Electrochemistry Communications 22 (2012) 81–84
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SnO2 encapsulated TiO2 hollow nanoﬁbers as anode material for lithium ion batteries Hyunjung Park a, 1, Taeseup Song b, 1, Hyungkyu Han b, Anitha Devadoss b, Junhan Yuh c, Changhwan Choi b, Ungyu Paik a,⁎ a b c
WCU Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea Department of Materials Science Engineering, Hanyang University, Seoul 133-791, Republic of Korea Applied Materials, Inc., Santa Clara, CA 95050, USA
a r t i c l e
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Article history: Received 9 May 2012 Received in revised form 29 May 2012 Accepted 29 May 2012 Available online 5 June 2012 Keywords: Lithium ion batteries Anode SnO2 TiO2
a b s t r a c t Nanoparticulate SnO2 was encapsulated into TiO2 hollow nanoﬁbers to achieve high energy density and robust electrochemical performance as an anode material for lithium ion batteries. The SnO2 encapsulated TiO2 hollow nanoﬁbers exhibit improved electrochemical performances over the TiO2 hollow nanoﬁbers, including a high discharge capacity of ~517 mAh g− 1 and doubled capacity at a 10 C rate. These improvements on electrochemical performances are attributed to favorable mechanics and kinetics associated with lithium. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Remarkable efforts have been devoted to expand the application of lithium ion batteries (LIBs) to electrical vehicles and power storage devices for renewable energy . Development of LIBs with high energy density, stable cycle life and fast charging/discharging rate performance is in great demand for broadening their applications. As the physicochemical properties of the electrode materials govern the electrochemical performances of LIBs, increasing attention has been paid to designing new electrode materials [2–4]. Anatase phase TiO2 is a promising anode material due to its abundance and structural stability associated with lithium . However, low power and energy densities of TiO2 limit its practical use. Extensive efforts have been devoted to improve the rate capability of TiO2 based electrodes using conducting metals and carbonaceous materials . Although signiﬁcant advancement in rate capability has been achieved in composite systems, the study on increasing the energy density has received little attention . SnO2 is also gaining momentum as a potential anode material owing to its high theoretical capacity (~800 mAh g− 1). The SnO2–TiO2 core–shell nanoparticles and carbon–TiO2–SnO2 nanocomposite electrodes have been reported to increase the energy density of TiO2 based electrodes [8,9]. However, these systems showed fast capacity fading since their electrode conﬁguration could not accommodate large volumetric change of SnO2 associated with lithium. In a previous study, we demonstrated that tailoring the morphology and surface ⁎ Corresponding author. Tel.: + 82 222200502. E-mail address: [email protected]
(U. Paik). 1 Both authors contributed equally to this work. 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.05.034
treatments, signiﬁcantly improves the mechanics and kinetics associated with lithium [10,11]. Here, we report SnO2 nanoparticles encapsulated in TiO2 hollow nanoﬁbers (SnO2/TiO2 HNFs) as anode materials for high energy density as well as high power density LIBs. The signiﬁcance of the electrode architecture and its impact on the electrochemical properties of the active materials were investigated in detail. 2. Experimental The coaxial electrospinning method was employed to synthesize SnO2/TiO2 HNFs . Two different precursor solutions were prepared. The inner solution containing a mixture of tetrabutyltin and mineral oil was stirred and heated at 130 °C for 2 days. The outer solution was prepared by mixing 3 g titanium (IV) isopropoxide (Ti(OiPr)4) with 2 ml acetic acid and 3 ml ethanol. The solution was added to the polymeric solution containing 0.3 g polyvinylpyrrolidone (Mw = 1,300,000) in 2 ml ethanol. The resultant solution was mixed for 20 min. The prepared inner and outer solutions were loaded into syringes connected to a metallic needle with the diameter of 1.2 mm (outer) and 0.5 mm (inner), respectively. The feeding rates of inner and outer solutions were set at 0.6 ml/h and 0.3 ml/h, respectively. This dual nozzle was connected to a high-voltage supply, and stainless steel (SUS) current collector was placed 15 cm away from the needle tip. A voltage of ~20 kV was applied to collect nanoﬁbers. Electrospun nanoﬁbers were transferred to a tube furnace and annealed at 500 °C in air for 1 h. Inductively coupled plasma atomic emission spectroscopy (ICP‐AES) results show that the molar ratio of Sn to Ti is 1:3. Coin-type half cells were fabricated to evaluate electrochemical properties of SnO2/TiO2 HNFs. The SnO2/TiO2 HNFs on the SUS foil was used as a working electrode
H. Park et al. / Electrochemistry Communications 22 (2012) 81–84
Scheme 1. ((a)–(c)) Morphological changes during lithiation/delithiation, ((d)–(f)) volumetric change of SnO2/TiO2 HNFs with different molar ratios of Sn to Ti during cycling.
