Two-Dimensional Unilamellar Cation-Deficient Metal Oxide

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Two-Dimensional Unilamellar Cation-Deficient Metal Oxide Nanosheet Superlattices for HighRate Sodium Ion Energy Storage ACS Nano 2018.12:12337-12346. Downloaded from pubs.acs.org by UNIV DE BARCELONA on 01/08/19. For personal use only.

Pan Xiong,†,‡ Xiuyun Zhang,§,∥ Fan Zhang,† Ding Yi,∥ Jinqiang Zhang,† Bing Sun,† Huajun Tian,† Devaraj Shanmukaraj,⊥ Teofilo Rojo,*,⊥ Michel Armand,*,⊥ Renzhi Ma,‡ Takayoshi Sasaki,*,‡ and Guoxiu Wang*,† †

Centre for Clean Energy Technology, School of Mathematical and Physical Sciences, University of Technology Sydney, Sydney, NSW 2007, Australia ‡ International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan § College of Physical Science and Technology, Yangzhou University, Yangzhou 225002, China ∥ Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea ⊥ CIC ENERGIGUNE, Parque Tecnológico de Á lava, Miñano 01510, Spain S Supporting Information *

ABSTRACT: Cation-deficient two-dimensional (2D) materials, especially atomically thin nanosheets, are highly promising electrode materials for electrochemical energy storage that undergo metal ion insertion reactions, yet they have rarely been achieved thus far. Here, we report a Tideficient 2D unilamellar lepidocrocite-type titanium oxide (Ti0.87O2) nanosheet superlattice for sodium storage. The superlattice composed of alternately restacked defective Ti0.87O2 and nitrogen-doped graphene monolayers exhibits an outstanding capacity of ∼490 mA h g−1 at 0.1 A g−1, an ultralong cycle life of more than 10000 cycles with ∼0.00058% capacity decay per cycle, and especially superior low-temperature performance (100 mA h g−1 at 12.8 A g−1 and −5 °C), presenting the best reported performance to date. A reversible Na+ ion intercalation mechanism without phase and structural change is verified by first-principles calculations and kinetics analysis. These results herald a promising strategy to utilize defective 2D materials for advanced energy storage applications. KEYWORDS: cation vacancies, unilamellar nanosheets, low-temperature sodium storage, lepidocrocite-type titanium oxide, superlattice

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storage kinetics and retains structural integrity during the reversible insertion of Li ions.8 Cation vacancies also determine the thermodynamic potentials of the metal ion insertion processes in electrochemical energy storage (e.g., rechargeable batteries and supercapacitors).9,10 Recently, intentional creation of cation vacancies in electrode materials has been tested to increase energy storage capabilities, where cation vacancies provide a thermodynamically favorable driving force for additional cation insertion.11−14 For example, the cation-deficient spinel γ-Fe2O3 and anatase TiO2 nanoparticles have demonstrated improved intercalation capacity because of additional cation intercalation

acancy engineering is desirable to rationally tailor the intrinsic properties and consequently tune the physicochemical performance (magnetic, electrical, optical, or catalytic) of functional nanomaterials. By controlling the concentration, distribution, and charge of the vacancies in lattice structures, it is expected to unlock extra potentials that are not attainable in perfect crystals.1−3 For example, both cation and anion vacancies can act as nucleation centers and anchoring sites to stabilize independent metal atoms for fabrication of single-atomic-site catalysts with high metal loading.4−6 Significantly enhanced electrocatalytic oxygen reduction and evolution activities have been achieved by tuning cation deficiencies in perovskite oxides. 7 The introduction of oxygen vacancies into the α-MoO3 lattice leads to a larger interlayer spacing that promotes faster charge © 2018 American Chemical Society

Received: August 14, 2018 Accepted: November 14, 2018 Published: November 14, 2018 12337

DOI: 10.1021/acsnano.8b06206 ACS Nano 2018, 12, 12337−12346

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Cite This: ACS Nano 2018, 12, 12337−12346

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Figure 1. (A) AFM image and corresponding height profiles of Ti0.87O2 nanosheets. (B) TEM image of Ti0.87O2 nanosheets. (C,D) HAADFSTEM images of a Ti0.87O2 nanosheet. (E) Line profile exhibits a clear intensity variation of atomic columns. The arrows indicate the Ti vacancies. (F) Effects of Ti vacancies and single-layer features for intercalation of Na+ ions. Top: from left to right, corresponding structures represent the interaction sites in anatase TiO2, stoichiometric (defect-free) lepidocrocite-type TiO2 single layer, and Ti-deficient lepidocrocite-type Ti0.87O2 single layer. Bottom: DFT-calculated intercalation energies for Na+ ions in different titanium oxide structures.

