Secondary battery, preparation method thereof and electric device

Abstract

The invention discloses a secondary battery, a preparation method thereof and an electric device. The secondary battery comprises a shell, a battery cell assembly and electrolyte, the battery cell assembly and the electrolyte are arranged in the shell, the battery cell assembly is located in the electrolyte, the battery cell assembly comprises a positive pole piece, a negative pole piece and a diaphragm, and the diaphragm is located between the positive pole piece and the negative pole piece; the water content of the positive pole piece is 80 ppm to 400 ppm, and / or the N-methyl pyrrolidone content of the positive pole piece is 20 ppm to 150 ppm, so that the service life of the secondary battery is prolonged, and the first charge-discharge efficiency of the secondary battery is improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to secondary batteries, their preparation methods, and electrical devices. Background Technology

[0002] In the fabrication of secondary batteries, the negative electrode, separator, and positive electrode are typically stacked or wound sequentially to form a cell assembly. The solvent content of the positive electrode significantly affects the performance of the secondary battery. Summary of the Invention

[0003] This application provides a secondary battery and its preparation method and electrical device, which reduces the solvent content in the positive electrode and improves the battery performance.

[0004] To address the aforementioned technical problems, the first aspect of this application provides a secondary battery, including a casing, a cell assembly disposed within the casing, and an electrolyte. The cell assembly is located in the electrolyte and includes a positive electrode, a negative electrode, and a separator, with the separator located between the positive and negative electrodes. The positive electrode has a water content of 80ppm-400ppm and / or an N-methylpyrrolidone content of 20ppm-150ppm.

[0005] The lower water and / or N-methylpyrrolidone content of the positive electrode extends the lifespan of the secondary battery and improves its initial charge-discharge efficiency.

[0006] In one embodiment, the water content of the positive electrode is 80ppm-150ppm.

[0007] The lower water content of the positive electrode sheet helps extend the life of the secondary battery and improves the initial charge and discharge efficiency.

[0008] In one embodiment, the content of N-methylpyrrolidone in the positive electrode is 20ppm-50ppm.

[0009] The lower content of N-methylpyrrolidone in the positive electrode sheet helps to extend the life of the secondary battery and improve the first charge and discharge efficiency of the secondary battery.

[0010] In one embodiment, the positive electrode includes a positive active layer, the positive active layer includes a positive active material, and the positive active material includes a lithium transition metal phosphate.

[0011] The positive electrode active material includes lithium transition metal phosphates. When the content of water and / or N-methylpyrrolidone in the positive electrode sheet is reduced by heat treatment, the lithium transition metal phosphates hardly undergo chemical reactions, thus maintaining the performance of the positive electrode active material while reducing the content of water and / or N-methylpyrrolidone in the positive electrode sheet.

[0012] In one embodiment, the surface of the lithium transition metal phosphate is coated with a carbon material.

[0013] By coating the surface of lithium transition metal-containing phosphates with carbon materials, a conductive network is formed on the surface of the phosphates, improving the conductivity of the material and enhancing the rate performance and cycle stability of the secondary battery. When the water and / or N-methylpyrrolidone content of the positive electrode is reduced through heat treatment, the carbon layer can act as a barrier, reducing the possibility of reactions occurring in the positive electrode active material during heat treatment, thereby improving the performance of the secondary battery.

[0014] In one embodiment, the positive electrode active layer further includes a binder and a conductive agent, and the mass ratio of the positive electrode active material, binder and conductive agent is (96.5-97.5):(1.2-2):(0.5-1.5).

[0015] By setting the above proportions of positive electrode active material, binder, and conductive agent, it is beneficial to extend the life of secondary batteries and improve the first charge and discharge efficiency of secondary batteries.

[0016] In one embodiment, the adhesive comprises polyvinylidene fluoride.

[0017] When heat treatment is used to reduce the water and / or N-methylpyrrolidone content of the positive electrode sheet, the binder is designed to include polyvinylidene fluoride (PVDF). In addition to reducing the water and / or N-methylpyrrolidone content in the positive electrode sheet, PVDF can also improve the coating state of the positive electrode active material particles, improve the surface dynamics of the positive electrode active material particles, which is beneficial to extending the life of the secondary battery and improving the first charge and discharge efficiency of the secondary battery.

[0018] In one embodiment, the thickness of the positive electrode active layer is 85 μm-95 μm.

[0019] By setting the thickness of the positive electrode active layer as described above, the secondary battery maintains high initial efficiency and cycle performance.

[0020] In one embodiment, the areal density of the positive electrode active layer is 20 mg / cm³. 2 -25mg / cm 2 .

[0021] By setting the areal density of the positive electrode active layer as described above, the secondary battery maintains high initial efficiency and cycle performance.

[0022] To address the aforementioned technical problems, a second aspect of this application provides a method for preparing a secondary battery, comprising: sequentially stacking a negative electrode sheet, a separator, and a positive electrode sheet to form a cell assembly; placing the cell assembly in a housing; and injecting an electrolyte into the housing and sealing it to form a secondary battery; wherein the method for preparing the positive electrode sheet comprises: depositing a positive active material slurry layer on at least one side of a positive current collector to form a first semi-finished product, the positive active material slurry layer comprising a positive active material, a binder, and a solvent; and drying and compacting the first semi-finished product to form... The second semi-finished product comprises a pre-formed positive electrode active layer formed by drying and compacting a slurry layer of positive electrode active material, the pre-formed positive electrode active layer comprising positive electrode active material, binder, and residual solvent; the residual solvent comprises at least one of water and N-methylpyrrolidone; the second semi-finished product is heat-treated to obtain a positive electrode sheet, wherein the pre-formed positive electrode active layer is heat-treated to form a positive electrode active layer; the water content of the positive electrode sheet is 80ppm-400ppm, and / or the N-methylpyrrolidone content of the positive electrode sheet is 20ppm-150ppm.

