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

Abstract

This invention relates to the field of lithium-ion battery cathode materials, specifically to a silicon-carbon coated lithium iron phosphate cathode material, its preparation method, cathode sheet, lithium-ion battery, and electrical device. The silicon-carbon coated lithium iron phosphate cathode material includes lithium iron phosphate and a coating layer covering the surface of the lithium iron phosphate. The coating layer includes a lithium-intercalated silicon-carbon material and an amorphous carbon material. The mass percentage (W1) of silicon-carbon material in the lithium-intercalated silicon-carbon material to the mass of lithium iron phosphate is 0.5%–3.5%. The mass percentage (W2) of silicon element in the lithium-intercalated silicon-carbon material to the mass of silicon-carbon material is 10%–80%. W1 and W2 satisfy 5 ≤ 350W1 + 3150W1 × W2 ≤ 65. The silicon-carbon coated lithium iron phosphate cathode material provided by this invention exhibits high capacity, high energy density, high initial efficiency, and excellent cycle performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, and more specifically, to a silicon-carbon coated lithium iron phosphate cathode material, its preparation method, cathode sheet, lithium-ion battery, and electrical equipment. Background Technology

[0002] Lithium-ion batteries are currently the most widely used high-performance energy storage devices capable of cyclic charging and discharging. A lithium-ion battery is a general term for batteries that use lithium-ion intercalation compounds as the positive electrode material. The internal structure of a lithium-ion battery consists of four parts: the positive electrode, the negative electrode, the electrolyte, and the separator. Essentially, a lithium-ion battery is a concentration cell; the positive and negative electrodes are lithium storage materials that allow the reversible insertion and extraction of lithium ions. The battery's operating voltage is determined by the lithium intercalation compound after lithium intercalation at the electrodes. The charging and discharging of a lithium-ion battery involves the insertion and extraction of lithium ions between the positive and negative electrode materials. During the insertion and extraction of lithium ions, an equivalent number of electrons are simultaneously inserted and extracted (conventionally, the positive electrode uses insertion or extraction, while the negative electrode uses insertion or extraction). During charging and discharging, lithium ions shuttle back and forth between the positive and negative electrodes, a process figuratively called a "rocking chair battery."

[0003] The performance of lithium-ion batteries is largely determined by the performance of the electrode materials (battery active materials). Excellent positive and negative electrode materials typically possess advantages such as long cycle life, high reversible specific capacity, low synthesis cost, and safe, pollution-free, and reliable performance. Since the first appearance of lithium-ion batteries, 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 all possess high redox electromotive forces, such as Li(Ni) 0.8 Co 0.15 Al 0.05 )O2 and Li(Ni 1 / 3 Co 1 / 3 Mn 1 / 3 Cathode materials containing lithium and low cobalt, such as LiCoO2, significantly reduce the manufacturing cost of batteries by lowering the proportion of LiCoO2; LiFePO4 cathode materials have an operating voltage of 3.25V; vanadium phosphate compounds include LiVPO4F and Li3V2P3O 12 It has a high charge / discharge platform and high energy density.

[0004] Currently, lithium iron phosphate (LiFePO4, or LFP) is the most widely used lithium battery cathode material, and it is widely used in energy storage and commercial vehicles. However, due to the characteristics of the material itself, the voltage platform of lithium iron phosphate is only about 3.3V. The low platform and low energy density limit its application environment.

[0005] And its next-generation material, lithium manganese iron phosphate / lithium manganese phosphate, is relative to Li + The electrode potential of Li increases with increasing manganese content, resulting in a high theoretical capacity density. Furthermore, the low cost of raw materials for synthesizing lithium manganese phosphate gives it significant market potential in the power supply field. However, during discharge, excessive Li... + Mn can embed into LiMn₂O₄, and may even form Li₂Mn₂O₄, which occurs when discharging to 3V. This process is accompanied by a transformation from a cubic to a tetragonal crystal phase, leading to an increase in cell volume. During discharge cycles, the electrode material collapses, causing a sharp decline in discharge capacity and severe Mn dissolution. Furthermore, both lithium iron manganese phosphate and lithium manganese phosphate suffer from excessively low conductivity, high discharge capacity reduction (DCR), and difficulty in fully utilizing their capacity.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The primary objective of this invention is to provide a silicon-carbon coated lithium iron phosphate cathode material that utilizes silicon-carbon to store active lithium, enabling the material to have its own lithium replenishment effect in the battery, compensating for lithium loss during SEI film formation on the negative electrode, increasing the total amount of reversible active lithium in the system, enhancing the partial capacity utilization of the lithium iron phosphate material, and improving the energy density of the cathode material.

