A negative electrode sheet, a preparation method thereof, and an application thereof

Abstract

The present invention provides a negative electrode sheet, a preparation method thereof and an application. The negative electrode sheet includes a current collector and active layers on both sides of the current collector. The active layer includes at least two coatings, and starting from the coating closer to the current collector and moving outwards in sequence, the content of the fast charging material in the coatings increases in sequence, while the content of the conductive agent decreases in sequence. The active layer of the present invention is composed of coatings with different contents of fast charging material and conductive agent, enabling the fast charging material to be combined with the conductive agent. While having less fast charging material on the inner side, compared with the outer part, it has more conductive agent, ensuring the lithium ion transmission of the inner coating, and at the same time maximizing the energy density of the electrode sheet. Therefore, while effectively exerting the advantages of the fast charging material, the present invention cooperates with the conductive agent, improves the compaction density of the negative electrode sheet, and avoids the problem that the compaction of the negative electrode sheet caused by simply coating the fast charging material on the surface layer is too low, which is not conducive to improving the energy density of the battery cell.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and relates to a negative electrode sheet, a preparation method thereof and an application thereof. Background Art

[0002] As the most promising secondary battery, lithium-ion batteries have been widely used in various fields of life, including electric vehicles, power tools and digital devices, etc. Different application scenarios have different requirements for the electrical performance characteristics of the batteries. For example, the cylindrical batteries commonly used in power tools generally achieve rapid ion and electron transmission by reducing the thickness of the positive and negative electrode sheets, and then achieve rapid charging and discharging to improve the rate performance. However, in the field of electric vehicles, to solve the range anxiety problem, it is necessary to ensure the fast charging performance of the battery, so that it can meet the fast charging within 30 minutes, or even within 10 minutes, while also ensuring the battery energy density and increasing the endurance. In the design of existing power batteries, to increase the battery energy density and ensure the endurance, in addition to using higher energy materials such as NCM811 positive electrodes and silicon-containing negative electrodes, thicker electrodes will also be designed, and some expensive conductive agents such as graphene and carbon nanotubes will be introduced or added to the positive or negative electrodes, or pore-forming agents will be added to increase the porosity of the electrodes to ensure the rapid ion and electron transmission in the system.

[0003] For example, CN 109167020A discloses a preparation method of a porous lithium-ion electrode sheet with high energy density and a lithium-ion battery. The preparation method includes the following steps: S1, dissolving a binder with N-methylpyrrolidone to obtain a glue solution, adding an active substance and a conductive agent to the glue solution and mixing evenly to obtain a slurry; S2, coating the mixed slurry on a current collector respectively to obtain an electrode sheet; S3, cold-pressing the coated electrode sheet, and then brushing a pore-forming agent on the surface of the cold-pressed electrode sheet. By means of a coating machine device, the pore-forming agent is evenly coated on the surface of the cold-pressed electrode sheet, and dried while coating. The drying temperature is set to be higher than the decomposition temperature of the pore-forming agent to obtain a porous lithium-ion electrode sheet with high energy density. By preparing a thick electrode with a relatively high surface density, the pore-forming agent is brushed on the surface of the electrode sheet after cold pressing to improve the porosity of the thick electrode sheet and improve the electrical performance. However, this method will increase the battery cost and produce some by-products that affect the battery performance, and it is impossible to take into account the fast charging performance and energy density of the battery cell.

[0004] In addition, the prior art improves the fast charging performance of the battery cell by introducing materials such as hard carbon into the negative electrode. Compared with the two-dimensional interlayer lithium deintercalation of traditional graphite (theoretical specific capacity ~372mAh / g), hard carbon has higher cycle stability and more excellent rate performance, and has a higher theoretical specific capacity of ~700mAh / g; however, hard carbon has a lower compaction density, and is generally introduced by a small amount of blending or surface coating. Moreover, only a small amount of blending cannot fully exert the fast charging characteristics of hard carbon, and the surface coating method will also lead to the problem of too low surface compaction, which is not conducive to improving the energy density of the battery cell.

