CN101265176A - A kind of purification method of lithium oxalate difluoroborate - Google Patents

Abstract

A method for purifying lithium oxalate difluoroborate, which comprises first dissolving the lithium oxalate difluoroborate (LiODFB) to be purified in a solvent with high solubility, then mixing the solution with a crystallizer, and passing solid-liquid Separation, and drying the crystallized solid matter in a vacuum oven to obtain a purified LiODFB product. The product obtained by the purification method of the present invention is confirmed to be LiODFB by ¹³ C, ¹¹ B and ¹⁹ F nuclear magnetic resonance spectra, and the water content of the product after one purification is 0.0020%. The mass percentages are respectively 0.0115%, 0.0032%, 0.0010%, 0.00045%, 0.0002%, 0.0001%. The present invention has the advantages of simple process, easy operation, mild conditions, low cost and high yield, and is suitable for industrial production .

Description

A kind of purification method of lithium oxalate difluoroborate

technical field

The invention relates to the field of crystallization chemistry, in particular to a method for removing impurities and purifying lithium oxalate difluoroborate that can be used in electrolytes of electrochemical devices such as lithium-ion batteries, and the invention also relates to a purification solvent used in the method.

Background technique

In recent years, due to the pressure of environmental pollution and energy shortage, countries are forced to find new green, environmentally friendly and sustainable energy sources. The green, high-energy and environmentally friendly lithium-ion battery that appeared in the 1990s has become one of the most eye-catching power sources due to its high energy density, long cycle life, and high operating voltage.

As an important part of the battery, the electrolyte of the lithium-ion battery is responsible for the transmission of ions between the positive and negative electrodes inside the battery. It has an important impact on the capacity, operating temperature range, cycle performance and safety performance of the battery. . LiPF ₆ and LiBF ₄ are commonly used lithium salts in current commercial lithium-ion batteries. LiPF ₆ is very sensitive to moisture, and hydrolyzes to generate corrosive gas HF. The rise of temperature will promote the above reaction. Therefore, the high temperature performance of LiPF ₆ is relatively poor, and the generation of gas also poses a safety hazard to lithium-ion batteries. In addition, the low-temperature conductivity of _LiPF6 -based electrolytes is low. Moreover, LiPF ₆ is usually combined with ethylene carbonate (EC) to make an electrolyte to form an effective SEI film on the negative electrode, but EC has a high melting point (37 ° C), which limits the low-temperature performance of the battery. LiBF ₄ as a lithium salt has a lower charge transfer resistance, which makes the battery have better low-temperature performance than LiPF ₆ . However, its oxidation potential is relatively low, and similar to LiPF ₆ , it is unstable to heat and moisture, which is not good for battery cycle life. In addition, the film-forming performance of LiBF ₄ is not good, which leads to a decrease in the high-rate discharge capacity of the battery and the first charge-discharge efficiency. Although lithium bisoxalate borate (LiBOB), which has attracted people's attention in recent years, has many advantages: it does not contain halogen in its molecular structure, does not corrode aluminum current collectors, has high electrical conductivity and electrochemical window, and can stabilize graphite negative electrodes in PC. There is almost no dissolution and corrosion for manganese and iron-based positive electrode materials. However, LiBOB is almost insoluble in some solvents with low dielectric constant (especially linear carbonates), and the insufficient concentration affects the specific energy of the battery. In addition, the LiBOB electrolyte will decompose and generate gas during use, which will increase the internal pressure of the battery and bring unsafe factors. Studies have also shown that the SEI film formed by LiBOB on the negative electrode has too high impedance, which will affect the low-temperature performance and discharge capacity of the battery. Therefore, the electrochemical performance of the lithium salt system currently used in the electrolyte is not ideal at high temperature and low temperature, and it is urgent to improve it or seek alternative materials with better performance and lower cost.

At present, the most eye-catching new lithium oxalate difluoroborate (LiODFB), which combines the advantages of two lithium salts LiBOB and LiBF ₄ , has become a new material to replace the existing electrolyte lithium salt. This lithium salt molecular structure includes half LiBOB and half _LiBF4 molecules. Since LiODFB has a partial structure of LiBF ₄ , the low-temperature performance has been improved. At the same time, it has a partial structure of LiBOB, which also has better high-temperature performance, and is not as sensitive to impurities and moisture as LiBOB, so it has a wider range of applications. temperature range. LiODFB has relatively high solubility in chain carbonate solvents, so it has high conductivity, and can form a stable SEI film on the surface of graphite negative electrode with PC, which provides a favorable premise for solving the problem of low-temperature use of batteries. LiODFB has good thermal stability to manganese-based and iron-based cathode materials. LiODFB can also improve the abuse resistance of Li-ion batteries.

