CN1614406A - Quantitatively analyzing method for fluohydric acid in lithium ion battery electrolyte - Google Patents

Abstract

A quantitative analysis method includes diluting lithium ion cell electrolyte in absolute ethyl alcohol or methyl alcohol, using MOH as titrant, applying automatic potentiometric titration, using (CoXV20)/(1000XM) to confirm titrimetric curve as Co referring to tritrant concentration, V referring to consumed tritrant volume ml, 20 referring to HF molecular weight and M referring to electrolyte weight, using potentiometric titrator to carry out second order derivation of titrimetric curve for cnofirming titrimetric end point.

Description

Quantitative analysis method of hydrofluoric acid in lithium-ion battery electrolyte

Technical field

The invention relates to the technical field of production and manufacture of lithium-ion battery electrolytes, in particular to a quantitative analysis method for hydrofluoric acid in lithium-ion battery electrolytes.

Background technique

Trace amounts of hydrofluoric acid in the non-aqueous electrolyte used in lithium-ion batteries have a great influence on the capacity, cycle life and safety of the battery. Therefore, the content of hydrofluoric acid must be strictly monitored during the production, storage, transportation and battery manufacturing process of lithium-ion battery electrolyte. However, currently reported technologies in this area are extremely rare. At present, tetrabutylammonium hydroxide (NBu ₄ OH) is usually used as titrant and bromothymol blue (BTB) as indicator. The reaction effect of butyl ammonium is used to measure hydrogen fluoride in non-aqueous system ( ), but the reaction is accompanied by side reactions ( ) brings certain errors to analytical results, and the price of tetrabutylammonium hydroxide is also very expensive (such as the assay method of MERCK company). In order to solve this problem, China's No. 01130017.5 invention patent application discloses a method using sodium methylate as titrant, which has certain improvements in reducing detection costs, but the common point between this method and the above-mentioned method is that BTB is used as titrant. Indicators, the end point is judged by visual inspection, which will produce certain systematic errors. In addition, in order to improve the stability of the electrolyte in the industry, some stabilizers are generally added to improve the stability of lithium hexafluorophosphate, inhibit the production of hydrogen fluoride and absorb hydrogen fluoride, and most of these stabilizers are Lewis bases, so that the electrolyte generally presents a weak Alkaline (pH value is about 7-9), and the pH value of BTB discoloration point is about 7, so the sudden point of solution pH value during titration is likely to be inconsistent with the discoloration point of BTB, which will bring large errors to the test results.

Contents of Invention

The object of the present invention is to provide a quantitative analysis method for hydrofluoric acid in the electrolyte of lithium-ion batteries in order to solve the above-mentioned deficiencies in the prior art. The method can overcome the problem of inconsistency between the abrupt point of the pH value of the solution and the discoloration point of the indicator in the current titration method, thereby greatly improving the accuracy of the measurement result.

The present invention is realized through the following technical scheme: the quantitative analysis method of hydrofluoric acid in the lithium-ion battery electrolyte is to dilute the lithium-ion battery electrolyte at a ratio of 2.5 to 3.0 g per 50 ml of solvent to an anhydrous solvent that does not need to be dried. In methanol or absolute ethanol, with MOH as the titrant, the titration curve is determined by the automatic potentiometric titration method, and the second-order derivation of the titration curve is carried out by the potentiometric titrator, thereby determining the titration end point; using the formula (C ₀ ×V20)/ (1000×m) to calculate the endpoint value, where C ₀ is the concentration mol/L of the titrant, V is the volume ml of the titrant consumed, 20 is the molecular weight of HF, and m is the weight of the electrolyte in g.

