Positive pole piece and preparation method thereof, battery and power utilization device

Abstract

The invention discloses a positive pole piece, a preparation method of the positive pole piece, a battery and a power utilization device. The positive pole piece comprises a positive current collector and a positive active layer arranged on at least one surface of the positive current collector, the positive active layer comprises a first active layer and a second active layer, and the second active layer is located between the positive current collector and the first active layer; the first active layer comprises a first positive electrode active material, the second active layer comprises a second positive electrode active material, the working voltage interval of the first positive electrode active material is larger than that of the second positive electrode active material, and the working voltage interval of the second positive electrode active material is smaller than or equal to 3.0 V. The first positive electrode active material begins to attenuate after a relatively large voltage platform, and a working voltage continuing effect is achieved by virtue of a relatively low working voltage interval of the second positive electrode active material, so that a high-power discharge voltage interval range is expanded, and meanwhile, the second positive electrode active material still has reaction activity in a relatively low charge state; therefore, the material has good discharge capability.

Description

Technical Field

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

[0002] In recent years, new energy vehicles have developed vigorously. The battery drive system is the main factor affecting the performance and cost of new energy vehicles. Secondary batteries have become the preferred solution for power batteries in the current new energy vehicle battery drive system due to their high energy density, low memory effect and high operating voltage.

[0003] A secondary battery cell generally includes a positive electrode sheet, a separator and a negative electrode sheet. The positive electrode active material in the positive electrode sheet generally decays rapidly after a certain discharge voltage platform, thereby affecting the discharge capacity of the battery. Summary of the invention

[0004] In view of the above problems, the present application provides a positive electrode plate and a preparation method thereof, a battery and an electrical device, aiming to solve the technical problem of how to make the positive electrode plate have good discharge capacity.

[0005] In a first aspect, an embodiment of the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active layer comprises a first active layer and a second active layer, and the second active layer is located between the positive electrode current collector and the first active layer;

[0006] The first active layer includes a first positive electrode active material, the second active layer includes a second positive electrode active material, the operating voltage range of the first positive electrode active material is greater than the operating voltage range of the second positive electrode active material, and the operating voltage range of the second positive electrode active material is less than or equal to 3.0V.

[0007] By matching the positive electrode active layer with positive electrode active materials of different working voltage ranges, specifically, the working voltage range of the first positive electrode active material in the first active layer away from the positive electrode current collector is greater than the working voltage range of the second positive electrode active material in the second active layer close to the positive electrode current collector. When the first positive electrode active material begins to decay after a larger voltage platform, the second positive electrode active material has a lower working voltage range of less than or equal to 3.0V, thereby achieving the effect of working voltage continuity, thereby expanding the high-power discharge voltage range of the positive electrode plate of the present application. At the same time, since the second positive electrode active material on the side close to the positive electrode current collector is still reactive under a lower charging state, such a positive electrode plate enables the battery to have a good discharge capacity.

[0008] In some embodiments, the operating voltage interval of the first positive electrode active material is greater than 3.0V, and optionally, the operating voltage interval of the first positive electrode active material is 3.1V to 4.0V; and / or,

[0009] The operating voltage range of the second positive electrode active material is 1.5V to 3.0V.

[0010] By selecting the working voltage range of the first positive electrode active material and the second positive electrode active material, the voltage connection function can be better played, thereby improving the discharge capacity of the battery.

[0011] In some embodiments, the first cathode active material includes at least one of lithium iron phosphate and lithium manganese iron phosphate; and / or,

[0012] The second positive electrode active material includes at least one of lithium iron phosphate and lithium iron manganese phosphate, and also includes at least one of lithium ferrous silicate and lithium vanadate. Optionally, the ratio of the total weight of lithium iron phosphate and lithium manganese phosphate in the second positive electrode active material to the total weight of lithium ferrous silicate and lithium vanadate is 1:1 to 9:1.

[0013] By selecting and matching the types of the first positive electrode active material and the second positive electrode active material, the operating voltage range of the first positive electrode active material and the operating voltage range of the second positive electrode active material can meet the range required by the present application.

[0014] In some embodiments, the particle size Dv50 of the first positive electrode active material is greater than the particle size Dv50 of the second positive electrode active material; optionally,

[0015] The particle size Dv50 of the first positive electrode active material is 300 nm to 2 μm, and the particle size Dv50 of the second positive electrode active material is 10 nm to 300 nm.

[0016] The positive electrode active material with smaller particle size not only has a high charge and discharge power density, but also has a lower voltage platform. Therefore, the combination of the first positive electrode active material and the second positive electrode active material with the above particle size can not only perform voltage connection well, but also is not easy to produce excessive voltage polarization, thereby further improving the power density.

[0017] In some embodiments, a weight ratio of the first positive electrode active material to the second positive electrode active material is 7:3 to 9:1.

[0018] At this weight ratio, the overall operating voltage platform of the positive electrode is higher, which helps to improve the battery energy density.

[0019] In some embodiments, the coating weight of the first active layer is 160 mg / 1540.25 mm 2~500mg / 1540.25mm 2 The coating weight of the second active layer is 16 mg / 1540.25 mm 2 ~100mg / 1540.25mm 2 and / or,

[0020] The total compaction density of the first active layer and the second active layer is 2.3-3.0 g / cm 3 .

[0021] Within this coating weight range and at a higher compaction density, the energy density of the battery can be significantly improved, and the battery has great application potential.

[0022] In some embodiments, the porosity of the first active layer is greater than the porosity of the second active layer; and / or,

[0023] The tortuosity of the first active layer is smaller than the tortuosity of the second active layer.

[0024] By matching the porosity and tortuosity of the first active layer and the second active layer, the positive electrode plate produces a gradient tortuosity and porosity distribution, thereby reducing the polarization of liquid phase concentration differences, which is conducive to high-power charging and discharging.

[0025] In some embodiments, a conductive coating is further disposed between the second active layer and the positive electrode current collector, and based on the total weight of the conductive coating being 100%, the conductive coating contains 50-95% of a conductive agent.

[0026] The conductive coating with a high content of conductive agent can improve the conductivity of the positive electrode active layer and the positive electrode current collector.

[0027] In a second aspect, the present application provides a method for preparing the above-mentioned positive electrode sheet, comprising:

[0028] Preparing the second active layer on at least one surface of the positive electrode current collector;

[0029] The first active layer is prepared on a surface of the second active layer away from the positive electrode current collector.

