Negative electrode sheet, secondary battery, battery module, battery pack, and power consumption device

Abstract

The present application provides a negative electrode sheet, a secondary battery, a battery module, a battery pack, and a power consumption device. The negative electrode sheet includes a negative electrode current collector, and a first negative electrode film layer, a second negative electrode film layer, and a functional coating layer, which are sequentially provided on at least one surface of the negative electrode current collector, the functional coating layer including a ferroelectric material, the second negative electrode film layer including a second negative electrode active material including hard carbon, and the first negative electrode film layer including a first negative electrode active material including graphite. The negative electrode sheet according to the present application allows the secondary battery to achieve both a high charge rate and a long cycle life on the premise of having a high energy density.

Description

[Technical Field]

[0001] The present application relates to the technical field of batteries, and more particularly to negative electrode sheets, secondary batteries, battery modules, battery packs, and power consuming devices. [Background technology]

[0002] In recent years, secondary batteries have been widely used in energy storage power systems such as hydroelectric, thermal, wind, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and many other fields. As secondary batteries become more widely used, the demands for their energy density, cycling performance, and high-rate charging performance are becoming increasingly high. As a key component of secondary batteries, the performance of the anode active material has a significant impact on the battery's performance. Graphite is one of the most commonly used anode active materials in secondary batteries, boasting low polarization and high cycling stability. However, its theoretical capacity per gram is only 372 mAh / g. Currently, the performance of commercial graphite has been almost fully developed, leaving very limited room for improvement in both its reversible capacity per gram and its energy density. At the same time, graphite's narrow interlayer spacing also limits its high-rate charging performance. Hard carbon, a novel negative electrode active material, has great potential for development because it can rapidly intercalate and deintercalate active ions during the charge and discharge process of secondary batteries. However, the compaction density and initial coulombic efficiency of commercial hard carbon are low, limiting the improvement of the energy density of secondary batteries. Summary of the Invention

[0003] The object of the present application is to provide a negative electrode sheet, a secondary battery, a battery module, a battery pack, and a power consumption device that achieve both a high charge rate and a long cycle life, provided that the secondary battery has a high energy density.

[0004] A first aspect of the present application provides a negative electrode sheet including a negative electrode current collector, and a first negative electrode film layer, a second negative electrode film layer, and a functional coating layer, which are sequentially provided on at least one surface of the negative electrode current collector, wherein the functional coating layer includes a ferroelectric material, the second negative electrode film layer includes a second negative electrode active material including hard carbon, and the first negative electrode film layer includes a first negative electrode active material including graphite.

[0005] In the present invention, a first negative electrode film layer containing graphite and a second negative electrode film layer containing hard carbon are sequentially applied to the surface of the negative electrode current collector, thereby compensating for the shortcomings of both graphite and hard carbon and highlighting their respective advantages. The large interlayer spacing of the hard carbon allows for a higher charging rate. The interposition of graphite between the hard carbon and the negative electrode current collector compensates for the initial Coulombic efficiency of the hard carbon. Furthermore, the insertion potential of active ions into the hard carbon micropore structure is approximately 0 V, close to the deposition potential of active ions on the graphite surface, preventing the hard carbon micropore structure from functioning effectively as an active ion storage site. However, the present inventors unexpectedly discovered that this problem can be solved by applying a functional coating layer containing a ferroelectric material to the surface of the negative electrode sheet. By controlling the deposition method of active ions, the ferroelectric material can highlight the advantages of the large capacity of the microporous structure of hard carbon and suppress the continuous reduction and deposition of active ions on the graphite surface, thereby improving the cycle life of the secondary battery. Therefore, a secondary battery using the negative electrode sheet of the present application has a long cycle life, can be charged at a high rate, has a high output voltage, and also has a high energy density.

[0006] In any embodiment of the present application, the thickness of the functional coating layer is H1 μm, the thickness of the second negative electrode film layer is H2 μm, the thickness of the first negative electrode film layer is H3 μm, and the negative electrode sheet satisfies H1 / (H2+H3) in the range of 0.01 to 0.15, preferably 0.01 to 0.08, thereby improving the overall performance of the secondary battery and realizing high-rate charging with high energy density.

[0007] In any embodiment of the present application, the thickness of the functional coating layer is H1 μm, where H1 is 2 to 10, and optionally 4 to 6. This improves the overall performance of the secondary battery and enables high-rate charging with high energy density.

[0008] In any embodiment of the present application, the thickness of the second negative electrode film layer is H2 μm, the thickness of the first negative electrode film layer is H3 μm, and the negative electrode sheet satisfies H2 / H3 in a range of 0.10 to 5, and optionally 0.5 to 4. This allows the second negative electrode film layer and the first negative electrode film layer to exhibit a better synergistic effect.

[0009] In any embodiment of the present application, the ferroelectric material has a volume average particle size Dv50 of d1 μm, where d1 is 1 or less and optionally 0.05 to 0.8, thereby improving the overall performance of the secondary battery, enabling high-rate charging with high energy density, and reducing production costs.

[0010] In any embodiment of the present application, the second negative electrode active material has a volume average particle diameter Dv50 of d2 μm, the first negative electrode active material has a volume average particle diameter Dv50 of d3 μm, and d2 / d3 is 0.1 to 1, optionally 0.2 to 0.8. In this case, the second negative electrode film layer is advantageous in improving the high-rate charging capability of the secondary battery, and the first negative electrode film layer is advantageous in fully improving the initial coulombic efficiency and cycle life of the secondary battery, thereby improving the overall performance of the secondary battery and enabling high-rate charging with a high energy density.

[0011] In any embodiment of the present application, the mass percentage of the ferroelectric material in the functional coating layer is W1, and W1 is 70% to 95%, preferably 80% to 95%, based on the total mass of the functional coating layer, thereby improving the overall performance of the secondary battery and realizing high-rate charging with high energy density.

[0012] In any embodiment of the present application, the mass percentage of hard carbon in the second negative electrode film layer is W2, and W2 is 68% or more, optionally 90% to 98%, based on the total mass of the second negative electrode film layer, which is advantageous for the secondary battery to have a higher charge rate.

[0013] In any embodiment of the present application, the mass percentage of graphite in the first negative electrode film layer is W3, and W3 is 78% or more, optionally 90% to 98%, based on the total mass of the first negative electrode film layer, which is advantageous for the secondary battery to have a higher initial coulombic efficiency and a longer cycle life.

[0014] In any embodiment of the present application, the second negative electrode active material has a volume average particle size Dv50 of d2 μm, where d2 is 3 to 11, and optionally 3 to 7. This is advantageous for improving the capacity and energy density of the secondary battery.

[0015] In any embodiment of the present application, the particle size distribution index (Dv90-Dv10) / Dv50 of the second negative electrode active material is α1, which is 0.6 to 5, and optionally 1 to 4. This is advantageous for improving the high-rate charging capability and charge / discharge efficiency of the secondary battery.

[0016] In any embodiment of the present application, the specific surface area of ​​the second negative electrode active material is 3 m 2 / g~7m 2 / g, selectable 4m 2 / g~6m 2This is advantageous for improving the high rate charging capability of the secondary battery.

[0017] In any embodiment of the present application, the powder compaction density of the second negative electrode active material at 20000 N is 0.9 g / cm 3 ~1.3g / cm 3 and can be selected as 1g / cm 3 ~1.2g / cm 3 This is advantageous for improving the energy density of the secondary battery.

[0018] In any embodiment of the present application, the second negative electrode active material includes primary particles, secondary particles, or a combination thereof, and optionally the proportion of primary particles in the second negative electrode active material is 90% to 100%, which is advantageous for improving the high-rate charging capability of the secondary battery.

[0019] In any embodiment of the present application, the volume average particle diameter Dv50 of the first negative electrode active material is d3 μm, and d3 is 9 to 18, and optionally 11 to 15. This is advantageous in that the secondary battery has high initial coulombic efficiency, high energy density, and long cycle life.

[0020] In any embodiment of the present application, the particle size distribution index (Dv90-Dv10) / Dv50 of the first negative electrode active material is α2, which is 0.2 to 5, and optionally 0.3 to 4. This is advantageous for improving the cycle performance of the secondary battery.

[0021] In any embodiment of the present application, the specific surface area of ​​the first negative electrode active material is 0.6 m 2 / g~1.5m 2 / g, selectable 0.8m 2 / g~1.4m 2 This is advantageous for improving the cycle performance of the secondary battery.

[0022] In any embodiment of the present application, the powder compaction density of the first negative electrode active material at 20000 N is 1.4 g / cm3 ~1.85g / cm 3 and selectable, 1.6g / cm 3 ~1.75g / cm 3 This is advantageous for improving the energy density of the secondary battery.

[0023] In any embodiment of the present application, the graphitization degree of the first negative electrode active material is 91% to 95%, and optionally 92% to 94%, which is advantageous for improving the cycle life of the secondary battery.

[0024] In any embodiment of the present application, the first negative electrode active material includes artificial graphite, natural graphite, or a combination thereof, and optionally, the surface of the artificial graphite has a carbon coating layer, which is advantageous for improving the cycle performance and high-rate charging capability of the secondary battery.

[0025] In any embodiment of the present application, the first negative electrode active material includes primary particles, secondary particles, or a combination thereof, and optionally the proportion of secondary particles in the first negative electrode active material is 80% to 100%, which is advantageous for improving the cycle performance, storage performance, and high-rate charging capability of the secondary battery.

[0026] In any embodiment of the present application, the dielectric constant of the ferroelectric material is 50 or more, and optionally 50-100,000.

[0027] In any embodiment of the present application, the ferroelectric material may include one or a combination of multiple materials selected from inorganic ferroelectric materials and organic ferroelectric materials. Optionally, the inorganic ferroelectric material may include one or a combination of multiple materials selected from perovskite-structure oxides, tungsten bronze-type compounds, bismuth oxide-type layer structure compounds, lithium niobate, lithium tantalate, lead metaniobate, and lead barium lithium niobate. Optionally, the organic ferroelectric material may include one or a combination of multiple materials selected from vinylidene fluoride homopolymer or copolymer, 2,5-dibromo-3,6-dihydroxy-p-benzoquinone, phenazine-chloranilic acid, and croconic acid.

[0028] In any embodiment of the present application, the functional coating layer further comprises an adhesive and / or a dispersing agent, and optionally the adhesive comprises one or more combinations selected from styrene butadiene rubber, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid, and carboxymethyl chitosan, and optionally the dispersing agent comprises sodium carboxymethyl cellulose.

[0029] A second aspect of the present application provides a secondary battery including the negative electrode sheet of the first aspect of the present application.

[0030] A third aspect of the present application provides a battery module including the secondary battery of the second aspect of the present application.

[0031] A fourth aspect of the present application provides a battery pack including one of the secondary battery of the second aspect of the present application and the battery module of the third aspect.

[0032] A fifth aspect of the present application provides a power consumption device including at least one of the secondary battery of the second aspect, the battery module of the third aspect, and the battery pack of the fourth aspect of the present application.

