A wide-temperature-range high-multiplying lithium iron phosphate positive electrode material, a preparation method and a lithium ion battery

Abstract

The application discloses a wide-temperature-range high-multiplying lithium iron phosphate positive electrode material, a preparation method and a lithium ion battery. The preparation method comprises the following steps: S1, uniformly mixing an iron source, a phosphorus source, a lithium source, a first carbon source, a transition metal-containing dopant and a dispersing agent, and performing first grinding; S2, performing spray drying on the slurry after the first grinding to obtain a first precursor; S3, performing first sintering on the first precursor under the protection of an inert gas atmosphere; S4, mixing the material obtained after the first sintering, a second carbon source and a dispersing agent, and performing second grinding; S5, performing spray drying on the slurry obtained after the second grinding to obtain a second precursor; and S6, performing second sintering on the second precursor under the protection of an inert gas atmosphere to obtain the lithium iron phosphate positive electrode material. The positive electrode material provided by the application can effectively reduce the side reaction of lithium iron phosphate and electrolyte after being assembled into a battery, has low-temperature high-multiplying performance, and has excellent high-temperature cycle performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a wide-temperature-range, high-rate lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, as energy storage and conversion devices, have experienced rapid development in recent years. The most crucial components of lithium-ion batteries are the cathode and anode materials. Among cathode materials, lithium iron phosphate (LFP) has become the most popular due to its advantages such as structural stability, good safety, excellent cycle performance, and low cost. The applications of LFP materials are mainly divided into two categories: energy-type and power-type. However, due to the low ionic and electronic conductivity of LFP materials, their low-temperature performance and rate performance are poor, limiting their application in various fields.

[0003] To address the above issues and make lithium iron phosphate (LFP) suitable for low-temperature or high-power applications, common methods to improve its rate capability and low-temperature performance include nano-sizing and carbon coating. By reducing the primary particle size of LFP materials, the lithium-ion transport distance is shortened. Combined with carbon coating to improve electronic conductivity, the discharge capability of LFP materials at low temperatures can be enhanced.

[0004] The conventional production process for lithium iron phosphate (LFP) involves mixing lithium, phosphorus, iron, carbon, and doping elements, followed by grinding, spray drying, and sintering. By controlling the particle size during grinding and the sintering temperature, nano-sized particles are achieved in a single step, shortening the lithium-ion transport distance and thus improving the low-temperature discharge capability of LFP materials. However, LFP produced through conventional methods exhibits rapid high-temperature cycle degradation and poor high-temperature calendar life after being assembled into batteries, making it unsuitable for high-temperature environments. Summary of the Invention

[0005] There are two main reasons why lithium-ion batteries assembled from lithium iron phosphate cathode materials obtained through conventional production processes are not suitable for high-temperature environments: ① Under conventional process conditions, when using a carbon source to coat lithium iron phosphate materials, the smaller the primary particles of lithium iron phosphate materials, the worse the uniformity of carbon coating. This leads to more side reactions when hydrofluoric acid produced by electrolyte decomposition comes into contact with the surface of the unevenly coated nano-lithium iron phosphate materials during long-term cycling in high-temperature environments, resulting in a decrease in the high-temperature cycling performance of the nano-lithium iron phosphate materials.

[0006] ② To obtain nano-sized lithium iron phosphate materials with good conductivity, current technologies use high carbon source content during lithium iron phosphate synthesis. Under high-temperature sintering conditions, this high carbon content inevitably leads to the formation of impurity phases such as iron phosphide and lithium phosphate when the carbon source forms a coating layer on the lithium iron phosphate surface due to its strong reducing properties. These impurities exhibit intensified side reactions with the electrolyte at high temperatures, resulting in reduced battery high-temperature cycling and calendar life. The specific reaction formula is as follows:

[0007] 6LiFePO4+8C→2Fe2P+2Li3PO4+2FeP+8CO2→3Fe2P+2Li3PO4+P↑.

[0008] The purpose of this invention is to provide a high-rate lithium iron phosphate cathode material that can be used in a wide temperature range; another purpose of this invention is to provide a method for preparing a high-rate lithium iron phosphate cathode material in a wide temperature range.

[0009] This invention discloses a method for preparing a wide-temperature-range, high-rate lithium iron phosphate cathode material, comprising the following steps:

[0010] S1: Mix the iron source, phosphorus source, lithium source, first carbon source, dopant containing transition metal and dispersant evenly, and then grind the evenly mixed slurry for the first time.

