Enhanced Structural and Electrochemical Stability of Self-Similar Rice

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Enhanced Structural and Electrochemical Stability of Self-similar Rice-shaped SnO2 Nanoparticles Du Pan, Ning Wan, Yong Ren, Weifeng Zhang, Xia Lu, Yuesheng Wang, Yong-Sheng Hu, and Ying Bai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00232 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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ACS Applied Materials & Interfaces

Enhanced Structural and Electrochemical Stability of Self-Similar Rice-Shaped SnO2 Nanoparticles Du Pan,†,‡,§ Ning Wan,†,‡,§ Yong Ren,‡,§ Weifeng Zhang,‡,§ Xia Lu,⊥ Yuesheng Wang,∥ Yong-Sheng Hu,∥ Ying Bai‡,§,* ‡

Henan Key Laboratory of Photovoltaic Materials and Laboratory of

Low-Dimensional Materials Science, Henan University, Kaifeng 475004, P.R. China §

School of Physics and Electronics, Henan University, Kaifeng 475004, P.R. China ⊥

College of Energy, Beijing University of Chemical Technology, Beijing 100029, P.R. China

∥

Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P.R. China

1. ABSTRACT: A facile one-pot hydrothermal strategy is applied to prepare Co and F co-doped SnO2 (Co-F/SnO2) nanoparticles, which exhibits a unique rice-shaped self-similar structure. Compared with the pristine and Co-doped counterparts (SnO2 and Co/SnO2), Co-F/SnO2 electrode demonstrates higher capacity, better cyclability and rate capability as anode material for lithium ion batteries (LIBs). A high charge capacity of 800 mAh g-1 can be successfully delivered after 50 cycles at 0.1 C, and a high reversible capacity of 700 mAh g-1 could be retained after 100 cycles at 5 C. The excellent lithium storage performances of the Co-F/SnO2 nanoparticles could be attributed to the synergetic effects of the doped Co and F, as well as the unique hierarchical self-similar structure with moderate oxygen defect and inactive pillars, which not only facilitates the fast diffusion of Li ions, but also stabilizes the structure 1

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during the electrochemical cycling. KEYWORDS: SnO2; co-doping; hierarchical structure; synergetic effect; Lithium ion batteries (LIBs)

2. INTRODUCTION Lithium ion batteries (LIBs), have been ubiquitous as power sources for portable electronic devices in our daily life for their prominent advantages.1-5 In the past decade, the fast development of electrical vehicles (EVs), hybrid electrical vehicles (HEVs) and the large-scale stationary energy storage devices pose great chance and challenge to present LIBs. As a result, it becomes focus of exploring low-cost LIBs with high energy density and cycling stability.6,7 As a matter of fact, the capacity bottleneck of LIBs, either in cathodes or in anodes, could still not meet the demanding power and energy density requirements. Therefore, great effort has been paid to explore high capacity electrode materials with desirable stability. Among many candidates, tin dioxide (SnO2) is one of the most intensively studied material owing to its significantly high specific capacity of 1494 mAh g−1.8-10 Concerning its lithium storage mechanism, it has been well established as an initial phase conversion reaction followed by a Li-Sn alloying reaction.11-13 Although the specific capacity is particularly attractive, SnO2 electrode suffers from essentially poor structural stability and electrochemical cycleability, which is mainly derived from extremely mechanical and structural instability,14,15 along with severe particle pulverization as of a ~300% volume change (expansion/shrinkage) upon 2


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lithiation/delithiation.16,17 Meanwhile, the formation of solid electrolyte interface (SEI) film, could be dynamically generated and evolved within cycling as the particle size changes, inevitably deteriorating the columbic efficiencies to some extent. Therefore, improvement on the SnO2 electrode comes into view regarding to the volume change and particle pulverization. On the other hand, SnO2 is intrinsically regarded as a n-type wide-band gap semiconductor (Eg: 3.6 eV), being indicative of the poor electronic conductivities. To obtain desirable Li storage and transport behaviors, more attention should be paid to constructing effectively conductive network to pursue the high performance of SnO2 electrode material. It is well-established that particle size and morphology of electrode materials demonstrate obvious influence on the electrochemical performances. For nano-sized electrode material, it is widely accepted that the diffusion path will be shortened, and thus the Li ion diffusion kinetics could be enhanced accordingly.18 As well, stress induced during the lithiation/delithiation process is easily to be accommodated and/or released for small size particles, which means that the tolerance to particle pulverization in nano-sized electrode materials can be expanded and the improvement of the structural stability, especially for huge volume change materials upon lithiation, eg. SnO2 could be achieved. Hydrothermal method has been proven to be a facile strategy to synthesize nano-sized metal oxides. Moreover, it is convenient to fabricate SnO2 nanoparticles with adjustable morphologies, which is of considerable interest in their application as energy storage materials.19,20 Furthermore, it has been reported that the electrochemical performances of SnO2 3