In order to better realize the present invention, M in the titrant MOH includes alkali metals such as lithium, sodium and potassium, especially KOH; the titrant concentration is 0.0100mol/L～0.1000mol/L; the solvent is Dehydrated ethanol or dehydrated methanol (water content 0.15～0.2%) without drying treatment, particularly preferably dehydrated alcohol without drying treatment; the lithium-ion battery electrolyte includes two parts, an organic solvent and a lithium salt, and the organic Solvents include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrolactone (γ-BL) , dimethoxyethane (DME), tetrahydrofuran (THF), one or more mixed solvents, the lithium salt includes LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN(CF ₃ SO ₂ ) ₂ in one or more than one mixture.

Compared with the prior art, the present invention has the following advantages and beneficial effects:

The present invention proposes an electrolyte suitable for lithium-ion batteries on the basis of respectively studying the influence of titrants and solvents on the quantitative analysis of hydrofluoric acid, and analyzing and comparing the test results of the indicator method and the automatic potentiometric titration method respectively. The accurate analysis method of hydrofluoric acid in medium. Compared with the prior art, the present invention greatly improves the measurement accuracy (the result measured by the prior art is on the low side, and the degree of deviation is larger after adding the organic amine stabilizer in the electrolytic solution, and the acidity is larger and the stabilizer content is higher. The results of the test in the electrolyte differ significantly, with a deviation of 18.2%).

Contrast experiments prove that the test results using tetrabutylammonium hydroxide as the titrant are larger than those of other titrants; the titration curve measured with sodium methoxide as the titrant is the gentlest, the jump is not obvious, and the measured parallel results The deviation is relatively large (this is mainly due to the weak alkalinity of sodium methoxide and the slow reaction with acid); the titration jumps using NaOH and KOH as titrants are obvious, and the accuracy of parallel determination results is higher than that of the above two methods. This titrant is good, but due to the high reactivity of NaOH with the solvent, the test result is slightly higher than that of KOH. KOH has strong alkalinity, reacts quickly with acid, and the jump of the curve is more obvious, and KOH has no reaction with anhydrous methanol and anhydrous ethanol. The test shows that the titration curve in absolute methanol is gentler than that in absolute ethanol; the presence of trace moisture in absolute ethanol without further drying not only does not affect the determination results, but also makes the titration jump more obvious, which is more conducive to end point judgment. Therefore, the preferred KOH of the present invention is the titrant, and dehydrated alcohol without further drying is used as the solvent, the titration endpoint is the easiest to judge, the test result is the most accurate, and the used titrant and solvent are cheap, and the titration process does not need to be carried out in a glove box. It is a test method very suitable for industrial analysis.

Description of drawings

Fig. 1 is the titration curve of four kinds of titrants in two kinds of solvents of absolute methanol and absolute ethanol in the embodiment of the present invention four;

Fig. 2 is the acidity titration curve of Example 4 of the present invention in ethanol solvents with different water contents.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings and embodiments.

Some reagents and instruments used in the examples include:

Methanol (chromatographically pure, produced by Merk, Germany); absolute ethanol (analytical pure); 3A molecular sieves; benzoic acid (standard substance), dried at 80°C for 8 hours in a vacuum; bromothymol blue (BTB), in a vacuum of 100 Dry at ℃ for 8 hours; 0.1mol/L sodium hydroxide ethanol solution (analytical pure, produced by Merk Company of Germany); 0.1mol/L potassium hydroxide ethanol solution (analytical pure, produced by German Merk Company); tetrabutylammonium hydroxide ; 0.1mol/L isopropanol solution (analytically pure, produced by Germany Merk Company); sodium methoxide (analytical pure).

EC, DMC, EMC: content ≥ 99.95%, water content < 10ppm. LiPF ₆ : content ≥ 99.95%, HF content < 100ppm. Organic base (indicated by N): content ≥ 99.9%, water content < 100ppm. Deionized water.

Metrohm798 potentiometric titrator (produced by Metrohm, Switzerland); long-life composite pH electrode; Metrohm83 1KF moisture analyzer (produced by Metrohm, Switzerland); electronic balance (accuracy 0.0001g, Sartorius); glove box: argon atmosphere , anhydrous and oxygen-free; the preparation and sampling of the electrolyte are carried out in the glove box; 10ml PE material syringe.

