Lithium secondary battery and electric device

Abstract

The invention provides a lithium secondary battery and a power utilization device. The lithium secondary battery comprises a positive pole piece and a negative pole piece, the first active layer is coated on at least one side of a positive current collector and contains a positive active material and a polycrystalline lithium supplement agent, the second active layer is coated on one side, far away from the current collector, of the first active layer and contains a positive active material, and the lithium supplement agent comprises at least one of Li < x > Ni < 1-y > M < y > O < 2 >, Li < x > RO < 2 > and Li < Ni < alpha > Co < beta > Mn < gamma > O < 2 >, m comprises one or more of Cu, Al and Mn; r comprises one of Ni and Mn; 1lt; xlt; 2, 0lt; y < = 0.01, alpha + beta + gamma = 1, and alpha > = 0.5. The lithium supplement agent can quickly release sufficient active lithium in the first charging process, so that the energy density of the secondary battery is improved; the lithium supplement agent can release active lithium permanently in the circulation process, and the attenuation of a battery cell is slowed down.

Description

Technical Field

[0001] The present application relates to the technical field of lithium batteries, and particularly to a lithium secondary battery and an electrical device. Background Art

[0002] Due to the advantages of high discharge voltage, high energy density, and long cycle life of lithium-ion batteries, they have now been widely used in various small portable electronic devices, such as laptop computers, mobile phones, and cameras, etc., and have also gradually been favored in high-tech fields, such as aerospace satellites, electric vehicles, and military, etc. In these applications, it is required that lithium-ion batteries have better capacity and cycle life.

[0003] The active lithium in a lithium-ion battery comes from the positive electrode material. During the first charge and discharge cycle of the lithium-ion battery, the surface of the negative electrode of the lithium-ion battery will consume active lithium to form a solid electrolyte interface film (SEI film), resulting in the loss of the first charge and discharge capacity (Initial Capacity Loss, ICL), leading to the capacity attenuation and reduced cycle efficiency of the lithium-ion battery. Therefore, it is of extremely important significance to develop a simple and efficient lithium supplementation technology. Summary of the Invention

[0004] The present application is made in view of the above problems, and its purpose is to provide a positive electrode tab, the positive electrode tab 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 supplementation 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 supplementation agent includes LixNi1-yMyO 2 、LixRO 2 、LiNiαCoβMnγO 2 at least one of them, wherein,

[0005] M includes one or more of Cu, Al, and Mn; R includes one of Ni and Mn;

[0006] 1 < x < 2, 0 < y ≤ 0.01, α + β + γ = 1 and α ≥ 0.5.

[0007] During the cycle of the secondary battery, the lithium supplementation agent in polycrystalline form in the first active layer has faster attenuation due to large concentration polarization during the first charge process, and rapidly releases sufficient active lithium to make up for the active lithium consumed on the surface of the negative electrode during the first charge and discharge cycle, improving the first cycle specific capacity of the secondary battery, and further increasing the energy density of the secondary battery; the lithium extracted from the lithium supplementation agent cannot be intercalated back into the lithium supplementation agent, and the positive electrode active material of the second active layer accepts the active lithium extracted from the lithium supplementation agent. Therefore, the lithium supplementation agent continuously releases all the lithium in it during the cycle process, significantly improving the cycle performance of the secondary battery, thereby slowing down the attenuation of the battery core.

[0008] In any embodiment, the lithium supplement includes LixNiO 2 , LixMnO 2 , LixNi1-yMyO 2 , LiNiαCoβMnγO 2 or one or more of them, where M includes one or more of Cu, Al, and 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. In any embodiment, the volume average particle size Dv50 of the lithium supplement is 2.5 to 12.5 μm, and optionally 3.5 to 11.5 μm. The Dv50 of the lithium supplement within the above range helps to control the amount of lithium deintercalation and the residual amount of the lithium supplement during the first charge-discharge cycle, so that the positive electrode sheet reaches a balance between the initial specific capacity and the late slow-release lithium amount. At the same time, the Dv50 of the lithium supplement within the above range helps to reduce the gas generation performance of the secondary battery and improve the storage performance of the secondary battery.

[0009] In any embodiment, the mass content p / q of the lithium supplement 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 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. The mass content p / q of the lithium supplement 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.

[0010] In any embodiment, based on the total mass of the first active layer and the second active layer of the positive electrode sheet, the mass content p of the lithium supplement 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 within the above dosage range can provide sufficient active lithium, while slowing down the attenuation rate of the secondary battery cell and improving the cycle performance of the secondary battery, and can also reduce the safety hazards caused by the side reactions of the lithium supplement.

[0011] 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%. 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 within a suitable range helps to improve the first-cycle specific capacity of the secondary battery, thereby increasing the energy density.

[0012] 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 one or more of lithium iron phosphate and its modified compounds.

[0013] In any embodiment, the positive electrode active material in the first active layer and the second active layer includes LizFe 1-k Me k PO 4 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.

[0014] 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.

[0015] 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 PO 4 (0.8≤z≤1.3, 0≤k≤0.7) and secondary batteries coated with modified compounds thereof can reduce the release of lithium supplement during the first cycle while meeting the cycle requirements of the secondary battery, which helps to slowly release the lithium supplement during the secondary battery cycle; it also helps to avoid high-voltage decomposition of the electrolyte.

