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

Abstract

The invention discloses a lithium secondary battery, a positive electrode active material, a preparation method of the positive electrode active material and a power utilization device, the positive electrode active material comprises an active substance LimAxFe (1-y) GyP (1-z) DzO (4-n) En, A comprises one or more elements of Zn, Al, Na, K and Mg; g comprises one or more of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V and Ti; d comprises one or more elements of B, S, Si and N; e comprises one or more elements of S, F, Cl and Br; m is equal to 0.5 to 1.15; x is equal to 0 to 0.1; y is equal to 0 to 0.5; z is equal to 0 to 0.5; n is equal to 0-0.5; the particle surface of the active material is coated with a carbonaceous material layer, and the ratio of the thickness of the carbonaceous material layer to the diameter of the primary particle of the positive electrode active material is 1 / 500-1 / 100, so that the problem of fast attenuation of the early cycle capacity retention rate of the battery can be improved.

Description

Technical Field

[0001] The present application relates to the field of batteries, and in particular, to lithium secondary batteries, positive electrode active materials, preparation methods thereof, and electrical devices. Background Art

[0002] Batteries are not only used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, but are also widely used in electric vehicles such as electric bicycles, electric motorcycles, electric cars, as well as military equipment and aerospace and other fields. Among them, lithium secondary batteries have shown broad application prospects in many fields due to their advantages such as high energy density, long life and environmental friendliness. However, at present, lithium secondary batteries with phosphate positive electrode active materials have the problem of fast attenuation rate of initial capacity retention rate. Summary of the invention

[0003] The first aspect of the present application provides a lithium secondary battery, the lithium secondary battery includes a positive electrode active material, the positive electrode active material includes an active substance, and the active substance is shown in formula (1):

[0004] Li m A x Fe 1-y G y P 1-z D z O 4-n E n (1);

[0005] 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; n=0-0.5;

[0006] The surface of the particles of the active substance is coated with a layer of carbonaceous material;

[0007] 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.

[0008] The phosphate active material provided in the present application has good electrochemical properties such as high safety and cycle performance. By coating the surface of the active material with a carbonaceous material layer, it can protect the interface and improve the interface stability. This is because the carbonaceous material layer is formed between the active material and the electrolyte to avoid the electrolyte and the active material from directly contacting the bulk phase to cause side reactions, thereby reducing the occurrence of side reactions, improving the structural stability of the positive electrode active material, reducing the damage of lithium ions, and slowing down the initial capacity decay rate. In addition, coating the carbonaceous material layer on the surface of the active material is conducive to improving the electronic conductivity of the positive electrode active material, constructing an electronic conductive network at the pole piece level, thereby accelerating the electron transfer rate, improving the electrochemical performance, and helping to reduce polarization during the battery charge and discharge process, thereby facilitating obtaining a higher capacity retention rate.

[0009] The present application satisfies the above conditions by 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, which is conducive to maintaining good dynamic performance and achieving the slowing effect of the attenuation of the early cycle capacity retention 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, the diameter of the primary particles is relatively large, and the protective effect on the contact interface between the positive electrode active material and the electrolyte is small, so the slowing effect of the attenuation of the battery capacity retention rate 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 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 will increase the transmission impedance of lithium ions and electrons and deteriorate the dynamic performance of the material; the larger the h / d, although it can better achieve the slowing effect of the attenuation of the early cycle capacity retention rate, but the dynamic deterioration amplitude will also increase, resulting in 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 present application, the phosphate active material can be doped and modified to improve the kinetic performance of the positive electrode active material, thereby improving the capacity of the battery, thereby achieving an effective balance between the capacity and capacity retention performance.

[0010] In some embodiments of the present application, the active material Li m A x Fe 1-y G y P 1-z D z O 4-n E n , at least one of the following (i) to (v) is satisfied:

[0011] (i) m = 0.95 to 1.05;

[0012] (ii) x = 0.001 to 0.005;

[0013] (iii) y = 0.001 to 0.1;

[0014] (iv) z = 0.001 to 0.1;

[0015] (v)n=0.001~0.1.

[0016] The present application can meet the requirements for improving battery performance by element doping, such as improving the kinetic performance of the positive electrode active material.

[0017] In some embodiments of the present 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, which is beneficial to slow down the attenuation rate of the battery's early cycle capacity retention rate.

[0018] In some embodiments of the present 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, which is conducive to better mitigating the attenuation rate of the battery's early cycle capacity retention rate.

[0019] In some embodiments of the present application, the thickness h of the carbonaceous material layer is 1nm to 5nm, and / or the diameter d of the primary particles of the positive electrode active material is 200nm to 600nm. The thickness of the carbonaceous material layer meets the above conditions, which is conducive to solving the problem of too fast decay rate of the battery's early cycle capacity retention rate, reducing the adverse effects on the dynamics of the positive electrode active material, and balancing the dynamic 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 conducive to improving the material's dynamic performance, increasing the powder compaction density of the positive electrode active material and the compaction density of the positive electrode sheet, and increasing the battery capacity.

[0020] In some embodiments of the present application, the thickness h of the carbonaceous material layer is 2nm to 4nm, and / or the diameter d of the primary particles of the positive electrode active material is 200nm to 400nm. The thickness of the carbonaceous material layer meets the above conditions, which is conducive to balancing the kinetic properties 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 conducive to improving the battery capacity.

[0021] In some embodiments of the present application, the thickness h of the carbonaceous material layer is 2nm to 3nm, and / or the diameter d of the primary particles of the positive electrode active material is 200nm to 300nm. The thickness of the carbonaceous material layer meets the above conditions, which is conducive to better balancing the kinetic properties 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 conducive to better improving the battery capacity.

[0022] In some embodiments of the present application, based on the total mass of the active material and the carbonaceous material layer, the mass proportion of the carbonaceous material layer is 1.0% to 2.0%, which is beneficial to balance the kinetic performance and sustained release effect of the positive electrode active material.

[0023] In some embodiments of the present application, the lithium secondary battery further comprises an electrolyte, and the electrolyte comprises lithium difluorophosphate. By adding lithium difluorophosphate additive to the electrolyte, the capacity of the battery cell can be improved and the side effects of excessively thick carbonaceous material layer coating can be reduced.

[0024] In some embodiments of the present application, based on the total mass of the electrolyte, the mass content of lithium difluorophosphate is 0.01% to 2%. By controlling the mass content of lithium difluorophosphate in the electrolyte to meet the above conditions, it is beneficial to effectively play the role of lithium difluorophosphate as a dual-functional additive, improve the battery capacity, and reduce the side effects of excessively thick carbonaceous material layers; at the same time, it is beneficial to mitigate the negative effects of excessive lithium difluorophosphate on the battery.

[0025] In some embodiments of the present application, the mass content of lithium difluorophosphate is 0.02% to 0.6% based on the total mass of the electrolyte, which is conducive to effectively exerting the role of lithium difluorophosphate as a dual-functional additive, improving the capacity of the battery cell, and reducing the side effects of excessively thick carbonaceous material layer coating.

[0026] In some embodiments of the present application, based on the total mass of the electrolyte, the mass content of lithium difluorophosphate is 0.05% to 0.3%, which is conducive to better exerting the role of lithium difluorophosphate as a dual-functional additive, improving the capacity of the battery cell, and reducing the side effects of excessively thick carbonaceous material layer coating.

[0027] In some embodiments of the present application, the lithium secondary battery further comprises an electrolyte, the electrolyte comprises vinylene carbonate, and 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 the capacity and cycle life of the battery.

[0028] In some embodiments of the present application, the lithium secondary battery further includes an electrolyte, the electrolyte includes vinylene carbonate, and the mass content of vinylene carbonate is 0.02% to 0.6% based on the total mass of the electrolyte. This is conducive to the effective function of vinylene carbonate in the electrolyte, and by increasing the battery capacity, the side effect of excessively thick carbonaceous material layer coating is reduced.

[0029] In some embodiments of the present application, the lithium secondary battery further includes an electrolyte, the electrolyte includes vinylene carbonate, and the mass content of vinylene carbonate is 0.05% to 0.3% based on the total mass of the electrolyte. This is conducive to vinylene carbonate to play a better role in the electrolyte, and by improving the battery capacity, the side effect of too thick carbonaceous material layer coating is reduced.

[0030] In some embodiments of the present application, the material of the carbonaceous material layer includes one or more of graphite, graphene, amorphous carbon, carbon fiber, carbon nanotubes, carbon dots, and carbon felt, which is beneficial to improve the conductivity of the positive electrode active material.

[0031] In some embodiments of the present application, the positive electrode active material satisfies at least one of the following (a) to (c):

[0032] (a) The BET specific surface area of ​​the positive electrode active material is 10 m 2 / g~15m 2 / g;

[0033] (b) the powder resistivity of the positive electrode active material at 25° C. is 1 Ω·m to 30 Ω·m;

[0034] (c) The compaction density of the positive electrode active material under 3T pressure is 2.35 g / cm 3 ~2.65g / cm 3 .

[0035] The BET specific surface area of ​​the positive electrode active material meets the above conditions, which is beneficial to improving the kinetic properties of the material.

[0036] The powder resistivity of the positive electrode active material at 25° C. satisfies the above conditions, which is conducive to obtaining a higher capacity retention rate.

[0037] The compaction density of the positive electrode active material under 3T pressure conditions meets the above conditions, which is beneficial to improving the energy density and capacity of the battery.