[0008] The second objective of this invention is to provide a method for preparing a silicon-carbon coated lithium iron phosphate cathode material. By using silicon-carbon coating and pre-lithiation, the prepared silicon-carbon coated lithium iron phosphate cathode material can have a higher lithium intercalation / deintercalation content compared with conventional lithium iron phosphate.

[0009] The third objective of this invention is to provide a positive electrode sheet that has advantages such as high capacity, high energy density, high initial efficiency, and excellent cycle performance.

[0010] The fourth objective of this invention is to provide a lithium-ion battery with high capacity, high energy density, high initial efficiency, and long cycle life.

[0011] The fifth objective of this invention is to provide an electrical appliance.

[0012] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0013] The present invention first provides a silicon-carbon coated lithium iron phosphate cathode material, comprising lithium iron phosphate and a coating layer coated on the surface of the lithium iron phosphate, wherein the coating layer comprises lithium-intercalated silicon-carbon material and amorphous carbon material;

[0014] Wherein, the mass percentage W1 of the silicon-carbon material in the lithium-intercalated silicon-carbon material is 0.5% to 3.5% of the mass of the lithium iron phosphate;

[0015] The percentage of silicon in the lithium-intercalated silicon-carbon material, W2, is 10% to 80% of the total mass of the silicon-carbon material.

[0016] The W1 and W2 satisfy the following relationship: 5≤350W1+3150W1×W2≤65.

[0017] Preferably, the particle size D of the silicon-carbon coated lithium iron phosphate cathode material is... 50 The value is 0.2–2 μm.

[0018] This invention further provides a method for preparing the silicon-carbon coated lithium iron phosphate cathode material, comprising the following steps:

[0019] Lithium iron phosphate, lithium source, carbon source and silicon carbide material are mixed and sintered.

[0020] Preferably, the carbon source includes at least one of citric acid, glucose, sucrose, and polyethylene glycol.

[0021] Preferably, the molar ratio of lithium iron phosphate to the lithium source is 1:0.02 to 0.3.

[0022] Preferably, the mass of the carbon source is 1% to 10% of the mass of the lithium iron phosphate.

[0023] Preferably, the mass of the silicon-carbon material is 0.5% to 3.5% of the mass of the lithium iron phosphate.

[0024] Preferably, the sintering is carried out in an oxygen-free atmosphere.

[0025] Preferably, the sintering temperature is 700–800°C, and the holding time is 2–12 hours.

[0026] Preferably, a dispersant is added during the mixing process, and the mixture is dried after mixing before sintering.

[0027] The present invention further provides a positive electrode sheet, comprising the silicon-carbon coated lithium iron phosphate positive electrode material.

[0028] The present invention also provides a lithium-ion battery, including the positive electrode sheet.

[0029] The present invention also provides an electrical device including the lithium-ion battery.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] (1) The silicon-carbon coated lithium iron phosphate cathode material provided by the present invention utilizes silicon-carbon to store active lithium, which enables the material to have its own lithium replenishment effect in the battery, compensate for the lithium loss of the SEI film of the negative electrode, increase the total amount of reversible active lithium in the system, enhance the partial capacity utilization of the lithium iron phosphate material, and improve the energy density of the cathode material.

[0032] (2) The silicon-carbon coated lithium iron phosphate cathode material provided by the present invention can store lithium in the negative electrode after the first charge and release it continuously during the cycle, making up for the reversible lithium loss of the battery system and significantly improving the cycle life of the battery.