sites for Li ion insertion provided by the cation vacancies.15,16 A high concentration of cation vacancies in hollow iron oxide nanoparticles has been proven to be responsible for enhanced intercalation and conversion capacities for fast Li storage.17 More importantly, the incorporation of cation vacancies has been demonstrated as an effective strategy to unlock reversible insertion of guest ions other than Li+ ions (e.g., Na+, Zn2+, Mg2+, Al3+, etc.).18−20 However, it should be noted that, in these reported cation-deficient electrode materials, most of the cation vacancies were present in the interior rather than on the surface, and hence they possibly may not be effectively accessible for significant cation insertion. The vacancy engineering of 2D materials, especially the atomically thin nanosheets, has attracted much attention in recent years.3,21,22 Compared with 3D solids, the effects of defects are often more accentuated in 2D nanosheets due to structural and electron confinement. For example, the introduction of sulfur vacancies can induce room-temperature ferromagnetism in nonmagnetic MoS2 nanosheets.23 In addition, the inert basal plane of MoS2 nanosheets can be activated and optimized for hydrogen evolution reactions by introducing sulfur vacancies.24,25 2D atomically thin nanosheets have been regarded as promising electrode materials in electrochemical energy storage applications because of their highly exposed surface active sites and atomic scale thickness.26,27 However, most of the previous studies mainly focused on anion vacancies,1,3 so research on cation-defective 2D nanosheets for energy storage have rarely been achieved

thus far. Based on the above discussion, it is of great interest to design and utilize cation-deficient 2D unilamellar nanosheets for ultimately enhanced intercalation capabilities in energy storage. Unilamellar nanosheets not only provide remarkably shortened diffusion lengths for fast charge transport but also enable the cation vacancies distributed on the surfaces to be fully accessed for ion insertion. Here, we report a 2D unilamellar lepidocrocite-type titanium oxide (Ti0.87O2) nanosheet with Ti vacancies in a superlattice structure as an anode material for sodium ion batteries. Compared with previously reported titanium oxides (anatase, rutile, bronze, and hollandite) (Figure S1), the Ti0.87O2 nanosheet is more promising for Na storage because of its single-layer morphology and cation-defective features. The superlattice is composed of intimately hybridized Ti0.87O2 and nitrogen-doped (N-doped) graphene nanosheets in an alternate restacking pattern and exhibits an ultrahigh rate capability, an excellent cycle life up to 10000 cycles, and a superior low-temperature capability. A reversible intercalation mechanism without phase and structural changes is investigated. Density functional theory (DFT) calculations and kinetics analysis are performed to assess the favorable Na+ ion intercalation processes in the superlattice structure.

RESULTS AND DISCUSSION Cation-Deficient Unilamellar Ti0.87O2 Nanosheets. The cation-deficient unilamellar Ti 0 . 8 7 O 2 nanosheets, Ti0.87□0.13O20.52−, where □ represents the Ti vacancies, were 12338


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Figure 2. (A) Illustration of the synthesis processes of the Ti0.87O2/N-doped graphene superlattice. (B) XRD patterns of (i) Ti0.87O2/PDDAgraphene superlattices and (ii) Ti0.87O2/N-doped graphene superlattices. (C) SEM and (D) HRTEM images of Ti0.87O2/PDDA-graphene superlattices. (E) SEM and (F) HRTEM images of Ti0.87O2/N-doped graphene superlattices. (G) High-resolution spectrum of N 1s in Ti0.87O2/N-doped graphene superlattices.

ions were probed using DFT calculations. Previous reports have shown that anatase TiO2 is a promising anode for Na ion batteries.29 We first compare the intercalation energies of Na+ ions in stoichiometric lepidocrocite-type TiO2 unilamellar nanosheet and anatase TiO2. Figure 1F shows the possible two Na host sites: the interstitial sites in anatase TiO2 and the surface sites in lepidocrocite nanosheets, which resulted in intercalation energies of −0.87 and −1.80 eV, respectively. The calculated results predict more favorable intercalation of Na+ ions in nanosheets. The effects of Ti vacancies on intercalation behavior of Na+ ions was further considered using the cationdeficient nanosheet, where the content of Ti vacancies is set based on the formula of Ti0.87□0.13O2, and the possible Na host sites are the Ti vacancies. The smallest intercalation energy of −4.82 eV was obtained among the studied structures, which indicates a more favorable Na ion intercalation within the defective nanosheets. These results suggest that the Tideficient Ti0.87O2 nanosheets are more promising hosts for the intercalation of Na+ ions. Ti0.87O2/Nitrogen-Doped Graphene Superlattice. Unilamellar nanosheets tend to restack, and cation-deficient structures tend to be less electronically conductive than the

prepared by a soft chemical exfoliation of a layered titanate crystal. The resulting Ti0.87O2 nanosheets possess an overall negative charge, which enables a stable colloidal suspension with noticeable Tyndall light scattering (Figure S2). The negatively charged nature of the Ti0.87O2 nanosheets was also confirmed by the ζ-potential measurements (Figure S3). A representative atomic force microscopy (AFM) image (Figure 1A) shows a uniform thickness of ∼1.1 nm, suggesting the formation of unilamellar nanosheets. A transmission electron microscopy (TEM) image (Figure 1B) presents an almost transparent sheet-like morphology, again confirming the ultrathin thickness of the as-obtained Ti0.87O2 nanosheets. The high-angle annular dark-field STEM (HAADF-STEM) images of a Ti0.87O2 nanosheet are shown in Figure 1C,D, which allows the direct visualization of the Ti vacancies. The variation in atomic column intensity corresponds to a variation in the atomic occupations.20,28 The intensity variation of atomic columns is shown in the line profile in Figure 1E, where the low intensity points as indicated by arrows are the Ti vacancies. The effects of single-layer features and Ti vacancies of the asobtained Ti0.87O2 nanosheet on intercalation behavior of Na 12339


