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• W2972468927 abstract "•Multilayer WS2 nanosheets coupled nitrogen-doped carbon nanosheets are synthesized•Hierarchical nitrogen-doped carbon hollow nanospheres are synthesized•Both the assembled NIHC and KIHC achieve high energy density Nonaqueous hybrid capacitors composed of battery-type anodes and electric double-layer capacitor (EDLC)-type cathodes have triggered much research interest due to the advantages of both high power and energy densities. Recently, urgent concern has been focused on the Na/K-ion hybrid energy-storage systems owing to the abundance and low cost of Na and K compared with Li. Unfortunately, the development of Na/K-ion hybrid capacitors is greatly plagued by the aggravated kinetics and capacity mismatch between anode and cathode. Herein, we designed high-performance Na/K-ion hybrid capacitors by simultaneously enhancing the kinetics of multilayer tungsten sulfide nanosheets coupled with nitrogen-doped carbon nanosheets (anode) and the specific capacity of hierarchical nitrogen-doped carbon hollow nanospheres (cathode). Our design philosophy of electrode materials could provide guidelines for the development of functional materials for highly efficient hybrid energy-storage systems. The kinetics and capacity mismatches between anode and cathode for the Na/K-ion hybrid capacitors (NIHC and KIHC) greatly hamper their overall performance output. Herein, we construct NIHC and KIHC using multilayer tungsten sulfide nanosheets anchored with nitrogen-doped carbon nanosheets (WS2@NCNs) composite as anode and hierarchical nitrogen-doped carbon hollow nanospheres (NCHS) as cathode. The strongly coupled sheet-on-sheet nanostructure of the anode can facilitate electron transport and alleviate volume change. In particular, the WS2-NCNs interface offers a more feasible channel for fast ion intercalation and deintercalation, which can improve kinetics. The hierarchical NCHS cathode provides numerous accessible active sites for reversible anion adsorption and desorption, dramatically enhancing the specific capacity. Consequently, due to the enhanced anode and cathode compatibility, the assembled NIHC and KIHC can deliver high energy densities of 134.7 and 103.4 Wh kg−1 at 117.5 and 235 W kg−1, respectively. Our electrode design strategy could offer guidance for the development of high-performance hybrid capacitors. The kinetics and capacity mismatches between anode and cathode for the Na/K-ion hybrid capacitors (NIHC and KIHC) greatly hamper their overall performance output. Herein, we construct NIHC and KIHC using multilayer tungsten sulfide nanosheets anchored with nitrogen-doped carbon nanosheets (WS2@NCNs) composite as anode and hierarchical nitrogen-doped carbon hollow nanospheres (NCHS) as cathode. The strongly coupled sheet-on-sheet nanostructure of the anode can facilitate electron transport and alleviate volume change. In particular, the WS2-NCNs interface offers a more feasible channel for fast ion intercalation and deintercalation, which can improve kinetics. The hierarchical NCHS cathode provides numerous accessible active sites for reversible anion adsorption and desorption, dramatically enhancing the specific capacity. Consequently, due to the enhanced anode and cathode compatibility, the assembled NIHC and KIHC can deliver high energy densities of 134.7 and 103.4 Wh kg−1 at 117.5 and 235 W kg−1, respectively. Our electrode design strategy could offer guidance for the development of high-performance hybrid capacitors. With the increase in demand for clean and renewable energy resources, the development of advanced electrochemical energy-storage devices with low cost, high energy and power densities, and long-term stability is becoming a