Deeply Nesting Zinc Sulfide Dendrites in Tertiary Hierarchical

Jun 7, 2019 - ... hybrid electric vehicles (EVs) attributed to their outstanding energy ... (11). To design advanced materials for KIBs to host the st...
0 downloads 0 Views 11MB Size
Download PDF
www.acsnano.org

Downloaded via BUFFALO STATE on July 17, 2019 at 07:08:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Deeply Nesting Zinc Sulfide Dendrites in Tertiary Hierarchical Structure for Potassium Ion Batteries: Enhanced Conductivity from Interior to Exterior Jianhua Chu,† Wei Alex Wang,*,‡,§ Jianrui Feng,† Cheng-Yen Lao,∥ Kai Xi,∥ Lidong Xing,† Kun Han,⊥ Qiang Li,§ Lei Song,† Ping Li,⊥ Xin Li,† and Yanping Bao*,† †

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China § John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States ∥ Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom ⊥ Beijing Advanced Innovation Center for Materials Genome Engineering, Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China ‡

S Supporting Information *

ABSTRACT: Transition metal sulfides are deemed as attractive anode materials for potassium-ion batteries (KIBs) due to their high theoretical capacities based on conversion and alloying reaction. However, the main challenges are the low electronic conductivity, huge volume expansion, and consequent formation of unstable solid electrolyte interphase (SEI) upon potassiation/depotassiation. Herein, zinc sulfide dendrites deeply nested in the tertiary hierarchical structure through a solvothermalpyrolysis process are designed as an anode material for KIBs. The tertiary hierarchical structure is composed of the primary ultrafine ZnS nanorods, the secondary carbon nanosphere, and the tertiary carbon-encapsulated ZnS subunits nanosphere structure. The architectural design of this material provides a stable diffusion path and enhances effective conductivity from the interior to exterior for both K+ ions and electrons, buffers the volume expansion, and constructs a stable SEI during cycling. A stable specific capacity of 330 mAh g−1 is achieved after 100 cycles at the current density of 50 mA g−1 and 208 mAh g−1 at 500 mA g−1 over 300 cycles. Using density functional theory calculations, we discover the interactions between ZnS and carbon interface can effectively decrease the K+ ions diffusion barrier and therefore promote the reversibility of K+ ions storage. KEYWORDS: tertiary hierarchical structure, zinc sulfide dendrites, from interior to exterior, anode, potassium-ion batteries

C

gained much interest recently due to the practically inexhaustible K resources (2.09 wt %) and the closer redox potential of K+/K (−2.93 V vs standard hydrogen electrode, SHE) to Li+/Li (−3.04 V vs SHE) than Na+/Na (−2.71 V vs SHE).9,10 Unfortunately, the much larger ionic radius of K+ ions (1.40 Å) than Li+ ions (0.76 Å) would lead to more severe volume expansion and electrode collapse upon cycling.11 To design advanced materials for KIBs to host the structural distortion, researchers have focused on transition metal sulfides

urrently, due to the rapid consumption of traditional fossil fuels, the development of affordable, sustainable, and safe electric storage batteries for large-scale applications has become a priority.1,2 Lithium-ion batteries (LIBs) have been widely applied to portable electronics and hybrid electric vehicles (EVs) attributed to their outstanding energy and power density.3−5 The relatively low natural abundance (only 0.0017 wt %) and uneven distribution of Li resources in the Earth’s crust lead to limited applications of LIBs.6 Sodium-ion batteries (NIBs) could be driven from a cost perspective due to the low cost and high abundance of sodium (2.3 wt %), but they suffer from low volumetric energy density.7,8 In contrast, potassium-ion batteries (KIBs) have © 2019 American Chemical Society

Received: March 5, 2019 Accepted: June 7, 2019 Published: June 7, 2019 6906

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916

Article

Cite This: ACS Nano 2019, 13, 6906−6916

Article

ACS Nano

Figure 1. Schematic illustration of the synthesis of the ZSC@C@RGO composite.

Figure 2. (a, b) FESEM images and (c) TEM image of the dendritic ZSC nanospheres. (d, e) FESEM images and (f) TEM image of the ZSC@C nanospheres. (g, h) FESEM images and (i) TEM image of the ZSC@C@RGO composite. (j) HRTEM image of the ZSC@C@RGO composite and (k) the corresponding SAED pattern. (l) HAADF-STEM image and the corresponding EDS mapping of the ZSC@C@RGO composite.

conductive carbon layer is used for coating on the sulfide surfaces or for supporting them, which can overcome particle agglomeration, accommodate the volume expansion, and simultaneously improve the electronic conductivity.18,19 In spite of this, the redundant SEI formation and side reaction remain unsolved because of the small particle size even after coating.20,21 Therefore, it is highly desirable to develop a strategy for fabricating multilevel hierarchical-structure materials to have the advantages of both small particles and reasonable side reaction. In this work, to fully utilize the merits of ZnS and overcome its shortcomings, all carbon protected uniform zinc sulfide dendrites with tertiary hierarchical structure are synthesized. The primary ultrafine ZnS nanorods can lower the absolute volume change and provide a stable diffusion path for K+ ions and electrons while the secondary carbon nanosphere can increase the electronic conductivity, buffer the volume variation during K+ ion intercalation/deintercalation, and prevent self-agglomeration of the dendritic-like ZnS. In the tertiary carbon-encapsulated ZnS subunits nanosphere struc-

such as MoS2, WS2, and ZnS due to their high reversible capacities, excellent physical properties, and fast ion diffusion rates.12,13 Among them, zinc sulfide (ZnS) is undoubtedly a very promising candidate for LIBs and NIBs due to its nontoxic, high natural abundance, and low cost.14,15 To the best of our knowledge, no research has been reported to explore the potential application of ZnS as an anode material for KIBs yet. Generally, in LIBs and NIBs, it presents poor cycling stability due to its low electronic conductivity and large volume expansion as well as unstable solid electrolyte interphase (SEI) growth on the active particles interface.16 To solve the key issues, there are some classically manipulated strategies to improve the electrochemical properties, so that the above-mentioned advantages of the active materials are fully utilized. Nanotechnology engineering is an effective way to enhance the ion transportation and reduce the volume expansion due to the small particles could expand in a ductile and multidimensional surrounding and increase the electrode− electrolyte contact area, but the nanoparticles suffer severe agglomeration owing to high surface energy.17 To settle this, a 6907

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916

Article

ACS Nano

Figure 3. (a) TEM image, (b, c) HRTEM images, and (d) corresponding SAED pattern of the dendritic ZSC nanospheres. The inset of (b) is the high-magnification TEM image of a selected area. (e) HAADF-STEM and (f) elemental mapping images of the dendritic ZSC nanospheres. (g) TEM and (h) HRTEM images of the ZSC@C nanospheres. (i) HAADF-STEM and (j, k) elemental mapping images of the ZSC@C nanospheres. (l, m) Line scanning curves of elemental distribution corresponding to Line 1 (e, ZSC) and Line 2 (i, ZSC@C), respectively.

