Electrolyte distribution measurement method

Abstract

The present application relates to the field of lithium-ion battery technology, and in particular to a method for determining electrolyte distribution, comprising: preparing multiple electrolytes with different lithium salt concentrations, subjecting each electrolyte to constant current charging in a three-electrode system, and calculating the capacitance S c , according to the capacitance S c Plot the standard S against the corresponding lithium salt concentration C in the electrolyte c ‑C relationship curve; the filled cell is disassembled to obtain the electrode to be tested, the electrode to be tested is punched into multiple units and then extracted with an organic solvent to obtain the corresponding extract, the extract corresponding to the j-th unit is placed in a three-electrode system for constant current charging to obtain the capacitance S cj , in standard S c Determine the capacitance S from the ‑C relationship curve cj The corresponding lithium salt concentration C j =The concentration of lithium salt in the extract corresponding to the jth unit cell, where j is a positive integer. This application can provide theoretical support for optimizing the injection process to improve the performance of the battery cell.

Description

Technical Field

[0001] The present application belongs to the technical field of lithium-ion batteries, and in particular relates to a method for measuring electrolyte distribution. Background Art

[0002] Lithium-ion batteries (LiBs) have outstanding advantages such as high energy density, good cycle life, high operating voltage, low self-discharge rate and environmental friendliness. They are the key to solving the intermittent characteristics of renewable energy and have therefore attracted much attention. Lithium-ion batteries mainly include positive electrode, negative electrode, separator and electrolyte, among which the electrolyte is lithium ion (Li + ) provides a good medium for movement inside the battery cell, but the electrolyte and the electrode will form a solid electrolyte interface (SEI film) by side reaction. The uniform and dense SEI film is Li + It is an excellent conductor and can effectively prevent the damage caused by the co-embedding of organic solvent molecules to the electrode material. However, the formation of SEI film consumes Li + If it is allowed to continue to form, the Coulombic efficiency of LiBs will decrease and the capacity will decay rapidly.

[0003] In pursuit of high-capacity lithium-ion batteries, anode materials with high theoretical capacity, such as silicon-based anodes, have become a research hotspot. However, silicon-based materials pulverize during cycling, exposing new surfaces. Excessive electrolyte content in these areas can lead to locally thicker SEI films. Therefore, the distribution of electrolyte affects the thickness and uniformity of the SEI film on the anode surface, thereby causing changes in various aspects of the battery cell's electrical performance. Research on the internal electrolyte distribution of lithium-ion batteries provides theoretical support for optimizing the injection process during battery production, and provides data support for improving battery cell electrical performance by optimizing the injection volume.

[0004] Battery cells are sealed during normal use or testing. Measuring the electrolyte using high-resolution 3D-CT is not only inconvenient and costly, but also difficult to determine its distribution. Due to the volatility of the electrolyte, measuring electrolyte distribution within the cell is particularly challenging. Therefore, there is an urgent need to develop a method that can simply and quickly characterize the electrolyte distribution within the cell. Summary of the Invention

[0005] The purpose of this application is to provide a method for measuring electrolyte distribution, aiming to solve the technical problems of complex and high cost of existing electrolyte distribution measurement technology inside battery cells.

[0006] To achieve the above application objectives, the technical solutions adopted in this application are as follows:

[0007] The present application provides a method for determining electrolyte distribution, comprising:

[0008] Prepare multiple electrolytes with different lithium salt concentrations, charge each electrolyte in a three-electrode system with constant current, and then calculate the capacitance S according to the following formula c , according to the capacitance S c Plot the standard S against the corresponding lithium salt concentration C in the electrolyte c -C relationship curve;

[0009]

[0010] Wherein, I is the charging current during constant current charging, t1 is the charging time during constant current charging, and U1 is the voltage during constant current charging;

[0011] The injected cell is disassembled to obtain the electrode to be tested. The electrode to be tested is punched into multiple units and then extracted with organic solvent to obtain the corresponding extract. The extract corresponding to the j-th unit is placed in a three-electrode system for constant current charging to obtain the capacitance S cj , in standard S c Determine the capacitance S from the -C relationship curve cj The corresponding lithium salt concentration C j That is, the lithium salt concentration in the extract corresponding to the j-th unit cell, where j is a positive integer.

