Hierarchical Hollow Microspheres Constructed by Carbon Skeleton

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Hierarchical Hollow Microspheres Constructed by Carbon Skeleton Supported TiO2-x Few-layer Nanosheets Enable High Rate Capability and Excellent Cycling Stability for Lithium Storage Huanlong Liu, Wei Zhao, Shaoning Zhang, Zheng Chang, Yufeng Tang, Meng Qian, zhi li, Wenli Zhao, Heliang Yao, Wei Ding, Jiantao Huang, and Fuqiang Huang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00331 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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ACS Applied Energy Materials

Hierarchical Hollow Microspheres Constructed by Carbon Skeleton Supported TiO2-x Few-layer Nanosheets Enable High Rate Capability and Excellent Cycling Stability for Lithium Storage Huanlong Liu, §† Wei Zhao, †* Shaoning Zhang, † Zheng Chang, † Yufeng Tang, † Meng Qian,

†

Zhi Li,† Wenli Zhao,

†

Heliang Yao,† Wei Ding,† Jiantao Huang,† Fuqiang

Huang †‡* § School of Material Science and Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, China † State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China. ‡ Beijing National Laboratory for Molecular Sciences and State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.

Abstract: Rational design and facile synthesis of TiO2 based hybrid electrodes with hierarchical microstructure have great advantages for exploration of advanced electrodes for lithium-ion batteries (LIBs). We design and synthesize a hierarchical hollow microspheres with inner carbon skeleton supported outer TiO2-x few-layer nanosheets (C@TiO2-x). The “core-shell” C@TiO2-x microspheres exhibit relatively high specific surface area and a remarkable electric conductivity (0.264 µS cm-1). The lithium kinetics of C@TiO2-x microspheres is significantly improved due to

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synergistic effects of few-layer TiO2-x nanosheets and conductive carbon skeleton. The C@TiO2-x microspheres manifest an excellent reversible capacity of 323 mAh g-1, together with an ultra-long cycling lifetime that the capacity shows ~220 mAh g-1 after 1000 cycles at 1.0 C. The C@TiO2-x microspheres also deliver a relative high performance in rate capacity (108 mAh g−1 at 20 C). When they are assembled into a hybrid lithium-ion capacitor, the relative high capacitance of 58 F g-1 is achieved that high power density reaches 14 kW kg-1. Keywords: hollow microspheres, TiO2-x few-layer nanosheets, carbon skeleton, lithium ion batteries, capacitor.

Introduction Titanium dioxide (TiO2) is being extensively investigated in various realms, such as water treatment, sensors, catalysis and energy storage etc.1-7 Due to its environmental benignancy, safety and chemical stability,6, 8 TiO2 is developed as a potential anode materials for LIBs9. Due to the relatively low molar mass, TiO2 could offer phase-dependent theoretical capacity (175~335 mAh g-1)10. In addition, TiO2 anode has a comparably high discharge platform that avoids forming solid electrolyte interface (SEI) films which cause a low irreversible capacity loss.11-12 However, the low conductivity and ions diffusivity restrict the rate performance and long cycling stability of semiconductive TiO2.13 Many efforts is developed to enhance electronic conductivity and structure stability of TiO2 including doping, coating, etc.3, 14-21 TiO2 with unique morphologies and porous structures would shorten the transport distance of lithium ions and electrons,

22-25

all of these can increase electrode/electrolyte

interfaces to improve lithium ions diffusivity.15, 26-29 Though the specific capacity of