As shown in Figure 2a and b, typical field-emission scanning electron microscopy (FESEM) images of the ZSC nanospheres structure with a uniform size of ≈120 nm are presented. The dendritic surface of ZnS nanorods embedded into the carbon spheres (Figure 2c) completely differs from the smooth surface of the ZIF-8 precursor (Figure S1). The organic linkers (2methylimidazole) in ZIF-8 can be self-assembled into single nanosphere skeletons due to the strong interaction force between the Zn2+ and sulfide through the solvothermal method.23 Interestingly, the carbon frame derived from the pyrolysis process of organic linkers at high temperatures supports ZnS nanoparticles. After the pyrolysis of glucose, the ZSC@C nanospheres are obtained (Figure 2d, e). Bright smooth areas at the edges from the surface of ZSC@C nanospheres can be observed (Figure 2f), revealing the dendritic ZSC nanospheres are well encapsulated by the amorphous carbon. Graphene oxide (Figure S2) is introduced during the synthetic process to support these nanospheres and further improve the electronic conductivity (Figure 2g−i). ZSC@RGO without the glucose treatment was also prepared with the same synthetic process for comparative research (Figure S3). Lattice spacing of 0.31 nm is clearly observed, corresponding to the ZnS (102) plane (Figure 2j). The crystal nature is also confirmed by selected area electron diffraction (SAED) pattern (Figure 2k). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) elemental mapping of ZSC@C@RGO show uniform distribution of ZSC@C on RGO (Figure 2l). In addition, the EDS spectrum (Figure S4a) and element content analysis (Table S1) of the

ture, the carbon surface can avoid direct exposure of ZnS interface to the electrolyte, thus reducing the excess SEI formation and side reaction. Consequently, the tertiary hierarchical structure wrapped in reduced graphene oxide (RGO) networks (noted as ZSC@C@RGO) delivers a high capacity of 330 mAh g−1 at 50 mA g−1 after 100 cycles and a stable cycling performance of maintaining 208 mAh g−1 at 500 mA g−1 over 300 cycles. This work sheds light on rational design of multilevel hierarchical-structure materials for highperformance energy storage.

RESULTS AND DISCUSSION The synthesis procedure of the ZSC@C@RGO nanoarchitecture is schematically shown in Figure 1. First, the well-defined ZIF-8 nanoparticles have regular polyhedron-like morphology with a size of about 500 nm by a precipitation method.22 The dendritic ZnS/carbon nanospheres composite (ZSC) was obtained through the subsequent solvothermalpyrolysis process. During this process, the formed ZnS nanorods were embedded into the carbon spheres, forming a dendritic structure by self-assembly under high temperature and high pressure. In the next similar solvothermal-carbonization process, glucose was converted to amorphous carbon and covered the dendritic ZnS (ZSC@C). To reduce the residual oxygen moieties, graphene oxide (GO) sheets were introduced and reduced to RGO sheets after annealing at 600 °C under Ar atmosphere. Finally, the ZSC@C nanospheres were well dispersed and tightly anchored onto the RGO sheets (ZSC@C@RGO). 6908

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916

Article

ACS Nano

Figure 4. (a) XRD patterns of the as-prepared dendritic ZSC nanospheres, ZSC@C nanospheres, and ZSC@C@RGO composite. (b) Raman spectrum of the dendritic ZSC nanospheres and ZSC@C@RGO composite, respectively. (c) N2 adsorption/desorption isotherms for the ZSC@C@RGO composite and (d) corresponding pore size distribution calculated by the density functional theory method. (e) XPS survey of the ZSC@C@RGO composite and its high-resolution spectra: (f) Zn 2p, (g) S 2p, (h) C 1s, and (i) N 1s.

content ≈ 14.26%) as well as the carbon sulfur analyzer data (Table S4). The as-prepared ZIF-8 nanocrystals display a typical X-ray diffraction (XRD) pattern (Figure S5) in the 2θ range of 10− 40°, which is in agreement with previously reported crystal structure data. After solvothermal and annealing treatment, the crystal phases of ZSC, ZSC@C, and ZSC@C@RGO are shown in Figure 4a. It shows clearly that all the diffraction peaks of the products agree well with the standard XRD patterns of ZnS (JCPDS No. 89-2156). The wide diffraction peaks of (102) plane of ZSC@C and ZSC@C@RGO are observed, which can be attributed to the typical carbon peak at 26°. Moreover, ZSC@RGO exhibits similar diffraction peaks (Figure S6) to those of ZSC@C@RGO (Figure S7a). And the tap density of ZSC@C@RGO is ca. 0.876 g cm−3, a slighter fall than that of ZSC and ZSC@C (Table S5). The Raman spectra of ZSC and ZSC@C@RGO (Figure 4b) show amorphous nature with the intensity ratio of the D to G band (ID/IG) being greater than 1.24 The Raman spectrum of RGO is also shown in Figure S7b. Additionally, the N2 adsorption−desorption isotherms show clearly a type IV isotherm (Figure 4c), and the hysteresis loops indicate that the ZSC@C@RGO composite possesses a typical mesoporous

ZSC@C@RGO composite show that the atomic ratio of Zn and S approaches 1:1 (C content ≈ 20.47%). More detailed structural information about the as-prepared samples is shown in Figure 3. Each ZnS dendrite is well crystallized with an average diameter of 5 nm (Figure 3a−d). Except for the ZnS dendrites embedded into the carbon surface, some ZnS nanorods also exist in the inner part of the carbon sphere (Figure 3c). The HAADF-STEM image (Figure 3e) and EDS mapping (Figure 3f) display well-defined distribution of Zn, S, C, and N elements across the whole dendritic ZSC nanosphere. The carbon content in ZSC is ≈3.85% according to the EDS spectrum (Figure S4b) and element content analysis (Table S2). After the pyrolysis of glucose, the ZnS dendrites are deeply nested in the amorphous carbon without the destruction of the dendrite shape (Figure 3g, h). The HAADF-STEM image (Figure 3i) and elemental mappings (Figure 3j, k) verify the uniform distribution of Zn, S, C and N elements. Noticeably, the carbon wrapped on the surface of the ZSC can be clearly seen in Figure 3j. And the line profiles (Figure 3l, m) strongly reveal that the amorphous carbon was successfully covered on the ZSC spheres after the pyrolysis of glucose, consistent with the EDS spectrum (Figure S4c) and the element content analysis (Table S3, carbon 6909

