Negative electrode sheet, secondary battery, and electric device

Abstract

The application discloses a negative electrode sheet, a secondary battery and an electric device. The negative electrode sheet comprises a current collector and a negative active layer arranged on at least one side of the current collector. The negative active layer comprises a first active layer and a second active layer. The first active layer is located between the second active layer and the current collector. The first active layer comprises a first negative active material, and the first negative active material comprises a metal phosphide. The second active layer comprises a second negative active material, and the second negative active material comprises hard carbon. The application arranges the negative active layer on at least one side of the current collector, and limits the first negative active material in the first active layer to comprise a metal phosphide and the second negative active material in the second active layer to comprise hard carbon, so that the energy density of a sodium ion battery can be effectively improved, and excellent fast charging performance and cycle stability can be maintained.

Description

Technical Field

[0001] This application relates to the field of energy storage technology, and in particular to a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0002] Against the backdrop of emission reduction and energy development, electric vehicles and lithium-ion batteries have experienced rapid growth. Simultaneously, the scarcity of lithium resources has led to a surge in the price of upstream raw materials for lithium-ion batteries, further driving up the price of lithium-ion batteries. Therefore, the industrialization of sodium-ion batteries has come into the public eye. Due to the availability of raw materials and the fact that sodium-ion batteries can use cheaper aluminum foil as the negative electrode current collector instead of more expensive copper foil, sodium-ion batteries have a low-cost advantage.

[0003] However, commercially available graphite used in lithium-ion battery anodes is unsuitable for sodium-ion batteries. Currently, hard carbon is more commonly used as the anode active material in the sodium-ion battery field. Hard carbon materials possess excellent kinetic performance, but their sodium storage capacity is relatively low, which is one of the important reasons why the energy density of sodium-ion batteries cannot be comparable to that of lithium-ion batteries.

[0004] Therefore, in order to enable sodium-ion batteries to simultaneously achieve high kinetics, safety, and high energy density, a new type of negative electrode must be developed. Summary of the Invention

[0005] This application aims to address at least one of the technical problems existing in the prior art. To this end, the first aspect of this application proposes a negative electrode sheet that can effectively improve the energy density and cycle stability of sodium-ion batteries.

[0006] A second aspect of this application also provides a secondary battery.

[0007] A third aspect of this application also provides an electrical appliance.

[0008] A first aspect of this application provides a negative electrode sheet, the negative electrode sheet including a current collector and a negative electrode active layer disposed on at least one side of the current collector, the negative electrode active layer including a first active layer and a second active layer; the first active layer is located between the second active layer and the current collector;

[0009] The first active layer includes a first negative electrode active material, which includes a metal phosphide.

[0010] The second active layer includes a second negative electrode active material, which includes hard carbon.

[0011] The negative electrode sheet according to the embodiments of this application has at least the following beneficial effects:

[0012] This application provides negative electrode active layers on at least one side of the current collector, and specifies that the first negative electrode active material in the first active layer includes metal phosphide, and the second negative electrode active material in the second active layer includes hard carbon. This effectively improves the energy density and cycle stability of sodium-ion batteries while ensuring fast charging. This is because the second active layer is made of hard carbon, which has better kinetic performance and can effectively limit the precipitation of metallic sodium on the electrode surface. The first active layer provides more specific capacity volatilization, further improving the energy density. Since the first active layer is located between the second active layer and the current collector, it mitigates the negative impact of metal phosphide on the precipitation of metallic sodium (sodium metal only precipitates on the electrode surface).

[0013] According to some embodiments of this application, the capacity ratio of the second active layer to the first active layer satisfies the following formula:

[0014] The ratio of (mass percentage of the second negative electrode active material in the second active layer * specific capacity of the second negative electrode active material * density of the second active layer) / (mass percentage of the first negative electrode active material in the first active layer * specific capacity of the first negative electrode active material * density of the first active layer) is in the range of 1 to 5.

[0015] When the capacity ratio of the second active layer to the first active layer is between 1 and 5, it is possible to further improve the energy density while ensuring cycle stability.

