Sb2O3 nanoparticles anchored on graphene sheets via alcohol

Engineering, Northwest Normal University, Lanzhou 730070, Gansu Province, P.R. China. .... the average particle size of Sb2O3 decreased greatly from c...
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Sb2O3 Nanoparticles Anchored on Graphene Sheets via Alcohol Dissolution−Reprecipitation Method for Excellent Lithium-Storage Properties Xiaozhong Zhou,* Zhengfeng Zhang, Xiaofang Lu, Xueyan Lv, Guofu Ma, Qingtao Wang, and Ziqiang Lei* Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, Gansu, P. R. China S Supporting Information *

ABSTRACT: Sb2O3 nanoparticles are uniformly anchored on reduced graphene oxide (rGO) sheets via a facile and ecofriendly route based on the alcohol dissolution−reprecipitation method. Such obtained Sb2 O 3 /rGO composite demonstrates a highly reversible specific capacity (1355 mA h g−1 at 100 mA g−1), good rate capability, and superior life cycle (525 mA h g−1 after 700 cycles at 600 mA g−1) when used an anode electrode for lithium-ion batteries (LIBs). The outstanding electrochemical properties of Sb2O3/rGO composite could be attributed to its unique structure in which the strong electronic coupling effect between Sb2O3 and rGO leads to an enhanced electronic conductivity, structure stability, and electrochemical activity during reversible conversion-alloying reactions. Also, these findings are helpful in both developing novel high-performance electrodes for LIBs and synthesizing functional materials in an ecofriendly and economical way. KEYWORDS: Sb2O3/rGO composite, alcohol dissolution−reprecipitation method, electronic coupling effect, lithium-ion batteries, lithium-storage properties

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first demonstrated the above reversible conversion-alloying reaction. However, the conversion reaction (eq 1) is usually irreversible for PTMOs due to the aggregation of the electrochemical inactive Li2O component during lithiation/ delithiation process, which leads to a large loss of capacity and a low initial Coulombic efficiency (1000 mA h g−1); therefore, they are being widely investigated as alternative anode materials for LIBs. Sb2O3, a cheap and commercial raw material, in particular, has been investigated as a high-performance anode for sodium-ion batteries16,17 and LIBs10−12 based on the reversible conversion-alloying reaction (eqs 1 and 2) Sb2 O3 + 6Li+ + 6e− ↔ 2Sb + 3Li 2O

(1)

2Sb + 6Li+ + 6e− ↔ 2Li3Sb

(2)

On the basis of the above two reactions, Sb2O3 could be assigned a high theoretical capacity of 1109 mA h g−1, which is nearly 3 times as high as that of graphite (372 mA h g−1).12 Also, Fu and co-workers12 have adopted a pulsed laser deposition approach to fabricate Sb2O3 thin film, which exhibited a reversible specific capacity of 794 mA h g−1, but © 2017 American Chemical Society

Received: July 12, 2017 Accepted: September 21, 2017 Published: September 21, 2017 34927

DOI: 10.1021/acsami.7b10107 ACS Appl. Mater. Interfaces 2017, 9, 34927−34936

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) X-ray diffraction (XRD) patterns of bulk Sb2O3, Sb2O3/rGO, and Sb2O3−rGO composites. (b) Raman spectra and (c) XPS surveys of bare rGO, Sb2O3/rGO, and Sb2O3−rGO composites. (d) High-resolution C 1s XPS spectra of Sb2O3/rGO and Sb2O3−rGO composites.

electrons and ions and large specific surface energy and/or lattice distortion energy.5,7,10,15−30 Recently, Zhou et al.10 embedded graphene nanosheets into the gap of Sb2O3 micro/ nanomaterial with a nest structure through a facile wet chemical method, which greatly improved the reversible capacity and cycling stability. Chemical synthesis is the most commonly used method to fabricate metal oxide-based composites.31,32 However, many unwanted alien ions from the raw reagents are left in liquid waste during chemical synthesis, causing wastage of resources and even environmental pollution. In this work, a novel recipe, in which Sb2O3 is first dissolved in ethylene glycol (EG) solvent and then reprecipitated on GO sheets in the subsequent solvothermal treatment, has been applied to synthesize Sb2O3/ rGO composite facilely and without any unwanted ions left over. It is well known that Sb2O3 can react with ethylene glycol (EG) at ca. 100 °C to form ethylene glycol antimony (EG-Sb), which can then be hydrolyzed to obtain Sb2O3 again in the presence of H2O, as shown in Figure S1a−e. Also, the average particle size of Sb2O3 decreased greatly from ca. 500 to ca. 100 nm through the dissolution and reprecipitation process in the EG solution (Figure S1f,d). During the solvothermal treatment, the oxygen-containing functional groups of the GO sheets as

