Controllable Preparation of Square Nickel Chalcogenide (NiS and

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Controllable Preparation of Square Nickel Chalcogenide (NiS and NiSe2) Nanoplates for Superior Li/Na ion Storage Properties Haosen Fan, Hong Yu, Xing-Long Wu, Yu Zhang, Zhong-Zhen Luo, Huanwen Wang, Yuanyuan Guo, Srinivasan Madhavi, and Qingyu Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07300 • Publication Date (Web): 25 Aug 2016 Downloaded from http://pubs.acs.org on August 28, 2016

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ACS Applied Materials & Interfaces

Controllable Preparation of Square Nickel Chalcogenide (NiS and NiSe2) Nanoplates for Superior Li/Na ion Storage Properties Haosen Fan,a, b Hong Yu,a Xinglong Wu,a Yu Zhang,a Zhongzhen Luo,a Huanwen Wang,a, b Yuanyuan Guo,a Srinivasan Madhavi a, b, * and Qingyu Yan a, b, * a

School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore Energy Research Institute@NTU, Nanyang Technological University, Research Techno Plaza, Singapore 637553, Singapore b

Supporting Information Placeholder ABSTRACT: A facile and bottom-up approach has been presented to prepare 2D Ni-MOFs based on cyanide-bridged hybrid coordination polymers. After thermally induced sulfurization and selenization processes, Ni-MOFs were successfully converted into NiS and NiSe2 nanoplates with carbon coating due to the decomposition of its organic parts. When evaluated as anodes of Li ion batteries (LIBs) and Na ion batteries (SIBs), NiS and NiSe2 nanoplates show high specific capacities, excellent rate capabilities and stable cycling stability. The NiS plates show good Li storage properties while NiSe2 plates show good Na storage properties as anode materials. The study of the diffusivity of Li+ in NiS and Na+ in NiSe2 shows consistent results with their Li/Na storage properties. These 2D MOFs derived NiS and NiSe2 nanoplates reported in this work explore a new approach for the large-scale synthesis of 2D metal sulfides or selenides with potential applications for advanced energy storage. KEYWORDS: 2D MOF, nickel chalcogenides, square nanoplate, Li ion batteries, Na ion batteries 1. Introduction In order to solving the shortage of fossil fuel and the increasing demand for energy resources, researchers have been devoting themselves to explore all kinds of new sustainable energy sources as well as energy storage devices.1-4 Among various electrochemical energy conversion technologies, lithium ions batteries were considered as most practical ones due to their high energy densities and high stability. Recently, Na-ion batteries (NIBs) have attracted great attention because of the abundance of sodium and low cost in comparison to lithium ion batteries.5-8 However, NIBs still have many technical issues as compared to LIBs, such as larger volume change of the electrode materials, lower specific capacities, poorer rate capability, as well as shorter cycling life due to larger ionic radius and molar mass of Na+ ions than that of Li+ ions.9-12 The large radius of Na+ hinders its movement into the host materials, resulting in the lack of suitable anode materials.13, 14 Traditional graphite anode materials for SIBs with a highly ordered structure have been demonstrated not suitable to accommodate Na+ because of the difficulty in formation of intercalation compounds.15,16 To date, many efforts have been attempted to explore various type anode materials with different mechanisms, such as materials by the intercalation mechanism, alloy-

type or conversion-type mechanism.17-24 But many of them are impeded by lower capacity or poor cycling life due to their large volume and structural changes or the sluggish charge transfer kinetics.25 Fortunately, the structural or morphological control of the electrode materials was demonstrated to be an effective way to improve Na storage properties.26, 27 Especially, two-dimensional (2D) structures exhibit great potentials to overcome these drawbacks because their high active surface area and short ion transport/diffusion paths.28-30 Recently, metal sulfides and selenides, have been attracting significant attention as electrode materials for NIBs because of their high conductivity and theoretical capacity.31-33 However, although lots of efforts have been demonstrated on synthesis of nanostructured metal sulfides or selenides,34, 35 the preparation of 2D structured metal sulfides and selenides are very challenge for those with nano-layered crystal structures (such as NiS or NiSe2). Many promising progress on synthesis of 2D structured metal sulfides and selenides are mainly demonstrated for preparing of layered crystal structures, such as MoS2, WS2, and WSe2.36-38 A general and simple strategy for preparation of 2D structured metal sulfides and selenides highly desirable. Recently, metal-organic frameworks (MOFs), also called coordination polymers, have been used as effective precursors to prepare metal oxides/sulfides with welldefined morphology (e.g. hollow spheres), which possess the merits of high specific surface areas and uniform micro-porous structures. Using the MOFs template approach to achieve 2D structured metal sulfides or selenides should be attractive, which has yet been demonstrated. Herein, we demonstrate a facile MOF-template approach to prepare 2D structured NiS or NiSe2. Firstly, a new 2D structured Nibased MOFs was synthesized with thickness of tens of nanometers through an aqueous solution cyanide-bridged coordination approach. Then, NiS and NiSe2 nanoplates were obtained by using the as-prepared 2D MOFs as templates through the sulfurization and selenization process. Based on this approach, we succeeded in preparation of 2D structured NiS and NiSe2 nanoplates despite their non-layer crystal structures. The lateral size of the asobtained NiS and NiSe2 nanoplates is in the range of 1-2 µm with the thickness of a few tens nanometers. There are carbon coating on these NiS and NiSe2 nanoplates due to the carbonization of cyano group. When used as anodes of LIBs and SIBs, NiS and

