Secondary battery and preparation method thereof, energy storage system and electric equipment

Abstract

The invention relates to the field of energy storage, and provides a secondary battery and a preparation method thereof, an energy storage system and electric equipment, the secondary battery comprises 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 internally comprises a carbon-coated composite material, the carbon-coated composite material comprises first lithium salt particles and second lithium salt particles which serve as inner cores, and the average particle size of the first lithium salt particles is larger than that of the second lithium salt particles; the first lithium salt particles and the second lithium salt particles are used for constructing a built-in electric field, and the built-in electric field is used for enabling electrons to flow from the second lithium salt particles to the first lithium salt particles; wherein the material of the first lithium salt particles comprises one or more of lithium manganese phosphate, lithium manganate or lithium vanadate; the material of the second lithium salt particles comprises one or more of lithium silicate or lithium borate; and the nitrogen-doped carbon layer is used as a carbon coating layer and is used for adsorbing lithium ions at the first voltage and releasing the lithium ions at the second voltage.

Description

Technical Field

[0001] This application relates to the field of energy storage, and particularly to a secondary battery, a preparation method thereof, an energy storage system, and an electrical device. Background Art

[0002] In recent years, with the rapid development of energy storage systems, higher requirements have been continuously put forward for various performances of lithium-ion batteries, among which the improvement of battery energy density is the most urgent. Under the existing lithium-ion battery system, on the one hand, the 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, etc.; on the other hand, through the iteration of the positive and negative electrode materials, such as using high-nickel ternary and high-voltage nickel-manganese materials for the positive electrode and high-capacity silicon and tin-based alloy anodes for the negative electrode, a large increase in the battery energy density can be achieved.

[0003] In addition, the lithium supplementation technology for lithium-ion batteries is also an important means to improve the battery energy density. During the first charging process of lithium-ion batteries, the organic electrolyte will be reduced and decomposed on the surface of the negative electrode such as graphite to form a solid electrolyte interface (Solid Electrolyte Interface, abbreviated as SEI) film, permanently consuming a large amount of lithium from the positive electrode, resulting in a low Coulomb efficiency (ICE) in the first cycle and reducing the capacity and energy density of the lithium-ion battery. In addition, there are also processes such as the inactivation of negative electrode material particles due to shedding and the irreversible deposition of lithium metal, all of which will consume the active lithium of the positive electrode and reduce the capacity and energy density of the battery.

[0004] Lithium supplementation is also called "pre-lithiation" and "pre-insertion of lithium", which is to add lithium to the inside of the battery to supplement lithium ions before the lithium-ion battery works. By pre-lithiating the electrode material to supplement lithium, the irreversible lithium loss is offset to improve the total capacity and energy density of the battery. Currently, the common lithium supplementation strategies are positive electrode lithium supplementation, negative electrode lithium supplementation, and electrolyte additive lithium supplementation. Positive electrode lithium supplementation is achieved by adding a lithium-rich compound to the positive electrode material, which releases lithium ions during charging. Negative electrode lithium supplementation is achieved by pre-storing lithium powder or lithium foil in the negative electrode, which supplements the lithium loss during discharge. Electrolyte lithium supplementation is achieved by directly providing lithium ions through the decomposition of additives.

[0005] However, the current lithium supplementation technology still needs to be improved. Summary of the Invention

[0006] The embodiments of this application provide a secondary battery, a preparation method thereof, an energy storage system, and an electrical device, which are at least beneficial to improving the lithium loss problem inside the secondary battery and increasing the total capacity and energy density of the battery.

[0007] According to some embodiments of the present application, on the one hand, an embodiment of the present application provides a secondary battery, including: a battery case and an electrolyte located within the battery case; a battery cell assembly, the battery cell assembly being located within the cavity and the battery cell assembly being 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 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, the carbon-coated composite material includes: a first lithium salt particle as a core and a second lithium salt particle, the average particle size of the first lithium salt particle is greater than the average particle size of the second lithium salt particle, and at least one of the second lithium salt particles 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, and the internal electric field is used to make electrons 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 manganate, 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 is used to adsorb lithium ions at a first voltage and release the lithium ions at a second voltage.

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

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

[0010] In some embodiments, 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 to 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 the present application, on the other hand, an embodiment of the present application provides a method for preparing a secondary battery, including: forming a positive electrode sheet, the positive electrode sheet including: 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, the carbon-coated composite material including: a first lithium salt particle and a second lithium salt particle as the core, 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 of the second lithium salt particles 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, and the internal electric field 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 manganate 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 the carbon-coated layer, the nitrogen-doped carbon layer covering the first lithium salt particle and the second lithium salt particle, and the nitrogen-doped carbon layer is used to adsorb lithium ions at a first voltage and release the lithium ions at a second voltage; providing a negative electrode sheet and a separator; stacking or winding the negative electrode sheet, the separator and the positive electrode sheet in sequence to obtain an electrode core assembly, putting the electrode core assembly into a battery case, injecting an electrolyte into the battery case, and then encapsulating to obtain a secondary battery.

[0014] In some embodiments, the process steps for forming the core composed of the first lithium salt particle and the second lithium salt particle include: forming the first lithium salt particle by the sol-gel method; dissolving a second lithium source material and a chelating agent in a mixed solution of ethanol and water according to a molar ratio of (0.3~0.5):(0.2~0.3) to form a lithium salt precursor; dropping an alkaline compound into the lithium salt precursor and stirring until a transparent solution is formed; adding the first lithium salt particle into 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; performing heat treatment on the dry gel, and calcining at a temperature of 600°C~800°C for 6h~10h in an inert gas atmosphere to form the core composed of the first lithium salt particle and the second lithium salt particle.

[0015] In some embodiments, the process steps for forming the nitrogen-doped carbon layer include: performing deoxidation treatment on the core composed of the first lithium salt particle and the second lithium salt particle; 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~1000°C for 30min~60min to enable the nitrogen atoms of the nitrogen source to be embedded into the carbon skeleton of the carbon source through cracking; continuing to introduce the inert gas until it cools 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 method further includes: performing a first formation process on the secondary battery using a first voltage, where 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 release part of the lithium ions, with part of the lithium ions released into the electrolyte; and the other part of the lithium ions adsorbed by the nitrogen-doped carbon layer; performing a second formation process on the secondary battery using a second voltage, where the second formation process is used to release the 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.

[0018] In some embodiments, the material of the first lithium salt particles is composed of lithium manganese phosphate, and the material of the second lithium salt particles is composed of lithium silicate; the first voltage is 3.0V~3.2V; the second voltage is 3.2V~3.5V and does not include 3.2V.

