A secondary battery and a method for preparing the same

Abstract

The present application provides a secondary battery and a method for preparing the same. The secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The positive electrode sheet includes a current collector, and an active layer is provided on one or both sides of the current collector. The active layer includes a first active layer and a second active layer. The first active layer is provided between the second active layer and the current collector. The first active layer includes carbon and graphene-coated particles. The carbon and graphene-coated particles are a core-shell structure consisting of a core and a shell surrounding the core. The shell is composed of a first shell close to the core and a second shell farther away from the core than the first shell. The first shell is a carbon layer and the second shell is a graphene layer. By changing the structure of the positive electrode sheet, the cycle life is improved, thereby solving the problem of low cycle life of the secondary battery. The present application also provides a method for preparing the above-mentioned secondary battery.

Description

Technical Field

[0001] The present application relates to the photovoltaic field, and in particular to a secondary battery and a preparation method thereof. Background Art

[0002] With the continuous development of new energy technologies, energy storage cells, as key components, have been widely used in electric vehicles, energy storage systems, and other fields. However, traditional energy storage cells often experience problems such as increased internal resistance and capacity decay after long-term cycling, seriously affecting battery performance and service life.

[0003] The problems of high internal resistance and low cycle life of batteries are primarily due to the material and structural design of the battery's positive electrode. Specifically, during the production process, the electrode components and structure are not properly selected. Inappropriate selection of lithium supplements can lead to unstable battery voltage when using the supplements, affecting battery performance and cycle life.

[0004] The present application provides a secondary battery and a preparation method thereof, which solve the problem of low cycle life of the secondary battery. Summary of the Invention

[0005] In order to solve the problem of low cycle life of secondary batteries, the present application provides a secondary battery and a preparation method thereof. The secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, the positive electrode sheet includes a current collector, an active layer is provided on one or both sides of the current collector, the active layer includes a first active layer and a second active layer, the first active layer is provided between the second active layer and the current collector, the first active layer includes carbon and graphene-coated particles, the carbon and graphene-coated particles are a shell-core structure consisting of a core and a shell surrounding the core, the shell material is carbon and graphene, the core material is one of lithium ferrite, lithium nickelate and lithium niobate, the shell is composed of a first shell layer close to the core and a second shell layer farther away from the core than the first shell layer, the first shell layer is a carbon layer, the second shell layer is a graphene layer, and the material forming the carbon layer includes a mixed carbon source, and the mixed carbon source includes ethanol, ethylene, benzene, hexachlorobenzene and ethylene.

[0006] The materials of the first active layer and the second active layer are different. The positive electrode sheet of the present application can simultaneously take into account the requirements of the energy storage battery for high specific energy and long cycle, and can simultaneously improve the cycle life, low-temperature performance and energy density of the energy storage battery. The first active layer uses graphene to improve the internal resistance and low-temperature performance of the battery; the carbon-coated particles reduce the internal resistance of the battery cell and improve the initial effect of the lithium supplement, reduce the use of the lithium supplement and maximize the capacity in the case of negative electrode redundancy; lithium niobate is used to supplement Li+, thereby improving the battery life. Compared with conventional lithium supplements, the lithium supplement has improved the problems of excessively high voltage, air instability, and too low amount of Li+ released, improved interface defects, and solved the problem of low cycle life of secondary batteries.

[0007] Optionally, the mass fraction of carbon and graphene-coated particles in the first active layer is 0.8% to 1.0%.

[0008] Optionally, the D50 particle size of the core particles is 5 μm to 10 μm, the thickness of the first shell layer is 60 nm to 100 nm, and the thickness of the second shell layer is 70 nm to 120 nm.

[0009] Optionally, the active layer includes lithium iron phosphate, the D50 particle size of the lithium iron phosphate is 0.6 μm to 1.6 μm, and the powder compaction density is ≥ 2.3 g / cm 3 , specific surface area 8m 2 / g~12m 2 / g.

[0010] Optionally, the thickness of the first active layer is 40 μm to 50 μm, and the thickness of the second active layer is 140 μm to 160 μm.

[0011] The present application also provides a method for preparing a secondary battery, which is used to prepare any of the secondary batteries described above, comprising: preparing carbon and graphene-coated particles using carbon, graphene and core materials; sequentially coating the carbon and graphene-coated particles and lithium iron phosphate on one or both sides of a positive electrode collector to obtain the positive electrode sheet; assembling the positive electrode sheet, negative electrode sheet, electrolyte and separator, and packaging them to obtain the secondary battery.

[0012] Optionally, the preparation method further includes: introducing an inert gas into a reaction vessel in which the core material powder is placed, heating up, and then adding a mixed carbon source to obtain a first coating material; cooling the first coating material, and transferring it into a bag filter to obtain a second coating material; ultrasonically dispersing the second coating material in water to obtain a first dispersion, ultrasonically dispersing graphene in water to obtain a second dispersion, mixing the first dispersion and the second dispersion and uniformly dispersing them by ultrasonication to obtain a mixed dispersion; drying the mixed dispersion at 150°C, and heat-treating it at 200°C to obtain the carbon and graphene coated particles.