[0030] The positive electrode plate is obtained by sequentially preparing a unique second active layer and a first active layer on the positive electrode current collector. Not only is the process simple, but the high-power discharge voltage range can be expanded through the relationship between the working voltage range of the second positive electrode active material in the second active layer and the first positive electrode active material in the first active layer. The positive electrode plate still has reaction activity under a lower charging state, so the prepared positive electrode plate can make the battery have a good discharge capacity.

[0031] In some embodiments, the step of preparing the second active layer on at least one surface of the positive electrode current collector comprises:

[0032] A conductive coating is first prepared on at least one surface of the positive electrode current collector, and then the second active layer is prepared on the surface of the conductive coating away from the positive electrode current collector: wherein, based on the total weight of the conductive coating being 100%, the conductive coating contains 50-95% of a conductive agent.

[0033] The conductive coating with a high content of conductive agent can improve the conductivity of the positive electrode active layer and the positive electrode current collector.

[0034] In a third aspect, an embodiment of the present application provides a battery, comprising the positive electrode sheet provided in the first aspect of the embodiment of the present application and / or the positive electrode sheet prepared by the preparation method provided in the second aspect of the embodiment of the present application.

[0035] Based on the characteristics of the positive electrode plate of the embodiment of the present application, the battery of the embodiment of the present application has a good discharge capacity.

[0036] In a fourth aspect, an embodiment of the present application provides an electrical device, comprising the battery provided in the third aspect of the present application.

[0037] By adopting the battery provided in the second aspect of the embodiment of the present application, such an electrical device has good charging and discharging performance and can work better.

[0038] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present application. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0040] Figure 1 A schematic diagram of the structure of the positive electrode sheet of an embodiment of the present application;

[0041] Figure 2 Another structural schematic diagram of the positive electrode sheet of an embodiment of the present application;

[0042] Figure 3 This is a rate discharge voltage curve of lithium iron phosphate in the positive electrode sheet of the embodiment of the present application;

[0043] Figure 4 A schematic diagram of a battery cell structure of an implementation mode of a secondary battery of an embodiment of the present application;

[0044] Figure 5 for Figure 4 A schematic diagram of an exploded view of a battery cell of a secondary battery shown;

[0045] Figure 6 This is a schematic structural diagram of an implementation scheme of a battery module according to an embodiment of the present application;

[0046] Figure 7 This is a schematic structural diagram of an implementation scheme of a battery pack according to an embodiment of the present application;

[0047] Figure 8 for Figure 7 A schematic diagram of the exploded structure of the battery pack shown;

[0048] Fig. 9 It is a schematic diagram of an implementation of an electrical device including the secondary battery of an embodiment of the present application as a power source.

[0049] Description of reference numerals:

[0050] 11-positive electrode current collector; 12-positive electrode active layer; 121-first active layer; 122-second active layer; 123-conductive coating;

[0051] 20 - battery cell; 21 - shell; 22 - top cover assembly; 23 - electrode assembly; 30 - battery module; 40 - battery pack; 41 - upper box; 42 - lower box. DETAILED DESCRIPTION

[0052] The following embodiments of the technical solution of the present application are described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0053] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

[0054] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0055] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0056] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0057] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two), and similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces). "At least one" refers to more than one (including one, two, three, etc.).

[0058] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, which are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the embodiments of the present application.

[0059] In the description of the embodiments of the present application, unless otherwise clearly specified and limited, technical terms such as "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0060] With the increasing depletion of traditional energy resources, the development of new energy storage devices has received more and more attention. Among them, secondary batteries have attracted much attention due to their high energy density, high theoretical capacity, good cycle stability and environmental protection characteristics. Secondary batteries can not only be applied to energy storage power systems such as hydropower, thermal power, wind power and solar power stations, but also be widely used in many fields such as electric bicycles, electric motorcycles, electric vehicles and other electric transportation tools. With the continuous expansion of the application fields of secondary batteries as power batteries, the market demand is also constantly increasing, and at the same time, the performance requirements for batteries such as cycle performance are getting higher and higher.

[0061] The power performance of secondary batteries is closely related to the positive polarization during the battery discharge process. A secondary battery monomer generally includes a positive electrode plate, a separator, and a negative electrode plate. If the thickness of the electrode plate is too large, it is easy to cause an increase in the degree of polarization. In order to reduce the positive electrode plate polarization, conventional battery designs generally design the positive electrode plate to have a relatively thin surface coating weight (C.W generally ranges from 0 to 400 g / 1540.25 mm 2 ), on the one hand, this design can reduce the diffusion path of active ions such as lithium ions and reduce the liquid-phase diffusion distance, on the other hand, it can shorten the electron transmission path along the thickness direction of the electrode plate and reduce the electron transmission impedance, ultimately achieving the effect of reducing the positive electrode plate polarization. However, due to the reduction of the surface coating weight, the volume ratio of non-active substances (such as current collectors, separators) in the battery increases significantly, reducing the space utilization rate of the positive active material, and then resulting in a decrease in the volume energy density, which is not conducive to the performance of the driving range of new energy vehicles. Moreover, the decrease in the volume energy density will also lead to an increase in the battery cost and an increase in the purchase cost of new energy vehicles.

[0062] With the continuous increase in the penetration rate of new energy vehicles, hybrid and range-extended vehicles have gradually entered the market, and gradually put forward higher requirements for the power performance of secondary batteries. Lithium-ion batteries, as one of the secondary batteries, have the characteristics of high energy density, long service life, energy conservation and environmental protection. In secondary batteries, the positive active material in the positive electrode plate generally decays rapidly after a certain discharge voltage platform. If there is no working voltage connection, it is easy to affect the discharge ability of the battery.

[0063] Based on the above considerations, in order to improve the discharge ability of secondary batteries, a positive electrode active layer including a first active layer and a second active layer is provided on at least one surface of the positive electrode current collector. Based on the matching of the working voltage ranges of the positive active materials in the first active layer and the second active layer, the discharge ability of the secondary battery can be improved. Thus, the following technical solutions are proposed.

[0064] Positive Electrode Plate and Its Preparation Method

[0065] In the first aspect, the embodiments of the present application provide a positive electrode plate. As some embodiments of the present application are summarized, such as Figure 1-Figure 2As shown, the positive electrode plate includes: (1) a positive electrode collector 11, and (2) a positive electrode active layer 12. The positive electrode active layer 12 is disposed on at least one surface of the positive electrode collector 11, that is, the positive electrode active layer 12 is disposed on one surface of the positive electrode collector 11, or the positive electrode active layer 12 is disposed on two opposite surfaces of the positive electrode collector 11.

