Lithium secondary battery and electric device

Abstract

A lithium secondary battery and an electric device. The lithium secondary battery comprises a positive electrode sheet, the positive electrode sheet comprises a first active layer coated on at least one side of a positive electrode current collector and containing a positive electrode active material and a polycrystalline lithium supplementing agent and a second active layer coated on the side of the first active layer away from the current collector and containing a positive electrode active material, and the lithium supplementing agent comprises at least one of LixNi1-yMyO2, LixRO2, and LiNiαCoβMnγO2, wherein M comprises one or more of Cu, Al, and Mn; R comprises one of Ni and Mn; 1<x<2, 0<y≤0.01, α+β+γ=1 and α≥0.5. The lithium supplementing agent rapidly releases sufficient active lithium during initial charge, thereby improving the energy density of a secondary battery, and persistently releases active lithium during a cycle, thereby slowing down the attenuation of a battery cell.

Description

Lithium Secondary Battery and Electrical Device

[0001] Cross - reference

[0002] This application claims the priority of Chinese Patent Application No. 202311075173.5, titled "Lithium Secondary Battery and Electrical Device", filed on August 24, 2023, which is hereby incorporated by reference in its entirety into this application. Technical Field

[0003] This application relates to the technical field of lithium batteries, and particularly to a lithium secondary battery and an electrical device. Background Art

[0004] Due to the advantages of high discharge voltage, high energy density, and long cycle life of lithium - ion batteries, they are now widely used in various small portable electronic devices, such as laptop computers, mobile phones, and cameras, etc., and are also gradually favored in high - tech fields, such as aerospace satellites, electric vehicles, and military applications, etc. In these applications, lithium - ion batteries are required to have better capacity and cycle life.

[0005] The active lithium in lithium - ion batteries comes from the positive electrode material. During the first charge - discharge cycle of a lithium - ion battery, the surface of the negative electrode of the lithium - ion battery consumes active lithium to form a solid electrolyte interface (SEI) film, resulting in an initial charge - discharge capacity loss (Initial Capacity Loss, ICL), leading to capacity attenuation and reduced cycle efficiency of the lithium - ion battery. Therefore, it is of great significance to develop simple and efficient lithium - supplementing technologies.

[0006] Summary of the Invention

[0007] This application is made in view of the above problems, and its purpose is to provide a positive electrode plate, which includes a first active layer coated on at least one side of a positive electrode current collector and containing a positive electrode active material and a lithium - supplementing agent in polycrystalline form, and a second active layer coated on the side of the first active layer away from the current collector and containing a positive electrode active material. The lithium - supplementing agent includes at least one of LixNi1 - yMyO2, LixRO2, LiNiαCoβMnγO2, where

[0008] M includes one or more of Cu, Al, Mn; R includes one of Ni and Mn;

[0009] 1 < x < 2, 0 < y ≤ 0.01, α + β + γ = 1 and α ≥ 0.5.

[0010] During the cycling process of the secondary battery, the lithium supplement agent with a polycrystalline morphology in the first active layer has faster attenuation due to a large concentration polarization during the first charge, quickly releasing sufficient active lithium to compensate for the active lithium consumed on the surface of the negative electrode during the first charge-discharge cycle, improving the first-cycle specific capacity of the secondary battery, and thus increasing the energy density of the secondary battery; the lithium removed from the lithium supplement agent cannot be intercalated back into the lithium supplement agent, and the positive electrode active material of the second active layer accepts the active lithium removed from the lithium supplement agent. Therefore, the lithium supplement agent continuously releases all of its lithium content during the cycling process, significantly improving the cycling performance of the secondary battery, thereby slowing down the attenuation of the battery cell.

[0011] In any embodiment, the lithium supplement agent includes one or more of LixNiO2, LixMnO2, LixNi1-yMyO2, LiNiαCoβMnγO2, where M includes one or more of Cu, Al, Mn; R includes one of Ni and Mn. Optionally, 1.1 < x < 1.8, 0 < y ≤ 0.01, α + β + γ = 1 and α ≥ 0.5, 0 < β < 1, 0 < γ < 1.

[0012] In any embodiment, the volume average particle size Dv50 of the lithium supplement agent is 2.5 to 12.5 μm, and optionally 3.5 to 11.5 μm. The Dv50 of the lithium supplement agent within this range helps to control the amount of de-lithiation and residual amount of the lithium supplement agent during the first charge-discharge cycle, enabling the positive electrode sheet to achieve a balance between the initial specific capacity and the late-stage slow-release lithium amount. At the same time, the Dv50 of the lithium supplement agent within this range helps to reduce the gas generation performance of the secondary battery and improve the storage performance of the secondary battery.

[0013] In any embodiment, the mass content p / q of the lithium supplement agent in the first active layer is not less than 0.5% and not more than 12%, and optionally not less than 2% and not more than 10%; where p is the ratio of the mass of the lithium supplement agent in the first active layer to the total mass of the first and second active layers of the positive electrode sheet; q is the ratio of the mass of the first active layer to the total mass of the first and second active layers of the positive electrode sheet. The mass content p / q of the lithium supplement agent in the first active layer within a suitable range helps to improve the capacity retention rate or the first-cycle specific capacity of the secondary battery.

[0014] In any embodiment, based on the total mass of the first and second active layers of the positive electrode sheet, the mass content p of the lithium supplement agent in the first active layer is not more than 5% and not less than 0.2%, and optionally 0.8% to 4%. The lithium supplement agent within this dosage range can provide a sufficient amount of active lithium, while slowing down the attenuation rate of the secondary battery cell and improving the cycling performance of the secondary battery, it can also reduce the safety hazards caused by the side reactions of the lithium supplement agent.

[0015] In any embodiment, the ratio q of the mass of the first active layer to the total mass of the first active layer and the second active layer of the positive electrode sheet is 10%-70%, and optionally 20%-60%. A suitable range of the ratio q of the mass of the first active layer to the total mass of the first active layer and the second active layer of the positive electrode sheet helps improve the first cycle gram capacity of the secondary battery, thereby increasing the energy density.

[0016] In any embodiment, the positive electrode active material in the first active layer and the second active layer includes one or more of olivine-structured lithium-containing phosphates, lithium transition metal oxides and their respective modified compounds, and can be optionally one or more of lithium iron phosphate and its modified compounds.

[0017] In any embodiment, the positive electrode active material in the first active layer and the second active layer includes LizFe 1-k Me k PO4 and its coated modified compounds, wherein Me is selected from one or more of Sc, V, Cr, Mn, Ti, Al, Co, Ni, Cu, and Zn, 0.8≤z≤1.3, and 0≤k≤0.7.

[0018] Lithium iron phosphate and its modified compounds have good stability, and secondary batteries using them as positive electrode active materials can have a longer cycle life.

