Negative electrode sheet, electrochemical device, and electronic apparatus

Abstract

The invention discloses a negative plate, an electrochemical device and electronic equipment. The negative plate comprises a negative current collector, and an upper negative material layer and a lower negative material layer are sequentially arranged on at least one surface of the negative current collector along the direction close to the negative current collector; the upper negative electrode material layer comprises an upper negative electrode active material, and the lower negative electrode material layer comprises a lower negative electrode active material; the negative plate satisfies the following conditions: 1.01 < = G < = 1.25; wherein G = d (002) upper / d (002) lower, d (002) upper is the d (002) interplanar spacing of the upper layer negative electrode active material, and d (002) lower is the d (002) interplanar spacing of the lower layer negative electrode active material; 1.2 < = D < = 6.0; wherein D = L < upper > / L < lower >, L < upper > is the length-diameter ratio of the upper-layer negative active material, and L < lower > is the length-diameter ratio of the lower-layer negative active material; 0.5 cc / g < = A < = 10 cc / g; wherein A = 5 (tau-epsilon) L < upper > / 19PL < lower >. According to the electrochemical device containing the negative plate, the rapid charging capacity is remarkably improved, and meanwhile, the electrochemical device also has relatively high energy density.

Description

Technical Field

[0001] The invention relates to a negative electrode sheet, an electrochemical device and an electronic device. Background Art

[0002] Sodium-ion batteries have become the focus of the industry due to their low cost and large sodium reserves. Amorphous carbon is the most suitable negative electrode material for sodium-ion batteries. Amorphous carbon usually includes hard carbon and / or soft carbon. However, the low energy density of sodium-ion batteries using amorphous carbon as the negative electrode material is a major shortcoming that limits their entry into the mainstream power market.

[0003] At present, the energy density of sodium ion batteries containing amorphous carbon negative electrode sheets is mainly improved by doping and increasing the number of pores on the market. For example, Chinese patent CN114695894A uses nitrogen doping, and Chinese patent CN116470044A increases the number of ultra-micropores by temperature regulation. The effect data recorded therein show that the gram capacity of the material is indeed improved, but the use of the material in practical applications is ignored, so that the energy density of the sodium ion battery obtained by the negative electrode sheet is not actually improved. In addition, the fast charging capability of current sodium ion batteries is also poor, which cannot meet people's demand for fast charging. Therefore, it is crucial to obtain a sodium ion battery that takes into account both high energy density and good fast charging capability. Summary of the invention

[0004] In order to solve the defect that the energy density and fast charging capability of sodium ion batteries in the prior art cannot be taken into account at the same time, the present invention provides a negative electrode sheet, an electrochemical device and an electronic device. When the negative electrode sheet provided by the present invention is applied to an electrochemical device, the energy density and fast charging capability can be improved simultaneously.

[0005] In a first aspect, the present invention provides a negative electrode sheet, comprising a negative electrode current collector, wherein an upper negative electrode material layer and a lower negative electrode material layer are sequentially arranged on at least one surface of the negative electrode current collector in a direction close to the negative electrode current collector; the upper negative electrode material layer comprises an upper negative electrode active material, and the lower negative electrode material layer comprises a lower negative electrode active material; the negative electrode sheet satisfies:

[0006] (1) 1.01≤G≤1.25; where G=d(002) 上 / d(002) 下 , d(002) 上 is the d(002) interplanar spacing of the upper negative electrode active material, d(002) 下 is the d(002) interplanar spacing of the lower negative electrode active material;

[0007] (2) 1.2 ≤ D ≤ 6.0; where D = L 上 / L 下 , L上 The aspect ratio of the upper negative electrode active material, L 下 is the aspect ratio of the lower negative electrode active material;

[0008] (3) 0.5 cc / g ≤ A ≤ 10 cc / g; where τ is the tortuosity of the negative electrode sheet, ε is the porosity of the negative electrode sheet, and P is the compaction of the negative electrode sheet.

[0009] In a second aspect, the present invention provides an electrochemical device comprising the negative electrode sheet as described above.

[0010] In a third aspect, the present invention provides an electronic device comprising the electrochemical device as described above.

[0011] On the basis of conforming to the common knowledge in the art, the above preferred conditions can be combined arbitrarily to obtain various preferred examples of the present invention.

[0012] The reagents and raw materials used in the present invention are all commercially available.

[0013] The positive progressive effect of the present invention lies in that:

[0014] The present invention provides a negative electrode sheet. By controlling and designing the interplanar spacing, aspect ratio, tortuosity, porosity, compaction, etc., the electrochemical device (especially a sodium ion battery) comprising the negative electrode sheet can significantly improve the fast charging ability while ensuring a relatively high energy density. Detailed Embodiments

[0015] The present invention will be further illustrated by the following examples, but the present invention is not limited to the scope of the examples described herein. The experimental methods without specific conditions noted in the following examples are carried out according to conventional methods and conditions, or selected according to the product specifications.

