Multi-element micro co-doped lithium iron phosphate / carbon composite positive electrode material, preparation method and application thereof

Abstract

The application provides a multi-element micro co-doped lithium iron phosphate / carbon composite positive electrode material, and the positive electrode material has a general formula of Li 1‑x M x Fe 1‑y TM y P 1‑z S z O4 / C,0

Description

Technical Field

[0001] This invention belongs to the field of energy storage battery technology, and specifically relates to a multi-element trace co-doped lithium iron phosphate / carbon composite cathode material, its preparation method, and its application in lithium / sodium hybrid ion batteries. Background Technology

[0002] Lithium-ion batteries are a type of green, high-energy battery that has seen rapid development in recent years. They are widely used in various portable electronic products and communication tools, and show promising application prospects in electric vehicles. However, lithium, the main component of lithium-ion batteries, is not abundant in nature and its distribution in the Earth's crust is uneven. With the large-scale application of lithium-ion batteries in energy storage devices, the demand for lithium resources is constantly expanding. Limited resource reserves and increasing costs severely restrict the application of lithium-ion batteries in large-scale energy storage systems. Sodium belongs to the same group as lithium and has similar physicochemical properties. Sodium is widely found in minerals such as albite (NaAlSi3O8), sodium chloride, sodium nitrate, and sodium carbonate, and exists in seawater as sodium ions. Sodium-ion batteries are considered one of the best complements and alternatives to lithium-ion batteries. However, the larger radius of sodium ions leads to shortcomings in energy density and power density. Lithium / sodium hybrid ion batteries combine the advantages of both lithium-ion and sodium-ion batteries, avoiding their shortcomings, and have become an emerging energy storage device. However, the lack of cathode materials remains a key issue in their development.

[0003] Lithium iron phosphate (LFP), as one of the cathode materials for lithium batteries, has been widely used due to its high cycle life and high safety. LFP possesses high energy density (its theoretical specific capacity is 170 mAh / g, and the actual specific capacity of products can approach 160 mAh / g). LFP exhibits good lattice stability; the insertion and extraction of lithium ions have little impact on the lattice, thus resulting in good cycle stability. Compared to ternary materials, LFP has a lower energy density and poorer conductivity. Common modification methods include carbon coating, nano-sizing, and metal doping. For example, invention patent (CN108258215A) discloses a method for preparing carbon-coated LFP material, which effectively improves the electrochemical performance of LFP material. Invention patent (CN104362341A) discloses a method for preparing high-density nano-LFP material; the secondary spherical nano-LFP material possesses high tap density and high specific density, exhibiting high specific capacity and good low-temperature performance, meeting the requirements of power batteries. Tu et al. (Journal of Materials Chemistry A, 2017, 5(32): 17021-17028) used Mg and Ti co-doping to effectively improve the rate performance of lithium iron phosphate microspheres.

[0004] However, in many cases, a single modification method cannot well improve the overall performance of lithium iron phosphate cathode materials. For example, Patent CN107359336A discloses a preparation method of lithium iron phosphate, which only uses single metal ion doping and cannot fundamentally solve the problem of poor lithium iron phosphate ion conductivity. Patent CN1401559A discloses a preparation method of carbon-coated lithium iron phosphate, which uses carbon black to coat the surface of lithium iron phosphate for modification. However, the modification method is single and the conductivity of the material is still very low, and the performance of the composite material needs to be improved. Therefore, combining multiple modification strategies to achieve a synergistic effect can further improve the comprehensive performance of lithium iron phosphate cathode materials and meet the performance requirements of the materials in battery applications.

[0005] In view of the above content, how to improve the capacity and rate performance of lithium iron phosphate while enhancing the stability and safety of the material is a challenging problem. In addition, how to develop a lithium iron phosphate-based cathode material for lithium / sodium mixed ion batteries and provide a simple preparation method has become an urgent problem to be solved. Summary of the Invention

[0006] In order to overcome the deficiencies and defects mentioned in the background technology, the present invention provides a multi-element trace co-doped lithium iron phosphate / carbon composite cathode material with high capacity and good cycle stability and its preparation method.

[0007] In addition, the present invention also provides the application of the above multi-element trace co-doped lithium iron phosphate / carbon composite cathode material in lithium / sodium mixed ion batteries to solve the key problem of the lack of cathode materials in lithium / sodium mixed ion batteries mentioned in the background technology.

[0008] To solve the above technical problems, the technical solution provided by the present invention is as follows:

[0009] A multi-element trace co-doped lithium iron phosphate / carbon composite cathode material, and the chemical formula of the multi-element trace co-doped lithium iron phosphate is: Li 1-x M x Fe 1-y TM y P 1-z S z O4, where 0 < x ≤ 0.05, 0 < y ≤ 0.05, 0 < z ≤ 0.005. The lithium-site doping element M is potassium and sodium, the iron-site doping element TM is two or more of aluminum, calcium, copper, chromium, magnesium, molybdenum, manganese, nickel, zinc, and tin, and the phosphorus-site dopant is sulfur element.

