Positive electrode active material, method for preparing same, secondary battery, battery module, battery pack, and power device

Abstract

The invention provides a positive electrode active material, a secondary battery, a battery module, a battery pack, and an electric device. The positive electrode active material is used as a positive electrode active material for a secondary battery and comprises an iron-based polyanion compound compounded by a carbon material and an aluminum-containing oxide, the iron-based polyanion compound has the following general formula: Na4Fe3-xMxAly (PO4) 2P2O7 / C, M comprises a transition metal element, x is more than or equal to 0 and less than or equal to 0.5, 0lt, and x is more than or equal to 0 and less than or equal to 0.5. Yt; Yt; and 0.2 part. The positive electrode active material has relatively low residual alkali content, and the battery with the positive electrode active material has excellent cycle performance and rate capability.

Description

[0001] This application is a divisional application based on the invention with application number 202310075495.3, application date January 16, 2023, applicant CATL, and invention title "Positive electrode active material, preparation method thereof, secondary battery, battery module, battery pack and power device". Technical Field

[0002] This application relates to the field of secondary battery technology, and in particular to a positive electrode active material, a method for preparing the same, a secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0003] Sodium batteries have great potential for large-scale energy storage due to their abundant reserves, low price, and wide operating temperature range.

[0004] Iron-based polyanionic compounds are popular cathode materials for novel sodium batteries, possessing significant advantages such as abundant resources, high operating voltage, good cycle stability, and environmental friendliness. However, current sodium batteries using iron-based polyanionic compounds as cathode materials still suffer from problems such as low electronic conductivity, low discharge capacity, and poor cycle performance, failing to meet the application needs of next-generation electrochemical systems. Summary of the Invention

[0005] This application is made in view of the above-mentioned issues, and its purpose is to provide a positive electrode active material in which aluminum is introduced to achieve the dual effects of lattice doping and composite modification, thereby reducing the residual alkali content of the positive electrode active material and improving the cycle performance and rate performance of the battery.

[0006] A first aspect of this application provides a positive electrode active material for secondary batteries, comprising an iron-based polyanionic compound composited with carbon materials and an aluminum-containing oxide, wherein the iron-based polyanionic compound has the following general formula: Na₄Fe 3-x M x Al y (PO4)2P2O7 / C, where M contains transition metal elements, 0≤x≤0.5, 0 <y<0.2。

[0007] Introducing aluminum into the positive electrode active material has two main advantages. First, some aluminum can be doped into the crystal lattice of the iron-based polyanion compound, replacing iron or transition metal elements and thus improving the ionic and electronic conductivity, enhancing its ion-conducting and electron-conducting properties. Second, some aluminum is distributed on the surface of the iron-based polyanion compound as aluminum oxides, reducing direct contact between the material surface and moisture in the environment, lowering residual alkali content, and improving the material's stability and processability. Furthermore, this positive electrode active material exhibits high specific capacity, which is beneficial for increasing the energy density of the battery. Introducing carbon materials into the positive electrode active material can effectively improve its conductivity and battery performance.

[0008] In any embodiment, the aluminum oxide is distributed on at least a portion of the surface of the primary particles of the iron-based polyanionic compound.

[0009] Aluminum oxides are distributed on part or all of the primary surface of the iron-based polyanionic compound particles, which can effectively reduce the direct contact between the surface of the positive electrode active material and the moisture in the environment, reduce the residual alkali content of the positive electrode active material, and improve the stability and processability of the positive electrode active material.

[0010] In any embodiment, based on the total mass of the positive electrode active material, the mass content of aluminum in the positive electrode active material is 0.03% to 0.9%.

[0011] Based on the total mass of the positive electrode active material, controlling the mass content of aluminum in the positive electrode active material to be 0.03%~0.9% is beneficial to improving the conductivity of the positive electrode active material, reducing the residual alkali on the material surface, improving the stability and processability of the positive electrode active material, increasing the specific capacity of the positive electrode active material and its capacity retention rate at the 3C rate of the battery, and improving the capacity and rate performance of the battery.

[0012] In any embodiment, M comprises one or more of Ni, Co, Mn, Cu, V, Ti, Mo, Nb, W, Cr, Zn, Zr, and Ca, and may be selected from one or more of Ni, Co, Mn, Cu, V, and Ca.

[0013] Introducing transition metal elements into iron-based polyanionic compounds can improve the voltage plateau of the positive electrode active material, enhance the ionic and electronic conductivity of the material, reduce battery polarization, and improve battery cycle performance and rate performance.

[0014] In any embodiment, M comprises at least two of Ni, Co, Mn, Cu, V, and Ca.

[0015] Controlling M to include at least two of Ni, Co, Mn, Cu, V, and Ca is beneficial for further improving the rate performance of the battery.

[0016] In any embodiment, the carbon material is coated on the surface of the iron-based polyanionic compound in the form of a carbon film, or the carbon material is distributed in the form of particles between the primary particles of the iron-based polyanionic compound.

[0017] Carbon materials can be coated on the surface of iron-based polyanionic compounds in the form of carbon films or distributed between the primary particles of iron-based polyanionic compounds in the form of carbon particles. Both methods can effectively improve the conductivity of the positive electrode active material and the battery performance.

[0018] In any embodiment, the carbon material is one or more of amorphous carbon, conductive carbon black, carbon nanotubes, and graphene.

[0019] The aforementioned carbon materials are easy to disperse and process, forming carbon composites with iron-based polyanionic compounds, thereby improving the conductivity of the positive electrode active material and battery performance. Among these, since conductive carbon black, carbon nanotubes, or graphene have higher crystallinity than amorphous carbon, carbon materials containing conductive carbon black, carbon nanotubes, or graphene exhibit better conductivity than carbon materials containing amorphous carbon.

[0020] In any embodiment, based on the total mass of the positive electrode active material, the mass content of the carbon material is 0.5% to 6%, optionally 1% to 3.6%.

[0021] Based on the total mass of the cathode active material, controlling the carbon content to 0.5%~6% is beneficial for achieving a balance between the specific capacity and conductivity of the cathode active material, while simultaneously obtaining higher specific capacity and better cycle performance and rate performance. Too low a carbon content leads to poor conductivity, which is detrimental to capacity volatilization; conversely, too high a carbon content results in a decrease in specific capacity. Based on the total mass of the cathode active material, controlling the carbon content to 1%~3.6% is beneficial for further improving the specific capacity of the cathode active material and thus further increasing the battery capacity.

[0022] In any embodiment, the specific capacity of the positive electrode active material is not less than 98 mAh / g.

[0023] Positive electrode active materials with a capacity of not less than 98 mAh / g are beneficial to improving the capacity of batteries and broadening their applications.

[0024] In any embodiment, based on the total mass of the positive electrode active material, the residual NaHCO3 alkali content on the surface of the positive electrode active material is less than 1.2%.

