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

Abstract

The invention relates to a positive pole piece, a preparation method thereof, a secondary battery and an electric device. The positive pole piece comprises a positive current collector, a positive active material layer is arranged on at least one side of the positive current collector, a lithium supplementing layer is arranged on at least one side of the positive active material layer, the positive active material layer comprises lithium iron phosphate, and the lithium supplementing layer comprises a ternary material. The lithium supplement layer containing the ternary material is arranged on at least one side of the positive active material layer of the positive pole piece, so that irreversible lithium supplement can be carried out on the negative pole piece, the lithium ion loss of the positive active material layer in the charge-discharge cycle process is effectively reduced, the capacity fading of the secondary battery is delayed, and the cycle life of the secondary battery is prolonged.

Description

Technical Field

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

[0002] Lithium-ion battery is a secondary battery widely used in electric vehicles, portable electronic products, aerospace and other fields. The commercial cathode materials mainly include ternary cathode materials, lithium cobalt oxide, lithium iron phosphate, etc. Among them, lithium iron phosphate has the advantages of low cost, low self-discharge rate and high safety. However, the cycle life of lithium iron phosphate still needs to be improved. Summary of the invention

[0003] Based on this, the present application provides a positive electrode plate, a preparation method thereof, a secondary battery and an electrical device to extend the cycle life of the secondary battery containing the positive electrode plate.

[0004] In a first aspect of the present application, a positive electrode plate is provided, which includes a positive electrode collector, at least one side of the positive electrode collector is provided with a positive electrode active material layer, at least one side of the positive electrode active material layer is provided with a lithium replenishing layer, the positive electrode active material layer includes lithium iron phosphate, and the lithium replenishing layer includes a ternary material.

[0005] A lithium replenishing layer is provided on at least one side of the positive electrode active material layer in the above-mentioned positive electrode plate, and the positive electrode active material layer includes lithium iron phosphate, and the lithium replenishing layer includes a ternary material. By providing a lithium replenishing layer including a ternary material, the amount of lithium ion loss in the positive electrode active material layer during the charge and discharge cycle is effectively reduced, thereby reducing the early capacity attenuation of the secondary battery and extending the cycle life of the secondary battery.

[0006] In some embodiments, the lithium replenishing layer is disposed between the positive electrode current collector and the positive electrode active material layer. Thus, the early capacity decay of the secondary battery is further reduced, and the cycle life of the secondary battery is extended. In addition, the bonding force between the positive electrode active material layer and the lithium replenishing layer is also improved, thereby improving the bonding force and conductivity of the positive electrode sheet, reducing the DC internal resistance of the secondary battery, and improving the storage performance of the secondary battery.

[0007] In some embodiments, the ternary material includes one or more of a ternary material containing nickel, cobalt and manganese and a ternary material containing nickel, cobalt and aluminum. Thus, the early capacity decay of the secondary battery is further reduced and the cycle life of the secondary battery is extended.

[0008] In some embodiments, the mass of the ternary material and the mass of the lithium iron phosphate are in a percentage of 1%-10%, and can be optionally 3%-5%. Thus, the initial capacity decay of the secondary battery is further reduced, and the cycle life of the secondary battery is extended.

[0009] In some embodiments, the thickness H2 of the lithium supplement layer on one side and the thickness H1 of the positive electrode active material layer on one side meet the following conditions: 5H2≤H1≤30H2. Thus, the early capacity decay of the secondary battery is further reduced and the cycle life of the secondary battery is extended. Optionally, the thickness H2 of the lithium supplement layer on one side and the thickness H1 of the positive electrode active material layer on one side meet the following conditions: 10H2≤H1≤15H2.

[0010] In some embodiments, the compaction density of the lithium supplement layer is 3 g / cm 3 -3.8g / cm 3 As a result, the initial capacity attenuation of the secondary battery is further reduced, and the cycle life of the secondary battery is extended.

[0011] In some embodiments, the weight fraction of the ternary material is 80%-97%, based on the weight of the lithium supplement layer, thereby further reducing the initial capacity decay of the secondary battery and extending the cycle life of the secondary battery.

[0012] In some embodiments, the weight fraction of the ternary material is 90%-95%, based on the weight of the lithium supplement layer, thereby further reducing the initial capacity decay of the secondary battery and extending the cycle life of the secondary battery.