[0016] According to some embodiments of this application, the specific capacity of the first negative electrode active material is 800–1200 mAh / g; the specific capacity of the second negative electrode active material is 250–450 mAh / g. Therefore, the battery exhibits good capacity and cycle performance.

[0017] According to some embodiments of this application, the areal density of the first active layer is 0.94–8 mg / cm³. 2 Therefore, when the areal density of the first active layer is between 0.94 and 8 mg / cm³, 2 At that time, it has a high battery capacity when the areal density is below 0.94 mg / cm³. 2 The effect on the overall battery capacity is not significant when the areal density is higher than 8 mg / cm³. 2 Increased battery internal resistance leads to decreased capacity cycle retention.

[0018] According to some embodiments of this application, the areal density of the second active layer is 8.32–23 mg / cm³. 2 Therefore, it has a good capacity retention rate.

[0019] The method for testing the areal density is as follows:

[0020] Weigh a substrate with a known area of ​​S to obtain its mass M1; weigh an electrode with the same area coated with active material to obtain its mass M2; surface density = (M2-M1) / S.

[0021] According to some embodiments of this application, the first active layer further includes a first conductive agent, a first binder, and a first dispersant. Based on the total mass of the first active layer, the first active layer comprises the following components by mass percentage:

[0022]

[0023] According to some embodiments of this application, the second active layer further includes a second conductive agent, a second binder, and a second dispersant. Based on the total mass of the second active layer, the second active layer comprises the following components in mass percentage:

[0024]

[0025] According to some embodiments of this application, the second negative electrode active material includes at least one of CaP3, CrP2, NiP2, VP2, CoP2, GaP, and ZnP4.

[0026] According to some embodiments of this application, the content of the first conductive agent in the first active layer is greater than the content of the second conductive agent in the second active layer. Metal phosphides have poor conductivity; introducing more conductive agent into the first active layer can appropriately improve electronic conductivity and cycle life. However, hard carbon has good conductivity, and excessive conductive agent has a negative effect on capacity.

[0027] According to some embodiments of this application, the first adhesive and the second adhesive are independently selected from at least one of polyvinylidene fluoride (PVDF), polyvinyl alcohol, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), or fluororubber. Therefore, the first adhesive and the second adhesive have better bonding performance.

[0028] According to some embodiments of this application, the first conductive agent and the second conductive agent are independently selected from at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, or carbon fiber. This results in better conductivity.

[0029] According to some embodiments of this application, the first dispersant and the second dispersant are independently selected from sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. This results in a more uniform dispersion of the first active layer and the second active layer.

[0030] According to some embodiments of this application, the current collector includes at least one of copper foil, aluminum foil, copper mesh, aluminum mesh, aluminum foil coated with a conductive carbon layer, copper foil coated with a conductive carbon layer, a polymer film coated with aluminum, a polymer film coated with copper, a conductive polymer film, or a conductive film that has corrosion stability when used in an electrolyte system.

[0031] According to some embodiments of this application, the method for preparing the negative electrode sheet includes the following steps:

[0032] S1. Stir the first negative electrode active material, the first conductive agent, the first binder and the first dispersant to obtain the first active layer; stir the second negative electrode active material, the second conductive agent, the second binder and the second dispersant to obtain the second active layer;

[0033] S2. The negative electrode sheet is obtained by coating the upper and lower surfaces of the current collector using a double-layer coating method, drying, rolling, and slitting.

[0034] A second aspect of this application provides a secondary battery comprising a positive electrode, an electrolyte, a separator, and a negative electrode as described above, wherein the separator is located between the positive electrode and the negative electrode, and the electrolyte fills the space between the positive electrode and the negative electrode and wets the separator.

[0035] According to some embodiments of this application, the positive electrode sheet includes a positive current collector and a positive active material layer formed on the surface of the positive current collector. The positive active material layer includes a positive active material, which includes at least one of sodium transition metal oxide, polyanionic compound, or Prussian blue compound.

[0036] According to some embodiments of this application, in sodium transition metal oxides, the transition metal can be at least one selected from Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. For example, sodium transition metal oxides are Na. x MO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, 0 <x≤1。

[0037] According to some embodiments of this application, the polyanionic compound may 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 price state.