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Figure 3. (A) Cycling performance at 0.1 A g−1 for 100 cycles and then 0.5 A g−1 for 1000 cycles. (B) Rate performance at various current densities from 0.1 to 51.2 A g−1 (∼155 C). (C) Ultra-long-term cycling performance at 5 A g−1 for 5000 cycles and then 10 A g−1 for 10000 cycles. (D) Comparison of rate capability between the superlattice anode and other reported titanium oxide-based anodes. (E) Capacity retentions at different temperatures (25 and −5 °C) at various current densities from 0.1 to 12.8 A g−1.

defect-free counterparts.10 To fully utilize the Ti-deficient unilamellar nanosheets for Na ion storage, a Ti0.87O2/N-doped graphene superlattice composed of alternately restacked unilamellar Ti0.87O2 and N-doped graphene nanosheets was designed and synthesized. The molecular-scale hybridized superlattice enables not only stabilization of the unilamellar morphology but also improvement of the overall conductivities. As illustrated in Figure 2A, the Ti0.87O2/N-doped graphene superlattice was synthesized via a solution-phase assembly method together with a postcalcination treatment. Stable suspensions of negatively charged unilamellar Ti0.87O2 nanosheets and positively charged poly(diallyldimethylammonium chloride) (PDDA)-modified reduced graphene oxide (PDDA-graphene) nanosheets were first mixed. Due to electrostatic attraction, these two oppositely charged nanosheets were attracted to each other and selfassembled into the Ti0.87O2/PDDA-graphene superlattice. Then, upon a postcalcination process in Ar atmosphere, the PDDA-graphene was carbonized to N-doped graphene and the Ti0.87O2/N-doped graphene superlattice was obtained. The positively charged PDDA-graphene nanosheets were prepared via modification of reduced graphene oxide (rGO) with PDDA during the chemical reduction of graphene oxide (GO). The presence of nitrogen in both the X-ray photoelectron spectroscopic (XPS) survey spectrum (Figure S4) and the energy-dispersive X-ray (EDX) spectrum (Figure S5) suggests the successful modification with PDDA. Compared with GO, the greatly suppressed peaks of the oxygencontaining groups in the C 1s spectrum of PDDA-graphene (Figure S4) indicate the reduction of GO. A TEM image (Figure S5) clearly presents an ultrathin sheet-like morphology of the PDDA-graphene. The height profiles of the AFM image further determine a thickness of ∼1.5 nm (Figure S6), which is larger than that of rGO nanosheets (∼0.6 nm),26,27 indicating the modification with PDDA. In addition, a charged N+ peak (Figure S4D) and ζ-potential measurements (Figure S3)

clearly showed the positively charged state of the PDDAgraphene. For a full face-to-face stacking, an optimized mass ratio between Ti0.87O2 and PDDA-graphene nanosheets was estimated to be ∼2.8 based on a hypothesized area matching model in the lateral dimensions of nanosheets (Figure S7).30 The suspensions of these two nanosheets were mixed in this ratio. A flocculation was induced immediately and the product of Ti0.87O2/PDDA-graphene was collected. X-ray diffraction (XRD) patterns of the Ti0.87O2/PDDA-graphene show a prominent peak with a basal spacing of ∼1.3 nm (line i in Figure 2B and Figure S8). The other two peaks at ∼49 and 63° can be ascribed to the in-plane 20 and 02 reflections of the Ti0.87O2 single layer (T20 and T02).26,27,30 The observed dspacing is half of the summed thicknesses of these two nanosheets, which suggests that the observed peak should be identified as the second-order reflection of the superlattice-like structure. XRD simulation based on the alternately restacked superlattice-like structure (Figure S9) shows an intensity of the second-order peak much stronger than that of the first-order one, matching with the measured pattern (Figure S10). Scanning electron microscopy (SEM) (Figure 2C) and TEM (Figure S11) images show a 3D porous structure composed of thin crumpled layers. The high-resolution TEM (HRTEM) image (Figure 2D) further reveals that the thin layers are restacked nanosheets with a repeating periodicity of ∼2.6 nm, which is twice the observed spacing in the XRD pattern. Upon a calcination treatment in Ar at 600 °C, the PDDAgraphene was carbonized to N-doped graphene, whereas the Ti0.87O2 monolayer was preserved.31 A new peak with a contracted basal spacing of ∼1.1 nm was observed for the Ti0.87O2/N-doped graphene (line ii in Figure 2B and Figure S8). This corresponds to the first-order peak of the Ti0.87O2/ N-doped graphene superlattice, although the broad peak suggests that the stacking order is rather limited. The inplane reflections of the unilamellar Ti0.87O2 nanosheets were 12340


