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Titanosilicate Derived SiO2/TiO2@C Nanosheets with Highly Distributed TiO2 Nanoparticles in SiO2 Matrix as Robust Lithium Ion Battery Anode li zhang, Xin Gu, Chunliu Yan, Shuo Zhang, Liangjun Li, Yingjie Jin, Shanlin Zhao, Haiyan Wang, and Xuebo Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16238 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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ACS Applied Materials & Interfaces

Titanosilicate Derived SiO2/TiO2@C Nanosheets with Highly Distributed TiO2 Nanoparticles in SiO2 Matrix as Robust Lithium Ion Battery Anode Li Zhang,† Xin Gu,*† Chunliu Yan,† Shuo Zhang,† Liangjun Li,† Yingjie Jin,ǂ Shanlin Zhao,ǂ Haiyan Wang,ǂ and Xuebo Zhao*†‡ † State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (East China), Qingdao, 266580, P. R. China. ‡ Institute of New Energy, China University of Petroleum (East China), Qingdao, 266580, P. R. China. ǂ College of Petrochemical and Technology, Liaoning Shihua University, Fushun, 113001, P.R. China. ABSTRACT: Carbon coated SiO2/TiO2 (SiO2/TiO2@C) nanosheets consist of TiO2 nanoparticles uniformly embedded in SiO2 matrix and carbon coating layer are fabricated by using acidified titanosilicate JDF-L1 nanosheets as template and precursor. SiO2/TiO2@C has unique structural features of sheet-like nanostructure, ultrafine TiO2 nanoparticles distributed in SiO2 matrix and carbon coating, which can expedite ion diffusion and electron transfer, and relieve volume expansion efficiently, and thus the synergetic combination of these advantages significantly enhance their Li storage capability. As anode of LIBs, SiO2/TiO2@C nanosheets exhibit a high

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capacity of 998 mAh g-1 at 100 mA g-1 after 100 cycles. Moreover, an ultrahigh capacity of 410 mAh g-1 retains at 2000 mA g-1 after 400 cycles. A mixed reaction mechanism of capacitance and diffusion-controlled intercalation is revealed by qualitative and quantitative analysis. KEYWORDS: SiO2 matrix, TiO2 nanoparticles, Carbon coating, Nanosheets, Lithium-ion batteries INTRODUCTION Lithium-ion batteries (LIBs) have come into extensive notice due to its high power density and energy density1,2 and they have been predominant in the fields of mobile power and electric vehicles.3 To resolve the problem of low cruising mileage of electric vehicles, LIBs with high power output and energy density are in required. However, the relatively low theoretical capacity of commercial graphite anode (372 mAh g-1) and poor electrochemical performance at high currents hinder its practical application in electric vehicles.4 Therefore, new and well-tailored anode materials with improved electrochemical performance have been developed, such as Sn,5,6 Ge,7,8 Si,9 SiO210 and MOx (M = Fe, CO, Ni, Ti, V and et al).11,12 Among these anode materials, SiO2 is regarded as a promising candidate, because of its potentially high theoretical capacity (1965 mAh g-1),13 low discharge potential and abundant resource.14 Nevertheless, the dielectric nature and volume expansion of SiO2 anode usually result in poor rate capability and short service life. In order to solve the above problems, several approaches have been proposed. One widely adopted approach is to design SiO2 anode with nanostructures to accommodate volume change, like SiO2 nanocubes15 multi-shell hollow SiO2 nanoparticles,16 mesoporous SiO2 nanoplates,17 hollow SiO2 spheres18 and so on. The nanostructured SiO2 can help to short the diffusion path of Li+ and buffer the release of stress.19 Another approach is to


