Manufacturing thick electrodes for Li-ion batteries is a challenging task to fulfill, but leads to higher energy densities inside the cell. Water-based processing even adds an extra level of complexity to the procedure. The focus of this work is to implement a multi-layered coating in an industrially relevant process, to overcome issues in electrode integrity and to enable high electrochemical performance. LiNi${}_{0.8}$Mn${}_{0.1}$Co${}_{0.1}$O${}_2$ (NMC811) was used as the active material to fabricate single- and multi-layered cathodes with areal capacities of 8.6 mA h cm⁻². A detailed description of the manufacturing process is given to establish thick defect-free aqueous electrodes. Good inter-layer cohesion and adhesion to the current collector foil are achieved by multi-layering, as confirmed by optical analysis and peel testing. Furthermore, full cells were assembled and rate capability tests were performed. These tests show that by multi-layering, an increase in specific discharge capacity (e.g., 20.7% increase for C/10) can be established for all tested C-rates.

Thick electrode production is a key enabler for realizing high energy density Lithium-ion batteries. Therefore, the investigation of tortuosity as a crucial limiting parameter was conducted in this work. A thickness threshold (>150 μm) for a drastic increase in tortuosity for aqueous processed LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) electrodes was determined. Symmetrical cells, under blocking conditions, were analyzed via electrochemical impedance spectroscopy. To counteract this phenomenon, multi-layer coated electrodes with varying binder concentrations were investigated. This novel coating method, combined with the reduction of binder material, leads to a tortuosity decrease of more than 80%, when compared to high-loading electrodes (>8.5 mAhcm⁻²) coated with the conventional doctor-blade technique. Additionally, a simplified transmission line model is opposed to a linear fitting method for analyzing the impedance data.

Multi-layer coating of electrodes with different material compositions helps unlock the full potential of high-loaded electrodes. Within this work, LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathodes with an areal capacity of >8.5 mA h cm⁻² and tuned binder concentrations were fabricated by using an industrially relevant roll-to-roll process. Rate capability tests revealed an increase in practical specific discharge capacity independent from the C-rate for cathodes with reduced binder concentration in the top layer. At high current densities (C-rate of 1C) an improved performance of up to 27% was achieved. Additionally, at lower C-rates, binder gradients perpendicular to the current collector have beneficial effects on thick electrodes. However, surface analysis and electrochemical impedance spectroscopy revealed that without an adequate connection between the active material particles through a carbon-binder domain, charge transfer resistance limits cycling performance at high current densities.

