Silicon-carbon-coated lithium iron phosphate positive electrode material and preparation method therefor, positive electrode sheet, lithium-ion battery, and electric device

Abstract

The present disclosure relates to the technical field of lithium-ion battery positive electrode materials, and specifically to a silicon-carbon-coated lithium iron phosphate positive electrode material and a preparation method therefor, a positive electrode sheet, a lithium-ion battery, and an electric device. The silicon-carbon-coated lithium iron phosphate positive electrode material comprises lithium iron phosphate and a coating layer coated on the surface of the lithium iron phosphate; the coating layer comprises a lithium-intercalated silicon-carbon material and an amorphous carbon material; the percentage W1 of the mass of a silicon-carbon material in the lithium-intercalated silicon-carbon material to the mass of the lithium iron phosphate is 0.5%-3.5%; the percentage W2 of the mass of a silicon element in the lithium-intercalated silicon-carbon material to the mass of the silicon-carbon material in the lithium-intercalated silicon-carbon material is 10%-80%; and W1 and W2 satisfy: 5≤350W1+3150W1×W2≤65. The silicon-carbon-coated lithium iron phosphate positive electrode material provided by the present disclosure has high capacity, high energy density, high initial Coulombic efficiency, and excellent cycle performance.

Description

Silicon-carbon-coated lithium iron phosphate positive electrode material, preparation method thereof, positive electrode sheet, lithium ion battery and electric device

[0001] Cross-reference to Related Applications

[0002] This application claims priority to the Chinese patent application No. 202410884851.0, filed on July 03, 2024, and entitled "Silicon-carbon-coated lithium iron phosphate positive electrode material, preparation method thereof, positive electrode sheet, lithium ion battery and electric device", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The present disclosure relates to the technical field of lithium ion battery positive electrode materials, in particular, to a silicon-carbon-coated lithium iron phosphate positive electrode material, a preparation method thereof, a positive electrode sheet, a lithium ion battery and an electric device. BACKGROUND

[0004] Lithium ion battery is the most widely used high-performance energy storage device that can be cycled for charging and discharging. Lithium ion battery refers to a battery with lithium ion intercalation compounds as positive electrode material. The internal structure of lithium ion battery is composed of four parts: positive electrode, negative electrode, electrolyte and separator. The essence of lithium ion battery is concentration cell. The positive and negative electrodes are lithium storage materials that allow lithium ions to be reversibly inserted and extracted. The working voltage of the battery is determined by the intercalated lithium compounds after lithium insertion. The charging and discharging of lithium ion battery is the insertion and extraction of lithium ions between positive and negative materials. During the insertion and extraction of lithium ions, the equivalent of electrons is also inserted and extracted (habitually, the positive electrode is represented by insertion or extraction, while the negative electrode is represented by insertion or extraction). During the charging and discharging process, lithium ions are inserted / extracted and inserted / extracted between the positive and negative electrodes, which is called "rocking chair battery" in a figurative sense.

[0005] The performance of lithium ion battery is largely determined by the performance of electrode materials (battery active materials). Excellent positive and negative electrode materials usually have long cycle life, high reversible specific capacity, low synthesis cost, and safe and pollution-free, reliable safety performance, etc. Since the first appearance of lithium ion battery, after decades of development and research, many lithium intercalation compounds that can be used as positive electrode materials for secondary lithium batteries have been discovered and have high redox potential, such as Li(Ni 0.8 Co 0.15 Al 0.05 )O2 and Li(Ni 1 / 3 Co 1 / 3 Mn 1 / 3O2, etc. This kind of material reduces the proportion of LiCoO2, greatly reduces the manufacturing cost of the battery; the LiFePO4 positive electrode material has a working voltage of 3.25V; the vanadium phosphate compound includes LiVPO4F and Li3V2P3O 12 , has a higher charge and discharge platform, high energy density, etc.

[0006] At present, lithium iron phosphate (LiFePO4, referred to as LFP) as the most widely used lithium battery positive electrode material is widely used in energy storage and commercial vehicle fields, but due to the characteristics of the material itself, the voltage platform of lithium iron phosphate material is only about 3.3V, the platform is low, the energy density is not high, which limits its use environment.

[0007] And its next-generation material, lithium manganese iron phosphate / lithium manganese phosphate, has a relatively high Li + / Li electrode potential with the increase of manganese content, high theoretical capacity density, low raw material cost of synthesizing lithium manganese phosphate, which has great market prospect in the field of power supply. However, too much Li + will be embedded in LiMn2O4, even Li2Mn2O4 will be formed, which will occur when discharging to 3V, at this time, the material will change from cubic crystal phase to tetragonal crystal phase, resulting in the increase of unit cell volume, the collapse of electrode material in the discharge cycle, thus the discharge capacity will be sharply attenuated, and Mn will be dissolved seriously. Moreover, lithium manganese iron phosphate / lithium manganese phosphate both have the problems of too low electrical conductivity, large DCR and difficult to fully exert the capacity.

