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Three-dimensional tungsten nitride nanowires as high performance anode material for lithium ion batteries
Journal of Power Sources 322 (2016) 163e168
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Three-dimensional tungsten nitride nanowires as high performance anode material for lithium ion batteries Min Zhang a, Yongfu Qiu a, c, Yi Han b, Yan Guo b, Faliang Cheng a, * a
Guangdong Engineering and Technology Research Center for Advanced Nanomaterials, Dongguan University of Technology, Guangdong 523808, PR China School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China c College of Chemistry and Environmental Engineering, Dongguan University of Technology, Guangdong 523808, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
WN nanowires were synthesized on carbon cloth by a facile hydrothermal process. The WNNW electrode could deliver a high capacity of 400 mAh g1 at 200 mA g1. The WNNW electrode also show a good rate and cyclic performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2015 Received in revised form 6 April 2016 Accepted 9 April 2016
Nanostructure materials often achieve low capacity when the active material mass loading is high. In this communication, high mass-loading tungsten nitride nanowires (WNNWs) were fabricated on a flexible carbon cloth by hydrothermal method and post annealing. The prepared electrode exhibited remarkable cyclic stability and attractive rate capability for lithium storage. It delivers at a current density of 200 mA g1, a high capacity of 418 mAh g1, which is higher than that of conventional graphite. This research opens more opportunity for the fabrication of three-dimensional metal nitrides as negative electrode material for flexible lithium ion batteries. © 2016 Published by Elsevier B.V.
Keywords: Tungsten nitride Nanowire High mass loading Three-dimensional Lithium ion battery
1. Introduction In the last two decades, lithium-ion batteries (LIBs) have been widely used in portable electronic devices and explored as power sources for electric or hybrid electric vehicles [1e5]. To achieve suitable LIBs for practical usage, new electrode materials with high performance such as high capacity, excellent stability and high rate
* Corresponding author. E-mail address: chengfl@dgut.edu.cn (F. Cheng). http://dx.doi.org/10.1016/j.jpowsour.2016.04.049 0378-7753/© 2016 Published by Elsevier B.V.
capability call for attention [6e8]. In this vein, transition metal nitrides with high electronic and thermal conductivity when compared to the matured transition metal oxides and sulfides [9e13] have also been used for different purposes such as in semiconductor industries, as electrocatalyst [14], energy storage [15e17] and coating material [18] and so on. Among these metal nitrides, tungsten nitrides (WN) hold great promise as anode for LIBs because they possesses significant advantages of moderately high capacity and good electrical conductivity [19,20]. However, WN is less frequently reported as LIB anode material due to the fact that it has high atomic weight, which could lead to low capacity
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[19]. Thus, it is necessary to fabricate WN with high capacity. Nanostructured materials have drawn intense attention when developing highly efficient electrochemical energy devices because of their high surface area, more active sites for the accommodation of ions, significantly enhanced kinetics, decreased huge volume changes and provide short diffusion pathways for ion insertion and extraction [21e26]. Among various nanostructures, threedimensional (3D) nanostructures have recently attracted intensive research interests because they have much higher surface area than low dimensional materials and supply enough absorption sites for all involved species in a small space [27e29]. Moreover, such 3D materials with high porosity could lead to better transportation of ions and electrons [25]. However, tungsten nitride 3D nanostructures are rarely reported. Furthermore, the most common problem of nanomaterials is the inability to achieve high specific capacity when the mass of the active materials is too high. High mass loading electrode materials are necessary for industrial applications in order to attain satisfactory capacity required to power a device. Hence, high mass loading electrodes call for urgent attention. Herein, we synthesized high mass loading tungsten nitride nanowires (WNNWs) with 8.0 mg/cm2 through hydrothermal process and post-annealing of tungsten oxide (WO3). When used as anode material for LIBs, they exhibited a high initial capacity of 418 mAh g1 at a current density of 200 mA g1, which is higher than that of conventional graphite (372 mAh g1). When the current density was increased to 4000 mA g1, the WNNWs could retain a capacity of 110 mAh g1 after 110 cycles. The lithium storage performance can be attributed to the synergistic effects of the nanowire nanostructure and robust 3D carbon cloth substrate. Even at a higher current density equivalent to 10 C, a capacity of about 100 mAh g1 can be recovered, which suggest its use as high performance anode for lithium ion batteries. Additionally, the growth of the WNNW on 3D flexible substrates also creates opportunity for the utilization of high mass loading electrode as flexible anode for lithium ion batteries. 2. Experimental section 2.1. Sample preparation All the reagents used were of analytical grade and were used directly without any purification. Free-standing WNNWs were grown on carbon cloth by a two-step process. WO3 nanowires were firstly synthesized on carbon cloth via a seed-assisted hydrothermal method. Carbon fabric cloth (2 cm 3 cm) was cleaned with ethanol and distilled water, followed by being immersed in a solution containing 0.695 g of Na2WO4$2H2O, 10 mL of 3 M HCl, and 2 mL of H2O2 (30 vol% aqueous solution) for 5 min and blow-dried with compressed air. The dried carbon cloth was further heated on a hotplate in air at 300 C for 5 min, forming WO3 nanoparticles on the carbon fabric cloth. 1.33 g of H2WO4 was dissolved in a mixed solution of 22 mL of distilled water and 6 mL of H2O2 (30 vol% aqueous solution), then 16 mL of ethanol and 0.04 g of NH4Cl were added into 4 mL of the previous solution and stirred into a clear solution. 20 mL of this clear solution mixture together with the carbon fabric cloth coated with WO3 nanoparticles were transferred to a Teflon-lined stainless-steel autoclave (25 mL volume). The sealed autoclave was heated in an electric oven at 180 C for 12 h, and then allowed to cool down slowly at room temperature. A light-blue WO3 film was uniformly coated on the carbon fabric cloth surface. The sample was thoroughly washed with deionized (DI) water and dried. Finally, the WO3 nanowires were converted to WN nanowires by annealing in N2 up to 600 C and then NH3 for 30 min.
