A positive electrode composite material with double coating layers and its preparation method and application

Abstract

The present disclosure relates to the technical field of sodium ion batteries, and specifically to a positive electrode composite material with a double coating layer, a preparation method thereof, and an application thereof, wherein the positive electrode composite material with a double coating layer comprises a sodium ferric manganese phosphate pyrophosphate / carbon composite material core, and an inorganic coating layer and an organic coating layer sequentially arranged on the surface of the sodium ferric manganese phosphate pyrophosphate / carbon composite material core from the inside to the outside; the material of the inorganic coating layer comprises sodium ferric phosphate pyrophosphate; the material of the organic coating layer comprises a conductive polymer. The present disclosure can not only enhance the ionic conductivity and electronic conductivity of the positive electrode material through the design of the organic / inorganic double coating layer, but also inhibit the dissolution of manganese ions in the sodium ferric manganese phosphate pyrophosphate, thereby effectively improving the cycle and rate performance of the positive electrode material.

Description

Technical Field

[0001] The present disclosure relates to the technical field of sodium ion batteries, and in particular to a positive electrode composite material with a double coating layer, and a preparation method and application thereof. Background Art

[0002] Energy crisis and environmental change are becoming increasingly serious, and countries around the world attach great importance to the development of efficient and sustainable energy storage technology. Lithium-ion batteries have been widely used in portable mobile electronic devices, electric vehicles (EVs) or plug-in hybrid vehicles (PHEVs) due to their advantages of high energy density and long cycle life. However, the limited storage of lithium resources on the earth restricts the large-scale sustainable development of lithium-ion batteries in the future. In comparison, sodium resources are more abundant in the earth's crust, and their reserves are far higher than lithium resources. Therefore, the development of sodium-ion battery technology using sodium resources as raw materials has important practical value for the development of long-term large-scale energy storage technology. In addition, sodium-ion batteries have excellent electrochemical properties, low-temperature properties and safety performance, and are expected to become the most competitive energy storage device in the "post-lithium era".

[0003] Among the many positive electrode materials for sodium-ion batteries, the polyanionic iron-based positive electrode material Na4Fe3(PO4)2(P2O7) has attracted extensive attention from academia and industry for its abundant raw materials, high structural stability, good thermal stability and excellent rate cycling performance. However, the voltage platform of the Na4Fe3(PO4)2(P2O7) positive electrode material is low (3.0V) and its electronic conductivity is poor, which seriously limits its development. By replacing part of the Fe element with the Mn element, the working voltage of the positive electrode material can be effectively increased, thereby effectively improving the energy density of the sodium-ion battery. However, the replacement of the Fe element with the Mn element will inevitably lead to problems such as the distortion of the Jan-Taylor structure, the intensification of polarization, and the dissolution of Mn in the positive electrode material during the sodium storage process, which ultimately affects the structural stability of the positive electrode material and causes the electrochemical performance of the battery to decay sharply.

[0004] Therefore, how to simultaneously improve the electronic conductivity and structural stability of the above-mentioned polyanionic iron-based positive electrode materials to achieve improved electrochemical performance has become a problem that needs to be solved urgently. Summary of the invention

[0005] In order to solve the above technical problems, the present disclosure provides a positive electrode composite material with a double coating layer, a preparation method and application thereof, wherein the obtained positive electrode composite material has a sodium manganese iron phosphate pyrophosphate / carbon composite material as the core, and through the design of an organic / inorganic double coating layer, the electronic conductivity, structural stability and electrochemical stability of the polyanionic iron-based positive electrode material are effectively improved; and the raw materials used are low in cost, high in earth abundance, and green and environmentally friendly, providing a solid material guarantee for the development of low-cost, high-energy density and high-power density sodium-ion batteries.

[0006] In a first aspect, the present disclosure provides a positive electrode composite material with a double coating layer, comprising a sodium manganese iron pyrophosphate phosphate / carbon composite material core, and an inorganic coating layer and an organic coating layer sequentially arranged on the surface of the sodium manganese iron pyrophosphate phosphate / carbon composite material core from the inside to the outside;

[0007] The chemical formula of the sodium manganese iron pyrophosphate / carbon composite material core is:

[0008] Na4Mn x Fe 3-x (PO4)2P2O7 / C, where 0.1≤x<3;

[0009] The material of the inorganic coating layer includes sodium ferric pyrophosphate;

[0010] The material of the organic coating layer includes a conductive polymer.

