Lithium secondary battery, positive electrode active material and preparation method therefor, and electric device

Abstract

A lithium secondary battery, a positive electrode active material and a preparation method therefor, and an electric device. The positive electrode active material comprises an active substance LimAxFe1-yGyP1-zDzO4-nEn, wherein A includes one or more elements of Zn, Al, Na, K, and Mg; G includes one or more elements of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, and Ti; D includes one or more elements of B, S, Si, and N; E includes one or more elements of S, F, Cl, and Br; m=0.5-1.15; x=0-0.1; y=0-0.5; z=0-0.5; and n=0-0.5. The surfaces of active material particles are each coated with a carbonaceous material layer. The ratio of the thickness of the carbonaceous material layer to the diameter of primary particles of the positive electrode active material ranges from 1 / 500 to 1 / 100.

Description

Lithium secondary battery, positive electrode active material, preparation method thereof and electric device

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the priority benefit of Chinese Patent Application No. 202510205850.3, filed on February 24, 2025, and incorporates it in its entirety. TECHNICAL FIELD

[0003] The present application relates to the field of batteries, in particular, to a lithium secondary battery, a positive electrode active material, a preparation method thereof and an electric device BACKGROUND

[0004] Batteries are not only applied to energy storage power systems such as hydroelectric, thermal, wind and solar power stations, but also widely used in electric vehicles such as electric bicycles, electric motorcycles, electric cars, electric transportation tools, military equipment, aerospace and other fields. Among them, lithium secondary batteries have shown broad application prospects in many fields due to their high energy density, long life and environmental friendliness. However, at present, lithium secondary batteries such as phosphate-based positive electrode active materials have the problem of fast capacity retention rate decay rate in the early stage. SUMMARY

[0005] The first aspect of the present application provides a lithium secondary battery, the lithium secondary battery comprising a positive electrode active material, the positive electrode active material comprising an active substance, the active substance being as shown in formula (1): Li m A x Fe 1-y G y P 1-z D z O 4-n E n (1);

[0006] Wherein, A comprises one or more elements of Zn, Al, Na, K, Mg; G comprises one or more elements of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti; D comprises one or more elements of B, S, Si, N; E comprises one or more elements of S, F, Cl, Br; m = 0.5-1.15; x = 0-0.1; y = 0-0.5; z = 0-0.5; n = 0-0.5;

[0007] The particle surface of the active substance is coated with a carbonaceous material layer;

[0008] The ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 500-1 / 100.

[0009] The phosphate active material provided in the application has high safety and good electrochemical performance such as cycle performance. The carbon material layer coated on the surface of the active material can protect the interface and improve the interface stability. This is because the carbon material layer is formed between the active material and the electrolyte to avoid the direct contact between the electrolyte and the bulk phase of the active material to generate side reactions, thereby reducing the occurrence of side reactions, improving the structural stability of the positive active material, reducing the damage of lithium ions, and slowing down the capacity decay rate in the early stage. In addition, the carbon material layer coated on the surface of the active material is beneficial to improve the electronic conductivity of the positive active material, construct an electronic conduction network at the level of the electrode sheet, thereby accelerating the electron transport rate, improving the electrochemical performance, and helping to reduce the polarization phenomenon during the battery charging and discharging process, thereby facilitating the obtaining of a higher capacity retention rate.

[0010] In the application, the ratio h / d of the thickness h of the carbon material layer to the diameter d of the primary particles of the positive active material meets the above conditions, which is beneficial to maintain good kinetic performance and achieve the effect of slowing down the capacity retention rate decay in the early stage. If the ratio h / d of the thickness h of the carbon material layer to the diameter d of the primary particles of the positive active material is too small, the carbon material layer is relatively thin, and the diameter of the primary particles is relatively large, so the protective effect of the carbon material layer on the contact interface between the positive active material and the electrolyte is small, and thus the effect of slowing down the capacity retention rate decay is relatively poor. Since lithium ions and electrons need to pass through the coating layer to act, if the ratio h / d of the thickness h of the carbon material layer to the diameter d of the primary particles of the positive active material is too large, the carbon material layer is relatively thick, and the diameter of the primary particles is relatively small, which will increase the transmission impedance of lithium ions and electrons and deteriorate the kinetic performance of the material. The larger the h / d is, the better the effect of slowing down the capacity retention rate decay in the early stage can be achieved, but the kinetic deterioration will also increase, causing excessive capacity loss of the battery, which is not conducive to the improvement of the overall performance of the battery. In the embodiments of the application, the kinetic performance of the positive active material can be improved by doping modification of the phosphate active material, the capacity of the battery can be improved, and the effective balance between the capacity and the capacity retention rate performance can be achieved.

[0011] In some embodiments of the application, the active material Li m A x Fe 1-y G y P 1-z D z O 4-n E n (i) to (v) are met:

[0012] (i) m = 0.95-1.05;

[0013] (ii) x = 0.001-0.005;

[0014] (iii) y = 0.001 ~ 0.1;

[0015] (iv) z = 0.001 ~ 0.1;

[0016] (v)n=0.001~0.1.

[0017] This application can meet the requirements for improving battery performance through element doping, such as improving the kinetic performance of the positive electrode active material.

[0018] In some embodiments of this application, the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 400 to 1 / 200. This helps to slow down the rate of capacity retention decay in the early stages of battery operation.

[0019] In some embodiments of this application, the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 350 to 1 / 300. This helps to better mitigate the rate of capacity retention decay in the early stages of battery cycling.

[0020] In some embodiments of this application, the lithium secondary battery satisfies at least one of the following (I) and (II):

[0021] (I) The thickness h of the carbonaceous material layer is 1 nm to 5 nm;

[0022] (II) The diameter d of the primary particles of the positive electrode active material is 200 nm to 600 nm.

[0023] The thickness of the carbonaceous material layer, meeting the above conditions, helps to address the problem of excessively rapid capacity retention decay in the early stages of battery cycling, mitigates the adverse effects on the kinetics of the positive electrode active material, and balances the kinetic performance and sustained-release effect of the positive electrode active material. The diameter of the primary particles of the positive electrode active material, meeting the above conditions, helps to improve the material's kinetic performance, increase the powder compaction density of the positive electrode active material and the compaction density of the positive electrode sheet, and ultimately improve battery capacity.

[0024] In some embodiments of this application, the lithium secondary battery satisfies at least one of the following (1) and (2):

[0025] (1) The thickness h of the carbonaceous material layer is 2nm to 4nm;

[0026] (2) The diameter d of the primary particles of the positive electrode active material is 200nm~400nm.

[0027] The thickness of the carbonaceous material layer meets the above conditions, which is beneficial for balancing the kinetic performance and sustained-release effect of the positive electrode active material. The diameter of the primary particles of the positive electrode active material meets the above conditions, which is beneficial for improving battery capacity.

[0028] In some embodiments of this application, the lithium secondary battery satisfies at least one of the following (A) and (B):

[0029] (A) The thickness h of the carbonaceous material layer is 2nm to 3nm;

[0030] (B) The diameter d of the primary particles of the positive electrode active material is 200nm to 300nm.

[0031] The thickness of the carbonaceous material layer meets the above conditions, which helps to better balance the kinetic performance and sustained-release effect of the positive electrode active material. The diameter of the primary particles of the positive electrode active material meets the above conditions, which helps to better improve the battery capacity.

[0032] In some embodiments of this application, the carbonaceous material layer accounts for 1.0% to 2.0% of the total mass of the active material and the carbonaceous material layer. This is beneficial for balancing the kinetic performance and sustained-release effect of the positive electrode active material.

[0033] In some embodiments of this application, the lithium secondary battery further includes an electrolyte comprising lithium difluorophosphate. Adding lithium difluorophosphate as an additive to the electrolyte can improve the cell capacity and reduce the side effects of excessively thick carbonaceous material coating.

[0034] In some embodiments of this application, the mass content of lithium difluorophosphate is 0.01% to 2% based on the total mass of the electrolyte. By controlling the mass content of lithium difluorophosphate in the electrolyte to meet the above conditions, it is beneficial to effectively exert the role of lithium difluorophosphate as a bifunctional additive, which can improve battery capacity and reduce the side effects of excessive carbon material layer coating; at the same time, it is beneficial to mitigate the negative impact of excessive lithium difluorophosphate on the battery.

[0035] In some embodiments of this application, the mass content of lithium difluorophosphate is 0.02% to 0.6% based on the total mass of the electrolyte. This is beneficial for effectively utilizing the bifunctional additive role of lithium difluorophosphate, which can improve the cell capacity and reduce the side effects of excessively thick carbon material coating.

[0036] In some embodiments of this application, the mass content of lithium difluorophosphate is 0.05% to 0.3% based on the total mass of the electrolyte. This is beneficial for better utilizing the bifunctional additive role of lithium difluorophosphate, improving the cell capacity, and reducing the side effects of excessively thick carbon material coating.

[0037] In some embodiments of this application, the lithium secondary battery further includes an electrolyte comprising vinylene carbonate, wherein the mass content of vinylene carbonate is 0.01% to 2% based on the total mass of the electrolyte. Vinylene carbonate (VC), as an electrolyte additive, can improve battery capacity and cycle life.

[0038] In some embodiments of this application, the lithium secondary battery further includes an electrolyte comprising vinylene carbonate, wherein the mass content of vinylene carbonate is 0.02% to 0.6% based on the total mass of the electrolyte. This facilitates the effective function of vinylene carbonate in the electrolyte and enhances battery capacity while reducing the side effects of excessively thick carbonaceous material coating.

[0039] In some embodiments of this application, the lithium secondary battery further includes an electrolyte comprising vinylene carbonate, wherein the mass content of vinylene carbonate is 0.05% to 0.3% based on the total mass of the electrolyte. This facilitates better function of vinylene carbonate in the electrolyte and enhances battery capacity while reducing the side effects of excessively thick carbonaceous material layers.

[0040] In some embodiments of this application, the carbonaceous material layer comprises one or more of graphite, graphene, amorphous carbon, carbon fiber, carbon nanotubes, carbon dots, and carbon felt. This is beneficial for improving the conductivity of the positive electrode active material.

[0041] In some embodiments of this application, the positive electrode active material satisfies at least one of the following (a) to (c):

[0042] (a) The BET specific surface area of ​​the positive electrode active material is 10m² 2 / g~15m 2 / g;

[0043] (b) The powder resistivity of the positive electrode active material at 25°C is 1Ω·m to 30Ω·m;

[0044] (c) The compaction density of the positive electrode active material under 3T pressure is 2.35 g / cm³. 3 ~2.65g / cm 3 .

[0045] The BET specific surface area of ​​the positive electrode active material meets the above conditions, which is beneficial to improving the kinetic performance of the material.

[0046] The powder resistivity of the positive electrode active material at 25°C meets the above conditions, which is beneficial for obtaining a high capacity retention rate.

[0047] The compaction density of the positive electrode active material under 3T pressure meets the above conditions, which is beneficial to improving the energy density and capacity of the battery.

[0048] The second aspect of this application discloses a method for preparing a positive electrode active material, comprising:

[0049] Provide lithium phosphate precursors;

[0050] The lithium phosphate precursor and carbon source were ground together.