[0025] Based on the total mass of the positive electrode active material, the residual NaHCO3 alkali content on the surface of the positive electrode active material is less than 1.2%, which can effectively improve its stability and processability.

[0026] A second aspect of this application provides a method for preparing a positive electrode active material for secondary batteries, comprising the following steps: Raw materials containing iron, sodium, phosphorus, aluminum and carbon sources are dissolved in water to obtain a mixed slurry. The raw materials may optionally include source M, which is a salt containing a transition metal. The mixed slurry is dried and then calcined to prepare the positive electrode active material. The positive electrode active material comprises an iron-based polyanionic compound and an aluminum-containing oxide. The iron-based polyanionic compound has the following general formula: Na4Fe 3-x M x Al y (PO4)2P2O7 / C Where M contains transition metal elements, 0 ≤ x ≤ 0.5, 0 <y<0.2。

[0027] The preparation method of the above-mentioned positive electrode active material is simple and has low production cost. The prepared positive electrode active material has a low residual alkali content and a high specific capacity, and the battery exhibits excellent cycle performance and rate performance.

[0028] In some implementations, 0 <x<0.5。

[0029] The introduction of transition metals into positive electrode active materials is beneficial to improving the ionic conductivity and electronic conductivity of the materials, thereby enhancing the cycle performance and rate performance of the battery.

[0030] In any embodiment, the step of drying the mixed slurry and then calcining it includes the following steps: The mixed slurry was dried to obtain precursor powder; The precursor powder is calcined in steps. The first step of calcination is carried out at a temperature of 300 ℃ to 400 ℃ for 3 h to 6 h, and the second step of calcination is carried out at a temperature of 500 ℃ to 600 ℃ for 8 h to 15 h to prepare the positive electrode active material.

[0031] Stepwise calcination facilitates the full reaction of precursor powders, improves the crystallinity and phase purity of the prepared cathode active material, reduces the residual alkali content on the surface of the cathode active material, improves the stability and processability of the cathode active material, increases the specific capacity of the cathode active material, as well as its battery cycle capacity retention rate and capacity retention rate at 3C rate, thereby improving the battery's capacity, cycle performance, and rate performance.

[0032] In any embodiment, the calcination temperature in the second step is 525 ℃~575 ℃, and can be selected as 550 ℃~575 ℃.

[0033] Controlling the second-step calcination temperature to 525℃~575℃ helps to further reduce the residual alkali content on the surface of the active material, improve the conductivity of the material, and enhance the cycle performance and rate performance of the battery. Further controlling the second-step calcination temperature to 550℃~575℃ helps to further improve the capacity retention rate after 200 cycles and at 3C rate, thus further enhancing the battery's cycle performance and rate performance.

[0034] In any embodiment, the calcination time in the second step is 10 h to 14 h.

[0035] Controlling the calcination time in the second step to 10 h to 14 h is beneficial for obtaining high-crystallinity and high-purity positive electrode active materials. At the same time, by controlling the heat preservation time, the particle size of the primary crystals and the amount of residual alkali on the material surface can be controlled, thereby improving the specific capacity of the material and the cycle rate performance of the battery.

[0036] In any embodiment, the M source includes one or more of nickel nitrate, nickel acetate, cobalt nitrate, manganese nitrate, cobalt acetate, manganese acetate, nickel oxalate, cobalt oxalate, manganese oxalate, nickel oxide, cobalt oxide, and manganese oxide.

[0037] In any embodiment, the aluminum source includes one or more of aluminum nitrate, aluminum acetylacetone, aluminum acetate, aluminum hydroxide, and aluminum oxide.

[0038] In any embodiment, the carbon source includes one or more of sucrose, tannic acid, polyethylene glycol, polyacrylonitrile, cellulose, polyvinylpyrrolidone, sucrose, oxalic acid, glucose, ascorbic acid, polyethylene, citric acid, conductive carbon black, carbon nanotubes, and graphene.

[0039] The aforementioned carbon sources, under high-temperature calcination, readily generate carbon materials that coat the surface of iron-based polyanionic compounds or are distributed among the primary particles of the iron-based polyanionic compounds, thereby improving the conductivity of the positive electrode active material. Due to the influence of calcination temperature, amorphous carbon formed from one or more carbon sources, including sucrose, tannic acid, polyethylene glycol, polyacrylonitrile, cellulose, polyvinylpyrrolidone, sucrose, oxalic acid, glucose, ascorbic acid, polyethylene, citric acid, conductive carbon black, carbon nanotubes, and graphene, exhibits lower crystallinity. In contrast, carbon materials formed from one or more carbon sources, including conductive carbon black, carbon nanotubes, and graphene, have higher crystallinity. Therefore, carbon materials formed from one or more carbon sources, including conductive carbon black, carbon nanotubes, and graphene, possess superior conductivity.

[0040] A third aspect of this application provides a secondary battery, including a positive electrode sheet, wherein the positive electrode sheet comprises the positive active material described in any embodiment or the positive active material prepared by the preparation method described in any embodiment.

[0041] In any embodiment, the secondary battery is a sodium battery without a negative electrode.

[0042] In any embodiment, the secondary battery further includes a negative electrode sheet, the negative electrode sheet including a negative current collector and a base coating disposed on at least one surface of the negative current collector, the base coating including one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles (Ag / C), and tin composite carbon nanoparticles (Sn / C).

[0043] The aforementioned base coating not only has excellent conductivity, but also facilitates the uniform deposition of metal ions on the current collector surface, thereby improving the battery's cycle performance and safety.

[0044] In any embodiment, the areal density of the base coating is 5 g / m³. 2 ~50 g / m 2 .

[0045] Surface density is 5 g / m 2 ~50 g / m 2 The base coating facilitates the uniform distribution of nucleation sites, promotes uniform metal deposition, and does not affect electron transport behavior.

[0046] In any embodiment, the thickness of the base coating is 2 μm to 100 μm.

[0047] Controlling the thickness of the base coating to 2 μm~100 μm can provide enough nucleation sites to facilitate the uniform deposition of metal ions and suppress dendrites.

[0048] A fourth aspect of this application provides a battery module, including the secondary battery of the third aspect of this application.

[0049] The fifth aspect of this application provides a battery pack, including a secondary battery according to the third aspect of this application or a battery module according to the fourth aspect of this application.

[0050] The sixth aspect of this application provides an electrical device, including at least one of the secondary battery of the third aspect of this application, the battery module of the fourth aspect of this application, and the battery pack of the fifth aspect of this application. Attached Figure Description

[0051] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application; Figure 2 yes Figure 1An exploded view of a secondary battery according to an embodiment of this application is shown. Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application; Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application; Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown; Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0052] Explanation of reference numerals in the attached figures: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0053] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its preparation method, secondary battery, battery module, battery pack, and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0054] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0055] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0056] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0057] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0058] Unless otherwise specified, the terms "comprising" and "including" as used in this application 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.

