COMMUNICATION DOI: 10.1002/asia.201301366

Preparation of Li4Ti5O12 Yolk–Shell Powders by Spray Pyrolysis and their Electrochemical Properties Kwang Min Yang,[a] You Na Ko,[a] Jung-Yeul Yun,[b] and Yun Chan Kang*[a]

Abstract: We have reported for the first time the preparation of yolk–shell-structured Li4Ti5O12 powders for use as anode materials in lithium-ion batteries. One Li4Ti5O12 yolk–shell-particle powder is directly formed from each droplet containing lithium, titanium, and carbon components inside the hot wall reactor maintained at 900 8C. The precursor Li4Ti5O12 yolk–shell-particle powders, which are directly prepared by spray pyrolysis, have initial discharge and charge capacities of 155 and 122 mA h g 1, respectively, at a current density of 175 mA g 1. Post-treatment of the yolk–shell-particle powders at temperatures of 700 and 800 8C improves the initial discharge and charge capacities. The initial discharge capacities of the Li4Ti5O12 powders with a yolk–shell structure and a dense structure post-treated at 800 8C are 189 and 168 mA h g 1, respectively. After 100 cycles, the corresponding capacities are 172 and 152 mA h g 1, respectively (retentions of 91 and 90 %).

attention in high power batteries, which demand rapid charging and discharging. Yu et al. have demonstrated an efficient template-based approach for the preparation of mesoporous Li4Ti5O12 hollow spheres with high quality.[23] The as-prepared Li4Ti5O12 hollow structures exhibited a remarkable rate capability up to 20 C and stable long-term capacity retention for over 300 cycles. Similarly, Huang and Jiang reported the preparation of hollow spherical Li4Ti5O12 by the macroemulsion method.[24] The hollow spherical Li4Ti5O12 thus obtained had a good capacity retention over 500 charge/discharge cycles at 2 C. In addition, the specific capacity stayed very stable at the value of 140 mA h g 1 with a loss of only 0.01 % per cycle. Yolk–shell particle powders with distinctive core@void@shell configuration are reported to have a high volumetric energy density, as well as offer the unique advantages of hollow materials, when used as the electrode material for LIBs.[25–32] The filled core of such yolk–shell-structured powders improves the rate capability as well as the volumetric energy density of the powders by increasing the weight fraction of the electrochemically active component. Thus far, various types of yolk–shell particle materials prepared by wet chemical methods have been used as anode and cathode materials for LIBs.[25–30] However, to the best our knowledge, the preparation of yolk–shell-structured Li4Ti5O12 powders has not been reported yet. Recent studies have successfully demonstrated the facile and scalable synthesis of yolk–shell-structured oxide materials with excellent electrochemical properties by spray pyrolysis of liquid droplets containing metal salts and a carbon source material.[31, 32] This method could be effectively applied for the preparation of several types of yolk–shell-structured powders, irrespective of the composition. In this study, yolk–shell-structured Li4Ti5O12 powders were prepared for the first time, and their electrochemical properties were compared to those of powders with a dense structure prepared under similar conditions. Figure 1 shows the morphology of the precursor powders, which were directly prepared by spray pyrolysis of spray solutions with and without sucrose. In this study, the excess amount of lithium dissolved into the spray solutions without and with sucrose to form the stoichiometric Li4Ti5O12 powders with the optimum electrochemical properties was found to be 5 and 20 %, respectively. In case of samples prepared by the addition of sucrose to the spray solution, the heat evolution due to rapid decomposition of sucrose increased

Spinel lithium titanate (Li4Ti5O12) is considered to be an attractive anode material for Li-ion batteries (LIBs) due to its good Li + insertion and de-insertion reversibility, zero strain volume change during charge and discharge cycling, and excellent safety performance.[1–9] However, Li4Ti5O12 has a poor rate capability and an initial capacity loss because of its low electrical conductivity of about 10 13 S cm 1.[10, 11] Recently, some studies have reported that the rate performance of Li4Ti5O12 could be improved by forming composites with conductive materials, such as carbon, graphene, carbon nanotubes, and metals.[11–15] In addition, nanostructuring of Li4Ti5O12 to form nanoparticles and hollow spheres has been reported to be an alternative strategy to improve its rate performance.[16–24] Especially, Li4Ti5O12 hollow spheres with large surface areas and open pores have attracted increased [a] K. M. Yang,+ Y. N. Ko,+ Prof. Y. C. Kang Department of Chemical Engineering Konkuk University 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701 (Korea) E-mail: [email protected] [b] Dr. J.-Y. Yun Powder Technology Department Korea Institute of Materials Science 797 Changwondaero, Seongsan-gu„ Changwon, Gyeongnam, 642-831 (Korea) [+] These two authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201301366.

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Figure 2. XRD patterns of the precursor and post-treated Li4Ti5O12 powders prepared from spray solutions with and without sucrose: a) with sucrose, precursor; (b) with sucrose, 700 8C; (c) with sucrose, 800 8C; (d) with sucrose, 900 8C; (e) without sucrose, 800 8C.

