A composite lithium iron phosphate positive electrode material and its preparation method and application

Abstract

The present invention provides a composite lithium iron phosphate positive electrode material and a preparation method and application thereof. The preparation method comprises the following steps: (1) subjecting vermiculite to a thermal expansion treatment and then mixing it with a solvent, and subjecting it to a cutting treatment to obtain a vermiculite nanosheet colloidal suspension; (2) mixing the vermiculite nanosheet colloidal suspension with a first lithium source solution to obtain a lithium intercalated vermiculite nanosheet suspension; (3) mixing iron phosphate, a second lithium source, a carbon source, and a lithium intercalated vermiculite nanosheet suspension with a solvent to obtain a mixed slurry, and subjecting the mixed slurry to drying and calcining to obtain the composite lithium iron phosphate positive electrode material. The present invention utilizes the expansion of vermiculite during high-temperature calcination to obtain a loose lithium iron phosphate material, increase the cavity, and improve the transmission rate of lithium ions; at the same time, by exchanging the cations between the layered vermiculite with lithium ions, that is, pre-setting lithium ions between the layers, the layers are used as a fast channel for lithium ion extraction / embedding.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries and relates to a composite lithium iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] Lithium iron phosphate cathode materials with olivine structure in lithium-ion batteries are in line with the development trend of commercial lithium-ion batteries because of their environmental friendliness, safety, long cycle life, environmental friendliness and wide source of raw materials.

[0003] Lithium iron phosphate is an olivine crystal structure, belonging to the orthorhombic Pnmb space group, and its structure includes FeO6 octahedron, LiO6 octahedron and PO4 tetrahedron. However, due to its special structure, Li + In LFP, Li is inserted / extracted in a one-dimensional form, which is affected by the tetrahedral structure of PO4. + The diffusion rate is poor.

[0004] CN116216686A discloses a method for preparing a lithium iron phosphate positive electrode material, which belongs to the technical field of lithium ion batteries. The method comprises: mixing a lithium source, an iron source, a phosphorus source and a carbon source with deionized water to form a first solution, mixing a dispersant with deionized water to form a second solution, and mixing the first solution with the second solution to form a first mixture; adding a modifier to the first mixture to form a compounding solution; sand-grinding the compounding solution to form a slurry; drying the slurry to obtain a precursor; and sintering the precursor to obtain a lithium iron phosphate positive electrode material.

[0005] CN117916196A discloses a method for preparing multi-porous lithium iron phosphate, which comprises the following steps: mixing a soy protein solution and an anionic polysaccharide solution and adjusting the pH value to be acidic to obtain a mixed solution; adding an iron source to the mixed solution and stirring, then adding phosphate and adjusting the pH value to react, filtering and washing after the reaction, collecting the filter residue after washing; freeze-drying the filter residue, calcining, and crushing to obtain a multi-porous iron phosphate precursor. The multi-porous iron phosphate precursor and the lithium source are mixed and wet-milled to obtain a ball-milled product; the ball-milled product and the carbon source are mixed and calcined to obtain a lithium iron phosphate material.

[0006] The lithium iron phosphate positive electrode material prepared by the above scheme cannot take into account both the diffusion rate and structural stability of lithium ions, which leads to a decrease in the rate performance or capacity of the lithium iron phosphate positive electrode material. Summary of the invention

[0007] The purpose of the present invention is to provide a composite lithium iron phosphate positive electrode material and a preparation method and application thereof. The present invention utilizes the expansion of vermiculite during high-temperature calcination to obtain a loose lithium iron phosphate material, increase the cavity, and improve the transmission rate of lithium ions; at the same time, by exchanging the cations between the layered vermiculite with lithium ions, that is, pre-setting lithium ions between the layers, the layers are used as a fast channel for lithium ion extraction / embedding.

[0008] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a method for preparing a composite lithium iron phosphate positive electrode material, the preparation method comprising the following steps:

[0010] (1) subjecting vermiculite to thermal expansion treatment, mixing the mixture with a solvent, and subjecting the mixture to cutting treatment to obtain a vermiculite nanosheet colloidal suspension;

[0011] (2) mixing the vermiculite nanosheet colloidal suspension, a first lithium source and a solvent to obtain a mixed slurry, and reacting to obtain a lithium-intercalated vermiculite nanosheet suspension;

[0012] (3) Mixing iron phosphate, a second lithium source, a carbon source, a suspension of lithium-intercalated vermiculite nanosheets and a solvent to obtain a mixed slurry, drying the mixed slurry and calcining it to obtain the composite lithium iron phosphate positive electrode material.

