Method for determining electrolyte distribution in lithium-ion battery cells

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a method for measuring electrolyte distribution in a lithium ion battery cell, which comprises the following steps: preparing multiple portions of electrolyte with different lithium salt concentrations, performing constant potential deposition treatment on each portion of electrolyte in a three-electrode system to obtain a current-time curve, and drawing a standard Q-C relationship curve according to the integral quantity Q of the current-time curve and the corresponding lithium salt concentration C in the electrolyte; disassembling the cell after electrolyte injection to obtain positive electrode sheets, negative electrode sheets or separators to be measured, stamping the positive electrode sheets, the negative electrode sheets or the separators into multiple unit sheets, respectively treating the unit sheets with organic solvent extraction to obtain corresponding extraction liquid, performing constant potential deposition treatment on the extraction liquid corresponding to the i-th unit sheet to obtain the integral quantity Q of the current-time curve i , and determining the integral quantity Q i corresponding to the lithium salt concentration C i in the extraction liquid corresponding to the i-th unit sheet in the standard Q-C relationship curve. The application can provide good theoretical support for electrolyte injection process optimization in the production process of lithium ion batteries.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a method for measuring electrolyte distribution in a lithium ion battery cell. BACKGROUND

[0002] A lithium ion battery (LiBs) is a kind of secondary battery mainly relying on the movement of lithium ions between the positive electrode and the negative electrode to work. The lithium ion battery is concerned due to its outstanding advantages such as high energy density, excellent cycle life, high working voltage, low self-discharge rate, and environmental friendliness. The cell of the lithium ion battery mainly includes a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte provides a good medium for the movement of lithium ions (Li + ) inside the cell, but the side reaction of the electrolyte with the negative electrode will form a solid electrolyte interface (SEI film). The uniform and dense SEI film is a good conductor of Li + , and can effectively prevent the destruction of the electrode material caused by the co-embedding of organic solvent molecules. Although the SEI film has many benefits, the formation of the SEI film consumes Li + , and if it is allowed to continue to form, it will reduce the coulombic efficiency of the LiBs and cause the capacity to decay quickly.

[0003] Currently, in order to pursue high-capacity lithium ion batteries, high-theoretical-capacity negative electrode materials such as silicon-based negative electrodes have become a research hotspot. However, the silicon-based material will pulverize during the cycle process, exposing a new surface. If the electrolyte content here is too high, it will cause the SEI film to be locally thick. Therefore, the distribution of the electrolyte will affect the film thickness and uniformity of the SEI film on the negative electrode surface, thereby causing changes in the electrical performance of the cell in all aspects. Research on the distribution of the electrolyte inside the lithium ion battery can provide theoretical support for optimizing the electrolyte injection process in the battery production process and provide data support for improving the electrical performance of the cell from the aspect of optimizing the electrolyte injection amount.

[0004] However, the cell is in a sealed state during normal use or testing, and generally needs to use high-resolution tomography 3D-CT to measure the electrolyte. This not only is inconvenient and costly, but also is difficult to obtain the distribution of the electrolyte. In addition, since the electrolyte has strong volatility, it is very difficult to measure the distribution of the electrolyte in the cell. Therefore, it is urgent to develop a method for simply and quickly characterizing the distribution of the electrolyte inside the cell. SUMMARY

[0005] The purpose of the present application is to provide a method for measuring the distribution of the electrolyte in a lithium ion battery cell, which aims to solve the technical problems of the existing electrolyte distribution measurement technology in the cell being complex and costly.

[0006] To achieve the above-mentioned application purposes, the technical solutions adopted by the present application are as follows:

[0007] The application provides a method for measuring electrolyte distribution in a lithium ion battery cell, comprising:

[0008] A plurality of electrolytes with different lithium salt concentrations are prepared, and each electrolyte is subjected to constant potential deposition treatment in a three-electrode system to obtain a current-time curve. The integral quantity Q of the current-time curve is calculated, wherein t1 is the reaction equilibrium time of the constant potential deposition treatment; and a standard Q-C relationship curve is drawn based on the integral quantity Q and the corresponding lithium salt concentration C in the electrolyte.

