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graphene aerogel composite as anode material for sodium and lithium ion batteries
Carbon 131 (2018) 86e93
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Antimony oxychloride/graphene aerogel composite as anode material for sodium and lithium ion batteries K.P. Lakshmi, K.J. Janas, M.M. Shaijumon* School of Physics, Indian Institute of Science Education and Research Thiruvananthapuram, Maruthamala PO, Thiruvananthapuram, Kerala, 695551, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 October 2017 Received in revised form 16 January 2018 Accepted 25 January 2018
Herein, we report on the synthesis of phase-pure antimony oxychloride (Sb4O5Cl2) microstructures and studies on their electrochemical properties as new and potential anodes for sodium and lithium ion batteries. We demonstrate that pristine Sb4O5Cl2 based electrode exhibits promising electrochemical behaviour, with a reversible discharge capacity of 830 mAh g1 for the first cycle when cycled against sodium at a current rate of 30 mA g1. Further, a composite of graphene aerogel is prepared with Sb4O5Cl2 microstructures, which are uniformly anchored on the graphene aerogel matrix (Sb4O5Cl2-GA), resulting in interconnected networks which facilitate better charge transfer and effective buffering to alleviate the structural variation of Sb4O5Cl2 during cycling. We show that Sb4O5Cl2-GA electrode exhibits excellent electrochemical properties with much improved cyclic stability and high rate capability. In addition, Sb4O5Cl2-GA delivers excellent performance as anode material for lithium ion batteries, with a reversible capacity of 600 mAh g1 obtained over 50 cycles, at a current rate of 50 mA g1. The obtained results are promising and demonstrate 3D networked antimony oxychloride/graphene composite as a potential anode material for both lithium and sodium ion batteries. © 2018 Elsevier Ltd. All rights reserved.
1. Introduction With the increase in energy demand and quest for efficient storage devices, lithium-ion batteries (LIBs) have successfully been demonstrated as potential power sources for various applications in portable electronics, transport sector, etc. However, with the concerns over limited lithium resources, several other battery chemistries are being explored and sodium-ion battery (SIB) seems to be an ideal alternative to LIB, owing to the abundance of sodium in the earth's crust and similar electrochemical properties to that of lithium [1e3]. There are several challenges ahead to realise the practical implementation of these battery systems. For instance, development of efficient electrode materials for Na-ion battery with higher energy and power densities along with long cycle life still remains a key issue. Graphite, the anode material used in commercial LIBs, however, is not suitable for sodium ion batteries due to its limited performance on sodium intercalation caused by the larger ionic radius of Na [1]. On the other hand, carbonaceous materials such as hard
* Corresponding author. E-mail address: [email protected] (M.M. Shaijumon). https://doi.org/10.1016/j.carbon.2018.01.095 0008-6223/© 2018 Elsevier Ltd. All rights reserved.
carbon [2], expanded graphite [3] has been studied as stable anodes for SIBs. Apart from the conventional choices of carbonaceous materials, considerable effort has been devoted to exploring metal oxides [4,5], and intermetallics that have the potential to deliver excellent gravimetric capacities against sodium and lithium [6e8]. Recently Sb based materials have attracted attention owing to their high theoretical capacity against sodium (660 mAh g1). Sb based materials such as Sb2O3 [9,10], Sb/C [11], Sb2O4 [12], Sb2S3 [13], and Sb based intermetallics such as SnSb [14], Bi-Sb [15] have been studied as promising anode materials for SIBs. Sb based materials typically undergo conversion and alloying reactions with Na, involving multiple electrons [16,17]. Huge volume expansion associated with these materials causes pulverisation and cracking of the material, resulting in poor cycling stability [18]. This motivates further exploration of Sb based materials for Na-ion batteries as an effort to improve their overall electrochemical performance. Recently, Xie et al., studied the electrochemical properties of antimony oxychloride based microrods, wherein the material consisted of a mixed phase of Sb4O5Cl2, Sb8O11Cl2 and Sb2S3 [19]. Achieving phase-pure growth of antimony oxychloride plays an important role in determining its electrochemical properties. Here we demonstrate that pure-phase antimony oxychloride (Sb4O5Cl2) microstructures, prepared via hydrothermal technique,
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exhibit promising electrochemical performances as potential electrode for both sodium and lithium-ion batteries. Lithium ion intercalation into MOCl (M ¼ Fe,V,Cr,Ti) was first reported by Whittingham [20,21], and recently, other metal oxychlorides such as Vanadium oxychloride [22,23] and Bismuth oxychloride [24] have been explored as electrode material for sodium and lithium ion batteries. Sb4O5Cl2 exibits excellent Na-ion storage performance by delivering a high initial reversible capacity of 830 mAh g1. Further, we prepared Sb4O5Cl2-graphene aerogel composite electrode (Sb4O5Cl2-GA) via low temperature self assembly process, which showed much improved rate capability by retaining a stable capacity of over 200 mAh g1 at 2 A g1 (~3 C; 1 C corresponding to 450 mA g1). Sb4O5Cl2 microstructures, which are uniformly anchored on the graphene aerogel matrix result in interconnected networks which facilitate better charge transfer and effective buffering to alleviate the structural variation of Sb4O5Cl2 during cycling. Further, we demonstrate that Sb4O5Cl2-GA delivers excellent performance as anode material for lithium ion batteries, with a reversible capacity of 600 mAh g1 obtained over 50 cycles, at a current rate of 50 mA g1.
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mixing the active material, Super-P and CMC binder (carboxymethyl cellulose) in the weight ratio of 70:20:10. For Sb4O5Cl2-GA, slurry was prepared with active material, Super-P and CMC binder in the ratio 80:10:10. The slurry was uniformly coated onto a Cu foil and vacuum dried at 120 C for 12 h. For the electrochemical studies, galvanostatic charge-discharge measurements on the fabricated electrodes were carried out in a CR2032 type coin cell assembled in an argon filled glove box with moisture and oxygen level maintained at less than 0.1 ppm. 1M NaClO4 in PC (with 10 wt % FEC) was used as the electrolyte and with Na metal as both reference and counter electrodes. Typical mass loading of the active material is 1.5 mg cm2. Cyclic voltammetry was performed for all the samples in the voltage range between 0.05 and 2 V versus Na/ Naþ. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a sinusoidal signal of 10 mV amplitude, between frequencies ranging from 10 mHz to 40 kHz. For lithium ion battery measurements, 1M LiPF6 in EC:DMC (1:1 vol ratio) was used as the electrolyte and all the electrochemical measurements were carried out in the voltage range between 0.01 and 3 V versus Li/Liþ. 3. Results and discussion
2. Experimental section 2.1. Synthesis of Sb4O5Cl2 microstructures SbCl3 (2 mmol) and PVP (315 mg) dissolved in 10 mL of DI water was stirred for 30 min to get a homogenous solution. Appropriate amount of 2M H2SO4 was added to the above solution and stirred well to get a uniform solution. The resultant solution was trans ferred to a Teflon-lined autoclave and was kept at 180 C for 2 h. The autoclave was allowed to cool down to room temperature and the product was washed several times with ethanol and DI water to remove the impurities. 2.2. Preparation of Sb4O5Cl2 -GA 20 mg of GO prepared by modified hummer's method was uniformly dispersed in 10 mL of DI water by ultrasonication for 1 h. 1.5 mL of 2 M ascorbic acid was added to the above solution and sonicated for few sec. The as-synthesized Sb4O5Cl2 microstructures were mixed with the above solution by stirring. The solution was kept in an oil bath at 60 C for 24 h without being disturbed. The black cylindrical aggregate thus formed was washed for several times in DI water. The product was freeze dried for 24 h to obtain Sb4O5Cl2-GA. 2.3. Morphological and structural characterization The surface morphology was examined by using scanning electron microscope (SEM; Nova NanoSEM 450, FEI) and transmission electron microscope (TEM; FEI TECNAI 30 G2 S-TWIN microscope with an accelerating voltage of 300 kV). Structural characterization has been done using X-ray diffraction (XRD) with Cu-Ka radiation within Bragg's angle ranging from 10 to 80 (Emperean, PANalytical XRD system). Thermogravimetric analysis of samples has been done in the presence of oxygen using SDT-Q600, TA Instruments. The porosity characteristics including BET surface area were studied using N2 adsorption-desorption isotherms measured at 77 K up to a maximum relative pressure of 1 bar, with the Micromeritics 3-Flex surface characterization analyzer. 2.4. Electrochemical characterization For pristine Sb4O5Cl2 samples, the electrodes were prepared by
