Preparation method of positive electrode material, positive electrode material and battery

Abstract

The application relates to a preparation method of a positive electrode material, the positive electrode material and a battery. The preparation method comprises the following steps: providing raw materials, mixing the raw materials in a solvent, drying to obtain a precursor, wherein the raw materials at least comprise a lithium source, an iron source, a phosphorus source and a first carbon source; placing the precursor in a second carbon source atmosphere to perform first-stage sintering to obtain first intermediate particles with a first coating layer; using an etching solution to etch the first coating layer of the first intermediate particles to obtain second intermediate particles, the specific surface area of the second intermediate particles being greater than that of the first intermediate particles; and placing the second intermediate particles in a third carbon source atmosphere to perform second-stage sintering to form a second coating layer on the surface of the first coating layer, thereby obtaining the positive electrode material. When the positive electrode material prepared by the preparation method of the positive electrode material is used in a battery, the battery has excellent cycle performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a method for preparing a cathode material, the cathode material, and a battery. Background Technology

[0002] Lithium-ion batteries, characterized by high energy density, excellent cycle performance, wide operating range, and low self-discharge, have become the dominant power source for portable electronic products such as mobile phones and laptops. The cathode material, a crucial raw material in lithium-ion batteries, directly determines the battery's performance indicators based on its cycle life. With the continuous development of the energy storage industry, increasing the number of battery cycles and extending battery life has become an important development direction. Summary of the Invention

[0003] In view of this, this application provides a method for preparing a cathode material, a cathode material, and a battery. When the cathode material prepared by the method is used in a battery, the battery exhibits excellent cycle performance.

[0004] This application provides a method for preparing a cathode material, the method comprising: providing raw materials; mixing the raw materials in a solvent and drying them to obtain a precursor, wherein the raw materials include at least a lithium source, an iron source, a phosphorus source and a first carbon source; placing the precursor in a second carbon source atmosphere for a first stage of sintering to obtain a first intermediate particle having a first coating layer; etching the first coating layer of the first intermediate particle with an etching solution to obtain a second intermediate particle, wherein the specific surface area of ​​the second intermediate particle is greater than that of the first intermediate particle; and placing the second intermediate particle in a third carbon source atmosphere for a second stage of sintering to form a second coating layer on the surface of the first coating layer to obtain the cathode material.

[0005] Furthermore, the etching solution is an acidic solution, and the concentration c of hydrogen ions in the etching solution satisfies the range: 0.01 mol / L ≤ c ≤ 0.06 mol / L.

[0006] Furthermore, the etching time for the etching solution to etch the first intermediate particle is t0, and t0 satisfies the range: 5min≤t0≤60min.

[0007] Further, the etching solution is selected from at least one of boric acid, formic acid, acetic acid, carbonic acid, and nitrous acid; and / or, the ratio α of the volume of the etching solution to the mass of the first intermediate particle satisfies the range: 4 mL / g ≤ α ≤ 6 mL / g; and / or, after etching the first coating layer of the first intermediate particle with the etching solution, the process further includes: washing the second intermediate particle with anhydrous ethanol.

[0008] Further, the first carbon source is selected from at least one of sucrose, glucose, starch, graphite powder, carbon nanotubes, and acetylene black; and / or, the second carbon source is selected from at least one of ethanol, isopropanol, ethyl acetate, toluene, and cyclohexane; and / or, the third carbon source is selected from at least one of ethanol, isopropanol, ethyl acetate, toluene, and cyclohexane.

[0009] Furthermore, the mass ratio A1 of the first carbon source to the phosphorus source satisfies the range: 1% ≤ A1 ≤ 5%; and / or, the mass ratio A2 of the second carbon source to the phosphorus source satisfies the range: 0.5% ≤ A2 ≤ 2.5%; and / or, the mass ratio A3 of the third carbon source to the phosphorus source satisfies the range: 0.5% ≤ A3 ≤ 2.5%.

[0010] Further, the step of placing the precursor in a second carbon source atmosphere for first-stage sintering to obtain first intermediate particles with a first coating layer includes: heating the precursor to a first temperature T1 at a heating rate v1 satisfying the range: 3℃ / min≤v1≤10℃ / min for first-stage sintering, wherein the first temperature T1 satisfies the range: 400℃≤T1≤600℃; and the first-stage sintering time t1 satisfies the range: 10h≤t1≤16h.

[0011] Further, the step of placing the second intermediate particles in a third carbon source atmosphere for a second stage of sintering to obtain a cathode material includes: heating the second intermediate particles to a second temperature T2 at a heating rate v2 satisfying the range: 3℃ / min≤v2≤10℃ / min for a second stage of sintering, wherein the second temperature T2 satisfies the range: 600℃≤T2≤800℃, and the second stage sintering time t2 satisfies the range: 10h≤t2≤16h.

[0012] This application also provides a cathode material, which is prepared by the preparation method provided in this application.

[0013] This application also provides a battery comprising: an electrolyte, a negative electrode, a separator, and a positive electrode. The negative electrode is at least partially immersed in the electrolyte. The separator is located on one side of the negative electrode and is at least partially immersed in the electrolyte. The positive electrode is disposed on the side of the separator opposite to the positive electrode and is at least partially immersed in the electrolyte. The positive electrode includes a positive current collector and a positive electrode material layer. The positive electrode material layer is disposed on the surface of the positive current collector and includes the positive electrode material provided in this application. In the positive electrode material layer, the mass fraction 'a' of the positive electrode material satisfies the range: 96% ≤ a ≤ 97.5%.

[0014] In the method for preparing the cathode material provided in this application, a first intermediate particle is obtained by first-stage sintering of the precursor formed by the lithium source, iron source, phosphorus source and the first carbon source. The lithium source, iron source and phosphorus source react to generate lithium iron phosphate, and the first carbon source forms first graphite carbon and is partially dispersed in the lithium iron phosphate to improve the conductivity of the lithium iron phosphate. At the same time, the second carbon source is carbonized to form the first coating layer, that is, the second carbon source forms second graphite carbon in the first-stage sintering and coats the outer periphery of the lithium iron phosphate to protect the lithium iron phosphate from oxygen. The first graphite carbon is also partially dispersed in the first coating layer. That is, the first coating layer includes the first graphite carbon formed by the first carbon source and the second graphite carbon formed by the second carbon source to improve the conductivity of the cathode material. Furthermore, the etching solution etches the first intermediate particles to obtain the second intermediate particles, removing some sites or impurities from the surface of the first coating layer. This results in more pores, grooves, or uneven surfaces on the surface of the first coating layer, making the specific surface area of ​​the second intermediate particles larger than that of the first intermediate particles. This provides more active sites and wider ion transport channels for active ions, and also increases the charge transport rate. When the positive electrode material is applied to the positive electrode sheet and assembled in a battery, the etched first coating layer facilitates the insertion and extraction of active ions and also facilitates charge transport, improving the active ion transport rate and charge transport rate. This slows down the capacity decay caused by impedance during battery cycling, thereby improving the battery's capacity retention rate and giving the battery better cycle performance. Furthermore, a second coating layer is formed on the surface of the second intermediate particles through a second sintering process. This second coating layer is not etched by the etching solution and is denser than the first coating layer. When the cathode material prepared by the aforementioned method is applied to the cathode electrode and assembled into a battery, the second coating layer can prevent the electrolyte from directly contacting the lithium iron phosphate, thereby mitigating the side reactions between the lithium iron phosphate and the electrolyte, reducing the consumption of active ions during cycling, and further improving the battery's capacity retention rate, resulting in superior cycle performance. The preparation method provided in this application is simple in process, uses widely available raw materials, and is suitable for large-scale industrial production and application. Attached Figure Description

[0015] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the implementation will be briefly introduced below. Obviously, the drawings described below are some implementations of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0016] Figure 1This is a schematic flowchart of a method for preparing a positive electrode material according to an embodiment of this application;

[0017] Figure 2 This is a scanning electron microscope (SEM) image of the positive electrode sheet of Embodiment 5 of this application;

[0018] Figure 3 This is a scanning electron microscope (SEM) image of the positive electrode of Embodiment 11 of this application;

[0019] Figure 4 The image shows the scanning electron microscope (SEM) image of the positive electrode of Comparative Example 1 of this application.

