Positive pole piece as well as preparation method and application thereof

Abstract

The invention provides a positive pole piece as well as a preparation method and application thereof. The positive pole piece comprises a positive current collector and a positive coating located on at least one side surface of the positive current collector, and the positive coating comprises a first lithium supplementing layer, a first positive active material layer and a boundary layer located between the first lithium supplementing layer and the first positive active material layer which are arranged in sequence; the first lithium supplementing layer is positioned on one side, close to a tab, of the positive current collector; the boundary layer comprises a second lithium supplementing layer and a second positive electrode active material layer which is positioned on the surface of one side, far away from the positive electrode current collector, of the second lithium supplementing layer. According to the invention, the first coulombic efficiency, the energy density and other performances of the battery can be improved.

Description

Technical Field

[0001] The invention relates to a positive electrode sheet and a preparation method and application thereof, belonging to the technical field of lithium ion batteries. Background Art

[0002] Existing lithium replenishment schemes adopt the method of coating the lithium replenishment material on the surface of the positive electrode material or the negative electrode material. However, coating the lithium replenishment material on the surface of the positive electrode material or the negative electrode material may affect the performance of the inner layer material (positive electrode material or negative electrode material), making the inner layer material unable to function, thereby causing a decrease in energy density. Alternatively, after the lithium ions in the lithium replenishment material are consumed, the lithium replenishment material after releasing the lithium ions will cause the thickness between the positive electrode sheet or the negative electrode sheet to increase, thereby affecting the lithium ion diffusion rate and affecting the performance such as the first coulomb efficiency and energy density. Summary of the invention

[0003] The present invention provides a positive electrode plate and a preparation method and application thereof, which are helpful to improve both the initial coulomb efficiency and energy density of a lithium-ion battery.

[0004] The present invention provides a positive electrode plate, comprising a positive electrode collector and a positive electrode coating located on at least one side of the surface of the positive electrode collector, wherein the positive electrode coating comprises a first lithium replenishing layer distributed along a first direction, a first positive electrode active material layer, and an interface layer located between the first lithium replenishing layer and the first positive electrode active material layer; the first direction is perpendicular to the thickness direction of the positive electrode plate; the interface layer comprises a second lithium replenishing layer and a second positive electrode active material layer located on a side of the second lithium replenishing layer away from the positive electrode collector.

[0005] Optionally, the positive electrode plate further includes a tab connected to the positive current collector, and the first lithium supplement layer is located on a side of the positive current collector close to the tab.

[0006] Optionally, the width of the first lithium replenishing layer is greater than 1 mm, and the ratio of the width of the first lithium replenishing layer to the width of the first positive electrode active material layer is 0.01-0.05; and / or, the ratio of the width of the interface layer to the width of the first lithium replenishing layer is 0.2-0.4; and / or, in the interface layer, the thickness of the second lithium replenishing layer tends to decrease along the direction from the first lithium replenishing layer to the first positive electrode active material layer; and / or, the thickness of at least part of the first lithium replenishing layer tends to increase along the direction from the first lithium replenishing layer to the first positive electrode active material layer; and / or, the thickness of at least part of the first lithium replenishing layer is the same as the thickness of the first positive electrode active material layer.

[0007] Optionally, the porosity of the first lithium supplement layer is smaller than the porosity of the interface layer; and the porosity of the interface layer is smaller than the porosity of the first positive electrode active material layer.

[0008] Optionally, a ratio of the resistance of the first lithium replenishing layer to the resistance of the first positive electrode active material layer is greater than 1.5.

[0009] Optionally, the first lithium replenishing layer includes a first lithium replenishing agent, and the second lithium replenishing layer includes a second lithium replenishing agent; the first lithium replenishing agent and the second lithium replenishing agent each independently include one or more of Li2MnO3, Li2NiO2, and Li5FeO4; and / or, in the first lithium replenishing layer, the mass percentage of the first lithium replenishing agent is 70% to 97%; in the second lithium replenishing layer, the mass percentage of the second lithium replenishing agent is 70% to 97%; and / or, the first lithium replenishing layer also includes a binder, and the mass percentage of the binder in the first lithium replenishing layer is 3% to 30%; and / or, the second lithium replenishing layer also includes a binder, and the mass percentage of the binder in the second lithium replenishing layer is 3% to 30%; and / or, the first positive electrode active material layer and the second positive electrode active material layer respectively include positive electrode active substances; the positive electrode active substances include ternary positive electrode active substances, and the mass percentage of nickel in the ternary positive electrode active substances is not less than 80%.

[0010] The present invention also provides a method for preparing the positive electrode plate as described above, comprising: sequentially coating the slurry of the first lithium replenishing layer, the slurry of the second lithium replenishing layer, and the slurry of the first positive electrode active material layer on at least one side surface of the positive electrode collector to form the first lithium replenishing layer, the second lithium replenishing layer, and the first positive electrode active material layer distributed sequentially along the first direction; then coating the slurry of the second positive electrode active material layer on the side surface of the second lithium replenishing layer away from the positive electrode collector to form the interface layer including the second lithium replenishing layer and the second positive electrode active material layer to obtain the positive electrode plate.

