Low-temperature rate type lithium iron phosphate cathode material and preparation method thereof

Abstract

The application provides a method for coating lithium iron phosphate with active ceramic material YTO, reducing Li + diffusion barrier, accelerating Li + diffusion rate, and significantly improving low-temperature and rate performance of the lithium iron phosphate. The prepared lithium iron phosphate has excellent low-temperature and rate performance, and the 1C initial discharge specific capacity can reach 155 mAh / g, and the 5C discharge specific capacity can reach 143 mAh / g; the 3Ah soft package battery is discharged at 1C rate under the condition of-30 DEG C, and the discharge capacity reaches 67% of the normal-temperature discharge capacity.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate cathode material technology, specifically to a low-temperature rate-capacity lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium iron phosphate (LFP) materials have achieved great success in the application of electric vehicles (EVs) and energy storage due to their environmental friendliness, long cycle life, and high energy density. However, LFP lithium-ion batteries have poorer low-temperature performance than batteries using other cathode materials. At -20°C, the capacity decays severely, often retaining less than 50% of its discharge capacity, limiting its application in cold regions and low-temperature applications. The poor low-temperature performance of LFP materials is related to their structural characteristics. Strong PO covalent bonds form delocalized three-dimensional chemical bonds, giving LFP strong thermodynamic / kinetic stability, while the migration rate of Li+ ions in this material is relatively slow.

[0003] Another major drawback of LFP is its poor conductivity, with a conductivity of (10⁻¹⁰ S·cm). -1 The low viscosity of the electrolyte means it is only suitable for charging and discharging at low current densities, resulting in poor rate performance. Furthermore, the increased viscosity of the electrolyte at low temperatures leads to increased polarization and overpotential during charging and discharging, causing lithium deposition at the anode during charging. During high-current discharge at low temperatures, the high polarization makes the battery prone to reaching its lower voltage limit, preventing it from discharging.

[0004] To improve the low-temperature and rate performance of lithium iron phosphate, existing technologies optimize electronic and ionic conductivity through elemental doping or carbon coating. For example, patent number 202211649263.6 involves doping with one or more of zirconium, titanium, manganese, magnesium, vanadium, and their compounds during the precursor mixing stage; patent number 202310803283.2 specifies that the dopant is preferably selected from one or more of titanium oxide, vanadium oxide, tungsten oxide, magnesium oxide, zirconium oxide, and aluminum oxide. These methods only improve the bulk ionic and electronic conductivity of the material by creating crystal defects through doping, and do not achieve the desired improvement in low-temperature and rate performance.

[0005] Patent CN107331852 B proposes a scheme to improve the surface impedance and cycle performance of ternary materials by surface modification with composite metal oxides. However, this scheme directly mixes metal oxides and then performs surface modification by high-temperature sintering. CN109802133 B also mentions a method of surface modification of lithium cobalt oxide with metal oxides to improve the cycle stability of the material. However, the above methods use a simple method of coating with a mixture of metal oxides, which have low activity, large particle size, poor coating effect, and poor improvement effect.

[0006] This invention is the first to employ a sol-gel method to coat the surface of lithium iron phosphate with a nanoscale mixture of metal hydroxides, followed by high-temperature sintering in an oxygen environment. This generates a uniform active ceramic oxide film (YTO) coated on the lithium iron phosphate surface, improving the lithium-ion diffusion rate at the lithium iron phosphate interface and reducing Li... + The diffusion barrier enhances Li + The diffusion rate is increased, which in turn significantly improves the low-temperature and rate performance of LFP. Summary of the Invention

[0007] To address the problems existing in the prior art, the present invention aims to provide a method for surface coating of LFP using the active ceramic material YTO to reduce Li... + diffusion barrier, accelerating Li + A method to improve the diffusion rate, thereby significantly enhancing the low-temperature and rate performance of LFP.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for preparing a low-temperature rate-controlled lithium iron phosphate cathode material, the method comprising the following steps:

[0010] 1) Lithium iron phosphate a is prepared by mixing lithium source, iron source, phosphorus source, dopant and carbon source and sintering under nitrogen atmosphere;

[0011] 2) Lithium iron phosphate a was placed in an ethanol solution of organic titanium salt and mixed thoroughly. Then, an aqueous solution of soluble yttrium salt was added. The mixture was stirred under heating and ammonia was added to obtain a sol-gel b of lithium iron phosphate coated with yttrium hydroxide and titanium hydroxide.