[0016] Negative electrode sheet

[0017] In the negative electrode sheet provided in the first aspect of the present invention, it comprises a negative electrode current collector, and an upper negative electrode material layer and a lower negative electrode material layer are sequentially arranged on at least one surface of the negative electrode current collector along the direction close to the negative electrode current collector; the upper negative electrode material layer comprises an upper negative electrode active material, and the lower negative electrode material layer comprises a lower negative electrode active material; the negative electrode sheet satisfies:

[0018] (1) 1.01 ≤ G ≤ 1.25; where G = d(002) 上 / d(002) 下 , d(002) 上 is the d(002) interplanar spacing of the upper negative electrode active material, and d(002) 下 is the d(002) interplanar spacing of the lower negative electrode active material;

[0019] (2) 1.2 ≤ D ≤ 6.0; where D = L 上 / L 下 , L 上 is the aspect ratio of the upper-layer negative active material, and L 下 is the aspect ratio of the lower-layer negative active material;

[0020] (3) 0.5 cc / g ≤ A ≤ 10 cc / g; where τ is the tortuosity of the negative electrode sheet, ε is the porosity of the negative electrode sheet, and P is the compaction of the negative electrode sheet.

[0021] In the present invention, G refers to the crystal plane spacing gradient, specifically the ratio of the crystal plane spacings of the upper-layer negative active material and the lower-layer negative active material. G can satisfy: 1.05 ≤ G ≤ 1.15, for example, 1.02, 1.11, 1.17 or 1.25.

[0022] In the present invention, the d(002) 上 can be optionally 0.37 - 0.43 nm, for example, 0.356 nm, 0.396 nm, 0.408 nm or 0.426 nm.

[0023] In the present invention, the d(002) 下 can be optionally 0.34 - 0.37 nm, for example, 0.341 nm, 0.35 nm, 0.351 nm or 0.357 nm.

[0024] In the present invention, the d(002) crystal plane spacing refers to the crystal plane spacing at the highest peak height of the 002 plane characteristic peak in the XRD pattern of the upper-layer negative active material or the lower-layer negative active material. In this specification, the XRD pattern is a fitted pattern of the XRD test results, and the "highest point" refers to the result shown by the fitted data in the XRD pattern. Individual noises will not affect the overall trend of the pattern. The fitting and interpretation of the XRD pattern are conventional methods in the art.

[0025] After research by the inventors, it is considered that by forming a double layer with the upper-layer negative active material and the lower-layer negative active material having different crystal plane spacings, a gradient structure in which the crystal plane spacing decreases sequentially from the upper negative electrode material layer to the lower negative electrode material layer can effectively increase the compaction of the negative electrode sheet and the reasonable distribution of pores. The upper negative electrode material layer ensures fast charging, and the lower negative electrode material layer takes into account the energy density; if G is too low, the overall pore abundance of the negative electrode sheet is low, which will deteriorate the kinetics of the negative electrode sheet; if G is too high, the overall pores of the negative electrode sheet are rich and the hardness is large, but the compaction will decrease.

[0026] In the present invention, the aspect ratio refers to the ratio of the longest diameter to the shortest diameter of the particles of the upper negative active material or the lower negative active material. The aspect ratio in the present invention is the average value obtained by statistical methods, and the test method can be: preparing a sample of the powder of the upper negative active material or the lower negative active material, taking pictures of 5 positions arbitrarily under SEM, each picture containing at least 30 particles, statistically calculating the aspect ratio of any 30 particles in each picture, and taking the average value. The D can satisfy: 1.5 ≤ D ≤ 4.0, for example, it is 1.3, 2.8 or 4.5.

[0027] Through research, the inventor believes that by combining the upper negative active material and the lower negative active material with different aspect ratios to form a gradient structure in which the aspect ratio decreases sequentially from the upper negative electrode material layer to the lower negative electrode material layer, the distribution of the compaction of the negative electrode sheet can be effectively increased. A larger aspect ratio is beneficial for fast charging, and a smaller aspect ratio has a higher energy compaction. The upper negative electrode material layer ensures fast charging, and the lower negative electrode material layer takes into account the energy density; if D is too low, the kinetics of the negative electrode sheet will deteriorate; if D is too high, the compaction will decrease.

[0028] In the present invention, the tortuosity τ refers to the ratio of the actual length of the pores in the upper negative electrode material layer or the lower negative electrode material layer to the shortest distance in the vertical direction. That is: when sodium ions are in the upper negative electrode material layer or the lower negative electrode material layer, the length of the path (average value of each path) traversing the upper negative electrode material layer or the lower negative electrode material layer is the ratio to the thickness of the corresponding upper negative electrode material layer or the lower negative electrode material layer. The τ can be selected from 2.14, 2.38, 2.52, 3.14, 3.21, 3.37, 3.8, 4.32, 5.71, 5.95 or 6.89.

