Genuine Unilamellar Metal Oxide Nanosheets Confined in a

Jan 22, 2018 - Different from previous reports, all unilamellar MnO2 nanosheets are separated and stabilized between the graphene monolayers, resultin...
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Genuine Unilamellar Metal Oxide Nanosheets Confined in a Superlattice-like Structure for Superior Energy Storage Pan Xiong, Renzhi Ma, Nobuyuki Sakai, and Takayoshi Sasaki* International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *

ABSTRACT: Two-dimensional (2D) metal oxide nanosheets can exhibit exceptional electrochemical performance owing to their shortened ion diffusion distances, abundant active sites, and various valence states. Especially, genuine unilamellar nanosheets with an atomic-scale thickness are expected to exhibit the ultimate energy storage capability but have not yet achieved their potential. Here, we demonstrate the utilization of genuine unilamellar MnO2 nanosheets for highperformance Li and Na storage using an alternately stacked MnO2/graphene superlattice-like structure. Different from previous reports, all unilamellar MnO2 nanosheets are separated and stabilized between the graphene monolayers, resulting in highly reversible 2D-confined conversion processes. As a consequence, large specific capacities of 1325 and 795 mA h g−1 at 0.1 A g−1, high-rate capacities of 370 and 245 mA h g−1 at 12.8 A g−1, and excellent cycling stabilities after 5000 cycles with ∼0.004% and 0.0078% capacity decay per cycle were obtained for Li and Na storage, respectively, presenting the best reported performance to date. KEYWORDS: genuine unilamellar nanosheets, superlattice-like structure, 2D-confined conversion processes, rate capability, cycling stability metal oxides.19−22 However, in general, the obtained metal oxides are not molecularly thin, even after carefully controlling the synthesis conditions. Owing to difficulties in controlling the anisotropic growth, single-layer metal oxide nanosheets have seldom been produced.23,24 The top-down exfoliation process using layered systems has been the most promising approach for obtaining monolayer nanosheets.25,26 In fact, a wide variety of stable colloidal suspensions of 2D metal oxide nanosheets with widths in the range of submicrometers to micrometers and thicknesses of a few nanometers have been successfully obtained via controlled intercalation-exfoliation processes.11,18,26,27 Although these nanosheets are promising for applications in electrochemical devices, one drawback is their tendency to restack during the electrode preparation process. The aggregation of nanosheets inevitably leads to massive losses of active sites, sluggish diffusion kinetics, and significant capacity decay.28−30 In principle, since the diffusion time of ions (t) is proportional to the square of the diffusion length (L), a much shorter diffusion time should be obtained if a single-layer nanosheet is utilized.

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ayered materials are important electrode materials for electrochemical energy storage because of their interlayer regions that can serve as the host matrix for ion intercalation.1−4 In particular, when the thickness of a layered material is reduced to its limit, the resulting 2D monoor few-layer sheets can exhibit ultimately enhanced properties, in theory, because of shortened diffusion distances and increased numbers of surface redox sites.3,4 To this point, a range of 2D nanosheets, such as graphene,5,6 transition metal dichalcogenides (TMDs),7−9 layered metal oxides,10−12 hydroxides,11,13 phosphorus,14,15 and transition metal carbides (MXenes),16 have been synthesized, and their promising electrochemical properties have been revealed. Among them, layered metal oxides have been recognized as an important class of 2D materials for electrochemical energy storage because of their variable valence states and rich redox activity.11,17,18 If fewlayer or even monolayer metal oxide nanosheets can be successfully isolated and truly used as a host for charge storage, we may expect that more exposed active sites, further shortened diffusion lengths, and reduced volume changes can be utilized to maximize electrochemical performance with a high specific capacity, ultrafast discharge/charge rate, and a long cycle life. Bottom-up synthesis based on chemical reactions, typically the hydrolysis of metal salts, can be employed for the synthesis of 2D nanosheets or flakes of both layered and nonlayered © XXXX American Chemical Society

Received: December 1, 2017 Accepted: January 5, 2018

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Figure 1. Schematic illustration of various metal oxide nanosheet/graphene structures for energy storage using MnO2 nanosheets as an example. (a) Normal MnO2 nanosheet/graphene composite. MnO2 nanosheets with thicknesses from several to tens of nanometers interact with graphene at the meso or nanoscale. (b) Conventional restacked MnO2 nanosheet/graphene composites. Large parts of MnO2 and graphene nanosheets are randomly self-restacked into several to tens of layers. (a, b) Only the limited region of MnO2 that is directly in contact with graphene (red line area) can be used for fast charge transport. (c) MnO2/graphene superlattice-like structure for molecular-scale interactions. Single-layer MnO2 and graphene nanosheets are alternately stacked on each other; thus, every single layer is involved for fast Li and Na storage.

