graphene nanostructure for lithium storage with high-rate performance

Electrochimica Acta 109 (2013) 389–394

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Li4 Ti5 O12 /graphene nanostructure for lithium storage with high-rate performance Song Gyun Ri a , Liang Zhan b,∗ , Yun Wang b , Lihui Zhou a , Jun Hu a,∗ , Honglai Liu a a

Department of Chemistry, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Department of Chemical Engineering, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China b

a r t i c l e

i n f o

Article history: Received 28 April 2013 Received in revised form 17 June 2013 Accepted 8 July 2013 Available online 19 July 2013 Keywords: Lithium titanate Graphene Ionic liquid Anode materials Lithium ion battery

a b s t r a c t Nano-crystalline Li4 Ti5 O12 with an average size of 18 nm was in situ grown on graphene sheets using ionic liquid of C12 H23 ClN2 [Omim]Cl as the exfoliated agent. Such unique nanostructure provides a high electrode/electrolyte area for the electron transport and the nanosized Li4 Ti5 O12 leads to a short path for the lithium ion transfer. When used as an anode material for lithium-ion battery, the Li4 Ti5 O12 /graphene nanostructure exhibits excellent reversibility (159 mAh g−1 at 0.5 C after 100 cycles) and high-rate performance (162 mAh g−1 at 0.2 C, 148.5 mAh g−1 at 20 C). Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction Lithium ion batteries, as power sources for portable electronic devices and electric/hybrid vehicles, have attracted tremendous attentions in the scientific and industrial fields due to their high electromotive force and high energy density [1]. To meet the increasing demand of batteries with high-rate performance and longer cycle life, many efforts have been made to develop new electrode materials [2]. Recently, Li4 Ti5 O12 (LTO) has been widely researched as an anode material for lithium storage owing to its high lithium intercalation–deintercalation potential, fast charge–discharge rate, excellent reversibility, structural stability and high safety [3]. Unfortunately, the low inherent electronic conductivity of LTO (<10−13 S cm−1 ) at room temperature largely prevent its applications, especially its high-rate performance [4]. So many researchers focus on enhancing its electron conductivity through designing novel nanostructures [5] and surface modification with conductive materials [6–8]. The nanostructured LTO has a high electrolyte/electrode contact area and provides a shorter path for lithium ion and electron transport, while the fabrications of nanometer-sized LTO by the conventional synthesis routes suffer from the severe aggregation of nanoparticles and high cost due to the post-treatment process at high temperatures [9,10]. The surface modifications of electrolyte/electrode interface were performed by ∗ Corresponding authors. Tel.: +86 21 64252922; fax: +86 21 64252914. E-mail addresses: [email protected] (L. Zhan), [email protected] (J. Hu).

means of carbon [11], metal [12] and inorganic/organic compounds [13]. However, the nature of the electronic conductivity and the ionic diffusivity in the internal bulk LTO has no obvious improvement [14,15]. Therefore, the synthesis of hybrids of nanostructured LTO and conductive additives is still challenging. Recently, Shen et al. [16] reported an interesting LTO/graphene hybrid nanostructure and got a capacity of 135 mAh g−1 at 10 C, however, the capacity decreased sharply with increasing the current density (0.1–60 C). Since the unavoidable restacking of graphene resulted in some LTO nanoparticles being deposited on the external surface of graphene and some aggregated into bulk LTO separated from graphene. The aggregated bulk LTO had a low electrode/electrolyte contact area and the deposited LTO nanoparticles might depart from the surface of graphene into electrolyte at high current density, which reduced the high-rate performance of LTO material. Additionally, the synthesis procedure was quite complicated. Herein, we report an one-step approach to synthesize a novel LTO/graphene nanostructure, where the graphene sheets, with a thickness of about 2 nm, were exfoliated by using ionic liquid (IL) of C12 H23 N2 Cl ([Omim]Cl) and LTO nanoparticles were in situ grown up on the surface of graphene sheets. The resulted LTO/graphene nanostructures were consisted of thin graphene sheets and LTO nanoparticles intercalated uniformly into the space of graphene sheets. Such unique features can not only provide a high electrolyte/electrode contact area and a short path for lithium ion and electron transport, but also make the intercalated LTO nanoparticles difficultly depart from the surface of graphene in the high-rate

0013-4686/$ – see front matter. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.07.059

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charge/discharge process. Therefore, this kind of LTO/graphene nanostructure exhibits an excellent reversibility and high-rate performance when used as anode materials for lithium-ion battery.