[0008] Therefore, the present disclosure is proposed.

[0009] Application content

[0010] The first object of the present disclosure is to provide a silicon-carbon-coated lithium iron phosphate positive electrode material, which can make the material itself have a lithium supplement effect in the battery, make up for the lithium loss of the negative electrode SEI film, improve the total amount of reversible active lithium in the system, improve the capacity of the iron lithium material, and improve the energy density of the positive electrode material.

[0011] The second object of the present disclosure is to provide a preparation method of a silicon-carbon-coated lithium iron phosphate positive electrode material, which adopts silicon-carbon-coating and pre-lithiation, so that the prepared silicon-carbon-coated lithium iron phosphate positive electrode material has a higher de-embeddable lithium content compared with conventional lithium iron phosphate.

[0012] The third object of the present disclosure is to provide a positive electrode sheet, which has the advantages of high capacity, high energy density, high initial efficiency, excellent cycle performance, etc.

[0013] The fourth object of the present disclosure is to provide a lithium ion battery, which has high capacity, high energy density, high initial efficiency and long cycle life.

[0014] A fifth object of the present disclosure is to provide a power-using device.

[0015] In order to achieve the above-mentioned objects of the present disclosure, the following technical solutions are adopted:

[0016] The present disclosure first provides a silicon-carbon-coated lithium iron phosphate positive electrode material, comprising lithium iron phosphate and a coating layer coated on the surface of the lithium iron phosphate, wherein the coating layer comprises lithium-embedded silicon-carbon material and amorphous carbon material.

[0017] The mass percentage W1 of the silicon-carbon material in the lithium-embedded silicon-carbon material in the lithium iron phosphate is 0.5% to 3.5%.

[0018] The mass percentage W2 of the silicon element in the lithium-embedded silicon-carbon material in the lithium iron phosphate is 10% to 80%.

[0019] The W1 and the W2 satisfy the following relationship: 5≤350W1+3150W1×W2≤65.

[0020] Further, the particle size D of the silicon-carbon-coated lithium iron phosphate positive electrode material is 0.2 to 2 microns. 50

[0021] The present disclosure further provides a preparation method of the silicon-carbon-coated lithium iron phosphate positive electrode material, comprising the following steps:

[0022] Mixing and sintering lithium iron phosphate, lithium source, carbon source and silicon-carbon material.

[0023] Further, the carbon source comprises at least one of citric acid, glucose, sucrose and polyethylene glycol.

[0024] Further, the molar ratio of the lithium iron phosphate and the lithium source is 1:0.02 to 0.3.

[0025] Further, the mass of the carbon source is 1% to 10% of the mass of the lithium iron phosphate.

[0026] Further, the mass of the silicon-carbon material is 0.5% to 3.5% of the mass of the lithium iron phosphate.

[0027] Further, the sintering is carried out in an oxygen-free atmosphere.

[0028] Further, the sintering temperature is 700 to 800℃, and the holding time is 2 to 12 hours.

[0029] Further, a dispersing agent is added during the mixing, and the mixture is dried before the sintering. ​

[0030] The present disclosure further provides a positive electrode sheet comprising the silicon-carbon-coated lithium iron phosphate positive electrode material.

[0031] The present disclosure further provides a lithium ion battery comprising the positive electrode sheet.

[0032] The present disclosure further provides an electric device comprising the lithium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the specific embodiments of the present disclosure or the technical solutions in the prior art, the drawings needed to be used in the specific embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are some embodiments of the present disclosure, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0034] FIG. 1 is a flowchart of the preparation method of the silicon-carbon-coated lithium iron phosphate positive electrode material of Example 1 provided by the present disclosure. DETAILED DESCRIPTION

[0035] The technical solutions of the present disclosure will be described clearly and completely in combination with the drawings and specific embodiments below. However, those skilled in the art will understand that the following described embodiments are part of the embodiments of the present disclosure, not all the embodiments, and are only used to illustrate the present disclosure, and should not be regarded as limiting the scope of the present disclosure. Based on the embodiments in the present disclosure, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present disclosure. The specific conditions not mentioned in the embodiments are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not marked with the manufacturer, which are all conventional products that can be purchased on the market.

[0036] If not specifically stated, in the present disclosure, "first aspect", "second aspect", "third aspect", "fourth aspect" and the like are only for description purposes, and cannot be understood as indicating or implying relative importance or quantity, nor can it be understood as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first", "second", "third", "fourth" and the like only serve the purpose of non-exhaustive enumeration and description, and should be understood as not constituting a closed limitation on the quantity.

