Negative electrode sheet and preparation method therefor, secondary battery, and electric device

Abstract

The present application relates to a negative electrode sheet and a preparation method therefor, a secondary battery, and an electric device. The negative electrode sheet comprises a negative current collector, an active material layer, and a passivation layer; the active material layer is arranged between the negative current collector and the passivation layer; the active material layer comprises a negative electrode active material; the passivation layer comprises an ion resin; and the ion resin comprises one or more of carboxymethyl cellulose and a carboxymethyl cellulose metal salt.

Description

Negative electrode sheet and preparation method thereof, secondary battery and electric device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on June 12, 2023, with application number 2023106915304 and invention name “Negative electrode sheet and preparation method thereof, secondary battery and electrical device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of secondary batteries, and in particular to a negative electrode plate and a preparation method thereof, a secondary battery and an electrical device. Background Art

[0003] Lithium-ion batteries are widely used in various large-scale power devices, energy storage systems and various consumer products due to their advantages such as high operating voltage, high energy density, no memory effect and environmental friendliness. In particular, they are widely used in new energy vehicles such as pure electric vehicles and hybrid electric vehicles.

[0004] With the widespread application and continuous development of lithium-ion batteries, the safety of lithium-ion batteries has also received increasing attention. After multiple cycles of lithium-ion batteries, metallic lithium will be deposited on the surface of the negative electrode, and metallic lithium dendrites will pose a considerable safety risk.

[0005] Therefore, seeking lithium-ion batteries with excellent cycle performance is one of the key areas of focus for those skilled in the art.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose includes providing a negative electrode plate and a preparation method thereof, a secondary battery and an electrical device.

[0008] In order to achieve the above-mentioned object, the first aspect of the present application provides a negative electrode plate, comprising a negative electrode current collector, an active material layer and a passivation layer, wherein the active material layer is provided between the negative electrode current collector and the passivation layer;

[0009] The active material layer includes a negative electrode active material, the passivation layer includes an ion resin, and the ion resin includes one or more of carboxymethyl cellulose and carboxymethyl cellulose metal salt.

[0010] The negative electrode provided in the present application has an active material layer and a passivation layer disposed on a current collector. The passivation layer includes an ionic resin containing cations that can be exchanged with lithium ions, thereby effectively driving the rapid migration and uniform distribution of lithium ions. The ionic resin also contains a large number of hydrogen bonds and can be cross-linked into a uniform and dense three-dimensional network structure. It has high deformation resistance, forms protection for the active material layer, effectively reduces the interfacial reaction between the negative electrode and the electrolyte during the cycle, and can also alleviate the impact of the volume change of the negative electrode active material during the battery cycle. The passivation layer in the negative electrode provided in the present application can serve as an artificial SEI, playing a role in inhibiting the generation and growth of lithium dendrites, thereby effectively improving the cycle performance of the battery.

[0011] In some embodiments, the molecular formula of the carboxymethyl cellulose is [C6H7O2(OH) 3- x1 (OCH2COOH) x1 ] n1 , n1 is the degree of polymerization, x1 is 0.4 to 0.7, and can be optionally 0.5 to 0.7; and / or,

[0012] The molecular formula of the carboxymethyl cellulose metal salt is [C6H7O2(OH) 3-x2 (OCH2COOM) x2 ] n2 , M includes one or more of lithium, sodium, potassium, rubidium and cesium, optionally, M includes lithium; n2 is the degree of polymerization, x2 is 0.4 to 0.7, and can be optionally 0.5 to 0.7.

[0013] In some embodiments, the Young's modulus of the ion resin is greater than or equal to 2.5 GPa, and can be optionally 18 GPa-27 GPa.

[0014] In some embodiments, the weight average molecular weight of the ionic resin is 50,000 Da-300,000 Da, and optionally 70,000-250,000 Da.

[0015] In some embodiments, the thickness of the passivation layer is 1 to 8 μm, and optionally 2 to 4 μm.

[0016] In some embodiments, the passivation layer is disposed around the active material layer to form a hollow closed structure.

[0017] Optionally, the area of ​​the hollow region accounts for 10% to 30% of the area of ​​the active material layer.

[0018] In some embodiments, the negative electrode active material includes at least one of graphite, soft carbon, hard carbon, silicon-based material, tin-based material and lithium titanate, and graphite can be selected.

[0019] A second aspect of the present application further provides a method for preparing a negative electrode sheet, comprising the following steps:

[0020] forming an active material layer on the negative electrode current collector, wherein the active material layer includes a negative electrode active material;

[0021] A passivation layer is formed on the active material layer. The passivation layer includes an ionic resin. The ionic resin includes one or more of carboxymethyl cellulose and a metal salt of carboxymethyl cellulose.

[0022] In some embodiments, the passivation layer is formed by physical vapor deposition, which may be evaporation or sputtering, or further may be vacuum evaporation.

[0023] In some embodiments, before forming the passivation layer on the active material layer, the method further includes:

[0024] A mask plate is provided so as to shield the middle portion of the active material layer to be processed, and an area of ​​the mask plate is smaller than an area of ​​the active material layer.

