Positive electrode sheet, electrode assembly, battery, and method for manufacturing battery

Abstract

The application relates to a positive electrode sheet, an electrode assembly, a battery and a preparation method of the battery. The positive electrode sheet comprises a positive electrode current collector, a first active material layer on the surface of the positive electrode current collector and a second active material layer on the first active material layer; wherein the first active material layer comprises a first active material, the first active material comprises a lithium iron phosphate core and a first lithium supplement layer covering the lithium iron phosphate core, and the delithiation potential of a first lithium supplement agent in the first lithium supplement layer is less than the delithiation potential of the lithium iron phosphate core; the second active material layer comprises a second active material, the second active material comprises a ternary material core and a second lithium supplement layer and a third lithium supplement layer successively covering the ternary material core from inside to outside, the delithiation potential of a second lithium supplement agent in the second lithium supplement layer is greater than the delithiation potential of the ternary material core, and the delithiation potential of a third lithium supplement agent in the third lithium supplement layer is less than the delithiation potential of the lithium iron phosphate core. In this way, the initial efficiency and the cycle life of the battery can be improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a positive electrode, an electrode assembly, a battery, and a method for preparing the battery. Background Technology

[0002] To meet the demand for long driving range in electric vehicles, high-nickel ternary materials with high theoretical specific capacity are widely used in the cathode of power batteries to improve battery energy density. However, in practical applications, high-nickel ternary materials suffer from two core drawbacks: poor thermal stability and low initial coulombic efficiency.

[0003] Specifically, on the one hand, high-nickel ternary materials have poor crystal structure stability under high temperature or overcharge conditions, and are prone to oxygen release. The released oxygen can quickly trigger exothermic side reactions when it comes into contact with the electrolyte, significantly increasing the risk of battery thermal runaway or even combustion and explosion. On the other hand, during the first charge and discharge process, high-nickel ternary materials suffer a large amount of irreversible lithium loss, resulting in generally low initial coulombic efficiency of the battery. It can also induce side reactions at the positive electrode interface, affecting the cycle life of the battery. Summary of the Invention

[0004] In view of this, the present application provides a positive electrode sheet, an electrode assembly, a battery, and a method for preparing the battery to solve at least one problem existing in the background art.

[0005] In a first aspect, embodiments of this application provide a positive electrode sheet, comprising a positive current collector, a first active material layer located on at least one surface of the positive current collector along its thickness direction, and a second active material layer located on the surface of the first active material layer away from the positive current collector; wherein... The first active material layer includes a first active material, which has a core-shell structure, including a lithium iron phosphate core and a first lithium replenishing layer covering the lithium iron phosphate core. The first lithium replenishing layer includes a first lithium replenishing agent, and the delithiation potential of the first lithium replenishing agent is lower than the delithiation potential of the lithium iron phosphate core. The second active material layer includes a second active material, which has a core-shell structure, comprising a ternary material core and a second lithium replenishing layer and a third lithium replenishing layer that sequentially cover the ternary material core from the inside out. The second lithium replenishing layer includes a second lithium replenishing agent, the delithiation potential of which is greater than that of the ternary material core. The third lithium replenishing layer includes a third lithium replenishing agent, the delithiation potential of which is less than that of the lithium iron phosphate core.

[0006] In conjunction with the first aspect of this application, in an alternative embodiment, The delithiation potential of the first lithium replenishing agent and / or the third lithium replenishing agent is greater than or equal to 3.0V and less than 3.4V; and / or, The delithiation potential of the second lithium replenishing agent is greater than or equal to 4.5V; and / or, The first lithium supplement and / or the third lithium supplement include one or more of lithium ferrite, lithium manganese oxide, lithium molybdenum oxide, lithium vanadium oxide, and lithium nickel oxide; and / or, The second lithium supplement includes one or more of lithium carbonate, lithium oxide, lithium hydroxide, lithium boron oxide, and lithium phosphorus oxide.

[0007] In conjunction with the first aspect of this application, in an alternative embodiment, The thickness ratio of the first lithium replenishment layer to the third lithium replenishment layer is 0.4~0.5; and / or, The thickness of the first lithium replenishment layer is 2nm~5nm; and / or, The thickness of the second lithium replenishment layer is 1 nm to 2 nm; and / or, The thickness of the third lithium replenishment layer is 5nm~10nm.

[0008] In conjunction with the first aspect of this application, in an alternative embodiment, The median particle size ratio of the first active material to the second active material is 1:(2~2.5); and / or, The median particle size of the first active material is 4 μm to 6 μm; and / or, The median particle size of the second active material is 10 μm to 12 μm.

[0009] In conjunction with the first aspect of this application, in an alternative embodiment, The first active material layer further includes a first binder, the first binder comprising carboxyl groups; and / or, The second active material layer also includes a second binder, which includes carboxyl groups.

[0010] Secondly, embodiments of this application provide an electrode assembly, the electrode assembly including a negative electrode sheet, a separator, and a positive electrode sheet as described in any one of the first aspects, wherein the separator is located between the positive electrode sheet and the negative electrode sheet.

[0011] In conjunction with a second aspect of this application, in an optional embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector along the thickness direction. The negative electrode active material layer includes a negative electrode body portion and a negative electrode thinning portion. The negative electrode thinning portion is located on at least one side of the negative electrode body portion along the width direction of the separator, and the thickness of the negative electrode thinning portion is less than that of the negative electrode body portion. The positive electrode further includes a third active material layer, which is located on at least one side of the second active material layer along the width direction of the separator. The thickness of the third active material layer is less than the thickness of the second active material layer. The second active material layer is disposed opposite to the negative electrode body portion, and the third active material layer is disposed opposite to the negative electrode thinning portion. The third active material layer includes the first active material.

[0012] In conjunction with the second aspect of this application, in an alternative embodiment, The dimension of the third active material layer located on one side of the second active material layer in the width direction of the separator is 5% to 15% of the dimension of the positive electrode sheet in the width direction of the separator; and / or, The positive electrode further includes a fourth active material layer, which is located between the second active material layer and the third active material layer, and the fourth active material layer includes the first active material and the second active material.

[0013] Thirdly, embodiments of this application provide a battery including the electrode assembly described in any of the second aspects.

[0014] Fourthly, embodiments of this application provide a method for preparing a battery, the method comprising: The electrode assembly described in any of the second aspects is placed in the housing; After injecting electrolyte into the casing and sealing it, formation is performed to obtain the battery; wherein the formation voltage is greater than or equal to the delithiation potential of the ternary material core and less than the delithiation potential of the second lithium replenishing agent.

[0015] Compared with the prior art, the embodiments of this application have the following beneficial effects: In the positive electrode provided in this application embodiment, the delithiation potentials of the first and third lithium replenishing agents are both lower than the delithiation potential of the lithium iron phosphate core. Under the low voltage conditions at the beginning of the battery's first charge, the first and third lithium replenishing agents can be activated to release lithium ions and participate in the formation of the negative electrode solid electrolyte interphase (SEI) film, thus avoiding irreversible consumption of lithium in the active material during the formation of the SEI film, thereby effectively improving the first coulombic efficiency (first efficiency). The delithiation potential of the second lithium replenisher is greater than that of the ternary material core. Within the normal cycling voltage window, the second lithium replenisher remains inert, allowing the second lithium replenishment layer to establish a stable protective barrier on the surface of the ternary material core, isolating the electrolyte and thus suppressing interfacial side reactions. This improves the phase transition, lattice oxygen evolution, and transition metal ion dissolution problems that easily occur in ternary materials during cycling, thereby ensuring the structural stability of the ternary material core during cycling and reducing the risk of capacity decay and thermal runaway. Moreover, after long-term cycling (in the middle and late stages of cycling), when the positive electrode interface degrades and active lithium is continuously lost, the battery internal resistance increases and the local potential rises. The second lithium replenisher can also be activated, slowly releasing lithium ions to compensate for cycle decay, thereby significantly improving the battery's cycle life. In this embodiment, the first active material layer disposed between the positive electrode current collector and the second active material layer has the following advantages: First, during discharge, the second active material in the second active material layer works first, followed by the first active material in the first active material layer, which can form a spontaneous voltage plateau compensation effect, making the mid-range discharge voltage of the battery more stable and the energy output efficiency higher. Second, the lithium iron phosphate core has high cycle stability, and the first active material layer has small volume changes during cycling, which can provide stable mechanical support and stress buffering for the relatively large volume expansion of the second active material layer. By absorbing the lattice stress generated by the ternary material core during lithium insertion / extraction, the pulverization of the overall electrode structure can be reduced, thereby improving the cycle life of the battery. That is, the positive electrode provided in this embodiment, through the synergistic cooperation of the first and second active material layers, can significantly improve the battery's initial efficiency and cycle life while ensuring the battery's energy density and safety performance.

