Ferrocene as a Novel Additive to Enhance the Lithium-Ion Storage

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Energy, Environmental, and Catalysis Applications

Ferrocene as a Novel Additive to Enhance the LithiumIon Storage Capability of SnO2/Graphene Composite Siyu Zhang, Beirong Liang, Yu Fan, Junjie Wang, Xianqing Liang, Haifu Huang, Dan Huang, Wen-Zheng Zhou, and Jin Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09363 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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ACS Applied Materials & Interfaces

Ferrocene as a Novel Additive to Enhance the Lithium-Ion Storage Capability of SnO2/Graphene Composite Siyu Zhang a, Beirong Liang a, Yu Fan a, Junjie Wang a, Xianqing Liang a,b,*, Haifu Huang a,b, Dan Huang a,b, Wenzheng Zhou a,b, Jin Guo a,b

a

Guangxi Key Laboratory for Relativistic Astrophysics, Guangxi Colleges and

Universities Key Laboratory of Novel Energy Materials and Related Technology, Guangxi Novel Battery Materials Research Center of Engineering Technology, School of Physical Science and Technology, Guangxi University, Nanning 530004, P. R. China b

Guangxi Collaborative Innovation Center of Structure and Property for New Energy

and Materials, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, P. R. China

*Email: [email protected] (Xianqing Liang).

KEYWORDS: tin dioxide, graphene, ferrocene, conversion reaction, lithium-ion battery

ABSTRACT: Improving the reversibility of conversion reaction is a promising way to enhance the lithium-ion storage capability of SnO2-based anodes. Herein, we report ferrocene as a novel additive to improve the Li-ion storage performance of SnO2/graphene (SnO2/G) composite. Through a simple mixing method, ferrocene can be uniformly dispersed into the SnO2/G electrode. It is found that ferrocene additive can effectively suppress the agglomeration of Sn/SnO2 and retain the nanoscale Sn/Li2O 1

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interface. Furthermore, metallic Fe is formed from ferrocene in the discharge process, and acts as catalyst to promote the reversible conversion between Sn/Li2O and SnO2. As a result, SnO2/G electrode with the addition of 10 wt% ferrocene (10%Fc-SnO2/G) exhibits a superior Li-ion storage performance. It displays a reversible capacity of up to 1084.5 mAh g−1 at 0.1 A g-1 after 150 cycles with a good rate capability (752 mAh g−1 at 1 A g-1). In addition, the 10%Fc-SnO2/G electrode can retain a capacity of 787.2 mAh g−1 at 0.5 A g-1 after 220 cycles. This work demonstrates the promising additive of ferrocene in enhancing the reversible capacity of SnO2-based anodes for lithium-ion batteries.

1. INTRODUCTION Lithium-ion batteries (LIBs) have been extensively applied as power source in our daily life.1-3 With the continuous development of electric vehicles and static storage, high-performance LIBs are consequently required.4-5 Hence, exploiting high-capacity electrode materials with good cyclability is a hot topics in the LIBs research.6 Recently, metal oxides have been extensively investigated as the potential anode materials. They are expected to replace the traditional carbon anodes, the theoretical capacity of which is merely 372 mAh g-1.7-9 Among them, tin dioxide (SnO2) material has promising prospects as the anode for next-generation LIBs, owing to its high Li-storage capacity and moderate working voltage.10-12 As is known, the Li-ion storage in SnO2 material generally contains two-step process:12 SnO2 + 4Li+ + 4e- ⇄ Sn + 2Li2O 2


(1)

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Sn + xLi+ + xe- ⇄ LixSn (0 ≤ x ≤ 4.4)

(2)