The performance of Li-ion batteries is highly influenced by the microstructure of the assembled electrodes. Typically, porosity is chosen as a standard characteristic parameter, describing the morphology of the material components in the coated layer. However, transport phenomena and concentration polarization resistance also depend on the tortuosity of the electrode coating [1]. Therefore, it is crucial to determine the relationship between coating parameters (such as coating speed and drying parameters) and their effect on the tortuosity. Especially for thick electrodes an elongation of ionic transport pathways can arise due to the inhomogeneous distribution of the materials. Dead ends, branches, and surface impurities can lead to an increase in effective tortuosity [2], which can be minimized by careful selection of the coating parameters. A high binder concentration at the layer surface, caused by binder particles migrating during the drying process, can lead to a blockage of Li-channels [3, 4]. This can not only diminish the mechanical stability of the electrode but also harm the cycling performance, due to limitations in mass and charge transport [5]. Electrochemical impedance spectroscopy can be used as a characterization technique to investigate the tortuosity of the electrode. Symmetric cells in combination with a non-intercalating electrolyte create a blocking condition of the cells. Extrapolating the spectrum's high and low-frequency regions, offsets in the real part of the impedance can be determined in the Nyquist plot (Figure 1a). The ionic resistance can be determined, and consequently, tortuosity can be calculated. A modified transmission line model can be used for in-depth analysis. In the case of homogenously distributed material, tortuosity should be an intrinsic value. In this work, a threshold value of the electrode thickness was determined, above which the tortuosity increases drastically (Figure 1b). This leads to the conclusion that when increasing the thickness, changes in the layer composition can arise.Additionally, the correlation between tortuosity and porosity was investigated and associated Bruggeman coefficients were compared. In a previous study, the multilayer coating technique has proven itself to be beneficial for increasing the electrochemical performance of high-loading electrodes containing cells [6]. Subsequent coating of multiple layers on top of each other enables the possibility to fine-tune material distributions along the layer thickness. Particularly, electrodes with an imposed binder gradient, of low concentration at the electrode separator interface, show a noteworthy decrease in tortuosity [7]. This study was done on cathode samples with NMC811 as active material. Processing was done with both water and NMP, to show the impact of the solvent on the electrode tortuosities. Acknowledgments This work was financially supported by the Austrian Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology (bmk). [1] B. Tjaden, D. J. L. Brett, and P. R. Shearing, “Tortuosity in electrochemical devices: a review of calculation approaches,” International Materials Reviews, vol. 63, no. 2, pp. 47–67, Feb. 2018, doi: 10.1080/09506608.2016.1249995. [2] J. Landesfeind, J. Hattendorff, A. Ehrl, W. A. Wall, and H. A. Gasteiger, “Tortuosity Determination of Battery Electrodes and Separators by Impedance Spectroscopy,” J Electrochem Soc, vol. 163, no. 7, pp. A1373–A1387, 2016, doi: 10.1149/2.1141607jes. [3] F. Font, B. Protas, G. Richardson, and J. M. Foster, “Binder migration during drying of lithium-ion battery electrodes: Modelling and comparison to experiment,” J Power Sources, vol. 393, no. December 2017, pp. 177–185, Jul. 2018, doi: 10.1016/j.jpowsour.2018.04.097. [4] S. Jaiser, M. Müller, M. Baunach, W. Bauer, P. Scharfer, and W. Schabel, “Investigation of film solidification and binder migration during drying of Li-Ion battery anodes,” J Power Sources, vol. 318, pp. 210–219, 2016, doi: 10.1016/j.jpowsour.2016.04.018. [5] M. Baunach, S. Jaiser, S. Schmelzle, H. Nirschl, P. Scharfer, and W. Schabel, “Delamination behavior of lithium-ion battery anodes: Influence of drying temperature during electrode processing,” Drying Technology, vol. 34, no. 4, pp. 462–473, 2016, doi: 10.1080/07373937.2015.1060497. [6] L. Neidhart, K. Fröhlich, N. Eshraghi, D. Cupid, F. Winter, and M. Jahn, “Aqueous Manufacturing of Defect-Free Thick Multi-Layer NMC811 Electrodes,” Nanomaterials, vol. 12, no. 3, pp. 1–15, 2022, doi: 10.3390/nano12030317. [7] L. Neidhart, K. Fröhlich, F. Winter, and M. Jahn, “Implementing Binder Gradients in Thick Water-Based NMC811 Cathodes via Multi-Layer Coating,” Batteries, vol. 9, no. 3, p. 171, Mar. 2023, doi: 10.3390/batteries9030171. Figure 1

Binder selection strongly influences silicon–graphite (Si–Gr) anode performance, yet the relationship between slurry processing, electrode structure, and electrochemical behaviour remains unclear. Here, we systematically investigate the influence of binder chemistry and binder pH on Si–Gr composite anodes using unmodified poly(acrylic acid) (PAA), lithiated PAA (LiPAA) at pH 5–7, and a conventional carboxymethyl cellulose (CMC) binder. Rheological, adhesion, zeta potential and microscopy analysis reveal that binder pH strongly governs slurry processability, binder–component interactions, and the resulting electrode structure. These differences were found to directly influence electrochemical performance in full cells versus NMC532. Among the binders studied, partially neutralised LiPAA at pH5 exhibited the most stable cycling performance, combining the favourable binder interactions associated with free acid PAA with the improved slurry processability required for practical electrode manufacturing. In contrast, unmodified PAA showed favourable electrochemical behaviour but poor rheological stability, while increasing binder pH resulted in progressively poorer electrode integrity, increased polarisation, and accelerated capacity fade. Differential capacity and electrochemical impedance analyses further revealed substantial differences in the electrochemical response and interfacial stability of the electrodes, while post‐mortem cross‐sectional scanning electron microscopy showed preservation of the electrode structure at pH 5 and severe structural degradation at pH 7 after 50 cycles. Overall, these results highlight the critical role of binder‐controlled processing in determining the stability of Si–Gr composite anodes by demonstrating how binder pH influences slurry interactions, electrode structure, and the subsequent electrochemical behaviour during cycling.