[0011] In the above chemical formula, the value of x is 0.1, 0.5, 1, 1.5, 2, 2.5 or 2.8, but is not limited to the listed values, and other values ​​not listed in the range are also applicable. The core of the sodium ferric manganese phosphate pyrophosphate / carbon composite material is in-situ composited with the sodium ferric manganese phosphate pyrophosphate material and the carbon material.

[0012] In the present disclosure, the presence of the inorganic coating layer can inhibit the distortion of the Jahn-Taylor structure and the dissolution of Mn, thereby improving the structural stability of the positive electrode material; and the presence of the organic coating layer can not only improve the electronic conductivity of the electrode material, but also reduce the side reactions between the positive electrode material and the electrolyte. The two-pronged design effectively improves the electrochemical performance of the resulting composite positive electrode material.

[0013] The following are preferred technical solutions of the present disclosure, but are not intended to be limitations of the technical solutions provided by the present disclosure. Through the following technical solutions, the technical objectives and beneficial effects of the present disclosure can be better achieved and realized.

[0014] As a preferred technical solution of the present invention, in the core of the sodium manganese iron phosphate pyrophosphate / carbon composite material, the carbon content is 1-4wt%, for example, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt% or 4wt%, etc., but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0015] As a preferred technical solution of the present invention, in the sodium manganese iron phosphate pyrophosphate / carbon composite material core and the inorganic coating layer, the content of the inorganic coating layer is 1-5wt%, for example, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt% or 5wt%, etc., but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0016] In the present disclosure, an appropriate content of the inorganic coating layer can effectively inhibit the dissolution of the Mn element. If the content is too high, that is, the inorganic coating layer is too thick, it will lead to poor sodium ion transmission and reduced capacity.

[0017] As a preferred technical solution of the present invention, in the positive electrode composite material with a double coating layer, the content of the organic coating layer is 0.5-5wt%, for example, 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt% or 5wt%, etc., but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0018] In the present disclosure, the content of the organic coating layer should not be too large, otherwise an overly thick organic coating layer will cause obstruction of sodium ion transmission and affect the material capacity.

[0019] As a preferred technical solution of the present disclosure, the conductive polymer includes any one of polypyrrole, polyacetylene, polyaniline or polythiophene, or a combination of at least two thereof, such as a combination of polypyrrole and polyacetylene, a combination of polyacetylene and polyaniline, etc.

[0020] In a second aspect, the present disclosure provides a method for preparing a positive electrode composite material having a double coating layer as described in the first aspect, the preparation method comprising the following steps:

[0021] (1) mixing a first sodium source, a manganese source, a first iron source, a first phosphorus source, a carbon source and a solvent, and then sequentially ball milling, drying and sintering to obtain a sodium manganese iron pyrophosphate / carbon composite material;

[0022] (2) mixing the sodium manganese iron pyrophosphate / carbon composite material obtained in step (1) with a sodium manganese iron pyrophosphate precursor solution, and then sequentially performing solid-liquid separation, drying and sintering to obtain a sodium manganese iron pyrophosphate / carbon positive electrode composite material with an inorganic coating layer;

[0023] (3) The sodium manganese iron pyrophosphate / carbon positive electrode composite material with an inorganic coating layer obtained in step (2), a conductive polymer monomer, a surfactant and an initiator are mixed to undergo an in-situ polymerization reaction, and then dried to obtain a positive electrode composite material with a double coating layer.

[0024] As a preferred technical solution of the present disclosure, in step (1), the first sodium source includes any one of sodium carbonate, sodium bicarbonate, sodium hydroxide, disodium hydrogen phosphate, sodium dihydrogen phosphate or sodium acetate, or a combination of at least two thereof, such as a combination of sodium carbonate and sodium bicarbonate, a combination of disodium hydrogen phosphate and sodium dihydrogen phosphate, etc.

[0025] Preferably, the manganese source in step (1) comprises any one of manganese carbonate, manganese dioxide, trimanganese tetraoxide, manganese phosphate, manganese dihydrogen phosphate, manganese oxalate or manganese sulfate, or a combination of at least two thereof, such as a combination of manganese carbonate and manganese dioxide, a combination of manganese phosphate and manganese dihydrogen phosphate, etc.