Sb4O5Cl2 microstructures were synthesized by a simple hydrothermal method. As-synthesized Sb4O5Cl2-GA composite is obtained as a black cylindrical aggregate with monoclinic Sb4O5Cl2 microstructures trapped inside three dimensionally connected thin reduced graphene oxide sheets (Fig. S1). Sb4O5Cl2 microstructures (~4 mm in size) exhibit a unique flower shaped morphology which is formed by the aggregation of Sb4O5Cl2 nanoflakes, ~200 nm in size, as shown in Fig. 1A (See Figs. S1A and B for details). Fig. 1C shows the SEM image of Sb4O5Cl2-GA composite clearly revealing Sb4O5Cl2 microstructures encapsulated within few-layered graphene sheets, forming three dimensionally oriented cross linked networks. Fig. 1B,D represent high resolution TEM (HRTEM) images of Sb4O5Cl2 microstructures and Sb4O5Cl2-GA composite, respectively, showing an inter planar distance of 0.31 nm corresponding to (002) plane of Sb4O5Cl2 [25]. Presence of few-layered graphene surrounding Sb4O5Cl2 domains is clearly evident in the HRTEM image (Fig. 1D). X-ray diffraction (XRD) was employed for the structural characterization of materials (Fig. 1E) and XRD patterns clearly indicate all the reflections which are indexed to the monoclinic phase of Sb4O5Cl2 (JCPDS No-30-0091) [25]. The graphitic peak at ~26 is not visible in the XRD pattern of Sb4O5Cl2-GA, which could be due to high crystallinity of the pristine material [26]. Thermogravimetric analysis (TGA) was per formed in a temperature range of 40 C-1000 C in presence of oxygen. Sb4O5Cl2 remains stable till ~400 C, beyond which it decomposes releasing SbCl3 gas (Fig. 1F) [27]. This in turn gets con verted to Sb2O3 phase above 475 C. An increase in sample weight can be observed above 600 C due to oxidation of Sb2O3 particles. TG curve of Sb4O5Cl2-GA shows a slight weight loss around 100 C due to evaporation of surface absorbed water. Weight loss till 400 C can be attributed to the elimination of carboxyl groups and surface absorbed moisture [18], as observed for the pure graphene aerogel (Fig. S4A). A drastic weight loss between 400 C to 600 C can be attributed to the thermal decomposition of rGO in air, in addition to the decomposition of Sb4O5Cl2 and the accompanied phase change of Sb4O5Cl2 to Sb2O3. A similar weight gain is observed beyond 600 C, corresponding to the oxidation of Sb2O3. Sb4O5Cl2-GA showed an altogether different thermal decomposition behaviour when compared to the pristine Sb4O5Cl2, indicating better stability of the graphene composite [28,29]. From the TG analysis, the amount of graphene in Sb4O5Cl2-GA was estimated to be ~10 wt%. It has been shown that pH of the reaction mixture is very critical for the controllable synthesis of Sb4O5Cl2, which are typically
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Fig. 1. (A) SEM and (B) HRTEM images of Sb4O5Cl2 microstructures. (C) SEM and (D) HRTEM images of Sb4O5Cl2-GA composite. (E) Powder XRD patterns of Sb4O5Cl2 microstructures, Sb4O5Cl2-GA composite. (F) TG curves of Sb4O5Cl2 and Sb4O5Cl2eGA. Graphene content in Sb4O5Cl2eGA is calculated from the TG analysis (~10 wt%).
formed in the pH range of 1e6. Further, the solvent composition plays a key role in obtaining the desired phases. For instance, Chen et al., investigated the effect of solvent composition on the selective synthesis of different phases of Sb2O3 [25]. Here, the reaction mechanism for the formation of Sb4O5Cl2 structures can be represented by the following equation [30]: SbCl3 þ H2O / SbOClþ 2HCl 4SbOCl þ H2O/Sb4O5Cl2þ 2HCl The reaction mixture is maintained at pH ¼ 1e5, as hydrolysis of SbCl3 gives acidic solution. Sb4O5Cl2-GA is formed by the hydrogelation process [31]. GO is uniformly dispersed in DI water by
ultrasonication. Addition of L-Ascorbic acid, a mild reductant, reduces the dispersed GO sheets [32] and hydrogelation of rGO sheets occur once the system is kept undisturbed for a prolonged period. The p-p interaction of the graphene sheets with a prolonged reaction time is responsible for the hydrogelation process. During the reaction, Sb4O5Cl2 microstructures are uniformly dispersed in the GO solution and get trapped inside the graphene sheets which are randomly oriented in all directions and finally form 3dimensionally networked structure during the self-assembly of graphene sheets. Such a composite with interconnected networks would facilitate better charge transfer and effectively minimize the structural variation of Sb4O5Cl2 during electrochemical cycling. Electrochemical performance studies on Sb4O5Cl2 and Sb4O5Cl2GA electrodes were conducted in a two-electrode coin cell
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assembly against metallic sodium employing cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance. Fig. 2A shows the CV curves of Sb4O5Cl2 electrode performed at a scan rate of 0.02 mV s1 in the potential range of 0.05 Ve2 V against Na/Naþ. In the first cathodic scan, reduction peaks are observed at 0.96 V and 0.36 V. Peak at 0.96 V, which could be due to the formation of SEI layer, was absent in the consecutive cycles, while the peak at 0.36 V is found to be slightly shifted. In the subsequent cycles, three reversible reduction peaks are observed at 1 V, 0.73 V and 0.43 V, indicating the reaction of Na with Sb4O5Cl2 in multiple stages, forming several intermediate alloys and oxides of Na and Sb. Similar profile has been reported with other metal oxychlorides which are studied as anodes for SIBs [22]. In the subsequent anodic scans, a sharp oxidation peak at 0.7 V and a broad peak between 1.4 V and 1.9 V were observed, corresponding to the de-alloying of Na based alloys and the reversible reactions of the other oxides of Na and Sb, respectively, indicating the reversible nature of the electrochemical reactions. Multiple conversion and alloying reactions is clearly evident from the CV profiles, similar to other reports in SIBs [33]. Apart from these redox peaks, a small peak at 0.06 V is observed for Sb4O5Cl2-GA electrode (Fig. S3C), due to the insertion of Naþ ions into the graphene layers, which has been confirmed from the CV measurements on pure graphene aerogel (Fig. S4C). In order to understand the redox behaviour and evaluate the electrochemical performance, galvanostatic charge/discharge measurements have been carried out on both the samples in the voltage range of 0.05 Ve2V against Na/Naþ. Figure 2B shows the charge/discharge curves of the Sb4O5Cl2 electrode at a current density of 30 mA g1. Sb4O5Cl2 electrode showed an initial
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discharge capacity of 830 mAh g1, indicating an irreversible capacity due to the SEI layer formation, consistent with the CV results. Charge/discharge profiles showed three distinct sloppy regions, in agreement with the redox peaks observed in CV curves. Pristine Sb4O5Cl2 electrode delivered reversible specific capacities of 528 mAh g1 and 320 mAh g1 upon 2nd and 50th discharge cycles, respectively. Further, voltage-composition curves were recorded (inset of Fig. 2B) which show a reversible capacity of ~10e11 Naþ ions during first charge-discharge cycle, indicating multi-stage conversion and alloying reactions in the pristine material. The electrode showed large polarization, indicating slower kinetics of Naþ insertion/de-insertion processes. Poor cyclic stability of pristine electrode (Fig. 2B) results from huge volume expansion of the material upon cycling, leading to pulverisation of the electrode and eventual loss of electrical contact with current collector, which is common for Sb-based systems. To overcome this issue, we prepared a composite electrode with a 3-D networked framework of graphene aerogel with Sb4O5Cl2 microstructures uniformly anchored on it (Sb4O5Cl2-GA). We performed the electrochemical characterization of the composite electrode following similar conditions as that for pristine Sb4O5Cl2 electrode. Galvanostatic charge-discharge voltage profiles obtained for Sb4O5Cl2-GA electrode (Fig. 2C) at a current density of 30 mA g1 in the voltage range of 0.05 Ve2 V show similar plateaus to that of pristine Sb4O5Cl2 electrode with an initial discharge capacity of 735 mAh g1. An irreversible capacity loss of 40%, due to the SEI layer formation, is observed and a reversible specific capacity of ~400 mAh g1 is obtained after 50 cycles. Sb4O5Cl2-GA electrode exhibited excellent electrochemical performances with a good
Fig. 2. (A) Cyclic Voltammograms of Sb4O5Cl2 electrode for the initial five cycles at a scan rate of 0.02 mV s1. (B) Galvanostatic discharge/charge curves for Sb4O5Cl2 electrode, cycled at a rate of 30 mA g1 vs. Na. Inset shows composition vs. voltage curves for Sb4O5Cl2 electrode. (C) Galvanostatic discharge/charge curves for Sb4O5Cl2-GA electrode cycled at a rate of 30 mA g1 vs. Na. (D) Capacity retention plot of Sb4O5Cl2 and Sb4O5Cl2-GA electrodes at 30 mA g1 vs. Na.
