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Characteristics of spherical-shaped Li4Ti5O12 anode powders prepared by spray pyrolysis
ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 70 (2009) 40–44
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Characteristics of spherical-shaped Li4Ti5O12 anode powders prepared by spray pyrolysis Seo Hee Ju, Yun Chan Kang Department of Chemical Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 March 2008 Received in revised form 4 August 2008 Accepted 2 September 2008
Spherical-shaped Li4Ti5O12 anode powders with a mean size of 1.5 mm were prepared by spray pyrolysis. The precursor powders obtained by spray pyrolysis had no peaks of crystal structure of Li4Ti5O12. The powders post-treated at temperatures of 800 and 900 1C had the single phase of spinel Li4Ti5O12. The powders post-treated at a temperature of 1000 1C had main peaks of the Li4Ti5O12 phase and small impurity peaks of Li2Ti3O7. The spherical shape of the precursor powders was maintained after posttreatment at temperatures below 800 1C. The Brunauer–Emmett–Teller (BET) surface areas of the Li4Ti5O12 anode powders post-treated at temperatures of 700, 800 and 900 1C were 4.9, 1.6 and 1.5 m2/g, respectively. The initial discharge capacities of Li4Ti5O12 powders were changed from 108 to 175 mAh/g when the post-treatment temperatures were changed from 700 to 1000 1C. The maximum initial discharge capacity of the Li4Ti5O12 powders was obtained at a post-treatment temperature of 800 1C, which had good cycle properties below current densities of 0.7 C. & 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics A. Electronic materials B. Chemical synthesis D. Electrochemical properties
1. Introduction In order to address the safety limitations of lithium-ion cells, particularly those containing nickel-based cathodes, alternative anodes to graphite have been suggested, such as the spinel Li4Ti5O12 that operates at approximately 1.5 V vs. Li0 [1–4]. The Li4Ti5O12 anode can be coupled with high-voltage cathode materials such as LiMn2O4 [5], LiCo0.4Fe0.4Mn3.2O8 [6] or Li2CoMn3O8 [7] for an approximately 2.5 V lithium-ion battery with high safety and reversibility. The spinel Li4Ti5O12 is a so-called zero-strain insertion material as the negative-electrode material of lithium secondary batteries [1]. One of the most important properties of it is that its lattice parameters are almost unchanged when lithium ion is inserted and extracted. This material accommodates Li with a theoretical capacity of 175 mAh/g, and the actual discharge capacity is 4160 mAh g1 [8]. Li4Ti5O12 does not react with electrolyte. It is cheap and easy to prepare. Compared with carbonaceous materials used as anode-active materials in commercial lithium ion batteries, Li4Ti5O12 has better electrochemical properties and higher safety. In most studies, Li4Ti5O12 has been synthesized using the solidstate reaction [9–16] and the sol–gel method [17–20]. The electrochemical performance of the electrode material in a secondary lithium battery is strongly affected by the properties
Corresponding author. Tel.: +82 2 2049 6010; fax: +82 2 458 3504.
E-mail address: [email protected] (Y.C. Kang). 0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.09.003
of the powders, such as the morphology, the specific surface area, the crystallinity and the composition of the materials [8,21–24]. With respect to the morphology of the electrode powders, the spherical-shaped powders with narrow size distributions show better electrochemical performance than the powders with irregular morphologies because of the high packing density of the spherical powders. Fine-sized electrode powders have been intensively investigated to improve the cyclability and rate capability of secondary lithium batteries. In this study, the spherical-shaped Li4Ti5O12 anode powders were prepared by spray pyrolysis. The morphological and electrochemical properties of the prepared Li4Ti5O12 anode powders post-treated at various temperatures were investigated.
