Tungsten nitride nanoplates as an anode material for lithium ion batteries

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CERAMICS INTERNATIONAL

Ceramics International 42 (2016) 1933–1942 www.elsevier.com/locate/ceramint

Tungsten nitride nanoplates as an anode material for lithium ion batteries Han-Chul Park, Si-Jin Kim, Min-Cheol Kim, Da-Mi Kim, Kyung-Won Parkn Department of Chemical Engineering, Soongsil University, Seoul 156-743, Republic of Korea Received 30 June 2015; received in revised form 30 August 2015; accepted 30 September 2015 Available online 9 October 2015

Abstract Well-defined mesoporous nanostructure electrodes have been known to have improved lithium ion reaction properties such as the lithium ion reaction, cyclability, and high rate performance. We suggest mesoporous tungsten nitride nanoplates prepared via a template-free synthesis for lithium-ion batteries. The as-prepared tungsten nitride (m-WN) exhibited a face-centered cubic WN phase, well-defined mesoporous structure. Furthermore, to investigate a formation mechanism of the well-defined mesoporous WN structure formed from two-dimensional WO3 nanoplates. Finally, the m-WN showed improved electrochemical reaction properties of lithium ions such as high specific capacity and high rate cycling performance due to low transport resistance and high lithium ion diffusion coefficient. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: WN; Nanoplates; Anode material; Lithium ion batteries

1. Introduction Recently, lithium-ion batteries (LIBs) have been intensively investigated as an essential energy storage device for portable electronics and electric vehicles. The LIBs consisting of a cathode, anode, separator, and electrolyte have exhibited attractive electrochemical properties such as high energy density and open-circuit voltage, low self-discharge rate, and long cyclability [1–3]. Among these properties, the active materials such as the anode and cathode require high ionic– electronic conductivity, and specific electrochemical active surface area, low cost, and electrochemical stability. In particular, since carbon-based materials used as a promising anode have shown a low theoretical capacity in LIBs, the ceramic materials such as transition metal oxide, nitride, and sulfide as an alternative electrode have been reported [4–7]. Among the candidate ceramic materials, the transition metal nitrides have been intensively investigated with low production cost, slight volume change during lithium ion insertion/ extraction, good structural stability, high electrical conductivity, and long cycle life in LIBs [8–10]. In the case of the n

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http://dx.doi.org/10.1016/j.ceramint.2015.09.163 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

transition metal nitrides in LIBs, a transition of MNx to M and lithium nitride (Li3N) i.e., the conversion reaction during the 1st discharge process occurs. In the subsequent cycles, the conversion reactions between Li3N and transition metal nitride exhibit a reversible insertion/extraction of lithium ion [9–20]. However, the bulk structure of most transition metal nitrides shows a low specific capacity and poor electrochemical properties in LIBs due to a low specific surface area and faster Li-ion diffusion process in comparison with the structurecontrolled electrodes [21]. To improve the lithium ion reaction properties, in particular, the mesoporous nanostructures with highly specific surface area and controlled pores and channels in the nanometer range have been suggested due to major advantages such as many lithium ion reaction sites and short diffusion distance of lithium ion for LIBs [22–25]. Furthermore, recently, Kundu and co-workers reported an electrochemical insertion behavior of lithium in the nanocrystalline VN with a high capacity upon electrochemical cycling, as opposed to a conversion electrochemical process [20,26]. In this work, the mesoporous tungsten nitride with a twodimensional nanostructure for LIBs was prepared using tungsten oxide nanoplates. The well-defined mesoporous tungsten nitride was synthesized using a template-free nitriding process with the tungsten oxide nanoplates. The structural and chemical analysis of