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NiSe2 nanoplates show high specific capacities, excellent rate capabilities and stable cycling stability. As anodes of LIBs, NiS nanoplates exhibit a high capacity of 972 mA h g-1 at a current density of 0.1 A g-1 and a reversible capacity of 468 mA h g-1 during the 100th cycle at 1.0 A g-1. This NiS nanoplates also exhibits good rate capability with a specific capacity of 213 mAh g-1 at 10 A g-1. As anodes of NIBs, NiSe2 nanoplates deliver a stable discharge capacity of 318 mAh g-1 after 100 cycles at 1A g-1 and excellent rate capability.

2. Experimental Section 2.1. Synthesis of 2D Ni-based and Cyanide-bridged MOFs. 2D Ni-MOFs precursor was prepared according to our previously reported coprecipitation method.39 Firstly, 0.024 g NiCl2·6H2O (0.1 mmol) and 0.45 g sodium dodecyl sulfate (SDS) were dissolved into 20 mL ultrapure water. After stirring for 10 min, a clear solution A was obtained. Meanwhile, 0.024 g K2[Ni(CN)4] • xH2O (0.1 mmol) was added into another 20 mL ultrapure water to obtain B solution after complete dissolution. Secondly, the B solution was dropwise added into A solution with continuous stirring for 30 min. As last, the mixed solution was kept stationary state for 24 hours at ambient temperature. The obtained light blue product was collected and completely washed with ethanol and pure water, following in being dried under vacuum oven at 60 °C for further use. 2.2. Conversion from Ni-MOFs into NiS and NiSe2 nanoplates. In a typical conversion procedure for NiS, 50 mg Ni-MOFs and 50 mg sulfur powders were thoroughly ground together. Then the mixture was place into a corundum boat and calcinated at 400 °C for 3 h in argon atmosphere (heating rate: 2 °C·min-1). For the preparation of NiSe2, 100 mg selenium powders were used instead of sulfur powders following the same procedure. 2.3. Characterization and electrochemical measurement. The morphological features of all the as-prepared samples were characterized by field emission scanning electron microscope (FESEM, JEOL 7600F) and high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2100F). X-ray diffractometry (XRD) was investigated on a Micscience M-18XHF with Cu kα radiation. The coin-type cells were assembled in an argon-filled glove-box. The electrodes were fabricated by mixing the electrode materials, acetylene black and poly(vinyldifluoride) (PVDF) at a weight ratio of weight ratio of 70:20:10 in N-methyl-2pyrrolidone (NMP) solvent. The slurry was then pasted on copper foils followed by vacuum-drying at 70 °C overnight to obtain the working electrodes. Lithium foil for LIBs (or sodium foil for SIBs) serving as both counter and reference electrodes. For LIBs, the electrolyte was 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, w/w). For SIBs, 1 M NaClO4 dissolved in propylene carbonate (PC) with 5% fluoroethylene carbonate (FEC) was used as electrolyte. The cells were assembled in an argonfilled glove box with both moisture and oxygen contents below 1.0 ppm. The electrochemical properties of the obtained working electrodes were characterized by two-electrode CR2032 (3 V) coin-type cells. Galvanostatic charge/discharge tests were carried out from a NEWARE battery tester at a voltage window of 0.0053.0 V. Cyclic voltammetry (CV) was studied from an electrochemical workstation (Solarton). For the calculation of battery capacity, only the mass of electrode materials was included. 3. Results and discussions

