Preparation method of high-purity LiF, coated diaphragm containing LiF and preparation method of coated diaphragm

Abstract

According to the preparation method of the high-purity LiF, provided by the invention, the oxygen content of the obtained high-purity LiF in a detection limit range of an energy dispersive spectrometer (EDS) or a wavelength dispersive spectrometer (WDS) is less than 100ppm. In addition, the invention further provides a coating diaphragm, which comprises a base film, a coating layer and a coating layer, the coating layer is formed on at least one side of the base film and comprises the high-purity LiF. In addition, the invention further provides a preparation method of the coated diaphragm.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery, in particular to a preparation method of high-purity LiF, and a coated separator containing the LiF and a preparation method thereof. BACKGROUND

[0002] With the exponential growth of demand for high-energy-density batteries in new energy vehicles, portable electronic devices, and energy storage systems, lithium metal batteries have become a key research direction for the next generation of energy storage technology due to the intrinsic advantages of their negative electrode materials: ultra-high theoretical specific capacity (3860 mAh / g, about 10 times that of graphite negative electrode) and the lowest electrochemical potential (-3.04 V vs. standard hydrogen electrode). However, lithium metal negative electrodes face key problems such as uncontrollable growth of lithium dendrites and dynamic instability of the solid-state electrolyte interface (SEI) during the cycling process, leading to decreased battery coulomb efficiency, shortened cycle life, and increased risk of thermal runaway. Although traditional polyolefin separators (such as PE and PP) have the advantage of chemical inertness, their inherent defects (electrolyte wetting angle > 120 degrees, lithium ion transference number < 0.3, and 150°C thermal shrinkage rate > 20%) severely restrict the ability to inhibit dendrite penetration and interfacial side reactions.

[0003] To address the above technical bottlenecks, the academic community has proposed a solution centered on the functional modification of separators. Existing research has mainly focused on the introduction of inorganic ceramic coatings (such as Al2O3 and boehmite), which can improve the thermal stability of the separator (150°C thermal shrinkage rate < 5%) and the affinity of the electrolyte (contact angle reduced to below 30 degrees). However, the nanoparticles can easily cause pore blockage (pore rate reduced by 40% to 60%), leading to blocked lithium ion transport paths (electrical conductivity reduced to 0.5 x 10 -3 S / cm), which in turn accelerates local nucleation of dendrites. It is worth noting that current commercial coating materials are still limited to oxide systems (aluminum oxide accounts for more than 80%), which cannot meet the subsequent development needs. SUMMARY

[0004] Lithium metal batteries face multiple technical challenges in the commercialization process. The most prominent problem is that lithium metal anode causes lithium ion to be deposited abnormally in local areas due to interface inhomogeneity during the cycling process, forming needle-shaped, mossy or dendritic crystals. These dendrites not only increase the interface of side reactions between the electrode and the electrolyte, leading to a significant decrease in coulombic efficiency (CE), but also can penetrate the separator and cause internal short circuit of the battery, greatly increasing the risk of thermal runaway. Although traditional polyolefin separators (such as PE and PP) have the advantage of chemical inertness, their inherent defects such as poor electrolyte wettability (water contact angle > 120 degrees), low lithium ion transference number (<0.3) and insufficient thermal stability (150℃ thermal shrinkage rate > 20%) make it difficult to effectively inhibit dendrite growth and interface side reactions. In addition, the purity problem of lithium fluoride (LiF) has long been ignored. The impurity content of LiF prepared by traditional process is high, which is difficult to meet the strict requirements of lithium metal battery on material purity.

[0005] To solve the above problems, the present application proposes an innovative solution. By using high-purity lithium fluoride (LiF) as the core component of the ceramic coating, the function of the separator is reconstructed from multiple dimensions: (1) The strong electronegative surface of LiF (ζ potential -45mV) regulates the deposition orientation of lithium ions through electrostatic repulsion, combined with optimized lithium ion flow distribution, achieving three-dimensional spatial inhibition of dendrite growth; (2) The excellent affinity of LiF for electrolyte significantly improves the wettability of the separator, while inhibiting the electron tunneling effect and enhancing the stability of the solid-state electrolyte interface (SEI); (3) The high melting point of LiF (845℃) and the self-passivation effect of fluorides work together to reduce the 150℃ thermal shrinkage rate of the separator to below 4%, significantly improving the thermal safety of the battery. Most importantly, the present application breaks through the bottleneck of LiF material purity and develops a preparation process with purity >99.9%, solving the problem of battery performance degradation caused by impurities in traditional LiF.

[0006] The first aspect of the present application is to propose a preparation method of high-purity LiF, which comprises: adding lithium salt to a dispersion solution to obtain a uniform suspension; adding a hydrofluoric acid aqueous solution to the uniform suspension for reaction; and separating the precipitate obtained by reaction, and washing the precipitate with anhydrous ethanol and / or ultrapure water at least once, and then drying the precipitate to obtain high-purity LiF.

[0007] The second aspect of the present application is to propose a preparation method of high-purity LiF, which comprises: dissolving lithium salt in ultrapure water to prepare a lithium salt solution; adding an NH4F solution to the lithium salt solution for reaction; and centrifuging the precipitate obtained by reaction, and sequentially washing the precipitate with anhydrous ethanol, an NH4HCO3 solution and ultrapure water, and drying the precipitate to obtain high-purity LiF.

[0008] The third aspect of the present application is to propose a high-purity LiF with an oxygen content of less than 100ppm.

[0009] A fourth aspect of the present application provides a coated separator, comprising: a base film; and a coating layer formed on at least one side of the base film and comprising high-purity LiF, the high-purity LiF having an oxygen content of less than 100 ppm.

[0010] A fifth aspect of the present application provides a method for preparing a coated separator, comprising: providing a coating slurry, the coating slurry comprising high-purity LiF, the high-purity LiF having an oxygen content of less than 100 ppm; and coating the coating slurry on at least one side of a base film and drying.