[0013] In some embodiments, the lithium replenishing layer further includes a conductive agent and a binder, thereby further improving the bonding force between the positive electrode active material layer and the lithium replenishing layer, thereby further improving the bonding force and conductivity of the positive electrode sheet, reducing the DC internal resistance of the secondary battery, and improving the storage performance of the secondary battery.

[0014] The second aspect of the present application provides a method for preparing the positive electrode sheet described in the first aspect of the present application, comprising the steps of forming the positive electrode active material layer and forming the lithium supplement layer.

[0015] The positive electrode sheet prepared by the above preparation method has at least the same advantages as the positive electrode sheet described in the first aspect of the present application. In addition, the above preparation method has the advantages of simple operation and easy large-scale production.

[0016] The third aspect of the present application provides a secondary battery, comprising the positive electrode sheet described in the first aspect of the present application or the positive electrode sheet prepared by the preparation method described in the second aspect of the present application.

[0017] The above-mentioned secondary battery has at least the same advantages as the positive electrode sheet described in the first aspect of the present application.

[0018] In some embodiments, the operating voltage of the secondary battery is 2 V to 3.8 V. Thus, the initial capacity decay of the secondary battery is further reduced, and the cycle life of the secondary battery is extended.

[0019] A fourth aspect of the present application provides an electrical device, comprising the secondary battery described in the third aspect of the present application.

[0020] The above-mentioned electrical device includes the secondary battery described in the third aspect of the present application, and thus has at least the same advantages as the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of a secondary battery according to one embodiment of the present application.

[0022] Figure 2 yes Figure 1 An exploded view of a secondary battery according to an embodiment of the present application is shown.

[0023] Figure 3 is a schematic diagram of a battery module according to an embodiment of the present application.

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

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

[0026] 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.

[0027] Description of reference numerals:

[0028] 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

[0029] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are described in detail below. In the following description, many specific details are set forth to facilitate a full understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present application, so the present application is not limited by the specific embodiments disclosed below.

[0030] "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.

[0031] If not otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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).

[0036] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of this application, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0038] Lithium iron phosphate has been widely used in the field of energy storage due to its advantages such as low cost, low self-discharge rate and high safety. Prolonging the cycle life of lithium iron phosphate can further promote its application in the field of energy storage. The reason for the attenuation of the cycle life of secondary batteries containing lithium iron phosphate is mostly due to the loss of active lithium ions in the system. In view of this, the present application provides a positive electrode plate, which includes a positive current collector, at least one side of the positive current collector is provided with a positive electrode active material layer, at least one side of the positive electrode active material layer is provided with a lithium replenishment layer, the positive electrode active material layer includes lithium iron phosphate, and the lithium replenishment layer includes a ternary material. At least one side of the positive electrode active material layer of the above-mentioned positive electrode plate is provided with a lithium replenishment layer containing a ternary material, which can irreversibly replenish the negative electrode plate with lithium, effectively reducing the amount of lithium ion loss in the positive electrode active material layer during the charge and discharge cycle, thereby delaying the capacity decay of the secondary battery and extending the cycle life of the secondary battery.

[0039] Positive electrode

[0040] An embodiment of the present application provides a positive electrode plate, including a positive electrode current collector, at least one side of the positive electrode current collector is provided with a positive electrode active material layer, at least one side of the positive electrode active material layer is provided with a lithium supplement layer, the positive electrode active material layer includes lithium iron phosphate, and the lithium supplement layer includes a ternary material.

[0041] In the above embodiment, since the ternary material has a faster cycle decay than lithium iron phosphate, the lithium replenishment layer can continuously irreversibly replenish lithium to the negative electrode plate during the cycle charging process, that is, the irreversible capacity formed in the negative electrode plate and the lithium ions lost in the SEI film during the cycle charging process are the lithium ions in the lithium replenishment layer, so that the lithium ions released by the positive electrode active material layer during the cycle charging process can return to the positive electrode active material layer to the greatest extent during the cycle discharge process, and the lithium ions embedded in the lithium replenishment layer in the negative electrode plate can play a slow release effect during the subsequent cycle discharge process, effectively reducing the amount of lithium ion loss in the positive electrode active material layer during the charge and discharge cycle, thereby reducing the early capacity decay of the secondary battery and extending the cycle life of the secondary battery. It should be noted that the lithium replenishment layer can be arranged between the positive current collector and the positive active material layer, and the lithium replenishment layer can also be arranged on the side of the positive active material layer away from the positive current collector. Of course, the lithium replenishment layer can be arranged between the positive current collector and the positive active material layer and on the side of the positive active material layer away from the positive current collector. It can be understood that the positive electrode active material layer includes lithium iron phosphate, which means that the positive electrode active material layer contains lithium iron phosphate, and the lithium supplement layer includes ternary material, which means that the lithium supplement layer contains ternary material.