[0023] In the secondary electrode preparation method provided in this application embodiment, the pre-formed positive electrode active layer is heat-treated to evaporate the residual solvent, thereby reducing the solvent content in the formed positive electrode active layer, reducing the water content and / or NMP content in the formed positive electrode sheet, and also reducing the amount of organic matter that affects battery life. In addition, by heat-treating the second semi-finished product, the coating state of the binder on the surface of the positive electrode active material particles is improved, and the surface dynamic properties of the positive electrode active material particles are improved, which is beneficial to extending the life of the secondary battery and improving the first charge and discharge efficiency of the secondary battery.

[0024] In one embodiment, the heat treatment temperature is 200℃-500℃ and the time is 10s-60s.

[0025] By setting the heat treatment temperature as described above, maintaining the temperature of the pre-formed positive electrode active layer after heating can evaporate the solvent in the pre-formed positive electrode active layer, reduce the amount of solvent in the pre-formed positive electrode active layer, and reduce the impact of the solvent on the performance of the secondary battery; at the same time, it reduces the probability of problems such as demolding caused by excessively high temperature.

[0026] By controlling the heat treatment time, the temperature of the pre-formed positive electrode active layer can be controlled. Maintaining the temperature of the pre-formed positive electrode active layer after heating can evaporate the solvent in the pre-formed positive electrode active layer, reducing the amount of solvent in the pre-formed positive electrode active layer. At the same time, the heat absorbed by the second semi-finished product is kept below the heat threshold of the binder losing its adhesive force, reducing problems such as demolding during the heat treatment process.

[0027] In one embodiment, the positive electrode active material slurry layer further includes a conductive agent; in the positive electrode active material slurry layer, the mass ratio of the positive electrode active material, binder, and conductive agent is (96.5-97.5):(1.2-2):(0.5-1.5); the solid content of the positive electrode active material slurry layer is 65%-69%.

[0028] By adopting the above settings, the positive electrode active material, binder, conductive agent, etc. are uniformly dispersed in the positive electrode active slurry, and the positive electrode active material slurry layer can be firmly attached to the positive electrode current collector, which is beneficial to improving the performance of the secondary battery.

[0029] To address the aforementioned technical problems, a third aspect of this application provides an electrical device comprising a secondary battery as described in any of the preceding claims or a secondary battery prepared by any of the preceding claims' methods. The electrical device possesses at least the same advantages as the secondary battery or the secondary battery prepared by the aforementioned methods.

[0030] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of the structure of the secondary battery provided in the embodiments of this application;

[0033] Figure 2 This is an exploded structural diagram of a battery cell provided in an embodiment of this application;

[0034] Figure 3 This is a schematic flowchart of the method for preparing a secondary battery provided in the embodiments of this application;

[0035] Figure 4 yes Figure 3 A schematic flowchart of the preparation method of the positive electrode sheet in the preparation method of the secondary battery shown.

[0036] Figure 5 This is a schematic diagram of the vehicle structure provided in an embodiment of this application. Detailed Implementation

[0037] To make the objectives, technical solutions, and effects of this application clearer and more explicit, the embodiments of the technical solutions of this application will be described in detail below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of this application, and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0039] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces), unless otherwise explicitly specified.

[0040] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0041] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0042] Quantities, ratios, and other numerical values ​​are presented in range format in this document. It should be understood that this range format is for convenience and brevity and should be interpreted flexibly to include not only numerical values ​​explicitly specified as range limits, but also all individual numerical values ​​or subranges covered within the range, as if each numerical value and subrange were explicitly specified.

[0043] Unless otherwise specified, all steps of this application may be performed sequentially, randomly, or in parallel, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially, or steps (a) and (b) may be performed simultaneously in parallel. For example, the method may also include step (c), indicating that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or steps (a), (c), and (b), or steps (c), (a), and (b), etc.

[0044] Lithium-ion batteries have advantages such as high voltage, light weight, long cycle life, no memory effect, and good safety, and have been widely used. A lithium-ion battery consists of a positive electrode, a negative electrode, a separator, and an electrolyte. The battery manufacturing process typically involves: sequentially stacking the negative electrode, separator, and positive electrode to form a cell assembly; placing the cell assembly in a casing; and injecting the electrolyte into the casing. The solvent content in the positive electrode affects the battery's performance.

[0045] Therefore, embodiments of this application provide a secondary battery, a method for preparing the same, and an electrical device thereof.

[0046] Please see Figure 1 , Figure 1 This is a schematic diagram of the structure of the secondary battery provided in the embodiments of this application.

[0047] The secondary battery 100 includes a housing 10 and a battery cell 20, with the battery cell 20 housed within the housing 10.

[0048] The housing 10 provides a space for accommodating the battery cell 20, and can adopt various structures. In some embodiments, the housing 10 may include a first part 11 and a second part 12, which overlap each other, and together define a space for accommodating the battery cell 20. The second part 12 may be a hollow structure with one open end, and the first part 11 may be a plate-like structure, covering the open side of the second part 12 so that the first part 11 and the second part 12 together define the space; alternatively, the first part 11 and the second part 12 may both be hollow structures with one open side, with the open side of the first part 11 covering the open side of the second part 12. Of course, the housing 10 formed by the first part 11 and the second part 12 can be of various shapes, such as a cylinder, a cuboid, etc.

[0049] In the secondary battery 100, there can be multiple battery cells 20, which can be connected in series, parallel, or in a mixed manner. A mixed connection means that multiple battery cells 20 are connected in both series and parallel. Multiple battery cells 20 can be directly connected in series, parallel, or in a mixed manner, and then the whole assembly of multiple battery cells 20 is housed in the housing 10. Of course, the secondary battery 100 can also be composed of multiple battery cells 20 first connected in series, parallel, or in a mixed manner to form a battery module, and then multiple battery modules are connected in series, parallel, or in a mixed manner to form a whole, which is also housed in the housing 10.

[0050] The secondary battery 100 may also include other structures, for example, the secondary battery 100 may also include a busbar component for realizing electrical connection between multiple battery cells 20.

[0051] Each battery cell 20 can be a lithium-ion battery or a sodium-ion battery, but is not limited to either. The battery cell 20 can be cylindrical, flat, cuboid, or other shapes.