[0013] Optionally, the preparation method further includes: pre-treating the core material powder and placing it in a reaction vessel, introducing an inert gas into the reaction vessel, heating it to 400°C~800°C, adding a mixed carbon source and reacting for 0.2h~3h to obtain a first coating material.

[0014] Optionally, the ratio of the carbon content of the mixed carbon source to the carbon content of the lithium iron phosphate in the active layer is 0.32-0.35:1.

[0015] Optionally, the mixed carbon source includes ethanol, ethylene, benzene, hexachlorobenzene and ethylene, and the mass ratio of ethanol, ethylene, benzene, hexachlorobenzene and ethylene in the mixed carbon source is 1:0.8~1.2:0.8~1.2:0.8~1.2:0.8~1.2.

[0016] Optionally, the method further includes: cooling the first coating material, and when the temperature is lower than 200° C., transferring the first coating material into a bag filter, and obtaining a second coating material by chemical vapor deposition.

[0017] The present application provides a secondary battery and a method for preparing the same. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The positive electrode comprises a current collector. An active layer is provided on one or both sides of the current collector. The active layer comprises a first active layer and a second active layer. The first active layer is provided between the second active layer and the current collector. The first active layer comprises carbon and graphene-coated particles. The carbon and graphene-coated particles are of a core-shell structure. The shell material is carbon and graphene. The core material is one of lithium ferrite, lithium nickelate, and lithium niobate. The shell of the carbon and graphene-coated particles consists of two layers. The first shell layer close to the core is a carbon layer, and the second shell layer is a graphene layer. By changing the structure of the positive electrode sheet, the cycle life is improved, thereby solving the problem of low cycle life of the secondary battery. The present application also provides a method for preparing the above-mentioned secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solution of the present application, the following is a brief introduction to the drawings required for use in the embodiments. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0019] Figure 1 Schematic diagram of the positive electrode structure provided in this application;

[0020] Figure 2 A disassembled diagram of the secondary battery structure provided in an embodiment of the present application;

[0021] Figure 3This is a schematic diagram of the winding core structure in an embodiment of the present application.

[0022] Illustration:

[0023] Among them, 1-aluminum foil; 2-first active layer; 3-second active layer. DETAILED DESCRIPTION

[0024] The following embodiments are described in detail, with examples illustrated in the accompanying drawings. When the following description refers to the drawings, identical numbers in different figures represent identical or similar elements unless otherwise indicated. The embodiments described in the following embodiments are not intended to represent all possible implementations consistent with the present application. They are merely examples of systems and methods consistent with certain aspects of the present application, as detailed in the claims.

[0025] The problems of high internal resistance and low cycle life in secondary batteries are mainly due to the material and structural design of the battery's positive electrode. Specifically, during the battery electrode manufacturing process, the selection of electrode components and structure is unreasonable; the improper selection of lithium supplements leads to unstable battery voltage when using the supplements, affecting battery performance and cycle life. Especially in extreme environments, the battery's cycle life and energy density often cannot meet the needs of actual applications. This application achieves an increase in cycle life by changing the electrode structure.

[0026] A battery's cycle life refers to the number of charge and discharge cycles a battery can undergo under specified charge and discharge conditions. It's usually expressed in cycles. When the battery capacity drops to a certain percentage of its initial capacity (e.g., 80%), it's considered to have reached the end of its cycle life. A battery's cycle life is affected by many factors, including charge and discharge rate, depth of charge and discharge, temperature, and battery materials. A long cycle life means the battery can maintain stable performance for a longer period, thereby extending the lifespan of the device and reducing its operating costs. This is particularly important in applications such as electric vehicles, energy storage systems, and devices requiring long-term operation. Battery cycle life is a key metric for evaluating battery performance. A long cycle life can extend the lifespan of equipment and reduce its operating costs.

[0027] To address the issue of low cycle life in secondary batteries, the present application provides a secondary battery and a method for preparing the same. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The positive electrode sheet comprises a current collector. An active layer is disposed on one or both sides of the current collector, comprising a first active layer and a second active layer.

[0028] like Figure 1As shown, the current collector is an aluminum foil 1, and an active layer is provided on one side of the aluminum foil 1. The active layer includes a first active layer 2 and a second active layer 3. The first active layer 2 is provided between the second active layer 3 and the aluminum foil 1; the first active layer 2 includes carbon and graphene-coated particles and lithium iron phosphate, and the second active layer 3 includes lithium iron phosphate.

[0029] The first active layer includes carbon and graphene-coated particles, and the carbon and graphene-coated particles are a core-shell structure consisting of a core and a shell surrounding the core. The shell is made of carbon and graphene, and the core is made of one of lithium ferrite, lithium nickelate, and lithium niobate. The shell is composed of a first shell close to the core and a second shell farther away from the core than the first shell. The first shell is a carbon layer, and the second shell is a graphene layer. The material forming the carbon layer includes a mixed carbon source, and the mixed carbon source includes ethanol, ethylene, benzene, hexachlorobenzene, and ethylene.