[0009] The positive plate, negative plate or separator to be measured is obtained by disassembling the cell filled with electrolyte, and the positive plate, negative plate or separator to be measured is punched into a plurality of unit plates and then subjected to organic solvent extraction treatment to obtain corresponding extraction liquid; the integral quantity Q of the current-time curve of the extraction liquid corresponding to the i-th unit plate is obtained by subjecting the extraction liquid to constant potential deposition treatment in a three-electrode system. i The integral quantity Q is determined in the standard Q-C relationship curve. i The corresponding lithium salt concentration C is determined. i The lithium salt concentration in the extraction liquid corresponding to the i-th unit plate is determined, wherein i is a positive integer.

[0010] The method for measuring electrolyte distribution in a lithium ion battery cell provided by the application first determines the standard Q-C relationship curve of the integral quantity Q of the current-time curve of the electrolyte subjected to constant potential deposition treatment in a three-electrode system and the lithium salt concentration C in the electrolyte, and then subjects the extraction liquid obtained by punching the positive plate, negative plate or separator of the cell to be measured into a plurality of unit plates to extraction to the integral quantity Q of the current-time curve obtained by subjecting the extraction liquid to constant potential deposition treatment in a three-electrode system. i The lithium salt concentration C in the extraction liquid corresponding to the unit plate is determined according to the previous standard Q-C relationship curve. i The method for measuring electrolyte distribution in a lithium ion battery cell provided by the application determines the lithium salt concentration C in the extraction liquid of the actual unit plate. i The method for measuring electrolyte distribution in a lithium ion battery cell provided by the application determines the lithium salt concentration C in the extraction liquid of the actual unit plate. BRIEF DESCRIPTION OF DRAWINGS

[0011] In order to more clearly illustrate the technical solutions in the embodiments of the application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.

[0012] Figure 1 is a flow chart of a method for measuring electrolyte distribution in a lithium ion battery cell according to an embodiment of the present application;

[0013] Figure 2 is a process for preparing an extraction solution in a method for measuring electrolyte distribution in a lithium ion battery cell according to an embodiment of the present application;

[0014] Figure 3 is a schematic diagram of an integral quantity Q of a current-time curve in a method for measuring electrolyte distribution in a lithium ion battery cell according to an embodiment of the present application;

[0015] Figure 4 is a test diagram of constant potential deposition treatment of different concentrations of electrolyte in a method for measuring electrolyte distribution in a lithium ion battery cell according to an embodiment of the present application, wherein C0 represents the central concentration C0 of lithium salt in the electrolyte, and a-k represent the current-time curves corresponding to the electrolyte concentrations of 0.02-0.7 times C0, respectively;

[0016] Figure 5 is a standard Q-C relationship curve obtained in a method for measuring electrolyte distribution in a lithium ion battery cell according to an embodiment of the present application;

[0017] Figure 6 is a current-time curve obtained by constant potential deposition treatment of extraction solutions of unit sheets at different positions in a method for measuring electrolyte distribution in a lithium ion battery cell according to an embodiment of the present application;

[0018] Figure 7 is an integral quantity Q obtained by testing unit sheets at different positions in a method for measuring electrolyte distribution in a lithium ion battery cell according to an embodiment of the present application. i and the extraction solution concentration diagram obtained by comparing with the standard Q-C relationship curve. DETAILED DESCRIPTION

[0019] In order to make the technical problems, technical solutions and beneficial effects of the present application clearer, the present application will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0020] In the present application, the term "and / or" describes the association relationship of the associated objects, which means that there can be three kinds of relationships, for example, A and / or B, which can represent the following cases: A exists alone, A and B exist together, and B exists alone. Wherein A and B can be singular or plural. The character " / " generally represents an "or" relationship between the associated objects before and after it.