preferential nucleation sites are favorable to the formation of nanosize particles, and GO could be simultaneously reduced by EG.33 The reduction of GO could be enhanced by L-ascorbic acid (L-AA), which is an environment-friendly reductant34,35 possessing a higher reduction efficiency with regard to the electrical conductivity.36 Benefiting from the exclusive nanostructure and good electrical conductivity, the well-defined Sb2O3/rGO composite, in which Sb2O3 nanoparticles are uniformly anchored onto rGO sheets, exhibits a high reversible specific capacity above 800 mA h g−1 over 120 cycles at 100 mA g−1, an excellent cyclic stability, and a high-rate performance as an alternative anode material for LIBs.

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EXPERIMENTAL SECTION

Materials. Ethylene glycol, natural flake graphite powder, Sb2O3, N-methyl pyrrolidinone (NMP), poly(vinylidene difluoride) (PVDF), and carbon black were purchased from Aladdin Biochemical Technology (Shanghai, China) and used without any additional purification. Electrolyte (1 M LiPF6 in a mixture of dimethyl carbonate (DMC), ethylene carbonate (EC), and ethylmethyl carbonate (EMC) in a 1:1:1 volume ratio) was provided by Shenzhen Capchem Technology Co., Ltd. (China). Synthesis of Sb2O3/rGO Composite. First, the GO sheets (Figure S2a,b) were prepared via modified Hummers’ method using 34928

DOI: 10.1021/acsami.7b10107 ACS Appl. Mater. Interfaces 2017, 9, 34927−34936

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ACS Applied Materials & Interfaces natural flake graphite powder as the raw material.37 A facile and ecofriendly solvothermal method based on the dissolution− reprecipitation strategy was employed to synthesize the Sb2O3/rGO composite, and the preparation process was similar to that of the Sb6O13/rGO composite.15 Typically, 0.3 g bulk Sb2O3 was first dispersed in 40 mL ethylene glycol (EG) to form a white suspension solution by ultrasonic and magnetic stirring, then 20 mL of GO aqueous solution (containing 0.1 g GO) was added. After 3.5 h of stirring, the color of the solution turned to brown and 2.0 g L-AA was then added dropwise, followed by continuous stirring for another 24 h. After that, the black suspension solution was obtained, indicating the partial reduction of GO, and then transferred to a 100 mL Teflon-lined autoclave with a stainless-steel shell and solvothermally treated at 140 °C for 12 h. The resulting black columned precipitate Sb2O3/rGO composite was collected by suction filtration and washed with deionized water and anhydrous ethanol for three times before being dried at 120 °C for 12 h. In contrast, bare rGO was obtained by the same procedure but without the addition of Sb2O3, and a Sb2O3−rGO composite also synthesized without the addition of L-AA. A co-ground Sb2O3 + rGO mixture was prepared using bulk Sb2O3 and bare rGO. Physical Characterization. X-ray diffraction (XRD, Rigaku D/ max 2400, Japan, operating with Cu Kα radiation of λ = 0.15416 nm, 40 kV, 150 mA, at a scanning rate of 8° min−1 from 2θ = 10 to 70°), thermogravimetric analysis (TGA, Mettler Toledo TGA2, Switzerland), X-ray photoelectron spectra (XPS, Kratos, Axis Ultra, Japan), Raman spectrometry (Renishaw inVia, Britain), field emission scanning electron microscopy (FESEM, Carl Zeiss, Ultra Plus, Germany, operating at 5 kV), and transmission electron microscopy (TEM, FEI Tecnai TF20, operating at 200 kV) were used to characterize the composition, structure, and morphology of the obtained samples. The elemental mapping was carried out by FESEM with an energy-dispersive X-ray spectroscopy system (Oxford X-Max 80, Britain). Characterization of Electrochemical Performance. To investigate the electrochemical lithium-storage performance of the obtained samples, CR2025-type coin half-cells were assembled in a glovebox fully filled with argon atmosphere using a lithium metal foil as the counter electrode and a Celgard 2400 polypropylene membrane as the separator. For preparing the testing electrode, typically, the asobtained active material was mixed together with PVDF binder and carbon black conducting additive (75:15:10 in weight) in NMP to form a homogeneous slurry, which was coated on a Cu foil uniformly. After being dried at 120 °C under vacuum for 24 h, the Cu foil coated with active material was cut into wafers having a diameter of 10 mm and ca. 1.0 mg cm−1 of loaded active material. A liquid solution of 1 M lithium hexafluorophosphate (LiPF6) dissolved in a solvent mixture of EMC, EC, and DMC (1:1:1 in volume) was used as the electrolyte. Galvanostatic discharge/charge (GSDC) tests and rate performances were investigated using a CT2001A battery tester (JINNUO Electronics Co., Ltd., Wuhan, China) in the voltage range of 0.001− 3.0 V (vs Li+/Li) at ambient temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on an Autolab PGSTAT128N electrochemical workstation (Metrohm, Switzerland) at ambient temperature. The CV tests were carried out at a scan rate of 0.2 mV s−1 in the voltage range of 0.001−3.0 V (vs Li+/Li), and EIS measurements were performed at open-circuit potentials with an excitation amplitude of 5 mV in the frequency range of 10−2−105 Hz.