[0019] In any embodiment, the cycle voltage window of the secondary battery prepared by the positive electrode sheet is 2.0 to 3.8 V or 2.0 to 4.1 V. The voltage window in the above range can be used for the positive electrode active material LizFe 1-k Me k Secondary batteries containing PO4 (0.8≤z≤1.3, 0≤k≤0.7) and its coated modified compounds reduce the release of lithium replenisher during the first cycle while meeting the cycle requirements of the secondary battery, which helps to slowly release the lithium replenisher during the secondary battery cycle; it also helps to avoid high-voltage decomposition of the electrolyte.

[0020] A second aspect of the present application provides a lithium secondary battery, which includes the positive electrode sheet provided in the first aspect.

[0021] In any embodiment, the cycle voltage window of the secondary battery prepared by the positive electrode sheet is 2.0 to 4.1 V. In any embodiment, the cycle voltage window of the secondary battery prepared by the positive electrode sheet is 2.0 to 3.8 V. In any embodiment, the upper limit voltage of the cycle voltage of the secondary battery prepared by the positive electrode sheet is 3.6 to 3.8 V, optionally 3.65 to 3.8 V. The upper limit voltage within the above range can make the lithium replenisher release the lithium replenisher at the beginning of the battery cycle and slowly release the lithium replenisher during the cycle, thereby improving the battery attenuation rate.

[0022] The third aspect of the present application provides an application of the lithium secondary battery of the second aspect.

[0023] In any embodiment, the cycle voltage window of the lithium secondary battery is 2.0 to 4.1 V. In any embodiment, the cycle voltage window of the lithium secondary battery is 2.0 to 3.8 V, or the upper voltage limit is 3.6 to 3.8 V, optionally 3.65 to 3.8 V. The voltage window within this range can avoid the rapid release of the lithium supplement and the high-voltage decomposition of the electrolyte, thereby slowing down the degradation of the secondary battery and improving the stability of the secondary battery electrolyte, thereby improving the cycle life of the secondary battery.

[0024] A fourth aspect of the present application provides an electrical device comprising the lithium secondary battery described in the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG1 is a schematic diagram of the mechanism of sustained release of active lithium by a lithium supplement agent in a prepared secondary battery;

[0026] Figure 2 shows the difference between the amount of lithium replenished and the amount of lithium removed in a lithium half-cell prepared using a lithium replenisher;

[0027] FIG3 is a battery capacity retention measurement result of the secondary batteries prepared in Example 1 and Comparative Example 3;

[0028] FIG4 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0029] FIG5 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG4 ;

[0030] FIG6 is a schematic diagram of a battery module according to an embodiment of the present application;

[0031] FIG7 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0032] FIG8 is an exploded view of the battery pack shown in FIG7 according to an embodiment of the present application;

[0033] 9 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present application;

[0034] FIG10 is a scanning electron microscope image of the lithium supplement prepared in Example 1 of the present application;

[0035] FIG11 is a scanning electron microscope image of the lithium supplement prepared in Comparative Example 2 of the present application.

[0036] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0037] Below, the embodiments of the positive electrode active material and its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack and electrical device of the present application are specifically disclosed in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0038] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0039] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0040] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0041] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0042] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0043] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0044] A lithium secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions (such as lithium ions) are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0045] During the first charge and discharge cycle of a lithium-ion battery, the surface of the negative electrode consumes active lithium to form an SEI film, resulting in a loss of initial charge and discharge capacity, leading to a decrease in the energy density of the lithium-ion battery, which in turn reduces the cycle capacity of the secondary battery, affecting the degradation and cycle efficiency of the secondary battery. The present application provides a positive electrode plate, which includes a lithium replenisher. The lithium replenisher replenishes active lithium to the positive electrode, thereby reducing the irreversible loss of active lithium in the secondary battery prepared from the positive electrode plate, thereby reducing the initial charge and discharge capacity loss of the secondary battery.

[0046] In some embodiments, the positive electrode plate includes a first active layer coated on at least one side of a positive current collector and a second active layer coated on a side of the first active layer away from the current collector. The first active layer comprises a positive electrode active material and a polycrystalline lithium supplement; the second active layer comprises a positive electrode active material.

[0047] In this application, the term "lithium replenisher" refers to a metal oxide containing lithium that can release active lithium and replenish active lithium in the positive electrode active material during the cycling of a secondary battery. During cycling, a lithium secondary battery containing such a positive electrode plate experiences greater concentration polarization in the lithium replenisher in the first active layer, resulting in a more rapid attenuation. Lithium released from the lithium replenisher cannot be re-intercalated into the lithium replenisher, allowing the lithium replenisher to continuously release all of its lithium. The positive electrode active material in the second active layer accepts the active lithium released from the lithium replenisher in the first active layer, slowing the decay rate of the battery cell and improving the cycling performance of the lithium-ion battery.

[0048] The lithium supplement in the first active layer generates a partial pore structure due to volume changes during the cycle, which is also beneficial to improving the dynamics of the positive electrode.

[0049] In this application, polycrystalline refers to secondary particles formed by agglomeration of multiple primary particles, and the polycrystalline form can be understood as a morphology similar to that of the secondary particles. The polycrystalline lithium supplement includes secondary particles formed by agglomeration, which are usually prepared by secondary particle agglomeration technology (for example, sintering). In the polycrystalline lithium supplement, the particle size distribution of the secondary particles in the lithium supplement powder is advantageous. Those skilled in the art can judge whether the lithium supplement is in a polycrystalline form based on common knowledge and ordinary technical knowledge in this field. In one example, the lithium supplement powder appears as agglomerated particles or substantially all as agglomerated particles under a scanning electron microscope.

[0050] Single crystal refers to a crystal of a primary particle, typically a single, tiny grain. Single crystal morphology can be understood as a morphology similar to that of a primary particle. A lithium supplement with a single crystal morphology contains primary particle crystals of the lithium supplement. In a lithium supplement with a single crystal morphology, the primary particles have a dominant particle size distribution within the lithium supplement powder. A person skilled in the art can determine whether a lithium supplement is in a single crystal morphology based on common knowledge and ordinary technical expertise in the field. In one example, the lithium supplement powder appears essentially entirely as primary particles under a scanning electron microscope.

[0051] In this application, primary particles are also referred to as primary particles, and aggregated particles formed by agglomeration of multiple primary particles or primary particles are referred to as "secondary particles." The forces that cause particles to agglomerate can be physical bonds (e.g., van der Waals forces) or chemical bonds (e.g., bridging oxygen bonds). Agglomeration can be achieved through external pressure or by the aid of additional substances (e.g., chemicals with adhesive properties).

[0052] Compared with single-crystal lithium replenishers, polycrystalline lithium replenishers have poor stability during battery cycling and are more prone to structural collapse, making it easy for active lithium to escape and difficult to reinsert. They can quickly replenish lithium during the first cycle and release lithium for a long time during subsequent cycles.

[0053] In some embodiments, the lithium supplement includes at least one of LixNi1-yMyO2, LixRO2, and LiNiαCoβMnγO2, where M includes one or more of Cu, Al, and Mn; R includes one of Ni and Mn;

[0054] 1 < x < 2, 0 < y ≤ 0.01, α + β + γ = 1 and α ≥ 0.5.