[0011] Compared with conventional polyolefin separators, the LiF-based separator of the present application achieves a leap in key performance indicators: electrolyte wettability is improved from a hydrophobic state of ~120 degrees to a highly hydrophilic state, lithium ion migration efficiency is significantly improved due to ion flow uniformization, and thermal shrinkage rate is reduced from more than 20% to less than 4%. More importantly, through the synergistic mechanism of electrochemical regulation and physical barrier, the dendrite suppression ability is upgraded from a simple physical barrier to a three-dimensional dynamic constraint, while considering the interface stability and thermal runaway protection. In summary, through material innovation and process breakthrough of the LiF-based separator, the present application systematically solves the core problems of uncontrollable growth of lithium metal battery dendrites, insufficient thermal safety, and lack of high-purity materials, and provides key technical support for the commercialization of lithium metal batteries. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 SEM photograph showing the LiF morphology of Example A7;

[0013] Figure 2 SEM photograph showing the LiF morphology of Comparative Example A1;

[0014] Figure 3 XRD results comparing the phase composition and crystal structure of LiF of Example A7, Example A12, Example B1, and Comparative Example A1;

[0015] Figure 4 DSC results showing the thermal stability of LiF of Example A7;

[0016] Figure 5 TGA results showing the thermal stability of LiF of Example A7. DETAILED DESCRIPTION

[0017] The specific embodiments of the present application are described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are merely intended to illustrate and explain the present application, and are not intended to limit the present application.

[0018] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as the exact dimensions are not critical to the invention. Any numeric range recited is intended to include all values from the lower value to the upper value, inclusive of both values, and to also include any value approximately or roughly between the lower value and the upper value. Numeric ranges can be expressed as "from X to Y," where X and Y are numbers. Numeric ranges can be expressed as "from X to Y," where X is a number and Y is infinity. Numeric ranges can be expressed as "from X to infinity," where X is a number. Numeric ranges can be expressed as "from X to Y," where X is a number and Y is a number, and "X or more" and "Y or less," as well as "X to Y," inclusive of both values.

[0019] A first embodiment of the present invention relates to a method for preparing high purity LiF, the method is specifically operated as follows:

[0020] First, a lithium salt is added to a dispersion solution to obtain a uniform suspension. Specifically, the lithium salt can be added to the dispersion solution, and a high-shear dispersion emulsifier is used to continuously disperse to form a uniform suspension. The suspension is transferred to a high-pressure reaction kettle and argon is introduced to replace the air in the kettle to ensure that the oxygen content is <5 ppm. In addition, the lithium salt is preferably one or more of Li2CO3, LiOH; the dispersion solution is preferably an ethanol-water mixed solvent, and the volume ratio between ethanol and water is 1:(0.5 to 2), and the volume ratio between the two is more preferably 1:(0.5, 1, 2 Any value or range formed by any two values).

[0021] Then, an aqueous hydrofluoric acid solution is added to the uniform suspension for reaction. Specifically, the hydrofluoric acid solution can be added to the reaction kettle at a rate of 0.1 mL / min to 1 mL / min, while maintaining the stirring rate at 100 rpm to 1000 rpm. After the addition is completed, the reaction system is sealed and continuously stirred in a constant temperature water bath for 6 h to 168 h. In addition, the addition rate of the hydrofluoric acid solution is preferably any one of 0.1 mL / min, 0.2 mL / min, 0.3 mL / min, 0.4 mL / min, 0.5 mL / min, 0.6 mL / min, 0.7 mL / min, 0.8 mL / min, 0.9 mL / min, 1 mL / min or a range formed by any two of them; the stirring rate of the hydrofluoric acid solution is preferably any one of 100 rpm, 200 rpm, 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1000 rpm or a range formed by any two of them; the continuous stirring time in the constant temperature water bath is preferably any one of 6 h, 24 h, 72 h, 120 h, 144 h, 168 h or a range formed by any two of them; the temperature of the constant temperature water bath is preferably 25°C to 120°C, more preferably any one of 25°C, 60°C, 90°C, 120°C or a range formed by any two of them.

[0022] In addition, the aqueous solution of hydrofluoric acid can be prepared by the following method: dissolving HF gas in deionized water to obtain the aqueous solution of hydrofluoric acid. Specifically, the HF gas can be passed into a pre-cooled circulating gas absorption device at a flow rate of 0.1 L / min to 1 L / min, and the HF gas is fully dissolved in the deionized water by a three-stage bubbling method. After the solution concentration reaches 30 wt% to 50 wt%, residual HF gas is removed by purging with high-purity argon gas to obtain the aqueous solution of hydrofluoric acid. In addition, the flow rate of the HF gas is preferably any one of 0.1 L / min, 0.2 L / min, 0.3 L / min, 0.4 L / min, 0.5 L / min, 0.6 L / min, 0.7 L / min, 0.8 L / min, 0.9 L / min, 1 L / min or a range formed by any two of them; and the solution concentration is preferably any one of 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt% or a range formed by any two of them.

[0023] Finally, the precipitate obtained by the reaction is subjected to at least one gradient washing with anhydrous ethanol and / or ultrapure water, and the precipitate is then dried to obtain high-purity LiF. Specifically, the precipitate obtained by the reaction can be quickly separated by a vacuum filtration system, and the precipitate is subjected to gradient washing with anhydrous ethanol and ultrapure water in sequence for multiple times, and the precipitate is then dried in air to obtain white high-purity LiF powder. Specifically, the gradient washing with anhydrous ethanol and ultrapure water can be performed for multiple times until the conductivity of the filtrate is less than 5 μS / cm. In addition, the obtained white high-purity LiF powder can be mechanically ground as needed to obtain particles with a desired particle size.

[0024] The second embodiment of the present application relates to a method for preparing high-purity LiF, and the specific operation process of the method is as follows:

[0025] First, a lithium salt is dissolved in ultrapure water to prepare a lithium salt solution. In addition, the lithium salt is preferably LiCl.

