent forming step. The rotation speed of the sand mill can be set to 1000-2000 r / min, and the working time can be set to 1-3 h for appropriate grinding.

[0084] The carbon dot material can be adsorbed on the surface of the hard carbon primary particles 111, the one-dimensional carbon material 112 and the graphene oxide 12 by π bond, and can be non-covalently modified on the one-dimensional carbon material 112 and the graphene oxide 12. As an active agent and a dispersant, it is beneficial to reduce the agglomeration of the one-dimensional carbon material 112 and the folding risk of the graphene oxide 12, fill the space gap, and thus improve the dispersibility and uniformity of the mixed solution as a whole.

[0085] Controlling the solid content of the mixed solution within the above range is beneficial to controlling the particle size of the secondary particles 10. Too high solid content will cause the secondary particles 10 to become larger, the pores between the particles to increase, and thus the conductivity and the compaction density to decrease.

[0086] In some embodiments, the mass ratio of the hard carbon primary particles 111, the carbon dot material, the one-dimensional carbon material 112 and the graphene oxide 12 is (40-50):1:(1-5):(1-5). Under this mass ratio, the one-dimensional carbon material 112 and the graphene oxide 12 are well dispersed, the π bond in the carbon dot acts as a bridge, and the one-dimensional carbon material 112 and the graphene oxide 12 are uniformly dispersed between the hard carbon primary particles 111.

[0087] In the second step, the mixed solution is spray dried to form secondary particles 10.

[0088] By virtue of the good flexibility and bending property of the graphene oxide 12, the spray drying process is used for secondary granulation, so that the graphene oxide 12 at least partially coats the hard carbon primary particles 111 and the one-dimensional carbon material 112.

[0089] In the spray drying process, in order to reduce the oxidation of the material by air, the spray drying process can be carried out in an inert protective gas atmosphere; at the same time, the spray drying process is carried out under appropriate temperature conditions, which is conducive to the uniformity and stability of the formation of the secondary particles 10, for example, the temperature of the feeding port of the spray drying equipment can be set to 180-250°C, and the temperature of the discharging port can be set to 100-150°C; in order to control the particle size and morphology of the secondary particles 10 in the spray drying process, the solid content of the mixed solution can be controlled to be between 30wt% and 50wt%.

[0090] Alternatively, after the secondary particles 10 are formed, the secondary particles 10 are further purified and washed to obtain purified secondary particles 10. The purpose of purification is to remove metal elements in the secondary particle 10 solution, and the purpose of washing is to make the solution pH=6.5-7.5 or neutral. If the formed secondary particle 10 solution is alkaline, it can be selected to be washed in an acid solution such as a hydrochloric acid solution, a nitric acid solution, a hydrofluoric acid solution and a sulfuric acid solution until the solution is neutral. If the formed secondary particle 10 solution is acidic, it can be selected to be washed in an alkali solution such as potassium hydroxide and sodium hydroxide until the solution is neutral.

[0091] In the third step, the secondary particles 10 are mixed with a NASICON-type solid electrolyte solution and sintered, so that the NASICON-type solid electrolyte coats at least part of the surface of the secondary particles 10. The structural formula of the NASICON-type solid electrolyte is Na 1+ x M2A x P 3–x O 12 , M is selected from any one of Ti, Zr, Ge and Sn, A is selected from any one of Si, V and Nb, 0

[0092] In the process of mixing, the solvent can be volatilized by continuous stirring at an appropriate temperature, and the mixed material can be sintered to form a coating layer 20 on the surface of the secondary particles 10, i.e., on the surface of the graphene oxide 12, to obtain the negative electrode material 100.

[0093] To achieve good coating effect, the appropriate coating thickness can be controlled, such as 10 nm to 30 nm. The coating process can control the mass ratio of the secondary particles 10 to the solid electrolyte precursor solution to be 100:(1-3), and is carried out in an inert protective gas atmosphere, the sintering temperature is controlled between 1100°C and 1400°C, and the holding time is controlled between 2h and 5h.

[0094] The scheme of the present application will be explained below in combination with examples. Those skilled in the art will understand that the following examples are only for explaining the present application and cannot be understood as limiting the present application. Unless otherwise stated, the reagents, software and instruments involved in the following examples, which are not specifically stated, are all conventional commercially available products or open source.