[0013] The first lithium source and the second lithium source of the present invention are only used for distinction and have no other limiting function.

[0014] The invention first heats and expands vermiculite, and then uses high-speed cutting and centrifugation in a solvent to obtain a nano vermiculite suspension; the vermiculite nano suspension is taken, mixed with a solvent and a lithium source, so that interlayer cations are replaced by lithium ions, wherein part of calcium ions and magnesium ions react with hydroxide to form precipitation, and filtered to obtain a replaced nano vermiculite suspension; the lithium intercalated vermiculite nano sheet suspension is mixed with iron phosphate, a lithium source, a carbon source and a solvent to prepare a slurry, and the composite lithium iron phosphate positive electrode material is obtained after spray drying and high-temperature calcination.

[0015] Preferably, the vermiculite in step (1) comprises raw vermiculite.

[0016] Preferably, the temperature of the thermal expansion treatment in step (1) is 350-400°C, for example, 350°C, 360°C, 380°C, 390°C or 400°C, etc., and is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0017] When the c° of the water molecule layer is 1.159nm, it is a single layer of water molecules; when c° is 0.902nm, it is completely dehydrated. After the vermiculite of the present invention is heated and dehydrated at 500°C, the vermiculite can absorb water and recover, but after exceeding 700°C, it does not have the recovery property. The hydration state of vermiculite is determined by the number of interlayer water layers. The degree of interlayer hydration is closely related to the radius and charge of the cation, and also affects the interlayer spacing of different types of vermiculite.

[0018] Preferably, the heating rate of the thermal expansion treatment in step (1) is 8 to 12°C / min, for example, 8°C / min, 9°C / min, 10°C / min, 11°C / min or 12°C / min, etc., and is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0019] Preferably, the holding time of the thermal expansion treatment in step (1) is 4 to 8 minutes, for example, 4 minutes, 5 minutes, 6 minutes, 7 minutes or 8 minutes, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0020] Preferably, the thermal expansion treatment in step (1) is followed by water washing and drying.

[0021] Preferably, the solvent in step (1) comprises water.

[0022] Preferably, the mass volume ratio of the vermiculite and the solvent in step (1) is 2:(30-40) g / mL, for example: 2:30 g / mL, 2:32 g / mL, 2:35 g / mL, 2:38 g / mL or 2:40 g / mL, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0023] Preferably, the cutting speed in step (1) is 10000-14000 rpm, for example, 10000 rpm, 11000 rpm, 12000 rpm, 13000 rpm or 14000 rpm, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0024] Preferably, the cutting treatment time in step (1) is 0.5 to 2 hours, for example, 0.5 hours, 0.8 hours, 1 hour, 1.5 hours or 2 hours, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0025] Preferably, the cutting process in step (1) is followed by centrifugation.

[0026] Preferably, the rotation speed of the centrifugal treatment is 6000-8000 rpm, for example: 6000 rpm, 6500 rpm, 7000 rpm, 7500 rpm or 8000 rpm, etc., not limited to the listed values, other values ​​not listed within the numerical range are also applicable.

[0027] The cutting speed is controlled at 10000-14000rpm, and the performance of the composite lithium iron phosphate positive electrode material is better. If the cutting speed is too slow, the crushing particle size is insufficient, and after centrifugal treatment, the yield of nano vermiculite is affected. If the cutting speed is too fast, the particle size is excessively crushed, resulting in an increase in the content of single-layer vermiculite, which ultimately affects the expansion effect and reduces the rate performance.

[0028] Preferably, the centrifugal treatment time is 5 to 10 minutes, for example, 5 minutes, 6 minutes, 7 minutes, 8 minutes or 10 minutes, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0029] Preferably, the length and width of the vermiculite nanosheets in the vermiculite nanosheet colloidal suspension in step (1) are independently 50 to 300 nm, for example, 50 nm, 80 nm, 100 nm, 200 nm or 300 nm, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0030] Preferably, taking the mass of the vermiculite after the thermal expansion treatment as 100%, the mass of the vermiculite nanosheets in the vermiculite nanosheet colloidal suspension is 50-60%, for example, 50%, 52%, 55%, 58% or 60%, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0031] Preferably, in step (2), the first lithium source comprises lithium hydroxide.

