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CNT-enhanced electrochemical property and sodium storage mechanism of Pb(NO3)2 as anode material for Na-ion batteries
Electrochimica Acta 169 (2015) 382–394
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
CNT-enhanced electrochemical property and sodium storage mechanism of Pb(NO3)2 as anode material for Na-ion batteries Xiaoting Lin, Peng Li, Lianyi Shao, Xi Zheng, Miao Shui, Nengbing Long, Dongjie Wang, Jie Shu * Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, Zhejiang Province, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 March 2015 Received in revised form 16 April 2015 Accepted 20 April 2015 Available online 22 April 2015
By using carbon nanotube (CNT), Pb(NO3)2/CNT is fabricated by a solution method and investigated for the first time as probable anode materials for sodium-ion batteries. For comparison, pristine Pb(NO3)2 and Pb(NO3)2/carbon black (CB) are also prepared by the same solution method. Electrochemical results show that Pb(NO3)2/CNT can deliver an initial charge capacity of 285.7 mA h g1, which is much higher than the pristine Pb(NO3)2 (203.8 mA h g1) and Pb(NO3)2/CB (252.1 mA h g1). After 50 cycles, Pb(NO3)2/ CNT still maintains a sodium storage capacity of 112.9 mA h g1. Furthermore, it also shows outstanding rate property compared with other two samples. All the enhanced results can be attributed to the introduction of crosslinked CNTs in the composite, which provide good electronic conductive pathways interconnecting Pb(NO3)2 particles and maintain the whole nano-micro structure upon repeated cycles. The reaction mechanism of Pb(NO3)2 with Na is studied by various in-situ and ex-situ techniques. It can be found that Pb(NO3)2/CNT irreversibly decomposes into Pb, NaNO3, NaN3, and Na2O, and then the resulting metal Pb will further react with Na to form NaxPb alloys during the initial discharge process. In contrast, the charge process is mainly associated with the de-alloying reaction of NaxPb to the formation of Pb. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Sodium-ion batteries Anode material Pb(NO3)2 Carbon nanotube Sodium storage mechanism
1. Introduction Since their commercialization two decades ago, lithium-ion batteries have essentially dominated the portable electronic market, and they are also considered to be the most potential candidate power of the next generation of electric vehicles and plug-in electric vehicles [1–6]. Although lithium-ion batteries have been largely successful in the portable electronic devices, the extensive utilization of lithium-ion batteries will inevitably lead to increasing cost of lithium resources, which is quite limited in the earth [7,8]. Recently, sodium ion batteries have strongly broken into the energy storage field and attracted growing attention as the most attractive alternative to lithium-ion batteries owning to the abundance of raw materials, low-cost and environmental benignity [9–13]. However, a Na ion intercalation and storage mechanism is also challenging because Na ions are about 55% larger than Li ions [14]. This makes it difficult to find a suitable host material to accommodate sodium ions that allows reversible and rapid ion insertion and extraction in the structure.
* Corresponding author. Tel.: +86 574 87600787; Fax: +86 574 87609987. E-mail addresses: [email protected], [email protected] (J. Shu). http://dx.doi.org/10.1016/j.electacta.2015.04.112 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Taking inspiration from lithium-ion batteries, a wide variety of cathode materials have been proposed for sodium ion batteries, including NaFePO4 [15–17], Na3V2(PO4)3 [18–22], Na2FePO4F [15,21,22], NaxCoO2 [23] and NaV6O15 [24]. In contrast, most previous reports on anode materials have limited in hard carbonbased materials because of their large interlayer distance and disordered structures [25–32]. However, the synthesis of a superior nanocarbon anode with a highly disordered state is still a challenge. Fortunately, significant improvements have been achieved in recent two years with more materials examined as suitable hosts for Na+ reversible insertion/extraction (such as Li4Ti5O12 [33–36], Li4-xCuxTi5O12 [37], Na0.66[Li0.22Ti0.78]O2 [38], Na0.8[Ni0.3Co0.2Ti0.5]O2 [39], Na3Ti2(PO4)3 [40], Na2C8H4O4 [41], MoSe2 [42], SnO2 [43], Bi [44]). Although these materials are promising for future sodium-ion batteries, they have so far not reached the high performances that have been obtained to-date for lithium-ion technology. In most recent, metal nitrates (Mx(NO3)y) with high specific capacity has been investigated as a novel anode material for lithium-ion batteries [45–47]. Among them, Pb(NO3)2/CNT presents high reversible capacity, excellent cycling property, outstanding rate capability [47]. Despite an excellent electrochemical performance in lithium-ion batteries, the electrochemical performance of Pb(NO3)2/CNT in a sodium ion environment would
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to bare Pb(NO3)2 and Pb(NO3)2/CB. Furthermore, in order to better understand the sodium storage mechanism in Pb(NO3)2/CNT, we utilized various in-situ and ex-situ techniques to make a careful study of the structural evolutions of Pb(NO3)2/CNT during initial charge-discharge cycles. 2. Experimental 2.1. Material preparation and characterization
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be of interest. Herein, the pristine Pb(NO3)2, Pb(NO3)2/CB and Pb (NO3)2/CNT were fabricated and extensively studied as advanced anode materials for novel sodium-ion batteries. It is found that Pb (NO3)2/CNT displays better electrochemical properties compared
In this experiment, commercial Pb(NO3)2 powder was analytical grade and used without further purification. Pb(NO3)2/CB and Pb(NO3)2/CNT composites were prepared by the following steps. Firstly, CB and CNT were treated by concentrated HNO3 for 5 h, washed by distilled water for several times and dried in a vacuum oven at 80 C. Then, 1.0 g Pb(NO3)2 powder was dissolved in 50 ml distilled water/ethanol mixed solution. The volume ratio of distilled water to ethanol is 1:9. Then, 0.1 g CB or CNT was added into the solution under vigorous stirring to obtain a suspension. After that, the solvent was evaporated in a vacuum oven at 120 C for 10 h to obtain the final products. For comparison, pristine Pb (NO3)2 powders was recrystallized according to the above procedure without adding carbon source. The phase identification and crystallinity analysis of samples were characterized by Bruker D8 Focus X-ray diffraction (XRD, diffractometer with Cu-Ka
Fig. 2. SEM images of (a, b) pristine Pb(NO3)2, (c, d) Pb(NO3)2/CB, and (e, f) Pb(NO3)2/CNT.
