Chirped excitation using frequency-swept wideline uniform rate smooth truncation (WURST) pulses in combination with Carr-Purcell-Meiboom-Gill acquisition (WCPMG) is currently the state-of-the-art method for the direct observation of the central transition (CT) in static ultra-wideline nuclear magnetic resonance (NMR) of half-integer spin quadrupolar nuclei. However, CT lineshape distortions and an inefficient, large number of transmitter steps in frequency-stepped acquisition are two major drawbacks. Here, we identify three main sources for lineshape distortions occurring in WCPMG NMR spectra of the CT: (I) distortions due to inaccurate setting of the radio frequency field strength, (II) chirped-excitation artifacts, and (III) distortions due to non-selective irradiation. A new and efficient approach for the acquisition minimizing these distortions is presented using low sweep rates (R ≤ 5 kHz/μs) and sweep widths (Δ ≤ 600 kHz). We further demonstrate that such an acquisition strategy also minimizes the number of transmitter steps in ultra-wideline NMR. This is achieved from numerical simulations and theoretical analysis of the orientational dependence for the quadrupolar-perturbed Zeeman states and their transition frequencies. The theoretically derived strategies are validated experimentally, allowing us to set up guidelines for the optimum recording of wideline and ultra-wideline WCPMG NMR spectra.
We report the 119 Sn and 7 Li solid-state nuclear magnetic resonance (NMR) spectroscopic characterization of all thermodynamically stable intermetallic phases of the binary Li−Sn system. The isotropic 119 Sn shifts (sum of the isotropic chemical and hyperfine shifts) of the Li−Sn intermetallics are found to be spread over a broad spectral range from 7300 to −500 ppm, allowing a clear Li−Sn phase identification. DFT calculations showed that the hyperfine interaction (Fermi-contact and spin-dipole contributions) constitutes the major 119 Sn magnetic shielding contribution for the Sn-rich Li−Sn intermetallics, which is significantly reduced for Li−Sn intermetallic phases with low and intermediate Sn-content. A full characterization of the effective 119 Sn magnetic shielding anisotropies for all Li−Sn intermetallic phases was achieved using the static broad-band 119 Sn Wideline Uniform Rate Smooth Truncation (WURST) Carr−Purcell−Meiboom−Gill (WCPMG) NMR experiment. These experiments further highlight the potential of the WCPMG NMR technique as it enables the acquisition of the full spectral range observed for the Li−Sn intermetallic phases in a single, static NMR experiment (B 0 up to 7 T), where information about crystallinity and local ordering is directly available from the 119 Sn NMR lineshapes. Such structural fingerprinting possibilities are clear advantages when compared to 7 Li NMR that will be of interest for studies of Sn-containing active materials in lithium-ion-based batteries, allowing a clear distinction between amorphous and crystalline (de)lithiation products in addition to the possibility to probe for amorphization during (dis)charge processes.
The exploration of promising renewable energy sources for the future is likely the most significant challenge for humanity. Hydrogen is considered to play a major role in the urgently required reorganization of our current energy sector. Water can be split into hydrogen and oxygen and therefore presents an in principle inexhaustible and environmentally friendly hydrogen source. However, electrochemical approaches for the cleavage of H 2 O remain challenging, especially considering that the experimentally required potential at which oxygen evolves is substantially higher than the theoretically required potential. This results in significant overpotentials (η) on the anode side, which limits the widespread applicability of this technique. Here, we have applied a two-step activation procedure of a Co-containing steel, which led to a significant reduction of η for the oxygen evolution reaction (OER) down to almost zero. The enhanced electrochemical behavior comes as a result of Li-ion doping, which leads to Li intercalation into the Co 3 O 4 containing surface layer of the steel-ceramic composite material. Thus, our results indicate that additional metal doping and resulting surface modification is a promising strategy for achieving substantial OER at pH-neutral conditions close to the thermodynamic limit.
Lithium alloying materials are promising candidates to replace the current intercalation-type graphite negative electrode materials in lithium-ion batteries (LIBs) due to their high specific capacity and relatively low cost. Here, we investigate the electrochemical performance of TiSnSb regarding its charge/discharge cycling stability as a negative electrode material in LIB cells. To assess a more practical performance with respect to a limited active lithium content in LIB full-cells, we evaluate the impact of pre-lithiation for TiSnSb with respect to the cycling stability in a NCM111||TiSnSb cell setup. The observation of the individual electrode potentials reveals comprehensive insights into the ongoing cell chemistry, showing that clear performance improvements can be achieved via pre-lithiation. Furthermore, the lithiation mechanism of TiSnSb is systematically studied via ex situ 7Li magic-angle spinning (MAS) nuclear magnetic resonance (NMR), ex situ X-ray diffraction, and static ex situ 119Sn wideband uniform rate smooth truncation Carr–Purcell Meiboom–Gill (CPMG) WCPMG NMR experiments. For comprehensive references regarding the isotropic 7Li shift of the Li–Sb intermetallic phases, all thermodynamically stable Li–Sb intermetallics of the binary Li–Sb systems have been synthesized and subsequently characterized by 7Li MAS NMR. Combined, our measurements for lithiated TiSnSb do not give any evidence for the formation of Li–Sn and Li–Sb intermetallics related to crystalline bulk phases (Li7Sn3, Li7Sn2, Li3Sb, and Li2Sb) as has been previously reported. In contrast, unique insights obtained from static ex situ 119Sn WCPMG NMR and ex situ XRD measurements reveal the formation of ternary Li–Sb–Sn species during lithiation, which can be assigned to the intermetallic phase Li2.8SbSn0.2. Additionally, the 7Li MAS NMR measurements combined with the observed discharge capacity reveal a second Li species, which we assign to an amorphous Li–Sn phase.
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