Hierarchical flower-like TiO2/MPCNFs as a free-standing anode with superior cycling reversibility and rate capability

Donghua Teng, Yunhua Yu and Xiaoping Yang*
State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, China. E-mail: yangxp@mail.buct.edu.cn

Received 16th December 2013 , Accepted 10th February 2014

First published on 10th February 2014


A hierarchical flower-like TiO2/MPCNFs web was cost-effectively fabricated by electrospinning, solvothermal treatment and calcination. The flower-like TiO2/MPCNFs as a free-standing anode possessed superior cycling reversibility and rate capability to nano-particulate and micro-particulate TiO2/MPCNFs because of its unique multiporous micro-/nano-architecture for synergistic lithium storage.


Rechargeable lithium ion batteries (LIBs) have been the most ubiquitous power source for portable electronic devices and burgeoning zero-emission electric vehicles (EVs). However, the energy density, rate capability and especially safety performance of LIBs still lie behind the stringent requirements for EVs.1,2 In this regard, it is still a formidable challenge to design and fabricate suitable electrode materials with high power density for next generation high-performance LIBs.

Anatase titanium dioxide (TiO2) has been envisioned as a promising high-power anode candidate owing to high lithiation potential (ca. 1.7 V vs. Li/Li+), high Li+ diffusion coefficient, low volume change (<5%) and environmental benignity.3–6 Furthermore, from both fundamental and practical standpoints, the electrochemical performance of TiO2 anodes can be undoubtedly improved by controlling their morphological feature, geometrical dimensionality and crystallographic orientation.7–14 Therefore, various hierarchical TiO2, such as solvothermally synthesized 1D nanowires,15 2D nanosheets13 and 3D microflowers,8,16 have been prudently exploited as advanced anodes for state-of-the-art LIBs. In particular, flower-like TiO2 can take the enthralling advantages of short Li+ diffusion distances endowed by a myriad of nanoscale building blocks (nanopetals) and large electrode/electrolyte interfaces proffered by multiporous microscale assemblies. However, the practical implementations of pristine TiO2 anodes are still hampered by severe thermodynamic aggregation and relatively low electronic conductivity.17 For circumventing the limitations, the combination of TiO2 and carbon nanomaterials, including carbon nanotubes (CNTs),18 graphenes19 and especially electrospun carbon nanofibers (CNFs),20–22 has been proposed as a pertinent strategy to deliver synergistic lithium storage effect. In this case, the nanostructured carbon matrix with a highly structural integrity can offer continuous e transportation pathways, and meanwhile the well-distributed TiO2 with a robust 3D open framework can confer fast Li+ percolation channels. Notwithstanding the above-mentioned enchanting advantages, little work has been reported on the combination of 3D hierarchical TiO2 and carbon nanomaterials as prospective anodes for high-performance LIBs to the best of our knowledge.

In this work, we employed an analogical route to prepare an intriguing hierarchical flower-like TiO2 interspersed multiporous carbon nanofibers (TiO2/MPCNFs) web following our previous work.21 For rationally designing the geometrical structure of TiO2, solvothermal treatment was adopted to replace phosphoric acid pre-impregnation activation. As a virtue of its peculiar multiporous hierarchy, the flower-like TiO2/MPCNFs as a free-standing anode demonstrated remarkable cycling reversibility and rate capability.