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the as-prepared samples was characterized using field-emission transmission electron microscopy (FE-TEM), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The specific surface area and pore size of the as-prepared electrodes were analyzed using a nitrogen sorption measurement. To evaluate the performance for LIBs, the charge–discharge and high rate curves, cyclic voltammograms (CVs), and electrochemical impedance spectroscopy of the electrodes were measured using lithium coin cells. 2. Experimental part 2.1. Synthesis of mesoporous tungsten nitride electrodes For tungsten oxide (WO3) nanoplates, 2 g ammonium tungstate ((NH4)10H2(W2O7)6) (99.99%, Sigma-Aldrich) was dissolved in 150 mL 5 M HCl (37%, Sigma-Aldrich) with constant stirring at 25 1C for 1 h and then reacted at 140 1C for 6 h. The resulting precipitate was washed several times with distilled water and dried in an oven at 50 1C. The tungsten oxide powder was heated at 700 1C for 4 h under the NH3 flow of 100 mL min  1. The product was cooled to 25 1C in flowing NH3 (99.999%, Seoul Speciality Gases Co., ltd.) followed by passivation for 2 h in air (denoted as m-WN) [27]. For comparison, typical tungsten nitride (denoted as c-WN) was prepared using a nitriding process with commercial WO3 (99%, Fluka) by means of heat treatment at 700 1C for 4 h under an NH3 atmosphere. 2.2. Structural characterization of the as-prepared electrodes The as-prepared samples were characterized by fieldemission TEM using a Tecnai G2 F30 system operating at 300 kV. The TEM samples were prepared by placing a drop of the nanoparticle suspension in ethanol on a carbon-coated copper grid. SEM images and EDX spectra were obtained using a JEOL JSM-6360A microscope operated at 20 kV. Structural analysis of the samples was carried out using an XRD method with a Bruker AXS D2 Phaser X-Ray Diffractometer with a Cu Kα radiation source of λ¼ 0.15418 nm with a Ni filter. Nitrogen adsorption and desorption isotherms were measured at 77 K using a Micromeritics ASAP 2020. Before the adsorption measurements, all samples were outgassed at 473 K for 360 min in the port of the adsorption analyzer. The starting relative pressure was 0.995 P/Po and ending relative pressure was 0.01 P/Po. XPS (XPS, Thermo VG, U.K.) analysis was carried out with an Al Kα X-ray source of 1486.6 eV at a chamber pressure below 1  10  8 Torr and a beam power of 200 W. All high resolution spectra were collected using pass energy of 46.95 eV. The step size and time per step were set at 0.025 eV and 100 ms, respectively. 2.3. Electrochemical characterization of the as-prepared electrodes The electrodes were prepared by mixing 80 wt% WN nanostructure electrode as an active material, 10 wt% Ketjen

black as a conducting agent, and 10 wt% polyvinylidene fluoride (PVDF) as a binder. To obtain the slurry, several drops of N-methyl pyrrolidone were added into the mixture of WN powder with Ketjen black and PVDF. The mixed slurries were cast onto a Cu foil current collector and dried in air at 110 1C for 12 h. The electrode with an area of 1.32 cm2 was dried at 70 1C in a vacuum oven. The average loading amount of active materials for all the electrodes was  0.7 mg cm  2. The electrodes were uniformly coated on 15 mm thick copper foil substrates and the thickness of the electrode was  45 μm after pressing. The electrodes were evaluated with respect to a lithium foil (FMC Corporation) counterelectrode. The coin cells were assembled inside an Ar-filled glove box (o 5 ppm, H2O and O2). The positive and negative electrodes of the cells were separated using a porous polypropylene membrane (Wellcos) and an electrolyte solution consisting of 1.1 M LiPF6 in ethylene carbonate:diethyl carbonate (1:1) solvent mixture (Techono Semichem). The electrochemical properties of the assembled cells were recorded with charge–discharge curves in a voltage window between 3.0 and 0.01 V vs. Li/ Li þ . The charge–discharge tests were galvanostatically performed between 3.0 and 0.01 V vs. Li/Li þ for 50 cycles at a current density of 100 mA g  1. The charge–discharge tests were also performed at various current densities from 100 to 4000 mA g  1 in order to evaluate the C-rate capability. Furthermore, The CVs of the electrodes were obtained at 0.5 mV s  1 between 0.01 and 3.0 V vs. Li/Li þ using a potentiostat (Eco Chemie, AUTOLAB). For electrochemical impedance spectroscopic measurements, the excitation potential applied to the cells was 5 mV and the frequency ranged from 100 kHz to 10 mHz. All the electrochemical measurements were carried out at 25 1C. 3. Results and discussion Fig. 1 shows SEM and TEM images of the samples prepared using a nitriding process of the tungsten oxides at 700 1C under an NH3 atmosphere. In particular, the FE-SEM and TEM images of the m-WN prepared with WO3 nanoplates are shown in Fig. 1a–c. The average size and thickness of the mWN were 210 nm and  15 nm, respectively (Fig. 1a). The asprepared sample exhibited a uniform mesoporous nanoplate and pore diameter of  8 nm (Fig. 1b). The highly crystalline nature of the m-WN can be clearly observed in the HR-TEM image, indicating a configuration toward the [200] direction due to a pseudomorphic reduction process (Fig. 1c). The fringes with interplanar spacing of 2.08 Å correspond to a (200) facet of WN based on a face-centered cubic (fcc) crystal structure. In contrast, the c-WN prepared using a nitriding process of commercial WO3 showed an irregular morphology with a non-homogeneous pore distribution as shown in Fig. 1d and e. The c-WN had an interplanar spacing of 0.252 nm, which corresponds to the {111} facets of WN phase (Fig. 1f). In the XRD patterns of the as-prepared samples, the diffraction peaks at 37.41, 43.71, 63.41, and 76.21 correspond to the (111), (200), (220), and (311) planes, respectively, indicating that the resulting products hold a pure WN phase with the unit cell

