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Enhanced Electrochemical Performances of Bi2O3/rGO Nanocomposite via Chemical Bonding as Anode Materials for Lithium Ion Batteries Zhuo Deng, Tingting Liu, Tao Chen, Jiaxiang Jiang, Wanli Yang, Jun Guo, Jianqing Zhao, Haibo Wang, and Lijun Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00996 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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ACS Applied Materials & Interfaces

Enhanced Electrochemical Performances of Bi2O3/rGO Nanocomposite via Chemical Bonding as Anode Materials for Lithium Ion Batteries Zhuo Deng,†,‡,∥ Tingting Liu,†,‡,ξ,∥ Tao Chen,†,‡ Jiaxiang Jiang,†,‡ Wanli Yang,†,‡ Jun Guo,ζ Jianqing Zhao,†,‡,* Haibo Wang,†,‡,§,* Lijun Gao†,‡,* †

Soochow Institute for Energy and Materials InnovationS, College of Physics,

Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China. ‡

Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies

of Jiangsu Province, Soochow University, Suzhou 215006, China. ζ

§

Testing and Analysis Center, Soochow University, Suzhou 215006, China Institute of Chemical Power Sources, Soochow University, Zhangjiagang 215600,

China ξ

Suzhou University of Science and Technology & Jiangsu Key Laboratory of

Environmental Science and Engineering, Suzhou 215001, China Corresponding Authors: Prof. Jianqing Zhao, Tel: +86-512-65229905; Fax: +86-512-65229905; E-mail: [email protected] Prof. Haibo Wang, Tel: +86-512-80158088; Fax: +86-512-80157713; E-mail: [email protected] Prof. Lijun Gao, Tel: +86-512-65229905; Fax: +86-512-65229905; E-mail: [email protected] ∥

These two authors equally contributed to this work.

KEYWORDS: Bi2O3/rGO nanocomposite, chemical bonding, alloying/dealloying process, lithium storage performance, anode material, lithium ion battery

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Bismuth oxide/reduced graphene oxide (marked as Bi2O3@rGO) nanocomposite has been facilely prepared by a solvothermal method via introducing chemical bonding that has been demonstrated by Raman and XPS spectra. Tremendous single-crystal Bi2O3 nanoparticles with an average size of ~5 nm are anchored and uniformly dispersed on rGO sheets. Such a nanostructure results in enhanced electrochemical reversibility and cycling stability of Bi2O3@rGO composite materials as anode for lithium ion batteries in comparison with agglomerated bare Bi2O3 nanoparticles. The Bi2O3@rGO anode material can deliver a high initial capacity of ~900 mAh/g at 0.1 C, and show excellent rate capability of ~270 mAh/g at 10 C rates (1 C = 600 mA/g). After 100 electrochemical cycling at 1 C, the Bi2O3@rGO anode material retains a capacity of 347.3 mAh/g with corresponding capacity retention of 79%, which is significantly better than that of bare Bi2O3 material. The lithium ion diffusion coefficient during lithiation/delithiation of Bi2O3@rGO nanocomposite has been evaluated to be around ~10-15-10-16 cm2/S. This work demonstrates the effects of chemical bonding between Bi2O3 nanoparticles and rGO substrate on enhanced electrochemical performances of Bi2O3@rGO nanocomposite, which can be severed as a promising anode alterative for superior lithium ion batteries.


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1. INTRODUCTION The rapid development of portable electronic devices and electric transportation system requires the higher energy and power densities and longer service life of lithium ion batteries that are highly dependent on the proper design of electrode structures for both anode and cathode materials.1-3 Among various anode materials, transition metal oxides and sulfides attract extensive research attention due to their stable chemical states and high specific capacities in a range of 600-1000 mAh/g,4-5 which is two to three times that of the conventional graphite anode material (372 mAh/g). Bi-based materials have demonstrated unique properties of high volumetric density and specific capacity as anode materials for lithium ion batteries. The metallic Bi0 material can deliver a volumetric capacity of 3760 mAh/cm3.6-7 Besides, bismuth sulfide (Bi2S3) shows desirable lithium storage performances with tailored morphologies, structures, and various compositions.5, 8-10 As reported in literatures,5, 9, 11

Bi2S3@CNT composite anode materials can deliver capacities above 630 mAh/g

with outstanding cycling stability. It is surprising that less attention has been paid to the works of bismuth oxide (Bi2O3), which also has the high theoretical capacity of 690 mAh/g.12 In similarity with other high-capacity anode materials that form LixMy alloys (M=Si, Sn, Sb, etc.) for reversible lithium storage,13-14 practical application of Bi2O3 for lithium ion batteries is significantly impeded by its poor cycling stability and rate capability. In order to improve cycling and high-rate performances, Luo et al. directly grew Bi2O3 nanoparticles on the nickel foam.12 As a result, the Bi2O3/Ni composite anode



