Tellurium

Three-Dimensional Hierarchical Reduced Graphene Oxide/Tellurium Nanowires: A HighPerformance Freestanding Cathode for Li−Te Batteries Jiarui He,† Yuanfu Chen,*,† Weiqiang Lv,‡ Kechun Wen,‡ Zegao Wang,† Wanli Zhang,† Yanrong Li,† Wu Qin,§ and Weidong He*,†,‡,∥ †

State Key Laboratory of Electronic Thin Films and Integrated Devices and ‡School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, PR China § National Engineering Laboratory for Biomass Power Generation Equipment, School of Renewable Energy Engineering, North China Electric Power University, Beijing 102206, PR China ∥ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China S Supporting Information *

ABSTRACT: Three-dimensional aerogel with ultrathin tellurium nanowires (TeNWs) wrapped homogeneously by reduced graphene oxide (rGO) is realized via a facile hydrothermal method. Featured with high conductivity and large flexibility, the rGO constructs a conductive threedimensional (3D) backbone with rich porosity and leads to a free-standing, binder-free cathode for lithium−tellurium (Li− Te) batteries with excellent electrochemical performances. The cathode shows a high initial capacity of 2611 mAh cm−3 at 0.2 C, a high retention of 88% after 200 cycles, and a highrate capacity of 1083 mAh cm−3 at 10 C. In particular, the 3D aerogel cathode delivers a capacity of 1685 mAh cm−3 at 1 C after 500 cycles, showing pronounced long-cycle performance at high current density. The performances are attributed to the well-defined flexible 3D architecture with high porosity and conductivity network, which offers highly efficient channels for electron transfer and ionic diffusion while compromising volume expansion of Te in charge/discharge. Owing to such advantageous properties, the reported 3D rGO/tellurium nanowire (3DGT) aerogel presents promising application potentials as a high-performance cathode for Li−Te batteries. KEYWORDS: lithium batteries, three-dimensional aerogel, free-standing cathode, reduced graphene oxide, tellurium

T

surface coating of separators, and the exploration of suitable electrolytes and additives.12−15 However, the application of Li−S batteries still faces the intrinsic disadvantages of sulfur.16 Selenium, another element in the chalcogen family, has also been proposed as a cathode material. Similarly, the low intrinsic electronic conductivity (∼10−6 S cm−1) and large volume expansion during the charge/discharge process lead to a low rate performance and poor cycling behavior of Li−selenium batteries.17

he increasing market of electric vehicles and consumer electronics has motivated extensive efforts of research on safer, lighter, and lower-cost power sources.1−5 Owing to a high theoretical specific capacity of 1675 mAh g−1 (3467 mAh cm−3), rechargeable lithium−sulfur (Li−S) batteries have been considered as a good candidate for the next-generation Liion batteries.6−8 However, the application of rechargeable Li−S batteries has been challenged by a few major issues including the large volume change of sulfur during the battery operation, the low electrical conductivity of elemental sulfur, and the high solubility of charge/discharge reaction intermediates (Li2Sn, n > 2) in the organic electrolyte, which results in the notorious shuttle effect and active mass loss.9−11 Many research efforts have been centered on addressing these issues, such as the impregnation of sulfur into various conductive porous matrices, © 2016 American Chemical Society

Received: July 12, 2016 Accepted: August 23, 2016 Published: August 23, 2016 8837

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Figure 1. (a) Illustration of the formation process of the 3DGT aerogel and schematics of fabricating a self-supporting electrode. The photograph of (b) 3DGT aerogel and (c) the disk of 3DGT with a diameter of 14 mm. (d) Bent 3DGT, indicating that it has good flexibility. (e) The 3DGT used as free-standing cathode.

electrochemical properties including a high capacity, a high rate capability, and a low impedance of the electrode, suggesting that 3DGT is a good candidate as high-capacity cathode for nextgeneration flexible Li−Te batteries.

