Enhanced electrochemical performance of carbon-coated TiO2 nanobarbed fibers as anode material for lithium-ion batteries

Enhanced electrochemical performance of carbon-coated TiO2 nanobarbed fibers as anode material for lithium-ion batteries

    Enhanced electrochemical performance of carbon-coated TiO 2 nanobarbed fibers as anode material for lithium-ion batteries De Pham-Con...

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    Enhanced electrochemical performance of carbon-coated TiO 2 nanobarbed fibers as anode material for lithium-ion batteries De Pham-Cong, Jae-Hyun Kim, Se-Young Jeong, Jun Hee Choi, Jinwoo Kim, Chae-Ryong Cho PII: DOI: Reference:

S1388-2481(15)00262-3 doi: 10.1016/j.elecom.2015.09.018 ELECOM 5550

To appear in:

Electrochemistry Communications

Received date: Revised date: Accepted date:

16 August 2015 20 September 2015 20 September 2015

Please cite this article as: De Pham-Cong, Jae-Hyun Kim, Se-Young Jeong, Jun Hee Choi, Jinwoo Kim, Chae-Ryong Cho, Enhanced electrochemical performance of carboncoated TiO2 nanobarbed fibers as anode material for lithium-ion batteries, Electrochemistry Communications (2015), doi: 10.1016/j.elecom.2015.09.018

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Enhanced electrochemical performance of carbon-coated TiO2 nanobarbed

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fibers as anode material for lithium-ion batteries

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De Pham-Conga, Jae-Hyun Kimb, Se-Young Jeonga, Jun Hee Choic, Jinwoo Kimd,*, Chae-

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Ryong Choa,*

Department of Nano-Fusion Technology and College of Nanoscience and Nanotechnology, Pusan National

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University, Busan 609–735, South Korea

Division of Nano-Bio Technology, Daegu Gyeongbuk Institute of Science and Technology, Daegu 711–873,

Samsung Advanced Institute of Technology, Samsung Electronics, Suwon 443–803, South Korea

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Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, IL 61801,

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South Korea

ABSTRACT

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We report the electrochemical performance of carbon-coated TiO2 nanobarbed fibers ([email protected] NBFs) as anode material for lithium-ion batteries. The [email protected] NBFs are composed of TiO2 nanorods grown on TiO2 nanofibers as a core, coated with a carbon shell. These nanostructures form a conductive network showing high capacity and C-rate performance due to fast lithium-ion diffusion and effective electron transfer. The [email protected] NBFs show a specific reversible capacity of approximately 170 mAh g−1 after 200 cycles at a 0.5 A g−1 current density, and exhibit a discharge rate capability of 4 A g−1 while retaining a capacity of about 70 mAh g−1. The uniformly coated amorphous carbon layer plays an important role to improve the electrical conductivity during the lithiation-delithiation process.

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ACCEPTED MANUSCRIPT Keywords: Li-ion batteries, Anode, Nanostructure, Carbon-coating Corresponding

authors.

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address:

[email protected]

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[email protected] (J. Kim)

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(C.R.

Cho),

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1. Introduction Titanium-based materials, such as TiO2 and Li4Ti5O12, have emerged as promising anode

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materials for rechargeable lithium-ion batteries (LIBs), owing to their low cost and structural

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stability during Li+ insertion-extraction. Their use is of particular interest in electric or hybrid

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electric vehicle systems that require batteries with long cycle life [1–3]. However, the practical use of TiO2 in LIBs is limited by its low theoretical capacity (335 mAh g−1), poor

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electron transport, and inherently low electrical conductivity [4]. To overcome the drawbacks of TiO2, many strategies have been investigated including increasing the Li+ diffusion length,

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and developing conductive networks, charge transport paths, and nanostructures [5–12]. Although considerable progress has been made in understanding the anodic structure and

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composition of various TiO2-based electrodes, there is an urgent need to integrate all these

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desirable characteristics into such electrodes. Several research results have been reported on

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hyperbranch designs developed to increase the specific surface area of metal oxide nanostructures [13–17]. However, the volume change occurring during the lithiationdelithiation process may still break the electrical contact pathways in metal oxide electrodes.