[0011] S2: Spray dry the slurry after the first grinding to obtain the first precursor;

[0012] S3: The first precursor is sintered for the first time under the protection of an inert gas atmosphere, and sintering is continued at 300-500℃ for 1-10 hours.

[0013] S4: Mix the material obtained after the first sintering, the second carbon source and the dispersant, and then perform a second grinding;

[0014] S5: Spray dry the slurry obtained from the second grinding to obtain the second precursor;

[0015] S6: The second precursor is sintered for the second time under the protection of an inert gas atmosphere, and sintered continuously at 600-800℃ for 1-10h to obtain a wide-temperature-range high-rate lithium iron phosphate cathode material.

[0016] Furthermore, the iron source is selected from one or more of anhydrous ferric phosphate, ferric phosphate dihydrate, and iron oxide.

[0017] Furthermore, the lithium source is selected from one or more of lithium carbonate, lithium dihydrogen phosphate, lithium hydroxide, lithium oxalate, and lithium phosphate.

[0018] The dispersant is deionized water. The heating rate for the first and second sintering is 1-10℃ / min.

[0019] Furthermore, in step S1, the added lithium source and iron source have a molar ratio of lithium to iron of 1.0-1.04:1.

[0020] Furthermore, the transition metal dopant is any one or a combination of two of the compounds containing Ti, V, Mg, Cr, Ni, and Mn; the content of each dopant element in the final lithium iron phosphate is less than ≤5000ppm.

[0021] The doping of transition metal elements gives lithium iron phosphate cathode materials a high discharge capacity at low temperatures.

[0022] Furthermore, in step S1, the first grinding is performed until the slurry D50 is 200-400nm.

[0023] Furthermore, in step S2, the carbon content in the first precursor powder is 1-3 wt%.

[0024] Furthermore, in step S3, the carbon content in the lithium iron phosphate after the first sintering is 0.5-2 wt%; the BET specific surface area is 8-20 m². 2 / g.

[0025] Furthermore, in step S4, the second grinding is performed until the slurry D50 is 100-300nm.

[0026] Furthermore, in step S5, the carbon content in the second precursor powder is 3-6 wt%.

[0027] By coating the precursor twice with two different carbon sources and performing two slurry grinding processes, the obtained nanoscale lithium iron phosphate particles are fully coated with carbon layers, forming a core-shell structure. This results in fewer side reactions between the lithium iron phosphate material and the electrolyte at high temperatures. Furthermore, the combination of a single low-temperature pre-sintering and a second high-temperature sintering avoids the drawbacks of conventional single-stage high-temperature sintering for lithium iron phosphate preparation. The high carbon content leads to the formation of numerous impurities such as iron phosphide and lithium phosphate during the high-temperature coating process due to the strong reducing properties of the carbon source. This significantly improves the material's high-temperature calendar life and cycle life.

[0028] Furthermore, the mass of the first carbon source added is greater than the mass of the second carbon source; the first carbon source and the second carbon source are each independently selected from one or more of glucose, sucrose, starch, polyethylene glycol, vapor-grown carbon fibers, cellulose, acetylene black, graphite, carbon nanotubes, and citric acid.

[0029] This invention also provides a wide-temperature-range, high-rate lithium iron phosphate cathode material, prepared by the method described above, wherein the carbon content in the lithium iron phosphate cathode material is 1.5-4.0 wt%; and the BET specific surface area is 10-25 m². 2 / g; the size of primary particles is 50-300nm.

[0030] A lithium-ion battery includes a negative electrode, a separator, and a positive electrode; the positive electrode includes a wide-temperature-range, high-rate lithium iron phosphate positive electrode material as described above.

[0031] This invention discloses a method for preparing a wide-temperature-range, high-rate lithium iron phosphate cathode material. It employs a two-stage sintering process and a secondary carbon source addition to avoid excessively high temperatures and carbon content during the first sintering stage, which would lead to the formation of large quantities of impurity particles such as iron phosphide and lithium phosphate. The secondary carbon coating ensures more uniform carbon coverage. When assembled into a lithium-ion battery, this method effectively reduces side reactions between nano-lithium iron phosphate and the electrolyte, resulting in not only high-rate performance at low temperatures but also excellent high-temperature cycling performance. Attached Figure Description

[0032] Figure 1 This is a SEM image of the wide-temperature-range, high-rate lithium iron phosphate material provided in Example 1 of this invention;

[0033] Figure 2 These are the low-temperature 1C discharge capacity results of Examples 1-3 and Comparative Examples 1-3 in this invention;

[0034] Figure 3 These are the discharge capacity results at -20℃ and 10C for Examples 1-3 and Comparative Examples 1-3 of this invention;

[0035] Figure 4 These are the high-temperature 45°C 1C / 1C cycle results of Examples 1-3 and Comparative Examples 1-3 of this invention.

