CN115832166B - Positive electrode sheet, secondary battery, battery module, battery pack, and power consumption device - Google Patents

Abstract

The application provides a positive pole piece, including coating and current collector, the coating is located at least one surface of current collector, the coating includes first coating and second coating, first coating coats on the current collector surface, the second coating coat in first coating surface, wherein, first coating tortuosity is 3.01 ~ 4.93, second coating tortuosity is 2.11 ~ 2.87.

Description

Positive electrode plates, secondary batteries, battery modules, battery packs and electrical devices

Technical field

The present application relates to the technical field of lithium batteries, and in particular to a positive electrode plate, a secondary battery, a battery module, a battery pack and an electrical device.

Background technique

In recent years, as lithium-ion batteries have become more and more widely used, lithium-ion secondary batteries are widely used in energy storage power systems such as hydraulic, thermal, wind and solar power stations, as well as power tools, electric bicycles, electric motorcycles, electric Automobiles, military equipment, aerospace and other fields. As lithium-ion secondary batteries have achieved great development, higher requirements have been placed on their energy density, cycle performance and safety performance. In addition, due to the bulk doping and surface coating of the positive electrode active material, the energy density modification effect of lithium-ion secondary batteries is limited. Therefore, the industry is currently focusing on optimizing the structural design of lithium-ion secondary batteries. For example, when the cell size and material chemistry are fixed, if the positive electrode sheet of a lithium-ion secondary battery has a higher coating weight, the proportion of active material can be significantly increased, thereby increasing the energy density of the cell.

However, as the coating amount of positive electrode sheets continues to increase, it is difficult to dry the electrode sheets of lithium-ion secondary batteries. Therefore, when producing lithium-ion secondary batteries, the baking time and soaking time will also increase accordingly. At the same time, it is difficult for the electrolyte to fully infiltrate the pole pieces, and the dynamic performance of the lithium-ion secondary battery is correspondingly deteriorated, which cannot meet the performance requirements of the lithium-ion secondary battery. Therefore, existing designs for positive electrode plates still need to be improved.

Contents of the invention

The technical problem to be solved by the invention

This application was made in view of the above problems, and its purpose is to provide a positive electrode sheet that can improve the rate performance, high and low temperature performance, and safety performance of a secondary battery using the positive electrode sheet, and at the same time improve its dynamic performance. .

Technical solutions to solve problems

In order to achieve the above object, the present application provides a positive electrode plate, a secondary battery, a battery module, a battery pack and a power device.

A first aspect of the present application provides a positive electrode sheet, which is characterized in that it includes a current collector and a coating coated on at least one surface of the current collector, and the coating includes a first coating and a second coating. layer, the first coating is coated on the surface of the current collector, the second coating is coated on the surface of the first coating, wherein the tortuosity of the first coating is 3.01 to 4.93, so The tortuosity of the second coating ranges from 2.11 to 2.87.

Therefore, this application comprehensively considers the processing convenience of the positive electrode piece, the electrochemical performance and energy density of the secondary battery, and realizes the counter electrode piece by adjusting the tortuosity of the first coating and the second coating of the positive electrode piece. Gradient adjustment of porosity. Although the mechanism is not yet clear, it is speculated that the tortuosity of the positive electrode piece is distributed in a gradient, which improves the wettability of the positive electrode piece to the electrolyte and shortens the drying time of the positive electrode piece, thereby improving the performance of the lithium-ion secondary battery. Chemical properties and energy density.

In any embodiment, the coating amount CW of the positive electrode sheet is greater than or equal to 400 mg/1540.25cm ² . Therefore, the coating amount CW of the positive electrode sheet can effectively increase the loading capacity of the positive electrode active material and ensure the energy density of the lithium-ion secondary battery.

In any embodiment, the first coating includes primary particles with Dv50=2~4μm and Dv90=4~8μm, the second coating includes primary particles with Dv50=2~7μm and Dv90=7~10μm and Dv50=4~12μm, Dv90=secondary particles of 13~30μm.

Therefore, when the primary particles and the secondary particles are mixed and the Dv50 and Dv90 of the primary particles and the secondary particles are within the above range, the primary particles can be effectively prevented from being broken during the cold pressing process, thereby improving the The problem of difficult cold pressing of positive electrode plates. It should be noted that the average particle size of the primary particles in the secondary particles refers to the average particle size of all primary particles in the 10K times scanning electron microscope image.

In any embodiment, relative to the total mass of the cathode active material, the mixing ratio of the primary particles in the second coating is 10 to 50%; the Dv50 of the secondary particles is the same as the primary particle. The Dv50 ratio is 1.5~10. Therefore, when the primary particles are within the above blending ratio range, the tortuosity of the positive electrode piece can be effectively controlled and the distribution of porosity of the positive electrode piece can be improved. When the ratio of the Dv50 of the secondary particles to the Dv50 of the primary particles is within the above range, the crushing of the secondary particles during the cold pressing process can be effectively improved, thereby improving the problem of difficulty in cold pressing of the positive electrode sheet.

In any embodiment, the thickness of the first coating layer is 20-140 μm, and the thickness of the second coating layer is 20-140 μm. Therefore, when the thickness of the first coating layer and the second coating layer is within the above range, the positive electrode sheet has better processing performance, which facilitates subsequent winding into a battery core and assembly into a secondary battery.

