Secondary battery and electric device

Abstract

The present application relates to the technical field of batteries, and relates to a secondary battery and an electric device. The secondary battery of the present application comprises a negative electrode sheet; the negative electrode sheet comprises a current collector and a coating arranged on at least one side surface of the current collector; the coating comprises a hard carbon layer, a sodium fast-ion conductor layer, and a soft carbon layer; and the sodium fast-ion conductor layer is arranged between the hard carbon layer and the soft carbon layer. The secondary battery has both good low-temperature fast-charging performance and excellent power performance.

Description

A secondary battery and an electrical device

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese application No. 202510374107.0, filed on March 26, 2025, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application belongs to the field of battery technology, specifically relating to a secondary battery and an electrical device. Background Technology

[0004] With the continuous growth of energy demand and the urgent need for sustainable energy storage technologies, sodium, which is abundant, widely distributed, and low in cost, has a broader application prospect in the power battery industry.

[0005] However, numerous technical challenges remain in the practical application of sodium-ion batteries. The low kinetics of hard carbon can negatively impact battery power performance in some cases. Therefore, this application is submitted. Summary of the Invention

[0006] The purpose of this application is to overcome the shortcomings of the prior art and provide a secondary battery and an electrical device.

[0007] To achieve the above objectives, the technical solution adopted in this application is as follows:

[0008] In a first aspect, this application provides a secondary battery, including a negative electrode sheet; the negative electrode sheet includes a current collector and a coating disposed on at least one surface of the current collector;

[0009] The coating comprises a hard carbon layer, a sodium fast ion conductor layer, and a soft carbon layer; the sodium fast ion conductor layer is disposed between the hard carbon layer and the soft carbon layer.

[0010] As an embodiment of this application, the hard carbon layer includes hard carbon, a dispersant, and a binder; the sodium fast ion conductor layer includes a sodium fast ion conductor, a dispersant, and a binder; and the soft carbon layer includes soft carbon, a dispersant, and a binder.

[0011] In some implementations, the secondary battery satisfies: 0.2 ≥ X3 / (X1+X3) ≥ 0.1;

[0012] X1% is the mass percentage of hard carbon in the hard carbon layer;

[0013] X3% represents the mass percentage of soft carbon in the soft carbon layer.

[0014] In some implementations, the secondary battery satisfies: 0.33 ≥ X2 / (X2+X3) ≥ 0.17;

[0015] X2% is the mass percentage of sodium fast ion conductors in the sodium fast ion conductor layer;

[0016] X3% represents the mass percentage of soft carbon in the soft carbon layer.

[0017] In some implementations, the hard carbon layer is in contact with the current collector.

[0018] In some implementations, the secondary battery satisfies at least one of the following:

[0019] a. The specific capacity of the hard carbon in the hard carbon layer is 330-360 mAh / g;

[0020] b. The sodium fast ion conductor in the sodium fast ion conductor layer includes M2O·Al2O3 and Na. l+x A2Si x P 3-x O 12 At least one of the following; M in M2O·Al2O3 is a monovalent, divalent, or trivalent cation, and Na... l+x A2Si x P 3-x O 12 In the equation, x = 0 to 3, and A represents a transition metal or a main group metal.

[0021] In some embodiments, the soft carbon in the soft carbon layer satisfies at least one of the following:

[0022] a. The specific capacity of the soft carbon is 260-290 mAh / g;

[0023] b. The particle size D of the soft carbon v50 The size is 5–10 μm;

[0024] c. The specific surface area of ​​the soft carbon is 2-7 m². 2 / g.

[0025] In some embodiments, the compaction density of the negative electrode sheet is 1.05–1.2 g / cm³. 3 .

[0026] In some implementations, the secondary battery satisfies at least one of the following:

[0027] a. The average thickness of the hard carbon layer is 80–100 μm;

[0028] b. The average thickness of the sodium fast ion conductor layer is 15–25 μm;

[0029] c. The average thickness of the soft carbon layer is 20–40 μm.

[0030] In some implementations, the secondary battery satisfies at least one of the following:

[0031] a. The mass percentage of the dispersant in the hard carbon layer is 1.3% to 1.5%, and the mass percentage of the binder is 1.9% to 2.3%.

[0032] b. The mass percentage of the dispersant in the sodium fast ion conductor layer is 1.1% to 1.3%, and the mass percentage of the binder is 2.3% to 2.7%.