Tellurium owns a substantially higher electronic conductivity (2 × 102 S m−1), resulting in higher utilization of active materials and higher rate capability compared with S and Se.18−20 In addition, tellurium has a high theoretical volumetric capacity (2621 mAh cm−3), which is an important advantage in practical applications.21 Furthermore, as a cathode material, tellurium has a theoretical potential of 1.96 V and an energy density of 823.2 Wh kg−1. Such an energy density is higher than that of LiFePO4 (578 Wh kg−1), which ensures sufficient energy as a power source.22 With these merits, tellurium is a good candidate of cathode for the Li−Te battery. Recently, Liu et al. reported pioneering work on tellurium/porous carbon (Te/C) as a cathode material for Li−Te batteries though the utilization of tellurium was not fully realized.18 Most recently, several strategies have been proposed to enhance the utilization of tellurium by using carbonaceous materials as the conductive matrix. However, the aforementioned composites still require conductive additives, polymeric binders, and metallic current collectors to prepare the cathode. The conductive additives, polymer binders, and metallic current collectors used in the batteries typically reduce the energy and power density of full batteries and decrease the electrochemical performance. In this study, a three-dimensional rGO/tellurium nanowire (3DGT) aerogel with ultrathin Te nanowires wrapped homogeneously with rGO was prepared, via a facile, costeffective, and environment friendly hydrothermal reaction. Owing to the advantageous architecture, the as-synthesized 3DGT exhibits advantageous characteristics: (i) 3DGT possesses high surface area and rich porosity usable for electrolyte/electrode interfaces, which effectively reduces the diffusion length of both electrons and ions. In addition, its high electrical conductivity ensures the fast transportation of electrons and low impedance of the electrode. (ii) Ultrathin Te nanowires with a nanoscale diameter size (∼40 nm) and a length of several micrometers exhibit higher rate performance and faster kinetics compared to the micrometer-sized materials because of the high surface-to-volume ratio which increases the reaction area between tellurium and the electrolyte.23 (iii) 3DGT can be employed as flexible, free-standing, and binder-free cathode for high-performance lithium−tellurium batteries. (iv) The synthesis of 3DGT is facile, cost-effective, and environment friendly. Therefore, the 3DGT demonstrates remarkably enhanced

RESULTS AND DISCUSSION The preparation of the 3DGT aerogel and the self-supporting electrode is shown in Figure 1a. The GO was effectively reduced by the ethanol during the hydrothermal treatment. In the mean time, the tellurium nanowires and rGO were assembled to construct robust 3DGT hydrogel. After freeze-drying, the assembled 3DGT aerogel (Figure 1b) was cut and compressed into circular pellet with a diameter of 14 mm for direct use as cathode for Li−Te battery (Figure 1c,e). The electrode can be bent without any protective layers and does not break during repeated bending, as shown in Figure 1d. The 3DGT aerogel has a highly porous, hierarchical architecture constructed with wrinkled and interconnected walls (Figure 2a). The interconnected walls in the hierarchical composite with thin rGO are wrapped with TeNWs (Figure 2b). The microstructure of the 3DGT was investigated with TEM. A

Figure 2. (a) Low- and (b) high-magnification SEM images of 3DGT. (c, d) TEM image of 3DGT. 8838

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Figure 3. (a) TGA profiles of 3DGT. (b) XRD patterns of rGO, TeNWs, and 3DGT. (c) Raman spectra of TeNWs and 3DGT. (d) N2 sorption isotherm and pore size distribution (inset) of 3DGT.

Figure 4. (a) CV curves of the 3DGT aerogel at a scan rate of 0.1 mV s−1 in the initial four cycles. (b) Discharge and charge curves of the 3DGT aerogel cathode at a 0.2 C. (c) Cyclic performance of the 3DGT cathode at 0.2 C for 200 cycles. (d) Rate performance at various C-rates for the 3DGT cathode. (e) Cycling stability of 3DGT cathode at 1 C for 500 cycles.

single Te nanowire wrapped by rGO is observed in the lowmagnification TEM image in Figure 2c. As shown in Figure 2d, the well-resolved lattice fringes with lattice spacings of 0.32 and 0.59 nm related with the (101) and (001) planes of hexagonal Te,

respectively. The inset of Figure 2c presents the selected area electron diffraction (SAED) pattern, which is consistent with Figure 2d. The results illustrate that the TeNWs exhibit pronounced crystallinity and are tightly wrapped by rGO. 8839