[0029] In the present invention, the porosity ε of the electrode sheet refers to the ratio of the volume of the pores in the porous material to the apparent volume (or total volume) of the porous material, generally expressed as a percentage. The ε can be selected from 15.6%, 16.8%, 19.2%, 21.4%, 23.4%, 23.8%, 26.1%, 27.3%, 29.3% or 31.2%.

[0030] Through research, the inventor believes that by adjusting the compaction and the electrode sheet design to form an electrode sheet with low tortuosity, high orientation and rich pores, ensuring that the size of A is within a certain range can effectively improve the fast charging ability of the double-layer electrode sheet of the material. The A can satisfy: 2 cc / g ≤ A ≤ 5 cc / g, for example, it is 1.5 cc / g, 1.8 cc / g, 2 cc / g, 2.2 cc / g, 2.5 cc / g, 2.8 cc / g, 3.3 cc / g, 3.7 cc / g or 6.1 cc / g.

[0031] In the present invention, P can satisfy: 0.9 g / cc ≤ P ≤ 1.25 g / cc, for example, 0.92 g / cc, 0.95 g / cc, 1.05 g / cc or 1.15 g / cc.

[0032] In the present invention, T refers to the ratio of the thickness of the upper negative electrode material layer to the thickness of the lower negative electrode material layer. T can satisfy: 4:6 ≤ T ≤ 6:4, for example, 5:5.

[0033] In a specific embodiment, the negative electrode sheet satisfies: G is 1.11, D is 2.8, and A is 2.0 cc / g.

[0034] In a specific embodiment, the negative electrode sheet satisfies: G is 1.17, D is 2.8, and A is 2.2 cc / g.

[0035] In a specific embodiment, the negative electrode sheet satisfies: G is 1.02, D is 2.8, and A is 2.5 cc / g.

[0036] In a specific embodiment, the negative electrode sheet satisfies: G is 1.11, D is 4.5, and A is 3.3 cc / g.

[0037] In a specific embodiment, the negative electrode sheet satisfies: G is 1.11, D is 1.3, and A is 1.8 cc / g.

[0038] In a specific embodiment, the negative electrode sheet satisfies: G is 1.11, D is 2.8, and A is 3.7 cc / g.

[0039] In a specific embodiment, the negative electrode sheet satisfies: G is 1.11, D is 2.8, and A is 1.8 cc / g.

[0040] In a specific embodiment, the negative electrode sheet satisfies: G is 1.25, D is 2.8, and A is 1.5 cc / g.

[0041] In a specific embodiment, the negative electrode sheet satisfies: G is 1.11, D is 6.0, and A is 6.1 cc / g.

[0042] In a specific embodiment, the negative electrode sheet satisfies: G is 1.11, D is 1.2, and A is 0.5 cc / g.

[0043] In a specific embodiment, the negative electrode sheet satisfies: G is 1.11, D is 6.0, and A is 10.0 cc / g.

[0044] In the present invention, the upper negative electrode active material preferably includes one or more of amorphous carbon, graphite, and mesophase microspheres.

[0045] In the present invention, the lower negative electrode active material preferably includes one or more of amorphous carbon, graphite, and mesophase microspheres.

[0046] Those skilled in the art generally understand that amorphous carbon includes hard carbon and / or soft carbon. The amorphous carbon described in the present invention contains at least hard carbon. Optionally, the mass percentage of hard carbon in amorphous carbon is 80% or more, and this mass percentage refers to the mass percentage of hard carbon in the overall amorphous carbon.

[0047] In some alternative embodiments, the negative electrode sheet can be prepared by conventional methods in the art. For example, the following method can be used: After mixing the upper negative electrode active material, conductive agent, thickening agent, and binder in a certain mass ratio, add a first solvent and mix evenly to obtain the upper negative electrode slurry; After mixing the lower negative electrode active material, conductive agent, thickening agent, and binder in a certain mass ratio, add a second solvent and mix evenly to obtain the lower negative electrode slurry; Then first evenly coat the lower negative electrode slurry on the negative electrode current collector to obtain the lower negative electrode material layer; Then coat the upper negative electrode slurry on the lower negative electrode material layer to obtain the upper negative electrode material layer; Then through processes such as drying, rolling, and cutting, the negative electrode sheet is prepared.

[0048] Among them, in the upper negative electrode material layer, the content of the amorphous carbon can be 80.0% - 99.9%, and the percentage is the mass percentage of the amorphous carbon in the upper negative electrode material layer.

[0049] Among them, in the lower negative electrode material layer, the content of the amorphous carbon can be 80.0% - 99.9%, and the percentage is the mass percentage of the amorphous carbon in the lower negative electrode material layer.

[0050] Among them, the first solvent can be conventional in the art, for example, deionized water.

[0051] Among them, the second solvent can be conventional in the art, for example, deionized water.

