Negative electrode sheet, secondary battery and electrical equipment

Abstract

The present invention discloses a negative electrode plate, a secondary battery, and an electrical device. The negative electrode plate includes a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode 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 negative electrode current collector. The first active layer includes a first negative electrode active material, and the second active layer includes a second negative electrode active material. The (002) crystal plane interlayer spacing d1 of the first negative electrode active material is greater than the (002) crystal plane interlayer spacing d2 of the second negative electrode active material. By limiting the (002) crystal plane interlayer spacing d1 of the first negative electrode active material to be greater than the (002) crystal plane interlayer spacing d2 of the second negative electrode active material, the present invention enables the negative electrode plate to improve the first cycle coulombic efficiency of the secondary battery and maintain a high battery capacity.

Description

Technical Field

[0001] The present invention relates to the field of energy storage technology, and in particular to a negative electrode plate, a secondary battery and an electrical device. Background Art

[0002] Currently, hard carbon is widely used as the negative electrode active material in sodium-ion batteries. However, the coulombic efficiency (ICE) of hard carbon electrodes is generally insufficient, which seriously compromises the cycling stability of sodium-ion batteries. To further enhance the application potential of hard carbon anodes in sodium-ion batteries, their ICE must be optimized.

[0003] Therefore, it is necessary to provide a negative electrode plate so that the sodium ion battery has a higher capacity and a higher first-cycle coulombic efficiency. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, in a first aspect, the present invention provides a negative electrode plate that can effectively improve the first-cycle coulombic efficiency and capacity.

[0005] A second aspect of the present invention also provides a secondary battery.

[0006] The third aspect of the present invention further provides an electrical device.

[0007] A first embodiment of the present invention provides a negative electrode plate, comprising a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector, wherein the negative electrode 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 negative electrode current collector;

[0008] The first active layer includes a first negative electrode active material, and the second active layer includes a second negative electrode active material;

[0009] The (002) interlayer spacing d1 of the first negative electrode active material is greater than the (002) interlayer spacing d2 of the second negative electrode active material.

[0010] The negative electrode sheet according to the embodiment of the present invention has at least the following beneficial effects:

[0011] By limiting the (002) interlayer spacing d1 of the first negative electrode active material to be larger than the (002) interlayer spacing d2 of the second negative electrode active material, the present invention enables the negative electrode plate to improve the first-cycle coulombic efficiency of a sodium-ion battery and maintain a high battery capacity. This is because the first active layer provides more sodium storage capacity, increasing battery capacity, while the second active layer reduces side reactions with the electrolyte, thereby improving the first-cycle coulombic efficiency.

[0012] According to some embodiments of the present invention, the (002) interlayer spacing d1 of the first negative electrode active material and the (002) interlayer spacing d2 of the second negative electrode active material satisfy the following: d1 / d2≤1.2. As a result, the first active layer and the second active layer have better compatibility. If the interlayer spacing differs too much, the stress changes during the charge and discharge process will differ significantly (the interlayer spacing affects the volume expansion during sodium ion insertion, and a large difference in interlayer spacing will result in a large difference in volume expansion), resulting in poor compatibility between the two and easy peeling of the second active layer.

[0013] According to some embodiments of the present invention, the (002) crystal plane interlayer spacing d1 of the first negative electrode active material satisfies According to some embodiments of the present invention, the (002) crystal plane interlayer spacing d1 of the first negative electrode active material is Or within the range consisting of any two of the above values.

[0014] According to some embodiments of the present invention, the (002) crystal plane interlayer spacing d2 of the second negative electrode active material satisfies According to some embodiments of the present invention, the (002) crystal plane interlayer spacing d2 of the second negative electrode active material is Or within the range consisting of any two of the above values.

[0015] According to some embodiments of the present invention, the interlayer spacing is characterized by transmission electron microscopy.

[0016] According to some embodiments of the present invention, the first negative electrode active material and the second negative electrode active material each independently contain heteroatoms, and the heteroatoms include at least two of nitrogen, oxygen, sulfur or boron.

[0017] According to some embodiments of the present invention, the heteroatom content in the first negative electrode active material is greater than the heteroatom content in the second negative electrode active material.

[0018] According to some embodiments of the present invention, the content of the heteroatoms is measured by inductively coupled plasma atomic emission spectrometry (ICP-OES) or scanning electron microscope-energy dispersive spectrometer (SEM-EDS).