worldwide hot topic. At present, state-of-the-art lithium-ion batteries (LIBs) with high energy density dominate the energy-storage markets. However, their low power densities can scarcely meet the ever-increasing power demands for the next-generation electrochemical energy-storage systems.1Etacheri V. Marom R. Elazari R. Salitra G. Aurbach D. Challenges in the development of advanced Li-ion batteries: a review.Energy Environ. Sci. 2011; 4: 3243-3262Crossref Scopus (4337) Google Scholar On the contrary, the supercapacitor stores energy based on the charge accumulation or transfer limited at the electrolyte/electrode interface, delivering high power density at the sacrifice of energy density.2Sharma P. Bhatti T. A review on electrochemical double-layer capacitors.Energy Convers. Manag. 2010; 51: 2901-2912Crossref Scopus (677) Google Scholar Therefore, simultaneously integrating the electrodes of LIBs and supercapacitors into a single energy-storage device, which is termed the Li-ion hybrid capacitor (LIHC), could be a promising strategy for achieving both high energy and power densities.3Ding J. Hu W. Paek E. Mitlin D. Review of hybrid ion capacitors: from aqueous to lithium to sodium.Chem. Rev. 2018; 118: 6457-6498Crossref PubMed Scopus (385) Google Scholar, 4Wang H. Zhang Y. Ang H. Zhang Y. Tan H.T. Zhang Y. Guo Y. Franklin J.B. Wu X.L. Srinivasan M. A high-energy lithium-ion capacitor by integration of a 3D interconnected titanium carbide nanoparticle chain anode with a pyridine-derived porous nitrogen-doped carbon cathode.Adv. Funct. Mater. 2016; 26: 3082-3093Crossref Scopus (272) Google Scholar, 5Wang H. Zhu C. Chao D. Yan Q. Fan H.J. Nonaqueous hybrid lithium-ion and sodium-ion capacitors.Adv. Mater. 2017; 29: 1702093Crossref Scopus (448) Google Scholar The proposed LIHC has attracted extensive research attention as the technological innovation can bridge the gap between supercapacitors and batteries. Nevertheless, due to the rarity and high exploitation cost of Li, researchers have started to turn their attention to the hybrid systems based on the earth-abundant elements. Na and K ions were discovered to be suitable for the aforementioned hybrid energy-storage devices owing to the low cost and similar electrochemical behaviors compared with Li counterparts. However, the imbalance of kinetics and capacity between anode and cathode greatly hinder the performance output of the hybrid full cells.6Wang X. Kajiyama S. Iinuma H. Hosono E. Oro S. Moriguchi I. Okubo M. Yamada A. Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors.Nat. Commun. 2015; 6: 6544Crossref PubMed Scopus (634) Google Scholar, 7Ding J. Li Z. Cui K. Boyer S. Karpuzov D. Mitlin D. Heteroatom enhanced sodium ion capacity and rate capability in a hydrogel derived carbon give record performance in a hybrid ion capacitor.Nano Energy. 2016; 23: 129-137Crossref Scopus (124) Google Scholar In particular, the worse kinetic properties of Na and K ions that result from the larger diameter than that of Li ion exacerbate the electrode mismatch, which further deteriorates the overall performance of the NIHC and KIHC. Consequently, enhancing the anode and cathode compatibility by rationally designing anode materials with rapid pseudocapacitance-based Na/K-ion storage and high-specific-capacity cathode materials will contribute to performance improvement of the NIHC and KIHC.8Tie D. Huang S. Wang J. Ma J. Zhang J. Zhao Y. Hybrid energy storage devices: advanced electrode materials and matching principles.Energy Storage Mater. 