WO2023050791A1 - 分析电芯内部电解液分布状态的方法及其应用 - Google Patents

分析电芯内部电解液分布状态的方法及其应用 Download PDF

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WO2023050791A1
WO2023050791A1 PCT/CN2022/089584 CN2022089584W WO2023050791A1 WO 2023050791 A1 WO2023050791 A1 WO 2023050791A1 CN 2022089584 W CN2022089584 W CN 2022089584W WO 2023050791 A1 WO2023050791 A1 WO 2023050791A1
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electrolyte
distribution
battery
battery cell
pole piece
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English (en)
French (fr)
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刘宇春
姚凌峰
鲍展栋
霍四平
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蜂巢能源科技股份有限公司
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
    • G01N30/96—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation using ion-exchange
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/058—Construction or manufacture
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the invention relates to the field of lithium ion batteries, in particular to a method for analyzing the concentration of electrolyte on the surface of a pole piece inside a battery cell and an application thereof.
  • Lithium-ion batteries are now one of the main directions for the development of new energy vehicle power batteries in my country. In order to realize the perfect replacement of fuel vehicles by electric vehicles, a lot of theoretical research and process development on battery technology are required.
  • lithium-ion batteries on the market are divided into cylindrical batteries, square shell batteries, soft pack batteries, etc. Batteries with different processes are designed to increase the specific capacity of the battery cell and reduce the dead volume.
  • different processes cause the internal pole pieces of the battery to present different states, and the pole pieces in different states will also affect the charging and discharging performance and safety performance of the battery.
  • One of the reasons is that the distribution of the electrolyte in the cell is different in different states.
  • the uniformity of the electrolyte distribution affects the formation of the SEI film on the surface of the negative electrode graphite, thereby causing changes in the performance of the battery in all aspects. Therefore, the research on the distribution state of the electrolyte inside the battery can better improve the process of the battery to improve the performance of the battery.
  • the specific analysis is as follows:
  • Lithium-ion batteries are full of electrolyte, even if the cell shell is opened, the electrolyte cannot be kept from being lost, and the electrolyte itself is also volatile.
  • the electrolyte density can be tested if the method of locally extracting a uniform volume sample is used, but the correlation between the electrolyte density and the electrolyte concentration on the electrode surface is not consistent. In addition to factors affecting the geometric structure of the cell, the electrolyte concentration on the surface of the pole piece will also be different due to the steps of homogenization, coating, and rolling in the pole piece process.
  • the main purpose of the present invention is to provide a method for analyzing the concentration of electrolyte on the surface of the pole piece inside the battery cell and its application, so as to better characterize the distribution state of the electrolyte inside the battery cell.
  • a method for analyzing the distribution state of the electrolyte inside the battery is provided. Obtain the distribution state of the electrolyte inside the cell.
  • the distribution state of the electrolyte inside the battery cell is obtained by qualitative and quantitative analysis of the lithium salt remaining on the surface of the pole piece inside the battery cell.
  • the distribution state of the electrolyte inside the battery is obtained, including: S1, the qualitative and quantitative test of the lithium salt remaining on the surface of the pole piece inside the battery, Obtain a distribution diagram of the electrolyte content on the surface of the pole piece; S2, analyze the distribution state of the electrolyte inside the cell through the distribution diagram of the electrolyte content; preferably, the distribution state includes the degree of uniform distribution and/or the degree of discrete distribution.
  • S1 includes: S11, taking several pole pieces inside the battery core and analyzing the lithium salts remaining at different positions on each pole piece Extraction into an aqueous solution; S12, quantitative analysis of the anions in the aqueous solution by ion chromatography.
  • S1 includes: extracting lithium salts remaining on the surface of discs with different positions but the same diameter on each of several pole pieces into the aqueous solution; quantitatively analyzing the anions in the aqueous solution by ion chromatography to obtain several pole pieces
  • the content of the lithium salt on the disc surface of each pole piece is calculated according to the following formula (1) to obtain the distribution map of the electrolyte content on the surface of the pole piece:
  • S2 includes: using the following formula (2), analyzing the dispersion degree S of the electrolyte distribution inside the cell according to the electrolyte content distribution diagram on the surface of the pole piece;
  • the lithium salt remaining on each pole piece is extracted into the aqueous solution by ultrasonic extraction; preferably, the lithium salt is lithium bisfluorosulfonyl imide.
  • a preparation process of the battery cell includes a roller coating diaphragm process or a spraying diaphragm process, and the electrolyte distribution state inside the battery cell is analyzed according to the above method, and the obtained The degree of dispersion and/or uniformity of the electrolyte distribution inside the battery core; according to the degree of dispersion and uniformity of the electrolyte distribution inside the battery core, the process parameters of the roller coating diaphragm process or the spraying diaphragm process are adjusted to prepare the battery core.
  • the process parameters include at least one of hot pressing parameters, liquid injection coefficient and aging time.
  • a method for characterizing the stability of the electrolyte inside the battery is provided, and the electrolyte inside the battery is characterized by the degree of dispersion and/or uniformity of the electrolyte distribution inside the battery Stability, wherein, the degree of dispersion and/or uniformity of the electrolyte distribution inside the battery cell is obtained by analyzing the above method;
  • the characterization method further includes: analyzing the stability of the electrolyte inside the cell by drawing a curve of electrochemical impedance spectroscopy.
  • this application takes the internal pole piece of the battery as the detection object, directly conducts qualitative and/or quantitative determination of the electrolyte composition remaining on the pole piece, and can directly obtain the electrolyte inside the battery cell according to the measured data. distribution status.
  • the stability of the electrolyte inside the battery can be further characterized according to the uniformity and dispersion of the electrolyte inside the battery, and the positive relationship between the uniformity and dispersion of the electrolyte inside the battery and the manufacturing process parameters Correlation is used to guide the optimization of process parameters and improve battery performance.
  • FIG. 1 shows a schematic diagram of the electrode sheet sampling site according to Embodiment 1 of the present invention.
  • Fig. 2 shows the graph of the LiFSI standard curve calibration report results according to Example 1 of the present invention.