[0019] According to some embodiments of the present application, on the other hand, the present application provides an energy storage system, including: the secondary battery as described in any one of the above embodiments or the secondary battery prepared by the preparation method of the secondary battery as described in any one of the above embodiments.

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

[0021] The technical solution provided by the embodiments of the present application has at least the following advantages: The present application provides a secondary battery. By forming a carbon-coated composite material within the positive electrode active material layer and using this as a lithium supplement composite material to perform the lithium supplement process, the carbon-coated composite material with the lithium supplement function is directly integrated into the positive electrode active layer. The structure is simple and compact, without the need for an additional complex lithium supplement process, and it can ensure that the lithium supplement agent is evenly distributed inside the battery and tightly combined with the positive electrode active material. The carbon layer wraps the core particles, preventing the internal first lithium salt particles and second lithium salt particles from directly contacting the electrolyte during the cycling process and undergoing continuous side reactions and excessive consumption, thereby improving the utilization efficiency and stability of the lithium supplement agent. At the same time, the carbon layer provides a good electronic conduction network to ensure the electron transfer required for the lithium supplement reaction.

[0022] The carbon-coated composite material includes a first lithium salt particle, a second lithium salt particle, and a nitrogen-doped carbon layer. The first lithium salt particle and the second lithium salt particle are used to construct an internal electric field, which is used to make electrons flow from the second lithium salt particle to the first lithium salt particle. The internal electric field will drive lithium ions to migrate from the second lithium salt particle to the electrolyte / interface. This provides an additional and directional driving force for the release of lithium ions, which can improve the lithium ion migration rate and lithium supplementation efficiency, significantly promote and accelerate the lithium supplementation process, and improve the lithium supplementation efficiency. The nitrogen-doped carbon layer is used to adsorb lithium ions at the first voltage and release lithium ions at the second voltage. At the first voltage, the nitrogen-doped carbon layer will adsorb the lithium ions released from the positive electrode active material or the lithium supplement agent. It can prevent lithium ions from diffusing prematurely into the electrolyte and being irreversibly consumed by the negative electrode or forming dead lithium, reducing the ineffective loss of the lithium supplement agent; at the second voltage, the nitrogen-doped carbon layer will release the lithium ions it adsorbs. Releasing lithium ions at this time can cooperate with the lithium ions generated by the decomposition of the second lithium salt particle itself to avoid redundancy due to a single rapid release of the first lithium salt particle and the second lithium salt particle, and the problem of lithium dendrites deposited on the negative electrode. At the same time, the ion channels formed based on the nitrogen-doped carbon layer ensure the uniform embedding of lithium ions into the positive electrode sheet, avoiding the problem of local stress concentration. Description of the Drawings

[0023] One or more embodiments are exemplarily illustrated by the figures in the corresponding drawings. These exemplary illustrations do not constitute a limitation on the embodiments. Unless otherwise stated, the figures in the drawings do not constitute a proportional limitation; in order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the following described drawings are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0024] Figure 1 It is a cross-sectional view of a battery cell assembly in a secondary battery provided by an embodiment of the present application.

[0025] Description of the Reference Numerals: 1. Positive electrode sheet; 2. Negative electrode sheet; 3. Separator. Detailed Embodiments

[0026] As can be seen from the background art, there is a lithium loss problem in the current secondary battery, and the total capacity and energy density of the secondary battery need to be further improved.

[0027] The embodiments of the present application provide a secondary battery, its preparation method, an energy storage system, and an 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-coated layer, a better lithium supplementation effect can be provided.

[0028] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity, specific order or primary-secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality" is more than two, unless otherwise clearly and specifically defined.

[0029] Referring to "embodiment" herein means that a specific feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present application. The phrase appearing at various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art will explicitly and implicitly understand that the embodiments described herein may be combined with other embodiments.

[0030] In the description of the embodiments of the present application, the term "and / or" is merely a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: the existence of A, the simultaneous existence of A and B, and the existence of B. In addition, the character " / " in this article generally represents an "or" relationship between the associated objects before and after.

[0031] In the description of the embodiments of the present application, the term "a plurality" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0032] In the description of the embodiments of the present application, the orientation or positional relationship indicated by technical terms such as "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the embodiments of the present application and simplifying the description, rather than indicating or implying that the indicated device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0033] In the description of the embodiments of the present application, unless otherwise clearly specified and limited, technical terms such as "installation", "connection", "connection", "fixation", etc. should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to specific situations.

[0034] In the accompanying drawings corresponding to the embodiments of the present application, for better understanding and convenience of description, the thickness and area of the layers are enlarged. When describing a component (such as a layer, film, region, or substrate) on or on the surface of another component, the component can be "directly" on the surface of the other component, or there can 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 provided on the surface of a component, it means that there is no third component between the two components. In addition, when describing a component "substantially" formed on another component, it means that the component is not formed on the entire surface (or front surface) of the other component, nor on a partial edge of the entire surface.

[0035] In the description of the embodiments of the present application, when a certain component "includes" another component, unless otherwise specified, other components are not excluded, and other components may further be included. In addition, when components such as layers, films, regions, or plates are referred to as "on / located on" another component, it can be "directly on" the other component (that is, located on the surface of the other component with no other components between them), or there can be another component between them. In addition, when components such as layers, films, regions, or plates are "directly located on" another component, or when components such as layers, films, regions, or plates are located on the surface of another component, it means that no other components are located between them.

[0036] The terms used in the description of the various embodiments herein are only for describing specific embodiments and are not intended to be limiting. As used in the description of the various embodiments and the appended claims, "the part" is also intended to include the plural form, unless the context clearly indicates otherwise. Among them, the components include components such as layers, films, regions, or plates.

[0037] The following will elaborate on the embodiments of the present application in conjunction with the accompanying drawings. However, those of ordinary skill in the art can understand that in the embodiments of the present application, many technical details are presented for the reader to better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0038] Figure 1 It is a cross-sectional view of a battery cell assembly in a secondary battery provided in an embodiment of the present application.

[0039] It should be noted that, for the purpose of illustration and to distinguish among the separator, the positive electrode sheet, and the negative electrode sheet, Figure 1 the separator is schematically shown by a dashed line, the negative electrode sheet is schematically shown by a thinner solid line, and the positive electrode sheet is schematically shown by a thicker solid line, but this does not represent the thickness relationship among the separator, the positive electrode sheet, and the negative electrode sheet. Secondly, Figure 1There are gaps between the middle separator, the positive electrode sheet, and the negative electrode sheet pairwise. This is to clearly show the winding correspondence relationship among the separator, the positive electrode sheet, and the negative electrode sheet. However, it does not mean that there must be gaps between the separator and the positive electrode sheet or between the separator and the negative electrode sheet. That is to say, the separator and the positive electrode sheet can be in contact with each other, or the separator and the negative electrode sheet can be in contact with each other.