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exhibited enhanced electrochemical reversibility, which can retain 782 mAh/g over 40 cycles at 100 mA/g, and also deliver 668 mAh/g at the higher current density of 800 mA/g. However, it is still imperative to further improve cycle and rate capabilities of Bi2O3 anode materials in addition to utilizing its high energy density. Various strategies have been used to effectively tune performances and properties of oxide-based electrode materials for superior lithium ion batteries, including fabrication of hierarchical structures,10 modifications of surface,15 and tuning phase and elemental composition.11,

16-17

Reduced graphene oxide (rGO) as a

two-dimensional carbon nanomaterial has been demonstrated as a versatile matrix to accommodate and support various oxides, due to its large surface area, remarkable mechanical strength, high electronic conductivity and excellent flexibility.18-20 As a result, different Bi2O3/rGO nanocomposite materials have demonstrated outstanding lithium/sodium ion storage performances. As reported in the literature,21 carbon-coated Bi2O3 nanoparticles/nitrogen-doped rGO hybrid as an anode material for lithium ion batteries reveals remarkable high-power capability and cycling performances. Furthermore, Bi2O3@rGO nanocomposite can also be severed as an anode material for sodium ion batteries, which shows acceptable capacity and cycling stability for sodium storage.22 According to reported literatures,23-25 the chemical bonding between oxides and the carbon-based matrix plays a critical role in improving electrochemical performances of active oxide materials. Therefore, introducing chemical bonding between Bi2O3 nanoparticles and rGO sheets within Bi2O3@rGO nanocomposite should result in significantly enhanced lithium storage


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performances of Bi2O3 materials. In this work, we report the facile synthesis and superior lithium storage performances of Bi2O3@rGO nanocomposite by combining Bi2O3 and rGO components via chemical bonding. Bi2O3@rGO composite material can be prepared in a one-step solvothermal approach, in which Bi2O3 nanoparticles with single crystal characteristics are intimately anchored on the rGO substrate. The ionic bonding forces between the Bi precursor (positively-charged) and GO sheets (negatively-charged) play a critical role in introducing chemical bonding to bridge as-prepared Bi2O3 nanoparticles and reduced rGO sheets within Bi2O3@rGO nanocomposite. Such a nanostructure should be favorable to achieve enhanced structural stability of Bi2O3 anode material in combination with rGO matrix. As a result, Bi2O3@rGO composite anode materials can deliver high capacities near 900 mAh/g at 0.1 C and 270 mAh/g at 10 C rates, respectively, and retain 347.3 mAh/g at 1 C after 100 electrochemical cycles. Electrochemical performances of Bi2O3@rGO nanocomposite demonstrates its great potential as an anode alternative for advanced lithium ion batteries. 2. EXPERIMENTAL SECTION Sample preparation. Bi2O3@rGO composite material was synthesized via a solvothermal method by using Bi(NO3)3·5H2O (Sinopharm) and graphene oxide (GO) as precursors, polyvinylpyrrolidone (PVP, Sinopharm) as the surfactant, and urea (Sinopharm) as the additive. In a typical run, two precursor solutions were prepared. One is with 10 mg GO dispersed in 25 mL ethylene glycol by using sonication, followed by adding 300 mg PVP and 80 mg urea. The other is with 182 mg



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Bi(NO3)3·5H2O dispersed in 5 mL of 0.4 M HNO3 solution. Afterwards, the Bi precursor solution was added dropwise into the GO precursor solution. The mixed solution was then transferred to a 100 mL Teflon-contained stainless steel autoclave and heated at 150 °C for 3 h. The resulting precipitate was collected by high-speed centrifugation and washed repeatedly with ethanol and water for several times. The final product was obtained after drying in vacuum at 60 °C overnight. For comparison, bare Bi2O3 sample was also prepared in the same synthetic conditions without adding the GO material. Characterizations. Crystallographic structures of as-prepared materials were identified by X-ray diffraction (XRD) on a Rigaku Dmax-2400 automatic diffractometer. Morphology and structure of different samples were observed by using scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai G2T20) at an acceleration voltage of 200 kV, respectively. The materials were further characterized by Raman spectra on a Horiba JY LabRAM ARAMIS equipment and X-ray photoelectron spectroscopic (XPS) measurements

on

an

ESCALAB

250Xi

XPS

equipment,

respectively.

