Influence of Sintering Temperature on the Structure of Layered Transition-Metal Oxide Cathodes and LLZO-Based Solid Electrolyte Composites for Solid-State Batteries Studied with Scanning Transmission Electron Microscopy
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Philipps-Universität Marburg
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To mitigate the consequences of the climate crisis, such as record temperatures and extreme weather events, the consumption of fossil fuels must decrease rapidly. Advances in battery technology are necessary to store electricity from renewable sources and to electrify transportation. As lithium-ion batteries are approaching their theoretical capacity limit, new cell concepts are being explored. A promising option is the use of solid electrolytes (SEs), which potentially offer advantages over liquid electrolytes such as higher energy density, safety, and lifespan.
However, several technical hurdles must be overcome for the widespread use of solid-state batteries (SSBs), such as the low ionic conductivity of SEs, the limited capacity of cathode active materials (CAMs), and material degradation. Interfacial issues are particularly critical, including voids between SE and CAM as well as unwanted side reactions during fabrication and operation. Solving these problems requires a deep understanding of processes on the micro- and nanoscale. (Scanning) transmission electron microscopy ((S)TEM), combining imaging, diffraction, and spectroscopy, offers excellent analytical capabilities. High-angle annular dark-field (HAADF)-STEM enables atomic-resolution imaging but is limited to small sample regions. Complementarily, scanning precession electron diffraction (SPED) provides structural information at the nanometer scale but relies on prior knowledge of crystallographic phases. Therefore, electron energy loss spectroscopy (EELS) is often required for verification, as it reveals chemical composition and bonding environment. The combination of these techniques allows for detailed analyses, such as distinguishing intact layered CAMs from degraded rock-salt structures.
These methods were used to investigate how thermal treatments affect the structure of SSB components. One study analyzed the effect of a two-step synthesis process on coarse-grained LiNiO2 (LNO) particles. After sintering at 800 °C, samples were annealed at either 600 °C (LNO-600) or 700 °C (LNO-700). The annealing smoothed particle surfaces, reduced reactive surface lithium, and increased lithium concentration within the crystal, which enhanced discharge capacity—more so for LNO-700 than for LNO-600. STEM, EELS, and SPED revealed a NiO-like rock-salt surface layer and low-angle misorientations within the particles, likely caused by lithiation or thermal stress during cooling. Despite these structural changes, no negative effects on cell performance were observed during the first cycle. However, further cycling experiments are necessary to determine whether these results hold over the battery's lifetime.
Another study examined LLZO-NCM composite cathodes. SPED identified a pronounced rock-salt phase in samples sintered at 600 °C, but not at 500 °C. Additionally, a nanometer-thin LaNiO3-like phase was detected at the LLZO/NCM interface. Numerous lattice defects in NCM near this interface were also observed. These findings underscore both the sensitivity of (S)TEM techniques and the importance of surface coatings to prevent adverse reactions between SE and CAM.
To analyze the LLZO-NCM interface, a new sample preparation method was developed. Since the particles were only loosely connected, conventional focused ion beam (FIB) techniques caused lamellae to fracture. This issue was resolved by introducing a protective frame structure, a method transferable to other sensitive materials.
To observe structural changes during heating, an in situ heating setup was developed that allows heating of samples in an oxygen atmosphere up to 1000 °C at 1 bar. A specialized preparation method enabled the placement of individual particles onto MEMS heating chips. Rapid switching between STEM imaging and 4D data acquisition allowed the observation of changes in LNO particles starting at 350 °C—such as the transformation of the LNO phase to NiO at the grain surfaces and the formation of a low amounts of lithium containing rock-salt phase in the interior. The transformation temperature was higher than previously reported for vacuum experiments but lower than detected using other less high-resolution techniques, highlighting the sensitivity of (S)TEM under realistic conditions.
In summary, the results show how thermal treatments influence the microstructure of cathodes and SE/CAM composites. High-resolution (S)TEM analyses enabled the detection of nanoscale phase transitions. These insights contribute to a better understanding of degradation mechanisms and support the development of mitigation strategies. Furthermore, new experimental setups and preparation techniques introduced in this work broaden the methodological toolkit for future research on solid-state batteries.
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Created: 2025Issued: 2025-08-06Updated: 2025-08-06
Faculty
Fachbereich Physik
Publisher
Philipps-Universität Marburg
Language
eng
Data types
DoctoralThesis
Keywords
Solid-state batteries
DFG-subjects
TEMin situLNOLLZOSTEMNCMSPEDFestkörperbatterien
DDC-Numbers
530
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Demuth, Thomas Herbert (0009-0002-9796-4959): Influence of Sintering Temperature on the Structure of Layered Transition-Metal Oxide Cathodes and LLZO-Based Solid Electrolyte Composites for Solid-State Batteries Studied with Scanning Transmission Electron Microscopy. : Philipps-Universität Marburg 2025-08-06. DOI: https://doi.org/10.17192/z2025.0487.