[0016] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0017] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a schematic diagram of the structure of the first active material in the positive electrode sheet provided in the embodiments of this application; Figure 2 This is a schematic diagram of the structure of the second active material in the positive electrode sheet provided in the embodiments of this application; Figure 3 This is a schematic diagram of the combination of a positive electrode, a separator, and a negative electrode in an electrode assembly provided in an embodiment of this application; Figure 4 This is a schematic flowchart illustrating the battery fabrication method provided in the embodiments of this application. Detailed Implementation

[0018] To make the technical solution and beneficial effects of the present invention more apparent and understandable, a detailed description is provided below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed under conventional experimental conditions. Unless otherwise specified, all reagents and raw materials used in this invention are commercially available.

[0019] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of this application. However, it will be apparent to those skilled in the art that this application can be practiced without one or more of these details. In other instances, to avoid confusion with this application, some technical features well-known in the art have not been described; that is, not all features of actual embodiments are described herein, nor are well-known functions and steps described in detail.

[0020] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. When used herein, the singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprising” and / or “including,” when used in this specification, identify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. When used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0021] To fully understand this application, detailed steps and structures will be presented in the following description to illustrate the technical solution of this application. Preferred embodiments of this application are described in detail below; however, in addition to these detailed descriptions, this application may have other implementation methods.

[0022] Unless otherwise defined, the technical and scientific terms used in this application have the same meanings as those in the technical and scientific field to which this application pertains.

[0023] Unless otherwise specified, the techniques or conditions described in the following embodiments are generally performed in accordance with conventional techniques or conditions described in the literature in this field, or in accordance with the product manual and the manufacturer's recommendations. All numerical ranges in the following embodiments include endpoint values.

[0024] In related technologies, one technical approach for cathode materials is to use lithium iron phosphate (LFP) as the active cathode material, which offers high safety. While LFP cathodes boast excellent thermal safety and long cycle life, their energy density (especially volumetric energy density) and voltage platform are significantly lower than those of high-nickel ternary materials, failing to meet the high range requirements of high-end electric vehicles and presenting a clear technical bottleneck. To achieve higher energy density, another technical approach is to use high-nickel ternary materials with high specific capacity as the active cathode material. To overcome the inherent structural and thermal instability, lattice oxygen evolution, and transition metal ion dissolution issues of high-nickel ternary materials, modification is typically employed. Specifically, this includes coating the surface of the high-nickel ternary material with an inert coating layer (such as Al2O3 or TiO2). While this can suppress interfacial side reactions and transition metal ion dissolution to some extent, the inert coating layer hinders lithium-ion diffusion, leading to a decrease in battery rate performance and failing to address the low thermal safety and low initial efficiency of the high-nickel ternary material itself. Alternatively, bulk doping of high-nickel ternary materials (e.g., doping with Al, Mg, etc.) can enhance their structural stability, but this offers limited improvement to the material's thermal safety and is complex, failing to address the low initial efficiency issue. Another approach is to add lithium replenishing agents to the cathode material to improve initial efficiency, but conventional high-voltage lithium replenishing agents (e.g., Li₂CO₃) do not react at the battery's normal operating voltage, while conventional medium-voltage lithium replenishing agents (e.g., Li₅FeO₄) cannot compensate for lithium loss in the later stages of cycling. More importantly, simply adding lithium replenishing agents cannot solve the high risk of thermal runaway in high-nickel ternary materials; that is, "high energy density" and "high safety" cannot be simultaneously achieved.

[0025] Based on this, embodiments of this application provide a positive electrode sheet, which includes a positive current collector, a first active material layer located on at least one side surface of the positive current collector along its thickness direction, and a second active material layer located on the side surface of the first active material layer away from the positive current collector; wherein, the first active material layer includes a first active material, please refer to Figure 1 The first active material has a core-shell structure, comprising a lithium iron phosphate core 100 and a first lithium replenishing layer 110 coating the lithium iron phosphate core. The first lithium replenishing layer 110 includes a first lithium replenishing agent, the delithiation potential of which is lower than that of the lithium iron phosphate core 100. The second active material layer comprises a second active material. Please refer to [reference needed]. Figure 2The second active material has a core-shell structure, including a ternary material core 200 and a second lithium replenishing layer 210 and a third lithium replenishing layer 220 that sequentially cover the ternary material core 200 from the inside out. The second lithium replenishing layer 210 includes a second lithium replenishing agent, the delithiation potential of which is greater than that of the ternary material core 200. The third lithium replenishing layer 220 includes a third lithium replenishing agent, the delithiation potential of which is less than that of the lithium iron phosphate core 100.

[0026] In this embodiment, the delithiation potentials of the first and third lithium replenishing agents are both lower than the delithiation potential of the lithium iron phosphate core 100. Under the low voltage conditions at the beginning of the battery's first charge, the first and third lithium replenishing agents can be activated to release lithium ions and participate in the formation of the negative electrode SEI film. This avoids irreversible consumption of lithium in the active materials (lithium iron phosphate core 100 and ternary material core 200) during the formation of the SEI film, thereby effectively improving the first-cycle efficiency. The delithiation potential of the second lithium replenisher is greater than that of the ternary material core 200. Within the normal cycling voltage window, the second lithium replenisher remains inert, allowing the second lithium replenishment layer 210 to establish a stable protective barrier on the surface of the ternary material core 200, isolating the electrolyte and thus suppressing interfacial side reactions. This improves the phase transition, lattice oxygen evolution, and transition metal ion dissolution problems that easily occur in ternary materials during cycling, thereby ensuring the structural stability of the ternary material core 200 during cycling and reducing the risk of capacity decay and thermal runaway. Moreover, after long-term cycling (in the middle and late stages of cycling), when the positive electrode interface degrades and active lithium is continuously lost, the battery internal resistance increases and the local potential rises. The second lithium replenisher can also be activated, slowly releasing lithium ions to compensate for cycle decay, thereby significantly improving the cycle life of the battery. In this embodiment, the first active material layer disposed between the positive electrode current collector and the second active material layer has the following advantages: First, during discharge, the second active material in the second active material layer works first, and the first active material in the first active material layer works later, which can form a spontaneous voltage plateau compensation effect, making the mid-range discharge voltage of the battery more stable and the energy output efficiency higher; Second, the lithium iron phosphate core 100 has high cycle stability (lithium iron phosphate has low surface chemical activity, weak side reaction with electrolyte, and the material itself has an olivine structure, which is structurally stable and has small volume change), and the first active material layer has small volume change during cycling, which can provide stable mechanical support and stress buffering for the second active material layer with relatively large volume expansion. By absorbing the lattice stress generated by the ternary material core 200 during lithium insertion and extraction, the pulverization of the overall electrode structure can be reduced, thereby improving the cycle life of the battery; that is, the positive electrode provided in this embodiment, through the synergistic cooperation of the first active material layer and the second active material layer, can significantly improve the battery's first efficiency and cycle life while ensuring the battery's energy density and safety performance.

[0027] It should be noted that in the positive electrode sheet of this application, the first active material layer and the second active material layer can be sequentially stacked on one side surface of the positive electrode current collector along the thickness direction, or they can be sequentially stacked on both sides surface of the positive electrode current collector along the thickness direction. The positive electrode current collector can be, for example, a commonly used positive electrode current collector in the art, such as aluminum foil, and this application does not impose any specific limitations.

[0028] In this embodiment, the first active material includes a lithium iron phosphate core 100 and a first lithium replenishment layer 110 covering the lithium iron phosphate core 100. The first lithium replenishment layer 110 can completely cover the surface of the lithium iron phosphate core 100. Here, "completely covered" can be understood as the first lithium replenishment layer 110 completely covering the surface of the lithium iron phosphate core 100 within the allowable error range of the coating process. The second active material includes a ternary material core 200 and a second lithium replenishment layer 210 and a third lithium replenishment layer 220 sequentially covering the ternary material core 200 from the inside out. That is, the second lithium replenishment layer 210 covers the ternary material core 200, and the third lithium replenishment layer 220 covers the second lithium replenishment layer 210. In other words, the second lithium replenishment layer 210 is located between the ternary material core 200 and the third lithium replenishment layer 220. Similarly, within the allowable error range of the coating process, the second lithium replenishment layer 210 can completely cover the ternary material core 200, and the third lithium replenishment layer 220 can completely cover the second lithium replenishment layer 210.