2.2. Material characterization Structure and morphology of the as-prepared samples were characterized by X-ray diffraction spectrometry (XRD, D8 ADVANCE) with Cu Ka radiation (l ¼ 1.5418 A), Raman spectrometry (Renishaw inVia) at room temperature with Arþ laser of 514.5 nm excitation, field-emission scanning electron microscopy (FESEM) (FESEM, JSM-6330 F), and transmission electron microscopy (TEM) (JEM2010-HR, 200 KV). 2.3. Electrochemical test The electrochemical tests were carried out with CR2032 coin type cells. The samples grown on carbon textile were first cut into many smaller square pieces with 1.0 cm2 area. Both the carbon textile with loading samples and bare carbon textile were weighed in a high-precision analytical balance (Sartorius, max. weight 5100 mg, d ¼ 0.001 mg). The reading difference was the exact mass of the coating on carbon textile. The reading difference was taken from the same carbon textile before and after the hydrothermal and annealing process of the WNNWs. The loading density of the active materials is 8.0 mg/cm2. The samples grown on carbon textile were used as the working electrodes. Coin cells were assembled in an argon-filled glove box [Mikrouna (China) Co., Ltd.] with the WNNWs sample used as the working electrode, Celgard 2400 as the separator, lithium foil as counter and reference electrode with an electrolyte consisting of 1 M LiPF6 in 1:1 by volume of ethylene carbonate (EC)/dimethyl carbonate (DMC). Charge and discharge measurements were carried out on a Neware battery tester (CT3008-164, Shenzhen, China) at a voltage range of 0.01e3 V (vs. Liþ/ Li). Cyclic voltammetry (CV) at a scan rate of 0.1 mV/s and cell impedances measurements for the two-electrode arrangement were conducted on an electrochemical work station (CHI 760d, Chenhua, Shanghai) in a frequency range of 101 Hz105 Hz. 2.4. Assembling of the flexible LIBs Flexible LIB was assembled by using the flexible WNNW as the anode (2 cm 3 cm), an Al foil coated with LiCoO2 as the cathode (2.5 cm 3 cm) and Celgard 2400 as the separator. The Cu foil was joined to the edge of the flexible WNNW anode and sticks to the carbon cloth with a conductive tape, and sealed by a sealing machine. The Cu foil joined to the edge of the flexible WNNW anode was used as the current conductor to enhanced easy sealing because the carbon textile is too thick to be sealed with the Al bag. The electrolyte was dropped on the electrodes in Al bag and sealed in argon-filled glove box. 3. Results and discussion Free-standing WNNWs were fabricated on a flexible carbon cloth in two steps. WO3 nanowires were first grown on the flexible carbon cloth as discussed in the experimental section. Images from the field-emission scanning electron microscope (FESEM) shows that the carbon cloth was uniformly coated with WO3 nanowires (Fig. 1a). The nanowires of the diameters ranged 50e80 nm (Fig. 1b). These nanowires were then subjected to annealing in N2 to 600 C at a ramping rate of 5 C per minute. At 600 C, the flowing gas was changed to NH3 gas for 30 min. After cooling, the product obtained was WNNWs. As observed in Fig. 1c and d, the nanowire morphology was maintained after annealing. Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) were carried out to reveal detailed morphology of the WNNWs. TEM image confirms that the WN are nanowire-like in nature with rough surface (Fig. 2a). The bright and
M. Zhang et al. / Journal of Power Sources 322 (2016) 163e168
Fig. 1. (a) SEM image of the WO3 nanowires (b) High magnification SEM image of the WO3 nanowires. (c and d) SEM images of the WNNW.