[0032] Preferably, the solvent in step (2) comprises water.

[0033] Preferably, the molar concentration of lithium element in the mixed slurry in step (2) is 0.4 to 1 mol / L, for example: 0.4 mol / L, 0.6 mol / L, 0.8 mol / L or 1 mol / L, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0034] Preferably, the mass percentage concentration of vermiculite nanosheets in the mixed slurry in step (2) is 20-35wt%, for example: 20wt%, 25wt%, 30wt% or 35wt%, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0035] Preferably, the reaction temperature in step (2) is 40-60°C, for example, 40°C, 45°C, 50°C, 55°C or 60°C, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0036] Preferably, stirring is performed during the reaction in step (2).

[0037] Preferably, the stirring speed is 80-120 rpm, for example, 80 rpm, 90 rpm, 100 rpm, 110 rpm or 120 rpm, etc., and is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0038] Preferably, the reaction time of step (2) is 4 to 8 hours, for example, 4 hours, 5 hours, 6 hours, 7 hours or 8 hours, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0039] Preferably, in step (3), the second lithium source comprises any one of lithium carbonate, lithium oxalate or lithium hydroxide, or a combination of at least two of them. Typical but non-limiting combinations include a combination of lithium carbonate and lithium hydroxide, a combination of lithium carbonate and lithium oxalate, or a combination of lithium oxalate and lithium hydroxide, etc.

[0040] Preferably, the carbon source in step (3) comprises any one of glucose, sucrose, carbon black, graphene, carbon nanotubes or citric acid, or a combination of at least two thereof. Typical but non-limiting combinations include a combination of glucose and sucrose, a combination of sucrose and carbon black, or a combination of graphene and carbon nanotubes, etc.

[0041] Preferably, the solvent in step (3) comprises water and / or ethanol.

[0042] Preferably, the molar ratio of the iron phosphate in step (3) to the lithium element in the second lithium source is 1:(0.99-1), for example: 1:0.99, 1:0.992, 1:0.995, 1:0.998 or 1:1, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0043] Preferably, the mass volume ratio of the iron phosphate and lithium intercalated vermiculite nanosheet suspension in step (3) is 1: (2-3) g / mL, for example: 1: 2 g / mL, 1: 2.2 g / mL, 1: 2.5 g / mL, 1: 2.8 g / mL or 1: 3 g / mL, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0044] Preferably, the mass ratio of the total mass of the iron phosphate and the lithium source to the carbon source in step (3) is 100:(8-10), for example: 100:8, 100:8.5, 100:9, 100:9.5 or 100:10, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0045] Preferably, the solid content of the mixed slurry in step (3) is 38-42%, for example: 38%, 39%, 40%, 41% or 42%, etc., not limited to the listed values, other unlisted values ​​within the numerical range are also applicable.

[0046] Preferably, the drying method in step (3) includes spray drying.

[0047] Preferably, the temperature of the calcination treatment in step (3) is 700-850°C, for example, 700°C, 720°C, 750°C, 800°C or 850°C, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0048] Preferably, the atmosphere of the calcination treatment in step (3) comprises nitrogen and / or argon.

[0049] Preferably, the calcination time in step (3) is 8 to 12 hours, for example, 8 hours, 9 hours, 10 hours, 11 hours or 12 hours, etc., not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0050] In a second aspect, the present invention provides a composite lithium iron phosphate positive electrode material, which is prepared by the method described in the first aspect.

[0051] In a third aspect, the present invention provides a positive electrode plate, wherein the positive electrode plate comprises the composite lithium iron phosphate positive electrode material as described in the second aspect.

[0052] In a fourth aspect, the present invention provides a lithium-ion battery, wherein the lithium-ion battery comprises the positive electrode sheet as described in the third aspect.

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] (1) The present invention uses lithium hydroxide to exchange cations between the layers of vermiculite after nano-forming the vermiculite, 2+ Mg 2+ 、Na + , K +Replaced with lithium ions, that is, pre-placed lithium ions between the layers, so that the layers serve as a fast channel for lithium ion deintercalation. The vermiculite expands during high-temperature calcination to form a fluffy lithium iron phosphate material, shortening the transmission distance of lithium ions and improving the rate performance of the battery.