[0037] If not specifically stated, "including" and "comprising" mentioned in the present disclosure mean open-ended, and can also be closed. For example, "including" and "comprising" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0038] Unless otherwise specified, in the present disclosure, "one or more" or "at least one" refers to any one, any two or any two or more of the listed items. Among them, "several" refers to any two or more.

[0039] In a first aspect, the present disclosure provides a silicon-carbon-coated lithium iron phosphate positive electrode material, which comprises lithium iron phosphate and a coating layer coated on the surface of the lithium iron phosphate.

[0040] The coating layer comprises lithium-embedded silicon-carbon material (a composite material in which lithium is embedded in silicon-carbon material) and amorphous carbon material.

[0041] In some specific embodiments, the amorphous carbon can be produced by decomposition of an added organic carbon source, which creates a reducing atmosphere for the reaction and uniformly coats the lithium iron phosphate material surface with the lithium-embedded silicon-carbon material.

[0042] The mass percentage W1 of the silicon-carbon material in the lithium-embedded silicon-carbon material in the lithium iron phosphate is 0.5% to 3.5%; including but not limited to any one of 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5% or a range value between any two of them.

[0043] The mass percentage W2 of the silicon element in the lithium-embedded silicon-carbon material (or the mass percentage of the silicon element in the silicon-carbon material) in the lithium-embedded silicon-carbon material is 10% to 80%; including but not limited to any one of 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80% or a range value between any two of them.

[0044] The W1 and the W2 satisfy the following relationship: 5≤350W1+3150W1×W2≤65. Wherein, the value of 350W1+3150W1×W2 includes but is not limited to any one of 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65 or a range value between any two of them.

[0045] The silicon-carbon-coated lithium iron phosphate positive electrode material provided by the present disclosure utilizes silicon-carbon to store active lithium, which can make the material itself have a lithium supplement effect in the battery, compensate for the lithium loss of the negative electrode SEI film, increase the total amount of reversible active lithium in the system, improve the capacity of the lithium iron material part, and improve the energy density of the positive electrode material, greatly compensating for the low energy density of LFP.

[0046] At the same time, the stored lithium can also be stored in the negative electrode after the first charge and continuously released during the cycle process, compensating for the reversible lithium loss of the battery system and significantly improving the cycle life of the battery.

[0047] In addition, the surface is uniformly coated with the fully pre-lithiated silicon-carbon material (i.e., lithium-embedded silicon-carbon material), which can release sufficient active lithium ions to compensate for the loss of negative electrode film formation during the initial charging process of the full battery, thereby increasing the total reversible lithium content of the battery system and solving the problem of low initial efficiency (i.e., first coulombic efficiency) of the lithium iron phosphate full battery.

[0048] The present disclosure can control the pre-lithium amount of the material to achieve the desired lithium supplement effect by adjusting the pre-lithium silicon-carbon coating amount and the silicon content. In the case of 350W1+3150W1xW2<5, the lithium supplement effect is weak and has no practical application value. In the range of 5-65, the lithium supplement effect gradually increases with the increase of the value. When in the range of 5-25, the value increases, and the energy density of the battery cell made of the material gradually increases. When in the range of 25-65, the value increases, and the lithium supplement amount exceeds the reversible value of the LFP material. The excess lithium is stored in the negative electrode and is released during the cycle process to compensate for the lithium consumed by the side reaction, thereby greatly improving the cycle performance of the material (the higher the value, the greater the cycle performance increase). When the formula value is >65, the lithium supplement amount is too large, and there is no practical application requirement.

[0049] In some specific embodiments, the pre-lithium silicon-carbon coated lithium iron phosphate positive electrode material provided by the present disclosure can increase the energy density of the full battery by more than 8% compared with the conventional LFP material.

[0050] In some specific embodiments, the particle size D 50 of the silicon-carbon coated lithium iron phosphate positive electrode material is 0.2-2 μm, including but not limited to any one of 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 1.3 μm, 1.5 μm, 1.8 μm, 2 μm, or a range value between any two of them.

[0051] The silicon-carbon coated lithium iron phosphate positive electrode material provided by the present disclosure has a small particle size, excellent rate performance, and uniform distribution.

[0052] In a second aspect, the present disclosure provides a preparation method of the above-mentioned silicon-carbon coated lithium iron phosphate positive electrode material, which comprises the following steps: mixing and sintering lithium iron phosphate, a lithium source, a carbon source, and a silicon-carbon material.

[0053] The present disclosure uses silicon-carbon coating and pre-lithium, which can make the prepared silicon-carbon coated lithium iron phosphate positive electrode material have a higher de-embeddable lithium content compared with the conventional lithium iron phosphate.