[0012] 3) Calcine b in air or oxygen atmosphere to obtain YTO-coated LFP precursor c;

[0013] 4) The YTO-coated LFP precursor c prepared above is coated with carbon by vapor deposition under high temperature conditions in an inert atmosphere to obtain a YTO and carbon composite coated LFP product.

[0014] In this invention, the lithium source in step 1) is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, and lithium dihydrogen phosphate.

[0015] In this invention, the iron source in step 1) is selected from one or more of ferric phosphate, ferrous oxalate, ferric nitrate, ferric oxide, and ferric chloride.

[0016] In this invention, the phosphorus source in step 1) is selected from one or more of phosphoric acid, lithium dihydrogen phosphate, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate.

[0017] In this invention, the dopant in step 1) is selected from one or more of titanium dioxide, magnesium oxide, magnesium hydroxide, niobium pentoxide, vanadium pentoxide, ammonium metavanadate, yttrium oxide, and zirconium oxide; the amount of dopant added is 0.2-1% of the mass of lithium iron phosphate.

[0018] In this invention, the carbon source in step 1) is selected from one or more of glucose, sucrose, PEG, PVP, PVA, starch, and cellulose.

[0019] In this invention, the molar ratio of lithium source, iron source, and phosphorus source in step 1) is (1.005-1.05):(1-1.05):(1.005-1.05), for example, a molar ratio of 1.005:1:1.005.

[0020] 1.005:1.05:1.05, 1:1:1, etc.; the amount of the dopant is 0.2%-2% of the mass of iron phosphate, and the amount of the carbon source is 8%-20% of the mass of iron phosphate.

[0021] In this invention, the sintering temperature in step 1) is 700-800℃, and the sintering time is 5-15h.

[0022] In this invention, after obtaining lithium iron phosphate a in step 1), a grinding step is further included, and after grinding, D 50 The particle size is 100-500nm, including but not limited to 100, 200, 300, 400, and 500nm.

[0023] In this invention, the organic titanium salt in step 2) is selected from one or more of tetrabutyl titanate, tetraethyl titanate, tetraisopropyl titanate, and acetylacetone titanium oxyacetate.

[0024] In this invention, the concentration of the ethanol solution of the organic titanium salt in step 2) is 1%-10% (mass concentration).

[0025] In this invention, the soluble yttrium salt in step 2) is selected from one or more of yttrium chloride, yttrium sulfate, yttrium nitrate, and yttrium acetate.

[0026] In this invention, the concentration of the aqueous solution of the soluble yttrium salt in step 2) is 1%-10% (mass concentration).

[0027] In this invention, the YTO precursor generated in step 2) accounts for 0.005%-1% of the total weight of lithium iron phosphate (LiFePO4), for example, including but not limited to proportions of 0.005%, 0.01%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, and 1%. The molar amount of ammonia is 2-10% of the molar amount of the organic titanium salt.

[0028] In this invention, the molar ratio of the organic titanium salt to the soluble yttrium salt in step 2) is 1:2.

[0029] In this invention, the calcination temperature in step 3) is 500-850℃, and the calcination time is 5-10h. Sintering temperatures include, but are not limited to, 500℃, 600℃, 700℃, 800℃, and 850℃. Calcination times include, but are not limited to, 5h, 6h, 7h, 8h, 9h, and 10h.

[0030] The sintering atmosphere in step 3) is an air or oxygen atmosphere, such as, but not limited to, an air atmosphere, 30% oxygen, 40% oxygen, 50% oxygen, 60% oxygen, 70% oxygen, 80% oxygen, 90% oxygen, and 100% oxygen.

[0031] In this invention, the inert atmosphere in step 4) is selected from nitrogen or argon.

[0032] In this invention, the carbon source in step 4) is selected from one or more of methanol, ethanol, acetone, acetylene, and ethane. The temperature for vapor deposition of carbon coating is 650℃-850℃, including but not limited to 650℃, 700℃, 750℃, 800℃, and 850℃. The holding time is 5-15 hours, including but not limited to 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, and 15 hours. The thickness of the carbon coating layer is 1 nm-5 nm.

[0033] Unless otherwise specified in this invention, all processes are conventional methods in the field of lithium iron phosphate cathode material preparation. Those skilled in the art will understand that conventional methods can be selected according to process requirements.