In any embodiment, the packed density of the first coating is greater than the packed density of the second coating. Therefore, by making the compaction density of the first coating layer greater than the compaction density of the second coating layer, the porosity and tortuosity of the positive electrode piece can be controlled, thereby improving the positive electrode performance. While improving the wettability of the sheet to the electrolyte, it also ensures that the positive electrode sheet has a high loading capacity of positive active material.

In any embodiment, the density of the first coating is 2.5-2.8g/cc, and the density of the second coating is 2.1-2.5g/cc. Therefore, when the compacted density of the first coating layer is within the above range, the first coating layer can load more positive electrode active materials, and the positive electrode active material particles can be fully contacted. When the compacted density of the second coating is within the above range, the electrolyte can be fully immersed in it, which improves the wettability of the positive electrode plate to the electrolyte.

In any embodiment, the first coating has a porosity of 16-20%; the second coating has a porosity of 26-30%. Therefore, when the first coating layer and the second coating layer are within the above range, the wettability of the positive electrode piece to the electrolyte can be effectively improved and the drying time of the positive electrode piece can be shortened.

In any embodiment, the _positive active _{material is LiNixCoyMnzFeaAlbPcO2} (wherein, _{0≤x≤1 ,} 0≤y≤1, 0≤z≤1, _0≤a ≤1, 0≤b≤0.8, 0≤c≤4). Therefore, by selecting the positive active material with high gram capacity and good cycle performance, the lithium ion secondary battery can have higher energy density and better electrochemical performance.

A second aspect of the application provides a secondary battery, including the positive electrode plate of the first aspect of the application.

A third aspect of the present application provides a battery module including the secondary battery of the second aspect of the present application.

A fourth aspect of the application provides a battery pack, including the battery module of the third aspect of the application.

A fifth aspect of the present application provides an electrical device, including at least one selected from the secondary battery of the second aspect of the present application, the battery module of the third aspect of the present application, or the battery pack of the fourth aspect of the present application. kind.

Invention effect

This application provides positive electrode plates, secondary batteries, battery modules, battery packs, and electrical devices. By mixing the primary particles and secondary particles of the positive active material, different cold pressing pressures are used for the first coating and the second coating to adjust the tortuosity of the first coating and the second coating of the positive electrode piece, so that The tortuosity of the first coating is greater than the tortuosity of the second coating, and the tortuosity of the positive electrode piece is gradient distributed, thereby improving the positive electrode piece's effect on electrolysis while ensuring a higher coating amount of the positive electrode piece. The wettability of the liquid shortens the drying time of the positive electrode sheet, thereby improving the electrochemical performance and dynamic performance of the lithium-ion secondary battery.

Description of the drawings

Figure 1 is a schematic diagram of the tortuosity of the positive electrode piece of the present application.

Figure 2 is a scanning electron microscope image of Example 2 of the present application.

FIG. 3 is a schematic diagram of a secondary battery according to an embodiment of the present application.

FIG. 4 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 3 .

Figure 5 is a schematic diagram of a battery module according to an embodiment of the present application.

Figure 6 is a schematic diagram of a battery pack according to an embodiment of the present application.

FIG. 7 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG. 6 .

FIG. 8 is a schematic diagram of a power consumption device using a secondary battery as a power source according to an embodiment of the present application.

Explanation of reference symbols:

L _t The actual path length of the substance in the porous medium; L ₀ medium distance; 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 Top cover assembly.

Detailed ways

Hereinafter, embodiments specifically disclosing the positive electrode tab, secondary battery, battery module, battery pack, and electrical device of the present application will be described in detail with reference to the accompanying drawings as appropriate. However, unnecessary detailed explanations may be omitted. For example, detailed descriptions of well-known matters may be omitted, or descriptions of substantially the same structure may be repeated. This is to prevent the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description 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.

"Ranges" disclosed herein are defined in terms of lower and upper limits. A given range is defined by selecting a lower limit and an upper limit that define the boundaries of the particular range. Ranges defined in this manner may be inclusive or exclusive of the endpoints, and may be arbitrarily combined, that is, any lower limit may be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, understand that ranges of 60-110 and 80-120 are also expected. Furthermore, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2- 3, 2-4 and 2-5. In this application, unless stated otherwise, the numerical range "a-b" represents an abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed in this article, and "0-5" is just an abbreviation of these numerical combinations. In addition, when stating that a certain parameter is an integer ≥ 2, it is equivalent to disclosing that the parameter is an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

If there is no special description, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

If there is no special description, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

If there is no special instructions, all steps of the present application can be performed sequentially or randomly, and are preferably performed 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, mentioning that the method may also include step (c) means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c). , may also include steps (a), (c) and (b), may also include steps (c), (a) and (b), etc.

If there is no special explanation, the words "include" and "include" mentioned in this application represent open expressions, which may also be closed expressions. For example, "comprising" and "comprising" may mean that other components not listed may also be included or included, or only the listed components may be included or included.

In this application, the term "or" is inclusive unless otherwise specified. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, condition "A or B" is satisfied by any of the following conditions: 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).

