A positive electrode sheet and its application

Abstract

The present application provides a positive electrode sheet and its application. The positive electrode sheet includes a positive electrode current collector, a first positive electrode active layer, and a second positive electrode active layer that are sequentially stacked. The thickness of the first positive electrode active layer is greater than or equal to 0.5 μm; in the first positive electrode active material of the first positive electrode active layer, the molar ratio of lithium element to active metal element is greater than 1; the full charge resistance R of the positive electrode sheet is not less than 1 Ω. The special composition and appropriate sheet resistance of this positive electrode sheet can effectively and stably improve the passing rate of the nail penetration test of the lithium-ion battery on the premise of ensuring the energy density of the lithium-ion battery.

Description

Technical Field

[0001] The present application relates to a positive electrode sheet, and in particular to a positive electrode sheet and its application, belonging to the technical field of secondary batteries. Background Art

[0002] With the advent of the era of electronization, digitization, and intelligentization, more and more products rely on energy storage devices such as batteries and power supplies. In this wave of development, lithium-ion batteries have developed rapidly due to their high energy density, good service life, and excellent environmental friendliness.

[0003] In the development of lithium-ion batteries, it is inevitable to pay attention to the safety performance during their use. With the large-scale popularization of lithium-ion batteries, the occasional problems of puncturing and burning batteries due to disassembly and collision in the market have made users pay more and more attention to the safety performance of batteries. The needle penetration test of a battery is to evaluate the safety performance of the battery under the scenarios of external force extrusion or disassembly and collision. It means that after the battery is fully charged with a certain current, a nail with a certain diameter is used to penetrate the battery at a fixed speed, and whether the battery smokes, catches fire and burns, or explodes is used as the test criterion.

[0004] At present, the improvement measures for needle penetration mainly include coating a ceramic layer on the surface of the negative electrode to prevent short circuit, or extending the empty current collectors of the positive and negative electrode sheets so that the short circuit mainly occurs between the empty current collectors of the positive and negative electrodes, but this will greatly reduce the energy density of the battery. In addition, at present, a piece of adhesive paper is often attached to the surface of the empty current collector to prevent the burrs of the empty current collector from piercing the separator, and the adhesive paper will reduce the probability of short circuit between the empty current collectors of the positive and negative electrode sheets. Therefore, the above improvement measures of making the short circuit occur between the empty current collectors of the positive and negative electrodes often have unstable effects and poor reproducibility of the test results. Summary of the Invention

[0005] The present application provides a positive electrode sheet and its application. The special composition and appropriate sheet resistance of the positive electrode sheet can effectively and stably improve the passing rate of the needle penetration test of the lithium-ion battery on the premise of ensuring the energy density of the lithium-ion battery.

[0006] The first aspect of the present application provides a positive electrode sheet, which includes a positive current collector, a first positive active layer, and a second positive active layer stacked in sequence. The thickness of the first positive active layer is greater than or equal to 0.5 μm;

[0007] In the first positive active material of the first positive active layer, the molar ratio of lithium element to active metal element is greater than 1;

[0008] The full charge resistance R of the positive electrode sheet is not less than 1 Ω.

[0009] In this application, by defining that the thickness of the first positive electrode active layer in the positive electrode sheet is greater than or equal to 0.5 μm and the full charge resistance is not less than 1 Ω, the positive electrode sheet has a lower short-circuit current. When the battery operates under harsh conditions such as impact and puncture, even if a short circuit occurs between the positive and negative electrodes, due to the low short-circuit current, the heat generation power during the short-circuit process is greatly reduced, ensuring the safety of the short-circuit process, improving the battery's ability to pass the puncture test, making the short-circuit phenomenon mild and controllable, and reducing the probability of the battery catching fire or exploding. In addition, the special structural and chemical compositions of the positive electrode sheet can ensure that there are more active lithium ions in the lithium-ion battery, thus helping to balance the energy density of the lithium-ion battery.

[0010] In one possible implementation, R ≤ 50 Ω.

[0011] In one possible implementation, the first positive electrode active material includes a compound with the chemical composition of Li x A y T z ;

[0012] wherein, x / y > 1, element A includes at least one of Fe, Ni, Co, Mn, Cu, Zn, Cr, and Al, and element T includes at least one of O, Cl, S, F, and P.

[0013] In one possible implementation, the second positive electrode active material includes at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium oxide phosphate, lithium-rich manganese-based material, lithium nickel cobalt aluminate, and lithium titanate.

[0014] In one possible implementation, in the second positive electrode active material, the molar ratio of lithium element to active metal element is less than or equal to 1.

[0015] In one possible implementation, the specific capacity of the first positive electrode active material is greater than that of the second positive electrode active material.