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could be enhanced through heterogeneous element doping.21 Recently, cobalt (Co) has been successfully doped into the SnO2 lattice and it was found that the doped samples exhibit a volume buffering effect and enhanced conductivity.22 In addition, the doped transition metal elements in electrode material can act as a catalytic media in corresponding electrochemical processes, reducing the reaction threshold and thus facilitating the reactions.23-27 In the field of microelectronic devices, element fluoride (F) has been widely applied to be doped into the lattice of SnO2 to generate F doped titanase oxide (FTO) as a universal substrate for various membrane deposition.28 In this regarding, it is found that insulated SnO2 turns to be partly conductive after appropriate F doping. In the meantime, it was reported that the F-doped SnO2 composite exhibited excellent long-term cycling stability and high-rate capability, which was attributed to the enhanced Li+ ion diffusion coefficient and the unique three dimensional porous structure.29 Recently, it was reported that La and F co-doping into anode material Li4Ti5O12 spinel anode effectively enhanced both the electron conductivity and the Li+ ion diffusion coefficient.30 In this work, a hydrothermal method was applied to synthesize Co and F co-doped SnO2 (Co-F/SnO2). For comparison, nano-sized SnO2 and Co-doped SnO2 (Co/SnO2) were also prepared. As a feature of this work, the Co-F/SnO2 was obtained by a facile one-pot hydrothermal method. Interestingly, it is found that the Co and F co-doped SnO2 material exhibits remarkably improved electrochemical properties.

3. EXPERIMENTAL SECTION 3.1 Material Synthesis 4


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To systematically compare the electrochemical performances of Co and F element doping, similar hydrothermal route was employed to prepare the pristine, Co-doped (Co/SnO2) and Co/F co-doped (Co-F/SnO2) samples. The synthesis of Co/SnO2 was carried out in a solvent of water starting from SnCl4·5H2O, Co(NO3)2·6H2O and NaOH. Typically, 40 mL of SnCl4·5H2O and Co(NO3)2·6H2O mixture solution was added into 40 mL of NaOH solution (56 mmol) with vigorous stirring. Next, the obtained solution was transferred into a 100 mL Teflon-lined autoclave and maintained at 180 °C for 24 h. Afterwards, the flaxen precipitate was collected, repeatedly washed with deionized water and ethanol to remove possible impurities, and then isolated by vacuum filtration. The pure SnO2 was obtained through the same route with the absence of Co(NO3)2·6H2O. To prepare Co-F/SnO2 sample, certain amount of SnCl4·5H2O, Co(BF4)2·6H2O and NaOH was homogeneously mixed followed by hydrothermally processed at 180 °C for 24 h in the same Teflon-lined autoclave. The obtained precipitation was thoroughly rinsed and dried to purify the as-prepared Co/F doped SnO2 nanoparticles. 3.2 Physical Characterizations The X-ray diffraction (XRD) patterns of the as-prepared samples were recorded on a Bruker D8 Advance diffractometer with slit size of 0.6 mm and Cu Kα radiation of λ ~ 0.15418 nm between 10° and 80° at a scan rate of 0.04°s−1. The morphologies were characterized by field emission scanning electron microscopy (FESEM, JEOL JSM-7001F), and electron dispersive spectroscopy (EDS) was applied to detect the element composition and distribution together with FESEM. The high-resolution 5



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transmission electron microscopy (HRTEM) images were recorded on a JEOL JEM 2010 instrument. Nitrogen adsorption measurements were performed on a Micromeritics ASAP 2020 adsorption analyzer. Specific surface areas were determined by the Brunaure-Emmert-Teller (BET) method. Pore volumes and sizes were estimated from pore size distribution curves from the adsorption isotherms using the Barrett-Joyner-Halenda (BJH) method. The Raman spectra were performed on a laser Raman spectrometer (RM-1000, Renishaw) with a 633 nm He-Ne laser. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Electron Corporation spectrometer with an Al Kα (1486.6 eV) radiation. 3.3 Electrochemical Characterizations Pristine SnO2, Co/SnO2, and Co-F/SnO2 powders were used as the active anode materials, respectively. Poly(vinylidene fluoride) (PVDF) and acetylene black (Super-P, TIMCAL Carbon) were used as the polymeric binder and the additive for conductivity enhancement. To prepare the anode slurry, a mixture of active material, acetylene black, and PVDF with a weight ratio of 7:2:1 was dispersed in N-methyl pyrrolidone (NMP). The obtained slurry was spread onto copper foil (thickness ~45 µm) and then dried in vacuum at 120 °C for 12 h. After that, the dried electrode was homogeneously cut into squares with area of 0.64 cm2. The electrodes were assembled into coin cells with fresh Li foil as the counter electrodes in an argon-filled glovebox. Galvanostatic cycling were collected by a tester (Neware CT-2001A, China) between 0.005 and 3.0 V (vs. Li/Li+) at a current density of 0.1 C. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) 6