[0055] (2) The specific surface area of ​​the composite lithium iron phosphate positive electrode material prepared by the method of the present invention can reach 21.8 m 2 / g or more, the 0.2C discharge capacity of the battery can reach more than 161mAh / g, the 1C discharge capacity can reach more than 156mAh / g, the 2C discharge capacity can reach more than 149mAh / g, the 5C discharge capacity can reach more than 142mAh / g, and the 10C discharge capacity can reach more than 131mAh / g. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is a SEM image of the composite lithium iron phosphate positive electrode material described in Example 1 of the present invention.

[0057] Figure 2 This is an SEM magnified image of the composite lithium iron phosphate positive electrode material described in Example 1 of the present invention. DETAILED DESCRIPTION

[0058] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0059] Example 1

[0060] This embodiment provides a composite lithium iron phosphate positive electrode material, and the preparation method of the composite lithium iron phosphate positive electrode material is as follows:

[0061] (1) taking raw vermiculite from Xinjiang Yuli Wvg-1 and placing it in a muffle furnace at room temperature, raising the temperature to 350° C. at a heating rate of 10° C. / min, and heating it for 5 min to obtain expanded vermiculite, and after cooling to room temperature, placing it in deionized water at a mass volume ratio of 2:35 g / mL, cutting it at a speed of 13000 rpm for 1.2 h, and then centrifuging it at 7000 rpm for 8 min to obtain a 50-300 nm vermiculite nanosheet colloidal suspension, wherein the vermiculite yield in the suspension obtained by the centrifugation is 52% of the input amount;

[0062] (2) adding deionized water to the above-mentioned vermiculite nanosheet colloidal suspension at 50° C. and stirring evenly, then adding lithium hydroxide to obtain a mixed slurry, wherein the molar concentration of lithium hydroxide in the mixed slurry is 0.8 mol / L, and the mass percentage concentration of the vermiculite nanosheets is 25 wt %. After stirring at 100 rpm for 5 h, the mixture is centrifuged to obtain a lithium intercalated vermiculite nanosheet suspension. Based on the property that lithium ions react with oxalic acid to generate a colorless precipitate of lithium oxalate under acidic conditions, the free lithium content in the lithium intercalated vermiculite suspension is determined by the oxalic acid method, and the intercalated Li content is detected to be 0.95 mmol / g;

[0063] (3) Mixing iron phosphate, glucose, lithium hydroxide and lithium intercalated vermiculite nanosheet suspension with water to prepare a slurry with a solid content of 40%, spray drying, placing it in a nitrogen atmosphere, and calcining it at 780°C for 10 hours to obtain the composite lithium iron phosphate positive electrode material. The molar ratio of lithium hydroxide to iron phosphate is 1:1, the mass of glucose is 10% of the total mass of lithium iron phosphate and lithium hydroxide, and the mass volume ratio of iron phosphate to lithium intercalated vermiculite nanosheet suspension is 1:2.5 g / mL.

[0064] The SEM image of the prepared composite lithium iron phosphate positive electrode material is as follows Figure 1-2 shown.

[0065] Example 2

[0066] This embodiment provides a composite lithium iron phosphate positive electrode material, and the preparation method of the composite lithium iron phosphate positive electrode material is as follows:

[0067] (1) taking raw vermiculite from Xinjiang Yuli Wvg-2 and placing it in a muffle furnace at room temperature, raising the temperature to 380° C. at a heating rate of 8° C. / min, and heating it for 6 min to obtain expanded vermiculite, and after cooling to room temperature, placing it in deionized water at a mass volume ratio of 2:30 g / mL, cutting it at a speed of 10000 rpm for 2 h, and centrifuging it at 6000 rpm for 10 min to obtain a 50-300 nm vermiculite nanosheet colloidal suspension, wherein the vermiculite yield in the suspension obtained by the centrifugation is 56% of the input amount;

[0068] (2) adding deionized water to the vermiculite nanosheet colloidal suspension at 40° C., stirring evenly, and then adding lithium hydroxide to obtain a mixed slurry, wherein the molar concentration of lithium hydroxide in the mixed slurry is 0.4 mol / L, and the mass percentage concentration of the vermiculite nanosheets is 25 wt %. After stirring at 120 rpm for 4 h, the mixture is centrifuged to obtain a lithium intercalated vermiculite nanosheet suspension;

[0069] (3) Mixing iron phosphate, sucrose, lithium oxalate and lithium intercalated vermiculite nanosheet suspension with water to prepare a slurry with a solid content of 38%, spray drying, placing it in a nitrogen atmosphere, and calcining it at 700°C for 12 hours to obtain the composite lithium iron phosphate positive electrode material. Among them, the molar ratio of lithium hydroxide to iron phosphate is 1:1, the mass of sucrose is 8% of the total mass of lithium iron phosphate and lithium hydroxide, and the mass volume ratio of iron phosphate to lithium intercalated vermiculite nanosheet suspension is 1:2g / mL.

