Secondary batteries and their preparation methods, energy storage systems and electrical equipment

Abstract

This application relates to the field of energy storage, providing a secondary battery and its preparation method, energy storage system, and electrical device. The secondary battery includes a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector. The positive electrode active layer includes a carbon-coated composite material, which includes: a first lithium salt particle and a second lithium salt particle as the core, wherein the average particle size of the first lithium salt particle is larger than the average particle size of the second lithium salt particle; the first lithium salt particle and the second lithium salt particle are used to construct an internal electric field, which is used to cause electrons to flow from the second lithium salt particle to the first lithium salt particle; wherein the material of the first lithium salt particle includes one or more of lithium manganese phosphate, lithium manganese oxide, or lithium vanadate; the material of the second lithium salt particle includes one or more of lithium silicate or lithium borate; and a nitrogen-doped carbon layer as the carbon coating layer, which is used to adsorb lithium ions at a first voltage and release lithium ions at a second voltage.

Description

Technical Field

[0001] This application relates to the field of energy storage, and in particular to a secondary battery and its preparation method, energy storage system, and electrical equipment. Background Technology

[0002] In recent years, with the rapid development of energy storage systems, higher requirements have been placed on the performance of lithium-ion batteries, among which improving battery energy density is the most urgent. Under the existing lithium-ion battery system, on the one hand, energy density can be improved by optimizing the battery structure, such as CTP (Cell to Pack) technology, CTC (Cell to Chassis) technology, or CTB (Cell to Body) technology; on the other hand, by iterating the positive and negative electrode materials, such as using high-nickel ternary or high-voltage nickel-manganese materials for the positive electrode and high-capacity silicon or tin-based alloy negative electrodes, the battery energy density can be significantly improved.

[0003] Furthermore, lithium replenishment technology is also an important means to improve the energy density of lithium-ion batteries. During the first charge of a lithium-ion battery, the organic electrolyte will reduce and decompose on the surface of the negative electrode, such as graphite, forming a solid electrolyte interface (SEI) film. This permanently consumes a large amount of lithium from the positive electrode, resulting in a low coulombic efficiency (ICE) in the first cycle, thus reducing the capacity and energy density of the lithium-ion battery. In addition, processes such as the deactivation of negative electrode material particles due to detachment and the irreversible deposition of lithium metal also consume the active lithium of the positive electrode, reducing the battery's capacity and energy density.

[0004] Lithium replenishment, also known as "pre-lithiation" or "pre-intercalation," involves adding lithium to the battery before it begins operation to replenish lithium ions. Pre-lithiation replenishes lithium in the electrode materials, offsetting irreversible lithium loss and improving the battery's overall capacity and energy density. Common lithium replenishment strategies include positive electrode replenishment, negative electrode replenishment, and electrolyte additive replenishment. Positive electrode replenishment involves adding lithium-rich compounds to the positive electrode material, releasing lithium ions during charging. Negative electrode replenishment involves pre-storing lithium powder or lithium foil in the negative electrode to compensate for lithium loss during discharge. Electrolyte replenishment directly provides lithium ions through the decomposition of additives.

[0005] However, current lithium replenishment technology still needs improvement. Summary of the Invention

[0006] This application provides a secondary battery and its preparation method, energy storage system, and electrical equipment, which at least helps to improve the lithium loss problem inside the secondary battery and increase the total capacity and energy density of the battery.

[0007] According to some embodiments of this application, one aspect of this application provides a secondary battery, including: a battery casing and an electrolyte located within the battery casing; a cell assembly located within the cavity and immersed in the electrolyte; the cell assembly includes a wound positive electrode sheet, a separator, and a negative electrode sheet, wherein the positive electrode sheet includes: a positive current collector and a positive active layer located on at least one surface of the positive current collector; the positive active layer includes a carbon-coated composite material, the carbon-coated composite material including: a first lithium salt particle as a core and a second lithium salt particle, the average particle size of the first lithium salt particle being greater than... The average particle size of the second lithium salt particle, at least one second lithium salt particle is located on the surface of the first lithium salt particle; the first lithium salt particle and the second lithium salt particle are used to construct an internal electric field, the internal electric field being used to cause electrons to flow from the second lithium salt particle to the first lithium salt particle; wherein, the material of the first lithium salt particle includes one or more of lithium manganese phosphate, lithium manganese oxide, or lithium vanadate; the material of the second lithium salt particle includes one or more of lithium silicate or lithium borate; a nitrogen-doped carbon layer as a carbon coating layer, the nitrogen-doped carbon layer being used to adsorb lithium ions at a first voltage and release the lithium ions at a second voltage.

[0008] In some embodiments, the first lithium salt particle is made of lithium manganese phosphate, and the second lithium salt particle is made of lithium silicate.

[0009] In some embodiments, the average particle size of the first lithium salt particles is 200 nm to 500 nm; the average particle size of the second lithium salt particles is 50 nm to 80 nm.

[0010] In some embodiments, the molar ratio of the content of the first lithium salt particles to the content of the second lithium salt particles is 1:(0.75~1).

[0011] In some embodiments, the thickness of the nitrogen-doped carbon layer is 5 nm to 10 nm.

[0012] In some embodiments, the nitrogen-doped carbon layer includes a pyridine nitrogen-doped carbon layer or a pyrrole nitrogen-doped carbon layer.

[0013] According to some embodiments of this application, another aspect of this application provides a method for preparing a secondary battery, comprising: forming a positive electrode sheet, the positive electrode sheet comprising: a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector; the positive electrode active layer comprising a carbon-coated composite material, the carbon-coated composite material comprising: a first lithium salt particle and a second lithium salt particle as a core, the average particle size of the first lithium salt particle being larger than the average particle size of the second lithium salt particle, at least one second lithium salt particle being located on the surface of the first lithium salt particle; the first lithium salt particle and the second lithium salt particle are used to construct an internal electric field, the internal electric field being used for electrons to escape from the second lithium salt particle. The lithium salt particles flow to the first lithium salt particles; wherein the material of the first lithium salt particles includes one or more of lithium manganese phosphate, lithium manganese oxide, or lithium vanadate; the material of the second lithium salt particles includes one or more of lithium silicate or lithium borate; a nitrogen-doped carbon layer serves as a carbon coating layer, the nitrogen-doped carbon layer covering the first lithium salt particles and the second lithium salt particles, the nitrogen-doped carbon layer being used to adsorb lithium ions at a first voltage and release the lithium ions at a second voltage; a negative electrode and a separator are provided; the negative electrode, separator, and positive electrode are stacked or wound sequentially to obtain a cell assembly, the cell assembly is placed in a battery case, electrolyte is injected into the battery case, and then it is encapsulated to obtain a secondary battery.

[0014] In some embodiments, the process steps for forming the core composed of the first lithium salt particles and the second lithium salt particles include: forming the first lithium salt particles by a sol-gel method; dissolving the second lithium source material and the chelating agent in a mixture of ethanol and water at a molar ratio of (0.3~0.5):(0.2~0.3) to form a lithium salt precursor; adding an alkaline compound dropwise to the lithium salt precursor and stirring until a transparent solution is formed; adding the first lithium salt particles to the transparent solution and adjusting the pH of the solution to 3~4, heating and stirring until a wet gel is formed; drying the wet gel to form a dry gel; and heat-treating the dry gel by calcining it at a temperature of 600℃~800℃ for 6h~10h in an inert gas atmosphere to form the core composed of the first lithium salt particles and the second lithium salt particles.

