Polyanion sodium ion battery positive electrode composite material

Abstract

The invention relates to a polyanion sodium ion battery positive electrode composite material, and belongs to the technical field of sodium ion batteries. The chemical expression of the positive electrode composite material is as follows: the positive electrode composite material is formed by compounding and sintering specifically designed carbon-doped potassium ferric pyrophosphate superfine powder and a ferric sodium pyrophosphate matrix. Wherein the carbon-doped potassium ferric pyrophosphate is prepared by adopting a structure control agent synthesized by p-ethyl thiophenol and triallylamine through a vacuum calcination process, the unique space symmetric structure of the carbon-doped potassium ferric pyrophosphate can be chelated with iron and potassium ions on a molecular level to form a uniform precursor, and an ultrastructure wrapped by edge brittle carbides is generated after calcination. According to the composite material, a continuous three-dimensional electronic conductive network and a long-range ion channel are constructed in an NFPP matrix, and an ion conductivity transition region is formed on a phase interface, so that the ion migration efficiency is remarkably improved, and the technical bottleneck of poor high-rate performance of the NFPP material is effectively solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium ion batteries, and particularly relates to a polyanion sodium ion battery positive electrode composite material. BACKGROUND

[0002] As a new emerging electrochemical energy storage technology, sodium ion batteries have shown great application potential in large-scale energy storage and low-speed electric vehicles due to their advantages of abundant raw material resources, low cost and environmental friendliness. Among the various components of sodium ion batteries, the positive electrode material is the key to determining the energy density, cycle life and safety performance. Among the many positive electrode material systems, polyanion compounds, especially sodium iron pyrophosphates (NFPP for short), have attracted much attention due to their unique advantages. The main elements of NFPP, sodium, iron and phosphorus, have a very high reserve in the earth's crust, and the raw material cost is much lower than that of cobalt and nickel in lithium ion batteries, making it have unparalleled competitiveness in the large-scale energy storage field which is extremely sensitive to cost. In addition, NFPP has a stable three-dimensional framework structure, and the volume change is small during sodium ion deintercalation, which provides it with an ultra-long cycle life. At the same time, its strong P-O covalent bond gives the material excellent thermal stability, strong anti-thermal runaway ability and outstanding safety performance. Therefore, NFPP has achieved a good balance between comprehensive performance and cost, and is praised by the industry as one of the most promising sodium ion battery positive electrode materials for industrialization.

[0003] However, the commercial application of NFPP still faces a key bottleneck: its intrinsic low ion and electron conductivity. This defect seriously limits the performance of the material under high-rate charging and discharging conditions, and the potential of the material in high-power application scenarios cannot be fully realized. To overcome this problem, existing technologies mainly proceed from two aspects: on the one hand, a conductive layer is constructed on the surface of the material by carbon coating, which is the most mature and effective method to improve the electronic conductivity at present, and can effectively reduce the interface charge transfer impedance. On the other hand, the lattice is optimized from the inside by element doping or defect engineering, for example, by introducing hetero-elements to widen the sodium ion migration channel or adjust the energy band structure, so as to improve the ion conductivity and rate performance within a certain range.

[0004] However, the existing technology system still has obvious deficiencies. Carbon coating mainly solves the problem of electronic conduction between particles, and has limited contribution to ion conduction. Traditional phase doping strategies can locally optimize the ion migration path, but it is difficult to construct a long-range and continuous ion conduction network inside the material. More seriously, when the doping concentration is too high, it will destroy the stable primary crystal structure of NFPP and cause the generation of impurities, which will have a negative impact on the capacity and structural stability of the material. Therefore, the existing technology has limited effect on improving the high-rate performance of NFPP, which has become a technical bottleneck restricting the development of NFPP as a high-power positive electrode material. SUMMARY

[0005] To solve the technical problems mentioned in the background art, the purpose of the present application is to provide a polyanion sodium ion battery positive electrode composite material.

