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

Abstract

The present application provides a positive electrode active material, a preparation method thereof, a secondary battery, a battery module, a battery pack, and a power utilization device. The positive electrode active material, as a positive electrode active material for a secondary battery, includes a carbon material-complexed iron-based polyanion compound and a magnesium-containing oxide, the iron-based polyanion compound having a general formula: Na4Fe 3‑x M x Mg y (PO4)2P2O7, wherein M contains a transition metal element, 0≤x≤0.5, 0

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a positive electrode active material, a preparation method thereof, a secondary battery, a battery module, a battery pack and a power utilization device. BACKGROUND

[0002] Sodium batteries have great application potential in large-scale energy storage due to their abundant reserves, low price and wide working temperature.

[0003] Iron-based polyanionic compounds are popular positive electrode materials for new sodium batteries, which have the advantages of abundant resources, high working voltage, good cycle stability and environmental friendliness. However, the sodium batteries using iron-based polyanionic compounds as positive electrode materials still have problems such as low electronic conductivity, low discharge capacity and poor cycle performance, which cannot meet the application needs of new electrochemical systems. SUMMARY

[0004] The present application is made in view of the above problems, and aims to provide a positive electrode active material, which introduces magnesium elements, realizes the dual effects of lattice doping and composite modification, and can effectively reduce the residual alkali content of the positive electrode active material and improve the cycle performance and rate performance of the battery.

[0005] In a first aspect, the present application provides a positive electrode active material for a secondary battery, which comprises an iron-based polyanionic compound and a magnesium-containing oxide, and the iron-based polyanionic compound has the general formula: Na4Fe 3-x M x Mg y (PO4)2P2O7 / C

[0006] wherein M comprises a transition metal element, 0≤x≤0.5, and 0<y<0.18.

[0007] The introduction of magnesium elements into the positive electrode active material can improve the ion conductivity and electronic conductivity of the positive electrode active material, and improve the ion conductivity and electronic conductivity of the positive electrode active material. On the other hand, part of the magnesium elements are distributed on the surface of the iron-based polyanionic compound in the form of magnesium-containing oxide, which reduces the direct contact between the surface of the positive electrode active material and the moisture in the environment, reduces the residual alkali content of the positive electrode active material, and improves the stability and processability of the positive electrode active material. Moreover, the positive electrode active material also has a high gram capacity, which is beneficial to improve the energy density of the battery.

[0008] The introduction of carbon material into the positive electrode active material can effectively improve the conductivity of the positive electrode active material and the performance of the battery.

[0009] In any embodiment, the magnesium-containing oxide is distributed on at least part of the surface of primary particles of the iron-based polyanionic compound.

[0010] The magnesium-containing oxide is distributed on part of the surface or the whole surface of the primary particles of the iron-based polyanionic compound, 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.

[0011] In any embodiment, the mass content of magnesium in the positive electrode active material is 0.02% to 0.7%, based on the total mass of the positive electrode active material.

[0012] Controlling the mass content of magnesium in the positive electrode active material to be 0.02% to 0.7%, based on the total mass of the positive electrode active material, 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, and improve the cycle performance and rate performance of the battery.

[0013] In any embodiment, the M comprises one or more of Ni, Co, Mn, Cu, V, Ti, Mo, Nb, W, Cr, Zn, Zr, and Ca, and optionally one or more of Ni, Co, Mn, Cu, V, and Ca.

[0014] The doping of the transition metal element in the iron-based polyanionic compound is beneficial to improve the voltage platform of the positive electrode active material, improve the ionic conductivity and electronic conductivity of the material, reduce the polarization of the battery, and improve the cycle performance and rate performance of the battery.

[0015] In any embodiment, the M comprises at least two of Ni, Co, Mn, Cu, V, and Ca.

[0016] Controlling the M to comprise at least two of Ni, Co, Mn, Cu, V, and Ca is beneficial to further improve the rate performance of the battery.

[0017] In any embodiment, the carbon material is in the form of a carbon film coated on the surface of the iron-based polyanionic compound, or the carbon material is in the form of particles distributed between the primary particles of the iron-based polyanionic compound.

[0018] The carbon material in the form of a carbon film coated on the surface of the iron-based polyanionic compound or the carbon material in the form of particles distributed between the primary particles of the iron-based polyanionic compound can effectively improve the conductivity of the positive electrode active material and the performance of the battery.

[0019] In any embodiment, the carbon material comprises one or more of amorphous carbon, conductive carbon black, carbon nanotubes, and graphene.

[0020] The carbon material is easy to disperse and process, and forms a carbon complex with the iron-based polyanion compound to improve the conductivity of the positive electrode active material and the battery performance. Since the crystallinity of the conductive carbon black, carbon nanotube or graphene is higher than that of the amorphous carbon, the conductivity of the carbon material including the conductive carbon black, carbon nanotube or graphene is better than that of the carbon material including the amorphous carbon.

[0021] In any embodiment, the mass content of the carbon material is 0.5% to 6%, or optionally 1% to 3.6%, based on the total mass of the positive electrode active material.

[0022] Controlling the mass content of the carbon material to be 0.5% to 6%, based on the total mass of the positive electrode active material, is beneficial to achieve a balance between the gram capacity and the conductivity of the positive electrode active material, while obtaining higher gram capacity and better cycle performance and rate performance. Avoiding too low mass content of the carbon material leads to poor conductivity of the positive electrode active material, which is not conducive to the capacity volatilization of the positive electrode active material, and avoiding too high mass content of the carbon material leads to reduced gram capacity and poor compaction performance of the positive electrode active material. Controlling the mass content of the carbon material to be 1% to 3.6%, based on the total mass of the positive electrode active material, is beneficial to further improve the gram capacity of the positive electrode active material and further improve the capacity of the battery.

[0023] In any embodiment, the gram capacity of the positive electrode active material is ≥98 mAh / g.

[0024] The positive electrode active material with a gram capacity of ≥98 mAh / g is beneficial to improve the capacity of the battery and broaden the application of the battery.

[0025] In any embodiment, the NaHCO3 residual alkali content of the positive electrode active material is less than 1.2%, based on the total mass of the positive electrode active material.

[0026] The NaHCO3 residual alkali content of the positive electrode active material is less than 1.2%, based on the total mass of the positive electrode active material, which can effectively improve the stability and processability of the positive electrode active material.

[0027] The second aspect of the present application provides a preparation method of a positive electrode active material for a secondary battery, comprising the following steps:

[0028] The raw materials including an iron source, a sodium source, a phosphorus source and a magnesium source are dissolved in water to obtain a mixed slurry, and the raw materials optionally include an M source including a salt containing a transition metal;

[0029] The mixed slurry is dried and calcined to prepare the positive electrode active material, and the positive electrode active material includes an iron-based polyanion compound and a magnesium-containing oxide, and the iron-based polyanion compound has the following general formula:

[0030] Na4Fe 3-x M x Mg y (PO4)2P2O7 / C

[0031] wherein M comprises a transition metal element, 0≤x≤0.5, 0<y<0.18.

[0032] The preparation method of the positive electrode active material is simple, and the production cost is low. The prepared positive electrode active material has a low residual alkali content and a high specific capacity, and the battery has excellent cycle performance and rate performance.