[0042] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode active material layer and / or the lithium supplement layer are arranged on any one or both of the two opposite surfaces of the positive electrode current collector.

[0043] In some embodiments, the positive electrode active material layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0044] 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.

[0045] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0046] In some embodiments, the lithium replenishment layer is disposed between the positive electrode current collector and the positive electrode active material layer. In a conventional positive electrode sheet, the positive electrode active material layer is in contact with the positive electrode current collector. Compared with the conventional positive electrode sheet, the positive electrode sheet in this embodiment has a lithium replenishment layer disposed between the positive electrode active material layer and the positive electrode current collector, which, on the one hand, reduces the early capacity decay during the cycle of the secondary battery and prolongs the cycle life of the secondary battery, and on the other hand, improves the bonding force between the positive electrode active material layer and the lithium replenishment layer, thereby improving the bonding force and conductivity of the positive electrode sheet, reducing the DC internal resistance (DCR) of the secondary battery, and improving the storage performance of the secondary battery.

[0047] In some embodiments, the ternary material includes one or more of a ternary material containing nickel, cobalt, and manganese elements and a ternary material containing nickel, cobalt, and aluminum elements.

[0048] The above-mentioned ternary material containing nickel, cobalt and manganese elements is not limited to lithium nickel cobalt manganese oxide, and lithium nickel cobalt manganese oxide can be, for example: LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 (Also referred to as NCM 333 )、LiNi 0.5 Co 0.2 Mn 0.3 O 2 (Also referred to as NCM 523 )、LiNi 0.5 Co 0.25 Mn 0.25 O 2 (Also referred to as NCM 211 )、LiNi 0.6 Co 0.2 Mn 0.2 O 2 (Also referred to as NCM 622 )、LiNi 0.8 Co 0.1 Mn 0.1 O 2 (Also referred to as NCM 811 ) and at least one of its modified compounds. The above-mentioned ternary material containing nickel, cobalt and aluminum includes but is not limited to lithium nickel cobalt aluminum oxide, and lithium nickel cobalt aluminum oxide can be, for example: LiNi 0.85 Co 0.15 Al 0.05 O 2 、LiNi 0.8 Co 0.1 Al 0.1 O 2 、LiNi0.6 Co 0.2 Al 0.2 O 2 、LiNi 0.5 Co 0.2 Al 0.3 O 2 and at least one of its modified compounds.

[0049] In some embodiments, the percentage of the mass of the ternary material to the mass of the lithium iron phosphate is 1%-10%. When the percentage of the mass of the ternary material to the mass of the lithium iron phosphate is too high, the capacity decay of the secondary battery is more serious due to the rapid decay of the ternary material, and the cycle life of the secondary battery decreases; when the percentage of the mass of the ternary material to the mass of the lithium iron phosphate is too low, the lithium ions released by the positive electrode active material layer during the cyclic charging process may not be able to return to the positive electrode active material layer during the cyclic discharge process, resulting in the lithium replenishment effect of the lithium replenishment layer being not obvious. Therefore, when the percentage of the mass of the ternary material to the mass of the lithium iron phosphate is within the above range, the amount of lithium ion loss of the positive electrode active material layer during the charge and discharge cycle is further effectively reduced, thereby reducing the early capacity decay of the secondary battery and extending the cycle life of the secondary battery. It can be understood that the percentage of the mass of the ternary material to the mass of the lithium iron phosphate includes but is not limited to: 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%. Optionally, the percentage of the mass of the ternary material to the mass of the lithium iron phosphate is 3%-5%.

[0050] In the present application, the percentage of the mass of the ternary material to the mass of the lithium iron phosphate refers to the mass of the ternary material / the mass of the lithium iron phosphate×100%.