[0034] On the other hand, the present invention provides a low-temperature rate-capacity lithium iron phosphate cathode material prepared by the above method.

[0035] The beneficial effects of this invention are as follows:

[0036] (1) The surface of LFP was modified with an active ceramic material YTO to reduce the diffusion barrier of Li+ and accelerate the diffusion rate of Li+, thereby significantly enhancing the low-temperature electrochemical performance and rate performance of LFP.

[0037] (2) In order to achieve a uniform coating layer, the following method was adopted: first synthesize the lithium iron phosphate core; then use the atomic-level mixed hydroxide method to coat the lithium iron phosphate core; then sinter at high temperature under oxygen-rich conditions to coat the nano-YTO layer; then reduce the core and simultaneously coat it with carbon to achieve a perfect core-shell structure material.

[0038] (3) The lithium iron phosphate prepared by this invention has excellent low-temperature performance. The specific capacity of the first discharge at 1C can reach 155mAh / g, and the specific capacity of the discharge at 5C can reach 143mAh / g. The 3Ah soft-pack cell can reach 67% of the discharge capacity at room temperature under the condition of -30℃ at 1C rate. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the synthesis process of the present invention.

[0040] Figure 2 TEM image of YTO-coated lithium iron phosphate precursor c prepared in Example 1 of this invention.

[0041] Figure 3 SEM image of lithium iron phosphate prepared in Example 1 of this invention.

[0042] Figure 4 The rate discharge curve of lithium iron phosphate prepared in Example 1 of this invention.

[0043] Figure 5 The -30℃ discharge curves of lithium iron phosphate prepared in Example 1 of this invention and Comparative Example 1 Detailed Implementation

[0044] To better understand the technical solution of the present invention, the following embodiments will further illustrate the method provided by the present invention. However, the present invention is not limited to the listed embodiments, but should also include any other known modifications within the scope of the claims of the present invention.

[0045] The present invention will be further explained and illustrated below through more specific embodiments, but these do not constitute any limitation.

[0046] The main raw materials for the following embodiments are sourced from the following sources:

[0047] Ferric phosphate: Yuntianhua;

[0048] Ferrous oxalate: Hefei Yalong;

[0049] Lithium carbonate: Tianqi Lithium, etc.;

[0050] Monoammonium phosphate: Guizhou Phosphorus Group;

[0051] Phosphoric acid: Shandong Zhenhua;

[0052] Glucose: Qinhuangdao Lihua;

[0053] PEG: Wanhua Chemical.

[0054] The performance characterization methods for the lithium iron phosphate cathode materials prepared in the examples and comparative examples are as follows:

[0055] SEM and TEM were performed using JEOL equipment from Japan.

[0056] The method for testing the coin cell rate specific capacity is as follows: Mix the above materials in a mass ratio of lithium iron phosphate:PVDF:conductive carbon black = 8:1:1 to form a slurry. Coat this slurry onto aluminum foil to form the positive electrode, and the negative electrode is a lithium sheet, thus assembling a coin cell. Set the charge / discharge current to 1C = 170mAh / g, and the charge / discharge cutoff voltage to 3.75V-2.0V.

[0057] The method for low-temperature testing of all-electric cells is as follows: Prepare a 3A soft-pack battery cell, fully charge it at 0.2C, place it in a low-temperature chamber, set the temperature to -30℃, leave it for 24 hours, and then discharge it to 2.0V at 1C.

[0058] Example 1

[0059] 1) Lithium carbonate, ferrous oxalate, and ammonium hydrogen phosphate were mixed with sucrose at a Li:Fe:P molar ratio of 1.01:1:1.005. The amount of sucrose added was 10% of the mass of the synthesized lithium iron phosphate. The dopant was titanium dioxide, and the amount of titanium dioxide added was 0.5% of the mass of the synthesized lithium iron phosphate. The mixture was milled to 100 nm and spray-dried. The inlet air temperature was 250℃ and the outlet air temperature was 90℃. The lithium iron phosphate was sintered at 550℃ for 5 h under a nitrogen atmosphere.

[0060] 2) Lithium iron phosphate a was placed in an ethanol solution of tetrabutyl titanate and mixed thoroughly. Then, an aqueous solution of yttrium nitrate was added at a molar ratio of yttrium to titanium of 2:1. The mixture was stirred under heating and ammonia was added. The amount of ammonia added was 2% of the mass of lithium iron phosphate a. This yielded a sol-gel b of lithium iron phosphate coated with yttrium hydroxide and titanium hydroxide, wherein the hydroxide accounted for 0.005% of the mass fraction of lithium iron phosphate a.