[0010] This invention employs multi-element composite doping at lithium, iron, and phosphorus sites, with the doping amount at each site controlled between 0 and 0.05. This synergistically improves the crystal structure stability of lithium iron phosphate, enhances electronic conductivity and lithium-ion diffusion rate, and suppresses the generation of Li / Fe antisite defects, thereby improving electrochemical performance and avoiding the negative effects of excessive element doping altering the material structure and hindering lithium-ion transport.

[0011] Preferably, the amount of multi-element trace co-doping is 0.005≤x≤0.02, 0.004≤y≤0.02, and 0.001≤z≤0.002.

[0012] This invention controls the carbon content in the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material to be 1.5–4.5 wt%, with the carbon composite being a surface coating. The thickness of the carbon coating layer increases with the increase of carbon addition. A thicker carbon layer can hinder lithium-ion transport, and adding too much carbon can easily generate inactive Fe2P impurities, affecting the specific capacity of lithium iron phosphate and the energy density of the composite cathode material. A lower carbon content cannot completely cover the surface of the lithium iron phosphate cathode material particles, thus failing to effectively improve the conductivity of the material.

[0013] The median particle size (D50) of the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material ranges from 2 to 6 μm, and the tap density of the material is 1.25 to 1.4 g / cm³. 3 .

[0014] This invention controls the median particle size of the material to 2–6 μm, which helps to maximize the electrochemical characteristics of lithium iron phosphate. Smaller lithium iron phosphate particles are prone to agglomeration, increasing the battery's internal resistance and reducing the solid-phase diffusion coefficient and electronic conductivity of lithium ions. Larger particles, on the other hand, lead to a longer diffusion path for lithium ions in the cathode material, resulting in poorer electrochemical performance of the cathode material.

[0015] This invention also provides a method for preparing the above-mentioned multi-element trace co-doped lithium iron phosphate / carbon composite cathode material, comprising the following steps:

[0016] (1) First, weigh the lithium source, iron source, phosphorus source and carbon source according to the stoichiometric ratio. The molar ratio of lithium, iron and phosphorus is 0.95≤Li<1, 0.95≤Fe<1, 0.95≤P<1. Add them to a high-speed inclined grinding mixer and grind them for 1 to 8 hours under the action of ball milling media. Then, under the protection of an inert or reducing atmosphere, heat the mixture to 300 to 500℃ at a heating rate of 2 to 15℃ / min and hold it for 1 to 6 hours. After cooling, mechanically crush it to obtain the intermediate product.

[0017] The lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium acetate; the iron source is one or more of iron oxide, iron phosphate, ferrous oxalate, and ferrous sulfate; the phosphorus source is one or more of phosphoric acid, iron phosphate, and ammonium dihydrogen phosphate; the carbon source is one or more of glucose, starch, and sucrose; the milling media is at least one of polyurethane balls, zirconia balls, agate balls, and alumina balls; and the inert or reducing atmosphere is at least one of argon, nitrogen, hydrogen / argon, and hydrogen / nitrogen.

[0018] (2) The intermediate product, M-containing compound, TM-containing compound, sulfur source and solvent are added to a high-speed planetary ball mill and mechanically ball-milled for 1 to 8 hours to obtain a slurry. The slurry is then placed in a vacuum box furnace and dried at 60 to 120°C for 8 to 24 hours. After drying, the mixture is mechanically mixed to obtain a powder. Finally, the powder mixture is placed in an atmosphere furnace and heated to 600 to 850°C at a heating rate of 2 to 10°C / min under the protection of an inert or reducing atmosphere. The temperature is held for 6 to 24 hours. After cooling, the mixture is mechanically pulverized and sieved to obtain a multi-element trace co-doped lithium iron phosphate / carbon composite cathode material.

[0019] The M-containing compound is one or more of potassium and sodium hydroxides, chlorides, sulfates, and carbonates; the TM-containing compound is one or more of aluminum, calcium, copper, chromium, magnesium, molybdenum, manganese, nickel, zinc, and tin oxides, hydroxides, chlorides, sulfates, and carbonates; the sulfur source is one or more of sulfur powder, thioacetamide, thiourea, thiourea dioxide, ammonium sulfide, ammonium sulfate, and ammonium hydrogen sulfate; and the solvent is at least one of deionized water and ethanol.

[0020] Preferably, the grinding time in the high-speed inclined grinding and mixing mill is 4 to 6 hours, the grinding media is polyurethane balls, the atmosphere is a nitrogen / hydrogen mixture, the heating rate is 5℃ / min, the heat treatment temperature is 400 to 450℃, and the holding time is 3 to 5 hours.

[0021] Preferably, the mechanical ball milling time in the high-speed planetary ball mill is 4 to 6 hours, the grinding media are zirconia balls and agate balls, the drying temperature in the box furnace is 80 to 100°C, the drying time is 10 to 12 hours, the high-temperature calcination temperature is 700 to 800°C, and the calcination time is 10 to 15 hours.

