Positive plate and lithium ion secondary battery

Abstract

The invention provides a positive plate and a lithium ion secondary battery. The positive plate comprises a positive current collector, the positive plate comprises an active area and an empty foil area which are sequentially arranged in the length direction of the positive current collector, the empty foil area is located on the edge of one side of the positive current collector, and the active area comprises a first module and a second module which are sequentially arranged in the width direction of the positive current collector; the first module is positioned on one side edge of the positive current collector in the width direction; in the thickness direction of the positive current collector, the first module comprises a positive current collector, a first active layer and a first coating, the first active layer and the first coating are stacked on at least one side of the positive current collector, and the second module comprises a positive current collector and a second active layer located on at least one side of the positive current collector. The lithium separation problem of the side edge of the negative electrode tab and the lithium separation problem of the negative electrode region corresponding to the positive electrode tab can be effectively improved, and the lithium separation window is widened, so that the rate capability and the cycle performance of the lithium ion secondary battery are favorably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a positive electrode sheet and a lithium-ion secondary battery including the positive electrode sheet. Background Art

[0002] With the advent of the 5G era and the large-scale use of electric vehicles, in the future, lithium-ion batteries are required to have higher energy density, faster charging speed and higher safety.

[0003] During high-rate charging of lithium-ion batteries, the speed of lithium deintercalation from the positive electrode is faster than the speed of lithium intercalation into the negative electrode. Lithium ions deintercalated from the positive electrode cannot be intercalated into the negative electrode in time, and lithium plating phenomenon will occur. In particular, the lithium plating problem at the two side edges of the negative electrode is the most serious; moreover, for a battery cell with tabs, there is also a relatively serious lithium plating problem in the negative electrode region corresponding to the tab position of the positive electrode sheet, resulting in the consumption of the electrolyte, the loss of active lithium and a large volume effect, causing an increase in the stress of the electrode sheet, and further resulting in a sharp reduction in the cycle life of the lithium-ion battery. Summary of the Invention

[0004] Aiming at the problems existing in the prior art, the present invention provides a positive electrode sheet, so that the lithium-ion secondary battery prepared by using the positive electrode sheet can effectively improve the lithium plating problems at the side edges of the negative electrode tab and the negative electrode region corresponding to the positive electrode tab, broaden the lithium plating window, thereby being beneficial to the improvement of the cycle performance of the lithium-ion secondary battery. The present invention also provides a lithium-ion secondary battery including the above positive electrode sheet.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] In a first aspect of the present invention, there is provided a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector, and along the length direction of the positive electrode current collector, the positive electrode sheet includes an active region and an empty foil region located at one side edge of the positive electrode current collector, which are arranged in sequence. Along the width direction of the positive electrode current collector, the active region includes a first module and a second module arranged in sequence, and the first module is located at one side edge of the positive electrode current collector in the width direction;

[0007] Along the thickness direction of the positive electrode current collector, the first module includes the positive electrode current collector and a first active layer and a first coating layer stacked on at least one side of the positive electrode current collector, and the second module includes the positive electrode current collector and a second active layer located at at least one side of the positive electrode current collector;

[0008] Along the width direction of the positive electrode current collector, the first module has a first groove, the second module has a second groove, and the positive electrode tab is located in the first groove and the second groove, and the positive electrode tab is electrically connected to the positive electrode current collector;

[0009] The first coating includes organic particles and / or inorganic particles and a binder;

[0010] The impedance of the positive current collector in the first groove > the impedance of the positive current collector in the second groove.

[0011] A second aspect of the present invention provides a lithium-ion secondary battery, which includes the positive electrode sheet of the first aspect of the present invention.

[0012] Through the above technical solutions, the present invention has at least the following beneficial effects compared with the prior art:

[0013] By providing a first coating with high impedance at the edge of the positive electrode sheet on the side close to the tab, and making the impedance of the positive current collector in the first groove contained in the first module greater than the impedance of the positive current collector in the second groove contained in the second module, the present invention can further effectively alleviate the lithium deposition in the negative electrode region corresponding to the positive tab while improving the lithium deposition at the edge of the negative tab side, broaden the lithium deposition window, and thus is beneficial to the improvement of the cycle performance of the lithium-ion secondary battery; in addition, the first coating can also act as a protective layer to avoid internal short circuit of the battery. Even if lithium deposition occurs at the edge under a relatively strict system design and harsh charging conditions, it can avoid the short circuit problem caused by the contact between the lithium dendrite and the negative electrode sheet when piercing the separator; further, the grooves electrically connected to the positive tab provided in both the first module and the second module of the present invention can solve the lithium deposition at the negative edge while avoiding the problem of poor welding of the positive tab, and further improve the cycle performance of the lithium-ion secondary battery.

[0014] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. Description of the Drawings

[0015] Figure 1 It is a cross-sectional view of the positive electrode sheet provided by an embodiment of the present invention along the width direction of the positive current collector;

[0016] Figure 2 It is a cross-sectional view of the tab groove position of the positive electrode sheet provided by an embodiment of the present invention along the width direction of the positive current collector;

[0017] Figure 3 It is a top view of the positive electrode sheet provided by an embodiment of the present invention;

[0018] Figure 4 It is a top view of the positive tab position of the positive electrode sheet provided by an embodiment of the present invention;

[0019] Figure 5 A cross-sectional view of a positive electrode sheet provided by an embodiment of the present invention along the width direction of the positive electrode current collector;

[0020] Figure 6 A cross-sectional view of a positive electrode sheet and a negative electrode sheet provided by an embodiment of the present invention along the width direction;

[0021] Figure 7 A schematic diagram of the winding core structure of a lithium-ion secondary battery provided by an embodiment of the present invention;

[0022] Figure 8 A SEM schematic diagram of the tab area of the positive electrode sheet provided by an embodiment of the present invention;

[0023] Figure 9 A schematic diagram of the structure of the negative electrode area corresponding to the positive tab provided by an embodiment of the present invention.

[0024] Explanation of reference numerals:

[0025] Positive electrode current collector 100, first active layer 111, second active layer 112, third active layer 113, first coating layer 120, second coating layer 130, first groove 140, second groove 150, positive tab 160, empty foil area 170, negative electrode sheet 200, insulating adhesive layer 210, separator 300, adhesive tape 400. Detailed description of the specific implementation

[0026] The following provides a detailed description of the specific implementation of the present invention. It should be understood that the specific implementation described herein is only used to illustrate and explain the present invention and is not used to limit the present invention.

[0027] In the specific implementation and the claims, a list of items connected by terms such as "at least one of", "at least one of", "at least one kind of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single element or multiple elements. Item B may include a single element or multiple elements. Item C may include a single element or multiple elements.

[0028] The first aspect of the present invention provides a positive electrode sheet, and the positive electrode sheet includes a positive electrode current collector 100, as Figure 1 shown Figure 1 A cross-sectional view of a positive electrode sheet provided by an embodiment of the present invention along the width direction of the positive electrode current collector;

[0029] The positive electrode sheet includes an active region and an empty foil region 170 located at one side edge of the positive electrode current collector in the length direction of the positive electrode current collector, arranged in sequence, as Figure 3 shown. Figure 3 This is a top view of the positive electrode sheet provided by an embodiment of the present invention. It should be noted that the empty foil region 170 is formed by the uncoated part of the positive electrode current collector, that is, any one side surface of the positive electrode current collector in the empty foil region 170 does not include a positive electrode active layer;

[0030] As Figure 1 shown, the active region includes a first module and a second module arranged in sequence in the width direction of the positive electrode sheet current collector. The first module is located at one side edge in the width direction of the positive electrode sheet current collector; in the thickness direction of the positive electrode current collector 100, the first module includes the positive electrode current collector 100 and a first active layer 111 and a first coating 120 stacked on at least one side of the positive electrode current collector 100. The second module includes the positive electrode current collector 100 and a second active layer 112 located on at least one side of the positive electrode current collector 100;

[0031] As Figure 2 shown, Figure 2 This is a sectional view of the tab groove position of the positive electrode sheet along the width direction of the positive electrode current collector provided by an embodiment of the present invention; in the width direction of the positive electrode sheet current collector; the first module contains a first groove 140, the second module contains a second groove 150, and the positive electrode tab 160 is located in the first groove 140 and the second groove 150. The positive electrode tab 160 is electrically connected to the positive electrode current collector 100;

[0032] The first coating includes organic particles and / or inorganic particles and a binder; the impedance of the positive electrode current collector in the first groove > the impedance of the positive electrode current collector in the second groove.