[0029] In some embodiments, the delithiation potential of the first and / or third lithium replenishing agents is greater than or equal to 3.0V and less than 3.4V. This voltage range is lower than the delithiation potential of the lithium iron phosphate core 100, thus ensuring that the first and third lithium replenishing agents are activated under the low voltage conditions during the initial charging of the battery, releasing lithium ions to participate in the formation of the negative electrode SEI film. At the same time, it effectively avoids irreversible consumption of lithium in the lithium iron phosphate core 100 during the formation of the SEI film, thereby effectively improving the battery's initial efficiency.

[0030] For example, the first lithium supplement and / or the third lithium supplement may include one or more of lithium ferrite (Li5FeO4, LFO), lithium manganese oxide (Li2MnO3), lithium molybdate (Li6MoO4), lithium vanadate (Li3VO4), and lithium nickel oxide (Li2NiO2). Specifically, the first lithium supplement and / or the third lithium supplement may be one or more of lithium ferrite, lithium manganese oxide, lithium molybdate, lithium vanadate, and lithium nickel oxide.

[0031] The first lithium replenishing agent and the third lithium replenishing agent can be the same or different. In one specific embodiment, the first lithium replenishing agent and the third lithium replenishing agent are the same. This helps to optimize the interfacial contact between the first active material layer and the second active material layer, enhances their synergistic effect, and better ensures the lithium replenishment effect.

[0032] In a preferred embodiment, both the first and third lithium replenishing agents are Li5FeO4. Li5FeO4 can provide a large amount of active lithium, has high lithium replenishment efficiency, and has relatively good tolerance to air and humidity, making it easy to handle. The reaction products (Fe-O compounds) are stable and harmless.

[0033] In some embodiments, the delithiation potential of the second lithium replenisher is greater than or equal to 4.5V. This ensures that the delithiation potential of the second lithium replenisher is greater than that of the ternary material core 200, allowing the second lithium replenisher to remain inert within the normal cycle voltage window of the battery. The second lithium replenishment layer 210 can establish a stable protective barrier on the surface of the ternary material core 200, isolating the electrolyte and thus suppressing interfacial side reactions. This improves the phase transition, lattice oxygen evolution, and transition metal ion dissolution problems that easily occur in ternary materials during cycling, thereby ensuring the structural stability of the ternary material core 200 during cycling and reducing the risk of capacity decay and thermal runaway.

[0034] For example, the second lithium supplement may include one or more of lithium carbonate (Li2CO3), lithium oxide (Li2O), lithium hydroxide (LiOH), lithium boron oxide (LBO), and lithium phosphorus oxide (LPO). LBO can be, for example, LiPO3. Specifically, the second lithium supplement may be one or more of lithium carbonate, lithium oxide, lithium hydroxide, lithium boron oxide, and lithium phosphorus oxide.

[0035] In a preferred embodiment, the second lithium replenishing agent is Li₂CO₃. Because trace amounts of Li₂CO₃ remain on the surface of the ternary material core 200, using Li₂CO₃ as the second lithium replenishing agent enhances the interfacial compatibility between the second lithium replenishing layer 210 and the ternary material core 200, reducing interfacial side reactions and resulting in a longer cycle life and higher initial efficiency. Furthermore, Li₂CO₃ can consume residual alkaline substances (such as LiOH) on the surface of the ternary material core 200, thereby reducing side reactions between alkaline substances and the electrolyte, preventing electrolyte and active lithium loss, stabilizing the SEI film, extending battery cycle life, reducing gas generation, lowering internal battery pressure, and improving battery safety.

[0036] The following section will further explain the lithium replenishment process using Li5FeO4 as the first and third lithium replenishing agents and Li2CO3 as the second lithium replenishing agent.

[0037] In the initial stage of the battery's first charge (when the voltage rises above 3.0V, but before reaching the delithiation voltage plateau of the lithium iron phosphate core 100 and the ternary material core 200), the first and third lithium replenishing agents (Li5FeO4) undergo decomposition reactions. The decomposition reaction of Li5FeO4 proceeds in steps, with the first step being Li5FeO4 → 2Li + +2e - +Li3FeO4, i.e., Li5FeO4, releases approximately 2 Li atoms. + The Fe-O framework undergoes a preliminary rearrangement during the initial decomposition of Li3FeO4 into a pseudo-cubic intermediate phase, accompanied by the oxidation of Fe and O. This lays the structural foundation for subsequent nanoporization. The second decomposition reaction is Li3FeO4 → 2Li + +2e - The process of Li3FeO4 undergoing further delithiation and structural reorganization, resulting in the release of O2 and its conversion into LiFeO2, involves the release of a large amount of lithium along with O2, leaving numerous vacancies and defects in the Fe-O framework. The Li released from Li5FeO4... + Migrating to the negative electrode, it preferentially participates in the formation of the SEI film, compensating for some of the unavoidable lithium loss required for SEI film formation, thus effectively improving the battery's initial efficiency. Within the normal charging voltage range (<4.5V), the secondary lithium replenisher (Li2CO3) remains essentially inert and does not undergo decomposition. During long-term cycling / overcharging, as cycling progresses, the battery's internal resistance increases, and the local potential rises. When the potential abnormally rises to the decomposition threshold of the secondary lithium replenisher, it undergoes a decomposition reaction: Li2CO3 → 2Li + +CO2 + 1 / 2O2 + 2e - This process slowly releases lithium ions, compensating for cycle degradation and significantly improving battery cycle life. The delithiation potential of Li₂CO₃ is above 4.8V. It should be noted that all potentials in this application refer to potentials relative to Li₂CO₃. + / Li reference electrode.

[0038] It is understandable that in the second active material, the third lithium replenishment layer 220 is located on the outermost layer of the ternary material core 200. Therefore, the transport paths of lithium ions and electrons released by the third lithium replenishment agent in the third lithium replenishment layer 220 are completely unobstructed, ensuring the maximization of lithium replenishment efficiency. Moreover, the third lithium replenishment agent is in direct contact with the electrolyte, and when it reacts, lattice oxygen is generated, resulting in defect sites. At the same time, the third lithium replenishment agent is a nanoscale coating, and in the actual coating process, it also undergoes heat treatment at a relatively low temperature. Therefore, the final product after the decomposition of the third lithium replenishment agent will form an "amorphous / nanocrystalline iron-based oxide network". For example, the Li5FeO4 generated in the second decomposition reaction can be further transformed into amorphous / nanocrystalline Fe-O3 under complete delithiation conditions. x In this network, the Fe oxidation state in the product is between +3 and +2, forming a non-stoichiometric oxide, denoted as Fe-O. x It is mainly composed of Fe2O3 and residual LiFeO2. During the delithiation process, the crystal structure of Li5FeO4 collapses and reorganizes, and its original micron-sized particles break down into fine particles with a size of about 10 nanometers. This nanostructuring process naturally introduces a large number of grain boundaries and interfaces, thereby forming abundant atomic-level channels and vacancies, i.e., forming porous Fe-O. x The network has abundant nanoscale channels and vacancies, Li + Rapid transitions along the pore surface or via vacancy mechanisms are possible for Li + It provides a continuous, high-speed migration channel. The metal oxide (Fe-O) formed after the decomposition of Li5FeO4 x The lithium-ion layer (Li5FeO4) itself is a good ionic conductor interface layer. Located between the second lithium-ion layer 210 and the electrolyte, it can serve as a functionally graded transition layer. On the one hand, it prevents the electrolyte from directly contacting the ion-phobic second lithium-ion layer 210; on the other hand, it optimizes the interface environment for lithium ion migration from the outside to the inside of the ternary material core 200, which helps reduce the overall interface impedance. If the third lithium-ion layer 220 is placed between the ternary material core 200 and the second lithium-ion layer 210, although the third lithium-ion agent (Li5FeO4) will also generate porous Fe-O upon decomposition... x The network, however, the dense outer second lithium replenishment layer 210 restricts the free expansion and structural recombination of the decomposition products, hindering or isolating the third lithium replenishment decomposition products from forming a continuous and ideal porous network.

[0039] The lithium iron phosphate (LFP) core 100 inherently has a high initial efficiency, with only a small portion (approximately 5%) of the lithium ions extracted from it being consumed in reactions such as SEI film formation, resulting in minimal irreversible capacity loss. In contrast, the ternary material core 200 inherently has a lower initial efficiency, with a higher proportion (approximately 12%–15%) of lithium ions extracted from it failing to return to the positive electrode after the first charge and being permanently consumed. Furthermore, the LFP core 100 possesses a stable olivine structure, exhibiting low surface activity and weaker side reactions with the electrolyte. The ternary material core 200, however, is unstable, prone to oxygen evolution, transition metal ion dissolution, and severe oxidative decomposition reactions with the electrolyte. Therefore, a thicker lithium replenishment layer can be deposited on the surface of the ternary material core 200, not only for lithium replenishment but also to utilize the reaction products of the lithium replenishment agent to construct a stable protective interface. Therefore, in some embodiments, the thickness ratio of the first lithium replenishment layer 110 to the third lithium replenishment layer 220 can be 0.4 to 0.5, for example, it can be 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, or any value within any two of the above ranges. This allows for effective lithium replenishment while better ensuring the stability of the ternary material core 200, promoting its capacity utilization, and thus further improving battery performance.