[0052] Please see Figure 2 , Figure 2 This is an exploded structural diagram of a battery cell provided in an embodiment of this application.

[0053] Battery cell 20 refers to the smallest unit that makes up the secondary battery 100. For example... Figure 2 As shown, the battery cell 20 includes an end cap 21, a housing 22, a cell assembly 23, and other functional components.

[0054] End cap 21 refers to a component that covers the opening of housing 22 to isolate the internal environment of battery cell 20 from the external environment. In any case, the shape of end cap 21 can be adapted to the shape of housing 22 to fit it. Optionally, end cap 21 can be made of a material with a certain hardness and strength (such as aluminum alloy), so that end cap 21 is less prone to deformation under pressure and impact, enabling battery cell 20 to have higher structural strength and improved stability.

[0055] The end cap 21 may be provided with functional components such as electrode terminals 21a. The electrode terminals 21a can be used to electrically connect with the cell assembly 23 for outputting or inputting electrical energy of the battery cell 20. In some embodiments, the end cap 21 may also be provided with a pressure relief mechanism for releasing internal pressure when the internal pressure or temperature of the battery cell 20 reaches a threshold.

[0056] The end cap 21 can be made of various materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc. This application embodiment does not impose any special restrictions on this.

[0057] In some embodiments, an insulating element may be provided on the inner side of the end cap 21. The insulating element can be used to isolate the electrical connection components within the housing 22 from the end cap 21 to reduce the risk of short circuits. For example, the insulating element may be made of plastic, rubber, etc.

[0058] The housing 22 is a component used to cooperate with the end cap 21 to form the internal environment of the battery cell 20. This internal environment can accommodate the cell assembly 23, electrolyte, and other components. The housing 22 and the end cap 21 can be independent components. An opening can be provided on the housing 22, and the end cap 21 can be used to close the opening to form the internal environment of the battery cell 20. Alternatively, the end cap 21 and the housing 22 can be integrated. Specifically, the end cap 21 and the housing 22 can form a common connecting surface before other components are inserted into the housing. When it is necessary to encapsulate the interior of the housing 22, the end cap 21 closes the housing 22. The housing 22 can be of various shapes and sizes, such as cuboid, cylindrical, hexagonal prism, etc. Specifically, the shape of the housing 22 can be determined according to the specific shape and size of the cell assembly 23. The material of the housing 22 can be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc. This application embodiment does not impose any special limitations on this.

[0059] The cell assembly 23 is the component in the battery cell 100 where the electrochemical reaction occurs. The casing 22 may contain one or more cell assemblies 23. The cell assembly 23 includes a positive electrode, a negative electrode, and a separator, with the separator located between the positive and negative electrode. The positive and negative electrode are wound or stacked to form the cell assembly 23. The portions of the positive and negative electrode with active material constitute the main body of the cell assembly 23, while the portions without active material each constitute a tab 23a. The positive and negative tabs may be located together at one end of the main body or at opposite ends. During the charging and discharging process of the secondary battery 100, the positive and negative active materials react with the electrolyte, and the tabs 23a connect to the electrode terminals 21a to form a current loop.

[0060] In this embodiment, the water content of the positive electrode is 80ppm-400ppm, and / or the N-methylpyrrolidone (NMP) content of the positive electrode is 20ppm-150ppm. The lower water content and / or N-methylpyrrolidone content of the positive electrode extends the lifespan of the secondary battery and improves its initial charge-discharge efficiency.

[0061] In one embodiment, the water content of the positive electrode is 80ppm-150ppm.

[0062] The lower water content of the positive electrode sheet helps extend the life of the secondary battery and improves the initial charge and discharge efficiency.

[0063] In one embodiment, the content of N-methylpyrrolidone in the positive electrode is 20ppm-50ppm.

[0064] The lower content of N-methylpyrrolidone in the positive electrode sheet helps to extend the life of the secondary battery and improve the first charge and discharge efficiency of the secondary battery.

[0065] In one embodiment, the positive electrode includes a positive active layer, the positive active layer includes a positive active material, and the positive active material includes a lithium transition metal phosphate.

[0066] The positive electrode active material includes lithium transition metal phosphates. When the content of water and / or N-methylpyrrolidone in the positive electrode sheet is reduced by heat treatment, the lithium transition metal phosphates hardly undergo chemical reactions, thus maintaining the performance of the positive electrode active material while reducing the content of water and / or N-methylpyrrolidone in the positive electrode sheet.

[0067] In one embodiment, the surface of the lithium transition metal phosphate is coated with a carbon material.

[0068] By coating the surface of lithium transition metal-containing phosphates with carbon materials, a conductive network is formed on the surface of the phosphates, improving the conductivity of the material and enhancing the rate performance and cycle stability of the secondary battery. When the water and / or N-methylpyrrolidone content of the positive electrode is reduced through heat treatment, the carbon layer can act as a barrier, reducing the possibility of reactions occurring in the positive electrode active material during heat treatment, thereby improving the performance of the secondary battery.

[0069] In one embodiment, the positive electrode active layer further includes a binder and a conductive agent, and the mass ratio of the positive electrode active material, binder and conductive agent is (96.5-97.5):(1.2-2):(0.5-1.5).

[0070] By setting the above proportions of positive electrode active material, binder, and conductive agent, it is beneficial to extend the life of secondary batteries and improve the first charge and discharge efficiency of secondary batteries.

[0071] In one embodiment, the adhesive comprises polyvinylidene fluoride.

[0072] When heat treatment is used to reduce the water and / or N-methylpyrrolidone content of the positive electrode sheet, the binder is designed to include polyvinylidene fluoride (PVDF). In addition to reducing the water and / or N-methylpyrrolidone content in the positive electrode sheet, PVDF can also improve the coating state of the positive electrode active material particles, improve the surface dynamics of the positive electrode active material particles, which is beneficial to extending the life of the secondary battery and improving the first charge and discharge efficiency of the secondary battery.

