Positive pole piece and preparation method thereof, secondary battery and electric device

Abstract

The invention relates to a positive pole piece, a preparation method of the positive pole piece, a secondary battery and an electric device. The positive pole piece comprises a positive current collector, a positive active material layer arranged on the surface of the positive current collector and a polymer layer arranged on the surface of the positive active material layer, and the material of the polymer layer comprises a polymer material; the solubility of the polymer material in a preset electrolyte is as follows: at the working temperature of the positive pole piece, more than 80% of the total mass of the polymer material can be dissolved in the preset electrolyte. The positive pole piece can effectively improve the cycle performance of the battery.

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 and an electrical device. Background Art

[0002] In recent years, as the application scope of batteries has become more and more extensive, batteries are widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields. As batteries have made great progress, higher requirements have been put forward for their energy density, cycle performance and safety performance.

[0003] The traditional positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer arranged on the surface of the positive electrode current collector. The cycle performance of the battery using this type of positive electrode sheet needs to be further improved. Summary of the invention

[0004] The present application provides a positive electrode sheet and a preparation method thereof, a secondary battery, and an electric device. The positive electrode sheet can effectively improve the cycle performance of the battery.

[0005] In a first aspect of the present application, a positive electrode plate is provided, comprising a positive electrode current collector, a positive electrode active material layer arranged on the surface of the positive electrode current collector, and a polymer layer arranged on the surface of the positive electrode active material layer, wherein the material of the polymer layer comprises a polymer material, and the solubility of the polymer material in a preset electrolyte is: at the operating temperature of the positive electrode plate, more than 80% of the total mass of the polymer material can be dissolved in the preset electrolyte.

[0006] The positive electrode plate is provided with a polymer layer on the surface of the positive electrode active material layer. The material of the polymer layer includes a polymer material that is soluble or easily soluble in a preset electrolyte, which can effectively improve the cycle life of the battery.

[0007] In some of the embodiments, the polymer material includes one or more of polymethyl methacrylate (PMMA), a block copolymer of polymethyl methacrylate and propylene sulfite, polyethylene oxide (PEO), polyacrylonitrile (PAN) and polystyrene (PS).

[0008] In some embodiments, the thickness of the polymer layer is D, D = 0.01 μm to 15 μm; optionally, D = 5 μm to 10 μm. Reasonable control of D can reduce the effect of the polymer material completely dissolved in the electrolyte on the viscosity and conductivity of the electrolyte while regulating the release rate of the active lithium in the formation of the polymer layer, thereby further improving the cycle performance.

[0009] In some embodiments, the weight percentage of the polymer material in the preset electrolyte is wt, wt=0.05% to 2%; optionally, wt=0.5% to 1.5%. Reasonable control of wt can reduce the influence of the polymer material on the viscosity and conductivity of the electrolyte after it is completely dissolved into the electrolyte while regulating the release rate of active lithium in the formation of the polymer layer, thereby further improving the cycle performance.

[0010] The second aspect of the present application provides a method for preparing the positive electrode sheet according to the first aspect, comprising the following steps:

[0011] forming the positive electrode active material layer on the surface of the positive electrode current collector;

[0012] The polymer layer is formed on the surface of the positive electrode active material layer.

[0013] The preparation method has simple steps and is easy to promote and apply in industrialization.

[0014] In some embodiments, forming the polymer layer on the surface of the positive electrode active material layer comprises:

[0015] mixing the polymer material with a solvent to form a polymer dispersion;

[0016] applying the polymer dispersion to the surface of the positive electrode active material layer, and drying to form the polymer layer;

[0017] Optionally, the application method includes one or more of coating, spraying and dipping;

[0018] Optionally, the solvent includes one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), 1,3-dioxolane, diphenyl carbonate, propene sultone, propenyl-1,3 sultone, triphenyl phosphate, triphenyl phosphite and sulfolane.

[0019] According to a third aspect of the present application, an electrode assembly is provided, comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the positive electrode sheet comprises the positive electrode sheet described in the first aspect.

[0020] In a fourth aspect of the present application, a secondary battery is provided, comprising a positive electrode plate, a negative electrode plate and an electrolyte, wherein the positive electrode plate comprises the positive electrode plate described in the first aspect, and the electrolyte comprises the preset electrolyte.

[0021] In some embodiments, the electrolyte includes one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), vinylene carbonate (VC), diethylene sulfate (DTD) and 1,3-propane sultone (PS).