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radiation, l = 1.5406 Å) with scattering angles of 10 -80 in a step of 0.02 . The surface morphology and particle size of samples were observed by Hitachi S4800 scanning electron microscopy (SEM). The sodiated and desodiated samples for high-resolution transmission electron microscopy (HRTEM) and corresponding selected area electron diffraction (SAED) observations were scraped from the cycled electrodes in argon-filled glove box, then dispersed in dimethyl carbonate using ultrasonic. After that, the sample was dropped onto copper grid, and evacuated for 5 h. The transfer of copper grid to the HRTEM chamber was performed within thirty seconds under argon blowing. The HRTEM and SAED patterns of the samples were performed with a JEOL JEM2100 transmission electron microscope. The sodiated and desodiated electrodes for X-ray photoelectron spectroscopy (XPS) measurement were washed by dimethyl carbonate and then vacuumed for 5 h before investigation. XPS investigation was measured by a focused and monochromatized Al Ka radiation with a Kratos Axis Ultra spectrometer. 2.2. Electrode preparation and battery characterization The electrochemical performances of pristine Pb(NO3)2, Pb (NO3)2/CB and Pb(NO3)2/CNT were evaluated by 2032 coin-type cells. For Pb(NO3)2/CB and Pb(NO3)2/CNT, the working electrodes were fabricated by mixing of 90 wt.% active material and 10 wt.% polyvinylidene difluoride (PVDF) as binder, and N-methyl-2pyrrolidone as solvent. For bare Pb(NO3)2, the electrode composition is composed of the weight ratio of 82.5:7.5:10 for active material, CB and PVDF. Next, the mixed viscous slurry was coated onto copper foil and dried at 100 C for 12 h in a vacuum oven, and then cut into discs with a diameter of 15 mm. In the coin-type cells, the as-prepared film was used as the working electrode, a sodium
metal foil was acted as counter electrode and the electrolyte was 1 M NaClO4 dissolved in propylene carbonate. For electrochemical measurements, charge-discharge behavior and rate performance of coin-type cells were measured by multichannel Land CT2001A battery test system between 0.0 and 2.8 V vs. Na/Na+ at a current density of 50 mA g1. Cyclic voltammetry (CV) test was carried out at a scan rate of 0.1 mV s1 from 0.0 to 3.0 V on a CHI 1000B electrochemical workstation at room temperature. Electrochemical impedance spectroscopy (EIS) patterns were carried out by using a three-electrode system to characterize the interfacial resistances of anode over the frequency range from 102 to 105 Hz on a CHI 660D electrochemical workstation. The in-situ structural evolutions of Pb(NO3)2/CNT were studied by in-situ X-ray diffraction technique using the same Bruker D8 Focus X-ray diffraction instrument as described above. The structure and equipment of the in-situ XRD battery were described in our group previous paper [48,49], and the characteristic XRD patterns were analyzed by using Fullprof program. Prior to the insitu X-ray diffraction tests, 30 milligrams of Pb(NO3)2/CNT powders were prepared in advance as the working electrode by directly putting on the beryllium window of the in-situ cell, and then separator, lithium metal, stainless steel disc and electrolyte are placed in turn in the in-situ cell chamber. All the coin-type cells and in-situ cells were assembled in an argon-filled glove box, where both moisture and oxygen levels were kept at less than 1 ppm. 3. Results and discussion The XRD patterns of the pristine Pb(NO3)2, Pb(NO3)2/CB and Pb (NO3)2/CNT powders are displayed in Fig. 1. All the diffraction
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Fig. 4. The charge-discharge curves (a-d), cycling properties (e) and coulombic efficiency (f) of pristine Pb(NO3)2, Pb(NO3)2/CB, and Pb(NO3)2/CNT at a current density of 50 mA g1.
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peaks of XRD patterns can be indexed to a cubic spinel structure of Pb(NO3)2 according to the JCPDS card No. 36-1462. No any diffraction peaks of carbon were detected, mainly due to its low content and amorphous nature. Moreover, the sharp diffraction peaks indicate that Pb(NO3)2 in the three samples are all highly crystalline, which demonstrate that recrystallization and the carbon-coating have no obvious impact on the crystal structure of spinel Pb(NO3)2. Fig. 2 illustrates the surface morphologies of pristine Pb(NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT. As shown in Fig. 2a and b, pristine Pb(NO3)2 sample displays irregular polyhedral particles with an average particle size of 2-6 mm, and their surfaces are glossy and smooth. With carbon black coating, a large number of CB particles are attached to the surface of Pb(NO3)2 (Fig. 2c and d). However, the CB particles would easily fall off from the Pb(NO3)2 particles when used as an electrode during charge-discharge cycling. In contrast, a superior appearance can be observed in Fig. 2e and f. It seems that Pb(NO3)2 particles are embedded in a three-dimensional network of CNTs, which would dramatically increase the conductive interconnection among the adjacent Pb(NO3)2 particles and form many conductive paths for electrons. Furthermore, it is a remarkable fact that the particle size of Pb(NO3)2/CNT and Pb (NO3)2/CB is about 2 mm, which is much smaller than that of the pristine Pb(NO3)2. It indicates that the coating of CB and CNT evidently inhibits the growth of Pb(NO3)2 particles. On the basis of the above results, it can be suggested that Pb(NO3)2/CNT may exhibit better electrochemical properties than the pristine Pb (NO3)2 and Pb(NO3)2/CB. Sodium ion insertion/extraction properties of the pristine Pb (NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT samples were investigated by cyclic voltammetry analysis. As shown in Fig. 3, three CV curves nearly coincide with each other, and the reduction/oxidation process during the first cycle is different from the ones observed in subsequent cycles. According to previous report, it is thought that the reactions between Pb(NO3)2 and Na+ are analogous to the Li+ reaction with Pb(NO3)2/Cu(NO3)2 in lithium-ion batteries [45,46]. In the first sodiation process, four sharp reduction peaks at 0.37, 0.59, 1.67, 2.09 V and three broad peaks at 0.99, 2.00, 2.28 V can be found clearly, which can be ascribed to a complex multiphase transition of Pb(NO3)2, alloying process of Pb to NaxPb and a partially irreversible solid electrolyte interphase (SEI) layer formation on the surface of the nanoparticles. In the subsequent desoliation process, four oxidation peaks centered at 0.31, 0.47, 0.58 and 2.27 V is corresponding to a multistep dealloying of Na-Pb alloys. As the electrochemical reaction of Pb(NO3)2 with Na
mayresult in the formation of metal PbO, NaNO3, Na2O, Pb, and NaxPb etc., the disappearance reduction peaks in the second cycle likely originates from the formation of irreversible NaNO3, Na2O or gaseous matters, and dead sodium in Na-Pb alloys. In all samples, the reduction and oxidation peaks in the subsequent cycles are unchanging, indicating that the formed SEI layer and the electrode material remained stable. Most importantly, the maintain of peak current during the initial three cycles indicates the better electrochemical properties of Pb(NO3)2/CNT than that of pristine Pb(NO3)2 and Pb(NO3)2/CB. The electrochemical performance of pristine Pb(NO3)2, Pb (NO3)2/CB and Pb(NO3)2/CNT were investigated by charge/discharge tests at the current density of 50 mA h g1. As depicted in Fig. 4a, five short potential plateaus appear at around 2.3, 1.7, 0.8, 0.5 and 0.1 V during the initial discharge processes, corresponding to the formation of NaNO3, Na2O, NaN3, Pb and its Na-Pb alloying reaction in CV curves. However, only three slopes (0.0-0.1 V, 0.11.0 V and 1.0-2.8 V) can be observed in the charge curves. The initial charge/discharge capacities of pristine Pb(NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT are 203.8/1330, 252.1/1437.6, and 285.7/1403.6 mA h g1, respectively. To our knowledge, the large irreversible capacity loss can be ascribed to the formation of SEI layer and irreversibility sodiated products during cathodic scan, thus, the capacity decay mainly originates from the initial cycles. In the subsequent cycles, the reversible sodium storage capacity of Pb (NO3)2 is mainly based on the reversible alloying and de-alloying of Na-Pb alloys. As a result, the following charge-discharge curves are different from the initial behavior as shown in Fig. 4b, which is in good agreement with the above CV results. After 50 cycle, Pb (NO3)2/CNT still maintains a reversible charge capacity of 112.9 mA h g1, which is much higher than that of bare Pb(NO3)2 (81.5 mA h g1) and Pb(NO3)2/CB (95.1 mA h g1). The cycling performance of three samples in Fig. 4c confirms that Pb(NO3)2/CNT exhibits much higher reversible capacity and stable sodium storage capability than that of pristine Pb(NO3)2 and Pb(NO3)2/CB. Furthermore, Pb(NO3)2/CNT also demonstrates a much improved average cycling coulombic efficiency over 50 cycles as exhibit in Fig. 4d. EIS measurements were conducted to elucidate and compared Na+ and electron kinetics in pristine Pb(NO3)2, Pb(NO3)2/CB and Pb (NO3)2/CNT electrodes. As can be observed from the Fig. 5, all the EIS curves are composed of a depressed semicircle in the high frequency region, and an inclined line in the low frequency region. The semicircle is approximately related to the charge transfer resistance (Rct) for sodium ion reaction at the interface of electrolyte and electrode material. The inclined line in the lowfrequency region can be ascribed to Warburg impedance, which is related to sodium-ion diffusion towards the bulk electrode. By using the equivalent circuit as inserted in Fig. 5, EIS spectra are simulated and calculated. The detailed calculated electrochemical parameters are listed in Table 1. It is obvious that the charge transfer resistance value of the Pb(NO3)2/CNT is 36.48 V, which is much lower than that of pristine Pb(NO3)2 (63.88 V) and Pb(NO3)2/ CB (46.88 V), indicating a smaller charge transfer resistance and the greater facilitation of electronic transportation during the electrochemical reactions. These results show that the addition of CNTs not only increases the electronic conductivity of the nanocomposite but also decreases the charge-transfer resistance. Since high rate performance is an important factor that need to be considered in fabricating power batteries in industry, further electrochemical evaluation are necessary. Rate performance of pristine Pb(NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT were evaluated at various current densities from 100 to 300 mA g1 as shown in Fig. 6a-e. Seen from the charge-discharge curves, the sodium storage behaviors at high rates are similar to those obtained at 50 mA g1. It can be found that the reversible capacities at the
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current density of 100 mA g1 are 89.5, 101.3 and 112.2 mA h g1 for pristine Pb(NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT, respectively. Increasing the current density to 200 mA g1, Pb(NO3)2/CNT can deliver a reversible capacity of 89.3 mA h g1(Fig. 6c). Even cycled at 300 mA g1, a sodium storage capacity of 75.3 mA h g1 can be maintained, whereas the pristine Pb(NO3)2 and Pb(NO3)2/CB only reveal the reversible capacities of 47.2 and 63.6 mA h g1 at 300 mA g1, respectively (Fig. 6e). As shown in Fig. 6f, the overall rate cycling results confirm again the importance of CNT coating towards good electrochemical performance in both low and high cycling rates for Pb(NO3)2. To illuminate the insertion/extraction behavior of Pb(NO3)2/ CNT during the first charge-discharge cycle, we made a thorough investigation about the structural evolutions by using an in-situ XRD technique. Fig. 7a illustrates the initial galvanostatic discharge/charge curves and corresponding in-situ XRD patterns of Pb(NO3)2/CNT. The in-situ XRD cell is discharged to 0.0 V at a
constant current density of 70 mA g1 with a specific charge capacity of 475.3 mA h g1. In the reverse desodiation process, a specific charge capacity of 132.7 mA h g1 can be delivered with a potential cutoff limitation of 2.8 V, and the shapes of the curve is similar to that obtained by coin-type cells. In the overall in-situ XRD patterns of Pb(NO3)2/CNT, the first black bold in-situ XRD pattern is taken at open-circuit potential before discharge. In the following observation, black and red curves represent the discharge and charge processes, respectively. In the in-situ XRD cell, beryllium disc is used as the X-ray transmission window and current collector. According to the JCPDS card No. 78-1557, the diffraction peaks at 38.70 , 41.36 , and 44.1 are attributed to beryllium oxide, which was oxidized during repeated usage. Viewed from Fig. 7a, it is clearly that the characteristic diffraction peaks of Pb(NO3)2/CNT located at 19.56 , 22.62 , 25.33 , 27.78 ,32.21, 37.94 and 39.69 (JCPDS card No. 36-1462) gradually vanish along with the discharge process.
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Fig. 7. (a) The initial galvanostatic discharge/charge curves and corresponding in-situ XRD patterns, (b, d, g) selected in-situ XRD patterns, and (c, e, h) images of change in intensity vs. 2 u in selected in-situ XRD patterns of Pb(NO3)2/CNT during the initial charge-discharge process.
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Fig. 10. XPS spectra of N in different sodiated and desodiated states. (a) The pristine Pab(NO3)2/CNT electrode, (b) discharge to 0.0 V, (c) charged to 2.8 V and (d) the overall comparative spectra.
Simultaneously, new diffraction peaks at 29.27 and 31.26 appear in the discharge process, corresponding to the formation of NaNO3 (JCPDS Card No. 85-1461) and Pb (JCPDS Card No. 04-0686), respectively. With more sodium ions insertion, the relative intensity and width of diffraction peak located at 31.26 increases as shown in Fig. 7e, which may contributed to the preliminary Naalloying formation of NaPb. Besides, one unknown diffraction peak at 36.34 appears in the early stage of discharge (Fig. 9g), and the width increases along with the discharge (Fig. 7h), which may be attributed to the appearance of unidentified NaxPb phases and mutual conversion. In the charge process, reverse evolutions of diffraction peaks for Pb and Na-Pb alloys can be observed by in-situ XRD patterns as shown in Fig. 7d and g. Though CV curves exhibit multistep dealloying peaks separated from each other, it is difficult to distinguish the appearance of each new phase during Na ion insertion and extraction with only the shifting and widen of diffraction peaks observed in in-situ XRD pattern, indicating that the structure of as-formed products, such as NaxPb phases, during complex phase transitions is quite similar with each other. At the same time, no signal of Pb(NO3)2 can be observed even recharged to 2.8 V as show in Fig. 8. It indicates that the structure evolutions of Pb(NO3)2 during charge-discharge cycles are partially reversible, which is consistent with the CV curves in Fig. 3.
Table. 1 EIS simulation parameters of the pristine Pb(NO3)2, Pb(NO3)2/CB, and Pb(NO3)2/ CNT. Sample
Re (V)
Rct (V)
CPE (mF)
W (V)
Pristine Pb(NO3)2 Pb(NO3)2/CB Pb(NO3)2/CNT
13.48 13.68 11.88
63.88 46.88 36.48
0.003947 0.0000233 0.0000354
0.003097 0.002148 0.002138
The XPS spectra of Pb, Na and N elements in different sodiated and desodiated states during the first cycles are shown in Figs. 9– 11, respectively. For the pristine sample, the strong peaks at 139.3 and 144.2 eV in the Pb 4f spectrum can be assigned to the presence of Pb+ in Pb(NO3)2/CNT as shown in Fig. 9a according to the Handbook of X-Ray Photoelectron Spectroscopy [50]. Additionally, the N1s (Fig. 10a) peak (N5+) at 406.1 eV can also be attributed to NO3 group in Pb(NO3)2/CNT. Compared with the peaks of Pb2+ in pristine Pb(NO3)2/CNT, the Pb 4f 7/2 peak at 141.4 eV and Pb 4f 5/2 peak at 136.5 eV can be observed for the full sodiated sample after a discharge to 0.0 V, which can be assigned to the formation of Pb0 in Li-Pb alloys as shown in Fig. 9b. No Pb2+ characteristic peaks (139.3 and 144.2 eV) can be detected, which shows the total transformation from Pb2+ to Pb0 during the first discharge process. Furthermore, a new and strong peak appears at 402.6 eV in Fig. 10b and corresponds to the existence of N3 according to the Handbook of X-Ray Photoelectron Spectroscopy [50], which suggests that the original NO3 group was partially transforms into a new compound. Besides, the formation of sodiated products is also confirmed by the appearance of Na 1s peak at 1071.5 eV as shown in Fig. 11b. It suggests that NaNO3 and NaN3 may generate after a full discharge to 0.0 V according to the Handbook of X-Ray Photoelectron Spectroscopy [50]. For the sample charged to 2.8 V, Pb0 featured peaks (141.4 and 136.5 eV) can be observed again in Fig. 9c, but no featured peaks of Pb2+ can be detected, suggesting the impossible appearance of Pb (NO3)2 or PbO after a reverse charge to 2.8 V, and the dealloying of Na-Pb phases is related to the formation of metal Pb. The featured peak of N3 at 402.3 eV in Fig. 10c becomes weak but does not disappear, and the N5+ peak (406.2 eV) become strong again, indicating the partial decomposition of NaN3 and the probable regeneration of NaNO3 (Fig. 11c).
