C composite as an anode material for lithium-ion batteries

C composite as an anode material for lithium-ion batteries

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Synthesis of Ti2Nb10O29/C composite as an anode material for lithium-ion batteries Guangyin Liu a, Bo Jin b,*, Ruixue Zhang a, Keyan Bao a, Haiquan Xie a, Jiali Guo a, Ming Wei a, Qing Jiang b a

College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China Key Laboratory of Automobile Materials, Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130022, China

b

article info

abstract

Article history:

A simple synthesis of Ti2Nb10O29/C (TNO/C) composite has been developed, based on a

Received 18 April 2016

solid state reaction method. The as-prepared TNO/C composite displays good discharge

Received in revised form

capacity, cycle stability and rate capability with the discharge capacity of 255.7 mAh g1

31 May 2016

over 100 cycles at 1 C, and even at 5 and 10 C, the discharge capacities of 224.8 and

Accepted 4 June 2016

194 mAh g1 can be obtained after 100 cycles, respectively. The good electrochemical

Available online 18 June 2016

performance demonstrates that TNO/C could be a promising candidate as an anode material for high-energy-density lithium-ion batteries.

Keywords:

© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Titanium-niobium oxides Anode material Solid state reaction method Lithium-ion batteries

Introduction Lithium-ion batteries (LIBs) have been focused on the various applications such as mobile phone, notebook, digital camera, video camera, electric vehicles (EVs) and hybrid electrical vehicles (HEVs) because of light weight, high-energy-density and long cycling lifetime [1e4]. However, in order to meet specially the requirement of applications in EVs and HEVs, it is necessary to explore novel electrode materials with high discharge capacity, high cycle stability and high rate capability. In commercial LIBs, graphite is often used as an anode material due to its easy Liþ insertion/de-insertion, long cycling lifetime and low cost. But the insertion voltage of the graphite is low, and charge at high current rate or overcharge may arouse

lithium plating or generation of lithium dendrites that will cause the safety questions of the cells [5e7]. Therefore, it is very important to develop novel anode materials with higher lithium storage capacity than that of graphite and assure these novel anode materials reacting with Li at potentials above 1.0 V within the voltage stability range of the electrolytes. Titanium-based oxides have drawn extensive attention due to relatively high redox voltage between 1.0 and 2.0 V [8e15]. In particular, zero-strain Li4Ti5O12 (LTO) has been extensively studied as an anode material for high-energydensity LIBs because of its high rate capability and long cycling lifetime [8,9,12e15]. However, the theoretical capacity of LTO is only 175 mAh g1. Recently, titanium-niobium oxides such as TiNb2O7 [16e25] and Ti2Nb10O29 (TNO) [26e29] have

* Corresponding author. E-mail address: [email protected] (B. Jin). http://dx.doi.org/10.1016/j.ijhydene.2016.06.017 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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literature on the electrochemical properties of TNO/C composite as anode material for high-energy-density LIBs. Herein, we report a solid state reaction method to prepare Ti2Nb10O29/C (TNO/C) composite and its electrochemical performance, and suggest a lithium storage mechanism. TNO/C composite displays higher discharge capacity, rate capability and cycle stability compared to pure TNO. This indicates the potential for the utilization of TNO/C composite as an anode material for high-energy-density LIBs.

Experimental Preparation of active materials Fig. 1 e XRD patterns of (a) pure TNO and (b) TNO/C composite.

been reported as the powerful competitor for LTO due to higher capacity and redox potentials between 1.0 and 2.0 V TiNb2O7 and TNO have high theoretical capacity of 388 and 396 mAh g1 due to Ti4þ/3þ, Nb5þ/4þ and Nb4þ/3þ redox couples, respectively [26e29]. Although the theoretical capacity of TNO is slightly higher than that of TiNb2O7, fewer studies have been performed for high-energy-density LIBs with TNO electrodes. Wu et al. report that TNO can deliver a discharge capacity of 247 mAh g1 at 0.1 C with a coulombic efficiency of 92% in the potential range of 1.0e2.5 V [26]. Cheng et al. report a direct one-step solid state reaction to fabricate TNO with commercial TiO2 and Nb2O5 precursors. At 10 C, the discharge capacity stabilizes at 144 mAh g1 after 800 cycles and the crystal structure can be kept upon long charge/discharge cycling [27]. However, its discharge capacity, rate capability and cycle stability have not been explored for high-energydensity LIBs due to low conductivity and great volume change ascribed to lithium ion intercalation/deintercalation during the discharge/charge processes. Recently, Wang et al. synthesized TNO/RGO composite by two-step solid state reaction method with superior electrochemical performance compared to the pure TNO [29]. However, RGO is expensive, and increases the cost of production. Carbon source is rich and cheap, carbon coating have been widely demonstrated as a feasible and effective method to enhance electrochemical properties of LIBs [17,30e32]. However, so far, there is no

TNO/C was synthesized from starting materials of Nb2O5, TiO2 and 5 wt.% C6H12O6 using a solid state reaction method. Nb2O5, TiO2 and C6H12O6 were added into suitable ethanol, and the mixture was ball-milled for 6 h. After drying, the mixture was heated at 1100  C for 24 h in a tube furnace at nitrogen atmosphere. For comparison, pure TNO was also synthesized at the same condition only in the absence of C6H12O6.

