Local Electric Field Facilitates HighPerformance Li-Ion Batteries Youwen Liu,†,§ Tengfei Zhou,‡,∥,§ Yang Zheng,‡ Zhihai He,† Chong Xiao,*,† Wei Kong Pang,‡ Wei Tong,⊥ Youming Zou,⊥ Bicai Pan,† Zaiping Guo,*,‡ and Yi Xie*,† †
Hefei National Laboratory for Physical Sciences at the Microscale, CAS Centre for Excellence in Nanoscience, iCHEM, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ‡ Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials (AIIM), and School of Mechanical, Materials and Mechatronics Engineering, Faculty of Engineering and Information Sciences, University of Wollongong, North Wollongong, NSW 2500, Australia ∥ Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Wuhan, 430074, People’s Republic of China ⊥ High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui 230031, People’s Republic of China S Supporting Information *
ABSTRACT: By scrutinizing the energy storage process in Li-ion batteries, tuning Li-ion migration behavior by atomic level tailoring will unlock great potential for pursuing higher electrochemical performance. Vacancy, which can effectively modulate the electrical ordering on the nanoscale, even in tiny concentrations, will provide tempting opportunities for manipulating Li-ion migratory behavior. Herein, taking CuGeO3 as a model, oxygen vacancies obtained by reducing the thickness dimension down to the atomic scale are introduced in this work. As the Li-ion storage progresses, the imbalanced charge distribution emerging around the oxygen vacancies could induce a local built-in electric field, which will accelerate the ions’ migration rate by Coulomb forces and thus have benefits for high-rate performance. Furthermore, the thusobtained CuGeO3 ultrathin nanosheets (CGOUNs)/graphene van der Waals heterojunctions are used as anodes in Li-ion batteries, which deliver a reversible specific capacity of 1295 mAh g−1 at 100 mA g−1, with improved rate capability and cycling performance compared to their bulk counterpart. Our findings build a clear connection between the atomic/ defect/electronic structure and intrinsic properties for designing high-efficiency electrode materials. KEYWORDS: CuGeO3, local electric field, oxygen vacancies, Li-ion migratory behavior, anode
T
been developed, in which the metal oxides could act as a matrix and buffer to disperse and accommodate the volume changes of the Ge nanoparticles formed in situ during charge/discharge process.17−20 Among the ternary germanates, CuGeO3 possesses a distinctive two-dimensional layered structure, which could provide a buffer space to deal with the shortcoming of volume expansion for long cycle life. In addition, the electrochemical reactions can be confined within the CuGeO3 layers as “microreaction pools” owning to its large interlayer spacing. Furthermore, before any systematic study of CuGeO3, we first utilized in situ synchrotron X-ray powder diffraction (SXRPD) technology21−23 to reveal the structure of CuGeO3, which seems
he ever-growing demands for electronic devices, ranging from large-scale energy storage systems to portable electronics, look toward the pursuit of lithium ion batteries (LIBs) with high power density, high energy density, and longer cycling life.1−6 Striving for superior anode materials is a significant direction in achieving the above expectations.7−11 Among the multitude of alternatives, Ge-based materials are attracting widespread attention due to their attractive features, including high theoretical specific capacity, fast lithium ion diffusivity, and high electrical conductivity.12−16 The root causes for the unsatisfactory electrochemical performance of the present germanium-based anode materials, however, can be summarized as follows: (i) devastating huge volume changes during Li-ion storage processes, leading to cracking and pulverization of the electrode materials, and (ii) aggregation of particles as cycling progresses, leading to increased resistance and longer diffusion paths for lithium ions. To reconcile the above inherent contradictions, ternary metal germanates have emerged and © 2017 American Chemical Society
Received: July 2, 2017 Accepted: July 26, 2017 Published: July 26, 2017 8519
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Figure 1. In situ synchrotron X-ray powder diffraction (SXRPD) of bulk CuGeO3 electrode. (a) SXRPD signal reflection of CuGeO3. (b) Crystal structure of CuGeO3. (c) SXRPD patterns from 10° to 11° containing (201) peak. (d) SXRPD data from 13° to 14.5°containing (011) and (210) peaks. Voltage profile for charge/discharge is superimposed. The detailed lithium process is shown in Supplementary Figure S2.