[0033] c. The mass percentage of the dispersant in the soft carbon layer is 1.2% to 1.4%, and the mass percentage of the binder is 2.2% to 2.6%.

[0034] Secondly, this application provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0035] Compared with the prior art, the beneficial effects of this application are as follows:

[0036] This application combines a hard carbon layer, a sodium fast ion conductor layer, and a soft carbon layer to form a composite coating, which, together with a current collector, forms the negative electrode. The sodium fast ion conductor in the sodium fast ion conductor layer located between the hard carbon layer and the soft carbon layer improves the diffusion of sodium ions, allowing sodium ions to diffuse rapidly from the soft carbon layer to the hard carbon layer during charging. This eliminates the potential difference inside the electrode caused by the difference in plateau dynamics between the hard carbon layer and the soft carbon layer, reduces the sodium deposition phenomenon that is prone to occur due to the low sodium ion diffusion rate in the hard carbon plateau region, and alleviates the side reactions caused by the excessively low potential on the negative electrode side. This helps to improve the low-temperature fast charging performance and power performance of the battery. Attached Figure Description

[0037] Figure 1 is a schematic diagram of the negative electrode sheet in the secondary battery of Example 1. Detailed Implementation

[0038] To better illustrate the purpose, technical solution, and advantages of this application, the following description, in conjunction with specific embodiments and comparative examples, aims to provide a detailed understanding of the content of this application, rather than limiting it. All other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of this application. Unless otherwise specified, the experimental reagents and instruments involved in the implementation of this application are all commonly used reagents and instruments.

[0039] In general, hard carbon has low kinetics, which can affect the power performance of batteries in some cases. For example, there is a risk of sodium deposition during high-rate charging, which is not conducive to high-rate charging of sodium-ion batteries at high state of charge (SOC), resulting in poor fast-charging performance and power performance of sodium-ion batteries.

[0040] According to a first aspect of this application, a secondary battery is provided, including a negative electrode sheet; the negative electrode sheet includes a current collector and a coating disposed on at least one surface of the current collector; the coating includes a hard carbon layer, a sodium fast ion conductor layer and a soft carbon layer; the sodium fast ion conductor layer is disposed between the hard carbon layer and the soft carbon layer.

[0041] This application combines a hard carbon layer, a sodium fast ion conductor layer, and a soft carbon layer to form a composite coating, which, together with a current collector, forms the negative electrode. The sodium fast ion conductor in the sodium fast ion conductor layer located between the hard carbon layer and the soft carbon layer improves the diffusion of sodium ions, allowing sodium ions to rapidly diffuse from the soft carbon layer to the hard carbon layer during charging. This eliminates the potential difference inside the electrode caused by the difference in plateau dynamics between the hard carbon layer and the soft carbon layer, reduces the sodium deposition phenomenon that is prone to occur due to the low sodium ion diffusion rate in the hard carbon plateau region, and alleviates the side reactions caused by excessively low overpotential on the negative electrode side. This helps to improve the low-temperature fast charging performance and power performance of the battery.

[0042] It should be noted that hard carbon layers contain hard carbon, and soft carbon layers contain soft carbon. The main differences between hard carbon and soft carbon are: 1) Graphitization ability: Soft carbon can be fully graphitized at high temperatures (above 2500℃) to form a graphite-like crystal structure, while hard carbon is difficult to fully graphitize even at high temperatures (above 2500℃), and its disordered structure is difficult to eliminate; 2) Microstructure: The interlayer spacing d(002) of soft carbon is smaller, usually 0.34–0.37 nm; while the interlayer spacing d(002) of hard carbon is larger, usually 0.37–0.42 nm. Generally, the hard carbon layer and soft carbon layer in the coating can be determined by observing the cross-section of the negative electrode sheet along the thickness direction using a scanning electron microscope (SEM) or by scraping powder from the upper and lower surfaces of the coating and combining it with X-ray diffraction (XRD).

[0043] The sodium fast ion conductor layer contains sodium fast ion conductors. Sodium fast ion conductors are a class of materials with rapid sodium ion conduction properties. They possess a stable crystal lattice structure capable of accommodating a large number of sodium ions and allowing them to move rapidly within the lattice. Simultaneously, sodium fast ion conductors exhibit high sodium ion conductivity, typically reaching 10⁻⁶ at room temperature. -3 The sodium fast ion conductor layer can generally be determined by combining scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to analyze the elemental distribution along the thickness direction of the cross-section of the negative electrode sheet.