[0061] 3) Calcine b at 850°C for 5 hours in air to obtain YTO-coated LFP precursor c;

[0062] 4) The prepared c was subjected to methanol vapor under a nitrogen atmosphere and kept at 650°C for 5 hours to form a carbon coating with a coating thickness of 2 nm. Simultaneously, the precursor in step 3) was reduced to lithium iron phosphate to obtain a YTO and carbon composite coated LFP product.

[0063] The lithium iron phosphate prepared in this embodiment has a first discharge specific capacity of 155 mAh / g at 1C and a discharge specific capacity of 143 mAh / g at 5C. The 3Ah soft-pack cell achieves 67% of its room temperature discharge capacity at -30℃ under 1C rate conditions.

[0064] Example 2

[0065] 1) Lithium hydroxide, iron nitrate, and phosphoric acid were mixed with glucose at a Li:Fe:P molar ratio of 1.005:1:1.005. The amount of glucose added was 12% of the mass of the synthesized lithium iron phosphate. The dopant was vanadium pentoxide, and the amount of vanadium pentoxide added was 0.2% of the mass of the synthesized lithium iron phosphate. The mixture was milled to 150 nm and spray-dried. The inlet air temperature was 200℃ and the outlet air temperature was 80℃. The mixture was sintered under a nitrogen atmosphere at 600℃ for 7 h.

[0066] 2) Lithium iron phosphate a was placed in an ethanol solution of tetraethyl titanate and mixed thoroughly. Then, an aqueous solution of yttrium sulfate was added at a molar ratio of yttrium to titanium of 2:1. The mixture was stirred under heating and ammonia was added. The amount of ammonia added was 1% of the mass of lithium iron phosphate a. This yielded a sol-gel b of lithium iron phosphate coated with yttrium hydroxide and titanium hydroxide, wherein the hydroxide precursor accounted for 0.02% of the mass of lithium iron phosphate a.

[0067] 3) Calcine b at 750°C for 6 hours in a 40% oxygen atmosphere to obtain YTO-coated LFP precursor c;

[0068] 4) The prepared c was subjected to ethanol vapor under a nitrogen atmosphere and kept at 700°C for 8 hours to form a carbon coating with a coating thickness of 3 nm. The precursor in step 3) was simultaneously reduced to lithium iron phosphate to obtain a YTO and carbon composite coated LFP product.

[0069] The lithium iron phosphate prepared in this embodiment exhibits excellent low-temperature performance, with a first discharge specific capacity of 153 mAh / g at 1C and a discharge specific capacity of 142 mAh / g at 5C. The 3Ah soft-pack cell achieves 63% of its room-temperature discharge capacity at -30℃ with a 1C rate.

[0070] Example 3

[0071] 1) Lithium dihydrogen phosphate, ferric oxide, and phosphoric acid were mixed with a combined carbon source of glucose and PEG at a Li:Fe:P molar ratio of 1.005:1:1.005. The amount of the combined carbon source added was 13% of the mass of the synthesized lithium iron phosphate. The dopant was magnesium oxide, and the amount of vanadium pentoxide added was 0.2% of the mass of the synthesized lithium iron phosphate. The mixture was milled to 250 nm and spray-dried at an inlet air temperature of 280°C and an outlet air temperature of 120°C. The mixture was sintered under a nitrogen atmosphere at 600°C for 7 h.

[0072] 2) Lithium iron phosphate a was placed in an ethanol solution of tetraisopropyl titanate and mixed thoroughly. Then, an aqueous solution of yttrium chloride was added at a molar ratio of yttrium to titanium of 2:1. The mixture was stirred under heating and ammonia was added. The amount of ammonia added was 5% of the mass of lithium iron phosphate a. This yielded a sol-gel b of lithium iron phosphate coated with yttrium hydroxide and titanium hydroxide, wherein the hydroxide precursor accounted for 0.4% of the mass of lithium iron phosphate a.

[0073] 3) Calcine b at 650°C for 7 hours in a 60% oxygen atmosphere to obtain YTO-coated LFP precursor c;

[0074] 4) The prepared c was subjected to acetone vapor under a nitrogen atmosphere and kept at 750°C for 11 hours to form a carbon coating with a coating thickness of 3 nm. The precursor in step 3) was simultaneously reduced to lithium iron phosphate to obtain a YTO and carbon composite coated LFP product.