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Although TiO2 nanobarbed fibers (TiO2 NBFs), which are composed of nanorods (NRs) on nanofibers (NFs) with high specific surface area, have been synthesized, they lacked a protective coating to maintain their structural integrity during Li+ insertion-extraction [18]. Thus, carbon-coated TiO2 NBFs ([email protected] NBFs) as an anode material for LIBs with both a high density of active sites and high-conductivity surface have not been reported so far. Here, we demonstrate the electrochemical performance of amorphous [email protected] NBFs as anode materials in LIBs. We also investigated the reaction mechanism of the electrodes through cycling stability and rate capability measurements.

2. Experimental 3

ACCEPTED MANUSCRIPT The TiO2 NBFs were synthesized according to the procedure described in our previous report [18]. The sample was annealed at 500 °C for 3 h in air. To coat the TiO2 NBFs with a

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carbon layer, 0.3 g of TiO2 NBFs and 0.25 M glucose in 40 mL of deionized water were

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sealed in a Teflon-lined autoclave and maintained at 180 °C for 3 h. The collected product

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was post-annealed in Ar at 500 °C for 3 h to increase the carbon density and minimize reaction between TiO2 and carbon, yielding the [email protected] NBFs. The electrochemical

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properties of the samples were investigated after preparing coin cells. The samples were mixed with carbon black and carboxymethyl cellulose in a weight ratio of 60:20:20,

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respectively, and coated on a copper foil. The mass of the active material in each working electrode disc with a diameter (D) of ~1.4 cm and thickness (t) of ~30 µm was calculated to

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be approximately 0.9–1.0 mg cm−2. The electrolyte was 1 M LiPF6 in ethylene

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carbonate/dimethyl carbonate (1:1 vol%) with addition of 1% vinylene carbonate; Li foil (D

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=~1.6 cm, t =~1.3 mm) served as the counter electrode.

3. Results and discussion

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3.1 Characterization of [email protected] NBFs Figure 1a shows the scanning electron microscopy (SEM) surface morphologies and a possible stepwise mechanism for the formation of the [email protected] NBFs. First, crystalline 200nm-diameter TiO2 nanofibers were obtained by annealing as-electrospun TiO2 nanofibers in air. Crystalline 600-nm-length TiO2 nanorods were long enough to maintain the shape and crosslinking of TiO2 NBFs. Finally, the TiO2 NBFs were coated with glucose by hydrothermal treatment. In Fig. 1b, the diffraction peaks are in good agreement with those of the X-ray diffraction (XRD) patterns of anatase/rutile (JCPDS card No. 21-1272/21-1276) TiO2. The rutile phase was observed on the TiO2 NBFs, which were attributed to the TiO2 nanorods. Fig. 1c shows four strong Raman peaks at 148, 396, 516, and 636 cm−1; they were 4

ACCEPTED MANUSCRIPT assigned to the Eg, B1g, and A1g modes of both TiO2 nanofibers and TiO2 NBFs [19]. Notably, a decrease in the intensity of these peaks was observed in the [email protected] NBF spectrum,

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resulting from the presence of the coated carbon layer (approximately 11 wt%) (Fig. 1d).