Fig. 1a–c show typical field emission scanning electron microscopy (FESEM) images of nano-particulate, micro-particulate and flower-like TiO2/MPCNFs. Partially aligned MPCNFs with a diameter distribution of 300–500 nm were deposited with plenteous TiO2 nanoparticles, microparticles and hierarchical micro-flowers, respectively. All the weight percentages of TiO2 were approximately 25% calculated from semi-quantitative energy dispersive X-ray (EDX) spectra (Fig. 1d) and quantitative thermogravimetric analysis (TGA, Fig. S1). The 3D hierarchical TiO2 micro-flowers with an average diameter of ∼2.5 μm were constructed from dozens of well-defined 2D interconnected multiporous nanopetals with an average thickness of ∼80 nm. The multiporous architecture was further validated by transmission electron microscopy (TEM) observation (Fig. 1e), demonstrating discernable dark (solid) and pale (porous) areas in both TiO2 and MPCNFs. As a consequence, the flower-like TiO2/MPCNFs had the highest Brunauer–Emmett–Teller (BET) specific surface area of 364 m2 g−1, while the nano-particulate and micro-particulate TiO2/MPCNFs had only 162 and 97 m2 g−1, respectively. Furthermore, as shown in the high-resolution transmission electron microscopy (HRTEM) image (Fig. 1f), the flower-like TiO2 emerged clear lattice fringes corresponding to the d101 (0.35 nm) and d002 (0.48 nm) spacings of polycrystalline anatase,7,19 which were well coincident with the select-area electron diffraction (SAED) pattern. The morphological formation and configurational construction of the flower-like TiO2/MPCNFs can be elucidated by interdiffusion–dissolution–nucleation-assembly growth mechanism. During the solvothermal stage, ethylene glycol (EG) and water (H2O) molecules can penetrate into stabilized nanofibers along nanopores to engender slow alcoholysis (or chelation) and fast hydrolyzation with titanium oxoacetate (TiO(OAc)2), respectively. Therefore, TiO(OAc)2 gradually evacuate from nanofibers to cause abundant micropores and then moderately nucleate with urea to self-assemble flower-like titanium coordination complexes via Ostwald ripening and anisotropic growth.7,8,23,24 Finally, after calcination at 600 °C, the titanium coordination complexes are dehydrated and transformed to TiO2 micro-flowers without distinct structural collapse, which are well crystallized and grown on polyacrylonitrile (PAN)-based MPCNFs to form the flower-like TiO2/MPCNFs.


image file: c3ra47685e-f1.tif
Fig. 1 FESEM images of (a) nano-particulate, (b) micro-particulate and (c) flower-like TiO2/MPCNFs. (d) EDX spectra of (i) nano-particulate, (ii) micro-particulate and (iii) flower-like TiO2/MPCNFs. (e) TEM and (f) HRTEM images of flower-like TiO2/MPCNFs with a corresponding SAED pattern (inset of f).

Fig. 2 shows X-ray diffraction (XRD) patterns of nano-particulate, micro-particulate and flower-like TiO2/MPCNFs. All samples emerged a similar broad diffraction peak near 2θ = 25° for (002) crystal plane of graphite (JCPDS 13-0148), indicating a conspicuous non-graphitizable disordered structure of the MPCNFs backbone.25 Moreover, the flower-like TiO2/MPCNFs exhibited the most pronounced diffraction peaks corresponding to crystallographic data for polycrystalline anatase TiO2 (JCPDS 21-1272), which should reveal a significant crystallographic improvement and were in good agreement with HRTEM and SAED analyses.26 The flower-like TiO2 had an average grain size of ∼25 nm according to Debye–Scherrer equation based on the half-width of the (101) reflection.


image file: c3ra47685e-f2.tif
Fig. 2 XRD patterns of (a) nano-particulate, (b) micro-particulate and (c) flower-like TiO2/MPCNFs.

Fig. 3 depicts representative cyclic voltammograms (CV) of as-obtained samples at a scan rate of 0.2 mV s−1 from 1 to 3 V. All samples exhibited virtually stable contour diagrams, indicating their excellent electrochemical reversibility. However, in comparison with nano-particulate TiO2/MPCNFs (Fig. 3a) and micro-particulate TiO2/MPCNFs (Fig. 3b), flower-like TiO2/MPCNFs (Fig. 3c) presented the strongest cathodic/anodic pair peaks at about 1.71/2.05 V, which should be ascribed to its architectural perfection and crystallographic improvement. Such the strongest Li+ intercalation/deintercalation fingerprint signals accompanied with the highest sloping plots of peak current vs. square root of scan rate (Fig. S2) unambiguously indicated that the flower-like TiO2/MPCNFs had the highest Li+ diffusion coefficient according to Randles–Sevcik equation,27 which was also demonstrated by Nyquist plots as shown in Fig. 3d. As for both fresh samples and cycled samples after five cycles at 50 mA g−1 from 1 to 3 V, the flower-like TiO2/MPCNFs showed the smallest radius of depressed semicircle at medium-frequency region and the greatest gradient of linear tail at low-frequency region, corresponding to the lowest charge-transfer resistance (Rct) and Warburg diffusion impedance (Zw), respectively.13


image file: c3ra47685e-f3.tif
Fig. 3 CV curves of (a) nano-particulate, (b) micro-particulate and (c) flower-like TiO2/MPCNFs at a scan rate of 0.2 mV s−1 from 1 to 3 V. (d) Nyquist plots of fresh and cycled (inset, after five cycles at 50 mA g−1) (i) nano-particulate, (ii) micro-particulate and (iii) flower-like TiO2/MPCNFs over 200 kHz to 0.1 Hz with a simplified Randles equivalent circuit.