[0021] In the present application, "at least one" means one or more, and "multiple" means two or more. "At least one of the following" or similar expressions means any combination of the items, including any combination of single or multiple items.

[0022] It should be understood that the size of the sequence number of the above processes in various embodiments of the present application does not mean the order of execution, and part or all of the steps can be executed in parallel or in sequence, and the execution order of the processes should be determined according to their functions and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0023] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "said" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0024] The weight of the related components mentioned in the embodiments of the present application can not only refer to the specific content of each component, but also represent the weight ratio relationship between each component, therefore, as long as the content of the related components in the embodiments of the present application is enlarged or reduced in proportion, it is within the scope disclosed in the embodiments of the present application. Specifically, the mass mentioned in the embodiments of the present application can be μg, mg, g, kg and other mass units commonly known in the chemical field.

[0025] The method for measuring the distribution of electrolyte in the lithium ion battery cell provided by the embodiments of the present application is as follows Figure 1 The method comprises the following steps:

[0026] S01: Prepare multiple portions of electrolyte with different lithium salt concentrations, and perform constant potential deposition treatment on each portion of electrolyte in a three-electrode system to obtain a current-time curve, calculate the integral quantity Q of the current-time curve according to , wherein t1 is the reaction equilibrium time of the constant potential deposition treatment, and draw a standard Q-C relationship curve with the integral quantity Q and the corresponding lithium salt concentration C in the electrolyte;

[0027] S02: Disassemble the cell after electrolyte injection to obtain the positive plate, negative plate or separator to be measured, stamp the positive plate, negative plate or separator to be measured into multiple unit plates, and then extract and treat the unit plates with an organic solvent to obtain corresponding extraction liquid, and place the extraction liquid corresponding to the i th unit plate in a three-electrode system to perform constant potential deposition treatment to obtain the integral quantity Q of the current-time curve i , wherein the integral quantity Q i corresponding to the lithium salt concentration C i is the lithium salt concentration in the extraction liquid corresponding to the i th unit plate, wherein i is a positive integer.

[0028] The application provides a method for determining the distribution of electrolyte in a lithium ion battery cell, which comprises drawing a standard Q-C relationship curve and determining the lithium salt concentration in the extraction solution of an actual unit piece. The method first determines the standard Q-C relationship curve between the integral quantity Q of the current-time curve and the lithium salt concentration C in the electrolyte in a three-electrode system subjected to constant potential deposition treatment. Then, the positive electrode piece, the negative electrode piece or the separator of the cell to be measured is punched into multiple unit pieces, and the integral quantity Q of the current-time curve is obtained by subjecting the extraction solution of the unit pieces at different positions to constant potential deposition treatment in a three-electrode system with the extraction solution as the electrolyte i , so as to determine the lithium salt concentration C in the extraction solution of the unit piece according to the previous standard Q-C relationship curve i . The application draws a standard Q-C relationship curve and determines the lithium salt concentration C in the extraction solution of an actual unit piece i . Both the drawing of the standard Q-C relationship curve and the determination of the lithium salt concentration C in the extraction solution of the actual unit piece are carried out by using the constant potential deposition method in a three-electrode system. In this way, the high sensitivity of the electrochemical workstation is utilized, and the deposition curve is integrated, so as to accurately characterize the lithium ion content in the electrolyte in the cell, thereby simply and quickly reflecting the distribution of the electrolyte in the cell. The method has the characteristics of low cost, and theoretically supports the optimization of the electrolyte injection process in the battery production process, and provides data support for improving the cycle performance of the cell from the optimization of the electrolyte injection amount.

[0029] The above step S01 is a standard Q-C relationship curve obtaining step.

[0030] Specifically, a series of electrolytes with the same volume and gradient distribution of lithium salt concentration can be prepared, and a standard Q-C relationship curve can be obtained by subjecting the electrolytes to constant potential deposition treatment in a three-electrode system. The deposited current-time curve is processed by matlab, and the integral electric quantity Q (such as Figure 3 ) and the equilibrium current I are combined to accurately characterize the lithium ion content in the cell.