[0026] Preferably, in step (1), the first iron source comprises any one of ferric phosphate, ferrous oxide, ferrous oxalate, ferric nitrate, ferrous oxalate or ferrous acetate, or a combination of at least two thereof, such as a combination of ferric phosphate and ferric nitrate, a combination of ferric oxalate and ferrous acetate, etc.

[0027] Preferably, in step (1), the first phosphorus source comprises any one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, manganese phosphate, manganese dihydrogen phosphate, sodium dihydrogen phosphate or disodium hydrogen phosphate, or a combination of at least two thereof, such as a combination of ammonium dihydrogen phosphate and diammonium hydrogen phosphate, a combination of manganese dihydrogen phosphate, sodium dihydrogen phosphate and disodium hydrogen phosphate, etc.

[0028] Preferably, the carbon source in step (1) comprises citric acid.

[0029] Preferably, the solvent in step (1) comprises water.

[0030] Preferably, the rotation speed of the ball mill in step (1) is 250-400 r / min, for example, 250 r / min, 300 r / min, 350 r / min or 400 r / min, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0031] Preferably, the ball milling time in step (1) is 6-15 h, for example, 6 h, 8 h, 10 h, 12 h or 15 h, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0032] Preferably, the drying temperature in step (1) is 70-150°C, such as 70°C, 80°C, 100°C, 120°C or 150°C, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0033] Preferably, the drying time in step (1) is 15-30 h, such as 15 h, 20 h, 25 h or 30 h, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0034] Preferably, the sintering in step (1) is carried out under a protective atmosphere.

[0035] Preferably, the sintering in step (1) includes a first stage sintering and a second stage sintering performed sequentially.

[0036] Preferably, the temperature of the first sintering stage is 300-400°C, such as 300°C, 320°C, 340°C, 360°C, 380°C or 400°C, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0037] Preferably, the first sintering time is 3-6 hours, such as 3 hours, 4 hours, 5 hours or 6 hours, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0038] Preferably, the temperature of the second sintering stage is 500-650°C, such as 500°C, 550°C, 600°C, 620°C or 650°C, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0039] Preferably, the second sintering time is 6-12 hours, such as 6 hours, 8 hours, 10 hours or 12 hours, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0040] As a preferred technical solution of the present disclosure, in step (2), the preparation process of the sodium iron pyrophosphate precursor solution includes: mixing a second sodium source, a second iron source and a second phosphorus source to obtain the sodium iron pyrophosphate precursor solution.

[0041] Preferably, the second sodium source includes any one of sodium carbonate, sodium dihydrogen phosphate, sodium acetate or sodium bicarbonate, or a combination of at least two thereof, such as a combination of sodium carbonate and sodium dihydrogen phosphate, a combination of sodium carbonate and sodium bicarbonate, and the like.

[0042] Preferably, the second iron source includes any one of ferrous oxalate, ferrous nitrate or ferrous acetate, or a combination of at least two of them, such as a combination of ferrous oxalate and ferrous acetate, a combination of ferrous nitrate and ferrous acetate, and the like.

[0043] Preferably, the second phosphorus source includes any one of diammonium phosphate, diammonium hydrogen phosphate or sodium dihydrogen phosphate, or a combination of at least two thereof, such as a combination of diammonium phosphate and diammonium hydrogen phosphate, a combination of diammonium phosphate, diammonium hydrogen phosphate and sodium dihydrogen phosphate, etc.

[0044] Preferably, the mixing in step (2) comprises ultrasonic treatment and stirring performed sequentially.

[0045] Preferably, the ultrasonic treatment time is 30-90 min, for example 30 min, 40 min, 50 min, 60 min, 70 min, 80 min or 90 min, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0046] Preferably, the stirring time is 5-30 h, for example 5 h, 10 h, 15 h, 20 h, 25 h or 30 h, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0047] Preferably, the drying temperature in step (2) is 80-150°C, for example, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C or 150°C, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0048] Preferably, the drying time in step (2) is 10-24 h, such as 10 h, 15 h, 20 h or 24 h, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0049] Preferably, the sintering in step (2) is carried out under a protective atmosphere.