[0026] Then, an NH4F solution is added to the lithium salt solution for reaction. Specifically, the lithium salt solution can be transferred to a flask, which is then placed in a constant-temperature water bath, and then the NH4F solution is slowly added to the flask, and stirring is continued after the addition is completed. The temperature of the constant-temperature water bath is preferably 25°C; and the stirring time is preferably any one of 10 min, 20 min, 30 min, 40 min, 50 min, 60 min or a range formed by any two of them.

[0027] In addition, the NH4F solution can be prepared by the following method: dissolving NH4F in ultrapure water to obtain the NH4F solution. Specifically, the NH4F can be dissolved in ultrapure water and ultrasonically oscillated until completely dissolved to obtain the NH4F solution.

[0028] Finally, the precipitate obtained from the reaction is centrifuged, and the precipitate is washed with anhydrous ethanol, an NH4HCO3 solution, and ultrapure water in sequence, and the precipitate is dried to obtain high-purity LiF. Specifically, the precipitate obtained from the reaction can be centrifuged at a speed of 6000 rpm to 10000 rpm for 5 min to 15 min, the precipitate is washed with anhydrous ethanol, an NH4HCO3 solution, and ultrapure water in sequence, and the precipitate is dried in a vacuum drying oven for 6 h to 16 h to obtain white high-purity LiF powder. The centrifugal speed is preferably any one value or a range formed by any two values of 6000 rpm, 7000 rpm, 8000 rpm, 9000 rpm, and 10000 rpm; the centrifugal time is preferably any one value or a range formed by any two values of 5 min, 10 min, and 15 min; and the drying time is preferably any one value or a range formed by any two values of 6 h, 8 h, 10 h, 12 h, 14 h, and 16 h. In addition, the white high-purity LiF powder obtained can be mechanically ground to obtain particles of a desired particle size according to needs.

[0029] A third embodiment of the present application relates to high-purity LiF having an oxygen content of less than 100 ppm. It should be noted that the high-purity LiF of the present embodiment can be obtained by using the preparation methods mentioned in the first embodiment or the second embodiment, but the high-purity LiF of the present embodiment can also be prepared by using other preparation methods not mentioned, and therefore should not be limited by the preparation methods mentioned in the first embodiment or the second embodiment. The high-purity LiF preferably has an oxygen content of 0 ppm to 100 ppm. In addition, the oxygen content can be detected by using an energy dispersive spectrometer (EDS) or a wave dispersive spectrometer (WDS). In addition, the high-purity LiF is preferably a single phase within the detection limit of XRD; the high-purity LiF preferably has a morphology of a block, a cube, or a strip. In addition, the high-purity LiF preferably does not undergo thermal decomposition or phase transition at 0°C to 800°C. In addition, the high-purity LiF preferably has a particle size D50 of 0.5 μm to 40 μm.

[0030] A fourth embodiment of the present application relates to a coated separator, which comprises: a base film; and a coating layer formed on at least one side of the base film and comprising the high-purity LiF mentioned in the third embodiment.

[0031] The base film can include one or more of a polymer, a ceramic material. The polymer preferably includes one or more of a polyamide, a polyimide, a polyolefin (the polyolefin is, for example, one or more of a polyethylene, a polypropylene, a polyvinylidene fluoride, a polytetrafluoroethylene), a polyacrylonitrile, a cellulose, a polyester (the polyester is, for example, a polyethylene terephthalate), an aramid base film. In addition, other lithium-ion battery base films can be used as desired. The base film can be in a single layer form, a double layer form, or a multi-layer form. When the base film is in a double layer form, or a multi-layer form, the material of each layer can be the same or different, and likewise, the thickness of each layer can be the same or different. When the base film is in a double layer form, or a multi-layer form, each layer can be combined together to form the base film by co-extrusion or lamination. In addition, the base film thickness can be in a range of 2 μm to 45 μm, preferably 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, 31 μm, 32 μm, 33 μm, 34 μm, 35 μm, 36 μm, 37 μm, 38 μm, 39 μm, 40 μm, 41 μm, 42 μm, 43 μm, 44 μm, 45 μm, or any range formed by any two of these values.

[0032] The coating layer can be formed on one side or both opposite sides. In addition to high-purity LiF, the coating layer can further contain one or more of a dispersant, a thickening agent, an adhesive, and a wetting agent. The dispersant can include one or more of polyacrylamide, sodium hexametaphosphate, and methylamyl alcohol; the thickening agent can include carboxymethyl cellulose (CMC); the adhesive can include one or more of polyvinylidene fluoride and its copolymer, polyvinyl alcohol, polyacrylic acid, styrene butadiene rubber, polyvinyl acetate, natural latex, chloroprene latex, and nitrile latex; and the wetting agent can include one or more of ethanol, propylene glycol, glycerol, polyoxyethylene alkylphenol ether, polyoxyethylene fatty alcohol ether, fatty acid ester sulfate, and vinyl triethoxy silane. In addition, the content of the dispersant can be 0.1 to 1 parts by mass, preferably any one of or a range formed by any two of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1 parts by mass, based on 100 parts by mass of high-purity LiF; the content of the thickening agent can be 1 to 2 parts by mass, preferably any one of or a range formed by any two of 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, and 2 parts by mass; the content of the adhesive can be 1 to 20 parts by mass, preferably any one of or a range formed by any two of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 parts by mass; and the content of the wetting agent can be 0.1 to 1 parts by mass, preferably any one of or a range formed by any two of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1 parts by mass. In addition, when the adhesive includes polyacrylate and polyvinyl acetate, the mass ratio between polyacrylate and polyvinyl acetate can be 1:(0.25 to 4), preferably any one of or a range formed by any two of 1:(0.25, 1, 4).

[0033] In addition, the thickness of the coating layer can be 1 to 5 μm, preferably any one of or a range formed by any two of 1, 2, 3, 4, and 5 μm. If the thickness of the coating layer is less than 1 μm, the heat dimensional stability, liquid retention, and capacity retention of the separator can be reduced. If the thickness of the coating layer is greater than 5 μm, the coating process can be difficult, and the liquid retention of the electrolyte and the mass of the separator can increase, resulting in a decrease in the energy density of the battery.