[0033] In the first module, it includes the positive electrode current collector 100 and a first active layer 111 and a first coating 120 stacked on at least one side of the positive electrode current collector 100. The first coating 120 can be arranged on the surface of the first active layer away from the positive electrode current collector, that is, the first coating 120 serves as the surface coating of the first active layer 111, as Figure 5 shown; the first coating 120 can also be directly arranged on the surface of the positive electrode current collector 100, and the first active layer 111 is arranged on the surface of the first coating 120 away from the positive electrode current collector 100, that is, the first coating 120 serves as the bottom coating of the first active layer 111, as Figure 1 shown.

[0034] In the present invention, the first module includes a positive current collector 100, a first active layer 111 and a first coating layer 120 which are stacked on at least one side of the positive current collector 100, and the first module has a first groove 140 which contains a positive tab 160, and the positive tab 160 is electrically connected to the positive current collector 100; it can be understood that the first module is located at one side edge in the width direction of the positive electrode sheet current collector, the first coating layer is arranged in the first module on the surface of the positive current collector on the same side as the positive tab, wherein the first groove of the first module is arranged on the first active layer and the first coating layer which are stacked, and in order to ensure that the positive electrode sheet and the positive current collector can be electrically connected, the positive active material layer and the first coating layer in the first groove will be removed as much as possible. As Figure 2 shown, after the first coating layer and the first active layer are removed, electrical connection is achieved between the positive tab and the positive current collector. The same applies to the second active layer in the second groove, which will not be elaborated here.

[0035] In the present invention, by providing a first coating layer including organic particles and / or inorganic particles and a binder on the edge of the positive electrode sheet near the tab, the first coating layer can have a relatively high impedance, and the impedance of the positive current collector in the first groove corresponding to the positive tab in the first module is greater than the impedance of the positive current collector in the second groove corresponding to the positive tab in the second module, which can increase the impedance of the edge of the positive electrode sheet on the positive tab side, effectively reduce the lithium deintercalation rate and migration rate of the edge of the positive electrode sheet on the tab side, thereby playing a role in improving lithium deposition on the edge of the negative tab corresponding to the positive electrode sheet; moreover, the setting of the positive tab will cause a decrease in the lithium intercalation uniformity at the connection between the positive tab and the positive current collector, and serious lithium deposition occurs in the negative region corresponding to the positive tab. By greatly increasing the impedance of the edge of the positive electrode sheet on the positive tab side, the present invention can, while improving lithium deposition on the edge of the negative tab, alleviate the lithium deposition problem in the negative region corresponding to the positive tab (a separator 300 is generally arranged on the surface of this negative region, and it is very easy for uneven lithium ion deintercalation to occur in this negative region, resulting in relatively serious lithium deposition at the edge of the adhesive tape 400, as Figure 9 shown, which is a schematic structural diagram of the negative region corresponding to the positive tab provided by an embodiment of the present invention. Figure 9 There is also a separator 300 between the positive electrode sheet and the negative electrode sheet), widen the lithium deposition window near the edge of the negative electrode on the tab side, avoid the deterioration of lithium deposition at the edge of the negative electrode sheet under extreme charging conditions, resulting in electrolyte consumption, loss of active lithium, and a large volume effect, thereby avoiding the attenuation of the cycle life of the lithium ion secondary battery and being beneficial to the improvement of the cycle performance of the lithium ion secondary battery.

[0036] In addition, the first coating on the side of the positive electrode plate close to the positive electrode tab can also serve as a protective layer for the edge of the tab side of the positive electrode plate. Even if lithium plating occurs at the edge under relatively strict system design and harsh charging conditions, it can prevent the lithium dendrites generated by lithium plating from contacting the negative electrode plate when piercing the separator, thus avoiding the short-circuit problem between the positive and negative electrode plates and improving the safety performance and cycle performance of the lithium-ion secondary battery.

[0037] Furthermore, in the present invention, by simultaneously providing grooves electrically connected to the positive electrode tab in the first module and the second module, the first groove is in the first module and the second groove is in the second module, and the positive electrode tab is electrically connected to the positive electrode current collector located in the first groove and the second groove, which can solve the problem of lithium plating on both sides of the negative electrode edge while avoiding the problem of poor welding of the positive electrode tab. This is because a part of the first coating remains in the first groove, while there is basically no residue of the positive electrode active material layer in the second groove. By extending the groove electrically connected to the positive electrode tab into the second module, the contact area between the positive electrode tab and the clean positive electrode current collector can be increased, thereby improving the connection strength between the positive electrode tab and the positive electrode current collector and avoiding the problem of poor welding of the positive electrode tab; at the same time, it can also achieve an excellent electrical connection effect between the positive electrode tab and the positive electrode current collector, further improving the lithium plating problem at the edge of the negative electrode tab side and the negative electrode region corresponding to the positive electrode tab, and enhancing the cycle performance of the lithium-ion secondary battery.

[0038] In one example, the mass content of the binder in the first coating is X1 and the mass content of the binder in the second active layer is X2, where X1 > X2.

[0039] In another example, 20 ≤ X1 / X2 ≤ 60, such as 20, 25, 30, 35, 40, 45, 50, 55 or 60.

[0040] The mass content of the binder in the first coating of the present invention is higher than that in the second active layer. By providing a first coating with a high mass content of the binder at the edge of the positive electrode plate near the positive electrode tab, the impedance of the edge of the positive electrode tab side of the positive electrode plate can be further increased. Moreover, by setting the mass content of the binder in the first coating to 20 to 60 times that of the binder in the second active layer, it can be ensured that the impedance of the edge of the positive electrode tab side (the first module) of the positive electrode plate is greater than that of the middle region (the second module) of the positive electrode plate and there is a certain difference in the impedance values. This can effectively reduce the lithium deintercalation rate and migration rate of the edge of the negative electrode tab corresponding to the positive electrode plate relative to the middle region of the positive electrode plate, thereby further improving the lithium deposition problem on the edge of the negative electrode tab corresponding to the positive electrode plate. However, the mass content of the binder in the first coating should not be too large and should not exceed 60 times that of the binder in the second active layer of the positive electrode. This is because if the impedance of the edge of the positive electrode plate is too large, the transmission of lithium ions at the edge will be blocked, resulting in difficulty for lithium ions to quickly and uniformly deintercalate from the positive electrode and intercalate into the negative electrode during charging, and also difficulty in smoothly returning from the negative electrode to the positive electrode during discharging, thereby reducing the charge-discharge efficiency of the battery and affecting the battery performance. At the same time, it may also lead to an accelerated decline in the battery capacity and shorten the service life of the battery.

[0041] In one example, the mass content of the binder in the first coating is 20 wt% to 50 wt%, such as 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt%, 30 wt%, 31 wt%, 32 wt%, 33 wt%, 34 wt%, 35 wt%, 36 wt%, 37 wt%, 38 wt%, 39 wt%, 40 wt%, 41 wt%, 42 wt%, 43 wt%, 44 wt%, 45 wt%, 46 wt%, 47 wt%, 48 wt%, 49 wt%, 50 wt%.

[0042] In one example, the mass content of the binder in the first coating is 30 wt% to 50 wt%.

[0043] In one example, the mass content of the binder in the second active layer is 0.5 wt% to 2.5 wt%, such as 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%.

[0044] In one example, the mass content of the binder in the second active layer is 0.5 wt% to 1.5 wt%.

[0045] It should be noted that the first active layer, the second active layer, and the third active layer are named for the convenience of distinguishing the position relationship of the active layers in the first module, the second module, and the third module. In the actual preparation process of the positive electrode sheet, the same positive electrode active paste is used, which contains the same positive electrode active material, and the types and mass contents of other components are also exactly the same.

[0046] In one example, the impedance of the positive electrode current collector in the empty foil area 170 is 0.01 Ω to 0.08 Ω, such as 0.01 Ω, 0.02 Ω, 0.03 Ω, 0.04 Ω, 0.05 Ω, 0.06 Ω, 0.07 Ω, or 0.08 Ω. Since no positive electrode active material layer is provided in the empty foil area, the impedance of the positive electrode current collector in the empty foil area can be used as the impedance of the positive electrode current collector without any coating.

