Positive electrode sheet and lithium-ion battery

Abstract

The present invention provides a positive electrode sheet and a lithium-ion battery. The positive electrode sheet of the present invention includes a current collector and an active material layer provided with at least one functional surface of the current collector. The present invention divides the positive electrode sheet into a first region that is difficult to wet and a second region that is easy to wet based on the different degrees of wettability of different regions of the positive electrode sheet. The first region is provided with a first active material layer including a first active material, and the second region is provided with a second active material layer including a second active material. The first active material is selected from lithium cobalt oxide or a mixture of lithium cobalt oxide and a ternary material, and the second active material is selected from a ternary material or a mixture of a ternary material and lithium cobalt oxide. Within the same unit volume range, the mass content of nickel in the first active material layer is less than the mass content of nickel in the second active material layer. The positive electrode sheet provided by the present invention effectively alleviates the lithium plating problem caused by the addition of a ternary material to the positive electrode sheet, and improves the cycle performance of the lithium-ion battery.

Description

Technical Field

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

[0002] Lithium-ion batteries are widely used in mobile phones, laptops, power tools, wearables, and other fields due to their long cycle life and light weight. With the widespread marketization of these consumer electronic products, there is a demand for lithium-ion batteries with lower costs to improve the market competitiveness of electronic products. Currently, the main positive electrode material of lithium-ion batteries used in consumer electronic products is lithium cobalt oxide. Compared with lithium cobalt oxide, ternary materials have lower costs. Therefore, in the design of positive electrode sheets, many battery manufacturers hope to reduce costs by using a mixture of ternary materials and lithium cobalt oxide, or using a single ternary material as the positive electrode material.

[0003] However, compared to lithium cobalt oxide, ternary materials consume electrolyte at a faster rate during battery cycling, making lithium plating more likely to occur in the later stages of the battery cycle. Therefore, it is of great significance to effectively solve the lithium plating problem that is prone to occur when ternary materials and lithium cobalt oxide, or when a single ternary material is used as the positive electrode material. Summary of the Invention

[0004] The present invention provides a positive electrode sheet, and designs the active layer composition of different areas of the positive electrode sheet according to the different wetting degrees of different areas of the positive electrode sheet, thereby alleviating the lithium plating problem that is easy to occur in the positive electrode sheet when the positive electrode material includes a ternary material.

[0005] The present invention also provides a lithium-ion battery, which includes the above-mentioned positive electrode sheet. Since the above-mentioned positive electrode sheet is not easy to deposit lithium, the lithium-ion battery has good cycle performance.

[0006] A first aspect of the present invention provides a positive electrode sheet, comprising a current collector and an active material layer provided on at least one functional surface of the current collector; the active material layer is provided on a coating area of ​​the functional surface, the coating area comprising a first region and a second region, the first region being located in an intermediate region of the coating area in a first direction of the positive electrode sheet and / or a second direction of the positive electrode sheet, and the second region being located in a non-intermediate region of the coating area in the first direction of the positive electrode sheet and / or the second direction of the positive electrode sheet; the first region is provided with a first active material layer, and the second region is provided with a second active material layer; the first active material layer comprises a first active material, and the second active material layer comprises a second active material, the first active material being selected from lithium cobalt oxide or a mixture of lithium cobalt oxide and a ternary material, and the second active material being selected from a ternary material or a mixture of a ternary material and lithium cobalt oxide;

[0007] Within the same unit volume range, the mass content of nickel in the first active material layer is less than the mass content of nickel in the second active material layer.

[0008] The functional surface of the present invention refers to the two surfaces of the current collector with the largest area, which are used for coating the active material layer.

[0009] The coating area as described above is in the middle area in the first direction of the positive electrode sheet and / or the second direction of the positive electrode sheet, wherein the middle area refers to the area whose symmetry axis in the first direction of the positive electrode sheet and / or the second direction of the positive electrode sheet overlaps with the symmetry axis of the positive electrode sheet in its first direction and / or second direction, and all areas except the middle area are non-middle areas.

[0010] Lithium-ion batteries primarily include spiral wound cells and laminated cells. For spiral wound cells, the electrolyte primarily wets the electrode sheets through channels along the upper and lower edges of the wound cell, relying on the adsorption of the separator and the capillary action of the electrode pores. Therefore, the electrolyte wettability in the central region of the positive electrode sheet in a spiral wound battery is poor, making lithium deposition more likely in the later stages of the battery cycle.

