Negative plate and lithium ion secondary battery

Abstract

The invention relates to the field of batteries, in particular to a negative plate and a lithium ion secondary battery comprising the negative plate. The negative plate comprises a negative current collector as well as a first active layer and a second active layer which are respectively arranged on two side surfaces of the negative current collector; the first active layer comprises a first negative electrode active material, and the first negative electrode active material comprises a carbon-based material; the second active layer comprises a second negative electrode active material, and the second negative electrode active material comprises a silicon-based material; the mass content of the element Si in the first active layer is c1, the mass content of the element Si in the second active layer is c2, and c1 and c2 meet the condition that 1.2% < = c2-c1 < = 15%. According to the negative plate, the dynamic performance and the structural stability of the positive plate can be improved by regulating and controlling the difference value of the silicon contents in the first active layer and the second active layer. Comprising the negative plate disclosed by the invention has relatively good rate capability and cycle performance while the energy density loss is relatively small.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of batteries, in particular to a negative electrode sheet and a lithium ion secondary battery comprising the same. BACKGROUND

[0002] With the continuous expansion of the electric vehicle market, the demand for high energy density batteries is increasing. Silicon negative electrode material as a kind of high energy density battery material has attracted widespread attention. However, the silicon negative electrode material itself has slow electron and ion transmission, and volume expansion and contraction occur during charging and discharging, which leads to the destruction of the electrode structure, poor rate performance of the battery, capacity decay and other problems. SUMMARY

[0003] The present application aims to overcome the problems of poor rate performance and capacity decay of batteries containing silicon negative electrode material in the prior art, and to provide a negative electrode sheet and a lithium ion secondary battery comprising the same. The negative electrode sheet of the present application can improve the kinetic performance and structural stability of the negative electrode sheet by regulating the difference in silicon content in the first active layer and the second active layer. The lithium ion secondary battery (hereinafter referred to as battery) comprising the negative electrode sheet of the present application can have better rate performance and cycle performance while having less energy density loss.

[0004] The first aspect of the present application provides a negative electrode sheet, comprising a negative electrode current collector and a first active layer and a second active layer on both sides of the negative electrode current collector, respectively; the first active layer comprises a first negative electrode active material, the first negative electrode active material comprises a carbon-based material; the second active layer comprises a second negative electrode active material, the second negative electrode active material comprises a silicon-based material; the mass content of element Si in the first active layer is c1, the mass content of element Si in the second active layer is c2, and c1 and c2 satisfy 1.2%≤c2-c1≤15%.

[0005] The second aspect of the present application provides a lithium ion secondary battery comprising the negative electrode sheet of the first aspect of the present application.

[0006] The above technical solution has at least the following advantages compared with the prior art:

[0007] (1) The negative electrode sheet of the present application can optimize the kinetic performance of the negative electrode sheet by regulating the difference in silicon content in the first active layer and the second active layer, and reduce the capacity loss caused by the insufficient kinetics of silicon material itself;

[0008] (2) The negative electrode sheet of the present application can improve the structural stability of the negative electrode sheet by regulating the difference in silicon content in the first active layer and the second active layer, and reduce the adverse effects of silicon material expansion / contraction on the battery;

[0009] (3) The lithium-ion secondary battery of the present invention can have better rate performance and cycle capacity retention rate, as well as lower cycle expansion rate, while having less energy density loss.

[0010] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0011] Figure 1 The figure shown is a cross-sectional schematic diagram of the negative electrode sheet in an embodiment of the present invention.

[0012] Figure 2 The image shown is a scanning electron microscope (SEM) image of the negative electrode sheet in an example of the present invention.

[0013] Figure 3 The image shown is a SEM image of the negative electrode in the related technology. Detailed Implementation

[0014] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0015] Related technologies suffer from poor rate performance and rapid capacity decay in silicon-containing batteries. The inventors of this invention discovered that the rate performance of a battery is closely related to the structural design of the electrodes. Optimizing the electrode structure can improve current distribution and transfer efficiency, thereby enhancing the battery's rate performance. Furthermore, they unexpectedly discovered that optimizing the structure of the negative electrode can also improve the problem of rapid cycle degradation in silicon-containing batteries. Based on this, the following solution is proposed:

[0016] A first aspect of the present invention provides a negative electrode sheet, which may include a negative electrode current collector and a first active layer and a second active layer respectively on both sides of the negative electrode current collector. Figure 1 The figure shows a cross-sectional schematic diagram of the negative electrode sheet in an embodiment of the present invention. As can be seen from the figure, the negative electrode sheet includes a negative current collector 1 and a first active layer 2 and a second active layer 3 on both sides of the negative current collector 1.

[0017] In this invention, the first active layer may include a first negative electrode active material, which may include a carbon-based material. The second active layer may include a second negative electrode active material, which may include a silicon-based material.

[0018] In this invention, the mass content of element Si in the first active layer is c1, and the mass content of element Si in the second active layer is c2. c1 and c2 satisfy 1.2% ≤ c2 - c1 ≤ 15%, for example, c2 - c1 is equal to 1.2%, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%.

[0019] The inventors of this invention have specially designed the negative electrode by controlling the difference in silicon content between the two active layers of the negative electrode to improve the rate performance and cycle performance of the battery. Specifically, the mass content of elemental Si in the first active layer is reduced, thereby improving the lithium-ion transport capability and enhancing the kinetic performance of the negative electrode. This makes the kinetic performance of the first active layer superior to that of the second active layer, allowing the first active layer to have a larger charge-discharge rate. Conversely, the content of elemental Si in the second active layer is increased, resulting in a higher specific capacity and expansion force, as well as a lower areal density. This reduces the tortuosity of lithium-ion transport to a certain extent, allowing the second active layer to tolerate a larger charge-discharge rate. Thus, without changing the overall charge-discharge rate, the battery's capacity decay rate is slowed down, improving capacity retention. Furthermore, the different mechanical properties of the active layers on both sides of the negative electrode can buffer the volume expansion of silicon, thereby improving the structural stability of the negative electrode and enhancing the rate performance and cycle performance of the battery.

[0020] like Figure 2 The image shown is a scanning electron microscope (SEM) image of the negative electrode sheet in an example of the present invention. Figure 3 The image shows a SEM image of a negative electrode sheet in related technologies. As can be seen from the image, in related technologies, the active layer on both sides of the negative electrode current collector has the same Si content, while in the negative electrode sheet of this invention, the active layer on both sides of the negative electrode current collector has different Si contents.

[0021] In this invention, the mass content of element Si in the first active layer and the mass content of element Si in the second active layer can be obtained by conventional methods in the art, such as inductively coupled plasma spectrometry (ICP), energy dispersive spectroscopy (EDS), atomic absorption spectrometry (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), X-ray fluorescence spectrometry (XRF), or atomic fluorescence spectrometry (AFS).

[0022] In one instance, 3% ≤ c2 - c1 ≤ 10%.

[0023] In one instance, 3.5% ≤ c2 - c1 ≤ 7%.

[0024] In one example, the first negative electrode active material also includes a silicon-based material.

[0025] In one example, the second negative electrode active material also includes a carbon-based material.