[0016] In one possible implementation, the de-lithiation potential of the first positive electrode active material is less than that of the second positive electrode active material.

[0017] In one possible implementation, the thickness of the first positive electrode active layer is 0.5 - 10 μm.

[0018] In one possible implementation, the first positive electrode active layer further includes a binder and a conductive agent, and the mass ratio of the binder to the conductive agent is (6:1) - (1:3).

[0019] In one possible implementation, the powder resistivity of the first positive electrode active material is not less than 1 Ω.

[0020] In a possible implementation, the powder resistivity of the first positive electrode active material is 10 to 200 Ω.

[0021] The second aspect of the present application provides a lithium-ion battery, and the lithium-ion battery includes the positive electrode sheet of the first aspect. This lithium-ion battery has excellent performance in terms of energy density and safety performance.

[0022] The third aspect of the present application provides an electronic device, and the electronic device includes the lithium-ion battery of the second aspect. This electronic device has excellent standby time and safety performance, and high user satisfaction. Description of the Drawings

[0023] Figure 1 It is a schematic structural diagram of the positive electrode sheet provided by the present application;

[0024] Figure 2a It is a schematic diagram before the nail penetration test on the battery cell of the present application;

[0025] Figure 2b It is a schematic diagram after the nail penetration test on the battery cell of the present application. Detailed Embodiments

[0026] To make the objectives, technical solutions, and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts shall fall within the protection scope of the present application.

[0027] As Figure 1 shown, the first aspect of the present application provides a positive electrode sheet, which includes a positive electrode current collector 10, a first positive electrode active layer 1, and a second positive electrode active layer 2 that are sequentially stacked. The thickness of the first positive electrode active layer is greater than or equal to 0.5 μm; in the first positive electrode active material of the first positive electrode active layer 1, the molar ratio of lithium element to active metal element is greater than 1; the full charge resistance R of the positive electrode sheet is not less than 1 Ω.

[0028] The positive electrode current collector 10 of the above positive electrode sheet includes two relatively arranged surfaces with the largest area, which are referred to as functional surfaces in the present application. The first positive electrode active layer 1 and the second positive electrode active layer 2 of the positive electrode sheet of the present application are sequentially arranged on the functional surface of the positive electrode current collector 10. Among them, the first positive electrode active layer 1 is arranged on the functional surface, and the second positive electrode active layer 2 is arranged on the surface of the first positive electrode active layer 1 away from the positive electrode current collector 10. It should be noted that in addition to Figure 1The first positive electrode active layer 1 and the second positive electrode active layer 2 shown are respectively disposed outside the two functional surfaces of the positive electrode current collector 10. In another embodiment, the first positive electrode active layer 1 and the second positive electrode active layer 2 may also be disposed only on one functional surface of the positive electrode current collector 10. The positive electrode current collector 10 may be common aluminum foil, nickel foil, etc. in the art. In this patent, aluminum foil is used as the positive electrode current collector.

[0029] Specifically, the first positive electrode active layer 1 includes a first positive electrode active material, a conductive agent, and a binder; the second positive electrode active layer 2 includes a second positive electrode active material, a conductive agent, and a binder. The present application does not limit the selection of the conductive agent in the first positive electrode active layer 1 and the conductive agent in the second positive electrode active layer 2. For example, the two can independently be selected from at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, single-walled carbon nanotube, multi-walled carbon nanotube, carbon fiber; the present application also does not limit the selection of the binder in the first positive electrode active layer 1 and the binder in the second positive electrode active layer 2. Exemplarily, the two can independently be selected from at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), lithium polyacrylate (PAALi).

[0030] In the positive electrode sheet of the present application, the first positive electrode active material and the second positive electrode active material are used to provide lithium ions that complete migration and insertion / extraction between the positive and negative electrodes of the lithium ion battery, thereby realizing the charge and discharge of the lithium ion battery. It can be understood that both the first positive electrode active material and the second positive electrode active material include lithium element. In addition, they also include active metal elements commonly used in the art, such as at least one of Mg, Fe, Co, Cu, Zn, Cr, Ti, Mn, Al, Te, W, Ni, Nb, Zr, La, Ce, Sr, Y, K, etc. Among them, in the first positive electrode active material, the molar ratio of lithium element to the total active metal element is greater than 1. Therefore, as a lithium-rich material, the first positive electrode active material can provide more lithium ions for the lithium ion battery in the first cycle, which is beneficial to making the lithium ion battery including the positive electrode sheet of the present application have a higher energy density. It should be noted that the present application does not limit the selection of the second positive electrode active material, which may be the same as or different from the first positive electrode active material.