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which are consistent with the d-spacing of (200) planes of cubic WN (JCPDS #65e2898) (Fig. 2b). Nevertheless, energy dispersive Xray spectroscopy (EDS) data reveal that the WNNW has the W: N: O ratio of 49: 42: 9. Note that the small amount of oxygen suggests the presence of WO3 and/or WOxNy in the WNNW resulting from the minor oxidation of the WN after annealing and cooling, but such oxidation could not affect the WN cubic phase. Similar reports on the presence of small amount of oxygen in the metal nitrides have also been reported TiN [30,31]. Additionally, EDS elemental mapping data show that these elements are uniformly distributed in the nanowire according to Fig. 2d-g. The distribution of W, N and O is represented in light blue, green and orange colour, respectively (Fig. 2d-g). X-Ray diffractometer (XRD) analysis was carried out to study the phase composition of the WNNWs. Fig. 3 displayed the XRD pattern and Raman spectra of the WNNWs. Significantly, XRD pattern confirm the transformation of WO3 (black line) to cubic WN (red line) after annealing (Fig. 3a). Intense peaks of WN were well observed, confirming that our as-prepared material is WN (JCPDS #65e2898). There were no traces of WO3 peaks further justifying that the sample is purely crystalline WN. Similarly, the Raman spectra of the sample shows Raman peaks around 210 and 800 cm1, which corresponds to the Raman peaks of WN [32,33] and also confirmed the successful fabrication of the tungsten nitride (Fig. 3b). The peaks beyond 1000 cm1 are attributed to the Raman peaks of the carbon cloth. In order to test the electrochemical performance of the prepared WNNW electrode for application in lithium ion batteries, several electrochemical analyses were conducted. Fig. 4a displays the cyclic voltammetry (CV) of the WNNW for the 1st-3rd cycle at a scan rate of 0.01 mV/s between 0.01 V and 3 V. There are many cathodic peaks at 1.93 V, 1.73 V, 1.44 V, 0.59 V and 0.27 V in the initial discharging process, while only two anodic peak at 0.37 V and 1.57 V in the charging process. The multi-peaks imply that the electrochemical behaviour of WN undergoes a complicated process, which means that there may be a series of insertion and decomposition reactions of WN with Li during the discharging and charging process. The cathodic peaks above 1.0 V in the first reduction process may be due to the formation of the insertion compound of LixWN, while those at 0.59 V and 0.27 V may be the formation process of metal W and reaction of the Li ion with the carbon cloth substrate. Similar reaction has been reported for other metal nitrides [31,34]. We proposed the reversibility process of Li insertion/desertion, which is common to most metal nitride as written below [11,16,31]:
xWN þ 3xLiþ þ 3xe 4xW þ xLi3 N
Fig. 2. (a) TEM image, (b) SAED pattern and (c) HRTEM image of the WNNW sample. (d-g) Elemental mapping of the same nanowire region, indicating spatial distribution of W (light blue), N (green), and O (orange), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
regular diffraction spotted via the SAED pattern revealed that the WNNW is polycrystalline (Fig. 2c). The high resolution TEM (HRTEM) image of Fig. 2a shows the lattice spacings to be 0.20 nm,
As can be seen from the charge-discharge profile, the first/initial discharge capacity of the electrode reaches 418 mAh g1 and charge capacity of 293 mAh g1 at the current density of 200 mA g1 (71% coulombic efficiency), which is higher than the theoretical capacity of the conventional graphite (Fig. 4b). During the second cycle, the discharge capacity decreases to 314 mAh g1. The reduction in capacity and slight loss in the coulombic efficiency can be attributed to the loss in the series of multi-peaks after the first lithiation process as a result of the formation of the solid electrolyte interface (SEI) layer, which is common to most LIB anode materials that exhibited conversion reactions [13,31]. Such peak loss can be clearly seen in 2nd cycle CV curve of the electrode (Fig. 4a). Thus, only the peaks at 0.81 V and 0.21 V were observed corresponding to the voltage peak of the WN and the carbon cloth, respectively. Taking into consideration the capacity contribution of the carbon cloth, we suggested that carbon cloth should contributed less than 9% of the total capacity because its contribution decreases with increasing mass loading [35e37]. Herein, our mass loading is
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Fig. 3. (a) XRD pattern and (b) Raman spectra of the WNNW sample.
Fig. 4. (a) CV curve in the potential window of 0.01e3 V at a scan rate of 0.01 mV/s and (b) charge-discharge profiles at a current density of 200 mA g1 of the WNNW electrode.