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3DGT cathode changed from 2562 mAh cm−3 (410.6 mAh g−1) to 1083 mAh cm−3 (173.6 mAh g−1), demonstrating a high rate performance. Moreover, the specific capacity is well recovered (2531 mAh cm−3) as the C-rate is restored to 0.2 C, indicating that the structure of the 3DGT electrode remains stable even under such high rate cycling. The excellent rate capability of the 3DGT is ascribed to the nanosized tellurium and thin rGO, leading to short diffusion/transport paths of Li ions and electrons as well as the highly porous microstructure providing rapid ion pathways. A high current density typically causes dramatic volume change of tellurium and retaining a high capacity at a high current density during cycling is challenging. Indeed, it has been a challenge to obtain tellurium-based cathodes with pronounced cycling stability at a high current density. To investigate the cyclic stability of the 3DGT aerogel cathode under a high current density, the long cycle performance has been studied at the current density of 1 C for 500 cycles, as shown in Figure 4e. The initial discharge and charge capacities of 3DGT aerogel cathodes are ∼2443 mAh cm−3 (391.5 mAh g−1) and ∼2358 mAh cm−3 (377.9 mAh g−1), respectively. It is worth noting that after the first cycle, the Columbic efficiencies rapidly reach 99%, and the discharge and charge capacities decrease only slightly during cycling, indicating the pronounced cycling stability of 3DGT aerogel cathodes at a high current density. After 500 cycles, both the discharge and charge capacities are stable at ∼1685 mAh cm−3 (270 mAh g−1). Such high capacities at a high current density during cycling are attributed to the advantageous structure of 3DGT aerogel. The binder-free 3DGT aerogel employed as cathode has a high conductivity allowing for efficient high current operation. In addition, the volume change of tellurium during cycling is effectively mitigated by the 3D porous structure and also the robust contact between TeNWs and rGO in the hierarchical structure of a high flexibility. To illustrate the electrochemical reaction mechanism of 3DGT, the chemical constituent and structure change of the active material at different discharge/charge cycles were characterized by in situ Raman spectroscopy. As shown in Figure 5, the characteristic peaks located at 120 and 140 cm−1 are slightly overlapped and correspond to A1 and E bond-stretching modes of Te, respectively. The characteristic Raman peak of Te chains (at ∼120 cm−1) becomes weak in the reduction process,

TeNW composition in the 3DGT was determined by thermogravimetric analysis (TGA) under N2 atmosphere, as shown in Figure 3a. The weight loss of the 3DGT in the temperature range of 450 °C ∼ 700 °C was observed, which corresponds to the evaporation of tellurium. The content of TeNWs in the 3DGT composite is 63.3%. The crystalline structures of the rGO, TeNWs and 3DGT were further characterized by XRD. The rGO shows a broad diffraction peak at 24.7° corresponding to the (002) peak of rGO, as shown in Figure 3b. The XRD pattern of TeNWs presents the characteristic peaks of hexagonal tellurium and agrees well with the standard data (JCPDS# 36-1452) as shown in Figure 3b. For the 3DGT aerogel, all the diffraction peaks are in good agreement with those of TeNWs except for the additional broad peak at 20°−30°. The peaks of rGO and TeNWs in the composite indicate that TeNWs are indeed wrapped by rGO. Raman spectroscopy was taken to investigate the structure of tellurium nanowires and rGO in the 3DGT composite. As shown in Figure 3c, two characteristic peaks at 120 and 140 cm−1 correspond to A1 and E bond-stretching modes, respectively.24 The peak at ∼1351 cm−1 (D band) is due to defects and disorders in the rGO, while peaks at ∼1593 cm−1 (G band) arise from the graphitic nature.12 Brunauer−Emmett−Teller (BET) data in Figure 3d indicate that the 3DGT has a surface area of 124.6 m2 g−1 with numerous mesopores centered at 2.2 nm. Such narrow pore size distribution also suggests a homogeneous dispersion of TeNWs in the interlayer of rGO. Cyclic voltammetry (CV) was conducted to demonstrate the electrochemical process of the 3DGT in the voltage range of 1−3 V at a sweep rate of 0.1 mV s−1 for six cycles. As shown in Figure 4a, two cathodic peaks appear for 3DGT in the initial sweep. The sharp cathodic peak is located at 1.56−1.68 V and corresponds to the formation of long-chain Li2Ten (4 ≤ n ≤ 8) and Li2Te2/ Li2Te, respectively.25,26 During the subsequent anodic scan, a sharp anodic peak is detected at 1.82 V and corresponds to its reverse process. The reaction mechanism between tellurium and lithium is proved by the in situ Raman shift and is subject to discussion as follows. After the third cycle, no obvious change is detected for the sharp redox peaks, indicating the pronounced electrochemical stability of 3DGT cathode. Such results are consistent with galvanostatic charge/discharge profiles (Figure 4b). Even after 200 cycles, the charge and discharge curves stay nearly the same, implying that the 3DGT has excellent capacity retention. The charge/discharge profiles also indicate the good capacity reversibility of the 3DGT cathode. The cyclic performance is a key factor for battery applications. As shown in Figure 4c, the 3DGT cathodes exhibit stable cyclic performances at 0.2 C with a high initial capacity of 2611 mAh cm−3 (theoretical capacity: 2621 mAh cm−3) and a final capacity of 2296 mAh cm−3 (368 mAh g−1) after 200 cycles, correlating to a capacity retention of 88%. The excellent cyclic performance of the 3DGT is attributed to the abundant voids present in the composite that provides enough space to compromise the volume change of TeNWs during cycling. It is known that the LiNO3-additive-free electrolyte only provides weak protection to Li anode. As shown in Figure 4c, the 3DGT exhibits a high Columbic efficiency (∼99%), suggesting that the resulted cathode effectively prevents the typical shuttling phenomenon. It is noted that 3DGT cathode not only exhibits good cycling stability but also possesses excellent rate performance. The rate performance of the 3DGT cathode was obtained under various C-rates from 0.2 to 10 C, as shown in Figure 4d. With the increase of the C-rate from 0.2 to 10 C, the specific capacity of the

