Positive electrode sheet, lithium secondary battery, method for manufacturing same, and electric device

Abstract

This application provides a positive electrode sheet, a lithium secondary battery, a method for preparing the same, and an electrical device thereof. The positive electrode sheet includes a positive current collector and a positive electrode film layer formed on the current collector. The positive electrode film layer sequentially includes a lower region and an upper region along a direction away from the current collector. Both the lower and upper regions include a positive electrode active material and a lithium replenishing agent. The lithium replenishing agent in the lower region accounts for 0 to 7% of the total mass of the lithium replenishing agent in the positive electrode film layer; and / or, the lithium replenishing agent in the upper region accounts for 93% to 100% of the total mass of the lithium replenishing agent in the positive electrode film layer. The positive electrode sheet of this application provides improved safety performance when used in lithium secondary batteries.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, and in particular to a positive electrode sheet, a lithium secondary battery and its preparation method, and an electrical device. Background Technology

[0002] In recent years, with the development of lithium-ion batteries, they have been widely used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and many other fields. However, during the first charge and discharge cycle of lithium-ion batteries, the formation of a solid electrolyte film leads to a reduction in active lithium ions, and this loss continues during subsequent cycles and storage, which is usually the main reason for the capacity decay of lithium-ion batteries. Therefore, various processes have been developed to replenish lithium-ion batteries. However, existing replenishment processes still suffer from stability issues such as gas generation by the replenishing agent.

[0003] Therefore, further improvements are still needed in lithium-ion battery refilling. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide a positive electrode sheet, a lithium secondary battery, a method for preparing the same, and an electrical device thereof, wherein the positive electrode sheet used as a lithium secondary battery has improved safety performance.

[0005] To achieve the above objectives, a first aspect of this application provides a positive electrode sheet, comprising a positive current collector and a positive electrode film layer formed on the positive current collector; the positive electrode film layer sequentially comprises a lower region and an upper region along a direction away from the positive current collector, wherein both the lower region and the upper region comprise a positive electrode active material and a lithium replenishing agent, the lithium replenishing agent in the lower region accounting for 0 to 7% of the total mass of the lithium replenishing agent in the positive electrode film layer; and / or, the lithium replenishing agent in the upper region accounting for 93% to 100% of the total mass of the lithium replenishing agent in the positive electrode film layer.

[0006] In this application, by setting the amount of lithium supplement in the upper region of the positive electrode film to be greater than that in the lower region, that is, by setting the majority of the lithium supplement in the upper region, it is beneficial to improve the stability of the positive electrode film, which in turn is beneficial to the capacity utilization and safety performance of the secondary battery.

[0007] In some embodiments, the lithium replenishing agent accounts for 0.9% to 4.4% of the total weight of the positive electrode active material in the positive electrode film. This allows for targeted lithium replenishment of the secondary battery, thereby achieving excellent lithium replenishment performance.

[0008] In some embodiments, the lithium replenishing agent accounts for 2.0% to 4.4% of the total weight of the positive electrode active material in the positive electrode film. This allows for more targeted lithium replenishment of the secondary battery, resulting in superior lithium replenishment performance.

[0009] In some embodiments, the thickness ratio of the upper region to the lower region is 2:8 to 4:6. This enables the overall stability of the film layer.

[0010] In some embodiments, the thickness of the upper region is 10 μm to 50 μm. This allows for easier dissipation of gases generated after the lithium replenishment decomposes, which is beneficial to the stability of the upper region and the positive electrode film. In some embodiments, both the lower and upper regions include a binder; the amount of binder in the lower region is less than that in the upper region. Because the upper region contains a larger amount of lithium replenishment, a larger amount of binder is required to achieve the stability of the film layer in the upper region.

[0011] In some embodiments, the amount of adhesive in the upper region is 3 wt% to 5 wt%; and the amount of adhesive in the lower region is 0.5 wt% to 2.5 wt%.

[0012] In some embodiments, both the lower region and the upper region include a conductive agent; the amount of conductive agent in the lower region is less than the amount of conductive agent in the upper region. Since the decomposition of the lithium replenishing agent requires a high decomposition potential, setting the amount of conductive agent in the lower region to be less than that in the upper region facilitates the decomposition of the lithium replenishing agent in the upper region at a relatively lower potential, i.e., delithiation, thus achieving a better lithium replenishment effect.

[0013] In some embodiments, the amount of conductive agent in the upper region is 4 wt% to 6 wt%; and the amount of conductive agent in the lower region is 0 wt% to 3 wt%.