[0011] Figure 1 This is a schematic structural diagram of a positive electrode sheet for a wound battery cell according to an embodiment of the present invention. Figure 1 As shown, the positive electrode sheet includes a current collector 100 and an active material layer 200 disposed on at least one functional surface of the current collector 100. The active material layer 200 is disposed on the coated area of ​​the positive electrode sheet. The positive electrode sheet also includes a non-coated area, which is a blank foil area for welding the positive electrode tab 300. The coated area includes a first region and a second region. The first region is located in the middle of the coated area in the first direction of the positive electrode sheet and has poor wetting performance. The second region is located in the non-middle area of ​​the coated area in the first direction of the positive electrode sheet and has better wetting performance. A first active material layer 2001 is disposed on the first region, and a second active material layer 2002 is disposed on the second region.

[0012] For laminated batteries, the electrolyte mainly penetrates the electrode along the direction from the edge of the electrode to the center of the electrode. Therefore, the electrode of the laminated battery is in the middle area between the first direction of the positive electrode and the second direction of the positive electrode (that is, the center area of ​​the positive electrode). The electrolyte has poor wetting performance, and lithium plating is prone to occur in the later stage of the battery cycle.

[0013] Figure 2 A schematic structural diagram of a positive electrode sheet for a laminated battery cell according to an embodiment of the present invention is shown in FIG. Figure 2As shown, the positive electrode sheet includes a current collector 100 and an active material layer 200 disposed on at least one functional surface of the current collector 100. The active material layer 200 is disposed on the coated area of ​​the positive electrode sheet. In the laminated battery cell, the functional surface of the current collector 100 coated with the active layer has no bare foil area. The coated area includes a first region and a second region. The first region is located between the coated area in the first and second directions of the positive electrode sheet and has poor wettability. The second region is located between the coated area in the first and second directions of the positive electrode sheet and has better wettability. A first active material layer 2001 is disposed on the first region, and a second active material layer 2002 is disposed on the second region.

[0014] The nickel element in the ternary material is more likely to consume electrolyte than cobalt and manganese elements during the battery cycle. The present invention limits the mass content of nickel in the first active material layer 2001 to be less than the mass content of nickel in the second active material layer 2002, which is beneficial to reducing the consumption rate of electrolyte in the first area that is difficult to infiltrate during the battery cycle and avoiding the occurrence of lithium plating on the positive electrode.

[0015] For the positive electrode sheet in the laminated battery cell, the first area is located in the middle area of ​​the coating area in the first direction and the second direction of the positive electrode sheet, and can also be located in the middle area of ​​the positive electrode sheet in either the first direction or the second direction of the positive electrode sheet. Figure 3 A schematic structural diagram of a positive electrode sheet for a laminated battery cell according to another embodiment of the present invention is shown in FIG. Figure 3 As shown, the first area is located in the middle area of ​​the coating area in the first direction of the positive electrode sheet. Figure 4 A schematic structural diagram of a positive electrode sheet for a laminated battery cell according to another embodiment of the present invention is shown in FIG. Figure 4 As shown, the first area is located in the middle area of ​​the coating area in the second direction of the positive electrode sheet. Figure 3 and Figure 4 In the embodiment shown, the first active material layer 2001 extends from the center of the electrode sheet to the edge of the electrode sheet in the first direction and the second direction of the positive electrode sheet, respectively. The first active material layer 2001 with a low nickel content not only covers the difficult-to-wet central area of ​​the electrode sheet, thereby avoiding the occurrence of lithium plating on the positive electrode sheet, but also extends from the central area of ​​the electrode sheet to both ends of the electrode sheet in the first direction or the second direction of the positive electrode sheet, thereby facilitating the coating of the first active material layer 2001.

[0016] In the present invention, the first direction of the positive electrode sheet is consistent with the extending direction of the positive electrode tab, and correspondingly, the second direction of the positive electrode sheet is perpendicular to the first direction of the positive electrode sheet.

[0017] In a specific embodiment, within the same unit volume range, the mass content of nickel in the first active material layer is M1, and the mass content of nickel in the second active material layer is M2, wherein M2-M1≥1%.