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capacity retention with much improved cycling stability and columbic efficiency compared to the pristine electrode (Fig. 2D). Sb4O5Cl2-GA electrode showed excellent rate performances when cycled at much higher current rates. Galvanostatic charge/ discharge voltage profiles have been obtained at various current densities of 100 and 200 mA g1 (Figs. S2 and S3) for both the pristine and composite electrodes. Sb4O5Cl2-GA sample showed very stable electrochemical performance compared to the pristine material for about 70 cycles at high current rates (Fig. 3A). As shown in Fig. 3B, Sb4O5Cl2-GA electrode could retain the lower rate performance after cycling at very high current rates of 5 A g1. Pure graphene aerogel cycled at a current density of 100 mA g1 for 100 cycles showed a stable capacity of 100 mAh g1 (Figure S4 B). With <10 wt% of graphene, the capacity contribution will be very less and the major redox activity towards the high capacity of Sb4O5Cl2-GA electrode could only come from Naþ storage in the Sb4O5Cl2 electrode by multiple conversion and alloying reactions [22]. The porosity characteristics including BET surface area were analyzed from N2 adsorption-desorption isotherms measured at 77 K (Fig. S7). Sb4O5Cl2-GA exhibit BET surface area of ~47 m2 g1, comparable to other aerogel-based anodes reported for sodium ion and lithium ion batteries [26,34]. Further, EIS measurements were carried out for both the samples before and after cycling to get better insight on the conductivity and reaction kinetics upon sodiation/de-sodiation. The Nyquist plots for both the samples show a semicircle in the high frequency region and a slanting line in the low frequency region (Fig. 3C). Improved rate capability of Sb4O5Cl2-GA electrode was further analyzed by using EIS spectra recorded after 100 chargedischarge cycles. A substantial drop in charge transfer resistance was observed for Sb4O5Cl2-GA sample (90.36 U), compared to pristine material (282.9 U) after 100 charge-discharge cycles. The best fitted equivalent circuit (Fig. S6) and the derived values of RC components (Table S1) are given in the supporting information. A small drop in the charge transfer resistance could be noted from the Nyquist plot for the Sb4O5Cl2-GA sample (29.16U) compared to the Sb4O5Cl2 sample (42.5U) recorded at the open circuit potential. The introduction of three dimensional wrapping of thin graphene sheets suppresses the associated volume expansion in Sb4O5Cl2. Further, the cross-linked 3D network of graphene sheets facilitate shorter diffusion path for Naþ ions and thus provide better ionic and electronic conductivity for the sodium ions, resulting in improved cycling performances. Thus an overall enhancement in electrochemical properties of the composite electrode is observed, owing to the synergistic effects from the redox behaviour of Sb4O5Cl2 microstructures as well as better electronic and ionic conductivities of 3D networked graphene structure. A comparison of electrochemical performance with other metal oxychloride based anodes for sodium ion batteries is given in the supporting information (Table S2). Ex-situ XRD studies have been done on the pristine Sb4O5Cl2 sample to obtain further insights on the Na-ion storage mechanism. XRD measurements were done on various samples with different depth of voltages during 2nd cycle of both discharge and charge processes. Fig. 4A shows the XRD patterns recorded for samples cycled to different charge/discharge potentials. Diffraction peaks corresponding to NaCl (PDF Card No-1000041) and Sb (PDF Card No-2310879) were found in the complete discharged state, indicating that Sb4O5Cl2 undergoes conversion reaction forming NaCl and Sb. Similar conversion reaction has been reported for other metal oxychlorides upon sodiation and lithiation [22]. The diffraction peaks corresponding to presence of Sb2O3 phase could not be detected from the XRD analysis which could be due to the formation of nanosized particles/amorphous phases similar to studies reported for other metal oxychlorides [23]. Diffraction peaks corresponding to pristine Sb4O5Cl2 were not found in the recharged
states because of the partial conversion of NaCl and Sb to Sb4O5Cl2. The diffraction peaks at the fully recharged state can be indexed to NaCl, Sb and other by-products. Similar results have been reported for other metal oxychlorides that shows partial conversion in fully recharged state [22,23]. X-ray photoelectron spectroscopy (XPS) was further employed to investigate the chemical composition and surface states of the pristine Sb4O5Cl2 as well as the electrode after complete discharge
Fig. 3. (A) Capacity retention plots for Sb4O5Cl2 and Sb4O5Cl2-GA electrodes cycled at a rate of 200 mA g1 vs Na. (B) Rate capability plots of Sb4O5Cl2, Sb4O5Cl2-GA electrodes obtained at different current rates vs Na. (C) Electrochemical impedance spectra of Sb4O5Cl2 and Sb4O5Cl2-GA electrodes after 100 discharge/charge cycles, measured in the frequency range of 100 kHz to 10 mHz. (A colour version of this figure can be viewed online.)
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Fig. 4. (A) Ex-situ XRD patterns for Sb4O5Cl2 electrode at different depth of charge/discharge edischarged to (a) 0.573 V (b) 0.369 V (c) 0.05 V and recharged to (d) 0.708 V (e) 0.97 V and (f) 2 V. (B) High resolution XPS spectra of (a) Sb4O5Cl2 (b) fully discharged Sb4O5Cl2 electrode (to 0.05 V), (c) fully charged Sb4O5Cl2 electrode (to 2 V). (A colour version of this figure can be viewed online.)