[0011] Optionally, the slurry of the first lithium replenishing layer, the slurry of the second lithium replenishing layer, and the slurry of the first positive electrode active material layer are sequentially coated on at least one side of the surface of the positive electrode collector, and then dried to form a first lithium replenishing layer precursor, a second lithium replenishing layer precursor, and a first positive electrode active material layer precursor, respectively; then the slurry of the second positive electrode active material layer is coated on the side of the second lithium replenishing layer precursor away from the positive electrode collector, and after drying, a second positive electrode active material layer precursor is formed on the surface of the second lithium replenishing layer precursor, and the second positive electrode active material layer precursor and the second lithium replenishing layer precursor form an interface layer precursor; and then after rolling, the positive electrode sheet is obtained; wherein, before rolling, the thickness of the first lithium replenishing layer precursor is greater than the thickness of the interface layer precursor, and the thickness of the interface layer precursor is greater than the thickness of the first positive electrode active material layer precursor.

[0012] The present invention also provides a lithium ion battery, comprising the positive electrode plate as described above or the positive electrode plate prepared according to the preparation method as described above.

[0013] The present invention also provides a preparation method of a lithium ion battery, comprising: packaging a battery cell comprising the positive electrode sheet as described above or the positive electrode sheet prepared according to the preparation method as described above into an aluminum-plastic film bag, drying the packaged battery cell and injecting an electrolyte, and then sequentially performing a first formation cycle, a second formation cycle, and a third formation cycle to obtain the lithium ion battery; wherein, in the first formation cycle, the formation voltage is 3.57-3.63V, and the battery is left to stand for 1.8-2.2h after formation; in the second formation cycle, the formation voltage is 3.87-3.93V, and the battery is left to stand for 3.8-4.2h after formation; in the third formation cycle, the formation voltage is 4.23-4.28V, and the battery is left to stand for 5.8-6.2h after formation.

[0014] The present invention provides a positive electrode plate and a preparation method and application thereof. In the interface layer, the second lithium replenishing layer is close to the positive electrode current collector, and the second positive electrode active material layer is located on the surface of the second lithium replenishing layer away from the positive electrode current collector, which effectively avoids the problem that the performance of the positive electrode material is affected and cannot function due to coating the lithium replenishing material on the surface of the positive electrode material. While replenishing lithium ions, the normal performance of the positive electrode active material is ensured, and a higher capacity is achieved, thereby improving the first coulomb efficiency and energy density, and maintaining a low internal resistance of the positive electrode plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A cross-sectional view of a positive electrode sheet along a first direction in some embodiments;

[0016] Figure 2 A cross-sectional view of a positive electrode sheet along a first direction in some embodiments;

[0017] Figure 3 is a cross-sectional view of the positive electrode sheet of Example 6 along the first direction;

[0018] Figure 4 Schematic diagram of the appearance of a lithium-ion battery.

[0019] Reference numerals:

[0020] 01-first positive electrode active material layer; 02-first lithium replenishing layer; 03-interface layer; 04-second lithium replenishing layer; 05-second positive electrode active material layer; 06-ear. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] An embodiment of the present invention provides a positive electrode plate, including a positive electrode current collector, and a positive electrode coating located on at least one side of the positive electrode current collector, the positive electrode coating including a first lithium replenishing layer distributed along a first direction, a first positive electrode active material layer, and an interface layer located between the first lithium replenishing layer and the first positive electrode active material layer; the above-mentioned first direction is perpendicular to the thickness direction of the positive electrode plate; the interface layer includes a second lithium replenishing layer, and a second positive electrode active material layer located on the side of the second lithium replenishing layer away from the positive electrode current collector.

[0023] According to the research and analysis of the inventors: the first lithium replenishment layer, the first positive electrode active material layer, and the interface layer located between the first lithium replenishment layer and the first positive electrode active material layer are sequentially distributed along the first direction (for example, the length direction or the width direction of the positive electrode plate), the second lithium replenishment layer is close to the positive electrode current collector in the interface layer, and the second positive electrode active material layer is located on the side of the second lithium replenishment layer away from the positive electrode current collector, which effectively avoids the problem that the performance of the positive electrode material is affected and cannot function due to the lithium replenishment material being coated on the surface of the positive electrode material. While the first lithium replenishment layer and the second lithium replenishment layer effectively replenish lithium ions, the performance of the first positive electrode active material layer and the second positive electrode active material layer is ensured to be normal, and a higher capacity is achieved, thereby improving the first coulomb efficiency and energy density and other performances, and taking into account the maintenance of a low internal resistance of the positive electrode plate. In addition, the first lithium replenishment layer and the second lithium replenishment layer have low conductivity after delithiation, and can play an insulating role, replacing the insulating layer (such as a ceramic layer) without lithium storage capacity, taking into account both lithium replenishment and insulation, which is conducive to further improving the energy density of the battery.

[0024] Therefore, the positive electrode plate provided in the embodiment of the present invention is helpful to improve the initial coulombic efficiency and energy density of the battery, while maintaining a low internal resistance of the positive electrode plate.

[0025] In some embodiments, the positive electrode plate further includes a tab 06 connected to the positive current collector, and the first lithium supplement layer is located on the side of the positive current collector close to the tab. Since the tab is conductive, the first lithium supplement layer is distributed on the side close to the tab. The first lithium supplement layer can play an insulating role after lithium removal, reduce the possibility of contact between the positive electrode and the negative electrode, prevent the positive and negative electrodes of the battery from short-circuiting, and ensure safety.

[0026] It can be understood that the first direction is substantially perpendicular to the thickness direction of the positive electrode sheet, for example, it may be the width direction or the length direction of the positive electrode sheet.

[0027] In some embodiments, the width of the first lithium replenishing layer in the first direction is greater than 1 mm to avoid the first lithium replenishing layer being too narrow, which would cause the insulating layer to be too narrow after delithiation, thereby ensuring that the insulating layer has sufficient width to ensure battery safety.

