Efficient Lithium-Ion Storage by Hierarchical Core–Shell TiO2

Jul 5, 2017 - Rational design and surface engineering are the key to synthesizing high-performance electrode materials for electrocatalysis and energy...
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Efficient Lithium-Ion Storage by Hierarchical Core−Shell TiO2 Nanowires Decorated with MoO2 Quantum Dots Encapsulated in Carbon Nanosheets Shitong Wang,† Zhongtai Zhang,† Yong Yang,*,‡ and Zilong Tang*,† †

State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China ‡ Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing, 100871, China S Supporting Information *

ABSTRACT: Rational design and surface engineering are the key to synthesizing highperformance electrode materials for electrocatalysis and energy conversion and storage applications. Herein, a novel three-dimensional (3D) nanoarchitecture of TiO2 nanowires decorated with MoO2 quantum dots encapsulated in carbon nanosheets was successfully synthesized by a simple polymerization method. Such a hierarchical nanostructure can not only exhibit the synergistic effect of structural stability of a 1D TiO2 substrate and high capacity of 0D MoO2 quantum dots but also prevent the aggregation and oxidation of MoO2. As a result, the novel 0D-1D-2D composite illustrates an initial discharge capacity of 470 mAh g−1 at a high current density of 500 mA g−1, especially a capacity retention of about 83% after 450 cycles. The present work highlights the designing strategy of nanoarchitectures containing high capacity materials for enhancing electrochemical performance of Ti-based materials. KEYWORDS: MoO2 quantum dots, TiO2 nanowires, carbon nanosheets, hierarchical architecture, lithium-ion batteries

1. INTRODUCTION As one of the most widely used green energy sources for portable digital products and electric vehicles (EVs), secondary lithium-ion batteries (LIBs) have attracted widespread attention in the world.1 Titanium dioxide (TiO2) is regarded as a competitive anode material for LIBs because of its outstanding Li-ion insertion/extraction reversibility with small structural change and enhanced safety over graphite.2−9 In particular, the monoclinic TiO2 bronze phase (often referred to as TiO2-B) with an open channel structure reveals a pseudocapacitive behavior during lithiation/delithiation, which makes fast charging/discharging of the electrode possible. However, TiO2-B material suffers from relatively low theoretical capacity and poor conductivity, which seriously hinder its potential applications for commercialization.10−13 Therefore, it is necessary to design and fabricate novel TiO2-based electrode materials which possess good conductivity and high capacity, so as to meet the increasing of demand. Combining high capacity materials, such as conversion (displacement) type materials (MoO2, Co3O4, Fe3O4, NiO, Cu2O, VO2, SnO2, WO2, etc.) or alloy-based materials (Sibased, Ga-based, Sb-based, etc.), is considered as a useful method to enhance the capacity of Ti-based electrodes.14−25 Among the materials mentioned above, molybdenum dioxide (MoO2), with relatively high capacity (840 mAh g−1), low electric resistivity (8.8 × 10−5 Ω cm at 300 K), and high density (6.5 g cm−3), has inevitably received considerable interest.26−30 © XXXX American Chemical Society

However, MoO2, including most of the alloy-based and conversion type materials, suffers from massive volume change during charge/discharge processes, leading to serious pulverization and poor cyclability.31,32 Decreasing the size of MoO2 to nanometer scale or even quantum dots (QDs) is a competent way to solve the problem.33−36 Nevertheless, the oxidation of nanosized-MoO2 is inevitable to some extent. The formation of insulating MoO3 layers on the surface of MoO2 results in the decrease of electrochemical activity.37 In this regard, it is highly desirable to prevent the surface oxidation by encapsulating MoO2 nanoparticles in carbon scaffolds, meanwhile increasing the conductivity and stability. Constructing hierarchical heterostructures has been generally employed to enhance the capacity and cyclability of electrode materials.38 A 1D nanostructure not only can relax the strain from volume change effectively but also can provide a large exposed surface as well as a short transfer path for lithium ions.39−41 For instance, Chen et al. synthesized elongated bending TiO2-based nanotubes, which exhibit an ultralong cycling life (10 000 cycles with ∼70% capacity retention) at a superior power rate (25 C), demonstrating its outstanding tolerance for fast ion insertion and extraction.42 What is more, two-dimensional (2D) materials have also been extensively Received: April 13, 2017 Accepted: June 23, 2017

