A negative electrode sheet and a battery

Abstract

The present invention provides a negative electrode sheet and a battery. The negative electrode sheet includes a current collector and an active layer. The active layer is disposed on both opposite surfaces of the current collector. The active layer includes a first functional material. In the direction away from the current collector, the content of the first functional material increases, so that the content of the first functional material in the first region of the active layer is less than the content of the first functional material in the second region of the active layer. The perpendicular distance from the first region to the current collector is less than the perpendicular distance from the second region to the current collector. The first functional material includes at least one of a silicon-based material, a metal oxide, and a metal sulfide. By adding the first functional material with a larger expansion rate and a higher lithium deintercalation / insertion ability to the active layer, while increasing the porosity of the active layer, the negative impact on the energy density of the negative electrode sheet is reduced, the migration rate of lithium ions in the active layer is increased, thereby improving the energy density and kinetics of the negative electrode sheet, and further improving the fast charging performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular, to a negative electrode sheet and a battery. Background Art

[0002] Lithium batteries are widely used in fields such as consumer electronics, new energy vehicles, military aerospace, etc. With the progress of technology, higher requirements are put forward for the charging ability and energy density of lithium-ion batteries.

[0003] In the prior art, generally, the energy density of the battery is improved by increasing the thickness of the electrode sheet. However, the increase in the thickness of the electrode sheet leads to an increase in the migration distance of lithium ions; or, the energy density of the battery is improved by increasing the compaction density of the electrode sheet. However, the increase in the compaction density leads to a decrease in the porosity of the electrode sheet. And the increase in the migration distance or the decrease in the porosity may both lead to a decrease in the charging ability of the lithium-ion battery.

[0004] It can be seen that there is a problem of poor battery performance in the prior art. Summary of the Invention

[0005] Embodiments of the present invention provide a negative electrode sheet and a battery to solve the problem of poor battery performance in the prior art.

[0006] Embodiments of the present invention provide a negative electrode sheet, including a current collector and an active layer. The active layer is disposed on two opposite side surfaces of the current collector. The active layer includes a first functional material. In the direction away from the current collector, the content of the first functional material increases, so that the content of the first functional material in the first region of the active layer is less than the content of the first functional material in the second region of the active layer. The perpendicular distance from the first region to the current collector is less than the perpendicular distance from the second region to the current collector. The first functional material includes at least one of a silicon-based material, a metal oxide, and a metal sulfide.

[0007] Optionally, the active layer further includes a second functional material. In the direction away from the current collector, the content of the second functional material increases, so that the content of the second functional material in the first region is less than the content of the second functional material in the second region.

[0008] Optionally, the conductivity of the second functional material is greater than the conductivity of the conductive agent in the active layer.

[0009] Optionally, the active layer at least includes a first sub-active layer, a second sub-active layer, and a third sub-active layer. The first sub-active layer is disposed on one side surface of the current collector. The second sub-active layer is disposed on the first sub-active layer. The third sub-active layer is disposed on the second sub-active layer.

[0010] The content of the first functional material increases in the direction from the first sub-active layer to the third sub-active layer.

[0011] Optionally, the active layer further includes a second functional material, and the content of the second functional material increases in the direction from the first sub-active layer to the third sub-active layer.

[0012] Optionally, the first sub-active layer includes a first active material, a first conductive agent, and a first binder, and the mass percentage content range ratio among the first active material, the first conductive agent, and the first binder is: (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%).

[0013] Optionally, the second sub-active layer includes a second active material, a second conductive agent, and a second binder, and the mass percentage content range ratio among the second active material, the second conductive agent, and the second binder is: (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%);

[0014] wherein, the second active material includes A1% of the first functional material, and the second conductive agent includes A2% of the second functional material.

[0015] Optionally, the third sub-active layer includes a third active material, a third conductive agent, and a third binder, and the mass percentage content range ratio among the third active material, the third conductive agent, and the third binder is: (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%);

[0016] wherein, the third active material includes B1% of the first functional material, and the third conductive agent includes B2% of the second functional material, and B1 is greater than A1, and B2 is greater than A2.

[0017] Optionally, the first sub-active layer, the second sub-active layer, and the third sub-active layer have the same thickness; or

[0018] the thickness of any one of the second sub-active layer and the third sub-active layer is less than the thickness of the first sub-active layer; or

[0019] the thickness of the third sub-active layer is less than the thickness of the second sub-active layer, and the thickness of the second sub-active layer is less than the thickness of the first sub-active layer.

