Secondary battery and electric device

Abstract

The invention relates to a secondary battery and an electric device, and belongs to the technical field of batteries. The secondary battery provided by the invention comprises a negative pole piece; the negative pole piece 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 good low-temperature fast charging performance and excellent power performance.

Description

Technical Field

[0001] This application belongs to the technical field of batteries, and particularly relates to a secondary battery and an electrical device. Background Art

[0002] With the continuous growth of energy demand and the urgent need for sustainable energy storage technologies, sodium, which is rich in resources, widely distributed, and low in cost, has a broader application prospect in the power battery industry.

[0003] However, in the actual application process of sodium-ion batteries, many technical challenges still remain. The kinetics of hard carbon is low, which affects the power performance of the battery in some cases. In view of this, this application is proposed. Summary of the Invention

[0004] The purpose of this application is to overcome the deficiencies of the prior art and provide a secondary battery and an electrical device.

[0005] To achieve the above purpose, the technical solution adopted in this application is as follows:

[0006] In the first aspect, this application provides a secondary battery, including a negative electrode plate; the negative electrode plate includes a current collector and a coating provided on at least one surface of the current collector;

[0007] 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 provided between the hard carbon layer and the soft carbon layer.

[0008] 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; the soft carbon layer includes soft carbon, a dispersant, and a binder.

[0009] As an embodiment of this application, the secondary battery satisfies: 0.2 ≥ X3 / (X1 + X3) ≥ 0.1;

[0010] X1% is the mass percentage of hard carbon in the hard carbon layer;

[0011] X3% is the mass percentage of soft carbon in the soft carbon layer.

[0012] As an embodiment of this application, the secondary battery satisfies: 0.33 ≥ X2 / (X2 + X3) ≥ 0.17;

[0013] X2% is the mass percentage of the sodium fast ion conductor in the sodium fast ion conductor layer;

[0014] X3% is the mass percentage of soft carbon in the soft carbon layer.

[0015] As an embodiment of the present application, the hard carbon layer is in contact with the current collector.

[0016] As an embodiment of the present application, at least one of the following is satisfied:

[0017] a. The specific capacity of the hard carbon in the hard carbon layer is 330 - 360 mAh / g;

[0018] b. The sodium fast ion conductor in the sodium fast ion conductor layer includes at least one of M2O·Al2O3, Na l+x A2Si x P 3-x O 12 ; M in M2O·Al2O3 is a monovalent, divalent or trivalent cation, and x in Na l+x A2Si x P 3-x O 12 is 0 - 3, and A is a transition group or main group metal.

[0019] As an embodiment of the present application, the soft carbon in the soft carbon layer satisfies at least one of the following:

[0020] a. The specific capacity of the soft carbon is 260 - 290 mAh / g;

[0021] b. The particle size D of the soft carbon v50 is 5 - 10 μm;

[0022] c. The specific surface area of the soft carbon is 2 - 7 m 2 / g.

[0023] As an embodiment of the present application, the compaction density of the negative electrode sheet is 1.05 - 1.2 g / cm 3 .

[0024] As an embodiment of the present application, at least one of the following is satisfied:

[0025] a. The average thickness of the hard carbon layer is 80 - 100 μm;

[0026] b. The average thickness of the sodium fast ion conductor layer is 15 - 25 μm;

[0027] c. The average thickness of the soft carbon layer is 20 - 40 μm.

[0028] As an embodiment of the present application, at least one of the following is satisfied:

[0029] a. The mass ratio of the dispersant in the hard carbon layer is 1.3% - 1.5%, and the mass ratio of the binder is 1.9% - 2.3%;

[0030] b. The mass ratio of the dispersant in the sodium fast ion conductor layer is 1.1% - 1.3%, and the mass ratio of the binder is 2.3% - 2.7%.

[0031] c. The mass ratio of the dispersant in the soft carbon layer is 1.2% - 1.4%, and the mass ratio of the binder is 2.2% - 2.6%.

[0032] In a second aspect, the present application provides an electrical device, including the above secondary battery, and the secondary battery serves as the power supply of the electrical device.