appeared in both composites, indicating negligible restacking of the rGO sheets after anchoring with Sb2O3. To investigate the structure of rGO in the as-obtained samples, laser Raman scattering method has been employed. As shown in Figure 1b, the two distinct scattering peaks at 1331 and 1599 cm−1 for the Sb2O3/rGO composite correspond to the D-band (corresponding to defects, structural disorders, or edges) and G-band (due to sp2 carbon vibration) structures of the rGO sheets, respectively.38,39 The shift in the G-band peak position in graphene-based composites relates to the charge transfer between graphene and other components induced by the doping effect and/or bonding formation.25,27,40,41 Thus, a distinct shift of 14 cm−1 from 1585 (Sb2O3−rGO composite) to 1599 cm−1 (Sb2O3/rGO composite) indicates an enhanced interfacial charge transfer due to the addition of L-AA, confirming a strong coupling effect between the rGO sheets and Sb2O3 nanoparticles in the Sb2O3/rGO composite but not doping effect.25 The G-band of the as-obtained Sb2O3−rGO composite (1585 cm−1) located near to that of the rGO (1582 cm−1) suggests a weak interaction between rGO and Sb2O3 in the absence of L-AA during the synthesis procedure. Meanwhile, two distinct peaks detected in both Sb2O3-based composites, located at 186 and 251 cm−1 respectively, are characteristics of cubic senarmoatilte Sb2O3,42,43 in good consistency with the XRD results of the well-crystallized Sb2O3 component. The intensity ratio of the D band to G band (ID/IG) was used to characterize the reduction degree and defects of the rGO-based materials.25,44 The ID/IG (1.24) of the as-obtained Sb2O3/rGO composite is much higher than that of the Sb2O3−rGO composite (ID/IG = 1.11), indicating a higher reduction degree and more defects and/or edges exposed for the rGO sheets in the Sb2O3−rGO composite due to the addition of L-AA in the synthesis procedure, which benefits the nucleation of Sb2O3 nanoparticles on the rGO sheets to form a stable nanostructure. Also, these results indicate that a welldefined Sb2O3/rGO composite was successfully synthesized via the dissolution−reprecipitation strategy by using bulk Sb2O3 and GO as the raw materials, which is a facile and ecofriendly solvothermal method. XPS was employed to further confirm the reduction of GO and the coupling effect between rGO and Sb2O3 in the asobtained samples. For the rGO obtained with L-AA, as shown in Figure S3, the relative intensities of oxygen-containing groups is much lower than that of GO due to the reduction by 34 15,33,45 L-AA and EG during the solvothermal treatment, which is well consistent with the Raman results. In addition, it is notable that the peak of C−O groups at 286.7 eV in GO shifts to a lower binding energy and splits into two overlapped peaks at around 286.3 and 285.4 eV, respectively, indicating new species from chemical conversion,46 which may originate from the hydrogen-bonding adsorption of the oxidized products of LAA on the rGO surfaces through the residual hydroxyl (C−O) groups, such as guluronic acid or oxalic acids.34 These adsorbed acids can prevent the agglomeration of rGO sheets and also act as nucleation sites for the formation of Sb2O3 nanoparticles. After composing with Sb2O3, strong Sb signals appeared in the XPS spectra of both Sb2O3−rGO and Sb2O3/rGO composites (Figure 1c), indicating the presence of Sb. To probe the chemical structure of the samples, the high-resolution C 1s peaks of both Sb2O3−rGO and Sb2O3/rGO composites were analyzed. As shown in Figure 1d, an intensive peak at 284.6 eV and two relatively small shoulder peaks are observed at 286.7 and 288.5 eV in both Sb2O3−rGO and Sb2O3/rGO composites,