[0018] When both the first active material layer and the second active material layer contain ternary materials and the types of ternary materials added to both are the same, and the difference between the mass content of the ternary material in the first active material layer and the mass content of the ternary material in the second active material layer within the same unit volume is greater than or equal to 1%, M2-M1≥1% can be achieved.

[0019] When the first active material layer does not contain the ternary material and only the second active material layer contains the ternary material, as long as the mass content of nickel per unit volume in the second active material layer is not less than 1%, M2-M1≥1% can be achieved.

[0020] Furthermore, M2-M1≥2%, the lower the mass content of nickel element in the first active material layer and the higher the content of ternary material in the second active material layer, the more conducive it is to increase the content of ternary material in the positive electrode sheet while avoiding lithium plating in the positive electrode sheet, thereby reducing the cost of the positive electrode sheet.

[0021] Taking into account the difference in wettability of the electrolyte to the areas where the first active material layer and the second active material layer are located, when the area content of the first active material layer in the coating area is too low or too high, the lithium plating situation of the positive electrode and the electrode cost cannot be effectively improved. Therefore, the area ratio of the first active material layer and the second active material layer to the coating area should be set within a reasonable range.

[0022] Specifically, when the first region is located in the middle region of the positive electrode sheet in the first direction or the second direction, and the second region is located in the non-middle region of the positive electrode sheet in the first direction or the second direction, the width ratio of the first active material layer to the second active material layer is (1:14) to (1:2), which can ensure that the first active material layer and the second active material layer have a relatively appropriate distribution ratio in the coating area. Figure 1 The width of the first active material layer is W1, the width of the second active material layer is 2W2, and W1:2W2=(1:14)~(1:2).

[0023] When the first region is located in the middle region between the first and second directions of the positive electrode sheet, and the second region is located in a non-middle region between the first and second directions of the positive electrode sheet, the first active material layer is located in the center region of the positive electrode sheet, and the second active material layer is evenly distributed around the first active material layer, and the coating area ratio of the first active material layer to the second active material layer is (1:14) to (1:2). Furthermore, in the first direction of the positive electrode sheet, the width ratio of the first active material layer to the second active material layer is (1:14) to (1:1), and in the second direction of the positive electrode sheet, the width ratio of the first active material layer to the second active material layer is (1:14) to (1:1).

[0024] Furthermore, the particle size of the ternary material used in the present invention is smaller than that of lithium cobalt oxide. On the one hand, large particles of lithium cobalt oxide are beneficial to reducing the consumption rate of the electrolyte during the cycle, and the large particles of lithium cobalt oxide material have a relatively slow lithium ion deintercalation rate during the charging cycle. Even if the electrolyte infiltration is difficult, the lithium plating problem in the middle area can be avoided as much as possible.

[0025] Specifically, the D50 particle size of ternary materials is 2μm to 7μm, and the D99 particle size is 8μm to 15μm; the D50 particle size of lithium cobalt oxide is 10μm to 20μm, and the D99 particle size is 30μm to 50μm. The D50 particle size, also known as the average particle size, refers to the particle size at which the cumulative particle size distribution percentage of the material particles reaches 50%; the D99 particle size, also known as the maximum particle size, refers to the particle size at which the cumulative particle size distribution percentage of the material particles reaches 99%.

[0026] In order to improve the consistency of the positive electrode sheet, the thickness of the first active material layer and the second active material layer can be set to be equal, and the thickness of the first active material layer and the second active material layer is 20 to 150 μm.

[0027] The positive electrode current collector of the present invention can be aluminum foil or carbon-coated aluminum foil, and its thickness can be 4-18μm. The positive electrode active material layer includes, in addition to the active material, a conductive agent and a binder. The first active material layer comprises, by weight, 92% to 98.4% of the first active material, 0.8% to 5% of the conductive agent, and 0.8% to 3% of the binder. Similarly, the second active material layer comprises, by weight, 92% to 98.4% of the second active material, 0.8% to 5% of the conductive agent, and 0.8% to 3% of the binder.

[0028] The lithium cobalt oxide and ternary materials used in the present invention may be doped or coated with metal elements such as Al, Mg, Ti, Zr, V, and Mn to improve material stability. The ternary materials of the present invention may be selected from commonly used ternary materials in the art, including but not limited to at least one of NCM111, NCM523, NCM613, and NCM811.