[0047] In one example, the impedance of the positive electrode current collector in the empty foil area is 0.02 Ω to 0.06 Ω.

[0048] In the present invention, the impedance of the positive electrode current collector in the first groove is 0.02 Ω to 0.15 Ω, such as 0.02 Ω, 0.03 Ω, 0.04 Ω, 0.05 Ω, 0.06 Ω, 0.07 Ω, 0.08 Ω, 0.09 Ω, 0.1 Ω, 0.11 Ω, 0.12 Ω, 0.13 Ω, 0.14 Ω, or 0.15 Ω.

[0049] In one example, the impedance of the positive electrode current collector in the first groove is 0.05 Ω to 0.12 Ω.

[0050] In the present invention, the impedance of the positive electrode current collector in the second groove is 0.02 Ω to 0.1 Ω, such as 0.02 Ω, 0.03 Ω, 0.04 Ω, 0.05 Ω, 0.06 Ω, 0.07 Ω, 0.08 Ω, 0.09 Ω, or 0.1 Ω.

[0051] In one example, the impedance of the positive electrode current collector in the second groove is 0.03 Ω to 0.08 Ω.

[0052] The test method for the impedance of the positive current collector in the first groove and the second groove is as follows: According to the four-probe test principle, a two-probe (diameter 1 mm) test device is self-assembled, and a pressure gauge and a HIOKI resistor 3560 are used. Test process: Cut the area to be tested of the electrode to be tested into a fixed square size, then place each area to be tested of the electrode to be tested under the two probes. The two probes are connected to the resistor through two poles. Rotate the test device, and the probes press the area to be tested on the surface of the electrode under a stable pressure. The pressure is controlled by the pressure gauge. After reaching a certain pressure, read the impedance data of the resistor. Test several times and obtain the average value. This data is the relative value of the impedance of the area to be tested of the corresponding electrode to be tested. The test method for the impedance of the positive current collector in the empty foil area is the same and will not be elaborated here.

[0053] When the impedance of the positive current collector in the first groove measured by the above test method is greater than 0.15 Ω, it will cause the impedance of the connection between the positive electrode tab side edge (first module) of the positive electrode plate and the positive current collector near the connection to be very large. The high impedance of the electrode plate will directly increase the internal resistance of the battery, thereby exacerbating the polarization phenomenon of the lithium-ion secondary battery; when the impedance of the positive current collector in the first groove measured by the above test method is less than 0.02 Ω, it will cause the impedance difference between the positive electrode tab side edge (first module) and the middle area of the positive electrode plate (second module) to be small, and the influence on the lithium deintercalation rate and migration rate at the positive electrode tab side edge of the positive electrode plate will not be obvious, resulting in the problem of lithium deposition on the negative electrode edge of the lithium-ion secondary battery not being effectively solved.

[0054] When the impedance of the positive current collector in the second groove measured by the above test method is greater than 0.1 Ω, it will cause the impedance of the middle area of the positive electrode plate (second module) to be very large. The impedance of the positive current collector in the second groove is too high, and the residual amount of the positive active material coating in the second module is too much, resulting in the inability to achieve excellent electrical connection between the positive electrode tab and the positive current collector, and then the problem of poor positive electrode tab welding occurs; when the impedance of the positive current collector in the second groove measured by the above test method is less than 0.02 Ω, it will cause the impedance difference between the positive electrode tab side edge (first module) and the middle area of the positive electrode plate (second module) to be too large, thereby affecting the lithium intercalation uniformity at the connection between the positive electrode tab and the positive current collector in the negative electrode plate area corresponding to the first groove and the second groove, resulting in the problem of lithium deposition on the negative electrode edge of the lithium-ion secondary battery not being effectively solved, and also causing the problem of lithium deposition in the negative electrode area corresponding to the positive electrode tab.

[0055] In addition, by directly adjusting the impedance of the positive current collector in the first groove and the second groove to a suitable range, the lithium-ion diffusion path in the positive and negative electrode plate areas corresponding to the electrode tab position can be improved, and the lithium deintercalation rate and lithium-ion diffusion rate at the positive electrode edge can be further adjusted, thereby further alleviating the problem of lithium deposition on the negative electrode edge of the lithium-ion battery.

[0056] Further, in the present invention, the impedance of the positive current collector in the first groove > the impedance of the positive current collector in the second groove > the impedance of the positive current collector in the empty foil area. By adjusting the impedance relationship of the positive current collectors in the first groove, the second groove, and the empty foil area, it is made that the impedance of the positive current collector in the first groove > the impedance of the positive current collector in the second groove > the impedance of the positive current collector in the empty foil area. Among them, the impedance of the positive current collector in the first groove > the impedance of the positive current collector in the second groove, which can reduce the lithium deintercalation speed and migration speed at the edge of the positive tab along the width direction of the positive electrode sheet, and can also reduce the charging current density in the coating area at the edge of the positive tab along the width direction during charging; moreover, it can also reduce the charging current density in the corresponding negative electrode sheet area, optimizing the lithium intercalation uniformity of the corresponding negative electrode sheet during charging; the impedance of the positive current collector in the second groove > the impedance of the positive current collector in the empty foil area, that is, the impedance of the second groove cannot be less than or equal to the impedance of the positive current collector in the empty foil area. By adjusting the impedance of these two areas in this way, it is possible to avoid too large a difference in impedance between the edge of the positive tab (the first module) and the middle area of the positive electrode sheet (the second module), thereby affecting the lithium intercalation uniformity in the negative electrode sheet area corresponding to the first groove and the second groove at the connection between the positive tab and the positive current collector, thus improving the problem of lithium deposition at the edge of the negative electrode tab of the lithium-ion secondary battery, broadening the lithium deposition window, and being beneficial to the improvement of the cycle performance of the lithium-ion secondary battery.

[0057] In the present invention, the first groove contains element M, and the element M includes at least one of Al, Si, Mg, Ti, Fe, Ca, Zr, Zn, Ba, F, C, Ni, Co, and Mn; the second groove contains element N, and the element N includes at least one of Co, Ni, Mn, Al, and Fe.

[0058] In the present invention, "the first groove contains element M / the second groove contains element N" means that: when performing an energy dispersive spectrometer (EDS) test on the first groove / the second groove, under the condition of an acceleration voltage of 15 kV, the first groove can be detected to contain element M, and the element M includes at least one of Al, Si, Mg, Ti, Fe, Ca, Zr, Zn, Ba, F, C, Ni, Co, and Mn, and the second groove contains element N, and the element N includes at least one of Co, Ni, Mn, Al, and Fe.

[0059] The M element contained in the first groove is the element contained in the residual first coating and the first active layer, and the N element contained in the second groove is the element contained in the residual second active layer. By controlling the types of elements contained in the first groove and the second groove, the present invention can ensure that the regions in the first groove and the second groove that are not electrically connected to the positive electrode tab can perform lithium deintercalation and the lithium ion migration speed, thereby reducing the charging current density in the positive electrode tab region of the positive electrode plate during charging, and can also optimize the lithium intercalation uniformity in the region of the negative electrode plate corresponding to the positive electrode tab position during charging, thereby further solving the problem of lithium deposition in the region of the negative electrode corresponding to the positive electrode tab position, which is beneficial to improving the cycle performance of the lithium ion secondary battery; in addition, by controlling the types of elements contained in the first groove and the second groove, the first coating and the first active layer can also have a good coating effect, thereby further inhibiting the transmission of lithium ions and avoiding the problem of lithium deposition at the edge of the negative electrode plate.

[0060] In the present invention, as Figure 4 shown, Figure 4 is a top view of the positive electrode tab position of the positive electrode plate provided by an embodiment of the present invention; along the width direction of the current collector of the positive electrode plate, the total width of the first groove 140 and the second groove 150 is W, and the width of the first groove 140 is W 1 , and the width of the region where the positive electrode tab is connected to the positive electrode current collector is W 3 (The way for the positive electrode tab to be electrically connected to the positive electrode current collector is generally welding, but it should be noted that the width W 3 of the region where the positive electrode tab and the positive electrode current collector can show electrical connection 3 does not mean that the entire W 3 region needs to be welded or provided with welding points, and there may be no welding points at the bottom of the positive electrode tab closest to the positive electrode active material layer), the width tolerance of the side edge of the positive electrode tab 160 located in the second groove 150 from the edge of the second groove 150 close to the second active layer 112 is d, and the width of the region where the W 4 coincides with the first groove is W 1 ; among them, W, W 3 , W 4 , W 1 and d satisfy the relational expression: W 3 ≤W-(W 4 ).

