A preparation method of lithium manganese iron phosphate modified by ternary materials

Abstract

The present invention provides a preparation method of lithium iron manganese phosphate modified by ternary positive electrode materials. The preparation method mainly comprises: placing a Li source, a Fe source, a Mn source, a P source and a ternary material in a ball mill, and ball milling with anhydrous ethanol as a solvent; drying the obtained slurry, and placing it in a tubular furnace for pre-firing to obtain a metal element-doped lithium iron manganese phosphate precursor; mixing the precursor with a carbon source and sintering to obtain a doped modified lithium iron manganese acid positive electrode material, wherein the doped metal element is two elements of nickel and cobalt or three elements of nickel, cobalt and aluminum derived from the ternary positive electrode material. In this invention, the ternary positive electrode material acts as a grinding aid to reduce the particle size of the raw material, making the physical phase more uniform, and at the same time participates in doping modification, and the two work together to greatly improve the discharge specific capacity, cycle performance and rate performance.

Description

Technical Field

[0001] The invention relates to the field of preparation of olivine-type positive electrode materials for lithium-ion batteries, and in particular to lithium manganese iron phosphate modified by using ternary materials and a preparation method thereof. Background Art

[0002] Goodenough et al. proposed a new olivine-type cathode material LiFePO in 1997. 4 , due to its excellent electrochemical performance, stable structure, low cost and green safety, it has been widely used in the field of new energy. 4 The positive electrode material has a lower energy density (theoretical discharge capacity 170mA hg -1 , discharge voltage 3.45V) is difficult to meet the requirements of high energy density batteries for new energy vehicles and other applications. From a material perspective, lithium manganese iron phosphate has the same olivine structure, theoretical capacity and safety characteristics as lithium iron phosphate, but compared with Li + The electrode potential of / Li is 4.1V, which is much higher than the 3.4V of lithium iron phosphate. The high potential of 4.1V makes lithium manganese iron phosphate have the advantage of high energy density. If the same battery cell technology is used, the actual energy density of lithium manganese iron phosphate battery will be 20-40% higher than that of lithium iron phosphate, while being able to give full play to its high safety characteristics.

[0003] There are two problems with olivine-type cathode materials, namely, low lithium ion diffusion coefficient and poor electronic conductivity. Usually, structural adjustment and surface modification of the materials are required to improve these problems, such as morphology and size control, specific ion doping and conductive material surface coating. The applicant team designed and developed fluorine-doped carbon coating technology to study the surface modification of olivine-type lithium manganese iron phosphate cathode materials, and found that fluorine-doped carbon can significantly improve the rate performance and cyclability of lithium manganese iron phosphate cathodes (ACS Sustainable Chemistry & Engineering, 2017, 4637; ZL201610343543.2), laying the foundation for the proposal of this patent. The applicant team studied in detail the effects of various ion doping on the performance of high-voltage lithium cobalt phosphate cathode materials (Chemical Industry Journal, 2021, 72: 6380-6387), and found that co-doping with Mn and Fe can help stabilize the crystal structure, prevent structural collapse, and broaden the transmission channel of lithium ions, improve the lithium ion diffusion coefficient, and thus improve the cycle performance.

[0004] Therefore, it is necessary to explore more effective methods for regulating the structure of lithium manganese iron phosphate to further improve the electrochemical performance of lithium manganese iron phosphate. Summary of the invention

[0005] According to the background technology, the purpose of the present invention is to provide a lithium iron manganese phosphate positive electrode material with a simple preparation method, optimized structure and excellent electrochemical performance. Specifically, a method for modifying lithium iron manganese phosphate material using a ternary positive electrode material is provided, wherein the ternary positive electrode material is used as a grinding aid, and the lithium iron manganese phosphate precursor raw material is formed by mixing and ball milling with a Li source, a Fe source, a Mn source and a P source. In this method, the spherical ternary positive electrode material can not only play a grinding aid role, which greatly reduces the raw material particles before firing; the elements such as Ni, Co and Al in the ternary positive electrode material can also play a doping effect, and the combined effect of the two makes the final product lithium iron manganese phosphate have excellent electrochemical properties.

[0006] The method mainly includes the following steps.