[0040] When the thickness of the first lithium replenishment layer 110 is too thin, its lithium replenishment effect is limited, resulting in limited improvement in initial efficiency. When the thickness of the first lithium replenishment layer 110 is too thick, it will affect the kinetic performance of the cathode and increase the Li-to-Li ratio. + The migration barrier leads to an increase in battery internal resistance and a decrease in rate performance. Therefore, in some embodiments, the thickness of the first lithium replenishment layer 110 can be 2nm to 5nm, for example, 2nm, 3nm, 4nm, 5nm, or any value between any two of the above ranges. This is beneficial for balancing the lithium replenishment effect and kinetic performance of the positive electrode, thereby achieving a better balance between the battery's initial efficiency and rate performance.

[0041] The second lithium replenishment layer 210 possesses high chemical stability and density, forming a protective layer on the surface of the ternary material core 200 to suppress interfacial side reactions. It can also be activated under abnormal conditions or at high potentials in the later stages of cycling, providing a small amount of lithium ions to "repair" cycle losses. When the thickness of the second lithium replenishment layer 210 is too thin, it hinders the formation of a continuous and dense film, weakening the electrolyte isolation effect and limiting the suppression of interfacial side reactions and further replenishment of lithium ions, thus offering limited improvement to battery cycle life. Conversely, when the thickness of the second lithium replenishment layer 210 is too thick, it can impede lithium ion transport to some extent, leading to increased charge transfer impedance and increased polarization. Therefore, in some embodiments, the thickness of the second lithium replenishment layer 210 can be 1 nm to 2 nm, for example, 1 nm, 1.1 nm, 1.2 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.7 nm, 1.8 nm, 1.9 nm, 2 nm, or any value between any two of the above ranges. This is beneficial for effectively protecting the ternary material core 200 while reducing the polarization of the positive electrode.

[0042] When the thickness of the third lithium replenishment layer 220 is too thin, its lithium replenishment effect is limited, resulting in limited improvement in initial efficiency. When the thickness of the third lithium replenishment layer 220 is too thick, it affects the kinetic performance of the cathode and the energy density of the battery; simultaneously, an excessively thick metal oxide interface layer formed after the decomposition of the third lithium replenisher leads to increased impedance. Therefore, in some embodiments, the thickness of the third lithium replenishment layer 220 can be 5nm to 10nm, for example, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, or any value between any two of the above ranges. This is beneficial for balancing the lithium replenishment effect and kinetic performance of the cathode, thereby achieving a better balance between the battery's initial efficiency and rate performance.

[0043] In the actual preparation process, the first lithium replenishment layer 110, the second lithium replenishment layer 210 and the third lithium replenishment layer 220 can be formed by atomic layer deposition. In this way, the number of deposition layers can be controlled more precisely by controlling the number of deposition cycles, that is, the thickness of the first lithium replenishment layer 110, the second lithium replenishment layer 210 and the third lithium replenishment layer 220 can be controlled more precisely.

[0044] In this embodiment, the median particle size of the first active material can be smaller than that of the second active material. This allows the smaller particles of the first active material in the lower layer (closer to the positive electrode current collector) to achieve high-density packing and strong adhesion, enabling the first active material layer to provide a solid substrate for the upper second active material layer and ensure support. Meanwhile, the larger particles of the upper second active material can achieve high capacity. If the particle sizes of the first and second active materials are similar, this functional division becomes blurred. During the coating and drying process of the positive electrode sheet, particles with similar sizes are more prone to interlayer mixing, affecting the interfacial properties of the first and second active material layers. If the particle size of the second active material is too large, resulting in a small median particle size ratio between the first and second active materials, the underlying first active material layer cannot provide uniform and stable support for the second active material layer under the rolling and cyclic mechanical stress during the preparation of the cathode. This leads to localized stress concentration, adhesion failure, and even delamination in the second active material layer. Furthermore, a large difference in particle size between the first and second active materials also hinders the formation of a tight bond at the interface, easily causing unconnected pores within the cathode, thus affecting electrolyte wetting and uniform lithium-ion transport. Therefore, in some embodiments, the median particle size ratio between the first and second active materials can be 1:(2~2.5), for example, 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, or any value within any two of these ranges. This is beneficial in ensuring the structural stability of the cathode while also considering electrolyte wetting and uniform lithium-ion transport, thus improving the synergistic effect of the first and second active material layers.

[0045] When the median particle size of the first active material is too large, the solid-state diffusion path of lithium ions within the particles is too long, leading to a decrease in the battery's rate performance and low-temperature performance. Furthermore, large particles are difficult to pack tightly during rolling, resulting in a lower compaction density of the positive electrode sheet and affecting its volumetric energy density. When the median particle size of the first active material is too small, the high specific surface area exacerbates side reactions between the first active material and the electrolyte. It also requires more binder during the preparation of the positive electrode slurry, resulting in high slurry viscosity, difficult coating, and a reduced proportion of active material, affecting the battery's energy density. Additionally, excessively small particles have high surface energy, making them prone to detaching from the positive electrode current collector during long-term cycling, especially at high temperatures, affecting cycle stability. Therefore, in some embodiments, the median particle size of the first active material can be 4μm to 6μm, for example, 4μm, 5μm, 6μm, or any value within any two of these ranges. This helps to balance the battery's rate performance, low-temperature performance, energy density, and cycle stability, while reducing the difficulty of the manufacturing process.

[0046] When the median particle size of the second active material is too large, the lattice stress of the ternary material core 200 is high during deep delithiation, and microcracks are more likely to form inside the large particles, leading to continuous exposure of fresh interfaces, intensified electrolyte erosion, and degradation of cycle performance. Simultaneously, the excessively long solid-state diffusion path of lithium ions within the particles results in a decrease in the battery's rate performance and low-temperature performance. When the median particle size of the second active material is too small, the excessively high specific surface area significantly catalyzes the oxidative decomposition of the electrolyte under high voltage, causing battery gas production and swelling, leading to safety and lifespan issues. Furthermore, small particles have low packing density and reduced compaction density, requiring more inactive material coating, which reduces the volumetric energy density of the positive electrode. Therefore, in some embodiments, the median particle size of the second active material can be 10μm to 12μm, for example, 10μm, 11μm, 12μm, or any value between any two of the above ranges. This is beneficial for balancing the battery's cycle performance, rate performance, low-temperature performance, energy density, and safety performance.

[0047] In some embodiments, the first active material layer further includes a first adhesive; the second active material layer further includes a second adhesive.

[0048] For example, the first adhesive and the second adhesive may each be independently selected from one or more of polyacrylic acid (PAA), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and polyvinylidene fluoride (PVDF).

[0049] Preferably, the first binder and / or the second binder may include carboxyl groups (-COOH). Thus, the carboxyl groups on the first binder and / or the second binder can form hydrogen bonds and / or ionic bonds with the lithium supplement. For example, the carboxyl groups can form strong hydrogen bonds with carbonate ions in Li2CO3 (the second lithium supplement), and the highly electronegative oxygen atoms in the carboxyl groups can form ionic bonds with lithium ions present on the surface of Li5FeO4 (the first and second lithium supplements), and can also form hydrogen bonds with oxygen atoms or hydroxyl groups on the surface of Li5FeO4. This better ensures the stability of the bonding between the materials inside the first and second active material layers and the stability of the connection between the two layers, thereby improving the cycle stability of the positive electrode.

[0050] In a preferred embodiment, the first adhesive and the second adhesive may be PAA.

[0051] Firstly, PAA molecules are rich in carboxyl groups, which, as mentioned above, can improve the cycle stability of the positive electrode. Secondly, PAA has better alkali resistance than other binders (such as CMC). Therefore, PAA is more tolerant to the alkaline environment of residual LiOH on the surface of the ternary material core 200 and the second lithium replenishment layer 210 containing Li2CO3, and is less prone to molecular chain breakage or cross-linking leading to slurry gelation. Thirdly, PAA has good dispersibility, which is conducive to the uniform dispersion of active materials and other materials with high solid content in the slurry, forming a stable three-dimensional network structure and preventing sedimentation. This is crucial for ensuring the uniformity of the electrode in double-layer coating or multi-material systems. Fourthly, PAA has appropriate flexibility, which is conducive to forming a film layer with high strength and certain toughness. This allows it to absorb the stress generated by the volume change of the active material during charging and discharging, maintaining the integrity of the electrode structure.