Embodiment one

The first step is to use a dry 10ml PE material syringe to take 8ml of lithium-ion battery electrolyte in the glove box, seal the needle with silica gel, and transfer the syringe to the air; the lithium-ion battery electrolyte is composed of diethyl carbonate (DEC), γ- Butyrolactone (γ-BL) mixed with LiCF ₃ SO ₃ and LiBF ₄ , where γ-BL:DEC=1:2(w), the concentration of the two lithium salts is 0.5mol/L;

In the second step, add 50.0ml of anhydrous methanol (no drying treatment, water content 0.15%) into the titration cup with a pipette, add 2 grams of electrolyte (weigh the amount of electrolyte injected with a balance, accurate to 0.1mg), start stirring;

The 3rd step selects concentration and is that the potassium hydroxide of 0.0100mol/L is titrant, starts titration, the consumption of electrolytic solution is input in the potentiometric titrator, input second-order derivation program (Switzerland Metrohm company provides) and acidity calculation formula ( C ₀ ×V20)/(1000×m), wherein, C ₀ is the concentration mol/L of the titrant, V is the volume ml of the titrant consumed, 20 is the molecular weight of HF, and m is the weight g of the electrolyte; The content of hydrogen fluoride in the liquid adjusts the titration step length, the titration step length is 1/10 to 1/20 of the volume of the titrant consumed at the end point of the titration, titrate at a constant speed, and when the end point appears and the titration curve is very smooth, stop the titration and take the final peak. Jump to the end;

The fourth step is to check whether the titration step length is 1/10 to 1/20 of the titrant volume consumed at the end of the titration according to the test results. If not, adjust the titration step length and repeat the third step; the results are measured in parallel 3 times, and the average value is taken.

Embodiment two

The first step is to use a dry 10ml PE material syringe to take 8ml of lithium-ion battery electrolyte in the glove box, seal the needle with silica gel, and transfer the syringe to the air; the lithium-ion battery electrolyte is composed of propylene carbonate (PC), dimethoxy Diethyl ethane (DME) and LiClO ₄ , LiAsF ₆ mixed composition, where PC: DME = 1:2 (w), the concentration of the two lithium salts are 0.5mol/L;

In the second step, add 50.0ml of absolute ethanol with a pipette in the titration cup (no drying treatment, water content 0.2%), add 2.5 grams of electrolyte (weigh the amount of electrolyte injected with a balance, accurate to 0.1mg), start stirring;

The 3rd step selects the sodium hydroxide that concentration is 0.0500mol/L to be titrant, starts titration, the consumption of electrolytic solution is input in the potentiometric titrator, input second-order derivation program (Switzerland Metrohm company provides) and acidity calculation formula ( C ₀ ×V20)/(1000×m), wherein, C ₀ is the concentration mol/L of the titrant, V is the volume ml of the titrant consumed, 20 is the molecular weight of HF, and m is the weight g of the electrolyte; The content of hydrogen fluoride in the liquid adjusts the titration step length, the titration step length is 1/10 to 1/20 of the volume of the titrant consumed at the end point of the titration, titrate at a constant speed, and when the end point appears and the titration curve is very smooth, stop the titration and take the final peak. Jump to the end;

The fourth step is to check whether the titration step length is 1/10 to 1/20 of the titrant volume consumed at the end of the titration according to the test results. If not, adjust the titration step length and repeat the third step; the results are measured in parallel 3 times, and the average value is taken.