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with a volume ratio of 1:1. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) profiles were obtained on an electrochemical workstation (CHI660D, Shanghai Chenhua) with a three-electrode system. The CV curves were recorded between 0.005 and 3.0 V at a scanning rate of 0.10 mV s−1. EIS measurements were performed on open circuit voltage (OCV) status over a frequency range from 100 kHz to 5 mHz.

4. RESULTS AND DISCUSSION 4.1 Structure and morphology analysis XRD profiles were collected to determine the phases of the as-prepared SnO2, Co/SnO2, and Co-F/SnO2 (Fig. 1). It is clear that all the samples are well-crystallized, and the diffraction peaks can be strictly indexed to the tetragonal rutile SnO2 (JCPDS No. 01-0657) with space group of P42/mnm (136). No other diffraction peaks could be observed as possible impurities after doping, indicating that the doping process does not change the structure of rutile SnO2. For comparison, the (101) diffractions are magnified in the right panel in Fig. 1, wherein the unambiguous (101) peak shift to higher angle could be observed as an indication of the structural contraction after doping process, especially for the Co/F co-doped SnO2 sample. This kind of structural changes are likely ascribed to the ion radius differences among the original Sn, doped Co and F. It has been reported that the ion radius of Sn4+ is 0.081 nm, which is higher than those of cobalt ions with different valences (Co2+ 0.0745 nm, Co3+ 0.061 nm and Co4+ 0.053 nm). On the other hand, the radius of F− ion is established to be smaller than that of O2- (0.133 nm versus 0.142 nm).31 Thus in turn, the peak shifts also serve 7



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as the solid evidence to verify the successful doping Co and F into the SnO2 rutile lattice as shown in Fig. 1. The corresponding lattice parameters are further calculated and listed in Table 1. Quantitatively, the lattice constants of different samples undergo an obvious shrinkage along the doping process to confirm the successful element doping of Co and Co/F. Moreover, using Scherrer’s formula the grain sizes were determined as shown in Table 1, which demonstrate the clear decrease from the pristine SnO2 to the Co/F co-doped SnO2 samples. Grain size decrease indicates the degradation of degree of order in crystal lattice, which has been widely reported in alien element doping samples.32-35 Generally, the specific surface area of a nanomaterial tends to increase along with the diminution of grain size, and in this case, increased surface atoms in ambient environment can easily contribute to the formation and agglomeration of various kinds of defects, such as ligands, unsaturated bonds, suspensions and vacancies. The small grain size of Co-F/SnO2 is supposed to be in favor of the kinetic behavior of Li+ diffusion, which will be confirmed in the following discussion. The surface morphologies of the pristine SnO2, Co/SnO2 and Co-F/SnO2 are characterized through FESEM and the results are illustrated in Fig. 2. As clearly shown in Fig. 2a, pristine SnO2 exhibits uniform submicron spheres (~ 500 nm in diameter) piled up by loosely connected nano-cubics with average diameter of ~ 80 nm. Deep insights reveal the rough surface of SnO2 nanoparticles. Significant difference could be observed for the morphologies for Co-doped sample (Fig. 2b). The main structure of micron sphere degrades to a great extent, and the original 8


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integrated hierarchical texture collapses to randomly agglomerated primary nanoparticles with average diameter of ~ 30 nm. It is noteworthy that SnO2 displays a novel self-similar fractal structure after Co and F co-doping (Fig. 2c). The major structure of submicron sized sphere has been perfectly retained with slightly increased diameter of ~ 800 nm, which is composed of dense-packed nano-units with a homogeneous size of ~ 100 nm. Within each nano-unit, smaller nanoparticles (~ 20 nm) agglomerate together, which benefits not only for the kinetics of Li+ diffusion but also for accommodating the strain and stress during electrochemical cycling to maintain the structure stability of rutile SnO2. The unique self-similar fractal texture with robust major structure and small primary particle size is anticipated to exhibit excellent electrochemical performances as discussed later. The energy dispersive X-ray analysis (EDAX) spectra and element mapping images of the Co-F/SnO2 samples are demonstrated in Fig. S1 (in Supporting Information), which not only proves the existence of Co and F, but also indicates a homogeneous distribution of the doped elements. HRTEM was applied to investigate the microstructures of the as-prepared samples and the images are compared in Fig. 3. As clearly seen in Fig. 3a, pure SnO2 demonstrates well-crystallized structure, with distinct fringes extending to the boundary of the particle. The fringe spacing is carefully determined to be 0.336 nm, coinciding with the (110) plane of rutile SnO2. After Co-doping and Co/F co-doping, the corresponding particles display similar crystallized structure, with decreased stripe spacing of 0.334 and 0.331 nm, plausibly in agreement with the slight diffraction peak 9