[0033] (3) The silicon-carbon coated lithium iron phosphate cathode material provided by the present invention has small particle size and uniform distribution, stable structure, and uniform surface coating with fully pre-lithiated silicon-carbon material. It 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, increase the total reversible lithium in the battery system, solve the problem of low first efficiency of lithium iron phosphate full battery, and increase the energy density of the full battery by more than 8%. Attached Figure Description

[0034] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0035] Figure 1 This is a schematic flowchart illustrating the preparation method of silicon-carbon coated lithium iron phosphate cathode material according to Example 1 of the present invention. Detailed Implementation

[0036] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0037] Unless otherwise specified, in this invention, terms such as "first aspect," "second aspect," "third aspect," and "fourth aspect" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, terms such as "first," "second," "third," and "fourth" serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on quantity.

[0038] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0039] Unless otherwise specified, in this invention, "one or more" or "at least one" refers to any one, any two, or any two or more of the listed items. "Several" refers to any two or more.

[0040] In a first aspect, the present invention provides a silicon-carbon coated lithium iron phosphate cathode material, including lithium iron phosphate and a coating layer coated on the surface of the lithium iron phosphate.

[0041] The coating layer includes lithium-intercalated silicon-carbon material (referring to a composite material in which lithium is intercalated into silicon-carbon material) and amorphous carbon material.

[0042] In some specific implementations, amorphous carbon can be produced by decomposing the added organic carbon source, which serves to create a reducing atmosphere for the reaction and to uniformly coat the surface of the lithium iron phosphate material with lithium-intercalated silicon carbon material.

[0043] Wherein, the percentage W1 of the mass of silicon-carbon material in the lithium-intercalated silicon-carbon material to the mass of 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 any range between two.

[0044] The percentage W2 of the mass of silicon element in the lithium-intercalated silicon-carbon material (or the mass of silicon element in the silicon-carbon material) to the mass of silicon-carbon material in the lithium-intercalated 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 any range between two.

[0045] The W1 and W2 satisfy the following relationship: 5 ≤ 350W1 + 3150W1 × W2 ≤ 65. The value of 350W1 + 3150W1 × W2 includes, but is not limited to, any one of the following values: 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, or a range between any two.

[0046] The silicon-carbon coated lithium iron phosphate cathode material provided by this invention utilizes silicon-carbon to store active lithium, enabling the material to have its own lithium replenishment effect in the battery, compensating for the lithium loss during the formation of the SEI film on the negative electrode, increasing the total amount of reversible active lithium in the system, enhancing the partial capacity utilization of the lithium iron phosphate material, improving the energy density of the cathode material, and greatly compensating for the low energy density of LFP.

[0047] Meanwhile, the stored lithium can also be stored in the negative electrode after the first charge and continuously released during cycling to compensate for the reversible lithium loss of the battery system and significantly improve the cycle life of the battery.

[0048] In addition, the surface is uniformly coated with fully pre-lithiated silicon-carbon material (i.e., lithium-intercalated silicon-carbon material), which can release sufficient active lithium ions during the initial charging process of the full cell to compensate for the loss of film formation on the negative electrode, increase the total reversible lithium in the battery system, and solve the problem of low first efficiency (i.e., first coulombic efficiency) of lithium iron phosphate full cells.

[0049] This invention controls the pre-lithiation amount of the material to achieve the required lithium replenishment effect by adjusting the pre-lithiation amount of silicon-carbon coating and silicon content. When 350W1 + 3150W1 × W2 < 5, the lithium replenishment effect is weak and has no practical application value. Within the range of 5-65, the lithium replenishment effect gradually improves with the increase of the value. When it is in the range of 5-25, the energy density of the battery cell made of the material gradually increases with the increase of the value. When it is in the range of 25-65, the lithium replenishment amount exceeds the reversible value of LFP material. The excess lithium will be stored in the negative electrode and continuously released during the cycle to make up for the lithium consumed by the side reaction and greatly improve the cycle performance of the material (the higher the value, the greater the increase in cycle performance). When the formula value > 65, the lithium replenishment amount is too large and there is no practical application requirement.