[0020] An embodiment of the present invention further provides a battery, including the negative electrode sheet described above.

[0021] In the embodiments of the present invention, by adding a first functional material to the active layer, the first functional material has a larger expansion rate and a higher lithium deintercalation / insertion capacity, which increases the porosity of the active layer while reducing the negative impact on the energy density of the negative electrode sheet. The increase in porosity improves the migration rate of lithium ions in the active layer, thereby improving the energy density and kinetics of the negative electrode sheet, and further improving the fast charging performance of the battery.

[0022] Moreover, in the direction away from the current collector, the content of the first functional material increases, so that the content of the first functional material in the first region is less than that in the second region to form a concentration gradient difference and enhance the mass transfer capacity of the active layer; a lower content of the first functional material is provided in the first region close to the current collector to reduce the electrical contact loss between the current collector and the active layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0024] Figure 1 is one of the schematic structural diagrams of the negative electrode sheet provided by the embodiments of the present invention;

[0025] Figure 2 is another schematic structural diagram of the negative electrode sheet provided by the embodiments of the present invention;

[0026] Figure 3 is yet another schematic structural diagram of the negative electrode sheet provided by the embodiments of the present invention;

[0027] Figure 4 is the schematic structural diagram of the positive electrode sheet provided by the embodiments of the present invention;

[0028] Figure 5 is yet another schematic structural diagram of the negative electrode sheet provided by the embodiments of the present invention;

[0029] Figure 6 is yet another schematic structural diagram of the negative electrode sheet provided by the embodiments of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0031] The terms "first", "second", etc. in the description and claims of the present invention are used to distinguish similar objects, rather than to describe a specific order or sequence. It should be understood that such structures can be interchanged under appropriate circumstances, so that the embodiments of the present invention can be implemented in an order other than those illustrated or described herein, and the objects distinguished by "first", "second", etc. are usually of the same type, and the number of objects is not limited. For example, the first object can be one or more. In addition, "and / or" in the description and claims means at least one of the connected objects, and the character " / ", generally represents an "or" relationship between the associated objects before and after.

[0032] An embodiment of the present invention provides a negative electrode sheet, as Figures 1 to 2 shown, including a current collector 10 and an active layer 20. The active layer 20 is disposed on both opposite side surfaces of the current collector 10. The active layer 20 includes a first functional material. In the direction away from the current collector 10, the content of the first functional material increases, so that the content of the first functional material in the first region of the active layer 20 is less than the content of the first functional material in the second region of the active layer 20. The vertical distance from the first region to the current collector 10 is less than the vertical distance from the second region to the current collector 10. The first functional material includes at least one of a silicon-based material, a metal oxide, and a metal sulfide.

[0033] In this embodiment, by adding a first functional material to the active layer 20, the first functional material composed of at least one of a silicon-based material, a metal oxide, and a metal sulfide has a larger expansion rate and a higher lithium deintercalation / insertion ability compared with other active substances in the active layer 20 (such as artificial graphite, natural graphite, soft carbon, hard carbon, etc.). While increasing the porosity of the active layer 20, it reduces the negative impact on the energy density of the negative electrode sheet. The increase in porosity improves the migration rate of lithium ions in the active layer 20, thereby improving the energy density and kinetics of the negative electrode sheet, and further improving the fast charging performance of the battery.

[0034] Moreover, in the direction away from the current collector 10, the content of the first functional material increases, so that the content of the first functional material in the first region is less than the content of the first functional material in the second region, so as to form a concentration gradient difference and enhance the mass transfer ability of the active layer 20; a lower content of the first functional material is provided in the first region close to the current collector 10 to reduce the electrical contact loss between the current collector 10 and the active layer 20.

[0035] Optionally, the average particle size Dv50 of the first functional material in the first region is equal to the average particle size Dv50 of the first functional material in the second region.

[0036] In this embodiment, the average particle size Dv50 can be the particle diameter at which the cumulative volume-based distribution is 50% as measured by a laser scattering particle size analyzer. The average particle size Dv50 of the first functional material in the first region and the second region of the active layer 20 is the same within the error range. In other words, the difference between the first functional material in the first region and the first functional material in the second region lies in the content. During the preparation of the active layer 20, the step of screening the particle size of the first functional material is reduced, thereby simplifying the process and improving the production efficiency.

[0037] Optionally, the active layer 20 may further include a second functional material, and in the direction away from the current collector 10, the content of the second functional material increases, so that the content of the second functional material in the first region is less than the content of the second functional material in the second region.