[0007] Step S1, Li source, Fe source, Mn source, and P source are lithium carbonate, iron oxalate, manganese carbonate, and ammonium dihydrogen phosphate, respectively. The raw materials are weighed according to the stoichiometric coefficients, added into a ball mill with a certain amount of ternary positive electrode material, and anhydrous ethanol is used as a dispersant to prepare a precursor slurry by ball milling.

[0008] Step S2, placing the obtained slurry in a vacuum drying oven for drying.

[0009] Step S3, placing the powder in a tube furnace for pre-sintering to obtain metal element-doped lithium manganese iron phosphate precursor powder.

[0010] Step S4, mixing the precursor and the carbon source in a ball milling tank for ball milling.

[0011] Step S5, placing the obtained slurry in a vacuum drying oven for drying.

[0012] Step S6, placing the positive electrode material powder in a tube furnace for sintering to obtain a modified acid manganese iron lithium positive electrode material, wherein the doped metal is nickel and cobalt or nickel, cobalt and aluminum elements from the ternary positive electrode material.

[0013] In step S1, ternary positive electrode materials are added at 0-3.75% and mixed.

[0014] Furthermore, in step S1, the volume of the dispersant is controlled to account for 1 / 5 vol% of the ball mill.

[0015] Furthermore, in steps S1 and S4, the rotation speed is controlled to be 500-600 rpm, the ball milling time is controlled to be 8-10 h, and the ball-to-material ratio is controlled to be 8:1.

[0016] Furthermore, in steps S2 and S5, the drying temperature is controlled to be 60-80°C.

[0017] [Further, in step S3, the pre-firing temperature is controlled to be 300-350°C, the holding time is 2-3h, the heating rate is 5°C / min, and the gas is an argon-hydrogen mixed gas (95 / 5vol%).

[0018] Further, in step S4, the speed is controlled at and time.

[0019] Furthermore, in step S4, the total mass of the carbon source is controlled to be 20-25 wt % of the positive electrode material.

[0020] Further, in step S6, the sintering process includes a first stage sintering and a second stage sintering performed sequentially, wherein the temperature of the first stage sintering is 300-350°C, and the time of the first stage sintering is 2-3 hours; the temperature of the second stage sintering is 650-700°C, and the time of the second stage sintering is 6-8 hours, and the gas is an argon-hydrogen mixed gas (95 / 5 vol%).

[0021] Further, in step S6, the coating carbon content after sintering is 5.6-7wt%.

[0022] Furthermore, the chemical formula of the modified lithium iron phosphate can be expressed as LiFe 0.25 Mn （0.75-x-y-z） Ni x Co y Al z PO 4 , where 0≤x<0.015, 0≤y<0.045, 0≤z<0.01, and z can be 0.

[0023] Furthermore, the average particle size of the modified lithium manganese iron phosphate material is 100nm~500nm.

[0024] The advantages of the present invention are embodied in the following aspects.

[0025] 1. The present invention uses ternary cathode materials as grinding aids. During the ball milling process, the ternary materials collide and recombine with the lithium iron manganese phosphate raw materials, which greatly reduces the particle size of the raw materials by about 1 / 5 to 1 / 10, and can prevent the raw materials from agglomerating during the ball milling process; ultimately, elements such as nickel, cobalt, manganese (or aluminum) as doping elements effectively improve the electrochemical properties of lithium iron manganese phosphate.

[0026] 2. Compared with the one-step method for preparing lithium manganese iron phosphate material, the present invention adopts a two-step ball milling method to obtain lithium manganese iron phosphate material with controllable composition, uniformity and finer particle size.

[0027] 3. The ball milling and sintering process is simple, easy to operate and suitable for industrial production, and the prepared lithium manganese iron phosphate material has excellent electrochemical properties.

[0028] 4. The lithium iron manganese phosphate material prepared by the method described in the present invention is prepared into a lithium ion battery, which has a high discharge specific capacity at 0.1C, stable cycling at 1C, and still has a high discharge specific capacity at a high rate of 10C. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a SEM morphology of the lithium manganese iron phosphate positive electrode material prepared in Example 1.

[0030] Figure 2 This is the cycle diagram of the button half-cell at 1C for preparing modified lithium manganese iron phosphate positive electrode material.