[0066] The positive electrode current collector 11 is a structure or part that collects current in the battery. The electrode active layer 12 refers to a film layer containing positive electrode active materials in the battery. When the battery is charged and discharged, the positive electrode active materials in the positive electrode active layer 12 can achieve the embedding and extraction of active metal ions.

[0067] The positive electrode active layer 12 includes a first active layer 121 and a second active layer 122. The second active layer 122 is located between the positive electrode current collector 11 and the first active layer 121. The first active layer 121 contains a first positive electrode active material, and the second active layer 121 contains a second positive electrode active material. There is a certain relationship between the working voltage ranges of the first positive electrode active material and the second positive electrode active material, that is, the working voltage range of the first positive electrode active material is greater than the working voltage range of the second positive electrode active material, and the working voltage range of the second positive electrode active material is less than or equal to 3.0V.

[0068] The working voltage refers to the reversible reaction potential range of the electrode material in the battery. For the working voltage range of the positive electrode active material in the positive electrode plate of the embodiment of the present application, it can specifically refer to the potential change range of the positive electrode active material during the charging and discharging process, and also reflects the maximum voltage platform that the battery can provide. Generally, the positive electrode material plates can be assembled into button-type half-cells. By testing the discharge energy E and capacity C of the button-type half-cell during charging and discharging, the charging and discharging voltage platform range of the positive electrode active material can be obtained using the formula V=E / C.

[0069] In the embodiment of the present application, the second positive electrode active material has a lower working voltage range of less than or equal to 3.0V. Therefore, when the first positive electrode active material begins to decay after a larger voltage platform, based on the existence of the second positive electrode active material, the lower working voltage range of the second positive electrode active material can play the role of working voltage continuity, thereby expanding the range of high-power discharge voltage range. At the same time, the second positive electrode active material is located in the second active layer 122, that is, close to the side of the positive electrode collector 11, so that the second positive electrode active material is still reactive in a lower charging state. Therefore, the positive electrode plate of the embodiment of the present application can enable the battery to have a good discharge capacity.

[0070] In some embodiments, the first positive electrode active material in the first active layer 121 first contacts the electrolyte and generally performs a discharge reaction first. Its working voltage interval is greater than 3.0V, so a discharge reaction with a larger discharge voltage can be performed first. Then the discharge voltage of the first positive electrode active material will decay rapidly after it is greater than the 3.0V platform. In this way, the addition of the second positive electrode active material with a lower working voltage interval supports the sudden drop in the voltage platform after 3.0V of the first positive electrode active material, which plays a role in the continuity of the working voltage, thereby extending the high-power discharge voltage interval range of the entire positive electrode active layer 12 and improving its discharge capacity. Optionally, the working voltage interval of the first positive electrode active material is 3.1V to 4.0V, and the first positive electrode active material can perform charge and discharge work within the working voltage interval of 3.1V to 4.0V.

[0071] In some embodiments, the operating voltage range of the second positive active material in the second active layer 121 is 1.5 V to 3.0 V. The second positive active material with a lower operating voltage range is arranged on the side of the positive active layer 12 close to the positive current collector 11, which can achieve the effect of operating voltage continuity.

[0072] In some embodiments, the working voltage range of the first positive electrode active material is 3.1V~4.0V, and the working voltage range of the second positive electrode active material is 1.5V~3.0V; by matching and selecting the working voltage ranges of the first positive electrode active material and the second positive electrode active material, the voltage connection function can be better played, thereby improving the discharge capacity of the battery.

[0073] In some embodiments, the first positive electrode active material includes at least one of lithium iron phosphate and lithium iron manganese phosphate. Both lithium iron phosphate and lithium iron manganese phosphate are positive electrode active materials with higher voltages. They are arranged on the side of the positive electrode active layer 12 away from the positive electrode current collector 11 to achieve high rate discharge.

[0074] In some embodiments, the second positive electrode active material includes at least one of lithium iron phosphate and lithium iron manganese phosphate, and at the same time includes at least one of lithium ferrous silicate and lithium vanadate. By matching at least one positive electrode active material of lithium iron phosphate and lithium iron manganese phosphate with at least one positive electrode active material of lithium ferrous silicate and lithium vanadate, a second positive electrode active material with a lower voltage can be realized, which is arranged on the side of the positive electrode active layer 12 close to the positive electrode current collector 11. When the voltage platform of the first positive electrode active material of high-rate discharge suddenly drops, the second positive electrode active material can realize the function of voltage continuity. At the same time, the second positive electrode active material still has the activity of reaction at a low state of charge (SOC). At this time, the second positive electrode active material has strong power and can still react, so that the battery has a good discharge capacity.

[0075] Take lithium iron phosphate as the first positive electrode active material as an example. Figure 3This is the high-rate discharge attenuation curve of lithium iron phosphate. The discharge voltage of lithium iron phosphate will decay rapidly after reaching the 3.3V platform. The reason for the decay is that as the discharge progresses, lithium ions are removed from the negative electrode and embedded in the lithium iron phosphate lattice, causing the lithium iron phosphate voltage to decrease. Since lithium iron phosphate is a phase change material, when fully embedded, the voltage drops suddenly. However, in addition to lithium iron phosphate, the second positive electrode active material also includes at least one of lithium ferrous silicate and lithium vanadate. For example, taking lithium ferrous silicate as an example, the presence of lithium ferrous silicate can continuously discharge to 2.0V or even 1.5V, realizing the function of voltage continuity.

[0076] In some embodiments, the ratio of the total weight of lithium iron phosphate and lithium iron manganese phosphate to the total weight of lithium ferrous silicate and lithium vanadate in the second positive electrode active material is 1: 1 to 9: 1. For example, taking the second positive electrode active material including lithium iron phosphate and lithium ferrous silicate as an example, the weight ratio between the two can be 1: 1, 2: 1, 3: 1, 4: 1, 6: 1, 8: 1, 9: 1, etc.

[0077] The slurry prepared by the second positive electrode active material within this weight ratio range has good stability and can be well coated, thereby improving the processing performance of the positive electrode sheet, and can be well compatible with the current mass production line drawing. At the same time, by selecting and matching the types of the first positive electrode active material and the second positive electrode active material, the operating voltage range of the first positive electrode active material and the operating voltage range of the second positive electrode active material can meet the range required by this application.

[0078] In some embodiments, the particle size Dv50 of the first positive active material is greater than the particle size Dv50 of the second positive active material.