performance improvements (Figure 1c). Besides, it should also be noted that, due to the conversion process, the initial 2D morphology of metal oxide nanosheet is usually decomposed after several cycles, leading to loss of active materials and thus capacity decay. Therefore, stabilization of these 2D nanosheets in a confined space is important for long-term cycling stability. Stacking different 2D nanosheets on top of each other in a precisely controlled sequence can produce a heterostructure or superlattice-like structure that may yield unusual properties and phenomena benefiting from the synergistic properties of these 2D materials via the high-quality heterointerfaces.40−43 However, to the best of our knowledge, reports regarding such superlattice-like structures with metal oxide nanosheets/ graphene for electrochemical energy storage do not exist. Here, using MnO2 nanosheets as an example, we demonstrate the utilization of genuine unilamellar metal oxide nanosheets for energy storage via molecular-scale hybridized MnO2/graphene superlattice. Different from previously reported structures, our MnO2/graphene composite is a superlattice-like structure composed of alternately stacked unilamellar MnO2 and graphene nanosheets. Besides, one should notice that the rather limited restacked number and loose structure of our superlattice-like structure compared with bulk layered materials may lead to more accessible active sites and promote ion migration. As a result, all unilamellar MnO2 nanosheets are separated and stabilized between the graphene monolayers thus

Another important obstacle that should be noted is the poor conductivity of metal oxides. Hybridization with graphene is confirmed to be effective in not only relieving the severe aggregation problems of nanosheets but also improving the electrical/ionic conductivity for effective charge transport.31−34 To this point, a number of metal oxide “nanosheets” have been hybridized with graphene, mostly using in situ growth methods.35−39 However, the metal oxides employed in these studies have not been based on molecularly thin unilamellar nanosheets but lamellar nanostructures with a typical thickness of several to tens of nanometers (Figure 1a). Conventional mixing of metal oxide nanosheets and graphene nanosheets usually leads to a randomly restacked morphology. Large parts of metal oxide and graphene nanosheets are self-restacked into several to tens of layers, resulting in the disappearance of merits of exfoliated nanosheets (Figure 1b). Only metal oxide/ graphene hybrids with meso or nanoscale intermixing have been achieved to date. In other words, a large portion of metal oxides have not been directly contacted with graphene. This lack of contact hinders fast charge transport and usually leads to inhomogeneous phase transitions and charge distributions, ultimately leading to severe pulverization with a sluggish rate capability and gradual capacity decay, especially at high discharge/charge rates. If unilamellar oxide nanosheets and graphene can be intimately integrated at the molecular-scale, the full utilization of oxides can be achieved, leading to further B

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Figure 2. Synthesis and structure of the MnO2/graphene superlattice. (a) Fabrication process via the electrostatic assembly of oppositely charged MnO2 nanosheets and rGO nanosheets. Typical AFM images and height profiles for the (b) MnO2 nanosheets and (c) rGO nanosheets. (d) TEM image of the MnO2/graphene superlattice. (e) HAADF-STEM image and corresponding elemental map of the MnO2/ graphene superlattice. (f) SAED pattern showing the in-plane reflections of the MnO2 nanosheets (M100 and M110) and rGO nanosheets (G100 and G110). (g) XRD patterns of the as-obtained MnO2/graphene superlattice and simulation results for the 00l basal series of the superlattice structure. M10 and M11 are the in-plane diffraction bands of the MnO2 nanosheets. (h) HRTEM image showing the alternative lamellar lattice fringes with different contrast.