[0015] In some embodiments, the process steps for forming the nitrogen-doped carbon layer include: deoxygenating the core composed of the first lithium salt particles and the second lithium salt particles; providing a carbon source and a nitrogen source, and introducing the nitrogen source and the carbon source into the reaction chamber through an inert gas; reacting at 700°C to 1000°C for 30 min to 60 min, so that the nitrogen atoms of the nitrogen source are embedded into the carbon skeleton of the carbon source through pyrolysis; and continuing to introduce the inert gas until cooling to room temperature.

[0016] In some embodiments, the volume ratio of the carbon source, the nitrogen source, and the inert gas is (0.7~1.4):(0.2~0.8):(4~10).

[0017] In some embodiments, after forming the secondary battery, the process further includes: performing a first formation process on the secondary battery using a first voltage, the first formation process being used to create an internal electric field between the first lithium salt particles and the second lithium salt particles, and releasing a portion of the lithium ions, with a portion of the lithium ions being released into the electrolyte; and another portion of the lithium ions being adsorbed by the nitrogen-doped carbon layer; performing a second formation process on the secondary battery using a second voltage, the second formation process being used to release the lithium ions from the first lithium salt particles into the electrolyte, and the nitrogen-doped carbon layer releasing the lithium ions adsorbed in the first formation process into the electrolyte.

[0018] In some embodiments, the first lithium salt particle is made of lithium manganese phosphate, and the second lithium salt particle is made of lithium silicate; the first voltage is 3.0V~3.2V; the second voltage is 3.2V~3.5V, excluding 3.2V.

[0019] According to some embodiments of this application, another aspect of this application provides an energy storage system, including: a secondary battery as described in any of the above embodiments or a secondary battery prepared by the method described in any of the above embodiments.

[0020] According to some embodiments of this application, another aspect of this application provides an electrical device, including: a secondary battery and a load, wherein the secondary battery is used to supply power to the load; the secondary battery is any one of the secondary batteries described in the above embodiments, a secondary battery prepared by the preparation method of any one of the secondary batteries described in the above embodiments, or an energy storage system as described in the above embodiments.

[0021] The technical solution provided in this application has at least the following advantages:

[0022] This application discloses a secondary battery that utilizes a carbon-coated composite material formed within the positive electrode active material layer as a lithium replenishment composite material for lithium replenishment processes. This direct integration of the lithium-replenishing carbon-coated composite material into the positive electrode active layer results in a simple and compact structure, eliminating the need for complex additional lithium replenishment processes. Furthermore, it ensures uniform distribution of the lithium replenisher within the battery and its tight bonding with the positive electrode active material. The carbon layer encapsulates the core particles, preventing the first and second lithium salt particles from directly contacting the electrolyte during cycling and undergoing continuous side reactions that could lead to excessive consumption. This improves the utilization efficiency and stability of the lithium replenisher. Simultaneously, the carbon layer provides a robust electronic conductivity network, ensuring the electron transport required for the lithium replenishment reaction.

[0023] The carbon-coated composite material comprises a first lithium salt particle, a second lithium salt particle, and a nitrogen-doped carbon layer. The first and second lithium salt particles are used to construct an internal electric field, which drives electrons to flow from the second lithium salt particle to the first lithium salt particle. This internal electric field drives lithium ions to migrate from the second lithium salt particle to the electrolyte / interface. This provides an additional, directional driving force for lithium ion release, which can improve the lithium ion migration rate and lithium replenishment efficiency, significantly promoting and accelerating the lithium replenishment process and improving lithium replenishment efficiency. The nitrogen-doped carbon layer is used to adsorb lithium ions at a first voltage and release lithium ions at a second voltage. At the first voltage, the nitrogen-doped carbon layer adsorbs lithium ions released from the positive electrode active material or lithium replenishing agent. This prevents lithium ions from prematurely diffusing into the electrolyte and being irreversibly consumed by the negative electrode or forming dead lithium, reducing the ineffective loss of the lithium replenishing agent. At the second voltage, the nitrogen-doped carbon layer releases the adsorbed lithium ions. The timely release of lithium ions can synergize with the lithium ions generated by the decomposition of the second lithium salt particles, thus avoiding the problem of redundancy caused by a single rapid release of the first and second lithium salt particles, which could lead to the deposition of lithium dendrites on the negative electrode. Simultaneously, the ion channels formed by the nitrogen-doped carbon layer ensure uniform embedding of lithium ions into the positive electrode, avoiding localized stress concentration. Attached Figure Description

[0024] One or more embodiments are illustrated by way of example with reference to the accompanying drawings. These illustrations do not constitute a limitation on the embodiments. Unless otherwise stated, the drawings in the accompanying drawings do not constitute a limitation on scale. In order to more clearly illustrate the technical solutions in the embodiments of this application or in the conventional art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a cross-sectional view of a cell assembly in a secondary battery provided in an embodiment of this application.

[0026] Explanation of reference numerals in the attached figures:

[0027] 1. Positive electrode plate; 2. Negative electrode plate; 3. Separator. Detailed Implementation

[0028] As can be seen from the background technology, current secondary batteries suffer from internal lithium loss issues, and their total capacity and energy density need to be further improved.

[0029] This application provides a secondary battery and its preparation method, energy storage system and electrical device. By forming a carbon-coated composite material in the positive electrode active layer of the positive electrode sheet, including a first lithium salt particle and a second lithium salt particle as the core and a nitrogen-doped carbon layer as the carbon coating layer, a better lithium replenishment effect is provided.

[0030] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0031] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0032] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A exists, A and B exist simultaneously, and B exists. In addition, the character " / " in this document generally indicates that the related objects before and after it have an "or" relationship.

[0033] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0034] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0035] In the description of the embodiments of this application, unless otherwise expressly specified and limited, the technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0036] In the accompanying drawings corresponding to the embodiments of this application, the thickness and area of ​​the layers are enlarged for better understanding and ease of description. When describing a component (such as a layer, film, region, or substrate) on or on the surface of another component, the component may be "directly" located on the surface of the other component, or there may be a third component between the two components. Conversely, when describing a component on the surface of another component, or when another component is formed or disposed on the surface of a component, it indicates that there is no third component between the two components. Furthermore, when describing a component as being "generally" formed on another component, it means that the component is not formed on the entire surface (or front surface) of the other component, nor is it formed on a portion of the edge of the entire surface.

[0037] In the description of the embodiments of this application, when a component "includes" another component, other components are not excluded unless otherwise stated, and other components may be further included. Furthermore, when a component such as a layer, film, region, or plate is referred to as being "on / located" on another component, it can be "directly on" the other component (i.e., located on the surface of the other component with no other components between them), or another component may be present therein. Moreover, when a component such as a layer, film, region, or plate is "directly located" on another component, or when a component such as a layer, film, region, or plate is located on the surface of another component, it indicates that no other components are located therein.

[0038] The terminology used in the description of the various embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the various embodiments and the appended claims, the term "part" is also intended to include the plural form unless the context clearly indicates otherwise. Components include layers, films, regions, or plates, etc.

[0039] The embodiments of this application will now be described in detail with reference to the accompanying drawings. However, those skilled in the art will understand that many technical details have been provided in the embodiments of this application to facilitate a better understanding of the application. However, the technical solutions claimed in this application can be implemented even without these technical details and various variations and modifications based on the following embodiments.

[0040] Figure 1 This is a cross-sectional view of a cell assembly in a secondary battery provided in an embodiment of this application.