[0073] In one embodiment, the thickness of the positive electrode active layer is 85 μm-95 μm.

[0074] By setting the thickness of the positive electrode active layer as described above, the secondary battery maintains high initial efficiency and cycle performance.

[0075] In one embodiment, the areal density of the positive electrode active layer is 20 mg / cm³. 2 -25mg / cm 2 .

[0076] By setting the areal density of the positive electrode active layer as described above, the secondary battery maintains high initial efficiency and cycle performance.

[0077] Please see Figure 3 and Figure 4 , Figure 3 This is a schematic flowchart of the secondary battery preparation method provided in the embodiments of this application. Figure 4 yes Figure 3 The flowchart illustrates the preparation method of the positive electrode in the secondary battery preparation method shown.

[0078] The method for preparing the secondary battery provided in this application can be used to prepare the secondary battery provided in the above embodiments. The method for preparing the secondary battery provided in this application specifically includes:

[0079] Step S01: The negative electrode, the separator and the positive electrode are stacked in sequence to form a cell assembly.

[0080] Specifically, the steps for forming a battery cell assembly include: sequentially preparing a positive electrode sheet and a negative electrode sheet, alternately stacking the positive and negative electrode sheets, and setting a separator between the positive and negative electrode sheets to achieve the function of isolation, thereby obtaining a battery cell assembly, or the battery cell assembly can be obtained by winding.

[0081] In this embodiment, the method for preparing the positive electrode sheet specifically includes:

[0082] Step S011: A positive electrode active material slurry layer is formed on at least one side of the positive electrode current collector to form a first semi-finished product. The positive electrode active material slurry layer includes a positive electrode active material, a binder, and a solvent.

[0083] In one embodiment, the positive electrode current collector may be a metal foil or a composite positive electrode current collector. Optionally, aluminum foil may be used as the metal foil. Optionally, the composite positive electrode current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite positive electrode current collector may be formed by forming a metal material on the polymer material substrate; the metal material may include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy; the polymer material may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0084] In one embodiment, the secondary battery is a lithium-ion battery, and the positive electrode active material includes one or more of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. This application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials may also be used. It should be noted that the modified compound of the positive electrode active material may be a doping modification and / or surface coating modification of the positive electrode active material.

[0085] Optionally, the positive electrode active material may include materials with the general formula Li a Ni b Co c M d O e A f One or more of lithium transition metal oxides and their modified compounds. 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M includes one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A includes one or more of N, F, S and Cl.

[0086] Optionally, the lithium transition metal oxide may include one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds.

[0087] Optionally, the lithium phosphate with an olivine structure may include one or more of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their respective modified compounds.

[0088] Optionally, the positive electrode active material may include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3O2(NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.85 Co 0.15 Al 0.05 One or more of O2, LiFePO4 and LiMnPO4.

[0089] In one embodiment, the secondary battery is a sodium-ion battery, and the positive electrode active material may include, but is not limited to, one or more of sodium-containing transition metal oxides, polyanionic materials, and Prussian blue materials.

[0090] Optionally, the polyanionic material includes one or more of phosphates, fluorophosphates, pyrophosphates, and sulfates.

[0091] Optionally, the positive electrode active material may include NaFeO2, NaCoO2, NaCrO2, NaMnO2, NaNiO2, NaNi 1 / 2 Ti 1 / 2O2, NaNi 1 / 2 Mn 1 / 2 O2, Na 2 / 3 Fe 1 / 3 Mn 2 / 3 O2, NaNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, NaFePO4, NaMnPO4, NaCoPO4, Prussian blue materials and materials with the general formula X p M' q (PO4) r O x Y 3-x One or more of the materials. In general formula X p M' q (PO4) r O x Y 3-x In the given condition, 0 < p ≤ 4, 0 < q ≤ 2, 1 ≤ r ≤ 3, 0 ≤ x ≤ 2, and X includes H. + Li + Na + K + and NH4 +One or more of the following, M' is a transition metal cation, and M' may include one or more of V, Ti, Mn, Fe, Co, Ni, Cu and Zn, and Y is a halide anion, and Y may include one or more of F, Cl and Br.

[0092] In one embodiment, the adhesive may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins.

[0093] In one embodiment, the solvent includes one or more of water and N-methylpyrrolidone (NMP).

[0094] In one embodiment, the positive electrode active material slurry layer further includes a conductive agent. The conductive agent includes one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0095] Step S012: The first semi-finished product is dried and compacted to form the second semi-finished product. The positive electrode active material slurry layer is dried and compacted to form a pre-formed positive electrode active layer. The pre-formed positive electrode active layer includes positive electrode active material, binder, and residual solvent. The residual solvent includes at least one of water and N-methylpyrrolidone.

[0096] Specifically, the positive electrode active material slurry layer is dried to remove the solvent, thus reducing the amount of solvent. Therefore, the amount of solvent in the pre-formed positive electrode active layer after drying is less than the amount of solvent in the positive electrode active material slurry layer. The pre-formed positive electrode active layer includes residual solvent. The process conditions for drying the positive electrode active material slurry layer can refer to existing technologies.

[0097] Step S013: The second semi-finished product is heat-treated to obtain a positive electrode sheet, wherein the pre-made positive electrode active layer is heat-treated to form a positive electrode active layer; the water content of the positive electrode sheet is 80ppm-400ppm, and / or the N-methylpyrrolidone content of the positive electrode sheet is 20ppm-150ppm.

[0098] Specifically, the pre-fabricated positive electrode active layer is heat-treated to remove residual solvent in the pre-fabricated positive electrode active layer, thereby further reducing the amount of solvent.

[0099] Step S02: Place the battery cell assembly in the housing.

[0100] Specifically, the casing is a component used to form an internal environment that can accommodate the battery cell assembly and electrolyte. The shape of the casing can be determined according to the specific shape and size of the battery cell assembly. The casing can be made of various materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and this application embodiment does not impose any special limitations on this.

[0101] Step S03: Inject electrolyte into the casing and seal it to form a secondary battery.