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shift shown in XRD analysis and proving the successful element doping of Co and F. Additionally, the particle size prominently decreases after Co/F co-doping (Fig. 3c), compared with pristine and Co-doped particle. And interestingly, the Co/F co-doped SnO2 sample exhibits a novel rice-shaped structure (Fig, 3c), which is homogeneously stacked together to form a unique factual texture (Fig. 2c) and should be helpful upon the lithiation/delithiation process. The microstructure information and textural properties of the as-prepared three samples were also compared. Fig. S2 (in Supporting Information) depicts the N2 adsorption/desorption isotherms and the corresponding pore size distributions, which are determined by the Barrett-Joyner-Halenda (BJH) model (inset of Fig. S2). The isotherms are of type IV adsorption and exhibit H4 hysteresis characterized by a well-defined and steep N2 uptake step in a wide relative pressure (P/P0) range of 0.60 – 0.90, which is the result of capillary condensation in mesoporous materials. The BET specific surface areas of pure SnO2, Co/SnO2, and Co-F/SnO2 are determined to be 123, 139, and 153 m2 g−1, respectively, indicating an increasing tendency after element doping. The enlarged specific surface areas of doped materials, particularly for the Co/F co-doped sample, will facilitate the Li+ diffusion kinetics and agrees with the grain size analysis in Fig. 1. Additionally, the three samples exhibit the BJH pore diameters at 8.7, 8.9, and 8.6 nm (inset of Fig. S2), and the corresponding pore volumes are determined to be 0.030, 0.037, and 0.041 cm3. The above mentioned parameters are summarized in Table 2. The notably increased BET surface area and pore volume for Co-F/SnO2 will facilitate the electrode kinetics during cycling. 10


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As a universally nondestructive testing strategy, Raman spectroscopy is generally applied to detect various states including solid, liquid and gas, which is of great significance for accurate determination of the materials’ structure and composition. Raman spectra of the as-prepared three samples are collected and the profiles are demonstrated in Fig. S3. As shown in Fig. S3, four prominent vibration peaks at 472 cm-1, 573 cm-1, 630 cm-1 and 774 cm-1 could be observed from the pristine SnO2, which could be assigned to the characteristic vibrations of SnO2 lattice as Eg, P, A1g and B2g modes.36 After Co doping and Co/F co-doping processes, the Raman curves of Co/SnO2 and Co-F/SnO2 exhibit no changes on the peak position, indicating the characteristic rutile Raman vibrations. However, it could be observed that the peak intensity of P band remarkably decrease in doping and co-doping processes (Figs. S3 b-c), compared with that of A1g band. It has been reported that the peak area ratio of P and A1g band is related with the particle size of nano-sized SnO2 materials.37 The continuous increase of A1g/P with element doping, as demonstrated in Table 3, may possibly correspond with the enlarged particle size as revealed in Fig. 2. Additionally, the fitting parameters including band position and FWHM (full width at half maximum) values of P bands for the three samples are also compared in Table 3. It is apparent that the peak red-shifts and broadens asymmetrically with doping, indicating the lowering of degree of order with element doping processes.38,39 It should be noted that the lowered degree of order is generally associated with increased defects. Corresponding relationship among crystallinity (high and low defect density), FWHM and peak shift have been systematically revealed in Ref. 40. In this case, the total 11



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defect density (lowered degree of order) is increased prominently in Co/F co-doping process, as supported by the grain size and vibration parameter analysis in Table 1 and Table 3. And interestingly, element F doping seems playing a more crucial role, as indicated in Table 3, where the FWHM and peak shift of P band change most significantly in the as-prepared three samples. X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface electronic states and the compositions of the doped SnO2 samples. The XPS surveys confirm the presence of Co element in both Co-SnO2 and Co/F-SnO2 samples, and the existence of F can be unambiguously found in the co-doped sample (688.8 eV for F 1s signal), as highlighted by the ellipse circle (Fig. 4a). In detail, the inset high-resolution Co 2p spectrum for Co-SnO2 exhibits two signals at 780.4 eV (Co 2p3/2) and 796.4 eV (Co 2p1/2), which could be ascribed to the trivalent and bivalent states of Co element. Different situation appears for Co/F co-doped SnO2, in which the signal corresponding to Co2+ seems almost absent in the XPS profile, while that for Co3+ remains unchanged. The valence state moves towards a higher valence, indicating chemical environment change of Co element after F doping. A recent study revealed that the roles played by the different valence states of cobalt in its oxides are different, and higher state of cobalt oxide exhibits higher conductivity,41-43 which is in favor of improving the battery performances as shown later. Subsequently, more attention has been paid to the XPS profiles of oxygen element as displayed in Fig. 4b. The normalized O 1s of pristine SnO2 (black line) demonstrates a prominent binding energy at 530.1 eV, which could be undoubtedly 12


