Positive electrode sheet and preparation method therefor, battery and electric device

Abstract

A positive electrode sheet and a preparation method therefor, a battery and an electric device. The positive electrode sheet comprises a positive current collector and a positive active layer arranged on at least one surface of the positive current collector, wherein 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; and the first active layer comprises a first positive active material, and the second active layer comprises a second positive active material, the operating voltage interval of the first positive active material being greater than the operating voltage interval of the second positive active material, and the operating voltage interval of the second positive active material being less than or equal to 3.0 V. The first positive active material starts to be attenuated after a relatively large voltage plateau, and the effect of continuing an operating voltage is achieved by means of the lower operating voltage interval of the second positive active material, thus expanding the voltage interval range of high-power discharge; in addition, the second positive active material still has reaction activity in a relatively low state of charge, and thus has a good discharge capacity.

Description

Positive electrode sheet and preparation method thereof, battery and electrical device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on November 23, 2023, with application number 202311580592.4 and invention name “Positive electrode sheet and preparation method thereof, battery and electrical device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] 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

[0003] In recent years, new energy vehicles have developed rapidly. The battery drive system is the main factor affecting the performance and cost of new energy vehicles. Secondary batteries have become the preferred power battery solution in the current new energy vehicle battery drive system due to their high energy density, low memory effect and high operating voltage.

[0004] A secondary battery cell generally consists of a positive electrode, a separator, and a negative electrode. The positive active material in the positive electrode generally decays rapidly after reaching a certain discharge voltage plateau, thus affecting the battery's discharge capacity.

[0005] Summary of the Invention

[0006] 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.

[0007] 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, wherein the second active layer is located between the positive electrode current collector and the first active layer;

[0008] 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.

[0009] 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. In this way, when the first positive electrode active material begins to decay after a larger voltage platform, due to the presence of the second positive electrode active material, with its lower working voltage range of less than or equal to 3.0V, the working voltage is continued, 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 in a lower charging state, such a positive electrode plate enables the battery to have a good discharge capacity.

[0010] In some embodiments, the operating voltage range of the first cathode active material is greater than 3.0 V. Optionally, the operating voltage range of the first cathode active material is 3.1 V to 4.0 V; and / or,

[0011] The operating voltage range of the second positive electrode active material is 1.5V to 3.0V.

[0012] 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 achieved, thereby improving the discharge capacity of the battery.

[0013] In some embodiments, the first cathode active material includes at least one of lithium iron phosphate and lithium manganese iron phosphate; and / or,

[0014] The second positive electrode active material includes at least one of lithium iron phosphate and lithium manganese iron 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 iron 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.

[0015] 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.

[0016] 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, the particle size Dv50 of the first positive electrode active material is 300nm~2μm, and the particle size Dv50 of the second positive electrode active material is 10nm~300nm.

[0017] 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 prone to excessive voltage polarization, thereby further improving the power density.

[0018] In some embodiments, a weight ratio of the first cathode active material to the second cathode active material is 7:3 to 9:1.

[0019] At this weight ratio, the overall operating voltage platform of the positive electrode is higher, which helps to improve the battery energy density.

[0020] 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,

[0021] The total compaction density of the first active layer and the second active layer is 2.3-3.0 g / cm 3 .

[0022] 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 strong application potential.

[0023] In some embodiments, the porosity of the first active layer is greater than the porosity of the second active layer; and / or,

[0024] The tortuosity of the first active layer is smaller than that of the second active layer.

[0025] 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.

[0026] In some embodiments, a conductive coating is further provided between the second active layer and the positive electrode current collector. Based on 100% of the total weight of the conductive coating, the conductive coating contains 50-95% of a conductive agent.

[0027] 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.

[0028] In a second aspect, the present invention provides a method for preparing the positive electrode sheet, comprising:

[0029] preparing the second active layer on at least one surface of the positive electrode current collector;

[0030] The first active layer is prepared on a surface of the second active layer away from the positive electrode current collector.

[0031] The positive electrode sheet 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 battery still has reaction activity at a lower charging state, so the prepared positive electrode sheet can make the battery have a good discharge capacity.