previous reports on single-layer MnO2 nanosheets.44 The positively charged rGO nanosheets were prepared via the chemical reduction of graphene oxide (GO) using hydrazine in the presence of a cationic polymer, poly(diallyldimethylammonium chloride) (PDDA). Figure S2b shows a homogeneous aqueous suspension of rGO nanosheets with clear Tyndall light scattering, similar to that of the original GO suspension (Figure S2a). An obvious color change from light brown (GO) (Figure S2a) to dark black (rGO) (Figure S2b) and a clear shift in the UV−vis absorption peak (Figure S2c) indicated the reduction of GO to rGO.45 Figure S3a shows an AFM image of the GO nanosheets, which shows an estimated thickness of ∼0.8 nm. In the control experiment, the rGO nanosheets without PDDA modification showed a smaller thickness of ∼0.6 nm (Figure S3b), implying the removal of the oxygen-containing functional groups of GO upon reduction. In contrast, the thickness of the rGO nanosheets after successful modification with PDDA increased to ∼1.5 nm (Figure 2c), and a uniform distribution of C, N, and O was observed (Figure S4). Zeta potential measurements indicated the cationic nature of the rGO nanosheets after PDDA modification (Figure S5). In contrast, the negatively charged nature of the MnO2 nanosheets was also confirmed. Different from previous assembly of metal oxide nanosheets and graphene nanosheets, in this study, to attain full

can work as the major active layer for Li/Na storage, while the graphene layers not only improve the overall conductivity but also provide 2D confined interspace for the highly reversible conversion process of unilamellar MnO2 nanosheets. The asfabricated MnO2/graphene superlattice exhibits a large reversible capacity, ultrahigh rate capability and excellent cycle performance, demonstrating great potential as a highperformance anode material for both Li and Na storage.

RESULT AND DISCUSSION Materials Synthesis and Characterization. The MnO2/ graphene superlattice was fabricated via solution-phase electrostatic assembly between negatively charged MnO2 nanosheets and positively charged reduced graphene oxide (rGO) nanosheets (Figure 2a). The MnO2 nanosheets were synthesized according to our previous report.30,44 A transmission electron microscopy (TEM) image of the nanosheets (Figure S1a) clearly presents a flat and almost transparent sheet-like morphology, demonstrating the ultrathin thickness. In addition, the stable colloidal suspension of MnO2 nanosheets showed clear Tyndall light scattering (Figure S1b) and a broad optical absorption peak centered at ∼380 nm (Figure S1c). Figure 2b displays an atomic force microscopy (AFM) image of the MnO2 nanosheets, which presents a uniform thickness of approximately 0.8 nm. All these results are consistent with our C

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Figure 3. Lithium storage performance of the MnO2/graphene superlattice. (a) Initial charge/discharge profiles of the MnO2/graphene superlattice at 0.1 A g−1. (b) Charge/discharge profiles of the MnO2/graphene superlattice at various current densities ranging from 0.1 to 12.8 A g−1. (c) Rate performance of the MnO2/graphene superlattice and the control sample (randomly restacked MnO2/graphene composite) at various current densities ranging from 0.1 to 12.8 A g−1. (d) Cycling performance of the MnO2/graphene superlattice at 0.5 A g−1 for 100 cycles. (e) Long-term cycling performance of the MnO2/graphene superlattice at 5 A g−1 for 5000 cycles.

hybridization via face-to-face restacking, the mixing ratio was calculated based on a hypothesized area-matching model using the in-plane unit cell area of both nanosheets.46−48 The mass ratio between the MnO2 nanosheets and rGO nanosheets was determined to be ∼2.8 (Figure S6). Then, the colloidal suspensions of these oppositely charged nanosheets were mixed in this ratio to produce a superlattice-like composite. The real mass ratio of MnO2 to rGO was estimated to be ∼2.2 (Figure S7), which is close to the calculated value. The scanning electron microscope (SEM) (Figure S8) and TEM images (Figure 2d) show a lamellar morphology for the resulting MnO2/graphene product. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and corresponding elemental mapping (Figure 2e) measurements indicate a uniform distribution of the C, N, O and Mn elements, implying the uniform and intimate mixing of the MnO2 and rGO nanosheets. The selected area electron diffraction (SAED) data (Figure 2f) reveal the in-plane diffraction rings of both the MnO2 and rGO nanosheets, again supporting for the intimate restacking. Moreover, the Xray diffraction (XRD) data of the as-prepared MnO2/graphene superlattice (the red trace in Figure 2g) shows a broad peak in the range of 1.06 to 1.14 nm. The other two peaks at ∼37° and 65° were ascribed to the in-plane 10 and 11 reflections of the unilamellar MnO2 nanosheets. Their anisotropic profiles have a long tail that extends toward the higher angle side, which indicates irregular sheet-to-sheet registry. The measured dspacing of ∼1.1 nm was attributed to the second-order reflections of the superlattice-like structure, because it is close to half of the sum of the thicknesses of the MnO2 (0.8 nm) and rGO (1.5 nm) nanosheets. XRD simulations (Figure 2g, black trace) based on the superlattice-like structure (Figure S9)