[0052] In some embodiments, the first active material layer further includes a first conductive agent; the second active material layer further includes a second conductive agent.

[0053] For example, the first conductive agent and the second conductive agent may each be independently selected from one or more of conductive carbon black, carbon nanotubes, graphene, and carbon fiber.

[0054] It is understandable that when the mass ratio of the first active material to the first binder is too high, the amount of the first binder added is too small, making it difficult to ensure sufficient coating of the first active material and the formation of a strong adhesive network. This leads to a decrease in electrode peel strength, and during subsequent coating of the second active material layer, rolling, and cycling, the first active material layer is prone to poor adhesion to the current collector or even peeling off from it. Insufficient binder also increases the brittleness of the electrode, making it prone to cracking during rolling. Furthermore, the first binder also serves to fix the first conductive agent; insufficient binder addition leads to uneven dispersion of the first conductive agent and an increase in local impedance of the electrode. Conversely, when the mass ratio of the first active material to the first binder is too small, excessive binder addition reduces the proportion of the first active material, affecting the volumetric energy density and capacity of the positive electrode. Simultaneously, excessive binder addition hinders the transport of lithium ions and electrons, leading to increased internal resistance and affecting rate performance. Therefore, in some embodiments, the mass ratio of the first active material to the first binder in the first active material layer can be (95~96):(2.5~3), for example, it can be 95:2.5, 95:3, 95.5:2.5, 95.5:3, 96:2.5, 96:3, or any value between any two of the above ranges. This helps to ensure the bonding strength between the first active material layer and the current collector, promotes the uniform dispersion of the first conductive agent, and simultaneously takes into account the energy density and rate performance of the positive electrode.

[0055] When the mass ratio of the second active material to the second binder is too high, the amount of second binder added is too small, making it difficult to ensure sufficient coating of the second active material and the formation of a strong bonding network. The ternary core material 200 in the second active material (such as lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide) undergoes significant volume changes during charge and discharge. If the amount of second binder added is insufficient, the particles of the second active material are prone to losing their connections, and the second binder cannot effectively buffer the stress of the second active material, leading to electrode pulverization and capacity decay. Conversely, when the mass ratio of the second active material to the second binder is too low, the amount of second binder added is excessive, resulting in a lower proportion of the second active material. This affects the volumetric energy density and capacity of the positive electrode. Furthermore, excessive second binder addition hinders the transport of lithium ions and electrons, leading to increased internal resistance and affecting rate performance. Therefore, in some embodiments, the mass ratio of the second active material to the second binder in the second active material layer can be (96~97):(1.5~2), for example, 96:1.5, 96:2, 96.5:1.5, 96.5:2, 97:1.5, 97:2, or any value between any two of the above ranges. This helps to ensure the structural stability of the second active material layer while also taking into account the energy density and rate performance of the positive electrode.

[0056] This application also provides an electrode assembly, which includes a negative electrode, a separator, and a positive electrode as described in any of the foregoing embodiments, with the separator located between the positive electrode and the negative electrode.

[0057] It is understood that the beneficial effects of the positive electrode sheet described in any of the foregoing embodiments are also applicable to the electrode assembly in the embodiments of this application. The electrode assembly in the embodiments of this application can be a wound electrode assembly or a stacked electrode assembly.

[0058] Figure 3 This is a schematic diagram illustrating the combination of a positive electrode, a separator, and a negative electrode in an electrode assembly provided in an embodiment of this application. The separator 300 is disposed between the positive electrode 100 and the negative electrode 200, serving to prevent short circuits between them while allowing active ions to pass through. Exemplarily, the separator 300 can be a single-layer polyethylene membrane or a polypropylene membrane, or it can be a composite separator 300, such as a polyethylene membrane or polypropylene membrane coated with a functional layer to improve the overall performance of the separator 300. The functional layer can be, for example, a ceramic coating or a polymer coating.

[0059] Please refer to Figure 3The negative electrode 200 includes a negative electrode current collector 210 and a negative electrode active material layer 220 located on at least one side surface of the negative electrode current collector 210 along the thickness direction. The negative electrode active material layer 220 includes a negative electrode main body portion 221 and a negative electrode thinning portion 222. The negative electrode thinning portion 222 is located on at least one side of the negative electrode main body portion 221 along the width direction of the separator 300, and the thickness of the negative electrode thinning portion 222 is less than that of the negative electrode main body portion 221. The positive electrode 100 includes a positive electrode current collector 110 and a first active material layer 121 located on at least one side surface of the positive electrode current collector 110 along the thickness direction, and a second active material layer 122 located on the side surface of the first active material layer 121 away from the positive electrode current collector 110.

[0060] In some embodiments, please refer to Figure 3 The positive electrode 100 may further include a third active material layer 123, which is located on at least one side of the second active material layer 122 along the width direction of the separator 300. The thickness of the third active material layer 123 is less than the thickness of the second active material layer 122. The second active material layer 122 is disposed opposite to the negative electrode body portion 221, and the third active material layer 123 is disposed opposite to the negative electrode thinning portion 222. The third active material layer 123 includes a first active material.

[0061] Please refer to Figure 3 The third active material layer 123 can be located on the surface of the first active material layer 121 away from the positive current collector 110, that is, it can be disposed in the same layer as the second active material layer 122. Of course, this application does not exclude the possibility that the third active material layer 123 is located on the positive current collector 110. For example, one side of the third active material layer 123 in the width direction of the separator 300 may be adjacent to one side of both the first active material layer 121 and the second active material layer 122 in the width direction of the separator 300.

[0062] In this embodiment, the second active material layer 122 is disposed opposite to the negative electrode body 221, and the central region of the positive electrode 100 carries the main capacity of the battery. Thus, the high specific capacity of the ternary material core in the second active material can be fully utilized, thereby ensuring the energy density of the battery. The third active material layer 123 is disposed opposite to the negative electrode thinning portion 222, which has the following advantages: First, the first active material in the third active material layer 123 has excellent thermal and structural stability, which can fundamentally suppress thermal runaway caused by rapid heat dissipation and high current density in the edge region of the positive electrode 100. Simultaneously, the intrinsic high-rate performance of the lithium iron phosphate core in the first active material is very suitable for rapid charging and discharging in the edge region. Second, the operating voltage platform of the lithium iron phosphate core in the first active material is much lower than the operating voltage platform of the ternary material core in the second active material. When the battery voltage reaches the cutoff voltage, the lithium iron phosphate core in the third active material layer 123 at the edge of the positive electrode 100 is already in a fully charged state, while the ternary material core in the second active material layer 122 in the middle is still working. This is equivalent to automatically reducing the local current in the edge region of the electrode, thereby greatly reducing the risk of lithium plating at the thinned part 222 of the negative electrode. Thirdly, using the first active material in the third active material layer 123 can save costs compared to using the second active material, because the first active material only covers the first lithium replenishment layer, eliminating the need for another lithium replenishment layer.

[0063] It should be noted that the thickness of the third active material layer 123 can be the same as or different from the thickness of the second active material layer 122. For some specific embodiments, please refer to... Figure 3 The thickness of the third active material layer 123 can be less than the thickness of the second active material layer 1222. That is, the third active material layer 123 serves as the thinned portion of the positive electrode, and the second active material layer 122 serves as the main body of the positive electrode. In this way, lithium deposition at the thinned portion 222 of the negative electrode edge can be better suppressed.

[0064] It should also be noted that, Figure 3 The illustration only shows, by way of example, the third active material layer 123 located on one side of the second active material layer 122 along the width direction of the diaphragm 300. In some other embodiments of this application, the third active material layer 123 may be located on both sides of the second active material layer 122 along the width direction of the diaphragm 300.

[0065] It is understood that the proportion of the third active material layer 123 in the positive electrode 100 needs to reach a certain range to effectively perform its functions. However, the capacity of the first active material is less than that of the second active material. Therefore, if the dimension of the third active material layer 123 in the width direction of the separator 300 is too large compared to the dimension of the positive electrode 100 in the width direction of the separator 300, it will lead to a decrease in the battery's capacity and energy density. Therefore, in some embodiments, the proportion of the dimension of the third active material layer 123 located on one side of the second active material layer 122 in the width direction of the separator 300 to the dimension of the positive electrode 100 in the width direction of the separator 300 can be 5% to 15%, for example, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or any value within any two of the above ranges. This is beneficial in ensuring the effective functioning of the third active material layer 123 while maintaining the battery's capacity and energy density.

[0066] In some embodiments, please refer to Figure 3 The positive electrode 100 may further include a fourth active material layer 124, which is located between the second active material layer 122 and the third active material layer 123, and includes a first active material and a second active material.