[0026] In this invention, the carbon-based material may include at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, and hard carbon. The silicon-based material may include at least one of silicon, silicon-carbon materials, silicon-oxygen materials, and silicon alloys. The silicon-carbon material may include, for example, at least one of silicon / graphite composite materials, silicon / mesophase carbon microsphere composite materials, silicon / carbon black composite materials, porous silicon-carbon, hollow silicon-carbon, silicon-carbon nanotube / wire composite materials, silicon-carbon gel composite materials, and silicon / graphene composite materials. The silicon-oxygen material may include, for example, at least one of pre-lithiated silicon-oxygen, silicon / silicon-oxygen composite materials, silicon-oxygen / carbon composite materials, porous silicon-oxygen materials, metal oxide-coated silicon-oxygen materials, and carbon-coated silicon-oxygen materials.

[0027] In one example, the porous silicon-carbon structure consists of elemental silicon distributed within a porous carbon framework.

[0028] The inventors of this invention have discovered that specific silicon-based materials can further improve the rate performance and cycle performance of batteries. For example, when the silicon-based material is porous silicon-carbon, nano-silicon can provide strong ion transport capabilities, the carbon framework can reserve expansion space for silicon, and the carbon framework can isolate the electrolyte, reducing the formation ratio of the SEI film, thereby improving the cycle performance and rate performance of the battery.

[0029] In this invention, based on the total mass of the first active layer, the content of the first negative electrode active material can be 70%-99.29%, for example, 70%, 75%, 80%, 85%, 90%, 95% or 99.29%.

[0030] In one example, the content of the first negative electrode active material is 96%-97% based on the total mass of the first active layer.

[0031] In this invention, based on the total mass of the second active layer, the content of the second negative electrode active material can be 70%-98.5%, for example, 70%, 75%, 80%, 85%, 90%, 95% or 98.5%.

[0032] In one example, the content of the second negative electrode active material is 94.5%-96.5% based on the total mass of the second active layer.

[0033] In this invention, c1 can be 0%-70%, for example, 0%, 5%, 10%, 20%, 30%, 40%, 50%, 60% or 70%.

[0034] In one instance, c1 ranges from 1% to 20%.

[0035] In one instance, c1 ranges from 2% to 7.5%.

[0036] In this invention, c2 can be 5%-100%, for example, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100%.

[0037] In one instance, c2 is 6%–15%.

[0038] In this invention, the porosity of the first active layer is k1, 40% ≤ k1 ≤ 70%, for example, k1 is 40%, 45%, 50%, 55%, 60%, 65% or 70%.

[0039] In one instance, 55% ≤ k1 ≤ 65%.

[0040] In this invention, the porosity of the second active layer is k2, 50% ≤ k2 ≤ 80%, for example, k2 is 50%, 55%, 60%, 65%, 70%, 75% or 80%.

[0041] In one instance, 60% ≤ k2 ≤ 70%.

[0042] In this invention, the porosity of the first active layer and the porosity of the second active layer can be tested by conventional methods in the art, such as liquid phase exclusion method (liquid expulsion method), mercury injection method, gas adsorption method, image analysis method or density method.

[0043] In this invention, k1 / k2 can be 1:(1.01-2), for example 1:1.01, 1:1.05, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9 or 1:2.

[0044] In one instance, k1 / k2 is 1: (1.1-1.15).

[0045] The inventors of this invention discovered that when k1 / k2 is within a specific range, it can further buffer the volume expansion of silicon-based materials and improve the structural stability of the negative electrode. When k1 / k2 is small (e.g., less than 1:2), the porosity of the second active layer is larger than that of the first active layer. Although the larger porosity can provide more expansion space for the volume expansion of silicon, it will lead to a decrease in the conductivity of the negative electrode, thereby affecting the rate performance of the battery. Conversely, when k1 / k2 is large (e.g., greater than 1:1.01), the porosity of the second active layer is smaller than that of the first active layer. The smaller porosity will lead to a larger expansion of the negative electrode, affecting the cycle performance of the battery.

[0046] In this invention, c1, c2, k1 and k2 satisfy 0≤(c1 / k1) / (c2 / k2)≤1, for example, (c1 / k1) / (c2 / k2) is 0, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1.

[0047] The inventors of this invention have discovered that when c1, c2, k1, and k2 satisfy a specific relationship, the buffering capacity of the negative electrode sheet against the volume expansion of silicon-based materials can be further improved, thereby reducing the cycle expansion rate of the battery.

[0048] In one instance, 0.4 ≤ (c1 / k1) / (c2 / k2) ≤ 0.6.

[0049] In this invention, the ratio of the median particle size Dv50 of the first negative electrode active material to the median particle size Dv50 of the second negative electrode active material can be (0.5-3.5):1, for example, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.03:1, 1.05:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 3:1 or 3.5:1.

[0050] In one example, the ratio of the median particle size Dv50 of the first negative electrode active material to the median particle size Dv50 of the second negative electrode active material is (1.1-1.15):1.

[0051] The inventors of this invention have discovered that when the ratio of the median particle size Dv50 of the first negative electrode active material to the median particle size Dv50 of the second negative electrode active material is within a specific range, it is possible to improve the performance of Li. + The high transmission efficiency allows the battery to balance rate performance and cycle performance.

[0052] In this invention, the median particle size Dv50 of the first negative electrode active material can be 6μm-15μm, for example, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm or 15μm.

[0053] In one example, the median particle size Dv50 of the first negative electrode active material is 11.5 μm-13 μm.

[0054] In this invention, the median particle size Dv50 of the second negative electrode active material can be 4μm-14μm, for example, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm or 14μm.

[0055] In one example, the median particle size Dv50 of the second negative electrode active material is 10 μm-12 μm.

[0056] In this invention, the median particle size Dv50 of the first negative electrode active material and the median particle size Dv50 of the second negative electrode active material can be obtained by conventional methods in the art, such as laser particle size analyzer.

[0057] In this invention, the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material can be 1:(1-1.8), for example, 1:1, 1:1.05, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7 or 1:1.8.

[0058] In one example, the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:(1.1-1.3).

[0059] The inventors of this invention have discovered that when the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is within a specific range, it is beneficial for the electrolyte to wet the first and second active layers, thereby improving the Li... + The high transmission efficiency allows the battery to balance rate performance and cycle performance.

[0060] In this invention, the specific surface area of ​​the first negative electrode active material can be 1 m². 2 / g-3.8m 2 / g, for example, 1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g, 3.5m 2 / g or 3.8m 2 / g.

[0061] In one example, the specific surface area of ​​the first negative electrode active material is 1.5 m². 2 / g-2m 2 / g.

[0062] In this invention, the specific surface area of ​​the second negative electrode active material can be 1.1 m². 2 / g-7m 2 / g, for example, 1.1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g、5m 2 / g, 5.5m 2 / g、6m 2 / g, 6.5m 2 / g or 7m 2 / g.

[0063] In one example, the specific surface area of ​​the second negative electrode active material is 1.5 m². 2 / g-2.5m 2 / g.

[0064] In this invention, the specific surface area of ​​the first negative electrode active material and the specific surface area of ​​the second negative electrode active material can be obtained by conventional methods in the art, such as the continuous flow method (dynamic method) or the static capacity method.

[0065] In this invention, the ratio of the areal capacity of the first active layer to the areal capacity of the second active layer can be 1:(1-1.5), for example, 1:1, 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05, 1:1.06, 1:1.07, 1:1.08, 1:1.09, 1:1.1, 1:1.2, 1:1.3, 1:1.4 or 1:1.5.

