Deciphering the Origin of High Electrochemical Performance in a Novel Ti-Substituted P2/O3 Biphasic Cathode for Sodium-Ion Batteries.

The layered Mn-based oxides (NaxMnO2), which is one of the most promising cathode families for rechargeable sodium-ion batteries, have received considerable attention because of their tunable electrochemical performances and low costs. Herein, a novel P2/O3 intergrown Li-containing Na0.8Li0.27Mn0.68Ti0.05O2 cathode material prepared by Ti-substitution into Mn-site is reported. Benefiting from the synergistic effects of the biphasic composite structure and inactive d0 element substitution, this P2/O3 electrode exhibits high initial charge/discharge capacity and excellent cycling performance. The combination of different characterization techniques including solid-state NMR, electron paramagnetic resonance, X-ray adsorption spectroscopy, and high-resolution transmission electron microscopy gives insights into the local electronic environment, the redox chemistry, and also the microstructure rigidity of these cathode materials upon cycling. On the basis of comprehensive comparison with the Ti-free P2/O3-Na0.8Li0.27Mn0.73O2, the observed improvement on the electrochemical performance is primarily attributed to the mitigation of notorious Mn3+/Mn4+ redox and the enhanced stability of the oxygen charge compensation behavior. From the viewpoint of structure evolution, Ti-substitution restrains the Li+ loss and irreversible structural degradation during cycling. This study provides an in-depth understanding of the electronic and crystal structure evolutions after inactive d0 element substitution and may shed light on the rational design of high-performance P2/O3 biphasic Mn-based layered cathodes.

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

This review will show how solid state nuclear magnetic resonance (NMR) has contributed to a better understanding of the borophosphate glass structure. Over the last fifteen years, 1D and 2D magic angle spinning (MAS)-NMR has been used to produce key information about both local and medium range organization in this type of glass. After a brief presentation on borophosphate glasses, the paper will focus on the description of the local order of phosphate and borate species obtained by 1D 31P-and 11B-MAS-NMR experiments, with a special emphasis on the improvements obtained at high magnetic fields on the borate speciation description. The last part of this review will show how correlation NMR provided new insights into the intermediate length scale order. Special attention will be paid to the quantitative data retrieved from 11B/31P REDOR-based NMR sequences and to the qualitative connectivity schemes observed on the 2D 11B/31P maps edited with the heteronuclear multiple quantum coherence (HMQC) NMR techniques.

Quantitative determination of the phosphorus environment in lithium aluminosilicate glasses using solid-state NMR techniques.

We investigated using solid-state NMR spectroscopy the short-range structural features in lithium aluminosilicate glasses with the addition of P2O5 and considering various Al2O3/Li2O ratios. The phosphorus environment is determined quantitatively using 31P Magic Angle Spinning NMR constrained by results obtained from 31P-27Al Multiple-Quantum Coherence-based NMR techniques. Phosphorus is mainly located as orthophosphate and pyrophosphate species in glasses with a low amount of Al2O3. These depolymerized units disappear with increasing Al2O3 content and a strong affinity of PO4 tetrahedra for aluminum is revealed, which reduces phase separation. The local environments of framework (Si and Al) and charge-balancing (Li) cations are also studied through NMR experiments to assess the influence of P2O5 addition. The Si environment is mostly modified by the presence of P2O5 in glasses containing a low amount of Al2O3, with an increase of Q4Si species in relation to phase separation phenomena observed in these compositions. Conversely, P2O5 addition does not have a significant influence on the 27Al NMR response. 7Li NMR spectra reflect a change in the structural role of Li when P2O5 or Al2O3 is added. The observed structural changes can be rationalized to improve our knowledge of the structure-property relationships, focusing, in particular, on phase separation and nucleation/crystallization processes that are strongly affected by the presence of P and the evolution of its local environment with composition.

Structure and thermal relaxation of network units and crystallization of lithium silicate based glasses doped with oxides of Al and B.

The current study reports on the relaxation behaviour of lithium silicate based glasses as probed by NMR spectroscopy. A total of four glass compositions were studied with the parent composition being 28Li2O-72SiO2, and added dopants of Al and B. All the compositions showed significant differences in the NMR spectra of both annealed and non-annealed glasses demonstrating the structural relaxation behaviour. We extended our binary statistical mechanical model to these complex compositions to study the relaxation behaviour. By the combined use of the extended statistical mechanical model and broken ergodicity, we shed light on the mechanism of structural relaxation as understood by NMR spectroscopy. We studied the crystallization behaviour of the glasses and reported on the variations of the residual glass composition changes in the crystallization fraction.

Selective Secondary Face Modification of Cyclodextrins by Mechanosynthesis.

α-, ß-, and γ-cyclodextrins (CDs) were modified on their secondary face by mechanosynthesis at room temperature using a laboratory-scale ball-mill. Mono-2-tosylated α-, ß-, and γ-CDs were obtained in good yield from mixtures of native α-, ß-, and γ-CDs, respectively, N-tosylimidazole, and an inorganic base, with each of them being in the solid state. The yields appeared to be dependent upon the nature of the base and the reaction time. A kinetic monitoring by (1)H NMR spectroscopy demonstrated that the highest yields in mono-2-tosyl-CDs were measured using KOH as a base in very short reaction times (up to 65% in 80 s). Mono-(2,3-manno-epoxide) α-, ß-, and γ-CDs were subsequently synthesized by ball-milling a mixture of monotosylated α-, ß-, and γ-CDs, respectively, and KOH. The characterization of the modified CDs was carried out by X-ray diffraction, mass spectrometry, solid-state NMR, and diffuse reflectance UV-vis (DR UV-vis) spectroscopies. Clues to the supramolecular arrangement of the molecules in the solid state provide information on the reaction mechanism.