exfoliated by the conventional liquid phase stripping method. Interestingly, self-adaptive oxygen vacancies were generated when the thickness of the CuGeO3 was reduced to the atomic level because more surface atoms could easily escape from the 2D lattice, which is confirmed by the electron spin resonance (ESR) spectra. On the basis of first-principles calculations, the obvious charge transfer phenomenon would occur, which will cause an imbalanced charge distribution and local electric field around the oxygen vacancy area. The tiny in-plane local electric field will accelerate ion/electron migration rates and promote charge transfer behavior of the electrode material. Meanwhile, by further incorporating graphene as a conductive matrix and buffer layer on the nanoscale, the cycling and rate performances of CGOUNs can be notably improved. Benefiting from this multiscale coordinated regulation, the present 2D CGOUNs/graphene van der Waals heterojunction delivers a reversible specific capacity of 1295 mAh g−1 at 100 mA g−1, with outstanding rate capability and cycling performance, revealing powerful competitiveness and potential as an anode material. More importantly, by manipulating the charge transfer behavior and migratory pathways in the electrode material, the self-adaptive vacancies in this work will provide a guide to the design of other high-efficiency electrode materials for advanced energy storage technology.
sufficiently stable under cycling, unlike the traditional conversion-type electrodes (Figure 1 and Figures S1−S3 in the Supporting Information with detailed discussion). During the first charge, the intensity of the (201) and (011) peaks reveals favorable structural stability and the reversibility of CuGeO3. On the basis of the interpretation of the SXRPD signals, unlike other conversion reaction based transition-metal oxides, a reversible charge−discharge reaction and crystalline structure of CuGeO3 are expected. Due to the inherent evolution of inorganic solids, self-adaptive vacancies may be generated as the thickness of two-dimensional (2D) materials is reduced to the atomic level.24−26 In response, vacancies, which could effectively adjust the electronic structure without radically changing the pristine lattice, would unlock potential for optimizing the performance of LIBs, especially rate capability.27−29 More interesting still, the 2D nanoarchitectures possess unrivaled “3 S” features, namely, stability by stress cushioning, high active surface area, and open shortened pathways for lithium storage, which interestingly, correspond exactly to the requirements for energy storage.30−34 In light of the above analysis, it is reasonably established that the ultrathin nanosheet architectures are an effective measure to synergistically optimize the electrochemical performance of CuGeO3. Meanwhile, graphene has been developed as an excellent substrate to host active nanomaterials, especially 2D ultrathin nanosheets to construct van der Waals heterojunctions for energy application, which provides opportunities profiting from the triple advantages of graphene, including its properties as an elastic buffer layer, electrical highway, and source of additional interface lithium storage sites.35−37 In this work, 2D CuGeO3 ultrathin nanosheets (CGOUNs)/ graphene van der Waals heterojunctions were fabricated and used as an anode in Li-ion batteries; the CGOUNs were
RESULTS AND DISCUSSION The CGOUNs were acquired from bulk CuGeO3 with fault configuration (Figure S4) by the conventional liquid phase exfoliation method in a strongly polar solvent of N-methyl-2pyrrolidone (NMP). The essential structural information on CGOUNs was probed by integrated characterizations, as shown in Figure 2. First, the X-ray diffraction (XRD) pattern of CGOUNs could be indexed and matches well with that of 8520
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Figure 2. Microstructural characterization of CGOUNs and CGOUNs/graphene van der Waals heterojunction. (a−c) XRD pattern, the AFM image and corresponding height profiles, and HAADF-STEM image of CGOUNs. (d−i) Raman spectra, STEM-HAADF image, and the corresponding EDS elemental mappings of CGOUNs/graphene.
