Industrial synthesis method of lithium iron phosphate / carbon nanotubes and its application

Abstract

The present invention belongs to the technical field of positive electrode materials for lithium ion batteries, and specifically relates to an industrial synthesis method of lithium iron phosphate / carbon nanotubes and its application. The present invention utilizes ferrocene to sublimate in the low temperature zone of a multi-temperature zone tubular furnace and then pass into the high temperature zone accompanied by nitrogen, and decomposes into elemental iron and carbon under heat, and reduces iron phosphate and lithium hydroxide to lithium iron phosphate, and 0-valent transition metal iron catalyzes the generation of carbon nanotubes, and a layer of uniform carbon nanotubes is coated on the surface of the synthesized lithium iron phosphate, so that the conductivity of lithium iron phosphate with poor conductivity is improved, and it has high structural stability, and the water generated by the reaction will also be evaporated at high temperature, and only the carbon dioxide generated by the reaction needs to be recovered later, so as to reduce the steps of lithium iron phosphate synthesis and reduce costs. As a positive electrode material for lithium ion batteries, lithium iron phosphate / carbon nanotubes have good rate performance and cycle performance, and solve the problems of poor conductivity of lithium iron phosphate and complex synthesis process.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion battery positive electrode materials, and specifically relates to an industrial synthesis method of lithium iron phosphate / carbon nanotubes and application thereof. Background Art

[0002] As the most mature new energy storage device at present, lithium-ion batteries are widely used in life because of their long cycle life, good discharge performance at low temperatures, high energy density, high specific capacity, and high green environmental protection performance.

[0003] The cathode material in lithium-ion batteries is mainly lithium iron phosphate (LiFePO 4 ) and ternary materials (Li(Ni x Co y Mn 1-x-y ) 2 ), the poor structural stability of ternary materials, the scarcity of rare metals such as nickel and cobalt, which leads to increased costs, and general safety performance have limited its development. Lithium iron phosphate has a significant reduction in cost because it does not contain precious metals, and due to its three-dimensional spatial network olivine structure, LiFePO 4 During the charge and discharge process, LiFePO 4 and FePO 4 The volume change rate is small, and the safety and stability of lithium-ion battery cathode are guaranteed. In addition, lithium iron phosphate cathode has excellent cycle performance and is one of the ideal lithium-ion battery cathode materials. However, the low energy density and power performance of lithium iron phosphate seriously hinder its further development. + The activation energy is between 0.3 and 0.5 eV, which results in a high conductivity and Li + The diffusion coefficient is low and the conductivity is 10 -9 ~10 -10 S / cm magnitude and Li + The diffusion coefficient is 10 -15 ~10 -12 cm 2 / s order of magnitude, low electronic conductivity and lithium ion diffusion coefficient lead to poor performance of lithium iron phosphate, which needs to be doped with other metal elements or carbon coated to improve its low conductivity, resulting in cumbersome process, complex process and greatly increased cost for synthesizing lithium iron phosphate with high conductivity. Summary of the invention

[0004] The purpose of the present invention is to explore an industrial-grade method for synthesizing lithium iron phosphate / carbon nanotubes for lithium-ion battery positive electrodes. The synthesis method has simple steps and low raw material costs. The conductivity of the synthesized lithium iron phosphate / carbon nanotubes is greatly improved compared to traditional lithium iron phosphate.

[0005] If a layer of dense carbon nanotubes can be coated on the surface of lithium iron phosphate through chemical vapor deposition technology during the process of synthesizing lithium iron phosphate, the conductivity and utilization rate of lithium iron phosphate can be effectively improved, and the cost of lithium-ion batteries can be reduced.