[0066] The inventors of this invention have discovered that when the areal capacity of the first active layer and the areal capacity of the second active layer are within a specific range, it is possible to ensure that the capacity of the active layers on both sides of the negative electrode current collector is consistent, which is beneficial to the cycle stability of the battery.

[0067] In this invention, the areal capacity of the first active layer can be 1.5 mAh / cm³. 2 -5mAh / cm 2 For example, 1.5mAh / cm³ 2 2mAh / cm 2 2.5mAh / cm 2 3mAh / cm 2 3.5mAh / cm 2 4mAh / cm 2 4.5mAh / cm 2 or 5mAh / cm 2 .

[0068] In this invention, the areal capacity of the second active layer can be 1.6 mAh / cm³. 2 -5.5mAh / cm 2 For example, 1.6mAh / cm³ 2 2mAh / cm 2 2.5mAh / cm 2 3mAh / cm 2 3.5mAh / cm 2 4mAh / cm 2 4.5mAh / cm 2 5mAh / cm 2 Or 5.5mAh / cm 2 .

[0069] In this invention, the areal capacity of the first active layer and the areal capacity of the second active layer can be obtained by methods conventional in the art, such as electrochemical performance testing.

[0070] In this invention, the thickness of the first active layer is h1, and the thickness of the second active layer is h2, satisfying 0.01μm≤h1-h2≤50μm. For example, h1-h2 can be 0.01μm, 0.05μm, 0.1μm, 0.5μm, 1μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, or 50μm.

[0071] In one instance, 7μm≤h1-h2≤9μm.

[0072] The inventors of this invention have discovered that when h1-h2 are within a specific range, the cycle capacity retention rate of the battery can be further improved.

[0073] In this invention, h1 can be 21μm-90μm, for example, 21μm, 30μm, 40μm, 50μm, 60μm, 70μm, 80μm or 90μm.

[0074] In one instance, h1 is 35μm-60μm.

[0075] In one instance, h1 is 40μm-55μm.

[0076] In this invention, h2 can be 16μm-80μm, for example, 16μm, 20μm, 30μm, 40μm, 50μm, 60μm, 70μm or 80μm.

[0077] In one instance, h2 is 30μm-55μm.

[0078] In one instance, h2 is 33μm-48μm.

[0079] In this invention, the first active layer may further include a first dispersant, and the second active layer may further include a second dispersant. The first dispersant may include at least one selected from carboxymethyl cellulose, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose. The second dispersant may include at least one selected from carboxymethyl cellulose, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose.

[0080] In one example, the first dispersant comprises at least one of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose.

[0081] In one example, the second dispersant comprises lithium carboxymethyl cellulose.

[0082] The inventors of this invention have discovered that when the second dispersant includes a specific substance, it is possible to improve Li + This improves the transmission capacity and dynamic performance of the negative electrode, thereby enhancing the rate performance of the battery.

[0083] In this invention, based on the total mass of the first active layer, the content of the first dispersant can be 0.2%-10%, for example, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0084] In one example, the content of the first dispersant is 0.5%-1% based on the total mass of the first active layer.

[0085] In this invention, based on the total mass of the second active layer, the content of the second dispersant can be 0.2%-10%, for example, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0086] In one example, the content of the second dispersant is 0.5%-1% based on the total mass of the second active layer.

[0087] In this invention, the first active layer may further include a first conductive agent, and the second active layer may further include a second conductive agent. The first conductive agent may include at least one of conductive carbon black, Ketjen black, conductive graphite, graphene, carbon aerogel, MXene material, carbon nanotubes, and carbon fibers. The second conductive agent may include at least one of conductive carbon black, Ketjen black, conductive graphite, graphene, carbon aerogel, MXene material, carbon nanotubes, and carbon fibers.

[0088] In one example, the second conductive agent comprises carbon nanotubes. The carbon nanotubes may include at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0089] In one example, the second conductive agent comprises single-walled carbon nanotubes.

[0090] The inventors of this invention have discovered that when the second conductive agent includes carbon nanotubes, the kinetic performance of the negative electrode can be improved, thereby increasing the rate performance of the battery.

[0091] In this invention, based on the total mass of the first active layer, the content of the first conductive agent can be 0.01%-10%, for example, 0.01%, 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0092] In one example, the content of the first conductive agent can be 0.05%-0.2% based on the total mass of the first active layer.

[0093] In this invention, based on the total mass of the second active layer, the content of the second conductive agent can be 0.01%-10%, for example, 0.01%, 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0094] In one example, the content of the second conductive agent can be 0.1%-0.5% based on the total mass of the second active layer.

[0095] In this invention, the first active layer may further include a first adhesive, and the second active layer may further include a second adhesive. The first adhesive may include at least one selected from polyacrylic acid, polyacrylate, styrene-butadiene rubber, styrene-acrylic rubber, polyurethane, polyvinylpyrrolidone, polyaniline, polyimide, polyamide-imide, polysiloxane, epoxy resin, polyester resin, polyurethane resin, and polyfluorene. The second adhesive may include at least one selected from polyacrylic acid, polyacrylate, styrene-butadiene rubber, styrene-acrylic rubber, polyurethane, polyvinylpyrrolidone, polyaniline, polyimide, polyamide-imide, polysiloxane, epoxy resin, polyester resin, polyurethane resin, and polyfluorene.

[0096] In one example, the first adhesive comprises at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, and styrene-acrylic rubber.

[0097] In one example, the first adhesive comprises polyacrylic acid.

[0098] In one example, the first adhesive comprises polyacrylate and styrene-butadiene rubber. The mass ratio of the polyacrylate to the styrene-butadiene rubber can be (1-2):1, for example, 1:1, 1.5:1 or 2:1.

[0099] In one example, the second adhesive comprises at least one of polyacrylate, styrene-butadiene rubber, styrene-acrylic rubber, polyurethane, and polyamide-imide.

[0100] In one example, the second adhesive comprises polyacrylic acid.

[0101] In one example, the second adhesive comprises polyacrylate and styrene-butadiene rubber. The mass ratio of the polyacrylate to the styrene-butadiene rubber can be (1-2):1, for example, 1:1, 1.5:1 or 2:1.

[0102] In this invention, based on the total mass of the first active layer, the content of the first adhesive can be 0.5%-10%, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0103] In one example, the content of the first adhesive is 2%-3% based on the total mass of the first active layer.

[0104] In this invention, based on the total mass of the second active layer, the content of the second adhesive can be 0.5%-10%, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0105] In one example, the content of the second adhesive is 2%-4% based on the total mass of the second active layer.

[0106] In this invention, the negative electrode current collector may include at least one of copper foil, nickel foil, and carbon-based current collector. The carbon-based current collector may include, for example, element C.

[0107] The negative electrode of the present invention has stable structural stability and good dynamic performance.

[0108] A second aspect of the present invention provides a lithium-ion secondary battery, wherein the lithium-ion secondary battery may include the negative electrode sheet described in the first aspect of the present invention.

[0109] In this invention, the lithium-ion secondary battery may further include a positive electrode.