[0033] In any embodiment, the calcining after drying the mixed slurry comprises the following steps:

[0034] The mixed slurry is subjected to drying treatment to obtain a precursor powder;

[0035] The precursor powder is subjected to step-by-step calcining, the first step calcining temperature is 300-400℃, the calcining time is 3-6h, the second step calcining temperature is 500-600℃, and the calcining time is 8-15h, to prepare the positive electrode active material.

[0036] The step-by-step calcining is beneficial to the full reaction of the precursor powder, improves the crystallinity and phase purity of the prepared positive electrode active material, reduces the residual alkali content on the surface of the positive electrode active material, improves the stability and processability of the positive electrode active material, and improves the specific capacity of the positive electrode active material and the cycle performance and rate performance of the battery.

[0037] In any embodiment, the second step calcining temperature is 525-575℃, which can be 550-575℃.

[0038] Controlling the second step calcining temperature to be 525-575℃ is beneficial to further reducing the residual alkali content on the surface of the active material, improving the conductivity of the material, and improving the cycle performance and rate performance of the battery. Further controlling the second step calcining temperature to be 550-575℃ is beneficial to more greatly improving the capacity retention rate of the battery after 200 cycles and the capacity retention rate under 3C rate, and more greatly improving the cycle performance and rate performance of the battery.

[0039] In any embodiment, the second step calcining time is 10-14h.

[0040] Controlling the second step calcining time to be 10-14h is beneficial to obtaining a positive electrode active material with high crystallinity and high purity, and by controlling the holding time, the particle size of primary grains and the residual alkali content on the surface of the material can be controlled, thereby improving the specific capacity of the material and the cycle and rate performance of the battery.

[0041] In any embodiment, the M source comprises 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, manganese oxide.

[0042] In any embodiment, the magnesium source comprises one or more of magnesium nitrate, magnesium carbonate, magnesium acetate, magnesium hydroxide, magnesium oxalate, magnesium oxide.

[0043] In any embodiment, the carbon source comprises 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 nanotube, graphene.

[0044] The carbon source described above is easy to produce carbon material coated on the surface of the iron-based polyanionic compound or distributed between the primary particles of the iron-based polyanionic compound under calcination at high temperature, thereby improving the conductivity of the positive electrode active material. Among them, due to the influence of the calcination temperature, the amorphous carbon formed by the carbon source comprising 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 nanotube, graphene has low crystallinity, while the carbon material generated by the carbon source comprising one or more of conductive carbon black, carbon nanotube, graphene has high crystallinity, so the carbon material generated by the carbon source comprising one or more of conductive carbon black, carbon nanotube, graphene has more excellent conductivity.

[0045] The third aspect of the present application provides a secondary battery comprising a positive electrode sheet, wherein the positive electrode sheet comprises the positive electrode active material described in any embodiment or the positive electrode active material prepared by the preparation method described in any embodiment.

[0046] In any embodiment, the secondary battery is a negative electrode-free sodium battery.

[0047] In any embodiment, the secondary battery further comprises a negative electrode sheet, wherein the negative electrode sheet comprises a negative electrode current collector and a primer layer disposed on at least one surface of the negative electrode current collector, and the primer layer comprises one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, tin composite carbon nanoparticles.

[0048] The primer layer described above not only has excellent conductivity, but also is beneficial to the uniform deposition of metal ions on the surface of the current collector, thereby improving the cycle performance and safety of the battery.

[0049] In any embodiment, the surface density of the primer layer is 5 g / m 2 ~ 50 g / m 2 .

[0050] 5 g / m 2 ~ 50 g / m 2 The undercoat layer with a surface density of 5 g / m

[0051] In any embodiment, the undercoat layer has a thickness of 2 μm to 100 μm.

[0052] Controlling the thickness of the undercoat layer to 2 μm to 100 μm can provide sufficient nucleation sites to facilitate uniform deposition of metal ions and inhibit dendrites.

[0053] A fourth aspect of the present application provides a battery module including the secondary battery of the third aspect of the present application.

[0054] A fifth aspect of the present application provides a battery pack including the secondary battery of the third aspect of the present application or the battery module of the fourth aspect of the present application.

[0055] A sixth aspect of the present application provides an electric device including at least one of the secondary battery of the third aspect of the present application, the battery module of the fourth aspect of the present application, and the battery pack of the fifth aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 is a schematic view of a secondary battery according to an embodiment of the present application;

[0057] Figure 2 is an exploded view of the secondary battery according to an embodiment of the present application shown in FIG. 1; Figure 1

[0058] Figure 3 is a schematic view of a battery module according to an embodiment of the present application;

[0059] Figure 4 is a schematic view of a battery pack according to an embodiment of the present application;

[0060] Figure 5 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG. 4; Figure 4

[0061] Figure 6 is a schematic view of an electric device using the secondary battery according to an embodiment of the present application as a power source.

[0062] REFERENCE NUMERALS

[0063] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 case; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0064] ​​Hereinafter, specific embodiments of the positive electrode active material of the present application, a method for producing the same, a secondary battery, a battery module, a battery pack, and an electric device will be described in detail with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known well, repetitive description of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0065] The ranges disclosed herein are defined by their lower and upper limits. Ranges can either be inclusive or exclusive of their endpoints, and both endpoints and the exclusion of either or both endpoints are appropriately resolvable by any of the combinations of ranges found in the listed ranges. If, for example, a range for a given parameter is stated to be 60-120 and 80-110, it is understood that a range of 60-110 and 80-120 are also contemplated. Furthermore, if a minimum range value of 1 and 2 is stated, and if a maximum range value of 3, 4, and 5 is stated, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, numerical ranges "a-b" are intended to specifically include any and all combinations of values within the range, unless the context clearly indicates otherwise. For example, the numerical range "0-5" is intended to specifically include every integer and fraction within the range of 0-5, even though those integers and fractions are not listed. Additionally, when a parameter is stated to be an integer ≥ 2, it is equivalent to state that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0066] All embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, unless otherwise specified.

[0067] All technical features and optional technical features of the present application can be combined with each other to form new technical solutions, unless otherwise specified.

[0068] If not specified otherwise, all steps of the present application can be carried out in sequence or randomly, preferably in sequence. For example, the method comprising steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method further comprising step (c) means that step (c) can be added to the method in any sequence, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0069] If not specified otherwise, the terms "comprising" and "including" mentioned in the present application mean open-ended, and can also mean closed-ended. For example, the terms "comprising" and "including" can mean that other components not listed can also be included, or can mean that only the listed components are included.

[0070] If not specified otherwise, in the present application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, any of the following satisfy the condition "A or B": 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).

[0071] The polyanionic compound in the prior art has become one of the popular sodium battery positive electrode materials due to sufficient resources, environmental friendliness, easy scale production, and open sodium ion diffusion channel, good thermal stability and cycle stability. However, in the actual production process, researchers found that the polyanionic compound material has a high residual alkali content on the surface, and exposure in the air also easily leads to an increase in residual alkali content on the material surface, causing capacity deterioration, thereby affecting the performance of the battery. Therefore, it is necessary to develop a positive electrode active material with low residual alkali content, excellent electrochemical performance and being applicable to high-rate batteries to meet the application needs of the new generation of electrochemical systems.