(to 0.05 V) and charge (to 2 V) processes (Fig. 4B). High resolution XPS spectra of pristine Sb4O5Cl2 show two strong peaks at 529.7 eV and 539.2 eV, corresponding to Sb 3d5/2 and Sb 3d3/2 of Sb (III) state, respectively (Fig. 4B a) [30]. High resolution XPS spectra in Fig. 4B b
reveal peaks corresponding to the presence of Sb (III) and Sb (0) states in the fully discharged electrode, with respective binding energies of 530.1 eV, 539.9 eV and 538.5 eV. This clearly indicates the formation of Sb particles with Sb (0) state and Sb2O3 particles
Fig. 5. (A) Cyclic Voltammograms of Sb4O5Cl2-GA electrode for the initial five cycles at a scan rate of 0.02 mV s1 (B) Galvanostatic discharge/charge curves for Sb4O5Cl2-GA electrode, cycled at a rate of 50 mA g1 vs.Li. (C) Capacity retention plot for Sb4O5Cl2-GA at 50 mA g1 for 50 cycles vs.Li. (D) Rate capability plots of Sb4O5Cl2-GA electrode at different current rates from 50 mA g1 to 5 A g1 vs.Li. (A colour version of this figure can be viewed online.)
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with Sb (III) state. Fig. 4B c shows high resolution XPS profiles for fully charged electrode indicating the presence of only Sb (III) oxidation state with peaks at 530.1 eV and 539.9 eV, corresponding to Sb 3d5/2 and Sb 3d3/2, thus clearly revealing the reversible conversion to Sb4O5Cl2. XPS spectrum of Sb4O5Cl2 sample indicating the presence of Sb, Cl, O and C elements is shown in Fig. S4C. The peaks observed at binding energies at around 33 eV and 200 eV can be attributed to Sb 4d and Cl 2p respectively, which correspond to þ3 oxidation state of Sb and 1 oxidation state of Cl. The O1s and C1s correspond to the presence of oxygen and carbon in the sample due to absorbed CO2 gas on the surface. Fig. S5D shows the high resolution XPS spectra of O1s of all the samples. In case of Sb4O5Cl2, O1s peaks are mostly overlapped with the Sb3þ peaks [35] as shown in the supporting information, Fig. S4 D. For the charged and discharged states, the O1s XPS spectra as shown in the supporting information mostly overlaps with the Sb 3d spectra and there is a slight shift in the peak positions due to the change in binding energy of the atoms. We further demonstrate Sb4O5Cl2-GA electrode as a potential anode for Li-ion batteries. Detailed electrochemical studies have been carried out including galvanostatic charge/discharge and cyclic voltammetry against metallic lithium (Fig. 5). Fig. 5A shows the CV curve of Sb4O5Cl2-GA electrode at a scan rate of 0.02 mV s1 in the potential range of 0.01e3 V against Li/Liþ. CV curves show redox peaks at subsequent cycles indicating the reversibility of the reactions. The irreversible oxidation peak that appears in the first cycle indicates the formation of SEI layer during the first discharge. The reversible reduction peak at 1.7 V and 0.75 V correspond to the conversion of Sb4O5Cl2 to Sb2O3, Sb, and LiCl via a two-step reaction. The reversible oxidation peaks that appear at 1 V and 1.3 V indicate the reversible conversion of these by-products to Sb4O5Cl2, which is analogous to reactions with Na. Fig. 5B shows the charge/ discharge voltage profiles of Sb4O5Cl2-GA electrode at a current density of 50 mA g1 in the voltage range of 0.01 Ve3 V against Li/ Liþ. Initial discharge capacity of ~1400 mAhg1 was obtained for Sb4O5Cl2-GA electrode. Reversible specific capacity of ~630 mAh g1 was obtained after 50 cycles, indicating good reversible nature of the redox reactions (Fig. 5C). Rate capability studies have also been done on the sample (Fig. 5D) and Sb4O5Cl2-GA electrode showed a stable electrochemical performance even at higher current rates indicating that Sb4O5Cl2-GA is a promising anode for Liion battery applications. 4. Conclusions In summary, synthesis and electrochemical performance of phase-pure Sb4O5Cl2 as a new anode for Na-ion battery was demonstrated. Various experimental tools were employed for the structural and morphological characterization of the sample. By using ex-situ XRD, HR-TEM and XPS analyses, the mechanism of Na-ion storage in Sb4O5Cl2 was investigated, which involved multiple reactions of conversion and alloying. Further, Sb4O5Cl2-GA composite with Sb4O5Cl2 microstructures anchored on completely interconnected 3D networks was prepared, which showed excellent electrochemical performance as anode for SIB, with improved cycling stability and rate capability compared to the pristine sample. Sb4O5Cl2-GA electrode delivered a stable reversible specific capacity of 300 mAh g1 at a very high current rate of 200 mA g1 for 70 cycles, which could have resulted from the synergistic effects provided by the interconnected graphene networks and good redox properties of Sb4O5Cl2. Sb4O5Cl2-GA was further studied as stable anode for LIBs. The present study thus reveals Sb4O5Cl2-GA as a promising anode for both sodium-ion and lithium ion batteries owing to its excellent electrochemical performance and facile synthesis route.
Acknowledgments Financial support from Technology Mission Division (TMD), Department of Science & Technology, Govt. of India [DST/TMD/ MES/2k16/114], is gratefully acknowledged. KPL acknowledges support from IISER Thiruvananthapuram and KJJ acknowledges Department of Science and Technology (DST), Government of India, for the INSPIRE Scholarship. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2018.01.095. References [1] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Sodium-ion batteries, Adv. Funct. Mater. 23 (26) (2013) 947e958. [2] E. Irisarri, A. Ponrouch, M.R. Palacin, Reviewdhard carbon negative electrode materials for sodium-ion batteries, J. Electrochem. Soc. 162 (14) (2015) A2476eA2482. [3] Y. Wen, K. He, Y. Zhu, F. Han, Y. Xu, I. Matsuda, Y. Ishii, J. Cumings, C. Wang, Expanded graphite as superior anode for sodium-ion batteries, Nat. Commun. 5 (2014) 4033. [4] D. Su, H.J. Ahn, G. Wang, SnO2@graphene nanocomposites as anode materials for Na-ion batteries with superior electrochemical performance, Chem. Commun. 49 (30) (2013) 3131e3133. [5] Y.X. Wang, Y.G. Lim, M.S. Park, S.L. Chou, J.H. Kim, H.K. Liu, S.X. Dou, Y.J. Kim, Ultrafine SnO2 nanoparticle loading onto reduced graphene oxide as anodes for sodium-ion batteries with superior rate and cycling performances, J. Mater. Chem. 2 (2) (2014) 529e534. [6] S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K. Kang, Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithiumion batteries, Adv. Energy Mater. 2 (7) (2012) 710e721. [7] L.P. Wang, L. Yu, X. Wang, M. Srinivasan, Z.J. Xu, Recent developments in electrode materials for sodium-ion batteries, J. Mater. Chem. 3 (18) (2015) 9353e9378. [8] J. Cabana, L. Monconduit, D. Larcher, M.R. Palacín, Beyond intercalation-based Li-Ion batteries: the state of the art and challenges of electrode materials reacting through conversion reactions, Adv. Mater. 22 (35) (2010) E170eE192. [9] K. Li, H. Liu, G. Wang, Sb2O3 nanowires as anode material for sodium-ion battery, Arabian J. Sci. Eng. 39 (9) (2014) 6589e6593. [10] D.-H. Nam, K.-S. Hong, S.-J. Lim, M.-J. Kim, H.-S. Kwon, High-performance Sb/ Sb2 O3 anode materials using a polypyrrole nanowire network for Na-Ion batteries, Small 11 (2015) 2885e2892. [11] J. Qian, Y. Chen, L. Wu, Y. Cao, X. Ai, H. Yang, High capacity Na-storage and superior cyclability of nanocomposite Sb/C anode for Na-ion batteries, Chem. Commun. 48 (56) (2012) 7070e7072. [12] Q. Sun, Q.Q. Ren, H. Li, Z.W. Fu, High capacity Sb2O4 thin film electrodes for rechargeable sodium battery, Electrochem. Commun. 13 (12) (2011) 1462e1464. [13] D.Y.W. Yu, P.V. Prikhodchenko, C.W. Mason, S.K. Batabyal, J. Gun, S. Sladkevich, A.G. Medvedev, O. Lev, High-capacity antimony sulphide nanoparticle-decorated graphene composite as anode for sodium-ion batteries, Nat. Commun. 