Figure 5. (a) CV curves of the 3DGT cathode in the third cycle. (b) Contour plot of Raman spectra of the 3DGT electrode recorded during the third cycle. 8840

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(85%, w/w%) and 3.5 mL aqueous ammonia solution were added to the solution. Then, the solution was transferred into a Teflon vessel (50 mL in volume) and maintained at 180 °C for 3 h before reaching the room temperature naturally.23 Synthesis of 3DGT. Graphene oxides (GOs) were synthesized with a modified Hummer’s method.27 To obtain 3DGT, TeNWs (1 g) was first dissolved in 30 mL ethanol, and then 30 mL GO suspension (6 mg mL−1) was added in the solution with vigorous magnetic stirring for 30 min. The solution was sealed in a 60 mL Teflon-lined stainless steel autoclave at 180 °C for 12 h. Black hydrogel was obtained as the autoclave cooled to the room temperature. The black hydrogel was fully immersed in water to remove the ethanol inside the hydrogel. The 3DGT aerogel was then obtained upon freeze-drying for 48 h. Characterizations. The component ratio of the composite was determined by TGA (TA Instruments TGA-Q50) in the temperature range of 50−800 °C at a heating rate of 10 °C min−1 under N2 atmosphere. The structure of the obtained samples was characterized by X-ray diffraction (XRD Rigaku D/MAX-rA diffractometer) using Cu Kα radiation. Raman spectra were acquired at the room temperature with laser excitation at 532 nm (Horiba). The surface area analysis was conducted using BET theory (Quantachrome, nova 2000e). The electrical conductivity was measured with a standard four-point probe. The morphology analysis was performed with a scanning electron microscope (SEM, JSM-7000F, JEOL) and a transmission electron microscope (TEM, Tecnai F20 at 200 kV). Electrochemical Measurements. The 3DGT aerogel was cut and compressed into a circular pellet with a diameter of 14 mm and used as electrode. TeNW mass loading in the cathode was 1.1 mg cm−2. Cointype (CR2025) cells were assembled in an argon-filled MBraun glovebox with oxygen and water content below 0.5 ppm, with lithium metal as an anode. Celgard 2400 was used as separator. 1.0 M lithium bis (trifluoromethanesulfonyl)imide (LiTFSI) in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (v/v = 1:1) (Zhangjiagang GuotaiHuarong New Chemical Materials Co., Ltd.) was used as electrolyte. Cyclic voltammetry (CV) and EIS were conducted by CHI 660D electrochemical workstation (CHI instrument). CV measurements were performed at a scan rate of 0.1 mV s−1 in the voltage range from 1 to 3 V. EIS tests were carried out at open-circuit potential in the frequency range between 100 kHz and 0.01 Hz. Galvanostatic charge−discharge cycles were tested by LAND CT2001A instrument (Wuhan Jinnuo Electronic Co. Ltd.) at various C-rates between 1 and 3 V (vs Li+/Li) at the room temperature. In this study, 1 C corresponds to a current density value of 420 mA g−1, and specific capacity values were calculated with known TeNW mass. In situ Raman spectra were collected simultaneously with CV test, using a special designed stainless steel cell with a small quartz transparent window. A 785 nm excitation laser was employed with a cumulative time of 50 s in the Raman spectrum acquirement.