[0019] According to some embodiments of the present invention, the compaction density of the negative electrode sheet is 1.1 mg / cm 3 ~1.7mg / cm 3 According to some embodiments of the present invention, the compaction density of the negative electrode sheet is 1.3 mg / cm 3 ~1.5mg / cm 3According to some embodiments of the present invention, the compaction density of the negative electrode sheet is 1.1 mg / cm 3 , 1.3mg / cm 3 , 1.5mg / cm 3 , 1.7mg / cm 3 Or it is within the range formed by any two of the above values. Thus, it has better first-cycle coulombic efficiency and dynamic performance.

[0020] According to some embodiments of the present invention, the first active layer further includes a first conductive agent, a first binder, and a first dispersant, and the second active layer further includes a second conductive agent, a second binder, and a second dispersant.

[0021] According to some embodiments of the present invention, the first active layer includes the following components in percentage by mass, calculated based on the total mass of the first active layer:

[0022]

[0023] According to some embodiments of the present invention, the second active layer includes the following components in percentage by mass, calculated based on the total mass of the second active layer:

[0024]

[0025]

[0026] According to some embodiments of the present invention, both the first negative electrode active material and the second negative electrode active material include at least one of hard carbon, soft carbon, and mesocarbon microbeads.

[0027] According to some embodiments of the present invention, the first binder and the second binder independently include at least one of polyvinylidene fluoride, polyvinyl alcohol, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer, sulfonated EPDM, styrene-butadiene rubber and fluororubber.

[0028] According to some embodiments of the present invention, the first conductive agent and the second conductive agent are independently selected from at least one of conductive carbon black, conductive graphite, carbon nanotubes, metal powder, or carbon fiber.

[0029] According to some embodiments of the present invention, the first dispersant and the second dispersant are independently selected from at least one of sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose.

[0030] According to some embodiments of the present invention, the negative electrode 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 having corrosion stability when used in an electrolyte system.

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

[0032] S1, stirring a first negative electrode active material, a first conductive agent, a first binder, and a first dispersant to obtain a first active layer; stirring a second negative electrode active material, a second conductive agent, a second binder, and a second dispersant to obtain a second active layer;

[0033] S2. Use a double-layer coating method to coat the upper and lower surfaces of the current collector, dry and roll-press to obtain the negative electrode sheet.

[0034] A second aspect of the present application provides a secondary battery, which includes a positive electrode sheet, an electrolyte, a separator, and the negative electrode sheet as described above, wherein the separator is located between the positive electrode sheet and the negative electrode sheet, and the electrolyte is filled between the positive electrode sheet and the negative electrode sheet and infiltrates the separator.

[0035] The secondary battery of the present application includes the negative electrode sheet of the possible embodiment of the first aspect of the present application. Since the negative electrode sheet of the first aspect of the present application effectively improves the capacity and first-cycle coulombic efficiency of the sodium ion battery, the secondary battery provided by the present application has a higher battery capacity and first-cycle coulombic efficiency.

[0036] According to some embodiments of the present application, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer formed on the surface of the positive electrode current collector, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of a sodium transition metal oxide, a polyanionic compound or a Prussian blue compound.

[0037] According to some embodiments of the present application, in the sodium transition metal oxide, the transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. For example, the sodium transition metal oxide is Na x MO2, wherein M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, 0 <x≤1。

[0038] According to some embodiments of the present application, the polyanionic compound may be a compound having sodium ions, transition metal ions and tetrahedral (YO4) 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- valence.

[0039] Polyanionic compounds can also be sodium ions, transition metal ions, tetrahedral (YO4) n- A class of compounds containing anion units and halogen 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- valence state; the halogen may be at least one of F, Cl and Br.

[0040] Polyanionic compounds can also be sodium ions, tetrahedral (YO4) n- Anion unit, polyhedron unit (ZO y ) m+ and an optional halogen anion. Y can be at least one of P, S and Si, and n represents (YO4) n- valence state; Z represents a transition metal, which may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce, and m represents (ZO y ) m+ valence state; the halogen may be at least one of F, Cl and Br.

[0041] Polyanionic compounds include NaFePO4, Na3V2(PO4)3, NaM'PO4F (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).