[0022] Preferably, the M compound is a potassium or sodium carbonate, the TM compound is a hydroxide or carbonate of aluminum, calcium, copper, chromium, magnesium, molybdenum, manganese, nickel, zinc, or tin, and the sulfur source is thioacetamide or thiourea.

[0023] Furthermore, the present invention provides a lithium / sodium hybrid ion battery, which consists of a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode is a thin sheet of lithium / sodium alloy, the separator is a polypropylene membrane, and the electrolyte is a mixed organic solution of lithium / sodium salts.

[0024] The preparation process of the lithium / sodium alloy sheet provided by this invention is as follows:

[0025] Equal masses of lithium and sodium sheets were weighed in a glove box filled with argon atmosphere and placed in a heating furnace to be heated to 180-250°C for melting. After cooling, the lithium / sodium alloy block was cut into thin sheets.

[0026] Furthermore, the specific steps for preparing the positive electrode sheet of the lithium / sodium hybrid ion battery are as follows:

[0027] The active material, conductive carbon black and polyvinylidene fluoride were weighed according to a mass ratio of 80:10:10 to 95:2:3. An appropriate amount of N-methylpyrrolidone was added, and the mixture was stirred evenly and then coated onto the current collector aluminum foil. After vacuum drying, the positive electrode was obtained.

[0028] The active material is a composite of multi-element trace-doped lithium iron phosphate / carbon composite powder and sodium-ion battery cathode material powder. The proportion of multi-element trace-doped lithium iron phosphate / carbon composite powder in the active material is 40-60 wt%, and the sodium-ion battery cathode material is NaMn. 1 / 3 Ni 1 / 2 Cu 1 / 6 O2, NaMn 1 / 3 Fe 1 / 3 Ni 1 / 3 O2, Na3V2(PO4)3, Na4MnV(PO4)3, Na4Fe 2.9 One of (PO4)2P2O7 and Na2Mn2(CN)6.

[0029] Preferably, the mass ratio of active material, conductive carbon black, and polyvinylidene fluoride is 90:5:5 to 95:2:3, and the positive electrode material of the sodium-ion battery is NaMn. 1 / 3 Ni 1 / 2 Cu 1 / 6 O2 and NaMn 1 / 3 Fe 1 / 3 Ni 1 / 3 O2.

[0030] It should be noted that the lithium / sodium hybrid ion battery provided by this invention has an initial discharge capacity of 100-120 mAh / g, calculated based on the mass of the cathode material.

[0031] As can be seen from the above technical solutions, compared with the prior art, the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material, its preparation method, and its application provided by the present invention have the following superior effects:

[0032] This invention employs multi-element synergistic doping and carbon coating to effectively improve the crystal structure stability, conductivity, and electrochemical performance of lithium iron phosphate. It combines easily industrialized mechanochemical and solid-state sintering methods, resulting in low cost and simple processes suitable for industrial production. Furthermore, the composite cathode material obtained by combining lithium iron phosphate with sodium-ion battery cathode materials can be applied to lithium / sodium hybrid ion batteries, effectively solving the problem of lithium resource scarcity and showing broad application prospects in the field of energy storage batteries. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0034] Figure 1 This is a flowchart illustrating the preparation process of the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material described in this invention.

[0035] Figure 2 The image shows the XRD pattern of the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material provided in Example 1.

[0036] Figure 3 The image shows a SEM image of the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material provided in Example 1.

[0037] Figure 4 The particle size distribution diagram is shown for the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material provided in Example 1.

[0038] Figure 5 The cycling performance diagram is shown for the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material provided in Example 1.

[0039] Figure 6 The cycling performance diagram is shown for the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material provided in Example 2.

[0040] Figure 7 The charge / discharge curves of the lithium / sodium hybrid ion battery provided in Example 10 are shown. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] Example 1

[0043] A Li 0.994 Na 0.005 K 0.001 Fe 0.996 Ca 0.002 Mg 0.002 P 0.999 S 0.001 Preparation methods of O4 / C composite cathode materials, such as Figure 1 As shown, it includes the following steps:

[0044] (1) Weigh FePO4 and Li2CO3 according to the Fe:Li molar ratio of 1:1, and then weigh glucose according to the carbon content of the product of 2wt%. Pour FePO4, Li2CO3 and glucose into a high-speed inclined grinding mixer, use polyurethane balls as the ball milling medium, and ball mill for 4 hours to obtain a mixture powder. Place the powder in an atmosphere furnace, and heat it to 400℃ at a heating rate of 5℃ / min under a nitrogen / hydrogen atmosphere. Hold it for 5 hours, and then cool it and mechanically crush it to obtain an intermediate product.