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916

Article

ACS Nano

Figure 5. (a) Galvanostatic charge/discharge profiles and (b) cycling performance of the ZSC@C@RGO at 50 mA g−1 in the voltage range of 0.01−2.5 V for KIBs. (c) Comparison of cycling performance of the as-prepared ZSC, ZSC@C, and ZSC@C@RGO at 100 mA g−1. (d) Comparison of rate performance at various current densities from 20 to 500 mA g−1. (e) Long-term cycling stability of the ZSC@C@RGO over 300 cycles at 500 mA g−1.

structure.25 The ZSC@C@RGO composite exhibits a Brunauer−Emmett−Teller (BET) surface area of 171.42 m2 g−1, which is much higher than that of the dendritic ZSC nanospheres (98.19 m2 g−1) (Figure S8a). And it is obviously observed that the pore size distribution of the ZSC@C@RGO composite mostly lies in the range of 1−2 nm (Figure 4d), which agrees well with that of the ZSC (Figure S8b). The mesoporous structure and favorable specific surface area of ZSC@C@RGO composite not only can buffer the volume changes during cycling but also can boost up the electronic conductivity, improving the electrochemical performance. X-ray photoelectron spectroscopy (XPS) measurements were conducted to investigate the chemical composition and analyze the surface physicochemical properties of ZSC@C@ RGO (Figure 4e−i), ZSC (Figure S9), and ZSC@C (Figure S10). The Zn 2p spectrum exhibits two peaks at 1021.7 and 1044.7 eV, corresponding to Zn 2p3/2 and Zn 2p1/2 spin−orbit peaks of ZnS, respectively (Figure 4f).26 And the two characteristic peaks of the S 2p spectrum at 161.6 and 162.8 eV are assigned to the S 2p3/2 and S 2p1/2 (Figure 4g), suggesting the existence of S2− in ZnS.27 The same peaks can also be observed in the Zn 2p and S 2p spectra of ZSC (Figure S9b and c) and ZSC@C (Figure S10b and c). Meanwhile, the C 1s spectra of ZSC@C@RGO (Figure 4h) not only show the peaks of C−C, C−O, CO, and O−CO originating from oxygen-functional groups of amorphous carbon and RGO,28 but also exhibit an obvious CN peak located at 285.6 eV, indicating the successful incorporation of N into carbon.10 Yet

the pure amorphous carbon shows only the peaks of C−C, CO, and CN for ZSC (Figure S9d) and ZSC@C (Figure S10d). Furthermore, the fitting results of the high-resolution N 1s spectrum exhibit the presence of pyridinic N, pyrrolic N and graphitic N in ZSC@C@RGO (Figure 4h), and the similar characteristic peaks of the N 1s spectrum are also observed for ZSC (Figure S9e) and ZSC@C (Figure S10e), which derive from the organic linkers (2-methylimidazole) in ZIF-8.29 The charge/discharge curves of the ZSC@C@RGO electrode in the voltage range of 0.01−2.5 V at a current density of 50 mA g−1 are shown in Figures S12 and 5a. Figure S12 shows a high initial discharge capacity (1023 mA h g−1) and a charge capacity (651 m Ah g−1), corresponding to a low Coulombic efficiency of 63.6% because of the irreversible formation of SEI on the surface of the ZSC@C@RGO electrode.30 A discharge voltage plateau at 0.6 V can be seen (Figure 5a), well agreeing with the Cyclic voltammetry (CV) results (Figure S11). And the capacity loss between the 50th and 100th cycles is much less, demonstrating a highly reversible reaction. The CV profiles and charge/discharge curves of ZSC@C are similar to those of ZSC@C@RGO (Figure S13). However, the ZSC electrode exhibits different CV profiles and charge/discharge curves, which may be due to the structural collapse during large-sized K ion intercalation/ deintercalation (Figure S14). Considering the highest initial discharge capacity of the ZSC@C@RGO electrode with the lowest content of ZnS in three samples, we can conclude that the rational architectural design of the well-defined tertiary 6910

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916

Article

ACS Nano

Figure 6. (a) Charge differences between the ZnS and Carbon. (b) Planar-average charge density along the direction of the z axis. (c) Migration barrier of K ions within the interface of ZnS/C. (d) Comparative migration barrier of the isolated ZnS without C.

200 mA g−1, respectively (Figure S15). The cycling stability of RGO can be observed in Figure S16. The amorphous carbon nanosphere was prepared by a hydrothermal-carbonation process (Figures S17). And its potassium storage performance is shown in Figures S18, which only exhibits a relatively low capacity of 81 mAh g−1 after 100 cycles at 100 mA g−1. Considering the small amount of the amorphous carbon layer in ZSC@C and ZSC@C@RGO, the capacity contribution is almost negligible to the total capacity of the composite electrode. Meanwhile, the ZSC@C@RGO electrode possesses the highest capacity at the same current densities (Figure 5d). Specifically, the ZSC@C@RGO electrode exhibits outstanding reversible capacities of 419, 362, 295, 223, and 162 mA h g−1 at 20, 50, 100, 200, and 500 mA g−1, respectively, and the ZSC@ C electrode displays slightly lower specific capacities at each current density. The fast capacity decay of ZSC@C@RGO electrode occurred in the initial several cycles is mainly ascribed to the consuming of a large amount of K ions in terms of high specific surface area for formation and stabilization of SEI film and activation process.20,23 In addition, the formation of nonconducting potassium polysulfides phase and the decomposition of electrolyte as well as dissolution of sulfur component also contribute to the irreversible capacity loss.12 Particularly, the specific capacity of the ZSC electrode almost decays to zero at high current densities. It is worth noting that the capacity of the ZSC@C@RGO electrode can still be restored to 376 mA h g−1 when the current density returns to 20 mA g−1. Furthermore, the long-term cycling stability of the ZSC@ C@RGO electrode was performed at 500 mA g−1 (Figure 5e). It exhibits a reversible discharge capacity of 208 mA h g−1 and a stable Coulombic efficiency around 100% over 300 cycles. Such outstanding cycling performance could be due to the highly structural stability of ZSC@C@RGO electrode during the successively charging/discharging processes. Excellent rate performance can be attributed to the rapid transport of ions