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large reversible specific capacity of the Ti0.87O2/N-doped graphene superlattice could still ensure a high energy density. Figure 3A further shows the cycling performance of the Ti0.87O2/N-doped graphene superlattice anodes at a current density of 0.5 A g−1. A large reversible specific capacity remained at ∼375 mA h g−1 was obtained after 1000 cycles. Ti0.87O2/N-doped graphene superlattice anode was further charged/discharged at various current densities ranging from 0.1 to 51.2 A g−1 (Figure S19). High reversible capacities of ∼490, 440, 380, 330, 280, 230, 200, 150, and 105 mA h g−1 were exhibited at current densities of 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, and 25.6 A g−1, respectively (Figure 3B). Even at an ultrahigh current density of 51.2 A g−1 (∼155 C, 1 C = 330 mA g−1), a specific capacity of ∼65 mA h g−1 was still maintained. After the high current density measurements, the capacity recovered its initial value when the current density was returned to 0.1 A g−1, indicating the high reversibility of the superlattice anodes. Furthermore, the Ti0.87O2/N-doped graphene superlattice anodes exhibited excellent long-term cycling stability at high current densities (Figure 3C). Almost no capacity decay was observed after 5000 cycles. Even at an ultrahigh current density of 10 A g−1, the Ti0.87O2/N-doped graphene superlattice can still deliver a reversible specific capacity of ∼155 mA h g−1 after 10000 cycles, corresponding to a capacity decay as low as ∼0.00058% per cycle. To the best of our knowledge, this is the best rate capability and cycling stability among all reported Ti-based anodes as well as their carbon-based hybrid anodes for sodium ion batteries (Figure 3D and Table S1).35,38−41 For comparison, a freeze-dried sample of restacked Ti0.87O2 nanosheets (Figure S20) and the Ti0.87O2/PDDA-graphene superlattice were also examined as anodes for sodium ion batteries. The restacked Ti0.87O2 nanosheets exhibited an initial specific capacity of ∼200 mA h g−1 at 0.1 A g−1, and the capacity decayed drastically to below 50 mA h g−1 after 100 cycles. Practically no capacity was delivered when the current density was increased to larger than 1.6 A g−1 (Figure S21). The Ti0.87O2/PDDA-graphene superlattice exhibited superior performance to the restacked Ti0.87O2 nanosheets (Figure S22), probably mainly due to the introduction of graphene, which contribute to a more favorable charge transfer process (Figure S23). A stable specific capacity of ∼300 mA h g−1 was obtained for the Ti0.87O2/PDDAgraphene superlattice anodes after 100 charge/discharge cycles at 0.1 A g−1 (Figure S22). However, the rate performance was much worse than that of the Ti0.87O2/N-doped graphene superlattice anodes, especially at large current densities. This difference should be ascribed to the improved conductivity of N-doped graphene and possible intimate chemical interaction among the Ti0.87O2/N-doped graphene superlattice, which is more favorable for fast charge transport than the electrostatically attracted Ti0.87O2/PDDA-graphene superlattice (Figure S23). Sodium ion batteries usually suffer from poor rate capability due to the large radius of Na+ ions, which usually results in sluggish Na+ ion diffusion within the host materials.42 Thus, it is desirable to develop electrode materials with the capability to operate at low temperatures and deliver a considerable sodium storage capability.43,44 Here, we evaluated the rate capacity of the Ti0.87O2/N-doped graphene superlattice anodes at a low temperature of −5 °C. As shown in Figure 3E and Figure S24, a comparable specific capacity of ∼500 mA h g−1 at 0.1 A g−1 was obtained at both 25 and −5 °C. More importantly, at a low temperature of −5 °C, the Ti0.87O2/N-