[0040] According to some embodiments of the present application, on the one hand, an embodiment of the present application provides a secondary battery, including: a battery case and an electrolyte located inside the battery case; a battery cell assembly, the battery cell assembly is located in the cavity, and the battery cell assembly is immersed in the electrolyte; the battery cell assembly includes a wound positive electrode sheet 1, a separator 3, and a negative electrode sheet 2. Among them, the positive electrode sheet 1 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, and the carbon-coated composite material includes: a first lithium salt particle as the core and a second lithium salt particle, the average particle size of the first lithium salt particle 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, and the internal electric field is used to make electrons flow from the second lithium salt particle to the first lithium salt particle; among them, the material of the first lithium salt particle includes one or more of lithium manganese phosphate, lithium manganate, 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 the carbon coating layer, and the nitrogen-doped carbon layer is used to adsorb lithium ions at the first voltage and release lithium ions at the second voltage.

[0041] The present application provides a secondary battery. By forming a carbon-coated composite material in the positive electrode active material layer and using it as a lithium supplement composite material to perform the lithium supplement process, the carbon-coated composite material with the lithium supplement function is directly integrated into the positive electrode active layer. The structure is simple and compact, without the need for an additional complex lithium supplement process, and it can ensure that the lithium supplement agent is evenly distributed inside the battery and tightly combined with the positive electrode active substance. The carbon layer wraps the core particles, preventing the internal first lithium salt particles and second lithium salt particles from directly contacting the electrolyte during the cycle and undergoing continuous side reactions and excessive consumption, improving the utilization efficiency and stability of the lithium supplement agent. At the same time, the carbon layer provides a good electron conductive network to ensure the electron transfer required for the lithium supplement reaction.

[0042] The carbon-coated composite material includes a first lithium salt particle, a second lithium salt particle, and a nitrogen-doped carbon layer. The first lithium salt particle and the second lithium salt particle are used to construct an internal electric field, which is used to make electrons flow from the second lithium salt particle to the first lithium salt particle. The internal electric field will drive lithium ions to migrate from the second lithium salt particle to the electrolyte / interface. This provides an additional and directional driving force for the release of lithium ions, which can improve the lithium ion migration rate and lithium supplementation efficiency, significantly promote and accelerate the lithium supplementation process, and improve the lithium supplementation efficiency. The nitrogen-doped carbon layer is used to adsorb lithium ions at the first voltage and release lithium ions at the second voltage. At the first voltage, the nitrogen-doped carbon layer will adsorb the lithium ions released from the cathode active material or the lithium supplement agent. It can prevent lithium ions from diffusing prematurely into the electrolyte and being irreversibly consumed by the anode or forming dead lithium, reducing the ineffective loss of the lithium supplement agent; at the second voltage, the nitrogen-doped carbon layer will release the lithium ions it adsorbed. Releasing lithium ions at this time can cooperate with the lithium ions generated by the decomposition of the second lithium salt particle itself to avoid redundancy due to a single rapid release of the first lithium salt particle and the second lithium salt particle, and the problem of lithium dendrites deposited on the anode. At the same time, the ion channels formed based on the nitrogen-doped carbon layer ensure that lithium ions are evenly embedded in the positive electrode sheet 1, avoiding the problem of local stress concentration.

[0043] Classified by shape, the prepared secondary battery can be divided into a square battery cell, a round battery cell, or a soft-pack battery cell. Classified by capacity, the secondary battery can be divided into models such as 50Ah, 100Ah, 150Ah, 200Ah, 280Ah, 306Ah, 314Ah, 500+Ah, 800+Ah, and 1000+Ah. Classified by the chemical composition and working principle of the battery cell components of the secondary battery, the secondary battery can be a lithium-ion battery, a lead-acid battery, a sodium-ion battery, or a nickel-metal hydride battery. In this application example, the preparation method of a lithium-ion battery is taken as an example. Those skilled in the art can replace the lithium ions in the positive electrode sheet 1, the negative electrode sheet 2, and the electrolyte with corresponding metal ions according to actual needs. For example, for a sodium-ion battery, replace the lithium transition metal oxide of the cathode active material in the following with any one of the corresponding layered metal oxides (such as NaFeO2), polyanion compounds (NaFePO4), and Prussian blue compound systems (such as NaMnFe(CN)6-zH2O), and replace the electrolyte with any one of an organic liquid electrolyte, a solid composite electrolyte, or a solid electrolyte.

[0044] Reference Figure 1 , the positive electrode sheet 1 serves as the cathode in the redox reaction. During discharge, the cathode material (lithium salt material) releases lithium ions and electrons through a chemical reaction; during charging, it reaccepts lithium ions.

[0045] The positive electrode sheet 1 includes a positive electrode current collector and a positive electrode active layer. The positive electrode current collector can be aluminum foil. The price of aluminum foil is lower than that of 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 conductivity through the tunneling effect.

[0046] In some embodiments, the positive electrode current collector of the positive electrode sheet 1 can also be a composite current collector. The composite current collector includes three stacked layers. The middle layer is an organic substance, and the upper and lower layers are copper-plated and aluminum-plated. The organic substance is PET (polyester), PP (polypropylene), PI (polyimide), etc.

[0047] The positive electrode active material in the positive electrode active layer is a kind of energy storage material, usually composed of metal oxides, metal sulfides or polymers, etc. Its main function is to chemically react with the lithium ions in the negative electrode during battery charging, thereby storing lithium ions, increasing the lithium ion concentration, and thus enabling the positive electrode of the battery to release charge current. At the same time, during battery discharge, the lithium ions stored in the positive electrode active material move to the negative electrode and react with the negative electrode material, forming a potential difference between the positive and negative electrodes of the battery, thereby generating current output.

[0048] A carbon-coated composite material is provided in the positive electrode active layer. The carbon-coated composite material serves as a lithium supplement agent. Integrating the carbon-coated composite material with lithium supplement function directly into the positive electrode active layer has a simple and compact structure, without the need for additional complex lithium supplement processes, and can ensure that the lithium supplement agent is evenly distributed inside the battery and tightly combined with the positive electrode active material. The carbon layer wraps the core particles, preventing the internal first lithium salt particles and second lithium salt particles from directly contacting the electrolyte during cycling and undergoing continuous side reactions and excessive consumption, improving the utilization efficiency and stability of the lithium supplement agent. At the same time, the carbon layer provides a good electron conduction network to ensure the electron transport required for the lithium supplement reaction.

