Electrospun TiO2−δ Nanofibers as Insertion Anode for Li-Ion Battery

Article pubs.acs.org/JPCC

Electrospun TiO2−δ Nanofibers as Insertion Anode for Li-Ion Battery Applications Jayaraman Sundaramurthy,†,§,⊥ Vanchiappan Aravindan,*,‡,⊥ Palaniswamy Suresh Kumar,§ Srinivasan Madhavi,*,‡,∥ and Seeram Ramakrishna*,† †

Center for Nanofibers and Nanotechnology, Department of Mechanical Engineering, National University of Singapore, 2 Engineering Drive 3, Singapore 117576 ‡ Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, Research Techno Plaza, 50 Nanyang Drive, Singapore 637553 § Environmental and Water Technology, Center of Innovation, Ngee Ann Polytechnic, 535, Clementi Road, Singapore 599489 ∥ School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, Nanyang Avenue, Singapore 639798 ABSTRACT: A scalable electrospinning technique is used to synthesize 1D TiO2 nanofibers for Li-ion battery applications. Oxygen deficiency (TiO2−δ) in anatase phases is created by treating the nanofibers in the H2 atmosphere with various temperature conditions. Structural and morphological features of both pure and oxygen deficient phases are analyzed by X-ray diffraction and scanning electron microscopy, respectively. Li-insertion properties of such nanofibers are evaluated in half-cell configuration at high current rate of 150 mA g−1 and found that there is no improvement in the reversible capacity after H2 treatment. Improved cycling profiles are noted for such deficient phases compared with pure TiO2 nanofibers. ac impedance spectra were also conducted to validate the electrical conductivity profiles of the electrospun TiO2 nanofibers.

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architecture with electrospun LiMn2O4 cathode.17 As expected, poor high-current performance results in full-cell assembly, which is mainly because of the inferior electrical conductivity of the insertion anode TiO2. Several attempts including metal decoration,18 carbon coating,19 composite with carbonaceous materials,20 and wrapping with graphene nanosheets are also carried out to overcome the inherent electronic conductivity of anatase phase.21 Unfortunately, blending or coating with carbonaceous materials dilutes further reduction in volumetric capacity.22 Therefore, search for the alternate approach to improve the electronic conductivity is highly warranted. One of the efficient ways is to create the oxygen deficiency in the anatase lattice, thereby increasing the electrical properties. Recently, Shin et al.23 succeeded by creating oxygen deficiency in commercially available anatase phases (TiO2−δ) with improved battery performance. Unfortunately, the tested commercial powders are electrochemically inactive. (It is able to deliver the maximum reversible capacity of ∼60 mAh g−1 only.) As a consequence, creating oxygen deficiency in inactive anatase phase results in the improvement in electrochemical profiles, but it is worth studying in the oxygen-deficient electrochemically active-phase materials. In this scenario, we

INTRODUCTION Nanostructured anatase TiO2 is considered to be a highcapacity (∼335 mAh g−1) insertion anode for Li-ion batteries (LIBs) compared with the other insertion materials like Li4Ti5O12 (∼175 mAh g−1),1,2 LiCrTiO4 (∼157 mAh g−1),3,4 TiP2O7 (∼121 mAh g−1),5,6 LiTi2(PO4)3 (∼138 mAh g−1),7,8 Li3V2(PO4)3 (∼132 mAh g−1),9 and so on. In addition, no surface film formation, ecofriendliness, and low cost make the anatase phase an attracting candidate for constructing high performance LIB. 10,11 However, the inferior electronic conductivity profile forbids us to employ them in practical cells that eventually affect the high current performance.10,11 It is well known that synthesis technique plays a vital role in determining the electrochemical activity of anatase-phase nanostructures, particularly the 1D structure. In general, the 1D structure offers salient features like high surface to volume ratio, facile Li diffusion, and good compatibility with current collectors. Therefore, such structures are preferred to construct high-performance LIB.12−14 Several techniques such as sol−gel, hydrothermal, electrospinning, template-assisted synthesis, and so on, have been used to produce 1D nanostructures. Among them, electrospinning technique is found to be appealing in terms of its simplicity and scalability.15,16 So far, several reports are available for the performance of such electrospun fibers in both half-cell and full-cell configurations; for example, very recently, we successfully demonstrated the performance of hollow structured anatase-phase TiO2 nanofibers in all 1D © 2014 American Chemical Society

Special Issue: Michael Grätzel Festschrift Received: December 30, 2013 Revised: February 12, 2014 Published: February 19, 2014 16776

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have prepared the electrochemically active anatase TiO2 phase and subsequently studied the influence of oxygen deficiency in such phases to extend the reversible capacity approaching one mole of Li. So, we have adopted a well-known and scalable electrospinning technique to prepare the 1D anatase TiO2phase nanofibers and subsequently created oxygen vacancies by treating them in a H2 atmosphere. The detailed structural and electrochemical studies are carried out and described in detail.