[0028] Furthermore, the ratio of the width of the first lithium replenishing layer in the first direction to the width of the first positive electrode active material layer in the first direction may be 0.01 to 0.05, such as 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045, 0.05 or any two thereof, preferably 0.015 to 0.03. This is beneficial for the first lithium replenishing layer to better exert the lithium replenishing effect and the insulating effect while taking into account the higher energy density.

[0029] In some embodiments, the ratio of the width of the interface layer in the first direction to the width of the first lithium replenishing layer in the first direction is 0.2 to 0.4, for example, 0.2, 0.3, 0.4 or a range composed of any two of them, to avoid the interface between the second lithium replenishing layer and the second positive electrode active material layer being too small (insufficient) due to the narrow width of the interface layer, causing cracking of the interface and failing to achieve a good isolation effect, and to avoid the reduction of battery performance due to the excessive width of the interface, thereby ensuring that the battery has better first coulombic efficiency and energy density and other performance.

[0030] The cross-sectional view of the positive electrode sheet along the first direction of some embodiments is as follows: Figure 1 As shown. Figure 1 It can be seen that in the interface layer 03, the thickness of the second lithium replenishing layer 04 tends to decrease (for example, gradually decreases) along the direction from the first lithium replenishing layer 02 to the first positive electrode active material layer 01, that is, the interface between the second lithium replenishing layer 04 and the second positive electrode active material layer 05 is in a downward arc shape, which helps to increase the contact area (the area of ​​the interface) between the second lithium replenishing layer 04 and the second positive electrode active material layer 05, avoid cracking of the interface, and help to improve the battery's initial coulombic efficiency, capacity, and energy density.

[0031] In some embodiments, in the interface layer 03, the thickness of the second positive electrode active material layer 05 tends to increase (for example, gradually increases) along the direction from the first lithium replenishing layer 02 to the first positive electrode active material layer 01, which helps to increase the contact area (the area of ​​the interface) between the second lithium replenishing layer 04 and the second positive electrode active material layer 05, avoid cracking of the interface, and help to improve the battery's initial coulombic efficiency, capacity, and energy density.

[0032] The cross-sectional view of the positive electrode sheet along the first direction of some embodiments is as follows: Figure 2 As shown. Figure 2 It can be seen that the thickness of at least part of the first lithium replenishing layer 02 tends to increase (for example, gradually increases) along the direction from the first lithium replenishing layer 02 to the first positive electrode active material layer 01, so that the first lithium replenishing layer 02 has a thinning area at the edge away from the first positive electrode active material layer, which can avoid the problem of edge cracking and powdering caused by excessive stress in the edge part, and ensure that the battery has a good first coulombic efficiency, capacity, and energy density.

[0033] Further, from Figure 2 It can also be seen that the thickness of at least part of the first lithium replenishment layer 02 shows an increasing trend (for example, gradually increasing) along the direction from the first lithium replenishment layer 02 to the first positive electrode active material layer 01. When its thickness increases to be the same as the thickness of the first positive electrode active material layer 01, the thickness of the remaining part of the first lithium replenishment layer remains the same as the thickness of the first positive electrode active material layer 01. That is, the first lithium replenishment layer includes at least a first region and a second region, wherein the thickness of the first lithium replenishment layer in the first region can show an increasing trend (for example, gradually increasing) along the direction from the first lithium replenishment layer 02 to the first positive electrode active material layer 01, and the thickness of the first lithium replenishment layer in the second region is the same as the thickness of the first positive electrode active material layer 01.

[0034] In some embodiments, the porosity of the first lithium supplementing layer is less than the porosity of the interface layer; and the porosity of the interface layer is less than the porosity of the first positive electrode active material layer.

[0035] In some embodiments, a ratio of a resistance of the first lithium replenishing layer to a resistance of the first positive electrode active material layer is greater than 1.5.

[0036] It can be understood that both the first lithium replenishing layer and the second lithium replenishing layer include a lithium replenishing agent. Specifically, the first lithium replenishing layer includes a first lithium replenishing agent, and the second lithium replenishing layer includes a second lithium replenishing agent.

[0037] In some embodiments, the first lithium supplement and the second lithium supplement each independently include one or more of Li2MnO3, Li2NiO2, and Li5FeO4. After the formation and delithiation of these lithium supplements, their structures will undergo phase changes, and their conductivity will be significantly reduced, thereby achieving both good lithium supplement and insulation effects.

[0038] In the first lithium replenishing layer, the mass percentage of the first lithium replenishing agent can be 70% to 97%, for example, 70%, 80%, 90%, 97% or a range consisting of any two thereof; in the second lithium replenishing layer, the mass percentage of the second lithium replenishing agent can be 70% to 97%, for example, 70%, 80%, 90%, 97% or a range consisting of any two thereof.

[0039] The first lithium replenishing layer may further include a binder, and the mass percentage of the binder in the first lithium replenishing layer may be 3% to 30%, for example, 3%, 10%, 20%, 30% or a range consisting of any two thereof.

[0040] The second lithium replenishing layer may also include a binder, and the mass percentage of the binder in the second lithium replenishing layer may be 3% to 30%, for example, 3%, 10%, 20%, 30% or a range consisting of any two thereof.

[0041] In some embodiments, the mass ratio of the first lithium replenishing agent to the binder in the first lithium replenishing layer is (70-97):(3-30), preferably (85-95):(5-15).

[0042] In the embodiment of the present invention, the first lithium replenishing layer and the second lithium replenishing layer do not contain a conductive agent, which can make the lithium in the first lithium replenishing layer and the second lithium replenishing layer be better released, and reduce the conductivity of the first lithium replenishing layer and the second lithium replenishing layer after lithium removal, so that they can better play an insulating role.