[0045] (2) Weigh Na2CO3, K2CO3, CaCO3, MgCO3, thiourea and the intermediate product obtained in step (1) according to the stoichiometric ratio, pour them into a planetary ball mill, add ethanol, use agate balls as the ball milling medium, and ball mill at 300 r / min for 6 hours to obtain a slurry. Place the slurry in a box furnace and dry it at 80℃ for 12 hours. Then, mechanically mix it evenly to obtain powder. Then, place the powder in an atmosphere furnace and heat it to 750℃ at a heating rate of 5℃ / min under a nitrogen / hydrogen atmosphere. Hold it for 10 hours. After cooling, mechanically crush and sieve through 300 mesh to obtain multi-element co-doped lithium iron phosphate / carbon composite cathode material.

[0046] The crystal structure of the multi-element co-doped lithium iron phosphate / carbon composite cathode material was analyzed using X-ray diffraction techniques, such as... Figure 2 As shown, its main diffraction peaks are consistent with those of the lithium iron phosphate standard card, indicating good crystallinity. The small amount of dopant ions in the crystal lattice mostly exist in the form of solid solution and do not affect the crystal structure.

[0047] The morphology of multi-element co-doped lithium iron phosphate / carbon composite cathode materials was investigated using scanning electron microscopy (SEM). The results were obtained from SEM images. Figure 3 As can be seen, the cathode material consists of irregular micron-sized particles, and the micron-sized primary particles are composed of secondary nanoparticles.

[0048] The elemental content of the multi-element co-doped lithium iron phosphate / carbon composite cathode material was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-OES / MS), as shown in Table 1. The test results show that the composition of the product is basically consistent with the chemical formula.

[0049] Table 1 shows the ICP measurement results of the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material provided in Example 1.

[0050]

[0051] The particle size distribution of multi-element co-doped lithium iron phosphate / carbon composite cathode materials was tested using a laser particle size analyzer, such as... Figure 4 As shown, the median particle size (D50) of the material is 2.7 μm.

[0052] The prepared multi-element co-doped lithium iron phosphate / carbon composite cathode material, super carbon black and polyvinylidene fluoride (PVDF) were mixed evenly in a mass ratio of 90:5:5. An appropriate amount of N-methylpyrrolidone (NMP) solution was added to make a slurry, which was then evenly coated on aluminum foil and vacuum dried at 120°C for 12 hours to obtain the cathode sheet.

[0053] Using a lithium metal sheet as the counter electrode, a Celgard 2400 polypropylene membrane as the separator, and a 1.0 M LiPF6 / EC-DEC-EMC solution (volume ratio 1:1:1) as the electrolyte, the battery was assembled in an argon-protected glove box. After the assembled battery was allowed to stand for 24 hours, its performance was tested using a LAND electrochemical analyzer. The charge-discharge test voltage range was 2.5–4.2 V, and the current density was 34 mA / g. The test results are as follows: Figure 5 As shown, the operating voltage platform is 3.4V, the initial discharge specific capacity is 135mAh / g, and the capacity remains at 120mAh / g after 200 cycles.

[0054] Example 2

[0055] A Li 0.993 Na 0.006 K 0.001 Fe 0.996 Mg 0.002 Al 0.002 P 0.998 S 0.002 The preparation method of O4 / C composite material includes the following steps:

[0056] (1) Weigh FePO4 and Li2CO3 according to the Fe:Li molar ratio of 1:1, and then weigh glucose according to the carbon content of the product of 3wt%. Pour FePO4, Li2CO3 and glucose into a high-speed inclined grinding mixer, use polyurethane balls as the ball milling medium, and ball mill for 6 hours to obtain a mixture powder. Place the powder in an atmosphere furnace, and heat it to 450°C at a heating rate of 5°C / min under a nitrogen / hydrogen atmosphere. Hold it at the temperature for 3 hours, and then cool it and mechanically crush it to obtain an intermediate product.

[0057] (2) Weigh Na2CO3, K2CO3, Al(OH)3, MgCO3, thiourea and the intermediate product obtained in step (1) according to the stoichiometric ratio, pour them into a planetary ball mill, add ethanol, use agate balls as the ball milling medium, and ball mill at 300 r / min for 5 hours to obtain a slurry. Place the slurry in a box furnace and dry it at 100℃ for 10 hours. Mechanically mix it evenly to obtain powder. Place the powder in an atmosphere furnace and heat it to 700℃ at a heating rate of 5℃ / min under a nitrogen / hydrogen atmosphere. Hold it at the temperature for 12 hours, cool it, mechanically crush it and sieve it through 300 mesh to obtain multi-element co-doped lithium iron phosphate / carbon composite cathode material.

[0058] The preparation of the positive electrode and the assembly of the battery are the same as in Example 1. Figure 6 As shown, the initial discharge specific capacity of the material is 123 mAh / g, and the discharge specific capacity remains stable at 148 mAh / g after 100 cycles.

[0059] Example 3

[0060] A Li 0.993 Na 0.006 K 0.001 Fe 0.995 Mo 0.003 Ni 0.002 P 0.999 S 0.001 The preparation method of O4 / C composite material includes the following steps:

[0061] (1) The preparation of the intermediate product is the same as in Example 2.