preserved, elucidating the preservation of the Ti 0.87 O 2 monolayer after the annealing process.31 The SEM (Figure 2E) and TEM (Figure S12) images confirmed the wellpreserved overall morphology after the calcination treatment. The HRTEM image of the Ti0.87O2/N-doped graphene superlattice (Figure 2F) exhibited a total repeated spacing of ∼1.1 nm, which is consistent with the observed d-spacing in the XRD pattern (Figure 2B). Moreover, it should be noted that the repeated parallel lattice fringes are composed of two different spacings in an alternating sequence, ∼0.7 and 0.4 nm, which should be attributed to Ti0.87O2 and N-doped graphene monolayers, respectively (HRTEM image in Figure 2A). The N 1s spectrum of the Ti0.87O2/N-doped graphene superlattice (Figure 2G) can be fitted to three main peaks of pyridinic N (∼398.8 eV), pyrrolic N (∼400.5 eV), and charged pyridinic N+ (∼402 eV),32 indicating the successful nitrogen doping. The N/C atomic ratio of the Ti0.87O2/N-doped graphene superlattice was measured to be ∼3.1 atomic %, which is close to that of other reported results.33 The content of N-doped graphene in the superlattice is determined to be ∼25 wt % (Figure S13) by thermal gravimetric analysis. Figure S14 compares the Ti 2p spectra of the Ti0.87O2/PDDA-graphene and Ti0.87O2/N-doped graphene. The same splitting value of 5.7 eV confirms that Ti is maintained in a Ti4+ chemical state in these two superlattices. A peak shift toward higher binding energy was observed in the Ti0.87O2/N-doped graphene superlattice, which may be ascribed to the possible formation of intimate interactions (N−Ti) between Ti0.87O2 and Ndoped graphene during the annealing process. The intimate interaction allows for fast charge transport and robust structural integrity, which contributes to superior rate capability and cycling stability.34 A N2 adsorption/desorption isotherm of Ti0.87O2/N-doped graphene superlattice is shown in Figure S15. A high Brunauer−Emmett−Teller surface area of ∼332 m2 g−1 was obtained. Sodium Ion Storage Performance. Figure S16 shows the initial charge/discharge profiles of Ti0.87O2/N-doped graphene superlattice anodes at 0.1 A g−1. After 100 charge/discharge cycles, a stable reversible capacity of ∼450 mA h g−1 was attained (Figure 3A). It should be noted that the N-doped graphene nanosheets exhibited a reversible capacity of ∼100 mA h g−1 at the same current density (Figure S17). As the mass ratio of Ti0.87O2 to N-doped graphene was ∼3, the contribution of the N-doped graphene to the capacity of the superlattice anodes was estimated as ∼25 mA h g−1. Thus, the capacity of the Ti0.87O2 nanosheets was ∼425 mA h g−1. This value is beyond the theoretical capacity of conventional TiO2 anodes and can be ascribed to the Ti vacancies which provide additional intercalation sites of Na ions for larger theoretical capacity (see calculation in Supporting Information). It is also noted that an average discharge voltage plateau at ∼1.2 V for Na insertion was observed in the case of Ti0.87O2/N-doped graphene superlattice anodes (Figure S18). This is higher than that of typical TiO2 anodes (∼0.8 V),29,35 which indicates a thermodynamically more favorable Na+ ion intercalation process in the Ti0.87O2/N-doped graphene superlattice.36 Previous works have revealed that the cation vacancies can make the Li+ ion intercalation process more thermodynamically favorable.17,37 By analogy, it is reasonable to conclude that the Ti vacancies here could also contribute to a strong thermodynamic driving force for a favorable Na+ ion intercalation. Although a slightly high operational voltage for an anode would sacrifice some energy density in full cells, the 12341


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Figure 4. (A) HRTEM and elemental mapping images of Ti0.87O2/N-doped graphene superlattice after Na+ intercalation. (B) Highresolution spectrum of Ti 2p of Ti0.87O2/N-doped graphene superlattice in initial, discharged, and charged states. (C) Cyclic voltammetry curves at various scan rates from 0.1 to 1 mV s−1. (D) Determination of the b value. (E) Contribution of the capacitive process at a scan rate of 1 mV s−1. (F) Contribution ratio of the capacitive process at different scan rates. (G) Schematic illustration of a possible reversible Na+ intercalation mechanism without phase and structural change in the Ti0.87O2/N-doped graphene superlattice anode.

nanosheets were preserved (Figure S26A), revealing no obvious phase change during Na+ intercalation/deintercalation. Figure 4B compares Ti 2p XPS spectra of the Ti0.87O2/Ndoped graphene superlattice anodes at the end of the charge and discharge cycles. Two characteristic peaks of Ti4+ without obvious evidence of Ti3+ or metallic Ti were observed, indicating no phase change in the superlattice anode. Compared with the initial state, the Ti 2p peaks slightly shifted (∼0.2 eV) to lower binding energy after Na + intercalation/deintercalation, which should be attributed to the interaction between negatively charged Ti0.87O2 nanosheets and intercalated Na+ ions. Due to a high specific surface area (Figure S15), it is reasonable that the partial of the intercalated Na+ ions are still trapped in this unilamellar nanosheet-based superlattice after the first charge cycle, resulting in a relatively low initial Coulombic efficiency, as shown in Figure S16. However, it is unreasonable to conclude that these intercalated Na+ ions consumed all the Ti vacancies, making them useless.

doped graphene superlattice anodes could still deliver a considerable specific capacity of ∼100 mA h g−1 at a high current density of 12.8 A g−1. Such a superior low-temperature capability for Na storage has not been previously reported for the metal oxide-based anodes. Reversible Na+ Intercalation Mechanism. Figure 4A shows HRTEM images of the Ti0.87O2/N-doped graphene superlattice anode after the discharge process. The maintained multilayered structure without structural collapse was observed. Moreover, an expanded basal spacing of ∼1.4 nm and well-distributed Na elemental traces suggest a possible intercalation of Na+ ions in the interlayer space of the superlattice. Figure S25 shows a TEM image after the charge process, where a maintained overall morphology is also confirmed. The HRTEM (Figure S26B) images illustrate a reduced spacing of ∼1.2 nm after the charge process, which suggest the Na+ deintercalation. Moreover, the in-plane reflections of both Ti0.87O2 nanosheets and N-doped graphene 12342