[0006] In order to achieve the above purpose, the present invention uses ferrocene as a carbon source, which reacts with lithium hydroxide and iron phosphate in the high temperature zone of a multi-temperature zone tube furnace to generate lithium iron phosphate / carbon nanotubes after sublimation into gas. The water generated during the reaction will evaporate at high temperature, and the reaction will not be accompanied by the generation of unnecessary by-products. 2 By recycling, we can get lithium iron phosphate / carbon nanotubes, which will not cause environmental pollution and is safe and environmentally friendly.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] An industrial synthesis method of lithium iron phosphate / carbon nanotubes comprises the following steps:

[0009] (1) Weigh a certain amount of lithium hydroxide and iron phosphate powder, mix them evenly, and place them in the high temperature zone of a multi-temperature zone tube furnace;

[0010] (2) Weigh a certain amount of ferrocene powder, grind it evenly, and place it in the low temperature zone of a multi-temperature zone tube furnace;

[0011] (3) introducing protective gas into the multi-temperature zone tubular furnace;

[0012] (4) Setting the low temperature zone of the multi-temperature zone tubular furnace at an appropriate temperature to ensure that ferrocene can sublime; setting the high temperature zone of the multi-temperature zone tubular furnace at 550-650°C, and maintaining the temperature for 2-3 hours after reaching the set temperature;

[0013] (5) After the reaction is completed, the mixture is cooled to room temperature to obtain lithium iron phosphate / carbon nanotubes.

[0014] Furthermore, the molar ratio of lithium hydroxide to iron phosphate is 1:(1-2), preferably 1:1.

[0015] Furthermore, the molar ratio of ferrocene to lithium hydroxide is (1-2):4.

[0016] Furthermore, in step (4), the heating rate of the multi-temperature zone tubular furnace is controlled at 5°C / min, the temperature of the low temperature zone of the multi-temperature zone tubular furnace is set at 180-200°C, the temperature of the high temperature zone of the multi-temperature zone tubular furnace is set at 550-650°C, and after heating to the set temperature, it is maintained for 3 hours.

[0017] Furthermore, in the step (3), nitrogen is introduced into the multi-temperature zone tubular furnace as a protective gas, and the flow rate of the nitrogen is 300 sccm.

[0018] The invention also discloses the application of the lithium iron phosphate / carbon nanotube obtained by the synthesis method in preparing positive electrode materials for lithium ion batteries.

[0019] Compared with the prior art, the technical solution of the present invention has the following advantages and beneficial effects:

[0020] 1. The synthesis process of the present invention is simple, and the carbon-coated lithium iron phosphate positive electrode can be synthesized by only one step of chemical vapor deposition. Compared with the traditional solid phase synthesis and hydrothermal synthesis methods, no tedious steps such as washing and drying are required.

[0021] 2. The synthesis method of the present invention only requires ferrocene as a carbon source. At the same time, ferrocene can also be used as an iron source after decomposition, without introducing other impurity elements, thereby improving the purity of lithium iron phosphate.

[0022] 3. The synthesis process of the present invention is safe and environmentally friendly. Ferrocene is used as a carbon source, and after decomposition, only CO2 with little environmental pollution is generated. 2 , using relevant technologies to recover CO 2 Compared with other carbon sources, such as organic matter, its decomposition at high temperature not only produces carbon, but also produces extremely dangerous H 2 , the safety risks are huge. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a theoretical schematic diagram of the present invention using chemical vapor deposition technology to plate carbon nanotubes on the surface of lithium iron phosphate;

[0024] Figure 2 This is a process flow chart of synthesizing lithium iron phosphate / carbon nanotubes according to Example 1 of the present invention;

[0025] Figure 3 This is a sample picture of lithium iron phosphate / carbon nanotubes synthesized in Example 1;

[0026] Figure 4 The XRD pattern of the lithium iron phosphate / carbon nanotube sample synthesized in Example 1 is compared with the lithium iron phosphate XRD standard card, and the characteristic peaks are consistent;

[0027] Figure 5 This is a SEM image of the lithium iron phosphate / carbon nanotube sample synthesized in Example 1. It can be seen that the outermost layer of the lithium iron phosphate powder obviously has a carbon coating layer;

[0028] Figure 6 The cycle performance and charge-discharge curve of the lithium iron phosphate / carbon nanotube sample synthesized in Example 1;

[0029] Figure 7 This is the cycle curve of the lithium iron phosphate / carbon nanotube sample synthesized in Example 2-5.