[0062] (2) Weigh Na2CO3, K2CO3, (NH4)2MoO4, Ni(OH)2, thiourea and the intermediate product of step (1) according to the stoichiometric ratio, pour them into a planetary ball mill, add a mixture of ethanol and deionized water, and then use agate balls as the ball milling medium to ball mill for 4 hours at a speed of 400 r / min to obtain a slurry. Place the slurry in a box furnace and dry it at 100℃ for 12 hours. Mechanically mix it evenly to obtain powder. Place the powder in an atmosphere furnace and heat it to 800℃ at a heating rate of 3℃ / min under a nitrogen / hydrogen atmosphere. Hold it for 10 hours. After cooling, mechanically crush it and sieve it through a 300-mesh sieve to prepare a multi-element co-doped lithium iron phosphate / carbon composite cathode material.

[0063] The preparation of the positive electrode and the assembly of the battery were the same as in Example 1. The initial discharge specific capacity of the material was 132 mAh / g, and the discharge specific capacity retention rate was higher than 92% after 15 cycles.

[0064] Example 4

[0065] A Li 0.994 Na 0.005 K 0.001 Fe 0.997 Ca 0.002 Cr 0.001 P 0.998 S 0.002 The preparation method of O4 / C composite material includes the following steps:

[0066] (1) The preparation of the intermediate product is the same as in Example 2.

[0067] (2) Weigh Na2CO3, K2CO3, CaCO3, Cr(OH)3, thiourea and the intermediate product of step (1) according to the stoichiometric ratio, pour them into a planetary ball mill, add a mixture of ethanol and deionized water, use agate balls as the ball milling medium, and ball mill at 300 r / min for 6 hours to obtain a slurry. Place the slurry in a box furnace and dry it at 80℃ for 12 hours. Mechanically mix it evenly to obtain powder. Place the powder in an atmosphere furnace and heat it to 700℃ at a heating rate of 2℃ / min under a nitrogen / hydrogen atmosphere. Hold it at the temperature for 10 hours. After cooling, mechanically crush it and sieve it through 300 mesh to obtain multi-element co-doped lithium iron phosphate / carbon composite cathode material.

[0068] The preparation of the positive electrode and the assembly of the battery were the same as in Example 1. The initial discharge specific capacity of the material was 150 mAh / g.

[0069] Example 5

[0070] A Li 0.994 Na 0.005 K 0.001 Fe 0.994 Mn0.004 Cu 0.002 P 0.998 S 0.002 The preparation method of O4 / C composite material includes the following steps:

[0071] (1) The preparation of the intermediate product is the same as in Example 2.

[0072] (2) Weigh Na2CO3, K2CO3, MnCO3, Cu(OH)2, thiourea and the intermediate product of step (1) according to the stoichiometric ratio, pour them into a planetary ball mill, add a mixture of ethanol and deionized water, use agate balls as the ball milling medium, and ball mill at a speed of 350 r / min for 6 hours to obtain a slurry. Place the slurry in a box furnace and dry it at 80°C for 12 hours. Mechanically mix it evenly to obtain powder. Place the powder in an atmosphere furnace and heat it to 750°C at a heating rate of 3°C / min under a nitrogen / hydrogen atmosphere. Hold it at the temperature for 12 hours. After cooling, mechanically crush it and sieve it through a 300-mesh sieve to obtain a multi-element co-doped lithium iron phosphate / carbon composite cathode material.

[0073] The preparation of the positive electrode and the assembly of the battery were the same as in Example 1. The initial discharge specific capacity of the material was 158 mAh / g.

[0074] Example 6

[0075] A Li 0.994 Na 0.005 K 0.001 Fe 0.996 Ca 0.002 Cr 0.001 Sn 0.001 P 0.999 S 0.001 The preparation method of O4 / C composite material includes the following steps:

[0076] (1) The preparation of the intermediate product is the same as in Example 2.

[0077] (2) Weigh Na2CO3, K2CO3, CaCO3, Cr(OH)3, SnO2, thiourea and the intermediate product of step (1) according to the stoichiometric ratio, pour them into a planetary ball mill, add a mixture of ethanol and deionized water, use agate balls as the ball milling medium, and ball mill at a speed of 400 r / min for 5 hours to obtain a slurry. Place the slurry in a box furnace and dry it at 100℃ for 10 hours. Mechanically mix it evenly to obtain powder. Place the powder in an atmosphere furnace and heat it to 750℃ at a heating rate of 5℃ / min under an argon atmosphere. Hold it at the temperature for 10 hours. After cooling, mechanically crush it and sieve it through a 300-mesh sieve to obtain a multi-element co-doped lithium iron phosphate / carbon composite cathode material.

[0078] The preparation of the positive electrode and the assembly of the battery were the same as in Example 1. The initial discharge specific capacity of the material was 129 mAh / g, and the discharge specific capacity remained essentially unchanged after 50 cycles.