[0061] Since the residue of the first coating will cause the decline of the electrical connection effect between the positive electrode tab and the positive electrode current collector, therefore, by controlling the positional relationship between the positive electrode tab and the positive electrode current collector to satisfy the above relationship, the present invention can make only part of the area where the positive electrode tab and the positive electrode current collector are electrically connected be in the first groove and the rest be in the second groove, thereby effectively avoiding the decline of the electrical connection effect between the positive electrode tab and the positive electrode current collector, avoiding the increase of the battery internal resistance, affecting the charge and discharge efficiency and cycle life of the battery; in addition, the first groove retaining the first coating can also serve as a buffer zone for the contact between the positive electrode tab and the positive electrode current collector, reducing the current density at the positive electrode tab, thereby effectively alleviating the problem of lithium deposition in the negative electrode region corresponding to the tab position and also increasing heat dissipation, avoiding the occurrence of battery thermal runaway, and improving the safety performance of the battery.

[0062] In one example, the d = 0.5 mm to 4 mm, such as 0.5 mm, 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm or 4 mm. d, as Figure 4 shown, refers to the width tolerance of the side edge of the positive electrode tab 160 located in the second groove 150 from the edge of the second groove 150 close to the edge of the second active layer 112. The reason for setting the width tolerance d is to prevent the electrical connection area of the positive electrode tab from extending into the second active layer and affecting the loading amount of the second active layer.

[0063] In one example, the W 4 = 0.1 mm to 5 mm, such as 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1 mm, 2 mm, 3 mm, 4 mm or 5 mm. W 4 As Figure 4 shown, refers to the width of the overlapping area between W 3 and the first groove, that is, the area where the positive electrode tab is actually welded to the first groove and electrically connected.

[0064] The parameters W, W 1 、W 3 、W 4 and d, W, W 1 and d can be obtained by observing the SEM image of the tab area of the positive electrode sheet and using software for measuring length for statistical calculation. As Figure 8 shown, it is the SEM schematic diagram of the tab area of the positive electrode sheet provided by an embodiment of the present invention. After the first groove and the second groove are cleaned, obvious traces of the first coating will still remain on the surface of the positive electrode current collector of the first module. Therefore, W 1can be directly obtained through observation; for the side edge of the second groove close to the second active layer, since the empty foils can be directly observed after the first groove and the second groove are cleaned, the boundary between the groove and the active layer is relatively clear, so W and d can also be directly obtained through observation; W 3 is the set width when welding the positive tab, and can be obtained by statistically calculating using software for measuring length after observing the cross-sectional SEM image of the positive tab area of the positive electrode sheet, W 4 is the width of the overlapping area between the first groove and W 3 That is, W 1 and W 3 is the width of the overlapping area between them, and can be obtained by calculation after obtaining W 1 and W 3

[0065] In the present invention, as Figure 1 shown, the active area further includes a third module along the width direction of the current collector of the positive electrode sheet, and the third module is located at one side edge of the current collector of the positive electrode sheet in the width direction away from the first module; the third module includes a positive current collector 100 and a third active layer 113 and a second coating 130 stacked on at least one side of the positive current collector 100, the second coating includes organic particles and / or inorganic particles and a binder, and the mass content of the binder in the second coating is X3, and the mass content of the binder in the second active layer is X2, and X3>X2.

[0066] In one example, 20≤X3 / X2≤60.

[0067] By also providing a second coating with a relatively large impedance at the edge on the side away from the tab in the present invention, the lithium deintercalation rate and migration rate at the edge on the side away from the tab of the positive electrode sheet can be further reduced, the charging current density in the area of the second coating during charging can be reduced, and the lithium intercalation uniformity on the other side edge of the corresponding negative electrode sheet during charging can be optimized, thereby improving the problem of lithium deposition on both side edges of the negative electrode sheet of the lithium ion secondary battery, which is beneficial to further improving the cycle performance of the lithium ion secondary battery.

[0068] In the present invention, the impedance of the first module and / or the third module is greater than the impedance of the second module. By simultaneously providing organic particles and / or inorganic particles in the second coating, the impedance on the other side edge of the tab of the positive electrode sheet can be increased, thereby effectively improving the problem of lithium deposition on both side edges of the positive electrode sheet.

[0069] In one example, the organic particles include at least one of polyurethane microspheres, polystyrene microspheres, and polyethylene microspheres.

[0070] ​In one example, the inorganic particles include at least one of silica, titanium oxide, aluminum oxide, magnesium oxide, zinc oxide, zirconium oxide, cerium oxide, ferrous oxide, barium sulfate, and boehmite.

[0071] In one example, the binder includes at least one of polyvinylidene fluoride, carboxymethyl cellulose, styrene-butadiene rubber, polyvinylpyrrolidone, polymethyl methacrylate, polyacrylonitrile, polyacrylic acid, polyvinyl alcohol, sodium alginate, β-cyclodextrin polymer, polypropylene emulsion, and polytetrafluoroethylene, as well as copolymers between functionalized derivatives or monomers of the above polymers.

[0072] In one example, the first active layer, the second active layer, and the third active layer contain a positive electrode active material, and the positive electrode active material includes at least one of lithium cobaltate, lithium nickel cobalt manganate, lithium manganate, lithium nickel manganate, lithium nickel cobalt aluminate, lithium iron phosphate, lithium nickelate, and lithium-rich manganese-based materials.

[0073] In one example, the first active layer, the second active layer, and the third active layer contain a positive electrode active material, and the positive electrode active material is lithium cobaltate. The positive electrode active material shows a bimodal distribution in a laser particle size test. The lithium cobaltate includes first lithium cobaltate particles and second lithium cobaltate particles, and the average particle size of the first lithium cobaltate particles > the average particle size of the second lithium cobaltate particles. The lithium cobaltate particles of the present invention include two types of lithium cobaltate particles with different particle sizes, large particle lithium cobaltate (first lithium cobaltate particles) and small particle lithium cobaltate (second lithium cobaltate particles). The particle size distribution of the positive electrode active material can be measured by a laser particle size analyzer. By using two types of lithium cobaltate particles with different particle sizes as the positive electrode active material, the present invention can effectively increase the tap density between the lithium cobaltate particles, thereby improving the energy density of the battery.

[0074] In one example, the particle size Dv50 of the organic particles and / or the inorganic particles ≤ the particle size Dv50 of the second lithium cobaltate particles. Controlling the particle size Dv50 of the organic particles and / or the inorganic particles to be less than or equal to the particle size Dv50 of the small particle lithium cobaltate, the second lithium cobaltate particles, can further adjust the impedance at both edges along the width direction of the positive electrode sheet (the first module / the third module), making the impedance difference between the first module / the third module and the second module more obvious, which is beneficial to improving the problem of lithium deposition at the negative electrode edge of the lithium-ion secondary battery, and further beneficial to improving the cycle performance of the lithium-ion secondary battery.

[0075] In one example, the particle size Dv50 of the organic particles and / or the inorganic particles is 10 nm to 1000 nm.

[0076] In one example, the particle size Dv50 of the second lithium cobaltate particles is 1 μm to 8 μm.

[0077] In one example, the Dv50 of the first lithium cobalt oxide particles is 8 μm to 15 μm.

[0078] The Dv50 of the second lithium cobalt oxide particles and the Dv50 of the first lithium cobalt oxide particles can be obtained from the normal distribution graph with a bimodal distribution of the positive electrode active material during the laser particle size test.

[0079] In one example, in the first module and / or the third module, as Figure 1 shown, the first coating 120 and the second coating 130 are disposed on the surface of the positive electrode current collector 100, the first active layer 111 is disposed on the surface of the first coating 120 away from the positive electrode current collector 100, the third active layer 113 is disposed on the surface of the second coating 130 away from the positive electrode current collector 100, the impedance of the positive electrode current collector in the first groove is 0.05 Ω to 0.12 Ω, and the impedance of the positive electrode current collector in the second groove is 0.03 Ω to 0.08 Ω. When the first coating 120 and the second coating 130 are used as the undercoatings of the first active layer 111 and the third active layer 113 respectively, there is an excessive amount of the first coating component remaining in the first groove, and the impedance of the positive electrode current collector in the first groove will increase relatively. By further controlling the groove impedance when the first coating and the second coating are used as the undercoatings of the first active layer and the third active layer respectively, the present invention can avoid an increase in the internal resistance of the battery caused by an excessive groove impedance, which affects the charge and discharge efficiency of the battery.

