Negative electrode sheet and battery

Abstract

A negative electrode sheet and a battery. The negative electrode sheet comprises a current collector (2) and an active material layer (1) provided on the surface of the current collector (2). The active material layer (1) comprises a silicon carbon material (11) and a first binder. The first binder contains a first group and a second group. The first group comprises a carboxyl group, and the second group comprises a cyano group and / or an amide group.

Description

Negative electrode and battery Technical Field

[0001] The present invention relates to the field of electrochemical device energy storage, and in particular to a negative electrode sheet and a battery. Background Art

[0002] Currently, silicon-doped anodes have the advantage of higher energy density compared to pure graphite and are gradually gaining widespread application. However, related technologies have drawbacks such as large thickness expansion and rapid capacity decay, which need to be addressed urgently.

[0003] Summary of the Invention

[0004] The present invention provides a negative electrode sheet and a battery, which at least solve the problems of large expansion rate and rapid capacity decay of silicon-doped negative electrodes in the prior art.

[0005] In one aspect of the present invention, a negative electrode sheet is provided, comprising a current collector and an active material layer disposed on the surface of the current collector, wherein the active material layer comprises a silicon-carbon material and a first binder, wherein the first binder comprises a first group and a second group, wherein the first group comprises a carboxyl group, and the second group comprises a cyano group and / or an amide group.

[0006] According to one embodiment of the present invention, the silicon-carbon material includes porous carbon and silicon material present in the pores of the porous carbon.

[0007] According to one embodiment of the present invention, the mass ratio of the porous carbon to the silicon material is 7:3 to 3:7; and / or the average pore diameter of the porous carbon is less than or equal to 20 nm.

[0008] According to one embodiment of the present invention, the particle size Dv50 of the silicon-carbon material is 5 μm to 15 μm; and / or the specific surface area of ​​the silicon-carbon material is less than or equal to 15 m 2 / g.

[0009] According to one embodiment of the present invention, the mass percentage of the silicon-carbon material in the active material layer is 3% to 20%, and / or the surface density of the negative electrode active material layer is 5 mg / cm 2 ~8mg / cm 2 .

[0010] According to one embodiment of the present invention, the carboxyl group includes a carboxylic acid group and / or a carboxylate group, and the carboxylate group includes one or more of -COOLi, -COONa, -COOK, and -COONH4; and / or, in the first binder, the molar number of the first group is M1, the molar number of the second group is M2, and 0.4≤M1 / (M1+M2)≤0.95.

[0011] According to one embodiment of the present invention, the weight average molecular weight of the first binder is 3,000 to 2,000,000.

[0012] According to one embodiment of the present invention, the mass percentage of the first binder in the active material layer is 1% to 10%.

[0013] According to one embodiment of the present invention, the active material layer also includes a carbon-based material, wherein the carbon-based material includes one or more of artificial graphite, natural graphite, mesophase carbon microbeads, soft carbon, hard carbon, carbon black, acetylene black, carbon fiber, single-walled carbon tubes, and multi-walled carbon tubes; and / or, in the active material layer, the mass percentage of the carbon-based material is 60% to 96%.

[0014] Another aspect of the present invention provides a battery comprising the above-mentioned negative electrode sheet.

[0015] In the present invention, the active material layer includes a silicon-carbon material, which has a higher capacity. At the same time, a first binder containing a first group and a second group is introduced, which has a strong bonding effect with the silicon-carbon material and can inhibit the silicon-carbon material from generating gaps through the expansion-contraction process during the battery cycle, thereby reducing the expansion rate of the negative electrode sheet, and then reducing the expansion rate of the battery, and at the same time can reduce the battery capacity attenuation rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG1 is a schematic structural diagram of a negative electrode sheet according to an embodiment of the present invention;

[0017] FIG2 is a pore size distribution curve of the porous carbon in Example 1;

[0018] FIG3 is a SEM image of the silicon-carbon material in Example 1;

[0019] FIG4 is a cross-sectional EDS line scan of the silicon-carbon material in Example 1.

[0020] Explanation of reference numerals: 1: active material layer; 2: current collector; 11: silicon-carbon material. DETAILED DESCRIPTION

[0021] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below. The specific embodiments listed below are only for describing the principles and features of the present invention. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts are within the scope of protection of the present invention.

[0022] In the description of the present invention, terms such as "first", "second", and "third" are used only for descriptive purposes, such as distinguishing between components to more clearly illustrate / explain the technical solution, and cannot be understood as indicating or implying the number of the indicated technical features or the order with substantial significance.

[0023] At present, negative electrode silicon-doped batteries generally have problems such as large expansion rate and poor capacity retention rate, which need to be solved urgently.

[0024] The electrode sheet is an important component of the battery. The negative electrode sheet with silicon materials as the active material has a relatively high capacity, but it is more prone to excessive expansion (during the battery cycle, the expansion rate of silicon material particles is large, and the expansion and contraction process produces excessive gaps), resulting in severe expansion of the silicon-doped negative electrode (mainly expansion in the thickness direction), affecting the battery's cyclability performance.

[0025] For example, the reversible lithium storage capacity of graphite is 372 mAh / g, while the reversible lithium storage capacity of silicon at room temperature is approximately 3600 mAh / g. Therefore, the use of silicon-based materials can significantly improve the energy density of lithium-ion batteries. However, the volume expansion rate of silicon when fully embedded with lithium typically exceeds 300%. This makes the active material layer using silicon-based materials as the active material fluffy, the conductive contact between the active material particles deteriorates, the material becomes more difficult to deintercalate lithium, and the precipitation of metallic lithium occurs easily, resulting in rapid battery capacity decay and a sharp increase in thickness expansion rate.