[0095] Na 1+x M2A x P 3–x O 12 Preparation of solid electrolyte:

[0096] For example, Na3Zr2Si2PO 12 , the preparation method is as follows: according to the stoichiometric ratio of Na3Zr2Si2PO 12 , NaNO3, C8H 20 O4Si, ZrO(NO3)2 and NH4H2PO4 are used as raw materials, C8H 20 O4Si is dispersed in ethanol, citric acid is added to adjust the pH to 1-1.5, then NaNO3, ZrO(NO3)2 and NH4H2PO4 are added in turn, and Na3Zr2Si2PO 12 solid electrolyte solution is prepared.

[0097] Example 1: This example provides a kind of negative electrode material and button cell based on the negative electrode material.

[0098] (1) The preparation method of the negative electrode material comprises:

[0099] S1, 1g graphene quantum dots are weighed and dispersed in 116mL pure water, 45g hard carbon powder tailings, 2g carbon nanotubes (average tube diameter is 1nm) and 2g graphene oxide (particle size D50 is 10μm, thickness is 7 layers, oxygen content is 15%) are added in turn during stirring, and the solution is continuously stirred and mixed uniformly, then it is introduced into a sand mill, the speed is 1500r / min, and grinding and dispersion are carried out for 2h to form a mixed solution;

[0100] S2, the mixed solution is spray dried under a nitrogen protective atmosphere, the solid content of the mixed solution is 30wt%, the feeding port temperature is 200℃, and the discharging port temperature is 120℃, to obtain secondary particles;

[0101] S3, the secondary particles are placed in a 12wt% hydrochloric acid solution, heated and stirred at 70℃ for 2h, then centrifuged, washed with pure water for several times until the pH is 7.0, and dried to obtain purified secondary particles;

[0102] S4, the purified secondary particles are added into a Na3Zr2Si2PO 12 The mass ratio of the two in the solid electrolyte solution is 100:2, the solvent is completely volatilized by heating and stirring at 100℃, to obtain secondary particle / solid electrolyte composite precursor powder, and the composite precursor powder is placed in a sintering furnace, heated to 1250℃ under a nitrogen atmosphere, and kept for 3h to obtain the negative electrode material.

[0103] The material of Example 1 is microscopically characterized by using a Hitachi S-4800 high-magnification scanning electron microscope: firstly, the secondary particles obtained in S2 are tested, please refer to Figure 3 , the scanning electron microscope spectrum shows that after spray drying granulation, the graphene oxide wraps the hard carbon tailings and carbon nanotubes, the outside is a wrinkled flexible graphene oxide wrapping layer, and the inside is the primary particle hard carbon tailings and carbon nanotubes; secondly, the negative electrode material obtained in S4 is tested, please refer to Figure 4 , the scanning electron microscope spectrum shows that the outside of the negative electrode material is a smooth solid electrolyte coating layer, and the inside is the secondary particles. The above results show that the preparation method of the application can successfully perform secondary granulation on the primary particle of the hard carbon material to form a negative electrode material with a graphene oxide wrapping layer and a solid electrolyte coating layer.

[0104] (2) The preparation method of the button cell includes: mixing the negative electrode material, conductive carbon black and LA133 binder in a mass ratio of 91:3:6 in pure water, coating on an aluminum foil current collector, vacuum drying at 80℃ to obtain an electrode sheet, and then assembling into a button cell in a glove box for testing, wherein the cathode uses a sodium sheet metal, the separator is a glass fiber with a model of GF / D from Whateman company, and the electrolyte is a 1.0M NaPF6 in DEGDME solution, to prepare a button cell.