[0038] The second aspect of the present application provides a method for preparing a positive electrode active material, comprising:

[0039] Providing lithium-containing phosphate precursors;

[0040] grinding a lithium-containing phosphate precursor and a carbon source;

[0041] The ground material is heated to a first temperature T1 at a first rate under a protective gas atmosphere and kept at the first temperature T1 for a first time t1; then heated to a second temperature T2 at a second rate and kept at the second temperature T2 for a second time t2, to obtain a positive electrode active material;

[0042] The positive electrode active material includes an active substance, and the active substance is shown in formula (1):

[0043] Li m A x Fe 1-y G y P 1-z D z O 4-n E n (1);

[0044] 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, 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; n=0-0.5;

[0045] The surface of the particles of the active substance is coated with a layer of carbonaceous material;

[0046] 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.

[0047] The present application adopts a solid phase method to prepare a positive electrode active material coated with a carbonaceous material layer, which is conducive to forming a uniform carbonaceous material coating layer on the surface of the active material; it can also control at least one parameter among the first rate, the first temperature T1, the first time t1, the second rate, the second temperature T2, and the second time t2 to regulate the crystallinity, particle size and distribution and other parameters of the positive electrode active material.

[0048] In some embodiments of the present application, the first rate is ≤4°C / min, which is beneficial for regulating the primary particle size and distribution of the positive electrode active material and improving the effect of improving the attenuation of the early cycle capacity retention rate of the battery.

[0049] In some embodiments of the present application, the first rate is 1°C / min to 4°C / min, which is beneficial to better control the primary particle size and distribution of the positive electrode active material and improve the effect of improving the rapid decay of the early cycle capacity retention rate of the battery.

[0050] In some embodiments of the present application, the first temperature T1 is ≥ 400° C. This is beneficial to improving the crystallinity of the material.

[0051] In some embodiments of the present application, the first temperature T1 is 400° C. to 600° C. This is beneficial to better improve the crystallinity of the material.

[0052] In some embodiments of the present application, the first temperature T1 is 450° C. to 550° C., which is beneficial to better improve the crystallinity of the material.

[0053] In some embodiments of the present application, the first time t1 is ≥ 1 h. The first time t1 can match the first rate to improve the particle size and distribution of the positive electrode active material.

[0054] In some embodiments of the present application, the first time t1 is 1 hour to 7 hours. The first time t1 can match the first rate to better improve the particle size and distribution of the positive electrode active material.

[0055] In some embodiments of the present application, the first time t1 is 3 hours to 5 hours. The first time t1 can match the first rate to better improve the particle size and distribution of the positive electrode active material.

[0056] In some embodiments of the present application, the second temperature T2 is ≥ 720° C. This is beneficial for regulating the primary particle size and distribution of the positive electrode active material to improve the capacity, powder compaction density, crystallinity and other properties of the positive electrode active material.

[0057] In some embodiments of the present application, the second temperature T2 is 720° C. to 850° C. This is conducive to better regulating the primary particle size and distribution of the positive electrode active material to improve the capacity, powder compaction density, crystallinity and other properties of the positive electrode active material.

[0058] In some embodiments of the present application, the second temperature T2 is 750° C. to 810° C. This is conducive to better regulating the primary particle size and distribution of the positive electrode active material to improve the capacity, powder compaction density, crystallinity and other properties of the positive electrode active material.

[0059] In some embodiments of the present application, the second rate is 1°C / min to 10°C / min; and / or the second time t2 is 5h to 20h. At least one parameter of the second rate and the second time t2 satisfies the above conditions, which is conducive to regulating the particle size and distribution of the primary particles of the positive electrode active material and obtaining higher kinetic performance.

[0060] In some embodiments of the present application, the second rate is 5°C / min to 9°C / min; and / or the second time t2 is 8h to 15h. At least one parameter of the second rate and the second time t2 satisfies the above conditions, which is conducive to better regulating the particle size and distribution of the primary particles of the positive electrode active material and obtaining higher kinetic performance.

[0061] In some embodiments of the present application, the carbon source includes a first carbon source, and the first carbon source includes a water-soluble polymer, which is beneficial to improve the density of the carbonaceous material and enhance the interface protection effect on the positive electrode active material.

[0062] In some embodiments of the present application, the water-soluble polymer includes one or more of polyethylene glycol, polyaniline, and their respective derivatives, which is beneficial to improve the graphitization degree and density of the carbonaceous material layer of the positive electrode active material and improve the interface stability of the positive electrode active material.

[0063] In some embodiments of the present application, based on the total weight of the carbon source, the weight content of the first carbon source is ≥ 50%, which is beneficial to better improve the interfacial stability of the carbonaceous material layer to the positive electrode active material.

[0064] In some embodiments of the present application, the weight content of the first carbon source is 50% to 70% based on the total weight of the carbon source, which is beneficial to better improve the interface stability of the carbonaceous material layer to the positive electrode active material.

[0065] In some embodiments of the present application, the carbon source further includes a second carbon source, and the second carbon source includes one or more of glucose, sucrose, lactose, and maltose. This is beneficial to reducing the cost of the carbon source; in addition, it is beneficial to improve the electrical properties and processing properties of the positive electrode active material.

[0066] In some embodiments of the present application, based on the total weight of the carbon source, the weight content of the second carbon source is ≤50%. By controlling the usage ratio of the first carbon source and the second carbon source, it is helpful to adjust the cost of the carbon source and the coating quality of the carbonaceous material layer.

[0067] In some embodiments of the present application, the weight content of the second carbon source is 30% to 50% based on the total weight of the carbon source. By controlling the usage ratio of the first carbon source and 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.

[0068] The third aspect of the present application proposes a positive electrode active material, which is the positive electrode active material described in the first aspect or the positive electrode active material prepared by the preparation method described in the second aspect. The proposed positive electrode active material has the beneficial effects of the positive electrode active materials described in the first and second aspects, which will not be described in detail here.

[0069] The fourth aspect of the present application provides an electric device, comprising the lithium secondary battery provided in the first aspect. The lithium secondary battery provided in the present application has a relatively high initial cycle capacity, and the initial cycle capacity of the battery decays relatively slowly.

[0070] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present application. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0072] Figure 1 is a schematic diagram of a battery according to one embodiment of the present application.

[0073] Figure 2 yes Figure 1 An exploded view of a battery according to an embodiment of the present application is shown.

[0074] Figure 3 is a schematic diagram of a battery module according to an embodiment of the present application.

[0075] Figure 4 It is a schematic diagram of a battery pack according to one embodiment of the present application.

[0076] Figure 5 yes Figure 4 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0077] Figure 6 Schematic diagram of an electrical device using a battery as a power source according to an embodiment of the present application.

[0078] Description of reference numerals:

[0079] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 battery; 51 shell; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0080] The following is a detailed description of the embodiments of the technical solution of the present application. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0081] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0082] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0083] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0084] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0085] At present, from the perspective of market development, the application of batteries is becoming more and more extensive. Batteries are not only used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, but also widely used in electric vehicles such as electric bicycles, electric motorcycles, electric cars, as well as military equipment and aerospace and other fields. With the continuous expansion of battery application areas, its market demand is also constantly expanding.

[0086] The positive electrode active material proposed in this application can be used in lithium secondary batteries, and further can be used in electrical devices that use batteries as power sources or various energy storage systems that use batteries as energy storage elements. Electrical devices may include, but are not limited to, mobile phones, tablets, laptops, electric toys, electric tools, battery cars, electric cars, ships, spacecraft, and the like. Among them, electric toys may include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, and the like, and spacecraft may include airplanes, rockets, space shuttles, and spacecraft, and the like.

[0087] During the charge and discharge process of lithium secondary batteries, the positive electrode active material will be accompanied by the deintercalation of lithium ions. During charging, lithium ions are removed from the lattice of the positive electrode active material and migrate to the negative electrode; during discharge, lithium ions are removed from the negative electrode and return to the positive electrode active material. In the early stage of the cycle of lithium secondary batteries, if the positive electrode active material is in direct contact with the electrolyte, side reactions may occur between the positive electrode active material and the electrolyte. These side reactions generate insoluble products, resulting in irreversible reactions, and the positive electrode active materials involved in the reaction lose their original structure; even such side reactions may cause the dissolution of non-lithium metal ions (such as transition metal ions) in the positive electrode active material, affecting the structural stability of the positive electrode active material, and the dissolved transition metal ions will have an adverse effect on the stability of the SEI film on the surface of the negative electrode, consuming more lithium ions. These adverse effects will cause the capacity retention rate of the battery to decay faster in the early stage.

[0088] In order to solve the problem of too fast decay rate of the initial cycle capacity retention rate of lithium secondary batteries, the embodiments of the present application achieve the effect of slowing down the decay rate of the initial cycle capacity retention rate of lithium secondary batteries by designing the iron phosphate-based positive electrode active material itself.

[0089] In a first aspect, the present application provides a lithium secondary battery. The lithium secondary battery includes a positive electrode active material. The positive electrode active material includes an active substance. The active substance is shown in formula (1):

[0090] Li m A x Fe 1-y G y P 1-z D z O 4-n E n (1);

[0091] 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; n=0-0.5;

[0092] The surface of the particles of the active substance is coated with a layer of carbonaceous material;

[0093] 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.

[0094] The active material is a compound represented by formula (1), which is a phosphate-based positive electrode active material, and can be undoped lithium iron phosphate or a doped and modified lithium iron phosphate.

[0095] 1) In formula (1), Li m A x Fe 1-y G y P 1-z D z O 4-n E n , m = 0.5 to 1.15; x = 0; y = 0; z = 0; n = 0, it represents undoped lithium iron phosphate, undoped material. 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 consisting of any two of the above values ​​or a value in the range.

