Positive electrode sheet, secondary battery, and electric device

Abstract

The present application solves the problem of high resistance to lithium ion intercalation and deintercalation in a single-sided region of an electrode sheet, which seriously affects the energy density of a battery cell. The present application provides a positive electrode sheet, a secondary battery, and an electric device. The positive electrode sheet comprises a current collector, and a lithium compensation layer and a positive electrode active material layer arranged in a stacked configuration on a surface of at least one side of the current collector, wherein the lithium compensation layer is arranged close to the current collector, and the positive electrode active material layer is arranged on a surface of a side of the lithium compensation layer facing away from the current collector; the lithium compensation layer comprises a pre-lithiation agent and a first positive electrode active material, and the positive electrode active material layer comprises a second positive electrode active material and does not contain the pre-lithiation agent; and a mass of the pre-lithiation agent accounts for 3.5%-10% of a total mass of the lithium compensation layer. The positive electrode sheet not only has a lithium compensating effect, but also exhibits good structural stability during charging and discharging cycles, thereby simultaneously improving the energy density and cycle performance of the battery.

Description

Positive electrode sheet, secondary battery and electric device

[0001] Cross-reference to related applications

[0002] The present application claims priority to the Chinese patent application No. 202410870270.1 filed on June 28, 2024, and entitled "Positive electrode sheet, secondary battery and electric device", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The present application relates to the technical field of batteries, in particular to a positive electrode sheet, a secondary battery and an electric device. BACKGROUND

[0004] In order to improve the energy density of the secondary battery, the industry often chooses to perform pre-lithiation treatment on the positive electrode or the negative electrode. However, the process conditions for supplementing lithium to the negative electrode are harsh, and the safety risk in the production process is relatively large. Compared with supplementing lithium to the negative electrode, the process of supplementing lithium to the positive electrode is simple and has low safety risk. At present, the industry generally simply mixes a lithium supplementing agent in the positive electrode active material layer, or separately sets a lithium supplementing layer composed of a lithium supplementing agent, a binder and a conductive agent. However, the above two positive electrode lithium supplementing schemes will cause large stress changes of the positive electrode during the charge and discharge cycles, which is not conducive to the cycle stability of the battery.

[0005] SUMMARY

[0006] In view of this, the embodiments of the present application provide a positive electrode sheet, a secondary battery and an electric device. The positive electrode sheet not only has a lithium supplementing effect, but also can exhibit good structural stability during the charge and discharge cycles, so as to simultaneously improve the energy density and the cycle performance of the battery.

[0007] A first aspect of the embodiments of the present application provides a positive electrode sheet, comprising a current collector and a lithium supplementing layer and a positive electrode active material layer which are sequentially arranged on at least one side surface of the current collector, the lithium supplementing layer is arranged close to the current collector, and the positive electrode active material is arranged on the side surface of the lithium supplementing layer away from the current collector.

[0008] The lithium supplementing layer comprises a lithium supplementing agent and a first positive electrode active material, the positive electrode active material layer comprises a second positive electrode active material, and the positive electrode active material layer does not contain the lithium supplementing agent; and the lithium supplementing agent accounts for 3.5%-10% of the total mass of the lithium supplementing layer.

[0009] In the above positive electrode sheet, the lithium supplement layer with the lithium supplement agent is arranged between the current collector and the positive active material layer, which can effectively reduce the risk of side reactions of the lithium supplement agent in the storage and subsequent assembly process of the positive electrode sheet, thereby improving the utilization rate of the lithium supplement agent and more conducive to improving the energy density of the final battery. In addition, the first positive active material is also dispersed in the lithium supplement layer, which can avoid the problem of structural change or even collapse of the lithium supplement layer caused by the empty space after the reaction of the lithium supplement agent in the related art, and can effectively improve the structural stability of the lithium supplement layer during the charge and discharge cycle process. Especially conducive to improving the peeling strength of the positive electrode sheet during long cycle process, so as to benefit the continuous stability of the conductive network in the positive electrode sheet and improve the electrochemical stability of the positive electrode sheet. Therefore, the above positive electrode sheet can be used to provide a secondary battery with higher energy density and better cycle performance.

[0010] The second aspect of the embodiments of the present application provides a secondary battery comprising the positive electrode sheet provided by the first aspect of the embodiments of the present application. Since the positive electrode sheet provided by the embodiments of the present application is used, the secondary battery can have higher energy density and better cycle performance.