[0006] The purpose of the present application can be achieved by the following technical solutions: A polyanion sodium ion battery positive electrode composite material is a composite sintered body of sodium iron pyrophosphate phosphate (NFPP) and specially prepared carbon-doped potassium iron pyrophosphate (KPFPP), and the chemical formula can be represented as: ; Therefore, the technical content of the present application includes the following two aspects: I, Preparation of the positive electrode composite material 1.1, dissolve benzoin dimethyl ether in an ethanol-acetone mixed solvent, add p-ethyl benzene mercaptan and triallylamine, mix well, irradiate with a UV mercury lamp and stir for 8-12 h, remove the mixed solvent by rotary evaporation after the reaction is complete, and obtain a structure control agent.

[0007] Among them, the raw material molar ratio of p-ethyl benzene mercaptan and triallylamine is 3:1, and the amount of benzoin dimethyl ether is 0.15-0.22wt% of the two; the reaction mechanism is as follows: under the initiation of ultraviolet irradiation, benzoin dimethyl ether and triallylamine undergo thiol-ene click addition reaction.

[0008] 1.2, grind and premix potassium carbonate, ferrous sulfate and ammonium dihydrogen phosphate, then add the structure control agent and dimethylformamide aqueous solution to the premix and grind to make a slurry, then stir and homogenize at 60-80℃ for 15-20h, vacuum dry to constant weight, then perform oxygen-free calcination, and finally grind and crush to obtain .

[0009] Among them, the amount of structure control agent is 18-22wt% of the premix.

[0010] Among them, the oxygen-free calcination process is divided into two stages, the first stage temperature is 320-350℃, the calcination time is 3-3.5h, and the heating rate is 1-5℃ / min; the second stage temperature is 600-650℃, the calcination time is 5.5-7h, and the heating rate is 10-15℃ / min.

[0011] II. Preparation of the positive electrode composite material 2.1, sodium dihydrogen phosphate and ferrous oxalate are prepared according to the molar ratio of Na:Fe:P of 4:3:4, then dry grinding and premixing of glucose is added to the mixture, wet grinding and mixing uniformly, and then placing for 24h before drying to constant weight, to prepare a composite precursor.

[0012] Among them, the amount of glucose is 3.5-4wt% of the mixture. ​

[0013] 2.2, the composite precursor and The mixed material is cold isostatic pressed, and then sintered in an argon atmosphere to obtain a positive electrode composite material.

[0014] The amount of the compound is 8.5-13.2wt% of the composite precursor. The cold isostatic pressing pressure is 30-35MPa.

[0015] The sintering temperature is 500-550℃, and the sintering time is 8-10h.

[0016] The present application has the following advantages:

[0017] The present application introduces an innovative design of carbon composite potassium ferric pyrophosphate as a functional doping phase, which is successfully combined with the NFPP matrix, and solves the inherent bottleneck of the NFPP material in electronic conductivity and ion conductivity, and realizes the leap of high-rate electrochemical performance. The core advantage is first derived from The unique microstructure of the material itself. In the preparation process, we innovatively use a molecule with a spatially centrosymmetric structure as a structure control agent. The sulfur and nitrogen atoms in the center of the molecule have a strong chelation effect on potassium ions and iron ions, achieving precise pre-enrichment of active elements at the molecular scale, ensuring the uniformity of the chemical composition of the final product. In the subsequent calcination process, the benzene ring system on the periphery of the molecule is carbonized to form a kind of edge brittle carbon composite. This structural characteristic makes the final product not need to undergo high-energy consumption and easy-to-introduce-defect strong mechanical crushing, and only needs to be broken by mild grinding to generate a large number of ultrafine particles composed of

[0018] crystal nucleus and edge carbon shell. These ultrafine particles not only greatly increase the contact area with the NFPP matrix, but also are more easily dispersed uniformly, laying a foundation for building a continuous conductive network throughout the electrode. In terms of ion transmission, the present application exhibits a multi-level and cross-scale synergistic enhancement mechanism. First, As a pyrophosphate salt, it has similar structural units and good lattice matching degree with NFPP, so it can realize excellent interface wetting and combination during solid phase sintering, forming a macroscopically dense composite structure. More importantly, the radius difference between potassium ions and iron ions can form a continuous ion transport channel in

[0019] ​​A large number of micropores and lattice distortions are induced inside the crystal grains. When these ultrafine particles are dispersed in the NFPP matrix, they themselves act as a "ion highway transfer station", and the pores and defects inside them provide a fast migration path for sodium ions, thereby constructing a long-range ion channel network in the NFPP matrix.