[0080] In this embodiment, the preparation process of the positive electrode sheet is as follows: First, the coating slurry is coated on the positive electrode current collector by a gravure coater to obtain the coating, and then the positive electrode active material layer slurry (the first active layer, the second active layer, and the third active layer are made of the same material and can be collectively referred to as the positive electrode active material layer slurry) is coated on the coating by a coater. Finally, the positive electrode sheet is obtained through rolling and slitting. It can be understood that in the positive electrode sheet obtained by this method, the coating is coated on the surface of the positive electrode current collector and is located between the active layer and the positive electrode current collector, that is, the coating is an undercoat formed based on the positive electrode current collector before the active layer is coated.

[0081] In one example, in the first module and / or the third module, as Figure 5 shown, Figure 5Cross-sectional view of the positive electrode sheet along the width direction of the positive electrode current collector provided by an embodiment of the present invention; the positive electrode active material layer 110 is disposed on the surface of the positive electrode current collector 100, the first coating layer 120 is disposed on the surface of the first active layer 111 away from the positive electrode current collector 100, and the second coating layer 130 is disposed on the surface of the third active layer 113 away from the positive electrode current collector 100; the impedance of the positive electrode current collector in the first groove is 0.03 Ω to 0.08 Ω, and the impedance of the positive electrode current collector in the second groove is 0.03 Ω to 0.08 Ω. When the first coating layer 120 and the second coating layer 130 are respectively used as the surface coating layers of the first active layer 111 and the third active layer 113, since the active layer is first coated on the positive electrode current collector and the active layer is easier to clean, the remaining components of the first coating layer in the first groove will be relatively less or even none, and the impedance of the positive electrode current collector in the first groove will be relatively smaller. Therefore, compared with the case where the first coating layer and the second coating layer are used as the bottom coating layers of the active layer, the improvement effect on the lithium deposition at the side edge of the negative electrode tab and the lithium deposition problem in the negative electrode region corresponding to the positive electrode tab will be reduced.

[0082] In this embodiment, the preparation process of the positive electrode sheet is as follows: First, a positive electrode active material layer slurry (the first active layer, the second active layer, and the third active layer are made of the same material and can be collectively referred to as the positive electrode active material layer slurry) is coated on the positive electrode current collector by a coater to obtain the positive electrode active material layer. Subsequently, a coating slurry is coated on the positive electrode active material layer by a gravure coater to form the coating layer. Finally, the positive electrode sheet is obtained through rolling and slitting. It can be understood that in the positive electrode sheet obtained by this method, the active layer slurry is coated on the surface of the positive electrode current collector, and the coating layer is coated on the surface of the positive electrode active material layer, that is, the coating layer is a surface coating layer formed based on the active layer after the active layer is coated.

[0083] In the present invention, along the width direction of the positive electrode current collector, the first coating or the second coating comprises a plurality of sub - coatings arranged in sequence, and the distance between adjacent sub - coatings is 10 μm to 5000 μm, for example, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 300 μm, 500 μm, 700 μm, 1000 μm, 1500 μm, 2000 μm, 2500 μm or 3000 μm. In one example, the distance between adjacent sub - coatings is 50 μm to 500 μm. The plurality of sub - coatings of the first coating on the positive electrode current collector or the first active layer may exhibit a uniform / non - uniform arrangement state, and the plurality of sub - coatings of the second coating on the positive electrode current collector or the third active layer exhibit a uniform / non - uniform arrangement state, and the distance between any two adjacent sub - coatings is 10 μm to 5000 μm. By further setting the coating patterns of the first coating and the second coating, the present invention can avoid the influence of the high - impedance coating on the charging speed of the edges of the positive and negative electrode sheets, and at the same time can ensure the improvement of the lithium deposition problem on both sides of the negative electrode sheet due to the increase in the coating impedance, avoid the adverse effects brought by setting the first coating and the second coating, and further improve the overall performance of the lithium - ion secondary battery.

[0084] The second aspect of the present invention provides a lithium - ion secondary battery, and the lithium - ion secondary battery includes the positive electrode sheet of the first aspect of the present invention.

[0085] In the present invention, the lithium-ion secondary battery further includes a negative electrode sheet, the negative electrode sheet includes a negative electrode active material, the negative electrode active material includes a silicon-based material, and the content of the silicon-based material is 0 wt% to 50 wt%, for example, 0 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt% or 50 wt%. By improving the structure of the positive electrode sheet, the charging current density in the middle region of the corresponding negative electrode sheet can be reduced, the lithium intercalation uniformity of the corresponding negative electrode sheet during charging is optimized, thereby improving the problem of lithium deposition on both sides of the negative electrode, and broadening the lithium deposition window. Further, when the negative electrode active material of the negative electrode sheet includes a silicon-based material, since the silicon-based material is prone to a large volume expansion during the alloying reaction with lithium, the silicon-based negative electrode material collapses or pulverizes during the cycling process, especially the structural stability of the region of the negative electrode sheet corresponding to the positive electrode tab decreases significantly, affecting the overall cycling stability and cycling life of the battery. However, in the present invention, by providing a first coating with a relatively large impedance on the edge of the positive electrode tab side of the positive electrode sheet, the expansion of the silicon-based material at the edge of the negative electrode and in the region corresponding to the positive electrode tab caused by the increase in the edge current density can be reduced, thereby significantly reducing the polarization effect at the edge of the negative electrode sheet, and thus avoiding the phenomenon that the silicon-based material structure collapses or pulverizes at the edge of the negative electrode sheet and in the region corresponding to the positive electrode tab to generate pulverized chips or the negative electrode active material layer directly detaches from the negative electrode current collector, improving the stability of the negative electrode sheet, and further improving the electrochemical performance and stability of the lithium-ion secondary battery.

[0086] In one example, the silicon-based material includes silicon carbide, and the silicon carbide includes a porous carbon skeleton and silicon particles deposited in the porous carbon skeleton.

[0087] In one example, along the width direction of the negative electrode sheet, as Figure 6 shown, Figure 6 is a cross-sectional view of the positive electrode sheet and the negative electrode sheet provided by an embodiment of the present invention along the width direction; the negative electrode sheet 200 includes a fourth module and a fifth module arranged in sequence, the fourth module is located in the non-overlapping region between the negative electrode sheet 200 and the positive electrode current collector 100, and the fifth module is located in the overlapping region between the negative electrode sheet 200 and the positive electrode current collector 100. By providing a non-overlapping region of the fourth module where the negative electrode sheet extends beyond the positive electrode current collector, the present invention can prevent lithium dendrites and their potential safety risks from appearing on both sides of the positive electrode sheet; and this part of the region of the negative electrode sheet provides additional space so that lithium ions can be safely deposited during charging, rather than forming lithium dendrites on the negative electrode surface, which helps to avoid internal short circuits and thermal runaway in the battery, and further contributes to the improvement of the safety performance of the lithium-ion secondary battery.

[0088] In one example, the lithium-ion secondary battery is a wound battery, as Figure 7As shown, an insulating adhesive layer 210 is provided on the surface of the negative electrode sheet 200 close to the winding center, and the end region of the positive current collector 100 close to the starting point of the winding center overlaps at least partially with the vertical projection of the insulating adhesive layer 210 in the length direction of the wound battery. By providing an insulating adhesive layer on the surface close to the winding center and making at least partial overlap between the insulating adhesive layer and the end region of the positive current collector close to the starting point of the winding center, the present invention can effectively fix the shape of the battery cell, ensure that the battery cell remains stable during subsequent assembly and use, and prevent performance problems or safety risks caused by changes in the shape of the battery cell; moreover, by providing the insulating adhesive layer, burrs on the positive electrode sheet and the positive electrode tab can be prevented from piercing the separator, and short circuit between the positive and negative electrode sheets caused by improper use can be prevented, thereby further improving the safety performance of the battery.