[0036] Figure 5 These are the results of high-temperature (60°C) storage in Examples 1-3 and Comparative Examples 1-3 of this invention. Detailed Implementation

[0037] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] Example 1

[0039] 50g of anhydrous iron phosphate, 12.8g of lithium carbonate, 5.4g of glucose, and 0.4g of ammonium metavanadate were dispersed in 180g of deionized water and then milled in a sand mill for 2 hours to control the particle size D50 of the slurry to 300-400nm. The milled slurry was then spray-dried to obtain the first precursor, which was found to have a carbon content of 2.8wt% by a carbon-sulfur analyzer. The first precursor was then placed in a graphite crucible and sintered for the first time under a nitrogen atmosphere. The temperature was increased to 400℃ at a rate of 5℃ / min and held for 4 hours to obtain the first-sintered lithium iron phosphate material with a specific surface area of ​​10.3m². 2 / g. The lithium iron phosphate material obtained after the first sintering was added to 150g of deionized water, and then 0.8g of PEG2000 was added as a second carbon source. The mixture was then milled for 2.5h using a sand mill to control the particle size D50 of the slurry to 200-300nm. The milled slurry was then transferred to a spray dryer to obtain the second precursor. Carbon and sulfur analysis showed that the carbon content of the second precursor was 3.5wt%. The second precursor was then placed back into a graphite crucible and sintered for the second time under a nitrogen atmosphere. The temperature was increased to 750℃ at a rate of 10℃ / min and held for 6h to obtain a wide-temperature-range high-rate lithium iron phosphate cathode material.

[0040] The wide-temperature-range, high-magnification lithium iron phosphate cathode material prepared in Example 1 was subjected to scanning electron microscopy (SEM) testing, such as... Figure 1 As shown, its primary particles can be as small as 50 nm.

[0041] Example 2

[0042] 62g of ferric phosphate dihydrate, 12.8g of lithium carbonate, 5.1g of sucrose, and 0.35g of ammonium metavanadate were dispersed in 180g of deionized water and then milled in a sand mill for 2 hours to control the particle size D50 of the slurry to 250-300nm. The milled slurry was then dried in a spray dryer to obtain the first precursor. The carbon content of the first precursor was measured to be 2.75wt% using a carbon-sulfur analyzer. The first precursor was placed in a graphite crucible and sintered for the first time under a nitrogen atmosphere. The temperature was increased to 400℃ at a rate of 5℃ / min and then held for 4 hours to obtain the first-sintered lithium iron phosphate material with a specific surface area of ​​14.6m². 2 / g. The lithium iron phosphate material obtained after the first sintering was added to 150g of deionized water, and then 2.5g of sucrose was added as a second carbon source. The mixture was further milled for 2.5h to control the particle size D50 of the slurry at 150-200nm. The milled slurry was transferred to a spray dryer to obtain the second precursor. The carbon content of the second precursor was determined to be 3.65wt% by carbon-sulfur analysis. The second precursor was reloaded into a graphite crucible and sintered a second time under a nitrogen atmosphere. The temperature was increased to 720℃ at a rate of 10℃ / min and held for 6h to obtain a wide-temperature-range high-rate lithium iron phosphate cathode material.

[0043] Example 3

[0044] 50g of anhydrous iron phosphate, 24g of lithium hydroxide, 6.0g of citric acid, 0.35g of ammonium metavanadate, and 0.2g of titanium dioxide were dispersed in 200g of deionized water and then milled in a sand mill for 2 hours to control the particle size D50 of the slurry to 300-350nm. The milled slurry was then dried in a spray dryer to obtain the first precursor. The carbon content of the first precursor was measured to be 3.0wt% using a carbon-sulfur analyzer. The first precursor was placed in a graphite crucible and sintered for the first time under a nitrogen atmosphere. The temperature was increased to 450℃ at a rate of 5℃ / min and then held for 6 hours to obtain the first-sintered lithium iron phosphate material with a specific surface area of ​​13.4m². 2 / g. The lithium iron phosphate material after the first sintering was added to 170g of deionized water, and 1.4g of carbon nanotubes were added as a second carbon source. The mixture was further milled for 2.0h to control the particle size D50 of the slurry to 150-200nm. The milled slurry was transferred to a spray dryer to obtain the second precursor. Carbon-sulfur analysis showed that the carbon content of the second precursor was 4.3%. The second precursor was put back into a graphite crucible and sintered a second time under a nitrogen atmosphere. The temperature was increased to 760℃ at a rate of 10℃ / min and held for 8h to obtain a wide-temperature-range high-rate lithium iron phosphate cathode material.

