Method for improving low-temperature performance of lithium iron phosphate-based positive electrode material

Abstract

The application discloses a method for improving low-temperature performance of a lithium iron phosphate-based positive electrode material and belongs to the technical field of lithium battery positive electrode materials. The method comprises the following steps: mixing, grinding and then low-temperature treating a lithium source, a phosphorus-iron source and a first carbon source, and then performing spray drying and secondary grinding to obtain a precursor; stirring and mixing the precursor and a second carbon source in ethanol, and evaporating ethanol; and performing high-temperature sintering on the obtained product, and the method is completed. The method reduces positive electrode polarization and lithium precipitation, enables the lithium iron phosphate-based positive electrode material to have good fluidity at low temperatures, and improves the low-temperature performance of the lithium iron phosphate battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery cathode material technology, and particularly relates to a method for improving the low-temperature performance of lithium iron phosphate-based cathode materials. Background Technology

[0002] With the explosive growth of electric vehicles and consumer electronics, lithium-ion battery technology continues to achieve breakthroughs. Currently, the mainstream cathode materials for lithium-ion batteries include lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), and ternary materials (LiNi). 1-y- x Co x Mn y Lithium iron phosphate (LiFePO4) and lithium iron phosphate (O2) are used in lithium-ion power batteries. Among them, LiFePO4 has advantages such as high energy density, ultra-long service life, stable operating voltage, good safety, abundant raw material reserves, and being green and pollution-free, and has good application prospects in the field of lithium-ion power batteries.

[0003] However, studies have found that at -30℃, with a charge-discharge rate of 0.2C, the cycle capacity of LiFePO4 only reaches about 69% of that at room temperature. Furthermore, at low temperatures, the cycle capacity of LiFePO4 exhibits severe decay and instability with increasing charge-discharge rate and cycle count, and the discharge voltage decreases with decreasing operating temperature, limiting its application in low-temperature environments. The deterioration of LiFePO4's electrochemical performance at low temperatures may be due to increased electrolyte viscosity and decreased ion diffusion rate; alternatively, it may be caused by poor low-temperature fluidity of the cathode material, cathode polarization, or lithium plating.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a method for improving the low-temperature performance of lithium iron phosphate-based cathode materials, reducing cathode polarization and lithium plating, enabling lithium iron phosphate cathode materials to have good fluidity at low temperatures, and improving the low-temperature performance of lithium iron phosphate batteries.

[0006] To achieve the above objectives, the present invention adopts the following specific technical solution:

[0007] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials includes the following steps:

[0008] S1: The lithium source, phosphorus iron source and first carbon source are mixed and ground, then treated at low temperature, spray dried, and ground a second time to obtain the precursor.

[0009] Preferably, the first carbon source includes one or more of polyethylene glycol, glucose, or sucrose.

[0010] Preferably, the lithium source includes Li2CO3 or LiOH; the phosphorus iron source includes FePO4.

[0011] Further, the molar ratio of the lithium source and the iron-phosphorus source, calculated as lithium to iron, is (1.02-1.08):1. Preferably, the molar ratio of the lithium source and the iron-phosphorus source, calculated as lithium to iron, is (1.04-1.05):1.

[0012] Furthermore, the mass of the first carbon source accounts for 0.5-1.5% of the sum of the masses of the lithium source and the iron phosphate source. If the mass percentage of the first carbon source is too low, it will not be very effective for subsequent low-temperature processing, and the grains will not be able to obtain the required orientation, resulting in poor flowability. If the mass percentage of the first carbon source is too high, the subsequent addition of the second carbon source will cause stacking after the composite lithium iron phosphate, thereby blocking the lithium-ion transport channels and reducing its rate performance.

[0013] Furthermore, the low-temperature treatment involves placing the mixed and ground product in a refrigeration device with temperatures of -80 to -60°C and -30 to -10°C at its two ends for orientation treatment. Freezing under a certain gradient after adding the first carbon source is beneficial for grain orientation at low temperatures, making it easier for lithium ions to transport and improving the fluidity of the cathode material itself; simultaneously, a memory effect is easily formed during the low-temperature process.

[0014] Preferably, the secondary grinding controls the particle size D50 of the material to be 2-10 μm.

[0015] Preferably, the spray drying temperature is 100-220℃.

[0016] S2: The precursor and the second carbon source are stirred and mixed in ethanol, and the ethanol is evaporated to dryness.

[0017] Preferably, the second carbon source comprises carbon nanotubes or graphene. The addition of a second carbon source provides both porosity and, with the help of the conductive properties of the coating agent itself, facilitates the transport of lithium ions.

[0018] Furthermore, the mass of the second carbon source accounts for 0.3-0.8% of the sum of the masses of the lithium source and the iron phosphorus source. If the mass percentage of the second carbon source is too high, it will cause a defect similar to that of the first carbon source if the mass percentage is too high; if its mass percentage is too low, it will lead to a decrease in conductivity.