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EXPERIMENTAL SECTION

The scalable electrospinning technique was adopted to synthesize anatase TiO2 phase nanofibers. First, 1.2 g of polyvinylpyrrolidone (PVP, MW: 1.30 × 105, Aldrich) was dissolved in 12 mL of absolute ethanol (99.8%, Aldrich) under continuous stirring for 6 h. Then, 1.5 g of titanium(IV) isopropoxide (99%, Aldrich) and acetic acid (1 mL of 99.7%, Sigma-Aldrich) were added dropwise to the above mixture under vigorous stirring overnight. Then, the mixture was transferred to a 5 mL syringe with a needle diameter of 11.9 mm. The procedure was performed in a controlled electrospinning setup (ELECTROSPUNRA, Microtools Pvt. Limited, Singapore). The distance between needle and aluminum foil static collector was maintained at 17 cm with an applied ac voltage of 17.5 kV and at a flow rate of 1 mL h−1 using a syringe pump (KDS 200). Finally, the resultant fiber mat with Al-foil was sintered at 400 °C for 2 h in air to yield single-phase anatase TiO2 nanofibers. Oxygen-deficient phase was prepared by treating the single-phase TiO2 nanofibers under the H2 atmosphere (5% H2 and 95% Ar) at various temperature conditions for 30 min. Powder X-ray diffraction (XRD) measurements were carried out using Bruker D8 Advance diffractometer equipped with CuKα radiation. Rietveld refinement was carried out using Topas V3 software. Morphological studies were performed using a scanning electron microscope equipped with a field emission gun source (JEOL FE-SEM 7600) and transmission electron microscope (JEOL 2100F). Standard 2016 coin-cell configuration was used to study the electrochemical properties. The composite electrode was formulated with accurately weighed 10 mg of active material TiO2 nanofibers, 1.5 mg of Super P, and 1.5 mg of teflonized acetylene black (TAB-2) using ethanol. The mixture was pressed onto a stainless-steel mesh (200 mm2 area, 0.25 mm thickness) that acts as current collector. The test cells were fabricated with composite electrode with Li metal (∼0.59 mm thick, Hohsen, Japan), which was separated by Whatman paper (cat. no. 1825-047, U.K.) and filled with 1 M LiPF6 in ethylene carbonate (EC)/ dimethyl carbonate (DMC) (1:1 wt %, Merck KGaA, Germany) as electrolyte solution. Cyclic voltammetric (CV) signatures were recorded at scan rate of 0.1 mV s−1 using Solartron, 1470E and SI 1255B impedance/gain-phase analyzer coupled to a potentiostat in two-electrode coin-cell configuration. Galvanostatic charge−discharge studies were conducted between 1 and 3 V versus Li at constant current density of 150 mA g−1 using Arbin 2000 battery tester under ambient temperature conditions.

Figure 1. (a) Powder XRD pattern of electrospun anatase TiO2 nanofibers treated at different atmospheres for 30 min. (I) 400 °C in air (pure nanofibers), (II) 400 °C in H2/Ar (5%/95%), (III) 450 °C in H2/Ar (5%/95%), and (IV) 500 °C in H2/Ar (5%/95%). (b) Lattice parameter values of the above fibers obtained by Rietveld refinement.

impurity traces like rutile phase. The observed reflections are indexed according to the tetragonal structure with I41/amd space group (Figure 1a,I). The lattice parameter values are calculated during the refinement and found to be a = 3.796 (4) Å and c = 9.508 (9) Å, which is consistent with the literature.10,17 Apparently, there is no variation in the diffraction peaks are noted for 400 and 450 °C treated anatase fibers under H2 environment. Unfortunately, increasing the temperature to 500 °C results in the phase transition from anatase to rutile, and the resultant nanofibers composed of powders consist of 49% rutile and 51% anatase phases. Although there is no variation in the diffraction peaks, a drastic reduction in the lattice parameter values is observed, which is clearly evident at 400 °C treatment (Figure 1b). Further increasing the temperature results in increasing lattice parameter values, which is contrary to TiO2 nanoparticles reported by Shin et al.23 Figure 2 represents the morphological features of the electrospun anatase phase TiO2 nanofibers treated with different temperatures. Figure 2a clearly shows the formation of randomly oriented fibrous with diameter of ∼1 μm. The calcination at 400 °C in an air atmosphere results in the reduction of fibrous morphology, which is mainly due to decomposition of the polymer backbone, PVP. The fibers exhibit the diameter of 200−400 nm after the calcination (Figure 2b), which has been further confirmed by TEM analysis (Figure 2c). The TEM pictures reveal the presence of fibrous morphology, which is composed of nanosized particulates with ∼14 nm in size (calculated using Sherrer formula). The crystallinity of the nanofibers is clearly supported from the selected area electron diffraction pattern (SAED), which showed well-defined concentric rings. It is interesting to note that there are no appreciable changes in the morphological features of H2-treated fibers, clearly evident in Figure 2d−f.