[0049] The first lithium salt particles and the second lithium salt particles serve as lithium supplement agents and also as positive electrode active materials.

[0050] The material of the first lithium salt particles includes one or more of lithium manganese phosphate, lithium manganate or lithium vanadate. Lithium manganese phosphate (working voltage is 4.1V), lithium manganate (working voltage is 4.0V), and lithium vanadate (working voltage is 2.5 - 3.5V) all have relatively high working voltages, while the working voltage of the second lithium salt particles (lithium silicate / lithium borate) is usually relatively low (working voltage < 2.0V). The heterogeneous structure formed by the two will generate a significant potential difference, which is conducive to establishing an internal electric field pointing from the second lithium salt particles to the first lithium salt particles at the interface.

[0051] The charge-discharge platform of the first lithium salt material is located in the conventional working voltage range of the battery (working voltage is 3.0V - 4.2V), and it can participate in charge and discharge as an active material by itself to provide capacity.

[0052] In some embodiments, the average particle size of the first lithium salt particles is 200 nm to 500 nm. In this way, the average particle size of the first lithium salt particles is relatively large. As the substrate, the first lithium salt particles can provide a stable support point for the small particle lithium supplement attached thereto, which helps to maintain the structural stability of the composite material, prevent the small particles from agglomerating or falling off, and ensure the durability of the lithium supplement 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.

[0053] The material of the second lithium salt particles includes one or more of lithium silicate or lithium borate.

[0054] The lithium supplement mechanism of the first lithium salt particles and the second lithium salt particles is as follows: at the initial stage of charging, the second lithium salt particles are preferentially decomposed to supplement lithium; as the voltage increases, the first lithium salt particles start to de-lithiate, and the released lithium ions can be supplied to the negative electrode synergistically with the lithium ions released by the lithium supplement agent. This staged lithium ion supply mechanism better meets the requirements of the negative electrode for forming the SEI film and intercalating lithium, and avoids side reactions caused by instantaneous excessive lithium ions.

[0055] 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 small average particle size and a large specific surface area, and are more likely to undergo a decomposition reaction at a lower voltage, releasing a large amount of lithium ions to compensate for the irreversible capacity loss of the negative electrode. The large particle first lithium salt particles not only form the basis of the built-in electric field, but also provide a stable support point for the small particle lithium supplement attached thereto, which helps to maintain the structural stability of the composite material, prevent the small particles from agglomerating or falling off, and ensure the durability of the lithium supplement function.

[0056] 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.

[0057] It should be noted that the particle size is the diameter of the particles. Among them, most particles are not standard spherical but irregular, and their particle sizes are represented by an average value. The physical meaning of the average particle size: It represents a scale of 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 when the cumulative particle size distribution percentage of a sample reaches 50%. D90: The particle size corresponding to when the cumulative particle size distribution number of a sample reaches 90%. Among them, the average particle sizes of the first lithium salt particles and the second lithium salt particles can be obtained by directly measuring the particle sizes through a scanning electron microscope or by a particle size analyzer.

[0058] The morphology of the second lithium salt particles is spherical or quasi-spherical particles.

[0059] 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). In this way, the second lithium salt particles provide sufficient active lithium while avoiding excessive side reactions. The built-in electric field drives lithium ions to flow into the electrolyte rather than diffuse disorderly inside the positive electrode. Among them, the molar ratio of the content 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.

[0060] In some embodiments, the second lithium salt particles are uniformly dispersed on the surface of the first lithium salt particles. In this way, volume expansion can be inhibited. The second lithium salt particles will have a 5% volume change during the delithiation process, while the volume expansion of the first lithium salt particles during the cycling process is less than 1%. The rigid structure of the first lithium salt particles can effectively buffer stress and prevent particle breakage. Therefore, the cycling stability is greatly improved. It can prevent particle agglomeration. The second lithium salt particles are uniformly dispersed on the surface of the first lithium salt particles, physically isolating the active substances and inhibiting agglomeration during high-temperature calcination or cycling. An internal electric field is formed at the interface between the first lithium salt particles and the second lithium salt particles, reducing the activation energy for lithium extraction and increasing the lithium ion migration rate, while also improving the lithium supplementation efficiency.

[0061] In some embodiments, the material of the first lithium salt particles is composed of lithium manganese phosphate, and the material of the second lithium salt particles is composed of lithium silicate. Lithium manganese phosphate controllably releases lithium at 3.0V - 3.3V, matching the voltage platform 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 smaller and are uniformly distributed on the surface of the lithium manganese phosphate. Moreover, an internal electric field is formed between the inner first lithium salt particles and the interface of the second lithium salt particles, increasing the lithium ion migration rate and the lithium supplementation efficiency.

[0062] The nitrogen doping in the nitrogen-doped carbon layer endows the carbon layer with special chemical activity, enabling it to have voltage-dependent lithium-ion adsorption / release ability to release lithium ions and capture lithium ions at different times, avoiding the lithium dendrite problem caused by excessive lithium ions on the surface of the negative electrode sheet 2. Secondly, the nitrogen-doped carbon layer has high stability, which can inhibit electrolyte corrosion and enhance electron conduction.

[0063] 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 <5 nm, the quantum tunneling effect will cause abnormal electron transitions, destroying the stability of the built-in electric field. And when the thickness of the nitrogen-doped carbon layer >10 nm, it will significantly increase the electron conduction resistance. This range allows electrons to pass through smoothly without disturbing the built-in electric field. The lithium silicate lithium supplement agent will have an 8% volume shrinkage when releasing lithium ions. The nitrogen-doped carbon layer is like an elastic mesh bag, which can absorb stress through its own micro-deformation. If it is too thin, it is easy to crack, and if it is too thick, it will limit the ion diffusion channel; this range can enable the nitrogen-doped carbon layer to accommodate the deformation of the second lithium salt particles without affecting the ion diffusion channel.

[0064] In some embodiments, the nitrogen-doped carbon layer includes a pyridine-nitrogen-doped carbon layer or a pyrrole-nitrogen-doped carbon layer.

[0065] The negative electrode sheet 2 serves as the anode in the redox reaction. During discharge, the negative electrode material (such as graphite, silicon-based material) accepts and stores lithium ions; during charging, lithium ions are released.