[0055] In some embodiments, the lithium supplement includes one or more of LixNiO2, LixMnO2, LixNi1-yMyO2, and LiNiαCoβMnγO2, where M includes one or more of Cu, Al, and Mn; R includes one of Ni and Mn; 1.1 < x < 1.8, 0 < y ≤ 0.01, α + β + γ = 1 and α ≥ 0.5, 0 < β < 1, 0 < γ < 1.

[0056] In some embodiments, the lithium supplement includes one or more of LixNiO2 (1 < x < 2). In some embodiments, 1.1 < x < 1.8, 1.5 < x < 1.8, 1.5 < x < 2, 1.1 < x < 1.5. In some embodiments, the lithium supplement can be Li 1.1 NiO2, Li 1.3 NiO2, Li 1.5 NiO2, Li 1.6 NiO2, Li 1.7 NiO2, Li 1.9 NiO2, one or more of Li

[0057] In some embodiments, the lithium supplement includes one or more of LixMnO2 (1 < x < 2). In some embodiments, 1.1 < x < 1.8, 1.5 < x < 1.8, 1.5 < x < 2, 1.1 < x < 1.5. In some embodiments, the lithium supplement can be Li 1.8 MnO2, Li 1.5 MnO2, Li 1.3 MnO2, Li 1.1 MnO2, Li 1.7 MnO2, Li 1.6 MnO2, one or more of Li

[0058] In some embodiments, the lithium supplement includes one or more of LixNi1-yMyO2, 1 < x < 2, 0 < y ≤ 0.01. In some embodiments, 1.1 < x < 1.8, 1.5 < x < 1.8, 1.5 < x < 2, 1.1 < x < 1.5. In some embodiments, the lithium supplement can be Li1.8 Ni 0.99 Cu 0.01 O2、Li 1.5 Ni 0.99 Cu 0.01 O2、Li 1.1 Ni 0.99 Cu 0.01 O2、Li 1.8 Ni 0.999 Cu 0.001 O2、Li 1.8 Ni 0.995 Cu 0.005 O2、Li 1.7 Ni 0.99 Cu 0.01 O2、Li 1.6 Ni 0.99 Cu 0.01 O2、Li 1.3 Ni 0.99 Cu 0.01 O2、Li 1.8 Ni 0.99 Al 0.01 O2、Li 1.5 Ni 0.99 Al 0.01 O2、Li 1.1 Ni 0.99 Al 0.01 O2、Li 1.8 Ni 0.999 Al 0.001 O2、Li 1.8 Ni 0.995 Al 0.005 O2、Li 1.7 Ni 0.99 Al 0.01 O2、Li 1.6 Ni 0.99 Al 0.01 O2、Li 1.3 Ni 0.99 Al 0.01 O2、Li 1.8 Ni 0.99 Mr 0.01 O2、Li 1.5 Ni 0.99 Mr 0.01 O2、Li 1.1 Ni 0.99 Mr 0.01 O2、Li 1.8 Ni 0.999 Mr 0.001 O2、Li 1.8 Ni 0.995 Mr 0.005O2、Li 1.7 Ni 0.99 Mn 0.01 O2、Li 1.6 Ni 0.99 Mn 0.01 O2、Li 1.3 Ni 0.99 Mn 0.01 O2.

[0059] In some embodiments, the lithium supplement comprises one or more of LiNiαCoβMnγO2, α+β+γ=1 and α≥0.5, 0<β<1, 0<γ<1. In some embodiments, the ratio of α:β:γ in LiNiαCoβMnγO2 is 5:2:3, 6:2:2 or 8:1:1. In some embodiments, the lithium supplement comprises LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523),LiNi 0.8 Co 0.1 Mn 0.1 One or more of O2 (NCM811).

[0060] In the lithium supplement agent of the present application, the number of oxygen atoms "2" in the chemical formula of the lithium supplement agent is not necessarily an integer of 2, taking into account possible crystal defects in the lithium supplement agent crystals. The number of oxygen atoms may vary to a degree acceptable to those skilled in the art. The number of oxygen atoms in the crystals can be measured or calculated using methods known in the art. Similarly, the atomic number of other atoms is not necessarily an integer.

[0061] The lithium supplement provided in the present application is not only simple in preparation process and relatively low in synthesis cost compared to lithium supplements with a lithium atom number x greater than 2, which helps to reduce the cost of secondary batteries, but also helps to reduce the residual alkaline substances (e.g., lithium hydroxide) in the lithium supplement (also known as "residual alkali"), improve the gelation phenomenon of the positive electrode slurry and reduce battery gas production, thereby reducing the deterioration of battery cell performance caused by gas production of lithium supplements with high lithium content.

[0062] In some embodiments, the release performance of the lithium supplement can be determined by any method known in the art. In some embodiments, the release performance can be determined by the following method:

[0063] A lithium replenisher: PVDF: SP is mixed in a weight ratio of 8:1:1, and a solvent N-methylpyrrolidone is added to the mixture to prepare a slurry. After thorough stirring, a coating machine is used to make a pole piece. The finished pole piece is dried, cold pressed, and cut into pieces to make a pole piece assembly for a lithium half-cell. A 0.1C test is used at 2.0-3.75V and a temperature of 25°C. The lithium deintercalation capacity of each cycle is recorded, and the obtained capacity is divided by the weight of the lithium replenisher to obtain the lithium deintercalation gram capacity of the lithium replenisher. The lithium deintercalation gram capacity of each cycle is subtracted from the lithium intercalation gram capacity, and the difference of each cycle is gradually increased with the number of cycles to obtain the limit lithium replenishment amount of the lithium replenisher for long-term lithium replenishment in the cycle.

[0064] Figure 1 of the present application shows a theoretical schematic diagram of the lithium supplement agent LixNiO2 (x=1.8), and Figure 2 shows the change curve of the number of cycles and gram capacity obtained by the lithium supplement agent release performance measurement method. It can be found that the lithium supplement agent continuously releases active lithium during multiple cycles, indicating that the polycrystalline lithium supplement agent added in the present application can continuously release active lithium in the secondary battery. In some embodiments, the lithium supplement agent of the present application can provide a lithium removal amount of not less than 400mAh / g. In some embodiments, the lithium supplement agent of the present application can provide a lithium removal amount of not less than 450, 455, 460, 465, 470 or 475mAh / g.

[0065] Therefore, the polycrystalline lithium supplement provided in this application can decay rapidly during the initial charge process, quickly releasing sufficient active lithium to compensate for the active lithium consumed on the negative electrode surface during the initial charge-discharge cycle, thereby improving the initial charge-discharge capacity and energy density of the secondary battery. The polycrystalline lithium supplement can also slowly release active lithium during the cycle, significantly improving the battery's cycle performance and reducing cell decay. The polycrystalline lithium supplement can be a commercially available product or prepared according to methods known or commonly used in the art.