The conversion reaction (Eq. (1)) and alloying reaction (Eq. (2)) of SnO2 offer Liion storage capacities of 711 and 783 mAh g-1, respectively. If both reactions could be fully reversible, a capacity high up to 1494 mAh g-1 would be obtained for the SnO2based materials. This high capacity will substantially meet the requirement of LIBs with high energy density. However, the reversibility of conversion reaction is usually poor due to the sluggish kinetics.13-14 Such poor reversibility would greatly limit the Li-ion storage capability of SnO2-based materials. In addition, the large volume variation (over 300%) comes with the Li+ insertion/extraction. This can cause the pulverization of electrode and the agglomeration of Sn/SnO2 nanoparticles with cycling.15-16 Moreover, the repeated volume variation results in the continuous growth of unstable solid electrolyte interphase (SEI). These intrinsic issues lead to the inferior Li-ion storage performance and significantly hinder the practical applications of SnO2-based anodes for LIBs. Therefore, it is essential to improve the reaction kinetics and structural stability of SnO2-based materials. Over the past decade, many efforts have been done toward enhancing the lithium storage properties of SnO2-based electrodes. One common and effective strategy is integrating nano-SnO2 with conductive carbon materials.15, 17-20 Lots of reports have illustrated that the carbon materials can not only mitigate the volume variation of SnO2 but also enhance the electrical conductivity of SnO2-carbon composites.21-24 Consequently, the composites exhibit greatly improved Li-storage performance as compared with the pure SnO2 materials. However, the sluggish kinetics of conversion 3



reaction still cannot gain sufficient improvement, and a gradual capacity fading still can be observed upon long-term cycling. Recently, several studies have demonstrated that incorporating metals into SnO2-based materials can significantly promote their Li-ion storage performances.13,

25-29

Especially, the transition metals can catalyze the

decomposition of Li2O, thereby promoting the electrochemical kinetics of conversion reaction between Sn/Li2O and SnO2.13,

25-26, 29

Nevertheless, the incorporation of

nanosized transition metals is almost by means of mechanical milling or chemical reduction, which require complicated process control. A recent research reported that the organometallic compound of ferrocene could be employed as the anode material in LIBs, and metallic Fe was formed in the discharge process.30 Based on these studies, we conceive that introducing ferrocene into SnO2/graphene (SnO2/G) composites should be capable of promoting the conversion reaction and structural stability, and hence induce a further enhancement of Li-ion storage performance. Reported for the first time, herein we utilize ferrocene as a novel additive to adequately exert the Li-ion storage capability of SnO2/G composite. The effects of ferrocene additive on the electrochemical properties of SnO2/G electrodes were investigated in detail. Results demonstrate that the addition of ferrocene can bring further improvement in the conversion reaction and cycling stability of SnO2/G composite. This work would offer a promising route to fabricate the high-performance SnO2 and other conversion/alloying-type anodes for use in LIBs. 2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. All reagents were used directly as received. Graphene oxide 4


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(GO) was fabricated through a modified Hummer’s method,31 and then dispersed in water by ultrasonication to form a uniform suspension. The SnO2/G precursor was synthesized by a hydrolysis approach. In brief, 1 g SnCl4·5H2O was first dissolved into 50 mL water, and then mixed with 250 mL GO suspension (2 mg/mL). Subsequently, the resultant solution was kept at 80 °C for 12 h under stirring. After that, the black precursor was harvested by centrifuging and washed by water. Finally, the precursor was thermally treated in Ar gas at 500 °C for 2 h, which yielded the SnO2/G composite. For comparison, the pure SnO2 material was also synthesized through the similar approach without GO. 2.2. Materials Characterization. The powder X-ray diffraction (XRD) was measured on a Rigaku Miniflex 600 diffractometer. Scanning electron microscope (SEM) images were acquired with a FEI Quanta 250 and elemental mappings were obtained on the JEOL JSM-6510A. Transmission electron microscope (TEM) was performed on a FEI TECNAI-G20 instrument. Thermogravimetric analysis (TGA) of the composite was taken on the LinseisSTA PT1600 from 30 to 800 °C with air flow. X-ray photoelectron spectroscopy (XPS) analysis was performed on the Thermo Escalab 250XI. The soft X-ray absorption spectroscopy (XAS) was conducted on the beamline 4B7B at Beijing Synchrotron Radiation Facility (BSRF). The total electron yield (TEY) mode was employed for data collection. 2.3. Electrochemical Characterization. Electrochemical properties were examined using coin-cells (CR2032) at ambient temperature. The SnO2/G electrode was prepared by mixing SnO2/G, acetylene black (AB) and polyvinylidene fluoride (PVDF) with a 5