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the TiO2 based electrode materials is effectively enhanced by these methods, introducing foreign conductive materials and modifying morphology cannot change the low conductivity of pristine TiO2. Therefore, the electrochemical performance of TiO2 electrode is far from satisfactory. Previous literature shows that Ti3+ is effectively reduced from TiO2 crystal by hydrogenation, aluminum or other metals (Mg, Ca etc.) and vacuum treatment30 to greatly improve its intrinsic conductivity.31-33 Recently, hierarchical porous TiO2 nanostructures have drawn widespread attention because of the excellent structural properties.34 As the anode of LIBs, such complex architecture could integrate two merits. The whole hierarchical microscale TiO2 results in efficient transportation of lithium ions in electrolyte or electrons in electrode.35 Besides, porous and nanostructured units are capable of storing lithium ions.36-37 Therefore, it is beneficial to design and prepare TiO2 with hierarchical architecture and higher intrinsic electronic conductivity to enhance the storage lithium capability for LIBs. Herein, we design an anti-wrapping strategy to prepare the hierarchical hollow microspheres constructed by carbon skeleton supported few-layer TiO2-x nanosheets (C@ TiO2-x) ( Scheme 1 ).38 The microspheres are composed of few-layer TiO2-x nanosheets as the outer shell and aggregated carbon nanoparticles as the inner skeleton. The hierarchical microstructure delivers a highly electrical conductivity (0. 264 µS cm-1) and porous network with improved lithium kinetics. Herein, the C@TiO2-x microspheres display a relative high specific capacity (320 mAh g−1 at 0.2 C), initial coulombic efficiency upto 89.7%, excellent rate performance and ultralong cycle life that the capacity maintains 223 mAh g−1 during 1000 cycling at 1.0 C.



Scheme 1. Schematic of preparation of C@TiO2-x hollow microspheres. The outer purple layers and inner white particles represent few-layer TiO2-x nanosheets and carbon skeleton, respectively.

Results and Discussion

Figure 1. (a) SEM of complete C@TiO2-x hollow microsphere hollow, (b) SEM of broken C@TiO2-x hollow microsphere hollow, (c) TEM images of the C@TiO2-x hollow microsphere and (d) the few-layer TiO2-x nanosheets. (e), (f) HRTEM images of the red regions marked in (d). (e) HRTEM image of few-layer TiO2-x nanosheets shows the two-dimensional with vertical interfacial spacing of 0.19 nm. The inset shows the FFT pattern highly indexed to the [001] zone. (f) HRTEM image shows lattice fringes of nanosheets edge under a lattice distance of 0.19 nm, (g) HAADF and EDS mapping of Ti, O, and C elements respectively shown in the C@TiO2-x hollow microsphere.


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The synthesis of the C@TiO2-x microspheres is illustrated in scheme 1. The Larginine has a low melting point (496 K) and solubility in anhydrous ethanol. In the solvothermal process, the solubility of L-arginine powder in anhydrous ethanol is increased and melts into spheres. The functional groups of L-arginine can react with titanium alkoxide to chemically reduce the reactivity to vapors. In addtion, the esterification reaction between ethanol solvent and L-arginine slowly produces trace amounts of moisture to assist the hydrolysis of titanium alkoxide. Some L-arginine is sacrificed from the spherical template and combined with the TiO2 matrix, forming hybrid hollow structures. Finally, the assembly nanosheets mesoporous TiO2 spheres wrapping carbon nanoparticles is obtained. The resultant hollow structure is then annealed to C@TiO2-x spheres. The SEM images shows a spherical structure surrounded by uniform nanosheets with a size of 800 nm, which is shown in Figure 1a. The high-magnification SEM images depicts in details that the ultrathin nanosheets evenly germinate at the surface of the TiO2-x sphere with carbon nanoparticles tightly attached to the inner wall of the sphere (Figure 1b and S1), forming a structure called ‘core-shell’. TEM images confirmed this ‘core-shell’ structure of C@TiO2-x spheres in the Figure 1c. The HRTEM image of C@TiO2-x presented in Figure 1d, the ultrathin nanosheets are made up of few layers with the interlayer spacing ~1.02 nm. In the Figure 1e and 1f, the fringe with one lattice spacing of both the TiO2 nanosheets and TiO2 folding boundary is averagely measured to attain 0.19 nm, supporting intensively for the d-spacing of the anatase TiO2 (200) and (020) planes, respectively. Moreover, the same region is observed via fast Fourier transform (FFT) in Figure 1e, indexing to the same diffraction spots as the [001] zone. The whole few-layer TiO2 nanosheets should be bounded with (001) facets. Figure 1g



presents the high-angle annular dark field (HAADF) of one single C@TiO2-x sphere via STEM. The C, O, and Ti elements and respective contents is proved to be existed in each single hollow sphere using the energy dispersed spectrum (EDS) (Figure S2 and Table S1). Corresponding elemental mapping results indicate that C, O, and Ti elements have relative uniform distribution in the sphere. Therefore, the hierarchical hollow C@TiO2-x spheres were prepared via solvothermal method.