[0014] In some embodiments, the compaction density of the positive electrode film is 2.4 g / cm³. 3 Up to 2.8 g / cm 3 Therefore, the gas generated after the lithium supplement decomposes is more easily discharged, which is more conducive to the stability of the positive electrode film.

[0015] In some embodiments, the lithium supplement includes Li a C b O c Where 0.5≤a≤3, 0.5≤b≤7, and 0.5≤c≤7. By adopting Li... a C b O c Type of lithium supplement, in which "C" b O c"Some of the elements can leave the electrode as gaseous CO2, leaving no residual elements in the electrode and reducing the impact on the electrode performance."

[0016] In some embodiments, the lithium replenishing agent includes at least one selected from Li₂C₂O₄, Li₂CO₃, Li₂C₃O₅, Li₂C₄O₄, and Li₂C₆O₆. These specific lithium replenishing agents have relatively high delithiation capacity, low initial efficiency, low delithiation potential, and are readily available.

[0017] In some embodiments, the positive electrode active material includes Li 1+x Fe 1-y A y P 1-z E z At least one of O4; wherein -0.1≤x≤0.1, 0≤y≤0.5, 0≤z≤0.1; A is selected from one or more of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, Ge; E is selected from one or more of B, Si, N, S, F, Cl, Br.

[0018] A second aspect of this application also provides a lithium secondary battery, including the positive electrode sheet described in the first aspect above.

[0019] The lithium secondary battery of this application includes the positive electrode provided in this application, therefore, the lithium secondary battery has excellent safety performance.

[0020] A third aspect of this application also provides a method for preparing a lithium secondary battery, comprising: S1, preparing a positive electrode sheet and an initial cell containing the positive electrode sheet; the positive electrode sheet includes a positive current collector and a positive electrode film layer formed on the positive current collector; the positive electrode film layer sequentially includes a lower region and an upper region along a direction away from the positive current collector, wherein both the lower region and the upper region include a positive electrode active material and a lithium replenishing agent, the lithium replenishing agent in the lower region accounting for 0 to 7% of the total mass of the lithium replenishing agent in the positive electrode film layer; and / or, the lithium replenishing agent in the upper region accounting for 93% to 100% of the total mass of the lithium replenishing agent in the positive electrode film layer; S2, performing a charge-discharge treatment on the initial cell to obtain the lithium secondary battery.

[0021] In some implementations, in step S2, the voltage is charged to 3.0V to 4.0V using a current of 0.01C to 0.1C, and then charged to 4.8V to 5V using a current of 0.05C to 0.1C.

[0022] The fourth aspect of this application also provides an electrical device, including the lithium secondary battery of the second aspect above or the lithium secondary battery prepared according to the method of the third aspect above.

[0023] The electrical device of this application includes the lithium secondary battery provided in this application, and therefore has at least the same advantages as the lithium secondary battery. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a battery cell according to one embodiment of this application;

[0025] Figure 2 yes Figure 1 An exploded view of a battery cell according to one embodiment of this application is shown.

[0026] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application;

[0027] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application;

[0028] Figure 5 yes Figure 4 An exploded view of a battery pack according to an embodiment of this application is shown;

[0029] Figure 6 This is a schematic diagram of an electrical device that uses a lithium secondary battery as a power source according to an embodiment of this application.

[0030] Explanation of reference numerals in the attached figures:

[0031] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0032] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode sheet, lithium secondary battery, preparation method thereof, and power application device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter described in this application.

[0033] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0034] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0035] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0036] Unless otherwise specified, all steps in this 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 it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0037] Unless otherwise specified, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0038] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, according to the testing methods given in this application.

[0039] Unless otherwise specified, in this application, the term "active ion" refers to ions that can be inserted and extracted back and forth between the positive and negative electrodes of a secondary battery, including but not limited to lithium ions.

[0040] In existing lithium replenishment processes, lithium replenishing agents are typically added to the entire film layer or its lower region. However, when lithium replenishing agents are added to the entire film layer or its lower region, the gases generated during the decomposition of the lithium replenishing agent can easily lead to stability and safety issues for the electrode. Especially when lithium oxalate-based materials are used as lithium replenishing agents, they require a high decomposition potential, thus necessitating significant optimization of the formulation. For example, this may involve drastically increasing the content of conductive agents or using super conductive agents to achieve decomposition and lithium release within the usable voltage range. Furthermore, the decomposition of these agents generates a large amount of CO2 gas, which, during its generation and emission from the electrode, will affect the structural stability of the electrode.

[0041] Based on this, this application proposes a positive electrode sheet, a lithium secondary battery, a method for preparing the same, and an electrical device thereof. The positive electrode sheet used as a lithium secondary battery has improved safety performance.