above results, the designed CGOUNs were obtained successfully. Naturally, to fully promote the lithium storage potential of CGOUNs, the 2D CGOUNs/graphene were designed conceptually and acquired experimentally through the electrostatic adsorption and morphology matching effect.39,40 The bright CGOUNs/graphene nanohybrids’ dispersion verified that CGOUNs and graphene formed a uniform phase (Figure S6). The TEM images of 2D CGOUNs/graphene shown in Figure S7 expressly demonstrate that the CGOUNs are homogeneously anchored on the graphene, which verifies that the 2D CGOUNs/ graphene van der Waals heterojunctions were successfully prepared. In addition, the Raman spectroscopy (Figure 2d) of the 2D CGOUNs/graphene was also conducted, and the strong intensity of the G peak in graphene and 2D CGOUNs/graphene suggests the good quality and high crystallinity of the synthesized graphene. Furthermore, the elemental mapping (Figure 2e−i) could provide visual evidence that the 2D CuGeO3 ultrathin nanosheets were evenly distributed on the graphene and formed van der Waals heterojunctions. From the thermogravimetric analysis (Figure S8), the amount of graphene in the CGOUNs/ graphene composite is about 4.5 wt %. Given the above, on the basis of all of the structural and morphology data, it is clear that the 2D CGOUNs/graphene van der Waals heterojunctions were successfully obtained.
orthorhombic CuGeO3, suggesting the high purity of the obtained nanosheets. What is interesting is that the peak intensity ratio (I(200)/I(120)) of the CGOUNs shows a dramatic enhancement relative to the bulk contrast, which indicates that (100) lattice planes were exposed in CGOUNs, in agreement with the crystal structure. Moreover, the Raman spectra of CGOUNs exhibited a slight peak shift toward lower wavenumber as compared to their bulk counterpart, further confirming the peculiarity of the ultrathin nanosheet regime (Figure S5a).38 Meanwhile, the transmission electron microscope (TEM) images made it possible to visually observe the ultrathin sheet configuration, and the atomic force microscope (AFM) image quantitatively detected the thickness to be about 4.2 nm (as seen in Figure 2b and Figure S5b). The detailed structure of CGOUNs was further revealed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Figure 2c). Generally, the HAADF detector collects electrons that undergo high-angle scattering, and the signal intensity is proportional to the atomic number, Z. As a consequence, the O atoms are invisible in the HAADF image, because they are not heavy enough to produce any contrast. As is shown in the inset in Figure 2c, the brighter spots represent Ge atoms, and the spots with general brightness represent Cu atoms, which is consistent with the schematic diagram of the CuGeO3 crystal structure viewed down the [100] zone axis (inset of Figure 2c). Given the 8521
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Figure 3. Electrochemical characterization. (a) Cyclic voltammetry. (b) Capacities at different current densities. (c) Rate capabilities. (d) Rate performance comparison of nanoscale Ge-based anode.15−18,20,42−44 Additional data and relevant references are listed in Table S1. (e) Longterm cycling performance.
initial charge capacity of CGOUNs/graphene is 1750 mAh g−1, which is obviously superior to the capacity of CGOUNs and their bulk counterpart. The first discharge capacity may be attributed to the large active surface area, additional interface lithium storage sites, and the formation of the solid electrolyte interphase (SEI) film. Interestingly, the charge platform of the 2D nanosheet anodes (CGOUNs and their van der Waals heterojunctions) differed from that of their bulk counterpart, which signifies that the 2D nanosheet configuration reveals distinctive lithium storage behavior derived from the modulation of the atomic arrangement. In addition, the rate performance is an important indicator for evaluating lithium storage systems. Hence, Figure 3b,c shows the rate capability of the as-prepared electrodes. As can be seen clearly, the CGOUNs/graphene electrode features superior high-rate performance compared to the other electrodes. The average charge capacity of the CGOUNs/graphene van der Waals heterojunctions is 1295, 1157, 1000, 850, 682, and 502 mAh g−1 at a current density of 100, 200, 500, 1000, 2000, and 5000 mA g−1, respectively. After a deep charge for 60 cycles, the capacity recovers to ∼1080 mAh g−1 with approximately 85% capacity retention when the current density goes back to 100 mA g−1. Furthermore, the separate CGOUNs’ electrode also exhibited decent rate capability (1164, 1010, 855, 661, 486, 231, and 855 mAh g−1 at the above specified current densities, respectively). In contrast, the bulk CuGeO3