[0041] It should be noted that, in order to explain and distinguish between the separator, the positive electrode plate, and the negative electrode plate, Figure 1 The diagram uses a dashed line to represent the separator, a thinner solid line to represent the negative electrode, and a thicker solid line to represent the positive electrode. However, this does not represent the thickness relationship between the separator, the positive electrode, and the negative electrode. Secondly, Figure 1 There are gaps between each pair of the separator, positive electrode, and negative electrode. This is to clearly illustrate the winding relationship between the separator, positive electrode, and negative electrode. However, it does not mean that there must be a gap between the separator and the positive electrode or the separator and the negative electrode. In other words, the separator and the positive electrode can be in contact with each other, or the separator and the negative electrode can be in contact with each other.

[0042] According to some embodiments of this application, one aspect of this application provides a secondary battery, including: a battery casing and an electrolyte located within the battery casing; a cell assembly located within a cavity and immersed in the electrolyte; the cell assembly includes a wound positive electrode 1, a separator 3, and a negative electrode 2, wherein the positive electrode 1 includes: a positive current collector and a positive active layer located on at least one surface of the positive current collector; the positive active layer includes a carbon-coated composite material, the carbon-coated composite material including: a first lithium salt particle as a core and a second lithium salt particle, the first lithium salt particle having an average particle size distribution of 100%. The diameter is larger than the average particle size of the second lithium salt particle, and at least one second lithium salt particle is located on the surface of the first lithium salt particle; the first lithium salt particle and the second lithium salt particle are used to construct an internal electric field, which is used to cause electrons to flow from the second lithium salt particle to the first lithium salt particle; wherein, the material of the first lithium salt particle includes one or more of lithium manganese phosphate, lithium manganese oxide, or lithium vanadate; the material of the second lithium salt particle includes one or more of lithium silicate or lithium borate; a nitrogen-doped carbon layer is used as a carbon coating layer to adsorb lithium ions at a first voltage and release lithium ions at a second voltage.

[0043] This application discloses a secondary battery that utilizes a carbon-coated composite material formed within the positive electrode active material layer as a lithium replenishment composite material for lithium replenishment processes. This direct integration of the lithium-replenishing carbon-coated composite material into the positive electrode active layer results in a simple and compact structure, eliminating the need for complex additional lithium replenishment processes. Furthermore, it ensures uniform distribution of the lithium replenisher within the battery and its tight bonding with the positive electrode active material. The carbon layer encapsulates the core particles, preventing the first and second lithium salt particles from directly contacting the electrolyte during cycling and undergoing continuous side reactions that could lead to excessive consumption. This improves the utilization efficiency and stability of the lithium replenisher. Simultaneously, the carbon layer provides a robust electronic conductivity network, ensuring the electron transport required for the lithium replenishment reaction.

[0044] The carbon-coated composite material comprises a first lithium salt particle, a second lithium salt particle, and a nitrogen-doped carbon layer. The first and second lithium salt particles are used to construct an internal electric field, which drives electrons to flow from the second lithium salt particle to the first lithium salt particle. This internal electric field drives lithium ions to migrate from the second lithium salt particle to the electrolyte / interface. This provides an additional, directional driving force for lithium ion release, which can improve the lithium ion migration rate and lithium replenishment efficiency, significantly promoting and accelerating the lithium replenishment process and improving lithium replenishment efficiency. The nitrogen-doped carbon layer is used to adsorb lithium ions at a first voltage and release lithium ions at a second voltage. At the first voltage, the nitrogen-doped carbon layer adsorbs lithium ions released from the positive electrode active material or lithium replenishing agent. This prevents lithium ions from prematurely diffusing into the electrolyte and being irreversibly consumed by the negative electrode or forming dead lithium, reducing the ineffective loss of the lithium replenishing agent. At the second voltage, the nitrogen-doped carbon layer releases the adsorbed lithium ions. The timely release of lithium ions can synergize with the lithium ions generated by the decomposition of the second lithium salt particles, thus avoiding the problem of redundancy caused by a single rapid release of the first and second lithium salt particles, which could lead to the deposition of lithium dendrites on the negative electrode. Simultaneously, the ion channels formed by the nitrogen-doped carbon layer ensure uniform embedding of lithium ions into the positive electrode 1, avoiding localized stress concentration.

[0045] According to their shape, the prepared secondary batteries can be classified into square cells, round cells, or pouch cells. According to their capacity, secondary batteries can be classified into models such as 50Ah, 100Ah, 150Ah, 200Ah, 280Ah, 306Ah, 314Ah, 500+Ah, 800+Ah, and 1000+Ah. According to the chemical composition and working principle of the battery cell components, secondary batteries can be lithium-ion batteries, lead-acid batteries, sodium-ion batteries, or nickel-metal hydride batteries. This application uses a lithium-ion battery preparation method as an example. Those skilled in the art can replace the lithium ions in the positive electrode 1, negative electrode 2, and electrolyte with corresponding metal ions according to actual needs. For example, in sodium-ion batteries, the lithium transition metal oxide of the positive electrode active material can be replaced with any of the following: a layered metal oxide (e.g., NaFeO2), a polyanionic compound (NaFePO4), or a Prussian blue compound system (e.g., NaMnFe(CN)6-zH2O), and the electrolyte can be replaced with any of the following: an organic liquid electrolyte, a solid composite electrolyte, or a solid electrolyte.

[0046] refer to Figure 1 The positive electrode 1 serves as the cathode in the redox reaction. During discharge, the positive electrode material (lithium salt material) releases lithium ions and electrons through a chemical reaction; during charging, it reaccompanies lithium ions.

[0047] The positive electrode 1 includes a positive current collector and a positive active layer. The positive current collector can be aluminum foil. Aluminum foil is cheaper than copper foil. A dense oxide film is formed on the surface of the aluminum foil, and the oxide film is very thin, which can improve the corrosion resistance of the aluminum foil, and electrons can achieve electrical conduction through the tunneling effect.

[0048] In some embodiments, the positive current collector of the positive electrode 1 can also be a composite current collector, which includes three stacked layers: an organic material in the middle layer, and copper-plated and aluminum-plated layers on the top and bottom. The organic material is PET (polyterephthalate), PP (polypropylene), PI (polyimide), etc.

[0049] The positive electrode active material within the positive electrode active layer is an energy storage material, typically composed of metal oxides, metal sulfides, or polymers. Its main function is to chemically react with lithium ions at the negative electrode during battery charging, thereby storing lithium ions, increasing the lithium ion concentration, and allowing the positive electrode to release charge current. Simultaneously, during battery discharge, the lithium ions stored in the positive electrode active material move towards the negative electrode, reacting with the negative electrode material, creating a potential difference between the positive and negative electrodes, and thus generating current output.

[0050] A carbon-coated composite material is incorporated into the positive electrode active layer. This carbon-coated composite material serves as a lithium replenishment agent, directly integrating itself into the positive electrode active layer. This results in a simple and compact structure, eliminating the need for complex additional lithium replenishment processes and ensuring uniform distribution and tight bonding of the lithium replenishment agent within the battery. The carbon layer encapsulates the core particles, preventing the first and second lithium salt particles from directly contacting the electrolyte during cycling and undergoing continuous side reactions that could lead to excessive consumption. This improves the utilization efficiency and stability of the lithium replenishment agent. Simultaneously, the carbon layer provides a robust electronic conductivity network, ensuring the electron transport required for the lithium replenishment reaction.