[0044] Furthermore, the current collector includes copper foil, aluminum foil, and copper foil or aluminum foil modified by corona treatment and coating techniques, with a foil thickness ranging from 6 to 15 μm. The aforementioned secondary battery includes at least one of the following: a wound pouch battery, a wound aluminum-cased battery, a wound cylindrical battery, a stacked pouch battery, and a stacked aluminum-cased battery.

[0045] The aforementioned secondary battery is a sodium-ion battery, which also includes a positive electrode sheet. The positive electrode sheet includes a current collector and an active material layer disposed on the surface of the current collector. The active material layer includes a positive electrode active material.

[0046] In one embodiment, the above-mentioned positive electrode active material can be a layered metal oxide, a polyanionic compound, or a Prussian blue-like compound; preferably, it is a layered metal oxide with the general chemical formula Na. x TMO2 (x≤1, "TM" is at least one transition metal such as Ni, Mn, Fe, Co, Cu); for example, it may specifically include Na. 0.74 Ni 0.43 Mn 0.6 O2, Na 0.67 Ni 0.33 Mn 0.59 Fe 0.1 O2, Na 0.76 Ni 0.33 Fe 0.1 Mn 0.57 O2, Na 0.86 Ni 0.33 Fe 0.1 Mn 0.545 O2, Na 0.867 Ni 0.33 Mn 0.42 Fe 0.2 Ti 0.048 O2, Na 0.87 Ni 0.33 Mn 0.4 Ti 0.21 At least one of O2.

[0047] In some embodiments, the hard carbon layer comprises hard carbon, a dispersant, and a binder; the sodium fast ion conductor layer comprises a sodium fast ion conductor, a dispersant, and a binder; and the soft carbon layer comprises soft carbon, a dispersant, and a binder.

[0048] In one embodiment, the dispersant in the hard carbon layer, sodium fast ion conductor layer, or soft carbon layer is independently selected from at least one of sodium hydroxymethyl cellulose, hydrogenated nitrile rubber, hydroxyethyl cellulose, and methyl cellulose; the binder in the hard carbon layer, sodium fast ion conductor layer, or soft carbon layer is independently selected from at least one of styrene-butadiene rubber, polyacrylic acid, sodium polyacrylate, polyvinyl alcohol, sodium alginate, and polyolefin.

[0049] In one embodiment, the hard carbon layer, the sodium fast ion conductor layer, and the soft carbon layer may further contain a conductive agent; wherein, the mass percentage of the conductive agent in the hard carbon layer is preferably 1.6% to 2.4%, specifically within the range of any one or any two of 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, and 2.4%; the mass percentage of the conductive agent in the sodium fast ion conductor layer is preferably 0.8% to 1.2%, specifically within the range of any one or any two of 0.8%, 0.9%, 1.0%, 1.1%, and 1.2%; and the mass percentage of the conductive agent in the soft carbon layer is preferably 1.4% to 2.2%, specifically within the range of any one or any two of 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, and 2.2%.

[0050] In some implementations, the secondary battery satisfies: 0.2 ≥ X3 / (X1+X3) ≥ 0.1;

[0051] X1% is the mass percentage of hard carbon in the hard carbon layer;

[0052] X3% represents the mass percentage of soft carbon in the soft carbon layer.

[0053] In one embodiment, X3 / (X1+X3) can specifically be any or any two of the following values: 0.1, 0.12, 0.14, 0.16, 0.18, and 0.2. Studies have found that when the secondary battery satisfies the above relationship, not only can the hard carbon in the hard carbon layer maintain a high specific capacity, but the soft carbon in the soft carbon layer can also promote the diffusion of sodium ions, thereby compensating for the insufficient SOC kinetics of the hard carbon layer and achieving a balance between specific capacity and kinetics. Generally, the current collector in the negative electrode sheet can be separated from the coating first, and then powder from the upper and lower surfaces of the coating can be taken separately. The resulting powders are then subjected to thermogravimetric analysis (TGA). By analyzing the thermogravimetric curves, the mass percentage of soft carbon in the soft carbon layer and the mass percentage of hard carbon in the hard carbon layer can be determined.