[0033] Compared with the prior art, the beneficial effects of the present application are as follows:

[0034] The present application forms a composite coating by combining a hard carbon layer, a sodium fast ion conductor layer, and a soft carbon layer, and forms a negative electrode plate together with the current collector. The sodium fast ion conductor in the sodium fast ion conductor layer disposed between the hard carbon layer and the soft carbon layer is used to improve the diffusion of sodium ions, so that sodium ions during the charging process rapidly diffuse from the soft carbon layer to the hard carbon layer, thereby eliminating the internal potential difference of the electrode plate caused by the platform kinetic difference between the hard carbon layer and the soft carbon layer, reducing the sodium deposition phenomenon that is likely to occur due to the low sodium ion diffusion rate in the hard carbon platform region, and alleviating the side reactions caused by the too low potential on the negative electrode side, thus contributing to improving the low-temperature fast charging performance and power performance of the battery. Description of the Drawings

[0035] Figure 1 It is a schematic structural diagram of the negative electrode plate in the secondary battery of Example 1. Detailed Embodiments

[0036] In order to better illustrate the purpose, technical solutions, and advantages of the present application, the present application will be further described below in conjunction with specific examples and comparative examples. The purpose is to understand the content of the present application in detail, rather than to limit the present application. All other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present application. The experimental reagents and instruments involved in the implementation of the present application are all common ordinary reagents and instruments unless otherwise specified.

[0037] Generally, the kinetics of hard carbon is low, which affects the power performance of the battery 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 SOC, resulting in poor fast charging performance and power performance of sodium-ion batteries.

[0038] According to the first aspect of the present application, a secondary battery is provided, including a negative electrode plate; the negative electrode plate 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.

[0039] In this application, a composite coating is formed by combining a hard carbon layer, a sodium fast ion conductor layer, and a soft carbon layer, and the composite coating and a current collector form a negative electrode tab. The sodium fast ion conductor in the sodium fast ion conductor layer disposed between the hard carbon layer and the soft carbon layer is used to improve the diffusion of sodium ions, so that sodium ions during the charging process rapidly diffuse from the soft carbon layer to the hard carbon layer, thereby eliminating the potential difference inside the tab caused by the platform kinetics difference between the hard carbon layer and the soft carbon layer, reducing the sodium deposition phenomenon that easily occurs due to the low sodium ion diffusion rate in the hard carbon platform region, and alleviating the side reactions caused by the too low overpotential on the negative electrode side, thus contributing to improving the low-temperature fast charging performance and power performance of the battery.

[0040] It should be noted that the hard carbon layer contains hard carbon, and the soft carbon layer contains soft carbon. The main differences between hard carbon and soft carbon are as follows: 1) Graphitization ability: Soft carbon can be fully graphitized at high temperatures (above 2500 °C) to form a crystal structure similar to graphite, while hard carbon is difficult to be fully graphitized even at high temperatures (above 2500 °C), and its disordered structure is difficult to eliminate; 2) Microstructure: The layer spacing d(002) of soft carbon is smaller, usually 0.34 - 0.37 nm; while the layer spacing d(002) of hard carbon is larger, usually 0.37 - 0.42 nm. Generally, the hard carbon layer and the soft carbon layer in the coating can be determined by observing the morphology of the cross-section of the negative electrode tab along the thickness direction through a scanning electron microscope (SEM) or scraping the powders on the upper and lower surfaces of the coating respectively and combining with X-ray diffraction (XRD).

[0041] The sodium fast ion conductor layer contains a sodium fast ion conductor. The sodium fast ion conductor is a type of material with fast sodium ion conduction performance. It has a stable lattice structure, can accommodate a large number of sodium ions and allow sodium ions to move rapidly in the lattice; at the same time, it has a high sodium ion conductivity, usually reaching above 10 -3 S / cm at room temperature. Generally, the sodium fast ion conductor layer can be determined by combining a scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS) and analyzing the element distribution of the cross-section of the negative electrode tab along the thickness direction.

[0042] In addition, the current collector includes copper foil, aluminum foil, and copper foil or aluminum foil modified by corona technology and coating technology. The thickness range of the foil material is 6 - 15 μm. The above secondary battery includes at least one of a wound soft-pack battery, a wound aluminum-shell battery, a wound cylindrical battery, a laminated soft-pack battery, a laminated aluminum-shell battery, etc.

[0043] The above secondary battery is a sodium ion battery, which further includes a positive electrode tab. The positive electrode tab 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.