[0059] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0060] Polyanionic compounds have become a popular cathode material for sodium batteries due to their abundant resources, environmental friendliness, ease of large-scale production, open sodium ion diffusion channels, and good thermal and cycle stability. However, in actual production, researchers have found that polyanionic compound materials have a high residual alkali content on their surface, and exposure to air easily leads to an increase in residual alkali, causing a deterioration in specific capacity and thus affecting battery performance. Therefore, there is a need to develop a cathode active material with low residual alkali content, excellent electrochemical performance, and applicability to high-rate batteries to meet the application needs of next-generation electrochemical systems.

[0061] [Positive electrode active material] Based on this, this application proposes a positive electrode active material for secondary batteries, which comprises an iron-based polyanionic compound composited with carbon materials and an aluminum-containing oxide. The iron-based polyanionic compound has the following general formula: Na₄Fe 3- x M x Al y (PO4)2P2O7 / C, where M contains transition metal elements, 0≤x≤0.5, 0 <y<0.2。

[0062] In this article, the term "transition metal element" refers to elements in groups IIIB to VIIB and VIII of the periodic table, as well as elements in groups IB to IIB.

[0063] In some implementations, x can be selected as 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5.

[0064] In some implementations, y can be selected as 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18 or 0.19.

[0065] Introducing aluminum into the positive electrode active material has two main advantages. First, some aluminum can be doped into the crystal lattice of the iron-based polyanion compound, replacing iron or transition metal elements and thus improving the electronic conductivity and electrical performance of the positive electrode active material. Second, some aluminum is distributed on the surface of the iron-based polyanion compound in the form of aluminum oxide, reducing direct contact between the material surface and moisture in the environment, lowering the residual alkali content, and improving the stability and processability of the positive electrode active material. Furthermore, this positive electrode active material also exhibits a high specific capacity, which is beneficial for increasing the energy density of the battery. Introducing carbon materials into the positive electrode active material can effectively improve its conductivity and battery performance.

[0066] In this article, the term "specific capacity" refers to the amount of electricity released per gram of positive electrode active material, which reflects the capacity of the battery and can be tested by any known method.

[0067] In some embodiments, aluminum oxide is distributed on at least a portion of the surface of the primary particles of the iron-based polyanionic compound.

[0068] In some embodiments, the aluminum oxide contains aluminum oxide.

[0069] In this paper, the term "primary particle" refers to the unaggregated particles of the iron-based polyanionic compound, with a particle size ranging from 30 nm to 120 nm.

[0070] In some embodiments, aluminum oxide is distributed on a portion of the surface of the primary particles of the iron-based polyanionic compound.

[0071] In some embodiments, aluminum oxide is distributed on the entire surface of the primary particles of the iron-based polyanionic compound.

[0072] Aluminum oxides are distributed on part or all of the surface of primary particles of iron-based polyanionic compounds, which can effectively reduce the direct contact between the surface of the positive electrode active material and the moisture in the environment, reduce the residual alkali content of the positive electrode active material, and improve the stability and processability of the positive electrode active material.

[0073] In some embodiments, the mass content of aluminum in the positive electrode active material is 0.03% to 0.9% based on the total mass of the positive electrode active material. In some embodiments, the mass content of aluminum in the positive electrode active material can be selected as 0.03%, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, or 0.9% based on the total mass of the positive electrode active material.

[0074] In some embodiments, the aluminum element in the positive electrode active material includes aluminum element in iron-based polyanionic compounds and aluminum element in aluminum-containing oxides.

[0075] The mass content of aluminum in the positive electrode active material can be tested using any method known in the art. For example, standards YS / T 1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015 can be referenced. Specifically, an inductively coupled plasma atomic emission spectrometer (Inductively Coupled Plasma Atomic Emission Spectrometry) (Thermo, ICAP7400; PerkinElmer, Avio200) can be used to determine the luminescence intensity of aluminum in the sample. Standard samples with different aluminum contents can be prepared, and their luminescence intensities can be measured to create a standard curve. The mass content of aluminum in the sample can then be determined from the standard curve based on the luminescence intensity of the aluminum in the sample.

[0076] Based on the total mass of the positive electrode active material, controlling the mass content of aluminum in the positive electrode active material to be 0.03%~0.9% is beneficial to reduce the residual alkali content of the positive electrode active material, improve the stability and processability of the positive electrode active material, increase the specific capacity of the positive electrode active material and its capacity retention rate at 3C rate, and improve the capacity and rate performance of the battery.

[0077] In some embodiments, M comprises one or more of Ni, Co, Mn, Cu, V, Ti, Mo, Nb, W, Cr, Zn, Zr, and Ca, and may be selected from one or more of Ni, Co, Mn, Cu, V, and Ca.

[0078] In some embodiments, M comprises Ni. In some embodiments, M comprises Co. In some embodiments, M comprises Mn. In some embodiments, M comprises Ca. In some embodiments, M comprises Cu.

[0079] Introducing transition metal elements into iron-based polyanionic compounds can improve the electronic and ionic conductivity of the positive electrode active material, reduce battery polarization, and enhance the battery's cycle and rate performance.

[0080] In some implementations, M comprises at least two of Ni, Co, Mn, Cu, V, and Ca.

[0081] In some embodiments, M comprises Ni and Mn. In some embodiments, M comprises Mn and Co. In some embodiments, M comprises Ni and Co. In some embodiments, M comprises Ni and Ca. In some embodiments, M comprises Mn, Ni, and Co.

[0082] Controlling M to include at least two of Ni, Co, Mn, Cu, V, and Ca is beneficial for further improving the rate performance of the battery.

[0083] In some embodiments, carbon material is coated on the surface of the iron-based polyanionic compound in the form of a carbon film, or carbon material is distributed in the form of particles between the primary particles of the iron-based polyanionic compound.

[0084] In some embodiments, carbon material is coated on the surface of the iron-based polyanionic compound in the form of a carbon film.

[0085] In some embodiments, the carbon material is distributed in particulate form between the primary particles of the iron-based polyanionic compound.

[0086] Carbon materials can be coated on the surface of iron-based polyanionic compounds in the form of carbon films or distributed between the primary particles of iron-based polyanionic compounds in the form of carbon particles. Both methods can effectively improve the conductivity of the positive electrode active material and the battery performance.

[0087] In some embodiments, the carbon material includes one or more of amorphous carbon, conductive carbon black, carbon nanotubes, and graphene.

[0088] In some embodiments, the carbon material includes amorphous carbon. In some embodiments, the carbon material includes conductive carbon black. In some embodiments, the carbon material includes carbon nanotubes. In some embodiments, the carbon material includes graphene.

[0089] The aforementioned carbon materials readily combine with iron-based polyanionic compounds, improving the conductivity of the positive electrode active material. Among these, since conductive carbon black, carbon nanotubes, or graphene have higher crystallinity than amorphous carbon, carbon materials containing conductive carbon black, carbon nanotubes, or graphene exhibit better conductivity than carbon materials containing amorphous carbon.

