Positive electrode sheet and preparation method thereof, secondary battery, battery module, battery pack, and electrical device

Abstract

The present application relates to a positive electrode sheet and a preparation method thereof, a secondary battery, a battery module, a battery pack, and an electric device. The positive electrode sheet comprises: a current collector, a first coating and a second coating; the first coating is disposed on the current collector, and the first active material in the first coating is selected from LiNi x Co y Mn z O2 and / or coated LiNi x Co y Mn z O2, x+y+z=1; coated LiNi x Co y Mn z O2 includes a first core and a first coating layer coated on the surface of the first core; the second coating layer is arranged on the surface of the first coating layer, the second active material in the second coating layer is a coated LiFePO4, the coated LiFePO4 includes a second core and a second coating layer coated on the surface of the second core, and the first coating layer and the second coating layer are conductive materials. The positive electrode plate can simultaneously give play to the advantages of lithium nickel cobalt manganese oxide and lithium iron phosphate materials, so that the secondary battery has both high battery energy density and cycle life.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a positive electrode plate and a preparation method thereof, a secondary battery, a battery module, a battery pack, and an electrical device. Background Art

[0002] Lithium-ion battery is a secondary battery (rechargeable battery) that mainly relies on Li + It works by moving between the positive and negative electrodes. During the charge and discharge process, Li + Insertion and deinsertion back and forth between the two electrodes: during charging, Li + It is deintercalated from the positive electrode and then embedded into the negative electrode through the electrolyte, and the negative electrode is in a lithium-rich state; the opposite is true during discharge. Lithium-ion batteries have high energy density, high average output voltage, low self-discharge, no memory effect, and a wide operating temperature range of -20℃ to 60℃. They have excellent cycle performance, can be charged and discharged quickly, have a charging efficiency of up to 100%, and have high output power.

[0003] Common positive electrode materials (or cathode materials) include LiCoO2, LiNi x Co y Mn z O2、 LiNi x Co y Al z O2 and other layered oxides, LiMn2O4, LiNi 0.5 Mn 1.5 O4 and other spinel oxides, LiFePO4 and other olivine polyanion oxides, as well as lithium-rich manganese-based positive electrode materials and other positive electrode materials. Different positive electrode materials have their own advantages and disadvantages. In actual production and application, it is found that it is difficult for current lithium-ion batteries and other secondary batteries to take into account both battery energy density and cycle life. Summary of the invention

[0004] Based on this, it is necessary to provide a positive electrode sheet and its preparation method, as well as a secondary battery, a battery module, a battery pack, and an electrical device that can simultaneously improve the energy density and cycle life of the battery.

[0005] The present application is implemented through the following technical solutions.

[0006] In a first aspect of the present application, a positive electrode sheet is provided, comprising:

[0007] current collector;

[0008] A first coating layer is provided on the current collector, wherein the first coating layer contains a first active material, and the first active material is selected from LiNi x Co y Mn zO2 and / or coated LiNi x Co y Mn z O2; the coated LiNi x Co y Mn z O2 includes a first core and a first coating layer coated on the surface of the first core, and the first core is LiNi x Co y Mn z O2, x+y+z=1, the first coating layer is a conductive material; and

[0009] The second coating is arranged on the surface of the first coating away from the current collector. The second coating contains a second active material. The second active material is coated LiFePO4. The coated LiFePO4 includes a second core and a second coating layer coated on the surface of the second core. The second core is LiFePO4 and the second coating layer is a conductive material.

[0010] The positive electrode sheet is formed by layering a first active material and a second active material of a specific type with similar or equivalent electrical conductivity on a current collector to form a first coating layer and a second coating layer, and the first active material is arranged in a lower layer close to the current collector, and the second active material is arranged in an upper layer away from the current collector, so that different active materials have better electrical conductivity matching, and can simultaneously play the role of LiNi x Co y Mn z The advantages of O2 and LiFePO4 enable the manufactured secondary battery to have both higher battery energy density and longer cycle life.

[0011] Compared with the positive electrode sheet prepared by directly mixing different positive electrode materials, the above positive electrode sheet avoids the problem of over-embedding or over-deintercalation of directly mixed positive electrode materials. It is applied to secondary batteries and has higher battery energy density and cycle life.