[0033] A secondary battery using the negative electrode sheet of the present application has a long cycle life, can be charged at a high rate, has a high output voltage, and has a high energy density. The battery module, battery pack, and power consumption device of the present application include the secondary battery of the present application, and therefore have at least the same advantages as the secondary battery. [Brief explanation of the drawings]

[0034] In order to more clearly explain the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application, and those skilled in the art can obtain other drawings based on the drawings without any creative efforts. [Figure 1] 1 is a schematic diagram of an embodiment of a negative electrode sheet according to the present application. FIG. [Figure 2] 1 is a schematic diagram of an embodiment of a secondary battery according to the present application. [Figure 3] 3 is an exploded schematic view of the embodiment of the secondary battery of FIG. 2. FIG. [Figure 4] 1 is a schematic diagram of an embodiment of a battery module according to the present application. [Figure 5] 1 is a schematic diagram of an embodiment of a battery pack according to the present application. [Figure 6] FIG. 6 is an exploded schematic view of the embodiment of the battery pack shown in FIG. 5. [Figure 7] 1 is a schematic diagram of an embodiment of a power consuming device that includes a secondary battery of the present application as a power source.

[0035] In the drawings, which are not necessarily drawn to scale, the reference numerals are as follows: 1 battery pack, 2 upper housing, 3 lower housing, 4 battery module, 5 secondary battery, 51 case, 52 electrode assembly, 53 cover plate, 10 negative electrode sheet, 101 functional coating layer, 102 second negative electrode film layer, 103 first negative electrode film layer, 104 negative electrode current collector. DETAILED DESCRIPTION OF THE INVENTION

[0036] Hereinafter, embodiments specifically disclosing the negative electrode sheet, secondary battery, battery module, battery pack, and power consumption device of the present application will be described in detail with appropriate reference to the drawings. However, unnecessary detailed description may be omitted. For example, detailed description of already well-known matters or redundant description of substantially identical configurations may be omitted. This is to avoid unnecessary redundancy in the following description and to facilitate understanding by those skilled in the art. Note that the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0037] The "ranges" disclosed herein are defined in the form of lower and upper limits, where a given range is defined by selecting one lower limit and one upper limit, and the selected lower and upper limits specifically define the boundaries of the range. Ranges defined in this manner may or may not include the endpoints and may be arbitrarily combined, i.e., any lower limit may be combined with any upper limit to form a single range. For example, reciting ranges of 60 to 120 and 80 to 110 for a particular parameter is understood to also contemplate ranges of 60 to 110 and 80 to 120. Furthermore, reciting minimum range values ​​of 1 and 2 and maximum range values ​​of 3, 4, and 5 contemplates ranges of 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. Unless otherwise specified, the numerical range "a to b" herein refers to a contraction of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0 to 5" indicates that all real numbers between "0 and 5" are listed in this specification, and "0 to 5" is a contraction of combinations of these numerical values. Furthermore, when a parameter is expressed as an integer of 2 or greater, this is equivalent to disclosing that the parameter is an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0038] Unless otherwise stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure content of the present application.

[0039] Unless otherwise stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure content of the present application.

[0040] Unless otherwise specified, all steps herein can be performed in order, can be performed randomly, and are preferably performed in order. For example, when a method includes steps (a) and (b), it means that the method can include steps (a) and (b) performed in order, or can include steps (b) and (a) performed in order. For example, when a method further includes step (c), it means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b), and (c), or can include steps (a), (c), and (b), or can further include steps (c), (a), and (b), etc.

[0041] Unless otherwise specified, the terms "comprise" and "include" referred to in this application may be open-ended or closed-ended. For example, "comprise" and "include" indicate that the term may comprise or include other components not further recited, or may comprise or include only the recited components.

[0042] Unless otherwise stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, any of the following conditions satisfy the condition "A or B": A is true (or exists) and B is false (or does not exist), A is false (or does not exist) and B is true (or exists), or A and B are both true (or exist).

[0043] As used herein, the terms "plurality" and "multiple types" refer to two or more types.

[0044] In this application, the terms "primary particles" and "secondary particles" have the meanings well known in the art. Primary particles refer to particles that are not in an agglomerated state, and secondary particles refer to particles in an agglomerated state formed by the agglomeration of two or more primary particles. Primary particles and secondary particles can be easily distinguished by taking SEM images using a scanning electron microscope.

[0045] Hard carbon refers to carbon that is resistant to graphitization, even at temperatures above 2500°C. Hard carbon is typically obtained by pyrolysis of a precursor, such as a polymer. During pyrolysis, the carbon atoms in the precursor are cross-linked, inhibiting the planar growth of the carbon layers. Consequently, the hard carbon structure contains numerous microcrystals with a disordered graphite-like structure (abbreviated as graphite microcrystals). The structure of hard carbon is complex, including not only graphite microcrystals but also defect structures (e.g., surface defects, lattice defects, etc.) and micropore structures (e.g., open pore structures, closed pore structures, etc.). Active ions can therefore be inserted and extracted from hard carbon at various angles, providing secondary batteries with excellent high-rate charging capabilities. Hard carbon offers unique advantages, particularly in the field of power batteries. Compared to graphite, the spacing between the (002) crystal plane layers of hard carbon is larger, resulting in greater structural stability and no significant volume expansion or contraction during charging and discharging.

[0046] Hard carbon has many defects, which catalyze the decomposition of the electrolyte, forming a thick solid electrolyte interface (SEI) film and increasing the irreversible loss of active ions. Furthermore, the surface of hard carbon has a sparse porous structure, which easily adsorbs moisture and oxygen gas from the air, forming various CH functional groups on the surface. These functional groups react with active ions, further increasing the irreversible loss of active ions. Therefore, compared to graphite, hard carbon has a lower initial coulombic efficiency and poor cycle performance. For example, the initial coulombic efficiency is typically less than 80%, preventing it from fully utilizing its high capacity advantage.

[0047] The inventors of the present application have conducted extensive research to provide a novel negative electrode sheet that can improve the capacity of hard carbon and provide a secondary battery with a high energy density, while also providing improved high-rate charging capability and cycle life. Negative electrode sheet

[0048] Specifically, an embodiment of the present application provides a negative electrode sheet, which includes a negative electrode current collector and a first negative electrode film layer, a second negative electrode film layer, and a functional coating layer sequentially provided on at least one surface of the negative electrode current collector, wherein the functional coating layer includes a ferroelectric material, the second negative electrode film layer includes a second negative electrode active material including hard carbon, and the first negative electrode film layer includes a first negative electrode active material including graphite.

[0049] Graphite has the advantages of high initial coulombic efficiency and high cycle stability, but its high-rate chargeability is limited. Hard carbon has the advantage of high active ion insertion and desorption rates, resulting in excellent high-rate chargeability, but its initial coulombic efficiency is low and its capacity decay is rapid, resulting in poor actual capacity of secondary batteries. Through practical research, the inventors of the present application discovered that a two-layer combination of graphite and hard carbon, sequentially applied to the surface of the negative electrode current collector, can compensate for the shortcomings of each. By placing graphite close to the negative electrode current collector, the initial coulombic efficiency of the hard carbon can be compensated for, while by placing the hard carbon farther from the negative electrode current collector, more active ions can be inserted into the negative electrode more quickly, thereby improving the high-rate chargeability of the negative electrode sheet and secondary battery.

[0050] However, in the course of further research, the inventors of the present application discovered that when graphite and hard carbon are sequentially provided on the surface of the negative electrode current collector to form a two-layer combination, the effect of improving the high-rate charge capacity and energy density of the secondary battery is not clear. After extensive research, the inventors discovered that the possible cause is that the formation of "dendrites" during charging of the secondary battery, especially in the final stage of charging, is a factor that affects the improvement of the high-rate charge capacity and energy density of the secondary battery.

[0051] During the charging process of a secondary battery, abnormal situations can occur, such as insufficient insertion space for negative electrode active ions, excessive resistance to their insertion into the negative electrode, or the active ions detaching from the positive electrode too quickly without being able to insert into the negative electrode in equal amounts. The active ions that do not insert into the negative electrode can only obtain electrons on the surface of the negative electrode, where they precipitate as simple metals, forming "dendrites." Therefore, the problem of "dendrites" becomes more serious as the charge rate of the secondary battery increases. The formed dendrites not only degrade secondary battery performance, for example by shortening cycle life, but also, in severe cases, develop sharp morphologies that pierce the separator, causing short circuits within the battery, potentially resulting in catastrophic consequences such as fire and explosion, thereby increasing the safety risk of the secondary battery. Furthermore, as dendrites continue to accumulate, they can shed from the surface of the negative electrode, losing electrical contact with the negative electrode current collector and therefore no longer participate in charge / discharge reactions or capacity contributions, thereby reducing the energy density of the secondary battery.

[0052] After graphite and hard carbon are sequentially applied to the surface of the negative electrode current collector to form a two-layer assembly, during the initial charging stage of the secondary battery, active ions are first adsorbed onto the surface defects and lattice defects of the hard carbon. This process corresponds to the "high potential slope region" in the hard carbon charge / discharge curve, where the negative electrode potential is typically between -2V and -0.1V. As charging continues, active ions intercalate between the graphite crystallites of the hard carbon, exhibiting graphite-like intercalation behavior. This process corresponds to the "low potential plateau region" in the hard carbon charge / discharge curve, where the negative electrode potential is typically between -0.1V and 0V. Because hard carbon has a more abundant microporous structure, during the second charge, when the negative electrode potential drops to approximately 0V, active ions are further stored in the microporous structure of the hard carbon, providing additional active ion storage sites and improving the capacity performance and initial coulombic efficiency of the hard carbon. Furthermore, the longer the discharge plateau in the low potential plateau region, the higher the corresponding reversible capacity of the hard carbon and the better its capacity performance. However, the potential in the low-potential plateau region of hard carbon is close to the dendrite formation potential of the graphite surface layer, both of which are around 0 V. Therefore, in order to prevent dendrite formation, the capacity of the hard carbon in the low-potential plateau region is limited, and the microporous structure of the hard carbon cannot fully function as an active ion storage site. At the same time, in order to prevent dendrite formation, the cutoff voltage of the negative electrode is usually set high, which reduces the output voltage of the entire secondary battery and further reduces the energy density of the secondary battery. Therefore, when a two-layer combination is formed by sequentially providing graphite and hard carbon on the surface of the negative electrode current collector, there is a limit to the effects of improving the high-rate charging capacity, energy density, and / or cycle life of the secondary battery.

[0053] The negative electrode sheet of the present application is designed by sequentially combining a first negative electrode film layer containing graphite and a second negative electrode film layer containing hard carbon on the surface of a negative electrode current collector, and a functional coating layer containing a ferroelectric material is further provided on the surface of the second negative electrode film layer. Ferroelectric materials exhibit spontaneous polarization. When electrons accumulate on the surface of dendrites, the ferroelectric material undergoes spontaneous polarization under the influence of an electric field, causing the positive charge center of the ferroelectric material to move to the electron accumulation region and surround the electron accumulation region. At the same time, the positive charge center of the ferroelectric material becomes positively charged, which repels the polarity of the positively charged active ions on the surface of the dendrites, balancing the electron density and reducing the concentration of active ions, thereby preventing the dendrites from continuing to grow perpendicular to the electrode sheet.