[0029] The conductive agents in the first active material layer and the second active material layer of the present invention may be of the same or different types, and may be selected from one or more of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, metal powder, and carbon fiber.

[0030] The contents of the binder in the first active material layer and the second active material layer of the present invention may be the same or different, and may be selected from one or more of polyvinylidene fluoride, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl pyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoroethylene or styrene-butadiene rubber.

[0031] The preparation method of the positive electrode sheet of the present invention can be carried out with reference to conventional preparation methods in the field. Specifically, the first active material, the conductive agent, and the binder can be evenly dispersed in a solvent in a certain proportion to obtain a first slurry, and the second active material, the conductive agent, and the binder can be evenly dispersed in a solvent in a certain proportion to obtain a second slurry. The first slurry and the second slurry are respectively coated on the first area and the second area of ​​at least one functional surface of the current collector. After drying, the first active material layer and the second active material layer of the present invention can be obtained, and then the positive electrode sheet of the present invention can be obtained.

[0032] In summary, the negative electrode sheet provided by the present invention is designed with a first active material layer located in a difficult-to-wet area and a second active material layer located in an easy-to-wet area according to the different electrolyte infiltration conditions, and by limiting the mass content of nickel in the first active material layer to be less than the mass content of nickel in the second active material layer, the lithium plating problem that occurs when the ternary material is added to the positive electrode sheet is alleviated, thereby improving the cycle performance of the lithium-ion battery.

[0033] A second aspect of the present invention provides a lithium-ion battery comprising any of the above-mentioned positive electrode sheets. The lithium-ion battery of the present invention may be a wound battery or a laminated battery.

[0034] The positive electrode sheet, separator and negative electrode sheet obtained in the first aspect of the present invention are stacked in sequence and then wound or laminated to obtain a battery core, and then packaged, injected, aged, formed and sorted to obtain the lithium ion battery of the present invention.

[0035] Among them, the negative electrode sheet includes a negative electrode current collector and a negative electrode layer arranged on at least one functional surface of the negative electrode current collector, and the negative electrode active layer includes a negative electrode active material, and the negative electrode active material is selected from one or more of graphite, hard carbon, soft carbon, silicon material, and lithium titanate.

[0036] Other materials used in lithium-ion batteries, such as diaphragms, electrolytes, and packaging materials, can all be made of commonly used materials in the art, and the present invention will not be further described herein.

[0037] The lithium ion battery provided by the present invention has good cycle performance because it includes the above-mentioned positive electrode sheet.

[0038] The implementation of the present invention has at least the following advantages:

[0039] 1. The negative electrode sheet provided by the present invention is designed with a first active material layer located in a difficult-to-wet area and a second active material layer located in an easily-wetted area according to the different electrolyte infiltration conditions. By limiting the mass content of nickel in the first active material layer to be less than the mass content of nickel in the second active material layer, the lithium plating problem that occurs when the ternary material is added to the positive electrode sheet is alleviated, thereby improving the cycle performance of the lithium-ion battery.

[0040] 2. The lithium-ion battery provided by the present invention has good cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0042] Figure 1 A schematic structural diagram of a positive electrode sheet for a wound battery cell provided by one embodiment of the present invention;

[0043] Figure 2 A schematic structural diagram of a positive electrode sheet for a laminated battery cell according to an embodiment of the present invention;

[0044] Figure 3 A schematic structural diagram of a positive electrode sheet for a laminated battery cell according to another embodiment of the present invention;

[0045] Figure 4 A schematic structural diagram of a positive electrode sheet for a laminated battery cell provided in yet another embodiment of the present invention.

[0046] Description of reference numerals:

[0047] 100-current collector;

[0048] 200-active material layer;

[0049] 2001-first active material layer;

[0050] 2002- second active material layer;

[0051] 300-positive electrode ear. DETAILED DESCRIPTION

[0052] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction 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 making creative efforts shall fall within the scope of protection of the present invention.