Embodiment three

The first step is to use a dry 10ml PE material syringe to take 8ml of lithium-ion battery electrolyte in the glove box, seal the needle with silica gel, and transfer the syringe to the air; the lithium-ion battery electrolyte is composed of PC, methyl carbonate (DMC), Tetrahydrofuran (THF) and LiN(CF ₃ SO ₂ ) ₂ mixed composition where PC:DMC:THF=2:3:1 (w), the concentration of lithium salt is 1mol/L;

The second step is to add 50.0ml of absolute ethanol (without drying process, water content 0.2%) into the titration cup with a pipette, and add 3 grams of electrolyte (weigh the amount of injected electrolyte with a balance, accurate to 0.1mg), start stirring;

The 3rd step selects the lithium hydroxide that concentration is 0.1000mol/L to be titrant, starts titration, the consumption of electrolytic solution is input in the potentiometric titrator, input second-order derivation program (Switzerland Metrohm company provides) and acidity calculation formula ( C ₀ ×V20)/(1000×m), wherein, C ₀ is the concentration mol/L of the titrant, V is the volume ml of the titrant consumed, 20 is the molecular weight of HF, and m is the weight g of the electrolyte; The content of hydrogen fluoride in the liquid adjusts the titration step length, the titration step length is 1/10 to 1/20 of the volume of the titrant consumed at the end point of the titration, titrate at a constant speed, and when the end point appears and the titration curve is very smooth, stop the titration and take the final peak. Jump to the end;

The fourth step is to check whether the titration step length is 1/10 to 1/20 of the titrant volume consumed at the end of the titration according to the test results. If not, adjust the titration step length and repeat the third step; the results are measured in parallel 3 times, and the average value is taken.

Embodiment four

For further illustrating the present invention, present embodiment adopts the mode that compares with prior art indicator method to carry out:

Comparative indicator method: use a dry 10ml PE material syringe to take 8ml of electrolyte in the glove box, seal the needle with silica gel, add 50ml of absolute ethanol and 4 to 5 drops of BTB into the titration cup with a pipette, and add the electrolyte 3 to 4 grams (accurate to 0.1 mg), mix well, shake while dripping until the solution turns blue, record the amount of titrant used, and measure 3 times in parallel.

Prepare the following Li-ion battery electrolytes in the glove box, but are not limited to the following formulations:

A. EC: DMC: EMC = 1:1:1 (w), LiPF ₆ : 1mol/L (is this expression possible)

B. EC: DMC: EMC = 1:1:1 (w), LiPF ₆ : 1mol/L, add stabilizer N0.1% (w)

C. Add a small amount of water to A to make the moisture content about 100ppm, and seal it at 45°C for 24 hours, then add stabilizer N0.5% (w)

D. EC:PC:DMC:EMC=2:1:2:3 (w), LiPF ₆ : 1mol/L, plus stabilizer N1% (w).

Dilute 0.1 mol/L sodium hydroxide ethanol solution and potassium hydroxide ethanol solution 10 times with dry ethanol (H ₂ O<20ppm) in the glove box, 0.1 mol/L tetrabutylammonium hydroxide isopropyl Dilute the alcoholic solution 10 times with methanol to prepare a 0.01mol/L methanolic solution of sodium methoxide. The four solutions were calibrated with reference benzoic acid.

Comparison of test methods: KOH is used as titrant, absolute ethanol is used as solvent, and electrolytes B and C are measured in parallel three times, one using potentiometric titration, and the other using indicator method.

Comparison of titrants and solvents: Use four kinds of titrants to measure electrolyte A in dry ethanol and methanol: use a 10ml syringe in the glove box to seal about 6ml of the sample, seal the syringe with silica gel, and measure it indoors. Each sample injection is about 2.5g, and the parallel measurement is performed three times. The test method is potentiometric titration. Compare the effects of titrants and solvents on the determination results.

Using KOH as the titrant, measure the electrolytes A and D in absolute ethanol (water content ~ 2000ppm) and further dried anhydrous ethanol (water content ~ 10ppm), respectively, and compare the influence of the presence of trace moisture on the test results.