[0054] Therefore, in the present invention, a first negative electrode film layer containing graphite and a second negative electrode film layer containing hard carbon are sequentially applied to the surface of the negative electrode current collector to create a combined design that compensates for the shortcomings of both graphite and hard carbon and highlights their respective advantages. The large interlayer spacing of the hard carbon allows for a higher charging rate. The interposition of graphite between the hard carbon and the negative electrode current collector compensates for the initial Coulombic efficiency of the hard carbon. Furthermore, the insertion potential of active ions into the micropore structure of hard carbon is approximately 0 V, which is close to the deposition potential of active ions on the graphite surface, preventing the micropore structure of hard carbon from functioning effectively as an active ion storage site. However, the present inventors unexpectedly discovered that this problem can be solved by applying a functional coating layer containing a ferroelectric material to the surface of the negative electrode sheet. By controlling the deposition method of active ions, ferroelectric materials can highlight the large capacity advantages of the microporous structure of hard carbon, and also suppress the continuous reduction and deposition of active ions on the graphite surface, thereby improving the cycle life of secondary batteries.

[0055] Specifically, during the initial charging stage of the secondary battery, active ions are rapidly adsorbed onto the surface defects and lattice defects of the hard carbon, resulting in high kinetic performance of the negative electrode sheet. During the later charging stage of the secondary battery, the functional coating layer precisely controls the overpotential, allowing a large amount of active ions to be stored in the micropore structure of the hard carbon, increasing the length of the low-potential plateau region and increasing the reversible capacity of the secondary battery, thereby improving the high-rate charging capability and energy density of the secondary battery. Furthermore, during overpotential, the functional coating layer generates a reverse electric field, reducing the concentration of active ions and inhibiting the continued growth of dendrites perpendicular to the electrode sheet, thereby improving the safety and cycle performance of the secondary battery.

[0056] Therefore, a secondary battery using the negative electrode sheet of the present invention has a long cycle life, can be charged at a high rate, has a high output voltage, and also has a high energy density.

[0057] In some embodiments, optionally, the dielectric constant of the ferroelectric material is greater than or equal to 50. The higher the dielectric constant of the ferroelectric material, the greater the effect of inhibiting dendrites from continuing to grow in a direction perpendicular to the polar sheet, but this effect does not always increase; at the same time, the higher the dielectric constant, the higher the requirements for the preparation process of the ferroelectric material, which also increases the production cost.

[0058] In some embodiments, the dielectric constant of the ferroelectric material may optionally be from 50 to 100,000, e.g., from 50 to 50,000, from 50 to 25,000, from 50 to 10,000, from 50 to 5,000, from 50 to 4,000, from 50 to 3,000, from 50 to 2,000, from 100 to 100,000, from 100 to 50,000, from 100 to 25,000, It may be 100 to 10,000, 100 to 5,000, 100 to 4,000, 100 to 3,000, 100 to 2,000, 200 to 100,000, 200 to 50,000, 200 to 25,000, 200 to 10,000, 200 to 5,000, 200 to 4,000, 200 to 3,000, 200 to 2,000, or 200 to 1,000.

[0059] In this application, the dielectric constant of a ferroelectric material refers to the dielectric constant at room temperature (25±5°C), has a meaning well known in the art, and can be tested using equipment and methods well known in the art. For example, after preparing a circular sample of the ferroelectric material, the capacitance C can be tested using an LCR meter and calculated using the formula: dielectric constant ε = (C × d) / (ε0 × A). C represents capacitance in farads (F), d represents sample thickness in cm, and A represents sample area in cm. 2 where ε0 is the vacuum dielectric constant, ε0=8.854×10 -14 F / cm. In this application, the test conditions may be 1KHz, 1.0V, and 25±5°C. The test standard may be in accordance with GB / T 11297.11-2015. When preparing samples, reference may be made to Chinese patent application CN114217139A.

[0060] In some embodiments, the ferroelectric material may include one or a combination of inorganic ferroelectric materials, organic ferroelectric materials, or optionally, inorganic ferroelectric materials.

[0061] In some embodiments, optionally, the inorganic ferroelectric material may include one or more combinations selected from perovskite structure oxides, tungsten bronzes, bismuth oxide type layered structure compounds, lithium niobate (LiNbO3), lithium tantalate (LiTaO3), lead metaniobate, and lead barium lithium niobate. More optionally, the inorganic ferroelectric material is selected from perovskite structure oxides.

[0062] Optionally, the perovskite structure oxide has the molecular formula Ba 1-x A x Ti 1-y B y O3. A may include one or more combinations selected from Pb, Sr, Ca, K, Na, and Cd, and B may include one or more combinations selected from Sn, Hf, Zr, Ce, Nb, and Th, with 0 ≦ x ≦ 1 and 0 ≦ y ≦ 1. For example, the perovskite structure oxide may include one or more combinations selected from BaTiO3, Ba 1-x1 Sr x1 TiO3 (0 ≦ x1 ≦ 1), SrTiO3, PbTiO3, PbZr y1 Ti 1-y1 O3 (0 ≦ y1 ≦ 1), BaZr y2 Ti 1-y2 O3 (0 < y2 < 1), KNbO3, and NaNbO3.

[0063] Optionally, the tungsten bronze type compound may have the molecular formula M z WO3. M may include one or more combinations selected from Na, K, Rb, and Cs, with 0 < z < 1. For example, the tungsten bronze type compound may include one or more combinations selected from Na z1 WO3 (0 < z1 < 1) and K z2 WO3 (0 < z2 < 1).

[0064] Optionally, the bismuth oxide type layered structure compound has the molecular formula (Bi2O2)(C n-1 Dn O 3n+1 ) C may contain one or a combination of several elements selected from Na, K, Ba, Sr, Pb, Ca, Ln, and Bi, and D may contain one or a combination of several elements selected from Zr, Cr, Nb, Ta, Mo, W, Fe, Ti, and V, where 2≦n≦5. For example, the bismuth oxide-type layered structure compound may be SrBi2Nb2O9, SrBi2Ta2O9, SrBi2Nb2O9, Bi4Ti3O 12 The above may be one type or a combination of multiple types.

[0065] In some embodiments, the organic ferroelectric material may include one or more combinations selected from vinylidene fluoride homopolymer or copolymer, 2,5-dibromo-3,6-dihydroxy-p-benzoquinone, phenazine-chloranilic acid, and croconic acid.

[0066] In some embodiments, the thickness of the functional coating layer is H1 μm, the thickness of the second negative electrode film layer is H2 μm, the thickness of the first negative electrode film layer is H3 μm, and the negative electrode sheet satisfies H1 / (H2+H3) in the range of 0.01 to 0.15, and optionally 0.01 to 0.08.

[0067] Through further investigation, the inventors found that when the thickness H1 μm of the functional coating layer, the thickness H2 μm of the second anode film layer, and the thickness H3 μm of the first anode film layer satisfy the relationship H1 / (H2 + H3) between 0.01 and 0.15, the overall performance of the secondary battery is improved, and high-rate charging is possible with high energy density. Furthermore, the following situation can be effectively avoided: When the thickness of the functional coating layer is low and the total thickness of the first anode film layer and the second anode film layer is high, the reverse electric field strength provided by the functional coating layer is insufficient, making it unable to balance the electron density and inhibit the continued growth of dendrites in the direction perpendicular to the electrode sheet. This increases the safety risk of the secondary battery when increasing the output voltage of the secondary battery to improve energy density, and when decreasing the output voltage of the secondary battery to reduce safety risks of the secondary battery, the micropore structure of the hard carbon cannot function sufficiently as an active ion storage site. If the thickness of the functional coating layer is high and the total thickness of the first and second negative electrode film layers is low, the functional coating layer will not have electrochemical activity and will not contribute to capacity, occupying a large volumetric space and mass, which will instead affect the energy density of the secondary battery.

[0068] In some embodiments, the functional coating layer has a thickness H1 μm, where H1 is 2 to 10 μm, and optionally 4 to 6 μm. When the functional coating layer has a suitable thickness, the overall performance of the secondary battery is improved, enabling high-rate charging with high energy density. Furthermore, the following situations can be effectively avoided: A thin functional coating layer results in insufficient reverse electric field strength, making it unable to balance the electron density and inhibit the continued growth of dendrites perpendicular to the electrode sheet. This increases the safety risk of the secondary battery when increasing the output voltage of the secondary battery to improve its energy density. Furthermore, when reducing the output voltage of the secondary battery to reduce its safety risk, the micropore structure of the hard carbon cannot fully function as an active ion storage site. A thick functional coating layer lacks electrochemical activity and cannot contribute to capacity, occupying a large volumetric space and mass, which actually affects the energy density of the secondary battery.

[0069] In some embodiments, the thickness of the second negative electrode film layer is H2 μm, the thickness of the first negative electrode film layer is H3 μm, and the negative electrode sheet satisfies H2 / H3 is 0.10 to 5, optionally 0.5 to 4.

[0070] Through further investigation, the inventors found that when the ratio of the thickness H2μm of the second anode film layer to the thickness H3μm of the first anode film layer is within an appropriate range, the second anode film layer and the first anode film layer can achieve a better synergistic effect. The second anode film layer mainly bears the pressure during high-rate charging, ensuring a high charge rate for the secondary battery, while the first anode film layer mainly provides a long cycle life while simultaneously ensuring a high initial coulombic efficiency, thereby improving the overall performance of the secondary battery and enabling high-rate charging with high energy density. Furthermore, the following situation can be effectively avoided: when the second anode film layer is thin and the first anode film layer is thick, the high-rate charging pressure borne by the hard carbon in the second anode film layer is reduced during charging. In this case, the graphite in the first anode film layer still plays a dominant role, and the effect of improving the charge rate of the secondary battery is not clear. If the second anode film layer is thick and the first anode film layer is thin, the hard carbon surface will contain a large number of inactive functional groups, which will increase the irreversible consumption of active ions, affecting the initial coulombic efficiency of the secondary battery and potentially reducing the cycle life of the secondary battery.

[0071] In some embodiments, the thickness of the second negative electrode film layer is H2 μm, where H2 is 10 to 120 μm, preferably 35 to 110 μm, and more preferably 40 to 100 μm. If the thickness of the second negative electrode film layer is within an appropriate range, it is advantageous for the secondary battery to have a high charge rate.

[0072] In some embodiments, the thickness of the first negative electrode film layer is H3 μm, where H3 is 20 to 100 μm, optionally 25 to 70 μm, and more preferably 28 to 60 μm. If the thickness of the first negative electrode film layer is within an appropriate range, it is advantageous for the secondary battery to have a high initial coulombic efficiency and a long cycle life.

[0073] The functional coating layer includes a ferroelectric material, and in some embodiments, the mass percentage of the ferroelectric material in the functional coating layer is W1, where W1 is 70% to 95%, and optionally 80% to 95%, based on the total mass of the functional coating layer.

[0074] Through further investigation, the inventors discovered that when the ferroelectric material content is within an appropriate range, the overall performance of the secondary battery is better and high-rate charging can be achieved with high energy density. Furthermore, the following situations can be effectively avoided: A low ferroelectric material content may result in insufficient reverse electric field strength, which may not balance the electron density or inhibit the continued growth of dendrites perpendicular to the electrode sheet. This increases safety risks when increasing the output voltage of the secondary battery to improve energy density, and prevents the micropore structure of the hard carbon from functioning adequately as active ion storage sites when reducing the output voltage of the secondary battery to reduce safety risks. A high ferroelectric material content reduces the content of other components (e.g., adhesive content) in the functional coating layer, which results in poor adhesion between the functional coating layer and the second negative electrode film layer and makes the functional coating layer more likely to detach from the second negative electrode film layer.