Thermogravimetric and corresponding differential thermal analysis (TG/DTA) were performed on a Seko TG/DTA-7300 analyzer at a heating rate of 10 °C/min from room temperature to 600 °C. Specific surface area of different powders was measured by nitrogen adsorption and desorption at 77 K on a Micromeritics Tristar II 3020 surface area analyzer. Electrochemical measurements. The working electrodes were composed of 70


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wt. % active materials, 20 wt. % Super-P (conductive carbon), and 10 wt. % polyvinylidenefluoride (PVDF) as the binder. The N-methyl-2-pyrrolidinone (NMP) was used as the solvent. The average loading mass of active materials in working electrodes is ~2.25 mg. The anodes were assembled into two-electrode CR2032-type coin cells in an Ar-filled glove box (MBraun) for electrochemical measurements, with the metallic lithium foil as the reference and counter electrodes and the glass microfiber (Whatman, Grade GF/B) as the separator. The concentrations of H2O and O2 in the glove box are both less than 5 ppm. The electrolyte was 1 M LiPF6 solution dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volumetric ratio of 1:1. Galvanostatic charge/discharge were performed at different current densities in a voltage range of 0.01-3.0 V vs. Li/Li+ on a LAND battery testing system (Jinnuo, China) at room temperature. Cyclic voltammetric (CV) profiles were recorded at various scanning rates in a voltage range of 0.01-3.0 V on a RST5200 electrochemical work station. 3. RESULTS AND DISCUSSION

Scheme 1. Schematics illustrating synthetic processes of Bi2O3@rGO nanocomposite. Scheme 1 schematically illustrates the synthesis processes of Bi2O3@rGO



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composite material. The ionic interaction between negatively-charged groups, such as COO- and OH- functional groups at the surface of graphene oxide (GO) and positively-charged Bi3+ cations plays a critical role in locating Bi3+ ions on original GO sheets. After dispersing GO sheets in ethylene glycol solvent by high-energy sonication, the Bi3+-based precursor is added to form GO-- Bi3+ intermediates as shown in the middle panel in Scheme 1. The formation of Bi2O3 nanoparticles and reduction of GO to rGO can be simultaneously achieved during the solvothermal process at 150 °C in ethylene glycol (as shown in the bottom panel in Scheme 1). According to reported literatures,26-27 the urea is always used as an additive, in order to improve the interaction between Bi3+ cations and GO sheets during the solvothermal preparation

of Bi2O3@rGO composite

material,

because

the

decomposition of urea yields OH- groups that can be easily adsorbed at the surface of GO sheets to attract Bi3+ cations. As a result, two Bi2O3 and rGO components are bridged via chemical bonding within as-prepared Bi2O3@rGO nanocomposite. For comparison, bare Bi2O3 material is also prepared by using the same solvothermal condition without adding GO sheets. Figure 1 depicts detailed morphologies and structures of as-prepared Bi2O3 nanoparticles and Bi2O3@rGO composite material. As shown in Figure 1a-c, bare Bi2O3 nanoparticles have a high tendency to agglomerate to form secondary nanospheres. SEM image in Figure 1a shows resulting nanospheres with an average diameter of ~150 nm and distinct characteristics of high dispersibility and uniformity. Each sphere is composed of numerous primary Bi2O3 nanoparticles with an even


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particle size around 5 nm (Figure 1b), which are single crystals in high crystallinity by showing clear lattice fringes in HRTEM observation (Figure 1c). HRTEM image in Figure 1c also reveals that all primary Bi2O3 nanoparticles are covered by thin layers of amorphous carbon material and closely aggregated to form the secondary spherical nanostructure. By contrast, due to the ionic bonding between Bi3+ precursors and GO sheets during the synthesis of Bi2O3@rGO nanocomposite (the upper panel in Scheme 1), the GO substrate supplies numerous reaction sites to capture Bi3+ cations at its surface (the middle panel in Scheme 1). As a result, Bi2O3 nanoparticles can be uniformly dispersed and tightly anchored on rGO sheets after the solvothermal process, during which Bi3+ precursors were converted to Bi2O3 nanoparticles and GO material was reduced to corresponding rGO material (the bottom panel in Scheme 1). Accordingly, SEM, TEM and HRTEM images of Bi2O3@rGO composite material reveal highly dispersed Bi2O3 nanoparticles on rGO sheets in different ranges of magnifications as shown in Figure 1d-f, respectively. In addition, HRTEM image in Figure 1f shows that the particle size of Bi2O3 nanoparticles is slightly reduced to less than 5 nm, while they still preserve the high crystallinity. The inset shows corresponding diffraction patterns obtained through live fast Fourier transformation (FFT), indicating singe crystal characteristics of Bi2O3 nanoparticles. It is noticeable that the low magnification SEM image of Bi2O3@rGO nanocomposite shown in the inset of Figure 1d illustrates remarkable flexibility of rGO sheets after loading Bi2O3 nanoparticles. Such a nanostructure should benefit for the structural stability of Bi2O3@rGO composite material.



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Figure 1. (a) SEM, (b) TEM and (c) HRTEM images of bare Bi2O3 nanoparticles; (d) SEM, (e) TEM and (f) HRTEM images of Bi2O3@rGO nanocomposite, and (g) SEM, (h) TEM and (i) HRTEM images of cycled Bi2O3@rGO electrode material after 100 cycles at 1 C.