[0038] Polyanionic compounds can also contain 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.

[0039] Polyanionic compounds can also be sodium-containing tetrahedral (YO4) compounds. 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- The 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.

[0040] Polyanionic compounds include, for example, NaFePO4, Na3V2(PO4)3, NaM'PO4F (where M' is one or more of V, Fe, Mn, and Ni) and Na3(VO y )2(PO4)2F 3-2y At least one of (0≤y≤1).

[0041] 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。

[0042] According to some embodiments of this application, the positive electrode active material layer may further include a conductive agent to improve the conductivity of the positive electrode. This application does not impose specific limitations on the type of conductive agent, which can be selected according to actual needs. As an example, the conductive agent may be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene, or carbon nanofibers.

[0043] According to some embodiments of this application, the positive electrode active material layer may further include a binder to firmly bond the positive electrode active material and optional conductive agent to the positive electrode current collector. This application does not specifically limit the type of binder, which can be selected according to actual needs. As an example, the binder may be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyvinyl alcohol (PVA), ethylene-vinyl acetate copolymer (EVA), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), sodium alginate (SA), polymethacrylic acid (PMA), or carboxymethyl chitosan (CMCS).

[0044] According to some embodiments of this application, the positive current collector is a conductive carbon sheet, a metal foil, a carbon-coated metal foil, a porous metal plate, or a composite current collector. The conductive carbon material of the conductive carbon sheet can be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene, or carbon nanofibers. The metal materials of the metal foil, carbon-coated metal foil, and porous metal plate can each be independently selected from at least one of copper, aluminum, nickel, and stainless steel. The composite current collector can be a composite current collector formed by combining a metal foil with a polymer base film.

[0045] According to some embodiments of this application, the separator can be any of the various materials suitable for secondary battery separators in the art, such as, but not limited to, at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0046] According to some embodiments of this application, the electrolyte comprises an organic solvent and an electrolyte sodium salt. For example, the organic solvent includes one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, methyl acetate, ethyl propionate, fluoroethylene carbonate, diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and methyl tert-butyl ether; the electrolyte sodium salt includes one or more of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium difluorophosphate, sodium perchlorate, and sodium chloride.

[0047] A third aspect of this application provides an electrical device, the electrical device including the secondary battery described above.

[0048] According to some embodiments of this application, the electrical equipment includes mobile phones, computers, wearable devices, power banks, electric vehicles, and energy storage devices, etc.

[0049] Other features and advantages of this application will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing this application. Detailed Implementation

[0050] The following are specific embodiments of this application, and the technical solutions of this application will be further described in conjunction with the embodiments, but this application is not limited to these embodiments.

[0051] Unless otherwise specified, the reagents, methods and equipment used in this application are all conventional reagents, methods and equipment in this technical field.

[0052] Example 1

[0053] Example 1 provides a negative electrode sheet, the preparation method of which is as follows:

[0054] S1. The second active layer uses hard carbon with a specific capacity of 250 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The first active material layer uses CoP2 with a specific capacity of 800 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0055] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the second and first layers of slurry, respectively. The density of the first active layer is 4.68 mg / cm³. 2 The density of the second active layer is 15 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 1, then dry, roll, and slit to obtain the negative electrode sheet.

[0056] Example 2

[0057] Example 2 also provides a negative electrode sheet, the preparation method of which is as follows:

[0058] S1. The second active layer uses hard carbon with a specific capacity of 250 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The second active layer slurry uses NiP2 with a specific capacity of 1200 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% CMC (sodium carboxymethyl cellulose), and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0059] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the second and first layers of slurry, respectively. The density of the first active layer is 4.68 mg / cm³. 2 The density of the second active layer is 22.46 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 1, then dry, roll, and slit to obtain the negative electrode sheet.

