Positive electrode active material, manufacturing method thereof, secondary battery, battery module, battery pack, and electric device

Abstract

The present application provides a cathode active material, a manufacturing method thereof, a secondary battery, a battery module, a battery pack, and an electric device. The cathode active material for a secondary battery includes an iron-based polyanion compound composited with a carbon material and a magnesium-containing oxide, and the iron-based polyanion compound is NaFe. 3-x M x Mg y It has the general formula (PO4)2P2O7, where 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, and in particular to a positive electrode active material and a manufacturing method thereof, a secondary battery, a battery module, a battery pack, and an electric device.

[0002] [Cross reference] This application cites a Chinese patent application filed on January 16, 2023, entitled "Positive electrode active material, and manufacturing method thereof, secondary battery, battery module, battery pack, and electrical device," with Chinese patent application number 202310070398.5, which is incorporated herein by reference in its entirety. [Background technology]

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

[0004] Iron-based polyanion compounds, the new trend in sodium battery cathode materials, have significant advantages, including abundant resources, high operating voltage, excellent cycling stability, and environmental friendliness. However, conventional sodium batteries using iron-based polyanion compounds as cathode materials still suffer from problems such as low electronic conductivity, low discharge capacity, and poor cycling performance, which cannot meet the application needs of new-generation electrochemical systems. Summary of the Invention

[0005] The present application has been made in view of the above-mentioned problems, and aims to provide a positive electrode active material that realizes the dual effects of crystal lattice doping and complex modification by introducing magnesium element, thereby effectively reducing the amount of residual alkali in the positive electrode active material and improving the cycle performance and rate performance of the battery.

[0006] In a first aspect of the present application, there is provided a positive electrode active material for a secondary battery, comprising a carbon material composite iron-based polyanion compound and a magnesium-containing oxide, wherein the iron-based polyanion compound is NaFe 3-x M x Mg y It has the general formula (PO4)2P2O7 / C, Wherein, M contains a transition metal element, 0≦x≦0.5, 0 <y<0.18である。

[0007] When magnesium is introduced into a positive electrode active material, a portion of the magnesium is doped into the crystal lattice of an iron-based polyanion compound, replacing the iron or transition metal elements in the crystal lattice of the iron-based polyanion, thereby improving the ionic and electronic conductivity of the positive electrode active material. Meanwhile, a portion of the magnesium is distributed on the surface of the iron-based polyanion compound in the form of a magnesium-containing oxide, reducing direct contact between the surface of the positive electrode active material and moisture in the environment, reducing the amount of residual alkali in the positive electrode active material, and improving the stability and processing performance of the positive electrode active material. Furthermore, the positive electrode active material has a higher gram capacity, which is advantageous for improving the energy density of batteries.

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

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

[0010] The magnesium-containing oxide is distributed on part or the entire surface of the primary particles of the iron-based polyanion compound, thereby effectively reducing direct contact between the surface of the positive electrode active material and moisture in the environment, reducing the amount of residual alkali in the positive electrode active material, and improving the stability and processing performance of the positive electrode active material.

[0011] In any embodiment, 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.

[0012] When the mass content of magnesium element in the positive electrode active material is controlled to 0.02% to 0.7% based on the total mass of the positive electrode active material, the amount of residual alkali in the positive electrode active material is reduced, the stability and processability of the positive electrode active material are improved, and it is advantageous to improve the cycle performance and rate performance of the battery.

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

[0014] Introducing doping of transition metal elements into iron-based polyanion compounds is advantageous for enhancing the voltage platform of the positive electrode active material, improving the ionic and electronic conductivity of the material, reducing the polarization of the battery, and improving the cycle performance and rate performance of the battery.

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

[0016] Controlling M to include at least two of Ni, Co, Mn, Cu, V, and Ca is advantageous in further improving the rate performance of the battery.

[0017] 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 is distributed in the form of particles among the primary ions of the iron-based polyanionic compound.

[0018] The carbon material can be coated on the surface of the iron-based polyanion compound in the form of a carbon film or distributed in the form of particles among the primary ions of the iron-based polyanion compound, both of which 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 materials are easy to disperse and process, and form carbon composites with iron-based polyanion compounds to improve the conductivity of the positive electrode active material and battery performance. Among them, conductive carbon black, carbon nanotubes, and graphene have higher crystallinity than amorphous carbon, so the conductivity of the carbon materials such as conductive carbon black, carbon nanotubes, and graphene is superior to that of amorphous carbon.

[0021] In any embodiment, 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.

[0022] Controlling the mass content of the carbon material between 0.5% and 6% based on the total mass of the positive electrode active material is advantageous for achieving a higher gram capacity, better cycle performance, and rate performance, while also achieving a balance between the gram capacity and conductivity of the positive electrode active material. A too low mass content of the carbon material reduces the conductivity of the positive electrode active material, avoiding the promotion of capacity volatilization. A too high mass content of the carbon material reduces the gram capacity of the positive electrode active material and avoids poor crimping performance. Controlling the mass content of the carbon material between 1% and 3.6% based on the total mass of the positive electrode active material is advantageous for further improving the gram capacity of the positive electrode active material and further improving the battery capacity.

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

[0024] A positive electrode active material with a gram capacity of ≥ 98 mAh / g is advantageous for improving the capacity of the battery and broadening the application of the battery.

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

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

[0027] In a second aspect of the present application, there is provided a method for producing a positive electrode active material for a secondary battery, the method comprising: A step of dissolving raw materials including an iron source, a sodium source, a phosphorus source, and a magnesium source in water to obtain a mixed slurry, wherein the raw materials optionally include an M source, and the M source includes a salt containing a transition metal; and drying the mixed slurry and then calcining it to produce the positive electrode active material, the positive electrode active material including an iron-based polyanion compound and a magnesium-containing oxide, the iron-based polyanion compound having the following general formula: NaFe 3-x M x Mg y (PO4)2P2O7 / C Wherein, M contains a transition metal element, 0≦x≦0.5, 0 <y<0.18である。

[0028] The positive electrode active material described above is simple to manufacture and has low manufacturing costs. The positive electrode active material produced has a lower residual alkali content and a higher gram capacity, and the resulting battery has excellent cycle performance and rate performance.

[0029] In any embodiment, the calcination of the mixed slurry after drying includes: drying the mixed slurry to obtain a precursor powder; and calcining the precursor powder in stages, the first stage being at a temperature of 300°C to 400°C for a time of 3 hours to 6 hours, and the second stage being at a temperature of 500°C to 600°C for a time of 8 hours to 15 hours, thereby producing the positive electrode active material.

[0030] The step-by-step calcination promotes complete reaction of the precursor powder, improves the crystallinity and phase purity of the produced positive electrode active material, reduces the amount of residual alkali on the surface of the positive electrode active material, improves the stability and processability of the positive electrode active material, increases the gram capacity of the positive electrode active material, and improves the cycle performance and rate performance of the battery.