[0044] In one embodiment, the above-mentioned positive electrode active material may be a layered metal oxide, a polyanion compound or a Prussian blue compound; preferably a layered metal oxide, and its chemical general formula is NaxTMO2 (x≤1, TM is at least one of transition metals such as Ni, Mn, Fe, Co, Cu, etc.); 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 0.4 2Fe 0.2 Ti 0.048 O2, Na 0.87 Ni 0.33 Mn 0.4 Ti 0.21 at least one of O2.

[0045] As an embodiment of the present 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; the soft carbon layer includes soft carbon, a dispersant and a binder.

[0046] In one embodiment, the dispersant in the above-mentioned hard carbon layer, sodium fast ion conductor layer or soft carbon layer is independently selected from at least one of sodium carboxymethyl cellulose, hydrogenated nitrile rubber, hydroxyethyl cellulose, and carboxymethyl cellulose; the binder in the above-mentioned hard carbon layer, sodium fast ion conductor layer or soft carbon layer is independently selected from at least one of styrene-butadiene rubber, polyacrylic acids, sodium polyacrylates, polyvinyl alcohol, sodium alginate, and polyolefins.

[0047] In one embodiment, the components of the above-mentioned hard carbon layer, sodium fast ion conductor layer and soft carbon layer may further contain a conductive agent; among them, the mass ratio of the conductive agent in the hard carbon layer is preferably 1.6% to 2.4%, specifically it can be any one or the range value of any two of 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%; the mass ratio of the conductive agent in the sodium fast ion conductor layer is preferably 0.8% to 1.2%, specifically it can be any one or the range value of any two of 0.8%, 0.9%, 1.0%, 1.1%, 1.2%; the mass ratio of the conductive agent in the soft carbon layer is preferably 1.4% to 2.2%, specifically it can be any one or the range value of any two of 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%.

[0048] As an embodiment of the present application, the secondary battery satisfies: 0.2 ≥ X3 / (X1 + X3) ≥ 0.1;

[0049] X1% is the mass percentage content of hard carbon in the hard carbon layer;

[0050] X3% is the mass percentage content of soft carbon in the soft carbon layer.

[0051] In one embodiment, X3 / (X1 + X3) can specifically be any one or the range value of any two of 0.1, 0.12, 0.14, 0.16, 0.18, 0.2. It has been found that when the secondary battery satisfies the above relationship, not only can the hard carbon in the hard carbon layer be used to maintain a relatively high specific capacity, but also the soft carbon in the soft carbon layer can be used to promote the diffusion of sodium ions, thereby compensating for the problem of insufficient high-SOC kinetics in the hard carbon layer and achieving the balance of specific capacity and kinetics. Generally, the current collector in the negative electrode plate can be separated from the coating first, then the powders on the upper and lower surfaces of the coating are taken respectively, and then the obtained powders are subjected to thermogravimetric analysis (TGA) tests. By analyzing the thermogravimetric curve, the mass percentage content of soft carbon in the soft carbon layer and the mass percentage content of hard carbon in the hard carbon layer can be determined.

[0052] As an embodiment of the present application, the secondary battery satisfies: 0.33 ≥ X2 / (X2 + X3) ≥ 0.17;

[0053] X2% is the mass percentage content of sodium fast ion conductor in the sodium fast ion conductor layer;

[0054] X3% is the mass percentage content of soft carbon in the soft carbon layer.

[0055] In one embodiment, X2 / (X2 + X3) can specifically be any one of 0.17, 0.19, 0.21, 0.23, 0.25, 0.27, 0.3, 0.33 or the range value of any two of them. It is found that when the secondary battery satisfies the above relationship, the sodium fast ion conductor layer can better transport the sodium ions diffused from the soft carbon layer to the hard carbon layer rapidly, thereby increasing the diffusion rate of sodium ions in the whole coating, alleviating or avoiding the sodium deposition caused by the low diffusion rate of sodium ions in the hard carbon plateau region, and reducing the side reactions caused by the too low overpotential on the negative electrode side.

[0056] As an embodiment of the present application, the hard carbon layer is in contact with the current collector.

[0057] Compared with the case where the soft carbon layer is in contact with 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 the sodium deposition caused by the insufficient kinetics in the hard carbon layer plateau region.