[0025] Optionally, the area of ​​the mask plate is 10% to 30% of the area of ​​the active material layer.

[0026] The third aspect of the present application provides a secondary battery, comprising the negative electrode sheet described in the first aspect of the present application.

[0027] A fourth aspect of the present application provides an electrical device comprising one or more secondary batteries selected from the third aspect of the present application.

[0028] The details of one or more embodiments of the present application are set forth in the following drawings and description. Other features, objects, and advantages of the present application will become apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to better describe and illustrate the embodiments or examples provided in this application, reference may be made to one or more of the accompanying drawings. The additional details or examples used to describe the accompanying drawings should not be considered as limiting the scope of the disclosed application, the embodiments or examples currently described, and any of the best modes currently understood for these applications. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the accompanying drawings:

[0030] FIG1 is a schematic structural diagram of a negative electrode sheet according to an embodiment of the present application.

[0031] FIG. 2 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0032] FIG. 3 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 2 .

[0033] FIG. 4 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0034] Explanation of the accompanying symbols: 1. Secondary battery; 11. Housing; 12. Electrode assembly; 13. Cover plate; 2. Power-consuming device; 10. Negative electrode current collector; 20. Active material layer; 30. Passivation layer. DETAILED DESCRIPTION

[0035] Below, some embodiments of the negative electrode sheet and its preparation method, secondary battery and electrical device of the present application are described in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0036] " scope " disclosed in the present application can be limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and any end value can be included or not included independently, and can be arbitrarily combined, that is, any lower limit can form a scope with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 listed, and if the maximum range value 3,4 and 5 are also listed, then the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is merely an abbreviation for a combination of these values. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For example, when a parameter is expressed as an integer selected from "2-10", this is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0037] In this application, "a plurality of" or "a plurality of" refers to a number greater than or equal to 2 unless otherwise specified. For example, "one or more" means one or more than or equal to two.

[0038] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0039] References to "embodiments" herein mean that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment or implementation of the present application. The appearance of such phrases in various locations in the specification does not necessarily refer to the same embodiment, nor does it necessarily refer to independent or alternative embodiments that are mutually exclusive with other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments. References to "implementations" herein have a similar understanding.

[0040] Those skilled in the art will appreciate that, in the methods of each embodiment or example, the order in which the steps are written does not imply a strict order of execution and does not constitute any limitation on the implementation process, and the detailed order of execution of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present application may be performed sequentially or randomly. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, it is mentioned that the method may also include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0041] In this application, open technical features or technical solutions described with words such as "contain," "include," and "includes" do not exclude additional members beyond the listed members unless otherwise specified, and can be regarded as providing both closed features or solutions consisting of the listed members and open features or solutions that include additional members beyond the listed members. For example, if A includes a1, a2, and a3, it may or may not include other members unless otherwise specified. This can be regarded as providing both the feature or solution of "A consists of a1, a2, and a3" and the feature or solution of "A includes not only a1, a2, and a3, but also other members."

[0042] In this application, unless otherwise specified, A (such as B) means that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0043] In this application, the terms "optionally," "optional," and "optional" are optional and refer to either option being present or absent. If a technical solution contains multiple "options," each option is considered independent unless otherwise specified and there are no conflicts or constraints.

[0044] The term "ionic resin" refers to any resin that carries an electrical charge, including resins that carry negatively charged ions and resins that carry positively charged ions.

[0045] At present, an important problem of lithium-ion batteries under long-term cycling and storage conditions is lithium dendrites. The lithium dendrites caused by the uneven deposition of lithium ions on the surface of the negative electrode will affect the interfacial stability of the graphite negative electrode and the electrolyte. As the number of cycles increases, the growth of lithium dendrites will continue to consume the electrolyte and cause irreversible deposition of active lithium, which will in turn produce more dead lithium and cause the battery's coulombic efficiency to decrease. At the same time, the continuous growth of lithium dendrites will bring considerable safety risks to the battery, which is mainly caused by lithium dendrites piercing the diaphragm. In the related art, inorganic salts are often used as negative electrode additives to construct artificial SEI films to improve the cycle performance of the battery. However, when inorganic salts are used as additives, the effect of improving the growth of lithium dendrites is not ideal.

[0046] Negative electrode

[0047] In order to solve the above technical problems, please refer to Figure 1. In the first aspect of the present application, a negative electrode plate is provided, including a negative electrode current collector 10 and a negative electrode film layer provided on at least one surface of the negative electrode current collector 10. The negative electrode film layer includes an active material layer 20 and a passivation layer 30, and the active material layer 20 is provided between the negative electrode current collector 10 and the passivation layer 30. The active material layer 20 includes a negative electrode active material, and the passivation layer 30 includes an ionic resin, and the ionic resin includes one or more of carboxymethyl cellulose and a metal salt of carboxymethyl cellulose. It should be noted that the negative electrode current collector 10 has two surfaces opposite to each other in its own thickness direction, and the negative electrode film layer is provided on any one or both of the two opposite surfaces of the negative electrode current collector 10.