[0096] 2) Li in formula (1) m A x Fe 1-y G y P 1-z D z O 4-n E n , m=0.5~1.15;0<x≤0.1;y=0;z=0;n=0, it means that the lithium iron phosphate is doped and modified at the Li position. 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 consisting of any two of the above values ​​or a value in the range.

[0097] 3) Li in formula (1) m A x Fe 1-y G y P 1-z D z O 4-n E n , m = 0.5 to 1.15; x = 0; 0 < y ≤ 0.5; z = 0; n = 0, indicating that the lithium iron phosphate is doped and modified at the Fe position. As an example, y can be 0.1, 0.2, 0.3, 0.4, 0.5, etc., or a range consisting of any two of the above values ​​or a value in the range.

[0098] 4) Li in formula (1) m A x Fe 1-y G y P 1-z D z O 4-n E n , m=0.5~1.15;x=0;y=0;0<z≤0.5;n=0, it means that the lithium iron phosphate is doped and modified at the P position. As an example, z can be 0.1, 0.2, 0.3, 0.4, 0.5, etc., or a range consisting of any two of the above values ​​or a value in the range.

[0099] 5) Li in formula (1) m A x Fe 1-y G y P 1-z D z O 4-n E n, m=0.5-1.15; x=0; y=0; z=0; 0<n≤0.5, it means that the lithium iron phosphate is doped at the O position. As an example, n can be 0.1, 0.2, 0.3, 0.4, 0.5, etc., or a range consisting of any two of the above values ​​or a value in the range.

[0100] When the lithium iron phosphate is doped and modified, any one of the Li position, Fe position, P position and O position may be doped and modified individually, or two or more of them may be doped and modified simultaneously.

[0101] Lithium iron phosphate or the doped and modified materials of lithium iron phosphate can be detected by inductively coupled plasma (ICP). The mass percentage of each element in the positive electrode active material is measured by ICP, and the mass percentage of each element is divided by their relative atomic mass to obtain the molar ratio of each element; the obtained molar ratio is simplified to the simplest integer ratio, which is the ratio of the number of atoms of the elements; based on the atomic number ratio, the chemical formula of the active substance is inferred.

[0102] The surface of the active material particles is coated with a carbonaceous material layer, which may be a full coating or a discontinuous coating, such as an island coating.

[0103] The "thickness h of the carbonaceous material layer" can be determined by conventional methods in the art, for example, by FIB (focused ion beam, abbreviated as Focused Ion Beam), and the specific method may include the following steps: randomly select a single particle from the positive electrode active material powder to be tested, cut a thin slice of about 100nm thickness from the middle position or near the middle position of the selected particle, and then perform TEM test on the thin slice to obtain the original TEM test image, and save the original image format (xx.dm3). The original image obtained by the above TEM test is opened in DigitalMicrograph software, and the coating layer is identified through the lattice spacing and angle information, and the thickness of the coating layer is measured. 3 to 5 positions are measured, and the values ​​with a value of 0 are excluded, and the average value is taken.

[0104] In the “diameter d of the primary particles of the positive electrode active material”, “primary particles” have a well-known meaning in the art, and refer to particles of the material formed after the active substance is coated with a carbonaceous material layer; as an example, the primary particle size statistics are performed by combining a scanning electron microscope and image analysis software. The specific operation method is as follows: obtain the positive electrode sheet, use a Zeiss Sigma300 scanning electron microscope to collect the film morphology image of the surface of the positive electrode film layer under a magnification of 10,000 times (10k) in the backscatter mode, and use Avizo software 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 the primary particles is fitted respectively, and the two diagonals of the quadrilateral are obtained. The longer of the two diagonals is the major diameter r1, and the shorter diagonal is the minor diameter r2. The particle size r = (r1+r2) / √2.

[0105] 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 may 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 1 / 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 a range consisting of any two of the above ratios, or a ratio within the range.

[0106] In the embodiment of the present application, the phosphate active material provided has good electrochemical properties such as high safety and cycle performance. By coating the surface of the active material with a carbonaceous material layer, it can protect the interface and improve the interface stability. This is because the carbonaceous material layer is formed between the active material and the electrolyte to avoid the electrolyte and the active material from directly contacting the bulk phase to cause side reactions, thereby reducing the occurrence of side reactions, improving the structural stability of the positive electrode active material, reducing the damage of lithium ions, and slowing down the initial capacity decay rate. In addition, coating the surface of the active material with a carbonaceous material layer is conducive to improving the electronic conductivity of the positive electrode active material, constructing an electronic conductive network at the pole piece level, thereby accelerating the electron transfer rate, improving the electrochemical performance, and helping to reduce polarization during the battery charge and discharge process, thereby facilitating obtaining a higher capacity retention rate.

[0107] In the embodiment of the present application, by 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, a carbonaceous material layer of a certain thickness is coated on the surface of the active material, and electrolyte infiltration is relatively difficult, which affects the lithium ion transmission efficiency in the early stage of the cycle, increases polarization, and slowly releases lithium ions, thereby slowing down the early capacity decay rate, thereby 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, the diameter of the primary particles is relatively large, and the protective effect on the contact interface between the positive electrode active material and the electrolyte is small, so the slowing effect on the attenuation of the battery capacity retention rate 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 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, the transmission impedance of lithium ions and electrons will be increased, and the kinetic properties of the material will be deteriorated; the larger the h / d, the better the slowing effect on the attenuation of the early cycle capacity retention rate can be achieved, but the kinetic deterioration amplitude will also increase, resulting in excessive capacity loss of the battery, which is not conducive to the improvement of the overall performance of the battery. In the embodiment of the present application, the phosphate active material can be doped and modified to improve the kinetic properties of the positive electrode active material, improve the capacity of the battery, and thus achieve an effective balance between the capacity and capacity retention rate performance.

[0108] In summary, the embodiments of the present application coat the phosphate active material with a carbonaceous material layer and limit 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, thereby providing good protection for the interface of the positive electrode active material and reducing the side reaction loss of the positive electrode active material, which is equivalent to hiding part of the initial capacity of the positive electrode active material, thereby enabling the positive electrode active material to achieve a sustained release of capacity.

[0109] In some embodiments, the active material Li m A x Fe 1-y G y P 1-z D z O 4-n E n , at least one of the following (i) to (v) is satisfied:

[0110] (i) M = 0.95 to 1.05;

[0111] (ii) X = 0.001 to 0.005;

[0112] (iii) Y = 0.001 to 0.1;

[0113] (iv) Z = 0.001 to 0.1;

[0114] (v)N=0.001~0.1.

[0115] In the embodiment of the present application, any one of the Li position, Fe position, P position, and O position in the phosphate active material can be doped and modified, or two or more of them can be doped and modified at the same time, thereby meeting the requirements for improving battery performance, such as improving the kinetic properties of the positive electrode active material.

[0116] As an example, when the Fe site in the phosphate active material is doped and modified, the lithium ion transmission channel can be broadened, the ion conductivity can be improved, the kinetic properties of the positive electrode active material can be improved, the capacity can be improved, and the overall performance of the battery can be better improved.

[0117] In some embodiments, a ratio h / d of a thickness h of the carbonaceous material layer to a diameter d of a primary particle of the positive electrode active material is 1 / 400 to 1 / 200.

[0118] In the embodiment of the present application, 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 mitigating the attenuation of the early cycle capacity retention rate.

[0119] In some embodiments, a ratio h / d of a thickness h of the carbonaceous material layer to a diameter d of a primary particle of the positive electrode active material is 1 / 350 to 1 / 300.

[0120] In the embodiment of the present application, 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 mitigating the attenuation of the early cycle capacity retention rate, and even achieve the effect of zero attenuation within a certain period of time.

[0121] In some embodiments, the thickness h of the carbonaceous material layer is 1 nm to 5 nm, and / or the diameter d of the primary particles of the positive electrode active material is 200 nm to 600 nm.

[0122] As an example, the thickness h of the carbonaceous material layer can be 1nm, 1.1nm, 1.2nm, 1.3nm, 1.4nm, 1.5nm, 1.6nm, 1.7nm, 1.8nm, 2nm, 2.1nm, 2.2nm, 2.3nm, 2.4nm, 2.5nm, 2.6nm, 2.7nm, 2.8nm, 2.9nm, 3nm, 3.1nm, 3.2nm, 3.3nm, 3.4nm, 3.5nm, 3.6nm, 3.7nm, 3.8nm, 4nm, 4.1nm, 4.2nm, 4.3nm, 4.4nm, 4.5nm, 4.6nm, 4.7nm, 4.8nm, 4.9nm, 5nm, etc., or a range composed of any two of the above values ​​or a value in the range composed of.

[0123] 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 composed of any two of the above values ​​or a value in the range of the composition.

[0124] In the embodiment of the present application, the greater the thickness of the carbonaceous material layer, the better the coating effect on the particles of the active material, the better the interface protection effect on the particles of the active material, which is conducive to avoiding the side reaction between the active material and the electrolyte, thereby the better the mitigation effect on the attenuation of the cycle capacity retention rate in the early stage of the battery cycle, and the more obvious the sustained release effect; at the same time, due to the increase in the thickness of the carbonaceous material layer, the transmission impedance of lithium ions and electrons will increase, and the kinetic properties of the material will be deteriorated. The thickness of the carbonaceous material layer meets the above conditions, which is conducive to mitigating the problem of too fast attenuation rate of the battery's early cycle capacity retention rate, reducing the adverse effects on the kinetics of the positive active material, and balancing the kinetic properties and sustained release effect of the positive active material.