[0020] Figure 5 The AC impedance spectra are of the batteries assembled from the cathode materials of Examples 5, 11 and Comparative Example 1 of this application. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0022] The terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0023] In this document, references to "embodiment" or "implementation" mean that a particular feature, structure, or characteristic described in connection with an embodiment or implementation may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0024] Lithium-ion batteries, characterized by high energy density, excellent cycle performance, wide operating range, and low self-discharge, have become the dominant power source for portable electronic products such as mobile phones and laptops. As a crucial raw material in lithium-ion batteries, the cycle life of the cathode material directly determines the battery's performance indicators. With the continuous development of the energy storage industry, increasing the number of battery cycles and extending battery life has become an important development direction. Therefore, improving the cycle life of cathode materials to extend battery life has broad development prospects and potential.

[0025] This application provides a battery comprising: an electrolyte, a negative electrode, a separator, and a positive electrode. The negative electrode is at least partially immersed in the electrolyte. The separator is located on one side of the negative electrode and is at least partially immersed in the electrolyte. The positive electrode is disposed on the side of the separator opposite to the positive electrode and is at least partially immersed in the electrolyte. The positive electrode includes a positive current collector and a positive electrode material layer. The positive electrode material layer is disposed on the surface of the positive current collector and includes the positive electrode material provided in this application. In the positive electrode material layer, the mass fraction 'a' of the positive electrode material satisfies the range: 96% ≤ a ≤ 97.5%.

[0026] Understandably, the positive electrode, the separator, and the negative electrode are arranged in sequence.

[0027] Understandably, in the battery, the electrolyte includes active ions, and when the battery is a lithium-ion battery, the active ions are lithium ions.

[0028] Specifically, the mass fraction 'a' of the cathode material can be, but is not limited to, 96%, 96.1%, 96.3%, 96.4%, 96.5%, 96.7%, 96.8%, 97%, 97.1%, 97.2%, 97.3%, 97.4%, and 97.5%.

[0029] Understandably, the positive electrode material layer is disposed on the surface of the positive electrode current collector. This can be either the positive electrode material layer being disposed on one surface of the positive electrode current collector, or the positive electrode material layer being disposed on one surface of the positive electrode current collector.

[0030] Understandably, if the mass fraction of the positive electrode material in the positive electrode material layer is a, then a is the ratio of the mass of the positive electrode material to the mass of the positive electrode material layer.

[0031] In the battery provided in this embodiment, the positive electrode sheet comprises a positive electrode material prepared by the preparation method provided in this application, and the mass fraction 'a' of the positive electrode material satisfies the range of 96% ≤ a ≤ 97.5%. The positive electrode material comprises a second coating layer, a first coating layer, and lithium iron phosphate. The first coating layer is etched by the etching solution, resulting in a surface with numerous pores, grooves, or unevenness. During the charge and discharge process of the battery, the first coating layer facilitates the provision of more active sites and wider ion transport channels for active ions in the electrolyte, while the second coating layer prevents the electrolyte from directly reacting with lithium iron phosphate and losing active ions, thus enabling the battery to have a high capacity retention rate and maintain excellent cycle performance.

[0032] Optionally, the battery can be one of cylindrical batteries, prismatic batteries, and pouch batteries.

[0033] Optionally, the positive electrode material layer further includes a positive electrode binder and a positive electrode conductive agent. In the positive electrode material layer, the positive electrode binder is used to bind the components in the positive electrode material layer to improve the overall performance of the positive electrode sheet; the positive electrode conductive agent can improve the conductivity of the positive electrode material layer.

[0034] Optionally, the positive electrode binder is selected from at least one of polyvinylidene fluoride, polyvinylidene fluoride (PVDF), and polytetrafluoroethylene.

[0035] Optionally, the positive electrode conductive agent is selected from at least one of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0036] Optionally, in the positive electrode material layer, the mass fraction of the positive electrode binder ranges from 2% to 2.5%. The mass fraction of the positive electrode binder can be, but is not limited to, 2%, 2.05%, 2.1%, 2.15%, 2.2%, 2.25%, 2.3%, 2.35%, 2.4%, 2.45%, and 2.5%.

[0037] Optionally, in the positive electrode material layer, the mass fraction of the positive electrode conductive agent ranges from 1.3% to 1.5%. The mass fraction of the positive electrode conductive agent can be, but is not limited to, 1.3%, 1.32%, 1.33%, 1.35%, 1.38%, 1.4%, 1.42%, 1.45%, 1.46%, 1.48%, and 1.5%.

[0038] This application provides a cathode material prepared by the method provided in this application. When the cathode material prepared by the method of this application is used as a cathode electrode and assembled in a battery, the second coating layer can prevent the electrolyte from directly contacting the lithium iron phosphate, thereby mitigating the side reactions between the lithium iron phosphate and the electrolyte, reducing the consumption of active ions during cycling, and further improving the battery's capacity retention rate, resulting in superior cycle performance of the battery.

[0039] Please see Figure 1 This application provides a method for preparing a cathode material, the method comprising:

[0040] S101, providing raw materials, mixing the raw materials in a solvent, and drying to obtain a precursor, wherein the raw materials include at least a lithium source, an iron source, a phosphorus source, and a first carbon source.

[0041] Understandably, the solvent is used to dissolve the lithium source, iron source, phosphorus source and the first carbon source, and to mix the lithium source, iron source, phosphorus source and the first carbon source evenly to form a mixed solution. The mixed solution is then dried by heating or other means to obtain the precursor, which is a solid phase.

[0042] Optionally, the lithium source is selected from one or more of lithium oxide, lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate, and lithium citrate.

[0043] Optionally, the iron source is selected from one or more of ferric nitrate, ferrous sulfate, ferric citrate, ferrous oxalate, ferric oxide, and ferrous phosphate.

[0044] Optionally, the phosphorus source is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, and iron phosphate.

[0045] Optionally, the first carbon source is selected from one or more of sucrose, glucose, starch, graphite powder, carbon nanotubes, and acetylene black.

[0046] Understandably, in the terminology of this application, "multiple" means two or more, and may be, but is not limited to, two, three, four, five or six kinds.

[0047] Optionally, in some embodiments, the solvent is water.

[0048] Understandably, the raw materials may also include substances other than lithium sources, iron sources, phosphorus sources and the first carbon source, such as other raw materials that can provide doping elements for lithium iron phosphate.

[0049] S102, the precursor is placed in a second carbon source atmosphere for first-stage sintering to obtain first intermediate particles with a first coating layer.

[0050] Understandably, in order to distinguish it from the graphite carbon formed by the second carbon source and the third carbon source, in this application specification, the graphite carbon formed by the first carbon source is referred to as the first graphite carbon, the graphite carbon formed by the second carbon source is referred to as the second graphite carbon, and the graphite carbon formed by the third carbon source is referred to as the third graphite carbon.

[0051] Understandably, during the first sintering process, the lithium source, iron source, and phosphorus source form lithium iron phosphate, the first carbon source forms first graphite carbon, the second carbon source forms second graphite carbon, the first carbon source is partially dispersed in the lithium iron phosphate and partially forms the first coating layer, and the second graphite carbon forms the first coating layer.

[0052] Understandably, the precursor is placed in the second carbon source atmosphere for a first stage of sintering, the second carbon source volatilizes into a gaseous carbon source and forms a second graphite carbon in the first stage of sintering, and the second graphite carbon is deposited on the surface of the lithium iron phosphate to form a first coating layer.

[0053] S103, an etching solution is used to etch the first coating layer of the first intermediate particle to obtain a second intermediate particle, wherein the specific surface area of ​​the second intermediate particle is greater than that of the first intermediate particle.

[0054] Understandably, the specific surface area of ​​the second intermediate particle is greater than that of the first intermediate particle. This can be interpreted as the specific surface area of ​​the first coating layer etched by the etching solution being greater than that of the first coating layer that has not been etched.

[0055] S104, the second intermediate particle is placed in a third carbon source atmosphere for a second stage of sintering to form a second coating layer on the surface of the first coating layer, thereby obtaining the cathode material.

[0056] Understandably, the cathode material prepared by the method of this application includes a plurality of cathode particles. Each cathode particle includes a second coating layer, a first coating layer, lithium iron phosphate, and a first graphite carbon. The second coating layer coats the outer periphery of the first coating layer, the first coating layer coats the outer periphery of the lithium iron phosphate, and the first graphite carbon is partially dispersed in the lithium iron phosphate and partially dispersed in the first coating layer.