[0012] In some embodiments, the first coating layer and the second coating layer are independently selected from at least one of a carbon material and a conductive polymer. Using these conductive materials as the coating layer material, the coating process is simple, and better conductivity matching can be achieved between different active materials.

[0013] In some embodiments, the conductive polymer is selected from at least one of polyacetylene, polypyrrole, polythiophene, polyphenylene, polyphenylene vinylene and polyaniline. x Co y Mn zThe electrical conductivity of O2 is comparable to or slightly lower than that of carbon-coated LiFePO4, which improves the electrical conductivity matching between the first active material and the second active material.

[0014] In some embodiments, the first coating layer is a carbon material or a conductive polymer;

[0015] The second coating layer is made of carbon material.

[0016] In some embodiments, the thickness of the first coating layer is 0.2 μm to 1.5 μm;

[0017] And / or, the thickness of the second coating layer is 0.001 μm to 0.2 μm.

[0018] When the thickness of the first coating layer and / or the second coating layer is within this range, the battery energy density and cycle life of the manufactured secondary battery can be further improved.

[0019] In some embodiments, the mass ratio of the first active material in the first coating layer to the second active material in the second coating layer is 1:(0.1-10).

[0020] In some embodiments, the mass content of the first active material in the first coating is 95wt% to 97wt%;

[0021] And / or, the mass content of the second active material in the second coating layer is 95wt% to 97wt%.

[0022] In a second aspect of the present application, a method for preparing a positive electrode sheet is provided. The positive electrode sheet is any of the positive electrode sheets described above. The preparation method comprises the following steps:

[0023] forming the first coating layer on the current collector; and

[0024] The second coating layer is formed on a surface of the first coating layer facing away from the current collector.

[0025] In a third aspect of the present application, a secondary battery is provided, comprising any of the above-mentioned positive electrode sheets.

[0026] A fourth aspect of the present application provides a battery module, comprising the secondary battery of the third aspect of the present application.

[0027] A fifth aspect of the present application provides a battery pack comprising the battery module of the fourth aspect of the present application.

[0028] In a sixth aspect of the present application, there is provided an electrical device comprising at least one selected from the secondary battery of the third aspect of the present application, the battery module of the fourth aspect of the present application, and the battery pack of the fifth aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a schematic diagram of a secondary battery according to one embodiment of the present application.

[0030] Figure 2 yes Figure 1 An exploded view of a secondary battery according to an embodiment of the present application is shown.

[0031] Figure 3 It is a schematic diagram of a battery module according to one embodiment of the present application.

[0032] Figure 4 It is a schematic diagram of a battery pack according to one embodiment of the present application.

[0033] Figure 5 yes Figure 4 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0034] Figure 6 FIG. 1 is a schematic diagram of an electric device using a secondary battery according to an embodiment of the present application as a power source.

[0035] Figure 7 The graph is a graph showing the capacity retention rate of Example 1, Example 3 and Comparative Example 1 at 25°C as a function of the number of cycles.

[0036] Description of reference numerals:

[0037] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0038] Below, the embodiments of the positive electrode plate and its manufacturing method, secondary battery, battery module, battery pack and electric device of the present application are specifically disclosed with appropriate reference to the drawings. However, there are cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following descriptions are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0039] The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. The range defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60 to 120 and 80 to 110 is listed for a particular parameter, it is understood that a range of 60 to 110 and 80 to 120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following ranges can all be expected: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4 and 2 to 5. In the present application, unless otherwise specified, the numerical range "a to b" represents an abbreviation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

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

[0041] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0042] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0043] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0044] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0045] The term "and / or" as used herein includes any and all combinations of one or more related listed items. In the description of the present application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the feature. In the description of the present application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0046] As described in the background art, different positive electrode materials have their own advantages and disadvantages. In actual production and application, it is found that it is difficult for current lithium-ion batteries and other secondary batteries to simultaneously take into account both battery energy density and cycle life.

[0047] For example, the positive electrode material of ternary lithium-ion batteries is generally LiNi x Co y Mn z O2、LiNi x Co y Al z Ternary cathode materials such as LiCoO2 have high energy density, but their volume energy density is lower than that of LiCoO2. These ternary cathode materials are high-nickel materials with high environmental sensitivity and poor cycle performance.