[0016] A second aspect of the present application provides a lithium secondary battery, the battery comprising the positive electrode plate provided in the first aspect.

[0017] In any embodiment, the upper limit voltage of the cycle voltage of the secondary battery prepared by the positive electrode sheet is 2.0 to 4.1V. In any embodiment, the upper limit voltage of the cycle voltage of the secondary battery prepared by the positive electrode sheet is 2.0 to 3.8V. 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 3.8V, which can be 3.65 to 3.8V. The upper limit voltage within the range can make the lithium supplement agent release the lithium supplement agent at the beginning of the battery cycle, and slowly release the lithium supplement agent during the cycle, thereby improving the battery attenuation rate.

[0018] The third aspect of the present application provides application of the lithium secondary battery of the second aspect.

[0019] 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 voltage upper limit voltage is 3.6 to 3.8 V, and can be 3.65 to 3.8 V. The voltage window in the above range can avoid the rapid release of the lithium supplement and the high-voltage decomposition of the electrolyte, thereby slowing down the attenuation of the secondary battery and improving the stability of the secondary battery electrolyte, thereby improving the cycle life of the secondary battery.

[0020] A fourth aspect of the present application provides an electrical device, which includes the lithium secondary battery described in the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the mechanism of continuous release of active lithium by the lithium supplement in the prepared secondary battery.

[0022] Figure 2 It is the difference between the amount of lithium replenished and the amount of lithium removed in the lithium half-cell prepared using a lithium replenisher.

[0023] Figure 3 The battery capacity retention measurement results of the secondary batteries prepared in Example 1 and Comparative Example 3 are shown.

[0024] Figure 4 is a schematic diagram of a secondary battery according to one embodiment of the present application.

[0025] Figure 5 yes Figure 4 An exploded view of a secondary battery according to an embodiment of the present application is shown.

[0026] Figure 6 is a schematic diagram of a battery module according to an embodiment of the present application.

[0027] Figure 7 It is a schematic diagram of a battery pack according to one embodiment of the present application.

[0028] Figure 8 yes Figure 7 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0029] Fig. 9 FIG. 1 is a schematic diagram of an electric device using a secondary battery according to an embodiment of the present application as a power source.

[0030] Fig.10 This is an electron microscope scanning image of the lithium supplement prepared in Example 1 of the present application.

[0031] Fig.11 This is an electron microscope scanning image of the lithium supplement prepared in Comparative Example 2 of the present application.

[0032] Description of reference numerals:

[0033] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 top cover assembly DETAILED DESCRIPTION

[0034] 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 with appropriate reference to the drawings. However, there are 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 descriptions 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.

[0035] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a 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 a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present 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 real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it 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.

[0036] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0037] 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.

[0038] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means 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), which means 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.

[0039] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0040] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": 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).

[0041] Lithium secondary batteries include positive electrode sheets, negative electrode sheets, electrolytes, and separators. During the battery charging and discharging process, active ions (such as lithium ions) are embedded and extracted back and forth between the positive electrode sheets and the negative electrode sheets. The electrolyte plays the role of conducting ions between the positive electrode sheets and the negative electrode sheets. The separator is set between the positive electrode sheets and the negative electrode sheets, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

[0042] During the first charge and discharge cycle of a lithium-ion battery, the surface of the negative electrode of the lithium-ion battery consumes active lithium to form an SEI film, resulting in a loss of the first charge and discharge capacity, leading to a reduction in the energy density of the lithium-ion battery, thereby reducing the cycle capacity of the secondary battery, affecting the attenuation and cycle efficiency of the secondary battery. The present application provides a positive electrode plate, the positive electrode plate comprising a lithium supplement, and the lithium supplement can reduce the loss of irreversible active lithium in the secondary battery prepared by the positive electrode plate by supplementing active lithium to the positive electrode, thereby reducing the first charge and discharge capacity loss of the secondary battery.

[0043] In some embodiments, the positive electrode plate includes a first active layer coated on at least one side of the positive current collector and a second active layer coated on a side of the first active layer away from the current collector, wherein the first active layer contains a positive electrode active material and a polycrystalline lithium supplement; and the second active layer contains a positive electrode active material.

[0044] In the present application, the "lithium supplement" refers to a metal oxide containing lithium, which can release active lithium and replenish active lithium to the positive electrode active material during the cycle of the secondary battery. During the cycle of the lithium secondary battery containing the positive electrode plate, the lithium supplement in the first active layer has a larger concentration polarization and loses the lithium insertion site faster, so it has a faster decay, and the lithium released from the lithium supplement cannot be inserted back into the lithium supplement, so the lithium supplement can permanently release all the lithium in it; the positive electrode active material of the second active layer accepts the active lithium released from the lithium supplement in the first active layer, slowing down the decay rate of the battery cell and improving the cycle performance of the lithium-ion battery.

[0045] The lithium supplement in the first active layer generates partial pore structure due to volume change during the cycle, which is also beneficial to improve the dynamics of the positive electrode.