[0042] Prussian blue compounds can be 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. Prussian blue compounds are, for example, Na a Me b Me' c (CN)6, wherein Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co and Zn, 0 <a≤2,0<b<1,0<c<1。

[0043] According to some embodiments of the present application, the positive electrode active material layer may further include a conductive agent to improve the conductivity of the positive electrode. The present application does not specifically limit the type of conductive agent, and the conductive agent may be selected based on actual needs. For 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.

[0044] According to some embodiments of the present application, the positive electrode active material layer may further include a binder to firmly bond the positive electrode active material and the optional conductive agent to the positive electrode current collector. The present application does not specifically limit the type of binder and can be selected according to actual needs. As an example, the binder can 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).

[0045] According to some embodiments of the present application, the positive electrode current collector is a conductive carbon sheet, metal foil, carbon-coated metal foil, 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 material of the metal foil, carbon-coated metal foil, and porous metal plate can each independently be 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.

[0046] According to some embodiments of the present application, the diaphragm can be various materials suitable for secondary battery diaphragms in the art, for example, including but not limited to polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and at least one of natural fibers.

[0047] According to some embodiments of the present application, the electrolyte includes 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, ethyl methyl carbonate, propylene carbonate, methyl acetate, ethyl propionate, fluoroethylene carbonate, ethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and methyl tert-butyl ether; and 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.

[0048] A third embodiment of the present application provides an electrical device, which includes the secondary battery described above.

[0049] Other features and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments with reference to the following drawings, in which:

[0051] Figure 1 Schematic diagram of the negative electrode sheet of Example 1;

[0052] Among them, 1 is the first active layer; 2 is the second active layer, and 3 is the negative electrode current collector. DETAILED DESCRIPTION

[0053] The following are specific embodiments of the present invention, and the technical solutions of the present invention are further described in conjunction with the embodiments, but the present invention is not limited to these embodiments.

[0054] Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0055] The raw materials used in the embodiments and comparative examples are as follows:

[0056] First negative electrode active material A: hard carbon (gram capacity 450 mAh / g, heteroatom ratio N: 5%, S: 2.5%, P: 1%, B: 0.5%, total ratio 9%); d1 is

[0057] First negative electrode active material B: hard carbon (gram capacity 400 mAh / g, wherein the heteroatom ratio is N: 2.5%, S: 1.5%, P: 0.5%, B: 0.5%, and the total ratio is 5%); d1 is

[0058] First negative electrode active material C: hard carbon (gram capacity 420 mAh / g, wherein the heteroatom accounts for N: 3.5%, S: 2%, P: 1%, B: 0.5%, and the total accounts for 7%); d1 is

[0059] The first negative electrode active material D: hard carbon (gram capacity 500 mAh / g, wherein the heteroatom accounts for N: 6%, S: 2.5%, P: 1.5%, B: 1%, and the total accounts for 11%); d1 is

[0060] Second negative electrode active material A: hard carbon (gram capacity 360 mAh / g, wherein the heteroatom ratio is N: 3%, S: 1%, P: 0.5%, B: 0.5%, and the total ratio is 5%), d2 is

[0061] Second negative electrode active material B: hard carbon (gram capacity 220 mAh / g, wherein the heteroatom ratios N: 0.75%, S: 0.25%, P: 0%, B: 0%, and the total ratio is 1%); d2 is

[0062] Second negative electrode active material C: hard carbon (gram capacity 350 mAh / g, wherein the heteroatom ratios N: 3%, S: 1%, P: 0.5%, B: 0%, and the total ratio is 4.5%); d2 is

[0063] The second negative electrode active material D: hard carbon (gram capacity 345 mAh / g, wherein the heteroatom ratio is N: 3%, S: 1%, P: 0%, B: 0.5%, and the total ratio is 4.5%); d2 is

[0064] Second negative electrode active material E: hard carbon (gram capacity 343 mAh / g, wherein the heteroatom ratio is N: 3%, S: 0.5%, P: 0.5%, B: 0.5%, and the total ratio is 4.5%); d2 is

[0065] Second negative electrode active material F: hard carbon (gram capacity 342 mAh / g, wherein the heteroatom ratio is N: 2.5%, S: 1%, P: 0.5%, B: 0.5%, and the total ratio is 4.5%); d2 is

[0066] Example 1

[0067] Example 1 provides a negative electrode plate, Figure 1 This is a schematic diagram of the structure of the negative electrode sheet of Example 1; wherein 1 is the first active layer; 2 is the second active layer; and 3 is the negative electrode current collector. The preparation method is as follows:

[0068] S1. 94% of the first negative electrode active material A, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a first active layer slurry. 94% of the second negative electrode active material A, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a second active layer slurry.