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Fig. 1. FE-SEM, TEM, and HR-TEM images of m-WN (a–c) and c-WN (d–f).

parameter of 4.13 Å and space group of Fm-3m (Fig. 2). In the case of WO3, a monoclinic structure is changed into an orthorhombic structure at the temperature of the nitriding process (Fig. 3a). Under an NH3 atmosphere for the nitriding process, the oxygen in WO3 is substituted with nitrogen (Fig. 3b). Through the complete nitriding process, WN with an FCC is formed with a contraction of the lattice (Fig. 3c). Fig. 4 shows wide and fine scan XPS spectra of the samples. The W 4f peaks for the m-WN and WO3 appeared at 32.8– 34.8 eV (Fig. 4b). The N 1s peaks corresponded to WN at 397.4 eV (Fig. 4d). The XPS spectrum of m-WN revealed the

elemental compositions of 53.4 at% W and 46.6 at% N. In addition, we prepared the samples with WO3 nanoplates as a function of reaction time at 700 1C under the NH3 flow of 100 mL min  1. As shown in Figs. 5 and 6, with increasing reaction time from 0.5 to 4 h, the samples were believed to be the more porous nitride structure with complete composition of WN. As a result, by combining XRD, XPS, and EDX data, the sample prepared using the nitriding process is found to be complete WN phase with an FCC structure. To characterize the porous structure of the as-prepared samples, the BET and pore size distribution curves were obtained using m-WN and c-WN

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Fig. 2. XRD patterns of (a) m-WN and (b) c-WN.

Fig. 3. Schematic illustration of the transition of WO3 to WN during the nitriding process.

(Fig. 7). The hysteresis loop of the m-WN showed a type IV curve that indicates a mesoporous structure [27]. The BET surface area and average pore size of the m-WN are  46.4 m2 g  1 and  8 nm, respectively (Fig. 7a). The surface area of the m-WN is relatively higher than that of the c-WN [25]. On the other hand, the c-WN exhibited an irregular pore structure and low specific surface area of  6.7 m2 g  1 (Fig. 7b) in contrast to the sample obtained previously by Kartachova et al. [28]. From commercial WO3 in ammonia at 700 1C with surface area of 45 m2 g  1. This implies that the nitriding process might be extremely sensitive to the experimental condition such as duration time, flow rate, and size of furnace as well as reaction temperature. The hysteresis loop of the cWN was a type II curve, indicating a non-porous solid structure. The c-WN exhibited the separated adsorption/desorption region at 0.6–1.0 of relative pressure due to the formation of secondary pores through the gaps between particles. In general, it is been well known that the tungsten nitride could be formed via a topotactic reaction using WO3 in NH3. During the present nitriding process, the increased oxygen vacancies in the tungsten nitride might be attributed to the rearrangement of the oxide structure. In particular, the as-prepared two-dimensional WO3 nanoplates with a nanometer scale thickness could produce a mesoporous tungsten nitride structure by forming pores in the