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construct carbon composite to improve the mechanical property and electronic conductivity,

including

silica/carbon

composite,20-22

free-standing

SiO2/carbon

nanofibers,23 SiO2/flake graphite nanocomposite24 and SiO2/[email protected] For example, Fu et al. reported N-doped ordered mesoporous carbon/silica composite exhibited a capacity of 800 mAh g-1 at 200 mA g-1 after 20 cycles, which is better than pure SiO2. The superior performance is confirmed to stem from nitrogen-doping, conductive carbon and unique mesoporous nanostructure.26 Most recently, the strategy of combining SiO2 with TiO2 has attracted much attention, due to their synergistic effect in which TiO2 can help to maintain structural integrity for its low strain property and SiO2 provides a high specific capacity. For example, carbon coated TiO2/SiO2 nanocomposites synthesized by a hydrothermal approach displayed a stable capacity of 233 mAh g-1 at 150 mA g-1.27 TiO2/SiO2/C film prepared by electrospinning approach delivered a value of 380 mAh g-1 at 200 mA g-1 after 700 cycles. 28 SiO2/TiO2/polypyrrole nanospheres synthesized by a hard templating method retained a capacity of 433 mAh g-1 after 50 cycles.29 However, the content of SiO2 in reported SiO2/TiO2 composites is usually below 35 wt. %, which is hard to meet the demands of high-energy LIBs.30,31 And these synthetic routes for SiO2/TiO2 composites cannot realize the highly distribution of SiO2 and TiO2 components due to the inherent heterogeneity of raw materials, and thus the synergistic effect between SiO2 and TiO2 cannot be optimized to the maximum extent. In this work, acidified titanosilicate JDF-L1 nanosheets with uniformly distributed Si and Ti elements at the atomic scale are employed as the template and precursor.32 The resulting carbon coated SiO2/TiO2 (SiO2/TiO2@C) nanosheets consist of highly distributed TiO2 nanoparticles in SiO2 matrix and carbon coating layer, which can suppress volume


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deformation of SiO2 to the maximum extent. Moreover, the full contact between SiO2 and TiO2 brings about abundant grain boundaries, which can further facilitate electron transfer and Li+ diffusion. As a result, SiO2/TiO2@C nanosheets exhibit an ultrahigh Li storage capacity of 998 mA h g-1 at 100 mA g-1 after 100 cycles. Even at 2000 mA g-1, an extremely stable cycling with a high capacity of 410 mAh g−1 can be achieved up to 400 cycles. Moreover, qualitative and quantitative analysis indicate a mixed reaction process of capacitance and diffusion-controlled intercalation for Li storage.

EXPERIMENTAL SECTION Material Synthesis. The synthetic strategies for SiO2/TiO2@C nanosheets are as follows. Firstly, titanosilicate JDF-L1 nanosheets were prepared at 200 °C for 24 h by a seeded method.

30,33

Secondly, the as-prepared JDF-L1 nanosheets were treated by a

hydrochloric aqueous solution (0.1 M) for 24 h to produce H-JDF-L1 (acidified JDF-L1) nanosheets. Finally, H-JDF-L1 and glucose with a mass ratio of 2:1 were grinded in mortar for 10 min and subsequently calcined at 600 °C in Ar flow for 3 h to achieve the SiO2/TiO2@C nanosheets. For comparison, SiO2/TiO2 nanosheets were prepared by annealing H-JDF-L1 at 600 °C in Ar flow for 3 h. Material Characterizations. The structure, morphology and composition of prepared samples were measured by using X-ray diffractometer (D8 Adv.), scanning electron microscopy (JSM-7500F), transmission electron microscopy (JEM-2100F), energydispersive X-ray spectroscopy (EDS), Raman spectra (NEXUS 670, 514 nm) and X-ray photoelectron spectroscopy (250X, ESCALAB). The C content in SiO2/TiO2@C was


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obtained by thermogravimetric analysis (SDTA851, Mettler Toledo TGA) at a ramp rate of 10 °C min-1 in air. Electrochemical Measurements. Li storage properties of SiO2/TiO2@C and SiO2/TiO2 nanosheets were achieved by using CR2032 coin cells. Active materials, super P and CMC (sodium carboxymethyl cellulose, MW=250 000) with a mass ratio of 8:1:1 were mixed to make working electrodes with an average mass loading of 1.0 mg cm-2. For the assembling of coin cells, lithium foil, polypropylene film (Celgard 2400) and 1 M LiPF6 in EC/DMC/EMC (1:1:1, volume ratio) were employed as counter, separator and electrolyte, respectively. The electrochemical properties were measured in the range of 0.01–3 V (vs. Li/Li+) on a battery test system (LAND CT-2001A). The cyclic voltammetry (CV) measurements and electrochemical impedance spectroscopy (EIS) were carried out on an electrochemistry workstation (CHI760E, China). EIS measurements were carried out using a perturbation voltage of 5 mV in a range of 105 Hz and 10-2 Hz. CV curves were measured over the range from 0.1 to 1.0 mV s-1 between 0.01 and 3 V (vs. Li/Li+).