[0075] The lithium iron phosphate prepared in this embodiment exhibits excellent low-temperature performance, with a first discharge specific capacity of 152.5 mAh / g at 1C and a discharge specific capacity of 141.5 mAh / g at 5C. The 3Ah soft-pack cell achieves 65% of its room-temperature discharge capacity at -30℃ with a 1C rate.

[0076] Example 4

[0077] 1) Lithium acetate, iron phosphate, and lithium dihydrogen phosphate were mixed with a carbon source of PVP and PEG in a Li:Fe:P molar ratio of 1.02:1:1.04 and a mass ratio of 1:1. The amount of carbon source added was 20% of the mass of the synthesized lithium iron phosphate. The dopant was ammonium metavanadate, and the amount of ammonium metavanadate added was 2% of the mass of the synthesized lithium iron phosphate. The mixture was milled to 400 nm and spray-dried. The inlet air temperature was 280℃ and the outlet air temperature was 120℃. The mixture was sintered under a nitrogen atmosphere at 700℃ for 10 h.

[0078] 2) Lithium iron phosphate a was placed in an ethanol solution of acetylacetone titanium oxide and mixed thoroughly. Then, an aqueous solution of yttrium acetate was added at a molar ratio of yttrium to titanium of 2:1. The mixture was stirred under heating and ammonia was added. The amount of ammonia added was 7% of the mass of lithium iron phosphate a. This yielded a sol-gel b of lithium iron phosphate coated with yttrium hydroxide and titanium hydroxide, wherein the hydroxide precursor accounted for 0.8% of the mass of lithium iron phosphate a.

[0079] 3) Calcine b at 550°C for 8 hours in an 80% oxygen atmosphere to obtain YTO-coated LFP precursor c;

[0080] 4) The prepared c was subjected to acetylene under a nitrogen atmosphere and heated at 800°C for 13 hours to form a carbon coating with a coating thickness of 3 nm by vapor deposition. The precursor in step 3) was simultaneously reduced to lithium iron phosphate to obtain a YTO and carbon composite coated LFP product.

[0081] The lithium iron phosphate prepared in this embodiment exhibits excellent low-temperature performance, with a first discharge specific capacity of 151 mAh / g at 1C and a discharge specific capacity of 140 mAh / g at 5C. The 3Ah soft-pack cell achieves 61.5% of its room-temperature discharge capacity at -30℃ with a 1C rate.

[0082] Example 5

[0083] 1) Lithium carbonate, ferric chloride, and ammonium dihydrogen phosphate were mixed in a Li:Fe:P molar ratio of 1.05:1.05:

[0084] 1.03 with starch carbon source, the amount of carbon source added is 20% of the mass of synthesized lithium iron phosphate, the dopant is a combination of yttrium oxide and zirconium oxide in a mass ratio of 1:1, the amount of dopant added is 1.5% of the mass of synthesized lithium iron phosphate, sand milled to 500nm, spray dried, the inlet air temperature is 210℃, the outlet air temperature is 110℃, sintered under nitrogen atmosphere, sintered at 750℃ for 12h for lithium iron phosphate a;

[0085] 2) Lithium iron phosphate a was placed in an ethanol solution of tetrabutyl titanate and tetraethyl titanate and mixed thoroughly. Then, an aqueous solution of yttrium sulfate and yttrium nitrate was added at a molar ratio of yttrium to titanium of 2:1. The mixture was stirred under heating and ammonia was added. The amount of ammonia added was 10% of the mass of lithium iron phosphate a. This yielded a sol-gel b of lithium iron phosphate coated with yttrium hydroxide and titanium hydroxide, wherein the hydroxide precursor accounted for 1% of the mass fraction of lithium iron phosphate a.

[0086] 3) Calcine b at 500°C for 10 h in a 100% oxygen atmosphere to obtain YTO-coated LFP precursor c;

[0087] 4) The prepared c was subjected to ethane under a nitrogen atmosphere and heated at 850°C for 15 hours to form a carbon coating with a coating thickness of 5 nm. Simultaneously, the precursor in step 3) was reduced to lithium iron phosphate to obtain a YTO and carbon composite coated LFP product.