[0105] Example 2:

[0106] The difference from Example 1 is that (1) the preparation method of the negative electrode material includes:

[0107] S1, 1g graphene quantum dots were weighed and dispersed in 43mL pure water, and 40g hard carbon tailings, 1g carbon nanotubes (average tube diameter 30nm) and 1g graphene oxide (particle size D50 20μm, layer number 10, oxygen content 20%) were sequentially added during stirring. After the solution was uniformly mixed by continuous stirring, it was introduced into a sand mill at a speed of 2000r / min and ground and dispersed for 1h to form a mixed solution;

[0108] S2, the above mixed solution was spray dried under a nitrogen protective atmosphere, the solid content of the mixed solution was 50wt%, the spray conditions were inlet temperature 180℃ and outlet temperature 100℃, and secondary particles formed by graphene oxide wrapped hard carbon tailings and carbon nanotubes were obtained;

[0109] S3, the above secondary particles were placed in a 10wt% hydrochloric acid solution, heated and stirred at 70℃ for 2h, then centrifuged and washed with pure water until the pH was 6.5, and then dried to obtain purified secondary particles;

[0110] S4, the above purified secondary particles were added to Na3Zr2Si2PO 12 The mass ratio of the two was 100:1, and the solvent was completely volatilized by heating and stirring at 80℃ to obtain a secondary particle / solid electrolyte composite precursor powder. The composite precursor powder was placed in a sintering furnace, heated to 1100℃ under a nitrogen atmosphere, and held for 5h to obtain a negative electrode material.

[0111] Example 3:

[0112] The difference from Example 1 is that (1) the preparation method of the negative electrode material comprises:

[0113] S1, 1g graphene quantum dots were weighed and dispersed in 142mL pure water, and 50g hard carbon tailings, 5g carbon nanotubes (average tube diameter 10nm) and 5g graphene oxide (particle size D50 5μm, layer number 5, oxygen content 18%) were sequentially added during stirring. After the solution was uniformly mixed by continuous stirring, it was introduced into a sand mill at a speed of 1000r / min and ground and dispersed for 3h to form a mixed solution;

[0114] S2, the above mixed solution was spray dried under a nitrogen protective atmosphere, the solid content of the mixed solution was 30wt%, the spray conditions were inlet temperature 250℃ and outlet temperature 150℃, and secondary particles formed by graphene oxide wrapped hard carbon tailings and carbon nanotubes were obtained;

[0115] S3, the secondary particles were placed in a 30wt% hydrochloric acid solution, heated and stirred at 70°C for 2h, then centrifuged, washed with pure water several times until the pH was 7.5, and dried to obtain purified secondary particles;

[0116] S4, the purified secondary particles were added to a Na3Zr2Si2PO 12 The mass ratio of the two was 100:3, heated and stirred at 100°C until the solvent completely evaporated to obtain secondary particle / solid electrolyte composite precursor powder, which was placed in a sintering furnace, heated to 1400°C under a nitrogen atmosphere, and held for 2h to obtain the negative electrode material.

[0117] Example 4:

[0118] The difference from Example 1 is that the preparation method of the negative electrode material includes:

[0119] S1, 1g of graphene quantum dots was weighed and dispersed in 75mL of pure water, 43g of hard carbon powder tailings, 3g of carbon nanotubes (average tube diameter 15nm) and 3g of graphene oxide (particle size D50 3μm, layer number 1, oxygen content 16%) were added in sequence during stirring, the solution was mixed uniformly and then introduced into a sand mill, the speed was 1200r / min, and grinding and dispersion were carried out for 2h to form a mixed solution;

[0120] S2, the mixed solution was spray dried under a nitrogen protective atmosphere, the solid content of the mixed solution was 40wt%, the inlet temperature was 200°C, and the outlet temperature was 130°C to obtain secondary particles formed by graphene oxide wrapped hard carbon tailings and carbon nanotubes;

[0121] S3, the secondary particles were placed in a 15wt% hydrochloric acid solution, heated and stirred at 70°C for 2h, then centrifuged, washed with pure water several times until the pH was 7.0, and dried to obtain purified secondary particles;

[0122] S4, the purified secondary particles were added to a Na3Zr2Si2PO 12 The mass ratio of the two was 100:3, heated and stirred at 100°C until the solvent completely evaporated to obtain secondary particle / solid electrolyte composite precursor powder, which was placed in a sintering furnace, heated to 1400°C under a nitrogen atmosphere, and held for 2h to obtain the negative electrode material.

[0123] Example 5:

[0124] The difference from Example 1 is that S3 is not performed in (1).

[0125] Example 6:

[0126] The difference from Example 1 is that in S4 of (1), the temperature is raised to 1000°C under a nitrogen atmosphere for 3h.