8.0 mg/cm2, which doubled that of the SnO2@TiO2 (with 4.0 mg/ cm2 mass loading and 9% carbon cloth capacity contribution) [37]. This indicated that the carbon cloth has lesser capacity contribution to the capacity of the WNNW electrode. The performance of the WNNW was compared with that of WN nanoparticles in order to study the morphological effect of the WN. According to Fig. 5a and at a low current density of 0.2 C, the WNNWs displayed a capacity of 450 mAh g1 after about 10 cycles, which is higher than that of the WN nanoparticles at 175 mAh g1 and even higher than the theoretical capacity of the conventional graphite. Furthermore, EIS measurements (details of the Nyquist plots will be discussed later in the manuscript) shown in Fig. 5b further confirmed that the impedance of the WNNW is smaller than that of the WN nanoparticles. These results suggest the better performance of the WNNW over that of the WN nanoparticles. Meanwhile, the WNNW electrode was cycled at various current densities ranging from 200 to 4000 mA g1 and back to initial current density of 200 mA g1 in order to study the rate performance of the electrode (Fig. 5c). During the rate performance test, the battery was continuously subjected to a long cycling of 50 cycles at current density 400 mA g1 to test the cycling stability of the WNNW electrode as shown in Fig. 5a.The reversible specific capacities of the WNNW electrode were 294 mAh g1, 207 mAh g1, 139 mAh g1, and 81 mAh g1 at 200 mA g1, 400 mA g1, 2000 mA g1 and 4000 mA g1 respectively. When the current density returned to 200 mA g1, the capacity of the WNNW electrode was also maintained at 330 mAh g1, confirming that the WNNW electrode exhibited excellent rate capability and reversibility. With an initial capacity at 228 mAh g1, the long cycling for
50 cycles shows that the electrode could maintain a capacity of 196 mAh g1, which means retained capacity retention of 86%, indicating excellent stability of the HMWWNW electrode. Additionally, another 10 cycles at current densities of 2000 mA g1 and 4000 mA g1 show discharge capacities of 130 mAh g1 and 73 mAh g1, which is above 90% of their initial capacities, respectively. This continuously affirmed the excellent stability and rate capability of the WNNW electrode. It should be noted that such remarkable results can be achieved for WN when the mass loading is 8.0 mg/cm2. Electrochemical impedance spectroscopy (EIS) was carried out so as to provide electrochemical insights. The WNNW cells Nyquist plots taken in frequency range of 0.1 Hze100 kHz at open circuit potential are shown in Fig. 5d. The EIS data before and after cycling are very similar with a semicircle at the high frequency region, middle frequency region and a spike at the low frequency region. The diameter of the first semicircle is assigned to the resistance and capacitance between the current collectors (Rpp), while that in the middle frequency region is attributed to the charge-transfer resistance (Rct) (inset of Fig. 5d) [38,39]. It was clearly observed that the Rp and Rct impedance after cyclization was much smaller than the impedance before cyclization. This reduction of the resistance led to a more rapid electron transport during the electrochemical lithium insertion/extraction, which was one of the reasons of the excellent performance of the WNNW electrode. The excellent performance of the WNNW electrode can further be attributed to the synergistic effect of the 1D nanostructure and the mechanical support from the 3D carbon cloth substrate, which in general could improve the kinetics of the electrode and thereby leads to excellent
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Fig. 5. (a) Cyclic performance at a current density of 0.2 C and (b) EIS measurements of the WNNW and WN nanoparticles. (c) Rate performance and cycling performance at different current densities up to 100 cycles and (d) Nyquist plot of the WNNW electrode. Inset is the equivalent circuit use to fit the plot.
lithium storage performance. Finally, since the carbon cloth is light, flexible and bendable, the applicability of such anodes in flexible electronics was demonstrated [40]. Using a commercially available cathode (LiCoO2) and
our as-fabricated anode (WNNW) as displayed in Fig. 6a, a largescale flexible full battery was further assembled and fully charged to 3 V (Fig. 6b). It is able to light red LED in the flat position (Fig. 6c) and bend position (Fig. 6d). The bending demonstration of the WNNW//LiCoO2 flexible battery ensures its potential for use in flexible electrochemical storage devices. 4. Conclusions In summary, we have successfully synthesized a high mass loading 3D WN nanowire as anode for LIBs. Because of the structural design, the electrode demonstrated excellent LIB performance. The rate capability of the WNNW electrode achieved a discharge capacity of 81 mAh g1 at a high current density of 4000 mA g1. The WNNW electrode also exhibited attractive high rate cycling performance, retaining a capacity of 86% after 50 cycles upon a high mass loading of 8.0 mg/cm2. Moreover, the WNNW electrode was synthesized on a carbon cloth, which could make the whole system a promising material to be utilized in the flexible lithium ion batteries. Our findings create a promising avenue to developing new metal nitride anodes with 3D nanostructures for efficient LIBS. Acknowledgments The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 21375016, 20475022, 21505019), the Natural Science Foundations of Guangdong Province (No. S2013010014324, 2015A030310272), Technology Planning Project of Guangdong Province (No. 2015B090927007).