  • Fig. 3 shows the results of the LiFSI standard curve according to Example 1 of the present invention.
  • Fig. 4 shows the ion chromatogram of the standard solution according to Example 1 of the present invention.
  • Fig. 5 shows a schematic diagram of the high-frequency Rs test results of different membrane injection aging processes according to Embodiment 3 of the present invention.
  • Fig. 6 shows a schematic diagram of the high-frequency Rs test results of the same roll-coated separator process-different hot-pressing parameters aging process according to Example 3 of the present invention.
  • the inventors of the present application conducted in-depth research on the detection of the electrolyte composition and distribution concentration inside the battery, and proposed a method for analyzing the concentration of the electrolyte on the surface of the pole piece inside the battery. Methods. The method can obtain the distribution state of the electrolyte inside the battery by qualitatively and quantitatively analyzing the components of the electrolyte remaining on the surface of the pole piece inside the battery.
  • the inventors try to analyze the electrolyte solution remaining on the surface of the pole piece inside the battery cell to obtain the distribution state of the electrolyte solution inside the battery cell. It has been verified by experiments that not only can the distribution state of the electrolyte inside the cell be obtained by analyzing the residual electrolyte on the surface of the pole piece, but also the distribution state of the electrolyte obtained by this method is positively correlated with the cell preparation process parameters and the stability of the cell. Correlation is more instructive for the optimization of cell preparation process parameters and the improvement of cell performance. Therefore, a series of protection schemes of the present application are proposed.
  • a method for analyzing the distribution state of the electrolyte inside the battery includes: performing qualitative and quantitative analysis on the electrolyte components remaining on the surface of the pole piece inside the battery, In this way, the distribution state of the electrolyte inside the cell is obtained.
  • the state of the electrolyte can be known according to the composition ratio of the components in the electrolyte.
  • the electrolyte remaining on the electrode surface is the electrolyte that is in direct contact with the electrode sheet, which can most accurately reflect the distribution state of the electrolyte around the electrode sheet.
  • the distribution state of the electrolyte inside the battery cell is obtained through qualitative and quantitative analysis of the lithium salt remaining on the surface of the pole piece inside the battery cell.
  • the electrolyte is mainly divided into three parts: organic solvent, lithium salt, and additives.
  • Lithium salt is an important component of the electrolyte. At present, lithium hexafluorophosphate, lithium difluorophosphate or lithium bisfluorosulfonimide (LiFSI) are mostly used in batteries. Lithium salt, based on lithium salt, develops a method for analyzing the distribution state of the electrolyte inside the battery cell, which has good versatility and can be applied to batteries of multiple manufacturers and models. It is also feasible to develop detection methods for components such as organic solvents and additives. According to different needs, one or more components in the electrolyte can be selected to develop a method for analyzing the distribution state of the electrolyte inside the cell.
  • the distribution state of the electrolyte inside the battery cell is obtained by qualitatively and quantitatively analyzing the lithium salt remaining on the surface of the internal pole piece of the battery cell: Perform qualitative and quantitative tests on the lithium salt remaining on the surface of the sheet to obtain a distribution map of the electrolyte content on the surface of the pole piece; S2, analyze the distribution state of the electrolyte inside the battery cell through the electrolyte content distribution map; preferably, the distribution state includes the degree of uniformity of distribution and/or or distribution dispersion.
  • sampling at multiple points on the electrode sheet can be carried out for detection and analysis respectively, so as to obtain a distribution map of the electrolyte content on the surface of the electrode sheet.
  • the distribution state such as the uniformity and dispersion of the electrolyte inside the battery core can be analyzed.
  • the lithium salt remaining on the surface of the pole pieces inside the battery is qualitatively and quantitatively tested by ion chromatography; preferably, S1 includes: S11, taking several pole pieces inside the battery and Extract the lithium salt remaining at different positions on each pole piece into the aqueous solution; S12, quantitatively analyze the anions in the aqueous solution by ion chromatography.
  • Ion chromatography is a method of measuring constant and trace amounts of anions and cations in liquids, and is a type of high performance liquid chromatography (HPLC). Compared with the use of large-scale equipment such as CT, the cost of detection by ion chromatography is low. According to the nature of the measured lithium salts being easily soluble in water, the lithium salts remaining on the surface of the pole piece inside the battery can be extracted into the aqueous solution, and after simple centrifugation, filtration, constant volume and other post-treatments, the lithium salts can be purified by ion chromatography. Quantitative analysis of anions in aqueous solution
  • the above S1 includes: extracting lithium salts remaining on the surface of discs with different positions but the same diameter on each pole piece among several pole pieces into the aqueous solution; performing ion chromatography on the anions in the aqueous solution Quantitative analysis obtains the content of lithium salt on the disc surface of each pole piece in several pole pieces, calculates and obtains the electrolyte content distribution figure of pole piece surface according to following formula (1):
  • the concentration of lithium salt extracted into the aqueous solution is only related to the difference in the distribution of the electrolyte, and the peak area given by ion chromatography and the prepared lithium
  • the standard curve of salt can be calculated according to the formula (1) to calculate the content of lithium salt on the surface of each pole piece. According to the lithium salt content of the circle obtained from different positions, the electrolyte content on the surface of the pole piece can be obtained. Distribution.
  • the above S2 includes: using the following formula (2), analyzing the dispersion degree S of the electrolyte distribution inside the battery cell according to the electrolyte content distribution diagram on the surface of the pole piece; using the following formula (3) to calculate Electrolyte distribution uniformity RSD%;
  • the dispersion degree of the electrolyte distribution inside the battery cell and the RSD% of the electrolyte distribution uniformity inside the battery cell can be calculated. According to the degree of dispersion and uniformity of distribution, the distribution of electrolyte on different electrode sheets can be clearly compared.
  • the above method uses an ultrasonic extraction method to extract the lithium salt remaining on each pole piece into an aqueous solution; preferably, the lithium salt is lithium bisfluorosulfonyl imide.