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The formation mechanism of Ni-based MOFs and its derived NiS and NiSe2 nanoplates from Ni-based MOFs precursor is schematically depicted in Figure 1. In the initial stage, Ni(CN)42- combines with Ni2+ to form Ni[Ni(CN)4] complexes. Then those complexes further combine the Ni(CN)42- and Ni2+ in turn to form a single layer MOF. In the assembly stage, these primary single layer MOFs gradually assemble into square nanoplates. Then, this Nibased and cyanide-bridged 2D MOFs were transformed into carbon-coated NiS and NiSe2 nanoplates through the simultaneous sulfurization and selenization process. During the annealing processes, nickel ions could be liberated from the MOFs and then reacted with sulfur or selenium powders to form plate-like nickel sulfides or selenides. Meanwhile, N-doped carbon coatings were formed on the surface of NiS or NiSe2 due to the decomposition of cyano group.

Figure 1. Schematic illustration of the preparation of Ni-based MOF and the derived NiS and NiSe2 nanoplates. As shown in Figure 2a and b, the as-synthesized Ni-based MOFs are square nanoplates with the side length of 1.5 µm and the thickness of tens of nanometers. TEM image (Figure 2c) further demonstrates its square shape with smooth surface. The crystal structure of the obtained Ni-MOFs was confirmed by XRD patterns in Figure 2d. All of the diffraction peaks could be indexed as the Hofmann-type coordination polymer with an orthorhombic system and a 2D layered structure.40, 41

Figure 2. SEM image (a, b), TEM image (c) and XRD pattern (d) of Ni-based MOFs precursor. Figure 3a exhibit the uniform NiS nanoplates are obtained by sulfurization process, which maintain the original square shape with a lateral size of about 1 µm and the thickness of about 100 nm, thicker than the original Ni-MOFs nanoplates owing to the


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formation of NiS with carbon-coating on the surface of nanoplates (Figure 3b). XRD pattern (Supporting Information, Figure S1) of NiS shows all of the diffraction peaks are in good agreement with the standard patterns of NiS (α-NiS, P63/mmc, JCPDS card no. 02-1280) with the peaks at 30.2°, 34.2°, 46.1°, 53.7° and 73.3°, which are assigned to the (100), (101), (102), (110) and (202) planes of the hexagonal phase of NiS. The sharp peaks indicate the good crystallinity of the as-synthesized NiS without detectable peaks from any impurity phase. TEM image of a single nanoplate further demonstrates that it is composed of many NiS nanocystals (Figure 3c). Figure 3d describes an enlarged TEM image of the edge of a NiS nanoplate, which demonstrates the NiS nanocrystals (red ring) coated by carbon layers (black arrows). The carbon content could be determined by TG analysis. As shown in Figure S2, assuming the complete conversion of NiS into NiO during thermal decomposition in air, the weight percentage of carbon in the nanoplates was calculated to be about 38%. NiS nanocrystal shows the clear lattice fringes with the lattice spacing of 0.29, 0.26 and 0.19 nm in HRTEM image (Figure 3e), which corresponds to the distance between (100), (101) and (102) planes, respectively. In addition, Elemental mapping reveals the uniform distribution of the Ni, S, C and N elements within the nanoplates (Figure 3f).

of NiSe2 nanoplates, revealing a homogeneous distribution of Ni, Se, C and N in the nanoplate (Figure 4f).

Figure 4. (a) SEM image, (b) XRD pattern and (c) TEM image of the as-obtained NiSe2 nanoplates. (d) TEM images of the NiSe2 nanocrystal coated with carbon layer. (e) HRTEM image of lattice structure of NiSe2 nanocrystal. (f) HAADF-STEM and elemental mapping of a single NiSe2 nanoplate. (The scale bar for a is 100 nm ).

Figure 3. (a-e) SEM and TEM images of as-prepared NiS nanoplates. (f) STEM and Elemental mapping of NiS nanoplates. (The scale bars for a and b are 100 nm ). Figure 4a shows the SEM image of the as-synthesized NiSe2 nanoplates, which retain the plate-shape of original Ni-MOFs precursor. X-ray diffraction pattern of NiSe2 (Figure 4b) is consistent with a cubic structure standard pattern (JCPDS no. 651843) with peaks indexed to the (200), (210), (211), (220), (311), and (321) planes, respectively. The high-resolution SEM images (Supporting Information, Figure S3) and TEM image (Figure 4c) show the nanoplates are composed of many NiSe2 nanocrystals with the size from 10nm to 100nm. TEM images (Figure 4d) further indicate that the NiSe2 is covered with a carbon layer of several nanometers thick. Assuming the complete conversion of NiSe2 into NiO during TG in air (Figure S4), the carbon content was calculated to be about 6%. Figure 4e shows the HRTEM image of the NiSe2 crystal structure having clear lattice fringes corresponding to the (200), (210) and (211) plane with an interspacing of 0.29, 0.26 and 0.24 nm, respectively. High-angle annular dark-field scanning TEM (HAADF-STEM) and selected area EDX mapping are performed to further confirm the composition

