A recent article in Nature Communications explored how crystal orientation affects interface properties in composite cathodes for all-solid-state batteries (ASSBs). The researchers addressed a key issue: maintaining the solid electrode-solid electrolyte interface during cycling. They investigated the influence of crystal orientation on interfacial reactions and stability during the co-sintering process.
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Background
ASSBs are considered promising for electric vehicles due to their high energy density and enhanced safety features. The recent growth in ASSBs is largely due to the discovery of different halide, oxide, and sulfide-based solid electrolytes, which offer lithium ionic conductivities similar to those in commercial lithium-ion batteries.
Despite advances in solid electrolyte materials, the practical use of ASSBs has been challenging. Ensuring a reliable interface between the electrode and solid electrolyte is crucial. This interface must maintain physical integrity and electrochemical and chemical stability during fabrication and throughout dynamic electrochemical cycling.
About the Research
The researchers investigated the interfacial properties of composite cathodes in ASSBs, focusing on how the crystal orientations of the cathode and solid electrolyte influence these properties. They observed that conventional composite cathodes with random particle orientations create complexities due to varying (electro)chemical compatibilities.
To address this, an epitaxial model system with precisely controlled crystal orientations for both the cathode and solid electrolyte was used. Interfaces were studied in real-time during co-sintering using in situ electron microscopy.
The study utilized lithium nickel cobalt manganese oxide (Li(Ni1/3Co1/3Mn1/3)O2 or (NCM)) as the layered cathode material and lithium lanthanum titanate (Li3xLa(2/3)-x⎕(1/3)−2xTiO3 or (LLTO)) as the perovskite-type oxide solid electrolyte.
Two types of epitaxial heterostructures were fabricated: one with no visible ion diffusion channels (closed ion pathway) and one with ion diffusion channels (open ion pathway). The interfacial properties were examined using in situ heating transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS).
Research Findings
The study revealed that interfacial reactions are strongly influenced by crystal orientation and the presence of open ion channels. In NCM cathodes, interfaces with open ion channels were more susceptible to interdiffusion but were stabilized by an early passivation layer. In contrast, interfaces with closed ion pathways remained stable at moderate temperatures but deteriorated at higher temperatures due to oxygen release, which led to increased interfacial resistance.
In situ heating TEM revealed that NCM(003), which had closed ion pathways, preserved a distinct interface structure up to around 600 °C. Above this temperature, structural degradation became noticeable. In contrast, NCM104, with open ion pathways, exhibited interface changes starting at 400 °C, though these changes did not significantly worsen with further heating.
High-resolution ex-situ TEM analysis found that NCM003 formed interfacial domains about 10-20 nm thick, mostly amorphous, even when heated to 600 °C. In contrast, NCM(104) developed byproducts like lithium titanium oxide (LiTiO2) and lanthanum transition metal oxide (LaTMO3) at 380 °C.
This early degradation was consistent with the blurred interface observed in situ, underscoring the vulnerability of the open interface in NCM(104) compared to the more stable closed interface in NCM(003). Nano-beam diffraction (NBD) analysis further showed significant differences between the bulk and interface domains at elevated temperatures.
EIS measurements indicated that the interfacial resistance (Rint) of NCM003 began to rise at 600 °C and continued to increase at higher temperatures. For NCM104, Rint started to increase at 400 °C but did not undergo significantly with further heating.
Applications
This research has significant implications for designing and optimizing composite cathodes in solid-state batteries. Understanding how crystal orientation affects interfacial reactions can help researchers and scientists develop strategies for reducing interfacial degradation. Possible approaches include using surface coatings or optimizing sintering conditions to improve the stability and performance of solid-state batteries.
The findings highlight the need to understand and separate interfacial properties to design solid-state batteries more effectively. Knowing how different interfacial behaviors impact performance can lead to the creation of composite cathodes with stable interfaces for high-performance ASSBs. For example, applying a surface coating with materials like LiTiO2 could help prevent degradation caused by oxygen release at the interface.
Conclusion
The study provided valuable insights into the design of interfaces between the electrode and solid electrolyte in composite cathodes for ASSBs. By examining model systems with controlled crystal orientations, the authors highlighted how crystal orientation affects interfacial reactions and charge transfer resistance. These insights are crucial for redesigning composite cathodes to achieve high-performance ASSBs.
Additionally, the real-time analysis of well-defined interfaces provides new opportunities to explore complex interfacial properties. Future work should focus on further optimizing these properties and investigating new materials and fabrication techniques to improve the performance and durability of solid-state batteries.
Journal Reference
Lee, S., et al. (2024). Unveiling crystal orientation-dependent interface property in composite cathodes for solid-state batteries by in situ microscopic probe. Nat Commun. DOI: 10.1038/s41467-024-52226-4, https://www.nature.com/articles/s41467-024-52226-4
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