[0031] In an optional embodiment, the second stage firing temperature is between 525°C and 575°C, optionally between 550°C and 575°C.

[0032] When the second-stage calcination temperature is controlled between 525°C and 575°C, the amount of residual alkali on the surface of the active material is further reduced, the conductivity of the material is improved, and the cycle performance and rate performance of the battery are improved.Furthermore, when the second-stage calcination temperature is controlled between 550°C and 575°C, the capacity retention rate of the battery after 200 cycles and the capacity retention rate at a 3C rate are significantly improved, and the cycle performance and rate performance of the battery are also significantly improved.

[0033] In any embodiment, the second stage firing time is 10 hours to 14 hours.

[0034] By controlling the calcination time in the second stage to 10 to 14 hours, a highly crystalline and pure positive electrode active material can be obtained. At the same time, by controlling the heat retention time, the particle size of the primary crystal particles and the amount of residual alkali on the material surface can be controlled, improving the gram capacity of the material and the cycle performance and rate performance of the battery.

[0035] 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.

[0036] In any embodiment, the magnesium source comprises one or more of magnesium nitrate, magnesium carbonate, magnesium acetate, magnesium hydroxide, magnesium oxalate, magnesium oxide.

[0037] 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 nanotubes, and graphene.

[0038] The carbon sources mentioned above can easily produce carbon materials by high-temperature calcination, which coat the surface of the iron-based polyanion compound or distribute among the primary particles of the iron-based polyanion compound, thereby improving the conductivity of the positive electrode active material. Due to the influence of the calcination temperature, amorphous carbons formed from carbon sources containing 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 have lower crystallinity, while carbon materials formed from carbon sources containing one or more of conductive carbon black, carbon nanotubes, and graphene have higher crystallinity. Therefore, carbon materials formed from carbon sources containing one or more of conductive carbon black, carbon nanotubes, and graphene have better conductivity.

[0039] In a third aspect of the present application, there is provided a secondary battery including a positive electrode piece, wherein the positive electrode piece includes a positive electrode active material described in any embodiment or a positive electrode active material produced by the production method described in any embodiment.

[0040] In any embodiment, the secondary battery is a non-anode sodium battery.

[0041] In any embodiment, the secondary battery further includes a negative electrode piece, the negative electrode piece including a negative electrode current collector and a primer layer disposed on at least one surface of the negative electrode current collector, the primer layer including one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, and tin composite carbon nanoparticles.

[0042] The primer layer not only has excellent electrical conductivity but also promotes uniform deposition of metal ions on the surface of the current collector, improving the cycle performance and safety of the battery.

[0043] In any embodiment, the areal density of the primer layer is 5 g / m 2 ~50g / m 2 is.

[0044] Surface density is 5g / m 2 ~50g / m 2 The primer layer favors a uniform distribution of nucleation sites and promotes uniform deposition of the metal without affecting the electron transport behavior.

[0045] In any embodiment, the thickness of the primer layer is from 2 μm to 100 μm.

[0046] Controlling the thickness of the primer layer to 2 μm to 100 μm provides sufficient nucleation sites, promotes uniform deposition of metal ions, and suppresses the occurrence of dendrites.

[0047] In a fourth aspect of the present application, there is provided a battery module including the secondary battery of the third aspect of the present application.

[0048] 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.

[0049] In a sixth aspect of the present application, there is provided an electric device including at least one of the secondary battery according to the third aspect of the present application, the battery module according to the fourth aspect of the present application, and the battery pack according to the fifth aspect of the present application. [Brief explanation of the drawings]

[0050] [Figure 1] 1 is a schematic diagram of a secondary battery according to an embodiment of the present application; [Figure 2] FIG. 2 is an exploded view of the secondary battery according to the embodiment of the present invention shown in FIG. [Figure 3] 1 is a schematic diagram of a battery module according to an embodiment of the present application; [Figure 4] 1 is a schematic diagram of a battery pack according to an embodiment of the present application. [Figure 5] FIG. 5 is an exploded view of the battery pack according to the embodiment of the present application shown in FIG. 4. [Figure 6] 1 is a schematic diagram of an electrical device that uses a secondary battery as a power source according to an embodiment of the present application. DETAILED DESCRIPTION OF THE INVENTION

[0051] Hereinafter, with reference to the drawings as appropriate, embodiments of the positive electrode active material, its manufacturing method, secondary battery, battery module, battery pack, and electric device specifically disclosed in the present application will be described in detail. However, unnecessary detailed description may be omitted. For example, detailed description of already well-known matters and repeated description of substantially identical configurations may be omitted. This is to avoid unnecessarily lengthening the following description and to facilitate understanding by those skilled in the art. Furthermore, the 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 scope of the claims.

[0052] The "ranges" disclosed herein are defined by lower and upper limits, and a given range is defined by selecting one lower limit and one upper limit, with the selected lower and upper limits defining the boundaries of that particular range. Such defined ranges may be inclusive or exclusive of both endpoints and may be arbitrarily combined; i.e., any lower limit can be combined with any upper limit to form a single range. For example, if ranges of 60 to 120 and 80 to 110 are recited for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. Furthermore, if the recited minimum range values ​​are 1 and 2, and the recited maximum range values ​​are 3, 4, and 5, then the ranges 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5 are all contemplated. Unless otherwise specified, the numerical range "a to b" herein represents a shorthand notation for any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0 to 5" indicates that all real numbers between "0 and 5" have already been listed in this specification, and "0 to 5" is simply an abbreviation for a combination of those numerical values. Note that when a parameter is described as an integer ≧2, this is equivalent to disclosing that the parameter is an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0053] Unless otherwise specified, all embodiments and optional embodiments in the present application can be combined with each other to form a new technical solution.

[0054] Unless otherwise specified, all technical features and optional technical features of the present application can be combined with each other to form a new technical solution.

[0055] Unless otherwise specified, all steps in the present application may be performed sequentially or randomly, but are preferably performed sequentially. For example, when the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, when the method described above may further include step (c), it means that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or may include steps (a), (c), and (b), or may include steps (c), (a), and (b), etc.

[0056] Unless otherwise specified, the terms "comprise" and "comprises" used in this application may be open or closed. For example, the terms "comprise" and "comprises" may further include or include other components not listed, or may include or include only the listed components.

[0057] Unless otherwise stated, in this 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 conditions 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).

[0058] In the prior art, polyanion 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 excellent thermal and cycling stability. However, during the actual manufacturing process, researchers have found that the amount of residual alkali on the surface of polyanion compound materials is high, and that exposure to air can easily increase the residual alkali on the material surface, resulting in a decrease in gram capacity and potentially affecting battery performance. Therefore, to meet the needs of applications in new-generation electrochemical systems, it is necessary to develop cathode active materials with low residual alkali, excellent electrochemical performance, and the ability to be used in high-power batteries.