[0050] Preferably, the sintering temperature in step (2) is 500-700°C, for example, 500°C, 550°C, 600°C, 650°C or 700°C, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0051] Preferably, the sintering time in step (2) is 8-15 h, such as 8 h, 10 h, 12 h or 15 h, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0052] As a preferred technical solution of the present disclosure, the mixing operation in step (3) includes: firstly mixing the sodium manganese iron pyrophosphate / carbon positive electrode composite material having an inorganic coating layer obtained in step (2) with a surfactant to obtain a first mixed solution; and then mixing the first mixed solution with a conductive polymer monomer and an initiator to obtain a second mixed solution.

[0053] Preferably, stirring is performed during the preparation of the first mixed liquid, and the stirring time is 1-5 hours, for example, 1 hour, 2 hours, 3 hours, 4 hours or 5 hours, etc., but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0054] Preferably, stirring is performed during the preparation of the second mixed liquid, and the stirring time is 2-8 hours, for example 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours or 8 hours, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0055] Preferably, the surfactant in step (3) comprises sodium dodecyl sulfate.

[0056] Preferably, the mass ratio of the surfactant in step (3) to the sodium manganese iron pyrophosphate / carbon positive electrode composite material having an inorganic coating layer is a:(100-a), wherein 1≤a≤5, for example, a is selected from 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0057] Preferably, the initiator in step (3) comprises ammonium persulfate.

[0058] Preferably, in step (3), the mass ratio of the initiator to the conductive polymer monomer is b:(100-b), wherein 0.5<b<5, for example, b is selected from 0.6, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 4.9, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0059] In the present disclosure, the type of the conductive polymer monomer is selected according to the material used for the conductive polymer coating layer.

[0060] Preferably, the drying temperature in step (3) is 50-100°C, for example, 50°C, 60°C, 70°C, 80°C, 90°C or 100°C, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0061] Preferably, the drying time in step (3) is 15-40 h, for example, 15 h, 20 h, 25 h, 30 h, 35 h or 40 h, but is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0062] In a third aspect, the present disclosure further provides an application of the positive electrode composite material with a double coating layer as described in the first aspect, wherein the positive electrode composite material with a double coating layer is used to prepare a sodium ion battery.

[0063] Compared with the prior art, the technical solution provided by the embodiments of the present disclosure has the following advantages:

[0064] (1) The positive electrode composite material with double coating layers provided in the present invention has a sodium manganese iron phosphate pyrophosphate / carbon composite material as the core, and is designed with an organic / inorganic double coating layer, which not only improves the electronic conductivity of the polyanionic iron-based positive electrode material in the prior art, but also inhibits the dissolution of the Mn element, while reducing the side reaction between the electrode active material and the electrolyte, thereby improving the structural stability and electrochemical performance of the positive electrode material.

[0065] (2) The preparation process of the positive electrode composite material with double coating layers provided by the present disclosure is simple, the coating is uniform, and the obtained material has excellent electrochemical stability and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

[0067] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0068] Figure 1 This is the XRD diagram of the positive electrode composite material with double coating layers described in Example 1 of the present disclosure.

[0069] Figure 2 This is a SEM image of the positive electrode composite material with double coating layers described in Example 1 of the present disclosure.

[0070] Figure 3 This is a graph showing the first charge and discharge curves of a half-cell assembled with the cathode composite material with double coating layers described in Example 1 of the present disclosure at a rate of 0.1C.

[0071] Figure 4 This is a cycle performance diagram of a half-cell assembled using the positive electrode composite material with double coating layers described in Example 1 of the present disclosure at a 1C rate.

[0072] Figure 5 This is a rate performance diagram of a half-cell assembled using the positive electrode composite material with double coating layers described in Example 1 of the present disclosure. DETAILED DESCRIPTION

[0073] In order to more clearly understand the above-mentioned objectives, features and advantages of the present disclosure, the scheme of the present disclosure will be further described below. It should be noted that the embodiments of the present disclosure and the features in the embodiments can be combined with each other without conflict.

[0074] In the following description, many specific details are set forth to facilitate a full understanding of the present disclosure, but the present disclosure may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present disclosure, rather than all of the embodiments.

