Preparation method of lithium iron phosphate / lithium manganese iron phosphate positive electrode material

Abstract

This invention belongs to the field of lithium-ion battery cathode material preparation, specifically a method for directly preparing lithium iron phosphate (LFP) / lithium manganese iron phosphate (MFP) from conventional aqueous solutions, and the subsequent preparation of LFP / MFP cathode materials. In a conventional (non-high temperature and high pressure) lithium-containing glycine aqueous solution system, this invention utilizes the coordination ability of glycine ligands for metal ions to control the coordination state of the metal ions, allowing direct preparation of LFP or MFP during phosphate precipitation. Carbon-coated sintering of LFP or MFP yields LFP or MFP cathode materials with excellent electrochemical performance. This method avoids the high temperature and high pressure conditions of hydrothermal methods, directly preparing the material from conventional aqueous solutions, significantly reducing the phase inversion reaction time, improving production efficiency, and lowering energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material preparation, and in particular, a method for directly preparing lithium iron phosphate / lithium manganese iron phosphate from conventional aqueous solutions, and the subsequent preparation of lithium iron phosphate / lithium manganese iron phosphate cathode materials. Background Technology

[0002] With the growing energy crisis and emerging environmental problems, lithium-ion batteries (LIBs) have broad development prospects due to their high operating voltage and energy density, as well as the low toxicity and environmental friendliness of their electrode materials. Among various cathode materials, the polyanionic cathode material LiFePO4 (LFP) exhibits a stable discharge platform (3.4V) and a high theoretical discharge specific capacity (170mAh·g). -1 With its good cycle performance, thermal stability, inexpensive and readily available raw materials, good safety and environmental friendliness, it is considered one of the most suitable cathode materials for lithium-ion batteries.

[0003] Currently, the main methods for preparing lithium iron phosphate / lithium manganese iron phosphate materials include high-temperature solid-state method, hydrothermal / solvothermal method, and sol-gel method.

[0004] The high-temperature solid-state method refers to using lithium salts (such as lithium hydroxide), iron salts (such as ferrous sulfate), and ammonium dihydrogen phosphate as raw materials, adding a carbon source (such as glucose), mixing them through high-energy ball milling for several hours, sintering at 600–800℃ for 10–15 hours under an inert atmosphere, and obtaining the final product through cooling, crushing, and grinding processes (see L. Yao, Facile Synthesis of LiFePO4 / C with high Tap-density as cathode for high performance lithium ion batteries, Int. J. Electrochem. Sci. (2017) 206–217.). The high-temperature solid-state method has a simple production process, high yield, and low cost, but the phase inversion reaction time is long, energy consumption is high, and the particle size is large and difficult to control.

[0005] The hydrothermal / solvothermal method involves dissolving lithium salts (such as lithium carbonate), iron sources (such as ferrous sulfate), and phosphorus sources (such as ammonium dihydrogen phosphate) in deionized water / organic solvents, reacting them in an autoclave at 180–200 °C for 12–24 h, and then naturally cooling to room temperature to obtain the product lithium iron phosphate or its precursor. The product is then sintered at 650–700 °C for 8–10 h to obtain the final product (see reference Z.Li, J.Yang, T.Guang, B.Fan, K.Zhu, X.Wang, Controlled hydrothermal / solvothermal synthesis of high-performance LiFePO4 for Li-ion batteries, Small Methods 5(6)(2021)e2100193.). This method produces particles with small and uniform particle size, but requires a high-temperature and high-pressure environment, resulting in low production capacity.

[0006] The sol-gel method uses lithium (e.g., lithium hydroxide), iron (e.g., ferrous sulfate), phosphorus (e.g., ammonium dihydrogen phosphate), and carbon (e.g., sucrose) as raw materials to form a wet gel. After drying and high-energy ball milling, the final product is obtained by sintering at 550–800℃ for 11–13 h under an argon atmosphere (see reference Li Lei. Effect of carbon addition on lithium iron phosphate in sol-gel method [J]. Journal of North China University of Technology, 2019, 227: 56-63). The material prepared by the sol-gel method has uniform particles and small particle size, but the phase inversion reaction is also relatively long, resulting in high energy consumption and low yield.

