Evaluation of electrolyte wettability for lithium batteries!

There are various methods for evaluating the wettability of electrolytes, including contact angle measurement, wetting time (static drop method), wetting height, surface tension measurement, and wetting balance method. The contact angle measurement method evaluates the wetting performance of the electrolyte by measuring the size of the contact angle formed by the electrolyte droplet on the solid surface; A contact angle close to 0 ° indicates good wetting of the electrolyte on the surface, while a contact angle close to 90 ° indicates poor wetting performance of the electrolyte on the surface.
The infiltration time method is to drop a certain amount of electrolyte onto a solid surface (such as a diaphragm) and test the time required for the electrolyte to fully penetrate. The shorter the time, the better the infiltration performance. The immersion height method is to completely immerse or immerse one end of a fixed size solid material (such as a diaphragm) into the electrolyte, and evaluate the immersion performance based on the quality of the electrolyte immersed in the electrode within a certain period of time. The surface tension measurement method refers to the force exerted on the liquid surface relative to the solid surface. The higher the surface tension of the electrolyte, the worse its wetting performance. By measuring the surface tension of the electrolyte, its wettability on solid surfaces can be indirectly evaluated.

1 Experiment
1.1 Experimental raw materials
The selected positive electrode for the experiment is lithium iron phosphate positive electrode. The proportions of lithium iron phosphate, binder, and conductive carbon black are 94.5%, 3.0%, and 2.5%, respectively. The single-sided surface density is 164g/m2, and the designed compaction density is 2.5g/cm3. The negative electrode selected for the experiment is artificial graphite, with proportions of 94.0%, 2.5%, 2.0%, and 1.5% of artificial graphite, binder, dispersant, and conductive carbon black. The single-sided density is 76g/m2, and the designed compaction density is 1.5g/cm3. The selected membrane for the experiment is a wet PE base film with a thickness of (12 ± 2) μ m.
1.2 Experimental formula
The formula design of the electrolyte used in this experiment is shown in Table 1:

1.3 Experimental Instruments
Experimental instruments: electronic balance, Mettler; Polar suspension device, self-made; Contact angle tension measuring instrument, LSA100 video optical contact angle tension measuring instrument.
1.4 Experimental Methods
The suspension method is used to test the climbing height of electrolyte. The operation process is as follows: place the device upright, inject electrolyte into its interior, and the liquid level is slightly lower than the height of the shell. Cut the sample to be tested into a long strip of 20mm × 260mm. The size of the experimental platform is 55mm (thickness) × 174mm (length) × 200mm (height), and the recommended sample cutting length is ≥ 1.2 times the height of the shell. Fix the sample in the grooves on both ends of the partition with double-sided tape. The fixing process should ensure that the sample is straight and not bent. Then slowly place the partition with the sample strip fixed into the shell, and record the time and the corresponding scale of the liquid level height. On the left side of each groove of the partition, there is a scale line with a division of 0.5mm, which serves as the starting point for the infiltration experiment. Then, slowly remove the pipette and seal the injection hole with a rubber stopper. Record the height Δ H of the electrolyte climbing inside the electrode at regular intervals thereafter.
The immersion balance method is similar to the suspension method for testing the climbing height of electrolyte. The electrode is hung under the balance, and the time and initial electrode mass are recorded. Subsequently, the mass changes caused by the electrode electrolyte are recorded at regular intervals.
The contact angle method and surface tension method are operated according to the general method of LSA100 video optical contact angle tension measuring instrument. Cell suction method, refer to the immersion balance method for operation, replace the electrode with a cell.

2 Results and Discussion
2.1 Immersion height method
Result analysis: The results of the infiltration height method are shown in Figure 1. As the amount of infiltration agents FB and DMC added increases, the infiltration height increases, which meets the actual situation. However, the numerical differences between different groups are small, the data deviation is large, and the results are difficult to reproduce, making it difficult to guarantee the consistency of the polarizer; The possible reason is due to various uncontrollable factors during the testing process, such as the uniformity of the electrolyte and the microstructure of the electrode surface, which may result in significant differences in the results of each test and make it difficult to obtain stable and reliable repeated results.

Unable to accurately evaluate the true infiltration situation: At the same time, the infiltration height method mainly relies on the quality of the electrode immersed in the electrolyte for a certain period of time to evaluate the infiltration performance, but this method may not fully reflect the true infiltration situation of the electrolyte inside the electrode, especially for electrodes with uneven infiltration or microstructural differences, the evaluation results may not be accurate enough. And it has a strong dependence on the size and shape of the polarizer: a fixed size polarizer is required during testing, which limits the applicability of this method on polarizer of different sizes and shapes. For non-standard sizes or special shapes of polarizer, additional processing and conversion may be required, increasing the complexity and uncertainty of testing. This type of method requires a longer testing time: in order to obtain more accurate evaluation results of infiltration performance, it is usually necessary to soak in the electrolyte for a period of time, which may result in a longer testing cycle and hinder timely evaluation during rapid research and development and production processes.
2.2 Wetting balance method
Result analysis: The wetting balance method shows that the overall numerical differences between different groups are small, the data deviation is large, and there are numerical anomalies, which cannot reflect the actual effect of the wetting agent, such as FB.  