[0030] It is understood that the first active layer is composed of an inner core, a first shell surrounding the inner core, and a second shell surrounding the first shell, i.e., a core-shell structure. The inner core is a lithium-supplementing material such as lithium ferrite, the first shell is a carbon layer, and the second shell is a graphene layer. Therefore, in this embodiment, the first active layer is defined as comprising carbon- and graphene-coated particles, and these carbon- and graphene-coated particles constitute the aforementioned core-shell structure. The materials of the first active layer and the second active layer are different. The positive electrode sheet of the present application can simultaneously take into account the energy storage battery's requirements for high specific energy and long cycle time, and can simultaneously improve the cycle life, low-temperature performance, and energy density of the energy storage battery. The first active layer uses graphene to improve the battery's internal resistance and low-temperature performance; the carbon-coated particles (i.e., the first shell layer) reduce the internal resistance of the energy storage battery and improve the initial effect of the lithium supplement, thereby reducing the use of the lithium supplement and maximizing the capacity in the case of negative electrode redundancy; lithium niobate is used to supplement Li+, thereby improving the life of the energy storage battery. Compared with conventional lithium supplements, the lithium supplement improves the problems of excessively high lithium supplement voltage, air instability, and too low a Li+ release amount, improves interface defects, and solves the problem of low cycle life of the secondary battery.

[0031] In some embodiments, the second active layer covers the entire surface of the first active layer.

[0032] In some embodiments, the mass fraction of the carbon and graphene-coated particles in the first active layer is 0.8% to 1.0%. The mass fraction of the carbon and graphene-coated particles in the first active layer refers to the percentage of the mass of the first active layer represented by the sum of the mass of all core-shell structures in the first active layer.

[0033] In some embodiments, the mass fraction of carbon and graphene-coated particles in the first active layer is 0.9%.

[0034] In some embodiments, the D50 particle size of the core particles is 5 μm to 10 μm, the thickness of the first shell layer is 60 nm to 100 nm, and the thickness of the second shell layer is 70 nm to 120 nm.

[0035] In some embodiments, the D50 particle size of the core particles is 6 μm to 9 μm, the thickness of the first shell layer is 70 nm to 95 nm, and the thickness of the second shell layer is 75 nm to 105 nm.

[0036] In some embodiments, the D50 particle size of the core particles is 7 μm, the thickness of the first shell layer is 80 nm, and the thickness of the second shell layer is 100 nm.

[0037] In some embodiments, based on the total mass percentage content of the carbon and graphene-coated particles as 100 mol%, that is, the total mass percentage content of a single core-shell structure is 100 mol%, the mass fraction of the first shell layer is 1.2% to 1.4%, the mass fraction of the second shell layer is 0.8% to 1.0%, and the mass fraction of the core is 97.6% to 98%.

[0038] In some embodiments, based on the total mass percentage content of the carbon and graphene-coated particles as 100 mol%, that is, the total mass percentage content of a single core-shell structure is 100 mol%, the mass fraction of the first shell layer is 1.32%~1.35%, the mass fraction of the second shell layer is 0.89%~0.95%, and the mass fraction of the core is 97.7%~97.9%.

[0039] In some embodiments, based on the total mass percentage content of the carbon and graphene-coated particles as 100 mol%, that is, the total mass percentage content of a single core-shell structure is 100 mol%, the mass fraction of the first shell is 1.34%, the mass fraction of the second shell is 0.9%, and the mass fraction of the core is 97.8%.

[0040] In some embodiments, the active layer comprises lithium iron phosphate, wherein the D50 particle size of the lithium iron phosphate is 0.6 μm to 1.6 μm, and the powder compaction density is ≥ 2.3 g / cm 3 , specific surface area 8m 2 / g~12m 2 / g.

[0041] Specifically, the first active layer may further include lithium iron phosphate; the second active layer may include lithium iron phosphate. The above powder compaction density is ≥ 2.3 g / cm 3 , specific surface area 8m 2 / g~12m2 / g refers to the parameters met by the lithium iron phosphate in the second active layer.

[0042] In some embodiments, the D50 particle size of the lithium iron phosphate is 0.7 μm to 1.5 μm.

[0043] In some embodiments, the D50 particle size of the lithium iron phosphate is 1.2 μm.

[0044] In some embodiments, the thickness of the first active layer is 40 μm to 50 μm, and the thickness of the second active layer is 140 μm to 160 μm.

[0045] In some embodiments, the thickness of the first active layer is 41 μm to 49 μm, and the thickness of the second active layer is 145 μm to 153 μm.

[0046] In some embodiments, the thickness of the first active layer is 47 μm, and the thickness of the second active layer is 147 μm.

[0047] This application also provides a method for preparing a secondary battery, which can be used to prepare any of the secondary batteries described above, comprising: preparing carbon and graphene-coated particles using carbon, graphene, and a core material; sequentially coating the carbon and graphene-coated particles and lithium iron phosphate on one or both surfaces of a positive electrode current collector to obtain the positive electrode sheet; and assembling the positive electrode sheet, negative electrode sheet, electrolyte, and separator, and encapsulating them to obtain the secondary battery. It should be noted that the corresponding descriptions in the aforementioned secondary battery embodiments also apply to the embodiments of the secondary battery preparation method of this application.