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Figure 5. (A) Illustration of the Na diffusion path along the [100] directions between the TiO2/graphene bilayers. (B) Energy of Na diffusion along the [100] direction in TiO2 bilayers and TiO2/graphene bilayers. (C) Illustration of the Na diffusion path along the [001] directions between the TiO2/graphene bilayers. (D) Energy of Na diffusion along the [001] direction in TiO2 bilayers and TiO2/graphene bilayers. (E) Illustration of a possible Na diffusion path in Ti0.87O2/graphene bilayers. (F) Energy of Na diffusion in Ti0.87O2 bilayers and Ti0.87O2/ graphene bilayers.

doped graphene superlattice. The contribution of the capacitive behavior can be further quantified by separating the peak current (i) at a fixed voltage into a capacitive process (proportional to v) and a diffusion-controlled process (proportional to v1/2): i = k1v + k2v1/2.49 By determining k1 and k2, we can quantify the fraction of the two processes. Figure 4E shows the typical CV profile at a scan rate of 1.0 mV s−1, in which a dominated contribution of 85% is capacitive. The contribution of the capacitive processes at other scan rates were also quantified (Figure 4F). The fraction gradually improved by increasing the scan rate and reached a maximum value of 98% at 5 mV s−1. The dominant capacitive-like process implies a fast sodiation/desodiation process without obvious structural change,49−51 which agrees with the excellent rate capability and cycle stability in the electrochemical measurements. A reversible capacity of 425 mA h g−1 was maintained after 100 charge/discharge cycles (Figure 3A), which is higher than the theoretical specific capacity (335 mA h g−1) of stoichiometric TiO2. This may suggest a reversible insertion of Na+ ion into the Ti vacancies of the Ti0.87O2/N-doped graphene superlattice, similar to the previous reports of reversible insertion of metal ions (Li+, Na+, Mg2+, Al3+, etc.) into cation vacancies of metal oxide anodes for rechargeable batteries.17,19,20 Based on the above results, a possible sodium storage mechanism is proposed as shown in Figure 4G. During the charge/discharge cycles, the reversible intercalation of Na+ ions occurs both into the Ti vacancies and between the interlayer spaces of the superlattice. The multilayered structure without phase or structural change is maintained during the sodiation/desodiation processes. The results presented above show superior sodium storage performance of ultrahigh rate capability, long-term cycling stability, and low-temperature capability in the Ti0.87O2/Ndoped graphene superlattice, which can be attributed to the unilamellar nanosheet-based superlattice structure. Theoretical calculation was performed to analyze the diffusion dynamics of

Moreover, STEM and corresponding elemental mapping images showed the preserved morphology after 1000 charge/ discharge processes (Figure S27). These results support a Na+ ion intercalation/deintercalation process without phase and structural change in the Ti0.87O2/N-doped graphene superlattice anode. This behavior is different from reported titanium oxide-based anodes, in which Na+ insertion/intercalation with Ti4+/Ti3+ redox couple and/or new phases of sodium titanate and metallic titanium were observed.35,38,45 It should be noted that here the superlattice anode is based on Ti0.87O2 single layer, which is a genuine unilamellar nanosheet without 3D ordering in the crystallographic structure. The negatively charged unilamellar nanosheets with cation vacancies are more capable of hosting Na+ ions (Figure 1F). Moreover, the interlayer distance between every Ti0.87O2 and adjacent Ndoped graphene nanosheet is ∼0.55 nm, which is large enough for Na+ intercalation.46−48 Therefore, we conclude a reversible Na+ intercalation mechanism without phase and structural change for the Ti0.87O2/N-doped graphene superlattice, which is similar to the carbon-based anodes with enough interlayer distances for sodium insertion/extraction.46,47 Kinetics analysis based on cyclic voltammetry (CV) measurements at different scan rates was carried out to further investigate the Na+ intercalation processes. Similar CV curves of a pair of cathodic/anodic peaks located at ∼1.2/1.3 V with small polarization were displayed at various scan rates from 0.1 to 10 mV s−1 (Figure S28 and Figure 4C), which is ascribed to the reversible Na+ ion insertion and extraction reactions.38 Based on the power law relationship between peak current (i) and scan rate (v), i = avb, the b value can be estimated by plotting log(i) against log(v).49 A b value of 0.5 indicates a slow diffusion-controlled process, whereas a value of 1.0 suggests a fast capacitive-like behavior. Figure 4D presents the plots of log(i) versus log(v), in which a b value of 0.94 and 0.95 is obtained for cathodic and anodic peaks, respectively, suggesting fast capacitive-like kinetics in the Ti0.87O2/N12343