  • Ultrasonic extraction method is used to oscillate the pole piece with high-frequency sound waves to accelerate the dispersion of residual lithium salt and dissolve it in the aqueous solution, so as to dissolve all the residual lithium salt on the electrode piece as much as possible to prevent detection due to insufficient extraction of lithium salt The result and subsequent electrolyte distribution is less accurate.
  • a preparation process of a battery cell includes a roller coating diaphragm process or a spraying diaphragm process.
  • the distribution state of the electrolyte inside the battery cell is used to obtain the degree of dispersion and/or uniformity of the electrolyte distribution inside the battery core; according to the degree of dispersion and uniformity of the electrolyte distribution inside the battery core, adjust the process parameters of the roller coating diaphragm process or the spraying diaphragm process for preparation , get the cell.
  • the degree of dispersion and/or uniformity of the electrolyte distribution inside the battery core can be detected, and the preparation of the battery core can be guided according to the detected degree of dispersion and/or uniformity of the distribution.
  • Quantitative detection values make it possible to guide the improvement and upgrading of the cell preparation process in a targeted manner, and also quantitatively compare the effects of the process methods and parameters such as the roller coating diaphragm process and the spraying diaphragm process on the distribution of the electrolyte inside the cell and even the performance of the cell Impact.
  • the above process parameters include at least one of hot pressing parameters, liquid injection coefficient and aging time.
  • the above-mentioned aging temperature can be 40-50° C., and can also be adjusted according to the difference between the electrode and the electrolyte.
  • a method for characterizing the stability of the battery cell is provided.
  • the stability of the battery cell can be characterized by the degree of dispersion and/or uniformity of the electrolyte distribution inside the battery cell.
  • the degree of dispersion and/or uniformity of the electrolyte distribution inside the core is obtained by analyzing the above-mentioned method for analyzing the distribution state of the electrolyte inside the battery core.
  • the degree of dispersion and/or uniformity of the electrolyte distribution inside the battery can be quantitatively characterized, which provides a new characterization method for the stability of the electrode and can be used to quantitatively evaluate different batteries. stability.
  • LiFSI lithium bisfluorosulfonimide
  • Lithium bisfluorosulfonyl imide is water-soluble, so ultrasonic extraction is used to extract it.
  • the above discs were weighed and placed in a 15 mL hard plastic test tube, 5 mL of ultrapure water was added and a plastic cover was placed on it. Put the 15mL hard plastic test tube in the plastic test tube rack, then move the plastic test tube rack to the ultrasonic cleaner, set the ultrasonic time to 60min, and start the ultrasonic extraction.
  • the ultrasonically extracted samples were taken out from the ultrasonic cleaner. Due to the action of ultrasonic waves, part of the slag of the positive and negative electrode materials of the pole piece falls off from the foil, so the aqueous solution appears turbid. Centrifuge to separate the solid and liquid in the aqueous solution, draw the aqueous solution with a plastic dropper and transfer it to a 10mL volumetric flask. To avoid lithium bisfluorosulfonyl imide (LiFSI) residues, rinse the 15 mL rigid plastic test tube with 3 mL of ultrapure water. Then dilute to the mark line of the 10mL volumetric flask with ultrapure water, and shake well.
  • LiFSI lithium bisfluorosulfonyl imide
  • LiFSI LiFSI into a 100mL beaker, accurate to 0.0001g, add ultrapure water to dissolve and transfer to a 100mL volumetric flask, dilute to volume with ultrapure water, shake well to obtain 5000mg/L LiFSI standard solution.
  • a pipette gun to pipette 0.1, 0.5, 1, 2, 3mL 5000mg/L LiFSI standard solution into a 100mL volumetric flask, add ultrapure water to volume, and shake well to obtain 5, 25, 50, 100, 150mg/L LiFSI standard curve solution.
  • the prepared standard curve solution is tested by anion liquid chromatography, and the correlation coefficient of the curve is ⁇ 0.997 (as shown in Figures 2, 3, and 4).
  • LiFSI lithium bisfluorosulfonyl imide
  • mi the mass of the i-th pole piece sample, g.
  • Embodiment 2 Analysis of the dispersion degree of electrolyte distribution
  • Example 3 Analysis of electrode sheets prepared with different process parameters
  • high-frequency Rs test is carried out on the electrode sheet prepared by the roller coating diaphragm process and the spraying diaphragm process to detect the stability of the battery cell.
  • the shorter the time for the high-frequency Rs/m ⁇ to level off the shorter the high-temperature aging time required for the battery cell, which proves that the electrolyte is more uniformly distributed, which is more conducive to improving the stability of the battery cell.
  • Table 3 LiFSI detection results of roller coating diaphragm process and spraying diaphragm process under the parameter of hot pressing pressure 4MPa
  • Table 4 High-frequency RS test results of roller coating diaphragm process and spraying diaphragm process under the parameters of hot pressing pressure 4MPa
  • Table 5 LiFSI detection results of roller coating diaphragm process under hot pressing pressure 3MPa and 4MPa parameters
  • the uniformity of electrolyte distribution in the cell affects the completion time of high-temperature aging of the electrolyte, and a more uniform electrolyte distribution can improve process efficiency and cell stability.
  • the application uses the pole piece inside the battery as the detection object of the electrolyte distribution state, and through several pole pieces of different levels and each Qualitative and/or quantitative measurements are made on the remaining electrolyte components at different positions of a pole piece, and the distribution state of the electrolyte inside the cell can be directly obtained according to the measured data.
  • the stability of the electrolyte inside the battery can be further characterized according to the uniformity and dispersion of the electrolyte inside the battery, and the positive relationship between the uniformity and dispersion of the electrolyte inside the battery and the manufacturing process parameters Correlation is used to guide the optimization of process parameters and improve battery performance.
  • the distribution state of the electrolyte is analyzed by quantitatively analyzing the content of lithium salt in the electrolyte remaining on the surface of the pole piece.