[0066] In some embodiments, the volume average particle size Dv50 of the lithium supplement agent is 2.5 to 12.5 μm. In some embodiments, the volume average particle size Dv50 of the lithium supplement agent is 2.5 to 11.5 μm, 3 to 12 μm, 3.5 to 11.5 μm, 4 to 12 μm, 5 to 12 μm, 6 to 12 μm, 7 to 12 μm, 4 to 11 μm, 5 to 11 μm, 6 to 11 μm, 7 to 11 μm, 5 to 10 μm, 6 to 10 μm or 7 to 10 μm. In some embodiments, the volume average particle size Dv50 of the lithium supplement agent is 3.5 to 11.5 μm.

[0067] As used herein, the term "Dv50" refers to the particle size at which the cumulative particle size distribution percentage of a test sample reaches 50%, and can also be understood as the median particle size. Dv50 can be obtained by measuring the particle size distribution of a test sample using equipment or instruments such as a laser particle size analyzer or a scanning electron microscope in accordance with industry operating requirements, and the Dv50 of the test sample can be calculated statistically.

[0068] In the present application, the test sample can be a powder particle of a lithium supplement agent, or a positive electrode plate containing a lithium supplement agent. For a lithium supplement agent in powder form, its Dv50 value can usually be measured using, for example, a laser particle size analyzer. For a positive electrode plate, for example, the lithium supplement agent in the active layer can be exposed, and an electronic scanning electron microscope can be used to scan and calculate its Dv50 value. For example, the particle size of the lithium supplement agent in the positive electrode plate can be measured by the following method:

[0069] Use a blade to cut a 100×100mm electrode sample, stick a special tape on the double-sided tape, use a pressure roller to roll it three times in the same direction, and use a stretcher to peel off the special tape to expose the first active layer containing the lithium supplement agent. According to industry testing specifications, use SEM (scanning electron microscope) and mapping to identify the lithium supplement agent material. A total of no less than 50 lithium supplement agent particles are measured. After statistical calculation, the Dv50 value of the lithium supplement agent in the electrode is obtained.

[0070] The volume average particle size Dv50 of the lithium supplement agent within the said range helps to quickly release the active lithium therein, and improve the kinetic performance and cycle performance (for example, capacity retention rate) of the secondary battery. When the Dv50 of the lithium supplement agent is within the appropriate range, it helps to control the amount of lithium removed and the residual amount of the lithium supplement agent during the first charge and discharge cycle, while improving the gram capacity of the first cycle, and also enables the positive electrode plate to reach a balance between the initial gram capacity and the amount of lithium slowly released in the later period. If the particle size of the lithium supplement agent is too small, the alkaline impurity residue (residual alkali) on the surface of the lithium supplement agent will increase sharply, which will easily cause the positive electrode slurry to gel during the preparation process, increasing the risk of gas production in the battery cell; and when the particle size of the lithium supplement agent is too large, the rate at which the lithium supplement releases lithium will slow down, affecting the kinetic performance of the battery cell and being unfavorable for improving the energy density.

[0071] In some embodiments, the second active layer does not include a lithium supplement agent. In this case, because the first active layer containing the lithium supplement agent is coated on the side closest to the positive electrode current collector and does not come into direct contact with the external processing environment, or the area in contact with the processing environment is very small, direct contact between the lithium supplement agent and moisture in the processing environment can be avoided or reduced, thereby improving the stability of the lithium supplement agent, helping to reduce safety risks of the lithium supplement agent and fully utilizing the gram capacity of the lithium supplement agent.

[0072] In some embodiments, based on the total mass of the first active layer and the second active layer of the positive electrode sheet, the mass content of the lithium supplement agent in the first active layer is not higher than 5% and not lower than 0.2%, and can be optionally 0.1% to 5%, 0.5% to 5%, 1% to 5%, 1.5% to 5%, 2% to 5%, 2.5% to 5%, 0.1% to 4.5%, 0.5% to 4.5%, 1% to 4.5%, 0.1% to 4%, 0.5% to 4%, 0.8% to 4% or 1% to 4%. In some embodiments, based on the total mass of the first active layer and the second active layer of the positive electrode sheet, the mass content of the lithium supplement agent is 0.2%, 0.4%, 0.6%, 0.8%, 1.2%, 1.4%, 1.6%, 1.8%, 2.2%, 2.7%, 3%, 3.3%, 3.5%, 3.7%, 4%, 4.2%, 4.4%, 4.6%, 4.8% or 5%, or a range between any two of the values. In some embodiments, based on the total mass of the first active layer and the second active layer of the positive electrode sheet, the mass content of the lithium supplement agent is 0.8% to 4%. The lithium supplement agent within the above dosage range can provide a sufficient amount of active lithium, which can slow down the decay rate of the secondary battery cell and improve the cycle performance of the secondary battery while reducing the safety hazards caused by the side reactions of the lithium supplement agent.

[0073] In some embodiments, the ratio q of the mass of the first active layer to the total mass of the first active layer and the second active layer of the positive electrode sheet is 10%-70%, optionally 20%-60%, 10%-60% or 20%-70%. In some embodiments, the ratio q of the mass of the first active layer to the total mass of the first active layer and the second active layer of the positive electrode sheet is 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65% or 70%, or a range between any two values. In some embodiments, the ratio q of the mass of the first active layer to the total mass of the first active layer and the second active layer of the positive electrode sheet is 20%-60%. When the ratio q is within the said range, it helps to improve the capacity retention rate and the first cycle gram capacity of the secondary battery, slow down the attenuation of the secondary battery, improve the cycle capacity of the secondary battery and increase the energy density of the secondary battery.

[0074] In some embodiments, the mass content p / q of the lithium supplement agent in the first active layer is not less than 0.5% and not more than 12%, and can be optionally 2% to 10%, 6% to 10%, 6% to 12%, or 2% to 6%. In some embodiments, the mass content p / q of the lithium supplement agent in the first active layer is 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11% 11.5% or 12%, or a range between any two values. In some embodiments, the mass content p / q of the lithium supplement agent in the first active layer is not less than 2% and not more than 10%. The mass content of the lithium supplement agent within the above range can improve the capacity retention rate or first cycle gram capacity of the secondary battery, improve the cycle capacity of the secondary battery, or increase the energy density.

[0075] In some embodiments, the positive electrode active material may include at least one of the following materials: an olivine-structured lithium-containing phosphate, a lithium transition metal oxide, and modified compounds thereof.

[0076] Here, the modification of the modifying compound may be one or more of doping modification and coating modification. These positive electrode active materials may be used alone or in combination of two or more.

[0077] Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05O2) and at least one of its modified compounds, etc.

