Raffael Ruess

Scanning electrochemical microscopy and a reaction–transport model reveal the effects of electrolyte additives, photoanode thickness, and light intensity on dye regeneration, and back electron transfer in dye-sensitized porous ZnO photoanodes.

Thiophosphate‐based solid‐state batteries (SSBs) with high‐nickel ternary cathode materials such as LiNi_0.83Co_0.11Mn_0.06O₂ (NCM) represent a promising next‐generation energy storage technology due to their expected high specific discharge capacity and improved safety. However, rapid capacity fading caused by contact loss through interphase and crack formation during cell cycling is a significant problem hindering stable SSB cycling and high‐energy‐density applications. In this work, a uniform coating of poly((4‐vinyl benzyl)trimethylammonium bis(trifluoromethanesulfonylimide)) (PVBTA‐TFSI) on NCM is obtained via a spray‐drying process. This exceptionally thin cationic polymer coating of only 2–4 nm thickness on NCM helps stabilize the interface between NCM and the Li₆PS₅Cl solid electrolyte (SE). Electrochemical tests confirm a significant improvement in long‐term cycling performance and active mass utilization compared to uncoated NCM. In addition, the polymer coating effectively suppresses the degradation of the NCM/SE interface, particularly the formation of oxygenated species, and reduces the extent of particle cracking. Overall, these results highlight a new approach to mitigate SSB degradation using a thin cationic polymer coating on NCM for SSBs.

Depositing ultrathin oxide coatings has been proven a successful approach to stabilize the surface of LiNi_xCo_yMn_zO₂ active cathode material in lithium‐ion batteries (LIB). The beneficial effect of Al₂O₃ coatings arises at least partly from spontaneous reactions between coating and liquid electrolyte. However, it remains unclear if comparable surface reactions occur for other oxide coatings. One difficulty is the characterization of reaction products at the cathode–electrolyte interface due to the multi‐phase properties of composite cathodes. Here, thin films are utilized as model systems to correlate surface reactions with the performance of Al₂O₃‐ and CeO₂‐coated nickel cobalt manganese oxides (NCM). Electrochemical characterization confirms that an Al₂O₃ coating improves long‐term cycling stability, while CeO₂‐coated thin films perform even worse than uncoated counterparts. The analysis of the surface reaction products using X‐ray photoelectron spectroscopy shows that both coatings are fluorinated upon contact with liquid electrolyte in agreement with thermodynamic considerations. The fluorinated Al₂O₃ coating is stable during cycling, resulting in the improved cell performance. In contrast, the fluorinated CeO₂ coating changes chemical composition, facilitating corrosion of the NCM surface. The results demonstrate the importance of a detailed analysis of surface reactions to evaluate the suitability of ultrathin oxide layers as protective coatings for LIBs.

To increase the specific capacity of LiNi_1-x-yCo_xMn_yO₂ (NCM) cathode active materials, academia and industry are aiming at a higher nickel content. The end member of this family, LiNiO₂ (LNO), offers the highest practical specific capacity, but suffers from morphological and structural instabilities during electrochemical cycling. Especially for the successful implementation in solid-state batteries, coarse-grain powders of single-crystalline LNO need to be synthesized to achieve high-performance cells. While NCM-type solid solutions with Co and Mn show sufficient structural stability during synthesis, LNO rapidly decomposes under the conventional conditions for the preparation of single crystals. We address this issue by presenting an alternative route towards single-crystalline LNO based on a flux-assisted synthesis. First, LNO is prepared from NiO and LiOH precursors, which yields agglomerated crystals of LNO with notable Li deficiency. Second, this raw LNO is transferred into a Li₂CO₃ melt, which induces crystal growth and particle separation, and further offers a high chemical potential of Li₂O for defect healing. With this method, well-separated and near-stoichiometric, large-grain LNO single crystals are obtained. When tested in solid-state battery cells, this material yields specific discharge capacities q _dis > 200 mAh g^-1 at room temperature and clearly outperforms the state-of-the-art polycrystalline LNO.

Festkörperbatterien (Solid‐State‐Batteries, SSBs) mit Poly(ethylenoxid) (PEO) und Hochvoltkathodenaktivmaterialien (CAMs) wie LiNi_1−‐x−yCo_xMn_yO₂ (NCM) zeigen beim Zyklisieren oftmals “Spannungsrauschen”, was mit einem Zellversagen verbunden ist. Darüber hinaus finden sich in der Literatur ganz unterschiedliche Aussagen zur Langzeit‐Zyklenperformance von PEO in Kontakt mit Hochvolt‐CAMs. In der vorliegenden Arbeit wird experimentell nachgewiesen, dass das Spannungsrauschen tatsächlich durch Lithiumdendriten, die den festen Polymerelektrolyten (SPE) penetrieren, verursacht wird. Eine Lösung für dieses Problem kann die Verwendung von PEO mit einem höheren Molekulargewicht sein, was zu einer verbesserten Zyklenstabilität führt. Zusätzlich kann mittels Röntgenphotoelektronenspektroskopie (XPS) gezeigt werden, dass es an der Kathodenseite zur Bildung von oxidativen Degradationsprodukten nach dem Zyklisieren mit NCM als CAM kommt, was mittels der üblichen Fourier‐Transformations‐Infrarot‐Spektroskopie (FTIR) aufgrund ihrer begrenzten Oberflächensensitivität nicht nachgewiesen werden kann. Damit tragen unsere Ergebnisse dazu bei, nicht nur die Stabilität von PEO‐basierten SSBs besser bewerten zu können, sondern zeigen auch, dass durch die Wahl eines höhermolekularem PEOs die Stabilität gegenüber Hochvolt‐CAM signifikant gesteigert werden kann.