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RESULTS AND DISCUSSION Structural Verification. The phase compositions of the asobtained samples were determined by XRD. As shown in Figure 1a, all of the diffraction peaks in the XRD patterns of bulk Sb2O3, Sb2O3/rGO, and Sb2O3−rGO composites can be well indexed according to the standard peaks of cubic senarmontite Sb2O3 (JCPDS No. 05-0534), indicating that the addition of the weak reductant L-AA has little effect on the Sb2O3 phase. No obvious diffraction peaks of rGO or GO 34929

DOI: 10.1021/acsami.7b10107 ACS Appl. Mater. Interfaces 2017, 9, 34927−34936

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Figure 2. FESEM images of (a) Sb2O3−rGO and (b) Sb2O3/rGO (insets: cross-sectional image and elemental mapping images) composites. TEM images of (c) Sb2O3−rGO and (d) Sb2O3/rGO composites. (e) HRTEM image and (f) indexed SAED pattern of the Sb2O3/rGO composite.

increased significantly with the addition of L-AA in the synthesis procedure, indicating a much stronger electronic coupling between Sb2O3 and rGO in the Sb2O3/rGO composite, which is consistent with the analysis result based on the Raman pattern and the previous reports.5,47−51 A stronger electronic coupling between Sb2O3 and rGO ensures a more stable structure for the lithium-storage performance. Morphology of the As-Obtained Samples. The morphological characteristics of the as-obtained samples

corresponding to the C−C/CC, C−O, and CO bonds of the rGO sheets, respectively, and the relative weak intensities of the peaks of oxygen-containing groups indicates the efficient reduction of GO during the synthesis procedure. Similar to that of the rGO obtained with L-AA, one more peak appears at 285.4 eV for the Sb2O3−rGO composite, which can be assigned to the C−O(−Sb) bond. It is worth noting that the peak for C−O(−Sb) bond shifted toward a lower binding energy (285.0 eV), and, indeed, the C−O(−Sb)-to-C−C/CC signals ratio 34930

DOI: 10.1021/acsami.7b10107 ACS Appl. Mater. Interfaces 2017, 9, 34927−34936

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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration for the Formation of Sb2O3/rGO and Sb2O3−rGO Composites

resolution transmission electron microscopy (HRTEM) image of the Sb2O3/rGO composite is shown in Figure 2e, which shows the lattice fringes of 0.32 and 0.28 nm in the particle, corresponding to the (222) and (400) planes of the cubic senarmontite Sb2O3 (JCPDS no. 05-0534), whereas the 0.37 nm fringes are the (002) planes of the rGO. Furthermore, selected area electron diffraction (SAED) pattern of the Sb2O3/ rGO composite (Figure 2f) can be well indexed according to the cubic senarmontite Sb2O3 (JCPDS no. 05-0534) and the (002) plane of the rGO, which agrees well with the XRD results shown in Figure 1a. On the basis of the aforementioned characterizations and analysis results, the Sb2O3/rGO composite with nanosized Sb2O3 particles anchored tightly and uniformly on the rGO sheets has been successfully synthesized. We confirmed that the addition of L-AA plays a vital role in forming the composite with unique structures. The conceivable formation mechanism of the as-obtained Sb2O3/rGO composite is briefly described in Scheme 1, consistent with our previous investigation.15 First, most of the oxygen-containing groups of GO were removed at room temperature by L-AA, and the oxidized products of L-AA and the generated H2O were adsorbed on the rGO surfaces by the hydrogen-bonding effect,34 which can provide much more nucleation sites for Sb2O3. During the following solvothermal treatment, bulk Sb2O3 particles were dissolved and transformed into ethylene glycol antimony (EG-Sb) through reaction with ethylene glycol. Meanwhile, the reduction of GO was further developed by EG solvent.33,45 Subsequently, the formed EG-Sb was hydrolyzed with the presence of H2O and transformed into Sb2O3 nanoparticles anchored tightly on the rGO sheets. In this process, oxygen-containing groups and defects on the rGO play a key role in controlling the size of the reprecipitated oxide particles and the bond strength between the rGO and particles.52 In comparison with the Sb2O3−rGO composite, the addition of L-AA in the synthesis procedure of Sb2O3/rGO composite not only strengthens the reduction degree of rGO

