Ionics https://doi.org/10.1007/s11581-018-2784-z
ORIGINAL PAPER
TiNb2O7/carbon nanotube composites as long cycle life anode for sodium-ion batteries Biao Shang 1 & Qimeng Peng 1 & Xun Jiao 1 & Guocui Xi 1 & Xuebu Hu 1 Received: 5 September 2018 / Revised: 23 October 2018 / Accepted: 31 October 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract TiNb2O7/carbon nanotubes composite (TNO/CNTs) was successfully synthesized by ultrasonic dispersion and a facile solvothermal method. Its physical properties were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TG), and scanning microscopy (SEM). As the anode material of sodium ion battery, its electrochemical performances including cyclic voltammograms (CVs), electrochemical impedance spectra (EIS), and galvanostatic charge-discharge cycles were detected and analyzed for the first time. Compared with pure TiNb2O7, a high-reversible capacity reaches 261.1 mAh g−1 at 50 mA g−1 after 200 cycles. In addition, a prominent rate capability maintains ~ 110 mAh g−1 at 500 mA g−1 with over 1000 cycles. The improvements have been explained by the corresponding kinetics analysis which demonstrates that the pseudocapacitive behavior in TNO/CNTs contributes a lot to the enhanced sodium storage capacities and rate performance. The results show the great potential of TNO/CNTs composites for sodium-ion battery with a long cycle life. Keywords TiNb2O7 . Carbon nanotubes . Sodium ion battery . Anode material . Pseudocapacitive behavior
Introduction The secondary batteries, especially rechargeable lithium-ion batteries (LIBs), are applied as a major power source in portable devices and electric vehicles is rapidly expanding. However, current LIBs technology is still facing several difficult challenges, such as low abundance and uneven distribution of lithium resources, these problems show the potential threat of the long-term and large-scale applications of lithiumion batteries [1]. Therefore, a new low cost and reliable electrochemical energy storage technologies are desirable and necessary. Sodium, which is located in the same family as lithium in the periodic table, shows some similar chemical properties as lithium, thus attracts enormous attention because of its low cost and abundant resource for alternative [2].
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11581-018-2784-z) contains supplementary material, which is available to authorized users. * Xuebu Hu
[email protected] 1
College of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400054, China
Transition-metal oxides have been widely investigated as high-capacity anode materials of sodium-ion batteries (SIBs) for their potential applications [3, 4]. Among the numerous transition metal oxides that can be used as the anode for rechargeable batteries, titanium- and niobium-based oxides continue to be considered promising candidates. TiNb2O7, in which Ti4+ and Nb5+ ions are disordered in octahedral sites that share corners and edges with a layered monoclinic structure, was first reported and proposed as a promising candidate LIBs anode by Goodenough in 2011 [5]. Because TNO anodes offer five electron transfers, compared to other titaniumbased oxide anodes, these make possible a higher theoretical capacity of 387.6 mAh g−1 during the electrochemical reaction. These findings have attracted much attention as a promising anode material for sodium-ion battery due to its hightheoretical capacity, involving five electron transfer in the process: Ti4+/Ti3+, Nb5+/Nb4+, and Nb4+/Nb3+ [6]. However, the low ionic and electronic conductivity of TiNb2O7 materials have limited the electrochemical performance [7, 8]. To achieve a practical application, the research is needed to overcome the hurdles of TNO including poor rate capability resulting from its low-electronic conductivity and poor ionic diffusivity which are mentioned above. An effective option is to incorporate CNTs both as the conductive substrate of TNO and as the buffer to accommodate the volume expansion [9].
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CNTs have long been considered as an ideal conductive additive because of their unique one-dimensional tubular structure, high-electronic conductivity, and large surface area [10, 11]. Of course, it can also be used in other fields, such as solar cell, supercapacitors, fuel cells, or even electromagnetic wave shielding; several research studies have been conducted on strain sensors based on CNTs for electronic applications in wearable strain sensors, robotics, artificial skin, and healthcare [12–18]. Both the strategies have the potential to improve the electrochemical performance [19]. Based on the above analysis, TNO/CNTs composite has been synthesized and has studied the electrochemical performance for a sodium-ion battery for the first time in this work. The incorporation of CNTs enhanced the specific capacities because of the improved electronic conduction and reduced electron/ion transfer resistance. The addition of CNTs not only improves the rate performance but also enhances the capacity, which provides a new idea for the development of highperformance sodium-ion batteries.
