Wide-temperature-range high-rate lithium iron phosphate positive electrode material, preparation method and lithium ion battery

Abstract

The invention discloses a wide-temperature-range high-rate lithium iron phosphate positive electrode material, a preparation method and a lithium ion battery, and 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 dispersant, and carrying out first grinding; s2, performing spray drying on the slurry subjected to primary grinding to obtain a first precursor; s3, sintering the first precursor for the first time under the protection of an inert gas atmosphere; s4, mixing a material obtained after primary sintering, a second carbon source and a dispersing agent, and then performing secondary grinding; s5, performing spray drying on the slurry obtained by secondary grinding to obtain a second precursor; and S6, sintering the second precursor for the second time under the protection of the inert gas atmosphere to obtain the lithium iron phosphate positive electrode material. After the positive electrode material provided by the invention is assembled into a battery, the side reaction of lithium iron phosphate and electrolyte can be effectively reduced, and the positive electrode material not only has low-temperature high-rate performance, but also has excellent high-temperature cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a wide-temperature range high-rate lithium iron phosphate positive electrode material, a preparation method and a lithium ion battery. Background Art

[0002] As a device for storing and converting energy, lithium-ion batteries have achieved rapid development in recent years. The most critical parts of lithium-ion batteries are positive electrode materials and negative electrode materials. Among positive electrode materials, lithium iron phosphate materials have become the most popular positive electrode materials due to their stable structure, good safety, excellent cycle performance, and low price. The application of lithium iron phosphate materials is mainly divided into two categories: energy type and power type. Due to the low ion and electronic conductivity of lithium iron phosphate materials themselves, their low temperature performance and rate performance are poor, which limits the application of lithium iron phosphate batteries in different fields.

[0003] In order to make lithium iron phosphate suitable for use in low temperature or high power scenarios, the current common method to improve the rate and low temperature performance of lithium iron phosphate is to nano-size and carbon-coat the material. By reducing the primary particles of lithium iron phosphate materials, shortening the lithium ion transmission distance, and combining carbon coating to improve electronic conductivity, the discharge capacity of lithium iron phosphate materials at low temperatures can be improved.

[0004] The conventional production process of lithium iron phosphate is: after mixing lithium source, phosphorus source, iron source, carbon source, doping elements, etc., grinding-spray drying-sintering is carried out. By controlling the particle size during grinding and the sintering temperature, the primary particles are nano-sized, shortening the lithium ion transmission distance, thereby improving the low-temperature discharge capacity of lithium iron phosphate materials. However, after the conventionally produced lithium iron phosphate is assembled into a battery, it decays quickly during high-temperature cycles and has a poor high-temperature calendar life, and is not suitable for high-temperature environments. Summary of the invention

[0005] There are two main reasons why lithium-ion batteries assembled from lithium iron phosphate positive electrode materials obtained through conventional production processes are not suitable for high-temperature environments: ① Under conventional process conditions, when a carbon source is used to coat the lithium iron phosphate material, the smaller the primary particles of the lithium iron phosphate material, the worse the uniformity of the carbon coating. This will result in more side reactions when the hydrofluoric acid produced by the decomposition of the electrolyte contacts the surface of the nano-lithium iron phosphate material with an uneven carbon coating during long-term cycling in a high-temperature battery environment, resulting in a decrease in the high-temperature cycling performance of the nano-lithium iron phosphate material.

[0006] ② In order to obtain nano-lithium iron phosphate materials with good conductivity, the existing technology uses a high amount of carbon source when synthesizing lithium iron phosphate. Under high-temperature sintering conditions, the high carbon content will inevitably produce impurity phases such as iron phosphide and lithium phosphate when the carbon source forms a coating layer on the surface of lithium iron phosphate due to its strong reducibility. The side reactions of such impurities with the electrolyte intensify under high-temperature environments, resulting in the battery's high-temperature cycle and calendar life attenuation. The specific reaction formula is:

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

[0008] An object of the present invention is to provide a high-rate lithium iron phosphate positive electrode material that can be used under a wide temperature range; another object of the present invention is to provide a method for preparing a high-rate lithium iron phosphate positive electrode material that can be used under a wide temperature range.