[0075] In some embodiments, the functional coating layer may further include an adhesive to bond the ferroelectric materials together and to bond the functional coating layer to the second negative electrode film layer. Optionally, the adhesive may include one or a combination of materials selected from 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).

[0076] In some embodiments, the functional coating layer may further include other additives, such as a dispersant, for example, sodium carboxymethylcellulose (CMC).

[0077] The second negative electrode film layer includes a second negative electrode active material, and the second negative electrode active material includes hard carbon. Optionally, the mass percentage of the hard carbon in the second negative electrode active material is 70% to 100%, preferably 80% to 98%, based on the total mass of the second negative electrode active material. This allows the second negative electrode film layer to contain a large amount of hard carbon, which is advantageous for the secondary battery to have a higher charge rate. In some embodiments, the second negative electrode active material may include only hard carbon. In other embodiments, the second negative electrode active material may further include another negative electrode active material other than hard carbon, such as graphite, soft carbon, mesocarbon microbeads, a silicon-based material, or a tin-based material. Optionally, the mass percentage of the other negative electrode active material in the second negative electrode active material is 30% or less, more preferably 20% or less, based on the total mass of the second negative electrode active material.

[0078] In some embodiments, the mass percentage of hard carbon in the second negative electrode film layer is W2, and W2 is 68% or more, optionally 90% to 98%, based on the total mass of the second negative electrode film layer. When the content of hard carbon is within an appropriate range, it is advantageous for the secondary battery to have a higher charge rate.

[0079] In some embodiments, the second negative electrode film layer may further include a conductive agent. This agent serves to collect microcurrent between the second negative electrode active material (e.g., hard carbon), reducing electrode contact resistance, accelerating electron transfer speed, and simultaneously reducing polarization, thereby improving the charge / discharge efficiency of the secondary battery. For example, the conductive agent may include one or a combination of materials selected from superconducting carbon, conductive graphite, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the conductive agent is 5% or less, based on the total mass of the second negative electrode film layer.

[0080] In some embodiments, the second negative electrode film layer may further include an adhesive to bond the second negative electrode active materials together, to bond the second negative electrode film layer to the functional coating layer, and to bond the second negative electrode film layer to the first negative electrode film layer. For example, the adhesive may include one or more of the following: styrene butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-soluble acrylic resin (e.g., polyacrylic acid (PAA), polymethacrylic acid (PMAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, the mass percentage of the adhesive is 5% or less, based on the total mass of the second negative electrode film layer.

[0081] In some embodiments, the second negative electrode membrane layer may further include other additives. For example, the other additives may include a thickener, such as sodium carboxymethyl cellulose (CMC), a PTC thermistor material, etc. In some embodiments, the mass percentage of the other additives is 2% or less, based on the total mass of the second negative electrode membrane layer.

[0082] The first negative electrode film layer includes a first negative electrode active material containing graphite. Optionally, the mass percentage of the graphite in the first negative electrode active material is 80% to 100%, preferably 90% to 98%, based on the total mass of the first negative electrode active material. This allows the first negative electrode film layer to contain a large amount of graphite, which is advantageous for the secondary battery to have a higher initial coulombic efficiency and a longer cycle life. In some embodiments, the first negative electrode active material may include only graphite. In some other embodiments, the first negative electrode active material may further include other negative electrode active materials other than graphite, such as hard carbon, soft carbon, mesocarbon microbeads, silicon-based materials, and tin-based materials. Optionally, the mass percentage of the other negative electrode active materials in the first negative electrode active material is 20% or less, more preferably 10% or less, based on the total mass of the first negative electrode active material.

[0083] In some embodiments, the mass percentage of graphite in the first negative electrode film layer is W3, and W3 is 78% or more, optionally 90% to 98%, based on the total mass of the first negative electrode film layer. When the graphite content is within an appropriate range, it is advantageous for the secondary battery to have a higher initial coulombic efficiency and a longer cycle life.

[0084] In some embodiments, the first negative electrode film layer may further include a conductive agent, which serves to collect microcurrent between the first negative electrode active material (e.g., graphite), thereby reducing electrode contact resistance, accelerating electron transfer speed, and simultaneously reducing polarization, thereby improving the charge / discharge efficiency of the secondary battery. For example, the conductive agent may include one or a combination of materials selected from superconducting carbon, conductive graphite, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the conductive agent is 5% or less, based on the total mass of the first negative electrode film layer.

[0085] In some embodiments, the first negative electrode film layer may further include an adhesive to bond the first negative electrode active materials together, to bond the first negative electrode film layer to the second negative electrode film layer, and to bond the first negative electrode film layer to the negative electrode current collector. For example, the adhesive may include one or more of the following: styrene butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-soluble acrylic resin (e.g., polyacrylic acid (PAA), polymethacrylic acid (PMAA), sodium polyacrylate (PAAS)), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, the mass percentage of the adhesive is 5% or less, based on the total mass of the first negative electrode film layer.

[0086] In some embodiments, the first negative electrode membrane layer may further include other additives. For example, the other additives may include a thickener, such as sodium carboxymethyl cellulose (CMC), a PTC thermistor material, etc. In some embodiments, the mass percentage of the other additives is 2% or less, based on the total mass of the first negative electrode membrane layer.

[0087] In some embodiments, the ferroelectric material has a volume average particle size Dv50 of d1 μm, where d1 is 1 or less, e.g., 0.8 or less, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less, 0.3 or less, or 0.2 or less. Optionally, d1 is 0.01 to 1, 0.02 to 1, 0.03 to 1, 0.04 to 1, 0.05 to 1, 0.06 to 1, 0.07 to 1, 0.08 to 1, 0.01 to 0.8, 0.02 to 0.8, 0.03 to 0.8, 0.04 to 0.8, 0.05 to 0.8, 0.06 to 0.8, 0.07 to 0.8, 0.08 to 0.8, 0.09 to 0.8, or 0.1 to 0.8.

[0088] When the Dv50 of a ferroelectric material is within an appropriate range, the overall performance of the secondary battery is improved, high energy density and high-rate charging can be achieved, and production costs can be reduced. Furthermore, the following situations can be effectively avoided: A high Dv50 of a ferroelectric material can result in significant interference from the reverse electric field, which can prevent electron density balancing and inhibit the continued growth of dendrites perpendicular to the electrode sheet. This can increase safety risks when increasing the output voltage of the secondary battery to improve energy density, and prevent the micropore structure of the hard carbon from functioning adequately as active ion storage sites when reducing the output voltage of the secondary battery to reduce safety risks. A low Dv50 of a ferroelectric material can complicate its preparation process and increase production costs.

[0089] In some embodiments, the second negative electrode active material has a volume average particle diameter Dv50 of d2 μm, the first negative electrode active material has a volume average particle diameter Dv50 of d3 μm, and d2 / d3 is 0.1 to 1, optionally 0.2 to 0.8.

[0090] In the negative electrode sheet of the present application, the first negative electrode film layer uses a first negative electrode active material with a large particle size, which increases the compaction density of the first negative electrode film layer, advantageously resulting in a secondary battery with a high initial coulombic efficiency, high energy density, and long cycle life. In the negative electrode sheet of the present application, the second negative electrode film layer uses a second negative electrode active material with a small particle size, which increases its specific surface area, increasing the contact area with the electrolyte and shortening the liquid-phase and solid-phase diffusion paths of active ions. At the same time, the integrity of the channel structure of the second negative electrode film layer can be better maintained, advantageously resulting in a secondary battery with a higher charge rate. In addition, the small particle size and high voltage resistance of the second negative electrode active material better maintain the micropore structure of the second negative electrode active material, particularly the hard carbon, which allows the micropore structure of the hard carbon to function fully as active ion storage sites, thereby improving the capacity and energy density of the secondary battery. At the same time, the compaction density of the second negative electrode film layer is low, which allows the electrolyte to quickly infiltrate into the first negative electrode film layer, which is advantageous for reducing polarization of the negative electrode and improving the charge / discharge efficiency of the secondary battery.

[0091] Through further investigation, the inventors have found that when the ratio d2 / d3 of the particle size of the second anode active material to the particle size of the first anode active material is within an appropriate range, it is advantageous for the second anode film layer to fully improve the high-rate charging capability of the secondary battery and for the first anode film layer to fully improve the initial coulombic efficiency and cycle life of the secondary battery, thereby improving the overall performance of the secondary battery and enabling high-rate charging with a high energy density.

[0092] In some embodiments, the second negative electrode active material has a volume average particle diameter Dv50 of d2 μm, where d2 is 3 to 11, optionally 3 to 9. When the particle diameter of the second negative electrode active material is within an appropriate range, its micropore structure is better maintained, thereby allowing the micropore structure of the hard carbon to fully function as active ion storage sites, thereby improving the capacity and energy density of the secondary battery. At the same time, when the particle diameter of the second negative electrode active material is within an appropriate range, the second negative electrode film layer has a smooth channel structure, which is advantageous for the electrolyte to smoothly pass through the channel structure of the second negative electrode film layer and quickly infiltrate into the first negative electrode film layer, thereby reducing polarization of the negative electrode and improving the charge / discharge efficiency of the secondary battery.

[0093] In some embodiments, the particle size distribution index (Dv90-Dv10) / Dv50 of the second negative electrode active material is α1, where α1 is 0.6 to 5, and optionally 1 to 4. When the particle size distribution index of the second negative electrode active material is within an appropriate range, it is advantageous to improve the processing performance of the second negative electrode film layer and ensure that the entire second negative electrode film layer has a highly consistent particle distribution and a smooth channel structure, thereby ensuring that different regions of the second negative electrode film layer have high active ion transport performance and further improving the high-rate charging capability of the secondary battery. At the same time, it is advantageous to allow the electrolyte to quickly infiltrate the first negative electrode film layer, reducing negative electrode polarization and improving the charge / discharge efficiency of the secondary battery.

[0094] In some embodiments, the specific surface area of ​​the second negative electrode active material is 3 m 2 / g~7m 2 / g, selectable 4m 2 / g~6m 2 / g. When the specific surface area of ​​the second negative electrode active material is within an appropriate range, the charge exchange impedance can be reduced, and the second negative electrode film layer has a smoother channel structure, which results in higher active ion transport performance, thereby further improving the high-rate charging capability of the secondary battery. When the specific surface area of ​​the second negative electrode active material is within an appropriate range, the SEI film formation efficiency is improved, the formation of an SEI film that is too thick is avoided, and irreversible loss of active ions is reduced, thereby further improving the capacity and cycle performance of the secondary battery.

[0095] In some embodiments, the powder compaction density of the second negative electrode active material at 20000 N is 0.9 g / cm 3 ~1.3g / cm 3 and can be selected as 1g / cm 3 ~1.2g / cm 3 If the compaction degree of the powder of the second negative electrode active material is within an appropriate range, it is advantageous for improving the energy density of the secondary battery.