[0053] In the following examples and comparative examples, the ternary material NCM523 (Ni element mass content of 30.39% accounting for 36.47%), the ternary material NCM613 (Ni element mass content of 36.47%), and lithium cobalt oxide LCO involved can all be obtained commercially. Among them, the D50 particle size of the ternary material NCM523 in Examples 1 to 7 and Comparative Examples 1 to 4 is 4.5 μm, and the D99 particle size is 9.8 μm; the D50 particle size of the ternary material NCM613 is 4.3 μm, and the D99 particle size is 10 μm; the D50 particle size of the lithium cobalt oxide LCO is 16 μm, and the D99 particle size is 30 μm.

[0054] Example 1

[0055] The positive electrode structure provided in this embodiment is referenced Figure 1 , including a current collector aluminum foil and an active material layer arranged on the surface of the aluminum foil. In the first direction of the positive electrode sheet, the active material layer includes a first active material layer located in the middle area and a second active material layer located on both sides of the first active material layer. The width ratio of the first active material layer and the second active material layer is 1:5.

[0056] The steps for preparing the positive electrode sheet and lithium-ion battery provided in this embodiment are as follows:

[0057] A. Preparation of positive electrode

[0058] 1) adding lithium cobalt oxide (LCO), a binder (PVDF), and a conductive agent (conductive carbon black) into deionized water and stirring and mixing to obtain a first slurry;

[0059] The ternary material NCM523 (the mass fraction of Ni element is 30.39%), the binder PVDF, and the conductive agent conductive carbon black are added into deionized water and stirred and mixed to obtain a second slurry.

[0060] 2) using a dual extrusion coating method, the first slurry is evenly coated on the middle area of ​​the aluminum foil surface, and the second slurry is evenly coated on the areas on both sides of the middle area of ​​the aluminum foil surface, and after drying, the first active material layer and the second active material layer are respectively obtained;

[0061] The first active material layer consists of 97% lithium cobalt oxide (LCO), 1.5% PVDF, and 1.5% conductive carbon black by weight. The second active material layer consists of 97% NCM523 (ternary material), 1.5% PVDF, and 1.5% conductive carbon black by weight. Both the first and second active material layers are 90 μm thick.

[0062] The mass content M2 of nickel element in the second active material layer per cubic centimeter is 29.47%, and the mass content M1 of nickel element in the first active material layer per cubic centimeter is 0, M2-M1=29.47%.

[0063] B. Preparation of negative electrode

[0064] Dispersing graphite as a negative electrode active material, conductive carbon black as a conductive agent, styrene-butadiene latex as a binder, and sodium carboxymethyl cellulose as a dispersant in deionized water, stirring evenly to obtain a negative electrode active slurry, coating the negative electrode active slurry on two functional surfaces of a current collector copper foil, and drying to obtain a negative electrode sheet containing a negative electrode active layer;

[0065] The negative electrode active layer includes 97% graphite, 1.2% styrene-butadiene latex, 0.5% conductive carbon black, and 1.3% sodium carboxymethyl cellulose according to mass content.

[0066] C. Preparation of lithium-ion batteries

[0067] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in sequence, and wound to obtain a roll core, which is placed in an aluminum-plastic film outer package, and an electrolyte is injected into the aluminum-plastic film outer package, and the lithium-ion battery is obtained through vacuum sealing, standing, forming, and shaping.

[0068] Among them, the electrolyte is a solution composed of lithium salt LiPF6 and non-aqueous organic solvents ethylene carbonate, propyl propionate, diethyl carbonate and propylene carbonate; the diaphragm is a polyethylene diaphragm coated with ceramic and polyvinylidene fluoride.

[0069] Example 2

[0070] The preparation method of the positive electrode sheet and the lithium-ion battery provided in this embodiment is basically the same as that in Example 1, except that the width ratio of the first active material layer to the second active material layer is 1:2.

[0071] Example 3

[0072] The preparation method of the positive electrode sheet and lithium-ion battery provided in this embodiment is basically the same as that of Example 1, except that in the preparation of the first slurry, the lithium cobalt oxide (LCO) is replaced with a mixture of lithium cobalt oxide (LCO) and the ternary material NCM 523. The mass ratio of lithium cobalt oxide (LCO) to the ternary material NCM 523 in the mixture is 1:1. The resulting first active material layer comprises, by weight, 48.5% NCM 523, 48.5% lithium cobalt oxide (LCO), 1.5% PVDF, and 1.5% conductive carbon black. The mass content (M2) of nickel per cubic centimeter in the second active material layer is 29.47%, and the mass content (M1) of nickel per cubic centimeter in the first active material layer is 14.74%, with M2 - M1 = 14.73%.

