A metal organic framework coated lithium supplementing agent, a preparation method and application thereof

Abstract

This invention discloses a lithium replenishing agent coated with a metal-organic framework (MOF), its preparation method, and its application, belonging to the field of lithium-ion batteries. The lithium replenishing agent comprises a matrix and a coating layer. The matrix is ​​lithium ferrite, and the coating layer is a MOF with a thickness of 0.5-10 nm. The MOF is prepared using fluorine-containing and carboxyl-containing compounds as ligands and zirconium tetrachloride as the metal source via a solvothermal method. The preparation method includes the following steps: preparing and activating a lithium ferrite intermediate; adding the activated lithium ferrite intermediate to a buffer solution containing dissolved dopamine hydrochloride to prepare lithium ferrite intermediate @PDA; performing in-situ growth of the MOF to construct a hydrophobic MOF-coated precursor; adding a lithium source to the precursor and performing programmed sintering to obtain the lithium replenishing agent. The lithium replenishing agent provided by this invention achieves excellent air stability, low gas production, and excellent conductivity.

Description

Technical Field

[0001] This invention relates to lithium-ion batteries, and more particularly to a metal-organic framework-coated lithium replenishing agent, its preparation method, and its application. Background Technology

[0002] In high-end electric vehicles and large-scale energy storage systems, there are stringent requirements for long battery life and high energy density. However, during the first charge and discharge process of lithium-ion batteries, an SEI film is formed on the negative electrode, which irreversibly loses a portion of the active lithium, directly leading to a decrease in the actual usable capacity of the battery. In order to actively compensate for this loss and improve battery performance, lithium replenishment technology has emerged. Among them, lithium iron phosphate, as a positive electrode lithium replenishment agent, has attracted much attention due to its high specific capacity and low cost. However, it has the following technical bottlenecks: (1) Poor conductivity: low intrinsic electronic conductivity, such as the conductivity of Li5FeO4 is about 10. -9 S / cm, resulting in high internal resistance and poor rate performance of the battery; (2) Poor stability: the material is easy to react with H2O and CO2 in the air to generate impurities such as LiOH and Li2CO3, which leads to battery capacity decay; (3) Severe gas filling: during the first charge, the O2 and CO2 generated by the decomposition of the lithium replenishment agent will cause the battery to swell, affecting safety. Summary of the Invention

[0003] Objectives of the invention: The objective of this invention is to provide a lithium replenishing agent coated with a metal-organic framework that exhibits excellent air stability, low gas production, and excellent conductivity; another objective of this invention is to provide a method for preparing the aforementioned lithium replenishing agent; a third objective of this invention is to provide an application of the aforementioned lithium replenishing agent in lithium-ion batteries.

[0004] Technical solution: The metal-organic framework-coated lithium supplement of the present invention includes a matrix and a coating layer, wherein the matrix is ​​lithium ferrite, the coating layer is a metal-organic framework, and the thickness of the coating layer is 0.5-10 nm.

[0005] Preferably, the thickness of the coating layer is 2-6 nm.

[0006] Preferably, the metal-organic framework is prepared by a solvothermal method using fluorinated compounds and carboxyl compounds as ligands and zirconium tetrachloride as the metal source; more preferably, the fluorinated compound is tetrafluoroterephthalic acid and the carboxyl compound is 4,4'-biphenyldicarboxylic acid.

[0007] The method for preparing the lithium supplement agent according to the present invention includes the following steps: (1) Preparation of lithium ferrite intermediate; (2) Activate the lithium ferrite intermediate to obtain lithium ferrite intermediate activated material; (3) Add the lithium ferrite intermediate activation material to a buffer solution containing dissolved dopamine hydrochloride to prepare lithium ferrite intermediate @PDA; (4) In situ growth of metal-organic frameworks to construct hydrophobic metal-organic framework-coated precursors: (5) Secondary lithium replenishment and carbonization: Lithium source is added to the precursor and sintering is performed to obtain lithium replenishment agent.

[0008] In step (1), the molar ratio of lithium to iron is 1-3:1; the iron source in lithium ferrite is ferric oxide, and the lithium source is lithium hydroxide.

[0009] In the activation process described in step (2), a mixed solution of citric acid and ethanol is used as the activation solution.

[0010] In step (5), after adding lithium source, the molar ratio of total lithium to iron is 5.1-6:1.