[0057] Understandably, the second intermediate particle is placed in the atmosphere of the third carbon source for a second stage of sintering. The third carbon source volatilizes into a gaseous carbon source and forms a third graphite carbon during the second stage of sintering. The third graphite carbon is deposited on the surface of the first coating layer and forms a second coating layer.

[0058] Understandably, in some embodiments, the first coating layer includes a first graphitic carbon and a second graphitic carbon, the second coating layer includes a third graphitic carbon, and there is no clear dividing line between the first coating layer and the second coating layer.

[0059] In the method for preparing the cathode material provided in this embodiment, a first intermediate particle is obtained by first-stage sintering of the precursor formed by the lithium source, iron source, phosphorus source and the first carbon source. The lithium source, iron source and phosphorus source react to generate lithium iron phosphate, and the first carbon source forms first graphite carbon and is partially dispersed in the lithium iron phosphate to improve the conductivity of the lithium iron phosphate. At the same time, the second carbon source carbonizes to form the first coating layer, that is, the second carbon source forms second graphite carbon in the first-stage sintering and coats the outer periphery of the lithium iron phosphate to protect the lithium iron phosphate from oxygen. The first graphite carbon is also partially dispersed in the first coating layer. That is, the first coating layer includes the first graphite carbon formed by the first carbon source and the second graphite carbon formed by the second carbon source to improve the conductivity of the cathode material. Furthermore, the etching solution etches the first intermediate particles to obtain the second intermediate particles, removing some sites or impurities from the surface of the first coating layer. This results in more pores, grooves, or uneven surfaces on the surface of the first coating layer, making the specific surface area of ​​the second intermediate particles larger than that of the first intermediate particles. This provides more active sites and wider ion transport channels for active ions, and also increases the charge transport rate. When the positive electrode material is applied to the positive electrode sheet and assembled in a battery, the etched first coating layer facilitates the insertion and extraction of active ions and also facilitates charge transport, improving the active ion transport rate and charge transport rate. This slows down the capacity decay caused by impedance during battery cycling, thereby improving the battery's capacity retention rate and giving the battery better cycle performance. Furthermore, a second coating layer is formed on the surface of the second intermediate particles through a second sintering process. This second coating layer is not etched by the etching solution and is denser than the first coating layer. When the cathode material prepared by the aforementioned method is applied to the cathode electrode and assembled into a battery, the second coating layer can prevent the electrolyte from directly contacting the lithium iron phosphate, thereby mitigating the side reactions between the lithium iron phosphate and the electrolyte, reducing the consumption of active ions during cycling, and further improving the battery's capacity retention rate, resulting in superior cycle performance. The preparation method provided in this application is simple in process, uses widely available raw materials, and is suitable for large-scale industrial production and application.

[0060] Optionally, during the first sintering stage, the precursor is also placed in an inert gas atmosphere, wherein the inert gas is selected from at least one of nitrogen, argon, and helium.

[0061] Understandably, in the terminology of this application, "at least one" means one or more.

[0062] In this embodiment, during the first sintering process, the precursor is placed in the inert gas atmosphere and the second carbon source atmosphere. The inert atmosphere is used to isolate oxygen, so that the first carbon source can form the first graphite carbon and the second carbon source can form the second graphite carbon and coat the surface of the lithium iron phosphate, so as to avoid the generation of other impurities in the lithium source, iron source, phosphorus source, first carbon source and second carbon source due to the influence of oxygen, thereby improving the purity of the cathode material.

[0063] Optionally, in the second sintering stage, the precursor is also placed in an inert gas atmosphere, wherein the inert gas is selected from at least one of nitrogen, argon, and helium.

[0064] In this embodiment, during the second sintering process, the precursor is placed in the inert gas atmosphere and the third carbon source atmosphere. The inert atmosphere is used to isolate oxygen, so that the third carbon source can form third graphite carbon and coat the surface of the first coating layer, thereby avoiding the generation of other impurities by the third carbon source due to the influence of oxygen, and improving the purity of the cathode material.

[0065] Optionally, the positive electrode material includes a plurality of positive electrode particles, wherein the particle size D50 of the positive electrode particles satisfies the range: 100nm≤D50≤1000nm.

[0066] Specifically, the particle size D50 of the positive electrode particles can be, but is not limited to, 100nm, 120nm, 150nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 750nm, 800nm, 850nm, 900nm, and 1000nm.

[0067] In some embodiments, the etching solution is an acidic solution, and the concentration c of hydrogen ions in the etching solution satisfies the range: 0.01 mol / L ≤ c ≤ 0.06 mol / L.

[0068] Understandably, the etching solution is an acidic solution and a weak acid.

[0069] Specifically, the concentration c of hydrogen ions in the etching solution can be, but is not limited to, 0.01 mol / L, 0.012 mol / L, 0.015 mol / L, 0.018 mol / L, 0.02 mol / L, 0.022 mol / L, 0.025 mol / L, 0.03 mol / L, 0.034 mol / L, 0.036 mol / L, 0.038 mol / L, 0.04 mol / L, 0.042 mol / L, 0.046 mol / L, 0.05 mol / L, 0.052 mol / L, 0.054 mol / L, 0.056 mol / L, and 0.06 mol / L.

[0070] In this embodiment, the etching solution is an acidic solution. When the concentration c of hydrogen ions in the etching solution meets the range of 0.01 mol / L ≤ c ≤ 0.06 mol / L, the acidity of the etching solution is within a reasonable range. The etching solution can etch the first coating layer of the first intermediate particle to increase the active sites and ion transport channels of the first coating layer, thereby improving the cycle performance of the battery when the cathode material is applied. In addition, it can avoid damage to the lithium iron phosphate in the first coating layer due to excessive acidity of the etching solution, and it can also avoid reducing the etching efficiency of the first intermediate particle due to insufficient acidity of the etching solution. This makes the preparation method of cathode material highly efficient, and the prepared cathode material has good performance. When the hydrogen ion concentration *c* in the etching solution is too high, the acidity of the etching solution is too strong. When the etching solution is used to etch the first intermediate particle, it may over-etch the first intermediate particle. That is, the etching solution may not only etch the first coating layer of the first intermediate particle, but also damage the crystal structure of lithium iron phosphate inside the first coating layer, thereby affecting the insertion and extraction of active ions, reducing the cycle performance of the battery, and potentially causing a decrease in the energy density of the cathode material. When the hydrogen ion concentration *c* in the etching solution is too low, the acidity of the etching solution is too weak. On the one hand, this reduces the efficiency of the etching solution in etching the first intermediate particle; on the other hand, it may lead to insufficient etching of the first intermediate particle, so that the active sites and ion transport channels of the first coating layer etched by the etching solution do not increase significantly, resulting in a weak improvement in the cycle performance of the battery when the cathode material is used in the battery.

[0071] Understandably, the etching solution cannot be a strong acid to prevent it from damaging the lithium iron phosphate.

[0072] Optionally, the etching solution etches the first intermediate particle, and the ratio α of the volume of the etching solution to the mass of the first intermediate particle satisfies the range: 4mL / g≤α≤6mL / g. In other words, the liquid-solid ratio α of the etching solution and the first intermediate particle satisfies the range: 4mL / g≤α≤6mL / g.

[0073] Specifically, the value of α, the ratio of the volume of the etching solution to the mass of the first intermediate particle, can be, but is not limited to, 4 mL / g, 4.1 mL / g, 4.2 mL / g, 4.3 mL / g, 4.5 mL / g, 4.6 mL / g, 4.8 mL / g, 5 mL / g, 5.1 mL / g, 5.2 mL / g, 5.3 mL / g, 5.4 mL / g, 5.5 mL / g, 5.6 mL / g, 5.7 mL / g, 5.8 mL / g, 5.9 mL / g, and 6 mL / g.