[0052] For the binder, it can be a component that helps the combination between the upper negative electrode active material or the lower negative electrode active material and the conductive agent and helps the combination between the lower negative electrode active material and the negative electrode current collector. It can generally be selected from polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene terpolymer (EPDM), sulfonated EPDM, styrene - butadiene rubber, fluororubber, and various copolymers.

[0053] For the conductive agent, it is a reagent used to ensure that the electrode has good charge and discharge performance. It can be arbitrarily selected from graphite materials such as natural graphite and artificial graphite, carbon black materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, etc., conductive fibers such as carbon fibers and metal fibers, metal powders such as carbon fluoride powder, aluminum powder, nickel powder, etc., conductive whiskers such as zinc oxide and potassium titanate, and conductive metal oxides such as titanium dioxide or polyphenylene derivatives.

[0054] In a specific embodiment, the mass ratio of the upper layer negative active material amorphous carbon, conductive agent carbon black (Super P), thickening agent sodium carboxymethyl cellulose, and binder styrene-butadiene rubber is 95.5:2.0:1.2:1.3.

[0055] In a specific embodiment, the mass ratio of the lower layer negative active material amorphous carbon, conductive agent carbon black (Super P), thickening agent sodium carboxymethyl cellulose, and binder styrene-butadiene rubber is 95.0:2.0:1.2:1.8.

[0056] In the present invention, the negative electrode current collector can be a current collector conventionally used for the negative electrode in the art, and can be a common current collector or a composite current collector. The negative electrode current collector can be used without limitation with materials that do not cause chemical changes and have conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum cadmium alloy can be used, or copper, stainless steel materials, or aluminum cadmium alloy surface-treated with carbon, nickel, titanium, or silver. In addition, in order to enhance the adhesion of the negative active material, micro-embossing can be formed on the surface of the negative electrode current collector. The negative electrode current collector can be used in various forms, such as a film, sheet, foil, net, or porous body, etc.

[0057] In some alternative embodiments, the thickness of the negative electrode current collector can be 5-10 μm.

[0058] In some specific embodiments, the negative electrode current collector is a copper foil with a thickness of 8 μm.

[0059] Electrochemical device

[0060] In the electrochemical device described in the second aspect of the present invention, it includes the negative electrode sheet as described above.

[0061] In the present invention, the electrochemical device can be a sodium ion battery. The sodium ion battery includes a positive electrode sheet, a separator, an electrolyte, and the negative electrode sheet as described above.

[0062] Among them, the negative electrode sheet is the same as that described above, so its detailed description is omitted, and the remaining components are described in detail.

[0063] Positive electrode sheet

[0064] In some embodiments, the positive electrode sheet is prepared by coating a positive electrode active material on a positive electrode current collector, and if necessary, a binder and a conductive agent may be further added.

[0065] For the positive electrode active material, it may be a positive electrode active material conventionally used for the positive electrode of a sodium ion battery in the art, and the positive electrode active material may be a sodium ion material such as a layered oxide, a polyanionic compound, a Prussian blue / white compound, etc.

[0066] Optionally, the general formula of the layered oxide is Na x [MFeMn]O2, where M is selected from at least one of Cu, Ni, Li, Mg, Al, Zn, Ti, Zr, and Sn, and optionally x ≤ 1.

[0067] Optionally, the general formula of the polyanionic compound is NaxMy(XaOb)zZw, where M is selected from one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Ca, Mg, Al, and Nb; X is selected from one or more of Si, S, P, As, B, Mo, W, and Ge; and Z is selected from F and / or OH.

[0068] Optionally, the general formula of the Prussian blue / white compound is Na x M1[M2(CN)6], where M is selected from one or more of Mn, Ni, Co, Zn, Cu, and Fe; optionally, 0 < x ≤ 2.

[0069] In a specific embodiment, the molecular formula of the positive electrode active material is NaNi 0.34 Fe 0.33 Mn 0.33 O2.

[0070] For the binder, it may be a component that helps the combination between the positive electrode active material and the conductive agent and helps the combination between the positive electrode active material and the positive electrode current collector. It can generally be selected from polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0071] For the conductive agent, it is a reagent used to ensure good charge and discharge performance of the electrode. It can be arbitrarily selected from graphite materials such as natural graphite and artificial graphite, carbon black materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black, conductive fibers such as carbon fibers and metal fibers, metal powders such as carbon fluoride powder, aluminum powder, and nickel powder, conductive whiskers such as zinc oxide and potassium titanate, and conductive metal oxides or polyphenylene derivatives such as titanium dioxide.

[0072] For the positive electrode current collector, materials that do not cause chemical changes and have high electrical conductivity can be used without limitation. For example, stainless steel, aluminum, nickel, titanium, or calcined carbon can usually be used, or aluminum or stainless steel materials surface-treated with carbon, nickel, titanium, silver, etc. In order to enhance the adhesion of the positive electrode active material, micro embossing can be formed on the surface of the positive electrode current collector. The positive electrode current collector can be used in various forms, such as films, sheets, foils, meshes, or porous bodies, etc.