Figure 2. XRD patterns of as-prepared Bi2O3 nanoparticles (black pattern) and Bi2O3@rGO nanocomposite (red pattern) in comparison with that after annealing at 450 °C in Ar. The standard XRD patterns of monoclinic Bi2O3 and metallic hexagonal Bi0 are referenced in green patterns.


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The crystallographic structures of as-prepared bare Bi2O3 and Bi2O3@rGO nanocomposite are further identified by XRD characterizations as shown in Figure 2. It is surprising to see that the two samples both exhibit poor crystallinity or even amorphous characteristics in XRD patterns, although conspicuous lattice fringes of Bi2O3 nanoparticles have been clearly captured in their HRTEM images (Figure 1c and 1f). Such inconsistent HRTEM and XRD results can be mostly attributed to effects of tiny particle size (~5 nm) of Bi2O3 synthesized in solvothermal processes. On the contrary, after annealing in inert Ar atmosphere the crystallinity of bare Bi2O3 particles are significantly improved (Navy pattern in Figure 2) due to the growth of particle size and corresponding crystal phase, which can be well indexed to the monoclinic phase of Bi2O3(ICDD #: 71-2274). Some expected tetragonal Bi2O2.33 phase is detected as marked by gray arrows, in consistent with reported Bi2O3 materials.28-29 However, annealing Bi2O3@rGO composite material in Ar flow at 450 °C results in the complete reduction of Bi2O3 to metallic Bi0 phase (Brown XRD pattern in Figure 2, ICDD #: 85-1330), which is attributed to the carbothermal reduction of rGO material during the heat treatment. The chemical environments and bonding conditions within Bi2O3@rGO nanocomposite are examined together with Raman scattering and X-ray photoelectron spectroscopic (XPS) measurements. Figure 3a shows a very broad wave number range of 200-500 cm-1 in the Raman response of as-prepared bare Bi2O3 nanoparticles (black spectrum). According to the literatures,30-31

this overlapping of different

models in crystalline Bi2O3 powder can be mainly attributed to high frequency



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broadening effects during the Raman scattering analysis. Additionally, it is difficult to orient a small biaxial crystal accurately in the cryostat during Raman characterizations of tiny nanoparticles. The small size of Bi2O3 nanoparticles with single crystal feature that has been demonstrated in HRTEM observation (Figure 1c) would further lead to the broad Raman peak of Bi2O3 nanoparticles at the low wave number. On the contrary, the Raman spectrum of Bi2O3@rGO nanocomposite (red spectrum) shows highly-resolved peaks of Bi2O3 lattice vibrations at 242, 321, 466, and 558 cm-1, although the particle size of Bi2O3 is even smaller than bare Bi2O3 sample (Figure 1f).32 This phenomenal difference in Raman spectra implies probable chemical bonding between well-dispersed Bi2O3 nanoparticles and the rGO substrate in Bi2O3@rGO composite material (as illustrated in Scheme 1). Meanwhile, two strong scattering bands at 1328 and 1591 cm-1 are related to characteristic graphitic sp3 (D) and sp2 (G) hybridizations from the rGO material,33-34 and another peaks located between 600-1200 cm-1 result from the testing background. Moreover, Figure 3b compares Bi 4f

5/2

and 4f

7/2

XPS peaks between bare Bi2O3 and Bi2O3@rGO

composite material. The bare Bi2O3 responds to characteristic Bi 4f

5/2

and 4f

7/2 XPS

peaks at 164.0 and 158.7 eV, respectively, in consistent with National Institute of Standards and Technology (NIST) XPS database. By contrast, Bi2O3@rGO reveals distinguishable peak shift of 0.7 eV to the higher binding energy. Such a phenomenon again demonstrates the chemical bonding between Bi2O3 nanoparticles and rGO sheets in Bi2O3@rGO composite material, in well agreement with Raman results (Figure 3a). The negatively-charged functional groups, such as COO- and OH-, tend to


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attract electrons from bridged Bi cations that charges chemical environment and relatively electronic distribution of Bi element, resulting in the peak shift of Bi 4f and 4f

7/2 XPS

5/2

peaks to the higher binding energy in Bi2O3@rGO nanocomposite. In

short summary, due to the strong chemical bonding, Bi2O3 nanoparticles can be uniformly dispersed and anchored at the surface of the rGO substrate within Bi2O3@rGO nanocomposite (Figure 1d-f), which is significantly different from closely-agglomerated bare Bi2O3 nanoparticles and its secondary nanospheres (Figure 1a-c).

Figure 3. (a) Raman spectra, (b) XPS spectra of characteristic Bi 4f 5/2 and 4f 7/2 peaks, (c) nitrogen adsorption/desorption isotherms with corresponding pore size contributions as the inset of as-prepared Bi2O3 nanoparticles and Bi2O3@rGO nanocomposite, and (d) TG profiles of Bi2O3@rGO annealed in Ar and air flow from room temperature to 500 °C.