[0054] Specifically, the use of silicon-carbon reserves active lithium in the material preparation process can make the material itself have a lithium supplement effect in the battery, make up for the lithium loss of negative electrode SEI film formation, improve the total amount of reversible active lithium in the system, and improve the capacity of iron-lithium material, so that the energy density of the full battery of the material is increased by more than 8% compared with the conventional LFP material. Moreover, the reserved lithium can also be stored in the negative electrode after the first charge and released continuously during the cycle process, making up for the reversible lithium loss of the battery system and significantly improving the cycle life of the battery.

[0055] It can be understood that the lithium source is anionically decomposed in the sintering process, and only lithium is embedded into the silicon-carbon to form a lithium-embedded silicon-carbon material. The organic carbon source is also decomposed in the sintering process, and only a small part forms amorphous carbon. Therefore, in the final obtained silicon-carbon coated lithium iron phosphate positive material, the mass percentage of the lithium-embedded silicon-carbon material in the mass of the silicon-carbon coated lithium iron phosphate positive material satisfies 0.5% to 3.5%, and the mass percentage of silicon in the lithium-embedded silicon-carbon material in the mass of the lithium-embedded silicon-carbon material satisfies 10% to 80%.

[0056] In some specific embodiments, the carbon source includes at least one of citric acid, glucose, sucrose, and polyethylene glycol.

[0057] In some specific embodiments, the molar ratio of the lithium iron phosphate and the lithium source is 1:0.02 to 0.3; including but not limited to any one of the point values of 1:0.02, 1:0.05, 1:0.08, 1:0.1, 1:0.15, 1:0.2, 1:0.25, 1:0.3 or a range value between any two of them.

[0058] The pre-lithiation effect is good with the above-mentioned molar ratio.

[0059] In some specific embodiments, the mass of the carbon source is 1% to 10% of the mass of the lithium iron phosphate; including but not limited to any one of the point values of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range value between any two of them.

[0060] In some specific embodiments, the mass of the silicon-carbon material is 0.5% to 3.5% of the mass of the lithium iron phosphate, including but not limited to any one of the point values of 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5% or a range value between any two of them.

[0061] In some specific embodiments, the sintering is carried out in an oxygen-free atmosphere, i.e., an atmosphere without oxygen.

[0062] In some embodiments, the sintering temperature is 700-800°C, including but not limited to any one of 700°C, 720°C, 750°C, 780°C, 800°C, or a range between any two of them.

[0063] In some embodiments, the sintering time is 2-12h, including but not limited to any one of 2h, 3h, 5h, 8h, 10h, 12h, or a range between any two of them.

[0064] In some embodiments, a dispersant is added during the mixing, and the mixture is dried before sintering.

[0065] In some embodiments, the dispersant includes one of deionized water, methanol solution, ethanol solution, acetone solution, and polyol solution.

[0066] In some embodiments, the dispersant is 5-10 times the mass of the lithium iron phosphate.

[0067] In some embodiments, the lithium source includes at least one of soluble lithium sources, such as lithium hydroxide, lithium carbonate, lithium chloride, lithium nitrate, lithium dihydrogen phosphate, and lithium acetate, but is not limited thereto.

[0068] In some embodiments, the raw materials are mixed uniformly by grinding.

[0069] In some embodiments, the gas used in the oxygen-free atmosphere includes one or more of nitrogen, argon, and carbon dioxide.

[0070] In some embodiments, the sintering is followed by a step of crushing, such as air flow crushing.

[0071] In some embodiments, the method for preparing the silicon-carbon-coated lithium iron phosphate cathode material specifically includes: mixing and grinding lithium iron phosphate, a lithium source, a carbon source, and a dispersant, the grinding being to a particle size D 50 of 0.2-1.5μm (including but not limited to any one of 0.2μm, 0.5μm, 0.8μm, 1μm, 1.3μm, 1.5μm, or a range between any two of them), then adding silicon-carbon material thereto, uniformly mixing, spray drying, and then sintering under an oxygen-free atmosphere, and crushing to a particle size D 50 of 0.2-2μm after cooling, to obtain the silicon-carbon-coated lithium iron phosphate cathode material.