[0073] In some alternative embodiments, the thickness of the positive electrode current collector can be 8 - 16 μm.

[0074] In some specific embodiments, the positive electrode current collector is an aluminum foil with a thickness of 16 μm.

[0075] Separator

[0076] In some embodiments, the separator can be a polypropylene film or a polyethylene film.

[0077] Among them, the thickness of the separator can be 12 μm.

[0078] Among them, the air permeability of the separator can be 180 - 380 s / 100 mL.

[0079] Among them, the porosity of the separator can be 30% - 50%.

[0080] In a specific embodiment, the separator is a polypropylene film; the thickness of the separator is 12 μm; the air permeability of the separator is 230 s / 100 mL; the porosity of the separator is 40%.

[0081] Electrolyte

[0082] In some embodiments, the electrolyte can be an electrolyte conventionally used in batteries in the art, generally including a non-aqueous solvent, a sodium salt, and an additive.

[0083] Among them, the non-aqueous solvent can be a conventional non-aqueous solvent in the art, preferably an ester solvent, more preferably a carbonate solvent. The carbonate solvent is preferably one or more of ethylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene sulfite (EC), propylene sulfite (PC), and butylene sulfite (BC).

[0084] In some preferred embodiments, the non-aqueous solvent is obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and propylene carbonate (PC) in a mass ratio of (2-4):(3-5):(2-4):(0-1).

[0085] Among them, the additive is preferably one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene vinylene carbonate (VEC), divinyl sulfate (DTD), vinylene sulfite, 1,3-propane sultone (PS), allyl sulfonic acid lactone, and 1,4-butane sultone.

[0086] Among them, the sodium salt can be a conventional sodium salt in the art, for example, NaPF6.

[0087] Among them, the content of the sodium salt can be 4%-24%, and the percentage is the mass percentage of the sodium salt in the total mass of the electrolyte.

[0088] In a specific embodiment, the electrolyte includes NaPF6, ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and propylene carbonate.

[0089] Among them, the mass ratio of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and propylene carbonate is preferably (2-4):(3-5):(2-4):(0-1). When the mass ratio is (2-4):(3-5):(2-4):0, it means that the non-aqueous solvent does not contain propylene carbonate.

[0090] In some embodiments, the electrolyte can be prepared by a conventional method in the art. Optionally, it can be prepared by the following method: In an argon atmosphere glove box with a water content <10 ppm, battery-grade ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and propylene carbonate are mixed according to the ratio to form an organic solvent, and then a sodium salt is added and mixed evenly to obtain the electrolyte.

[0091] In the present invention, the preparation method of the sodium ion battery can be a conventional preparation method in the art. It can be that the positive electrode sheet, the negative electrode sheet, and the separator are wound to obtain an electric core, and then packaged in a packaging shell and injected with the electrolyte; it can also be that the negative electrode sheet, the separator, the positive electrode sheet, and the separator are stacked in sequence to obtain an electric core, and then packaged in a packaging shell and injected with the electrolyte.

[0092] In some embodiments, the method for preparing the sodium-ion battery comprises the following steps: laminating the positive electrode sheet, the separator, and the negative electrode sheet in sequence, with the separator positioned between the positive electrode sheet and the negative electrode sheet to play an isolation role, then wrapping with an aluminum-plastic film, transferring to a vacuum oven for drying at 120 °C, injecting 3.0 g / Ah of electrolyte and then sealing, performing electrolytic liquefaction, and finally preparing a ternary system soft-pack battery with a capacity of 1 Ah. The electrolyte injection coefficient in the sodium-ion battery is 3 g / Ah, the cell capacity is 1 Ah, and the mass of the electrolyte is 3 g.

[0093] Among them, the electrolytic liquefaction comprises the following steps: under a hot-pressing environment of 0.1 MPa, the sodium-ion battery after injecting the electrolyte is charged at 0.02 C for 17 min at 45 °C in a static state, after standing for 5 min, it is charged to 0.3 Ah at 0.02 C, then the air bag is cut off and vacuum packaged, and left standing at room temperature for 48 h, so that the electrolyte is completed for forming.

[0094] Electronic device

[0095] In the electronic device provided in the third aspect of the present invention, it includes the electrochemical device as described above.

[0096] Exemplarily, the electronic device of the present invention can be, but is not limited to, a mobile device (such as a mobile phone, a tablet computer, a laptop computer, a video recorder, a portable printer / copier, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, a backup power supply, etc.

[0097] On the basis of conforming to common general knowledge in the art, the above preferred conditions can be combined arbitrarily to obtain various preferred examples of the present invention.

[0098] The reagents and raw materials used in the present invention are all commercially available.