Experimental section Synthesis of TiNb2O7/carbon nanotubes For the fabrication of the TiNb2O7/carbon nanotubes composite, two major steps are involved. The first step is the pretreatment of carbon nanotubes; the carbon nanotubes with an average diameter of 40–60 nm (Shenzhen Nanotech Port Co. Ltd.) were added into a 6 M HNO3 solution, nitrified for 0.5 h, vacuum filtrated, washed by deionized water, and vacuum dried at 80 °C. The second step is the synthesis of TiNb2O7/carbon nanotubes in a typical solvothermal procedure; 1.0808 g (4 mmol) niobium chloride (NbCl5; 99.9%, Alfa) was added to 40 mL of ethanol, and then 0.5684 g (2 mmol) of titanium (IV) isopropoxide (C12H28O4Ti, TIP; 97%, Aldrich) was added. After the solution was gently stirred for nearly 30 min, 0.46 g carbon nanotubes were added into it and dispersed by supersonic for 15 min. The reaction solution was then transferred to a 100-mL Teflon-lined stainless steel autoclave and kept in an electric oven at 200 °C for 24 h. After, cool naturally to room temperature. The black precipitate was washed several times with deionized water and ethanol and dried at 80 °C overnight. All of the products were calcined at 300 °C in air atmosphere for 2 h with a heating rate of 2 °C min−1 to evaporate moisture and then were calcined at 700 °C in air atmosphere for 2 h with a heating rate of 3 °C min−1 to obtain a highly crystalline monoclinic phase. For comparison, pure TiNb2O7 was synthesized with the similar manner except for calcination in an air atmosphere and addition of carbon nanotubes. These as-prepared products were denoted as BTNO/CNTs^ and BTNO^ respectively.
Characterization For material characterizations, XRD patterns were collected on a Shimadzu-XRD-7000S X-ray diffractometer with CuKα radiation (λ = 1.54056 Å) at a voltage of 40 kV and a current of 40 mA. TG was conducted under NETZSCH STA 2500 in a temperature range of 30 to 900 °C at air atmospheres with a heated rate of 10 °C min−1. SEM images were acquired on S4800 microscopes.
Electrochemical measurement The as-prepared active materials were mixed and ground with carboxymethyl cellulose (CMC) as a binder and super P conductive carbon as the conductive additive material at a respective weight ratio of 80:10:10 in deionized water. The asprepared mixture was uniformly spread and pressed onto a copper foil and dried in a vacuum oven at 90 °C overnight to be used as the working electrode. The electrolyte was 1 M NaClO4 in ethylene carbonate/diethyl carbonate (EC/DEC, volume ratio was 1:1) solution with the addition of 5% (by mass) fluoroethylene carbonate (FEC). A sodium disk was served as the counter and reference electrode, a glass fiber (whatman GF/D) was used as the separator. The CR2032 coin cells were assembled in an argon-filled dry glove box (DELLIX, LS800S). Cyclic voltammetry (CV) was conducted on an electrochemical analyzer (Autolab PGSTAT 128 N) in the voltage range of 0.01 ~ 3.0 V vs. Na/Na + for Na-ion cells at a scan rate of 0.1 mV s−1. The galvanostatic charge/ discharge tests were performed on a LAND-CT2001 battery tester with a cutoff voltage of 0.01 ~ 3.0 V under different current densities. All tests were done at room temperature.