[0022] In a fifth aspect of the present application, there is provided an electrical device comprising the electrode assembly described in the third aspect, the secondary battery described in the fourth aspect, the battery module described in the fifth aspect, or the battery pack described in the sixth aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of a secondary battery according to an embodiment of the present application;

[0024] Figure 2 for Figure 1 An exploded view of a secondary battery according to an embodiment of the present application is shown;

[0025] Figure 3 is a schematic diagram of a battery module according to an embodiment of the present application;

[0026] Figure 4 A schematic diagram of a battery pack according to an embodiment of the present application;

[0027] Figure 5 for Figure 4 An exploded view of a battery pack according to an embodiment of the present application is shown;

[0028] Figure 6 A schematic diagram of an electrical device using a secondary battery according to an embodiment of the present application as a power source;

[0029] Description of reference numerals:

[0030] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover plate; 6 electrical device. DETAILED DESCRIPTION

[0031] Below, the embodiments of the positive electrode sheet and its preparation 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.

[0032] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0033] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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).

[0038] Some examples of the present application provide a positive electrode plate, including a positive electrode current collector, a positive electrode active material layer arranged on the surface of the positive electrode current collector, and a polymer layer arranged on the surface of the positive electrode active material layer, the material of the polymer layer includes a polymer material, and the solubility of the polymer material in a preset electrolyte is: at the working temperature of the positive electrode plate, more than 80% of the total mass of the polymer material can be dissolved in the preset electrolyte.

[0039] The above-mentioned positive electrode plate has a polymer layer disposed on the surface of the positive electrode active material layer, and the material of the polymer layer includes a polymer material that is soluble or easily soluble in a preset electrolyte, that is, the polymer material will gradually dissolve in the electrolyte after contacting with the electrolyte (such as during battery assembly and injection and subsequent cyclic use). By adjusting the thickness of the polymer layer, the release rate of active lithium can be adjusted, so that it is not easy to escape when the initial polarization phenomenon of the cycle is more obvious, and more is pre-stored in the positive electrode plate. As the battery cycle proceeds, the polymer material gradually dissolves, the polarization phenomenon gradually decreases, and the active lithium pre-stored in the positive electrode plate is gradually released, gradually replenishing the consumption of active lithium during the battery cycle. This can effectively improve the cycle life of the battery.

[0040] It can be understood that the number of polymer layers can be one or more, the materials of the polymer layers can be the same or different, and the same polymer layer can use one polymer material or multiple polymer materials.

[0041] It can be understood that the preset electrolyte refers to the electrolyte pre-set in the secondary battery that adopts the positive electrode plate.

[0042] It can be understood that the working temperature of the positive electrode plate refers to the battery temperature when the positive electrode plate is assembled into a secondary battery and subjected to charge and discharge cycles, which is not limited and can be exemplified by -10°C to 70°C.

[0043] In some examples, the polymer material includes one or more of polymethyl methacrylate (PMMA), a block copolymer of polymethyl methacrylate and propylene sulfite, polyethylene oxide (PEO), polyacrylonitrile (PAN), and polystyrene (PS).

[0044] In some of the examples, the thickness of the polymer layer is D, D = 0.01μm ~ 2μm. Reasonable control of D, while forming a polymer layer to regulate the release rate of active lithium, can reduce the influence of the polymer material on the viscosity and conductivity of the electrolyte after it is completely dissolved into the electrolyte, and further improve the cycle performance. Specifically, the value of D includes but is not limited to: 0.01μm, 0.05μm, 0.1μm, 0.3μm, 0.5μm, 0.7μm, 1μm, 1.2μm, 1.5μm, 2μm or any range between the two. Further, D = 0.5μm ~ 1μm.

[0045] In some of the examples, the weight percentage of the polymer material in the preset electrolyte is wt, wt=0.05%~2%. Reasonable control of wt, while forming a polymer layer to regulate the release rate of active lithium, can reduce the influence of the polymer material on the viscosity and conductivity of the electrolyte after it is completely dissolved into the electrolyte, and further improve the cycle performance. Specifically, the value of wt includes but is not limited to: 0.05%, 0.1%, 0.3%, 0.5%, 0.7%, 1%, 1.2%, 1.5%, 1.7%, 2% or any range between the two. Further, wt=1.5%~2%.