[0011] The third aspect of the embodiments of the present application provides a power consumption device comprising the secondary battery provided by the second aspect of the embodiments of the present application. Since the secondary battery provided by the embodiments of the present application is used to supply power, the power consumption device has strong endurance and strong market competitiveness. BRIEF DESCRIPTION OF DRAWINGS

[0012] The drawings described herein are used to provide further understanding of the present application, and form a part of the present application. The schematic embodiments of the present application and their descriptions are used to explain the present application, and do not constitute an improper limitation on the present application. In the drawings:

[0013] FIG. 1 is a schematic structural diagram of the cross section of the positive electrode sheet provided by an embodiment of the present application;

[0014] FIG. 2 is a schematic structural diagram of the cross section of the positive electrode sheet provided by another embodiment of the present application;

[0015] Explanation of reference numerals: 1-positive electrode sheet; 10-current collector; 20-lithium supplement layer; 201-first lithium supplement sub-layer; 20n-nth lithium supplement sub-layer; 30-positive active material layer; 301-first positive active material sub-layer; 30m-mth positive active material sub-layer. DETAILED DESCRIPTION

[0016] Please refer to FIG. 1, the embodiments of the present application provide a positive electrode sheet 1, which comprises a current collector 10 and a lithium supplement layer 20 and a positive active material layer 30 arranged in layers on at least one side surface of the current collector 10, the lithium supplement layer 20 is arranged close to the current collector 10, and the positive active material layer 30 is arranged on the side surface of the lithium supplement layer 20 away from the current collector 10;

[0017] The lithium supplement layer 20 comprises a lithium supplement agent and a first positive electrode active material, the positive electrode active material layer 30 comprises a second positive electrode active material, and the positive electrode active material layer 30 does not contain a lithium supplement agent; the lithium supplement agent accounts for 3.5%-10% of the total mass of the lithium supplement layer 20.

[0018] In the embodiments of the present application, the first positive electrode active material and the second positive electrode active material can be the same or different. In some embodiments of the present application, the first positive electrode active material and the second positive electrode active material each independently comprise one or more of modified or unmodified lithium iron phosphate, modified or unmodified lithium manganese iron phosphate, modified or unmodified lithium vanadium phosphate, modified or unmodified lithium cobalt phosphate, modified or unmodified lithium cobaltate, modified or unmodified lithium manganate, modified or unmodified lithium nickelate, modified or unmodified lithium nickel manganate, modified or unmodified lithium nickel manganese cobaltate, modified or unmodified lithium nickel manganese aluminate, etc., but are not limited thereto. The above-mentioned modification includes physical modification and chemical modification, wherein the physical modification includes but is not limited to coating of conductive material, coating of fast ion conductor, etc.; the chemical modification includes but is not limited to chemical group modification, element doping, etc.

[0019] It can be understood that the positive electrode lithium supplement agent commonly used in the industry is generally relatively active in chemical properties compared with the positive electrode active material, and is prone to side reactions with moisture in the air, etc. In the positive electrode plate provided in the embodiments of the present application, the lithium supplement layer with the lithium supplement agent is arranged between the current collector and the positive electrode active material layer, which can effectively reduce the risk of side reactions of the lithium supplement agent in the storage and subsequent assembly process of the positive electrode plate, thereby improving the utilization rate of the lithium supplement agent and more favorably improving the energy density of the final battery. In addition, the first positive electrode active material is also dispersed in the lithium supplement layer, which can avoid the problem of structural change or even collapse of the lithium supplement layer caused by the vacancies after the reaction of the lithium supplement agent in the related art, and can effectively improve the structural stability of the lithium supplement layer in the charge and discharge cycle process, especially favorably improving the peeling strength of the positive electrode plate in the long cycle process, thereby favorably maintaining the stability of the conductive network in the positive electrode plate and improving the electrochemical stability of the positive electrode plate. Therefore, the above-mentioned positive electrode plate can be used to provide a secondary battery with relatively high energy density and relatively optimal cycle performance.

[0020] In the embodiments of the present application, the mass percentage of the lithium supplement agent in the lithium supplement layer can be but is not limited to 3.5%, 3.8%, 4%, 4.2%, 4.5%, 5%, 6%, 7%, 8%, 9%, 10%. If the content of the lithium supplement agent is too high, in order to reduce the risk of lithium precipitation in the negative electrode, the N / P ratio of the final secondary battery needs to be correspondingly increased, which causes the first cycle efficiency of the secondary battery to decrease. In addition, in the subsequent cycle, the active lithium stored in the negative electrode is gradually consumed, which causes the N / P to further gradually increase, the negative electrode side is shallowly charged and discharged, and the positive electrode side is prone to overcharge, which causes safety risks. If the content of the lithium supplement agent is too low, the pre-stored lithium is insufficient, and the cycle performance is poor.

[0021] In some embodiments of the present application, the lithium supplement agent includes, but is not limited to, at least one of Li5FeO4, Li2NiO2, Li6CoO4, Li2MoO3, lithium-rich manganese-based compounds, Li3N, Li2O, Li2CO3, and LiF.