[0110] In this invention, the positive electrode sheet may include a positive current collector and a positive electrode coating on at least one side surface of the positive current collector. The positive current collector may include positive current collectors conventionally used in the art, such as aluminum foil. The thickness of the positive current collector may be 1 μm-15 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, or 15 μm. The positive electrode coating may include a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode active material may include at least one of lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium iron phosphate (LFP), ternary materials (e.g., lithium nickel cobalt manganese oxide NCM or lithium nickel cobalt aluminum oxide NCA), and lithium aluminate. The positive electrode conductive agent may include at least one of conductive carbon black, conductive graphite, graphene, and carbon nanotubes. The positive electrode binder may include at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, styrene-acrylate copolymer, styrene-butadiene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0111] In this invention, based on the total mass of the positive electrode coating, the content of the positive electrode active material can be 80%-99.8% (e.g., 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, 96%, 98%, 99%, or 99.8%), the content of the positive electrode conductive agent can be 0.1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%), and the content of the positive electrode binder can be 0.1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%).

[0112] In one example, based on the total mass of the positive electrode coating, the content of the positive electrode active material is 95%-98.5%, the content of the positive electrode conductive agent is 0.1%-3%, and the content of the positive electrode binder is 0.1%-5%.

[0113] In this invention, the lithium-ion secondary battery may further include a separator. The separator may include a substrate layer. The substrate layer may include at least one selected from polyethylene, polypropylene, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyethylene terephthalate, polyimide, and aramid.

[0114] In this invention, the thickness of the diaphragm can be 3μm-12μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm or 12μm.

[0115] In this invention, the diaphragm may further include a porous layer located on at least one side surface of the substrate layer. The porous layer may include inorganic particles and a binder. The inorganic particles may include at least one of alumina (Al₂O₃), silicon dioxide (SiO₂), magnesium oxide (MgO), titanium dioxide (TiO₂), hafnium dioxide (HfO₂), tin oxide (SnO₂), cerium dioxide (CeO₂), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO₂), yttrium oxide (Y₂O₃), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder may include at least one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, carboxymethyl cellulose salt, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The pores of the diaphragm have a diameter of 0.01 μm to 1 μm.

[0116] In this invention, the diaphragm may further include an adhesive layer. The adhesive layer is located on at least one outer surface of the diaphragm. The adhesive layer may include the adhesive.

[0117] In this invention, the lithium-ion secondary battery may further include an electrolyte. The electrolyte may include at least one selected from fluoroether, fluoroethylene carbonate, and ether nitrile.

[0118] In this invention, the electrolyte may further comprise a lithium salt. The lithium salt may comprise at least one of lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate. In the electrolyte, the concentration of the lithium salt may be 1 mol / L to 2 mol / L, for example, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, or 2 mol / L.

[0119] In one example, the lithium salt comprises lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate.

[0120] In one example, the lithium salt contains lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate in a mass ratio of (0.06-5):1.

[0121] In this invention, the electrolyte may further comprise a non-aqueous solvent. The non-aqueous solvent may comprise at least one of carbonate compounds, carboxylic acid ester compounds, and ether compounds. The carbonate compounds may comprise at least one of chain carbonate compounds, cyclic carbonate compounds, and fluorocarbonate compounds. The chain carbonate compounds may comprise at least one of diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and methyl ethyl carbonate (MEC). The cyclic carbonate compounds may comprise at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), and vinyl ethylene carbonate (VEC). The fluorocarbonate compound may include at least one selected from 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, and trifluoromethylethylene carbonate. The carboxylic acid ester compound may include at least one selected from methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, caprolactone, and methyl formate. The ether compound may include at least one selected from dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, and tetrahydrofuran. The non-aqueous solvent may also include at least one selected from dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphate esters.

[0122] The lithium-ion secondary battery of the present invention can improve cycle retention rate and reduce cycle expansion rate while minimizing the loss of basic energy density.

[0123] The present invention also provides an electronic device, which may include the lithium-ion secondary battery of the present invention. The electronic device may include at least one of the following: laptop computer, pen input computer, mobile computer, e-book player, portable telephone, portable fax machine, portable copier, portable printer, stereo headset, video recorder, LCD TV, portable cleaner, portable CD player, mini CD, transceiver, electronic notebook, calculator, memory card, portable recorder, radio, backup power supply, motor, automobile, motorcycle, electric bicycle, bicycle, drone, lighting fixture, toy, game console, clock, power tool, flashlight, camera, household large-capacity battery, and lithium-ion capacitor.

[0124] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0125] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0126] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0127] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0128] Example 1

[0129] The battery is prepared according to the following method:

[0130] Preparation of negative electrode:

[0131] The first negative electrode active material (artificial graphite and porous silicon carbon, with a mass ratio of artificial graphite to porous silicon carbon of 95:5) has a Dv50 of 13 μm and a specific surface area of ​​1.6 m². 2 A mixture of polyacrylic acid, lithium carboxymethyl cellulose, and single-walled carbon nanotubes in a mass ratio of 97:2.2:0.75:0.05 was added to deionized water to form a first slurry. A 6μm thick copper foil was used as the negative electrode current collector, and the first slurry was coated on one side of the negative electrode current collector to form a first active layer.

[0132] The second negative electrode active material (artificial graphite and porous silicon carbon, with a mass ratio of artificial graphite to porous silicon carbon of 87:13) has a Dv50 of 11.8 μm and a specific surface area of ​​1.8 m². 2A mixture of polyacrylic acid, lithium carboxymethyl cellulose, and single-walled carbon nanotubes in a mass ratio of 96.25:2.8:0.8:0.15 was added to deionized water to form a second slurry. The second slurry was then coated onto the other side of the negative electrode current collector to form a second active layer. After drying, cold pressing, and slitting, the negative electrode sheet was obtained.

[0133] Among them, c1 is 2.4%, c2 is 6.3%, and c1-c2 is 3.9%; the porosity k1 of the first active layer is 55%, the porosity k2 of the second active layer is 62%, and the ratio of k1 to k2 is 1:1.13; (c1 / k1) / (c2 / k2) is 0.44; the ratio of the Dv50 of the first negative electrode active material to the Dv50 of the second negative electrode active material is 1.1:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1.13; and the areal capacity of the first active layer is 2.9 mAh / cm³. 2 The areal capacity of the second active layer is 3 mAh / cm². 2 The ratio of the areal capacity of the first active layer to that of the second active layer is 1:1.03; the thickness of the first active layer is 53.1 μm, the thickness of the second active layer is 45.3 μm, and h1-h2 is 7.9 μm.

[0134] Positive electrode preparation: Lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride were added to an N-methylpyrrolidone (NMP) solution at a mass ratio of 97.6:1.1:1.3 to form a positive electrode slurry; a 9μm thick aluminum foil was used as the positive electrode current collector, and the positive electrode slurry was coated on both sides of the positive electrode current collector; after drying, cold pressing, and slitting, the positive electrode sheet was obtained.

[0135] Preparation of the diaphragm: The substrate layer is 8μm thick polyethylene. A 2μm thick porous layer (including alumina and polyvinylidene fluoride) is coated on both sides of the substrate layer. A 1.5μm thick adhesive layer (including polyvinylidene fluoride) is coated on both sides of the porous layer. After drying, the pores of the diaphragm have a diameter of 0.01μm-1μm.

[0136] Preparation of electrolyte: In an environment with a water content of less than 10 ppm, EC, PC, PP and DEC were prepared into a non-aqueous organic solvent in a mass ratio of 1:1:1:1. Lithium hexafluorophosphate was added, wherein the concentration of lithium hexafluorophosphate was 1.15 mol / L. 10% of fluoroethylene carbonate based on the total mass of the electrolyte was added.