[0090] In some embodiments, the mass content of carbon material, based on the total mass of the positive electrode active material, is 0.5% to 6%, and optionally 1% to 3.6%. In some embodiments, the mass content of carbon material, based on the total mass of the positive electrode active material, can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 3.6%, 4%, 4.5%, 5%, 5.5%, or 6%.

[0091] Based on the total mass of the positive electrode active material, controlling the carbon content to 0.5%~6% is beneficial for improving the conductivity of the positive electrode active material, while ensuring a high specific capacity, thus improving the battery's capacity, cycle performance, and rate performance. Too low a carbon content leads to poor conductivity, hindering capacity volatilization, while too high a carbon content results in a decrease in specific capacity. Based on the total mass of the positive electrode active material, controlling the carbon content to 1%~3.6% is beneficial for further improving the specific capacity of the positive electrode active material, thereby further increasing the battery's capacity.

[0092] In some implementations, the specific capacity of the positive electrode active material is not less than 98 mAh / g.

[0093] In some embodiments, the specific capacity of the positive electrode active material can be selected as 98mAh / g, 100mAh / g, 105mAh / g, 110mAh / g, 115mAh / g, 120mAh / g, 125mAh / g or 129mAh / g.

[0094] The specific capacity of the positive electrode active material can be tested using any method known in the art. As an example, at 25°C and normal pressure, a coin cell is charged at a constant current rate of 0.1C to a voltage of 4V, then charged at a constant voltage of 4V until the current drops to 0.05C. The charging specific capacity at this point is recorded as the initial sodium deintercalation capacity. Subsequently, it is discharged at a constant current rate of 0.1C to a voltage of 1.5V, and the discharging specific capacity at this point is recorded as the initial sodium intercalation capacity. The specific capacity of the positive electrode active material is thus the initial sodium intercalation capacity.

[0095] Positive electrode active materials with a capacity of not less than 98 mAh / g are beneficial to improving the capacity of batteries and broadening their applications.

[0096] In some implementations, the residual NaHCO3 content on the surface of the positive electrode active material is less than 1.2% based on the total mass of the positive electrode active material.

[0097] In some embodiments, based on the total mass of the positive electrode active material, the residual NaHCO3 content on the surface of the positive electrode active material can be selected as 0.01%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, or 1.15%.

[0098] The residual NaHCO3 content of the positive electrode active material can be tested using any method known in the art. As an example, it can be determined by chemical reagent potentiometric titration, for instance, referring to standard GB / T 9725-2007. Specifically, a Metrohm 905 Titrando potentiometric titrator can be used. The positive electrode active material is dissolved in deionized water, and a standard titrant is used to titrate the aqueous solution of the positive electrode active material. For each 0.1 ml of standard titrant solution added, the potential or pH value is recorded. Titration is stopped when the change in potential or pH value is minimal. The volume of the added standard titrant solution and the measured potential or pH value are recorded. The titration endpoint is determined using a graphical method or a second derivative method, and the titration volume of the standard solution is determined. The calculated mass of NaHCO3 is divided by the mass of the positive electrode active material to obtain the residual NaHCO3 content of the positive electrode active material.

[0099] Based on the total mass of the positive electrode active material, the residual NaHCO3 content of the positive electrode active material is less than 1.2%, which can effectively improve its stability and processability.

[0100] This application also proposes a method for preparing a positive electrode active material for secondary batteries, comprising the following steps: Raw materials containing iron, sodium, phosphorus and aluminum sources are dissolved in water to obtain a mixed slurry. The raw materials may optionally include source M, which is a salt containing a transition metal. The mixed slurry is dried and then calcined to prepare the positive electrode active material. The positive electrode active material includes an iron-based polyanionic compound and an aluminum-containing oxide. The iron-based polyanionic compound has the following general formula: Na4Fe 3-x M x Al y (PO4)2P2O7 / C Where M contains transition metal elements, 0 ≤ x ≤ 0.5, 0 <y<0.2。

[0101] The preparation method of the above-mentioned positive electrode active material is simple and has low production cost. The prepared positive electrode active material has a low residual alkali content and a high specific capacity, and the battery exhibits excellent cycle performance and rate performance.

[0102] In some implementations, 0 <x≤0.5。

[0103] The introduction of transition metals into positive electrode active materials is beneficial to improving the ionic conductivity and electronic conductivity of the materials, thereby enhancing the cycle performance and rate performance of the battery.

[0104] In some embodiments, calcining the mixed slurry after drying includes the following steps: The mixed slurry was dried to obtain precursor powder; The precursor powder was calcined in steps. The first step was calcined at 300 ℃~400 ℃ for 3 h~6 h, and the second step was calcined at 500 ℃~600 ℃ for 8 h~15 h to prepare the positive electrode active material.

[0105] Stepwise calcination facilitates the full reaction of precursor powders, improves the crystallinity and phase purity of the prepared cathode active material, reduces the residual alkali content on the surface of the cathode active material, improves the stability and processability of the cathode active material, increases the specific capacity of the cathode active material, and improves the battery cycle performance and rate performance.

[0106] In some embodiments, the calcination temperature in the second step is 525 ℃~575 ℃, and can be selected as 550 ℃~575 ℃.

[0107] Controlling the second-step calcination temperature to 525℃~575℃ helps to further reduce the residual alkali content on the surface of the active material, improve the conductivity of the material, and enhance the cycle performance and rate performance of the battery. Further controlling the second-step calcination temperature to 550℃~575℃ further enhances the cycle performance and rate performance of the battery to an even greater extent.

[0108] In some implementations, the second calcination step takes 10 h to 14 h.

[0109] Controlling the second-step calcination time to 10 h to 14 h is beneficial for obtaining high-crystallinity and high-purity positive electrode active materials. At the same time, by controlling the heat preservation time, the particle size of the primary crystals and the amount of residual alkali on the material surface can be controlled, thereby improving the specific capacity of the material and the cycle rate performance of the battery.

[0110] In some embodiments, the M source includes one or more of nickel nitrate, nickel acetate, cobalt nitrate, manganese nitrate, cobalt acetate, manganese acetate, nickel oxalate, cobalt oxalate, manganese oxalate, nickel oxide, cobalt oxide, and manganese oxide.

[0111] In some embodiments, the aluminum source includes one or more of aluminum nitrate, aluminum acetylacetone, aluminum acetate, aluminum hydroxide, and aluminum oxide.

[0112] In some embodiments, the carbon source includes one or more of sucrose, tannic acid, polyethylene glycol, polyacrylonitrile, cellulose, polyvinylpyrrolidone, sucrose, oxalic acid, glucose, ascorbic acid, polyethylene, citric acid, conductive carbon black, carbon nanotubes, and graphene.