[0054] In some implementations, the secondary battery satisfies: 0.33 ≥ X2 / (X2+X3) ≥ 0.17;

[0055] X2% is the mass percentage of sodium fast ion conductors in the sodium fast ion conductor layer;

[0056] X3% represents the mass percentage of soft carbon in the soft carbon layer.

[0057] In one embodiment, X2 / (X2+X3) can specifically be any or both of the following values: 0.17, 0.19, 0.21, 0.23, 0.25, 0.27, 0.3, and 0.33. Studies have found that when the secondary battery satisfies the above relationship, the fast sodium ion conductor layer can better and more rapidly transport sodium ions diffused from the soft carbon layer to the hard carbon layer, thereby increasing the diffusion rate of sodium ions throughout the coating. This alleviates or avoids sodium deposition due to the low sodium ion diffusion rate in the hard carbon plateau region and reduces side reactions caused by excessively low overpotential on the negative electrode side.

[0058] In some implementations, the hard carbon layer is in contact with the current collector.

[0059] Compared to the contact between the soft carbon layer and the current collector, when the hard carbon layer is in contact with the current collector, the potential difference between the hard carbon layer and the soft carbon layer can be better utilized to promote the diffusion and transport of sodium ions, thereby alleviating or avoiding sodium precipitation caused by insufficient kinetics in the plateau region of the hard carbon layer.

[0060] In some implementations, the secondary battery satisfies at least one of the following:

[0061] a. The specific capacity of the hard carbon in the hard carbon layer is 330-360 mAh / g;

[0062] b. The sodium fast ion conductor in the sodium fast ion conductor layer includes M2O·Al2O3 and Na. l+x A2Si x P 3-x O 12 At least one of the following; M in M2O·Al2O3 is a monovalent, divalent, or trivalent cation, and Na... l+x A2Si x P 3-x O 12 In the equation, x = 0 to 3, and A represents a transition metal or a main group metal.

[0063] In one embodiment, the specific capacity of the hard carbon can specifically be any or both of the following: 330 mAh / g, 340 mAh / g, 350 mAh / g, and 360 mAh / g. High specific capacity hard carbon is more conducive to improving battery capacity, discharge voltage plateau, and energy density. The aforementioned specific capacity of hard carbon not only benefits the system's capacity utilization and energy density improvement but also improves the kinetics and compaction density of the plateau region, thus balancing the material's compaction density, high specific capacity utilization, and kinetic capabilities.

[0064] In one embodiment, M in M2O·Al2O3 can specifically be Na. + K + 、Rb + Li +At least one of them, such as Na2O· 11 Al2O3, Na2Li 0.3 Al 10.66 O 17.14 etc;Na l+x A2Si x P 3-x O 12 The A in the formula can be at least one of Zr, Mg, Ca, Ge, and Ce, preferably NaZr. 2( PO4)3, Na3Zr2Si2PO 12 Na 3.4 Zr 1.8 Ca 0.2 Si2PO 12 Na 3.1 Zr 1.95 Mg 0.05 Si2PO 12 Na3Zr 1.9 Ce 0.1 Si2PO 12 At least one of them.

[0065] As an embodiment of this application, the soft carbon in the soft carbon layer satisfies at least one of the following:

[0066] a. The specific capacity of the soft carbon is 260-290 mAh / g;

[0067] b. The particle size D of the soft carbon v50 The size is 5–10 μm;

[0068] c. The specific surface area of ​​the soft carbon is 2-7 m². 2 / g.

[0069] In one embodiment, the specific capacity of the soft carbon can specifically be any or both of the following: 260 mAh / g, 270 mAh / g, 280 mAh / g, and 290 mAh / g. Soft carbon with this specific capacity range can better balance high compaction density and high specific capacity.

[0070] In one embodiment, the particle size D of the soft carbon v50 Specifically, the particle size can be any or both of the following ranges: 5μm, 6μm, 7μm, 8μm, 9μm, and 10μm. Studies have found that the particle size of soft carbon affects its specific surface area, compaction density, and processing properties. The aforementioned particle size D... v50 The range of soft carbon can better balance high compaction density and homogenization performance, which is beneficial to improving the electrical performance of the electrode.

[0071] In one embodiment, the specific surface area of ​​the soft carbon can be 2m². 2 / g、3m2 / g、4m 2 / g、5m 2 / g、6m 2 / g、7m 2 The study found that the specific surface area of ​​soft carbon affects slurry performance and formation efficiency. Soft carbon within the above specific surface area range can achieve better slurry and coating performance without significantly deteriorating the formation efficiency.