[0032] In some embodiments, the step of preparing the second active layer on at least one surface of the positive electrode current collector includes:

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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

[0040] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present application. The same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0041] FIG1 is a schematic structural diagram of a positive electrode sheet according to an embodiment of the present application;

[0042] FIG2 is another schematic structural diagram of a positive electrode sheet according to an embodiment of the present application;

[0043] FIG3 is a graph showing a rate discharge voltage curve of lithium iron phosphate in a positive electrode sheet according to an embodiment of the present application;

[0044] FIG4 is a schematic diagram of a battery cell structure of an embodiment of a secondary battery according to an embodiment of the present application;

[0045] FIG5 is an exploded schematic diagram of a battery cell of the secondary battery shown in FIG4 ;

[0046] FIG6 is a schematic structural diagram of an embodiment of a battery module according to the present application;

[0047] FIG7 is a schematic structural diagram of an embodiment of a battery pack according to the present application;

[0048] FIG8 is a schematic diagram of the exploded structure of the battery pack shown in FIG7 ;

[0049] FIG9 is a schematic diagram of an embodiment of an electric device including a secondary battery according to an embodiment of the present application as a power source.

[0050] Explanation of the reference numerals: 11 - positive electrode current collector; 12 - positive electrode active layer; 121 - first active layer; 122 - second active layer; 123 - conductive coating; 20 - battery cell; 21 - housing; 22 - top cover assembly; 23 - electrode assembly; 30 - battery module; 40 - battery pack; 41 - upper case; 42 - lower case. DETAILED DESCRIPTION

[0051] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0052] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art 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" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0053] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0054] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0055] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0056] In the description of the embodiments of this application, the term "plurality" refers to two or more (including two). Similarly, "multiple groups" refers to two or more (including two groups), and "multiple sheets" refers to two or more (including two sheets). "At least one" refers to more than one (including one, two, three, etc.).

[0057] 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., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and 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 device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0058] In the description of the embodiments of the present application, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connected," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; internal connections between two components or interactions between two components. Those skilled in the art can understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0059] With the increasing depletion of traditional energy resources, the development of new energy storage devices is gaining increasing attention. Secondary batteries, in particular, have attracted considerable attention due to their high energy density, high theoretical capacity, excellent cycle stability, and environmentally friendly properties. Secondary batteries are not only used in energy storage power systems such as hydropower, thermal power, wind power, and solar power stations, but are also widely used in a variety of fields, including electric vehicles such as electric bicycles, electric motorcycles, and electric vehicles. As the application areas of secondary batteries as power batteries continue to expand, their market demand is also growing, and the requirements for battery performance, such as cycle performance, are becoming increasingly stringent.

[0060] The power performance of secondary batteries is closely related to the polarization of the positive electrode during the battery discharge process. Secondary battery cells generally include a positive electrode sheet, a separator, and a negative electrode sheet. If the thickness of the electrode sheet is too large, it is easy to cause the degree of polarization to increase. In order to reduce the polarization of the positive electrode sheet, conventional battery designs generally design the positive electrode sheet to be thinner (CW is generally between 0 and 400g / 1540.25mm). 2 ), such a design can, on the one hand, thin the diffusion path of active ions such as lithium ions and reduce the liquid phase diffusion distance, and on the other hand, shorten the electron transmission path along the thickness direction of the electrode, reduce the electron transmission impedance, and ultimately reduce the polarization of the positive electrode. However, due to the reduction in surface coating weight, the volume proportion of inactive substances (such as current collectors and separators) in the battery increases significantly, reducing the space utilization rate of the positive electrode active material, which in turn leads to a decrease in volume energy density, which is not conducive to the mileage of new energy vehicles. Moreover, the reduction in volume energy density will also lead to an increase in battery costs and increase the purchase cost of new energy vehicles.

[0061] As the penetration rate of new energy vehicles continues to climb, hybrid and extended-range vehicles are gradually entering the market, placing higher demands on the power performance of secondary batteries. Lithium-ion batteries, as a type of secondary battery, offer high energy density, long service life, energy conservation, and environmental protection. In secondary batteries, the positive electrode active material in the positive electrode plate generally degrades rapidly after reaching a certain discharge voltage plateau. Without a continuous operating voltage, this can easily affect the battery's discharge capacity.

[0062] Based on the above considerations, in order to improve the discharge capacity of a secondary battery, a positive electrode active layer comprising a first active layer and a second active layer is provided on at least one surface of the positive electrode current collector. The discharge capacity of the secondary battery can be improved by matching the operating voltage ranges of the positive electrode active materials in the first and second active layers. Therefore, the following technical solution is proposed.