[0074] In this embodiment, when the ratio α of the volume of the etching solution to the mass of the first intermediate particle satisfies the range of 4 mL / g ≤ α ≤ 6 mL / g, both the volume of the etching solution and the mass of the first intermediate particle are within a reasonable range. The etching solution can fully wet the first intermediate particle and etch it to increase its specific surface area, increase the number of active sites in the first coating layer, and widen the ion transport channels, thereby improving the cycle performance of the battery when the cathode material is used. When the value of α is too large, the volume of the etching solution is too large given the fixed mass of the first intermediate particle. On the one hand, this may cause the etching solution to over-etch the first intermediate particle and corrode the lithium iron phosphate in the first coating layer, thereby affecting the insertion and extraction of active ions and reducing the energy density of the cathode material; on the other hand, it may cause waste of the etching solution and increase the preparation cost of the cathode material. When the value of α is too small, given a fixed mass of the first intermediate particle, the volume of the etching solution is too small. The etching solution may not be able to wet the first intermediate particle, resulting in insufficient etching of the first intermediate particle. This makes it difficult to increase the specific surface area of ​​the first coating layer of the first intermediate particle, and the active sites and ion transport channels of the first coating layer etched by the etching solution do not increase significantly. When the cathode material is applied to a battery, the cycle performance of the battery is difficult to improve.

[0075] In some embodiments, the etching time for the etching solution to etch the first intermediate particle is t0, where t0 satisfies the range: 5 min ≤ t0 ≤ 60 min.

[0076] Specifically, the etching time t0 for the etching solution to etch the first intermediate particle can be, but is not limited to, 5 min, 8 min, 10 min, 12 min, 15 min, 18 min, 20 min, 22 min, 25 min, 28 min, 30 min, 32 min, 38 min, 40 min, 45 min, 48 min, 50 min, 55 min, and 60 min.

[0077] In this embodiment, when the etching time t0 of the etching solution on the first intermediate particle meets the range of 5 min ≤ t0 ≤ 60 min, the etching time of the etching solution on the first intermediate particle is within a reasonable range. On the one hand, the etching solution can etch the first coating layer to increase the active sites and widen the active ion transport channels of the first coating layer, thereby improving the cycle performance of the battery when the cathode material is applied to the battery. On the other hand, it can avoid the first intermediate particle being over-etched due to excessive etching time, and also avoid the first intermediate particle being under-etched due to excessive etching time, thereby improving the performance of the cathode material prepared by the preparation method. When the etching time t0 of the etching solution on the first intermediate particle is too long, that is, the etching time is too long, it may lead to the etching solution over-etching the first intermediate particle. That is, the etching solution may not only etch the first coating layer of the first intermediate particle, but also destroy the crystal structure of lithium iron phosphate inside the first coating layer, thereby affecting the insertion and extraction of active ions, and may cause a decrease in the energy density of the cathode material. When the etching time t0 of the etching solution on the first intermediate particle is too short, that is, the etching time is too short, it may result in insufficient etching of the first intermediate particle by the etching solution. As a result, the active sites and ion transport channels of the first coating layer etched by the etching solution do not increase significantly, and the effect of improving the cycle performance of the battery when the cathode material is applied to the battery is weak.

[0078] In some embodiments, the etching solution is selected from at least one of boric acid, formic acid, acetic acid, carbonic acid, and nitrous acid.

[0079] In this embodiment, the etching solution is selected from at least one of boric acid, formic acid, acetic acid, carbonic acid, and nitrous acid. The etching solution is a weak acid to avoid the etching solution being too acidic and damaging the lithium iron phosphate in the first coating layer. This allows the etching solution to etch the first coating layer while avoiding etching the lithium iron phosphate, thereby improving the cycle performance of the battery when the cathode material is used in the battery, while maintaining the energy density of the battery.

[0080] Optionally, after etching the first coating layer of the first intermediate particle with the etching solution, the method further includes washing the second intermediate particle with anhydrous ethanol. Anhydrous ethanol can be used to remove residual etching solution from the surface of the first intermediate particle to prevent the etching solution from remaining on the surface of the first intermediate particle and causing insufficient uniform etching.

[0081] In some embodiments, the first carbon source is selected from at least one of sucrose, glucose, starch, graphite powder, carbon nanotubes, and acetylene black; and / or, the second carbon source is selected from at least one of ethanol, isopropanol, ethyl acetate, toluene, and cyclohexane; and / or, the third carbon source is selected from at least one of ethanol, isopropanol, ethyl acetate, toluene, and cyclohexane.

[0082] Understandably, the first carbon source is a solid carbon source. The second carbon source is a liquid carbon source, which is more likely to volatilize into a gaseous carbon source compared to the first carbon source; the third carbon source is a liquid carbon source, which is more likely to volatilize into a gaseous carbon source compared to the first carbon source.

[0083] In this embodiment, the first carbon source is mixed with the lithium source, iron source, and phosphorus source to form a precursor, and first graphite carbon is formed during the first stage of sintering to disperse in lithium iron phosphate and improve the conductivity of the lithium iron phosphate. The first carbon source is selected from at least one of sucrose, glucose, starch, graphite powder, carbon nanotubes, and acetylene black, which facilitates the full dissolution of the first carbon source in the solvent and its uniform mixing with the lithium source, iron source, and phosphorus source, thereby improving the uniformity of the first carbon source dispersion in the lithium iron phosphate. In addition, the second carbon source and the third carbon source are both selected from at least one of ethanol, isopropanol, ethyl acetate, toluene, and cyclohexane.

[0084] In some embodiments, the ratio A1 of the mass of the first carbon source to the mass of the phosphorus source satisfies the range: 1% ≤ A1 ≤ 5%.

[0085] Specifically, the ratio A1 of the mass of the first carbon source to the mass of the phosphorus source can be, but is not limited to, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 4%, 4.5%, 4.7%, 4.9%, and 5%.

[0086] In this embodiment, when the ratio A1 of the mass of the first carbon source to the mass of the phosphorus source satisfies the range of 1% ≤ A1 ≤ 5%, both the mass of the first carbon source and the mass of the phosphorus source are within a reasonable range. This ensures that the mass fraction of the first coating layer and the mass of lithium iron phosphate in the cathode material are both within a reasonable range, resulting in a cathode material with high energy density and good conductivity. When the cathode material is applied to a battery, the battery exhibits both high energy density and good cycle performance. Conversely, when the ratio of the mass of the first carbon source to the mass of the phosphorus source is too large, given a fixed mass of phosphorus source, the mass of the first carbon source is excessively large. This results in an excessively large mass fraction of the first coating layer in the cathode material, and consequently, a excessively small mass fraction of lithium iron phosphate. This reduces the content of active material in the cathode material, leading to a lower energy density when the cathode material is applied to a battery. When the ratio of the mass of the first carbon source to the mass of the phosphorus source is too small, the mass of the first carbon source is too small when the mass of the phosphorus source is constant. This results in a small mass fraction of the first coating layer in the cathode material, which reduces the conductivity of the cathode material. Consequently, when the cathode material is applied to a battery, the battery's cycle performance is poor.

[0087] In some embodiments, the ratio A2 of the mass of the second carbon source to the mass of the phosphorus source satisfies the range: 0.5% ≤ A2 ≤ 2.5%.

[0088] Specifically, the ratio A2 of the mass of the second carbon source to the mass of the phosphorus source can be, but is not limited to, 0.5%, 0.6%, 0.8%, 0.9%, 1%, 1.2%, 1.3%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, and 2.5%.

[0089] Understandably, in the first stage of sintering, the quality of the second carbon source can be controlled by the rate, concentration, and time of introducing the second carbon source atmosphere.

[0090] In this embodiment, when the ratio A2 of the mass of the second carbon source to the mass of the phosphorus source satisfies the range of 0.5% ≤ A2 ≤ 2.5%, the mass of both the second carbon source and the phosphorus source is within a reasonable range. Therefore, in the first sintering stage, the thickness of the first coating layer formed by the second carbon source on the surface of the lithium iron phosphate is within a reasonable range. On the one hand, the first coating layer can protect the internal lithium iron phosphate, preventing the etching solution from etching the lithium iron phosphate during further etching, thus maintaining the performance of the lithium iron phosphate. On the other hand, it can prevent the first coating layer from becoming too thick, which would increase the etching time of the first coating layer and reduce the efficiency of etching the first intermediate particles. When the ratio A2 of the mass of the second carbon source to the mass of the phosphorus source is too large, the thickness of the first coating layer formed by the second carbon source on the surface of the lithium iron phosphate during the first sintering stage is too large. On the one hand, this increases the difficulty of further etching the first coating layer, prolongs the etching time, and reduces the efficiency of etching the first intermediate particles. On the other hand, the excessive thickness of the first coating layer results in a small mass fraction of lithium iron phosphate when the mass of the cathode material is constant. Although the first coating layer can improve the conductivity of the cathode material, it also reduces the content of active material in the cathode material and reduces the energy density of the cathode material. When the ratio A2 of the mass of the second carbon source to the mass of the phosphorus source is too small, the thickness of the first coating layer formed by the second carbon source on the surface of the lithium iron phosphate is too small during the first stage of sintering. When the first intermediate particle is sintered with an etching solution, the etching solution not only etches the first coating layer, but may also enter the first coating layer and corrode it, thereby destroying the crystal structure of the lithium iron phosphate, affecting the insertion and extraction of active ions, and potentially causing a decrease in the energy density of the cathode material.