[0050] In some specific embodiments, the pre-lithium silicon-carbon coated lithium iron phosphate cathode material provided by the present invention improves the full-cell energy density by more than 8% compared with conventional LFP materials.

[0051] In some specific embodiments, the particle size D of the silicon-carbon coated lithium iron phosphate cathode material is... 50 The value is 0.2 to 2 μm, including but not limited to point values ​​or ranges between any one of 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 1.3 μm, 1.5 μm, 1.8 μm, and 2 μm.

[0052] The silicon-carbon coated lithium iron phosphate cathode material provided by this invention has a small particle size, excellent rate performance, and uniform distribution.

[0053] Secondly, the present invention provides a method for preparing the above-mentioned silicon-carbon coated lithium iron phosphate cathode material, comprising the following steps: mixing lithium iron phosphate, lithium source, carbon source and silicon-carbon material and sintering them.

[0054] This invention employs silicon-carbon coating and pre-lithiation, which enables the resulting silicon-carbon coated lithium iron phosphate cathode material to have a higher lithium intercalation / deintercalation content compared to conventional lithium iron phosphate.

[0055] Specifically, by utilizing silicon-carbon to store active lithium during the material preparation process, the material itself can replenish lithium within the battery, compensating for lithium loss during SEI film formation at the negative electrode, increasing the total amount of reversible active lithium in the system, and enhancing the capacity utilization of lithium iron phosphate materials. This results in a full-cell energy density that is more than 8% higher than that of conventional LFP materials. Furthermore, the stored lithium can be stored at the negative electrode after the first charge and continuously released during cycling to compensate for reversible lithium loss in the battery system, significantly improving the battery's cycle life.

[0056] Understandably, during the sintering process, the lithium source decomposes into anions, leaving only lithium to intercalate into silicon-carbon, forming a lithium-intercalated silicon-carbon material. The organic carbon source also decomposes during sintering, with only a small portion forming amorphous carbon. Therefore, in the final silicon-carbon coated lithium iron phosphate cathode material, the mass percentage of the lithium-intercalated silicon-carbon material accounts for 0.5% to 3.5% of the total mass of the silicon-carbon coated lithium iron phosphate cathode material, and the mass percentage of silicon in the lithium-intercalated silicon-carbon material accounts for 10% to 80% of the total mass of the lithium-intercalated silicon-carbon material.

[0057] In some specific embodiments, the carbon source includes at least one of citric acid, glucose, sucrose, and polyethylene glycol.

[0058] In some specific embodiments, the molar ratio of lithium iron phosphate to the lithium source is 1:0.02 to 0.3; including but not limited to any one 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 any range between the two.

[0059] The above molar ratio results in good pre-lithiation.

[0060] 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 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any range between two.

[0061] 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 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5% or a range between any two.

[0062] In some specific embodiments, the sintering is carried out in an oxygen-free atmosphere, i.e., an atmosphere that does not contain oxygen.

[0063] In some specific embodiments, the sintering temperature is 700-800°C, including but not limited to any one of 700°C, 720°C, 750°C, 780°C, and 800°C, or a range between any two.

[0064] In some specific embodiments, the sintering holding time is 2 to 12 hours; including but not limited to any one of 2 hours, 3 hours, 5 hours, 8 hours, 10 hours, and 12 hours, or any range between two of them.

[0065] In some specific embodiments, a dispersant is added during the mixing process, followed by drying and then sintering.

[0066] In some specific embodiments, the dispersant includes one of deionized water, methanol solution, ethanol solution, acetone solution, and polyol solution.

[0067] In some specific embodiments, the mass of the dispersant is 5 to 10 times the mass of lithium iron phosphate.

[0068] In some specific embodiments, the lithium source includes a soluble lithium source, such as at least one of lithium hydroxide, lithium carbonate, lithium chloride, lithium nitrate, lithium dihydrogen phosphate, and lithium acetate, but is not limited thereto.

[0069] In some specific implementations, grinding is used to mix the raw materials evenly.

[0070] In some specific embodiments, the gas used in the oxygen-free atmosphere includes one or more of nitrogen, argon, and carbon dioxide.