[0051] The ground material is heated to a first temperature T1 at a first rate under a protective gas atmosphere and held at the first temperature T1 for a first time t1; then it is heated to a second temperature T2 at a second rate and held at the second temperature T2 for a second time t2 to obtain the positive electrode active material.

[0052] The positive electrode active material includes an active substance, which is shown in formula (1): Li m A x Fe 1-y G y P 1-z D z O 4-n E n (1);

[0053] Wherein, A includes one or more elements selected from Zn, Al, Na, K, and Mg; G includes one or more elements selected from Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, and Ti; D includes one or more elements selected from B, S, Si, and N; E includes one or more elements selected from S, F, Cl, and Br; m = 0.5–1.15; x = 0–0.1; y = 0–0.5; z = 0–0.5; n = 0–0.5;

[0054] The surface of the active substance particles is coated with a layer of carbonaceous material;

[0055] The ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 500 to 1 / 100.

[0056] This application employs a solid-state method to prepare a positive electrode active material with a carbonaceous material coating, which facilitates the formation of a uniform carbonaceous material coating layer on the surface of the active material. Furthermore, parameters such as the crystallinity, particle size, and distribution of the positive electrode active material can be controlled by adjusting at least one of the following parameters: first rate, first temperature T1, first time t1, second rate, second temperature T2, and second time t2.

[0057] In some embodiments of this application, the first rate is ≤4℃ / min. This facilitates the control of the primary particle size and distribution of the positive electrode active material, thereby improving the effect on the reduction of early-stage cycle capacity retention in the battery.

[0058] In some embodiments of this application, the first rate is 1°C / min to 4°C / min. This facilitates better control of the primary particle size and distribution of the positive electrode active material, improving the effect of mitigating the rapid decay of the battery's early cycle capacity retention.

[0059] In some embodiments of this application, the first temperature T1 ≥ 400℃. This is beneficial for improving the crystallinity of the material.

[0060] In some embodiments of this application, the first temperature T1 is 400℃~600℃. This is beneficial for improving the crystallinity of the material.

[0061] In some embodiments of this application, the first temperature T1 is 450°C to 550°C. This is beneficial for improving the crystallinity of the material.

[0062] In some embodiments of this application, the first time t1 is ≥ 1h. The first time t1 can be matched with the first rate to improve the particle size and distribution of the positive electrode active material.

[0063] In some embodiments of this application, the first time t1 is 1h to 7h. The first time t1 can be matched with the first rate to better improve the particle size and distribution of the positive electrode active material.

[0064] In some embodiments of this application, the first time t1 is 3h to 5h. The first time t1 can be matched with the first rate to better improve the particle size and distribution of the positive electrode active material.

[0065] In some embodiments of this application, the second temperature T2 ≥ 720℃. This is beneficial for controlling the primary particle size and distribution of the positive electrode active material, thereby improving the capacity, powder compaction density, crystallinity, and other properties of the positive electrode active material.

[0066] In some embodiments of this application, the second temperature T2 is 720℃~850℃. This facilitates better control of the primary particle size and distribution of the positive electrode active material, thereby improving the capacity, powder compaction density, crystallinity, and other properties of the positive electrode active material.

[0067] In some embodiments of this application, the second temperature T2 is 750℃~810℃. This facilitates better control of the primary particle size and distribution of the positive electrode active material, thereby improving the capacity, powder compaction density, crystallinity, and other properties of the positive electrode active material.

[0068] In some embodiments of this application, the preparation method satisfies at least one of the following (α) and (β):

[0069] (α) The second rate is 1℃ / min to 10℃ / min;

[0070] (β) The second time t2 is 5h to 20h.

[0071] If at least one parameter in the second rate and the second time t2 satisfies the above conditions, it is beneficial to control the particle size and distribution of the primary particles of the positive electrode active material and obtain higher kinetic performance.

[0072] In some embodiments of this application, the preparation method satisfies at least one of the following (①) and (②):

[0073] (①) The second rate is 5℃ / min~9℃ / min;

[0074] (②) The second time t2 is 8h to 15h.

[0075] If at least one parameter in the second rate and the second time t2 satisfies the above conditions, it is beneficial to better control the particle size and distribution of the primary particles of the positive electrode active material and obtain higher kinetic performance.

[0076] In some embodiments of this application, the carbon source includes a first carbon source, which comprises a water-soluble polymer. This is beneficial for increasing the density of the carbonaceous material and enhancing its interfacial protection effect on the positive electrode active material.

[0077] In some embodiments of this application, the water-soluble polymer includes one or more of polyethylene glycol, polyaniline, and their respective derivatives. This is beneficial for increasing the graphitization degree and density of the carbonaceous material layer of the positive electrode active material, thereby improving the interfacial stability of the positive electrode active material.

[0078] In some embodiments of this application, the weight content of the first carbon source is ≥50% based on the total weight of the carbon source. This is beneficial for improving the interfacial stability of the carbon material layer with the positive electrode active material.

[0079] In some embodiments of this application, the weight content of the first carbon source is 50% to 70% based on the total weight of the carbon source. This is beneficial for improving the interfacial stability of the carbon material layer with the positive electrode active material.

[0080] In some embodiments of this application, the carbon source further includes a second carbon source, which includes one or more of glucose, sucrose, lactose, and maltose. This helps reduce the cost of the carbon source; furthermore, it helps improve the electrical and processing properties of the positive electrode active material.

[0081] In some embodiments of this application, the weight content of the second carbon source is ≤50% based on the total weight of the carbon sources. By controlling the ratio of the first carbon source to the second carbon source, it is beneficial to adjust the cost of the carbon sources and the coating quality of the carbonaceous material layer.

[0082] In some embodiments of this application, the weight content of the second carbon source is 30% to 50% based on the total weight of the carbon sources. By controlling the ratio of the first carbon source to the second carbon source, it is beneficial to better adjust the cost of the carbon source and the coating quality of the carbonaceous material layer.

[0083] The third aspect of this application proposes a positive electrode active material, which is either the positive electrode active material described in the first aspect or a positive electrode active material prepared by the preparation method described in the second aspect. The proposed positive electrode active material possesses the beneficial effects of the positive electrode active materials described in the first and second aspects, which will not be elaborated further here.

[0084] The fourth aspect of this application discloses an electrical device comprising the lithium secondary battery described in the first aspect. The lithium secondary battery provided by this application has a high initial cycle capacity and a slow initial cycle capacity decay.

[0085] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0086] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0087] Figure 1 is a schematic diagram of a battery according to one embodiment of this application.

[0088] Figure 2 is an exploded view of the battery according to one embodiment of this application shown in Figure 1.

[0089] Figure 3 is a schematic diagram of a battery module according to one embodiment of this application.

[0090] Figure 4 is a schematic diagram of a battery pack according to one embodiment of this application.

[0091] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4.

[0092] Figure 6 is a schematic diagram of an electrical device in which a battery is used as a power source according to an embodiment of this application.

[0093] Explanation of reference numerals in the attached drawings: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0094] The present application will be further described below with reference to specific embodiments. It should be understood that these specific embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0095] In this application, the reference to "embodiment" means that a specific 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 mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this application can be combined with other embodiments.

[0096] The "range" disclosed in this application is defined in the form of a lower limit and / or an upper limit. A given range is defined by selecting a lower limit and / or an upper limit, which defines the boundary of the particular range. Ranges defined in this way may or may not include endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form an undefined range, and any lower limit can be combined with other lower limits to form an undefined range. Similarly, any upper limit can be combined with any other upper limit to form an undefined range. Furthermore, each individually disclosed point or single value can itself serve as a lower limit or upper limit, combined with any other point or single value, or combined with other lower limits or upper limits to form an undefined range.

[0097] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0098] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0099] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps S1 and S2, indicating that the method may include steps S1 and S2 performed sequentially, or it may include steps S2 and S1 performed sequentially. For example, the method may also include step S3, indicating that step S3 may be added to the method in any order. For example, the method may include steps S1, S2, and S3, or it may include steps S1, S3, and S2, or it may include steps S3, S1, and S2, etc.

[0100] Unless otherwise specified, 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: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0101] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0102] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in this application is for the purpose of describing particular embodiments only and is not intended to limit this application; the terms "comprising" and "having," and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion. Unless otherwise stated, the terms used in this application have their commonly understood meanings as understood by one of ordinary skill in the art. Unless otherwise stated, the numerical values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0103] Currently, judging from market trends, battery applications are becoming increasingly widespread. Batteries are not only used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, but also extensively in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. With the continuous expansion of battery applications, market demand is also constantly increasing.

[0104] The positive electrode active material proposed in this application can be used in lithium secondary batteries, and further in electrical devices that use batteries as a power source or in various energy storage systems that use batteries as energy storage elements. Electrical devices can include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., while spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0105] During the charging and discharging process of a lithium-ion rechargeable battery, lithium ions undergo intercalation and deintercalation in the positive electrode active material. During charging, lithium ions are extracted from the lattice of the positive electrode active material and migrate to the negative electrode; during discharging, lithium ions are extracted from the negative electrode and return to the positive electrode active material. In the early stages of cycling a lithium-ion rechargeable battery, if the positive electrode active material is in direct contact with the electrolyte, side reactions may occur between them. These side reactions generate insoluble products, leading to irreversible reactions and causing the positive electrode active material to lose its original structure. Furthermore, these side reactions can cause the dissolution of non-lithium metal ions (such as transition metal ions) from the positive electrode active material, affecting its structural stability. The dissolved transition metal ions can also adversely affect the stability of the SEI film on the negative electrode surface, consuming more lithium ions. All these adverse effects accelerate the rate of capacity retention decay in the early stages of battery operation.

[0106] To address the issue of excessively rapid capacity retention decay in the early stages of lithium-ion batteries, this application's embodiments utilize a design to mitigate the rate of capacity retention decay in the early stages of lithium-ion batteries.

[0107] The first aspect of this application proposes a lithium secondary battery, which includes a positive electrode active material, the positive electrode active material including an active substance as shown in formula (1): Li m A x Fe 1-y G y P 1-z D z O 4-n E n (1);

[0108] Wherein, A includes one or more elements selected from Zn, Al, Na, K, and Mg; G includes one or more elements selected from Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, and Ti; D includes one or more elements selected from B, S, Si, and N; E includes one or more elements selected from S, F, Cl, and Br; m = 0.5–1.15; x = 0–0.1; y = 0–0.5; z = 0–0.5; n = 0–0.5;

[0109] The surface of the active substance particles is coated with a layer of carbonaceous material;

[0110] The ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 500 to 1 / 100.

[0111] The active material is a compound represented by formula (1), which is a phosphate-based positive electrode active material. It can be undoped lithium iron phosphate or a material that has been doped and modified from lithium iron phosphate.

[0112] 1) In equation (1), Li m A x Fe 1-y G y P 1-z D z O 4-n E n When m = 0.5–1.15; x = 0; y = 0; z = 0; n = 0, it represents undoped lithium iron phosphate, an undoped substance. As an example, m can be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, etc., or a range of any two of the above values, or a value within that range.