[0059] [Cathode active material] Based on this, the present application proposes a positive electrode active material for a secondary battery, which comprises a carbon material composite iron-based polyanion compound and a magnesium-containing oxide, and the iron-based polyanion compound is NaFe 3-x M x Mg y It has the general formula (PO4)2P2O7 / C, where M contains a transition metal element, 0≦x≦0.5, 0 <y<0.18である。

[0060] As used herein, the term "transition metal elements" refers to elements in groups IIIB-VIIB, VIII, and IB-IIB of the Periodic Table of the Elements.

[0061] In some embodiments, x is optionally 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or 0.5.

[0062] In some embodiments, y is optionally 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, or 0.17.

[0063] When magnesium is introduced into a positive electrode active material, a portion of the magnesium is doped into the crystal lattice of an iron-based polyanion compound, replacing the iron or transition metal elements in the crystal lattice of the iron-based polyanion, thereby improving the ionic and electronic conductivity of the positive electrode active material. Meanwhile, a portion of the magnesium is distributed on the surface of the iron-based polyanion compound in the form of a magnesium-containing oxide, reducing direct contact between the surface of the positive electrode active material and moisture in the environment, reducing the amount of residual alkali in the positive electrode active material, and improving the stability and processing performance of the positive electrode active material. Furthermore, the positive electrode active material has a higher gram capacity, which is advantageous for improving the energy density of batteries.

[0064] Introducing carbon materials into the positive electrode active material can effectively improve the conductivity of the positive electrode active material and the battery performance.

[0065] As used herein, the term "gram capacity" refers to the amount of electricity delivered per gram of positive electrode active material, reflecting the capacity of the battery, which can be tested by any known method.

[0066] In some embodiments, the magnesium-containing oxide is distributed on at least a portion of the surface of the primary particles of the iron-based polyanionic compound.

[0067] In this specification, the term "primary particles" refers to particles of an iron-based polyanionic compound before aggregation, and the particle size range of the primary particles of the iron-based polyanionic compound is 30 nm to 120 nm.

[0068] In some embodiments, the magnesium-containing oxide comprises magnesium oxide.

[0069] In some embodiments, the magnesium-containing oxide is distributed on a portion of the surface of the primary particles of the iron-based polyanionic compound.

[0070] In some embodiments, the magnesium-containing oxide is distributed over the entire surface of the primary particles of the iron-based polyanionic compound.

[0071] The magnesium-containing oxide is distributed on part or the entire surface of the primary particles of the iron-based polyanion compound, thereby effectively reducing direct contact between the positive electrode active material and moisture in the environment, reducing the amount of residual alkali in the positive electrode active material, and improving the stability and processing performance of the positive electrode active material.

[0072] In some embodiments, the mass content of elemental magnesium in the positive electrode active material is 0.02% to 0.7%, based on the total mass of the positive electrode active material. In some embodiments, the mass content of elemental magnesium in the positive electrode active material is optionally 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 electrode active material.

[0073] In some embodiments, the elemental magnesium in the positive electrode active material includes Mg in an iron-based polyanion compound and magnesium in a magnesium-containing oxide.

[0074] The mass content of magnesium element in the positive electrode active material can be tested using any method known in the art. For example, see standards YS / T 1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015. Specifically, an inductively coupled plasma optical emission spectrometer (Thermo, ICAP7400, PerkinElmer, Avio200) is used to measure the emission intensity of magnesium element in the sample to be measured. Standard samples with different magnesium content can be prepared and their emission intensities measured as a calibration curve. The mass content of magnesium element in the sample to be measured can be determined on the calibration curve based on the emission intensity of the magnesium element.

[0075] When the mass content of magnesium in the positive electrode active material is controlled to 0.02% to 0.7%, based on the total mass of the positive electrode active material, the amount of residual alkali in the positive electrode active material is reduced, the stability and processing performance of the positive electrode active material are improved, the gram capacity of the positive electrode active material and the capacity retention rate of the battery at a 3C rate are improved, and this is beneficial to improving the cycle performance and rate performance of the battery.

[0076] In some embodiments, M includes one or more of Ni, Co, Mn, Cu, V, Ti, Mo, Nb, W, Cr, Zn, Zr, and Ca, and optionally includes one or more of Ni, Co, Mn, Cu, V, and Ca.

[0077] 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.

[0078] Introducing doping of transition metal elements into iron-based polyanion compounds is advantageous for enhancing the voltage platform of the positive electrode active material, improving the ionic and electronic conductivity of the material, reducing the polarization of the battery, and improving the cycle performance and rate performance of the battery.

[0079] In some embodiments, M comprises at least two of Ni, Co, Mn, Cu, V, and Ca.

[0080] 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.

[0081] Controlling M to include at least two of Ni, Co, Mn, Cu, V, and Ca is advantageous in further improving the rate performance of the battery.

[0082] In some embodiments, the carbon material is coated on the surface of the iron-based polyanionic compound in the form of a carbon film or distributed in the form of particles among the primary ions of the iron-based polyanionic compound.

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

[0084] In some embodiments, the carbon material is distributed in the form of particles among the primary ions of the iron-based polyanionic compound.

[0085] The carbon material can be coated on the surface of the iron-based polyanion compound in the form of a carbon film or distributed in the form of particles among the primary ions of the iron-based polyanion compound, both of which can effectively improve the conductivity of the positive electrode active material and the performance of the battery.

[0086] In some embodiments, the carbon material comprises one or more of amorphous carbon, conductive carbon black, carbon nanotubes, and graphene.

[0087] 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.

[0088] The carbon materials can be easily combined with iron-based polyanion compounds to improve the conductive properties of the positive electrode active material. Among them, conductive carbon black, carbon nanotubes, and graphene have higher crystallinity than amorphous carbon, so the conductivity of the conductive carbon black, carbon nanotubes, and graphene carbon materials is superior to that of amorphous carbon.

[0089] In some embodiments, the mass content of the carbon material is 0.5% to 6%, and 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.

[0090] Controlling the mass content of the carbon material between 0.5% and 6% based on the total mass of the positive electrode active material is advantageous for achieving higher gram capacity, better cycle performance, and rate performance, while also achieving a balance between the gram capacity and conductivity of the positive electrode active material. A too low mass content of the carbon material reduces the conductivity of the positive electrode active material, avoiding the promotion of capacity volatilization of the positive electrode active material. A too high mass content of the carbon material also reduces the gram capacity of the positive electrode active material and avoids the effects of compaction density. Controlling the mass content of the carbon material between 1% and 3.6% based on the total mass of the positive electrode active material is advantageous for further improving the gram capacity of the positive electrode active material and further improving the battery capacity.

[0091] In some embodiments, the gram capacity of the positive electrode active material is ≧98 mAh / g.