[0038] In this embodiment, by adding the second functional material to the active layer 20, the content of the second functional material is increased synchronously with the content of the first functional material to compensate for the influence on the conductivity of the active layer 20 after adding the silicon-based material, thereby enhancing the conductivity of the negative electrode sheet.

[0039] Among them, the conductivity of the second functional material is greater than the conductivity of the conductive agent in the active layer 20. For example, the second functional material may include at least one of carbon nanotubes (or carbon nanotubes) and graphene. The second functional material including carbon nanotubes and / or graphene has a larger contact area compared to the conductive agent (such as conductive carbon black) in the active layer 20 to form a linear or planar conductive network, thereby enhancing the conductivity; at the same time, in combination with the first functional material having a larger expansion rate and higher lithium deintercalation / insertion ability, the migration rate of lithium ions in the active layer 20 is enhanced, thereby enhancing the fast charging performance of the battery.

[0040] Among them, the silicon-based material may include at least one of silicon particles, silicon carbide, and silicon oxide;

[0041] The metal oxide may include at least one of tin oxide, nickel oxide, and cobalt oxide;

[0042] The metal sulfide may include at least one of tin sulfide, nickel sulfide, and cobalt sulfide.

[0043] Optionally, the active layer 20 at least includes a first sub-active layer 201, a second sub-active layer 202, and a third sub-active layer 203. The first sub-active layer 201 is disposed on one surface of the current collector 10, the second sub-active layer 202 is disposed on the first sub-active layer 201, and the third sub-active layer 203 is disposed on the second sub-active layer 202;

[0044] The content of the first functional material increases in the direction from the first sub-active layer 201 to the third sub-active layer 203.

[0045] In some optional embodiments, argon ion polishing is performed in the thickness direction of the negative electrode sheet. Through a scanning electron microscope (SEM), it can be observed that the cross-section of the negative electrode sheet provided by the present invention includes a current collector 10, and active layers 20 are respectively disposed on the two surfaces of the current collector 10 facing away from each other. The active layer 20 includes a first sub-active layer 201, a second sub-active layer 202, and a third sub-active layer 203. Different concentration gradients of a first functional material can be provided in the first sub-active layer 201 to the third sub-active layer 203.

[0046] In this way, by adding a first functional material with a larger expansion rate and a higher lithium deintercalation / insertion ability in the active layer 20, when the thickness of the active layer 20 is the same, the energy density of the negative electrode sheet is increased; at the same time, the silicon-based material has a larger expansion rate compared to other active substances in the active layer 20, thereby increasing the porosity of the active layer 20. The increase in porosity improves the migration rate of lithium ions in the active layer 20, thereby improving the energy density and conductivity of the negative electrode sheet, and further improving the performance of the battery.

[0047] Moreover, the content of the first functional material increases along the direction from the first sub-active layer 201 to the third sub-active layer 203 to form a concentration gradient difference, enhancing the mass transfer ability of the active layer 20; a lower content of the first functional material is provided in the first sub-active layer 201 to reduce the electrical contact loss between the current collector 10 and the active layer 20.

[0048] Among them, a second functional material is further included in the active layer 20, and the content of the second functional material increases along the direction from the first sub-active layer 201 to the third sub-active layer 203.

[0049] By adding the second functional material in the active layer 20, the second functional material has a higher conductivity compared to the conductive agent in the active layer 20. The content of the second functional material increases synchronously with the content of the first functional material to compensate for the influence on the conductive performance after adding the silicon-based material in each sub-active layer, thereby improving the conductivity of the negative electrode sheet.

[0050] It should be noted that according to the actual preparation process, the active layer 20 can be set to more sub-active layers, so that the contents of the first functional material and the second functional material increase along the direction away from the current collector 10, and the same technical effects can be achieved. To avoid repetition, it will not be elaborated here.

[0051] Optionally, the first sub-active layer 201 does not contain the first functional material and the second functional material.

[0052] In some alternative embodiments, the expansion of the silicon-based material loosens the electrode sheet, thereby improving the ion transport performance. What really needs to be loosened is the part of the active layer 20 away from the current collector 10, that is, the third sub-active layer 203 that really needs to be loosened. The first functional material is not provided in the first sub-active layer 201 to reduce the electrical contact loss between the current collector 10 and the active layer 20. The concentration gradient of the first functional material and the second functional material corresponding to improving the conductivity increases from the second sub-active layer 202 to the third sub-active layer 203, improving the fast charging performance of the battery without losing the electrical contact on the side of the current collector 10.