[0060] Example 3

[0061] Example 3 also provides a negative electrode sheet, the preparation method of which is as follows:

[0062] S1. The second active layer uses hard carbon with a specific capacity of 450 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The second active layer slurry uses CoP2 with a specific capacity of 800 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% CMC (sodium carboxymethyl cellulose), and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0063] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the second and first layers of slurry, respectively. The density of the first active layer is 4.68 mg / cm³. 2 The density of the second active layer is 8.32 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 1, then dry, roll, and slit to obtain the negative electrode sheet.

[0064] Example 4

[0065] Example 4 also provides a negative electrode sheet, the preparation method of which is as follows:

[0066] S1. The second active layer uses hard carbon with a specific capacity of 450 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The second active layer slurry uses NiP2 with a specific capacity of 1200 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% CMC (sodium carboxymethyl cellulose), and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0067] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the second and first layers of slurry, respectively. The density of the first active layer is 4.68 mg / cm³. 2 The density of the second active layer is 12.48 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 1, then dry, roll, and slit to obtain the negative electrode sheet.

[0068] Example 5

[0069] Example 5 provides a negative electrode sheet, the preparation method of which is as follows:

[0070] S1. The second active layer uses hard carbon with a specific capacity of 250 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The first active material layer uses CoP2 with a specific capacity of 800 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0071] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the second and first layers of slurry, respectively. The density of the first active layer is 3.13 mg / cm³. 2 The density of the second active layer is 10 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 1, then dry, roll, and slit to obtain the negative electrode sheet.

[0072] Example 6

[0073] Example 6 provides a negative electrode sheet, the preparation method of which is as follows:

[0074] S1. The second active layer uses hard carbon with a specific capacity of 250 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The first active material layer uses CoP2 with a specific capacity of 800 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0075] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the second and first layers of slurry, respectively. The density of the first active layer is 7.2 mg / cm³. 2 The density of the second active layer is 23 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 1, then dry, roll, and slit to obtain the negative electrode sheet.

[0076] Example 7

[0077] Example 7 also provides a negative electrode sheet, the preparation method of which is as follows:

[0078] S1. The second active layer uses hard carbon with a specific capacity of 450 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The second active layer slurry uses NiP2 with a specific capacity of 1200 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% CMC (sodium carboxymethyl cellulose), and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0079] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the second and first layers of slurry, respectively. The density of the first active layer is 1.56 mg / cm³. 2 The density of the second active layer is 12.48 mg / cm³. 2Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 3. Dry, roll, and slit to obtain the negative electrode sheet.

[0080] Example 8

[0081] Example 8 also provides a negative electrode sheet, the preparation method of which is as follows:

[0082] S1. The second active layer uses hard carbon with a specific capacity of 450 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The second active layer slurry uses NiP2 with a specific capacity of 1200 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% CMC (sodium carboxymethyl cellulose), and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0083] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the first and second layers of slurry, respectively. The density of the second active layer is 12.48 mg / cm³. 2 The density of the first active layer is 0.936 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 5. Dry, roll, and slit to obtain the negative electrode sheet.

[0084] Example 9

[0085] Example 9 also provides a negative electrode sheet, the preparation method of which is as follows:

[0086] S1. The second active layer uses hard carbon with a specific capacity of 450 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The second active layer slurry uses NiP2 with a specific capacity of 1200 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% CMC (sodium carboxymethyl cellulose), and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0087] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the first and second layers of slurry, respectively. The density of the second active layer is 12.48 mg / cm³.2 The density of the first active layer is 7.78 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 0.6. Dry, roll, and slit to obtain the negative electrode sheet.

[0088] Example 10

[0089] Example 10 also provides a negative electrode sheet, the preparation method of which is as follows:

[0090] S1. The second active layer uses hard carbon with a specific capacity of 450 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The second active layer slurry uses NiP2 with a specific capacity of 1200 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% CMC (sodium carboxymethyl cellulose), and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0091] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the first and second layers of slurry, respectively. The density of the second active layer is 12.48 mg / cm³. 2 The density of the first active layer is 0.85 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 5.5. Dry, roll, and slit to obtain the negative electrode sheet.

