High-Field One-Dimensional and Two-Dimensional 27Al Magic-Angle Spinning Nuclear Magnetic Resonance Study of θ-, δ-, and γ-Al2O3 Dominated Aluminum Oxides: Toward Understanding the Al Sites in γ-Al2O3.

Herein, a detailed analysis was carried out using high-field (19.9 T) 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) on three specially prepared aluminum oxide samples where the γ-, δ-, and θ-Al2O3 phases are dominantly expressed through careful control of the synthesis conditions. Specifically, two-dimensional (2D) multiquantum (MQ) MAS 27Al was used to obtain high spectral resolution, which provided a guide for analyzing quantitative 1D 27Al NMR spectra. Six aluminum sites were resolved in the 2D MQ MAS NMR spectra, and seven aluminum sites were required to fit the 1D spectra. A set of octahedral and tetrahedral peaks with well-defined quadrupolar line shapes was observed in the θ-phase dominant sample and was unambiguously assigned to the θ-Al2O3 phase. The distinct line shapes related to the θ-Al2O3 phase provided an opportunity for effectively deconvoluting the more complex spectrum obtained from the δ-Al2O3 dominant sample, allowing the peaks/quadrupolar parameters related to the δ-Al2O3 phase to be extracted. The results show that the δ-Al2O3 phase contains three distinct AlO sites and three distinct AlT sites. This detailed Al site structural information offers a powerful way of analyzing the most complex γ-Al2O3 spectrum. It is found that the γ-Al2O3 phase consists of Al sites with local structures similar to those found in the δ-Al2O3 and θ-Al2O3 phases albeit with less ordering. Spin-lattice relaxation time measurement further confirms the disordering of the lattice. Collectively, this study uniquely assigns 27Al features in transition aluminas, offering a simplified method to quantify complex mixtures of aluminum sites in transition alumina samples.

Multinuclear NMR Study of the Solid Electrolyte Interface Formed in Lithium Metal Batteries.

The composition of the solid electrolyte interphase (SEI) layers formed in Cu|Li cells using lithium bis(fluorosulfonyi)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2-dimethoxyethane (DME) electrolytes is determined by a multinuclear solid-state MAS NMR study at high magnetic field. It is found that the "dead" metallic Li is largely reduced in the SEI layers formed in a 4 M LiFSI-DME electrolyte system compared with those formed in a 1 M LiFSI-DME electrolyte system. This finding relates directly to the safety of Li metal batteries, as one of the main safety concerns for these batteries is associated with the "dead" metallic Li formed after long-term cycling. It is also found that a large amount of LiF, which exhibits superior mechanical strength and good Li+ ionic conductivity, is observed in the SEI layer formed in the concentrated 4 M LiFSI-DME and 3 M LiTFSI-DME systems but not in the diluted 1 M LiFSI-DME system. Quantitative 6Li MAS NMR results indicate that the SEI associated with the 4 M LiFSI-DME electrolyte is denser than those formed in the 1 M LiFSI-DME and 3 M LiTFSI-DME systems. These studies reveal the fundamental mechanisms behind the excellent electrochemical performance associated with higher concentration LiFSI-DME electrolyte systems.

Sealed rotors for in situ high temperature high pressure MAS NMR.

Here we present the design of reusable and perfectly sealed all-zirconia MAS rotors. The rotors are used to study AlPO4-5 molecular sieve crystallization under hydrothermal conditions, high temperature high pressure cyclohexanol dehydration reaction, and low temperature metabolomics of intact biological tissue.

Following the transient reactions in lithium-sulfur batteries using an in situ nuclear magnetic resonance technique.

A fundamental understanding of electrochemical reaction pathways is critical to improving the performance of Li-S batteries, but few techniques can be used to directly identify and quantify the reaction species during disharge/charge cycling processes in real time. Here, an in situ (7)Li NMR technique employing a specially designed cylindrical microbattery was used to probe the transient electrochemical and chemical reactions occurring during the cycling of a Li-S system. In situ NMR provides real time, semiquantitative information related to the temporal evolution of lithium polysulfide allotropes during both discharge/charge processes. This technique uniquely reveals that the polysulfide redox reactions involve charged free radicals as intermediate species that are difficult to detect in ex situ NMR studies. Additionally, it also uncovers vital information about the (7)Li chemical environments during the electrochemical and parasitic reactions on the Li metal anode. These new molecular-level insights about transient species and the associated anode failure mechanism are crucial to delineating effective strategies to accelerate the development of Li-S battery technologies.