[0096] In some embodiments, the second negative electrode active material comprises primary particles, secondary particles, or a combination thereof, and optionally the proportion of primary particles in the second negative electrode active material is 90% to 100%. When the second negative electrode active material comprises a suitable proportion of primary particles, the second negative electrode film layer advantageously has a short active ion transport path, which can further improve the high-rate charging capability of the secondary battery, reduce polarization of the negative electrode and side reactions in the electrolyte, and further improve the cycle performance and storage performance of the secondary battery.

[0097] In some embodiments, the volume average particle diameter Dv50 of the first negative electrode active material is d3 μm, where d3 is 9 to 18, and optionally 11 to 15. When the particle diameter of the first negative electrode active material is within an appropriate range, it is advantageous for the first negative electrode film layer to have a high degree of compaction, so that the secondary battery has a high initial coulombic efficiency, a high energy density, and a long cycle life.

[0098] In some embodiments, the particle size distribution index (Dv90-Dv10) / Dv50 of the first negative electrode active material is α2, which is 0.2 to 5, and optionally 0.3 to 4. When the particle size distribution index of the first negative electrode active material is within an appropriate range, it is advantageous to improve the processing performance of the first negative electrode film layer and ensure that the entire first negative electrode film layer has a highly consistent particle distribution and a smooth channel structure, thereby ensuring that different regions of the first negative electrode film layer have high active ion transport performance and further improving the cycle performance of the secondary battery.

[0099] In some embodiments, the specific surface area of ​​the first negative electrode active material is 0.6 m 2 / g~1.5m 2 / g, selectable 0.8m 2 / g~1.4m 2 When the specific surface area of ​​the first negative electrode active material is within an appropriate range, the charge exchange impedance can be reduced, and the first negative electrode film layer has a smoother channel structure, which results in higher active ion transport performance, thereby further improving the cycle performance of the secondary battery.

[0100] In some embodiments, the powder compaction density of the first negative electrode active material at 20000 N is 1.4 g / cm 3 ~1.85g / cm 3 and selectable, 1.6g / cm 3 ~1.75g / cm 3 If the compaction degree of the powder of the first negative electrode active material is within an appropriate range, it is advantageous for improving the energy density of the secondary battery.

[0101] In some embodiments, the graphitization degree of the first negative electrode active material is 91% to 95%, and optionally 92% to 94%. When the graphitization degree of the first negative electrode active material is within an appropriate range, it is advantageous to improve the cycle performance of the secondary battery.

[0102] In some embodiments, the first negative electrode active material comprises artificial graphite, natural graphite, or a combination thereof. Optionally, the mass percentage of the artificial graphite in the first negative electrode active material is 50% or more, and optionally 60% to 100%, based on the total mass of the first negative electrode active material. Optionally, a carbon coating layer is further provided on the surface of the artificial graphite, thereby further reducing the impedance of charge transfer and improving the cycle performance and high-rate charging capability of the secondary battery. Optionally, the carbon coating layer comprises amorphous carbon.

[0103] In some embodiments, the first negative electrode active material comprises primary particles, secondary particles, or a combination thereof, and optionally the secondary particles comprise 80% to 100% of the first negative electrode active material. When the first negative electrode active material comprises a suitable proportion of secondary particles, its isotropy is improved, which is advantageous for the first negative electrode film layer to have more active ion transport paths, thereby further improving the high-rate charging capability of the secondary battery; and at the same time, the first negative electrode film layer has a high compaction ratio, which is advantageous for improving the energy density of the secondary battery, and also for reducing polarization of the negative electrode and side reactions in the electrolyte, thereby further improving the cycle performance and storage performance of the secondary battery.

[0104] In some embodiments, the negative electrode sheet has a compaction ratio of 1.4 g / cm 3 ~1.85g / cm 3 and selectable 1.6g / cm 3 ~1.75g / cm 3 If the compaction degree of the negative electrode sheet is within an appropriate range, it is advantageous for improving the high-rate charging capability, cycle performance, and energy density of the secondary battery.

[0105] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. Examples of the metal foil include copper foil and copper alloy foil. The composite current collector may include a polymeric material base layer and a metal material layer formed on at least one surface of the polymeric material base layer. For example, the metal material may include one or more combinations selected from copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. The polymeric material base layer may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0106] The negative electrode current collector has two surfaces facing each other in the thickness direction thereof, and the first negative electrode film layer, the second negative electrode film layer, and the functional coating layer may be provided on either or both of the two facing surfaces of the negative electrode current collector.

[0107] The parameters of the first negative electrode film layer, the second negative electrode film layer, and the functional coating layer provided in this application all refer to the parameter range of the coating layer on one side of the negative electrode current collector. When the first negative electrode film layer, the second negative electrode film layer, and the functional coating layer are provided on two surfaces of the negative electrode current collector, if the parameters of the coating layer on either surface meet the provisions of this application, it is considered to be within the protection scope of this application.

[0108] The negative electrode sheet of the present application will be described below with reference to the drawings. FIG. 1 is a schematic diagram of one embodiment of a negative electrode sheet 10 of the present application. The negative electrode sheet 10 includes a negative electrode current collector 104, a first negative electrode film layer 103 provided on each of the two surfaces of the negative electrode current collector 104, a second negative electrode film layer 102 provided on the first negative electrode film layer 103, and a functional coating layer 101 provided on the second negative electrode film layer 102. Of course, the first negative electrode film layer 103, the second negative electrode film layer 102, and the functional coating layer 101 may be provided on only one surface of the negative electrode current collector 104.

[0109] In this application, the Dv90, Dv50, and Dv10 of a material have meanings known in the art and can be measured using instruments and methods known in the art. For example, see GB / T 19077-2016, Laser Diffraction Method for Particle Size Distribution, and they can be easily tested using a laser particle size analyzer, such as Malvern Instruments' Mastersizer 2000E laser particle size analyzer. Dv90 is the particle size corresponding to the cumulative volume distribution percentage of the material reaching 90%, Dv50 is the particle size corresponding to the cumulative volume distribution percentage of the material reaching 50%, and Dv10 is the particle size corresponding to the cumulative volume distribution percentage of the material reaching 10%.

[0110] In this application, the specific surface area of ​​a material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, the specific surface area can be measured using the nitrogen gas adsorption specific surface area analysis test method described in GB / T 19587-2017 and calculated using the Brunauer Emmett Teller (BET) method. The nitrogen gas adsorption specific surface area analysis test can be performed using a Tri-Star 3020 specific surface area pore size analyzer manufactured by Micromeritics, Inc., USA.

[0111] In this application, the graphitization degree of a material has a meaning known in the art and can be measured by an apparatus and method known in the art. For example, the graphitization degree of a material can be measured by an X-ray diffractometer (e.g., Bruker D8 Discover) in accordance with JIS K 0131-1996 and JB / T 4220-2011. 002 After obtaining the formula g=(0.344-d 002 The graphitization degree of the material was calculated from the formula: ) / (0.344-0.3354)×100%. 002 is the layer spacing of the (002) crystal plane in the crystal structure of the material, expressed in nanometers (nm).

[0112] In this application, the powder compaction degree of a material has a meaning known in the art and can be measured using equipment and methods known in the art. For example, it can be tested using an electronic pressure tester (e.g., UTM7305 type) with reference to GB / T24533-2009. One exemplary test method is to weigh 1 g of material and measure the powder compaction degree of a material having a base area of ​​1.327 cm. 2 The method includes the steps of placing the powder in a mold, pressurizing it to 2000 kg (equivalent to 20,000 N), holding the pressure for 30 seconds, releasing the pressure, holding it for 10 seconds, and then recording and calculating the compaction degree of the powder at an applied force of 20,000 N of the material.

[0113] In this application, the quantitative proportions of primary particles and secondary particles can be measured using instruments and methods known in the art, such as a scanning electron microscope. To ensure the accuracy of the test results, multiple (e.g., five or more) different areas of the test sample can be randomly selected and scanned at a certain magnification (e.g., 1000x or more). The percentage of the number of primary particles and secondary particles in each area relative to the total number of particles, i.e., the quantitative proportions of primary particles and secondary particles in each area, can be calculated. To ensure the accuracy of the test results, the above test can be repeated using multiple test samples (e.g., 10 or more), and the average value of each test sample can be used as the final test result. For test standards, see JY / T010-1996.

[0114] In this application, the thicknesses of the first negative electrode film layer, the second negative electrode film layer, and the functional coating layer have meanings known in the art and can be measured using equipment and methods known in the art, such as a scanning electron microscope (e.g., ZEISS Sigma300), which allows for more accurate determination of the boundary area between the functional coating layer and the second negative electrode film layer and the boundary area between the second negative electrode film layer and the first negative electrode film layer. One exemplary test method involves cutting a negative electrode sheet into a test sample of a certain size (e.g., 2 cm x 2 cm), fixing the negative electrode sheet to a sample stand with paraffin, placing the sample stand in a sample holder and locking it in place, and turning on an argon ion cross-section polishing device (e.g., IB-19500CP) to draw a vacuum (e.g., 10ー4 The steps include setting the argon gas flow rate (e.g., 0.15 MPa), voltage (e.g., 8 KV), and polishing time (e.g., 2 hours), adjusting the sample stand to swing mode, and starting polishing. The test standard can be found in JY / T010-1996. To ensure the accuracy of the test results, a scan test can be performed by randomly selecting multiple (e.g., five or more) different areas from the sample to be measured, and reading the thicknesses of the first negative electrode film layer, the second negative electrode film layer, and the functional coating layer in the scale test area at a certain magnification (e.g., 500x or more). For test accuracy, multiple test areas can be used and the average value can be used.

[0115] In this application, the compaction degree of the negative electrode sheet has a meaning known in the art and can be measured using equipment and methods known in the art. The compaction degree of the negative electrode sheet = areal density of the negative electrode sheet / thickness of the coating layer on one side of the negative electrode current collector. The areal density of the negative electrode sheet has a meaning known in the art and can be measured using equipment and methods known in the art. One exemplary test method involves punching a negative electrode sheet coated on one side and cold-pressing it (if the negative electrode sheet is coated on two sides, the coating layer on one side can be wiped off first) into a small wafer with an area of ​​S1, weighing the weight and recording it as M1, wiping off the coating layer from the weighed negative electrode sheet, and weighing the weight of the negative electrode current collector and recording it as M0, so that the areal density of the negative electrode sheet = (M1 - M0) / S1.