Lithium-ion batteries have very strict requirements on electrolytes, and usually directly synthesized products cannot be directly used as electrolytes for lithium-ion batteries due to their high impurities and moisture content. It must therefore undergo a purification step to reduce the impurity content.

S.Tsujioka etc. react in dimethyl carbonate with oxalic acid, lithium tetrafluoroborate and aluminum chloride or silicon tetrachloride in European patent EP1308449A2 or react with oxalic acid, lithium tetrafluoroborate, lithium fluoride and boron trichloride Or the reaction of trimethoxyboron in dimethyl carbonate realized the synthesis of LiODFB. But there is no mention of how to purify LiODFB.

S.S. Zhang et al. synthesized LiODFB by other methods in Electrochemistry Communications 8 (2006) 1423-1428, and purified LiODFB by extraction and recrystallization using dimethyl carbonate as a solvent. Because the solubility of dimethyl carbonate is relatively large in this purification method, it is difficult to crystallize out and cause the productive rate to be very low.

Contents of the invention

The object of the invention is to provide a method for purifying the electrolyte lithium oxalate difluoroborate that meets the needs of lithium-ion batteries.

The present invention adopts the principle of dissolution and crystallization, and first dissolves the LiODFB to be purified in a solvent with high solubility to make a high-concentration solution (including a saturated solution), and then mixes the solution with a crystallization agent. The low solubility in the solvent leads to the supersaturation of the mixed solvent, and LiODFB is precipitated. The low-content components such as impurities remain in the solution because they cannot reach supersaturation in the mixed solvent, and the purified LiODFB product is obtained by solid-liquid separation.

The purification process steps of the present invention are as follows: 1) dissolving the lithium oxalate difluoroborate to be purified in a solvent; 2) then mixing the solution with a crystallization agent, and separating the solid and liquid to obtain a solid substance; 3) The purified lithium oxalate difluoroborate was obtained by vacuum drying.

Repeat steps 1) and 2) to obtain the solid substance obtained after solid-liquid separation for multiple purifications.

The solubility of lithium oxalate difluoroborate in the solvent is not less than 15 grams.

The dissolution temperature of lithium oxalate difluoroborate is from room temperature to the boiling point of the solvent.

The solubility of lithium oxalate difluoroborate in the crystallization agent is not more than 5 grams.

After the crystallized solid matter is filtered, it can also be rinsed with lithium oxalate difluoroborate in a crystallizer with a solubility of not more than 5 grams.

The mass ratio of the amount of the crystallization agent used for rinsing to the crystallized solid matter to be leached is 5% to 20%.

Concrete purification steps of the present invention are:

First, dissolve the LiODFB to be purified in a solvent with a solubility greater than 15 grams at a certain temperature (normal temperature to the boiling point of the solvent), and then mix it in the solution according to the ratio of solvent and crystallization agent 1:100 to 10:1 (LiODFB The solubility is less than 5 grams), and then the solid matter is obtained through solid-liquid separation, and the solid matter is rinsed with a crystallization agent with a solubility of less than 5 grams according to the mass ratio of 5% to 20%, and finally the vacuum degree is 0.05 to 0.095Mpa Dry in a vacuum box, the drying temperature ranges from room temperature to 150°C, and the drying time ranges from 12 to 48 hours.

If further purification is required, the above process can be repeated several times.

The LiODFB to be purified above can be pre-dissolved in a solvent with high solubility, or it can be a lithium oxalate difluoroborate solution that is filtered, evaporated and concentrated after the synthesis reaction is completed. The solution concentrated by evaporation should ensure that the content of lithium oxalate difluoroborate is greater than 15% without precipitation.

The purification solvent of the present invention is preferably: THF, propylene carbonate, dimethyl carbonate, diethyl carbonate, butylene carbonate, acetonitrile, propionitrile, butyronitrile, acetone, N, N-dimethylformamide, sulfolane , dimethyl sulfoxide, etc., or a mixture thereof.