[0051] In some embodiments, the single-side thickness H2 of the lithium replenishment layer and the single-side thickness H1 of the positive electrode active material layer satisfy the following conditions: 5H2≤H1≤30H2. When H1 is too thin relative to H2, the capacity decay of the secondary battery is more serious due to the rapid decay of the ternary material, and the cycle life of the secondary battery is reduced; when H1 is too thick relative to H2, the lithium ions released by the positive electrode active material layer during the cyclic charging process may not be able to return to the positive electrode active material layer during the cyclic discharge process, resulting in the lithium replenishment effect of the lithium replenishment layer being not obvious. Therefore, H1 and H2 satisfy the above conditions to further reduce the early capacity decay of the secondary battery and extend the cycle life of the secondary battery. Further, the single-side thickness H2 of the lithium replenishment layer and the single-side thickness H1 of the positive electrode active material layer satisfy the following conditions: 10H2≤H1≤15H2. It can be understood that the single-side thickness of the lithium replenishment layer refers to the thickness of the lithium replenishment layer located on one side of the positive current collector, and the single-side thickness of the positive electrode active material layer refers to the thickness of the positive electrode active material layer located on one side of the positive current collector.

[0052] In some embodiments, the compaction density of the lithium supplementation layer is 3 g / cm 3 -3.8g / cm 3 Controlling the compaction density of the lithium supplement layer to the above range can reduce the internal resistance of the secondary battery, reduce polarization loss, and further extend the cycle life of the secondary battery. It can be understood that the compaction density of the lithium supplement layer includes but is not limited to: 3g / cm 3 、3.1g / cm 3 、3.2g / cm 3 、3.3g / cm 3 、3.4g / cm 3 , 3.5g / cm 3 、3.6g / cm 3 、3.7g / cm 3 、3.8g / cm 3 .

[0053] In some embodiments, the weight fraction of the ternary material is 80%-97%, based on the weight of the lithium replenishing layer. Controlling the weight fraction of the ternary material in the lithium replenishing layer to the above range further reduces the early capacity decay during the secondary battery cycle and extends the cycle life of the secondary battery. It is understood that the weight fraction of the ternary material includes but is not limited to: 80%, 82%, 85%, 87%, 90%, 92%, 95%, 97%. The weight fraction of the ternary material can be optionally 90%-95%, based on the weight of the lithium replenishing layer.

[0054] In some embodiments, the lithium replenishment layer further includes a conductive agent and a binder. The conductive agent and binder contained in the above-mentioned lithium replenishment layer further improve the bonding force between the positive electrode active material layer and the lithium replenishment layer, thereby further improving the bonding force and conductivity of the positive electrode plate, reducing the DC internal resistance (DCR) of the secondary battery, and improving the storage performance of the secondary battery. The mass ratio of the ternary material, the conductive agent and the binder in the above-mentioned lithium replenishment layer can be, for example, 80:8:12, 85:6:9, 90:4:6 or 95:2:3. 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. 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.

[0055] Another embodiment of the present application provides a method for preparing the above-mentioned positive electrode plate, comprising the steps of forming a positive electrode active material layer and forming a lithium supplement layer.

[0056] The positive electrode sheet prepared by the above preparation method has at least the same advantages as the above positive electrode sheet. In addition, the above preparation method has the advantages of simple operation and easy large-scale production.

[0057] In some embodiments, the positive electrode sheet can be prepared by the following method: dispersing lithium iron phosphate, a conductive agent, a binder and any other components in a solvent (such as N-methylpyrrolidone) to form a lithium replenishing slurry; coating the lithium replenishing slurry on the positive electrode collector and drying to form a lithium replenishing layer; dispersing lithium iron phosphate, a conductive agent, a binder and any other components in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on the side of the lithium replenishing layer away from the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0058] In other embodiments, the positive electrode sheet can be prepared by the following method: dispersing lithium iron phosphate, a conductive agent, a binder and any other components in a solvent (such as N-methylpyrrolidone) to form a lithium replenishing slurry; coating the lithium replenishing slurry on the positive electrode collector and drying to form a lithium replenishing layer; dispersing lithium iron phosphate, a conductive agent, a binder and any other components in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on the side of the lithium replenishing layer away from the positive electrode collector, and drying to form a positive electrode active material layer; coating the lithium replenishing slurry on the side of the positive electrode active material layer away from the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0059] In other embodiments, the positive electrode sheet can be prepared by the following method: dispersing lithium iron phosphate, a conductive agent, a binder and any other components in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on the positive electrode collector and drying to form a positive electrode active material layer; dispersing lithium iron phosphate, a conductive agent, a binder and any other components in a solvent (such as N-methylpyrrolidone) to form a lithium replenishing slurry; coating the lithium replenishing slurry on the side of the positive electrode active material layer away from the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0060] Another embodiment of the present application provides a secondary battery, comprising the above-mentioned positive electrode sheet or the positive electrode sheet prepared by the above-mentioned preparation method.