A

DOI: 10.1021/acsami.7b05194 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

analyzer (449 F3). The specific surface area was calculated using an automated vapor sorption analyzer (Autosorb-iQ2-MP, Quanta Chrome) at 77.4 K under vacuum by the Brunauer−Emmett−Teller (BET) method. X-ray photoelectron spectroscopy (XPS) data were obtained using an Escalab 250XI system (Thermo Fisher Scientific, U.S.). Electrochemical Measurements. For the electrochemical measurements, the active materials, Super P and poly(vinylidene fluoride) binder at a weight ratio of 8:1:1, were weighed and added into N-methyl-2-pyrrolidinone solvent with stirring, forming a homogeneous slurry. The obtained slurry was casted via scraper machine on a 15 μm thickness Cu foil, then dried at 110 °C in a vacuum oven overnight. For 2032-coin-type cells, lithium foil as the counter electrode and a microporous membrane (Celguard 2400, USA) as the separator were used. The electrolyte was composed of 80 μL of 1.0 M LiPF6 in a mixture (1:1 volume ratio) of ethylene carbonate (EC) and dimethylcarbonate (DMC). The mass loading of the electrode in this work is ∼1.0 mg cm−2. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) from 10 mHz to 100 kHz were performed by an IM6 (Bas-Zahner, Germany) electrochemical workstation, with a perturbation of 5 mV applied. Before the potentiostatic intermittent titration technique (PITT) measurement, the cell was charged and discharged between 0.05 and 3.00 V for 5 cycles at a relatively low current density of 100 mA g−1. The applied potential step jump is 30 mV with the relaxation time of 3 h, followed by charging to the next voltage at the current density of 50 mA g−1. All tests were measured on a LAND 2001A Cell test system between 0.05 and 3.00 V at ambient temperature.

studied as electrodes for LIBs due to the large specific surface, fast ion diffusion along the short paths, and plentiful ion insertion/extraction channels.43 The most well-known 2D materials for energy storage are sheetlike carbon species, such as graphene.44 However, fabrication of such complicated 0D1D-2D nanoarchitectures with high capacity and superstability is still a challenge. Herein, novel hierarchical core−shell TiO2 nanowires decorated with MoO2 QDs encapsulated in carbon nanoarchitectures were obtained through a facile method. MoO2 tiny dots with the diameter of around 1 nm are uniformly embedded into 2D carbon matrixes, and the carbon nanosheets are constructed on the surface of TiO2-B nanowires. The synergetic effect mentioned above can be summarized as follows: (i) 0D MoO2 QDs can greatly enhance the specific capacity; (ii) 1D TiO2 substrate can provide structural stability for lithium insertion/extraction as well as large exposed surface; (iii) the 2D carbon network can avoid surface oxidation of MoO2 QDs, contribute to the conductivity as well as prevent the aggregation for both two functional materials 0D MoO2 and 1D TiO2. Consequently, the novel 0D-1D-2D nanoarchitecture exhibits outstanding electrochemical performance as a highcapacity anode material for LIBs. The present study highlights the importance of rationally designing hierarchically multicomponent electrode materials, which combines three different dimensions of the material together so as to give full play to their advantages.

3. RESULTS AND DISCUSSION The 3D hierarchical core−shell TiO2@MoO2/C architecture was synthesized via a three-step method as exhibited in Scheme 1. First, hydrogen titanate nanowires (H2Ti2O5·H2O, HTO)