[0113] 2) In equation (1), Li m A x Fe 1-y G y P 1-z D z O 4-n E n When m = 0.5–1.15; 0 < x ≤ 0.1; y = 0; z = 0; and n = 0, it indicates that the lithium iron phosphate has undergone Li-site doping modification. As an example, x can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, etc., or a range of any two of the above values, or a value within that range.

[0114] 3) In equation (1), Li m A x Fe 1-y G y P 1-z D z O 4-n E n When m = 0.5–1.15; x = 0; 0 < y ≤ 0.5; z = 0; and n = 0, it indicates that the lithium iron phosphate has undergone Fe-site doping modification. As an example, y can be 0.1, 0.2, 0.3, 0.4, 0.5, etc., or a range of any two of the above values, or a value within that range.

[0115] 4) In equation (1), Li m A x Fe 1-y G y P 1-z D z O 4-n E n When m = 0.5–1.15; x = 0; y = 0; 0 < z ≤ 0.5; and n = 0, it indicates that the lithium iron phosphate has undergone P-site doping modification. As an example, z can be 0.1, 0.2, 0.3, 0.4, 0.5, etc., or a range of any two of the above values, or a value within that range.

[0116] 5) In equation (1), Li m A x Fe 1-y G y P 1-z D z O 4-n E nWhen m = 0.5–1.15; x = 0; y = 0; z = 0; and 0 < n ≤ 0.5, it indicates that the lithium iron phosphate has undergone O-site doping modification. As an example, n can be 0.1, 0.2, 0.3, 0.4, 0.5, etc., or a range of any two of the above values, or a value within that range.

[0117] When modifying lithium iron phosphate with doping, one of the Li, Fe, P, and O sites can be doped individually, or two or more of them can be doped simultaneously.

[0118] The detection of lithium iron phosphate or its doped and modified forms can be performed using inductively coupled plasma (ICP). ICP measures the mass percentage of each element in the positive electrode active material. Dividing the mass percentage of each element by its relative atomic mass yields the molar ratio of each element. This molar ratio is then simplified to its simplest integer proportion, which represents the atomic ratio of the elements. Based on this atomic ratio, the chemical formula of the active material can be deduced.

[0119] The surface of the active substance particles is coated with a carbonaceous material layer, which can be a full coating or a discontinuous coating, such as an island-like coating.

[0120] The thickness h of the carbonaceous material layer can be determined using conventional methods in the field, such as FIB (Focused Ion Beam). A specific method may include the following steps: randomly selecting a single particle from the positive electrode active material powder to be tested; cutting a thin slice approximately 100 nm thick from the center or near the center of the selected particle; performing TEM on the slice to obtain the original TEM image; and saving the original image in xx.dm3 format. Opening the original TEM image in DigitalMicrograph software, identifying the coating layer using lattice spacing and angle information, measuring the thickness of the coating layer at 3-5 locations, excluding values ​​of 0, and taking the average.

[0121] In the phrase "diameter d of primary particles of the positive electrode active material," "primary particle" has a well-known meaning in the art, referring to the particles of the material formed after the active material is coated with a carbonaceous material layer. As an example, the primary particle size is statistically analyzed using a combination of scanning electron microscopy (SEM) and image analysis software. The specific operation method is as follows: A positive electrode sheet is obtained, and the surface of the positive electrode film layer is imaged using a Zeiss Sigma 300 SEM at 10,000x (10k) magnification in backscatter mode. Avizo software is used to identify the primary particles on the surface of the positive electrode film layer. Based on the identified primary particles, the optimal circumscribed quadrilateral of each primary particle is fitted, resulting in two diagonals. The longer diagonal is the major axis r1, and the shorter diagonal is the minor axis r2. The particle size is then determined.

[0122] As an example, the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material can be 1 / 500, 1 / 490, 1 / 480, 1 / 470, 1 / 460, 1 / 450, 1 / 440, 1 / 430, 1 / 420, 1 / 410, 1 / 400, 1 / 390, 1 / 380, 1 / 370, 1 / 360, 1 / 350, 1 / 340, 1 / 330, 1 / 320, 1 / 3 10, 1 / 300, 1 / 290, 1 / 280, 1 / 270, 1 / 260, 1 / 250, 1 / 240, 1 / 230, 1 / 220, 1 / 210, 1 / 200, 1 / 190, 1 / 180, 1 / 170, 1 / 160, 1 / 150, 1 / 140, 1 / 130, 1 / 120, 1 / 110, 1 / 100, etc., or any range of any two of the above ratios, or ratios within the range of ratios formed.

[0123] In this embodiment, the provided phosphate-based active material exhibits high safety, excellent cycle performance, and other good electrochemical properties. By coating the surface of this active material with a carbonaceous material layer, the interface can be protected, improving interface stability. This is because the carbonaceous material layer forms between the active material and the electrolyte, preventing direct contact between the electrolyte and the active material in the bulk phase and thus avoiding side reactions. This reduces the occurrence of side reactions, improves the structural stability of the positive electrode active material, reduces lithium-ion damage, and slows down the early capacity decay rate. Furthermore, coating the surface of the active material with a carbonaceous material layer improves the electronic conductivity of the positive electrode active material, constructing an electronic conductive network at the electrode layer level, thereby accelerating electron transport rate, improving electrochemical performance, and helping to reduce polarization during battery charging and discharging, ultimately contributing to a higher capacity retention rate.

[0124] In this embodiment, by limiting the ratio h / d of the thickness h of the carbon material layer to the diameter d of the primary particles of the positive electrode active material to meet the above conditions, a carbon material layer of a certain thickness is coated on the surface of the active material. This makes electrolyte wetting relatively difficult, affecting the lithium ion transport efficiency in the early stage of cycling, increasing polarization, and causing lithium ions to slowly escape, thereby slowing down the early capacity decay rate and achieving a slowing effect on the early cycle capacity retention rate decay rate. If the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is too small, the carbonaceous material layer is relatively thin and the diameter of the primary particles is relatively large, resulting in less protection of the interface between the positive electrode active material and the electrolyte, and thus a relatively poor effect on mitigating the capacity retention rate decay. Since lithium ions and electrons need to pass through the coating layer to interact, if the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is too large, the carbonaceous material layer is relatively thick and the diameter of the primary particles is relatively small, which increases the transport impedance of lithium ions and electrons and deteriorates the kinetic performance of the material. A larger h / d ratio can better mitigate the capacity retention rate decay in the early cycle, but the magnitude of kinetic degradation will also increase, resulting in excessive capacity loss and hindering the improvement of the overall battery performance. In the embodiments of this application, the kinetic performance of the positive electrode active material can be improved by doping and modifying the phosphate-based active material, thereby improving the battery's capacity utilization and achieving an effective balance between capacity and capacity retention performance.

[0125] In summary, the embodiments of this application, by coating the phosphate-based active material with a carbonaceous material layer and limiting the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material, provide good protection for the interface of the positive electrode active material, reduce the side reaction losses of the positive electrode active material, and effectively hide part of the initial capacity of the positive electrode active material, thereby enabling the positive electrode active material to achieve a capacity-slow-release effect.

[0126] In some embodiments, the active substance Li m A x Fe 1-y G y P 1-z D z O 4-n E n In, at least one of the following (i) to (v) is satisfied:

[0127] (i) M = 0.95–1.05;

[0128] (ii) X = 0.001~0.005;

[0129] (iii) Y = 0.001 to 0.1;

[0130] (iv) Z = 0.001–0.1;

[0131] (v)N=0.001~0.1.

[0132] In the embodiments of this application, the battery performance can be improved by doping any one of the Li, Fe, P, and O sites in the phosphate active material, or by doping two or more of them simultaneously, thereby meeting the requirements for improving battery performance, such as improving the kinetic performance of the cathode active material.

[0133] As an example, when Fe sites in phosphate-based active materials are doped and modified, lithium-ion transport channels can be broadened, ionic conductivity can be improved, the kinetic performance of the cathode active material can be enhanced, capacity utilization can be improved, and the overall performance of the battery can be better improved.

[0134] In some embodiments, the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 400 to 1 / 200.

[0135] In this embodiment, by further limiting the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material to meet the above conditions, it is beneficial to maintain better kinetic performance and achieve a better effect of slowing down the decay of early cycle capacity retention rate.

[0136] In some embodiments, the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 350 to 1 / 300.

[0137] In this embodiment, by further limiting the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material to meet the above conditions, it is beneficial to maintain better kinetic performance, achieve a better effect of slowing down the decay of early cycle capacity retention, and even achieve a zero decay effect for a certain period of time.

[0138] In some embodiments, the lithium secondary battery satisfies at least one of the following (I) and (II):

[0139] (I) The thickness h of the carbonaceous material layer is 1 nm to 5 nm;

[0140] (II) The diameter d of the primary particles of the positive electrode active material is 200 nm to 600 nm.

[0141] As an example, the thickness h of the carbonaceous material layer can be 1 nm, 1.1 nm, 1.2 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.7 nm, 1.8 nm, 2 nm, 2.1 nm, 2.2 nm, 2.3 nm, 2.4 nm, 2.5 nm, 2.6 nm, 2.7 nm, 2.8 nm, 2.9 nm, 3 nm, 3.1 nm, 3.2 nm, 3.3 nm, 3.4 nm, 3.5 nm, 3.6 nm, 3.7 nm, 3.8 nm, 4 nm, 4.1 nm, 4.2 nm, 4.3 nm, 4.4 nm, 4.5 nm, 4.6 nm, 4.7 nm, 4.8 nm, 4.9 nm, 5 nm, etc., or a range of any two of the above values, or a value within that range.

[0142] As an example, the diameter d of the primary particles of the positive electrode active material can be 200nm, 210nm, 220nm, 230nm, 240nm, 250nm, 260nm, 270nm, 280nm, 290nm, 300nm, 310nm, 320nm, 330nm, 340nm, 350nm, 360nm, 370nm, 380nm, 390nm, 400nm, 410nm, 420nm, 430nm, 440nm, 450nm, 460nm, 470nm, 480nm, 490nm, 500nm, 510nm, 520nm, 530nm, 540nm, 550nm, 560nm, 570nm, 580nm, 590nm, 600nm, etc., or a range of any two of the above values, or a value within the range.

[0143] In this embodiment, a greater thickness of the carbonaceous material layer results in better coating of the active material particles and better interfacial protection, thus preventing side reactions between the active material and the electrolyte. This leads to a better mitigation of capacity retention decay in the early stages of battery cycling and a more significant slow-release effect. However, increased carbonaceous material layer thickness also increases the transport impedance of lithium ions and electrons, deteriorating the material's kinetic performance. A carbonaceous material layer thickness that meets these conditions helps mitigate the problem of excessively rapid capacity retention decay in the early stages of battery cycling, reduces the adverse effects on the kinetics of the cathode active material, and balances the kinetic performance and slow-release effect of the cathode active material.