[0125] In the embodiment of the present application, the smaller the primary particle diameter of the positive electrode active material, the shorter the diffusion path of lithium ions inside the particle, which is conducive to improving the diffusion rate of lithium ions and the kinetic properties of the material. The smaller the primary particle diameter of the positive electrode active material, the more it helps to improve the close contact between the primary particles, thereby increasing the powder compaction density and the pole piece compaction density. In this way, within the compaction range allowed by the material, the greater the powder compaction density and the pole piece compaction density, the higher the capacity of the battery. This is because a large compaction density means a higher content of active substances per unit volume, thereby being able to store more electrical energy. The diameter of the primary particles of the positive electrode active material meets the above conditions, which is conducive to improving the kinetic properties of the material, increasing the powder compaction density of the positive electrode active material and the compaction density of the positive pole piece, and increasing the battery capacity.

[0126] In some embodiments, the thickness h of the carbonaceous material layer is 2 nm to 4 nm, and / or the diameter d of the primary particles of the positive electrode active material is 200 nm to 400 nm.

[0127] In the embodiment of the present application, by controlling the thickness of the carbonaceous material layer to be h in the range of 2nm to 4nm, it is helpful to alleviate the problem of too fast attenuation rate of the battery's early cycle capacity retention rate, reduce the adverse effects on the kinetics of the positive electrode active material, and balance the kinetic performance and sustained release effect of the positive electrode active material.

[0128] In the embodiment of the present application, the diameter d of the primary particles of the positive electrode active material is 200nm to 400nm, which is beneficial to improving the material kinetic properties, increasing the powder compaction density of the positive electrode active material and the compaction density of the positive electrode sheet, and increasing the battery capacity.

[0129] In some embodiments, the thickness h of the carbonaceous material layer is 2 nm to 3 nm, and / or the diameter d of the primary particles of the positive electrode active material is 200 nm to 300 nm.

[0130] In the embodiment of the present application, by controlling the thickness of the carbonaceous material layer to h of 2nm to 3nm, it is helpful to better alleviate the problem of too fast attenuation rate of the battery's early cycle capacity retention rate, reduce the adverse effects on the kinetics of the positive electrode active material, and balance the kinetic performance and sustained release effect of the positive electrode active material.

[0131] In the embodiment of the present application, the diameter d of the primary particles of the positive electrode active material is 200nm to 300nm, which is conducive to better improving the material kinetic properties, increasing the powder compaction density of the positive electrode active material and the compaction density of the positive electrode sheet, and increasing the battery capacity.

[0132] In some embodiments, based on the total mass of the active material and the carbonaceous material layer, the mass of the carbonaceous material layer accounts for 1.0% to 2.0%.

[0133] In the embodiment of the present application, the mass proportion of the above-mentioned carbonaceous material layer refers to the mass proportion of the carbonaceous material layer relative to the particles of the entire positive electrode active material. The mass proportion of the carbonaceous material layer can be determined by a method known in the art, such as the carbon content analysis method, that is, the infrared absorption method. Specific method: carbon and sulfur content analyzer C / S content analyzer; equipment model Dekai HCS-140. According to the determination of the total carbon and sulfur content of steel, the infrared absorption method (conventional method) after combustion in a high-frequency induction furnace is used to test the carbon content in the powder. The sample is burned in oxygen to convert carbon and sulfur into CO2 and SO2, which are converted into corresponding signals by the detector after entering the absorption cell. This signal is sampled by a computer, converted into a numerical value proportional to the concentration of CO2 and SO2 after linear correction, and then the values ​​of the entire analysis process are accumulated. After the analysis is completed, this accumulated value is divided by the weight value in the computer, multiplied by the correction coefficient, and the blank is deducted to obtain the percentage of carbon and sulfur in the sample. Thus, the mass proportion of the carbonaceous material layer is obtained.

[0134] As an example, based on the total mass of the active substance and the carbonaceous material layer, the mass proportion 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 consisting of any two of the above values ​​or a value within the range.

[0135] In the embodiment of the present application, the higher the mass proportion of the carbonaceous material layer, the greater the thickness of the carbonaceous material layer coated on the surface of the particles of the active material, the greater the thickness of the carbonaceous material layer, the better the coating effect on the particles of the active material, the better the interface protection effect on the particles of the active material, and the better the side reaction between the active material and the electrolyte, thereby the better the mitigation effect on the attenuation of the cycle capacity retention rate in the early stage of the battery cycle, and the more obvious the sustained release effect; at the same time, due to the increase in the thickness of the carbonaceous material layer, the transmission impedance of lithium ions and electrons will increase, and the kinetic properties of the material will be deteriorated. The mass proportion of the carbonaceous material layer meets the above conditions, which is conducive to mitigating the problem of too fast attenuation rate of the battery's early cycle capacity retention rate, reducing the adverse effects on the kinetics of the positive active material, and balancing the kinetic properties and sustained release effect of the positive active material.

[0136] Furthermore, based on the total mass of the active material and the carbonaceous material layer, the mass proportion of the carbonaceous material layer is 1.0% to 1.8%.

[0137] Furthermore, based on the total mass of the active material and the carbonaceous material layer, the mass of the carbonaceous material layer accounts for 1.1% to 1.6%.

[0138] In the implementation mode of the present application, by further controlling the mass proportion of the carbonaceous material layer, it is helpful to better alleviate the problem of too fast attenuation rate of the battery's early cycle capacity retention rate, reduce the adverse effects on the kinetics of the positive electrode active material, and balance the kinetic performance and sustained release effect of the positive electrode active material.

[0139] In some embodiments, the lithium secondary battery further includes an electrolyte, and the electrolyte includes lithium difluorophosphate.

[0140] In the embodiment of the present application, in order to improve the slowing effect of the battery's early cycle capacity retention rate attenuation, the thickness of the carbonaceous material layer will be increased, but the increase in the thickness of the carbonaceous material layer will have side effects on the kinetic properties of the material and reduce the capacity of the battery. Therefore, in the embodiment of the present application, by adding a lithium difluorophosphate additive to the electrolyte, the capacity of the battery cell can be improved and the side effects of the carbonaceous material layer being too thick can be reduced. This is because lithium difluorophosphate can form a thin and strong SEI / CEI film at the positive / negative electrode interface of the battery (SEI is the abbreviation of "Solid Electrolyte Interphase", i.e., "solid electrolyte interface film"; CEI is the abbreviation of "Chemical Electrolyte Interface", i.e., "chemical electrolyte interface film"), reduce the interface film (SEI) impedance, improve the battery's low temperature performance and rate performance, and thus improve the problem of low capacity due to the large initial polarization of the positive electrode.

[0141] In some embodiments, based on the total mass of the electrolyte, the mass content of lithium difluorophosphate is 0.01% to 2%.

[0142] The type and content of the inorganic component lithium difluorophosphate in the electrolyte are well-known in the art and can be measured by equipment and methods well-known in the art, such as the standard JY / T020-1996 "General Rules for Ion Chromatography Analysis Methods" can be used to qualitatively or quantitatively analyze the lithium difluorophosphate in the electrolyte by ion chromatography analysis methods. In the embodiment of the present application, the battery that has been fully discharged (discharged to the lower limit cut-off voltage so that the charged state of the battery is about 0% SOC) can be reversely disassembled, and the free electrolyte obtained from the battery is used as a sample and detected by ion chromatography analysis. Specifically: weigh a quantitative electrolyte (the concentration of the diluent is in the middle of the standard curve), make it dilute to 100mL with ultrapure water, and perform ion chromatography automatic sampling detection. According to the peak position of the chromatogram, the corresponding inorganic type is compared, and the corresponding inorganic ion concentration is calculated according to the peak area. The mass of the electrolyte can be calculated The inorganic mass in the electrolyte.

[0143] 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 consisting of any two of the above values ​​or a value in the range.

[0144] In the embodiment of the present application, lithium difluorophosphate, as a bifunctional additive, can form a stable solid electrolyte interface (SEI) film on the surface of the negative electrode, and can also generate a stable interface film on the surface of the positive electrode, which can effectively inhibit the oxidative decomposition of the electrolyte and protect the integrity of the electrode structure. By controlling the mass content of lithium difluorophosphate in the electrolyte to meet the above conditions, it is beneficial to effectively play the role of the bifunctional additive of lithium difluorophosphate, which can improve the battery capacity and reduce the side effects of the over-thick coating of the carbonaceous material layer; at the same time, it is beneficial to mitigate the negative impact of excessive lithium difluorophosphate on the battery.

[0145] In some embodiments, based on the total mass of the electrolyte, the mass content of lithium difluorophosphate is 0.02% to 0.6%.

[0146] In the embodiment of the present application, the mass content of lithium difluorophosphate in the electrolyte is controlled to be 0.02% to 0.6%, which is conducive to effectively exerting the role of lithium difluorophosphate as a dual-functional additive, can improve the capacity of the battery cell, and reduce the side effects of excessively thick carbonaceous material layer coating.

[0147] In some embodiments, based on the total mass of the electrolyte, the mass content of lithium difluorophosphate is 0.05% to 0.3%.

[0148] In the embodiment of the present application, 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-functional additive, thereby improving the battery capacity and reducing the side effects of excessively thick carbonaceous material layer coating.

[0149] In some embodiments, the lithium secondary battery further includes an electrolyte, the electrolyte includes vinylene carbonate, and the mass content of vinylene carbonate is 0.01% to 2% based on the total mass of the electrolyte.