[0046] In the present 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 secondary particles. The polycrystalline lithium supplement includes secondary particles formed by agglomeration, which are usually prepared by secondary particle agglomeration technology (e.g., sintering). In the polycrystalline lithium supplement, the particle size distribution of the secondary particles in the lithium supplement powder is dominant. Those skilled in the art can judge whether the lithium supplement is in a polycrystalline form based on common knowledge and common technical knowledge in the field. In one example, the lithium supplement powder appears as agglomerated particles or substantially all as agglomerated particles under a scanning electron microscope.

[0047] Single crystal refers to the crystal of primary particles, usually a single small grain. The single crystal form can be understood as a morphology similar to that of primary particles. The lithium supplement in single crystal form has primary particle crystals of the lithium supplement. In the lithium supplement in single crystal form, the primary particles have an advantage in the particle size distribution in the lithium supplement powder. Those skilled in the art can judge whether the lithium supplement is in single crystal form based on common knowledge and common technical knowledge in the field. In one example, the lithium supplement powder is basically all primary particles under a scanning electron microscope.

[0048] In this application, primary particles are also called original particles, and agglomerated particles formed by agglomeration of multiple original particles or primary particles are called "secondary particles". The force for agglomeration between particles can be physical bonding (such as van der Waals force) or chemical bonding (such as bridging oxygen bond); the method for agglomeration between particles can be external pressure or another substance (such as a chemical substance with bonding properties) to assist agglomeration.

[0049] Compared with single-crystal lithium supplements, polycrystalline lithium supplements 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 supplement lithium during the first cycle and release lithium for a long time during subsequent cycles.

[0050] In some embodiments, the lithium supplement includes LixNi1-yMyO 2 , LixRO 2 , LiNiαCoβMnγO 2 and at least one of them, where M includes one or more of Cu, Al, and Mn; R includes one of Ni and Mn;

[0051] 1 < x < 2, 0 < y ≤ 0.01, α + β + γ = 1 and α ≥ 0.5.

[0052] In some embodiments, the lithium supplement includes LixNiO 2 , LixMnO 2 , LixNi1-yMyO 2 , LiNiαCoβMnγO 2 and one or more of them, 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.

[0053] In some embodiments, the lithium supplement includes LixNiO 2 (1 < x < 2) and one or more of them. Optionally, 1.1 < x < 1.8, 1.5 < x < 1.8, 1.5 < x < 2, 1.1 < x < 1.5. Exemplarily, the lithium supplement can be Li 1.1 NiO 2 , Li 1.3 NiO 2 , Li 1.5 NiO 2 , Li 1.6 NiO 2 , Li 1.7 NiO 2 , Li 1.9 NiO 2 and one or more of them.

[0054] In some embodiments, the lithium supplement includes LixMnO 2 (1 < x < 2) and one or more of them. Optionally, 1.1 < x < 1.8, 1.5 < x < 1.8, 1.5 < x < 2, 1.1 < x < 1.5. Exemplarily, the lithium supplement can be Li 1.8 MnO 2 , Li 1.5 MnO 2 , Li 1.3 MnO 2 , Li 1.1 MnO2 、 Li 1.7 MnO 2 、 Li 1.6 MnO 2 or one or more of them.

[0055] In some embodiments, the lithium supplement includes LixNi1-yMyO 2 , where 1 < x < 2, 0 < y ≤ 0.01, or one or more of them. Optionally, 1.1 < x < 1.8, 1.5 < x < 1.8, 1.5 < x < 2, 1.1 < x < 1.5. Exemplarily, the lithium supplement can be Li 1.8 Ni 0.99 Cu 0.01 O 2 、 Li 1.5 Ni 0.99 Cu 0.01 O 2 、 Li 1.1 Ni 0.99 Cu 0.01 O 2 、 Li 1.8 Ni 0.999 Cu 0.001 O 2 、 Li 1.8 Ni 0.995 Cu 0.005 O 2 、 Li 1.7 Ni 0.99 Cu 0.01 O 2 、 Li 1.6 Ni 0.99 Cu 0.01 O 2 、 Li 1.3 Ni 0.99 Cu 0.01 O 2 、 Li 1.8 Ni 0.99 Al 0.01 O 2 、 Li 1.5 Ni 0.99 Al 0.01 O 2 、 Li 1.1 Ni 0.99 Al 0.01 O 2 、 Li 1.8 Ni 0.999 Al 0.001 O 2 、 Li 1.8 Ni 0.995 Al 0.005 O2 , Li 1.7 Ni 0.99 Al 0.01 O 2 , Li 1.6 Ni 0.99 Al 0.01 O 2 , Li 1.3 Ni 0.99 Al 0.01 O 2 , Li 1.8 Ni 0.99 Mn 0.01 O 2 , Li 1.5 Ni 0.99 Mn 0.01 O 2 , Li 1.1 Ni 0.99 Mn 0.01 O 2 , Li 1.8 Ni 0.999 Mn 0.001 O 2 , Li 1.8 Ni 0.995 Mn 0.005 O 2 , Li 1.7 Ni 0.99 Mn 0.01 O 2 , Li 1.6 Ni 0.99 Mn 0.01 O 2 , Li 1.3 Ni 0.99 Mn 0.01 O 2 .

[0056] In some embodiments, the lithium supplement comprises LiNiαCoβMnγO 2 One or more of, α+β+γ=1 and α≥0.5, 0<β<1, 0<γ<1. In some embodiments, LiNiαCoβMnγO 2 In some embodiments, the lithium supplement includes LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523), LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) or more.