[0012] The electrolyte distribution determination method provided in this application first determines the standard S obtained by constant current charging of the electrolyte in a three-electrode system. c -C relationship curve, then the electrode of the cell to be tested is punched into multiple unit pieces and the extracted liquid is placed in a three-electrode system and charged with the same constant current to obtain the capacitance S cj , so according to the previous standard S c -C relationship curve to determine the lithium salt concentration C in the extract corresponding to the unit piece j This application adopts a constant current charging method in a three-electrode system to conduct capacitance S cj Determination, so the high sensitivity of the electrochemical workstation is used and the standard S is fitted. c -C relationship curve can accurately characterize the lithium ion content in the extract, and thus infer the distribution state of the electrolyte in the battery cell. Therefore, this application can simply and quickly reflect the distribution of the electrolyte in the battery cell, and provide good theoretical support for the optimization of the injection process in the lithium-ion battery production process, thereby improving the battery cell performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0014] Figure 1 Schematic diagram of the flow of the electrolyte distribution measurement method provided in the embodiment of the present application;

[0015] Figure 2 This is the constant current charging mechanism in the electrolyte distribution determination method provided in the embodiment of the present application;

[0016] Figure 3 This is a schematic diagram of the electrolyte distribution measurement method provided in an embodiment of the present application;

[0017] Figure 4 : This is a test diagram of electrolytes of different concentrations using the electrolyte distribution determination method provided in an embodiment of the present application, wherein the constant current charging current during the test is 0.2 mA, the voltage window is 0-1 V, a is the charge-discharge curve of the electrolyte with different lithium salt concentrations (1C represents the lithium salt concentration of a certain electrolyte provided by the applicant company), and b is the capacitance value of each electrolyte calculated based on the charge-discharge curve of a;

[0018] Figure 5 It is the standard S in the electrolyte distribution determination method provided in the embodiment of the present application. c -C relationship curve, the constant current charging current during the test is 4μA, and the charging time and discharge time are 20s respectively;

[0019] Figure 6 Figure 1 is a test diagram of a unit cell at different positions in the electrolyte distribution determination method provided in an embodiment of the present application, wherein the constant current charging current during the test is 4 μA, the charging time and the discharging time are each 20 s, a is the charge and discharge curve of the unit cell extract at different positions, and b is the capacitance value of each extract calculated based on the charge and discharge curve of a;

[0020] Figure 7 The capacitance value of the extract of the unit cell at different positions in the electrolyte distribution measurement method provided in the embodiment of the present application is Figure 5 Standard S c -C relationship curve comparison to obtain the extract concentration;

[0021] Figure 8 It is the capacitance value of the extract of the unit sheet at different positions in the electrolyte distribution measurement method provided in the embodiment of the present application. DETAILED DESCRIPTION

[0022] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0023] In this application, the term "and / or" describes the relationship between associated objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0024] In this application, "at least one" means one or more, "more than one" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items.

[0025] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0026] The terms used in the embodiments of the present application are for the purpose of describing specific embodiments only and are not intended to limit the present application. The singular forms "a", "an", "the" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0027] The weights of the relevant components mentioned in the examples of this application may not only refer to the specific content of each component, but also represent the weight ratio between the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the examples of this application, it is within the scope disclosed in the examples of this application. Specifically, the mass described in the examples of this application may be a mass unit known in the chemical industry, such as μg, mg, g, kg, etc.

[0028] The terms "first" and "second" are used solely for descriptive purposes to distinguish objects, such as substances, from one another and should not be understood to indicate or imply relative importance or to implicitly specify the quantity of the technical features being referred to. For example, without departing from the scope of the embodiments of this application, a first XX may also be referred to as a second XX, and similarly, a second XX may also be referred to as a first XX. Thus, features defined as "first" or "second" may explicitly or implicitly include one or more of such features.