The lithium storage performance of the constructed anode material was inspected in half-cells, and the detailed process is described in the Experimental Section. The Li-ion storage behavior of the CGOUNs/graphene was initially investigated using cyclic voltammetry (CV). Figure 3a displays representative CV curves for the first five cycles at a scan rate of 0.1 mV s−1 in the voltage range of 0.01−3.0 V (vs Li+/Li). The CV curves show multipeak features, demonstrating that the reaction of the CGOUNs/graphene with Li proceeds in multiple steps. The CV curves display a trend that is in good agreement with the discharge−charge voltage profile in Figure 3a. There are three peaks arising from the delithiation reaction. The potential peak at 0.538 V corresponds to LixGe delithiation to Ge (dealloying), while the two peaks at 1.192 to 1.870 V of the charge profile are consistent with multistep oxidation of Cu to CuOx.41 It is noteworthy that, from the second cycle onward, both the reduction and the oxidation peaks in the CV curves overlap very well, proving that the electrode exhibits good reversibility and stability during the electrochemical reactions. The CV curves of CGOUNs and bulk CuGeO3 showed similar potential peaks (Figure S9). Meanwhile, cyclic voltammograms at different scanning rates also were collected and are shown in Figure S10, which verify the high activity of the CGOUNs/graphene electrode. The discharge voltage profiles were first investigated at a current density of 100 mA g−1, as shown in Figure S11. The 8522
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Figure 4. Atomic level vacancy characterization and corresponding electronic structure calculations. (a) ESR results. (b, c) Difference charge density around VO••. Charge density distribution of oxygen-vacancy-free CuGeO3 (d) and oxygen vacancy areas at different sites: site 1 (e) and site 2 (f). (g, h) Schematic diagram of CGOUNs/graphene (g) and oxygen vacancy in CGOUNs (h). (i) Schematic illustration of charge transfer behavior around the oxygen vacancy region. (j) Summary of the enhanced high rate performance mechanism due to local electric fields.
could provide comprehensive advantages for cycling stability and capacity in the anode material. From this point of view, the rate capability, which reflects the lithium ion migration behavior during the charging−discharging process, should attract particular attention. The atomic arrangement and derived electronic structure are deemed to be accountable for the chemical properties, including the charge transfer behavior and migratory pathways. The atomic vacancies will be a nonnegligible factor for enhancing lithium ion storage behavior on account of the self-adaptive local built-in electric field derived from the lopsided charge distribution around the vacancy area. In this regard, low-temperature ESR, which has been widely accepted as a powerful technique to detect oxygen vacancy, was utilized in this work, and the results are displayed in Figure 4a. An evident oxygen vacancy signal could be observed in the ESR spectrum of CuGeO3 ultrathin nanosheets.45,46 In consideration of the lower formation energy of an oxygen vacancy, the oxygen will preferentially escape from the CuGeO3 primitive lattice with the greater proportion of surface-exposed atoms in the 2D ultrathin nanosheets and generate an oxygen vacancy. The charge distribution around the oxygen vacancy, which may be located in two possible sites, was simulated based on first-principles calculation. First, the apparent charge transfer
delivered a poor rate capacity of 969, 538, 397, 273,165, and 81 mAh g−1 with increasing current density from 100 to 5000 mA g−1, respectively. Meanwhile, the exceptional rate capability of the CGOUNs/graphene anode provides opportunities to satisfy the requirements for a high-performing anode material (Figure 3d).15−18,20,42−44 More importantly, the CGOUNs/graphene anode could still deliver a discharge capacity of 693 mAh g−1 after 500 cycles at 1000 mA g−1 (Figure 3e), which showed prolonged cycling capability. As a contrast, the discharge capacity of bulk CuGeO3 had a noticeable decay at 100 mA g−1 over 100 cycles (Figure S12). The optimization of the cycling stability in the constructed van der Waals heterojunctions may be attributed to the widely recognized stress degradation of nanosheets and the supporting effect of graphene. The integrated electrochemical curves indicate that the constructed 2D van der Waals heterojunctions achieved the simultaneous optimization of multiple constraints (capacity, cycling stability, and rate capability) as an anode material. Deeply understanding the mechanism under multiple constraints in the constructed anode material is the prerequisite for pursuing a more efficient 2D anode material. As is widely recognized, the nature of the nanoscale 2D ultrathin morphology and microscale graphene-based van der Waals heterojunctions 8523
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electron migration behavior proposed in this work could broaden horizons for establishing more efficient electrode materials.