[0070] Example 3

[0071] This embodiment provides a composite lithium iron phosphate positive electrode material, and the preparation method of the composite lithium iron phosphate positive electrode material is as follows:

[0072] (1) taking raw vermiculite from Xinjiang Yuli Wvg-3 and placing it in a muffle furnace at room temperature, raising the temperature to 400° C. at a heating rate of 12° C. / min, and heating it for 4 min to obtain expanded vermiculite, and after cooling to room temperature, placing it in deionized water at a mass volume ratio of 2:40 g / mL, high-speed cutting at a speed of 14000 rpm for 0.5 h, and centrifuging it at 8000 rpm for 5 min to obtain a 50-300 nm vermiculite nanosheet colloidal suspension, wherein the vermiculite yield in the suspension obtained by the centrifugation is 50% of the input amount;

[0073] (2) adding deionized water to the vermiculite nanosheet colloidal suspension at 60° C. and stirring the mixture evenly, and then adding lithium hydroxide to obtain a mixed slurry, wherein the molar concentration of lithium hydroxide in the mixed slurry is 1 mol / L, and the mass percentage concentration of the vermiculite nanosheets is 25 wt %. The mixture is stirred at 80 rpm for 8 h, and then centrifuged to obtain a lithium intercalated vermiculite nanosheet suspension;

[0074] (3) Mixing iron phosphate, carbon nanotubes, lithium oxalate and lithium intercalated vermiculite nanosheet suspension with water to prepare a slurry with a solid content of 42%, spray drying, placing it in a nitrogen atmosphere, and calcining it at 850° C. for 8 hours to obtain the composite lithium iron phosphate positive electrode material. The molar ratio of lithium hydroxide to iron phosphate is 1:1, the mass of the carbon nanotubes is 9% of the total mass of lithium iron phosphate and lithium hydroxide, and the mass volume ratio of the iron phosphate to the lithium intercalated vermiculite nanosheet suspension is 1:3 g / mL.

[0075] Example 4

[0076] The only difference between this embodiment and embodiment 1 is that the molar concentration of lithium hydroxide in the mixed slurry in step (2) is 0.2 mol / L mass percentage concentration, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0077] Example 5

[0078] The only difference between this embodiment and embodiment 1 is that the molar concentration of lithium hydroxide in the mixed slurry in step (2) is 1.2 mol / L mass percentage concentration, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0079] Example 6

[0080] The difference between this embodiment and embodiment 1 is that the mass volume ratio of the iron phosphate and lithium intercalated vermiculite nanosheet suspension in step (3) is 1:1 g / mL, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0081] Example 7

[0082] The difference between this embodiment and embodiment 1 is that the mass volume ratio of the iron phosphate and lithium intercalated vermiculite nanosheet suspension in step (3) is 1:4 g / mL, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0083] Comparative Example 1

[0084] The only difference between this comparative example and Example 1 is that no vermiculite is added, and iron phosphate, a carbon source, a lithium source and a solvent are directly used to prepare a slurry, and then a composite lithium iron phosphate positive electrode material is prepared by calcining after spray drying.

[0085] Comparative Example 2

[0086] The only difference between this comparative example and Example 1 is that step (2) is not performed, and the other conditions and parameters are exactly the same as those in Example 1.

[0087] Comparative Example 3

[0088] The difference between this comparative example and Example 1 is that in step (1), the vermiculite is simply crushed to obtain millimeter-level vermiculite, which is then mixed with iron phosphate, a carbon source, and a lithium source to obtain a slurry, which is then spray-dried and calcined to obtain a composite lithium iron phosphate positive electrode material.