[0029] The present invention provides a method for measuring electrolyte distribution. Figure 1 As shown, the following steps are included:

[0030] T01: Prepare multiple electrolytes with different lithium salt concentrations, charge each electrolyte in a three-electrode system with constant current, and then calculate the capacitance S according to the following formula: c , according to the capacitance S c Plot the standard S against the corresponding lithium salt concentration C in the electrolyte c -C relationship curve;

[0031]

[0032] Wherein, I is the charging current during constant current charging, t1 is the charging time during constant current charging, and U1 is the voltage during constant current charging;

[0033] T02: Disassemble the battery cell that has been filled with liquid to obtain the electrode to be tested, punch the electrode to be tested into multiple units, and then extract them with organic solvents to obtain the corresponding extracts. Place the extract corresponding to the j-th unit in a three-electrode system for constant current charging to obtain the capacitance S cj , in standard S c Determine the capacitance S from the -C relationship curve cj The corresponding lithium salt concentration C j That is, the lithium salt concentration in the extract corresponding to the j-th unit cell, where j is a positive integer.

[0034] The electrolyte distribution determination method provided in the embodiment of the present application includes the standard S c -C relationship curve and the actual unit cell extract concentration determination, specifically, first determine the electrolyte in the three-electrode system constant current charging treatment obtained by the standard S c -C relationship curve, then the electrode of the cell to be tested is punched into multiple unit pieces and the extracted liquid is placed in a three-electrode system and charged with the same constant current to obtain the capacitance S cj , so according to the previous standard S c -C relationship curve to determine the lithium salt concentration C in the extract corresponding to the unit piece j This application adopts a constant current charging method in a three-electrode system to conduct capacitance S c Determination, so the high sensitivity of the electrochemical workstation is used and the standard S is fitted. c -C relationship curve can accurately characterize the lithium ion content in the extract, and thus infer the distribution state of the electrolyte in the battery cell. Therefore, this application can simply and quickly reflect the distribution of the electrolyte in the battery cell, and provide good theoretical support for the optimization of the injection process in the lithium-ion battery production process, thereby providing data support for improving the cycle performance of the battery cell from the perspective of optimizing the injection amount.

[0035] The above step T01 is standard S c -C relationship curve acquisition steps.

[0036] Specifically, a series of electrolytes with the same volume and a gradient distribution of lithium salt concentration can be prepared, and constant current charging can be performed in a three-electrode system. The capacitance S can be fitted using Minitab. c and electrolyte lithium salt concentration curve, according to the actual measured capacitance S of the extract cj Accurately characterize the lithium ion content in the battery cell. The number of electrolytes is expressed in S, which can fully reflect the actual electrolyte concentration of the battery cell. c -C relationship curve shall prevail.

[0037] like Figure 2 As shown, a constant current charging test is performed in a three-electrode system. The working electrode and the counter electrode are equivalent to forming an electric double layer supercapacitor (EDLC). By charging with a constant current of charging current I, the capacitance can be calculated using the time parameter t1 and the voltage parameter U1. Specifically, we can fix two parameters in the formula and use the change of the remaining parameter to obtain the capacitance S of the electrolyte with different lithium salt concentrations under constant current charging. c .

