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Maximizing the electrode thickness is an inevitable target to reach when improving the energy density of Li-ion batteries [1,2]. Unfortunately, the fabrication of electrodes of several hundreds of micrometer thickness is often accompanied by detrimental side-effects. A critical process step in thick electrode production is the drying of the coated layer. During the evaporation of the solvent, light components of the slurry such as binder and carbon black can be dragged to the top of the coating [3,4]. The lack of liquid linkage throughout the layer leads to binder particles being evenly spread across the coating after the drying process is concluded. Inhomogeneous distribution of binder can not only cause delamination, but also negatively influences the Li-ion transport pathways through the electrode. To ensure ideal mechanical stability and satisfactory electrochemical performance, binder migration needs to be suppressed effectively. Multi-layering has proven itself as a technique within aqueous processing to overcome the limitations arising for thick electrodes. Subsequent coating of layers offers opportunities to establish selective material compositions at specific regions within the electrode (Figure 1a). This novel coating method can be considered a steppingstone to further optimize high loading electrodes. We have previously demonstrated that by multi-layering an increase of 40% in adhesion can be achieved compared to single-layer electrodes. Moreover, the assembled cells show superior performances at rate capability tests. Especially for low current densities (0.2C) an increase of up to 20% in specific discharge capacity was reached through multi-layering (Figure 1b) [5]. Within aqueous electrode fabrication, the use of sodium carboxy-methyl cellulose (CMC) as binder material is common, due to its role as a thickening agent of the slurry and solubility in water. However, due to its brittleness, the usage of CMC requires additional binders to maintain flexibility of the coating. The ratio of active and inactive materials within the cell is strongly correlated to its capacity and the addition of these binders ultimately results in a decreased energy density of the cell. Besides keeping the particles together, the main purpose of the binder is to guarantee good adhesion to the substrate. Therefore, a high concentration is not necessarily needed at the top of the coating and the overall amount of binder material should be kept at a minimum. Bi-layered cathodes consisting of Ni-rich lithium nickel manganese cobalt oxides (NMC811) as active material were fabricated. The concentration of poly(methyl)acrylate (PMA) binder was changed in the top layer and its effect on structural properties and electrochemical performance was investigated. Water-based processed cathodes with high loadings of more than 8 mAhcm-2 were tested in full cell configurations. Reduction of the binder content leads to an increase in specific discharge capacity of up to 40% compared to single-layered and 25% to unmodified multi-layered electrodes at a C-rate of 1C (Figure 1b). Acknowledgements This work was financially supported by the Austrian Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology (bmk). [1] Z. Du, D.L.W. Iii, C.D.S. Kalnaus, J. Li, Understanding limiting factors in thick electrode performance as applied to high energy density Li-ion batteries, J. Appl. Electrochem. 47 (2017) 405–415. https://doi.org/10.1007/s10800-017-1047-4. [2] Y. Kuang, C. Chen, D. Kirsch, L. Hu, Thick Electrode Batteries: Principles, Opportunities, and Challenges, Adv. Energy Mater. 9 (2019). https://doi.org/10.1002/aenm.201901457. [3] F. Font, B. Protas, G. Richardson, J.M. Foster, Binder migration during drying of lithium-ion battery electrodes: Modelling and comparison to experiment, J. Power Sources. 393 (2018) 177–185. https://doi.org/10.1016/j.jpowsour.2018.04.097. [4] M. Müller, L. Pfaffmann, S. Jaiser, M. Baunach, V. Trouillet, F. Scheiba, P. Scharfer, W. Schabel, W. Bauer, Investigation of binder distribution in graphite anodes for lithium-ion batteries, J. Power Sources. 340 (2017) 1–5. https://doi.org/10.1016/j.jpowsour.2016.11.051. [5] L. Neidhart, K. Fröhlich, N. Eshraghi, D. Cupid, F. Winter, M. Jahn, Aqueous Manufacturing of Defect-Free Thick Multi-Layer NMC811 Electrodes, Nanomaterials. 12 (2022) 1–15. https://doi.org/10.3390/nano12030317. Figure 1