[0042] Positive electrode sheet

[0043] A first aspect of this application provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive electrode film layer formed on the positive current collector; the positive electrode film layer sequentially comprises a lower region and an upper region along a direction away from the positive current collector, wherein both the lower region and the upper region comprise a positive electrode active material and a lithium replenishing agent, the lithium replenishing agent in the lower region accounting for 0 to 7% of the total mass of the lithium replenishing agent in the positive electrode film layer; and / or, the lithium replenishing agent in the upper region accounting for 93% to 100% of the total mass of the lithium replenishing agent in the positive electrode film layer.

[0044] In this application, the positive electrode film layer is divided into a lower region and an upper region, with the amount of lithium additive in the upper region being greater than that in the lower region. Compared to when the lithium additive is distributed throughout the entire positive electrode film layer, this configuration primarily locates the lithium additive in the upper region. Therefore, the gas generated during the lithium additive's operation is more easily expelled, and most of the vacancies created after the lithium additive's operation are located in the upper region. This localizes the impact of the lithium additive on the positive electrode film layer, which is beneficial to the stability of the positive electrode film layer and, consequently, to the capacity utilization and safety performance of the secondary battery.

[0045] In some embodiments, the lithium replenishing agent in the aforementioned region is the same lithium replenishing agent as the lithium replenishing agent in the lower region. For example, the lithium replenishing agent is either lithium oxalate or lithium carbonate.

[0046] In some embodiments, the lithium replenishing agent accounts for 0.9% to 4.4% of the total weight of the positive electrode active material in the positive electrode film layer, optionally 2.0% to 4.4%. In this application, by setting the amount of lithium replenishing agent added within the above range, lithium replenishment of the secondary battery can be targeted, thereby achieving excellent lithium replenishment effect.

[0047] In some embodiments, the thickness ratio of the upper region to the lower region is 2:8 to 4:6. Setting the thickness ratio of the upper and lower regions within this range is beneficial to the stability of the positive electrode film.

[0048] In some embodiments, the thickness of the upper region is between 10 μm and 50 μm. A thickness within this range allows for easier venting of gases generated after the decomposition of the lithium replenisher, which is beneficial to the stability of the upper region and the positive electrode film. In some embodiments, both the lower and upper regions include a binder; the amount of binder in the lower region is less than that in the upper region. Because the upper region contains a higher amount of lithium replenisher, a larger amount of binder is required to achieve stability of the upper region film.

[0049] In some embodiments, the amount of adhesive in the upper region is 3 wt% to 5 wt%; and the amount of adhesive in the lower region is 0.5 wt% to 2.5 wt%.

[0050] In some embodiments, both the lower region and the upper region include a conductive agent; the amount of conductive agent in the lower region is less than the amount of conductive agent in the upper region. Since the decomposition of the lithium replenishing agent requires a high decomposition potential, setting the amount of conductive agent in the lower region to be less than that in the upper region facilitates the decomposition of the lithium replenishing agent in the upper region at a relatively lower potential, i.e., delithiation, thus achieving a better lithium replenishment effect.

[0051] In some embodiments, the amount of conductive agent in the upper region is 4 wt% to 6 wt%; and the amount of conductive agent in the lower region is 0 wt% to 3 wt%.

[0052] In some embodiments, the compaction density of the positive electrode film is 2.4 g / cm³. 3 Up to 2.8 g / cm 3 The compaction density of the positive electrode film is within the above range, which makes it easier for the gas generated after the lithium supplement decomposes to be discharged, thus benefiting the stability of the positive electrode film.

[0053] In some embodiments, the lithium supplement includes Li a C b O cWhere 0.5≤a≤3, 0.5≤b≤7, and 0.5≤c≤7. By adopting Li... a C b O c Type of lithium supplement, in which "C" b O c "Some of the elements can leave the electrode in the form of gaseous CO2, leaving no residual elements in the electrode, which is beneficial to the electrode performance."

[0054] In some embodiments, the lithium replenishing agent includes at least one selected from Li₂C₂O₄, Li₂CO₃, Li₂C₃O₅, Li₂C₄O₄, and Li₂C₆O₆. These specific lithium replenishing agents have relatively high delithiation capacity, low initial efficiency, relatively low delithiation potential, and are readily available.

[0055] In some embodiments, the lithium supplement includes Li2C2O4.

[0056] In some embodiments, the lithium replenishing agent includes Li2CO3.

[0057] In some embodiments, the lithium supplement includes Li2C3O5.

[0058] In some embodiments, the lithium supplement includes Li2C4O4.

[0059] In some embodiments, the lithium supplement includes Li2C6O6.