captured by FESEM are shown in Figure 2a,b, revealing that the Sb2O3 nanoparticles of size around 100−200 nm were intimately and uniformly wrapped by the rGO sheets in the Sb2O3−rGO composite (Figure 2b). Completely different from the Sb2O3−rGO composite, the Sb2O3/rGO composite presents an agglomerated bulk structure composed of overlapped rGO sheets (cross-sectional image shown in Figure 2b inset) and several Sb2O3 particles of size around 100 nm dispersed on the surface. Furthermore, the elemental mapping images shown in Figure 2b insets demonstrate the presence of uniformly distributed C, O, and Sb elements in the designated area, further indicating the uniform anchoring of the Sb2O3 nanoparticles on the rGO sheets throughout the Sb2O3/rGO composite. Such a compact structure, beneficial for achieving a high initial Coulombic efficiency for the rGO-based composite electrode, can be derived from the effect of L-AA in the synthesis process. The morphological characteristics of the rGO samples prepared in different conditions are shown in Figure S4. Bare rGO with accordion-like wrinkles could be obtained without the addition of L-AA (Figure S4a). Also, the morphological characteristics of GO could be maintained well after 24 h stirring at room temperature with the addition of LAA (Figure S4b). However, it is surprising that the rGO sheets were overlapped to form flat plate-like particles after the following solvothermal treatment (Figure S4c), which is similar to that of the Sb2O3/rGO composite. These results indicate that the addition of L-AA plays a vital role in the morphological characteristics of bare rGO and Sb2O3/rGO composite. The microstructural details of Sb2O3−rGO and Sb2O3/rGO were further investigated by TEM. Compared with the Sb2O3−rGO composite (Figure 2c), the Sb2O3/rGO composite had much smaller Sb2O3 nanoparticles (∼30 nm, as shown in Figure 2d) uniformly anchored on the rGO sheets, which is attributed to the oxygen-containing groups and the defects functioning as the anchor sites of the Sb2O3 particles. Also, these results are also consistent with the Raman and XPS results. A typical high34931

DOI: 10.1021/acsami.7b10107 ACS Appl. Mater. Interfaces 2017, 9, 34927−34936

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ACS Applied Materials & Interfaces but also introduces much more nucleation sites, which benefits the uniform reprecipitation of the Sb2O3 particles of a smaller size and enhances the bonding effects between Sb2O3 and rGO. It should be emphasized that such a unique structure of the Sb2O3/rGO composite ensures an effective accommodation for the volumetric change and a good electrical contact between Sb2O3 and rGO during the lithiation/delithiation process, thus leading to a high reversible specific capacity and an extraordinary cycle stability during lithium storage.53 To verify the electrical conductivity of the obtained samples, electrochemical impedance spectroscopy (EIS) measurements for bulk Sb2O3, Sb2O3/rGO, and Sb2O3−rGO electrodes before cycling were conducted. As shown in Figure 3, the Sb2O3−rGO

Figure 4. Thermogravimetric analysis (TGA) patterns of bulk Sb2O3 and Sb2O3/rGO composite.

and degree of volatilization and the simultaneous oxidation of Sb2O3 to Sb2O4 are minimally affected by particle size, surface area, heating rate, and method of preparation.42 Therefore, according to the TGA analysis results, it can be calculated that the Sb2O3 content in the Sb2O3/rGO composite is ca. 73.0 wt % on the basis of eq 3, which is in accordance with the result of the XPS analysis (ca. 72.2 wt %). Wtotal − WH 2O ‐ absorbed − WrGo − WSb2O3 ‐ volatilized WSb2O3% = (Wtotal − WH 2O ‐ absorbed) × (1 − 21.9%) Figure 3. Nyquist plots of the as-prepared Sb2O3/rGO (with L-AA) and Sb2O3/rGO (without L-AA) composites electrode and the bulk Sb2O3 electrode at an open-circuit potential.