[0061] 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.

[0062] In some embodiments, the operating voltage of the secondary battery is 2V-3.8V. The operating voltage of the ternary material is generally 2.8V-4.25V, and the maximum operating voltage of lithium iron phosphate is 3.8V. Since the operating voltage ranges of the ternary material and lithium iron phosphate are different, the simultaneous use of the ternary material and lithium iron phosphate can easily cause the lithium iron phosphate to be overcharged, causing its structural damage, and affecting the cycle performance of the secondary battery containing lithium iron phosphate. In this embodiment, the operating voltage of the secondary battery is set to 2V-3.8V, so that the ternary material operates in the voltage range of 2.8V-3.8V, which does not affect the structure of the lithium iron phosphate in the positive electrode active material layer, and can also achieve irreversible lithium supplementation for the negative electrode plate, thereby further reducing the early capacity decay of the secondary battery and extending the cycle life of the secondary battery.

[0063] Negative electrode

[0064] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material.

[0065] 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 either or both of the two facing surfaces of the negative electrode current collector.

[0066] In some 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.).

[0067] 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.

[0068] In some embodiments, the negative electrode active material layer may further include a binder, which 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).

[0069] 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.

[0070] In some embodiments, the negative electrode active material layer may further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0071] 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.

[0072] Electrolytes

[0073] The electrolyte plays the role of 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. For example, the electrolyte can be liquid, gel or all-solid.

[0074] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0075] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0076] 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.

[0077] 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.

[0078] Isolation film

[0079] 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.

[0080] 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.

[0081] 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.

[0082] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0083] In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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, electric vehicles, electric trains, ships and satellites, energy storage systems, etc., but are not limited to these. Among them, the mobile device may be, for example, a mobile phone, a laptop computer, etc.; the electric vehicle may be, for example, a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc., but are not limited to these.

[0092] As the electrical device, a secondary battery, a battery module or a battery pack may be selected according to its usage requirements.

[0093] Figure 6The 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.

[0094] 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.

[0095] 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.

[0096] Example 1

[0097] (1) Preparation of positive electrode sheet

[0098] (1.1) The ternary material LiNi 0.5 Mn 0.3 Co 0.2 O 2 , polyvinylidene fluoride (PVDF), and conductive carbon black are mixed in a weight ratio of 90:4:6, and a solvent N-methylpyrrolidone (NMP) is added, and the mixture is fully stirred and mixed to obtain a lithium supplement slurry, which is then coated on both surfaces of an aluminum foil, and then placed in an oven and dried at 120° C. for 30 minutes, and the single-side thickness of the lithium supplement layer is 10 μm;

[0099] (1.2) LiFePO 4 ), polyvinylidene fluoride, and conductive carbon black are mixed in a weight ratio of 96:2:2, and a solvent N-methylpyrrolidone is added, and the mixture is stirred and mixed to obtain a positive electrode slurry. The positive electrode slurry is coated on the lithium supplement layer. The single-side thickness of the positive electrode active material layer is 150 μm. The positive electrode sheet is made through drying, cold pressing and other processes. The compaction density of the positive electrode active material layer is 2.65 g / cm 3 The compaction density of the lithium supplement layer is 3.5g / cm 3 ;

[0100] (2) Preparation of negative electrode sheet

[0101] Artificial graphite, conductive carbon black, and styrene-butadiene latex (SBR) were mixed in a weight ratio of 96:2:2, and deionized water was added as a solvent. The mixture was fully stirred and mixed to obtain a negative electrode slurry, which was coated on both surfaces of the copper foil. The negative electrode sheet was made through drying, cold pressing and other processes. The compaction density of the negative electrode active material layer was 1.45 g / cm 3 ;

[0102] (3) Assembling lithium-ion batteries

[0103] The positive electrode and negative electrode are matched with a 12 μm thick PE separator and wound to form a lithium-ion battery cell. The cell is packaged into a metal shell to form a lithium-ion battery, and then injected into an electrolyte after drying. The electrolyte is composed of 1M LiPF 6 And the solvent DMC:EC:EMC=1:1:1Vol% composition, after the electrolyte is fully entered, the formation and capacity tests are carried out.