Positive electrode piece

In one embodiment of the present application, the present application proposes a positive electrode sheet, including a current collector and a coating coated on at least one surface of the current collector. The coating includes a first coating and a second coating. layer, the first coating is coated on the surface of the current collector, the second coating is coated on the surface of the first coating, wherein the tortuosity of the first coating is 3.01 to 4.93, so The tortuosity of the second coating ranges from 2.11 to 2.87.

Referring to Figure 1, tortuosity is the ratio of the actual path length L _t of a substance in the porous medium to the medium distance (thickness) L ₀ . In the positive electrode sheet, L _t is the actual path length of lithium ions passing through the positive electrode active material particles, and L ₀ is the thickness of the coating layer of the positive electrode sheet.

Although the mechanism is not yet clear, the applicant unexpectedly discovered that if the tortuosity of the positive electrode piece is higher, it is difficult for the electrolyte to infiltrate the positive electrode piece, and the diffusion performance of lithium ions therein is also relatively poor. On the contrary, the lower the tortuosity of the positive electrode piece, the electrolyte can fully infiltrate the positive electrode piece, and the diffusion performance of lithium ions in it is better. However, the content of the positive active material in the positive electrode piece is lower, so the lower tortuosity is used. The positive electrode piece will reduce the energy density of lithium-ion secondary battery to a certain extent.

This application provides a first coating and a second coating on the surface of the current collector, and adopts corresponding control means (such as the mixing of primary particles and secondary particles, different cold compaction densities) to make the first coating tortuous. The degree of tortuosity is greater than that of the second coating, so that the positive electrode piece of the present application has both high active material loading and excellent dynamic properties: the first coating has a greater tortuosity, ensuring that the positive electrode piece has a higher The loading capacity of the positive electrode active material significantly increases the energy density of the lithium-ion secondary battery; at the same time, the second coating has a smaller tortuosity, allowing the electrolyte to fully infiltrate the positive electrode sheet, which is conducive to the diffusion of lithium ions in the positive electrode sheet. , ensuring that the lithium-ion secondary battery has better kinetic properties, thereby improving its cycle performance and other electrochemical properties.

In some embodiments, from the perspective of increasing the coating amount of the positive active material of the positive electrode sheet and thereby increasing the energy density of the lithium-ion secondary battery, the coating amount CW of the positive electrode sheet is greater than or equal to 400 mg/1540.25cm ² .

In some embodiments, the first coating includes primary particles with Dv50=2~4μm and Dv90=4~7μm, the second coating includes primary particles with Dv50=2~7μm and Dv90=7~10μm and Dv50=4~12μm, Dv90=secondary particles of 13~30μm.

Referring to Figure 2, Figure 2 is a scanning electron microscope (SEM) image of the cross section of the positive electrode piece of the present application. The primary particle size of the cathode active material in the first coating layer is significantly smaller than the secondary particle size of the cathode active material in the second coating layer. It should be noted that the average particle size of the primary particles in the secondary particles refers to the average particle size of all primary particles in the 10K times scanning electron microscope image. Therefore, when the primary particles and secondary particles are mixed, and the Dv50 and Dv90 of the primary particles and secondary particles are within the above range, it can not only prevent the secondary particles from being broken during the cold pressing process, but also solve the problem of the positive electrode plate It solves the problem of difficulty in cold pressing. At the same time, the tortuosity of the coating can be adjusted so that the tortuosity of the first coating is greater than the tortuosity of the second coating, so that the cathode plate has both high loading capacity of cathode active material and excellent wettability. .

In some embodiments, in the second coating, the blending ratio of the primary particles is 10 to 50% relative to the total mass of the cathode active material, and the ratio of the Dv50 of the secondary particles to the Dv50 of the primary particles is 1.5 ~10. Therefore, when the blending ratio of the primary particles is within the above range, the cold pressing performance of the positive electrode piece can be significantly improved. When the ratio of the Dv50 of the secondary particles to the Dv50 of the primary particles is within the above range, the crushing of the secondary particles during the cold pressing process can be effectively improved, thereby improving the problem of difficulty in cold pressing of the positive electrode sheet.

In some embodiments, the thickness of the first coating layer is 20-140 μm, and the thickness of the second coating layer is 20-140 μm. Therefore, when the thickness of the first coating layer and the second coating layer is within the above range, the positive electrode sheet has good processing performance, which facilitates subsequent winding into a battery core and assembly into a secondary battery.

In some embodiments, from the perspective of regulating the tortuosity and porosity of the first coating and the second coating, the compacted density of the first coating is greater than the compacted density of the second coating.

Therefore, by making the compaction density of the first coating layer greater than the compaction density of the second coating layer, the porosity and tortuosity of the positive electrode piece can be controlled, thereby improving the positive electrode performance. While ensuring the wettability of the sheet to the electrolyte, it also ensures that the positive electrode sheet has a high loading capacity of positive active materials.

In some embodiments, the density of the first coating is 2.5-2.8g/cc, and the density of the second coating is 2.1-2.5g/cc.