[0092] In some embodiments, the gram capacity of the positive electrode active material is optionally 98 mAh / g, 100 mAh / g, 105 mAh / g, 110 mAh / g, 115 mAh / g, 120 mAh / g, or 125 mAh / g.

[0093] The gram capacity of the positive electrode active material can be tested using any means known in the art. For example, at 25°C and in a normal pressure environment, a button battery is charged at a constant current of 0.1 C until the voltage reaches 4 V, and then charged at a constant voltage of 4 V until the current drops to 0.05 C. The charge specific capacity at this time, i.e., the initial sodium release capacity, is recorded, and then the battery is discharged at a constant current rate of 0.1 C until the voltage reaches 1.5 V. The discharge specific capacity at this time is recorded and is defined as the initial sodium storage capacity. The gram capacity of the positive electrode active material is the initial sodium storage capacity.

[0094] A positive electrode active material with a gram capacity of ≥ 98 mAh / g is advantageous for improving the capacity of the battery and broadening the application of the battery.

[0095] In some embodiments, the amount of residual alkali of NaHCO3 in the positive electrode active material is less than 1.2%, based on the total mass of the positive electrode active material.

[0096] In some embodiments, the amount of residual alkalinity of NaHCO in the positive electrode active material is optionally 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.

[0097] The amount of residual alkalinity in the positive electrode active material, NaHCO3, can be tested using any means known in the art. For example, it can be measured by potentiometric titration of chemical reagents, as described in GB / T 9725-2007. Specifically, the positive electrode active material is dissolved in deionized water using a Metrohm 905 Titrando potentiometric titrator. The aqueous solution of the positive electrode active material is titrated using a standard titration solution. 0.1 mL of the standard titration solution is added to each drop, recording the potential or pH value. The titration is terminated when the potential or pH value no longer changes significantly. The amount of titration standard added and the measured potential or pH value are recorded, and the titration endpoint is determined by a graphical or second-order derivative method, along with the titration volume of the titration standard. The calculated mass of NaHCO3 divided by the mass of the positive electrode active material is used to determine the amount of residual alkalinity in the NaHCO3 positive electrode active material.

[0098] Based on the total mass of the positive electrode active material, the residual alkali content of NaHCO3 in the positive electrode active material is less than 1.2%, which can effectively improve the stability and processability.

[0099] The present application provides a method for producing a positive electrode active material for a secondary battery, the method comprising: A step of dissolving raw materials including an iron source, a sodium source, a phosphorus source, and a magnesium source in water to obtain a mixed slurry, wherein the raw materials optionally include an M source, and the M source includes a salt containing a transition metal; and drying the mixed slurry and then calcining it to produce a positive electrode active material, the positive electrode active material including an iron-based polyanion compound and a magnesium-containing oxide, the iron-based polyanion compound having the following general formula: NaFe 3-x M x Mg y (PO4)2P2O7 / C Wherein, M contains a transition metal element, 0≦x≦0.5, 0 <y<0.18である。

[0100] The positive electrode active material described above is simple to manufacture and has low manufacturing costs. The positive electrode active material produced has a lower residual alkali content and a higher gram capacity, and the resulting battery has excellent cycle performance and rate performance.

[0101] The introduction of transition metals into the positive electrode active material is advantageous for improving the ionic and electronic conductivity of the material and enhancing the cycle and rate capabilities of the battery.

[0102] In some embodiments, the feedstock further comprises a carbon source.

[0103] The addition of a carbon source is advantageous in that conductive carbon elements are introduced into the positive electrode active material, thereby improving the conductivity of the positive electrode active material.

[0104] In some embodiments, drying the mixed slurry and then calcining the mixed slurry comprises: drying the mixed slurry to obtain a precursor powder; and calcining the precursor powder in stages, the first stage being at a temperature of 300°C to 400°C for 3 hours to 6 hours, and the second stage being at a temperature of 500°C to 600°C for 8 hours to 15 hours, to produce a positive electrode active material.

[0105] The step-by-step calcination promotes complete reaction of the precursor powder, improves the crystallinity and phase purity of the produced positive electrode active material, reduces the amount of residual alkali on the surface of the positive electrode active material, improves the stability and processability of the positive electrode active material, increases the gram capacity of the positive electrode active material, and improves the cycle performance and rate performance of the battery.

[0106] In some embodiments, the second stage firing temperature is between 525°C and 575°C, optionally between 550°C and 575°C.

[0107] The second-stage calcination temperature can be controlled to 525°C to 575°C, which is advantageous for further reducing the amount of residual alkali 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-stage calcination temperature to 550°C to 575°C is advantageous for greater improvement in the cycle performance and rate performance of the battery.

[0108] In some embodiments, the second stage firing time is 10 hours to 14 hours.

[0109] By controlling the calcination time in the second stage to 10 to 14 hours, a highly crystalline and pure positive electrode active material can be obtained. At the same time, by controlling the heat retention time, the particle size of the primary crystal particles and the amount of residual alkali on the material surface can be controlled, improving the gram capacity of the material and the cycle performance and rate performance of the battery.

[0110] In some embodiments, 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.

[0111] In some embodiments, the magnesium source comprises one or more of magnesium nitrate, magnesium carbonate, magnesium acetate, magnesium hydroxide, magnesium oxalate, magnesium oxide.

[0112] 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 nanotubes, and graphene.

[0113] 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, and citric acid.

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

[0115] The carbon sources mentioned above can easily produce carbon materials by high-temperature calcination, which coat the surface of the iron-based polyanion compound or distribute among the primary particles of the iron-based polyanion compound, thereby improving the conductivity of the positive electrode active material. Due to the influence of the calcination temperature, amorphous carbons formed from carbon sources containing 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 have lower crystallinity, while carbon materials formed from carbon sources containing one or more of conductive carbon black, carbon nanotubes, and graphene have higher crystallinity. Therefore, carbon materials formed from carbon sources containing one or more of conductive carbon black, carbon nanotubes, and graphene have better conductivity.

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

[0117] The positive electrode active material layer may contain a conductive agent to improve the conductivity of the positive electrode, which may be one or more of Super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphite, graphene, and carbon nanofibers.

[0118] The positive electrode active material layer may further include an adhesive for firmly adhering the positive electrode active material and any conductive agent to the positive electrode current collector, and the adhesive may include 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).

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

[0120] In some embodiments, the positive electrode pieces can be manufactured by a method in which the above-mentioned components for manufacturing the positive electrode pieces, such as the positive electrode active material, conductive agent, adhesive, and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry, and the positive electrode slurry is applied onto a positive electrode current collector, followed by processes such as drying and cold pressing to obtain the positive electrode pieces.

[0121] [Separator film] In some embodiments, the secondary battery further includes a separator film. The present application does not particularly limit the type of separator film, but any separator film having a known porous structure and good chemical and mechanical stability can be selected.