[0072] In some embodiments, the compaction density of the negative electrode sheet is 1.05–1.2 g / cm³. 3 Optionally, the compaction density of the negative electrode sheet can specifically be 1.05 g / cm³. 3 1.1g / cm 3 1.15g / cm 3 1.2g / cm 3 The range of values ​​for any one or both of these. This compaction density of the negative electrode sheet is beneficial for increasing energy density.

[0073] In some implementations, the secondary battery satisfies at least one of the following:

[0074] a. The average thickness of the hard carbon layer is 80–100 μm;

[0075] b. The average thickness of the sodium fast ion conductor layer is 15–25 μm;

[0076] c. The average thickness of the soft carbon layer is 20–40 μm.

[0077] In one embodiment, the average thickness of the hard carbon layer can be any or both of the following: 80 μm, 85 μm, 90 μm, 95 μm, and 100 μm.

[0078] In one embodiment, the average thickness of the sodium fast ion conductor layer can be any or both of the following: 15 μm, 16 μm, 17 μm, 18 μm, 20 μm, 23 μm, and 25 μm.

[0079] In one embodiment, the average thickness of the soft carbon layer can be any or both of the following: 20 μm, 23 μm, 25 μm, 28 μm, 30 μm, 32 μm, 35 μm, 38 μm, and 40 μm.

[0080] In some implementations, the secondary battery satisfies at least one of the following:

[0081] a. The mass percentage of dispersant in the hard carbon layer is 1.3% to 1.5%, and the mass percentage of binder is 1.9% to 2.3%.

[0082] b. The mass percentage of dispersant in the sodium fast ion conductor layer is 1.1% to 1.3%, and the mass percentage of binder is 2.3% to 2.7%.

[0083] c. The mass percentage of dispersant in the soft carbon layer is 1.2% to 1.4%, and the mass percentage of binder is 2.2% to 2.6%.

[0084] In one embodiment, the mass percentage of the dispersant in the hard carbon layer can be any one or both of 1.3%, 1.4%, and 1.5%, and the mass percentage of the binder can be any one or both of 1.9%, 2.0%, 2.1%, 2.2%, and 2.3%.

[0085] In one embodiment, the mass percentage of the dispersant in the sodium fast ion conductor layer can be any one or both of 1.1%, 1.2%, and 1.3%, and the mass percentage of the binder can be any one or both of 2.3%, 2.4%, 2.5%, 2.6%, and 2.7%.

[0086] In one embodiment, the mass percentage of the dispersant in the soft carbon layer can be any one or both of 1.2%, 1.3%, and 1.4%, and the mass percentage of the binder can be any one or both of 2.2%, 2.3%, 2.4%, 2.5%, and 2.6%.

[0087] The amount of binder and dispersant added mainly affects the uniformity and stability of the coating slurry. The above-mentioned mass ratio of binder and dispersant is more conducive to improving the peel force and cohesion of the negative electrode sheet, thereby making the conductive network of the negative electrode sheet more stable during long-term cycling, thus improving cycle life and safety.

[0088] Secondly, this application provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0089] To clearly understand the technical solution of this application, the following detailed description of this application is provided in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application.

[0090] Example 1

[0091] This embodiment 1 provides a secondary battery, the preparation method of which includes the following steps:

[0092] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator in the middle of the positive and negative electrode. The inner core is formed by winding, hot pressing and shaping, and the tabs are welded to obtain the bare cell. The bare cell is then placed in the outer packaging aluminum-plastic film and baked in an oven. The electrolyte is injected into the dried battery, and the battery is allowed to stand, form, and be capacity tested to obtain a secondary battery.

[0093] 1) The preparation of the above negative electrode sheet includes the following steps:

[0094] S1. Hard carbon (specific capacity of 340 mAh / g), acetylene black, carbon nanotubes, binder (styrene-butadiene rubber) and dispersant (sodium hydroxymethyl cellulose) are added to a vacuum mixing tank at a mass ratio of 94:2.2:0.2:1.3:2.3 and mixed evenly. Then, deionized water is added as a solvent and stirred in the vacuum mixing tank until homogeneous to obtain slurry 1 (solid content of 49% to 51%).