×

MSb2O3 MSb2O4

× 100% (3)

Electrochemical Performances of the As-Obtained Samples. Taking advantage of the unique anchoring configuration through the bonding interaction, we can expect excellent lithium-storage performance for the as-obtained Sb2O3/rGO composite when used as an alternative electrode material for LIBs. Electrochemical properties of the as-obtained electrode materials for LIBs were measured in the potential range of 0.001−3.0 V (vs Li+/Li). First, to evaluate the electrochemical activity of the Sb2O3/rGO composite, the CV curves were obtained for the initial three cycles at a scan rate of 0.2 mV s−1, as shown in Figure 5a. Similar to those of bulk Sb 2 O3 and the Sb 2 O 3−rGO composite (Figure S5a,b, respectively), the reduction peak of the Sb2O3/rGO composite, located at 1.15 V in the first cycle, can be assigned to the lithiation of Sb2O3 to form nanosized Sb and Li2O;55 the shifts of ca. 1.53 V in the peaks in subsequent cycles indicate an activation process in the initial lithiation process. The redox pairs at ca. 0.73 V during reduction and ca. 1.18 V during oxidation derive from the alloying and dealloying reaction between Sb and Li.10,11,13,15 During the reversed anodic scan, the peak around 1.48 V, which is not detected for bulk Sb2O3 electrode (Figure S5a), originates from the oxidation of Sb to form a nano-Sb2O3.10 It means that the nanosized Sb2O3 particles anchored on the rGO sheets can be decomposed and reconstructed reversibly during subsequent lithiation/ delithiation processes according to the conversion reactions (eq 1). It is worth noting that both the pairs of redox peaks of 1.53/ 1.48 V and 0.73/1.18 V for conversion and alloying lithiumstorage mechanism, respectively, can be located stably, and that

composite delivers a noticeably smaller semicircle in the highand medium-frequency regions compared with bulk Sb2O3, indicating a smaller charge-transfer resistance and, therefore, an improved electrical conductivity.54 It is worth noting that the Sb2O3/rGO composite, obtained with the addition of L-AA, exhibits the smallest charge-transfer resistance, indicating that the electrical contact between Sb2O3 and rGO is further enhanced through a stronger electronic coupling effect and a great increase in the reduction degree of GO with the addition of L-AA, thus leading to excellent high-rate of lithium storage and high reversibility of the conversion-alloying lithium storage. To determine the percentage content of Sb2O3 in the Sb2O3/ rGO composite, the obtained bulk Sb2O3 and Sb2O3/rGO composite were analyzed by thermogravimetric analysis (TGA) in ambient atmosphere. As shown in Figure 4, when the bulk cubic senarmontite Sb2O3 is heated at 10 °C min−1, there is a sharp mass loss of ca. 21.9% between 450 and 625 °C due to the evaporation of Sb2O3 and oxidation to Sb2O4.10,52 Heating the remaining products from 625 to 800 °C does not result in any further weight change, and the final product is α-Sb2O4.7,31 Furthermore, for the Sb2O3/rGO composite, the mass loss of around 1.6% below 200 °C can be mainly attributed to the evaporation of water absorbed on the surface of the sample, whereas the mass loss of ca. 39.2% between 200 and 600 °C is ascribed to multiple factors including the decomposition of the residual oxygen-containing groups of the rGO, the combustion of the rGO component, the volatilization of Sb2O3, and the oxidation of Sb2O3 to form Sb2O4. It is reported that the rate 34932

DOI: 10.1021/acsami.7b10107 ACS Appl. Mater. Interfaces 2017, 9, 34927−34936

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Figure 5. (a) Cyclic voltammograms of initial three cycles at a scan rate of 0.2 mV s−1 and (b) galvanostatic discharge/charge curves in the potential range of 0.001−3.0 V (vs Li+/Li) at a current density of 100 mA g−1 for as-obtained Sb2O3/rGO electrode.

Figure 6. TEM images of Sb2O3/rGO electrodes in full-lithiated (a) and full-delithiated (b) conditions in initial cycle (inset: corresponding to SAED patterns of (a) and (b), respectively).

dealloying reaction of Li3Sb to Sb and the reconstruction of Sb2O3 phase. Corresponding to those in the CV curves, the obviously declined region between 0.5 and 0.01 V in the discharge curves could be ascribed to multiple lithium-storage sites in the Sb2O3/rGO electrode.56,47−49 It is interesting that the discharge voltage plateau corresponding to the decomposition of Sb2O3 becomes steep, revealing that the transformation of Sb2O3 crystalline to Sb2O3 nanocrystalline during the initial charge process can lead to the modification of the lithiation reaction conditions for Sb2O3 phase.12 Also, these results agree well with the previous CV results. To further attest the highly reversible conversion-alloying mechanism for electrochemical lithium storage in Sb2O3/rGO electrode, the morphology and composition of the electrodes at full-lithiated and delithiated conditions in the initial cycle were observed using TEM and SAED measurements. As shown in Figure 6, high-contrast nanoparticles uniformly anchored on pleat-like rGO sheets in low contrast can be distinctly detected for the electrodes in both conditions. SAED pattern for the electrode in a full-lithiated condition displayed three obvious diffraction rings (inset in Figure 6a), corresponding to the (002), (102), and (110) crystal planes of Li3Sb (JCPDS no. 653515), respectively. Notably, two distinctly diffraction rings can be also detected for the electrode in a full-delithiated condition (inset in Figure 6b) and indexed to (400) and (331) crystal