2018; https://doi.org/10.1016/j.ensm.2018.12.018Crossref Scopus (72) Google Scholar To surmount the difficulties of Na/K-ion diffusion within the battery-type anode, some efforts have been dedicated to exploring appropriate materials with sizable channels for Na and K ions, such as carbonaceous materials,9Chen Y. Li X. Park K. Lu W. Wang C. Xue W. Yang F. Zhou J. Suo L. Lin T. Nitrogen-doped carbon for sodium-ion battery anode by self-etching and graphitization of bimetallic MOF-based composite.Chem. 2017; 3: 152-163Abstract Full Text Full Text PDF Scopus (149) Google Scholar, 10Fan L. Lin K. Wang J. Ma R. Lu B. A nonaqueous potassium-based battery-supercapacitor Hybrid Device.Adv. Mater. 2018; 30: 1800804Crossref Scopus (201) Google Scholar, 11Wang Y. Wang Z. Chen Y. Zhang H. Yousaf M. Wu H. Zou M. Cao A. Han R.P. Hyperporous sponge interconnected by hierarchical carbon nanotubes as a high-performance potassium-ion battery anode.Adv. Mater. 2018; 30: 1802074Crossref Scopus (182) Google Scholar transition metal oxides/sulfides/phosphides,12Deng J. Gong Q. Ye H. Feng K. Zhou J. Zha C. Wu J. Chen J. Zhong J. Li Y. Rational synthesis and assembly of Ni3S4 nanorods for enhanced electrochemical sodium-ion storage.ACS Nano. 2018; 12: 1829-1836Crossref PubMed Scopus (72) Google Scholar, 13Longoni G. Pena Cabrera R.L. Polizzi S. D’Arienzo M. Mari C.M. Cui Y. Ruffo R. Shape-controlled TiO2 nanocrystals for Na-ion battery electrodes: the role of different exposed crystal facets on the electrochemical properties.Nano Lett. 2017; 17: 992-1000Crossref PubMed Scopus (127) Google Scholar, 14Jin R. Li X. Sun Y. Shan H. Fan L. Li D. Sun X. Metal–organic frameworks-derived Co2[email protected]@rGO with dual protection layers for improved sodium storage.ACS Appl. Mater. Interfaces. 2018; 10: 14641-14648Crossref PubMed Scopus (67) Google Scholar, 15Dong C. Guo L. He Y. Chen C. Qian Y. Chen Y. Xu L. Sandwich-like Ni2P nanoarray/nitrogen-doped graphene nanoarchitecture as a high-performance anode for sodium and lithium ion batteries.Energy Storage Mater. 2018; 15: 234-241Crossref Scopus (104) Google Scholar, 16Ge 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: 1801477Crossref Scopus (226) Google Scholar, 17Bai J. Xi B. Mao H. Lin Y. Ma X. Feng J. Xiong S. One-step construction of N, P-codoped porous carbon sheets/CoP hybrids with enhanced lithium and potassium storage.Adv. Mater. 2018; 30: 1802310Crossref Scopus (249) Google Scholar, 18Gao H. Zhou T. Zheng Y. Zhang Q. Liu Y. Chen J. Liu H. Guo Z. CoS quantum dot nanoclusters for high-energy potassium-ion batteries.Adv. Funct. Mater. 2017; 27: 1702634Crossref Scopus (274) Google Scholar alloys, and organic molecules.19Qin J. Wang T. Liu D. Liu E. Zhao N. Shi C. He F. Ma L. He C. A top-down strategy toward SnSb in-plane nanoconfined 3D N-doped porous graphene composite microspheres for high performance Na-ion battery anode.Adv. Mater. 2018; 30: 1704670Crossref Scopus (133) Google Scholar, 20Xie H. Tan X. Luber E.J. Olsen B. Kalisvaart W.P. Jungjohann K.L. Mitlin D. Buriak J.M. β-SnSb for sodium ion battery anodes: phase transformations responsible for enhanced cycling stability revealed by in-situ TEM.ACS Energy Lett. 2018; 3: 1670-1676Crossref Scopus (55) Google Scholar, 21Thangavel R. Kaliyappan K. Kim D.-U. Sun X. Lee Y.-S. All-organic sodium hybrid capacitor: a new, high-energy, high-power energy storage system bridging batteries and capacitors.Chem. Mater. 2017; 29: 7122-7130Crossref Scopus (33) Google Scholar Among them, the two-dimensional (2D) metal dichalcogenides with layered structures, e.g., MoS2, VS2, VSe2, and WS2, have triggered increasing interest due to their tunable interlayer spacing and relatively high theoretical capacity.22Wang L. Zhang Q. Zhu J. Duan X. Xu Z. Liu Y. Yang H. Lu B. Nature of extra capacity in MoS2 electrodes: molybdenum atoms accommodate with lithium.Energy Storage Mater. 