[0046] It can be understood that the thickness of the polymer layer is D, in μm; the injection weight of the preset electrolyte is m, in g; the weight percentage of the polymer material to the preset electrolyte is wt(%); the area of ​​the polymer layer is a, in mm 2 ; The true density of the polymer material is ρ, in g / cm 3 ; The area coverage of the polymer layer relative to the surface of the positive electrode active material layer is c, in %; At the working temperature of the positive electrode plate, the mass percentage of the polymer material that can be dissolved in the preset electrolyte (based on the total mass of the polymer material) is d, in %. D, m, wt, a, ρ, c and d meet the following conditions:

[0047] D=m*wt*100000 / (2a*ρ) / c / d.

[0048] It can be understood that when there are multiple polymer layers, the total thickness D0 of the polymer layers can be correspondingly:

[0049] D0=m*wt1*100000 / (2a1*ρ1) / c1 / d1+m*wt2*100000 / (2a2*ρ2) / c2 / d2+……+m*wt n *100000 / (2a n *ρ n ) / c n / d n.

[0050] Among them, wt1 to wt n , a1 to a n , ρ1 to ρ n , c1 to c n , d1 to d n Parameters corresponding to the polymer material of the first polymer layer to the polymer material of the nth polymer layer are respectively shown.

[0051] In some examples, the polymer layer includes one or two layers. When there are two layers, it includes a first polymer layer and a second polymer layer stacked in sequence. Without limitation, the material of the first polymer layer includes one or more of polymethyl methacrylate (PMMA), a block copolymer of polymethyl methacrylate-propylene sulfite, polyethylene oxide (PEO), polyacrylonitrile (PAN) and polystyrene (PS); the material of the second polymer layer includes one or more of polymethyl methacrylate (PMMA), a block copolymer of polymethyl methacrylate-propylene sulfite, polyethylene oxide (PEO), polyacrylonitrile (PAN) and polystyrene (PS).

[0052] Some other examples of the present application provide a method for preparing the positive electrode sheet as described above, comprising the following steps:

[0053] forming the positive electrode active material layer on the surface of the positive electrode current collector;

[0054] The polymer layer is formed on the surface of the positive electrode active material layer.

[0055] The preparation method has simple steps and is easy to promote and apply in industrialization.

[0056] In some examples, forming the polymer layer on the surface of the positive electrode active material layer includes:

[0057] mixing the polymer material with a solvent to form a polymer dispersion;

[0058] The polymer dispersion is applied to the surface of the positive electrode active material layer and dried to form the polymer layer.

[0059] The polymer material is solid and cannot be directly disposed on the surface of the positive electrode active material layer. Therefore, the polymer material is first mixed with a solvent and then applied, which can be applicable to a variety of processing techniques. Without limitation, the application method includes one or more of coating, spraying and immersion. Without limitation, the coating can be ordinary direct coating or can be coated with the aid of tools such as micro-gravure.

[0060] In some of the examples, the application method includes one or both of direct coating and spraying. The polymer layer prepared in this way can better improve the cycle performance of the battery, and the process efficiency is high, which can meet the needs of industrial application.

[0061] In some examples, the solvent includes one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), 1,3-dioxolane, diphenyl carbonate, propene sultone, propenyl-1,3 sultone, triphenyl phosphate, triphenyl phosphite and sulfolane.

[0062] In some further examples of the present application, a secondary battery is provided, comprising a positive electrode plate, a negative electrode plate and an electrolyte, wherein the positive electrode plate comprises the positive electrode plate as described above, and the electrolyte comprises the preset electrolyte as described above.

[0063] In some examples, the electrolyte includes one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), vinylene carbonate (VC), diethylene sulfate (DTD) and 1,3-propane sultone (PS).

[0064] In some further examples of the present application, an electric device is provided, comprising the secondary battery as described above.

[0065] 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.

[0066] In one embodiment of the present application, a secondary battery is provided.

[0067] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. During the battery charging and discharging process, active ions are embedded and released 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 set 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.

[0068] Positive electrode

[0069] The positive electrode is as described above.

[0070] As an example, the positive electrode current collector has two surfaces facing each other in its thickness direction, and the positive electrode active material layer is disposed on any one or both of the two facing surfaces of the positive electrode current collector.