indicate that the second-order peak has a much stronger intensity than the first-order one. This result strongly supports the fact that the MnO2 and rGO nanosheets are alternately stacked into a superlattice-like structure, although the broad nature of the peak suggests that the stacking order is not perfect. The high-resolution TEM (HRTEM) image (Figure 2h) shows parallel lattice fringes with two different spacings in an alternating sequence, ∼0.8 and 1.4 nm, which may be attributable to the MnO2 and rGO nanosheets, respectively. The repeating thickness of 2.2 nm agrees well with the XRD result and further suggests that the monolayer MnO 2 nanosheets are alternately stacked with rGO nanosheets. The superlattice repeating distance is ∼2.2 nm, meaning that the intersheet spacing between MnO2 and adjacent graphene is ∼1.1 nm, which is much larger than that of bulk layered MnO2 (∼0.7 nm).30 Therefore, the as-obtained MnO2/graphene superlattice with enlarged interlayer spacing could be considered as promising host for reversible Li and Na storage. On the other hand, mixing of MnO2 nanosheets and rGO under mass ratio other than the designated one might result in segregation in addition to the targeted superlattice structure (Figure S10). For comparison, a control MnO2/graphene composite was also prepared by mixing the MnO2 nanosheets and control rGO nanosheets in a PDDA solution. In contrast, diffraction features from an ordered stacking structure were not observed for the obtained sample (Figure S11a). In this process, both the MnO2 nanosheets and control rGO nanosheets were negatively charged, which suggests that these two kinds of nanosheets may have been restacked in a random manner. This random stacking was revealed in the SEM image and corresponding elemental analysis (Figure S11b). D

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Figure 4. Microstructural characterizations of the MnO2/graphene superlattice during the first lithiation process. (a, b) TEM images of the discharged MnO2/graphene superlattice. (c) SAED pattern of the discharged MnO2/graphene superlattice. (d−f) HRTEM image showing the formation of metallic Mn and MnO after lithiation. Supercells of (g) MnO2 from the c-axis, (h) FCC MnO from the 111-zone-axis and (i) BCC Mn0 from the 111-zone-axis. (j) A possible 2D-confined conversion process of the MnO2/graphene superlattice anode during the first lithiation process. The robust rGO layers were stable, while the MnO2 nanosheets could transform into MnO and metallic Mn layers, which are homogeneously confined between the rGO layers.

of MnO2 (1230 mA h g−1). This result should be the largest value among most Mn-based oxides as well as their carbonbased hybrid anodes (Table S1) and suggests the possibly full utilization of the unilamellar MnO2 nanosheets for efficient energy storage. The MnO2/graphene superlattice was further charged/discharged at various current densities ranging from 0.1 to 12.8 A g−1 (Figure 3b). A rate performance with high reversible capacities of ∼1325, 1195, 1075, 945, 805, 680, and 545 mA h g−1 was exhibited at current densities of 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, and 6.4 A g−1, respectively. Even at a high current density of 12.8 A g−1, a specific capacity of ∼370 mA h g−1, which is comparable to the theoretical capacity of a conventional graphite anode (372 mA h g−1), was maintained (Figure 3c). Importantly, after the high current density measurements, the capacity recovered its initial value when the current density decreased to 0.1 A g−1, indicating the high reversibility of the composites. The nominal tap density (ρ) of our MnO2/graphene superlattice was estimated to be ∼0.75 g

Lithium Ion Battery Performance. The MnO2/graphene superlattice was first tested as an anode for lithium-ion batteries (LIBs). Figure 3a shows the charge/discharge profiles for the first five cycles at 0.1 A g−1. Initial discharge and charge capacities of ∼2575 and 1395 mA h g−1 were obtained, respectively, corresponding to a Coulombic efficiency of 54%. The irreversible capacity loss may be mainly attributed to the formation of the solid electrolyte interface (SEI) layer. The Coulombic efficiency increased and reached up to ∼95% during the second cycle, and it remained at ∼100% after 3 cycles. After five charge/discharge cycles, a stable reversible capacity of ∼1325 mA h g−1 was attained (Figure 3a). It should be noted that the rGO nanosheet anode exhibited a reversible capacity of ∼510 mA h g−1 at the same current density (Figure S12). Since the mass ratio of MnO2 to rGO was ∼2.8, the contribution of rGO to the capacity of the composite electrodes was estimated as ∼135 mA h g−1. Thus, the capacity of the MnO2 nanosheet was ∼1190 mA h g−1, which is close to the theoretical capacity E