[0079] The size of the granular material is called particle size, the percentage of particles in different particle size ranges in the total is called particle size distribution, and the volume distribution particle size is the particle size calculated cumulatively based on the particle volume. For example, Dv50 represents the particle size corresponding to when the cumulative volume particle size distribution percentage in a sample reaches 50%. In a specific embodiment, a particle size tester can be used to test the average particle size.

[0080] At the end of discharge, the electrochemical reaction area gradually moves down from the first active layer 121 to the second active layer 122. Because the second active layer 122 is coated with a second positive electrode active material with a smaller particle size, the total solid phase diffusion time is reduced, which is conducive to the rapid occurrence of solid phase reaction, thereby reducing the electrochemical polarization behavior at the end of high-power discharge, helping to improve the battery power performance and extend the discharge time. Specifically, for the second positive electrode active material and the first positive electrode active material distributed with a particle size gradient along the thickness direction of the positive electrode plate, during the discharge process at a low charge state, due to the influence of the liquid phase concentration difference polarization of the electrode plate, the first positive electrode active material far away from the positive electrode collector 11 at a high charge state has completed the reaction, and the electrochemical reaction area is mainly the second positive electrode active material close to the positive electrode collector 11. At this time, the main area where the lithium insertion reaction occurs is the second active layer 122. Since the particle size of the second positive electrode active material in the second active layer 122 is relatively small, the total solid phase diffusion time is reduced. At this time, the solid phase lithium insertion reaction occurs rapidly, thereby reducing the electrochemical polarization behavior at the end of high-power discharge, which helps to improve the battery power performance and extend the discharge power under low charge state conditions.

[0081] Therefore, the positive electrode active material with smaller particle size not only has a high charge and discharge power density, but also has a lower voltage platform. Therefore, the combination of the first positive electrode active material and the second positive electrode active material with the above particle size can not only perform voltage connection well, but also is not easy to produce excessive voltage polarization, thereby further improving the power density.

[0082] In some embodiments, the particle size Dv50 of the first positive electrode active material is 300nm to 2μm, and the particle size Dv50 of the second positive electrode active material is 10nm to 300nm. Exemplarily, the particle size Dv50 of the first positive electrode active material can be 300nm, 500nm, 800nm, 900nm, 1μm, 1.2μm, 1.5μm, 1.8μm, 2μm, etc., and the particle size Dv50 of the second positive electrode active material can be 10nm, 40nm, 50nm, 80nm, 100nm, 150nm, 180nm, 200nm, 220nm, 250nm, 280nm, 300nm, etc. The first positive electrode active material within the above particle size range enables the battery to have good charge and discharge kinetics, and the corresponding slurry processing performance is good, while the second positive electrode active material within the above particle size range can achieve a lower voltage platform and be matched with the first positive electrode active material for voltage connection.

[0083] In some embodiments, the weight ratio of the first cathode active material to the second cathode active material is 7:3 to 9:1. Exemplarily, the weight ratio of the two can be 7:3, 8:2, 9:1, etc. The first cathode active material and the second cathode active material combined under this weight ratio condition result in a relatively high working voltage plateau for the overall cathode electrode sheet, which helps to improve the energy density of the battery.

[0084] In some embodiments, the coating weight of the first active layer is 160 mg / 1540.25 mm 2 to 500 mg / 1540.25 mm 2 , and the coating weight of the second active layer is 16 mg / 1540.25 mm 2 to 100 mg / 1540.25 mm 2 . Exemplarily, the coating weight of the first active layer can be 160 mg / 1540.25 mm 2 , 200 mg / 1540.25 mm 2 , 250 mg / 1540.25 mm 2 , 300 mg / 1540.25 mm 2 , 350 mg / 1540.25 mm 2 , 400 mg / 1540.25 mm 2 , 450 mg / 1540.25 mm 2 , 500 mg / 1540.25 mm 2 etc.; the coating weight of the second active layer can be 16 mg / 1540.25 mm 2 , 25 mg / 1540.25 mm 2 , 30 mg / 1540.25 mm 2 , 40 mg / 1540.25 mm 2 , 50 mg / 1540.25 mm 2 , 60 mg / 1540.25 mm 2 , 70 mg / 1540.25 mm 2 , 80 mg / 1540.25 mm 2 , 90 mg / 1540.25 mm 2 etc. The total compaction density of the first active layer and the second active layer can be 2.3 to 3.0 g / cm 3 .

[0085] Within the above coating weight range and at a relatively high compaction density, the energy density of the battery cell can reach 400 to 470 Wh / L, showing strong application potential.

[0086] The above coating weight is a single-side coating weight, which is the unit area weight of a single side of the active layer slurry coated on the positive electrode current collector 11 after drying. The positive electrode sheet of the embodiment of the present application can achieve a high coating weight and a high compaction density at the same time, and the power density is not easy to decay.

[0087] Porosity refers to the percentage of the pore volume in a porous material to the total volume of the material in its natural state. The ratio of the total volume of interconnected tiny voids in a porous material to the surface volume of the porous material is called effective porosity. The ratio of the total volume of all interconnected and uninterconnected micro-voids in a porous material to the surface volume of the porous medium is called absolute porosity or total porosity. The porosity of the active layer in the present embodiment refers to the effective porosity of the active layer material.

[0088] In some embodiments, the porosity of the first active layer 121 is greater than the porosity of the second active layer 122. By matching the porosity of the first active layer 121 and the second active layer 122, a gradient porosity distribution is generated in the positive electrode sheet: that is, the porosity of the positive electrode active layer 12 decreases from the side away from the positive electrode current collector 11 to the side close to the positive electrode current collector 11. This reduces the polarization of the liquid phase concentration difference, which is conducive to high-power charging and discharging.

[0089] Tortuosity is also called tortuosity: In the porous structure of lithium-ion battery pole pieces, the natural bending of the pores themselves leads to the generation of liquid phase concentration polarization. In order to quantify and define this phenomenon, tortuosity is defined as the ratio of the actual path length through which the electrolyte diffuses in the lithium-ion battery pole piece to the thickness of the pole piece's macroscopic coating. The overall tortuosity of the pole piece can be measured by electrochemical impedance spectroscopy (EIS), and the tortuosity comparison between the first active layer and the second active layer in the pole piece layer can be measured by SEM tomography.

[0090] In some embodiments, the tortuosity of the first active layer 121 is smaller than that of the second active layer 122. By matching the porosity and tortuosity of the first active layer 121 and the second active layer 122, a gradient tortuosity distribution is generated in the positive electrode sheet: that is, the tortuosity of the positive electrode active layer 12 increases from the side away from the positive electrode current collector 11 to the side close to the positive electrode current collector 11. This reduces the polarization of the liquid phase concentration difference, which is conducive to high-power charging and discharging.