[0022] It can be understood that, in order to formability and structural stability of the positive electrode sheet, the lithium supplement layer and the positive active material layer further include a binder; that is, the lithium supplement layer further includes a first binder, and the positive active material layer further includes a second binder. In some specific embodiments, the mass percentage of the first binder in the lithium supplement layer is 1%-10%; in this way, the lithium supplement layer is easy to prepare and has good structural stability and strong interfacial adhesion with the positive active material layer. Specifically, the mass percentage of the first binder in the lithium supplement layer can be, but is not limited to, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%. In some specific embodiments, the mass percentage of the second binder in the positive active material layer can be, but is not limited to, 1.5%-10%. Specifically, the mass percentage of the second binder in the positive active material layer can be, but is not limited to, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%.

[0023] In the embodiments of the present application, the first binder and the second binder described above can be selected from any binder suitable for the positive electrode sheet known in the art. For example, the first binder and the second binder described above can be selected from at least one of modified or unmodified polyvinylidene fluoride and modified or unmodified polyvinylidene chloride. The first binder and the second binder can be the same or different.

[0024] It can be understood that, in order to ensure the electronic conductivity of the positive electrode sheet, the lithium supplement layer and the positive active material layer further include a conductive agent; specifically, the lithium supplement layer further includes a first conductive agent, and the positive active material layer further includes a second conductive agent.

[0025] In some embodiments of the present application, the areal density of the first conductive agent in the lithium supplement layer is greater than the areal density of the second conductive agent in the positive active material layer. The areal density of the first conductive agent = the mass of the first conductive agent / the area of the lithium supplement layer; the areal density of the second conductive agent = the mass of the second conductive agent / the area of the positive active material layer. In the embodiments of the present application, the first conductive agent and the second conductive agent can be selected from any conductive agent suitable for the positive electrode plate known in the art, including but not limited to at least one of conductive carbon black, acetylene black, graphite, graphene, carbon nanotubes and carbon nanofibers. The first conductive agent and the second conductive agent can be the same or different.

[0026] In some embodiments of the present application, the mass percentage of the first conductive agent in the lithium supplement layer is 3%-30%. A suitable first conductive agent percentage not only facilitates the construction of the conductive network in the lithium supplement layer, but also takes into account the high energy density of the final battery; when the content of the first conductive agent is within the above range, the higher the mass percentage of the first conductive agent, the better the electronic conductivity of the lithium supplement layer. Specifically, the mass percentage of the first conductive agent in the lithium supplement layer can be but is not limited to 3%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%. It should be noted that the above mass percentage is the mass percentage of the first conductive agent in the lithium supplement layer before the first charge and discharge of the positive electrode plate.

[0027] In some embodiments of the present application, the mass percentage of the first positive active material in the lithium supplement layer is 50%-90%. In this way, the lithium supplement layer can also provide a certain capacity during subsequent charge and discharge cycles, and is conducive to maintaining the structural stability of the lithium supplement layer during the charge and discharge cycles. Specifically, the mass percentage of the first positive active material in the lithium supplement layer can be but is not limited to 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%. In some embodiments of the present application, the mass percentage of the second positive active material in the positive active material layer is 80%-96%. Specifically, the mass percentage of the second positive active material in the lithium supplement layer can be but is not limited to 80%, 85%, 90%, 95%.

[0028] In some embodiments of the present application, the mass percentage of the second conductive agent in the positive active material layer is 2%-10%. In this way, it is conducive to the performance of the final battery. Specifically, the mass percentage of the second conductive agent in the positive active material layer can be but is not limited to 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%.

[0029] In consideration of the fact that, after the first charging, the decomposition of the lithium supplement agent and the volume shrinkage will form vacancies in the lithium supplement layer, which will increase the difference at the interface between the lithium supplement layer and the positive active material layer and thus aggravate the stress change inside the lithium supplement layer. In order to improve the above technical problems and further improve the structural stability of the positive electrode sheet, in some embodiments of the present application, the lithium supplement layer comprises a plurality of lithium supplement sub-layers arranged in layers, and the mass fraction of the lithium supplement agent in the plurality of lithium supplement sub-layers decreases layer by layer in the direction of the current collector pointing to the positive active material layer, and the mass fraction of the lithium supplement agent in each lithium supplement sub-layer is ≥ 3.5%. Specifically, the mass fraction of the lithium supplement agent in each lithium supplement sub-layer may, for example, be 3.5%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc. In some specific embodiments, in order to optimize the conductive network in the positive electrode sheet, so that the electrons are more easily transmitted from the surface of the positive electrode sheet to the surface of the current collector during the charging and discharging cycle, reduce the electrochemical polarization of the positive electrode sheet, and optimize the cycle performance of the battery, the mass fraction of the first conductive agent in the plurality of lithium supplement sub-layers increases layer by layer in the direction of the current collector pointing to the positive active material layer. In the embodiments of the present application, the mass fraction of the first positive active material in the direction of the current collector pointing to the positive active material layer may be constant or increase layer by layer.

