Infusion and infiltration are the key steps to ensure the consistency and cycle life of Li-ion batteries. We have done a lot of work to improve the infiltration effect of the electrolyte. However, due to the sealed structure design of Li-ion batteries, it is very difficult to observe in real time To the infiltration of electrolyte, so these are mainly based on experience, often less effective results in recent years, with the detection technology, more and more new detection methods have been developed, so that we have the opportunity to be able to For example, some time ago we reported that engineers such as WJ Weydanz from Germany's Bosch used neutron imaging technology to analyze the infiltration process of electrolyte inside the cell.
Neutron penetration is strong, and very sensitive to Li atoms and H atoms, so the electrolyte has a strong absorption of neutrons, neutron diffraction is to detect the electrolyte in the lithium-ion battery infiltration process ideal means Munich Institute of Technology Thomas Knoche et al. Recently used neutron imaging technology to study the process of infiltrating the electrolyte in the cell during the filling process of the flexible battery. As shown in the figure above, the equipment used in the experiment mainly includes a vacuum Cans, sample cells, priming and sealing equipment, as well as neutron emitting and receiving equipment.
The experiment uses the battery as the flexible package laminated battery, 4 positive electrodes and 5 negative electrodes. The separator adopts Z-shaped laminations, the porosity of positive and negative electrodes is about 30%, and the electrolyte is EC: EMC = 3: 7 Mixed solvents, LiPF6 is not added to the electrolyte for safety reasons.
In general, we believe that the vacuum infusion can effectively promote the infiltration of electrolyte inside the lithium-ion battery, in order to verify the effect of vacuum on the infiltration of electrolyte, Thomas Knoche uses two modes of pressure control procedures (as shown below Both modes are vacuum infusion (both modes have different degrees of vacuum), then the cell is sealed after the normal pressure and vacuum cycles are resumed, several vacuum evacuation steps after the closure to further promote the electrolyte at Battery infiltration (in order to reduce the evaporation of the electrolyte, each time the vacuum is shortened as much as possible).
During the infusion process, the neutron beam was taken every 15 seconds to take a picture of the battery to record the electrolyte infiltration inside the battery. Thomas Knoche then used the image processing software to process the obtained image and recorded the infiltrated The number of pixels, the number of infiltrated pixels divided by the total number of pixels to obtain the value is the 'infiltration rate'.
The following chart shows the infiltration of 0s, 75s and 585s after the electrolyte in the battery infiltration, can be seen from the figure at the moment of infusion only a small amount of electrolyte along the upper end of the cell into the cell interior, most of the electrolysis The liquid flows to the lower part of the battery.When the capillary action, the electrolyte is gradually absorbed inside the cell, in the picture we can clearly see that the front of the electrolyte infiltration presents a U-shaped curve, infiltration of both sides of the cell Fastest, the middle of the cell infiltration slowest.
After the battery is sealed, the pressure of the vacuum tank is restored to normal pressure. Due to the pressure difference between the inside and the outside of the battery, under the action of atmospheric pressure, the electrolyte is driven into the inside of the cell to promote the infiltration of the electrolyte in the cell .
The following figure shows the relationship between the infiltration rate and the infiltration time of a different liquid injection system. It can be seen that in the A system with less evacuated cycles, the average infiltration rate of the battery is 73.18% at 850s, The number of vacuum cycles B system at 850s when the average infiltration rate of 78.73%, indicating that after injection of multiple vacuum, pressure cycle is conducive to improving the infiltration of lithium-ion batteries.
The following figure shows the infiltration of the battery under different pressure conditions. It can be seen that the pressure of the electrolyte has a significant effect on the infiltration of the electrolyte in the battery. The lower the pressure at the time of injection, the more the infiltration rate of the electrolyte High, the final infiltration rates of the batteries injected at 50 mbar, 400 mbar and 900 mbar were 82.3%, 77.9% and 70.1%, respectively.
Thomas Knoche study also found that the low pressure seal can promote more uniform infiltration of the electrolyte inside the battery, mainly because the low pressure seal can increase the pressure difference between inside and outside the battery, the atmospheric pressure can promote the electrolyte outside the cell into the cell Internal, thus promoting the electrolyte in the cell more uniform infiltration.
In order to model the electrolyte infiltration in Li-ion batteries, Thomas Knoche established the following model based on the LWE of porous material and the influence of gravity and electrolyte viscosity on the infiltration process.
The above constants a and b can be respectively calculated by the following formula, where R is the capillary pore diameter and u is the viscosity of the electrolyte.
The figure below shows the simulation results of the infiltration process using the above model. It can be seen that the model can better simulate the infiltration process of the electrolyte in the cell compared with the LWE model.
Thomas Knoche's research has made a very important contribution to our understanding of the wetting process of electrolytes and the improvement of the wettability of the electrolyte. We need to keep the gas pressure as low as possible during the filling process (however, the boiling point of the electrolyte is taken into consideration and the electrolyte Volatile) to promote the infiltration of the electrolyte in the cell.In order to ensure the infiltration effect of the electrolyte, it is necessary to carry out multiple vacuum evacuation and pressurization cycles before the cell sealing to promote the infiltration of the electrolyte. Low pressure, to use the pressure difference between the inside and outside the battery drive electrolyte into the interior of the cell to improve the infiltration of electrolyte.