[0067] Please refer to Figure 3 The fourth active material layer 124 can be located on the surface of the first active material layer 121 away from the positive electrode current collector 110, i.e., it can be disposed in the same layer as the second active material layer 122. In this embodiment, the fourth active material layer 124 can serve as a transition region between the second active material layer 122 and the third active material layer 123, mitigating the abrupt change in interface impedance caused by the different active materials of the second active material layer 122 and the third active material layer 123, and ensuring the continuity of lithium-ion and electronic conductivity. The thickness of the fourth active material layer 124 can be the same as the thickness of the second active material layer 122, i.e., it can be part of the positive electrode body, thus better ensuring the capacity utilization of the positive electrode sheet 100. Of course, this application does not exclude the possibility that the thickness of the fourth active material layer is less than the thickness of the second active material layer, i.e., the fourth active material layer can be part of the thinned portion of the positive electrode.

[0068] In some specific embodiments, the mass ratio of the first active material and the second active material in the fourth active material layer 124 can be (3:1) to (1:3), for example, 3:1, 3:2, 1:1, 1:2, 1:3, or any value within any two of the above ranges. This helps to ensure that the fourth active material layer 124 better mitigates the abrupt change in interfacial impedance between the second active material layer 122 and the third active material layer 123 caused by the different active materials.

[0069] For the negative electrode sheet in the electrode assembly, the negative electrode current collector can be a copper foil or other negative electrode current collector well known to those skilled in the art. The negative electrode active material in the negative electrode active material layer can be at least one of graphite, hard carbon, soft carbon, and silicon-based materials.

[0070] In some embodiments, the method for preparing an electrode assembly may include: firstly, stacking a positive electrode, a separator, and a negative electrode, wherein the separator is placed between the positive and negative electrode, and then preparing an electrode assembly by winding or stacking.

[0071] The preparation methods for the positive and negative electrode sheets mainly include: first, mixing the corresponding active materials, binders, and conductive agents in a solvent to prepare a coating slurry; then, coating the corresponding slurry onto a current collector; and finally, drying and rolling to obtain the positive and negative electrode sheets. For cases where the positive electrode sheet also includes a third and fourth active material layer, a zoned coating process can be used, coating different areas of the first active material layer with the corresponding slurry to form the second, third, and fourth active material layers.

[0072] Since the preparation of the slurry and the coating process are both conventional preparation processes in this field, the specific steps will not be described here.

[0073] This application also provides a battery that includes the electrode assembly described in any of the foregoing embodiments.

[0074] It is understood that the beneficial effects of the electrode assembly described in any of the foregoing embodiments are also applicable to the battery of the embodiments of this application.

[0075] This application also provides a method for preparing a battery, please refer to... Figure 4 The battery manufacturing method includes the following steps: S1: Place the electrode assembly described in any of the foregoing embodiments into the housing; S2: After injecting electrolyte into the casing and sealing it, formation is carried out to obtain a battery; wherein the formation voltage is greater than or equal to the delithiation potential of the ternary material core and less than the delithiation potential of the second lithium replenishing agent.

[0076] In step S1, the housing may have a receiving cavity and an opening, the electrode assembly is placed in the receiving cavity of the housing, and the opening of the housing may be sealed by a top cover.

[0077] In step S2, the electrolyte may include a lithium salt, a solvent, and optionally an additive; wherein the lithium salt may be selected from at least one of LiPF6, LiFSI, LiTFSI, LiBOB, and LiDFOB, the solvent may be selected from at least two of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl acetate (MA), and ethyl acetate (EA), and the additive may be selected from at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sulpholactone (PS), vinyl sulfate (DTD), tris(trimethylsilane) phosphite (TMSP), tris(trimethylsilane) phosphate (TTSP), and lithium difluorophosphate (LiPO2F2).

[0078] In this embodiment, the formation voltage is greater than or equal to the delithiation potential of the ternary material core and less than the delithiation potential of the second lithium replenishing agent. This ensures that the ternary material core is fully activated and drives the decomposition of the first and third lithium replenishing agents to release a large number of lithium ions (also known as sacrificial lithium) to replenish the consumption of active lithium in the formation of the SEI film.

[0079] If the formation voltage is too low, the ternary material core will not be sufficiently delithiated, and the battery capacity will not reach the design value. Simultaneously, the first and third lithium replenishing agents are prone to incomplete decomposition, resulting in their lithium replenishment function not being fully utilized, and the battery's initial efficiency not being significantly improved. If the formation voltage is too high, it can easily induce the following problems: First, the electrolyte in the first active material layer region undergoes oxidation and decomposition, resulting in severe gas production; second, the ternary material core structure undergoes over-oxidation, leading to lattice oxygen loss and a decrease in cycle life; third, the negative electrode SEI film thickens and becomes unstable due to excessive reduction. In short, this will lead to deterioration of battery cycle performance, gas buildup, and increased safety risks.

[0080] In some embodiments, the voltage formed can be 4.15V to 4.25V, for example, 4.15V, 4.20V, 4.25V or any value between any two of the above ranges.

[0081] In the positive electrode, the delithiation potential of the second lithium replenishing agent in the second lithium replenishing layer of the second active material is greater than that of the ternary material core. Therefore, when the battery reaches the mid-cycle stage, considering the loss of active lithium due to the continuous formation of the SEI film and other side reactions, a "stimulation voltage" higher than the normal operating voltage of the battery can be applied to cause the second lithium replenishing agent in the second lithium replenishing layer to undergo controlled decomposition, releasing the stored lithium ions to compensate for the loss of active lithium in the system and extend the cycle life of the battery. Specifically, for example, after the 400th cycle, the charging cutoff voltage can be stepped up to 4.6V for 1-2 cycles of "high-voltage stimulation cycling," and then the cycle can continue at the normal voltage window. Of course, even without applying an additional "stimulation voltage" higher than the normal operating voltage of the battery, after long-term cycling, when the positive electrode interface degrades and active lithium is continuously lost, the battery internal resistance increases, the local potential rises, and the second lithium replenishing agent can be activated, slowly releasing lithium ions to compensate for cycle decay, thereby significantly improving the cycle life of the battery.

[0082] The technical solution of this application will be further described below with reference to several embodiments and comparative examples.

[0083] Example 1 The steps for preparing the battery in this embodiment include: Step S101: Preparation of the first active material and the second active material: Lithium iron phosphate (LFP) powder was placed in a vacuum drying oven and dried at 120°C for 12 hours to completely remove physically adsorbed water. 10 grams of the dried LFP powder was then loaded into a fluidized bed atomic layer deposition reactor. Ferrocene (FeCp2) and lithium tert-butyl oxide (LiOtBu) sources were placed in bubblers and heated to 120°C. The reactor temperature was set to 600°C, O2 was introduced at a flow rate of 100 sccm, and the system pressure was maintained at 300 Pa. Pure argon was used as the carrier gas. The flow rates of the two reaction gases were adjusted to ensure that the gas flow rate ratio delivered to the reaction chamber was 8:1. The reactor was turned on, the fluidization speed was 150 rpm, and the deposition time was 30 min. A 3.5 nm thick LFO (first lithium replenishment layer) was coated on the LFP surface. After the reaction was completed, the material was heat-treated at 300 °C in an argon atmosphere for 1.5 h to obtain the first active material, denoted as LFP@LFO. Lithium nickel cobalt manganese oxide (NCM) powder was placed in a vacuum drying oven and dried at 120°C for 12 hours to completely remove physically adsorbed water. 10 grams of the dried NCM powder was then loaded into a fluidized bed atomic layer deposition reactor. FeCp2 and LiOtBu were placed in a bubbler and heated to 120°C. The reactor temperature was set to 300°C, and O2 and CO2 were introduced at flow rates of 100 sccm and 50 sccm, respectively. High-purity argon was used as the carrier gas. The reaction gas flow rate was adjusted to 40 sccm through the bubbler containing LiOtBu. After 20 minutes of deposition, a 1.5 nm thick Li2CO3 layer (second lithium replenishment layer) was coated onto the NCM surface. The reactor temperature was raised to 350°C, and O2 was introduced at a flow rate of 100 sccm. The flow rates of the two reaction gases were adjusted using a bubbler equipped with LiOtBu and a bubbler equipped with FeCp2 to ensure that the gas flow rate ratio delivered to the reaction chamber was 8:1. After 46 minutes of deposition, a 7.7 nm thick LFO layer (third lithium replenishment layer) was coated onto the surface of the second lithium replenishment layer to obtain the second active material, denoted as NCM@Li2CO3 / LFO. Step S102: Prepare the positive electrode sheet: The first active material obtained in step S101, the composite conductive agent (a mixture of conductive carbon black, carbon nanotubes, and graphene, wherein the mass ratio of conductive carbon black to carbon nanotubes is 1:1, and the mass fraction of graphene in the composite conductive agent is 0.05%), and the first binder PAA are added to N-methylpyrrolidone at a mass ratio of 95:2:3 and mixed evenly to obtain the first positive electrode slurry; the second active material obtained in step S101, the composite conductive agent (the same composite conductive agent as described above), and the second binder PAA are added to N-methylpyrrolidone at a mass ratio of 97:1:2 and mixed evenly to obtain the second positive electrode slurry; A first positive electrode slurry is coated onto the surface of the positive electrode current collector aluminum foil, and after drying, a first active material layer is formed. A second positive electrode slurry is coated onto the middle region of the first active material layer along the width direction of the positive electrode current collector, and after drying, a second active material layer is formed. The first positive electrode slurry is coated onto the first active material layers on both sides of the second active material layer along the width direction of the positive electrode current collector, and after drying, a third active material layer is formed, resulting in a positive electrode sheet. The size ratio of the third active material layer located on one side of the second active material layer to the second active material layer along the width direction of the positive electrode current collector is 5:90. The thickness of the first active material layer is 110 μm, and the thicknesses of the second and third active material layers are 360 ​​μm. Step S103: Preparation of negative electrode sheet: Add graphite, conductive carbon black (conductive agent), and styrene-butadiene rubber (binder) to deionized water at a mass ratio of 97:1:2, mix evenly to obtain negative electrode slurry; coat the negative electrode slurry onto the negative electrode current collector copper foil, and after drying, obtain negative electrode sheet; Step S104: Battery preparation: The positive electrode sheet, separator, and negative electrode sheet prepared in step S102 are stacked in sequence and wound to obtain an electrode assembly. The electrode assembly is placed in the housing and then subjected to steps such as liquid injection, formation, aging, sealing nail welding, and helium detection to obtain the battery. The formation voltage is 4.20V.