[0051] The first and second lithium salt particles serve as lithium replenishing agents and also as positive electrode active materials.

[0052] The first lithium salt particle is made of one or more of lithium manganese phosphate, lithium manganese oxide, or lithium vanadate. Lithium manganese phosphate (operating voltage 4.1V), lithium manganese oxide (operating voltage 4.0V), and lithium vanadate (operating voltage 2.5~3.5V) all have high operating voltages, while the second lithium salt particle (lithium silicate / lithium borate) typically has a lower operating voltage (operating voltage <2.0V). The heterostructure formed by the two will generate a significant potential difference, which is conducive to establishing an internal electric field at the interface pointing from the second lithium salt particle to the first lithium salt particle.

[0053] The first lithium salt material's charge-discharge platform is located within the battery's conventional operating voltage range (3.0V~4.2V), and it can itself act as an active material to participate in charge-discharge and provide capacity.

[0054] In some embodiments, the average particle size of the first lithium salt particles is 200 nm to 500 nm. This relatively large average particle size allows the first lithium salt particles to serve as a substrate, providing stable support for the small lithium supplementation particles attached to them. This helps maintain the structural stability of the composite material, prevents particle agglomeration or detachment, and ensures the durability of the lithium supplementation function. The average particle size of the first lithium salt particles is 200 nm to 250 nm, 250 nm to 300 nm, 300 nm to 350 nm, 350 nm to 400 nm, 400 nm to 450 nm, or 450 nm to 500 nm. The average particle size of the first lithium salt particles can be 210 nm, 230 nm, 260 nm, 290 nm, 310 nm, 330 nm, 358 nm, 394 nm, 421 nm, 466 nm, 478 nm, or 500 nm.

[0055] The material of the second lithium salt particle includes one or more of lithium silicate or lithium borate.

[0056] The lithium replenishment mechanism between the first and second lithium salt particles is as follows: In the initial stage of charging, the second lithium salt particles preferentially decompose to replenish lithium; as the voltage increases, the first lithium salt particles begin to delithigate, and the lithium ions released can synergistically supply the negative electrode with the lithium ions released by the lithium replenishing agent. This staged lithium ion supply mechanism better meets the needs of the negative electrode for forming an SEI film and lithium intercalation, avoiding side reactions caused by instantaneous excess of lithium ions.

[0057] The average particle size of the first lithium salt particles is larger than that of the second lithium salt particles. The second lithium salt particles have a smaller average particle size and a larger specific surface area, making them more likely to decompose at lower voltages and release a large number of lithium ions to compensate for the irreversible capacity loss of the negative electrode. The large first lithium salt particles not only form the basis of the built-in electric field but also provide stable support points for the small lithium replenishing particles attached to them. This helps maintain the structural stability of the composite material, prevents the aggregation or detachment of small particles, and ensures the durability of the lithium replenishment function.

[0058] The average particle size of the second lithium salt particles is 50 nm to 80 nm. The average particle size of the second lithium salt particles can be 50 nm to 58 nm, 58 nm to 66 nm, 66 nm to 73 nm, or 73 nm to 80 nm. The average particle size of the second lithium salt particles can be 51 nm, 53 nm, 62 nm, 68 nm, 72 nm, 75 nm, or 80 nm.

[0059] It should be noted that particle size refers to the diameter of the particle. Most particles are not perfectly spherical but irregular in shape, and their particle size is expressed as an average value. The physical meaning of average particle size is: it represents a scale indicating the geometric size of a dispersed solid particle group. For example, the average particle size can be D50 or D90. D50: the particle size corresponding to a cumulative particle size distribution percentage of 50% in a sample. D90: the particle size corresponding to a cumulative particle size distribution percentage of 90% in a sample. The average particle size of the first lithium salt particles and the second lithium salt particles can be obtained by directly measuring the particle size using a scanning electron microscope or by using a particle size analyzer.

[0060] The second lithium salt particles are spherical or near-spherical in shape.

[0061] In some embodiments, the molar ratio of the first lithium salt particles to the second lithium salt particles is 1:(0.75~1). This ensures that the second lithium salt particles provide sufficient active lithium while avoiding excessive amounts that could trigger side reactions. The built-in electric field drives lithium ions to flow into the electrolyte, rather than allowing them to diffuse randomly within the positive electrode. The molar ratio of the first lithium salt particles to the second lithium salt particles can be 1:0.75, 1:0.8, 1:0.85, 1:0.9, 1:0.95, or 1:1.

[0062] In some embodiments, the second lithium salt particles are uniformly dispersed on the surface of the first lithium salt particles. This suppresses volume expansion, as the second lithium salt particles undergo a 5% volume change during delithiation, while the first lithium salt particles experience less than 1% volume expansion during cycling. The rigid structure of the first lithium salt particles effectively buffers stress and prevents particle breakage, thus significantly improving cycle stability. Particle agglomeration is also prevented, as the uniform dispersion of the second lithium salt particles on the surface of the first lithium salt particles physically isolates the active material, inhibiting agglomeration during high-temperature calcination or cycling. An internal electric field is formed at the interface between the first and second lithium salt particles, reducing the lithium extraction activation energy, increasing the lithium-ion migration rate, and simultaneously improving lithium replenishment efficiency.

[0063] In some embodiments, the first lithium salt particle is made of lithium manganese phosphate, and the second lithium salt particle is made of lithium silicate. Lithium manganese phosphate releases lithium controllably at 3.0V~3.3V, matching the voltage plateau of LFP (lithium iron phosphate, chemical formula LiFePO4) (the charge / discharge voltage window of LFP is 2.5V~3.6V). The lithium silicate particles are small and uniformly distributed on the surface of the lithium manganese phosphate, and an internal electric field is formed between the interface of the first and second lithium salt particles in the inner layer, improving the lithium-ion migration rate and lithium replenishment efficiency.

[0064] Nitrogen doping in the nitrogen-doped carbon layer endows the carbon layer with special chemical activity, enabling it to possess voltage-dependent lithium-ion adsorption / release capabilities. This allows for the release and capture of lithium ions at different times, avoiding lithium dendrite formation on the surface of the negative electrode 2 due to excessive lithium ions. Secondly, the nitrogen-doped carbon layer exhibits high stability, suppressing electrolyte corrosion and enhancing electron conduction.

[0065] In some embodiments, the thickness of the nitrogen-doped carbon layer is 5 nm to 10 nm. The thickness of the nitrogen-doped carbon layer can be 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, or 10 nm. When the thickness of the nitrogen-doped carbon layer is < 5 nm, the quantum tunneling effect can cause abnormal electron transitions, disrupting the stability of the built-in electric field. Conversely, a thickness > 10 nm significantly increases electron conduction resistance. This range allows electrons to pass through smoothly without interfering with the built-in electric field. Lithium silicate supplements experience an 8% volume shrinkage when releasing lithium ions. The nitrogen-doped carbon layer acts like an elastic net, absorbing stress through its own micro-deformation. Too thin a layer is prone to cracking, while too thick a layer restricts ion diffusion channels. This range allows the nitrogen-doped carbon layer to accommodate the deformation of the second lithium salt particles without affecting the ion diffusion channels.

[0066] In some embodiments, the nitrogen-doped carbon layer includes a pyridine nitrogen-doped carbon layer or a pyrrole nitrogen-doped carbon layer.

[0067] The negative electrode 2 serves as the anode in the redox reaction. During discharge, the negative electrode material (such as graphite or silicon-based materials) accepts and stores lithium ions; during charging, it releases lithium ions.