Figure 2. (a) XRD patterns of C@TiO2-x and pure TiO2, (b) Nitrogen adsorption/desorption

isotherms, inset: corresponding pore size distribution, (c) Raman spectra patterns (100-2000 cm-1), and insets are the magnification of marked area by gray dotted box, (d) TGA and DTA curves of C@TiO2-x spheres at 723K.

As shown in the Figure 2a, the obtained XRD pattern fits anatase TiO2 well on the characteristic diffraction peaks (JCPDS 89-4921). Showing similar peaks in the Figure 2c, Raman spectroscopy of all samples confirms the existence of anatase phase TiO2. However, in the case of the Eg peak (inset Figure 2c), C@TiO2-x has higher semi-high width that shifts from 142 cm-1 to 154 cm-1. This Raman spectral change is mainly ascribing to previous introduction of oxygen vacancies after the hydrogenation


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treatment. Meanwhile, Raman vibrational mode, especially Eg peak and A1g peak, plays a vital role in determining the proportion of exposed anatase TiO2 {001} facets.39 The calculated intensity ratio from the Eg peaks to A1g peaks could offer a numerical percentage of the facets above. In the inset of Figure 2c, the A1g peaks hardly exist, indicating the exposed {001} facets in anatase TiO2-x nanosheets. A typeIV curve was observed via Nitrogen sorption isotherms which is shown in Figure 2b. The specific surface area is measured as 325 m2 g-1, exceeding that of TiO2 (167 m2 g1

). The pore size distribution of the C@TiO2-x spheres mainly within 10 nm due to the

accumulation of nanosheets and carbon nanoparticles. The carbon content of C@TiO2-x spheres was checked out via thermogravimetric analysis (TGA), which is presented in Figure 2d. The weight loss is obvious below 423 K mainly due to moisture elimination of C@TiO2-x spheres interiorly, and the other weight loss is caused by the carbon oxidation exposed to air. The calculated carbon content is 12.9%.

Figure 3. (a) High-resolution XPS spectra of Ti2p and (b) C1s of C@TiO2-x spheres. (c) EPR



spectra and (d) the electrical conductivity of TiO2 and C@TiO2-x spheres.

X-ray photoelectron spectroscopic (XPS) full spectrum results (Figure S3) reveal the C, O, and Ti elemental states and contents of as-obtained samples. Same as those of ordinary anatase TiO2, the binding energies of samples corresponding to the peak centers of Ti 2p3/2 and Ti 2p1/2 are 458.5 eV and 464.3 eV, respectively (Figure 3a).40 In addition, according to C1s spectra in the Figure 3b, the sp2 type carbon is dominant due to relatively high annealing temperature. The high content of sp2 type carbon improves the conductivity of C@TiO2-x spheres. The EPR is proved to be effective for probing the existence of oxygen vacancy and Ti3+ of defective TiO2. A strong EPR peak (g = 2.003) of C@TiO2-x spheres is ascribed to the unpaired electrons from the reduction to surface Ti3+ (Figure 3c).41-42 In contrast, the EPR spectra of TiO2 exhibits a straight line, suggesting that there is no Ti3+ existing.32 Therefore, due to the extra vacancy resulted from the reduction to Ti3+, the conductivity of C@TiO2-x reaches 0.264 µS cm-1, exceeding which TiO2 does (Figure 3d). The synthesis of the C@TiO2-x spheres heavily depends on the templating agent and the control of annealing conditions. The isoelectric point of the amino acid can seriously affect the formation of TiO2 morphology where the amino acid takes reaction with titanium alkoxide precursor,38 and the amino group is significant to the formation of TiO2 (001) facets (Figure S4).43 Different annealing temperatures can cause the phase transition and morphology collapse of C@TiO2-x spheres. Annealed at 723K, amorphous carbon is converted to graphitized carbon by the catalytic effect of TiO2, and such morphology preserves. Annealed below 723K, the TiO2-x nanosheets gradually stack, agglomerate as the outer shell of microspheres, and beyond 723K, gradually transfers from anatase phase to rutile phase (Figure S5 and Figure S6).