Fig. 6. (a) Optical images of the WNNW anode and LiCoO2 cathode before assembling the flexible Li-ion battery. (b) Image of the assembled flexible LIB device during charging. Demonstration of the WNNW//LiCoO2 LIB device powering a red LED at the (c) flat position and (d) bending position. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Three-dimensional tungsten nitride nanowires as high performance anode material for lithium ion batteries Min Zhang a, Yongfu Qiu a, c, Yi Han b, Yan Guo b, Faliang Cheng a, * a
Guangdong Engineering and Technology Research Center for Advanced Nanomaterials, Dongguan University of Technology, Guangdong 523808, PR China School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China c College of Chemistry and Environmental Engineering, Dongguan University of Technology, Guangdong 523808, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
WN nanowires were synthesized on carbon cloth by a facile hydrothermal process. The WNNW electrode could deliver a high capacity of 400 mAh g1 at 200 mA g1. The WNNW electrode also show a good rate and cyclic performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2015 Received in revised form 6 April 2016 Accepted 9 April 2016
Nanostructure materials often achieve low capacity when the active material mass loading is high. In this communication, high mass-loading tungsten nitride nanowires (WNNWs) were fabricated on a flexible carbon cloth by hydrothermal method and post annealing. The prepared electrode exhibited remarkable cyclic stability and attractive rate capability for lithium storage. It delivers at a current density of 200 mA g1, a high capacity of 418 mAh g1, which is higher than that of conventional graphite. This research opens more opportunity for the fabrication of three-dimensional metal nitrides as negative electrode material for flexible lithium ion batteries. © 2016 Published by Elsevier B.V.
Keywords: Tungsten nitride Nanowire High mass loading Three-dimensional Lithium ion battery
1. Introduction In the last two decades, lithium-ion batteries (LIBs) have been widely used in portable electronic devices and explored as power sources for electric or hybrid electric vehicles [1e5]. To achieve suitable LIBs for practical usage, new electrode materials with high performance such as high capacity, excellent stability and high rate
* Corresponding author. E-mail address: chengfl@dgut.edu.cn (F. Cheng). http://dx.doi.org/10.1016/j.jpowsour.2016.04.049 0378-7753/© 2016 Published by Elsevier B.V.
capability call for attention [6e8]. In this vein, transition metal nitrides with high electronic and thermal conductivity when compared to the matured transition metal oxides and sulfides [9e13] have also been used for different purposes such as in semiconductor industries, as electrocatalyst [14], energy storage [15e17] and coating material [18] and so on. Among these metal nitrides, tungsten nitrides (WN) hold great promise as anode for LIBs because they possesses significant advantages of moderately high capacity and good electrical conductivity [19,20]. However, WN is less frequently reported as LIB anode material due to the fact that it has high atomic weight, which could lead to low capacity
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[19]. Thus, it is necessary to fabricate WN with high capacity. Nanostructured materials have drawn intense attention when developing highly efficient electrochemical energy devices because of their high surface area, more active sites for the accommodation of ions, significantly enhanced kinetics, decreased huge volume changes and provide short diffusion pathways for ion insertion and extraction [21e26]. Among various nanostructures, threedimensional (3D) nanostructures have recently attracted intensive research interests because they have much higher surface area than low dimensional materials and supply enough absorption sites for all involved species in a small space [27e29]. Moreover, such 3D materials with high porosity could lead to better transportation of ions and electrons [25]. However, tungsten nitride 3D nanostructures are rarely reported. Furthermore, the most common problem of nanomaterials is the inability to achieve high specific capacity when the mass of the active materials is too high. High mass loading electrode materials are necessary for industrial applications in order to attain satisfactory capacity required to power a device. Hence, high mass loading electrodes call for urgent attention. Herein, we synthesized high mass loading tungsten nitride nanowires (WNNWs) with 8.0 mg/cm2 through hydrothermal process and post-annealing of tungsten oxide (WO3). When used as anode material for LIBs, they exhibited a high initial capacity of 418 mAh g1 at a current density of 200 mA g1, which is higher than that of conventional graphite (372 mAh g1). When the current density was increased to 4000 mA g1, the WNNWs could retain a capacity of 110 mAh g1 after 110 cycles. The lithium storage performance can be attributed to the synergistic effects of the nanowire nanostructure and robust 3D carbon cloth substrate. Even at a higher current density equivalent to 10 C, a capacity of about 100 mAh g1 can be recovered, which suggest its use as high performance anode for lithium ion batteries. Additionally, the growth of the WNNW on 3D flexible substrates also creates opportunity for the utilization of high mass loading electrode as flexible anode for lithium ion batteries. 2. Experimental section 2.1. Sample preparation All the reagents used were of analytical grade and were used directly without any purification. Free-standing WNNWs were grown on carbon cloth by a two-step process. WO3 nanowires were firstly synthesized on carbon cloth via a seed-assisted hydrothermal method. Carbon fabric cloth (2 cm 3 cm) was cleaned with ethanol and distilled water, followed by being immersed in a solution containing 0.695 g of Na2WO4$2H2O, 10 mL of 3 M HCl, and 2 mL of H2O2 (30 vol% aqueous solution) for 5 min and blow-dried with compressed air. The dried carbon cloth was further heated on a hotplate in air at 300 C for 5 min, forming WO3 nanoparticles on the carbon fabric cloth. 1.33 g of H2WO4 was dissolved in a mixed solution of 22 mL of distilled water and 6 mL of H2O2 (30 vol% aqueous solution), then 16 mL of ethanol and 0.04 g of NH4Cl were added into 4 mL of the previous solution and stirred into a clear solution. 20 mL of this clear solution mixture together with the carbon fabric cloth coated with WO3 nanoparticles were transferred to a Teflon-lined stainless-steel autoclave (25 mL volume). The sealed autoclave was heated in an electric oven at 180 C for 12 h, and then allowed to cool down slowly at room temperature. A light-blue WO3 film was uniformly coated on the carbon fabric cloth surface. The sample was thoroughly washed with deionized (DI) water and dried. Finally, the WO3 nanowires were converted to WN nanowires by annealing in N2 up to 600 C and then NH3 for 30 min.