hierarchy with RGO can dramatically facilitate the diffusion of K ions and electrons from interior to exterior in the entire electrode, thus enhancing the electrochemical reaction kinetics,12 which coincides with the EIS results (Figure S19a). Besides, the high specific surface area of ZSC@C@ RGO could also enhance K+ adsorption/desorption and result in high initial capacity. The cycling performance and the corresponding Coulombic efficiency of the ZSC@C@RGO electrode at a current density of 50 mA g−1 are shown in Figure 5b. High capacity and much improved Coulombic efficiency are maintained after the first 10 cycles. Although the capacity drop of the ZSC@C@RGO electrode keeps for 20−60 cycles, it still maintains an excellent discharge capacity of 330 mA h g−1 with a high Coulombic efficiency of nearly 100% after 100 cycles. It is necessary to point that the huge capacity drop can be ascribed to the formation of nonconducting potassium polysulfides phase and the decomposition of electrolyte as well as stabilization of the interface between electrode and electrolyte.27 It is worth noting that the improved Coulombic efficiency and cycling stability of the ZSC@C@RGO electrode are closely related to the protection of dual-carbon layer and its good electronic conductivity in the tertiary hierarchical structure, which can avoid direct exposure of fresh active material to the electrolyte to reduce the excess formation of SEI and other irreversible side reactions.31 In contrast, the cycling performance of the as-prepared samples at 100 mA g−1 is depicted in Figure 5c. The ZSC@ C@RGO and ZSC@C electrode deliver a relatively high discharge capacity of 254 and 202 mA h g−1 after 100 cycles, respectively, which are much higher than that of the ZSC (28 mA h g−1) electrode. The reason for the lower capacity and poor cycling stability of ZSC electrode can be attributed to their large pulverization, poor conductivity, and low surface area. In addition, the ZSC@RGO electrode only shows the capacities of 96 mA h g−1and 60 mA h g−1 at 100 mA g−1 and 6911

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916

Article

ACS Nano

Figure 7. Ex situ XPS spectra of (a) Zn 2p and (b) S 2p at different charge/discharge states of the ZSC@C@RGO electrode. (c, d) TEM images of the ZSC@C@RGO electrode after 300 cycles at 500 mA g−1. (e) Log(i) versus log(v) plots based on specific peak currents from the CV curves at different scan rates in the range of 0.1−5 mV s−1 in KIBs. (f) Contribution ratio of the capacitive capacity and diffusionlimited capacity at different scan rates.

color (accumulation of electron) occupies the C zone and the green color (loss of electron) occupies the ZnS zone neighboring the interface. The planar-average charge density is shown in Figure 6b with a vertical dash line denoting the interface of the two zones. On the left side of the dashed line, the values are negative, implying the loss of electrons, while the values on the right side are positive, which denotes the increase of electron density in the C zone. It is reasonable because the electrostatic potential of the C zone is lower than that of the ZnS zone (Figure S21). It concludes that the electron transfer between ZnS and C can enhance the electron conductivity and tune the electronic properties of the ZnS phase. Meanwhile, the diffusion barrier of K ions is strongly related to the electronic properties of ZnS. In this regard, we performed the nudged elastic band method to calculate the barrier of K ion migration in the interface of ZnS/C (Figure 6c). Compared with the model of ZnS without C (Figure 6d), the diffusion barrier of the interface model is very low. It is because of the stabilization of the transition state of K ion in the diffusion pathway by the interaction between ZnS and C. The transition state is denoted as 2 in the figure, and from the structure conformation we can deduce that the K ion interacts with C and ZnS at the same time to stabilize the transition state. Therefore, both electronic and ionic transport are boosted. To reveal the reaction mechanism, we carried out ex situ XPS analysis calibrated with C 1s (284.5 eV) of the ZSC@C@ RGO in the intermediate (discharge to 0.8 V), fully potassiated (discharge to 0.01 V) and fully depotassiated (charge to 2.5 V) states, respectively. Figure 7a shows the high-resolution Zn 2p XPS spectra at different charge/discharge states. When discharged to 0.8 V, two major peaks located at 1021.5 and 1044.5 eV are observed, which correspond to the Zn 2p3/2 and

and electrons, which is consistent with the results of electrochemical impedance spectroscopy (EIS) in Figure S19. It shows the Nyquist plots of the AC impedance spectra for the three samples before cycling. The ZSC@C@RGO electrode demonstrates obviously reduced semicircle at high medium frequency, indicating enhanced kinetics of the electrochemical reaction (Figure S19a). And the line relationship between real impedance and reciprocal square root of frequencies at the low-frequency region (Figure S19b) is used to calculate the K+ ions diffusion coefficient (D k+) according to eq 132 D k+ = 0.5(RT /AF 2σwC)2

(1)

where R is the gas constant, T is the absolute temperature, A is the surface area of the electrode, F is the Faraday constant, C is the concentration of K+ ions in active material, andσw can be derived by the above-mentioned line relationship. As shown in Table S6,D k+ of ZSC@C@RGO electrode is 1.88 × 10−17, which is much higher than that of the ZSC and ZSC@C electrodes. The superior K+ ions diffusion coefficient of ZSC@ C@RGO electrode can be attributed to the ultrafine ZnS nanorod and all carbon-protected tertiary hierarchical structure. Furthermore, compared with ZSC (7.39 × 103 S m−1) and ZSC@C (8.56 × 103 S m−1), a significantly enhanced electronic conductivity (9.60 × 103 S m−1) of ZSC@ C@RGO was obtained by the four-probed method (Figure S20). First-principle calculations were performed to gain insight into improved potassium storage capacity of ZSC@C. To depict the interface between ZnS and C, the heterojunction model of ZnS/C was built as shown in Figure 6a and b. The charge density difference between ZnS and C manifests that the electron is transferred from ZnS to C, in which the red 6912

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916

Article

ACS Nano

Figure 8. Schematic illustration during the potassiation/depotassiation processes of (a) ZnS nanorods, (b) ZSC, (c) ZSC@C, and (d) ZSC@ C@RGO.