[0071] In some specific embodiments, the sintering process includes a pulverization step, preferably using air jet pulverization.

[0072] In some specific embodiments, the preparation method of 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, wherein the grinding is carried out until the particle size D of the mixture is... 50The particle size is 0.2–1.5 μm (including but not limited to point values ​​or ranges between any two of 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 1.3 μm, and 1.5 μm), then silicon carbide material is added, mixed evenly, spray-dried, sintered in an oxygen-free atmosphere, cooled, and pulverized to a particle size D. 50 The silicon-carbon coated lithium iron phosphate cathode material was obtained with a thickness of 0.2–2 μm.

[0073] In some specific embodiments, lithium iron phosphate is prepared using a hydrothermal method. The preparation method includes: weighing soluble lithium source, iron source, and phosphorus source according to a molar ratio of lithium:iron:phosphorus = 1.01–1.05 (including but not limited to any one of 1.01, 1.02, 1.03, 1.04, 1.05 or any range between two): 0.99–1:1, then pouring them into a solvent and stirring on an automatic stirrer for 0.5–1 hour to obtain a precursor solution; transferring the precursor solution into a hydrothermal reactor under an oxygen-free gas atmosphere, and heating at 100–200°C (including but not limited to 100°C, 130°C, 150°C, 180°C). The lithium iron phosphate is obtained by holding the material at a temperature of 100-200℃ (including but not limited to the values ​​of 100℃, 130℃, 150℃, 180℃, 200℃, or any value between 2 and 2) for 2-12 hours, cooling it to room temperature, filtering to obtain a solid material, washing it with anhydrous ethanol, and then vacuum drying it at a temperature of 100-200℃ (including but not limited to the values ​​of 100℃, 130℃, 150℃, 180℃, 200℃, or any value between 2 and 2) for 2-10 hours (including but not limited to the values ​​of 2 hours, 3 hours, 5 hours, 8 hours, 10 hours, or any value between 2 and 2) to obtain the lithium iron phosphate.

[0074] The lithium source includes soluble lithium sources, such as one or more of lithium hydroxide, lithium carbonate, lithium chloride, lithium nitrate, lithium dihydrogen phosphate, and lithium acetate, but is not limited thereto.

[0075] The iron source includes, but is not limited to, one or more of soluble iron sources, such as ferrous sulfate, ferrous nitrate, and ferrous chloride.

[0076] The phosphorus source includes, but is not limited to, soluble phosphorus sources such as phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and lithium dihydrogen phosphate.

[0077] Thirdly, the present invention provides a positive electrode sheet comprising the above-mentioned silicon-carbon coated lithium iron phosphate positive electrode material.

[0078] The positive electrode provided by this invention has advantages such as high capacity, high energy density, high initial efficiency, and excellent cycle performance.

[0079] Optionally, the positive electrode may further include a binder and / or a conductive agent, which is not limited in this invention.

[0080] Fourthly, the present invention provides a lithium-ion battery comprising the above-mentioned positive electrode sheet.

[0081] The lithium-ion battery provided by this invention has high capacity, high energy density, high initial efficiency, and long cycle life.

[0082] Optionally, the lithium-ion battery may also include a negative electrode, a separator, and an electrolyte, but the present invention does not limit these components.

[0083] Fifthly, the present invention provides an electrical device including the aforementioned lithium-ion battery.

[0084] The electrical equipment mentioned above includes any device or apparatus that uses the lithium-ion battery, such as electric vehicles, electric motorcycles, electric bicycles, power tools, starting power supplies, energy storage devices, electronic products, and office equipment. This invention does not limit the scope of such devices.

[0085] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0086] Example 1

[0087] The method for preparing silicon-carbon coated lithium iron phosphate cathode material provided in this embodiment includes the following steps:

[0088] (1) Hydrothermal synthesis of lithium iron phosphate: Weigh 2.45g of lithium hydroxide (lithium source), 15.00g of ferrous sulfate (iron source), and 9.80g of solid phosphoric acid (phosphorus source) and pour them into 150ml of deionized water. Stir on an automatic stirrer for 1h to mix evenly and completely dissolve into a solution to obtain a precursor solution. Transfer the precursor solution into a hydrothermal reactor in an oxygen-free gas atmosphere (nitrogen) and keep it at 200℃ for 6h. After cooling to room temperature, filter to obtain solid material, wash with anhydrous ethanol, and vacuum dry at 150℃ for 4h to obtain lithium iron phosphate.