[0091] After the positive electrode sheet is manufactured, the comparative sizes of the tortuosity and porosity of the first active layer and the second active layer can be known by observing and comparing the cross section of the positive electrode sheet with a high-resolution scanning electron microscope.

[0092] In some embodiments, the first active layer 121 includes a first positive electrode active material, a conductive agent, and a binder, wherein the mass ratio of the first positive electrode active material can be 95%-100%, that is, the conductive agent and the binder can be added or not. Specifically, the mass ratio of the first positive electrode active material, the conductive agent, and the binder in the first active layer 121 can be (95-99): (0.5-2.5): (0.5-2.5). The second active layer 122 includes a second positive electrode active material, a conductive agent, and a binder, wherein the mass ratio of the second positive electrode active material can be 95%-100%, that is, the conductive agent and the binder can be added or not. Specifically, the mass ratio of the second positive electrode active material, the conductive agent, and the binder in the second active layer 122 can be (97-99): (0.5-1.5): (0.5-1.5).

[0093] 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. 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.

[0094] In some embodiments, the positive electrode current collector 11 may be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil or stainless steel foil may be used. 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 polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0095] In some embodiments, a conductive coating 123 is further provided between the second active layer 122 and the positive current collector 11. The conductive coating contains 50-95% of a conductive agent, based on the total weight of the conductive coating 123 being 100%. The conductive coating 123 with a high content of conductive agent can improve the conductivity of the positive active layer 12 and the positive current collector 11.

[0096] Specifically, the conductive coating 123 is composed of a conductive agent and a binder. The conductive agent in the second active layer 123 may include at least one of conductive carbon black, single-walled carbon nanotubes, and multi-walled carbon nanotubes. The binder in the second active layer 123 may include one or more of polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), carboxymethyl cellulose, styrene-butadiene rubber, and calcium hydroxide. The weight proportion of the conductive agent is as high as 50-95%, so as to improve the conductivity between the positive electrode active layer 12 and the positive electrode current collector 11.

[0097] In a second aspect, the present application provides a method for preparing the above-mentioned positive electrode sheet, comprising:

[0098] S01: preparing a second active layer 122 on at least one surface of the positive electrode current collector 11;

[0099] S02: preparing a first active layer 121 on a surface of the second active layer 122 away from the positive electrode current collector 11 .

[0100] The positive electrode active layer 12 of the positive electrode plate is obtained by sequentially preparing a unique second active layer 122 and a first active layer 122 on the positive electrode current collector 11. Not only is the process simple, but also the high-power discharge voltage range can be expanded through the relationship between the working voltage range of the second positive electrode active material in the second active layer 122 and the first positive electrode active material in the first active layer 121. It still has reaction activity under a lower charging state, so the prepared positive electrode plate can make the battery have a good discharge capacity.

[0101] Specifically, the specific material types and combinations of the positive electrode current collector 11 , the second active layer 122 , and the first active layer 121 are as described above.

[0102] In some embodiments, the preparation method of the positive electrode active layer 12 includes: coating the second positive electrode slurry containing the second positive electrode active material, the conductive agent and the binder in the above ratio on the positive electrode current collector 11, and drying to obtain the second active layer 122 of the positive electrode active layer 12, and then coating the first positive electrode slurry containing the first positive electrode active material, the conductive agent and the binder in the above ratio on the second active layer 122, and drying to obtain the first active layer 121 of the positive electrode active layer 12. The first active layer 121 and the second active layer 122 constitute the positive electrode active layer 12.

[0103] In some embodiments, the step of preparing the second active layer 121 on at least one surface of the positive electrode current collector 11 includes:

[0104] First, a conductive coating 123 is prepared on at least one surface of the positive electrode current collector 11, and then a second active layer 122 is prepared on the surface of the conductive coating 123 away from the positive electrode current collector 11: wherein, based on the total weight of the conductive coating 123 being 100%, the conductive coating 123 contains 50-95% of a conductive agent. The material type selection of the specific conductive coating 123 is as described above. The conductive coating 123 with a high content of conductive agent can improve the conductivity of the positive electrode active layer 12 and the positive electrode current collector 11.

[0105] Subsequently, the positive electrode product is prepared by conventional electrode preparation methods such as cold pressing and die-cutting, and then assembled with the negative electrode, isolation membrane, and electrolyte to prepare the battery.

[0106] Battery

[0107] In a third aspect, an embodiment of the present application provides a battery, comprising the positive electrode sheet provided in the first aspect of the embodiment of the present application and / or the positive electrode sheet prepared by the preparation method provided in the second aspect of the embodiment of the present application.

[0108] The battery provided in the embodiment of the present application uses a positive electrode plate unique to the embodiment of the present application. Based on the characteristics of the positive electrode plate in the embodiment of the present application, the battery in the embodiment of the present application has a good discharge capacity.

[0109] Specifically, the battery may be a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet is the positive electrode sheet provided in the first aspect of the embodiment of the present application and / or the positive electrode sheet prepared by the preparation method provided in the second aspect of the embodiment of the present application.

[0110] In some embodiments, the secondary battery includes a lithium-ion battery. During the battery charge and discharge process, active lithium ions are inserted and removed back and forth between the positive electrode and the negative electrode. The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode. The separator is arranged between the positive electrode and the negative electrode, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing lithium ions to pass through.

[0111] The electrolyte plays a role in conducting ions between the positive electrode and the negative electrode. The present application embodiment 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.

[0112] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector. 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 a 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 polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0113] The negative electrode active layer contains a negative electrode active material, including at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, etc. It may also optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS). The negative electrode active layer may also optionally include 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.

[0114] In some embodiments, the negative electrode active layer may also optionally include other additives, such as a dispersant, a thickener (such as sodium carboxymethyl cellulose), and the like.

[0115] In some embodiments, the electrolyte is an electrolyte. The electrolyte includes an electrolyte salt and a solvent. For a secondary battery that is a lithium ion battery, 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.

[0116] In some embodiments, the solvent in the electrolyte 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.

[0117] 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.

[0118] In some embodiments, the secondary battery of the present application may include any one of a battery cell, a battery module, and a battery pack.

[0119] The battery cell refers to a battery housing and an electrode assembly encapsulated in the battery housing. The shape of the battery cell is not particularly limited and can be cylindrical, square or any other shape. Figure 4 The battery cell 20 shown has a square structure.