[0031] In addition to optimizing the energy density of a lithium-ion battery through the positive electrode sheet with the above chemical composition and structural composition, the positive electrode sheet of the present application also has a full charge resistance R of not less than 1 Ω. The inventor found that under external forces such as impact and collision, the positive electrode sheet can significantly reduce the probability of the lithium-ion battery catching fire or exploding. Especially during the nail penetration test of the lithium-ion battery, it can significantly and stably improve the passing rate of the nail penetration test. The inventor speculated that the reason might be that the relatively high full charge resistance of the electrode sheet including the first positive electrode active layer with a thickness of not less than 0.5 μm can reduce the heat generation power during the battery short-circuit process, thus making the short-circuit phenomenon mild and controllable, and reducing the probability of the battery catching fire or exploding. Moreover, compared with the method of short-circuiting the positive and negative empty current collectors covered with adhesive tape, the efficiency of using the positive electrode sheet of the present application to improve the safety performance of the lithium-ion battery is higher and the stability is stronger.

[0032] It is worth mentioning that during the actual application process, the full charge resistance of the positive electrode sheet can also be adjusted according to the application requirements and application environment of the battery, so as to make the lithium-ion battery more adaptable to the current application. For example, for consumer batteries, the full charge resistance R can be minimized on the basis of not less than 1 Ω, while for power batteries, the full charge resistance R can be moderately increased on the basis of not less than 1 Ω to further ensure safety.

[0033] It should be added that the above full charge resistance refers to the resistance of the positive electrode sheet obtained after disassembling the lithium-ion battery when the formed battery is charged with constant current-constant voltage until its SOC is 100%. The specific constant current-constant voltage charging regime is not overly limited in the present application.

[0034] In addition, the present application does not overly limit the preparation method of the positive electrode sheet. For example, after coating the prepared first positive electrode slurry on the functional surface of the positive electrode current collector 10 and drying it, then coating the second positive electrode slurry on the surface of the first positive electrode active layer 1 formed by drying and drying to obtain the second positive electrode active layer 2, and then through post-treatments such as pressing and cutting, the positive electrode sheet of the present application is obtained. Of course, during the preparation process, relevant parameters need to be controlled so that the positive electrode sheet has a full charge resistance of not less than 1 Ω. For example, the adjustment of the full charge resistance R is achieved by controlling the selection and mutual combination between the first positive electrode active material and the second positive electrode active material.

[0035] Furthermore, considering the relevant electrical properties of the lithium-ion battery including rate performance, low-temperature cycle performance, etc., the full charge resistance R of the positive electrode sheet is ≤ 50 Ω. The inventor found that when the full charge resistance R is too high, the resistance inside the battery cell will increase, thereby causing a certain degree of reduction in the electrical performance of the battery. Therefore, when the full charge resistance is not higher than 50 Ω, the positive electrode sheet can enable the lithium-ion battery to simultaneously take into account excellent safety performance, energy density, and electrical performance.

[0036] In a specific embodiment, the first positive electrode active material includes a compound with a chemical composition of Li x A y T z ; wherein, x / y > 1, x > 1, element A includes at least one of Fe, Ni, Co, Mn, Cu, Zn, Cr, Al, and element T includes at least one of O, Cl, S, F, P.

[0037] In a specific embodiment, the second positive electrode active material includes at least one of lithium cobalt oxide, lithium manganate, lithium nickel cobalt manganate, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium oxide phosphate, lithium-rich manganese-based material, lithium nickel cobalt aluminate, and lithium titanate, and the present application does not make excessive limitations.

[0038] In order to further ensure the energy density of the lithium-ion battery, a first positive electrode active material with a higher specific capacity than the second positive electrode active material can be selected to further improve the energy density of the lithium-ion battery by increasing the initial efficiency of the lithium-ion battery.

[0039] As described above, the first positive electrode active material is a lithium-rich material. Therefore, in order to maximize the initial efficiency of the lithium-ion battery, a more suitable first positive electrode active material and second positive electrode active material can also be selected with the de-lithiation potential as a parameter. Specifically, by making the de-lithiation potential of the first positive electrode active material lower than that of the second positive electrode active material, the first positive electrode active material can release more lithium ions during the first charge, thereby further improving the energy density of the lithium-ion battery by increasing the initial efficiency.

[0040] In the specific implementation process of the present application, the thickness of the first positive electrode active layer is generally controlled to be 0.5 μm or more. Reasonably controlling the thickness of the first positive electrode active layer is beneficial to further improving the performance of the lithium-ion battery. The inventors have found that as the first positive electrode active layer increases within a certain range, the safety performance of the lithium-ion battery gradually improves, and the energy density first shows an increasing trend and then basically remains unchanged or slightly decreases. Therefore, considering that the lithium-ion battery has better performance, the thickness of the first positive electrode active layer is generally controlled within 0.5 - 10 μm.