[0048] The negative electrode provided in the embodiment of the present application includes an active material layer 20 and a passivation layer 30. The passivation layer 30 includes an ionic resin. The ionic resin contains cations that can be exchanged with lithium ions, thereby effectively driving the rapid migration and uniform distribution of lithium ions. The ionic resin also contains a large number of hydrogen bonds, which can be cross-linked into a uniform and dense three-dimensional network structure. It has high deformation resistance, forming protection for the active material layer 20, effectively reducing the interfacial reaction between the negative electrode and the electrolyte during the cycle, and can also alleviate the impact of the volume change of the negative electrode active material during the battery cycle. The passivation layer 30 in the negative electrode provided in the embodiment of the present application can act as an artificial SEI, playing a role in suppressing the generation and growth of lithium dendrites, thereby effectively improving the cycle performance of the battery.

[0049] Carboxymethyl cellulose and its salts are cellulose ethers in which some of the hydroxyl groups on the anhydroglucose ring in cellulose are replaced by carboxymethyl groups.

[0050] In some embodiments, the molecular formula of the carboxymethyl cellulose is [C6H7O2(OH) 3- x1 (OCH2COOH) x1 ] n1 , n1 is the degree of polymerization, x1 is 0.4 to 0.7, for example, 0.5 to 0.7. x1 represents the "degree of substitution", which can be defined as the average number of hydroxyl groups per glucose unit of cellulose replaced by carboxymethyl groups.

[0051] In the above “x1 is 0.4 to 0.7”, the value of x1 includes the minimum and maximum values ​​of the range, and every value between the minimum and maximum values. Examples include but are not limited to the point values ​​in the embodiments and the following point values: 0.42, 0.44, 0.46, 0.48, 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.62, 0.64, 0.66, 0.68; or a range consisting of any two values.

[0052] In some embodiments, the molecular formula of the carboxymethyl cellulose metal salt is [C6H7O2(OH) 3- x2 (OCH2COOM) x2 ] n2 , M includes one or more of lithium, sodium, potassium, rubidium, and cesium, for example, M includes lithium; n2 is the degree of polymerization, and x2 is 0.4 to 0.7, for example, 0.5 to 0.7. x2 represents the "degree of substitution," which can be defined as the average number of hydroxyl groups per glucose unit of cellulose replaced by carboxymethyl metal groups.

[0053] In the above “x2 is 0.4 to 0.7”, the value of x2 includes the minimum and maximum values ​​of the range, and every value between the minimum and maximum values. Examples include but are not limited to the point values ​​in the embodiments and the following point values: 0.42, 0.44, 0.46, 0.48, 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.62, 0.64, 0.66, 0.68; or a range consisting of any two values.

[0054] It can be understood that n1 and n2 may be the same or different, and x1 and x2 may be the same or different.

[0055] The values ​​of x1 and x2 affect the film-forming quality of the passivation layer 30. The higher the values ​​of x1 and x2, the fewer hydrogen bonds between molecules in the ionic resin and the lower the viscosity, which is not conducive to cross-linking between molecules. The formed passivation layer 30 has poor density and poor coating properties on the active material layer 20.

[0056] x1 and x2 can be measured using instruments and methods known in the art, such as the ashing method, which can be referred to as follows: Weigh 0.2000 grams (g) of the sample to be tested (accurate to 0.1 milligrams (mg)), place it in a 30mL porcelain crucible, and after ashing, place it in a muffle furnace and calcine at 700°C for 1 hour. Remove it after cooling; use distilled water to transfer all the residue in the porcelain crucible to a 150mL Erlenmeyer flask, add 3 to 4 drops of methyl orange indicator solution to the Erlenmeyer flask, and titrate with sulfuric acid standard solution. When the solution turns red, continue to add 10mL of sulfuric acid standard solution, record the total volume of sulfuric acid standard solution used, then heat to boiling. After 10 minutes, titrate with sodium hydroxide standard solution until the solution color changes from red to yellow, and record the volume of sodium hydroxide standard solution used. The formula for substitution degree is as follows:

[0057] Where DS is the degree of substitution; M H2SO4 is the molar concentration of sulfuric acid standard solution, mol / L; M NaOH is the molar concentration of sodium hydroxide standard solution, mol / L; V H2SO4 V is the total volume of the sulfuric acid standard solution used for titration, mL; NaOH is the total volume of sodium hydroxide standard solution used, mL; m is the mass of the sample to be tested, g.

[0058] In some embodiments, the Young's modulus of the ion resin is greater than or equal to 2.5 GPa, for example, between 18 GPa and 27 GPa. A Young's modulus of the ion resin within this range can enhance the deformation resistance of the passivation layer 30, further improving the battery's cycling performance. A higher Young's modulus improves the ability to block lithium dendrites during cycling, improving cycling performance. However, a higher Young's modulus is not recommended.

[0059] The values ​​of the above-mentioned Young's modulus include the minimum and maximum values ​​of the above-mentioned range, and each value between such minimum and maximum values. Examples include but are not limited to the point values ​​in the embodiments and the following point values: 10 Gpa, 11 Gpa, 12 Gpa, 13 Gpa, 14 Gpa, 15 Gpa, 16 Gpa, 17 Gpa, 18 Gpa, 19 Gpa, 20 Gpa, 21 Gpa, 22 Gpa, 23 Gpa, 24 Gpa, 25 Gpa, and 26 Gpa.