2. Experimental The spray pyrolysis system consists of a droplet generator, a quartz reactor, and a powder collector. A 1.7 MHz ultrasonic spray generator with six vibrators was used to generate a large amount of droplets, which are carried into the high-temperature tubular reactor by a carrier gas. The droplets and powders evaporated, decomposed and/or crystallized in the quartz reactor. The length and diameter of the quartz reactor are 1200 and 50 mm, respectively. The reactor temperature was maintained at 800 1C. The flow rate of the air used as the carrier gas was 40 l/min. The precursor solution was prepared by dissolving a stoichiometric amount of lithium nitrate (LiNO3) and titanium(IV)
ARTICLE IN PRESS S.H. Ju, Y.C. Kang / Journal of Physics and Chemistry of Solids 70 (2009) 40–44
size distribution. Fig. 2 shows the TG and DSC curves of the precursor powders. In this study, the TG/DSC curves were analyzed to investigate the characteristics of the precursor powders obtained by spray pyrolysis. The short residence time of the powders inside the hot wall reactor is an important factor in the large production of powders by spray pyrolysis. Therefore, there was no complete decomposition of precursors or additives in the spray pyrolysis. In the TG curve, the total weight loss of the precursor powders obtained at a temperature of 800 1C was 36.6 wt%. There are two steps of weight loss. The first step of weight loss of 12.19%, observed between room temperature and 200 1C, is mainly due to the vaporization of water and impurities. Then, the decomposition process mainly occurred at the temperature range between 450 and 530 1C with a weight loss of 24.43%. The endothermic peak at 135.4 1C in the DSC curve was attributed to the evaporation of adsorbed water molecules in the powder. The endothermic peaks at 255.9 and 528 1C could be attributed to the decomposition of precursors contained in the powders. The precursor powders obtained by spray pyrolysis had no peaks of crystal structure of Li4Ti5O12 as shown in Fig. 3. The reaction between Li and Ti components to form the Li4Ti5O12 phase did not occur because of the short residence time of the powders inside the hot wall reactor. Therefore, the precursor powders had the main peaks of anatase TiO2. The crystal 100
TG (%)
-24.43 %
60 40
0 135.4 °C 255.9 °C
20 -2
-528.0 °C
0
200
400 600 Temperature (°C)
800
(440)
(531)
(511)
o
o Li2Ti3O7 # Rutile TiO2 ∗ Anatase TiO2 (331)
(311) (222)
Intensity (arbitrary uint)
(400)
Fig. 2. TG/DSC curves of the Li4Ti5O12 precursor powders.
A 1000 A 900
A 800 A 700 #
#
# #
# ∗ A 600
#
P 700
∗
10
20
30
40
50
60
2θ Fig. 1. SEM photograph of the Li4Ti5O12 precursor powders.
DSC (mW/mg)
80
3. Results and discussion The mean size and morphology of the precursor powders obtained by spray pyrolysis are affected by the preparation conditions and the characteristics of the spray solution. Fig. 1 shows the SEM photograph of the precursor powders obtained by spray pyrolysis. The precursor powders had spherical shape and non-aggregation characteristics. One precursor particle was formed from one droplet with several micron sizes. However, the precursor powders had large and broad size distribution. The mean size of the precursor powders measured from the SEM photograph was 1.5 mm. In this work, the residence time of the droplets or powders inside the hot wall reactor was as short as 0.4 s. Therefore, the high drying and decomposition rates of droplets resulted in the precursor powders with large and broad
2
-12.19 %
(111)
tetraisopropoxide (TTIP, Ti[OCH(CH3)2]4) in distilled water. The total concentration of Li and Ti components dissolved in spray solution was 0.5 M. The as-prepared powders obtained by spray pyrolysis at a preparation temperature of 800 1C were post-treated in the box furnace at the temperature range between 600 and 1000 1C for 12 h in air atmosphere. The crystal structures of the as-prepared and post-treated powders were investigated using X-ray diffractometry (XRD, RIGAKU DMAX-33) using Cu Ka radiation (l ¼ 1.5418 1010 m) at room temperature in the 2y range 10–801. The morphological characteristics of the powders were investigated using scanning electron microscopy (SEM, JEOL JSM-6060). Surface area measurements were carried out by the Brunauer–Emmett–Teller (BET) method using N2 as the adsorbate gas. Measurement of the thermal properties of the precursor powders was performed on a thermo-analyzer (TG-DSC, Netzsch, STA409C, Germany) in the temperature range from 40 to 900 1C (10 1C/min). The charge/ discharge capacities of the prepared Li4Ti5O12 anode materials were measured. The anode electrode was made of 12 mg of Li4Ti5O12 compounds mixed with 4 mg of a conductive binder (3.2 mg of teflonized acetylene black and 0.8 mg of graphite). The lithium metal and polypropylene film were used as the counterelectrode and the separator, respectively. The electrolyte (TECHNO Semichem. Co.) was 1 M LiPF6 in a 1:1 mixture by volume of EC/DEC. The entire cell was assembled in a glove box under an argon atmosphere. The charge/discharge characteristics of the samples were measured through cycling in the 0.5–2.5 V potential range at various current densities.