[0089] In addition to the positive electrode sheet, the lithium-ion battery of the present invention further includes an electrolyte, a negative electrode sheet 200, and a separator 300 (as Figure 9 shown). Among them, the composition of the negative electrode sheet 200 can refer to the conventional negative electrode in the art, and the separator 300 can also adopt the separators commonly used in the art, such as PP film, PE film, etc.

[0090] The lithium-ion battery of the present invention can be prepared by a conventional method in the art. Specifically, the positive electrode, the separator, and the negative electrode can be stacked in sequence, and then the battery cell can be obtained through a stacking or winding process, and then the above battery can be obtained through processes such as baking, liquid injection, formation, and encapsulation.

[0091] Hereinafter, the positive electrode sheet and the lithium-ion battery provided by the present invention will be introduced in detail through specific examples.

[0092] Example 1

[0093] The lithium-ion battery of the present invention is obtained in the following manner: the positive electrode sheet is of an STP structure (ear-centered structure, with the positive electrode tab disposed in the first groove and the second groove); the width of the positive electrode sheet is 80.8 mm, and the width of the negative electrode sheet is 82.2 mm.

[0094] Preparation of the positive electrode sheet

[0095] Preparation of the first coating / second coating: Take raw materials with a mass ratio of silicon dioxide (particle size Dv50 = 80 nm): polymethyl methacrylate (PMMA) = 60:40 (the mass content X1 of the binder in the first coating is 40%); then add deionized water and ethanol to the above raw materials to prepare a slurry with a solid content of 5%; then use a gravure coater to coat the first coating and the second coating on the positive current collector; the width of the first coating is 9 mm, the coating width of the second coating is 9 mm, and the coating length is 1460 mm; the thickness of the first coating / second coating is ~3 ± 0.5 μm;

[0096] Preparation of the first active layer, the second active layer, and the third active layer

[0097] Raw materials with a mass ratio of lithium cobaltate (the particle size Dv50 of the first lithium cobaltate particles is 12 μm, and the particle size Dv50 of the second lithium cobaltate particles is 5 μm): conductive carbon black: polyvinylidene fluoride = 97.2:1.5:1.3 (the mass content of the binder in the first active layer, the second active layer, and the third active layer is equal to 1.3%, X2 = 1.3%, X1 / X2 = 30.77) are added to a stirring tank, and then NMP solvent is added. Stir well according to the known batching process, and pass through a 200-mesh sieve to prepare a positive electrode active material slurry with a solid content of 73%; then, use a coater to continue coating the positive electrode active material slurry on the above positive electrode current collector. The coating thickness of the positive electrode active material slurry corresponding to the first module / third module is 26 ± 0.5 μm, and the coating thickness of the positive electrode active material slurry corresponding to the second module is 29 ± 0.5 μm; the coating length is 1432 mm, the positive electrode current collector is aluminum foil with a length of 1498 mm, and there is an empty foil area at the end of the aluminum foil;

[0098] Then, clean out the groove that can accommodate the positive electrode tab. After cleaning the groove, roll press the prepared positive electrode sheet and perform primary slitting and secondary slitting (the slitting width is 3 mm, which is equivalent to the width of the first coating / second coating being 6 mm) to obtain the positive electrode sheet.

[0099] Among them, the total width W of the first groove and the second groove is 24 mm, the width W of the first groove 1 = 3 mm, the width W of the area where the positive electrode tab is connected to the positive electrode current collector 3 = 10 mm, the width W 3 coinciding with the first groove is 1 mm, and the width tolerance d of the side edge of the positive electrode tab located in the second groove from the edge of the second groove close to the second active layer is 2 mm. 4 = 1 mm, and the width tolerance d of the side edge of the positive electrode tab located in the second groove from the edge of the second groove close to the second active layer is 2 mm.

[0100] Preparation of the negative electrode sheet

[0101] Take raw materials with a mass ratio of artificial graphite: conductive agent: styrene-butadiene rubber: lithium carboxymethyl cellulose = 96%: 1.5%:

[0102] 1.5%; 1%; then add deionized water as a solvent to the above raw materials to prepare a negative electrode active material layer slurry with a solid content of 45.5%; then use a coater to coat the negative electrode active coating slurry on both surfaces of the negative electrode current collector respectively, with a coating thickness of 40 μm for each layer and a coating length of 1428 mm; then dry at a temperature of 100 °C to obtain the negative electrode sheet.

[0103] Preparation of the lithium-ion battery

[0104] The negative electrode sheet obtained in the above steps, the positive electrode sheet obtained in the above steps, and the separator (the separator is a polyethylene film with a thickness of 9 μm) are wound to form a core, which is packaged with an aluminum-plastic film. After baking to remove moisture, electrolyte is injected (the preparation of the electrolyte is as follows: raw materials with a mass percentage of propylene carbonate: ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate = 1:1:0.5:1 are mixed, and 1 mol / L of LiPF 6 is added and mixed evenly), and the lithium-ion battery is obtained by adopting a hot pressing formation process.

[0105] Example 2 group

[0106] This example is carried out with reference to Example 1. The only difference is that the mass content X1 of the binder in the first coating and the mass content X2 of the binder in the second active layer are changed. Specifically:

[0107] For Example 2-1, preparation of the first coating: Raw materials with a mass ratio of silicon dioxide: polymethyl methacrylate (PMMA) = 80:20 are taken, and the mass content X1 of the binder in the first coating = 20%;

[0108] Preparation of the second active layer: Raw materials with a mass ratio of lithium cobaltate: conductive carbon black: polyvinylidene fluoride = 97.5:1.5:1 are added to a stirring tank. The mass content X2 of the binder in the second active layer = 1%, and X1 / X2 = 20;

[0109] For Example 2-2, preparation of the first coating: Raw materials with a mass ratio of silicon dioxide: polymethyl methacrylate (PMMA) = 50:50 are taken, and the mass content X1 of the binder in the first coating = 50%;

[0110] Preparation of the second active layer: Raw materials with a mass ratio of lithium cobaltate: conductive carbon black: polyvinylidene fluoride = 96:1.5:2.5 are added to a stirring tank. The mass content X2 of the binder in the second active layer = 0.5%, and X1 / X2 = 20;

[0111] For Example 2-3, preparation of the first coating: Raw materials with a mass ratio of silicon dioxide: polymethyl methacrylate (PMMA) = 70:30 are taken, and the mass content X1 of the binder in the first coating = 30%;

[0112] Preparation of the second active layer: Raw materials with a mass ratio of lithium cobaltate: conductive carbon black: polyvinylidene fluoride = 98:1.5:0.5 are added to a stirring tank. The mass content X2 of the binder in the second active layer = 0.5%, and X1 / X2 = 60;

[0113] For Example 2-4, preparation of the first coating: Raw materials with a mass ratio of silicon dioxide: polymethyl methacrylate (PMMA) = 50:50 are taken, and the mass content X1 of the binder in the first coating = 50%;

[0114] Preparation of the second active layer: Raw materials with a mass ratio of lithium cobaltate: conductive carbon black: polyvinylidene fluoride = 97:1.5:1.5 are added to a stirring tank. The mass content X2 of the binder in the second active layer is 1.3%, and X1 / X2 = 33.33;

[0115] Examples 2-5: Preparation of the second active layer: Raw materials with a mass ratio of lithium cobaltate: conductive carbon black: polyvinylidene fluoride = 96:1.5:2.5 are added to a stirring tank. The mass content X2 of the binder in the second active layer is 2.5%, and X1 / X2 = 16;

[0116] Example 2-6: Preparation of the second active layer: Raw materials with a mass ratio of lithium cobaltate: conductive carbon black: polyvinylidene fluoride = 98:1.5:0.5 are added to a stirring tank. The mass content X2 of the binder in the second active layer is 0.5%, and X1 / X2 = 80;

[0117] See Table 1 for details.

[0118] Example 3

[0119] The lithium-ion battery of the present invention is obtained in the following manner: The positive electrode sheet has an STP structure (center-tab structure, with the positive electrode tab disposed in the first groove and the second groove); the width of the positive electrode sheet is 80.8 mm, and the width of the negative electrode sheet is 82.2 mm.