[0091] In some embodiments, the ratio A3 of the mass of the third carbon source to the mass of the phosphorus source satisfies the range: 0.5% ≤ A3 ≤ 2.5%.

[0092] Specifically, the ratio A3 of the mass of the third carbon source to the mass of the phosphorus source can be, but is not limited to, 0.5%, 0.6%, 0.8%, 0.9%, 1%, 1.2%, 1.3%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, and 2.5%.

[0093] Understandably, in the second sintering stage, the quality of the third carbon source can be controlled by the rate, concentration, and time of introducing the third carbon source atmosphere.

[0094] In this embodiment, when the ratio A3 of the mass of the third carbon source to the mass of the phosphorus source is within the range of 0.5% ≤ A3 ≤ 2.5%, the mass of the third carbon source is within a reasonable range. Therefore, during the second sintering stage, the thickness of the second coating layer formed by the third carbon source on the surface of the first coating layer is within a reasonable range, and the second coating layer can protect the first coating layer. When the cathode material is applied to a battery, the second coating layer can prevent the electrolyte from entering the interior of the first coating layer and undergoing side reactions with lithium iron phosphate, thereby avoiding the consumption of active ions in the lithium iron phosphate and enabling the battery to have better cycle performance when the cathode material is applied. When the ratio A3 of the mass of the third carbon source to the mass of the phosphorus source is too large, the thickness of the second coating layer formed by the third carbon source on the surface of the first coating layer is too large during the second sintering stage. This results in an excessively large mass fraction of the second coating layer when the mass of the cathode material is constant. Consequently, the mass fraction of lithium iron phosphate is too small. Although the second coating layer can improve the conductivity of the cathode material and prevent the electrolyte from contacting the electrolyte after passing through the first coating layer, it also reduces the content of active material in the cathode material and reduces the energy density of the cathode material. When the ratio A3 of the mass of the third carbon source to the mass of the phosphorus source is too small, the thickness of the second coating layer formed by the third carbon source on the surface of the first coating layer during the second sintering stage will be too small. As a result, when the cathode material is applied to the battery, the second coating layer will be difficult to form a dense coating layer on the outer periphery of the first coating layer. This will allow the electrolyte to pass through the second coating layer in sequence, and the pores formed after the first coating layer is etched will enter the first coating layer and come into contact with the electrolyte. This will increase the consumption of active ions caused by the side reaction between the lithium iron phosphate and the electrolyte, and reduce the cycle performance of the battery when the cathode material is applied to the battery.

[0095] Optionally, the temperature at which the second intermediate particle is sintered in the second stage is higher than the temperature at which the precursor is sintered in the first stage.

[0096] In this embodiment, after the precursor undergoes a first-stage sintering, the second carbon source forms a first coating layer on the surface of the lithium iron phosphate, and the specific surface area of ​​the first coating layer is further increased by etching. Furthermore, the second intermediate particles undergo a second-stage sintering at a higher temperature than the first-stage sintering temperature of the precursor, resulting in a denser second coating layer on the surface of the first coating layer. This second coating layer protects both the first coating layer and the lithium iron phosphate. When the cathode material prepared by this method is applied to a battery, it prevents the electrolyte from sequentially passing through the first and second coating layers to enter the first coating layer and react with the lithium iron phosphate, thus avoiding the loss of active material in the cathode material. Correspondingly, the first coating layer is more porous than the second coating layer, facilitating etching by the etching solution and improving the efficiency of the cathode material preparation method.

[0097] In some embodiments, the step of placing the precursor in a second carbon source atmosphere for first-stage sintering to obtain first intermediate particles with a first coating layer includes: heating the precursor to a first temperature T1 at a heating rate v1 satisfying the range: 3℃ / min≤v1≤10℃ / min for first-stage sintering, wherein the first temperature T1 satisfies the range: 400℃≤T1≤600℃; and the first-stage sintering time t1 satisfies the range: 10h≤t1≤16h.

[0098] Specifically, the heating rate v1 can be, but is not limited to, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, 5.5℃ / min, 6℃ / min, 6.5℃ / min, 7℃ / min, 7.5℃ / min, 8℃ / min, 8.5℃ / min, 9℃ / min, 9.5℃ / min, and 10℃ / min.

[0099] Specifically, the value of the first temperature T1 can be, but is not limited to, 400℃, 410℃, 420℃, 440℃, 450℃, 460℃, 480℃, 500℃, 520℃, 530℃, 540℃, 550℃, 560℃, 580℃, 590℃, and 600℃.

[0100] Specifically, the value of the sintering time t1 for the first stage can be, but is not limited to, 10h, 10.2h, 10.5h, 10.8h, 11h, 11.5h, 11.8h, 12h, 12.2h, 12.5h, 12.8h, 13h, 13.5h, 14h, 14.5h, 14.8h, 15h, 15.5h, 15.8h, and 16h.

[0101] In this embodiment, the precursor is sintered in a second carbon source atmosphere for the first stage, so that the lithium source, iron source, and phosphorus source form lithium iron phosphate. The first carbon source is carbonized to form first graphite carbon, and the second carbon source is carbonized to form second graphite carbon, which coats the surface of the lithium iron phosphate to protect it. When the heating rate v1 of the precursor meets the range of 3℃ / min ≤ v1 ≤ 10℃ / min, the heating rate of the first stage sintering of the precursor is within a reasonable range, so that the precursor can be heated to the sintering temperature quickly, thereby improving the full reaction of the lithium source, iron source, and phosphorus source, and the full carbonization of the first carbon source and the second carbon source. This is beneficial to improving the efficiency of the cathode material preparation method for preparing the cathode material.

[0102] When the first temperature T1 is within the range of 400℃≤T1≤600℃ and the first sintering time t1 is within the range of 10h≤t1≤16h, the first temperature and the first sintering time are within a reasonable range. During the first sintering of the precursor, the lithium source, iron source, and phosphorus source react fully to generate lithium iron phosphate, preventing the lithium iron phosphate from decomposing due to excessively high first temperature or excessively long first sintering time, thus ensuring the amount of active material in the cathode material. Furthermore, the thickness of the first coating layer formed by the carbonization of the second carbon source is within a reasonable range, protecting the lithium iron phosphate while preventing the first coating layer from being too thick and reducing etching efficiency. This allows the cathode material to possess both high energy density and numerous active ion transport channels. When the cathode material is applied to a battery, the battery exhibits both high energy density and good cycle performance. When the first temperature T1 is too high and the first sintering time t1 is too long, on the one hand, the lithium iron phosphate generated by the reaction of the lithium source, iron source, and phosphorus source may decompose, thereby reducing the amount of active material in the cathode material; on the other hand, the first coating layer formed by the second carbon source is too thick, increasing the difficulty of further etching the first coating layer, increasing the etching time of the first coating layer, and reducing the efficiency of etching the first intermediate particles, thereby reducing the efficiency of the preparation method for preparing the cathode material. When the first temperature T1 is too low and the first sintering time t1 is too short, on the one hand, the lithium source, iron source, and phosphorus source are difficult to form lithium iron phosphate or the reaction of the lithium source, iron source, and phosphorus source is insufficient, resulting in too little active material in the cathode material, thereby affecting the insertion and extraction of active ions; on the other hand, the first carbon source is difficult to carbonize to form the first graphitic carbon, and the second carbon source is difficult to carbonize to form the second graphitic carbon, resulting in a decrease in the conductivity of the cathode material. Furthermore, the second carbon source is difficult to form a first coating layer on the surface of the lithium iron phosphate, or the formed first coating layer is too loose, making it difficult to prevent the electrolyte from contacting the lithium iron phosphate. Also, the first coating layer is too thin or too loose, making it difficult to perform further etching, which in turn makes it difficult to improve the specific surface area of ​​the first coating layer and the cycle performance of the battery when the cathode material is used in the battery.

