Facile synthesis of nano-Li4 Ti5O12 for high-rate Li-ion battery anodes
© Jin et al; licensee Springer. 2012
Received: 2 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
One of the most promising anode materials for Li-ion batteries, Li4Ti5O12, has attracted attention because it is a zero-strain Li insertion host having a stable insertion potential. In this study, we suggest two different synthetic processes to prepare Li4Ti5O12 using anatase TiO2 nanoprecursors. TiO2 powders, which have extraordinarily large surface areas of more than 250 m2 g-1, were initially prepared through the urea-forced hydrolysis/precipitation route below 100°C. For the synthesis of Li4Ti5O12, LiOH and Li2CO3 were added to TiO2 solutions prepared in water and ethanol media, respectively. The powders were subsequently dried and calcined at various temperatures. The phase and morphological transitions from TiO2 to Li4Ti5O12 were characterized using X-ray powder diffraction and transmission electron microscopy. The electrochemical performance of nanosized Li4Ti5O12 was evaluated in detail by cyclic voltammetry and galvanostatic cycling. Furthermore, the high-rate performance and long-term cycle stability of Li4Ti5O12 anodes for use in Li-ion batteries were discussed.
Li4Ti5O12 is one of the most promising anode materials for Li-ion batteries even though it has lower specific capacity (175 mAh g-1) than does graphite (372 mAh g-1). One of the unique properties of Li4Ti5O12 is the negligible lattice change in the Li-ion insertion/desertion process, which provides good high-rate cycling stability . The electrochemical properties of Li4Ti5O12 are dependent on its method of preparation. The conventional solid-state, sol-gel , hydrothermal , spray pyrolysis , and combustion  methods have been proposed for Li4Ti5O12 synthesis. Among these, the solid-state process is a simple method that is well suited for production scale-up. However, the solid-state process using TiO2 as a starting precursor requires lengthy heating with Li salts at high temperatures in order to obtain highly crystalline Li4Ti5O12. As a result, particle size control is more difficult than that in hydrothermal or sol-gel method, and the resultant larger particles lead to poor capacity retention and rate capability.
Herein, we demonstrate the preparation of highly crystalline nanosized Li4Ti5O12 [nano-Li4Ti5O12] with a uniform particle size via a urea-mediated wet process, in which a TiO2 precursor with a large surface area is initially formed, followed by wet and solid-state processes with different Li sources, LiOH and Li2CO3, respectively. After subsequent heat treatment, the electrochemical performance of the resultant Li4Ti5O12 as an anode for Li-ion batteries is evaluated and discussed.
Preparation of TiO2 precursor
TiO2 nanoparticles with an anatase structure were prepared using the urea-mediated precipitation method , in which 0.015 M titanium trichloride (20% in 3% hydrochloric acid, TiCl3, Alfa Aesar, Ward Hill, MA, USA) and 3.0 M urea (99.3%, (NH2)2CO, Alfa Aesar, Ward Hill, MA, USA) were dissolved in deionized [DI] water at room temperature. The solution was heated at 90°C to 100°C for 4 h with magnetic stirring. Precipitates were obtained by centrifugation and repeated washing (five times with DI water and once with anhydrous ethanol). The powders were dried at 100°C for several hours in a vacuum oven.
Preparation of Li4Ti5O12
Stoichiometric amounts of the prepared TiO2 nanopowder were dispersed in DI water by sonication for 2 h. A stoichiometric amount of LiOH (98%, Sigma-Aldrich, St. Louis, MO, USA) was then dissolved in the solution with stirring. The resulting white-colored suspensions were heated at 110°C to evaporate water. Finally, the powder was calcined at various temperatures in air to afford Li4Ti5O12.
For the solid-state process, Li2CO3 (99%, Sigma-Aldrich, St. Louis, MO, USA) was chosen as the Li source. The stoichiometric mixture was agitated for 24 h with a zirconia ball in absolute ethanol, dried, and calcined at various temperatures in air.
Characterization of TiO2 precursors and Li4Ti5O12 nanoparticles
The powders were characterized by X-ray powder diffraction [XRD] (D/max-2500 V, Rigaku, Tokyo, Japan), Brunauer-Emmett-Teller [BET] (Belsorp-mini II, BEL Japan Inc., Osaka, Japan) surface area determination, high-resolution transmission electron microscopy [HRTEM] (JEM-3000F, JEOL, Tokyo, Japan) at an accelerating voltage of 300 kV, and field-emission scanning electron microscopy [FESEM] (JSM-6700F, JEOL, Tokyo, Japan).
A mixture consisting of 70 wt.% of the active materials, 15 wt.% Super P carbon black (MMM Carbon, Brussels, Belgium), and 15 wt.% Kynar 2801 binder (PVDF-HFP, Arkema Inc., King of Prussia, PA, USA) was dissolved in 1-methyl-2-pyrrolidinone (Sigma-Aldrich, St. Louis, MO, USA) solvent for uniform dispersion of the active materials on a Cu foil to obtain positive electrodes. Then, the solvent was evaporated in a vacuum oven at 100°C. A Swagelok-type cell was assembled in an Ar-filled glove box in order to protect the cell from oxidation and moisture. A Li metal foil (negative electrode) and the prepared mixture (positive electrode) were saturated with a liquid electrolyte obtained by dissolving 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1 by volume, Techno Semichem Co., Ltd., Sungnam, South Korea). Li4Ti5O12 powders were analyzed by the galvanostatic discharge/charge cycling method and cyclic voltammetry [CV] measurements with a battery cycler (WBCS 3000, WonATech, Seoul, South Korea). Each cell was cycled through a voltage range of 1.0 to 2.5 V versus Li/Li+.
Results and discussion
Figure 4b shows the galvanostatic cycling characteristics of nano-Li4Ti5O12 powders that were prepared through the solid-state process. The first discharge capacity was 154 mAh g-1 over a voltage window of 1.0 to 2.5 V at a current rate of 1 C (175 mAh g-1; here, C is defined as three Li ions per hour and per formula unit of Li4Ti5O12 on the basis of the above equation). The reversible capacities were observed to be 135, 133, 131, 130, and 128 mAh g-1 after 100, 200, 300, 400, and 500 cycles, respectively. Indeed, it is interesting to note that the nano-Li4Ti5O12 electrode in this study shows superior long-term cyclability and negligible variation in reversible capacity upon cycling (0.013% fading per cycle between 100 and 500 cycles).
In summary, spinel-type nano-Li4Ti5O12 particles were synthesized by a solid-state process from a large-surface-area TiO2 precursor and subsequent calcination at 700°C. The average particle size of these nano-Li4Ti5O12 particles was 50 to 100 nm. High Li electroactivity was confirmed by CV experiments. The nano-Li4Ti5O12 particles calcined at 700°C showed a high Li storage capacity of 128 mAh g-1 after 500 cycles at 1 C and superior cycle performance (112 mAh g-1) even at a high rate of 20 C. The enhanced reversible capacity and cycling performance were attributed to the formation of highly crystalline, uniform nanoparticles, which make this nano-Li4Ti5O12 a potential host material for high-powder Li-ion batteries.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST; No. 2010-0029617 & 2011-0005776) and was completed with Ajou University Research Fellowship of 2011 (S-2011-G0001-00070).
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