[0072] [Positive electrode active material]

[0073] Based on this, the present application provides a positive electrode active material for a secondary battery, which comprises a carbon material composite iron-based polyanionic compound and a magnesium-containing oxide, and the iron-based polyanionic compound has the general formula: Na4Fe 3- x M x Mg y (PO4)2P2O7 / C, wherein M comprises a transition metal element, 0≤x≤0.5, and 0<y<0.18.

[0074] In the present text, the term "transition metal elements" refers to the elements of groups III B to VII B, VIII of the periodic table, and the elements of groups IB to IIB.

[0075] In some embodiments, x can be selected from 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5.

[0076] In some embodiments, y can be selected from 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16 or 0.17.

[0077] In the positive electrode active material, on the one hand, part of the magnesium elements can be doped into the iron-based polyanion compound lattice, replacing the iron elements or transition metal elements in the iron-based polyanion compound lattice, thereby improving the ionic conductivity and electronic conductivity of the positive electrode active material, and improving the ion conductivity and electronic conductivity of the positive electrode active material. On the other hand, part of the magnesium elements are distributed in the form of magnesium-containing oxides on the surface of the iron-based polyanion compound, reducing the direct contact of the positive electrode active material surface with the moisture in the environment, reducing the residual alkali content of the positive electrode active material, and improving the stability and processability of the positive electrode active material. Moreover, the positive electrode active material also has a high specific capacity, which is beneficial to improve the energy density of the battery.

[0078] In the positive electrode active material, the introduction of carbon material can effectively improve the conductivity and battery performance of the positive electrode active material.

[0079] In the present text, the term "specific capacity" refers to the amount of electricity released per gram of positive electrode active material, which can reflect the capacity of the battery, which can be tested by any known method.

[0080] In some embodiments, the magnesium-containing oxides are distributed on at least part of the surface of the primary particles of the iron-based polyanion compound.

[0081] In the present text, the term "primary particle" refers to the particles of the iron-based polyanion compound before agglomeration, and the particle size of the primary particles of the iron-based polyanion compound ranges from 30 nm to 120 nm.

[0082] In some embodiments, the magnesium-containing oxides include magnesium oxide.

[0083] In some embodiments, the magnesium-containing oxides are distributed on part of the surface of the primary particles of the iron-based polyanion compound.

[0084] In some embodiments, the magnesium-containing oxides are distributed on the entire surface of the primary particles of the iron-based polyanion compound.

[0085] The magnesium-containing oxide is distributed on part or all of the surface of the primary particles of the iron-based polyanionic compound, which can effectively reduce the direct contact between the positive active material and moisture in the environment, reduce the residual alkali content of the positive active material, improve the stability and processability of the positive active material, and improve the specific capacity and capacity retention rate of the positive active material at a 3C rate, and improve the cycle performance and rate performance of the battery.

[0086] In some embodiments, the mass content of magnesium in the positive active material is 0.02% to 0.7%, based on the total mass of the positive active material. In some embodiments, the mass content of magnesium in the positive active material can be selected as 0.02%, 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%, or 0.7%, based on the total mass of the positive active material.

[0087] In some embodiments, the magnesium in the positive active material includes Mg in the iron-based polyanionic compound and magnesium in the magnesium-containing oxide.

[0088] The mass content of magnesium in the positive active material can be tested by any means known in the art. As an example, reference can be made to the standards YS / T 1006.2-2014, GB / T23367.2-2009, or YS / T 1028.5-2015. Specifically, the luminescence intensity of magnesium in the sample to be tested can be determined by an inductively coupled plasma emission spectrometer (Thermo, ICAP7400; PerkinElmer, Avio200), and standard samples with different contents of magnesium are configured to determine their luminescence intensity as a standard curve, and the mass content of magnesium in the sample to be tested is determined in the standard curve according to the luminescence intensity.

[0089] Controlling the mass content of magnesium in the positive active material to be 0.02% to 0.7%, based on the total mass of the positive active material, is beneficial to reduce the residual alkali content of the positive active material, improve the stability and processability of the positive active material, improve the specific capacity of the positive active material and the capacity retention rate at a 3C rate, and improve the cycle performance and rate performance of the battery.

[0090] In some embodiments, M contains one or more of Ni, Co, Mn, Cu, V, Ti, Mo, Nb, W, Cr, Zn, Zr, and Ca, and can contain one or more of Ni, Co, Mn, Cu, and V.

[0091] In some embodiments, M contains Ni. In some embodiments, M contains Co. In some embodiments, M contains Mn. In some embodiments, M contains Ca. In some embodiments, M contains Cu.

[0092] The doping of the transition metal element in the iron-based polyanion compound is beneficial to improve the voltage platform of the positive active material, improve the ionic conductivity and electronic conductivity of the material, reduce the polarization of the battery, and improve the cycle performance and rate performance of the battery.

[0093] In some embodiments, M comprises at least two of Ni, Co, Mn, Cu, V, and Ca.

[0094] 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.

[0095] Controlling M to comprise at least two of Ni, Co, Mn, Cu, V, and Ca is beneficial to further improve the rate performance of the battery.

[0096] In some embodiments, the carbon material is in the form of a carbon film coated on the surface of the iron-based polyanion compound or the carbon material is in the form of particles distributed between primary ions of the iron-based polyanion compound.

[0097] In some embodiments, the carbon material is in the form of a carbon film coated on the surface of the iron-based polyanion compound.

[0098] In some embodiments, the carbon material is in the form of particles distributed between primary ions of the iron-based polyanion compound.

[0099] The carbon material in the form of a carbon film coated on the surface of the iron-based polyanion compound or the carbon material in the form of particles distributed between primary ions of the iron-based polyanion compound can effectively improve the conductivity of the positive active material and the performance of the battery.

[0100] In some embodiments, the carbon material comprises one or more of amorphous carbon, conductive carbon black, carbon nanotubes, and graphene.

[0101] In some embodiments, the carbon material comprises amorphous carbon. In some embodiments, the carbon material comprises conductive carbon black. In some embodiments, the carbon material comprises graphene. In some embodiments, the carbon material comprises carbon nanotubes.

[0102] The above-mentioned carbon material is easy to be compounded with the iron-based polyanion compound, thereby improving the conductivity of the positive active material. Among them, since the crystallinity of the conductive carbon black, the carbon nanotubes, or the graphene is higher than that of the amorphous carbon, the conductivity of the carbon material comprising the conductive carbon black, the carbon nanotubes, or the graphene is superior to that of the carbon material comprising the amorphous carbon.

[0103] In some embodiments, the mass content of the carbon material is 0.5% to 6%, optionally 1% to 3.6%, based on the total mass of the positive electrode active material. In some embodiments, the mass content of the carbon material is optionally 0.5%, 1%, 1.1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 3.6%, 4%, 4.5%, 5%, 5.5%, or 6%, based on the total mass of the positive electrode active material.