[0026] In view of the above problems, an embodiment of the present invention provides a negative electrode sheet, as shown in Figure 1, the negative electrode sheet includes a current collector 2 and an active material layer 1 arranged on the surface of the current collector 2, the active material layer 1 includes a silicon-carbon material 11 and a first binder, the first binder includes a first group and a second group, the first group includes a carboxyl group, and the second group includes a cyano group and / or an amide group.

[0027] In this way, silicon-carbon material is used as an active material with higher capacity. At the same time, a first binder comprising a first group and a second group is introduced, which has a strong bonding effect with the silicon-carbon material and can inhibit the silicon-carbon material from generating excessive voids when lithium is inserted and expanded, thereby effectively inhibiting the expansion of the negative electrode sheet, reducing the expansion rate of the negative electrode sheet, and then reducing the expansion rate of the battery. At the same time, it can reduce the battery capacity decay rate and improve the battery cycle performance. Studies have shown that the capacity retention rate of the battery after 200 cycles (or 200 times) is more than 95.5%, and the thickness expansion rate is less than 6%. The capacity retention rate after 400 cycles is more than 91.5%, and the thickness expansion rate is not higher than 8.1%. The capacity retention rate after 600 cycles is more than 87.5%, and the thickness expansion rate is less than 10%. The capacity retention rate after 800 cycles is more than 81%, and the thickness expansion rate is not higher than 11.1%, showing high energy density and high capacity retention rate.

[0028] Generally, the silicon-carbon material 11 is in a granular form, and its macroscopic appearance is powder, and serves as the negative electrode active material of the active material layer 1 .

[0029] In some embodiments, the silicon-carbon material 11 may specifically include a carbon material and a silicon material composited with a carbon material, wherein the carbon material may specifically be a porous carbon having a porous structure, that is, it has pores, and the silicon material exists in the pores of the porous carbon, that is, the silicon material fills the pores of the porous carbon, and specifically can fill the pores of the porous carbon (that is, all the pores of the porous carbon are basically filled with silicon material), which can prevent the electrolyte from penetrating into the pores of the porous carbon and reduce the occurrence of side reactions.

[0030] Specifically, the pores of the porous carbon may include mesopores and / or micropores. In some embodiments, the average pore size of the porous carbon may be less than or equal to 20 nm, specifically 0.5 nm to 20 nm, such as 0.5 nm, 1 nm, 3 nm, 5 nm, 8 nm, 10 nm, 12 nm, 15 nm, 18 nm, 20 nm or a range consisting of any two of them. According to the inventor's research, if the pore size of the porous carbon is too large (greater than 20 nm), the size of the silicon material is large, which will affect the structural stability of the silicon-carbon material 11 to a certain extent. Therefore, controlling the pore size of the porous carbon to be less than or equal to 20 nm can further improve the structural stability of the silicon-carbon material 11 and avoid the occurrence of side reactions and expansion. At the same time, the pore size of the porous carbon is less than or equal to 20 nm, which can have a good bonding effect with the first binder and effectively alleviate the expansion of silicon.

[0031] The embodiment of the present invention can measure the average pore size of the porous carbon in the silicon-carbon material 11 by conventional methods in the art. For example, the silicon-carbon material can be immersed in an alkaline aqueous solution (such as a sodium hydroxide solution) with a pH of 9-12 to react until no bubbles are generated to remove the silicon therein; then it is dried to obtain a porous carbon sample; and then the specific surface area and total pore volume of the porous carbon sample are measured by nitrogen adsorption method, and the average pore size of the porous carbon is calculated based on the measured specific surface area and total pore volume of the porous carbon sample.

[0032] In general, porous carbon has a large specific surface area, usually greater than 100m 2 / g, and when the silicon material is filled in the pores, the specific surface area of ​​the formed silicon-carbon material 11 is usually significantly reduced relative to the specific surface area of ​​the porous carbon.

[0033] Studies have shown that when the specific surface area of ​​silicon carbon material 11 is too large (greater than 15m 2 / g), the amount of side reactions on the surface of the silicon-carbon material 11 is large. Therefore, it is generally preferred that the specific surface area of ​​the silicon-carbon material 11 is less than or equal to 15m 2 / g, specifically 1m2 / g~15m 2 / g, for example 1m 2 / g, 2m 2 / g、3m 2 / g、4m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g、8m 2 / g、9m 2 / g、10m 2 / g、11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g、15m 2 / g or a range consisting of any two thereof.

[0034] During specific implementation, the specific surface area of ​​the silicon-carbon material 11 can be measured by conventional methods in the art, such as nitrogen adsorption method.

[0035] In addition, the silicon material in the silicon-carbon material 11 is filled in the pores of the porous carbon, and the particle size of the silicon-carbon material 11 is substantially consistent with that of the porous carbon.

[0036] In some embodiments, the particle size Dv50 of the silicon-carbon material 11 can be 5 μm to 15 μm, for example, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 15 μm, or a range consisting of any two thereof. Dv50 is the median particle size of the silicon-carbon material 11 based on the volume density distribution, which represents the particle size at which the volume accumulation of the silicon-carbon material particles reaches 50% from the small particle size side in the volume-based particle size distribution. Relatively speaking, if the particle size of the silicon-carbon material 11 is too small (Dv50 < 5 μm), the surface side reaction of the silicon-carbon material 11 is large, while if the particle size of the silicon-carbon material 11 is too large (Dv50 > 15 μm), the volume expansion of the silicon-carbon material 11 is excessive. Therefore, controlling the particle size Dv50 of the silicon-carbon material 11 to 5 μm to 15 μm can further improve the performance of the negative electrode sheet and reduce its expansion rate.