[0058] As an embodiment of the present application, at least one of the following is satisfied:

[0059] a. The specific capacity of the hard carbon in the hard carbon layer is 330 - 360 mAh / g;

[0060] b. The sodium fast ion conductor in the sodium fast ion conductor layer includes at least one of M2O·Al2O3, Na l+x A2Si x P 3-x O 12 ; M in M2O·Al2O3 is a monovalent, divalent or trivalent cation, and x in Na l+x A2Si x P 3-x O 12 is 0 - 3, and A is a transition group or main group metal.

[0061] In one embodiment, the specific capacity of the hard carbon can specifically be any one of 330 mAh / g, 340 mAh / g, 350 mAh / g, 360 mAh / g or the range value of any two of them. The hard carbon with a high specific capacity is more beneficial to improving the capacity, discharge voltage platform and energy density of the battery. The hard carbon with the above specific capacity is not only beneficial to the capacity and energy density improvement of the system, but also can improve the kinetics and compaction density in the plateau region, thus taking into account the compaction density of the material, the high specific capacity performance and the kinetics ability.

[0062] 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 In the formula, A can specifically be at least one of Zr, Mg, Ca, Ge, 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.

[0063] As an embodiment of the present application, the soft carbon in the soft carbon layer satisfies at least one of the following:

[0064] a. The specific capacity of the soft carbon is 260 - 290 mAh / g;

[0065] b. The particle size D of the soft carbon v50 is 5 - 10 μm;

[0066] c. The specific surface area of the soft carbon is 2 - 7 m 2 / g.

[0067] In one embodiment, the specific capacity of the soft carbon can specifically be any one of 260 mAh / g, 270 mAh / g, 280 mAh / g, 290 mAh / g or the range value of any two of them. The soft carbon with this specific capacity range can better balance the high tap density and high specific capacity.

[0068] In one embodiment, the particle size D of the soft carbon v50 can specifically be any one of 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or the range value of any two of them. It is found that the particle size of the soft carbon will affect its specific surface area, tap density and processing performance. The soft carbon with the above particle size D v50 range can better balance the high tap density and slurry performance, thus being beneficial to improving the electrical performance of the electrode sheet.

[0069] In one embodiment, the specific surface area of the soft carbon can specifically be 2 m 2 / g, 3 m2 / g, 4m 2 / g, 5m 2 / g, 6m 2 / g, 7m 2 The range values of any one or any two of them in / g. It is found that the specific surface area of the soft carbon will affect the homogenization performance and the initial efficiency of formation. The soft carbon within the above specific surface area range can obtain better slurry and coating performance without significantly deteriorating the initial efficiency.

[0070] As an embodiment of the present application, the compaction density of the negative electrode plate is 1.05 - 1.2 g / cm 3 . Optionally, the compaction density can specifically be 1.05 g / cm 3 , 1.1 g / cm 3 , 1.15 g / cm 3 , 1.2 g / cm 3 The range values of any one or any two of them. The negative electrode plate with this compaction density is beneficial to improving the energy density.

[0071] As an embodiment of the present application, at least one of the following is satisfied:

[0072] a. The average thickness of the hard carbon layer is 80 - 100 μm;

[0073] b. The average thickness of the sodium fast ion conductor layer is 15 - 25 μm;

[0074] c. The average thickness of the soft carbon layer is 20 - 40 μm.

[0075] In one embodiment, the average thickness of the hard carbon layer can specifically be any one or any two of the range values of 80 μm, 85 μm, 90 μm, 95 μm, 100 μm.

[0076] In one embodiment, the average thickness of the sodium fast ion conductor layer can specifically be any one or any two of the range values of 15 μm, 16 μm, 17 μm, 18 μm, 20 μm, 23 μm, 25 μm.

[0077] In one embodiment, the average thickness of the soft carbon layer can specifically be any one or any two of the range values of 20 μm, 23 μm, 25 μm, 28 μm, 30 μm, 32 μm, 35 μm, 38 μm, 40 μm.

[0078] As an embodiment of the present application, at least one of the following is satisfied:

[0079] a. The mass ratio of the dispersant in the hard carbon layer is 1.3% - 1.5%, and the mass ratio of the binder is 1.9% - 2.3%;

[0080] b. The mass ratio of the dispersant in the sodium fast ion conductor layer is 1.1% to 1.3%, and the mass ratio of the binder is 2.3% to 2.7%.

[0081] c. The mass ratio of the dispersant in the soft carbon layer is 1.2% to 1.4%, and the mass ratio of the binder is 2.2% to 2.6%.