[0063] Positive electrode sheet and preparation method thereof

[0064] In a first aspect, the embodiments of the present application provide a positive electrode sheet. As a summary of some embodiments of the present application, as shown in Figures 1 and 2, the positive electrode sheet includes: (1) a positive electrode collector 11, and (2) a positive electrode active layer 12. The positive electrode active layer 12 is provided on at least one surface of the positive electrode collector 11, that is, the positive electrode active layer 12 is provided on one surface of the positive electrode collector 11, or the positive electrode active layer 12 is provided on two opposite surfaces of the positive electrode collector 11.

[0065] The positive electrode current collector 11 is the structure or component that collects current in the battery. The positive electrode active layer 12 is the film layer containing the positive electrode active material in the battery. The positive electrode active material in the positive electrode active layer 12 can embed and extract active metal ions during battery charging and discharging.

[0066] 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 122 contains a second positive electrode active material. There is a certain relationship between the operating voltage ranges of the first positive electrode active material and the second positive electrode active material, that is, 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.

[0067] The operating voltage refers to the reversible reaction potential range of the electrode material in the battery. For the operating 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.

[0068] In the embodiment of the present application, the second positive electrode active material has a relatively low operating voltage range of less than or equal to 3.0V. Therefore, when the first positive electrode active material begins to decay after reaching a relatively high voltage plateau, the presence of the second positive electrode active material, leveraging its relatively low operating voltage range, can provide a continuous operating voltage, thereby expanding the high-power discharge voltage range. Furthermore, the second positive electrode active material is located in the second active layer 122, i.e., on the side close to the positive electrode current collector 11. This allows the second positive electrode active material to remain reactive even at relatively low charge states. Therefore, the positive electrode sheet of the embodiment of the present application can provide the battery with excellent discharge capacity.

[0069] In some embodiments, the first positive electrode active material in the first active layer 121 first contacts the electrolyte and generally undergoes a discharge reaction first. Its operating voltage range is greater than 3.0V, so a discharge reaction with a larger discharge voltage can be carried out first. Then, the discharge voltage of the first positive electrode active material will rapidly decay after exceeding the 3.0V platform. In this way, the addition of the second positive electrode active material with a lower operating voltage range supports the sudden drop in the voltage platform after 3.0V of the first positive electrode active material, playing the role of continuing the operating voltage, thereby extending the high-power discharge voltage range of the entire positive electrode active layer 12 and improving its discharge capacity. Optionally, the operating voltage range of the first positive electrode active material is 3.1V to 4.0V. Charging and discharging can be performed by the first positive electrode active material within the operating voltage range of 3.1V to 4.0V.

[0070] In some embodiments, the operating voltage range of the second positive electrode active material in the second active layer 122 is 1.5 V to 3.0 V. By disposing the second positive electrode active material with a lower operating voltage range on the side of the positive electrode active layer 12 close to the positive electrode current collector 11, the operating voltage can be connected.

[0071] In some embodiments, the operating voltage range of the first positive electrode active material is 3.1V~4.0V, and the operating voltage range of the second positive electrode active material is 1.5V~3.0V; by matching the operating voltage ranges of the above-mentioned 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.

[0072] 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 have relatively high voltages and are disposed on the side of the positive electrode active layer 12 away from the positive electrode current collector 11 to achieve high-rate discharge.

[0073] In some embodiments, the second positive electrode active material includes at least one of lithium iron phosphate and lithium manganese iron phosphate, and also includes at least one of lithium ferrous silicate and lithium vanadate. By combining at least one of lithium iron phosphate and lithium manganese iron phosphate with at least one of lithium ferrous silicate and lithium vanadate, a second positive electrode active material with a lower voltage can be achieved. The second positive electrode active material 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 suddenly drops during high-rate discharge, the second positive electrode active material can achieve the function of voltage continuity. At the same time, the second positive electrode active material still has reaction activity 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.

[0074] Taking lithium iron phosphate as the first positive electrode active material as an example, as shown in Figure 3, which is a 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.

[0075] 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.

[0076] The slurry prepared with 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, while also being well compatible with the wire drawing of current mass production lines. 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.

[0077] 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.

