Li₈SiN₄ (Tetragonal structure; a = 10.21₇ Åand c = 9.53₆ Å[1], space group; P4/ncc or P4₂nm [2]) has been expected as an electrode material of the Li secondary battery because it includes eight Li atoms in unit cell.

Li₈SiN₄ is synthesized by direct reaction between Li₃N (powder, 99.5% pure) and Si (powder, 99.999% pure). These elements with the molar ratio of 2.5:1 are charged in Ta crucible. The charged crucible is synthesized at 1073 K for 30 minutes under a nitrogen atmosphere of about 700 Torr. As-grown crystals, which are light orange in color, are confirmed to be single phase of Li₈SiN₄ with a = 10.30₀ Åand c = 9.50₉ Å by a powder x-ray diffraction method. Four Raman spectra are observed at 216, 431, 586 and 766 cm^-1. The 586 cm^-1 Raman spectrum is the broad one with a full width at half maximum of 144 cm^-1, suggesting the disordered structure between Li and Si as observed in antifluorite type LiMgN, which is the disordered structure between Li and Mg [3]. The band gap of Li₈SiN₄ evaluated using optical absorption and photoacoustic spectroscopy is about 2.3 eV. This value is slightly larger than the d-d electron transition in spinel LiMn₂O₄ (1.63 and 2.00 eV [4]), which has been used for cathode materials.

The 5×5 mm² sized lithium secondary battery of Li₈SiN₄ cathode / propylene carbonate + LiClO₄ electrolyte / Li anode structure is charged at 10 μA for 20 minutes with the charge voltage of about 2.8 V. This battery shows a capacity of 66.7 μAh/cm² for a discharge current of 10 µA.

[1] H. Yamane, S. Kikkawa and M. Koizumi, Solid State Ionics, 25, 183 (1987).

[2] J. David, J.-P. Charlot and J. Lang, Rev. Chim. miner, 11, 415 (1974).

[3] K. Kuriyama, Y. Yamashita, T. Ishikawa and K. Kushida, Phys. Rev. B75, 233204

(2007).

[4] K. Kushida and K. Kuriyama, Appl. Phys. Lett. ,77, 4154 (2000).

*Corresponding author: e-mail: kuri@ionbeam.hosei.ac.jp

1. FULLTEXT: Synthesis and physical properties of Li_{8}SiN_{4} as a cathode material of lithium secondary batteries, Zip archive data, at least v2.0 to extract, 0MB

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