Fig. 4a–c show typical galvanostatic discharge/charge curves for the initial three consecutive cycles of as-prepared nanocomposite anodes at 50 mA g−1 from 1 to 3 V. Similar to the aforementioned CV results, flower-like TiO2/MPCNFs (Fig. 4c) rendered the most pronounced discharge/charge pair plateaus at around 1.71/2.06 V, which were related to expectantly enhanced Li+ insertion/extraction into TiO2 accompanied with repetitious biphasic transitions between tetragonal and orthorhombic structures.19 Benefiting from incremental electrode/electrolyte interfaces for Li+ flux and electroactive sites for Li+ insertion, the flower-like TiO2/MPCNFs delivered the highest initial discharge/charge capacities of 280.9/213.2 mA h g−1, corresponding to a desirable coulombic efficiency of 75.9%. Fig. 4d compares galvanostatic cycling performance of three samples at 50 mA g−1 from 1 to 3 V. It is clearly demonstrated that the flower-like TiO2/MPCNFs delivered the highest reversible capacity of 186.8 mA h g−1 after 200 cycles, which was attributed to its unique spatial configuration for Li+ diffusion and appropriate crystallization for Li+ insertion.8 Moreover, the flower-like TiO2/MPCNFs also showed the best high-rate capability (Fig. 4e), delivering gravimetric reversible capacities of about 187.3, 174.8, 160.5, 144.6, 125.1, 105.2 and 188.6 mA h g−1 after every sequential 20 cycles at 50, 100, 200, 500, 1000, 2000 and again 50 mA g−1, respectively. In principle, the flower-like TiO2/MPCNFs can be used in a broader cut-off voltage window of 0.001–3 V (Fig. S3), which emerged superior cycling performance and rate capability to the Ti3O5/TiP2O7@MPCNFs counterpart reported in our previous work.21


image file: c3ra47685e-f4.tif
Fig. 4 Discharge/charge curves for the initial three cycles of (a) nano-particulate, (b) micro-particulate and (c) flower-like TiO2/MPCNFs anodes at 50 mA g−1 from 1 to 3 V. (d) Cycling performance at 50 mA g−1 and (e) rate capability of (i) nano-particulate, (ii) micro-particulate and (iii) flower-like TiO2/MPCNFs anodes from 1 to 3 V.

The outstanding electrochemical performance of flower-like TiO2/MPCNFs can be attributed to its well-developed multiporous micro-/nano-hierarchy, favorable crystal structure and appropriate heterogeneous composition. As expected, the interconnected pores and interspaces as electrolyte buffering reservoirs definitely boost electrolyte accessibility and facilitate Li+ diffusion, resulting in a superior rate capability.3,10,28 Furthermore, the uniformly distributed 3D hierarchical flower-like TiO2 are constructed from many well-crystallized 2D nanopetals, which play a pivot role to enhance the specific capacity by providing copious highly-exposed nanocrystal lattices as electroactive sites for Li+ intercalation.8,16 Additionally, the flower-like TiO2 also effectively confer abundant intrinsic crystal channels for fluent Li+ transportation, and meanwhile the interlaced MPCNFs framework with a high structural integrity and mechanical flexibility render a continuous e transportation pathway to alleviate polarization phenomenon at high rate.20–22 Therefore, such the synergistic effect is further contributory to the excellent thermodynamically cycling reversibility and rate capability of the flower-like TiO2/MPCNFs.

Conclusions

In summary, a 3D hierarchical flower-like TiO2/MPCNFs monolith with well-developed micro-/nano-architecture and multiporous interconnectivity was rationally designed and simultaneously accomplished via EG–H2O solvothermal treatment and subsequent calcination of electrospun Ti(OC4H9)4/PAN hybrid nanofibers. The flower-like TiO2/MPCNFs as a self-supporting and binder-free anode exhibited superior cycling performance and rate capability in virtue of the synergistic incorporation of highly Li+ diffusional TiO2 and well-conductive MPCNFs. The fascinating flower-like TiO2/MPCNFs as well as the scalable route may shed some light on the development and application of various 3D hierarchical multiporous micro-/nanomaterials with a spectrum of advanced functions.

Acknowledgements

The authors gratefully acknowledge the financial supports from National Natural Science Foundation of China (no. 51072013 and no. 51272021) and Natural Science Foundation of Jiangsu Province (no. BK20131147).

Notes and references

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47685e

This journal is © The Royal Society of Chemistry 2014