[0137] Preparation of lithium-ion batteries: The positive electrode, separator and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as a separator, and then wound to obtain the electrode assembly; the electrode assembly is placed in the outer packaging aluminum-plastic film, and after the moisture is removed at 80°C, the above-mentioned electrolyte is injected and sealed. The battery is obtained through processes such as formation, degassing, shaping and capacity testing.

[0138] Example 2

[0139] The procedure was carried out in accordance with Example 1, except that the preparation of the negative electrode was changed, as follows:

[0140] Preparation of negative electrode:

[0141] The first negative electrode active material (artificial graphite and porous silicon carbon, with a mass ratio of artificial graphite to porous silicon carbon of 90:10) has a Dv50 of 12.5 μm and a specific surface area of ​​1.72 m². 2 Lithium polyacrylate, styrene-butadiene rubber, lithium carboxymethyl cellulose, and single-walled carbon nanotubes were added to deionized water in a mass ratio of 96.3:1.4:1.4:0.8:0.1 to form a first slurry. A 6μm thick copper foil was used as the negative electrode current collector, and the first slurry was coated on one side of the negative electrode current collector to form a first active layer.

[0142] The second negative electrode active material (artificial graphite and porous silicon carbon, with a mass ratio of 80:20) has a Dv50 of 11 μm and a specific surface area of ​​2.2 m². 2 Lithium polyacrylate, styrene-butadiene rubber, lithium carboxymethyl cellulose, and single-walled carbon nanotubes were added to deionized water in a mass ratio of 95.65:1.65:1.65:0.8:0.25 to form a second slurry. The second slurry was then coated onto the other side of the negative electrode current collector to form a second active layer. After drying, cold pressing, and slitting, the negative electrode sheet was obtained.

[0143] Among them, c1 is 4.8%, c2 is 9.6%, and c1-c2 is 4.8%; the porosity k1 of the first active layer is 60%, the porosity k2 of the second active layer is 66%, and the ratio of k1 to k2 is 1:1.11; (c1 / k1) / (c2 / k2) is 0.56; the ratio of the Dv50 of the first negative electrode active material to the Dv50 of the second negative electrode active material is 1.14:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1.28; and the areal capacity of the first active layer is 2.9 mAh / cm³. 2 The areal capacity of the second active layer is 3.03 mAh / cm². 2 The ratio of the areal capacity of the first active layer to that of the second active layer is 1:1.04; the thickness of the first active layer is 48.7 μm, the thickness of the second active layer is 41.1 μm, and h1-h2 is 7.6 μm.

[0144] Example 3

[0145] The procedure was carried out in accordance with Example 1, except that the preparation of the negative electrode was changed, as follows:

[0146] Preparation of negative electrode:

[0147] The first negative electrode active material (artificial graphite and porous silicon carbon, with a mass ratio of artificial graphite to porous silicon carbon of 85:15) has a Dv50 of 11.8 μm and a specific surface area of ​​2 m². 2 Lithium polyacrylate, styrene-butadiene rubber, lithium carboxymethyl cellulose and single-walled carbon nanotubes were added to deionized water in a mass ratio of 96.05:2:1:0.8:0.15 to form the first slurry; a 6μm thick copper foil was used as the negative electrode current collector, and the first slurry was coated on one side of the negative electrode current collector to form the first active layer.

[0148] The second negative electrode active material (artificial graphite and porous silicon carbon, with a mass ratio of 70:30) has a Dv50 of 10.5 μm and a specific surface area of ​​2.4 m². 2 Lithium polyacrylate, styrene-butadiene rubber, lithium carboxymethyl cellulose, and single-walled carbon nanotubes were added to deionized water in a mass ratio of 94.6:2.7:1.3:1:0.4 to form a second slurry. The second slurry was then coated onto the other side of the negative electrode current collector to form a second active layer. After drying, cold pressing, and slitting, the negative electrode sheet was obtained.

[0149] Among them, c1 is 7.2%, c2 is 14.2%, and c1-c2 is 7%; the porosity k1 of the first active layer is 63%, the porosity k2 of the second active layer is 70%, and the ratio of k1 to k2 is 1:1.11; (c1 / k1) / (c2 / k2) is 0.56; the ratio of the Dv50 of the first negative electrode active material to the Dv50 of the second negative electrode active material is 1.12:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1.2; and the areal capacity of the first active layer is 2.9 mAh / cm³. 2 The areal capacity of the second active layer is 3.03 mAh / cm². 2 The ratio of the areal capacity of the first active layer to that of the second active layer is 1:1.04; the thickness of the first active layer is 43.8 μm, the thickness of the second active layer is 35.4 μm, and h1-h2 is 8.4 μm.

[0150] Example 4 group

[0151] This set of examples is used to verify the impact of the change in "c1-c2".

[0152] This set of embodiments is based on Embodiment 1, except that c1-c2 are changed by altering the second active layer, as detailed below:

[0153] Example 4a: The second negative electrode active material (artificial graphite and porous silicon carbon, wherein the mass ratio of artificial graphite to porous silicon carbon is 88.5:11.5, the Dv50 of the second negative electrode active material is 12.2 μm, and the specific surface area of ​​the second negative electrode active material is 1.72 m². 2 Lithium carboxymethyl cellulose (LCLC) and single-walled carbon nanotubes (SCH) are dissolved in deionized water in a mass ratio of 96.3:2.8:0.8:0.1 to form a second slurry. The second slurry is coated on the other side of the negative electrode current collector to form a second active layer. After drying, cold pressing, and slitting, the negative electrode sheet is obtained.

[0154] Among them, c2 is 5.5%, c1-c2 is 3.1%; the porosity k2 of the second active layer is 60%, k1 / k2 is 1:1.08; (c1 / k1) / (c2 / k2) is 0.47; the ratio of Dv50 of the first negative electrode active material to that of the second negative electrode active material is 1.07:1; the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material is 1:1.08; the areal capacity of the second active layer is 2.9 mAh / cm³. 2 The ratio of the areal capacity of the first active layer to that of the second active layer is 1:1; the thickness of the second active layer is 47.6 μm, and h1-h2 is 5.6 μm.

[0155] Example 4b uses a second negative electrode active material (artificial graphite and porous silicon carbon, wherein the mass ratio of artificial graphite to porous silicon carbon is 73:27, the Dv50 of the second negative electrode active material is 10.7 μm, and the specific surface area of ​​the second negative electrode active material is 2.4 m²). 2 A mixture of polyacrylic acid, lithium carboxymethyl cellulose, and single-walled carbon nanotubes in a mass ratio of 94.8:3.8:1:0.4 was added to deionized water to form a second slurry. The second slurry was then coated onto the other side of the negative electrode current collector to form a second active layer. After drying, cold pressing, and slitting, the negative electrode sheet was obtained.

[0156] Among them, c2 is 12.3%, c1-c2 is 9.9%; the porosity k2 of the second active layer is 70%, k1 / k2 is 1:1.27; (c1 / k1) / (c2 / k2) is 0.25; the ratio of Dv50 of the first negative electrode active material to that of the second negative electrode active material is 1.21:1; the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material is 1:1.5; and the areal capacity of the second active layer is 3.03 mAh / cm². 2The ratio of the areal capacity of the first active layer to that of the second active layer is 1:1.04; the thickness of the second active layer is 35.7 μm, and h1-h2 is 17.5 μm.