[0092] Example 11

[0093] Example 11 also provides a negative electrode sheet, the preparation method of which is as follows:

[0094] S1. The second active layer uses hard carbon with a specific capacity of 450 mAh / g, accounting for 94%, with the remainder being 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The first active layer uses NiP2 with a specific capacity of 1200 mAh / g, accounting for 92%, with the remainder being 5% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0095] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the first and second layers of slurry, respectively. The density of the second active layer is 12.48 mg / cm³. 2 The density of the first active layer is 1.59 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 3. Dry, roll, and slit to obtain the negative electrode sheet.

[0096] Example 12

[0097] Example 12 also provides a negative electrode sheet, the preparation method of which is as follows:

[0098] S1. The second active layer uses hard carbon with a specific capacity of 450 mAh / g, accounting for 99.85%, with the remainder being 0.05% SuperP (conductive carbon black), 0.05% sodium carboxymethyl cellulose, and 0.05% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The second active layer slurry uses NiP2 with a specific capacity of 1200 mAh / g, accounting for 65%, with the remainder being 15% SuperP (conductive carbon black), 5% CMC (sodium carboxymethyl cellulose), and 15% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0099] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the second and first layers of slurry, respectively. The density of the first active layer is 2.39 mg / cm³. 2 The density of the second active layer is 12.48 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 3. Dry, roll, and slit to obtain the negative electrode sheet.

[0100] Example 13

[0101] Example 13 also provides a negative electrode sheet, the preparation method of which is as follows:

[0102] S1. The second active layer uses hard carbon with a specific capacity of 450 mAh / g, accounting for 65%, with the remainder being 15% Super P (conductive carbon black), 5% sodium carboxymethyl cellulose, and 15% binder (styrene-butadiene rubber). The solvent is deionized water, and the first active layer slurry is prepared by stirring. The first active layer uses NiP2 with a specific capacity of 1200 mAh / g, accounting for 99.85%, with the remainder being 0.05% Super P (conductive carbon black), 0.05% CMC (sodium carboxymethyl cellulose), and 0.05% binder (styrene-butadiene rubber). The solvent is deionized water, and the second active layer slurry is prepared by stirring.

[0103] S2. Apply the coating using a double-layer coating machine. The upper and lower dies correspond to the second and first layers of slurry, respectively. The density of the first active layer is 1.02 mg / cm³. 2 The density of the second active layer is 12.48 mg / cm³. 2 Ensure that the value of (percentage of second negative electrode active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of first negative electrode active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) is 3. Dry, roll, and slit to obtain the negative electrode sheet.

[0104] Comparative Example 1

[0105] Comparative Example 1 provides a negative electrode sheet, the preparation steps of which are as follows:

[0106] S1. The active material is 250mAh / g hard carbon, accounting for 94%, and the remainder is 3% Super P (conductive carbon black), 1.5% CMC (sodium carboxymethyl cellulose), 1.5% binder (styrene-butadiene rubber), and the solvent is deionized water. The slurry is prepared by stirring.

[0107] S2. Apply the coating using a double-layer coating machine, using the slurry from step 1 on both the upper and lower dies, ensuring a total coating surface density of 19.68 mg / cm². 2 (With the same total coating density as in Example 1), the electrode is dried, rolled, and slit to obtain the negative electrode sheet.

[0108] Comparative Example 2

[0109] Comparative Example 2 provides a negative electrode sheet, the preparation steps of which are as follows:

[0110] S1. The active material is 800mAh / g CoP2, which accounts for 94% of the total. The remainder consists of 3% Super P (conductive carbon black), 1.5% sodium carboxymethyl cellulose, and 1.5% binder (styrene-butadiene rubber). The solvent is deionized water. The slurry is prepared by stirring.

[0111] S2. Apply the coating using a double-layer coating machine, using the slurry from step 1 on both the upper and lower dies, ensuring a total coating surface density of 19.68 mg / cm². 2 (With the same total coating density as in Example 1), the electrode is dried, rolled, and slit to obtain the negative electrode sheet.