4 (2013) 2922. [14] L. Xiao, Y. Cao, J. Xiao, W. Wang, L. Kovarik, Z. Nie, J. Liu, High capacity reversible alloying reactions in SnSb/C nanocomposites for Na-ion battery applications, Chem. Commun. 48 (27) (2012) 3321e3323. [15] Y. Zhao, A. Manthiram, High-capacity, high-rate BieSb alloy anodes for lithium-ion and sodium-ion batteries, Chem. Mater. 27 (8) (2015) 3096e3101. [16] M. Hu, Y. Jiang, W. Sun, H. Wang, C. Jin, M. Yan, Reversible conversion-alloying of Sb 2 O 3 as a high-capacity, high- rate, and durable anode for sodium ion batteries, ACS Appl. Mater. Interfaces 6 (21) (2014) 19449e19455. [17] K.-S. Hong, D.-H. Nam, S.-J. Lim, D. Sohn, T.-H. Kim, H. Kwon, Electrochemically synthesized Sb/Sb2O3 composites as high-capacity anode materials utilizing a reversible conversion reaction for Na-Ion batteries, ACS Appl. Mater. Interfaces 7 (31) (2015) 17264e17271. [18] N. Li, S. Liao, Y. Sun, H.W. Song, C.X. Wang, Uniformly dispersed selfassembled growth of Sb2O3/Sb@graphene nanocomposites on a 3D carbon sheet network for high Na-storage capacity and excellent stability, J. Mater. Chem. 3 (11) (2015) 5820e5828. [19] J. Xie, Y. Pei, L. Liu, S. Guo, J. Xia, M. Li, Y. Ouyang, X. Zhang, X. Wang, Hydrothermal synthesis of antimony oxychlorides submicron rods as anode materials for lithium-ion batteries and sodium-ion batteries, Electrochim. Acta 254 (2017) 246e254. [20] M.S. Whittingham, Chemistry of intercalation compounds: metal guests in chalcogenide hosts, Prog. Solid State Chem. 12 (1) (1978) 41e99. [21] Whittingham, M.S., Electrochemical cells with cathode-active materials of layered compounds. US patent 4049887, 1977. [22] P. Gao, C. Wall, L. Zhang, M.A. Reddy, M. Fichtner, Vanadium oxychloride as
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Antimony oxychloride/graphene aerogel composite as anode material for sodium and lithium ion batteries K.P. Lakshmi, K.J. Janas, M.M. Shaijumon* School of Physics, Indian Institute of Science Education and Research Thiruvananthapuram, Maruthamala PO, Thiruvananthapuram, Kerala, 695551, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 October 2017 Received in revised form 16 January 2018 Accepted 25 January 2018
Herein, we report on the synthesis of phase-pure antimony oxychloride (Sb4O5Cl2) microstructures and studies on their electrochemical properties as new and potential anodes for sodium and lithium ion batteries. We demonstrate that pristine Sb4O5Cl2 based electrode exhibits promising electrochemical behaviour, with a reversible discharge capacity of 830 mAh g1 for the first cycle when cycled against sodium at a current rate of 30 mA g1. Further, a composite of graphene aerogel is prepared with Sb4O5Cl2 microstructures, which are uniformly anchored on the graphene aerogel matrix (Sb4O5Cl2-GA), resulting in interconnected networks which facilitate better charge transfer and effective buffering to alleviate the structural variation of Sb4O5Cl2 during cycling. We show that Sb4O5Cl2-GA electrode exhibits excellent electrochemical properties with much improved cyclic stability and high rate capability. In addition, Sb4O5Cl2-GA delivers excellent performance as anode material for lithium ion batteries, with a reversible capacity of 600 mAh g1 obtained over 50 cycles, at a current rate of 50 mA g1. The obtained results are promising and demonstrate 3D networked antimony oxychloride/graphene composite as a potential anode material for both lithium and sodium ion batteries. © 2018 Elsevier Ltd. All rights reserved.
1. Introduction With the increase in energy demand and quest for efficient storage devices, lithium-ion batteries (LIBs) have successfully been demonstrated as potential power sources for various applications in portable electronics, transport sector, etc. However, with the concerns over limited lithium resources, several other battery chemistries are being explored and sodium-ion battery (SIB) seems to be an ideal alternative to LIB, owing to the abundance of sodium in the earth's crust and similar electrochemical properties to that of lithium [1e3]. There are several challenges ahead to realise the practical implementation of these battery systems. For instance, development of efficient electrode materials for Na-ion battery with higher energy and power densities along with long cycle life still remains a key issue. Graphite, the anode material used in commercial LIBs, however, is not suitable for sodium ion batteries due to its limited performance on sodium intercalation caused by the larger ionic radius of Na [1]. On the other hand, carbonaceous materials such as hard
* Corresponding author. E-mail address: [email protected] (M.M. Shaijumon). https://doi.org/10.1016/j.carbon.2018.01.095 0008-6223/© 2018 Elsevier Ltd. All rights reserved.
carbon [2], expanded graphite [3] has been studied as stable anodes for SIBs. Apart from the conventional choices of carbonaceous materials, considerable effort has been devoted to exploring metal oxides [4,5], and intermetallics that have the potential to deliver excellent gravimetric capacities against sodium and lithium [6e8]. Recently Sb based materials have attracted attention owing to their high theoretical capacity against sodium (660 mAh g1). Sb based materials such as Sb2O3 [9,10], Sb/C [11], Sb2O4 [12], Sb2S3 [13], and Sb based intermetallics such as SnSb [14], Bi-Sb [15] have been studied as promising anode materials for SIBs. Sb based materials typically undergo conversion and alloying reactions with Na, involving multiple electrons [16,17]. Huge volume expansion associated with these materials causes pulverisation and cracking of the material, resulting in poor cycling stability [18]. This motivates further exploration of Sb based materials for Na-ion batteries as an effort to improve their overall electrochemical performance. Recently, Xie et al., studied the electrochemical properties of antimony oxychloride based microrods, wherein the material consisted of a mixed phase of Sb4O5Cl2, Sb8O11Cl2 and Sb2S3 [19]. Achieving phase-pure growth of antimony oxychloride plays an important role in determining its electrochemical properties. Here we demonstrate that pure-phase antimony oxychloride (Sb4O5Cl2) microstructures, prepared via hydrothermal technique,
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exhibit promising electrochemical performances as potential electrode for both sodium and lithium-ion batteries. Lithium ion intercalation into MOCl (M ¼ Fe,V,Cr,Ti) was first reported by Whittingham [20,21], and recently, other metal oxychlorides such as Vanadium oxychloride [22,23] and Bismuth oxychloride [24] have been explored as electrode material for sodium and lithium ion batteries. Sb4O5Cl2 exibits excellent Na-ion storage performance by delivering a high initial reversible capacity of 830 mAh g1. Further, we prepared Sb4O5Cl2-graphene aerogel composite electrode (Sb4O5Cl2-GA) via low temperature self assembly process, which showed much improved rate capability by retaining a stable capacity of over 200 mAh g1 at 2 A g1 (~3 C; 1 C corresponding to 450 mA g1). Sb4O5Cl2 microstructures, which are uniformly anchored on the graphene aerogel matrix result in interconnected networks which facilitate better charge transfer and effective buffering to alleviate the structural variation of Sb4O5Cl2 during cycling. Further, we demonstrate that Sb4O5Cl2-GA delivers excellent performance as anode material for lithium ion batteries, with a reversible capacity of 600 mAh g1 obtained over 50 cycles, at a current rate of 50 mA g1.