[0078] The size of a granular material is called particle size. The percentage of particles within different size ranges is called particle size distribution. Volume distribution particle size is the cumulative particle size calculated based on particle volume. For example, Dv50 represents the particle size at which the cumulative volume particle size distribution percentage in a sample reaches 50%. In specific embodiments, the average particle size can be measured using a particle size analyzer.

[0079] At the end of discharge, the electrochemical reaction area gradually moves downward 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 the 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, the second positive electrode active material and the first positive electrode active material with a particle size gradient distribution along the thickness direction of the positive electrode plate, during the discharge process in the low charge state, due to the influence of the liquid phase concentration difference polarization of the electrode plate, the first positive electrode active material away from the positive electrode collector 11 at the high charge state has completed the reaction, and the electrochemical reaction area is mainly the second positive electrode active material in the area 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. Because 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.

[0080] 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 prone to excessive voltage polarization, thereby further improving the power density.

[0081] In some embodiments, 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. For example, the particle size Dv50 of the first positive electrode active material can be 300 nm, 500 nm, 800 nm, 900 nm, 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, etc., and the particle size Dv50 of the second positive electrode active material can be 10 nm, 40 nm, 50 nm, 80 nm, 100 nm, 150 nm, 180 nm, 200 nm, 220 nm, 250 nm, 280 nm, 300 nm, etc. The first positive electrode active material within the above particle size range enables the battery to have excellent charge and discharge kinetics and good corresponding slurry processing performance. The second positive electrode active material within the above particle size range can achieve a lower voltage platform and be used in voltage connection with the first positive electrode active material.

[0082] In some embodiments, the weight ratio of the first positive electrode active material to the second positive electrode active material is 7:3 to 9:1. For example, the weight ratio can be 7:3, 8:2, 9:1, etc. The combination of the first and second positive electrode active materials at this weight ratio results in a higher operating voltage platform for the overall positive electrode sheet, which helps improve the battery's energy density.

[0083] 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 16mg / 1540.25mm 2 ~100mg / 1540.25mm 2 For example, the coating weight of the first active layer can be 160 mg / 1540.25 mm 2 , 200mg / 1540.25mm 2 、250mg / 1540.25mm 2 、300mg / 1540.25mm 2 、350mg / 1540.25mm 2 , 400mg / 1540.25mm 2 、450mg / 1540.25mm 2 、500mg / 1540.25mm 2 etc.; the coating weight of the second active layer can be 16mg / 1540.25mm 2 , 25mg / 1540.25mm 2 、30mg / 1540.25mm 2、40mg / 1540.25mm 2 、50mg / 1540.25mm 2 、60mg / 1540.25mm 2 、70mg / 1540.25mm 2 、80mg / 1540.25mm 2 、90mg / 1540.25mm 2 The total compaction density of the first active layer and the second active layer can be 2.3 to 3.0 g / cm 3 .

[0084] Within the above coating weight range and at a relatively high compaction density, the energy density of the battery cell can reach 400-470Wh / L, which has strong application potential.

[0085] The above coating weight is the single-sided coating weight, which is the weight per unit area of ​​the active layer slurry coated and dried on the positive electrode current collector 11. The positive electrode sheet of the embodiment of the present application can achieve both a high coating weight and a high compaction density without significantly reducing the power density.

[0086] 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 microvoids within a porous material to the external volume of the porous material is called effective porosity, denoted by φ_e. The ratio of the total volume of all interconnected and non-interconnected microvoids within a porous material to the external volume of the porous medium is called absolute porosity or total porosity, denoted by φ_T. The porosity of the active layer in the present embodiment refers to the effective porosity φ_e of the active layer material.

[0087] In some embodiments, the porosity of the first active layer 121 is greater than that of the second active layer 122. The porosity of the first and second active layers 121, 122 is matched to create a gradient porosity distribution in the positive electrode sheet: that is, the porosity of the positive active layer 12 decreases from the side away from the positive current collector 11 to the side close to the positive current collector 11. This reduces polarization caused by differences in liquid phase concentration, facilitating high-power charging and discharging.

[0088] Tortuousness, also known as tortuosity, is a phenomenon in the porous structure of lithium-ion battery electrodes. The natural curvature of the pores leads to concentration polarization in the liquid phase. To quantify and define this phenomenon, tortuosity is defined as the ratio of the actual path length through which the electrolyte diffuses in a lithium-ion battery electrode to the thickness of the electrode's macroscopic coating. The overall electrode tortuosity can be measured using electrochemical impedance spectroscopy (EIS), and the comparative relationship between the tortuosity of the first and second active layers within the electrode layer can be measured using SEM tomography.