[0069] S2: Double-layer coating is used on the upper and lower surfaces of the negative electrode current collector. The density of the first active layer and the second active layer is the same, both of which are 20 mg / cm 2 , drying, rolling and cutting to obtain the negative electrode sheet with a compaction density of 1.5g / cm 3 .

[0070] Examples 2 to 9

[0071] Examples 2 to 9 provide a series of negative electrode sheets, the preparation methods and component contents of which are the same as those of Example 1, except that the first negative electrode active material and the second negative electrode active material are different, as shown in Table 1.

[0072] Table 1 Components of Examples 1 to 9

[0073]

[0074]

[0075] Example 10

[0076] Example 10 provides a negative electrode plate, and the preparation method thereof is as follows:

[0077] S1. 94% of the first negative electrode active material B, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a first active layer slurry. 94% of the second negative electrode active material B, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a second active layer slurry.

[0078] S2: Double-layer coating is used on the upper and lower surfaces of the current collector. The density of the first active layer and the second active layer is the same, both of which are 20 mg / cm 2 , drying, rolling and cutting to obtain the negative electrode sheet with a compaction density of 1.0g / cm 3 .

[0079] Example 11

[0080] Example 11 provides a negative electrode plate, and the preparation method thereof is as follows:

[0081] S1. 94% of the first negative electrode active material B, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a first active layer slurry. 94% of the second negative electrode active material B, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a second active layer slurry.

[0082] S2: Double-layer coating is used on the upper and lower surfaces of the current collector. The density of the first active layer and the second active layer is the same, both of which are 20 mg / cm 2 , drying, rolling and cutting to obtain the negative electrode sheet with a compaction density of 1.1g / cm 3 .

[0083] Example 12

[0084] Example 12 provides a negative electrode plate, and the preparation method thereof is as follows:

[0085] S1. 94% of the first negative electrode active material B, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a first active layer slurry. 94% of the second negative electrode active material B, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a second active layer slurry.

[0086] S2: Double-layer coating is used on the upper and lower surfaces of the current collector. The density of the first active layer and the second active layer is the same, both of which are 20 mg / cm 2 , drying, rolling and cutting to obtain the negative electrode sheet with a compaction density of 1.7g / cm 3 .

[0087] Example 13

[0088] Example 13 provides a negative electrode plate, and the preparation method thereof is as follows:

[0089] S1. 94% of the first negative electrode active material B, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a first active layer slurry. 94% of the second negative electrode active material B, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a second active layer slurry.

[0090] S2: Double-layer coating is used on the upper and lower surfaces of the current collector. The density of the first active layer and the second active layer is the same, both of which are 20 mg / cm 2 , drying, rolling and cutting to obtain the negative electrode sheet with a compaction density of 1.9g / cm 3 .

[0091] Example 14

[0092] Example 14 provides a negative electrode plate, and the preparation method thereof is as follows:

[0093] S1. 99.85% of the first negative electrode active material A, 0.05% of conductive carbon black, 0.05% of styrene-butadiene rubber, and 0.05% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a first active layer slurry. 65% of the second negative electrode active material A, 15% of conductive carbon black, 15% of styrene-butadiene rubber, and 5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a second active layer slurry.

[0094] S2: Double-layer coating is used on the upper and lower surfaces of the negative electrode current collector. The density of the first active layer and the second active layer is the same, both of which are 20 mg / cm 2 , drying, rolling and cutting to obtain the negative electrode sheet with a compaction density of 1.5g / cm 3 .

[0095] Example 15

[0096] Example 15 provides a negative electrode plate, and the preparation method thereof is as follows:

[0097] S1. 65% of the first negative electrode active material A, 15% of conductive carbon black, 15% of styrene-butadiene rubber, and 5% of sodium carboxymethyl cellulose were stirred in deionized water to obtain a first active layer slurry. 99.85% of the second negative electrode active material A, 0.05% of conductive carbon black, 0.05% of styrene-butadiene rubber, and 0.05% of sodium carboxymethyl cellulose were stirred in deionized water to obtain a second active layer slurry.