[0120] In some embodiments, Figure 5 As shown, the outer packaging of the battery cell 20 may include a shell 21 and a top cover assembly 22. The shell 21 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 21 has an opening connected to the receiving cavity, and the top cover assembly 22 is used to cover the opening to close the receiving cavity. The positive electrode sheet, the isolation membrane and the negative electrode sheet contained in the secondary battery of the embodiment of the present application can form an electrode assembly 23 through a winding process and / or a lamination process. The electrode assembly 23 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 23. The number of electrode assemblies 23 contained in the battery cell 20 may be one or more, which can be adjusted according to actual needs.

[0121] The preparation method of the battery cell 20 is well known. In some embodiments, the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte can be assembled to form the battery cell 20. As an example, the positive electrode sheet, the separator and the negative electrode sheet can be formed into an electrode assembly 23 through a winding process or a lamination process, and the electrode assembly 23 is placed in an outer package, dried and injected with electrolyte, and then vacuum packaged, left to stand, formed, shaped and other processes are performed to obtain the battery cell 20.

[0122] The battery module is assembled from the battery cells 20 , that is, it may contain a plurality of the battery cells 20 , and the specific number can be adjusted according to the application and capacity of the battery module.

[0123] In some embodiments, Figure 6 3 is a schematic diagram of an exemplary battery module 30. In the battery module 30, a plurality of battery cells 20 may be arranged in sequence along the length direction of the battery module 30. Of course, they may also be arranged in any other manner. Further, the plurality of battery cells 20 may be fixed by fasteners.

[0124] Optionally, the battery module 30 may further include a housing having an accommodation space, and the plurality of battery cells 20 may be accommodated in the accommodation space.

[0125] A battery pack is composed of the above-mentioned battery cells 20, that is, it may contain multiple battery cells 20, wherein multiple battery cells 20 may be assembled into the above-mentioned battery module 30. The specific number of battery cells 20 or battery modules 30 contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0126] As in the embodiment, Figure 7 and Figure 8 4 is a schematic diagram of an exemplary battery pack 40. The battery pack 40 may include a battery box and a plurality of battery modules 30 disposed in the battery box. The battery box includes an upper box body 41 and a lower box body 42, wherein the upper box body 41 is used to cover the lower box body 42 and form a closed space for accommodating the battery modules 30. The plurality of battery modules 30 may be arranged in the battery box in any manner.

[0127] Electrical devices

[0128] In a fourth aspect, the present application also provides an electric device, which includes the battery provided in the third aspect of the above application. The battery can be used as a power source for the electric device, or as an energy storage unit for the electric device. Therefore, the charging and discharging performance of the electric device in the present application can work well.

[0129] The electrical device may be, but is not limited to, a mobile device (such as a mobile phone, a laptop computer, etc.), an electric vehicle (such as 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.), an electric train, a ship and a satellite, an energy storage system, etc. The electrical device may select a secondary battery, a battery module or a battery pack according to its use requirements.

[0130] Fig. 9 Schematic diagram of an electric device as an example. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the requirements of the electric device for high power and high energy density, a battery pack or a battery module may be used.

[0131] As another example, the electric device may be a mobile phone, a tablet computer, a notebook computer, etc. The electric device is usually required to be light and thin, and a secondary battery may be used as a power source.

[0132] Example

[0133] 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.

[0134] 1. Positive electrode sheet and preparation method thereof

[0135] Example A1

[0136] A positive electrode sheet, comprising a positive electrode current collector and a second active layer and a first active layer sequentially arranged on the positive electrode current collector; wherein the positive electrode current collector is a 15um aluminum foil, and the material of the second active layer (total weight 100%) is 48% D v 50 = 100nm lithium iron phosphate, 48% D v 50 = 200nm lithium ferrous silicate, 2% acetylene black, 2% PVDF, the material of the second active layer (total weight 100%) is 96% D v 50 = 0.3 μm lithium iron phosphate, 2% acetylene black, 2% PVDF; the weight ratio of the second active layer to the first active layer is 10:90.

[0137] The method for preparing the positive electrode sheet of the embodiment of the present application comprises the following steps:

[0138] D v 50 is 100nm lithium iron phosphate, D v 50 is 200nm lithium ferrous silicate, conductive agent acetylene black, and binder PVDF are stirred and evenly mixed in N-methylpyrrolidone solvent at a final coating percentage of 48%, 48%, 2%, and 2% to obtain a coating slurry for the second active layer; D v 50 is 0.3μm lithium iron phosphate, conductive agent acetylene black, and binder PVDF are stirred and evenly mixed in N-methylpyrrolidone solvent at a final coating percentage ratio of 96%, 2%, and 2% to obtain a first active layer coating slurry; then the second active layer coating slurry and the first active layer coating slurry are passed through a double-layer coating die head and evenly coated on a 15μm aluminum foil positive electrode collector in an extrusion spraying weight ratio of 10:90 to form a second active layer and a first active layer stacked in sequence, which is the positive electrode active layer. The total weight of the single-sided coating is 430mg / 1540.25mm after final drying. 2 After drying, the 3 Cold pressing and slitting are performed to obtain positive electrode sheets.

[0139] Example A2

[0140] A positive electrode sheet and a preparation method thereof, which differs from Example A1 in that before preparing the second active layer and the first active layer on the aluminum foil positive electrode current collector, a conductive coating is first prepared on the aluminum foil positive electrode current collector, specifically: acetylene black, CMC, PAA, and calcium hydroxide are stirred and mixed in an N-methylpyrrolidone solvent at a final coating percentage ratio of 85%, 5%, 5%, and 5%, to obtain a conductive coating coating slurry, and then the conductive coating coating slurry is uniformly coated on a 15μm aluminum foil positive electrode current collector by extrusion spraying, and then dried to prepare a conductive coating on the aluminum foil positive electrode current collector. The second active layer and the first active layer are then prepared.

[0141] Example A3

[0142] A positive electrode plate, the difference between which and Example A1 is shown in Table 1.

[0143] Example A4

[0144] A positive electrode plate, the difference between which and Example A1 is shown in Table 1.

[0145] Example A5

[0146] A positive electrode plate, the difference between which and Example A1 is shown in Table 1.

[0147] Example A6

[0148] A positive electrode plate, the difference between which and Example A1 is shown in Table 1.

[0149] Example A7

[0150] A positive electrode plate, the difference between which and Example A1 is shown in Table 1.

[0151] Example A8

[0152] A positive electrode plate, the difference between which and Example A1 is shown in Table 1.

[0153] Embodiment A9

[0154] A positive electrode plate, the difference between which and Example A1 is shown in Table 1.