[0020] Secondly, in the sintering process, Controllable potassium-sodium ion mutual diffusion and solid-phase ion exchange occur at the phase and NFPP phase interface. Since the radius of potassium ions is larger than that of sodium ions, after diffusing into the near-surface region of the NFPP lattice, the potassium ions can effectively support the interlayer spacing of the sodium layer like "pillars", widening the migration channel of sodium ions. However, unlike traditional high-concentration bulk doping, the present application forms a potassium concentration gradient "ion conductivity transition zone" in the local area of NFPP adjacent to the second phase by means of second phase compounding. The existence of the transition zone makes the conduction ability of sodium ions change smoothly and continuously from the NFPP bulk phase with low intrinsic mobility to the composite phase interface with high ion mobility, rather than changing steeply and abruptly. This ingenious design effectively reduces the interface ion accumulation effect caused by the large difference in ion conductivity between the heterogeneous materials, significantly reduces the interface ion transmission impedance, and enables sodium ions to achieve more smooth and efficient global transport from the bulk phase to the interface.

[0021] In terms of electron conduction, The in-situ carbonized layer at the edge of the particles plays a crucial role. These carbon layers are firmly combined with the core and, after compounding with the NFPP, can act as "conductive bridges" to connect individual isolated NFPP particles, forming a continuous and stable three-dimensional electron conduction network inside the entire electrode. This network and the above-mentioned ion channel network interweave and complement each other, together solving the "double low" conduction problem of NFPP materials. This effectively overcomes the limitations of existing single carbon coating or bulk doping technology, greatly improves the rate performance of the NFPP materials without sacrificing their high capacity, long life and high safety, and provides strong technical support for promoting the industrialization process of high-performance sodium ion batteries. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of the present application.

[0023] ​Example 1, preparation of positive electrode composite material, the specific implementation method as follows: One, Preparation of 1.1, according to the 3:1 mole ratio of raw materials p-ethyl benzene thiophenol and triallylamine, the amount of raw material 0.15wt% of photoinitiator benzoin dimethyl ether, with anhydrous ethanol and acetone as mixed solvent and take 3 times the mass of raw materials according to 1:1 volume ratio; first benzoin dimethyl ether is added to the mixed solvent, then p-ethyl benzene thiophenol and triallylamine are added and mixed, irradiated with 100W 365nm ultraviolet mercury lamp, stirred for 12h, after the reaction is completed, the mixed solvent is removed by rotary evaporation, and the structure control agent is prepared.

[0024] 1.2, preparation powder, with x=0.5, using potassium carbonate, ferrous sulfate and ammonium dihydrogen phosphate as raw materials, according to K:Fe:P mole ratio of 3:0.5:2, the raw materials are mixed and ground, then 18wt% of the structure control agent and 2 times the mass of dimethylformamide aqueous solution (volume fraction of 45%) are added to the premixed material, and the uniform slurry is prepared by grinding, then the temperature is raised to 60℃ and stirred for 20h, then the slurry is vacuum dried to constant weight, then sent to nitrogen atmosphere furnace for anaerobic calcination, the calcination process parameters are set as follows: the first stage, the temperature rises at a rate of 3℃ / min, the temperature reaches 320℃ and calcines for 3.5h, the second stage, the temperature rises at a rate of 10℃ / min, the temperature reaches 600℃ and calcines for 7h; after cooling, the airflow pulverizer is used for grinding and crushing, and the powder is prepared.

[0025] Two, preparation of positive electrode composite material 2.1, sodium dihydrogen phosphate and ferrous oxalate as raw materials, according to Na:Fe:P mole ratio of 4:3:4, then add 3.5wt% of glucose to the mixture and dry grind to mix, then add 1.2 times the mass of anhydrous ethanol to the mixture and mix uniformly, then dry to constant weight after standing for 24h, and prepare the composite precursor.