framework of the tungsten nitride. Thus, it can be concluded that the formation of the m-WN might be related to atomic substitution, condensation, and two-dimensional nanostructure in the nitriding process compared to c-WN with micrometer scale in size [29–31]. Similarly, Oyama and Thompson reported pseudomorphic transformations upon ammonia reduction of V2O5 nanoparticles and obtained fine vanadium nitride powders [32,33]. Fig. 8a and b shows the charge–discharge curves of m-WN and c-WN measured using coin-type half-cells at a current density of 100 mA g  1 between 3.0 and 0.01 V vs. Li/Li þ . The 1st and 2nd discharge capacities of the m-WN (1014 and 484 mA h g  1, respectively) were higher than those of the c-WN (530 and 229 mA h g  1, respectively). The high specific surface area and well-defined pore structure of the m-WN might be responsible for a high specific capacity for both insertion/extraction and conversion reaction compared to the c-WN. In addition, the porous nanostructure of the electrode for LIBs may affect cycle life and high rate performance. Accordingly, to investigate the cycling performance of the electrodes, the discharge–charge curves were obtained at a constant current density of 100 mA g  1 for 50 cycles (Fig. 8c and d). The m-WN with a well-defined pore structure displayed much improved cell performance with high reversible specific capacity compared to the c-WN. Furthermore, to characterize high rate cycling performance of the electrodes, the discharge rates increased stepwise from 100 to 4000 mA g  1

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Fig. 4. (a) Wide scan, (b) N 1s, (c) O 1s, and (d) W 4f XPS peaks of the samples.

Fig. 5. FE-SEM images of the as-prepared samples prepared with WO3 nanoplates for (a) 0.5, (b) 1, (c) 2, and (d) 4 h at 700 1C under the NH3 flow of 100 mL min  1.

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Fig. 6. SEM images and EDX spectra of the as-prepared samples prepared with WO3 nanoplates for (a) 0.5, (b) 1, (c) 2, and (d) 4 h at 700 1C under the NH3 flow of 100 mL min  1.

(Fig. 8e and f). The specific discharge capacities of m-WN and cWN are 444.7 and 189.3 mA h g  1 at 100 mA g  1, respectively; 336 and 127.3 mA h g  1 at 200 mA g  1, respectively; 250.2 and 95.1 mA h g  1 at 400 mA g  1, respectively; 176.2 and 64.4 mA h g  1 at 1000 mA g  1, respectively. The specific discharge capacities of m-WN and c-WN are 444.7 and 189.3 mA h g  1 at 100 mA g  1, respectively; 336 and 127.3 mA h g  1 at 200 mA g  1, respectively; 250.2 and 95.1 mA h g  1 at 400 mA g  1, respectively; 176.2 and 64.4 mA h g  1 at 1000 mA g  1,

respectively; 123.5 and 47 mA h g  1 at 2000 mA g  1, respectively; and 82.6 and 35.7 mA h g  1 at 4000 mA g  1, respectively. With increasing discharge rate, the specific discharge capacities of lithium ion of the m-WN in the entire ranges are higher than those of the c-WN. The specific discharge capacity ratios of m-WN to cWN are 2.35 at 100 mA g  1; 2.64 at 200 mA g  1; 2.63 at 400 mA g  1; 2.73 at 1000 mA g  1; 2.62 at 2000 mA g  1, and 2.31 at 4000 mA g  1. This demonstrates that even at the increased current rates from 100 mA h g  1 to 4000 mA h g  1, the m-WN

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Fig. 7. Nitrogen gas adsorption–desorption isotherms and pore size distributions of (a) m-WN and (b) c-WN.