[0079] Example 7

[0080] A Li 0.993 Na 0.006 K 0.001 Fe 0.995 Mg 0.002 Al 0.001 Zn 0.002 P 0.996 S 0.004 The preparation method of O4 / C composite material includes the following steps:

[0081] (1) The preparation of the intermediate product is the same as in Example 2.

[0082] (2) Weigh Na2CO3, K2CO3, MgCO3, Al(OH)3, Zn(OH)2, thiourea and the intermediate product of step (1) according to the stoichiometric ratio, pour them into a planetary ball mill, add a mixture of ethanol and deionized water, use agate balls as the ball milling medium, and ball mill at a speed of 400 r / min for 6 hours to obtain a slurry. Place the slurry in a box furnace and dry it at 80℃ for 12 hours, and mechanically mix it evenly. Place the mixed powder in an atmosphere furnace, and heat it to 700℃ at a heating rate of 5℃ / min under an argon atmosphere. Hold it at the temperature for 12 hours. After cooling, mechanically crush and sieve through 300 mesh to obtain multi-element co-doped lithium iron phosphate / carbon composite cathode material.

[0083] The preparation of the positive electrode and the assembly of the battery were the same as in Example 1. The initial discharge specific capacity of the material was 128 mAh / g.

[0084] Example 8

[0085] A Li 0.994 Na 0.005 K 0.001 Fe 0.992 Ca 0.002 Mn 0.004 Cu 0.002 P 0.998 S 0.002 The preparation method of O4 / C composite material includes the following steps:

[0086] (1) The preparation of the intermediate is the same as in Example 2.

[0087] (2) Weigh Na2CO3, K2CO3, CaCO3, MnCO3, Cu(OH)2, thiourea and the intermediate product of step (1) according to the stoichiometric ratio, pour them into a planetary ball mill, add ethanol, use agate balls as the ball milling medium, and ball mill at 300 r / min for 6 hours to obtain a slurry. Place the slurry in a box furnace and dry it at 100℃ for 12 hours, then mechanically mix it evenly. Place the mixed powder in an atmosphere furnace and heat it to 700℃ at a heating rate of 5℃ / min under a nitrogen atmosphere. Hold it at the temperature for 12 hours. After cooling, mechanically crush and sieve through 300 mesh to obtain multi-element co-doped lithium iron phosphate / carbon composite cathode material.

[0088] The preparation of the positive electrode and the assembly of the battery were the same as in Example 1. The initial discharge specific capacity of the material was 116 mAh / g, and the discharge specific capacity retention rate was higher than 98% after 50 cycles.

[0089] Example 9

[0090] A Li 0.993 Na 0.006 K 0.001 Fe 0.996 Cu 0.001 Ni 0.002 Mo 0.001 P 0.999 S 0.001 The preparation method of O4 / C composite material includes the following steps:

[0091] (1) The preparation of the intermediate product is the same as in Example 2.

[0092] (2) Weigh Na2CO3, K2CO3, (NH4)2MoO4, Cu(OH)2, Ni(OH)2, thiourea and the intermediate product of step (1) according to the stoichiometric ratio, pour them into a planetary ball mill, add a mixture of ethanol and deionized water, use agate balls as the ball milling medium, and ball mill at 300 r / min for 6 hours to obtain a slurry. Place the slurry in a box furnace and dry it at 80°C for 12 hours, then mechanically mix it evenly. Place the mixed powder in an atmosphere furnace and heat it to 750°C at a heating rate of 5°C / min under a nitrogen atmosphere. Hold it at the temperature for 10 hours. After cooling, mechanically crush and sieve through 300 mesh to obtain multi-element co-doped lithium iron phosphate / carbon composite cathode material.

[0093] The preparation of the positive electrode and the assembly of the battery were the same as in Example 1. The initial discharge specific capacity of the material was 109 mAh / g.

[0094] Example 10

[0095] A Li 0.994 Na 0.005 K 0.001 Fe0.996 Ca 0.002 Mg 0.002 P 0.999 S 0.001 O4 / C and NaNi 1 / 2 Mn 1 / 3 Cu 1 / 6 The preparation method of O2 composite cathode material and its lithium / sodium hybrid ion battery includes the following steps:

[0096] (1) Weigh the Li prepared in Example 1 at a mass ratio of 1:1. 0.994 Na 0.005 K 0.001 Fe 0.996 Ca 0.002 Mg 0.002 P 0.99 9S 0.001 O4 / C cathode material and NaNi 1 / 2 Mn 1 / 3 Cu 1 / 6 O2 was poured into a high-speed inclined grinding and mixing machine, and ball milled for 4 hours using polyurethane balls as the grinding medium to obtain composite cathode material powder.

[0097] (2) The composite positive electrode material is mixed with super carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 92:4:4. An appropriate amount of N-methylpyrrolidone (NMP) solution is added to make a slurry, which is then uniformly coated on aluminum foil and vacuum dried at 120°C for 12 hours to obtain the positive electrode sheet.