[0082] In one embodiment, the mass ratio of the dispersant in the hard carbon layer can specifically be any one of 1.3%, 1.4%, 1.5% or the range value of any two of them, and the mass ratio of the binder can specifically be any one of 1.9%, 2.0%, 2.1%, 2.2%, 2.3% or the range value of any two of them.

[0083] In one embodiment, the mass ratio of the dispersant in the sodium fast ion conductor layer can specifically be any one of 1.1%, 1.2%, 1.3% or the range value of any two of them, and the mass ratio of the binder can specifically be any one of 2.3%, 2.4%, 2.5%, 2.6%, 2.7% or the range value of any two of them.

[0084] In one embodiment, the mass ratio of the dispersant in the soft carbon layer can specifically be any one of 1.2%, 1.3%, 1.4% or the range value of any two of them, and the mass ratio of the binder can specifically be any one of 2.2%, 2.3%, 2.4%, 2.5%, 2.6% or the range value of any two of them.

[0085] The addition amounts of the binder and the dispersant mainly affect the homogenization uniformity and stability of the coating slurry. The binder and the dispersant with the above mass ratios are more conducive to improving the peel strength and cohesion of the negative electrode sheet, thereby making the conductive network of the negative electrode sheet more stable during long-term cycling, and thus improving the cycle life and safety.

[0086] In a second aspect, the present application provides an electrical device, including the above secondary battery, and the secondary battery serves as the power supply of the electrical device.

[0087] To clearly understand the technical solution of the present application, the following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0088] Example 1

[0089] This example provides a secondary battery, and its preparation method includes the following steps:

[0090] Stack the positive electrode sheet, separator, and negative electrode sheet in sequence, with the separator in the middle of the positive and negative electrode sheets, wind them to form an inner core, perform hot pressing and shaping, and obtain a bare battery cell after welding the tabs; then place the bare battery cell in an outer packaging aluminum-plastic film and bake it in an oven, inject the electrolyte into the dried battery, and let it stand, age, and grade-capacity test to obtain a secondary battery.

[0091] 1) The preparation of the above-mentioned negative electrode sheet includes the following steps:

[0092] S1. Add hard carbon (gram capacity of 340 mAh / g), acetylene black, carbon nanotubes, binder (styrene-butadiene rubber), and dispersant (sodium carboxymethyl cellulose) into a vacuum stirring tank according to a mass ratio of 94:2.2:0.2:1.3:2.3, mix evenly, then add deionized water as a solvent, and stir in the vacuum stirring tank until uniform to obtain Slurry 1 (solid content is 49% - 51%);

[0093] Add sodium fast ion conductor (sodium-aluminum oxide (Na2O·Al2O3)), acetylene black, binder (styrene-butadiene rubber), and dispersant (sodium carboxymethyl cellulose) into a vacuum stirring tank according to a mass ratio of 95:1.2:1.1:2.7, mix evenly, then add deionized water as a solvent, and stir in the vacuum stirring tank until uniform to obtain Slurry 2 (solid content is 50% - 52%);

[0094] Add soft carbon (gram capacity of 270 mAh / g), acetylene black, binder (styrene-butadiene rubber), and dispersant (sodium carboxymethyl cellulose) into a vacuum stirring tank according to a mass ratio of 94:2.2:1.2:2.6, mix evenly, then add deionized water as a solvent, and stir in the vacuum stirring tank until uniform to obtain Slurry 3 (solid content is 50% - 52%);

[0095] S2. Using a copper foil as the negative electrode current collector, coat Slurry 1 and Slurry 2 in S1 on the surface of the copper foil with a double-layer coater and dry at 80 - 90 °C for 15 - 20 min to form a slightly wet hard carbon layer (in contact with the copper foil) and a sodium fast ion conductor layer; then coat Slurry 3 in S1 on the sodium fast ion conductor layer and dry at 95 - 110 °C to obtain a three-layer composite negative electrode sheet (as Figure 1 shown), and obtain the negative electrode sheet through processes such as rolling, tab shaping, slitting, and blanking.