[0030] For the convenience of description, the mass fraction of the lithium supplement agent in the i-th lithium supplement sub-layer is denoted as Ai, and Ai-Ai+1≤5%; wherein i is any positive integer in 1 to (n-1), and n≥2; please refer to FIG. 2, the lithium supplement sub-layer close to the current collector 10 is the first lithium supplement sub-layer 201, and the lithium supplement sub-layer close to the positive active material layer is the n-th lithium supplement sub-layer 20n. Suitable gradient setting can effectively reduce the stress difference inside the lithium supplement layer, thereby further improving the structural stability of the positive electrode sheet during the charging and discharging cycle, and further optimizing the cycle performance. Specifically, Ai-Ai+1 may, for example, be 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.8%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, 4.2%, 4.5%, 4.8%, 5.0%. In addition, it is also beneficial to build a more complete conductive network in the positive electrode sheet. Similarly, the mass fraction of the first conductive agent in the i-th lithium supplement sub-layer is denoted as Bi, and Bi+1-Bi≤3%. It should be noted that Bi is the mass fraction of the first conductive agent in the corresponding lithium supplement sub-layer before the first charging and discharging cycle of the positive electrode sheet. Specifically, Bi+1-Bi may, for example, be 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.8%, 3.0%.

[0031] In some embodiments of the present application, in order to further reduce the reaction difference at the interface between the positive electrode active material layer and the lithium supplement layer, the positive electrode active material layer comprises a plurality of positive electrode active material sub-layers arranged in layers, and the mass fraction of the second positive electrode active material in the plurality of positive electrode active material sub-layers increases layer by layer in the direction of the current collector pointing to the positive electrode active material layer, and the mass fraction of the second conductive agent decreases layer by layer.

[0032] Similarly, the mass fraction of the second positive electrode active material in the kth positive electrode active material sub-layer is denoted as Ck, and Ck+1-Ck≤5%; wherein k is any positive integer in 1 to (m-1), and m≥2; the positive electrode active material sub-layer close to the lithium supplement layer is the first positive electrode active material sub-layer 301, and the positive electrode active material sub-layer 30m as the outer surface of the positive electrode active material layer is the mth positive electrode active material sub-layer. Specifically, Ck+1-Ck may, for example, be 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.8%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, 4.2%, 4.5%, 4.8%, 5.0%. The mass fraction of the second conductive agent in the kth positive electrode active material sub-layer is denoted as Dk, and Dk-Dk+1≤2%. In this way, the performance of the positive electrode sheet can be better utilized. Specifically, Dk-Dk+1 may, for example, be 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%.

[0033] In some specific embodiments, in the first positive electrode active material sub-layer, the mass fraction of the second conductive agent is 2-5%, the mass fraction of the second active material is 85-96%, and the mass fraction of the second binder is 2-5%; in the nth lithium supplement sub-layer, the mass fraction of the first conductive agent is 5-30%, the mass fraction of the first active material is 50-80%, the mass fraction of the lithium supplement agent is 3.5-5%, and the mass fraction of the first binder is 2-5%. In this way, the interface reaction difference between the lithium supplement layer and the positive electrode active material layer during the charging and discharging cycle can be reduced, and the interface reaction difference between the two during the first charging and discharging process can be reduced.

[0034] In some embodiments of the present application, the number of lithium supplement sub-layers is 2-5, and the number of positive electrode active material sub-layers is 2-5. In this way, it is easy to prepare, and after the first cycle, the material components of each layer in the positive electrode sheet can also be more smoothly over to the target content, which is beneficial to build a more gentle internal stress change gradient in the positive electrode sheet after the first cycle, and optimize the cycle performance. Specifically, the number of lithium supplement sub-layers may, but not limited to, be 2, 3, 4, 5. The number of positive electrode active material sub-layers may, but not limited to, be 2, 3, 4, 5.

[0035] In the embodiments of the present application, the thickness of the lithium supplement sub-layer is not specifically limited, and those skilled in the art can confirm it according to the total thickness of the lithium supplement layer and the gradient construction. Similarly, the thickness of the positive active material sub-layer is also not specifically limited in the embodiments of the present application. When multiple lithium supplement sub-layers are contained, the thicknesses of the multiple lithium supplement sub-layers can be equal or not equal. When multiple positive active material sub-layers are contained, the thicknesses of the multiple positive active material sub-layers can be equal or not equal.

[0036] In some embodiments of the present application, the thickness of the lithium supplement layer is 1 μm-30 μm. The suitable thickness of the lithium supplement layer can provide a suitable amount of lithium supplement for the final battery, and the space occupied in the battery is also relatively suitable, which is beneficial to improve the cost performance of the energy density of the final battery. Specifically, the thickness of the lithium supplement layer can be, but is not limited to, 1 μm, 2 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, 28 μm, 30 μm. In some specific embodiments, the thickness of the lithium supplement layer is 10 μm-30 μm. In some specific embodiments, the thickness of the lithium supplement layer is 20 μm-30 μm.