[0019] S3: The product obtained in step S2 is sintered at high temperature to obtain the final product.

[0020] Furthermore, the high-temperature sintering is sintering at 650-750℃ for 10-18 hours.

[0021] Compared with existing technologies, this invention incorporates a first carbon source during the lithium iron phosphate preparation process for low-temperature orientation, which facilitates lithium-ion transport and improves the fluidity of the cathode material itself, reducing cathode polarization and lithium plating. Simultaneously, the subsequent addition of a second carbon source increases conductivity. The resulting lithium iron phosphate battery exhibits excellent low-temperature performance. Detailed Implementation

[0022] The embodiments of the present invention will be described in detail below with reference to specific examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0023] Example 1

[0024] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0025] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 2.29g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with a refrigerator and dry ice connected to both ends respectively. Control the temperature of the refrigerator to -30℃ and the dry ice to -79℃, cool and orient for 2h, and then spray dry at 200℃. After taking it out, grind it to D50=5μm to obtain the precursor.

[0026] (2) Place the precursor and 1.14g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0027] (3) The product obtained in step (2) is heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain the modified lithium iron phosphate-based cathode material.

[0028] Example 2

[0029] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0030] (1) Weigh 79.8g Li2CO3, 151g FePO4 and 2.31g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with a refrigerator and dry ice connected to both ends respectively. Control the temperature of the refrigerator to -30℃ and the dry ice to -79℃, cool and orient for 2h, and then spray dry at 200℃. After taking it out, grind it to D50=5μm to obtain the precursor.

[0031] (2) Place the precursor and 1.15g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0032] (3) The product obtained in step (2) is heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain the modified lithium iron phosphate-based cathode material.

[0033] Example 3

[0034] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0035] (1) Weigh 75.3g Li2CO3, 151g FePO4 and 2.26g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with a refrigerator and dry ice connected to both ends respectively. Control the temperature of the refrigerator to -30℃ and the dry ice to -79℃, cool and orient for 2h, and then spray dry at 200℃. After taking it out, grind it to D50=5μm to obtain the precursor.

[0036] (2) Place the precursor and 1.13g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0037] (3) The product obtained in step (2) is heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain the modified lithium iron phosphate-based cathode material.

[0038] Example 4

[0039] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0040] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 2.29g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with two refrigerators connected to both ends. Control the temperature of one refrigerator to -20℃ and the temperature of the other refrigerator to -60℃, cool and orient for 2h, and then spray dry at 200℃. After taking it out, grind it to D50=5μm to obtain the precursor.

[0041] (2) Place the precursor and 1.14g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0042] (3) The product obtained in step (2) is heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain the modified lithium iron phosphate-based cathode material.

[0043] Example 5

[0044] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0045] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 2.29g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with two refrigerators connected to both ends. Control the temperature of one refrigerator to -20℃ and the temperature of the other refrigerator to -60℃, cool and orient for 2h, and then spray dry at 180℃. After taking it out, grind it to D50=5μm to obtain the precursor.

[0046] (2) Place the precursor and 1.14g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0047] (3) The product obtained in step (2) is heated to 750°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 10 hours to obtain the modified lithium iron phosphate-based cathode material.

[0048] Example 6

[0049] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0050] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 2.29g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with two refrigerators connected to both ends. Control the temperature of one refrigerator to -20℃ and the temperature of the other refrigerator to -60℃, cool and orient for 2h, and then spray dry at 150℃. After taking it out, grind it to D50=5μm to obtain the precursor.

[0051] (2) Place the precursor and 1.14g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0052] (3) The product obtained in step (2) is heated to 650°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain the modified lithium iron phosphate-based cathode material.

[0053] Example 7

[0054] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0055] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 2.29g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with two refrigerators connected to both ends. Control the temperature of one refrigerator to -20℃ and the temperature of the other refrigerator to -60℃, cool and orient for 2h, and then spray dry at 200℃. After taking it out, grind it to D50=8μm to obtain the precursor.

[0056] (2) Place the precursor and 1.14g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0057] (3) The product obtained in step (2) is heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain the modified lithium iron phosphate-based cathode material.

[0058] Example 8

[0059] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0060] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 2.29g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with two refrigerators connected to both ends. Control the temperature of one refrigerator to -20℃ and the temperature of the other refrigerator to -60℃, cool and orient for 2h, and then spray dry at 200℃. After taking it out, grind it to D50=2μm to obtain the precursor.

[0061] (2) Place the precursor and 1.14g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0062] (3) The product obtained in step (2) is heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain the modified lithium iron phosphate-based cathode material.