[0043] In some embodiments, the first positive electrode active material layer and the second positive electrode active material layer respectively include positive electrode active materials; the positive electrode active materials may include ternary positive electrode active materials, and the mass percentage of nickel in the ternary positive electrode active materials is not less than 80%, LiNi 0.8 Co 0.1 Mn 0.1 O2、LiNi 0.9 Co 0.05 Mn 0.05 O2、LiNi 0.92 Co 0.04 Mn 0.04 O2、LiNi 0.9 Co 0.0.04 Mn 0.04 Al 0.02 In the system of the positive electrode sheet including the above-mentioned ternary positive electrode active material, the performance of the battery such as the first coulomb efficiency and energy density can be better improved.

[0044] An embodiment of the present invention also provides a method for preparing the above-mentioned positive electrode plate, comprising: sequentially coating a slurry of a first lithium replenishing layer, a slurry of a second lithium replenishing layer, and a slurry of a first positive electrode active material layer on at least one side surface of a positive electrode current collector to form a first lithium replenishing layer, a second lithium replenishing layer, and a first positive electrode active material layer sequentially distributed along a first direction; then coating a slurry of a second positive electrode active material layer on a side surface of the second lithium replenishing layer away from the positive electrode current collector to form an interface layer including the second lithium replenishing layer and the second positive electrode active material layer to obtain the above-mentioned positive electrode plate.

[0045] According to the inventor's research and analysis: the above preparation method can obtain a positive electrode sheet with good coulombic efficiency and energy density for the first time.

[0046] In a specific implementation, the slurry of the first lithium replenishing layer, the slurry of the second lithium replenishing layer, and the slurry of the first positive electrode active material layer can be sequentially coated on at least one side surface of the positive electrode current collector, and then dried to form a first lithium replenishing layer precursor, a second lithium replenishing layer precursor, and a first positive electrode active material layer precursor, respectively; then the slurry of the second positive electrode active material layer is coated on the side surface of the second lithium replenishing layer precursor away from the positive electrode current collector, and after drying, a second positive electrode active material layer precursor is formed on the surface of the second lithium replenishing layer precursor, and the second positive electrode active material layer precursor and the second lithium replenishing layer precursor form an interface layer precursor; and then after rolling, a positive electrode sheet is obtained.

[0047] In some embodiments, before the above-mentioned rolling, the thickness of the first lithium replenishing layer precursor is greater than the thickness of the interface layer precursor, and the thickness of the interface layer precursor is greater than the thickness of the first positive electrode active material layer precursor. After rolling, the thickness of the first lithium replenishing layer, the thickness of the interface layer and the thickness of the first positive electrode active material layer are substantially equal and remain consistent. In addition, since the porosity of the first lithium replenishing layer precursor, the porosity of the interface layer precursor and the porosity of the first positive electrode active material layer precursor are similar before rolling, after rolling, the porosity of the first lithium replenishing layer can be made smaller than the porosity of the interface layer; the porosity of the interface layer is smaller than the porosity of the first positive electrode active material layer.

[0048] In some embodiments, along the direction from the first lithium replenishing layer to the first positive electrode active material layer, by controlling the coating amount of the second lithium replenishing layer to decrease, the thickness of the second lithium replenishing layer can be reduced, that is, the interface between the second lithium replenishing layer and the second positive electrode active material layer in the junction layer is arc-shaped downward. Similarly, along the direction from the first lithium replenishing layer to the first positive electrode active material layer, by controlling the coating amount of the second positive electrode active material layer to increase, the thickness of the second positive electrode active material layer can be increased, that is, the interface between the second lithium replenishing layer and the second positive electrode active material layer in the junction layer is arc-shaped downward.

[0049] In some embodiments, along the direction from the first lithium replenishment layer to the first positive electrode active material layer, by controlling the coating amount of the first lithium replenishment layer to increase, the thickness of at least part of the first lithium replenishment layer increases, so that the first lithium replenishment layer is away from the first positive electrode active material layer. There is a thinning area at the edge. This can avoid the problem of edge cracking and powdering caused by excessive stress in the edge part, thereby ensuring that the battery has a good first coulombic efficiency, capacity, and energy density.

[0050] In some embodiments, the first lithium replenishment layer is thinned at an edge away from the first positive electrode active material layer so that the thickness of at least a portion of the first lithium replenishment layer increases along the direction from the first lithium replenishment layer to the first positive electrode active material layer, that is, there is a thinned area of ​​the first lithium replenishment layer away from the edge away from the first positive electrode active material layer.

[0051] In a specific implementation, during the rolling process, a cold pressing process can be used to obtain the positive electrode sheet.

[0052] In some embodiments, the slurry of the first lithium replenishing layer can be prepared by the following process: the first lithium replenishing agent and the binder are dispersed in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and the mixture is stirred to form a uniform slurry of the first lithium replenishing layer. Similarly, the slurry of the second lithium replenishing layer can be prepared by the following process: the second lithium replenishing agent and the binder are dispersed in an appropriate amount of solvent, and the mixture is stirred to form a uniform slurry of the second lithium replenishing layer.

[0053] The binder in the first lithium replenishing layer and the second lithium replenishing layer may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, carboxymethyl cellulose, and styrene-butadiene rubber.

[0054] In a specific implementation, the raw material formula of the first lithium replenishing layer and the second lithium replenishing layer may be the same, that is, the slurry of the two is the same.