all of the peaks well overlapped during the subsequent cycle, indicating a good reversibility and an excellent cyclability of the Sb2O3/rGO composite based on the conversion-alloying mechanism during lithium storage. The relative intensities ratio of the reduction peak at 0.73 V in the second cycle and the initial cycle for the Sb2O3/rGO composite indeed increased significantly versus that of the Sb2O3−rGO composite, indicating that enhanced electrochemical activity of Sb2O3 in the Sb2O3/rGO composite due to the stronger electronic coupling effect can improve the reversibility and specific capacity. The broad reduction peak between 0.5 and 0.01 V for both Sb2O3/rGO and Sb2O3−rGO composites overlap well in the subsequent cycles, shown in Figures 5a and S5b, respectively, indicating that lithium storage in defects, interfaces, and/or other active sites in the Sb2O3/rGO electrode can lead to a higher actual specific capacity than that expected based on the traditional theory.56,47−50 The galvanostatic discharge/charge curves at a current density of 100 mA g−1 of the as-obtained Sb2O3/rGO electrode show correlative plateau regions corresponding to the peaks of CV curves, as shown in Figure 5b. During the initial discharge process, two smooth voltage plateaus at 1.29 and 0.87 V can be attributed to the decomposition of Sb2O3 and the alloying reaction of Sb, respectively. Also, the two charge voltage plateaus at ca. 1.05 and 1.30 V, respectively, correspond to the 34933

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Figure 7. (a) Cycling performance (left y axis) and Coulombic efficiencies (right y axis) of the Sb2O3/rGO electrode at 100 mA g−1, in comparison with Sb2O3−rGO composite, Sb2O3−rGO mixture, and bulk Sb2O3. (b) Rate performances at various current densities of Sb2O3/rGO and Sb2O3− rGO composites. (c) Long-term cycling performance of the Sb2O3/rGO electrode at a current density of 600 mA g−1.

planes of Sb2O3 (JCPDS no. 05-0534), respectively, indicating a highly reversible conversion-alloying reaction in the Sb2O3/ rGO electrode based on eqs 1 and 2. The cycling performance of the as-obtained electrodes is investigated at 100 mA g−1, as shown in Figure 7a. The Sb2O3/ rGO electrode exhibits an initial Coulombic efficiency (I.E.) of 60% and a reversible specific capacity of 1355 mA g−1, both of which are much higher than those for the Sb2O3−rGO composite (I.E. 53% and reversible capacity of 793 mA h g−1 ), which is attributed to the enhancement of the electrochemical activity and reversibility for lithium storage in the Sb2O3/rGO composite through the nanocrystallization and the strong electronic coupling effect between Sb2O3 and rGO. In spite of a rapid decline to 896 mA h g−1 in the initial 20 cycles, the reversible specific capacity above 808 mA h g−1 can be achieved after 120 cycles, corresponding to the capacity retention of 90.2% based on that of the 20th cycle, which is much higher than that for the Sb2O3−rGO composite, Sb2O3 + rGO mixture, bulk Sb2O3 (Figure 7a), rGO (Figure S6), and previously reported Sb2O3-based electrodes (Table S1).10−12 The marked capacity decay in the initial cycles are mainly due to the pulverization of those unfixed larger particles (as shown in Figure 2b), as well as the continuous formation of SEI layer