[0089] Performance Test:

[0090] After the specific surface area test of the composite lithium iron phosphate positive electrode material obtained by the BET method in the embodiment and the comparative example, a uniform slurry was prepared according to the mass ratio of the positive electrode material: acetylene black: PVDF at 75:15:10, and evenly coated on an aluminum foil substrate as the positive electrode of the simulated battery. The negative electrode of the simulated battery uses a lithium sheet, a polypropylene porous membrane as a diaphragm, and an electrolyte of 1 mol LiPF6 dissolved in a mixed solvent of 1L EC and DMC (volume ratio 1:1). The positive electrode, negative electrode, electrolyte, and diaphragm are assembled into a battery in an argon-protected glove box.

[0091] Simulate battery rate and cycle test steps:

[0092] First, charge to 4.2V with constant current, then discharge to 2.0V with rate current, the released capacity is the discharge capacity at this rate, after discharge, discharge to 2.0V with constant current again. Then, test at the next rate, the test results are shown in Table 1:

[0093] Table 1

[0094]

[0095]

[0096] As can be seen from Table 1, from Examples 1-3, the specific surface area of ​​the composite lithium iron phosphate positive electrode material prepared by the method of the present invention can reach 21.8 m 2 / g or more, the 0.2C discharge capacity of the battery can reach more than 161mAh / g, the 1C discharge capacity can reach more than 156mAh / g, the 2C discharge capacity can reach more than 149mAh / g, the 5C discharge capacity can reach more than 142mAh / g, and the 10C discharge capacity can reach more than 131mAh / g.

[0097] By comparing Example 1 with Example 4-5, it can be seen that in the preparation process of the composite lithium iron phosphate positive electrode material of the present invention, the amount of lithium hydroxide added to the vermiculite nanosheet suspension for preparing the lithium intercalation layer in step (2) will affect the replacement amount of lithium ions between the vermiculite nanosheet layers, thereby affecting the lithium ion transmission performance. The molar concentration of lithium in the mixed slurry is controlled between 0.4 and 1 mol / L, and the performance of the composite lithium iron phosphate positive electrode material is better. If the concentration of lithium is too small, the exchange amount of the layer ions will be too small; if the concentration of lithium is too large, the lithium hydroxide concentration will be too high and the lithium ion exchange will be too fast, resulting in blocking the interlayer channels of the vermiculite nanosheets, so that the ions inside the vermiculite cannot be replaced.

[0098] By comparing Example 1 with Examples 6-7, it can be seen that in the preparation process of the composite lithium iron phosphate positive electrode material of the present invention, the amount of vermiculite added will affect its performance. The mass volume ratio of the vermiculite nanosheet suspension with iron phosphate and lithium intercalation is controlled at 1: (2-3) g / mL, and the performance of the composite lithium iron phosphate positive electrode material is better. If the amount of vermiculite added is too low, its effect becomes worse and the rate performance is reduced. If the amount of vermiculite added is too high, the specific capacity of the battery will be reduced.

[0099] From the comparison between Example 1 and Comparative Example 1, it can be seen that without adding intercalated vermiculite nanomaterials, the specific surface area and rate performance of the prepared lithium iron phosphate material both show poor electrochemical performance.

[0100] From the comparison between Example 1 and Comparative Example 2, it can be seen that if no lithium ion exchange treatment is performed, the transmission of lithium ions will be affected, and the rate performance of the composite lithium iron phosphate positive electrode material will be reduced.

[0101] From the comparison between Example 1 and Comparative Example 3, it can be seen that millimeter-level vermiculite cannot affect the structure of the micro-nano-level positive electrode material, so the rate performance is reduced and the specific surface area is decreased.

[0102] The applicant declares that the above is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention shall fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a composite lithium iron phosphate positive electrode material, characterized in that: The preparation method comprises the following steps: (1) subjecting vermiculite to thermal expansion treatment, mixing the mixture with a solvent, and subjecting the mixture to cutting treatment to obtain a vermiculite nanosheet colloidal suspension; (2) mixing the vermiculite nanosheet colloidal suspension, a first lithium source and a solvent to obtain a mixed slurry, and reacting to obtain a lithium-intercalated vermiculite nanosheet suspension; (3) mixing iron phosphate, a second lithium source, a carbon source, a suspension of lithium-intercalated vermiculite nanosheets and a solvent to obtain a mixed slurry, drying the mixed slurry and subjecting it to calcination to obtain the composite lithium iron phosphate positive electrode material; The length and width of the vermiculite nanosheets in the vermiculite nanosheet colloidal suspension of step (1) are independently 50 to 300 nm, and the mass of the vermiculite nanosheets in the vermiculite nanosheet colloidal suspension is 50 to 60% based on the mass of the vermiculite after the thermal expansion treatment being 100%; Step (2) The first lithium source includes lithium hydroxide, the molar concentration of lithium element in the mixed slurry is 0.4-1 mol / L, and the mass percentage concentration of vermiculite nanosheets in the mixed slurry is 20-35wt%.