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Figure 3c reveals porous properties and characteristics of two different samples analyzed by nitrogen adsorption/desorption measurements. The specific surface area of Bi2O3@rGO is calculated to be 14.89 m2/g by using the Brunauer-Emmett-Teller (BET) method, which is more than twice that of the agglomerated Bi2O3 nanoparticles (7.14 m2/g). The introduction of rGO sheets and reduced particle size of Bi2O3 material together contribute to considerably increased surface area of Bi2O3@rGO nanocomposite. As shown in Figure 3c, the hysteresis in two isothermals both indicates macro/meso porosities of Bi2O3 nanoparticles (black lines) and Bi2O3@rGO composite material (red lines). Such structural characteristics are mostly resulted from the interstitial space among Bi2O3 nanoparticles in two different samples, which contributes to porous structures. As a result, the agglomerated Bi2O3 nanoparticles and Bi2O3@rGO composite materials both show wide range of pore size distribution between 4 - 65 nm with similar mesoporous characteristics (the inset in Figure 3c), and corresponding pore volumes of Bi2O3 and Bi2O3@rGO are 0.034 cm3/g and 0.087 cm3/g, respectively. The higher dispersity of Bi2O3 nanoparticles in Bi2O3@rGO composite surely can result in larger surface area, higher pore volume and wider pore size distribution range in comparison with agglomerated bare Bi2O3 nanoparticles, in agreement with the SEM and TEM results. In order to calculate the content of rGO in Bi2O3@rGO composite material, TG analyses were performed in air and Ar atmospheric conditions, respectively. Two corresponding TG records are compared in Figure 3d. There is a distinct mass loss in the temperature range of 200-425 °C when Bi2O3@rGO was annealed in air flow,


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which is attributed to the complete removal of the rGO carbon material. Accordingly, the content of rGO is approximately estimated to 25 wt. %. The slightly weight decrease less than 3 wt.% before 200 °C probably results from loss of hydrates that are trapped within original Bi2O3@rGO material. However, it is noticeable that TG curve obtained in Ar flow shows mass changes occur especially between 200 and 275 °C. This significant reduction of weight can be mostly ascribed to the removal of functional groups (-COO-, -CO-, -OH- and hydrates) on rGO sheets,35 because in the inert atmosphere carbon materials should be well preserved. The subsequent weight loss is due to the reduction of Bi2O3 to metallic Bi0, in consistent with XRD result (Figure 2). In this work, in order to favorably keep chemical bonding between Bi2O3 and rGO substrate within Bi2O3@rGO nanocomposite as demonstrated by Raman (Figure 3a) and XPS spectra (Figure 3b), two electrode materials are directly used for electrochemical measurements without any heat treatments. Post-annealing effects on tailoring

crystallinity,

composition,

structure

and

relative

electrochemical

performances of Bi2O3@rGO composite materials will be published in another work. Figure 4 compares electrochemical performances between bare Bi2O3 nanoparticles and Bi2O3@rGO nanocomposite as anode materials for lithium ion batteries, with metallic Li0 as the counter and reference electrodes in the two-electrode testing system for CV measurements. The lithium storage performances of Bi2O3-based anodes are evaluated by CV and galvanostatic charge/discharge measurements in a voltage range of 0.01-3.0 V vs. Li/Li+. Figure 4a shows CV profiles of bare Bi2O3 nanoparticles in the first three cycles at 0.1 mV/s, revealing corresponding



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lithiation/delithiation mechanism of Bi2O3 anode material. Three cathodic peaks at 1.83, 1.72 and 1.35 V in the initial lithiation segment can be probably attributed to electrochemical conversions of monoclinic Bi2O3 and tetragonal Bi2O2.33 to BiO and then to metallic Bi0 and Li2O,12, 36 respectively. As reported in literatures,12, 36 such a reaction is irreversible, leading to the highly irreversible capacity loss in the first cycle similar to SnO2-based anode materials.14,

37-39

Subsequently, the reduced Bi0

undergoes reversible alloying with Li ions to form LiBi and Li3Bi in two steps.36, 40 Because of the simultaneous formation of solid electrolyte interface (SEI) below 1.0 V that increases the resistance of the electrode, CV peaks corresponds to two-step Li-Bi alloying cannot be resolved in the initial discharge process, leading to a broad cathodic peak between 0.25 and 1.0 V centered at around 0.63 V. The expected peak splitting can be clearly seen in the second cycle at 0.62 and 0.47 V. Correspondingly, in the initial charge process, the as-alloyed Li3Bi is subsequently dealloyed to LiBi and Bi0, resulting in a wide anodic peak at 0.97 V. The absence of two resolved CV peaks related to reactions of Li3Bi to LiBi and continuous LiBi to Bi0 may be attributed to low electronic conductivity of bare Bi2O3 anode material. The other reason is that the oxidization potentials of these two dealloying reactions are too close to distinguish with each other, leading to the broad anodic peak. The existence of an anodic peak at 1.57 V corresponding to the cathodic peak at 1.35 V as marked by brown asterisks in the first CV cycle is possibly resulted from the partial oxidization of the metallic Bi0 back to BiO (Bi2+) in bare Bi2O3 anode material,12 followed by transferring to Bi2O3 (a broad anodic peak between 2.0 and 3.0 V). In the second cycle,