[0122] In some embodiments, the separator film is made of at least one material selected from the group consisting of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers. The separator film may be a single-layer thin film or a multilayer composite thin film, without any particular limitations. When the separator film is a multilayer composite thin film, the materials of the layers may be the same or different, without any particular limitations.

[0123] In some embodiments, the positive electrode strips, negative electrode strips, and separator film can be fabricated into an electrode assembly by a winding or lamination process.

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

[0125] In some embodiments, the exterior packaging of the secondary battery may be a hard casing, such as a hard plastic casing, an aluminum case, a steel case, etc. The exterior packaging of the secondary battery may be a pouch, such as a bag-type pouch. The pouch may be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0126] [Secondary battery] The secondary battery includes a positive electrode piece, which includes the positive electrode active material of some embodiments or the positive electrode active material produced by the production method of some embodiments.

[0127] In some embodiments, the secondary battery further comprises a negative electrode piece, a separator, and an electrolyte.

[0128] In some embodiments, the secondary battery is a non-anode sodium battery.

[0129] Anode-less sodium batteries do not include a pre-deposited anode active material and only include anode current collectors. During initial charging, sodium ions gain electrons on the anode side, depositing metallic sodium on the surface of the current collector to form a sodium metal phase. During discharge, metallic sodium is converted back to sodium ions and returned to the cathode, thus enabling repeated charging and discharging. Anode-less sodium batteries are not limited by the anode material, allowing for higher energy density. Anode-less sodium batteries lack sufficient sodium metal as anode material to provide sufficient sodium element to the battery. Therefore, the application of the cathode active material provided herein can more effectively improve the cycle performance and high-rate performance of secondary batteries.

[0130] In some embodiments, the CB value of the anode-less sodium battery is 0.1 or less.

[0131] The CB value is calculated by dividing the unit area capacity of the negative electrode pieces in a secondary battery by the unit area capacity of the positive electrode pieces. Since a negative electrode-less battery does not contain a negative electrode active material, the unit area capacity of the negative electrode pieces is small, and the CB value of the secondary battery is 0.1 or less.

[0132] In some embodiments, the secondary battery includes a negative electrode piece, which may include only a negative electrode current collector and may not include a negative electrode active material. In some embodiments, the negative electrode piece may have a metallic phase pre-deposited on the negative electrode current collector.

[0133] In some embodiments, the negative electrode current collector can be a metal foil piece or a composite current collector. Examples of the metal foil piece include aluminum foil and copper foil. The composite current collector can include a polymeric base layer and a metal layer formed on at least one surface of the polymeric substrate. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy) on a polymeric substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE)).

[0134] In some embodiments, the negative electrode piece includes a negative electrode current collector and a primer layer disposed on at least one surface of the negative electrode current collector, the primer layer including one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, and tin composite carbon nanoparticles.

[0135] The primer layer not only has excellent electrical conductivity, but also has advantages in uniform deposition of metal ions on the surface of the negative electrode-less battery current collector, and in improving the cycle performance and safety of the battery.

[0136] In some embodiments, the areal density of the primer layer is 5 g / m 2 ~50g / m 2 is.

[0137] In some embodiments, the areal density of the primer layer is optionally 5 g / m 2 , 10g / m 2 , 15g / m 2 , 20g / m 2 , 25g / m 2 , 30g / m 2 , 35g / m 2 , 40g / m 2 , 45g / m 2 , or 50 g / m 2 is.

[0138] Surface density is 5g / m 2~50g / m 2 A primer layer having a thickness of 0.1 to 100 nm favors a uniform distribution of anode nucleation sites in anode-less batteries and promotes uniform deposition of metal without affecting electron transport behavior.

[0139] In some embodiments, the thickness of the primer layer is between 2 μm and 100 μm.

[0140] In some embodiments, the thickness of the primer layer is optionally 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.

[0141] Although the thickness of the primer layer is too thin to be used as a negative electrode active material, in a non-negative electrode sodium battery, the primer layer reduces the nucleation overpotential of sodium, which is beneficial for the uniform deposition of metal ions and suppresses dendrites.

[0142] The present application does not particularly limit the shape of the secondary battery, but it may be cylindrical, rectangular, or any other shape. For example, Fig. 1 shows an example of a rectangular secondary battery 5, and Fig. 2 is an exploded view of the secondary battery 5.

[0143] In some embodiments, referring to FIG. 2 , the exterior packaging may include a casing 51 and a cover plate 53. The casing 51 includes a base plate and a side plate connected to the base plate, which together form a surrounding accommodating cavity. The casing 51 has an opening communicating with the accommodating cavity, and the cover plate 53 can be installed over the opening to seal the accommodating cavity. The positive electrode piece, the negative electrode piece, and the separator film can be formed into an electrode assembly 52 by a winding or lamination process. The electrode assembly 52 is sealed within the accommodating cavity. A non-Newtonian fluid electrolyte composition permeates the electrode assembly 52. ​​The secondary battery 5 may include one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0144] In some embodiments, the secondary batteries can be assembled into a battery module, and the number of secondary batteries included in the battery module 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 module.

[0145] 3 shows a battery module 3 as an example. Referring to FIG. 3, in the battery module 3, the plurality of secondary batteries 5 may be arranged in sequence along the length of the battery module 3. Of course, they may be arranged in any other manner. Furthermore, the plurality of secondary batteries 5 may be fixed by fastening members.

[0146] Optionally, the battery module 3 may further include an exterior case having an accommodating space, and the plurality of secondary batteries 5 are accommodated in the accommodating space.

[0147] In some embodiments, the battery modules can also be assembled into a battery pack, and the number of battery modules included 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.

[0148] 4 and 5 show a battery pack 1 as an example. Referring to FIGS. 4 and 5, the battery pack 1 may include a battery box and a plurality of battery modules 4 installed in the battery box. The battery box includes an upper case 2 and a lower case 3, and the upper case 2 is attached to the lower case 3 as a cover, forming a sealed space for accommodating the battery modules 4. A plurality of battery modules 4 may be installed in the battery box as desired.

[0149] [Electrical Equipment] In one embodiment of the present application, there is provided an electric device including at least one of the secondary battery according to any of the embodiments, the battery module according to any of the embodiments, or the battery pack according to any of the embodiments.

[0150] The electric device includes at least one of the secondary battery, battery module, or battery pack according to the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device or as an energy storage unit for the electric device. The electric device includes, but is 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.

[0151] As the electrical device, a secondary battery, a battery module, or a battery pack can be selected depending on the application.

[0152] 6 shows an example of an electric device, such as a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the demand for high power and high energy density from secondary batteries, a battery pack or a battery module can be used.

[0153] As another example, the device is a mobile phone, a tablet, a laptop, etc. Typically, the device is required to be thin and lightweight, and can use a secondary battery as a power source.