Solid‐state batteries (SSBs) with high‐voltage cathode active materials (CAMs) such as LiNi_1−x−yCo_xMn_yO₂ (NCM) and poly(ethylene oxide) (PEO) suffer from “noisy voltage” related cell failure. Moreover, reports on their long‐term cycling performance with high‐voltage CAMs are not consistent. In this work, we verified that the penetration of lithium dendrites through the solid polymer electrolyte (SPE) indeed causes such “noisy voltage cell failure”. This problem can be overcome by a simple modification of the SPE using higher molecular weight PEO, resulting in an improved cycling stability compared to lower molecular weight PEO. Furthermore, X‐ray photoelectron spectroscopy analysis confirms the formation of oxidative degradation products after cycling with NCM, for what Fourier transform infrared spectroscopy is not suitable as an analytical technique due to its limited surface sensitivity. Overall, our results help to critically evaluate and improve the stability of PEO‐based SSBs.

ASSBs (all-solid-state batteries) are promoted as an energy dense and safe alternative to current Li-ion batteries (LIBs) and attract great interest from academia and industry. In contrast to LIBs, which employ a liquid organic electrolyte, they utilize a solid electrolyte. This substitution promises to eliminate the flammability of the battery and to simplify the cell design. While recent research efforts have concentrated on miniaturizing and eventually even removing the anode host material in batteries, the relative portion of the cathode needs to be maximized, as cathodes are the only component that can increase energy density by increasing its fraction. In a simplified view, the cathode kinetics are determined by the cathode microstructure, the volume fractions of the constituents and the properties of electrolyte and cathode active material (CAM). Liquid electrolytes can easily penetrate porous composite cathodes, but rigid SEs can not do the same, resulting in residual porosity in the cathode. This porosity can lower active interface area between CAM and SE, and increase tortuosity of ionic and electronic charge transport pathways. Sufficient ionic and electronic transport pathways in composite cathode structures are, however, essential because cathode active material particles that are either electronically or ionically isolated cannot contribute to the charging or discharging process. We analyse the requirements for SSB cathodes and determine charge transport bottlenecks by impedance spectroscopy of a reference system consisting of a thiophosphate based solid electrolyte and a nickel rich layered CAM. Different cathode microstructures are analysed and their charge transport properties are quantified as partial conductivities. From the obtained partial conductivities, we calculated tortuosity factors and correlated them to cell performance with complementary cycling data of all-solid-state batteries in order to determine charge transport bottlenecks. We find, that ionic charge transport and consequently cathode kinetics are highly dependent on the SE particle size distribution In addition, we analyse the requirements for CAMs for SSBs and develop design principles for different CAM types that aim to further increase cathode performance.

Solid‐state batteries (SSBs) currently attract great attention as a potentially safe electrochemical high‐energy storage concept. However, several issues still prevent SSBs from outperforming today's lithium‐ion batteries based on liquid electrolytes. One major challenge is related to the design of cathode active materials (CAMs) that are compatible with the superionic solid electrolytes (SEs) of interest. This perspective, gives a brief overview of the required properties and possible challenges for inorganic CAMs employed in SSBs, and describes state‐of‐the art solutions. In particular, the issue of tailoring CAMs is structured into challenges arising on the cathode‐, particle‐, and interface‐level, related to microstructural, (chemo‐)mechanical, and (electro‐)chemical interplay of CAMs with SEs, and finally guidelines for future CAM development for SSBs are proposed.

Single-crystalline Ni-rich LiNi_1-x-y Co _x Mn _y O₂ (SC-NCM) cathode active materials promise to increase the lifetime of high energy Li-ion batteries. SC-NCM consist of large primary particles that offer low surface area, limiting detrimental chemical reactions while exhibiting high morphological stability. A typical SC-NCM synthesis starts from the same Ni_1-x-y Co _x Mn _y (OH)₂ and LiOH∙H₂O precursors commonly used for conventional spherical poly-crystalline NCM (PC-NCM), but requires higher temperatures and additional post-processing. Consequently, the cost and environmental impact of the production of Ni-rich SC-NCM is higher compared to the production of PC-NCM. In this study, we demonstrate a synthesis of SC-NCM that does not require the same highly engineered precursors as used for PC-NCM. We propose a more energy-efficient and cost-effective route that involves simple blending of NiO, MnO, Co₃O₄ and Li₂CO₃ which yields single-crystalline LiNi_0.83Co_0.11Mn_0.06O₂ with 2–3 μm particle size and good structural quality. It is shown by in situ XRD during synthesis that—while the reaction suffers from slow kinetics—the elevated temperature and longer reaction time, which are in any case required for the crystal growth, are sufficient to also complete the reaction. Furthermore, it is shown that this material is structurally and electrochemically equivalent to the material commonly synthesized from hydroxide-based precursors.

With the ongoing transformation to e-mobility, lithium all-solid-state batteries are promising candidates for advanced mobile energy storage. Other than in conventional lithium ion cells, the rigid solid electrolyte entails its own morphology and does not wet residual voids in composite electrodes, which can limit the cell performance. We therefore take a closer look at the influence of microstructural characteristics on different scales in composite cathodes by means of electrochemical simulation using the finite element method. Cathode active material particle arrangements are constructed to validate the model against experimental data. We highlight the significance of the active material particle size distribution and state-of-charge dependent input parameters, such as the lithium diffusion coefficient in NCM811 and the exchange current density at the interface of NCM811 and Li ₆ PS ₅ Cl. We zoom in on that interface under the presence of void space that can result from manufacturing or arise from inter-particle contact loss upon volume changes. In a 1-particle-void model, the impact of the active surface area covered by voids is studied as well as the influence of the void distribution and the void size on the electrochemical performance. Beyond that, we simulate a tortuosity-optimized structured electrode and provide first guidelines for laser-patterned all-solid-state cathodes.