[0068] The negative electrode 2 includes a negative electrode current collector and a negative electrode active layer, with the negative electrode active layer located on the negative electrode current collector.

[0069] The negative electrode current collector can be copper foil. Copper foil has low conductivity, which allows for high electron transport capability. Furthermore, copper foil has weak lithium intercalation capability, capturing fewer lithium ions and thus effectively reducing lithium ion loss. In other embodiments, the negative electrode current collector can also be a foamed copper current collector, a copper mesh current collector, or a three-dimensional nano-copper array current collector.

[0070] In some embodiments, the negative electrode current collector can be a composite current collector, including a polymer material layer and a metal coating. The metal coating is located on the upper and lower sides of the polymer material layer. The polymer material layer is PET (polyterephthalate), PP (polypropylene), PI (polyimide), etc., and the metal coating is a copper layer.

[0071] In some embodiments, the negative electrode current collector can also be a carbon-based current collector, i.e., a conductive carbon layer is formed on the copper foil. The conductive carbon layer acts as a protective layer, effectively protecting the negative electrode current collector from corrosion and thus improving its lifespan. Secondly, the conductive carbon layer itself has low resistivity, thereby minimizing electrical losses. The conductive carbon layer can be made of materials such as flake graphite, spherical graphite, carbon nanotubes, or graphene.

[0072] In some embodiments, the negative electrode active material particles in the negative electrode active layer are the carriers for the oxidation reaction of the battery cell. The negative electrode active material can be divided into two main categories: carbon materials and non-carbon materials. Carbon materials include graphite materials (natural graphite, artificial graphite, and mesophase carbon spheres) and other carbon materials (hard carbon, soft carbon, and graphene). Non-carbon materials can be further subdivided into titanium-based materials, silicon-based materials, tin-based materials, nitrides, and lithium metal, etc.

[0073] In some embodiments, the negative electrode active layer may include a first negative electrode active layer and a second negative electrode active layer, with the first negative electrode active layer located between the negative electrode current collector and the second negative electrode active layer. By setting the first and second active layers on the negative electrode current collector, the arrangement of these two active material layers serves two purposes: first, the first negative electrode active layer is closer to the negative electrode current collector, resulting in better stability and conductivity of the lower surface where the negative electrode current collector connects to the active coating, thereby improving the electrochemical performance and cycle performance of the negative electrode; second, the second negative electrode active layer, being farther from the negative electrode current collector, is in contact with the separator 3, and its corresponding negative electrode side surface is prone to lithium plating. Therefore, the second negative electrode active layer in contact with the separator 3 can focus on stability and conductivity, which facilitates the smooth insertion and extraction of lithium ions within the positive and negative electrode materials.

[0074] The separator 3 is located between the positive electrode 1 and the negative electrode 2 to prevent short circuits caused by physical contact between the positive electrode 1 and the negative electrode 2. At the same time, it allows lithium ions to freely shuttle through the micropores while hindering electron transport, so that ions and electrons can form a circuit during the charging and discharging process of the battery. As a carrier of electrolyte, it adsorbs electrolyte to ensure ion transport efficiency. At high temperatures, it closes the pores to block ion flow and prevent thermal runaway.

[0075] The separator 3 can be any of the following: microporous membrane, modified microporous membrane, nonwoven membrane 3, and composite membrane 3. A microporous membrane is a separator 3 with pore sizes in the micrometer range, mainly including polyolefin microporous membranes and other polymer microporous membranes. A modified microporous membrane is a separator 3 obtained by modifying a microporous membrane; common modification methods include surface treatment, chemical grafting, and surface coating. A nonwoven membrane 3 has a small fiber diameter and typically exhibits higher porosity than other types of separators 3. A composite membrane 3 is prepared by coating or filling inorganic materials into a microporous membrane or nonwoven membrane 3, and exhibits higher thermal stability and electrolyte wettability compared to other types of separators 3.

[0076] In some embodiments, the separator 3 may be coated with polyvinylidene fluoride (PVDF) material to give it excellent adhesion and flexibility. Due to its excellent adhesion, the separator 3 can achieve good contact performance with the positive electrode 1 or the negative electrode 2, thereby reducing the assembly time of the energy storage cell and effectively lowering the overall production cost. The excellent flexibility of the separator 3 also improves its strength, thus enhancing its impact resistance.

[0077] In some embodiments, the secondary battery further includes a positive electrode tab and a negative electrode tab, which are metallic conductors that lead out from the positive electrode plate 1 / negative electrode plate 2 of the energy storage cell. The positive electrode tab / negative electrode tab is the contact point between the positive electrode plate 1 / negative electrode plate 2 and an external contact component when the cell is charging and discharging. The external contact component can be a terminal post.

[0078] The electrolyte is the carrier of electrons between the positive and negative electrodes in a battery. In some embodiments, the electrolyte can be a liquid electrolyte, which consists of a solvent, a lithium salt, and additives. The solvent is used to dissolve the lithium salt and can include cyclic carbonates (PC, EC); chain carbonates (DEC, DMC, EMC); and carboxylic acid esters (MF, MA, EA, MA, MP, etc.). The lithium salt can be LiPF6, LiClO4, LiBF4, LiAsF6, etc. The additives can be one or more of the following: film-forming additives, conductive additives, flame-retardant additives, overcharge protection additives, additives for controlling the H2O and HF content in the electrolyte, additives for improving low-temperature performance, and multifunctional additives.

[0079] In some embodiments, the secondary battery may further include a top cover, an adapter plate, and terminals. The top cover engages with the outer casing. The adapter plate is located within the cavity and is electrically connected to the tabs. The terminals pass through the top cover, with one end electrically connected to the adapter plate.

[0080] The adapter includes at least a first adapter and a second adapter, and the terminal includes a positive terminal and a negative terminal. The first adapter is electrically connected to the positive electrode tab and the positive terminal of the positive electrode 1, and the second adapter is electrically connected to the negative electrode tab and the negative terminal of the negative electrode 2.

[0081] This application discloses a secondary battery that utilizes a carbon-coated composite material formed within the positive electrode active material layer as a lithium replenishment composite material for lithium replenishment processes. This direct integration of the lithium-replenishing carbon-coated composite material into the positive electrode active layer results in a simple and compact structure, eliminating the need for complex additional lithium replenishment processes. Furthermore, it ensures uniform distribution of the lithium replenisher within the battery and its tight bonding with the positive electrode active material. The carbon layer encapsulates the core particles, preventing the first and second lithium salt particles from directly contacting the electrolyte during cycling and undergoing continuous side reactions that could lead to excessive consumption. This improves the utilization efficiency and stability of the lithium replenisher. Simultaneously, the carbon layer provides a robust electronic conductivity network, ensuring the electron transport required for the lithium replenishment reaction.