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RESULTS AND DISCUSSION Figure 1a represents the XRD patterns of anatase TiO2 nanofibers prepared by electrospinning. The TiO2 nanofibers exhibit the phase-pure structure without any noticeable 16777

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Figure 2. (a) Electrospun green fibers (as-spun) of anatase phase TiO, (b) the electrospun fibers calcined at 400 °C in air atmosphere (inset: magnified view), (c) TEM pictures of the electrospun fibers calcined at 400 °C in air atmosphere (inset: SAED pattern), (d) 400 °C in H2/Ar (5%/ 95%), (e) 450 °C in H2/Ar (5%/95%), and (f) 500 °C in H2/Ar (5%/95%).

Li, except mixed rutile and anatase phase nanofibers. We strongly believe that lower OCV is mainly because of the presence of rutile content. All cells are exhibiting sharp peaks at ∼1.68 and ∼2.03 V versus Li during cathodic and anodic sweeps, respectively. The cathodic and anodic peaks are associated with the reduction of Ti4+ to Ti3+ and subsequent oxidation of Ti3+ to Ti4+, respectively. However, the variation in peak currents depends on the conducting nature of the TiO2

Li-insertion properties are evaluated in both potentiostatic and galvanostatic measurements in half-cell configuration between 1 and 3 V versus Li under ambient conditions (Figure 3). The CV studies are performed at a slow scan rate of 0.1 mV s−1 for the half-cell assembly, in which metallic Li acts as both reference and counter electrode. It is worth mentioning that all test cells are fabricated with 10 mg of active material. All test materials exhibit the open circuit voltage (OCV) ∼3 V versus 16778

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Figure 3. Cyclic voltammetry (CV) curves of electrospunTiO2 nanofibers in half-cell assembly cycled between 1 and 3 V versus Li at slow scan rate of 0.1 mV s−1 (a) 400 °C in air (pure nanofibers), (b) 400 °C in H2/Ar (5%/95%), (c) 450 °C in H2/Ar (5%/95%), and (d) 500 °C in H2/Ar (5%/ 95%) in which metallic lithium acts as both counter and reference electrode.

nanofibers prepared by electrospinning. The overall Liinsertion/extraction mechanism can be described according to the following equilibrium TiO2 + Li + e− ↔ LixTiO2. Except for the first cycle, CV traces are consistent with the subsequent cycles irrespective of the nanofibers, which correspond to the excellent reversibility of the system. Galvanostatic cycling studies are performed at current density of 150 mA g−1 and given in Figure 4. Figure 4a represents the initial charge−discharge curves of various electrospun TiO2 nanofibers. As observed in the first cycle of CV studies, different kinds of charge−discharge curves are noted. The cells delivered the discharge capacity of ∼195, ∼193, ∼183, and ∼199 mAh g−1 for pure TiO2 fibers and TiO2−δ fibers obtained at 400, 450, and 500 °C under a H2 atmosphere, respectively. The irreversible capacity is another important issue for the anatase-phase TiO2 nanostructures. In the present case, the irreversible capacity of ∼20, ∼22, ∼32, and ∼54 mAh g−1 for pure TiO2 fibers and TiO2−δ fibers is obtained at 400, 450, and 500 °C under a H2 atmosphere, respectively. The Li-insertion into anatase-phase TiO2 induces the phase transition from tetragonal (I41/amd) to orthorhombic (Li0.5TiO2, space group Pnm21) because of the loss of symmetry in y direction. Furthermore, such transition occurs along with a spontaneous phase separation of lithium-poor (Li0.01TiO2) into lithium-rich (Li0.5TiO2) phases and reported by several researchers.24−26 In general, during Li-insertion into anatase phase leads to the formation of solid-solution (dropping of OCV to ∼1.7 V vs Li), followed by Li-insertion according to the two-phase reaction mechanism (evident from long distinct flat plateau at ∼1.7 V vs Li) and interfacial storage (monotonous curves observed from ∼1.7 to 1 V vs Li) as well. Mainly, the two-phase region is the characteristic of anatasephase TiO2.27 The introduction of H2 treatment shortens the