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two weak cathodic peaks at 1.94 and 1.35 V can be assigned to reduction reactions of recovered Bi2O3 and BiO material, respectively. The alloying and dealloying of Li-Bi alloys result in a wide cathodic peak between 0.25 and 1.0 V and anodic peak between 0.75 and 1.4 V. The increased polarity is aggravated during alloying and dealloying reactions in the third cycle, which gives rise to a significant voltage gap ∆V=0.84 V, indicating poor electrochemical reversibility of bare Bi2O3 anode material. Accordingly, Figure 4b shows charge and discharge curves in the first three cycles, in consistent with CV results in Figure 4a. The bare Bi2O3 anode material delivers initial discharge and charge capacities of 781.4 and 489.4 mAh/g with a low Coulombic efficiency of 62.6 % at 0.1 C, respectively. A charge capacity of 451.4 mAh/g can be reserved in the third cycle due to the apparent electrochemical polarity.

Figure 4. (a) CV profiles recorded at 0.1 mV/s and (b) charge and discharge curves



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cycled at 0.1 C both in the first three cycles of bare Bi2O3 nanoparticles, (c) CV profiles recorded at 0.1 mV/s and (d) charge and discharge curves cycled at 0.1 C both in the first three cycles of Bi2O3@rGO nanocomposite; and (e) high-rate performances at different C rates and (f) cycling performance at 1 C of Bi2O3 and Bi2O3@rGO anode materials after first cycled at 0.1 C for three cycles in a voltage range of 0.01-3.0 V vs. Li/Li+. 1 C responds to a current density of 600 mA/g. CV and galvanostatic charge and discharge curves of Bi2O3@rGO composite anode material are presented in Figure 4c and 4d, respectively, in order to evaluate effects of rGO incorporation on enhanced electrochemical performances for Bi2O3-based anode materials. HRTEM, Raman and XPS results have demonstrated chemical bonding between Bi2O3 and rGO in Bi2O3@rGO nanocomposite, which should be favorable to increase electronic conductivity of the entire anode, stabilize active anode materials (Bi, LiBi and Li3Bi) and release reaction strains during alloying and dealloying processes of Li-Bi alloys. Additionally, the rGO substrate can act as the buffer for volumetric changes of various Li-Bi alloys. As a result, Bi2O3@rGO anode material reveals better electrochemical stability and reversibility as validated in CV and charge/discharge measurements. In comparison with CV profiles of bare Bi2O3 (Figure 4a), Bi2O3@rGO composite material gives a very similar initial CV cycle, showing a broad cathodic peak at 1.93 V for the reduction of Bi2O3 to metallic Bi0 and a redox pair located at 0.53 and 0.98 V for alloying and dealloying of reduced Bi0. The redox couple near 1.5 V is highly suppressed in Bi2O3@rGO anode material as compared with the bare Bi2O3 electrode (Figure 4a), which can be attributed to


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enhanced electronic conductivity of Bi2O3@rGO nanocomposite that is favorable for electrochemical conversion of Bi2O3 to the metallic Bi0. It is also noticeable that the second CV performance of Bi2O3@rGO demonstrates two-step alloying processes of Bi0 to form LiBi and Li3Bi by resolving two distinct cathodic peaks at 0.70 and 0.59 V. The corresponding anodic peak is positioned at 0.97 V with less chemical polarity and higher current density as compared with that in the first cycle. Furthermore, the CV profile in the third cycle is identical to the second cycle, especially with a small voltage gap ∆V=0.26 V during lithiation and delithiation of Bi0, indicating excellent chemical reversibility of Bi2O3@rGO anode material. Accordingly, the Bi2O3@rGO anode delivers much higher initial discharge and charge capacities of 1512.6 and 899.1 mAh/g in comparison with the bare Bi2O3 anode material. The lower Coulombic efficiency of 59.5 % could be associated with the irreversible conversion of Bi2O3 together with the formation of SEI film at the surface of rGO sheets. During this discharge reaction, the chemical bonding between Bi2O3 and rGO should be broken, leaving Bi0 nanoparticles on the rGO substrate but still in high dispersity and good electronic connection. On the contrary, this reduction reaction probably breaks as-prepared bare Bi2O3 nanospheres (Figure 1a), due to the drastic volumetric change and associated severe reaction strains during the reduction process of Bi2O3 to metallic Bi0, which would lead to instable structure of the electrode and inferior electrochemical performances of bare Bi2O3 (Figure 4a, 4b). As a result, Bi2O3@rGO anode material can retain charge capacity of 810 mAh/g after three cycles as shown in Figure 4d, much higher than the 451.4 mAh/g of bare Bi2O3 anode (Figure 4a), and it