[0053] It should be noted that during the preparation process of the negative electrode sheet, through the rolling process, part of the first functional material and the second functional material in the second sub-active layer 202 can be transferred to the first sub-active layer 201, causing a change in the substances in the first sub-active layer 201. In other words, the first sub-active layer 201 contains a small amount of the first functional material and the second functional material embedded from the second sub-active layer 202, which can achieve the same technical effect and will not be elaborated here.

[0054] Optionally, the first sub-active layer 201, the second sub-active layer 202, and the third sub-active layer 203 have the same thickness; or

[0055] The thickness of any one of the second sub-active layer 202 and the third sub-active layer 203 is less than the thickness of the first sub-active layer 201; or

[0056] The thickness of the third sub-active layer 203 is less than the thickness of the second sub-active layer 202, and the thickness of the second sub-active layer 202 is less than the thickness of the first sub-active layer 201.

[0057] In some alternative embodiments, the first sub-active layer 201, the second sub-active layer 202, and the third sub-active layer 203 have the same thickness. The first functional material has a larger expansion rate and a higher lithium deintercalation / insertion ability, so that when the thickness of the active layer 20 is the same, the negative electrode sheet has an energy density. In other words, the active layer 20 with the first functional material added has the characteristic of high specific capacity. When the energy density of the negative electrode sheet is the same, the designed thickness of the negative electrode sheet can be reduced. The first sub-active layer 201, the second sub-active layer 202, and the third sub-active layer 203 have the same thickness, which is convenient for adjusting the content of the first functional material in the first sub-active layer 201, the second sub-active layer 202, and the third sub-active layer 203 respectively, so that the content of the first functional material increases along the direction from the first sub-active layer 201 to the third sub-active layer 203 to form a concentration gradient difference, enhancing the mass transfer ability of the active layer 20 and improving the preparation efficiency of the negative electrode sheet.

[0058] In some other alternative embodiments, the thickness of either the second sub-active layer 202 or the third sub-active layer 203 is less than that of the first sub-active layer 201. The first functional material may not be provided in the first sub-active layer 201 to reduce the electrical contact loss between the current collector 10 and the active layer 20; while the second sub-active layer 202 and the third sub-active layer 203 provided with the first functional material have the characteristic of high specific capacity. Under the same energy density, the thickness of either the second sub-active layer 202 or the third sub-active layer 203 can be less than that of the first sub-active layer 201, thereby reducing the designed thickness of the negative electrode sheet, increasing the proportion of the active material in the negative electrode sheet, and further improving the battery performance.

[0059] Further, the content of the first functional material in the third sub-active layer 203 is greater than that in the second sub-active layer 202. Under the same energy density of the second sub-active layer 202 and the third sub-active layer 203, the thickness of the third sub-active layer 203 can be less than that of the second sub-active layer 202. Similarly, the thickness of the second sub-active layer 202 can be less than that of the first sub-active layer 201, thereby further reducing the designed thickness of the negative electrode sheet, increasing the proportion of the active material in the negative electrode sheet, and improving the battery performance.

[0060] The preparation process of the first sub-active layer 201 can be referred to the following description:

[0061] Optionally, the first sub-active layer 201 includes a first active material, a first conductive agent, and a first binder. The mass percentage content range ratio among the first active material, the first conductive agent, and the first binder can be: (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%).

[0062] The first active material, the first conductive agent, and the first binder are configured according to the mass percentage content range ratio of (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%), and a solvent is added and stirred to form a first slurry. The solid content in the first slurry can be 40wt% - 45wt%. The first slurry is respectively coated on the two opposite side surfaces of the current collector 10 to form the first sub-active layer 201.

[0063] The first active material can be a carbon-based material, such as graphite, mesophase carbon microspheres, etc.; the first conductive agent can be conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, metal powder, and carbon fiber, etc.; the first binder can be styrene-butadiene latex, polyacrylic acid, polyacrylate, sodium polyacrylate, polyvinylidene fluoride, polytetrafluoroethylene, and lithium polyacrylate, etc.

[0064] For example, the first active material is selected as graphite; the first conductive agent is selected as conductive carbon black; the first binder is selected as styrene-butadiene latex. Graphite, conductive carbon black and styrene-butadiene latex are configured according to the mass percentage ratio of 95.5wt%:1.5wt%:3wt%, and a solvent is added and stirred to prepare the first slurry. The solid content in the first slurry can be 40%.