[0038] Specifically, the charging current I of the constant current charging process remains unchanged. In the actual operation process, the voltage U1 can be fixed, and the charging time t1 required for the constant current charging of electrolytes with different lithium salt concentrations to reach the voltage U1 is calculated. C For example, for an electrolyte with a high lithium salt concentration, the fixed voltage U1 is the window voltage, and the charging time t1 required to reach the window voltage U1 after constant current charging is calculated as S C Alternatively, the charging time t1 can be fixed, and the voltage U1 reached after constant current charging t1 of electrolytes with different lithium salt concentrations can be used to calculate the S of different electrolytes. C In the actual electrolyte distribution measurement process, due to the low lithium salt concentration of the extract, the window voltage is generally reached quickly, while the standard S c The constant current charging treatment in the drawing of the -C relationship curve is consistent with the constant current charging treatment conditions of the subsequent actual measurement of the extract. Therefore, it is preferred to use a fixed charging time t1 and calculate S according to the voltage U1 obtained after constant current charging t1 of the electrolyte with different lithium salt concentrations. C , which can better characterize the electrolyte distribution in the actual battery cell; finally, the capacitance S calculated by constant current charging of electrolytes with different lithium salt concentrations C , draw S C -C relationship curve.

[0039] In one embodiment, the above obtained standard SC The type of lithium salt and solvent in the electrolyte prepared for the C-C relationship curve are the same as those in the electrolyte for subsequent cell injection. Specifically, the lithium salt in the prepared electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(oxalatoborate), and lithium bis(fluorosulfonyl)imide. Furthermore, the solvent in the electrolyte is selected from at least one of dimethyl carbonate (DMC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC), and the organic solvent in the subsequent extraction process of the test unit is the same as the solvent in the electrolyte.

[0040] In one embodiment, the steps of preparing multiple electrolytes with different lithium salt concentrations include:

[0041] The porosity of the positive electrode sheet P1, the porosity of the negative electrode sheet P2 and the porosity of the diaphragm P3 before the battery cell assembly are measured to obtain the porosity ratio of the positive electrode sheet P a % = [P1 / (P1+P2+P3)] × 100% and the negative electrode porosity ratio P b % = [P2 / (P1+P2+P3)] × 100%; then calculate the lithium salt center concentration C0 after the electrode is theoretically punched into a plurality of the unit sheets according to the following formula;

[0042]

[0043] Where S1 is the area of ​​a unit cell, S2 is the area of ​​the entire electrode, m is the mass of lithium salt in the electrolyte to be injected, M is the molar mass of lithium salt in the electrolyte, and V is the volume of one portion of electrolyte;

[0044] A plurality of electrolytes with the same volume and a gradient of lithium salt concentration are prepared with the lithium salt central concentration C0 as the center.

[0045] This application can measure the electrolyte distribution of the positive or negative electrode sheet. Therefore, in the above process, the porosity ratio of the negative electrode sheet and the porosity ratio of the positive electrode sheet can be measured and the average value can be taken to calculate C0. c -C relationship curve, based on the porosity, calculate the concentration C0 formed by the theoretical electrolyte distribution when the electrode is punched into a unit piece as the lithium salt center concentration, and use this ligand lithium salt concentration to form a series of electrolytes with a gradient. Specifically, the concentration C0 is the lithium salt center concentration, with the upper and lower limits of 10 -1 Prepare 6 to 10 parts of electrolyte with the volume of V in multiples of the order of magnitude, so that the standard S c -C relationship curve to simplify the standard S c The volume V of each electrolyte can be 10-25 ml, and the volume of the extract of each unit cell in subsequent actual testing is the same as the volume V of the electrolyte.

[0046] In one embodiment, the method for measuring the porosity P1 of the negative electrode sheet, the porosity P2 of the positive electrode sheet, and the porosity P3 of the separator before the battery cell is assembled includes: punching the negative electrode sheet, the positive electrode sheet, or the separator before the battery cell is assembled into a test piece with the same area as the unit cell, and then calculating the porosity of the test piece by immersion in an alkane solution. The calculation formula is as follows:

[0047]

[0048] Where S1 is the area of ​​the test piece and d is the thickness of the test piece.

[0049] Specifically, the positive and negative dry electrodes and the separator before winding were punched into test pieces using a punching machine, and their thickness d was measured respectively. 3 ) were immersed and the porosity was tested.