mass ratio of 70/15/15 in N-methyl-2-pyrrolidone (NMP) to get a slurry. For the SnO2/G electrodes with ferrocene additive, two different compositions of SnO2/G, ferrocene, AB and PVDF were first mixed with the weight ratios of 63/7/15/15 and 56/14/15/15 in a mortar. Then, the mixtures were stirred with a certain amount of NMP to form slurries. They were labeled as 10%Fc-SnO2/G and 20%Fc-SnO2/G, respectively. The resultant slurries were uniformly pasted onto Cu foil and then vacuum-dried at 90 °C overnight to obtain electrode sheets. These electrode sheets were punched and assembled into coin-cells. The electrolyte was 1 mol L-1 LiPF6 in carbonate-based solvent (EC/DMC, 1:1 vol), and Li plate was used as counter electrode. The cyclic performance of cells was characterized on a LAND CT2001A battery test system. The cutoff voltages for discharge/charge were 0.01/3.0 V (vs. Li/Li+). Cyclic voltammetric (CV) curves and electrochemical impedance spectra (EIS) were collected on a Gamry Reference-1000 electrochemical workstation.

3. RESULTS AND DISCUSSION The crystalline phases of the pure SnO2, reduced GO and SnO2/G composite are analyzed by XRD characterization. As illustrated in Figure 1a, the similar XRD patterns can be seen for pure SnO2 and SnO2/G composite. The diffraction peaks of both samples correspond to the tetragonal rutile-like SnO2 (JCPDS 41-1445).32 The broad peaks suggest the small crystalline sizes of SnO2 particles. Besides, there is no characteristic diffraction peaks of reduced GO in the SnO2/G composite, implying the negligible restacking of graphene sheets with the decoration of SnO2 particles.33 Figure 6


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1b displays the TGA curve of SnO2/G composite obtained under air condition. The curve exhibits two weight losses, which correspond to the release of absorbed water and the combustion of graphene sheets, respectively.34-36 Based on the analysis above, the mass fraction of SnO2 in the SnO2/G composite is evaluated to be ~64.8%. Figure 1c presents the C K-edge XAS spectra of GO and SnO2/G samples. These two spectra are characterized by three main peaks. Peak A at ~285.4 eV is corresponding to the graphitic π* states, peak C at ~292.7 eV to the graphitic σ* states, and peak B at ~288.2 eV to the sp3-hybridized states of carbons bonded with oxygenated groups.37-39 As can be seen, SnO2/G shows a significantly enhanced peak A and a weak peak B compared with GO. This observation indicates the effective removal of oxygenated groups and restoration of graphitic electronic structure after thermal treatment. Thus, the reduced GO is able to improve the electrical transport across the SnO2/G composite. The Sn M5,4-edge XAS spectrum is displayed in Figure 1d, where two groups of triplet peaks are observed. They are the characteristic peaks of rutile-like SnO2 as found in previous studies.40-41 It further manifests the formation of rutile SnO2 nanoparticles in the reduced GO matrix. The typical SEM images of SnO2/G composite are shown in Figure 2a and b. A sheet-like structure with curled and wrinkled morphology is clearly presented. No noticeable particles appear in the SEM images, implying that the SnO2 nanoparticles are highly dispersed in the composite. The microstructures of SnO2/G composite were further characterized by TEM measurements. As shown in Figure 2c, there are many SnO2 nanoparticles loaded uniformly on the graphene sheets. Besides, void spaces can 7