[0098] The preparation process of the lithium / sodium alloy sheet provided by the present invention is as follows: equal masses of lithium and sodium sheets are weighed in a glove box filled with argon atmosphere and placed in a heating furnace and heated to 180-250°C for melting. After cooling, the lithium / sodium alloy block is cut into thin sheets.

[0099] The battery was assembled in a glove box using a lithium / sodium alloy sheet as the negative electrode, a Celgard 2400 polypropylene membrane as the separator, and a solution obtained by dissolving 1.0 mol of LiPF6 / NaPF6 mixed salt in 1 L of EC-DEC-EMC (volume ratio 1:1:1) solvent as the electrolyte.

[0100] The assembled battery was left to stand for 24 hours, and its performance was tested using a LAND electrochemical tester. The charge / discharge test voltage range was 2.0–4.3V, and the current density was 20 mA / g. The battery performance was as follows: Figure 7 As shown, the initial discharge specific capacity is 108 mAh / g, and the discharge specific capacity retention rate is higher than 90% after 10 cycles.

[0101] Comparative Example 1

[0102] A lithium iron phosphate / carbon composite cathode material and its preparation method, comprising the following steps:

[0103] (1) Weigh FePO4 and Li2CO3 according to the Fe:Li molar ratio of 1:1.02, then weigh glucose according to the carbon content of 3wt% in the product, pour them into a planetary ball mill, add ethanol and deionized water solution, use agate balls as the ball milling medium, and ball mill at 300 r / min for 6 hours to obtain a slurry. Place the slurry in a box furnace and dry it at 80℃ for 12 hours. Then, mechanically mix it evenly to obtain an intermediate product.

[0104] (2) The intermediate product was placed in an atmosphere furnace and heated to 400°C at a heating rate of 5°C / min under a nitrogen atmosphere. The temperature was held for 2 hours, then heated to 750°C and held for 10 hours. After cooling, the product was mechanically crushed and sieved through a 300-mesh sieve to obtain lithium iron phosphate / carbon composite cathode material.

[0105] The preparation of the positive electrode and the assembly of the battery were the same as in Example 1. The initial discharge specific capacity of the material was 129 mAh / g, and the capacity remained at 129 mAh / g after 100 cycles.

[0106] Comparative Example 2

[0107] A lithium iron phosphate / carbon composite cathode material and its preparation method, comprising the following steps:

[0108] (1) Weigh FePO4 and Li2CO3 according to the Fe:Li molar ratio of 1:1.02, then weigh glucose according to the carbon content of 4wt% in the product, pour them into a planetary ball mill, add ethanol and deionized water solution, use agate balls as the ball milling medium, and ball mill at 300r / min for 6 hours to obtain a slurry. Place the slurry in a box furnace and dry it at 80℃ for 12 hours. Then, mechanically mix it evenly to obtain an intermediate product.

[0109] (2) The intermediate product was placed in an atmosphere furnace and heated to 400°C at a heating rate of 5°C / min under a nitrogen atmosphere. The temperature was held for 2 hours, then heated to 750°C and held for 10 hours. After cooling, the product was mechanically crushed and sieved through a 300-mesh sieve to obtain lithium iron phosphate / carbon composite cathode material.

[0110] The preparation of the positive electrode and the assembly of the battery were the same as in Example 1. The initial discharge specific capacity of the material was 113 mAh / g, and the capacity remained at 114 mAh / g after 100 cycles.

[0111] analyze:

[0112] Metal ion K + Na +It can improve the specific capacity of lithium iron phosphate because the substitution of metal ions at the Li site leads to an equilibrium of the compound's valence, resulting in a higher Li content in the LiFePO4 lattice. + site defects and a certain amount of Fe 2+ / Fe 3+ Coexistence state, thus benefiting Li + Diffusion in the solid phase improves the electrical conductivity of the crystal. Cr 3+ Mn 2+ Trace doping increases the interplanar spacing of lithium iron phosphate, resulting in good cycle stability. 2+ Mg 2+ Lithium iron phosphate materials doped with iron sites have the highest specific capacity, but due to their large ionic radius and poor crystal order, their cycle stability is generally poor. Comparatively, more carbon coating is not necessarily better; excessive carbon coating reduces the material's tap density, causes agglomeration, affects particle size and distribution, and a thicker carbon layer can hinder lithium-ion transport, thus reducing the material's electrochemical performance.

[0113] In summary, this invention prepares a multi-element co-doped lithium iron phosphate / carbon composite cathode material through a mechanochemical method and a solid-state sintering method, and then combines it with a sodium-ion battery cathode material to prepare a composite cathode material that can be used in lithium / sodium hybrid ion batteries. The preparation method is simple to operate, has low energy consumption, and is easy to industrialize.