The main problem with testing lies in the difficulty of controlling the testing conditions: the infiltration quality method requires precise control of the testing conditions, such as electrolyte temperature, concentration, electrode size, etc. Any small changes may affect the testing results, increasing the difficulty and complexity of the testing; At the same time, the testing time is relatively long: in order to obtain accurate infiltration quality data, it is usually necessary to immerse the electrode piece in the electrolyte for a long time, which may result in a long testing cycle and is not conducive to rapid research and development and timely evaluation in the production process; And the results are affected by the surface state of the electrode: factors such as the microstructure and roughness of the electrode surface may affect the wetting behavior of the electrolyte, thereby affecting the accuracy of the test results. Therefore, strict pre-treatment and characterization of the electrode surface are required before testing; And the requirements for the electrolyte are high: the infiltration quality method testing requires high uniformity and stability of the electrolyte, and it is necessary to ensure that the electrolyte remains in a constant state throughout the entire testing process, otherwise it may affect the reliability of the test results; At the same time, the equipment dependence is strong: the infiltration quality method testing usually requires the use of specific measuring equipment, such as high-precision scales, constant temperature and humidity chambers, etc. The accuracy and stability of these devices directly affect the accuracy of the test results.
Result analysis: The surface tension test results show that the surface tension of the electrolyte increases with the addition of wetting agents. However, the overall difference in values between different groups is small, and the data deviation is large, making it impossible to draw a regular conclusion. The reason for this is that the infiltration process of the electrolyte into the electrode not only includes interfacial interactions, but also the siphon effect generated by the internal voids of the electrode.

2.3 Contact angle method and immersion time method
Result analysis: As shown in Figure 4, there are two problems with the static drop test: (1) the electrolyte droplets will quickly spread on the surface of the electrode, making it difficult to maintain stability; (2) It is difficult to ensure that the contact time and speed between the liquid and the electrode are completely consistent when measuring the contact angle using the seat drop method; Therefore, the infiltration time is difficult to define; At the same time, the contact angle test results are shown in Figure 5, and the contact angle cannot remain relatively stable for a period of time.

Although the contact angle method is a commonly used method for testing the wettability of polarizer, it also has some drawbacks, mainly including: (1) the influence of surface roughness: the actual surface is often rough or uneven, which can lead to hysteresis in the contact angle, that is, the forward angle and backward angle are not consistent, thereby affecting the accuracy of the test results. This unevenness may make it difficult for the test results to accurately reflect the true wetting performance of the polarizer; (2) Uncertainty of compression effect: During contact angle testing, if the compression effect of the polarizer is poor, it may lead to poor contact between the droplets and the polarizer surface during the testing process, thereby affecting the reliability of the test results. For example, powdered materials may roll during testing, making the droplet shape unstable and difficult to accurately measure the contact angle; (3) Operational complexity and error: Contact angle testing requires precise operation and measurement techniques, with high demands on operators. At the same time, there may be multiple sources of error during the testing process, such as instrument accuracy, changes in environmental temperature and humidity, which can all have an impact on the test results; (4) Dependence on polarizer materials: Different materials may have significant differences in polarizer surface properties, which requires testing methods to have high adaptability and flexibility. However, the testing effect of contact angle method on certain specific materials may not be ideal, and it needs to be comprehensively evaluated in combination with other testing methods.
2.4 Cell liquid absorption method
Result analysis: The test results of the cell liquid absorption method show that the amount of liquid absorption increases after adding the wetting agent to the electrolyte; The more wetting agent is added, the more liquid the battery cell absorbs, and the wetting effect of fluorobenzene is better than that of dimethyl carbonate. The overall testing error is small, the repeatability is good, and it is suitable for evaluating electrolyte wettability.

3 Conclusion
Conventional methods such as contact angle measurement, immersion time measurement, immersion height measurement, surface tension measurement, and wetting balance measurement are affected by electrode consistency and have large errors, making it difficult to meet the evaluation of cell wettability. The cell suction method for testing electrolyte wettability has a series of significant advantages, as it can directly reflect the wetting effect. By directly measuring the absorption of electrolyte by the cell, the cell suction method can intuitively and accurately reflect the wetting effect of electrolyte in the cell.
This method avoids the errors that may arise from indirect measurements, making the evaluation results more reliable; Easy to operate, compared to other complex testing methods, the operation steps of the cell suction method are relatively simple and do not require complex equipment or professional skills. This makes the method easier to promote and use in practical applications; The cost is low, and due to the relatively simple equipment and materials required for the cell suction method, its testing cost is also relatively low. This is an undeniable advantage for large-scale production and research and development; The liquid absorption method for multiple types of battery cells is not only applicable to common types of battery cells such as lithium-ion batteries, but also to other types of battery cells. This makes the method widely applicable and able to meet the needs of different fields and industries; Helps optimize the electrolyte formula. By testing the infiltration effect of different electrolyte formulas on the battery cells using the cell liquid absorption method, the electrolyte formula with better infiltration performance can be selected. This is of great significance for improving battery performance and extending battery life; Directly related to battery performance.
The wettability of battery cells directly affects the performance of batteries. Good wettability helps to evenly distribute the electrolyte in the battery cell, improve ion transport efficiency, and thus enhance the rate performance, discharge capacity, and service life of the battery. Therefore, the cell liquid absorption method, as an effective means of evaluating wettability, plays an important role in improving the overall performance of batteries.
In summary, the cell suction method for testing electrolyte wettability has the advantages of directly reflecting the wetting effect, easy operation, low cost, applicability to various cell types, helping to optimize electrolyte formulations, and being directly related to battery performance. These advantages make the cell suction method one of the important methods for evaluating electrolyte wettability.