X. Lin et al. / Electrochimica Acta 169 (2015) 382–394
To further clarify the Na storage mechanism in Pb(NO3)2/CNT, ex-situ HRTEM and SAED techniques were performed on the sodiated and desodiated samples. The TEM in Fig. 12a confirms that Pb(NO3)2 and CNTs are interconnected tightly, forming a threedimensional hierarchical nanostructure. The inset of Fig. 12b shows an inter planar spacing of 0.19 nm, which is in good agreement with the (4 1 0) plane of Pb(NO3)2. Furthermore, the SAED pattern in Fig. 12c exhibits clear diffraction rings and can be well indexed as a pure cubic spinel Pb(NO3)2 phase. After a discharge to 0.0 V, the fringe spacings in the HRTEM image (Fig. 12e) shows can be measured to be 2.85, 3.03, 2.87, 2.83, 2.75 and 2.89 Å corresponding to the (1 0 0) plane of Pb (JCPDS card No. 87-0663), (1 0 4) plane of NaNO3 (JCPDS card No. 89-2828), (111) plane of NaN3 (JCPDS card No. 75-0670), (3 3 2) plane of Na15Pb4 (JCPDS card No. 65-3168), (11 6) and (3 2 1) plane of NaPb (JCPDS card No. 06-0714). Besides, the well-defined diffraction rings can be assigned to the (74 3) and (4 3 9) planes of NaPb, (4 2 2) plane of Na2O (JCPDS card no.77-2148), (111) planes of Pb, (5 4 1) plane of Na15Pb4, (11 3) plane of NaN3 and (0 2 4) plane of NaNO3 as the SAED pattern shown in Fig. 12f. These results are in accordance with the observations in XPS and in-situ XRD. When Pb(NO3)2/CNT is recharged to 2.8 V, the fringe spacings are found to be 3.03, 2.87, 3.68, and 2.77 Å, which can be attributed to the (1 0 4) plane of NaNO3, (111) plane of Pb, (2 1 3) and (11 6) plane of NaPb (Fig. 12h). In addition, the d-spacing values calculated from the rings at 2.12, 2.53 and 2.49 Å can correspond to the (2 0 2), (0 0 6) and (2 0 0) planes of NaNO3, NaN3 and Pb in Fig. 12i. These results state clearly that the fully sodiated products consist of NaNO3, Pb, NaN3, NaPb
391
and Na15Pb4, and the oxidized products consist of NaNO3, Pb, NaPb and NaN3. Here, the existence of some NaPb alloys oxidized products may results from the dead sodium in Na-Pb alloys. Moreover, the observation of NaNO3, Pb, NaN3 NaxPb in the lithiated electrode provides the evidence to support the electrochemical decomposition mechanism as proposed by ex-XPS and in-situ XRD analysis. According to above results and previous studies, the electrochemical reactions of Pb(NO3)2 with Na can be proposed as follows: 2PbðNO3 Þ2 þ 20Naþ þ 20e
discharge
!
discharge
yNaþ þ ye þ electrolyte
Pb þ xNaþ þ xe
discharge
!
!
2Pb þ NaNO3 þ NaN3 þ 9Na2 O
SEI
Nax Pb
charge
NaN3 þ Na2 O ! 2NaNO3 þ Naþ
charge
Nax Pb ! Pb þ xNaþ þ xe
+
(a)
(b)
Intensity (a.u.)
Intensity (a.u.)
Na
1064
1068
1072
1076
1080
1064
2 Theta (Degree)
1068
1072
1076
1080
Binding Energy (eV)
+
Pristine Pb(NO3)2/CNT
(c)
(d)
Discharge to 0.0 V Charged to 2.8 V
Intensity (a.u.)
Intensity (a.u.)
Na
1064
1068
1072
1076
Binding Energy (eV)
1080
1060
1064
1068 1072 1076 Binding Energy (eV)
1080
Fig. 11. XPS spectra of Na in different sodiated and desodiated states. (a) The pristine Pb(NO3)2/CNT electrode, (b) discharge to 0.0 V, (c) charged to 2.8 V and (d) the overall comparative spectra.
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Fig. 12. TEM, HRTEM and SAED images for (a-c) pristine Pb(NO3)2/CNT, (d-f) Pb(NO3)2/CNT electrode discharged to 0.0 V and (g-i) Pb(NO3)2/CNT electrode recharged to 2.8 V.
X. Lin et al. / Electrochimica Acta 169 (2015) 382–394
4. Conclusion In this paper, pristine Pb(NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT are successfully prepared by a simple solution route, and reported for the first time as sodium storage materials. It is observed that CB fail to form effective conductive network for Pb(NO3)2. As a result, Pb(NO3)2/CB do not show obviously improved electrochemical performances compared to bare Pb(NO3)2. For comparison, CNTs provide cross-linking conductive cages for Pb(NO3)2 particles. It can be found that CNT coating can significantly enhance the electrochemical property of Pb(NO3)2. Pb(NO3)2/CNT delivers a reversible charge capacity of 285.7 mA h g1 at a current density of 50 mA g1, which is much higher than that of bare Pb(NO3)2 (203.8 mA h g1) and Pb(NO3)2/CB (252.1 mA h g1). Furthermore, Pb(NO3)2/CNT shows smaller charge-transfer resistance and higher rate performance compared with other two samples. Besides, the in-situ XRD, ex-situ HRTEM, ex-situ SAED and ex-situ XPS results show that the electrochemical reaction of Pb(NO3)2 with Na can be ascribe to the formation of Pb, NaNO3, NaN3, Na2O and NaxPb in the sodiation process. During the reverse desodiation process, only the de-alloying reaction of NaxPb to the formation of Pb can be observed. Acknowledgements This work is sponsored by Ningbo Key Innovation Team (2014B81005), Ningbo Natural Science Foundation (2014A610042) and Opening Project of Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS (PCOM201408). The work is also supported by Outstanding Dissertation Growth Foundation (No. PY2014004) and K.C. Wong Magna Fund in Ningbo University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2015.04.112. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652– 657. [2] P.P. Lv, H.L. Zhao, Z.P. Zeng, J. Wang, T.H. Zhang, X.W. Li, Facile preparation and electrochemical properties of carbon coated Fe3O4 as anode material for lithium-ion batteries, Journal of Power Sources 259 (2014) 92–97. [3] X.X. Jiang, K.Q. Wu, L.Y. Shao, M. Shui, X.T. Lin, M.M. Lao, N.B. Long, Y.L. Ren, J. Shu, Lithium storage mechanism in superior high capacity copper nitrate hydrate anode material, Journal of Power Sources 260 (2014) 218–224. [4] L. Tan, S. Zhang, C. Deng, Fast lithium intercalation chemistry of the hierarchically porous Li2FeP2O7/C composite prepared by an iron-reduction method, Journal of Power Sources 275 (2015) 6–13. [5] C. Deng, S. Zhang, L. Ma, Y.H. Sun, S.Y. Yang, B.L. Fu, F.L. Liu, Q. Wu, Effects of precipitator on the morphological, structural and electrochemical characteristics of Li[Ni1/3Co1/3Mn1/3]O2 prepared via carbonate coprecipitation, Journal of Alloys and Compounds 509 (2011) 1322–1327. [6] S. Zhang, C. Deng, H. Gao, F.L. Meng, M. Zhang, Li2+xMn1xPxSi1xO4/C as novel cathode materials for lithium ion batteries, Electrochimica Acta 107 (2013) 406–412. [7] L. Li, K.H. Seng, D. Li, Y.Y. Xia, H.K. Liu, Z.P. Guo, SnSb@carbon nanocable anchored on graphene sheets for sodium ion batteries, Nano Research 7 (2014) 1466–1476. [8] Y.J. Zhu, X.G. Han, Y.H. Xu, Y.H. Liu, S.Y. Zheng, K. Xu, L.B. Hu, C.S. Wang, Electrospun Sb/C fibers for a stable and fast sodium-ion battery anode, ACS Nano 7 (2013) 6378–6386. [9] Z.C. Yan, L. Liu, H.B. Shu, X.K. Yang, H. Wang, J.L. Tan, Q. Zhou, Z.F. Huang, X.Y. Wang, A tightly integrated sodium titanate-carbon composite as an anode material for rechargeable sodium ion batteries, Journal of Power Sources 274 (2015) 8–14. [10] H. Wang, B.J. Yang, X.Z. Liao, J. Xu, D.Z. Yang, Y.S. He, Z.F. Ma, Electrochemical properties of P2-Na2/3[Ni1/3Mn2/3]O2 cathode material for sodium ion batteries when cycled in different voltage ranges, Electrochimica Acta 113 (2013) 200– 204.
