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Articles you may be interested inExperimental and computational studies on structural transitions in the LiBH 4 -LiI pseudobinary system

Solid state lithium conductors are attracting much attention for their potential applications to solid-state batteries and supercapacitors of high energy density to overcome safety issues and irreversible capacity loss of the currently commercialized ones. Recently, we discovered a new class of lithium super ionic conductors based on lithium borohydride (LiBH(4)). LiBH(4) was found to have conductivity as high as 10(-2) Scm(-1) accompanied by orthorhombic to hexagonal phase transition above 115 degrees C. Polarization to the lithium metal electrode was shown to be extremely low, providing a versatile anode interface for the battery application. However, the high transition temperature of the superionic phase has limited its applications. Here we show that a chemical modification of LiBH(4) can stabilize the superionic phase even below room temperature. By doping of lithium halides, high conductivity can be obtained at room temperature. Both XRD and NMR confirmed room-temperature stabilization of superionic phase for LiI-doped LiBH(4). The electrochemical measurements showed a great advantage of this material as an extremely lightweight lithium electrolyte for batteries of high energy density. This material will open alternative opportunities for the development of solid ionic conductors other than previously known lithium conductors.

Some of the authors have reported that a complex hydride, Li(BH(4)), with the (BH(4))(-) anion exhibits lithium fast-ion conduction (more than 1 x 10(-3) S/cm) accompanied by the structural transition at approximately 390 K for the first time in 30 years since the conduction in Li(2)(NH) was reported in 1979. Here we report another conceptual study and remarkable results of Li(2)(BH(4))(NH(2)) and Li(4)(BH(4))(NH(2))(3) combined with the (BH(4))(-) and (NH(2))(-) anions showing ion conductivities 4 orders of magnitude higher than that for Li(BH(4)) at RT, due to being provided with new occupation sites for Li(+) ions. Both Li(2)(BH(4))(NH(2)) and Li(4)(BH(4))(NH(2))(3) exhibit a lithium fast-ion conductivity of 2 x 10(-4) S/cm at RT, and the activation energy for conduction in Li(4)(BH(4))(NH(2))(3) is evaluated to be 0.26 eV, less than half those in Li(2)(BH(4))(NH(2)) and Li(BH(4)). This study not only demonstrates an important direction in which to search for higher ion conductivity in complex hydrides but also greatly increases the material variations of solid electrolytes.

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The short-range structure of "invert" glasses along the pseudobinary join MgSiO(3)-Mg(2)SiO(4) has been studied using (29)Si and (25)Mg MAS NMR spectroscopy. The results indicate a progressive compositional evolution in Q speciation that approximately follows a statistical distribution. The Mg(2)SiO(4) glass shows an abrupt deviation from this trend with the presence of nearly 40% of the Si atoms as (Si(2)O(7))(6-) dimers, i.e., Q(1) species. Mg(2+) ions are present in predominantly octahedral coordination in all glasses. When taken together, these results indicate that glasses with MgO contents between 50 and 60 mol % are characterized by a structure consisting primarily of at least three types of Q species and MgO(6) octahedra. On the other hand, the structure of glasses with >60 mol % MgO appears to consist of Q(0) and Q(1) species with structural connectivity being primarily provided by the MgO(6) octahedra. The possible consequences of such compositional evolution of structure on the ability of glass formation in this system are discussed.

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LiBH 4 exhibits lithium superionic conduction accompanied by structural transition at around 390 K. Addition of LiCl to LiBH4 drastically affects both the transition and electrical conductivity: Transition from low-temperature (LT) to high-temperature (HT) phases in LiBH4 is observed at 370 K upon heating and the HT phase can be retained at 350–330 K upon cooling. Further, the conductivity in the LT phase is more than one or two orders of magnitude higher than that of pure LiBH4. These properties could be attributed to the dissolution of LiCl into LiBH4, suggested by in situ x-ray diffraction measurement.