[0055] In some embodiments, the slurry of the first positive electrode active material layer (or the slurry of the second positive electrode active material layer) can be prepared by the following process: the positive electrode active material, the conductive agent and the binder are dispersed in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and the mixture is stirred and mixed to form a uniform slurry of the first positive electrode active material layer (or the slurry of the second positive electrode active material layer). In a specific embodiment, the first positive electrode active material layer (or the second positive electrode active material layer) includes 70-99wt% of the positive electrode active material, 0.5-15wt% of the conductive agent, and 0.5-15wt% of the binder according to the mass percentage, and further includes 80-98wt% of the positive electrode active material, 1-10wt% of the conductive agent, and 1-10wt% of the binder.

[0056] In a specific implementation, the raw material formula of the first positive electrode active material layer and the second positive electrode active material layer may be the same, that is, the slurry of the two is the same.

[0057] The embodiments of the present invention do not strictly limit the positive electrode active material in the positive electrode sheet, and may be a positive electrode active material commonly used in lithium-ion batteries, such as at least one composite oxide of lithium and metals of cobalt, manganese, nickel, and combinations thereof. Specifically, it may be one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganate, lithium manganate, nickel-cobalt-manganese ternary material, nickel-cobalt-aluminum ternary material, lithium iron phosphate (LFP), lithium nickel manganate, lithium-rich manganese-based material, and the like.

[0058] In the embodiment of the present invention, the material of the positive electrode current collector may be at least one of aluminum foil and nickel foil. In the first positive electrode active material layer (or the second positive electrode active material layer), the conductive agent may be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; the binder may be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride (PVDF), polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinyl pyrrolidone, and polyurethane.

[0059] The embodiment of the present invention further provides a lithium ion battery, comprising the above-mentioned positive electrode sheet or the positive electrode sheet prepared according to the above-mentioned preparation method. The lithium ion battery has the same advantages as the above-mentioned positive electrode sheet, which will not be described in detail.

[0060] Specifically, the above lithium-ion battery also includes a negative electrode plate, a separator and an electrolyte.

[0061] The negative electrode sheet of the embodiment of the present invention specifically includes a negative electrode current collector and a negative electrode active layer formed of a negative electrode active material and arranged on the surface of the negative electrode current collector.

[0062] When preparing the negative electrode sheet, the negative electrode active material, the conductive agent and the binder can be dispersed in an appropriate amount of deionized water, and the mixture is stirred and mixed to form a uniform negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode current collector, and the negative electrode sheet is obtained after drying, rolling and cutting. In a specific embodiment, the negative electrode active layer includes 70-99wt% of the negative electrode active material, 0.5-15wt% of the conductive agent, and 0.5-15wt% of the binder according to the mass percentage, and further includes 80-98wt% of the negative electrode active material, 1-10wt% of the conductive agent, and 1-10wt% of the binder.

[0063] Among them, the material of the negative electrode current collector layer can be at least one of copper foil, nickel foam, and copper foam; the conductive agent can be selected from at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and graphene; the binder can be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate.

[0064] The embodiments of the present invention do not strictly limit the negative electrode active material in the negative electrode sheet, and can be at least one of the negative electrode active materials commonly used in lithium-ion batteries, such as graphite, hard carbon, soft carbon, mesophase carbon microbeads, silicon-based negative electrode materials (mainly including silicon oxide, silicon-carbon negative electrode), tin-based negative electrode materials (mainly including tin, tin alloy), etc.

[0065] The embodiments of the present invention do not strictly limit the selection of the electrolyte, and may include one or more of the solvents commonly used in the current lithium-ion battery electrolyte, and the electrolyte lithium salts commonly used in the current lithium-ion electrolyte, for example: the solvent may be ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, ethyl methyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, cyclopentane, γ-butyrolactone, etc.; the lithium salt may be, for example, one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0066] The embodiments of the present invention do not strictly limit the material selection of the diaphragm, which can be a diaphragm material commonly used in lithium-ion batteries, such as a polypropylene diaphragm (PP), a polyethylene diaphragm (PE), a polypropylene / polyethylene double-layer composite film (PP / PE), a polyimide electrospinning diaphragm (PI), a polypropylene / polyethylene / polypropylene three-layer composite film (PP / PE / PP), a cellulose non-woven fabric diaphragm, and a diaphragm with a ceramic coating.

[0067] An embodiment of the present invention also provides a method for preparing a lithium-ion battery, comprising: packaging a battery cell including the above-mentioned positive electrode plate or the positive electrode plate prepared according to the above-mentioned preparation method into an aluminum-plastic film bag, drying the packaged battery cell and injecting an electrolyte, and then sequentially placing, forming, and re-packaging the battery cell to obtain the above-mentioned lithium-ion battery, wherein after the above-mentioned formation is completed, the formed lithium-ion battery needs to be left to stand for at least 2 hours to ensure that the lithium ions have sufficient diffusion time, thereby suppressing the problem of lithium plating at the negative electrode.

[0068] The battery cell can be formed by winding a positive electrode sheet, a separator and a negative electrode sheet, and then the battery cell is packaged in an aluminum-plastic film bag, and then the packaged battery cell is dried and injected with electrolyte, and then placed in sequence to obtain a battery to be formed.

[0069] In some embodiments, the above-mentioned formation process includes: sequentially performing a first formation cycle, a second formation cycle, and a third formation cycle on the battery to be formed to obtain a lithium ion battery, wherein, in the first formation cycle, the formation voltage is 3.57-3.63V, and the formation is allowed to stand for 1.8-2.2h after formation; in the second formation cycle, the formation voltage is 3.87-3.93V, and the formation is allowed to stand for 3.8-4.2h after formation; in the third formation cycle, the formation voltage is 4.23-4.28V, and the formation is allowed to stand for 5.8-6.2h after formation. As the formation voltages of the first formation cycle, the second formation cycle, and the third formation cycle gradually increase, the standing time after formation is also gradually extended, which is more conducive to suppressing the problem of lithium precipitation at the negative electrode. This is because as the formation voltage increases, the potential of the negative electrode is closer and closer to the electrode potential, and thus the problem of lithium precipitation is more likely to occur. The above operation can ensure that lithium ions have sufficient diffusion time, thereby effectively suppressing the problem of lithium precipitation at the negative electrode.