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CNT-enhanced electrochemical property and sodium storage mechanism of Pb(NO3)2 as anode material for Na-ion batteries Xiaoting Lin, Peng Li, Lianyi Shao, Xi Zheng, Miao Shui, Nengbing Long, Dongjie Wang, Jie Shu * Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, Zhejiang Province, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 March 2015 Received in revised form 16 April 2015 Accepted 20 April 2015 Available online 22 April 2015
By using carbon nanotube (CNT), Pb(NO3)2/CNT is fabricated by a solution method and investigated for the first time as probable anode materials for sodium-ion batteries. For comparison, pristine Pb(NO3)2 and Pb(NO3)2/carbon black (CB) are also prepared by the same solution method. Electrochemical results show that Pb(NO3)2/CNT can deliver an initial charge capacity of 285.7 mA h g1, which is much higher than the pristine Pb(NO3)2 (203.8 mA h g1) and Pb(NO3)2/CB (252.1 mA h g1). After 50 cycles, Pb(NO3)2/ CNT still maintains a sodium storage capacity of 112.9 mA h g1. Furthermore, it also shows outstanding rate property compared with other two samples. All the enhanced results can be attributed to the introduction of crosslinked CNTs in the composite, which provide good electronic conductive pathways interconnecting Pb(NO3)2 particles and maintain the whole nano-micro structure upon repeated cycles. The reaction mechanism of Pb(NO3)2 with Na is studied by various in-situ and ex-situ techniques. It can be found that Pb(NO3)2/CNT irreversibly decomposes into Pb, NaNO3, NaN3, and Na2O, and then the resulting metal Pb will further react with Na to form NaxPb alloys during the initial discharge process. In contrast, the charge process is mainly associated with the de-alloying reaction of NaxPb to the formation of Pb. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Sodium-ion batteries Anode material Pb(NO3)2 Carbon nanotube Sodium storage mechanism
1. Introduction Since their commercialization two decades ago, lithium-ion batteries have essentially dominated the portable electronic market, and they are also considered to be the most potential candidate power of the next generation of electric vehicles and plug-in electric vehicles [1–6]. Although lithium-ion batteries have been largely successful in the portable electronic devices, the extensive utilization of lithium-ion batteries will inevitably lead to increasing cost of lithium resources, which is quite limited in the earth [7,8]. Recently, sodium ion batteries have strongly broken into the energy storage field and attracted growing attention as the most attractive alternative to lithium-ion batteries owning to the abundance of raw materials, low-cost and environmental benignity [9–13]. However, a Na ion intercalation and storage mechanism is also challenging because Na ions are about 55% larger than Li ions [14]. This makes it difficult to find a suitable host material to accommodate sodium ions that allows reversible and rapid ion insertion and extraction in the structure.
* Corresponding author. Tel.: +86 574 87600787; Fax: +86 574 87609987. E-mail addresses: [email protected], [email protected] (J. Shu). http://dx.doi.org/10.1016/j.electacta.2015.04.112 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Taking inspiration from lithium-ion batteries, a wide variety of cathode materials have been proposed for sodium ion batteries, including NaFePO4 [15–17], Na3V2(PO4)3 [18–22], Na2FePO4F [15,21,22], NaxCoO2 [23] and NaV6O15 [24]. In contrast, most previous reports on anode materials have limited in hard carbonbased materials because of their large interlayer distance and disordered structures [25–32]. However, the synthesis of a superior nanocarbon anode with a highly disordered state is still a challenge. Fortunately, significant improvements have been achieved in recent two years with more materials examined as suitable hosts for Na+ reversible insertion/extraction (such as Li4Ti5O12 [33–36], Li4-xCuxTi5O12 [37], Na0.66[Li0.22Ti0.78]O2 [38], Na0.8[Ni0.3Co0.2Ti0.5]O2 [39], Na3Ti2(PO4)3 [40], Na2C8H4O4 [41], MoSe2 [42], SnO2 [43], Bi [44]). Although these materials are promising for future sodium-ion batteries, they have so far not reached the high performances that have been obtained to-date for lithium-ion technology. In most recent, metal nitrates (Mx(NO3)y) with high specific capacity has been investigated as a novel anode material for lithium-ion batteries [45–47]. Among them, Pb(NO3)2/CNT presents high reversible capacity, excellent cycling property, outstanding rate capability [47]. Despite an excellent electrochemical performance in lithium-ion batteries, the electrochemical performance of Pb(NO3)2/CNT in a sodium ion environment would
(440) (531) (600) (620)
(331) (420) (422) (511)
(400)
(311) (222)
(a)
(200) (210) (211) (220)
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to bare Pb(NO3)2 and Pb(NO3)2/CB. Furthermore, in order to better understand the sodium storage mechanism in Pb(NO3)2/CNT, we utilized various in-situ and ex-situ techniques to make a careful study of the structural evolutions of Pb(NO3)2/CNT during initial charge-discharge cycles. 2. Experimental 2.1. Material preparation and characterization
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be of interest. Herein, the pristine Pb(NO3)2, Pb(NO3)2/CB and Pb (NO3)2/CNT were fabricated and extensively studied as advanced anode materials for novel sodium-ion batteries. It is found that Pb (NO3)2/CNT displays better electrochemical properties compared
In this experiment, commercial Pb(NO3)2 powder was analytical grade and used without further purification. Pb(NO3)2/CB and Pb(NO3)2/CNT composites were prepared by the following steps. Firstly, CB and CNT were treated by concentrated HNO3 for 5 h, washed by distilled water for several times and dried in a vacuum oven at 80 C. Then, 1.0 g Pb(NO3)2 powder was dissolved in 50 ml distilled water/ethanol mixed solution. The volume ratio of distilled water to ethanol is 1:9. Then, 0.1 g CB or CNT was added into the solution under vigorous stirring to obtain a suspension. After that, the solvent was evaporated in a vacuum oven at 120 C for 10 h to obtain the final products. For comparison, pristine Pb (NO3)2 powders was recrystallized according to the above procedure without adding carbon source. The phase identification and crystallinity analysis of samples were characterized by Bruker D8 Focus X-ray diffraction (XRD, diffractometer with Cu-Ka
Fig. 2. SEM images of (a, b) pristine Pb(NO3)2, (c, d) Pb(NO3)2/CB, and (e, f) Pb(NO3)2/CNT.