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Characteristic peaks located at 1338 and 1588 cm−1 were assigned to disordered, or

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amorphous, carbon (D band) and graphitic carbon (G band), respectively. The microstructure of the [email protected] NBFs was investigated using high-resolution transmission electron

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microscopy (HRTEM). The amorphous carbon layer with a thickness of 3.1 ± 0.5 nm collected from the region outlined by the red square was uniform, and the TiO2 nanorods had

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a high density and a uniform distribution (bottom left of Fig. 1e). The selected area electron diffraction (SAED) lattice pattern of the TiO2 nanorod region (bottom right of Fig. 1e)

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obtained using Fourier transformation of the diffraction pattern confirmed the presence of

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rutile phase single-crystalline TiO2, which agreed with the XRD results. The specific surface areas and total pore volumes of the TiO2 nanofibers, TiO2 NBFs, and [email protected] NBFs

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measured using the Brunauer-Emmett-Teller method were 17, 100, and 110 m2 g−1 and 0.13, 0.82, and 0.97 cm3 g−1, respectively (data not shown). The increased specific surface area of

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the [email protected] NBFs may be attributed to the defective and low-crystallinity carbon layer [20].

3.2 Electrochemical performance of TiO2-based-nanostructures Figure 2a shows that the samples have only modest capacity retention over the first 200 charge-discharge cycles in the 1–3 V potential range at a 0.5 A g−1 current density. However, the reversible capacity value of the [email protected] NBFs (170 mAh g−1) dramatically exceeds those of both the TiO2 NBFs (135 mAh g−1) and TiO2 nanofibers (48 mAh g−1), which is attributed to the unique nanostructure and carbon layer. The initial irreversible capacity loss of the electrodes may be likely due to the inevitable formation of a solid-electrolyte interphase (SEI) layer and electrolyte decomposition [21]. The fast capacity fading of the [email protected] 5

ACCEPTED MANUSCRIPT NBFs compared to the TiO2 NBFs may be caused by several factors. First, carbon layers have similar properties to graphite, which formed an SEI layer during the cycling test, although it

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can improve electron mobility during charge-discharge processes [22]. Second, the carbon

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layer with a thickness of ~3.1 nm may be hardly strong enough to suppress the 4% volume

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expansion of TiO2. In addition, charge-discharge cycling tests with a lower positive limit of 2.5 V were also performed. The discharge capacity of the [email protected] NBFs was retained up to

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50 cycles at ~185 mAh g−1; this value is still higher than that of the TiO2 NBFs (125 mAh g−1) and 2.5 times that of the TiO2 nanofibers (~52 mAh g−1). These values are lower than

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those obtained with the limit of 3 V because of the lower charge voltage. Furthermore, galvanostatic charge-discharge curves for [email protected] NBFs with several representative cycles

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are shown in Fig. 2b. The voltages during the charge-discharge process remain constant,

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keeping the plateau aside from slightly decreasing capacity with increasing cycles. The redox couple (Ti4+/Ti3+) reactions of the Li+ insertion-extraction for the [email protected] NBFs are highly

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reversible at a scan rate of 0.1 mV s−1, as described by the cyclic voltammetry curves (inset of Fig. 2b), where cathodic Li+ insertion occurs at 1.67 V vs. Li/Li+ and anodic extraction at 2.0

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V vs. Li/Li+. The electrochemical redox reaction can be expressed as follows: TiO2 + xLi+ + xe− ↔ LixTiO2 (x ≤ 1) [23]. In Fig. 2c, the charge-discharge capacity of the TiO2 NBFs is approximately 200 mAh g−1 under a 0.05 A g−1 current density, nearly two times higher than that of the TiO2 nanofibers. Although the high contact area between the TiO2 NBFs and the electrolyte can support a Li+ ion transfer faster than that observed in TiO2 nanofibers, the NBFs still possess limited charge-discharge capacity due to their low conductivity at high current densities. Conversely, in the case of the [email protected] NBFs, a maximum discharge capacity of 250 mAh g−1 was achieved with a minimum capacity of 70 mAh g−1 in the 0.05–4 A g−1 current density range. The superior capacity of [email protected] NBFs compared to TiO2 NBFs may be due to high conductivity as well as a large specific surface area, resulting from the 6

ACCEPTED MANUSCRIPT presence of the uniform carbon coating layer [20]. After applying a high C-rate and then returning to a value of 0.15 C, the value remained almost constant for the [email protected] NBFs.