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ACS Nano cm−3 based on the measured weight and geometry of our electrode before being assembled into the cell.49 It should be noted that, a volumetric capacities of ∼994 mA h cm−3 was obtained for our MnO2/graphene superlattice at a current density of 0.1 A g−1, which is larger than that of commercial graphite anode and comparable to that of some available representative Si-based anodes (Table S2). Figure 3d shows the cycling performance of the MnO2/ graphene superlattice at a current density of 0.5 A g−1. The specific capacity gradually increased, and then, it remained at ∼1250 mA h g−1 during the end of the 100th cycle. The gradual increase in the capacity was attributed to the reversible electrochemical reactions of the polymeric/gel-like layer and activation of the internal materials after deep charge/discharge cycles, which have been widely reported in transition metal oxide-based anodes.34,50,51 Furthermore, the MnO2/graphene superlattice exhibited excellent long-term cycling stability at a high current density of 5 A g−1 (Figure 3e). After 5000 cycles, a reversible capacity was remained at ∼480 mA h g−1 with a capacity decay as low as ∼0.004% per cycle. According to a comprehensive summary (Table S1), this is the best cycling capability among most reported Mn-based oxide anode materials as well as their carbon-based hybrids for LIBs, to the best of our knowledge. Moreover, the electrochemical performance with different mass loading (∼1.9 and 2.8 mg cm−2) was also tested as shown in Figure S13. At 0.1 mA cm−2, a reversible areal capacity reached ∼3.5 mA h cm−2, similar to, if not higher than, that of commercial graphite, some representative Si-based anodes and high-rate metal oxidebased anodes (Table S3). For comparison, the MnO2 nanosheets (Figure S14) and control composites with randomly restacked MnO2 and graphene (Figure S15) were also examined as anodes for LIBs. Reversible capacities of ∼170 and 450 mA h g−1 at 0.1 A g−1 were obtained for the MnO2 nanosheets and the composites, respectively. The composites exhibited much improved performance than the MnO2 nanosheets, probably because of the addition of conductive graphene. However, a poor specific capacity of less than 10 mA h g−1 was observed at a current density of 3.2 A g−1, and practically no capacity was measured when the current density increased to 6.4 A g−1, obviously worse than that of the MnO2/graphene superlatticelike composite. This result suggests the superiority of the superlattice-like alternating structure at a molecular level, which should contribute to a more favorable charge transfer process (Figure S16) and, hence, better rate capability. 2D-Confined Conversion Process. To understand the superior electrochemical performance of the composite, further studies were carried out on the conversion reactions and structural variations in the MnO2/graphene superlattice during the charge/discharge cycles. Figure S17 shows the cyclic voltammetry (CV) curves of the MnO2/graphene superlattice during the initial five cycles at a scan rate of 0.1 mV s−1. In the first cycle, the peak at approximately 2.4 V should be attributed to the decomposition of the electrolyte. The cathodic peak at approximately 0.5 V is assigned to the formation of an SEI layer, which disappeared in the second and subsequent cycles. The two cathodic peaks at approximately 0.7 and 0.1 V correspond to a possible two-step reduction process, Mn3+/4+ to Mn2+ and Mn2+ to Mn0, which shifted to approximately 1.0 and 0.2 V, respectively, during the second discharge cycle.52 The two main anodic peaks at approximately 1.1 and 1.9 V were attributed to a two-step oxidation process of the Mn species.51