[0060] In some embodiments, the positive electrode active material includes Li 1+x Fe 1-y A y P 1-z E z O4; wherein -0.1≤x≤0.1, 0≤y≤0.5, 0≤z≤0.1; wherein A is selected from one or more of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, and Ge; and E is selected from one or more of B, Si, N, S, F, Cl, and Br. In some embodiments, A is selected from one or more of Mn, Ti, V, Ni, Co, and Mg. In some embodiments, E is selected from one or more of B, Si, N, and S. Therefore, this application is applicable to a wide range of positive electrode active materials and is easy to apply in industrial applications.

[0061] In some embodiments, the positive electrode active material includes LiFePO4.

[0062] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this application, the molar Li content refers to the initial state of the material, i.e., before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0063] In the examples of positive electrode active materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0064] In some embodiments, the adhesive may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin, but is not limited thereto.

[0065] In some embodiments, 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, but is not limited thereto.

[0066] In some embodiments, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0067] In some embodiments, the positive 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 substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (e.g., aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy) on the polymer substrate, wherein the polymer substrate is, for example, polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0068] Lithium secondary batteries

[0069] A second aspect of the embodiments of this application provides a lithium secondary battery, which includes the positive electrode sheet provided in the above embodiments.

[0070] In this article, the term "secondary battery" refers to a single battery cell, battery module, or battery pack. Figure 1 This is a schematic diagram of a battery cell according to one embodiment of this application; Figure 2 yes Figure 1 An exploded view of a battery cell according to one embodiment of this application is shown. Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application;

[0071] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application; Figure 5 yes Figure 4 The diagram shown is an exploded view of a battery pack according to an embodiment of this application. The following is in conjunction with... Figure 1 and Figure 5 The lithium secondary battery in this application will be described.

[0072] In some embodiments, the lithium secondary battery provided in this application also includes a negative electrode.

[0073] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0074] As an example, the negative electrode current collector has two surfaces opposite each other in its own 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.

[0075] 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 substrate and a metal layer formed on at least one surface of the polymer 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0076] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. 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, and 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, this application is not limited to these materials, and other conventional 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.

[0077] In some embodiments, the negative electrode film layer may optionally 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).

[0078] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0079] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners like sodium carboxymethyl cellulose (CMC-Na).

[0080] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0081] In some embodiments, the lithium secondary battery provided in this application also includes an electrolyte.

[0082] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0083] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0084] 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 difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0085] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0086] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0087] In some embodiments, the lithium secondary battery provided in this application also includes a separator.

[0088] In some embodiments, the battery cell also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0089] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven 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.

[0090] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0091] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0092] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0093] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 A quasi-square battery cell 5 is shown as an example.

[0094] In some implementations, refer to Figure 2The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0095] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0096] Figure 3 Battery module 4 is shown as an example. (Refer to...) Figure 3 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0097] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0098] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0099] Figure 4 and Figure 5 Battery pack 1 is shown as an example. (Refer to...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0100] Preparation method of lithium secondary battery

[0101] A third aspect of the embodiments of this application also provides a method for preparing a lithium secondary battery, comprising the following steps:

[0102] S1, preparing a positive electrode sheet and an initial battery cell containing the positive electrode sheet; the positive electrode sheet includes a positive current collector and a positive electrode film layer formed on the positive current collector; the positive electrode film layer sequentially includes a lower region and an upper region in a direction away from the positive current collector, wherein both the lower region and the upper region include a positive electrode active material and a lithium replenishing agent, the lithium replenishing agent in the lower region accounts for 0 to 7% of the total mass of the lithium replenishing agent in the positive electrode film layer; and / or, the lithium replenishing agent in the upper region accounts for 93% to 100% of the total mass of the lithium replenishing agent in the positive electrode film layer;

[0103] S2, perform a charge-discharge process on the initial cell to obtain the lithium secondary battery.

[0104] In some embodiments, step S1, preparing the positive electrode sheet includes: preparing a first positive electrode slurry and a second positive electrode slurry, both of which include a positive electrode active material and a lithium supplement agent; the lithium supplement agent in the first positive electrode slurry accounts for 0 to 7% of the total mass of the lithium supplement agent in the first and second positive electrode slurries, and the lithium supplement agent in the second positive electrode slurry accounts for 93% to 100% of the total mass of the lithium supplement agent in the first and second positive electrode slurries; coating the first positive electrode slurry onto the positive electrode current collector to form a lower region; coating the second positive electrode slurry below the lower region to form an upper region, wherein the upper region and the lower region constitute a positive electrode film layer.

[0105] In step S1, the coating speed of the positive electrode film is 10 m / min to 50 m / min.