[0120] Preparation of the positive electrode sheet

[0121] Preparation of the first active layer, the second active layer, and the third active layer

[0122] Raw materials with a mass ratio of lithium cobaltate: conductive carbon black: polyvinylidene fluoride = 97.2:1.5:1.3 are added to a stirring tank, and then NMP solvent is added. Thorough stirring is carried out according to the well-known batching process, and then it is passed through a 200-mesh sieve to form a positive electrode active material slurry. The solid content of the positive electrode active material slurry is 73%; subsequently, the positive electrode active material slurry is coated on the above positive electrode current collector by a coater. The coating thickness of the positive electrode active material slurry corresponding to the first module / third module is 26 ± 0.5 μm, and the coating thickness of the positive electrode active material slurry corresponding to the second module is 29 ± 0.5 μm; the coating length is 1432 mm;

[0123] Preparation of the first coating / second coating: Raw materials with a mass ratio of silicon dioxide: polymethyl methacrylate (PMMA) = 60:40 are taken, and then deionized water and ethanol are added to the above raw materials to prepare a slurry with a solid content of 5%; subsequently, the first coating and the second coating are continuously coated on the active layer by a gravure coater; the width of the first coating is 9 mm, the coating width of the second coating is 9 mm, and the coating length is 1460 mm; the first coating /

[0124] The thickness of the second coating is ~3 ± 0.5 μm; the prepared positive electrode sheet is obtained by rolling, and first and second slitting operations (the slitting width is 3 mm, which is equivalent to the width of the first coating / second coating being 6 mm), as Figure 5 shown.

[0125] The preparation of the negative electrode sheet and the preparation process of the lithium-ion battery are the same as those in Example 1.

[0126] Comparative Example 1

[0127] This comparative example is carried out with reference to Example 1. The only difference is that the mass content X1 of the binder in the first coating and / or the mass content X2 of the binder in the second active layer are changed. Specifically: Preparation of the first coating: Raw materials with a mass ratio of silicon dioxide: polymethyl methacrylate (PMMA) = 90:10 are taken, the mass content X1 of the binder in the first coating is 10%, and X1 / X2 = 7.69;

[0128] See Table 1 for details.

[0129] Comparative Example 2

[0130] This comparative example is carried out with reference to Example 1. The only difference is that a conductive agent is added to the first coating. Specifically: Preparation of the first coating: Raw materials with a mass ratio of silicon dioxide: polymethyl methacrylate (PMMA): conductive carbon black = 58.5:40:1.5 are taken, the mass content X1 of the binder in the first coating is 40%, and X1 / X2 = 30.77;

[0131] See Table 1 for details.

[0132] Table 1

[0133]

[0134] Example 4 group

[0135] This example is carried out with reference to Example 1. The only difference is that the width of the first coating in the first module is changed, so that the width W 1 of the first groove changes, thereby changing the width W 4 where the tab connection area coincides with the first groove. This is because the welding position of the positive tab and the width of the tab connection area are unchanged. When the width W 1 of the first groove becomes larger, the overlapping area between the tab connection area and the first groove will also become larger. See Table 2 for details.

[0136] Table 2

[0137]

[0138]

[0139] Example 5

[0140] The lithium-ion battery of the present invention is obtained in the following manner: the positive electrode sheet has an STP structure (center-tab structure, the positive tab is arranged in the first groove and the second groove); the width of the positive electrode sheet is 80.8 mm, and the width of the negative electrode sheet is 82.2 mm.

[0141] Preparation of the positive electrode sheet

[0142] Preparation of the first coating: Take raw materials with a mass ratio of silicon dioxide: polymethyl methacrylate (PMMA) = 60:40, and then add deionized water and ethanol to the above raw materials to prepare a slurry with a solid content of 5%; then use a gravure coater to coat the first coating on the positive current collector; the width of the first coating is 9 mm, and the coating length is 1460 mm; the thickness of the first coating is ~3 ± 0.5 μm;

[0143] Preparation of the first active layer, the second active layer, and the third active layer

[0144] Take raw materials with a mass ratio of lithium cobaltate: conductive carbon black: polyvinylidene fluoride = 97.2:1.5:1.3 and add them to a stirring tank, then add NMP solvent, and stir thoroughly according to the well-known batching process, and pass through a 200-mesh sieve to prepare a positive electrode active material slurry with a solid content of 73%; then use a coater to continue coating the positive electrode active material slurry on the above positive current collector. The coating thickness of the positive electrode active material slurry corresponding to the first module is 26 ± 0.5 μm, and the coating thickness of the positive electrode active material slurry corresponding to the second module / third module is 29 ± 0.5 μm; the coating length is 1432 mm; the prepared positive electrode sheet is obtained by rolling, primary slitting, and secondary slitting (the slitting width is 3 mm, which is equivalent to the width of the first coating being 6 mm). The positive electrode sheet is provided with the first coating only on one edge on the side of the positive tab.

[0145] The preparation of the negative electrode sheet and the preparation process of the lithium-ion battery are the same as those in Example 1.

[0146] Example 6

[0147] This example is carried out with reference to Example 1. The only difference is that the particle size of lithium cobaltate in the first active layer, the second active layer, and the third active layer is changed. Two sizes of lithium cobaltate particles are not set, and only lithium cobaltate with one particle size is used, and the particle size Dv50 of lithium cobaltate is 8 μm.

[0148] Example 7

[0149] This example is carried out with reference to Example 1. The only difference is that by changing the coating procedure of the first coating and changing the coating method of the first coating, the first coating is set as several sub-coatings, and the distance between adjacent sub-coatings is 100 μm.

[0150] Test example

[0151] The above-mentioned examples and comparative examples were tested as follows, and the experimental results were recorded in Table 3.

[0152] 1. Lithium deposition test: The batteries prepared from the examples and comparative examples were subjected to lithium deposition test. The specific method is as follows: Under the condition of 25°C, the charging process is: first, constant current charging at 4C to 4.48V, and then constant voltage charging until the current drops to 0.05C; the discharging process is: constant current discharging at 1C to 3.0V; finally, charging at 0.3C until full; after repeating 20 cycles, the battery was disassembled, and the lithium deposition situation and powder falling situation in the negative electrode area corresponding to the area around the positive electrode tab were observed; the degree of lithium deposition is divided into four levels: no lithium deposition, slight lithium deposition, lithium deposition, and severe lithium deposition:

[0153] No lithium deposition means that there is no lithium deposition phenomenon at the top and bottom edges of the negative electrode sheet and the negative electrode area corresponding to the area around the positive electrode tab;

[0154] Slight lithium deposition means that there is a linear lithium deposition phenomenon in the negative electrode area corresponding to the area around the positive electrode tab of the negative electrode sheet;

[0155] Lithium deposition means that on the basis of the linear lithium deposition in the negative electrode area at the top and bottom edges and around the positive electrode tab of the negative electrode sheet, it slightly spreads to other areas;

[0156] Severe lithium deposition means that it has severely spread from the negative electrode areas corresponding to the top and bottom edges and the area around the positive electrode tab to other areas;

[0157] Among them, lithium deposition is a lithium deposition situation between slight lithium deposition and severe lithium deposition, and the experimental results were recorded in Table 3.

[0158] 2. Cycle test: The lithium-ion battery was subjected to a cycle test on a Blue Power test cabinet. The test conditions were 25°C ± 2°C, charging at 3.7C to 4.37V, charging at 3.0C to 4.37V, charging at 2C to 4.53V, charging at 1.5C to 4.58V, cutoff at 0.05C; discharging at 0.7C to 3V.

[0159] The specific charging regime is as follows: Let it stand for 5 min at 25°C ± 2°C, discharge at 0.2C to the lower limit voltage; let it stand for 5 min, charge at 0.7C to the upper limit voltage, cut off at 0.025C, let it stand for 5 min, discharge at 0.2C to the lower limit voltage; (perform the initial capacity test), let it stand for 5 min, charge at 3.7C to 4.37V, charge at 3.0C to 4.37V, charge at 2C to 4.53V, charge at 1.5C to 4.58V, cut off at 0.05C, measure and record the data of the full charge state, voltage and thickness; let it stand for 5 min at 25°C ± 2°C, discharge at 0.7C to 3V, let it stand for 5 min, charge at 3.7C to 4.37V, charge at 3.0C to 4.37V, charge at 2C to 4.53V, charge at 1.5C to 4.58V, cut off at 0.05C, let it stand for 5 min, discharge at 0.7C to 3V, let it stand for 5 min, perform 8 - 91200 cycles, and repeat steps 3 - 4 at 25°C for capacity test every 100 cycles. For the first 200T: measure the voltage and thickness of the full charge cell every 50T, after 200T: measure the voltage and thickness of the full charge cell every 100T. Then test the capacity retention rate of the lithium-ion battery at 600T and record the experimental results in Table 3.