[0034] A fifth embodiment of the present application relates to a method of making a coated separator, comprising: providing a coating slurry, the coating slurry comprising the high purity LiF of the third embodiment; and coating the coating slurry on at least one side of a base film and drying.

[0035] The coating slurry can further include one or more of a dispersant, a thickening agent, a binder, a wetting agent, water in addition to the high-purity LiF. The dispersant can include one or more of polyacrylamide, sodium hexametaphosphate, methylamyl alcohol; the thickening agent can include carboxymethyl cellulose (CMC); the binder can include one or more of polyvinylidene fluoride and its copolymer, polyvinyl alcohol, polyacrylic acid, styrene butadiene rubber, polyvinyl acetate, natural latex, chloroprene latex, nitrile latex; the wetting agent can include one or more of ethanol, propylene glycol, glycerol, polyoxyethylene alkyl phenol ether, polyoxyethylene fatty alcohol ether, fatty acid ester sulfate, vinyl triethoxy silane. In addition, the content of the dispersant can be 0.1 to 1 parts by mass, preferably 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 parts by mass, or a range formed by any two of the values; the content of the thickening agent can be 1 to 2 parts by mass, preferably 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2 parts by mass, or a range formed by any two of the values; the content of the binder can be 1 to 20 parts by mass, preferably 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 parts by mass, or a range formed by any two of the values; the content of the wetting agent can be 0.1 to 1 parts by mass, preferably 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 parts by mass, or a range formed by any two of the values; the content of the water can be 50 to 500 parts by mass, preferably 50, 100, 150, 200, 250, 300, 350, 400, 450, 500 parts by mass, or a range formed by any two of the values. In addition, when the binder includes polyacrylate and polyvinyl acetate, the mass ratio between the polyacrylate and the polyvinyl acetate can be 1:(0.25 to 4), preferably 1:(0.25, 1, 4, or a range formed by any two of the values).

[0036] The coating slurry can be prepared as follows:

[0037] First, high-purity LiF, dispersant, thickening agent are added into water in the required mass fraction, and stirring is performed to obtain a first slurry. Ball milling is performed on the first slurry to obtain a second slurry. In addition, the stirring speed can be 1000 rpm to 5000 rpm, preferably any one of 1000 rpm, 2000 rpm, 3000 rpm, 4000 rpm, 5000 rpm or a range formed by any two of them; the stirring time can be 1 h to 12 h, preferably any one of 1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h or a range formed by any two of them; the ball milling speed can be 300 rpm to 1200 rpm, preferably any one of 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm or a range formed by any two of them; the ball milling time can be 0.5 h to 6 h, preferably any one of 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or a range formed by any two of them.

[0038] Next, the second slurry is filtered, and an adhesive and a wetting agent are added to the filtered second slurry, and stirring is performed to obtain a coating slurry. In addition, the stirring speed can be 500 rpm to 2000 rpm, preferably any one of 500 rpm, 1000 rpm, 1500 rpm, 2000 rpm or a range formed by any two of them; the stirring time can be 0.5 h to 6 h, preferably any one of 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or a range formed by any two of them.

[0039] In addition, the coating can be performed by one or more of electrostatic spraying, blade coating, extrusion coating, transfer coating, dip coating, gravure or microgravure coating.

[0040] In addition, the drying temperature can be 60℃ to 200℃, preferably any one of 60℃, 70℃, 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃ or a range formed by any two of them; the drying time can be 5 min to 60 min, preferably any one of 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min or a range formed by any two of them.

[0041] The application is exemplarily demonstrated by the following examples:

[0042] Example A1

[0043] The method for synthesizing high-purity LiF powder of the present embodiment comprises the following steps:

[0044] (1) Preparation and purification of hydrofluoric acid solution

[0045] In an inert gas-protected glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), high-purity HF gas (99.999%) was passed into a pre-cooled circulating gas absorption device (polytetrafluoroethylene material) at a flow rate of 0.5 L / min, and was fully dissolved in deionized water by a three-stage bubbling method. After the solution concentration reached 40 wt%, high-purity argon gas (99.999%) was used for purging for 30 min to remove residual HF gas, and a clear hydrofluoric acid aqueous solution was obtained, which was sealed and stored in a fluorinated ethylene propylene (FEP) container for use.

[0046] (2) Construction of lithium carbonate precursor dispersion system

[0047] High-purity lithium carbonate (Li2CO3, 99.99%) powder was weighed and added to an ethanol-water mixed solvent (volume ratio 1:1, pre-deoxidized), and a high-shear dispersion emulsifier was used for continuous dispersion for 30 min to form a uniform suspension. The suspension was transferred to a high-pressure reaction kettle lined with polytetrafluoroethylene, a magnetic stirring rotor was installed, and argon was introduced to replace the air in the kettle three times to ensure that the oxygen content was <5 ppm.

[0048] (3) Controllable reaction and crystallization control

[0049] The hydrofluoric acid solution prepared in step (1) was added dropwise to the reaction kettle at a rate of 0.2 mL / min through a constant-pressure dropping funnel, while the magnetic stirring rate was maintained at 500 rpm. After the addition was completed, the reaction system was sealed and continuously stirred in a 25°C constant-temperature water bath for 6 h, and the pH value change of the reaction liquid was monitored by an online pH meter until it reached the stable interval.

[0050] (4) Separation and post-treatment of the product

[0051] After the reaction was completed, the precipitate was quickly separated by a vacuum filtration system, and gradient washing was performed with anhydrous ethanol and ultrapure water multiple times until the conductivity of the filtrate was <5 μS / cm. The obtained solid was dried in air to obtain a white powder.

[0052] Example A2

[0053] In Example A1, step (3) was changed from 6 h to 24 h, and the rest was the same as Example A1.

[0054] Example A3

[0055] In Example A1, step (3) was changed from 6 h to 72 h, and the rest was the same as Example A1.

[0056] Example A4

[0057] Change 6h in step (3) of Example Al to 120h, and the rest is the same as Example Al.

