Given the irreversible effects on the global climate, there is a collective societal challenge to reduce CO₂ emissions. Research in battery technology has the potential to provide a solution for carbon-neutral mobility by increasing the efficiency as well as expanding the utility of storage options for electrical energy [1]. In this context, high capacity lithium-ion batteries have the potential to transform the mobility industry in the short and medium term by replacing fossil fuels. As a result, the demand for large-format battery cells with high specific capacities and power has increased rapidly in recent years [2]. The implementation of the EU regulation for CO₂ reduction in the automotive sector indicates that this trend will likely continue [3].

Thus, this rising demand necessitates more efficient battery production in order to save time, material, and the associated costs. Battery cost drivers include, in addition to raw material acquisition costs, inefficient manufacturing processes [4]. The process of electrolyte filling, due to long storage times of cells, represents a promising entry point for cost savings ^[5][6][5,6]. This process step is critical for quality, as insufficient wetting of the cell stack materials can result in poor cell performance [7].

Although the potential for savings has been widely recognized in both research and industry, findings covering electrolyte filling remain fragmented. There are complicated interactions between process–structure–characteristics and their relationships, which are often related to prior process steps. Existing research focuses on highly application-specific industrial processes, covers simulation studies without experimental foundation, or investigates isolated aspects such as material or process influences separately. However, a comprehensive scientific analysis of experimental results on material and process parameters is lacking.

2. Trends

Figure 1 shows an increase in publications over the last ten years, with a peak in 2020, reflecting the growing importance of battery production over the last decade. This is particularly due to increasing efforts and political measures towards sustainability, which have a strong impact on the automotive industry. It is also notable that there has been an increasing number of contributions focusing on improved process parameters, as their efficient and effective design is crucial for the implementation of large-scale industrial production. From 2011 to 2017, the focus was on material-centered publications related to electrolyte filling, such as studies on the wetting properties of newly developed materials and the effects of upstream process steps in electrode production, such as calendering or laser patterning, on the wetting properties of electrodes ^[8][9][10][48,49,50]. Since 2017 there has been a noticeable increase in publications investigating the electrolyte filling process itself, in addition to the development of new materials and the investigation of their wetting properties. Production-related issues are now also becoming a focus of scientific investigation. Researchers mapped the distribution of the labeled contents in a diagram (cf. Figure 2) according to their frequency. From the inside out, the sub-labels became more and more specific. Several labels of a contribution were therefore represented in several different labelings, so the sum of all labels exceeded the total number of examined publications by far. Approximately half of the publications (24/52) were related to materials, while roughly 20–25% were related to process parameters (13/52) and other processes (15/52). The category of Materials mainly referred to contributions that investigate the wetting behavior of different cell composite materials. The wetting behavior often characterized only one of many parameters used to dimension the cell composite materials (electrode, separator, and electrolyte). In most cases, however, the focus was on modifying the cell composite materials. The Process Parameters topic block included articles that specifically examined the electrolyte-filling process. Here, process parameters and certain product parameters such as cell design (hardcase vs. pouch cells) or cell geometry played a dominant role. The topic block of Further Processes dealt with research contributions on the influence of processes other than electrolyte filling on wetting. In addition to the divisions into topic blocks with corresponding sub-labels, it is also indicated which level the research addressed: the Material Level, which included analyses of individual materials of the cell stack without a cell composite present; or the Cell Level, where analyses were performed on assembled cell composites. Table 1, Table 2 and Table 3 below illustrate the categorization made of the research articles selected and analyzed in full text. Research also focused on the modification of electrolyte composition, primarily involving carbonate-based electrolytes with LiPF6 as the conducting salt ^[12][13][20][31,34,54]. Only a few studies, such as the work of Davoodabadi et al., investigated specific variations in electrode formulation, using a water-based binder in their formulation ^[18][20][52,54]. Simple material characterization methods, such as the droplet spread test or contact angle measurement, were commonly used. In contrast, there was a lack of investigation into the transfer of findings to the cell level through cell performance tests, indicating a research gap. The analysis also revealed several publications that addressed the main process parameters of temperature and pressure ^[20][31][32][15,16,54]. While temperature experiments were conducted at both the material and cell levels, they were never conducted in the same experiment, leaving a lack of information on the possible mutual influence of the process parameters on each other and the influence of different material parameters. Again, the material and cell levels were only investigated in isolation without analyzing the transfer of findings from the material level to the cell level. The most commonly used measurement methods were electrochemical impedance spectroscopy and neutron radiography. Cell design and geometry also played a role in the dispensing and wetting conditions between pouch and hardcase cells. However, the evacuation process and the pressure it set were not fully considered in the literature. It is unclear whether the pressure applied on the process side also affects the pore system or just the dead volume of the cell. This is especially important for hardcase cells, which have narrow, coiled designs, where the diffusion paths for gas from the pores correspond to half the cell composite height. The cell geometries discussed in the literature showed little variation. While the size and number of compartments may vary, the cell geometries often have similar aspect ratios with the same length-to-width ratio. With the trend in the automotive industry is towards blade cells for use in the cell-to-pack battery, the impact of varying the electrode area and shape, particularly the aspect ratio, has not been explored. The analyzed literature covered various studies on upstream and downstream processes. Some contributions focused on the influence of calendering and laser structuring processes on the wetting properties of electrodes. Calendering, as the final process step in electrode manufacturing, significantly affects the pore morphology of the electrode. However, there is a lack of a comprehensive view that takes into account the influence of mixing, coating, and drying parameters on the resulting pore morphology in addition to calendering parameters. This would allow for more general conclusions to be drawn about the relationship between the electrode manufacturing process parameters, pore morphology, and wetting behavior of the electrode. Additionally, it would be useful to consider the materials used, as they have a major influence on both the resulting pore morphology and wetting properties. Laser structuring is an additional or alternative process step in the manufacturing process chain that is used to improve ion diffusion into electrodes, such as for fast charging processes. However, this process step also leads to a decrease in specific volumetric capacity ^[39][67]. The introduced structures also provide additional channels for electrolyte to spread better in the direction of the electrode thickness and create punctual starting points for wetting in otherwise remote regions of the electrode. Many publications studied the influence of this process step on wetting at both the material and cell levels. Other articles occasionally addressed other processes, such as lamination of electrode-separator composites, or process improvements for electrolyte filling processes such as roll pressing, in which the electrolyte is distributed in the filled and sealed cell by a roller ^[40][44][68,72]. The bar chart in Figure 38 illustrates the frequency with which different measurement methods were used in the publications studied. It distinguishes between investigations at the cell level (in dark blue) and those at the material level (in light blue). While contact angle measurement and wetting balance tests are commonly used at the material level, electrochemical impedance spectroscopy and neutron radiography are most frequently used at the cell level. The choice of measurement method is often dictated by the level at which it operates; contact angle measurement can only be performed between a solid and a liquid, so it cannot be used to image the cell level. Similarly, chronoamperometry and electrochemical impedance spectroscopy require a complete cell, which precludes measurements on individual cell stack materials. Therefore, no dominant measurement method emerged.