Figure 2 shows the XRD patterns of the precursor and post-treated Li4Ti5O12 powders. The yolk–shell powders were post-treated at temperatures between 700 and 900 8C, and the dense powders were post-treated at 800 8C. The yolk–shell powders were directly prepared by spray pyrolysis for a short residence time of the powders (24 s) inside the hot wall reactor maintained at 900 8C. The resulting sample had mainly the crystal structure of Li4Ti5O12, with negligible impurity peaks corresponding to the lithium-deficient Li2Ti3O7 phase. On the other hand, the XRD pattern of the post-treated Li4Ti5O12 yolk–shell powders had no impurity peaks, irrespective of the post-treatment temperatures. The XRD pattern of the powders with a dense structure post-treated at 800 8C also have a phase-pure Li4Ti5O12 structure. The mean crystallite sizes of the Li4Ti5O12 yolk– shell powders post-treated at 700, 800, and 900 8C, calculated from the full width half maximum (FWHM) of the (111) peak by using Scherrers equation, were found to be 56, 84, and 67 nm, respectively. The mean crystallite size of the Li4Ti5O12 powders with a dense structure post-treated at 800 8C was estimated to be 84 nm. Figure 3 shows the morphology of the Li4Ti5O12 yolk–shell powders post-treated at 800 8C, for which the best electrochemical properties were observed. The spherical shape and yolk–shell structure of the precursor powders were maintained even after post-treatment at 800 8C. Clear core parts of the yolk–shell powders were observed in TEM and dotmapping images (Figure 3). The representative high-resolution TEM image (Figure 3 d) indicated the high crystallinity and large grain size of the post-treated Li4Ti5O12 powders. Figure S4 in the the Supporting Information shows the SEM images of the Li4Ti5O12 powders post-treated at 700 and 900 8C. The powders post-treated at 700 8C had a morphology similar to that of the precursor yolk–shell powders, as shown in Figure 1 b. On the other hand, the powders post-treated at 900 8C had a non-spherical shape and hardly aggregated. Sintering and partial decomposition of the Li4Ti5O12 powders occurred at a high post-treatment temperature of 900 8C. Figure S5 in the Supporting Information shows the morphology of the Li4Ti5O12 powders prepared without su-

Figure 1. Morphologies of the precursor powders directly prepared by spray pyrolysis. (a) SEM image of the precursor powders prepared from a spray solution without sucrose. (b–d) SEM and TEM images of the precursor powders prepared from a spray solution with sucrose.

the evaporation rate of the lithium component during the formation of the Li4Ti5O12 yolk–shell powders. The precursor powders prepared without sucrose in the spray solution had a spherical shape, dense structure, and clean surface (Figure 1 a and Figure S2 in the Supporting Information). By contrast, the precursor powders prepared with sucrose in the spray solution had a spherical shape, yolk–shell structure with a single shell, and a rough surface (Figure 1 b–d). The mechanism underlying the formation of yolk–shell powders during spray pyrolysis has already been rationalized in our previous studies reported elsewhere.[31, 32] Typically, due to the rapid drying and decomposition of metal salts and sucrose during spray pyrolysis, an intermediate composite powder containing Li, Ti, and carbon components was first formed in the front region of the reactor maintained at 900 8C. Subsequently, combustion of the outer surface and contraction of the inner part, followed by combustion of the inner part in the rear region of the reactor, resulted in the formation of yolk–shell powders. The rapid combustion of the carbon source increased the temperature of the powder during the formation of the yolk–shell particle structure. Therefore, crystallization of the powders during the formation of the yolk–shell particle structure resulted in Li4Ti5O12 powders with a rough surface. High-resolution TEM imaging of the precursor powders (Figure 1 d) clearly reveals a lattice fringe spacing of 0.48 nm, which corresponds to the (111) plane of spinel Li4Ti5O12 (JCPDS Card No. 26-1198). The mean particle size of the yolk–shell and dense powders, as measured from the SEM images, was estimated to be 1.0 and 0.7 mm, respectively. The difference in the mean size is likely due to changes in the internal structure of the powders prepared with and without sucrose. Figure S3 in the Supporting Information illustrates the formation of Li4Ti5O12 yolk–shell powders in the spray pyrolysis process.

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Figure 3. Morphologies and dot-mapping images of the Li4Ti5O12 yolkshell powders post-treated at 800 8C. (a) SEM image. (b,c) TEM images. (d) High-resolution TEM image. (e) Dot-mapping images.

Figure 4. Electrochemical properties of the Li4Ti5O12 powders prepared from the spray solutions with and without sucrose at a constant current density of 175 mA g 1. (a) Initial discharge and discharge curves. (b) Cycling performances.