[0127] Example 7:

[0128] The difference from Example 1 is that in S4 of (1), the mass ratio of the secondary particles to the solid electrolyte precursor solution is 100:10.

[0129] Example 8:

[0130] The difference from Example 1 is that in S1 of (1), the average tube diameter of the carbon nanotubes is 100nm.

[0131] Comparative Example 1:

[0132] The difference from Example 1 is that in S4 of (1), no solid electrolyte solution is added, and only the obtained secondary particles are heated to 1250°C under a nitrogen atmosphere for 3h to obtain a hard carbon material.

[0133] Comparative Example 2:

[0134] The difference from Example 1 is that the hard carbon crushing tailings are directly taken for S3, that is, 45g of hard carbon crushing tailings are placed in a 12wt% hydrochloric acid solution, heated and stirred at 70°C for 2h, and then centrifuged and washed with pure water several times until the pH is 7.0. The hard carbon tailing purification material is obtained after drying; the hard carbon tailing purification material is placed in a sintering furnace and heated to 1250°C under a nitrogen atmosphere for 3h to obtain a hard carbon material.

[0135] Comparative Example 3:

[0136] The difference from Example 1 is that in S1 of (1), graphene oxide with an oxygen content of 50% is used for preparation.

[0137] Comparative Example 4:

[0138] The difference from Example 1 is that in S1 of (1), the particle size D50 of the graphene oxide is 1μm.

[0139] Comparative Example 5:

[0140] The difference from Example 1 is that in S1 of (1), the particle size D50 of the graphene oxide is 50μm.

[0141] Comparative Example 6:

[0142] The difference from Example 1 is that in S1 of (1), the number of layers of the graphene oxide is 20 layers.

[0143] Comparative Example 7:

[0144] The difference from Example 1 is that the solid content of the mixed solution in S2 of (1) is 70 wt%.

[0145] It should be noted that the same button cell preparation method as in Example 1 is used in Examples 2-8 and Comparative Examples 1-7, and based on the negative electrode material or hard carbon material obtained respectively, the corresponding button cell is provided.

[0146] The negative electrode materials of Examples 1-8 and the hard carbon materials of Comparative Examples 1-7 are subjected to a plurality of tests, including: (1) particle size test of the materials by Malvern 3000 laser particle size analyzer; (2) specific surface area test of the materials by TriStar II 3030 analyzer (USA, Micromeritics Corporation); (3) compaction density test of the materials by automatic compaction density instrument UTM7305, and the compaction density of each material measured under 3T pressure is taken as the test result; (4) electrical conductivity test of the materials by powder electrical conductivity tester, and the electrical conductivity of each material measured under 20kN pressure is taken as the test result. The above test results are shown in Table 1.

[0147] Table 1. Part of the parameter and physical property test results of the negative electrode materials of Examples 1-8 and the hard carbon materials of Comparative Examples 1-7

[0148]

[0149] The GITT test method is used to test the sodium ion diffusion coefficient D Na+ of the button cells of Examples 1-8 and Comparative Examples 1-7, and the specific test process includes: first cycle charging and discharging of the button cell at 0.1C current, recording the charging capacity as xmAh / g; discharging the activated battery at 0.1C constant current for 10min, then standing for 30min to reach the equilibrium state, continuing the cycle until the voltage is 1mV, then charging at 0.1C constant current for 10min, then standing to reach the equilibrium state, continuing the cycle until the voltage is 1mV. The relaxation time τ, the total voltage change ΔEs caused by the pulse, the voltage change ΔEt of the constant current charging and discharging, are measured, and the sodium ion diffusion coefficient D is calculated according to the formula:

[0150]

[0151] m where n m is the number of moles, V Na+ is the molar volume, and S is the contact area of the electrode and the electrolyte.

[0152] The application also tests the electrochemical performance of the button cells of Examples 1-8 and Comparative Examples 1-7. First, the Wuhan Blue Electric High-precision Battery Test System M310A is used to perform the first cycle charge-discharge test on the battery, with a voltage range of 0.001-2.0 V. Second, the rate performance test is performed, including charging the above-mentioned first cycle charge-discharge activated battery at 0.33 C, and discharging at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 3 C and 5 C, each cycle for 1 cycle. The above test results are shown in Table 2.