Zn 2p1/2 in the Zn2+ state, respectively. No obvious conversion reaction happens at this stage. However, two main peaks of Zn 2p3/2 (1020.8 eV) and Zn 2p1/2 (1043.9 eV) can be assigned to metal in the fully potassiated state, indicating a conversionalloying reaction from Zn2+ to KZnx alloy.23,33 When fully depotassiated, the binding energy of Zn 2p3/2 and Zn 2p1/2 centered at 1021.6 and 1044.6 eV, respectively, corresponding to the intercalated Zn in ZnS. In the meantime, the dominant S 2p3/2 and S 2p1/2 peaks located at 161.8 and 163.0 eV when discharged to 0.8 V (Figure 7b), respectively, which is considered to be the potassium polysulfides.34 And the two peaks centered at 161.2 and 162.5 eV are assigned to K2S when fully potassiated.35 After being fully depotassiated, the S 2p XPS spectrum of S 2p3/2 and S 2p1/2 is in agreement with the pristine state, corresponding to the S2− species of ZnS. In addition, a wide peak appears at around 168.0 to 169.0 eV throughout the potassiation/depotassiation process, which is assigned to sulfate originates from electrolyte or the oxidized active material in air.8 Based on the above discussion, the electrochemical reaction of ZSC@C@RGO electrode can be described by the following equation: ZnS + K+ + e− ↔ K 2S + KZn x

is further estimated quantitatively according to the equation of i = k1v + k2v1/2, where k1v and k2v1/2 correspond to the relative contribution from the capacitive process and the intercalation process, respectively.2 As shown in Figure S23b, capacitive charge contributes 43.81% of the total capacity at the sweep rate of 0.1 mV s−1. With the increase of the sweep rates, the contribution of capacitive charge rises (Figure 7f) and finally reaches a maximum value of 71.86% at a high scan rate of 5 mV·s−1, which is induced by the well-developed tertiary hierarchical structure with RGO that can enhance K + adsorption. It is worth noting that at a low sweep rate the capacity contribution mainly derives from diffusion-controlled process via a conversion and alloy reaction of ZnS dendrites.38 Based on the above, the superior potassium storage performance of the ZSC@C@RGO electrode could be closely related to the well-defined tertiary hierarchical structure and the supporting effect of RGO. Figure 8 illustrates the structural change during the potassiation/depotassiation processes. In general, ultrafine subunits have the advantages of shortened diffusion path and large electrode−electrolyte contact area but suffer from agglomeration and pulverization during cycling, which inevitably leads to repeated formation of SEI film (Figure 8a).39 To solve this problem, the carbon coating on the surface of the active material can be carried out to prevent the particle from cracking (Figure 8b). Nevertheless, the partial carbon coating cannot fully curb the inhomogeneous volume change during the potassiation/depotassiation processes, causing the excessive formation of the SEI layer and rapid capacity decay. In contrast, the dual carbon-protected tertiary structure (Figure 8c) can restrict the agglomeration of ultrafine active particles during discharge/charge process to maintain the structure integrity. Notably, the robust outer carbon shell further improves the electronic conductivity and the stability of the SEI layer. The introduction of RGO further enhances the electronic conductivity and inhibits the agglomeration. The well-defined tertiary hierarchical ZSC@C@RGO (Figure 8d) not only dramatically facilitates potassium ions and electrons fast diffusion throughout the entire electrode during cycling, but also supplies enough space to accommodate the accumulated strain. Moreover, the hybrid structure can also boost the conductivity of ions and electrons from the interior to exterior.

(2)

Furthermore, there was no apparent morphology change of ZSC@C@RGO electrode after 300 cycles, (Figure 7c, d), indicating its high structural stability. As shown in the TEM image (Figure S22a) of ZSC electrode, the outer of ZnS nanorods almost disappeared and only carbon spheres remain after 100 cycles. Whereas the cycled ZSC@C do not present any electrode collapse (Figure S22b). To further prove the contribution of the capacitive behavior, CV curves at various sweep rates from 0.1 to 5 mV s−1 in a voltage range from 0.01 to 2.5 V were recorded (Figure S23a). The current response and sweep rate follow the formula i = avb, where i is the current, v is the scan rate, and a and b stand for empirical parameters.4 A typical diffusion-controlled intercalation corresponds to b = 0.5, whereas b = 1.0 indicates a surface-induced capacitive behavior.36 Figure 7e shows the log(i) versus log(v) profile, and the b-values of the cathodic and the anodic peaks are calculated to be 0.55 and 0.43, respectively, indicating that the redox process is dominated by both the diffusion-controlled and pseudocapacitive-controlled behaviors.37 The contribution of the surface capacitive capacity 6913

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916

Article

ACS Nano

range of 10−90° with a scan rate of 10° min−1 under 40 kV and 25 mA. Raman spectra were measured by using a Micro-Raman spectrometer (Lab RAM-HR Evolution) with an argon laser with a wavelength of 532 nm. XPS was investigated by using an AXIS ULTRADLD Scientific instrument to obtain a spectrum with an Al Κα (hυ = 1486.69 eV) X-ray source. Nitrogen adsorption/desorption isotherms were measured with an ASAP 2460 accelerated surface area and porosimetry instrument using the Barrett−Joyner−Halenda (BJH) method. A carbon sulfur analyzer by EMIA-920 V2 was applied to test the carbon and sulfur content in the respective samples. The density and tap density of samples were measured by using a densitometer (TD-2200) and tap density meter (LABULK0335) with 3000 vibrations, respectively. The electronic conductivity of samples was measured through a four-point probe method at the current of 0.02 mA (Keithley 4200-SCS). Four parallel gold electrodes were used with a spacing distance of ∼1 mm between each adjacent electrode. Electrochemical Measurements. Electrochemical measurements were performed with CR2032 coin-type cells. The negative electrode is composed of 90 wt % active material and a binder of 10 wt % polyvinylidene fluoride (PVDF). The mixed slurry was coated onto the copper foil and further dried at 80 °C overnight in vacuum oven, and then the coated copper foil was punched into pellets in a diameter of 12 mm (Figure S24a) with a mass loading of ca. 0.8−1.2 mg cm−2 (Figure S24b). The 2032-type coin cells were assembled in an argon-filled glovebox (Mikrouna, Super 1200/750), where the concentrations of moisture and oxygen were maintained below 0.1 ppm. Potassium metal was applied as the anode. A Whatman GF/D glass fiber filter was used as the separator, and the electrolyte was 1.0 M KPF6 in ethylene carbon/diethyl carbonate (EC/DEC, 1:1 v/v) solution. The cells were aged for 12 h before measurements to ensure complete electrode wetting by the electrolyte. Galvanostatic charge− discharge cycle tests were performed with a Land CT2001A battery testing system in the fixed voltage window from 0.01 to 2.5 V versus K+/K at room temperature. CV measurements with various scan rates from 0.1 to 5 mV s−1 in the voltage range of 0−2.5 V (vs K+/K), and EIS (frequency range from 100 kHz to 0.1 Hz) of coin cells was performed on a CHI 660 electrochemical workstation (Shanghai Chenhua Corp.) Computation. All the first-principle calculations were performed by using the Vienna ab initio simulation package (VASP). For optimization of the geometries, we used projector augmented waves (PAW)40 and the function of Perdew, Burke, and Ernzerhof (PBE)41 for depiction of ion-electron interaction and the exchange correlation potential, respectively, with 5 × 5 × 1 Monkhorst−Pack42 sampled k points and a cutoff energy of 400 eV. The criteria of convergence were set to 1 × 10−5 eV and 0.01 eV/Å for the self-consistent field (SCF) and ion steps, respectively. In all the calculations, the spin polarization was considered. The nudged elastic band (NEB) method was adopted for estimating the Li migration barrier. The heterojunction model of ZnS/C interface was built by 2 × 2 ZnS (0 0 1) and 3 × 3 graphite (001). The lattice parameters were 7.50 and 12.99 Å for a and b, respectively. The vacuum slab was more than 12 Å in the direction of the c axis.