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Figure 4. (a) The electrochemical performance from 0.2 C to 20 C. (b) Cycle stability of C@TiO2x

spheres and TiO2 up to 1000 cycles at 1.0 C. (c) Charging/discharging curves of the C@TiO2-x

spheres at 1.0 C. (d) Nyquist plots of C@TiO2-x spheres and TiO2 after 3 cycles. With unique advantages mentioned above, the hierarchical hollow C@TiO2-x spheres become ideal for the lithium intercalation/deintercalation. Figure 4a shows that the rate performance of C@TiO2-x electrode and TiO2 electrode was evaluated as an auxiliary electrode in half cell with metallic lithium. The initial capacity of the asobtained samples fades because of the side effects between the electrode and the electrolyte. The capacities of C@TiO2-x spheres are 321, 262, 219, 180, 163, 141, 121, and 108 mAh g−1 with different rates of 0.2, 0.5, 1, 2, 5, 10, 15, and 20 C, respectively. The capacity of C@TiO2-x spheres is clearly higher than the TiO2 electrode, especially at high current rate, demonstrating impressive rate performance for the LIBs. Furthermore, when the current rates back to 0.2 C, the capacity returns to 292 mAh g−1, which is close to the initial value. In addition, it is presented in the Figure S11 that the rate capacity of as-prepared samples with different temperature indicates that the electrochemical performance improves as the annealing temperature increases. However, the presence of rutile TiO2 decreases the specific capacity of



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C@TiO2-x spheres. The stability of C@TiO2-x electrode is evaluated by the ultra-long cycle testing. The C@TiO2-x electrode can still remain a relative high capacity of 223 mAh g−1 at 1.0 C after 1000 cycles, and the coulombic efficiencies are stably retained near 100% (Figure 4b). In contrast, the TiO2 electrode fades after 350 cycles. The Figure S10 shows the Coulombic efficiency of C@TiO2-x and TiO2 electrodes during 1000 cycling. Their Coulombic efficiency did not change significantly, while the Coulomb efficiency of TiO2 is some fluctuated in the early stage, which may be related to the destruction of the TiO2 nanosheets with electrochemical reaction. In the Figure 4c, the charging/discharging curves of the 1st, 100th, 500th, and 1000th are shown for the C@TiO2-x spheres electrode at 1.0 C where voltage window remains 1.0~3 V vs. Li+/Li. Similarly as the previous report has shown18,

44

, the voltage

platform comes up in the vicinity of potential at 1.74/1.92 V, corresponding to the insertion/extraction process of Li-ion to the anatase TiO2 lattice. There is comparison of electrochemical properties of TiO2 in various morphology in the literature is shown in Table S2. In the Figure 4d, the Nyquist plots of C@TiO2-x spheres and TiO2 consists of one semicircle in the mid-frequency, and then turns to straight line in low frequency. They are corresponding to a charge-transfer reaction resistance and a Warburg diffusion process (lithium diffusion process) occurring on the electrodes, respectively. When it comes to high frequency region, the electrode consisting of C@TiO2-x spheres presents a lower resistance compare to the TiO2 electrode. According to the semicircle, the C@TiO2-x electrode delivers a lower charge transfer resistance (Rct 38.9 Ohms) than TiO2 electrode (42.8 Ohms). As low frequency region shows, there is a larger linear slope of C@TiO2-x spheres comparing with TiO2 electrode, indicating that lithium ion diffusion rate becomes faster in the solid electrode.


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Figure 5. Electrochemical performance of C@TiO2-x spheres: (a) cyclic voltammetry curves

and (b) the relationship between log-peak current and log-sweep rate of the electrode from 0.2 to 10 mV s-1. (c) Typical charging/discharging curves and (d) specific capacity proportions for each characteristic region of the insertion process and surface-related capacitance at 0.2 C and 1.0 C.