2019; 16: 37-45Crossref Scopus (145) Google Scholar, 23Yang C. Feng J. Lv F. Zhou J. Lin C. Wang K. Zhang Y. Yang Y. Wang W. Li J. Metallic graphene-like VSe2 ultrathin nanosheets: superior potassium-ion storage and their working mechanism.Adv. Mater. 2018; 30: e1800036Crossref PubMed Scopus (231) Google Scholar, 24Sun R. Wei Q. Sheng J. Shi C. An Q. Liu S. Mai L. Novel layer-by-layer stacked VS2 nanosheets with intercalation pseudocapacitance for high-rate sodium ion charge storage.Nano Energy. 2017; 35: 396-404Crossref Scopus (205) Google Scholar, 25Choi S.H. Kang Y.C. Sodium ion storage properties of WS2-decorated three-dimensional reduced graphene oxide microspheres.Nanoscale. 2015; 7: 3965-3970Crossref PubMed Google Scholar Unfortunately, the rapid performance decay induced by the large volume change resulting from the ion intercalation/deintercalation and conversion reaction prevents them being ideal anode materials in NIHC and KIHC.26Cui J. Yao S. Lu Z. Huang J.Q. Chong W.G. Ciucci F. Kim J.K. Revealing pseudocapacitive mechanisms of metal dichalcogenide SnS2/graphene-CNT aerogels for high-energy Na hybrid capacitors.Adv. Energy Mater. 2018; 8: 1702488Crossref Scopus (102) Google Scholar In addition, the kinetics of intercalation and deintercalation of Na/K ion in the 2D metal dichalcogenides need to be further enhanced to provide more pseudocapacitance-type charge storage. Nanoarchitecture is regarded as an effective way to reduce the diffusion pathway of Na/K ion and expose more active sites for surface redox reactions, thus boosting the rate performance. Moreover, their incorporation with conductive carbon materials as the substrates can accelerate the electron transfer, and the coupled rigid carbon matrix can effectively mitigate the volume change of 2D metal dichalcogenides during the charge/discharge process.27Wang Y. Kong D. Shi W. Liu B. Sim G.J. Ge Q. Yang H.Y. Ice templated free-standing hierarchically WS2/CNT-rGO aerogel for high-performance rechargeable lithium and sodium ion batteries.Adv. Energy Mater. 2016; 6: 1601057Crossref Scopus (211) Google Scholar For the cathode utilized in the hybrid full cell, improving the specific capacity is the top priority. However, in most research the commercial active carbon (C-AC) is taken as the cathode material in the hybrid systems, and less attention has been focused on the exploitation of advanced carbon materials with appropriate microstructures for enhancement of specific capacity. Although the C-AC possesses a high specific surface area, the intrinsically blocked pore texture greatly decreases its contact area with electrolyte, resulting in relatively low specific capacity.28Li Y. Wang G. Wei T. Fan Z. Yan P. Nitrogen and sulfur co-doped porous carbon nanosheets derived from willow catkin for supercapacitors.Nano Energy. 2016; 19: 165-175Crossref Scopus (803) Google Scholar Based on the equation 1/Cfull cell = 1/Canode + 1/Ccathode, the specific capacity of the hybrid full cell heavily depends on the cathode, and the low specific capacity of cathode will aggravate the electrode mismatch of the hybrid systems, leading to low energy density.29Li H. Lang J. Lei S. Chen J. Wang K. Liu L. Zhang T. Liu W. Yan X. A high-performance sodium-ion hybrid capacitor constructed by metal-organic framework-derived anode and cathode materials.Adv. Funct. Mater. 