[0041] Of course, using different first or second positive electrode active materials, conductive agents, or binders in the positive electrode sheet, or even using different negative electrode active materials, electrolytes, or separators will affect the final performance of the lithium-ion battery. Therefore, generally speaking, when the thickness of the first positive electrode active layer is controlled within 0.5 - 3 μm, the performance of the lithium-ion battery can be basically optimized.

[0042] The present application does not limit the adjustment method of the full charge resistance R of the positive electrode sheet and the implementation method of making R ≥ 1 Ω, or 1 Ω ≤ R ≤ 50 Ω.

[0043] In a specific embodiment, the full charge resistance R can be adjusted by adjusting the mass ratio of the conductive agent to the binder in the first positive electrode active layer. Specifically, when the mass ratio of the binder to the conductive agent in the first positive electrode active layer is (6:1) to (1:3), the limitation of the full charge resistance R of the positive electrode sheet of the present application can be achieved. Further, when the mass ratio of the binder to the conductive agent in the first positive electrode active layer is (3:1) to (1:1), it is beneficial to further reduce the internal resistance of the battery cell on the premise of taking into account the passing rate of the needle penetration test.

[0044] As is well known in the art, in order to improve the cycle performance or rate performance of the battery, the positive electrode active material is often subjected to carbon coating treatment or doping treatment with trace metals (such as Mg, Ti, Al, Ni, Cu, Cr, Pt, La, Nb) or oxides of trace metal elements, and different carbon coating amounts or doping amounts of trace elements will affect the powder conductivity of the positive electrode active material. In another specific embodiment, the full charge resistance R can be adjusted by the powder resistivity of the first positive electrode active material. Specifically, when the powder resistivity of the first positive electrode active material is not less than 1 Ω, R ≥ 1 Ω can be achieved. Further, when the powder resistivity of the first positive electrode active material is 10 to 200 Ω, 1 Ω ≤ R ≤ 50 Ω.

[0045] The positive electrode sheet of the present application can not only provide more lithium ions but also has a relatively high full charge resistance. Therefore, it can stably improve the safety performance of the lithium ion battery without reducing the energy density of the lithium ion battery, especially the needle penetration test has a stable and excellent passing rate.

[0046] The second aspect of the present application provides a lithium ion battery, which includes the positive electrode sheet of the foregoing first aspect.

[0047] In addition to the positive electrode sheet, the lithium ion battery further includes a negative electrode sheet, an electrolyte, and a separator. Among them, the separator is located between the positive and negative electrodes, and the electrolyte fills the battery cell. The specific structure of the lithium ion battery is not different from that of the existing lithium ion batteries in the art and will not be described in detail here.

[0048] The present application does not make excessive limitations on the selection of the negative electrode sheet, the electrolyte, and the separator.

[0049] In a specific embodiment, the negative electrode precursor includes a negative electrode current collector and a negative electrode active layer provided on at least one functional surface of the current collector. Among them, the negative electrode current collector can be a common aluminum foil in the art. The negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent. The negative electrode active material is, for example, an amorphous carbon material, specifically including hard carbon materials, soft carbon materials, etc. Further, the negative electrode active material is selected from hard carbon materials such as resin carbon, organic polymer pyrolysis carbon, carbon black, etc. The conductive agent is selected from but not limited to at least one of super-P, conductive carbon black, carbon nanotubes, and acetylene black. The binder is selected from but not limited to one of polyvinylidene fluoride (PVDF) or polyethylene oxide (PEO).

[0050] In a specific embodiment, the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride.

[0051] In a specific embodiment, the electrolyte includes at least an organic solvent and a lithium salt. Among them, the organic solvent can be selected from at least one of ethylene carbonate, butylene carbonate, propylene carbonate, ethyl methyl carbonate, vinylene carbonate, ethylene vinyl carbonate, fluoroethylene carbonate, fluoroethyl methyl carbonate, difluoroethylene carbonate, fluorodimethyl carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate; the lithium salt can include but not limited to being selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(fluorosulfonyl)imide (LiFSI).

[0052] The lithium-ion battery of the present application includes the aforementioned positive electrode sheet, and thus has excellent performance in terms of safety performance and energy density.

[0053] The third aspect of the present application provides an electronic device. Among them, the electronic device can include but not limited to mobile or fixed terminals with batteries such as mobile phones, tablet computers, laptop computers, ultra-mobile personal computers (UMPCs), handheld computers, walkie-talkies, netbooks, POS machines, personal digital assistants (PDAs), wearable devices, virtual reality devices, base stations, energy storage devices, etc.