[0089] (2) Pre-lithium silicon-carbon coated lithium iron phosphate: Weigh 10.00g of lithium iron phosphate and 0.08g of lithium hydroxide obtained in step (1) (i.e., the molar ratio of lithium iron phosphate to the lithium source is 1:0.052), pour them into deionized water, and then add 0.8g of glucose (i.e., the mass of the carbon source is 8% of the mass of lithium iron phosphate). Grind the mixture using a sand mill to control the particle size D. 50 To achieve a particle size of 0.5 μm, add 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, the mass fraction of silicon in the silicon-carbon material is 50%, and the silicon-carbon material is a customized version of Lanxi Zhide SO310 silicon-carbon material). After mixing evenly, spray dry, and then sinter at 750℃ for 6 hours in an oxygen-free atmosphere (nitrogen). After cooling, the coated material is subjected to air jet milling to a particle size D. 50 The thickness was 0.7 μm, resulting in a silicon-carbon coated lithium iron phosphate cathode material. This silicon-carbon coated lithium iron phosphate cathode material comprises lithium iron phosphate and a coating layer on the surface of the lithium iron phosphate. The coating layer includes lithium-intercalated silicon-carbon material and amorphous carbon material.

[0090] Figure 1 This is a schematic flowchart illustrating the preparation method of the silicon-carbon coated lithium iron phosphate cathode material in this embodiment.

[0091] Example 2

[0092] The method for preparing silicon-carbon coated lithium iron phosphate cathode material provided in this embodiment includes the following steps:

[0093] (1) Hydrothermal synthesis of lithium iron phosphate: Weigh 3.87g of lithium carbonate, 18.00g of ferrous nitrate and 14.9g of ammonium phosphate solid and pour them into 200ml of 50% ethanol solution. Stir on an automatic stirrer for 1h to mix evenly and completely dissolve into a solution to obtain a precursor solution. Transfer the precursor solution into a hydrothermal reactor in an oxygen-free gas atmosphere (nitrogen) and keep it at 170℃ for 6h. After cooling to room temperature, filter to obtain solid material, wash with anhydrous ethanol and vacuum dry at 110℃ for 7h to obtain lithium iron phosphate.

[0094] (2) Pre-lithium silicon-carbon coated lithium iron phosphate: Weigh 10.00g of lithium iron phosphate and 0.18g of lithium hydroxide obtained in step (1) (i.e., the molar ratio of lithium iron phosphate to the lithium source is 1:0.12), pour them into deionized water, and then add 0.6g of glucose (i.e., the mass of the carbon source is 6% of the mass of lithium iron phosphate). Grind the mixture using a sand mill to control the particle size D. 50To achieve a particle size of 0.3 μm, 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, the mass fraction of silicon in the silicon-carbon material is 30%, and the silicon-carbon material is a customized version of Lanxi Zhide SO310 silicon-carbon material) is added. After uniform mixing, the mixture is spray-dried and then sintered at 730℃ for 7 hours in an oxygen-free atmosphere (nitrogen). After cooling, the coated material is subjected to air jet milling to a particle size D. 50 With a thickness of 0.5 μm, silicon-carbon coated lithium iron phosphate cathode material was obtained.

[0095] Example 3

[0096] The method for preparing silicon-carbon coated lithium iron phosphate cathode material provided in this embodiment includes the following steps:

[0097] (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 solid were weighed and poured into 170 ml of 40% methanol solution. The mixture was stirred on an automatic stirrer for 1.2 h to mix evenly and completely dissolve into a solution to obtain a precursor solution. The precursor solution was transferred into a hydrothermal reactor in an oxygen-free gas atmosphere (argon) and kept at 150 °C for 9 h. After cooling to room temperature, the solid material was obtained by filtration, washed with anhydrous ethanol and vacuum dried at 140 °C for 5.5 h to obtain lithium iron phosphate.