[0104] Example 2-14

[0105] The method is basically the same as Example 1, except that the composition, single-side thickness, and compaction density of the lithium supplement layer in step (1.1), the composition, single-side thickness, and compaction density of the positive electrode active material layer in step (1.2), and the operating voltage of the secondary battery are different from those in Example 1.

[0106] Comparative Example 1

[0107] Basically the same as Example 1, except that step (1.1) is not performed, and in step (1.2), the positive electrode slurry is coated on both surfaces of the aluminum foil.

[0108] Comparative Example 2

[0109] The method is basically the same as that in Example 1, except that: the method for preparing the positive electrode sheet is different from that in Example 1;

[0110] The positive electrode sheet of this comparative example is prepared as follows: the ternary material LiNi 0.5 Mn 0.3 Co 0.2 O 2 , lithium iron phosphate, polyvinylidene fluoride, and conductive carbon black are mixed in a weight ratio of 5:90:2:3, and a solvent N-methylpyrrolidone is added. The mixture is stirred and mixed to obtain a positive electrode slurry, which is then coated on both surfaces of the aluminum foil. The single-side thickness of the positive electrode active material layer is 160 μm. The positive electrode sheet is made through drying, cold pressing and other processes. The compaction density of the positive electrode active material layer is 2.65 g / cm 3 .

[0111] Comparative Example 3

[0112] Basically the same as Example 1, except that: step (1.1) is not performed, and in step (1.2), the positive electrode slurry is coated on both surfaces of the aluminum foil; the operating voltage range of the lithium-ion battery is 2-4V.

[0113] Comparative Example 4

[0114] The method is basically the same as that in Example 1, except that: the method for preparing the positive electrode sheet is different from that in Example 1;

[0115] The positive electrode sheet of this comparative example is prepared as follows: the ternary material LiNi 0.5 Mn 0.3 Co 0.2 O 2 , lithium iron phosphate, polyvinylidene fluoride, and conductive carbon black are mixed in a weight ratio of 5:90:2:3, and a solvent N-methylpyrrolidone is added. The mixture is stirred and mixed to obtain a positive electrode slurry, which is then coated on both surfaces of the aluminum foil. The single-side thickness of the positive electrode active material layer is 160 μm. The positive electrode sheet is made through drying, cold pressing and other processes. The compaction density of the positive electrode active material layer is 2.65 g / cm 3 ;The operating voltage range of lithium-ion batteries is 2-4V.

[0116] The product parameters of the lithium-ion batteries obtained in the embodiments and comparative examples are shown in Table 1.

[0117] Table 1 Product parameters of each embodiment and comparative example

[0118]

[0119]

[0120]

[0121] Performance Testing

[0122] (1) Test method for adhesion

[0123] Take the electrode to be tested, cut a sample with a width of 30mm and a length of 100mm-160mm with a blade, and stick it on the steel plate with a special double-sided tape, with the test surface facing down, and then roll it three times in the same direction with a roller, insert a paper tape with the same width as the electrode under the electrode and fix it. Fix the steel plate and paper tape on the tensile machine with a clamp, pull the paper tape upward at a speed of 50mm / min until the displacement is greater than 70mm, and read the tensile machine reading at this time as the bonding force between the active material layer or lithium supplement layer and the current collector.

[0124] (2) Test method of DC internal resistance

[0125] At room temperature, the lithium-ion battery is charged at a rate of 1 / 3C to the upper limit operating voltage (3.8V or 4V), that is, 100% SOC, and then the lithium-ion battery is discharged to 50% SOC. After standing for 5 minutes, it is discharged at a rate of 4C for 10 seconds to obtain the DC internal resistance DCR of the lithium-ion battery at 50% SOC at room temperature.