41
Fig. 3. XRD spectra of the Li4Ti5O12 anode powders.
70
80
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Fig. 4. SEM photographs of the Li4Ti5O12 anode powders at different post-treatment temperatures: (a) A600; (b) A700; (c) A800; (d) A900 and (e) A1000.
ARTICLE IN PRESS S.H. Ju, Y.C. Kang / Journal of Physics and Chemistry of Solids 70 (2009) 40–44
2.5
Voltage (V)
2.0
1.5
1.0 A 800
0.5
A 600
0
30
A 700
60 90 120 Capacity (mAh/g)
A 900
A 1000
150
180
Fig. 5. Initial charge/discharge curves of the Li4Ti5O12 anode powders obtained at different post-treatment temperatures.
Capacity (mAh/g)
200
0.3C
0.2C
0.1C
0.5C
0.7C
1C
150
100
A 800
A 1000
50
A 900
0
A 700
0
5
10
15 20 Cycle Number
25
30
Fig. 6. Current density performances of the Li4Ti5O12 anode powders obtained at different post-treatment temperatures.
160
Capacity (mAh/g)
structures of the post-treated powders at temperatures between 600 and 1000 1C are shown in Fig. 3. The peaks of the Li4Ti5O12 phase appeared at a post-treatment temperature of 600 1C. However, the main peaks of the powders post-treated at a temperature of 600 1C were those of rutile TiO2. The powders post-treated at a temperature of 700 1C had the main peaks of the Li4Ti5O12 phase and the small impurity peaks of rutile TiO2. The powders post-treated at temperatures of 800 and 900 1C had the single phase of spinel Li4Ti5O12. The mean crystallite sizes of the Li4Ti5O12 powders post-treated at temperatures of 800 and 900 1C were 37 and 46 nm, respectively. The powders post-treated at a temperature of 1000 1C had main peaks of the Li4Ti5O12 phase and small impurity peaks of Li2Ti3O7. Decomposition of the Li4Ti5O12 phase occurred at a high post-treatment temperature of 1000 1C. The effects of the post-treatment temperatures on the morphologies of the Li4Ti5O12 anode powders are investigated in Fig. 4. The spherical shape of the precursor powders was maintained after post-treatment at temperatures below 800 1C. On the other hand, the powders post-treated at a temperature of 900 1C had spherical-like shape and aggregated morphology. The spherical shape of the precursor powders disappeared after post-treatment at a temperature of 1000 1C. The BET surface areas
43
120
80
40
0
0
5
10
15 20 Cycle Number
25
30
Fig. 7. Cycle property of the Li4Ti5O12 anode powders at a post-treatment temperature of 800 1C.