[0057] In the lithium supplement of the present application, considering the crystal defects that may exist in the lithium supplement crystal, the number of oxygen atoms "2" in the chemical structure of the lithium supplement is not necessarily an integer 2, and the number of oxygen atoms varies to an acceptable degree to those skilled in the art. The number of oxygen atoms in the crystal can be measured or calculated using methods known in the art. Similarly, the atomic number of other atoms is not necessarily an integer.

[0058] The lithium supplement provided in the present application not only has a simple preparation process and a relatively low synthesis cost, compared with the lithium supplement 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 called "residual alkali"), improve the gelation phenomenon of the positive electrode slurry and reduce the gas production of the battery, and reduce the deterioration of the performance of the battery cell caused by the gas production of the lithium supplement with a high lithium content.

[0059] The release performance of the lithium supplement can be obtained by any method known in the art. For example, it can be measured by the following method:

[0060] 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 being fully stirred, a coating machine is used to make a pole piece, and 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 performed 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.

[0061] This application Figure 1 The lithium supplement LixNiO 2 Theoretical schematic diagram of (x=1.8), Figure 2 The curves of the number of cycles and gram capacity obtained by the lithium supplement release performance determination method are shown. It can be found that the lithium supplement continuously releases active lithium during multiple cycles, indicating that the polycrystalline lithium supplement added in the present application can continuously release active lithium in the secondary battery. The lithium supplement of the present application can provide a lithium release amount of not less than 400 mAh / g, and optionally, not less than 450, 455, 460, 465, 470 or 475 mAh / g.

[0062] Therefore, the polycrystalline lithium supplement provided by the present application can decay rapidly during the first charge process, quickly release sufficient active lithium, and make up for the active lithium consumed on the negative electrode surface during the first charge and discharge cycle, which can improve the first charge and discharge capacity of the secondary battery and increase the energy density of the battery; and slowly release active lithium during the cycle, significantly improving the cycle performance of the battery and improving the decay of the battery cell. The polycrystalline lithium supplement can be a commercialized product, or it can be prepared according to a method known or commonly used in the art.

[0063] In some embodiments, the volume average particle size Dv50 of the lithium supplement is 2.5-12.5 μm, which can be optionally 2.5-11.5 μm, 3-12 μm, 3.5-11.5 μm, 4-12 μm, 5-12 μm, 6-12 μm, 7-12 μm, 4-11 μm, 5-11 μm, 6-11 μm, 7-11 μm, 5-10 μm, 6-10 μm or 7-10 μm. In some embodiments, the volume average particle size Dv50 of the lithium supplement is 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 10.5 μm, 11 μm, 11.5 μm or 12 μm, or a range between any two of the values.

[0064] In this application, the term "Dv50" refers to the particle size corresponding to the cumulative particle size distribution percentage in the test sample reaching 50%, which can also be understood as the median particle size. Dv50 can be obtained by measuring the particle size distribution of the test sample using a laser particle size analyzer or an electronic scanning electron microscope or other equipment or instruments according to industry operating requirements, and the Dv50 of the test sample is obtained by statistics.

[0065] 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 count 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:

[0066] 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 stretching machine 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 (surface scanning) to identify the lithium supplement agent material, and measure no less than 50 lithium supplement agent particles in total. After statistical calculation, the Dv50 value of the lithium supplement agent in the electrode is obtained.

[0067] The volume average particle size Dv50 of the lithium supplement agent within the 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 a suitable 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 increasing 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 released later. 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 rise sharply, which will easily cause the positive electrode slurry to gel during the preparation process, increasing the risk of gas production of the battery cell; and when the particle size of the lithium supplement agent is too large, the rate at which the lithium supplement agent releases lithium will slow down, affecting the kinetic performance of the battery cell and being unfavorable for improving the energy density.

[0068] In some embodiments, the second active layer does not include a lithium supplement. In this case, since the first active layer containing the lithium supplement is coated on the side close to the positive electrode current collector, it is not in 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 and moisture in the processing environment can be avoided or reduced, the stability of the lithium supplement is improved, and it is helpful to reduce the safety hazards of the lithium supplement and fully utilize the gram capacity of the lithium supplement.

[0069] 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 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% or 4.8%, or a range between any two of the values.

[0070] The lithium supplement agent within the dosage range can provide a sufficient amount of active lithium, which can reduce the safety hazards caused by the side reactions of the lithium supplement agent while slowing down the decay rate of the secondary battery cell and improving the cycle performance of the secondary battery.

[0071] 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 30%, 35%, 40%, 45%, 50%, 55% or 65%, or a range between any two values. When the ratio q is within the 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.

[0072] 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 selected from 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 3%, 4%, 5%, 7%, 8%, 9% or 11%, or a range between any two values. The mass content of the lithium supplement agent within the range can improve the capacity retention rate or the first cycle gram capacity of the secondary battery, improve the cycle capacity of the secondary battery or increase the energy density.