[0031] In an embodiment, the lithium salt and the solvent in the electrolyte prepared for obtaining the standard Q-C relationship curve are the same as the electrolyte injected in the subsequent cell. Specifically, the lithium salt in the prepared electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bisoxalate borate and lithium bisfluorosulfonylimide. Further, the solvent in the electrolyte is selected from at least one of dimethyl carbonate (DMC), ethylene carbonate (EC) and methyl ethyl carbonate (EMC), and the organic solvent in the extraction treatment of the subsequent test unit piece is the same as the solvent in the electrolyte.

[0032] In an embodiment, the step of preparing multiple portions of electrolytes with different lithium salt concentrations comprises:

[0033] The porosity P1 of the negative electrode sheet, the porosity P2 of the positive electrode sheet and the porosity P3 of the separator before assembling the battery cell are measured, and the porosity ratio P% of the negative electrode sheet is obtained, i.e., P% = [P1 / (P1+P2+P3)]*100%; then the lithium salt center concentration C0 (mol / L) of the negative electrode sheet after theoretically being punched into multiple unit sheets is calculated according to the following formula:

[0034]

[0035] wherein S1 is the area of one unit sheet (unit: mm 2 ), S2 is the area of the entire negative electrode sheet (unit: mm 2 ), m is the mass of lithium salt in the electrolyte to be injected (unit: g), M is the molar mass of lithium salt in the electrolyte (unit: g / mol), and V is the volume of one portion of electrolyte (unit: ml);

[0036] A plurality of portions of electrolyte with the same volume and a gradient of lithium salt concentration are configured with the lithium salt center concentration C0 as the center.

[0037] In the above process, if the electrolyte distribution of the positive electrode sheet is measured, the porosity ratio of the positive electrode sheet is calculated; if the electrolyte distribution of the negative electrode sheet is measured, the porosity ratio of the negative electrode sheet is calculated; and if the electrolyte distribution of the separator is measured, the porosity ratio of the separator is calculated. In the embodiments of the present application, the electrolyte distribution of the negative electrode sheet is preferably measured, and thus the porosity ratio P% of the negative electrode sheet is calculated, i.e., P% = [P1 / (P1+P2+P3)]*100%. In order to more simply and quickly obtain the Q-C relationship curve, the concentration C0 formed by the theoretical electrolyte distribution amount of the negative electrode sheet punched into unit sheets is calculated as the lithium salt center concentration, and a series of electrolyte with a gradient of lithium salt concentration is configured with the lithium salt center concentration C0 as the center, specifically, 6-10 portions of electrolyte with the same volume V are prepared by multiplying the order of magnitude of 10 -1 above and below the lithium salt center concentration C0. In this way, the standard Q-C relationship curve can be quickly drawn to simplify the process of obtaining the standard Q-C relationship curve. The volume V of each portion of electrolyte can be 15-25 ml, and the volume of the extraction liquid of each unit sheet in the subsequent actual test is the same as the volume V of the electrolyte.

[0038] In an 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 assembling the battery cell includes: punching the negative electrode sheet, the positive electrode sheet or the separator before assembling the battery cell into a test sheet with the same area as the actual test unit sheet, and then testing the porosity of the test sheet by using the alkane solution immersion method, and the calculation formula is as follows:

[0039]

[0040] wherein S1 is the area of the test sheet (the same as the area of the unit sheet), and d is the thickness of the test sheet.

[0041] Specifically, the positive and negative dry electrodes and the separator before winding are punched into test pieces by a punching machine, and the thickness d is measured respectively, and the test porosity is measured by immersing in n-hexadecane (density = 0.7733 g / cm3).