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is even higher than the theoretical capacity of Bi2O3 material (690 mAh/g).12 The extra capacity may be from reversible absorption and desortion of lithium ions at the surface of rGO material, since there is no other detectable redox reaction in CV records of Bi2O3@rGO anode material except for Bi2O3 active material (Figure 4c). Such an excellent lithium storage capability of Bi2O3@rGO composite material can be attributed to chemical bonding that bridges Bi2O3 nanoparticles on rGO sheets, which benefits for even dispersing and well accommodating the reduced Bi0 at the surface of rGO substrate, and thus results in better electrochemical stability and reversibility during alloying and dealloying with lithium ions. In addition to delivering higher specific capacities, Bi2O3@rGO composite anode material also exhibits much better high-rate performances in comparison with Bi2O3 without rGO sheets. Figure 4e compares rate capabilities of bare Bi2O3 and Bi2O3@rGO anode materials up to 10 C rates. The Bi2O3@rGO exhibits specific charge capacities of 713.1, 604.0, 539.2, 493.8 409.0 and 270.0 mAh/g at 0.2, 0.5, 1, 2, 5 and 10 C, respectively. At each rates, Bi2O3@rGO contributes to additional ~200 mAh/g as compared with bare Bi2O3 material (Figure 4e). It is also notable that a high capacity of 539.0 mAh/g can be recovered when Bi2O3@rGO anode was cycled back at 0.5 C, which is higher than 305.8 mAh/g of bare Bi2O3. Figure 4f compares cycling performances of these two anode materials at 1 C after first cycled at 0.1 C for three cycles. Bi2O3@rGO anode material reveals remarkable preserved capacity of 347.3 mAh/g with corresponding capacity retention of 79% after 100 electrochemical cycles, significantly better than the 169.5 mAh/g and 43 % of the bare Bi2O3 anode material.


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This work can be comparable and even with superior performance than reported Bi-based composite materials as anode for lithium ion batteries, such as Bi2O3/Ni, Bi2S3-CNT and Bi2S3/C.5, 8-9, 12 Figure 1g-i show resulting morphology and structure of cycled Bi2O3@rGO electrode material, revealing well-preserved nanostructure of Bi2O3@rGO composite material in comparison with its initial state (Figure 1d-f). The reduced metallic Bi0 nanoparticles from original Bi2O3 have an average particle size of ~5 nm and high crystallinity (Figure 1i), which are still uniformly dispersed in the rGO matrix (Figure 1g, h). Such a reserved nanostructure can be attributed to the chemical bonding between Bi2O3 and rGO components (Figure 3a, b), resulting in its remarkable lithium storage performances (Figure 4e-f). Superior electrochemical performances and high kinetics of Bi2O3@rGO nanocomposite are further analyzed by CV measurements at different scanning rates. As displayed in Figure 5a, Bi2O3@rGO anode material can continuously sustain a series of potential sweeps in CV trials in a wide scanning rate range of 0.1-10 mV/s. Each record retains distinct redox pair related to alloying and dealloying of Li-Bi alloys even at a high rate of 10 mV/s. Such CV results indicate high rate capability of Bi2O3@rGO anode material, which is in high accordance to its rate performances shown in Figure 4e. The kinetics, i.e., lithium ion diffusion coefficient ( ) during electrochemical lithium storage in this composite material has been evaluated by using a relationship between anodic/cathodic peak current ( / ) and the square root of scanning rate ( / ) collected in CV curves as the following equation: 41-42 /

= 2.69 × 10 × / /



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Where is the peak current of the highest anodic/cathodic peaks (A) in CV curves, is electrons per molecule lithium ion ( = 3), is the scanning rate of cyclic voltammetry (V/s), is the concentration change of lithium ions during the formation of Li3Bi alloy that can be calculated as follows: 43-44 c=

Mole of lithium ions 3 mol = )*+,-) )./+/ ∗ Volume of Li Bi unit cell 6.02 ∗ 10 ∗ 303.75

= 1.64 × 107 mol/cm . is the actual surface area of working electrode (8 ), which can be roughly estimated equal to the surface area of active material in the working electrode: A = BET surface area of active material ∗ loading mass in working electrode = 14.89 m /g ∗ 2.25 mg = 3.4 × 10 cm .

Figure 5. (a) a series of CV patterns of Bi2O3@rGO nanocomposite cycled continuously from a low scanning rate at 0.1 mV/s to high 10 mV/s in a voltage range of 0.01-3.0 V vs. Li/Li+, and (b) corresponding relationships between anodic/cathodic peak currents ( / ) and the square root of scanning rates ( / ).