of the Li4Ti5O12 anode powders post-treated at temperatures of 700, 800 and 900 1C were 4.9, 1.6 and 1.5 m2/g, respectively. Therefore, the density of the Li4Ti5O12 anode powders with spherical shape increased on increasing the post-treatment temperature. Fig. 5 shows the initial charge–discharge curves between 0.5 and 2.5 V of Li4Ti5O12 powders obtained at different posttreatment temperatures. The profile of the discharge curves exhibited an extremely flat operating voltage of about 1.5 V, which is consistent with the results of previous studies [12,25]. During the charge process, a very flat change curve at around 1.6 V can be observed. This suggests a two-phase reaction based on the Ti4+/Ti3+ redox couple. The initial discharge capacities of Li4Ti5O12 powders were changed from 108 to 175 mAh/g when the posttreatment temperatures were changed from 700 to 1000 1C. The maximum initial discharge capacity of the Li4Ti5O12 powders was obtained at a post-treatment temperature of 800 1C. The powders obtained at a post-treatment temperature of 700 1C had a low discharge capacity of 108 mAh/g because of the impurity phase as shown in Fig. 3. Starting of decomposition of the Li4Ti5O12 phases at high post-treatment temperatures of 900 and 1000 1C decreased the discharge capacities of the anode powders. Fig. 6 shows the cycle properties of the Li4Ti5O12 powders by changing the current density from 0.1 to 1 C when the post-treatment temperatures were changed from 700 to 1000 1C. The Li4Ti5O12 powders obtained at post-treatment temperatures of 900 and 1000 1C showed poor cycle properties at high current densities. On the other hand, the Li4Ti5O12 powders obtained at a posttreatment temperature of 800 1C had good cycle properties below current densities of 0.7 C. Fig. 7 shows the cycle properties of Li4Ti5O12 powders obtained at a post-treatment temperature of 800 1C at a constant current density of 0.1 C after the 30th cycle test by changing the current density as shown in Fig. 6. The discharge capacities of the Li4Ti5O12 powders dropped from 154 to 142 mAh/g by the 30th cycle.
4. Conclusions The morphologies and crystal structures of the as-prepared and post-treated Li4Ti5O12 anode powders obtained by spray pyrolysis were investigated. The precursor powders with a spherical shape had the main peaks of anatase TiO2. The singlephase Li4Ti5O12 anode powders with a spherical shape and fine
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size were obtained at an optimum post-treatment temperature of 800 1C. The initial discharge capacities of Li4Ti5O12 powders were strongly affected by the post-treatment temperatures. The maximum initial discharge capacity of the Li4Ti5O12 anode powders was 175 mAh/g at a post-treatment temperature of 800 1C. The Li4Ti5O12 powders obtained at a post-treatment temperature of 800 1C had good cycle properties at various current densities. References [1] [2] [3] [4] [5]
T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc. 142 (5) (1995) 1431. K.M. Colbow, J.R. Dahn, R.R. Haering, J. Power Sources 26 (1989) 397. M.M. Thackeray, J. Am. Ceram. Soc. 82 (12) (1999) 3347. E. Ferg, R.J. Gummow, A. de Kock, J. Electrochem. Soc. 141 (11) (1994) L147. P.P. Prosini, R. Mancini, L. Petrucci, V. Contini, P. Villano, Solid State Ionics 144 (2001) 185. [6] S. Panero, D. Satolli, M. Salomon, B. Scosati, Electrochem. Commun. 2 (2000) 810. [7] R.I. Eglitis, G. Borstel, Phys. Stat. Sol. (a) R13 (2005) 202.