[0089] In some embodiments, the tortuosity of the first active layer 121 is smaller than that of the second active layer 122. The porosity and tortuosity of the first and second active layers 121, 122 are matched to create a gradient tortuosity distribution in the positive electrode sheet: that is, the tortuosity of the positive active layer 12 increases from the side away from the positive current collector 11 to the side close to the positive current collector 11. This reduces polarization caused by differences in liquid phase concentration, facilitating high-power charging and discharging.

[0090] 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 using a high-resolution scanning electron microscope.

[0091] 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).

[0092] As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a 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.

[0093] In some embodiments, the positive electrode current collector 11 may be a metal foil or a composite current collector. For example, aluminum foil or stainless steel foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. 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.

[0094] In some embodiments, a conductive coating 123 is further provided between the second active layer 122 and the positive electrode current collector 11. The conductive coating 123 contains 50-95% of a conductive agent, based on 100% of the total weight of the conductive coating 123. The provision of a conductive coating 123 containing a high content of conductive agent can improve the electrical conductivity of the positive electrode active layer 12 and the positive electrode current collector 11.

[0095] Specifically, the conductive coating 123 is composed of a conductive agent and a binder. The conductive agent in the conductive coating 123 may include at least one of conductive carbon black, single-walled carbon nanotubes, and multi-walled carbon nanotubes. The binder in the conductive coating 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.

[0096] In a second aspect, the present invention provides a method for preparing the positive electrode sheet, comprising:

[0097] S01: preparing a second active layer 122 on at least one surface of the positive electrode current collector 11;

[0098] S02: preparing a first active layer 121 on a surface of the second active layer 122 away from the positive electrode current collector 11 .

[0099] 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 121 on the positive electrode current collector 11. 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 122 and the first positive electrode active material in the first active layer 121. The reaction activity is still maintained at a lower charging state, so the prepared positive electrode plate can enable the battery to have a good discharge capacity.

[0100] 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.

[0101] In some embodiments, the preparation method of the positive electrode active layer 12 includes: applying a second positive electrode slurry containing the second positive electrode active material, the conductive agent, and the binder in the above-mentioned proportions on the positive electrode current collector 11, and drying to obtain the second active layer 122 of the positive electrode active layer 12; then applying a first positive electrode slurry containing the first positive electrode active material, the conductive agent, and the binder in the above-mentioned proportions 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.

[0102] In some embodiments, the step of preparing the second active layer 122 on at least one surface of the positive electrode current collector 11 includes:

[0103] A conductive coating 123 is first formed on at least one surface of the positive electrode current collector 11. A second active layer 122 is then formed on the surface of the conductive coating 123 facing away from the positive electrode current collector 11. The conductive coating 123 contains 50-95% of a conductive agent, based on the total weight of the conductive coating 123 as 100%. The specific material selection for the conductive coating 123 is described above. Preparing a conductive coating 123 with a high content of conductive agent improves the conductivity of the positive electrode active layer 12 and the positive electrode current collector 11.

[0104] 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.

[0105] Battery

[0106] 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.

[0107] 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.

[0108] 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.

[0109] In some embodiments, the secondary battery comprises a lithium-ion battery. During the battery's charge and discharge processes, active lithium ions are intercalated and released between the positive and negative electrodes. The electrolyte acts as an ion conductor between the positive and negative electrodes. A separator is positioned between the positive and negative electrodes, primarily preventing short circuits between the positive and negative electrodes while allowing lithium ions to pass through.

[0110] The electrolyte conducts ions between the positive and negative electrodes. The present invention does not specifically limit the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0111] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode active layer provided 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, copper foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0112] 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, and tin-based materials. 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.

[0113] In some embodiments, the negative electrode active layer may optionally include other additives, such as a dispersant, a thickener (such as sodium carboxymethyl cellulose), and the like.

[0114] In some embodiments, the electrolyte is an electrolyte. The electrolyte includes an electrolyte salt and a solvent. If the secondary battery 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.

[0115] 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, methylpropyl carbonate, ethylpropyl 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.

[0116] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include 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, and additives that improve battery high or low temperature performance.

[0117] 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.