[0150] The type and content of the organic component vinylene carbonate in the electrolyte are well known in the art and can be detected by using equipment and methods well known in the art. For example, reference can be made to GB / T9722-2006 "General Rules for Gas Chromatography of Chemical Reagents" to conduct qualitative and quantitative analysis of the organic components in the electrolyte by gas chromatography. In the embodiment of the present application, a battery that has been fully discharged (discharged to the lower limit cut-off voltage so that the battery's charged state is about 0% SOC) can be reversely disassembled, and the free electrolyte obtained from the battery is used as a sample, and the gas phase spectrum analysis method is used for detection. The free electrolyte was diluted 3 to 10 times with acetonitrile to obtain the electrolyte dilution to be tested. The electrolyte dilution was placed in the instrument for full scan qualitative analysis using a GC-MS 3100 organic component gas chromatograph. The injection port temperature was 250°C and the scanning range was 35 μm to 270 μm. After the test, an organic ion chromatogram was obtained. The corresponding organic species were compared according to the peak position of the chromatogram. The corresponding content percentage of each organic matter was calculated according to the peak area. The mass of the inorganic matter in the electrolyte could be calculated through the mass of the electrolyte.

[0151] 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 consisting of any two of the above values ​​or a value in the range.

[0152] In the embodiment of the present application, vinylene carbonate (VC) as an electrolyte additive can improve the capacity and cycle life of the battery. This is because VC can improve the performance of the electrolyte, making it stable under a wider range of temperature conditions, thereby enhancing the overall performance of the battery. In addition, VC can also improve the high and low temperature performance of the electrolyte, which is crucial for improving the reliability and durability of the battery in extreme environments. As an electrolyte additive, VC also has the effect of improving the specific capacity and cycle life of the battery. This is because VC can be better compatible with the positive electrode material, reduce the occurrence of side reactions, and thus maintain the performance of the battery. At the same time, VC also has an overcharge protection effect, which helps prevent damage to the battery when overcharged, further enhancing the safety of the battery. It can play a synergistic effect with the carbonaceous material layer to mitigate the side effects of the carbonaceous material layer being too thick.

[0153] Therefore, the role of vinyl carbonate in the electrolyte includes increasing the capacity and cycle life of the battery, improving the high and low temperature performance of the electrolyte, increasing the specific capacity and cycle life of the battery, and having an overcharge protection effect. Vinyl carbonate can increase the battery capacity and reduce the side effects of the carbonaceous material layer being too thick.

[0154] In the embodiment of the present application, by controlling the mass content of vinylene carbonate in the electrolyte to meet the above conditions, vinylene carbonate is conducive to playing a role in the electrolyte, improving the capacity and cycle life of the battery, improving the high and low temperature performance of the electrolyte, improving the specific capacity and cycle life of the battery, and having an overcharge protection effect; vinylene carbonate is used to improve the battery capacity and reduce the side effects of the excessively thick carbonaceous material layer. In addition, as the mass content of vinylene carbonate increases, the thickness of the formed SEI film will increase, and the internal resistance of the battery will tend to increase. Controlling the mass content of vinylene carbonate in the electrolyte to meet the above conditions is conducive to mitigating the negative impact caused by the increase in the thickness of the SEI film.

[0155] In some embodiments, the lithium secondary battery further includes an electrolyte, the electrolyte includes vinylene carbonate, and the mass content of vinylene carbonate is 0.02% to 0.6% based on the total mass of the electrolyte.

[0156] In the embodiment of the present application, 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 reduce the side effects of excessively thick carbonaceous material layer coating by improving battery capacity.

[0157] In some embodiments, the lithium secondary battery further includes an electrolyte, the electrolyte includes vinylene carbonate, and the mass content of vinylene carbonate is 0.05% to 0.3% based on the total mass of the electrolyte.

[0158] In the embodiment of the present application, the mass content of vinylene carbonate in the electrolyte is controlled to be 0.05% to 0.3%, which is conducive to the vinylene carbonate to play a better role in the electrolyte and reduce the side effects of excessively thick carbonaceous material layer coating by improving the battery capacity.

[0159] In some embodiments, the material of the carbonaceous material layer includes one or more of graphite, graphene, amorphous carbon, carbon fiber, carbon nanotubes, carbon dots, and carbon felt.

[0160] In the embodiment of the present application, the material of 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.

[0161] In some embodiments, the positive electrode active material satisfies at least one of the following (a) to (c):

[0162] (a) The BET specific surface area of ​​the positive electrode active material is 10 m 2 / g~15m 2 / g;

[0163] (b) the powder resistivity of the positive electrode active material at 25° C. is 1 Ω·m to 30 Ω·m;

[0164] (c) The compaction density of the positive electrode active material under 3T pressure is 2.35 g / cm 3 ~2.65g / cm 3 .

[0165] Specific surface area has a well-known meaning in the art and refers to the total area per unit mass of a substance, with a dimension of m 2 / g. The BET specific surface area can be measured by methods known in the art, such as the determination of the specific surface area of ​​solid substances by the gas adsorption BET method according to GB / T 19587-2004. Specifically, the BET specific surface area is measured by a Micromeritics ASAP 2020 specific surface area tester, which is based on the BET (Brunauer-Emmett-Teller) method to calculate the specific surface area of ​​the sample.

[0166] As an example, the BET specific surface area of ​​the positive electrode active material can be 10 m 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.

[0167] Powder resistivity has a well-known meaning in the art, and refers to the ratio of the electric field strength of the current inside the positive electrode active material to the steady-state current density, that is, the volume resistance per unit volume. It is an important physical quantity that describes the conductive properties of the positive electrode material powder, and reflects the resistance when the current passes through the powder material. The lower the powder resistivity, the stronger the conductivity of the material, and the smaller the obstacle for the current to pass through the material. The test can be carried out using methods well known in the art, such as referring to GB / T30835-2014 "Carbon Composite Lithium Iron Phosphate Positive Electrode Materials for Lithium Ion Batteries", and using the ST-2722 powder resistivity analyzer of Suzhou Jingge Electronics Co., Ltd. for determination. Specifically: The powder resistivity test method of the positive electrode active material at 25°C and 20MPa is: dry the positive electrode active material powder, weigh an appropriate amount of powder, and then use a powder resistivity tester (equipment model Suzhou Jingge ST2722); place the dry powder sample in the mold / sample bin of the resistivity tester, the sample bin depth is 20mm, and the cross-sectional area is 1cm 2 , then slowly apply pressure from small to large, manually collect data, and record the corresponding powder resistivity test results at different pressure points.

[0168] 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 consisting of any two of the above values, or a value in a range of the above values.

[0169] The compaction density has a well-known meaning in the art, which refers to the density of the positive electrode active material after being compacted under specific conditions, and affects the energy density of the positive electrode active material. The test can be carried out by methods well-known in the art, such as the test method: a certain amount of powder is placed on a special compaction mold with a known diameter (for example, the special mold can be Sansi Zongheng UTM7305), with a metal sheet on the top and bottom of the mold, and the powder is placed in the middle. By applying a pressure of 226.0738Mpa (i.e. 3T) and testing the corresponding powder thickness at the same time, the compaction density is calculated using the formula ρ=m / v. The specific operation can refer to the standard: GB / T24533-2009.

[0170] As an example, the compaction density of the positive electrode active material under 3T pressure conditions can be 2.35 g / cm 3 , 2.36g / cm 3 , 2.37g / cm 3 , 2.38g / cm 3 , 2.39g / cm 3 , 2.40g / cm 3 , 2.41g / cm 3 , 2.42g / cm 3 , 2.43g / cm 3 , 2.44g / cm 3 , 2.45g / cm 3 , 2.46g / cm 3 , 2.47g / cm 3 , 2.48g / cm 3 , 2.49g / cm 3 , 2.50g / cm 3 , 2.51g / cm 3 , 2.52g / cm 3 , 2.53g / cm 3 , 2.54g / cm 3 , 2.55g / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm 3 , 2.60g / cm 3 , 2.61g / cm 3 , 2.62g / cm 3 , 2.63g / cm 3 , 2.64g / cm 3 , 2.65g / cm 3 Etc., or a range consisting of any two of the above values, or a value within the range.

[0171] In the embodiment of the present application, the positive electrode active material has a higher BET surface area. The larger the BET specific surface area of ​​the positive electrode active material, the smaller the particles of the positive electrode active material, and the shorter the diffusion path of lithium ions inside the particles, which is beneficial to improving the diffusion rate of lithium ions and improving the kinetic properties of the material.

[0172] In the embodiment of the present application, the positive electrode active material has a lower 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 transfer rate, improves the electrochemical performance, helps to reduce polarization during the battery charging and discharging process, and further facilitates obtaining a higher capacity retention rate.

[0173] In the embodiment of the present application, the positive electrode active material has a higher 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 materials can be encapsulated in the battery. Thus, within the compaction range allowed by the material, the greater the powder compaction density and the electrode compaction density, the higher the energy density and capacity of the battery.

[0174] In some embodiments, the lithium secondary battery further comprises a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator; the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer, and the positive electrode film layer comprises the above-mentioned positive electrode active material.

[0175] Generally, lithium secondary batteries include a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery charging and discharging process, active ions are embedded and removed back and forth between the positive electrode and the negative electrode. The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode. The separator is set between the positive electrode and the negative electrode, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

[0176] [Positive electrode]

[0177] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, and the positive electrode film layer includes the positive electrode active material described in the first aspect of the present application.

[0178] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0179] In some embodiments, the positive electrode 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 material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0180] In some embodiments, the positive electrode film layer may also optionally include a binder. As an example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0181] In some embodiments, the positive electrode film layer may further include a conductive agent, for example, one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0182] In some embodiments, the positive electrode sheet can be prepared in the following manner: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0183] [Negative electrode]

[0184] 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, wherein the negative electrode film layer includes a negative electrode active material.