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radiation, l = 1.5406 Å) with scattering angles of 10 -80 in a step of 0.02 . The surface morphology and particle size of samples were observed by Hitachi S4800 scanning electron microscopy (SEM). The sodiated and desodiated samples for high-resolution transmission electron microscopy (HRTEM) and corresponding selected area electron diffraction (SAED) observations were scraped from the cycled electrodes in argon-filled glove box, then dispersed in dimethyl carbonate using ultrasonic. After that, the sample was dropped onto copper grid, and evacuated for 5 h. The transfer of copper grid to the HRTEM chamber was performed within thirty seconds under argon blowing. The HRTEM and SAED patterns of the samples were performed with a JEOL JEM2100 transmission electron microscope. The sodiated and desodiated electrodes for X-ray photoelectron spectroscopy (XPS) measurement were washed by dimethyl carbonate and then vacuumed for 5 h before investigation. XPS investigation was measured by a focused and monochromatized Al Ka radiation with a Kratos Axis Ultra spectrometer. 2.2. Electrode preparation and battery characterization The electrochemical performances of pristine Pb(NO3)2, Pb (NO3)2/CB and Pb(NO3)2/CNT were evaluated by 2032 coin-type cells. For Pb(NO3)2/CB and Pb(NO3)2/CNT, the working electrodes were fabricated by mixing of 90 wt.% active material and 10 wt.% polyvinylidene difluoride (PVDF) as binder, and N-methyl-2pyrrolidone as solvent. For bare Pb(NO3)2, the electrode composition is composed of the weight ratio of 82.5:7.5:10 for active material, CB and PVDF. Next, the mixed viscous slurry was coated onto copper foil and dried at 100 C for 12 h in a vacuum oven, and then cut into discs with a diameter of 15 mm. In the coin-type cells, the as-prepared film was used as the working electrode, a sodium
metal foil was acted as counter electrode and the electrolyte was 1 M NaClO4 dissolved in propylene carbonate. For electrochemical measurements, charge-discharge behavior and rate performance of coin-type cells were measured by multichannel Land CT2001A battery test system between 0.0 and 2.8 V vs. Na/Na+ at a current density of 50 mA g1. Cyclic voltammetry (CV) test was carried out at a scan rate of 0.1 mV s1 from 0.0 to 3.0 V on a CHI 1000B electrochemical workstation at room temperature. Electrochemical impedance spectroscopy (EIS) patterns were carried out by using a three-electrode system to characterize the interfacial resistances of anode over the frequency range from 102 to 105 Hz on a CHI 660D electrochemical workstation. The in-situ structural evolutions of Pb(NO3)2/CNT were studied by in-situ X-ray diffraction technique using the same Bruker D8 Focus X-ray diffraction instrument as described above. The structure and equipment of the in-situ XRD battery were described in our group previous paper [48,49], and the characteristic XRD patterns were analyzed by using Fullprof program. Prior to the insitu X-ray diffraction tests, 30 milligrams of Pb(NO3)2/CNT powders were prepared in advance as the working electrode by directly putting on the beryllium window of the in-situ cell, and then separator, lithium metal, stainless steel disc and electrolyte are placed in turn in the in-situ cell chamber. All the coin-type cells and in-situ cells were assembled in an argon-filled glove box, where both moisture and oxygen levels were kept at less than 1 ppm. 3. Results and discussion The XRD patterns of the pristine Pb(NO3)2, Pb(NO3)2/CB and Pb (NO3)2/CNT powders are displayed in Fig. 1. All the diffraction
X. Lin et al. / Electrochimica Acta 169 (2015) 382–394
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peaks of XRD patterns can be indexed to a cubic spinel structure of Pb(NO3)2 according to the JCPDS card No. 36-1462. No any diffraction peaks of carbon were detected, mainly due to its low content and amorphous nature. Moreover, the sharp diffraction peaks indicate that Pb(NO3)2 in the three samples are all highly crystalline, which demonstrate that recrystallization and the carbon-coating have no obvious impact on the crystal structure of spinel Pb(NO3)2. Fig. 2 illustrates the surface morphologies of pristine Pb(NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT. As shown in Fig. 2a and b, pristine Pb(NO3)2 sample displays irregular polyhedral particles with an average particle size of 2-6 mm, and their surfaces are glossy and smooth. With carbon black coating, a large number of CB particles are attached to the surface of Pb(NO3)2 (Fig. 2c and d). However, the CB particles would easily fall off from the Pb(NO3)2 particles when used as an electrode during charge-discharge cycling. In contrast, a superior appearance can be observed in Fig. 2e and f. It seems that Pb(NO3)2 particles are embedded in a three-dimensional network of CNTs, which would dramatically increase the conductive interconnection among the adjacent Pb(NO3)2 particles and form many conductive paths for electrons. Furthermore, it is a remarkable fact that the particle size of Pb(NO3)2/CNT and Pb (NO3)2/CB is about 2 mm, which is much smaller than that of the pristine Pb(NO3)2. It indicates that the coating of CB and CNT evidently inhibits the growth of Pb(NO3)2 particles. On the basis of the above results, it can be suggested that Pb(NO3)2/CNT may exhibit better electrochemical properties than the pristine Pb (NO3)2 and Pb(NO3)2/CB. Sodium ion insertion/extraction properties of the pristine Pb (NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT samples were investigated by cyclic voltammetry analysis. As shown in Fig. 3, three CV curves nearly coincide with each other, and the reduction/oxidation process during the first cycle is different from the ones observed in subsequent cycles. According to previous report, it is thought that the reactions between Pb(NO3)2 and Na+ are analogous to the Li+ reaction with Pb(NO3)2/Cu(NO3)2 in lithium-ion batteries [45,46]. In the first sodiation process, four sharp reduction peaks at 0.37, 0.59, 1.67, 2.09 V and three broad peaks at 0.99, 2.00, 2.28 V can be found clearly, which can be ascribed to a complex multiphase transition of Pb(NO3)2, alloying process of Pb to NaxPb and a partially irreversible solid electrolyte interphase (SEI) layer formation on the surface of the nanoparticles. In the subsequent desoliation process, four oxidation peaks centered at 0.31, 0.47, 0.58 and 2.27 V is corresponding to a multistep dealloying of Na-Pb alloys. As the electrochemical reaction of Pb(NO3)2 with Na
mayresult in the formation of metal PbO, NaNO3, Na2O, Pb, and NaxPb etc., the disappearance reduction peaks in the second cycle likely originates from the formation of irreversible NaNO3, Na2O or gaseous matters, and dead sodium in Na-Pb alloys. In all samples, the reduction and oxidation peaks in the subsequent cycles are unchanging, indicating that the formed SEI layer and the electrode material remained stable. Most importantly, the maintain of peak current during the initial three cycles indicates the better electrochemical properties of Pb(NO3)2/CNT than that of pristine Pb(NO3)2 and Pb(NO3)2/CB. The electrochemical performance of pristine Pb(NO3)2, Pb (NO3)2/CB and Pb(NO3)2/CNT were investigated by charge/discharge tests at the current density of 50 mA h g1. As depicted in Fig. 4a, five short potential plateaus appear at around 2.3, 1.7, 0.8, 0.5 and 0.1 V during the initial discharge processes, corresponding to the formation of NaNO3, Na2O, NaN3, Pb and its Na-Pb alloying reaction in CV curves. However, only three slopes (0.0-0.1 V, 0.11.0 V and 1.0-2.8 V) can be observed in the charge curves. The initial charge/discharge capacities of pristine Pb(NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT are 203.8/1330, 252.1/1437.6, and 285.7/1403.6 mA h g1, respectively. To our knowledge, the large irreversible capacity loss can be ascribed to the formation of SEI layer and irreversibility sodiated products during cathodic scan, thus, the capacity decay mainly originates from the initial cycles. In the subsequent cycles, the reversible sodium storage capacity of Pb (NO3)2 is mainly based on the reversible alloying and de-alloying of Na-Pb alloys. As a result, the following charge-discharge curves are different from the initial behavior as shown in Fig. 4b, which is in good agreement with the above CV results. After 50 cycle, Pb (NO3)2/CNT still maintains a reversible charge capacity of 112.9 mA h g1, which is much higher than that