[0095] Sodium fast ion conductor (sodium-alumina (Na2O·Al2O3)), acetylene black, binder (styrene-butadiene rubber) and dispersant (sodium hydroxymethyl cellulose) were added to a vacuum mixing tank at a mass ratio of 95:1.2:1.1:2.7 and mixed evenly. Then, deionized water was added as a solvent and stirred in the vacuum mixing tank until homogeneous to obtain slurry 2 (solid content of 50% to 52%).

[0096] Soft carbon (capacity 270mAh / g), acetylene black, binder (styrene-butadiene rubber) and dispersant (sodium hydroxymethyl cellulose) were added to a vacuum mixing tank at a mass ratio of 94:2.2:1.2:2.6 and mixed evenly. Then, deionized water was added as a solvent and stirred in the vacuum mixing tank until homogeneous to obtain slurry 3 (solid content 50%~52%).

[0097] S2. Using copper foil as the negative electrode current collector, slurry 1 and slurry 2 from S1 are coated onto the surface of the copper foil using a double-layer coating machine and dried at 80-90℃ for 15-20 minutes to form a slightly moist hard carbon layer (in contact with the copper foil) and a sodium fast ion conductor layer. Then, slurry 3 from S1 is coated onto the sodium fast ion conductor layer and dried at 95-110℃ to obtain a three-layer composite negative electrode sheet (as shown in Figure 1). The negative electrode sheet is obtained through rolling, tab shaping, slitting and cutting processes.

[0098] 2) The preparation of the above positive electrode sheet includes the following steps:

[0099] Layered oxide positive electrode active material (Na) 0.74 Ni 0.43 Mn 0.6O2), binder (PVDF) and acetylene black are mixed in a mass ratio of 95:1.8:3.2, and N-methylpyrrolidone (NMP) is added as a solvent. The mixture is stirred in a vacuum mixing tank until a homogeneous slurry (solid content of 60%) is formed. The slurry is then coated onto aluminum foil, baked (95-120℃), and then rolled, tab shaped, slitting and cutting processes are used to obtain the positive electrode sheet.

[0100] 3) The electrolyte used above is a commercially available sodium ion electrolyte;

[0101] 4) The above-mentioned diaphragm is a 7+3+5 composite PE-based diaphragm (i.e., the thickness of the PE base film is 7μm, the thickness of the ceramic coating is 3μm, and the thickness of the PVDF coating is 5μm).

[0102] Examples 2-3

[0103] Examples 2 and 3 respectively provide a secondary battery. The difference between the secondary battery and Example 1 is that the thicknesses of the hard carbon layer, the sodium fast ion conductor layer and the soft carbon layer in the negative electrode sheet are different. At the same time, in the preparation process of the negative electrode sheet, by adjusting the mass ratio of hard carbon in slurry 1, the mass ratio of sodium fast ion conductor in slurry 2 and the mass ratio of soft carbon in slurry 3 in step S1, X3 / (X1+X3) and X2 / (X2+X3) are basically the same as those in Example 1.

[0104] Examples 4-7

[0105] Examples 4 to 7 provide a secondary battery, which differs from Example 1 in that, during the preparation of the negative electrode sheet, the mass ratio of hard carbon in slurry 1, the mass ratio of sodium fast ion conductor in slurry 2, and the mass ratio of soft carbon in slurry 3 in step S1 are adjusted to make X3 / (X1+X3) and X2 / (X2+X3) different.

[0106] Examples 8-11

[0107] Examples 8-11 each provide a secondary battery, the difference between the secondary battery and Example 1 being that the particle size D of the soft carbon in the soft carbon layer of the negative electrode is... v50 The specific surface area S is different.

[0108] Example 12

[0109] Example 12 provides a secondary battery, which differs from Example 1 in that the sodium fast ion conductor in the sodium fast ion conductor layer of the negative electrode is NaZr(PO4)3.

[0110] Comparative Example 1

[0111] Comparative Example 1 provides a secondary battery, which differs from Example 1 in that the negative electrode does not have a sodium fast ion conductor layer and a soft carbon layer.

[0112] Comparative Example 2

[0113] Comparative Example 2 provides a secondary battery that differs from Example 1 in that the negative electrode does not have a hard carbon layer and a sodium fast ion conductor layer.

[0114] Comparative Example 3

[0115] Comparative Example 3 provides a secondary battery that differs from Example 1 in that the negative electrode does not have a sodium fast ion conductor layer.