Figure 4. (a) Galvanostatic charge−discharge curves of electrospunTiO2 nanofibers in half-cell assembly cycled between 1 and 3 V versus Li at current density of 150 mAg−1 under ambient temperature conditions. (b) Plot of discharge capacity versus cycle number for above cell assembly.

two-phase region with increase in temperature. Along with the shortened two-phase region, increase in polarization of the oxygen-deficient TiO2 nanofibers is noted while the temperature is increased (400 to 500 °C). The observed results were 16779

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completely contrary to the previous report by Shin et al.,23 in which much higher electrochemical activity was noticed for the oxygen-deficient phases. The plot of discharge capacity versus cycle number is given in Figure 4b. It is worth noting that the native-phase TiO2 nanofibers delivered very stable capacity behavior compared with the deficient-phase nanofibers for the reported 500 cycles. The test cell renders ∼82, ∼85, ∼79, and ∼91% of initial reversible capacity after 500 cycles for pure TiO2 fibers and TiO2−δ fibers obtained at 400, 450, and 500 °C under a H2 atmosphere, respectively. Although the nanofibers treated at 500 °C under a H2 atmosphere exhibit lower reversible capacity, they retain better cyclability than the rest of the phases. We believe that such stability is mainly due to the presence of a large amount of robust rutile phase, and it is convincingly proven by us in our previous work.20 This study clearly illustrates that mild H2 treatment may help the capacity retention characteristics (ex. 400 °C under H2 atmosphere treatment), but the reversible capacity has to be sacrificed. Electrochemical impedance measurement is a unique technique to study electrical properties of the materials and its interfaces. In general, the impedance spectra are recorded using applied frequency with respect to exciting signal. The analysis provides quantitative information about the conductivity, the dielectric coefficient, the static properties of the interfaces, and its dynamic change because of either adsorption or charge-transfer (CT) phenomena. In the present case, to validate the influence of oxygen deficiency toward electrical profiles of electrospun anatase-phase nanofibers with different temperature conditions, we performed ac impedance, given in Figure 5. The impedance measurement is performed in the

clearly suggests that creating oxygen deficiency in the anatase lattice did not provide the improvement in electrical conductivity of the electrospun fibers.

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CONCLUSIONS Oxygen-deficient anatase-phase TiO2 nanofibers were successfully synthesized via electrospinning and followed by H2 treatment with various temperature conditions. Unlike that of anatase-phase TiO2 nanoparticles, there was no improvement in the electrical profiles noted for the reduced nanofibers. As a result, decrease in Li-insertion properties was noted. Oxygen deficiency certainly provides good capacity retention characteristics compared with the pure TiO2 fibers, although it exhibits lower capacity. This study clearly showed that creating oxygen deficiency did not provide the desirable improvement in the electrochemical performance of the 1D nanofibers compared with nanoparticles.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (V. Aravindan). *E-mail: [email protected] (S. Madhavi). *E-mail: [email protected] (S. Ramakrishna). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Author Contributions ⊥

J.S. and V.A. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.A. and S.M. thank the National Research Foundation (NRF, Singapore) for financial support through the Competitive Research Programme (CRP, Grant no. NRF-CRP4-2008-03).

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REFERENCES

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Figure 5. ac impedance spectra of electrospun TiO2 nanofibers in halfcell configuration at applied ac amplitude of 10 mV between 10 kHz and 1 mHz.

same 2016 coin-cell configuration by applied amplitude of 10 mV. In general, the Nyquist plot is composed of three main regions: (i) first, the presence of high-frequency semicircle, which is attributed to the surface film formation, that is, solid electrolyte interface or contact resistance; (ii) second, the appearance of medium-frequency region, which is associated with the CT kinetics of the electrode/electrolyte interface; and (iii) finally, the 45° inclined vertical tail toward the real axis corresponding to the lithium diffusion kinetics toward the electrodes (so-called as Warburg tail). No obvious difference between the diameters of the semicircles is noted for the electrospun pure and TiO2−δ nanofibers, that is, lower CT impedance compared with the fibers treated at 500 °C. This 16780

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