[0058] Example A5

[0059] Change 6h in step (3) of Example Al to 144h, and the rest is the same as Example Al.

[0060] Example A6

[0061] Change 6h in step (3) of Example Al to 168h, and the rest is the same as Example Al.

[0062] Example A7

[0063] Change 6h in step (3) of Example Al to 120h, and 25°C to 60°C, and the rest is the same as Example Al.

[0064] Example A8

[0065] Change 6h in step (3) of Example Al to 120h, and 25°C to 90°C, and the rest is the same as Example Al.

[0066] Example A9

[0067] Change high purity lithium carbonate (Li2CO3, 99.99%) in step (2) of Example Al to high purity lithium hydroxide (LiOH, 99.99%), and the rest is the same as Example Al.

[0068] Example A10

[0069] Change 6h in step (3) of Example A9 to 120h, and the rest is the same as Example A9.

[0070] Example Al 1

[0071] Change 6h in step (3) of Example A9 to 120h, and 25°C to 60°C, and the rest is the same as Example A9.

[0072] Example A12

[0073] Change 6h in step (3) of Example A9 to 120h, and 25°C to 90°C, and the rest is the same as Example A9.

[0074] Example A13

[0075] Change 6h in step (3) of Example A9 to 72h, and 25°C to 120°C, and the rest is the same as Example A9.

[0076] Comparative Example Al

[0077] LiF raw material purchased on the market (Shanghai Zhongli Real Industry Co., Ltd.).

[0078] Example B1

[0079] The method for synthesizing high-purity LiF powder provided in this example includes the following steps:

[0080] (1) Weigh NH4F and dissolve it in ultrapure water, and ultrasonically oscillate until completely dissolved to obtain a 0.8 mol / L NH4F solution. Weigh LiCl and dissolve it in 500 mL of ultrapure water, and stir until clear to prepare a 0.8 mol / L LiCl solution.

[0081] (2) After transferring the LiCl solution to a flask containing a polytetrafluoroethylene liner, place it in a constant-temperature water bath and set the temperature to 25°C. Then install a mechanical stirrer (rotation speed 300 rpm) and slowly add the NH4F solution to the flask through a constant-flow pump. After the addition is complete, continue stirring for 30 minutes.

[0082] (3) Transfer the reaction solution to a high-speed centrifuge and centrifuge at 8000 rpm for 10 min. Discard the supernatant and wash the precipitate with anhydrous ethanol, NH4HCO3 solution, and ultrapure water in sequence. After each washing, centrifuge and detect the Cl - / NO3 - residue in the filtrate.

[0083] (4) Transfer the precipitate to a vacuum drying oven and dry for 12 h to obtain white LiF powder.

[0084] Example C1

[0085] The separator provided in this example includes a base film and a coating layer coated on both sides of the base film.

[0086] The base film is a 9 μm thick porous polyethylene (PE), and the thickness of the coating layer on both sides of the base film is 2 μm, and the total thickness of the separator is 13 μm.

[0087] The raw materials for preparing the coating layer include, by mass fraction, 100 parts of LiF particles (average particle size 0.5 μm, taken from Examples A7, A12, or B1), 5 parts of polyacrylate, 1.5 parts of thickening agent (CMC), 0.5 parts of dispersing agent (polyacrylamide), and 0.1 parts of wetting agent (polyoxyethylene alkyl phenol ether).

[0088] The method for preparing the separator includes:

[0089] S1: According to the above preparation of raw material dosage, LiF particles, dispersant, thickening agent were added into 300 parts of deionized water, mechanical stirring at 2000 rpm for 2 h, to obtain the first slurry; the first slurry was uniformly dispersed in the ball mill tank by ball milling, the ball milling rate was 500 rpm, the ball milling time was 1 h, to obtain the second slurry.

[0090] S2: The second slurry was filtered, 5 parts of polyacrylate and 0.1 parts of wetting agent were added into the filtered second slurry, mechanical stirring for 1 h, the stirring speed was 500 rpm, to obtain the coating slurry.

[0091] S3: The above coating slurry was uniformly coated on both sides of the base film on the coating machine, dried at 60℃ for 5 min, to obtain the separator with coating on both sides of the base film.

[0092] Example C2

[0093] The difference between this example and Example C1 is that the amount of adhesive is 1 part.

[0094] Example C3

[0095] The difference between this example and Example C1 is that the amount of adhesive is 3 parts.

[0096] Example C4

[0097] The difference between this example and Example C1 is that the amount of adhesive is 10 parts.

[0098] Example C5

[0099] The difference between this example and Example C1 is that the amount of adhesive is 15 parts.

[0100] Example C6

[0101] The difference between this example and Example C1 is that only 2 μm coating is provided on one side surface of the base film.

[0102] Example C7

[0103] The difference between this example and Example C6 is that the thickness of the coating is 3 μm.

[0104] Example C8

[0105] The difference between this example and Example C6 is that the thickness of the coating is 5 μm.

[0106] Example C9

[0107] The difference between this example and Example C1 is that the adhesive is polyvinyl acetate.

[0108] Example C10

[0109] The difference between this example and Example C1 is that the adhesive is polyacrylate and polyvinyl acetate, which are added in a mass ratio of 1:4, respectively.

[0110] Example C11

[0111] The difference between this example and Example C1 is that the adhesive is polyacrylate and polyvinyl acetate, which are added in a mass ratio of 1:1, respectively.

[0112] Example C12

[0113] The difference between this example and Example C1 is that the adhesive is polyacrylate and polyvinyl acetate, which are added in a mass ratio of 4:1, respectively.

[0114] Comparative Example C1

[0115] The difference between this comparative example and Example C1 is that the base film surface is not provided with a coating.

[0116] Comparative Example C2

[0117] The difference between this comparative example and Example C1 is that the thickness of the base film is changed to 13 μm.

[0118] Comparative Example C3

[0119] The difference between this comparative example and Example C1 is that the thickness of the base film is changed to 15 μm.