[0104] Controlling the mass content of the carbon material to be 0.5% to 6%, based on the total mass of the positive electrode active material, is conducive to achieving a balance between the gravimetric capacity and the electrical conductivity of the positive electrode active material, while achieving higher gravimetric capacity and better cycle performance and rate performance. Avoiding too low mass content of the carbon material leads to poor electrical conductivity of the positive electrode active material, which is not conducive to capacity volatilization of the positive electrode active material, and also avoiding too high mass content of the carbon material leads to reduced gravimetric capacity and impact on the tap density of the positive electrode active material. Controlling the mass content of the carbon material to be 1% to 3.6%, based on the total mass of the positive electrode active material, is conducive to further improving the gravimetric capacity of the positive electrode active material and further improving the capacity of the battery.

[0105] In some embodiments, the positive electrode active material has a gravimetric capacity of ≥98 mAh / g.

[0106] In some embodiments, the positive electrode active material has a gravimetric capacity of 98 mAh / g, 100 mAh / g, 105 mAh / g, 110 mAh / g, 115 mAh / g, 120 mAh / g, or 125 mAh / g.

[0107] The gravimetric capacity of the positive electrode active material can be tested by any means known in the art. As an example, a coin cell is charged at a rate of 0.1C to a voltage of 4V under ambient pressure at 25°C, and then charged at a constant voltage of 4V until the current drops to 0.05C, and the charge specific capacity at this time is recorded, which is the first sodium extraction capacity. Then, the coin cell is discharged at a rate of 0.1C to a voltage of 1.5V, and the discharge specific capacity at this time is recorded, which is the first sodium intercalation capacity. The gravimetric capacity of the positive electrode active material is the first sodium intercalation capacity.

[0108] The positive electrode active material having a gravimetric capacity of ≥98 mAh / g is conducive to improving the capacity of the battery and widening the application of the battery.

[0109] In some embodiments, the NaHCO3 residual alkali content of the positive electrode active material is less than 1.2%, based on the total mass of the positive electrode active material.

[0110] In some embodiments, the NaHCO3 residual alkali content of the positive electrode active material can be selected to be 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%, based on the total mass of the positive electrode active material.

[0111] The NaHCO3 residual alkali content of the positive electrode active material can be tested by any means known in the art. As an example, it can be determined by chemical reagent potentiometric titration, for example, according to the standard GB / T 9725-2007. Specifically, a Swiss Metrohm 905 Titrando potentiometric titrator can be used, the positive electrode active material is dissolved in deionized water, and the positive electrode active material aqueous solution is titrated with a standard titration solution. Each 0.1 ml of standard titration solution is added, and the potential or pH value is recorded. When the potential or pH value changes little, the titration is stopped. The volume of the added standard titration solution and the measured potential or pH value are recorded, and a graphic method or a second derivative method is used to determine the titration end point and determine the titration volume of the standard solution. The mass of the calculated NaHCO3 is divided by the mass of the positive electrode active material to obtain the NaHCO3 residual alkali content of the positive electrode active material as a mass content.

[0112] The NaHCO3 residual alkali content of the positive electrode active material is less than 1.2%, based on the total mass of the positive electrode active material, which can effectively improve the stability and processability thereof.

[0113] The present application also provides a preparation method of a positive electrode active material for a secondary battery, comprising the following steps:

[0114] The raw materials including an iron source, a sodium source, a phosphorus source, and a magnesium source are dissolved in water to obtain a mixed slurry. The raw materials can optionally include an M source, and the M source includes a salt containing a transition metal.

[0115] The mixed slurry is dried and calcined to prepare a positive electrode active material, and the positive electrode active material includes an iron-based polyanion compound and a magnesium-containing oxide. The iron-based polyanion compound has the following general formula:

[0116] Na4Fe 3-x M x Mg y (PO4)2P2O7 / C

[0117] wherein M contains a transition metal element, 0≤x≤0.5, and 0

[0118] The preparation method of the positive electrode active material is simple and has low manufacturing cost. The prepared positive electrode active material has a low residual alkali content and a high specific capacity, and the battery has excellent cycle performance and rate performance.

[0119] The introduction of transition metals in the positive electrode active material is beneficial to improve the ionic conductivity and electronic conductivity of the material, and to improve the cycle performance and rate performance of the battery.

[0120] In some embodiments, the raw materials further include a carbon source.

[0121] The addition of the carbon source is beneficial to introduce conductive carbon elements in the positive electrode active material, and to improve the conductivity of the positive electrode active material.

[0122] In some embodiments, the calcination after drying the mixed slurry includes the following steps:

[0123] The mixed slurry is subjected to drying treatment to obtain a precursor powder;

[0124] The precursor powder is subjected to step-by-step calcination, the first step calcination temperature is 300-400℃, the calcination time is 3-6h, the second step calcination temperature is 500-600℃, and the calcination time is 8-15h, to prepare a positive electrode active material.

[0125] The step-by-step calcination is beneficial to the full reaction of the precursor powder, to improve the crystallinity and phase purity of the prepared positive electrode active material, to reduce the residual alkali amount on the surface of the positive electrode active material, to improve the stability and processability of the positive electrode active material, and to improve the specific capacity of the positive electrode active material and the cycle performance and rate performance of the battery.

[0126] In some embodiments, the second step calcination temperature is 525-575℃, which can be selected as 550-575℃.

[0127] Controlling the second step calcination temperature to be 525-575℃ is beneficial to further reduce the residual alkali amount on the surface of the active material, to improve the conductivity of the material, and to improve the cycle performance and rate performance of the battery. Further controlling the second step calcination temperature to be 550-575℃ is beneficial to further improve the cycle performance and rate performance of the battery.

[0128] In some embodiments, the second step calcination time is 10-14h.

[0129] Controlling the second step calcination time to be 10-14h is beneficial to obtain a positive electrode active material with high crystallinity and high purity, and by controlling the holding time, the particle size of the primary grains and the residual alkali amount on the surface of the material can be controlled, thereby improving the specific capacity of the material and the cycle rate performance of the battery.

[0130] 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.

[0131] In some embodiments, the magnesium source comprises one or more of magnesium nitrate, magnesium carbonate, magnesium acetate, magnesium hydroxide, magnesium oxalate, magnesium oxide.

[0132] In some embodiments, the carbon source comprises 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 nanotube, graphene.

[0133] In some embodiments, the carbon source comprises one or more of sucrose, tannic acid, polyethylene glycol, polyacrylonitrile, cellulose, polyvinylpyrrolidone, sucrose, oxalic acid, glucose, ascorbic acid, polyethylene, citric acid.

[0134] In some embodiments, the carbon source comprises one or more of conductive carbon black, carbon nanotube, graphene.

[0135] The carbon source described above is easy to produce carbon material coated on the surface of the iron-based polyanionic compound or distributed between the primary particles of the iron-based polyanionic compound under calcination at high temperature, thereby improving the conductivity of the positive electrode active material. Among them, due to the influence of the calcination temperature, the carbon source comprising 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 nanotube, graphene forms amorphous carbon with low crystallinity, while the carbon material generated by the carbon source comprising one or more of conductive carbon black, carbon nanotube, graphene has high crystallinity, so the carbon material generated by the carbon source comprising one or more of conductive carbon black, carbon nanotube, graphene has more excellent conductivity.