[0088] The lithium iron phosphate prepared in this embodiment exhibits excellent low-temperature performance, with a first discharge specific capacity of 153 mAh / g at 1C and a discharge specific capacity of 142.5 mAh / g at 5C. The 3Ah soft-pack cell achieves 65.8% of its room-temperature discharge capacity at -30℃ with a 1C rate.

[0089] Comparative Example 1:

[0090] Lithium carbonate, ferrous oxalate, and ammonium hydrogen phosphate were mixed with sucrose at a Li:Fe:P molar ratio of 1.01:1:1.005, with titanium dioxide as the dopant. The mixture was milled to 100 nm, spray-dried, and sintered under a nitrogen atmosphere. The lithium iron phosphate (LiFePO4) was sintered at 550 °C for 5 h without YTO coating.

[0091] The initial discharge capacity at 1C is only 125.3 mAh / g, and the discharge capacity at 5C is 102.5 mAh / g. The discharge capacity of the 3Ah soft-pack cell at 1C rate under -30℃ conditions is only 26.7% of the discharge capacity at room temperature.

[0092] Comparative Example 2:

[0093] The third step of air atmosphere sintering was not performed, and no YTO film coating was formed. The initial discharge specific capacity at 1C was only 141 mAh / g, and the discharge specific capacity at 5C was 129 mAh / g; the discharge capacity of the 3Ah soft-pack cell at 1C rate under -30℃ conditions was only 39.5% of the discharge capacity at room temperature.

[0094] Comparative Example 3:

[0095] All steps were the same as in Example 3, except that the molar ratio of yttrium to titanium in the coating layer was 1:1, and a perfect YTO film coating was not formed, resulting in poor performance. The initial discharge capacity at 1C was only 127.4 mAh / g, and the discharge capacity at 5C was 104.8 mAh / g. The discharge capacity of the 3Ah soft-pack cell at 1C rate under -30℃ conditions was only 27.5% of the discharge capacity at room temperature.

[0096] Comparative Example 4:

[0097] 1) Lithium acetate, iron phosphate, and lithium dihydrogen phosphate were mixed with a carbon source of PVP and PEG in a Li:Fe:P molar ratio of 1.02:1:1.04 and a mass ratio of 1:1. The amount of carbon source added was 20% of the mass of the synthesized lithium iron phosphate. The dopant was ammonium metavanadate, and the amount of ammonium metavanadate added was 2% of the mass of the synthesized lithium iron phosphate. The mixture was milled to 400 nm and spray-dried. The inlet air temperature was 280℃ and the outlet air temperature was 120℃. The mixture was sintered under a nitrogen atmosphere at 700℃ for 10 h.

[0098] 2) Lithium iron phosphate a was placed in an ethanol solution of acetylacetone titanium oxide and mixed thoroughly. Then, an aqueous solution of zirconium nitrate was added at a molar ratio of zirconium to titanium of 1:1. The mixture was stirred under heating and ammonia was added. The amount of ammonia added was 7% of the mass of lithium iron phosphate a. This yielded a sol-gel b of lithium iron phosphate coated with yttrium hydroxide and titanium hydroxide, wherein the hydroxide precursor accounted for 0.8% of the mass fraction of lithium iron phosphate a.

[0099] 3) Calcine b at 550°C for 8 hours in an 80% oxygen atmosphere to obtain YTO-coated LFP precursor c;

[0100] 4) The prepared c was subjected to acetylene under a nitrogen atmosphere and heated at 800°C for 13 hours to form a carbon coating with a coating thickness of 3 nm by vapor deposition. The precursor in step 3) was simultaneously reduced to lithium iron phosphate to obtain a YTO and carbon composite coated LFP product.

[0101] The lithium iron phosphate prepared in this embodiment has poor low-temperature performance, with a first discharge specific capacity of only 131 mAh / g at 1C and a discharge specific capacity of 110 mAh / g at 5C. The 3Ah soft-pack cell achieves 31.5% of its room temperature discharge capacity at -30℃ with a 1C rate.

[0102] The comprehensive performance of the lithium iron phosphate cathode materials prepared in Examples 1-5 and Comparative Examples 1-4 is compared, and the results are shown in the table below:

[0103]

[0104] As shown in the table above, the method provided by this invention can produce lithium iron phosphate materials with excellent low-temperature rate performance. Additionally, [the following is a separate section]. Figure 2 The image shows a TEM photograph of YTO-coated lithium iron phosphate precursor c. As can be seen from the image, the YTO coating is very uniform. Figure 3 A well-defined carbon coating is also clearly visible. (Attached) Figure 5 The examples in Example 1 and the comparative examples more clearly demonstrate the significant effects of this technical solution.