[0050] In one embodiment, the working electrode and the counter electrode in the three-electrode system are both inert electrodes, preferably Pt (foil electrode, optional size 2x2cm 2 , thickness is 0.1~0.2mm, the distance between the two electrodes is 0.5~0.8cm), the reference electrode is saturated Ag + The working electrode and counter electrode of the three-electrode system are inert foil electrodes, thus forming a symmetrical electric double-layer supercapacitor (EDLC) operating in constant current mode.

[0051] Furthermore, the constant current of the constant current charging treatment in the three-electrode system can be 1μA to 1mA, such as 1mA, 0.5mA, 0.4mA, 0.2mA, 0.1mA, 50μA, 20μA, 10μA, 4μA, etc. The current of the constant current charging treatment is constant during actual testing. For example, in a volume of 10mL with different concentrations of electrolyte, a Pt electrode is used as the working electrode and the counter electrode, and an Ag electrode is used as the counter electrode. + In the three-electrode system with AgCl electrode as reference electrode, constant current charging was performed with a current of 4 μA to obtain the capacitance S c , draw S c -C relationship curve.

[0052] The above step T02 is a step for testing the electrode sheets in the actual liquid-filled battery cell.

[0053] The target cell that has been injected with liquid is disassembled to obtain the positive electrode sheet or negative electrode sheet to be tested. The positive electrode sheet or negative electrode sheet to be tested is punched into multiple unit sheets and then extracted with an organic solvent to obtain the corresponding extract. Among them, the smaller the area of ​​each unit sheet, the more accurately it can reflect the electrolyte concentration distribution of the cell injection at the unit sheet position. Specifically, the unit sheet is a round sheet with a diameter of ≤20mm, such as a round sheet with a diameter of 5 to 17mm. The total number of unit sheets N is based on the actual lithium-ion battery model to reflect the electrolyte concentration distribution of each unit sheet at different positions of the positive and negative electrodes as much as possible. For example, for an 18650 model lithium-ion battery, the width of the electrode sheet is generally about 56 to 58mm, and the winding length is about 700 to 900mm. Specifically, according to actual needs, 100 to 800 unit sheets with the same area and different positions can be punched out for testing to reflect the electrolyte distribution at different positions in the battery cell.

[0054] In one embodiment, in order to ensure that the lithium salt concentration test accurately represents the actual distribution of the electrolyte in the battery cell, it is preferred to disassemble the battery cell with SOC (State of charge) of 0. Figure 3 As shown, the negative electrode sheet obtained by disassembling the target battery cell with fresh liquid injection and SOC=0 is then punched into multiple φ17 small discs using a φ17 punching machine. The small discs are placed in an organic solvent for extraction and then vacuum filtered to remove the fallen material powder and the volume is fixed to 10 (ml) to obtain an extract, which contains lithium ions left by the injected electrolyte. The extraction can be ultrasonic extraction, the ultrasonic time is 40 to 60 minutes, and the ultrasonic temperature is 20 to 40°C. After ultrasonic extraction, the centrifugal treatment is performed at a centrifugal speed of 8000r / min and a time of 30 minutes to obtain an extract. Furthermore, the step of disassembling the battery cell that has been injected includes: first aging the battery cell at 30 to 40°C, and then exposing the electrode sheet to a dew point of -30 to -20°C for drying. Specifically, to ensure sufficient electrolyte penetration, the injected cells are aged at 30-40°C for 8-16 hours. The cells are then disassembled to obtain fresh battery electrodes. These fresh electrodes are then exposed to an ambient dew point of -30-20°C until the solvent in the electrodes is completely evaporated, resulting in dried electrodes. The dried electrodes are then disassembled and pressed a second time to obtain multiple dry small unit cells.

[0055] After the target cell is disassembled to obtain the electrode to be tested and punched into N unit pieces, the extracts of the N unit pieces at different positions are placed in the previous three-electrode system for constant current charging to obtain the capacitance S cj The corresponding lithium salt concentration C j That is, the concentration of lithium salt in the extract corresponding to the jth unit cell, in the standard S c -C relationship curve to find the capacitance S cjThe corresponding concentration C j , and obtain the lithium salt concentration in all unit cells.