Therefore, when the compaction density of the first coating and the second coating is within the above range, the first coating has a high tortuosity, a high content of positive active material, and close contact between particles, and the second coating has a high tortuosity. The tortuosity is small, and the electrolyte can fully infiltrate the positive electrode piece, thereby significantly improving the dynamic performance of the positive electrode piece.

In some embodiments, the first coating has a porosity of 16-20%; the second coating has a porosity of 26-30%.

Therefore, when the porosity of the first coating layer and the porosity of the second coating layer are within the above range, the performance of the positive electrode plate has both a high active material coating amount and excellent electrolyte wettability. . Therefore, not only the energy density of the lithium-ion secondary battery is significantly improved, but also the diffusion ability of lithium ions in the positive electrode sheet is improved, thereby improving the dynamic performance of the lithium-ion secondary battery.

In some embodiments, the _positive active _material is _{LiNixCoyMnzFeaAlbPcO2} (wherein, 0≤x≤1 _{, 0≤y≤1} , 0≤z≤1 _{, 0≤a} ≤1, 0≤b≤0.8, 0≤c≤4).

As a result, the performance of the positive electrode sheet can better fit the characteristics of the electrochemical system, thereby meeting the performance requirements of lithium-ion secondary batteries.

In addition, the average volume distribution particle size Dv50 refers to the particle size corresponding to when the cumulative volume distribution percentage of the cathode active material reaches 50%. The average volume distribution particle size Dv90 refers to the particle size corresponding to when the cumulative volume distribution percentage of the positive active material reaches 90%. In this application, the volume average particle size Dv50 of the cathode active material can be measured using laser diffraction particle size analysis. For example, refer to the standard GB/T19077-2016 and use a laser particle size analyzer (such as MalvernMaster Size 3000) for measurement.

Liquid absorption rate test method:

Use a capillary tube to absorb 2mm electrolyte, place it vertically on the cold-pressed pole piece and let it rest for 200 seconds, and observe the residual height of the electrolyte in the capillary tube;

Liquid absorption rate = (2mm - residual height) * capillary cross-sectional area * 1.0g/cm ³ (electrolyte density) / time tortuosity test method:

In this application, the tortuosity of the positive electrode piece can be tested by the mercury intrusion method, for example, with reference to the standard GB/T21650.1-2008, using a pore size analyzer (for example, poremaster60GT).

The tortuosity is calculated according to the following formula:

τ＝(2.23-1.13Vρ _Hg )(0.92y) ^1+E

in, V: volume of mercury; ρ _Hg : density of mercury; S: total area of positive electrode plate;

△V _i : pore volume change; D _i : average pore diameter; E: pore index.

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

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

Typically, a secondary battery includes a positive electrode plate, a negative electrode plate, an electrolyte and a separator. During the charging and discharging process of the battery, active ions are inserted and detached back and forth between the positive and negative electrodes. The electrolyte plays a role in conducting ions between the positive and negative electrodes. The isolation film is placed between the positive electrode piece and the negative electrode piece. It mainly prevents the positive and negative electrodes from short-circuiting and allows ions to pass through.

[Positive pole piece]

The positive electrode piece is the positive electrode piece of the first aspect of the present application.

[Negative pole piece]

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

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

In the secondary battery of the present application, the negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, copper foil can 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 base material. The composite current collector can be formed by forming metal materials (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as polypropylene (PP), polyterephthalate It is formed on substrates such as ethylene glycol ester (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

In the secondary battery of the present application, the negative active material may be a negative active material for batteries known in the art. As an example, the negative 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, and the like. 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 battery negative electrode active materials can also be used. Only one type of these negative electrode active materials may be used alone, or two or more types may be used in combination.

In the secondary battery of the present application, the negative electrode film layer optionally includes a binder. The binder can be selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polysodium acrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), poly At least one of methacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

In the secondary battery of the present application, the negative electrode film layer optionally includes a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

In the secondary battery of the present application, the negative electrode film layer may optionally include other auxiliaries, such as thickeners (such as sodium carboxymethyl cellulose (CMC-Na)), etc.

In the secondary battery of the present application, the negative electrode sheet can be prepared in the following manner: the above-mentioned components used to prepare the negative electrode sheet, such as negative active materials, conductive agents, binders 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 current collector, and after drying, cold pressing and other processes, the negative electrode piece can be obtained.

[electrolyte]

The electrolyte plays a role in conducting ions between the positive and negative electrodes. There is no specific restriction on the type of electrolyte in this application, and it can be selected according to needs. For example, the electrolyte can be liquid, gel, or completely solid.

In the secondary battery of the present application, an electrolyte solution is used as the electrolyte. The electrolyte solution includes electrolyte salts and solvents.

In the secondary battery of the present application, the electrolyte salt can be selected from the group consisting of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonimide, lithium bistrifluoromethanesulfonimide, and trifluoromethanesulfonimide. At least one of lithium fluomethanesulfonate, lithium difluorophosphate, lithium difluoroborate, lithium dioxaloborate, lithium difluorodioxalate phosphate and lithium tetrafluoroxalate phosphate.