[0185] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0186] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0187] In some embodiments, the negative electrode active material may adopt the negative electrode active material for the battery known in the art. 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, etc. The silicon-based material may be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. When the battery is a lithium-ion battery, the titanate uses lithium titanate; when the battery is a sodium-ion battery, the titanate uses sodium titanate. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0188] In some embodiments, the negative electrode film layer may further 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).

[0189] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0190] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0191] In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0192] [Electrolytes]

[0193] The electrolyte plays a role in conducting ions between the positive electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs.

[0194] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0195] In some embodiments, the electrolyte salt may include one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0196] In some embodiments, the solvent can be selected from one or more of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0197] In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

[0198] [Isolation film]

[0199] In some embodiments, the battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

[0200] In some embodiments, the isolation membrane includes a porous substrate. The material of the porous substrate may include one or more of glass fiber, non-woven 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.

[0201] In some embodiments, the isolation membrane may further include a coating layer located on at least one surface of the porous substrate. Specifically, the coating layer includes one or more of inorganic heat-resistant particles and organic heat-resistant particles.

[0202] In some embodiments, the porosity of the isolation film is 10% to 40%.

[0203] In some embodiments, the thickness of the isolation film may be 3 μm to 20 μm.

[0204] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0205] In some embodiments, the battery may include an outer packaging, which may be used to encapsulate the electrode assembly and the electrolyte.

[0206] In some embodiments, the outer packaging of the battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0207] The present application has no particular limitation on the shape of the battery, which may be cylindrical, square or any other shape. For example, Figure 1 The battery 5 is a square structure as an example.

[0208] In some embodiments, reference Figure 2 , the outer package may include a shell 51 and a cover plate 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0209] In some embodiments, batteries may be assembled into a battery module. The number of batteries contained in the battery module may be one or more. The specific number may be selected by those skilled in the art according to the application and capacity of the battery module.

[0210] Figure 3 4 is an example of a battery module. Figure 3 In the battery module 4, the multiple batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other manner. Further, the multiple batteries 5 can be fixed by fasteners.

[0211] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of batteries 5 are received in the receiving space.

[0212] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery pack.

[0213] Figure 4 and Figure 5 1 is a battery pack 1 as an example. Figure 4 and Figure 5 The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0214] The second aspect of the embodiment of the present application provides a method for preparing a positive electrode active material, comprising:

[0215] Providing lithium-containing phosphate precursors;

[0216] grinding a lithium-containing phosphate precursor and a carbon source;

[0217] The ground material is heated to a first temperature T1 at a first rate under a protective gas atmosphere and kept at the first temperature T1 for a first time t1; then heated to a second temperature T2 at a second rate and kept at the second temperature T2 for a second time t2, to obtain a positive electrode active material;

[0218] The positive electrode active material includes an active substance, and the active substance is shown in formula (1):

[0219] Li m A x Fe 1-y G y P 1-z D z O 4-n E n (1);

[0220] 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; n=0-0.5;

[0221] The surface of the particles of the active substance is coated with a layer of carbonaceous material;

[0222] 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.

[0223] In the embodiment of the present application, a positive electrode active material coated with a carbonaceous material layer is prepared by a solid phase method. First, a lithium phosphate precursor is ground with a carbon source. Through grinding, the precursor and the carbon source can be mixed to form a uniform mixture, creating conditions for subsequent heat treatment and carbon coating; the grinding process can refine the particles of the precursor and the carbon source, which helps to adjust and control their particle size distribution, and helps to improve the uniformity of carbon coating during the subsequent heat treatment. The ground material is heat treated. During the heat treatment, the carbon source decomposes and deposits on the surface of the precursor particles to form a carbonaceous material layer. During the heat treatment, a section is set during the heating process to maintain a constant temperature, that is, keep the first temperature T1 for a first time t1, as the temperature for the formation of the phosphate active substance lattice, so that the relevant side reaction products can be fully and timely discharged, avoiding the generation of other impurities, which is conducive to improving the crystallinity. As an example, if a lithium-containing phosphate precursor is prepared by a metal source of the metal contained therein (such as a lithium source, etc.), at a first temperature T1, dissolution of the lithium source and embedding of lithium ions will occur, accompanied by the generation and discharge of reaction by-products (such as H2O, CO2, H2, CO, etc.).

[0224] In addition, by controlling at least one parameter among the first rate, the first temperature T1, the first time t1, the second rate, the second temperature T2, and the second time t2, the parameters such as the crystallinity, particle size and distribution of the positive electrode active material can be regulated.

[0225] In the embodiment of the present application, the active material (i.e., phosphate active material) is coated with a carbonaceous material layer by the above-mentioned 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-mentioned conditions, so that part of the initial capacity of the positive electrode active material is hidden, thereby enabling the positive electrode active material to have a capacity sustained release effect.

[0226] In the embodiment of the present application, the lithium-containing phosphate precursor can be synthesized during the preparation process of the positive electrode active material, or can be a purchased product. As an example, the precursor can be purchased iron phosphate, a bulk chemical product.

[0227] In some embodiments, the lithium-containing phosphate precursor includes a lithium-containing transition metal phosphate having a composition of the following general formula:

[0228] Li a Fe b P c O d Q e ;

[0229] Among them, 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, Br, 0.8≤a≤1.15, 0.9≤b≤1, 0.95≤c≤1, 3.5≤d≤4, 0<e≤0.1.

[0230] 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 consisting of any two of the above a's, or a numerical value in the range consisting of the above two a's.

[0231] 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 consisting of any two of the above x, or a value in the range consisting of.

[0232] 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 in a range consisting of the above values.

[0233] 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 in the range consisting of.

[0234] 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's or a value in the range consisting of the above two e's.

[0235] In some embodiments, the modifying element Q may exist in the lithium-containing transition metal phosphate particles in the form of a doping element, and may also exist in the coating layer of the lithium-containing transition metal phosphate particles in the form of a coating element.

[0236] In some embodiments, Q includes one or more of Zn, Al, Na, K, Mg.

[0237] 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 lithium insertion and extraction, improve the structural stability of the material, and reduce the oxygen activity on the particle surface, thereby increasing the gram capacity of the material.

[0238] In some embodiments, Q includes one or more of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, and Ti.

[0239] 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.

[0240] In some embodiments, Q includes Ti. This helps to improve the kinetic performance of the secondary battery.

[0241] In some embodiments, Q includes one or more of B, S, Si, and N.

[0242] 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 lithium insertion and extraction, improve the structural stability of the material, and reduce the oxygen activity on the particle surface, thereby increasing the gram capacity of the material.

[0243] In some embodiments, Q includes one or more of S, F, Cl, and Br, which is beneficial to improve the electrochemical performance of the positive electrode active material.

[0244] In some embodiments, the mass content of Q is 4000 ppm to 6000 ppm based on the total mass of the active material.

[0245] As an example, based on the total mass of the active substance, the mass content of Q can be 4000 ppm, 4200 ppm, 4400 ppm, 4600 ppm, 4800 ppm, 5000 ppm, 5200 ppm, 5400 ppm, 5600 ppm, 5800 ppm, 6000 ppm, etc., or a range consisting of any two of the above mass contents or a numerical value in the range.

[0246] In the implementation of this application, the type and content of the elements in the active material can be tested by any known method in the art. As an example, the corresponding elements and content are tested by inductively coupled plasma emission spectrometry with reference to Appendix C of GB / T 33822-2017.

[0247] In some embodiments, the first rate is ≤ 4°C / min.

[0248] 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 .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 consisting of any two of the above values, or a value in the range.

[0249] In the embodiment of the present application, the heating rate will affect the crystal growth rate, thereby affecting the particle size and particle size distribution of the obtained 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 improvement effect of the early cycle capacity retention rate attenuation of the battery.

[0250] In some embodiments, the first rate is 1°C / min to 4°C / min.

[0251] By controlling the first rate to 1°C / min to 4°C / min, it is helpful to better regulate the primary particle size and distribution of the positive electrode active material, thereby improving the attenuation of the early cycle capacity retention rate of the battery.

[0252] In some embodiments, the first temperature T1 ≥ 400°C.

[0253] 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 consisting of any two of the above values, or a value in the range.

[0254] In the implementation manner of the present application, the first temperature T1 is used as the temperature for the formation of the lattice of the phosphate active material, so that the related side reaction products can be fully and timely discharged, avoiding the generation of other impurities, which is conducive to improving the crystallinity.

[0255] In some embodiments, the first temperature T1 is 400°C to 600°C.

[0256] In the embodiment of the present application, by controlling the first temperature T1 to be 400° C. to 600° C., the crystallinity of the active material is improved; in addition, it is also beneficial to subsequently control the particle size and distribution of the primary particles of the positive electrode active material.

[0257] In some embodiments, the first temperature T1 is 450°C to 550°C.

[0258] In the embodiment of the present application, by controlling the first temperature T1 to be 450° C. to 550° C., the crystallinity of the active material is effectively improved, which facilitates the subsequent better regulation of the particle size and distribution of the primary particles of the positive electrode active material.

[0259] In some embodiments, the first time t1 ≥ 1 h.

[0260] As an example, the first time t1 is 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 composed of any two of the above values ​​or a value in the composed range.

[0261] In the embodiment of the present application, the first time t1 can match the first rate, affecting the rate of crystal growth, thereby affecting the size and particle size distribution of the obtained positive electrode active material particles. 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 improvement effect of the early cycle capacity retention rate attenuation of the battery.

[0262] In some embodiments, the first time t1 is 1 hour to 7 hours.

[0263] In the embodiment of the present application, by controlling the first time t1 to be 1 hour to 7 hours, it is beneficial to regulate the primary particle size and distribution of the positive electrode active material, thereby enhancing the improvement effect on the attenuation of the early cycle capacity retention rate of the battery.