[0011] The application of the lithium replenishing agent described in this invention in a soft-pack battery includes the following steps: preparing a positive electrode sheet by using a positive electrode active material, a lithium replenishing agent, a conductive agent, and a binder as a positive electrode slurry; preparing a negative electrode sheet by using a negative electrode active material, a conductive agent, a thickener, and a binder as a negative electrode slurry; and assembling the positive electrode sheet, the negative electrode sheet, the electrolyte, and the separator into a lithium-ion soft-pack battery as required.

[0012] The application of the lithium replenishing agent described in this invention in a coin cell includes the following steps: preparing an electrode by combining a lithium replenishing agent, a conductive agent, and a binder, and assembling it into a coin cell, wherein the counter electrode is a lithium metal sheet.

[0013] Invention Principle: This invention aims to provide a lithium replenishment material for batteries. It involves introducing a metal-organic framework (MOF) containing fluorine functional groups into lithium-poor materials, loading a hydrophobic MOF layer onto the surface of the lithium-poor material using a solvothermal method, and then adding a lithium source followed by a second sintering process. This results in a lithium replenishment material with excellent air stability, low gas production, and excellent conductivity. Firstly, lithium-rich lithium ferrite is prepared by constructing intermediates, preferentially generating lithium-poor intermediate nuclei at low temperatures. This allows for control of the nucleus size, resulting in uniformly sized lithium-rich lithium ferrite. Furthermore, the intermediate material is stable and, after activation, can be directly used as a matrix in the precursor solution for MOF synthesis. The solvothermal method allows the fluorine-functionalized MOF to grow in situ on the lithium-poor material, avoiding the stringent environmental requirements associated with operations on lithium-rich lithium ferrite. Second, the MOF introduced in this invention has a porous structure, which provides lithium channels for secondary lithium replenishment and space for sintering and phase formation, allowing lithium oxide to diffuse through the channels to the crystal nuclei and react with intermediates to form lithium-rich lithium ferrite. Furthermore, the high specific surface area and functionalizable channels of the MOF material enable it to selectively adsorb and lock gas molecules (such as O2 and CO2) generated during the charging and discharging process of the lithium replenishment material, preventing them from triggering subsequent chain reactions. Third, this invention introduces fluorine functional groups into the MOF material through fluorine-containing ligands, which helps increase the hydrophobicity of the MOF material. This hydrophobic MOF material, coating the surface of lithium-rich lithium ferrite, can significantly improve the air stability of the lithium replenishment material. Fourth, this invention involves the controlled thermal decomposition of MOF-coated precursor materials under an inert atmosphere. The organic ligands in the MOF are carbonized into a porous carbon matrix, while the zirconium clusters are converted into ZrO2 and uniformly embedded in the carbon skeleton. This preserves the high specific surface area and porous structure of the MOF material, which can significantly improve the air stability of the material, effectively suppress oxygen release during cycling, and improve the conductivity of the material. Ultimately, a hydrophobic, structurally stable, and highly conductive core-shell lithium replenishment material is obtained.

[0014] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The present invention loads MOF material onto the surface of lithium iron ferrite, which helps to suppress the generation of gas by the lithium supplement during charging and discharging. The gas generation is as low as 10.1 mL, which is 60.5% lower than that of the lithium supplement without MOF material; (2) The present invention introduces fluorine functional groups into MOF material, which can significantly improve the air stability of the lithium supplement. The water absorption rate is as low as 2.0 μg / (g·s), which is 65.5% lower than that of the lithium supplement without fluorine functional groups. Attached Figure Description

[0015] Figure 1 Transmission electron microscopy image of the lithium replenishing agent prepared in Example 1; Figure 2Transmission electron microscopy image of the lithium replenishing agent prepared in Example 2; Figure 3 The image shows a transmission electron microscope (TEM) image of the lithium replenishing agent prepared in Example 3. Detailed Implementation

[0016] The technical solution of the present invention will be further described below with reference to the embodiments.

[0017] Example 1 The metal-organic framework-coated lithium supplement of the present invention includes a matrix and a coating layer. The matrix is ​​lithium ferrite, the coating layer is MOF, and the thickness of the coating layer is 3.6 nm.