[0009] The present invention discloses a method for preparing a wide temperature range high rate lithium iron phosphate positive electrode material, comprising the following steps:

[0010] S1: uniformly mixing an iron source, a phosphorus source, a lithium source, a first carbon source, a dopant containing a transition metal and a dispersant, and performing a first grinding on the uniformly mixed slurry;

[0011] S2: spray drying the slurry after the first grinding to obtain a first precursor;

[0012] S3: sintering the first precursor for the first time under the protection of an inert gas atmosphere at 300-500° C. for 1-10 hours;

[0013] S4: mixing the material obtained after the first sintering, the second carbon source and the dispersant, and then grinding for the second time;

[0014] S5: spray drying the slurry obtained by the second grinding to obtain a second precursor;

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

[0016] Furthermore, the iron source is selected from one or more of anhydrous ferric phosphate, dihydrate ferric phosphate, 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 rates of the first sintering and the second sintering are 1-10°C / min.

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

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

[0021] Doping with transition metal elements enables lithium iron phosphate positive electrode materials to have a higher discharge capacity in low temperature environments.

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

[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 0.5-2wt%; the BET specific surface area is 8-20m 2 / g.

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

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

[0027] By coating the precursor with two different carbon sources twice and grinding the slurry twice, the nano-scale lithium iron phosphate material particles can be fully coated with a carbon layer to form a shell-core structure, thereby reducing the side reactions of the lithium iron phosphate material itself with the electrolyte under high temperature. At the same time, the combination of a low-temperature pre-sintering and a secondary high-temperature sintering avoids the disadvantage of the conventional high-temperature sintering directly preparing lithium iron phosphate, in which the high carbon content causes the carbon source to form a coating layer on the surface of lithium iron phosphate at high temperature, resulting in more impurity phase metal magnetic foreign matter such as iron phosphide and lithium phosphate due to its strong reducibility. This greatly improves the high-temperature calendar life and cycle life of the material itself.

[0028] Furthermore, the mass of the added first carbon source 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 fiber, cellulose, acetylene black, graphite, carbon nanotubes, and citric acid.

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

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

[0031] The invention discloses a method for preparing a wide temperature range high rate lithium iron phosphate positive electrode material, which adopts two sintering and secondary carbon source addition to avoid excessively high temperature and carbon content during the first sintering, and the massive generation of impurity particles such as iron phosphide and lithium phosphate, and adopts secondary carbon coating to make the carbon coating more uniform. After being assembled into a lithium-ion battery, the side reaction between nano lithium iron phosphate and the electrolyte can be effectively reduced, and it not only has low temperature high rate performance, but also has excellent high temperature cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0033] Figure 2 1C discharge capacity results of Examples 1-3 and Comparative Examples 1-3 of the present invention;

[0034] Figure 3 10C discharge capacity results at -20°C for Examples 1-3 and Comparative Examples 1-3 of the present invention;

[0035] Figure 4 The results of the high temperature 45°C 1C / 1C cycle of Examples 1-3 and Comparative Examples 1-3 of the present invention are shown in FIG.

[0036] Figure 5 It is a diagram of the high temperature 60°C storage results of Examples 1-3 and Comparative Examples 1-3 in the present invention. DETAILED DESCRIPTION

[0037] In order to make the technical solution of the present invention clearer, the present invention is 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 transferred to a sand mill for sand milling for 2h, so that the slurry particle size D50 was controlled at 300-400nm; the sand milled slurry was transferred to a spray dryer for spray drying to obtain a first precursor, and the carbon content of the first precursor was tested by a carbon sulfur analyzer to be 2.8wt%. The first precursor was placed in a graphite crucible and sintered for the first time in a nitrogen atmosphere. After heating to 400℃ at a rate of 5℃ / min, it was kept warm for 4h to obtain a lithium iron phosphate material after the first sintering, with a specific surface area of ​​10.3m 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 the second carbon source, and then sanded with a sand mill for 2.5h to control the slurry particle size D50 at 200-300nm; the sand-milled slurry was transferred to a spray dryer to obtain a second precursor, and the carbon content of the second precursor was 3.5wt% through carbon-sulfur analysis. The second precursor was reloaded into a graphite sagger, and the second sintering was performed under a nitrogen atmosphere, and the temperature was raised to 750℃ at a rate of 10℃ / min, and the temperature was kept for 6h to obtain a wide temperature range and high-rate lithium iron phosphate positive electrode material.

[0040] The wide temperature range high rate lithium iron phosphate positive electrode material prepared in Example 1 was tested by scanning electron microscope. Figure 1 As shown, the minimum primary particle size can reach 50nm.