[0144] In the embodiments of this application, the smaller the diameter of the primary particles of the positive electrode active material, the shorter the diffusion path of lithium ions within the particles, which is beneficial for increasing the diffusion rate of lithium ions and improving the kinetic performance of the material. A smaller diameter of the primary particles of the positive electrode active material also helps to improve the close contact between the primary particles, thereby increasing the powder compaction density and electrode compaction density. Thus, within the allowable compaction range of the material, the higher the powder compaction density and electrode compaction density, the higher the battery capacity. This is because a higher compaction density means a higher content of active material per unit volume, thereby enabling the storage of more electrical energy. The diameter of the primary particles of the positive electrode active material satisfying the above conditions is beneficial for improving the material's kinetic performance, increasing the powder compaction density of the positive electrode active material and the compaction density of the positive electrode sheet, and ultimately increasing the battery capacity.

[0145] In some embodiments, the lithium secondary battery satisfies at least one of the following (1) and (2):

[0146] (1) The thickness h of the carbonaceous material layer is 2nm to 4nm;

[0147] (2) The diameter d of the primary particles of the positive electrode active material is 200nm~400nm.

[0148] In this embodiment, by controlling the thickness h of the carbonaceous material layer to be 2nm to 4nm, it is beneficial to mitigate the problem of excessively rapid decay rate of battery capacity retention in the early cycle stage, reduce the adverse effects on the kinetics of the positive electrode active material, and balance the kinetic performance and slow-release effect of the positive electrode active material.

[0149] In this embodiment, the diameter d of the primary particles of the positive electrode active material is 200nm to 400nm, which is beneficial to improve the material's kinetic properties, increase the powder compaction density of the positive electrode active material and the compaction density of the positive electrode sheet, and improve the battery capacity.

[0150] In some embodiments, the lithium secondary battery satisfies at least one of the following (A) and (B):

[0151] (A) The thickness h of the carbonaceous material layer is 2nm to 3nm;

[0152] (B) The diameter d of the primary particles of the positive electrode active material is 200nm to 300nm.

[0153] In this embodiment, by controlling the thickness h of the carbonaceous material layer to be 2nm to 3nm, it is beneficial to better mitigate the problem of excessively rapid decay rate of battery capacity retention in the early cycle stage, reduce the adverse effects on the kinetics of the positive electrode active material, and balance the kinetic performance and slow-release effect of the positive electrode active material.

[0154] In this embodiment, the diameter d of the primary particles of the positive electrode active material is 200nm to 300nm, which is beneficial to improve the material's kinetic properties, increase the powder compaction density of the positive electrode active material and the compaction density of the positive electrode sheet, and improve the battery capacity.

[0155] In some embodiments, the carbonaceous material layer accounts for 1.0% to 2.0% of the total mass of the active material and the carbonaceous material layer.

[0156] In this embodiment, the mass percentage of the carbonaceous material layer refers to the mass percentage of the carbonaceous material layer relative to the total mass of the positive electrode active material particles. This can be determined using methods known in the art, such as carbon content analysis, specifically infrared absorption. The method involves a C / S content analyzer (model: Dekai HCS-140). Based on the determination of total carbon and sulfur content in steel, the infrared absorption method after combustion in a high-frequency induction furnace (a conventional method) GBT 20123-2006 is used to test the carbon content in the powder. The sample is burned in oxygen, converting carbon and sulfur into CO2 and SO2, which are then converted into corresponding signals by a detector after entering the absorption cell. This signal is sampled by a computer, linearly corrected, and converted into a value proportional to the CO2 and SO2 concentrations. The values ​​from the entire analysis process are then accumulated. After the analysis, this accumulated value is divided by the weight value in the computer, multiplied by a correction factor, and the blank is subtracted to obtain the carbon and sulfur percentages in the sample. Thus, the mass percentage of the carbonaceous material layer is obtained.

[0157] As an example, based on the total mass of the active material and the carbonaceous material layer, the mass percentage of the carbonaceous material layer can be 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, etc., or a range of any two of the above values, or a value within the range of the above values.

[0158] In this embodiment, a higher mass percentage of the carbonaceous material layer indicates a greater thickness of the carbonaceous material layer coating the surface of the active material particles. A thicker carbonaceous material layer provides better coating of the active material particles and better interfacial protection, thus preventing side reactions between the active material and the electrolyte. This results in a better mitigation of capacity retention decay in the early stages of battery cycling and a more significant slow-release effect. However, increased carbonaceous material layer thickness also increases the transport impedance of lithium ions and electrons, deteriorating the material's kinetic performance. A mass percentage of the carbonaceous material layer that meets these conditions helps mitigate the problem of excessively rapid capacity retention decay in the early stages of battery cycling, reduces the adverse effects on the kinetics of the positive electrode active material, and balances the kinetic performance and slow-release effect of the positive electrode active material.

[0159] Furthermore, based on the total mass of the active material and the carbonaceous material layer, the mass percentage of the carbonaceous material layer is 1.0% to 1.8%.

[0160] Furthermore, based on the total mass of the active material and the carbonaceous material layer, the mass percentage of the carbonaceous material layer is 1.1% to 1.6%.

[0161] In this embodiment, by further controlling the mass ratio of the carbonaceous material layer, it is beneficial to better mitigate the problem of excessively rapid decay rate of battery capacity retention in the early cycle stage, reduce the adverse effects on the kinetics of the positive electrode active material, and balance the kinetic performance and slow-release effect of the positive electrode active material.

[0162] In some embodiments, the lithium secondary battery also includes an electrolyte, which includes lithium difluorophosphate.

[0163] In this embodiment, to improve the mitigation effect on the rapid decline in battery capacity retention during early cycles, the thickness of the carbonaceous material layer is increased. However, increasing the thickness of the carbonaceous material layer can have adverse effects on the material's kinetic properties, reducing the battery's capacity utilization. Therefore, in this embodiment, by adding lithium difluorophosphate to the electrolyte, the cell's capacity utilization can be improved, and the adverse effects of excessively thick carbonaceous material layer coating can be reduced. This is because lithium difluorophosphate can form a thin and robust SEI / CEI film (SEI is an abbreviation for "Solid Electrolyte Interphase"; CEI is an abbreviation for "Chemical Electrolyte Interface") at the positive / negative electrode interface, reducing the interface film (SEI) impedance, improving the battery's low-temperature performance and rate performance, and thus improving the problem of low capacity utilization caused by high initial polarization of the positive electrode.

[0164] In some embodiments, the mass content of lithium difluorophosphate is 0.01% to 2% based on the total mass of the electrolyte.

[0165] The types and contents of lithium difluorophosphate, an inorganic component in the electrolyte, are well-known in the art and can be determined using well-known equipment and methods. For example, the lithium difluorophosphate in the electrolyte can be qualitatively or quantitatively analyzed by ion chromatography using standard JY / T020-1996, "General Rules for Ion Chromatography Analysis". In the embodiments of this application, a fully discharged battery (discharged to the lower limit cutoff voltage so that the battery's state of charge is approximately 0% SOC) can be disassembled in reverse. The free electrolyte obtained from the battery is used as a sample and detected by ion chromatography. Specifically: a quantitative amount of electrolyte (the concentration of the diluent is in the middle of the standard curve) is weighed and diluted to 100 mL with ultrapure water. The sample is automatically injected by ion chromatography. The corresponding inorganic substances are compared according to the peak positions in the chromatogram, and the corresponding inorganic ion concentration is calculated based on the peak area. The mass of inorganic substances in the electrolyte can be calculated from the mass of the electrolyte.

[0166] As an example, based on the total mass of the electrolyte, the mass content of lithium difluorophosphate can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, etc., or a range of any two of the above values, or a value within a range of the above values.

[0167] In this embodiment, lithium difluorophosphate, as a bifunctional additive, can form a stable solid electrolyte interphase (SEI) film on the negative electrode surface and a stable interfacial film on the positive electrode surface, effectively inhibiting the oxidative decomposition of the electrolyte and protecting the integrity of the electrode structure. By controlling the mass content of lithium difluorophosphate in the electrolyte to meet the above conditions, the bifunctional additive effect of lithium difluorophosphate can be effectively utilized, improving battery capacity and reducing the side effects of excessive carbon material layer coating; at the same time, it helps to mitigate the negative impact of excessive lithium difluorophosphate on the battery.

[0168] In some embodiments, the mass content of lithium difluorophosphate is 0.02% to 0.6% based on the total mass of the electrolyte.

[0169] In this embodiment, the mass content of lithium difluorophosphate in the electrolyte is controlled to be 0.02% to 0.6%, which is beneficial to effectively exert the role of lithium difluorophosphate as a dual-function additive, improve the cell capacity, and reduce the side effects of excessive carbon material coating.

[0170] In some embodiments, the lithium difluorophosphate content is 0.05% to 0.3% based on the total mass of the electrolyte.

[0171] In this embodiment, the mass content of lithium difluorophosphate in the electrolyte is controlled to be 0.05% to 0.3%, which is conducive to better exerting the role of lithium difluorophosphate as a dual-function additive, improving battery capacity, and reducing the side effects of excessive carbon material coating.

[0172] In some embodiments, the lithium secondary battery further includes an electrolyte comprising vinylene carbonate, wherein the mass content of vinylene carbonate is 0.01% to 2% based on the total mass of the electrolyte.

[0173] The types and contents of vinylene carbonate, an organic component in the electrolyte, are well-known in the art and can be detected using equipment and methods known in the art. For example, the organic components in the electrolyte can be qualitatively and quantitatively analyzed by gas chromatography using GB / T9722-2006 "General Rules for Gas Chromatography of Chemical Reagents". In the embodiments of this application, a battery that has been completely discharged (discharged to the lower limit cutoff voltage so that the battery's state of charge is approximately 0% SOC) can be disassembled in reverse, and the free electrolyte obtained from the battery can be used as a sample for detection by gas chromatography analysis. The above free electrolyte was diluted 3 to 10 times with acetonitrile to obtain the electrolyte dilution to be tested. Using a GC-MS 3100 organic component gas chromatograph, the above electrolyte dilution was placed in the instrument for full-scan qualitative analysis. The injection port temperature was 250℃, and the scanning range was 35μm to 270μm. After the test was completed, an organic ion chromatogram was obtained. The corresponding organic species were identified by the peak positions in the chromatogram, and the percentage content of each organic species was calculated based on the peak area. The mass of inorganic matter in the electrolyte could be calculated from the mass of the electrolyte.

[0174] As an example, based on the total mass of the electrolyte, the mass content of vinylene carbonate can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, etc., or a range of any two of the above values, or a value within that range.

[0175] In this embodiment, vinylene carbonate (VC), as an electrolyte additive, can improve battery capacity and cycle life. This is because VC improves electrolyte performance, making it stable over a wider range of temperature conditions, thereby enhancing overall battery performance. Furthermore, VC can improve the high and low temperature performance of the electrolyte, which is crucial for improving battery reliability and durability in extreme environments. As an electrolyte additive, VC also enhances battery specific capacity and cycle life. This is because VC is more compatible with the cathode material, reducing side reactions and maintaining battery performance. Simultaneously, VC provides overcharge protection, helping to prevent battery damage during overcharging and further enhancing battery safety. It can also synergistically work with the carbonaceous material layer, mitigating the side effects of excessively thick carbonaceous material coating.

