Method for testing activity of lithium ions in electrolyte

Abstract

The invention provides a method for testing the activity of lithium ions in an electrolyte, and relates to the technical field of lithium ion activity testing. The method comprises the following steps: testing components and contents of a solvent and a lithium salt of a to-be-tested electrolyte; then preparing a solvent again according to a result, and preparing gradient electrolyte with different lithium salt concentrations; testing the voltage of the gradient electrolyte, drawing and performing linear fitting by taking the natural logarithm value lna of the mass concentration of the lithium salt as an abscissa and the voltage E as an ordinate to obtain a fitting straight line; and measuring the voltage Es of the electrolyte to be measured, and calculating the lithium ion activity coefficient gamma s and the activity as of the electrolyte to be measured according to the formula and the intercept and the slope obtained by fitting the straight line. The invention provides the operable method for testing the activity of the lithium ions in the electrolyte, a voltage method is used for testing, the method can accurately evaluate the activity of the lithium ions in the electrolyte, and the result is stable.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion activity testing, and in particular to a method for testing the activity of lithium ions in an electrolyte. Background Art

[0002] After years of rapid development, China's new energy industry has achieved remarkable results in the global industrial chain. The demand for lithium-ion batteries has increased rapidly. In order to cope with different battery application scenarios, such as low temperature scenarios, high altitude scenarios, and long-range scenarios, R&D personnel have developed different system materials to meet the needs. Key materials in power batteries, such as positive electrode materials, negative electrode materials, diaphragm materials, electrolyte materials, etc., are the most basic units that affect battery performance, especially the reverse analysis of batteries. By analyzing the chemical composition, physical properties, structure and electrical properties of disassembled battery materials, the reactions of battery materials during battery use can be inferred, thereby guiding the research of battery materials and optimizing battery structure. And a suitable electrolyte is crucial to improving battery performance.

[0003] Activity is one of the most important properties of electrolyte solutions. It refers to the effective concentration of solutes in the solution. Activity reflects the interaction between ions and between ions and solvent molecules in a specified solvent. It is a hot topic in solution thermodynamics research and has important significance for the theoretical research and application of ion solvation, ion association and solution structure change. Activity is a measure of the concentration of ions in a solution and their ability to participate in reactions. This concentration is usually called the "activity coefficient" and is a characteristic value that reflects the actual reaction capacity of a chemical equation. The importance of activity lies in that it indicates the direction and rate of the reaction. In many chemical equations, the actual activity of the reactants is different from their concentration, so the calculation of activity is very important for correctly defining the reaction chemical equation. On the contrary, if concentration is used to describe activity, when the reaction conditions change, the actual rate and direction of the chemical reaction will be affected.

[0004] Therefore, it is very important to accurately measure the lithium ion activity in the electrolyte. However, the complex composition of the electrolyte leads to inaccurate and unstable results in the determination of lithium ion activity. Summary of the invention

[0005] In view of the above problems, the present invention provides a method for evaluating the activity of lithium ions in an electrolyte by using a voltage method.

[0006] The object of the present invention is to provide a method for testing the activity of lithium ions in an electrolyte.

[0007] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0008] The present invention provides a method for testing the activity of lithium ions in an electrolyte, comprising the following steps:

[0009] (a) testing the solvent composition and content of the electrolyte to be tested, and testing the lithium salt composition and content of the electrolyte to be tested;

[0010] (b) re-preparing the solvent according to the solvent composition and content tested in step (a), and dividing it into at least 5 portions, adding lithium salts of different mass concentrations to each portion and stirring evenly to form a gradient electrolyte, wherein the mass concentration a of the lithium salt ranges from 0.1% to 1.3%;

[0011] (c) pouring each gradient electrolyte of step (b) into the electrolytic cell in turn, connecting the working electrode and the reference electrode of the electrolytic cell with the positive and negative electrodes of the electrochemical workstation, and recording the voltage of each gradient electrolyte; wherein the working electrode is a Pt sheet; and the reference electrode is an oxidized Ag wire;

[0012] (d) plotting the natural logarithm value lna of the mass concentration a of the lithium salt in the gradient electrolyte as the abscissa and the voltage E of the gradient electrolyte as the ordinate, and performing a linear fit to obtain a fitted straight line;

[0013] (e) Measure the voltage E of the electrolyte to be tested according to the method of step (c) s , calculate the lithium ion activity a of the electrolyte to be tested according to formula I and formula II s :

[0014]

[0015] a s =γ s *m s Formula II,

[0016] in, is the slope of the fitted straight line, is the intercept of the fitted line, m s is the mass concentration of the lithium salt tested in step (a).