[0030] Figure 8 The cycle curves of the samples synthesized in Comparative Examples 1 and 2 are shown. DETAILED DESCRIPTION

[0031] The specific process of one-step synthesis of lithium iron phosphate / carbon nanotubes of the present invention is described in detail below in conjunction with specific embodiments and drawings of the specification.

[0032] The one-step synthesis process and principle of lithium iron phosphate / carbon nanotubes of the present invention are as follows: Figure 2 and Figure 1 As shown, ferrocene is heated and sublimated into gas in the low temperature zone of the multi-temperature zone tubular furnace. The ferrocene gas enters the high temperature zone of the multi-temperature zone tubular furnace along with the protective gas nitrogen and is decomposed into elemental iron and carbon by heat. Under high temperature conditions, carbon and lithium hydroxide and iron phosphate mixed powder undergo redox reaction to generate lithium iron phosphate. Under the catalysis of zero-valent transition metal iron, carbon atoms are deposited and crystallized on the surface of lithium iron phosphate metal particles to form a carbon nanotube structure. The carbon nanotube forms a dense coating on the surface of the lithium iron phosphate. The lithium iron phosphate / carbon nanotube is naturally cooled to obtain a positive electrode material for lithium-ion batteries.

[0033] Example 1 An industrial synthesis method of lithium iron phosphate / carbon nanotubes, the steps are as follows:

[0034] 1) First weigh lithium hydroxide (LiOH) powder and iron phosphate (FePO 4 ) powders are mixed in a molar ratio of 1:1 (in this embodiment, lithium hydroxide is 4 mmol), and the two powders are fully ground and stirred for 2 hours until the two powders are completely mixed, and placed in the high temperature zone of a multi-temperature zone tubular furnace.

[0035] 2) Weigh ferrocene (Fe(C 5 H 5 ) 2 ) powder, the molar ratio of ferrocene powder to lithium hydroxide and ferric phosphate is 1:4:4, the ferrocene powder is fully ground for 1 hour and placed in the low temperature zone of a multi-temperature zone tubular furnace.

[0036] 3) Nitrogen was introduced into the multi-temperature zone tubular furnace as a protective gas, the flow rate of nitrogen was 300 sccm, the temperature of the low temperature zone of the multi-temperature zone tubular furnace was set to 180°C, the temperature of the high temperature zone was set to 650°C, the heating rate was 5°C / min, and the temperature was raised to 650°C for 3 hours to recover the carbon dioxide (CO 2 ), the obtained solid is cooled to room temperature (25°C, the same below) to obtain the target product lithium iron phosphate / carbon nanotubes (with a carbon content of 1.48%), which can be used as a positive electrode material for lithium ion batteries.

[0037] In the reaction, ferrocene sublimates into gas in the low temperature zone of the multi-temperature zone tubular furnace, and then decomposes into elemental iron and carbon in the high temperature zone. Carbon reacts with lithium hydroxide and iron phosphate to generate lithium iron phosphate. The specific reaction formula is as follows: 4LiOH+4FePO 4 +C→4LiFePO 4 +CO 2 +2H 2 O.

[0038] Example 2 An industrial synthesis method of lithium iron phosphate / carbon nanotubes, the steps are as follows:

[0039] The synthesis method of this embodiment is carried out with reference to that of embodiment 1, except that the temperature of the low temperature zone of the multi-temperature zone tubular furnace is set to 100°C, the temperature of the high temperature zone is set to 800°C, and the other reaction conditions are the same as those of embodiment 1, to obtain lithium iron phosphate / carbon nanotubes (carbon content 0.3%).