[0070] The present invention is described in more detail below through specific embodiments.

[0071] Example 1

[0072] 1. Preparation of positive electrode sheet

[0073] Disperse lithium nickelate (Li2NiO2) and PVDF in NMP at a mass ratio of 80:20, and stir and mix to form a uniform slurry of the first lithium replenishing layer (slurry of the second lithium replenishing layer);

[0074] The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2, carbon black, and PVDF are dispersed in NMP at a mass ratio of 95:2.5:2.5, and stirred and mixed to form a uniform slurry of the first positive electrode active material layer (slurry of the second positive electrode active material layer);

[0075] The slurry of the first lithium replenishing layer, the slurry of the second lithium replenishing layer, and the slurry of the first positive electrode active material layer are sequentially coated on both sides of the aluminum foil, and then dried to form a first lithium replenishing layer precursor, a second lithium replenishing layer precursor, and a first positive electrode active material layer precursor respectively; the slurry of the second positive electrode active material layer is then coated on the side of the second lithium replenishing layer precursor away from the aluminum foil, and after drying, a second positive electrode active material layer precursor is formed on the surface of the second lithium replenishing layer precursor, and the second positive electrode active material layer precursor and the second lithium replenishing layer precursor form an interface layer precursor; and then cold pressed to obtain a positive electrode sheet;

[0076] Among them, during the coating process, by adjusting the amount of the slurry of the first lithium replenishing layer, the slurry of the second lithium replenishing layer, the slurry of the first positive electrode active material layer and the slurry of the second positive electrode active material layer, the thickness of the first lithium replenishing layer precursor is greater than the thickness of the interface layer precursor, and the thickness of the interface layer precursor is greater than the thickness of the first positive electrode active material layer precursor. At the same time, the width of the first lithium replenishing layer precursor, the width of the interface layer precursor, and the width of the first positive electrode active material layer precursor are in line with expectations, and the thickness of the second lithium replenishing layer in the interface layer increases along the direction from the first lithium replenishing layer to the first positive electrode active material layer. The first lithium replenishing layer has a thinning area at the edge away from the first positive electrode active material layer. The cross-sectional view of the positive electrode sheet along the first direction is as shown in FIG. Figure 2 As shown;

[0077] 2. Preparation of negative electrode sheet

[0078] Dispersing hard carbon, carbon black and carboxymethyl cellulose in deionized water at a mass ratio of 96:1:3, and fully stirring and mixing to form a uniform negative electrode slurry; uniformly coating the negative electrode slurry on the negative electrode collector, and obtaining a negative electrode sheet through drying, rolling and slitting;

[0079] 3. Diaphragm

[0080] Polypropylene diaphragm

[0081] 4. Preparation of lithium-ion batteries

[0082] The positive electrode sheet, the separator, and the negative electrode sheet are wound to form a battery cell, and the battery cell is packaged in an aluminum-plastic film bag. The packaged battery cell is dried and then injected with electrolyte, and then sequentially set aside to obtain a battery to be formed. The battery to be formed is sequentially subjected to a first formation cycle, a second formation cycle, and a third formation cycle to obtain a lithium-ion battery. Its appearance schematic diagram is shown in FIG. Figure 4 As shown; wherein, in the first formation cycle, the formation voltage is 3.6V, and the formation is allowed to stand for 2 hours; in the second formation cycle, the formation voltage is 3.9V, and the formation is allowed to stand for 4 hours; in the third formation cycle, the formation voltage is 4.25V, and the formation is allowed to stand for 6 hours.

[0083] With reference to Example 1, lithium ion batteries of Examples 2 to 12 and Comparative Examples 1 to 5 were prepared. Parameters such as the width of the first lithium supplement layer, the width of the interface layer / the width of the first lithium supplement layer, the width of the first lithium supplement layer / the width of the first positive electrode active material layer, the first lithium supplement agent and the second lithium supplement agent, the position of the second positive electrode active material layer in the interface layer, the thickness change trend of the second lithium supplement layer in the interface layer along the direction from the first lithium supplement layer to the first positive electrode active material layer, whether there is a thinning area at the edge of the first lithium supplement layer away from the first positive electrode active material layer, and the schematic diagram are shown in Table 1; parameters such as the formation voltage / V and standing time / h of the first formation cycle of the battery, the formation voltage / V and standing time / h of the second formation cycle, and the formation voltage / V and standing time / h of the third formation cycle are shown in Table 2. Among them, Figure 3 It is a cross-sectional view of the positive electrode sheet of Example 6 along the first direction.