[0102] Specifically, the electrolyte affects the performance of a secondary battery. It provides a path for movement between the positive and negative electrodes, facilitating the charging and discharging process. The specific composition of the electrolyte is designed according to requirements.

[0103] Electrolyte is injected into the casing and sealed to obtain a single battery cell. Multiple battery cells are integrated to form a battery module, which can provide higher voltage and capacity, and has specific output functions. The battery module is then installed in a battery housing, usually with the addition of a battery management system, to form a battery pack, which is typically provided to the user. Alternatively, multiple battery cells can be directly installed in a housing to form a battery pack.

[0104] In the secondary electrode preparation method provided in this application embodiment, the pre-formed positive electrode active layer is heat-treated to evaporate the residual solvent, thereby reducing the solvent content in the formed positive electrode active layer and reducing the amount of organic matter (e.g., carboxyl matrix molecules) that affects battery life. In addition, by heat-treating the second semi-finished product, the coating state of the binder (e.g., PVDF) on the surface of the positive electrode active material particles is improved, and the surface dynamic properties of the positive electrode active material particles are improved, which is beneficial to extending the life of the secondary battery and improving the first charge and discharge efficiency of the secondary battery.

[0105] In one embodiment, the heat treatment temperature is 200℃-500℃ and the time is 10s-60s.

[0106] By setting the heat treatment temperature as described above, maintaining the pre-formed positive electrode active layer at its heated temperature allows the solvent in the pre-formed positive electrode active layer to evaporate, reducing the amount of solvent in the pre-formed positive electrode active layer and minimizing the solvent's impact on the secondary battery performance. Simultaneously, it reduces the probability of problems such as demolding due to excessively high temperatures. By controlling the heat treatment time, the temperature of the pre-formed positive electrode active layer can be controlled, maintaining the heated temperature of the pre-formed positive electrode active layer to evaporate the solvent, reducing the amount of solvent in the pre-formed positive electrode active layer. At the same time, it ensures that the heat absorbed by the second semi-finished product does not exceed the heat threshold for the binder to lose its adhesive strength, reducing the likelihood of demolding during the heat treatment process.

[0107] Optionally, in step S013, the heat treatment of the second semi-finished product specifically includes: raising the temperature of the heating equipment to a set temperature; moving the second semi-finished product from one side of the heating equipment to the other side for heating; wherein the set temperature is 200℃-500℃; and / or, the heating rate of the heating equipment is 30℃ / s-100℃ / s. It should be noted that raising the temperature to the set temperature is only necessary when using the heating equipment for the first time; when the heating equipment is constantly in operation and heat-treating another second semi-finished product, the step of raising the temperature to the set temperature is not required.

[0108] Heating is achieved by moving the second semi-finished product from one side of the heating equipment to the other side, while maintaining a constant heating temperature throughout the process. This ensures consistent heating across the second semi-finished product, reduces the formation of hot spots at fixed points, lowers the probability of localized overheating or insufficient heating, optimizes heat absorption, reduces heat loss, and improves heating efficiency.

[0109] By setting the heating equipment temperature as described above, maintaining the pre-formed positive electrode active layer at the pre-formed temperature can evaporate the solvent in the pre-formed positive electrode active layer, reducing the amount of solvent in the pre-formed positive electrode active layer and reducing the impact of the solvent on the performance of the secondary battery; at the same time, it reduces the probability of problems such as excessive temperature of the second semi-finished product leading to demolding.

[0110] The set temperature can be 200℃, 250℃, 300℃, 350℃, 400℃, 450℃, 500℃, etc., or a range consisting of any two of the above values, such as 200℃-500℃, 400℃-500℃, etc. The heating rate can be 30℃ / s, 40℃ / s, 50℃ / s, 60℃ / s, 70℃ / s, 80℃ / s, 90℃ / s, 100℃ / s, or a range consisting of any two of the above values, such as 30℃ / s-60℃ / s, 40℃ / s-80℃ / s, etc.

[0111] Optionally, the second semi-finished product is moved at a constant speed from one side of the heating equipment to the other side.

[0112] Optionally, the heating device is a muffle furnace. The heating device can be designed according to needs, and this application does not limit it.

[0113] Optionally, the heating equipment includes a heating zone; moving the second semi-finished product from one side of the heating equipment to the other side of the heating equipment for heating specifically includes: moving the second semi-finished product from one side of the heating zone to the other side of the heating zone for heating in 10s-60s.

[0114] By controlling the time it takes for the second semi-finished product to move from one side of the heating zone to the other, the temperature of the second semi-finished product can be controlled. Maintaining the temperature of the pre-formed positive electrode active layer after heating can evaporate the residual solvent in the pre-formed positive electrode active layer, reducing the amount of solvent in the pre-formed positive electrode active layer. At the same time, the heat absorbed by the second semi-finished product is kept below a threshold (the boundary where the adhesive loses its bonding strength), reducing problems such as demolding during the heating process of the second semi-finished product.

[0115] The time it takes for the second semi-finished product to move from one side of the heating zone to the other side can be 10s, 20s, 30s, 40s, 50s, 60s, etc., or it can be a range of any two of the above values, such as 30s-50s, 20s-60s, etc.

[0116] Optionally, the width of the heating zone is greater than or equal to the width of the second semi-finished product, and the direction of the width is perpendicular to the direction of movement of the second semi-finished product.

[0117] By designing the width of the heating zone to be greater than or equal to the width of the second semi-finished product, it can be ensured that the edges of the second semi-finished product can also be fully heated, maintaining the uniformity and consistency of heating throughout the second semi-finished product. This helps to maintain the stability and uniformity of the temperature throughout the second semi-finished product after heating, reduces the internal thermal stress of the second semi-finished product, and lowers the probability of problems such as cracking and demolding.

[0118] It should be noted that the heating time of the second semi-finished product on the heating equipment is T, the length of the heating area of ​​the heating equipment is L, the direction of the length is parallel to the moving direction of the second semi-finished product, and the speed of the second semi-finished product through the heating equipment is S, where S = L / T.