[0098] S2: Double-layer coating is used on the upper and lower surfaces of the negative electrode current collector. The density of the first active layer and the second active layer is the same, both of which are 20 mg / cm 2 , drying, rolling and cutting to obtain the negative electrode sheet with a compaction density of 1.5g / cm 3 .

[0099] Comparative Example 1

[0100] Comparative Example 1 provides a negative electrode sheet, and its preparation method is as follows:

[0101] S1. 94% of the first negative electrode active material A, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are mixed with deionized water as the solvent and stirred to obtain an active layer slurry;

[0102] S2: Double-layer coating is used on the upper and lower surfaces of the current collector, with a surface density of 20 mg / cm 2 The upper and lower dies are both made of the first active layer slurry, which is dried, rolled and cut to obtain the negative electrode sheet with a compaction density of 1.5g / cm 3 .

[0103] Comparative Example 2

[0104] Comparative Example 2 provides a negative electrode sheet, and its preparation method is as follows:

[0105] S1. 94% of the second negative electrode active material A, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water as a solvent to obtain a second active layer slurry;

[0106] S2: Double-layer coating is used on the upper and lower surfaces of the current collector, with a surface density of 20 mg / cm 2 The upper and lower dies are both made of active layer slurry, which is dried, rolled and cut to obtain the negative electrode sheet with a compaction density of 1.5g / cm 3 ;

[0107] Comparative Example 3

[0108] Comparative Example 3 provides a negative electrode sheet, and its preparation method is as follows:

[0109] S1. 94% of the second negative electrode active material B, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a first active layer slurry. 94% of the first negative electrode active material B, 3% of conductive carbon black, 1.5% of styrene-butadiene rubber, and 1.5% of sodium carboxymethyl cellulose are stirred in deionized water to obtain a second active layer slurry.

[0110] S2: Double-layer coating is used on the upper and lower surfaces of the current collector. The density of the first active layer and the second active layer is the same, both of which are 20 mg / cm 2 , drying, rolling and cutting to obtain the negative electrode sheet with a compaction density of 1.5g / cm 3 .

[0111] Performance Testing

[0112] Preparation of sodium ion batteries:

[0113] (1) Electrolyte: 1M NaPF6 dissolved in ethylene carbonate (EC): diethyl carbonate (DEC) = 3:7

[0114] (2) Diaphragm: PP film;

[0115] (3) Components of the positive electrode sheet: The positive electrode material (Na3V2(PO4)2F3, accounting for 96%), the conductive agent (SP, accounting for 3%), and the binder (PVDF, accounting for 1%) are uniformly mixed in N-methylpyrrolidone to obtain a positive electrode sheet slurry. The positive electrode sheet slurry is applied to the positive electrode current collector, dried, rolled, and cut to obtain the positive electrode sheet.

[0116] (4) Assembly of sodium ion batteries

[0117] The negative electrode sheets, positive electrode sheets, separators and structural parts prepared in Examples 1 to 15 and Comparative Examples 1 to 3 were assembled, wound in the same number of layers, and then subjected to processes such as liquid injection, formation, aging and sealing to obtain a sodium ion battery.

[0118] The obtained sodium ion battery was subjected to a 0.5A current charge and discharge test using a Xinwei power battery test system to obtain the battery capacity, and the data are recorded in Table 2.

[0119] First-cycle coulombic efficiency: First-cycle coulombic efficiency = first charge capacity / first discharge capacity * 100%; record the data in Table 2.

[0120] 80% capacity retention: charge and discharge at a current of 0.5C, and record the number of cycles at room temperature until 80% capacity retention (80% SOH) is achieved.

[0121] Table 2 Data of Examples and Comparative Examples

[0122]

[0123]

[0124] The test results in Table 1 show that Comparative Example 1 has the highest capacity, but its Coulombic efficiency and cycle life both fall short of the standards. This shows that using only hard carbon with a high element doping level does not yield ideal performance. Comparative Example 2, while having a lower level of impurity doping, exhibits high cycle performance and initial efficiency, but its capacity falls short.

