Electrolyte is an important part of lithium-ion battery. The main function of lithium-ion battery is to conduct ions between positive and negative electrodes. As the capacity of lithium-ion battery continues to increase, the size of the battery also continues to increase. How to ensure that electrolysis Liquid inside the lithium-ion battery is fully and evenly infiltration is particularly important because of the closed lithium-ion battery structure so that we can not directly observe the electrolyte inside the lithium-ion battery infiltration, only through the dissection to determine the electrolysis Liquid is impregnated inside the cell is sufficient, recently Germany's Bosch engineers WJ Weydanz neutron diffraction imaging technology through the electrolyte inside the cell infiltration process conducted a full study found that the vacuum can be infiltration of electrolyte time Reduce by 50% and increase the amount of fluid by 10%.
In general, the injection process can be divided into two steps: 1) injection, this usually takes only a few seconds to inject electrolyte into the battery; 2) infiltration, this step is to absorb the injected electrolyte into the electricity In the core, this is usually a very time-consuming process that usually takes hours and, in order to achieve a good wetting effect, the pouring and wetting often require several iterations, all in a dry room environment In, greatly pulled up the lithium-ion battery production costs.
Traditional detection methods can not real-time monitoring electrolyte infiltration inside the lithium-ion battery, WJ Weydanz use of Li for neutron has a strong absorption of this feature, the use of neutron diffraction technology in the electrolyte solution in the lithium-ion battery that The infiltration process was studied in detail.
In the experiment, WJ Weydanz used the Al-shell square battery used on HEV, the size of the battery is 120mm * 91.5mm * 12.5mm, the battery uses seven graphite negative electrodes (coating amount 8mg / cm2, porosity 35%), NCM111 material cathode (coating amount 15.8mg / cm2, porosity 35%) and 12 polymer membranes (thickness 20um, porosity 48%).
After the battery is filled with fluid, WJWeydanz shoots a photo every 15 seconds using a seed diffraction method, takes a photo every 60 seconds after 10 minutes and takes a photo every 120 seconds after 30 minutes. Sub-diffraction photographs, the black parts on both sides of the picture is the battery electrolyte gap, the electrode is slightly darker in color is the electrolyte has infiltrated the location of the lighter color is not yet infiltrated electrolyte area.
After the above images have been specially processed by software, the cells can be divided into a wetting area (black) and a non-wetting area (white) so that the software can be used to count and process the wetting of the electrolyte.
The following figure shows the infiltration of the battery (Figure a, b, c) and the battery filled under normal pressure (d, e, f) in a vacuum, the injection can be seen 2min After most of the electrolyte is also in the outer space of the cell, the edge of the cell began to infiltrate in 47min infiltration, the battery infused in a vacuum almost completely infiltrated cells, residual electrolyte outside the cell Significantly reduced.However, under normal pressure, there is still a considerable part of the batteries infused battery is not infiltrated, leaving a large amount of free electrolyte outside the battery.
In order to study the relationship between electrolyte infiltration rate and time, WJ Weydanz analyzed the infiltration rate of the electrolyte in the four directions of upper, lower, left and right of the cell. The results are shown in the following figure (vacuum environment liquid injection). From Figure can be noted that within 5min after the battery infusion, the electrolyte infiltration area reached 52%, and then after 5min, the infiltration area increased 19% to 71%, indicating that the infiltration rate of the electrolyte over time Increase and decrease, the battery is basically completed after 51min infiltration.From the figure we can also notice that the electrolyte from the cell, the infiltration rate in the next two directions is almost the same, we can see the infiltration of gravity on the impact of electrolyte is minimal .
WJ Weydanz analyzed the infiltration data in four directions and found that the infiltration rate of the electrolyte showed a logarithmic decay relationship with time but WJ Weydanz found that in the first 30 minutes the infiltration rate of the electrolyte Showing a trend of logarithmic decay, but a significant increase in infiltration rate after this time, which can be explained by the distance relationship between each edge of the infiltration zone in the infiltration area in the figure below. When infiltration for 33 minutes ), The infiltration front on the left is almost the same distance from the left, upper and lower ends of the cells, so initially the electrolyte infiltrated to the left is replenished mainly from the left with new electrolyte but after 8 minutes left The distance between the infiltrated edge and the upper and lower ends is obviously shorter than the distance from the left end. Therefore, the infiltration on the left side of the cell is replenished mainly from the upper and lower ends, thereby accelerating the infiltration of the electrolyte on the left and right sides.
At the same time WJWeydanz study also found that the vacuum infusion can effectively reduce the infiltration time, the following picture shows the infiltration rate of the electrolyte in the left and right directions after normal pressure (blue curve) and vacuum (red curve) , You can see the vacuum infusion of the battery infiltration rate significantly faster than the battery under normal pressure, pressure injection under the battery needs 101min to complete the infiltration, but the vacuum infusion of the battery only 51min Complete the infiltration, the infiltration time reduced by 50%.
In addition to being able to effectively reduce the wetting time of the electrolyte, WJ Weydanz also found that under vacuum, the electrolyte can make the cell absorb more 10% of the electrolyte, and these multiple absorption of the electrolyte is mainly filled in those not infiltrated Among the micropores in the electrode.
Due to the limitation of the sealing structure of the Li-ion battery, in the past, our understanding of the electrolyte infiltration was mainly based on experience. For the first time, WJ Weydanz's work allowed us to have a 'visual' understanding of the infiltration process of the electrolyte. Infiltration rate and time show a declining logarithm of the relationship.At the same time let us see that the electrolyte from the cell at the upper and lower infiltration rate is the same, the gravity of the infiltration of electrolyte infiltration is minimal, the real infiltration of the electrolyte Affect the larger is the vacuum, in a vacuum environment, liquid infusion can be reduced by 50% of the infiltration of electrolyte, batteries to absorb the amount of electrolyte increased by 10%.