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mixing the active material, Super-P and CMC binder (carboxymethyl cellulose) in the weight ratio of 70:20:10. For Sb4O5Cl2-GA, slurry was prepared with active material, Super-P and CMC binder in the ratio 80:10:10. The slurry was uniformly coated onto a Cu foil and vacuum dried at 120 C for 12 h. For the electrochemical studies, galvanostatic charge-discharge measurements on the fabricated electrodes were carried out in a CR2032 type coin cell assembled in an argon filled glove box with moisture and oxygen level maintained at less than 0.1 ppm. 1M NaClO4 in PC (with 10 wt % FEC) was used as the electrolyte and with Na metal as both reference and counter electrodes. Typical mass loading of the active material is 1.5 mg cm2. Cyclic voltammetry was performed for all the samples in the voltage range between 0.05 and 2 V versus Na/ Naþ. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a sinusoidal signal of 10 mV amplitude, between frequencies ranging from 10 mHz to 40 kHz. For lithium ion battery measurements, 1M LiPF6 in EC:DMC (1:1 vol ratio) was used as the electrolyte and all the electrochemical measurements were carried out in the voltage range between 0.01 and 3 V versus Li/Liþ. 3. Results and discussion
2. Experimental section 2.1. Synthesis of Sb4O5Cl2 microstructures SbCl3 (2 mmol) and PVP (315 mg) dissolved in 10 mL of DI water was stirred for 30 min to get a homogenous solution. Appropriate amount of 2M H2SO4 was added to the above solution and stirred well to get a uniform solution. The resultant solution was trans ferred to a Teflon-lined autoclave and was kept at 180 C for 2 h. The autoclave was allowed to cool down to room temperature and the product was washed several times with ethanol and DI water to remove the impurities. 2.2. Preparation of Sb4O5Cl2 -GA 20 mg of GO prepared by modified hummer's method was uniformly dispersed in 10 mL of DI water by ultrasonication for 1 h. 1.5 mL of 2 M ascorbic acid was added to the above solution and sonicated for few sec. The as-synthesized Sb4O5Cl2 microstructures were mixed with the above solution by stirring. The solution was kept in an oil bath at 60 C for 24 h without being disturbed. The black cylindrical aggregate thus formed was washed for several times in DI water. The product was freeze dried for 24 h to obtain Sb4O5Cl2-GA. 2.3. Morphological and structural characterization The surface morphology was examined by using scanning electron microscope (SEM; Nova NanoSEM 450, FEI) and transmission electron microscope (TEM; FEI TECNAI 30 G2 S-TWIN microscope with an accelerating voltage of 300 kV). Structural characterization has been done using X-ray diffraction (XRD) with Cu-Ka radiation within Bragg's angle ranging from 10 to 80 (Emperean, PANalytical XRD system). Thermogravimetric analysis of samples has been done in the presence of oxygen using SDT-Q600, TA Instruments. The porosity characteristics including BET surface area were studied using N2 adsorption-desorption isotherms measured at 77 K up to a maximum relative pressure of 1 bar, with the Micromeritics 3-Flex surface characterization analyzer. 2.4. Electrochemical characterization For pristine Sb4O5Cl2 samples, the electrodes were prepared by
Sb4O5Cl2 microstructures were synthesized by a simple hydrothermal method. As-synthesized Sb4O5Cl2-GA composite is obtained as a black cylindrical aggregate with monoclinic Sb4O5Cl2 microstructures trapped inside three dimensionally connected thin reduced graphene oxide sheets (Fig. S1). Sb4O5Cl2 microstructures (~4 mm in size) exhibit a unique flower shaped morphology which is formed by the aggregation of Sb4O5Cl2 nanoflakes, ~200 nm in size, as shown in Fig. 1A (See Figs. S1A and B for details). Fig. 1C shows the SEM image of Sb4O5Cl2-GA composite clearly revealing Sb4O5Cl2 microstructures encapsulated within few-layered graphene sheets, forming three dimensionally oriented cross linked networks. Fig. 1B,D represent high resolution TEM (HRTEM) images of Sb4O5Cl2 microstructures and Sb4O5Cl2-GA composite, respectively, showing an inter planar distance of 0.31 nm corresponding to (002) plane of Sb4O5Cl2 [25]. Presence of few-layered graphene surrounding Sb4O5Cl2 domains is clearly evident in the HRTEM image (Fig. 1D). X-ray diffraction (XRD) was employed for the structural characterization of materials (Fig. 1E) and XRD patterns clearly indicate all the reflections which are indexed to the monoclinic phase of Sb4O5Cl2 (JCPDS No-30-0091) [25]. The graphitic peak at ~26 is not visible in the XRD pattern of Sb4O5Cl2-GA, which could be due to high crystallinity of the pristine material [26]. Thermogravimetric analysis (TGA) was per formed in a temperature range of 40 C-1000 C in presence of oxygen. Sb4O5Cl2 remains stable till ~400 C, beyond which it decomposes releasing SbCl3 gas (Fig. 1F) [27]. This in turn gets con verted to Sb2O3 phase above 475 C. An increase in sample weight can be observed above 600 C due to oxidation of Sb2O3 particles. TG curve of Sb4O5Cl2-GA shows a slight weight loss around 100 C due to evaporation of surface absorbed water. Weight loss till 400 C can be attributed to the elimination of carboxyl groups and surface absorbed moisture [18], as observed for the pure graphene aerogel (Fig. S4A). A drastic weight loss between 400 C to 600 C can be attributed to the thermal decomposition of rGO in air, in addition to the decomposition of Sb4O5Cl2 and the accompanied phase change of Sb4O5Cl2 to Sb2O3. A similar weight gain is observed beyond 600 C, corresponding to the oxidation of Sb2O3. Sb4O5Cl2-GA showed an altogether different thermal decomposition behaviour when compared to the pristine Sb4O5Cl2, indicating better stability of the graphene composite [28,29]. From the TG analysis, the amount of graphene in Sb4O5Cl2-GA was estimated to be ~10 wt%. It has been shown that pH of the reaction mixture is very critical for the controllable synthesis of Sb4O5Cl2, which are typically
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Fig. 1. (A) SEM and (B) HRTEM images of Sb4O5Cl2 microstructures. (C) SEM and (D) HRTEM images of Sb4O5Cl2-GA composite. (E) Powder XRD patterns of Sb4O5Cl2 microstructures, Sb4O5Cl2-GA composite. (F) TG curves of Sb4O5Cl2 and Sb4O5Cl2eGA. Graphene content in Sb4O5Cl2eGA is calculated from the TG analysis (~10 wt%).