be observed between SnO2 nanoparticles, which are beneficial for the volume change and electrolyte penetration. The inset of Figure 2c shows the selected area electron diffraction (SAED) pattern of SnO2/G composite. It contains four clear diffraction rings, which are corresponding to the (110), (101), (211) and (301) planes of tetragonal rutilelike SnO2.12 The high-resolution TEM image (Figure 2d) reveals that the sizes of SnO2 nanoparticles in the composite are about 4−7 nm, and the clear lattice fringes ((110) planes with lattice spacing of 0.335 nm) indicate the good crystallinity of SnO2 nanoparticles. Figure 3a and b present the representative SEM images of the fresh SnO2/G and 10%Fc-SnO2/G electrode, which display a rough and rugged surface. As we can see, the SnO2/G composite is well mixed with acetylene black in both electrodes. To understand more about the distribution of SnO2/G and ferrocene in the 10%Fc-SnO2/G electrode, energy-dispersive spectroscopy (EDS) characterization was performed with SEM. As shown in Figure S1, the C, O, F, Sn and Fe peaks can be seen in the EDS spectrum. In the elemental mappings of 10%Fc-SnO2/G electrode (Figure 3c−e), both Sn and Fe elements are evenly present, signifying the uniform distribution of SnO2/G composite and ferrocene additive throughout the electrode. To further confirm the presence of ferrocene in 10%Fc-SnO2/G electrode, XPS measurements were performed. As seen from Figure 4a, the spectrum could be deconvoluted into two peaks at ~716.4 and 708.1 eV, which correspond to Sn 3p3/2 and Fe 2p3/2, respectively, indicating the presence of Fe().42-43 In addition, the Fe 3p signal was also detected as shown in Figure 4b, providing further evidence of ferrocene in the 10%Fc-SnO2/G electrode.42 8


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It is worth to note that ferrocene has good solubility in the NMP solvent. As a result, the ferrocene additive can be homogeneously dispersed in the slurry and subsequent electrode. The above observations demonstrate that the ferrocene-added SnO2/G electrode with well-dispersed structure can be easily fabricated by a simple mixing route. To evaluate the effects of ferrocene additive on the lithium storage properties of SnO2/G composite, various electrochemical measurements were performed. Figure 5 displays the first five CV curves of SnO2/G and 10%Fc-SnO2/G electrodes scanned at 0.1 mV s-1. According to Figure 5a, the first discharge curve of SnO2/G is characterized by two broad peaks, indicative of the multiple reactions between Li+ and SnO2. The first peak at ~0.88 V is assigned to the conversion of SnO2 to Sn (Eq. (1)) along with the formation of SEI.12, 35 The second peak at ~0.09 V is ascribed to the Sn alloying to form LixSn (Eq. (2)).12 Compared to SnO2/G electrode, the 10%Fc-SnO2/G (Figure 5b) and 20%Fc-SnO2/G (Figure S2a) electrodes show two additional peaks at ~1.60 and 0.73 V. According to the references, the peak at ~1.60 V is ascribed to the formation of SEI, which disappears in the following cycles.30, 44 This peak can also be found in the first discharge scan of the ferrocene electrode (Figure S2b). Another peak at ~0.73 V could refer to the electrochemical reaction of ferrocene with Li+.30 In the subsequent charge process, an intense peak between 0.36 and 0.92 V is related to the reversible dealloying of LixSn to Sn.12, 45 In addition, two mirror peaks appear at ~1.28 and 1.94 V, which are corresponding to the reconversion of Sn to SnO2.12, 46-47 In the following four cycles, the CV curves of 10%Fc-SnO2/G and 20%Fc-SnO2/G electrodes show 9



more overlapped than those of SnO2/G electrode, suggesting the better structural stability and higher reversible reaction after the addition of ferrocene. Figure 6a shows the cycling performances of 10%Fc-SnO2/G and 20%Fc-SnO2/G electrodes at 0.1 A g−1. Besides, those of SnO2/G, 10%Fc-SnO2 and SnO2 were also investigated as controls. As seen from Figure 6a, the SnO2 electrode exhibits a rapid capacity fading within 50 cycles, which is due to the large volume variation and particle agglomeration. In comparison, the SnO2/G electrode shows a much better cycling performance than SnO2 electrode, indicating the substantial improvement of cycling stability by graphene. However, its reversible capacity still decreases gradually and declines to 671.6 mAh g−1 over 100 cycles, retaining only 69.5% of the 2nd cycle. Strikingly, with the addition of ferrocene, the reversible capacities of 10%Fc-SnO2/G and 20%Fc-SnO2/G electrodes show slight decline within the first 32 cycles, and then gradually increase in the following cycles. This phenomenon is also observed in the other metal oxide related anodes.48-50 It might be ascribed to the continuous activation of electrode and the reversible formation of a gel-like polymer from electrolyte.48, 51-53 The 10%Fc-SnO2/G and 20%Fc-SnO2/G electrodes deliver reversible capacities up to 1084.5 and 961.5 mAh g−1 upon 150 cycles, retaining 99.9% and 109.6% of the 2nd cycle, respectively. The cycling performance of ferrocene was also conducted and shown in Figure S3a. It delivers a reversible capacity of 193.2 mAh g-1 after 100 cycles, which is much lower than that of the SnO2/G composite. Thus, the 20%Fc-SnO2/G electrode exhibits superior capacity retention, however, with a bit lower capacity due to the higher content of ferrocene additive in the electrode. Note that the coulombic 10