  • the lithium salt is preferably lithium bisfluorosulfonyl imide (LiFSI), which has the advantages of high stability, no decomposition below 200 ° C, good hydrolytic stability and more environmentally friendly, while lithium bisfluorosulfonyl imide ( LiFSI) can effectively reduce the high and low temperature resistance of the SEI layer formed on the surface of the electrode plate at low temperature, reduce the capacity loss of the lithium battery during placement, thereby providing high battery capacity and electrochemical performance of the battery, which will be used in future electrolysis Liquid will gradually get more applications.
  • LiFSI lithium bisfluorosulfonyl imide
  • choosing lithium bisfluorosulfonyl imide for qualitative and quantitative analysis will help to make the analysis results more accurate, so as to be closer to the real distribution state of the electrolyte inside the battery, and provide more accurate results for battery performance testing and process improvement. Guiding significance.
  • the present application establishes a model of the degree of dispersion and uniformity of electrolyte distribution, and compares it with the high-frequency Rs/m ⁇ test results during the high-temperature aging process of batteries manufactured with different process parameters. It can be seen that there is a positive correlation between the two. That is, the degree of dispersion and uniformity of the electrolyte distribution inside the cell obtained by the method of the present application can represent the stability of the cell and improve the process efficiency.
  • the anion FSI in the electrolyte is detected to realize the analysis, and the analysis can also be performed by quantitatively analyzing other anions or organic components in the electrolyte.
  • by quantitatively analyzing the anion FSI by quantitatively analyzing the anion FSI, calculating the distribution of the residual electrolyte of the pole piece, and then characterizing the hot pressure parameters and the injection coefficient in the cell manufacturing process through the method of quantitative analysis of the residual electrolyte content , high temperature aging time and other factors on the battery performance, so as to guide the optimization of process parameters and improve battery performance.
  • the stability of the electrolyte inside the cell can also be analyzed by drawing a curve using electrochemical impedance spectroscopy (EIS).

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Abstract

一种分析电芯内部极片表面电解液浓度的方法及其应用。其中，方法包括：通过对电芯内部极片表面残留的电解液成分进行定性定量分析，从而获得电芯内部电解液的分布状态。通过以电芯内部极片为检测对象，直接对极片上残留的电解液成分进行定性和/或定量测定，根据测定所得数据即可直接获得电芯内部电解液的分布状态。根据电芯内部电解液的分布均匀程度和分布离散程度能够进一步表征电芯内部电解液的稳定性，通过电芯内部电解液的分布均匀程度和分布离散程度与电芯制备工艺参数之间的正相关性，用于指导优化工艺参数，改善电芯性能。