[0072] In some specific embodiments, the lithium iron phosphate is prepared by a hydrothermal method, the preparation method of the lithium iron phosphate comprising: weighing a soluble lithium source, an iron source and a phosphorus source according to a molar ratio of lithium element: iron element: phosphorus element = 1.01-1.05 (including but not limited to any one of 1.01, 1.02, 1.03, 1.04, 1.05 or a range value between any two of them): 0.99-1:1, pouring into a solvent, stirring on an automatic blender for 0.5-1 h to mix uniformly to obtain a precursor solution; transferring the precursor solution into a hydrothermal reaction kettle in an oxygen-free gas atmosphere, incubating at a temperature of 100-200 ℃ (including but not limited to any one of 100 ℃, 130 ℃, 150 ℃, 180 ℃, 200 ℃ or a range value between any two of them) for 2-12 h (including but not limited to any one of 2 h, 3 h, 5 h, 8 h, 10 h, 12 h or a range value between any two of them), cooling to room temperature, filtering to obtain a solid material, washing with anhydrous ethanol, and vacuum drying at a temperature of 100-200 ℃ (including but not limited to any one of 100 ℃, 130 ℃, 150 ℃, 180 ℃, 200 ℃ or a range value between any two of them) for 2-10 h (including but not limited to any one of 2 h, 3 h, 5 h, 8 h, 10 h or a range value between any two of them) to obtain the lithium iron phosphate.

[0073] The lithium source includes a soluble lithium source, for example, one or more of lithium hydroxide, lithium carbonate, lithium chloride, lithium nitrate, lithium dihydrogen phosphate, lithium acetate, but is not limited thereto.

[0074] The iron source includes a soluble iron source, for example, one or more of ferrous sulfate, ferrous nitrate, ferrous chloride, but is not limited thereto.

[0075] The phosphorus source includes a soluble phosphorus source, for example, one or more of phosphoric acid, monammonium phosphate, diammonium phosphate, ammonium phosphate, lithium dihydrogen phosphate, but is not limited thereto.

[0076] In a third aspect, the present disclosure provides a positive electrode sheet, comprising the silicon-carbon-coated lithium iron phosphate positive electrode material described above.

[0077] The positive electrode sheet provided by the present disclosure has the advantages of high capacity, high energy density, high initial efficiency and excellent cycle performance.

[0078] Further, the positive electrode sheet further comprises a binder and / or a conductive agent, which is not limited by the present disclosure.

[0079] In a fourth aspect, the present disclosure provides a lithium ion battery comprising the positive electrode sheet described above.

[0080] The lithium ion battery provided by the present disclosure has high capacity, high energy density, high initial efficiency and long cycle life.

[0081] Further, the lithium ion battery further comprises a negative electrode sheet, a separator and an electrolyte, which are not limited by the present disclosure.

[0082] In a fifth aspect, the present disclosure provides a power consuming device comprising the lithium ion battery.

[0083] The power consuming device includes any device or apparatus using the lithium ion battery, such as an electric vehicle, an electric motorcycle, an electric bicycle, an electric tool, a starting power supply, an energy storage device, an electronic product, an office equipment, and the like, which are not limited by the present disclosure.

[0084] The embodiments of the present disclosure will be described in detail below with reference to the examples, but those skilled in the art will understand that the following examples are only for illustration of the present disclosure and should not be regarded as limiting the scope of the present disclosure. The specific conditions are not specified in the examples, and the conventional conditions or the conditions recommended by the manufacturer are used. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be obtained on the market.

[0085] Example 1

[0086] The preparation method of the silicon-carbon coated lithium iron phosphate positive electrode material provided in the present embodiment includes the following steps:

[0087] (1) Synthesis of lithium iron phosphate by hydrothermal method: 2.45 g of lithium hydroxide (lithium source), 15.00 g of ferrous sulfate (iron source), and 9.80 g of phosphoric acid solid (phosphorus source) were weighed and sequentially added into 150 ml of deionized water. The mixture was stirred for 1 h on an automatic stirrer to obtain a solution, and then the precursor solution was obtained. The precursor solution was transferred into a hydrothermal reactor in an oxygen-free gas atmosphere (nitrogen) and incubated at 200°C for 6 h. After cooling to room temperature, the solid material was obtained by filtration, washed with anhydrous ethanol, and then vacuum dried at 150°C for 4 h to obtain lithium iron phosphate.

[0088] (2) Pre-lithium silicon-carbon coated lithium iron phosphate: 10.00 g of lithium iron phosphate obtained in step (1) and 0.08 g of lithium hydroxide (i.e. the molar ratio of lithium iron phosphate to lithium source is 1:0.052) were weighed and sequentially added into deionized water. Then, 0.8 g of glucose (i.e. the mass of the carbon source is 8% of the mass of the lithium iron phosphate) was added, and the mixture was ground using a sand mill to control the particle size D 50 to 0.5 μm. Then, 0.05 g of silicon-carbon material (i.e. the mass of the silicon-carbon material is 0.5% of the mass of the lithium iron phosphate, and the mass fraction of silicon in the silicon-carbon material is 50%, and the silicon-carbon material is Lanxi Zhide S0310 silicon-carbon ratio customized) was added, and the mixture was uniformly mixed and spray dried. Subsequently, the coated material was sintered at 750°C for 6 h under an oxygen-free atmosphere (nitrogen), and then cooled. Finally, the coated material was subjected to airflow crushing to obtain a powder with a particle size D50 0.7 pm, to obtain the silicon-carbon-coated lithium iron phosphate positive electrode material. The silicon-carbon-coated lithium iron phosphate positive electrode material comprises lithium iron phosphate and a coating layer coated on the surface of the lithium iron phosphate, and the coating layer comprises lithium-embedded silicon-carbon material and amorphous carbon material.