[0048] In some embodiments, the preparation method further includes: introducing an inert gas into a reaction vessel where the core material powder is placed, heating up, and then adding a mixed carbon source to obtain a first coating material; cooling the first coating material and transferring it to a bag filter to obtain a second coating material; ultrasonically dispersing the second coating material in water to obtain a first dispersion, ultrasonically dispersing graphene in water to obtain a second dispersion, mixing the first dispersion and the second dispersion and uniformly dispersing them by ultrasonication to obtain a mixed dispersion; drying the mixed dispersion at 150°C, and heat treating it at 200°C to obtain the carbon and graphene coated particles.

[0049] In some embodiments, the preparation method further includes: pre-treating the core material powder and placing it in a reaction vessel, introducing an inert gas into the reaction vessel, heating it to 400°C~800°C, adding a mixed carbon source and reacting for 0.2h~3h to obtain a first coating material.

[0050] In some embodiments, the ratio of the carbon content of the mixed carbon source to the carbon content of the lithium iron phosphate in the active layer is 0.32-0.35:1.

[0051] In some embodiments, the mixed carbon source includes ethanol, ethylene, benzene, hexachlorobenzene and ethylene, and the mass ratio of ethanol, ethylene, benzene, hexachlorobenzene and ethylene in the mixed carbon source is 1:0.8~1.2:0.8~1.2:0.8~1.2:0.8~1.2.

[0052] In some embodiments, the method further includes: cooling the first coating material, and when the temperature is lower than 200° C., transferring the first coating material into a bag filter, and obtaining a second coating material by chemical vapor deposition.

[0053] In some embodiments, as Figure 2 The secondary battery provided in this application uses a square shell (aluminum shell) battery cell. The square shell (aluminum shell) battery cell has high mechanical strength, is conducive to mass production, has a high system grouping rate, can accommodate sufficient electrolyte, and is conducive to a long life.

[0054] In some embodiments, as Figure 3 As shown, a winding needle is used to wind the positive electrode sheet, negative electrode sheet and separator provided in the present application into a bare battery cell, and the alignment consistency of the positive electrode sheet and the negative electrode sheet is ensured by correction.

[0055] For ease of understanding, the following examples describe the present disclosure in more detail. These examples are intended for illustrative purposes only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios mentioned in the following examples are by weight.

[0056] Example 1:

[0057] After pretreatment, lithium ferrite powder was placed in a reaction vessel. An inert gas was introduced into the reaction vessel, the temperature was raised to 400°C, and a mixed carbon source was added for a reaction period of 0.2 hours to produce a first coating material. The mass ratio of ethanol, ethylene, benzene, hexachlorobenzene, and ethylene in the mixed carbon source was 1:0.8:1.2:0.8:0.8. The carbon content of the mixed carbon source was compared to the carbon content of the lithium iron phosphate in the active layer in a ratio of 0.32:1. The lithium ferrite particles had a D50 particle size of 10 μm. The shell of the carbon- and graphene-coated particles consisted of two layers: a first carbon layer adjacent to the core and a second graphene layer. The thickness of the first shell was 60 nm, and the thickness of the second shell was 70 nm. The first coating material was cooled. Once the temperature dropped below 200°C, it was transferred to a bag filter and a second coating material was obtained by chemical vapor deposition.

[0058] The second coating material is ultrasonically dispersed in water to obtain a first dispersion, and the graphene is ultrasonically dispersed in water to obtain a second dispersion. The first and second dispersions are mixed and uniformly dispersed by ultrasonication to obtain a mixed dispersion. The mixed dispersion is dried at 150°C and heat-treated at 200°C to obtain first carbon- and graphene-coated particles. Based on the total mass percentage of the first carbon- and graphene-coated particles as 100 mol%, the mass fraction of the first shell is 1.2%, the mass fraction of the second shell is 0.8%, and the mass fraction of the core is 98%.

[0059] Active layers are provided on both sides of the positive electrode current collector to obtain a first positive electrode sheet. The thickness of the first active layer is 40μm, and the thickness of the second active layer is 140μm. Specifically, the first active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, PVDF as the binder, and the above-mentioned first carbon and graphene-coated particles as lithium supplements; and in the first active layer, the mass fraction of the positive electrode active material is 96.1%, the mass fraction of the conductive agent is 1.1%, the mass fraction of the binder is 2%, and the mass fraction of the lithium supplement is 0.8%. The second active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, and PVDF as the binder; and in the second active layer, the mass fraction of the positive electrode active material is 96.6%, the mass fraction of the conductive agent is 1.2%, and the mass fraction of the binder is 2.2%. The D50 particle size of the lithium iron phosphate in the active layer is 0.6μm, and the powder compaction density is 2.3g / cm 3 , specific surface area 8m 2 The first positive electrode sheet, the negative electrode sheet, the electrolyte and the separator are assembled and packaged to obtain the first secondary battery.