Material characterization The crystalline structures were inspected with X-ray diffraction (XRD, Dmax/2500PC, Rigaku, Japan) with Cu Ka radiation (l ¼ 1.5406  A). Morphologies of samples were observed by scanning electron microscope (SEM, Hitachi SU8010) and transmission electron microscope (TEM, JEM-2100F, an accelerating voltage of 200 kV). Thermogravimetric analysis (TGA, SDT Q600) was carried out to estimate the amount of carbon in TNO/C under air flow.

Electrochemical measurements In order to prepare the working electrode, TNO/C, acetylene black and polyvinylidene fluoride were mixed in N-methyl-2pyrrolidinone in a weight ratio of 70:20:10. The slurry was prepared by being coated onto copper foil, and dried at 120  C for 12 h in a vacuum oven. The loading amount of the active material was controlled to about 1.5 mg cm2. LIR2025 coin type cell was assembled in an argon-filled glove box, and Celgard polypropylene was used as the separator, and lithium

Fig. 2 e SEM images of (a) pure TNO and (b) TNO/C composite.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 4 8 0 7 e1 4 8 1 2

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Fig. 3 e TEM images of (a) pure TNO and (c) TNO/C composite. HRTEM images of (b) pure TNO and (d) TNO/C composite. foil as both the counter electrode and the reference electrode. 1 M LiPF6 in ethylene carbonate and dimethyl carbonate with a volume ratio of 50:50 was utilized as an electrolyte. As a comparison, pure TNO electrode was also synthesized by the same method. The battery test system (LAND CT2001A) was used to conduct the galvanostatic discharge/charge tests in a voltage range of 1.0e2.5 V at 1, 5 and 10 C (1 C ¼ 396 mA g1) at room

temperature, and rate performance was also inspected at different current rates in the same potential range. The electrochemical workstation (CHI650D, Shanghai Chenhua Instruments Ltd.) was used to carry out cyclic voltammogram (CV) measurements at a scan rate of 0.1 mV s1 from 1.0 to 2.5 V.

Results and discussion

Fig. 4 e TGA curve of TNO/C composite.

XRD patterns of TNO/C composite and pure TNO are shown in Fig. 1. As for two samples, the diffraction peaks are in accordance with standard TNO (JCPDS No. 40-0039) with monoclinic ReO3 structure (the space group of C2/m) [33], and no impurity phases are detected. There are no apparent diffraction peaks of carbon in TNO/C composite because of its low quantity and amorphous state. The amorphous carbon derived from the thermal decomposition of glucose does not alter the crystallization structure of TNO. Fig. 2 presents SEM images of TNO/C composite and pure TNO. As shown in Fig. 2, both the size and morphology of two samples are almost identical and the average particle sizes of both samples are around 1 mm. TNO/C composite has relative uniformity and dispersibility than pure TNO.

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Fig. 5 e Cyclic voltammograms curves of pure TNO and TNO/C composite at a scan rate of 0.1 mV s¡1 in the potential range of 1.0e2.5 V. In order to demonstrate the amorphous carbon in TNO/C composite, TEM and HRTEM observation were performed, and the observation results are presented in Fig. 3. Fig. 3a and b show TEM and HRTEM images of pure TNO, respectively. As shown in Fig. 3b, there is no coating layer on the surface of

pure TNO. For TNO/C composite, we can see that an amorphous layer is coated on TNO particle surfaces with the thickness of about 2e4 nm (Fig. 3d). It is due to formation of the amorphous carbon onto the surfaces of TNO particle by pyrolytic process of glucose during calcination at 1100  C. As shown in Fig. 3d, the well-resolved lattice fringe (an interplanar distance of 0.374 nm) can be observed, ascribing to the d-spacing of the (011) plane of TNO. TGA analysis was carried out in air to estimate the content of carbon in TNO/C composite, and TGA curve is given in Fig. 4. Fig. 4 displays significant weight loss of TNO/C composite over 50e750  C, corresponding to the oxidation of carbon except for the evaporation of the absorbed water. The content of carbon in TNO/C composite is calculated to be around 2.2%. In order to investigate the redox kinetic properties of TNO and TNO/C composite, cyclic voltammogram measurements were carried out at a scan rate of 0.1 mV s1 in the potential range of 1.0e2.5 V. CV curves of TNO and TNO/C composite is displayed in Fig. 5. As shown in Fig. 5, two curves depict a pair of pronounced cathodic/anodic peaks at 1.57/1.72 V and 1.62/ 1.71 V for TNO and TNO/C composite, respectively, which is attributed to Nb5þ/Nb4þ redox couple [26,29,34]. The pair of wide redox shoulder peaks at 1.88/1.92 V for two electrodes

Fig. 6 e Galvanostatic discharge/charge curves of (a) pure TNO and (b) TNO/C composite in the first cycle at the different current rates. (c) Cycling performance of pure TNO and TNO/C composite at 10 C. (d) Cycling performance of TNO/C composite at 1 and 5 C.