[0089] FIG. 1 is a flowchart of the preparation method of the silicon-carbon-coated lithium iron phosphate positive electrode material according to the embodiment.

[0090] Embodiment 2

[0091] The preparation method of the silicon-carbon-coated lithium iron phosphate positive electrode material provided in the embodiment comprises the following steps:

[0092] (1) Synthesis of lithium iron phosphate by a hydrothermal method: 3.87 g of lithium carbonate, 18.00 g of ferrous nitrate, and 14.9 g of ammonium phosphate solid were sequentially added into 200 ml of 50% ethanol solution, and stirred for 1 h on an automatic stirrer to mix uniformly and completely dissolve into a solution, to obtain a precursor solution; the precursor solution was transferred into a hydrothermal reaction kettle in an oxygen-free gas atmosphere (nitrogen), and incubated at 170°C for 6 h; after cooling to room temperature, the solid material was obtained by filtration, washed with anhydrous ethanol, and then vacuum dried at 110°C for 7 h, to obtain lithium iron phosphate.

[0093] (2) Pre-lithium silicon-carbon-coated lithium iron phosphate: 10.00 g of the lithium iron phosphate obtained in step (1) and 0.18 g of lithium hydroxide (i.e., the molar ratio of lithium iron phosphate to lithium source is 1:0.12) were sequentially added into deionized water, and 0.6 g of glucose (i.e., the mass of the carbon source is 6% of the mass of the lithium iron phosphate) was further added; the mixture was ground by a sand mill to control the particle size D 50 0.3 pm, and then 0.20 g of silicon-carbon material (i.e., the mass of the silicon-carbon material is 2% of the mass of the lithium iron phosphate, and the mass fraction of silicon in the silicon-carbon material is 30%, and the silicon-carbon material is Lanxi Zhide S0310 silicon-carbon proportion customized) was further added; the mixture was uniformly mixed and then spray dried, and then sintered at a high temperature of 730°C for 7 h under an oxygen-free atmosphere (nitrogen); after cooling, the coated material was subjected to airflow crushing to a particle size D 50 0.5 pm, to obtain the silicon-carbon-coated lithium iron phosphate positive electrode material.

[0094] Embodiment 3

[0095] The preparation method of the silicon-carbon-coated lithium iron phosphate positive electrode material provided in the embodiment comprises the following steps:

[0096] (1) Hydrothermal synthesis of lithium iron phosphate: 10.39 g of lithium dihydrogen phosphate, 25.20 g of ferrous chloride, and 11.5 g of ammonium dihydrogen phosphate were weighed and sequentially added to 170 ml of 40% methanol solution. The mixture was stirred in an automatic stirrer for 1.2 h to obtain a solution, and then the precursor solution was transferred to a hydrothermal reactor in an oxygen-free atmosphere (argon) and heated at 150°C for 9 h. After cooling to room temperature, the solid material was obtained by filtration, washed with anhydrous ethanol, and then vacuum dried at 140°C for 5.5 h to obtain lithium iron phosphate.

[0097] (2) Pre-lithiated silicon-carbon coated lithium iron phosphate: 10.00 g of lithium iron phosphate obtained in step (1) and 0.47 g of lithium carbonate (i.e., the molar ratio of lithium iron phosphate to lithium source is 1:0.1) were sequentially added to 60% ethanol solution, and then 0.5 g of glucose (i.e., the mass of carbon source is 5% of the mass of lithium iron phosphate) was added. The mixture was ground using a sand mill to control the particle size D 50 to 0.4 μm, and then 0.15 g of silicon-carbon material (i.e., the mass of silicon-carbon material is 1.5% of the mass of lithium iron phosphate, and the mass fraction of silicon in the silicon-carbon material is 60%, and the silicon-carbon material is Lanxi Zhide S0310 silicon-carbon custom-made) was added. The mixture was uniformly mixed and then spray dried, and then sintered at 760°C for 10 h in an oxygen-free atmosphere (argon). After cooling, the coated material was subjected to airflow crushing to obtain a particle size D 50 of 0.8 μm, thereby obtaining a silicon-carbon coated lithium iron phosphate positive electrode material.