[0112] Performance testing

[0113] Preparation of sodium-ion batteries:

[0114] (1) Electrolyte: 1M NaPF6 dissolved in ethylene carbonate (EC): diethyl carbonate (DEC) = 3:7;

[0115] (2) Separator: PP film;

[0116] (3) Composition of the positive electrode sheet: The positive electrode material (Na3V2(PO4)2F3, accounting for 96%), conductive agent (SP, accounting for 3%), and binder (PVDF, accounting for 1%) are uniformly mixed in N-methylpyrrolidone to obtain the positive electrode sheet slurry. The positive electrode sheet slurry is coated on the positive electrode current collector, and then dried, rolled, and slit to obtain the positive electrode sheet.

[0117] (4) Assembly of sodium-ion batteries

[0118] The negative electrode, positive electrode, separator, and structural components prepared in Examples 1-13 and Comparative Examples 1-2 were assembled, wound with the same number of layers, and then subjected to processes such as liquid injection, formation, aging, and sealing to obtain a sodium-ion battery.

[0119] The obtained sodium-ion batteries were tested using the Xinwei Power Battery Testing System at a current of 0.5A to obtain the battery capacity, which was recorded in Table 1. In addition, 1C current is defined as battery capacity; the higher the battery capacity, the higher the energy density.

[0120] The sodium-ion battery was charged to the upper limit voltage using a 4C current using the Xinwei Power Battery Testing System, and then discharged to the lower limit voltage using a 1C current, and the charge and discharge cycle was repeated 10 times. Then the battery was fully charged with a 0.5A current. Finally, the battery was disassembled and the sodium deposition on the negative electrode surface was observed. If sodium was deposited, it indicated that fast charging was not supported, and the results were recorded in Table 1.

[0121] Charge the battery at a constant current of 2C to the upper limit voltage, then maintain the voltage at a constant current until the current reaches 0.05C, and then discharge it at a current of 1C to the lower limit voltage. Repeat this process at 25°C, and record the number of cycles in which the battery capacity decays to 80% of the initial capacity. Record the data in Table 1.

[0122] Table 1. Data from Examples 1-13 and Comparative Examples 1-2

[0123] Capacity / Ah Sodium precipitation Cycling at 25°C @ 80% SOH Example 1 3.05 Sodium not precipitated 1300 Example 2 4.57 Sodium not precipitated 1150 Example 3 3.05 Sodium not precipitated 1800 Example 4 4.51 Sodium not precipitated 1500 Example 5 2.83 Sodium not precipitated 1800 Example 6 4.68 Sodium not precipitated 1000 Example 7 3.03 Sodium not precipitated 1800 Example 8 2.84 Sodium not precipitated 1850 Example 9 6.09 Sodium not precipitated 850 Example 10 2.70 Sodium not precipitated 1500 Example 11 3.06 Sodium not precipitated 2200 Example 12 3.06 Sodium not precipitated 2200 Example 13 2.78 Sodium not precipitated 1500 Comparative Example 1 2.00 Sodium not precipitated 2250 Comparative Example 2 6.41 Severe sodium precipitation 150

[0124] As can be seen from the test results in Table 1, Examples 1-4 all maintained a constant density of the first active layer and a fixed capacity ratio between the second and first layers of 1. It can be observed that the capacity decay in Examples 3 and 4 was slower. When the areal density of the second active layer is large, the composite electrode is thicker, resulting in a longer distance for sodium ions to travel to the first active layer. This leads to a rapid increase in internal resistance and accelerated decay during long-term cycling.

[0125] Comparing Examples 1, 5, and 6, the areal density design was adjusted while maintaining a fixed interlayer capacity ratio of 1 and unchanged material selection. It is evident that a higher areal density results in higher battery capacity, but also accelerates cycle degradation.