[0066] The negative electrode sheet 2 includes a negative electrode current collector and a negative electrode active layer, and the negative electrode active layer is located on the negative electrode current collector.

[0067] The negative electrode current collector can be a copper foil. The copper foil has low conductivity, can have high electron transport ability, and has weak lithium-ion intercalation ability, capturing fewer lithium ions, thus effectively reducing the loss of lithium ions. In some other embodiments, the negative electrode current collector can also be a foam copper current collector, a copper mesh current collector, and a three-dimensional nano-copper array current collector.

[0068] In some embodiments, the negative electrode current collector can be a composite current collector, including a polymer material layer and a metal coating layer. The metal coating layer is located on both the upper and lower sides of the polymer material layer. The polymer material layer is PET (polyethylene terephthalate), PP (polypropylene), PI (polyimide), etc., and the metal coating layer is a copper layer.

[0069] In some embodiments, the negative electrode current collector can also be a carbon-based current collector, that is, there is a layer of conductive carbon layer on the copper foil. The conductive carbon layer can serve as a protective layer to effectively protect the negative electrode current collector to prevent corrosion of the metal current collector, thereby improving the lifespan of the negative electrode current collector; secondly, the conductive carbon layer itself has a low resistivity, so that excessive electrical losses will not be generated. Among them, the material of the conductive carbon layer can be flake graphite, spherical graphite, carbon nanotubes, graphene, etc.

[0070] 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 major categories: carbon materials and non-carbon materials. Carbon-based materials include graphite materials (natural graphite, artificial graphite, and mesophase carbon microspheres) and other carbon-based 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 metallic lithium, etc.

[0071] In some embodiments, the negative electrode active layer can include a first negative electrode active layer and a second negative electrode active layer. The first negative electrode active layer is located between the negative electrode current collector and the second negative electrode active layer. By providing the first active layer and the second active layer on the negative electrode current collector, firstly, based on the fact that the first negative electrode active layer is closer to the negative electrode current collector, the stability and conductivity of the lower surface where the negative electrode current collector is connected to the active coating are better, thereby improving the electrochemical performance and cycling performance of the negative electrode; secondly, for the second negative electrode active layer far from the negative electrode current collector, which is in contact with the separator 3, lithium plating is likely to occur on the corresponding surface on the negative electrode side. Therefore, the second negative electrode active layer in contact with the separator 3 can focus on stability and conductivity, which is conducive to the smooth insertion and extraction of lithium ions inside the positive and negative electrode materials.

[0072] The separator 3 is located between the positive electrode sheet 1 and the negative electrode sheet 2 to prevent short-circuit problems caused by physical contact between the positive electrode sheet 1 and the negative electrode sheet 2. At the same time, it allows lithium ions to freely shuttle through the micropores and hinders electron transmission, so that ions and electrons form a circuit during the charge and discharge process of the battery; as a carrier of the electrolyte, it adsorbs the electrolyte to ensure the ion transport efficiency; it closes the pores at high temperatures to block ion flow and prevent thermal runaway.

[0073] The separator 3 can be any one of a microporous membrane, a modified microporous membrane, a non-woven fabric separator 3, and a composite separator 3. The microporous membrane is a separator 3 with pore diameters in the micron range, mainly including polyolefin microporous membranes and other polymer microporous membranes. The modified microporous membrane is a separator 3 obtained by modifying the microporous membrane. Common modification methods include surface treatment, chemical grafting, surface coating, etc. The non-woven fabric separator 3 has a small fiber diameter and usually exhibits a higher porosity than other types of separators 3. The composite separator 3 is prepared by coating or filling inorganic materials in the microporous membrane or the non-woven fabric separator 3, and has higher thermal stability and electrolyte wettability compared to other types of separators 3.

[0074] In some embodiments, the separator 3 may be coated with a polyvinylidene fluoride (PVDF) material, so that the separator 3 has excellent adhesiveness and flexibility. Due to the excellent adhesiveness of the separator 3, good contact performance can be achieved between the separator 3 and the positive electrode sheet 1 or between the separator 3 and the negative electrode sheet 2, thereby reducing the assembly time of the energy storage battery cell and effectively reducing the overall production cost of the energy storage battery cell. Due to the excellent flexibility of the separator 3, the strength of the separator 3 is improved, which is beneficial to effectively improving the impact resistance of the separator 3.

[0075] In some embodiments, the secondary battery further includes a positive electrode tab and a negative electrode tab. The positive electrode tab / negative electrode tab is a metal conductor that leads out the positive electrode sheet 1 / negative electrode sheet 2 of the energy storage battery cell from the battery cell. The positive electrode tab / negative electrode tab is the contact point between the positive electrode sheet 1 / negative electrode sheet 2 and the external contact component during the charge and discharge of the battery cell. The external contact component may be a terminal post.

[0076] The electrolyte is a carrier for conducting electrons between the positive and negative electrodes of the battery. In some embodiments, the electrolyte may be an electrolyte solution, which is composed of a solvent, a lithium salt, and an additive. The solvent is used to dissolve the lithium salt, and the solvent may include cyclic carbonates (PC, EC); chain carbonates (DEC, DMC, EMC); carboxylic acid esters (MF, MA, EA, MA, MP, etc.). The lithium salt may be LiPF6, LiClO4, LiBF4, LiAsF6, etc. The additive may be one or more of a film-forming additive, a conductive additive, a flame retardant additive, an overcharge protection additive, an additive for controlling the content of H2O and HF in the electrolyte solution, an additive for improving low-temperature performance, and a multifunctional additive.

[0077] In some embodiments, the secondary battery may further include a top cover, a connecting piece, and a terminal post. The top cover is engaged with the outer shell. The connecting piece is located in the chamber and is electrically connected to the tab. The terminal post passes through the top cover, and one end of the terminal post is electrically connected to the connecting piece.

[0078] Among them, the connecting piece at least includes a first connecting piece and a second connecting piece, the terminal post includes a positive electrode terminal post and a negative electrode connecting post, the first connecting piece is electrically connected to the positive electrode tab of the positive electrode sheet 1 and the positive electrode terminal post respectively, and the second connecting piece is electrically connected to the negative electrode tab of the negative electrode sheet 2 and the negative electrode connecting post respectively.