[0073] 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 their respective modified compounds. In this article, the modification of the modified compound may be one or more of a doping modification and a coating modification. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO 2 ), lithium nickel oxide (such as LiNiO 2 ), lithium manganese oxide (such as LiMnO 2 、LiMn 2 O 4 ), 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 O 2 (also referred to as NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O 2 (also referred to as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O 2 (also referred to as NCM211), LiNi0.6 Co 0.2 Mn 0.2 O 2 (also referred to as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O 2 (also referred to as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O 2 ) and at least one of its modified compounds. The olivine-structured lithium-containing phosphate includes 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 PO 4 , 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, for example, can be lithium manganese iron phosphate. The lithium iron phosphate and its doped and modified compounds can also be coated and modified, such as carbon coating. Examples of lithium-containing phosphates may include, but are not limited to, lithium iron phosphate (such as LiFePO 4 (also referred to as LFP), composite materials of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO 4 ), 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 PO 4 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.

[0074] The positive electrode active material has high thermal stability, which is beneficial for the battery to obtain higher high temperature safety performance. In addition, the battery can also have both a longer high temperature cycle life and higher high temperature storage performance.

[0075] The present application also provides a lithium secondary battery prepared using the above-mentioned positive electrode sheet and the application of the lithium secondary battery. The lithium secondary battery provided by the present application has good cycle performance, especially good high-temperature cycle performance, and the battery attenuation is significantly improved compared with the secondary battery without adding lithium supplement. In addition, the secondary battery reduces the loss of active lithium in the first charge and discharge cycle and has improved energy density.

[0076] In some embodiments, the cycle voltage window of the lithium secondary battery is 2.0 to 4.1 V, and an exemplary secondary battery includes a secondary battery whose positive electrode active material is lithium iron manganese phosphate. In some embodiments, the cycle voltage window of the lithium secondary battery is 2.0 to 3.8 V, optionally 2.0 to 3.65 V, and an exemplary secondary battery includes a secondary battery whose positive electrode active material is lithium iron phosphate.

[0077] 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 plate 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, which helps to increase the initial capacity, but 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, 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.

[0078] In some embodiments, the upper limit voltage of the cycle voltage of the lithium secondary battery is 3.6-3.8V, which can be 3.65-3.8V, 3.7-3.8V, 3.6-3.7V, 3.7-3.8V or 3.65-3.7V. An exemplary secondary battery can be a secondary battery whose positive electrode active material is lithium iron phosphate.

[0079] The voltage window in 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.

[0080] In the lithium secondary battery provided in the present application, the positive electrode current collector is a carrier that carries the positive electrode coating layer, which can not only carry the positive electrode active material, but also collect and output the current generated by the positive electrode active material.

[0081] 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 any one or both of the two opposite surfaces of the positive electrode current collector.

[0082] 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.).

[0083] 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), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0084] In some embodiments, the positive electrode film layer may further include a conductive agent, which may include, for example, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0085] In some embodiments, the positive electrode sheet can be prepared in the following manner: 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 collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0086] The lithium secondary battery comprises a negative electrode plate, wherein the negative electrode plate comprises a negative electrode current collector and a negative electrode film layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a negative electrode active material.

[0087] 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 any one or both of the two opposite surfaces of the negative electrode current collector.

[0088] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. 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 material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0089] In some embodiments, the negative electrode active material may adopt the negative electrode active material for the battery 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 oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, 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.

[0090] 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).

[0091] 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.

[0092] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0093] In some embodiments, the negative electrode sheet can be prepared in the following manner: 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 collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0094] The lithium secondary battery includes an electrolyte, which plays a role in conducting ions between the positive electrode plate and the negative electrode plate. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

[0095] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0096] In some embodiments, the electrolyte salt can 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.

[0097] 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, methyl propyl carbonate, ethyl propyl 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 sulfone, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0098] In some embodiments, the electrolyte may further include additives, such as 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, additives that improve battery high or low temperature performance, etc.

[0099] 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 stability and mechanical stability can be selected.

[0100] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0101] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0102] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0103] In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0104] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or any other shape. Figure 4 The secondary battery 5 is a square structure as an example.

[0105] In some embodiments, reference Figure 5 , the outer packaging may include a shell 51 and a cover plate 53. Among them, 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 form 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.

[0106] In some embodiments, secondary batteries may be assembled into a battery module. The number of secondary batteries contained in the battery module may be one or more, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery module.

[0107] Figure 6 4 is an example of a battery module. Figure 6 In the battery module 4, the plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

[0108] 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.

[0109] 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 according to the application and capacity of the battery pack.

[0110] Figure 7 and Figure 8 1 is a battery pack 1 as an example. Figure 7 and Figure 8The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0111] In addition, the present application also provides an electrical 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 electrical device, and can also be used as an energy storage unit for the electrical device. The electrical 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 are not limited thereto.

[0112] As the electrical device, a secondary battery, a battery module or a battery pack may be selected according to its usage requirements.

[0113] Fig. 9 The power consumption device is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the high power and high energy density requirements of the power consumption device for the secondary battery, a battery pack or a battery module can be used.

[0114] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

[0115] Example

[0116] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0117] 1. Preparation method

[0118] Example 1

[0119] 1) Preparation of positive electrode

[0120] Preparation of the first active layer:

[0121] The positive electrode active material lithium iron phosphate (purchased from Defang Nano, DF-5) and the lithium supplement Li 1.8 NiO 2, 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. After充分 stirring and mixing evenly, a positive electrode slurry with good fluidity and not easily gelling is obtained, which is coated on one side of the current collector aluminum foil.