[0040] Example 3 An industrial synthesis method of lithium iron phosphate / carbon nanotubes, the steps are as follows:

[0041] The synthesis method of this embodiment is carried out with reference to that of embodiment 1, except that the molar ratio of ferrocene powder to lithium hydroxide and iron phosphate is 1:8:4 (in this embodiment, lithium hydroxide is 8 mmol), and other reaction conditions are the same as those of embodiment 1 to obtain lithium iron phosphate / carbon nanotubes.

[0042] Example 4 An industrial synthesis method of lithium iron phosphate / carbon nanotubes, the steps are as follows:

[0043] The synthesis method of this embodiment is carried out with reference to that of embodiment 1, except that the molar ratio of ferrocene powder to lithium hydroxide and iron phosphate is 1:4:8, and other reaction conditions are the same as those of embodiment 1, to obtain lithium iron phosphate / carbon nanotubes.

[0044] Example 5 An industrial synthesis method of lithium iron phosphate / carbon nanotubes, the steps are as follows:

[0045] The synthesis method of this embodiment is carried out with reference to that of embodiment 1, except that the molar ratio of ferrocene powder to lithium hydroxide and iron phosphate is 1:2:2, and other reaction conditions are the same as those of embodiment 1, to obtain lithium iron phosphate / carbon nanotubes.

[0046] Comparative Example 1 An industrial synthesis method of lithium iron phosphate / carbon nanotubes, the steps are as follows:

[0047] 1) First weigh lithium hydroxide (LiOH) powder and iron phosphate (FePO 4 ) powders are mixed in a molar ratio of 1:1 (in this comparative example, the lithium hydroxide is 4 mmol), and the two powders are fully ground and stirred for 2 hours until the two powders are completely mixed, and then placed in the high temperature zone of a multi-temperature zone tubular furnace.

[0048] 2) Weigh ferrocene (Fe(C 5 H 5 ) 2 ) powder, the molar ratio of ferrocene powder to lithium hydroxide and ferric phosphate is 0.5:4:4. After fully grinding the ferrocene powder for 1 hour, it is ultrasonically mixed with 5 ml of xylene (99%) for 10 minutes and placed in the low temperature zone of a multi-temperature zone tubular furnace.

[0049] 3) Nitrogen was introduced into the multi-temperature zone tubular furnace as a protective gas, the flow rate of nitrogen was 300 sccm, the temperature of the low temperature zone of the multi-temperature zone tubular furnace was set to 180°C, the temperature of the high temperature zone was set to 650°C, the heating rate was 5°C / min, and the temperature was raised to 650°C for 3 hours to recover the carbon dioxide (CO 2 ), and the obtained solid is cooled to room temperature to obtain lithium iron phosphate / carbon nanotubes.

[0050] Comparative Example 2 An industrial synthesis method of lithium iron phosphate / carbon nanotubes, the steps are as follows:

[0051] 1) First weigh lithium hydroxide (LiOH) powder and iron phosphate (FePO 4 ) powders are mixed in a molar ratio of 1:1 (in this comparative example, the lithium hydroxide is 4 mmol), and the two powders are fully ground and stirred for 2 hours until the two powders are completely mixed, and then placed in the high temperature zone of a multi-temperature zone tubular furnace.

[0052] 2) Using xylene (99%) as a carbon source, 5 ml of xylene was placed in the low temperature zone of a multi-temperature zone tubular furnace.

[0053] 3) Nitrogen was introduced into the multi-temperature zone tubular furnace as a protective gas, the flow rate of nitrogen was 300 sccm, the temperature of the low temperature zone of the multi-temperature zone tubular furnace was set to 180°C, the temperature of the high temperature zone was set to 650°C, the heating rate was 5°C / min, and the temperature was raised to 650°C for 3 hours to recover the carbon dioxide (CO 2 ), and the obtained solid is cooled to room temperature to obtain lithium iron phosphate / carbon nanotubes.