[0056] In one embodiment, according to the lithium salt concentration C j , use the following formula to get the electrolyte volume V for different unit cells j电 :

[0057]

[0058] Where V is the volume of the extract corresponding to the jth (j≤N) unit cell, C 电 is the lithium salt concentration of the electrolyte during injection. By quantitatively testing the residual lithium salt in the electrode inside the battery cell, the above formula can be used to obtain the distribution diagram of the electrolyte volume content in the electrode.

[0059] In one embodiment, according to the volume of the electrolyte V j电 , the discrete degree of the electrolyte after injection is obtained using the following formula:

[0060]

[0061] Among them, S is the discrete degree of electrolyte distribution after injection, N is the number of unit cells, is the average volume of electrolyte on the cell. Using the electrode electrolyte volume distribution diagram, the above formula can be used to further analyze the uniformity and dispersion of the electrolyte distribution within the cell, thereby characterizing the electrolyte stability in the cell.

[0062] In summary, the present application provides a method for determining the electrolyte distribution in lithium-ion battery cells based on electrochemical testing. The target cell with fresh electrolyte injection is disassembled, and the electrode sheet is punched out using a punching machine to obtain a unit sheet. The electrolyte lithium salt in the unit sheet is extracted using the same organic solvent in the electrolyte as the extractant. The extract is used as the electrolyte for constant current charging in a three-electrode system to calculate the capacitance S. c Get standard S c -C relationship curve. Finally, the actual test extraction results of each unit cell were compared with the standard S obtained based on the theoretical electrolyte distribution. c -C relationship curve is compared to obtain the lithium salt concentration in the extract, and then the electrolyte distribution amount of the unit sheet at different positions is deduced. Constant current charging is adopted, and the high sensitivity characteristics of the electrochemical workstation are utilized, combined with Minitab to fit the capacitance and electrolyte lithium salt concentration curve. According to the fitting curve, the lithium ion content in the battery cell electrode is obtained, and the distribution state of the electrolyte in the fresh battery cell is obtained by reverse deduction. This method has the advantages of low cost, environmental friendliness, simplicity and standardization. Moreover, the distribution state and discreteness of the electrolyte inside the battery cell are obtained according to the measurement results, which can further optimize the injection process and improve the battery cell performance.

[0063] The following describes the details in conjunction with specific embodiments.

[0064] Example 1

[0065] A method for determining the distribution of negative electrode electrolyte in a lithium-ion battery cell comprises the following steps:

[0066] Step 01: Standard S c -C relationship curve acquisition

[0067] Porosity determination: The positive and negative electrode dry sheets and diaphragms before fresh assembly of the battery cell were punched into φ17 (17 mm diameter) discs using a punching machine. The porosity was calculated and measured using the n-hexadecane immersion method to obtain the negative electrode porosity, positive electrode porosity, and diaphragm porosity. The proportion of the negative electrode porosity in the total porosity (P%) was then calculated, as shown in Table 1 below:

[0068]

[0069] The density of hexadecane is ρ1 = 0.7733 g / cm 3 , S1 is the area of ​​the φ17 disc, and d is the thickness of the φ17 disc.

[0070] Table 1

[0071]

[0072]

[0073] Calculate the lithium salt center concentration C0: Calculate the positive electrode porosity ratio P according to Table 1 a % and the negative electrode porosity ratio P b %, and the lithium salt center concentration C0 is determined using the following formula:

[0074]

[0075] Where:

[0076] S1------φ17 disc area, mm 2 ;

[0077] S2------total area of ​​negative electrode, mm 2 ;

[0078] ------LiPF4 molar mass, 151.9 g / mol;

[0079] V------the volume of one electrolyte, 10ml.