[0098] Example 4

[0099] The preparation method of the silicon-carbon coated lithium iron phosphate positive electrode material provided in this example is basically the same as that of Example 1, except that in step (2), the mass of lithium hydroxide is replaced by 0.40 g (i.e., the molar ratio of lithium iron phosphate to lithium source is 1:0.26), and the amount of silicon-carbon material added is replaced by 0.25 g (i.e., the mass of silicon-carbon material is 2.5% of the mass of lithium iron phosphate).

[0100] Example 5

[0101] The preparation method of the silicon-carbon coated lithium iron phosphate positive electrode material provided in this example is basically the same as that of Example 1, except that in step (2), the glucose is replaced by an equal mass of citric acid.

[0102] Comparative Example 1

[0103] The positive electrode material provided in this comparative example is the lithium iron phosphate prepared in step (1) of Example 1.

[0104] Comparative Example 2

[0105] The preparation method of the positive electrode material provided by the present comparative example is basically the same as that of Example 1, except that in step (2), lithium hydroxide is not added.

[0106] Comparative Example 3

[0107] The preparation method of the positive electrode material provided by the present comparative example is basically the same as that of Example 1, except that in step (2), lithium hydroxide is not added.

[0108] Comparative Example 4

[0109] The preparation method of the positive electrode material provided by the present comparative example is basically the same as that of Example 1, except that in step (2), the mass of lithium hydroxide is replaced by 0.80 g, i.e., the molar ratio of lithium iron phosphate to the lithium source is 1:0.52; and the amount of silicon-carbon material added is replaced by 0.50 g (i.e., the mass of silicon-carbon material is 5.0% of the mass of lithium iron phosphate). The values of W1, W2 and 350W1+3150W1xW2 in the positive electrode materials prepared in each example and each comparative example are shown in Table 1.

[0110] Table 1 Values of W1, W2 and 350W1+3150W1xW2

[0111] Experimental Example

[0112] The positive electrode materials obtained in each example and each comparative example are assembled into full cells in a glove box, a conventional graphite negative electrode is used as the negative electrode, the positive electrode formulation and the surface density of the negative electrode are kept the same, the surface density of the negative electrode is adjusted by charging NP ratio 1.04, and then the electrochemical performance is tested by a new Wei test system. The electrical performance test results are shown in Table 2.

[0113] Among them, the working voltage range of the electrochemical test is 2.0-3.8V.

[0114] Table 2 Full cell electrical performance test results

[0115] As can be seen from Table 2, the silicon-carbon coated lithium iron phosphate positive electrode material prepared in each example has excellent 0.33C specific capacity performance, far exceeding LFP (related to the pre-lithium silicon-carbon coating amount and silicon content), and has consistent performance at high rate, indicating that the process has excellent capacity improvement effect.

[0116] As can be seen from Example 3, when the pre-lithium silicon-carbon coating lithium supplement amount is higher than the negative electrode film formation lithium consumption amount, the total amount of reversible lithium in the battery system is higher than the reversible amount of lithium iron phosphate, and the remaining reversible lithium will be stored in the negative electrode during the cycle process, continuously releasing to make up for the loss of reversible lithium in the battery system, so that the capacity does not decay during the long cycle process of the battery, and the cycle life of the battery is significantly improved.

[0117] By comparing Example 1 and Comparative Example 2, it can be seen that the addition of the pre-lithiated lithium source can embed lithium into the silicon-carbon material through sintering, thereby achieving the effect of lithium supplementation. If no lithium source is added, the silicon-carbon material alone has no effect.

[0118] By comparing Example 1 and Comparative Example 3, it can be seen that the coated silicon-carbon material can become a carrier of lithium, so that the pre-lithiated silicon-carbon material achieves the effect of lithium supplementation. If no silicon-carbon material is added, the lithium source alone cannot be sintered into the material, and has no effect.

[0119] By comparing Example 2, 3, 4 and Comparative Example 4, it can be seen that when the relationship 350W1+3150W1×W2 is greater than 25, the gram capacity does not increase but decreases, the energy density does not increase, and the excess lithium is reserved in the negative electrode, which improves the cycle. When 350W1+3150W1×W2 is greater than 65, the gram capacity decreases greatly, and the negative electrode reserves too much lithium, which has no practical application significance.

[0120] In summary, the silicon-carbon coated lithium iron phosphate positive electrode material provided by the present disclosure can effectively improve the capacity and cycle stability by coating a specific coating layer on the lithium iron phosphate and adjusting the amount of silicon-carbon material and silicon.

[0121] Compared with the prior art, the present disclosure has the following advantages:

[0122] (1) The silicon-carbon coated lithium iron phosphate positive electrode material provided by the present disclosure uses silicon-carbon to reserve active lithium, which can make the material itself have the effect of lithium supplementation in the battery, compensate for the loss of lithium in the formation of SEI in the negative electrode, increase the total amount of reversible active lithium in the system, improve the capacity of the lithium iron material, and improve the energy density of the positive electrode material.