[0045] Comparative Example 1

[0046] The comparative example used the same amount of raw materials as Example 1.

[0047] 50g of anhydrous iron phosphate, 12.8g of lithium carbonate, 5.4g of glucose, 0.8g of PEG2000, and 0.4g of ammonium metavanadate were dispersed in 180g of deionized water and then milled in a sand mill for 8 hours to control the particle size D50 of the slurry to 200-300nm. The milled slurry was then dried in a spray dryer to obtain a precursor. The carbon content in the precursor was measured to be 3.6wt% using a carbon-sulfur analyzer. The precursor was placed in a graphite crucible and sintered under a nitrogen atmosphere, with the temperature increased to 750℃ at a rate of 10℃ / min and held for 6 hours to obtain the lithium iron phosphate cathode material.

[0048] Comparative Example 2

[0049] Comparative Example 2 and Example 2 used the same amount of raw materials.

[0050] 62g of iron phosphate dihydrate, 12.8g of lithium carbonate, 7.3g of sucrose, and 0.35g of ammonium metavanadate were dispersed in 180g of deionized water and then milled in a sand mill for 8 hours to control the particle size D50 of the slurry to 150-200nm. The milled slurry was then dried in a spray dryer to obtain a precursor. The carbon content in the precursor was measured to be 3.65wt% using a carbon-sulfur analyzer. The precursor was then placed in a graphite crucible and sintered under a nitrogen atmosphere, with the temperature increased to 720℃ at a rate of 10℃ / min and held for 6 hours to obtain the lithium iron phosphate cathode material.

[0051] Comparative Example 3

[0052] Comparative Example 3 and Example 3 used the same amount of raw materials.

[0053] 50g of anhydrous iron phosphate, 24g of lithium hydroxide, 6.0g of citric acid, 1.4g of carbon nanotubes, 0.35g of ammonium metavanadate, and 0.2g of titanium dioxide were dispersed in 200g of deionized water and then milled in a sand mill for 7 hours to control the particle size D50 of the slurry to 150-200nm. The milled slurry was then transferred to a spray dryer for granulation to obtain a precursor. The carbon content in the precursor was measured to be 4.4wt% using a carbon-sulfur analyzer. The precursor was then placed in a graphite crucible and sintered under a nitrogen atmosphere, with the temperature increased to 760℃ at a rate of 10℃ / min and held for 8 hours to obtain the lithium iron phosphate cathode material.

[0054] Performance testing and analysis

[0055] The lithium iron phosphate cathode materials obtained in the above embodiments and comparative examples were characterized by powder physicochemical testing and coin cell electrical performance testing. The results are shown in the table below.

[0056] Table 1 Performance test results of Examples 1-3 and Comparative Examples 1-3

[0057]

[0058]

[0059] As shown in Table 1, the rate performance of the lithium iron phosphate materials prepared in Examples 1-3 is better than that of the corresponding comparative examples, and they contain fewer magnetic metal particles, which is more beneficial to high-temperature cycling and storage performance.

[0060] The lithium iron phosphate materials prepared in Examples 1-3 and Comparative Examples 1-3 were used as the main cathode materials for full-cell assembly and evaluation. The process is as follows:

[0061] 1) Positive electrode sheet preparation: Appropriate amounts of positive electrode material, conductive agent, and binder in a mass ratio of 95:2:3 are added to N-methylpyrrolidone solvent and dispersed at high speed to form a homogenized slurry. The solid content of the slurry is adjusted to 55%. The positive electrode material is lithium iron phosphate as described in the above examples or comparative examples, the conductive agent is carbon black (ECP), and the binder is polyvinylidene fluoride (PVDF). The dispersed slurry is coated onto aluminum foil using a coating machine, and after rolling and drying, a positive electrode sheet is obtained.