[0005] Based on the above research, it is necessary to provide a negative electrode sheet that can take into account the high-rate fast charging characteristics while also improving the compaction density of the negative electrode sheet to ensure the energy density of the battery. Summary of the invention

[0006] The object of the present invention is to provide a negative electrode sheet and a preparation method and application thereof, wherein the active layer of the negative electrode sheet is gradiently doped with a fast-charging material that can improve the fast-charging performance, so that the negative electrode sheet can have good fast-charging performance without reducing the energy density of the negative electrode sheet.

[0007] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a negative electrode sheet, comprising a current collector and an active layer on both sides of the current collector, wherein the active layer comprises at least two coating layers, and along the coating layer close to the current collector toward the outside, the content of fast charging material in the coating layer increases successively, and the content of conductive agent decreases successively.

[0009] The active layer described in the present invention is composed of coatings with different contents of fast-charging materials and conductive agents, and from the side close to the current collector to the outside, the content of fast-charging materials in different coatings increases and the content of conductive agents decreases, that is, the fast-charging material content on the innermost side is the least to ensure the energy density of the electrode, and the fast-charging material content in the outermost coating is high, which can quickly transfer lithium ions and avoid the accumulation of lithium ions on the surface of the negative electrode to cause lithium precipitation. At the same time, the conductive agent is combined so that the inner side has less fast-charging material and more conductive agent than the outer side, thereby ensuring the lithium ion transmission of the inner coating and maximizing the energy density of the electrode. Therefore, while effectively exerting the advantages of the fast-charging material, it cooperates with the conductive agent to improve the compaction density of the negative electrode, avoiding the problem of too low compaction of the negative electrode due to simply coating the fast-charging material on the surface, which is not conducive to improving the energy density of the battery cell.

[0010] The active layer includes at least two coatings, for example, it can be two, three, four, or five layers, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable; Exemplarily, when the active material includes two coatings, the active layer includes a first coating and a second coating stacked in sequence, the content of the fast charging material in the first coating is less than that in the second coating, and the content of the conductive agent is greater than that in the second coating; When the active material includes three coatings, the active layer includes a first coating, a second coating, and a third coating stacked in sequence. From the first coating to the third coating, the content of the fast charging material increases in sequence, and the content of the conductive agent decreases in sequence.

[0011] Preferably, the fast charging material includes any one or at least two combinations of hard carbon, soft carbon, or graphene. Typical but non-limiting combinations include a combination of hard carbon and soft carbon, a combination of soft carbon and graphene, or a combination of hard carbon and graphene.

[0012] The fast charging material described in the present invention has a high lithium deintercalation ability, which can greatly improve the fast charging performance of the battery. For example, hard carbon and soft carbon have no fixed morphological structure and are mainly composed of graphite microcrystals and amorphous regions. They store lithium through micropore adsorption and interlayer insertion, thus having more excellent rate performance. In addition, graphene is a single-layer graphite and belongs to two-dimensional materials. It has extremely strong conductivity, and the transmission rate of lithium ions in graphene is faster. Therefore, it can improve the fast charging performance of the battery.

[0013] Preferably, the active layer includes a first coating and a second coating stacked in sequence, where the first coating is closer to the current collector.

[0014] The active layer described in the present invention is preferably two layers. More than three layers will increase the preparation difficulty, and moreover, it will not significantly improve the fast charging performance and energy density of the material, and the performance of the electrode obtained with two coatings is not much different.

[0015] Preferably, the content of the binder in the first coating is greater than the content of the binder in the second coating.

[0016] In the present invention, the content of the binder in the first coating closer to the current collector is higher, and the content of the binder in the outer second coating is lower, which can solve the problems of low peeling force and material dropping of the electrode caused by the floating of the binder during the baking process, and to a certain extent, improve and ensure the safety performance and electrochemical performance of the electrode during long-term use.

[0017] Preferably, the content of the active material in the first coating is not less than the content of the active material in the second coating.