[0042] In an embodiment, the working electrode in the three-electrode system is a nickel foam (1x 1.5cm 2 , thickness 1mm), the counter electrode is a platinum electrode, and the reference electrode is an Ag + electrode in a tetrabutylammonium hexafluorophosphate solution (the solvent can be dimethyl sulfoxide DMSO). The working electrode of the three-electrode system is a circular nickel foam piece with improved surface organic solution affinity after complete immersion in a DMSO environment. A series of electrolyte concentrations with C0 as the center concentration is configured, and in different concentrations of electrolyte with a volume of V (ml), the I-t curve is obtained by constant potential deposition with the above three-electrode system, the integral quantity Q is calculated, and the Q-C relationship curve and the equilibrium current I1-C relationship curve are drawn. Further, the constant potential deposition treatment conditions include: constant potential -3V~ -5V, time 5~15min. By using the high sensitivity characteristics of the electrochemical workstation and combining with the deposition curve processed by matlab, the lithium ion content in the battery is accurately characterized according to the integral electric quantity Q and the equilibrium current I of the deposition curve, so as to inversely deduce the distribution of the electrolyte in the fresh battery according to the concentration of the lithium salt in the electrolyte.

[0043] The above step S02 is the actual liquid injection battery test step.

[0044] The target battery after liquid injection is disassembled to obtain the positive electrode sheet, negative electrode sheet or separator to be tested, and the positive electrode sheet, negative electrode sheet or separator to be tested is punched into a plurality of unit sheets and then extracted with an organic solvent to obtain corresponding extraction liquid. The smaller the area of each unit sheet, the more accurately the electrolyte concentration distribution of the unit sheet position in the battery can be reflected. Specifically, the unit sheet is a circular sheet with a diameter of ≤20mm, for example, a circular sheet with a diameter of 5~17mm. The total number N of unit sheets is subject to the actual lithium ion battery model, so as to reflect the electrolyte concentration distribution of each different position unit sheet of the positive electrode sheet, negative electrode sheet or separator as much as possible, for example, for a 18650 type lithium ion battery, the width of the electrode sheet or separator is generally about 56~58mm, and the winding length is about 700~900mm. According to actual needs, 100~800 unit sheets with the same area and different positions can be punched and tested to reflect the electrolyte distribution at different positions in the battery.

[0045] In order to ensure that the lithium salt concentration test can as accurately as possible reflect the actual distribution of the liquid injection electrolyte in the battery, the battery with SOC (State of charge, i.e. state of charge) of 0 is preferably disassembled. Taking the negative electrode sheet test as an example, as shown in Figure 2As shown, the negative plate is obtained by disassembling the target battery cell with fresh injected electrolyte and SOC=0, and then is punched into a plurality of φ17 small round plates by using a φ17 plate punching machine. The small round plates are placed in an organic solvent to obtain an extraction liquid containing lithium ions left by the injected electrolyte. The extraction can be ultrasonic extraction, and the ultrasonic time is 40-60 min and the ultrasonic temperature is 20-40℃. After ultrasonic extraction, the fallen negative electrode powder is removed by vacuum filtration, and the extraction liquid is obtained by constant volume to V(ml).

[0046] In an embodiment, the step of disassembling the injected battery cell includes: first, aging the battery cell at 30-40℃, and then disassembling the negative plate to expose it to an environment with a dew point of-30 to-20℃ for drying. Specifically, to make the injected electrolyte fully infiltrate, the injected battery cell is aged at 30-40℃ for 8-16h, and then the battery cell is disassembled to obtain a fresh negative plate. The fresh negative plate is exposed to an external dew point of-30 to-20℃, and the dry negative group is obtained after the solvent of the negative plate is completely volatilized.

[0047] Taking the negative plate as an example, the target battery cell is disassembled to obtain N unit plates, and the extraction liquids of the N unit plates at different positions are placed in the three-electrode system to perform constant potential deposition, and the I-t curve is obtained by integrating Q i , and the concentration C i corresponding to Q i is searched in the standard Q-C relationship curve to obtain the lithium salt concentration in all unit plates.