The CV curves nearly overlapped during the second through fifth cycle, again indicating the high electrochemical reversibility. The occurrence of a reversible conversion reaction was also reflected in the TEM and X-ray photoelectron spectroscopy (XPS) analysis. Figure 4a shows the TEM image after the first lithiation. The overall lamellar geometry was preserved probably because of the well-maintained rGO backbone. The diffraction rings of ∼0.21 and 0.12 nm attributed to the rGO 100 and 110 in-plane reflections were clearly observed in the SAED patterns (Figure 4c). In contrast, the MnO2 nanosheets appeared to change into small dark domains (Figure 4b). The HRTEM image (Figure 4d) shows these dark domains with lattice fringes that are different from those of the MnO2. The lattice spacing of ∼0.27 nm (Figure 4e) and the SAED rings in Figure 4c are indicative of the {111} planes of face-centered cubic (FCC) MnO (Figure 4h). Other lattice planes with d spacing values of ∼0.22 nm were predominantly observed (Figure 4f), which may correspond to the {111} lattice planes of body-centered cubic (BCC) Mn0 (Figure 4i). Owing to its similar diffraction position, the SAED signal for metallic Mn likely overlapped with the reflections from the rGO (Figure 4c). The XPS results in Figure S18 also indicate the formation of Mn2+ and metallic Mn0 after the first lithiation. The Li 1s XPS spectra shown in Figure S19 indicate the formation of a separate Li2O phase after the first lithiation. All these results agree well with the possible two-step reduction proposed from the CV analysis. On the basis of the results shown above, a possible 2D-confined conversion reaction is proposed, as shown in Figure 4j. The MnO2 nanosheet (Figure 4g) is converted into MnO and then Mn layer, which is homogeneously dispersed between the rGO bilayers during the first lithiation process. Thus, the conversion processes are well confined in the 2D galleries between the rGO networks, resulting in preserved high electrochemical activity and, hence, a long cycling stability. After the first delithiation, the overall morphology was preserved by rGO, as suggested by its persisting in-plane 100 and 110 reflections in the SAED patterns (Figure S20). The nanodomains of the Mn-based species with a decreased particle size were still homogeneously dispersed between the 2D galleries of the rGO layers. Owing to the further decreased particle size and low crystallinity, obvious lattice spacing was not observed, but the corresponding SAED pattern indicates the possible presence of Mn3+/4+O2. The XPS signal of Mn2+ practically disappeared and no evidence of metallic Mn was observed (Figure S18), also suggesting the oxidation of Mn0 and Mn2+ to higher valence states. The XPS signal of Li2O almost disappeared after the first lithiation (Figure S19) suggests the reversible formation of the Li2O phase. These observations indicate the highly reversible conversion mechanism of the MnO2/graphene superlattice anodes, which is well consistent with the CV analysis. Furthermore, the TEM images and elemental mapping results show the retained morphology and homogeneous distribution of the C, N, O and Mn elements after 100 cycles, further demonstrating the excellent cycling stability of the MnO2/ graphene superlattice anodes (Figure S21). In view of its excellent performance in half cells, the MnO2/graphene superlattice was further examined as anodes in full cells using commercial LiFePO4 powders (Figure S22) as cathodes. Preliminary results from the full-cell (Figure S23) indicated an initial reversible capacity of 135 mA h g−1 and a capacity retention of 75% at a current density of 50 mA g−1 for 100 cycles, suggesting promising potential for the commercial F

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Figure 5. Sodium storage performance of the MnO2/graphene superlattice. (a) Charge/discharge profiles of the MnO2/graphene superlattice at various current densities ranging from 0.1 to 12.8 A g−1. (b) Rate performance of the MnO2/graphene superlattice and control sample (randomly restacked MnO2/graphene composite) at various current densities ranging from 0.1 to 12.8 A g−1. (c) Comparison of the rate capability of the MnO2/graphene superlattice anode with other metal oxide-based anodes reported recently. (d) Cycling performance of the MnO2/graphene superlattice at 0.5 A g−1 for 100 cycles. (e) Long-term cycling performance of the MnO2/graphene superlattice at 5 A g−1 for 5000 cycles.

density of 5 A g−1, the reversible capacity was ∼185 mA h g−1, corresponding to an ∼0.0078% capacity decay per cycle (Figure 5e). In contrast, the MnO2 nanosheets themselves (Figure S25) showed a reversible charge capacity of only 15 mA h g−1. The control composites (Figure S26) exhibited clearly improved specific capacities of 135 mA h g−1 compared to the MnO2 nanosheets. However, the rate performance of the control composites was far lower than that of the MnO2/graphene superlattices (Figure 5b). These results further demonstrated that the superlattice-like structure provides a favorable charge transfer process for Na storage.