[0073] Example 4

[0074] The preparation method of the positive electrode sheet and lithium-ion battery provided in this embodiment is basically the same as that of Example 1, except that in the preparation of the first slurry, the lithium cobalt oxide (LCO) is replaced with a mixture of lithium cobalt oxide (LCO) and the ternary material NCM 523. The mass ratio of lithium cobalt oxide (LCO) to the ternary material NCM 523 in the mixture is 3:97. The resulting first active material layer comprises, by weight, 94.09% NCM 523, 2.91% lithium cobalt oxide (LCO), 1.5% PVDF, and 1.5% conductive carbon black. The mass content (M2) of nickel per cubic centimeter in the second active material layer is 29.47%, and the mass content (M1) of nickel per cubic centimeter in the first active material layer is 28.59%, with M2 - M1 = 0.88%.

[0075] Example 5

[0076] The preparation method of the positive electrode sheet and lithium-ion battery provided in this embodiment is basically the same as that in Example 1, except that:

[0077] In the preparation of the first slurry, the ternary material NCM523 is replaced by a mixture of lithium cobalt oxide LCO and ternary material NCM613 (the mass fraction of Ni element is 36.47%), the mass ratio of lithium cobalt oxide LCO and ternary material NCM613 in the mixture is 1:1, and the obtained first active material layer includes 48.5% of ternary material NCM613, 48.5% of lithium cobalt oxide LCO, 1.5% of PVDF, and 1.5% of conductive carbon black according to the mass content;

[0078] In the preparation of the second slurry, lithium cobalt oxide LCO is replaced by a mixture of lithium cobalt oxide LCO and ternary material NCM613. The mass ratio of lithium cobalt oxide LCO and ternary material NCM613 in the mixture is 2:3. The obtained second active material layer includes 58.2% of ternary material NCM613, 38.8% of lithium cobalt oxide LCO, 1.5% of PVDF, and 1.5% of conductive carbon black by mass content.

[0079] The mass content M2 of nickel element in the second active material layer per cubic centimeter is 21.22%, the mass content M1 of nickel element in the first active material layer per cubic centimeter is 17.69%, and M2-M1=3.53%.

[0080] Example 6

[0081] The preparation method of the positive electrode sheet and lithium-ion battery provided in this embodiment is basically the same as that in Example 1, except that:

[0082] In the preparation of the first slurry, the ternary material NCM523 is replaced by a mixture of lithium cobalt oxide LCO and the ternary material NCM613. The mass ratio of lithium cobalt oxide LCO to the ternary material NCM613 in the mixture is 1:1.35. The obtained first active material layer includes 55.7% of the ternary material NCM613, 41.3% of the lithium cobalt oxide LCO, 1.5% of PVDF, and 1.5% of conductive carbon black according to the mass content;

[0083] In the preparation of the second slurry, lithium cobalt oxide LCO is replaced by a mixture of lithium cobalt oxide LCO and ternary material NCM613. The mass ratio of lithium cobalt oxide LCO and ternary material NCM613 in the mixture is 2:3. The obtained second active material layer includes 58.2% of ternary material NCM613, 38.8% of lithium cobalt oxide LCO, 1.5% of PVDF, and 1.5% of conductive carbon black by mass content.

[0084] The mass content M2 of nickel element in the second active material layer per cubic centimeter is 21.22%, the mass content M1 of nickel element in the first active material layer per cubic centimeter is 20.31%, and M2-M1=0.90%.

[0085] Example 7

[0086] The preparation method of the positive electrode sheet and lithium-ion battery provided in this embodiment is basically the same as that in Example 1, except that:

[0087] In the preparation of the first slurry, the ternary material NCM523 is replaced with a mixture of lithium cobalt oxide LCO and the ternary material NCM523. The mass ratio of lithium cobalt oxide LCO to the ternary material NCM523 in the mixture is 2:3. The obtained first active material layer includes 58.2% of the ternary material NCM523, 38.8% of the lithium cobalt oxide LCO, 1.5% of PVDF, and 1.5% of conductive carbon black according to the mass content;

[0088] In the preparation of the second slurry, lithium cobalt oxide LCO is replaced by a mixture of lithium cobalt oxide LCO and ternary material NCM613. The mass ratio of lithium cobalt oxide LCO and ternary material NCM613 in the mixture is 2:3. The obtained second active material layer includes 58.2% of ternary material NCM613, 38.8% of lithium cobalt oxide LCO, 1.5% of PVDF, and 1.5% of conductive carbon black by mass content.