[0018] The method for preparing the lithium supplement agent according to the present invention includes the following steps: (1) Preparation of lithium ferrite intermediate: Weigh lithium hydroxide monohydrate and battery-grade ferric oxide according to the molar ratio of Li:Fe 1.5:1. Add lithium hydroxide monohydrate to an appropriate amount of deionized water at room temperature and dissolve it completely. Then add ferric oxide and stir to prepare a uniform slurry. Put the slurry into a sand mill for sand milling. After the slurry particle size is reduced to 0.5μm, collect the slurry and spray dry it to obtain powder. Sinter the powder at 550℃ for 5 hours in air atmosphere to obtain lithium ferrite intermediate.

[0019] (2) Preparation of lithium ferrite intermediate activated material: Take 50g of lithium ferrite intermediate obtained in step (1) and sonicate it in 100mL of citric acid-ethanol mixed solution (citric acid to ethanol volume ratio of 1:3) for 30min. After centrifugation, dry it in an oven at 60℃ for 3h to obtain lithium ferrite intermediate activated material. The purpose of this operation is to remove contaminants from the material surface and increase the surface roughness. (3) Preparation of lithium ferrite intermediate @PDA: Dissolve 1.25g of dopamine hydrochloride in 250mL of 10mM Tris buffer, then add 50g of lithium ferrite intermediate activating material prepared in step (2), stir at 25℃ for 5 hours, centrifuge to obtain lithium ferrite intermediate @PDA. The purpose of polydopamine (PDA) coating is to introduce active sites. PDA can form a thin film on the surface of the material through self-polymerization. This film contains abundant hydroxyl and amino functional groups, which can themselves serve as active sites.

[0020] (4) Construction of hydrophobic MOF-coated precursor: 0.183 g benzoic acid (1.5 mmol) was added to 100 mL N,N-dimethylamide and sonicated until completely dissolved to obtain a transparent solution; 0.536 g tetrafluoroterephthalic acid (2.25 mmol), 0.545 g 4,4'-biphenyl dicarboxylic acid (2.25 mmol) and 0.350 g zirconium tetrachloride (1.5 mmol) were weighed into the above transparent solution and sonicated for half an hour until completely dissolved. Then, 50 g of lithium ferrite intermediate @PDA obtained in step (3) was added, heated at 100 °C for 20 h, washed three times with ethanol, and dried at 60 °C. The MOF was then generated in situ on the surface of the lithium ferrite intermediate, and the hydrophobic MOF-coated precursor was obtained. The in situ growth process of MOF is as follows: hydroxyl and amino functional groups are used to synthesize the metal ions required for MOF (such as Zr in the MOF of this scheme). 4+ These anchored metal ions have extremely strong chelating ability, forming high-concentration "hot spots" on the surface of the PDA substrate. This greatly promotes the heterogeneous nucleation of MOF crystals, which means that MOF crystals tend to nucleate and grow uniformly on the PDA coating rather than spontaneously nucleating in the solution to form stray crystals. This is beneficial for forming a dense and uniform MOF layer.

[0021] (5) Secondary lithium replenishment and carbonization: Lithium oxide is added to the precursor obtained in step (4) so ​​that the total Li element and Fe element molar ratio is Li:Fe=5.1:1. After mixing, it is sintered under nitrogen atmosphere. The temperature is raised to 520℃ at 3℃ / min and held for 3h, then raised to 750℃ and held for 10h to finally obtain the hydrophobic MOF-coated lithium replenishment agent Li5FeO4@MOF.

[0022] The application of the lithium replenishing agent described in this invention in a soft-pack battery includes the following steps: The positive electrode active material LiFePO4 (LFP), the lithium replenishing agent prepared in Example 1, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are uniformly mixed in an N-methylpyrrolidone (NMP) solution at a mass ratio of 95:3:1:1 to obtain a positive electrode slurry. This slurry is then prepared into a positive electrode sheet through homogenization, coating, drying, and cutting operations. The negative electrode active material graphite, the conductive agent Super P, the thickener carboxymethyl cellulose (CMC), and the binder styrene-butadiene rubber (SBR) are uniformly mixed in deionized water at a mass ratio of 95:2:0.5:2.5 to obtain a negative electrode slurry. This negative electrode slurry is coated onto a current collector copper foil, and after drying, rolling, and a second drying process, a negative electrode sheet is formed. The positive electrode sheet, negative electrode sheet, electrolyte, and separator are then assembled into a lithium-ion soft-pack battery as required.