[0120] Comparative Example C4

[0121] The difference between this comparative example and Example C1 is that the LiF in the coating is changed to the same particle size of aluminum oxide.

[0122] Comparative Example C5

[0123] The difference between this comparative example and Example C1 is that the LiF in the coating is changed to the same particle size of boehmite.

[0124] Comparative Example C6

[0125] The difference between this comparative example and Example C1 is that the LiF in the coating is the same particle size of LiF in Comparative Example A1.

[0126] X-ray Diffraction Test

[0127] The phase composition and crystal structure of the material are characterized by X-ray diffraction (XRD) technology. The atoms in the crystal material can become a grating of X-ray diffraction due to periodic arrangement. X-rays produce diffraction through the crystal grating, and the superposition of diffracted waves makes the intensity of the rays increase in some directions and decrease in other directions. By analyzing the diffraction pattern, the crystal structure of the material can be determined. The interplanar spacing of the crystal can be calculated by the formula, and then the crystal structure of the material is analyzed.

[0128] In this paper, by comparing the XRD results of the sample with the diffraction peak position and intensity of the standard card of the substance, the phase composition and crystal structure of the sample are determined, and the phase content is determined by refinement. In order to eliminate the residual stress and grain preferred orientation that may exist in the sample, powder sample test is used.

[0129] Scanning electron microscope test

[0130] In this paper, scanning electron microscope (SEM) is used to observe the microstructure of the sample, and energy dispersive spectrometer (EDS) is used for element analysis.

[0131] Differential scanning calorimetry test

[0132] Differential scanning calorimetry (DSC) is a method for observing the change of heat flow power difference between sample end and reference end with temperature or time under the control of certain temperature program (rise / fall / constant temperature), so as to obtain the information of heat effect such as endothermic, exothermic, specific heat change of sample in the process of temperature program, and calculate the heat effect of endothermic and exothermic quantity (enthalpy) and characteristic temperature (starting point, peak, end point, etc.). In this paper, DSC is used to determine whether the synthesized substance will undergo phase change in the use temperature range.

[0133] Thermogravimetric analysis test

[0134] Thermogravimetric analysis (TGA) is a technique for studying the thermal behavior of materials by monitoring the change of mass with temperature or time. In this paper, TGA is used to detect the thermal stability of the synthesized substance.

[0135] The performance test method of the diaphragm is as follows:

[0136] (1) Shrinkage: The separator was cut into multiple 120mm x 100mm (length x width) pieces in the longitudinal MD and transverse TD directions. An A4 paper was placed on the surface of the separator, and the separator was placed in an oven at 130°C, 150°C, and 180°C, respectively, for 1 hour. The size of the separator after heat shrinkage was measured, and the shrinkage ratio was calculated.

[0137] (2) Peeling strength: The peeling strength between the coating and the base film was tested on a universal tensile tester. The coating separator was cut into 150mm x 30mm (length x width) sample strips using a sampler, and the sample strips were adhered to the double-sided tape of the test plate. A 200mm x 20mm (length x width) transparent tape was then adhered above the sample strips, and a cylindrical roller was used to naturally press the separator in the same direction. When the separator was coated on one side, the corresponding base film faced downward, and the coating faced upward. The transparent tape was peeled off from the coating side of the separator to a length of 80mm, and the free end of the transparent tape was folded. The free end of the transparent tape and the test plate were clamped on the upper and lower clamps, respectively, and continuous peeling was performed at a stretching speed of 50mm / min using a tensile tester in the same environment until the coating and the base film were completely separated. The peeling strength of the separator coating was directly read and recorded.

[0138] (3) Air permeability and puncture strength: The standard of GBT36363-2018 was referred to for testing; the thickness was measured using a Malvern film thickness meter.

[0139] (4) Water contact angle: The contact angle measurement instrument was used to test the water contact angle according to the standard of GBT30693-2014.

[0140] (5) Ionic conductivity: Five pieces of separator matching the resistance test mold were cut, and the separator was placed in a lithium hexafluorophosphate (LiPF6) electrolyte with a concentration of 1mol / L. The solvent of the electrolyte was ethylene carbonate (EC), methyl ethyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1. The separator was sealed and soaked for 2h. The electrochemical workstation was used to test and record the electrochemical impedance spectrum at 25°C and a frequency range of 0.01Hz to 1x10 6 Hz. The slope and linear fitting degree of the curve were calculated with the number of separator layers as the abscissa and the resistance of the separator as the ordinate. When the linear fitting degree was greater than 0.99, the slope k was the impedance value of the separator. The ionic conductivity of the separator was calculated by the formula σ=d / kS, where σ was the ionic conductivity of the separator, d was the thickness of the separator, k was the impedance value of the separator, and S was the use area of the separator during testing.

[0141] (6) Diaphragm liquid absorption rate: cut the diaphragm into a standard size (e.g. 50mm x 50mm or a circular sample), ensure the surface is clean and undamaged, then place it in a vacuum drying oven at 80°C to remove water and volatile substances, after cooling, weigh the initial mass W0 with an analytical balance with an accuracy of 0.1mg, then completely immerse it in an electrolyte (e.g. LiPF6 / EC-DMC system) at a constant temperature and stand until saturated absorption, after taking it out, wipe off the residual liquid on the surface and quickly measure the mass W1 after absorption, finally calculate the result by the formula: absorption rate (%) = (W1-W0) x 100% / W0.

[0142] Purity analysis of synthesized LiF:

[0143] Figure 1 The SEM results of Example A7 show that the morphology of the synthesized LiF is cubic, which is related to its cubic crystal structure. Figure 2 The SEM results of Comparative Example A1 show the morphology of the LiF, which is different from Figure 1 Example A7, the particle size of the LiF synthesized in Example A7 is significantly reduced. In addition, Figure 2 The LiF crystals of Comparative Example A1 are large and irregular, and the crystal growth process is significantly inferior to Example A7, resulting in a lower uniformity than Example A7. Moreover, Figure 2 The LiF particles of Comparative Example A1 are large and irregular, and the growth is poor, which is more likely to cause the mixing of water molecules and oxygen molecules, resulting in more impurities.