[0031] Figure 3 This is a SEM morphology image of the modified lithium manganese iron phosphate positive electrode material prepared in Example 2.

[0032] Figure 4 This is the XRD spectrum of the lithium manganese iron phosphate positive electrode material prepared in Example 2.

[0033] Figure 5 This is a cycle diagram of a button-type half-cell of lithium manganese iron phosphate positive electrode material prepared in Example 2 at 10C.

[0034] Figure 6 These are the first cycle discharge curves of the button-type half-cell with different rates for the lithium manganese iron phosphate positive electrode material prepared in Example 2. DETAILED DESCRIPTION

[0035] Example 1.

[0036] Weigh the chemicals according to the ratio of Li source: Fe source: Mn source: P source of 1:0.25:0.75:1, weigh Li 2 CO 3 The mass is 738.9 mg, FeC 2 O 4 ·2H 2 O mass is 900 mg, MnCO 3 Mass is 1724mg, NH 4 H 2 PO 4 The mass is 2300mg, added to a 100ml ball mill, 10ml of anhydrous ethanol as a dispersant, ball milled for 10h, the slurry was dried, and pre-calcined at 300℃ for 2h; the precursor and mixed carbon source (sucrose and PVDF) were placed in a ball mill, ball milled for 10h, and dried at 60℃; during the carbonization process, the first stage sintering temperature was 300℃, the first stage sintering time was 2h, the second stage sintering temperature was 650℃, the second stage sintering time was 8h, and the gas was argon hydrogen mixed gas (95 / 5vol%). The morphology of the lithium iron phosphate positive electrode material is obtained, as shown Figure 1The above-prepared lithium manganese iron phosphate and lithium metal were assembled into a 2032 battery, and the discharge capacity at 1C was 126.7 mA h g -1 ,like Figure 2 shown.

[0037] Example 2.

[0038] The difference between this embodiment and embodiment 1 is that the ratio of Li source: Fe source: Mn source: P source is 1:0.25:0.725:1, the medicine is weighed, and 1.25% by mass fraction of ternary NCM622 positive electrode material is added, and the positive electrode material is prepared according to the above method to obtain a lithium iron phosphate positive electrode material, whose morphology is as follows Figure 3 As shown, the particle size is 1 / 10-1 / 5 times that of Example 1. The XRD spectrum is as follows Figure 4 As shown, it conforms to the crystal structure of lithium manganese iron phosphate. Figure 2 As shown, the discharge capacity at 1C is 138.4 mA h g -1 ;like Figure 5 As shown, after 500 cycles at 10C, the specific capacity remains 107 mA h g -1 , the capacity retention rate is 94.1%. Figure 6 This is the first-cycle discharge curve at different rates.

[0039] Example 3.

[0040] The difference between this embodiment and embodiment 1 is that the ratio of Li source: Fe source: Mn source: P source is 1:0.25:0.7:1, the drugs are weighed, and 2.5% by mass fraction of ternary NCM622 positive electrode material is added, and the positive electrode material is prepared according to the above method, such as Figure 2 As shown, the discharge capacity at 1C is 133.3 mA h g -1 .

[0041] Example 4.

[0042] The difference between this embodiment and embodiment 1 is that the ratio of Li source: Fe source: Mn source: P source is 1:0.25:0.7:1, the drugs are weighed, and 3.75% by mass fraction of ternary NCM622 positive electrode material is added, and the positive electrode material is prepared according to the above method, such as Figure 2 As shown, the discharge capacity at 1C is 88.4 mA h g -1 .

[0043] Example 5.

[0044] The difference between this embodiment and embodiment 1 is that the ratio of Li source: Fe source: Mn source: P source is 1:0.25:0.725:1, the drugs are weighed, and 1.25% by mass fraction of ternary NCM811 positive electrode material is added, the positive electrode material is prepared according to the above method, and finally a modified lithium manganese iron phosphate positive electrode material is prepared.

[0045] Example 6.

[0046] The difference between this embodiment and embodiment 1 is that the ratio of Li source: Fe source: Mn source: P source is 1:0.25:0.725:1, the drugs are weighed, and 1.25% by mass fraction of ternary NCM111 positive electrode material is added, the positive electrode material is prepared according to the above method, and finally a modified lithium manganese iron phosphate positive electrode material is prepared.