[0176] Therefore, the role of vinylene carbonate in electrolytes includes improving battery capacity and cycle life, enhancing the high and low temperature performance of the electrolyte, increasing the battery's specific capacity and cycle life, and providing overcharge protection. Vinylene carbonate can improve battery capacity utilization and reduce the side effects of excessively thick carbonaceous material coatings.

[0177] In this embodiment, by controlling the mass content of vinylene carbonate in the electrolyte to meet the above conditions, the vinylene carbonate can effectively function in the electrolyte, improving battery capacity and cycle life, enhancing the high and low temperature performance of the electrolyte, increasing the battery's specific capacity and cycle life, and providing overcharge protection. The increased battery capacity due to vinylene carbonate also reduces the side effects of excessively thick carbonaceous material coating. Furthermore, as the mass content of vinylene carbonate increases, the thickness of the SEI film increases, leading to a tendency for increased battery internal resistance. Controlling the mass content of vinylene carbonate in the electrolyte to meet the above conditions helps mitigate the negative impacts of increased SEI film thickness.

[0178] In some embodiments, the lithium secondary battery further includes an electrolyte comprising vinylene carbonate, wherein the mass content of vinylene carbonate is 0.02% to 0.6% based on the total mass of the electrolyte.

[0179] In this embodiment, the mass content of vinylene carbonate in the electrolyte is controlled to be 0.02% to 0.6%, which is beneficial for vinylene carbonate to play an effective role in the electrolyte and to improve battery capacity while reducing the side effects of excessive carbon material layer coating.

[0180] In some embodiments, the lithium secondary battery further includes an electrolyte comprising vinylene carbonate, wherein the mass content of vinylene carbonate is 0.05% to 0.3% based on the total mass of the electrolyte.

[0181] In this embodiment, the mass content of vinylene carbonate in the electrolyte is controlled to be 0.05% to 0.3%, which is beneficial for vinylene carbonate to play a better role in the electrolyte and to improve battery capacity while reducing the side effects of excessive carbon material layer coating.

[0182] In some embodiments, the carbonaceous material layer comprises one or more of graphite, graphene, amorphous carbon, carbon fiber, carbon nanotubes, carbon dots, and carbon felt.

[0183] In this embodiment, the carbonaceous material layer can be selected from one or more of graphite, graphene, amorphous carbon, carbon fiber, carbon nanotubes, carbon dots, and carbon felt. The synthesis cost is relatively low, and a good coating effect can be obtained, which is beneficial to improving the conductivity of the positive electrode active material.

[0184] In some embodiments, the positive electrode active material satisfies at least one of the following (a) to (c):

[0185] (a) The BET specific surface area of ​​the positive electrode active material is 10m² 2 / g~15m 2 / g;

[0186] (b) The powder resistivity of the positive electrode active material at 25°C is 1Ω·m to 30Ω·m;

[0187] (c) The compaction density of the positive electrode active material under 3T pressure is 2.35 g / cm³. 3 ~2.65g / cm 3 .

[0188] Specific surface area, as is known in the field, refers to the total surface area per unit mass of a substance, with dimensions in meters (m). 2 / g. The BET specific surface area can be determined using methods known in the art, such as the determination of the specific surface area of ​​solid substances by gas adsorption BET method according to GB / T 19587-2004. Specifically, the BET specific surface area is tested using a Micromeritics ASAP 2020 specific surface area analyzer, and the method is based on the BET (Brunauer-Emmett-Teller) method to calculate the specific surface area of ​​the sample.

[0189] As an example, the BET specific surface area of ​​the positive electrode active material can be 10 m². 2 / g, 10.2m 2 / g, 10.4m 2 / g, 10.6m 2 / g, 10.8m 2 / g、11m 2 / g, 11.2m 2 / g, 11.4m 2 / g, 11.6m 2 / g, 11.8m 2 / g、12m 2 / g, 12.2m 2 / g, 12.4m 2 / g, 12.6m 2 / g, 12.8m 2 / g、13m 2 / g, 13.2m 2 / g, 13.4m 2 / g, 13.6m 2 / g, 13.8m 2 / g、14m 2 / g, 14.2m 2 / g, 14.4m 2 / g, 14.6m 2 / g, 14.8m 2 / g, 15m 2 / g etc., or a range consisting of any two of the above values, or a value within the range.

[0190] Powder resistivity, as understood in the art, refers to the ratio of the electric field intensity to the steady-state current density within a positive electrode active material, i.e., the volume resistance per unit volume. It is an important physical quantity describing the conductivity of positive electrode powder, reflecting the resistance to current passing through the powder material. The lower the powder resistivity, the stronger the material's conductivity, and the smaller the obstacle to current flow. It can be tested using methods known in the art, such as referring to GB / T30835-2014 "Carbon Composite Lithium Iron Phosphate Positive Electrode Materials for Lithium-ion Batteries," using the ST-2722 powder resistivity analyzer from Suzhou Jingge Electronics Co., Ltd. Specifically: the powder resistivity test method for positive electrode active materials at 25℃ and 20MPa is as follows: Dry the positive electrode active material powder, weigh an appropriate amount of powder, and then use a powder resistivity analyzer (equipment model Suzhou Jingge ST2722); place the dried powder sample in the mold / sample chamber of the resistivity analyzer, with a sample chamber depth of 20mm and a cross-sectional area of ​​1cm². 2 Then, the pressure is applied slowly from small to large, and data is collected manually to record the powder resistivity test results at different pressure points.

[0191] As an example, the powder resistivity of the positive electrode active material at 25°C can be 1Ω·m, 2Ω·m, 3Ω·m, 4Ω·m, 5Ω·m, 6Ω·m, 7Ω·m, 8Ω·m, 9Ω·m, 10Ω·m, 11Ω·m, 12Ω·m, 13Ω·m, 14Ω·m, 15Ω·m, 16Ω·m, 17Ω·m, 18Ω·m, 19Ω·m, 20Ω·m, 21Ω·m, 22Ω·m, 23Ω·m, 24Ω·m, 25Ω·m, 26Ω·m, 27Ω·m, 28Ω·m, 29Ω·m, 30Ω·m, etc., or a range of any two of the above values, or a value within a range.

[0192] Compacted density, as understood in the art, refers to the density of a positive electrode active material after compaction under specific conditions, affecting its energy density. It can be tested using methods known in the art, such as: applying a certain amount of powder to a compaction mold of known diameter (e.g., a Sansi Zongheng UTM7305 mold), with a metal sheet at the top and bottom of the mold, placing the powder in the middle, and applying a pressure of 226.0738 MPa (3T) while simultaneously measuring the corresponding powder thickness. The compacted density is then calculated using the formula ρ = m / v. Specific procedures can be performed according to standard GB / T24533-2009.

[0193] As an example, the compaction density of the positive electrode active material under 3T pressure can be 2.35 g / cm³. 3 2.36 g / cm 3 2.37 g / cm 3 2.38g / cm 3 2.39 g / cm 3 2.40 g / cm 3 2.41 g / cm 3 2.42 g / cm 3 2.43 g / cm 3 2.44 g / cm 3 2.45g / cm 3 2.46 g / cm 3 2.47 g / cm 3 2.48 g / cm 3 2.49 g / cm 3 2.50g / cm 3 2.51g / cm 3 2.52g / cm 3 2.53 g / cm 3 2.54 g / cm 3 2.55g / cm 3 2.56 g / cm 32.57g / cm 3 2.58g / cm 3 2.59g / cm 3 2.60g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm 3 2.64 g / cm 3 2.65g / cm 3 etc., or a range consisting of any two of the above values, or values ​​within the range.

[0194] In this embodiment, the positive electrode active material has a high BET surface area. The larger the BET specific surface area of ​​the positive electrode active material, the smaller the particle size of the positive electrode active material, and the shorter the diffusion path of lithium ions inside the particle, which is beneficial to improving the diffusion rate of lithium ions and improving the kinetic performance of the material.

[0195] In this embodiment, the positive electrode active material has a low powder resistivity. The lower the powder resistivity of the positive electrode active material, the higher the electronic conductivity of the positive electrode active material, which accelerates the electron transport rate, improves electrochemical performance, helps reduce polarization during battery charging and discharging, and thus facilitates obtaining a higher capacity retention rate.

[0196] In this embodiment, the positive electrode active material has a high compaction density, which is beneficial to improving the energy density and capacity of the battery. This is because the powder compaction density reflects the mass of the active material per unit volume. A higher compaction density means that more active material can be encapsulated in the battery. Thus, within the allowable compaction range of the material, the greater the powder compaction density and electrode compaction density, the higher the energy density and capacity of the battery.

[0197] In some embodiments, the lithium secondary battery further includes a positive electrode, a negative electrode, an electrolyte, and a separator; the positive electrode includes a positive current collector and a positive electrode film, the positive electrode film including the aforementioned positive electrode active material.

[0198] Typically, a lithium-ion secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0199] [Positive electrode tablets]

[0200] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including the positive electrode active material described in the first aspect of this application.

[0201] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0202] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0203] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0204] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0205] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive electrode active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0206] [Negative electrode plate]

[0207] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material.

[0208] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0209] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0210] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include one or more of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and titanates. Silicon-based materials may be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. When the battery is a lithium-ion battery, lithium titanate is used; when the battery is a sodium-ion battery, sodium titanate is used. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0211] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0212] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0213] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0214] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0215] [Electrolytes]

[0216] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not specify any particular type of electrolyte; it can be selected according to requirements.

[0217] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0218] In some embodiments, the electrolyte salt may include one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0219] In some embodiments, the solvent may be selected from one or more of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0220] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0221] [Isolation membrane]

[0222] In some embodiments, the battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0223] In some embodiments, the separator includes a porous substrate. The porous substrate may be made of one or more of the following materials: glass fiber, nonwoven fabric, polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyester, and polyimide. The porous substrate may be a single-layer film or a multi-layer composite film, without particular limitation. When the porous substrate is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0224] In some embodiments, the separator may further include a coating on at least one surface of the porous substrate. Specifically, the coating includes one or more of inorganic heat-resistant particles and organic heat-resistant particles.

[0225] In some embodiments, the porosity of the separator is 10% to 40%.

[0226] In some embodiments, the thickness of the separator can be 3 μm to 20 μm.

[0227] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0228] In some embodiments, the battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0229] In some implementations, the battery's outer packaging can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The battery's outer packaging can also be a soft pack, such as a pouch. The soft pack can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0230] This application does not impose any particular limitation on the shape of the battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square battery 5 as an example.

[0231] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0232] In some implementations, the batteries can be assembled into battery modules, and the number of batteries contained in a battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0233] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple batteries 5 can be fixed in place using fasteners.

[0234] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of batteries 5 are received.

[0235] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0236] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0237] The second aspect of this application provides a method for preparing a positive electrode active material, including:

[0238] Provide lithium phosphate precursors;

[0239] The lithium phosphate precursor and carbon source were ground together.

[0240] The ground material is heated to a first temperature T1 at a first rate under a protective gas atmosphere and held at the first temperature T1 for a first time t1; then it is heated to a second temperature T2 at a second rate and held at the second temperature T2 for a second time t2 to obtain the positive electrode active material.