CONCLUSIONS In summary, all carbon-protected ZSC@C@RGO with tertiary hierarchical structure for reversible K-ion storage is synthesized via a solvothermal-pyrolysis strategy. In this structure, the ZnS dendrites undergo a conversion and alloying reaction with good reversibility. As a result, high capacity (330 mAh g−1 at 50 mA g−1) and long cycle life (208 mAh g−1 at 500 mA g−1 after 300 cycles) are achieved. DFT calculations disclose that the ZnS/carbon interface can effectively decrease the K+ ions diffusion barrier and improve K+ ions storage stability. The good electrochemical performance is ascribed to the synergistic effect of the tertiary hierarchical structure, that is the primary ultrafine ZnS nanorods, the secondary carbon nanosphere, and the tertiary carbon-encapsulated ZnS subunits nanosphere structure. Within the material, the K+ ions and electrons can easily transfer from the interior to exterior of the electrode with limited volume variation and stable SEI. This work provides valuable guidance for the synthesis of multilevel hierarchicalstructure materials for energy storage. EXPERIMENTAL SECTION Synthesis of ZIF-8 Template. In a typical synthesis process, methanolic solutions of zinc nitrate hexahydrate (2.43 g, 120 mL) and methanolic solutions of 2-methylimidazole (1.578 g, 120 mL) were mixed under stirring. After stirring vigorously for 20 min, the mixture was aged at room temperature for 24 h. The resulting white precipitates were centrifuged and washed with methanolic four times and then freeze-dried for 12 h. Synthesis of ZSC@C. For the synthesis of ZSC@C, 0.03 g of ZIF8 and 5 mL of methanolic were loaded into a Teflon-lined stainlesssteel autoclave with 50 mL capacity and stirred for 1 h to form a white solution at room temperature. Then, 0.03 g of L-cysteine, 0.025 g of urea, and 30 mL of an aqueous solution containing 0.15 g of glucose were added to the above-mentioned solution under continuous stirring for 1 h. After that, the autoclave was sealed, maintained at 120 °C for 24 h, and cooled naturally to room temperature. The asderived precipitate was washed with ethanol several times and dried in vacuum at 80 °C overnight, and subsequently annealed at 600 °C for 2 h in Ar atmosphere with a heating rate of 10 °C min−1. The dendritic ZSC was also prepared following the above procedure without the presence of glucose. Synthesis of ZSC@C@RGO. An amount of 0.015 g of ZSC@C was dispersed in 20 mL of ethanol solutions under ultrasonicatio for 10 min. Subsequently, the above mixtures were added to 5 mL of 0.001 g mL−1 GO suspension under stirring for 5 h. Then, the solid product was recovered by centrifugation and then vacuum-dried at 70 °C overnight. Finally, the as-prepared composites were calcined in a tube furnace for 2 h at 600 °C with a heating rate of 10 °C min−1 under the atmosphere of Ar gas to remove surface organics and some oxygen-containing groups. For comparison, ZSC@RGO was synthesized under the same conditions just replacing ZSC@C with ZSC. Synthesis of Amorphous Carbon Nanosphere. Here, 0.5 g of glucose was dissolved 30 mL deionized water under vigorous stirring for 1 h form a homogeneous transparent solution, and then the solution was loaded into a 50 mL Teflon-lined stainless-steel autoclave maintained at 180 °C for 10 h. The as-acquired precipitate was centrifuged with ethanol several times and then dried in vacuum at 70 °C overnight. Finally, the obtained powders were calcined in Ar gas at 600 °C for 2 h with a ramp rate of 10 °C min−1. Materials Characterization. The morphology was investigated by FESEM (Hitachi SU8010) operating at 5 kV and TEM and highresolution TEM (HRTEM) on a JEOL JEM-2100F operating at 200 kV. SAED patterns were collected by using a Gatan charge-coupled device camera in digital format. Atomic resolved HRTEM images and EDS mapping were obtained by HAADF-STEM. The crystal structures and phases of the as-prepared materials were characterized by XRD (Smartlab, RIGAKU) using Cu Kα radiation recorded over a

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b01773. Figures showing FESEM and TEM results, EDS spectrum, XRD pattern, Raman spectrum, surface area and corresponding pore size distributions, XPS spectra, CV curves, discharge/charge profiles, cycling performance, Coulombic efficiency, Nyquist plots, schematic illustration of the electronic conductivity and specific value, DFT calculations results, pseudocapacitive contributions; digital photographs show the diameter and the mass loading of the electrode; and table showing 6914