[0065] The preparation process of the second sub-active layer 202 can be referred to the following description:

[0066] Optionally, the second sub-active layer 202 includes a second active material, a second conductive agent and a second binder. The range ratio of the mass percentage content between the second active material, the second conductive agent and the second binder can be: (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%);

[0067] Among them, the second active material includes A1% of the first functional material, and the second conductive agent includes A2% of the second functional material.

[0068] The second active material including A1% of the first functional material, the second conductive agent including A2% of the second functional material and the second binder are configured according to the range ratio of the mass percentage content of (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%), and a solvent is added and stirred to prepare the second slurry. The solid content in the second slurry can be 40wt% - 45wt%. Among them, A1% of the first functional material can be at least one of 0% to 30% of silicon-based materials, metal oxides and metal sulfides, and A2% of the second functional material can be 0.3wt% of carbon nanotubes. The second slurry is applied on the surface of the first sub-active layer 201 to form the second sub-active layer 202.

[0069] The second active material can be a carbon-based silicon-doped material; the second conductive agent can be conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, metal powder, carbon fiber, etc.; the second binder can be styrene-butadiene latex, polyacrylic acid, polyacrylate, sodium polyacrylate, polyvinylidene fluoride, polytetrafluoroethylene, lithium polyacrylate, etc.

[0070] For example, the second active material is selected as silicon-doped graphite; the second conductive agent is selected as conductive carbon black, and carbon nanotubes with better conductivity are added; the second binder is selected as styrene-butadiene latex. Silicon-doped graphite, conductive carbon black, carbon nanotubes and styrene-butadiene latex are configured according to the mass percentage ratio of 95.5wt%:1.2wt%:0.3wt%:3wt%, and a solvent is added and stirred to prepare the second slurry. The solid content in the second slurry can be 40%. The second active material includes 6% of silicon-based materials, that is, the percentage value of the carbon-based material to the silicon-based material is 94%:6%.

[0071] The preparation process of the third sub-active layer 203 can be referred to the following description:

[0072] Optionally, the third sub-active layer 203 includes a third active material, a third conductive agent, and a third binder. The mass percentage range ratio among the third active material, the third conductive agent, and the third binder can be: (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%);

[0073] Among them, the third active material includes B1% of a first functional material, the third conductive agent includes B2% of a second functional material, and B1 is greater than A1, and B2 is greater than A2.

[0074] The third active material including B1% of the first functional material, the third conductive agent including B2% of the second functional material, and the third binder are configured according to the mass percentage range ratio of (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%), and a solvent is added and stirred to form a third slurry. The solid content in the third slurry can be 40wt% - 45wt%. Among them, B1% of the first functional material can be at least one of 0% to 30% of silicon-based materials, metal oxides, and metal sulfides, and B1 is greater than A1; B2% of the second functional material can be 0.5wt% of carbon nanotubes. The third slurry is applied on the surface of the second sub-active layer 202 to form the third sub-active layer 203.

[0075] The third active material can be a carbon-based silicon-doped material; the third conductive agent can be conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, metal powder, carbon fiber, etc.; the third binder can be styrene-butadiene latex, polyacrylic acid, polyacrylate, sodium polyacrylate, polyvinylidene fluoride, polytetrafluoroethylene, lithium polyacrylate, etc.

[0076] For example, the third active material is selected as silicon-doped graphite; the third conductive agent is selected as conductive carbon black, and carbon nanotubes with better conductivity are added; the third binder is selected as styrene-butadiene latex. Silicon-doped graphite, conductive carbon black, carbon nanotubes, and styrene-butadiene latex are configured according to the mass percentage ratio of 95.5wt%:1wt%:0.5wt%:3wt%, and a solvent is added and stirred to form a third slurry. The solid content in the third slurry can be 40%. The third active material includes 9% of silicon-based materials, that is, the percentage value of the carbon-based material to the silicon-based material is 91%:9%.

[0077] The embodiment of the present invention also provides a battery, including the above-mentioned negative electrode sheet.

[0078] It should be noted that the implementation methods of the above-mentioned negative electrode sheets are equally applicable to the embodiments of this battery and can achieve the same technical effects, which will not be elaborated here.

[0079] Next, based on multiple groups of experiments, the effects of the battery prepared with the negative electrode sheet provided by the present invention will be described.