2018; : 1800757https://doi.org/10.1002/adfm.201800757Crossref Scopus (146) Google Scholar As a consequence, exploring advanced porous carbon materials with enhanced specific capacity is another key solution to further boost the performance output of hybrid full cells. In this work, we fabricated high-energy-density and high-power-density NIHC and KIHC assembled with the multilayer WS2@NCNs composite as anode and hierarchical NCHS as cathode. For the anode, the 2D WS2 nanosheets are strongly coupled with the 2D conductive NCNs. The self-assembled sheet-on-sheet architecture can enhance the electron transfer from the WS2 nanosheets to the NCN substrate and effectively alleviate the volume expansion and shrinkage of WS2 matrix during Na/K-ion intercalation/deintercalation and conversion reaction. Specifically, the interface formed between the WS2 nanosheets and NCNs contributes to fast ion migration, which can enhance the pseudocapacitive charge storage. Benefiting from the aforementioned structure advantages, the rate performance and stability of the WS2@NCNs composite anode are greatly improved. For the cathode, the hierarchical NCHS with high specific surface area is assembled with vertically aligned ultrathin carbon nanosheets. The unique hollow nanostructure can maximize the utilization of active materials. In addition, numerous in-plane nanopores are generated on the 2D ultrathin carbon nanosheets, which can provide abundant active sites for reversible anion adsorption/desorption. As a result, the specific capacity of the NCHS is effectively boosted compared with the C-AC. Owing to the optimized kinetics and capacity compatibility between the anode and cathode, the assembled NIHC and KIHC can obtain high energy densities of 134.7 and 103.4 Wh kg−1 at the power densities of 117.5 and 235 W kg−1, respectively. Even at a high power density of 4,700 W kg−1, the NIHC can still achieve a high energy density of 65.8 Wh kg−1. The preparation process of the multilayer WS2@NCNs composite is briefly illustrated in Figure 1A. The synthesis of the hierarchical and strongly coupled WS2@NCNs composite nanoarchitecture was first proposed through a facile metal-chelate-assisted method, followed by a sulfuration conversion process. Typically, a certain amount of sodium dodecyl sulfate (SDS) and sodium tungstate (Na2WO4) were successively added into deionized water under continuous agitation. After adding dopamine hydrochloride (DH), the multilayer polydopamine-tungstate (PD-W) organic/inorganic hybrid ensembles were generated through the chelation of tungstate ions (WO42−) with organic ligands of dopamine and the self-polymerization of dopamine with the help of SDS. Finally, the organic/inorganic PD-W chelates were facilely converted into multilayer WS2@NCNs composite through one-step sulfuration under 600°C for 2 h. As shown in scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Figures 1B and S1), the WS2@NCNs composite presents a symmetrical multilayer structure along a central axis. The open framework contributes to electrolyte permeation, maximizing the contact area between the WS2@NCNs composite and electrolyte. The zoomed-in TEM image on the edge reveals that the WS2@NCNs composite is composed of multiple ultrathin nanosheets (Figure S2A). A closer TEM image in Figure 1C clearly displays the few-layers WS2 nanosheet anchored on the ultrathin carbon nanosheets. In addition, the lateral view of single WS2@NCNs composite nanosheet further confirms that the WS2 nanosheet is tightly coupled with the carbon layer (Figure S2B). The sheet-on-sheet nanostructure can facilitate the electron transfer and alleviate the volume change of WS2 matrix during ion intercalation/deintercalation. In particular, the interface formed between the WS2 and NCNs nanosheets can act as the “highway” for ion transport, which effectively enhances the pseudocapacitive charge storage. High-resolution TEM (HRTEM) imaging shows the edge of the WS2 nanosheet (Figure 1D). The in-plane lattice fringes with a spacing of 0.27 nm, corresponding to the (100) plane of the WS2 (tungstenite-2H). The interlayer distance is measured to be 0.68 nm, which is assigned to the (002) crystal plane of WS2.30Su D. Dou S. Wang G. WS2@graphene nanocomposites as anode materials for Na-ion batteries with enhanced electrochemical performances.Chem. Commun. 