[0116] The various parameters of the first negative electrode active material, the second negative electrode active material, and the ferroelectric material may be tested by sampling before application, or by sampling from the negative electrode sheet after cold pressing. When the test sample is sampled from the cold-pressed negative electrode sheet, for example, the steps are as follows: (1) arbitrarily select a coating layer on one side of the cold-pressed negative electrode current collector to sample the ferroelectric material (for example, a blade can be used to scrape off the powder for sampling), with the scraping depth not exceeding the boundary region between the functional coating layer and the second negative electrode film layer; and (2) sample the second negative electrode active material (for example, a blade can be used to scrape off the powder for sampling), with the scraping depth not exceeding the boundary region between the second negative electrode film layer and the first negative electrode film layer. During the cold-pressing process, there may be an inter-melted layer at the boundary region between the functional coating layer and the second negative electrode film layer. Therefore, for the accuracy of the test, when sampling the second negative electrode active material, the inter-melted layer should first be scraped off, and then the second negative electrode active material should be sampled. (3) sampling the first negative electrode active material (for example, by using a blade to scrape off the powder for sampling). During the cold pressing process, a mutually fused layer may be present at the boundary region between the second negative electrode film layer and the first negative electrode film layer. Therefore, for the sake of test accuracy, when sampling the first negative electrode active material, the mutually fused layer is first scraped off, and then the powder of the first negative electrode active material is scraped off for sampling; and (4) the collected ferroelectric material, first negative electrode active material, and second negative electrode active material are each placed in deionized water, followed by suction filtration and drying. The dried powder is then sintered at a certain temperature and time (for example, 400°C, 2 hours) to remove adhesives, conductive agents, etc., to obtain a test sample through sampling. [Preparation method]

[0117] The method for preparing the negative electrode sheet of the present application is known. In some embodiments, the negative electrode sheet can be prepared by a method of dispersing a first negative electrode active material, a conductive agent, an adhesive, and any other components in a solvent (e.g., deionized water) to form a first slurry, dispersing a second negative electrode active material, a conductive agent, an adhesive, and any other components in a solvent (e.g., deionized water) to form a second slurry, dispersing a ferroelectric material, an adhesive, and any other components in a solvent (e.g., deionized water) to form a third slurry, applying the first slurry to a negative electrode current collector and drying to form a first negative electrode film layer, applying the second slurry to the first negative electrode film layer and drying to form a second negative electrode film layer, applying the third slurry to the second negative electrode film layer, and then drying, cold pressing, and other processes to obtain the negative electrode sheet. secondary battery

[0118] An embodiment of the present application further provides a secondary battery including the negative electrode sheet of the first aspect of the present application.

[0119] A secondary battery, also known as a rechargeable battery or storage battery, is a battery that can continue to be used after discharging by recharging to activate the active material. A secondary battery includes an electrode assembly and an electrolyte. The electrode assembly typically includes a positive electrode sheet, a negative electrode sheet, and a separator. The separator is disposed between the positive electrode sheet and the negative electrode sheet and primarily serves to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through. The electrolyte serves to conduct active ions between the positive electrode sheet and the negative electrode sheet. The secondary battery of the present application may be a lithium-containing secondary battery, particularly a lithium-ion secondary battery. [Negative electrode sheet]

[0120] The negative electrode sheet used in the secondary battery of the present application is the negative electrode sheet of any of the embodiments of the first aspect of the present application. [Positive electrode sheet]

[0121] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, for example, the positive electrode current collector has two surfaces facing each other in its thickness direction, and the positive electrode film layer is disposed on one or both of the two facing surfaces of the positive electrode current collector.

[0122] The positive electrode film layer includes a positive electrode active material, and the positive electrode active material can be a positive electrode active material for secondary batteries known in the art. For example, the positive electrode active material may include one or a combination of two or more selected from lithium transition metal oxides, lithium-containing phosphates, and their modified compounds. Examples of lithium transition metal oxides include lithium cobalt oxide (e.g., LiCoO2), lithium nickel oxide (e.g., LiNiO2), lithium manganese oxide (e.g., LiMnPO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, and lithium nickel cobalt manganese oxide (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.5 Co 0.25 Mn 0.25 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2), lithium nickel cobalt aluminum oxide (e.g., LiNi 0.85 Co 0.15 Al 0.05It may contain one or a combination of two or more selected from O2) and each of its modified compounds. Examples of the lithium-containing phosphate include lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and one or a combination of two or more selected from these modified compounds.

[0123] In some embodiments, in order to further increase the energy density of the secondary battery, the positive electrode active material may contain one or a combination of two or more selected from the lithium transition metal oxide represented by Formula 1 and its modified compounds.

[0124] Li a Ni b Co c M d O e A f Formula 1

[0125] In Formula 1, 0.8 ≦ a ≦ 1.2, 0.5 ≦ b < 1, 0 < c < 1, 0 < d < 1, 1 ≦ e ≦ 2, 0 ≦ f ≦ 1, M may contain one or a combination of two or more selected from Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, and B, and A may contain one or a combination of two or more selected from N, F, S, and Cl.

[0126] In the present application, the modified compound of each of the above positive electrode active materials may be obtained by performing doping modification or / and surface coating modification on the positive electrode active material.

[0127] In some embodiments, the positive electrode film layer may further include a conductive agent, which serves to collect microcurrent between the positive electrode active material, reduce electrode contact resistance, accelerate electron transfer, and reduce polarization, thereby improving the charge / discharge efficiency of the secondary battery. For example, the conductive agent may include one or a combination of superconducting carbon, conductive graphite, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the conductive agent is 5% or less, based on the total mass of the positive electrode film layer.

[0128] In some embodiments, the positive electrode film layer may further include an adhesive for bonding the positive electrode active materials together and for bonding the positive electrode film layer to the positive electrode current collector. For example, the adhesive may include one or more selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene ternary copolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene ternary copolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin. In some embodiments, the mass percentage of the adhesive is 5% or less, based on the total mass of the positive electrode film layer.

[0129] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. Examples of the metal foil include aluminum foil or aluminum alloy foil. The composite current collector may include a polymeric substrate and a metal layer formed on at least one surface of the polymeric substrate. For example, the metal may include one or more combinations selected from aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. The polymeric substrate may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0130] The positive electrode film layer is typically formed by applying a positive electrode slurry to a positive electrode current collector, drying, and cold pressing. The positive electrode slurry is typically formed by dispersing a positive electrode active material, a conductive agent, an adhesive, and any other components in a solvent and stirring the mixture uniformly. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP). [Electrolytes]

[0131] In the present application, the type of the electrolyte is not particularly limited and can be selected according to needs. For example, the electrolyte may be at least one selected from a solid electrolyte, a gel electrolyte, and a liquid electrolyte (i.e., an electrolytic solution).

[0132] In some embodiments, the electrolyte employs an electrolytic solution, the electrolytic solution including a lithium salt and a solvent.

[0133] For example, the lithium salt may include one or more combinations selected from lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium hexafluoroarsenate (LiAsF), lithium bisfluorosulfonylimide (LiFSI), lithium bistrifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium bisoxalatoborate (LiBOB), lithium difluorophosphate (LiPOF), lithium difluorobisoxalatophosphate (LiDFOP), and lithium tetrafluorooxalatophosphate (LiTFOP).

[0134] By way of example, the organic solvent may comprise at least one or a combination of more than one of 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), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0135] In some embodiments, the electrolyte may include an additive, such as a negative electrode film-forming additive or a positive electrode film-forming additive, and may further include an additive that can improve certain performance of the battery, such as an additive that improves the overcharge performance of the battery, or an additive that improves the high-temperature or low-temperature performance of the battery. [Separator]

[0136] In the present application, the type of the separator is not particularly limited, and any known porous structure separator having good chemical stability and mechanical stability can be selected.

[0137] In some embodiments, the separator may be made of a material selected from the group consisting of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride, or a combination of two or more materials. The separator may be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of the layers may be the same or different.

[0138] In some embodiments, the positive electrode sheet, the separator, and the negative electrode sheet can be prepared into an electrode assembly by a winding process or a stacking process.

[0139] In some embodiments, the secondary battery may include an exterior case that can be used to seal the electrode assembly and the electrolyte.

[0140] In some embodiments, the exterior of the secondary battery may be a hard case, such as a hard plastic case, an aluminum case, a steel case, etc. The exterior of the secondary battery may be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, such as one or a combination of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0141] In the present application, the shape of the secondary battery is not particularly limited, and may be cylindrical, rectangular, or any other shape. For example, Fig. 2 shows a secondary battery 5 having a rectangular structure as an example.

[0142] In some embodiments, as shown in FIG. 3 , the exterior may include a case 51 and a cover plate 53. The case 51 includes a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate are enclosed to form a storage chamber. The case 51 has an opening communicating with the storage chamber, and the cover plate 53 covers the opening to close the storage chamber. The positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is sealed in the storage chamber. The electrode assembly 52 is impregnated with an electrolyte. The number of electrode assemblies 52 included in the secondary battery 5 may be one or more and may be adjusted according to needs.

[0143] Methods for preparing the secondary battery of the present application are known. In some embodiments, a secondary battery can be formed by assembling a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte. For example, the positive electrode sheet, the separator, and the negative electrode sheet can be wound or stacked to form an electrode assembly, which can then be placed in a housing and dried. The electrolyte can then be injected, and the secondary battery can be obtained through processes such as vacuum sealing, standing, chemical formation, and shaping.

[0144] In some embodiments of the present application, the secondary battery according to the present application may be assembled into a battery module, and the number of secondary batteries included in the battery module may be multiple, and the specific number may be adjusted according to the application and capacity of the battery module.

[0145] Fig. 4 is a schematic diagram of an example battery module 4. As shown in Fig. 4, in the battery module 4, the plurality of secondary batteries 5 may be arranged in order along the longitudinal direction of the battery module 4. Of course, they may be arranged in any other manner. Furthermore, the plurality of secondary batteries 5 may be fixed with fasteners.

[0146] Optionally, the battery module 4 further includes a housing having an accommodating space, and the plurality of secondary batteries 5 are accommodated in the accommodating space.

[0147] In some embodiments, the battery modules may be assembled into a battery pack, and the number of battery modules included in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0148] 5 and 6 are schematic diagrams of an example battery pack 1. As shown in FIGS. 5 and 6, the battery pack 1 may include a battery box and a plurality of battery modules 4 provided in the battery box. The battery box includes an upper housing 2 and a lower housing 3, and the upper housing 2 is covered by the lower housing 3, forming an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the battery box in any manner. power consumption equipment

[0149]

[0006] An embodiment of the present application further provides a power consuming device including at least one of the secondary battery, battery module, and battery pack of the present application. The secondary battery, battery module, and battery pack may be used as a power source for the power consuming device or as an energy storage unit for the power consuming device. The power consuming device may be, but is not limited to, a mobile device (e.g., a mobile phone, a laptop, etc.), an electric vehicle (e.g., 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, etc.

[0150] The power consumption device can select a secondary battery, a battery module, or a battery pack according to its usage needs.

[0151] 7 is a schematic diagram of an example power consuming device, such as a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, which may employ a battery pack or battery module to meet the high power and high energy density demands of the power consuming device.