[0106] In step S1, the drying temperature after the positive electrode film layer is coated is 100°C to 120°C.

[0107] In some embodiments, the preparation method of a lithium secondary battery further includes: preparing a negative electrode sheet.

[0108] In some embodiments, in step S1, the positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to isolate them. Then, they are wound to form a bare cell, tabs are welded, aluminum-plastic film is inserted to assemble a soft-pack cell, and standard electrolyte is injected to form an initial cell.

[0109] In some embodiments, in step S2, the battery is charged to 3.0V to 4.0V using a current of 0.01C to 0.1C, and then charged to 4.8V to 5V using a current density of 0.05C to 0.1C. In this application, the first step of the charge-discharge process uses a smaller current to charge the battery, causing the electrolyte to decompose and generate gas during the initial charge, forming a solid electrolyte interphase (SEI) film. The second step uses a smaller current to charge the battery, making the solid electrolyte film denser and enhancing its stability.

[0110] In some implementations, after step S2, the initial battery cell after charging and discharging is degassed, replenished with liquid, and sealed to form a lithium secondary battery.

[0111] In some embodiments, prior to step S1, the preparation method of this application further includes: determining the amount of lithium replenishing agent added to the positive electrode film layer.

[0112] In some implementations, the amount of lithium replenishing agent required to fill the vacancies after the first delithiation of the positive electrode active material can be calculated based on the first-efficiency relationship between the positive and negative electrode active materials of the lithium secondary battery. For example, taking a lithium secondary battery with lithium iron phosphate as the positive electrode active material and lithium oxalate as the lithium replenishing agent, the amount of lithium oxalate added satisfies the following relationship.

[0113] The initial delithiation amount of the positive electrode of a lithium secondary battery = the initial delithiation amount of lithium iron phosphate * (1 - the amount of lithium oxalate added) + the initial delithiation amount of lithium oxalate * the amount of lithium oxalate added.

[0114] The first-cycle reversible capacity of a lithium secondary battery = the amount of lithium de-lithiated from the positive electrode in the first cycle * [1 - (the amount of lithium inserted into the positive electrode in the first cycle - the specific capacity of the negative electrode after lithium de-lithiated) / the amount of lithium inserted into the negative electrode in the first cycle * the excess ratio of the negative electrode capacity to the positive electrode capacity per unit area between the positive and negative electrode plates];

[0115] The number of vacancies in a lithium secondary battery after the first delithiation is equal to the amount of lithium iron phosphate delithiation in the first cycle * the initial coulombic efficiency of the positive electrode active material * (1 - the amount of lithium oxalate added) + the amount of lithium oxalate intercalation in the first cycle * the amount of lithium oxalate added.

[0116] The first-cycle lithium insertion capacity of lithium oxalate refers to the first-cycle discharge capacity measured when a coin cell is assembled using lithium oxalate as the positive electrode active material. The first-cycle delithiation capacity of the positive electrode active material refers to the first-cycle charging capacity measured when a coin cell is assembled using lithium iron phosphate as the positive electrode active material. The excess capacity of the negative electrode per unit area between the positive and negative electrode plates is the Cell Balance (CB) value.

[0117] For example, based on the first-efficiency relationship between the lithium iron phosphate material and graphite used in this technology, the average lithium oxalate content of the electrode is calculated to be 3.1% when the reversible specific capacity is replenished to the maximum.

[0118] In addition, this application also provides an electrical device, which includes the secondary battery provided in this application. The secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0119] As the electrical device, a single battery cell, a battery module, or a battery pack can be selected according to its usage requirements.

[0120] Figure 6 This is a schematic diagram of an electrical device using a lithium secondary battery as a power source according to one embodiment of this application. The electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this electrical device, a battery pack or battery module can be used.

[0121] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0122] Example

[0123] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0124] Example 1

[0125] Preparation of positive electrode sheet

[0126] Preparation of the first positive electrode slurry: Lithium iron phosphate (LFP), conductive carbon black, and PVDF are mixed together in a ratio of 97:2:1. After adding N-methylpyrrolidone solvent and stirring, a first positive electrode slurry with a solid content of 70% is formed.

[0127] Preparation of the second positive electrode slurry: Based on the first-efficiency relationship between the lithium iron phosphate material and graphite used, the average lithium oxalate addition amount of the electrode sheet was calculated to be 3.1% when the reversible specific capacity was maximized. LFP, lithium oxalate, conductive carbon black, and PVDF were mixed together in a ratio of 76.6:14.4:5:4. After adding N-methylpyrrolidone solvent and stirring, a second positive electrode slurry with a solid content of 70% was formed. That is, the lithium oxalate content (W1) in the first positive electrode slurry accounts for a percentage of the total mass of lithium oxalate in the positive electrode film (W... 总 The proportion of lithium oxalate in the second cathode slurry (W2) is 0, and the content of lithium oxalate in the second cathode slurry accounts for 0% of the total mass of lithium oxalate in the cathode film (W2). 总 The percentage is 100%.