[0048] In an embodiment, according to the above lithium salt concentration C i , the following formula can be further used to obtain the electrolyte volume V i电 injected in different unit plates:

[0049]

[0050] Wherein, V is the extraction liquid volume corresponding to the i(i≤N) unit plate, C 电 is the lithium salt concentration of the electrolyte when injected. By quantitatively testing the residual lithium salt in the negative plate inside the battery cell, the above formula can be used to obtain the electrolyte volume content distribution diagram of the negative plate.

[0051] In an embodiment, according to the above electrolyte volume V i电 , the following formula can be further used to obtain the dispersion degree of the injected electrolyte:

[0052]

[0053] Wherein, S is the dispersion degree of the injected electrolyte distribution, N is the number of unit plates, The average value of the electrolyte volume on the unit cell sheet. Through the electrolyte volume content distribution map of the negative electrode sheet, the above formula can be used to further analyze the uniformity and dispersion degree of the electrolyte in the battery cell, so as to characterize the stability of the electrolyte in the battery cell.

[0054] In summary, the method for determining the distribution of electrolyte in the lithium ion battery cell based on electrochemical test provided by the application, by disassembling the target battery cell after fresh injection, and using the punching machine to punch the unit sheet, the same organic solvent in the electrolyte is used as the extractant to extract the lithium salt in the electrolyte of the unit sheet, and the extract is used as the electrolyte in the three-electrode system for constant potential deposition, and the integral quantity Q i , the equilibrium current is I1. Finally, the obtained results are compared with the standard Q-C relationship curve obtained according to the theoretical electrolyte distribution amount, to obtain the lithium salt concentration in the extract, and then the electrolyte distribution amount of the unit sheet at different positions is calculated. The constant potential deposition is used, the high sensitivity characteristics of the electrochemical workstation are used, and the deposition curve is processed by matlab, the lithium ion content in the electrode sheet is accurately characterized according to the integral quantity Q of the deposition curve and the equilibrium current I, so that the dispersion state of the electrolyte in the fresh battery cell is obtained by reverse calculation according to the lithium salt concentration of the electrolyte. The method has the advantages of low cost, environmental friendliness, simple method and standardization, and according to the determination results, the distribution state and dispersion degree of the electrolyte in the battery cell are obtained, which can further optimize the injection process and improve the performance of the battery cell.

[0055] The following will be described with specific examples.

[0056] Example 1

[0057] The method for determining the distribution of electrolyte in the lithium ion battery cell, comprising the following steps:

[0058] Step 01: Obtain the standard Q-C relationship curve

[0059] Porosity determination: The positive and negative electrode dry sheets and the separator before fresh assembly of the battery cell are punched into φ17(diameter 17mm) round sheets by the punching machine, and the porosity is calculated and measured by the n-hexadecane soaking method, to obtain the negative electrode porosity, the positive electrode porosity, the separator porosity, and then the proportion P% of the negative electrode porosity in the total porosity is obtained, as shown in Table 1 below:

[0060]

[0061] Wherein the density of hexadecane ρ1 is 0.7733g / cm3, S1 is the area of φ17 round sheet, and d is the thickness of φ17 round sheet.

[0062] Table 1

[0063]

[0064] Calculate the lithium salt center concentration C0: Based on the negative electrode porosity percentage P% in Table 1, determine the lithium salt center concentration C0 using the following formula.

[0065]

[0066] In the formula:

[0067] S1------Area of ​​φ17 circular piece, mm 2 ;

[0068] S2 represents the total area of ​​the negative electrode, in mm². 2 ;

[0069] ------Molar mass of LiPF4, 151.9 g / mol;

[0070] V------ One volume of electrolyte, 20ml.

[0071] Plot a standard QC relationship curve: Prepare a series of electrolytes with the same volume V (20 ml, solvent composed of DMC, EC, and EMC in a volume ratio of 2:1:1) of lithium LiPF4 salt concentration gradient, centered at the above C0 concentration. Use nickel foam (1 x 1.5 cm) as the core. 2 The working electrode is 1 mm thick, and the counter electrode is Pt. Ag in DMSO solution of tetrabutylammonium hexafluorophosphate... + -4V constant potential deposition was performed in a three-electrode system with the reference electrode as the reference electrode. The obtained It curve was then analyzed based on... Calculate the integral Q (e.g.) Figure 3 (as shown); finally, based on the Q and corresponding C of each electrolyte, a standard QC relationship curve is plotted.