Results and discussion Physical properties XRD patterns of as-obtained TNO/CNTs and TNO samples are shown in Fig. 1a. All diffraction peaks of the samples can be well indexed as the standard pattern (JCPDS card No. 391407). The main diffraction peaks are narrow and intensive, which manifest good crystallinity of the as-synthesized TNO nanoparticles. Besides the TNO peaks, carbon nanotube diffraction peaks overlapped with TNO at 25.9° was observed. The characteristic diffraction peak of TNO/CNTs at 2θ = 25.9° shows a stronger intensity than standard pure TNO peak in the composite due to the content of ~ 40% CNTs with respect to the overwhelming diffraction signals from TNO phase. Figure 1b shows the TGA curve of the TNO/CNTs. Clearly, the weight ratio of the TNO nanoparticles to the carbon nanotube was determined to be around 60:40 which
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Fig. 1 XRD diffraction patterns of a carbon nanotube, TNO, and TNO/CNT, bars on bottom are standard TiNb2O7 (red) diffractions peaks, b TG curve of TNO/CNTs composites
matches well with the original weight ratio. The TNO/CNTs powders experienced a slight weight loss prior to ∼ 420 °C, corresponding to the loss of absorptive water. The ~ 40 wt.% weight loss occurred mainly from 500 to 700 °C which was related to the removal of combustion of CNTs. The morphologies of the samples were characterized by SEM. As shown in Fig. 2a, the TNO particles exhibit homogeneous spherical morphology. As seen in Fig. 2b, the TNO particles and CNTs are wrapped around each other which Fig. 2 SEM image of a TNO and b TNO/CNTs composites; c low and d high magnification TEM images of TNO/CNTs composites
form a network structure, which is beneficial to prevent the TNO particles from agglomerating and enables good dispersion of TNO particles. Besides, it can be found that TNO microspheres are dispersed homogeneously and partly twined and connected with the CNTs to form the network structure. For understanding the distribution of CNTs and TNO in the composites, TEM image is shown in Fig. 2c and d, it can be seen that the fluffy CNTs network are wrapped on the surface of TNO particles clearly.
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Electrochemical performance The sodium ion extraction/insertion behavior of the TNO and TNO/CNTs composite was investigated by CV and galvanostatic discharge-charge cycling. Figure 3a shows the CV curves of TNO. The first sodiated curve of TNO and TNO/ CNTs show little difference with the 2nd and 3rd cycle below 0.8 V, which suggest the decomposition of the electrolyte to form a solid electrolyte interphase (SEI) film [20, 21]. Besides, there is no obvious difference between the 2nd cycle and the later 50th cycle was observed, which indicates good stability. Figure 3b displays the 1st, 2nd, and 50th CV curves of TNO/CNTs composite samples. The sharp anodic peaks near 0 V agree well with redox peaks of active carbon (Super P and CNTs), they cannot be attributed to TNO [20].
Fig. 3 CV profiles of a TNO and b TNO/CNTs at 0.1 mV/s; charge/ discharge curves of c TNO and d TNO/CNT at 50 mA g−1; e cycling performance of TNO and TNO/CNT at 50 mA g−1; f rate capabilities of
A broad peak in the wide potential range of 0.2–1.1 V supports a capacitive contribution to the overall charge storage capacity in the TNO and TNO/CNTs, involving the valence changes of the Nb5+/Nb4+ and Nb4+/Nb3+ during the intercalation process [22–24]. The peaks located at ~ 1.5 and ~ 1.0 V, which show in three cycles in Fig. 3a and b can be ascribed to the Ti4+/Ti3+ redox couple [25–27]. After the 50th cycles, long slopes were shown in the curves, it demonstrates that the Na+ storage in the amorphous parts and/or nanocrystals is mainly featured by a capacitive behavior in the full voltage window which well consists with subsequent kinetics analysis [28, 29]. Figure 3c and d presents the 1st, 2nd, and 50th dischargecharge profiles of the TNO and TNO/CNTs composite used as a sodium-ion electrode at 50 mAh g −1 , respectively. Obviously, no well-defined voltage vs. capacity plateau was