[0154] Example The following examples are provided for illustrative purposes only and are not intended to limit the scope of the present application. Unless specific techniques or conditions are described in the examples, they may be performed in accordance with techniques or conditions described in technical documents or in accordance with the product instructions. Unless the manufacturer of the reagents or equipment used is specified, they may be commercially available products.

[0155] 1. Manufacturing method Example 1 1) Preparation of positive electrode active material 0.1mol sodium pyrophosphate, 0.3mol ferrous oxalate, 0.2mol ammonium dihydrogen phosphate, 0.01mol glucose and 0.006mol magnesium acetate were added to 200mL deionized water and ground for 10 hours using a sand mill 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 tubular furnace and nitrogen gas was passed through as the protective gas. The first stage of calcination was carried out, at 300°C and kept at that temperature for 4 hours. The second stage of calcination was carried out, at 550°C and kept at that temperature for 12 hours, to obtain a positive electrode active material.

[0156] 2) Manufacturing of positive electrode pieces 10 wt% of polyvinylidene fluoride adhesive was completely dissolved in N-methylpyrrolidone (NMP), and then 10 wt% of carbon black conductive agent and 80 wt% of the above positive electrode active material were added and mixed uniformly to obtain a positive electrode slurry. The slurry was evenly applied to the surface of an aluminum foil current collector and transferred to a vacuum drying oven for complete drying. The dried magnetic pole sheet was roll-pressed and punched to obtain a positive electrode piece.

[0157] 3) Manufacturing of negative electrode pieces 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 then applied to the surface of the negative electrode current collector copper foil to form a primer layer, which was then transferred to a vacuum drying oven to dry completely and then punched to form a primer layer with a thickness of 20 μm and an areal density of 25 g / m. 2 A negative electrode piece with no negative electrode structure was obtained.

[0158] 4) Electrolyte In an argon atmosphere glove case (H2O<0.1 ppm, O2<0.1 ppm), sodium hexafluorophosphate (NaPF6) was dissolved in ethylene glycol dimethyl ether (DME) as an organic solvent and stirred uniformly to obtain an electrolyte solution with a sodium salt concentration of 1 mol / L.

[0159] 5) Separator film A polypropylene film was used as the separator film.

[0160] 6) Button battery manufacturing A metallic sodium sheet was used as the counter electrode, a Celgard 2400 separator film was used, and the electrolyte was injected to assemble a button cell.

[0161] 7) Manufacturing of all batteries The positive electrode pieces, separator, and negative electrode pieces were stacked in this order, with the separator film interposed between them to provide isolation, and the above-mentioned electrolyte was added to assemble the stacked battery, resulting in the negative electrode-free sodium battery product of Example 1.

[0162] Examples 2 to 6 The manufacturing methods of the batteries of Examples 2 to 6 are similar to those of the battery of Example 1, but the mass content of carbon element in the positive electrode active material is adjusted. The specific parameters are shown in Table 1.

[0163] Example 7 The manufacturing method of the battery of Example 7 is the same as that of the battery of Example 1, but the manufacturing method of the positive electrode active material is changed, and the manufacturing method is specifically as follows.

[0164] 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 (M source), and 0.006 mol magnesium acetate were added to 200 mL of deionized water and ground for 10 hours using a sand mill to obtain a mixed slurry. The mixed slurry was then dried using a spray dryer to obtain a dried precursor powder. The precursor powder was placed in a tubular furnace, nitrogen gas was passed through as a protective gas, and the temperature was raised to 300°C and maintained at that temperature for 4 hours, and then further raised to 550°C and maintained at that temperature for 12 hours to obtain a positive electrode active material.

[0165] Examples 8 to 12 The manufacturing methods for the batteries of Examples 8 to 12 were the same as that for Example 7, but the x value of the Mn element in the iron-based polyanion compound was changed. Specific parameters are shown in Table 1.

[0166] Examples 13 to 17 The manufacturing methods of the batteries of Examples 13 to 17 are similar to those of the battery of Example 7, but the mass content of magnesium in the positive electrode active material is adjusted. The specific parameters are shown in Table 1.

[0167] Examples 18 to 25 The manufacturing methods for the batteries of Examples 18 to 25 were the same as those for the battery of Example 7, but the firing temperature and firing time in the second stage were changed. Specific parameters are shown in Table 1.

[0168] Examples 26 to 31 The manufacturing methods for the batteries of Examples 26 to 31 were the same as those for the battery of Example 7, but the types and combinations of transition metal elements in the iron-based polyanion compound were changed. Specific parameters are shown in Table 1.

[0169] Comparative Example 1 The manufacturing method of the battery of Comparative Example 1 is the same as that of Example 1, but the manufacturing method of the positive electrode active material is changed, and the specific manufacturing method is as follows.

[0170] 0.1 mol sodium pyrophosphate, 0.3 mol ferrous oxalate, and 0.2 mol ammonium dihydrogen phosphate were added to 200 mL of deionized water and ground for 10 hours using a sand mill to obtain a mixed slurry. The mixed slurry was then dried using a spray dryer to obtain dried precursor powder. The precursor powder was then placed in a tubular furnace and, using nitrogen gas as the protective gas, the first stage of calcination was carried out, heating to 300°C and keeping the temperature for 4 hours. The second stage of calcination was carried out, heating to 550°C and keeping the temperature for 12 hours, to obtain the positive electrode active material.

[0171] Comparative Example 2 The manufacturing method of the battery of Comparative Example 2 is the same as that of the battery of Comparative Example 1, except that a carbon material is introduced into the positive electrode active material, and the specific parameters are shown in Table 1.

[0172] Comparative Example 3 The manufacturing method of the battery of Comparative Example 3 is similar to that of the battery of Comparative Example 2, but magnesium element is introduced into the positive electrode active material, and the specific parameters are shown in Table 1.

[0173] Comparative Examples 4-5 The manufacturing methods of the batteries of Comparative Examples 4 and 5 are the same as that of Comparative Example 1, but Mn elements with different x values ​​are introduced into the positive electrode active material, and the specific parameters are shown in Table 1.

[0174] 2. Performance test 1. Performance test of positive electrode active material 1) Magnesium element content test The composition of the positive electrode active material is determined by inductively coupled plasma (ICP) spectroscopy, for example, see standards YS / T 1006.2-2014, GB / T 23367.2-2009, or YS / T 1028.5-2015. Specifically, an inductively coupled plasma optical emission spectrometer (Thermo, ICAP7400, PerkinElmer, Avio200) is used to measure the emission intensity of magnesium in the sample to be measured, and standard samples with different magnesium contents are prepared and their emission intensities are measured as a calibration curve. The mass content of magnesium in the sample to be measured is determined on the calibration curve based on the emission intensity of the magnesium.