[0037] In some embodiments of the present application, the thickness of the positive active material layer is 10 μm-80 μm. In this way, it is beneficial to the performance of the battery. Specifically, the thickness of the positive active material layer can be, but is not limited to, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm. In some specific embodiments, the thickness of the positive active material layer is 10 μm-40 μm. In some specific embodiments, the thickness of the positive active material layer is 20 μm-40 μm.

[0038] In some embodiments of the present application, the preparation of the positive electrode sheet comprises: S01, forming a lithium supplement layer and a positive active material layer arranged in layers on the surface of the current collector. Specifically, the process of forming the lithium supplement layer and the positive active material layer comprises coating, and the specific process of coating can be a process known to those skilled in the art.

[0039] In the embodiments of the present application, the current collector can be any current collector suitable for the positive electrode of a lithium ion battery, for example, an aluminum foil.

[0040] In some cases, a multi-layer coating process, for example, a double-layer coating process, can be used. Specifically, a coating device that can simultaneously realize double-layer coating is used to simultaneously form a lithium supplement layer and a positive active material layer on the surface of the current collector. In other cases, the lithium supplement layer and the positive active material layer can also be sequentially coated and formed on the surface of the current collector.

[0041] In the embodiments of the present application, the lithium supplement layer slurry and the positive active material layer slurry can be disposed on the current collector, and then dried, rolled and cut to obtain the positive electrode sheet. When the to-be-prepared positive electrode sheet includes multiple lithium supplement sub-layers and / or multiple positive active material sub-layers, the lithium supplement sub-layer slurries with different mass ratios of lithium supplement agent and different mass ratios of first conductive agent, and / or the positive active material sub-layer slurries of the positive active material sub-layers with different mass ratios of second positive active material and different mass ratios of second conductive agent, are prepared first. The mass content of each component in the multiple lithium supplement sub-layer slurries and the multiple positive active material sub-layer slurries is determined according to the to-be-prepared positive electrode sheet.

[0042] In some embodiments of the present application, the lithium supplement layer slurry includes but is not limited to a first solvent, a first binder, a first conductive agent, a lithium supplement agent and a first positive active material. The positive active material layer slurry includes but is not limited to a second solvent, a second binder, a second conductive agent and a second positive active material. The first solvent and the second solvent described above can be agents suitable for positive electrode slurries known in the art, including but not limited to N-methyl pyrrolidone (NMP).

[0043] The embodiments of the present application also provide a secondary battery including the aforementioned positive electrode sheet provided by the embodiments of the present application. Since the positive electrode sheet provided by the embodiments of the present application is used, the secondary battery can have both high energy density and excellent cycle performance.

[0044] In the embodiments of the present application, the secondary battery described above can be a liquid secondary battery using a liquid electrolyte, a solid-state secondary battery using a solid-state electrolyte, a semi-solid secondary battery using a gel-state electrolyte, or a semi-solid secondary battery including both a solid-state electrolyte and a liquid electrolyte.

[0045] The embodiments of the present application also provide an electric device including the secondary battery provided by the embodiments of the present application. Since the secondary battery provided by the embodiments of the present application is used to supply power, the electric device has strong endurance and strong market competitiveness.

[0046] In some embodiments of the present application, the electric device described above includes but is not limited to 3C electronic devices, power vehicles, energy storage systems, etc. The power vehicles include but are not limited to new energy vehicles, power-assisted bicycles, etc.

[0047] The technical solutions of the present application are further described in the following embodiments.

[0048] Embodiment 1

[0049] A positive electrode sheet includes a current collector (specifically, an aluminum foil) having a lithium supplement layer and a positive electrode active material layer stacked on opposite surfaces of the current collector, the lithium supplement layer being disposed close to the current collector. The lithium supplement layer has a thickness of 25 μm, and the positive electrode active material layer has a thickness of 45 μm. The lithium supplement layer is composed of 6% lithium supplement (specifically, Li5FeO4), 2.5% binder (specifically, polyvinylidene fluoride), 10% conductive agent (specifically, conductive carbon black), and 81.5% positive electrode active material (specifically, lithium iron phosphate) by mass. The positive electrode active material layer is composed of 95.5% positive electrode active material (specifically, lithium iron phosphate), 2% conductive agent (specifically, conductive carbon black), and 2.5% binder (specifically, polyvinylidene fluoride) by mass.

[0050] Example 2

[0051] A positive electrode sheet includes a current collector (specifically, an aluminum foil) having a lithium supplement layer and a positive electrode active material layer stacked on opposite surfaces of the current collector, the lithium supplement layer being disposed close to the current collector. The lithium supplement layer has a thickness of 25 μm, and the positive electrode active material layer has a thickness of 45 μm. The lithium supplement layer is composed of 6% lithium supplement (specifically, Li5FeO4), 2.5% binder (specifically, polyvinylidene fluoride), 10% conductive agent (specifically, conductive carbon black), and 81.5% positive electrode active material (specifically, lithium iron phosphate) by mass. The positive electrode active material layer is composed of 95.5% positive electrode active material (specifically, lithium iron phosphate), 2% conductive agent (specifically, conductive carbon black), and 2.5% binder (specifically, polyvinylidene fluoride) by mass.