[0128] Subsequently, a double-layer coating process was performed. The first positive electrode slurry was coated onto a 13μm aluminum foil to form the lower region, and then the second positive electrode slurry was coated onto this lower region to form the upper region, thus obtaining a positive electrode film layer comprising the lower and upper regions. The coating width was 87mm, and the thickness ratio of the upper to lower regions was 2:8. After coating, the film was rolled and compacted to a compaction density of 2.5g / cm³. 3 The absolute thickness of the single-sided positive electrode film is 69 μm.

[0129] Preparation of negative electrode sheet

[0130] Preparation of negative electrode slurry: Graphite, negative electrode conductive carbon black, and styrene-butadiene rubber are mixed together in a ratio of 90:3:7. After adding deionized water as a solvent and stirring, a negative electrode slurry with a solid content of 55% is formed.

[0131] The obtained negative electrode slurry was coated on both sides of a 6μm copper current collector. After drying and rolling, a negative electrode sheet with a coating width of 93mm was obtained. The length of the negative electrode sheet was 1017.91mm, and the surface density of the single-sided coating was 0.082mg / mm². 2 .

[0132] Electrolyte preparation

[0133] Ethylene carbonate and diethyl carbonate were mixed in a mass ratio of 3:7, and lithium hexafluorophosphate (LiPF6) was added. After complete dissolution, a 1 mol / L LiPF6 solution was prepared as the electrolyte.

[0134] Separating membrane

[0135] A 12μm thick polypropylene diaphragm was used as the separator.

[0136] Preparation of lithium secondary batteries

[0137] The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to isolate them. They are then wound to form a bare cell, tabs are welded, and aluminum-plastic film is inserted to assemble a soft-pack cell. Standard electrolyte is injected, and the cell is charged to 3.5V using a constant current of 0.05C. Then, it is activated by charging to 4.8V using a constant current of 0.1C. After degassing, electrolyte replenishment, and sealing, a lithium secondary battery is obtained.

[0138] Example 2

[0139] The lithium secondary battery was prepared using the same method as in Example 1, except that both the first and second positive electrode slurries for preparing the positive electrode sheet contained lithium oxalate. The first positive electrode slurry was formed by mixing LFP, lithium oxalate, conductive carbon black, and PVDF in a ratio of 96.8:0.2:2:1, and the second positive electrode slurry was formed by mixing LFP, lithium oxalate, conductive carbon black, and PVDF in a ratio of 77.32:13.68:5:4. Specifically, the lithium oxalate content (W1) accounted for a percentage of the total mass of lithium oxalate in the positive electrode film (W2). 总 The proportion of lithium oxalate in the second cathode slurry was 5.8%, and the content of lithium oxalate in the second cathode slurry (W2) accounted for 5.8% of the total mass of lithium oxalate in the cathode film layer (W3). 总 The proportion was 94.2%.

[0140] Comparative Example 1

[0141] LFP, lithium oxalate, conductive carbon black, and PVDF were mixed in a ratio of 88.2:2.8:5:4. After adding N-methylpyrrolidone solvent and stirring, a positive electrode slurry with a solid content of 70% was formed. This slurry was then coated onto a 13μm aluminum foil with a coating width of 87mm. Following coating, the slurry was rolled and compacted to a density of 2.5g / cm³. 3 The absolute thickness of the single-sided positive electrode film is 69 μm.

[0142] Subsequently, the negative electrode and electrolyte were prepared in the same manner as in Example 1, and the lithium secondary battery was assembled in the same way.

[0143] Comparative Example 2

[0144] The lithium secondary battery was prepared using the same method as in Example 1, except that in the preparation of the positive electrode sheet, a double-layer coating was used. The second positive electrode slurry was coated on a 13μm aluminum foil to obtain the lower region, and the first positive electrode slurry was coated on the lower region to obtain the upper region.

[0145] Table 1 below shows the relevant parameters of the positive electrode plates in Examples 1 and 2, as well as Comparative Examples 1 and 2.

[0146] Table 1:

[0147]

[0148] Lithium secondary battery performance test

[0149] 1. Charge and discharge test

[0150] The lithium secondary batteries prepared in Examples 1 and 2, as well as Comparative Examples 1 and 2, were subjected to charge-discharge tests according to the following test procedure. Specifically, at a test temperature of 25°C, steps 1) to 7) of the following charge-discharge tests were performed.