[0118] A battery cell is a battery housing and an electrode assembly encapsulated within the housing. The shape of a battery cell is not particularly limited and can be cylindrical, square, or any other shape. A square-shaped battery cell 20 is shown in FIG4 .

[0119] In some embodiments, as shown in Figure 5, 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 be formed into 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 can be one or more, which can be adjusted according to actual needs.

[0120] The preparation method of the battery cell 20 is well known. In some embodiments, the positive electrode sheet, separator, and negative electrode sheet can be assembled with an electrolyte to form the battery cell 20. As an example, the positive electrode sheet, separator, and negative electrode sheet can be wound or laminated to form an electrode assembly 23. The electrode assembly 23 is then placed in an outer package, dried, and then injected with electrolyte. The battery cell 20 is then vacuum packaged, allowed to stand, formed, and shaped.

[0121] A battery module is assembled from the battery cells 20 , that is, it may contain a plurality of battery cells 20 , and the specific number can be adjusted according to the application and capacity of the battery module.

[0122] In some embodiments, FIG6 is a schematic diagram of an exemplary battery module 30. In the battery module 30, multiple battery cells 20 may be arranged sequentially along the length of the battery module 30. Of course, they may also be arranged in any other manner. Furthermore, the multiple battery cells 20 may be secured together using fasteners.

[0123] 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.

[0124] A battery pack is assembled from the battery cells 20 described above, and may contain multiple battery cells 20, wherein multiple battery cells 20 may be assembled into the battery module 30 described above. The specific number of battery cells 20 or battery modules 30 contained in a battery pack may be adjusted according to the application and capacity of the battery pack.

[0125] In the embodiment, Figures 7 and 8 are schematic diagrams of an exemplary battery pack 40. The battery pack 40 may include a battery box and multiple battery modules 30 disposed within the battery box. The battery box includes an upper box body 41 and a lower box body 42. The upper box body 41 covers the lower box body 42 and forms an enclosed space for accommodating the battery modules 30. The multiple battery modules 30 may be arranged in any manner within the battery box.

[0126] Electrical devices

[0127] Fourthly, embodiments of the present application further provide an electrical device comprising the battery provided in the third aspect of the present application. The battery can serve as both a power source and an energy storage unit for the electrical device. Therefore, the electrical device of the present application embodiment can maintain excellent charge and discharge performance.

[0128] Electrical devices may include, but are not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc. These electrical devices may use secondary batteries, battery modules, or battery packs based on their usage requirements.

[0129] Figure 9 is a schematic diagram of an exemplary electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of this device, a battery pack or battery module may be used.

[0130] As another example, the electric device may be a mobile phone, a tablet computer, a laptop computer, etc. Such an electric device is usually required to be lightweight and thin, and may use a secondary battery as a power source.

[0131] Example

[0132] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0133] 1. Positive Electrode Sheet and Preparation Method Example

[0134] Example A1

[0135] 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 15 μm 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.

[0136] The method for preparing the positive electrode sheet of the embodiment of the present application includes the following steps:

[0137] D v 50 is 100nm lithium iron phosphate, D v 50 is 200nm lithium ferrous silicate, conductive agent acetylene black, binder PVDF in the final coating percentage ratio of 48%, 48%, 2%, 2% in N-methylpyrrolidone solvent to obtain the coating slurry of 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 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 current 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 the positive electrode sheet.

[0138] Example A2

[0139] A positive electrode sheet and preparation method thereof, differing from Example A1 in that, before forming 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 in a ratio of 85%, 5%, 5%, and 5% to obtain a conductive coating slurry. The conductive coating slurry is then evenly applied to a 15μm aluminum foil positive electrode current collector by extrusion spraying, and then dried to form a conductive coating on the aluminum foil positive electrode current collector. The second active layer and the first active layer are then prepared.

[0140] Example A3

[0141] A positive electrode plate, the differences from Example A1 are shown in Table 1.

[0142] Example A4

[0143] A positive electrode plate, the differences from Example A1 are shown in Table 1.

[0144] Example A5

[0145] A positive electrode plate, the differences from Example A1 are shown in Table 1.

[0146] Example A6

[0147] A positive electrode plate, the differences from Example A1 are shown in Table 1.

[0148] Example A7

[0149] A positive electrode plate, the differences from Example A1 are shown in Table 1.