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efficiency (CE) of 10%Fc-SnO2/G electrode increases quickly to 99% after the initial several cycles. Additionally, the influence of the addition of ferrocene to SnO2 electrode was also investigated. As shown in Figure 6a, the 10%Fc-SnO2 electrode shows enhanced cycling performance compared to SnO2 electrode. These results indicate that the ferrocene additive has a positive effect on the Li-ion storage capability of SnO2based materials. In addition, we compare the lithium storage performance of 10%FcSnO2/G with some reported SnO2-containing composites. As listed in Table S1, the present 10%Fc-SnO2/G electrode shows remarkable specific capacity and cycling stability as compared with the previously reported SnO2-containing composites. More importantly, this study provides a facile but effective route to improve the Li-storage capability of SnO2-based materials. Figure 6b and c depict the discharge-charge voltage curves of SnO2/G and 10%Fc-SnO2/G electrodes in typical cycles at 0.1 A g-1. As we can see, two electrodes have the similar discharge-charge profiles. There are several voltage plateaus in the curves, which coincide with the CV tests. The first charge/discharge capacities of SnO2/G and 10%Fc-SnO2/G electrodes are 959.7/1497.8 and 1060.8/1687.2 mAh g-1, corresponding to the initial CEs of 64.1% and 62.9%, respectively. The initial capacity loss is mainly due to the generation of SEI layers and the incomplete delithiation of active materials.54 It should be noted that the first charge and discharge capacities of ferrocene are merely 186.1 and 359.3 mAh g-1 (Figure S3b). Thus, the enhancement in the initial capacity of 10%Fc-SnO2/G electrode should be attributed to the synergistic lithium storage effect between SnO2 and ferrocene. With the increase of cycle number, 11



the voltage plateaus of SnO2/G electrode become shorter, whereas those of 10%FcSnO2/G electrode are relatively stable. It means that the addition of ferrocene has positive effects on the electrochemical performance of SnO2/G electrode. To further understand the function of ferrocene additive, the differential charge-capacity plots (DCPs) of SnO2/G and 10%Fc-SnO2/G are compared in Figure 6d and e. It can be seen that the DCPs present two sets of peaks, which correspond to two electrochemical reaction processes. According to the previous literatures, the intense peaks at 0.01−1.0 V correspond to the dealloying reaction from LixSn to Sn, while the broad peaks at 1.0−3.0 V to the conversion reaction from Sn to SnO2.27, 29, 45, 55-57 As we can see, the intensities of two peaks remain quite stable for 10%Fc-SnO2/G even after 150 cycles. On the other hand, the peaks of SnO2/G become weaker with cycling. These results signify the enhanced structural stability and reaction reversibility of SnO2/G composite after adding ferrocene. Based on the DCPs analysis, we divide the charge capacities into two parts: dealloying reaction at 0.01−1.0 V and conversion reaction at 1.0−3.0 V. Figure 6f presents the plot of these two charge capacities versus cycle number for SnO2/G and 10%Fc-SnO2/G electrodes. Although the graphene can suppress the aggregation of nanoparticles in a certain extent, the coalescence of Sn/SnO2 particles still exists during cycling. Thus, for SnO2/G electrode, the dealloying/conversion reaction capacities slowly decrease from ~449.7/510 mAh g-1 of the 1st cycle to ~304.2/357.4 mAh g-1 of the 100th cycle. In contrast, the dealloying reaction capacity of 10%Fc-SnO2/G is quite stable and maintains at ~448 mAh g-1 after complete activation. This might be due to the controlled agglomeration of Sn nanoparticles by 12