[0155] Embodiment A10

[0156] A positive electrode plate, the difference between which and Example A1 is shown in Table 1.

[0157] Comparative Example A1

[0158] A positive electrode sheet and a preparation method thereof, which differ from Example A1 in that:

[0159] Only the first active layer is prepared on the 15 μm aluminum foil positive electrode current collector. The coating weight of the first active layer is 430 mg / 1540.25 mm after drying.2 After drying, the 3 Cold pressing and slitting are performed to obtain positive electrode sheets.

[0160] Comparative Example A2

[0161] A positive electrode sheet and a preparation method thereof, which differ from Example A1 in that:

[0162] Only one positive electrode active layer is prepared, specifically: D v 50 is 100nm lithium iron phosphate, D v 50 is 0.3μm lithium iron phosphate, acetylene black, PVDF in the final positive electrode active layer percentage ratio of 9.6%, 86.4%, 2%, 2% in N-methylpyrrolidone solvent to obtain a positive electrode coating slurry, and then the positive electrode coating slurry is evenly coated on the 15μm primer substrate by extrusion spraying to form a positive electrode active layer, and the coating weight of the positive electrode active layer is 430mg / 1540.25mm after drying. 2 After drying, the 3 Cold pressing and slitting are performed to obtain positive electrode sheets.

[0163] 2. Secondary battery cell example

[0164] Example B1 to Example B10 and Comparative Example B1 to Comparative Example B2;

[0165] The present embodiment B1 to embodiment B10 and comparative examples B1 to comparative examples B2 respectively provide a secondary battery cell, each of which includes a bare battery cell formed by a positive electrode plate, a separator and a negative electrode plate, and also includes an electrolyte. Among them, the positive electrode plates of embodiments B1 to embodiment B10 and comparative examples B1 to comparative examples B2 correspond to the positive electrode plates provided by embodiments A1 to embodiment A10 and comparative examples A1 to comparative examples A2, respectively. Among them, the positive electrode plate in the above embodiment A1 is used as the positive electrode plate in the battery cell of the secondary battery embodiment B1, the positive electrode plate in the embodiment A2 is used as the positive electrode plate in the battery cell of the secondary battery embodiment B2, and so on, the positive electrode plate in comparative example A10 is used as the positive electrode plate in the battery cell of the secondary battery comparative example B10.

[0166] The method for preparing a secondary battery monomer comprises:

[0167] Preparation of positive electrode sheets: refer to the positive electrode sheets provided in Examples A1 to A10 and Comparative Examples A1 to A2.

[0168] Negative electrode sheet preparation:

[0169] Natural graphite, artificial graphite, single-walled carbon nanotubes, acetylene black, CMC, and SBR are uniformly stirred and mixed in a percentage ratio of 42%, 42%, 10%, 2.0%, 1.8%, and 2.2% to obtain a negative electrode slurry; the slurry is then uniformly coated on a 6um copper foil current collector by extrusion spraying, and then dried, cold pressed, and cut to obtain a negative electrode sheet.

[0170] Preparation of electrolyte:

[0171] In an argon atmosphere glove box (H 2 O<0.1ppm, O 2 <0.1ppm), mix the organic solvent ethylene carbonate (EC) / ethyl methyl carbonate (EMC) in a volume ratio of 3 / 7, add 12.5% ​​LiPF 6 The lithium salt is dissolved in an organic solvent and stirred evenly to obtain an electrolyte.

[0172] Isolation film: Polypropylene film is used as the isolation film.

[0173]

Battery assembly

[0174] Secondary battery assembly: The prepared negative electrode sheet, positive electrode sheet, and polypropylene porous isolation membrane are stacked in sequence, and an electrode assembly with a theoretical capacity of 30Ah is obtained through a winding process. Then, it is packaged, injected with electrolyte, formed, sorted, and made into a cylindrical lithium-ion secondary battery, which is a secondary battery monomer.

[0175] Performance Testing

[0176] The positive electrode sheets and secondary battery cells of the above-mentioned embodiments and comparative examples were tested respectively.

[0177] (1) Positive electrode

[0178]

Particle size test

[0179] Positive electrode active material D v 50 test steps:

[0180] Disperse 10 mg of positive electrode active material in ethanol solution, start ultrasonic dispersion, and use Malvern particle size analyzer Master 3000 to test the particle size by laser testing method. After the computer processes the data, the particle size distribution curve of the positive electrode active material can be obtained, and the particle size distribution of 50% of the particles is obtained, which is Dv50.

[0181]

Voltage platform test

[0182] Voltage platform calculation: Assemble the positive electrode sheet containing positive electrode active materials, the separator and the metal lithium negative electrode into a button half-cell, charge the button half-cell at 0.33C until 3.8V (LFP) or 4.2V (LMFP), then let it stand for 10 minutes, and discharge the button half-cell at 0.33C until the button half-cell voltage reaches 1.5V. During the 0.33C discharge process, record the discharge energy E and capacity C of the battery cell, and use the formula V=E / C to obtain the charging and discharging voltage platform range of the battery cell, which is the working voltage range of the positive electrode active material.

[0183]

Coating tortuosity test

[0184] Measurement of the tortuosity of the entire positive electrode active layer (electrochemical impedance spectroscopy): The electrode containing the positive electrode active material and the isolation membrane are assembled into a soft-pack symmetrical battery. 60uL of 50mM tetrabutylammonium perchlorate electrolyte (solvent EC:DMC=1:1, lithium ion conductivity is 1.7mS / cm) is added to each battery, and the electrochemical impedance spectroscopy is tested in the frequency range of 200kHz to 50mHz. Using τ / ε=R ion ×S×k int / l can calculate the tortuosity. Where τ is the tortuosity, ε is the porosity, l is the thickness of the electrode, S is the area of ​​the electrode, and k int is the lithium ion conductivity of the electrolyte, R ion is the lithium ion impedance. ion =3×(R h -R l ), R h is the high frequency intercept of the impedance spectrum, R l is the low frequency intercept.

[0185] Comparative measurement of the tortuosity of the first active layer and the second active layer (tortuosity calculation of tortuosity of tomographic SEM electron microscope images): Import the tomographic SEM image into the ImagJ software, adjust the grayscale and contrast to obtain the ion path without active substances, and use imaging methods to count the ion flow path in the path in the thickness direction of the electrode, and then compare the tortuosity with the ratio of the coating thickness of the electrode.