[0136] [Positive electrode sheet]

[0137] The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer formed on at least part of the surface of the positive electrode current collector, and the positive electrode active material layer comprises the positive electrode active material in some embodiments.

[0138] The positive electrode active material layer can further comprise a conductive agent to improve the conductivity of the positive electrode. The conductive agent can be selected from one or more of Super P, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene, and carbon nanofibers.

[0139] The positive electrode active material layer can further include a binder to firmly bind the positive electrode active material and the optional conductive agent to the positive electrode current collector. The binder can be at least one of 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).

[0140] The positive electrode current collector can be an electrically conductive carbon sheet, a metal foil, a carbon-coated metal foil, a porous metal plate, or a composite current collector. The electrically conductive carbon material of the electrically conductive carbon sheet can be 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, the carbon-coated metal foil, and the porous metal plate is independently selected from at least one of copper, aluminum, nickel, and stainless steel, and the composite current collector can be a composite current collector formed by combining a metal foil and a polymer-based film.

[0141] In some embodiments, the positive electrode tab can be prepared by dispersing the components described above for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on the positive electrode current collector, and then drying, cold pressing, and the like to obtain the positive electrode tab.

[0142] [Separator]

[0143] In some embodiments, the secondary battery further includes a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0144] 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 fiber. The separator can be a single-layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0145] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator can be used to make an electrode assembly by a winding process or a stacking process.

[0146] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0147] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, and the like. The outer package of the secondary battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, and polybutylene succinate, and the like can be listed.

[0148] [Secondary battery]

[0149] The secondary battery includes a positive electrode sheet, which includes the positive active material in some embodiments or the positive active material prepared by the preparation method in some embodiments.

[0150] In some embodiments, the secondary battery further includes a negative electrode sheet, a separator, and an electrolyte.

[0151] In some embodiments, the secondary battery is a negative electrode-free sodium battery.

[0152] In the negative electrode-free sodium battery, no negative active material is pre-deposited, and only a negative current collector is included. During the first charging, sodium ions obtain electrons on the anode side to deposit metal sodium on the surface of the current collector to form a sodium metal phase. During discharging, the metal sodium can be converted into sodium ions to return to the positive electrode, realizing cyclic charging and discharging. Compared with other sodium secondary batteries, the negative electrode-free sodium battery can obtain higher energy density due to the limitation of the negative electrode material. In the negative electrode-free sodium battery, there is not enough sodium metal as a negative electrode material to provide enough sodium elements to the battery. Therefore, the application of the positive active material provided by the present application can more effectively improve the cycle performance and high-rate performance of the secondary battery.

[0153] In some embodiments, the CB value of the negative electrode-free sodium battery is less than or equal to 0.1.

[0154] The CB value is the capacity per unit area of the negative electrode sheet divided by the capacity per unit area of the positive electrode sheet in the secondary battery. Since the negative electrode-free battery does not include negative active material, the capacity per unit area of the negative electrode sheet is small, and the CB value of the secondary battery is less than or equal to 0.1.

[0155] In some embodiments, the secondary battery includes a negative electrode sheet, which can only include a negative current collector and does not include negative active material. In some embodiments, the negative electrode sheet can also pre-deposit a metal phase on the negative current collector.

[0156] In some embodiments, the negative current collector can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil or a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer such as a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0157] In some embodiments, the negative electrode tab includes a negative current collector and a primer layer disposed on at least one surface of the negative current collector, the primer layer including one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, and tin composite carbon nanoparticles.

[0158] The primer layer described above not only has excellent electrical conductivity, but also facilitates uniform deposition of metal ions on the surface of the negative current collector of the anode-free battery, thereby improving the cycle performance and safety of the battery.

[0159] In some embodiments, the primer layer has an area density of 5 g / m 2 to 50 g / m 2 .

[0160] In some embodiments, the primer layer has an area density of 5 g / m 2 , 10 g / m 2 , 15 g / m 2 , 20 g / m 2 , 25 g / m 2 , 30 g / m 2 , 35 g / m 2 , 40 g / m 2 , 45 g / m 2 , or 50 g / m 2 .

[0161] The primer layer having an area density of 5 g / m 2 to 50 g / m 2 facilitates uniform distribution of nucleation sites of the negative electrode in the anode-free battery, promotes uniform deposition of metal, and does not affect the transmission behavior of electrons.

[0162] In some embodiments, the primer layer has a thickness of 2 μm to 100 μm.

[0163] In some embodiments, the thickness of the undercoat layer can be selected to be 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.

[0164] The undercoat layer described above is too thin to be used as a negative active material, but can be used as an undercoat layer in a sodium-free negative electrode battery to reduce the nucleation overpotential of sodium and facilitate uniform deposition of metal ions and inhibit dendrites.

[0165] The shape of the secondary battery is not particularly limited in the present application, and can be cylindrical, square, or any other shape. For example, Figure 1 is a square structure as an example of a secondary battery 5, Figure 2 is an exploded view of the secondary battery 5.

[0166] In some embodiments, referring to Figure 2 , the outer package can include a shell 51 and a cover plate 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate form an accommodation cavity. The shell 51 has an opening communicating with the accommodation cavity, and the cover plate 53 can be provided on the opening to close the accommodation cavity. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the accommodation cavity. The non-Newtonian fluid electrolyte composition is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and the skilled person can select according to the specific actual needs.

[0167] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by the skilled person according to the application and capacity of the battery module.

[0168] Figure 3 is a battery module 3 as an example. Referring to Figure 3 , in the battery module 3, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 3. Of course, it can also be arranged in any other way. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0169] Optionally, the battery module 3 can further include a housing having an accommodation space, and the plurality of secondary batteries 5 are accommodated in the accommodation space.

[0170] In some embodiments, the above battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0171] Figure 4 and Figure 5 is a battery pack 1 as an example. Referring to Figure 4 and Figure 5 In the battery pack 1, a battery box and a plurality of battery modules 4 arranged in the battery box can be included. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be arranged on the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0172] [Power consuming device]

[0173] In one embodiment of the present application, a power consuming device is provided, which includes at least one of the secondary battery of any embodiment, the battery module of any embodiment, or the battery pack of any embodiment.

[0174] The power consuming device includes at least one of the secondary battery, the battery module, or the battery pack provided by the present application. The secondary battery, the battery module, or the battery pack can be used as a power source of the power consuming device, or can be used as an energy storage unit of the power consuming device. The power consuming device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0175] As the power consuming device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirements thereof.

[0176] Figure 6 is a power consuming device as an example. The power consuming device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirements of high power and high energy density of the secondary battery for the power consuming device, the battery pack or the battery module can be used.

[0177] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and the secondary battery can be used as a power source.

[0178] Embodiment

[0179] Hereinafter, the examples of the present application will be described. The examples described below are illustrative and are intended to be purely exemplary of the application and are not intended to limit the application. Unless otherwise indicated, the techniques or conditions described in the examples are those that would be described in the literature or according to the manufacturer's instructions. Unless otherwise indicated, the reagents or instruments used are conventional products that can be obtained commercially.