[0054] The binder and the conductive agent were mixed with the lithium iron phosphate / carbon nanotube powder of the embodiment and the comparative example at a mass ratio of 1:1:8, and N-methylpyrrolidone was added to make a slurry, which was evenly coated on an aluminum foil, vacuum dried, rolled and punched into a circular electrode sheet, and assembled into a button battery in a glove box. The button battery was tested for constant current charge and discharge cycles, and the charge and discharge voltage was 2.60-3.65V. The cycle performance and charge and discharge curves of the embodiment and the comparative example at 0.1C were tested, and the results are shown in Table 1.

[0055] Table 1

[0056]

[0057] The samples synthesized in Example 1 were made into electrodes, and the cycle performance and charge-discharge curves of the assembled batteries were as follows: Figure 6 As shown, after 75 cycles at 0.1C, the capacity retention rate is as high as 99.6%, and the charge and discharge platform voltage remains basically stable.

[0058] Example 2-5 The cycle performance of the synthesized samples was verified by changing the temperature of the tube furnace and the ratio of lithium hydroxide to iron phosphate. The results are shown in Figure 7 . In Example 2, due to the low temperature of the tubular furnace in the low temperature zone, ferrocene cannot be completely sublimated, and only the ferrocene on the outermost surface will decompose a small amount. At the same time, the temperature of the tubular furnace in the high temperature zone is too high, resulting in layered destruction of the generated lithium iron phosphate positive electrode, so it fails after 30 cycles. In Example 3 and Example 4, due to excessive lithium hydroxide or iron phosphate, the generated lithium iron phosphate powder will be doped with impurities, resulting in a decrease in specific capacity and unstable cycles. In Example 5, the performance of the carbon-coated lithium iron phosphate was not improved by adding excessive ferrocene, and it also caused a waste of raw materials and increased the cost of synthesis.

[0059] Comparative Examples 1 and 2 verify the cycle performance of the synthesized samples by using different carbon sources. The results are shown in Figure 8 In Comparative Example 1, xylene and ferrocene were used as carbon sources together. However, since the content of ferrocene was not enough to completely generate carbon-coated lithium iron phosphate, the battery material short-circuited after 53 cycles, and the specific capacity dropped directly to zero. In Comparative Example 2, xylene was used to completely replace ferrocene as a carbon source. Since it could not be decomposed by heat to produce zero-valent transition metal as a catalyst, a carbon nanotube coating layer could not be formed on the surface of lithium iron phosphate, resulting in extremely poor specific capacity and coulombic efficiency of the generated sample after less than 40 cycles.

Claims

1. An industrial synthesis method of lithium iron phosphate / carbon nanotubes, comprising the following steps: (1) Weighing a certain amount of lithium hydroxide and iron phosphate powder, mixing them evenly, and placing them in the high temperature zone of a multi-temperature zone tube furnace, wherein the molar ratio of lithium hydroxide to iron phosphate is 1:(1-2); (2) Weighing a certain amount of ferrocene powder, grinding it evenly, and placing it in the low temperature zone of a multi-temperature zone tube furnace, wherein the molar ratio of ferrocene to lithium hydroxide is (1-2):4; (3) Nitrogen was introduced into the multi-temperature zone tubular furnace as a protective gas, and the flow rate of nitrogen was 300 sccm; (4) Set the low temperature zone of the multi-temperature zone tubular furnace at an appropriate temperature to ensure that ferrocene can sublime; set the high temperature zone of the multi-temperature zone tubular furnace at 550-650°C, and keep it at the set temperature for 2-3 hours; (5) After the reaction is completed, the mixture is cooled to room temperature to obtain lithium iron phosphate / carbon nanotubes.

2. The industrial synthesis method of lithium iron phosphate / carbon nanotubes according to claim 1, characterized in that: The heating rate of the multi-temperature zone tubular furnace in (4) is controlled at 5°C / min, and the temperature of the low temperature zone of the multi-temperature zone tubular furnace is set at 180-200°C. After heating to the set temperature, it is maintained for 3 hours.

3. Use of lithium iron phosphate / carbon nanotubes obtained by the industrial synthesis method according to any one of claims 1 to 2 in the preparation of positive electrode materials for lithium ion batteries.

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