[0082] The carbon-coated composite material comprises a first lithium salt particle, a second lithium salt particle, and a nitrogen-doped carbon layer. The first and second lithium salt particles are used to construct an internal electric field, which drives electrons to flow from the second lithium salt particle to the first lithium salt particle. This internal electric field drives lithium ions to migrate from the second lithium salt particle to the electrolyte / interface. This provides an additional, directional driving force for lithium ion release, which can improve the lithium ion migration rate and lithium replenishment efficiency, significantly promoting and accelerating the lithium replenishment process and improving lithium replenishment efficiency. The nitrogen-doped carbon layer is used to adsorb lithium ions at a first voltage and release lithium ions at a second voltage. At the first voltage, the nitrogen-doped carbon layer adsorbs lithium ions released from the positive electrode active material or lithium replenishing agent. This prevents lithium ions from prematurely diffusing into the electrolyte and being irreversibly consumed by the negative electrode or forming dead lithium, reducing the ineffective loss of the lithium replenishing agent. At the second voltage, the nitrogen-doped carbon layer releases the adsorbed lithium ions. The timely release of lithium ions can synergize with the lithium ions generated by the decomposition of the second lithium salt particles, thus avoiding the problem of redundancy caused by a single rapid release of the first and second lithium salt particles, which could lead to the deposition of lithium dendrites on the negative electrode. Simultaneously, the ion channels formed by the nitrogen-doped carbon layer ensure uniform embedding of lithium ions into the positive electrode 1, avoiding localized stress concentration.

[0083] According to some embodiments of this application, another aspect of this application provides a method for preparing a secondary battery, comprising: forming a positive electrode sheet 1, the positive electrode sheet 1 comprising: a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector; the positive electrode active layer comprising a carbon-coated composite material, the carbon-coated composite material comprising: a first lithium salt particle and a second lithium salt particle as a core, the average particle size of the first lithium salt particle being larger than the average particle size of the second lithium salt particle, at least one second lithium salt particle being located on the surface of the first lithium salt particle; the first lithium salt particle and the second lithium salt particle are used to construct an internal electric field, the internal electric field being used for electron flow from the second lithium salt particle. The process involves: first lithium salt particles, wherein the material of the first lithium salt particles includes one or more of lithium manganese phosphate, lithium manganese oxide, or lithium vanadium oxide; second lithium salt particles, wherein the material of the second lithium salt particles includes one or more of lithium silicate or lithium borate; a nitrogen-doped carbon layer serving as a carbon coating layer, covering the first lithium salt particles and the second lithium salt particles, the nitrogen-doped carbon layer being used to adsorb lithium ions at a first voltage and release lithium ions at a second voltage; a negative electrode 2 and a separator 3 are provided; the negative electrode 2, the separator 3, and the positive electrode 1 are stacked and wound sequentially to obtain a cell assembly; the cell assembly is placed in a battery casing, electrolyte is injected into the battery casing, and then it is encapsulated to obtain a secondary battery.

[0084] In some embodiments, the process steps for forming the core composed of the first lithium salt particles and the second lithium salt particles include: forming the first lithium salt particles by a sol-gel method. The sol-gel method can provide uniform first lithium salt particles with a narrow particle size distribution, resulting in a more uniform internal electric field subsequently formed.

[0085] In some embodiments, the process steps for forming the first lithium salt particles include: providing a first lithium source material and a chelating agent, dissolving them in dilute nitric acid at a molar ratio of (0.1~0.3):(0.15~0.3), adjusting the solution pH to 3~4, heating and stirring until a wet gel is formed; drying the wet gel to form a dry gel; and heat-treating the dry gel by calcining it at a temperature of 600℃~800℃ for 10h~12h in an inert gas atmosphere to form the first lithium salt particles.

[0086] The second lithium source material and the chelating agent are dissolved in a mixture of ethanol and water at a molar ratio of (0.3~0.5):(0.2~0.3) to form a lithium salt precursor. An alkaline compound is added dropwise to the lithium salt precursor and stirred until a transparent solution is formed. The first lithium salt particles are added to the transparent solution and the pH of the solution is adjusted to 3~4. The solution is heated and stirred until a wet gel is formed. The wet gel is dried to form a dry gel. The dry gel is heat-treated by calcining at a temperature of 600℃~800℃ for 6h~10h in an inert gas atmosphere to form a core composed of the first lithium salt particles and the second lithium salt particles.

[0087] In some embodiments, the process steps for forming a nitrogen-doped carbon layer include: deoxygenating the core composed of the first lithium salt particles and the second lithium salt particles; providing a carbon source and a nitrogen source, and introducing the nitrogen source and carbon source into the reaction chamber through an inert gas; reacting at 700°C to 1000°C for 30 min to 60 min, so that the nitrogen atoms of the nitrogen source are embedded into the carbon skeleton of the carbon source through pyrolysis; and continuing to introduce an inert gas until cooling to room temperature.

[0088] In some embodiments, the volume ratio of carbon source, nitrogen source and inert gas is (0.7~1.4):(0.2~0.8):(4~10).

[0089] In some embodiments, after forming the secondary battery, the process further includes: performing a first formation process on the secondary battery using a first voltage, wherein the first formation process is used to form an internal electric field between the first lithium salt particles and the second lithium salt particles, and to release some lithium ions, wherein some lithium ions are released into the electrolyte; and other lithium ions are adsorbed by the nitrogen-doped carbon layer; and performing a second formation process on the secondary battery using a second voltage, wherein the second formation process is used to release lithium ions from the first lithium salt particles into the electrolyte, and the nitrogen-doped carbon layer releases the lithium ions adsorbed in the first formation process into the electrolyte.

[0090] In some embodiments, the first lithium salt particle is made of lithium manganese phosphate, and the second lithium salt particle is made of lithium silicate; the first voltage is 3.0V~3.2V; the second voltage is 3.2V~3.5V, excluding 3.2V.

[0091] In a specific example, the first formation process involves preferential lithium release at a low potential (3.0V~3.2V). When the charging voltage rises to 3.0V, lithium begins to be released from the interface between the LiMnPO4 core and the NC layer in the doped carbon layer of the outer shell. Lithium ions are adsorbed at pyridine nitrogen defect sites in the outer shell, forming a localized high-concentration region. The specific lithium release pathway is as follows: lithium ions rapidly migrate through the Li4SiO4 intermediate layer to the NC surface, where some are temporarily captured by pyridine nitrogen, and the remainder enters the electrolyte. A portion of the released lithium is used for the formation of the SEI film on the negative electrode, reducing irreversible losses during the first cycle. The pyridine nitrogen layer limits excessive lithium release, preventing lithium deposition on the negative electrode.

[0092] The second formation process involves deep lithium release at a mid-potential (3.2V~3.5V). When the voltage is further increased to above 3.2V, the core LiMnPO4 begins to delithigate, while the pyridine nitrogen sites in the carbon-doped layer release adsorbed lithium ions due to the increased potential, forming a secondary lithium source. The simultaneously released lithium improves the lithium replenishment efficiency. This prevents lithium redundancy caused by a rapid single release, which could lead to lithium dendrites deposited on the negative electrode. This step compensates for the loss of active lithium at the positive electrode, improving the initial efficiency of the secondary battery to over 95%.

[0093] According to some embodiments of this application, another aspect of this application provides an energy storage system, including: a secondary battery as described in any of the above embodiments or a secondary battery prepared by a method for preparing a secondary battery as described in any of the above embodiments.

[0094] According to some embodiments of this application, another aspect of this application provides an electrical device, including: a secondary battery and a load, wherein the secondary battery is used to supply power to the load; the secondary battery is any of the secondary batteries in the above embodiments, a secondary battery prepared by the preparation method of any of the secondary batteries in the above embodiments, or an energy storage system as described in the above embodiments.

[0095] The beneficial effects of the embodiments of this application will be further illustrated below with reference to examples and comparative examples.