[0084] Example 2 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: 1) In step S101, when depositing the first lithium replenishment layer and the third lithium replenishment layer, the iron source ferrocene is replaced with the manganese source manganese acetylacetone, that is, the lithium replenishing agent of the first lithium replenishment layer and the third lithium replenishment layer is replaced with Li2MnO3. 2) In step S101, when depositing the second lithium replenishment layer, the gas introduced is adjusted to water vapor, that is, the lithium replenishment agent of the second lithium replenishment layer is replaced with Li2O.

[0085] Example 3 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S101, when depositing the first lithium replenishment layer, the iron source ferrocene is replaced with the manganese source manganese acetylacetone, that is, the lithium replenishing agent of the first lithium replenishment layer is replaced with Li2MnO3.

[0086] Example 4 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: 1) In the process of preparing the first active material in step S101, the deposition time of LFO is adjusted to 17 min, and LFO with a thickness of 2 nm is coated on the surface of LFP; 2) In the process of preparing the second active material in step S101, the deposition time of LFO is adjusted to 30 min, and LFO with a thickness of 5 nm is coated on the surface of the second lithium replenishment layer.

[0087] Example 5 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: 1) In the process of preparing the first active material in step S101, the deposition time of LFO is adjusted to 43 min, and LFO with a thickness of 5 nm is coated on the LFP surface; 2) In the process of preparing the second active material in step S101, the deposition time of LFO is adjusted to 60 min, and LFO with a thickness of 10 nm is coated on the surface of the second lithium replenishment layer.

[0088] Example 6 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S102, a second positive electrode slurry is coated on the entire surface of the first active material layer to form a second active material layer; that is, a third active material layer is not formed.

[0089] Example 7 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S104, the voltage of the generated circuit is adjusted to 4.15V.

[0090] Example 8 The battery preparation steps in this embodiment are basically the same as those in Example 1, with the main difference being: In step S104, the voltage of the generated circuit is adjusted to 4.25V.

[0091] Comparative Example 1 The steps for preparing the battery in this comparative example include: NCM powder, composite conductive agent (same as the composite conductive agent in Example 1), and binder PAA were added to N-methylpyrrolidone at a mass ratio of 97:1:2 and mixed evenly to obtain a positive electrode slurry. The positive electrode slurry was coated on the surface of the positive electrode current collector aluminum foil and dried to form a positive electrode active material layer, thus obtaining a positive electrode sheet. The thickness of the positive electrode active material layer was 470 μm.

[0092] Comparative Example 2 The steps for preparing the battery in this comparative example include: LFP powder, composite conductive agent (same as the composite conductive agent in Example 1), and binder PAA were added to N-methylpyrrolidone at a mass ratio of 95:2:3 and mixed evenly to obtain a positive electrode slurry. The positive electrode slurry was coated on the surface of the positive electrode current collector aluminum foil and dried to form a positive electrode active material layer, thus obtaining a positive electrode sheet. The thickness of the positive electrode active material layer was 470 μm.

[0093] Comparative Example 3 The steps for preparing the battery in this comparative example include: LFP powder, composite conductive agent (same as the composite conductive agent in Example 1), and binder PAA were added to N-methylpyrrolidone at a mass ratio of 95:2:3 and mixed evenly to obtain a first slurry. NCM powder, composite conductive agent (same as the composite conductive agent mentioned above), and binder PAA were added to N-methylpyrrolidone at a mass ratio of 97:1:2 and mixed evenly to obtain a second slurry. The first slurry and the second slurry were layered and coated on the surface of the aluminum foil with the first slurry closer to the positive electrode current collector aluminum foil. After drying, a first active material layer and a second active material layer were sequentially stacked on the surface of the aluminum foil to obtain a positive electrode sheet. The thicknesses of the first active material layer and the second active material layer were 110 μm and 360 μm, respectively.

[0094] Comparative Example 4 The steps for preparing the battery in this comparative example are basically the same as those in Example 1, with the following differences: In step S101, during the preparation of the second active material, LFO is first deposited on the NCM surface, and then Li2CO3 is deposited on the LFO surface; that is, the positions of the second and third lithium replenishment layers are interchanged.

[0095] Comparative Example 5 The steps for preparing the battery in this comparative example are basically the same as those in Example 1, with the following differences: In step S101, during the preparation of the second active material, LFO is deposited only on the NCM surface for 55 minutes and with a thickness of 9.2 nm. That is, the second lithium replenishment layer is replaced with the third lithium replenishment layer, which is equivalent to two layers of the third lithium replenishment layer being stacked.

[0096] Comparative Example 6 The steps for preparing the battery in this comparative example are basically the same as those in Example 1, with the following differences: 1) In the process of preparing the first active material in step S101, Li2CO3 is deposited on the LFP surface for 46 min and the deposition thickness is 3.5 nm; that is, the lithium replenishing agent in the first lithium replenishing layer is replaced by Li2CO3 instead of LFO.

[0097] 2) In the process of preparing the second active material in step S101, Li2CO3 is deposited only on the NCM surface for 120 min and the deposition thickness is 9.2 nm; that is, the third lithium replenishment layer is replaced by the second lithium replenishment layer, which is equivalent to two layers of the second lithium replenishment layer being superimposed.

[0098] Comparative Example 7 The steps for preparing the battery in this comparative example are basically the same as those in Example 1, with the following differences: In step S104, the voltage of the generated circuit is adjusted to 4.80V.

[0099] The performance of the batteries prepared in the above embodiments and comparative examples was tested, and the specific tests are as follows: (1) Initial first-cycle coulombic efficiency: The battery capacity is tested and cycled 3 times. The average of the three charging and discharging capacities is taken. The average of the discharging capacity is divided by the average charging capacity to obtain the initial first-cycle coulombic efficiency of the battery. (2) Coulomb efficiency after cycling: After the 400th cycle, the charging cut-off voltage is stepped up to 4.6V and two cycles of "stimulation cycling" are performed. Then, the voltage is restored to the normal voltage window and the cycling continues. The coulomb efficiency of the battery is measured again in the manner described in step (1) above and recorded as the coulomb efficiency after cycling. (3) Capacity retention rate after 2000 cycles at room temperature: After battery formation, charge and discharge tests were performed at room temperature: ① Charged at a constant current of 0.33C to 3.65V; charged at a constant voltage of 0.05C; ② Let stand for 30 minutes; ③ Discharged at a constant current of 1C to 2V; ④ Let stand for 30 minutes; Repeat steps ①-④ until 3 cycles, and record the average discharge capacity of 3 cycles as C0; Repeat steps ①-④ until 2000 cycles, and record the capacity at this time as C2000. The capacity retention rate after 2000 cycles at room temperature = (C2000 / C0) * 100%; (4) Capacity retention rate after 2000 cycles at high temperature: The test method is the same as the test method in step (3) above, except that the test temperature is adjusted to 60℃; (5) High-temperature storage capacity retention rate: Charged at a constant current of 0.33C to 3.65V; charged at a constant voltage of 0.05C; left to stand for 30 minutes to reach 100% SOC (State of Charge); the battery was stored in a 60℃ temperature cabinet for 120 days. The battery capacity was tested before and after storage and recorded as C1 and C2. High-temperature storage capacity retention rate = (C2 / C1) * 100%; (6) DC internal resistance (DCR) growth rate after 2000 cycles: At room temperature, charge at a constant current of 0.33C to 3.65V; charge at a constant voltage of 0.05C; let stand for 30 min; discharge at a constant current of 1C to 50% SOC; let stand for 30 min; discharge at 3C for 10 s, and record the voltages before and after the 3C discharge as V0 and V1, respectively. DCR = (V0 - V1) / I 3C , among which, I 3C This refers to the current value during 3C discharge. The DCR for the first cycle is calculated using the method described above and recorded as DCR0. After 2000 cycles, the DCR for the first cycle is calculated and recorded as DCR0. 2000 The DCR growth rate after 2000 cycles = ((DCR) 2000 -DCR0) / DCR0)*100%.