[0241] The positive electrode active material includes an active substance, which is shown in formula (1): Li m A x Fe 1-y G y P 1-z D z O 4-n E n (1);

[0242] Wherein, A includes one or more elements selected from Zn, Al, Na, K, and Mg; G includes one or more elements selected from Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, and Ti; D includes one or more elements selected from B, S, Si, and N; E includes one or more elements selected from S, F, Cl, and Br; m = 0.5–1.15; x = 0–0.1; y = 0–0.5; z = 0–0.5; n = 0–0.5;

[0243] The surface of the active substance particles is coated with a layer of carbonaceous material;

[0244] The ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 500 to 1 / 100.

[0245] In this embodiment, a positive electrode active material coated with a carbonaceous material layer is prepared using a solid-state method. First, a lithium phosphate precursor and a carbon source are ground. Grinding mixes the precursor and carbon source to form a homogeneous mixture, creating conditions for subsequent heat treatment and carbon coating. Grinding refines the particles of the precursor and carbon source, helping to adjust and control their particle size distribution and improve the uniformity of carbon coating during subsequent heat treatment. The ground material is then heat-treated. During heat treatment, the carbon source decomposes and deposits a carbonaceous material layer on the surface of the precursor particles. During the heat treatment process, a temperature range is maintained at a constant temperature (T1) for a first time (t1), which serves as the temperature for phosphate-based active material lattice formation. This allows for the timely and sufficient removal of by-reaction products, preventing the formation of other impurities and improving crystallinity. As an example, if a lithium phosphate precursor is prepared by a metal source containing a metal (such as a lithium source), at a first temperature T1, the lithium source will dissolve and lithium ions will be inserted, accompanied by the generation and discharge of reaction byproducts (such as H2O, CO2, H2, CO, etc.).

[0246] Furthermore, by controlling at least one of the following parameters—first rate, first temperature T1, first time t1, second rate, second temperature T2, and second time t2—the crystallinity, particle size, and distribution of the positive electrode active material can be adjusted.

[0247] In this embodiment, the active material (i.e., phosphate active material) is coated with a carbonaceous material layer by the above method, and the ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material satisfies the above condition, so that part of the initial capacity of the positive electrode active material is hidden, thereby enabling the positive electrode active material to achieve a capacity-slow release effect.

[0248] In this embodiment, the lithium phosphate precursor can be synthesized during the preparation of the positive electrode active material, or it can be a purchased product. As an example, the precursor can be purchased iron phosphate, a bulk chemical product.

[0249] In some embodiments, the lithium-containing phosphate precursor comprises a lithium-containing transition metal phosphate having the following general formula:

[0250] Li a Fe b P c O d Q e ;

[0251] Wherein, Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, with 0.8≤a≤1.15, 0.9≤b≤1, 0.95≤c≤1, 3.5≤d≤4, and 0<e≤0.1.

[0252] As an example, 'a' can be 0.8, 0.85, 0.9, 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, etc., or a range formed by any two of the above 'a's, or a value within the range formed by the above 'a's.

[0253] As an example, b can be 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, etc., or a range formed by any two of the above x, or a value within the range formed.

[0254] As an example, c can be 0.95, 0.98, 1.0, or a range consisting of any two of the above y values, or a value within that range.

[0255] As an example, d can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, or a range consisting of any two of the above d, or a value within that range.

[0256] As an example, e can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or a range consisting of any two of the above e, or a value within that range.

[0257] In some embodiments, the modifying element Q may exist in the lithium-containing transition metal phosphate particles as a dopant element, or in the coating layer of the lithium-containing transition metal phosphate particles as a coating element.

[0258] In some implementations, Q includes one or more of Zn, Al, Na, K, and Mg.

[0259] When Q includes one or more of Zn, Al, Na, K, and Mg, it is beneficial to improve the lattice change rate of the positive electrode active material during the lithium insertion / extraction process, improve the structural stability of the material, and reduce the oxygen activity on the particle surface, thereby increasing the specific capacity of the material.

[0260] In some embodiments, Q includes one or more of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, and Ti.

[0261] When Q includes one or more of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, and Ti, it helps to improve the kinetic performance of the secondary battery.

[0262] In some implementations, Q includes Ti. This helps improve the kinetic performance of the secondary battery.

[0263] In some implementations, Q includes one or more of B, S, Si, and N.

[0264] When Q includes one or more of B, S, Si, and N, it is beneficial to improve the lattice change rate of the positive electrode active material during the lithium insertion / extraction process, improve the structural stability of the material, and reduce the oxygen activity on the particle surface, thereby increasing the specific capacity of the material.

[0265] In some embodiments, Q includes one or more of S, F, Cl, and Br. This is beneficial for improving the electrochemical performance of the positive electrode active material.

[0266] In some embodiments, the mass content of Q is 4000 ppm to 6000 ppm based on the total mass of the active substance.

[0267] As an example, based on the total mass of the active substance, the mass content of Q can be 4000ppm, 4200ppm, 4400ppm, 4600ppm, 4800ppm, 5000ppm, 5200ppm, 5400ppm, 5600ppm, 5800ppm, 6000ppm, etc., or a range of any two of the above mass contents, or a value within the range of contents.

[0268] In this application, the types and contents of elements in the active substance can be tested using any method known in the art. As an example, inductively coupled plasma atomic emission spectrometry (ICP-AES) is used to test the corresponding elements and their contents, referring to Appendix C of GB / T33822-2017.

[0269] In some implementations, the first rate is ≤4°C / min.

[0270] As an example, the first rates are 4°C / min, 3.9°C / min, 3.8°C / min, 3.7°C / min, 3.6°C / min, 3.5°C / min, 3.4°C / min, 3.3°C / min, 3.2°C / min, 3.1°C / min, 3 .0℃ / min, 2.9℃ / min, 2.8℃ / min, 2.7℃ / min, 2.6℃ / min, 2.5℃ / min, 2.4℃ / min, 2.3℃ / min, 2.2℃ / min, 2.1℃ / min, 2.0℃ / min, 1 0.9℃ / min, 1.8℃ / min, 1.7℃ / min, 1.6℃ / min, 1.5℃ / min, 1.4℃ / min, 1.3℃ / min, 1.2℃ / min, 1.1℃ / min, 1.0℃ / min, 0.9℃ / min, 0.8℃ / min, 0.7℃ / min, 0.6℃ / min, 0.5℃ / min, 0.4℃ / min, 0.3℃ / min, 0.2℃ / min, 0.1℃ / min, etc., or a range of any two of the above values, or values ​​within a range.

[0271] In this embodiment, the heating rate affects the crystal growth rate, which in turn affects the particle size and particle size distribution of the resulting positive electrode active material. By controlling the first rate to meet the above conditions, it is beneficial to regulate the primary particle size and distribution of the positive electrode active material, thereby improving the effect on the reduction of early cycle capacity retention in the battery.

[0272] In some implementations, the first rate is 1°C / min to 4°C / min.

[0273] By controlling the first rate to be 1℃ / min to 4℃ / min, it is easier to better regulate the particle size and distribution of the primary particles of the positive electrode active material, thereby improving the effect of reducing the capacity retention rate decay in the early cycle of the battery.

[0274] In some implementations, the first temperature T1 is ≥ 400°C.

[0275] As an example, the first temperature T1 is 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, 460℃, 470℃, 480℃, 490℃, 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, etc., or a range of any two of the above values, or a value within a range.

[0276] In this embodiment, the first temperature T1 is used as the temperature at which the lattice of phosphate-based active substances forms, so that the relevant by-reaction products can be discharged in a timely manner, avoiding the generation of other impurity phases and improving crystallinity.

[0277] In some implementations, the first temperature T1 is 400°C to 600°C.

[0278] In this embodiment, controlling the first temperature T1 to be 400℃~600℃ is beneficial to improving the crystallinity of the active material; in addition, it is also beneficial to subsequently regulate the particle size and distribution of the primary particles of the positive electrode active material.

[0279] In some implementations, the first temperature T1 is 450°C to 550°C.

[0280] In this embodiment, by controlling the first temperature T1 to be 450℃~550℃, it is beneficial to effectively improve the crystallinity of the active material, which facilitates better subsequent control of the particle size and distribution of the primary particles of the positive electrode active material.

[0281] In some implementations, the first time t1 ≥ 1h.

[0282] As an example, the first time t1 can be 1h, 1.2h, 1.4h, 1.6h, 1.8h, 2h, 2.2h, 2.4h, 2.6h, 2.8h, 3h, 3.2h, 3.4h, 3.6h, 3.8h, 4h, 4.2h, 4.4h, 4.6h, 4.8h, 5h, 5.2h, 5.4h, 5.6h, 5.8h, 6h, 6.2h, 6.4h, 6.6h, 6.8h, 7h, 7.2h, 7.4h, 7.6h, 7.8h, 8h, etc., or a range of any two of the above values, or a value within the range.

[0283] In this embodiment, the first time t1 can be matched with the first rate, affecting the crystal growth rate and thus the particle size and particle size distribution of the resulting positive electrode active material. By controlling the first time t1 to meet the above conditions, it is beneficial to regulate the primary particle size and distribution of the positive electrode active material, thereby improving the effect on the reduction of early cycle capacity retention of the battery.

[0284] In some implementations, the first time t1 is 1h to 7h.

[0285] In this embodiment, by controlling the first time t1 to be 1h to 7h, it is beneficial to regulate the particle size and distribution of the primary particles of the positive electrode active material, thereby improving the effect on the degradation of the battery's early cycle capacity retention rate.

[0286] In some implementations, the first time t1 is 3h to 5h.

[0287] In this embodiment, by controlling the first time interval t1 to be 3h to 5h, it is beneficial to better regulate the particle size and distribution of the primary particles of the positive electrode active material, thereby improving the effect on the reduction of the battery's early cycle capacity retention rate.

[0288] In some implementations, the second temperature T2 is ≥ 720°C.

[0289] As an example, the second temperature T2 is 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃, 800℃, 810℃, 820℃, 830℃, 840℃, 850℃, 860℃, 870℃, 880℃, 890℃, 900℃, etc., or a range of any two of the above values, or a value within the range.

[0290] In this embodiment of the application, by controlling the second temperature T2 to meet the above conditions, it is beneficial to regulate the primary particle size and distribution of the positive electrode active material, so as to improve the capacity, powder compaction density, crystallinity and other properties of the positive electrode active material.

[0291] In some implementations, the second temperature T2 is 720°C to 850°C.

[0292] In this embodiment, by controlling the second temperature T2 to be between 720°C and 850°C, it is beneficial to regulate the primary particle size and distribution of the positive electrode active material, thereby improving the capacity, powder compaction density and other properties of the positive electrode active material.

[0293] In some implementations, the second temperature T2 is 750°C to 810°C.

[0294] In this embodiment, by controlling the second temperature T2 to be between 750°C and 810°C, it is beneficial to better regulate the primary particle size and distribution of the positive electrode active material, so as to improve the capacity, powder compaction density and other properties of the positive electrode active material.