Description

分析电芯内部电解液分布状态的方法及其应用
本申请是以CN申请号为202111166817.2，申请日为2021年9月30日的中国申请为基础，并主张其优先权，该CN申请的公开内容再次作为整体引入本申请中。
技术领域
本发明涉及锂离子电池领域，具体而言，涉及一种分析电芯内部极片表面电解液浓度的方法及其应用。
背景技术
20世纪以来，随着全球工业化的扩张和人类消费水平的提高，油气资源的大量消耗带来了日益严重的大气污染和全球变暖等生态问题。随着这些问题由发达国家扩散到发展中国家，其严重性逐渐得到各个国家的重视，并为解决这一全球性危机而制定出了相应的产业升级规化。我国也已发布了《中国传统燃油汽车退出时间表研究》，报告指出传统燃油车最晚于2030前后退出中国市场。为填补传统燃油车的庞大市场和发展可持续能源，电动汽车行业由国家牵头展开了飞速的发展。
锂离子电池现为我国发展新能源汽车动力电池的主要方向之一。为实现电动汽车完美替代燃油汽车，则需要在电池技术上进行大量理论研究和工艺开发。现如今，市面上锂离子电池分为圆柱电池、方壳电池、软包电池等。不同工艺的电池则是为了提高电芯比容量，减少无效死体积。然而不同的工艺造成电芯内部极片呈现不同状态，不同状态下的极片也会影响电芯充放电性能和安全性能。其中原因之一就是电解液在不同状态下在电芯内部的分布状态不同。电解液分布均匀程度影响负极石墨表面SEI膜的形成，从而引发电芯各方面性能的变化。所以对于电芯内部电解液分布状态的研究可以更好地改善电芯工艺以提高电芯性能。具体分析如下：
1.目前市场上电芯类型多种多样，主要有方壳、圆柱、软包等形式，不同形式的电芯对工艺也有着不同的要求。为测试及改善电芯性能，需要进行测试方法的开发，利用原材料的特性和分析技术进行表征。电芯在进行注液、化成、二次注液等工艺步骤后，电解液在其内部浓度分布产生差异。电芯正常使用或性能测试过程中，电芯呈现密封状态，需借助电子计算机断层扫描(CT)等大型设备从外部分析电解液的分布情况。
2.锂离子电池内部充满了电解液，即使是打开电芯外壳，也无法保持电解液不流失，电解液本身也具有挥发性。如果采用局部抽取均一体积样品的方法可以进行电解液密度的测试，但是电解液密度与极片表面电解液浓度的相关性不一致。极片表面电解液浓度除了与电芯几何结构影响因素外，也会因为极片工艺中的匀浆、涂布、辊压等步骤产生差异。
综上所述，目前亟需开发一种能够对电芯内部电解液分布浓度进行检测和/或表征的方案。
发明内容
本发明的主要目的在于提供一种分析电芯内部极片表面电解液浓度的方法及其应用，以便更好地表征电芯内部电解液分布状态。
为了实现上述目的，根据本发明的一个方面，提供了一种分析电芯内部电解液分布状态的方法，该方法包括：通过对电芯内部极片表面残留的电解液成分进行定性定量分析，从而获得电芯内部电解液的分布状态。
进一步地，通过对电芯内部极片表面残留的锂盐进行定性定量分析，从而获得电芯内部电解液的分布状态。
进一步地，通过对电芯内部极片表面残留的锂盐进行定性定量分析，从而获得电芯内部电解液的分布状态包括：S1，对电芯内部极片表面残留的锂盐进行定性定量测试，得到极片表面电解液含量分布图；S2，通过电解液含量分布图分析电芯内部电解液的分布状态；优选地，分布状态包括分布均匀程度和/或分布离散程度。
进一步地，通过离子色谱法对电芯内部极片表面残留的锂盐进行定性定量测试，优选地，S1包括：S11，取电芯内部若干极片并对每一极片上不同位置残留的锂盐萃取到水溶液中；S12，通过离子色谱法对水溶液中的阴离子进行定量分析。
进一步地，S1包括：对若干极片中每一极片上不同位置但相同直径的圆片表面残留的锂盐萃取到水溶液中；通过离子色谱法对水溶液中的阴离子进行定量分析，得到若干极片中每一极片的圆片表面的锂盐的含量，根据如下公式(1)计算得到极片表面电解液含量分布图：
Figure PCTCN2022089584-appb-000001
式中：x i——第i个样品LiFSI质量，mg；
C——离子色谱法测试的LiFSI含量，mg/kg；
m i——第i个极片样品质量，g。
进一步地，S2包括：利用如下公式(2)，根据极片表面电解液含量分布图分析电芯内部电解液分布离散程度S；
Figure PCTCN2022089584-appb-000002
公式(2)中：S——电解液分布离散程度；
n——取样数量；
x i——第i个样品LiFSI质量，mg；
x——n个样品LiFSI质量算术平均值，mg；
i——样品序号；
利用如下公式(3)计算电芯内部电解液分布均匀程度RSD％；
Figure PCTCN2022089584-appb-000003
公式(3)中：RSD％——电解液分布均匀程度；
S——电解液分布离散程度；
x——n个样品LiFSI质量算术平均值，mg。
进一步地，采用超声波萃取法将每一极片上残留的锂盐萃取到水溶液中；优选地，锂盐为双氟磺酰亚胺锂。
为了实现上述目的，根据本发明的第二个方面，提供了一种电芯的制备工艺，该制备工艺包括辊涂隔膜工艺或喷涂隔膜工艺，根据上述方法分析电芯内部电解液分布状态，得到电芯内部电解液分布离散程度和/或均匀程度；根据电芯内部电解液分布离散程度和均匀程度，调整辊涂隔膜工艺或喷涂隔膜工艺的工艺参数进行制备，得到电芯。
进一步地，工艺参数包括热压参数、注液系数及老化时间中的至少一种。
为了实现上述目的，根据本发明的第三个方面，提供了一种电芯内部电解液稳定性的表征方法，通过电芯内部电解液分布离散程度和/或均匀程度来表征电芯内部电解液稳定性，其中，电芯内部电解液分布离散程度和/或均匀程度通过述方法分析得到；
进一步地，可选地，表征方法还包括：通过电化学阻抗谱绘制曲线分析电芯内部电解液稳定性。
应用本发明的技术方案，本申请通过以电芯内部极片为检测对象，直接对极片上残留的电解液成分进行定性和/或定量测定，根据测定所得数据即可直接获得电芯内部电解液的分布状态。根据电芯内部电解液的分布均匀程度和分布离散程度能够进一步表征电芯内部电解液的稳定性，通过电芯内部电解液的分布均匀程度和分布离散程度与电芯制备工艺参数之间的正相关性，用于指导优化工艺参数，改善电芯性能。
附图说明