2. EXPERIMENTAL SECTION Material Preparation. First, a typical hydrothermal process was used to synthesize the hydrogen titanate nanowires (HTO) precursor. The well-mixed anatase TiO2 and concentrated NaOH solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and then placed in an oven at 180 °C for 48 h, following the ion substitution progress of Na+ with H+ in HNO3 solution. Second, 30 mg of HTO powder, 10 mg of ammonium molybdate ((NH4)6Mo7O24), and certain 15 mg of dopamine hydrochloride (Dopamine·HCl) with a certain amount of ammonium hydroxide were added in a mixture (1:2 volume ratio) of distilled water and ethanol. After continuous stirring for 6 h, the orange red HTO@Mo-polydopamine (hereinafter referred to as HTO@Mo-PDA) product was harvested. After heat treatment at 450 °C for 2 h under a N2 atmosphere, HTO@ Mo-PDA was transferred to TiO2(B)/MoO2@C (hereinafter referred to as TiO2/MoO2 @C). For comparison, carbon coated TiO 2 nanowires (hereinafter referred to as TiO2@C) were obtained without adding ammonium molybdate; TiO2 nanowires (hereinafter referred to as TiO2) were prepared without adding ammonium molybdate and dopamine hydrochloride, while all the other experimental procedures remain unchanged. Material Characterization. The phase of the as-prepared samples was determined on a Rigaku D/Max-B X with Cu Kα radiation (λ = 1.5418 Å) for X-ray diffraction (XRD). The morphology and size of the samples were characterized by scanning electron microscopy (SEM, MERLIN VP Compact) and transmission electron microscopy (TEM, Hitachi-HT7700). High-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field (HAADF)scanning transition electron microscopy (STEM) were conducted on a JEM-2100F equipped with selected-area electron diffraction (SAED), and an energy dispersive spectrometer (EDS). The Raman spectrum was recorded on an HR800 Raman spectrometer (HORIBA, France) using the 633 nm line of a helium−neon as the excitation beam. The content of molybdenum element was determined by inductively coupled plasma mass spectroscopy (ICP-MS) analysis (Thermo Fisher Scientific, U.S.). Thermogravimetric analysis (TGA) of the obtained sample was performed at a heating rate of 10 °C min−1 between room temperature and 600 °C on a NETZSCH-STA thermogravimetric

Scheme 1. Overall Fabrication Procedure of the TiO2@ MoO2/C Nanoarchitecture

were synthesized via a simple hydrothermal method between anatase TiO2 powders and concentrated NaOH solution.45 Second, dopamine hydrochloride and ammonium molybdate were chosen to obtain a carbon coating with Mo-polydopamine nanosheets around the surface of the HTO nanowires (HTO@ Mo-PDA), which can be illustrated by SEM and TEM in Figure S1. After annealing HTO@Mo-PDA precursor in a N2 atmosphere, the HTO substrate converted to TiO2, and the Mo-PDA precursor transformed in situ to MoO2 QDs along with carbonization of dopamine, resulting in the 0D-1D-2D TiO2@MoO2/C nanoarchitecture. The introduction of Mo could change the growth behavior of pure PDA, and ultimately resulted in the formation of hierarchical Mo-PDA inorganic/ organic hybrid composite. As shown in Figure S2, only solid carbonaceous spheres with smooth surfaces were yielded without the adding of Mo precursor. In this case, the polymerization of dopamine resulted in the isotropic growth B

DOI: 10.1021/acsami.7b05194 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces of PDA, and finally transformed into a spherical shape. On the contrary, upon the addition of Mo7O242− anions into the dopamine solution, the original colorless solution immediately turned dark red, showing the chelation of Mo with PDA molecules under weakly alkaline solution. In the next polymerization, the presence of the chelation of Mo with PDA molecules reduced the surface energy of certain crystal planes, leading to the differences of growth speed and promoting the polymer to evolve into a unique 2D structure. For comparison, carbon coated TiO2 nanowires (TiO2@C) were obtained without adding Mo precursor, and similarly, TiO2 nanowires (TiO2) were prepared without adding Mo precursor and carbon precursor, while keeping the other heating experimental variables fixed. The phase structure of as-synthesized TiO2@MoO2/C was characterized by XRD analysis. As depicted in Figure 1a, the

The morphology and microstructure of the 0D-1D-2D TiO2@MoO2/C nanoarchitecture are further characterized by SEM and HRTEM. The SEM images (Figure 1b,c) show a 1D nanowires shape with the diameter of 150−400 nm. Besides, 2D nanosheets with a thickness of 5−10 nm are uniformly distributed around the surface of 1D nanowires, which can be further demonstrated by HRTEM in Figure 2a,b and Figure

Figure 2. (a−c) HRTEM images and the corresponding SAED pattern of TiO2@MoO2/C composite. (d) HRTEM image of MoO2 QDs embedded in carbon nanosheets. (e, f) TEM-EDS mapping of 0D-1D2D TiO2@MoO2/C nanoarchitecture.