[0152] Other examples of power consuming devices include mobile phones, tablet computers, notebook computers, etc. Such power consuming devices are usually required to be thin and can employ secondary batteries as their power source. Example

[0153] The following examples will more specifically illustrate the contents of the present disclosure, and these examples are merely used for illustrative purposes. Various modifications and variations within the scope of the contents of the present disclosure will be apparent to those skilled in the art. Unless otherwise specified, all parts, percentages, and ratios described in the following examples are calculated by mass, and all reagents used in the examples can be obtained commercially or synthesized according to conventional methods and can be used directly without further treatment. In addition, all devices used in the examples can be obtained commercially. Example 1-1 Step (1): Preparation of negative electrode sheet S10: Preparation of artificial graphite

[0154] Raw coke powder, which is non-needle coke from petroleum, is pretreated to remove impurities, then mixed with coal pitch and granulated to obtain secondary particles with a Dv50 of 10 μm. The granules are then placed in an Acheson graphitization furnace and graphitized at 3000°C for 24 hours to obtain graphite particles. The resulting graphite particles are mixed with petroleum pitch and then carbonized at 1000°C for 15 hours to obtain artificial graphite. The artificial graphite has a Dv50 of 12 μm, a Dv90 of 20 μm, a Dv10 of 6.5 μm, a degree of graphitization of 92%, and a specific surface area of ​​0.86 m. 2 / g, and the powder compaction density is 1.6g / cm 3 The morphology was secondary particles. S20: Preparation of hard carbon

[0155] The phenolic resin was heat-treated at 500°C for 30 minutes to obtain a precursor, which was then crushed and heat-treated at 1200°C for 20 hours under a nitrogen gas atmosphere. The hard carbon was then crushed again to obtain a hard carbon. The hard carbon had a Dv50 of 5 μm, a Dv90 of 10 μm, a Dv10 of 2.5 μm, and a specific surface area of ​​5 m. 2 / g, and the morphology was primary particles. S30: Preparation of first slurry

[0156] The artificial graphite prepared above, conductive carbon black as a conductive agent, styrene butadiene rubber (SBR) as an adhesive, and sodium carboxymethyl cellulose (CMC) as a thickener were mixed in a mass ratio of 96.8:0.8:1.2:1.2, and then added to deionized water. The mixture was stirred with a vacuum mixer until the system became uniform, yielding a first slurry with a solids content of 66%. S40: Preparation of second slurry

[0157] The hard carbon prepared above, conductive carbon black as a conductive agent, styrene butadiene rubber (SBR) as an adhesive, and sodium carboxymethyl cellulose (CMC) as a thickener were mixed in a mass ratio of 95:1.5:3.1:0.4, and then added to deionized water. The mixture was stirred under a vacuum mixer until the system became uniform, yielding a second slurry with a solids content of 66%. S50: Preparation of third slurry

[0158] Barium titanate (dielectric constant of 3000 or more) with a Dv50 of 100 nm, styrene butadiene rubber (SBR) as an adhesive, and sodium carboxymethyl cellulose (CMC) as a dispersant were mixed in a mass ratio of 85:10:5, and then added to deionized water. The mixture was stirred with a vacuum mixer until the system became uniform, yielding a third slurry with a solid content of 40%. S60: Slurry application

[0159] The first slurry was applied to one surface of an 8 μm-thick copper foil negative electrode current collector and dried to form a first negative electrode film layer. The second slurry was applied to the first negative electrode film layer and dried to form a second negative electrode film layer. The third slurry was applied to the second negative electrode film layer and dried, after which a functional coating layer was formed. The above steps were then repeated on the other surface of the copper foil negative electrode current collector, and after further processes such as cold pressing, a negative electrode sheet was obtained. The thickness of the first negative electrode film layer on one side of the negative electrode current collector was 59 μm, the thickness of the second negative electrode film layer was 82 μm, and the thickness of the functional coating layer was 2 μm. The compaction density of the negative electrode sheet was 1.55 g / cm. 3 It was. Step (2): Preparation of the positive electrode sheet

[0160] LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2 (NCM811), conductive carbon black as a conductive agent, and polyvinylidene fluoride (PVDF) as an adhesive were mixed in a mass ratio of 98:1:1, and then added to NMP as a solvent. The mixture was stirred using a vacuum mixer until the system was uniform, yielding a positive electrode slurry with a solid content of 75%. The positive electrode slurry was then evenly applied to the two surfaces of 13 μm thick aluminum foil, dried at 90°C, and cold pressed to obtain a positive electrode sheet with a positive electrode film layer thickness of 114 μm on one side of the positive electrode current collector. Step (3): Preparation of the electrolyte

[0161] In an environment with a water content of less than 10 ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then thoroughly dried LiPF6 was dissolved in the organic solvent to prepare an electrolyte solution with a concentration of 1 mol / L. Step (4): Preparation of separator

[0162] A porous polyethylene film was used as the separator. Step (5): Preparation of secondary battery

[0163] The positive electrode sheet, separator, and negative electrode sheet were stacked in this order and wound up to obtain an electrode assembly. The electrode assembly was placed in a packaging and dried, after which an electrolyte solution was injected, and the secondary battery was obtained through processes such as vacuum sealing, standing, chemical formation, and shaping. Examples 1-2 to 1-7

[0164] A secondary battery was prepared in the same manner as in Example 1-1, except that the coating thickness of the third slurry in the preparation of the negative electrode sheet and the thickness of the resulting functional coating layer were different. See Table 1 for specific parameters. Comparative Example 1-1

[0165] A secondary battery was prepared in the same manner as in Example 1-1, except that the third slurry was not applied in preparing the negative electrode sheet. Comparative Example 1-2

[0166] The secondary battery was prepared in a similar manner to Example 1-1, except that the third slurry was not applied to the negative electrode sheet. Instead, the second slurry was applied to one surface of an 8-μm-thick copper foil negative electrode current collector and dried to form a first negative electrode film layer. The first slurry was then applied to the first negative electrode film layer and dried to form a second negative electrode film layer. The same steps were then repeated on the other surface of the copper foil negative electrode current collector, followed by cold pressing and other processes to obtain a negative electrode sheet. The thickness of the first negative electrode film layer on one side of the negative electrode current collector was 82 μm, and the thickness of the second negative electrode film layer was 59 μm. Comparative Examples 1-3

[0167] A secondary battery was prepared in the same manner as in Example 1-1, except that the second and third slurries were not applied in preparing the negative electrode sheet.

[0168] The first slurry was applied to both surfaces of an 8 μm-thick copper foil negative electrode current collector, followed by drying, cold pressing, and other processes to obtain a negative electrode sheet. The thickness of the negative electrode film layer on one side of the negative electrode current collector was 141 μm. Comparative Examples 1-4

[0169] A secondary battery was prepared in the same manner as in Example 1-1, except that the first slurry and the third slurry were not applied in preparing the negative electrode sheet.

[0170] The second slurry was applied to both surfaces of an 8 μm-thick copper foil negative electrode current collector, followed by drying and cold pressing to obtain a negative electrode sheet. The thickness of the negative electrode film layer on one side of the negative electrode current collector was 141 μm. Comparative Examples 1-5

[0171] A secondary battery was prepared in a similar manner to Example 1-1, except that the negative electrode sheet preparation process was different.

[0172] S10 to S20 are the same as in Example 1-1. S30: Preparation of slurry

[0173] The artificial graphite and hard carbon prepared above, conductive carbon black as a conductive agent, styrene butadiene rubber (SBR) as an adhesive, and sodium carboxymethyl cellulose (CMC) as a thickener were mixed in a mass ratio of 40:55:1.5:3.1:0.4, and then added to deionized water. The mixture was stirred with a vacuum mixer until the system became uniform, yielding a slurry with a solid content of 66%. S40: Slurry application

[0174] The slurry was applied to both surfaces of an 8 μm thick copper foil negative electrode current collector, and after drying and cold pressing, a negative electrode sheet was obtained. The thickness of the negative electrode film layer on one side of the negative electrode current collector was 141 μm. Comparative Examples 1-6

[0175] A secondary battery was prepared in a similar manner to Example 1-1, except that the negative electrode sheet preparation process was different.

[0176] S10 to S30 are the same as in the first embodiment. S40: Preparation of second slurry

[0177] The hard carbon prepared above, barium titanate (Dv50 of 100 nm, dielectric constant of 3000 or more), conductive carbon black as a conductive agent, styrene butadiene rubber (SBR) as an adhesive, and sodium carboxymethyl cellulose (CMC) as a thickener were mixed in a mass ratio of 93:1:1.5:3.1:0.4, and then added to deionized water. The mixture was stirred with a vacuum mixer until the system was uniform, yielding a second slurry with a solids content of 66%. S50: Slurry application

[0178] The first slurry was applied to one surface of an 8-μm-thick copper foil negative electrode current collector and dried to form a first negative electrode film layer. The second slurry was then applied to the first negative electrode film layer and dried to form a second negative electrode film layer. The above steps were then repeated on the other surface of the copper foil negative electrode current collector. After further cold pressing and other processes, a negative electrode sheet was obtained. The first negative electrode film layer on one side of the negative electrode current collector was 59 μm thick, and the second negative electrode film layer was 82 μm thick. Test part (1) Maximum charging rate test

[0179] The secondary battery prepared above was fully discharged at a 0.33C rate at 25°C, fully charged at a 0.33C rate, and then allowed to stand for 5 minutes. The discharge capacity obtained when the fully charged secondary battery was fully discharged at a 0.33C rate was designated C0, the actual capacity of the secondary battery at a 0.33C rate. The secondary battery was fully charged at a rate of xC0 (a gradient charge rate, e.g., 1C0, 1.1C0, 1.2C0, 1.3C0, 1.4C0, etc.), allowed to stand for 5 minutes, and then fully discharged at 1C. After 10 cycles in this manner, the secondary battery was fully charged at 1C. The secondary battery was disassembled and the lithium deposition on the surface of the negative electrode sheet was observed. If no lithium was deposited on the surface of the negative electrode sheet, the charge rate was increased and the measurement was repeated until lithium was deposited on the surface of the negative electrode sheet. The maximum charge rate at which no lithium was deposited on the surface of the negative electrode sheet was recorded. The higher the maximum charge rate of the secondary battery, the better the high-rate charging capability. (2) Energy density test

[0180] The secondary battery prepared above was fully charged at a rate of 0.33 C at 25°C and fully discharged at a rate of 0.33 C three times, and then the discharge energy of the secondary battery was recorded. Energy density of secondary battery = discharge energy of secondary battery / mass of secondary battery. In each example and comparative example of the present application, the energy density of the secondary battery prepared in Comparative Example 1-1 is set to 100%, and the energy densities of the secondary batteries of the other examples and comparative examples are shown. (3) Cycle life test

[0181] The secondary battery prepared above was fully discharged at a 1C rate at 25°C, fully charged at a 1C rate, and then allowed to stand for 5 minutes before being fully discharged at a 1C rate. The discharge capacity C0 obtained when the fully charged secondary battery was fully discharged at a 1C rate was defined as the initial capacity of the secondary battery. The secondary battery was fully charged at a 3C rate and fully discharged at a 1C rate, and a charge-discharge cycle test was conducted. The discharge capacity after each cycle was recorded until the secondary battery's discharge capacity decayed to 80% of its initial capacity. The number of cycles at this point indicates the cycle life of the secondary battery. It is expected that the higher the cycle count of a secondary battery, the longer its cycle life will be.

[0182] [Table 1]

[0183] Summarizing the test results of Examples 1-1 to 1-7 and Comparative Examples 1-1 to 1-6, it was found that secondary batteries using negative electrode sheets with the three-layer structure design of the present invention simultaneously exhibited a high charge rate and a long cycle life. In Comparative Example 1-1, a graphite layer and a hard carbon layer were sequentially formed on the surface of the negative electrode current collector, but a functional coating layer was not formed. This made it easy for lithium to precipitate during the long-term charge / discharge cycle of the secondary battery, affecting the cycle life of the secondary battery. In Comparative Example 1-2, a hard carbon layer and a graphite layer were sequentially formed on the surface of the negative electrode current collector, but a functional coating layer was not formed. This made it difficult for the secondary battery to achieve both a high charge rate and a long cycle life. In Comparative Example 1-3, a graphite layer was formed only on the surface of the negative electrode current collector, making it difficult for the secondary battery to achieve a high charge rate. In Comparative Example 1-4, a hard carbon layer was provided only on the surface of the negative electrode current collector, allowing the secondary battery to have a high charge rate but difficulty in achieving a long cycle life. In Comparative Example 1-5, a mixed layer of graphite and hard carbon was provided on the surface of the negative electrode current collector, making it difficult for the secondary battery to achieve both a high charge rate and a long cycle life. Compared to Comparative Example 1-1, the hard carbon layer of Comparative Example 1-6 also contains barium titanate, but barium titanate has a poor effect in inhibiting the continued growth of lithium dendrites in the direction perpendicular to the electrode sheet, making it difficult to significantly improve the charge rate and cycle life of the secondary battery.