In the secondary battery of the present application, the solvent can be selected from the group consisting of ethylene carbonate, propylene carbonate, methylethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, and ethylenepropylene carbonate. Ester, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, butyl At least one of ethyl acid ester, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

In the secondary battery of the present application, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain properties of the battery, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[Isolation film]

In the secondary battery of the present application, the secondary battery further includes a separator. There is no particular restriction on the type of isolation membrane in this application. Any well-known porous structure isolation membrane with good chemical stability and mechanical stability can be used.

In the secondary battery of the present application, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation film can be a single-layer film or a multi-layer composite film, with no special restrictions. When the isolation film is a multi-layer composite film, the materials of each layer can be the same or different, and there is no particular limitation.

In the secondary battery of the present application, the positive electrode piece, the negative electrode piece and the separator film can be made into an electrode assembly through a winding process or a lamination process.

In the secondary battery of the present application, the secondary battery may include an outer packaging. The outer packaging can be used to package the above-mentioned electrode assembly and electrolyte.

In the secondary battery of the present application, 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 bag, such as a bag-type soft bag. The material of the soft bag may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.

This application has no particular limitation on the shape of the secondary battery, which can be cylindrical, square or any other shape. For example, FIG. 3 shows a square-structured secondary battery 5 as an example.

In some embodiments, referring to FIG. 4 , the outer package may include a housing 51 and a cover 53 . The housing 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The housing 51 has an opening communicating with the accommodation cavity, and the cover plate 53 can cover the opening to close the accommodation cavity. The positive electrode piece, the negative electrode piece and the isolation film can be formed into the electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is packaged in the containing cavity. The electrolyte soaks into 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.

In some embodiments, secondary batteries can be assembled into battery modules, and the number of secondary batteries contained in the battery module can be one or more. Those skilled in the art can select the specific number according to the application and capacity of the battery module.

Figure 5 is a battery module 4 as an example. Referring to FIG. 5 , in the battery module 4 , a plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4 . Of course, it can also be arranged in any other way. Furthermore, the plurality of secondary batteries 5 can be fixed by fasteners.

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

In some embodiments, the above-mentioned battery modules can also be assembled into a battery pack. The number of battery modules contained in the battery pack can be one or more. Those skilled in the art can select the specific number according to the application and capacity of the battery pack.

6 and 7 show the battery pack 1 as an example. Referring to FIGS. 6 and 7 , 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 2 and a lower box 3 . The upper box 2 can be covered with the lower box 3 and form a closed space for accommodating the battery module 4 . Multiple battery modules 4 can be arranged in the battery box in any manner.

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 by the present application. The secondary battery, battery module, or battery pack may be used as a power source for the electrical device, or may be used as an energy storage unit for the electrical device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, and electric golf carts). , electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited to these.

As the power-consuming device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

Figure 8 is an electrical device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the secondary battery for the electrical device, a battery pack or battery module can be used.

As another example, the device may be a mobile phone, a tablet, a laptop, etc. The device is usually required to be thin and light, and a secondary battery can be used as a power source.

Example

Hereinafter, examples of the present application will be described. The embodiments described below are illustrative and are only used to explain the present application and are not to be construed as limitations of the present application. If specific techniques or conditions are not specified in the examples, the techniques or conditions described in literature in the field or product instructions will be followed. If the manufacturer of the reagents or instruments used is not indicated, they are all conventional products that can be purchased commercially.

Example 1

Mix the cathode active material LiFePO ₄ of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After stirring and mixing evenly, it is coated on Al foil, dried and cold pressed to obtain the first coating layer.

Then, the cathode active material LiFePO ₄ of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After fully stirring and mixing, apply extrusion coating or transfer coating on the first coating, dry and cold-press to obtain a positive electrode piece.

In the first coating, the primary particles have Dv50=2.0μm, Dv90=4.0μm, compacted density: 2.6g/cc, coating layer thickness 22.5μm, coating weight CW=90mg/1540.25cm ² , cold pressure for 40T.

In the second coating, the primary particles have Dv50 = 5.5 μm and Dv90 = 8.2 μm, the secondary particles have Dv50 = 7.3 μm and Dv90 = 25 μm. The blending ratio of primary particles is 30%, and the compaction density is: 2.2g/ cc, the coating layer thickness is 91.5μm, the coating weight CW=310mg/1540.25cm ² , and the cold pressure is 10T.

Example 2

Mix the cathode active material LiFePO ₄ of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After stirring and mixing evenly, apply extrusion coating or transfer coating on the Al foil, dry it, and cold press to obtain the first coating.

Then, the cathode active material LiFePO ₄ of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After thoroughly stirring and mixing, the mixture is applied on the first coating layer, dried and cold-pressed to obtain a positive electrode piece.

Among them, in the first coating, the primary particles have Dv50=2.0μm, Dv90=4.0μm, the compacted density is: 2.6g/cc, the coating layer thickness is 22.5μm, and the coating weight CW=90mg/1540.25cm ² , The cold pressure is 40T.

In the second coating, the Dv50=5.5μm and Dv90=8.2μm of the primary particles, the Dv50=7.3μm and Dv90=25μm of the secondary particles, the blending ratio of the primary particles is 40%, and the compacted density is: 2.2g/cc , the thickness of the coating layer is 91.5μm, the coating weight CW=310mg/1540.25cm ² , and the cold pressure is 20T.