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The cycle testing results at 0.15 C show a stable and constant value. Subsequently,

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electrochemical impedance spectroscopy analysis of the three samples before cycling was

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conducted in the 0.05 Hz to 100 kHz frequency range by applying an AC voltage of 10 mV amplitude (Fig. 2d). The Nyquist plots show that the charge transfer resistance follows a

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descending order: TiO2 nanofibers (188 Ω) < TiO2 NBFs (160 Ω) < [email protected] NBFs (52 Ω). This suggests that the electrical conductivity of the [email protected] NBFs is greater than those of the

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TiO2 NBFs and TiO2 nanofibers. The rate capability of [email protected] NBF electrodes is comparable to those reported in the literature for different systems (Fig. 2e), such as

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nitridated TiO2 hollow-nanofibers [6], TiO2 with nanoglues [7], rutile-TiO2-functionalized

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graphene sheets [24], c-SWCNT-TiO2 [25], and CNT-TiO2 film [26]. Figure 2f presents a

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schematic of the charge transfer mechanism accounting for the dramatically improved capacity, rate capability, and cycling performance of the [email protected] NBFs. Several factors may explain the enhanced lithium storage properties of the [email protected] NBFs. First, the carbon

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coating layer on both the TiO2 nanofibers and nanorods acts as a charge transfer pathway. The contact points provided by the TiO2 nanorods between the [email protected] NBFs help form a continuous path, improving the electrical conductivity during the lithiation-delithiation process. Furthermore, the space between the nanorods on the NBFs may allow the fast transfer of Li+ ions in the depth direction of the electrode, enhancing the anode electrochemical performance, even under high current density conditions.

4. Conclusions In summary, although the electrochemical performance of the TiO2 NBFs was enhanced,

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ACCEPTED MANUSCRIPT compared to that of the TiO2 nanofibers, the successfully synthesized [email protected] NBFs with the carbon layer showed long-term cycling stability up to 200 cycles, as well as a high capacity

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of 170 mAh g−1 at a 0.5 A g−1 current density. This approach may be applicable for high-

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performance anodes for next-generation LIBs.

Conflict of interest

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There is no conflict of interest.

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Acknowledgements

This work was supported by a research program (grant NRF-2015R1D1A3A01018611)

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through the Ministry of Education and DGIST R&D Program (No. 15-EN-01) funded by the

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Figures and Figure Captions

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standing and binder-free Li-ion anode, J. Mater. Chem. A 2 (2014) 2701-2707.

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Fig. 1. (a) Schematic of the formation process of [email protected] NBFs and corresponding SEM images for each step. (b) XRD patterns and (c) Raman spectra of three samples. (d)

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Thermogravimetric weight loss curve of [email protected] NBFs. (e) TEM image of [email protected] NBFs (top); HRTEM image of the region in the red square (bottom left), and its SAED lattice

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pattern (bottom right).

Fig. 2. (a) Cycling performance and Coulombic efficiency of three electrodes at a 0.5 A g−1 current density. (b) Charge-discharge profiles of [email protected] NBFs at a 0.5 A g−1 current density; the inset shows the cyclic voltammetry curves at a scan rate of 0.1 mV s−1. (c) Specific capacity of three electrodes at various current densities from 0.05 A g−1 to 4 A g−1. (d) Nyquist plots for three electrodes before cycling test. (e) Comparison of the electrochemical data of our [email protected] NBFs and reported TiO2-based electrodes. (f) Schematic showing the charge-transfer mechanism in [email protected] NBFs during Li-ion charge-discharge.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights TiO2 nanobarbed fibers were prepared using electrospinning and hydrothermal process.



Carbon layer was coated on TiO2 nanobarbed fiber by using glucose as carbon source.



Carbon linking networks were considered for high performance lithium ion batteries.



The physical and electrochemical properties of hierarchical nanostructure were studied.

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