[0122] Among them, the lithium supplement agent is prepared by a method including the following steps:

[0123] Lithium hydroxide (volume average particle size Dv50 is 6-7μm) and nickel hydroxide precursor (volume average particle size Dv50 is 8-10μm) with a molar ratio of 1.02x:1 are mixed in a ball mill under a nitrogen atmosphere. The mixed material is transferred to a crucible and calcined at 790℃-850℃ for 10-12h under a nitrogen atmosphere, with a heating rate of 4℃ / min, and then cooled to obtain a sintered material. The sintered material is crushed and sieved under a nitrogen atmosphere to obtain a lithium supplement agent LixNiO with a volume average particle size Dv50 (measured by laser diffraction method for particle size distribution according to GB / T19077-2016) of 8.2μm 2 (1 < x < 2). The value of x is obtained by the method of "Determination of metal elements in the lithium supplement agent" under the performance test item. Fig.10 The scanning electron microscope image of this lithium supplement agent is shown, which has obvious agglomeration phenomenon. It is formed by the agglomeration of multiple primary particles into secondary particles, and the prepared lithium supplement agent is in a polycrystalline form.

[0124] Preparation of the second active layer:

[0125] The positive electrode 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 stirring and mixing evenly, a positive electrode slurry is obtained, which is coated on the side of the first active layer away from the current collector.

[0126] The current collector coated with the first active layer and the second active layer is dried at 85℃ and then cold-pressed. After drying in a vacuum at 85℃ for 4h, it is trimmed, sliced, and slit to make a positive electrode plate. Among them, the ratio q of the mass of the first active layer to the sum of the masses of the first active layer and the second active layer is 40%, and the ratio p of the mass of the lithium supplement agent in the first active layer to the sum of the masses of the first active layer and the second active layer is 2.4%.

[0127] 2) Preparation of the negative electrode plate

[0128] The negative electrode active material graphite, conductive agent Super P, thickener sodium carboxymethyl cellulose (CMC-Na), and adhesive styrene butadiene rubber (SBR) are mixed evenly in deionized water to prepare a negative electrode slurry with a solid content of 30wt%. Among them, the mass ratio of graphite, Super P, CMC-Na and SBR is 80:15:3:2. The negative electrode slurry is coated on the current collector copper foil and dried at 85°C. After cold pressing, trimming, cutting, and striping, it is dried at 120°C under vacuum conditions for 12 hours to prepare a negative electrode sheet.

[0129] 3) Isolation film

[0130] A 16 μm polyethylene film (PE) was used as the isolation membrane.

[0131] 4) Preparation of electrolyte

[0132] In an argon atmosphere glove box (H2O content < 0.1 ppm, O 2 content <0.1ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a ratio of 3:7 to obtain a mixed solvent. Then, lithium hexafluorophosphate (LiPF6) with a molar concentration of 1 mol / L was added. 6 ) is dissolved in the above mixed solvent and stirred evenly to prepare an electrolyte.

[0133] 5) Preparation of batteries

[0134] The positive electrode sheet, the isolation film, and the negative electrode sheet are stacked in order, so that the isolation film 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, and the pole ears are welded. The bare battery cell is placed in an outer package, and the prepared electrolyte 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.

[0135] The secondary batteries of Examples 2 to 15 are similar to the secondary battery preparation method of Example 1, but the composition and product parameters of the battery pole pieces are adjusted, as shown in Table 1. In the lithium supplement preparation method of Example 6, the volume average particle size of lithium hydroxide is Dv50 2-3 μm, the volume average particle size of the nickel hydroxide precursor is Dv50 5-6 μm, and the heating rate during calcination is 5°C / min. In Example 7, the volume average particle size of lithium hydroxide is Dv50 of 2-3μm, the volume average particle size of the nickel hydroxide precursor is Dv50 of 6-8μm, and the heating rate during the calcination process is 5°C / min; in Example 8, the volume average particle size of lithium hydroxide is Dv50 of 8-9μm, the volume average particle size of the nickel hydroxide precursor is Dv50 of 11-12μm, and the heating rate during the calcination process is 3°C / min; in Example 9, the volume average particle size of lithium hydroxide is Dv50 of 8-9μm, the volume average particle size of the nickel hydroxide precursor is Dv50 of 12-13μm, and the heating rate during the calcination process is 3°C / min.

[0136] 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 materials of the first active layer and the second active layer are both LiFe 0.4 Mn 0.6 PO 4 .

[0137] The secondary batteries of Examples 17 to 19 are prepared in similar ways 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.

[0138] 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).

[0139] The preparation method of the secondary battery of Example 22 is similar to that of the secondary battery of Example 1, 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 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.

[0140] The secondary battery of Example 23 was prepared in a similar manner to that of Example 15, but the raw materials for preparing the lithium supplement were adjusted. Among them, a nickel hydroxide precursor and a copper hydroxide precursor with a Ni:Cu molar ratio of 99:1.02 (volume average particle size Dv50 of 20 - 30 μm) were added.

[0141] The secondary battery of Example 24 was prepared in a similar manner to that of Example 14, but the raw materials for preparing the lithium supplement were adjusted. Among them, 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 were added.