without the conductive agent and binder. For the comparison study, a TiO2 nanoparticle (Degussa, P-25) electrode was prepared. The TiO2 nanoparticles were mixed with carbon black and a poly(vinylidene ﬂuoride) at a weight ratio of 80:10:10, respectively, in a solvent (N-methyl2-pyrrolidone). 1 M LiPF6 in a solvent mixture of ethylene carbonate and diethylene carbonate (1:1 vol.%) and lithium metal were used as an electrolyte and a counter electrode, respectively. 3. Results and discussion Scheme 1 illustrates geometric details of SnO2/TiO2 HNFs during cycling. The outer TiO2 shell maintains its mechanical stability during cycling as the volumetric change associated with lithium is negligible (b4%). The mechanical rigidness of the TiO2 shell and hollow inner space induces conﬁned volume expansion of encapsulated SnO2 nanoparticles during cycling, which beneﬁts the mechanics for the improvement in cycle retention and reversible morphological change. The SnO2 decomposes into a highly conducting Sn metal and inactive Li2O after the
ﬁrst cycle . These Sn metal particles, decorated on the inner surface of TiO2 hollow nanoﬁbers, could improve the rate capability via fast electron transport along 1D geometry of the electrode. Therefore, ideal architecturing of the geometry and modulating the composition of active materials enable improved structural stability and electrochemical kinetics together with an increased energy density. The ratio of Sn to Ti is important to achieve both a stable cycle performance and an increase in the capacity. Scheme 1(d), (e) and (f) display the schematic illustration of volumetric change of SnO2/TiO2 HNFs with different molar ratios of Sn to Ti. Based on electron microscopy images (Fig. 1) and ICP-AES results, the SnO2/TiO2 HNFs have a 30 nm thick TiO2 layer and a 13 nm thick SnO2 layer. In the case of low Sn content (Scheme 1(d)), the cycle retention is improved since the hollow inner space sufﬁciently accommodates the volume expansion of the SnO2. However, a noticeable capacity increase is not possible. In the optimum ratio of Sn to Ti (Scheme 1(e)), SnO2/TiO2 HNFs enable an increase in capacity and stable cycle performance. In the case of high Sn content (Scheme 1(f)), a signiﬁcant capacity increase can be achieved. However, large amounts
Fig. 1. (a) SEM image (inset: TEM image), (b) XRD patterns, ((c)–(f)) STEM image and the corresponding elemental mapping images of SnO2/TiO2 HNFs. ((g)–(i)) STEM image and the corresponding elemental mapping images of TiO2 HNFs.
H. Park et al. / Electrochemistry Communications 22 (2012) 81–84
Fig. 2. Electrochemical performances of TiO2 and SnO2/TiO2 HNFs. (a) First charge–discharge proﬁles of the TiO2 nanoparticles, TiO2 HNFs and SnO2/TiO2 HNFs electrodes at a rate of 0.2 C. (b) Cycle retentions and coulombic efﬁciencies. (c) Rate capability of TiO2 and SnO2/TiO2 HNFs electrodes at different discharge rates.