[0103] In some embodiments, the step of placing the second intermediate particles in a third carbon source atmosphere for second-stage sintering to obtain a cathode material includes: heating the second intermediate particles at a heating rate v2 satisfying the range: 3℃ / min≤v2≤10℃ / min, to a second temperature T2 for second-stage sintering, wherein the second temperature T2 satisfies the range: 600℃≤T2≤800℃, and the second-stage sintering time t2 satisfies the range: 10h≤t2≤16h.

[0104] Specifically, the heating rate v2 can be, but is not limited to, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, 5.5℃ / min, 6℃ / min, 6.5℃ / min, 7℃ / min, 7.5℃ / min, 8℃ / min, 8.5℃ / min, 9℃ / min, 9.5℃ / min, and 10℃ / min.

[0105] Specifically, the value of the second temperature T2 can be, but is not limited to, 600℃, 620℃, 630℃, 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, 720℃, 735℃, 750℃, 760℃, 770℃, 780℃, 790℃, and 800℃.

[0106] Specifically, the value of the second sintering time t2 can be, but is not limited to, 10h, 10.2h, 10.5h, 10.8h, 11h, 11.5h, 11.8h, 12h, 12.2h, 12.5h, 12.8h, 13h, 13.5h, 14h, 14.5h, 14.8h, 15h, 15.5h, 15.8h, and 16h.

[0107] In this embodiment, the second intermediate particles are placed in a third carbon source atmosphere for a second-stage sintering process. This allows the third carbon source to carbonize and form the third graphite carbon, which then coats the surface of the first coating layer to form a second coating layer. The second temperature is higher than the first temperature, making the second coating layer denser than the first coating layer. The second coating layer protects both the first coating layer and the lithium iron phosphate. Furthermore, when the heating rate v2 for the second intermediate particles meets the range of 3℃ / min ≤ v2 ≤ 10℃ / min, the heating rate is within a reasonable range. This avoids increasing the total sintering time due to excessive heating time and ensures that the third carbon source is fully carbonized to form the second coating layer, thus improving the efficiency of the cathode material preparation method. When the cathode material prepared by this method is applied to a battery, the second coating layer prevents the electrolyte in the battery from reacting with the lithium iron phosphate after passing through both the second and first coating layers, ensuring the lithium iron phosphate content in the cathode material.

[0108] When the second temperature T2 is within the range of 600℃≤T2≤800℃ and the second sintering time t2 is within the range of 10h≤t2≤16h, the second temperature and the second sintering time are within a reasonable range, which is beneficial for the further crystallization of the lithium iron phosphate, the first graphite carbon, and the second graphite carbon, thereby improving the performance of the lithium iron phosphate and the conductivity of the first graphite carbon and the second graphite carbon. Furthermore, the third carbon source forms a second coating layer during the second sintering process. When the second temperature and the second sintering time are appropriate, the density and thickness of the second coating layer are within a reasonable range. This avoids both an excessively thick second coating layer that reduces the lithium iron phosphate content of the cathode material and an excessively sparse second coating layer that allows electrolyte to enter the first coating layer. This ensures that when the cathode material is used in a battery, the battery possesses both high energy density and good cycle performance. When the second temperature T2 is too high and the second sintering time t2 is too long, on the one hand, the preparation method becomes more difficult, and both the high temperature and long sintering time increase energy loss, thus increasing the cost of preparing cathode particles. On the other hand, if the second temperature is too high and the second sintering time is too long, the lithium iron phosphate generated by the reaction of the lithium source, iron source, and phosphorus source may decompose, thereby reducing the amount of active material in the cathode material. When the second temperature T2 is too low and the second sintering time t2 is too short, the graphitization degree of the second coating layer formed by the third carbon source is low, and the second coating layer is not dense enough. If the second coating layer is not dense enough, and the first coating layer has been etched and has pores, when the cathode material is applied to a battery, the electrolyte in the battery can easily pass through the second coating layer and the first coating layer to enter the first coating layer and contact the lithium iron phosphate, resulting in the loss of active ions in the lithium iron phosphate and reducing the content of active material in the cathode material.

[0109] The technical solution of this application will be further described below with reference to several embodiments.

[0110] Examples 1 to 12, Comparative Example 1:

[0111] 1. Preparation of cathode materials:

[0112] Step 1: Mix ferric nitrate (i.e., iron source), ammonium dihydrogen phosphate (i.e., phosphorus source), and lithium hydroxide (i.e., lithium source) in a ratio of 0.95:1.00:1.04. Add deionized water (i.e., solvent) as a dispersant. The weight ratio of deionized water to the iron source, phosphorus source, and lithium source is 12%. At the same time, add glucose as the first carbon source to obtain a mixed solution. The mass ratio A1 of the first carbon source to the phosphorus source is shown in Table 1.

[0113] Step 2: The mixture is dried and crushed to obtain a precursor. The precursor is placed in a tube furnace and heated to a first temperature T1 of 500°C at a heating rate of 3°C / min under an inert gas atmosphere (e.g., nitrogen atmosphere) and a second carbon source atmosphere. The precursor is sintered for the first stage for 10 hours to obtain the first intermediate particles. The ratio A2 of the mass of the second carbon source to the mass of the phosphorus source is shown in Table 1.

[0114] Step 3: Take etching solutions of different concentrations, where the ratio α of the volume of the etching solution to the mass of the first intermediate particle is 5 mL / g. Etch the first intermediate particle with the etching solution to obtain the second intermediate particle. After etching, filter and wash several times with anhydrous ethanol to remove residual weak acid. The concentration of hydrogen ions in the etching solution is c, and the etching time is t1. The values ​​of hydrogen ion concentration c and etching time t0 in the etching solutions of Examples 1 to 12 and Comparative Example 1 are shown in Table 1.

[0115] Step 4: The washed second intermediate particles are dried at 80°C for 4 hours, and then heated to a second temperature T2 of 700°C at a heating rate of 3°C / min under an inert gas atmosphere (e.g., nitrogen atmosphere) and a third carbon source atmosphere. The second intermediate particles are sintered for a second stage for 14 hours. After cooling, they are pulverized by air jet milling to obtain the cathode material. The ratio A2 of the mass of the third carbon source to the mass of the phosphorus source is shown in Table 1.

[0116] The prepared cathode material was tested for particle size D50, specific surface area, and performance parameters of the carbon coating layer. The relevant instruments were as follows: Particle size D50 was determined by SEM analysis using a Regulus 8100 microscope, and the particle diameter was subsequently counted. Specific surface area was measured using a dynamic nitrogen adsorption specific surface area analyzer (JW-DX model). The thickness of the first coating layer and the sum of the thicknesses of the second coating layer (carbon coating layer) were measured using a transmission electron microscope (TEM) (JEM-2100 model).

[0117] 2. Preparation of the positive electrode sheet:

[0118] The positive electrode materials from Examples 1 to 12 and Comparative Example 1 were used. The positive electrode materials, polyvinylidene fluoride (PVDF), and carbon black were mixed in a mass ratio of 96.5%, 2.2%, and 1.3%, respectively. A certain amount of N-methylpyrrolidone was added and stirred thoroughly to obtain a positive electrode slurry. The positive electrode slurry was dried on the surface of the positive electrode current collector at 120°C using a coating machine to form a film. Subsequently, it was rolled in a roller press to obtain a positive electrode sheet. Specifically, the positive electrode material from Example 1 yielded the positive electrode sheet of Example 1, the positive electrode material from Example 2 yielded the positive electrode sheet of Example 2, the positive electrode material from Comparative Example 1 yielded the positive electrode sheet of Comparative Example 1, and so on.

[0119] 3. Battery fabrication:

[0120] Using the positive electrode sheets prepared in Examples 1 to 12 and Comparative Example 1, and graphene as the negative electrode sheet, the positive electrode sheet, separator, and negative electrode sheet were coated, sliced, rolled, slit, dried, and coated with adhesive tape to obtain a wound battery cell. The cell was dried at 80°C for 48 hours, and then the electrolyte was used to fill and seal the battery. After standing for 24 hours, formation, first final sealing, aging, and second final sealing were performed to prepare the battery. The electrolyte can be any electrolyte suitable for lithium-ion batteries in the industry; no limitation is placed on the type of electrolyte. Specifically, the positive electrode sheet of Example 1 was used to prepare Example 1, the positive electrode sheet of Example 2 was used to prepare Example 2, and the positive electrode sheet of Comparative Example 1 was used to prepare Control Battery 1.