[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 transferred to a sand mill for sand milling for 2h, so that the slurry particle size D50 was controlled at 250-300nm; the sand milled slurry was transferred to a spray dryer for drying to obtain a first precursor, and the carbon content of the first precursor was tested by a carbon sulfur analyzer to be 2.75wt%. The first precursor was placed in a graphite crucible and sintered for the first time in a nitrogen atmosphere. After heating to 400℃ at a rate of 5℃ / min, it was kept warm for 4h to obtain a lithium iron phosphate material after the first sintering, with a specific surface area of ​​14.6m 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 the second carbon source, and further sand-milled for 2.5h to control the slurry particle size D50 to 150-200nm; the sand-milled slurry was transferred to a spray dryer to obtain a second precursor, and the carbon content of the second precursor was 3.65wt% tested by carbon-sulfur analysis. The second precursor was reloaded into a graphite sagger, sintered for the second time in a nitrogen atmosphere, heated to 720℃ at a rate of 10℃ / min, and kept warm for 6h to obtain a wide temperature range high-rate lithium iron phosphate positive electrode 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 transferred to a sand mill for sand milling for 2h, so that the slurry particle size D50 was controlled at 300-350nm; the sand milled slurry was transferred to a spray dryer for drying to obtain a first precursor, and the carbon content of the first precursor was 3.0wt% tested by a carbon sulfur analyzer. The first precursor was placed in a graphite crucible and sintered for the first time in a nitrogen atmosphere. After heating to 450℃ at a rate of 5℃ / min, it was kept warm for 6h to obtain a lithium iron phosphate material after the first sintering, with a specific surface area of ​​13.4m 2 / g. The lithium iron phosphate material after the first sintering was added to 170g of deionized water, and 1.4g of carbon nanotubes was added as the second carbon source, and further sand-milled for 2.0h to control the slurry particle size D50 to 150-200nm; the sand-milled slurry was transferred to a spray dryer to obtain a second precursor, and the carbon content of the second precursor was 4.3% through carbon-sulfur analysis. The second precursor was reloaded into a graphite sagger and sintered for the second time in a nitrogen atmosphere, heated to 760℃ at a rate of 10℃ / min, and kept warm for 8h to obtain a wide temperature range high-rate lithium iron phosphate positive electrode material.

[0045] Comparative Example 1

[0046] The comparative example and Example 1 use the same amount of raw materials

[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 transferred to a sand mill for sand milling for 8h, so that the slurry particle size D50 was controlled at 200-300nm; the sand milled slurry was transferred to a spray dryer for drying to obtain a precursor, and the carbon content in the precursor was tested by a carbon sulfur analyzer to be 3.6wt%. The precursor was placed in a graphite sagger, sintered in a nitrogen atmosphere, heated to 750℃ at a rate of 10℃ / min, and kept warm for 6h to obtain a lithium iron phosphate positive electrode material.

[0048] Comparative Example 2

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

[0050] 62g of ferric 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 transferred to a sand mill for sand milling for 8h, so that the slurry particle size D50 was controlled at 150-200nm; the sand milled slurry was transferred to a spray dryer for drying to obtain a precursor, and the carbon content in the precursor was 3.65wt% tested by a carbon sulfur analyzer. The precursor was placed in a graphite sagger, sintered in a nitrogen atmosphere, heated to 720℃ at a rate of 10℃ / min, and kept warm for 6h to obtain a lithium iron phosphate positive electrode material.

[0051] Comparative Example 3

[0052] Comparative Example 3 and Example 3 use 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 transferred to a sand mill for sand milling for 7h, so that the slurry particle size D50 was controlled at 150-200nm; the sand milled slurry was transferred to a spray dryer for granulation and a precursor was obtained, and the carbon content in the precursor was tested by a carbon sulfur analyzer to be 4.4wt%. The precursor was placed in a graphite sagger, sintered in a nitrogen atmosphere, heated to 760℃ at a rate of 10℃ / min, and kept warm for 8h to obtain a lithium iron phosphate positive electrode material.

[0054] Performance testing and analysis

[0055] The lithium iron phosphate positive electrode materials obtained in the above embodiments and comparative examples were subjected to powder physical and chemical test characterization and button half-cell electrical performance test, and the results are shown in the following table.