Purification crystallization agent of the present invention is preferably: any one in ethyl acetate, normal hexane, benzene, toluene, xylene, γ-butyrolactone, ether, carbon tetrachloride, dioxolane etc. or their mixture.

The product obtained by the purification method of the present invention is confirmed to be LiODFB by ¹³ C, ¹¹ B and ¹⁹ F NMR spectra.

According to the present invention, the water content of the LiODFB product after one purification is 0.0020% based on Karl Fischer coulometric titration. The mass percent contents of sodium, potassium, aluminum, iron, calcium and zinc of metal ions measured by ICP were 0.0115%, 0.0032%, 0.0010%, 0.00045%, 0.0002%, 0.0001%, respectively.

The invention has the advantages of simple process, easy operation, mild condition, low cost and high yield, and is suitable for industrialized production. The purified product has a good application effect in lithium-ion batteries.

Description of drawings

Fig. 1 is the NMR spectra of ¹³ C, ¹¹ B and ¹⁹ F of the product obtained in Example 1 of the present invention.

Fig. 2 is that the product obtained in Example 1 of the present invention is prepared into 1.2mol/L LiODFBPC/EC/DMC (1:1:3) electrolyte and unpurified LiODFBPC/EC/DMC (1:1:3) electrolysis The solution was applied to Li/graphite battery for charging and discharging curves. The charge and discharge cut-off voltage of the battery is 2.000V～0.000V, the charge and discharge current density is 30mA·g ^-1 , and the test temperature is 25°C. The ordinate in the figure is the battery voltage, the unit is V. 1, 15, and 30 in the figure represent the charge-discharge curves of the 1st, 15th, and 30th cycles, respectively.

Detailed ways

The present invention will be described in further detail below in conjunction with the examples, but these examples should not be construed as limiting the protection scope of the present invention. The present invention can be realized in any mode described in the summary of the invention.

Example 1

Add 15 g of the LiODFB product to be purified to prepare a saturated solution by adding acetonitrile. Add 200g of ether into a 500ml Erlenmeyer flask, slowly add the saturated acetonitrile solution of LiODFB and keep stirring, white crystals are precipitated. After the addition was completed, filter with a Buchner funnel, rinse with 10ml of ether three times, and vacuum-dry at 120°C (vacuum degree: 0.05-0.095Mpa) for 36h after suction filtration to obtain 12.9g of purified LiODFB solid. The yield was 86%. The molecular structure of the purified product was determined by NMR test, as shown in FIG. 1 . The obtained product was formulated into 1.2 mol L LiODFBPC/EC/DMC (1:1:3) electrolyte and applied to Li/graphite battery for charge and discharge test. The battery charge and discharge cut-off voltage is 2.000V～0.000V, the current density is 30mA·g ^-1 , and the test temperature is 25°C. The test results are shown in Figure 2. It can be seen from the charge-discharge test that the purified LiODFB has better charge-discharge capacity and excellent capacity retention than the Li/graphite battery in the electrolyte prepared by unpurified LiODFB at room temperature.

Example 2

Add 30 g of the LiODFB product to be purified to prepare a saturated solution by adding dimethyl carbonate. Add 400g of ether into a 1000ml Erlenmeyer flask, slowly add the saturated dimethyl carbonate solution of LiODFB and keep stirring, white crystals are precipitated. After the addition is completed, filter with a Buchner funnel, rinse with 20ml of ether three times, and vacuum-dry at 150°C (vacuum degree: 0.05-0.095Mpa) for 48h after suction filtration to obtain 24g of purified LiODFB solid. The yield was 80%.

Example 3

Add 10 g of the LiODFB product to be purified to prepare a saturated solution by adding propylene carbonate. Add 150g of carbon tetrachloride to a 500ml Erlenmeyer flask, slowly add the saturated propylene carbonate solution of LiODFB and keep stirring, white crystals are precipitated. After the addition is complete, filter with a Buchner funnel, rinse with 10ml of carbon tetrachloride three times, and vacuum-dry at 100°C (vacuum degree: 0.05-0.095Mpa) for 16 hours to obtain purified LiODFB solid 8.5 g. The yield was 85%.