[0080] Draw Standard S c-C relationship curve: With the above C0 as the center concentration, a series of electrolytes with the same volume V (10 ml, composed of DMC, EC, and EMC with a volume ratio of 2:1:1) of LiPF4 lithium salt concentration gradient are prepared, and placed in a Pt electrode as the working electrode and the counter electrode, Ag + In a three-electrode system with a / AgCl electrode as the reference electrode, constant current charging is performed at a current of 4uA, and the charging and discharging times are each 20s. The capacitance S is calculated according to the following formula c :

[0081]

[0082] Wherein, I is the charging current of the constant current charging process;

[0083] t1 is the predetermined charging time for constant current charging;

[0084] U1 is the voltage at which the constant current charging process reaches the predetermined charging time.

[0085] According to the S calculated in lithium salt solutions with different concentrations c ;like Figure 4 As shown in the figure, by testing with a current of 0.2mA and a voltage window of 0-1V, the charge-discharge curves and capacitance values ​​of different LiPF4 lithium salt concentration electrolytes (1C, 0.8C, 0.6C, 0.4C, 0.2C) and DMC solvents can be obtained. However, this condition quickly reaches the window voltage. Therefore, for the convenience of the experiment, a constant current charge treatment of 4μA is further selected, with a charging time and a discharge time of 20s each. The capacitance value S is obtained based on the charge-discharge curve. c . Draw the standard S c -C relationship curve, such as Figure 5 shown.

[0086] Step 02: Actual injection electrolyte distribution measurement

[0087] Extract preparation: Freshly injected 18650 cells were discharged and aged at 30°C for 10 hours. The cells were then disassembled to obtain fresh negative electrode sheets. These were exposed to a dew point of -30°C to -20°C until the solvent in the negative electrode sheets completely evaporated, resulting in dried negative electrode sheets. The dried negative electrode sheets were punched into multiple φ17 discs and ultrasonically extracted in a mixed organic solvent (DMC, EC, and EMC in a volume ratio of 2:1:1). Finally, vacuum filtration was used to remove the fallen negative electrode powder and the volume was fixed to 10ml to obtain the extract corresponding to each φ17 disc.

[0088] Electrolyte distribution measurement: The extracts of φ17 discs at different positions were placed in the three-electrode system previously used for constant current charging (the parameters were the same as those obtained). Figure 5Standard S c -C relationship curve) and obtain S cj , in standard S c -C relationship curve to find S cj The corresponding concentration C j , get the electrolyte lithium salt concentration in all φ17 discs; Figure 6-8 As shown, the discs at different positions have corresponding extract charge-discharge curves and capacitance values, so the corresponding concentration C can be obtained. j Moreover, according to this method, the electrolyte concentration of the electrode at different upper and lower positions of the inner ring, middle ring and outer ring of the battery cell can be further reflected.

[0089] The following formula is further used to infer the distribution volume of the electrolyte in the negative electrode sheet:

[0090]

[0091] Where:

[0092] C j ------Li measured in the extract of the j-th φ17 disc + Concentration, mol / L;

[0093] V------The volume of the test extract is 10ml;

[0094] C 电 ----Electrolyte Li during injection + Concentration, mol / L;

[0095] V j电 ----The volume content of electrolyte on the φ17 disc.

[0096] The following formula is further used to analyze the uniformity and dispersion of the electrolyte distribution inside the battery cell through the electrode electrolyte content distribution diagram:

[0097]

[0098] Where:

[0099] S------the degree of dispersion of electrolyte distribution after injection;

[0100] N------the total number of φ17 discs;

[0101] ------The average volume of electrolyte on the φ17 disc.