[0060] The Young's modulus of the ionic resin has a well-known meaning in the art and can be measured using instruments and methods known in the art, such as the AFM force-displacement curve measurement method, using an NSG-10 single-crystal silicon probe with a tip curvature radius of approximately 10 nm and an elastic modulus (Kc) ranging from 3.1 N / m to 37.6 N / m. The Kc value of the probe is determined by thermal modulation before measurement. TM The two-dimensional topography of the sample was measured in this mode, and the force-displacement curve (referred to as the force curve) was recorded (at a rate of 2 μm / s). All tests were performed at 25°C and 40% relative humidity. The data from the effective force curves were analyzed using NanoScope Analysis software, and the compressive Young's modulus of the sample was calculated based on the elastic contact model.

[0061] In some embodiments, the weight average molecular weight of the ionic resin is 50,000 Da to 300,000 Da, for example, 70,000 Da to 250,000 Da. A weight average molecular weight within the above range has better film-forming properties. A weight average molecular weight that is too large or too small is not conducive to film formation.

[0062] The above values ​​of "50000Da-300000Da" include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Examples include but are not limited to the point values ​​in the examples and the following point values: 60000Da, 70000Da, 80000Da, 90000Da, 100000Da, 110000Da, 120000Da, 130000Da, 140000Da, 150000Da, 160000Da, 170000Da, 180000Da, 190000Da, 200000Da, 210000 Da, 220000 Da, 230000 Da, 240000 Da, 250000 Da, 260000 Da, 270000 Da, 280000 Da, 290000 Da; or a range consisting of any two values.

[0063] The weight average molecular weight of the ionic resin has a well-known meaning in the art and can be measured using instruments and methods known in the art. For example, the viscosity of its aqueous solution can be measured according to ASTM D 1439-03, and the molecular weight can be estimated from the viscosity of the standard resin aqueous solution. According to one method, the molecular weight of the ionic resin can be estimated using the viscosity according to the following formula:

[0064] η[Pa S]=8.91×10 -4 +1.30×10 -5 cMw 0.9 +5.33×10 -8 c2 M w 1.8 +4.60×10 -15 c 4.34 M w 3.91 , where η is the viscosity, c is the concentration of the CMC, and Mw is the molecular weight, as described by Kulicke in Polymer, Vol. 37, No. 13, pp. 2723-2731, 1996.

[0065] The carboxymethyl cellulose or carboxymethyl cellulose metal salt of the present application can be prepared or obtained by commercial means or methods known in the art. In some embodiments, other carboxymethyl cellulose metal salts other than sodium carboxymethyl cellulose can be prepared by the following method:

[0066] Treating sodium carboxymethyl cellulose with acid to obtain carboxymethyl cellulose; and

[0067] The MOH solution was added to the obtained carboxymethyl cellulose and heated.

[0068] It should be noted that the amount of MOH solution is excessive relative to the amount of carboxymethyl groups in the carboxymethyl cellulose to ensure that all carboxyl hydrogens in the carboxymethyl groups are replaced by M. M can be one or more of lithium, sodium, potassium, rubidium and cesium.

[0069] In some embodiments, the thickness of the passivation layer 30 is 1 to 8 microns (μm). It is understandable that the thickness of the passivation layer may include but is not limited to: 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm. Further, for example, it is 2μm to 4μm. When the thickness of the passivation layer is within the above range, the film-forming effect and interface dynamics performance are better. If the thickness of the passivation layer 30 is too small, the coating effect on the active material layer 20 is poor, and if the thickness of the passivation layer 30 is too large, the interface transport performance of Li+ will be reduced.

[0070] In some embodiments, the thickness of the active material layer 20 is greater than 40 μm, for example, 40 μm to 79 μm. It can be understood that the thickness of the active material layer 20 may include but is not limited to: 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm.

[0071] The thickness of the negative electrode film layer is 40 μm to 80 μm.

[0072] The thickness of the passivation layer 30, the active material layer 20, and the negative electrode film layer can be measured using a micrometer, for example, a Mitutoyo 293-100 with an accuracy of 0.1 μm. It should be noted that the thickness of the passivation layer 30, the active material layer 20, and the negative electrode film layer in this application refers to the thickness of the passivation layer 30, the active material layer 20, and the negative electrode film layer in the negative electrode sheet after cold pressing and used for assembly of the battery.

[0073] The negative electrode active material described in this application is not particularly limited and may include but is not limited to at least one of graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials and lithium titanate. The graphite may include at least one of natural graphite and artificial graphite. The silicon-based material may be at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites and silicon alloys. The tin-based material may be at least one of elemental tin, tin oxide compounds and tin alloys. However, this application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0074] The weight ratio of the negative electrode active material in the active material layer 20 is 70-100 weight %, based on the total weight of the active material layer.