[0153] For Example 1, please refer to Figure 5 , the reversible capacity of the button cell of Example 1 is 363.8 mAh / g, and the first coulombic efficiency is 93.08%, please refer to Figure 6 , the button cell of Example 1 has good capacity retention rate at different discharge rates, and the capacity retention rate at 5C rate is still as high as 65.8%, showing excellent electrochemical performance.

[0154] Table 2. Performance test results of button cells of Examples 1-8 and Comparative Examples 1-7 of the application

[0155]

[0156] Please refer to Tables 1 and 2, Examples 1-8 are prepared by secondary granulation and surface coating of primary particles and one-dimensional carbon materials, using specific conditions of graphene oxide and solid electrolyte coating layer, so that the obtained negative electrode material has suitable particle size, relatively small specific surface area (≤10 m 2 / g), high powder conductivity (≥48 S / cm) and high compaction density (≥1.07 g / cm 3 ), which improves the problems of large specific surface area of primary particles, low powder conductivity and low compaction density. The button cells based on the negative electrode materials of Examples 1-8 have relatively high sodium ion diffusion coefficient, charge specific capacity, first coulombic efficiency, cycle capacity retention rate and high-rate discharge capacity retention rate.

[0157] In comparison, the hard carbon material of Comparative Example 1 is not coated with solid electrolyte, so the specific surface area of the material is large, and there are more side reactions between the electrolyte and the secondary particles, which affects the capacity release and the first coulombic efficiency of the battery. In addition, the lack of a solid electrolyte shell causes the structure of the material to collapse at a large rate, so the rate performance of the battery is poor. The hard carbon material of Comparative Example 2 is not doped with carbon nanotube material and is not wrapped with graphene oxide, so the final specific surface area is significantly larger, the compaction density is significantly lower, and the powder conductivity is significantly lower. The battery formed consumes more sodium ions to form irreversible SEI films during the charging and discharging process, resulting in a lower first coulombic efficiency. The hard carbon material is amorphous carbon with low graphitization degree, and has poor ion and electron conductivity, so without the improvement process of the present application, its capacity, rate and long cycle performance are all poor. The above results show that coating the hard carbon material with graphene oxide is beneficial to the formation of secondary particles with uniform particle size, thereby improving the compaction density of the material or reducing the specific surface area of the material. The carbon nanotubes in the secondary particles are distributed between and on the surface of the hard carbon material, forming a conductive network structure, which is beneficial to improving the electronic conductivity of the hard carbon material and increasing the electron migration speed.

[0158] In comparison with Examples 1-8, the negative electrode material of Comparative Example 3 has a higher oxygen content of graphene oxide. The higher oxygen content causes more defect regions to form in the graphene oxide, which is not conducive to the conductivity of the material after the formation of the secondary particles. In addition, during the charging and discharging process of the negative electrode material, more electrolyte may be consumed, resulting in lower first coulombic efficiency, cycle stability and capacity maintenance rate of the battery. The negative electrode materials of Comparative Examples 4-6 have graphene oxide with particle sizes that are too small, too large, and too thick in layers, respectively, which are all not conducive to the formation of secondary particles, resulting in a larger specific surface area of the negative electrode material. The reversible capacity and first coulombic efficiency of the battery during the charging and discharging process are both low. In addition, graphene oxide with a particle size that is too small, too large, or too thick in layers may aggregate in the hard carbon primary particles, hindering the transport of sodium ions and affecting the rate performance of the battery. The above results show that controlling the oxygen content, particle size and layer thickness of graphene oxide is beneficial to improving the performance of the negative electrode material and the battery.

[0159] In addition, in comparison with Examples 1-8, the negative electrode material of Comparative Example 7 has a too high solid content of the mixed solution during preparation, resulting in a higher average particle size and lower compaction density of the obtained negative electrode material, which leads to lower performance of the battery than Examples 1-8.

[0160] The above embodiments are only used to illustrate the technical solutions of the present application and not to limit it. Although the present application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application.