[0084] Table 1

[0085]

[0086]

[0087] Table 2

[0088]

[0089]

[0090] Test example

[0091] 1. The following parameters of the above embodiments and comparative examples were tested:

[0092] 1) Resistance of the positive electrode sheet: Take a sample with an area of ​​4cm*25cm along the longitudinal direction of the positive electrode sheet, use a four-probe film resistance tester, select the single-point mode, and the test area of ​​the input terminal is 154.02mm 2 , pressure 0.4t, time interval 15s, place the sample (electrode) to be tested in the middle of the probe, first click the run button on the software and then turn the reversing valve downward, automatically collect a data after 15s, repeat the above test process when changing a point (test position), and change to another group after testing 20 points (test positions). Also note that the two probes tested are in contact with the junction, and the other two probes are in contact with or active area. Test 20 samples in parallel in each group and take the average value;

[0093] 2) First coulombic efficiency: After the lithium ion batteries of each embodiment and comparative example were allowed to stand for 4 hours at 25°C, the first charge and discharge capacity test was performed. The test conditions were as follows: charging at 0.1C to 4.25V, charging at a constant voltage to 0.05C and then cutting off, standing for 3 minutes, and then discharging at 0.1C to 2.8V. The charge and discharge curves were obtained, and the first charge gram capacity C0 and the first discharge gram capacity D0 at 2.8 to 4.25V were recorded respectively. The first coulombic efficiency was calculated according to D0 / C0.

[0094] 3) Energy density: Using a battery charge and discharge tester, the lithium ion batteries of the embodiments and comparative examples were charged to 4.25 V at 0.5 C constant current at 25° C., and then charged at a constant voltage until the current dropped to 0.02 C. After standing for 5 min, the batteries were discharged to 2.8 V at 0.5 C constant current. The first discharge capacity Q discharge and the first discharge energy E discharge of the battery were recorded. The mass of the battery was weighed and recorded as W, and the mass energy density ED = E discharge / W was calculated.

[0095] 4) Lithium deposition: (1) The battery is placed in a constant temperature box at 25℃±2℃ and allowed to stand for 2 hours; (2) The battery is charged, specifically: charged to 3.6V at 1C constant current and allowed to stand for 5 minutes; charged to 3.9V at 1.0C constant current and allowed to stand for 8 minutes; charged to 4.28V at 1C constant current; and charged to 0.05C constant voltage and allowed to stand for 10 minutes; (3) The battery is discharged, specifically: discharged to 2.8V at 1C constant current; (4) The battery is disassembled and the surface of the negative electrode active material in the negative electrode plate area corresponding to the lithium supplement layer is checked for lithium deposition; if a gray area is observed on the surface of the negative electrode active material, it is lithium deposition; if there is no gray area, there is no lithium deposition. Among them, the degree of lithium deposition is divided into slight lithium deposition, moderate lithium deposition and severe lithium deposition; slight lithium deposition means that the lithium deposition area is less than 0.5% of the whole area of ​​the negative electrode plate, moderate lithium deposition means that the lithium deposition area is 0.5% to 5.0% of the whole area of ​​the negative electrode plate, and severe lithium deposition means that the lithium deposition area is greater than 5.0% of the whole area of ​​the negative electrode plate; the specific results are shown in Table 1.

[0096] 5) Porosity: The porosity of the first lithium replenishment layer, the junction layer, and the first positive electrode active material layer is detected according to the following method: the positive electrode sheet is cut according to the first lithium replenishment layer, the junction layer, and the first positive electrode active material layer to obtain the positive electrode sheet samples corresponding to the first lithium replenishment layer, the junction layer, and the first positive electrode active material layer, the positive electrode sheet samples are spread flat on a glass table, cut into a certain size, and the thickness of the positive electrode sheet samples is measured with a micrometer to calculate the volume V1 of the positive electrode sheet samples; then the positive electrode sheet samples are placed in the test equipment, the gas valve is opened, helium is introduced, and the true volume V2 of the positive electrode sheet samples is tested; the porosity of the positive electrode sheet samples is obtained according to the formula (V1-V2) / V1*100%; in addition, the porosity of each layer of slurry before cold pressing is measured according to the above method; the specific results are shown in Table 3.

[0097] 2. Test results

[0098] Table 3 Resistance, first coulombic efficiency, energy density, and lithium precipitation of the positive electrode

[0099]

[0100]

[0101] Data Analysis:

[0102] Compared with Example 5, the second positive electrode active material layer of Comparative Example 1 is located between the second lithium replenishing layer and the positive electrode current collector and in the interface layer, and the thickness of the second lithium replenishing layer increases along the direction from the first lithium replenishing layer to the first positive electrode active material layer. The various properties of the positive electrode sheet of Comparative Example 1 are worse than those of Example 5.

[0103] Compared with Example 5, the second positive electrode active material layer of Comparative Example 2 is located between the second lithium supplement layer and the positive electrode current collector and in the interface layer. The performance of the positive electrode sheet of Comparative Example 2 is worse than that of Example 5.

[0104] Compared with Examples 10-12, the second positive electrode active material layer of Comparative Example 3-5 is located between the second lithium supplement layer and the positive electrode current collector and in the interface layer, and the various properties of the positive electrode sheet of Comparative Example 3-5 are inferior to those of Examples 10-12;

[0105] The embodiments of the present invention are helpful to improve the resistance, first coulomb efficiency, energy density, lithium precipitation, etc. of the positive electrode sheet.

[0106] Table 4 Porosity

[0107]

[0108]

[0109] Data Analysis:

[0110] The porosity of each layer of slurry before cold pressing and the porosity of each layer after cold pressing in each embodiment are compared. The porosity of each layer is significantly reduced after cold pressing, and the porosity of the first lithium supplement layer is smaller than the porosity of the interface layer; the porosity of the interface layer is smaller than the porosity of the first positive electrode active material layer.

[0111] It can be seen from Table 4 that the positive electrode sheet of the embodiment of the present invention meets the following conditions: the porosity of the first lithium supplement layer is smaller than the porosity of the interface layer; and the porosity of the interface layer is smaller than the porosity of the first positive electrode active material layer.