[0264] In some embodiments, the first time t1 is 3 hours to 5 hours.

[0265] In the embodiment of the present application, by controlling the first time t1 to be 3h to 5h, it is beneficial to better control the primary particle size and distribution of the positive electrode active material, and enhance the improvement effect of the early cycle capacity retention rate attenuation of the battery.

[0266] In some embodiments, the second temperature T2 is ≥ 720°C.

[0267] 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 consisting of any two of the above values ​​or a value in the range.

[0268] In the embodiment of the present 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 to improve the capacity, powder compaction density, crystallinity and other properties of the positive electrode active material.

[0269] In some embodiments, the second temperature T2 is 720°C to 850°C.

[0270] In the embodiment of the present application, by controlling the second temperature T2 to meet 720° C. to 850° C., it is beneficial to regulate the primary particle size and distribution of the positive electrode active material to improve the capacity, powder compaction density and other properties of the positive electrode active material.

[0271] In some embodiments, the second temperature T2 is 750°C to 810°C.

[0272] In the embodiment of the present application, by controlling the second temperature T2 to meet 750°C to 810°C, it is beneficial to better regulate the primary particle size and distribution of the positive electrode active material to improve the capacity, powder compaction density and other properties of the positive electrode active material.

[0273] In some embodiments, the second rate is 1° C. / min to 10° C. / min; and / or the second time t2 is 5 h to 20 h.

[0274] 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 consisting of any two of the above values, or a value in the range.

[0275] 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 consisting of any two of the above values ​​or a value in the range.

[0276] In the embodiment of the present application, controlling the second rate will affect the growth rate of the material, the second time t2 can match the second rate, and at least one parameter of the second rate and the second time t2 satisfies the above conditions, which is beneficial to regulating the particle size and distribution of the primary particles of the positive electrode active material to obtain higher kinetic performance.

[0277] In some embodiments, the second rate is 5°C / min to 9°C / min; and / or the second time t2 is 8h to 15h.

[0278] In the embodiment of the present application, by controlling at least one parameter in the second rate or t2 to meet the above conditions, it is beneficial to better regulate the particle size and distribution of the primary particles of the positive electrode active material and obtain higher kinetic performance.

[0279] In some embodiments, the carbon source comprises a first carbon source comprising a water soluble polymer.

[0280] Water-soluble polymers, also known as water-soluble resins or water-soluble macromolecules, are hydrophilic polymer materials that can dissolve or swell in water to form solutions and dispersions. In the embodiment of the present application, the material selected for the first carbon source includes a water-soluble polymer, which is conducive to obtaining a carbonaceous material layer with a high degree of graphitization, which is conducive to increasing the density of the carbonaceous material and improving the interface protection effect on the positive electrode active material.

[0281] In some embodiments, the water-soluble polymer includes one or more of polyethylene glycol, polyaniline, and their respective derivatives.

[0282] In the embodiment of the present application, the water-soluble polymer is selected from one or a combination of two or more of the above materials, which is beneficial to improving the graphitization degree and density of the carbonaceous material layer of the positive electrode active material and improving the interface stability of the positive electrode active material.

[0283] In some embodiments, based on the total weight of the carbon source, the weight content of the first carbon source is ≥ 50%.

[0284] 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 consisting of any two of the above values ​​or a value in the range.

[0285] In the embodiment of the present application, 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 interface stability of the positive electrode active material.

[0286] In some embodiments, the weight content of the first carbon source is 50% to 70% based on the total weight of the carbon source.

[0287] In the embodiment of the present application, 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 facilitating cost control.

[0288] In some embodiments, the carbon source further comprises a second carbon source, and the second carbon source comprises one or more of glucose, sucrose, lactose, and maltose.

[0289] In the embodiment of the present application, the second carbon source is used in combination with the first carbon source as the carbon source of the carbonaceous material layer, and the material selected for the second carbon source includes one or more of glucose, sucrose, lactose, and maltose, which is conducive to reducing the cost of the carbon source. In addition, it will also affect the electrical properties and processing properties of the positive electrode active material obtained after coating; the coating process is liquid phase coating, which needs to be soluble in water to carry out the process, otherwise it is not conducive to uniform coating on the surface of the material, such as glucose, sucrose, lactose, maltose, one or more of which are soluble in water as the second carbon source, which is conducive to the uniform coating of the carbonaceous material layer on the surface of the active substance.

[0290] In some embodiments, based on the total weight of the carbon source, the weight content of the second carbon source is ≤50%,

[0291] 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 consisting of any two of the above values ​​or a value in the range.

[0292] In the embodiment of the present application, a second carbon source is used in combination with the first carbon source as the carbon source of the carbonaceous material layer, and the cost of the carbon source and the coating quality of the carbonaceous material layer, such as the degree of graphitization and density, are adjusted by controlling the dosage ratio of the first carbon source and the second carbon source.

[0293] Furthermore, the sum of the weight content of the first carbon source and the weight content of the second carbon source is 100%.

[0294] In some embodiments, the weight content of the second carbon source is 30% to 50% based on the total weight of the carbon source.

[0295] In the embodiment of the present application, a second carbon source is used in combination with the first carbon source as the carbon source of the carbonaceous material layer. By controlling the dosage ratio of the first carbon source and the second carbon source to meet the above conditions, the graphitization degree and density of the carbonaceous material layer can be effectively improved, and the cost of the carbon source can be reduced.

[0296] A third aspect of the embodiments of the present application proposes a positive electrode active material, which is the positive electrode active material described in the first aspect or the positive electrode active material prepared by the preparation method proposed in the second aspect.

[0297] In the embodiment of the present application, the positive electrode active material proposed has the beneficial effects of the positive electrode active material described in the first aspect and the second aspect above, which will not be described in detail here.

[0298] A fourth aspect of the embodiments of the present application provides an electrical device, comprising the lithium secondary battery provided in the first aspect.

[0299] The electrical device includes one or more of the batteries, battery modules, and battery packs provided in the present application. The battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as 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., but are not limited thereto.

[0300] As the electrical device, a battery, a battery module or a battery pack may be selected according to its usage requirements.

[0301] Figure 6 The power consumption device is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the high power and high energy density requirements of the power consumption device for the battery, a battery pack or a battery module can be used.

[0302] As another example, the device may be a mobile phone, a tablet computer, a notebook computer, etc. The device is usually required to be light and thin, and a battery may be used as a power source.

[0303] In order to make the technical problems, technical solutions and beneficial effects solved by the embodiments of the present application clearer, the following will be further described in detail with reference to the embodiments and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present application and its applications. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0304] 1. Preparation of lithium-ion batteries

[0305] Example 1

[0306] (1) Preparation of positive electrode active materials

[0307] FePO4, Li2CO3, glucose and polyethylene glycol are mixed evenly, a small amount of water is added, and the mixture is spray-dried after grinding; the spray-dried powder is placed in a sintering furnace, and the temperature is increased from 25°C to 500°C (first temperature T1) at a rate of 2°C / min (first rate) under a nitrogen atmosphere and kept at this temperature for 3 hours (first time t1); then the temperature is increased to 780°C (second temperature T2) at a rate of 5°C / min (second rate) and kept at this temperature for 10 hours (second time t2); after the temperature is lowered, the mixture is crushed by air flow to obtain the positive electrode active material, i.e., carbon-coated LiFePO4.

[0308] The carbon source is a mixture of glucose and polyethylene glycol (based on the total mass of the carbon source, the weight content of glucose is 50%, and the weight content of polyethylene glycol is 50%), and the amount of carbon source added is such that the weight content of residual carbon of the carbon source (i.e., the weight content m of the carbonaceous material layer) is 1.29%, based on the total weight of the prepared positive electrode active material.

[0309] (2) Preparation of batteries

[0310] 1) Preparation of positive electrode sheet:

[0311] The positive electrode active material, carbon black SP and polyvinylidene fluoride (PVDF) were uniformly mixed in an N-methylpyrrolidone solvent system at a weight ratio of 92:2.5:5.5, and then coated on an aluminum foil, dried and cold pressed to obtain a positive electrode sheet.

[0312] 2) Preparation of negative electrode sheet:

[0313] The negative electrode active materials graphite, acetylene black, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) were mixed uniformly in deionized water at a weight ratio of 95:2:2:1, coated on a copper foil, dried, and cold pressed to obtain a negative electrode sheet.

[0314] 3) Preparation of electrolyte:

[0315] Ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then fully dried LiPF6 is dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0316] 4) Battery isolation film:

[0317] A polyethylene isolation film with a thickness of 8 μm is used.

[0318] 5) Preparation of battery:

[0319] Using a polyethylene (PE) porous polymer film as a separator, the positive electrode sheet, separator, and negative electrode sheet are stacked in order, so that the separator is placed between the positive and negative electrodes to play a role of isolation, and then wound to obtain an electrode assembly. The electrode assembly is placed in an outer package, injected with electrolyte and packaged, and after processes such as formation, a battery is obtained.

[0320] Example 2-Example 34

[0321] The batteries provided in Examples 2 to 34 are prepared using the method of Example 1, and the specific parameters are shown in Tables 1 and 2.

[0322] Comparative Example 1

[0323] The battery provided in Comparative Example 1 adopts the preparation method of Example 1, except that glucose and polyethylene glycol are not added in the preparation step of the positive electrode active material. Specific parameters are shown in Tables 1 and 2.

[0324] Comparative Example 2

[0325] The battery provided in Comparative Example 2 adopts the preparation method of Example 1, except that 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 / 600. Specific parameters are shown in Tables 1 and 2.