[0023] The application of the lithium replenishing agent described in this invention in a coin cell includes the following steps: homogenizing the lithium replenishing agent, conductive agent Super P and binder PVDF obtained in Example 1 into a slurry at a mass ratio of 8:1:1 to prepare an electrode and assembling it into a coin cell in a glove box, wherein the counter electrode is a lithium metal sheet.

[0024] Example 2 The similarities with Example 1 will not be repeated here. The difference is that the amounts of benzoic acid, tetrafluoroterephthalic acid, 4,4'-biphenyl dicarboxylic acid and zirconium tetrachloride added in step (4) are changed to 3 mmol, 4.5 mmol, 4.5 mmol and 3 mmol, respectively.

[0025] Example 3

[0026] The similarities with Example 1 will not be repeated here. The difference is that the amounts of benzoic acid, tetrafluoroterephthalic acid, 4,4'-biphenyl dicarboxylic acid and zirconium tetrachloride added in step (4) are changed to 0.75 mmol, 1.125 mmol, 1.125 mmol and 0.75 mmol, respectively.

[0027] Example 4

[0028] The similarities with Example 1 will not be repeated here. The difference is that the maximum temperature of the sintering process in step (5) is increased to 900°C.

[0029] Example 5

[0030] The similarities with Example 1 will not be repeated here. The difference is that the maximum temperature of the sintering process in step (5) is reduced to 600°C.

[0031] Comparative Example 1 (1) Preparation of lithium ferrite intermediate: Same as step (1) in Example 1.

[0032] (2) Secondary lithium replenishment and sintering: Lithium oxide is added to the lithium ferrite intermediate obtained in step (1) so that the total Li element and Fe element molar ratio is Li:Fe=5.1:1. After mixing, sintering is carried out under nitrogen atmosphere. The temperature is raised to 520℃ at 3℃ / min and held for 3h, then raised to 750℃ and held for 10h to obtain lithium-rich lithium ferrite.

[0033] (3) Solid carbon coating: Add 5 wt% carbon black to the lithium iron ferrite obtained in step (2), mix evenly by ball milling, transfer to nitrogen atmosphere and sinter at 450℃ for 5h to obtain solid carbon coated lithium iron ferrite material.

[0034] Comparative Example 2 The similarities with Example 1 will not be repeated here. The difference is that the ligand tetrafluoroterephthalic acid (2.25 mmol) in step (4) is replaced with terephthalic acid (2.25 mmol).

[0035] Residual alkali test method: Using anhydrous ethanol as solvent, the hydroxide and carbonate contents of the lithium supplement are calculated by acid-base titration, and the total amount of LiOH and Li2CO3 is calculated.

[0036] Powder resistance test: The resistance of the lithium supplement is tested using a powder resistance meter.

[0037] Moisture resistance test method: Place 5g of lithium supplement in a constant temperature and humidity chamber (25℃, relative humidity 30±2%) for 20min, record the values ​​after moisture absorption, and calculate the moisture absorption rate of each lithium supplement using the following formula. Moisture absorption rate calculation formula: V=(w t -w0) / w0*t, unit: μg / (g·s). Where t represents time, w0 represents the initial mass, and w t This represents the mass at time t.

[0038] The test results are shown in Table 1.

[0039] Electrochemical performance tests were conducted on coin cells assembled using the lithium replenishing agents obtained in various experiments. The test voltage range was 2.5-4.2V, and the charge / discharge rate was 0.05C. The test results are shown in Table 1, “Lithium Replenishment Capacity”.

[0040] For the pouch cells assembled using the lithium replenishing agents obtained in various experiments, a formation gas production test was conducted at 45°C. The steps were as follows: constant current charging at 0.1C to 3.2V; constant current charging at 0.2C to 3.4V; constant current and constant voltage charging at 0.33C to 3.55V, with a cutoff current of 0.05C; constant current charging at 0.05C to 4.05V; constant current charging at 0.02C to 4.05V; and constant current charging at 0.01C to 4.05V. The battery volume V1 was measured using the water displacement method; vacuuming was performed for 8 seconds, followed by heat sealing at 195°C for 5 seconds; the volume V2 after venting was measured using the water displacement method, yielding the formation gas production V. 产气 =V1-V2, the test results are shown in Table 1 under "Chemicalization and Gas Production".

[0041] Table 1 Test results of each experiment According to the experimental data in Table 1, the lithium supplement prepared by this method has excellent air stability, low gas production, and excellent conductivity.