[0037] In addition, in the silicon-carbon material 11, the mass ratio of the carbon material (porous carbon) to the silicon material can be 7:3 to 3:7, for example, 7:3, 6:4, 5:5, 4:6, 3:7 or a range consisting of any two thereof. Relatively speaking, if the mass ratio of the carbon material to the silicon material in the silicon-carbon material 11 is too large (greater than 7:3), the first coulombic efficiency of the silicon-carbon material 11 is low. If the mass ratio of the carbon material to the silicon material is too small (less than 3:7), the silicon content in the silicon-carbon material 11 is too high, affecting the structural stability of the silicon-carbon material 11. Therefore, controlling the mass ratio of the carbon material to the silicon material to be 7:3 to 3:7 can further improve its first coulombic efficiency and structural stability, thereby facilitating the silicon-carbon material 11 to function as an electrode active material.

[0038] The silicon-carbon material 11 can be prepared by chemical vapor deposition. In some specific embodiments, the preparation process of the silicon-carbon material 11 may include: placing a porous carbon material in a vapor deposition furnace, introducing a mixed gas of inert gas and silane into the mixed gas, then heating the material to 400°C to 600°C for reaction, and the reaction time is 4h to 20h. During the reaction, silane is cracked to form silicon element which is deposited in the pores filled in the porous carbon material to form the silicon-carbon material 11. The porous carbon material used can be commercially available or homemade, such as porous carbon purchased from Aladdin; the volume ratio of the inert gas to the silane in the mixed gas can be (10-1):1, and the inert gas includes, for example, argon; the silane used can include one or more of monosilane, disilane, and trichlorosilane.

[0039] In addition, the current collector 2 has two opposite surfaces (i.e., the front and back surfaces), one of which may have the active material layer 1, or both surfaces may have the active material layer 1. In specific implementation, it can be set as needed.

[0040] In some embodiments, the mass percentage of the silicon-carbon material 11 in the active material layer 1 can be 3% to 20% (i.e., the mass of the silicon-carbon material 11 in the active material layer 1 accounts for 3% to 20% of the total mass of the active material layer 1), for example, 3%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, or any combination thereof. Relatively speaking, if the content of the silicon-carbon material 11 is too low (less than 3%), it will affect the capacity of the negative electrode sheet to a certain extent, while if the content of the silicon-carbon material 11 is too high (greater than 50%), it will also increase the expansion rate of the negative electrode sheet to a certain extent. Therefore, controlling the content of the silicon-carbon material 11 to 3% to 20% can enable the active material layer 1 to have a higher specific capacity, thereby improving the capacity of the negative electrode sheet while also reducing the expansion rate of the negative electrode sheet.

[0041] The introduction of the first binder into the active material layer 1 can improve the bonding between the components in the negative electrode active material layer 1, as well as the bonding between the negative electrode active material layer 1 and the current collector. The first binder has a strong bonding effect with the above-mentioned silicon-carbon material 11, which can inhibit the silicon-carbon material 11 from generating excessive voids when lithium is inserted and expanded, thereby reducing the expansion rate of the negative electrode sheet and the battery.

[0042] Specifically, the carboxyl group in the first binder may include a carboxylic acid group (—COOH) and / or a carboxylate group. The carboxylate group, for example, includes one or more of —COOLi, —COONa, —COOK, and —COONH 4 .

[0043] In addition, the second group in the first binder includes a cyano group (-CN) and / or an amide group, and the amide group is, for example, -CONH2.

[0044] Specifically, the first binder can be formed by polymerizing a monomer containing a first group and a monomer containing a second group. The monomer containing the first group and the monomer containing the second group respectively contain unsaturated bonds, specifically double bonds, so that a polymerization reaction can occur through the unsaturated bonds to generate the first binder containing the first group and the second group.

[0045] Illustratively, monomers containing a first group include CH2=CHCOOH (the first group is -COOH) and / or CH2=CHCOOM, where M is a metal ion, for example, M is Li (the first group is -COOLi), Na (the first group is -COONa), K (the first group is -COOK) or NH4 (the first group is -COONH4), etc., and monomers containing a second group include CH2=CHCN (the second group is -CN) and / or CH2=CHCONH2 (the second group is CONH2).

[0046] In some embodiments, in the above-mentioned first binder, the molar number of the first group is M1, the molar number of the second group is M2, 0.4≤M1 / (M1+M2)≤0.95, and correspondingly, 0.05≤M2 / (M1+M2)≤0.6. In this way, the bonding effect of the first binder on components such as the silicon-carbon material 11 in the active material layer 1 can be improved, and the preparation of the negative electrode sheet can be facilitated.

[0047] Illustratively, M1 / (M1+M2) may be 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or a range consisting of any two thereof.

[0048] Specifically, the embodiment of the present invention can prepare the negative electrode sheet by a conventional coating method in the art, that is, a first binder, a silicon-carbon material 11, and other materials used to form the active material layer 1 are placed in a first solvent to form a slurry, and then the slurry is applied to the current collector 2. The negative electrode sheet is then prepared through processes such as drying, rolling, and slitting (i.e., shearing into negative electrode sheets of a preset shape). Studies have shown that in the preparation process of the negative electrode sheet, when M1 / (M1+M2) is less than 0.4, the slurry applied to the current collector is not easy to dry, which increases the occurrence of side reactions. When M1 / (M1+M2) is greater than 0.95, the first binder has a weaker bonding effect on the silicon-carbon material 11, and the active material layer 1 formed after the slurry applied to the current collector 2 is dried has a greater hardness (i.e., is relatively hard), which is prone to powdering (coating powder falling off) and other phenomena during subsequent slitting and other processes. Therefore, controlling 0.4≤M1 / (M1+M2)≤0.95 can reduce the above problems and make the slurry stable and easy to process, thereby facilitating the preparation of the negative electrode sheet and improving the preparation yield and performance of the negative electrode sheet.