In order to evaluate the lithium storage ability of NiS and NiSe2, coin-type half cells with Li metal as counter-electrode were assembled and the electrochemical measurements were performed at room temperature in the voltage window of 0.005-3.0 V. The cyclic voltammetric (CV) curves of the cell with NiS and NiSe2 anodes tested at a scan rate of 0.1 mV s-1 are shown in Figure 5a and Figure S5 (Supporting information). The second and the third CV curves of both two electrodes almost overlap with each other, indicating good stability and reversibility of the redox reactions of the two electrodes. For NiS battery, during the first cathodic scan, there are several cathodic peaks from 1.0 to 2.0 V. Diverse peaks on the CV curves are attributed to different states of reactions. These intermediate multiple redox stages can be shown with the following reactions (Equations 1-3):42, 43 3NiS + 2Li ↔ 4Ni3S2 + Li2S (1) Ni3S2 + 4Li ↔ 3Ni + 2Li2S (2) 2Li2S ↔ 4Li + 2S (3) The first galvanostatic charge-discharge profiles of the NiS and NiSe2 LIBs were obtained between 0.005 and 3.0 V at a current density of 0.1 A g-1. Figure S6 describe the Nyquist plots of NiS and NiSe2 lithium ion batteries after 100 cycles 1 A g-1. The two electrodes both show a semicircle in the high-frequency region and a straight line in the low-frequency region, which are attributed to the charge-transfer resistance of the electrode (Rct) and the internal resistance (Rs), respectively. From Figure 5b, the NiS electrode shows the initial discharge and charge capacities of as high as 1311 and 972 mAh g-1, respectively, which are much higher than that of NiSe2 electrode (e.g. 992 mAh g-1 for discharge and 758 mAh g-1 for charge). This low initial Coulombic efficiency is possibly related to the formation of solid electrolyte interphase layer accompanying the electrolyte decomposition. As shown in Figure 5c, when increasing the discharge rates, NiS exhibits decent capacity retentions: 710 mAh g-1 (0.2 A g-1), 611 mAh g-1 (0.5 A g-1), 538 mAh g-1 (1 A g-1), and 352 mAh g-1 (10 A g-1). When back to 0.1 A g-1, NiS electrode still can keep a high



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capacity of 754 A g-1. For NiSe2, it exhibit a capacity of 597 mA h g-1 at a current density of 0.2 A g-1 and the capacity of 214 mA h g-1 at 10 A g-1.The cycling stability of the NiS and NiSe2 electrode at a constant current rate at 1 A g-1 up to 100 cycles is depicted in Figure 5d. NiS electrode exhibits good cycling performance with the discharge capacity of 468 mAh g-1 even after 100 cycles of charge/discharge. The NiSe2 electrode shows a discharge capacity of 286 mAh g-1 after 100 cycles. Figure 6. (a) CV curves of NiS electrode at various scan rates from 0.1 mV s-1 to 2.0 mV s-1; (b) The linear relationship of the cathodic peak current (ip) and the anodic peak current (ip) between the square root of scan rate (ν1/2) for NiS LIB. discussion mentioned above, the peak current variation of CV curves for the two batteries is depended on the scan rates and Li+ diffusion rates. According to the fitting results (Figure 6b), the slopes of the fitting lines are AD1/2, which can be used to illustrate the diffusion coefficient of Li+ in the cells. It can be found that NiS shows the slope of 2.21 for positive peaks and 2.18 for negative peaks, respectively, which indicate a high apparent diffusion coefficient of Li+ into NiS electrode.

Figure 5. (a) CV curves of LIB with NiS electrode at a scan rate of 0.1 mV s-1 within a potential range of 0.001 to 3.0 V; (b) The 1st charge-discharge profiles of the LIBs of NiS and NiSe2 electrodes at a current density of 0.1 A g-1; (c) Rate capability of the NiS and NiSe2 LIBs; (d) Cycling performances of the LIBs of NiS and NiSe2 electrode at 1 A g-1. To further understand the good Li storage properties of NiS, Figure 6a shows the CV curves of the NiS anode at different scan rates ranging from 0.1 to 2 mV s-1. The intensity of the cathodic and anodic peaks increases with increasing scan rates, which demonstrates a good fast CV response. Theoretically, the peak area divided by the scan rate yields the capacity of the electrode, which is considered to be constant. The dependence of the cathodic and anodic peak currents on the square root of the scan rate (ν1/2) is presented in Figure 6b. It can be clearly seen that the both cathodic and anodic peak show a linear relationship with the square root of scan rate. Herein, the classical Randles-Sevchik equation (Equation 4) for a semi-infinite diffusion of Li+ into NiS anode can be used to further explain this phenomenon.44, 45