[8] E.M. Sorensen, S.J. Barry, H. Jung, H.K. Jung, J.R. Rondinelli, Chem. Mater. 18 (2006) 482. [9] K. Zaghib, M. Simoneau, M. Armand, M. Gauthier, J. Power Sources 81–82 (1999) 300. ¨ oring, W. Weppner, Solid State Ionics 118 (1999) 149. [10] P. Birke, F. Salam, S. D [11] K. Zaghib, M. Armand, M. Gauthier, J. Elecrochem. Soc. 9 (1998) 3135. [12] G.X. Wang, D.H. Bradhurst, S.X. Dou, H.K. Liu, J. Power Sources 83 (1999) 156. [13] K. Kanamra, T. Umegaki, H. Naito, J. Appl. Electrochem. 31 (2001) 73. [14] M. Masatoshi, U. Satoshi, Y. Eriko, K. Keiji, I. Shinji, J. Power Sources 101 (2001) 53. [15] S. Scharner, W. Weppner, P. Shmi-Beurmann, J. Electrochem. Soc. 146 (3) (1999) 857. [16] O. Tsutomu, U. Atsushi, Y. Norihiro, I. Yasunobu, J. Power Sources 54 (1995) 99. [17] S. Bach, J.P. Pereira-Ramos, N. Baffier, J. Power Sources 81–82 (1999) 273. [18] C.M. Shen, X.G. Zhang, Y.K. Zhou, H. Li, Mater. Chem. Phys. 78 (2002) 437. [19] Y.H. Rho, K. Kanamura, M. Fujisaki, J. Hamagami, S. Suda, T. Umegaki, Solid State Ionics 151 (2002) 151. [20] K.N. Jung, S.I. Pyun, S.W. Kim, J. Power Sources 119–121 (2003) 637. [21] K. Matsuda, I. Taniguchi, J. Power Sources 132 (2004) 156. [22] I. Taniguchi, Mater. Chem. Phys. 92 (2005) 172. [23] Y.K. Sun, D.W. Kim, Y.M. Choi, J. Power Sources 79 (1999) 231. [24] Z. Bakenov, I. Taniguchi, Solid State Ionics 176 (2005) 1027. [25] Y.J. Hao, Q.Y. Lai, J.Z. Lu, H.Li. Wang, Y.D. Chen, X.Y. Ji, J. Power Sources 158 (2006) 1358.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Characteristics of spherical-shaped Li4Ti5O12 anode powders prepared by spray pyrolysis Seo Hee Ju, Yun Chan Kang Department of Chemical Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 March 2008 Received in revised form 4 August 2008 Accepted 2 September 2008
Spherical-shaped Li4Ti5O12 anode powders with a mean size of 1.5 mm were prepared by spray pyrolysis. The precursor powders obtained by spray pyrolysis had no peaks of crystal structure of Li4Ti5O12. The powders post-treated at temperatures of 800 and 900 1C had the single phase of spinel Li4Ti5O12. The powders post-treated at a temperature of 1000 1C had main peaks of the Li4Ti5O12 phase and small impurity peaks of Li2Ti3O7. The spherical shape of the precursor powders was maintained after posttreatment at temperatures below 800 1C. The Brunauer–Emmett–Teller (BET) surface areas of the Li4Ti5O12 anode powders post-treated at temperatures of 700, 800 and 900 1C were 4.9, 1.6 and 1.5 m2/g, respectively. The initial discharge capacities of Li4Ti5O12 powders were changed from 108 to 175 mAh/g when the post-treatment temperatures were changed from 700 to 1000 1C. The maximum initial discharge capacity of the Li4Ti5O12 powders was obtained at a post-treatment temperature of 800 1C, which had good cycle properties below current densities of 0.7 C. & 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics A. Electronic materials B. Chemical synthesis D. Electrochemical properties
1. Introduction In order to address the safety limitations of lithium-ion cells, particularly those containing nickel-based cathodes, alternative anodes to graphite have been suggested, such as the spinel Li4Ti5O12 that operates at approximately 1.5 V vs. Li0 [1–4]. The Li4Ti5O12 anode can be coupled with high-voltage cathode materials such as LiMn2O4 [5], LiCo0.4Fe0.4Mn3.2O8 [6] or Li2CoMn3O8 [7] for an approximately 2.5 V lithium-ion battery with high safety and reversibility. The spinel Li4Ti5O12 is a so-called zero-strain insertion material as the negative-electrode material of lithium secondary batteries [1]. One of the most important properties of it is that its lattice parameters are almost unchanged when lithium ion is inserted and extracted. This material accommodates Li with a theoretical capacity of 175 mAh/g, and the actual discharge capacity is 4160 mAh g1 [8]. Li4Ti5O12 does not react with electrolyte. It is cheap and easy to prepare. Compared with carbonaceous materials used as anode-active materials in commercial lithium ion batteries, Li4Ti5O12 has better electrochemical properties and higher safety. In most studies, Li4Ti5O12 has been synthesized using the solidstate reaction [9–16] and the sol–gel method [17–20]. The electrochemical performance of the electrode material in a secondary lithium battery is strongly affected by the properties
Corresponding author. Tel.: +82 2 2049 6010; fax: +82 2 458 3504.