[0048] For example, lithium iron phosphate battery is a lithium-ion battery that uses lithium iron phosphate (LiFePO4) as the positive electrode material and carbon as the negative electrode material. The LiFePO4 material used in this battery is low in cost, has good safety performance, and a long cycle life, but has poor conductivity, low energy density, and poor low-temperature performance.

[0049] Therefore, how to make lithium-ion batteries have both battery energy density and cycle life is an urgent problem to be solved.

[0050] At present, the industry often utilizes the characteristics of different cathode materials and mixes them to achieve high energy density and long cycle life. However, when multiple different cathode materials are mixed, there may be problems of over-embedding or over-de-embedding of one of the cathode materials, which seriously affects the cycle life of the cathode materials.

[0051] In order to solve the problem of how to make lithium-ion batteries take into account both battery energy density and cycle life, an embodiment of the present application provides a positive electrode sheet. The positive electrode sheet includes a current collector, a first coating layer and a second coating layer.

[0052] The first coating layer is disposed on the current collector. The first coating layer contains a first active material, and the first active material is selected from LiNi x Co y Mn z O2 (lithium nickel cobalt manganese oxide) and / or coated LiNi x Co y Mn z O2. Where x+y+z=1.

[0053] Among them, coated LiNi x Co y Mn z O2 includes a first core and a first coating layer coated on the surface of the first core, and the first core is LiNi x Co y Mn z O2, the first coating layer is a conductive material.

[0054] The second coating layer is disposed on the surface of the first coating layer away from the current collector. The second coating layer contains a second active material, which is a coated LiFePO4 (lithium iron phosphate).

[0055] The coated LiFePO4 comprises a second core and a second coating layer coated on the surface of the second core, the second core is LiFePO4, and the second coating layer is a conductive material.

[0056] It is understandable that the active material in the first coating is not limited to the first active material mentioned above, and may also contain other active materials; the active material in the second coating is also not limited to the second active material mentioned above, and may also contain other active materials.

[0057] The positive electrode sheet is formed by layering a first active material and a second active material of a specific type with similar or equivalent electrical conductivity on a current collector to form a first coating layer and a second coating layer, and the first active material is arranged in a lower layer close to the current collector, and the second active material is arranged in an upper layer away from the current collector, so that different active materials have better electrical conductivity matching, and can simultaneously play the role of LiNi x Co y Mn z The advantages of O2 and LiFePO4 enable the manufactured secondary battery to have both higher battery energy density and longer cycle life.

[0058] Compared with the positive electrode sheet prepared by directly mixing different positive electrode materials, the above positive electrode sheet avoids the problem of over-embedding or over-deintercalation of directly mixed positive electrode materials. It is applied to secondary batteries and has higher battery energy density and cycle life.

[0059] In some embodiments, the conductive material is selected from at least one of carbon and a conductive polymer. In other words, the first coating layer and the second coating layer are each independently selected from at least one of a carbon material and a conductive polymer. By using these conductive materials as the coating layer material, the coating process is simple and can achieve good conductivity matching between different active materials.

[0060] Preferably, the first active material contains coated LiNi x Co y Mn z O2.

[0061] In some embodiments, the coated LiNi x Co y Mn z O2 is carbon-coated LiNi x Co y Mn z O2 and conductive polymer coated LiNi x Co y Mn z At least one of O2.

[0062] It is understandable that coated LiNi x Co y Mn z O2 can be purchased or made at home.

[0063] In some embodiments, the carbon-coated LiNi x Co y Mn z O2 can be prepared by the following method: LiNi x Co y Mn z O2 is mixed with an organic carbon source in a solvent, taken out, dried, and calcined to obtain carbon-coated LiNi x Co y Mn z O2. Further, the organic carbon source may be glucose or the like. Further, the solvent may be an organic solvent such as N-methylpyrrolidone (NMP). Further, the mixing is performed by ball milling, for example, ball milling at 800 rpm for 2 hours. Further, the calcination condition is to treat at 600°C for 1 hour, and the calcination atmosphere is a protective atmosphere such as nitrogen.

