The reversible capacities of (A, green) pristine and (B, red) nitridated Li4Ti5O12 with different charge/discharge current densities during cycling. Click to enlarge. Credit: ACS
Researchers at Korea’s Samsung Advanced Institute of Technology (SAIT) and Sungkyunkwan University report that the thermal nitridation of lithium titanate (Li4Ti5O12, LTO) to modify the oxide structure of the material to introduce a thin conductive film can result in an increase in capacity of up to 6 times (around 120 mAh g-1) at a high charge/discharge current density (10C) compared to pristine LTO material.
At low current densities below the 1C-rate, the capacities after the nitridation do not differ much from those for pristine Li4Ti5O12—around 150 mAh g-1. A paper on their work was published online 15 October in the Journal of the American Chemical Society.
A number of battery makers, such as EnerDel, Toshiba, and AltairNano, are using lithium titanate as an anode material because of its improved thermal stability compared to conventional carbon anodes, and hence improved fast-charge and safety characteristics. The downside of LTO is a lower cell voltage (2.5V vs 3.7V, according to the DOE), and a low capacity density of about half that of graphite.
Thermal nitridation has been known to change the oxygen stoichiometry in some oxides and to introduce nitride thin films. Thus, modification of the surface structure of Li4Ti5O12 via annealing in NH3 should change Li insertion/extraction behaviors, and it should also make bonding between surface Ti and N simultaneous. To address this issue, we introduced thermal nitridation to Li4Ti5O12 and found, for the first time to our knowledge, that it can introduce a mixed-valent intermediate phase, Li4+δTi5O12, and a surface conductive layer, TiN, with a significant enhancement in the battery performances.
—Park et al. (2008)
The ammonia gas decomposes the surface LTO into a conductive layer of TiN and Li2CO3 without a change in the bulk lattice parameter; it may also modify the surface structure in a way to accommodate the single phase Li insertion and extraction, the researchers suggested.
- Kyu-Sung Park, Anass Benayad, Dae-Joon Kang, and Seok-Gwang Doo (2008) Nitridation-Driven Conductive Li4Ti5O12 for Lithium Ion Batteries. ASAP J. Am. Chem. Soc., doi: 10.1021/ja806104n