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designated to the contribution of lattice oxygen. After Co doping (red line), no obvious difference could be observed. Nevertheless, the corresponding peak experiences sharp decrease after Co/F co-doping (blue line), though the binding energy keep unchanged. As discussed in the above Raman analysis, element F doping induced the augmentation of total defect density. Combined the XPS results of lattice oxygen herein, the defect could be most possibly identified as oxygen vacancy, which stemmed from the charge balance in the F doping process. To further verify this assumption, the amount of Co(BF4)2·6H2O precursor was doubled to provide more F source and the similar XPS test was performed (shown as the green line in the same figure). Unsurprisingly, the normalized peak of O 1s further lowers its intensity, implying the continuing increase of oxygen vacancy, which is possibly benefit for the enhancement of electrochemical properties for electrode materials. 44 4.2 Electrochemical performances The initial galvanostatic profiles for the three samples are collected between 3.0 and 0.005 V at 0.1 C. As a typical metal oxide anode material, it has been widely accepted that SnO2 experiences the following reactions in lithiation/delithiation process.11-13 SnO2 + 4Li+ + 4e- → 2Li2O + Sn

(1)

Sn + xLi+ + xe- ↔ LixSn (0 ≤ x ≤ 4.4）

(2)

As clearly displayed in Fig. 5a, similar discharge/charge profiles have been exhibited, corresponding to the reactions in Equations 1-2. Especially, the metal Sn undergoes two separate oxidation steps, from Sn0 to Sn2+, then Sn2+ to Sn4+, as 13



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highlighted in the circles in Fig. 5a and 5b. The samples of pristine, Co-doped and Co/F co-doped SnO2 samples deliver initial discharge capacities of 1214.9, 1597.3 and 2397.2 mA h g-1 at 0.1 C, respectively. It could be found that the discharge capacities after doping are extraordinarily high, particularly for the Co/F co-doped sample, which could be partly attributed to the increased BET specific surface areas after doping processes. In addition to the formation of solid-electrolyte interphase (SEI) film in the initial discharge, the doped F is supposed to react with inserted Li ions to generate LiF, which homogeneously distributed in the SnO2 matrix in the following cycles. The resultant LiF in bulk structure could contribute not only to an extra initial discharge capacity, but also to the structural stability as an inactive inorganic pillars in repeated cycling. On the other hand, initial columbic efficiencies of 56.9%, 58.1% and 63.1% are demonstrated for the pristine, Co-doped and Co/F co-doped SnO2. It is well known that the initial columbic efficiency is of great significance for an anode, the increased columbic efficiency herein, indicating the enhancement of reversibility. To find subtle differences among the initial charge profiles of the three as-prepared samples, the related curves are clearly demonstrated in Fig. 5b. It is clear that the Co-doped SnO2 remains a similar shape with that of pristine material, while for Co and F co-doped sample, the charge profile changes prominently. For specific in the magnified insets of this figure, it can be seen unambiguously that more capacity is delivered after Co and F co-doping into SnO2 electrode above 2.0 V, probably coming from the improvement in the electronic conductivity.45 In this work, the extra capacity 14


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of the Co/F doped SnO2 electrode above 2.0 V, is likely originated from F doping to form the good electronic conductors as FTO (F-doped SnO2). Lithium storage properties of the as-prepared three samples were tested up to one hundred cycles over the potential range from 0.005 V to 3.0 V (Fig. 5c). The capacity retention of the initial 50 cycles at a constant current density of 0.1 C is displayed in Fig. 5c, from which the pristine SnO2 electrode undergoes fast capacity decay with cycling. However, the doped samples exhibit greatly improved capacity retention compared with pristine counterpart. Remarkably, the Co-F/SnO2 electrode still remains a high specific charge capacity of 820 mA h g-1 after 50 cycles, much higher than those of Co-doped and pristine SnO2 (480 and 118 mA h g-1, respectively). It should be noted that the charge capacity of Co-F/SnO2 electrode keeps pretty stable after the initial 20 cycles, with only a capacity decrease of 40 mA h g -1 within the following 30 cycles. After 50 cycles, the same cells were further evaluated on rate capability as shown in Fig. 5d. It can be seen that the Co and F co-doped electrode demonstrates a best rate performance. A high and stable charge capacity of 664 mA h g-1 can still be obtained for Co-F/SnO2 electrode at 5 C, which is almost two times that of commercial graphite anode. When the current rate goes back to 0.1 C after high current cycles, a stable high capacity of 786 mA h g-1 can still be maintained, indicating good structural stability of Co and F co-doped SnO2 material, which could be possibly attributed to the reduced grain size and the pillaring effect of inactive LiF formed in the initial discharge process. To identify the electrode reactions during charge/discharge for the three samples, 15