[0026] 2.2, add 13.2wt% of powder to the composite precursor, send it to the ball mill for dry ball milling, mix the mixture, send it to the cold isostatic pressing machine for tabletting at 30MPa, then send it to the argon atmosphere furnace, and sinter at 500℃ for 10h, then crush and grind after cooling, and get the positive electrode composite material.

[0027] Example 2, preparation of positive electrode composite material, the specific implementation method as follows: One, Preparation of 1.1, Accurately measure raw materials p-ethylbenzenethiol and triallylamine according to a 3:1 molar ratio, measure 0.18wt% of the photoinitiator benzoin dimethyl ether, mix anhydrous ethanol and acetone according to a 1:1 volume ratio as mixed solvents and take 3 times the mass of the raw materials as standby; first add benzoin dimethyl ether to the mixed solvent for dissolution, then add p-ethylbenzenethiol and triallylamine and mix well, irradiate with a 100W 365nm ultraviolet mercury lamp, stir for 10h, remove the mixed solvent by rotary evaporation after the reaction is complete, and prepare the structure control agent.

[0028] 1.2, Preparation of powder, with x=0.7, using potassium carbonate, ferrous sulfate and ammonium dihydrogen phosphate as raw materials, according to a K:Fe:P molar ratio of 2.6:0.7:2, the pre-mixed material is added with 20wt% of the structure control agent and 2 times the mass of dimethylformamide aqueous solution (volume fraction 45%), and the uniform slurry is prepared by grinding, then the temperature is raised to 70℃ and stirred for 18h, then the slurry is vacuum dried to constant weight, then it is sent to a nitrogen atmosphere furnace for anaerobic calcination, the calcination process parameters are set as follows: the first stage, the temperature is raised at a rate of 5℃ / min, the temperature reaches 320℃ and is calcined for 3.2h, the second stage, the temperature is raised at a rate of 12℃ / min, the temperature reaches 620℃ and is calcined for 6.5h; after cooling, the material is ground by air jet mill, and K 4-2x Fe x P2O7 / C powder is prepared.

[0029] II. Preparation of positive electrode composite material 2.1, Using sodium dihydrogen phosphate and ferrous oxalate as raw materials, according to a Na:Fe:P molar ratio of 4:3:4, the mixture is added with 3.8wt% of glucose for dry grinding pre-mixing, then the mixture is added with 1.4 times the mass of anhydrous ethanol for wet grinding and mixing, and the composite precursor is prepared by drying to constant weight after standing for 24h.

[0030] 2.2, Add 11.5wt% of powder to the composite precursor, put it into a ball mill for dry ball milling, and then put the mixture into a cold isostatic pressing machine to press into a tablet at 32MPa, then put it into an argon atmosphere furnace for solid phase sintering at 510℃ for 9.5h, and then crush and grind after cooling to obtain the positive electrode composite material.

[0031] Example 3, preparation of positive electrode composite material, the specific implementation method is as follows: I. Preparation of 1.1, Accurately measure raw materials p-ethylbenzenethiol and triallylamine according to a 3:1 molar ratio, measure 0.22wt% of the photoinitiator benzoin dimethyl ether of the raw materials, mix anhydrous ethanol and acetone according to a 1:1 volume ratio as mixed solvents and take 3 times the mass of the raw materials for standby; first add benzoin dimethyl ether to the mixed solvent for dissolution, then add p-ethylbenzenethiol and triallylamine and mix well, irradiate with a 100W 365nm ultraviolet mercury lamp, stir for 8h, remove the mixed solvent by rotary evaporation after the reaction is complete, and prepare the structure control agent.

[0032] 1.2, Preparation of powder, measure the ingredients with x=1.0, use potassium carbonate, ferrous sulfate, and ammonium dihydrogen phosphate as raw materials, and mix according to a K:Fe:P molar ratio of 2:1:2 and grind to pre-mix, then add 22wt% of the structure control agent to the pre-mixed material and 2.5 times the mass of dimethylformamide aqueous solution (45% by volume), grind to make a uniform slurry, then heat to 80℃ and stir for 15h, then vacuum dry the slurry to constant weight, then send it to a nitrogen atmosphere furnace for anaerobic calcination, with the following process parameters: first stage, heating rate 2℃ / min, temperature reaches 350℃ calcination for 3h, second stage, heating rate 15℃ / min, temperature reaches 650℃ calcination for 5.5h; after cooling, grind with an air jet mill to prepare powder.