[0157] Example 4c: The second negative electrode active material (artificial graphite and porous silicon carbon, wherein the mass ratio of artificial graphite to porous silicon carbon is 65:35, the Dv50 of the second negative electrode active material is 10 μm, and the specific surface area of ​​the second negative electrode active material is 2.5 m²) 2 A mixture of polyacrylic acid, lithium carboxymethyl cellulose, and single-walled carbon nanotubes in a mass ratio of 94:4.5:1:0.5 is added to deionized water to form a second slurry. The second slurry is then coated onto the other side of the negative electrode current collector to form a second active layer. After drying, cold pressing, and slitting, the negative electrode sheet is obtained.

[0158] Among them, c2 is 16.5%, c1-c2 is 14.1%; the porosity k2 of the second active layer is 71%, k1 / k2 is 1:1.29; (c1 / k1) / (c2 / k2) is 0.19; the ratio of Dv50 of the first negative electrode active material to that of the second negative electrode active material is 1.3:1; the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material is 1:1.56; and the areal capacity of the second active layer is 3.02 mAh / cm². 2 The ratio of the areal capacity of the first active layer to that of the second active layer is 1:1.04; the thickness of the second active layer is 34.7 μm, and h1-h2 is 18.5 μm.

[0159] In Example 4d, the second negative electrode active material (artificial graphite and porous silicon carbon, wherein the mass ratio of artificial graphite to porous silicon carbon is 90:10, the Dv50 of the second negative electrode active material is 12.5 μm, and the specific surface area of ​​the second negative electrode active material is 1.72 m²) was used. 2 A mixture of polyacrylic acid, lithium carboxymethyl cellulose, and single-walled carbon nanotubes in a mass ratio of 96.3:2.8:0.8:0.1 was added to deionized water to form a second slurry. The second slurry was then coated onto the other side of the negative electrode current collector to form a second active layer. After drying, cold pressing, and slitting, the negative electrode sheet was obtained.

[0160] Among them, c2 is 4.8%, c1-c2 is 2.4%; the porosity k2 of the second active layer is 60%, k1 / k2 is 1:1.08; (c1 / k1) / (c2 / k2) is 0.54; the ratio of Dv50 of the first negative electrode active material to that of the second negative electrode active material is 1.04:1; the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material is 1:1.08; the areal capacity of the second active layer is 2.9 mAh / cm³. 2The ratio of the areal capacity of the first active layer to that of the second active layer is 1:1; the thickness of the second active layer is 47.7 μm, and h1-h2 is 5.5 μm.

[0161] Example 5 group

[0162] This set of examples is used to verify the impact of changes in "silicon-based materials".

[0163] This set of embodiments is based on Embodiment 1, except that the silicon-based material in the first negative electrode active material and / or the silicon-based material in the second negative electrode active material are changed, as follows:

[0164] Example 5a: The "porous silicon-carbon" in the first negative electrode active material was replaced with the same mass of "carbon-coated silicon-oxygen material". The first negative electrode active material had a Dv50 of 14 μm and a specific surface area of ​​1.4 m². 2 / g, c1 is 2.9%, c1-c2 is 3.4%; the porosity of the first active layer k1 is 53%, k1 / k2 is 1:1.17; (c1 / k1) / (c2 / k2) is 0.54; the ratio of Dv50 of the first negative electrode active material to that of the second negative electrode active material is 1.19:1; the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material is 1:1.29; the thickness of the first active layer is 52.1μm, h1-h2 is 6.8μm;

[0165] In Example 5b, the "porous silicon-carbon" in the second negative electrode active material was replaced with the same mass of "carbon-coated silicon-oxygen material," wherein the Dv50 of the second negative electrode active material was 12.6 μm, and the specific surface area of ​​the second negative electrode active material was 1.8 m². 2 / g, c2 is 7.5%, c1-c2 is 5.1%; the porosity of the second active layer k2 is 58%, k1 / k2 is 1:1.05; (c1 / k1) / (c2 / k2) is 0.34; the ratio of Dv50 of the first negative electrode active material to Dv50 of the second negative electrode active material is 1.03:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1.13; the thickness of the second active layer is 47.1μm, h1-h2 is 6μm;

[0166] In Example 5c, the "porous silicon-carbon" in the first negative electrode active material and the "porous silicon-carbon" in the second negative electrode active material were replaced with the same mass of "carbon-coated silicon-oxygen material". The first negative electrode active material had a Dv50 of 14 μm and a specific surface area of ​​1.4 m². 2 / g, the Dv50 of the second negative electrode active material is 12.6μm, and the specific surface area of ​​the second negative electrode active material is 1.8m².2 / g; c1 is 2.9%, c2 is 7.5%, c1-c2 is 4.6%; the porosity k1 of the first active layer is 53%, the porosity k2 of the second active layer is 58%, k1 / k2 is 1:1.09; (c1 / k1) / (c2 / k2) is 0.42; the ratio of Dv50 of the first negative electrode active material to that of the second negative electrode active material is 1.11:1; the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material is 1:1.29; the thickness of the first active layer is 52.1μm, the thickness of the second active layer is 47.1μm, and h1-h2 is 5μm.

[0167] Example 6

[0168] This embodiment is used to verify the effects of the change in the "second dispersant".

[0169] This embodiment is based on Embodiment 1, except that lithium carboxymethyl cellulose is replaced with sodium carboxymethyl cellulose of the same mass.

[0170] Example 7

[0171] This embodiment is used to verify the impact of the change in the "second conductive agent".

[0172] This embodiment is based on Embodiment 1, except that single-walled carbon nanotubes are replaced with multi-walled carbon nanotubes of the same mass.

[0173] Example 8 group

[0174] This set of examples is used to verify the impact of changes in the "median particle size of the first negative electrode active material" or the "median particle size of the second negative electrode active material".

[0175] This set of embodiments is based on Embodiment 1, except that the "median particle size of the first negative electrode active material" or the "median particle size of the second negative electrode active material" is changed, as follows:

[0176] In Example 8a, the Dv50 of the second negative electrode active material is 4 μm, and the specific surface area of ​​the second negative electrode active material is 2.05 m². 2 / g, wherein the ratio of Dv50 of the first negative electrode active material to Dv50 of the second negative electrode active material is 3.25:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1.28; the thickness of the second active layer is 47.2μm, and h1-h2 is 6μm;

[0177] In Example 8b, the Dv50 of the second negative electrode active material is 14 μm, and the specific surface area of ​​the second negative electrode active material is 1.71 m². 2 / g, wherein the ratio of Dv50 of the first negative electrode active material to Dv50 of the second negative electrode active material is 0.93:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1.07; the thickness of the second active layer is 45.9μm, and h1-h2 is 7.2μm;

[0178] In Example 8c, the Dv50 of the first negative electrode active material is 6 μm, and the specific surface area of ​​the first negative electrode active material is 1.8 m². 2 / g, wherein the ratio of Dv50 of the first negative electrode active material to Dv50 of the second negative electrode active material is 0.51:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1; the thickness of the first active layer is 54.2μm, and h1-h2 is 8.9μm;

[0179] In Example 8d, the Dv50 of the first negative electrode active material was 15 μm, and the specific surface area of ​​the first negative electrode active material was 1.53 m². 2 / g, wherein the ratio of Dv50 of the first negative electrode active material to Dv50 of the second negative electrode active material is 1.27:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1.18; the thickness of the first active layer is 52.9μm, and h1-h2 is 7.6μm.