[0054] The electronic device of the present application, due to using the aforementioned lithium-ion battery as a driving source or an energy storage unit, has excellent standby time and safety performance, and high user satisfaction.

[0055] Hereinafter, the positive electrode sheet and the lithium-ion battery of the present application will be introduced in detail through specific examples.

[0056] Example 1

[0057] The preparation method of the positive electrode sheet in this embodiment includes the following steps:

[0058] 1) Using aluminum foil as the positive electrode current collector, uniformly coat the first positive electrode active slurry (including 96 wt% Li 1.5 FePO 4 , 3 wt% PVDF and 1 wt% conductive carbon black) on the two functional surfaces of the aluminum foil, and dry it at 85 °C to form the first positive electrode active layer; among them, the carbon coating amount of Li 1.5 FePO 4 is 3 wt%, and 2000 ppm of Mg element is doped therein (the doping amount is calculated based on the atomic weight of Fe).

[0059] 2) Continuously coat the second positive electrode active slurry (including 97.8 wt% LiCoO 2 , 0.8 wt% PVDF and 1.4 wt% conductive carbon black) on the two surfaces of the first positive electrode active layer, and dry it at 85 °C to form the second positive electrode active layer; among them, the specific capacity of LiCoO 2 is 176 mAh / g.

[0060] 3) After successively cold pressing, cutting and slitting the structure in step 2), the positive electrode sheet of this embodiment is obtained, wherein the thickness of the first positive electrode active layer is 2 μm, and the thickness of the second positive electrode active layer is 104 μm.

[0061] The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0062] Example 2

[0063] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, the difference is that Li 2 MnO 3 is used to replace Li 1.5 FePO 4 . The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0064] Example 3

[0065] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, the difference is that Li 2.5 CuO 2 is used to replace Li 1.5 FePO 4 . The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0066] Example 4

[0067] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, the difference is that Li 2 Mn 0.5 Ni0.5O3 Replace Li 1.5 FePO 4 The relevant parameters of the positive electrode sheet in this example are shown in Table 1.

[0068] Example 5

[0069] The preparation method of the positive electrode sheet in this example is basically the same as that in Example 1, except that in the first positive electrode active paste, the mass ratio of PVDF to conductive carbon black is 6:1. The relevant parameters of the positive electrode sheet in this example are shown in Table 1.

[0070] Example 6

[0071] The preparation method of the positive electrode sheet in this example is basically the same as that in Example 1, except that in the first positive electrode active paste, the mass ratio of PVDF to conductive carbon black is 5:1. The relevant parameters of the positive electrode sheet in this example are shown in Table 1.

[0072] Example 7

[0073] The preparation method of the positive electrode sheet in this example is basically the same as that in Example 1, except that in the first positive electrode active paste, the mass ratio of PVDF to conductive carbon black is 4:1. The relevant parameters of the positive electrode sheet in this example are shown in Table 1.

[0074] Example 8

[0075] The preparation method of the positive electrode sheet in this example is basically the same as that in Example 1, except that in the first positive electrode active paste, the mass ratio of PVDF to conductive carbon black is 2:1. The relevant parameters of the positive electrode sheet in this example are shown in Table 1.

[0076] Example 9

[0077] The preparation method of the positive electrode sheet in this example is basically the same as that in Example 1, except that in the first positive electrode active paste, the mass ratio of PVDF to conductive carbon black is 1:1. The relevant parameters of the positive electrode sheet in this example are shown in Table 1.

[0078] Example 10

[0079] The preparation method of the positive electrode sheet in this example is basically the same as that in Example 1, except that in the first positive electrode active paste, the mass ratio of PVDF to conductive carbon black is 1:2. The relevant parameters of the positive electrode sheet in this example are shown in Table 1.

[0080] Example 11

[0081] The preparation method of the positive electrode sheet in this example is basically the same as that in Example 1, except that in the first positive electrode active paste, the mass ratio of PVDF to conductive carbon black is 1:3. The relevant parameters of the positive electrode sheet in this example are shown in Table 1.

