Model-based investigation and optimization of electrolyte filling using laser structured electrodes

The wetting of battery electrodes with electrolyte is a time- and cost-intensive process step. One of the biggest problems is the time it takes for the liquid electrolyte to be absorbed into the porous electrode. To reduce this wetting time, laser structured electrodes can be used. The resulting grooves facilitate deeper penetration of the electrolyte during the wetting process, leading to faster wetting. Multiphysics simulations and measurement data will be used to optimize the wetting process and to investigate the influence of the structuring geometry on the wetting time. In addition to modelling the structured electrode, achieving a suitable meshing is crucial. Moreover, the physical behavior of the wetting process will be represented by selecting appropriate and realistic boundary conditions. Capillary effects and fluid flow in porous media will be considered to describe the wetting process. The computer model will be validated using measurement data. In this paper it is shown that the wetting time can be significantly reduced by using structured electrodes. It is also shown that the wetting time is further reduced for smaller distances between the grooves. The software COMSOL MULTIPHYSICS will be used to create the model.


Introduction
The structuring of electrodes for lithium batteries has gained significance in recent years as it affects both battery manufacturing and battery performance [1].By using structural modifications, a 3D electrode is created, enhancing lithium-ion transport and leading to higher battery cell performance.This opens up the possibility of combining high-power and high-energy cells [2].Furthermore, structured electrodes offer advantages in battery manufacturing.They enable faster and more energy-efficient drying processes and thus improving this process step.Furthermore, the wetting time during the electrolyte filling process can be significantly reduced [3].Wetting is a time-consuming and cost-intensive step, as the liquid electrolyte needs to penetrate the porous electrode structure [4,5,6].Due to the small pore size and low porosity of the electrode material, permeability is low, resulting in long wetting times.However, the use of structured electrodes can substantially decrease this wetting time.The grooves in structured electrodes have a larger diameter than the pores in the porous material.As a result, the grooves have a higher permeability, allowing the electrolyte to penetrate quickly into the electrode.Figure 1 shows a microscope image of unstructured and structured electrodes.So far, there have been a few studies on the wetting of structured electrodes, mostly based on experimental investigations that give limited insight into the exact distribution within the electrode.In the previous investigations, the wetting of the unstructured electrode with DMC could not be fully described and a comparison of structured and unstructured electrodes was only possible to a limited extent.Furthermore, only the width of the grooves has been varied so far, but not the distance between two grooves.This work focuses on detailed multiphysics simulations to investigate the influence of structured electrodes on the wetting process.This study analyses the distribution of the electrolyte within the electrode during the wetting process.The wetting of structured and unstructured electrodes with DMC is investigated and the influence of the groove spacing on the wetting is analysed.In chapter 2, some important basics of the wetting process are explained.The simulation setup and the experimental setup are described in chapter 3. Furthermore, important material parameters and boundary conditions are defined.In chapter 4 the results are discussed and in chapter 5 the content of this paper is summarised.

Fundamentals
The wetting of battery electrodes is significantly determined by the capillary effects that occur in capillaries and porous structures.Capillaries are small tubes with a narrow diameter, while porous structures consist of a network of pores.Capillary forces are based on the phenomena of permeability (1) and capillary pressure (2), with: where  is the porosity, rpore is the pore radius, pec is the entry capillary pressure, λ is the pore size distribution, sw is the saturation of the wetting phase, σ is the surface tension and ϴ is the contact angle [7, 8].Darcy's law is a fundamental concept in fluid mechanics that describes the flow of liquids through porous media.The volumetric flow rate (u) through a porous medium is proportional to the pressure gradient (Δp) and the permeability (K) of the medium, with the proportionality constant being the inverse of the fluid's viscosity coefficient (μ).The Darcy's law can be described by the following equation [7,8]: Capillary forces play a crucial role in the wetting and distribution of liquids within porous structures.
The pores and channels in porous materials allow the liquid to penetrate and fill the structure through capillary action.Capillary action is influenced by the properties of the pores, such as diameter, shape, and surface characteristics.
Understanding capillary forces and fluid flow is essential for investigating the wetting process in porous electrode structures.By applying Darcy's law, relationships between pressure gradient, permeability of the porous medium, and volumetric flow rate can be established.This concept is utilized in this study to analyze and optimize the impact of structured electrodes on the wetting behaviour.