[0078] Lithium-containing phosphates with an olivine structure include one or more of lithium iron phosphate and its modified compounds. The modified compound of lithium iron phosphate may be, for example, a compound in which Fe is partially replaced by a metal element other than Fe and having an atomic radius similar to that of Fe, such as Sc, V, Cr, Mn, Ti, Al, Co, Ni, Cu, Zn, etc. Examples of modified compounds of lithium iron phosphate include LizFe 1-k Me k PO4, wherein Me is selected from one or more of Sc, V, Cr, Mn, Ti, Al, Co, Ni, Cu, and Zn, 0.8≤z≤1.3, and 0≤k≤0.5. The doped and modified lithium iron phosphate may be, for example, lithium iron manganese phosphate. Lithium iron phosphate and its doped and modified compounds may also be coated and modified, such as carbon coated. Examples of lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. In some embodiments, the positive electrode active material is selected from one or more of lithium iron phosphate and its modified compounds; optionally, the positive electrode active material includes LizFe 1-k Me k PO4 and its coated modified compounds, wherein Me is selected from one or more of Sc, V, Cr, Mn, Ti, Al, Co, Ni, Cu, and Zn, 0.8≤z≤1.3, and 0≤k≤0.7.

[0079] The positive electrode active material has high thermal stability, which is beneficial for the battery to achieve higher high-temperature safety performance. In addition, the battery can also have a longer high-temperature cycle life and higher high-temperature storage performance.

[0080] This application also provides a lithium secondary battery prepared using the aforementioned positive electrode sheet and applications of the lithium secondary battery. The lithium secondary battery provided herein exhibits excellent cycling performance, particularly high-temperature cycling performance, and significantly improves battery degradation compared to secondary batteries without a lithium supplement. Furthermore, the secondary battery reduces active lithium loss during the initial charge and discharge cycle, resulting in improved energy density.

[0081] In some embodiments, the cycling voltage window of the lithium secondary battery prepared using the above-mentioned positive electrode sheet is 2.0 to 4.1 V, and exemplary secondary batteries include secondary batteries whose positive electrode active material is lithium iron manganese phosphate. In some embodiments, the cycling voltage window of the lithium secondary battery prepared using the above-mentioned positive electrode sheet is 2.0 to 3.8 V, optionally 2.0 to 3.65 V, and exemplary secondary batteries include secondary batteries whose positive electrode active material is lithium iron phosphate.

[0082] In the present application, the "cycle battery window" is the operating voltage range of the secondary battery. When the upper limit of the operating voltage is too low, the lithium supplement in the positive electrode cannot be released, which will also result in a low initial capacity. When the upper limit of the operating voltage is too high, the lithium supplement is released quickly. Although it helps to increase the initial capacity, it cannot achieve the effect of slow and sustained release, which is not conducive to improving the attenuation of the secondary battery. The cycle battery window can make the lithium supplement release active lithium at the beginning of the cycle within the range, but not all of it is released, so the starting capacity of the secondary battery can be improved; at the same time, the lithium supplement slowly releases the active lithium therein during the cycle, which improves the attenuation rate of the secondary battery.

[0083] In some embodiments, the upper limit voltage of the cycle voltage of the lithium secondary battery is 3.6 to 3.8 V. In some embodiments, the upper limit voltage of the cycle voltage of the lithium secondary battery is 3.65 to 3.8 V, 3.7 to 3.8 V, 3.6 to 3.7 V, 3.7 to 3.8 V, or 3.65 to 3.7 V. An exemplary secondary battery may be a secondary battery whose positive electrode active material is lithium iron phosphate.

[0084] The voltage window within the above range can avoid the rapid release of the lithium supplement during the first cycle, improve the sustained release effect of the lithium supplement, and help avoid high-voltage decomposition of the electrolyte, thereby improving the stability of the secondary battery electrolyte and improving the cycle life of the secondary battery.

[0085] In the lithium secondary battery provided in the present application, the positive electrode current collector is a carrier that supports the positive electrode coating layer. It can not only support the positive electrode active material, but also collect and output the current generated by the positive electrode active material.

[0086] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0087] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0088] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0089] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0090] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0091] The lithium secondary battery includes a negative electrode plate, which includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0092] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0093] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0094] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0095] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0096] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0097] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0098] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0099] The lithium secondary battery includes an electrolyte that conducts ions between the positive and negative electrodes. The present application does not specifically limit the type of electrolyte, and the electrolyte may be selected based on needs. For example, the electrolyte may be liquid, gel, or solid.

[0100] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0101] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0102] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0103] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0104] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0105] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0106] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0107] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0108] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0109] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG4 shows a secondary battery 5 having a square structure as an example.

[0110] In some embodiments, referring to Figure 5, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0111] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0112] Figure 6 shows an example battery module 4. Referring to Figure 6 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured using fasteners.

[0113] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0114] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0115] Figures 7 and 8 illustrate an example battery pack 1. Referring to Figures 7 and 8 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0116] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0117] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0118] Figure 9 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0119] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0120] Example

[0121] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0122] 1. Preparation method

[0123] Example 1

[0124] 1) Preparation of positive electrode sheet

[0125] Preparation of the first active layer:

[0126] The positive electrode active material lithium iron phosphate (purchased from Defang Nano, DF-5) and lithium supplement Li 1.8 NiO2, conductive agent acetylene black and binder PVDF (polyvinylidene fluoride) are mixed in a weight ratio of 91:6:0.7:2.3, and solvent N-methylpyrrolidone is added. The mixture is thoroughly stirred and mixed to obtain a positive electrode slurry with good fluidity and non-gelling properties, which is then coated on one side of the current collector aluminum foil.

[0127] The lithium supplement is prepared by a method comprising the following steps:

[0128] Lithium hydroxide with a molar ratio of 1.02x:1 (volume average particle size Dv50 of 6 - 7 μm) and a nickel hydroxide precursor (volume average particle size Dv50 of 8 - 10 μm) are mixed in a ball mill under a nitrogen atmosphere. The mixed material is transferred to a crucible and calcined at 790 °C - 850 °C for 10 - 12 h under a nitrogen atmosphere with a heating rate of 4 °C / min, then cooled to obtain a sintered material. The sintered material is crushed and sieved under a nitrogen atmosphere to obtain a lithium - supplemented agent LixNiO2 (1 < x < 2) with a volume average particle size Dv50 (measured by laser diffraction method for particle size distribution in accordance with GB / T19077 - 2016) of 8.2 μm. The value of x is obtained by the method of "determination of metal elements in the lithium - supplemented agent" under performance testing. Figure 10 shows the scanning electron microscope image of this lithium - supplemented agent, which has obvious agglomeration phenomenon. It is formed by the agglomeration of multiple primary particles into secondary particles, and the prepared lithium - supplemented agent is in a polycrystalline form.

[0129] Preparation of the second active layer:

[0130] The cathode active material lithium iron phosphate, conductive agent acetylene black, and binder PVDF are mixed in a weight ratio of 98:0.7:1.3, and solvent N - methylpyrrolidone is added. After充分搅拌混合均匀得到正极浆料,涂布在第一活性层远离集流体的一侧。(The mixture is stirred thoroughly to obtain a uniform cathode slurry, which is coated on the side of the first active layer away from the current collector.)