[0160] Table 3

[0161] Group Lithium plating situation in the area corresponding to the negative electrode around the positive electrode tab Capacity retention rate of 600T during cycle test / % Example 1 No lithium plating 90.45 Example 2-1 Slight lithium plating 89.67 Example 2-2 Slight lithium plating 89.89 Example 2-3 Slight lithium plating 89.35 Example 2-4 No lithium plating 90.13 Example 2-5 Slight lithium plating 86.78 Example 2-6 Slight lithium plating 85.67 Example 3 Lithium plating 87.65 Comparative example 1-1 Severe lithium plating 83.13 Comparative example 1-2 Severe lithium plating 83.67 Example 4-1 No lithium plating 89.72 Example 4-2 No lithium plating 88.88 Example 4-3 Slight lithium plating 86.54 Example 4-4 Slight lithium plating 87.13 Example 5 Lithium plating 84.56 Example 6 Slight lithium plating 87.68 Example 7 Slight lithium plating 87.25

[0162] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet includes a positive electrode current collector, the positive electrode sheet includes an active area arranged in sequence along the length direction of the positive electrode current collector and a hollow foil area located at one side edge of the positive electrode current collector, the active area includes a first module and a second module arranged in sequence along the width direction of the positive electrode current collector, and the first module is located at one side edge of the positive electrode current collector in the width direction; Along the thickness direction of the positive electrode current collector, the first module includes the positive electrode current collector and a first active layer and a first coating layer stacked on at least one side of the positive electrode current collector, and the second module includes the positive electrode current collector and a second active layer located on at least one side of the positive electrode current collector; Along the width direction of the positive electrode current collector, the first module comprises a first groove, the second module comprises a second groove, the positive electrode tab is located in the first groove and the second groove, and the positive electrode tab is electrically connected to the positive electrode current collector; The first coating layer comprises organic particles and / or inorganic particles and a binder; The impedance of the positive electrode current collector in the first groove is greater than the impedance of the positive electrode current collector in the second groove.

2. The positive electrode sheet according to claim 1, wherein: The mass content of the binder in the first coating layer is X1, the mass content of the binder in the second active layer is X2, and X1>X2; Preferably, 20≤X1 / X2≤60.

3. The positive electrode sheet according to claim 1, wherein: The impedance of the positive electrode current collector in the empty foil area is 0.01Ω to 0.08Ω; And / or, the impedance of the positive electrode current collector in the first groove is 0.02Ω to 0.15Ω; And / or, the impedance of the positive electrode current collector in the second groove is 0.02Ω to 0.1Ω; And / or, the impedance of the positive electrode current collector in the first groove>the impedance of the positive electrode current collector in the second groove>the impedance of the positive electrode current collector in the empty foil area.

4. The positive electrode sheet according to claim 1, wherein: Along the width direction of the positive electrode sheet current collector, the total width of the first groove and the second groove is W, the width of the first groove is W1, the width of the area where the positive electrode ear is connected to the positive electrode current collector is W3, the width tolerance of the edge of the positive electrode ear located in the second groove from the edge of the second groove close to the second active layer is d, and the width of the area where W3 overlaps with the first groove is W4; wherein W, W1, W3, W4 and d satisfy the relationship: W1≤W-(W3+d)+W4; Preferably, d=0.5mm-4mm; Preferably, W4=0.1mm~5mm.

5. The positive electrode sheet according to claim 1, wherein: The active area further includes a third module along the width direction of the positive electrode sheet current collector, and the third module is located at the edge of one side of the positive electrode sheet current collector away from the first module in the width direction; the third module includes the positive electrode current collector and a third active layer and a second coating layer stacked on at least one side of the positive electrode current collector, the second coating layer includes organic particles and / or inorganic particles and a binder, the mass content of the binder in the second coating layer is X3 and the mass content of the binder in the second active layer is X2, X3>X2; preferably, 20≤X3 / X2≤60; Preferably, the impedance of the first module and / or the third module is greater than the impedance of the second module.

6. The positive electrode sheet according to claim 5, wherein: The organic particles include at least one of polyurethane microspheres, polystyrene microspheres and polyethylene microspheres; and / or, the inorganic particles include at least one of silicon oxide, titanium oxide, aluminum oxide, magnesium oxide, zinc oxide, zirconium oxide, cerium oxide, ferrous oxide, barium sulfate and boehmite; And / or, the binder includes at least one of polyvinylidene fluoride, carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl pyrrolidone, polymethyl methacrylate, polyacrylonitrile, polyacrylic acid, polyvinyl alcohol, sodium alginate, β-cyclodextrin polymer, polypropylene emulsion and polytetrafluoroethylene, and functionalized derivatives of the above polymers or copolymers between monomers.

7. The positive electrode sheet according to claim 5, wherein: The first active layer, the second active layer and the third active layer comprise positive electrode active materials, the positive electrode active materials comprise lithium cobalt oxide, the positive electrode active materials present a bimodal distribution in a laser particle size test, the lithium cobalt oxide comprises first lithium cobalt oxide particles and second lithium cobalt oxide particles, and the particle size Dv50 of the first lithium cobalt oxide particles is greater than the particle size Dv50 of the second lithium cobalt oxide particles; Preferably, the particle size Dv50 of the organic particles and / or inorganic particles is ≤ the particle size Dv50 of the second lithium cobalt oxide particles; Preferably, the particle size Dv50 of the organic particles and / or inorganic particles is 10 nm to 1000 nm; Preferably, the particle size Dv50 of the second lithium cobalt oxide particles is 1 μm to 8 μm.

8. The positive electrode sheet according to claim 5, wherein: In the first module and / or the third module, the first coating and the second coating are arranged on the surface of the positive electrode current collector, the first active layer is arranged on the surface of the first coating away from the positive electrode current collector, and the third active layer is arranged on the surface of the second coating away from the positive electrode current collector, the impedance of the positive electrode current collector in the first groove is 0.05Ω to 0.12Ω, and the impedance of the positive electrode current collector in the second groove is 0.03Ω to 0.08Ω; And / or, in the first module and / or the third module, the first active layer and the third active layer are arranged on the surface of the positive electrode current collector, the first coating is arranged on the surface of the first active layer away from the positive electrode current collector, the second coating is arranged on the surface of the third active layer away from the positive electrode current collector, the impedance of the positive electrode current collector in the first groove is 0.03Ω~0.08Ω, and the impedance of the positive electrode current collector in the second groove is 0.03Ω~0.08Ω; And / or, along the width direction of the positive electrode sheet current collector, the first coating layer or the second coating layer includes a plurality of sub-coating layers arranged in sequence, and the spacing between adjacent sub-coating layers is 10 μm to 5000 μm, preferably 50 μm to 500 μm.

9. A lithium ion secondary battery, characterized in that: The lithium-ion secondary battery comprises the positive electrode sheet according to any one of claims 1 to 8.

10. The lithium ion secondary battery according to claim 9, wherein The lithium-ion secondary battery also includes a negative electrode sheet: The negative electrode sheet includes a negative electrode active material, the negative electrode active material includes a silicon-based material, the content of the silicon-based material is 0wt% to 50wt%, the silicon-based material includes silicon carbon, and the silicon carbon includes a porous carbon skeleton and silicon particles deposited in the porous carbon skeleton; And / or, along the width direction of the negative electrode sheet, the negative electrode sheet comprises a fourth module and a fifth module arranged in sequence, the fourth module is located in a non-overlapping area of ​​the negative electrode sheet and the positive electrode collector, and the fifth module is located in an overlapping area of ​​the negative electrode sheet and the positive electrode collector; And / or, the lithium-ion secondary battery is a wound battery, an insulating rubber layer is provided on the surface of the negative electrode sheet near the winding center, and the end area of ​​the positive electrode collector near the starting point of the winding center at least partially overlaps with the vertical projection of the insulating rubber layer in the length direction of the wound battery.

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