Example 3

Mix the cathode active material LiFePO ₄ of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After stirring and mixing evenly, it is coated on Al foil, dried and cold pressed to obtain the first coating layer.

Then, the cathode active material LiFePO ₄ of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After thoroughly stirring and mixing, the mixture is applied on the first coating layer, dried and cold-pressed to obtain a positive electrode piece.

Among them, in the first coating, the primary particles have Dv50=2.0μm, Dv90=4.0μm, the compacted density is: 2.6g/cc, the coating layer thickness is 22.5μm, and the coating weight CW=90mg/1540.25cm ² , The cold pressure is 40T.

In the second coating, the primary particles have Dv50 = 5.5 μm and Dv90 = 8.2 μm, the secondary particles have Dv50 = 7.3 μm and Dv90 = 25 μm, the primary particle blending ratio is 20%, and the compacted density is: 2.2g/cc , the thickness of the coating layer is 91.5μm, the coating weight CW=310mg/1540.25cm ² , and the cold pressure is 30T.

Example 4

Mix the cathode active material LiFePO ₄ of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After stirring and mixing evenly, apply extrusion coating or transfer coating on the Al foil, dry it, and cold press to obtain the first coating.

Then, the cathode active material LiFePO ₄ of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After fully stirring and mixing, apply extrusion coating or transfer coating on the first coating, dry and cold-press to obtain a positive electrode piece.

Among them, in the first coating, the primary particles have Dv50=2.0μm, Dv90=4.0μm, the compacted density is: 2.6g/cc, the coating layer thickness is 70μm, the coating weight CW=279mg/1540.25cm ² , cold The pressure is 30T.

In the second coating, the primary particles have Dv50 = 5.5 μm and Dv90 = 8.2 μm, the secondary particles have Dv50 = 7.3 μm and Dv90 = 25 μm, the primary particle blending ratio is 30%, and the compacted density is: 2.2g/cc , the thickness of the coating layer is 91.5μm, the coating weight CW=121mg/1540.25cm ² , and the cold pressure is 15T.

Example 5

Mix the cathode active material LiFePO ₄ of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After stirring and mixing evenly, apply extrusion coating or transfer coating on the Al foil, dry it, and cold press to obtain the first coating.

Then, the cathode active material LiFePO ₄ of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After thoroughly stirring and mixing, the mixture is applied on the first coating layer, dried and cold-pressed to obtain a positive electrode piece.

Among them, in the first coating, the primary particles have Dv50=2.0μm, Dv90=4.0μm, the compacted density is: 2.7g/cc, the coating layer thickness is 62μm, the coating weight CW=258mg/1540.25cm ² , cold The pressure is 55T.

In the second coating, the primary particles have Dv50 = 5.5 μm and Dv90 = 8.2 μm, the secondary particles have Dv50 = 7.3 μm and Dv90 = 25 μm, the primary particle blending ratio is 30%, and the compacted density is: 2.2g/cc , the thickness of the coating layer is 42μm, the coating weight CW=142mg/1540.25cm ² , and the cold pressure is 50T.

Comparative example 1

Mix the cathode active material LiFePO ₄ of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After stirring and mixing evenly, it is coated on Al foil, dried and cold pressed to obtain the first coating layer.

Then, the cathode active material LiFePO ₄ of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After fully stirring and mixing, apply extrusion coating or transfer coating on the first coating, dry and cold-press to obtain a positive electrode piece.

Among them, in the first coating, the primary particles have Dv50=2.0μm, Dv90=4.0μm, the compacted density is: 2.6g/cc, the coating layer thickness is 22.5μm, and the coating weight CW=90mg/1540.25cm ² , The cold pressure is 70T.

In the second coating, the primary particles have Dv50 = 5.5 μm and Dv90 = 8.2 μm, the secondary particles have Dv50 = 7.3 μm and Dv90 = 25 μm, the primary particle blending ratio is 5%, and the compacted density is: 2.2g/cc , the thickness of the coating layer is 91.5μm, the coating weight CW=310mg/1540.25cm ² , and the cold pressure is 21T.

Comparative example 2

Mix the cathode active material LiFePO ₄ of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After stirring and mixing evenly, apply extrusion coating or transfer coating on the Al foil, dry it, and cold press to obtain the first coating.

Then, the cathode active material LiFePO ₄ of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97:2:1 in an N-methylpyrrolidone (NMP) solvent system. After fully stirring and mixing, apply extrusion coating or transfer coating on the first coating, dry and cold-press to obtain a positive electrode piece.

Among them, in the first coating, the primary particles have Dv50=2.0μm, Dv90=4.0μm, the compacted density is: 2.6g/cc, the coating layer thickness is 22.5μm, and the coating weight CW=90mg/1540.25cm ² , The cold pressure is 70T.

In the second coating, the primary particles have Dv50 = 5.5 μm and Dv90 = 8.2 μm, the secondary particles have Dv50 = 7.3 μm and Dv90 = 25 μm, the blending ratio of primary particles is 60%, and the compacted density is: 2.2g/cc , the thickness of the coating layer is 91.5μm, the coating weight CW=310mg/1540.25cm ² , and the cold pressure is 60T.