[0075] The present disclosure provides a method for preparing a positive electrode composite material having a double coating layer, the preparation method comprising the following steps:

[0076] (1) Based on Na4Mn x Fe 3-x (PO4)2P2O7 / C, wherein 0.1≤x<3, dissolving a first sodium source, a manganese source, a first iron source, a first phosphorus source and citric acid in deionized water according to corresponding molar ratios, ball milling at a speed of 250-400r / min for 6-15h, and then drying at 70-150°C for 15-30h; then calcining at 300-400°C for 3-6h in an argon atmosphere, and then sintering at 500-650°C for 6-12h to obtain a sodium ferromanganese phosphate pyrophosphate / carbon composite material, wherein the content of carbon in the obtained sodium ferromanganese phosphate pyrophosphate / carbon composite material is 1-4wt%;

[0077] (2) According to Na4Fe3(PO4)2P2O7, a second sodium source, a second iron source and a second phosphorus source are mixed in a corresponding molar ratio to obtain a sodium iron phosphate pyrophosphate precursor solution; the sodium iron manganese phosphate pyrophosphate / carbon composite material obtained in step (1) is mixed with the sodium iron phosphate pyrophosphate precursor solution, and then ultrasonically treated for 30-90 minutes, stirred for 5-30 hours, then filtered, washed with pure water, and dried at 80-150°C for 10-24 hours; the powder obtained after drying is sintered at 500-700°C in a nitrogen atmosphere for 8-15 hours, and then cooled to room temperature (25±1°C) to obtain a sodium iron manganese phosphate pyrophosphate / carbon positive electrode composite material with an inorganic coating layer, wherein the content of the inorganic coating layer in the obtained sodium iron manganese phosphate pyrophosphate / carbon positive electrode composite material with an inorganic coating layer is 1-5wt%;

[0078] (3) Mixing the sodium manganese ferric pyrophosphate phosphate / carbon cathode composite material with an inorganic coating layer obtained in step (2) with an aqueous solution of a surfactant (sodium dodecyl sulfate) at 30° C. and stirring for 1-5 hours to obtain a first mixed solution; wherein the mass ratio of the surfactant to the sodium manganese ferric pyrophosphate phosphate / carbon cathode composite material with an inorganic coating layer is a:(100-a), 1≤a≤5;

[0079] Then, the first mixed liquid is mixed with a conductive polymer monomer and an initiator (ammonium persulfate), and the mass ratio of the initiator to the conductive polymer monomer is controlled to be b:(100-b), 0.5<b<5, and stirred for 2-8 hours. After washing with deionized water, the mixture is dried at 50-100°C for 15-40 hours to obtain a positive electrode composite material with a double coating layer; in the obtained positive electrode composite material with a double coating layer, the content of the organic coating layer is 0.5-5wt%.

[0080] Examples 1-5 of the present disclosure respectively provide a method for preparing a positive electrode composite material with a double coating layer. The preparation method refers to the preparation method in the specific implementation manner, and the specific preparation parameters are shown in Table 1.

[0081] Table 1

[0082]

[0083]

[0084] The positive electrode composite material with double coating layers obtained in Example 1 was characterized.

[0085] The XRD pattern of the positive electrode composite material with double coating layers obtained in Example 1 is as follows: Figure 1 As shown. Figure 1 It can be seen that the XRD pattern of the positive electrode composite material with double coating layers corresponds to that with orthogonal Pn twenty one a NASICON structure of the space group.

[0086] The SEM image of the positive electrode composite material with double coating layers obtained in Example 1 is as follows Figure 2 As shown. Figure 2 It can be seen that the positive electrode composite material with double coating layers has an irregular block morphology, and the presence of the coating layer makes the particle surface rougher.

[0087] Comparative Example 1

[0088] This comparative example provides a sodium manganese iron pyrophosphate phosphate / carbon positive electrode composite material, which is prepared by referring to the method of step (1) in Example 1.

[0089] Comparative Example 2

[0090] This comparative example provides a sodium manganese ferric pyrophosphate phosphate / carbon positive electrode composite material with an inorganic coating layer, and the sodium manganese ferric pyrophosphate phosphate / carbon positive electrode composite material with an inorganic coating layer is prepared by referring to the methods of steps (1) and (2) in Example 1.