[0064] In some embodiments, the conductive polymer is selected from at least one of polyacetylene, polypyrrole, polythiophene, polyphenylene, polyphenylene vinylene and polyaniline. x Co y Mn z The conductivity of O2 is comparable to or slightly lower than that of carbon-coated LiFePO4, which improves the conductivity matching between the first active material and the second active material. The structures and conductivities of these conductive polymers are shown in Table 1 below:

[0065] Table 1

[0066]

[0067]

[0068] In some embodiments, the conductive polymer-coated LiNi x Co y Mn z O2 can be prepared by the following method: LiNi x Co y Mn z O2 is mixed with the conductive polymer solution, taken out and dried to obtain the conductive polymer-coated LiNi x Co y Mn z O2.

[0069] In some embodiments, the thickness of the first coating layer is 0.2 μm to 1.5 μm. More preferably, the thickness of the first coating layer is 1.0 μm. When the thickness of the first coating layer is within this range, the battery energy density and cycle life of the secondary battery can be further improved.

[0070] It is understood that the coated LiFePO4 can be obtained by purchasing or by making it yourself. The coating method can be the same as that for preparing the coated LiNi x Co y Mn z O2 is similar.

[0071] Furthermore, carbon-coated LiFePO4 can be purchased by oneself. Currently, industrial LiFePO4 powder is coated with carbon, which has higher electrical conductivity.

[0072] In some embodiments, the thickness of the second coating layer is 0.001 μm to 0.2 μm. When the thickness of the second coating layer is within this range, the battery energy density and cycle life of the secondary battery can be further improved.

[0073] In some specific examples, the first coating layer is a carbon material or a conductive polymer; and the second coating layer is a carbon material.

[0074] In some embodiments, the mass ratio of the first active material in the first coating layer to the second active material in the second coating layer is 1:(0.1-10). Further, the mass ratio of the first active material in the first coating layer to the second active material in the second coating layer is 1:(1-10); further, 1:(2-8), and further preferably (1-3):8.

[0075] In some embodiments, the mass content of the first active material in the first coating layer is 95 wt % to 97 wt %. It is understood that the first coating layer may also include other components such as a conductive agent and a binder.

[0076] In some embodiments, the mass content of the second active material in the second coating layer is 95 wt % to 97 wt %. It is understood that the second coating layer may also include other components such as a conductive agent and a binder.

[0077] In some embodiments, the first core is NCM333 (LiNi 0.3 Co 0.3 Mn 0.3 O2), NCM523(LiNi 0.5 Co 0.2 Mn 0.3 O2), NCM622(LiNi 0.6 Co 0.2 Mn 0.2 O2) or NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 O2). Further, the current collector for the positive electrode plate may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0078] Another embodiment of the present application further provides a method for preparing the above-mentioned positive electrode sheet, which includes the following steps S10 to S20.

[0079] Step S10: forming a first coating layer on the current collector.

[0080] Step S20: forming a second coating layer on the surface of the first coating layer facing away from the current collector.

[0081] In some embodiments, step S10 of forming a first coating on the current collector includes steps S11 to S12:

[0082] Step S11: providing a first positive electrode slurry, wherein the first positive electrode slurry comprises the first active material, a conductive agent, a binder and a solvent;

[0083] Step S12: coating the first positive electrode slurry on the current collector.

[0084] It is understood that the coating described in the present application includes but is not limited to printing coating, blade coating, spin coating or inkjet coating.

[0085] In some embodiments, step S20 of forming a second coating layer on a surface of the first coating layer facing away from the current collector includes steps S21 to S22:

[0086] Step S21: providing a second positive electrode slurry, wherein the second positive electrode slurry includes the second active material, a conductive agent, a binder and a solvent;

[0087] Step S22: coating a second positive electrode slurry on a surface of the first coating layer facing away from the current collector.

[0088] As an example, in the first positive electrode slurry and the second positive electrode slurry, 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.

[0089] As an example, in the first positive electrode slurry and the second positive electrode slurry, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.

[0090] Further, in the first positive electrode slurry and the second positive electrode slurry, the solvent is independently selected from at least one of N-methylpyrrolidone (NMP), N,N-dimethylformamide, ethanol, ethylene glycol, methanol and isopropanol. Specifically, the solvent is selected from N-methylpyrrolidone (NMP). In some embodiments, the positive electrode sheet can be prepared in the following manner: the components for preparing the positive electrode sheet, such as active materials, conductive agents, binders and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0091] In addition, the secondary battery, battery module, battery pack, and electric device of the present application will be described below with reference to the drawings as appropriate.

[0092] In one embodiment of the present application, a secondary battery is provided.