[0131] The current collector coated with the first and second active layers is dried at 85 °C and then cold - pressed. It is dried under vacuum at 85 °C for 4 h, and after trimming, cutting, and slitting, a cathode electrode sheet is made. Among them, the ratio q of the mass of the first active layer to the sum of the masses of the first and second active layers is 40%, and the ratio p of the mass of the lithium - supplemented agent in the first active layer to the sum of the masses of the first and second active layers is 2.4%.

[0132] 2) Preparation of the anode electrode sheet

[0133] The anode active material graphite, conductive agent Super P, thickening agent sodium carboxymethyl cellulose (CMC - Na), and binder styrene - butadiene rubber (SBR) are mixed uniformly in deionized water to prepare an anode slurry with a solid content of 30 wt%. Among them, the mass ratio of graphite, Super P, CMC - Na, and SBR is 80:15:3:2. The anode slurry is coated on the current collector copper foil and dried at 85 °C, cold - pressed, trimmed, cut, and slit, and then dried under vacuum at 120 °C for 12 h to make an anode electrode sheet.

[0134] 3) Separator

[0135] A 16 - μm polyethylene film (PE) is used as the separator.

[0136] 4) Preparation of the electrolyte

[0137] In an argon atmosphere glove box (H2O content <0.1ppm, O2 content <0.1ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a ratio of 3:7 to prepare a mixed solvent. Lithium hexafluorophosphate (LiPF6) was then added to the mixed solvent at a molar concentration of 1 mol / L and dissolved. The mixture was stirred to prepare an electrolyte.

[0138] 5) Preparation of batteries

[0139] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is placed in the middle of the positive and negative electrode sheets to isolate the positive and negative electrode sheets, and the bare battery cell is wound to obtain the bare battery cell. The tabs are welded and the bare battery cell is placed in an outer package. The electrolyte prepared above is injected into the dried battery cell. After packaging, standing, formation, shaping, capacity testing and other processes, the lithium secondary battery product of Example 1 is obtained.

[0140] The secondary batteries of Examples 2 to 15 were prepared similarly to the secondary battery of Example 1, except that the composition and product parameters of the battery electrodes were adjusted, as detailed in Table 1. Specifically, in the lithium supplement preparation method of Example 6, the volume average particle size of the lithium hydroxide was 2-3 μm (Dv50), the volume average particle size of the nickel hydroxide precursor was 5-6 μm (Dv50), and the heating rate during calcination was 5°C / min. In Example 7, the volume average particle size of lithium hydroxide Dv50 is 2-3 μm, the volume average particle size of the nickel hydroxide precursor Dv50 is 6-8 μm, and the heating rate during the calcination process is 5°C / min; in Example 8, the volume average particle size of lithium hydroxide Dv50 is 8-9 μm, the volume average particle size of the nickel hydroxide precursor Dv50 is 11-12 μm, and the heating rate during the calcination process is 3°C / min; in Example 9, the volume average particle size of lithium hydroxide Dv50 is 8-9 μm, the volume average particle size of the nickel hydroxide precursor Dv50 is 12-13 μm, and the heating rate during the calcination process is 3°C / min.

[0141] The secondary battery of Example 16 is prepared in a similar manner to the secondary battery of Example 1, but the positive electrode active material of the electrode is adjusted, wherein the positive electrode active material of the first active layer and the second active layer are both LiFe 0.4 Mn 0.6 PO4.

[0142] The secondary batteries of Examples 17 to 19 are prepared in a similar manner to the secondary batteries of Examples 1, 15, and 14, respectively, except that the raw material for preparing the lithium supplement is changed from a nickel hydroxide precursor to a manganese hydroxide precursor.

[0143] The secondary batteries of Examples 20 and 21 are prepared in a similar manner to the secondary battery of Example 1, except that Example 20 uses the lithium supplement NCM523 (purchased from Rongbai New Energy Technology Co., Ltd., S700), and Example 21 uses the lithium supplement NCM811 (purchased from Rongbai New Energy Technology Co., Ltd., S700).

[0144] The secondary battery of Example 22 is prepared in a similar manner to the secondary battery of Example 1, except that the raw materials for preparing the lithium supplement are adjusted, wherein a nickel hydroxide precursor (volume average particle size Dv50 of 8-10 μm) and a copper hydroxide precursor (volume average particle size Dv50 of 20-30 μm) with a Ni:Cu molar ratio of 99:1.02 are added.

[0145] The secondary battery of Example 23 is prepared in a similar manner to that of Example 15, but the raw materials for preparing the lithium supplement are adjusted, wherein a nickel hydroxide precursor and a copper hydroxide precursor (volume average particle size Dv50 is 20-30 μm) with a Ni:Cu molar ratio of 99:1.02 are added.

[0146] The secondary battery of Example 24 is prepared in a similar manner to the secondary battery of Example 14, except that the raw materials for preparing the lithium supplement are adjusted, wherein a nickel hydroxide precursor (volume average particle size Dv50 of 8-10 μm) and a copper hydroxide precursor (volume average particle size Dv50 of 20-30 μm) having a Ni:Cu molar ratio of 99:1.02 are added.

[0147] The preparation method of the secondary battery of Example 25 is similar to that of the secondary battery of Example 22, but the preparation raw materials of the lithium supplement are adjusted, wherein a nickel hydroxide precursor (volume average particle size Dv50 is 8-10 μm) and an aluminum hydroxide precursor (volume average particle size Dv50 is 20-30 μm) with a Ni:Al molar ratio of 999:1.02 are added.

[0148] The secondary battery of Example 26 is prepared by a method similar to that of Example 22, except that the raw materials for preparing the lithium supplement are adjusted, wherein a nickel hydroxide precursor (volume average particle size Dv50 of 8-10 μm) and a manganese hydroxide precursor (volume average particle size Dv50 of 20-30 μm) having a Ni:Mn molar ratio of 995:5.1 are added.

[0149] The secondary battery of Comparative Example 1 uses commercially available LiPO 4 as a lithium supplement, and the rest of the preparation process is similar to that of Example 1.

[0150] The secondary battery of Comparative Example 2 is substantially similar to that of Example 1, wherein the lithium supplement is prepared according to the following method:

[0151] Lithium hydroxide with a molar ratio of 1.08x:1 (volume average particle size Dv50 of 6 - 7 μm) and a nickel hydroxide precursor (volume average particle size Dv50 of 8 - 10 μm) were mixed in a ball mill under a nitrogen atmosphere. The mixed material was transferred to a crucible and calcined at 900 °C - 950 °C for 10 - 12 h under a nitrogen atmosphere with a heating rate of 4 °C / min, and then cooled to obtain a sintered material. The sintered material was pulverized and sieved under a nitrogen atmosphere to obtain a lithium - supplement agent LixNiO2 (1 < x < 2) with a volume average particle size Dv50 (measured by laser diffraction method for particle size distribution according to GB / T19077 - 2016) of approximately 8.2 μm. As shown in Figure 11, the lithium - supplement agent shows dispersed primary - particle crystals under a scanning electron microscope and has a single - crystal morphology.