[0295] In some embodiments, the preparation method satisfies at least one of the following (α) and (β):

[0296] (α) The second rate is 1℃ / min to 10℃ / min;

[0297] (β) The second time t2 is 5h to 20h.

[0298] As an example, the second rate is 1°C / min, 1.2°C / min, 1.4°C / min, 1.6°C / min, 1.8°C / min, 2°C / min, 2.2°C / min, 2.4°C / min, 2.6°C / min, 2.8°C / min, 3°C / min, 3.2°C / min. min, 3.4℃ / min, 3.6℃ / min, 3.8℃ / min, 4℃ / min, 4.2℃ / min, 4.4℃ / min, 4.6℃ / min, 4.8℃ / min, 5℃ / min, 5.2℃ / min, 5.4℃ / min, 5.6℃ / min, 5.8℃ / min, 6℃ / min, 6.2℃ / min, 6.4℃ / min, 6.6℃ / min, 6.8℃ / min, 7℃ / min, 7.2℃ / min, 7.4℃ / min, 7.6℃ / min, 7.8℃ / min, 8℃ / min, 8.2℃ / min, 8.4℃ / min, 8.6℃ / min, 8.8℃ / min, 9℃ / min, 9.2℃ / min, 9.4℃ / min, 9.6℃ / min, 9.8℃ / min, 10℃ / min, etc., or a range of any two of the above values, or values ​​within a range of values.

[0299] As an example, the second time t2 is 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h, 12h, 12.5h, 13h, 13.5h, 14h, 14.5h, 15h, 15.5h, 16h, 16.5h, 17h, 17.5h, 18h, 18.5h, 19h, 19.5h, 20h, etc., or a range of any two of the above values, or a value within the range.

[0300] In this embodiment, controlling the second rate affects the growth rate of the material, and the second time t2 can be matched with the second rate. At least one parameter of the second rate and the second time t2 satisfies the above conditions, which is beneficial to control the particle size and distribution of the primary particles of the positive electrode active material and obtain higher kinetic performance.

[0301] In some embodiments, the preparation method satisfies at least one of the following (①) and (②):

[0302] (①) The second rate is 5℃ / min~9℃ / min;

[0303] (②) The second time t2 is 8h to 15h.

[0304] In this embodiment of the application, by controlling at least one parameter of the second rate or t2 to satisfy the above conditions, it is beneficial to better control the particle size and distribution of the primary particles of the positive electrode active material and obtain higher kinetic performance.

[0305] In some embodiments, the carbon source includes a first carbon source, which includes a water-soluble polymer.

[0306] Water-soluble polymers, also known as water-soluble resins or water-soluble macromolecules, are hydrophilic polymeric materials that can dissolve or swell in water to form solutions and dispersions. In the embodiments of this application, the first carbon source is selected from materials including water-soluble polymers, which is beneficial for obtaining a carbonaceous material layer with a high degree of graphitization, improving the density of the carbonaceous material, and enhancing the interfacial protection of the positive electrode active material.

[0307] In some embodiments, the water-soluble polymer includes one or more of polyethylene glycol, polyaniline, and their respective derivatives.

[0308] In this embodiment, the water-soluble polymer is selected from one or more of the above-mentioned materials, which is beneficial to improve the graphitization degree and density of the carbonaceous material layer of the positive electrode active material and improve the interfacial stability of the positive electrode active material.

[0309] In some implementations, the weight content of the first carbon source is ≥50% based on the total weight of the carbon sources.

[0310] As an example, based on the total weight of the carbon source, the weight content of the first carbon source is 50%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 66%, 68%, 70%, 72%, 74%, 76%, 78%, 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, 96%, 98%, 100%, etc., or a range of any two of the above values, or a value within the range.

[0311] In this embodiment, the higher the weight content of the first carbon source, the higher the graphitization degree and density of the carbonaceous material layer of the positive electrode active material, which is more beneficial to improving the interfacial stability of the positive electrode active material.

[0312] In some implementations, the weight content of the first carbon source is 50% to 70% based on the total weight of the carbon sources.

[0313] In this embodiment, by controlling the weight content of the first carbon source to meet the above conditions, it is beneficial to obtain a higher degree of graphitization and density of the carbonaceous material layer, while also facilitating cost control.

[0314] In some embodiments, the carbon source also includes a second carbon source, which includes one or more of glucose, sucrose, lactose, and maltose.

[0315] In this embodiment, the second carbon source is used in combination with the first carbon source as the carbonaceous material layer. The material selected for the second carbon source includes one or more of glucose, sucrose, lactose, and maltose, which helps to reduce the cost of the carbon source. Furthermore, it also affects the electrical and processing properties of the resulting positive electrode active material after coating. The coating process is a liquid-phase coating, requiring water solubility to carry out the process; otherwise, it is not conducive to uniform coating on the material surface. For example, one or more of glucose, sucrose, lactose, and maltose are used as the second carbon source, which are soluble in water, facilitating uniform coating of the carbonaceous material layer on the surface of the active material.

[0316] In some implementations, the weight content of the second carbon source is ≤50% based on the total weight of the carbon sources.

[0317] As an example, based on the total weight of the carbon source, the weight content of the first carbon source is 50%, 48%, 46%, 44%, 42%, 40%, 38%, 36%, 34%, 32%, 30%, 28%, 26%, 24%, 22%, 20%, 18%, 16%, 14%, 12%, 10%, 8%, 6%, 4%, 2%, 0%, etc., or a range of any two of the above values, or a value within the range.

[0318] In this embodiment, a combination of a second carbon source and a first carbon source is used as the carbon source for the carbon material layer. By controlling the ratio of the first carbon source to the second carbon source, the cost of the carbon source and the coating quality of the carbon material layer, such as the degree of graphitization and density, are adjusted.

[0319] Furthermore, the sum of the weight content of the first carbon source and the weight content of the second carbon source is 100%.

[0320] In some implementations, the weight content of the second carbon source is 30% to 50% based on the total weight of the carbon source.

[0321] In this embodiment, a combination of a second carbon source and a first carbon source is used as the carbon source for the carbonaceous material layer. By controlling the ratio of the first carbon source to the second carbon source to meet the above conditions, it is beneficial to effectively improve the graphitization degree and density of the carbonaceous material layer, while also reducing the cost of the carbon source.

[0322] The third aspect of this application provides a positive electrode active material, which is the positive electrode active material described in the first aspect above or a positive electrode active material prepared by the preparation method proposed in the second aspect above.

[0323] In the embodiments of this application, the proposed positive electrode active material has the beneficial effects of the positive electrode active material described in the first and second aspects above, which will not be repeated here.

[0324] The fourth aspect of this application provides an electrical device including the lithium secondary battery described in the first aspect above.

[0325] Electrical devices include one or more of the batteries, battery modules, and battery packs provided in this application. The battery, battery module, or battery pack can be a power source for the electrical device or an energy storage unit for the electrical device. Electrical devices may include, but are not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0326] As the electrical device, a battery, battery module, or battery pack can be selected according to its usage requirements.

[0327] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of this device, a battery pack or battery module can be used.

[0328] Another example device could be a mobile phone, tablet, laptop, etc. These devices typically require a slim and lightweight design and can use a battery as their power source.

[0329] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0330] I. Preparation of Lithium-ion Batteries

[0331] Example 1

[0332] (1) Preparation of positive electrode active material

[0333] FePO4, Li2CO3, glucose, and polyethylene glycol were mixed evenly, a small amount of water was added, and the mixture was ground and then spray-dried. The spray-dried powder was placed in a sintering furnace and heated from 25°C to 500°C (first temperature T1) at a rate of 2°C / min under a nitrogen atmosphere and held at that temperature for 3 hours (first time t1). Then, the temperature was increased to 830°C (second temperature T2) at a rate of 5°C / min and held at that temperature for 10 hours (second time t2). After the process was completed, the mixture was cooled and broken up by airflow to obtain the positive electrode active material, namely carbon-coated LiFePO4.

[0334] The carbon source is a mixture of glucose and polyethylene glycol (50% by weight of glucose and 50% by weight of polyethylene glycol based on the total mass of the carbon source). The amount of carbon source added is such that the residual carbon content of the carbon source (i.e., the weight content m of the carbon material layer) is 1.29%, based on the total weight of the prepared positive electrode active material.

[0335] (2) Battery manufacturing

[0336] 1) Preparation of positive electrode:

[0337] The above-mentioned positive electrode active material was mixed evenly with carbon black SP and polyvinylidene fluoride (PVDF) in an N-methylpyrrolidone solvent system at a weight ratio of 92:2.5:5.5. The mixture was then coated onto aluminum foil, dried, and cold-pressed to obtain the positive electrode sheet.

[0338] 2) Preparation of negative electrode:

[0339] The negative electrode active materials graphite, acetylene black, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) are mixed evenly in deionized water at a weight ratio of 95:2:2:1, coated onto copper foil, dried, and cold-pressed to obtain the negative electrode sheet.

[0340] 3) Preparation of electrolyte:

[0341] Ethyl carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. The first additive (lithium dichlorophosphate) was added, and then fully dried LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0342] 4) Battery separator:

[0343] A polyethylene separator with a thickness of 8μm is used.

[0344] 5) Battery fabrication:

[0345] Using a porous polyethylene (PE) polymer film as the separator, the positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an outer packaging, injected with electrolyte, and sealed. After formation and other processes, the battery is obtained.

[0346] Examples 2-34

[0347] The batteries provided in Examples 2-34 were prepared using the method of Example 1. The specific differences in various parameters are shown in Tables 1 and 2.

[0348] Comparative Example 1

[0349] The battery provided in Comparative Example 1 was prepared using the method of Example 1, except that glucose and polyethylene glycol were not added in the preparation step of the positive electrode active material. Specific parameters are shown in Tables 1 and 2.

[0350] Comparative Example 2

[0351] The battery provided in Comparative Example 2 was prepared using the method of Example 1, with the difference that the ratio of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material, h / d, is 1 / 600. Specific parameters are shown in Tables 1 and 2.

[0352] Comparative Example 3

[0353] The battery provided in Comparative Example 3 was prepared using the method of Example 1, with the difference that the ratio of the thickness h of the carbon material layer to the diameter d of the primary particles of the positive electrode active material, h / d, is 1 / 50. Specific parameters are shown in Tables 1-1, 1-2, and 2.

[0354] Table 2

[0355] II. Performance Testing

[0356] (I) Performance Testing Methods

[0357] (1) Before the cycle begins, the capacity and power need to be tested as the initial cycle capacity and power values ​​of the battery cell. The process is as follows:

[0358] 1) Let the battery stand at 25℃ for 120 minutes;

[0359] 2) Discharge the battery to 2.5V at 0.5P;

[0360] 3) Discharge the battery to 2.0V at 0.1P;

[0361] 4) Let the battery stand at 25℃ for 30 minutes;

[0362] 5) Charge at a constant current of 0.5C to 3.8V;

[0363] 6) Charge to 3.8V at a constant current of 0.04C;

[0364] 7) Let the battery stand at 25℃ for 30 minutes;

[0365] 8) Discharge at a constant current of 0.5C to 2.5V, and record the discharge power of this step as P;

[0366] 9) Discharge at a constant current of 0.04C to 2.0V, and record the sum of the capacities in steps 8) and 9) as the initial discharge capacity C of the cycle;

[0367] 10) Let the battery stand at 25°C for 30 minutes.