[0047] Example 7.

[0048] The difference between this embodiment and embodiment 1 is that the ratio of Li source: Fe source: Mn source: P source is 1:0.25:0.725:1, the medicine is weighed, and 1.25% by mass fraction of ternary NCA positive electrode material is added, and the positive electrode material is prepared according to the above method, and finally a modified lithium manganese iron phosphate positive electrode material is prepared.

[0049] Comparative example 1.

[0050] Weigh the chemicals according to the ratio of Li source: Fe source: Mn source: P source of 1:0.25:0.75:1, weigh Li 2 CO 3 The mass is 738.9 mg, FeC 2 O 4 ·2H 2 O mass is 900 mg, MnCO 3 Mass is 1724mg, NH 4 H 2 PO 4 A mass of 2300 mg, 20wt% sucrose is added to a 100ml ball mill, 10ml anhydrous ethanol is used as a dispersant, ball milled for 10 hours, dried at 60°C, and calcined. The temperature of the first sintering stage is 300°C, and the time of the first sintering stage is 2 hours; the temperature of the second sintering stage is 650°C, and the time of the second sintering stage is 6 hours. The gas is an argon-hydrogen mixed gas (95 / 5vol%).

[0051] Research has found that due to the use of single ball milling and the absence of grinding aids, the lithium manganese iron phosphate particles are coarse and the coating is uneven. In addition, the failure to use a mixed carbon source makes lithium ion transmission difficult, thereby reducing the electrochemical performance.

Claims

1. A method for preparing lithium manganese iron phosphate modified by ternary materials, characterized in that: The method comprises the following steps: Step S1, weighing Li source, Fe source, Mn source, and P source raw materials according to the stoichiometric ratio of lithium iron manganese phosphate, adding them together with an appropriate amount of ternary positive electrode material into a ball milling jar, using anhydrous ethanol as a solvent, and performing ball milling to prepare a precursor slurry; Step S2, placing the obtained slurry in a vacuum drying oven for drying; Step S3, placing the powder in a tube furnace for pre-sintering to obtain a doped and modified lithium manganese iron phosphate precursor powder; Step S4, mixing the precursor with an appropriate amount of carbon source and placing them in a ball mill for ball milling; Step S5, placing the obtained slurry in a vacuum drying oven for drying; Step S6, placing the positive electrode material powder in a tube furnace for sintering to obtain a modified lithium manganese iron phosphate positive electrode material; The ternary positive electrode material is a ternary NCM622 positive electrode material, and the added mass fraction is 1.25%.

2. The method according to claim 1, characterized in that In step S1, the amount of raw materials added is controlled to account for 10-30% of the volume of the ball mill.

3. The method according to claim 2, characterized in that: In step S1, the amount of raw materials added is controlled to account for 15-20% of the volume of the ball mill.

4. The method according to claim 1, characterized in that In steps S1 and S4, the ball milling time is controlled to be 6-15 hours.

5. The method according to claim 4, characterized in that In steps S1 and S4, the ball milling time is controlled to be 8-10 hours.

6. The method according to claim 1, characterized in that In step S3, the pre-firing temperature is controlled to be 250-450°C.

7. The method according to claim 6, characterized in that In step S3, the pre-firing temperature is controlled to be 300-350°C.

8. The method according to claim 1, characterized in that In step S3, the holding time is 2-5 hours, the heating rate is 3-10°C / min, and the protective gas is an argon-hydrogen mixed gas with a volume ratio of 95:

5.

9. The method according to claim 1, characterized in that In step S3, the holding time is 3-4 hours and the heating rate is 4-6°C / min.

10. The method according to claim 1, characterized in that In step S6, the sintering process includes a first stage sintering and a second stage sintering performed sequentially, wherein the temperature of the first stage sintering is 250-450°C, and the time of the first stage sintering is 2-5h; the temperature of the second stage sintering is 600-700°C, and the time of the second stage sintering is 6-8h, and the gas is an argon-hydrogen mixed gas with a volume ratio of 95:5vol%.

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