2014; 50: 4192-4195Crossref PubMed Scopus (195) Google Scholar Energy-dispersive spectroscopy (EDS) measurement was used to characterize the elemental distribution. As displayed in Figures 1E–1H, the elements C, W, and S are uniformly distributed throughout the nanosheet without apparent agglomeration. The weak mapping intensity of N indicates the presence of a small amount of N, which is derived from the dopamine precursor. The content of WS2 in the WS2@NCNs composite was determined to be 63.9 wt % using inductively coupled plasma mass spectrometry (ICP-MS). Moreover, the nitrogen gas (N2) adsorption-desorption isotherm and pore-size distribution curve were tested to investigate the specific surface area and pore texture. The specific surface area of the WS2@NCNs composite is calculated to be 245.7 m2 g−1. The WS2@NCNs composite displays a typical type IV adsorption/desorption isotherm characteristic of a mesoporous structure (Figure S3A).31Yang Y. Luo M. Xing Y. Wang S. Zhang W. Lv F. Li Y. Zhang Y. Wang W. Guo S. A universal strategy for intimately coupled carbon nanosheets/MoM nanocrystals (M=P, S, C, and O) hierarchical hollow nanospheres for hydrogen evolution catalysis and sodium-ion storage.Adv. Mater. 2018; 30: 1706085Crossref PubMed Scopus (116) Google Scholar The pore-size distribution of the WS2@NCNs composite based on the adsorption branch of the N2 adsorption-desorption isotherm is calculated according to the density functional theory (DFT) model, and the pore size is mainly concentrated below 4 nm (Figure S3B). The open and mesoporous structure of the multilayer WS2@NCNs composite nanosheets is beneficial for electrolyte transport. The crystal structure of the WS2@NCNs composite was investigated by X-ray diffraction (XRD) (Figure 1I). The diffraction peaks located at 14.1°, 28.8°, 32.8°, 39.5°, 43.9°, 49.7°, 58.4°, 60.5°, and 68.7° correspond to the (002), (004), (100), (103), (006), (105), (110), (112), and (200) planes of WS2, respectively (PDF #08-0237). No other impurity peaks indicate the complete conversion of WS2 during the sulfuration treatment. The surface chemical states of the WS2@NCNs composite were further exploited using X-ray photoelectron spectroscopy (XPS) measurement. Figure S4A shows the wide-scan XPS spectrum of the WS2@NCNs composite within the range of 0–1,000 eV, which contains C 1s, N 1s, O 1s, S 2p, and W-relative peaks. The high-resolution W 4f XPS spectrum for the multilayer WS2@NCNs composite is displayed in Figure 1J. The peaks at 32.6, 34.7, 35.2, and 38.1 eV are assigned to W 4f7/2, W 4f5/2, W 5p5/2, and W 5p3/2, respectively, indicating that the W element in the composite is W4+.32Wang X. Huang J. Li J. Cao L. Hao W. Xu Z. Improved Na storage performance with the involvement of nitrogen-doped conductive carbon into WS2 nanosheets.ACS Appl. Mater. Interfaces. 2016; 8: 23899-23908Crossref PubMed Scopus (51) Google Scholar Two peaks of S 2p3/2 and S 2p1/2 located at 162.3 and 163.8 eV are characteristic of S2− in WS2 (Figure S4B).33Von Lim Y. Wang Y. Kong D. Guo L. Wong J.I. Ang L. Yang H.Y. Cubic-shaped WS2 nanopetals on a Prussian blue derived nitrogen-doped carbon nanoporous framework for high performance sodium-ion batteries.J. Mater. Chem. A. 2017; 5: 10406-10415Crossref Google Scholar The deconvoluted C 1s XPS spectrum of the WS2@NCNs composite exhibits a dominating peak at 284.4 eV, which is attributed to the sp2-C, and the other five peaks at 285.1, 285.7, 285.9, 286.8, and 288.9 eV correspond to the sp3-C, C-OH, C-N, C-O-C, and O-C=O species, respectively (Figure S4C).34Li Y. Cao D. Wang Y. Yang S. Zhang D. Ye K. Cheng K. Yin J. Wang G. Xu Y. Hydrothermal deposition of manganese dioxide nanosheets on electrodeposited graphene covered nickel foam as a high-performance electrode for supercapacitors.J. Power Sources. 