[0368] (2) The loop test process is as follows:

[0369] 1) Let the battery stand at 25℃ for 120 minutes;

[0370] 2) Discharge the battery to 2.5V at 0.5P;

[0371] 3) Let the battery stand at 25℃ for 30 minutes;

[0372] 4) Charge the battery to 3.8V at a constant power of 0.5P;

[0373] 5) Let stand at 25℃ for 1 minute;

[0374] 6) Charge to 3.8V with 0.05P constant power;

[0375] 7) Let the battery stand at 25℃ for 30 minutes;

[0376] 8) Discharge the battery to 2.5V at 0.5P;

[0377] 9) Let the battery stand at 25℃ for 5 minutes;

[0378] 10) Steps 4) to 8) above constitute one charge-discharge cycle of the battery. This process is repeated continuously until the battery capacity decays to 80% of its initial value. The number of cycles at this point is recorded, which is the cycle life, measured in cls, as shown in Table 2. The capacity retention rate after the nth cycle = (discharge capacity of the nth cycle / initial discharge capacity of the cycle) × 100%.

[0379] Capacity retention rate at the 300th cycle; Capacity retention rate at the 300th cycle = (Discharge capacity at the 300th cycle / Initial discharge capacity at the start of the cycle) × 100%.

[0380] Cyclic climbing difficulty:

[0381] Record the number of cycles in the first 300 cycles where the capacity retention rate is continuously higher than 100%. If the number of cycles continuously higher than 100% is ≥10, it is determined that the cycle process has exhibited cyclic ramping, and the maximum capacity retention rate when the capacity retention rate is higher than 100% is recorded. If the number of cycles continuously higher than 100% is <10, it is determined that the cycle process has not exhibited cyclic ramping, and is recorded as "-".

[0382] Specifically, the cycle ramp rate is calculated as follows: (Cycle to 300cls capacity - Initial cycle capacity) / Initial cycle capacity × 100%.

[0383] (II) Performance Test Results

[0384] The performance test results of the batteries provided in Examples 1-34 and Comparative Examples 1-3 are shown in Table 3.

[0385] Table 3

[0386] As shown in Table 3, the lithium secondary battery provided in this application embodiment exhibits a high cycle capacity retention rate in the initial stage of cycling and shows a cycle ramping phenomenon, indicating that the lithium secondary battery can gradually reach its optimal performance state in the initial stage of cycling. In contrast, the lithium secondary batteries provided in Comparative Examples 1-3 show a low cycle capacity retention rate in the initial stage of cycling, and the cycle capacity retention rate of the battery decays too quickly, without showing a cycle ramping phenomenon.

[0387] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A lithium secondary battery, wherein, The lithium secondary battery includes a positive electrode active material, which includes an active substance as shown in formula (1): Li m A x Fe 1-y G y P 1-z D z About 4-n E n (1); Wherein, A includes one or more elements selected from Zn, Al, Na, K, and Mg; G includes one or more elements selected from Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, and Ti; D includes one or more elements selected from B, S, Si, and N; E includes one or more elements selected from S, F, Cl, and Br; m = 0.5–1.15; x = 0–0.1; y = 0–0.5; z = 0–0.5; n = 0–0.5; The surface of the active substance particles is coated with a carbonaceous material layer; The ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 500 to 1 / 100.

2. The lithium secondary battery according to claim 1, wherein, The active substance Li m A x Fe 1-y G y P 1-z D z O 4-n E n In, at least one of the following (i) to (v) is satisfied: (i)m = 0.95–1.05; (ii) x = 0.001~0.005; (iii) y = 0.001 ~ 0.1; (iv) z = 0.001 ~ 0.1; (v)n=0.001~0.

1.

3. The lithium secondary battery according to claim 1 or 2, wherein, The ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 400 to 1 / 200.

4. The lithium secondary battery according to claim 1 or 2, wherein, The ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 350 to 1 / 300.

5. The lithium secondary battery according to any one of claims 1 to 4, wherein, Satisfy at least one of the following (I) and (II): (I) The thickness h of the carbonaceous material layer is 1 nm to 5 nm; (II) The diameter d of the primary particles of the positive electrode active material is 200 nm to 600 nm.

6. The lithium secondary battery according to any one of claims 1 to 4, wherein, Satisfy at least one of the following (1) and (2): (1) The thickness h of the carbonaceous material layer is 2nm to 4nm; (2) The diameter d of the primary particles of the positive electrode active material is 200nm to 400nm.

7. The lithium secondary battery according to any one of claims 1 to 4, wherein, At least one of (A) and (B) below must be satisfied: (A) The thickness h of the carbonaceous material layer is 2nm to 3nm; (B) The diameter d of the primary particles of the positive electrode active material is 200 nm to 300 nm.

8. The lithium secondary battery according to any one of claims 1 to 7, wherein, Based on the total mass of the active material and the carbonaceous material layer, the mass percentage of the carbonaceous material layer is 1.0% to 2.0%.

9. The lithium secondary battery according to any one of claims 1 to 8, wherein, The lithium secondary battery also includes an electrolyte, which includes lithium difluorophosphate.

10. The lithium secondary battery according to claim 9, wherein, Based on the total mass of the electrolyte, the lithium difluorophosphate content is 0.01% to 2%.

11. The lithium secondary battery according to claim 9, wherein, Based on the total mass of the electrolyte, the lithium difluorophosphate content is 0.02% to 0.6%.

12. The lithium secondary battery according to claim 9, wherein, Based on the total mass of the electrolyte, the lithium difluorophosphate content is 0.05% to 0.3%.

13. The lithium secondary battery according to any one of claims 1 to 12, wherein, The lithium secondary battery also includes an electrolyte comprising vinylene carbonate, wherein the mass content of the vinylene carbonate is 0.01% to 2% based on the total mass of the electrolyte.

14. The lithium secondary battery according to any one of claims 1 to 12, wherein, The lithium secondary battery also includes an electrolyte comprising vinylene carbonate, wherein the mass content of the vinylene carbonate is 0.02% to 0.6% based on the total mass of the electrolyte.

15. The lithium secondary battery according to any one of claims 1 to 12, wherein, The lithium secondary battery also includes an electrolyte comprising vinylene carbonate, wherein the mass content of vinylene carbonate is 0.05% to 0.3% based on the total mass of the electrolyte.

16. The lithium secondary battery according to any one of claims 1 to 15, wherein, The carbonaceous material layer includes one or more of the following: graphite, graphene, amorphous carbon, carbon fiber, carbon nanotubes, carbon dots, and carbon felt.

17. The lithium secondary battery according to any one of claims 1 to 16, wherein, The positive electrode active material satisfies at least one of the following (a) to (c): (a) The BET specific surface area of ​​the positive electrode active material is 10 m². 2 / g~15m 2 / g; (b) The powder resistivity of the positive electrode active material at 25°C is 1 Ω·m to 30 Ω·m; (c) The compaction density of the positive electrode active material under 3T pressure is 2.35 g / cm³. 3 ~2.65g / cm 3 .

18. A method for preparing a positive electrode active material, wherein, include: Provide lithium phosphate precursors; The lithium phosphate precursor was ground with a carbon source; The ground material is heated to a first temperature T1 at a first rate under a protective gas atmosphere and held at the first temperature T1 for a first time t1. Then, the temperature is increased to a second temperature T2 at a second rate and held at the second temperature T2 for a second time t2 to obtain the positive electrode active material; The positive electrode active material includes an active substance, which is shown in formula (1): Li m A x Fe 1-y G y P 1-z D z About 4-n E n (1); Wherein, A includes one or more elements selected from Zn, Al, Na, K, and Mg; G includes one or more elements selected from Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, and Ti; D includes one or more elements selected from B, S, Si, and N; E includes one or more elements selected from S, F, Cl, and Br; m = 0.5–1.15; x = 0–0.1; y = 0–0.5; z = 0–0.5; n = 0–0.5; The surface of the active substance particles is coated with a carbonaceous material layer; The ratio h / d of the thickness h of the carbonaceous material layer to the diameter d of the primary particles of the positive electrode active material is 1 / 500 to 1 / 100.

19. The preparation method according to claim 18, wherein, The first rate is ≤4℃ / min.

20. The preparation method according to claim 18, wherein, The first rate is 1℃ / min to 4℃ / min.

21. The preparation method according to any one of claims 18 to 20, wherein, The first temperature T1 ≥ 400℃.

22. The preparation method according to any one of claims 18 to 20, wherein, The first temperature T1 is 400℃~600℃.

23. The preparation method according to any one of claims 18 to 20, wherein, The first temperature T1 is 450℃~550℃.

24. The preparation method according to any one of claims 18 to 23, wherein, The first time t1 ≥ 1h.

25. The preparation method according to any one of claims 18 to 23, wherein, The first time t1 is from 1 hour to 7 hours.

26. The preparation method according to any one of claims 18 to 23, wherein, The first time t1 is 3h to 5h.

27. The preparation method according to any one of claims 18 to 26, wherein, The second temperature T2 ≥ 720℃.

28. The preparation method according to any one of claims 18 to 26, wherein, The second temperature T2 is 720℃~850℃.

29. The preparation method according to any one of claims 18 to 26, wherein, The second temperature T2 is 750℃~810℃.

30. The preparation method according to any one of claims 18 to 29, wherein, Satisfy at least one of the following (α) and (β): (α) The second rate is 1℃ / min to 10℃ / min; (β) The second time t2 is 5h to 20h.

31. The preparation method according to any one of claims 18 to 29, wherein, Satisfy at least one of the following (①) and (②): (①) The second rate is 5℃ / min~9℃ / min; (②) The second time t2 is 8h to 15h.

32. The preparation method according to any one of claims 18 to 31, wherein, The carbon source includes a first carbon source, which includes a water-soluble polymer.

33. The preparation method according to claim 32, wherein, The water-soluble polymer includes one or more of polyethylene glycol, polyaniline, and their respective derivatives.

34. The preparation method according to claim 32 or 33, wherein, Based on the total weight of the carbon sources, the weight content of the first carbon source is ≥50%.

35. The preparation method according to claim 32 or 33, wherein, Based on the total weight of the carbon sources, the weight content of the first carbon source is 50% to 70%.

36. The preparation method according to any one of claims 32 to 35, wherein, The carbon source also includes a second carbon source, which includes one or more of glucose, sucrose, lactose, and maltose.

37. The preparation method according to claim 36, wherein, Based on the total weight of the carbon sources, the weight content of the second carbon source is ≤50%.

38. The preparation method according to claim 36, wherein, Based on the total weight of the carbon sources, the weight content of the second carbon source is 30% to 50%.

39. A positive electrode active material, wherein the positive electrode active material is the positive electrode active material in the lithium secondary battery according to any one of claims 1 to 17 or the positive electrode active material prepared by the preparation method according to any one of claims 18 to 38.

40. An electrical appliance, wherein, The lithium secondary battery includes any one of claims 1 to 17.

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