[0075] In some embodiments, the active material layer 20 further includes a binder. The binder may be at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The weight ratio of the binder in the active material layer is 0-30% by weight, based on the total weight of the active material layer.

[0076] In some embodiments, the active material layer 20 further includes a conductive agent. The conductive agent can be at least one of superconducting carbon, carbon black (e.g., acetylene black or Ketjen black), carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the active material layer is 0-20% by weight, based on the total weight of the active material layer.

[0077] In some embodiments, the active material layer 20 further includes other additives, such as a thickener (e.g., sodium carboxymethyl cellulose (CMC-Na)), etc. The weight ratio of the other additives in the active material layer is 0-15% by weight, based on the total weight of the active material layer.

[0078] In some embodiments, the passivation layer 30 is disposed around the active material layer 20 to form a hollow closed structure.

[0079] During the battery cycle, the polarization distribution of the negative electrode is uneven due to uneven temperature and expansion force. The temperature in the middle is high (affected by heat dissipation), the expansion force is small, and the polarization is small; the temperature at the head and bottom is low, the expansion force is large, and the polarization is large. The uneven polarization can easily lead to lithium deposition. The passivation layer 30 is arranged circumferentially around the active material layer 20 to form a hollow closed structure, which can effectively solve the lithium deposition problem.

[0080] In some embodiments, the area of ​​the hollow region accounts for 10% to 30% of the area of ​​the active material layer 20, and may also include but is not limited to 15%, 20%, 22%, 24%, 26%, 28%, and 30%. Furthermore, the area of ​​the hollow region accounts for 10% to 20% of the area of ​​the active material layer 20.

[0081] A second aspect of the present application provides a method for preparing a negative electrode sheet, comprising:

[0082] S10, forming an active material layer 20 on the negative electrode current collector 10, wherein the active material layer 20 includes a negative electrode active material;

[0083] S30 , forming a passivation layer 30 on the active material layer 20 , wherein the passivation layer 30 includes an ionic resin, and the ionic resin includes one or more of carboxymethyl cellulose and carboxymethyl cellulose metal salt.

[0084] In some embodiments, the passivation layer 30 is formed by physical vapor deposition.

[0085] Physical vapor deposition methods include at least one of evaporation and sputtering. Evaporation methods include, but are not limited to, at least one of vacuum evaporation, thermal evaporation, and electron beam evaporation (EBEM). Sputtering methods include, but are not limited to, magnetron sputtering.

[0086] In some embodiments, the passivation layer 30 is formed by vacuum evaporation. In some embodiments, the deposition rate of the vacuum evaporation method is 0.05 to 0.2 μ / min, and further, for example, 0.07 to 0.15 μ / min. The deposition rate affects the film formation effect of the passivation layer. A lower deposition rate will cause the deposited film to be loose and prone to forming large particles, affecting the density of the passivation layer. However, a deposition rate that is too high will increase the internal stress of the film, resulting in increased defects in the film layer, and in severe cases, it will cause the film layer to rupture.

[0087] In some embodiments, before forming the passivation layer 30 on the active material layer 20 , the method further includes step S20 :

[0088] S20 , setting a mask plate so that the mask plate covers the middle portion of the active material layer 20 to be processed, and the area of ​​the mask plate is smaller than the area of ​​the active material layer 20 .

[0089] It should be noted that the middle portion is intended to form a hollow area and is not limited to the exact middle portion in a strict sense, as long as the periphery of the active material layer 20 can be exposed.

[0090] In some embodiments, the area of ​​the mask is 10% to 30% of the area of ​​the active material layer 20 .

[0091] The third aspect of the present application further provides a secondary battery, comprising the negative electrode sheet described in the first aspect of the present application.

[0092] Therefore, the obtained battery has good cycle performance.

[0093] The secondary battery and the electric device of the present application will be described below with reference to the accompanying drawings as appropriate.

[0094] In one embodiment of the present application, a secondary battery is provided.

[0095] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0096] Positive electrode

[0097] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0098] As a non-limiting example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0099] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be obtained by forming a metal material on a polymer material substrate. Among them, the metal material may include but is not limited to aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc. The polymer material substrate may include but is not limited to one or more substrates of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0100] In some embodiments, the positive electrode active material may include a positive electrode active material for a battery known in the art. As non-limiting examples, the positive electrode active material may include one or more of the following materials: an olivine-structured lithium-containing phosphate, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials; other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their modified compounds. Non-limiting examples of olivine-structured lithium-containing phosphates may include, but are not limited to, one or more of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. Non-limiting examples of lithium cobalt oxides may include LiCoO2; non-limiting examples of lithium nickel oxides may include LiNiO2; non-limiting examples of lithium manganese oxides may include LiMnO2, LiMn2O4, etc.; non-limiting examples of lithium nickel cobalt manganese oxides may include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ) etc. Non-limiting examples of lithium nickel cobalt aluminum oxide may include LiNi 0.8 Co 0.15 Al 0.05 O2.

[0101] The weight ratio of the positive electrode active material in the positive electrode film layer is 80-100 weight %, based on the total weight of the positive electrode film layer.