[0007] Invention patent CN105633401A discloses a method for synthesizing high-energy-density lithium manganese iron phosphate by adding an active ion buffer. First, an iron source, a phosphorus source, and an active ion buffer are mixed and added to a solvent. After stirring in a water bath and drying, the mixture is calcined in a muffle furnace at 150–300°C for 3–12 hours. Next, a manganese source is added and mixed into the solvent. The mixture is stirred in a water bath at 60–100°C until the water evaporates, dried in an oven, and then calcined in a muffle furnace at 300–600°C for 5–15 hours. Finally, a lithium source and a carbon source are mixed and added to the solvent. The mixture is then placed in a tube furnace and protectively sintered at 600–900°C for 5–24 hours to obtain carbon-coated lithium manganese iron phosphate material. This method involves cumbersome steps, a complex process, requires multiple sintering cycles, has a long phase inversion reaction time, and consumes a lot of energy.

[0008] Invention patent CN104752720A discloses a method for preparing lithium manganese iron phosphate. The method involves mixing a first solution containing water-soluble divalent manganese, water-soluble divalent iron, and water-soluble phosphorus sources with a second solution containing a water-soluble lithium source in a parallel flow. The mixture is then hydrothermally reacted at 140–200°C and 0.1–3.5 MPa for 6–10 hours to obtain lithium manganese iron phosphate. This is followed by mixing with an organic carbon source, spray drying at 230–270°C, and calcining at 550–750°C for 4–10 hours. This method requires not only a high-temperature and high-pressure environment for preparing lithium manganese iron phosphate, but also subsequent carbon-coating calcination, which still requires high temperatures and long processing times. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides a method for directly preparing lithium iron phosphate or lithium manganese iron phosphate (LiFe) from conventional aqueous solutions. x Mn 1-x The method for preparing the corresponding cathode material is described, along with the method for preparing PO4 (0 < x < 1). This preparation method does not require a high-temperature and high-pressure environment, has high safety, significantly reduces the subsequent carbon coating sintering time, and produces materials with uniform particle size.

[0010] To achieve the above objectives, the technical solution adopted in this invention is as follows: In a conventional (non-high temperature and high pressure) lithium-containing glycine aqueous solution system, the coordination ability of glycine ligands for metal ions is utilized to control the coordination state of the metal ions, allowing lithium iron phosphate or lithium manganese iron phosphate to be directly produced during phosphate precipitation. Carbon-coated sintering of lithium iron phosphate or lithium manganese iron phosphate yields lithium iron phosphate or lithium manganese iron phosphate cathode materials with excellent electrochemical performance. This avoids the high temperature and high pressure conditions of the hydrothermal method, directly producing the material from a conventional aqueous solution, greatly reducing the phase inversion reaction time, improving production efficiency, and reducing energy consumption.

[0011] A method for preparing a lithium iron phosphate / lithium manganese iron phosphate cathode material includes the following steps:

[0012] (1) Lithium hydroxide and a soluble metal salt are added to a reactor, and a protective gas is introduced. An aqueous solution of glycine is added to the reactor, and the mixture is heated and stirred until dissolved. After complete dissolution, a phosphoric acid solution is added to the reactor at a uniform rate to begin precipitation. After the phosphoric acid solution is completely pumped into the reactor, the mixture is aged, and solid-liquid separation is performed to obtain lithium iron phosphate (LiFePO4) or lithium manganese iron phosphate (LiFe). x Mn 1-x PO4, 0 < x < 1.