[0114] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily 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 invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A multi-element trace co-doped lithium iron phosphate / carbon composite cathode material, characterized in that, The general formula of the cathode material is: Li 1-x M x Fe 1-y TM y P 1-z S z O4 / C, 0.005≤x≤0.02, 0.004≤y≤0.02, 0.001≤z≤0.002, where M represents sodium and potassium, TM represents two or more of aluminum, calcium, copper, chromium, magnesium, molybdenum, manganese, nickel, zinc, and tin, and the carbon content is 1.5~4.5wt%, with carbon composited through surface coating; the median particle size of the material is 2~6μm, and the tap density of the material is 1.25~1.4g / cm³. 3 ; The preparation method of the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material includes the following steps: (1) First, weigh out the lithium source, iron source, phosphorus source and carbon source according to the stoichiometric ratio. The molar ratio of lithium, iron and phosphorus is 0.95≤Li<1, 0.95≤Fe<1, 0.95≤P<1. Add them to a high-speed inclined grinding mixer and grind them for 1 to 8 hours under the action of ball milling media. Then, under the protection of an inert or reducing atmosphere, heat them to 300 to 500℃ at a heating rate of 2 to 15℃ / min for 1 to 6 hours. After cooling, mechanically crush them to obtain the intermediate product. (2) The intermediate product, M-containing compound, TM-containing compound, sulfur source and solvent are added to a high-speed planetary ball mill and mechanically ball-milled for 1 to 8 hours; the slurry is placed in a vacuum box furnace and dried at 60 to 120°C for 8 to 24 hours and then mechanically mixed; under the protection of an inert or reducing atmosphere, the temperature is raised to 600 to 850°C at a heating rate of 2 to 10°C / min and calcined for 6 to 24 hours; after cooling, it is pulverized and sieved to obtain multi-element trace co-doped lithium iron phosphate / carbon composite cathode material.

2. The multi-element trace co-doped lithium iron phosphate / carbon composite cathode material according to claim 1, characterized in that, The milling media is at least one of polyurethane balls, zirconia balls, agate balls, and alumina balls, and the inert or reducing atmosphere is at least one of argon, nitrogen, hydrogen / argon, and hydrogen / nitrogen.

3. The multi-element trace co-doped lithium iron phosphate / carbon composite cathode material according to claim 1, characterized in that, In step (1), the lithium source is at least one of lithium carbonate, lithium hydroxide, and lithium acetate; the iron source is at least one of iron oxide, iron phosphate, ferrous oxalate, and ferrous sulfate; the phosphorus source is at least one of phosphoric acid, iron phosphate, and ammonium dihydrogen phosphate; and the carbon source is at least one of glucose, starch, and sucrose.

4. The multi-element trace co-doped lithium iron phosphate / carbon composite cathode material according to claim 1, characterized in that, The M-containing compound mentioned in step (2) is at least one of potassium or sodium hydroxide, chloride, sulfate, or carbonate; the TM-containing compound is at least one of aluminum, calcium, copper, chromium, magnesium, molybdenum, manganese, nickel, zinc, or tin oxide, hydroxide, chloride, sulfate, or carbonate; the sulfur source is at least one of sulfur powder, thioacetamide, thiourea, thiourea dioxide, ammonium sulfide, ammonium sulfate, or ammonium hydrogen sulfate; and the solvent is at least one of deionized water and ethanol.

5. The application of the multi-element trace co-doped lithium iron phosphate / carbon composite cathode material as described in claim 1 in lithium / sodium hybrid ion batteries.

6. The application according to claim 5, characterized in that, The lithium / sodium hybrid ion battery is composed of a positive electrode, a negative electrode, a separator, and an electrolyte; and the lithium / sodium hybrid ion battery uses a thin sheet of lithium / sodium alloy as the negative electrode, a mixed organic solution of lithium / sodium salt as the electrolyte, and a polypropylene membrane as the separator.

7. The application according to claim 6, characterized in that, The method for preparing the positive electrode is as follows: Weigh out the active material, conductive carbon black and polyvinylidene fluoride according to the mass ratio, add an appropriate amount of N-methylpyrrolidone, stir evenly and coat it on the current collector aluminum foil, and then dry it under vacuum to obtain the positive electrode. The active material is a composite of the multi-element trace co-doped lithium iron phosphate / carbon composite material powder as described in claim 1 and sodium-ion battery cathode material powder.

8. The application according to claim 7, characterized in that, The mass ratio is 80:10:10~95:2:3, the proportion of multi-element trace co-doped lithium iron phosphate / carbon composite material powder in the composite is 40~60wt%, and the sodium-ion battery cathode material is NaMn. 1 / 3 Ni 1 / 2 Cu 1 / 6 O2, NaMn 1 / 3 Fe 1 / 3 Ni 1 / 3 O2, Na3V2(PO4)3, Na4MnV(PO4)3, Na4Fe 2.9 (PO4)2P2O7 or Na2Mn2(CN)6.

9. The application according to claim 6, characterized in that, The preparation process of the aforementioned lithium / sodium alloy sheet is as follows: Equal masses of lithium and sodium sheets are weighed in a glove box filled with argon atmosphere and placed in a heating furnace at 180-250°C for melting. After cooling, the lithium / sodium alloy block is cut into thin sheets to obtain the final product.

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