[0326] Comparative Example 3

[0327] The battery provided in Comparative Example 3 adopts the preparation method of Example 1, except that 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 / 50. The specific parameters are shown in Tables 1-1, 1-2 and 2.

[0328]

[0329]

[0330] Table 2

[0331]

[0332] 2. Performance Test

[0333] (I) Performance test method

[0334] (1) Before the cycle begins, the capacity and power must be tested as the initial cycle capacity and power values ​​of the battery cell. The process is as follows:

[0335] 1) Leave the battery at 25°C for 120 minutes;

[0336] 2) Discharge the battery to 2.5V at 0.5P;

[0337] 3) Discharge the battery to 2.0V at 0.1P;

[0338] 4) Leave the battery at 25°C for 30 minutes;

[0339] 5) 0.5C constant current charging to 3.8V;

[0340] 6) 0.04C constant current charging to 3.8V;

[0341] 7) Leave the battery at 25°C for 30 minutes;

[0342] 8) 0.5C constant current discharge to 2.5V, and the discharge power of this step is recorded as P;

[0343] 9) 0.04C constant current discharge to 2.0V, and the sum of the capacities of step 8) and step 9) is recorded as the cycle initial discharge capacity C;

[0344] 10) Leave the battery at 25°C for 30 minutes.

[0345] (2) The cycle test process is as follows:

[0346] 1) Leave the battery at 25°C for 120 minutes;

[0347] 2) Discharge the battery to 2.5V at 0.5P;

[0348] 3) Leave the battery at 25°C for 30 minutes;

[0349] 4) Charge the battery to 3.8V at 0.5P constant power;

[0350] 5) Stand at 25℃ for 1 min;

[0351] 6) 0.05P constant power charging to 3.8V;

[0352] 7) Leave the battery at 25°C for 30 minutes;

[0353] 8) Discharge the battery to 2.5V at 0.5P;

[0354] 9) Leave the battery at 25°C for 5 minutes;

[0355] 10) The above steps 4) to 8) are a charge and discharge cycle of the battery, which is repeated continuously until the battery capacity decays to 80% of the initial value. The number of cycles at this time is recorded, which is the cycle life, in units of cls, as shown in Table 2.

[0356] Capacity retention rate at the nth cycle = (discharge capacity at the nth cycle / discharge capacity at the initial stage of the cycle) × 100%.

[0357] Capacity retention rate at the 300th cycle; Capacity retention rate at the 300th cycle = (discharge capacity at the 300th cycle / discharge capacity at the beginning of the cycle) × 100%.

[0358] Cycle climbing degree:

[0359] Record the number of cycles in which the capacity retention rate is continuously higher than 100% during the first 300 cycles. If the number of cycles continuously higher than 100% is ≥10, it is determined that the cycle process has a cycle climbing degree, and the maximum capacity retention rate when the capacity retention rate is higher than 100% during the cycle process is recorded. If the number of cycles continuously higher than 100% is <10, it is determined that the cycle process has no cycle climbing degree, and it is recorded as "-".

[0360] Specifically, the cycle ramp degree is calculated and expressed by the following method: (capacity after cycling to 300 cls - capacity at the initial cycle) / capacity at the initial cycle×100%.

[0361] (II) Performance test results

[0362] The performance test results of the batteries provided in Examples 1-34 and Comparative Examples 1-3 are shown in Table 3.

[0363] Table 3

[0364]

[0365] As shown in Table 3, the lithium secondary battery provided in the embodiment of the present application exhibits a high cycle capacity retention rate at the beginning of the cycle, and there is a cycle climbing phenomenon, indicating that the lithium secondary battery can gradually reach the optimal performance state at the beginning of the cycle. However, the lithium secondary batteries provided in Comparative Examples 1 to Comparative Examples 3 have a low cycle capacity retention rate at the beginning of the cycle, and the cycle capacity retention rate of the battery decays too quickly, and there is no cycle climbing phenomenon.

[0366] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and they should all be included in the scope of the claims and specification of the present application. In particular, as long as there is no structural conflict, the various technical features mentioned in the various embodiments can be combined in any way. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims

1. A lithium secondary battery, characterized in that: The lithium secondary battery includes a positive electrode active material, and the positive electrode active material includes an active substance, and the active substance 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 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; n=0-0.5; The surface of the particles of the active substance is coated with a carbonaceous material layer; A ratio h / d of a thickness h of the carbonaceous material layer to a diameter d of primary particles of the positive electrode active material is 1 / 500 to 1 / 100.

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

1.

3. The lithium secondary battery according to claim 1 or 2, characterized in that: A ratio h / d of a thickness h of the carbonaceous material layer to a diameter d of 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, characterized in that: A ratio h / d of a thickness h of the carbonaceous material layer to a diameter d of 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, characterized in that: The thickness h of the carbonaceous material layer is 1 nm to 5 nm, and / or 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, characterized in that: The thickness h of the carbonaceous material layer is 2 nm to 4 nm, and / or the diameter d of the primary particles of the positive electrode active material is 200 nm to 400 nm.

7. The lithium secondary battery according to any one of claims 1 to 4, characterized in that: The thickness h of the carbonaceous material layer is 2 nm to 3 nm, and / or 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, characterized in that: Based on the total mass of the active material and the carbonaceous material layer, the mass proportion 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, characterized in that: The lithium secondary battery further includes an electrolyte, and the electrolyte includes lithium difluorophosphate.

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

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

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

13. The lithium secondary battery according to any one of claims 1 to 12, characterized in that: The lithium secondary battery further includes an electrolyte, wherein the electrolyte includes vinylene carbonate, and based on the total mass of the electrolyte, the mass content of the vinylene carbonate is 0.01% to 2%.

14. The lithium secondary battery according to any one of claims 1 to 12, characterized in that: The lithium secondary battery further includes an electrolyte, wherein the electrolyte includes vinylene carbonate, and based on the total mass of the electrolyte, the mass content of the vinylene carbonate is 0.02% to 0.6%.

15. The lithium secondary battery according to any one of claims 1 to 12, characterized in that: The lithium secondary battery further includes an electrolyte, wherein the electrolyte includes vinylene carbonate, and based on the total mass of the electrolyte, the mass content of the vinylene carbonate is 0.05% to 0.3%.

16. The lithium secondary battery according to any one of claims 1 to 15, characterized in that: The material of the carbonaceous material layer includes one or more of graphite, graphene, amorphous carbon, carbon fiber, carbon nanotube, carbon dot, and carbon felt.

17. The lithium secondary battery according to any one of claims 1 to 16, characterized in that: 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 m 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 / cm 3 ~2.65g / cm 3 .

18. A method for preparing a positive electrode active material, characterized in that: include: Providing lithium-containing phosphate precursors; grinding the lithium-containing phosphate precursor and a carbon source; The ground material is heated to a first temperature T1 at a first rate under a protective gas atmosphere and maintained at the first temperature T1 for a first time t1; Then, the temperature is raised to a second temperature T2 at a second rate and kept at the second temperature T2 for a second time t2 to obtain a positive electrode active material; The positive electrode active material includes an active substance, and the active substance 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 of Zn, Al, Na, K, and Mg; G includes one or more elements of Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, 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; n=0-0.5; The surface of the particles of the active substance is coated with a carbonaceous material layer; A ratio h / d of a thickness h of the carbonaceous material layer to a diameter d of primary particles of the positive electrode active material is 1 / 500 to 1 / 100.

19. The preparation method according to claim 18, characterized in that: The first rate is ≤4°C / min.

20. The preparation method according to claim 18, characterized in that: The first rate is 1°C / min to 4°C / min.

21. The preparation method according to any one of claims 18 to 20, characterized in that: The first temperature T1 is ≥ 400°C.

22. The preparation method according to any one of claims 18 to 20, characterized in that: The first temperature T1 is 400°C to 600°C.

23. The preparation method according to any one of claims 18 to 20, characterized in that: The first temperature T1 is 450°C to 550°C.

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

25. The preparation method according to any one of claims 18 to 23, characterized in that: The first time t1 is 1 hour to 7 hours.

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

27. The preparation method according to any one of claims 18 to 26, characterized in that: The second temperature T2 ≥ 720°C.

28. The preparation method according to any one of claims 18 to 26, characterized in that: The second temperature T2 is 720°C to 850°C.

29. The preparation method according to any one of claims 18 to 26, characterized in that: The second temperature T2 is 750°C to 810°C.

30. The preparation method according to any one of claims 18 to 29, characterized in that: The second rate is 1°C / min to 10°C / min; and / or the second time t2 is 5h to 20h.

31. The preparation method according to any one of claims 18 to 29, characterized in that: The second rate is 5°C / min to 9°C / min; and / or the second time t2 is 8h to 15h.

32. The preparation method according to any one of claims 18 to 31, characterized in that The carbon source includes a first carbon source, and the first carbon source includes a water-soluble polymer.

33. The preparation method according to claim 32, characterized in that: 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, characterized in that: Based on the total weight of the carbon source, the weight content of the first carbon source is ≥50%.

35. The preparation method according to claim 32 or 33, characterized in that: Based on the total weight of the carbon source, 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, characterized in that The carbon source also includes a second carbon source, and the second carbon source includes one or more of glucose, sucrose, lactose, and maltose.

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

38. The preparation method according to claim 36, characterized in that: Based on the total weight of the carbon source, the weight content of the second carbon source is 30% to 50%. 39 . A positive electrode active material, which is the positive electrode active material in the lithium secondary battery according to claim 1 or the positive electrode active material prepared by the preparation method according to claim 18 or 38.

40. An electrical device, characterized in that: A lithium secondary battery comprising the lithium secondary battery according to any one of claims 1 to 17.

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