[0089] The mass content M2 of nickel element in the second active material layer per cubic centimeter is 21.22%, the mass content M1 of nickel element in the first active material layer per cubic centimeter is 17.69%, and M2-M1=3.53%.

[0090] Example 8

[0091] The positive electrode structure provided in this embodiment is referenced Figure 2 The positive electrode sheet comprises a current collector aluminum foil and an active material layer disposed on the surface of the aluminum foil. The first active material layer is located in the center of the positive electrode sheet, and the second active material layer is distributed around the first active layer. In the first direction of the positive electrode sheet, the width ratio of the first active material layer to the second active material layer is 1:3. In the second direction of the positive electrode sheet, the width ratio of the first active material layer to the second active material layer is also 1:3.

[0092] The steps for preparing the positive electrode sheet and the lithium-ion battery of this embodiment are basically the same as those of Example 1, except that the battery cell of the lithium-ion battery of this embodiment is prepared by lamination.

[0093] Example 9

[0094] The positive electrode structure provided in this embodiment is referenced Figure 3 , including a current collector aluminum foil and an active material layer disposed on the surface of the aluminum foil. In the first direction of the positive electrode sheet, the active material layer includes a first active material layer located in the middle region and a second active material layer located on either side of the first active material layer. In the second direction of the positive electrode sheet, the width ratio of the first active material layer to the second active material layer is 1:3. In the first direction of the positive electrode sheet, the width ratio of the first active material layer to the second active material layer is 1:1.

[0095] The steps for preparing the positive electrode sheet and the lithium-ion battery of this embodiment are basically the same as those of Example 1, except that the battery cell of the lithium-ion battery of this embodiment is prepared by lamination.

[0096] Example 10

[0097] The positive electrode structure provided in this embodiment is referenced Figure 4 , including a current collector aluminum foil and an active material layer disposed on the surface of the aluminum foil. In the first direction of the positive electrode sheet, the active material layer includes a first active material layer located in the middle region and a second active material layer located on either side of the first active material layer. In the second direction of the positive electrode sheet, the width ratio of the first active material layer to the second active material layer is 1:1. In the first direction of the positive electrode sheet, the width ratio of the first active material layer to the second active material layer is 1:3.

[0098] The steps for preparing the positive electrode sheet and the lithium-ion battery of this embodiment are basically the same as those of Example 1, except that the battery cell of the lithium-ion battery of this embodiment is prepared by lamination.

[0099] Comparative Example 1

[0100] The preparation method of the positive electrode sheet and the lithium-ion battery provided in this embodiment is basically the same as that in Example 1, except that the positive electrode materials used in the preparation of the first slurry and the second slurry are both the ternary material NCM523.

[0101] Comparative Example 2

[0102] The preparation method of the positive electrode sheet and the lithium-ion battery provided in this embodiment is basically the same as that in Example 1, except that the positive electrode materials used in the preparation of the first slurry and the second slurry are both the ternary material NCM613.

[0103] Comparative Example 3

[0104] The preparation method of the positive electrode sheet and lithium-ion battery provided in this embodiment is basically the same as that in Example 1, except that the positive electrode materials used in the preparation of the first slurry and the second slurry are a mixture of lithium cobalt oxide LCO and the ternary material NCM613, and the mass ratio of lithium cobalt oxide LCO and the ternary material NCM613 in the mixture is 2:3. The obtained first active material layer and second active material layer each include 58.2% of the ternary material NCM613, 38.8% of lithium cobalt oxide LCO, 1.5% of PVDF, and 1.5% of conductive carbon black in terms of mass content.

[0105] Comparative Example 4

[0106] The preparation method of the positive electrode sheet and the lithium-ion battery provided in this embodiment is basically the same as that in Example 1, except that the positive electrode materials used in the preparation of the first slurry and the second slurry are both lithium cobalt oxide materials.