[0071] In some of the embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0072] In some of the embodiments, the positive electrode active material may adopt the positive electrode active material for batteries known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 )、LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 )、LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 )、LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 )、LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05O2) and its modified compounds, etc. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0073] In some embodiments, the positive electrode active material layer may further include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0074] In some embodiments, the positive electrode active material layer may further include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0075] Negative electrode

[0076] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material.

[0077] As an example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode active material layer is disposed on any one or both of the two facing surfaces of the negative electrode current collector.

[0078] In some of the embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0079] In some of the embodiments, the negative electrode active material may be a negative electrode active material for a 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.

[0080] In some embodiments, the negative electrode active material 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).

[0081] In some embodiments, the negative electrode active material 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.

[0082] In some of the embodiments, the negative electrode active material layer may further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0083] In some of the 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.

[0084] Electrolyte

[0085] The electrolyte plays a role in conducting ions between the positive electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs.

[0086] In some of these embodiments, the electrolyte includes an electrolyte salt and a solvent.

[0087] In some of the embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate and lithium tetrafluorooxalatophosphate.

[0088] In some of the 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.

[0089] 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.

[0090] Isolation film

[0091] 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.

[0092] 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.

[0093] In some of the embodiments, the positive electrode sheet, the negative electrode sheet and the separator can be formed into an electrode assembly by a winding process or a lamination process.

[0094] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0095] 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 as the plastic, polypropylene, polybutylene terephthalate, and polybutylene succinate can be listed.

[0096] 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.

[0097] In some of these 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 enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0098] In some of the embodiments, the secondary batteries may be assembled into a battery module. The number of the 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.

[0099] Figure 3 4 is an example of a battery module. 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.

[0100] 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.

[0101] In some of the embodiments, the battery modules may be assembled into a battery pack. The battery pack may contain one or more battery modules. The specific number may be selected by those skilled in the art according to the application and capacity of the battery pack.

[0102] 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.

[0103] In addition, the present application also provides an electrical device, which includes at least one of the secondary batteries, battery modules, or battery packs provided in the present application. The secondary batteries, battery modules, or battery packs can be used as power sources for electrical devices, or as energy storage units for electrical devices. Electrical devices may include mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto. Among them, mobile devices may be, for example, mobile phones, laptop computers, etc.; electric vehicles may be, for example, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc., but are not limited thereto.

[0104] As an electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0105] Figure 6 The power consumption device 6 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.

[0106] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

[0107] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0108] Example 1

[0109] (1) Preparation of positive electrode sheet

[0110] Step 1: Mix the positive electrode active material lithium iron phosphate (gram capacity of 139 mAh / g), the conductive agent acetylene black, and the binder PVDF in a weight ratio of 94:4:2, add the solvent N-methylpyrrolidone, stir and mix well to obtain the positive electrode slurry, and then apply it on the two surfaces of the positive electrode current collector aluminum foil, wherein the coating weight of the positive electrode slurry is 0.224g / 1540.25mm 2 (based on weight excluding solvent), and then dried and cold pressed to obtain a positive electrode active material layer.

[0111] Step 2: 100g PMMA (polymer material, ρ = 1.2g / cm 3) was dissolved in 900 g of 1,3-dioxolane to obtain a PMMA solution with a mass fraction of 10%.

[0112] Step 3: Spray the PMMA solution prepared in step 2 onto the surface of the positive electrode active material layer prepared in step 1, and dry it at 25°C for 30 minutes to form a polymer layer with a weight of 50 mg and a coverage area of ​​416666 mm on the surface of the positive electrode active material layer. 2 (a), the coverage is 50% (c), at the working temperature of the positive electrode sheet, PMMA is finally dissolved 100% (d) in the electrolyte to obtain a positive electrode sheet.

[0113] (2) Preparation of negative electrode sheet

[0114] The negative electrode active material artificial graphite (gram capacity of 340 mAh / g), the conductive agent acetylene black, the binder SBR, and the binder CMC were mixed according to a weight ratio of 95:1.5:3.1:0.4, and the solvent deionized water was added, and the mixture was fully stirred to obtain a negative electrode slurry, which was then coated on both surfaces of the negative electrode current collector copper foil, wherein the coating weight of the negative electrode slurry was 0.108 g / 1540.25 mm 2 (measured by weight excluding solvent), and after drying and cold pressing, a negative electrode sheet is obtained.