[0116] Table 1. Relevant parameters of the negative electrode sheet of the secondary battery in each embodiment and comparative example.

[0117] In Table 1, h1 is the average thickness of the hard carbon layer in the negative electrode sheet, in μm;

[0118] h2 is the average thickness of the sodium fast ion conductor layer in the negative electrode plate, in μm;

[0119] h3 is the average thickness of the soft carbon layer in the negative electrode sheet, in μm;

[0120] D v50 The particle size D of soft carbon v50 , μm;

[0121] S is the specific surface area of ​​soft carbon, m 2 / g;

[0122] "Compacted density" refers to the compacted density of the negative electrode sheet, in g / cm³. 3 ;

[0123] X1 represents the mass percentage of hard carbon in the hard carbon layer, in %;

[0124] X2 represents the mass percentage of sodium fast ion conductors in the sodium fast ion conductor layer, in %;

[0125] X3 represents the mass percentage of soft carbon in the soft carbon layer, in %.

[0126] Performance testing

[0127] The secondary batteries in each embodiment and comparative example were first subjected to a 0.33C constant-capacity test, followed by a performance test. The test results are shown in Table 2. The specific test methods are as follows:

[0128] 1) 0.33C constant capacity test: Adjust the temperature to 25℃, let stand for 30 minutes, charge at a constant rate of 0.33C to 4.0V, then maintain a constant voltage of 4.0V to 0.05C, let stand for 30 minutes, discharge at a constant rate of 0.33C to 1.5V, repeat the above steps 3 times to obtain the 0.33C capacity C0;

[0129] 2) 25℃ DC internal resistance (DCR) test: Adjust the temperature to 25℃, charge at a constant rate of 0.33C0, adjust the state of charge to 50% SOC, let stand for 60min to obtain the starting voltage U0, discharge at a constant rate of 3C0 (current is I) for 30s to obtain the ending voltage U1, and obtain the DCR by (U0-U1) / I.

[0130] 3) -20℃ discharge retention rate test: Adjust the temperature to 25℃, let stand for 30 min, charge at a constant rate of 0.33C0 to 4.0V, then maintain a constant voltage of 4.0V to 0.05C0, let stand for 30 min, and discharge at a constant rate of 1C0 to 1.5V to obtain the standard capacity C1; let stand for 30 min, charge at a constant rate of 0.33C0 to 4.0V, then maintain a constant voltage of 4.0V to 0.05C0, adjust the temperature to -20℃, let stand for 180 min, and discharge at 1C0 to 1.5V to obtain the capacity C2. The -20℃ discharge retention rate is obtained by C2 / C1×100%.

[0131] 4) Sodium deposition determination and capacity retention rate after 20 cycles at -20℃ and -0.1C / 0.33C: After standing for 180 min, charge at a constant rate of 0.1C to 4.0V, then maintain a constant voltage of 4.0V to 0.05C, stand for 30 min, discharge at a constant rate of 0.33C to 1.5V, stand for 30 min, and repeat this charge-discharge cycle 20 times. Then charge at a constant rate of 0.1C to 4.0V, and maintain a constant voltage of 4.0V to 0.05C. After the cell is removed from the cabinet, disassemble the fully charged interface to determine the sodium deposition situation; Capacity retention rate = Discharge capacity of the 20th cycle / Discharge capacity of the 1st cycle × 100%.

[0132] Table 2 shows the performance of the secondary batteries in each embodiment and comparative example.

[0133] According to the data in Table 2, the DCR of the secondary batteries in Examples 1 to 12 is less than or equal to 800mΩ at 25℃, and the discharge retention rate is greater than or equal to 95% at -20℃. At the same time, the capacity retention rate after 20 cycles at -20℃ is greater than or equal to 94% and no sodium precipitation occurs, indicating that the secondary batteries of this application have good low-temperature fast charging performance and excellent power performance.

[0134] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, comprising a negative electrode sheet; the negative electrode sheet comprising a current collector and a coating disposed on at least one surface of the current collector; The coating comprises a hard carbon layer, a sodium fast ion conductor layer, and a soft carbon layer; the sodium fast ion conductor layer is disposed between the hard carbon layer and the soft carbon layer.

2. The secondary battery of claim 1, wherein the hard carbon layer comprises hard carbon, a dispersant, and a binder; the sodium fast ion conductor layer comprises a sodium fast ion conductor, a dispersant, and a binder; and the soft carbon layer comprises soft carbon, a dispersant, and a binder.