[0060] Example 2:

[0061] After pretreatment, lithium nickelate powder was placed in a reaction vessel. An inert gas was introduced into the reaction vessel, the temperature was raised to 800°C, and a mixed carbon source was added and reacted for 3 hours to obtain a first coating material. The mass ratio of ethanol, ethylene, benzene, hexachlorobenzene, and ethylene in the mixed carbon source was 1:1.2:0.8:1.2:1.2. The carbon content of the mixed carbon source was compared to the carbon content of the lithium iron phosphate in the active layer in a ratio of 0.35:1. The D50 particle size of the lithium nickelate was 5 μm. The shell of the carbon and graphene-coated particles consisted of two layers: a first shell adjacent to the core, a carbon layer, and a second shell, a graphene layer. The thickness of the first shell was 100 nm, and the thickness of the second shell was 120 nm. The first coating material was cooled. Once the temperature dropped below 200°C, it was transferred to a bag filter and the second coating material was obtained by chemical vapor deposition.

[0062] The second coating material is ultrasonically dispersed in water to obtain a first dispersion, and the graphene is ultrasonically dispersed in water to obtain a second dispersion. The first and second dispersions are mixed and uniformly dispersed by ultrasonication to obtain a mixed dispersion. The mixed dispersion is dried at 150°C and heat-treated at 200°C to obtain second carbon- and graphene-coated particles. Based on the total mass percentage of the second carbon- and graphene-coated particles as 100 mol%, the mass fraction of the first shell is 1.4%, the mass fraction of the second shell is 1%, and the mass fraction of the core is 97.6%.

[0063] Active layers are provided on both sides of the positive electrode current collector to obtain a second positive electrode sheet. The thickness of the first active layer is 50μm, and the thickness of the second active layer is 160μm. Specifically, the first active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, PVDF as the binder, and the above-mentioned second carbon and graphene-coated particles as lithium replenishers; and in the first active layer, the mass fraction of the positive electrode active material is 95.9%, the mass fraction of the conductive agent is 1.1%, the mass fraction of the binder is 2%, and the mass fraction of the lithium replenisher is 1.0%. The second active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, and PVDF as the binder; and in the second active layer, the mass fraction of the positive electrode active material is 96.6%, the mass fraction of the conductive agent is 1.2%, and the mass fraction of the binder is 2.2%. The D50 particle size of the lithium iron phosphate in the active layer is 1.6μm, and the powder compaction density is 2.5g / cm 3 , specific surface area 12m 2 The second positive electrode sheet, the negative electrode sheet, the electrolyte and the separator are assembled and packaged to obtain the second secondary battery.

[0064] Example 3:

[0065] After pretreatment, lithium niobate powder was placed in a reaction vessel. An inert gas was introduced into the reaction vessel, the temperature was raised to 500°C, and a mixed carbon source was added for a reaction of 1 hour to obtain a first coating material. The mass ratio of ethanol, ethylene, benzene, hexachlorobenzene, and ethylene in the mixed carbon source was 1:1.1:1.0:0.9:1.0. The carbon content of the mixed carbon source was 0.33:1 relative to the carbon content of the lithium iron phosphate in the active layer. The lithium niobate particles had a D50 particle size of 7 μm. The shell of the carbon- and graphene-coated particles consisted of two layers: a first shell adjacent to the core, a carbon layer, and a second shell, a graphene layer. The thickness of the first shell was 80 nm, and the thickness of the second shell was 100 nm. The first coating material was cooled. Once the temperature dropped below 200°C, it was transferred to a bag filter and a second coating material was obtained by chemical vapor deposition.

[0066] The second coating material is ultrasonically dispersed in water to obtain a first dispersion, and the graphene is ultrasonically dispersed in water to obtain a second dispersion. The first and second dispersions are mixed and uniformly dispersed by ultrasonication to obtain a mixed dispersion. The mixed dispersion is dried at 150°C and heat-treated at 200°C to obtain third carbon- and graphene-coated particles. Based on the total mass percentage of the third carbon- and graphene-coated particles as 100 mol%, the mass fraction of the first shell is 1.3%, the mass fraction of the second shell is 0.9%, and the mass fraction of the core is 97.8%.

[0067] Active layers are provided on both sides of the positive electrode current collector to obtain a third positive electrode sheet. The thickness of the first active layer is 41μm, and the thickness of the second active layer is 145μm. Specifically, the first active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, PVDF as the binder, and the above-mentioned third carbon and graphene-coated particles as lithium supplements; and in the first active layer, the mass fraction of the positive electrode active material is 96%, the mass fraction of the conductive agent is 1.1%, the mass fraction of the binder is 2%, and the mass fraction of the lithium supplement is 0.9%. The second active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, and PVDF as the binder; and in the second active layer, the mass fraction of the positive electrode active material is 96.6%, the mass fraction of the conductive agent is 1.2%, and the mass fraction of the binder is 2.2%. The D50 particle size of the lithium iron phosphate in the active layer is 1.2μm, and the powder compaction density is 2.4g / cm 3 , specific surface area 10m 2 The third positive electrode sheet, the negative electrode sheet, the electrolyte and the separator are assembled and packaged to obtain the third secondary battery.