2. The preparation method according to claim 1, characterized in that The vermiculite in step (1) includes raw vermiculite.

3. The preparation method according to claim 1, characterized in that: The temperature of the thermal expansion treatment in step (1) is 350-400°C.

4. The preparation method according to claim 1, characterized in that: The heating rate of the thermal expansion treatment in step (1) is 8 to 12°C / min.

5. The preparation method according to claim 1, characterized in that: The holding time of the thermal expansion treatment in step (1) is 4 to 8 minutes.

6. The preparation method according to claim 1, characterized in that: The heat expansion treatment in step (1) is followed by water washing and drying.

7. The preparation method according to claim 1, characterized in that: The solvent in step (1) includes water.

8. The preparation method according to claim 1, characterized in that: The mass volume ratio of the vermiculite and the solvent in step (1) is 2:(30-40) g / mL.

9. The preparation method according to claim 1, characterized in that: The cutting process speed in step (1) is 10000-14000 rpm.

10. The preparation method according to claim 1, characterized in that: The cutting process in step (1) takes 0.5 to 2 hours.

11. The preparation method according to claim 1, characterized in that: The cutting process in step (1) is followed by centrifugation.

12. The preparation method according to claim 11, characterized in that: The rotation speed of the centrifugal treatment is 6000-8000 rpm.

13. The preparation method according to claim 11, characterized in that: The centrifugal treatment time is 5 to 10 minutes.

14. The preparation method according to claim 1, characterized in that: The solvent in step (2) includes water.

15. The preparation method according to claim 1, characterized in that: The reaction temperature in step (2) is 40-60°C.

16. The preparation method according to claim 1, characterized in that: Stirring is performed during the reaction in step (2).

17. The preparation method according to claim 16, characterized in that: The stirring speed is 80-120 rpm.

18. The preparation method according to claim 1, characterized in that: The reaction time of step (2) is 4 to 8 hours.

19. The preparation method according to claim 1, characterized in that: Step (3) The second lithium source includes any one of lithium carbonate, lithium oxalate or lithium hydroxide, or a combination of at least two of them.

20. The preparation method according to claim 1, characterized in that: The carbon source in step (3) includes any one of glucose, sucrose, carbon black, graphene, carbon nanotubes or citric acid, or a combination of at least two of them.

21. The preparation method according to claim 1, characterized in that: The solvent in step (3) includes water and / or ethanol.

22. The preparation method according to claim 1, characterized in that: The molar ratio of the iron phosphate to the lithium element in the second lithium source in step (3) is 1:(0.99-1).

23. The preparation method according to claim 1, characterized in that: The mass volume ratio of the iron phosphate and lithium intercalated vermiculite nanosheet suspension in step (3) is 1:(2-3) g / mL.

24. The preparation method according to claim 1, characterized in that: In step (3), the mass ratio of the total mass of the iron phosphate and the second lithium source to the carbon source is 100:(8-10).

25. The preparation method according to claim 1, characterized in that: The solid content of the mixed slurry in step (3) is 38-42%.

26. The preparation method according to claim 1, characterized in that: The drying method in step (3) includes spray drying.

27. The preparation method according to claim 1, characterized in that: The temperature of the calcination treatment in step (3) is 700-850°C.

28. The preparation method according to claim 1, characterized in that: The atmosphere for the calcination treatment in step (3) includes nitrogen and / or argon.

29. The preparation method according to claim 1, characterized in that: The calcination time in step (3) is 8 to 12 hours.

30. A composite lithium iron phosphate positive electrode material, characterized in that: The composite lithium iron phosphate positive electrode material is prepared by the method according to any one of claims 1 to 29.

31. A positive electrode plate, characterized in that: The positive electrode plate comprises the composite lithium iron phosphate positive electrode material as described in claim 30.

32. A lithium ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet as claimed in claim 31.

Images