[0151] 1) Let it stand for 5 minutes;

[0152] 2) Discharge at a constant current of 0.33C to 2.0V;

[0153] 3) Let it stand for 5 minutes;

[0154] 4) Charge at a constant current of 0.33C to 3.65V, and then charge at a constant voltage of 3.65V to 0.05C. Record the charging capacity C0.

[0155] 5) Let it stand for 5 minutes;

[0156] 6) Discharge at a constant current of 0.33C to 2.0V, and record the discharge capacity C1;

[0157] 7) Let it stand for 5 minutes.

[0158] The actual amount of lithium removed from the positive electrode is the capacity C0 measured in step 4), and the actual reversible specific capacity of the positive electrode is the capacity C1 measured in step 6).

[0159] Battery capacity (Ah) = Actual reversible specific capacity of the positive electrode C1 * Mass of the positive electrode active material m.

[0160] The mass of the positive electrode active material m = 2 * the surface density of the single-sided coating of the positive electrode * the length of the positive electrode sheet * the width of the positive electrode sheet.

[0161] 2. Shear force test

[0162] Disassemble the battery and obtain the positive electrode. Apply die-cut double-sided tape to one side of the positive electrode and a sanded stainless steel plate to the other side. Place it in an oven and heat at 80°C for 5 minutes. Fix the stainless steel plate and the area of ​​the electrode without double-sided tape on a tensile testing machine. Turn on the tensile testing machine, set the parameters, start the test and record the data to obtain the shear force (MPa·s) of the positive electrode.

[0163] The charge / discharge data and shear force data of the lithium secondary batteries in Examples 1 and 2, as well as Comparative Examples 1 and 2, are shown in Table 2 below.

[0164] Table 2:

[0165] project Capacity / Ah Positive electrode shear force (MPa·s) Example 1 3.85 0.78 Example 2 3.85 0.76 Comparative Example 1 3.85 0.65 Comparative Example 2 3.85 0.61

[0166] As shown in Table 2, after lithium replenishment to achieve the same capacity, compared to Comparative Example 1 (where the mass percentage of lithium oxalate in the upper and lower regions is the same) and Comparative Example 2 (where the content of lithium oxalate in the lower region is greater than that in the upper region), Examples 1 and 2 of this application significantly improve the shear force of the positive electrode by setting the content of lithium oxalate in the lower region to be less than that in the upper region. This indicates that the positive electrode of this application can resist greater shear deformation, reflecting that the positive electrode film has good cohesion and structural strength, that is, the stability of the positive electrode film is high, which is beneficial to the safety performance of lithium secondary batteries.

[0167] Examples 3-5

[0168] The lithium secondary battery was prepared using the same method as in Example 1, except that the thickness ratio of the upper and lower regions was different. Please refer to Table 3 for details.

[0169] Table 3:

[0170]

[0171] The charge / discharge data and shear force data of the lithium secondary batteries in Examples 3-5 are shown in Table 4 below. Additionally, for ease of comparison, the charge / discharge data and shear force data of Example 1 are also shown in Table 4.

[0172] Table 4:

[0173] project Capacity / Ah Positive electrode shear force (MPa·s) Example 1 3.85 0.78 Example 3 3.85 0.79 Example 4 3.85 0.78 Example 5 3.85 0.75

[0174] As shown in Table 4, when the same capacity is achieved after lithium replenishment, the positive electrode exhibits greater shear force when the thickness ratio of the upper to lower regions is between 2:8 and 4:6. This indicates that the positive electrode can resist significant shear deformation, thus its positive electrode film layer possesses better cohesion and structural strength, i.e., higher stability, which is beneficial to the safety performance of lithium secondary batteries.

[0175] Examples 6 to 8

[0176] The lithium secondary battery was prepared in the same manner as in Example 1, except that the ratio of the second positive electrode slurry was adjusted according to Table 5 below.

[0177] Table 5 below shows the parameter information of the positive electrode sheets of Examples 6 to 8.

[0178] Table 5:

[0179]

[0180] Lithium secondary battery performance test

[0181] Theoretical cathode delithiation capacity = coin cell delithiation capacity of LFP as cathode active material * mass percentage of LFP in cathode electrode + coin cell delithiation capacity of lithium oxalate as cathode active material * mass percentage of lithium oxalate in cathode electrode.

[0182] Theoretical reversible capacity of the positive electrode = coin cell lithium intercalation capacity of LFP as the positive electrode active material * mass percentage of LFP in the positive electrode sheet + coin cell lithium intercalation capacity of lithium oxalate as the positive electrode active material * mass percentage of lithium oxalate in the positive electrode sheet.