2. Experimental 2.1. Synthesis The overall synthetic procedure of LTO/graphene nanostructure is shown in Scheme 1, which mainly includes four steps. In briefly, graphene oxide (GO) was firstly fabricated via the modified Hummers method [17]. Then the vacuum-dried GO (0.1 g) was thoroughly dispersed in ethyleneglycol (EG, 40 ml) by the ultrasonification for 3 h. Subsequently, 8 ml of EG/GO solution, 0.5 g of [Omim]Cl, 0.39 g of LiOH·H2 O, 2 mL of H2 O2 , and 1.5 g of Ti(OBu)4 were added into 40 ml of H2 O under stirring at room temperature. After stirring for 2 h, the dark yellow solution was transferred into a Teflon-lined heat-resistant plastic autoclave and treated by microwave-assisted hydrothermal reaction at 180 ◦ C for 20 min, and then annealed at 550 ◦ C for 6 h under N2 atmosphere. The final product was denoted as LTO/graphene-IL and the intermediate graphene exfoliated by [Omim]Cl was denoted as graphene-IL. For comparison, the microwave-assisted hydrothermal reaction was carried out without adding GO and IL, or with GO but without

IL, where the products were denoted as LTO and LTO/graphene, respectively. 2.2. Characterization The morphology and microstructure of the samples were systematically investigated by TEM (JEM-1400), HRTEM (Field Emission JEOL 2100), AFM (Digital Instrument Nanoscope IIIA), XRD (Rigaku D/Max Ultima II Powder X-ray diffractometer) and N2 adsorption–desorption measurements. Electrochemical experiments were carried out in 2032 coin-type cells. The working electrodes were prepared by mixing the active material (LTO/graphene-IL, LTO/graphene or LTO), carbon black and poly(vinyl difluoride) (PVDF) at a weight ratio of 80:10:10 and pasted on pure copper foil (99.6%, Goodfellow). Pure lithium foil (Aldrich) was used as the counter electrode. The obtained slurry was then pasted onto copper foil with the electrode thickness of 150 ␮m. The electrolyte was a solution of 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1:1:1, v/v) and the separator was Celgard 2400. The coin-type cell was assembled in an argon-filled glove box, using Li metal as the counter electrode. Galvanostatic charge/discharge tests were conducted in the voltage range of 1.0–3.0 V versus Li/Li+ on the CT2001A LAND Battery Tester. Cyclic voltammograms (CV) were recorded between 1.0 and 3.0 V at different scan rates of 0.1, 0.2, 0.5, 1 mV s−1 on an Arbin BT2000 electrochemical workstation.

Scheme 1. The synthesis strategy of LTO/graphene-IL nanostructure from LTO precursors, GO and ionic liquid using microwave-assisted hydrothermal reaction process.

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Fig. 1. (a) N2 adsorption/desorption isotherms and pore size distribution and (b) XRD patterns of the synthesized LTO, LTO/graphene and LTO/graphene-IL samples. (c) TEM and (d) AFM images of LTO/graphene-IL sample. The inserts in (c) and (d) correspond to the HRTEM image taken from the square in (c) and the thickness analysis taken around the blue line in (d). (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)

3. Results and discussion The microstructures of the graphene and graphene-IL intermediates were initially analyzed with TG, TEM, AFM, Raman and XPS (see Support Information, Figs. S1–S5). The results indicate that the graphene-IL has been successfully exfoliated with a thickness of about 2 nm. The thickness of graphene-IL is obviously thinner than that of graphene without [Omim]Cl, which illustrates that ionic liquid of [Omim]Cl plays an important role in obtaining thin graphene sheets due to its special imidazole functional group and long alkyl chain [18–20]. The pore structures of the LTO, LTO/graphene and LTO/graphene-IL samples were analyzed by nitrogen adsorption–desorption isotherms as shown in Fig. 1a. The specific surface areas of LTO, LTO/graphene and LTO/graphene-IL samples are 18.6, 33.4 and 75.6 m2 g−1 , respectively. The N2 adsorption/desorption hysteresises at the P/P0 of 0.6 and 0.4 illustrate the mesoporous structure of the LTO/graphene and LTO/graphene-IL samples, respectively. The pore size distributions further indicate that the average mesoporous size of LTO/graphene-IL is larger than that of LTO/graphene, which is beneficial to the LTO intercalated into the space between graphene layers, forming well-dispersed LTO nanoparticles. According to the TG curves (see Support Information, Fig. S1), the contents of graphene and IL in the resulted LTO/graphene-IL product are 2.10 and 0.49%, respectively. To further elucidate the morphology and microstructure of LTO, LTO/graphene and LTO/graphene-IL samples, XRD, TEM, HRTEM and AFM were combinedly carried out. The XRD patterns of