[0033] II. Preparation of positive electrode composite material 2.1, Use sodium dihydrogen phosphate and ferrous oxalate as raw materials, mix according to a Na:Fe:P molar ratio of 4:3:4, then add 4wt% of glucose to the mixture for dry grinding and pre-mixing, then add 1.5 times the mass of anhydrous ethanol to the mixture for wet grinding and mixing, let stand for 24h, then dry to constant weight to prepare the composite precursor.

[0034] 2.2, Add 8.5wt% of powder to the composite precursor, send it to a dry ball mill for mixing, press the mixture into a tablet in a cold isostatic press at 35MPa, then send it to an argon atmosphere furnace for solid phase sintering at 550℃ for 8h, then crush and grind after cooling to obtain the positive electrode composite material.

[0035] Example 4, preparation of positive electrode composite material, the specific implementation method is as follows: I. preparation of 1.1, accurately measure raw materials p-ethylbenzenethiol and triallylamine according to a 3:1 molar ratio, measure 0.20wt% of the photoinitiator benzoin dimethyl ether, mix anhydrous ethanol and acetone according to a 1:1 volume ratio as mixed solvents and take 3 times the mass of the raw materials for standby; first add benzoin dimethyl ether to the mixed solvent for dissolution, then add p-ethylbenzenethiol and triallylamine and mix well, irradiate with a 100W 365nm ultraviolet mercury lamp, stir for 11h, remove the mixed solvent by rotary evaporation after the reaction is complete, and prepare the structure control agent.

[0036] 1.2, preparation powder, with x=0.8, using potassium carbonate, ferrous sulfate and ammonium dihydrogen phosphate as raw materials, according to a K:Fe:P molar ratio of 2.4:0.8:2, the raw materials are mixed and ground, then 21wt% of the structure control agent and 2 times the mass of dimethylformamide aqueous solution (volume fraction 45%) are added to the premixed material, and the uniform slurry is ground, then heated to 75°C and stirred for 16h, then vacuum dried to constant weight, then sent to a nitrogen atmosphere furnace for anaerobic calcination, the calcination process parameters are set as follows: first stage, heating rate 3°C / min, temperature reaches 330°C, calcination for 3.5h, second stage, heating rate 12°C / min, temperature reaches 630°C, calcination for 6.5h; after cooling, the material is ground by air jet mill to prepare powder.

[0037] II. Preparation of positive electrode composite material 2.1, using sodium dihydrogen phosphate and ferrous oxalate as raw materials, according to a Na:Fe:P molar ratio of 4:3:4, then adding 3.8wt% of glucose to the mixture for dry grinding and pre-mixing, then adding 1.4 times the mass of anhydrous ethanol to the mixture for wet grinding and mixing uniformly, drying to constant weight after standing for 24h, to prepare the composite precursor.

[0038] 2.2, add 10.8wt% of powder to the composite precursor, send it to a ball mill for dry ball milling, mix the mixture, send it to a cold isostatic press for tabletting at 33MPa, then send it to an argon atmosphere furnace for solid phase sintering at 530°C for 9h, and then crush and grind after cooling to obtain the positive electrode composite material.

[0039] Comparative example, using NFPP as the positive electrode material, to avoid the influence of the process on the product, strictly according to the preparation method of example 4, without adding , the rest is the same.

[0040] Electrode sheet preparation: The above prepared positive electrode composite, conductive acetylene black, polyvinylidene fluoride binder were mixed in a mass ratio of 8:1:1 in N-methyl pyrrolidone solvent, stirred to form a uniform slurry. Then, the slurry was uniformly coated on an aluminum foil current collector using a doctor blade, dried in a vacuum oven at 100°C for 12 hours, and finally, the electrode sheet was punched into a round sheet using a sheet punching machine.