[0013] (2) The lithium iron phosphate (LiFePO4) or lithium manganese iron phosphate (LiFe) obtained in step (1) x Mn 1-x After mixing PO4 with a carbon source and sintering under a protective atmosphere, carbon-coated lithium iron phosphate or carbon-coated lithium manganese iron phosphate cathode materials are obtained.

[0014] The protective gas used in the above-mentioned dissolution, precipitation and sintering processes is nitrogen or an inert gas.

[0015] In step (1), the soluble metal salt is a soluble ferrous salt, or a mixture of a soluble ferrous salt and a manganese (II) salt.

[0016] The ratio of lithium hydroxide, soluble metal salt and phosphoric acid solution is lithium:iron / (iron+manganese):phosphorus (1.1~3):1:(1~1.2), and the concentration of ferrous ions or the total concentration of manganese(II) and iron(II) ions in the solution is controlled at 0.5~1.2mol / L.

[0017] Furthermore, the concentration of the phosphoric acid solution is 2.5–3.5 mol / L; the phosphoric acid solution is added at a rate of phosphoric acid solution volume (mL) / time (min), that is, the phosphoric acid solution is added completely at a uniform rate over 0.5–1 h;

[0018] Furthermore, the volume of the glycine aqueous solution is calculated based on the theoretical amount of lithium iron phosphate / lithium manganese iron phosphate that can be generated, and it is added at a liquid-to-solid ratio (mL / g) of 5:1 to 10:1, with a concentration of 1.5 to 4 mol / L.

[0019] Furthermore, the heating temperature is 80–100°C;

[0020] Furthermore, the aging time is 0.5–2 hours;

[0021] Furthermore, the stirring speed for both the precipitation and aging reactions was 100–500 r / min;

[0022] In step (2), the sintering temperature is 400-600℃ and the sintering time is 1-5h.

[0023] The beneficial effects of this invention are as follows:

[0024] By avoiding the high temperature and high pressure conditions of the hydrothermal method, lithium iron phosphate or lithium manganese iron phosphate can be directly prepared from conventional aqueous solutions, which greatly reduces the phase inversion reaction time, significantly shortens the sintering time, and lowers the temperature of subsequent high-temperature sintering, thereby improving production efficiency, reducing energy consumption, and producing uniform lithium iron phosphate materials. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the process flow of the present invention.

[0026] Figure 2 These are the XRD patterns of lithium iron phosphate (LiFePO4) before sintering (a) and after sintering (b) in Example 1.

[0027] Figure 3These are SEM images of lithium iron phosphate (LiFePO4) before sintering (a) and after sintering (b) in Example 1.

[0028] Figure 4 This is a rate performance diagram of the carbon-coated lithium iron phosphate cathode material (LiFePO4 / C) in Example 1.

[0029] Figure 5 This is a cycle performance diagram of the carbon-coated lithium iron phosphate cathode material (LiFePO4 / C) in Example 1.

[0030] Figure 6 Lithium manganese iron phosphate (LiFe) in Example 2 0.5 Mn 0.5 XRD patterns of PO4 before sintering (a) and after sintering (b).

[0031] Figure 7 Lithium manganese iron phosphate (LiFe) in Example 2 0.5 Mn 0.5 SEM images of PO4 before sintering (a) and after sintering (b).

[0032] Figure 8 The carbon-coated lithium manganese iron phosphate cathode material (LiFe) in Example 2 0.5 Mn 0.5 Rate performance diagram of PO4 / C.

[0033] Figure 9 The carbon-coated lithium manganese iron phosphate cathode material (LiFe) in Example 2 0.5 Mn 0.5 Cyclic performance graph of PO4 / C. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0035] Preparation process such as Figure 1 As shown.