[0049] In specific implementation, the amount of the monomer containing the first group and the monomer containing the second group used in preparing the first binder can be adjusted to control the content of the first group and the second group in the formed first binder, thereby obtaining a first binder that meets the preset M1 / (M1+M2) value.

[0050] In addition, the weight-average molecular weight of the first binder can be 3,000 to 2,000,000 (i.e., 3,000 to 2,000,000), for example, 3,000, 5,000, 8,000, 10,000, 50,000, 100,000, 200,000, 300,000, 400,000, 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, 1,200,000, 1,500,000, 1,800,000, and 2,000,000. In this way, the first binder has both good dispersibility and adhesion (when the weight-average molecular weight of the first binder is less than 3,000, the cohesion between the molecules is weak and the adhesion effect is poor; when the weight-average molecular weight of the first binder is greater than 2,000,000, the molecules are severely entangled and the dispersion effect is poor).

[0051] In some embodiments, the mass percentage of the first binder in the active material layer 1 is 1% to 10% (i.e., the mass of the first binder accounts for 1% to 10% of the total mass of the active material layer 1), for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range consisting of any two thereof. Relatively speaking, if the content of the first binder is too low (less than 1%), the binding force between the material particles in the active material layer 1 is too weak, while if the content of the first binder is too high (greater than 10%), the conductivity of the negative electrode sheet is poor. Therefore, controlling the mass percentage of the first binder to 1% to 10% can take into account both the binding force between the material particles in the active material layer 1 and the conductivity of the negative electrode sheet.

[0052] In addition, the active material layer 1 may also include carbon-based materials, which may specifically include one or more of artificial graphite, natural graphite, mesocarbon microbeads, soft carbon, hard carbon, carbon black, acetylene black, carbon fiber, single-walled carbon tubes, and multi-walled carbon tubes.

[0053] In some specific embodiments, the carbon-based material may include a first carbon-based material and / or a second carbon-based material, the first carbon-based material including one or more of graphite (artificial graphite and / or natural graphite), mesophase carbon microspheres, soft carbon, and hard carbon, which can be used with silicon-carbon materials as active substances, that is, the active material layer can also include other active substances (i.e., the first carbon-based material) in addition to silicon-carbon materials, and the second carbon-based material including one or more of carbon black, acetylene black, carbon fiber, single-walled carbon tube, and multi-walled carbon tube, which mainly serve as a conductive agent for the active material layer. The second carbon-based material may specifically include zero-dimensional carbon-based materials (such as the above-mentioned carbon black and / or acetylene black, etc.) and / or one-dimensional carbon-based materials (such as the above-mentioned carbon fiber and / or single-walled carbon tube, etc.). The gram capacity of the above-mentioned graphite in the range of 0.005 to 0.7V can generally be 345 to 355 mAh / g, and the first coulomb efficiency can be 91% to 95%.

[0054] Generally, the mass percentage of the first carbon-based material in the active material layer 1 (the ratio of the mass of the first carbon-based material to the mass of the active material layer 1) can be greater than the mass percentage of the silicon-carbon material 11 (the ratio of the mass of the silicon-carbon material 11 to the mass of the active material layer 1), that is, the mass ratio of the first carbon-based material in the active material layer 1 to the silicon-carbon material 11 is greater than 1, and specifically can be (3 to 32):1, for example, 3:1, 5:1, 8:1, 10:1, 12:1, 15:1, 18:1, 20:1, 22:1, 25:1, 28:1, 30:1, 32:1 or a range consisting of any two of them.

[0055] In some embodiments, in the active material layer 1, the mass percentage of the carbon-based material can be 60% to 96% (that is, the mass of the carbon-based material accounts for 70% to 96% of the total mass of the active material layer 1), for example, 60%, 62%, 65%, 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 93%, 96% or a range composed of any two of them, which is beneficial to further cooperate with the silicon-carbon material 11 and other components in the active material layer 1 to improve the conductivity, stability and other properties of the electrode.

[0056] Among them, in the active material layer 1, the mass percentage of the first carbon-based material can be 60% to 96%, for example, 60%, 62%, 65%, 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 93%, 96% or a range consisting of any two of them, and the mass percentage of the second carbon-based material can be 0 to 10%, for example, 0, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range consisting of any two of them.

[0057] In addition, the surface density of the active material layer 1 can be 5 mg / cm 2 ~8mg / cm 2 , for example 5mg / cm 2 , 5.5mg / cm 2 , 6mg / cm 2 、6.5mg / cm 2 , 7mg / cm 2 , 7.5mg / cm 2 , 8mg / cm 2 Or a range consisting of any two thereof, but not limited thereto.

[0058] In some embodiments, the thickness of the active material layer 1 can be 15 μm to 60 μm, for example, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, or a range consisting of any two thereof. Relatively speaking, if the thickness of the active material layer 1 is too small (less than 15 μm), the capacity of the negative electrode sheet is low, and at the same time, during the preparation of the negative electrode sheet, undesirable phenomena such as coating scraping and deformation of the current collector during rolling are likely to occur. If the thickness of the active material layer 1 is too large (greater than 60 μm), the polarization phenomenon during battery charging and discharging is more severe, and the non-uniformity of lithium insertion and extraction is greater. Therefore, controlling the thickness of the active material layer 1 to 15 μm to 60 μm can further improve the performance of the negative electrode sheet and the battery.