[0175] 2) Testing the amount of residual alkali in NaHCO3 The residual alkalinity of the prepared positive electrode active material was tested. In this application, the residual alkalinity of NaHCO3 refers to the mass content of the measured mass of NaHCO3 in the positive electrode active material. This is determined by potentiometric titration of a chemical reagent, for example, see standard GB / T 9725-2007. Specifically, the positive electrode active material was dissolved in deionized water using a Metrohm 905 Titrando potentiometric titrator. The aqueous solution of the positive electrode active material was titrated using a standard titration solution. 0.1 mL of the standard titration solution was added to each drop, and the potential or pH value was recorded. The titration was terminated when the potential or pH value no longer changed significantly. The amount of titration standard solution added and the measured potential or pH value were recorded, and the titration endpoint was determined by a graphical or second-order derivative method, and the titration volume of the titration standard solution was determined. The mass content calculated by dividing the mass of NaHCO3 by the mass of the positive electrode active material was determined as the residual alkalinity of NaHCO3 in the positive electrode active material.

[0176] 2. Battery performance testing 1) Testing the gram capacity of button batteries The test procedure for the gram capacity of a button battery is as follows: At 25°C and under normal pressure, the button battery was charged at a constant current of 0.1C until the voltage reached 4V, then charged at a constant voltage of 4V until the current dropped to 0.05C, and the charge specific capacity at this time, i.e., the initial sodium release capacity, was recorded; it was then discharged at a constant current rate of 0.1C until the voltage reached 1.5V, and the discharge specific capacity at this time was recorded, which was taken as the initial sodium absorption capacity. The gram capacity of the positive electrode active material is the initial sodium absorption capacity.

[0177] 2) Testing the cycle capacity retention rate of all batteries The test procedure for total battery capacity retention was as follows: At 25°C, all the fabricated batteries were charged to 3.7V at a constant current of 1C, then charged at a constant voltage of 3.7V until the current dropped to 0.05C, and then discharged to 1.5V at 1C. The resulting capacity was recorded as the initial capacity (C0). The same procedure was repeated for the same batteries, and the discharge capacity (Cn) of the total battery after the nth cycle was recorded. The total battery capacity retention after each cycle was calculated as Pn = Cn / C0 × 100%. The 200 values ​​(P1, P2...200) were used as the ordinate and the corresponding cycle number as the abscissa, to generate a curve graph of total battery capacity retention versus cycle number. In this 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 total battery capacity retention data for the Examples and Comparative Examples in Table 1 is the data measured after 200 cycles under the above test conditions, i.e., the P200 value. The test process for the Comparative Examples and other Examples was the same as above.

[0178] 3) Rate performance test The test procedure for rate performance was as follows: The fabricated battery was placed in a 25°C thermostatic case and allowed to stand for 30 minutes until the battery reached a constant temperature. After reaching this temperature, the battery was charged at 25°C with a constant current of 0.33C to 3.7V, then charged to 0.05C with a constant voltage of 3.7V, and allowed to stand for 5 minutes. After that, the battery was discharged at a constant current of 0.33C to 1.5V. This resulted in a 0.33C discharge capacity C1. The battery was then charged at a constant current of 0.33C to 3.7V, then charged to 0.05C with a constant voltage of 3.7V, and allowed to stand for 5 minutes. This was followed by a 3C discharge capacity C2. The capacity retention at a 3C rate, R, was calculated as C2 / C1 × 100%. The test procedures for the comparative example and other examples were the same as described above.

[0179] 3. Analysis of the test results of each example and comparative example Batteries of each example and comparative example were manufactured according to the above method, and various performance parameters were measured. The results are shown in Table 1 below.

[0180] [Table 1-1] [Table 1-2]

[0181] As can be seen from the above results, all of Examples 1 to 31 are positive electrode active materials for secondary batteries containing an iron-based polyanion compound and a magnesium-containing oxide. The iron-based polyanion compound is NaFe 3-x M x Mg y It has the general formula (PO4)2P2O7 / C, where M contains a transition metal element, 0≦×≦0.5, 0 <y<0.18であった。

[0182] As can be seen from the comparison between Examples 1 to 31 and Comparative Examples 1 to 2, the doping of the magnesium element in the iron-based polyanion compound and the residue of the oxide derived from the magnesium source on the surface of the iron-based polyanion compound effectively reduce the amount of residual alkali in the positive electrode active material, improve the gram capacity of the positive electrode active material, and improve the capacity retention rate of the battery under high rates.

[0183] As can be seen from the comparison between Examples 1 to 31 and Comparative Example 3, by controlling the y value in the positive electrode active material to be 0 < y value < 0.18, the gram capacity of the positive electrode active material can be improved, and the capacity retention rate of the battery at 200 cycles and the capacity retention rate at a 3C rate can be improved.

[0184] As can be seen from the comparison between Examples 7 to 31 and Comparative Examples 4 to 5, compared with the conventional positive electrode active material containing only transition metal elements, the positive electrode active material containing magnesium-containing oxide and Mg-doped iron-based polyanion compound is advantageous for 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 at 200 cycles and the capacity retention rate at a 3C rate.

[0185] As can be seen from the comparison between Examples 1 to 6 and Comparative Example 1, based on the total mass of the positive electrode active material, controlling the mass content of the carbon material to be 0.5% to 6% is advantageous for improving the conductivity 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 at 200 cycles and the capacity retention rate at a 3C rate. As can be seen from the comparison between Examples 2 to 4 and Examples 1, 5 to 6, 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 advantageous for further improving the gram capacity of the positive electrode active material.

[0186] As can be seen from the comparison between Examples 7 to 12 and Comparative Examples 4 to 5, controlling the x value of the transition metal element to be ≤ 0.5 is advantageous for reducing the amount of residual alkali in the positive electrode active material, improving the gram capacity of the positive electrode active material, and improving the capacity retention rate of the battery at 20 cycles and the capacity retention rate at a 3C rate

[0187] As can be seen from the comparison between Examples 7, 13 to 17 and Comparative Example 2, introducing magnesium element and magnesium-containing oxide into the positive electrode active material and controlling the total mass content of magnesium element in the positive electrode active material to 0.02% to 0.7% based on the total mass of the positive electrode active material is advantageous in reducing the amount of residual alkali in the positive electrode active material, improving the gram capacity of the positive electrode active material, and improving the capacity retention rate of the battery at a 3C rate.