[0144] The EDS results of Table 1 show that the synthesized LiF has no oxygen element within the detection limit, and the purity is close to 100% (the Li element cannot be displayed due to the low reflection factor), which is consistent with Figure 3 The XRD results of Example A7. Although the raw material of Comparative Example A1 shows a single phase by XRD, the EDS results show that there is 2.2% oxygen element, indicating that the LiF of Comparative Example A1 is contaminated by oxygen and has insufficient purity. Figure 4 The DSC and Figure 5 The TGA results of Example A7 show that the synthesized high-purity LiF has good thermal stability within the measured temperature range. Therefore, the high-purity LiF (purity about 100%) synthesized by the present application is suitable for the field of lithium metal batteries.

[0145] Table 1

[0146] Sample O, at% F, at% Total, at% Example A7 not detected 100 100 Example A12 not detected 100 100 Example B1 not detected 100 100 Comparative Example A1 2.2 97.8 100

[0147] Comprehensive analysis of diaphragm performance:

[0148] Table 2 is the test results of Examples C1 to C12 and Comparative Examples C1 to C6.

[0149] (1) Heat resistance and safety:

[0150] Under the condition of coating thickness of 4 μm and total thickness of the separator no more than 13 μm, the shrinkage of the separator can be controlled within 3% after heat treatment at 180℃ for 1 hour, and no particle shedding phenomenon is observed. This shows that the adhesive remains effective under this condition, significantly improving the thermal stability and safety of the separator, meeting the application requirements.

[0151] (2) The influence of the adhesive dosage and the coating thickness on the heat resistance and the peeling strength:

[0152] Comparative examples C1 to C8 show that increasing the adhesive dosage and the coating thickness can simultaneously improve the heat resistance of the separator and the peeling strength of the coating. The mechanism is as follows:

[0153] a. Increase of the adhesive dosage: enhances the interaction force between the adhesive and the LiF particles, making them more closely combined.

[0154] b. Increase of the coating thickness: equivalent to increasing the content of the adhesive and the LiF particles in unit area, thereby strengthening the adhesion between the LiF particles and the base film. This enhanced adhesion can effectively inhibit the shrinkage of the separator at high temperature, thereby improving its heat resistance.

[0155] (3) The mechanism of improving the ionic conductivity and the wettability:

[0156] Compared with Comparative Examples C1 to C3, the LiF-coated separator provided by the present patent embodiment exhibits significantly higher ionic conductivity, and the ionic conductivity increases with the increase of the coating thickness. This advantage is due to:

[0157] a. The structural characteristics of the LiF coating: the PE base film itself has poor wettability, resulting in poor electrolyte retention and liquid absorption. The LiF coating has a highly developed porous structure, and the LiF particles themselves have good liquid affinity, both of which synergistically improve the wettability of the separator (the contact angle is reduced from about 116° to below 40°).

[0158] b. The benefits of improved wettability: effectively shortens the electrolyte immersion time during battery assembly; improves the electrolyte retention capacity of the separator; and is more conducive to the efficient transmission of ions in the working state of the battery, thereby inhibiting the growth of lithium dendrites.

[0159] c. The double-sided nature of the coating thickness: thicker coating can accommodate more electrolyte (increased retention and liquid absorption), which is beneficial to the ionic conductivity. However, a too thick coating (such as Example C8) will increase the ion transmission path length, which will actually lead to a decrease in ionic conductivity.

[0160] d. The negative effects of excessive adhesive: too high adhesive content not only leads to difficult slurry coating and easy agglomeration, but also significantly increases the coating air permeability value (Gurle value). The increase in air permeability value will hinder ion transmission, thereby reducing the ionic conductivity.

[0161] (4) Influence of the type of adhesive:

[0162] Comparative examples C1, C9 to C12 show that, at the same amount of addition, the heat resistance and wettability (the smaller the contact angle, the better) of the separator using polyvinyl acetate (solution type adhesive) are superior to those of the separator using polyacrylate (emulsion type adhesive); however, the peel strength of the polyvinyl acetate system separator is lower. Therefore, the combined use of the two types of adhesives can produce a synergistic effect and obtain better comprehensive performance.

[0163] (5) Comparison of coating materials:

[0164] Under the same conditions, the heat resistance of the LiF-coated separator of example C1 is slightly worse than that of the alumina-coated separator and comparable to that of the boehmite-coated separator. However, the wettability, liquid absorption rate and ionic conductivity of the LiF-coated separator are significantly better than those of the other two materials.

[0165] (6) Importance of LiF purity:

[0166] Comparing example C1 with comparative example C6, it can be seen that the wettability and ionic conductivity of the former are slightly higher. This shows that the contamination of LiF by oxygen affects the wettability of the electrolyte and thus reduces the ionic conductivity, meaning that the purity of LiF has an important influence on the performance of the battery.

[0167] Table 2

[0168]

[0169]

[0170] In summary, the synthesis method provided by the present application directly prepares high-purity LiF, solving the problem of high impurity content in traditional methods that cannot be applied to lithium metal batteries. In addition, the material innovation and process breakthrough of the LiF-coated separator provided by the present application systematically solve the core problems of lithium metal batteries, such as uncontrolled growth of dendrites, insufficient thermal safety and poor affinity of electrolyte, while having high peel strength, ionic conductivity, puncture strength and air permeability, providing key technical support for the commercial application of lithium metal batteries.

[0171] The above content related to common knowledge is not described in detail, and those skilled in the art can understand it.

[0172] The above only describes some specific embodiments of the present application and does not limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application. The technical scope of the present application is not limited to the content in the specification, and the technical scope must be determined according to the scope of claims.

Claims

1. A method for preparing high-purity LiF, characterized in that, include: A uniform suspension was obtained by adding lithium salt to the dispersion solution; An aqueous solution of hydrofluoric acid is added to the homogeneous suspension to carry out the reaction; as well as The precipitate obtained from the separation reaction is washed at least once with anhydrous ethanol and / or ultrapure water in a gradient, and then dried to obtain high-purity LiF.