of SnO2 in the hollow space lead to fast capacity fading since the hollow inner space could not accommodate the volume expansion of the SnO2. The nanoﬁbers have an average diameter of ~ 250 nm. (Fig. 1(a)) The SnO2/TiO2 HNFs have an inner diameter of ~200 nm and outer shell thickness of ~30 nm. X-ray diffraction (XRD) analysis showed that both TiO2 and SnO2 are anatase and tetragonal rutile phases, respectively. Spatial distribution of Ti, Sn, and O elements were analyzed using STEM elemental mapping (Fig. 1(c–f)). The Ti and O element signals were uniformly observed over the whole region of the hollow nanoﬁber. Conversely, Sn element signals were detected only in the hollow region. Elemental mapping of hollow TiO2 nanoﬁber clearly supports that only Ti and O are present. The electrochemical performances of TiO2 HNFs and SnO2/TiO2 HNFs were evaluated over a potential window of 0.01–3 V vs. Li/Li +. Fig. 2(a) shows the voltage proﬁles of the TiO2 nanoparticles, TiO2 HNFs and SnO2/TiO2 HNFs electrodes at the ﬁrst cycle. The voltage plateaus around 1.75 V and 1.90 V correspond to Li+ ion intercalation into and de-intercalation from the interstitial octahedral sites of anatase
TiO2. The receded voltage plateaus in the SnO2/TiO2 HNFs are attributed to the incorporation of the SnO2 phase. The SnO2/TiO2 HNFs showed a voltage plateau around 0.5 V during discharging, which is related to the dealloying process of LixSn. TiO2 HNFs exhibit the ﬁrst charge and discharge capacities of ~636 mAh g− 1 and ~433 mAh g− 1, respectively. The TiO2 nanoparticle electrode exhibits a charge capacity of ~694 mAh g − 1 and the initial coulombic efﬁciency of ~38%. These capacities of TiO2 electrodes are higher than that of typical TiO2 material evaluated in the voltage window of 1.5–3 V. In this experiment, the potential window was set from 0.01 V to 3 V to fully utilize the SnO2. The TiO2 [email protected]
composite material was also evaluated in the same voltage window to utilize CNT as an anode material . The SnO2/TiO2 HNFs showed increased ﬁrst charge and discharge capacities of ~802 mAh g − 1 and ~517 mAh g − 1, respectively. The discharge capacity (~517 mAh g − 1) of SnO2/TiO2 HNFs is much higher compared to those of previously reported TiO2 based composite electrode . The SnO2/TiO2 hollow nanoﬁbers with the ratio of Sn to Ti = 1:4 showed the discharge capacity of 476 mAh g − 1 and excellent cycle performance. (The data is not shown here.) Both TiO2 and SnO2/ TiO2 HNFs exhibit the stable cycle performance over 100 cycles. The SnO2/TiO2 HNFs showed improved cycle retention compared to previously reported SnO2–TiO2 composite electrodes such as mesoporous C–TiO2–SnO2 nanocomposites, [email protected]
hollow spheres and SnO2 nanocrystal coated TiO2 nanotube [8,9,15]. The cycle performance results reveal that the void space in hollow nanoﬁber and mechanically rigid TiO2 shell enables the improvement in the mechanics of SnO2 associated with cycling. The rate capabilities of TiO2 and SnO2/TiO2 HNFs electrodes were evaluated (Fig. 2(c)). The SnO2/TiO2 HNFs electrode delivers discharge capacities of ~496, ~380 and 244 mAh g − 1 at rates of 0.5, 2 and 10 C, respectively. The SnO2/TiO2 HNFs exhibit improved rate performance compared to that of TiO2 HNFs, which is attributed to fast electron transport along the Sn particulates. As described in Scheme 1((a) and (b)), the SnO2 completely transforms into highly conducting Sn and Li2O after the ﬁrst cycle. Fig. 3 shows the structural changes of SnO2/TiO2 HNFs during the ﬁrst cycle. Despite the tremendous volume change, the fully lithiated SnO2/TiO2 HNFs maintained its morphology and dimension without mechanical degradations. A remarkable contrast in the brightness, with lighter edges and a dark center, is observed in the TEM image of the fully lithiated SnO2/TiO2 HNFs, which implies that the hollow space of nanoﬁbers accommodates volume expansion of SnO2 and rigid TiO2 shell induces its volume expansion toward the hollow space. HR-TEM image and selective area electron diffraction (SAED) pattern reveal that TiO2 shell retains its crystalline structure. At the fully lithiated state, it is difﬁcult to index the ﬁnal phases for the LixTiO2 and Lix Sn due to the overlap of electron patterns. After the ﬁrst cycle, its morphological features are identical with those of pristine ones. Electron diffraction patterns of the SnO2 in SAED image (Fig. 3(f)) disappear after cycling, which indicates the decomposition of the crystalline SnO2. 4. Conclusions The SnO2/TiO2 HNFs enable favorable mechanical properties and kinetics associated with lithiation/delithiation and show superior electrochemical properties. Our electrode design strategy could be applied to other types of alloy electrode materials, which experience large volume change associated with lithium. Acknowledgment This work was ﬁnancially supported by the National Research Foundation of Korea through grant no. K20704000003TA050000310, the Global Research Laboratory Program provided by the Korean Ministry of Education, Science and Technology in 2011, the International Cooperation program of the Korea Insitute of Energy Technology Evaluation and
H. Park et al. / Electrochemistry Communications 22 (2012) 81–84
Fig. 3. TEM images of (a) pristine SnO2/TiO2 HNFs, (b) fully lithiated SnO2/TiO2 HNFs, (c) SnO2/TiO2 HNFs after 1 cycle, and (d–f) HR-TEM image and SAED pattern, respectively.
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