The electrochemical behaviors of coin cells are investigated during 1.0 to 3.0 V. In the Figure 5a, cyclic voltammetry (CV) curves of C@TiO2-x spheres are displayed within various sweep rates of 0.2~10 mV s-1. With increasing sweep rates, the separation between anodic peak and cathodic peak increases, implying more polarized voltage at high sweep rates due to the lithium kinetic limits.18,

45-46

Despite the

noticeable peak shift, the current peaks value remains about the same, suggesting lithium kinetics. Figure 5b presents a plot of log (i) vs. log (v) of cathodic peaks and anodic peaks during 0.2~10 mV s-1, respectively. The obtained points are linearly dependent, and the linear slopes range between 0.5 and 1, indicating the diffusioncontrolled distinction during lithium reaction in C@TiO2-x spheres electrode.47-49 The



charging/discharging profiles of C@TiO2-x spheres at 0.2 C and 1.0 C which are presented in Figure 5c. Same as the platform analysis above, an obvious high voltage platform is presented from charging/discharging curves, indicating there are both anatase and lithium titanate phases in the electrode. The discharge curves are divided into two distinct regions: The region A is mainly from the plateau region, controlled by the bulk diffusion in the lithium intercalation. In the region B, a voltage decreases after the platform suggesting surface-related capacitance prevails.39 From the respective discharge curve regions, the derivation value of capacity is shown in Figure 5d. The value indicate that the portion of the platform capacity is mainly reduced with the current increasing. In addition, the charging/discharging curves of different rates shows the platform gradually disappeared when the charge/discharge current increases, and the surface-related capacity plays dominant role in high rates (Figure S7).15 Therefore, the C@TiO2-x spheres electrode has an impressive rate performance at high current density.

Figure 6. (a) Schematic of the lithium-ion insertion/extraction process. The inner carbon

nanoparticle skeleton supporting TiO2-x nanosheets, keeping the C@TiO2-x spheres a robust structure under cycle. The TEM images of the C@TiO2-x spheres (b) before and (c) after 1000 cycling test. Scale bars stay 400 nm in all TEM images above.


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Figure 6a demonstrates the schematic of lithium-ion intercalation/deintercalation process in C@TiO2-x spheres. In the charging process, there are plenty of lithium ions embedding in the crystal lattice of TiO2-x. During discharging process, lithium-ion is released from the TiO2-x nanosheets and then are introduced to the electrolyte. Meanwhile, electrons are compensated to maintain the charge balance during electrochemical reaction. According to TEM images (Figure 6c), the C@TiO2-x spheres after 1000 cycles still keeps the spherical structure. This phenomenon reveals that the TiO2-x nanosheets have the ability to keep structure stable with charging/discharging process due to inner carbon skeleton supporting. The volume expansion percentage of TiO2 is less than 4%. However, the few-layer TiO2-x nanosheets could collapse into particles with cycling. The existence of the internal carbon skeleton avoids the occurrence of agglomeration and maintains relative unchanged high specific surface area of C@TiO2-x electrode, which contributes to the intercalation of lithium-ion. In the Figure S8, the TiO2 sheets suffer from the agglomeration and collapse. The original spherical shape is completely lost after ultralong cycling test.



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Figure 7. (a) Schematic of the hybrid LIC with the C@TiO2-x microspheres as anode and AC as

cathode in electrolyte. CV curves (b), charging/discharging curves (c), specific capacitances with different current densities (d), and (e) Ragone plot of the LIC with the voltage window of 0~2.7 V. Inset: a red light emitting diodes (LEDs) are light up by a LIC.

The outstanding performance of such designed C@TiO2-x spheres in half-cells can be utilized in hybrid lithium ions capacitor (LIC). Commercialized activated carbon (AC) was used as a cathode to couple with the C@TiO2-x spheres anode. Due to the fact that each electrode has various reaction mechanisms, the applied current distributes to two electrodes. Therefore, it is necessary to balance the charge so that a higher energy density can be obtained in LIC configuration. In addition, the stored charge has a direct relationship with capacity. Based on the capacity value for the C@TiO2-x spheres and AC, the optimal mass ratio is mAC // m


C@TiO2-x =

4 in the LIC.