[0125] As in Examples 1 to 6, the first active layer is kept unchanged, and the width of the (002) crystal plane is adjusted by adjusting the heteroatom content of the second active layer. The battery capacity and first efficiency can meet the standards. In addition, as in Examples 1, 3 to 6, the effects of various impurity elements are further compared. Based on Example 1, Examples 3 to 6 reduce the boron, phosphorus, sulfur, and nitrogen doping amounts by 0.5% each, so as to judge the effect of each element on battery performance. Because the capacity differences between the groups in Examples 3 to 6 are basically small (<0.05), it is obvious that the reduction of nitrogen and sulfur elements is more conducive to improving the first efficiency and cycle stability.

[0126] Examples 2, 7, and 9 maintain the second active layer unchanged, while modifying the first active layer. The design parameters of Examples 2, 7, and 8 all fall within the specifications protected by the claims, and their performance is excellent. However, the first active layer of Example 9 utilizes hard carbon with a greater (002) plane width. This reaches the limits of hard carbon materials, resulting in a prone to layered structure collapse and accelerated cyclic decay.

[0127] The effect of compaction density (compression) on battery performance is compared in Examples 7, 10-13. At a reasonable compaction density, the battery performance is good. However, if the compaction density is too low, the pores are larger and there are more side reactions, which is not conducive to coulombic efficiency. Therefore, the first efficiency of Example 10 is relatively low. If the compaction density is too high, the risk of material damage increases. When the compaction density reaches 1.9, the battery performance decreases.

[0128] Compared with Example 1, Example 14 increases the proportion of active materials in the first active layer, and the capacity is improved to a certain extent, and there is no significant difference in coulombic efficiency and cycle stability. It can be seen that reducing the proportion of inactive materials is a feasible approach. Compared with Example 1, Example 15 reduces the proportion of active materials in the first active layer, and the capacity is significantly reduced. From the perspective of increasing capacity, reducing the proportion of active materials has no obvious advantage.

[0129] Comparative Example 3 and Example 7 interchange the positions of the active materials, resulting in d1 / d2 < 1 for Comparative Example 3. Obviously, the capacity, cycle stability and first efficiency of Comparative Example 3 are not ideal.

[0130] The above is a detailed description of the embodiments of the present invention, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the scope of the present invention.

Claims

1. A negative electrode plate, characterized in that: The negative electrode plate includes a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector, wherein the negative electrode active layer is composed of a first active layer and a second active layer; the first active layer is located between the second active layer and the negative electrode current collector; The first active layer includes a first negative electrode active material, and the second active layer includes a second negative electrode active material; The first negative electrode active material and the second negative electrode active material each independently contain heteroatoms, wherein the heteroatoms include at least two of nitrogen, oxygen, sulfur or boron; The heteroatom content of the first negative electrode active material is greater than the heteroatom content of the second negative electrode active material; The first negative electrode active material and the second negative electrode active material both include at least one of hard carbon, soft carbon, and mesocarbon microbeads; The (002) interlayer spacing d1 of the first negative electrode active material is greater than the (002) interlayer spacing d2 of the second negative electrode active material; the (002) interlayer spacing d1 of the first negative electrode active material and the (002) interlayer spacing d2 of the second negative electrode active material satisfy the following conditions: (i) d1 / d2≤1.2; (ii) 3.77 Å≤d1≤3.98 Å; (iii) 3.32 Å≤d2≤3.83 Å.

2. The negative electrode sheet according to claim 1, characterized in that: The compaction density of the negative electrode sheet is 1.1 mg / cm 3 ~1.7 mg / cm 3 .

3. The negative electrode sheet according to claim 1, characterized in that: The first active layer further includes a first conductive agent, a first binder, and a first dispersant, and the second active layer further includes a second conductive agent, a second binder, and a second dispersant.

4. The negative electrode sheet according to claim 3, characterized in that: The first conductive agent and the second conductive agent each independently include at least one of conductive carbon black, conductive graphite, carbon nanotubes, metal powder or carbon fiber; and / or, the first binder and the second binder each independently comprise at least one of polyvinylidene fluoride, polyvinyl alcohol, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer, sulfonated EPDM, styrene-butadiene rubber, and fluororubber; And / or, the first dispersant and the second dispersant each independently include at least one of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose.

5. A secondary battery, characterized in that: The invention comprises a positive electrode sheet, an electrolyte, a separator and a negative electrode sheet as claimed in any one of claims 1 to 4.

6. An electrical device, characterized in that: The secondary battery according to claim 5 is included.

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