[0180] Example 9 group

[0181] This set of examples is used to verify the impact of changes to "h1" or "h2".

[0182] This set of embodiments is based on Embodiment 1, except that "h1" or "h2" is changed, as follows:

[0183] In Example 9a, the second negative electrode active material, styrene-butadiene rubber, lithium carboxymethyl cellulose, and single-walled carbon nanotubes were added to deionized water in a mass ratio of 95.05:4:0.8:0.15; wherein the thickness of the second active layer was 52.5 μm, h1-h2 was 0.6 μm, c2 was 6.2%, and c2-c1 was 3.8%;

[0184] In Example 9b, the first negative electrode active material, styrene-butadiene rubber, lithium carboxymethyl cellulose and single-walled carbon nanotubes were added to deionized water in a mass ratio of 97.7:1.5:0.75:0.05; wherein the thickness of the first active layer was 60 μm and h1-h2 was 14.8 μm.

[0185] Example 10 group

[0186] This set of examples is used to verify the impact of the change in "c1".

[0187] This set of embodiments is based on embodiment 3, except that “c1” is changed, as follows:

[0188] Example 10a: The first negative electrode active material (artificial graphite, with a Dv50 of 13 μm and a specific surface area of ​​1.6 m²) was used. 2 / g), styrene-butadiene rubber and lithium carboxymethyl cellulose were added to deionized water in a mass ratio of 97.6:1.4:1; wherein c1 is 0, c1-c2 is 14.2%; the porosity k1 of the first active layer is 40%, k1 / k2 is 1:1.75; (c1 / k1) / (c2 / k2) is 0; the ratio of Dv50 of the first negative electrode active material to Dv50 of the second negative electrode active material is 1.24:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1.5; the thickness of the first active layer is 60μm, and h1-h2 is 24.6μm;

[0189] Example 10b uses a first negative electrode active material (artificial graphite and porous silicon carbon, wherein the mass ratio of artificial graphite to porous silicon carbon is 75:25, the Dv50 of the first negative electrode active material is 10.9 μm, and the specific surface area of ​​the first negative electrode active material is 2.3 m²). 2 A mixture of polyacrylic acid, lithium carboxymethyl cellulose, and single-walled carbon nanotubes in a mass ratio of 94.8:3.8:1:0.4 was added to deionized water. The composition of C1 was 12.2%, and C1-C2 was 2%. The porosity k1 of the first active layer was 69%, and the ratio of k1 / k2 was 1:1.01. The ratio of (c1 / k1) / (c2 / k2) was 0.87. The ratio of the Dv50 of the first negative electrode active material to that of the second negative electrode active material was 1.04:1. The ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material was 1:1.04. The areal capacity of the first active layer was 3.03 mAh / cm³. 2 The ratio of the areal capacity of the first active layer to that of the second active layer is 1:1; the thickness of the first active layer is 35.5 μm, and h1-h2 is 0.1 μm.

[0190] Example 11

[0191] This embodiment is used to verify the impact of the change in "c2".

[0192] This embodiment is based on Embodiment 3, except that "c2" is changed, specifically as follows: the second negative electrode active material (artificial graphite and porous silicon carbon, wherein the mass ratio of artificial graphite to porous silicon carbon is 55:45, the Dv50 of the second negative electrode active material is 9.9 μm, and the specific surface area of ​​the first negative electrode active material is 2.6 m². 2The following components were dissolved in deionized water at a mass ratio of 89.8:8:1.2:1:g (e.g., 1 g), polyacrylic acid, lithium carboxymethyl cellulose, and single-walled carbon nanotubes; c2 was 20.2%, c1-c2 was 13%; the porosity k1 of the second active layer was 73%, k1 / k2 was 1:1.16; (c1 / k1) / (c2 / k2) was 0.41; the ratio of Dv50 of the first negative electrode active material to that of the second negative electrode active material was 1.19:1; the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material was 1:1.3; the areal capacity of the second active layer was 3 mAh / cm³. 2 The ratio of the areal capacity of the first active layer to that of the second active layer is 1:1.03; the thickness of the first active layer is 32.9 μm, and h1-h2 is 11 μm.

[0193] Comparative Example 1

[0194] The procedure was carried out in accordance with Example 1, except that the preparation of the negative electrode was changed, as follows:

[0195] Preparation of negative electrode:

[0196] The negative electrode active material (artificial graphite and porous silicon carbon, with a mass ratio of artificial graphite to porous silicon carbon of 91:9) has a Dv50 of 13.2 μm and a specific surface area of ​​1.65 m². 2 A mixture of polyacrylic acid, lithium carboxymethyl cellulose, and single-walled carbon nanotubes (in mass ratios of 96.5:2.6:0.8:0.1) was added to deionized water to form a negative electrode slurry. A 6μm thick copper foil was used as the negative electrode current collector. The negative electrode slurry was coated onto both sides of the current collector. After drying, cold pressing, and slitting, a negative electrode sheet was obtained. The porosity of the negative electrode active coating was 60%, and the areal capacity was 2.9 mAh / cm³. 2 The thickness is 47.7 μm, and the mass content of elemental Si in the negative electrode active coating is 4.3%.

[0197] Comparative Example 2

[0198] The procedure is carried out in accordance with Example 1, except that the second active layer is changed, as follows:

[0199] The second negative electrode active material (artificial graphite and porous silicon carbon, with a mass ratio of artificial graphite to porous silicon carbon of 93:7) has a Dv50 of 13.8 μm and a specific surface area of ​​1.62 m². 2A mixture of polyacrylic acid, lithium carboxymethyl cellulose, and single-walled carbon nanotubes in a mass ratio of 96.75:2.4:0.75:0.1 was added to deionized water to form a second slurry. The second slurry was then coated onto the other side of the negative electrode current collector to form a second active layer. After drying, cold pressing, and slitting, the negative electrode sheet was obtained.

[0200] Among them, c2 is 3.4%, c1-c2 is 1%; the porosity of the second active layer k2 is 59%, k1 / k2 is 1:1.06; (c1 / k1) / (c2 / k2) is 0.76; the ratio of Dv50 of the first negative electrode active material to Dv50 of the second negative electrode active material is 0.94:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1.01; the thickness of the second active layer is 51.6μm, and h1-h2 is 1.5μm.

[0201] Comparative Example 3

[0202] The procedure is carried out in accordance with Example 1, except that the second active layer is changed, as follows:

[0203] The second negative electrode active material (artificial graphite and porous silicon carbon, with a mass ratio of 50:50) has a Dv50 of 9.6 μm and a specific surface area of ​​2.7 m². 2 A mixture of polyacrylic acid, lithium carboxymethyl cellulose, and single-walled carbon nanotubes in a mass ratio of 89.3:8.5:1.2:1 was added to deionized water to form a second slurry. The second slurry was then coated onto the other side of the negative electrode current collector to form a second active layer. After drying, cold pressing, and slitting, the negative electrode sheet was obtained.