构成本申请的一部分的说明书附图用来提供对本发明的进一步理解，本发明的示意性实施例及其说明用于解释本发明，并不构成对本发明的不当限定。在附图中：
图1示出了根据本发明实施例1的电极片取样部位示意图。
图2示出了根据本发明实施例1的LiFSI标准曲线校正报告结果图。
图3示出了根据本发明实施例1的LiFSI标准曲线结果图。
图4示出了根据本发明实施例1的标准溶液离子色谱图。
图5示出了根据本发明实施例3的不同隔膜注液老化过程高频Rs测试结果示意图。
图6示出了根据本发明实施例3的相同辊涂隔膜工艺-不同热压参数老化过程高频Rs测试结果示意图。
具体实施方式
需要说明的是，在不冲突的情况下，本申请中的实施例及实施例中的特征可以相互组合。下面将结合实施例来详细说明本发明。
如背景技术所述提到的，由于现有的分析电池电解液的方法需要CT等大型设备，检测成本高，使用不便。为提高电芯内部电解液分布状态检测效率，本申请的发明人对现有的电芯内部电解液成分和分布浓度的检测进行深入研究，提出了一种分析电芯内部极片表面电解液浓度的方法。该方法可以通过对电芯内部极片表面残留的电解液成分进行定性定量分析，从而获得电芯内部电解液的分布状态。
因而，在本申请中发明人尝试分析电芯内部极片表面残留的电解液，来获得电芯内部电解液的分布状态。经试验验证发现，通过分析极片表面残留的电解液不仅能够获得电芯内部电解液的分布状态，而且这种方法获得的电解液的分布状态与电芯制备工艺参数及电芯的稳定性呈正相关性，为电芯制备工艺参数的优化及电芯性能的改善更具指导意义。因而提出了本申请的一系列保护方案。
在本申请的第一种典型的实施方式中，提供了一种分析电芯内部电解液分布状态的方法，该方法包括：通过对电芯内部极片表面残留的电解液成分进行定性定量分析，从而获得电芯内部电解液的分布状态。
通过定性、定量分析电芯内部极片表面残留的电解液的一种或多种成分，根据成分在电解液中的组成比例即可得知电解液的状态。而电极表面残留的电解液是与电极片直接接触的电解液，最能准确的体现与电极片周围的电解液的分布状态。
在一种优选的实施例中，在上述方法中，通过对电芯内部极片表面残留的锂盐进行定性定量分析，从而获得电芯内部电解液的分布状态。
电解液主要分为有机溶剂、锂盐、添加剂三部分，锂盐是电解液的重要组成成分，目前电池中多采用六氟磷酸锂、二氟磷酸锂或双氟磺酰亚胺锂(LiFSI)等物质作为锂盐，根据锂盐开发分析电芯内部电解液分布状态的方法，具有较好的广泛性，可以适用于多厂家、多型 号的电池。而对于有机溶剂和添加剂等成分开发检测方法也是可行的，根据不同的需求，可以选用一种或多种电解液中的成分进行分析电芯内部电解液分布状态的方法的开发。
在一种优选的实施例中，在上述方法中，通过对电芯内部极片表面残留的锂盐进行定性定量分析，从而获得电芯内部电解液的分布状态包括：S1，对电芯内部极片表面残留的锂盐进行定性定量测试，得到极片表面电解液含量分布图；S2，通过电解液含量分布图分析电芯内部电解液的分布状态；优选地，分布状态包括分布均匀程度和/或分布离散程度。
在上述分析电芯内部电解液分布状态的方法中，可以在电极片上进行多点取样，分别检测分析，从而得到极片表面电解液含量分布图。通过获得的极片表面电解液含量分布图，可以分析出电芯内部电解液的均匀程度和分布离散程度等分布状态。
在一种优选的实施例中，在上述S1中，通过离子色谱法对电芯内部极片表面残留的锂盐进行定性定量测试；优选地，S1包括：S11，取电芯内部若干极片并对每一极片上不同位置残留的锂盐萃取到水溶液中；S12，通过离子色谱法对水溶液中的阴离子进行定量分析。
离子色谱法是一种通过检测液体中阴阳离子的常量以及痕量的测量方法，是高效液相色谱法(HPLC)的一种。相较于利用CT等大型设备，利用离子色谱法进行检测的成本低。根据所测锂盐易溶于水的性质，可以将电芯内部极片表面残留的锂盐萃取到水溶液中，经过简单的离心、过滤、定容等后处理后，即可通过离子色谱法对水溶液中的阴离子进行定量分析
在一种优选的实施例中，上述S1包括：对若干极片中每一极片上不同位置但相同直径的圆片表面残留的锂盐萃取到水溶液中；通过离子色谱法对水溶液中的阴离子进行定量分析，得到若干极片中每一极片的圆片表面的锂盐的含量，根据如下公式(1)计算得到极片表面电解液含量分布图：
Figure PCTCN2022089584-appb-000004
式中：x i——第i个样品LiFSI质量，mg；
C——离子色谱法测试的LiFSI含量，mg/kg；
m i——第i个极片样品质量，g。
通过从极片的不同位置取相同直径即相同表面积的极片原片，控制萃取到水溶液中的锂盐浓度仅与电解液分布的差异相关，通过离子色谱法给出的峰面积和制备的锂盐的标准曲线，即可根据公式(1)计算出每一极片的圆片表面的锂盐的含量，根据从不同位置取得的圆片的锂盐含量，即可获得极片表面电解液含量分布图。
在一种优选的实施例中，上述S2包括：利用如下公式(2)，根据极片表面电解液含量分布图分析电芯内部电解液分布离散程度S；利用如下公式(3)计算电芯内部电解液分布均匀程度RSD％；
Figure PCTCN2022089584-appb-000005
公式(2)中：S——电解液分布离散程度；
n——取样数量；
x i——第i个样品LiFSI质量，mg；
x——n个样品LiFSI质量算术平均值，mg；
i——样品序号；
Figure PCTCN2022089584-appb-000006
公式(3)中：RSD％——电解液分布均匀程度；
S——电解液分布离散程度；
x——n个样品LiFSI质量算术平均值，mg。
通过上述获得的从不同位置取得的圆片的锂盐含量，利用公式(2)和(3)，可以计算获得电芯内部电解液分布离散程度和电芯内部电解液分布均匀程度RSD％。根据分布离散程度和分布均匀程度，可以清楚地比较不同电极片上电解液的分布情况。
在一种优选的实施例中，上述方法采用超声波萃取法将每一极片上残留的锂盐萃取到水溶液中；优选地，锂盐为双氟磺酰亚胺锂。
利用超声萃取法，使用高频率的声波对极片进行震荡，加速残留的锂盐分散并溶解于水溶液中，尽可能得溶解电极片上所有残留的锂盐，防止由于锂盐提取不充分，导致检测结果以及后续电解液的分布情况准确度降低。
在本申请的第二种典型的实施方式中，提供了一种电芯的制备工艺，该制备工艺包括辊涂隔膜工艺或喷涂隔膜工艺，根据上述分析电芯内部电解液分布状态的方法，分析电芯内部电解液分布状态，得到电芯内部电解液分布离散程度和/或均匀程度；根据电芯内部电解液分布离散程度和均匀程度，调整辊涂隔膜工艺或喷涂隔膜工艺的工艺参数进行制备，得到电芯。