of bare Pb(NO3)2 (81.5 mA h g1) and Pb(NO3)2/CB (95.1 mA h g1). The cycling performance of three samples in Fig. 4c confirms that Pb(NO3)2/CNT exhibits much higher reversible capacity and stable sodium storage capability than that of pristine Pb(NO3)2 and Pb(NO3)2/CB. Furthermore, Pb(NO3)2/CNT also demonstrates a much improved average cycling coulombic efficiency over 50 cycles as exhibit in Fig. 4d. EIS measurements were conducted to elucidate and compared Na+ and electron kinetics in pristine Pb(NO3)2, Pb(NO3)2/CB and Pb (NO3)2/CNT electrodes. As can be observed from the Fig. 5, all the EIS curves are composed of a depressed semicircle in the high frequency region, and an inclined line in the low frequency region. The semicircle is approximately related to the charge transfer resistance (Rct) for sodium ion reaction at the interface of electrolyte and electrode material. The inclined line in the lowfrequency region can be ascribed to Warburg impedance, which is related to sodium-ion diffusion towards the bulk electrode. By using the equivalent circuit as inserted in Fig. 5, EIS spectra are simulated and calculated. The detailed calculated electrochemical parameters are listed in Table 1. It is obvious that the charge transfer resistance value of the Pb(NO3)2/CNT is 36.48 V, which is much lower than that of pristine Pb(NO3)2 (63.88 V) and Pb(NO3)2/ CB (46.88 V), indicating a smaller charge transfer resistance and the greater facilitation of electronic transportation during the electrochemical reactions. These results show that the addition of CNTs not only increases the electronic conductivity of the nanocomposite but also decreases the charge-transfer resistance. Since high rate performance is an important factor that need to be considered in fabricating power batteries in industry, further electrochemical evaluation are necessary. Rate performance of pristine Pb(NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT were evaluated at various current densities from 100 to 300 mA g1 as shown in Fig. 6a-e. Seen from the charge-discharge curves, the sodium storage behaviors at high rates are similar to those obtained at 50 mA g1. It can be found that the reversible capacities at the
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current density of 100 mA g1 are 89.5, 101.3 and 112.2 mA h g1 for pristine Pb(NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT, respectively. Increasing the current density to 200 mA g1, Pb(NO3)2/CNT can deliver a reversible capacity of 89.3 mA h g1(Fig. 6c). Even cycled at 300 mA g1, a sodium storage capacity of 75.3 mA h g1 can be maintained, whereas the pristine Pb(NO3)2 and Pb(NO3)2/CB only reveal the reversible capacities of 47.2 and 63.6 mA h g1 at 300 mA g1, respectively (Fig. 6e). As shown in Fig. 6f, the overall rate cycling results confirm again the importance of CNT coating towards good electrochemical performance in both low and high cycling rates for Pb(NO3)2. To illuminate the insertion/extraction behavior of Pb(NO3)2/ CNT during the first charge-discharge cycle, we made a thorough investigation about the structural evolutions by using an in-situ XRD technique. Fig. 7a illustrates the initial galvanostatic discharge/charge curves and corresponding in-situ XRD patterns of Pb(NO3)2/CNT. The in-situ XRD cell is discharged to 0.0 V at a
constant current density of 70 mA g1 with a specific charge capacity of 475.3 mA h g1. In the reverse desodiation process, a specific charge capacity of 132.7 mA h g1 can be delivered with a potential cutoff limitation of 2.8 V, and the shapes of the curve is similar to that obtained by coin-type cells. In the overall in-situ XRD patterns of Pb(NO3)2/CNT, the first black bold in-situ XRD pattern is taken at open-circuit potential before discharge. In the following observation, black and red curves represent the discharge and charge processes, respectively. In the in-situ XRD cell, beryllium disc is used as the X-ray transmission window and current collector. According to the JCPDS card No. 78-1557, the diffraction peaks at 38.70 , 41.36 , and 44.1 are attributed to beryllium oxide, which was oxidized during repeated usage. Viewed from Fig. 7a, it is clearly that the characteristic diffraction peaks of Pb(NO3)2/CNT located at 19.56 , 22.62 , 25.33 , 27.78 ,32.21, 37.94 and 39.69 (JCPDS card No. 36-1462) gradually vanish along with the discharge process.
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Fig. 7. (a) The initial galvanostatic discharge/charge curves and corresponding in-situ XRD patterns, (b, d, g) selected in-situ XRD patterns, and (c, e, h) images of change in intensity vs. 2 u in selected in-situ XRD patterns of Pb(NO3)2/CNT during the initial charge-discharge process.
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Simultaneously, new diffraction peaks at 29.27 and 31.26 appear in the discharge process, corresponding to the formation of NaNO3 (JCPDS Card No. 85-1461) and Pb (JCPDS Card No. 04-0686), respectively. With more sodium ions insertion, the relative intensity and width of diffraction peak located at 31.26 increases as shown in Fig. 7e, which may contributed to the preliminary Naalloying formation of NaPb. Besides, one unknown diffraction peak at 36.34 appears in the early stage of discharge (Fig. 9g), and the width increases along with the discharge (Fig. 7h), which may be attributed to the appearance of unidentified NaxPb phases and mutual conversion. In the charge process, reverse evolutions of diffraction peaks for Pb and Na-Pb alloys can be observed by in-situ XRD patterns as shown in Fig. 7d and g. Though CV curves exhibit multistep dealloying peaks separated from each other, it is difficult to distinguish the appearance of each new phase during Na ion insertion and extraction with only the shifting and widen of diffraction peaks observed in in-situ XRD pattern, indicating that the structure of as-formed products, such as NaxPb phases, during complex phase transitions is quite similar with each other. At the same time, no signal of Pb(NO3)2 can be observed even recharged to 2.8 V as show in Fig. 8. It indicates that the structure evolutions of Pb(NO3)2 during charge-discharge cycles are partially reversible, which is consistent with the CV curves in Fig. 3.
Table. 1 EIS simulation parameters of the pristine Pb(NO3)2, Pb(NO3)2/CB, and Pb(NO3)2/ CNT. Sample
Re (V)
Rct (V)
CPE (mF)
W (V)
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63.88 46.88 36.48
0.003947 0.0000233 0.0000354
0.003097 0.002148 0.002138
The XPS spectra of Pb, Na and N elements in different sodiated and desodiated states during the first cycles are shown in Figs. 9– 11, respectively. For the pristine sample, the strong peaks at 139.3 and 144.2 eV in the Pb 4f spectrum can be assigned to the presence of Pb+ in Pb(NO3)2/CNT as shown in Fig. 9a according to the Handbook of X-Ray Photoelectron Spectroscopy [50]. Additionally, the N1s (Fig. 10a) peak (N5+) at 406.1 eV can also be attributed to NO3 group in Pb(NO3)2/CNT. Compared with the peaks of Pb2+ in pristine Pb(NO3)2/CNT, the Pb 4f 7/2 peak at 141.4 eV and Pb 4f 5/2 peak at 136.5 eV can be observed for the full sodiated sample after a discharge to 0.0 V, which can be assigned to the formation of Pb0 in Li-Pb alloys as shown in Fig. 9b. No Pb2+ characteristic peaks (139.3 and 144.2 eV) can be detected, which shows the total transformation from Pb2+ to Pb0 during the first discharge process. Furthermore, a new and strong peak appears at 402.6 eV in Fig. 10b and corresponds to the existence of N3 according to the Handbook of X-Ray Photoelectron Spectroscopy [50], which suggests that the original NO3 group was partially transforms into a new compound. Besides, the formation of sodiated products is also confirmed by the appearance of Na 1s peak at 1071.5 eV as shown in Fig. 11b. It suggests that NaNO3 and NaN3 may generate after a full discharge to 0.0 V according to the Handbook of X-Ray Photoelectron Spectroscopy [50]. For the sample charged to 2.8 V, Pb0 featured peaks (141.4 and 136.5 eV) can be observed again in Fig. 9c, but no featured peaks of Pb2+ can be detected, suggesting the impossible appearance of Pb (NO3)2 or PbO after a reverse charge to 2.8 V, and the dealloying of Na-Pb phases is related to the formation of metal Pb. The featured peak of N3 at 402.3 eV in Fig. 10c becomes weak but does not disappear, and the N5+ peak (406.2 eV) become strong again, indicating the partial decomposition of NaN3 and the probable regeneration of NaNO3 (Fig. 11c).