[0036] Example 1

[0037] 0.21 mol of lithium hydroxide monohydrate and 0.07 mol of ferrous sulfate heptahydrate were added to a reactor under nitrogen protection. 100 mL of a 3 mol / L aminoacetic acid aqueous solution was added to the reactor, and the mixture was stirred at 95 °C until completely dissolved. While maintaining the temperature, 28.5 mL of a 2.82 mol / L phosphoric acid solution was pumped into the reactor at a constant rate over 35 min to initiate precipitation (the molar ratio of the components in the solution was lithium:iron:phosphorus = 3:1:1.15). The stirring speed was maintained at 400 r / min throughout the process. After the phosphoric acid solution was completely pumped into the reactor, the mixture was aged for 1 h, and solid-liquid separation yielded lithium iron phosphate. The lithium iron phosphate was mixed with glucose and sintered at 550 °C for 3 h under an argon atmosphere to obtain carbon-coated lithium iron phosphate cathode material.

[0038] Figure 2 Image (a) shows the XRD pattern of lithium iron phosphate directly prepared from a conventional aqueous solution in Example 1. It can be seen that the diffraction peaks of the material correspond well to the standard card for lithium iron phosphate, indicating that the prepared material is indeed lithium iron phosphate. Figure 2 (b) is the XRD pattern of lithium iron phosphate after carbon coating and sintering. The diffraction peaks of the material correspond completely with the standard card, indicating that there are no impurities. The diffraction peaks are also very sharp, indicating that the crystallinity is very good after sintering at the temperature and time in Example 1.

[0039] Figure 3 Image (a) is a SEM image of lithium iron phosphate directly prepared from a conventional aqueous solution in Example 1. It can be seen that the material has a complete structure, consisting of slender, sheet-like particles. Figure 3 (b) is a SEM image of lithium iron phosphate after carbon coating and sintering. The material has a complete structure and consists of nano-sized particles. Nano-sized particles can effectively improve the electrochemical performance of the material.

[0040] Figure 4 The rate performance of the carbon-coated lithium iron phosphate cathode material prepared in Example 1 is shown. This material exhibits excellent rate performance, with a discharge specific capacity of 147.90 mAh / g at 0.1C and 104.56 mAh / g at a high current of 20C. Furthermore, there is no capacity decay when the discharge current is adjusted back to 0.1C.

[0041] Figure 5 The cycling performance of the carbon-coated lithium iron phosphate cathode material prepared in Example 1 is as follows: after 200 cycles at 1C, the discharge specific capacity is 133.47 mAh / g, and the capacity retention rate is 98.03%.

[0042] Example 2

[0043] 0.21 mol lithium hydroxide monohydrate, 0.035 mol ferrous sulfate heptahydrate, and 0.035 mol anhydrous manganese sulfate were added to a reactor under nitrogen protection. 100 mL of a 3 mol / L aminoacetic acid aqueous solution was added to the reactor, and the mixture was stirred at 90 °C until completely dissolved. While maintaining the temperature, 26 mL of a 2.83 mol / L phosphoric acid solution was pumped into the reactor at a constant rate over 35 min to initiate precipitation (the molar ratio of the components in the solution was lithium:(manganese+iron):phosphorus = 3:1:1.05; manganese:iron = 1:1). The mixture was stirred at 300 r / min throughout the process. After the phosphoric acid solution was completely added, the reaction was allowed to continue for 1 h. Solid-liquid separation yielded lithium manganese iron phosphate (LiFe). 0.5 Mn 0.5 PO4). Lithium manganese iron phosphate was mixed with glucose and sintered at 500°C for 2 hours under an argon atmosphere to obtain carbon-coated lithium manganese iron phosphate cathode material.

[0044] Figure 6 Image (a) shows the XRD pattern of lithium manganese iron phosphate directly prepared from a conventional aqueous solution in Example 2. It can be seen that the diffraction peaks of the material correspond well with the standard card for lithium manganese iron phosphate, indicating that the prepared material is indeed lithium manganese iron phosphate and has excellent crystallinity. Figure 6 (b) is the XRD pattern of lithium manganese iron phosphate after carbon coating and sintering. The diffraction peak positions of the material correspond one-to-one with the standard card, and the diffraction peaks are very sharp. The peak intensities also correspond completely with the standard card, indicating that the crystallinity is very good after sintering at the temperature and time of Example 2.