利用上述分析电芯内部电解液分布状态的方法，可以检测电芯内部电解液分布离散程度和/或均匀程度，根据检测到的分布离散程度和/或均匀程度，可以指导电芯的制备。定量的检测数值使得可以有针对性地指导电芯制备工艺进行改进和升级，也可以定量地比较辊涂隔膜工艺、喷涂隔膜工艺等工艺方法和参数对于电芯内部电解液分布状态乃至电芯性能的影响。
在一种优选的实施例中，上述工艺参数包括热压参数、注液系数及老化时间中的至少一种。上述老化的温度可以为40～50℃，也可以根据电极和电解液的差异进行调整。
在本申请的第三种典型的实施方式中，提供了一种电芯稳定性的表征方法，电芯稳定性可以通过电芯内部电解液分布离散程度和/或均匀程度来表征，其中，电芯内部电解液分布离散程度和/或均匀程度通过上述分析电芯内部电解液分布状态的方法分析得到。
利用上述分析电芯内部电解液分布状态的方法，可以定量表征电芯内部电解液分布离散程度和/或均匀程度，给电极稳定性提供了新的表征方法，可以用于定量地评价不同电芯的稳定性。
下面将结合具体的实施例来进一步详细解释本申请的有益效果。
实施例1：极片双氟磺酰亚胺锂残留量的检测
1.负极极片取样：
以电解液中含有锂盐(双氟磺酰亚胺锂，LiFSI)的方壳电芯为例，现将方壳电芯内部极片取出，取出不同层的极片若干片。现场进行打孔器将极片上若干分布位置(如图1)的极片裁剪成直径为10mm的圆片。该圆片即为进行定量分析的样品。
2.超声波萃取：
双氟磺酰亚胺锂具有水溶性，所以采用超声波萃取法将其进行萃取。将上述圆片进行称重操作并放置在15mL硬质塑料试管中，加入5mL超纯水并盖上塑料盖。将15mL硬质塑料试管放在塑料试管架中，再将塑料试管架移至超声波清洗机中，设置超声时间为60min，并开始超声波萃取。
3.萃取溶液转移并定容：
将超声波萃取完的样品从超声波清洗机中取出。由于超声波的作用，部分极片正负极材料碎渣从箔材上脱落，所以水溶液呈现浑浊。离心使水溶液中固液分离，用塑料滴管吸取水溶液并转移至10mL容量瓶中。为避免双氟磺酰亚胺锂(LiFSI)残留，用3mL超纯水冲洗15mL硬质塑料试管。之后用超纯水定容至10mL容量瓶刻度线，摇匀。
4.离子色谱仪曲线绘制：
称取0.5g LiFSI于100mL烧杯中，精确至0.0001g，加超纯水溶解并转移至100mL容量瓶，超纯水定容，摇匀，得5000mg/L LiFSI标准溶液。用移液枪分别移取0.1、0.5、1、2、3mL 5000mg/L LiFSI标准溶液于100mL容量瓶中，加超纯水定容，摇匀，得5、25、50、100、150mg/L LiFSI标准曲线溶液。
配制好的标准曲线溶液通过阴离子液相色谱仪进行测试，曲线相关系数≥0.997(如图2、3、4)。
5.电芯内部电解液分布浓度数据分析：
将定容后的样品注入阴离子液相色谱仪进行测试，利用峰面积和上述标准曲线，可得双氟磺酰亚胺锂(LiFSI)含量。整理不同层次不同位置若干圆片的测试数据，可分析出方壳电芯内电解液分布浓度。
计算极片样品残留LiFSI质量：
Figure PCTCN2022089584-appb-000007
式(1)中：x i——第i个样品LiFSI质量，mg；
C——离子色谱仪测试LiFSI含量，mg/kg；
mi——第i个极片样品质量，g。
表1：极片LiFSI残留量分布
Figure PCTCN2022089584-appb-000008
实施例2：电解液分布离散程度的分析
电解液分布离散程度S计算：
Figure PCTCN2022089584-appb-000009
式(2)中：S——电解液分布离散程度；
n——取样数量；
xi——第i个样品LiFSI质量，mg；
x——n个样品LiFSI质量算术平均值，mg；
i——样品序号。
电解液分布均匀程度RSD％计算：
Figure PCTCN2022089584-appb-000010
式中：RSD％——电解液分布均匀程度；
S——电解液分布离散程度；
x——n个样品LiFSI质量算术平均值，mg。
表2：电解液分布分析
  分布离散程度S 分布均匀程度RSD％
样品A 0.0105 5.14％
样品B 0.0094 4.73％
样品C 0.0107 5.13％
上表中，电解液的分布离散程度和分布均匀程度的数值越小，表示电解液在电芯内的分布越均匀。
实施例3：对于不同工艺参数制备的电极片的分析
1.在热压压力4MPa下，对辊涂隔膜工艺和喷涂隔膜工艺制备的电极片进行高频Rs测试，检测电芯的稳定性。在电芯高温老化过程中，高频Rs/mΩ趋于平缓时间越短，电芯所需高温老化时间越短，则证明电解液分布更均匀，更有利于提高电芯稳定性。
表3：辊涂隔膜工艺和喷涂隔膜工艺在热压压力4MPa参数下LiFSI检测结果
Figure PCTCN2022089584-appb-000011
Figure PCTCN2022089584-appb-000012
表4：辊涂隔膜工艺和喷涂隔膜工艺在热压压力4MPa参数下高频RS测试结果
  分布离散程度 分布均匀程度 高温老化完成时间
辊涂隔膜4MPa 0.0110 5.48％ 5.5h
喷涂隔膜4MPa 0.0076 2.70％ 1.5h
通过高频Rs测试结果可知，在热压压力4MPa下，喷涂隔膜高频Rs/mΩ在1.5h趋于平缓，辊涂隔膜高频Rs/mΩ在5.5h趋于平缓(如图5)。
2.在热压压力4MPa和3Mpa下，对辊涂隔膜工艺制备的电极片进行高频Rs测试，检测电芯的稳定性。
表5：辊涂隔膜工艺在热压压力3MPa和4MPa参数下LiFSI检测结果
Figure PCTCN2022089584-appb-000013
表6：辊涂隔膜工艺在热压压力3MPa和4MPa参数下高频Rs测试结果
  分布离散程度 分布均匀程度 高温老化完成时间
辊涂隔膜3MPa 0.0064 2.06％ 1.5h
辊涂隔膜4MPa 0.0113 5.40％ 5.5h
通过上述表5和表6的高频Rs测试结果可知，辊涂隔膜在热压压力3MPa下高频Rs/mΩ在1.5h趋于平缓，辊涂隔膜在热压压力4MPa下高频Rs/mΩ在5.5h趋于平缓(如图6)。
通过以上测试步骤得出结论，电芯内电解液分布均匀程度影响电解液高温老化完成时间，更均匀的电解液分布可提高工艺效率以及电芯稳定性。