[0017] Principle description:

[0018] 1. The electrode potential E of metallic lithium in the electrolyte and the ion activity in the electrolyte satisfy the following formula (1):

[0019]

[0020] Where, E is the electromotive force, V;

[0021] E 0 is the standard electromotive force, V;

[0022] R is the gas constant, 8.31441 J / (K*mol);

[0023] T is absolute temperature, K;

[0024] F is the Faraday constant, 96487 J / (V*mol);

[0025] n is the charge transfer number, for this system, n = 1;

[0026] At a temperature of 293K, RT / nF = 25.24mv;

[0027] a is the activity of lithium ions in the electrolyte, a=γ*m, γ is the activity coefficient, and m is the concentration, mol / L.

[0028] During the test, a stable reference electrode is set up and the voltage V between them is measured:

[0029]

[0030] Since the reference electrode and the test system remain unchanged, equation (2) can be written as:

[0031]

[0032] In the formula, Is a constant.

[0033] On the other hand, in the electrolyte, when the electrolyte concentration is infinitely diluted, the activity is equal to the concentration, and equation (3) becomes:

[0034]

[0035] Therefore, under the condition that the solvent remains unchanged, extremely dilute solutions of different concentrations are prepared, and their voltages are tested. A straight line can be obtained with the voltage as the ordinate and ln m as the abscissa. The intercept is

[0036] 2. For a specific sample, its concentration is m s , the activity coefficient is γ s , formula (3) is rewritten as:

[0037]

[0038] According to formula (5), as long as the voltage V of the sample is tested s , we can calculate the activity coefficient γ of the sample s .

[0039] Based on the above principles, the present invention is obtained:

[0040] Step (a) Testing of electrolyte solvent composition and lithium salt composition:

[0041] The solvent composition and content of the electrolyte to be tested are tested (i.e. qualitative and quantitative) using a gas chromatography-mass spectrometer GC-MS;

[0042] The lithium salt composition and content of the electrolyte to be tested are tested using an ion chromatograph (the ion chromatograph tests anions, thereby qualitatively corresponding to different anion lithium salts, and quantifies anions, converting the anion content into lithium salt content).

[0043] Step (b) Preparation of extremely dilute electrolyte:

[0044] In a glove box, various solvent components are configured according to the tested content ratio, stirred evenly, to form a new electrolyte solvent; the new electrolyte solvent is divided into at least 5 parts (M 1 、M 2 、M 3 、M 4 、M 5 …), each portion is 20g, and different concentrations of lithium salt are added to each portion of electrolyte solvent (for example, taking 5 portions as an example, the concentration mass ratio of lithium salt is a 1 =0.10%, a 2 =0.25%, a 3 =0.40%, a 4 =0.78%, a 5 =1.23%), stir for at least 12 hours after adding the lithium salt to ensure that the lithium salt is completely dissolved and evenly distributed to form a gradient extremely dilute electrolyte, i.e., M 1 +a 1 Lithium salt, M 2 +a 2 Lithium salt, M 3 +a 3 Lithium salt, M 4 +a 4 Lithium salt, M 5 +a 5 Lithium salts….

[0045] Step (c) Voltage test of extremely dilute electrolyte:

[0046] Transfer the prepared gradient extremely dilute electrolyte to the clean room, insert the working electrode and the reference electrode into the electrolytic cell, assemble the electrolytic cell, pour the gradient extremely dilute electrolyte into the electrolytic cell in turn, connect the working electrode and the reference electrode with the positive and negative electrodes of the electrochemical workstation, measure and record the voltage of each extremely dilute electrolyte, that is, E 1 、E 2 、E 3 、E 4 、E 5 ….

[0047] Since the electrolyte has a complex composition and contains organic solvents, it is easy to react with the working electrode and cause corrosion, resulting in inaccurate and unstable results when measuring using the voltage method.

[0048] To this end, the present invention uses a Pt sheet as a working electrode and an Ag wire as a reference electrode. At the same time, the Ag wire is oxidized (heat treated in air) before use to improve its stability.

[0049] Step (d) E-lna linear fitting:

[0050] According to the above formula (4), a straight line can be obtained by plotting the natural logarithm value lna of the lithium salt mass concentration of the gradient electrolyte as the horizontal coordinate and the voltage E of the gradient electrolyte as the vertical coordinate.