[0096] Example 1:

[0097] (1) Preparation of the first lithium salt particles: 0.1 mol of the first lithium source material (lithium carbonate, Li2CO3) was dissolved in 40 mL of dilute nitric acid (0.1 mol / L); 0.1 mol of the manganese source (manganese acetate, Mn(CH3COO)2·4H2O) was dissolved in deionized water, and citric acid was added as a chelating agent, with the amount of citric acid being 0.15 mol. The solutions were stirred evenly. 0.1 mol of the phosphorus source (ammonium dihydrogen phosphate, NH4H2PO4) was slowly added dropwise to the above manganese source solution, and stirring was continued for 1 h. The lithium source solution was slowly poured into the manganese-phosphorus mixture, and the pH was adjusted to 3 with ammonia. Then, the solution was heated in an 80°C water bath and stirred for 3 h until a transparent sol was formed. The water bath temperature was raised to 120°C, and stirring was continued until the sol was converted into a wet gel. The wet gel was placed in an oven and dried at 80°C for 18 h to obtain a dry gel. The dried gel was calcined in a muffle furnace, heated to 400°C at a rate of 5°C / min, and held at that temperature for 3 hours to remove organic matter and residual nitrates. It was then transferred to a tube furnace and calcined at 700°C for 11 hours under an inert atmosphere (nitrogen). After natural cooling to room temperature, LiMnPO4 was obtained. The average particle size of LiMnPO4 was 300 nm.

[0098] (2) Preparation of the core composed of the first lithium salt particles and the second lithium salt particles: 0.4 mol of lithium nitrate (LiNO3) was dissolved in 100 ml of ethanol-water mixed solvent (volume ratio 1:1), stirred evenly, and 0.3 mol of citric acid was added as a chelating agent. 0.1 mol of tetraethyl orthosilicate (TEOS, Si(OC2H5)4) was slowly added dropwise to the above mixed solution and stirred until a transparent solution was formed. 0.1 mol of LiMnPO4 was added to the above solution, and the pH of the solution was adjusted to 3 with dilute nitric acid. The solution was heated and stirred in a water bath at 100 °C until a wet gel was formed. The wet gel was placed in an oven and dried at 80 °C for 20 h to obtain a dry gel. The dry gel was calcined in a muffle furnace and heated to 400 °C at a rate of 5 °C / min, and held for 4 h to remove organic matter and residual nitrates. The mixture was then transferred to a tube furnace and calcined at 800°C for 8 hours under an inert atmosphere (nitrogen) to obtain the LiMnPO4-Li4SiO4 composite material. The average particle size of Li4SiO4 was 50 nm.

[0099] (3) Preparation of nitrogen-doped carbon layer: The LiMnPO4-Li4SiO4 composite material was laid flat on a quartz boat with a thickness of <1 mm; then placed in the center of the isothermal zone of the CVD furnace. The reaction tube was sealed, and argon gas was introduced. The pipeline was flushed for 10 min to ensure an oxygen-free environment. The temperature was increased to the target temperature (900℃) at 10℃ / min, and the Ar atmosphere was maintained (50 sccm) for 30 min to remove adsorbates on the substrate surface. Methane gas was introduced using CH4 as the carbon source; liquid pyridine was evaporated at high temperature (100℃) and introduced into the reaction zone by Ar bubbling method. The ratio of the mixed gas was CH4:pyridine vapor:Ar = 1:0.5:6 (volume ratio). The target temperature (900℃) was maintained for 50 min to allow nitrogen atoms in the pyridine to be embedded into the carbon skeleton through pyrolysis. The reaction gas was turned off, and the Ar atmosphere was maintained to avoid air oxidation. The sample was removed after cooling to room temperature. The thickness of the nitrogen-doped carbon layer was 8 nm.

[0100] (4) Preparation of positive electrode sheet: During the positive electrode slurry preparation and homogenization process, the composite material is directly incorporated into LFP positive electrode powder (PVDF / NMP system) at a mass ratio of 3% to 8%, and then coated and dried to form a positive electrode sheet.

[0101] (5) Assemble the secondary battery.

[0102] (6) The first formation process is carried out when the charging voltage is raised to 3.0V; the second formation process is carried out when the charging voltage is raised to 3.2V or above.

[0103] Example 2:

[0104] The difference from Example 1 is that the first lithium salt particles are replaced with lithium manganese oxide.

[0105] Example 3:

[0106] The difference from Example 1 is that the first lithium salt particles are replaced with lithium vanadate.

[0107] Example 4:

[0108] The difference from Example 1 is that the second lithium salt particles are replaced with lithium borate.

[0109] Example 5:

[0110] The difference from Example 1 is that the first lithium salt particles are replaced with lithium manganese oxide, and the second lithium salt particles are replaced with lithium borate.

[0111] Example 6:

[0112] The difference from Example 1 is that the average particle size of LiMnPO4 is 500 nm and the average particle size of Li4SiO4 is 80 nm.

[0113] Example 7:

[0114] The difference from Example 1 is that the thickness of the nitrogen-doped carbon layer is 5 nm.

[0115] Comparative Example 1:

[0116] The difference from Example 1 is that it does not contain second lithium salt particles.

[0117] Comparative Example 2:

[0118] The difference from Example 1 is that it does not contain the first lithium salt particles.

[0119] Comparative Example 3:

[0120] The difference from Example 1 is that it does not contain a nitrogen-doped carbon layer.

[0121] Comparative Example 4:

[0122] The difference from Example 1 is that the first lithium salt particle is lithium manganese phosphate, the second lithium salt particle is lithium manganese oxide, and there is no built-in electric field between the two.

[0123] Comparative Example 5:

[0124] The difference from Example 1 is that the average particle size of the first lithium salt particles is equal to the average particle size of the second lithium salt particles.

[0125] Comparative Example 6:

[0126] The difference from Example 1 is that the thickness of the nitrogen-doped carbon layer is 3 nm.

[0127] Comparative Example 7:

[0128] The difference from Example 1 is that the thickness of the nitrogen-doped carbon layer is 13 nm.

[0129] Comparative Example 8:

[0130] The difference from Example 1 is that there is no first formation process and no second formation process.

[0131] Electrochemical performance tests were performed on the above-mentioned embodiments and comparative examples in sequence, and the test results were summarized and recorded in Table 1.

[0132] Electrochemical performance testing: The test temperature was 25±2℃. The battery was charged at a constant current of 0.5C to 3.65V; allowed to rest for 10 minutes; then discharged at a constant current of 0.5C until the cutoff voltage reached 2.5V. This capacity was recorded as the initial capacity for the rate test. The battery was then charged at a constant current of 1C to 3.65V; allowed to rest for 10 minutes; and then discharged at a constant current of 1C until the cutoff voltage reached 2.5V. This capacity was recorded as the 1C rate capacity. Here, C-rate (C for short) is a value relative to the battery's rated capacity. For example, if a battery's rated capacity is 200Ah, then 1C is equivalent to a charging or discharging rate of 200A.

[0133] The capacity retention rate of the rate capacity was obtained after 100 cycles at a 1C / 1C charge / discharge rate.

[0134] Table 1

[0135]

[0136] Analysis of the data from Examples 1-5, Comparative Examples 1-2, and Comparative Example 4 shows that the core composed of the first lithium salt particle and the second lithium salt particle can improve the initial capacity of the secondary battery and maintain a high capacity retention rate after 100 cycles, thereby improving the battery performance of the secondary battery. Analysis of the data from Examples 1, 6, and Comparative Example 5 shows that setting the average particle size of the second lithium salt particle to be smaller and the average particle size of the first lithium salt particle to be larger, so that the second lithium salt particle is located on the surface of the first lithium salt particle and fills the first lithium salt particle, can achieve a greater integration density, thus resulting in better initial capacity and capacity retention rate.