formed in the pH range of 1e6. Further, the solvent composition plays a key role in obtaining the desired phases. For instance, Chen et al., investigated the effect of solvent composition on the selective synthesis of different phases of Sb2O3 [25]. Here, the reaction mechanism for the formation of Sb4O5Cl2 structures can be represented by the following equation [30]: SbCl3 þ H2O / SbOClþ 2HCl 4SbOCl þ H2O/Sb4O5Cl2þ 2HCl The reaction mixture is maintained at pH ¼ 1e5, as hydrolysis of SbCl3 gives acidic solution. Sb4O5Cl2-GA is formed by the hydrogelation process [31]. GO is uniformly dispersed in DI water by
ultrasonication. Addition of L-Ascorbic acid, a mild reductant, reduces the dispersed GO sheets [32] and hydrogelation of rGO sheets occur once the system is kept undisturbed for a prolonged period. The p-p interaction of the graphene sheets with a prolonged reaction time is responsible for the hydrogelation process. During the reaction, Sb4O5Cl2 microstructures are uniformly dispersed in the GO solution and get trapped inside the graphene sheets which are randomly oriented in all directions and finally form 3dimensionally networked structure during the self-assembly of graphene sheets. Such a composite with interconnected networks would facilitate better charge transfer and effectively minimize the structural variation of Sb4O5Cl2 during electrochemical cycling. Electrochemical performance studies on Sb4O5Cl2 and Sb4O5Cl2GA electrodes were conducted in a two-electrode coin cell
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assembly against metallic sodium employing cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance. Fig. 2A shows the CV curves of Sb4O5Cl2 electrode performed at a scan rate of 0.02 mV s1 in the potential range of 0.05 Ve2 V against Na/Naþ. In the first cathodic scan, reduction peaks are observed at 0.96 V and 0.36 V. Peak at 0.96 V, which could be due to the formation of SEI layer, was absent in the consecutive cycles, while the peak at 0.36 V is found to be slightly shifted. In the subsequent cycles, three reversible reduction peaks are observed at 1 V, 0.73 V and 0.43 V, indicating the reaction of Na with Sb4O5Cl2 in multiple stages, forming several intermediate alloys and oxides of Na and Sb. Similar profile has been reported with other metal oxychlorides which are studied as anodes for SIBs [22]. In the subsequent anodic scans, a sharp oxidation peak at 0.7 V and a broad peak between 1.4 V and 1.9 V were observed, corresponding to the de-alloying of Na based alloys and the reversible reactions of the other oxides of Na and Sb, respectively, indicating the reversible nature of the electrochemical reactions. Multiple conversion and alloying reactions is clearly evident from the CV profiles, similar to other reports in SIBs [33]. Apart from these redox peaks, a small peak at 0.06 V is observed for Sb4O5Cl2-GA electrode (Fig. S3C), due to the insertion of Naþ ions into the graphene layers, which has been confirmed from the CV measurements on pure graphene aerogel (Fig. S4C). In order to understand the redox behaviour and evaluate the electrochemical performance, galvanostatic charge/discharge measurements have been carried out on both the samples in the voltage range of 0.05 Ve2V against Na/Naþ. Figure 2B shows the charge/discharge curves of the Sb4O5Cl2 electrode at a current density of 30 mA g1. Sb4O5Cl2 electrode showed an initial
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discharge capacity of 830 mAh g1, indicating an irreversible capacity due to the SEI layer formation, consistent with the CV results. Charge/discharge profiles showed three distinct sloppy regions, in agreement with the redox peaks observed in CV curves. Pristine Sb4O5Cl2 electrode delivered reversible specific capacities of 528 mAh g1 and 320 mAh g1 upon 2nd and 50th discharge cycles, respectively. Further, voltage-composition curves were recorded (inset of Fig. 2B) which show a reversible capacity of ~10e11 Naþ ions during first charge-discharge cycle, indicating multi-stage conversion and alloying reactions in the pristine material. The electrode showed large polarization, indicating slower kinetics of Naþ insertion/de-insertion processes. Poor cyclic stability of pristine electrode (Fig. 2B) results from huge volume expansion of the material upon cycling, leading to pulverisation of the electrode and eventual loss of electrical contact with current collector, which is common for Sb-based systems. To overcome this issue, we prepared a composite electrode with a 3-D networked framework of graphene aerogel with Sb4O5Cl2 microstructures uniformly anchored on it (Sb4O5Cl2-GA). We performed the electrochemical characterization of the composite electrode following similar conditions as that for pristine Sb4O5Cl2 electrode. Galvanostatic charge-discharge voltage profiles obtained for Sb4O5Cl2-GA electrode (Fig. 2C) at a current density of 30 mA g1 in the voltage range of 0.05 Ve2 V show similar plateaus to that of pristine Sb4O5Cl2 electrode with an initial discharge capacity of 735 mAh g1. An irreversible capacity loss of 40%, due to the SEI layer formation, is observed and a reversible specific capacity of ~400 mAh g1 is obtained after 50 cycles. Sb4O5Cl2-GA electrode exhibited excellent electrochemical performances with a good
Fig. 2. (A) Cyclic Voltammograms of Sb4O5Cl2 electrode for the initial five cycles at a scan rate of 0.02 mV s1. (B) Galvanostatic discharge/charge curves for Sb4O5Cl2 electrode, cycled at a rate of 30 mA g1 vs. Na. Inset shows composition vs. voltage curves for Sb4O5Cl2 electrode. (C) Galvanostatic discharge/charge curves for Sb4O5Cl2-GA electrode cycled at a rate of 30 mA g1 vs. Na. (D) Capacity retention plot of Sb4O5Cl2 and Sb4O5Cl2-GA electrodes at 30 mA g1 vs. Na.
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capacity retention with much improved cycling stability and columbic efficiency compared to the pristine electrode (Fig. 2D). Sb4O5Cl2-GA electrode showed excellent rate performances when cycled at much higher current rates. Galvanostatic charge/ discharge voltage profiles have been obtained at various current densities of 100 and 200 mA g1 (Figs. S2 and S3) for both the pristine and composite electrodes. Sb4O5Cl2-GA sample showed very stable electrochemical performance compared to the pristine material for about 70 cycles at high current rates (Fig. 3A). As shown in Fig. 3B, Sb4O5Cl2-GA electrode could retain the lower rate performance after cycling at very high current rates of 5 A g1. Pure graphene aerogel cycled at a current density of 100 mA g1 for 100 cycles showed a stable capacity of 100 mAh g1 (Figure S4 B). With <10 wt% of graphene, the capacity contribution will be very less and the major redox activity towards the high capacity of Sb4O5Cl2-GA electrode could only come from Naþ storage in the Sb4O5Cl2 electrode by multiple conversion and alloying reactions [22]. The porosity characteristics including BET surface area were analyzed from N2 adsorption-desorption isotherms measured at 77 K (Fig. S7). Sb4O5Cl2-GA exhibit BET surface area of ~47 m2 g1, comparable to other aerogel-based anodes reported for sodium ion and lithium ion batteries [26,34]. Further, EIS measurements were carried out for both the samples before and after cycling to get better insight on the conductivity and reaction kinetics upon sodiation/de-sodiation. The Nyquist plots for both the samples show a semicircle in the high frequency region and a slanting line in the low frequency region (Fig. 3C). Improved rate capability of Sb4O5Cl2-GA electrode was further analyzed by using EIS spectra recorded after 100 chargedischarge cycles. A substantial drop in charge transfer resistance was observed for Sb4O5Cl2-GA sample (90.36 U), compared to pristine material (282.9 U) after 100 charge-discharge cycles. The best fitted equivalent circuit (Fig. S6) and the derived values of RC components (Table S1) are given in the supporting information. A small drop in the charge transfer resistance could be noted from the Nyquist plot for the Sb4O5Cl2-GA sample (29.16U) compared to the Sb4O5Cl2 sample (42.5U) recorded at the open circuit potential. The introduction of three dimensional wrapping of thin graphene sheets suppresses the associated volume expansion in Sb4O5Cl2. Further, the cross-linked 3D network of graphene sheets facilitate shorter diffusion path for Naþ ions and thus provide better ionic and electronic conductivity for the sodium ions, resulting in improved cycling performances. Thus an overall enhancement in electrochemical properties of the composite electrode is observed, owing to the synergistic effects from the redox behaviour of Sb4O5Cl2 microstructures as well as better electronic and ionic conductivities of 3D networked graphene structure. A comparison of electrochemical performance with other metal oxychloride based anodes for sodium ion batteries is given in the supporting information (Table S2). Ex-situ XRD studies have been done on the pristine Sb4O5Cl2 sample to obtain further insights on the Na-ion storage mechanism. XRD measurements were done on various samples with different depth of voltages during 2nd cycle of both discharge and charge processes. Fig. 4A shows the XRD patterns recorded for samples cycled to different charge/discharge potentials. Diffraction peaks corresponding to NaCl (PDF Card No-1000041) and Sb (PDF Card No-2310879) were found in the complete discharged state, indicating that Sb4O5Cl2 undergoes conversion reaction forming NaCl and Sb. Similar conversion reaction has been reported for other metal oxychlorides upon sodiation and lithiation [22]. The diffraction peaks corresponding to presence of Sb2O3 phase could not be detected from the XRD analysis which could be due to the formation of nanosized particles/amorphous phases similar to studies reported for other metal oxychlorides [23]. Diffraction peaks corresponding to pristine Sb4O5Cl2 were not found in the recharged
states because of the partial conversion of NaCl and Sb to Sb4O5Cl2. The diffraction peaks at the fully recharged state can be indexed to NaCl, Sb and other by-products. Similar results have been reported for other metal oxychlorides that shows partial conversion in fully recharged state [22,23]. X-ray photoelectron spectroscopy (XPS) was further employed to investigate the chemical composition and surface states of the pristine Sb4O5Cl2 as well as the electrode after complete discharge
Fig. 3. (A) Capacity retention plots for Sb4O5Cl2 and Sb4O5Cl2-GA electrodes cycled at a rate of 200 mA g1 vs Na. (B) Rate capability plots of Sb4O5Cl2, Sb4O5Cl2-GA electrodes obtained at different current rates vs Na. (C) Electrochemical impedance spectra of Sb4O5Cl2 and Sb4O5Cl2-GA electrodes after 100 discharge/charge cycles, measured in the frequency range of 100 kHz to 10 mHz. (A colour version of this figure can be viewed online.)