[0091] Comparative Example 3

[0092] This comparative example provides a sodium manganese ferric pyrophosphate phosphate / carbon positive electrode composite material with an organic coating layer, and the sodium manganese ferric pyrophosphate phosphate / carbon positive electrode composite material with an organic coating layer is prepared by referring to the methods of step (1) and step (3) in Example 1.

[0093] The positive electrode composite materials obtained in Examples 1-5 and Comparative Examples 1-3 were respectively used to assemble half-cells, and the specific assembly steps were as follows:

[0094] The positive electrode sheet made of the obtained positive electrode composite material (80wt%), Super P conductive carbon black (10wt%) and polyvinylidene fluoride (10wt%) was used as the positive electrode of the sodium ion battery, and the metal sodium sheet was used as the counter electrode. 5vol.% of fluoroethylene carbonate was added to the NaClO4 / propylene carbonate solution with a concentration of 1mol / L as the electrolyte, and glass fiber was used as the diaphragm to assemble the button battery in a high-purity argon atmosphere glove box.

[0095] The electrochemical properties of the half-cells prepared in Examples 1-5 and Comparative Examples 1-3 were measured.

[0096] Specifically include: voltage range: 1.8-4.2V, charge and discharge at various rates, select:

[0097] (1) Discharge specific capacity at 0.1C;

[0098] (2) Calculate the discharge capacity retention rate after 100 cycles at 1C rate;

[0099]

[0100] (3) Discharge specific capacity at 20C rate.

[0101] The test results are shown in Table 2.

[0102] also, Figure 3 The first charge and discharge curve of the half-cell prepared in Example 1 at a rate of 0.1C is shown.

[0103] Figure 4 The cycling performance of the half-cell prepared in Example 1 at a 1C rate is demonstrated.

[0104] Figure 5 The rate performance of the half-cell prepared in Example 1 is demonstrated.

[0105] Table 2

[0106]

[0107] As can be seen from Table 2, the batteries prepared using the cathode composite materials disclosed in Examples 1-5 have a discharge capacity of more than 90 mAh / g at 0.1C, a discharge capacity retention rate of more than 82.1% after 100 cycles at a rate of 1C, and a discharge capacity of more than 35 mAh / g at a rate of 20C.

[0108] In comparative example 1, no organic / inorganic coating was performed, in comparative example 2, only the inorganic layer was coated, and in comparative example 3, only the organic layer was coated, all of which resulted in a significant decrease in the discharge capacity retention rate of the battery after 100 cycles at a 1C rate.

[0109] It should be noted that, in this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0110] The above description is only a specific embodiment of the present disclosure, so that those skilled in the art can understand or implement the present disclosure. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure will not be limited to the embodiments described herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a positive electrode composite material having a double coating layer, characterized in that: The preparation method comprises the following steps: (1) mixing a first sodium source, a manganese source, a first iron source, a first phosphorus source, a carbon source and a solvent, and then sequentially ball milling, drying and sintering to obtain a sodium manganese iron pyrophosphate / carbon composite material; The sintering includes a first stage sintering and a second stage sintering performed sequentially; The temperature of the first sintering stage is 300-400°C; the temperature of the second sintering stage is 500-650°C; (2) mixing the sodium manganese iron pyrophosphate / carbon composite material obtained in step (1) with a sodium manganese iron pyrophosphate precursor solution, and then sequentially performing solid-liquid separation, drying and sintering to obtain a sodium manganese iron pyrophosphate / carbon positive electrode composite material with an inorganic coating layer; The sintering temperature in step (2) is 500-700°C; (3) mixing the sodium manganese iron pyrophosphate / carbon positive electrode composite material with an inorganic coating layer obtained in step (2), a conductive polymer monomer, a surfactant and an initiator, performing an in-situ polymerization reaction, and then drying to obtain a positive electrode composite material with a double coating layer; The positive electrode composite material with double coating layers comprises a sodium manganese iron pyrophosphate phosphate / carbon composite material core, and an inorganic coating layer and an organic coating layer sequentially arranged on the surface of the sodium manganese iron pyrophosphate phosphate / carbon composite material core from the inside to the outside; The chemical formula of the sodium manganese iron pyrophosphate / carbon composite material core is: Na4Mn x Fe 3-x (PO4)2P2O7 / C, where 0.1≤x<3; The material of the inorganic coating layer includes sodium ferric pyrophosphate; The material of the organic coating layer includes a conductive polymer; In the core of the sodium manganese iron pyrophosphate / carbon composite material, the carbon content is 1-4wt%; In the sodium manganese iron pyrophosphate / carbon composite material core and the inorganic coating layer, the content of the inorganic coating layer is 1-5wt%; In the positive electrode composite material with double coating layers, the content of the organic coating layer is 0.5-5wt%.