E-mail address: [email protected] (Y.C. Kang). 0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.09.003
of the powders, such as the morphology, the specific surface area, the crystallinity and the composition of the materials [8,21–24]. With respect to the morphology of the electrode powders, the spherical-shaped powders with narrow size distributions show better electrochemical performance than the powders with irregular morphologies because of the high packing density of the spherical powders. Fine-sized electrode powders have been intensively investigated to improve the cyclability and rate capability of secondary lithium batteries. In this study, the spherical-shaped Li4Ti5O12 anode powders were prepared by spray pyrolysis. The morphological and electrochemical properties of the prepared Li4Ti5O12 anode powders post-treated at various temperatures were investigated.
2. Experimental The spray pyrolysis system consists of a droplet generator, a quartz reactor, and a powder collector. A 1.7 MHz ultrasonic spray generator with six vibrators was used to generate a large amount of droplets, which are carried into the high-temperature tubular reactor by a carrier gas. The droplets and powders evaporated, decomposed and/or crystallized in the quartz reactor. The length and diameter of the quartz reactor are 1200 and 50 mm, respectively. The reactor temperature was maintained at 800 1C. The flow rate of the air used as the carrier gas was 40 l/min. The precursor solution was prepared by dissolving a stoichiometric amount of lithium nitrate (LiNO3) and titanium(IV)
ARTICLE IN PRESS S.H. Ju, Y.C. Kang / Journal of Physics and Chemistry of Solids 70 (2009) 40–44
size distribution. Fig. 2 shows the TG and DSC curves of the precursor powders. In this study, the TG/DSC curves were analyzed to investigate the characteristics of the precursor powders obtained by spray pyrolysis. The short residence time of the powders inside the hot wall reactor is an important factor in the large production of powders by spray pyrolysis. Therefore, there was no complete decomposition of precursors or additives in the spray pyrolysis. In the TG curve, the total weight loss of the precursor powders obtained at a temperature of 800 1C was 36.6 wt%. There are two steps of weight loss. The first step of weight loss of 12.19%, observed between room temperature and 200 1C, is mainly due to the vaporization of water and impurities. Then, the decomposition process mainly occurred at the temperature range between 450 and 530 1C with a weight loss of 24.43%. The endothermic peak at 135.4 1C in the DSC curve was attributed to the evaporation of adsorbed water molecules in the powder. The endothermic peaks at 255.9 and 528 1C could be attributed to the decomposition of precursors contained in the powders. The precursor powders obtained by spray pyrolysis had no peaks of crystal structure of Li4Ti5O12 as shown in Fig. 3. The reaction between Li and Ti components to form the Li4Ti5O12 phase did not occur because of the short residence time of the powders inside the hot wall reactor. Therefore, the precursor powders had the main peaks of anatase TiO2. The crystal 100
TG (%)
-24.43 %
60 40
0 135.4 °C 255.9 °C
20 -2
-528.0 °C
0
200
400 600 Temperature (°C)
800
(440)
(531)
(511)
o
o Li2Ti3O7 # Rutile TiO2 ∗ Anatase TiO2 (331)
(311) (222)
Intensity (arbitrary uint)
(400)
Fig. 2. TG/DSC curves of the Li4Ti5O12 precursor powders.
A 1000 A 900
A 800 A 700 #
#
# #
# ∗ A 600
#
P 700
∗
10
20
30
40
50
60
2θ Fig. 1. SEM photograph of the Li4Ti5O12 precursor powders.