[0082] Example 12

[0083] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that in the first positive electrode active paste, the carbon coating amount of Li 1.5 FePO 4 is adjusted from 3 wt% to 4 wt%, and the doping amount of Mg remains unchanged at 2000 ppm, so as to obtain the first positive electrode active material with a powder resistivity of 5 Ω. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0084] Embodiment 13

[0085] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that in the first positive electrode active paste, the carbon coating amount of Li 1.5 FePO 4 is adjusted from 3 wt% to 4.5 wt%, and the doping amount of Mg remains unchanged at 2000 ppm, so as to obtain the first positive electrode active material with a powder resistivity of 3 Ω. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0086] Embodiment 14

[0087] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that in the first positive electrode active paste, the carbon coating amount of Li 1.5 FePO 4 is adjusted from 3 wt% to 5 wt%, and the doping amount of Mg remains unchanged at 2000 ppm, so as to obtain the first positive electrode active material with a powder resistivity of 1 Ω. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0088] Embodiment 15

[0089] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that in the first positive electrode active paste, the carbon coating amount of Li 1.5 FePO 4 is adjusted from 3 wt% to 2.7 wt%, and the doping amount of Mg is reduced to 1500 ppm, so as to obtain the first positive electrode active material with a powder resistivity of 20 Ω. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0090] Embodiment 16

[0091] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that in the first positive electrode active paste, the carbon coating amount of Li 1.5 FePO 4 is adjusted from 3 wt% to 2.3 wt%, and the doping amount of Mg is reduced to 1300 ppm, so as to obtain the first positive electrode active material with a powder resistivity of 50 Ω. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0092] Embodiment 17

[0093] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that in the first positive electrode active paste, the carbon coating amount of Li 1.5 FePO 4 is adjusted from 3 wt% to 2.0 wt%, and the doping amount of Mg is reduced to 1300 ppm, so as to obtain a first positive electrode active material with a powder resistivity of 100 Ω. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0094] Embodiment 18

[0095] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that in the first positive electrode active paste, the carbon coating amount of Li 1.5 FePO 4 is adjusted from 3 wt% to 1.5 wt%, and the doping amount of Mg is reduced to 1000 ppm, so as to obtain a first positive electrode active material with a powder resistivity of 150 Ω. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0096] Embodiment 19

[0097] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that in the first positive electrode active paste, the carbon coating amount of Li 1.5 FePO 4 is adjusted from 3 wt% to 1.0 wt%, and the doping amount of Mg is reduced to 1000 ppm, so as to obtain a first positive electrode active material with a powder resistivity of 200 Ω. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0098] Embodiment 20

[0099] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that the thickness of the first positive electrode active layer is 1 μm by reducing the mass of the first positive electrode paste. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0100] Embodiment 21

[0101] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that the thickness of the first positive electrode active layer is 0.5 μm by reducing the mass of the first positive electrode paste. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0102] Embodiment 22

[0103] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that the thickness of the first positive electrode active layer is 3 μm by increasing the mass of the first positive electrode paste. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0104] Embodiment 23

[0105] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that the thickness of the first positive electrode active layer is 5 μm by increasing the mass of the first positive electrode slurry. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0106] Example 24

[0107] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Embodiment 1, except that the thickness of the first positive electrode active layer is 10 μm by increasing the mass of the first positive electrode slurry. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0108] Example 25

[0109] The preparation method of the positive electrode sheet in this comparative example is basically the same as that in Embodiment 1, except that Li 1.2 Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 is used to replace Li 1.5 FePO 4 . The relevant parameters of the positive electrode sheet in this comparative example are shown in Table 1.

[0110] Example 26

[0111] The preparation method of the positive electrode sheet in this comparative example is basically the same as that in Embodiment 1, except that in the first positive electrode active slurry, the mass ratio of PVDF to conductive carbon black is 7:1. The relevant parameters of the positive electrode sheet in this comparative example are shown in Table 1.

[0112] Example 27

[0113] The preparation method of the positive electrode sheet in this comparative example is basically the same as that in Embodiment 1, except that in the first positive electrode active slurry, the carbon coating amount of Li 1.5 FePO 4 is adjusted from 3 wt% to 0.8 wt%, and the doping amount of Mg is reduced to 800 ppm, so as to obtain a first positive electrode active material with a powder resistivity of 250 Ω. The relevant parameters of the positive electrode sheet in this embodiment are shown in Table 1.

[0114] Example 28

[0115] The preparation method of the positive electrode sheet in this comparative example is basically the same as that in Embodiment 1, except that the thickness of the first positive electrode active layer is 15 μm by increasing the mass of the first positive electrode slurry. The relevant parameters of the positive electrode sheet in this comparative example are shown in Table 1.

[0116] Comparative Example 1

[0117] The preparation method of the positive electrode sheet in this comparative example includes: using aluminum foil as the positive electrode current collector, and using the second positive electrode active slurry in Embodiment 1 (including 97.8 wt% LiCoO2 , 0.8 wt% PVDF and 1.4 wt% conductive carbon black) were directly coated on the two functional surfaces of the aluminum foil, then dried at 85 °C, and subsequently passed through cold pressing, cutting, and slitting in sequence. The positive electrode sheet of this comparative example was obtained (each functional performance of the current collector had only one positive electrode active layer), and the thickness of the single positive electrode active layer was 107 μm. The relevant parameters of the positive electrode sheet of this comparative example are shown in Table 1.