[0079] This application provides a secondary battery. By forming a carbon-coated composite material within the positive electrode active material layer and using this as a lithium supplement composite material for the lithium supplementation process, the carbon-coated composite material with the lithium supplementation function is directly integrated into the positive electrode active layer. The structure is simple and compact, without the need for an additional complex lithium supplementation process, and it can ensure that the lithium supplement agent is evenly distributed inside the battery and tightly combined with the positive electrode active material. The carbon layer wraps the core particles, preventing the internal first lithium salt particles and second lithium salt particles from directly contacting the electrolyte during the cycling process and undergoing continuous side reactions and excessive consumption, thereby improving the utilization efficiency and stability of the lithium supplement agent. At the same time, the carbon layer provides a good electronic conductive network to ensure the electron transfer required for the lithium supplementation reaction.

[0080] The carbon-coated composite material includes first lithium salt particles, second lithium salt particles, and a nitrogen-doped carbon layer. The first lithium salt particles and the second lithium salt particles are used to construct an internal electric field. The internal electric field is used to make electrons flow from the second lithium salt particles to the first lithium salt particles, and the internal electric field will drive lithium ions to migrate from the second lithium salt particles to the electrolyte / interface. This provides an additional and directional driving force for the release of lithium ions, which can increase the lithium ion migration rate and lithium supplementation efficiency, significantly promote and accelerate the lithium supplementation process, and improve the lithium supplementation efficiency. The nitrogen-doped carbon layer is used to adsorb lithium ions at the first voltage and release lithium ions at the second voltage. At the first voltage, the nitrogen-doped carbon layer will adsorb the lithium ions released from the positive electrode active material or the lithium supplement agent. This can prevent 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 supplement agent; at the second voltage, the nitrogen-doped carbon layer will release the lithium ions it has adsorbed. Releasing lithium ions at this time can cooperate with the lithium ions generated by the decomposition of the second lithium salt particles themselves to avoid redundancy caused by the one-time rapid release of the first lithium salt particles and the second lithium salt particles, and the problem of lithium dendrites deposited on the negative electrode. At the same time, the ion channels formed based on the nitrogen-doped carbon layer ensure the uniform embedding of lithium ions into the positive electrode sheet 1, avoiding the problem of local stress concentration.

[0081] According to some embodiments of the present application, on the other hand, an embodiment of the present application provides a method for preparing a secondary battery, including: forming a positive electrode sheet 1, the positive electrode sheet 1 including: 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, the carbon-coated composite material including: a first lithium salt particle as the 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 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, and the internal electric field 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 manganate 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 the carbon-coated layer, the nitrogen-doped carbon layer covering the first lithium salt particle and the second lithium salt particle, and the nitrogen-doped carbon layer is used to adsorb lithium ions at a first voltage and release lithium ions at a second voltage; providing a negative electrode sheet 2 and a separator 3; stacking the negative electrode sheet 2, the separator 3 and the positive electrode sheet 1 in sequence and winding them to obtain an electrode assembly, placing the electrode assembly into a battery case, injecting an electrolyte into the battery case, and then encapsulating to obtain a secondary battery.

[0082] In some embodiments, the process steps for forming the core composed of the first lithium salt particle and the second lithium salt particle include: forming the first lithium salt particle by the sol-gel method. The sol-gel method can provide the first lithium salt particle with uniform and narrow particle size distribution, and the subsequent formed internal electric field is more uniform.

[0083] In some embodiments, the process steps for forming the first lithium salt particle include: providing a first lithium source material and a chelating agent, dissolving them in dilute nitric acid according to a molar ratio of (0.1~0.3):(0.15~0.3), 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; performing heat treatment on the dry gel, and calcining it at a temperature of 600°C~800°C for 10h~12h in an inert gas atmosphere to form the first lithium salt particle.

[0084] Dissolve the second lithium source material and the chelating agent in a mixed solution of ethanol and water according to a molar ratio of (0.3~0.5):(0.2~0.3) to form a lithium salt precursor; dropwise add an alkaline compound to the lithium salt precursor and stir until a transparent solution is formed; add the first lithium salt particle to the transparent solution and adjust 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; performing heat treatment on the dry gel, and calcining it at a temperature of 600°C~800°C for 6h~10h in an inert gas atmosphere to form the core composed of the first lithium salt particle and the second lithium salt particle.

[0085] In some embodiments, the process steps for forming the nitrogen-doped carbon layer include: performing deoxidation treatment on 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 cracking; continuously introducing the inert gas until it cools down to room temperature.

[0086] In some embodiments, the volume ratio of the carbon source, the nitrogen source, and the inert gas is (0.7 to 1.4):(0.2 to 0.8):(4 to 10).

[0087] In some embodiments, after forming the secondary battery, it further includes: performing a first formation process on the secondary battery using a first voltage, and 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 release part of the lithium ions, and part of the lithium ions are released into the electrolyte; another part of the lithium ions are adsorbed by the nitrogen-doped carbon layer; performing a second formation process on the secondary battery using a second voltage, and 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.

[0088] In some embodiments, the material of the first lithium salt particles is composed of lithium manganese phosphate, and the material of the second lithium salt particles is composed of lithium silicate; the first voltage is 3.0 V to 3.2 V; the second voltage is 3.2 V to 3.5 V and does not include 3.2 V.

[0089] In a specific example, the first formation process: lithium is preferentially released at a low potential (3.0 V to 3.2 V). When the charging voltage rises to 3.0 V, the lithium at the N-C interface between LiMnPO4 in the core and the doped carbon layer of the outer shell starts to escape, and the pyridine nitrogen defect sites in the outer shell adsorb lithium ions, forming a locally high-concentration region. The specific lithium release path is that lithium ions rapidly migrate to the N-C surface through the Li4SiO4 intermediate layer, part of which is temporarily captured by pyridine nitrogen, and the rest enter the electrolyte. Among them, part of the released lithium is used for the formation of the negative electrode SEI film, reducing the irreversible loss of the first cycle. The pyridine nitrogen layer restricts the excessive release of lithium and prevents lithium deposition on the negative electrode.

[0090] The second formation process: lithium is deeply released at a medium potential (3.2 V to 3.5 V). When the voltage further rises above 3.2 V, the LiMnPO4 in the core starts to release lithium, and the pyridine nitrogen sites in the doped carbon layer release the adsorbed lithium ions due to the increase in potential, forming a secondary lithium source. The synchronously released lithium improves the lithium replenishment efficiency. Avoiding the redundancy caused by the rapid release of lithium at one time and depositing lithium dendrites on the negative electrode. This step replenishes the loss of active lithium in the positive electrode, and the first efficiency of the secondary battery is increased to more than 95%.

[0091] According to some embodiments of the present application, on the other hand, an energy storage system is provided, including: a secondary battery as described in any one of the above embodiments or a secondary battery prepared by the preparation method of the secondary battery as described in any one of the above embodiments.