[0119] Optionally, moving the second semi-finished product from one side of the heating device to the other side of the heating device for heating specifically includes: moving the second semi-finished product from one side of the heating device to the other side of the heating device in an air state for heating.

[0120] The second semi-finished product is heat-treated in air, which can directly utilize the surrounding environment and requires less heating equipment, thus reducing the structural complexity of the heating equipment and helping to reduce costs.

[0121] In one embodiment, the positive electrode active material slurry layer further includes a conductive agent; in the positive electrode active material slurry layer, the ratio of positive electrode active material, binder, and conductive agent is (96.5-97.5):(1.2-2):(0.5-1.5); the solid content of the positive electrode active material slurry layer is 65%-69%.

[0122] By adopting the above settings, the positive electrode active material, binder, conductive agent, etc. are uniformly dispersed in the positive electrode active slurry, and the positive electrode active material slurry layer can be firmly attached to the positive electrode current collector, which is beneficial to improving the performance of the secondary battery.

[0123] In one embodiment, the thickness of the positive electrode active layer is 85 μm-95 μm.

[0124] By setting the thickness of the positive electrode active layer as described above, the secondary battery maintains high initial efficiency and cycle performance.

[0125] In one embodiment, the areal density of the positive electrode active layer is 20 mg / cm³. 2 -25mg / cm 2 .

[0126] By setting the areal density of the positive electrode active layer as described above, the secondary battery maintains high initial efficiency and cycle performance.

[0127] In one embodiment, the positive electrode active material comprises a lithium transition metal phosphate.

[0128] When the positive electrode active material includes lithium transition metal phosphates, such as lithium iron phosphate (LFP), the iron element is theoretically divalent. However, due to the synthesis process of lithium transition metal phosphates (e.g., LFP synthesis by carbon reduction), some iron element will actually be zero-valent or trivalent. Heating the second semi-finished product oxidizes the zero-valent iron element to trivalent iron, reducing the content of metallic iron in the positive electrode active material. This helps maintain the consistency of charge and discharge, capacity, etc. of secondary batteries produced in batches.

[0129] In one embodiment, the surface of the lithium transition metal phosphate is coated with a carbon material.

[0130] By coating the surface of lithium transition metal-containing phosphates with carbon materials, a conductive network is formed on the surface of the phosphates, improving the conductivity of the material and enhancing the rate performance and cycle stability of the secondary battery. The carbon layer can also act as a barrier, protecting the positive electrode active material from electrolyte corrosion and reducing side reactions, thereby improving the performance of the secondary battery.

[0131] In one embodiment, the adhesive comprises polyvinylidene fluoride (PVDF).

[0132] By heat-treating the pre-formed positive electrode active layer, residual water and / or NMP are evaporated, reducing the water content and / or NMP content in the formed positive electrode sheet. In addition, by heat-treating the second semi-finished product, the coating state of PVDF on the surface of the positive electrode active material particles is improved, the surface dynamic properties of the positive electrode active material particles are improved, which is beneficial to extending the life of the secondary battery and improving the first charge and discharge efficiency of the secondary battery.

[0133] In one embodiment, the first semi-finished product is dried at 100°C-130°C. Optionally, during the drying process, the first semi-finished product passes through multiple ovens, with the temperature of each oven gradually increasing in the order in which the first semi-finished product passes through the ovens. Optionally, the first semi-finished product is dried at a constant temperature. The drying temperature of the first semi-finished product can be 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, etc., or it can be a range consisting of any two of the above values, such as 100°C-120°C, 105°C-125°C, etc.

[0134] In one specific embodiment, the positive electrode active material includes LFP; the heating device is heated to 200℃-500℃, and the second semi-finished product is moved from one side of the heating device to the other side of the heating device in 10s-60s, resulting in a positive electrode sheet with low water content and NMP content, and high battery consistency.

[0135] For example, the heating device is heated to 500°C, and the second semi-finished product is moved from one side of the heating device to the other side of the heating device in 30 seconds to form a positive electrode sheet.

[0136] The batteries disclosed in this application can be used in electrical devices that use batteries as a power source or in various energy storage systems that use batteries as energy storage elements. Electrical devices include the secondary batteries provided in the above embodiments. Electrical devices can be, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0137] For ease of explanation, the following embodiments use a vehicle 1000 as an example of an electrical device according to an embodiment of this application. Please refer to... Figure 5 , Figure 5 This is a schematic diagram of the vehicle structure provided in an embodiment of this application.

[0138] Vehicle 1000 can be a gasoline-powered vehicle, a natural gas-powered vehicle, or a new energy vehicle. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. A secondary battery 100 is installed inside vehicle 1000, which can be located at the bottom, front, or rear of vehicle 1000. The secondary battery 100 can be used to power vehicle 1000; for example, it can serve as the operating power source for vehicle 1000. Vehicle 1000 may also include a controller 200 and a motor 300. The controller 200 controls the secondary battery 100 to supply power to the motor 300, for example, to meet the power needs of vehicle 1000 during starting, navigation, and driving.

[0139] In some embodiments of this application, the secondary battery 100 can not only serve as the operating power source for the vehicle 1000, but also as the driving power source for the vehicle 1000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000.

[0140] This application also investigates the performance of secondary batteries prepared using the secondary battery preparation method provided in the embodiments of this application.

[0141] The preparation process of Example 1 is as follows:

[0142] (1) Preparation of negative electrode sheet:

[0143] (a) Obtain a negative electrode current collector copper foil with a thickness of 6 μm.

[0144] (b) A negative electrode active material (graphite), a conductive agent (super-P conductive carbon black), a binder (styrene-butadiene rubber (SBR), and a dispersant (sodium carboxymethyl cellulose)) are mixed in a mass percentage ratio of 96.1%:0.7%:2%:1.2% and dispersed in a solvent of water to prepare a negative electrode active slurry. The negative electrode active slurry is then coated onto a copper foil current collector to form an areal density of 12 mg / cm². 2 A negative electrode active material layer with a thickness of 70 μm.