[0102] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A method for measuring electrolyte distribution, characterized in that: include: Prepare multiple electrolytes with different lithium salt concentrations, perform constant current charging on each of the electrolytes in a three-electrode system, and then calculate the capacitance S according to the following formula: c , according to the capacitance S c Plot the standard S with the corresponding lithium salt concentration C in the electrolyte c -C relationship curve; Wherein, I is the charging current of the constant current charging process, t1 is the charging time of the constant current charging process, and U1 is the voltage during the constant current charging process; The battery cell that has been filled with liquid is disassembled to obtain the electrode to be tested, and the electrode to be tested is punched into multiple unit pieces, which are then extracted with organic solvents to obtain corresponding extracts. The extract corresponding to the j-th unit piece is placed in the three-electrode system and subjected to the constant current charging process to obtain the capacitor S cj , in the standard S c -C relationship curve to determine the capacitance S cj The corresponding lithium salt concentration C j That is, the lithium salt concentration in the extract corresponding to the j-th unit cell, where j is a positive integer.

2. The electrolyte distribution measurement method according to claim 1, wherein The steps of preparing multiple electrolytes with different lithium salt concentrations include: The porosity of the positive electrode sheet P1, the porosity of the negative electrode sheet P2 and the porosity of the diaphragm P3 before the battery cell assembly are measured to obtain the porosity ratio of the positive electrode sheet P a % = [P1 / (P1+P2+P3)] × 100% and the negative electrode porosity ratio P b % = [P2 / (P1+P2+P3)] × 100%; then calculate the lithium salt center concentration C0 after the electrode is theoretically punched into a plurality of the unit sheets according to the following formula; Where S1 is the area of ​​a unit cell, S2 is the area of ​​the entire electrode, m is the mass of lithium salt in the electrolyte to be injected, M is the molar mass of lithium salt in the electrolyte, and V is the volume of one portion of electrolyte; A plurality of electrolytes with the same volume and a gradient of lithium salt concentration are prepared with the lithium salt central concentration C0 as the center.

3. The electrolyte distribution measurement method according to claim 2, wherein: The method for determining the porosity P1 of the negative electrode sheet, the porosity P2 of the positive electrode sheet, and the porosity P3 of the separator before cell assembly includes: punching the negative electrode sheet, the positive electrode sheet, or the separator before cell assembly into a test piece with the same area as the unit sheet, and then calculating the porosity of the test piece by an alkane solution immersion method. The calculation formula is as follows: Wherein, m1 is the weight of the test piece after immersion, m2 is the weight of the test piece before immersion, S1 is the area of ​​the test piece, and d is the thickness of the test piece.

4. The electrolyte distribution measurement method according to claim 1, wherein Also includes: According to the lithium salt concentration C j , use the following formula to get the electrolyte volume V for different unit cells j电 : Where V is the volume of the extract corresponding to the jth unit piece, C 电 is the lithium salt concentration of the electrolyte during injection.

5. The electrolyte distribution measurement method according to claim 4, wherein: Also includes: According to the volume of electrolyte V j电 , the discrete degree of the electrolyte after injection is obtained using the following formula: Among them, S is the discrete degree of electrolyte distribution after injection, N is the number of unit cells, is the average volume of electrolyte on the unit cell.

6. The electrolyte distribution measurement method according to claim 1, wherein: The working electrode and the counter electrode in the three-electrode system are both inert electrodes, and the reference electrode is Ag. + / AgCl.

7. The electrolyte distribution measurement method according to claim 6, wherein: The constant current of the constant current charging process in the three-electrode system is 1 μA to 1 mA.

8. The method for measuring electrolyte distribution according to any one of claims 1 to 7, wherein: The lithium salt in the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(oxalatoborate) and lithium bis(fluorosulfonyl)imide; and / or, The solvent in the electrolyte is selected from at least one of dimethyl carbonate, ethylene carbonate and ethyl methyl carbonate, and the organic solvent in the extraction process is the same as the solvent in the electrolyte.

9. The method for measuring electrolyte distribution according to any one of claims 1 to 7, wherein: The steps of disassembling the battery cell that has been filled with liquid include: first aging the battery cell at 30-40°C, then disassembling the electrode pieces and exposing them to an environment with a dew point of -30--20°C for drying.

10. The method for measuring electrolyte distribution according to any one of claims 1 to 7, wherein: The unit piece is a round piece with a diameter of ≤20 mm.

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