RESULTS AND DISCUSSION


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Figure 1. Schematic illustration for fabricating SiO2/TiO2@C nanosheets.

The synthetic route for SiO2/TiO2@C nanosheets is illustrated schematically in Figure 1. First, titanosilicate JDF-L1 with uniformly distributed Si and Ti atoms are successfully synthesized by a seeded secondary growth method, which can be revealed by XRD pattern (Figure S1a) that corresponds well with the standard card (PDF#54-0227).34 SEM image (Figure S2a) shows that JDF-L1 is composed of uniform nanosheets. Second, JDF-L1 nanosheets are transformed to acidized JDF-L1 (H-JDF-L1) nanosheets by acid treatment. In this stage, sodium ions in JDF-L1 are replaced by H+, which is crucial for the subsequent synthesis of SiO2/TiO2@C. The inherited sheet-like morphology of H-JDF-L1 is demonstrated by the SEM image (Figure S2b). The changed phase and composition of HJDF-L1 are verified by the XRD pattern (Figure S1b) and EDS spectrum (Figure S3). As shown in Figure S1, several diffraction peaks of H-JDF-L1 shift towards high angle or low angle, compared to those of JDF-L1. This phenomenon may be correlated with the different


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ionic radiuses of H+ and Na+.35 Moreover, the broadening of diffraction peaks indicates the formation of stacking disorder after acid treatment. Herein, the two weak peaks at about 15-20° of H-JDF-L1 can be corresponded to the peaks at ~17° and ~21° of JDF-L1, as marked in Figure S1. The EDS result reveals that very few sodium ions are still existed in H-JDF-L1, thus it can be ignored in subsequent studies. Finally, carbon coated SiO2/TiO2@C nanosheets are fabricated by annealing a mixture of H-JDF-L1 nanosheets and glucose. In this annealing step, a relatively low temperature of 600 °C was applied to produce amorphous SiO2 with weak bonds, according to the thermal evolution of JDF-L1, which is beneficial for the lithium storage properties.36

Figure 2. (a) XRD patterns of SiO2/TiO2@C and SiO2/TiO2 nanosheets. (b) EDS spectrum of SiO2/TiO2@C nanosheets. (c) Raman spectra of SiO2/TiO2@C and SiO2/TiO2 nanosheets. (d) TGA curve of SiO2/TiO2@C nanosheets in air.


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As shown in the XRD patterns of SiO2/TiO2@C and SiO2/TiO2 (Figure 2a), several typical diffraction peaks centered at 25.4°, 37.9°, 48.0° and 54.9° can be observed, corresponding to the (101), (004), (200) and (211) planes of anatase TiO2 (PDF#89-4921),37 respectively. A weak and broad peak at 21.6° confirms the successful synthesis of amorphous SiO2, because of the relatively low treating temperature of 600 oC.38 EDS spectrum of SiO2/TiO2@C (Figure 2b) indicates that it is composed of four elements, including Si, Ti, C and O (Cu element detected in EDS spectrum is the signal of the base copper). The calculated atomic ratio of Si/Ti is well consistent with that in H-JDF-L1 precursor (Figure S3). As presented in the Raman spectrum of SiO2/TiO2 (Figure 2c), the peaks located at 145, 196, 394, 508, and 621 cm-1 are associated with anatase TiO2.39 As for SiO2/TiO2@C, only two peaks located at 1326 and 1583 cm-1 were detected, corresponding to the disordered carbon (D-band) and ordered graphitic carbon (G-band).40 The intensity of D-band is higher than that of G-band (ID/IG=1.03), indicating the disordered carbon structure in SiO2/TiO2@C, which can provide extra reaction sites for Li+ storage and facilitate the diffusion of Li+.41 It is worth mentioning that the peaks of TiO2 were not detected in SiO2/TiO2@C sample, which may be caused by the masking effect of carbon coating layer. To determine the content of carbon in SiO2/TiO2@C, thermogravimetric analysis (TGA) was conducted. As shown in Figure 2d, two weight losses are observed in the TGA profile. The weight loss before 300 oC is attributed to the removal of adsorbed water (3.5 wt. %), whereas the weight loss after 300 oC is the character of carbon decomposition. Herein, the content of carbon in SiO2/TiO2@C is calculated to


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be 7.9 wt. %. The carbon content is relatively low, so its contribution for the capacity can be negligible.