[0099] Examples 1-11 and Comparative Examples 1-8

[0100] Preparation of the negative electrode sheet

[0101] For the preparation methods of the negative electrode sheets in Examples 1-11 and Comparative Examples 1-8 (the relevant parameters of the upper negative electrode active material, the lower negative electrode active material, and the negative electrode sheets in Examples 1-11 and Comparative Examples 1-8 are listed in Table 1), the following steps are adopted:

[0102] The upper-layer negative active material amorphous carbon, conductive agent carbon black (Super P), thickening agent sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) were mixed at a mass ratio of 95.5:2.0:1.2:1.3 (a total of 100 mass parts), and then 82 mass parts of deionized water was added and mixed evenly to obtain the upper-layer negative electrode slurry (mass concentration of 52%); the lower-layer negative active material amorphous carbon, conductive agent carbon black (Super P), thickening agent sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) were mixed at a mass ratio of 95.0:2.0:1.2:1.8 (a total of 100 mass parts), and then 82 mass parts of deionized water was added and mixed evenly to obtain the lower-layer negative electrode slurry (mass concentration of 52%); then the lower-layer negative electrode slurry was first evenly coated on an 8-μm copper foil to obtain the lower negative electrode material layer; then the upper-layer negative electrode slurry was coated on the lower negative electrode material layer to obtain the upper negative electrode material layer; and then through drying, rolling, die-cutting, and punching, the negative electrode sheets of Examples 1-11 and Comparative Examples 1-8 were prepared, where the rolling pressure was 5T.

[0103] Among them, the crystal plane spacing, aspect ratio, tortuosity of the electrode sheet, and porosity of the electrode sheet of the upper-layer negative active material and the lower-layer negative active material were tested by the following methods:

[0104] 1. Crystal plane spacing test

[0105] Using the upper-layer negative active material or the lower-layer negative active material as the sample to be tested.

[0106] The test method of the d(002) crystal plane spacing refers to the records in GB / T 24533-2019. Specifically: Weigh 0.15 g of silicon powder and 0.35 g of the sample to be tested into an agate mortar and grind them thoroughly for 10 min. Take 100 mg of the sample and put it into the sample holder and flatten it. Use an X-ray diffractometer to scan the sample, with the conditions of 25°-30° and a step size of 0.02°. Use the silicon 111 peak to correct the 002 peak of the sample to be tested to obtain the 2θ value of the 002 peak, and calculate the crystal plane spacing of the d(002) crystal plane spacing through the Bragg equation.

[0107] 2. Aspect ratio test

[0108] Using the upper-layer negative active material or the lower-layer negative active material as the sample to be tested.

[0109] The sample to be tested was ground and prepared into a sample. Take 1 g of the sample to be tested and randomly take pictures of 5 positions under SEM. Each picture contains at least 30 particles. The aspect ratios of 30 randomly selected particles within the shooting range of each picture were statistically analyzed, and the average value was calculated, which is the required aspect ratio. The aspect ratio of the upper-layer negative active material is denoted as L 上 , and the aspect ratio of the lower-layer negative active material is denoted as L 下 .

[0110] 3. Tortuosity Test of the Negative Electrode Sheet

[0111] Refer to the literature of Ebner M, Wood V. Tool for Tortuosity Estimation in Lithium Ion Battery Porous Electrodes[J]. Journal of the Electrochemical Society, 2014, 162(2): A3064 - A3070 and conduct the test by the image method.

[0112] 4. Porosity Test of the Negative Electrode Sheet

[0113] Conduct the test with reference to the national standard GB21650.1 - 2008.

[0114] 5. Compaction Test of the Negative Electrode Sheet

[0115] Use a micrometer to measure the thickness of the punched negative electrode sheet, record the thickness d, then cut a unit - area negative electrode sheet, subtract the mass of the copper foil it contains, and record the mass as w. The compaction of the negative electrode sheet is P = w / d. The compaction of the negative electrode sheet is an index to measure the energy density of the negative electrode sheet.

[0116] The results of the above tests are shown in Table 1.

[0117] Table 1

[0118]

[0119]

[0120] Effect Example 1

[0121] Preparation of Sodium - Ion Battery:

[0122] (1) Preparation of the Positive Electrode Sheet

[0123] Mix the positive active material NaNi 0.34 Fe 0.33 Mn 0.33 O2, conductive agent carbon black (Super P), carbon nanotubes (CNT), and binder polyvinylidene fluoride (PVDF) in a mass ratio of 94:2:1:3, stir in a vacuum mixer until the system becomes homogeneous to obtain the positive electrode slurry; uniformly coat the positive electrode slurry on an aluminum foil with a thickness of 16μm, dry the aluminum foil at room temperature and then transfer it to an oven for drying, and then obtain the positive electrode sheet through cold pressing and slitting.