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916

Article

ACS Nano

Catalyzing of Zinc Sulfide for High Performance Lithium-Sulfur Battery. Nano Energy 2018, 51, 73−82. (14) Su, D.; Kretschmer, K.; Wang, G. Improved Electrochemical Performance of Na-Ion Batteries in Ether-Based Electrolytes: A Case Study of ZnS Nanospheres. Adv. Energy Mater. 2016, 6, 1501785. (15) Cao, L.; Zhang, B.; Ou, X.; Wang, C.; Peng, C.; Zhang, J. Synergistical Coupling Interconnected ZnS/SnS2 Nanoboxes with Polypyrrole-Derived N/S Dual-Doped Carbon for Boosting HighPerformance Sodium Storage. Small 2019, 15, 1804861. (16) Fan, X.; Gao, T.; Luo, C.; Wang, F.; Hu, J.; Wang, C. Superior Reversible Tin Phosphide-Carbon Spheres for Sodium Ion Battery Anode. Nano Energy 2017, 38, 350−357. (17) Liu, B.; Zhang, Q.; Jin, Z.; Zhang, L.; Li, L.; Gao, Z.; Wang, C.; Xie, H.; Su, Z. Uniform Pomegranate-Like Nanoclusters Organized by Ultrafine Transition Metal Oxide@Nitrogen-Doped Carbon Subunits with Enhanced Lithium Storage Properties. Adv. Energy Mater. 2018, 8, 1702347. (18) Xu, X.; Liu, J.; Liu, Z.; Shen, J.; Hu, R.; Liu, J.; Ouyang, L.; Zhang, L.; Zhu, M. Robust Pitaya-Structured Pyrite as High Energy Density Cathode for High-Rate Lithium Batteries. ACS Nano 2017, 11, 9033−9040. (19) Han, K.; Liu, Z.; Li, P.; Yu, Q.; Wang, W.; Lao, C.-Y.; He, D.; Zhao, W.; Suo, G.; Guo, H.; Song, L.; Qin, M.; Qu, X. HighThroughput Fabrication of 3D N-Doped Graphenic Framework Coupled with Fe3C@Porous Graphite Carbon for Ultrastable Potassium Ion Storage. Energy Storage Mater. 2019, DOI: 10.1016/ j.ensm.2019.01.016. (20) Li, Z.; Zhao, H.; Lv, P.; Zhang, Z.; Zhang, Y.; Du, Z.; Teng, Y.; Zhao, L.; Zhu, Z. Watermelon-Like Structured SiOx-TiO2@C Nanocomposite as a High-Performance Lithium-Ion Battery Anode. Adv. Funct. Mater. 2018, 28, 1605711. (21) Szczech, J. R.; Jin, S. Nanostructured Silicon for High Capacity Lithium Battery Anodes. Energy Environ. Sci. 2011, 4, 56−72. (22) Tang, J.; Salunkhe, R. R.; Liu, J.; Torad, N. L.; Imura, M.; Furukawa, S.; Yamauchi, Y. Thermal Conversion of Core-Shell Metalorganic Frameworks: A New Method for Selectively Functionalized Nanoporous Hybrid Carbon. J. Am. Chem. Soc. 2015, 137, 1572−80. (23) Dong, S.; Li, C.; Ge, X.; Li, Z.; Miao, X.; Yin, L. ZnS-Sb2S3@C Core-Double Shell Polyhedron Structure Derived from Metal-Organic Framework as Anodes for High Performance Sodium Ion Batteries. ACS Nano 2017, 11, 6474−6482. (24) Chu, J.; Yu, Q.; Yang, D.; Xing, L.; Lao, C.-Y.; Wang, M.; Han, K.; Liu, Z.; Zhang, L.; Du, W.; Xi, K.; Bao, Y.; Wang, W. ThicknessControl of Ultrathin Bimetallic Fe−Mo Selenide@N-doped Carbon Core/Shell “Nano-Crisps” for High-Performance Potassium-Ion Batteries. Appl. Mater. Today 2018, 13, 344−351. (25) Xiong, P.; Bai, P.; Tu, S.; Cheng, M.; Zhang, J.; Sun, J.; Xu, Y. Red Phosphorus Nanoparticle@3D Interconnected Carbon Nanosheet Framework Composite for Potassium-Ion Battery Anodes. Small 2018, 14, 1802140. (26) Liu, X.; Liu, Y.; Feng, M.; Fan, L.-Z. MOF-Derived and Nitrogen-Doped ZnSe Polyhedra Encapsulated by Reduced Graphene Oxide as the Anode for Lithium and Sodium Storage. J. Mater. Chem. A 2018, 6, 23621−23627. (27) Dong, S.; Li, C.; Li, Z.; Zhang, L.; Yin, L. Mesoporous Hollow Sb/ZnS@C Core-Shell Heterostructures as Anodes for HighPerformance Sodium-Ion Batteries. Small 2018, 14, 1704517. (28) Xing, L.; Yu, Q.; Jiang, B.; Chu, J.; Lao, C.-Y.; Wang, M.; Han, K.; Liu, Z.; Bao, Y.; Wang, W. Carbon-Encapsulated Ultrathin MoS2 Nanosheets Epitaxially Grown on Porous Metallic TiNb2O6 Microspheres with Unsaturated Oxygen Atoms for Superior Potassium Storage. J. Mater. Chem. A 2019, 7, 5760−5768. (29) Li, W.; Hu, S.; Luo, X.; Li, Z.; Sun, X.; Li, M.; Liu, F.; Yu, Y. Confined Amorphous Red Phosphorus in MOF-Derived N-Doped Microporous Carbon as a Superior Anode for Sodium-Ion Battery. Adv. Mater. 2017, 29, 1605820. (30) Lei, K.; Wang, C.; Liu, L.; Luo, Y.; Mu, C.; Li, F.; Chen, J. A Porous Network of Bismuth Used as the Anode Material for High-