Example 4

Add 12 g of the LiODFB product to be purified to prepare a saturated solution by adding propylene carbonate. Add 180g of γ-butyrolactone into a 500ml Erlenmeyer flask, slowly add the saturated propylene carbonate solution of LiODFB and keep stirring, white crystals are precipitated. After the addition is completed, filter with a Buchner funnel, rinse with 12ml of γ-butyrolactone three times, and vacuum-dry at 120°C (vacuum degree: 0.05-0.095Mpa) for 20h to obtain purified LiODFB solid 10.08 g. The yield was 84%.

Example 5

10 g of the LiODFB product to be purified was added into a mixed solution of propylene carbonate and tetrahydrofuran (volume ratio 1:1) to prepare a saturated solution. Add 100g of γ-butyrolactone into a 500ml Erlenmeyer flask, slowly add the saturated solution of LiODFB and keep stirring, white crystals precipitate out. After the addition is complete, filter with a Buchner funnel, rinse with 10ml of γ-butyrolactone three times, and vacuum-dry at 130°C (vacuum degree: 0.05-0.095Mpa) for 16 hours to obtain purified LiODFB solid 7.6 g. The yield was 76%.

Claims (13)

1. A purification method for lithium oxalate difluoroborate, characterized in that: 1) the lithium oxalate difluoroborate to be purified is dissolved in a solvent; 2) then the solution is mixed with a crystallizer, and separated by solid-liquid separation , the solid substance after crystallization; 3) the purified lithium oxalate difluoroborate is obtained by vacuum drying. 2. A method for purifying lithium oxalate difluoroborate according to claim 1, characterized in that the solubility of lithium oxalate difluoroborate in the solvent is not less than 15 grams. 3. A method for purifying lithium oxalate difluoroborate as claimed in claim 2, characterized in that: the dissolution temperature of lithium oxalate difluoroborate is from room temperature to the boiling point of the solvent. 4. A method for purifying lithium oxalate difluoroborate as claimed in claim 1, 2 or 3, wherein the solvent is tetrahydrofuran, propylene carbonate, dimethyl carbonate, diethyl carbonate, butylene carbonate , acetonitrile, propionitrile, butyronitrile, acetone, N,N-dimethylformamide, sulfolane, dimethyl sulfoxide, or a mixture thereof. 5. A method for purifying lithium oxalate difluoroborate as claimed in claim 1, characterized in that the solubility of lithium oxalate difluoroborate in the crystallization agent is not greater than 5 grams. 6. A method for purifying lithium oxalate difluoroborate as claimed in claim 5, the crystallization agent added is ethyl acetate, n-hexane, benzene, toluene, xylene, γ-butyrolactone, diethyl ether, tetra Any one of carbon chloride and dioxolane or a mixture thereof. 7. A method for purifying lithium oxalate difluoroborate as claimed in claim 1, characterized in that the mass ratio of crystallization agent added to solvent is 1:100 to 10:1. 8. Any one of the purification methods of lithium oxalate difluoroborate as described in claims 2-7 is characterized in that: the solid substance obtained after solid-liquid separation is repeated steps 1) and 2) for multiple purifications . 9. A method for purifying lithium oxalate difluoroborate as claimed in claim 1, characterized in that: after solid-liquid separation of the crystallized solid matter, use lithium oxalate difluoroborate to dissolve in a solution with a solubility of not more than 5 grams. Crystallization agent rinse. 10. A method for purifying lithium oxalate difluoroborate as claimed in claim 9, characterized in that: the mass ratio of the amount of crystallization agent used for rinsing to the crystallized solid matter being leached is 5 %~20%. 11. The purification method according to claim 1, characterized in that: the vacuum degree of the vacuum drying is 0.05-0.095Mpa, the drying temperature is from room temperature to 150°C, and the drying time is 12-48h. 12. A method for purifying lithium oxalate difluoroborate as claimed in claim 1, characterized in that the crude product of LiODFB to be purified is solid lithium oxalate difluoroborate, or obtained by filtration, evaporation and concentration after the synthesis reaction is completed. Lithium oxalate difluoroborate solution. 13. A method for purifying lithium oxalate difluoroborate as claimed in claim 12, characterized in that: the solution concentrated by evaporation should ensure that the content of lithium oxalate difluoroborate is greater than 15% without precipitation.

Images