[0115] (3) Preparation of electrolyte

[0116] In an argon atmosphere glove box with a water content of <10ppm, EC, PC, and DMC were mixed in a weight ratio of EC:PC:DMC=3:3:3, and then LiPF6, VC, DTD, and PS were added to the mixed organic solvent and stirred evenly to obtain an electrolyte, in which the concentration of LiPF6 in the lithium-ion battery electrolyte was 1mol / L, and the mass percentages of VC, DTD, and PS were 3%, 1%, and 1%, respectively.

[0117] (4) Preparation of isolation membrane

[0118] A polyethylene porous membrane was used as the isolation membrane.

[0119] (5) Preparation of lithium-ion secondary batteries

[0120] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is 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, 10g (m) of the prepared electrolyte is injected, and the package is formed to obtain a lithium-ion secondary battery.

[0121] The secondary batteries of Examples 2-6 are prepared in a similar manner to that of Example 1, with the main difference being that the thickness of the polymer layer is changed accordingly by changing m, wt, c and d (the value of (2a*ρ) is kept constant).

[0122] The preparation method of the secondary battery of Examples 7-8 is similar to that of the secondary battery of Example 2, the main difference being that the type of polymer material is changed.

[0123] The preparation method of the secondary battery of Examples 9-10 is similar to that of the secondary battery of Example 2, the main difference being that two polymer layers made of different polymer materials are provided.

[0124] The preparation method of the secondary battery of Example 11 is similar to that of the secondary battery of Example 2, except that the polymer layer is formed by direct coating. The steps are as follows:

[0125] Step 3: directly coating the PMMA solution prepared in step 2 on the surface of the positive electrode active material layer prepared in step 1, and drying at 25° C. for 30 minutes to form a polymer layer, thereby obtaining a positive electrode sheet.

[0126] The preparation method of the secondary battery of Example 12 is similar to that of the secondary battery of Example 2, except that the polymer layer is formed by infiltration. The steps are as follows:

[0127] Step 3: The positive electrode active material layer prepared in step 1 is immersed in the PMMA solution prepared in step 2, then taken out, and air-dried at 25° C. for 30 minutes to form a polymer layer, thereby obtaining a positive electrode sheet.

[0128] The secondary battery of Example 13 is prepared in a similar manner to the secondary battery of Example 2, except that the polymer layer is formed by micro-gravure coating. The steps are as follows:

[0129] Step 3: The PMMA solution prepared in step 2 is transferred and coated onto the surface of the positive electrode active material layer prepared in step 1 by gravure equipment, and dried at 25° C. for 30 minutes to form a polymer layer, thereby obtaining a positive electrode sheet.

[0130] The preparation method of the secondary battery of Comparative Example 1 is similar to that of the secondary battery of Example 2, the main difference being that the polymer layer is not prepared, that is, step 2 and step 3 are not performed.

[0131] The main parameters of the embodiments and comparative examples are summarized in Table 1 below.

[0132] Among them, the test steps of d are as follows:

[0133] At the battery operating temperature, a polymer material with a mass of m1 is placed in a corresponding electrolyte and stirred for 48 hours. The polymer material is filtered and weighed, and the mass of the undissolved polymer material is m2, where d = 1-m2 / m1.

[0134] In Example 1:

[0135] wt=50mg / 10g=0.5%, D=m*wt*100000 / (2a*ρ) / c / d=10*0.5%*100000 / (2*416666*1.2) / 50% / 100%=0.01μm.

[0136] In Example 2:

[0137] wt=49.5mg / 33g=1.5%, D=m*wt*100000 / (2a*ρ) / c / d=33*1.5%*100000 / (2*416666*1.2) / 10% / 100%=0.5μm.

[0138] In Example 3:

[0139] wt=49.5mg / 33g=1.5%, D=m*wt*100000 / (2a*ρ) / c / d=33*1.5%*100000 / (2*416666*1.2) / 5% / 100%=1μm.

[0140] In embodiment 4:

[0141] wt=99mg / 66g=1.5%, D=m*wt*100000 / (2a*ρ) / c / d=66*1.5%*100000 / (2*416666*1.2) / 5% / 100%=2μm.

[0142] In embodiment 5:

[0143] wt=5mg / 10g=0.05%, D=m*wt*100000 / (2a*ρ) / c / d=10*0.05%*100000 / (2*416666*1.2) / 5% / 100%=0.01μm.