TNO and TNO/CNTs at different current densities; and g long-term cycling performance of TNO/CNTs at 500 mA g−1
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Fig. 3 (continued)
displayed during the discharge-charge profiles of TNO which well matched with the dQ/dV curves. In the 1st cycle, a discharge capacity of 285.0 mAh g−1 was recorded but this then dropped to 163.3 mAh g−1 after 50 cycles. However, the corresponding values in TNO/CNTs were 466.6 mAh g−1 and 279.4 mAh g−1, respectively. As illustrated in Fig. 3e, the cycling performance of TNO and TNO/CNTs at 50 mA g−1 were investigated, and the reversible capacities still remain 261.1 mAh g −1 after 200 cycles for TNO/CNTs but 156.3 mAh g−1 after 100 cycles for TNO. Moreover, the coulombic efficiency during the 200 cycles has remained above 99%, indicating a good long-term cycling stability. Figure 3f shows the side-by-side comparison of the rate and cycling performance of TNO and TNO/CNTs composites. The TNO/CNTs anode shows a better rate performance and cycling stability than TNO. TNO/CNTs can deliver a capacity of ~ 170 mA g−1 at a high-current density of 1000 mA g−1. And the specific capacity can be recovered to ~ 300 mAh g−1 (at 50 mA g−1) after cycling at 50 mA g−1, 100 mA g−1, 200 mA g−1, 500 mA g−1, and 1000 mA g−1 for 50 cycles. In comparison, the reversible specific capacity of TNO is only ~ 40 mAh g−1 at 1000 mA g−1, which indicates a very poor rate performance. After 50 cycles, the specific capacity fades to ~ 40 mAh g−1 rapidly. Although the TNO returns back to 200 mAh g−1, the capacity is fading with the cycle number increasing. It is believed that the CNTs network structure is the
main reason for a good electrochemical performance, which offers a high surface area for sufficient sodiation and desodiation [30]. The capacity enhancement can be attributed to the following two aspects. In the voltage range of sodiumion ex/insertion (0.01–3.0 V vs. Na/Na+), CNTs takes part in the energy storage mechanism and makes contribution to the sodium-ion storage capacity to some extent (Fig. S1); benefiting from the synergistic effects of their combination, the experimental specific capacity of TNO/CNTs is much higher than the sum of the specific capacity of pure TNO and CNTs in their relative ratios. Moreover, compared with TNO, the rate performance has been improved at the same time. In order to further evaluate the rate performance of the TNO/CNTs composite electrode applied in sodium-ion half cells, the long-term cycling performance of the TNO/CNTs composite was compared at a current density of 500 mA g−1. As shown in Fig. 3g, the capacity of the TNO/CNTs composite electrode can be maintained around 110 mAh g−1 after the 1000 discharge-charge cycles. These satisfied results imply that the incorporation of the specific network structure leads to the composite electrode with a markedly higher sodium-ion extraction/insertion capacity while improving the overall rate performance of the electrode. For comparison, some recent reports of different titanoniobate-based electrodes for sodium-ion battery were listed in Table 1. Obviously, TNO/ CNTs shows one of the best performances among these
Ionics Table 1 Electrode materials, rate capacity, and capacity retention of different titanoniobate-based electrodes for sodium-ion battery in reported works
Ref.
Electrode materials
This work
TiNb2O7/carbon nanotube
206.5 (200)
112.5 (1000th, 500)
Ball milling TiNb2O7 TiNb2O7/graphene
174.7 (500) ~ 80 (200) ~ 60 (500) ~ 210 (200) ~ 160 (500)
267.3 (70th, 50) ~ 70 (500th, 200)
[23]
Layered HTi2NbO7
~ 60 (200) ~ 50 (400)
~ 65 (2000th, 100)
[31]
Pb-Ti-niobate
~ 50 (331)
~ 70 (450th, 33.1)
Al-Ti-niobate Ba-Ti-niobate
– –
~ 50 (70th, 50) ~ 60 (100th, 50)
Sb-Ti-niobate KTiNbO5 Nb0.25Ti0.75O2
– – ~ 60 (200) ~ 130 (330)
[20] [21]
[32] [33]
Rate capacity* (current density**)
Nb doped TiO2
Capacity retention* (cycle number, current density**)
~ 210 (70th, 200)
~ 55 (100th, 50) ~ 45 (30th, 50) ~ 50 (500th, 1000) ~ 175 (100th, 33)
*Unit: mAh g−1 **Unit: mA g−1