DSC (mW/mg)
80
3. Results and discussion The mean size and morphology of the precursor powders obtained by spray pyrolysis are affected by the preparation conditions and the characteristics of the spray solution. Fig. 1 shows the SEM photograph of the precursor powders obtained by spray pyrolysis. The precursor powders had spherical shape and non-aggregation characteristics. One precursor particle was formed from one droplet with several micron sizes. However, the precursor powders had large and broad size distribution. The mean size of the precursor powders measured from the SEM photograph was 1.5 mm. In this work, the residence time of the droplets or powders inside the hot wall reactor was as short as 0.4 s. Therefore, the high drying and decomposition rates of droplets resulted in the precursor powders with large and broad
2
-12.19 %
(111)
tetraisopropoxide (TTIP, Ti[OCH(CH3)2]4) in distilled water. The total concentration of Li and Ti components dissolved in spray solution was 0.5 M. The as-prepared powders obtained by spray pyrolysis at a preparation temperature of 800 1C were post-treated in the box furnace at the temperature range between 600 and 1000 1C for 12 h in air atmosphere. The crystal structures of the as-prepared and post-treated powders were investigated using X-ray diffractometry (XRD, RIGAKU DMAX-33) using Cu Ka radiation (l ¼ 1.5418 1010 m) at room temperature in the 2y range 10–801. The morphological characteristics of the powders were investigated using scanning electron microscopy (SEM, JEOL JSM-6060). Surface area measurements were carried out by the Brunauer–Emmett–Teller (BET) method using N2 as the adsorbate gas. Measurement of the thermal properties of the precursor powders was performed on a thermo-analyzer (TG-DSC, Netzsch, STA409C, Germany) in the temperature range from 40 to 900 1C (10 1C/min). The charge/ discharge capacities of the prepared Li4Ti5O12 anode materials were measured. The anode electrode was made of 12 mg of Li4Ti5O12 compounds mixed with 4 mg of a conductive binder (3.2 mg of teflonized acetylene black and 0.8 mg of graphite). The lithium metal and polypropylene film were used as the counterelectrode and the separator, respectively. The electrolyte (TECHNO Semichem. Co.) was 1 M LiPF6 in a 1:1 mixture by volume of EC/DEC. The entire cell was assembled in a glove box under an argon atmosphere. The charge/discharge characteristics of the samples were measured through cycling in the 0.5–2.5 V potential range at various current densities.
41
Fig. 3. XRD spectra of the Li4Ti5O12 anode powders.
70
80
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Fig. 4. SEM photographs of the Li4Ti5O12 anode powders at different post-treatment temperatures: (a) A600; (b) A700; (c) A800; (d) A900 and (e) A1000.
ARTICLE IN PRESS S.H. Ju, Y.C. Kang / Journal of Physics and Chemistry of Solids 70 (2009) 40–44
2.5
Voltage (V)
2.0
1.5
1.0 A 800
0.5
A 600
0
30
A 700
60 90 120 Capacity (mAh/g)
A 900
A 1000
150
180
Fig. 5. Initial charge/discharge curves of the Li4Ti5O12 anode powders obtained at different post-treatment temperatures.
Capacity (mAh/g)
200
0.3C
0.2C
0.1C
0.5C
0.7C
1C
150
100
A 800
A 1000
50
A 900
0
A 700
0
5
10
15 20 Cycle Number
25
30
Fig. 6. Current density performances of the Li4Ti5O12 anode powders obtained at different post-treatment temperatures.