[0180] I. Preparation method

[0181] Example 1

[0182] 1) Preparation of positive electrode active material

[0183] 0.1 mol of sodium pyrophosphate, 0.3 mol of ferrous oxalate, 0.2 mol of ammonium dihydrogen phosphate, 0.01 mol of glucose, and 0.006 mol of magnesium acetate were added to 200 ml of deionized water, and ground for 10 h using a sand mill to obtain a mixed slurry. The mixed slurry was dried by a spray drying apparatus to obtain a dried precursor powder. The precursor was placed in a tube furnace, and nitrogen gas was used as a protective gas. The first step of calcination was performed by heating to 300°C for 4 h, and the second step of calcination was performed by heating to 550°C for 12 h to obtain a positive electrode active material.

[0184] 2) Preparation of positive electrode sheet

[0185] 10 wt% of polyvinylidene fluoride binder was sufficiently dissolved in N-methyl pyrrolidone (NMP), and then 10 wt% of carbon black conductive agent and 80 wt% of the above positive electrode active material were added and stirred to obtain a positive electrode slurry. The slurry was uniformly coated on the surface of the current collector aluminum foil, and then transferred to a vacuum drying oven for complete drying. The dried electrode sheet was roll-pressed and punched to obtain a positive electrode sheet.

[0186] 3) Preparation of negative electrode sheet

[0187] Carbon nanotubes and carboxymethyl cellulose (CMC) were added to deionized water in a mass ratio of 1:0.4 and stirred to form a uniform slurry. The slurry was coated on the surface of the negative electrode current collector copper foil to form a primer layer, and then transferred to a vacuum drying oven for complete drying. The primer layer had a thickness of 20 μm and a surface density of 25 g / m 2 . A negative electrode sheet without a negative electrode structure was obtained.

[0188] 4) Electrolyte

[0189] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), sodium salt sodium hexafluorophosphate NaPF6 was dissolved in an organic solvent ethylene glycol dimethyl ether (DME) and stirred to obtain an electrolyte with a sodium salt concentration of 1 mol / L.

[0190] 5) separator film

[0191] The polypropylene film is used as the separator film.

[0192] 6) Preparation of button cell

[0193] The button cell is assembled by using sodium sheet as the counter electrode, Celgard 2400 separator film and electrolyte.

[0194] 7) Preparation of full cell

[0195] The positive electrode sheet, the separator film and the negative electrode sheet are stacked in sequence, the separator film is between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and the above electrolyte is added to assemble the stacked cell. The negative electrode-free sodium battery product of Example 1 is obtained.

[0196] Examples 2-6

[0197] The batteries of Examples 2-6 are prepared in a similar manner to the battery of Example 1, but the mass content of carbon element in the positive electrode active material is adjusted, and the specific parameters are shown in Table 1.

[0198] Example 7

[0199] The battery of Example 7 is prepared in a similar manner to the battery of Example 1, but the preparation method of the positive electrode active material is adjusted, and the preparation method is as follows:

[0200] 0.1 mol of sodium pyrophosphate, 0.28 mol of ferrous oxalate, 0.2 mol of ammonium dihydrogen phosphate, 0.04 mol of glucose, 0.02 mol of manganese oxalate (M source) and 0.006 mol of magnesium acetate are added to 200 ml of deionized water, and a sand mill is used for grinding for 10 h to obtain a mixed slurry; the mixed slurry is dried by a spray drying device to obtain a dried precursor powder; the precursor powder is placed in a tube furnace, nitrogen gas is used as a protective gas, the temperature is raised to 300°C for 4 h, and then the temperature is raised to 550°C for 12 h to obtain the positive electrode active material.

[0201] Examples 8-12

[0202] The batteries of Examples 8-12 are prepared in a similar manner to the battery of Example 7, but the x value of the Mn element in the iron-based polyanion compound is adjusted, and the specific parameters are shown in Table 1.

[0203] Examples 13-17

[0204] The batteries of Examples 13-17 are prepared in a similar manner to the battery of Example 7, but the mass content of magnesium element in the positive electrode active material is adjusted, and the specific parameters are shown in Table 1.

[0205] Examples 18-25

[0206] The batteries of Examples 18-25 were prepared in a similar manner to the batteries of Example 7, but the second-step calcination temperature and calcination time were adjusted, respectively, and the specific parameters are shown in Table 1.

[0207] Examples 26-31

[0208] The batteries of Examples 26-31 were prepared in a similar manner to the batteries of Example 7, but the type and combination of transition metal elements in the iron-based polyanion compound were adjusted, and the specific parameters are shown in Table 1.

[0209] Comparative Example 1

[0210] The batteries of Comparative Example 1 were prepared in a similar manner to the batteries of Example 1, but the preparation method of the positive active material was adjusted, and the preparation method is as follows:

[0211] 0.1 mol of sodium pyrophosphate, 0.3 mol of ferrous oxalate, and 0.2 mol of ammonium dihydrogen phosphate were added to 200 ml of water, and ground for 10 h using a sand mill to obtain a mixed slurry; the mixed slurry was dried by a spray drying device to obtain a dried precursor powder; the precursor was placed in a tube furnace, nitrogen gas was used as a protective gas, and the first-step calcination was performed by heating to 300°C for 4 h, and the second-step calcination was performed by heating to 550°C for 12 h to obtain the positive active material.

[0212] Comparative Example 2

[0213] The batteries of Comparative Example 2 were prepared in a similar manner to the batteries of Comparative Example 1, but the carbon material was introduced into the positive active material, and the specific parameters are shown in Table 1.

[0214] Comparative Example 3

[0215] The batteries of Comparative Example 3 were prepared in a similar manner to the batteries of Comparative Example 2, but the magnesium element was introduced into the positive active material, and the specific parameters are shown in Table 1.

[0216] Comparative Examples 4-5

[0217] The batteries of Comparative Examples 4-5 were prepared in a similar manner to the batteries of Comparative Example 1, but the Mn element with different x values was introduced into the positive active material, and the specific parameters are shown in Table 1.

[0218] II. Performance Test

[0219] 1. Positive Active Material Performance Test

[0220] 1) Magnesium Element Content Test

[0221] The composition of the positive electrode active material is determined by inductively coupled plasma (ICP) spectroscopy analysis, for example, refer to the standards YS / T 1006.2-2014, GB / T 23367.2-2009 or YS / T 1028.5-2015. Specifically, the luminescence intensity of magnesium element in the sample to be tested can be determined by an inductively coupled plasma emission spectrometer (Thermo, ICAP7400; PerkinElmer, Avio200), and the luminescence intensity of magnesium element standard samples with different contents is determined as a standard curve, and the mass content of magnesium element in the sample to be tested is determined in the standard curve according to the luminescence intensity.