[0093] Typically, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. During the battery charge and discharge process, active ions are embedded and removed back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through. Among them, the positive electrode sheet adopts any of the above-mentioned positive electrode sheets of this application.

[0094] In some examples, the secondary battery is a lithium-ion battery.

[0095] [Negative electrode]

[0096] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0097] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0098] 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 material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0099] In some embodiments, the negative electrode active material may adopt the negative electrode active material for the battery known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0100] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

[0101] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0102] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0103] In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0104] [Electrolytes]

[0105] The electrolyte plays the role of conducting ions between the positive electrode plate and the negative electrode plate. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

[0106] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0107] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0108] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane sulfone, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0109] In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

[0110] [Isolation film]

[0111] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

[0112] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0113] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0114] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0115] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0116] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or any other shape. Figure 1 The secondary battery 5 is a square structure as an example.

[0117] In some embodiments, reference Figure 2 , the outer package may include a shell 51 and a cover plate 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate are enclosed to form a accommodating cavity. The shell 51 has an opening connected to the accommodating cavity, and the cover plate 53 can be covered on the opening to close the accommodating cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the accommodating cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0118] In some embodiments, secondary batteries may be assembled into a battery module. The number of secondary batteries contained in the battery module may be one or more, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery module.

[0119] Figure 3 This is a battery module 4 as an example. Figure 3 In the battery module 4, the plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

[0120] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0121] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery pack.

[0122] Figure 4 and Figure 5 1 is a battery pack 1 as an example. Figure 4 and Figure 5 The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0123] In addition, the present application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0124] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0125] Figure 6 The power consumption device is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the power consumption device's requirements for high power and high energy density of secondary batteries, a battery pack or a battery module can be used.

[0126] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be light and thin, and a secondary battery may be used as a power source.

[0127] In order to make the purpose, technical solutions and advantages of the present application more concise and clear, the present application is described with the following specific embodiments, but the present application is by no means limited to these embodiments. The embodiments described below are only preferred embodiments of the present application and can be used to describe the present application, and cannot be understood as limiting the scope of the present application. It should be pointed out that any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

[0128] In order to better illustrate the present application, the present application is further described below in conjunction with the embodiments. The following are specific embodiments.

[0129] Comparative Example 1: Preparation of secondary battery

[0130] Positive electrode sheet: Provide the following components by mass percentage: 96.2% positive electrode active material, 1.1% PVDF, and 2.7% conductive carbon. Among them, the positive electrode active material is NCM523 and carbon-coated LFP (abbreviation of lithium iron phosphate material) in a mass ratio of 2:8. Dissolve the above components in NMP (N-methylpyrrolidone) solution, stir evenly, apply on aluminum foil, dry, and cold press to obtain a positive electrode sheet.

[0131] Negative electrode sheet: artificial graphite as negative electrode active material, conductive agent acetylene black, binder styrene butadiene rubber (SBR) and thickener sodium carbon methyl cellulose (CMC) are fully stirred and mixed in a deionized water solvent system in a weight ratio of 90:5:2:2:1, coated on copper foil, dried and cold pressed to obtain a negative electrode sheet.

[0132] A porous polymer film made of polyethylene (PE) is used as an isolation membrane. The electrolyte contains ethylene carbonate and ethyl methyl carbonate in a mass ratio of 3:7, and also contains LiPF6 at a concentration of 1 mol / L.

[0133] The positive electrode sheet, the separator and the negative electrode sheet are stacked in order, the separator is placed between the positive and negative electrodes to play a role of isolation, and then wound to obtain a bare cell. The bare cell is placed in an outer package, the electrolyte used in the preparation is injected and packaged to obtain a secondary battery.

[0134] The positive electrode current collector, coated LFP, negative electrode sheet, separator electrolyte and other processes used in the following embodiments are the same as those in Comparative Example 1, and the only difference is the preparation of the active material layer on the positive electrode sheet. The preparation of the positive electrode sheet in the secondary battery of each embodiment will be described in detail below.

[0135] Embodiment 1:

[0136] The following components are provided by mass percentage: 96.2% of the first active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, applied on aluminum foil, dried, and cold pressed to form a first coating.

[0137] The following components are provided by mass percentage: 96.2% of the second active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, and then coated on the first coating, dried, and cold pressed to form a second coating.