[0096] 2) The preparation of the above-mentioned positive electrode sheet includes the following steps:

[0097] The 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 then N-methylpyrrolidone (NMP) is added as a solvent. The mixture is stirred in a vacuum stirring tank until a uniform slurry (solid content of 60%) is obtained. Then, the slurry is coated on an aluminum foil. After baking (95 - 120 °C), it is further processed through processes such as rolling, tab forming, slitting, and die cutting to obtain the positive electrode sheet.

[0098] 3) The above electrolyte selects a commercial sodium ion electrolyte;

[0099] 4) The above separator is a 7 + 3 + 5 composite PE-based separator (i.e., the thickness of the PE-based film is 7 μm, the thickness of the ceramic coating is 3 μm, and the thickness of the PVDF coating is 5 μm).

[0100] Examples 2 - 3

[0101] Examples 2 - 3 provide a secondary battery. The difference between this secondary battery and Example 1 is that the thicknesses of the hard carbon layer, sodium fast ion conductor layer, and soft carbon layer in the negative electrode sheet are different. At the same time, during the preparation process of the negative electrode sheet, by adjusting the mass ratio of hard carbon in Slurry 1 in step S1, the mass ratio of sodium fast ion conductor in Slurry 2, and the mass ratio of soft carbon in Slurry 3, so that X3 / (X1 + X3) and X2 / (X2 + X3) are basically the same as those in Example 1.

[0102] Examples 4 - 7

[0103] Examples 4 - 7 provide a secondary battery. The difference between this secondary battery and Example 1 is that during the preparation process of the negative electrode sheet, by adjusting the mass ratio of hard carbon in Slurry 1 in step S1, the mass ratio of sodium fast ion conductor in Slurry 2, and the mass ratio of soft carbon in Slurry 3, X3 / (X1 + X3) and X2 / (X2 + X3) are different.

[0104] Examples 8 - 11

[0105] Examples 8 - 11 provide a secondary battery. The difference between this secondary battery and Example 1 is that the particle size D of the soft carbon in the soft carbon layer of the negative electrode sheet v50 and the specific surface area S are different.

[0106] Example 12

[0107] Example 12 provides a secondary battery. The difference between this secondary battery and Example 1 is that the sodium fast ion conductor in the sodium fast ion conductor layer of the negative electrode sheet is NaZr(PO4)3.

[0108] Comparative Example 1

[0109] Comparative Example 1 provides a secondary battery, which is different from Example 1 in that there is no sodium fast ion conductor layer and soft carbon layer in the negative electrode sheet.

[0110] Comparative Example 2

[0111] Comparative Example 2 provides a secondary battery, which is different from Example 1 in that there is no hard carbon layer and sodium fast ion conductor layer in the negative electrode sheet.

[0112] Comparative Example 3

[0113] Comparative Example 3 provides a secondary battery, which is different from Example 1 in that there is no sodium fast ion conductor layer in the negative electrode sheet.

[0114] Table 1 Negative electrode sheets of secondary batteries in each example and comparative example

[0115]

[0116]

[0117]

[0118] In Table 1, h1 is the average thickness of the hard carbon layer in the negative electrode sheet, μm;

[0119] h2 is the average thickness of the sodium fast ion conductor layer in the negative electrode sheet, μm;

[0120] h3 is the average thickness of the soft carbon layer in the negative electrode sheet, μm;

[0121] D v50 is the particle size D of the soft carbon v50 , μm;

[0122] S is the specific surface area of the soft carbon, m 2 / g;

[0123] The tap density is the tap density of the negative electrode sheet, g / cm 3 ;

[0124] X1 is the mass percentage of the hard carbon in the hard carbon layer, %;

[0125] X2 is the mass percentage of the sodium fast ion conductor in the sodium fast ion conductor layer, %;

[0126] X3 is the mass percentage of the soft carbon in the soft carbon layer, %.

[0127] Performance test

[0128] The secondary batteries in each example and comparative example were first subjected to a constant volume test at 0.33C and then subjected to a performance test. The test results are shown in Table 2. The specific test method is as follows:

[0129] 1) 0.33C Constant-Volume Test: Adjust the temperature to 25°C, let it stand for 30 min, charge at a constant rate of 0.33C to 4.0V, then hold at 4.0V at a constant voltage until 0.05C, let it stand for 30 min, discharge at a constant rate of 0.33C to 1.5V, and repeat the above steps 3 times to obtain the partial capacity C0 at 0.33C;

[0130] 2) 25°C - DCR Test: Adjust the temperature to 25°C, adjust 0.33C0 to 50% SOC, let it stand for 60 min to obtain the initial voltage U0, discharge at 3C0 (current is I) for 30 s to obtain the end voltage U1, and obtain DCR through (U0 - U1) / I.