[0222] 2) NaHCO3 residual alkali content test

[0223] The prepared positive electrode active material is subjected to residual alkali content test. The NaHCO3 residual alkali content in the present application refers to the mass of NaHCO3 determined in the positive electrode active material. It is determined by chemical reagent potentiometric titration method, for example, refer to the standard GB / T 9725-2007. Specifically, the positive electrode active material can be dissolved in deionized water by using a Swiss WTW905Titrando potentiometric titrator, and the positive electrode active material aqueous solution is titrated by using a standard titration solution. Each 0.1 ml of standard titration solution is added, and the potential or pH value is recorded. When the potential or pH value changes little, the titration is stopped. The volume of the added standard titration solution and the measured potential or pH value are recorded, and the titration end point is determined by graphic method or secondary derivative method, and the titration volume of the titration standard solution is determined. The mass of NaHCO3 obtained by calculation is divided by the mass of the positive electrode active material, and the mass content is taken as the NaHCO3 residual alkali content of the positive electrode active material.

[0224] 2, battery performance test

[0225] 1) Button cell gram capacity test

[0226] The button cell gram capacity test process is as follows: at 25°C, under normal pressure, the button cell is charged at 0.1C rate to 4V, and then charged at 4V constant voltage until the current drops to 0.05C, and the charge specific capacity at this time is recorded, which is the first sodium extraction capacity; then discharged at 0.1C rate to 1.5V, and the discharge specific capacity at this time is recorded, which is the first sodium intercalation capacity. The gram capacity of the positive electrode active material is the first sodium intercalation capacity.

[0227] 2) Full battery cycle capacity retention rate test

[0228] The full battery capacity retention test process is as follows: at 25°C, the prepared full battery is charged at 1C constant current to 3.7V, then charged at 3.7V constant voltage until the current drops to 0.05C, then discharged at 1C to 1.5V, and the obtained capacity is recorded as the initial capacity (C0). The above steps are repeated for the same full battery, and the discharge capacity of the full battery after the nth cycle (Cn) is recorded at the same time. The full battery capacity retention Pn = Cn / C0 x 100% after each cycle, and the curve of full battery capacity retention and cycle number is obtained with P1, P2, …, 200 as the vertical coordinate and the corresponding cycle number as the horizontal coordinate. In the test process, the first cycle corresponds to n = 1, the second cycle corresponds to n = 2, …, and the 200th cycle corresponds to n = 200. The full battery capacity retention data of the examples or comparative examples in Table 1 is the data measured after 200 cycles under the above test conditions, i.e. the value of P200. The test process of the comparative examples and other examples is the same as above.

[0229] 3) Rate performance test

[0230] The rate performance test process is as follows: at 25°C, the prepared battery is placed in a 25°C constant temperature oven and allowed to stand for 30 minutes to reach a constant temperature. The battery is charged at 0.33C constant current to 3.7V, then charged at 3.7V constant voltage to 0.05C, allowed to stand for 5 minutes, then discharged at 0.33C constant current to 1.5V, allowed to stand for 5 minutes, and the 0.33C discharge capacity C1 is obtained. Then the battery is charged at 0.33C constant current to 3.7V, then charged at 3.7V constant voltage to 0.05C, allowed to stand for 5 minutes, then discharged at 3C constant current to 1.5V, allowed to stand for 5 minutes, and the 3C discharge capacity C2 is obtained. The capacity retention at 3C rate is R = C2 / C1 x 100%. The test process of the comparative examples and other examples is the same as above.

[0231] III. Analysis of test results of examples and comparative examples

[0232] The batteries of the examples and comparative examples were prepared according to the above method, and various performance parameters were measured. The results are shown in Table 1 below.

[0233] Table 1

[0234]

[0235]

[0236] According to the above results, examples 1-31 are all positive electrode active materials for secondary batteries, which include a iron-based polyanionic compound and a magnesium-containing oxide. The iron-based polyanionic compound has the following general formula: Na4Fe 3-x M x Mg y(PO4)2P2O7 / C, wherein M comprises a transition metal element, 0≤x≤0.5, 0

[0237] From the comparison of Examples 1-31 and Comparative Examples 1-2, it can be seen that the doping of magnesium element in the iron-based polyanionic compound and the residue of the oxide derived from the magnesium source on the surface of the iron-based polyanionic compound effectively reduce the residual alkali content of the positive active material, increase the gram capacity of the positive active material, and increase the capacity retention rate of the battery at high rate.

[0238] From the comparison of Examples 1-31 and Comparative Example 3, it can be seen that controlling the value of y in the positive active material to be greater than 0 and less than 0.18 can increase the gram capacity of the positive active material, increase the capacity retention rate of the battery after 200 cycles, and increase the capacity retention rate at 3C rate.

[0239] From the comparison of Examples 7-31 and Comparative Examples 4-5, it can be seen that compared to the conventional positive active material containing only transition metal elements, the positive active material including the magnesium-containing oxide and the Mg-doped iron-based polyanionic compound is beneficial to reduce the residual alkali content of the positive active material, increase the gram capacity of the positive active material, increase the capacity retention rate of the battery after 200 cycles, and increase the capacity retention rate at 3C rate.

[0240] From the comparison of Examples 1-6 and Comparative Example 1, it can be seen that controlling the mass content of the carbon material to be 0.5%-6% based on the total mass of the positive active material is beneficial to improve the conductivity of the positive active material, increase the gram capacity of the positive active material, and increase the capacity retention rate of the battery after 200 cycles and the capacity retention rate at 3C rate. From the comparison of Examples 2-4 and Examples 1, 5-6, it can be seen that controlling the mass content of the carbon material to be 1%-3.6% based on the total mass of the positive active material is beneficial to further increase the gram capacity of the positive active material.

[0241] From the comparison of Examples 7-12 and Comparative Examples 4-5, it can be seen that controlling the value of x of the transition metal element to be less than or equal to 0.5 is beneficial to reduce the residual alkali content of the positive active material, increase the gram capacity of the positive active material, and increase the capacity retention rate of the battery after 200 cycles and the capacity retention rate at 3C rate.

[0242] From the comparison of Examples 7, 13-17 and Comparative Example 2, it can be seen that the positive active material is introduced with magnesium element and magnesium-containing oxide, and controlling the total mass content of the magnesium element in the positive active material to be 0.02%-0.7% based on the total mass of the positive active material is beneficial to reduce the residual alkali content of the positive active material, increase the gram capacity of the positive active material, and increase the capacity retention rate of the battery at 3C rate.

[0243] From the comparison of Examples 7, 18-21 and Comparative Example 2, it can be seen that controlling the calcination temperature to be 500-600°C is conducive to reducing the residual alkali content of the positive electrode active material, improving the gram capacity of the positive electrode active material, and improving the capacity retention rate of the battery after 200 cycles and the capacity retention rate at 3C rate. From the comparison of Examples 7, 19-20 and Examples 18, 21, it can be seen that controlling the calcination temperature to be 525-575°C is conducive to further improving the gram capacity of the positive electrode active material, improving the capacity retention rate of the battery after 200 cycles and the capacity retention rate at 3C rate. From the comparison of Examples 7, 20 and Examples 18-19, 21, it can be seen that controlling the calcination temperature to be 550-575°C is conducive to further improving the capacity retention rate of the battery after 200 cycles and the capacity retention rate at 3C rate.