[0068] Example 4:

[0069] The fourth positive electrode sheet is obtained by setting an active layer only on one side of the positive electrode current collector. The thickness of the first active layer is 49μm, and the thickness of the second active layer is 153μm. Specifically, the first active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, PVDF as the binder, and the above-mentioned third carbon and graphene-coated particles as lithium supplements; and in the first active layer, the mass fraction of the positive electrode active material is 96%, the mass fraction of the conductive agent is 1.1%, the mass fraction of the binder is 2%, and the mass fraction of the lithium supplement is 0.9%. The second active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, and PVDF as the binder; and in the second active layer, the mass fraction of the positive electrode active material is 96.6%, the mass fraction of the conductive agent is 0.9%, and the mass fraction of the binder is 2.2%. The D50 particle size of the lithium iron phosphate in the active layer is 1.2μm, and the powder compaction density is 2.4g / cm 3, specific surface area 10m 2 The fourth positive electrode sheet, the negative electrode sheet, the electrolyte and the separator are assembled and packaged to obtain the fourth secondary battery.

[0070] Embodiment 5:

[0071] Active layers are provided on both sides of the positive electrode current collector to obtain the fifth positive electrode sheet. The thickness of the first active layer is 47μm, and the thickness of the second active layer is 147μm. Specifically, the first active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, PVDF as the binder, and the above-mentioned third carbon and graphene-coated particles as lithium supplements; and in the first active layer, the mass fraction of the positive electrode active material is 96.1%, the mass fraction of the conductive agent is 1.1%, the mass fraction of the binder is 2%, and the mass fraction of the lithium supplement is 0.8%. The second active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, and PVDF as the binder; and in the second active layer, the mass fraction of the positive electrode active material is 96.6%, the mass fraction of the conductive agent is 1.2%, and the mass fraction of the binder is 2.2%. The D50 particle size of the lithium iron phosphate in the active layer is 1.2μm, and the powder compaction density is 2.4g / cm 3 , specific surface area 10m 2 The fifth positive electrode sheet, the negative electrode sheet, the electrolyte and the separator are assembled and packaged to obtain the fifth secondary battery.

[0072] Example 6:

[0073] Active layers are provided on both sides of the positive electrode current collector to obtain the sixth positive electrode sheet. The thickness of the first active layer is 45μm, and the thickness of the second active layer is 146μm. Specifically, the first active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, PVDF as the binder, and the above-mentioned third carbon and graphene-coated particles as lithium supplements; and in the first active layer, the mass fraction of the positive electrode active material is 95.9%, the mass fraction of the conductive agent is 1.1%, the mass fraction of the binder is 2%, and the mass fraction of the lithium supplement is 1.0%. The second active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, and PVDF as the binder; and in the second active layer, the mass fraction of the positive electrode active material is 96.6%, the mass fraction of the conductive agent is 1.2%, and the mass fraction of the binder is 2.2%. The D50 particle size of the lithium iron phosphate in the active layer is 1.2μm, and the powder compaction density is 2.4g / cm 3 , specific surface area 10m 2 The sixth positive electrode sheet, the negative electrode sheet, the electrolyte and the separator are assembled and packaged to obtain the sixth secondary battery.

[0074] Comparative Example 1:

[0075] After pretreatment, lithium niobate powder was placed in a reaction vessel. An inert gas was introduced into the reaction vessel, the temperature was raised to 850°C, and a mixed carbon source was added for 0.15 hours to produce a first coating material. The mass ratio of ethanol, ethylene, benzene, hexachlorobenzene, and ethylene in the mixed carbon source was 1:0.7:0.7:0.7. The carbon content of the mixed carbon source was compared to the carbon content of the lithium iron phosphate in the active layer at a ratio of 0.37:1. The D50 particle size of the lithium niobate powder was 9 μm. The shell of the carbon- and graphene-coated particles consisted of two layers: a first shell adjacent to the core, a carbon layer, and a second shell, a graphene layer. The thickness of the first shell was 130 nm, and the thickness of the second shell was 130 nm. The first coating material was cooled. Once the temperature dropped below 200°C, it was transferred to a bag filter and the second coating material was produced by chemical vapor deposition.

[0076] The second coating material is ultrasonically dispersed in water to obtain a first dispersion, and the graphene is ultrasonically dispersed in water to obtain a second dispersion. The first and second dispersions are mixed and uniformly dispersed by ultrasonication to obtain a mixed dispersion. The mixed dispersion is dried at 150°C and heat-treated at 200°C to obtain first comparative carbon and graphene-coated particles. Based on the total mass percentage of the first comparative carbon and graphene-coated particles as 100 mol%, the mass fraction of the first shell layer is 1.2%, the mass fraction of the second shell layer is 0.8%, and the mass fraction of the core is 98%.

[0077] Active layers are provided on both sides of the positive electrode current collector to obtain a first comparative positive electrode sheet. The thickness of the first active layer is 42μm, and the thickness of the second active layer is 140μm. Specifically, the first active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, PVDF as the binder, and the above-mentioned first comparative carbon and graphene-coated particles as lithium supplements; and in the first active layer, the mass fraction of the positive electrode active material is 96.3%, the mass fraction of the conductive agent is 1.1%, the mass fraction of the binder is 2%, and the mass fraction of the lithium supplement is 0.6%. The second active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, and PVDF as the binder; and in the second active layer, the mass fraction of the positive electrode active material is 96.6%, the mass fraction of the conductive agent is 1.2%, and the mass fraction of the binder is 2.2%. The D50 particle size of the lithium iron phosphate in the active layer is 1.7μm, and the powder compaction density is 2.0g / cm 3 , specific surface area 13m 2 The first comparative positive electrode sheet, negative electrode sheet, electrolyte and separator were assembled and packaged to obtain the first comparative secondary battery.