[0098] (2) Pre-lithium silicon-carbon coated lithium iron phosphate: Weigh 10.00g of lithium iron phosphate and 0.47g of lithium carbonate obtained in step (1) (i.e., the molar ratio of lithium iron phosphate to the lithium source is 1:0.1), pour them into a 60% ethanol solution, and then add 0.5g of glucose (i.e., the mass of the carbon source is 5% of the mass of lithium iron phosphate). Grind the mixture using a sand mill to control the particle size D. 50 To achieve a particle size of 0.4 μm, 0.15 g of silicon-carbon material (i.e., the mass of the silicon-carbon material is 1.5% of the mass of the lithium iron phosphate, the mass fraction of silicon in the silicon-carbon material is 60%, and the silicon-carbon material is a customized version of Lanxi Zhide SO310 silicon-carbon material) is added. After uniform mixing, the mixture is spray-dried and then sintered at 760℃ for 10 hours in an oxygen-free atmosphere (argon). After cooling, the coated material is subjected to air jet milling to a particle size D. 50 The thickness was 0.8 μm, resulting in a silicon-carbon coated lithium iron phosphate cathode material.

[0099] Example 4

[0100] The preparation method of silicon-carbon coated lithium iron phosphate cathode material provided in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the mass of lithium hydroxide is replaced with 0.40g, that is, the molar ratio of lithium iron phosphate and the lithium source is 1:0.26; and the amount of silicon-carbon material added is replaced with 0.25g (that is, the mass of silicon-carbon material is 2.5% of the mass of lithium iron phosphate).

[0101] Example 5

[0102] The preparation method of silicon-carbon coated lithium iron phosphate cathode material provided in this embodiment is basically the same as that in Example 1. The difference is that in step (2), glucose is replaced with an equal mass of citric acid.

[0103] Comparative Example 1

[0104] The cathode material provided in this comparative example is lithium iron phosphate obtained in step (1) of Example 1.

[0105] Comparative Example 2

[0106] The preparation method of the cathode material provided in this comparative example is basically the same as that in Example 1, except that lithium hydroxide is not added in step (2).

[0107] Comparative Example 3

[0108] The preparation method of the cathode material provided in this comparative example is basically the same as that in Example 1, except that no silicon-carbon material is added in step (2).

[0109] Comparative Example 4

[0110] The preparation method of the cathode material provided in this comparative example is basically the same as that in Example 1, except that in step (2), the mass of lithium hydroxide is replaced with 0.80g, that is, the molar ratio of lithium iron phosphate and the lithium source is 1:0.52; and the amount of silicon carbon material added is replaced with 0.50g (that is, the mass of silicon carbon material is 5.0% of the mass of lithium iron phosphate). The values ​​of W1, W2 and 350W1+3150W1×W2 in the cathode materials prepared in each example and each comparative example are shown in Table 1.

[0111] Table 1 shows the values ​​of W1, W2, and 350W1 + 3150W1 × W2.

[0112] Group <![CDATA[W1]]> <![CDATA[W2]]> <![CDATA[350W1+3150W1×W2]]> Example 1 0.5％ 50％ 9.625 Example 2 2.0％ 30％ 25.9 Example 3 1.5％ 60％ 33.6 Example 4 2.5％ 50％ 48.12 Example 5 0.5％ 50％ 9.625 Comparative Example 1 0％ 0％ 0 Comparative Example 2 0％ 50％ 0 Comparative Example 3 0％ 0％ 0 Comparative Example 4 5％ 50％ 96.25

[0113] Experimental Example

[0114] The positive electrode materials obtained from each embodiment and comparative example were assembled into a full battery in a glove box. The negative electrode was a conventional graphite negative electrode. The positive electrode formulation and areal density were kept the same as the negative electrode formulation. The areal density of the negative electrode coating was adjusted by a charging NP ratio of 1.04. Then, the electrochemical performance was tested using the Xinwei testing system. The electrochemical performance test results are shown in Table 2.

[0115] The operating voltage range for electrochemical testing is 2.0-3.8V.

