Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
Lithium-titanium oxide (Li4Ti5O12) is a lithium ionic conductor of good structural stability, with an almost negligible volume change during the Li+ insertion and extraction processes. Due to this it can be used as a negative electrode (anode) in lithium-ion secondary batteries with theoretically unlimited cycle life what prompts recently an increasing interest in investigation of this material. In electronic transport point of view, Li4Ti5O12 is a wide band gap semiconductor material (Eg = 3.6 eV), so pure, undoped crystals have very high electronic resistivity, of the order of 1012 Ωcm. We tried to overcome this problem by doping the material with silver.
The three-step solid state synthesis including ball-milling process was used to obtain the lithium titanium-oxide powder of spinel structure modified with Ag nanoparticles. It was measured optically with use of Raman scattering, absorption and time-resolved photoluminescence (PL) and electrically: I-V characteristics and impedance spectroscopy.
Raman spectroscopy revealed differences between pristine and electrochemically charged samples showing that lithium can outdiffuse from the material. The delithiation of this compound caused the disappearance of typical Li4Ti5O12 spinel spectra and the new modes arrival which were located at 192, 330 and 631 cm-1.
The PL spectra consisted of weak band-to-band emission at about 3.6 eV and strong deep defect band with peaks at 2.2 and 2.6 eV. The energy of this band increased with temperature what was probably due to activation of Li transport. The PL lifetime of this band was of the order of few ns and decrease after extraction of lithium. The PL decays were of Langevin type what was due to charge carrier diffusion.
Impedance spectroscopy was measured in the range 1 Hz - 100 kHz. It was found that in low frequency range the impedance was characteristic for electronic transport but in high frequencies (f >100 Hz) signal was dominated by Warburg impedance (Re(Z) = AWΩ-1/2), what was due to lithium diffusion.Measurements of conductivity in function of temperature revealed thermal activation with energy EA = 0.7 eV. This activation energy is most probably related to diffusion of lithium through Li4Ti5O12 lattice.
In conclusion, using optical and electrical methods we have observed charge transport in lithium-titanium oxide and determined diffusion parameters (Warburg constant, activation energy).
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