Ionic conductivities of lithium borohydride-lithium nitride composites

The x LiBH 4 -Li 3 N ( x   =   1, 2, 4) composites were investigated on their lithium ionic conductivities . They maintained the original phase structures of orthorhombic LiBH 4 and hexagonal β-Li 3 N during ball milling according to the analyses by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Electrochemical impedance spectra (EIS) tests showed that LiBH 4 -Li 3 N composites lift the lithium ionic conductivities from that of bulk LiBH 4 by nearly 100 times at low temperature. LiBH 4 -Li 3 N offers 1.06   ×   10 −   5   S   cm −   1 of conductivity while bulk LiBH 4 possesses only 2.05   ×   10 −   7   S   cm −   1 at 323   K. Cyclic voltammetry (CV) tests indicated that the voltage window of composites could attain 3   V, which is much more stable than Li 3 N (<   1   V). No thermodynamic incident appears in DSC curves except for the orthorhombic-hexagonal transition endothermic peak of LiBH 4 in the x LiBH 4 -Li 3 N composites. It confirms the phases stabilities of the x LiBH 4 -Li 3 N ( x   =   1, 2, 4) composites before and after CV tests. In-situ solid state Nuclear Magnetic Resonance (NMR) tests on LiBH 4 -Li 3 N illustrated that the high conductivity in low temperature should not come from high-temperature phase of LiBH 4 . Therefore, the enhancement of lithium ionic mobility of the composite might be attributed to both Li 3 N and the interfaces between small crystallites where transmission electron microscope (TEM) morphology showed the appearance of amorphous grain boundary.