[0126] (3) Storage performance test method

[0127] At 25°C, the lithium-ion battery was stored in a constant temperature box for 365 days. The battery capacity was measured every 30 days. After storage, it was charged to the upper limit operating voltage (3.8V or 4V) with a constant current of 0.5C, then charged to 0.05C with a constant voltage, and then discharged to 2.5V with a current of 0.5C, and then discharged to 2.0V with a current of 0.1C, and the ratio of the capacity at this time to the initial capacity was recorded.

[0128] (4) Cycle performance test method

[0129] The effect of lithium supplementation can be verified by a cycle test of a lithium-ion battery. The cycle test conditions are: at 25°C, charge at a constant current of 0.5C to the upper limit operating voltage (3.8V or 4V), then charge at a constant voltage to 0.05C, let stand for 10 minutes, then discharge at 0.5C to 2.5V, then discharge at 0.1C to 2.0V, until the discharge capacity of the lithium-ion battery decays to 80% of the initial capacity, and record the number of cycles at this time.

[0130] Table 2 Performance parameters of positive electrode sheets and lithium-ion batteries of various embodiments and comparative examples

[0131]

[0132] By analyzing Table 1 and Table 2, and comparing Examples 1-10, Examples 12-14, and Comparative Example 1-2, it can be seen that the DC internal resistance of the secondary battery prepared in Example 1-10 is significantly smaller, and the number of cycles attenuated to 80% of the initial capacity is relatively large; by comparing Example 11 and Comparative Examples 3-4, it can be seen that the DC internal resistance of the secondary battery prepared in Example 11 is significantly smaller, and the number of cycles attenuated to 80% of the initial capacity is relatively large; this indicates that in Example 1-14, a lithium supplement layer is provided on at least one side of the positive electrode active material layer, which prolongs the cycle life of the secondary battery and reduces the DC internal resistance of the secondary battery.

[0133] It can be seen from Example 1, Examples 9-10 and Comparative Examples 1-2 that the lithium supplement layer disposed between the positive electrode current collector and the positive electrode active material layer improves the bonding force between the lithium supplement layer and the current collector and reduces the DC internal resistance of the secondary battery.

[0134] 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.

[0135] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the patent application. 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 application shall be subject to the attached claims, and the specification and drawings may be used to interpret the contents of the claims.

Claims

1. A positive electrode sheet, characterized in that: It comprises a positive electrode current collector, at least one side of which is provided with a positive electrode active material layer, at least one side of which is provided with a lithium supplement layer, the positive electrode active material layer comprises lithium iron phosphate, and the lithium supplement layer comprises a ternary material.

2. The positive electrode sheet according to claim 1, characterized in that: The lithium supplement layer is arranged between the positive electrode current collector and the positive electrode active material layer; Optionally, the ternary material includes one or more of a ternary material containing nickel, cobalt and manganese elements and a ternary material containing nickel, cobalt and aluminum elements.

3. The positive electrode sheet according to claim 1 or 2, characterized in that: The percentage of the mass of the ternary material to the mass of the lithium iron phosphate is 1%-10%, and can be optionally 3%-5%.

4. The positive electrode sheet according to claim 1 or 2, characterized in that: The single-side thickness H2 of the lithium supplement layer and the single-side thickness H1 of the positive electrode active material layer satisfy the following condition: 5H2≤H1≤30H2, which may be 10H2≤H1≤15H2.

5. The positive electrode sheet according to claim 1 or 2, characterized in that: The compaction density of the lithium supplement layer is 3g / cm 3 -3.8g / cm 3 .

6. The positive electrode sheet according to claim 1, characterized in that: The weight fraction of the ternary material is 80%-97%, based on the weight of the lithium supplement layer; Optionally, the weight fraction of the ternary material is 90%-95%, based on the weight of the lithium replenishing layer.

7. The positive electrode sheet according to claim 1 or 6, characterized in that: The lithium supplement layer also includes a conductive agent and a binder.

8. The method for preparing a positive electrode sheet according to any one of claims 1 to 7, characterized in that: include: The steps of forming the positive electrode active material layer and forming the lithium replenishing layer.

9. A secondary battery, characterized in that: A positive electrode sheet comprising the positive electrode sheet according to any one of claims 1 to 7 or the positive electrode sheet prepared by the preparation method according to claim 8; Optionally, the operating voltage of the secondary battery is 2V-3.8V.

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

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