Magnesium-ion batteries (MIBs) represent a promising chemistry to potentially substitute lithium-ion technologies in the e-mobility and stationary energy storage applications. This is due to the favourable properties of metallic Mg, such as: abundancy, non-toxic nature, high recycling rate1, low redox potential (-2.37 vs SHE), safety (smooth Mg2+ electrodeposition), as well as divalent character of Mg2+ cations which leads to higher theoretical volumetric capacity (3833 mAh/cm3) than Li (2046 mAh/cm3) and commercial graphite (760 mAh/cm3).2 However, the major obstacle in the further development of MIBs is the incompatibility of Mg metal anode with conventional electrolyte solutions, which are formed by mixing simple Mg-based salts (e.g Mg(TFSI)2, Mg(ClO4)2, etc.) and polar aprotic solvents (e.g. acetonitrile, carbonates, etc.). These solutions decompose at the surface of metallic Mg forming an electronic and ionic insulating layer, leading to the passivation of the Mg anode and poor performance of the overall cell. Conversely, organoborate (Mg-tetrakis(hexafluorosisopropyloxy)borate in monoglyme, MgBOR)3 or organoaluminate (1:2 AlCl3:PhMgCl in THF, APC)4 ethereal solutions are known to prevent the passivation of the Mg metal anode, allowing the reversible electrochemical Mg2+ electrodeposition onto its surface. Despite a great effort has been done in the development of MIB,5 very little is known about the formation, evolution and degradation of the solid electrolyte interphase (SEI) formed at the interface between metallic Mg and electrolyte. This work, therefore, aims to investigate the interactions between Mg metal and passivating (Mg(TFSI)2 in monoglyme:diglyme) and non-passivating (MgBOR and APC) electrolytes combining ex-situ and in-situ spectroscopic and microscopic techniques with electrochemical testing. The properties of the SEIs will be evaluated at different states of charge (ex-situ) and during cell cycling (in-situ). Raman, Fourier transformed infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies are used to identify the composition of the electrolyte interphases, as well as monitor their changes upon cell discharge-charge cycles. Scanning electron microscopy (SEM) is performed to analyse the interphase morphologies, whereas scanning microwave microscopy (SMM)6,7 locally probes the impedance of the SEI layer. Atomic force microscopy (AFM) is also employed to evaluate the roughness of the Mg metal electrodes. Cyclic voltammetry (CV) and galvanostatic cycling with potential limitation (GCPL) are carried out in order to determine the electrochemical performance of bare Mg metal or covered with SEI layers. Furthermore, electrochemical impedance spectroscopy (EIS) is employed to probe the Mg2+ diffusion coefficients through the SEI layers at different state of charge (e.g. open circuit voltage, etc.) and determine charge transfer evolution with cycling time of the Mg metal anode. As the first step, a successful polishing method was developed to remove the native oxide layer form the surface of Mg discs allowing to expose a bare Mg metal to the electrolyte solutions and to evaluate their interactions. The polishing method also enabled to perform SMM imaging of the Mg metal since a roughness between 1-2.5 µm was achieved. The Mg discs were then immersed in the electrolyte solutions and an initial deposition of interfacial species (few nanometre thickness) was observed by SEM when Mg(TFSI)2 in monoglyme:diglyme was used, whereas a smooth surface was detected with MgBOR and APC electrolytes. This resulted in different electrochemical behaviours. In fact, symmetric cells (Mg||Mg) with MgBOR electrolyte showed a significantly higher cycling stability (> 250 h) than those with Mg(TFSI)2 in monoglyme:diglyme solution. In addition, when the latter electrolyte was used, fluorinated by-products were identified by XPS. In order to study the SEI formation and growth further, in-situ spectroscopic techniques (e.g. Raman and SMM) were employed to establish a correlation between the chemical composition of the electrolyte, the voltage range of the electrochemical tests and cycling time. In particular, the SMM method was applied to MIB technologies for the first time in this work. References I. R. P. United Nations Environment Programme, (available at https://wedocs.unep.org/20.500.11822/8702); J. Niu, Z. Zhang, D. Aurbach, Adv. Energy Mater., 2020, 10, 2000697; Z. Zhao-Karger, M. E. Gil Bardaji, O. Fuhr, M. Fichtner, J. Mater. Chem. A, 2017, 5, 10815–10820; D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich, E. Levi, Nature, 2000, 407, 724; R. Dominko, J. Bitenc, R. Berthelot, M. Gauthier, G. Pagot, V. Di Noto, J. Power Sources, 2020, 478, 229027; A. Buchter, J. Hoffmann, A. Delvallée, E. Brinciotti, D. Hapiuk, C. Licitra, K. Louarn, A. Arnoult, G. Almuneau, F. Piquemal, M. Zeier, F. Kienberger, Rev. Sci. Instrum., 2018, 89, 23704; J. Hoffmann, M. Wollensack, M. Zeier, J. Niegemann, H. Huber, F. Kienberger, in 2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO), pp. 1–4.