[0018] Preferably, the thickness of the first coating is not less than the thickness of the second coating.

[0019] The thickness of the coating of the present invention can cooperate with the content of the active material. The thickness of the first coating near the inner side of the current collector is not less than that of the outer layer, and the content of the active material is not less than that of the second coating. On the one hand, it can make the relative content of the active material in the negative electrode sheet higher, ensuring the energy density of the negative electrode sheet. On the other hand, it does not affect the function of the fast charging material and improves the transmission rate of lithium ions.

[0020] Preferably, in the first coating, the content of the conductive agent is 2-8 wt%, for example, it can be 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt% or 8 wt%, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0021] Preferably, in the second coating, the content of the conductive agent is 1-7 wt%, for example, it can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt% or 7 wt%, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0022] Preferably, in the first coating, the content of the fast charging material is 1-20 wt%, for example, it can be 1 wt%, 3 wt%, 5 wt%, 7 wt%, 9 wt%, 11 wt%, 13 wt%, 15 wt%, 17 wt%, 19 wt% or 20 wt%, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0023] Preferably, in the second coating, the content of the fast charging material is 5-30 wt%, for example, it can be 5 wt%, 8 wt%, 10 wt%, 12.5 wt%, 15 wt%, 17.5 wt%, 20 wt%, 22.5 wt%, 25 wt%, 27.5 wt% or 30 wt%, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0024] The content of the fast charging material in the second coating and the first coating of the present invention should be matched with each other to achieve the improvement of the fast charging performance of the material while ensuring the energy density.

[0025] Preferably, the thickness ratio of the first coating to the second coating is (4-9):(1-6), for example, it can be 4:1, 5:2, 6:5 or 9:6, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0026] Preferably, the thickness of the first coating is 90-162 μm, for example, it can be 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm or 162 μm, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0027] Preferably, in the first coating, the content of the binder is 2-4 wt%, for example, it can be 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt% or 4 wt%, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0028] Preferably, in the second coating, the content of the binder is 1-3 wt%, for example, it can be 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt% or 3 wt%, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0029] Preferably, in the first coating, the content of the active material is 70-85 wt%, for example, it can be 70 wt%, 72.5 wt%, 75 wt%, 77.5 wt%, 80 wt%, 82.5 wt% or 85 wt%, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0030] Preferably, in the second coating, the content of the active material is 70-85 wt%, for example, it can be 70 wt%, 72.5 wt%, 75 wt%, 77.5 wt%, 80 wt%, 82.5 wt% or 85 wt%, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0031] Preferably, the active material includes artificial graphite and / or natural graphite.

[0032] Preferably, the conductive agent includes Super-P and / or conductive graphite.

[0033] Preferably, the binder includes carboxymethyl cellulose (CMC) and / or styrene-butadiene rubber (SBR).

[0034] Preferably, the current collector includes copper foil.

[0035] In a second aspect, the present invention provides a method for preparing the negative electrode sheet as described in the first aspect, and the preparation method includes the following steps:

[0036] Coat the active layer on the surface of the current collector according to the formula amount, and obtain the negative electrode sheet after drying.

[0037] Preferably, the coating method includes double-layer coating.

[0038] In a third aspect, the present invention provides a lithium-ion battery, and the lithium-ion battery includes the negative electrode sheet as described in the first aspect.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] (1) By setting the active layer as at least two coatings, and arranging the content of the fast-charging material to increase successively from the inner coating outwards and the content of the conductive agent to decrease successively, the present invention avoids the problem that the fast-charging material cannot function when directly incorporated or the energy density of the negative electrode sheet decreases when directly coated. It enables the fast-charging material and the conductive agent to cooperate with each other, ensuring both the energy density of the negative electrode sheet and excellent fast-charging performance.

[0041] (2) In order to further ensure the energy density and fast-charging performance of the active layer during long-term use, the present invention sets a binder with a relatively high content on the inner side close to the current collector, avoiding the problem of the binder floating up and causing cracking and powder falling off of the negative electrode sheet. Therefore, it not only further improves the energy density but also ensures the fast-charging performance of the negative electrode sheet.

