A Li K-edge XANES study of salts and minerals

Lithium-Induced Reorientation of Few-Layer MoS2 Films

Molybdenum disulfide (MoS2) few-layer films have gained considerable attention for their possible applications in electronics and optics and also as a promising material for energy conversion and storage. Intercalating alkali metals, such as lithium, offers the opportunity to engineer the electronic properties of MoS2. However, the influence of lithium on the growth of MoS2 layers has not been fully explored. Here, we have studied how lithium affects the structural and optical properties of the MoS2 few-layer films prepared using a new method based on one-zone sulfurization with Li2S as a source of lithium. This method enables incorporation of Li into octahedral and tetrahedral sites of the already prepared MoS2 films or during MoS2 formation. Our results discover an important effect of lithium promoting the epitaxial growth and horizontal alignment of the films. Moreover, we have observed a vertical-to-horizontal reorientation in vertically aligned MoS2 films upon lithiation. The measurements show long-term stability and preserved chemical composition of the horizontally aligned Li-doped MoS2.

Controlled Experiments and Optimized Theory of Absorption Spectra of Li Metal and Salts

Investigation of Li metal and ionic compounds through experimental and theoretical spectroscopy has been of tremendous interest due to their prospective applications in Li-metal and Li-ion batteries. Li K-edge soft X-ray absorption spectroscopy (sXAS) provides the most direct spectroscopic characterization; unfortunately, due to the low core-level energy and the highly reactive surface, Li-K sXAS of Li metal has been extremely challenging, as evidenced by many controversial reports. Here, through controlled and ultra-high energy resolution experiments of two kinds of in situ prepared samples, we report the intrinsic Li-K sXAS of Li-metal that displays a prominent leading peak that has not been revealed before. Furthermore, theoretical simulations show that, due to the low number of valence electrons in Li, the Li-K sXAS is strongly affected by the response of the valence electrons to the core hole. We successfully reproduce the Li-K sXAS by state-of-the-art calculations with considerations of a number of relevant parameters such as temperature, energy resolution, and, especially, contributions from transitions which are forbidden in the single-particle treatment. Such a comparative experimental and theoretical investigation is further extended to a series of Li ionic compounds, which highlight the importance of considering the total and single-particle energies for obtaining an accurate alignment of the spectra. Our work provides the first reliable Li-K sXAS of the Li metal surface with advanced theoretical calculations. The experimental and theoretical results provide a critical benchmark for studying Li chemistry in both metallic and ionic states.

Superionic Conductors via Bulk Interfacial Conduction

Superionic conductors with ionic conductivity on the order of mS cm^-1 are expected to revolutionize the development of solid-state batteries (SSBs). However, currently available superionic conductors are limited to only a few structural families such as garnet oxides and sulfide-based glass/ceramic. Interfaces in composite systems such as alumina in lithium iodide have long been identified as a viable ionic conduction channel, but practical superionic conductors employing the interfacial conduction mechanism are yet to be realized. Here we report a novel method that creates continuous interfaces in the bulk of composite thin films. Ions can conduct through the interface, and consequently, the inorganic phase can be ionically insulating in this type of bulk interface superionic conductors (BISCs). Ionic conductivities of lithium, sodium, and magnesium ion BISCs have reached 1.16 mS cm^-1, 0.40 mS cm^-1, and 0.23 mS cm^-1 at 25 °C in 25 μm thick films, corresponding to areal conductance as high as 464 mS cm^-2, 160 mS cm^-2, and 92 mS cm^-2, respectively. Ultralow overpotential and stable long-term cycling for up to 5000 h were obtained for solid-state Li metal symmetric batteries employing Li ion BISCs. This work opens new structural space for superionic conductors and urges for future investigations on detailed conduction mechanisms and material design principles.

Soft X-ray absorption spectroscopy in the low-energy region explored using an argon gas window

The soft X-ray region below 200 eV is important for investigating chemical and biological phenomena since it covers K-edges of Li and B and L-edges of Si, P, S and Cl. Helium gas is generally used as the soft X-ray transmission window for soft X-ray absorption spectroscopy (XAS) under atmospheric conditions. However, the helium gas window cannot be applied to XAS in the low-energy region since transmitted soft X-rays mostly consist of high-order X-rays due to the low transmission of first-order X-rays. In this study, the argon gas window is proposed as a new soft X-ray transmission window in the low-energy region. High-order X-rays are removed by the absorption of the Ar L-edge (240 eV), and first-order X-rays become the major contribution of transmitted soft X-rays in the low-energy region. Under atmospheric argon conditions, the double-excitation Rydberg series of helium gas (60 eV), Si L-edge XAS of an Si3N4 membrane (100 eV) and S L-edge XAS of dimethyl sulfoxide gas (170 eV) are successfully measured, indicating that the argon gas window is effective for soft X-ray transmission in the low-energy region from 60 eV to 240 eV.