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The electrochemical properties of the C@TiO2-x //AC LIC during 0~ 2.7 V is shown in Figure 7. The hybrid LIC gradually diverges from the standard rectangular mode with scan rate increasing from 2.0 unto 50 mV s-1 (Figure 7b). There are two different energy storage mechanisms involved to account for the deviation related to the synergistic effect: electrical double layer capacitor (EDLC) processes and pseudocapacitance reactions. As shown in the Figure 7c, the charging/discharging curves corresponding to two electrodes with changed current densities indicate that there is no simply linear relationship of the slopes especially when it reduces to low current densities, suggesting the joint behaviors of capacitor and battery in the hybrid LIC performance. To figure out the relation, take specific capacitance as dependent variables and current density as independent variables, which is shown in Figure 7d. With current increasing, specific capacitance decreases because the electrochemical reaction only occurs on the surface at high current rates. The maximum of capacitance is 192 F g-1 at 0.5 A g-1 and remains 57.8 F g-1 as current density increases to 10 A g-1. Meanwhile, the charging/discharging time decreases to 8.2 s. Moreover, the LIC performs acceptable cycling performance in the Figure S9, and the capacitance retention measured to be 72.2% during 1000cycling. The electrochemical performance of C@TiO2-x spheres //AC LIC are calculated (Figure 7e). The energy density decreases from 51.9 to 15.2 Wh kg-1 with the power density increasing to 14 from 0.7 kW kg-1. The power density value of LIC reaches 14 kW kg-1 proving that charge/discharge fulfills 8.2 s. Besides, a red LED can be charged and illuminated by the hybrid LIC, which is photographed in inset of Figure 7e. Again the experiment also indicates the C@TiO2-x spheres is promising for LIBs with high power density. The significant electrochemical performance that hierarchical hollow C@TiO2-x spheres exhibits in LIBs is ascribed to several aspects. First, the C@TiO2-x spheres



have unique features, including high specific surface area, different pore diameter distribution of mesopores (~2.2 and 6.7 nm), the robust hollow structure and fewlayer TiO2-x nanosheets. All of these not only provide a large number of active sites, but also contribute to the diffusion and access of lithium ions. Second, the outer fewlayer nanosheets of C@TiO2-x spheres can decreases lithium-ion diffusion distance and increases the contact interfaces of electrolyte. The highly conductive few-layer TiO2-x nanosheets and carbon nanoparticles significantly enhance lithium ions diffusion dynamics and improve the lithium ion reversibility. Third, the carbon nanoparticles adhering to the inner wall function as the skeleton. The skeleton can maintain previous volume and porous ‘core-shell’ structures, thereby improving the reaction area of C@TiO2-x microspheres. Fourth, large layer spacing of few-layer TiO2-x nanosheets is beneficial for lithium ions intercalation/deintercalation process. Therefore, the joint effect between the conductive few-layer TiO2-x nanosheets and inner conductive carbon skeleton significantly improve the rate performance and cycle lifetime.

Conclusion In conclusion, we designed an anti-wrapping strategy to prepare hierarchical hollow C@TiO2-x spheres with unique features. The high specific surface area, mesoporous and robust hollow structure, few-layer TiO2-x nanosheets and the carbon nanoparticles firmly encapsulated forming outer shell and inner skeleton, respectively. The C@TiO2-x spheres delivered excellent storage lithium capability due to synergistic effects between unique hierarchical hollow features with conductive TiO2-x


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nanosheets and inner carbon skeleton supporting. To satisfy an anode for LIBs, the C@TiO2-x spheres indicated an excellent reversible capacity reaching 321 mAh g-1 at 0.2 C compared with the TiO2 electrode. In addition, this electrode shows outstanding rate capability because of the enhanced conductance and surface-related capacitance contribution. The C@TiO2-x electrode also exhibited an ultra-long cycle stability (221 mAh g−1 after 1000cycling at 1.0 C). Assembled into a LIC with AC, the C@TiO2x//AC

LIC revealed a specific energy density of 51.9 Wh kg-1 and high capacitance of

192 F g-1 at an high power density of 700 W kg-1. The energy density of LIC can maintain 15.2 Wh kg-1 even the power density reaches 14 kW kg-1. These results suggest hierarchical TiO2 hybrid nanostructure has a great potential for high power density LIBs.