applications of MnO2/graphene superlattice anode for LIBs, although further optimization is needed. Sodium Ion Battery Performance. The sodium storage capability of the MnO2/graphene superlattice was also investigated. As shown in Figure S24, the first discharge profile showed a plateau between 1.5 and 1.0 V, which is attributable to the formation of the SEI layer and the reduction of MnO2. After the first cycle, charge/discharge profiles without obvious plateaus were observed, similar to previous reports.53,54 After five discharge/charge cycles, a reversible specific capacity of ∼795 mA h g−1 and a Coulombic efficiency up to ∼100% were delivered. Figure 5a shows the charge/discharge profiles of the MnO2/graphene superlattice at various current densities ranging from 0.1 to 12.8 A g−1. The MnO2/graphene superlattice exhibited high reversible capacities of ∼795, 680, 595, 520, 445, 375, 305, and 245 mA h g−1 at current densities of 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, and 12.8 A g−1, respectively (Figure 5b). Owing to the larger ionic radius of Na+ compared to Li+, the kinetics of Na+ diffusion should have been more sluggish compared to the Li+ diffusion. However, the MnO2/ graphene superlattice showed a comparable rate performance for the Na storage and Li storage, which may be ascribed to its molecular-scale hybridized superlattice-like structure. To the best of our knowledge, this is the best rate performance among recently reported metal oxide-based anodes for SIBs (Figure 5c and Table S4).21,55−58 The cycling performance of our MnO2/graphene superlattice for SIBs was also impressively high. As shown in Figure 5d, a reversible specific capacity of ∼500 mA h g−1 was obtained at the end of the 100th cycle, corresponding to a capacity retention of ∼90%. Moreover, after 5000 cycles at a current

CONCLUSIONS This study revealed the distinguished electrochemical performance of MnO2/graphene superlattice-like composites for both Li and Na storage. The superlattice-like structure of the alternately stacked MnO2/graphene clearly contributes to its superior properties. Here, in contrast to the previously reported metal oxide/graphene hybrids, the unilamellar nanosheets of MnO2 and graphene are alternately stacked with each other at the molecular scale, thus leading to the possibility to fully utilize the redoxable MnO2 nanosheets to achieve high energy storage capabilities that approach the theoretical value. The unilamellar MnO2 nanosheets effectively shortened the diffusion pathways of Li+/Na+ ions, leading to promoted electrochemical kinetics. The intimate hybridization with graphene nanosheets improved the conductivity of the whole hybrid for fast charge transport (Figure S16). Compared with the bulk layered materials and randomly restacked composite, the superlattice-like structure provides more 2D channels for fast Li/Na diffusion and thus resulted in a superior rate capability. The kinetics analysis based G

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mass ratio based on a hypothetical area-matching model (Supporting Information). The flocculate was recovered after centrifugation at 6000 rpm and dried in air. In a control experiment, control rGO nanosheets and MnO2 nanosheets were directly added in the same ratio into a PDDA solution (10 g dm−3) to form the control MnO2/ graphene composites. Characterization. Powder XRD data were recorded using a Rigaku Rint-2200 diffractometer equipped with monochromatic Cu Kα radiation (λ = 0.15405 nm). A JEOL JSM-6700F SEM and a JEOL JEM-3000F TEM were employed to observe the microtexture and morphology of the samples. A Seiko SPA 400 AFM was used to examine the topography of the nanosheets deposited onto a Si wafer substrate. The zeta potential was determined using an ELS-Z zetapotential analyzer. UV−vis absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. Electrochemical Measurements. All electrochemical measurements were carried out using a half-cell system with a 2032-type coin cell. The working electrode was prepared by casting a slurry of active material, carbon black, and polyvinylidene difluoride (PVDF) 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 1.0−2.8 mg cm−2. For the LIBs, lithium foil was used as the counter electrode, and Celgard 2500 was used as the separator. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC) (1:1 in volume). For the SIBs, sodium foil was used as the counter electrode. Waltman glassy fibers were used as the separator. The electrolyte was 1 M NaClO4 in propylene carbonate (PC) with a 10 wt % fluoroethylene carbonate (FEC) additive. The CV tests were carried out using a Solartron SI 1287 electrochemical workstation. EIS was performed with a 10 mV AC amplitude and frequencies ranging from 10 mHz to 20 kHz. The galvanostatic charge/discharge tests were performed on a Hokuto charging/discharging system (HJ1001SD8). In the case of the Li-ion full cell, MnO2/graphene and commercial LiFePO4 were used as the anodes and cathodes, respectively. The prelithiation procedure of the MnO2/graphene superlattice anodes was performed according to a reported method.60,61 Then, 1 M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC) (1:1 in volume) was used as electrolyte, and Celgard 2500 was used as the separator. The specific capacity was calculated based on the mass loading of the cathode material.