[0116] Table 2. Results of Full Battery Electrical Performance Tests

[0117]

[0118]

[0119] As can be seen from Table 2, the silicon-carbon coated lithium iron phosphate cathode materials prepared in each embodiment have excellent 0.33C specific capacity performance, which is far superior to LFP (related to the amount of pre-lithiated silicon-carbon coating and silicon content), and the performance is consistent at high rates, indicating that the process has a superior capacity improvement effect.

[0120] As can be seen from Example 3, when the amount of lithium replenishment from pre-lithiated silicon-carbon coating is higher than the amount of lithium consumed in the formation of the negative electrode film, the total amount of reversible lithium in the battery system is higher than the reversible amount of lithium iron phosphate. The remaining reversible lithium will be stored in the negative electrode and continuously released during the cycle to make up for the reversible lithium loss of the battery system, so that the capacity of the cell does not decay during long-term cycling and significantly improves the cycle life of the battery.

[0121] By comparing Example 1 and Comparative Example 2, it can be seen that adding a pre-lithiated lithium source can embed lithium into silicon-carbon materials through sintering, thus achieving a lithium replenishment effect. If no lithium source is added, silicon-carbon materials alone have no effect.

[0122] By comparing Example 1 and Comparative Example 3, it can be seen that the coated silicon-carbon material can serve as a lithium carrier, enabling the pre-lithiated silicon-carbon to achieve a lithium replenishment effect. Without the addition of silicon-carbon material, the lithium source alone cannot be sintered into the material and has no effect.

[0123] By comparing Examples 2, 3, 4 and Comparative Example 4, it can be seen that when the relationship 350W1 + 3150W1 × W2 is greater than 25, the specific capacity decreases instead of increasing, the energy density does not increase, and the excess lithium is reserved in the negative electrode for cycle improvement. When 350W1 + 3150W1 × W2 is greater than 65, the specific capacity decreases significantly, and the amount of lithium reserved in the negative electrode is too high, which has no practical application significance.

[0124] In summary, the silicon-carbon coated lithium iron phosphate cathode material provided by this invention can effectively improve capacity and cycle stability by coating lithium iron phosphate with a specific coating layer and controlling the amount of silicon-carbon material and silicon.

[0125] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A silicon-carbon coated lithium iron phosphate cathode material, characterized in that, It includes lithium iron phosphate and a coating layer covering the surface of the lithium iron phosphate, the coating layer including lithium-intercalated silicon-carbon material and amorphous carbon material; Wherein, the mass percentage W1 of the silicon-carbon material in the lithium-intercalated silicon-carbon material is 0.5% to 3.5% of the mass of the lithium iron phosphate; The percentage of silicon in the lithium-intercalated silicon-carbon material, W2, is 10% to 80% of the total mass of the silicon-carbon material. The W1 and W2 satisfy the following relationship: 5≤350W1+3150W1×W2≤65.

2. The silicon-carbon coated lithium iron phosphate cathode material according to claim 1, characterized in that, The particle size D of the silicon-carbon coated lithium iron phosphate cathode material 50 The value is 0.2–2 μm.

3. The method for preparing silicon-carbon coated lithium iron phosphate cathode material as described in claim 1 or 2, characterized in that, Includes the following steps: Lithium iron phosphate, lithium source, carbon source and silicon carbide material are mixed and sintered.

4. The method for preparing silicon-carbon coated lithium iron phosphate cathode material according to claim 3, characterized in that, The carbon source includes 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 must be met: (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 for preparing silicon-carbon coated lithium iron phosphate cathode material according to claim 3, characterized in that, At least one of the following conditions must be met: (1) The sintering is carried out in an oxygen-free atmosphere; (2) The sintering temperature is 700-800℃ and the holding time is 2-12h.

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

8. A positive electrode plate, characterized in that, Includes the silicon-carbon coated lithium iron phosphate cathode material as described in claim 1 or 2.

9. A lithium-ion battery, characterized in that, Includes the positive electrode as described in claim 8.

10. An electrical appliance, characterized in that, Including the lithium-ion battery as described in claim 9.

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