[0042] (3) The thickness of the coating of the present invention is matched with the content of the active material, such that the thickness and the content of the active material on the inner side are not lower than those on the outer side, thereby ensuring the energy density of the negative electrode sheet. At the same time, it can be matched with the higher content of the conductive agent on the inner side to ensure the transmission of lithium ions and improve the fast-charging performance of the battery. Description of the Drawings

[0043] Figure 1 is a schematic structural diagram of the negative electrode sheet described in Embodiment 1 of the present invention;

[0044] Wherein, 1 - the first coating, 2 - the second coating, 3 - the active layer, 4 - the current collector. Detailed Embodiments

[0045] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations to the present invention.

[0046] Embodiment 1

[0047] This embodiment provides a negative electrode sheet as Figure 1 shown, the negative electrode sheet includes a current collector 4 and active layers 3 on both sides of the current collector 4. The current collector 4 is a copper foil, and the active layer 3 is composed of a first coating 1 and a second coating 2. The first coating 1 is close to the current collector 4, and the content of hard carbon in the first coating 1 is less than the content of hard carbon in the second coating 2, and the content of Super-P in the first coating 1 is greater than the content of Super-P in the second coating 2;

[0048] The content of SBR in the first coating 1 is greater than the content of SBR in the second coating 2, the content of artificial graphite in the first coating 1 is greater than the content of artificial graphite in the second coating 2, and the thickness of the first coating 1 is the same as the thickness of the second coating 2;

[0049] In the first coating 1, the content of hard carbon is 18.75 wt%, the content of Super-P is 3.25 wt%, the content of SBR is 3 wt%, the content of artificial graphite is 75 wt%, and the thickness of the first coating 1 is 90 μm. Among them, the mass of hard carbon accounts for 20% of the total mass of hard carbon and artificial graphite;

[0050] In the second coating 2, the content of hard carbon is 27 wt%, the content of Super-P is 1 wt%, the content of SBR is 2 wt%, the content of artificial graphite is 70 wt%, the thickness ratio of the first coating 1 to the second coating 2 is 1:1, and the thickness of the second coating 2 is 90 μm. Among them, the mass of hard carbon accounts for 27.8% of the total mass of hard carbon and artificial graphite;

[0051] The preparation method of the negative electrode sheet includes the following steps:

[0052] Prepare the slurries of the first coating 1 and the second coating 2 according to the formula amounts, and coat the slurries of the first coating 1 and the second coating 2 on the current collector 4 by a double-layer coating method, wherein the slurry of the first coating 1 is coated close to the current collector to obtain the negative electrode sheet.

[0053] Example 2

[0054] This example provides a negative electrode sheet, which includes a current collector and active layers on both sides of the current collector. The current collector is a copper foil, and the active layer is composed of a first coating and a second coating. The first coating is close to the current collector, and the content of hard carbon in the first coating is less than that in the second coating, and the content of conductive graphite in the first coating is greater than that in the second coating;

[0055] The content of CMC in the first coating is greater than that in the second coating, the content of natural graphite in the first coating is greater than that in the second coating, and the thickness of the first coating is the same as that of the second coating;

[0056] In the first coating, the content of hard carbon is 11 wt%, the content of conductive graphite is 2 wt%, the content of natural graphite is 85 wt%, the content of CMC is 2 wt%, and the thickness is 126 μm. Among them, the mass of hard carbon accounts for 11.4% of the total mass of hard carbon and natural graphite;

[0057] In the second coating, the content of hard carbon is 28 wt%, the content of conductive graphite is 1 wt%, the content of CMC is 1 wt%, the content of natural graphite is 70 wt%, the thickness ratio of the first coating to the second coating is 6:4, and the thickness of the second coating is 54 μm. Among them, the mass of hard carbon accounts for 28.6% of the total mass of hard carbon and natural graphite;

[0058] The preparation method of the negative electrode sheet includes the following steps:

[0059] Prepare the first coating slurry and the second coating slurry according to the formulation amounts, and coat the first coating slurry and the second coating slurry on the current collector by means of double-layer coating, wherein the first coating slurry is coated closer to the current collector to obtain the negative electrode sheet.