[0186] (2) Secondary battery cells

[0187] Electrochemical performance test: The secondary battery cells prepared in the above embodiments and comparative examples were subjected to room temperature discharge DCR (internal resistance) test: At 25°C, the cells were fully charged to 1 / 3, then discharged to 50% SOC at 1 / 3C, left to stand for 30 minutes, and the voltage at this time was recorded as V0. Then, they were discharged at a discharge current I of 4C for 10 seconds, and the voltage at this time was recorded as V1. Then, DCR = [(V0-V1) / I].

[0188] Table 1 Positive electrode active layer ratio

[0189]

[0190]

[0191] Table 2 Test results

[0192]

[0193] As can be seen from Table 2, based on the embodiment of the present application, the positive active layer of the positive electrode sheet includes two layers: the second active layer and the first active layer sequentially arranged on the positive current collector, the working voltage range of the first positive active material is 3.1-4.0V, which is greater than the working voltage range of the second positive active material 2.0-3.0V, and the overall tortuosity is less than the comparative example with only one active layer. This makes the DCR internal resistance of the battery in the embodiment of the present application smaller, and the discharge end voltage tested under the same conditions (50% SOC, 25°C, 5KW discharge) is larger, that is, the battery discharge capacity of the embodiment of the present application is better.

[0194] Among them, the positive electrode plate of the battery in Example B2 has a conductive coating between the second active layer and the current collector, which further improves the discharge capacity of the battery. The positive electrode plate of the battery in Example B3 has a conductive coating between the second active layer and the current collector, which further improves the discharge capacity of the battery. The positive electrode plate of the battery in Example B3 has a conductive coating between the second active layer and the current collector, which further improves the discharge capacity of the battery. The positive electrode plate of the battery in Example B4 has a conductive coating between the second active layer and the current collector, which further improves the discharge capacity of the battery. The positive electrode plate of the battery in Example B5 has a conductive coating between the second active layer and the current collector, which further improves the discharge capacity of the battery. The positive electrode plate of the battery in Example B6 uses lithium vanadate in the second active layer, which increases the ion transmission path due to the three-dimensional ion transmission channel of lithium vanadate. The positive electrode plate of the battery in Example B7 uses lithium iron manganese phosphate in the first active layer, which has a higher voltage, so the terminal voltage of the battery is higher and the discharge capacity is further enhanced. Compared with Example B1, the positive electrode plate of the battery in Example B8 uses lithium iron phosphate with a smaller particle size in the first active layer, so the lithium ion diffusion path is reduced and the discharge capacity is enhanced. Compared with Example B1, the positive electrode plate of the battery in Example B9 uses lithium iron manganese phosphate with a larger particle size in the first active layer, so the lithium ion diffusion path is increased and the discharge capacity is weakened. Compared with Example B1, the positive electrode plate of the battery in Example B10 has an increased particle size of active materials, so the lithium ion diffusion path is increased and the discharge capacity is weakened.

[0195] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and they should all be included in the scope of the claims and specification of the present application. In particular, as long as there is no structural conflict, the various technical features mentioned in the various embodiments can be combined in any way. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims

1. A positive electrode sheet, comprising a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector, It is characterized in that The positive electrode active layer comprises a first active layer and a second active layer, wherein the second active layer is located between the positive electrode current collector and the first active layer; The first active layer includes a first positive electrode active material, the second active layer includes a second positive electrode active material, the operating voltage range of the first positive electrode active material is greater than the operating voltage range of the second positive electrode active material, and the operating voltage range of the second positive electrode active material is less than or equal to 3.0V.

2. The positive electrode sheet according to claim 1, It is characterized in that The operating voltage range of the first positive electrode active material is greater than 3.0V. Optionally, the operating voltage range of the first positive electrode active material is 3.1V to 4.0V; and / or, The operating voltage range of the second positive electrode active material is 1.5V to 3.0V.

3. The positive electrode sheet according to claim 1 or 2, It is characterized in that The first positive electrode active material includes at least one of lithium iron phosphate and lithium manganese iron phosphate; and / or, The second positive electrode active material includes at least one of lithium iron phosphate and lithium iron manganese phosphate, and also includes at least one of lithium ferrous silicate and lithium vanadate. Optionally, the ratio of the total weight of lithium iron phosphate and lithium manganese phosphate in the second positive electrode active material to the total weight of lithium ferrous silicate and lithium vanadate is 1:1 to 9:

1.

4. The positive electrode sheet according to any one of claims 1 to 3, It is characterized in that The particle size Dv50 of the first positive electrode active material is greater than the particle size Dv50 of the second positive electrode active material; optionally, The particle size Dv50 of the first positive electrode active material is 300 nm to 2 μm, and the particle size Dv50 of the second positive electrode active material is 10 nm to 300 nm.

5. The positive electrode sheet according to any one of claims 1 to 4, It is characterized in that The weight ratio of the first positive electrode active material to the second positive electrode active material is 7:3 to 9:

1.

6. The positive electrode sheet according to claim 5, It is characterized in that The coating weight of the first active layer is 160 mg / 1540.25 mm 2 ~500mg / 1540.25mm 2 The coating weight of the second active layer is 16 mg / 1540.25 mm 2 ~100mg / 1540.25mm 2 and / or, The total compaction density of the first active layer and the second active layer is 2.3-3.0 g / cm 3 .

7. The positive electrode sheet according to any one of claims 1 to 6, It is characterized in that The porosity of the first active layer is greater than the porosity of the second active layer; and / or, The tortuosity of the first active layer is smaller than the tortuosity of the second active layer.

8. The positive electrode sheet according to any one of claims 1 to 7, It is characterized in that A conductive coating is also provided between the second active layer and the positive electrode current collector. The conductive coating contains 50-95% of a conductive agent based on 100% of the total weight of the conductive coating.

9. A method for preparing a positive electrode sheet according to any one of claims 1 to 7, It is characterized in that include: Preparing the second active layer on at least one surface of the positive electrode current collector; The first active layer is prepared on a surface of the second active layer away from the positive electrode current collector.

10. The preparation method according to claim 9, It is characterized in that The step of preparing the second active layer on at least one surface of the positive electrode current collector comprises: A conductive coating is first prepared on at least one surface of the positive electrode current collector, and then the second active layer is prepared on the surface of the conductive coating away from the positive electrode current collector: wherein, based on the total weight of the conductive coating being 100%, the conductive coating contains 50-95% of a conductive agent.

11. A battery, It is characterized in that It comprises the positive electrode sheet as described in any one of claims 1 to 8 and / or the positive electrode sheet prepared by the preparation method as described in any one of claims 9 to 10.

12. An electrical device, It is characterized in that Comprising the battery of claim 11.

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