Determination results of hydrofluoric acid:

(1) Concentration calibration of titrant

The concentration of four kinds of titrants in table 1 (unit: mol/L) Titrant 1 2 3 average result NBu4OH 0.0098 0.0010 0.0098 0.0099 NaOCH ₃ 0.0097 0.0099 0.0098 0.0098 NaOH 0.0101 0.0102 0.0101 0.0101 KOH 0.0102 0.0103 0.0102 0.0102

(2) Comparison of test methods

In industrial applications, in order to improve the stability of the electrolyte, a certain amount of Lewis base (such as organic amine) is generally added as a stabilizer to make the electrolyte close to neutral or alkaline (pH _0.7-9 ). None of the existing technologies has considered this problem, so it is likely that the measured end point pH is not within the discoloration range of the indicator by using the existing technology, so there will be a large error. If the alkalinity of the stabilizer is strong or the content is high, the electrolysis The pH ₀ of the solution is greater, so the deviation of the result may be greater.

Table 2 is that the present invention (automatic potentiometric titration method) compares with prior art (indicator method) test electrolyte A and B result, and table 3 is the electrical result comparison of two kinds of methods test electrolyte C, and wherein pH _z is a test end point When the pH value was measured, the titrant was KOH, the solvent was undried ethanol, and the parallel measurement was performed 3 times.

The impact of table 2 stabilizer on the quantitative analysis of hydrofluoric acid Electrolyte sample pH ₀ m ₁ (ppm) pH _z1 m ₂ (ppm) pH _z2 (M ₂ -M ₁ )/M ₂ ×100％ A 5.4 13.9 6.3 15.0 6.7 8.5 14.5 6.5 15.6 6.9 14.3 6.4 16.1 6.7 B 6.8 14.0 7.2 16.3 7.5 8.8 14.2 7.1 15.9 7.6 15.0 7.2 15.3 7.5

Note: m ₁ is the result of indicator method test, M ₁ is its average value, m ₂ is the test result of potentiometric titration method, and M ₂ is its average value.

Table 3 Two methods test the results of the electrolyte of electrolyte C

pH ₀ m ₁ (ppm) pH _z1 m ₂ (ppm) pH _z2 (M ₂ -M ₁ )/M ₂ ×100%

259.7 7.6 305.6 7.9

7.2 230.4 7.5 298.9 7.8 18.2

249.7 7.6 300.3 7.9

Note: m ₁ is the result of indicator method test, M ₁ is its average value, m ₂ is the test result of potentiometric titration method, and M ₂ is its average value.

It can be seen from the data in Table 2 that the addition of the stabilizer has almost no effect on the total acidity of the electrolyte, but only increases the pH value of the electrolyte. In conjunction with the data analysis of Table 3, the result determined by the indicator method is smaller than the result determined by the potentiometric titration method, and this deviation is small (8.5%) in the electrolyte solution with low acidity content (<30ppm); The difference between the test results of the two methods increases greatly after the injection, especially in the electrolyte with high acidity (≥50ppm) and high stabilizer content (≥0.5), the difference between the test results is very obvious (18.2%), and the indication The accuracy of the test results of the reagent method is very poor, indicating that the indicator method is not suitable for the acidity test in the current electrolyte system at this time.

(3) Comparative analysis of titrant and solvent

Table 4 is respectively in methanol and ethanol, adopts tetrabutylammonium hydroxide, sodium hydroxide, potassium hydroxide and sodium methylate four kinds of titrants to test the result of 3 parallel tests of electrolyte A, and the test method is automatic potentiometric titration Law. As shown in Figure 1, the titration jump in ethanol B is more obvious than in methanol A (a, tetrabutylammonium hydroxide, b, sodium hydroxide, c, potassium hydroxide, d, sodium methoxide). The data in Table 4 show that KOH has the best accuracy among the four titrants. The test results of tetrabutylammonium hydroxide as the titrant are larger than those of the other three titrants. The jump of sodium methoxide is very flat, which makes it difficult to choose the end point of the jump. The accuracy of the test results with KOH as titrant is the best, whether methanol or ethanol is used as solvent, the standard deviation is less than 1%.