[0059] Generally, the thickness of the active material layer 1 is greater than the maximum size of the material particles such as the silicon-carbon material 11 therein (i.e., the maximum particle size, which can specifically be the Dv99 of the material particles in the active material layer 1, where Dv99 represents the particle size from the small particle size side to the cumulative volume of 99%), so as to facilitate the coating process during the preparation of the negative electrode sheet, avoid scraping during the coating process, and avoid excessive pressure on the current collector 2 during rolling, as well as the resulting compression deformation of the current collector 2.

[0060] The above-mentioned current collector (negative electrode current collector) 2 may include copper foil, which may include pure copper foil formed only by copper metal or copper foil containing a first doping element. The first doping element may include carbon element and / or a first metal element. The first metal element may include, for example, one or more of nickel, tin, zinc, lead, silver, chromium, tungsten, iron, and cobalt.

[0061] In addition, the current collector 2 may further include a first coating material present on the surface of the copper foil. Specifically, the first coating material may be a first coating layer provided on the surface of the copper foil. The first coating material may include one or more of amorphous carbon, silver, and gold.

[0062] In addition, the thickness of the current collector 2 may be in the range of 3 μm to 15 μm, for example, 3 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, or any two thereof.

[0063] The negative electrode sheet of the embodiment of the present invention can be prepared by conventional methods in the field such as coating method. For example, the silicon-carbon material 11, the binder, the carbon-based material and the first solvent (such as water) are mixed to form a slurry; the slurry is coated on the surface of the current collector 2, sliced ​​after drying, and then rolled, slitting, and sheeting are carried out to obtain the negative electrode sheet.

[0064] In some embodiments, the preparation process of the negative electrode sheet may include: mixing the silicon-carbon material and at least part of the carbon-based material, then adding the first solvent and the first binder thereto, stirring evenly, and if there is any remaining carbon-based material, then adding the remaining carbon-based material, stirring evenly to obtain a slurry, and then applying the slurry to the surface of the current collector 2, drying and slicing, and then undergoing processes such as rolling, slitting, and sheeting to obtain a negative electrode sheet.

[0065] For example, in some specific embodiments, the carbon-based materials used include graphite, zero-dimensional carbon-based materials (such as carbon black, etc.), and one-dimensional carbon-based materials (such as single-walled carbon tubes, etc.). The preparation process of the negative electrode sheet can include mixing the silicon-carbon material, graphite and zero-dimensional carbon-based material, and then adding a first solvent and a first binder thereto, stirring evenly, and then adding the one-dimensional carbon-based material, stirring evenly to obtain a slurry; then applying the slurry to the surface of the current collector 2, drying and slicing, and then undergoing rolling, slitting, sheet making and other processes to obtain the negative electrode sheet.

[0066] The battery provided by the embodiment of the present invention includes the above-mentioned negative electrode sheet. The advantages of the battery over the prior art are the same as those of the above-mentioned negative electrode sheet, which will not be described in detail.

[0067] Specifically, the battery may include a lithium-ion battery.

[0068] Specifically, the above-mentioned battery also includes a positive electrode sheet, a diaphragm, an electrolyte and a shell. The diaphragm is located between the positive electrode sheet and the negative electrode sheet to separate / isolate the positive electrode sheet and the negative electrode sheet to prevent the positive electrode sheet and the negative electrode sheet from short circuiting. The positive electrode sheet, the diaphragm and the negative electrode sheet form a battery cell, which can be formed into a stacked battery cell by stacking the sheets or formed into a wound battery cell by winding. The shell is enclosed to form a cavity, the battery cell is located in the cavity, and the electrolyte exists in the cavity.

[0069] The battery of the embodiment of the present invention can be manufactured by conventional methods in the art. For example, after welding the positive electrode tab to the positive electrode sheet and the negative electrode tab to the negative electrode sheet, the positive electrode sheet, separator, and negative electrode sheet are stacked in sequence and then wound to form a wound battery cell. The battery cell is placed in a cavity enclosed by a casing and dried, and then an electrolyte is injected into the cavity. The battery is then manufactured through processes such as packaging, formation, secondary sealing, and sorting. The casing can include one or more of, but is not limited to, an aluminum-plastic film, an aluminum shell, and a steel shell.

[0070] Specifically, the negative electrode tab may include a nickel tab, and the positive electrode tab may include an aluminum tab.

[0071] Among them, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer arranged on the surface of the positive electrode current collector, and the positive electrode active material layer includes a positive electrode active material, a conductive agent and a second binder. Among them, the mass percentage of the positive electrode active material can be 90% to 99%, the mass percentage of the conductive agent can be 0.5% to 5%, and the mass percentage of the second binder can be 0.5% to 5%.

[0072] The positive electrode active material may include a lithium-containing active material, for example, one or more of lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium iron silicate, lithium cobalt oxide, nickel-cobalt-manganese ternary material, nickel-manganese binary material, cobalt-manganese binary material, nickel-cobalt binary material, lithium manganate, lithium nickelate and lithium-rich manganese-based material.

[0073] The conductive agent may be a conventional conductive material in the art, for example, one or more of conductive carbon black, acetylene black, carbon nanotubes, conductive graphite, and graphene.

[0074] The second binder can be a conventional bonding material in the art, for example, one or more of polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose (CMC-Na), polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0075] The positive electrode current collector has two opposite surfaces (ie, front and back surfaces), one of which may have a positive electrode active material layer, or both of which may have a positive electrode active material layer.

[0076] The positive electrode current collector may include aluminum foil, which may include pure aluminum foil formed only of aluminum metal or doped aluminum foil containing a second doping element, the second doping element may include silicon element and / or a second metal element, and the second metal element may include, for example, one or more of copper, magnesium, zinc, manganese, nickel, iron, titanium, chromium, and lithium.