[0150] Example A8

[0151] A positive electrode plate, the differences from Example A1 are shown in Table 1.

[0152] Example A9

[0153] A positive electrode plate, the differences from Example A1 are shown in Table 1.

[0154] Example A10

[0155] A positive electrode plate, the differences from Example A1 are shown in Table 1.

[0156] Comparative Example A1

[0157] A positive electrode sheet and a preparation method thereof, which differ from Example A1 in that:

[0158] Only the first active layer was prepared on the 15 μm aluminum foil positive electrode current collector. The coating weight of the first active layer was 430 mg / 1540.25 mm after drying. 2 After drying, the 3 Cold pressing and slitting are performed to obtain the positive electrode sheet.

[0159] Comparative Example A2

[0160] A positive electrode sheet and a preparation method thereof, which differ from Example A1 in that:

[0161] Only one positive electrode active layer is prepared, specifically: D v 50 is 100nm lithium iron phosphate, D v Lithium iron phosphate, acetylene black, and PVDF with a final positive electrode active layer content of 9.6%, 86.4%, 2%, and 2% are stirred and evenly mixed in N-methylpyrrolidone solvent to obtain a positive electrode coating slurry. The positive electrode coating slurry is then evenly coated on a 15 μm primer substrate by extrusion spraying to form a positive electrode active layer. The coating weight of the positive electrode active layer is 430 mg / 1540.25 mm after drying. 2 After drying, the 3 Cold pressing and slitting are performed to obtain the positive electrode sheet.

[0162] 2. Secondary Battery Cell Example

[0163] Example B1 to Example B10 and Comparative Example B1 to Comparative Example B2;

[0164] The present embodiments B1 to B10 and comparative examples B1 to B2 respectively provide a secondary battery cell, each of which includes a bare cell formed by a positive electrode plate, a separator, and a negative electrode plate, and also includes an electrolyte. The positive electrode plates of embodiments B1 to B10 and comparative examples B1 to B2 respectively correspond to the positive electrode plates provided in embodiments A1 to A10 and comparative examples A1 to A2. The positive electrode plate in embodiment A1 above serves as the positive electrode plate in the battery cell of secondary battery embodiment B1, the positive electrode plate in embodiment A2 serves as the positive electrode plate in the battery cell of secondary battery embodiment B2, and so on. The positive electrode plate in comparative example A10 serves as the positive electrode plate in the battery cell of secondary battery comparative example B10.

[0165] The method for preparing a secondary battery cell includes:

[0166] Preparation of positive electrode sheets: refer to the positive electrode sheets provided in Examples A1 to A10 and Comparative Examples A1 to A2.

[0167] Negative electrode preparation:

[0168] Natural graphite, artificial graphite, single-walled carbon nanotubes, acetylene black, CMC, and SBR are stirred and evenly 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 evenly coated on a 6um copper foil current collector by extrusion spraying, and then dried, cold pressed, and cut to obtain a negative electrode sheet.

[0169] Preparation of electrolyte:

[0170] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), organic solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 3 / 7, 12.5% ​​LiPF6 lithium salt was added and dissolved in the organic solvent, and stirred to obtain an electrolyte.

[0171] Isolation film: Polypropylene film is used as the isolation film.

[0172]

Battery assembly

[0173] 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, after packaging, electrolyte injection, formation, and sorting, a cylindrical lithium-ion secondary battery is made, which is a secondary battery cell.

[0174] Performance Testing

[0175] The positive electrode sheets and secondary battery cells of the above embodiments and comparative examples were tested respectively.

[0176] (1) Positive electrode

[0177]

Particle size test

[0178] Positive electrode active material D v 50 test steps:

[0179] Disperse 10 mg of the positive electrode active material in an ethanol solution, activate ultrasonic dispersion, and perform particle size analysis using a Malvern Master 3000 particle size analyzer using laser particle size measurement. After computer processing of the data, the particle size distribution curve for the positive electrode active material is obtained, and the particle size distribution at 50% is defined as Dv50.

[0180]

Voltage platform test

[0181] Voltage plateau calculation: Assemble a coin-shaped half-cell containing the positive electrode active material, a separator, and a lithium metal anode. Charge the cell at 0.33C to 3.8V (LFP) or 4.2V (LMFP). After standing for 10 minutes, discharge the cell at 0.33C until the voltage reaches 1.5V. During the 0.33C discharge, record the discharge energy E and capacity C of the cell. Use the formula V = E / C to calculate the charge and discharge voltage plateau range, which is the operating voltage range of the positive electrode active material.