[0183] The charge / discharge data and shear force data of the lithium secondary batteries in Examples 6-8 are shown in Table 6 below. Additionally, for ease of comparison, the charge / discharge data and shear force data of Example 1 are also shown in Table 6.

[0184] Table 6:

[0185]

[0186] As can be seen from Tables 5 and 6, when the mass percentage of binder in the second positive electrode slurry is 3wt% to 5wt% and the mass percentage of conductive agent is 4wt% to 5wt%, the positive electrode sheets all have large shear forces, that is, high stability, while the reversible capacity of the lithium secondary battery is also high.

[0187] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A positive electrode plate, characterized in that, Includes a positive current collector and a positive electrode film layer formed on the positive current collector; The positive electrode film layer includes a lower region and an upper region sequentially along the direction away from the positive electrode current collector, wherein both the lower region and the upper region include positive electrode active material and lithium supplementation agent. The lithium replenishing agent in the lower region accounts for more than 0 and less than or equal to 7% of the total mass of the lithium replenishing agent in the positive electrode film layer; The lithium replenishing agent in the upper region accounts for 93% to 100% of the total mass of the lithium replenishing agent in the positive electrode film layer; Both the lower region and the upper region include an adhesive; the amount of adhesive in the lower region is less than the amount of adhesive in the upper region.

2. The positive electrode sheet according to claim 1, characterized in that, The lithium supplement agent accounts for 0.9% to 4.4% of the total weight of the positive electrode active material in the positive electrode film layer.

3. The positive electrode sheet according to claim 2, characterized in that, The lithium supplement agent accounts for 2.0% to 4.4% of the total weight of the positive electrode active material in the positive electrode film layer.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The thickness ratio of the upper region to the lower region is 2:8 to 4:

6.

5. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The thickness of the upper region is 10 μm to 50 μm.

6. The positive electrode sheet according to claim 1, characterized in that, The amount of adhesive in the upper region is 3 wt% to 5 wt%; the amount of adhesive in the lower region is 0.5 wt% to 2.5 wt%.

7. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, Both the lower region and the upper region include a conductive agent; the amount of conductive agent in the lower region is less than the amount of conductive agent in the upper region.

8. The positive electrode sheet according to claim 7, characterized in that, The amount of conductive agent in the upper region is 4 wt% to 6 wt%; the amount of conductive agent in the lower region is 0 wt% to 3 wt%.

9. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The compaction density of the positive electrode film is 2.4 g / cm³. 3 Up to 2.8 g / cm 3 .

10. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The lithium supplement includes Li a C b O c Where 0.5≤a≤3, 0.5≤b≤7, and 0.5≤c≤7.

11. The positive electrode sheet according to claim 10, characterized in that, The lithium supplement includes at least one of Li2C2O4, Li2CO3, Li2C3O5, Li2C4O4, and Li2C6O6.

12. The positive electrode sheet according to any one of claims 1 to 3, characterized in that, The positive electrode active material includes Li 1+x Fe 1-y A y P 1-z E z O4; Where -0.1≤x≤0.1, 0≤y≤0.5, 0≤z≤0.1; A is selected from one or more of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, and Ge; E is selected from one or more of B, Si, N, S, F, Cl, and Br.

13. A lithium secondary battery, characterized in that, The positive electrode sheet includes any one of claims 1 to 12.

14. A method for preparing a lithium secondary battery, characterized in that, Includes the following steps: S1, preparing a positive electrode sheet and an initial battery cell containing the positive electrode sheet; the positive electrode sheet includes a positive current collector and a positive electrode film layer formed on the positive current collector; the positive electrode film layer sequentially includes a lower region and an upper region along a direction away from the positive current collector, wherein both the lower region and the upper region include a positive electrode active material and a lithium supplement agent, the lithium supplement agent in the lower region accounts for more than 0 and less than or equal to 7% of the total mass of the lithium supplement agent in the positive electrode film layer; the lithium supplement agent in the upper region accounts for 93% to 100% of the total mass of the lithium supplement agent in the positive electrode film layer; both the lower region and the upper region include a binder; the amount of binder in the lower region is less than the amount of binder in the upper region; S2, perform a charge-discharge process on the initial cell to obtain the lithium secondary battery.

15. The method according to claim 14, characterized in that, In step S2, the voltage is charged to 3.0V to 4.0V using a current of 0.01C to 0.1C, and then charged to 4.8V to 5V using a current of 0.05C to 0.1C.

16. An electrical device comprising the lithium secondary battery of claim 13 or the lithium secondary battery prepared by the method according to claim 14 or 15.

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