[0052] Example 3

[0053] The difference from Example 1 is that the lithium supplement is Li2NiO2.

[0054] Example 4

[0055] The difference from Example 1 is that the lithium supplement layer and the positive electrode active material layer are both replaced with lithium manganese oxide instead of lithium iron phosphate.

[0056] Example 5

[0057] The difference from Example 1 is that the lithium supplement layer has a thickness of 10 μm, and the positive electrode active material layer has a thickness of 30 μm.

[0058] Example 6

[0059] The difference from Example 1 is that the lithium supplement layer is composed of 10% lithium supplement (specifically, Li5FeO4), 2.5% binder (specifically, polyvinylidene fluoride), 10% conductive agent (specifically, conductive carbon black), and 77.5% positive electrode active material (specifically, lithium iron phosphate) by mass.

[0060] Example 7

[0061] The difference from Example 1 is that the lithium supplement layer is composed of a first lithium supplement sub-layer, a second lithium supplement sub-layer and a third lithium supplement sub-layer which are sequentially stacked, the three lithium supplement sub-layers have the same thickness, the total thickness of the lithium supplement layer is 25 μm; and the first lithium supplement sub-layer is arranged close to the current collector. The first lithium supplement sub-layer is composed of 8% lithium supplement agent (specifically Li5FeO4) by mass content, 2.5% binder (specifically polyvinylidene fluoride) by mass content, 10% conductive agent (specifically conductive carbon black) by mass content and 79.5% positive electrode active material (specifically lithium iron phosphate) by mass content; the second lithium supplement sub-layer is composed of 6% lithium supplement agent (specifically Li5FeO4) by mass content, 2.5% binder (specifically polyvinylidene fluoride) by mass content, 10% conductive agent (specifically conductive carbon black) by mass content and 81.5% positive electrode active material (specifically lithium iron phosphate) by mass content; and the third lithium supplement sub-layer is composed of 4% lithium supplement agent (specifically Li5FeO4) by mass content, 2.5% binder (specifically polyvinylidene fluoride) by mass content, 10% conductive agent (specifically conductive carbon black) by mass content and 83.5% positive electrode active material (specifically lithium iron phosphate) by mass content.

[0062] Example 8

[0063] The difference from Example 7 is that the positive electrode active material layer includes a first positive electrode active material sub-layer and a second positive electrode active material sub-layer which are sequentially stacked, the two positive electrode active material sub-layers have the same thickness, the total thickness of the positive electrode active material layer is 45 μm; and the first positive electrode active material sub-layer is arranged close to the lithium supplement layer. The first positive electrode active material sub-layer is composed of 90% positive electrode active material (specifically lithium iron phosphate) by mass content, 7% conductive agent (specifically conductive carbon black) by mass content and 3% binder (specifically polyvinylidene fluoride) by mass content. The second positive electrode active material sub-layer is composed of 95% positive electrode active material (specifically lithium iron phosphate) by mass content, 2% conductive agent (specifically conductive carbon black) by mass content and 3% binder (specifically polyvinylidene fluoride) by mass content.

[0064] In order to highlight the beneficial effects of the embodiments of the present application, the following comparative examples are provided.

[0065] Comparative Example 1

[0066] A positive electrode sheet includes a current collector (specifically, an aluminum foil) having a lithium supplement layer and a positive electrode active material layer stacked on opposite surfaces of the current collector, respectively, the lithium supplement layer being disposed close to the current collector. The lithium supplement layer has a thickness of 25 μm, and the positive electrode active material layer has a thickness of 45 μm. The lithium supplement layer is composed of 1.5% by mass of a lithium supplement agent (specifically, Li5FeO4), 2.5% by mass of a binder (specifically, polyvinylidene fluoride), 10% by mass of a conductive agent (specifically, conductive carbon black), and 86% by mass of a positive electrode active material (specifically, lithium iron phosphate). The positive electrode active material layer is composed of 95.5% by mass of a positive electrode active material (specifically, lithium iron phosphate), 2% by mass of a conductive agent (specifically, conductive carbon black), and 2.5% by mass of a binder (specifically, polyvinylidene fluoride).

[0067] Comparative Example 2

[0068] A positive electrode sheet includes a current collector (specifically, an aluminum foil) and a positive electrode active material layer disposed on opposite surfaces of the current collector. The positive electrode active material is composed of 1.5% by mass of a lithium supplement agent (specifically, Li5FeO4), 2.5% by mass of a binder (specifically, polyvinylidene fluoride), 2% by mass of a conductive agent (specifically, conductive carbon black), and 94% by mass of a positive electrode active material (specifically, lithium iron phosphate). The positive electrode active material layer has a thickness of 70 μm.