S8a,b. HAADF-STEM further verifies the existence of numerous MoO2 atomic clusters with good dispersibility in a hybrid platelike carbon network (Figure 1d and Figure S8c). The representative HRTEM image as well as the relevant inset SAED patterns (Figure 2 c) indicate single crystallinity of individual TiO2 backbone nanowires. The lattice fringes of 0.58 and 0.36 nm for the backbone nanowires in the HRTEM image correspond to the (200) and (110) atomic planes of TiO2-B, respectively. At a higher magnification (Figure 2d), the 0D-1D2D nanoarchitectures of TiO2@MoO2/C are clearly observed, demonstrating that the well-dispersed amorphous MoO2 QDs with a typical size around 1 nm are encapsulated in the 2D carbon nanosheets. Elemental mapping (Figure 2e) and the corresponding EDX line scanning (Figure S9) were conducted on the nanostructures using HAADF-STEM mode, indicating the uniform distribution of Ti, Mo, O, and C elements. It is worth noting that MoO2 QDs are well-wrapped in the carbon nanosheets instead of on the surface. Therefore, the carbon hybrid nanosheets not only can prevent the aggregation and oxidation of MoO2 QDs but also can avoid the abruption of MoO2 QDs during the charge/discharge process, keeping the integrity of the nanoarchitecture. The surface chemical state and composition of the TiO2@ MoO2/C composite are investigated by XPS. Several distinct peaks located at 531.6 (O 1s), 459.6 (Ti 2p), 416.6 (Mo 3p1/2), 399.6 (Mo 3p3/2), 285.6 (C 1s), and 233.6 (Mo 3d) eV in the typical survey XPS spectrum (Figure 3a) indicate the existence of molybdenum oxide, titanium oxide, and carbon species. From high-resolution XPS for the Mo 3d peak (Figure 3b), the Mo 3d3/2 and 3d5/2 peaks are located at 232.8 and 229.6 eV, respectively, indicating the Mo(IV) oxidation state of MoO2.26,47 Besides, two peaks at 235.8 and 230.8 eV could

Figure 1. (a) XRD pattern with TiO2-B (JCPDS No. 74-1940) in black line. (b, c) SEM images and (d) STEM image of 0D-1D-2D TiO2@MoO2/C nanostructure.

identification diffraction peaks are well assigned to the monoclinic TiO2-B with space group of C2m (JCPDS No. 74-1940). However, there were no MoO2 diffraction peaks in the XRD pattern, verifying the amorphous nature of the hybrids, which could be associated with the low crystalline nature under relatively mild calcination conditions in the N2 atmosphere. The Raman spectrum (Figure S3) was further conducted to support the predication above. Considering that some peaks of MoO2 might be covered by those of TiO2 with high crystallinity, the MoO2/C hybrid material was prepared using the same procedure without adding HTO precursor (TEM, HRTEM image and XRD pattern of MoO2/C are shown in Figures S4−S6). On the basis of the Raman spectrum in Figure S3, Raman peaks at 560 and 720 cm−1 are characteristic for the telescopic vibration mode of molybdenum oxide. The rest of the peaks including 191, 317, 390, 664, 788, and 866 cm−1 could be assigned to the characteristic peaks of MoO2.46 The MoO2 content in TiO2@MoO2/C composite determined from the ICP-MS was about 19 wt %. Besides, two broad peaks assigned to the typical D band (1360 cm−1) and G band (1560 cm−1) implied the amorphous nature of carbon. The content of carbon was estimated to be approximately 29 wt % by TGA (Figure S7), and the corresponding weight loss could be attributed to the carbon decomposition and the oxidation of MoO2. C

DOI: 10.1021/acsami.7b05194 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

materials and electrolyte, which could lead to outstanding highrate performance. To elucidate the electrochemical performances of the product as an anode of LIBs, standard half-coin cells were assembled, as illustrated in Figure 4. Figure 4a presents the galvanostatic discharge−charge voltage profiles of the 0D-1D2D TiO2@MoO2/C nanostructure for the first five cycles. During the first cycle, it exhibits an irreversible reduction peak around 0.75 V in the first cathode sweep. This might be caused by the generating of the solid electrolyte interphase (SEI) layer as well as the adverse reactions of electrolytes,47 resulting in a relatively low Coulombic efficiency of about 49% (Figure S11). In the following cycles, a pair of redox peaks at about 1.69 V/ 1.53 V can be identified, which might correspond to the redox reactions of Ti4+/Ti3+ of the TiO2-B phase (eq 1). Another two indistinct reversible peaks at 1.49 V/1.27 V correspond to the redox reaction of Mo6+/Mo4+, which are associated with reversible phase transitions between LixMoO2 and MoO2 (eq 2).52,53 It is noted that, when the potential is discharged below 1.0 V, LixMoO2 would slowly convert to Mo and Li2O especially when the particle size is ultrasmall (e.g.,