[0077] In addition, the positive electrode current collector may further include a second coating present on the surface of the aluminum foil. Specifically, the second coating may be a second coating layer provided on the surface of the aluminum foil. The second coating may include one or more of amorphous carbon, silver, and gold.

[0078] The positive electrode sheet of the embodiment of the present invention can be prepared by conventional methods in the field such as coating method. For example, the positive electrode active material, binder, conductive agent and second solvent (such as N-methylpyrrolidone (NMP)) are mixed to prepare positive electrode slurry; the positive electrode slurry is coated on the surface of the current collector, sliced ​​after drying, and then rolled, slit, and sheeted to obtain the positive electrode sheet.

[0079] The diaphragm includes a polymer film, which can be a single-layer film formed by one or more polymers, or a composite film formed by stacking at least two sides of films. The polymer film can specifically include polyethylene and / or polypropylene, etc. For example, the above-mentioned diaphragm is, for example, a polyethylene film formed by polyethylene, etc.

[0080] In addition, the diaphragm may further include a third coating layer provided on the surface of the polymer film. The third coating layer may include a polymer and / or a ceramic material, for example, one or more of polyvinylidene fluoride, aluminum oxide, and boehmite.

[0081] The electrolyte includes a third solvent and a lithium salt, the third solvent includes an organic solvent, specifically an ester solvent, for example, one or more of ethylene carbonate (EC), propylene carbonate (PC), propyl propionate (PP), diethyl carbonate (DEC), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC); the lithium salt may include one or more of LiPF6, LiBF4, LiSbF6, LiClO4, LiCF3SO3, LiAlO4, LiAlCl4, Li(CF3SO2)2N, LiBOB and LiDFOB.

[0082] The present invention will be further described below with reference to specific examples and comparative examples.

[0083] Example 1

[0084] 1. Preparation of silicon-carbon materials

[0085] The porous carbon was purchased from commercial sources with a Dv50 of 7 μm and a specific surface area of ​​1289 m 2 / g. The pore size distribution curve of the porous carbon was measured using a pore analyzer TriStar II from Micromeritics Instrument Corp. based on the nitrogen adsorption method, as shown in Figure 2. From the pore size distribution curve, it can be seen that the porous carbon contains abundant micropores and mesopores, and the volume contained in the pores increases with decreasing pore diameter. The average pore diameter calculated by the BJH model is 2.4 nm.

[0086] Preparation of silicon-carbon material: 500 g of the above-mentioned porous carbon was placed in a vapor deposition furnace, and a mixed gas of argon and silane with a volume ratio of 8:2 was introduced. Then the temperature was raised to 500°C to decompose the silane into silicon and deposit it to fill the porous carbon. The reaction lasted for 10 hours to obtain the silicon-carbon material.

[0087] The SEM image of the silicon-carbon material measured using a scanning electron microscope (SEM) is shown in Figure 3. It can be seen that the silicon in the silicon-carbon material is filled in the pores of the porous carbon, so that the pores of the porous carbon in the silicon-carbon material are basically filled with silicon, and the pores of the porous carbon in the silicon-carbon material are basically not observed in the SEM image of the silicon-carbon material.

[0088] In addition, the silicon-carbon material particles were cut by argon ion milling to obtain a particle cross-section. The SEM image of the cross-section measured by SEM and the EDS line scan curve measured by X-ray energy dispersive spectrometer (EDS) are shown in Figure 4. From the EDS line scan curve, it can be seen that the C and Si elements in the silicon-carbon material particles are evenly distributed. This is because Si is filled in the pores of the porous carbon.

[0089] In addition, the mass ratio of porous carbon to silicon was measured to be 4.4:5.6; the particle size of the silicon-carbon material was Dv50 = 7 μm, and the specific surface area of ​​the silicon-carbon material was 5.2 m 2 / g.

[0090] 2. First binder

[0091] The first binder is formed by polymerization of CH2=CHCOOH and CH2=CHCN. In the first binder, the molar number of -COOH is M1 and the molar number of -CN is M2, satisfying M1 / (M1+M2)=0.8 and M2 / (M1+M2)=0.2; the weight average molecular weight of the first binder is 500,000.

[0092] 3. Preparation of negative electrode sheet

[0093] By weight, 85.86 parts of artificial graphite, 9.54 parts of the above silicon-carbon material and 0.5 parts of carbon black were fully mixed, and then deionized water and 4 parts of the above first binder were added thereto and stirred evenly. Then, 0.1 parts of single-walled carbon nanotubes were added and stirred evenly to obtain a negative electrode slurry; the negative electrode slurry was evenly coated on the front and back surfaces of a pure copper foil with a thickness of 8 μm, and dried to form a surface density of 6.5 mg / cm 2 Anode coating;

[0094] Then the copper foil with the negative electrode coating was transferred to a vacuum oven at 100°C and dried for 10 hours. After cooling naturally, the copper foil was heated to 1.65 g / cm 3 The negative electrode coating on both sides of the copper foil is rolled to form a negative electrode active material layer, which is then cut to obtain a negative electrode sheet. The compaction density of the negative electrode active material layer reaches a stable value of about 1.57 g / cm after about 48 hours. 3 The thickness of the negative electrode active material layer is about 40 μm, and the surface density of the negative electrode active material layer is about 6.487 mg / cm 2 (Compared to the surface density of the negative electrode coating (6.5 mg / cm 2 ) are basically the same, and usually the surface density of the negative electrode active material layer formed after roller pressing is reduced by about 0.2% relative to the negative electrode coating before roller pressing).