[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 contains fewer magnetic metal particles, which is more conducive 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 positive electrode materials to carry out full battery assembly and evaluation, and the process was as follows:

[0061] 1) Preparation of positive electrode sheet: Take an appropriate amount of positive electrode material, conductive agent, and binder with a mass ratio of 95:2:3 and add them to N-methylpyrrolidone solvent for high-speed dispersion and homogenization, and adjust the slurry solid content to 55%. The positive electrode material is lithium iron phosphate in the above embodiments or comparative examples, the conductive agent is carbon black ECP, and the binder is polyvinylidene fluoride (PVDF). The dispersed slurry is applied to aluminum foil by a coating machine, and the positive electrode sheet is obtained by rolling and drying.

[0062] 2) Negative electrode sheet production: Take an appropriate amount of negative electrode material, conductive agent, and binder with a mass ratio of 93:3:4 and add them to N-methylpyrrolidone solvent for high-speed dispersion and homogenization, and adjust the slurry solid content to 40%. The negative electrode material is conventional artificial graphite, the conductive agent is carbon black, and the binder is styrene-butadiene rubber latex (SBR). The dispersed slurry is applied to copper foil by a coating machine, and the negative electrode sheet is obtained after rolling and drying.

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

[0064] 4) The battery is subjected to -20℃ 1C charge and discharge test, -30℃ 1C charge and discharge test, high temperature 45℃ cycle 1C test, and high temperature 60℃ storage test.

[0065] The test results of the battery cells of each embodiment and comparative example are shown in Table 2 below.

[0066] Table 2 Test results of battery cells of Examples 1-3 and Comparative Examples 1-3

[0067]

[0068] like Figure 2-3 As shown in Figure 2 and Table 2, the discharge capacity of each embodiment at low temperature -20°C and -30°C is better than that of the comparative example; and the capacity retention rate at -20°C and high rate 10C discharge is also better than that of the comparative example. It is proved that the lithium iron phosphate prepared by implementing the technical solution has low temperature and high rate discharge capability.

[0069] Table 3 High temperature cycle 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 is superior to the control group in terms of capacity retention rate of high temperature cycle and high temperature storage, which proves that the lithium iron phosphate prepared by implementing the technical solution has excellent high temperature performance.

[0072] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for preparing a wide temperature range high rate lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: S1: uniformly mixing an iron source, a phosphorus source, a lithium source, a first carbon source, a dopant containing a transition metal and a dispersant, and performing a first grinding on the uniformly mixed slurry; S2: spray drying the slurry after the first grinding to obtain a first precursor; S3: sintering the first precursor for the first time under the protection of an inert gas atmosphere at 300-500° C. for 1-10 hours; S4: mixing the material obtained after the first sintering, the second carbon source and the dispersant, and then grinding for the second time; S5: spray drying the slurry obtained by the second grinding to obtain a second precursor; S6: sintering the second precursor for a second time under the protection of an inert gas atmosphere, and continuously sintering at 600-800°C for 1-10h to obtain a wide temperature range and high rate lithium iron phosphate positive electrode material.

2. The method for preparing a wide temperature range high rate lithium iron phosphate positive electrode material according to claim 1, characterized in that: In the step S1, the lithium source and the iron source are added, wherein the molar ratio of lithium element to iron element is 1.0-1.04:1; the transition metal-containing 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 doping element in the finally obtained lithium iron phosphate is less than ≤5000ppm.

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

4. The method for preparing a wide temperature range high rate lithium iron phosphate positive electrode material according to claim 1, characterized in that: In the step S2, the carbon content in the first precursor powder is 1-3 wt%.

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

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

7. The method for preparing a wide temperature range high rate lithium iron phosphate positive electrode material according to claim 1, characterized in that: In the step S5, the carbon content in the second precursor powder is 3-6 wt %.

8. The method for preparing a wide temperature range high rate lithium iron phosphate positive electrode material according to claim 1, characterized in that: The mass of the added first carbon source 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 fiber, cellulose, acetylene black, graphite, carbon nanotubes, and citric acid.

9. A wide temperature range high rate lithium iron phosphate positive electrode material, characterized in that: Obtained by the preparation method according to any one of claims 1 to 9, the carbon content in the material is 1.5-4.0wt%; the BET specific surface area is 10-25m 2 / g; the size of the primary particles is 50-300nm.

10. A lithium ion battery, characterized in that: It comprises a negative electrode sheet, a separator, and a positive electrode sheet; the positive electrode sheet comprises the wide temperature range and high rate lithium iron phosphate positive electrode material as claimed in claim 9.

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