[0102] In some embodiments, the positive electrode film layer further includes a binder. As a non-limiting example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin. The weight ratio of the binder in the positive electrode film layer is 0-20 weight%, based on the total weight of the positive electrode film layer.

[0103] In some embodiments, the positive electrode film layer further includes a conductive agent. As non-limiting examples, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the positive electrode film layer is 0-20% by weight, based on the total weight of the positive electrode film layer.

[0104] In some embodiments, the positive electrode sheet can be prepared in the following manner: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent to form a positive electrode slurry, and the positive electrode slurry is coated on at least one side of the positive electrode collector. After drying, cold pressing and other processes, the positive electrode sheet can be obtained. Cold pressing can be performed using a cold rolling mill. The type of solvent can be selected from but not limited to any one of the aforementioned embodiments, such as N-methylpyrrolidone (NMP). The surface of the positive electrode collector coated with the positive electrode slurry can be on a single surface of the positive electrode collector or on both surfaces of the positive electrode collector. The solid content of the positive electrode slurry can be 40wt% to 80wt%. The viscosity of the positive electrode slurry at room temperature can be adjusted to 5000mPa·s-25000mPa·s. When applying the positive electrode slurry, the coating unit surface density based on dry weight (excluding solvent) can be 2mg / cm 2 -2.7mg / cm 2 , for example 2.2 g / cm 3 -2.4g / cm 3The thickness of the positive electrode membrane is 51μm-191μm.

[0105] The thickness T of the positive electrode film layer can be measured using a micrometer, for example, a Mitutoyo 293-100 with an accuracy of 0.1 μm. It should be noted that the thickness of the positive electrode film layer described in this application refers to the thickness of the positive electrode film layer in the positive electrode sheet after cold pressing and used for battery assembly.

[0106] Negative electrode

[0107] The negative electrode plate adopts the negative electrode plate provided in the first aspect of the present application.

[0108] electrolytes

[0109] The electrolyte conducts ions between the positive and negative electrodes. This application does not specifically limit the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0110] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0111] In some embodiments, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium bis(oxalatoborate) (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP), and lithium tetrafluorooxalatophosphate (LiTFOP). The concentration of the electrolyte salt is typically 0.5-5 mol / L.

[0112] In some embodiments, the solvent may include fluoroethylene carbonate (FEC), ethylene carbonate (EC, ), propylene carbonate (PC, ), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC, ), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

[0113] In some embodiments, the electrolyte further includes additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives capable of improving certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0114] In some embodiments, the additives in the electrolyte may include, but are not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethylethylene carbonate (TFPC), and the like.

[0115] Isolation film

[0116] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0117] In some embodiments, the material of the separator can include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0118] In some embodiments, the isolation film has a thickness of 5-30 μm, for example, 7-18 μm.

[0119] In some embodiments, the air permeability of the isolation membrane may be 100s / 100mL-300s / 100mL; for example, the air permeability of the isolation membrane may be 150s / 100mL-250s / 100mL, as tested according to national standard GB / T 36363-2018.

[0120] The thickness of the isolation can be measured using a micrometer, for example, a Mitutoyo 293-100 micrometer with an accuracy of 0.1 μm.

[0121] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0122] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0123] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft shell, such as a pouch-type soft shell. The material of the soft shell can be plastic. Further, non-limiting examples of plastic can include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0124] A secondary battery includes at least one battery cell. A secondary battery may include one or more battery cells.

[0125] In this application, unless otherwise specified, a "battery cell" refers to a basic unit that can achieve the mutual conversion of chemical energy and electrical energy. Further, generally speaking, it includes at least a positive electrode plate, a negative electrode plate, and an electrolyte. During the battery's charge and discharge process, active ions are embedded in and out of the positive and negative electrode plates. The electrolyte plays the role of conducting active ions between the positive and negative electrode plates.

[0126] The present application has no particular limitation on the shape of the battery cell, which may be cylindrical, square, or any other shape. For example, FIG2 shows a secondary battery 1 having a square structure as an example.

[0127] In some embodiments, referring to Figure 3, the outer packaging may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 11 has an opening connected to the receiving cavity, and the cover plate 13 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 12 through a winding process or a lamination process. The electrode assembly 12 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 12. The number of electrode assemblies 22 contained in the secondary battery 1 can be one or more, and those skilled in the art can select according to actual needs.

[0128] In some embodiments, the secondary batteries 1 can be assembled into a battery module. The battery module can contain one or more secondary batteries 1 , and the number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0129] In the battery module, the plurality of secondary batteries 1 can be arranged in sequence along the length of the battery module. Of course, they can also be arranged in any other manner. Furthermore, the plurality of secondary batteries 1 can be fixed by fasteners.

[0130] In some embodiments, the battery module may further include a housing having a receiving space, and the plurality of secondary batteries 1 may be received in the receiving space.

[0131] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0132] A battery pack may include a battery box and multiple battery modules disposed within the box. The battery box comprises an upper case and a lower case. The upper case can be placed over the lower case to form an enclosed space for accommodating the battery modules. The multiple battery modules can be arranged in any manner within the battery box.