[0095] 4. Preparation of positive electrode

[0096] Lithium cobalt oxide, polyvinylidene fluoride, and carbon black were mixed in a mass ratio of 96:2:2, N-methylpyrrolidone was added thereto, and the mixture was stirred in a vacuum mixer until a uniform positive electrode slurry was formed;

[0097] The positive electrode slurry was evenly coated on the front and back surfaces of an aluminum foil with a thickness of 10 μm and dried to form a surface density of 17 mg / cm 2 positive electrode coating;

[0098] The aluminum foil with the positive electrode coating was placed in a vacuum oven at 120°C and dried for 8 hours. 3 The positive electrode coating on both sides of the aluminum foil is rolled to form a positive electrode active material layer, which is then cut into pieces to obtain a positive electrode sheet.

[0099] 5. Preparation of lithium-ion batteries

[0100] Under an inert atmosphere (nitrogen), EC, PC, PP, LiPF6, FEC, and PS were mixed in a mass ratio of 13:13:50:15:5:4 and stirred to obtain an electrolyte;

[0101] An aluminum tab is welded on the positive electrode sheet, and a nickel tab is welded on the negative electrode sheet; the positive electrode sheet, a polyethylene separator with a thickness of 8 μm, and the negative electrode sheet are then stacked in sequence and wound to form a wound battery cell; the wound battery cell is placed in a cavity surrounded by an aluminum-plastic film and dried, and the electrolyte is injected into the battery cell. After packaging, formation, secondary sealing, sorting and other processes, a lithium-ion battery is produced.

[0102] Example 2 to Example 17

[0103] Referring to the process of Example 1, the silicon-carbon material, the first binder, the negative electrode sheet, the positive electrode sheet and the lithium-ion battery of Examples 2 to 17 were prepared. The differences between Examples 2 to 17 and Example 1 are shown in Table 1. Except for the differences shown in Table 1, the other conditions are the same.

[0104] In addition, the silicon-carbon materials of Examples 1 to 17 were used to measure the gram capacity and first coulombic efficiency of the silicon-carbon materials of each Example in the range of 0.005-0.7V through the following process. The results are shown in Table 1:

[0105] A negative electrode sheet was prepared by referring to the preparation method of the negative electrode sheet in Example 1 (using silicon-carbon material as the negative electrode active material, omitting the addition of graphite, and other conditions being the same as in Example 1);

[0106] The negative electrode sheet is used as the working electrode and the lithium metal sheet is used as the counter electrode to assemble into a button cell;

[0107] Based on the weight of the silicon-carbon material, the discharge was performed at a current of 50 mA / g (i.e., the ratio of current to the mass of the silicon-carbon material was 50 mA: 1 g) to 0.005 V to obtain the discharge capacity. The material was allowed to stand for 10 minutes, and then charged at a current of 50 mA / g to 0.7 V to obtain the charging capacity. The charging capacity was taken as the gram capacity of the silicon-carbon material, and the ratio obtained by dividing the charging capacity by the discharge capacity was taken as the first coulombic efficiency.

[0108] In addition, referring to the test process of the gram capacity and the first coulombic efficiency of the above silicon-carbon material, the artificial graphite used in the negative electrode sheet of each embodiment was measured to have a gram capacity of 352 mAh / g in the range of 0.005-0.7V and a first coulombic efficiency of 93.9%.

[0109] Table 1

[0110] Comparative Example 1

[0111] The difference between Comparative Example 1 and Example 1 is that the silicon-carbon material is replaced by a silicon-oxygen-carbon material. The silicon-oxygen-carbon material is composed of pre-lithiated silicon oxide and amorphous carbon coated on the surface of the pre-lithiated silicon oxide. The mass of the amorphous carbon is 2.1% of the total mass of the silicon-oxygen-carbon material. The particle size Dv50 of the silicon-oxygen-carbon material is 7 μm, and the specific surface area is 4.1 m2 / g, the gram capacity in the range of 0.005-0.7V is 1240mAh / g, and the first coulombic efficiency is 76.1%; in the preparation process of the negative electrode sheet of Comparative Example 2, the surface density of the negative electrode coating is 7.15mg / cm 2 The remaining conditions are the same as those in Example 1.

[0112] Comparative Example 2

[0113] The difference between Comparative Example 2 and Example 1 is that the silicon-carbon material is replaced by a silicon-oxygen-carbon material. The silicon-oxygen-carbon material is composed of pre-lithiated silicon oxide and amorphous carbon coated on the surface of the pre-lithiated silicon oxide. The mass of the amorphous carbon is 44% of the total mass of the silicon-oxygen-carbon material. The particle size Dv50 of the silicon-oxygen-carbon material is 7 μm, and the specific surface area is 4.1 m 2 / g, the gram capacity in the range of 0.005-0.7V is 730mAh / g, and the first coulomb efficiency is 64.8%; in the preparation process of the negative electrode sheet of Comparative Example 3, the surface density of the negative electrode coating is 7.95mg / cm 2 The remaining conditions are the same as those in Example 1.

[0114] Comparative Example 3

[0115] The difference between Comparative Example 3 and Example 1 is that 1.9 parts of sodium carboxymethyl cellulose and 2.1 parts of styrene-butadiene rubber are used as binders to replace 4 parts of the first binder in Example 1, and the other conditions are the same as those in Example 1.