[0080] Example 1:

[0081] Prepare a negative electrode sheet: Select a copper foil as the negative electrode current collector 3001. Select graphite as the first active material, conductive carbon black as the first conductive agent, and styrene-butadiene latex as the first binder. Configure graphite, conductive carbon black, and styrene-butadiene latex according to a mass percentage ratio of 95.5 wt%: 1.5 wt%: 3 wt%, and add a solvent for stirring to make a first slurry. The solid content in the first slurry can be 45%. Use a coater to coat the first slurry on the two opposite surfaces of the negative electrode current collector 3001 to form a first active layer 3002.

[0082] Then, select silicon-doped graphite as the second active material, conductive carbon black as the second conductive agent, and add carbon nanotubes with better conductivity, and select styrene-butadiene latex as the second binder. Configure silicon-doped graphite, conductive carbon black, carbon nanotubes, and styrene-butadiene latex according to a mass percentage ratio of 95.5 wt%: 1.2 wt%: 0.3 wt%: 3 wt%, and add a solvent for stirring to make a second slurry. The solid content in the second slurry can be 45%. The silicon-doped graphite contains 6% of the silicon-based material, that is, the percentage value of graphite to silicon in the silicon-doped graphite is 94%: 6%. Use a coater to coat the second slurry on the surface of the first active layer 3002 to form a second active layer 3003. Compared with the first active layer 3002, the second active layer 3003 increases the silicon-based material and simultaneously increases carbon nanotubes with better conductivity.

[0083] Then, graphite doped with silicon is selected as the third active material, conductive carbon black is selected as the third conductive agent, carbon nanotubes with better conductivity are added, and styrene-butadiene latex is selected as the third binder. Graphite doped with silicon, conductive carbon black, carbon nanotubes and styrene-butadiene latex are configured according to the mass percentage ratio of 95.5wt%:1wt%:0.5wt%:3wt%, and a solvent is added and stirred to make the third slurry. The solid content in the third slurry can be 45%. The graphite doped with silicon contains 9% of silicon-based material, that is, the percentage value of graphite to silicon in the graphite doped with silicon is 91%:9%. The third slurry is coated on the surface of the second active layer 3003 by a coater to form the third active layer 3004. Compared with the second active layer 3003, the third active layer 3004 further increases the silicon-based material and simultaneously further increases the carbon nanotubes with better conductivity. The concentration gradient of the silicon-based material and the carbon nanotubes corresponding to the improved conductivity increases from the first active layer 3002 to the third active layer 3004.

[0084] After drying at 120°C and rolling, a negative electrode sheet is obtained, and its structure is as Figure 3 shown.

[0085] Preparation of the positive electrode sheet: Lithium cobaltate, acetylene black and polyvinylidene fluoride are added to a stirring tank according to the mass ratio of 97.2:1.5:1.3, and after adding N-methylpyrrolidone solvent and stirring, it is passed through a 200-mesh sieve to prepare a positive electrode active slurry with a solid content of 70wt% to 75wt%; the slurry is coated on the positive electrode current collector 4001 (aluminum foil is selected) by a coater to form the positive electrode active material layer 4002. After drying at 120°C and rolling, the positive electrode sheet is obtained, and its structure is as Figure 4 shown.

[0086] Battery assembly: The negative electrode sheet, positive electrode sheet and separator prepared above are wound together to form a core (width 62mm), packaged with an aluminum-plastic film, baked to remove moisture, and then injected with electrolyte and hot-pressed to obtain a battery.

[0087] Example 2:

[0088] Preparation of the negative electrode sheet: Copper foil is selected as the negative electrode current collector 5001. Graphite doped with silicon is selected as the negative electrode active material, conductive carbon black is selected as the conductive agent, and styrene-butadiene latex is selected as the binder. Graphite doped with silicon, conductive carbon black and styrene-butadiene latex are configured according to the mass percentage ratio of 95.5wt%:1.5wt%:3wt%, and a solvent is added and stirred to make the negative electrode slurry. The solid content in the negative electrode slurry can be 45%. The graphite doped with silicon contains 5% of silicon-based material, that is, the percentage value of graphite to silicon in the graphite doped with silicon is 95%:5%. The negative electrode slurry is coated on the two opposite side surfaces of the negative electrode current collector 5001 by a coater to form the negative electrode active layer 5002.

[0089] Then, it is dried at 120°C and roll-pressed to obtain a negative electrode sheet, the structure of which is as Figure 5 shown.