references, and its rate property and capacity retention were significantly better than that of these references. To further explore the better electrochemical performance of TNO/CNTs than that of TNO, their EIS measurements were conducted and the resultant Nyquist plots are illustrated in Fig. 4. The EIS can be fitted by two semicircles and a line, and the equivalent circuit was also fitted and given (the inset of Fig. 4). The circuit was made by Rb (electrolyte resistance), R1 (SEI film resistance, fitted from high-frequency semicircle), R2 (Faraday resistance, fitted from middle frequency semicircle), CPE1 (SEI film capacitance), CPE2 (Faraday capacitance), and W (Warburg impedance). All the fitted results are listed in Table 2. R2 of TNO/CNTs and TNO was fitted as
144.1 Ω and 203.0 Ω. Lower R2 of TNO/CNTs indicated that the CNTs improved the effective electronic conductivity of TNO, which facilitates the electronic transfers. Further, to explain the high-rate performance of TNO/CNTs, the kinetic characterizations of electrodes were investigated by separating the diffusion-controlled capacity and capacitive capacity. The current response (i) at a certain scanning rate (v) can be described as the combination of two separate mechanisms, namely capacitive effects (k1v) and diffusion controlled insertion (k2v0.5), based on Eq. 3. Figure 5a exhibits CV curves of TNO/ CNTs at 0.4, 0.6, 0.8, and 1.0 mV s−1, and the well-defined shapes of each curve are observed, indicating the reversible electrochemical reaction at increasing sweep rates. i ¼ avb
ð1Þ
log ðiÞ ¼ b log ðvÞ þ logðaÞ
ð2Þ
I ðV Þ ¼ k 1 v þ k 2 v0:5
ð3Þ
Based upon the Dunn’ reports (Eqs. 1 and 2), the main contribution of the charge/discharge mechanism can be determined [34]. Considering Eq. 1, the b value approaching to 1 implies a surface-controlled pseudocapacitive behavior, while b = 0.5 corresponds to an ionic-diffusion process. Figure 5b Table 2
EIS fitted results of TNO and TNO/CNTs electrodes
Sample name
Fig. 4 Comparison of electrochemical impedance spectroscopy (EIS) plots for TNO and TNO/CNTs
TNO TNO/CNTs
Rb (Ω)
R1 (Ω)
R2 (Ω)
10.9 11.5
89.4 93.2
203.0 144.1
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Fig. 5 CV curves of a TNO/CNTs at various sweep rates from 0.1 to 1.0 mV s−1; b calculation of b value during charge-discharge processes by the relationship of the sweep rate and peak current; c separation of
capacitive and diffusion currents in TNO/CNTs at the sweep rate of 1.0 mV s−1; d contribution ratio of capacitive- and diffusion-controlled processes at different scan rates
shows the b values are 0.9856 and 1.0038, suggesting that the electrochemical reaction of TNO/CNTs is mainly controlled by the pseudocapacitive behaviors, bringing about excellent rate capabilities. This result is well consistent with the better rate capability of the TNO/CNTs (Fig. 3f and g). Typically, the CV curves at 1.0 mV s−1 of the fraction of capacitive contribution (84.2%) are found (green region) in Fig. 5c, indicating that the pseudocapacitive charge storage amount occupies a dominant position on the contribution of capacity at highcurrent density. From Fig. 5d, the capacitive ratio gradually increases with the increasing sweep rates and finally even reaches a maximum value of 84.2%. It means that the fast charge-discharge process mainly occurs on the surface of active materials, which provides a favorable evidence for excellent rate performance of TNO/CNTs and the electrochemical performance was optimized greatly compared with TNO. Figure 6 shows the galvanostatic intermittent titration technique (GITT) profiles to analysis the diffusion of ions in solid phases, this method has been recognized as a good technique to determine the alkali metal ions chemical diffusion coefficient of intercalation-based compounds [35]. In this study, GITT measurement is performed on both the TNO and