[0072] The results are as follows Figure 4 As shown, It curves for electrolytes with different LiPF4 lithium salt concentrations (0.02CO, 0.04CO, 0.06CO, 0.08CO, 0.1CO, 0.2CO, 0.3CO, 0.4CO, 0.5CO, 0.6CO, 0.7CO) can be obtained; further, standard QC relationship curves can be plotted, as shown... Figure 5 As shown.

[0073] Step 02: Measurement of actual electrolyte distribution during injection

[0074] Preparation of extract: After discharging freshly injected 18650 battery cells, they were aged at 30℃ for 10 hours. The cells were then disassembled to obtain fresh negative electrode sheets. These fresh negative electrode sheets were exposed to an external dew point of -30 to -20℃ until the solvent completely evaporated, resulting in dried negative electrode sheets. The dried negative electrode sheets were pressed into multiple φ17 discs. These discs were placed in a mixed organic solvent (DMC, EC, and EMC in a volume ratio of 2:1:1) for ultrasonic extraction. Finally, the detached negative electrode powder was removed by vacuum filtration, and the volume was adjusted to 20 ml to obtain the extract for each φ17 disc.

[0075] Electrolyte distribution determination: Extracts from φ17 discs at different locations were placed in the previously prepared three-electrode system for constant potential deposition (deposition parameters were the same as those obtained). Figure 5 The It curve obtained is obtained by using the same potentiostatic deposition parameters as the standard QC relationship curve, as shown in the figure. Figure 6 The image shows the extraction solution deposition results of φ17 discs at different locations within the battery cell; based on... Figure 6 Calculate the integral Q from the It curve i Find Q in the standard QC relationship curve i The corresponding concentration C i ,like Figure 7 As shown, the lithium salt concentration of the electrolyte in the φ17 disc at different positions was obtained, which can further reflect the LiPF4 concentration of the electrolyte in different positions of the negative electrode in the inner, middle and outer rings of the cell.

[0076] The following formula can be used to further deduce the volumetric distribution of the electrolyte on the negative electrode:

[0077]

[0078] In the formula:

[0079] C i ------Li content measured in the extract of the i-th φ17 disc + Concentration, mol / L;

[0080] V------ The volume of the extract used in the test was 20 ml;

[0081] C 电 ----Electrolyte Li during injection + Concentration, mol / L;

[0082] V i电 ----The volume content of electrolyte on the φ17 disc.

[0083] The following formula is used to further analyze the uniformity and dispersion of electrolyte distribution within the battery cell using an electrode electrolyte content distribution diagram:

[0084]

[0085] In the formula:

[0086] S - dispersion degree of electrolyte distribution after liquid injection;

[0087] N - total number of φ17 wafers;

[0088] S - mean value of electrolyte volume on φ17 wafers.

[0089] The above only describes the preferred embodiments of the present application and is not used to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method for determining the distribution of electrolyte in a lithium-ion battery cell, characterized by, The application further relates to a battery cell prepared by the method. Preparation of electrolyte with different lithium salt concentration, and then constant potential deposition treatment of each electrolyte in three-electrode system to obtain current-time curve, and then Calculation of integral quantity Q of current-time curve, wherein t1 is reaction equilibrium time of constant potential deposition treatment; and then drawing of standard Q-C relationship curve with integral quantity Q and corresponding lithium salt concentration C in electrolyte; The liquid injected battery cell is disassembled to obtain a to-be-tested positive plate, a to-be-tested negative plate or a separator, the to-be-tested positive plate, the to-be-tested negative plate or the separator is punched into a plurality of unit plates, and then the to-be-tested positive plate, the to-be-tested negative plate or the separator is treated by organic solvent extraction to obtain corresponding extraction liquid, and the integral quantity Q of the current-time curve is obtained by placing the extraction liquid corresponding to the i th unit plate in the three-electrode system for the constant potential deposition treatment i The integral quantity Q is determined in the standard Q-C relationship curve i The corresponding lithium salt concentration C i That is, the lithium salt concentration in the extraction liquid corresponding to the i th unit plate, wherein i is a positive integer.