[0063] Example 9

[0064] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0065] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 2.29g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with two refrigerators connected to both ends. Control the temperature of one refrigerator to -10℃ and the temperature of the other refrigerator to -70℃, cool and orient for 2h, and then spray dry at 200℃. After taking it out, grind it to D50=5μm to obtain the precursor.

[0066] (2) Place the precursor and 1.14g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0067] (3) The product obtained in step (2) is heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain the modified lithium iron phosphate-based cathode material.

[0068] Example 10

[0069] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0070] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 1.14g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with two refrigerators connected to both ends. Control the temperature of one refrigerator to -20℃ and the temperature of the other refrigerator to -60℃, cool and orient for 2h, and then spray dry at 200℃. After taking it out, grind it to D50=5μm to obtain the precursor.

[0071] (2) Place the precursor and 1.83g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0072] (3) The product obtained in step (2) is heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain the modified lithium iron phosphate-based cathode material.

[0073] Example 11

[0074] A method for improving the low-temperature performance of lithium iron phosphate-based cathode materials:

[0075] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 3.43g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, and then place them in a rectangular device with two refrigerators connected to both ends. Control the temperature of one refrigerator to -20℃ and the temperature of the other refrigerator to -60℃, cool and orient for 2h, and then spray dry at 200℃. After taking it out, grind it to D50=5μm to obtain the precursor.

[0076] (2) Place the precursor and 0.69g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0077] (3) The product obtained in step (2) is heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain the modified lithium iron phosphate-based cathode material.

[0078] Comparative Example 1

[0079] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 2.29g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, then spray dry at 200℃, take out and grind to D50=5μm to obtain the precursor.

[0080] (2) Place the precursor and 1.14g of carbon nanotubes in ethanol, stir and mix, and then evaporate the ethanol at 80°C.

[0081] (3) The product obtained in step (2) is heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain lithium iron phosphate-based cathode material.

[0082] Comparative Example 2

[0083] (1) Weigh 77.6g Li2CO3, 151g FePO4 and 2.29g polyethylene glycol and dissolve them in deionized water, control the solid content to 30%, mix and grind, then spray dry at 200℃, take out and grind to D50=5μm to obtain the precursor.

[0084] (2) The precursor was heated to 700°C at a rate of 4°C / s under a nitrogen atmosphere and sintered for 12 hours to obtain lithium iron phosphate-based cathode material.

[0085] Table 1. Comparison of key parameters and performance of lithium iron phosphate cathode materials obtained in Examples 1-11 and Comparative Examples 1-2

[0086]

[0087] As shown in Table 1, the low-temperature performance of lithium iron phosphate materials is significantly improved by adding the first carbon source and performing cooling orientation compared to those without cooling orientation. The low-temperature performance is further enhanced by adding the second carbon source. Based on the addition of both the first and second carbon sources and the cooling orientation, the optimal results are achieved when the temperatures at the two cooling ends are -60℃ and -20℃, respectively.

[0088] In addition, the lithium-iron ratio, precursor particle size, spray drying temperature, and sintering temperature may also have some influence.

[0089] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

[0090] Furthermore, those skilled in the art will understand that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are meant to be within the scope of the invention and form different embodiments. For example, in the foregoing claims, any of the claimed embodiments can be used in any combination. The information disclosed in this background section is intended only to enhance the understanding of the general background of the invention and should not be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

Claims

1. A method for improving the low temperature performance of a lithium iron phosphate-based positive electrode material, characterized in that, The method comprises the following steps: S1: mixing and grinding a lithium source, a phosphorus-iron source and a first carbon source, then low-temperature treatment, spray drying and secondary grinding to obtain a precursor; the low-temperature treatment is to place the product after the mixing and grinding in a refrigeration device with the temperature of-80~-60℃ and-30~-10℃ at both ends for orientation treatment; the spray drying temperature is 180-200℃; the mass of the first carbon source accounts for 1.0-1.5% of the sum of the mass of the lithium source and the phosphorus-iron source; S2: stirring and mixing the precursor with a second carbon source in ethanol and evaporating ethanol; the second carbon source comprises carbon nanotubes or graphene; the mass of the second carbon source accounts for 0.3-0.8% of the sum of the mass of the lithium source and the phosphorus-iron source; S3: high-temperature sintering the product obtained in step S2 to obtain the product. The high-temperature sintering is sintering at 700-750℃ for 10-18h.

2. The method of claim 1, wherein, In step S1, the first carbon source comprises one or more of polyethylene glycol, glucose or sucrose.

3. The method of claim 1, wherein, In step S1, the lithium source comprises Li2CO3 or LiOH; the phosphorus-iron source comprises FePO4. Further, the molar ratio of lithium element to iron element in the lithium source and the phosphorus-iron source is (1.02-1.08):

1.

4. The method of claim 1, wherein, In step S1, the secondary grinding controls the particle size D50 of the material to be 2-10μm.

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