Claims

1. A negative electrode material, characterized in that, Includes secondary particles and a coating layer covering at least a portion of the surface of the secondary particles; The secondary particles include a secondary particle core and graphene oxide covering at least a portion of the surface of the secondary particle core. The secondary particle core includes hard carbon primary particles and one-dimensional carbon material dispersed around the hard carbon primary particles. The graphene oxide has an oxygen content of less than or equal to 25%, a particle size D50 of 3 μm to 20 μm, and a number of layers of 1 to 10. The coating layer includes a NASICON-type solid electrolyte, the structural formula of which is Na. 1+ x M2A x P 3–x O 12 M is selected from any one of Ti, Zr, Ge and Sn, and A is selected from any one of Si, V and Nb, where 0 < x < 3.

2. The negative electrode material as described in claim 1, characterized in that, The hard carbon primary particles satisfy at least one of the following conditions: (1) The average particle size of the hard carbon primary particles is less than or equal to 3 μm; (2) The median particle size D50 of the hard carbon primary particles is 1.2 μm to 1.8 μm; (3) The specific surface area of ​​the hard carbon primary particles is greater than 10 m². 2 / g.

3. The negative electrode material as described in claim 1, characterized in that, The one-dimensional carbon material includes at least one of carbon nanotubes and carbon fibers, wherein the carbon nanotubes satisfy at least one of the following conditions: (1) The average diameter of the carbon nanotubes is 1 nm to 30 nm; (2) The length of the carbon nanotubes is 1 μm to 10 μm.

4. The negative electrode material as described in claim 1, characterized in that, The secondary particles also include carbon dot materials, which are at least disposed on the surfaces of the one-dimensional carbon material and the graphene oxide. The carbon dot materials include at least one of carbon quantum dots, graphene quantum dots, and polymer quantum dots.

5. The negative electrode material as described in claim 1, characterized in that, The covering layer satisfies at least one of the following conditions: (1) The chemical formula of the NASICON-type solid electrolyte is Na3Zr2Si2PO4. 12 ; (2) The thickness of the coating layer is 10 nm to 30 nm; (3) The coating layer accounts for less than 10% of the mass of the negative electrode material.

6. The negative electrode material as described in claim 1, characterized in that, The negative electrode material satisfies at least one of the following conditions: (1) The specific surface area of ​​the negative electrode material is less than or equal to 10 m². 2 / g; (2) The compaction density of the negative electrode material at a test tonnage of 3 tons is 1.05 g / cm³. 3 Up to 1.12 g / cm 3 ; (3) The average particle size of the negative electrode material is 4.5 μm to 6.5 μm; (4) The powder conductivity of the negative electrode material is greater than or equal to 48 S / cm.

7. A method for preparing a negative electrode material, characterized in that, include: Hard carbon, carbon dot material, one-dimensional carbon material, and graphene oxide solution are mixed and ground. The oxygen content of the graphene oxide is less than or equal to 25%, the particle size D50 of the graphene oxide is 3 μm to 20 μm, and the number of layers of the graphene oxide is 1 to 10. The carbon dot material is adsorbed on the surface of the hard carbon, the one-dimensional carbon material, and the graphene oxide to obtain a mixed solution with a solid content of 30 wt% to 50 wt%. The mixed solution is spray-dried to form secondary particles; The secondary particles are mixed with a NASICON-type solid electrolyte solution and sintered, so that the NASICON-type solid electrolyte coats at least a portion of the surface of the secondary particles. The structural formula of the NASICON-type solid electrolyte is Na. 1+ x M2A x P 3–x O 12 M is selected from any one of Ti, Zr, Ge and Sn, and A is selected from any one of Si, V and Nb, where 0 < x < 3, to obtain the negative electrode material.

8. The preparation method according to claim 7, characterized in that, The preparation method satisfies at least one of the following conditions: (1) The average diameter of the one-dimensional carbon material is 1 nm to 30 nm; (2) The mass ratio of the hard carbon, carbon dot material, one-dimensional carbon material and graphene oxide is (40-50):1:(1-5):(1-5); (3) The thickness of the coating is 10 nm to 30 nm.

9. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, characterized in that, The negative electrode active material layer includes the negative electrode material as described in any one of claims 1-6, or includes the negative electrode material prepared by the preparation method as described in any one of claims 7-8.

10. A secondary battery, characterized in that, Includes the negative electrode as described in claim 9.

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