[0105] Although the present invention has been described in detail through the preferred embodiments described above, it should be understood that the above description should not be considered as a limitation of the present invention. Those skilled in the art will understand that modifications or adjustments can be made to the present invention based on the teachings of this specification. These modifications or adjustments should also be within the scope defined by the claims of the present invention.

Claims

1. A method for preparing a low-temperature rate-controlled lithium iron phosphate cathode material, the method comprising the following steps: 1) Lithium iron phosphate a is prepared by mixing lithium source, iron source, phosphorus source, dopant and carbon source and sintering under nitrogen atmosphere; 2) Lithium iron phosphate a was placed in an ethanol solution of organic titanium salt and mixed thoroughly. Then, an aqueous solution of soluble yttrium salt was added. The mixture was stirred under heating and ammonia was added to obtain a sol-gel b of lithium iron phosphate coated with yttrium hydroxide and titanium hydroxide. 3) Calcine b in air or oxygen atmosphere to obtain YTO-coated LFP precursor c; 4) The YTO-coated LFP precursor c prepared above is coated with carbon by vapor deposition under high temperature conditions in an inert atmosphere to obtain a YTO and carbon composite coated LFP product.

2. The preparation method according to claim 1, characterized in that, In step 1), the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, and lithium dihydrogen phosphate; and / or, in step 1), the iron source is selected from one or more of ferric phosphate, ferrous oxalate, ferric nitrate, ferric oxide, and ferric chloride; and / or, in step 1), the phosphorus source is selected from one or more of phosphoric acid, lithium dihydrogen phosphate, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate.

3. The preparation method according to claim 2, characterized in that, In step 1), the dopant is selected from one or more of titanium dioxide, magnesium oxide, magnesium hydroxide, niobium pentoxide, vanadium pentoxide, ammonium metavanadate, yttrium oxide, and zirconium oxide, and the amount of dopant added is 0.2-1% of the mass of lithium iron phosphate; and / or, in step 1), the carbon source is selected from one or more of glucose, sucrose, PEG, PVP, PVA, starch, and cellulose; and / or, in step 1), the elemental molar ratio of lithium source, iron source, and phosphorus source is (1.005-1.05):(1-1.05):(1.005-1.05).

4. The preparation method according to any one of claims 1-3, characterized in that, The sintering temperature in step 1) is 700-800℃ and the sintering time is 5-15h; and / or, after obtaining lithium iron phosphate a in step 1), a grinding step is also included, and the D50 particle size after grinding is 100-500nm.

5. The preparation method according to any one of claims 1-4, characterized in that, In step 2), the organic titanium salt is selected from one or more of tetrabutyl titanate, tetraethyl titanate, tetraisopropyl titanate, and acetylacetonate oxytitanium; and / or, the mass concentration of the ethanol solution of the organic titanium salt in step 2) is 1%-10%; and / or, the soluble yttrium salt in step 2) is selected from one or more of yttrium chloride, yttrium sulfate, yttrium nitrate, and yttrium acetate; and / or, the mass concentration of the aqueous solution of the soluble yttrium salt in step 2) is 1%-10%.

6. The preparation method according to any one of claims 1-5, characterized in that, In step 2), the YTO precursor generated accounts for 0.005%-1% of the total weight of lithium iron phosphate a; and / or, the molar amount of ammonia is 2-10% of the molar amount of organic titanium salt.

7. The preparation method according to any one of claims 1-6, characterized in that, Step 2) The molar ratio of the organic titanium salt to the soluble yttrium salt is 1:

2.

8. The preparation method according to any one of claims 1-7, characterized in that, In step 3), the calcination temperature is 500-850℃, the calcination time is 5-10h, and / or the sintering atmosphere is air or oxygen.

9. The preparation method according to any one of claims 1-8, characterized in that, In step 4), the carbon source is selected from one or more of methanol, ethanol, acetone, acetylene, and ethane. The temperature for carbon deposition is 650℃-850℃, the holding time is 5-15h, and the thickness of the carbon coating layer is 1nm-5nm.

10. The low-temperature rate-capacity lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 1-9.

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