[0142] The secondary battery of Example 25 was prepared in a similar manner to that of Example 22, but the raw materials for preparing the lithium supplement were adjusted. Among them, a nickel hydroxide precursor (volume average particle size Dv50 of 8 - 10 μm) and an aluminum hydroxide precursor (volume average particle size Dv50 of 20 - 30 μm) with a Ni:Al molar ratio of 999:1.02 were added.

[0143] The secondary battery of Example 26 was prepared in a similar manner to that of Example 22, but the raw materials for preparing the lithium supplement were adjusted. Among them, 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) with a Ni:Mn molar ratio of 995:5.1 were added.

[0144] In the secondary battery of Comparative Example 1, commercially available LiPO 4 was used as the lithium supplement, and the other preparation processes were similar to those of Example 1.

[0145] The secondary battery of Comparative Example 2 was basically similar to that of Example 1, and the lithium supplement was prepared according to the following method:

[0146] Lithium hydroxide (volume average particle size Dv50 of 6 - 7 μm) and a nickel hydroxide precursor (volume average particle size Dv50 of 8 - 10 μm) with a molar ratio of 1.08x:1 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 LixNiO with a volume average particle size Dv50 (measured by laser diffraction method for particle size distribution according to GB / T19077 - 2016) of basically 8.2 μm 2 (1 < x < 2). As Fig.11 shown, this lithium supplement showed dispersed primary particle crystals and a single crystal morphology under a scanning electron microscope.

[0147] The preparation method of the secondary battery of Comparative Example 3 is similar to that of Example 1, but without adding a lithium supplement, the positive electrode active material lithium iron phosphate, the conductive agent acetylene black and the binder PVDF (polyvinylidene fluoride) are mixed in a weight ratio of 97:0.7:2.3 to prepare the first active layer.

[0148] The preparation method of the secondary battery of Comparative Example 4 is similar to that of the secondary battery of Comparative Example 3, but the positive electrode active material is LiFe 0.4 Mn 0.6 PO 4 .

[0149] 2. Performance Test

[0150] 1. Capacity retention rate test

[0151] 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 first cycle of the battery (C0); repeat the above steps for the same battery, and the process capacity of the battery after the 800th cycle (C1), the capacity retention rate after 800 cycles = C1 / C0×100%. The test process of the comparative example and other embodiments is the same as above. Calculate the ratio of the capacity retention rate improvement of Examples 1-15 and Comparative Examples 1-2 relative to Comparative Example 3 (capacity retention rate improvement rate).

[0152] 2. Secondary battery first cycle capacity test

[0153] 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, and after standing for 5 minutes, 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 first cycle of the battery (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). Calculate 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).

[0154] 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, it is discharged to 2.0V at a constant current of 1C, and 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 first cycle of the battery (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). Calculate the ratio of the first cycle gram capacity improvement of Example 16 relative to Comparative Example 4 (first cycle gram capacity improvement rate).

[0155] 3. Gas production performance test

[0156] The battery prepared in the embodiment is assembled into a hard shell secondary battery, and the prepared hard shell battery core is welded with a gas-producing nail from the injection hole; then the inside of the Teflon tube is filled with oil, and the Teflon tube filled with oil is connected to the gas-producing nail of the battery core; then a pressure gauge is taken, and the pressure gauge is also filled with oil, and finally the pressure gauge is connected to the other end of the Teflon tube, and the battery core with a pressure gauge is completed and left to stand for 24 hours. The prepared gas-producing battery core is cycled for 200 cycles according to the method in the above "1. Capacity retention rate test", and the pressure gauge reading is recorded online in real time with the number of cycles. Calculate 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), as well as the ratio of the gas production increase of Example 16 relative to Comparative Example 4.

[0157] 4. Morphology test of lithium supplement

[0158] 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.

[0159] 5. Determination of metal elements in lithium supplements

[0160] Using an inductively coupled plasma emission spectrometer, according to EPA test method 6010D-2014, weigh the test sample, add digestion reagent and aqua regia, use the plate digestion method, completely digest the sample at no more than 200°C, test the digestion solvent by an excitation light source, and determine the element content in the sample by the characteristics and emission intensity of different emission lines. The element content (mass fraction w / w%) = element mass / weighed sample is used to determine the proportion of metal elements in the sample.

[0161] III. Analysis of test results of various embodiments and comparative examples

[0162] The batteries of the embodiments and comparative examples were prepared according to the above method, and various performance parameters were measured. The results are shown in Table 1 below.

[0163] Table 1: Test results of Examples 1 to 26 and Comparative Examples 1 to 2

[0164]

[0165] According to the above results, Examples 1-26 all achieved good results. Compared with Comparative Example 3 or 4 without adding a lithium supplement, the capacity retention rate of the secondary battery was significantly improved after adding a lithium supplement, indicating that the decay rate of the secondary battery cell during the cycle was significantly slowed down; or the first cycle capacity was increased, which improved the cycle performance and energy density of the secondary battery. The secondary batteries prepared in Examples 1-26 have good high-temperature cycle performance. Figure 3 As shown, the absolute capacity slope of the secondary battery with the lithium supplement added is slower, which indicates that the cell attenuation of the secondary battery prepared in Example 1 is significantly improved during the cycle.