[0145] (c) After being compacted by a cold press, a negative electrode sheet is obtained.

[0146] (2) Preparation of positive electrode sheet:

[0147] (a) Obtain the positive current collector aluminum foil with a thickness of 13 μm.

[0148] (b) A positive electrode active material, lithium iron phosphate, a conductive agent, super-P conductive carbon black, and a binder, polyvinylidene fluoride, were mixed in a mass percentage ratio of 97.5%:1%:1.5% and dispersed in a solvent, N-methylpyrrolidone, to prepare a positive electrode active slurry. The positive electrode active slurry was then coated onto a positive electrode current collector aluminum foil to form an areal density of 25 mg / cm².2 A 90μm thick slurry layer of positive electrode active material was formed to obtain the first semi-finished product.

[0149] (c) The first semi-finished product from step (b) is dried at 115°C and then subjected to a cold pressing process to form a pre-fabricated positive electrode active layer with a thickness of 90 μm, thus obtaining the second semi-finished product.

[0150] (d) The second semi-finished product is heated at 500°C for 30s in a muffle furnace to obtain a positive electrode sheet; during the heating process, the second semi-finished product moves from one side of the heating area of ​​the muffle furnace to the other side.

[0151] (3) Preparation of electrolyte:

[0152] Lithium salt LiPF6 was added to a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and vinylene carbonate (VC) in a mass ratio of 35:64.5:0.5 and mixed thoroughly to obtain an electrolyte, wherein the molar concentration of LiPF6 in the electrolyte was 1 mol / L.

[0153] (4) Diaphragm:

[0154] A 12μm polyethylene membrane was used as the diaphragm.

[0155] It should be noted that the prepared negative electrode and positive electrode are stacked in sequence, with the separator placed between the positive and negative electrode to provide isolation. The cells are then wound to obtain a bare cell, which is then inserted into the battery casing. The battery is obtained through processes such as baking, electrolyte injection, settling, encapsulation, formation, and capacity testing.

[0156] The differences between Examples 2 to 6 and Example 1 are: the heating temperature and heating time of the second semi-finished product are different during the preparation of the positive electrode sheet; and the positive electrode active material is different.

[0157] The preparation process of Example 7 is as follows:

[0158] (1) Preparation of negative electrode sheet:

[0159] (a) Obtain a negative electrode current collector copper foil with a thickness of 6 μm.

[0160] (b) A negative electrode active material (hard carbon), a conductive agent (super-P conductive carbon black), a binder (styrene-butadiene rubber (SBR), and a dispersant (sodium carboxymethyl cellulose)) were mixed in a mass percentage ratio of 96.1%:0.7%:2%:1.2% and dispersed in a solvent of water to prepare a negative electrode active slurry. The negative electrode active slurry was then coated onto a copper foil used as a negative electrode current collector to form an areal density of 12 mg / cm². 2 A negative electrode active material layer with a thickness of 70 μm.

[0161] (c) After being compacted by a cold press, a negative electrode sheet is obtained.

[0162] (2) Preparation of positive electrode sheet:

[0163] (a) Obtain the positive current collector aluminum foil with a thickness of 13 μm.

[0164] (b) A positive electrode active material Na4Fe3(PO4)2(P2O7), a conductive agent (super-P conductive carbon black), and a binder (polyvinylidene fluoride) were mixed in a mass percentage ratio of 97.5%:1%:1.5% and dispersed in a solvent (N-methylpyrrolidone) to prepare a positive electrode active slurry. The positive electrode active slurry was then coated onto a positive electrode current collector aluminum foil to form an areal density of 25 mg / cm³. 2 A 90μm thick slurry layer of positive electrode active material was formed to obtain the first semi-finished product.

[0165] (c) The first semi-finished product from step (b) is dried at 115°C and then subjected to a cold pressing process to form a pre-fabricated positive electrode active layer with a thickness of 90 μm, thus obtaining the second semi-finished product.

[0166] (d) The second semi-finished product is heated at 500°C for 30s in a muffle furnace to obtain a positive electrode sheet; during the heating process, the second semi-finished product moves from one side of the heating area of ​​the muffle furnace to the other side.

[0167] (3) Preparation of electrolyte:

[0168] Sodium salt NaPF6 was added to a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and vinylene carbonate (VC) in a mass ratio of 35:64.5:0.5 and mixed thoroughly to obtain an electrolyte, wherein the molar concentration of LiPF6 in the electrolyte was 1 mol / L.

[0169] (5) Diaphragm:

[0170] A 12μm polyethylene membrane was used as the diaphragm.

[0171] It should be noted that the prepared negative electrode and positive electrode are stacked in sequence, with the separator placed between the positive and negative electrode to provide isolation. The cells are then wound to obtain a bare cell, which is then inserted into the battery casing. The battery is obtained through processes such as baking, electrolyte injection, settling, encapsulation, formation, and capacity testing.

[0172] The difference between Comparative Example 1 and Example 1 is that step (d) was not performed during the preparation of the positive electrode sheet.

[0173] The difference between Comparative Example 2 and Example 7 is that step (d) was not performed during the preparation of the positive electrode sheet.

[0174] The relevant parameter tests for each embodiment and comparative example are as follows:

[0175] 1. NMP content test.

[0176] NMP detection employs a headspace gas phase method: the sample is directly cut into strips, placed in a headspace vial, and heated. After the volatile components in the solid sample reach equilibrium in a very stable, sealed container, the ratio of volatile components in the gas phase and the solid phase remains constant. The vapor in the upper part of the solid after equilibrium is automatically separated and detected by gas chromatography, and the quantification method is the external standard method.

[0177] 2. Water content test.

[0178] 1) Dry the sample vials at 120-160℃ for 3-6 hours. When taking them out, the vials must be immediately capped and sealed for storage. The blank moisture content of the sample vials should be less than 10μg.

[0179] 2) Cut the electrode into 0.5cm pieces, ensuring that the injection volume is not less than 0.5g, and put them into the sample vial.