In the field of secondary batteries, the lithium-ion battery (LIB) is one of the most promising candidates for future electric vehicle (EV) applications, due to its high specific energy and good specific power [1]. However, to meet the demands in large scale storage, not only higher energy and power density and lower cost are needed, but environmentally friendly processing of LiBs is getting more in the focus of research. To meet the desirable requirements, one necessity is to tune the cathode in terms of sustainable production and materials. Ni-rich layered LiNi x Mn y Co1−x−y O2 (NMCs, x ≥ 0.8) cathodes are poised to be the dominating cathode materials for lithium-ion batteries for the foreseeable future due to its high energy density [2]. The state-of-the-art NMC based cathode production in both industrial and lab scale is with polyvinylidene fluoride (PVDF) in organic solvent N-methyl-2-pyrrolidone (NMP). NMP is a toxic, expensive, and highly flammable organic solvent [3]. Replacing NMP with water in the production of LiB cathodes is critical in terms of process cost savings and environmental concerns. Aqueous processes of NMC based cathodes are highly favorable as well as challenging due to the high reactivity of NMC particles. During the slurry preparation, the surface of Ni-rich particles undergoes Li leaching via exchange between H+ from water and Li+ from the active material which leads to increase in pH of the slurry. Li leaching leads to capacity fade and reduces the specific capacity of the cathode. The high pH of the slurry causes corrosion on the aluminum current collector. Surface treatment of NMC particles is a way to prevent Li leaching as well as using carbon coated Al current collector to resist the corrosion, but in terms of production cost these methods are not favorable. In this study, our fundamental aim is to demonstrate the manufacturing of large scale water based NMC811-PAA cathodes. We therefore employ a well-known binder material, polyacrylic acid (PAA), which is water soluble and an anionic polymer. During the mixing processes of the slurry, H+ from PAA can drive Li+ leaching, the leached Li+ reacts with carboxylic groups of PAA and binds to the PAA backbone. Thus, the reaction of H+ and OH- neutralizes the pH of the slurry. Bonded Li+ on PAA creates an active binder in which these Li ions can participate as an additional Li source to aid to prevent capacity decrease. This method has been successfully implemented in the lab scale [4]. Our main target is to investigate the slurry behavior and scalability of the cathodes with a PAA binder. Herein, we report high solid content of slurry behavior of NMC with PAA and the comparison to the conventionally produced slurry for the first time. Also, we show how the slurry behavior is affected by the mixing techniques from lab scale to pilot scale, which is an important parameter for later transfer to industrial production. The electrodes and successfully formed LiPAA as binder are shown by Fourier transformed infrared (FTIR) and Raman spectroscopy. The electrochemical tests are performed both in half and full cell configuration for NMC811-PAA and NMC811-PVDF cathodes. References [1] Tao, Yanqiu, et al. "Second life and recycling: Energy and environmental sustainability perspectives for high-performance lithium-ion batteries." Science advances 7.45 (2021): eabi7633. https://doi.org10.1126/sciadv.abi7633. [2] Lu, Yanying, et al. "Single‐crystal LiNixMnyCo1− x− yO2 cathodes for extreme fast charging." Small 18.12 (2022): 2105833. https://doi.org/10.1002/smll.202105833. [3] Zou, Feng, and Arumugam Manthiram. "A review of the design of advanced binders for high‐performance batteries." Advanced Energy Materials 10.45 (2020): 2002508. https://doi.org/10.1002/aenm.202002508. [4] Hawley, W. Blake, Harry M. Meyer III, and Jianlin Li. "Enabling aqueous processing for LiNi0. 80Co0. 15Al0. 05O2 (NCA)-based lithium-ion battery cathodes using polyacrylic acid." Electrochimica Acta 380 (2021): 138203. https://doi.org/10.1016/j.electacta.2021.138203.