[0113] In some embodiments, the carbon source includes one or more of sucrose, tannic acid, polyethylene glycol, polyacrylonitrile, cellulose, polyvinylpyrrolidone, sucrose, oxalic acid, glucose, ascorbic acid, polyethylene, and citric acid.

[0114] In some implementations, the carbon source includes one or more of conductive carbon black, carbon nanotubes, and graphene.

[0115] The aforementioned carbon sources, when calcined at high temperatures, readily produce carbon materials that coat the surface of iron-based polyanionic compounds or distribute between the primary particles of the iron-based polyanionic compounds, thereby improving the conductivity of the positive electrode active material. However, due to the influence of calcination temperature, amorphous carbon formed from organic carbon sources has lower crystallinity, while carbon materials formed from one or more inorganic carbon sources, including conductive carbon black, carbon nanotubes, and graphene, have higher crystallinity. Therefore, carbon materials formed from one or more carbon sources, including conductive carbon black, carbon nanotubes, and graphene, exhibit superior conductivity.

[0116] [Positive electrode plate] The positive electrode includes a positive current collector and a positive active material layer formed on at least a portion of the surface of the positive current collector, the positive active material layer including the positive active material in some embodiments.

[0117] The positive electrode active material layer may also include a conductive agent to improve the conductivity of the positive electrode. The conductive agent may be one or more of SuperP, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene, and carbon nanofibers.

[0118] The positive electrode active material layer may also include a binder to firmly bond the positive electrode active material and optional conductive agent to the positive electrode current collector. The binder may be at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyvinyl alcohol (PVA), ethylene-vinyl acetate copolymer (EVA), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), sodium alginate (SA), polymethacrylic acid (PMA), and carboxymethyl chitosan (CMCS).

[0119] The positive electrode current collector can be made of conductive carbon sheet, metal foil, carbon-coated metal foil, porous metal plate, or composite current collector. The conductive carbon material of the conductive carbon sheet can be selected from one or more of Super P, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene, and carbon nanofibers. The metal material of the metal foil, carbon-coated metal foil, and porous metal plate can be independently selected from at least one of copper, aluminum, nickel, and stainless steel. The composite current collector can be a composite current collector formed by combining metal foil with a polymer base film.

[0120] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0121] [Isolation membrane] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0122] In some embodiments, the material of the separator can be selected from at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0123] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0124] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0125] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0126] [Rechargeable Battery] A secondary battery includes a positive electrode sheet, which includes a positive electrode active material in some embodiments or a positive electrode active material prepared by a preparation method in some embodiments.

[0127] In some embodiments, the secondary battery also includes a negative electrode, a separator, and an electrolyte.

[0128] In some implementations, the secondary battery is a sodium battery without a negative electrode.

[0129] The anode-less sodium battery does not pre-deposit negative electrode active material, but only contains a negative electrode current collector. During the first charge, sodium ions gain electrons on the anode side and deposit metallic sodium on the surface of the current collector to form a sodium metal phase. During discharge, the metallic sodium can be converted back into sodium ions and return to the positive electrode, achieving cycle charging and discharging. Compared with other sodium rechargeable batteries, the anode-less sodium battery can achieve a higher energy density because it is not limited by the negative electrode material. The anode-less sodium battery does not have enough sodium metal as a negative electrode material to provide sufficient sodium element to the battery. Therefore, the application of the positive electrode active material provided in this application in the anode-less sodium battery can more effectively improve its cycle performance and high-rate performance.

[0130] In some implementations, the CB value of a sodium-free battery is less than or equal to 0.1.

[0131] The CB value is the capacity per unit area of ​​the negative electrode in a secondary battery divided by the capacity per unit area of ​​the positive electrode. Since batteries without a negative electrode do not contain negative electrode active material, the capacity per unit area of ​​the negative electrode is relatively small, and the CB value of the secondary battery is less than or equal to 0.1.

[0132] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil or copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0133] In some embodiments, the negative electrode sheet includes a negative current collector and a base coating disposed on at least one surface of the negative current collector, the base coating including one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, and tin composite carbon nanoparticles.

[0134] The aforementioned base coating not only has excellent conductivity, but also facilitates the uniform deposition of metal ions on the surface of the negative electrode current collector in a negative electrode-less sodium battery, thereby improving the battery's cycle performance and safety.

[0135] In some embodiments, the areal density of the base coating is 5 g / m³. 2 ~50 g / m 2 .

[0136] In some embodiments, the areal density of the base coating may be selected as 5 g / m³. 2 10 g / m 2 15 g / m 2 20 g / m 225g / m 2 30 g / m 2 35 g / m 2 40 g / m 2 45 g / m 2 Or 50 g / m 2 .

[0137] In some embodiments, the thickness of the base coating is 2 μm to 100 μm.

[0138] In some embodiments, the thickness of the base coating can be selected as 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm or 100 μm.

[0139] The aforementioned base coating has low surface density and thin thickness, making it unsuitable for use as a negative electrode active material. However, in a negative electrode-free sodium battery, it can be used as a base coating to reduce the overpotential for sodium nucleation, which is beneficial for the uniform deposition of metal ions and suppresses dendrite formation.

[0140] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 Here is an example of a square-structured secondary battery, 5. Figure 2 This is an exploded view of secondary battery 5.

[0141] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 via a winding or stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. A non-Newtonian fluid electrolyte composition is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0142] [Battery Module] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0143] Figure 3 This is battery module 3, shown as an example. (See reference...) Figure 3In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0144] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0145] [Battery Pack] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0146] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0147] [Electrical appliances] In one embodiment of this application, an electrical device is provided, including at least one of a secondary battery, a battery module, or a battery pack according to any embodiment.

[0148] Electrical devices include at least one of the secondary batteries, battery modules, or battery packs provided in this application. The secondary batteries, battery modules, or battery packs can be the power source of the electrical device or the energy storage unit of the electrical device. Electrical devices may include, but are not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0149] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0150] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0151] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0152] Example The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0153] I. Preparation Method Example 1 1) Preparation of positive electrode active materials 0.1 mol sodium pyrophosphate, 0.3 mol ferrous oxalate, 0.2 mol ammonium dihydrogen phosphate, 0.01 mol glucose, and 0.005 mol aluminum nitrate were added to 200 ml of deionized water and ground in a sand mill for 10 h to obtain a mixed slurry. The mixed slurry was then dried using a spray dryer to obtain a dried precursor powder. The precursor was placed in a tube furnace, and nitrogen was used as a protective gas for the first calcination step, which was performed at 300 °C for 4 h. The second calcination step was then performed at 550 °C for 12 h to obtain the positive electrode active material.