[0180] 3) Before testing the sample, test the blank moisture content of 1-2 empty bottles as blank values ​​and fill them into the "Blank Deduction" option in the test settings.

[0181] 4) Before formally testing the sample, the current drift should be kept below 0.5 μg / s and stable without any obvious upward or downward trend.

[0182] 5) After completing one sample, remove the bottle containing the sample from the Karl Fischer furnace and reinsert it into the drift bottle to allow the instrument to stabilize before completing the next sample.

[0183] 3. Initial charge and discharge efficiency.

[0184] The initial charge-discharge efficiency (EDE) is the ratio of the capacity of a secondary battery during its first charge and discharge, often referred to as the initial efficiency. The specific test procedure is as follows: At 25°C, the battery is charged at a constant current rate of 0.2C for 1 hour, recorded as IGC0; then discharged at a rate of 0.1C to 2.0V, recorded as AGD0; allowed to stand for 5 minutes; then the secondary battery is charged at a rate of 0.5C to 3.65V, recorded as AGC0; allowed to stand for 5 minutes; then discharged at a rate of 0.1C to 2.5V, recorded as AGD1; where C0 is IGC0 - AGD0 + AGC0; D0 is AGD1; the initial efficiency is calculated as D0 / C0.

[0185] 4. Cyclic performance.

[0186] At 25°C, the above-mentioned secondary battery was charged to 3.65V at a rate of 0.5C, then charged at a constant voltage until the current was below 0.05C. Then it was discharged to 2.5V at a rate of 1C. This full charge and discharge cycle test was carried out until the battery's discharge capacity decayed to 80% of its initial capacity. The number of cycles at this point was recorded.

[0187] 5. Storage performance at 60℃.

[0188] After storing the battery in a 60℃ oven for a period of time t (days), test the battery capacity (Ct). The ratio of the battery capacity (Ct) to the initial capacity (C0), Ct / C0, is the storage capacity retention rate. Record the number of storage days when the capacity retention rate is 80%.

[0189] Table 1. Test data of relevant parameters for each embodiment and comparative example.

[0190]

[0191] Comparing Examples 1 to 6 with Comparative Example 1, it can be seen that heat treatment of the second semi-finished product can reduce the water content and NMP content in the positive electrode of the lithium-ion battery, thereby improving the battery's initial efficiency, cycle performance, and 60°C storage performance. Comparing Example 7 with Comparative Example 2, it can be seen that heat treatment of the second semi-finished product can reduce the water content and NMP content in the positive electrode of the sodium-ion battery, thereby improving the battery's initial efficiency, cycle performance, and 60°C storage performance.

[0192] The above description is merely an embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A secondary battery, characterized in that, The device includes a housing, a battery cell assembly disposed within the housing, and an electrolyte. The battery cell assembly is located in the electrolyte. The battery cell assembly includes a positive electrode, a negative electrode, and a separator, with the separator located between the positive and negative electrode. The positive electrode has a water content of 80ppm-400ppm and / or an N-methylpyrrolidone content of 20ppm-150ppm.

2. The secondary battery according to claim 1, characterized in that, The water content of the positive electrode is 80ppm-150ppm.

3. The secondary battery according to claim 1 or 2, characterized in that, The positive electrode sheet contains 20ppm-50ppm of N-methylpyrrolidone.

4. The secondary battery according to any one of claims 1 to 3, characterized in that, The positive electrode sheet includes a positive active layer, the positive active layer includes a positive active material, and the positive active material includes a lithium transition metal phosphate.

5. The secondary battery according to claim 4, characterized in that, The lithium transition metal phosphate is coated with carbon material.

6. The secondary battery according to any one of claims 4 or 5, characterized in that, The positive electrode active layer further includes a binder and a conductive agent, and the mass ratio of the positive electrode active material, the binder, and the conductive agent is (96.5-97.5):(1.2-2):(0.5-1.5).

7. The secondary battery according to any one of claims 4 to 6, characterized in that, The adhesive includes polyvinylidene fluoride.

8. The secondary battery according to any one of claims 4 to 7, characterized in that, The thickness of the positive electrode active layer is 85μm-95μm.

9. The secondary battery according to any one of claims 4 to 8, characterized in that, The areal density of the positive electrode active layer is 20 mg / cm³. 2 -25mg / cm 2 .

10. A method for preparing a secondary battery, characterized in that, include: A battery cell assembly is formed by sequentially stacking negative electrode plates, separator membranes, and positive electrode plates; The battery cell assembly is placed in the housing; as well as An electrolyte is injected into the casing and sealed to form the secondary battery; The method for preparing the positive electrode sheet includes: A positive electrode active material slurry layer is disposed on at least one side of the positive electrode current collector to form a first semi-finished product, wherein the positive electrode active material slurry layer includes a positive electrode active material, a binder and a solvent; The first semi-finished product is dried and compacted to form a second semi-finished product. The positive electrode active material slurry layer is dried and compacted to form a pre-formed positive electrode active layer. The pre-formed positive electrode active layer includes the positive electrode active material, the binder, and the residual solvent. The residual solvent includes at least one of water and N-methylpyrrolidone. The second semi-finished product is subjected to heat treatment to obtain a positive electrode sheet, wherein the pre-made positive electrode active layer is formed into a positive electrode active layer through the heat treatment; the water content of the positive electrode sheet is 80ppm-400ppm, and / or the N-methylpyrrolidone content of the positive electrode sheet is 20ppm-150ppm.

11. The method for preparing a secondary battery according to claim 10, characterized in that, The heat treatment temperature is 200℃-500℃, and the time is 10s-60s.

12. The method for preparing a secondary battery according to claim 10 or 11, characterized in that, The positive electrode active material slurry layer further includes a conductive agent; in the positive electrode active material slurry layer, the mass ratio of the positive electrode active material, the binder, and the conductive agent is (96.5-97.5):(1.2-2):(0.5-1.5); the solid content of the positive electrode active material slurry layer is 65%-69%.

13. An electrical appliance, characterized in that, This includes secondary batteries prepared by any one of the methods described in claims 1-9 or 10-12.

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