从以上的描述中，可以看出，本发明上述的实施例实现了如下技术效果：本申请通过以电芯内部极片为电解液分布状态的检测对象，通过对若干不同层次的极片及每一极片不同位置上残留的电解液成分进行定性和/或定量测定，根据测定所得数据即可直接获得电芯内部电 解液的分布状态。根据电芯内部电解液的分布均匀程度和分布离散程度能够进一步表征电芯内部电解液的稳定性，通过电芯内部电解液的分布均匀程度和分布离散程度与电芯制备工艺参数之间的正相关性，用于指导优化工艺参数，改善电芯性能。
在本申请优选的实施例中，通过定量分析极片表面残留的电解液中的锂盐的含量来分析电解液分布状态。其中，锂盐优选为双氟磺酰亚胺锂(LiFSI)，其具有稳定性高，在200℃以下不分解、水解稳定性好和环境更友好等优点，同时双氟磺酰亚胺锂(LiFSI)能够有效降低形成在电极板表面上的SEI层在低温下的高低温电阻，降低锂电池在放置过程中的容量损失，从而提供高电池容量和电池的电化学性能，其在将来的电解液中将逐步得到更多的应用。因而，选择双氟磺酰亚胺锂进行定性定量分析，有助于使得分析结果更准确，从而更接近于电芯内部电解液的真实分布状态，为电芯性能检测及工艺改善提供更准确的指导意义。
进一步地，本申请通过建立电解液分布离散程度、分布均匀程度模型，与不同工艺参数制造的电芯高温老化过程中高频Rs/mΩ测试结果进行对比，可知两者之间存在正相关性。即本申请的方法得到的电芯内部的电解液分布离散程度和均匀程度可表征电芯稳定性和提高工艺效率。
需要说明的是，本申请优选实施例中是检测电解液中的阴离子FSI以实现分析，也可通过定量分析电解液中的其他阴离子或是有机物成分进行分析。而且，本申请优选的实施例中，通过定量分析阴离子FSI，计算极片残留电解液分布，通过对电解液残留含量的定量分析的方法，进而表征电芯制造工艺中热压参数、注液系数、高温老化时间等因素对于电池性能的影响，从而指导优化工艺参数，提升电池性能。除了本申请优选实施例中的方法来表征电芯性能外，还可通过电化学阻抗谱(EIS)绘制曲线分析电芯内部电解液稳定性。
以上所述仅为本发明的优选实施例而已，并不用于限制本发明，对于本领域的技术人员来说，本发明可以有各种更改和变化。凡在本发明的精神和原则之内，所作的任何修改、等同替换、改进等，均应包含在本发明的保护范围之内。

Claims (10)

  1. 一种分析电芯内部电解液分布状态的方法，其特征在于，所述方法包括：
    通过对电芯内部极片表面残留的电解液成分进行定性定量分析，从而获得所述电芯内部电解液的分布状态。
  2. 根据权利要求1的方法，其特征在于，通过对电芯内部极片表面残留的锂盐进行定性定量分析，从而获得所述电芯内部电解液的分布状态。
  3. 根据权利要求2的方法，其特征在于，通过对电芯内部极片表面残留的锂盐进行定性定量分析，从而获得所述电芯内部电解液的分布状态包括：
    S1，对所述电芯内部极片表面残留的锂盐进行定性定量测试，得到所述极片表面电解液含量分布图；
    S2，通过所述电解液含量分布图分析所述电芯内部电解液的分布状态；
    优选地，所述分布状态包括分布均匀程度和/或分布离散程度。
  4. 根据权利要求3的方法，其特征在于，在所述S1中，通过离子色谱法对所述电芯内部极片表面残留的锂盐进行定性定量测试，
    优选地，所述S1包括：
    S11，取所述电芯内部若干极片并对每一所述极片上不同位置残留的所述锂盐萃取到水溶液中；
    S12，通过所述离子色谱法对所述水溶液中的阴离子进行定量分析。
  5. 根据权利要求4的方法，其特征在于，所述S1包括：
    对若干极片中每一所述极片上不同位置但相同直径的圆片表面残留的所述锂盐萃取到水溶液中；
    通过所述离子色谱法对所述水溶液中的阴离子进行定量分析，得到若干极片中每一所述极片的所述圆片表面的所述锂盐的含量，根据如下公式(1)计算得到所述极片表面电解液含量分布图：
    Figure PCTCN2022089584-appb-100001
    式(1)中：x i——第i个样品LiFSI质量，mg；
    C——离子色谱法测试的LiFSI含量，mg/kg；
    m i——第i个极片样品质量，g。
  6. 根据权利要求5的方法，其特征在于，所述S2包括：
    利用如下公式(2)，根据所述极片表面电解液含量分布图分析所述电芯内部电解液分布离散程度S；
    Figure PCTCN2022089584-appb-100002
    公式(2)中：S——电解液分布离散程度；
    n——取样数量；
    x i——第i个样品LiFSI质量，mg；
    x——n个样品LiFSI质量算术平均值，mg；
    i——样品序号；
    利用如下公式(3)计算所述电芯内部电解液分布均匀程度RSD％；
    Figure PCTCN2022089584-appb-100003
    公式(3)中：RSD％——电解液分布均匀程度；
    S——电解液分布离散程度；
    x——n个样品LiFSI质量算术平均值，mg。
  7. 根据权利要求4至6中任一项所述的方法，其特征在于，采用超声波萃取法将每一所述极片上残留的所述锂盐萃取到水溶液中；
    优选地，所述锂盐为双氟磺酰亚胺锂。
  8. 一种电芯的制备工艺，所述制备工艺包括辊涂隔膜工艺或喷涂隔膜工艺，其特征在于，
    根据权利要求1至7中任一项所述的方法分析所述电芯内部电解液分布状态，得到所述电芯内部电解液分布离散程度和/或均匀程度；
    根据所述电芯内部电解液分布离散程度和均匀程度，调整所述辊涂隔膜工艺或喷涂隔膜工艺的工艺参数进行制备，得到所述电芯。
  9. 根据权利要求8所述的制备工艺，其特征在于，所述工艺参数包括热压参数、注液系数及老化时间中的至少一种。
  10. 一种电芯内部电解液稳定性的表征方法，其特征在于，通过电芯内部电解液分布离散程度和/或均匀程度来表征所述电芯内部电解液稳定性，其中，所述电芯内部电解液分布离散程度和/或均匀程度通过权利要求1至7中任一项所述的方法分析得到；
    可选地，所述表征方法还包括：通过电化学阻抗谱绘制曲线分析所述电芯内部电解液稳定性。
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