[0184] Summarizing the test results of Examples 1-1 to 1-7, it was found that the maximum charge rate and cycle life of the secondary battery both increased with an increase in the thickness of the functional coating layer. A possible reason for this is that the functional coating layer increases the reverse electric field strength, which increases the effect of suppressing the continued growth of lithium dendrites in the direction perpendicular to the electrode sheet, thereby improving the high-rate charge capacity and cycle performance of the secondary battery.

[0185] To summarize the test results of Examples 1-1 to 1-7, when H1 / (H2+H3) is smaller than 0.01, the functional coating layer is set to be small relative to the total thickness of the first negative electrode film layer and the second negative electrode film layer, and the effect of inhibiting the continued growth of lithium dendrites in the direction perpendicular to the electrode sheet is not obvious, and therefore the effect of improving the maximum charging rate of the secondary battery is not obvious.

[0186] Summarizing the test results of Examples 1-6 and 1-7, when H1 is greater than 10, the functional coating layer does not sustainably increase its inhibitory effect on the continued growth of lithium dendrites in the direction perpendicular to the electrode sheet, and the maximum charge rate of the secondary battery does not sustainably increase. At the same time, the functional coating layer does not have electrochemical activity and cannot contribute to capacity, so when its thickness exceeds 10 μm, the energy density of the secondary battery is significantly reduced.

[0187] Next, the inventors investigated the effect of the mass percentage of the ferroelectric material in the functional coating layer on the performance of the secondary battery. The secondary batteries of Examples 2-1 to 2-6 were prepared in a similar manner to Example 1-2, except for the mass percentage of the ferroelectric material in the functional coating layer.

[0188] [Table 2]

[0189] Summarizing the test results for Examples 1-2, 2-1 to 2-6, it was found that when the thickness of the functional coating layer was fixed at 4 μm, the maximum charge rate and cycle life of the secondary battery both increased as the mass percentage of the ferroelectric material in the functional coating layer increased. A possible reason for this is that in this case, the reverse electric field strength of the functional coating layer increased, which in turn increased the effect of suppressing the continued growth of lithium dendrites in the direction perpendicular to the electrode sheet, thereby improving both the high-rate charge capacity and cycle performance of the secondary battery.

[0190] To summarize the test results of Comparative Example 1-1 and Example 2-1, it was found that when the mass percentage of the ferroelectric material in the functional coating layer is small, the functional coating layer obtained thereby does not have a clear effect of inhibiting the continued growth of lithium dendrites in the direction perpendicular to the electrode sheet, and therefore does not have a clear effect of improving the maximum charging rate of the secondary battery.

[0191] Summarizing the test results of Examples 2-5 and 2-6, it was found that when the mass percentage of the ferroelectric material in the functional coating layer exceeded 95%, the cycle performance of the secondary battery was significantly deteriorated. The possible reason for this is that the adhesive content in the functional coating layer was too low, which could cause the functional coating layer to peel off from the surface of the second negative electrode film layer during long-term charge-discharge cycles of the secondary battery.

[0192] Next, the inventors investigated the influence of the volume average particle diameter Dv50 of the ferroelectric material in the functional coating layer on the performance of the secondary battery. The secondary batteries of Examples 3-1 to 3-6 were prepared in a similar manner to Example 1-2, except that the volume average particle diameter Dv50 of the ferroelectric material in the functional coating layer was different.

[0193] [Table 3]

[0194] Summarizing the test results of Examples 1-2 and 3-1 to 3-6, it was found that when the thickness of the functional coating layer was fixed at 4 μm and the mass percentage of the ferroelectric material was fixed at 85%, the maximum charge rate and cycle life of the secondary battery both decreased as the volume average particle diameter Dv50 of the ferroelectric material increased. A possible reason for this is that as the volume average particle diameter Dv50 of the ferroelectric material increased, the interference of the reverse electric field increased, which reduced the effectiveness of suppressing the continued growth of lithium dendrites in the direction perpendicular to the electrode sheet, resulting in a decrease in the maximum charge rate and cycle life of the secondary battery.

[0195] Next, the inventors investigated the influence of the thickness of the first negative electrode film layer and the thickness of the second negative electrode film layer on the performance of the secondary battery. The secondary batteries of Examples 4-1 to 4-7 were prepared in a similar manner to Example 1-2, except that the thicknesses of the first negative electrode film layer and the second negative electrode film layer were different.

[0196] [Table 4]

[0197] Summarizing the test results of Examples 4-1 to 4-7, it was found that the maximum charge rate of the secondary battery increases when the thickness of the second negative electrode film layer increases.

[0198] Summarizing the test results of Examples 4-1 to 4-7, it was found that when the thickness H2 μm of the second anode film layer and the thickness H3 μm of the first anode film layer satisfy the condition that H2 / H3 is between 0.25 and 4, the secondary battery can achieve both a high maximum charge rate and a long cycle life. When H2 / H3 is less than 0.25, the second anode film layer is thin, and the effect of improving the maximum charge rate of the secondary battery is not clear. When H2 / H3 is greater than 4, the second anode film layer is thick, and the cycle performance of the hard carbon itself is poor, resulting in poor cycle performance of the secondary battery. Summarizing the test results of Examples 4-7 and Comparative Examples 1-4, it was found that when H2 / H3 is greater than 4, the improvement in cycle life of the secondary battery is not clear.

[0199] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are merely illustrative, and any embodiment that has substantially the same configuration as the technical idea within the technical scope of the present application and that achieves similar effects is included within the technical scope of the present application. In addition, various modifications that a person skilled in the art can conceive of can be applied to the embodiments, and other forms constructed by combining some of the components of the embodiments are also included within the scope of the present application, as long as they do not deviate from the spirit of the present application.

Claims

1. A negative electrode sheet, a negative electrode current collector; a first negative electrode film layer, a second negative electrode film layer, and a functional coating layer, which are sequentially provided on at least one surface of the negative electrode current collector; the functional coating layer comprises a ferroelectric material; the second negative electrode film layer includes a second negative electrode active material including hard carbon, the first negative electrode film layer includes a first negative electrode active material including graphite; The thickness of the functional coating layer is H 1 μm, the thickness of the second negative electrode film layer is H 2 μm, the thickness of the first negative electrode film layer is H 3 μm, and the negative electrode sheet satisfies the following: H 1 is 1 to 10, H 1 / (H 2 +H 3 ) is 0.01 to 0.15, and H 2 / H 3 is 0.25 to 4; the mass percentage of the ferroelectric material in the functional coating layer is W1, and W1 is 70% to 95% based on the total mass of the functional coating layer; Negative electrode sheet.

2. The negative electrode sheet is H １ / (H ２ +H ３ 2. The negative electrode sheet according to claim 1, wherein the value of (a) is 0.01 to 0.

08.

3. H １ The negative electrode sheet according to claim 1, wherein is 2 to 10.

4. The volume average particle diameter Dv50 of the ferroelectric material is d １ μm, and d １ The negative electrode sheet according to claim 1 , wherein is 1 or less.

5. The volume average particle diameter Dv50 of the second negative electrode active material is d ２ μm, and the volume average particle diameter Dv50 of the first negative electrode active material is d ３ μm, and d ２ / d ３ The negative electrode sheet according to claim 1, wherein is 0.1 to 1.

6. A negative electrode sheet as described in claim 1, wherein W1 is 80% to 95%.

7. The mass percentage of hard carbon in the second negative electrode film layer is W2, and W2 is 68% or more based on the total mass of the second negative electrode film layer; and / or 2. The negative electrode sheet according to claim 1, wherein a mass percentage of graphite in the first negative electrode film layer is W3, and W3 is 78% or more based on the total mass of the first negative electrode film layer.

8. The negative electrode sheet according to claim 1, wherein the second negative electrode active material satisfies at least one of the following conditions (1) to (5): (1) The volume average particle diameter Dv50 of the second negative electrode active material is d ２ μm, and d ２ is 3 to 11. (2) The particle size distribution index (Dv90-Dv10) / Dv50 of the second negative electrode active material is α １ and α １ is 0.6 to 5. (3) The specific surface area of ​​the second negative electrode active material is 3 m ２ / g~7m ２ / g. (4) The powder compaction density of the second negative electrode active material at 20,000 N is 0.9 g / cm ３ ~1.3 g / cm ３ is. (5) The second negative electrode active material includes primary particles, secondary particles, or a combination thereof.

9. The negative electrode sheet according to claim 1, wherein the first negative electrode active material satisfies at least one of the following conditions (1) to (7): (1) The volume average particle diameter Dv50 of the first negative electrode active material is d ３ μm, and d ３ is 9 to 18. (2) The particle size distribution index (Dv90-Dv10) / Dv50 of the first negative electrode active material is α ２ and α ２ is 0.2 to 5. (3) The specific surface area of ​​the first negative electrode active material is 0.6 m ２ / g to 1.5m ２ / g. (4) The powder compaction density of the first negative electrode active material at 20,000 N is 1.4 g / cm ３ ~1.85g / cm ３ is. (5) The degree of graphitization of the first negative electrode active material is 91% to 95%. (6) The first negative electrode active material includes artificial graphite, natural graphite, or a combination thereof. (7) The first negative electrode active material includes primary particles, secondary particles, or a combination thereof.

10. The negative electrode sheet according to claim 1 , wherein the ferroelectric material has a dielectric constant of 50 or more.

11. The negative electrode sheet according to claim 1 , wherein the ferroelectric material comprises one or a combination of a plurality of ferroelectric materials selected from inorganic ferroelectric materials and organic ferroelectric materials.

12. The inorganic ferroelectric material comprises one or a combination of a plurality of compounds selected from perovskite structure oxides, tungsten bronze type compounds, bismuth oxide type layer structure compounds, lithium niobate, lithium tantalate, lead metaniobate, and lead barium lithium niobate; The organic ferroelectric material may include one or a combination of a plurality of materials selected from a homopolymer or copolymer of vinylidene fluoride, 2,5-dibromo-3,6-dihydroxy-p-benzoquinone, phenazine-chloranilic acid, and croconic acid. The negative electrode sheet according to claim 11.

13. The negative electrode sheet according to claim 1 , wherein the functional coating layer further comprises an adhesive and / or a dispersant.

14. A secondary battery comprising the negative electrode sheet according to claim 1.

15. A battery module comprising the secondary battery according to claim 14.

16. A battery pack comprising one of the secondary battery according to claim 14 and the battery module according to claim 15.

17. A power consuming device comprising at least one of the secondary battery according to claim 14, the battery module according to claim 15, and the battery pack according to claim 16.

Images