crose in the spray solution after post-treatment at 800 8C. The low- and high-resolution TEM images of the powders showed the formation of a dense structure without voids and a well-faceted crystal structure. The BET surface areas of the Li4Ti5O12 powders with a yolk–shell and a dense structure post-treated at 800 8C were 5 and 3 m2 g 1, respectively. Figure 4 a shows the initial charge–discharge curves of the Li4Ti5O12 powders with a yolk–shell and a dense structure at a constant current density of 175 mA g 1. Both the yolk– shell and the dense powders had similar voltage profiles with plateaus at the potential of around 1.5 V, which corresponds to the two-phase reaction between Li4Ti5O12 and Li7Ti5O12.[33] The precursor Li4Ti5O12 yolk–shell powders had initial discharge and charge capacities of 155 and 122 mA h g 1, respectively. Post-treatment of the yolk–shell powders at temperatures of 700 and 800 8C improved the initial discharge and charge capacities. However, the aggregated powders post-treated at 900 8C had lower initial discharge and charge capacities compared to those of the precursor powders. The initial discharge capacity of the Li4Ti5O12 powders with a yolk–shell structure and a dense structure posttreated at 800 8C was 189 and 168 mA h g 1, respectively. Figure 4 b shows the cycling performances of the Li4Ti5O12 powders with a yolk–shell structure and a dense structure posttreated at various temperatures. The yolk–shell-structured Li4Ti5O12 powders had good cycling performances, irrespective of the post-treatment temperatures. The discharge capacity of the yolk–shell powders before and after post-treatment at 700 and 800 8C after 100 cycles were 117, 141, and 172 mA h g 1, respectively, and the corresponding capacity retentions were 75, 89, and 91 %. However, the discharge capacity of the aggregated Li4Ti5O12 powders post-treated at 900 8C was 101 mA h g 1 after 100 cycles. The discharge capacity of the Li4Ti5O12 powders with a dense structure post-

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treated at 800 8C after 100 cycles was found to be 152 mA h g 1. The yolk–shell powders having a high contact area with the electrolyte had higher capacities at a current density of 175 mA g 1 compared to the powders with the dense structure. In summary, we report the facile synthesis of yolk–shellstructured Li4Ti5O12 powders by spray pyrolysis applying liquid droplets containing metal salts and sucrose. Yolk– shell-structured Li4Ti5O12 powders with a high crystallinity were directly prepared by spray pyrolysis, even at a short residence time of the powders inside the hot wall reactor maintained at 900 8C. However, post-treatment of the powders below 800 8C improved the electrochemical properties of the yolk–shell powders. The yolk–shell-structured Li4Ti5O12 powders having a high contact area with the electrolyte exhibited higher discharge and charge capacities than the powders with a dense structure prepared under similar conditions.

Experimental Section Li4Ti5O12 powders with either a yolk–shell structure or a dense structure were prepared by ultrasonic spray pyrolysis. A schematic illustration of the ultrasonic spray pyrolysis system used for the synthesis of Li4Ti5O12 powders is shown in Figure S1 in the Supporting Information. It consisted of a 1.7 MHz ultrasonic spray generator with twenty resonators to generate droplets. A quartz reactor with a length of 200 cm and a diameter of 10 cm was used. The reactor temperature was maintained at 900 8C. The flow rate of the air used as the carrier gas was 10 L min 1. Titanium(IV) tetraisopropoxide (TiACHTUNGRE[OCHACHTUNGRE(CH3)2]4, Junsei, 98 %) and lithium nitrate (LiNO3, Junsei, 98 %) were used as the source materials of Ti and Li components. The total concentration of the Li and Ti components was

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fixed at 0.2 m. The concentration of sucrose, which was used as the carbon source material for the preparation of yolk–shell Li4Ti5O12 powders, was fixed at 0.7 m. The excess amount of lithium dissolved in the spray solution without and with sucrose for the formation of stoichiometric Li4Ti5O12 powders was 5 and 20 %, respectively. To improve the crystallinity and remove potential impurity phases, the as-prepared powders obtained by the spray pyrolysis procedure were post-treated in a box furnace at temperatures between 700 and 900 8C for 3 h under an air atmosphere.

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The crystal structures of the powders were investigated by X-ray diffractometry (XRD, X’Pert PRO MPD) using Cu Ka radiation (l = 1.5418 ) at the Korea Basic Science Institute (Daegu). The morphological characteristics of the powders were investigated using field-emission transmission electron microscopy (FETEM, JEOL, JEM-2100F). The surface areas of the powders were measured by the Brunauer–Emmett–Teller (BET) method using N2 as the adsorbate gas. The anode was prepared by mixing Li4Ti5O12, carbon black, and polyvinylidene fluoride (PVDF) in the weight ratio of 7:2:1. Lithium metal and a microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte was 1 m LiPF6 in a 1:1 mixture (by volume) of ethylene carbonate–dimethyl carbonate (EC–DMC) with 2 % vinylene carbonate. The entire cell was assembled in a glove box under an argon atmosphere. The charge–discharge characteristics of the samples were determined by performing potential cycling in the range 1–3 V at a current density of 175 mA g 1.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012R1A2A2A02046367). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1B3002382).

Keywords: anode materials · lithium-ion batteries · lithium titanate · nanoparticles · spray pyrolysis

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Chem. Asian J. 2014, 9, 443 – 446

Yun Chan Kang et al.

Received: October 11, 2013 Published online: November 26, 2013

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Preparation of Li4Ti5O12 yolk-shell powders by spray pyrolysis and their electrochemical properties.

We have reported for the first time the preparation of yolk-shell-structured Li4Ti5O12 powders for use as anode materials in lithium-ion batteries. On...
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