[0138] The first active material is NCM523, the second active material is carbon-coated LFP, and the mass ratio of NCM523 in the first coating layer to the coated LFP in the second coating layer is 2:8.

[0139] Embodiment 2:

[0140] The following components are provided by mass percentage: 96.2% of the first active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, applied on aluminum foil, dried, and cold pressed to form a first coating.

[0141] The following components are provided by mass percentage: 96.2% of the second active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, and then coated on the first coating, dried, and cold pressed to form a second coating.

[0142] The first active material is 0.5 μm carbon-coated NCM523, the second active material is carbon-coated LFP, and the mass ratio of the carbon-coated NCM523 in the first coating layer to the carbon-coated LFP in the second coating layer is 2:8.

[0143] Embodiment 3:

[0144] The following components are provided by mass percentage: 96.2% of the first active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, applied on aluminum foil, dried, and cold pressed to form a first coating.

[0145] The following components are provided by mass percentage: 96.2% of the second active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, and then coated on the first coating, dried, and cold pressed to form a second coating.

[0146] The first active material is 1.0 μm carbon-coated NCM523, the second active material is carbon-coated LFP, and the mass ratio of the carbon-coated NCM523 in the first coating layer to the carbon-coated LFP in the second coating layer is 2:8.

[0147] Embodiment 4:

[0148] The following components are provided by mass percentage: 96.2% of the first active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, applied on aluminum foil, dried, and cold pressed to form a first coating.

[0149] The following components are provided by mass percentage: 96.2% of the second active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, and then coated on the first coating, dried, and cold pressed to form a second coating.

[0150] The first active material is 1.5 μm carbon-coated NCM523, the second active material is carbon-coated LFP, and the mass ratio of the carbon-coated NCM523 in the first coating layer to the carbon-coated LFP in the second coating layer is 2:8.

[0151] Embodiment 5:

[0152] The following components are provided by mass percentage: 96.2% of the first active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, applied on aluminum foil, dried, and cold pressed to form a first coating.

[0153] The following components are provided by mass percentage: 96.2% of the second active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, and then coated on the first coating, dried, and cold pressed to form a second coating.

[0154] The first active material is 1.0 μm carbon-coated NCM523, the second active material is carbon-coated LFP, and the mass ratio of the carbon-coated NCM523 in the first coating layer to the carbon-coated LFP in the second coating layer is 2:8.

[0155] Embodiment 6:

[0156] The following components are provided by mass percentage: 96.2% of the first active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, applied on aluminum foil, dried, and cold pressed to form a first coating.

[0157] The following components are provided by mass percentage: 96.2% of the second active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, and then coated on the first coating, dried, and cold pressed to form a second coating.

[0158] The first active material is 1.0 μm polythiophene coated NCM523, the second active material is carbon coated LFP, and the mass ratio of carbon coated NCM523 in the first coating layer to carbon coated LFP in the second coating layer is 2:8.

[0159] Embodiment 7:

[0160] The following components are provided by mass percentage: 96.2% of the first active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, applied on aluminum foil, dried, and cold pressed to form a first coating.

[0161] The following components are provided by mass percentage: 96.2% of the second active material, 1.1% of PVDF, and 2.7% of conductive carbon. The above components are dissolved in NMP (N-methylpyrrolidone) solution, stirred evenly, and then coated on the first coating, dried, and cold pressed to form a second coating.

[0162] The first active material is 1.0 μm polyaniline coated NCM523, and the second active material is carbon coated LFP. The mass ratio of carbon coated NCM523 in the first coating layer to carbon coated LFP in the second coating layer is 2:8.

[0163] Example 8

[0164] Example 8 is basically the same as Example 3, with the only difference being that in the positive electrode sheet, the mass ratio of the carbon-coated NCM523 in the first coating layer to the carbon-coated LFP in the second coating layer is 1:8.

[0165] Example 9

[0166] Example 9 is basically the same as Example 3, with the only difference being that in the positive electrode sheet, the mass ratio of the carbon-coated NCM523 in the first coating layer to the carbon-coated LFP in the second coating layer is 3:8.