LTO, LTO/graphene and LTO/graphene-IL samples are compared in Fig. 1b. After annealing, all the products show the spinel LTO structure detected as JCPDS: 49-0207 [21,22]. In the case of LTO/graphene-IL sample, the precursors were completely transferred into Li4 Ti5 O12 , while some crystalline anatase TiO2 (JCPDS: 21-1272) was detected in the LTO/graphene sample due to the existence of carboxyl groups in GO restricting the interaction between Li ion and the precursor of Ti. TEM image (Fig. 1c) of LTO/graphene-IL shows the two-dimensional graphene sheets with the size of 500 nm to several micrometers coated with LTO nanoparticles. The particle size of anchored LTO nanoparticles distributes in a narrow range of 18–20 nm. In the case of synthesis of LTO/graphene without [Omim]Cl, only a small part of LTO nanoparticles anchored on the surface of graphene, while most of them are aggregated and separated from graphene sheets (see Support Information, Fig. S6). The (1 1 1) lattice plane of the LTO nanoparticle can be clearly observed (in the inset HRTEM, Fig. 1c), consistent with the calculation by Debye–Scherrer equation (D = 0.89/ˇcos()) [23] from the (1 1 1) lattice plane in the XRD patterns (Fig. 1a), suggesting the LTO nanoparticles have a good crystallization. The AFM analyses (Fig. 1d) further shows the microstructure of LTO/graphene-IL with a total thickness of about 20 nm, and LTO nanoparticles grows uniformly on the thin graphene sheets with a particle size of about 18 nm. Cyclic voltammetry experiments were conducted to evaluate the electrochemical performance of the LTO/graphene-IL electrode at different scan rates of 0.1, 0.2, 0.5, 1 mV s−1 over the voltage

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Fig. 2. Cycle voltammograms of LTO/graphene-IL (a) and bare LTO (b) at different scan rate of 0.1, 0.2, 0.5 and 1 mV s−1 (voltage range: 1.0–3.0 V).

range from 1.0 to 3.0 V. For comparison, bare LTO electrode was also tested under the same electrochemical conditions. As shown in Fig. 2a, only one pair of redox peaks appears in all the cycles which coincides with the reversible phase transition potential between Li4 Ti5 O12 and Li7 Ti5 O12 [24]. At a scan rate of 0.1 mV s−1 , one reduction peak at 1.48 V and one oxidation peak at 1.66 V are observed for LTO/graphene-IL, corresponding to the Li+ insertion (reduction) and extraction (oxidation) processes. With increasing the scan rate, the CV curves become broaden and display good symmetry between two peaks, where the ratios of current peaks to anodic current peak are closed to 1. Even at a high scan rate of 1 mV s−1 , two peaks of LTO/graphene-IL display at a narrower voltage window compared to bare LTO (Fig. 2b), suggesting a lower electrode polarization for LTO/graphene-IL. To further investigate the electrode process, the measured current data were fitted by the typical Cottrell’s equation and a power law relationship [25,28]. The results (see Supporting Information, Fig. S7) indicate that the

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ion diffusion in LTO/graphene-IL is much faster than that of bare LTO, i.e., the ion diffusion pathway in LTO/graphene-IL is shorter than that of bare LTO. Moreover, the optimal conductive network formed by graphene can improve the electric conductivity of LTO nanoparticles, resulting in a fast electron transfer. To investigate the effects of [Omim]Cl in improving the rate performance of the LTO/graphene-IL electrode, the galvanostatic discharge–charge profiles of electrodes between 1 and 3.0 V at a current density of 0.5 C were recorded. Fig. 3a depicts the voltage profiles of the 1st, 2nd, 50th and 100th cycles for the LTO/grapheneIL electrode. All the curves display a charge plateau potential at 1.62 V and a discharge one at 1.52 V, corresponding to the reversible Li ion insertion/extraction process as mentioned above. The first discharge capacity is 177 mAh g−1 , which corresponds to the increase of 1.1% compared with the theoretical capacity (175 mAh g−1 ) [26]. After 100 cycles, the reversible capacity can still maintain at 159 mAh g−1 , suggesting that the LTO/graphene-IL

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Fig. 3. (a) Galvanostatic charge/discharge profiles of LTO/graphene-IL electrode during 1st, 2nd, 50th and 100th cycles at 0.5 C. (b) Cyclic performance of LTO, LTO/graphene and LTO/graphene-IL electrodes at 0.5 C. (c) High-rate performance of LTO, LTO/graphene and LTO/graphene-IL electrodes. (d) High rate performance of LTO/graphene-IL electrode.