[0092] According to some embodiments of the present application, on yet another aspect, an electrical device is provided, including: a secondary battery and a load, the secondary battery is used to supply power to the load; the secondary battery is a secondary battery as described in any one of the above embodiments, a secondary battery prepared by the preparation method of the secondary battery as described in any one of the above embodiments, or the energy storage system as described in the above embodiments.

[0093] Hereinafter, the beneficial effects of the embodiments of the present application will be further described in conjunction with examples and comparative examples.

[0094] Example 1: (1) Preparation of the first lithium salt particles: Dissolve 0.1 mol of the first lithium source material (lithium carbonate, Li2CO3) in 40 mL of dilute nitric acid (0.1 mol / L); dissolve 0.1 mol of the manganese source (manganese acetate, Mn(CH3COO)2·4H2O) in deionized water, add citric acid as a chelating agent, and the amount of citric acid is 0.15 mol. Stir evenly respectively. Slowly drop 0.1 mol of the phosphorus source (ammonium dihydrogen phosphate, NH4H2PO4) into the above manganese source solution, and continue to stir for 1 h. Slowly pour the lithium source solution into the manganese-phosphorus mixture, and adjust the pH = 3 with ammonia water. Then heat in a water bath at 80 °C and stir for 3 h until a transparent sol is formed. Raise the temperature of the water bath to 120 °C, and continuously stir until the sol is converted into a wet gel. Place the wet gel in an oven and dry it at 80 °C for 18 h to obtain a dry gel. Put the dry gel into a muffle furnace, heat it at 5 °C / min to 400 °C, and keep it warm for 3 h to remove organic substances and residual nitrates. Subsequently, transfer it to a tubular furnace, place it under an inert atmosphere (nitrogen), and calcine it at 700 °C for 11 h. Naturally cool to room temperature to obtain LiMnPO4. The average particle size of LiMnPO4 is 300 nm.

[0095] (2) Preparation of the core composed of the first lithium salt particles and the second lithium salt particles: Dissolve 0.4 mol of lithium nitrate (LiNO3) in 100 ml of ethanol-water mixed solvent (volume ratio 1:1), stir evenly, add citric acid as a chelating agent, and the amount of citric acid is 0.3 mol. Using tetraethyl orthosilicate (TEOS, Si(OC2H5)4) as the silicon source, slowly drop 0.1 mol of TEOS into the above mixed solution and stir until a transparent solution is formed. Add 0.1 mol of LiMnPO4 to the above solution and adjust the pH of the solution to 3 with dilute nitric acid. Heat and stir in a water bath at 100 °C until a wet gel is formed. Place the wet gel in an oven and dry it at 80 °C for 20 h to obtain a dry gel. Put the dry gel into a muffle furnace, heat it to 400 °C at a rate of 5 °C / min, and keep it for 4 h to remove organic substances and residual nitrates. Subsequently, transfer it to a tubular furnace, place it under an inert atmosphere (nitrogen), and calcine it at 800 °C for 8 h to obtain the LiMnPO4-Li4SiO4 composite material. The average particle size of Li4SiO4 is 50 nm.

[0096] (3) Preparation of the nitrogen-doped carbon layer: Lay the LiMnPO4-Li4SiO4 composite material flat on a quartz boat with a thickness of <1 mm; then place it in the center of the constant temperature zone of a CVD furnace. Seal the reaction tube, introduce argon, and first flush the pipeline for 10 min to ensure an oxygen-free environment. Heat it to the target temperature (900 °C) at a rate of 10 °C / min, maintain the Ar atmosphere (50 sccm) for 30 min to remove the adsorbed substances on the substrate surface. Use CH4 as the carbon source and introduce methane gas; evaporate liquid pyridine at a high temperature (100 °C) and introduce it into the reaction zone through the carrier gas Ar bubbling method for reaction. Among them, the ratio of the mixed gas is CH4:pyridine vapor:Ar = 1:0.5:6 (volume ratio). Keep it at the target temperature (900 °C) for 50 min to embed the nitrogen atoms in pyridine into the carbon skeleton through cracking. Turn off the reaction gas, maintain the Ar atmosphere, avoid air oxidation, and take out the sample after cooling to room temperature. The thickness of the nitrogen-doped carbon layer is 8 nm.

[0097] (4) Preparation of the positive electrode sheet: For positive electrode pulping, during the homogenization process, directly incorporate the composite material into the LFP positive electrode powder (PVDF / NMP system) at a mass ratio of 3% - 8%, and form a positive electrode sheet after coating and drying.

[0098] (5) Assemble a secondary battery.

[0099] (6) Raise the charging voltage to 3.0 V for the first formation process; conduct the second formation process when the charging voltage rises above 3.2 V.

[0100] Example 2: The difference from Example 1 is that the first lithium salt particles are replaced with lithium manganate.

[0101] Example 3: The difference from Example 1 is that the first lithium salt particle is replaced by lithium vanadate.

[0102] Example 4: The difference from Example 1 is that the second lithium salt particle is replaced by lithium borate.

[0103] Example 5: The difference from Example 1 is that the first lithium salt particle is replaced by lithium manganate, and the second lithium salt particle is replaced by lithium borate.

[0104] Example 6: 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.

[0105] Example 7: The difference from Example 1 is that the thickness of the nitrogen-doped carbon layer is 5 nm.

[0106] Comparative Example 1: The difference from Example 1 is that the second lithium salt particle is not included.

[0107] Comparative Example 2: The difference from Example 1 is that the first lithium salt particle is not included.

[0108] Comparative Example 3: The difference from Example 1 is that the nitrogen-doped carbon layer is not included.

[0109] Comparative Example 4: The difference from Example 1 is that the first lithium salt particle is lithium manganese phosphate, the second lithium salt particle is lithium manganate, and there is no built-in electric field between them.

[0110] Comparative Example 5: The difference from Example 1 is that the average particle size of the first lithium salt particle is equal to the average particle size of the second lithium salt particle.

[0111] Comparative Example 6: The difference from Example 1 is that the thickness of the nitrogen-doped carbon layer is 3 nm.

[0112] Comparative Example 7: The difference from Example 1 is that the thickness of the nitrogen-doped carbon layer is 13 nm.

[0113] Comparative Example 8: The difference from Example 1 is that there is no first formation process and second formation process.

[0114] The above examples and comparative examples were successively subjected to electrochemical performance tests, and the test results were summarized and recorded in Table 1.