[0051] According to the lithium salt mass concentration in step (b) (such as a 1 =0.10%, a 2 =0.25%, a 3 =0.40%, a 4 =0.78%, a 5 =1.23%) Take the natural logarithm ln(lna 1 =-2.23, lna 2 =-1.37, lna 3 =-0.90, lna 4 =-0.23, lna 5 =0.21), as the abscissa; the voltage E of the electrolyte of each lithium salt concentration obtained according to step (c), in mV, as the ordinate, plotted according to several E-lna points, and fitted into a straight line to obtain the intercept of the fitted straight line and slope

[0052] The closer the slope is to the theoretical value of 25.24, the more reasonable the test method is and the solution configuration is correct. If it exceeds ±0.5 of the theoretical value, the solution needs to be reconfigured for testing.

[0053] Step (e) Voltage test of the electrolyte to be tested and calculation of lithium ion activity:

[0054] Transfer the electrolyte to be tested to a clean room, insert the working electrode and the reference electrode into the electrolytic cell, assemble the electrolytic cell, weigh 20g of the electrolyte to be tested and pour it into the electrolytic cell, connect the working electrode and the reference electrode with the positive and negative electrodes of the electrochemical workstation (same as step (c)), and measure and record the voltage E of the electrolyte to be tested. s .

[0055] Calculation of lithium ion activity of the electrolyte to be tested:

[0056] First, calculate the lithium ion activity coefficient γ of the electrolyte to be tested according to formula I s :

[0057]

[0058] in, is the slope of the fitted line in step (d), is the intercept of the fitted line in step (d), m s is the mass concentration of the lithium salt in step (a);

[0059] Then calculate the lithium ion activity a of the electrolyte to be tested according to formula II s :

[0060] a s =γ s *m s Formula II. a s The unit is mol / L, which is converted into molar mass.

[0061] Beneficial effects:

[0062] Through the test of this experiment, effective and accurate lithium ion activity can be obtained. A corrosion-resistant electrolytic cell is used in the experiment, and the electrolytic cell can be sealed to ensure the stability of the electrolyte test environment. The electrodes used in the experiment are pre-treated, and the voltage of the low-concentration solution is configured to make it easier to test.

[0063] The invention provides an operable method for testing the activity of lithium ions in an electrolyte. The voltage method is used for testing. The method can accurately evaluate the activity of lithium ions in the electrolyte, and the result is stable.

[0064] The present invention has been described in detail above, but the above embodiments are only illustrative in nature and are not intended to limit the present invention. In addition, this article is not limited by any theory described in the above prior art or invention content or the following examples. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 is the relationship curve between the solution activity and voltage of the electrolyte A to be tested in the embodiment (T 293K);

[0066] Figure 2 It is the relationship curve between solution activity and voltage of electrolyte A to be tested in comparative example (T 293K). DETAILED DESCRIPTION

[0067] The present invention is further described below in conjunction with examples. It should be noted that the following examples are provided for illustrative purposes only and do not constitute a limitation on the scope of protection claimed for the present invention.

[0068] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the examples are conventional raw materials, reagents, methods in the art.

[0069] Example Testing the lithium ion activity in a certain electrolyte A

[0070] 1. Analysis of electrolyte solvent composition and lithium salt composition: First, the solvent composition and content of the electrolyte A to be tested were tested using a gas phase mass spectrometer GC-MS; then the lithium salt composition and content of the electrolyte A to be tested were tested using an ion chromatograph. The results are shown in Table 1.

[0071] Table 1

[0072] Element EMC EC DMC PS DEC <![CDATA[LiFP 6 ]]> Content wt% 59.46 26.21 0.19 0.12 0.25 13.77

[0073] 2. Preparation of extremely dilute gradient electrolyte: In a glove box, various solvents were mixed and stirred evenly according to the solvent composition and content ratio in Table 1 to prepare a new electrolyte solvent; the new electrolyte solvent was divided into 5 portions, each with 20 g, and the lithium salt mass was 0.02 g, 0.05 g, 0.08 g, 0.16 g, and 0.25 g (corresponding to the lithium salt mass concentration of a 1 =0.10%, a 2 =0.25%, a 3 =0.40%, a 4 =0.78%, a 5 =1.23%), different concentrations of lithium salt were added to each solvent, and stirred for at least 12 h to ensure that the lithium salt was completely dissolved and evenly distributed.