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To further clarify the Na storage mechanism in Pb(NO3)2/CNT, ex-situ HRTEM and SAED techniques were performed on the sodiated and desodiated samples. The TEM in Fig. 12a confirms that Pb(NO3)2 and CNTs are interconnected tightly, forming a threedimensional hierarchical nanostructure. The inset of Fig. 12b shows an inter planar spacing of 0.19 nm, which is in good agreement with the (4 1 0) plane of Pb(NO3)2. Furthermore, the SAED pattern in Fig. 12c exhibits clear diffraction rings and can be well indexed as a pure cubic spinel Pb(NO3)2 phase. After a discharge to 0.0 V, the fringe spacings in the HRTEM image (Fig. 12e) shows can be measured to be 2.85, 3.03, 2.87, 2.83, 2.75 and 2.89 Å corresponding to the (1 0 0) plane of Pb (JCPDS card No. 87-0663), (1 0 4) plane of NaNO3 (JCPDS card No. 89-2828), (111) plane of NaN3 (JCPDS card No. 75-0670), (3 3 2) plane of Na15Pb4 (JCPDS card No. 65-3168), (11 6) and (3 2 1) plane of NaPb (JCPDS card No. 06-0714). Besides, the well-defined diffraction rings can be assigned to the (74 3) and (4 3 9) planes of NaPb, (4 2 2) plane of Na2O (JCPDS card no.77-2148), (111) planes of Pb, (5 4 1) plane of Na15Pb4, (11 3) plane of NaN3 and (0 2 4) plane of NaNO3 as the SAED pattern shown in Fig. 12f. These results are in accordance with the observations in XPS and in-situ XRD. When Pb(NO3)2/CNT is recharged to 2.8 V, the fringe spacings are found to be 3.03, 2.87, 3.68, and 2.77 Å, which can be attributed to the (1 0 4) plane of NaNO3, (111) plane of Pb, (2 1 3) and (11 6) plane of NaPb (Fig. 12h). In addition, the d-spacing values calculated from the rings at 2.12, 2.53 and 2.49 Å can correspond to the (2 0 2), (0 0 6) and (2 0 0) planes of NaNO3, NaN3 and Pb in Fig. 12i. These results state clearly that the fully sodiated products consist of NaNO3, Pb, NaN3, NaPb
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and Na15Pb4, and the oxidized products consist of NaNO3, Pb, NaPb and NaN3. Here, the existence of some NaPb alloys oxidized products may results from the dead sodium in Na-Pb alloys. Moreover, the observation of NaNO3, Pb, NaN3 NaxPb in the lithiated electrode provides the evidence to support the electrochemical decomposition mechanism as proposed by ex-XPS and in-situ XRD analysis. According to above results and previous studies, the electrochemical reactions of Pb(NO3)2 with Na can be proposed as follows: 2PbðNO3 Þ2 þ 20Naþ þ 20e
discharge
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Nax Pb
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NaN3 þ Na2 O ! 2NaNO3 þ Naþ
charge
Nax Pb ! Pb þ xNaþ þ xe
+
(a)
(b)
Intensity (a.u.)
Intensity (a.u.)
Na
1064
1068
1072
1076
1080
1064
2 Theta (Degree)
1068
1072
1076
1080
Binding Energy (eV)
+
Pristine Pb(NO3)2/CNT
(c)
(d)
Discharge to 0.0 V Charged to 2.8 V
Intensity (a.u.)
Intensity (a.u.)
Na
1064
1068
1072
1076
Binding Energy (eV)
1080
1060
1064
1068 1072 1076 Binding Energy (eV)
1080
Fig. 11. XPS spectra of Na in different sodiated and desodiated states. (a) The pristine Pb(NO3)2/CNT electrode, (b) discharge to 0.0 V, (c) charged to 2.8 V and (d) the overall comparative spectra.
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Fig. 12. TEM, HRTEM and SAED images for (a-c) pristine Pb(NO3)2/CNT, (d-f) Pb(NO3)2/CNT electrode discharged to 0.0 V and (g-i) Pb(NO3)2/CNT electrode recharged to 2.8 V.
X. Lin et al. / Electrochimica Acta 169 (2015) 382–394
4. Conclusion In this paper, pristine Pb(NO3)2, Pb(NO3)2/CB and Pb(NO3)2/CNT are successfully prepared by a simple solution route, and reported for the first time as sodium storage materials. It is observed that CB fail to form effective conductive network for Pb(NO3)2. As a result, Pb(NO3)2/CB do not show obviously improved electrochemical performances compared to bare Pb(NO3)2. For comparison, CNTs provide cross-linking conductive cages for Pb(NO3)2 particles. It can be found that CNT coating can significantly enhance the electrochemical property of Pb(NO3)2. Pb(NO3)2/CNT delivers a reversible charge capacity of 285.7 mA h g1 at a current density of 50 mA g1, which is much higher than that of bare Pb(NO3)2 (203.8 mA h g1) and Pb(NO3)2/CB (252.1 mA h g1). Furthermore, Pb(NO3)2/CNT shows smaller charge-transfer resistance and higher rate performance compared with other two samples. Besides, the in-situ XRD, ex-situ HRTEM, ex-situ SAED and ex-situ XPS results show that the electrochemical reaction of Pb(NO3)2 with Na can be ascribe to the formation of Pb, NaNO3, NaN3, Na2O and NaxPb in the sodiation process. During the reverse desodiation process, only the de-alloying reaction of NaxPb to the formation of Pb can be observed. Acknowledgements This work is sponsored by Ningbo Key Innovation Team (2014B81005), Ningbo Natural Science Foundation (2014A610042) and Opening Project of Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS (PCOM201408). The work is also supported by Outstanding Dissertation Growth Foundation (No. PY2014004) and K.C. Wong Magna Fund in Ningbo University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2015.04.112. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652– 657. [2] P.P. Lv, H.L. Zhao, Z.P. Zeng, J. Wang, T.H. Zhang, X.W. Li, Facile preparation and electrochemical properties of carbon coated Fe3O4 as anode material for lithium-ion batteries, Journal of Power Sources 259 (2014) 92–97. [3] X.X. Jiang, K.Q. Wu, L.Y. Shao, M. Shui, X.T. Lin, M.M. Lao, N.B. Long, Y.L. Ren, J. Shu, Lithium storage mechanism in superior high capacity copper nitrate hydrate anode material, Journal of Power Sources 260 (2014) 218–224. [4] L. Tan, S. Zhang, C. Deng, Fast lithium intercalation chemistry of the hierarchically porous Li2FeP2O7/C composite prepared by an iron-reduction method, Journal of Power Sources 275 (2015) 6–13. [5] C. Deng, S. Zhang, L. Ma, Y.H. Sun, S.Y. Yang, B.L. Fu, F.L. Liu, Q. Wu, Effects of precipitator on the morphological, structural and electrochemical characteristics of Li[Ni1/3Co1/3Mn1/3]O2 prepared via carbonate coprecipitation, Journal of Alloys and Compounds 509 (2011) 1322–1327. [6] S. Zhang, C. Deng, H. Gao, F.L. Meng, M. Zhang, Li2+xMn1xPxSi1xO4/C as novel cathode materials for lithium ion batteries, Electrochimica Acta 107 (2013) 406–412. [7] L. Li, K.H. Seng, D. Li, Y.Y. Xia, H.K. Liu, Z.P. Guo, SnSb@carbon nanocable anchored on graphene sheets for sodium ion batteries, Nano Research 7 (2014) 1466–1476. [8] Y.J. Zhu, X.G. Han, Y.H. Xu, Y.H. Liu, S.Y. Zheng, K. Xu, L.B. Hu, C.S. Wang, Electrospun Sb/C fibers for a stable and fast sodium-ion battery anode, ACS Nano 7 (2013) 6378–6386. [9] Z.C. Yan, L. Liu, H.B. Shu, X.K. Yang, H. Wang, J.L. Tan, Q. Zhou, Z.F. Huang, X.Y. Wang, A tightly integrated sodium titanate-carbon composite as an anode material for rechargeable sodium ion batteries, Journal of Power Sources 274 (2015) 8–14. [10] H. Wang, B.J. Yang, X.Z. Liao, J. Xu, D.Z. Yang, Y.S. He, Z.F. Ma, Electrochemical properties of P2-Na2/3[Ni1/3Mn2/3]O2 cathode material for sodium ion batteries when cycled in different voltage ranges, Electrochimica Acta 113 (2013) 200– 204.
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