[0133] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0134] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0135] Figure 4 shows an example of an electric device. This device can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0136] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0137] Example

[0138] In order to make the technical problems, technical solutions and beneficial effects solved by this application clearer, the application will be further described in detail below with reference to the embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present application and its applications. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0139] If the techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in the field or the product instructions were followed. If the manufacturers of the reagents or instruments are not specified, they are all commercially available conventional products.

[0140] 1. Preparation of Carboxymethyl Cellulose Metal Salt

[0141] Sodium carboxymethyl cellulose was dispersed in an ethanol / water mixture (volume ratio 95:5), and then a 20 wt% HCl solution was added. The mixture was acid-treated at 35°C for 2 hours, filtered, washed three times with an ethanol / water mixture (volume ratio 85:15), and dried at 105°C for 12 hours to obtain carboxymethyl cellulose.

[0142] Carboxymethyl cellulose was dispersed in an ethanol / water mixture (90:10 by volume) and then added to a 7 wt% aqueous solution of LiOH. The mixture was reacted at 50°C for 2 hours. The mixture was then washed three times with an ethanol / water mixture (85:15 by volume) and dried at 105°C for 12 hours to obtain lithium carboxymethyl cellulose.

[0143] The aqueous solution of LiOH can be replaced with KOH, RbOH, CsOH, or Ca(OH)2 as needed to obtain potassium carboxymethyl cellulose, rubidium carboxymethyl cellulose, cesium carboxymethyl cellulose, and calcium carboxymethyl cellulose for subsequent preparation of negative electrode sheets. It should be noted that when replacing the aqueous solution with other alkali solutions, the amount of the alkali solution used is in excess relative to the carboxylic acid groups in the carboxymethyl cellulose.

[0144] 2. Preparation of Secondary Batteries

[0145] Example 1

[0146] 1. Preparation of positive electrode sheet

[0147] The positive electrode active material lithium iron phosphate (gram capacity of 139 mAh / g), the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 94:4:2, added to the solvent N-methylpyrrolidone (NMP), and stirred thoroughly to obtain the positive electrode slurry; then the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and then dried, cold pressed, and cut to obtain the positive electrode sheet.

[0148] 2. Preparation of negative electrode sheet

[0149] The negative electrode active material, artificial graphite (with a gram capacity of 340 mAh / g), the conductive agent, acetylene black, the binder, styrene-butadiene rubber (SBR), and the thickener, sodium carboxymethyl cellulose (CMC), were mixed in a weight ratio of 95:1.5:3.1:0.4. Deionized water was added as a solvent and thoroughly stirred to form an active material layer slurry. The active material layer slurry was applied to the negative electrode current collector copper foil and dried to form an active material layer. The coating weight of the negative electrode slurry was 0.132 g / 1540.25 mm² (based on weight excluding solvent), and the coating thickness was 80 μm.

[0150] The copper foil with the active material layer was placed in a vacuum coating machine, dried carboxymethyl cellulose was placed in the evaporation boat, the chamber was evacuated to 5×10-5Torr, and the temperature was gradually raised to 110°C so that the film deposition rate was 0.15μ / min. Carboxymethyl cellulose with a thickness of 5μm was deposited on the active material layer to form a passivation layer.

[0151] 3. Preparation of Electrolyte

[0152] Ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed in a volume ratio of 3:7 to obtain an organic solvent; LiPF6 is dissolved in the above organic solvent, and then fluoroethylene carbonate (FEC) is added and mixed evenly to obtain an electrolyte; wherein the concentration of LiPF6 is 1 mol / L, and the mass percentage of fluoroethylene carbonate (FEC) based on the total mass of the electrolyte is 2%.

[0153] 4. Preparation of isolation membrane

[0154] PE porous film is used as the isolation membrane.

[0155] 5. Preparation of lithium-ion secondary batteries

[0156] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrodes to provide insulation. The cells are then wound to form a bare cell. The tabs are then welded to the bare cell and placed in an aluminum shell. The cells are then baked at 80°C to remove moisture. The electrolyte is then injected and sealed to create an uncharged battery. The uncharged battery then undergoes a series of processes, including resting, hot and cold pressing, formation, shaping, and testing, to produce a lithium-ion secondary battery.

[0157] Examples 2 to 18

[0158] The preparation method is basically the same as that of Example 1, except that the negative electrode plate material and its parameters are replaced with the corresponding parameters in Example 1, as shown in Table 1.

[0159] Comparative Example 1

[0160] The preparation method is basically the same as that of Example 1, except that the passivation layer is omitted in the negative electrode plate.

[0161] Comparative Example 2

[0162] The preparation method is basically the same as that of Example 1, except that sodium hydroxymethyl cellulose is used instead of carboxymethyl cellulose.

[0163] 3. Performance Testing

[0164] 1. Cycle performance test

[0165] (1) Cyclic test at room temperature (25°C)

[0166] The test battery was placed in a high-temperature chamber at 25°C. It was then charged at a constant current rate of 1C to 3.65V, allowed to rest for 5 minutes, and then charged at a constant voltage rate of 3.65V to a current of 0.05C, allowed to rest for 30 minutes. After charging, it was discharged at a constant current of 1C to 2.5V. This constituted one charge-discharge cycle. The battery was cyclically charged and discharged according to this method until the discharge capacity decayed to 80%. The number of cycles was recorded.