2015; 279: 138-145Crossref Scopus (58) Google Scholar Figure S4D displays the high-resolution N 1s XPS spectrum of the WS2@NCNs composite. The N content is about 2.3 wt % and the N 1s peak can be deconvoluted into four peaks at 398.2, 399.8, 401.2, and 402.4 eV, which are assigned to the pyridinic N (N-6), pyrrolic N (N-5), quaternary N (N-Q), and oxidized N (N-X), respectively.35Gu J. Du Z. Zhang C. Yang S. Pyridinic nitrogen-enriched carbon nanogears with thin teeth for superior lithium storage.Adv. Energy Mater. 2016; 6: 1600917Crossref Scopus (91) Google Scholar The Na/K-ion storage properties of the WS2@NCNs composite were investigated by means of cyclic voltammetry (CV) and galvanostatic charge/discharge measurements. Figure 2A displays the CV curves of the WS2@NCNs composite anode at a scan rate of 0.1 mV s−1 within the potential range of 0.01–3.0 V versus Na/Na+. In the first cathodic scan, there are three reduction peaks located at 1.05, 0.6, and 0.24 V, which correspond to the insertion of Na ions into WS2 interlayers to form NaxWS2, conversion reaction of WS2 with Na ions to generate metallic W and Na2S matrix, and formation of solid electrolyte interface (SEI) layers at the surface of electrode, respectively.36Zhu C. Kopold P. Li W. van Aken P.A. Maier J. Yu Y. Engineering nanostructured electrode materials for high performance sodium ion batteries: a case study of a 3D porous interconnected WS2/C nanocomposite.J. Mater. Chem. A. 2015; 3: 20487-20493Crossref Google Scholar In the first anodic scan, the oxidation peak at about 1.80 V is ascribed to the Na-ion extraction and oxidation of W to WS2. In the following cycles the redox peaks almost overlap, indicating the excellent stability of the formed SEI layer. Note that the reduction peaks in the first cycle are replaced by two new broad peaks at 1.7 and 1.1 V, which are attributed to the multistep reactions of the formation of NaxWS2 and Na2S, respectively.37Li X. Zhang J. Liu Z. Fu C. Niu C. WS2 nanoflowers on carbon nanotube vines with enhanced electrochemical performances for lithium and sodium-ion batteries.J. Alloys Compd. 2018; 766: 656-662Crossref Scopus (18) Google Scholar Kinetic analysis based on CV measurements was conducted to gain deep insight into the electrochemical behavior of Na ion in the WS2@NCNs composite anode. CV curves at different scan rates from 0.1 to 50 mV s−1 in a potential range of 0.01–3.0 V are shown in Figure S5A according to the relationship between the current density (i) and the scan rate (v):38Kurra N. Alhabeb M. Maleski K. Wang C.-H. Alshareef H.N. Gogotsi Y. Bistacked titanium carbide (MXene) anodes for hybrid sodium-ion capacitors.ACS Energy Lett. 2018; 3: 2094-2100Crossref Scopus (82) Google Scholari = avb.(Equation 1) The b value can be obtained by the slope of the log(v)-log(i) curve. A b value of 1 suggests that the electrode undergoes a capacitive process, whereas a b value of 0.5 indicates ion diffusion-controlled behavior. The log(v)-log(i) curve for the WS2@NCNs composite anode is displayed in Figure S5B. The b value for the cathodic current can be quantified as about 0.92 at the scan rates from 0.1 to 10 mV s−1, which shows typical capacitive behavior.39Augustyn V. Come J. Lowe M.A. Kim J.W. Taberna P.-L. Tolbert S.H. Abruña H.D. Simon P. Dunn B. High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance.Nat. Mater. 