[0060] Example 3

[0061] This example provides a negative electrode sheet, which includes a current collector and active layers on both sides of the current collector. The current collector is a copper foil, and the active layer is composed of a first coating and a second coating. The first coating is closer to the current collector, and the content of hard carbon in the first coating is less than that in the second coating, and the content of conductive graphite in the first coating is greater than that in the second coating;

[0062] The content of CMC in the first coating is greater than that in the second coating, the content of natural graphite in the first coating is greater than that in the second coating, and the thickness of the first coating is the same as that of the second coating;

[0063] In the first coating, the content of hard carbon is 10 wt%, the content of conductive graphite is 4 wt%, the content of natural graphite is 82 wt%, the content of CMC is 4 wt%, and the thickness is 126 μm. Among them, the mass of hard carbon accounts for 10.9% of the total mass of hard carbon and natural graphite;

[0064] In the second coating, the content of hard carbon is 18.75 wt%, the content of conductive graphite is 3.25 wt%, the content of natural graphite is 75 wt%, the content of CMC is 3 wt%, the thickness ratio of the first coating to the second coating is 6:4, and the thickness of the second coating is 54 μm. Among them, the mass of hard carbon accounts for 20% of the total mass of hard carbon and natural graphite;

[0065] The preparation method of the negative electrode sheet includes the following steps:

[0066] Prepare the first coating slurry and the second coating slurry according to the formulation amounts, and coat the first coating slurry and the second coating slurry on the current collector by means of double-layer coating, wherein the first coating slurry is coated closer to the current collector to obtain the negative electrode sheet.

[0067] Example 4

[0068] This example provides a negative electrode sheet, which includes a current collector and active layers on both sides of the current collector. The current collector is a copper foil, and the active layer is composed of a first coating and a second coating. The first coating is closer to the current collector, and the content of hard carbon in the first coating is less than that in the second coating, and the content of conductive graphite in the first coating is greater than that in the second coating;

[0069] The content of CMC in the first coating is greater than that in the second coating, the content of natural graphite in the first coating is greater than that in the second coating, and the thickness of the first coating is the same as that of the second coating;

[0070] In the first coating, the content of hard carbon is 5 wt%, the content of conductive graphite is 8 wt%, the content of natural graphite is 83 wt%, the content of CMC is 4 wt%, and the thickness is 126 μm. Among them, the mass of hard carbon accounts for 5.7% of the total mass of hard carbon and natural graphite;

[0071] In the second coating, the content of hard carbon is 10 wt%, the content of conductive graphite is 6 wt%, the content of natural graphite is 82 wt%, the content of CMC is 2 wt%. The thickness ratio of the first coating to the second coating is 6:4, and the thickness of the second coating is 54 μm. Among them, the mass of hard carbon accounts for 10.9% of the total mass of hard carbon and natural graphite;

[0072] The preparation method of the negative electrode sheet includes the following steps:

[0073] Prepare the first coating slurry and the second coating slurry according to the formula amounts, and use the double-layer coating method to coat the first coating slurry and the second coating slurry on the current collector. Among them, the first coating slurry is coated closer to the current collector to obtain the negative electrode sheet.