[0112] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode sheet, characterized in that: It includes a positive electrode current collector and a positive electrode coating located on at least one side of the positive electrode current collector, wherein the positive electrode coating includes a first lithium replenishing layer, a first positive electrode active material layer, and an interface layer located between the first lithium replenishing layer and the first positive electrode active material layer distributed along a first direction; the first direction is perpendicular to the thickness direction of the positive electrode sheet; The interface layer includes a second lithium replenishing layer and a second positive electrode active material layer located on a surface of the second lithium replenishing layer away from the positive electrode current collector.

2. The positive electrode sheet according to claim 1, characterized in that: The positive electrode plate further includes a tab connected to the positive current collector, and the first lithium supplement layer is located on a side of the positive current collector close to the tab.

3. The positive electrode sheet according to claim 1, characterized in that: The width of the first lithium replenishing layer is greater than 1 mm, and the ratio of the width of the first lithium replenishing layer to the width of the first positive electrode active material layer is 0.01 to 0.05; And / or, the ratio of the width of the interface layer to the width of the first lithium replenishing layer is 0.2 to 0.4; And / or, in the boundary layer, the thickness of the second lithium replenishing layer decreases along the direction from the first lithium replenishing layer to the first positive electrode active material layer; and / or, the thickness of at least a portion of the first lithium replenishing layer increases along the direction from the first lithium replenishing layer to the first positive electrode active material layer; And / or, the thickness of at least a portion of the first lithium replenishing layer is the same as the thickness of the first positive electrode active material layer.

4. The positive electrode sheet according to claim 1, characterized in that: The porosity of the first lithium supplement layer is smaller than the porosity of the interface layer; and the porosity of the interface layer is smaller than the porosity of the first positive electrode active material layer.

5. The positive electrode sheet according to claim 1, characterized in that: The ratio of the resistance of the first lithium replenishing layer to the resistance of the first positive electrode active material layer is greater than 1.

5.

6. The positive electrode sheet according to claim 1, characterized in that: The first lithium replenishing layer includes a first lithium replenishing agent, and the second lithium replenishing layer includes a second lithium replenishing agent; the first lithium replenishing agent and the second lithium replenishing agent each independently include one or more of Li2MnO3, Li2NiO2, and Li5FeO4; And / or, in the first lithium replenishing layer, the mass percentage of the first lithium replenishing agent is 70% to 97%; in the second lithium replenishing layer, the mass percentage of the second lithium replenishing agent is 70% to 97%; And / or, the first lithium replenishing layer further comprises a binder, and the mass percentage of the binder in the first lithium replenishing layer is 3% to 30%; And / or, the second lithium replenishing layer further comprises a binder, and the mass percentage of the binder in the second lithium replenishing layer is 3% to 30%; And / or, the first positive electrode active material layer and the second positive electrode active material layer respectively include positive electrode active materials; the positive electrode active materials include ternary positive electrode active materials, and the mass percentage of nickel in the ternary positive electrode active materials is not less than 80%.

7. A method for preparing a positive electrode sheet according to any one of claims 1 to 6, characterized in that: include: The slurry of the first lithium replenishing layer, the slurry of the second lithium replenishing layer, and the slurry of the first positive electrode active material layer are sequentially coated on at least one side surface of the positive electrode current collector to form the first lithium replenishing layer, the second lithium replenishing layer, and the first positive electrode active material layer distributed in sequence along the first direction; then the slurry of the second positive electrode active material layer is coated on the side surface of the second lithium replenishing layer away from the positive electrode current collector to form the interface layer including the second lithium replenishing layer and the second positive electrode active material layer to obtain the positive electrode sheet.

8. The preparation method according to claim 7, characterized in that: The slurry of the first lithium replenishing layer, the slurry of the second lithium replenishing layer, and the slurry of the first positive electrode active material layer are sequentially coated on at least one side of the positive electrode current collector, and then dried to form a first lithium replenishing layer precursor, a second lithium replenishing layer precursor, and a first positive electrode active material layer precursor, respectively; Then, coating the slurry of the second positive electrode active material layer on the surface of the second lithium replenishing layer precursor away from the positive electrode current collector, and after drying, forming a second positive electrode active material layer precursor on the surface of the second lithium replenishing layer precursor, and the second positive electrode active material layer precursor and the second lithium replenishing layer precursor form an interface layer precursor; After rolling, the positive electrode sheet is obtained; Wherein, before the rolling, the thickness of the first lithium supplement layer precursor is greater than the thickness of the interface layer precursor, and the thickness of the interface layer precursor is greater than the thickness of the first positive electrode active material layer precursor.

9. A lithium ion battery, characterized in that: The positive electrode sheet comprises the positive electrode sheet as described in any one of claims 1 to 6 or the positive electrode sheet prepared according to the preparation method as described in claim 7 or 8.

10. A method for preparing a lithium ion battery, characterized in that: include: Encapsulating a battery cell comprising the positive electrode sheet according to any one of claims 1 to 6 or the positive electrode sheet prepared by the preparation method according to claim 7 or 8 into an aluminum-plastic film bag, drying the encapsulated battery cell and injecting an electrolyte, and then sequentially performing a first formation cycle, a second formation cycle, and a third formation cycle to obtain the lithium-ion battery; Among them, in the first formation cycle, the formation voltage is 3.57-3.63V, and the formation is allowed to stand for 1.8-2.2 hours; in the second formation cycle, the formation voltage is 3.87-3.93V, and the formation is allowed to stand for 3.8-4.2 hours; in the third formation cycle, the formation voltage is 4.23-4.28V, and the formation is allowed to stand for 5.8-6.2 hours.

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