[0121] Table 1: Process parameters of the preparation method of cathode material and performance parameters of the prepared cathode material.

[0122]

[0123] Understandably, in Comparative Example 1, the concentration of hydrogen ions in the etching solution is 0 mol / L and the etching time is 0 min. Therefore, in the preparation process of Comparative Example 1, no etching solution was used to etch the first intermediate particle.

[0124] Battery performance testing:

[0125] 1. Cyclic performance test:

[0126] Battery 1 to battery 12 and control battery 1 were subjected to charge-discharge cycles at a constant power of 0.5P. The charging and discharging voltage range was 2.5V to 3.65V. Repeated charge-discharge cycles were performed. Cyclic cycles at room temperature were conducted in a 25℃ constant temperature chamber, and cyclic cycles at high temperature were conducted in a 45℃ constant temperature chamber. The capacity retention rate of battery 1 to battery 12 and control battery 1 after different numbers of cycles was calculated. The capacity retention rate (%) after N cycles was calculated as follows: (Discharge capacity of the Nth cycle / Initial discharge capacity) × 100%, where N is the number of cycles.

[0127] The cycle parameters of implementation batteries 1 to 12 and control battery 1 at 25°C are shown in Table 2, and the cycle parameters at 45°C are shown in Table 3. The cycle parameters at 25°C characterize the cycle performance of the battery under normal temperature conditions, while the cycle parameters at 45°C characterize the cycle performance of the battery under high temperature conditions.

[0128] 2. Morphology test of the positive electrode sheet:

[0129] After cycling the experimental batteries 5, 11, and 1 (comparative example) at 25°C for 500 cycles, the batteries were disassembled to obtain the positive electrode sheets of Examples 5, 11, and 1 (comparative example). The positive electrode sheets were observed using a scanning electron microscope (SEM). The SEM image of the positive electrode sheet of Example 5 is shown below. Figure 2 As shown, the scanning electron microscope (SEM) image of the positive electrode sheet of Example 11 is as follows. Figure 3 As shown, the scanning electron microscope (SEM) image of the positive electrode of Comparative Example 1 is as follows. Figure 4 As shown.

[0130] 3. Impedance test:

[0131] The positive electrode sheets from Examples 5, 11, and Comparative Example 1 were assembled into batteries and subjected to relevant tests to obtain the AC impedance spectra of the positive electrode materials from Examples 5, 11, and Comparative Example 1. Figure 5 As shown.

[0132] Among them, the slope of the line in the low-frequency region of the AC impedance spectrum can characterize the Warburg impedance of the cathode material. Warburg impedance refers to the diffusion impedance in the electrochemical reaction. The smaller the slope of the line in the low-frequency region of the AC impedance spectrum, the smaller the Warburg impedance of the cathode material, and the faster the active ions diffuse in the cathode material.

[0133] The semicircles that appear in the mid-to-high frequency region of the AC impedance spectrum reflect the charge transfer resistance of the cathode material. The larger the semicircle, the greater the charge transfer resistance in the cathode material, that is, the slower the charge diffusion in the cathode material.

[0134] Table 2: Cyclic parameters of implementation batteries 1 to 12 and comparison battery 1 at 25°C.

[0135]

[0136] Table 3: Cycling parameters of experimental batteries 1 to 12 and comparison battery 1 at 45°C

[0137]

[0138]

[0139] As shown in Tables 1 to 3, the particle size D50 of the cathode materials in Examples 1 to 8 and Comparative Example 1 meets the range of 100nm ≤ D50 ≤ 1000nm. The sum of the thickness of the first coating layer and the thickness of the second coating layer is also within a reasonable range. The thickness of the first coating layer or the thickness of the second coating layer meets the range of 2nm to 5nm, and the total thickness meets the range of 4nm to 8.8nm, which gives the cathode material good conductivity. In addition, in the preparation process of the cathode material in Comparative Example 1, no etching solution was used to etch the first intermediate particles, which resulted in the cycle performance of the control battery 1 at room temperature being lower than that of the control batteries 1 to 8 at room temperature. The specific surface area of ​​the second intermediate particles of the cathode material in Comparative Example 1 is smaller than that of the second intermediate particles of the cathode materials in Examples 1 to 8, and the cycle performance of the control battery 1 at high temperature is lower than that of the control batteries 1 to 8 at high temperature. This is because: an appropriate amount of etching solution etches the first intermediate particle to obtain the second intermediate particle, thereby removing some positions or impurities on the surface of the first coating layer. This results in more pores, grooves, or uneven surfaces on the surface of the first coating layer, making the specific surface area of ​​the second intermediate particle larger than that of the first intermediate particle. This provides more active sites and wider ion transport channels for active ions, and also increases the charge transport rate. As can be seen from the table data, the specific surface area of ​​the second intermediate particle of the cathode material in Examples 1 to 8 is significantly increased. This results in higher capacity retention rates for Implemented Batteries 1 to 8 at both room temperature and high temperature. In other words, the cycle performance of Implemented Batteries 1 to 8 is better than that of Controlled Batteries 1. Therefore, by etching the first intermediate particle, the cycle performance of the battery at both room temperature and high temperature can be effectively improved. This allows the cathode material prepared by the preparation method provided in this application to have better overall performance when applied to a battery.

[0140] Understandably, in the embodiments of this application, the specific surface area of ​​the second intermediate particle satisfies the range: 11.7 m². 2 / g to 14.1m 2 / g. The specific surface area of ​​the second intermediate particle is within a reasonable range, that is, the specific surface area of ​​the second coating layer of the second intermediate particle is within a reasonable range, so as to provide more active sites and wider ion transport channels for active ions, and also increase the charge transport rate. When the positive electrode material is applied to the positive electrode sheet and assembled into a battery, the etched first coating layer facilitates the insertion and extraction of active ions, and also facilitates charge transport, thereby improving the active ion transport rate and charge transport rate, slowing down the capacity decay of the battery due to impedance during cycling, thereby improving the battery's capacity retention rate and giving the battery better cycle performance.

[0141] Further, referring to Examples 1 to 6, the concentration c of hydrogen ions in the etching solution all satisfy the range of 0.01 mol / L ≤ c ≤ 0.06 mol / L, and the etching time t0 all satisfy the range of 5 min ≤ t1 ≤ 60 min. With the etching time t0 remaining constant, as the concentration of hydrogen ions in the etching solution continuously increases, the specific surface area of ​​the second intermediate particles in Examples 1 to 6 gradually increases, and the capacity retention rate of Implemented Batteries 1 to 6 shows a trend of first increasing and then decreasing. This is because: as the concentration of hydrogen ions in the etching solution increases, the acidity of the etching solution gradually increases, and the etching solution can etch the first coating layer of the first intermediate particles to increase the number of active sites in the first coating layer and widen the ion transport channels, thereby gradually increasing the specific surface area of ​​the second intermediate particles of the cathode material in Examples 1 to 5, and thus gradually increasing the capacity retention rate of Implemented Batteries 1 to 5. However, as the concentration of hydrogen ions in the etching solution continuously increases, the capacity retention rate of the battery will not increase indefinitely. Conversely, if the concentration of hydrogen ions in the etching solution is too high, the etching solution may over-etch the first intermediate particle. That is, the etching solution may not only etch the first coating layer of the first intermediate particle but also damage the crystal structure of lithium iron phosphate within the first coating layer, thereby affecting the insertion / extraction of active ions, reducing the battery's cycle performance, and consequently decreasing the battery's capacity retention. Furthermore, if the specific surface area of ​​the second intermediate particle continues to increase after reaching a certain value, it may also increase side reactions between the electrolyte and the cathode material, thereby reducing the battery's cycle performance.