[0204] Among them, c2 is 22.3%, c1-c2 is 19.9%; the porosity of the second active layer k2 is 73%, k1 / k2 is 1:1.33; (c1 / k1) / (c2 / k2) is 0.14; the ratio of Dv50 of the first negative electrode active material to Dv50 of the second negative electrode active material is 1.35:1; the ratio of the specific surface area of ​​the first negative electrode active material to the specific surface area of ​​the second negative electrode active material is 1:1.69; the thickness of the second active layer is 31.8 μm, and h1-h2 is 21.4 μm.

[0205] Test case

[0206] (1) Cyclic performance test

[0207] The batteries prepared in the examples and comparative examples were subjected to cycle performance testing. The specific testing methods are as follows:

[0208] The thickness of the battery sample was tested at 25℃±5℃; then:

[0209] 1. Let stand at 25℃±5℃ for 10 minutes;

[0210] 2. Discharge at 0.2C to the lower limit voltage (3.0V), and let stand for 10 minutes;

[0211] 3. Charge at 0.7C to the upper limit voltage (4.5V), stop at 0.025C, and let stand for 10 minutes;

[0212] 4. Discharge at 0.2C to the lower limit voltage (3.0V) (perform initial capacity test);

[0213] 5. Let stand for 10 minutes;

[0214] 6. Charge at 4.25V with 2C (until 1.5C), charge at 4.3V with 1.5C (until 1C), charge at 4.5V with 1C (until 0.25C), and test the data and thickness of the first full charge.

[0215] 7. Let stand at 25℃±5℃ for 10 minutes;

[0216] 8. Discharge at 0.7C to the lower limit voltage (3.0V), and let stand for 10 minutes;

[0217] 9. Charge at 4.25V using 2C (cut off at 1.5C), charge at 4.3V using 1.5C (cut off at 1C), charge at 4.5V using 1C (cut off at 0.25C), and let stand for 10 minutes;

[0218] Steps 8 and 9 were repeated 500 times, and the results of the cycle capacity retention rate and thickness expansion rate were recorded in Table 1.

[0219] (2) Ratio Performance Test

[0220] The batteries prepared in the examples and comparative examples were subjected to rate performance testing. The specific testing methods are as follows:

[0221] 1. Let stand at 25℃±5℃ for 10 minutes;

[0222] 2. Discharge at 0.2C to the lower limit voltage (3.0V);

[0223] 3. Let stand for 10 minutes;

[0224] 4. Charge to the upper limit voltage (4.5V) at a certain rate (specific rate is as follows), with a cut-off current of 0.025C;

[0225] 5. Let stand for 10 minutes;

[0226] 6. Discharge at 0.2C to the lower limit voltage (3.0V);

[0227] 7. Let stand for 10 minutes;

[0228] Repeat steps 4-7 until all charging rate tests are completed; charging rate: 0.2C / 3C; capacity retention rate = capacity at 3C rate / capacity at 0.2C rate, and record the results in Table 1.

[0229] (3) Energy density test

[0230] The volumetric energy density of the batteries prepared in the examples and comparative examples was tested, and the calculation formula is as follows:

[0231] Volumetric energy density = capacity × platform voltage / battery volume, where capacity is the capacity at 0.2C rate in the above rate performance test, and the results are recorded in Table 1.

[0232] Table 1

[0233]

[0234]

[0235] As shown in Table 1, compared with the comparative example, the lithium-ion secondary battery prepared by the negative electrode sheet of the present invention, with a similar sum of silicon content in the first and second active layers, exhibits superior rate performance and cycle performance while experiencing less energy density loss. For example, compared with Comparative Example 1, the batteries in Examples 1, 6, 7, 8, and 9 achieve both good cycle performance and rate performance with less energy density loss. Compared with Comparative Example 2, Examples 4a and 4d exhibit higher energy density while maintaining similar cycle performance and rate performance.

[0236] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A negative electrode sheet characterized by comprising: The negative electrode sheet comprises a negative electrode current collector and first and second active layers on both sides of the negative electrode current collector respectively; the first active layer comprises a first negative electrode active material, the first negative electrode active material comprising a carbon-based material; the second active layer comprises a second negative electrode active material, the second negative electrode active material comprising a silicon-based material; the mass content of element Si in the first active layer is c1, the mass content of element Si in the second active layer is c2, and c1 and c2 satisfy 1.2%≤c2-c1≤15%.

2. The negative electrode sheet according to claim 1, wherein 3.5%≤c2-c1≤7%; And / or, c1 is 0%-70%; preferably 2%-7.5%; And / or, c2 is 5%-100%; preferably 6%-15%.

3. The negative electrode sheet according to claim 1 or 2, wherein The porosity of the first active layer is k1, and the porosity of the second active layer is k2, and 40%≤k1≤70%; preferably, 55%≤k1≤65%; And / or, 50%≤k2≤80%; preferably, 60%≤k2≤70%.

4. The negative electrode sheet according to claim 3, wherein k1 / k2 is 1:(1.01-2); preferably 1:(1.1-1.15); And / or, c1, c2, k1 and k2 satisfy 0≤(c1 / k1) / (c2 / k2)≤1; preferably, 0.4≤(c1 / k1) / (c2 / k2)≤0.

6.

5. The negative electrode sheet according to claim 1 or 2, wherein The ratio of the median particle size Dv50 of the first negative electrode active material to the median particle size Dv50 of the second negative electrode active material is (0.5-3.5):1; preferably (1.1-1.15):1; And / or, the median particle size Dv50 of the first negative electrode active material is 6 μm-15 μm; And / or, the median particle size Dv50 of the second negative electrode active material is 4 μm-14 μm.

6. The negative electrode sheet according to claim 1 or 2, wherein The ratio of the specific surface area of the first negative electrode active material to the specific surface area of the second negative electrode active material is 1:(1-1.8); preferably 1:(1.1-1.3); Preferably, the specific surface area of the first negative electrode active material is 1 m 2 / g-3.8 m 2 / g; Preferably, the specific surface area of the second negative electrode active material is 1.1 m 2 / g-7 m 2 / g.

7. The negative electrode sheet according to claim 1 or 2, wherein The ratio of the surface capacity of the first active layer to the surface capacity of the second active layer is 1:(1-1.5); and / or the surface capacity of the first active layer is 1.5 mAh / cm 2 - 5 mAh / cm 2 ; and / or the surface capacity of the second active layer is 1.6 mAh / cm 2 - 5.5 mAh / cm 2 .

8. The negative electrode sheet according to claim 1 or 2, wherein The thickness of the first active layer is h1, and the thickness of the second active layer is h2, satisfying 0.01 μm≤h1-h2≤50 μm; preferably, 7 μm≤h1-h2≤9 μm; And / or, h1 is 21 μm-90 μm; preferably 40 μm-55 μm; And / or, h2 is 16 μm-80 μm; preferably 33 μm-48 μm.

9. The negative electrode sheet according to claim 1 or 2, wherein The first active layer further comprises a first dispersant, and the second active layer further comprises a second dispersant; preferably, the first dispersant comprises at least one of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose, and the second dispersant comprises lithium carboxymethyl cellulose; And / or, the first active layer further comprises a first conductive agent, and the second active layer further comprises a second conductive agent; preferably, the second conductive agent comprises carbon nanotubes.

10. A lithium-ion secondary battery, characterized by comprising: The lithium ion secondary battery comprises the negative electrode sheet according to any one of claims 1-9.

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