Figure 3. (a) FESEM image of SiO2/TiO2 nanosheets. (b) FESEM image of SiO2/TiO2@C nanosheets, (c) TEM image of SiO2/TiO2@C nanosheets, and (d) HRTEM image of SiO2/TiO2@C nanosheets. (e-h) TEM image and corresponding elemental mapping images of SiO2/TiO2@C nanosheets.

The morphologies and microstructures of SiO2/TiO2@C and SiO2/TiO2 were investigated by SEM, TEM and HRTEM measurements. As shown in Figure 3a and 3b, SiO2/TiO2 and SiO2/TiO2@C exhibit the same sheet-like nanostructure as H-JDF-L1 precursor (Figure S2b). This result indicates that there is almost no structural destruction and collapse during the processes of structure transformation and carbonization of glucose at high temperature. The apparent contrast in TEM image (Figure 3c and Figure 4Sa) suggests that large amounts of ultrafine nanoparticles are uniformly distributed in the SiO2/TiO2@C nanosheet, which can be attributed to the minor component of TiO2. In the HRTEM image


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of SiO2/TiO2@C (Figure 3d and Figure 4Sb), the lattice fringes with spacings of 0.357 nm and 0.231 nm can be indexed as the (101) and (004) planes of anatase TiO2. This result indicates the successful introduction of TiO2 nanoparticles into SiO2 matrix. In addition, a carbon coating layer with a thickness of about 12 nm is detected, demonstrating the formation of carbon coated SiO2/TiO2 nanostructure. The elemental distribution information for SiO2/TiO2@C (Figure 3e-h) further illustrate the highly uniform distribution of Ti and Si elements in SiO2/TiO2@C nanosheets. The highly distributed TiO2 in SiO2 matrix will be beneficial for the inhibition of volume expansion of SiO2, and thus improve the cycling stability. The relatively nonuniform distribution of C element can be attributed to the physical blending of raw materials.


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Figure 4. High-resolution XPS spectra of (a) Si 2p, (b) Ti 2p, (c) C 1s and (d) O 1s of SiO2/TiO2@C.

The surface characteristics of SiO2/TiO2@C was investigated by XPS measurement. The XPS survey spectrum of SiO2/TiO2@C (Figure S5) reveals the chemical composition of Si, Ti, C and O elements in SiO2/TiO2@C and the atomic ratio of Si/Ti is calculated to be 4.98. As shown in the Si 2p spectrum (Figure 4a), the peak at 103.8 eV is associated with the Si-O bond of SiO2.25 As for Ti 2p spectrum (Figure 4b), two peaks located at 459.6 and 465.2 eV can be corresponded to Ti 2p3/2 and Ti 2p1/2 in TiO2.42 C 1s peaks (Figure 4c) at 284.6, 285.8, and 288.6 eV can be assigned to the bonds of C−C, C−O, and C=O, respectively. O 1s spectrum (Figure 4d) can be fitted into three peaks centered at 533.1, 531.5 and 530.9 eV, corresponding to Si–O in SiO2, C–O and Ti–O in TiO2,43 respectively.

SiO2+ 4Li+ + 4e  2Li2O + Si

(1)

2SiO2+ 4Li+ + 4e  Li4SiO4+ Si

(2)

5SiO2+ 4Li+ + 4e  2Li2Si2O5+ Si

(3)

Si+ xLi+ + xe  LixSi

(4)

yLi+ + TiO2+ ye  LiyTiO2

(5)


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Figure 5. Electrochemical properties of SiO2/TiO2@C and SiO2/TiO2 electrodes. (a) CV curves, (b) discharge/charge profiles of SiO2/TiO2@C electrode for different cycles. (c) Cycling performances of SiO2/TiO2@C and SiO2/TiO2 electrodes at 100 mA g-1 and the


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corresponded CE of SiO2/TiO2@C electrode. (d) Rate performances of SiO2/TiO2@C and SiO2/TiO2 electrodes. (e) Nyquist plots of SiO2/TiO2@C at different cycles at 1000 mA g1.