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the addition of ferrocene. Besides, the conversion reaction capacity of 10%Fc-SnO2/G shows a decrease in the initial 20 cycles and then gradually increases with cycling. These results reveal that the ferrocene additive play an important role in promoting the Li-ion storage capability of SnO2/G composite. According to the recent research, ferrocene can react with Li+ ions to form metallic Fe in the discharge process.30 More importantly, the metallic Fe can act as a catalyst to facilitate the decomposition of Li2O and regeneration of SnOx in the charge process.25 Thus, the conversion reaction in 10%Fc-SnO2/G electrode is becoming more reversible with cycling as shown in Figure 6f. This might be one of the reasons for the capacity increase phenomenon. Besides, the ferrocene additive should be helpful for suppressing the Sn/SnO2 nanoparticles from agglomeration and maintaining the nanoscale Sn/Li2O interface during long-term cycling. These factors will greatly promote the reversibility of conversion reaction from Sn/Li2O to SnO2. Therefore, the Li-storage capacity and cycling stability of SnO2/G composite can be effectively enhanced by the addition of ferrocene. To further highlight the advantages of ferrocene addition, rate capability of the electrodes was also studied. Figure 7a displays the rate performances of SnO2, 10%FcSnO2, SnO2/G, 10%Fc-SnO2/G, 20%Fc-SnO2/G electrodes at different current densities. Clearly, the ferrocene-added electrodes show obvious improvement in the rate capability as compared to their counterparts. As expected, the 10%Fc-SnO2 electrode exhibits the best rate capability among these electrodes. It delivers reversible capacities of 1033.8, 969.1, 861.4, and 752 mAh g−1 when tested at 0.1, 0.2, 0.5 and 1 A g−1. Notably, the capacity can be resumed to 993.7 mAh g−1 at the 50th cycle as the current 13



returning to 0.1 A g−1. The capacity retention is 96.1% for 10%Fc-SnO2/G, which is much higher than that for SnO2/G (81.4%), implying the decent recoverability of 10%Fc-SnO2/G electrode. Furthermore, to explore the superiority of the ferrocene additive, the long-term cyclability of 10%Fc-SnO2/G electrode at high current density is also evaluated. As shown in Figure 7b, the cycling performance is examined at a current density of 0.5 A g−1 for 220 cycles after activation at 0.1 and 0.2 A g−1. The 10%Fc-SnO2/G electrode delivers a specific capacity of 787.2 mAh g−1 at the 230th cycle, retaining 88.6% of the 11th cycle. The high current density could lead to a larger volume change rate, which would result in a less homogenous and thicker SEI layer with cycling. The thicker SEI layer would consume a part of Li+ ions and suppress the diffusion of Li+ ions, leading to the incomplete lithiation/delithiation of active materials. These might be the reasons why the capacity increase phenomenon does not appear in the 10%Fc-SnO2/G electrode at 0.5 A g−1. The above electrochemical results manifest that the ferrocene additive can remarkably enhance the cycling stability and rate capability of SnO2/G composite. To gain insight into the effects of ferrocene addition on the electrochemical kinetics, the CV curves of SnO2/G and 10%Fc-SnO2/G electrode at different scan rates were tested. As shown in Figure 8a and b, the characteristic peaks of 10%Fc-SnO2/G electrode display higher intensity than those of SnO2/G electrode. This observation implies that the electrochemical reversibility of SnO2/G can be effectively improved by adding ferrocene.58 In addition, the peak currents (Ip) under different scan rates (ν) can be obtained from the CV data. The correlation between Ip and ν can be described by the 14


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Randles-Sevcik equation:59-61