[0167] (2) High temperature (45°C) cycle test

[0168] The test battery was placed in a high-temperature oven at 45°C. It was then charged at a constant current rate of 1C to 3.65V, allowed to rest for 5 minutes, and then charged at a constant voltage rate of 3.65V to a current of 0.05C, allowed to rest for 30 minutes. After charging, it was discharged at a constant current of 1C to 2.5V. This constituted one charge-discharge cycle. The battery was cyclically charged and discharged according to this method until the discharge capacity decayed to 80%. The number of cycles was recorded.

[0169] (3) High temperature (60°C) cycle test

[0170] The test battery was placed in a high-temperature oven at 60°C. It was then charged at a constant current rate of 1C to 3.65V, allowed to rest for 5 minutes, and then charged at a constant voltage rate of 3.65V to a current of 0.05C, allowed to rest for 30 minutes. After charging, it was discharged at a constant current of 1C to 2.5V. This constituted one charge-discharge cycle. The battery was cyclically charged and discharged according to this method until the discharge capacity decayed to 80%. The number of cycles was recorded.

[0171] 2. Lithium dendrites

[0172] After cycling each of the secondary batteries prepared above for 200 cycles according to the charge-discharge method described under "1. Cycling Performance Test," the secondary batteries were disassembled in an argon atmosphere glove box (H2O <0.1ppm, O2 <0.1ppm). The surface morphology of the negative electrode plates was visually observed to determine whether lithium dendrites had formed. The absence of white spots on the negative electrode plates indicated no lithium dendrites, the presence of sporadic white spots indicated mild lithium dendrites, and the presence of dense white spots indicated severe lithium dendrites.

[0173] The test results are shown in Table 1 below.

[0174] Table 1

[0175] The above description of the various embodiments tends to emphasize the differences between the various embodiments. The same or similar aspects can be referenced with each other and will not be repeated herein for the sake of brevity.

[0176] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector, an active material layer and a passivation layer, wherein the active material layer is disposed between the negative electrode current collector and the passivation layer; in, The active material layer includes a negative electrode active material, the passivation layer includes an ion resin, and the ion resin includes one or more of carboxymethyl cellulose and a carboxymethyl cellulose metal salt.

2. The negative electrode sheet according to claim 1, wherein: The molecular formula of the carboxymethyl cellulose is [C6H7O2(OH) 3-x1 (OCH2COOH) x1 ]n1, n1 is the degree of polymerization; x1 is 0.4 to 0.7, and can be optionally 0.5 to 0.7; and / or, The molecular formula of the carboxymethyl cellulose metal salt is [C6H7O2(OH) 3-x2 (OCH2COOM) x2 ] n2 , M includes one or more of lithium, sodium, potassium, rubidium and cesium, optionally, M includes lithium; n2 is the degree of polymerization; x2 is 0.4 to 0.7, optionally 0.5 to 0.

7.

3. The negative electrode sheet according to claim 1 or 2, wherein: The Young's modulus of the ion resin is greater than or equal to 2.5 Gpa, and can be selected to be 18 Gpa-27 Gpa.

4. The negative electrode sheet according to any one of claims 1 to 3, wherein: The weight average molecular weight of the ionic resin is 50000Da-300000Da, and can be optionally 70000-250000Da.

5. The negative electrode sheet according to any one of claims 1 to 4, wherein: The thickness of the passivation layer is 1 to 8 μm, and can be optionally 2 to 4 μm.

6. The negative electrode sheet according to any one of claims 1 to 5, wherein: The passivation layer is arranged around the active material layer to form a hollow closed structure; optionally, the area of ​​the hollow region accounts for 10% to 30% of the area of ​​the active material layer.

7. The negative electrode sheet according to any one of claims 1 to 6, wherein: The negative electrode active material includes at least one of graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials and lithium titanate, and graphite may be selected.

8. A method for preparing a negative electrode sheet according to any one of claims 1 to 7, comprising the following steps: forming an active material layer on the negative electrode current collector, wherein the active material layer includes a negative electrode active material; A passivation layer is formed on the active material layer, wherein the passivation layer includes an ionic resin, and the ionic resin includes one or more of carboxymethyl cellulose and a carboxymethyl cellulose metal salt.

9. The preparation method according to claim 8, wherein: The passivation layer is formed by a physical vapor deposition method, which may be an evaporation method or a sputtering method, and may further be a vacuum evaporation method.

10. The preparation method according to claim 9, wherein: Before forming a passivation layer on the active material layer, the method further includes: Setting a mask plate so that the mask plate covers the middle part of the active material layer to be processed, and the area of ​​the mask plate is smaller than the area of ​​the active material layer; Optionally, the area of ​​the mask plate is 10% to 30% of the area of ​​the active material layer.

11. A secondary battery comprising the negative electrode sheet according to any one of claims 1 to 7.

12. An electric device comprising the secondary battery according to claim 11.

Images