[0154] 2) Preparation of positive electrode sheet Dissolve 10 wt% polyvinylidene fluoride adhesive thoroughly in N In methylpyrrolidone (NMP), 10 wt% carbon black conductive agent and 80 wt% of the above-mentioned positive electrode active material are added and mixed evenly to obtain a positive electrode slurry. The slurry is evenly coated on the surface of the current collector aluminum foil and then transferred to a vacuum drying oven for complete drying. The dried electrode is then rolled and punched to obtain the positive electrode sheet.

[0155] 3) Preparation of negative electrode sheet Carbon nanotubes and carboxymethyl cellulose (CMC) were added to deionized water at a mass ratio of 1:0.4 and stirred to form a homogeneous slurry. This slurry was then coated onto the surface of the copper foil used as the negative electrode current collector. Afterward, it was transferred to a vacuum drying oven for complete drying, followed by die cutting. The thickness of the base coating was 20 μm, and the areal density was 25 g / m³. 2 This yields a negative electrode sheet without a negative electrode structure.

[0156] 4) Electrolyte In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), sodium hexafluorophosphate (NaPF6) was dissolved in the organic solvent dimethyl ethylene glycol (DME) and stirred until homogeneous to obtain an electrolyte with a sodium salt concentration of 1 mol / L.

[0157] 5) Separating membrane Polypropylene film is used as the separator.

[0158] 6) Preparation of button cells A coin cell was assembled by using a sodium metal sheet as the counter electrode, employing a Celgard 2400 separator, and injecting electrolyte.

[0159] 7) Preparation of full cells The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrolyte is then added to assemble the stacked battery.

[0160] The sodium metal battery product without a negative electrode of Example 1 was obtained.

[0161] Examples 2-6 The batteries in Examples 2-6 were prepared using a similar method to those in Example 1, but the mass content of carbon in the positive electrode active material was adjusted. The specific parameters are shown in Table 1.

[0162] Example 7 The battery in Example 7 is prepared using a method similar to that in Example 1, but the preparation method of the positive electrode active material is adjusted, and the preparation method is as follows: 0.1 mol sodium pyrophosphate, 0.28 mol ferrous oxalate, 0.2 mol ammonium dihydrogen phosphate, 0.04 mol glucose, 0.02 mol manganese oxalate (source M), and 0.005 mol aluminum nitrate were added to 200 ml of deionized water and ground in a sand mill for 10 h to obtain a mixed slurry. The mixed slurry was dried using a spray dryer to obtain a dried precursor powder. The precursor was placed in a tube furnace, and nitrogen was used as a protective gas. The temperature was raised to 300 ℃ and held for 4 h, then raised to 550 ℃ and held for 12 h to obtain the positive electrode active material.

[0163] Examples 8-12 The batteries in Examples 8-12 were prepared using a method similar to that in Example 1, but the x value of the Mn element in the iron-based polyanionic compound was adjusted. The specific parameters are shown in Table 1.

[0164] Examples 13-17 The batteries in Examples 13-17 were prepared using methods similar to those in Example 7, but the mass content of aluminum in the positive electrode active material was adjusted. The specific parameters are shown in Table 1.

[0165] Examples 18-25 The batteries in Examples 18-25 were prepared using methods similar to those in Example 7, but the calcination temperature and calcination time in the second step were adjusted. The specific parameters are shown in Table 1.

[0166] Examples 26-31 The batteries in Examples 26-31 are prepared using methods similar to those in Example 7, but the types and combinations of transition metal elements in the iron-based polyanionic compound are adjusted. The specific parameters are shown in Table 1.

[0167] Comparative Example 1 The battery in Comparative Example 1 is prepared using a method similar to that in Example 1, but the preparation method of the positive electrode active material has been adjusted, and the preparation method is as follows: 0.1 mol sodium pyrophosphate, 0.3 mol ferrous oxalate, and 0.2 mol ammonium dihydrogen phosphate were added to 200 ml of water and ground in a sand mill for 10 h to obtain a mixed slurry. The mixed slurry was dried by spray drying to obtain a dried precursor powder. The precursor was placed in a tube furnace and calcined in the first step with nitrogen as a protective gas, heated to 300 ℃ and held for 4 h, and then calcined in the second step with the temperature raised to 550 ℃ and held for 12 h to obtain the positive electrode active material.

[0168] Comparative Example 2 The battery in Comparative Example 2 was prepared in a similar manner to that in Comparative Example 1, but carbon material was introduced into the positive electrode active material. The specific parameters are shown in Table 1.

[0169] Comparative Example 3 The battery in Comparative Example 3 was prepared in a similar manner to that in Comparative Example 2, but aluminum was introduced into the positive electrode active material. The specific parameters are shown in Table 1.

[0170] Comparative Examples 4-5 The batteries in Comparative Examples 4 and 5 were prepared using similar methods to those in Comparative Example 1, but Mn elements with different x values ​​were introduced into the positive electrode active material. The specific parameters are shown in Table 1.

[0171] II. Performance Testing 1. Performance testing of positive electrode active materials 1) Aluminum content test The composition of the positive electrode active material is determined by inductively coupled plasma (ICP) spectroscopy, for example, by referring to standards YS / T 1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015. Specifically, an inductively coupled plasma emission spectrometer (Thermo, ICAP7400; PerkinElmer, Avio200) can be used to measure the luminescence intensity of aluminum in the sample. Standard samples with different aluminum contents are prepared, and their luminescence intensities are measured to create a standard curve. The mass content of aluminum in the sample is then determined based on the luminescence intensity from the standard curve.

[0172] 2) Residual alkali content test The residual alkali content of the prepared positive electrode active material is tested. In this application, the residual alkali content of NaHCO3 refers to the mass content of NaHCO3 relative to the mass of the positive electrode active material. This is determined by chemical reagent potentiometric titration, for example, referring to standard GB / T 9725-2007. Specifically, a Metrohm 905 Titrando potentiometric titrator can be used. The positive electrode active material is dissolved in deionized water, and a standard titrant solution is used to titrate the aqueous solution of the positive electrode active material. For each drop of 0.1 ml of standard titrant solution added, the potential or pH value is recorded. Titration is stopped when the change in potential or pH value is minimal. The volume of the added standard titrant solution and the measured potential or pH value are recorded. The titration endpoint is determined using a graphical method or a second derivative method, and the titration volume of the standard solution is determined. The calculated mass of NaHCO3 is divided by the mass of the positive electrode active material, and the mass content is taken as the residual alkali content of NaHCO3 in the positive electrode active material.

[0173] 2. Full battery performance test 1) Button cell capacity test The specific capacity test process for coin cell batteries is as follows: Under normal temperature and pressure, the coin cell battery is charged at a constant current rate of 0.1C to a voltage of 4V, and then charged at a constant voltage of 4V until the current drops to 0.05C. The charging specific capacity at this point is recorded, which is the initial sodium deintercalation capacity. Afterwards, it is discharged at a constant current rate of 0.1C to a voltage of 1.5V, and the discharge specific capacity at this point is recorded, which is the initial sodium intercalation capacity. The specific capacity of the positive electrode active material is the initial sodium intercalation capacity.