I P  (2.69 105 )n3/ 2 ADLi1/ 2CLi v1/ 2 Where n, A, CLi and DLi are the number of electrons per molecule, electrode area, Li+ concentration and Li+ diffusion coefficient, respectively. It is used to reflect the electrochemical reaction kinetics at the electrolyte/electrode interface and the Li+ diffusion in the active material. Figure 8c and d show the plots of Ip vs. ν1/2 with the fitted lines for peak 1 and peak 2 of SnO2/G and 10%Fc-SnO2/G electrodes. According to the Randles-Sevcik equation, DLi is proportional to the square of the slope of Ip–ν1/2 fitted line. As illustrated in Figure 8c and d, the slopes for 10%Fc-SnO2/G electrode are larger than those for SnO2/G electrode, suggesting the enhanced Li+ diffusion kinetics of electrode with the addition of ferrocene. The electrochemical kinetics of SnO2/G and 10%Fc-SnO2/G electrodes was also studied by EIS measurements after 10 cycles. As seen from the Nyquist plots (Figure S4), 10%Fc-SnO2/G electrode shows a smaller diameter of the semicircle at high frequencies compared to SnO2/G electrode. Besides, the slope of the straight line for 10%Fc-SnO2/G electrode at low frequencies is larger than that for SnO2/G electrode. These observations further confirm that the addition of ferrocene can effectively enhance the transfer kinetics of electrons and Liions, which brings in the substantial improvement of Li-ion storage capability for SnO2/G composite.62 The microstructure and composition of the SnO2/G and 10%Fc-SnO2/G electrodes after cycling were characterized by SEM-EDS, XPS and TEM. The cycled electrodes were obtained by disassembling the cells and washing with DMC solution. As displayed 15



in Figure 9a, the SnO2/G electrode shows apparent agglomeration and large particles after cycling 100 times. For the 10%Fc-SnO2/G electrode (Figure 9b), no obvious agglomeration phenomenon can be observed even after 150 cycles, indicating the good structural integrity of electrode. Besides, EDS elemental mappings for Sn and Fe (Figure 9c and d) exhibit the compositional homogeneity of the 10%Fc-SnO2/G electrode after long-term cycling. Figure 9e shows the Fe 3p XPS spectrum of the 10%Fc-SnO2/G electrode after 150 cycles, further demonstrating the existence of Fe in the cycled electrode. To examine the structural integrity, the TEM images of 10%FcSnO2/G after 150 cycles are conducted. It can be seen from Figure 9f that the active materials are still evenly attached on the graphene sheets without obvious aggregation. In the high-resolution TEM image of Figure 9g, lattice fringes are difficult to be found in the cycled 10%Fc-SnO2/G, implying that the charged products are almost amorphous. These observations confirm that the ferrocene additive can effectively inhibit the aggregation of Sn/SnO2 nanoparticles, thereby maintaining high-density Sn/Li2O interfaces. To further prove the effect of ferrocene additive on the conversion reaction, the Sn M5,4-edge XAS spectra of 10%Fc-SnO2/G electrode before cycling and after 150 cycles were recorded for comparison. It can be seen from Figure S5 that the spectral profile of the 10%Fc-SnO2/G electrode after 150 cycles is similar to that of the fresh 10%Fc-SnO2/G electrode, indicating the good reoxidation of Sn to SnO2 in the 10%FcSnO2/G electrode. Overall, the above results demonstrate that the ferrocene additive can help prevent the Sn/SnO2 nanoparticles from detrimental agglomeration, and consequently keep the 16


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structural integrity of SnO2/G composite. The good structural stability of electrode is beneficial for the formation of stable SEI layers and the transport of electron/Li-ion, thereby enabling the reversible reactions between lithium and SnO2 during cycling. Furthermore, as illustrated in Figure 9h, metallic Fe is formed from ferrocene in the discharge process, and acts as the catalyst to facilitate the decomposition of Li2O, promoting the conversion reaction kinetics between Sn/Li2O and SnO2. Therefore, the ferrocene-added SnO2/G electrode possesses high Li-storage capacity and superior cycling stability, which are the desirable characteristics for the high-performance LIBs in practical applications.