[0042] As can be seen from the test results of Examples 1-5 and Comparative Example 1 in Table 1, this application can effectively reduce the residual alkali, powder resistance and water absorption rate of the material by combining the lithium replenishing agent lithium iron ferrite material with a hydrophobic metal-organic framework. In addition, the formation gas production of the battery assembled using the lithium replenishing agent prepared by this invention is also effectively reduced, and the lithium replenishing capacity is also effectively utilized.

[0043] As can be seen from the test results of Examples 1-5 and Comparative Example 2 in Table 1, this application can effectively improve the hydrophobicity of metal-organic framework materials by introducing fluorine functional groups, thereby improving the moisture resistance of lithium supplementation agents.

[0044] As can be seen from the test results of Examples 1 and 2 in Table 1, increasing the proportion of hydrophobic organometallic framework in the composite material can further reduce the residual alkali, powder resistance, water absorption rate and formation gas production of the material, but the corresponding proportion of active material decreases, thus reducing the lithium replenishment capacity.

[0045] As can be seen from the test results of Examples 1, 4 and 5 in Table 1, the secondary sintering temperature also has a significant impact on the composite lithium replenishing agent. If the sintering temperature is too high (above 900℃), the porous structure of the hydrophobic MOF will partially collapse, and the gas generation capacity of the adsorbent material will decrease. If the sintering temperature is too low (below 600℃), the crystallinity of the material will be affected, thereby leading to a decrease in the lithium replenishing capacity of the material.

[0046] according to Figure 1-3 It can be seen that the coating thicknesses of the lithium replenishing agents prepared in Examples 1-3 are 3.6-3.7 nm, 5.4-5.5 nm, and 2.3-3.0 nm, respectively. (Referring to Table 1 and...) Figure 1-3 It is known that the coating thickness of MOF in composite materials has a significant impact on material performance: increasing the coating amount can further reduce the residual alkali, powder resistivity, water absorption rate and formation gas production of the material, but the corresponding proportion of active material decreases, thus reducing the lithium replenishment capacity; while excessively reducing the coating amount will correspondingly increase the residual alkali, powder resistivity, water absorption rate and formation gas production of the composite material, which will also affect the material's capacity.

Claims

1. A lithium supplement agent coated with a metal-organic framework, characterized in that, It includes a substrate and a coating layer, wherein the substrate is lithium ferrite, the coating layer is a metal-organic framework, and the thickness of the coating layer is 0.5-10 nm.

2. The lithium supplement agent according to claim 1, characterized in that, The metal-organic framework is prepared by a solvothermal method using fluorine-containing compounds and carboxyl-containing compounds as ligands and zirconium tetrachloride as the metal source.

3. The lithium supplement agent according to claim 2, characterized in that, The fluorinated compound is tetrafluoroterephthalic acid, and the carboxyl compound is 4,4'-biphenyldicarboxylic acid.

4. A method for preparing the lithium supplement agent according to any one of claims 1-3, characterized in that, Includes the following steps: (1) Preparation of lithium ferrite intermediate; (2) Activate the lithium ferrite intermediate to obtain lithium ferrite intermediate activated material; (3) Add the lithium ferrite intermediate activation material to a buffer solution containing dissolved dopamine hydrochloride to prepare lithium ferrite intermediate @PDA; (4) In situ growth of metal-organic frameworks to construct precursors coated with hydrophobic metal-organic frameworks; (5) Secondary lithium replenishment and carbonization: Lithium source is added to the precursor and sintering is performed to obtain lithium replenishment agent.

5. The preparation method according to claim 4, characterized in that, In step (1), the molar ratio of lithium to iron is 1-3:1; the iron source in lithium ferrite is ferric oxide, and the lithium source is lithium hydroxide.

6. The preparation method according to claim 4, characterized in that, In the activation process described in step (2), a mixed solution of citric acid and ethanol is used as the activation solution.

7. The preparation method according to claim 4, characterized in that, In step (5), after adding lithium source, the molar ratio of total lithium to iron is 5.1-6:

1.

8. The preparation method according to claim 4, characterized in that, In step (5), the sintering temperature is 600-900℃.

9. The application of a lithium replenishing agent according to any one of claims 1-3 or a lithium replenishing agent obtained by the preparation method according to any one of claims 4-8 in a lithium-ion battery.

10. The application according to claim 9, characterized in that, The lithium-ion battery is either a pouch cell or a button cell.

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