[0090] Preparation of the positive electrode sheet: Lithium cobaltate, acetylene black, and polyvinylidene fluoride are added to a stirring tank according to a mass ratio of 97.2:1.5:1.3, and after adding N-methylpyrrolidone solvent and stirring, it is passed through a 200-mesh sieve to prepare a positive electrode active paste with a solid content of 70 wt% to 75 wt%; the paste is coated onto a positive electrode current collector 4001 (aluminum foil is selected) using a coater to form a positive electrode active material layer 4002. It is dried at 120°C and roll-pressed to obtain a positive electrode sheet, the structure of which is as Figure 4 shown.

[0091] Battery assembly: The negative electrode sheet, positive electrode sheet, and separator prepared above are wound together to form a core (width: 62 mm), packaged with an aluminum-plastic film, baked to remove moisture, and then electrolyte is injected, followed by hot pressing to obtain a battery.

[0092] Comparative Example 1:

[0093] Preparation of the negative electrode sheet: Prepare a paste; among them, the paste consists of the negative electrode active material graphite, and the paste composition is: 95.5 wt% graphite, 1.5 wt% conductive carbon black, 3 wt% styrene-butadiene latex, and its solid content is 45 wt%; the paste is coated onto a negative electrode current collector 6001 (copper foil is selected) using a coater to form a negative electrode active material layer 6002. It is dried at 120°C and roll-pressed to obtain a negative electrode sheet, the structure of which is as Figure 6 shown.

[0094] Preparation of the positive electrode sheet: Lithium cobaltate, acetylene black, and polyvinylidene fluoride are added to a stirring tank according to a mass ratio of 97.2:1.5:1.3, and after adding N-methylpyrrolidone solvent and stirring, it is passed through a 200-mesh sieve to prepare a positive electrode active paste with a solid content of 70 wt% to 75 wt%; the paste is coated onto a positive electrode current collector 4001 (aluminum foil is selected) using a coater to form a positive electrode active material layer 4002. It is dried at 120°C and roll-pressed to obtain a positive electrode sheet, the structure of which is as Figure 4 shown.

[0095] Battery assembly: The negative electrode sheet, positive electrode sheet, and separator prepared above are wound together to form a core (width: 62 mm), packaged with an aluminum-plastic film, baked to remove moisture, and then electrolyte is injected, followed by hot pressing to obtain a battery.

[0096] The relevant parameters in Examples 1 to 2 and Comparative Example 1 are shown in Table 1 below.

[0097] Table 1:

[0098]

[0099] The lithium deposition rate tests were carried out on Example 1, Example 2 and Comparative Example 1. The test process can be as follows: at 25 °C, the lithium-ion battery is charged to 4.48 V at a constant current of 3C, then charged at a constant voltage of 4.48 V until the current is 0.05C, left standing for 2 min, and then discharged at a constant current of 1C to 3.0 V and left standing for 2 min. This is taken as one cycle. After repeating 10 cycles, the lithium-ion battery is disassembled to obtain the electrode assembly. The electrode assembly is laid flat. If it is found that any area on the negative electrode plate larger than 2 mm 2 shows lithium deposition, it is determined that the negative electrode plate has lithium deposition. The severity of lithium deposition indicates the size of the lithium deposition area and the thickness of the lithium deposition. The test results are shown in Table 2 below.

[0100] Table 2:

[0101]

[0102]

[0103] By comparing Example 2 and Comparative Example 1: Since the negative electrode plate is thinner after doping with silicon, the transmission distance of lithium ions from the separator to the current collector side is reduced, and the kinetic performance is improved; the charge conduction ability of the thinner electrode plate from the current collector side to the separator side is better. Therefore, during high-rate charging, the lithium deposition of the thinner negative electrode plate (Example 2, Example 1) after doping with silicon is greatly reduced; the expansion of the silicon in the silicon-doped electrode plate is slightly larger than that of graphite. After the first charge and discharge, voids will be formed, and the penetration of the electrolyte will greatly improve the ion transport performance, thereby improving the fast charging performance of the battery.

[0104] By comparing Example 1 and Example 2: The expansion of the silicon particles after doping with silicon will loosen the electrode plate and improve the ion transport performance. Among them, the part of the active layer that really needs to be loosened is far from the current collector. The overall mixing (Example 2) will cause the loss of electrical contact in the overall electrode plate. Compared with Example 2, Example 1 only forms a gradient porosity effect near the surface, and improves the fast charging performance on the surface without losing the electrical contact on the current collector side, and there is no lithium deposition during 3C high-rate charging.