Methods and Modelling
To investigate the wetting of structured and unstructured electrodes a simulation model was developed in the software COMSOL MULTIPHYSICS.Figure 2 illustrates the schematic representation of the model setup for a segmented electrode.In the simplified model, the electrode is divided into two regions.Section (I) describes the porous structure of the electrode, while section (II) represents the capillary groove resulting from laser structuring.At the lower edge of the electrode (III), the liquid electrolyte is in contact under ambient conditions.Capillary action causes the electrode to wet in the y-direction (IV).Due to the different sections of the electrode, an inhomogeneous distribution of the electrolyte is expected, aiming to significantly improve the wetting time compared to unstructured electrodes.
To reduce the simulation time to a reasonable level, the model was simplified to a 2D model.The physical models of Darcy's Law and Phase Transport in Porous Media were used to create a multiphysics model and gravity has been considered.A physics-oriented mesh was applied.
The investigated electrode represents an electrode with a porosity of 30 %, a pore diameter of 1.5 µm, and a pore distribution of 2. The liquid electrolyte solvent is DMC with a dynamic viscosity of 0.00063 Pa*s, a surface tension of 0.0288 N/m, and a density of 1060 kg/m³ [2].For simplification purposes, the contact angle between the electrolyte and the electrode material was set to 0° in this simulation.In case of structured electrodes, the groove wide is 50 µm and because the grooves are wedge shaped the average pore diameter of the grooves is 33 µm.The distance between two grooves is 150 µm.The investigated electrode has a high of 40 mm.In order to validate the simulation with measurement data, wetting tests with similar electrodes and electrolytes were carried out in the laboratory.Figure 3 shows the schematic experimental setup of the wetting tests for structured and unstructured electrodes.The wetting tests were carried out in a closed system to minimise the influence of the evaporation of the electrolyte.With a ruler, the capillary rise can be determined for different time steps.The main simulation result is a saturation value, while the result value of the experiment is the current capillary rise.Assuming that the electrode below the liquid front is completely saturated, the capillary rise can approximately be converted into a saturation (related to the maximum sample height of 40 mm).The saturation (s) can be calculated with equation ( 5) where h is the capillary rise and hsample is the maximum high of the electrode.With this simplified conversion, the experimental data can be compared with the simulation data.

Simulation and Results
Using the created model in COMSOL Multiphysics, the wetting behavior of a laser-structured electrode was simulated and compared with the wetting of an unstructured electrode.Figure 4 illustrates the electrolyte saturation profile for an unstructured electrode (left) and a structured electrode (right) for a specific time step.It can be observed that the unstructured electrode exhibits a very homogenously wetting behavior, while the structured electrode has a highly heterogeneous wetting pattern.The liquid electrolyte wets the grooves faster than the porous structure of the electrode.Consequently, the electrolyte, starting from the grooves, distributes within the porous electrode, resulting in a faster wetting process.Figure 5 shows the wetting time of an unstructured and a structured electrode for the simulation and the measurement data.The simulation result shows that the wetting time of a structured electrode is about much faster than the wetting time of an unstructured electrode.The structured electrode reaches the maximum saturation after about 14 s, while the unstructured electrode hast a saturation of 0,23 after 14 s.It takes over 950 s reach full saturation for the unstructured electrode.The result agrees well with the measured values, in which the wetting time was 16 s for the structured electrode and 840 s for the unstructured electrode.Wetting is slightly faster in the simulation than in the experiment.On the one hand, it is possible that the evaporation of electrolyte still has a small influence on the wetting, although a closed system was used for the experimental measurement.On the other hand, some material data such as pore size and pore distribution are not known exactly, so that inaccurate assumptions in the simulation can lead to different results.Some results from the literature show similar results in the same order of magnitude.However, the results are difficult to compare because the literature mostly used different materials, geometries and boundary conditions [2, 3].To investigate the influence of the structuring geometry on the wetting behavior and wetting time, different structuring geometries were simulated.To save electrode material, the parameter study was only investigated using simulations.In this study the distance between two grooves were varied from 150 µm up to 600 µm.Figure 6 illustrates the results of the parameter study.The graph shows that the wetting time increases continuously with increasing the groove distance.The wetting of the liquid electrolyte is slower in the porous structure of the electrode than in the capillaries of the grooves.Therefore, the porous material is supplied with electrolyte through the grooves during the wetting process.If the groove spacing is increased while the groove width remains the same, a single capillary must supply a larger area of the porous material.In addition, longer distances must be covered within the porous material for complete wetting.These effects lead to a longer wetting time.The wetting time of a structured electrode with a groove distance of 600 µm is about 19s slower than the wetting time of a structured electrode with a groove distance of 150 µm.When comparing the influence of the structuring geometry on the wetting time with the wetting time of an unstructured electrode, it can be concluded that the exact geometry plays a secondary role in the investigated cases and that any groove is better than no groove.