2. The method for preparing a positive electrode composite material having a double coating layer according to claim 1, characterized in that: The conductive polymer includes any one of polypyrrole, polyacetylene, polyaniline or polythiophene, or a combination of at least two of them.

3. The preparation method according to claim 1, characterized in that: Step (1) the first sodium source includes any one of sodium carbonate, sodium bicarbonate, sodium hydroxide, disodium hydrogen phosphate, sodium dihydrogen phosphate or sodium acetate, or a combination of at least two thereof; And / or, the manganese source in step (1) includes any one of manganese carbonate, manganese dioxide, manganese tetraoxide, manganese phosphate, manganese dihydrogen phosphate, manganese oxalate or manganese sulfate, or a combination of at least two thereof; And / or, in step (1), the first iron source comprises any one of ferric phosphate, ferrous oxide, ferrous oxalate, ferric nitrate, ferric oxalate or ferrous acetate, or a combination of at least two thereof; And / or, in step (1), the first phosphorus source comprises any one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, manganese phosphate, manganese dihydrogen phosphate, sodium dihydrogen phosphate or disodium hydrogen phosphate, or a combination of at least two thereof; And / or, the carbon source in step (1) includes citric acid; and / or, the solvent in step (1) comprises water; And / or, the drying temperature in step (1) is 70-150°C; And / or, the drying time in step (1) is 15-30 hours; And / or, the sintering in step (1) is carried out under a protective atmosphere.

4. The preparation method according to claim 1, characterized in that: The first sintering time is 3-6h; And / or, the second sintering time is 6-12 hours.

5. The preparation method according to claim 1, characterized in that: In step (2), the preparation process of the sodium iron phosphate pyrophosphate precursor solution includes: mixing a second sodium source, a second iron source and a second phosphorus source to obtain a sodium iron phosphate pyrophosphate precursor solution; And / or, the drying temperature in step (2) is 80-150°C; And / or, the drying time in step (2) is 10-24 hours; And / or, the sintering in step (2) is carried out under a protective atmosphere; And / or, the sintering time in step (2) is 8-15 hours.

6. The preparation method according to claim 5, characterized in that: The second sodium source includes any one of sodium carbonate, sodium dihydrogen phosphate, sodium acetate or sodium bicarbonate, or a combination of at least two thereof; And / or, the second iron source includes any one of ferrous oxalate, ferric nitrate or ferrous acetate, or a combination of at least two thereof; And / or, the second phosphorus source includes any one of diammonium phosphate, diammonium hydrogen phosphate or sodium dihydrogen phosphate, or a combination of at least two thereof.

7. The preparation method according to claim 1, characterized in that: The mixing operation in step (3) comprises: firstly mixing the sodium manganese iron pyrophosphate / carbon positive electrode composite material with an inorganic coating layer obtained in step (2) with a surfactant to obtain a first mixed solution; and then mixing the first mixed solution with a conductive polymer monomer and an initiator to obtain a second mixed solution; and / or, the surfactant in step (3) comprises sodium dodecyl sulfate; And / or, the mass ratio of the surfactant in step (3) to the sodium manganese iron pyrophosphate / carbon positive electrode composite material having an inorganic coating layer is a:(100-a), wherein 1≤a≤5; and / or, the initiator in step (3) comprises ammonium persulfate; And / or, in step (3), the mass ratio of the initiator to the conductive polymer monomer is b:(100-b), wherein 0.5<b<5; And / or, the drying temperature in step (3) is 50-100°C; And / or, the drying time in step (3) is 15-40 hours.

8. The preparation method according to claim 7, characterized in that: Stirring is performed during the preparation of the first mixed solution, and the stirring time is 1-5 hours; The second mixed solution is stirred during the preparation, and the stirring time is 2-8 hours.

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