TNO/CNTs samples to determine its sodium chemical diffusion coefficient as a function of voltage throughout the entire charge-discharge cycle. Figure 6a and d show the GITT curves of TNO and TNO/CNTs anode from 0.01 to 3.0 V. During the GITT tests, the sodiation and desodiation were carried out at a constant current density for an interval of 30 min, this operation is repeated until the end of the voltage window. Figure 6b and e describes a single-titration profile, with schematic labeling of different parameters. Es refers to the steady-state potential after a 30-min relaxation period, while Eτ denotes the cell potential after a 5-min current pulse. The chemical diffusion coefficient can be computed by solving Fick’s second law of diffusion. After considering numerous assumptions and simplifications, the following Eq. 4 is obtained [28]: 0 12 4 mB V M 2 @ ΔEs A L2 ð4Þ τ≪ DNaþ ¼ DN aþ π MWS pτffiffi τ dE d τ
Herein, DNa+ refers to the sodium chemical diffusion coefficient (cm2 s−1), mB is the mass loading (g), VM is the molar
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Fig. 6 GITT measurements of a TNO and d TNO/CNTs electrode galvanostatic charge-discharge profile of GITT measurement. A magnified single step during GITT measurement marked with τ, ΔEs,
and ΔEτ parameters of b TNO and e TNO/CNTs. Diffusion coefficients calculated from the GITT potential profiles of c TNO and f TNO/CNTs
volume (cm3 mol−1), Mw is the molecular weight (g mol−1), S is the contact area of the electrode (cm2), and τ is the time when the current pulse is applied (s) [36]. When the cell voltage variation during titration is found to exhibit a linear relationship with τ1/2, Eq. 4 can be further simplified to Eq. 5 below: 4 mB V M 2 ΔE s 2 ð5Þ DNaþ ¼ ΔE τ πτ M B S
lower than that of the TNO/CNTs at the corresponding voltage (Fig. 6f), indicating higher polarizations compared to the composite material. In other words, the sodium chemical diffusion coefficient for the TNO/CNTs sample is about one order of magnitude higher than that TNO sample, which explains the better rate performance of TNO/CNTs.
where ΔEs is the voltage difference between the steady-state potentials before and after the current pulse is applied and ΔEτ refers to the voltage difference between the cell potential at the start and end of the current pulse. Since the cell voltage and τ1/2 exhibits a linear relationship, with R2 values of 0.9912 and 0.9855 as shown in Fig. S2 a and b, Eq. 5 can be employed to determine the sodium chemical diffusion coefficient for both two electrodes [37]. The sodium chemical diffusion coefficient values of TNO/CNTs (Fig. 6c) are calculated to be 3.7 × 10−10 cm2 s−1 at the beginning of discharge process and it unevenly decreases with the discharging depth and recorded as 4.6 × 10−12 cm2 s−1, indicating that sodium ion faced a high-polarization effect. During the charge process, DNa+ values increases from 7.2 × 10−12 to 9.4 × 10−11 cm 2 s−1 and the potential profile show that overpotential gradually increases with the extraction of sodium ions. The electrode material faces a high-reaction resistance towards the charging and discharging depths. During the discharge and charge processes of TNO, DNa+ values are
Conclusions The TNO/CNTs composite has been successfully synthesized. As an anode material of sodium-ion batteries for the first time, a reversible capacity of ~ 300 mAh g−1 at 50 mA g−1 was obtained; moreover, this composites exhibit a superior rate performance of ~ 110 mAh g−1 at 500 mA g−1 after 1000 charge-discharge cycles, even at a higher current density of 1000 mA g−1, a stable capacity of ~ 170 mAh g−1 is still retained. Through synergistic effects of their combination, the total specific capacity of TNO/CNTs is improved compared with the sum of the specific capacity of pure TNO and carbon nanotube in their relative ratios. Detailed electrochemical kinetic analysis based on CV reveals that energy storage in TNO/CNTs is dominated by a pseudocapacitive process, which is confirmed by the results of GITT tests. The enhanced electrochemical properties of TNO/CNTs composites are attributed to the positive effects of CNTs. These results demonstrated that TNO/CNTs composites could be potential anode materials with a long cycle life for sodium-ion batteries.
Ionics Acknowledgements This work was supported by the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJ1709217) and the Scientific Research Innovation Team of Chongqing University of Technology (No. cqut2015srim).
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