[0137] Analysis of the data from Examples 1 and 7, as well as Comparative Examples 3, 6 and 7, shows that although nitrogen-doped carbon layers can increase conductivity, excessive thickness of the nitrogen-doped carbon layer can hinder the migration of lithium ions and the wetting of the electrolyte, thereby reducing the initial capacity and capacity retention.

[0138] Analysis of the data from Examples 1 to 7 and Comparative Example 8 shows that setting the first formation process and the second formation process can activate the positive and negative electrode active materials in the secondary battery, thereby making the secondary battery composed of the first lithium salt particles and the second lithium salt particles perform better.

[0139] Those skilled in the art will understand that the above embodiments are specific examples of implementing this application, and in practical applications, various changes in form and detail can be made without departing from the spirit and scope of this application. Any person skilled in the art can make various alterations and modifications without departing from the spirit and scope of this application; therefore, the scope of protection of this application should be determined by the scope defined in the claims.

Claims

1. A secondary battery, characterized in that, include: Battery casing and electrolyte located inside the battery casing; A battery cell assembly, wherein the battery cell assembly is immersed in the electrolyte; The battery cell assembly includes a wound positive electrode sheet, a separator, and a negative electrode sheet, wherein the positive electrode sheet includes: a positive current collector and a positive active layer located on at least one surface of the positive current collector; the positive active layer includes a carbon-coated composite material, the carbon-coated composite material comprising: The first lithium salt particle and the second lithium salt particle form the core. The average particle size of the first lithium salt particle is larger than that of the second lithium salt particle, and at least one second lithium salt particle is located on the surface of the first lithium salt particle. The first lithium salt particle and the second lithium salt particle are used to construct an internal electric field, which is used to cause electrons to flow from the second lithium salt particle to the first lithium salt particle, so that lithium ions from the second lithium salt particle flow from the second lithium salt particle to the electrolyte. The material of the first lithium salt particle includes one or more of lithium manganese phosphate, lithium manganese oxide, or lithium vanadate. The material of the second lithium salt particle includes one or more of lithium silicate or lithium borate. The nitrogen-doped carbon layer, serving as a carbon coating layer, is used to adsorb lithium ions at a first voltage and release the lithium ions at a second voltage.

2. The secondary battery according to claim 1, characterized in that, The first lithium salt particle is made of lithium manganese phosphate, and the second lithium salt particle is made of lithium silicate.

3. The secondary battery according to claim 1 or 2, characterized in that, The average particle size of the first lithium salt particles is 200nm~500nm; the average particle size of the second lithium salt particles is 50nm~80nm.

4. The secondary battery according to claim 3, characterized in that, The molar ratio of the content of the first lithium salt particle to the content of the second lithium salt particle is 1:(0.75~1).

5. The secondary battery according to claim 1, characterized in that, The thickness of the nitrogen-doped carbon layer is 5 nm to 10 nm.

6. The secondary battery according to claim 1, characterized in that, The nitrogen-doped carbon layer includes a pyridine nitrogen-doped carbon layer or a pyrrole nitrogen-doped carbon layer.

7. A method for preparing a secondary battery, characterized in that, include: A positive electrode sheet is formed, the positive electrode sheet comprising: a positive current collector and a positive active layer located on at least one surface of the positive current collector; the positive active layer comprises a carbon-coated composite material, the carbon-coated composite material comprising: The first lithium salt particle and the second lithium salt particle serve as the core, with the average particle size of the first lithium salt particle being larger than that of the second lithium salt particle, and at least one second lithium salt particle located on the surface of the first lithium salt particle; the first lithium salt particle and the second lithium salt particle are used to construct an internal electric field, which is used for electrons to flow from the second lithium salt particle to the first lithium salt particle; wherein, the material of the first lithium salt particle includes one or more of lithium manganese phosphate, lithium manganese oxide, or lithium vanadate; the material of the second lithium salt particle includes one or more of lithium silicate or lithium borate. As a carbon coating layer, a nitrogen-doped carbon layer covers the first lithium salt particle and the second lithium salt particle. The nitrogen-doped carbon layer is used to adsorb lithium ions at a first voltage and release the lithium ions at a second voltage. A negative electrode and a separator are provided; the negative electrode, separator and positive electrode are stacked and wound in sequence to obtain a cell assembly; the cell assembly is placed in a battery case, electrolyte is injected into the battery case, and then it is packaged to obtain a secondary battery.

8. The method for preparing a secondary battery according to claim 7, characterized in that, The process steps for forming the core composed of the first lithium salt particle and the second lithium salt particle include: The first lithium salt particles were formed by the sol-gel method; The second lithium source material and the chelating agent are dissolved in a mixture of ethanol and water at a molar ratio of (0.3~0.5):(0.2~0.3) to form a lithium salt precursor. An alkaline compound was added dropwise to the lithium salt precursor and stirred until a transparent solution was formed; The first lithium salt particles are added to the transparent solution, and the solution pH is adjusted to 3-4. The solution is heated and stirred until a wet gel is formed. The wet gel is dried to form a dry gel; The dry gel is heat-treated by calcining at 600°C to 800°C for 6 to 10 hours in an inert gas atmosphere to form a core composed of the first lithium salt particles and the second lithium salt particles.

9. The method for preparing a secondary battery according to claim 8, characterized in that, The process steps for forming the nitrogen-doped carbon layer include: The core composed of the first lithium salt particles and the second lithium salt particles is subjected to deoxygenation treatment. A carbon source and a nitrogen source are provided, and the nitrogen source and the carbon source are introduced into the reaction chamber by an inert gas; The reaction is carried out at 700℃~1000℃ for 30min~60min, so that the nitrogen atoms of the nitrogen source are inserted into the carbon skeleton of the carbon source through cleavage; Continue to introduce the inert gas until it cools to room temperature.

10. The method for preparing a secondary battery according to claim 9, characterized in that, The volume ratio of the carbon source, the nitrogen source, and the inert gas is (0.7~1.4):(0.2~0.8):(4~10).

11. The method for preparing a secondary battery according to claim 7, characterized in that, After forming the secondary battery, the following steps are also included: The secondary battery is subjected to a first formation process using a first voltage. The first formation process is used to form an internal electric field between the first lithium salt particles and the second lithium salt particles, and to release some of the lithium ions. Some of the lithium ions are released into the electrolyte; the other part of the lithium ions are adsorbed by the nitrogen-doped carbon layer. The secondary battery is subjected to a second formation process using a second voltage. The second formation process is used to cause the first lithium salt particles to release lithium ions into the electrolyte, and the nitrogen-doped carbon layer to release the lithium ions adsorbed in the first formation process into the electrolyte.

12. The method for preparing a secondary battery according to claim 11, characterized in that, The first lithium salt particle is made of lithium manganese phosphate, and the second lithium salt particle is made of lithium silicate; the first voltage is 3.0V~3.2V; the second voltage is 3.2V~3.5V, excluding 3.2V.

13. An energy storage system, characterized in that, include: The secondary battery prepared by the method of any one of claims 1 to 6 or any one of claims 7 to 12.

14. An electrical appliance, characterized in that, include: A secondary battery and a load, wherein the secondary battery is used to supply power to the load; the secondary battery is a secondary battery as described in any one of claims 1 to 6, a secondary battery prepared by the method of preparing a secondary battery as described in any one of claims 7 to 12, or an energy storage system as described in claim 13.

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