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CV measurements were performed and the plots are presented in Figs. 6(a)-(c), in which similar electrochemical redox process could be observed. In the initial discharge, the CV profiles for all the electrodes show one apparent reduction peak at around ~ 0.95 V, which could be derived from the Li2O formation when the SnO2 nanomaterials react with Li+, as described in Equation 1. The following reduction peaks at approximate ~ 0.24 V originate from the alloying reaction of metal Sn with extra Li+ in deep lithiation state, coinciding with the reaction described in Equation 2. In the opposite delithiation process, the electrodes undergo prominent oxidation reactions at ~ 0.5 V, which could be ascribed to the dealloying of LixSn. The other two anodic peaks at ~ 1.25 and ~ 1.83 V could be attributed to the multistep oxidations of partial Sn to Sn2+ (Equation 3), and then to Sn4+ (Equation 4).46 Sn + Li2O → SnO + 2Li+ + 2e−

(3)

SnO + Li2O → SnO2 + 2Li+ + 2e−

(4)

To further understand the CV plots in Figs. 6(a)-(c), the potential difference of the alloying/dealloying redox (D-value) and the FWHM of the dealloying peaks for the three electrodes are compared in Fig. 6d. Concerning the electrode polarization, as indicated by the D-values of alloying/dealloying redox (top graph of Fig. 6d), it slightly decreases after Co-doping in the initial cycle, and experiences no prominent variation for both the pristine and Co-doped electrodes in cycling. After Co and F co-doping, the corresponding electrode exhibits much smaller polarization in both the initial cycle and in the following several cycles. On the other hand, the FWHM values of dealloying reaction, as an indicator of electrode reaction kinetic, show similar 16


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tendency for the three electrodes (bottom plots of Fig. 6d), increases mildly with cycling. Nevertheless, it could be clearly observed that the doping effectively accelerates the electrode reaction kinetic. Especially, the Co and F co-doped electrode demonstrates the lowest value, for each cycling in the initial cycles. 4.3 Mechanism exploration To clarify the origin of superior electrode kinetics revealed in Fig. 6d, Nyquist plots of the three electrodes were measured at open circuit voltage (OCV) over a frequency range from 100 kHz to 5 mHz. As shown in Fig. 7a, the interception at the Zreal axis at high frequency corresponds to the Ohmic resistance (Rs), which represents the total resistance of the electrolyte, separator, and electrical contacts. The semicircle in the middle frequency range indicates the charge transfer resistance (Rct), and the slope

region

in

the

low-frequency

range

represents

the

Warburg

impedance. Quantitatively, the fitted resistances through the equivalent circuit (Fig. 7a) are listed in Table 4.47 It can be clearly observed that the Rs values are similar among the three electrodes. Nevertheless, the charge transfer resistances and Warburg impedances are distinctly different, which exhibit minimum values of 131.25 and 241.62 Ω for Co-F/SnO2 electrode. Undoubtedly, this could be attributed to the co-doping of Co and F into the SnO2 nanocrystals, which not only decrease the charge transfer resistance between the solid and liquid interface, but also effectively accelerate the Li+ diffusion in bulk material. To elucidate the effect of Co and F co-doping on Li ion diffusion in the electrode, the Li+ diffusion coefficients (DLi) of the three samples were further determined from 17



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the Nyquist plots (Fig. 7a) according to the following formulas: 𝑍𝑟𝑒 = 𝐴 − 𝜎𝜔 −1/2 𝑅2 𝑇 2