2.2. Material characterization Structure and morphology of the as-prepared samples were characterized by X-ray diffraction spectrometry (XRD, D8 ADVANCE) with Cu Ka radiation (l ¼ 1.5418 A), Raman spectrometry (Renishaw inVia) at room temperature with Arþ laser of 514.5 nm excitation, field-emission scanning electron microscopy (FESEM) (FESEM, JSM-6330 F), and transmission electron microscopy (TEM) (JEM2010-HR, 200 KV). 2.3. Electrochemical test The electrochemical tests were carried out with CR2032 coin type cells. The samples grown on carbon textile were first cut into many smaller square pieces with 1.0 cm2 area. Both the carbon textile with loading samples and bare carbon textile were weighed in a high-precision analytical balance (Sartorius, max. weight 5100 mg, d ¼ 0.001 mg). The reading difference was the exact mass of the coating on carbon textile. The reading difference was taken from the same carbon textile before and after the hydrothermal and annealing process of the WNNWs. The loading density of the active materials is 8.0 mg/cm2. The samples grown on carbon textile were used as the working electrodes. Coin cells were assembled in an argon-filled glove box [Mikrouna (China) Co., Ltd.] with the WNNWs sample used as the working electrode, Celgard 2400 as the separator, lithium foil as counter and reference electrode with an electrolyte consisting of 1 M LiPF6 in 1:1 by volume of ethylene carbonate (EC)/dimethyl carbonate (DMC). Charge and discharge measurements were carried out on a Neware battery tester (CT3008-164, Shenzhen, China) at a voltage range of 0.01e3 V (vs. Liþ/ Li). Cyclic voltammetry (CV) at a scan rate of 0.1 mV/s and cell impedances measurements for the two-electrode arrangement were conducted on an electrochemical work station (CHI 760d, Chenhua, Shanghai) in a frequency range of 101 Hz105 Hz. 2.4. Assembling of the flexible LIBs Flexible LIB was assembled by using the flexible WNNW as the anode (2 cm 3 cm), an Al foil coated with LiCoO2 as the cathode (2.5 cm 3 cm) and Celgard 2400 as the separator. The Cu foil was joined to the edge of the flexible WNNW anode and sticks to the carbon cloth with a conductive tape, and sealed by a sealing machine. The Cu foil joined to the edge of the flexible WNNW anode was used as the current conductor to enhanced easy sealing because the carbon textile is too thick to be sealed with the Al bag. The electrolyte was dropped on the electrodes in Al bag and sealed in argon-filled glove box. 3. Results and discussion Free-standing WNNWs were fabricated on a flexible carbon cloth in two steps. WO3 nanowires were first grown on the flexible carbon cloth as discussed in the experimental section. Images from the field-emission scanning electron microscope (FESEM) shows that the carbon cloth was uniformly coated with WO3 nanowires (Fig. 1a). The nanowires of the diameters ranged 50e80 nm (Fig. 1b). These nanowires were then subjected to annealing in N2 to 600 C at a ramping rate of 5 C per minute. At 600 C, the flowing gas was changed to NH3 gas for 30 min. After cooling, the product obtained was WNNWs. As observed in Fig. 1c and d, the nanowire morphology was maintained after annealing. Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) were carried out to reveal detailed morphology of the WNNWs. TEM image confirms that the WN are nanowire-like in nature with rough surface (Fig. 2a). The bright and
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Fig. 1. (a) SEM image of the WO3 nanowires (b) High magnification SEM image of the WO3 nanowires. (c and d) SEM images of the WNNW.