4. CONCLUSION In summary, we use a simple ferrocene addition approach to improve the lithium storage capability of SnO2/G composite. It is demonstrated that the ferrocene additive can inhibit the agglomeration of Sn/SnO2 nanoparticles, and generate metallic Fe that acts as catalyst to improve the conversion reaction. Thus, the ferrocene-added SnO2/G electrodes exhibit superior Li-storage capacity, rate capability and cyclic performance. The facile fabrication method and excellent Li-storage performance demonstrate that the addition of ferrocene could provide a new route to design promising SnO2-based anodes for high-performance LIBs.

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ASSOCIATED CONTENT Supporting Information. EDS spectrum of the fresh 10%Fc-SnO2/G electrode; Initial five CV curves of 20%Fc-SnO2/G and ferrocene electrodes scanned at 0.1 mV s-1; Cycling performance and discharge/charge voltage curves of ferrocene electrode at 0.1 A g-1; EIS spectra of the SnO2/G and 10%Fc-SnO2/G electrodes after 10 cycles; Sn M5,4-edge XAS spectra of the 10%Fc-SnO2/G electrode before cycling and after 150 cycles.

AUTHOR INFORMATION Corresponding Authors *X. Liang. Email : [email protected]. Notes The authors declare no conflicts of interest.

ACKNOWLEDGEMENT This research was supported by the National Natural Science Foundation of China (No. 11465003) and Innovation-Driven Development Foundation of Guangxi Province (No. AA17204063).

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FIGURES

Figure 1. (a) XRD patterns of SnO2, reduced GO and SnO2/G; (b) TGA curve of SnO2/G composite; (c) C K-edge XAS spectra of GO and SnO2/G; (d) Sn M5,4-edge XAS spectrum of SnO2/G.

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Figure 2. (a, b) SEM and (c, d) TEM images of the SnO2/G composite (The inset in (c) shows the SAED pattern of SnO2/G).

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Figure 3. SEM images of the fresh (a) SnO2/G and (b) 10%Fc-SnO2/G electrodes; Elemental mappings of (d) Sn and (e) Fe based on the (c) SEM image of the fresh 10%Fc-SnO2/G electrode.

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Figure 4. XPS spectra of (a) Fe 2p and (b) Fe 3p regions of the fresh 10%Fc-SnO2/G electrode.

Figure 5. The CVs of (a) SnO2/G and (b) 10%Fc-SnO2/G electrodes scanned at 0.1 mV s-1.

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Figure 6. (a) Cyclic performance of SnO2, 10%Fc-SnO2, SnO2/G, 10%Fc-SnO2/G, 20%Fc-SnO2/G electrodes along with the CE of 10%Fc-SnO2/G; (b) Selected discharge/charge voltage curves of (b) SnO2/G and (c) 10%Fc-SnO2/G electrodes; Differential charge-capacity plots of (d) SnO2/G and (e) 10%Fc-SnO2/G; (f) Charge capacities divided into two parts: 0.01−1.0 V for dealloying reaction and 1.0−3.0 V for conversion reaction.

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Figure 7. (a) Rate capability of SnO2, 10%Fc-SnO2, SnO2/G, 10%Fc-SnO2/G, 20%FcSnO2/G electrodes; (b) Cyclic performance of 10%Fc-SnO2/G electrode at 0.5 A g-1.

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Figure 8. The CVs of (a) SnO2/G and (b) 10%Fc-SnO2/G electrodes scanned at different rates; Ip–ν1/2 plots for (c) peak 1 and (d) peak 2 of SnO2/G and 10%Fc-SnO2/G electrodes.

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Figure 9. SEM images of (a) SnO2/G electrode after 100 cycles and (b) 10%Fc-SnO2/G electrode after 150 cycles; (c, d) Elemental mapping images of Sn and Fe, (e) XPS spectrum of Fe 3p and (f, g) TEM images for the 10%Fc-SnO2/G electrode after 150 cycles; (h) Schematic illustration for the discharge/charge process of 10%Fc-SnO2/G electrode.

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Ferrocene as a Novel Additive to Enhance the Lithium-Ion Storage

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