element content via EDS mapping, carbon sulfur analyzer results, density and tap density, comparison of kinetic parameters (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wei Alex Wang: 0000-0001-9360-2745 Kai Xi: 0000-0003-0508-7910 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51274029) and State Key Laboratory of Advanced Metallurgy Foundation of China (No. 41602014). REFERENCES (1) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. (2) Augustyn, V.; Simon, P.; Dunn, B. Pseudocapacitive Oxide Materials for High-Rate Electrochemical Energy Storage. Energy Environ. Sci. 2014, 7, 1597−1614. (3) Lu, J.; Chen, Z.; Ma, Z.; Pan, F.; Curtiss, L. A.; Amine, K. The Role of Nanotechnology in the Development of Battery Materials for Electric Vehicles. Nat. Nanotechnol. 2016, 11, 1031−1038. (4) Lou, S.; Cheng, X.; Zhao, Y.; Lushington, A.; Gao, J.; Li, Q.; Zuo, P.; Wang, B.; Gao, Y.; Ma, Y.; Du, C.; Yin, G.; Sun, X. Superior Performance of Ordered Macroporous TiNb2O7 Anodes for Lithium Ion Batteries: Understanding from the Structural and Pseudocapacitive Insights on Achieving High Rate Capability. Nano Energy 2017, 34, 15−25. (5) Lou, S.; Cheng, X.; Gao, J.; Li, Q.; Wang, L.; Cao, Y.; Ma, Y.; Zuo, P.; Gao, Y.; Du, C.; Huo, H.; Yin, G. Pseudocapacitive Li+ Intercalation in Porous Ti2Nb10O29 Nanospheres Enables Ultra-Fast Lithium Storage. Energy Storage Mater. 2018, 11, 57−66. (6) Grey, C. P.; Tarascon, J. M. Sustainability and In Situ Monitoring in Battery Development. Nat. Mater. 2017, 16, 45−56. (7) Ge, J.; Fan, L.; Wang, J.; Zhang, Q.; Liu, Z.; Zhang, E.; Liu, Q.; Yu, X.; Lu, B. MoSe2/N-Doped Carbon as Anodes for Potassium-Ion Batteries. Adv. Energy Mater. 2018, 8, 1801477. (8) Xiao, Y.; Su, D.; Wang, X.; Wu, S.; Zhou, L.; Shi, Y.; Fang, S.; Cheng, H.-M.; Li, F. CuS Microspheres with Tunable Interlayer Space and Micropore as a High-Rate and Long-Life Anode for Sodium-Ion Batteries. Adv. Energy Mater. 2018, 8, 1800930. (9) Liu, Y.; Tai, Z.; Zhang, J.; Pang, W. K.; Zhang, Q.; Feng, H.; Konstantinov, K.; Guo, Z.; Liu, H. K. Boosting Potassium-Ion Batteries by Few-layered Composite Anodes Prepared via SolutionTriggered One-Step Shear Exfoliation. Nat. Commun. 2018, 9, 3645. (10) Yang, J.; Ju, Z.; Jiang, Y.; Xing, Z.; Xi, B.; Feng, J.; Xiong, S. Enhanced Capacity and Rate Capability of Nitrogen/Oxygen DualDoped Hard Carbon in Capacitive Potassium-Ion Storage. Adv. Mater. 2018, 30, 1700104. (11) Bin, D. S.; Lin, X. J.; Sun, Y. G.; Xu, Y. S.; Zhang, K.; Cao, A. M.; Wan, L. J. Engineering Hollow Carbon Architecture for HighPerformance K-Ion Battery Anode. J. Am. Chem. Soc. 2018, 140, 7127−7134. (12) Zhao, Y.; Zhu, J.; Ong, S. J. H.; Yao, Q.; Shi, X.; Hou, K.; Xu, Z. J.; Guan, L. High-Rate and Ultralong Cycle-Life Potassium Ion Batteries Enabled by In Situ Engineering of Yolk-Shell FeS2@C Structure on Graphene Matrix. Adv. Energy Mater. 2018, 8, 1802565. (13) Xu, J.; Zhang, W.; Fan, H.; Cheng, F.; Su, D.; Wang, G. Promoting Lithium Polysulfide/Sulfide Redox Kinetics by the 6915

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916

Article

ACS Nano Energy-Density Potassium-Ion Batteries. Angew. Chem., Int. Ed. 2018, 57, 4687−4691. (31) Ge, P.; Li, S.; Xu, L.; Zou, K.; Gao, X.; Cao, X.; Zou, G.; Hou, H.; Ji, X. Hierarchical Hollow-Microsphere Metal-Selenide@Carbon Composites with Rational Surface Engineering for Advanced Sodium Storage. Adv. Energy Mater. 2019, 9, 1803035. (32) Xu, M.; Xia, Q.; Yue, J.; Zhu, X.; Guo, Q.; Zhu, J.; Xia, H. Rambutan-Like Hybrid Hollow Spheres of Carbon Confined Co3O4 Nanoparticles as Advanced Anode Materials for Sodium-Ion Batteries. Adv. Funct. Mater. 2019, 29, 1807377. (33) Yan, C.; Gu, X.; Zhang, L.; Wang, Y.; Yan, L.; Liu, D.; Li, L.; Dai, P.; Zhao, X. Highly Dispersed Zn Nanoparticles Confined in a Nanoporous Carbon Network: Promising Anode Materials for Sodium and Potassium Ion Batteries. J. Mater. Chem. A 2018, 6, 17371−17377. (34) Yu, X.; Manthiram, A. Room-Temperature Sodium−Sulfur Batteries with Liquid-Phase Sodium Polysulfide Catholytes and Binder-Free Multiwall Carbon Nanotube Fabric Electrodes. J. Phys. Chem. C 2014, 118, 22952−22959. (35) Yu, X.-R.; Liu, F.; Wang, Z.-Y.; Chen, Y. Auger Parameters for Sulfur-Containing Compounds Using a Mixed Aluminum-Silver Excitation Source. J. Electron Spectrosc. Relat. Phenom. 1990, 50, 159−166. (36) Wang, J.; Polleux, J.; Lim, J.; Dunn, B. Pseudocapacitive Contributions to Electrochemical Energy Storage in TiO2 (Anatase) Nanoparticles. J. Phys. Chem. C 2007, 111, 14925−14931. (37) Liang, Q.; Zheng, Y.; Du, C.; Luo, Y.; Zhang, J.; Li, B.; Zong, Y.; Yan, Q. General and Scalable Solid-State Synthesis of 2D MPS3(M = Fe, Co, Ni) Nanosheets and Tuning Their Li/Na Storage Properties. Small Methods 2017, 1, 1700304. (38) Yu, D.; Pang, Q.; Gao, Y.; Wei, Y.; Wang, C.; Chen, G.; Du, F. Hierarchical Flower-Like VS2 Nanosheets-A High Rate-Capacity and Stable Anode Material for Sodium-Ion Battery. Energy Storage Mater. 2018, 11, 1−7. (39) Liu, Q.; Fan, L.; Ma, R.; Chen, S.; Yu, X.; Yang, H.; Xie, Y.; Han, X.; Lu, B. Super Long-Life Potassium-Ion Batteries Based on an Antimony@Carbon Composite Anode. Chem. Commun. 2018, 54, 11773−11776. (40) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (41) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (42) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192.

6916

DOI: 10.1021/acsnano.9b01773 ACS Nano 2019, 13, 6906−6916