[0078] Comparative Example 2:

[0079] After pretreatment, lithium niobate powder was placed in a reaction vessel. An inert gas was introduced into the reaction vessel, the temperature was raised to 370°C, and a mixed carbon source was added for a reaction of 3.5 hours to produce a first coating material. The mass ratio of ethanol, ethylene, benzene, hexachlorobenzene, and ethylene in the mixed carbon source was 1:0.7:0.7:1.5:0.7. The carbon content of the mixed carbon source was compared to the carbon content of the lithium iron phosphate in the active layer in a ratio of 0.30:1. The lithium niobate powder had a D50 particle size of 6 μm. The shell of the carbon- and graphene-coated particles consisted of two layers: a first carbon layer adjacent to the core and a second graphene layer. The thickness of the first shell was 110 nm, and the thickness of the second shell was 60 nm. The first coating material was cooled. Once the temperature dropped below 200°C, it was transferred to a bag filter and the second coating material was produced by chemical vapor deposition.

[0080] The second coating material is ultrasonically dispersed in water to obtain a first dispersion, and the graphene is ultrasonically dispersed in water to obtain a second dispersion. The first and second dispersions are mixed and uniformly dispersed by ultrasonication to obtain a mixed dispersion. The mixed dispersion is dried at 150°C and heat-treated at 200°C to obtain second comparative carbon and graphene-coated particles. Based on the total mass percentage of the second comparative carbon and graphene-coated particles as 100 mol%, the mass fraction of the first shell is 1.4%, the mass fraction of the second shell is 1%, and the mass fraction of the core is 97.6%.

[0081] Active layers are provided on both sides of the positive electrode current collector to obtain a second comparative positive electrode sheet. The thickness of the first active layer is 44μm, and the thickness of the second active layer is 158μm. Specifically, the first active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, PVDF as the binder, and the above-mentioned second comparative carbon and graphene-coated particles as lithium supplements; and in the first active layer, the mass fraction of the positive electrode active material is 96.2%, the mass fraction of the conductive agent is 1.1%, the mass fraction of the binder is 2%, and the mass fraction of the lithium supplement is 0.7%. The second active layer of the positive electrode sheet uses lithium iron phosphate as the positive electrode active material, carbon black as the conductive agent, and PVDF as the binder; and in the second active layer, the mass fraction of the positive electrode active material is 96.6%, the mass fraction of the conductive agent is 1.2%, and the mass fraction of the binder is 2.2%. The D50 particle size of the lithium iron phosphate in the active layer is 1.7μm, and the powder compaction density is 2.0g / cm 3 , specific surface area 13m 2 The second comparative positive electrode sheet, negative electrode sheet, electrolyte and separator were assembled and packaged to obtain the second comparative secondary battery.

[0082] The first efficiency, internal resistance, and energy retention of the first, second, third, fourth, fifth, sixth, first, and second comparative secondary batteries were tested. Specifically, the test environment was adjusted to 25°C, and the batteries were charged at a constant power of 0.5P to a voltage of 3.65V, allowed to stand for 10 minutes, and then discharged at a constant power of 0.5P to 2.5V, allowed to stand for 10 minutes, for a total of 100 charge and discharge cycles.

[0083] The formula for calculating the initial efficiency is: Initial Efficiency = First Discharge Capacity / First Charge Capacity. Internal resistance refers to the internal resistance measured by an ohmmeter after the first full charge. Energy Retention = 100th Cycle Discharge Energy / 1st Cycle Discharge Energy × 100%. The experimental results are shown in Table 1.

[0084] The first, second, and third secondary batteries all have first and second active layers on both sides of the positive electrode sheet. Based on the third secondary battery, the fourth secondary battery is constructed by only having the first and second active layers on one side of the positive electrode sheet. Based on the third secondary battery, the fifth and sixth secondary batteries are constructed by varying the content of carbon and graphene-coated particles in the first active layer.

[0085] Table 1 Battery energy retention rate experimental results

[0086] First efficiency (%) Internal resistance (mΩ) Energy retention rate (25℃, 0.5P, 100 cycles, %) First and second batteries 94.67 0.21 99.2 Second secondary battery 94.76 0.23 98.6 Third secondary battery 94.84 0.20 99.3 Fourth secondary battery 91.55 0.38 96.1 Fifth secondary battery 94.32 0.22 98.7 Sixth secondary battery 94.21 0.24 98.3 First comparison of secondary batteries 92.38 0.31 96.9 Second comparative secondary battery 92.31 0.32 96.6

[0087] Experimental results show that the secondary battery provided by this application has high initial efficiency, low internal resistance, and high energy retention, resulting in a longer cycle life. Specifically, when the content of carbon and graphene-coated particles in the first active layer is 0.9%, the secondary battery has a longer cycle life; and compared to secondary batteries with active layers on both sides of the positive electrode current collector, the cycle life is also longer.