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Fig. 4. (A) Ex-situ XRD patterns for Sb4O5Cl2 electrode at different depth of charge/discharge edischarged to (a) 0.573 V (b) 0.369 V (c) 0.05 V and recharged to (d) 0.708 V (e) 0.97 V and (f) 2 V. (B) High resolution XPS spectra of (a) Sb4O5Cl2 (b) fully discharged Sb4O5Cl2 electrode (to 0.05 V), (c) fully charged Sb4O5Cl2 electrode (to 2 V). (A colour version of this figure can be viewed online.)
(to 0.05 V) and charge (to 2 V) processes (Fig. 4B). High resolution XPS spectra of pristine Sb4O5Cl2 show two strong peaks at 529.7 eV and 539.2 eV, corresponding to Sb 3d5/2 and Sb 3d3/2 of Sb (III) state, respectively (Fig. 4B a) [30]. High resolution XPS spectra in Fig. 4B b
reveal peaks corresponding to the presence of Sb (III) and Sb (0) states in the fully discharged electrode, with respective binding energies of 530.1 eV, 539.9 eV and 538.5 eV. This clearly indicates the formation of Sb particles with Sb (0) state and Sb2O3 particles
Fig. 5. (A) Cyclic Voltammograms of Sb4O5Cl2-GA electrode for the initial five cycles at a scan rate of 0.02 mV s1 (B) Galvanostatic discharge/charge curves for Sb4O5Cl2-GA electrode, cycled at a rate of 50 mA g1 vs.Li. (C) Capacity retention plot for Sb4O5Cl2-GA at 50 mA g1 for 50 cycles vs.Li. (D) Rate capability plots of Sb4O5Cl2-GA electrode at different current rates from 50 mA g1 to 5 A g1 vs.Li. (A colour version of this figure can be viewed online.)
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with Sb (III) state. Fig. 4B c shows high resolution XPS profiles for fully charged electrode indicating the presence of only Sb (III) oxidation state with peaks at 530.1 eV and 539.9 eV, corresponding to Sb 3d5/2 and Sb 3d3/2, thus clearly revealing the reversible conversion to Sb4O5Cl2. XPS spectrum of Sb4O5Cl2 sample indicating the presence of Sb, Cl, O and C elements is shown in Fig. S4C. The peaks observed at binding energies at around 33 eV and 200 eV can be attributed to Sb 4d and Cl 2p respectively, which correspond to þ3 oxidation state of Sb and 1 oxidation state of Cl. The O1s and C1s correspond to the presence of oxygen and carbon in the sample due to absorbed CO2 gas on the surface. Fig. S5D shows the high resolution XPS spectra of O1s of all the samples. In case of Sb4O5Cl2, O1s peaks are mostly overlapped with the Sb3þ peaks [35] as shown in the supporting information, Fig. S4 D. For the charged and discharged states, the O1s XPS spectra as shown in the supporting information mostly overlaps with the Sb 3d spectra and there is a slight shift in the peak positions due to the change in binding energy of the atoms. We further demonstrate Sb4O5Cl2-GA electrode as a potential anode for Li-ion batteries. Detailed electrochemical studies have been carried out including galvanostatic charge/discharge and cyclic voltammetry against metallic lithium (Fig. 5). Fig. 5A shows the CV curve of Sb4O5Cl2-GA electrode at a scan rate of 0.02 mV s1 in the potential range of 0.01e3 V against Li/Liþ. CV curves show redox peaks at subsequent cycles indicating the reversibility of the reactions. The irreversible oxidation peak that appears in the first cycle indicates the formation of SEI layer during the first discharge. The reversible reduction peak at 1.7 V and 0.75 V correspond to the conversion of Sb4O5Cl2 to Sb2O3, Sb, and LiCl via a two-step reaction. The reversible oxidation peaks that appear at 1 V and 1.3 V indicate the reversible conversion of these by-products to Sb4O5Cl2, which is analogous to reactions with Na. Fig. 5B shows the charge/ discharge voltage profiles of Sb4O5Cl2-GA electrode at a current density of 50 mA g1 in the voltage range of 0.01 Ve3 V against Li/ Liþ. Initial discharge capacity of ~1400 mAhg1 was obtained for Sb4O5Cl2-GA electrode. Reversible specific capacity of ~630 mAh g1 was obtained after 50 cycles, indicating good reversible nature of the redox reactions (Fig. 5C). Rate capability studies have also been done on the sample (Fig. 5D) and Sb4O5Cl2-GA electrode showed a stable electrochemical performance even at higher current rates indicating that Sb4O5Cl2-GA is a promising anode for Liion battery applications. 4. Conclusions In summary, synthesis and electrochemical performance of phase-pure Sb4O5Cl2 as a new anode for Na-ion battery was demonstrated. Various experimental tools were employed for the structural and morphological characterization of the sample. By using ex-situ XRD, HR-TEM and XPS analyses, the mechanism of Na-ion storage in Sb4O5Cl2 was investigated, which involved multiple reactions of conversion and alloying. Further, Sb4O5Cl2-GA composite with Sb4O5Cl2 microstructures anchored on completely interconnected 3D networks was prepared, which showed excellent electrochemical performance as anode for SIB, with improved cycling stability and rate capability compared to the pristine sample. Sb4O5Cl2-GA electrode delivered a stable reversible specific capacity of 300 mAh g1 at a very high current rate of 200 mA g1 for 70 cycles, which could have resulted from the synergistic effects provided by the interconnected graphene networks and good redox properties of Sb4O5Cl2. Sb4O5Cl2-GA was further studied as stable anode for LIBs. The present study thus reveals Sb4O5Cl2-GA as a promising anode for both sodium-ion and lithium ion batteries owing to its excellent electrochemical performance and facile synthesis route.
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