on the CV curves further suggests that the MnO2/graphene superlattice (Figure S27) had favorable charge transfer kinetics compared with the MnO2 nanosheet (Figure S28) and control MnO2/graphene composite (Figure S29). In addition, the majority of the charge storage process of the MnO2/graphene superlattice anode was capacitive and capable of an ultrahigh rate performance. The superlattice-like structure of the alternately stacked MnO2/graphene superlattice also contributed to the ultralong cycling stability. The robust carbon network of graphene nanosheets accommodates the rather drastic changes of MnO2 nanosheets during the Li/Na uptake and release processes, thus resulting in a well-maintained overall morphology after the charge/discharge cycles (Figure S21). The 2D gallery between the graphene monolayers can act as a nanoreactor for the highly reversible 2D-confined conversion processes. Although the initial nanosheet morphology of MnO2 no longer existed after the initial several cycles, the resulting ultrafine nanodomains were still well stabilized and confined between the graphene monolayers. The Nyquist impedance plots of the MnO2/graphene composites after the first, fifth, 10th, 20th, 40th, 60th, 80th, and 100th cycles are shown in Figure S30a. The impedance spectra were fit to an equivalent electrical circuit,14,59 a stable Rsf+ct circuit after the initial several cycles was observed (Figure S30b), indicating well-maintained electrical contact, which again explains the stable cycling of the MnO2/graphene composites. In summary, the utilization of genuine unilamellar MnO2 nanosheets with an atomic-scale thickness as anodes for both Li and Na ion batteries was demonstrated using a MnO2/ graphene superlattice-like composite. In this structure, every unilamellar MnO2 nanosheet was stabilized between the graphene monolayers, enabling excellent Li/Na ion storage based on a highly reversible 2D-confined conversion process. The graphene monolayers served as the robust backbone, not only improving the overall conductivity but also maintaining the overall structural integrity. As a consequence, specific capacities of 1325 and 795 mA h g−1 at 0.1 A g−1 and 370 and 245 mA h g−1 at 12.8 A g−1, were obtained for Li and Na storage, respectively. More importantly, an ultralong cyclability with 0.004% and 0.0078% capacity decay per cycle up to 5000 cycles was achieved for Li and Na storage, respectively, outperforming previously reported metal oxide-based anodes to date. Our findings are important for further utilizing 2D nanosheets, especially the use of genuine unilamellar nanosheets with atomic-scale thicknesses for advanced energy storage and conversion applications.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b08522. TEM, AFM, UV−vis, zeta potential, SEM characterizations of unilamellar nanosheets and control samples, additional characterizations and electrochemical measurements of MnO2/graphene superlattice, and comparison of the electrochemical performance (PDF)

EXPERIMENTAL SECTION

AUTHOR INFORMATION

Synthesis. The MnO2 nanosheets were prepared via the delamination of layered birnessite according to our previous reports.30,44 The positively charged rGO nanosheets were prepared by modifying rGO with PDDA. First, 200 cm3 of a GO suspension (0.2 g dm−3), prepared by the Hummers’ method, was mixed with 1.5 cm3 of a PDDA solution (20 wt %). Subsequently, 15 mm3 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 high-speed centrifugation at 20 000 rpm, and the recovered sediment was redispersed in H2O. A stable suspension of rGO was obtained after collecting the supernatant after centrifugation at 6000 rpm. Normal rGO samples without PDDA modification were also prepared as the control rGO nanosheets. To obtain the MnO2/graphene superlattice-like composite, suspensions of rGO modified with PDDA and MnO2 nanosheets were mixed dropwise under continuous stirring using a determined

Corresponding Author

*E-mail: [email protected]. ORCID

Pan Xiong: 0000-0001-9483-6535 Renzhi Ma: 0000-0001-7126-2006 Nobuyuki Sakai: 0000-0002-9395-6751 Takayoshi Sasaki: 0000-0002-2872-0427 Author Contributions

T. S. and P. X. conceived the project and designed the experiment; P. X. carried out the sample synthesis, structural characterization and electrochemical measurements; R. M. and N. S. assisted with the ex situ characterizations, discussed the results and gave constructive advice on this work. H

DOI: 10.1021/acsnano.7b08522 ACS Nano XXXX, XXX, XXX−XXX

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The authors declare no competing financial interest.

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