Francis Wang

Hydrolysis of octahydrotriborate in cold acidic methanol--water solutions. Preparation of B/sub 3/H/sub 7/OH/sub 2/ and B/sub 3/H/sub 7/OH/sup -/

In methanol--water solutions of HCl at -78/sup 0/, the octahydrotriborate ion reacts to form 1 mol of hydrogen and a solution of B/sub 3/H/sub 7/, which we formulate as B/sub 3/H/sub 7/OH/sub 2/. The rate of this reaction, for (H/sup +/) less than or equal to 1.25M, is given by the expression -d(B/sub 3/H/sub 8//sup -/)/dt = k/sub 1/(B/sub 3/H/sub 8//sup -/)(H/sup +/), where k/sub 1/ = 2.9 x 10/sup -3/M/sup -1/min/sup -1/. When a solution of B/sub 3/H/sub 7/OH/sub 2/ is warmed to -45/sup 0/C, complete hydrolysis to boric acid and hydrogen occurs. The rate of the latter reaction, for (H/sup +/) less than or equal to l.5M, is given by the expression -d(B/sub 3/H/sub 7/OH/sub 2/)/dt = k/sub 2/(B/sub 3/H/sub 7/OH/sub 2/), where k/sub 2/ = 3.0 x 10/sup -3/min/sup -1/. The hydroxyheptahydrotriborate ion, B/sub 3/H/sub 7/OH/sup -/, is formed by the addition of hydroxide to a B/sub 3/H/sub 7/OH/sub 2/ solution at -78/sup 0/C. The anion undergoes a quantitative base-catalyzed disproportionation to borate and borohydride at a temperature above -65/sup 0/C: B/sub 3/H/sub 7/OH/sup -/ + 2OH/sup -/ + H/sub 2/O ..-->.. 2BH/sub 4//sup -/ + B(OH)/sub 4//sup -/. 5 figures, 2 tables.

7 Li MAS NMR Studies of Lithiated Manganese Dioxide Tunnel Structures: Pyrolusite and Ramsdellite

The one-dimensional 1×1 and 1×2 tunnel structures of manganese dioxides, pyrolusit(β-MnO 2 ) and ramsdellite (R-MnO 2 ), respectively, were chemically intercalated with LiI. Two 7 Li resonances were observed in lithiated pyrolusite. One isotropic resonance arising at 110 ppm shows a short spin-lattice relaxation time (T 1 ∼ 3 ms) and was assigned to Li + ions in the 1×1 tunnel structure. The other isotropic resonance arising at 4 ppm shows a long spin-lattice relaxation time (T1 ∼ 100 ms) and was assigned to Li + ions in diamagnetic local environments in the form of impurities such as Li 2 O or on the surface of the MnO 2 particles. Three 7 Li resonances were observed in lithiated ramsdellite at very different frequencies (600, 110 and 0 ppm). The resonance at 600 ppm, which is observed at low lithium intercalation levels, is assigned toLi + ions coordinated to both Mn(III) and Mn(IV) ions in the 1×2 tunnels, while the resonanceat 110 ppm is due to Li + ions coordinated to Mn(III) ions and appears at higher Li levels. The resonance at 0 ppm is associated with a long spin-lattice relaxation time (T1 ∼ 100 ms) and is also assignedto Li + ions in diamagnetic impurities.

Study of Fluoride Ion Motions in PbSnF4 and BaSnF4 Compounds With Molecular Dynamics Simulation and Solid State NMR Techniques

A combined approach, using solid state NMR and Molecular Dynamics (MD) simulations, has been employed in this work to investigate fluoride-ion motion in the PbSnF 4 family of anionic conductors, materials that contain double layers of Sn 2+ and M 2+ cations. 19 F MAS NMR spectra of PbSnF 4 and BaSnF 4 show that the fluoride ions are mobile on the NMR timescale (10 −4 s), even at room temperature. In the case of BaSnF 4 , two different groups of fluoride ions were observed, one group corresponding to fluorine atoms between the layers of Ba 2+ cations, and the other set, corresponding to mobile fluoride ions undergoing exchange between sites in the Ba-Sn and Sn-Sn layers. The 119 Sn NMR suggests a highly distorted Sn environment in these compounds, consistent with the presence of stereoactive Sn lone pairs. MD simulations, using the Polarizable Ion Model, have been carried out to probe the conduction mechanism. These simulations are able to reproduce elements of the structure such as the reduction in the occupancy of the fluorine ions between the Sn-Sn layers. Anisotropic conductivity, involving primarily motion in the M-Sn layers, is predicted, consistent with the NMR results. In the case of BaSnF 4 , no motion involving the fluoride ions in the Ba-Ba layers is observed on the simulation timescale (10 −12 s) and a cyclic mechanism of fluoride-ion motion involving two types of fluoride ions in the Ba-Sn layers is proposed.

(sup 6)Li and (sup 7)MAS NMR and In Situ X-Ray Diffraction Studies of Lithium Manganate Cathode Materials

{sup 6}Li MAS NMR spectra of lithium manganese oxides with differing manganese oxidation states (LiMn{sub 2}O{sub 4}, Li{sub 4}Mn{sub 5}O{sub 12}, Li{sub 2}Mn{sub 4}O{sub 9}, and Li{sub 2}Mn{sub 2}O{sub 4}) are presented. Improved understanding of the lithium NMR spectra of these model compounds is used to interpret the local structure of the Li{sub x}Mn{sub 2}O{sub 4} cathode materials following electrochemical Li{sup +} deintercalation to various charging levels. In situ x-ray diffraction patterns of the same material during charging are also reported for comparison. Evidence for two-phase behavior for x <0.4 (Li{sub x}Mn{sub 2}O{sub 4}) is seen by both NMR and diffraction.