[0100] The test results are shown in Table 1.

[0101] Table 1

[0102] As can be seen from the data in Table 1, in Comparative Examples 1 and 2, the overall electrochemical performance of the battery is poor when a single active material layer (the active material is a single LFP or NCM) is used in the positive electrode. In Comparative Example 3, a double active material layer (lower active material LFP, upper active material NCM) is used, but the combination of the two active materials still cannot effectively improve the overall electrochemical performance of the battery.

[0103] The batteries prepared in Examples 1 to 8 all achieved an initial coulombic efficiency of over 95%, a coulombic efficiency of over 97% after cycling, a capacity retention rate of over 82% after 2000 cycles at room temperature, a capacity retention rate of over 75% after 2000 cycles at high temperature, and a capacity retention rate of over 90% during high-temperature storage. At the same time, the DCR growth rate after 2000 cycles was decreasing, indicating a significant improvement in the overall electrochemical performance of the batteries.

[0104] In Comparative Examples 4 to 6, the positions of the second and third lithium replenishment layers in the second active material were interchanged, or only a single second lithium replenishment layer or only a single third lithium replenishment layer was used. The excessively high delithiation potential of the first lithium replenishing agent in the first active material resulted in an ineffective improvement in the overall electrochemical performance of the battery. In Comparative Example 7, directly increasing the formation voltage to 4.80V irreversibly damaged the crystal structure of the ternary material core NCM. Oxygen in the lattice was released as oxygen, leading to oxygen vacancies and structural collapse. Large amounts of nickel, cobalt, and manganese ions dissolved and migrated to the negative electrode, damaging the SEI film and causing "cross-contamination." Furthermore, the severe delithiation of the NCM caused stress concentration within the particles, resulting in penetrating microcracks. The electrolyte was severely oxidized, producing a large amount of HF, which corroded the positive electrode surface. The resulting byproducts (such as LiF and polymers) blocked lithium-ion channels, ultimately leading to poor cycle life and storage performance, and a high DCR growth rate.

[0105] A comparison of the data from Examples 1 and 3 in Table 1 shows that replacing the lithium replenishing agent LFO in the first lithium replenishing layer with Li2MnO3 resulted in a decrease in the overall electrochemical performance of the battery. This is likely due to the dissolution of manganese ions from the Li2MnO3 lithium replenishing agent, which to some extent affected the lithium replenishment effect and thus the electrochemical performance of the battery. Therefore, using LFO as the first lithium replenishing agent in the first lithium replenishing layer is a better technical solution.

[0106] In this application, the synergistic innovation of "lithium iron phosphate / ternary material double-layer coating technology" combined with "gradient functionalized lithium replenishment agent coating technology" solves the problems of intrinsic safety defects, low initial coulombic efficiency and capacity decay under long cycle life of high energy density ternary batteries.

[0107] It should be noted that the positive electrode sheet embodiments, electrode assembly embodiments, battery embodiments, and battery preparation method embodiments provided in this application belong to the same concept; the technical features in the technical solutions described in each embodiment can be arbitrarily combined without conflict.

[0108] It should be understood that the above embodiments are exemplary and not intended to encompass all possible implementations. Various modifications and changes can be made to the above embodiments without departing from the scope of this disclosure. Similarly, the various technical features of the above embodiments can be arbitrarily combined to form other embodiments of the present invention that may not be explicitly described. Therefore, the above embodiments only illustrate several implementations of the present invention and do not limit the scope of protection of this patent.

Claims

1. A positive electrode plate, characterized in that, It includes a positive electrode current collector, a first active material layer located on at least one surface of the positive electrode current collector along its thickness direction, and a second active material layer located on the surface of the first active material layer away from the positive electrode current collector; wherein, The first active material layer includes a first active material, which has a core-shell structure, including a lithium iron phosphate core and a first lithium replenishing layer covering the lithium iron phosphate core. The first lithium replenishing layer includes a first lithium replenishing agent, and the delithiation potential of the first lithium replenishing agent is lower than the delithiation potential of the lithium iron phosphate core. The second active material layer includes a second active material, which has a core-shell structure, comprising a ternary material core and a second lithium replenishing layer and a third lithium replenishing layer that sequentially cover the ternary material core from the inside out. The second lithium replenishing layer includes a second lithium replenishing agent, the delithiation potential of which is greater than that of the ternary material core. The third lithium replenishing layer includes a third lithium replenishing agent, the delithiation potential of which is less than that of the lithium iron phosphate core.

2. The positive electrode sheet according to claim 1, characterized in that, The delithiation potential of the first lithium replenishing agent and / or the third lithium replenishing agent is greater than or equal to 3.0V and less than 3.4V; and / or, The delithiation potential of the second lithium replenishing agent is greater than or equal to 4.5V; and / or, The first lithium supplement and / or the third lithium supplement include one or more of lithium ferrite, lithium manganese oxide, lithium molybdenum oxide, lithium vanadium oxide, and lithium nickel oxide; and / or, The second lithium supplement includes one or more of lithium carbonate, lithium oxide, lithium hydroxide, lithium boron oxide, and lithium phosphorus oxide.

3. The positive electrode sheet according to claim 1, characterized in that, The thickness ratio of the first lithium replenishment layer to the third lithium replenishment layer is 0.4~0.5; and / or, The thickness of the first lithium replenishment layer is 2nm~5nm; and / or, The thickness of the second lithium replenishment layer is 1 nm to 2 nm; and / or, The thickness of the third lithium replenishment layer is 5nm~10nm.

4. The positive electrode sheet according to claim 1, characterized in that, The median particle size ratio of the first active material to the second active material is 1:(2~2.5); and / or, The median particle size of the first active material is 4 μm to 6 μm; and / or, The median particle size of the second active material is 10 μm to 12 μm.

5. The positive electrode sheet according to claim 1, characterized in that, The first active material layer further includes a first binder, the first binder comprising carboxyl groups; and / or, The second active material layer also includes a second binder, which includes carboxyl groups.

6. An electrode assembly, characterized in that, The electrode assembly includes a negative electrode, a separator, and a positive electrode as described in any one of claims 1-5, wherein the separator is located between the positive electrode and the negative electrode.

7. The electrode assembly according to claim 6, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer located on at least one side surface of the negative electrode current collector along the thickness direction. The negative electrode active material layer includes a negative electrode body portion and a negative electrode thinning portion. The negative electrode thinning portion is located on at least one side of the negative electrode body portion along the width direction of the separator, and the thickness of the negative electrode thinning portion is less than that of the negative electrode body portion. The positive electrode further includes a third active material layer, which is located on at least one side of the second active material layer along the width direction of the separator. The thickness of the third active material layer is less than the thickness of the second active material layer. The second active material layer is disposed opposite to the negative electrode body portion, and the third active material layer is disposed opposite to the negative electrode thinning portion. The third active material layer includes the first active material.

8. The electrode assembly according to claim 7, characterized in that, The dimension of the third active material layer located on one side of the second active material layer in the width direction of the separator is 5% to 15% of the dimension of the positive electrode sheet in the width direction of the separator; and / or, The positive electrode further includes a fourth active material layer, which is located between the second active material layer and the third active material layer, and the fourth active material layer includes the first active material and the second active material.

9. A battery, characterized in that, Includes the electrode assembly as described in any one of claims 6-8.

10. A method for preparing a battery, characterized in that, The method includes: The electrode assembly according to any one of claims 6-8 is placed in the housing; After injecting electrolyte into the casing and sealing it, formation is performed to obtain the battery; wherein the formation voltage is greater than or equal to the delithiation potential of the ternary material core and less than the delithiation potential of the second lithium replenishing agent.

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