Enrico Trevisanello

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.

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.

Polymers in the form of composite electrolytes, as binders and as protective coatings are used to modify interfaces and to facilitate the manufacture of lithium solid-state batteries with inorganic solid electrolytes.

The use of solid electrolytes in lithium batteries promises to increase their power and energy density, but several challenges still need to be overcome. One critical issue is capacity-fading, commonly ascribed to various degradation reactions in the composite cathode. Chemical, electrochemical as well as chemo-mechanical effects are discussed to be the cause, yet no clear understanding of the mechanism of capacity fading is established. In this work, a model is proposed to interpret the low-frequency impedance of the cathode in terms of lithium diffusion within an ensemble of LiNi_1−x−y Co _x Mn _y O₂ (NCM) cathode active material particles with different particle sizes. Additionally, an electrochemical technique is developed to determine the electrochemically active mass in the cathode, based on the estimation of the state-of-charge via open circuit potential-relaxation. Tracking the length of lithium diffusion pathways and active mass over 40 charge-discharge cycles demonstrates that the chemo-mechanical evolution in the composite cathode is the major cause for cell capacity fading. Finally, it is shown that single-crystalline NCM is far more robust against chemo-mechanical degradation compared to polycrystalline NCM and can maintain a high cycling stability.

Representatives of the Li_xNi_1−y−zCo_yMn_zO₂ (NCM) family of cathode active materials (CAMs) with high nickel content are becoming the CAM of choice for high performance lithium‐ion batteries. In addition to high specific capacities, these layered oxides offer high specific energy, power, and long cycle life. Recently, the development of single crystalline particles of NCM has enabled even longer lifetimes due to achieving higher Coulomb efficiencies. In this work, the performance of NCM materials with different particle size and morphology is explored in terms of key parameters such as the charge‐transfer resistance and the chemical diffusion coefficient of lithium. Cracking of secondary particles leads to liquid electrolyte infiltration in the CAM, lowering the charge‐transfer resistance and increasing the apparent diffusion coefficient by more than one order of magnitude. In contrast, these effects are not observed with single‐crystalline NCM, which is mostly free of cracks after cycling. Consequently, severe kinetic limitations are observed when cycling large “uncracked” secondary particles at low potential and capacity. These results demonstrate that cracking of polycrystalline particles of NCM is not solely detrimental but helps to achieve high reversible capacities and rate capability. Thus, optimization of CAMs size and morphology is decisive to achieve good rate capability with high‐nickel NCMs.