indicating that the lithium ions have been effectively lithiated in Te and transformed to Li2Te. When oxidized, the intensities of A1 and E modes increase, confirming the reversible reaction of Li2Te to Te. In addition, the rapid decreases of the Raman peak intensity at the reduction CV peak and the quick increase of the Raman peak intensity at the oxidization CV peak indicate the fast intercalation and deintercalation of Li+ into 3DGT. Electrochemical impedance spectroscopy (EIS) was taken on the cell at an open circuit potential after 500th cycles, as shown in Figure 6. Both plots display a semicircle at the high frequency

Figure 6. EIS spectra of 3DGT fresh cells and those after 500th cycles.

assigned to the charge-transfer resistance (Rct) of the electrode and the straight line at the low-frequency region corresponding to a semi-infinite Warburg diffusion process (Zw). Moreover, the junction of the semicircle at the real axis in the high frequency corresponds to the internal resistance (Rs). It is noted that even after 500 cycles, the cell still maintains a low impedance, which illustrates pronounced structural stability of the 3DGT. The reason for this is that 3DGT possesses a higher conductivity and the interconnected pores inside, which facilitates the electron and ion transportation.

CONCLUSION In summary, three-dimensional rGO/tellurium nanowire aerogel with ultrathin Te nanowires wrapped homogeneously with rGO was fabricated via a simple hydrothermal reaction. The 3DGT aerogel exhibits a highly porous network architecture and sufficient electrically conducting pathways, which can be cut and compressed into pellets for direct use as cathode of Li−Te battery without requiring any metallic current collectors, binder, and conductive additive. Due to its advantageous structure, the 3DGT exhibits high electrochemical performances: It displays a high discharge capacity with a high initial capacity of 2611 mAh cm−3 (418.4 mAh g−1) at 0.2 C and a high retention of 88% after 200 cycles. The high-rate capacity up to 10 C is as large as 1083 mAh cm−3 (173.6 mAh g−1). After 500 cycles, the 3DGT aerogel cathode maintains a capacity of ∼1685 mAh cm−3 (270 mAh g−1) at a 1 C, showing pronounced long-cycle performances at a high current density. The 3DGT is promising as high-capacity cathode for flexible lithium−tellurium batteries.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04622. Computational calculations (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The research was supported by the National Natural Science Foundation of China (grant nos. 51372033, 21403031, 51202022, and 61378028), the Fundamental Research Funds for the Central Universities (grant nos. ZYGX2013Z001,

EXPERIMENTAL SECTION Synthesis of Tellurium Nanowires (TeNWs). One g polyvinylpyrrolidone (PVP) and 0.089 g Na2TeO3 (0.4 mmol) were dissolved in 35 mL of deionization water. 1.65 mL hydrazine hydrate 8841

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ZYGX2014J088, and ZYGX2015Z003), National High Technology Research and Development Program of China (grant no. 2015AA034202), the 111 Project (grant no. B13042), the Specialized Research Fund for the Doctoral Program of Higher Education (grant no. 20120185120011), the Science & Technology Support Funds of Sichuan Province (grant no. 2016GZ0151), Sichuan Youth Science and Technology Innovation Research Team Funding (grant no. 2011JTD0006), and the Sino-German Cooperation PPP Program of China (grant no. 201400260068).

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