[0174] 2) Full battery cycle capacity retention test The full-cell capacity retention rate test process is as follows: At 25°C, the prepared battery is charged at a constant current of 1C to 3.7V, then charged at a constant voltage of 3.7V until the current drops to 0.05C, and then discharged at 1C to 1.5V. The resulting capacity is recorded as the initial capacity (C0). The above steps are repeated for the same battery, and the discharge capacity (Cn) of the battery after the nth cycle is recorded. The battery capacity retention rate after each cycle is Pn = Cn / C0 × 100%. A curve is obtained by plotting the 200 points (P1, P2...200) on the ordinate and the corresponding cycle number on the abscissa. In this test, the first cycle corresponds to n=1, the second cycle to n=2, ..., the 200th cycle to n=200. The battery capacity retention rate data corresponding to the examples or comparative examples in Table 1 are the data measured after 200 cycles under the above test conditions, i.e., the value of P200. The test process for the comparative examples and other examples is the same as above.

[0175] 3) Ratio Performance Test The rate performance test process is as follows: The prepared battery was placed in a 25°C constant temperature chamber and left to stand for 30 minutes to reach a constant temperature. The battery was then charged at a constant current of 0.33C to 3.7V at 25°C, followed by constant voltage charging to 0.05C at 3.7V, left to stand for 5 minutes, and then discharged at a constant current of 0.33C to 1.5V, left to stand for 5 minutes, yielding the capacity C1 at 0.33C discharge. Next, the battery was charged at a constant current of 0.33C to 3.7V, followed by constant voltage charging to 0.05C at 3.7V, left to stand for 5 minutes, and then discharged at a constant current of 3C to 1.5V, left to stand for 5 minutes, yielding the capacity C2 at 3C discharge. The capacity retention rate at 3C rate is R = C2 / C1 × 100%. The test process for the comparative example and other embodiments is the same.

[0176] III. Analysis of Test Results for Each Embodiment and Comparative Example Batteries for each embodiment and comparative example were prepared according to the above method, and various performance parameters were measured. The results are shown in Table 1 below.

[0177] Table 1

[0178] Based on the above results, it can be seen that Examples 1 to 31 are all positive electrode active materials for secondary batteries, including iron-based polyanionic compounds and aluminum-containing oxides. The iron-based polyanionic compounds have the following general formula: Na₄Fe 3-x M x Al y(PO4)2P2O7 / C, where M contains transition metal elements, 0≤x≤0.5, 0 <y<0.2。

[0179] As can be seen from the comparison between Examples 1-31 and Comparative Examples 1-2, the doping of Al element in iron-based polyanionic compounds and the residual of Al-derived oxides on the surface of iron-based polyanionic compounds effectively reduce the residual alkali content of the positive electrode active material, increase the specific capacity of the positive electrode active material, and improve the capacity retention rate of the battery at high rates.

[0180] As can be seen from the comparison between Examples 1-31 and Comparative Example 3, controlling the y value in the positive electrode active material to be greater than 0 and less than 0.2 is beneficial to improving the specific capacity of the positive electrode active material, improving the capacity retention rate after 200 battery cycles and the capacity retention rate at 3C rate, and broadening the application of the battery.

[0181] As can be seen from the comparison between Examples 7-31 and Comparative Examples 4-5, compared with traditional cathode active materials containing only transition metal elements, cathode active materials including iron-based polyanionic compounds containing aluminum oxide and Al doping are beneficial to reduce the residual alkali content of cathode active materials, increase the specific capacity of cathode active materials, and improve the capacity retention rate after 200 battery cycles and the capacity retention rate at 3C rate.

[0182] A comparison of Examples 1-6 with Comparative Example 1 shows that, based on the total mass of the positive electrode active material, controlling the carbon material content to be 0.5%-6% is beneficial for improving the specific capacity of the positive electrode active material, the capacity retention rate after 200 battery cycles, and the capacity retention rate at 3C rate. A comparison of Examples 2-4 with Examples 1, 5-6 shows that, based on the total mass of the positive electrode active material, controlling the carbon material content to be 1%-3.5% is beneficial for further improving the specific capacity of the positive electrode active material.

[0183] As can be seen from the comparison between Examples 7-12 and Comparative Examples 4-5, the introduction of carbon materials, the doping of Al elements in iron-based polyanionic compounds, and the residual of Al-derived oxides on the surface of iron-based polyanionic compounds effectively reduce the residual alkali content of the positive electrode active material, increase the specific capacity of the positive electrode active material, and improve the capacity retention rate of the battery after 200 cycles and the capacity retention rate at 3C rate.

[0184] As can be seen from the comparison between Examples 7-12 and Example 3, the introduction of transition metal elements and the control of the x value of the transition metal elements to be less than or equal to 0.5 are beneficial to further improve the capacity retention rate of the battery at 3C rate.

[0185] As can be seen from the comparison of Examples 7, 13-17 with Comparative Example 2, the introduction of aluminum and aluminum oxide into the positive electrode active material, based on the total mass of the positive electrode active material, and controlling the total mass content of aluminum in the positive electrode active material to be 0.03%~0.9%, is beneficial to reduce the residual alkali content of the positive electrode active material, increase the specific capacity of the positive electrode active material, and improve the capacity retention rate of the battery at 3C rate.

[0186] A comparison of Examples 7, 18-21 with Comparative Example 2 shows that controlling the calcination temperature to 500℃-600℃ is beneficial for reducing the residual alkali content of the positive electrode active material, increasing the specific capacity of the positive electrode active material, and improving the capacity retention rate after 200 battery cycles and the capacity retention rate at 3C rate. A comparison of Examples 7, 19-20 with Examples 18, 21 shows that controlling the calcination temperature to 525℃-575℃ is beneficial for further improving the specific capacity of the positive electrode active material, and improving the capacity retention rate after 200 battery cycles and the capacity retention rate at 3C rate. A comparison of Examples 7, 20 with Examples 18-19, 21 shows that controlling the calcination temperature to 550℃-575℃ is beneficial for further improving the capacity retention rate after 200 battery cycles and the capacity retention rate at 3C rate.

[0187] A comparison of Examples 7, 22-25 with Comparative Example 2 shows that controlling the calcination time to 8-15 h is beneficial for reducing the residual alkali content of the positive electrode active material, increasing the specific capacity of the positive electrode active material, and improving the capacity retention rate after 200 battery cycles and the capacity retention rate at 3C rate. A comparison of Examples 7, 23-24 with Comparative Examples 22, 25 shows that controlling the calcination time to 10-14 h is beneficial for further improving the specific capacity of the positive electrode active material, and improving the capacity retention rate after 200 battery cycles and the capacity retention rate at 3C rate.