160
Capacity (mAh/g)
structures of the post-treated powders at temperatures between 600 and 1000 1C are shown in Fig. 3. The peaks of the Li4Ti5O12 phase appeared at a post-treatment temperature of 600 1C. However, the main peaks of the powders post-treated at a temperature of 600 1C were those of rutile TiO2. The powders post-treated at a temperature of 700 1C had the main peaks of the Li4Ti5O12 phase and the small impurity peaks of rutile TiO2. The powders post-treated at temperatures of 800 and 900 1C had the single phase of spinel Li4Ti5O12. The mean crystallite sizes of the Li4Ti5O12 powders post-treated at temperatures of 800 and 900 1C were 37 and 46 nm, respectively. The powders post-treated at a temperature of 1000 1C had main peaks of the Li4Ti5O12 phase and small impurity peaks of Li2Ti3O7. Decomposition of the Li4Ti5O12 phase occurred at a high post-treatment temperature of 1000 1C. The effects of the post-treatment temperatures on the morphologies of the Li4Ti5O12 anode powders are investigated in Fig. 4. The spherical shape of the precursor powders was maintained after post-treatment at temperatures below 800 1C. On the other hand, the powders post-treated at a temperature of 900 1C had spherical-like shape and aggregated morphology. The spherical shape of the precursor powders disappeared after post-treatment at a temperature of 1000 1C. The BET surface areas
43
120
80
40
0
0
5
10
15 20 Cycle Number
25
30
Fig. 7. Cycle property of the Li4Ti5O12 anode powders at a post-treatment temperature of 800 1C.
of the Li4Ti5O12 anode powders post-treated at temperatures of 700, 800 and 900 1C were 4.9, 1.6 and 1.5 m2/g, respectively. Therefore, the density of the Li4Ti5O12 anode powders with spherical shape increased on increasing the post-treatment temperature. Fig. 5 shows the initial charge–discharge curves between 0.5 and 2.5 V of Li4Ti5O12 powders obtained at different posttreatment temperatures. The profile of the discharge curves exhibited an extremely flat operating voltage of about 1.5 V, which is consistent with the results of previous studies [12,25]. During the charge process, a very flat change curve at around 1.6 V can be observed. This suggests a two-phase reaction based on the Ti4+/Ti3+ redox couple. The initial discharge capacities of Li4Ti5O12 powders were changed from 108 to 175 mAh/g when the posttreatment temperatures were changed from 700 to 1000 1C. The maximum initial discharge capacity of the Li4Ti5O12 powders was obtained at a post-treatment temperature of 800 1C. The powders obtained at a post-treatment temperature of 700 1C had a low discharge capacity of 108 mAh/g because of the impurity phase as shown in Fig. 3. Starting of decomposition of the Li4Ti5O12 phases at high post-treatment temperatures of 900 and 1000 1C decreased the discharge capacities of the anode powders. Fig. 6 shows the cycle properties of the Li4Ti5O12 powders by changing the current density from 0.1 to 1 C when the post-treatment temperatures were changed from 700 to 1000 1C. The Li4Ti5O12 powders obtained at post-treatment temperatures of 900 and 1000 1C showed poor cycle properties at high current densities. On the other hand, the Li4Ti5O12 powders obtained at a posttreatment temperature of 800 1C had good cycle properties below current densities of 0.7 C. Fig. 7 shows the cycle properties of Li4Ti5O12 powders obtained at a post-treatment temperature of 800 1C at a constant current density of 0.1 C after the 30th cycle test by changing the current density as shown in Fig. 6. The discharge capacities of the Li4Ti5O12 powders dropped from 154 to 142 mAh/g by the 30th cycle.
4. Conclusions The morphologies and crystal structures of the as-prepared and post-treated Li4Ti5O12 anode powders obtained by spray pyrolysis were investigated. The precursor powders with a spherical shape had the main peaks of anatase TiO2. The singlephase Li4Ti5O12 anode powders with a spherical shape and fine
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size were obtained at an optimum post-treatment temperature of 800 1C. The initial discharge capacities of Li4Ti5O12 powders were strongly affected by the post-treatment temperatures. The maximum initial discharge capacity of the Li4Ti5O12 anode powders was 175 mAh/g at a post-treatment temperature of 800 1C. The Li4Ti5O12 powders obtained at a post-treatment temperature of 800 1C had good cycle properties at various current densities. References [1] [2] [3] [4] [5]
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