[0116] In the above-mentioned embodiments and comparative examples, the surface density of the negative electrode coating was adjusted to the above-mentioned values, so that the positive electrode working conditions of these embodiments and comparative examples were similar. The cycling performance of the lithium-ion batteries of the embodiments and comparative examples was tested using a LAND test system at a test temperature of 45°C through the following process:

[0117] The battery was charged at a constant current of 1.5C to 4.48V, charged at a constant voltage to 0.05C, allowed to stand for 10 minutes, and discharged at 0.7C to 3.0V. The discharge capacity was taken as the initial capacity, and the discharge energy was taken as the initial energy (the test results of Examples 1 to 7 are shown in Table 2. The test results of Examples 8 to 17 are similar to those of Example 1 and will not be repeated here);

[0118] Charge at a constant current of 1.5C to 3.82V, charge at a constant voltage to 0.025C, let stand for 10 minutes, and test the thickness of the battery. This thickness is taken as the initial thickness, the product of the initial thickness and the length and width of the battery is taken as the initial volume of the battery, and the initial energy divided by the initial volume is taken as the energy density of the battery (the initial thickness and energy density test results of Examples 1 to 7 are shown in Table 2, and the initial thickness and energy density test results of Examples 8 to 17 are similar to those of Example 1 and will not be repeated here);

[0119] The battery was discharged at 0.7C to 3.0V, allowed to stand for 10 minutes, charged at a constant current of 1.5C to 4.48V, charged at a constant voltage to 0.05C, and allowed to stand for 10 minutes. The cyclic charge and discharge test was performed in this manner; the ratio of each discharge capacity divided by the initial capacity was taken as the capacity retention rate, and the cycle was repeated 800 times; every 200 cycles, the thickness of the fully charged battery was tested, and the ratio of this thickness divided by the initial thickness was taken as the thickness expansion rate (the thickness expansion rate test results of Examples 1 to 17 are shown in Table 3).

[0120] Table 2

[0121] Table 3

[0122] It can be seen that the lithium-ion batteries of Examples 1 to 17 have the advantages of high energy density (greater than 730Wh / L), high capacity retention rate (capacity retention rate after 800 cycles is higher than 79%), and low thickness expansion rate (thickness expansion rate after 800 cycles is not higher than 13.1%).

[0123] In addition, compared with Comparative Examples 1 and 2 (using a silicon-oxygen material including pre-lithiated silicon oxide and amorphous carbon coated on the surface of the pre-lithiated silicon oxide as the negative electrode active material), the lithium-ion batteries of Examples 1 to 17 achieve higher capacity retention and lower thickness expansion while improving energy density.

[0124] In addition, compared with Comparative Example 3 (using sodium carboxymethyl cellulose and styrene-butadiene rubber as binders), the lithium-ion batteries of Examples 1 to 17 have higher capacity retention and lower thickness expansion while improving energy density.

[0125] In addition, it can be seen from Example 1 and Examples 4 to 7 that, relative to Example 6 and Example 7, the molar number M1 of the first group and the molar number M2 of the second group in the first binder used in Example 1, Example 4 and Example 5 satisfy 0.4≤M1 / (M1+M2)≤0.95, and have better capacity retention and lower thickness expansion rate.

[0126] In addition, it can be seen from Example 1 and Examples 11 to 14 that, relative to Example 14, the average pore size of the porous carbon in Example 1 and Examples 11 to 13 is no more than 20 nm, and their batteries have a higher capacity retention rate and a lower thickness expansion rate.

[0127] In addition, it can be seen from Example 1 and Examples 15 to 17 that, relative to Example 17, the specific surface areas of the silicon-carbon materials in Examples 1, 15, and 16 are all no greater than 15 m2 / g, and its battery has higher capacity retention and lower thickness expansion rate.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet, characterized in that: It includes a current collector and an active material layer arranged on the surface of the current collector, the active material layer includes a silicon-carbon material and a first binder, the first binder contains a first group and a second group, the first group includes a carboxyl group, and the second group includes a cyano group and / or an amide group.

2. The negative electrode sheet according to claim 1, characterized in that: The silicon-carbon material includes porous carbon and a silicon material present in pores of the porous carbon.

3. The negative electrode sheet according to claim 2, characterized in that: The mass ratio of the porous carbon to the silicon material is 7:3 to 3:7; And / or, the average pore size of the porous carbon is less than or equal to 20 nm.

4. The negative electrode sheet according to any one of claims 1 to 3, characterized in that: The particle size Dv50 of the silicon-carbon material is 5 μm to 15 μm; And / or, the specific surface area of ​​the silicon-carbon material is less than or equal to 15 m 2 / g.

5. The negative electrode sheet according to any one of claims 1 to 3, characterized in that: In the active material layer, the mass percentage of the silicon-carbon material is 3% to 20%; And / or, the surface density of the active material layer is 5 mg / cm 2 ~8mg / cm 2 .

6. The negative electrode sheet according to claim 1, characterized in that: The carboxyl group includes a carboxylic acid group and / or a carboxylate group, and the carboxylate group includes one or more of -COOLi, -COONa, -COOK, and -COONH4; And / or, in the first binder, the molar number of the first group is M1, the molar number of the second group is M2, and 0.4≤M1 / (M1+M2)≤0.

95.

7. The negative electrode sheet according to claim 1 or 6, characterized in that: The weight average molecular weight of the first binder is 3,000 to 2,000,000.

8. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: In the active material layer, the mass percentage of the first binder is 1% to 10%.

9. The negative electrode sheet according to claim 1, characterized in that: The active material layer also includes a carbon-based material, wherein The carbon-based material includes one or more of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, hard carbon, carbon black, acetylene black, carbon fiber, single-walled carbon tubes, and multi-walled carbon tubes; And / or, in the active material layer, the mass percentage of the carbon-based material is 60% to 96%.

10. A battery, characterized in that: A negative electrode sheet comprising any one of claims 1 to 9.

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