[0167] Each secondary battery prepared in the comparative example and the embodiment was charged to 4.25V at 1C at 2.45-4.25V under a constant temperature environment of 25°C, then charged at a constant voltage at 4.25V to a current of ≤0.05mA, left to stand for 5min, then discharged to 2.5V at 4C, and the above operation was repeated for 400 cycles to determine the capacity fading value; the energy density, rate performance and capacity retention rate after 400 cycles were obtained, as shown in Table 2 below. The energy density is calculated as follows: gravimetric energy density = capacity (Ah) * voltage platform (V) / cell weight (kg).

[0168] Some key parameters of each embodiment and comparative example are shown in Table 2 below:

[0169] Table 2

[0170]

[0171]

[0172] The lower layer is the first coating layer close to the current collector, and the upper layer is the second coating layer away from the current collector. LPF is the abbreviation of lithium iron phosphate material. The mass ratio in Table 2 is the mass ratio of the active material in the first coating layer to the active material in the second coating layer.

[0173] As can be seen from Table 2, the secondary battery prepared in the embodiment of the present application has higher energy density, rate performance and capacity retention rate than the secondary battery prepared in Comparative Example 1, indicating that it has improved both energy density and cycle performance and improved cycle life.

[0174] like Figure 7 As shown, the capacity retention rate of Example 1, Example 3 and Comparative Example 1 at 25°C varies with the number of cycles, wherein the abscissa is the number of cycles (Cycle No.) and the ordinate is the capacity retention rate (CapacityRetention). Figure 1 It can also be seen that the capacity retention rate of Example 3 is better than that of Example 1, and the capacity retention rate of Example 1 is better than that of Comparative Example 1.

[0175] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0176] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims, and the description and drawings may be used to interpret the contents of the claims.

Claims

1. A positive electrode sheet, characterized in that: include: current collector; A first coating layer is provided on the current collector, wherein the first coating layer contains a first active material, and the first active material is selected from LiNi x Co y Mn z O2, x+y+z=1; and The second coating is arranged on the surface of the first coating away from the current collector. The second coating contains a second active material. The second active material is coated LiFePO4. The coated LiFePO4 includes a second core and a second coating layer coated on the surface of the second core. The second core is LiFePO4 and the second coating layer is a conductive material.

2. The positive electrode sheet according to claim 1, characterized in that: The LiNi x Co y Mn z O2 includes coated LiNi x Co y Mn z O2, the coated LiNi x Co y Mn z O2 includes a first core and a first coating layer coated on the surface of the first core, and the first core is LiNi x Co y Mn z O2, the first coating layer is a conductive material.

3. The positive electrode sheet according to claim 2, characterized in that: The first coating layer and the second coating layer are each independently selected from at least one of a carbon material and a conductive polymer.

4. The positive electrode sheet according to claim 3, characterized in that: The conductive polymer is selected from at least one of polyacetylene, polypyrrole, polythiophene, polyphenylene, polyphenylene vinylene and polyaniline.

5. The positive electrode sheet according to claim 3, characterized in that: The first coating layer is a carbon material or a conductive polymer; The second coating layer is a carbon material.

6. The positive electrode sheet according to claim 2, characterized in that: The thickness of the first coating layer is 0.2 μm to 1.5 μm; And / or, the thickness of the second coating layer is 0.001 μm to 0.2 μm.

7. The positive electrode sheet according to claim 1, characterized in that: The mass ratio of the first active material in the first coating layer to the second active material in the second coating layer is 1:(0.1-10).

8. The positive electrode sheet according to any one of claims 1 to 7, characterized in that: The mass content of the first active material in the first coating is 95wt% to 97wt%; And / or, the mass content of the second active material in the second coating layer is 95wt% to 97wt%.

9. A method for preparing a positive electrode sheet, characterized in that: The positive electrode sheet is the positive electrode sheet according to any one of claims 1 to 8, and the preparation method comprises the following steps: forming the first coating layer on the current collector; and The second coating layer is formed on a surface of the first coating layer facing away from the current collector.

10. A secondary battery, characterized in that: Contains the positive electrode sheet as described in any one of claims 1 to 8.

11. A battery module, characterized in that: The secondary battery as claimed in claim 10 is included.

12. A battery pack, characterized in that: Comprising the battery module as claimed in claim 11.

13. An electrical device, characterized in that: The battery comprises at least one selected from the group consisting of the secondary battery as claimed in claim 10, the battery module as claimed in claim 11, and the battery pack as claimed in claim 12.

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