The relevant parameters of the positive electrode plates of the above-mentioned Examples 1 to 5, Comparative Example 1, and Comparative Example 2 are as shown in Table 1.

Table 1: Parameters of positive electrode plates of Examples 1 to 5 and Comparative Examples 1 and 2

The liquid absorption rate and tortuosity of the positive electrode plates of the above-mentioned Examples 1 to 5, Comparative Example 1, and Comparative Example 2 were tested respectively. The test results are shown in Table 2 below.

Table 2: Test results of liquid absorption rate and tortuosity of Examples 1 to 5 and Comparative Examples 1 and 2

In addition, the positive electrode sheets of the above-mentioned Examples 1 to 5, Comparative Example 1, and Comparative Example 2 were prepared into secondary batteries as follows, and performance tests were performed. The test results are shown in Table 3 below.

(1) Preparation of secondary batteries

As the positive electrode piece, the positive electrode piece in the above-mentioned embodiments and comparative examples was used.

The artificial graphite, conductive agent acetylene black, binder styrene-butadiene rubber (SBR) and thickener carbon methyl cellulose sodium (CMC) as negative electrode active materials are removed in a weight ratio of 90:5:2:2:1. After being thoroughly stirred and evenly mixed in the ionic water solvent system, it is coated on copper foil, dried, and cold-pressed to obtain a negative electrode piece.

A porous polymeric film made of polyethylene (PE) is used as the isolation membrane. The positive electrode sheet, the isolation film and the negative electrode sheet are overlapped in order, so that the isolation film acts as an isolation between the positive and negative electrodes, and the bare battery core is obtained by winding. The bare battery core was placed in the outer package, electrolyte was injected and packaged to obtain a secondary battery using the positive electrode sheet in each example and comparative example.

(2) Secondary battery AC impedance test

Use an electrochemical workstation (model: VMP3, manufacturer: Bio-logic, France), set the frequency range of normal full battery testing to 500kHz to 30mHz, and set the amplitude to 5mV to test the electrochemical impedance of the secondary battery prepared above.

(3) Secondary battery discharge rate test

Each secondary battery prepared above was left to stand for 30 minutes in a constant temperature environment of 25°C. Discharge to 2.5V with a constant current of 0.33C, discharge to 2.0V with a constant current of 0.33C, then let it stand for 1 hour, charge with a constant current of 0.33C to 3.65V, charge with a constant voltage, the cut-off current is 0.05C, let it stand for 30 minutes, follow the instructions 0.33C constant current discharge to 2.5V, then 0.33C constant current discharge to 2.0V, record the discharge capacity C ₀ , let it stand for 1 hour, 0.33C constant current charge to 3.65V, constant voltage charge, cut-off current 0.05C, let it stand 30 minutes.

Discharge to 2.5V according to 1C, discharge to 2.0V at 1C, record the discharge capacity C ₁ , then let it stand for 1 hour, charge with a constant current of 0.33C to 3.65V, charge with a constant voltage, cut off current 0.05C, and let it stand for 30 minutes. 1C discharge capacity retention rate (C ₁ /C ₀ )*100%.

Discharge to 2.5V at 2C and 2.0V at 2C, record the discharge capacity C ₂ , and then let it sit for 30 minutes. 2C discharge capacity retention rate (C ₂ /C ₀ )*100%.

(4) High and low temperature performance test of secondary battery

25°C performance test: Place each secondary battery prepared above in a high and low temperature chamber at 25°C (model: SM-012PF, manufacturer: Guangdong Sanmu Technology Co., Ltd.), discharge to 2.5V at 1C, and discharge at 1C to 2.0V, then let it sit for 5 minutes, charge to 3.65V according to 1C constant current, constant voltage charging, the cut-off current is 0.05C, record the discharge capacity C ₀ , then let it stand for 30 minutes, discharge to 2.5V according to 1C, discharge at 1C to 2.0V, then let it sit for 30 minutes, charge to 3.65V according to 1C constant current, constant voltage charging, with a cut-off current of 0.05C, and then let it stand for 5 minutes.

The discharge capacity retention rate at 25°C is (C ₀ /C ₀ )*100%.

-25°C performance test: adjust the temperature of the high and low temperature oven to -25°C, let each secondary battery prepared above stand for 120 minutes, discharge to 2.5V at 1C, discharge to 2.0V at 1C, record the discharge capacity C ₁ , Then let it sit for 5 minutes.

-25℃ discharge capacity retention rate (C ₁ /C ₀ )*100%.

60℃ performance test: Adjust the high and low temperature oven to 25℃, let each secondary battery prepared above stand for 120 minutes, charge it to 3.65V according to 1C constant current, constant voltage charging, the cut-off current is 0.05C, and then let it stand for 5 minute. Adjust the temperature of the high and low temperature oven to 60°C. Let each secondary battery prepared above stand for 120 minutes. Discharge to 2.5V at 1C and 2.0V at 1C. Record the discharge capacity C ₂ and then let it stand for 5 minutes.

The discharge capacity retention rate at 60°C is (C ₂ /C ₀ )*100%.