[0107] Test example

[0108] The energy density and cycle performance of the lithium-ion batteries prepared in the above examples and comparative examples were tested using the following test methods. The test results are shown in Table 1:

[0109] (1) Energy density test method:

[0110] The energy density is calculated according to energy density (Wh / L) = cell energy / (length*width*thickness). The test method for cell energy includes: at room temperature of 25°C, charging to a full state (100% SOC) using a charging current of 0.5C, and then cutting off the current to 0.05C; after standing still for 10 minutes, discharging the current at a current of 0.2C to the energy released at 0% SOC; during the test, the discharge energy is recorded by the charge and discharge test equipment.

[0111] (2) Cyclic performance test method:

[0112] At room temperature (25°C), charge at a constant current of 1C, with a cutoff current of 0.05C, rest for 10 minutes, and discharge at 0.7C for 300, 500, 800, and 1000 cycles, respectively. Disassemble the fully charged lithium-ion battery to confirm the lithium deposition in the first active material layer of the positive electrode sheet and calculate the capacity retention (%) and cycle expansion (%) after 1000 cycles. The upper charging voltage limit is 4.4V. The passing criteria for the cycle performance test are: 1000-cycle capacity retention ≥ 80%, and cycle expansion thickness ≤ 10%.

[0113] Table 1

[0114]

[0115] The mass content of the Co element in Table 1 refers to the mass content of Co in the positive electrode active materials in the first positive electrode active material layer and the second active material layer.

[0116] As can be seen from the data in Table 1, the present invention designs the active layer composition of the corresponding areas according to the different degrees of wetting in different areas of the positive electrode sheet, which can effectively avoid the occurrence of lithium plating when the positive electrode material includes a ternary material, so that the lithium-ion battery has both low cost and excellent cycle performance.

[0117] 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 above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above 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: The positive electrode sheet includes a current collector and an active material layer provided on at least one functional surface of the current collector; the active material layer is provided on a coating area of ​​the functional surface, the coating area including a first region and a second region, the first region being located in a middle region of the coating area in a first direction of the positive electrode sheet and / or a second direction of the positive electrode sheet, and the second region being located in a non-middle region of the coating area in the first direction of the positive electrode sheet and / or the second direction of the positive electrode sheet; the first region is provided with a first active material layer, and the second region is provided with a second active material layer; The first active material layer includes a first active material, and the second active material layer includes a second active material, the first active material is selected from lithium cobalt oxide or a mixture of lithium cobalt oxide and a ternary material, and the second active material is selected from a ternary material or a mixture of a ternary material and lithium cobalt oxide; Within the same unit volume range, the mass content of nickel in the first active material layer is less than the mass content of nickel in the second active material layer; The first region is located in the middle region of the coating region in the first direction or the second direction of the positive electrode sheet, the second region is located in the non-middle region of the coating region in the first direction or the second direction of the positive electrode sheet, and the width ratio of the first active material layer to the second active material layer is (1:14) to (1:2).

2. The positive electrode sheet according to claim 1, characterized in that Within the same unit volume range, the mass content of nickel in the first active material layer is M1, and the mass content of nickel in the second active material layer is M2, wherein M2-M1≥1%.

3. The positive electrode sheet according to claim 2, characterized in that: M2-M1≥2%.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The first region is located in the middle region of the coating region in the first direction and the second direction of the positive electrode sheet, the second region is located in the non-middle region of the coating region in the first direction and the second direction of the positive electrode sheet, and the coating area ratio of the first active material layer to the second active material layer is (1:14) to (1:2).

5. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The particle size of the ternary material is smaller than the particle size of the lithium cobalt oxide.

6. The positive electrode sheet according to claim 5, characterized in that: The D50 particle size of the ternary material is 2 μm to 7 μm, and the D99 particle size is 8 μm to 15 μm; The D50 particle size of the lithium cobalt oxide is 10 μm to 20 μm, and the D99 particle size is 30 μm to 50 μm.

7. The positive electrode sheet according to any one of claims 1 to 3 and 6, characterized in that: The thickness of the first active material layer and the second active material layer are both 20 to 150 μm, and the thickness of the first active material layer and the second active material layer are equal.

8. The positive electrode sheet according to any one of claims 1 to 3 and 6, characterized in that: The ternary material is selected from at least one of NCM111, NCM523, NCM613, and NCM811.

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

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