2. The method of claim 1, wherein the lithium-ion battery cell is a 18650 cell. The application further relates to a battery cell prepared by the method. According to the lithium salt concentration C i , the electrolyte volume V on different unit chips after injection is obtained by the following formula i电 : Wherein, V is the extraction liquid volume corresponding to the i th unit piece, C 电 is the electrolyte lithium salt concentration when injecting.

3. The method of claim 2, wherein the lithium-ion battery cell is a 18650 cell. The step of preparing multiple portions of electrolyte with different lithium salt concentrations comprises the following steps: According to the volume V of the electrolyte i电 The dispersion degree of the electrolyte after injection is obtained by the following formula: Wherein, S is the dispersion degree of electrolyte distribution after injection, N is the number of unit pieces, is the mean value of electrolyte volume on the unit piece.

4. The method of claim 1, wherein the lithium-ion battery cell is a 18650 cell. The negative electrode sheet porosity P1, the positive electrode sheet porosity P2 and the diaphragm porosity P3 before assembling the battery cell are determined, and the negative electrode sheet porosity ratio P% is obtained, that is, P%=[P1 / (P1+P2+P3)]*100%; then the lithium salt central concentration C0 of the negative electrode sheet after being punched into multiple unit sheets is calculated according to the following formula: Wherein, S1 is the area of a unit sheet, S2 is the area of the whole negative electrode sheet, 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. The multiple portions of electrolyte with the same volume and gradient lithium salt concentrations are arranged around the lithium salt central concentration C0. The method for determining the negative electrode sheet porosity P1, the positive electrode sheet porosity P2 and the diaphragm porosity P3 before assembling the battery cell comprises the following steps: the negative electrode sheet, the positive electrode sheet or the diaphragm before assembling the battery cell is punched into a test sheet with the same area as the unit sheet, and then the porosity of the test sheet is obtained by using an alkane solution immersion method according to the following formula:

5. The method of claim 4, wherein the lithium-ion battery cell is a 18650 cell. Wherein, S1 is the area of a unit sheet, S2 is the area of the whole negative electrode sheet, 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. The constant potential deposition treatment condition in the three-electrode system comprises the following steps: constant potential -3V to -5V, time 10-15 min.

6. The method of claim 1, wherein the lithium-ion battery cell is a 18650 cell. The working electrode in the three-electrode system is a nickel foam, the counter electrode is a platinum electrode, and the reference electrode is Ag in a tetrabutylammonium hexafluorophosphate solution environment + electrode.

7. The method of claim 6, wherein the lithium-ion battery cell is a 18650 cell. The unit sheet is a circular sheet with a diameter of less than or equal to 20 mm.

8. The method of any one of claims 1-7, wherein the lithium-ion battery cell is a 18650 cell. The step of disassembling the battery cell after injecting the electrolyte comprises the following steps: first, the battery cell is subjected to aging treatment at 30-40 DEG C, and then the negative electrode sheet is disassembled and exposed to an environment with a dew point of -30 to -20 DEG C for drying.

9. The method of any one of claims 1-7, wherein the lithium-ion battery cell is a 18650 cell. The lithium salt in the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bisoxalate borate and lithium bisfluorosulfonylimide; and / or, 10. The method of any one of claims 1-7, wherein the lithium-ion battery cell is a 18650 cell. The solvent in the electrolyte is selected from at least one of dimethyl carbonate, ethylene carbonate and methyl ethyl carbonate, and the organic solvent in the extraction treatment is the same as the solvent in the electrolyte. ​

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