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the titanate crystals in 0.5 mol L−1 HCl solution at room temperature for 2 days. The acid solution was replaced daily with a fresh one. The protonic titanate crystals were collected by filtration, washed with copious amounts of pure water, and air-dried. Finally, the protonic titanate crystals were treated by shaking in a tetrabutylammonium hydroxide solution ((C4H9)4NOH; hereafter TBAOH). The TBAOH concentration was in a 1:1 molar ratio with respect to the exchange protons in the titanate. The PDDA-graphene nanosheets were prepared by modifying rGO nanosheets with PDDA. First, 200 mL of a GO suspension (0.2 mg mL−1), prepared by the Hummers’ method, was mixed with 1.5 mL of a PDDA solution (20 wt %). Subsequently, 15 μL of hydrazine monohydrate (98 wt %) was added, and the suspension was heated to 90 °C and stirred for 3 h. The resulting slurry was subjected to highspeed centrifugation at 20000 rpm, and the recovered sediment was redispersed in H2O. A stable suspension of PDDA-graphene was obtained by collecting the supernatant after centrifugation at 6000 rpm. To obtain the Ti0.87O2/PDDA-graphene superlattice, suspensions of PDDA-graphene and Ti0.87O2 nanosheets were mixed dropwise under continuous stirring at a determined mass ratio based on a hypothetical area-matching model. The flocculate was recovered and washed after centrifugation at 6000 rpm and then dried in air. The Ti0.87O2/N-doped graphene superlattice was prepared by annealing the as-obtained Ti0.87O2/PDDA-graphene superlattice in Ar at 600 °C for 2 h. The freeze-dried sample of restacked Ti0.87O2 nanosheets was prepared by freeze-drying the slurry obtained by high-speed centrifugation of the colloidal suspensions of Ti0.87O2 nanosheets. Material Characterization. Powder XRD data were recorded using a Bruker D8 Discover diffractometer equipped with monochromatic Cu Kα radiation (λ = 0.15405 nm). A field-emission scanning electron microscope (Zeiss Supra 55VP) and a JEOL JEMARM200F TEM instrument were employed to observe the microtextures and morphologies of the samples. For the atomic-scale HRTEM observation of the Ti0.87O2 nanosheets, the dilute suspension of nanosheets was dropped onto a holey carbon film on a TEM grid, which was then illuminated under ultraviolet light in air for 2 h before the observation. The acceleration voltage was set to 80 kV to decrease knock-on damage in the high-resolution observation. A Dimension 3100 SPM instrument was used to examine the topography of the nanosheets deposited onto Si wafer substrates. The ζ-potentials of nanosheet suspensions were determined using an ELS-Z ζ-potential analyzer. Thermogravimetric analysis measurements were carried out using a SDT 2960 instrument. XPS measurements were performed using an ESCALAB250Xi (Thermo Scientific, UK) equipped with monochromated Al Kα (energy = 1486.68 eV). Electrochemical Measurements. The sodium ion battery measurements were carried out using a half-cell system in CR2032type coin cells. The working electrodes were prepared by casting a slurry of active material, carbon black, and polyvinylidene difluoride in a weight ratio of 80:15:5 onto a copper foil and drying under vacuum at 80 °C for 24 h. The mass loading of the active material was approximately 1.2 mg cm−2. Sodium foil was used as the counter electrode. Whatman glassy fibers were used as the separator. The electrolyte was 1 M NaClO4 in ethylene carbonate/propylene carbonate with a 5 wt % fluoroethylene carbonate additive. The CV and electrochemical impedance spectroscopy tests were carried out using a VMP3 electrochemical workstation (Bio-Logic Inc.). The galvanostatic charge/discharge tests were performed on Neware battery testers. Theoretical Calculations. First-principles calculations were performed in the Vienna ab initio simulation package (VASP).54 The ion−electron interactions were described by the projectoraugmented wave potential (PAW)55 and the generalized gradient approximation (GGA) with Perdew−Burke−Ernzerholf (PBE) functional56 including van der Waals corrections (DFT-D2 method57) was employed. The energy cutoff for the plane-wave expansion was set to 450 eV. The Brillouin zone integration was sampled with a Monkhorst−Pack grid of 3 × 2 × 1 for geometry optimization. Moreover, the Coulomb interaction effect (Hubbard U) is also

Na+ ions in the Ti0.87O2/N-doped graphene superlattice and explain the superiority of the superlattice structure for ultrafast sodium storage. For an approximate calculation, the ideal graphene was used in the superlattice structure and the restacked bilayer model was used to represent the multilayered structure. We first investigated the sodiation dynamics in the bilayer structure of unilamellar nanosheets, which is a typical model for conventional nanosheet-based structures prepared by the exfoliation−restacking method. For the defect-free lepidocrocite-type TiO2 nanosheet bilayers (TiO2/TiO2), the simulation was performed specifically with two typical paths, path i along the [100] direction (Figure S29A) and path ii along the [001] direction (Figure S29B), which are assumed to be the most probable channels for Na+ ion diffusion. The energy barriers for paths i and ii in TiO2/TiO2 bilayers are determined to be ∼0.77 and 0.43 eV, respectively. The same simulations were considered in the TiO2/graphene bilayers (Figure 5A,C). The energy barriers for paths i and ii were obviously reduced to ∼0.54 and 0.16 eV, respectively (Figure 5B,D), suggesting a much more favorable diffusion of Na+ ions. For the defect systems (Ti0.87O2/Ti0.87O2 and Ti0.87O2/ graphene bilayers), we investigated the Na diffusion from the Ti vacancies into the interlayer spaces of the bilayers (Figure 5E and Figure S30A). The calculated energy barrier for Na atoms in Ti0.87O2/Ti0.87O2 bilayers is ∼2.1 eV (Figure S30B). Importantly, the energy barrier remarkably decreased to ∼1.4 eV in the Ti0.87O2/graphene bilayers (Figure 5F). These simulation results show a much lower diffusion energy barrier for the Na+ ion diffusion in the TiO2/graphene and Ti0.87O2/ graphene, demonstrating the superiority of the superlattice structure for ultrafast sodium storage.