Conclusion
The electrolyte filling process is a crucial step in battery production.In this study, the filling process was investigated and optimized using multiphysics simulations for both unstructured and structured electrodes.The results demonstrate that structured electrodes significantly reduce the wetting time.Furthermore, it was shown that selecting an appropriate structuring geometry further reduces the wetting time.Through detailed multiphysics simulations of the wetting process, the wetting behavior was observed and understood.The grooves in the structured electrodes exhibit better permeability compared to the porous structure of the electrode.As a result, the grooves facilitate the rapid flow of electrolyte into the electrodes and its distribution within the electrode.Using a structured electrode with a groove diameter of 50 µm and a groove spacing of 150 µm reduced the wetting time to 14 s, which is only about 1.5 % of the wetting time for the unstructured electrode (950 s).The choice of structuring parameters can further influence the wetting time.A smaller groove distance lead to faster wetting.For a groove distance of 150 µm, the wetting time was reduced by 19 s compared to a groove distance of 600 µm.Overall, this study has demonstrated that the use of structured electrodes can significantly reduce the wetting time, making it a promising approach in this process step.Detailed simulations have also enhanced the understanding of the wetting process of structured electrodes and investigated the influence of structuring geometry.A good understanding of the wetting of structured electrodes is important to further optimise this process and to quantify the influence of different parameters on the wetting process.With the help of the simulation model, many investigations can be carried out without using electrode material.This can save material and time.In the future, further parameter studies can be carried out to further optimise the wetting of electrodes and thus the battery manufacturing.

Figure 1 .
Figure 1.Microscope image of an unstructured cathode (left) and a structured cathode (right).

Figure 4 .
Figure 4. Saturation profile of unstructured (left) and structured (right) electrodes for a specific time step in COMSOL MULTIPHYSICS.

Figure 5 .
Figure 5. Saturation plot of unstructured and structured electrodes.

Figure 6 .
Figure 6.Saturation plot of structured electrodes for different groove distances.
Pfleging et al. 2014 A new approach for rapid electrolyte wetting in tape cast electrodes for lithium-ion batteries Journal of Materials Chemistry A doi: 10.1039/c4a02353f [3] W Pfleging et al. 2014 Laser generated microstructures in tape cast electrodes for rapid electrolyte wettingnew technical approach for cost efficient battery manufacturing Green Photonics Best Paper Award doi: 10.1117/12.2039635[4] J B Habedank et al. 2019 Rapid electrolyte wetting of lithium-ion batteries containing laser structured electrodes: in situ visualization by neutron radiography The International Journal of Advanced Manufacturing Technology https://doi.org/10.1007/s00170-019-03347-4[5] M P Lautenschlaeger et al. 2022 Understanding Electrolyte Filling of Lithium-Ion Battery Electrodes on the Pore Scale Using the Lattice Boltzmann Method Batteries & Supercaps doi.org/10.1002/batt.202200090[6] A Shodiev et al. 2021 Insight on electrolyte infiltration of lithium ion battery electrodes by means of a new three-dimensional-resolved lattice Boltzmann model Energy Storage Materials https://doi.org/10.1016/j.ensm.2021.02.029 [7] G Schaefer et al. 2004 Modelling Two-Phase Incompressible Flow in Porous Media Using Mixed