[0045] Figure 7 Image (a) is a SEM image of lithium manganese iron phosphate directly prepared from a conventional aqueous solution in Example 2. It can be seen that the material has a complete structure consisting of regular nanoscale rhombic particles. Figure 7 (b) is a SEM image of carbon-coated lithium manganese iron phosphate after sintering. The material particles are intact and appear as nanospheres. These nanospheres can significantly improve the electrochemical performance of the material.

[0046] Figure 8 The rate performance of the carbon-coated lithium manganese iron phosphate cathode material prepared in Example 2 is shown. This material exhibits excellent rate performance, with a discharge specific capacity of 151.91 mAh / g at 0.1C and 101.12 mAh / g at a high current of 20C. Furthermore, when adjusted back to 0.1C, the capacity did not decrease.

[0047] Figure 9 The cycling performance of the carbon-coated lithium manganese iron phosphate cathode material prepared in Example 2 is as follows: after 200 cycles at 1C, the discharge specific capacity is 136.65 mAh / g, and the capacity retention rate is 98.15%.

[0048] The embodiments described above are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments. Any obvious improvements, substitutions or modifications that can be made by those skilled in the art without departing from the essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate / lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: (1) Lithium hydroxide and a soluble metal salt are added to the reactor, and a protective gas is introduced. An aqueous solution of glycine is added to the reactor, and the mixture is heated and stirred to dissolve. After complete dissolution, a phosphoric acid solution is added to the reactor at a constant rate to begin precipitation. After the phosphoric acid solution is completely pumped into the reactor, the mixture is aged, and solid-liquid separation is performed to obtain lithium iron phosphate (LiFePO4) or lithium manganese iron phosphate (LiFe). x Mn 1- x PO4, 0 < x < 1; Among them, the soluble metal salt is a soluble ferrous salt, or a mixture of soluble ferrous salt and manganese (II) salt; The ratio of lithium hydroxide, soluble metal salt and phosphoric acid solution is lithium:iron / (iron + manganese):phosphorus (1.1~3):1:(1~1.2), and the concentration of ferrous ions or the total concentration of manganese(II) and iron(II) ions in the solution is controlled at 0.5~1.2 mol / L; The volume of the glycine aqueous solution is calculated based on the theoretical amount of lithium iron phosphate / lithium manganese iron phosphate that can be generated, and it is added at a liquid-to-solid ratio (mL / g) of 5:1 to 10:

1. The concentration of the glycine aqueous solution is 1.5 to 4 mol / L. (2) The lithium iron phosphate (LiFePO4) or lithium manganese iron phosphate (LiFe) obtained in step (1) x Mn 1-x After mixing PO4 with a carbon source and sintering under a protective atmosphere, carbon-coated lithium iron phosphate or carbon-coated lithium manganese iron phosphate cathode materials are obtained.

2. The preparation method according to claim 1, characterized in that, In step (1), the concentration of phosphoric acid solution is 2.5~3.5 mol / L; the phosphoric acid solution is added at a constant rate over a period of 0.5~1h.

3. The preparation method according to claim 1, characterized in that, In step (1), the heating temperature is 80~100℃.

4. The preparation method according to claim 1, characterized in that, In step (1), the aging time is 0.5~2h.

5. The preparation method according to claim 1, characterized in that, In step (1), the stirring speed for both precipitation and aging reactions is 100~500 r / min.

6. The preparation method according to claim 1, characterized in that, In step (2), the sintering temperature is 400~600℃ and the sintering time is 1~5h.

7. The preparation method according to claim 1, characterized in that, The protective gas used in the dissolution, precipitation, or sintering process is nitrogen or an inert gas.

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