[0144] In Example 6:

[0145] wt=200mg / 10g=2%, D=m*wt*100000 / (2a*ρ) / c / d=10*2%*100000 / (2*416666*1.2) / 5% / 100%=0.4μm.

[0146] Table 1

[0147]

[0148]

[0149] Test example:

[0150] (1) Room temperature cycle performance test of lithium-ion secondary batteries

[0151] At 25°C, the lithium-ion secondary battery is first charged at a constant current of 1C (i.e., the current value that completely discharges the theoretical capacity within 1 hour) to a voltage of 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C. After standing for 5 minutes, the lithium-ion secondary battery is discharged at a constant current of 1C to a voltage of 2.5V. This is a charge and discharge cycle process, and the discharge capacity this time is the discharge capacity of the first cycle. The lithium-ion secondary battery is subjected to multiple cycle charge and discharge tests according to the above method until the discharge capacity of the lithium-ion secondary battery decays to 80%, and the number of cycles of the lithium-ion secondary battery is recorded.

[0152] (2) High temperature cycle performance test of lithium-ion secondary batteries

[0153] At 60°C, the lithium-ion secondary battery is first charged at a constant current of 1C (i.e., the current value that completely discharges the theoretical capacity within 1 hour) to a voltage of 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C. After standing for 5 minutes, the lithium-ion secondary battery is discharged at a constant current of 1C to a voltage of 2.5V. This is a charge and discharge cycle process, and the discharge capacity this time is the discharge capacity of the first cycle. The lithium-ion secondary battery is subjected to multiple cycle charge and discharge tests according to the above method, and the discharge capacity of the 500th cycle is detected.

[0154] Capacity retention rate of the lithium ion secondary battery after 500 cycles at 60° C.=(discharge capacity at the 500th cycle / discharge capacity at the first cycle)×100%.

[0155] The test results are shown in Table 2 below.

[0156] Table 2

[0157]

[0158] From the comparison between Examples 1 to 13 and Comparative Example 1, it can be seen that the cycle performance of the battery can be effectively improved by providing the polymer layer.

[0159] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode sheet, characterized in that: It includes a positive electrode current collector, a positive electrode active material layer arranged on the surface of the positive electrode current collector, and a polymer layer arranged on the surface of the positive electrode active material layer. The material of the polymer layer includes a polymer material. The solubility of the polymer material in a preset electrolyte is: at the working temperature of the positive electrode plate, more than 80% of the total mass of the polymer material can be dissolved in the preset electrolyte.

2. The positive electrode sheet according to claim 1, characterized in that: The polymer material includes one or more of polymethyl methacrylate, a block copolymer of polymethyl methacrylate-propylene sulfite, polyethylene oxide, polyacrylonitrile and polystyrene.

3. The positive electrode sheet according to claim 1 or 2, characterized in that: The thickness of the polymer layer is D, D=0.01 μm to 2 μm; optionally, D=0.5 μm to 1 μm.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The weight percentage of the polymer material to the preset electrolyte is wt, wt=0.05% to 2%; optionally, wt=1.5% to 2%.

5. The method for preparing a positive electrode sheet according to any one of claims 1 to 4, characterized in that: The steps include: forming the positive electrode active material layer on the surface of the positive electrode current collector; The polymer layer is formed on the surface of the positive electrode active material layer.

6. The method for preparing a positive electrode sheet according to claim 5, characterized in that: Forming the polymer layer on the surface of the positive electrode active material layer comprises: mixing the polymer material with a solvent to form a polymer dispersion; applying the polymer dispersion to the surface of the positive electrode active material layer, and drying to form the polymer layer; Optionally, the application method includes one or more of coating, spraying and dipping; Optionally, the solvent includes one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,3-dioxolane, diphenyl carbonate, propene sultone, propenyl-1,3 sultone, triphenyl phosphate, triphenyl phosphite and sulfolane.

7. An electrode assembly, characterized in that: It comprises a positive electrode sheet, a negative electrode sheet and a separator, wherein the positive electrode sheet comprises the positive electrode sheet according to any one of claims 1 to 4.

8. A secondary battery, characterized in that: It comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet comprises the positive electrode sheet according to any one of claims 1 to 4, and the electrolyte comprises the preset electrolyte.

9. The secondary battery according to claim 8, characterized in that: The electrolyte includes one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, vinylene carbonate, vinyl sulfate and 1,3-propane sultone.

10. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to claim 8 or 9.

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