[0131] 3) -20°C Discharge Retention Rate Test: Adjust the temperature to 25°C, let it stand for 30 min, charge at a constant rate of 0.33C0 to 4.0V, then hold at 4.0V at a constant voltage until 0.05C0, let it stand for 30 min, discharge at a constant rate of 1C0 to 1.5V to obtain the standard capacity C1; let it stand for 30 min, charge at a constant rate of 0.33C0 to 4.0V, then hold at 4.0V at a constant voltage until 0.05C0, adjust the temperature to -20°C, let it stand for 180 min, and discharge at 1C0 to 1.5V to obtain the capacity C2, and obtain the -20°C discharge retention rate through C2 / C1×100%.

[0132] 4) -20°C - 0.1C / 0.33C Cycling 20 Cycles Sodium Deposition Judgment and Capacity Retention Rate: Let it stand for 180 min, charge at a constant rate of 0.1C to 4.0V, then hold at 4.0V at a constant voltage until 0.05C, let it stand for 30 min, discharge at a constant rate of 0.33C to 1.5V, let it stand for 30 min, repeat the charge and discharge steps 20 times, and then charge at a constant rate of 0.1C to 4.0V and hold at 4.0V at a constant voltage until 0.05C. After the battery cell is taken out of the cabinet, disassemble the fully charged interface to judge the sodium deposition situation; Capacity retention rate = Discharge capacity of the 20th cycle / Discharge capacity of the 1st cycle × 100%.

[0133] Table 2 Performance of Secondary Batteries in Each Example and Comparative Example

[0134]

[0135] According to the data in Table 2, it can be seen that the DCR of the secondary batteries in Examples 1 - 12 at 25°C is less than or equal to 800 mΩ, and the discharge retention rate at -20°C is greater than or equal to 95%; at the same time, the capacity retention rate of cycling 20 times at -20°C is greater than or equal to 94% and no sodium deposition occurs, indicating that the secondary batteries of this application have good low-temperature fast charging performance and excellent power performance.

[0136] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application rather than to limit the protection scope of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present application can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A secondary battery, characterized in that: It comprises a negative electrode plate; the negative electrode plate comprises a current collector and a coating provided 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; the sodium fast ion conductor layer is arranged between the hard carbon layer and the soft carbon layer.

2. The secondary battery according to 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 according to claim 1 or 2, characterized in that: 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% is the mass percentage of the soft carbon in the soft carbon layer.

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

5. The secondary battery according to claim 1, wherein: The hard carbon layer is in contact with the current collector.

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

7. The secondary battery according to any one of claims 1, 2 and 5, characterized in that: The soft carbon in the soft carbon layer satisfies at least one of the following: a. The gram capacity of the soft carbon is 260 to 290 mAh / g; b. The particle size D of the soft carbon v50 5~10μm; c. The specific surface area of ​​the soft carbon is 2 to 7 m 2 / g.

8. The secondary battery according to any one of claims 1, 2 and 5, characterized in that: The compaction density of the negative electrode plate is 1.05-1.2 g / cm 3 .

9. The secondary battery according to any one of claims 1, 2 and 5, characterized in that: Satisfy at least one of the following: a. The average thickness of the hard carbon layer is 80 to 100 μm; b. The average thickness of the sodium fast ion conductor layer is 15 to 25 μm; c. The average thickness of the soft carbon layer is 20 to 40 μm.

10. The secondary battery according to claim 2, wherein: Satisfy at least one of the following: a. The mass proportion of the dispersant in the hard carbon layer is 1.3% to 1.5%, and the mass proportion of the binder is 1.9% to 2.3%; b. The mass proportion of the dispersant in the sodium fast ion conductor layer is 1.1% to 1.3%, and the mass proportion of the binder is 2.3% to 2.7%; c. The mass proportion of the dispersant in the soft carbon layer is 1.2% to 1.4%, and the mass proportion of the binder is 2.2% to 2.6%.

11. An electrical device, characterized in that: The invention comprises the secondary battery according to any one of claims 1 to 10, wherein the secondary battery is used as a power supply for the electrical device.

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