3. The secondary battery as described in claim 1 or 2, wherein the secondary battery satisfies: 0.2 ≥ X3 / (X1+X3) ≥ 0.1; X1% is the mass percentage of hard carbon in the hard carbon layer; X3% represents the mass percentage of soft carbon in the soft carbon layer.

4. The secondary battery as described in claim 1 or 2, wherein the secondary battery satisfies: 0.33 ≥ X2 / (X2+X3) ≥ 0.17; X2% is the mass percentage of sodium fast ion conductors in the sodium fast ion conductor layer; X3% represents the mass percentage of soft carbon in the soft carbon layer.

5. The secondary battery as described in claim 3, wherein the secondary battery satisfies: 0.33≥X2 / (X2+X3)≥0.17; X2% is the mass percentage of sodium fast ion conductors in the sodium fast ion conductor layer; X3% represents the mass percentage of soft carbon in the soft carbon layer.

6. The secondary battery of claim 1, wherein the hard carbon layer is in contact with the current collector.

7. The secondary battery according to any one of claims 1, 2, 5-6, wherein the secondary battery satisfies at least one of the following: a. The specific capacity of the hard carbon in the hard carbon layer is 330-360 mAh / g; b. The sodium fast ion conductor in the sodium fast ion conductor layer includes M2O·Al2O3 and Na. l+x A2Si x P 3-x O 12 At least one of the following; M in M2O·Al2O3 is a monovalent, divalent, or trivalent cation, and Na... l+x A2Si x P 3-x O 12 In the equation, x = 0 to 3, and A represents a transition metal or a main group metal.

8. The secondary battery according to any one of claims 1, 2, 5-6, wherein the soft carbon in the soft carbon layer satisfies at least one of the following: a. The specific capacity of the soft carbon is 260-290 mAh / g; b. The particle size D of the soft carbon v50 The size is 5–10 μm; c. The specific surface area of ​​the soft carbon is 2-7 m². 2 / g.

9. The secondary battery of claim 7, wherein the soft carbon in the soft carbon layer satisfies at least one of the following: a. The specific capacity of the soft carbon is 260-290 mAh / g; b. The particle size D of the soft carbon v50 The size is 5–10 μm; c. The specific surface area of ​​the soft carbon is 2-7 m². 2 / g.

10. The secondary battery according to any one of claims 1, 2, 5-6, and 9, wherein the compaction density of the negative electrode sheet is 1.05–1.2 g / cm³. 3 .

11. The secondary battery as claimed in claim 8, wherein the compaction density of the negative electrode sheet is 1.05–1.2 g / cm³. 3 .

12. The secondary battery according to any one of claims 1, 2, 5-6, 9, and 11, wherein the secondary battery satisfies at least one of the following: a. The average thickness of the hard carbon layer is 80–100 μm; b. The average thickness of the sodium fast ion conductor layer is 15–25 μm; c. The average thickness of the soft carbon layer is 20–40 μm.

13. The secondary battery according to any one of claims 1, 2, 5-6, 9, and 11, wherein the secondary battery satisfies at least one of the following: a. The mass percentage of the dispersant in the hard carbon layer is 1.3% to 1.5%, and the mass percentage of the binder is 1.9% to 2.3%. b. The mass percentage of the dispersant in the sodium fast ion conductor layer is 1.1% to 1.3%, and the mass percentage of the binder is 2.3% to 2.7%. c. The mass percentage of the dispersant in the soft carbon layer is 1.2% to 1.4%, and the mass percentage of the binder is 2.2% to 2.6%.

14. The secondary battery of claim 12, wherein the secondary battery satisfies at least one of the following: a. The mass percentage of the dispersant in the hard carbon layer is 1.3% to 1.5%, and the mass percentage of the binder is 1.9% to 2.3%. b. The mass percentage of the dispersant in the sodium fast ion conductor layer is 1.1% to 1.3%, and the mass percentage of the binder is 2.3% to 2.7%. c. The mass percentage of the dispersant in the soft carbon layer is 1.2% to 1.4%, and the mass percentage of the binder is 2.2% to 2.6%.

15. An electrical device comprising a secondary battery as described in any one of claims 1 to 14, wherein the secondary battery serves as a power supply for the electrical device.

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