[0069] Comparative Example 3

[0070] The difference from Comparative Example 1 is that the positive electrode active material is composed of 2.5% by mass of a binder (specifically, polyvinylidene fluoride), 2% by mass of a conductive agent (specifically, conductive carbon black), and 95.5% by mass of a positive electrode active material (specifically, lithium iron phosphate).

[0071] Comparative Example 4

[0072] The difference from Comparative Example 1 is that the lithium supplement layer and the positive electrode active material in the positive electrode active material layer are both replaced from lithium iron phosphate to lithium manganate.

[0073] Performance Test

[0074] (1) High-temperature cycle test

[0075] The positive electrode sheet of each of the above examples and comparative examples was assembled with a negative electrode sheet to form a test battery, wherein the negative electrode sheet included a current collector (specifically, a copper foil) and a negative electrode material layer disposed on the surface of the current collector, the negative electrode material layer was composed of graphite, a conductive agent, and a binder at a mass ratio of 95:3:2, and the electrolyte was a lithium hexafluorophosphate organic solution with a concentration of 1.0 mol / L, wherein the solvent was EC:DMC:EMC at a mass ratio of 1:1:1. After formation, a plurality of test batteries of each of the examples and comparative examples were obtained. The formation process was as follows: 0.1C charging for 3h, standing for 10 min, and 0.2C charging to 3.8V.

[0076] (2) Normal temperature cycle performance test: the test batteries of each of the examples and comparative examples were subjected to charge-discharge cycles at 45±2°C: constant current charging to 3.8V at a current density of 1.0C, standing for 5min, constant current discharging to 2.0V at a current density of 1.0C, standing for 5min; the above was one cycle, and the cycle was repeated for 500 times. The capacity retention rate of the test battery after the 100th cycle and the 500th cycle was recorded, and the results are shown in Table 1.

[0077] (3) Peeling strength test of the positive electrode sheet

[0078] The positive electrode sheet before assembly, the positive electrode sheet removed from the test battery after 10 cycles, and the positive electrode sheet removed from the test battery after 100 cycles were cut into samples with a length of 400mm and a width of 10mm;

[0079] The above samples were pasted on the surface of a steel plate with a length of 200mm and a width of 40mm using double-sided tape, and the positive electrode active material layer was attached to the surface of the steel plate to obtain test samples;

[0080] The above test samples were fixed on a universal tensile testing machine, and the peeling strength of each positive electrode sheet was tested at a peeling speed of 50mm / min. The results are shown in Table 2.

[0081] (4) Waterproof performance test of the positive electrode sheet

[0082] The positive electrode plate of the examples and the comparative examples was stored in an environment with a relative humidity of 20% and a temperature of 25°C for a period of time, and then the positive electrode plate was assembled into a test battery. The cycle capacity retention rate of the test battery after the test positive electrode plate was stored in the above environment for different time was tested. The assembly method of the test battery is described in the high-temperature cycle test. The cycle test was performed on the test battery of each example and comparative example at 25±2°C: constant current charging to 3.8V at a current density of 1.0C, standing for 5min, then constant current discharging to 2.0V at a current density of 1.0C, standing for 5min; the above is one cycle, and the cycle is 500 times. The capacity retention rate of the test battery after the 200th cycle and the 500th cycle was recorded respectively, and the results are shown in Table 3.

[0083] Table 1

[0084] Table 2

[0085] Table 3

[0086] From the data in Tables 1 to 3, it can be seen that the battery using the positive electrode plate provided by the examples of the present application has a better high-temperature cycle and room-temperature cycle retention rate, which indicates that the lithium supplement in the positive electrode plate provided by the examples of the present application does not affect or does not significantly affect the structural stability of the positive electrode plate after decomposition during the first charge and discharge process, fully demonstrating the effect of the positive electrode plate of the examples of the present application. Further, from the data in Table 3, it can be seen that the water resistance of the positive electrode plate provided by the examples of the present application is better, which indicates that the special structural design of the positive electrode plate of the examples of the present application can improve the structural stability of the positive electrode plate during storage and assembly. Specifically, by comparing the data of Example 1 and Comparative Example 1, it can be found that if the proportion of the lithium supplement in the lithium supplement layer is too low, it will significantly affect the high-temperature cycle performance and room-temperature cycle performance of the battery. Further, by comparing the data of Example 2 and Example 7, it can be found that when the lithium supplement layer includes a plurality of lithium supplement sub-layers in accordance with the further suggestions of the present application, it is more conducive to the performance of the battery; by comparing the data of Example 7 and Example 8, it can be seen that when the positive active material layer includes a plurality of positive active material sub-layers in accordance with the further suggestions of the examples of the present application, the comprehensive performance of the battery is better.