phenomenon could be observed around both potential oxygen vacancy sites, which may induce an imbalanced charge distribution in the CuGeO3 layer plane (Figure 4b,c). As expected, the above phenomena were verified by the charge density distribution simulation, which suggested that the skewing arising from the vacancies induces the skewing of other atoms, especially around the vacancy sites, compared to the vacancy-free CuGeO3 primitive lattice (Figure 4d−f). For Li-ion batteries, the lopsided charge distribution will intrinsically induce an in-plane built-in electric field, which would provide an adventitious Coulomb force to accelerate Li-ion migration and boost the rate capability of LIBs.47−49 On the basis of the complete atomic structure characterization and electronic structure simulation, the outstanding Li storage performance of our CGOUNs/graphene may be derived from triscale stepwise coordinated regulation (Figure 4g−j). Concretely speaking, the graphene layers provide an elastic buffer layer to accommodate the swell of CGOUNs and thus are conducive to long cycle life. Moreover, the abundant interface between the CGOUNs and graphene will afford additional lithium storage sites (Figure 4g). Beyond the “3 S” features of CGOUNs, the oxygen vacancies were generated on the CuGeO3 layer following the atomic evolution law (Figure 4h). Relative to the virgin lattice, a noticeable charge transfer phenomenon and imbalanced charge distribution will occur (Figure 4i), which will induce a positive area in the oxygen vacancy center and a corresponding negatively charged area around the oxygen vacancy. In particular, on the atomic level, during the discharging process, the electric field pointing to the negatively charged area from the nonvacancy area will accelerate the migration of Li ions and cause them to gather around the vacancy area because of the Coulomb attractive force (Figure 4j). After full lithiation, the originally negatively charged area will tend to be electrically neutral. In this regard, during the charging, the secondary electric field pointing from the positive area of the oxygen vacancy center to the electroneutral lithiation layer will facilitate Li+ migration, promoting Li+ extraction in the charge process. In short, the finetuning of the migratory behavior of the Li-ions facilitates the insertion/extraction speed for high rate performance.
EXPERIMENTAL SECTION Synthesis of Bulk CuGeO3. In a typical procedure, 1 mmol of GeO2 and 1 mmol of CuO were adequately ground and mixed. Then, the mixture was calcinated at 1000 °C for 48 h in air. The system was then allowed to cool to room temperature at a rate of 5 °C/min. The obtained blue powders were collected for further characterization. Synthesis of CGOUNs. In a typical procedure, 100 mg of bulk CuGeO3 powders and 100 mL of NMP were added into a 300 mL threenecked flask and refluxed at 95 °C for 24 h. After the expansion process by the refluxing operation, the mixed system was then subjected to a strong ultrasonic treatment for 24 h. The resultant dispersion was centrifuged at 500 rpm for 20 min. After centrifugation, the supernatant (about three-fourths of the centrifuged dispersion) was collected by pipet. The final product was collected by centrifuging the mixture at 12 000 rpm for 10 min and then drying it under vacuum overnight for further characterization. Synthesis of CGOUNs/Graphene. The graphene used in our experiments was synthesized according to the procedure used in previous studies.39,40 Thus, in our work, we adopted dimethylformamide (DMF) to disperse the graphene nanosheets. The DMF dispersions of graphene and NMP dispersions of CGOUNs were stirred and mixed overnight to further form a homogeneous dispersion. In the same way, the final product was collected by centrifuging the mixture, washing the product with absolute ethanol many times, and then drying under vacuum overnight for further characterization. Characterization. XRD patterns were recorded on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.541 78 Å). TEM was performed on a H7650 instrument at an acceleration voltage of 100 kV. The HAADF-STEM images were collected on a JEOL JEMARF200F atomic resolution analytical microscope. The energy dispersive spectroscopy (EDS) elemental mappings were acquired on a JEM 2100F (field emission) transmission electron microscope equipped with an Oxford INCA x-sight EDS Si (Li) detector at an acceleration voltage of 200 kV. AFM in the present work was carried out by means of a Veeco DI Nanoscope MultiMode V system. Raman spectra were detected with a RenishawRM3000 Micro-Raman system. ESR measurements were performed in a JSE-FA200 EPR spectrometer in the X-band (9 GHz) at 10 K. Electrochemical Measurements. The electrochemical tests were carried out via CR2032 coin cells. The working electrodes were prepared by mixing the as-prepared materials, Super P, and sodium carboxymethyl cellulose/poly(acrylic acid) (1:1) in a weight ratio of 70:20:10. The resultant slurry was pasted on Cu foil and dried in a vacuum oven at 80 °C for 12 h, followed by pressing at 300 kg/cm2. The loading of the materials on individual electrodes was about 1.0 ± 0.2 mg cm−2. Electrochemical measurements were carried out using twoelectrode coin cells with Li metal as counter and reference electrode. Celgard (product 2400) was used as the separator. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1/1; v/v). Electrochemical impedance spectroscopy and cyclic voltammetry were conducted on a VMP-3 electrochemical workstation at a scan rate of 0.1 mV s−1. The cells were galvanostatically charged and discharged over the voltage range of 0.01−3.0 V versus Li/ Li+ at different constant current densities, based on the weight of the samples, on a Land CT2001A battery tester. At least three parallel cells were tested for each electrochemical measurement, in order to make sure that the results were reliable and represented the typical behavior of the samples. Operando Synchrotron X-ray Powder Diffraction. A customized CR2032 coin cell for use in in situ SXRPD experiments was made. The cell was galvanostatically charged and discharged over the range of 0.1−3.0 V (vs Li) at constant current (equivalent to ∼100 mA/g) during data collection. SXRPD experiments were conducted on the Powder Diffraction beamline at the Australian Synchrotron, where the SXRPD data for the cell were collected every 3.4 min during charge and discharge using a MYTHEN microstrip detector. The wavelength used
CONCLUSION In conclusion, by focusing on the intrinsic factor of Li-ion migration behavior for higher performance LIBs, we have experimentally acquired oxygen vacancies confined in CGOUNs/graphene van der Waals heterojunctions as a high performance anode material. In this system, the atomic thickness of the nanosheet configuration offers abundant active sites to promote high capacity and release structural stress to resist collapse. More importantly, the self-adaptive atomic-scale oxygen vacancies will induce an in-plane built-in electric field, which could accelerate ion/electron migration rates and lead to high rate performance on both sides of the electrochemical reactions (charge and discharge). Beyond that, the graphene substrate could act as an elastic buffer layer, electrical highway, and source of additional interface lithium storage for further optimizing the lithium ion storage properties. As a typical example, profiting from this multiscale stepwise regulation strategy, the CGOUNs/graphene showed a high specific capacity of 1295 mAh g−1 at a current density of 100 mA g−1, satisfactory rate capability, and prolonged cycling performance, which makes our thus-constructed van der Waals heterojunctions attractive and promising as a high-performance anode material. More significantly, the strategy of optimizing by tuning the ion/ 8524
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Research Council (ARC) (DP170102406, FT150100109, and FT160100251) is gratefully acknowledged. The authors are grateful to the staff at Powder Diffraction beamline at the Australian Synchrotron, Victoria, Australia, for their support during the operando synchrotron powder diffraction experiment. Furthermore, we also thank Dr. T. Silver for critical reading of the manuscript.
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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.7b04617. Supplementary SXRPD data and the detailed crystal structure of CuGeO3; SEM images of bulk CuGeO3; TEM and Raman of CGOUNs; digital photographs of sample dispersion; TEM images of CGOUNs/graphene; TGA curves of the CGOUNs and CGOUNs/graphene; comparison of electrochemical rate performance of Gebased anode materials (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Tengfei Zhou: 0000-0002-7364-0434 Chong Xiao: 0000-0002-6134-6086 Zaiping Guo: 0000-0003-3464-5301 Yi Xie: 0000-0002-1416-5557 Author Contributions
Y.W.L., T.F.Z., C.X., Z.P.G., and Y.X. conceived the idea and cowrote the paper. Y.W.L. carried out the sample synthesis and characterization. T.F.Z., Y.Z., and W.K.P. implemented electrochemical measurements. Theoretical simulations were performed and interpreted by Z.H.H., B.C.P., Y.W.L., and T.F.Z. All the authors discussed the results and commented on and revised the manuscript. Author Contributions §
Y. Liu and T. Zhou contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (21622107, 21401182, 21331005, 11321503, and U1532265), National Basic Research Program of China (2015CB932302), the Youth Innovation Promotion Association CAS (2016392), and the Fundamental Research Funds for the Central University (WK2340000075 and WK2340000063). Financial support provided by the Australian 8525
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ACS Nano
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DOI: 10.1021/acsnano.7b04617 ACS Nano 2017, 11, 8519−8526