(f) the relationship between Z′ and ω-1/2 within 105˗10-2 Hz. (g) Long-term cycling

performances of SiO2/TiO2@C electrode at 1000 and 2000 mA g-1.

Figure 5a presents the first four cyclic voltammograms (CVs) of SiO2/TiO2@C electrode recorded at a scan rate of 0.2 mV s-1 with a potential range of 0.01–3.0 V (vs. Li/Li+). In the first cathodic scan, two weak peaks (1.22 V and 0.88 V) and a sharp slope below 0.4 V are observed. The peaks at 1.22 V and 0.88 V enlarged in Figure 6Sa may correspond to the irreversible reactions between SiO2 and Li+ (eqn. 1, 2) and the formation of solid electrolyte interphase (SEI) layer.44 The sharp slope is ascribed to Li+ insertion into SiO2 (eqn. 3, 4). In the first anodic process, two peaks at 0.52 V and 1.55 V are detected, which can be attributed to the delithiation of LixSi to Si and the delithiation of Li2Si2O5 to SiO2, respectively.13 In addition, the weak redox peaks of 1.74 V/2.18 V (Figure 6Sb) should be corresponded to the reversible reaction between TiO2 and Li+ (eqn. 5).45 The reaction mechanism of SiO2/TiO2@C electrode is also confirmed by ex-situ XRD and XPS measurements, as presented in Figure S7. For the second cycle, a new cathodic peak appears at 0.48 V, due to the structure reconstruction in the first scan.46 In the subsequent scans, the CVs almost overlap with each other, indicating the good reversibility of SiO2/TiO2@C electrode. Unsurprisingly, SiO2/TiO2 electrode also exhibits similar Li storage behaviors to SiO2/TiO2@C electrode, as presented in the CVs (Figure S8a). Figure 5b shows the discharge/charge profiles of SiO2/TiO2@C electrode at a current density of 100 mA g-1. As shown, the characters of the discharge/charge profiles are in


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good agreement with CV curves. An initial discharge capacity of 1016 mAh g-1 is obtained for SiO2/TiO2@C electrode, accompanying a Coulombic efficiency (CE) of 66%. The low initial CE was mainly caused by the formation of SEI layer and irreversible side reaction. Compared with SiO2/TiO2@C, SiO2/TiO2 delivers a much lower initial discharge capacity of 566 mAh g1 and a low CE of 54 % (Figure S8b). Figure 5c presents the cycling performances of SiO2/TiO2@C and SiO2/TiO2 electrodes at 100 mA g-1. As shown, SiO2/TiO2@C displays a reversible capacity of 998 mAh g-1 after 100 cycles, superior to that of SiO2/TiO2 (300 mAh g-1). The CE of SiO2/TiO2@C electrode keeps higher than 99% after the first several cycles. Moreover, it is worth noting that both SiO2/TiO2@C and SiO2/TiO2 electrodes display extremely stable cycleability up to 100 cycles, demonstrating that the strategy of introducing TiO2 into SiO2 works well. The gradually increasing capacities of SiO2/TiO2@C and SiO2/TiO2 upon cycling may be correlated with the slow activation of SiO2, which has been reported in many SiO2-based anodes.16, 28 Figure 5d exhibits the rate capability of SiO2/TiO2@C and SiO2/TiO2 at 100-2000 mA g-1. Compared to SiO2/TiO2, SiO2/TiO2@C delivers much higher capacities of 681, 665, 589, 458 and 408 mAh g-1 at 100, 200, 500, 1000 and 2000 mA g-1, respectively. When the current density goes back to 100 mA g-1, the capacity recovers to 734 mAh g-1, indicating the high stability of SiO2/TiO2@C electrode at high current densities. The improved rate capability reveals that carbon coating strategy plays a significant role in the improvement of Li storage capability. Electrochemical impedance spectra (EIS) were measured to gain insight into electrochemical kinetics of SiO2/TiO2@C electrode upon cycling, as shown in Figure 5e. All the spectra of SiO2/TiO2@C at different cycles shows typical Nyquist plots with a