[0074] Example 5

[0075] This example provides a negative electrode sheet, which includes a current collector and active layers on both sides of the current collector. The current collector is a copper foil, and the active layer is composed of a first coating, a second coating and a third coating. The first coating is close to the current collector, and the content of hard carbon in the first coating is less than that in the second coating, and the content of hard carbon in the second coating is less than that in the third coating. The content of Super-P in the first coating is greater than that in the second coating, and the content of Super-P in the second coating is greater than that in the third coating;

[0076] The content of SBR in the first coating is greater than that in the second coating, the content of SBR in the second coating is greater than that in the third coating, the content of artificial graphite in the first coating is greater than that in the second coating, and the content of artificial graphite in the second coating is greater than that in the third coating;

[0077] In the first coating, the content of hard carbon is 5 wt%, the content of Super-P is 8 wt%, the content of natural graphite is 83 wt%, the content of SBR is 4 wt%, and the thickness is 126 μm. Among them, the mass of hard carbon accounts for 5.7% of the total mass of hard carbon and natural graphite;

[0078] In the second coating, the content of hard carbon is 10 wt%, the content of Super-P is 6 wt%, the content of natural graphite is 82 wt%, the content of SBR is 2 wt%, the thickness ratio of the first coating to the second coating is 6:4, and the thickness of the second coating is 30 μm. Among them, the mass of hard carbon accounts for 10.9% of the total mass of hard carbon and natural graphite;

[0079] In the third coating, the content of hard carbon is 18.75 wt%, the content of Super-P is 4.75 wt%, the content of natural graphite is 75 wt%, the content of SBR is 1.5 wt%, the thickness of the third coating is 24 μm. Among them, the mass of hard carbon accounts for 20% of the total mass of hard carbon and natural graphite;

[0080] The preparation method of the negative electrode sheet includes the following steps:

[0081] Prepare the first coating slurry, the second coating slurry and the third coating slurry according to the formula amount, and use the three-layer coating method to coat the first coating slurry, the second coating slurry and the third coating slurry on the current collector, wherein the first coating slurry is coated close to the current collector to obtain the negative electrode sheet.

[0082] Example 6

[0083] This example provides a negative electrode sheet. The difference between this negative electrode sheet and that in Example 1 is that in the second coating, the content of hard carbon is 39 wt%, the content of Super-P is 1 wt%, the content of SBR is 2 wt%, the content of artificial graphite is 58 wt%, the thickness of the first coating is 162 μm, the thickness ratio of the first coating to the second coating is 9:1, and the thickness of the second coating is 18 μm. Among them, the mass of hard carbon accounts for 40.2% of the total mass of hard carbon and artificial graphite, and the rest are the same as those in Example 1.

[0084] Example 7

[0085] This example provides a negative electrode sheet. The difference between this negative electrode sheet and that in Example 1 is that in the second coating, the content of hard carbon is 39 wt%, the content of Super-P is 1 wt%, the content of SBR is 2 wt%, the content of artificial graphite is 58 wt%, the thickness ratio of the first coating to the second coating is 1:1, and the thickness of the second coating is 90 μm. Among them, the mass of hard carbon accounts for 40.2% of the total mass of hard carbon and artificial graphite, and the rest are the same as those in Example 1.

[0086] Example 8

[0087] This embodiment provides a negative electrode sheet. The difference between this negative electrode sheet and that of Embodiment 1 is that the thickness of the first coating is 70 μm, the thickness of the second coating is 110 μm, and the thickness ratio of the two is 1:1.5. Except for this, the rest are the same as those of Embodiment 1.

[0088] Embodiment 9

[0089] This embodiment provides a negative electrode sheet. The difference between this negative electrode sheet and that of Embodiment 1 is that in the first coating, the content of Super-P is 4.25 wt%, and the content of SBR is 2 wt%, so that the content of the binder in the first coating and the second coating is the same. Except for this, the rest are the same as those of Embodiment 1.

[0090] Embodiment 10

[0091] This embodiment provides a negative electrode sheet. The difference between this negative electrode sheet and that of Embodiment 1 is that in the first coating, the content of Super-P is 5.25 wt%, and the content of SBR is 1 wt%, so that the content of the binder in the first coating is lower than that in the second coating. Except for this, the rest are the same as those of Embodiment 1.

[0092] Embodiment 11

[0093] This embodiment provides a negative electrode sheet. The difference between this negative electrode sheet and that of Embodiment 1 is that the hard carbon is replaced with soft carbon in equal mass. Except for this, the rest are the same as those of Embodiment 1.