Fig. 8. Charge–discharge curves and specific discharge capacities vs. cycle number of (a, c) m-WN and (b, d) c-WN measured using coin-type half-cells at a current density of 100 mA g  1 between 3.0 and 0.01 V vs. Li/Li þ . Variation in discharge capacities vs. cycle numbers for (e) m-WN and (f) c-WN at different current rates between 0 and 3 V vs. Li/Li þ .

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Fig. 9. CVs of (a) m-WN and (b) c-WN obtained at a scan rate of 0.5 mV s  1 between 0 and 3 V vs. Li/Li þ .

Fig. 10. Schematic illustration of transition of m-WN as an electrode during a charge and discharge process.

Fig. 11. (a) Nyquist plots of the samples in the frequency ranging between 100 kHz and 10 mHz. (b) Relationship between ZRe and ω  1/2 in the low frequency range.

displayed an excellent rate cycling performance. The improved high rate performance for the m-WN might be mainly attributed to facilitating lithium ion insertion in the mesoporous nanostructure. To further identify the electrochemical properties of the electrodes, CVs were obtained as shown in Fig. 9. In the 1st CV curves, the as-prepared samples exhibited irreversible reactions due to the formation of a solid electrolyte interface layer and Li3N phase: WNþ 3Li þ þ 3e  -Wþ LixWyNþ Li3N [15–17,34,35]. In the

1st charge and subsequent cycles, electrons are mainly located on the metallic tungsten, while Li-ions are inserted on Li3N as an ionic conducting material, resulting in a charge separation: Wþ LixWyNþ Li3N2(1 x)Wþ WxNþ 3Li þ þ 3e  (Fig. 10) [9–19,36,37]. In the CVs for the as-prepared electrodes, the peaks corresponding to the oxidation of W to WN appear at  1.3 V in the 1st and 2nd charge curves [37–40]. Most metal nitrides including VN as an anode for LIBs have been shown to react

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with lithium by conversion reaction resulting in the formation of metal nanoparticles in Li3N matrix. However, recently, it was reported that the VN nanostructures as an anode for LIBs exhibited an insertion/deinsertion mechanism in contrast to the conversion electrochemical process. In particular, to the best of my knowledge, firstly, we reported Li behavior of tungsten nitride as an anode for LIBs. However, more evident evidences of the reaction mechanism in WN should be obtained using more characterization of electrode material at various stages of charge–discharge. Fig. 11a shows Nyquist plots of the m-WN and c-WN obtained between 100 kHz and 10 mHz. The value of the diameter of the semicircle on the Zreal axis is related to the charge transfer resistance (Rct). The values of Rct of the m-WN and c-WN are 110.8 and 223.23 Ω, respectively, representing much improved charge transport for the m-WN. As indicated in Fig. 11b, using a relationship between ZRe and the square root of frequency (ω  1/2) in the low frequency range, the Liion diffusion coefficients of the m-WN and c-WN are determined as 3.39  10  15 and 0.198  10  15 cm2 s  1, respectively, exhibiting much faster Li-ion diffusion process for the m-WN. As a result, the improved lithium-ion reaction properties of the m-WN (i.e., High specific capacity and excellent rate cycling performance) may be attributed to the low transport resistance, high diffusion coefficient, and short diffusion length of the lithium ion in the mesoporous nanostructure electrode [40–42]. 4. Conclusions In summary, we demonstrated the mesoporous tungsten nitride synthesized via a topotactic reaction using a nitriding process under an NH3 atmosphere. The as-prepared m-WN showed a well-defined mesoporous nanostructure resulting from the atomic substitution, condensation, and the twodimensional nanostructure in the nitriding process. The improved electrochemical reaction properties of lithium ion such as high specific capacity and excellent rate cycling performance in the m-WN might be attributed to the low transport resistance and high diffusion coefficient for lithium ions compared to the c-WN. Acknowledgments This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF2013R1A1A2012541). References [1] J.-M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [2] M.S. Whittingham, Electrical energy storage and intercalation chemistry, Science 192 (1976) 1126–1127. [3] B. Dunn, H. Kamath, J.-M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928–935. [4] N. Nitta, G. Yushin, High-capacity anode materials for lithium-ion batteries: choice of elements and structures for active particles, Part. Syst. Charact. 31 (2014) 317–336.

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