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Fig. 4. (a) Nyquist-type plots of different electrodes obtained at 1.7 V after 20 cycles; (a) LTO, LTO/graphene and LTO/graphene-IL. (b) The fitted plot of LTO/graphene-IL electrode. The inset figures in (a) and (b) show the Randle and modified Randle models, respectively.

electrode has a high reversible capacity, structural stability and excellent cycling stability. Compared with LTO/graphene-IL, the charge/discharge plateaus of LTO/graphene are shorter and the potential difference between two plateaus is larger, showing the poor charge transfer ability and the high polarization. The cyclic performance of LTO, LTO/graphene and LTO/graphene-IL electrodes during 100 cycles at 0.5 C are shown in Fig. 3b. The capacity of LTO/graphene-IL electrode is much more stable and the reversible capacity retains up as high as 159 mAh g−1 after 100 cycles. For comparison, the first discharge capacity of LTO and LTO/graphene are lower as 116.2 and 120 mAh g−1 . Even worse, the reversible capacities of both LTO and LTO/graphene decrease sharply during the initial 10 cycles and then gradually stabilize at 71 and 103 mAh g−1 , respectively. Fig. 3c shows the rate performance of the electrodes at 0.2–20 C. As for the LTO/graphene-IL electrode, the reversible capacities at 0.2, 0.5, 1, 2, 5, 10 and 20 C are 162, 159, 157.6, 155.6, 153.9, 152 and 148.5 mAh g−1 , which are 97.5, 97.3, 96, 95, 93.8 and 91.5% of the initial discharge capacity at 0.2 C, respectively. And when the current rate is again reduced back to 0.2 C, the reversible capacity can be absolutely recovered and maintain at 162 mAh g−1 . In the case of the bare LTO and LTO/graphene electrodes, the reversible capacity at 20 C only present 15% and 20% of their corresponding capacity at 0.2 C, respectively. To understand why LTO/graphene-IL electrode exhibits such a high-rate performance compared to bare LTO or LTO/graphene electrodes, AC impedance measurements were performed at 1.7 V vs Li/Li+ after 20 cycles. The Nyquist plots (Fig. 4) show that the diameter of the semicircle for LTO/graphene-IL electrode in the high-medium frequency region is much smaller than those of both bare LTO and LTO/graphene electrodes, illustrating that the LTO/graphene-IL electrode possesses a lower contact and charge-transfer impedance. The second semicircle of the LTO/graphene-IL electrode at medium frequency region should be attributed to the well-dispersed LTO nanoparticles anchored on the surface of graphene sheets. The kinetic variations of the electrodes were further investigated, in which the AC impendence spectra of LTO and LTO/graphene electrodes were analyzed by the Randle equivalent circuit (as shown in the inset figure, Fig. 4a) [27]. Meanwhile, the AC impendence spectrum of the LTO/graphene-IL electrode was analyzed by the modified Randle equivalent circuit (as shown in the inset figure, Fig. 4b). As shown in Table S1, in the case of LTO/graphene-IL electrode, the values of film resistance Rs and charge-transfer resistance Rct are 4.54 and 17.7 , respectively, which are significantly lower than those of bare LTO (7.66 and 82.1 ) and LTO/graphene electrodes (7.53 and 78.4 ). Additionally, the diffusion coefficient D of LTO/graphene-IL (1.0 × 10−10 cm2 s−1 ) is much larger than that of bare LTO (2.8 × 10−12 cm2 s−1 ) and LTO/graphene electrodes (3.8 × 10−12 cm2 s−1 ). The above results should be attributed to the

unique microstructure of LTO/graphene-IL that the LTO nanoparticles intercalated into the space between very thin graphene sheets. It is important to mention that the high surface area of graphene can make LTO nanoparticles dispersed uniformly, resulting in a high electrode/electrolyte contact area. Furthermore, the existence of [Omim]Cl is beneficial for LTO nanoparticles to intercalate into the space between graphene layers. The function of surface energy between graphene layers will inhibit the growth of LTO nanoparticles into bulk, importantly; it can prevent the formed LTO nanoparticles departing from the surface of graphene at high charge–discharge rate. 4. Conclusion The one-step synthesis strategy of LTO/graphene-IL nanostructure was developed through a microwave-assisted hydrothermal reaction, in which [Omim]Cl was used as exfoliated agent to control the microstructures of graphene sheets as well as the inducer for LTO growth on graphene sheets. The resulted LTO/grapheneIL exhibited a two-dimensional structure with a total thickness of about 20 nm and LTO nanoparticles uniformly anchored on the surface of thin graphene sheets (2 nm). Such unique microstructure provided a high electrode/electrolyte contact area for electron transport and the nanosized LTO led to a short path for lithium ion transfer. When LTO/graphene-IL was used as the anode for the lithium-ion battery, it showed an excellent high-rate performance. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21176066 and 51002051), the 111 Project of Ministry of Education of China (No. B08021) and the Fundamental Research Funds for the Central Universities of China. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2013.07.059. References [1] B. Dunn, H. Kamath, J.M. Tarascon, Tarascon. Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928–935. [2] N.S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.K. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G. Bruce, Challenges facing lithium batteries and electrical double-layer capacitors, Angewandte Chemie International Edition 51 (2012) 9994–10024. [3] G.N. Zhu, Y.G. Wang, Y.Y. Xia, Ti-based compounds as anode materials for Li-ion batteries, Energy and Environmental Science 5 (2012) 6652–6667.

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