[0094] Embodiment 12

[0095] This embodiment provides a negative electrode sheet. The difference between this negative electrode sheet and that of Embodiment 1 is that the hard carbon is replaced with graphene in equal mass. Except for this, the rest are the same as those of Embodiment 1.

[0096] Comparative Example 1

[0097] This comparative example provides a negative electrode sheet. The negative electrode sheet includes a current collector and active layers on both sides of the current collector. In the active layer, the content of artificial graphite is 93.75 wt%, the content of Super-P is 3.25 wt%, the content of SBR is 3 wt%, and the thickness of the active layer is 180 μm.

[0098] Comparative Example 2

[0099] This comparative example provides a negative electrode sheet. The negative electrode sheet includes a current collector and active layers on both sides of the current collector. In the active layer, the content of hard carbon is 5 wt%, the content of artificial graphite is 88.75 wt%, the content of Super-P is 3.25 wt%, the content of SBR is 3 wt%, the thickness of the active layer is 180 μm, and the mass of hard carbon accounts for 5.7% of the total mass of hard carbon and artificial graphite.

[0100] Comparative Example 3

[0101] This comparative example provides a negative electrode sheet, which includes a current collector and active layers on both sides of the current collector. In the active layer, the content of artificial graphite is 93.75 wt%, the content of Super-P is 3.25 wt%, the content of SBR is 3 wt%, and the thickness of the active layer is 170 μm;

[0102] A layer of hard carbon layer is also coated on the surface of the active layer. In the hard carbon layer, the content of hard carbon is 93.75 wt%, the content of Super-P is 3.25 wt%, the content of SBR is 3 wt%, and the thickness of the active layer is 10 μm.

[0103] Comparative Example 4

[0104] This comparative example provides a negative electrode sheet, the difference between this negative electrode sheet and Example 1 is that in the first coating, the content of Super-P is 1 wt% and the content of SBR is 5 wt%, and the rest are the same as in Example 1.

[0105] The negative electrode sheets of the above examples and comparative examples were tested for their tap density, and then assembled into batteries with NCM811 positive electrode sheets, PE diaphragms and lithium hexafluorophosphate electrolytes. After normal formation, aging and grading, their negative electrode specific capacity, battery energy density, 6C fast charge capacity retention rate, and whether there is lithium deposition after full charge at 5C fast charge were tested; among them, the test conditions were: charge and discharge voltage range: 2.5V - 4.2V, and the test results are shown in Table 1.

[0106] Table 1

[0107]

[0108] It can be seen from Table 1 as follows:

[0109] (1) The fast charge materials added in the negative electrode sheet of the present invention enable the battery to not only have good fast charge performance and be able to inhibit lithium deposition on the negative electrode surface, but also have a high energy density; among them, on the basis of Example 4, the active layer of Example 5 includes three coatings, and the total thickness is the same as that of Example 4, but the performance is similar to that of Example 4, and the coating difficulty increases; from Example 1 and Examples 6 - 7, it can be seen that there is too much fast charge material in the second coating of Example 6. Although it is paired with a thinner thickness of the second coating, it will still reduce the tap density and negative electrode specific capacity of the negative electrode sheet. If too much fast charge material in Example 7 is paired with an excessive thickness, it will further reduce the tap density, negative electrode specific capacity and energy density of the negative electrode sheet, which is not conducive to the improvement of comprehensive performance; from Example 1 and Example 8, it can be seen that the thickness of the first coating is greater than that of the second coating, which is beneficial to improving the specific capacity, tap density and energy density of the negative electrode sheet.