[0118] Comparative Example 2

[0119] The preparation method of the positive electrode sheet of this comparative example includes the following steps:

[0120] 1) Aluminum foil was used as the positive electrode current collector, and the first positive electrode active paste (including 96 wt% LiFePO 4 , 1% conductive carbon black, and 3% PVDF) was uniformly coated on the two functional surfaces of the aluminum foil and dried at 85 °C to form the first positive electrode active layer;

[0121] 2) The second positive electrode active paste in Example 1 (including 97.8 wt% LiCoO 2 , 0.8 wt% PVDF, and 1.4 wt% conductive carbon black) was continuously coated on the two surfaces of the first positive electrode active layer and dried at 85 °C to form the second positive electrode active layer;

[0122] 3) The structure in step 2) was sequentially passed through cold pressing, cutting, and slitting to obtain the positive electrode sheet of this comparative example. Among them, the thickness of the first positive electrode active layer was 2 μm, and the thickness of the second positive electrode active layer was 104 μm.

[0123] The relevant parameters of the positive electrode sheet of this comparative example are shown in Table 1.

[0124] Comparative Example 3

[0125] The preparation method of the positive electrode sheet of this comparative example was basically the same as that of Example 1, except that in the first positive electrode active paste, the mass ratio of PVDF to conductive carbon black was 1:4. The relevant parameters of the positive electrode sheet of this comparative example are shown in Table 1.

[0126] Comparative Example 4

[0127] The preparation method of the positive electrode sheet of this comparative example was basically the same as that of Example 1, except that in the first positive electrode active paste, the carbon coating amount of Li 1.5 FePO 4 was adjusted from 3 wt% to 6 wt%, and the doping amount of Mg remained unchanged at 2000 ppm, thereby obtaining the first positive electrode active material with a powder resistivity of 0.5 Ω. The relevant parameters of the positive electrode sheet of this example are shown in Table 1.

[0128] Comparative Example 5

[0129] The preparation method of the positive electrode sheet of this comparative example is basically the same as that of Example 1, except that the thickness of the first positive electrode active layer is 0.2 μm by reducing the mass of the first positive electrode slurry. The relevant parameters of the positive electrode sheet of this comparative example are shown in Table 1.

[0130] Test example

[0131] The positive electrode sheets of the above-mentioned embodiments and comparative examples are respectively welded and glued with the negative electrode sheets and then wound. The positive and negative electrode sheets are separated by a diaphragm to form a bare battery cell. The bare battery cell is subjected to the processes of top sealing, side sealing, coding, vacuum drying, electrolyte injection, high-temperature standing, formation, exhaust, side cutting, capacity, high-temperature standing, folding, etc. to obtain the battery cells of the embodiments and comparative examples.

[0132] Among them, the preparation method of the negative electrode sheet includes: using copper foil as the negative electrode current collector, coating the negative electrode active slurry (including 97.7wt% graphite, 1.2wt% CMC and 1.1wt% SBR) on the two functional surfaces of the copper foil, and then drying at 85°C, and then cold pressing, cutting and slitting in sequence to obtain the negative electrode sheet.

[0133] The following parameters were tested on the battery cell and the first positive electrode active material prepared as above, and the results are shown in Table 1.

[0134] 1. Energy density W test

[0135] Place the battery cell in an environment of 25℃±3℃ for 2h, charge it at a constant current of 0.5C to a cut-off voltage of 4.48V, then perform a constant voltage test until the current decreases to 0.025C, place it at rest for 10min, and discharge it at a cross current of 0.2C to 3.0V. The discharge energy at this time is the energy of the battery cell.

[0136] Charge the battery cell to the shipping voltage of 4.0V, then measure the thickness of the battery cell with PPG, measure the length and width of the battery cell with a constant force caliper, and calculate the battery cell energy density with the following formula. When the battery cell energy density W ≥ 680Wh / L, it proves that the battery cell energy density is excellent.

[0137] Energy density W = discharge energy / (cell length * width * thickness)

[0138] 2. Cell internal resistance R1

[0139] Place the battery cell in an environment of 25℃±3℃ for more than 12 hours, and then use the HIOKI internal resistance tester to measure the internal resistance R1 of the battery cell, where the internal resistance frequency is set to 1KHz and the measurement accuracy is ±0.01mohm. When the internal resistance R1 of the battery cell is ≤30mohm, it proves that the battery cell has excellent electrical performance.