[0123] (2) The silicon-carbon coated lithium iron phosphate positive electrode material provided by the present disclosure reserves lithium, which can be stored in the negative electrode after the first charge and continuously released during the cycle process, thereby compensating for the loss of reversible lithium in the battery system and significantly improving the cycle life of the battery.

[0124] (3) The silicon-carbon coated lithium iron phosphate positive electrode material provided by the present disclosure has small particle size and uniform distribution, stable structure, and uniform surface coating of completely pre-lithiated silicon-carbon material. It can release sufficient active lithium ions in the initial process of the first charge of the material full battery to compensate for the loss of negative electrode film formation, improve the total amount of reversible lithium in the battery system, solve the problem of low first efficiency of lithium iron phosphate full battery, and improve the energy density of the full battery by more than 8%.

[0125] Although the present disclosure has been illustrated and described with reference to specific embodiments, it is noted that these are only for illustration and description, and are not intended to limit the technical solutions of the present disclosure; it should be understood by those skilled in the art that the technical solutions recorded in the foregoing embodiments can be modified, or some or all of the technical features can be replaced equivalently, without departing from the spirit and scope of the present disclosure; 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 disclosure; therefore, this means that all these replacements and modifications within the scope of the present disclosure are included in the appended claims. Industrial applicability

[0126] The silicon-carbon-coated lithium iron phosphate positive electrode material provided by the present disclosure can effectively improve the capacity and cycle stability by coating a specific coating layer on the lithium iron phosphate and adjusting the amount of silicon-carbon material and silicon.

Claims

1. A silicon-carbon-coated lithium iron phosphate cathode material, characterized in that, The silicon-carbon coated lithium iron phosphate cathode material comprises lithium iron phosphate and a coating layer coated on the surface of the lithium iron phosphate, wherein the coating layer comprises lithium intercalation silicon-carbon material and amorphous carbon material. The mass percentage of the silicon-carbon material in the lithium intercalation silicon-carbon material in the lithium iron phosphate is W1, and W1 is 0.5% to 3.5%. The mass percentage of the silicon element in the lithium intercalation silicon-carbon material in the lithium intercalation silicon-carbon material is W2, and W2 is 10% to 80%. The W1 and W2 satisfy the following relationship: 5≤350W1+3150W1×W2≤65.

2. The silicon-carbon-coated lithium iron phosphate positive electrode material of claim 1, wherein the silicon-carbon-coated lithium iron phosphate positive electrode material has a tap density of 1.5 to 2.5 g / cm3. The particle size D of the silicon-carbon-coated lithium iron phosphate positive electrode material is 0.2-2 μm. 50 is 0.2-2 μm.

3. The preparation method of the silicon-carbon-coated lithium iron phosphate positive electrode material according to claim 1 or 2, characterized in that, The method comprises the following steps: Mixing and sintering lithium iron phosphate, a lithium source, a carbon source, and a silicon-carbon material.

4. The method for preparing silicon-carbon coated lithium iron phosphate cathode material according to claim 3, characterized in that, The carbon source comprises at least one of citric acid, glucose, sucrose, and polyethylene glycol.

5. The method for preparing silicon-carbon coated lithium iron phosphate cathode material according to claim 3, characterized in that, At least one of the following conditions is satisfied: (1) The molar ratio of the lithium iron phosphate to the lithium source is 1:0.02 to 0.3; (2) The mass of the carbon source is 1% to 10% of the mass of the lithium iron phosphate; (3) The mass of the silicon-carbon material is 0.5% to 3.5% of the mass of the lithium iron phosphate.

6. The method of claim 3, wherein the silicon-carbon-coated lithium iron phosphate positive electrode material is prepared by the steps of: mixing lithium iron phosphate, silicon, and carbon to form a mixture; and heating the mixture at a temperature of 600-800°C for 2-10 hours in a non-oxidizing atmosphere. At least one of the following conditions is satisfied: (1) The sintering is performed in an oxygen-free atmosphere; (2) The sintering temperature is 700 to 800°C, and the holding time is 2 to 12 hours.

7. The method for preparing silicon-carbon coated lithium iron phosphate cathode material according to claim 3, characterized in that, A dispersant is added during the mixing, and the mixture is dried after the mixing and then sintered.

8. A positive electrode sheet characterized by comprising: The silicon-carbon coated lithium iron phosphate cathode material according to claim 1 or 2.

9. A lithium-ion battery, characterized by The positive electrode sheet according to claim 8.

10. An electric device, characterized by The lithium ion battery according to claim 9.

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