2013; 12: 518Crossref PubMed Scopus (2506) Google Scholar The relatively high b value at low scan rates manifests a facile Na-ion intercalation into the WS2@NCNs composite interlayers. A decreased b value of 0.58 at scan rates above 10 mV s−1 reflects the transformation of electrochemical behavior from the surface-controlled kinetics to diffusion-controlled ones.40Hu Z. Sayed S. Jiang T. Zhu X. Lu C. Wang G. Sun J. Rashid A. Yan C. Zhang L. Self-assembled binary organic granules with multiple lithium uptake mechanisms toward high-energy flexible lithium-ion hybrid supercapacitors.Adv. Energy Mater. 2018; 8: 1802273Crossref Scopus (41) Google Scholar For further investigation of the capacity contributions from the capacitive effect and diffusion-controlled process, Equation 1 can be rewritten as follows41Wang X. Li Q. Zhang L. Hu Z. Yu L. Jiang T. Lu C. Yan C. Sun J. Liu Z. Caging Nb2O5 nanowires in PECVD-derived graphene capsules toward bendable sodium-ion hybrid supercapacitors.Adv. Mater. 2018; 30: e1800963Crossref PubMed Scopus (123) Google Scholar:i = k1v + k2v1/2,(Equation 2) Where i is the current density, v represents the scan rate. k1 and k2 are constants under a specific potential derived from different scan rates and can be determined from the slope and y axis intercept point, respectively. The current density can be divided into capacitive (k1v) and diffusion-controlled parts (k2v1/2). According to Equation 2, it can be calculated that the capacitive contribution is about 41.3% of the total capacity of the WS2@NCNs composite anode at a scan rate of 0.1 mV s−1 (Figure 2B). In particular, the ratio of capacitive contribution can reach 88.6% when the scan rate increases to 5 mV s−1 (Figure 2C). The high capacitive contribution of the WS2@NCNs composite anode could be attributed to the strongly coupled sheet-on-sheet hybrid nanostructure that can offer a more feasible channel for Na-ion intercalation/deintercalation at the interface. To gain further insight into the kinetics behavior of Na ion in the WS2@NCNs composite, we analyzed the diffusion barriers of Na ion in various models (surface, interlayer, and interface) using DFT calculations (Figure 2D). The model of surface structure can be regarded as the expanded interlayer in the 2D WS2 matrix. The migration trajectory of Na ion is set from the hollow octahedral (Oh) to the top tetrahedral (Td) and then to the Oh sites. The relative energies of ion diffusion along the three pathways are shown in Figure 2E. The energy barrier of Na-ion diffusion based on the climbing-image nudged elastic band method (NEB) calculations in the interlayer of WS2 matrix is about 1.3 eV, much higher than that on the surface of 2D WS2 layers (0.03 eV). This result indicates that Na ion diffuses faster on the layers than in the interlayers of WS2 nanosheets. It is noteworthy that the activation energy barrier of Na-ion diffusion in the interface between the NCNs and WS2 nanosheets (0.35 eV) is greatly decreased compared with that in the interlayers of WS2 matrix. The decreased activation energy barrier demonstrates that interfacial coupling can enhance Na-ion transport kinetics, which contributes to providing more pseudocapacitive energy storage.42Chen C. Wen Y. Hu X. Ji X. Yan M. Mai L. Hu P. Shan B. Huang Y. Na+ intercalation pseudocapacitance in graphene-coupled titanium oxide enabling ultra-fast sodium storage and long-term cycling.Nat. Commun. 2015; 6: 6929Crossref PubMed Scopus (803) Google Scholar The galvanostatic charge/discharge curves of the WS2@NCNs composite anode under different current densities are shown in Figure S6, and the rate per" @default.
• W2972468927 created "2019-09-19" @default.
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• W2972468927 date "2019-10-01" @default.
• W2972468927 modified "2023-10-16" @default.
• W2972468927 title "Enhanced Cathode and Anode Compatibility for Boosting Both Energy and Power Densities of Na/K-Ion Hybrid Capacitors" @default.
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