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semicircle in the high frequency region and an inclined line in the low frequency region. The semicircle is associated with two overlapped interface impedances (SEI and chargetransfer impedance, Rct). The inclined line is affected by ion diffusion (Warburg impedance, Zw). Based on the equivalent circuit, the Rct of SiO2/TiO2@C electrode at different cycles decrease upon cycling, indicating the enhanced charge transfer. Figure 5f shows the relationship between Z' (ZRe) and ω-1/2 within 105−10-2 Hz, in which Warburg factor (σ) (the slope of the lines) is related to the lithium ion diffusion coefficient (DLi). DLi can be calculated based on the following equations:

Z Re= Rct + RΩ + σω-1/2 DLi =

R 2T 2 2 A2n 4 F 4CLi 2σ 2

(6)

(7)

where ω is the angular frequency, R is the gas constant, T is the absolute temperature, A is the surface area of the electrode, n is the number of electrons transferred for the redox couple, F is the Faraday constant, and CLi is the concentration of lithium ions in the solid. It can be seen that Warburg factor gradually reduced as the cycle increases, reflecting the increase of lithium ion diffusion coefficient (eqn. 7). The increased lithium ion diffusion coefficient and enhanced charge transfer upon cycling can account for the capacity growing phenomenon of SiO2/TiO2@C electrode in Figure 5c. Figure 5g shows the long-term cycling performance of SiO2/TiO2@C electrode, which was first activated at 100 mA g-1 for 10 cycles (that was not shown in Figure. 5g), and then cycled at high rates. As a result, SiO2/TiO2@C electrode delivers a stable cycling over 400 cycles at 1000 mA g-1 with a capacity of 590 mAh g-1 retained, and an ultrahigh capacity of 410 mAh g-1 after 400 cycles at 2000 mA g-1. This superior Li storage capability of SiO2/TiO2@C


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is almost close to the properties of SiOX-TiO2@C nanoparticles with the best performance reported,47 and is obviously superior to most of SiO2-based materials, as summarized in Table S1.

Figure 6. (a) CV profiles of SiO2/TiO2@C electrode at different scan rates. (b) The current response plotted against scan rates of SiO2/TiO2@C at various potentials. (c) The determination of the k1 using the relationship between v1/2 and i/v1/2. (d) Capacitive contributions at different scan rates for SiO2/TiO2@C electrode.

To further understand the possible reason for the outstanding electrochemical performance of SiO2/TiO2@C electrode, CV measurements at 0.1 to 1 mV s-1 were performed to investigate the detailed electrochemical kinetics. The CVs of SiO2/TiO2@C electrode at different scan rates are shown in Figure 6a. It is notable that the CVs display the same shape, and the current increases gradually with the increasing scan rate.


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Meanwhile, small electrode polarizations can be observed for cathodic and anodic peaks. Generally, the correlation between the current (i) and the scan rate (v) can be calculated based on the following equation: i = avb

(8)

Qualitatively, the b value of 1 indicates a surface-driven capacitive behaviour, while the b value of 0.5 reveals a diffusion-controlled intercalation process. As exhibited in Figure 6b, the b values of all these peaks are in the range of 0.5 and 1, indicating a mixed reaction process of capacitance and diffusion-controlled intercalation for SiO2/TiO2@C electrode. Quantitatively, the total electrochemical reaction can be divided into capacitive behaviour (k1v) and diffusion-controlled intercalation process (k2v1/2) at a fixed potential based on the following equations: i(v) = k1v + k2v1/2 i(v)/v1/2 = k1v1/2 + k2

(9) (10)

where k1 can be determined as the slope of eqn. 10 (Figure 6c). Figure 6d clearly exhibits the calculated results of capacitive contributions at different rates. A shown, the capacitive contribution from surface-driven process increases from 70.2% to 90.5% with the increasing scan rates from 0.2 to 1 mV s-1. The high capacitive contribution in SiO2/TiO2@C electrode can be ascribed to its unique structural features, including nanosheet-like morphology, abundant grain boundaries and carbon coating layer, which can effectively facilitate ion diffusion and accelerate electron transfer.


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Figure 7. SEM images of SiO2/TiO2@C electrode (a) before cycling and (b) after 120 cycles at 1000 mA g-1. (c) Schematic illustration of lithiation and delithiation process of SiO2/TiO2@C electrode.