Experiment:

Sample preparation: The chemicals were bought from Sinopharm Chemical Reagent Co., Ltd. The L-arginine was ground and sieved, and ethanol was treated with anhydrous molecular sieve. The hybrid of L-arginine (0.6 g) and anhydrous ethanol (30 mL) was added in a Teflon lining of 50 mL by stirred for 30 min at room temperature. Then, the titanium n-butoxide (1.0 mL) was added to the hybrid, stirred for another 20 min, and assembled. The hydrothermal reactor heated to 473 K and remained 12 h, along with the furnace cooling. The product was washed three times with anhydrous ethanol, and freeze-dried 12 h. As-obtained sample was annealed at a gas flow of Ar/H2 (10:1) at different temperatures (573K, 673K, 723K, and 773K) for 6 h, labeled C@TiO2-x spheres. A sample without L-arginine was annealed in air at 723 K and remained 6 h, labeled TiO2.



Materials Characterization: The physical structure of samples and morphology were measured by using X- XRD (Bruker D8 ADVANCE, Cu Ka radiation). The samples morphology was measured by SEM (JSM 6510) and TEM (JEM 2100F). The thermal analysis of materials were measured by TG-DTA/DSC equipment (STA-409PC Luxx, Neutsch) with flowing air. The BET and pore-size distributions of as-obtained materials were carried out by Micrometitics Tristar 3000 system. EPR spectra was measured by Bruker EMX-8 spectrometer at 9.44 GHz and 300 K.

Electrochemical measurements: The electrochemical performances of electrode was evaluated using the 2016-type coin cell. The electrode contained active material, acetylene black, and PVDF (the ratio is 8:1:1) solvating onto Cu foil and then dried at 343 K overnight. The coin cell was assembled in an argon-filled glove box (both H2O and O2 contents below 0.1 ppm) by using electrode as the working electrode, the lithium tablets is used counter electrode. The separator model is Celgard 2600. The electrolyte is 1.0 mol/L LiPF6 with dissolved in ethylene carbonate and dimethyl carbonate (1:1 volume ratio). The discharging/charging tests were performed in the CT2001A battery testing system (Land®, China) at 298 K during 1.0–3.0 V. The Cyclic Voltammetry, charging/discharging curve and Electrochemical Impedance Spectroscopy were conducted on a CHI electrochemical workstation. All hybrid lithium ions capacitors were assembled with AC and C@TiO2-x spheres. The AC is cathode and C@TiO2-x spheres is used as anode. The assembly conditions are as above.


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ASSOCIATED CONTENT Supporting Information Available: Additional SEM, EDS, XPS full spectrum, Fourier infrared spectra, XRD, charging/discharging cures and cycle performance of as-obtained samples, the capacitance calculation method for LIC.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. ORCID Fuqiang Huang: 0000-0001-7727-0488

Notes The authors declare no competing financial interest

Acknowledgements This work was financially supported by National key research and development program (Grant 2016YFB0901600), Science and Technology Commission of Shanghai (Grants No. 16ZR1440400 and 16JC1401700), National Science



Foundation of China (Grant 51672301), the Key Research Program of Chinese Academy of Sciences (Grants No. QYZDJ-SSW-JSC013 and KGZD-EW-T06), CAS Center for Excellence in Superconducting Electronics, and Youth Innovation Promotion Association CAS.

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The hollow C@TiO2-x microspheres exhibited a high specific surface area (325 m2 g1

), a bimodal size distribution of mesopore (∼2.2 and 6.7 nm) and a remarkable

electric conductivity (0.264 µS cm-1) due to the joint effects between the few-layer TiO2-x nanosheets and conductive carbon skeleton. And the C@TiO2-x electrode revealed the excellent electrochemical performance.


Hierarchical Hollow Microspheres Constructed by Carbon Skeleton

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