Solid State NMR: A Powerful Tool for the Characterization of Borophosphate Glasses

Ab initio study of stability and quadrupole coupling constants in borophosphates

The DFT method was used to predict the formation energies and quadrupole coupling constants CQ in a series of borophosphates: Li3BP2O8, Li2NaBP2O8, Na3BP2O8, Li2B3PO8, Na5B2P3O13, LiNa2B5P2O14 and Na3B6PO13 composed of different networks and different amounts of borate and phosphate units. The change in formation energies with increasing number of B atoms in this series is attributed to the multiplicity of boron sites and is explained by density of states calculations. The calculated CQ values of 7Li, 23Na and 11B are correlated with the coordination and distortion of polyhedra to elucidate the influence of local and more distant environments. As for the CQ of 11B, it should be in the ranges of 0.26-0.36, 0.48-0.84 and ∼1 MHz for boron tetrahedral distortion indices of 0.004-0.013, 0.015-0.019 and 0.033, respectively, whereas CQ ∼3.0 MHz corresponds to boron in a triangular site. The obtained numerical relationships make it possible to predict the quadrupole frequencies for these nuclei based only on their local environment, and vice versa, to propose structural models from NMR data. These results provide guidance for studying similar characteristics of other borophosphates, the structure of which varies depending on the initial reaction, composition and temperature.

Lewis Acid Probe for Basicity of Sulfide Electrolytes Investigated by 11B Solid-State NMR

Sulfide-based solid-state lithium-ion batteries (SSLIB) have attracted a lot of interest globally in the past few years for their high safety and high energy density over the traditional lithium-ion batteries. However, sulfide electrolytes (SEs) are moisture-sensitive which pose significant challenges in the material preparation and cell manufacturing. To the best of our knowledge, there is no tool available to probe the types and the strength of the basic sites in sulfide electrolytes, which is crucial for understanding the moisture stability of sulfide electrolytes. Herein, we propose a new spectral probe with the Lewis base indicator BBr3 to probe the strength of Lewis basic sites on various sulfide electrolytes by 11B solid-state NMR spectroscopy (11B-NMR). The active sulfur sites and the corresponding strength of the sulfide electrolytes are successfully evaluated by the proposed Lewis base probe. The probed strength of the active sulfur sites of a sulfide electrolyte is consistent with the results of DFT (density functional theory) calculation and correlated with the H2S generation rate when the electrolyte was exposed in moisture atmosphere. This work paves a new way to investigate the basicity and moisture stability of the sulfide electrolytes.

The Structure Evolution of Hydroxyl-Reacted LiBC to Improve the Capacity for Li-Ion Batteries

As a graphite-like material, the LiBC can deliver a high capacity up to 500 mA h g^-1 in Li-ion batteries, which is dependent on the carbon precursor, the high-temperature treatment, and the lithium insufficiency. However, the underlying mechanism is still not clear for the electrochemical reactions of LiBC. In this work, the pristine LiBC was reacted with aqueous solutions of different alkalinity, which was delithiated chemically and retained the layered structure. According to the XPS and NMR results, the B-B bond might be produced through the aqueous reaction or the initial charge process, which can be oxidized (charged) and reduced (discharged) in the electrochemical measurements. In the Li-ion battery, the reversible capacity of LiBC increases evidently with the alkalinity of aqueous solution and significantly rises to a similar value of ca. 285 mA h g^-1 under 200 cycles. Therefore, the specific capacity of LiBC should be contributed by the active sites of B-B bonds, which can be significantly increased through the reaction with the hydroxyl ions, and this strategy might be adopted to activate more graphite-like materials.

NMR spectroscopy probes microstructure, dynamics and doping of metal halide perovskites

Solid-state magic-angle spinning NMR spectroscopy is a powerful technique to probe atomic-level microstructure and structural dynamics in metal halide perovskites. It can be used to measure dopant incorporation, phase segregation, halide mixing, decomposition pathways, passivation mechanisms, short-range and long-range dynamics, and other local properties. This Review describes practical aspects of recording solid-state NMR data on halide perovskites and how these afford unique insights into new compositions, dopants and passivation agents. We discuss the applicability, feasibility and limitations of ¹H, ¹³C, ¹⁵N, ¹⁴N, ¹³³Cs, ⁸⁷Rb, ³⁹K, ²⁰⁷Pb, ¹¹⁹Sn, ¹¹³Cd, ²⁰⁹Bi, ¹¹⁵In, ¹⁹F and ²H NMR in typical experimental scenarios. We highlight the pivotal complementary role of solid-state mechanosynthesis, which enables highly sensitive NMR studies by providing large quantities of high-purity materials of arbitrary complexity and of chemical shifts calculated using density functional theory. We examine the broader impact of solid-state NMR on materials research and how its evolution over seven decades has benefitted structural studies of contemporary materials such as halide perovskites. Finally, we summarize some of the open questions in perovskite optoelectronics that could be addressed using solid-state NMR. We, thereby, hope to stimulate wider use of this technique in materials and optoelectronics research.

A significant enhancement of sodium ion conductivity in phosphate glasses by addition of WO3 and MoO3: the effect of mixed conventional-conditional glass-forming oxides

Ion conducting oxide glasses are attractive materials for application in various electrochemical devices and an understanding of the structure-transport properties relationship is crucial for their development. An interesting effect of glass structure on the dynamics of mobile ions is the mixed glass-former effect which causes a non-linear change of ionic conductivity when glass-forming oxides get gradually substituted. Here, we report a strong, positive effect of structural changes on the conductivity of sodium ions in two glass systems 40Na2O-xMoO3-(60-x)P2O5 and 40Na2O-xWO3-(60-x)P2O5; x = 0-50 mol% where a conventional glass-forming oxide (P2O5) is gradually replaced by WO3/MoO3 which are conditional ones. In both glass systems, the compositional change in DC conductivity is non-linear, with the maximal increase of four orders of magnitude in the case of WO3 and three orders of magnitude in the case of MoO3. This significant enhancement of ionic conductivity is related to the formation of mixed phosphate-tungstate and phosphate-molybdate units in the glass network. The facilitating effect of these structural units on sodium ion dynamics is also observed in the changes of the shape of frequency-dependent conductivity and in the values of typical spatial extent of diffusion of sodium ions known as the Sidebottom length.