[0110] (2) As can be seen from Example 1 and Examples 9-10, there is no difference in the binder content in the first coating and the second coating, which has little impact on the fast charging performance. When the binder content in the second coating is relatively high, the corresponding conductive agent increases, and the fast charging performance becomes better. However, there is a risk of material loss and deterioration in the long-term performance, resulting in a decrease in the safety performance and electrochemical performance of the battery during long-term use. Therefore, a reasonable binder content needs to be ensured to simultaneously guarantee the fast charging performance and energy density, as well as the safety performance and electrical performance during long-term use; as can be seen from Example 1 and Examples 11-12, soft carbon and graphene can also be used as fast charging materials to improve the fast charging performance of the battery. Although graphene has better rate performance and energy density, it will significantly increase the preparation cost; as can be seen from Example 1 and Comparative Example 1, the battery obtained from the negative electrode sheet of the present invention has good fast charging performance and can also guarantee the energy density of the battery; as can be seen from Example 1 and Comparative Example 2, Comparative Example 2 uses a conventional method to directly incorporate a fast charging material into the active layer, but the fast charging material cannot play its role and cannot effectively improve the fast charging performance of the battery; as can be seen from Example 1 and Comparative Example 3, using the method of directly coating a fast charging material layer on the surface of the active layer can improve the fast charging performance, but it cannot guarantee the energy density of the battery; as can be seen from Example 1 and Example 4, when the conductive agent content in the first coating and the second coating is the same and there is no difference, it cannot be matched with the fast charging material, and therefore, the fast charging performance of the battery cannot be significantly improved.

[0111] In summary, the present invention provides a negative electrode sheet, a preparation method thereof, and an application thereof. The negative electrode sheet can take into account the characteristics of fast charging at high rates while improving the compaction density of the negative electrode sheet and ensuring the energy density of the battery.

[0112] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a current collector and active layers on both sides of the current collector. The active layers include at least two coatings, and starting from the coating closer to the current collector and moving outwards in sequence, the content of the fast charging material in the coatings increases in sequence, while the content of the conductive agent decreases in sequence; The active layer includes a first coating and a second coating stacked in sequence, with the first coating closer to the current collector; The first coating includes an active material, a fast charging material, and a conductive agent. In the first coating, the content of the fast charging material is 1-20 wt%, and the content of the conductive agent is 2-8 wt%; The second coating includes an active material, a fast charging material, and a conductive agent. In the second coating, the content of the fast charging material is 5-30 wt%, and the content of the conductive agent is 1-7 wt%; The thickness of the first coating is 90-162 μm, and the thickness ratio of the first coating to the second coating is (4-9):(1-6).

2. The negative electrode sheet according to claim 1, wherein, The fast charging material includes any one or a combination of at least two of hard carbon, soft carbon, or graphene.

3. The negative electrode sheet according to claim 1, characterized in that, The content of the binder in the first coating is greater than the content of the binder in the second coating.

4. The negative electrode sheet according to claim 1, characterized in that, The content of the active material in the first coating is not less than the content of the active material in the second coating.

5. The negative electrode sheet according to claim 1, characterized in that, The thickness of the first coating is not less than the thickness of the second coating.

6. The negative electrode sheet according to claim 1, characterized in that, In the first coating, the content of the binder is 2-4 wt%.

7. The negative electrode sheet according to claim 1, characterized in that, In the second coating, the content of the binder is 1-3 wt%.

8. The negative electrode sheet according to claim 1, characterized in that, In the first coating, the content of the active material is 70-85 wt%.

9. The negative electrode sheet according to claim 1, characterized in that, In the second coating, the content of the active material is 70-85 wt%.

10. The negative electrode sheet according to claim 1, wherein, The active material includes artificial graphite and / or natural graphite.

11. The negative electrode sheet according to claim 1, characterized in that, The conductive agent includes Super-P and / or conductive graphite.

12. The negative electrode sheet according to claim 3, wherein, The binder includes carboxymethyl cellulose and / or styrene-butadiene rubber.

13. The negative electrode sheet according to claim 1, characterized in that, The current collector includes copper foil.

14. A method for preparing a negative electrode sheet according to any one of claims 1-13, characterized in that, The preparation method includes the following steps: Coat the active layer on the surface of the current collector according to the formula amount, and obtain the negative electrode sheet after drying.

15. A lithium-ion battery, characterized in that, The lithium-ion battery includes the negative electrode sheet according to any one of claims 1-13.

Images