[0088] The present application provides a secondary battery and a method for preparing the same. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. The positive electrode comprises a current collector. An active layer is provided on one or both sides of the current collector. The active layer comprises a first active layer and a second active layer. The first active layer is provided between the second active layer and the current collector. The first active layer comprises carbon and graphene-coated particles. The carbon and graphene-coated particles are of a core-shell structure. The shell material is carbon and graphene. The core material is one of lithium ferrite, lithium nickelate, and lithium niobate. The shell of the carbon and graphene-coated particles consists of two layers. The first shell layer close to the core is a carbon layer, and the second shell layer is a graphene layer. By changing the structure of the positive electrode sheet, the cycle life is improved, thereby solving the problem of low cycle life of the secondary battery. The present application also provides a method for preparing the above-mentioned secondary battery.

[0089] Similar parts between the embodiments provided in this application can be referenced to each other. The specific implementation methods provided above are only a few examples under the overall concept of this application and do not constitute a limitation on the scope of protection of this application. For those skilled in the art, any other implementation methods expanded based on the scheme of this application without expending creative work shall fall within the scope of protection of this application.

Claims

1. A method for preparing a secondary battery, characterized in that: include: The core material powder is pre-treated and placed in a reaction vessel, an inert gas is introduced into the reaction vessel, the temperature is raised to 400° C. to 800° C., a mixed carbon source is added and reacted for 0.2 h to 3 h to obtain a first coating material, wherein the mixed carbon source includes ethanol, ethylene, benzene, hexachlorobenzene and ethylene; Cooling the first coating material until the temperature is below 200° C., transferring the first coating material into a bag filter, and obtaining a second coating material by chemical vapor deposition; Ultrasonic dispersion of the second coating material in water to obtain a first dispersion, ultrasonic dispersion of graphene in water to obtain a second dispersion, and mixing the first dispersion and the second dispersion to obtain a mixed dispersion; The mixed dispersion is dried at 150° C. and heat-treated at 200° C. to obtain carbon and graphene-coated particles; The carbon and graphene coated particles and lithium iron phosphate are sequentially coated on one side or both sides of a positive electrode current collector to obtain a positive electrode sheet, wherein the positive electrode sheet includes a current collector, and an active layer is arranged on one side or both sides of the current collector, wherein the active layer includes a first active layer and a second active layer, wherein the first active layer is arranged between the second active layer and the current collector, and the first active layer includes carbon and graphene coated particles, and the carbon and graphene coated particles are a core-shell structure consisting of a core and a shell surrounding the core, wherein the material of the core is one of lithium ferrite, lithium nickelate and lithium niobate, and the shell consists of a first shell layer close to the core and a second shell layer farther away from the core than the first shell layer, wherein the first shell layer is a carbon layer and the second shell layer is a graphene layer, and the D50 particle size of the core particles is 5 μm to 10 μm, the thickness of the first shell layer is 60 nm to 100 nm, and the thickness of the second shell layer is 70 nm to 120 nm; The positive electrode sheet, the negative electrode sheet, the electrolyte and the separator are assembled and packaged to obtain the secondary battery.

2. The method according to claim 1, characterized in that The ratio of the carbon content of the mixed carbon source to the carbon content of the lithium iron phosphate in the active layer is 0.32-0.35:1, the mixed carbon source includes ethanol, ethylene, benzene, hexachlorobenzene and ethylene, and the mass ratio of ethanol, ethylene, benzene, hexachlorobenzene and ethylene in the mixed carbon source is 1:0.8-1.2:0.8-1.2:0.8-1.2:0.8-1.

2.

3. A secondary battery prepared by the preparation method according to any one of claims 1 or 2, wherein the secondary battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, the positive electrode sheet comprises a current collector, an active layer is provided on one or both sides of the current collector, the active layer comprises a first active layer and a second active layer, the first active layer is provided between the second active layer and the current collector, the first active layer comprises carbon and graphene coated particles, the carbon and graphene coated particles are a core-shell structure consisting of a core and a shell surrounding the core, the shell material is carbon and Graphene, the material of the core is one of lithium ferrite, lithium nickelate and lithium niobate, the shell is composed of a first shell layer close to the core and a second shell layer farther away from the core than the first shell layer, the first shell layer is a carbon layer, the second shell layer is a graphene layer, the material forming the carbon layer includes a mixed carbon source, the mixed carbon source includes ethanol, ethylene, benzene, hexachlorobenzene and ethylene, the D50 particle size of the core particles is 5μm to 10μm, the thickness of the first shell layer is 60nm to 100nm, and the thickness of the second shell layer is 70nm to 120nm.

4. The secondary battery according to claim 3, wherein The mass fraction of carbon and graphene-coated particles in the first active layer is 0.8% to 1.0%.

5. The secondary battery according to claim 3, wherein The active layer includes lithium iron phosphate, the D50 particle size of the lithium iron phosphate is 0.6 μm to 1.6 μm, and the powder compaction density is ≥2.3 g / cm 3 , specific surface area 8m 2 / g~12m 2 / g.

6. The secondary battery according to claim 3, wherein The thickness of the first active layer is 40 μm to 50 μm, and the thickness of the second active layer is 140 μm to 160 μm.

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