The structure durability of SiO2/TiO2@C nanosheets was also confirmed by SEM images before and after cycling, as shown in Figure 7a and 7b. Although the thickness of nanosheets has increased from a few tens of nanometers to several hundred nanometers after 120 cycles at 1000 mA g-1, the sheet-like nanostructure remains intact, indicating the reason behind the excellent performance of SiO2/TiO2@C electrodes. The structural stabilization of SiO2/TiO2@C active particle is mainly attributed to its unique structure feature, as illustrated in Figure 7c. During the lithiation/delithiation process, the ultrafine TiO2 nanoparticles highly distributed in SiO2 matrix can produce homogeneous mechanical


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stress and buffering power to minimize volume expansion, sustaining the structural integrity. And the lithiated TiO2 (LiyTiO2) can effectively enhance the electron and ion diffusion.48 The outside carbon layer is able to avoid the direct contact between active material and electrolyte, improve electronic conductivity and alleviate volume changes, further improving the cycleability of SiO2/TiO2@C. Moreover, the nanosheet structure of SiO2/TiO2@C can help to shorten lithium ion diffusion path, relieve stress and buffer volume expansion. In short, SiO2/TiO2@C electrode maintains the structure integrity and exhibits the superior electrochemical performance under the effect of the above advantages.

CONCLUSIONS In summary, SiO2/TiO2@C nanosheets with evenly distributed TiO2 particles in SiO2 matrix were prepared by using acidified titanosilicate JDF-L1 as precursor. Electrochemical measurements suggest that SiO2/TiO2@C nanosheets exhibit an outstanding Li storage capability, which is ascribed to its unique nanostructure. The TiO2 particles uniformly embedded in SiO2 matrix can maintain the structure integrity. The nanosheet-like morphology and abundant grain boundaries can facilitate ion diffusion, and the carbon coating layer can accelerate electron transfer. As lithium-ion battery anode, SiO2/TiO2@C nanosheets display a high capacity of 998 mAh g-1 after 100 cycles at 100 mA g-1, and an excellent cycling stability at 2000 mA g-1 up to 400 cycles. In addition, it is a good attempt for the application of titanosilicate to construct high energy density SiO2based anode for LIBs.

ASSOCIATED CONTENT Supporting Information


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XRD patterns of JDF-L1 and H-JDF-L1; SEM images of JDF-L1 and H-JDF-L1; Energydispersive X-ray spectrum of H-JDF-L1; TEM image and HRTEM image of SiO2/TiO2@C; XPS survey spectrum of SiO2/TiO2@C; Partial-enlarged CV curves of SiO2/TiO2@C electrode; XRD patterns and Si 2p XPS spectra of SiO2/TiO2@C electrode at different lithiated/delithiated states; CV curves and discharge/charge profiles of SiO2/TiO2electrode at 100 mA g-1; Comparasion of SiO2-based anode materials for LIBs.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Xin Gu: 0000-0002-6074-4146 Xuebo Zhao: 0000-0002-5352-0953 Notes There are no conflicts to declare.

ACKNOWLEDGEMENTS This work was supported by Fundamental Research Funds for the Central Universities (17CX02039A), the Key research and development plan of Shandong province (2018GGX102017), New Faculty Start-up funding in China University of Petroleum (East China) (YJ201601023), Special Project Fund of “Taishan Scholars” of Shandong Province (ts201511017), and Natural Science Foundation of China (21473254).


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Abstract Graphic

SiO2/TiO2@C nanosheets with TiO2 nanoparticles uniformly embedded in SiO2 matrix and carbon coating layer are fabricated by using acidified titanosilicate JDF-L1 nanosheets as template and precursor. Its unique structure induces several advantages during the lithiation/delithiation process: 1) The TiO2 nanoparticles highly distributed in SiO2 matrix can produce homogeneous mechanical stress and buffering power to minimize volume expansion; 2) The nanosheet structure can shorten lithium ion diffusion path and relieve stress; 3) The carbon layer is able to effectively improve electronic conductivity and alleviate volume changes. As a result, SiO2/TiO2@C nanosheets exhibit an excellent cycling stability at 2000 mA g-1 over 400 cycles with an ultrahigh capacity of 410 mAh g-1 retained.


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TiO2@C Nanosheets ... - ACS Publications

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