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Table 1. Lithium ion diffusion coefficient ( ) in Bi2O3@rGO nanocomposite. Scan rate v (mv/s)

0.1

0.2

0.5

1

2

5

10

V (V)

0.01

0.01414

0.02236

0.03162

0.04472

0.07071

0.1

1/2

Anodic peak current ianodic (A)

1.220e-3

2.270e-3

3.860e-3

6.170e-3

9.210e-3

1.416e-2

1.981e-2

Corresponding peak voltage (V)

0.984

1.002

1.046

1.092

1.170

1.337

1.509

Cathodic peak current icathodic (A)

-8.480e-4

-1.180e-3

2.250e-3

4.090e-3

6.870e-3

1.227e-2

1.803e-2

Corresponding peak voltage (V)

0.529

0.581

0.552

0.529

0.489

0.368

0.279

Lithium ion diffusion coefficient Dli+ (Anodic)/(cathodic) (cm2/s)

6.28e-16/7.04e-16

Figure 5b plots collected peak currents of anodic and cathodic peaks as a function of the square root of scanning rates together with linearly-fixed lines, and all corresponding data are summarized in Table 1. It is notable that two adjusted R square (R2) values of the linear fixing are both approximately equal to 1, indicating excellent chemical reversibility of Bi2O3@rGO composite material during alloying and dealloying processes of Li-Bi alloys even at high-rate situations. Accordingly, the of alloying and dealloying reactions is calculated to be 7.04 × 107E cm2/S and 6.28 × 107E cm2/S, respectively. It has to mention that such a calculated lithium diffusion coefficient is highly dependent on the value used for the actual surface area of working electrode. If the geometric area of the working electrode is used in our case for the , the can be considerably improved to ~10-11-10-12 cm2/S. Overall, Bi2O3@rGO nanocomposite significantly demonstrates effects of bridging rGO sheets via chemical bonding on enhanced electrochemical performances of Bi2O3 nanoparticles as anode materials for lithium ion batteries. The ionic bonding between Bi3+ precursor and GO material in negative charge (Scheme 1) during the solvothermal synthesis results in favorable chemical bonding between Bi2O3 nanoparticles and as-reduced rGO sheets within final Bi2O3@rGO nanocomposite



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(Figure 3a and 3b). Tremendous Bi2O3 nanoparticles can be tightly anchored and uniformly distributed at the surface of the rGO sheets (Figure 1d-f). Such a combination should be favorable for supporting reduced Bi0 after the initial discharge. The rGO substrate can offer versatile functions by reducing charge transfer resistance, shortening lithium ion diffusion pathway and releasing reaction strains from drastic volumetric changes during lithiation/delithiation of metallic Bi0. As a result, Bi2O3@rGO anode material shows considerably increased specific capacities, better cycling stability and rate capability in comparison with agglomerated bare Bi2O3 nanoparticles. 4.

CONCLUSION

Bi2O3@rGO nanocomposite has been prepared in a facile solvothermal synthesis process at low reaction temperature. The favorable chemical bonding between the two Bi2O3 and rGO components has been demonstrated by Raman and XPS characterizations. Bi2O3 nanoparticles have an average particle size of ~5 nm, which are uniformly anchored at the surface of rGO sheets. As compared with bare Bi2O3 nanosphere that is composed of numerous agglomerated Bi2O3 nanoparticles in similar size and crystallinity, Bi2O3@rGO reveals significantly increased capacity, and enhanced cycling and high-rate performances as anode materials for lithium ion batteries. Bi2O3@rGO anode materials can deliver high reversible discharge capacity of ~900 and 270 mAh/g at 0.1 and 10 C, respectively, with remarkable cycling stability and rate capability. This work provides a facile approach to fabricate Bi2O3-based nanocomposite with rGO sheets for promising lithium storage


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performances that should be adoptable for other oxide-based anode materials for superior lithium ion batteries. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (grant #: U1401248) and Natural Science Foundation of Jiangsu Province, China (grant #: BK20151227). REFERENCES (1) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414 (6861), 359-367. (2) Chabi, S.; Peng, C.; Hu, D.; Zhu, Y. Ideal Three-Dimensional Electrode Structures for Electrochemical Energy Storage. Adv. Mater. 2014, 26 (15), 2440-2445. (3) Gao, X. P.; Yang, H. X. Multi-Electron Reaction Materials for High Energy Density Batteries. Energy Environ. Sci. 2010, 3 (2), 174-189. (4) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nano-Sized Transition-Metal Oxides as Negative-Electrode Materials for Lithium-Ion Batteries. Nature 2000, 407 (6803), 496-499. (5) Zhao, Y.; Liu, T.; Xia, H.; Zhang, L.; Jiang, J.; Shen, M.; Ni, J.; Gao, L. Branch-Structured Bi2S3-CNT Hybrids with Improved Lithium Storage Capability. J. Mater. Chem. A 2014, 2 (34), 13854-13858. (6) Park, C.-M.; Yoon, S.; Lee, S.-I.; Sohn, H.-J. Enhanced Electrochemical Properties



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Table of Contents


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rGO

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