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which are consistent with the d-spacing of (200) planes of cubic WN (JCPDS #65e2898) (Fig. 2b). Nevertheless, energy dispersive Xray spectroscopy (EDS) data reveal that the WNNW has the W: N: O ratio of 49: 42: 9. Note that the small amount of oxygen suggests the presence of WO3 and/or WOxNy in the WNNW resulting from the minor oxidation of the WN after annealing and cooling, but such oxidation could not affect the WN cubic phase. Similar reports on the presence of small amount of oxygen in the metal nitrides have also been reported TiN [30,31]. Additionally, EDS elemental mapping data show that these elements are uniformly distributed in the nanowire according to Fig. 2d-g. The distribution of W, N and O is represented in light blue, green and orange colour, respectively (Fig. 2d-g). X-Ray diffractometer (XRD) analysis was carried out to study the phase composition of the WNNWs. Fig. 3 displayed the XRD pattern and Raman spectra of the WNNWs. Significantly, XRD pattern confirm the transformation of WO3 (black line) to cubic WN (red line) after annealing (Fig. 3a). Intense peaks of WN were well observed, confirming that our as-prepared material is WN (JCPDS #65e2898). There were no traces of WO3 peaks further justifying that the sample is purely crystalline WN. Similarly, the Raman spectra of the sample shows Raman peaks around 210 and 800 cm1, which corresponds to the Raman peaks of WN [32,33] and also confirmed the successful fabrication of the tungsten nitride (Fig. 3b). The peaks beyond 1000 cm1 are attributed to the Raman peaks of the carbon cloth. In order to test the electrochemical performance of the prepared WNNW electrode for application in lithium ion batteries, several electrochemical analyses were conducted. Fig. 4a displays the cyclic voltammetry (CV) of the WNNW for the 1st-3rd cycle at a scan rate of 0.01 mV/s between 0.01 V and 3 V. There are many cathodic peaks at 1.93 V, 1.73 V, 1.44 V, 0.59 V and 0.27 V in the initial discharging process, while only two anodic peak at 0.37 V and 1.57 V in the charging process. The multi-peaks imply that the electrochemical behaviour of WN undergoes a complicated process, which means that there may be a series of insertion and decomposition reactions of WN with Li during the discharging and charging process. The cathodic peaks above 1.0 V in the first reduction process may be due to the formation of the insertion compound of LixWN, while those at 0.59 V and 0.27 V may be the formation process of metal W and reaction of the Li ion with the carbon cloth substrate. Similar reaction has been reported for other metal nitrides [31,34]. We proposed the reversibility process of Li insertion/desertion, which is common to most metal nitride as written below [11,16,31]:
xWN þ 3xLiþ þ 3xe 4xW þ xLi3 N
Fig. 2. (a) TEM image, (b) SAED pattern and (c) HRTEM image of the WNNW sample. (d-g) Elemental mapping of the same nanowire region, indicating spatial distribution of W (light blue), N (green), and O (orange), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
regular diffraction spotted via the SAED pattern revealed that the WNNW is polycrystalline (Fig. 2c). The high resolution TEM (HRTEM) image of Fig. 2a shows the lattice spacings to be 0.20 nm,
As can be seen from the charge-discharge profile, the first/initial discharge capacity of the electrode reaches 418 mAh g1 and charge capacity of 293 mAh g1 at the current density of 200 mA g1 (71% coulombic efficiency), which is higher than the theoretical capacity of the conventional graphite (Fig. 4b). During the second cycle, the discharge capacity decreases to 314 mAh g1. The reduction in capacity and slight loss in the coulombic efficiency can be attributed to the loss in the series of multi-peaks after the first lithiation process as a result of the formation of the solid electrolyte interface (SEI) layer, which is common to most LIB anode materials that exhibited conversion reactions [13,31]. Such peak loss can be clearly seen in 2nd cycle CV curve of the electrode (Fig. 4a). Thus, only the peaks at 0.81 V and 0.21 V were observed corresponding to the voltage peak of the WN and the carbon cloth, respectively. Taking into consideration the capacity contribution of the carbon cloth, we suggested that carbon cloth should contributed less than 9% of the total capacity because its contribution decreases with increasing mass loading [35e37]. Herein, our mass loading is
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Fig. 3. (a) XRD pattern and (b) Raman spectra of the WNNW sample.
Fig. 4. (a) CV curve in the potential window of 0.01e3 V at a scan rate of 0.01 mV/s and (b) charge-discharge profiles at a current density of 200 mA g1 of the WNNW electrode.