2. The method for preparing high-purity LiF according to claim 1, characterized in that, The method for preparing the hydrofluoric acid aqueous solution includes: HF gas is introduced into a pre-cooled circulating gas absorption device at a flow rate of 0.1 L / min to 1 L / min. The HF gas is fully dissolved in deionized water by a three-stage bubbling method. After the solution concentration reaches 30 wt% to 50 wt%, the residual HF gas is purged with high-purity argon gas to obtain the hydrofluoric acid aqueous solution.

3. The method for preparing high-purity LiF according to claim 1, characterized in that, The steps for obtaining the homogeneous suspension include: The lithium salt is added to the dispersion solution and continuously dispersed using a high-shear dispersion emulsifier to form a uniform suspension. The suspension is then transferred to a high-pressure reactor and argon gas is introduced to replace the air inside the reactor, ensuring that the oxygen content is <5ppm.

4. The method for preparing high-purity LiF according to claim 1, characterized in that, The step of adding the hydrofluoric acid aqueous solution to the homogeneous suspension includes: The hydrofluoric acid solution was added to the reaction vessel at a rate of 0.1 mL / min to 1 mL / min, while maintaining a stirring rate of 100 rpm to 1000 rpm. After the addition was complete, the reaction system was sealed and stirred continuously in a constant temperature water bath for 6 h to 168 h.

5. A method for preparing high-purity LiF, characterized in that, include: Lithium salts were dissolved in ultrapure water to prepare lithium salt solutions. An NH4F solution is added to the lithium salt solution for reaction. as well as The precipitate obtained by centrifugation was washed successively with anhydrous ethanol, NH4HCO3 solution and ultrapure water, and then dried to obtain high-purity LiF.

6. The method for preparing high-purity LiF according to claim 5, characterized in that, The lithium salt is LiCl.

7. The method for preparing high-purity LiF according to claim 5, characterized in that, The step of adding the NH4F solution to the lithium salt solution includes: After transferring the lithium salt solution to a flask, place it in a constant temperature water bath, and then slowly add the NH4F solution to the flask. After the addition is complete, continue stirring. The temperature of the constant temperature water bath is 25°C, and the stirring time is 10 min to 60 min.

8. A high-purity LiF, characterized in that, Oxygen content is less than 100 ppm.

9. The high-purity LiF according to claim 8, characterized in that, It is a single phase within the XRD detection limit; its morphology is blocky, cubic, or elongated; it does not undergo thermal decomposition or phase transformation at temperatures ranging from 0°C to 800°C; and / or its particle size D50 is 0.5 μm to 40 μm.

10. A coated diaphragm comprising: One base film; And a coating layer formed on at least one side of the base film and comprising high-purity LiF according to any one of claims 8 and 9.

11. The coated diaphragm according to claim 10, characterized in that, The coating layer further contains one or more of the following: dispersant, thickener, adhesive, and wetting agent.

12. The coated diaphragm according to claim 11, characterized in that, The dispersant includes one or more of polyacrylamide, sodium hexametaphosphate, and methylpentanol; the thickener includes carboxymethyl cellulose; the adhesive includes one or more of polyvinylidene fluoride and its copolymers, polyvinyl alcohol, polyacrylic acid, styrene-butadiene rubber, polyvinyl acetate, natural latex, chloroprene cream, and nitrile latex; and the wetting agent includes one or more of ethanol, propylene glycol, glycerin, polyoxyethylene alkylphenol ether, polyoxyethylene fatty alcohol ether, fatty acid ester sulfate, and vinyltriethoxysilane.

13. The coated diaphragm according to claim 12, characterized in that, Based on 100 parts by weight of the high-purity LiF, the content of the dispersant is 0.1 to 1 part by weight, the content of the thickener is 1 to 2 parts by weight, the content of the adhesive is 1 to 20 parts by weight, and the content of the wetting agent is 0.1 to 1 part by weight.

14. The coated diaphragm according to claim 10, characterized in that, The thickness of the coating layer is 1 μm to 5 μm.

15. A method for preparing a coated diaphragm, characterized in that, include: A coating slurry is provided, the coating slurry comprising high-purity LiF according to any one of claims 8 and 9; as well as The coating slurry is applied to at least one side of a base film and dried.

16. The method for preparing the coated diaphragm according to claim 15, characterized in that, The coating slurry further contains one or more of the following: dispersant, thickener, adhesive, wetting agent, and water.

17. The method for preparing the coated diaphragm according to claim 16, characterized in that, The dispersant includes one or more of polyacrylamide, sodium hexametaphosphate, and methylpentanol; the thickener includes carboxymethyl cellulose; the adhesive includes one or more of polyvinylidene fluoride and its copolymers, polyvinyl alcohol, polyacrylic acid, styrene-butadiene rubber, polyvinyl acetate, natural latex, chloroprene cream, and nitrile latex; and the wetting agent includes one or more of ethanol, propylene glycol, glycerin, polyoxyethylene alkylphenol ether, polyoxyethylene fatty alcohol ether, fatty acid ester sulfate, and vinyltriethoxysilane.

18. The method for preparing the coated diaphragm according to claim 17, characterized in that, Based on 100 parts by weight of the high-purity LiF, the content of the dispersant is 0.1 to 1 part by weight, the content of the thickener is 1 to 2 parts by weight, the content of the adhesive is 1 to 20 parts by weight, the content of the wetting agent is 0.1 to 1 part by weight, and the content of water is 50 to 500 parts by weight.

19. The method for preparing the coated diaphragm according to claim 16, characterized in that, The method for preparing the coating slurry includes: The high-purity LiF, the dispersant, and the thickener are added to the water and stirred to obtain a first slurry. The first slurry is then ball-milled to obtain a second slurry. The second slurry is filtered, and the adhesive and the wetting agent are added to the filtered second slurry. The mixture is stirred to obtain the coating slurry.

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