[0062] 2) Anode Sheet Fabrication: A suitable amount of anode material, conductive agent, and binder in a mass ratio of 93:3:4 are added to N-methylpyrrolidone solvent and dispersed at high speed to form a homogenized slurry. The solid content of the slurry is adjusted to 40%. The anode material is conventional artificial graphite, the conductive agent is carbon black, and the binder is styrene-butadiene rubber latex (SBR). The dispersed slurry is coated onto copper foil using a coating machine, and after rolling and drying, the anode sheet is obtained.

[0063] 3) Assemble the positive and negative electrode sheets with the polyethylene separator and inject electrolyte to form a soft-pack battery with a rated capacity of 4Ah.

[0064] 4) Perform 1C charge / discharge tests at -20℃ and -30℃, 1C charge / discharge tests at -30℃, 1C cycling tests at 45℃, and storage tests at 60℃ on the battery.

[0065] The test results of the various embodiments and comparative examples of battery cells are shown in Table 2 below.

[0066] Table 2. Cell test results of Examples 1-3 and Comparative Examples 1-3

[0067]

[0068] like Figure 2-3 As shown in Figure 1 and Table 2, the discharge capacity of each embodiment is superior to that of the comparative example at low temperatures of -20°C and -30°C; and the capacity retention rate is also superior to that of the comparative example under high-rate 10C discharge at -20°C. This demonstrates that the lithium iron phosphate prepared by this technical solution has low-temperature high-rate discharge capability.

[0069] Table 3. High-temperature cycling performance results of Examples 1-3 and Comparative Examples 1-3

[0070]

[0071] As shown in Table 3 and Figure 4-5 As shown, each embodiment outperforms the control group in terms of capacity retention during high-temperature cycling and high-temperature storage. This demonstrates that the lithium iron phosphate prepared using this technical solution possesses excellent high-temperature performance.

[0072] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing a wide-temperature-range, high-rate lithium iron phosphate cathode material, characterized in that, Includes the following steps: S1: Mix the iron source, phosphorus source, lithium source, first carbon source, dopant containing transition metal and dispersant evenly, and then grind the evenly mixed slurry for the first time. S2: Spray dry the slurry after the first grinding to obtain the first precursor; S3: The first precursor is sintered for the first time under the protection of an inert gas atmosphere, and sintering is continued at 300-500℃ for 1-10 hours. S4: Mix the material obtained after the first sintering, the second carbon source and the dispersant, and then perform a second grinding; S5: Spray dry the slurry obtained from the second grinding to obtain the second precursor; S6: The second precursor is sintered for the second time under the protection of an inert gas atmosphere, and sintered continuously at 600-800℃ for 1-10h to obtain a wide temperature range high rate lithium iron phosphate cathode material. In step S2, the carbon content in the first precursor powder is 1-3 wt%. In step S5, the carbon content in the second precursor powder is 3-6 wt%. In step S1, the added lithium source and iron source have a molar ratio of lithium to iron of 1.0-1.04:1; the transition metal dopant is any one or a combination of two of the compounds containing Ti, V, Mg, Cr, Ni, and Mn; and the content of each dopant element in the final lithium iron phosphate is less than ≤5000ppm.

2. The method for preparing a wide-temperature-range, high-rate lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1, the first grinding is performed until the slurry D50 is 200-400 nm.

3. The method for preparing a wide-temperature-range, high-rate lithium iron phosphate cathode material according to claim 1, characterized in that, In step S3, the carbon content in the lithium iron phosphate after the first sintering is 0.5-2 wt%; the BET specific surface area is 8-20 m². 2 / g.

4. The method for preparing a wide-temperature-range, high-rate lithium iron phosphate cathode material according to claim 1, characterized in that, In step S4, the second grinding is performed until the slurry D50 is 100-300 nm.

5. The method for preparing a wide-temperature-range, high-rate lithium iron phosphate cathode material according to claim 1, characterized in that, The mass of the first carbon source added is greater than the mass of the second carbon source; the first carbon source and the second carbon source are each independently selected from one or more of glucose, sucrose, starch, polyethylene glycol, vapor-grown carbon fibers, cellulose, acetylene black, graphite, carbon nanotubes, and citric acid.

6. A wide-temperature-range, high-rate lithium iron phosphate cathode material, characterized in that, Obtained by the preparation method according to any one of claims 1-5, the material contains 1.5-4.0 wt% carbon and has a BET specific surface area of ​​10-25 m². 2 / g; the size of primary particles is 50-300 nm.

7. A lithium-ion battery, characterized in that, It includes a negative electrode, a separator, and a positive electrode; the positive electrode includes the wide-temperature-range high-rate lithium iron phosphate positive electrode material as described in claim 6.

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