8.0 mg/cm2, which doubled that of the SnO2@TiO2 (with 4.0 mg/ cm2 mass loading and 9% carbon cloth capacity contribution) [37]. This indicated that the carbon cloth has lesser capacity contribution to the capacity of the WNNW electrode. The performance of the WNNW was compared with that of WN nanoparticles in order to study the morphological effect of the WN. According to Fig. 5a and at a low current density of 0.2 C, the WNNWs displayed a capacity of 450 mAh g1 after about 10 cycles, which is higher than that of the WN nanoparticles at 175 mAh g1 and even higher than the theoretical capacity of the conventional graphite. Furthermore, EIS measurements (details of the Nyquist plots will be discussed later in the manuscript) shown in Fig. 5b further confirmed that the impedance of the WNNW is smaller than that of the WN nanoparticles. These results suggest the better performance of the WNNW over that of the WN nanoparticles. Meanwhile, the WNNW electrode was cycled at various current densities ranging from 200 to 4000 mA g1 and back to initial current density of 200 mA g1 in order to study the rate performance of the electrode (Fig. 5c). During the rate performance test, the battery was continuously subjected to a long cycling of 50 cycles at current density 400 mA g1 to test the cycling stability of the WNNW electrode as shown in Fig. 5a.The reversible specific capacities of the WNNW electrode were 294 mAh g1, 207 mAh g1, 139 mAh g1, and 81 mAh g1 at 200 mA g1, 400 mA g1, 2000 mA g1 and 4000 mA g1 respectively. When the current density returned to 200 mA g1, the capacity of the WNNW electrode was also maintained at 330 mAh g1, confirming that the WNNW electrode exhibited excellent rate capability and reversibility. With an initial capacity at 228 mAh g1, the long cycling for
50 cycles shows that the electrode could maintain a capacity of 196 mAh g1, which means retained capacity retention of 86%, indicating excellent stability of the HMWWNW electrode. Additionally, another 10 cycles at current densities of 2000 mA g1 and 4000 mA g1 show discharge capacities of 130 mAh g1 and 73 mAh g1, which is above 90% of their initial capacities, respectively. This continuously affirmed the excellent stability and rate capability of the WNNW electrode. It should be noted that such remarkable results can be achieved for WN when the mass loading is 8.0 mg/cm2. Electrochemical impedance spectroscopy (EIS) was carried out so as to provide electrochemical insights. The WNNW cells Nyquist plots taken in frequency range of 0.1 Hze100 kHz at open circuit potential are shown in Fig. 5d. The EIS data before and after cycling are very similar with a semicircle at the high frequency region, middle frequency region and a spike at the low frequency region. The diameter of the first semicircle is assigned to the resistance and capacitance between the current collectors (Rpp), while that in the middle frequency region is attributed to the charge-transfer resistance (Rct) (inset of Fig. 5d) [38,39]. It was clearly observed that the Rp and Rct impedance after cyclization was much smaller than the impedance before cyclization. This reduction of the resistance led to a more rapid electron transport during the electrochemical lithium insertion/extraction, which was one of the reasons of the excellent performance of the WNNW electrode. The excellent performance of the WNNW electrode can further be attributed to the synergistic effect of the 1D nanostructure and the mechanical support from the 3D carbon cloth substrate, which in general could improve the kinetics of the electrode and thereby leads to excellent
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Fig. 5. (a) Cyclic performance at a current density of 0.2 C and (b) EIS measurements of the WNNW and WN nanoparticles. (c) Rate performance and cycling performance at different current densities up to 100 cycles and (d) Nyquist plot of the WNNW electrode. Inset is the equivalent circuit use to fit the plot.
lithium storage performance. Finally, since the carbon cloth is light, flexible and bendable, the applicability of such anodes in flexible electronics was demonstrated [40]. Using a commercially available cathode (LiCoO2) and
our as-fabricated anode (WNNW) as displayed in Fig. 6a, a largescale flexible full battery was further assembled and fully charged to 3 V (Fig. 6b). It is able to light red LED in the flat position (Fig. 6c) and bend position (Fig. 6d). The bending demonstration of the WNNW//LiCoO2 flexible battery ensures its potential for use in flexible electrochemical storage devices. 4. Conclusions In summary, we have successfully synthesized a high mass loading 3D WN nanowire as anode for LIBs. Because of the structural design, the electrode demonstrated excellent LIB performance. The rate capability of the WNNW electrode achieved a discharge capacity of 81 mAh g1 at a high current density of 4000 mA g1. The WNNW electrode also exhibited attractive high rate cycling performance, retaining a capacity of 86% after 50 cycles upon a high mass loading of 8.0 mg/cm2. Moreover, the WNNW electrode was synthesized on a carbon cloth, which could make the whole system a promising material to be utilized in the flexible lithium ion batteries. Our findings create a promising avenue to developing new metal nitride anodes with 3D nanostructures for efficient LIBS. Acknowledgments The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 21375016, 20475022, 21505019), the Natural Science Foundations of Guangdong Province (No. S2013010014324, 2015A030310272), Technology Planning Project of Guangdong Province (No. 2015B090927007).
Fig. 6. (a) Optical images of the WNNW anode and LiCoO2 cathode before assembling the flexible Li-ion battery. (b) Image of the assembled flexible LIB device during charging. Demonstration of the WNNW//LiCoO2 LIB device powering a red LED at the (c) flat position and (d) bending position. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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