and the incomplete extraction of lithium. Certainly, these specific capacity values for electrodes were obtained based on the total mass of composites, including both Sb2O3 and rGO sheets. To present the contribution of the rGO sheets or the Sb2O3 particles to a specific capacity, another way for cycling performance is shown in Figure S7 for the obtained Sb2O3/ rGO electrode, hypothesizing that the contribution to specific capacity was fixed at 744 mA h g−1 for the rGO sheets and 1109 mA h g−1 for Sb2O3. When the capacity contribution of rGO was hypothetically fixed at its theoretical value (744 mA h g−1), the specific capacities delivered by Sb2O3 component (green dots in Figure S7) during the 120 cycles are approximately equal to those for Sb2O3/rGO electrode (red dots in Figure S7), indicating that the theoretical specific capacity value can be achieved for rGO component in the Sb2O3/rGO electrodee. Due to the unique structure of the Sb2O3/rGO composite, a promising rate performance is also expected. As shown in Figure 5b, the rate capability of Sb2O3/rGO improved significantly, and high reversible specific capacities of 1025 mA h g−1 at 0.2 A g−1, 496 mA h g−1 at 2 A g−1, and 188 mA h g−1 even at 5 A g−1 can be delivered. Moreover, after these high-rate measurements, the reversible capacity of 759 mA h g−1 at 0.2 A g−1 can be recovered, much higher than that of the 34934

DOI: 10.1021/acsami.7b10107 ACS Appl. Mater. Interfaces 2017, 9, 34927−34936

Research Article

ACS Applied Materials & Interfaces Sb2O3−rGO electrode (363 mA h g−1), demonstrating great improvement in the structural stability of the Sb2O3/rGO composite.51 It is surprising that the Sb2O3/rGO composite also presents a superior long-term cycling performance at the current density of 600 mA g−1 (Figure 7c). The obtained Sb2O3/rGO electrode exhibits a remarkable reversible specific capacity of 525 mA h g−1 after 700 cycles, and the average capacity decay rate was only 0.0136% per cycle between cycles 50 and 700 at the current density of 600 mA g−1. These outstanding electrochemical properties can be attributed to the unique structure of the Sb2O3/rGO composite. Both the Sb2O3 nanoparticles tightly anchored on the rGO sheets and the strong electronic coupling effect between Sb 2 O 3 and rGO can enhance the electronic conductivity, structure stability, and electrochemical lithiumstorage activity, which benefits the achievement of highly reversible lithium storage based on the conversion-alloying mechanism, leading to such a superior performance for the Sb2O3/rGO composite used in LIBs.

ORCID

Xiaozhong Zhou: 0000-0001-5366-0545 Ziqiang Lei: 0000-0001-9195-4472 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (grant nos. 51462032 and 21664012); the Fundamental Research Funds for universities in Gansu province; the program for Changjiang Scholars and Innovative Research Team in University (IRT15R56); and the Innovation Group of Basic Research in Gansu Province (1606RJIA324).

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CONCLUSIONS In summary, a facile and ecofriendly solvothermal method based on the dissolution−reprecipitation strategy is utilized to synthesize a Sb2O3/rGO composite for high-performance LIBs. L-AA plays a key role in decreasing the particle size, improving the reduction degree of GO, and strengthening the electronic coupling effect between Sb2O3 and rGO; therefore, with the addition of L-AA, we obtained a much more stable architecture of the Sb2O3/rGO composite showing a high electrochemical lithium-storage activity. Also, such Sb2O3/rGO composite demonstrates a high specific capacity (1355 mA h g−1 at 100 mA g−1) based on the conversion-alloying mechanism, good rate capability, and superior cycle lifespan (525 mA h g−1 after 700 cycles) when used as an alternative electrode for LIBs. We believe that these findings are helpful in developing novel highperformance electrodes for LIBs and synthesizing functional materials in an ecofriendly and economical way.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10107. Digital camera images for the procedure of dissolution and reprecipitation of Sb2O3 in ethylene glycol (Figure S1a−e), SEM images for bulk Sb2O3 (Figure S1f,g) and rGO (Figure S4) obtained in different conditions, SEM images with different magnifications of the as-prepared GO (Figure S2), high-resolution C 1s XPS spectra (Figure S3), cyclic voltammograms for bulk Sb2O3 and Sb2O3−rGO electrodes (Figure S5), cycling performance and Coulombic efficiencies of the bare rGO electrode (Figure S6), summary of the SbxOy-based electrodes materials for LIB applications (Table S1), and capacity contribution from Sb2O3 or rGO during cycling in the Sb2O3/rGO nanocomposite (Figure S7) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel/Fax: +86 931 7972663 (X.Z.). *E-mail: [email protected]. Tel/Fax: +86 931 7971261 (Z.L.). 34935

DOI: 10.1021/acsami.7b10107 ACS Appl. Mater. Interfaces 2017, 9, 34927−34936

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