[0105] Through the above comparison, the silicon-based material of the first functional material has a larger expansion rate and higher lithium deintercalation / insertion ability. While increasing the porosity of the active layer 20, it reduces the negative impact on the energy density of the negative electrode plate. The increase in porosity improves the migration rate of lithium ions in the active layer 20, thereby improving the energy density and conductivity of the negative electrode plate, and further improving the performance of the battery.

[0106] Moreover, in the direction away from the current collector 10, the content of the first functional material increases, such that the content of the first functional material in the first region is less than that in the second region, so as to form a concentration gradient difference. With the design of gradient concentration silicon doping, the effect of gradient porosity design is finally achieved, enhancing the mass transfer ability of the active layer 20. A lower content of the first functional material is provided in the first region close to the current collector 10 to reduce the electrical contact loss between the current collector 10 and the active layer 20, which has a good effect in the design of high energy density fast charging lithium ion batteries.

[0107] It should be noted that, in this document, the terms "comprising", "including" or any other variant thereof are intended to cover non-exclusive inclusion, such that a process, method, article or device comprising a series of elements not only includes those elements but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the phrase "comprising a..." does not exclude the presence of additional identical elements in the process, method, article or device comprising such element. In addition, it should be pointed out that the scope of the methods and devices in the embodiments of the present invention is not limited to performing functions in the order discussed, but may also include performing functions in a substantially simultaneous manner or in a reverse order according to the functions involved. For example, the described methods may be performed in an order different from that described, and various steps may be added, omitted, or combined. Additionally, features described with reference to certain examples may be combined in other examples.

[0108] The embodiments of the present invention have been described above in conjunction with the accompanying drawings. However, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the spirit of the present invention and the scope protected by the claims, and all of them fall within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, It includes a current collector and an active layer. The active layer is disposed on two opposite surface sides of the current collector. The active layer includes a first functional material. In the direction away from the current collector, the content of the first functional material increases, so that the content of the first functional material in the first region of the active layer is less than the content of the first functional material in the second region of the active layer. The perpendicular distance from the first region to the current collector is less than the perpendicular distance from the second region to the current collector. The first functional material includes at least one of a silicon-based material, a metal oxide, and a metal sulfide; Wherein, the average particle size of the first functional material in the first region is equal to the average particle size of the first functional material in the second region; The active layer further includes a second functional material. In the direction away from the current collector, the content of the second functional material increases, so that the content of the second functional material in the first region is less than the content of the second functional material in the second region; The conductivity of the second functional material is greater than the conductivity of the conductive agent in the active layer.

2. The negative electrode sheet according to claim 1, characterized in that, The active layer at least includes a first sub-active layer, a second sub-active layer, and a third sub-active layer. The first sub-active layer is disposed on one surface side of the current collector. The second sub-active layer is disposed on the first sub-active layer. The third sub-active layer is disposed on the second sub-active layer; The content of the first functional material increases in the direction from the first sub-active layer to the third sub-active layer.

3. The negative electrode sheet according to claim 2, wherein The active layer further includes a second functional material. The content of the second functional material increases in the direction from the first sub-active layer to the third sub-active layer.

4. The negative electrode sheet according to claim 2, wherein The first sub-active layer includes a first active material, a first conductive agent, and a first binder. The mass percentage content range ratio among the first active material, the first conductive agent, and the first binder is: (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%).

5. The negative electrode sheet according to claim 3, characterized in that, The second sub-active layer includes a second active material, a second conductive agent, and a second binder. The mass percentage content range ratio among the second active material, the second conductive agent, and the second binder is: (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%); Wherein, the second active material includes A1% of the first functional material, and the second conductive agent includes A2% of the second functional material.

6. The negative electrode sheet according to claim 3, wherein The third sub-active layer includes a third active material, a third conductive agent, and a third binder. The mass percentage content range ratio among the third active material, the third conductive agent, and the third binder is: (70wt% - 99wt%):(0.5wt% - 15wt%):(0.5wt% - 15wt%); Wherein, the third active material includes B1% of the first functional material, and the third conductive agent includes B2% of the second functional material, and B1 is greater than A1, and B2 is greater than A2.

7. The negative electrode sheet according to claim 2, characterized in that, The thicknesses of the first sub-active layer, the second sub-active layer, and the third sub-active layer are the same; or The thickness of any one of the second sub-active layer and the third sub-active layer is less than the thickness of the first sub-active layer; or The thickness of the third sub-active layer is less than the thickness of the second sub-active layer, and the thickness of the second sub-active layer is less than the thickness of the first sub-active layer.

8. A battery, characterized in that, Comprising the negative electrode sheet according to any one of claims 1 to 7.

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