[0244] From the comparison of Examples 7, 22-25 and Comparative Example 2, it can be seen that controlling the calcination time to be 8-15h is conducive to reducing the residual alkali content of the positive electrode active material, improving the gram capacity of the positive electrode active material, and improving the capacity retention rate of the battery after 200 cycles and the capacity retention rate at 3C rate. From the comparison of Examples 7, 23-24 and Comparative Examples 22, 25, it can be seen that controlling the calcination time to be 10-14h is conducive to further improving the gram capacity of the positive electrode active material, improving the capacity retention rate of the battery after 200 cycles and the capacity retention rate at 3C rate.

[0245] From the comparison of Examples 7, 26-31 and Comparative Examples 4-5, it can be seen that controlling the transition metal elements in the positive electrode active material to contain one or more of Ni, Co, Mn and Ca is conducive to reducing the residual alkali content of the positive electrode active material, improving the gram capacity of the positive electrode active material, and improving the capacity retention rate of the battery after 200 cycles and the capacity retention rate at 3C rate.

[0246] From the comparison of Example 26 and Examples 7, 29, and Example 27 and Examples 7, 30, it can be seen that compared to the transition metal elements in the positive electrode active material containing only one of Ni, Co and Mn, controlling the transition metal elements in the positive electrode active material to contain two of Ni, Co and Mn is conducive to further improving the capacity retention rate of the battery at 3C rate. From the comparison of Example 28 and Examples 26-27, it can be seen that compared to the transition metal elements in the positive electrode active material containing only two of Ni, Co and Mn, controlling the transition metal elements in the positive electrode active material to contain all of Ni, Co and Mn is conducive to further improving the capacity retention rate of the battery at 3C rate.

[0247] Note that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration, function, and effect as the technical idea of the present application are included in the technical scope of the present application. Furthermore, other modes constructed by applying various modifications to the embodiments, or by combining part of the configurations of the embodiments, which can be conceived by those skilled in the art, without departing from the spirit of the present application, are also included in the scope of the present application.

Claims

1. A positive electrode active material for secondary batteries, characterized in that, The positive electrode active material comprises a carbon material composite iron-based polyanion compound and a magnesium-containing oxide, the iron-based polyanion compound has the following general formula: Na4Fe 3-x M x Mg y (PO4)2P2O7 / C Wherein, M comprises a transition metal element, 0≤x≤0.5, 0 2. The positive electrode active material according to claim 1, characterized by The magnesium-containing oxide is distributed on at least part of the surface of primary particles of the iron-based polyanion compound.

3. The positive electrode active material according to claim 1 or 2, characterized by The mass content of magnesium element in the positive electrode active material is 0.02% to 0.7% based on the total mass of the positive electrode active material.

4. The positive electrode active material according to any one of claims 1 to 3, characterized by, The M comprises one or more of Ni, Co, Mn, Cu, V, Ti, Mo, Nb, W, Cr, Zn, Zr, and Ca.

5. The positive electrode active material according to claim 4, characterized by The M comprises one or more of Ni, Co, Mn, Cu, V, and Ca.

6. The positive electrode active material according to any one of claims 1 to 5, characterized by, The M comprises at least two of Ni, Co, Mn, Cu, V, and Ca.

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

8. The positive electrode active material according to any one of claims 1 to 7, characterized by, The carbon material comprises one or more of amorphous carbon, conductive carbon black, carbon nanotubes, and graphene.

9. The positive electrode active material according to any one of claims 1 to 8, characterized by, The mass content of the carbon material is 0.5% to 6% based on the total mass of the positive electrode active material.

10. The positive electrode active material according to claim 9, characterized by The mass content of the carbon material is 1% to 3.6% based on the total mass of the positive electrode active material.

11. The positive electrode active material according to any one of claims 1 to 10, characterized by The specific capacity of the positive electrode active material is ≥98 mA·h / g.

12. The positive electrode active material according to any one of claims 1 to 11, characterized in that, The NaHCO3 residual alkali content of the positive electrode active material is less than 1.2% based on the total mass of the positive electrode active material.

13. A method for producing a positive electrode active material for a secondary battery, characterized by, Comprising the following steps: The raw materials including an iron source, a sodium source, a phosphorus source, a magnesium source, and a carbon source are dissolved in water to obtain a mixed slurry, the raw materials include an M source, and the M source includes a salt containing a transition metal; The mixed slurry is dried and calcined to prepare the positive electrode active material, the positive electrode active material comprises an iron-based polyanion compound and a magnesium-containing oxide, and the iron-based polyanion compound has the following general formula: Na4Fe 3-x M x Mg y (PO4)2P2O7 / C Wherein, M comprises a transition metal element, 0≤x≤0.5, 0 14. The method of claim 13, wherein, The mixed slurry is dried and calcined to prepare the positive electrode active material, the positive electrode active material comprises an iron-based polyanion compound and a magnesium-containing oxide, and the iron-based polyanion compound has the following general formula: Wherein, M comprises a transition metal element, 0≤x≤0.5, 0 The mixed slurry is dried and calcined to prepare the positive electrode active material, the positive electrode active material comprises an iron-based polyanion compound and a magnesium-containing oxide, and the iron-based polyanion compound has the following general formula:

15. The preparation method according to claim 14, characterized in that, The first step calcination temperature is 300°C to 400°C, and the time is 3 hours to 6 hours, the second step calcination temperature is 500°C to 600°C, and the time is 8 hours to 15 hours to prepare the positive electrode active material.

16. The method of claim 15, wherein, The second step calcination temperature is 525°C to 575°C.

17. The production method according to any one of claims 14 to 16, characterized by, The second step calcination temperature is 550°C to 575°C.

18. The production method according to any one of claims 13 to 17, characterized by, The second step calcination time is 10 hours to 14 hours.

19. The production method according to any one of claims 13 to 18, characterized by, 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. The magnesium source includes one or more of magnesium nitrate, magnesium carbonate, magnesium acetate, magnesium hydroxide, magnesium oxalate, and magnesium oxide.

20. The production method according to any one of claims 13 to 19, characterized by, 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, graphene.

21. A secondary battery characterized by comprising: The secondary battery includes a cathode electrode sheet including the cathode active material of any one of claims 1 to 12 or the cathode active material prepared by the preparation method of any one of claims 13 to 20.

22. The secondary battery according to claim 21, characterized by The secondary battery is a sodium battery.

23. The secondary battery according to claim 21 or 22, characterized by The secondary battery is a sodium battery without an anode.

24. The secondary battery according to any one of claims 21 to 23, characterized by, The secondary battery further includes an anode electrode sheet including an anode current collector and a primer layer disposed on at least one surface of the anode current collector, the primer layer including one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, tin composite carbon nanoparticles.

25. The secondary battery according to claim 24, characterized by The areal density of the primer coat is 5 g / m 2 50 g / m 2 .

26. The secondary battery according to claim 24 or 25, characterized by The thickness of the primer layer is 2 μm to 100 μm.

27. A battery module, comprising: The secondary battery of any one of claims 21 to 26.

28. A battery pack, characterized by The secondary battery of any one of claims 21 to 26 or the battery module of claim 27.

29. An electrical device, comprising: At least one of the secondary battery of any one of claims 21 to 26, the battery module of claim 27, or the battery pack of claim 28.

Images