Optical band gap of Li8SiN4 with disordered structure as a cathode material of lithium secondary batteries.

The band gap of and disordered structure in Li8SiN4 (lattice constant: a = 10.148 Å, c = 9.567 Å) are studied by optical absorption, photoacoustic spectroscopy, X-ray diffraction, and Raman scattering spectroscopy. Li8SiN4 is a semiconductor with a band gap of 2.4 eV, suggesting the transition from the N-2p valence band to the conduction band mainly consisting of Si-3s and/or -3p orbitals. A broad Raman peak is observed at 586 cm-1, indicating the homogenously random distribution of Li and Si atoms. Li8SiN4 crystallizes in a superstructure of eight face-centered N sublattices. In each sublattice, one Li and one Si atom are randomly and diagonally occupied at two tetrahedral sites next to N, while six Li atoms reside at the other sites next to N. The possible locations of one remaining Li atom are also discussed. 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. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]