[0152] The preparation method of the secondary battery of Comparative Example 3 was similar to that of the secondary battery of Example 1, but without adding a lithium - supplement agent. The cathode active material lithium iron phosphate, conductive agent acetylene black, and binder PVDF (polyvinylidene fluoride) were mixed in a weight ratio of 97:0.7:2.3 to prepare the first active layer.

[0153] The preparation method of the secondary battery of Comparative Example 4 was similar to that of the secondary battery of Comparative Example 3, but the cathode active material used LiFe 0.4 Mn 0.6 PO4.

[0154] II. Performance Testing

[0155] 1. Capacity Retention Rate Testing

[0156] In a constant - temperature environment of 60 °C, after standing for 2 h, the prepared secondary battery was charged at a constant current of 0.5C to 3.65V, then charged at a constant voltage of 3.65V until the current dropped to 0.05C, stood for 10 min, and then discharged at a constant current of 1C to 2.0V, stood for 5 min, and then discharged at 0.05C to 2.0V. This was the first charge / discharge cycle of the battery, and the discharge capacity at this time was recorded as the discharge capacity (C0) of the first cycle of the battery; the above steps were repeated for the same battery. After the 800th cycle, the process capacity (C1) of the battery was measured. The capacity retention rate after 800 cycles = C1 / C0×100%. The testing process for the comparative examples and other examples was the same as above. Calculate the ratio of the increase in the capacity retention rate of Examples 1 - 15 and Comparative Examples 1 - 2 relative to Comparative Example 3 (capacity retention rate increase rate).

[0157] 2. First - Cycle Gram Capacity Testing of Secondary Battery

[0158] The positive electrode active material is lithium iron phosphate: in a constant temperature environment of 60°C, after standing for 2 hours, the prepared secondary battery is charged to 3.65V at a constant current of 0.5C, then charged at a constant voltage of 3.65V until the current drops to 0.05C, and after standing for 10 minutes, discharged to 2.0V at a constant current of 1C. After standing for 5 minutes, it is discharged to 2.0V at 0.05C. This is the first charge / discharge cycle of the battery, and the discharge capacity of this time is recorded as the discharge capacity of the battery in the first cycle (C0). The gram capacity is the ratio of C0 to the sum of the mass of the positive electrode active material and the lithium supplement (mAh / g). The ratio of the first cycle gram capacity improvement of Examples 1-15 and Comparative Examples 1-2 relative to Comparative Example 3 (first cycle gram capacity improvement rate) is calculated.

[0159] The positive electrode active material is lithium manganese iron phosphate: in a constant temperature environment of 60°C, after standing for 2 hours, the prepared secondary battery is charged to 4.1V at a constant current of 0.5C, then charged at a constant voltage of 4.1V until the current drops to 0.05C, and after standing for 10 minutes, discharged to 2.0V at a constant current of 1C. After standing for 5 minutes, it is discharged to 2.0V at 0.05C. This is the first charge / discharge cycle of the battery, and the discharge capacity of this time is recorded as the discharge capacity of the battery in the first cycle (C0). The gram capacity is the ratio of C0 to the sum of the mass of the positive electrode active material and the lithium supplement (mAh / g). The ratio of the first cycle gram capacity improvement of Example 16 relative to Comparative Example 4 (first cycle gram capacity improvement rate) is calculated.

[0160] 3. Gas production performance test

[0161] The batteries prepared in the examples were assembled into hard-shell secondary batteries. The gas-producing pins were welded to the prepared hard-shell battery cells through the liquid injection holes. The inside of the Teflon tube was then filled with oil, and the oil-filled Teflon tube was connected to the gas-producing pins of the battery cells. A pressure gauge was then taken and filled with oil. Finally, the pressure gauge was connected to the other end of the Teflon tube. The battery cells with pressure gauges were completed and allowed to stand for 24 hours. The prepared gas-producing battery cells were cycled for 200 cycles according to the method described in "1. Capacity Retention Rate Test" above. The pressure gauge readings were recorded online in real time with the number of cycles. The ratio of the gas production increase of Examples 1-15, 17-26 and Comparative Examples 1-2 relative to Comparative Example 3 (gas production increase rate) was calculated, as well as the ratio of the gas production increase of Example 16 relative to Comparative Example 4.

[0162] 4. Morphology test of lithium supplement

[0163] The lithium supplement was tested using a ZEISS sigma 300 scanning electron microscope, and the sample morphology was observed in accordance with the standard JY / T010-1996.

[0164] 5. Determination of metal elements in lithium supplements

[0165] Using an inductively coupled plasma optical emission spectrometer in accordance with EPA test method 6010D-2014, the test sample was weighed, a digestion reagent and aqua regia were added, and the sample was completely digested at no more than 200°C using the plate digestion method. The digestion solvent was tested by an excitation light source, and the element content in the sample was determined by the characteristics and emission intensity of different emission lines. The element content (mass fraction w / w%) = element mass / weighed sample was used to determine the proportion of metal elements in the sample.

[0166] 3. Analysis of test results of various embodiments and comparative examples

[0167] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in Table 1 below.

[0168] Table 1: Test results of Examples 1 to 26 and Comparative Examples 1 to 2

[0169] Based on the above results, Examples 1-26 all achieved good results. Compared to Comparative Examples 3 or 4, which did not add a lithium supplement, the capacity retention rate of the secondary battery was significantly improved after adding the lithium supplement, indicating that the cell decay rate of the secondary battery during the cycle was significantly slowed; or the first cycle gram capacity was increased, improving the cycle performance and energy density of the secondary battery. The secondary batteries prepared in Examples 1-26 had good high-temperature cycle performance. As shown in Figure 3, the absolute capacity slope of the secondary battery with the lithium supplement was slower, indicating that the cell decay condition of the secondary battery prepared in Example 1 was significantly improved during the cycle.

[0170] As shown in Examples 1-5, compared with Comparative Example 3, a lithium supplementation agent content in the first active layer of 0.5% to 12% by mass can improve the capacity retention rate or first cycle gram capacity of the secondary battery. Specifically, a lithium supplementation agent content in the first active layer of 2% to 10% by mass can also improve both the capacity retention rate and the first cycle gram capacity of the secondary battery.

[0171] Examples 1-5 also show that when the mass ratio of the lithium supplement agent in the positive electrode solid material (excluding the current collector) is 0.2%-4.8%, the capacity retention rate or the first cycle gram capacity of the secondary battery can be improved. Specifically, when the mass ratio of the lithium supplement agent in the positive electrode solid material (excluding the current collector) is 0.8%-4%, both the capacity retention rate and the first cycle gram capacity of the secondary battery can be improved.