𝐷𝐿𝑖 = 2𝐴2 𝑛4𝐹4 𝐶 2 𝜎2 𝐿𝑖

(5) (6)

where Zre is the real component of the impedance, which has a linear relationship with ω−1/2 (ω = 2πf); σ is the Warburg factor and obtained from the slope of the Zre − ω−1/2 line as illustrated in Fig. 7a; DLi is calculated by substituting the gas constant (R), the absolute temperature (T), electrode area (A), the number of electrons transferred in the half-reaction for the redox couple (n), the Faraday’s constant (F), the concentration of Li+ in solid (CLi = 4.213 × 10−2 mol cm−3) and σ. Fig. 7b. displays the Zre − ω−1/2 plots of the pristine SnO2, Co/SnO2 and Co-F/SnO2 electrodes. The σ values are calculated to be 31.9, 26.5 and 16.3 Ω rad1/2 s−1/2, for the electrodes of pristine SnO2, Co/SnO2 and Co-F/SnO2, respectively. And the DLi for the Co-F/SnO2 electrode is determined to be 4.15 ×10−13 cm2 s−1, about two times higher than that for the Co/SnO2 electrode (1.57 × 10−13 cm2 s−1) and three times higher than that for the pure SnO2 electrode (1.08 × 10−13 cm2 s−1). The obtained DLi values are comparable to those for other alloy- and conversion-type electrode materials.48 It is clear that Li+ has a remarkable higher diffusion kinetic in the Co-F/SnO2 electrode compared with the other two electrodes, and verifies that Co and F co-doping effectively enhances the Li+ diffusion kinetics in SnO2.

5. CONCLUSION In summary, a composite of Co and F co-doped SnO2 with a unique rice-shaped self-similar fractal structure was synthesized using Co(BF4)2·6H2O as the source of 18


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Co and F through a facile one-pot hydrothermal process. Physical characterizations proved the successfully co-doping of Co and F, which not only decreases the grain size and the degree of order, but also increases the BET surface area and introduces a certain content of oxygen deficiency. The as-prepared Co-F/SnO2 exhibits greatly improved electrochemical performances, delivering a high specific capacity of 820 mA h g−1 after 50 cycles. It was also found that the co-doped electrode demonstrates prominently enhanced Li+ kinetics behavior, with a high Li+ diffusion coefficient of 4.15 ×10−13 cm2 s−1. As illustrated in Fig. 8, the advantages of Co/F co-doping could be summarized as follows: (1) The reduced grain size of Co-F/SnO2, originated from the co-doping process, could reduce the Li ion diffusion distance and release the accumulated stress to afford the enhanced structure reversibility as well; (2) The generated oxygen vacancy will benefit for the Li storage and transport; (3) The doped F ions play a role not only in enhancing the electron conductivity of composite material, but also acting as a stable structure framework in repeated cycling with the form of LiF. To sum up, the strategy of Co and F co-doping proposed in this work effectively enhanced the structural reversibility and electrochemical stability of SnO2. This contribution, undoubtedly, will provide significant guidance for designing the SnO2 material itself and the community of LIBs as well with higher structural and electrochemical stability. It is also being expected to exciting immediate interest in a very broad research area, such as electrode fabrication, material functionalization and hybrid material engineering. 19



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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figure S1 ： The EDAX spectrum and element mapping images of Co-F/SnO2. Morphology of selected area (a), EDAX spectrum (b), Co and F element mapping images (c-d), and integrated mapping signals of the above two elements (e). Figure S2：Adsorption-desorption isotherms measured at 77 K under N2 atmosphere for the as-prepared samples (a) pure SnO2, (b) Co/SnO2, and (c) Co-F/SnO2 samples. The inset is the BJH pore-size distribution of the corresponding materials. Figure S3：Raman spectra of the as-prepared SnO2 a), Co/SnO2 b) and Co-F/SnO2 c).

AUTHOR INFORMATION Corresponding Author *Tel: +86 (0371)23881602, e-mail: [email protected] (Ying Bai). ORCID Ying Bai: 0000-0001-7835-7067 Author Contributions D.P., N.W. contributed equally. ACKNOWLEDGMENT Authors Du Pan, Ning Wan, Yong Ren, Weifeng Zhang and Ying Bai received funding from the National Natural Science Foundation of China (50902044, 51672069), the 20


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863 Program of China (2015AA034201), the Program for Science and Technology Innovation Talents in Universities of Henan Province (16HASTIT042) and the International Cooperation Project of Science and Technology Department of Henan Province (162102410014). Authors Yuesheng Wang, Yong-Sheng Hu and Ying Bai received funding from the National Natural Science Foundation of China (51472268). Prof. X. Lu thanks the funding resource from the State Key Laboratory of Organic-Inorganic Composites (oic-201701011). Notes The authors declare no competing financial interest.

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27



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Table 1 Lattice parameters and grain sizes of as-prepared SnO2, Co/SnO2 and Co-F/SnO2 samples. Sample

a/Å

c/Å

Grain size/nm

SnO2

4.7655

3.1843

34.0

Co/SnO2

4.7451

3.1839

32.1

Co-F/SnO2

4.7278

3.1761

19.5

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Table 2 Specific surface area, pore volume and average pore size comparison of the three as-prepared samples. BET/m2 g-1

Pore volume/cm3

Average pore size/nm

SnO2

123

0.030

8.7

Co/SnO2

139

0.037

8.9

Co-F/SnO2

153

0.041

8.6

Sample

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Table 3 Peak parameters for the Raman spectra of as-prepared samples. A1g/P

Peak centerP/cm-1 a

FWHMP/cm-1 b

SnO2

0.52

576.6

60.9

Co/SnO2

0.85

570.2

66.5

Co-F/SnO2

1.88

560.5

76.1

Sample

a: peak center of P band;

b: FWHM of P band.