[0140] 3. Acupuncture test

[0141] Specific operations such asFigure 2a and Figure 2b As shown. Take 10 battery cells B of each example and comparative example, place the battery cells B in an environment of 25°C ± 3°C and let them stand for 2 h, charge at a constant current of 0.5C until the cut-off voltage of 4.48V, and then perform a constant voltage test until the current decreases to 0.025C. Then, use a steel needle A with a diameter of 4 ± 0.06 mm, a taper of 15 ± 2°, and a total length of 100 mm to perform a needle puncture test on the battery cell B at room temperature. The needle puncture speed is 30 mm / s, and the puncture position is the exact middle of the battery cell B. The penetration depth of the nail is based on the steel needle A passing through the battery cell B. When the passing rate ≥ 5 / 10 (that is, at least 5 out of 10 samples do not catch fire, explode, etc.), the needle puncture test is qualified.

[0142] 4. Test of the full-charge internal resistance R of the electrode sheet

[0143] Place the battery cell in an environment of 25°C ± 3°C and let it stand for 2 h, charge at a constant current of 0.5C until the cut-off voltage of 4.48V, then perform a constant voltage test until the current decreases to 0.025C, let it stand for 10 min, disassemble it and take out the positive electrode sheet, use a punching machine to make samples with a diameter of 14 mm, and use Yuaneng Technology BER2500 to test the full-charge internal resistance R of the electrode sheet samples. The test pressure is set at 5 MPa, and the measurement accuracy is 0.5% FS.

[0144] 5. Test of the powder conductivity

[0145] Press 5 ± 1 g of the first positive electrode active material powder into small round tablets with a diameter of 2 mm using a tablet press, and then use a four-probe resistivity meter to test the conductivity of the small round tablets. Test 16 pcs of samples, and take the average value as the conductivity of the first positive electrode active material.

[0146] 6. Test of the de-lithiation potential

[0147] Using a lithium sheet as the counter electrode, after preparing the positive electrode active material to be tested into a button cell, charge and discharge the cell at a current of 0.01C. The potential reaching the rated gram capacity is the de-lithiation potential of the positive electrode active material. Test the de-lithiation potentials of the first positive electrode active material and the second positive electrode active material respectively, and compare the magnitude relationship C of the de-lithiation potentials of the first positive electrode active material and the second positive electrode active material. If C is greater, the de-lithiation potential of the first positive electrode active material is greater than that of the second positive electrode active material; if C is less, the de-lithiation potential of the first positive electrode active material is less than that of the second positive electrode active material.

[0148] Table 1

[0149]

[0150]

[0151] As can be seen from Table 1, compared with the comparative example, the positive electrode sheet of the present application has excellent performance in terms of the passing rate of the needle punching performance and the energy density.

[0152] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and 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 application.

Claims

1. A positive electrode sheet, characterized in that, the positive electrode sheet comprises a positive electrode current collector, a first positive electrode active layer and a second positive electrode active layer which are sequentially stacked, and the thickness of the first positive electrode active layer is 0.5 μm to 10 μm; in the first positive electrode active material of the first positive electrode active layer, the molar ratio of lithium element to active metal element is greater than 1; in the second positive electrode active material of the second positive electrode active layer, the molar ratio of lithium element to active metal element is less than or equal to 1; the full charge resistance R of the positive electrode sheet is not less than 1 Ω; the de-lithiation potential of the first positive electrode active material is less than the de-lithiation potential of the second positive electrode active material, and the specific capacity of the first positive electrode active material is greater than the specific capacity of the second positive electrode active material; the powder resistivity of the first positive electrode active material is 10 to 200 Ω; the first positive electrode active layer further comprises a binder and a conductive agent, and the mass ratio of the binder to the conductive agent is (6:1) to (1:3).

2. The positive electrode sheet according to claim 1, characterized in that, R ≤ 50 Ω.

3. The positive electrode sheet according to claim 1, characterized in that, The first positive electrode active material includes a compound having a chemical composition of Li x A y T z ; wherein, x / y > 1, element A includes at least one of Fe, Ni, Co, Mn, Cu, Zn, Cr, Al, and element T includes at least one of O, Cl, S, F, P.

4. The positive electrode sheet according to claim 1, characterized in that, the second positive electrode active material includes at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium oxide phosphate, lithium-rich manganese-based material, lithium nickel cobalt aluminate and lithium titanate.

5. A lithium-ion battery, characterized in that, the lithium-ion battery includes the positive electrode sheet according to any one of claims 1-4.

6. An electronic device, characterized in that, the electronic device includes the lithium-ion battery according to claim 5.

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