[0074] 3. Voltage test of extremely dilute gradient electrolyte: Transfer the above 5 prepared electrolytes to a clean room, insert the working electrode (Pt sheet) and the reference electrode (oxidized Ag wire) into a sealable electrolytic cell, assemble the electrolytic cell, pour the above 5 electrolytes into the electrolytic cell in turn, connect the working electrode and the reference electrode of the electrolytic cell with the positive and negative poles of the electrochemical workstation, and record the voltage of each extremely dilute electrolyte.

[0075] 4. The natural logarithm of the mass concentration of lithium salt in the extremely dilute gradient electrolyte, lna 1 =-2.23, lna 2 =-1.37, lna 3 =-0.90, lna 4 =-0.23, lna 5 =0.21 as the horizontal axis; the voltage (mV) of the corresponding extremely dilute electrolyte is used as the vertical axis to draw a graph and fit it into a straight line. The result is as follows Figure 1 As shown, the fitting result is E = 24.899lna + 307.07, the slope RT / nF = 24.899, which is close to the theoretical slope of 25.24, and the intercept It is 307.07.

[0076] 5. Voltage test of electrolyte A to be tested: transfer electrolyte A to a clean room, insert the working electrode (Pt sheet) and the reference electrode (oxidized Ag wire) into the electrolytic cell, assemble the electrolytic cell, weigh 20g of electrolyte A and pour it into the electrolytic cell, connect the working electrode and the reference electrode of the electrolytic cell with the positive and negative electrodes of the electrochemical workstation, and record the voltage E of electrolyte A. s .

[0077] 6. Calculation: E s Substitute into the formula In which RT / nF=24.899, m s =13.77%, and the activity coefficient γ is obtained s =0.2791;

[0078] Then according to formula a s =γ s *m s , converted into molar mass to obtain activity a s =0.2054mol / L.

[0079] Comparative Example

[0080] The difference from the embodiment is that in step 3, the electrodes use a working electrode (Pt sheet) and a reference electrode (Ag wire without oxidation treatment) to perform test fitting. The fitting results are as follows: Figure 2 shown.

[0081] The fitting result of step 4 is E=39.831lna+524.19, which does not conform to the theoretical slope.

[0082] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents, without departing from the spirit and essence of the claims of the present invention; and these modifications or replacements are still within the scope defined by the claims of the present invention.

Claims

1. A method for testing the activity of lithium ions in an electrolyte, characterized in that: The following steps are involved: (a) testing the solvent composition and content of the electrolyte to be tested, and testing the lithium salt composition and content of the electrolyte to be tested; (b) re-preparing the solvent according to the solvent composition and content tested in step (a), and dividing it into at least 5 portions, adding lithium salts of different mass concentrations to each portion and stirring evenly to form a gradient electrolyte, wherein the mass concentration a of the lithium salt ranges from 0.1% to 1.3%; (c) pouring each gradient electrolyte of step (b) into the electrolytic cell in turn, connecting the working electrode and the reference electrode of the electrolytic cell with the positive and negative electrodes of the electrochemical workstation, and recording the voltage of each gradient electrolyte; The working electrode is a Pt sheet, and the reference electrode is an oxidized Ag wire. (d) plotting the natural logarithm value lna of the mass concentration a of the lithium salt in the gradient electrolyte as the abscissa and the voltage E of the gradient electrolyte as the ordinate, and performing a linear fit to obtain a fitted straight line; (e) Measure the voltage E of the electrolyte to be tested according to the method of step (c) s , calculate the lithium ion activity a of the electrolyte to be tested according to formula I and formula II s : a s = γ s * m s Formula II, in, is the slope of the fitted straight line, is the intercept of the fitted line, m s is the mass concentration of the lithium salt tested in step (a).

2. The method according to claim 1, characterized in that In step (a), the solvent composition and content of the electrolyte to be tested are tested using a gas chromatography-mass spectrometer GC-MS.

3. The method according to claim 1, characterized in that In step (a), an ion chromatograph is used to test the lithium salt composition and content of the electrolyte to be tested.

4. The method according to claim 1, characterized in that: In step (b), the solvent is divided into 5 portions, and lithium salt is added to each portion at mass concentrations of 0.10%, 0.25%, 0.40%, 0.78% and 1.23%, respectively.

5. The method according to claim 1, characterized in that In step (b), the lithium salt is added and stirred for at least 12 hours.

6. The method according to claim 1, characterized in that In step (d), the slope of the fitting line is 25.24±0.5.

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