[0172] As shown in Examples 6-9, when the lithium supplement agent's particle size DV50 is in the range of 2.5-12.5 μm, it helps improve the secondary battery's capacity retention rate or first cycle gram capacity. When the lithium supplement agent's particle size DV50 is in the range of 2.5-11.2 μm, it can simultaneously improve the secondary battery's capacity retention rate and first cycle gram capacity. However, the secondary battery prepared with a lithium supplement agent with a particle size DV50 of 2.5 μm has a significant increase in gas production and reduced gas production performance. When the lithium supplement agent's particle size DV50 is in the range of 3.5-11.2 μm, the secondary battery can achieve both improved capacity retention rate and first cycle gram capacity, while also having good gas production performance.

[0173] As can be seen from Examples 10-13, when the mass ratio of the first active layer in the positive electrode sheet solid material (excluding the current collector) is 10%-70%, the capacity retention rate or the first cycle gram capacity of the secondary battery can be improved. Specifically, when the mass ratio of the first active layer in the positive electrode sheet solid material (excluding the current collector) is 20%-70%, both the capacity retention rate and the first cycle gram capacity of the secondary battery can be improved. When the mass ratio of the first active layer in the positive electrode sheet solid material (excluding the current collector) is 20%-60%, both the capacity retention rate or the first cycle gram capacity of the secondary battery can be improved, while also having good gas production performance.

[0174] From Examples 1, 14-15, it can be seen that polycrystalline Li 1.1 NiO2、Li 1.5 NiO2、Li 1.8 NiO2 can be used as a lithium supplement for lithium secondary batteries to improve the capacity retention rate or first cycle capacity of secondary batteries. 1.1 NiO2、Li 1.5 NiO2、Li 1.8 NiO2 not only improves the capacity retention rate and first cycle gram capacity of the secondary battery, but also has excellent gas production performance, indicating that the secondary battery prepared with the lithium supplement has good storage performance.

[0175] From Examples 1 and 16, it can be seen that the solution of adding lithium supplement in this application can be applied to secondary batteries whose positive electrode active material is lithium manganese iron phosphate, and at the same time improves the capacity retention rate and first cycle gram capacity of the secondary battery. In addition, the gas production performance is also greatly improved. Compared with Comparative Example 2, the single crystal Li 1.8 NiO2 has a more stable crystalline state, and the rate of lattice destruction and decay during cycling is relatively lower than that of polycrystalline Li 1.8 NiO2 has little or no effect on the improvement of the first cycle capacity; at the same time, the active lithium in it is always released slowly during the subsequent cycles, which ultimately leads to the improvement of the capacity retention rate of the secondary battery being significantly worse than that of polycrystalline Li1.8 NiO2. Polycrystalline Li 1.8 NiO2 is more helpful in improving the cycle performance of secondary batteries.

[0176] As can be seen from Examples 17-19, polycrystalline Li 1.1 MnO2、Li 1.5 MnO2、Li 1.8 MnO2 can be used as a lithium supplement for lithium secondary batteries, and can significantly improve the capacity retention rate and significantly slow down the attenuation of secondary batteries. 1.5 MnO2、Li 1.8 MnO2 can simultaneously improve the first cycle gram capacity and cycle capacity retention rate of secondary batteries, thereby improving the energy density of secondary batteries while improving attenuation.

[0177] As shown in Examples 20-21, the ternary lithium supplements NCM523 and NCM811 can significantly improve capacity retention, helping to slow the degradation of secondary batteries. Furthermore, NCM523 and NCM811 have low gas production increase rates and excellent gas production performance, helping to improve the storage life of secondary batteries.

[0178] As shown in Examples 22-26, lithium nickelate-based lithium supplements doped with Cu, Al, and Mn can significantly improve capacity retention and significantly slow the degradation of secondary batteries. Furthermore, they can improve the initial cycle gram capacity and cycle capacity retention of secondary batteries, increasing the energy density of the secondary batteries while mitigating degradation. Furthermore, the doping treatment also helps improve the gas production rate of the secondary batteries.

[0179] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode sheet, wherein: The positive electrode sheet includes a first active layer coated on at least one side of the positive current collector and containing a positive active material and a lithium supplement agent in polycrystalline form, and a second active layer coated on the side of the first active layer away from the current collector and containing a positive active material; The lithium supplement agent includes at least one of LixNi1-yMyO2, LixRO2, and LiNiαCoβMnγO2, where M includes one or more of Cu, Al, and Mn; R includes one of Ni and Mn; 1 < x < 2, 0 < y ≤ 0.01, α + β + γ = 1 and α ≥ 0.

5.

2. The positive electrode sheet according to claim 1, wherein: The lithium supplement agent includes one or more of LixNiO2, LixMnO2, LixNi1-yMyO2, and LiNiαCoβMnγO2, where M includes one or more of Cu, Al, and Mn; R includes one of Ni and Mn; 1.1 < x < 1.8, 0 < y ≤ 0.01, α + β + γ = 1 and α ≥ 0.5, 0 < β < 1, 0 < γ < 1.

3. The positive electrode sheet according to claim 1 or 2, wherein: The volume average particle size Dv50 of the lithium supplement agent is 2.5 to 12.5 μm, and can be 3.5 to 11.5 μm.

4. The positive electrode sheet according to any one of claims 1 to 3, wherein: The mass content p / q of the lithium supplement agent in the first active layer is not less than 0.5% and not more than 12%, and can be not less than 2% and not more than 10%; Wherein, p is the ratio of the mass of the lithium supplement agent in the first active layer to the total mass of the first active layer and the second active layer of the positive electrode sheet; q is the ratio of the mass of the first active layer to the total mass of the first active layer and the second active layer of the positive electrode sheet.

5. The positive electrode sheet according to any one of claims 1 to 4, wherein: The p is not more than 5% and not less than 0.2%, and can be 0.8% to 4%.

6. The positive electrode sheet according to any one of claims 1 to 5, wherein: The q is 10% - 70%, and can be 20% - 60%.

7. The positive electrode sheet according to any one of claims 1 to 6, wherein: The positive active materials in the first active layer and the second active layer include one or more of lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds, and can be one or more of lithium iron phosphate and its modified compounds.

8. The positive electrode sheet according to any one of claims 1 to 7, wherein: The positive electrode active material in the first active layer and / or the second active layer includes LizFe 1-k Me k PO4 and its coated modified compounds, wherein Me includes one or more of Sc, V, Cr, Mn, Ti, Al, Co, Ni, Cu, and Zn, 0.8≤z≤1.3, and 0≤k≤0.

7.

9. The positive electrode sheet according to any one of claims 1 to 8, wherein: The cyclic voltage window of the secondary battery prepared from the positive electrode sheet is 2.0 to 3.8V or 2.0 to 4.1V.

10. A lithium secondary battery, wherein: Including the positive electrode sheet according to any one of claims 1-9.

11. Use of the lithium secondary battery according to claim 10, wherein: The cyclic voltage window of the lithium secondary battery is 2.0 to 4.1V.

12. The use according to claim 11, wherein: The cyclic voltage window of the lithium secondary battery is 2.0 to 3.8V, or the upper limit voltage of the cyclic voltage is 3.6 to 3.8V, and can be 3.65 to 3.8V.

13. An electrical device, wherein: Including the lithium secondary battery according to claim 10.

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