[0166] It can be seen from Examples 1-5 that, compared with Comparative Example 3, the mass content of the lithium supplement in the first active layer is in the range of 0.5%-12%, which can improve the capacity retention rate or the first cycle gram capacity of the secondary battery. Among them, when the mass content of the lithium supplement in the first active layer is in the range of 2%-10%, the capacity retention rate or the first cycle gram capacity of the secondary battery can be improved at the same time.

[0167] It can also be seen from Examples 1-5 that when the mass ratio of the lithium supplement in the positive electrode sheet 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. Among them, when the mass ratio of the lithium supplement in the positive electrode sheet solid material (excluding the current collector) is 0.8%-4%, the capacity retention rate and the first cycle gram capacity of the secondary battery can be improved at the same time.

[0168] It can be seen from Examples 6-9 that when the particle size DV50 of the lithium supplement agent is in the range of 2.5-12.5 μm, it helps to improve the capacity retention rate or the first cycle gram capacity of the secondary battery; when the particle size DV50 of the lithium supplement agent is in the range of 2.5-11.2 μm, the capacity retention rate and the first cycle gram capacity of the secondary battery can be improved at the same time, however, the gas production of the secondary battery prepared by the lithium supplement agent with a particle size DV50 of 2.5 μm increases significantly, and the gas production performance decreases. When the particle size DV50 of the lithium supplement agent is in the range of 3.5-11.2 μm, the secondary battery can have both improved capacity retention rate and first cycle gram capacity, and good gas production performance.

[0169] It can be seen from Examples 10-13 that when the mass ratio of the first active layer in the solid material of the positive electrode sheet (excluding the current collector) is 10%-70%, the capacity retention rate or the first cycle gram capacity of the secondary battery can be improved. Among them, when the mass ratio of the first active layer in the solid material of the positive electrode sheet (excluding the current collector) is 20%-70%, the capacity retention rate and the first cycle gram capacity of the secondary battery can be improved at the same time; when the mass ratio of the first active layer in the solid material of the positive electrode sheet (excluding the current collector) is 20%-60%, the capacity retention rate or the first cycle gram capacity of the secondary battery can be improved at the same time, and good gas production performance can be achieved.

[0170] As can be seen from Examples 1, 14-15, polycrystalline Li 1.1 NiO 2 , Li 1.5 NiO 2 , Li 1.8 NiO 2 All of them can be used as lithium supplements for lithium secondary batteries to improve the capacity retention rate or first cycle capacity of secondary batteries. 1.1 NiO 2 , Li 1.5 NiO 2 , Li 1.8 NiO 2 Not only the capacity retention rate and the first cycle gram capacity of the secondary battery are improved, but also the gas production performance is excellent, indicating that the secondary battery prepared by the lithium supplement has good storage performance.

[0171] It can be seen from Examples 1 and 16 that the solution of adding a lithium supplement in the present 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, Example 1 shows that the single crystal Li 1.8 NiO 2 It 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 NiO 2 The effect of improving the capacity of the first cycle is not obvious or has no effect; at the same time, the active lithium 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 the polycrystalline Li 1.8 NiO 2 Polycrystalline Li 1.8 NiO 2 It is more helpful to improve the cycle performance of secondary batteries.

[0172] As can be seen from Examples 17-19, polycrystalline Li 1.1 MnO2 , Li 1.5 MnO 2 , Li 1.8 MnO 2 All of them can be used as lithium supplements for lithium secondary batteries, and can significantly improve the capacity retention rate and significantly slow down the attenuation of secondary batteries. 1.5 MnO 2 , Li 1.8 MnO 2 It can simultaneously improve the first cycle gram capacity and cycle capacity retention rate of the secondary battery, and improve the energy density of the secondary battery while improving the attenuation.

[0173] From Examples 20-21, it can be seen that the ternary lithium supplement NCM523 and NCM811 can significantly improve the capacity retention rate, which helps to slow down the attenuation of the secondary battery. At the same time, NCM523 and NCM811 have a low gas production increase rate and excellent gas production performance, which helps to improve the storage life of the secondary battery.

[0174] It can be seen from Examples 22-26 that the lithium nickelate lithium supplement after Cu, Al, and Mn doping can significantly improve the capacity retention rate and significantly slow down the attenuation of the secondary battery. At the same time, it can also improve the first cycle gram capacity and cycle capacity retention rate of the secondary battery, and improve the energy density of the secondary battery while improving the attenuation. In addition, the doping treatment also helps to improve the gas production rate of the secondary battery.

[0175] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and the same effect as the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode plate 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, characterized in that: 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, characterized in that: 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, characterized in that: 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 plate; 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 plate.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that: 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, characterized in that: The q is 10% - 70%, and can be 20% - 60%.

7. The positive electrode sheet according to any one of claims 1 to 6, characterized in that: The positive active materials in the first active layer and the second active layer include one or more of lithium-containing phosphates with 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, characterized in that: 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, characterized in that: The cyclic voltage window of the secondary battery prepared from the positive electrode plate is 2.0 to 3.8V or 2.0 to 4.1V.

10. A lithium secondary battery, characterized in that: Including the positive electrode plate according to any one of claims 1-9.

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

12. The use according to claim 11, characterized in that: 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, characterized in that: Including the lithium secondary battery according to claim 10.

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