[0124] (2) Preparation of the Electrolyte

[0125] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and propylene carbonate (PC) are uniformly mixed in a mass ratio of (2 - 4):(3 - 5):(2 - 4):(0 - 1). Then, an appropriate amount of NaPF6 is dissolved in the above mixed solution to prepare an electrolyte with a concentration of 4% - 24%.

[0126] (3) Preparation of separator

[0127] The separator is a polypropylene film; the thickness of the separator is 12 μm; the gas permeability of the separator is 230 s / 100 mL; the porosity of the separator is 40%.

[0128] (4) Preparation of sodium-ion battery

[0129] The positive electrode sheet, separator, electrolyte prepared in the above steps, and the negative electrode sheets prepared in Examples 1 - 11 and Comparative Examples 1 - 8 are assembled into a sodium-ion battery, so that the positive electrode sheet, separator, negative electrode sheet, and separator are alternately combined, and a soft-pack battery with a capacity of 1 Ah is obtained through formation, and its performance is tested.

[0130] 1. Fast charging time test method

[0131] The battery cell is directly charged at a current of 0.33C until it reaches 8% SOC state. Then, according to the actual measurement of the three-electrode window of the battery cell, the charging windows at 10%, 20%, 30%, 40%, 50%, 60%, 70%, and 80% are C1, C2, C3, C4, C5, C6, C7, and C8 respectively. It is charged step by step to 80%, that is, from 8% - 10% using C1, from 10% to 20% using C2, and so on. The charging time from 8% to 80% SOC state is recorded as the measurement standard for fast charging ability. The calculation formula is T = (0.02 / C1 + 0.1 / C2 + 0.1 / C3 + 0.1 / C4 + 0.1 / C5 + 0.1 / C6 + 0.1 / C7 + 0.1 / C8) × 60.

[0132] 2. Cycle number (cycle retention rate of 80%) test method

[0133] Under the condition of 25°C, in the voltage range of 2.0 - 4.1V, it is cycled with a 1C / 1C charge-discharge regime, and the number of cycles until the measured capacity reaches 80% of the initial capacity is recorded, which is the cycle number.

[0134] 3. Storage time test method

[0135] At 25°C, it is constant-volume charged with a current of 0.33C and recorded as C0. Then, the battery cell is stored at a high temperature of 60°C. After that, the battery cell is taken out every 7 days to measure the capacity at room temperature and recorded as C1, C2…Cn. The number of days when Cn reaches 80% of C0 is the storage time.

[0136] 4. Test Method for Specific Capacity (mAh / g)

[0137] Under a constant temperature environment of 25 °C, charge at a constant current and constant voltage at a rate of 0.33C until 4.0V (the constant voltage cut-off current is 0.05C), and then discharge at a constant current at a rate of 0.33C until 1.5V to obtain the 0.33C energy. Weigh the mass of the battery cell with a balance, and the ratio of the 0.33C energy to the mass of the battery cell is the specific capacity.

[0138] The above effect data are shown in Table 2 specifically.

[0139] Table 2

[0140]

[0141]

[0142] According to Table 1 and Table 2, it can be seen that for the negative electrode sheets prepared in Examples 1-11, the crystal plane spacing gradient G is between 1.02 and 1.25, D is between 1.2 and 6.0, and A is between 0.5 and 10.0 cc / g. For the sodium-ion battery containing this negative electrode sheet, its specific capacity can reach more than 297 mAh / g, and the fast charging time can be as low as 29.3 min or less, indicating excellent fast charge and discharge capabilities of the battery. Further, the electrode sheet compaction of the negative electrode sheet is between 0.92 and 1.25 g / cc. Applying the negative electrode sheets of Examples 1-11 to sodium-ion batteries can achieve both high energy density and good fast charging performance.

[0143] The differences between Comparative Examples 1, 5, and 6 and Examples 1-11 lie in the crystal plane spacing gradient G, that is, d(002) 上 / d(002) 下 The value is not within the scope of the present invention, being too large or too small. From the data in Table 2, it can be seen that its energy density or fast charging ability is poor, and it is impossible to balance the fast charging ability and the energy density. This may be because when the crystal plane spacing gradient is 1, a gradient structure in which the crystal plane spacing of the upper negative electrode material layer decreases sequentially to the crystal plane spacing of the lower negative electrode material layer cannot be formed, which is not conducive to increasing the electrode sheet compaction and reasonable pore distribution of the negative electrode sheet. The overall pore abundance of the negative electrode sheet is low, which will deteriorate the kinetics of the negative electrode sheet; if the crystal plane spacing gradient is too large, the overall pores of the negative electrode sheet are rich and the hardness is large, but the compaction will decrease.

[0144] The difference between Comparative Example 2 and Examples 1-11 lies in D, that is, L 上 / L 下 The value is not within the scope of the present invention, being too small, and its fast charging ability is poor. This may be because when D is 1, a gradient structure in which the aspect ratio of the upper negative electrode material layer decreases sequentially to the aspect ratio of the lower negative electrode material layer cannot be formed, thus deteriorating the kinetics of the negative electrode sheet and unable to ensure good fast charging ability.