[0142] Further, referring to Examples 7 to 12, the concentration c of hydrogen ions in the etching solution all satisfy the range of 0.01 mol / L ≤ c ≤ 0.06 mol / L, and the etching time t0 all satisfy the range of 5 min ≤ t1 ≤ 60 min. With the concentration of hydrogen ions in the etching solution remaining constant, as the etching time t0 increases, the specific surface area of ​​the second intermediate particles of the cathode material in Examples 7 to 12 gradually increases, and the capacity retention rate of Embodiments 7 to 12 shows a trend of first increasing and then decreasing. This is because: as the etching time t0 increases, the etching solution can more fully etch the first coating layer of the first intermediate particles, thereby increasing the active sites of the first coating layer and widening the ion transport channels, thus gradually increasing the specific surface area of ​​the second intermediate particles of the cathode material in Examples 7 to 11, and thus gradually increasing the capacity retention rate of Embodiments 7 to 11. However, as the etching time t0 continues to increase, the capacity retention rate of the battery will not increase indefinitely. Conversely, if the etching time is too long, the etching solution may over-etch the first intermediate particle. That is, the etching solution may not only etch the first coating layer of the first intermediate particle but also damage the crystal structure of lithium iron phosphate within the first coating layer, thereby affecting the insertion / extraction of active ions, reducing the battery's cycle performance, and consequently decreasing the battery's capacity retention. Furthermore, if the specific surface area of ​​the second intermediate particle continues to increase after reaching a certain value, it may also increase side reactions between the electrolyte and the cathode material, thereby reducing the battery's cycle performance.

[0143] Please see Figures 2 to 4 After 500 cycles at room temperature, the positive electrode sheets of Example 5 and Example 11 still showed relatively intact morphology of the positive electrode material and relatively rounded particle size. However, cracks appeared on the surface of the positive electrode material in the positive electrode sheet of Comparative Example 1. This is because the positive electrode material prepared by the method provided in the embodiments of this application has better performance and can effectively resist the strain and structural damage caused during cycling. This is due to the etching treatment of the first intermediate particles by the etching solution, which makes it easier for active ions in the battery to diffuse in the positive electrode material, thereby reducing the impact of volume deformation on the structure of the positive electrode material and ultimately improving the cycle capacity retention rate of the battery.

[0144] Please see Figure 5In Examples 5 and 11, the positive electrode materials used in the positive electrode sheets were etched using the etching solution during the preparation process, while the positive electrode material used in Comparative Example 1 was not etched using the etching solution during the preparation process. Compared to Comparative Example 1, the slope of the oblique lines in the low-frequency region of Examples 5 and 11 is smaller, meaning that the Warburg impedance of the positive electrode material of Examples 5 and 11 is lower than that of the positive electrode material of Comparative Example 1, indicating that the active ions have a faster transport rate in the positive electrode materials of Examples 5 and 11. Similarly, compared to Comparative Example 1, the semicircles appearing in the mid-to-high frequency region of Examples 5 and 11 are smaller, indicating that the charge has a faster transport rate in the positive electrode materials of Examples 5 and 11. This is because, during the preparation of the cathode material in Example 5 and Example 11, the etching solution etches the first intermediate particles, resulting in a wider ion transport channel in the first coating layer. Simultaneously, the etching solution reduces impurities on the carbon material and lithium iron phosphate crystal surface of the first coating layer, thereby increasing the charge transport rate. The increased diffusion rate of active ions and the increased charge transport rate in the cathode material improve the capacity decay caused by impedance, thus improving the capacity retention rate. Furthermore, as the third carbon source is sintered in the second stage to form a dense second coating layer on the surface of the first coating layer, the side reactions caused by direct contact between lithium iron phosphate and the electrolyte are reduced, and the consumption of active lithium during cycling is reduced, further improving the cycle performance of the battery. Therefore, the capacity retention rates of Example 5 and Example 11 are greater than that of the control battery 1.

[0145] In this application, the terms "embodiment" and "implementation" mean that a specific feature, structure, or characteristic described in connection with an embodiment can be included in at least one embodiment of this application. The appearance of these phrases in various locations throughout the specification does not necessarily refer to the same embodiment, nor are they independent or alternative embodiments mutually exclusive with other embodiments. Those skilled in the art will understand, explicitly and implicitly, that the embodiments described in this application can be combined with other embodiments. Furthermore, it should be understood that the features, structures, or characteristics described in the various embodiments of this application can be arbitrarily combined to form another embodiment that does not depart from the spirit and scope of the technical solution of this application, provided there is no contradiction between them.

[0146] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application should not depart from the spirit and scope of the technical solutions of this application.

Claims

1. A method for preparing a positive electrode material, characterized in that, The preparation method includes: The raw materials are provided, mixed in a solvent, and dried to obtain a precursor. The raw materials include at least a lithium source, an iron source, a phosphorus source, and a first carbon source. The lithium source, iron source, and phosphorus source react to generate lithium iron phosphate. The precursor is placed in a second carbon source atmosphere for first-stage sintering to obtain first intermediate particles with a first coating layer. An etching solution is used to etch the first coating layer of the first intermediate particle to obtain a second intermediate particle, the specific surface area of ​​the second intermediate particle being greater than that of the first intermediate particle; wherein, the etching solution is an acidic solution, and the concentration c of hydrogen ions in the etching solution satisfies the range: 0.01 mol / L ≤ c ≤ 0.06 mol / L; the etching time for the first intermediate particle is t0, and t0 satisfies the range: 5 min ≤ t0 ≤ 60 min; the etching solution is selected from at least one of boric acid, formic acid, acetic acid, carbonic acid, and nitrous acid; the ratio α of the volume of the etching solution to the mass of the first intermediate particle satisfies the range: 4 mL / g ≤ α ≤ 6 mL / g; and The second intermediate particle is placed in a third carbon source atmosphere for a second stage of sintering to form a second coating layer on the surface of the first coating layer, thereby obtaining the cathode material.

2. The preparation method according to claim 1, characterized in that, After etching the first coating layer of the first intermediate particle with an etching solution, the method further includes washing the second intermediate particle with anhydrous ethanol.

3. The preparation method according to claim 1, characterized in that, The first carbon source is selected from at least one of sucrose, glucose, starch, graphite powder, carbon nanotubes, and acetylene black; And / or, the second carbon source is selected from at least one of ethanol, isopropanol, ethyl acetate, toluene, and cyclohexane; And / or, the third carbon source is selected from at least one of ethanol, isopropanol, ethyl acetate, toluene, and cyclohexane.

4. The preparation method according to claim 1, characterized in that, The ratio A1 of the mass of the first carbon source to the mass of the phosphorus source satisfies the range: 1% ≤ A1 ≤ 5%; And / or, the ratio A2 of the mass of the second carbon source to the mass of the phosphorus source satisfies the range: 0.5% ≤ A2 ≤ 2.5%; And / or, the ratio A3 of the mass of the third carbon source to the mass of the phosphorus source satisfies the range: 0.5% ≤ A3 ≤ 2.5%.

5. The preparation method according to claim 1, characterized in that, The step of placing the precursor in a second carbon source atmosphere for a first stage of sintering to obtain first intermediate particles with a first coating layer includes: The precursor is heated to a first temperature T1 at a heating rate v1 that meets the range of 3℃ / min≤v1≤10℃ / min for the first stage of sintering. The first temperature T1 meets the range of 400℃≤T1≤600℃. The sintering time t1 of the first stage meets the range of 10h≤t1≤16h.

6. The preparation method according to claim 1, characterized in that, The step of placing the second intermediate particles in a third carbon source atmosphere for a second-stage sintering process to obtain the cathode material includes: The second intermediate particles are heated at a rate v2 that meets the range of 3℃ / min≤v2≤10℃ / min, and then heated to a second temperature T2 for second-stage sintering. The second temperature T2 meets the range of 600℃≤T2≤800℃, and the second-stage sintering time t2 meets the range of 10h≤t2≤16h.

7. A positive electrode material, characterized in that, The cathode material is prepared by the preparation method according to any one of claims 1 to 6.

8. A battery, characterized in that, The battery includes: Electrolyte: A negative electrode sheet, wherein the negative electrode sheet is at least partially immersed in the electrolyte; A separator, located on one side of the negative electrode, and at least partially immersed in the electrolyte; and A positive electrode sheet is disposed on the side of the separator opposite to the positive electrode sheet and is at least partially immersed in the electrolyte. The positive electrode sheet includes a positive current collector and a positive electrode material layer. The positive electrode material layer is disposed on the surface of the positive current collector and includes the positive electrode material as described in claim 7. In the positive electrode material layer, the mass fraction a of the positive electrode material satisfies the range: 96% ≤ a ≤ 97.5%.

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