562015-03-24 , 法政大学大学院理工学・工学研究科
Li8SiN4 (tetragonal structure) is expected as a cathode material of Li secondary batteries because it includes eight Li atoms in its 1/8-sublattice. Li8SiN4 is synthesized by the direct reaction between Li3N and Si with the molar ratio of 2.5:1 at 1073 K for 30 minutes under a nitrogen atmosphere of about 700 Torr. As grown crystals, which are light orange color, are confirmed to be single phase of Li8SiN4 with a = 10.148 Å and c = 9.567 Å by the X-ray diffraction method. A broad Raman peak observed at 586 cm-1 suggests the disordered structure between Li and Si. The band gap of Li8SiN4 evaluated using optical absorption and photoacoustic spectroscopy is about 2.4 eV. Li8SiN4 is a direct transition, which is slightly larger than the d-d electron transitions in the typical cathode materials LiMn2O4 and LiCoO2. For a 5 × 5 mm2 sized lithium secondary battery with the Li8SiN4 + Acetylene Black cathode / Propylene Carbonate + LiClO4 electrolyte / Li anode structure, the capacity for the discharge current of 10 μA is between 2.44 μAh/cm2 and 3.56 μAh/cm2 for a potential range between 2.8 V and 3.8 V.