[0126] Examples 4, 7, and 8 compare the impact of the difference in the capacity ratio between the second and first layers (percentage of active material in the second active layer * specific capacity of the second active layer * surface density of the second active layer) / (percentage of active material in the first active layer * specific capacity of the first active layer * surface density of the first active layer) on battery performance. The capacity ratios between the second and first layers in Examples 4, 7, and 8 are 1, 3, and 5, respectively, maintaining a consistent coating surface density of the second active layer. It is evident that when the capacity ratio between each layer is at a suitable value, such as in Example 7, the overall battery performance can be better maintained. Comparing Examples 4 and 7, as the surface density of the first active layer gradually decreases, the battery capacity shows a decreasing trend, while the capacity retention rate shows an increasing trend. This is because although the first active layer can contribute more capacity, its conductivity is biased. Reducing the surface density of the first active layer can reduce the battery's internal resistance. However, when the surface density of the first active layer is further reduced, as in Examples 7 and 8, the effect is not significant, and the long cycle life of the battery can still be guaranteed. But when the difference in the capacity ratio between the second and first layers deviates from the specified range, such as in Examples 9 and 10... Either the capacity is significantly lower than that of Example 4 (Example 10), or the circulation effect is not ideal (Example 9).

[0127] Example 11 improves battery performance by optimizing the active material loading through adjusting the proportion of Super P (conductive carbon black). This is because the first active material layer has poor conductivity. Introducing more conductive carbon black into the first layer can appropriately increase electronic conductivity, resulting in slower cycle degradation.

[0128] Examples 7, 12, and 13 modified the electrode formulation. Compared to Example 7, Example 12 had more conductive agent in the first active layer, which optimized impedance and resulted in better cycle stability. Compared to Example 7, Example 13 had an increased amount of conductive agent in the second active layer. However, because hard carbon has good conductivity, excessive conductive agent (inactive material) has a negative effect on capacity.

[0129] The sodium-ion battery obtained in Comparative Example 1 had the lowest capacity, while the metal phosphide anode in Comparative Example 2 had the highest capacity. However, due to its poor conductivity, surface pulverization, poor kinetics, extremely rapid cycle decay, and severe sodium deposition, it also exhibited poor performance.

[0130] The embodiments of this application have been described in detail above, but this application is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of this application.

Claims

1. A negative electrode sheet for a sodium-ion battery, characterized in that, The negative electrode sheet includes a current collector and a negative electrode active layer disposed on at least one side of the current collector. The negative electrode active layer includes a first active layer and a second active layer. The first active layer is located between the second active layer and the current collector. The first active layer includes a first negative electrode active material, which is a metal phosphide; the metal phosphide includes at least one of CaP3, CrP2, NiP2, VP2, CoP2, GaP, and ZnP4. The second active layer includes a second negative electrode active material, which is hard carbon; the specific capacity of the first negative electrode active material is 800~1200mAh / g; the specific capacity of the second negative electrode active material is 250~450mAh / g. The areal density of the first active layer is 0.94~8 mg / cm³. 2 The areal density of the second active layer is 8.32~23 mg / cm³. 2 ; The capacity ratio of the second active layer to the first active layer satisfies the following formula: The ratio of (mass percentage of the second negative electrode active material in the second active layer * specific capacity of the second negative electrode active material * density of the second active layer) / (mass percentage of the first negative electrode active material in the first active layer * specific capacity of the first negative electrode active material * density of the first active layer) is in the range of 1 to 5.

2. The negative electrode of the sodium-ion battery according to claim 1, characterized in that, The first active layer further includes a first conductive agent, and the second active layer further includes a second conductive agent, wherein the content of the first conductive agent in the first active layer is greater than the content of the second conductive agent in the second active layer.

3. The negative electrode of the sodium-ion battery according to claim 2, characterized in that, The first conductive agent and the second conductive agent each independently include at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder or carbon fiber.

4. The negative electrode of the sodium-ion battery according to claim 1, characterized in that, The first active layer further includes a first adhesive, and the second active layer further includes a second adhesive. The first adhesive and the second adhesive are independently selected from at least one of polyvinylidene fluoride, polyvinyl alcohol, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer, sulfonated EPDM, styrene-butadiene rubber, or fluororubber.

5. A sodium-ion secondary battery, characterized in that, It includes a positive electrode, an electrolyte, a separator, and a negative electrode as described in any one of claims 1 to 4 for a sodium-ion battery.

6. An electrical appliance, characterized in that, Including the sodium-ion secondary battery as described in claim 5.

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