[0087] The above is an exemplary embodiment of the present application. It should be noted that those skilled in the art can make some improvements and refinements without departing from the principles of the present application, and these improvements and refinements are also considered within the protection scope of the present application.

Claims

1. A positive electrode (1), characterized in that, It includes a current collector (10) and a lithium replenishment layer (20) and a positive electrode active material layer (30) stacked on at least one side surface of the current collector (10), wherein the lithium replenishment layer (20) is disposed close to the current collector (10) and the positive electrode active material layer (30) is disposed on the side surface of the lithium replenishment layer (20) away from the current collector (10); The lithium replenishing layer (20) includes a lithium replenishing agent and a first positive electrode active material, the positive electrode active material layer (30) includes a second positive electrode active material, and the positive electrode active material layer (30) does not contain the lithium replenishing agent; the mass of the lithium replenishing agent accounts for 3.5%-10% of the total mass of the lithium replenishing layer (20).

2. The positive electrode sheet (1) according to claim 1, characterized in that, The lithium replenishment layer (20) further includes a first conductive agent, and the positive electrode active material layer (30) further includes a second conductive agent; the first conductive agent accounts for 3%-30% of the mass of the lithium replenishment layer (20), and the second conductive agent accounts for 2%-10% of the mass of the positive electrode active material layer (30).

3. The positive electrode sheet (1) according to claim 1 or 2, characterized in that, The lithium replenishment layer (20) includes a plurality of lithium replenishment sub-layers stacked together; along the direction from the current collector (10) to the positive electrode active material layer (30), the mass percentage of the lithium replenishing agent in the plurality of lithium replenishment sub-layers decreases layer by layer, and the mass percentage of the first conductive agent increases layer by layer; and, in each lithium replenishment sub-layer, the mass percentage of the lithium replenishing agent is ≥3.5%; And / or, the positive electrode active material layer (30) includes a plurality of positive electrode active material sublayers stacked together. Along the direction from the current collector (10) to the positive electrode active material layer (30), the mass percentage of the second positive electrode active material in the plurality of positive electrode active material sublayers increases layer by layer, and the mass percentage of the second conductive agent decreases layer by layer.

4. The positive electrode sheet (1) according to claim 3, characterized in that, The difference in the mass percentage of the lithium replenishing agent between any two adjacent lithium replenishing sublayers is ≤5%; the difference in the mass percentage of the second positive electrode active material between any two adjacent positive electrode active material sublayers is ≤5%.

5. The positive electrode sheet (1) according to claim 4, characterized in that, The number of lithium-supplementing sublayers is 2-5; the number of positive electrode active material sublayers is 2-5.

6. The positive electrode sheet (1) according to any one of claims 1-5, characterized in that, The thickness of the lithium replenishment layer (20) is 1μm-30μm; the thickness of the positive electrode active material layer (30) is 10μm-80μm.

7. The positive electrode sheet (1) according to any one of claims 1-6, characterized in that, The areal density of the first conductive agent is greater than that of the second conductive agent.

8. The positive electrode sheet (1) according to any one of claims 1-7, characterized in that, The first positive electrode active material accounts for 50%-90% of the mass of the lithium replenishment layer (20); the second positive electrode active material accounts for 80%-96% of the mass of the positive electrode active material layer (30).

9. The positive electrode sheet (1) according to any one of claims 1-8, characterized in that, The lithium supplement includes at least one of Li5FeO4, Li2NiO2, Li6CoO4, Li2MoO3, lithium-rich manganese-based compounds, Li3N, Li2O, Li2CO3, and LiF.

10. The positive electrode sheet (1) according to any one of claims 1-9, characterized in that, The lithium replenishment layer further includes a first binder, and the positive electrode active material layer further includes a second binder. The mass percentage of the first binder in the lithium replenishment layer is 1%-10%, and the mass percentage of the second binder in the positive electrode active material layer is 1.5%-10%.

11. The positive electrode sheet (1) according to claim 10, characterized in that, The first adhesive and the second adhesive may be selected from at least one of modified or unmodified polyvinylidene fluoride and modified or unmodified polyvinylidene chloride.

12. The positive electrode sheet (1) according to any one of claims 1-9, characterized in that, The first positive electrode active material and the second positive electrode active material include one or more of the following: modified or unmodified lithium iron phosphate, modified or unmodified lithium manganese iron phosphate, modified or unmodified lithium vanadium phosphate, modified or unmodified lithium cobalt phosphate, modified or unmodified lithium cobalt oxide, modified or unmodified lithium manganese oxide, modified or unmodified lithium nickel oxide, modified or unmodified lithium nickel manganese oxide, modified or unmodified lithium nickel manganese cobalt oxide, and modified or unmodified lithium nickel manganese aluminum oxide.

13. A secondary battery, characterized in that, Includes the positive electrode sheet (1) as described in any one of claims 1-12.

14. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 13.

Images