[0115] Electrochemical performance test: The test temperature is 25 ± 2 °C. It is charged at a constant current of 0.5C to 3.65V; left standing for 10 min, and then discharged at a constant current of 0.5C until the cut-off voltage is 2.5V. Record this capacity and use it as the initial capacity in the rate test. Then, charge it at a constant current of 1C to 3.65V; leave it standing for 10 min; then discharge it at a constant current of 1C until the cut-off voltage is 2.5V, and record this capacity and use it as the rate capacity at 1C. Among them, the C-rate (abbreviated as C) is a value relative to the rated capacity of the battery. For example, if the rated capacity of a battery is 200 Ah, then 1C is equivalent to a charging or discharging rate of 200 A.

[0116] At a charge-discharge rate of 1C / 1C, after 100 cycles, the capacity retention rate of the rate capacity is obtained.

[0117] Table 1

[0118] From the data analysis of Examples 1 to 5, Comparative Examples 1 to 2, and Comparative Example 4, it can be seen that the core composed of the first lithium salt particles and the second lithium salt particles can improve the initial capacity of the secondary battery, and after 100 cycles, it can still have a high capacity retention rate, thereby improving the battery performance of the secondary battery. From the data analysis of Example 1, Example 6, and Comparative Example 5, it can be seen that by setting the average particle size of the second lithium salt particles to be smaller and the average particle size of the first lithium salt particles to be larger, so that the second lithium salt particles are located on the surface of the first lithium salt particles and fill the first lithium salt particles, a larger integration density can be obtained, thereby bringing better initial capacity and capacity retention rate.

[0119] From the data analysis of Example 1, Example 7, and Comparative Examples 3, 6, and 7, it can be seen that although the nitrogen-doped carbon layer can increase the conductivity, if the thickness of the nitrogen-doped carbon layer is too thick, it will hinder the migration of lithium ions and the infiltration of the electrolyte, thereby reducing the initial capacity and capacity retention rate.

[0120] From the data analysis of Examples 1 to 7 and Comparative Example 8, it can be seen that setting the first formation process and the second formation process can activate the positive and negative active materials in the secondary battery, thereby making the battery performance of the secondary battery composed of the first lithium salt particles and the second lithium salt particles better.

[0121] Those of ordinary skill in the art can understand that the above embodiments are specific embodiments for implementing the present application. In actual applications, various changes can be made in form and details without departing from the spirit and scope of the present application. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the protection scope of the present application should be determined by the scope defined in the claims.

Claims

1. A secondary battery, characterized in that, Comprising: A battery case and an electrolyte located within the battery case; A battery cell assembly, the battery cell assembly being immersed in the electrolyte; The battery cell assembly includes a wound positive electrode sheet, a separator, and a negative electrode sheet. Among them, the positive electrode sheet 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, and the carbon-coated composite material includes: A first lithium salt particle and a second lithium salt particle as the core, the average particle size of the first lithium salt particle being larger than that of the second lithium salt particle, and at least one of the second lithium salt particles 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 a built-in electric field, and the built-in electric field is used to make electrons flow from the second lithium salt particle to the first lithium salt particle; among them, the material of the first lithium salt particle includes one or more of lithium manganese phosphate, lithium manganate, 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 the 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.

2. The secondary battery according to claim 1, wherein The material of the first lithium salt particle is composed of lithium manganese phosphate, and the material of the second lithium salt particle is composed 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 particle is 200 nm to 500 nm; the average particle size of the second lithium salt particle is 50 nm to 80 nm.

4. The secondary battery according to claim 3, wherein 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, wherein The thickness of the nitrogen-doped carbon layer is 5 nm to 10 nm.

6. The secondary battery according to claim 1, wherein 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, Comprising: Form a positive electrode sheet, the positive electrode sheet 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, and the carbon-coated composite material includes: A first lithium salt particle and a second lithium salt particle as the core, the average particle size of the first lithium salt particle being larger than that of the second lithium salt particle, and at least one of the second lithium salt particles 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 a built-in electric field, and the built-in electric field is used for electrons to flow from the second lithium salt particle to the first lithium salt particle; among them, the material of the first lithium salt particle includes one or more of lithium manganese phosphate, lithium manganate, 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 the carbon coating layer, the nitrogen-doped carbon layer covering the first lithium salt particle and the second lithium salt particle, and the nitrogen-doped carbon layer is used to adsorb lithium ions at a first voltage and release the lithium ions at a second voltage; Provide a negative electrode sheet and a separator; stack the negative electrode sheet, the separator, and the positive electrode sheet in sequence and wind them to obtain a battery cell assembly, place the battery cell assembly into a battery case, inject electrolyte into the battery case, and then encapsulate to obtain a secondary battery.

8. The method for preparing a secondary battery according to claim 7, wherein 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 the sol-gel method; Dissolving the second lithium source material and the chelating agent in a mixed solution of ethanol and water according to a molar ratio of (0.3 - 0.5):(0.2 - 0.3) to form a lithium salt precursor; Dropping the alkaline compound into the lithium salt precursor and stirring until a transparent solution is formed; Adding the first lithium salt particles to the transparent solution, 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; Performing heat treatment on the dry gel, and calcining at a temperature of 600°C - 800°C for 6h - 10h under an inert gas atmosphere to form the core composed of the first lithium salt particles and the second lithium salt particles.

9. The manufacturing method of the secondary battery according to claim 8, characterized in that, The process steps for forming the nitrogen-doped carbon layer include: Performing deoxygenation treatment on 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 - 1000°C for 30min - 60min to enable the nitrogen atoms of the nitrogen source to be embedded into the carbon skeleton of the carbon source through cracking; Continuing 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, it further includes: Performing a first formation process on the secondary battery using a first voltage, where 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 release part of the lithium ions. Part of the lithium ions are released into the electrolyte; another part of the lithium ions are adsorbed by the nitrogen-doped carbon layer; Performing a second formation process on the secondary battery using a second voltage, where the second formation process is used to release the 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.

12. The manufacturing method of the secondary battery according to claim 11, wherein The material of the first lithium salt particles is composed of lithium manganese phosphate, and the material of the second lithium salt particles is composed of lithium silicate; the first voltage is 3.0V - 3.2V; the second voltage is 3.2V - 3.5V and does not include 3.2V.

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

14. An electrical device, characterized in that, It includes: A secondary battery and a load, where the secondary battery is used to supply power to the load; the secondary battery is the secondary battery according to any one of claims 1 - 6, the secondary battery prepared by the preparation method of the secondary battery according to any one of claims 7 - 12, or the energy storage system according to claim 13.

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