[0188] As can be seen from a comparison between Examples 7, 18 to 21 and Comparative Example 2, controlling the firing temperature to 500°C to 600°C reduces the amount of residual alkali in the positive electrode active material, improves the gram capacity of the positive electrode active material, and is advantageous for improving the battery's capacity retention rate after 200 cycles and the capacity retention rate at a 3C rate. As can be seen from a comparison between Examples 7, 19 to 20 and Examples 18 and 21, controlling the firing temperature to 525°C to 575°C further improves the gram capacity of the positive electrode active material, and is advantageous for improving the battery's capacity retention rate after 200 cycles and the capacity retention rate at a 3C rate. As can be seen from a comparison between Examples 7, 20 and Examples 18 to 19 and 21, controlling the firing temperature to 550°C to 575°C is advantageous for improving the battery's capacity retention rate after 200 cycles and the capacity retention rate at a 3C rate. As can be seen from a comparison between Examples 7, 22-25 and Comparative Example 2, controlling the firing time to 8-15 hours reduces the amount of residual alkali in the positive electrode active material, improves the gram capacity of the positive electrode active material, and is advantageous in improving the capacity retention rate of the battery after 200 cycles and the capacity retention rate at a 3C rate. As can be seen from a comparison between Examples 7, 23-24 and Comparative Examples 22 and 25, controlling the firing time to 10-14 hours further improves the gram capacity of the positive electrode active material, and is advantageous in improving the capacity retention rate of the battery after 200 cycles and the capacity retention rate at a 3C rate.

[0189] As can be seen from the comparison between Examples 7, 26 to 31 and Comparative Examples 4 and 5, controlling the transition metal element in the positive electrode active material to include one or more of Ni, Co, Mn, and Ca is advantageous in reducing the amount of residual alkali in 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 a 3C rate.

[0190] As can be seen from a comparison of Example 26 with Examples 7 and 29, and Example 27 with Examples 7 and 30, controlling the transition metal elements in the positive electrode active material to contain two of Ni, Co, and Mn is advantageous for further improving the capacity retention rate of the battery at a 3C rate, compared to when the transition metal elements in the positive electrode active material contain only one of Ni, Co, and Mn. As can be seen from a comparison of Example 28 with Examples 26 and 27, controlling the transition metal elements in the positive electrode active material to contain three of Ni, Co, and Mn is advantageous for significantly improving the capacity retention rate of the battery at a 3C rate, compared to when the transition metal elements in the positive electrode active material contain only two of Ni, Co, and Mn.

[0191] As should be understood, the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments that have substantially the same configuration as the technical idea of ​​the present application and that exhibit the same functions and effects are also included in the technical scope of the present application. In addition, various modifications conceivable by those skilled in the art can be made to the embodiments, or parts of the components of the embodiments can be combined to form other forms, within the scope of the gist of the present application, and these embodiments are also included in the scope of the present application. [Explanation of symbols]

[0192] 1 battery pack, 2 upper case, 3 lower case, 4 battery module, 5 secondary battery, 51 casing, 52 electrode assembly, 53 cover plate

Claims

1. A positive electrode active material for a secondary battery, the positive electrode active material comprising a carbon material composite iron-based polyanion compound and a magnesium-containing oxide, the iron-based polyanion compound having the following general formula: Na 4 Fe 3-x M x MM y (2O 4 ) 2 P 2 O 7 / C wherein M contains a transition metal element, and 0≦x≦0.5 and 0<y<0.18; Positive electrode active material.

2. the magnesium-containing oxide is distributed on at least a part of the surface of the primary particles of the iron-based polyanion compound; The positive electrode active material of claim 1 .

3. 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. The positive electrode active material according to claim 1 or 2.

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

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

6. 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 among primary ions of the iron-based polyanionic compound. The positive electrode active material according to any one of claims 1 to 5.

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

8. The mass content of the carbon material is 0.5% to 6%, preferably 1% to 3.6%, based on the total mass of the positive electrode active material; The positive electrode active material according to any one of claims 1 to 7.

9. The gram capacity of the positive electrode active material is ≧98 mAh / g; The positive electrode active material according to any one of claims 1 to 8.

10. The positive electrode active material NaHCO 3 The amount of residual alkali is less than 1.2% based on the total mass of the positive electrode active material. The positive electrode active material according to any one of claims 1 to 9.

11. A method for producing a positive electrode active material for a secondary battery, comprising: A step of dissolving raw materials including an iron source, a sodium source, a phosphorus source, a magnesium source, and a carbon source in water to obtain a mixed slurry, wherein the raw materials selectively contain an M source, and the M source includes a salt containing a transition metal; and drying the mixed slurry and then calcining it to produce the positive electrode active material, the positive electrode active material including an iron-based polyanion compound and a magnesium-containing oxide, the iron-based polyanion compound having the following general formula: Na 4 Fe 3-x M x MM y (2O 4 ) 2 P 2 O 7 / C wherein M contains a transition metal element, and 0≦x≦0.5 and 0<y<0.18; Manufacturing method.

12. Drying the mixed slurry and then firing it includes: drying the mixed slurry to obtain a precursor powder; and calcining the precursor powder in stages to prepare the positive electrode active material, wherein the calcination temperature in the first stage is 300°C to 400°C and the calcination time is 3 hours to 6 hours, and the calcination temperature in the second stage is 500°C to 600°C and the calcination time is 8 hours to 15 hours. The method of claim 11.

13. The firing temperature in the second step is 525°C to 575°C, optionally 550°C to 575°C. The method of claim 12.

14. The second stage firing time is 10 to 14 hours. The method according to claim 12 or 13.

15. 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 manufacturing method according to any one of claims 11 to 14.

16. The magnesium source includes one or more of magnesium nitrate, magnesium carbonate, magnesium acetate, magnesium hydroxide, magnesium oxalate, and magnesium oxide. The manufacturing method according to any one of claims 11 to 15.

17. 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; The manufacturing method according to any one of claims 11 to 16.

18. A secondary battery comprising a positive electrode piece, the positive electrode piece comprising the positive electrode active material according to any one of claims 1 to 10, or a positive electrode active material obtained by the manufacturing method according to any one of claims 11 to 17. Secondary battery.

19. The battery includes a sodium battery. The secondary battery according to claim 18.

20. It is characterized in that it is a negative electrode-less sodium battery, 20. The secondary battery according to claim 18 or 19.

21. The battery further includes a negative electrode piece, the negative electrode piece including a negative electrode current collector and a primer layer disposed on at least one surface of the negative electrode current collector, the primer layer including one or more of carbon nanotubes, graphite, graphene, silver composite carbon nanoparticles, and tin composite carbon nanoparticles. The secondary battery according to any one of claims 18 to 20.

22. The surface density of the primer layer is 5 g / m 2 ~50g / m 2 characterized in that The secondary battery according to claim 21.

23. The thickness of the primer layer is 2 μm to 100 μm.

23. The secondary battery according to claim 21 or 22.

24. A battery module, characterized in that it includes the secondary battery according to any one of claims 18 to 23. Battery module.

25. A battery pack, comprising the secondary battery according to any one of claims 18 to 23 or the battery module according to claim 24. Battery pack.

26. An electrical device, comprising 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. Electrical equipment.

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