Artistic impression of lithium ions whizzing around at an solid-state electrolyte surface being probed by extreme ultraviolet second harmonic generation spectroscopy where an incoming femtosecond XUV pulse (red) gets frequency doubled (blue) at the interface. Illustration: Ella Maru Studio.
Shedding light on the surface of a battery material
The current generation of batteries used in electric vehicles, laptops, and other electronic devices rely on liquid electrolytes to transport ions between the electrodes. Batteries using liquid electrolytes are volatile in-part due to a process called dendrite formation that can short the battery leading to catastrophic accidents. Solid-state electrolytes, on the other hand, offer prospects for improved performance at increased safety and lifetime.
In a new study recently published in Nature Materials, an international research team, led by Professor of Chemistry Michael Zuerch, focuses on studying Lithium Lanthanum Titanium Oxide (LLTO), which is a promising candidate material for solid-state electrolytes. LLTO is classified as an ABO3 perovskite material, consisting of an alternating arrangement of La-rich and La-poor layers, or equivalently, lithium vacancy-poor and rich layers. It is known to have one of the highest ionic conductivities for Li-ion containing oxides, comparable to liquid electrolytes, and can participate in fast Li+ ion transport. However, LLTO and other solid-state electrolyte materials face challenges in physical contact and interfacial impedances.
Measuring these interfacial properties and their impact on ion dynamics is critical to device design and future material discovery. Unfortunately, studying such buried interfaces in operando is extremely difficult with most of the published research conducting post-mortem analysis of the devices. A significant step towards in operando studies of solid-state electrolyte surfaces and interfaces with the capability to infer ionic transport properties is at the core of this new study.
Accessing element-specific properties at the surface with nonlinear XUV spectroscopy
For this study, the researchers employed extreme ultraviolet second harmonic generation (XUV-SHG) to retrieve spectral signatures that specifically contain the contribution of lithium ions at the surface of the material. Second harmonic generation occurs when two light waves of frequency ω, called the fundamental, mix to produce a wave at twice the frequency, 2ω.
This process requires breaking of the material’s inversion symmetry. This symmetry is always broken at material interfaces, hence, recording a spectrum of this second harmonic radiation can inform us about the material properties specifically at the interface layer. Combining this approach with extreme ultraviolet radiation further enables isolating the response to the interfacial properties of a single atomic element. That this approach is feasible has only recently been demonstrated by researchers of the College of Chemistry who have since become leading in this new field.       
One caveat for broad application of this novel method is the requirement for intense XUV pulses with femtosecond pulse durations which are currently only available at large scale facilities that grant only a few days access at a time. The present study on LLTO represents the first application of this novel spectroscopic method to study solid-state electrolyte materials. The measurements have been conducted at the free-electron laser SACLA in Japan. (http://xfel.riken.jp/eng/)
Crystal vibrations matter
In the experiments, Zuerch’s team tuned the free-electron laser such that it would take a combination of two photons to excite a lithium core-electron into unoccupied energy levels. When the electron then relaxes its energy back to its original state it emits a single photon of twice the energy giving rise to second harmonic radiation that is then carefully detected. By slightly varying the photon energy of the free-electron laser the researchers obtained a spectrum, that due to the symmetry-breaking requirement, could only stem from lithium ions at the surface.
Prof. Zuerch describes the difficulty of the experiment, “One can imagine that it takes a very high density of photons to create conditions such that two photons can simultaneously interact with the same electron which we call a nonlinear response. While such experiments are commonly done with visible light lasers, they are extremely challenging in other spectral regions such as the extreme ultraviolet requiring large scale facilities.”
To learn how the ions behave differently at the surface as compared to the crystal bulk, in a separate experiment the linear response of the material was measured at the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory. In this case, the radiation from the ALS was tuned such that it only took a single photon to excite the electron which is a widely applied method to obtain element-specific properties of a material. In this case, the experiment provided information about the lithium within the bulk crystal.
By comparing the obtained bulk and surface spectra, the researchers were able to determine differences in the bulk and surface characteristics of LLTO specific to the lithium ions. Spectral shifts at the Lithium K-edge for both techniques were observed and interpreted using first-principles electronic structure simulations. In combination with ab initio molecular dynamics simulations, they revealed that the lithium dynamics are significantly suppressed at the surface due to lack of low-frequency vibrational lattice modes, resulting in reduced lithium entropy and mobility leading to the observed surface spectra, and hence the large interfacial resistance in the material.
“We are excited about our findings, as they could pave the way for safer, more efficient batteries,” said Prof. Zuerch. “Our research provides a better understanding of the surface and interface characteristics of solid-state electrolyte materials and the molecular-level interactions at play at an interface that limit the ion mobility. Understanding such phenomena enables us to focus on designing better interfaces in the future and also provides impetus for guided design of future solid-state electrolytes.”
This work was funded in part by the California Interfacial Science Institute (CISI) which is a University of California Office of the President an initiative funded since 2021 and that was just renewed for another three years.
Professor Zuerch comments, “It is great to see that our new initiative is bearing fruit. When we founded CISI, the core idea was to bring together researchers working on experimental and numerical methods pertaining to interfaces. The present work on studying LLTO interfaces is a great example for this collaborative approach in which CISI-funded students at UC Berkeley and UC San Diego were involved in the experiments and numerical simulations respectively”,
Contact: Michael Zuerch firstname.lastname@example.org
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