(5) Secondary battery DC impedance test

Each secondary battery prepared above was placed in a constant temperature environment of 25°C for 30 minutes, discharged at a constant current of 0.33C to 2.5V, left for 30 minutes, charged at a constant current of 0.33C to 3.65V, and then charged at a constant voltage of 0.33C. Charge, cut off current 0.05C, and then let it sit for 30 minutes. Discharge at 0.33C to 2.5V, record the discharge capacity C ₀ , let it sit for 30 minutes, charge at a constant current of 0.33C to 3.65V, then charge at a constant voltage, cut off the current at 0.05C, and then let it stand for 5 minutes. Discharge according to 0.33C, cut-off current 0.5C ₀ (this step adjusts the 50% SOC of each secondary battery prepared above), let it stand for 1 hour, record the voltage U ₁ at this time, discharge according to the current I = 5C for 30 seconds, record this time voltage U ₂ and then let sit for 5 minutes. DCR resistance = (U ₁ -U ₂ )/I.

Table 2: Performance test results of Examples 1 to 5 and Comparative Examples 1 and 2

According to the above results, it can be seen that in the positive electrode sheets in Examples 1 to 5, the first coating uses small particles of positive active material and high cold compaction density, so that the tortuosity of the first coating is higher than that of the second coating. This makes the porosity of the positive electrode plate have a gradient distribution, thus achieving good results in the rate performance, high and low temperature performance, and electrolyte wettability of the secondary battery. Moreover, the dynamic performance of the lithium-ion secondary battery is also improved.

In contrast, the primary particle blending ratio of the second coating in the positive electrode sheet obtained in Comparative Example 1 is only 5%, and the primary particle blending ratio of the second coating in the positive electrode sheet obtained in Comparative Example 2 is 5%. As high as 60%, the tortuosity of the positive electrode piece is relatively large. Therefore, the diffusion resistance R _f of Comparative Example 1 and Comparative Example 2 is large, and no effective improvement is achieved in terms of dynamic performance.

In addition, comparing Example 1 with Example 2 and Example 3, it can be seen that although Example 2 and Example 3 have a smaller diffusion resistance R _f , their dynamic performance, high and low temperature performance, and rate performance are all improved. However, continuing to increase the blending ratio of primary particles in the second coating has no obvious improvement effect on the kinetic performance of lithium-ion secondary batteries.

It should be noted that the present application is not limited to the above-described embodiment. The above-mentioned embodiments are only examples. Within the scope of the technical solution of the present application, embodiments that have substantially the same structure as the technical idea and exert the same functions and effects are included in the technical scope of the present application. In addition, within the scope that does not deviate from the gist of the present application, various modifications to the embodiments that can be thought of by those skilled in the art, and other forms constructed by combining some of the constituent elements in the embodiments are also included in the scope of the present application. .

Claims (13)

1. A positive electrode sheet, characterized in that it includes a current collector and a coating coated on at least one surface of the current collector, the coating including a first coating and a second coating, the first The coating is coated on the surface of the current collector, the second coating is coated on the surface of the first coating, wherein the tortuosity of the first coating is 3.01 to 4.93, and the tortuosity of the second coating is The degree is 2.11~2.87. 2. The positive electrode piece according to claim 1, characterized in that the total coating amount CW of the positive electrode piece is greater than or equal to 400 mg/ ^1540.25cm2 . 3. The positive electrode plate according to any one of claims 1 or 2, characterized in that the first coating includes primary particles with Dv50=2~4μm and Dv90=4~8μm, and the second coating The layer includes primary particles with Dv50=2 to 7 μm and Dv90=7 to 10 μm, and secondary particles with Dv50=4 to 12 μm and Dv90=13 to 30 μm. 4. The positive electrode sheet according to claim 3, characterized in that, in the second coating, the blending ratio of the primary particles is 10 to 50% relative to the total mass of the positive active material, and the secondary particles The ratio of secondary particle Dv50 to primary particle Dv50 is 1.5 to 10. 5. The positive electrode sheet according to claim 1, 2 or 4, wherein the thickness of the first coating layer is 20-140 μm, and the thickness of the second coating layer is 20-140 μm. 6. The positive electrode sheet according to claim 1, 2, or 4, wherein the compacted density of the first coating layer is greater than the compacted density of the second coating layer. 7. The positive electrode sheet according to claim 6, wherein the compacted density of the first coating is 2.5-2.8g/cc, and the compacted density of the second coating is 2.1-2.5g. /cc. 8. The positive electrode piece according to claim 1 or 2 or 4 or 7, characterized in that the porosity of the first coating layer is 16-20%; the porosity of the second coating layer is 26-20%. 30%. 9. The cathode plate according to claim 1, wherein the cathode active material is _LiNix Co _y Mn _z Fe _a Al _b P _c O ₂ , where 0≤x≤1, 0≤y≤ 1, 0≤z≤1, 0≤a≤1, 0≤b≤0.8, 0≤c≤4. 10. A secondary battery, characterized by comprising the positive electrode tab according to any one of claims 1 to 9. 11. A battery module, comprising the secondary battery according to claim 10. 12. A battery pack, comprising the battery module according to claim 11. 13. An electrical device, characterized by comprising at least one selected from the group consisting of the secondary battery of claim 10, the battery module of claim 11, or the battery pack of claim 12.