[0041] Battery assembly: In an argon-filled glove box, a CR2032 type button cell was assembled with a metal sodium sheet as the counter electrode, a glass fiber membrane as the separator, 1M NaClO4 dissolved in EC / PC (volume ratio 1:1) and adding 5% FEC as the electrolyte.

[0042] Constant current charge-discharge tests were performed on all assembled button cells, with a voltage range of 1.7-4.3V. The rate performance tests were all carried out in a constant temperature environment of 25°C, and the specific test data are shown in Table 1: Table 1 0.2C first cycle discharge capacity (mAh / g) 5C rate capacity retention (%) 20C rate capacity retention (%) 50C rate capacity retention (%) Example 1 112 93.3 87.5 70.3 Example 2 118.9 91.2 84.2 66.5 Example 3 106.3 91.8 81.7 72.8 Example 4 121.5 92.5 85.9 69.4 Comparative Example 103.7 79.1 55.4 26.2 Note: The capacity retention in the data is the percentage relative to the discharge capacity at 0.2C.

[0043] As can be seen from the test results in Table 1, the capacity retention of the example at 5C rate is above 90% relative to the capacity at 0.2C, and the capacity retention at high rates of 20C and 50C is significantly higher than that of the comparative example, indicating that the positive electrode composite material of the present application has extremely excellent rate performance.

[0044] In the description of the specification, the description of the reference terms "one embodiment", "example", "specific example" and the like means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are contained in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0045] The above is only an example and description of the present application, and those skilled in the art can make various modifications or supplements to the described specific embodiments or use similar ways to replace, as long as they do not deviate from the invention or exceed the scope defined by the present claims, which shall belong to the protection scope of the present application.

Claims

1. A polyanionic sodium-ion battery cathode composite, characterized in that, The chemical formula of the positive electrode composite is: x = 0.5-1.0; The Prepared by the method: Step A1: dissolve benzpinacol in ethanol-acetone mixed solvent, add p-ethyl phenyl mercaptan and triallylamine, mix well, and perform click addition reaction under ultraviolet irradiation to prepare a structure control agent; Step A2: The potassium carbonate, ferrous sulfate and ammonium dihydrogen phosphate are premixed and ground, then the premix is added with the structure control agent and dimethylformamide aqueous solution to make pulp, which is homogenized, dried to constant weight, then subjected to anaerobic calcination and grinding to obtain .

2. The polyanionic sodium-ion battery cathode composite of claim 1, wherein, The raw material molar ratio of p-ethyl phenyl mercaptan and triallylamine is 3:1, and the amount of benzpinacol is 0.15-0.22wt% of the two.

3. The polyanionic sodium-ion battery cathode composite of claim 2, wherein, The amount of the structure control agent is 18-22wt% of the premix.

4. The polyanionic sodium-ion battery cathode composite of claim 3, wherein, The oxygen-free roasting process is divided into two stages, the first stage temperature is 320-350℃, the roasting time is 3-3.5h, and the heating rate is 1-5℃ / min; the second stage temperature is 600-650℃, the roasting time is 5.5-7h, and the heating rate is 10-15℃ / min.

5. The polyanionic sodium-ion battery cathode composite of claim 1, wherein, The positive electrode composite material is prepared by the following method: Step B1: according to the molar ratio of Na:Fe:P of 4:3:4, sodium dihydrogen phosphate and ferrous oxalate are dosed, then dry grinding pre-mixing of glucose is added, wet grinding mixing with anhydrous ethanol is performed, and after standing for 24h, the composite precursor is prepared by drying to constant weight. Step B2: The composite precursor and The mixture was mixed by dry ball milling, and the mixture was cold isostatic pressed into a sheet. The sheet was sintered in an argon atmosphere, and the sintered sheet was crushed and ground to obtain the positive electrode composite material.

6. A polyanionic sodium-ion battery cathode composite according to claim 5, wherein, in an amount of 8.5-13.2 wt% of the composite precursor.

7. A polyanionic sodium-ion battery cathode composite according to claim 6, wherein, The cold isostatic pressing pressure is 30-35MPa.

8. The polyanionic sodium-ion battery cathode composite of claim 7, wherein, The solid phase sintering temperature is 500-550℃, and the time is 8-10h.

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