CONCLUSIONS In summary, a cation-deficient unilamellar metal oxide nanosheet superlattice composed of alternately restacked Ti0.87O2 and N-doped graphene nanosheets was demonstrated as an anode material for sodium ion batteries. We achieved a large reversible capacity of 490 mA h g−1 at 0.1 A g−1 and an ultralong cycle life of more than 10000 cycles. More importantly, the superlattice maintained a superior rate capacity of 100 mA h g−1 at 12.8 A g−1 even at a low temperature of −5 °C, displaying the best performance among all Ti-based anodes for sodium storage thus far. A reversible Na+ intercalation mechanism without phase and structural change was proposed. The DFT calculations indicated the superiority of the unilamellar Ti-deficient Ti0.87O2 nanosheets compared with previously reported titanium oxides. The superlattice structure provides a much lower diffusion energy barrier for fast sodium intercalation/deintercalation. Our findings could highlight the opportunity of using 2D materials with defects as building blocks for advanced energy storage applications. EXPERIMENTAL SECTION Material Synthesis. Colloidal suspensions of Ti0.87O2 nanosheets were prepared based on exfoliation of layered titanate crystals.52,53 The nonstoichiometry formula means the presence of Ti vacancies and the metal contents of layered titanates were determined by ICP spectrophotometry based on previous report.53 The layered titanate crystals of K0.8Ti1.73Li0.27O4 were obtained by solid-state calcination. TiO2, K2CO3, and Li2CO3 in a molar ratio of 1.73:0.4:0.14 were mixed and calcinated at a high temperature (above 900 °C) for 20 h. Then, the protonic form, H1.07Ti1.73O4·H2O, was obtained by stirring 12344


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ACS Nano considered to describe the magnetism of the Ti atoms, and the GGA + U with U = 4.2 eV was used (4.2 eV has previously be used to model lithium intercalation in anatase TiO2 systems20). The vacuum region was set at 15 Å to eliminate artificial interactions in adjacent unit cells due to the periodic boundary condition. All the atoms are allowed to be fully relaxed until Hellmann−Feynman forces on each atom were smaller than 0.01 eV Å−1, and spin-polarization was taken into account during the calculations. To evaluate the diffusion energy barrier of sodium, the nudged elestic band method was adopted.58

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b06206. Details of characterizations, electrochemical measurements, and theoretical calculations, and table for comparison of electrochemical performance (PDF)

AUTHOR INFORMATION Corresponding Authors

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[email protected]. [email protected]. [email protected]. [email protected].

ORCID

Pan Xiong: 0000-0001-9483-6535 Jinqiang Zhang: 0000-0001-5476-0134 Teofilo Rojo: 0000-0003-2711-8458 Michel Armand: 0000-0002-1303-9233 Renzhi Ma: 0000-0001-7126-2006 Takayoshi Sasaki: 0000-0002-2872-0427 Guoxiu Wang: 0000-0003-4295-8578 Author Contributions

P.X. and G.W. conceived the idea. P.X. and F.Z. carried out the sample synthesis and electrochemical measurement. P.X., J.Z., B.S., H.T., D.S., and T.R. assisted with structural characterization and data analysis. X.Z. and D.Y. conducted the theoretical calculations. The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Australian Research Council through the ARC Discovery projects (DP160104340 and DP170100436). This work is partly supported by the World Premier International Research Center Initiative on Materials Nanoarchitectonics (WPI-MANA), MEXT, Japan. REFERENCES (1) Li, G.; Blake, G. R.; Palstra, T. T. M. Vacancies in Functional Materials for Clean Energy Storage and Harvesting: The Perfect Imperfection. Chem. Soc. Rev. 2017, 46, 1693−1706. (2) Ball, J. M.; Petrozza, A. Defects in Perovskite-Halides and Their Effects in Solar Cells. Nat. Energy 2016, 1, 16149. (3) Liu, Y.; Xiao, C.; Li, Z.; Xie, Y. Vacancy Engineering for Tuning Electron and Phonon Structures of Two-Dimensional Materials. Adv. Energy Mater. 2016, 6, 1600436. (4) Gong, X.-Q.; Selloni, A.; Dulub, O.; Jacobson, P.; Diebold, U. Small Au and Pt Clusters at the Anatase TiO2(101) Surface: Behavior at Terraces, Steps, and Surface Oxygen Vacancies. J. Am. Chem. Soc. 2008, 130, 370−381. 12345


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Two-Dimensional Unilamellar Cation-Deficient Metal Oxide

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