30


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Table 4 Fitting results of EIS data for pure SnO2, Co/SnO2, and Co-F/SnO2. Rs/Ω

Rct/Ω

Ws/Ω

Pure SnO2

3.09

217.86

318.13

Co/SnO2

2.47

181.73

285.77

Co-F/SnO2

2.12

131.25

241.62

Sample

31



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Figure Captions Fig. 1. XRD patterns of the as-prepared samples. a) Pristine SnO2, b) Co/SnO2 and c) Co-F/SnO2 nanocrystals. The prominent diffraction peaks of the three samples can be identified to tetragonal SnO2 (JCPDS card No. 01-0657) (left main figure). Right box magnifies the selected (101) diffraction peak to distinguish the position difference. Fig. 2. Surface morphologies of the as-prepared samples. SnO2 a1, a2), Co/SnO2 b1, b2) and Co-F/SnO2 c1, c2). Fig. 3. Microstructures of the as-prepared SnO2 a), Co/SnO2 b) and Co-F/SnO2 c,d). Fig. 4. XPS survy of Co/SnO2 and Co-F/SnO2 (inset shows Co 2p signals) a) and the lattice O profile comparison among different samples b). The asterisk indicates the Co/F co-doped sample with even higher F content than that used in this work. Fig. 5. Initial charge-discharge curves of the as-prepared samples between 3.0 and 0.005 V a) and the highlighted charge profile above 2.0 V with the same capacity gap (horizontal region) of 100 mA h g-1 b). Galvonostatic cycling performances at 0.1 C c) and rate capabilities d) of the as-prepared samples. (“dis” and “cha” represent the states of discharge and charge upon cycling) Fig. 6. CV curves of the as-prepared pure SnO2 a), Co/SnO2 b) and Co-F/SnO2 c) electrodes at a scanning rate of 0.10 mV s−1 and quantitative analysis of the electrode reactions d). Fig. 7. Nyquist plots of pure SnO2, Co/SnO2, and Co-F/SnO2 (inset shows the equivalent circuit) obtained at OCV a) and the Zre − ω−1/2 plots for the corresponding three electrodes b). Fig. 8. Structural schematic illustrations of pure and Co/F co-doped SnO2 in cycling. 32


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Fig. 1. XRD patterns of the as-prepared samples. a) Pristine SnO2, b) Co/SnO2 and c) Co-F/SnO2 nanocrystals. The prominent diffraction peaks of the three samples can be identified to tetragonal SnO2 (JCPDS card No. 01-0657) (left main figure). Right box magnifies the selected (101) diffraction peak to distinguish the position difference.

33



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Fig. 2. Surface morphologies of the as-prepared samples. SnO2 a1, a2), Co/SnO2 b1, b2) and Co-F/SnO2 c1, c2).

34


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Fig. 3. Microstructures of the as-prepared SnO2 a), Co/SnO2 b) and Co-F/SnO2 c,d).

35



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Fig. 4. XPS survey of Co/SnO2 and Co-F/SnO2 (inset shows Co 2p signals) a) and the lattice O profile comparison among different samples b). The asterisk indicates the Co/F co-doped sample with even higher F content than that used in this work.

36


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Fig. 5. Initial charge-discharge curves of the as-prepared samples between 3.0 and 0.005 V a) and the highlighted charge profile above 2.0 V with the same capacity gap (horizontal region) of 100 mA h g-1 b). Galvonostatic cycling performances at 0.1 C c) and rate capabilities d) of the as-prepared samples. (“dis” and “cha” represent the states of discharge and charge upon cycling)

37



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Fig. 6. CV curves of the as-prepared pure SnO2 a), Co/SnO2 b) and Co-F/SnO2 c) electrodes at a scanning rate of 0.10 mV s−1 and quantitative analysis of the electrode reactions d).

38


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Fig. 7. Nyquist plots of pure SnO2, Co/SnO2, and Co-F/SnO2 (inset shows the equivalent circuit) obtained at OCV a) and the Zre − ω−1/2 plots for the corresponding three electrodes b).

39



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Fig. 8. Structural schematic illustrations of pure and Co/F co-doped SnO2 in cycling.

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Table of contents

41


Enhanced Structural and Electrochemical Stability of Self-Similar Rice

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