[0145] The difference between Comparative Example 3 and Examples 1-11 lies in D and A, that is, A = (5(τ - ε)L 上 ) / (19PL 下 ) is not within the scope of the present invention and is too large, resulting in poor fast charging ability and being unable to balance fast charging ability and energy density. This may be because if D is too high, the tap density will decrease, which is not conducive to forming a cathode with low tortuosity, high orientation, and rich pores. This further illustrates that ensuring the magnitudes of D and A within a certain range can effectively improve the fast charging ability of the bilayer cathode of the material.

[0146] The difference between Comparative Example 4 and Examples 1-11 is that the negative electrode sheet in Comparative Example 4 is provided with a single-layer negative electrode material layer prepared by blending the upper negative electrode active material and the lower negative electrode active material, rather than the bilayer negative electrode material layer formed by the combination of the upper negative electrode material layer and the lower negative electrode material layer in the present invention. Its fast charging ability is poor and it cannot balance fast charging ability and energy density, indicating that in the present invention, by combining the upper negative electrode active material and the lower negative electrode active material with different aspect ratios and crystal plane spacings in a bilayer structure, a gradient structure in which the aspect ratio and crystal plane spacing decrease sequentially from the upper negative electrode material layer to the lower negative electrode material layer can effectively increase the tap density of the negative electrode sheet and the reasonable distribution of pores, thereby ensuring fast charging ability and balancing energy density. If the blending method in Comparative Example 4 is adopted, it cannot be balanced.

[0147] The difference between Comparative Examples 7 and 8 and Examples 1-11 lies in that the value of A is not within the scope of the present invention, being too small or too large, resulting in poor fast charging ability and being unable to balance fast charging ability and energy density. This may be because when the value of A is too large or too small, it is impossible to form a cathode with low tortuosity, high orientation, and rich pores, thus unable to effectively improve the fast charging ability of the bilayer cathode of the material and further unable to balance fast charging ability and energy density.

[0148] Although the specific embodiments of the present invention have been described above, those skilled in the art should understand that this is only for illustration purposes. The protection scope of the present invention is defined by the appended claims. Without departing from the principles and essence of the present invention, those skilled in the art can make various changes or modifications to these embodiments, but these changes and modifications all fall within the protection scope of the present invention.

Claims

1. A negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector, and an upper negative electrode material layer and a lower negative electrode material layer are sequentially arranged on at least one surface of the negative electrode current collector along a direction close to the negative electrode current collector; the upper negative electrode material layer comprises an upper negative electrode active material, and the lower negative electrode material layer comprises a lower negative electrode active material, wherein, The negative electrode sheet satisfies the following: (1) 1.01 ≤ G ≤ 1.25; where G = d(002) 上 / d(002) 下 , d(002) 上 is the d(002) interplanar spacing of the upper-layer negative active material, and d(002) 下 is the d(002) interplanar spacing of the lower-layer negative active material; (2) 1.2 ≤ D ≤ 6.0; where D = L 上 / L 下 , L 上 is the aspect ratio of the upper-layer negative active material, and L 下 is the aspect ratio of the lower-layer negative active material; (3) 0.5 cc / g ≤ A ≤ 10 cc / g; where, τ is the tortuosity of the negative electrode sheet, ε is the porosity of the negative electrode sheet, and P is the compaction of the negative electrode sheet.

2. The negative electrode sheet according to claim 1, characterized in that, 1.05≤G≤1.15。 3. The negative electrode sheet according to claim 1, wherein, 1.5≤D≤4.0。 4. The negative electrode sheet according to claim 1, characterized in that, 2 cc / g ≤ A ≤ 5 cc / g.

5. The negative electrode sheet according to claim 1, wherein 0.9 g / cc ≤ P ≤ 1.25 g / cc.

6. The negative electrode sheet according to claim 1, wherein, The d(002) 上 is 0.37 - 0.43 nm.

7. The negative electrode sheet according to claim 1, wherein, The d(002) 下 is 0.34 - 0.37 nm.

8. The negative electrode sheet according to claim 1, wherein, It satisfies one or more of the following conditions (a)-(c): (a) 4:6 ≤ T ≤ 6:4; where T = t 上 / t 下 , t 上 is the thickness of the upper negative electrode material layer, and t 下 is the thickness of the lower negative electrode material layer; (b) The upper-layer negative electrode active material includes one or more of amorphous carbon, graphite, and mesophase microspheres; (c) The lower-layer negative electrode active material includes one or more of amorphous carbon, graphite, and mesophase microspheres.

9. An electrochemical device, characterized in that, It includes the negative electrode sheet according to any one of claims 1-8.

10. An electronic device, characterized in that, It includes the electrochemical device according to claim 9.

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