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Table 1 e Initial discharge capacities of TNO and TNO/C composite at various current rates. Electrode Discharge Discharge Discharge material capacity at 1 C capacity at 5 C capacity at 10 C (mAh g1) (mAh g1) (mAh g1) TNO TNO/C

253 295.5

183 224.8

175.2 204

could be contributed to Ti4þ/Ti3þ redox couple [26,34]. The wide bump in the potential range of 1.0e1.4 V could be ascribed to Nb4þ/Nb3þ redox couple [29,34]. As for TNO and TNO/C composite, the difference of redox potential is 0.15 and 0.09 V, respectively. The decreased redox potential difference demonstrates that the TNO/C composite possesses better kinetic reversibility than that of pure TNO [29]. Galvanostatic discharge/charge curves of TNO and TNO/C composite in the first cycle at the different current rates are presented in Fig. 6a and b. As shown in Fig. 6a and b, and Table 1, TNO can deliver the discharge capacities of 253 mAh g1 at 1 C, 183 mAh g1 at 5 C and 175.2 mAh g1 at 10 C, respectively, however, in the case of TNO/C composite, the discharge capacities are 295.5 mAh g1 at 1 C, 224.8 mAh g1 at 5 C and 204 mAh g1 at 10 C, respectively. It is evident that the charge voltage platform of TNO/C composite is more flat at 1, 5 and 10 C than that of TNO, confirming the improved kinetics of TNO/C composite because the cell may run at high current rates. The charge voltage platform of two samples gradually increases with increasing current rates due to the electrode polarization. In addition, when the current rates are increasing, the charge and discharge capacities of two samples are decreased, however, TNO/C composite displays much less electrode polarization and greater capacity than TNO. Such prominent enhancement of rate capability in TNO/C composite is ascribed to the enhancement in electronic conductivity by introduction of the amorphous carbon. Cycling performance of TNO and TNO/C composite at 10 C is displayed in Fig. 6c, and cycling performance of TNO/C composite at 1 and 5 C is presented in Fig. 6d. As shown in Fig. 6c, the discharge capacity of TNO/C composite is 204 mAh g1 in the initial cycle, gradually increases over the initial few cycles and finally immobilizes at 194 mAh g1 after

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100 cycles. However, TNO can only deliver the discharge capacities of 175.2 mAh g1 in the initial cycle and 141.3 mAh g1 after 100 cycles, respectively. As shown in Fig. 6d, TNO/C composite exhibits good cycling stability at 1 and 5 C. The discharge capacities are 295.5 mAh g1 in the first cycle and 255.7 mAh g1 after 100 cycles at 1 C, respectively. Even at 5 C, TNO/C composite can still display the discharge capacities of 224.8 mAh g1 in the initial cycle and 214.3 mAh g1 after 100 cycles, respectively. Such prominent enhancement of cycling performance in TNO/C composite is ascribed to the improvement in electronic conductivity by introduction of the amorphous carbon. Rate performance of TNO and TNO/C composite at various current rates is presented in Fig. 7. As shown in Fig. 7, TNO/C composite displays better high-rate capability than TNO. The discharge capacities of TNO/C composite are 275.8 mAh g1 at 1 C, 233 mAh g1 at 5 C and 200.9 mAh g1 at 10 C, respectively. Especially, the discharge capacity at 20 C stabilizes at 180.3 mAh g1. Even at 30 C, TNO/C composite can exhibit the discharge capacity of 145 mAh g1, however, the discharge capacity of TNO is only 38.3 mAh g1.

Conclusions In short, we have successfully prepared TNO/C composite by a solid state reaction method. The as-prepared TNO/C composite displays good discharge capacity, cycle stability and rate capability with the discharge capacity of 255.7 mAh g1 over 100 cycles at 1 C, and even at 5 and 10 C, the discharge capacities of 224.8 and 194 mAh g1 can be obtained after 100 cycles, respectively. The prominent enhancement of discharge capacity, cycle stability and rate capability in TNO/C composite is ascribed to the improvement in electronic conductivity by introduction of the amorphous carbon. These results demonstrate that TNO/C composite can be utilized as a potential anode material of high-energy-density LIBs.

Acknowledgements This study was supported by The Program for Innovative Talent in University of Henan Province (16HASTIT010), Henan Province Project Education Fund (15A150019) and Special Fund for Industrial Innovation in Jilin Province (No. 2016C039).

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Fig. 7 e Rate performance of pure TNO and TNO/C composite at various current rates.

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