[0188] As can be seen from the comparison of Examples 7, 26-31 with Comparative Examples 4-5, controlling the transition metal element in the positive electrode active material to include one or more of Ni, Co, Mn, and Ca is beneficial to reducing the residual alkali content of the positive electrode active material, increasing the specific capacity of the positive electrode active material, and improving the capacity retention rate after 200 battery cycles and the capacity retention rate at 3C rate.

[0189] A comparison of Examples 26 with Examples 7 and 29, and Examples 27 with Examples 7 and 30, shows that, compared to the positive electrode active material containing only one of the transition metal elements Ni, Co, and Mn, controlling the presence of two of the transition metal elements Ni, Co, and Mn in the positive electrode active material is beneficial for further improving the capacity retention rate of the battery at 3C rate. A comparison of Examples 28 with Examples 26-27 shows that, compared to the positive electrode active material containing only two of the transition metal elements Ni, Co, and Mn, controlling the presence of all three transition metal elements Ni, Co, and Mn in the positive electrode active material is beneficial for further improving the capacity retention rate of the battery at 3C rate.

[0190] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A positive electrode active material for secondary batteries, characterized in that, The positive electrode active material comprises a carbon-based composite iron-based polyanionic compound and an aluminum-containing oxide, wherein the iron-based polyanionic compound has the following general formula: Na4Fe 3-x M x Al y (PO4)2P2O7 / C Where M contains transition metal elements, 0 ≤ x ≤ 0.5, 0 <y<0.2。 2. The positive electrode active material according to claim 1, characterized in that, The aluminum oxide is distributed on at least a portion of the surface of the primary particles of the iron-based polyanionic compound.

3. The positive electrode active material according to any one of claims 1 to 2, characterized in that, Based on the total mass of the positive electrode active material, the mass content of aluminum in the positive electrode active material is 0.03%~0.9%.

4. The positive electrode active material according to any one of claims 1 to 3, characterized in that, The M includes one or more of Ni, Co, Mn, Cu, V, Ti, Mo, Nb, W, Cr, Zn, Zr, and Ca, and may be selected from one or more of Ni, Co, Mn, Cu, V, and Ca.

5. The positive electrode active material according to any one of claims 1 to 4, characterized in that, The M includes at least two of Ni, Co, Mn, Cu, V, and Ca.

6. The positive electrode active material according to any one of claims 1 to 5, characterized in that, The carbon material is coated on the surface of the iron-based polyanionic compound in the form of a carbon film, or the carbon material is distributed in the form of particles between the primary particles of the iron-based polyanionic compound.

7. The positive electrode active material according to any one of claims 1 to 6, characterized in that, The carbon material includes one or more of amorphous carbon, conductive carbon black, carbon nanotubes, and graphene.

8. The positive electrode active material according to any one of claims 1 to 7, characterized in that, Based on the total mass of the positive electrode active material, the mass content of the carbon material is 0.5% to 6%, and can be selected as 1% to 3.6%.

9. The positive electrode active material according to any one of claims 1 to 8, characterized in that, The specific capacity of the positive electrode active material is not less than 98 mAh / g.

10. The positive electrode active material according to any one of claims 1 to 9, characterized in that, Based on the total mass of the positive electrode active material, the residual NaHCO3 content of the positive electrode active material is less than 1.2%.

11. A method for preparing a positive electrode active material for a secondary battery, characterized in that, Includes the following steps: Raw materials containing iron, sodium, phosphorus, aluminum and carbon sources are dissolved in water to obtain a mixed slurry. The raw materials may optionally include source M, which is a salt containing a transition metal. The mixed slurry is dried and then calcined to prepare the positive electrode active material. The positive electrode active material comprises an iron-based polyanionic compound and an aluminum-containing oxide. The iron-based polyanionic compound has the following general formula: Na4Fe 3-x M x Al y (PO4)2P2O7 / C Where M contains transition metal elements, 0 ≤ x ≤ 0.5, 0 <y<0.2。 12. The preparation method according to claim 11, characterized in that, The process of drying the mixed slurry and then calcining it includes the following steps: The mixed slurry was dried to obtain precursor powder; The precursor powder is calcined in steps. The first step of calcination is carried out at a temperature of 300 ℃ to 400 ℃ for 3 h to 6 h, and the second step of calcination is carried out at a temperature of 500 ℃ to 600 ℃ for 8 h to 15 h to prepare the positive electrode active material.

13. The preparation method according to claim 12, characterized in that, The calcination temperature in the second step is 525 ℃~575 ℃, which can be selected as 550 ℃~575 ℃.

14. The preparation method according to claim 12 or 13, characterized in that, The second calcination step takes 10 to 14 hours.

15. The preparation method according to any one of claims 11 to 14, characterized in that, The M source includes one or more of nickel nitrate, nickel acetate, cobalt nitrate, manganese nitrate, cobalt acetate, manganese acetate, nickel oxalate, cobalt oxalate, manganese oxalate, nickel oxide, cobalt oxide, and manganese oxide.

16. The preparation method according to any one of claims 11 to 15, characterized in that, The aluminum source includes one or more of aluminum nitrate, aluminum acetylacetone, aluminum acetate, aluminum hydroxide, and aluminum oxide.

17. The preparation method according to any one of claims 11 to 16, characterized in that, The carbon source includes one or more of the following: sucrose, tannic acid, polyethylene glycol, polyacrylonitrile, cellulose, polyvinylpyrrolidone, sucrose, oxalic acid, glucose, ascorbic acid, polyethylene, citric acid, conductive carbon black, carbon nanotubes, and graphene.

18. A secondary battery, characterized in that, It includes a positive electrode sheet, wherein the positive electrode sheet comprises the positive electrode active material according to any one of claims 1 to 10 or the positive electrode active material prepared by the preparation method according to any one of claims 11 to 17.

19. The secondary battery according to claim 18, characterized in that, The secondary battery includes a sodium battery.

20. The secondary battery according to claim 18 or 19, characterized in that, The secondary battery is a sodium-ion battery without a negative electrode.

21. The secondary battery according to any one of claims 18 to 20, characterized in that, The secondary battery further includes a negative electrode sheet, which includes a negative current collector and a base coating disposed on at least one surface of the negative current collector. The base coating includes one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, and tin composite carbon nanoparticles.

22. The secondary battery according to claim 21, characterized in that, The areal density of the base coating is 5 g / m³. 2 ~50g / m 2 .

23. The secondary battery according to claim 21 or 22, characterized in that, The thickness of the base coating is 2 μm to 100 μm.

24. A battery module, characterized in that, The secondary battery includes any one of claims 18 to 23.

25. A battery pack, characterized in that, It includes the secondary battery according to any one of claims 18 to 23 or the battery module according to claim 24.

26. An electrical appliance, characterized in that, It includes at least one of the secondary battery according to any one of claims 18 to 23, the battery module according to claim 24, or the battery pack according to claim 25.

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