[0182]

Coating tortuosity test

[0183] Measurement of the tortuosity of the entire positive electrode active layer (electrochemical impedance spectroscopy): The electrode containing the positive electrode active material and the separator are assembled into a soft-pack symmetrical cell. 60uL of 50mM tetrabutylammonium perchlorate electrolyte (wherein the solvent EC:DMC=1:1, lithium ion conductivity is 1.7mS / cm) is added to each cell, and the electrochemical impedance spectroscopy is measured in the frequency range of 200kHz to 50mHz. Using τ / ε=R ion ×S×k int / l can be used to 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, k int is the lithium ion conductivity of the electrolyte, R ion is the lithium ion resistance. ion =3×(R h -R l ), R h is the high-frequency intercept of the impedance spectrum, R l is the low-frequency intercept.

[0184] Comparative measurement of the tortuosity of the first active layer and the second active layer (tortuosity calculation of tomographic SEM electron microscope images): Import the tomographic SEM image into 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.

[0185] (2) Secondary battery cells

[0186] Electrochemical performance test: The secondary battery cells prepared in the above examples 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 at 1 / 3C to 50% SOC, left to stand for 30 minutes, and the voltage at this time was recorded as V0. Then, the cells 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].

[0187] The test results are shown in Table 2.

[0188] Table 1 Positive electrode active layer ratio

[0189] Table 2 Test results

[0190] As shown in Table 2, based on the embodiment of the present application, the positive electrode active layer of the positive electrode sheet includes two layers: a second active layer and a first active layer sequentially disposed on the positive electrode current collector. The operating voltage range of the first positive electrode active material is 3.1-4.0V, which is greater than the operating voltage range of the second positive electrode active material of 2.0-3.0V. Moreover, the overall tortuosity is less than that of the comparative example with only one active layer. This results in a lower DCR internal resistance for the battery of the embodiment of the present application, and a higher end-of-discharge voltage under the same conditions (50% SOC, 25°C, 5KW discharge). In other words, the battery of the embodiment of the present application has a better discharge capacity.

[0191] The positive electrode plate of the battery in Example B2, compared to Example B1, has a conductive coating between the second active layer and the current collector, further improving the battery's discharge capacity. The positive electrode plate of the battery in Example B3, compared to Example B1, omits the conductive agent in the second active layer, and the tortuosity of the two active layers is essentially the same, resulting in a slight reduction in the battery's discharge capacity. The positive electrode plate of the battery in Example B4, compared to Example B1, increases the proportion of the second positive electrode active material, which can extend the discharge capacity in the low-voltage range, thereby improving the battery's discharge capacity. The positive electrode plate of the battery in Example B5, compared to Example B1, reduces the proportion of lithium ferrous silicate in the second active layer, resulting in a decrease in the discharge capacity in the low-voltage range, thereby reducing the battery's discharge capacity. The positive electrode plate of the battery in Example B6, compared to Example B1, uses lithium vanadate in the second active layer. The three-dimensional ion transport channels of lithium vanadate increase the number of ion transport pathways, thereby improving the discharge capacity. The positive electrode plate of the battery in Example B7, compared to Example B1, uses lithium iron manganese phosphate in the first active layer. Lithium iron manganese phosphate has a higher voltage, resulting in a higher terminal voltage of the battery cell and further enhancing the discharge capacity. Compared to Example B1, the positive electrode plate of the battery of Example B8 uses lithium iron phosphate with a smaller particle size in the first active layer, which reduces the lithium ion diffusion path and enhances the discharge capacity. Compared to Example B1, the positive electrode plate of the battery of Example B9 uses lithium manganese iron phosphate with a larger particle size in the first active layer, which increases the lithium ion diffusion path and weakens the discharge capacity. Compared to Example B1, the positive electrode plate of the battery of Example B10 increases the particle size of the active material, which increases the lithium ion diffusion path and weakens the discharge capacity.

[0192] 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 make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions 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, wherein: 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; 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, wherein: 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, wherein: 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, wherein: 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, wherein: 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, wherein: 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, wherein: 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, wherein: 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, wherein: 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, wherein 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, wherein: 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, wherein: Comprising the battery of claim 11.

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