The test results (unit: ppm) of different titrants in methanol and ethanol respectively in table 4 Titrant tetrabutylammonium hydroxide sodium hydroxide Potassium hydroxide Sodium methoxide Methanol Measurements 28.3, 27.7, 27.0 16.8, 21.8, 12.4 14.4, 16.3, 15.8 15.2, 11.0, 16.9 Standard DeviationS(%) 0.65 4.70 0.98 3.04 ethanol Measurements 22.8, 18.6, 16.9 16.5, 16.9, 17.3 11.7, 11.3, 12.0 16.6, 18.0, 15.1 Standard DeviationS(%) 3.04 0.40 0.35 1.45

(4) Influence of trace moisture on test results

In the prior art, ethanol is dried through molecular sieves. In this embodiment, 3A molecular sieves are used to dry ethanol, and the ethanol before and after drying is used as a solvent for testing. The tested electrolytes are A (Figure A) and D (Figure B). Table 5 shows the test results, which show that the results of the ethanol test before and after drying are not much different, and the accuracy of the test results with undried ethanol as the solvent is better. As shown in Figure 2, curve a is the test titration curve for ethanol (H ₂ O: 2171.6ppm); curve b is the test titration curve for ethanol (H ₂ O: 23.6ppm). It can be seen that the sudden jump of the titration curve in undried ethanol is steeper than that in dried ethanol, and the data in Table 5 shows that the accuracy of the results measured in undried ethanol is also higher. From this result, it can be seen that the influence of trace moisture (0.15-0.2%) in the analysis of pure ethanol on the test results can be ignored, and the experimental results show that the existence of these moisture is beneficial to the judgment of the end point.

Table 5 The influence of solvent water content on hydrofluoric acid quantitative analysis Moisture content (ppm) Electrolyte Acidity (ppm) RSD(%) 2171.6 A 15.0, 13.0, 13.6 1.03 D. 115.3, 117.1, 114.4 1.37 23.6 A 15.2, 13.2, 14.5 1.01 D. 114.0, 118.3, 116.7 2.17

As described above, the present invention can be preferably realized.

Claims (7)

1. A quantitative analysis method for hydrofluoric acid in lithium ion battery electrolyte is characterized in that the lithium ion battery electrolyte is diluted to a solvent without drying treatment according to the proportion that 2.5-3.0 g of the lithium ion battery electrolyte is added into 50ml of the solventIn water methanol or absolute ethanol, MOH is used as a titrant, an automatic potentiometric titration method is adopted to determine a titration curve, and a potentiometric titrator is used for carrying out second-order derivation on the titration curve, so that a titration end point is determined; using the formula (C)₀X V20)/(1000 x m) calculating the end point value, where C₀Is the concentration mol/L of the titrant, V is the volume ml of the titrant consumed, 20 is the molecular weight of HF, and m is the weight g of the electrolyte.

2. The method of claim 1, wherein M in the titrant MOH comprises lithium, sodium and potassium as alkali metals.

3. The method of claim 1 or 2, wherein the titrant is KOH.

4. The method of claim 1, wherein the titrant concentration is 0.0100mol/L to 0.1000 mol/L.

5. The method of claim 1, wherein the solvent is anhydrous ethanol or anhydrous methanol without drying treatment, and the water content is 0.15-0.2%.

6. The method for quantitatively analyzing the hydrofluoric acid in the lithium ion battery electrolyte according to claim 1 or 5, wherein the solvent is absolute ethyl alcohol without drying treatment, and the water content is 0.15-0.2%.

7. The method of claim 1, wherein the electrolyte comprises an organic solvent and a lithium salt, and the organic solvent comprises ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ -butyrolactone, dimethyl carbonate, or a mixture thereofOne or more mixed solvents of oxyethane and tetrahydrofuran, wherein the lithium salt comprises LiCF₃SO₃、LiPF₆、LiBF₄、LiClO₄、LiAsF₆、LiN(CF₃SO₂)₂One or more than one mixture.

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