i p = (2.69 × 105 ) n3/ 2 SD1/2Cν 1/ 2

(4)

i

in which the p is the peak current (A), n is the charge-transfer number, S is the electrode area, D is the diffusion coefficient of Li+ (cm2 s-1), C is the concentration of lithium ions, and ν is the potential scan rate (V s-1). Since all the electrodes were prepared and tested from the same procedure, the Randles-Sevchik equation can be simplified as:

i p = AD1/2ν 1/2

(5)

where A is regarded as a constant for the LIBs, and AD1/2 is defined as the apparent diffusion coefficient of Li+ in the cells, which can be calculated by fitting the linear curves. Based on the

Figure 7. (a) CV curves of a fresh SIB with NiSe2 electrode at a scan rate of 0.1 mV s-1 within a potential range of 0.001 to 3.0 V (vs. Na/Na+); (b) The 1st charge-discharge profiles of the SIBs of NiS and NiSe2 electrodes at a current density of 0.1 A g-1; (c) Rate capabilities of the SIBs of NiS and NiSe2 electrodes; (d) Cycling performances of the SIBs of NiS and NiSe2 electrodes at 1 A g-1. To evaluate the sodium storage ability, coin-type half cells were assembled using NiS/NiSe2 nanoplates and the electrochemical measurements were performed at room temperature. The cyclic voltammetric (CV) curves of the cell with NiSe2 anode tested at a scan rate of 0.1 mV s-1 are shown in Figure 7a. During the first scan, there are two cathodic peaks at around 1.75 and 1.93 V and several anodic peaks. According to the previous reports about sulfides and selenides,46, 32 the charge peaks may be illustrated the formation of intermediate product NaxNiSe2 and fully charged product NiSe2, respectively (Equation 6-7). The discharge peaks can indicate the formation of NaxNiSe2, NiSe and Na2Se, and Ni and Na2Se, respectively (Equation 8-10). The charge and discharge processes may be possibly as follows: Charge process: Ni + 2Na2Se → NaxNiSe2 + (4-x)Na+ +(4-x)e(6) + NaxNiSe2 → NiSe2 + xNa + xe (7) Discharge process: NiSe2 + xNa+ + xe- → NaxNiSe2 (8)


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NaxNiSe2 + (2-x)Na+ + (2-x)e- → NiSe + Na2Se (9) (10) NiSe + 2Na+ + 2e- → Ni + Na2Se The CV curves for the following four cycles overlap well with each other, indicating the excellent reversible sodium storage of NiSe2 after the initial activation cycle, as well as the NiS SIB (Figure S7, Supporting information). Figure 7b shows the first galvanostatic charge-discharge profiles of the SIBs of NiS and NiSe2 electrodes between 0.005 and 3.0 V at a current density of 0.1 A g-1. The discharge plateau of NiSe2 electrode located at around 1.95 V that corresponds to the intercalation of Na+ into NiSe2 is clearly observed, and this is well consistent with the CV results (Figure 7a). The first discharge capacity is 1008 and 763 mAh g-1 for NiSe2 and NiS, accompanied by the first charge capacitis of 517 and 381 mAh g-1, respectively. Owing to SEI formation and electrolyte decomposition also contribute to the first discharge capacity for SIBs, which heavily influence the coulombic efficiency in the first charge/discharge process.47 The different charge/discharge capacity of NiS and NiSe2 electrodes can be well explained by the difference in their chemical composition and charge carrier transfer kinetics. In comparison to NiS, the relatively fast sodium diffusion kinetics is response for the higher capacity of NiSe2. Besides, the larger atomic radius of Se may also contribute to its faster sodium diffusion and larger capacity of NiSe2. Figure 7c presents the rate performance of the two SIBs. It is clearly shows that the SIB of NiSe2 electrode exhibits good rate capabilities. The 2nd-cycle discharge capacities of the NiSe2 electrode are 390, 345, 309, 281, 249, and 213 mAh g-1 at current densities of 0.2, 0.5, 1, 2, 5, and 10 A g-1, respectively. For NiS, the 2nd-cycle charge capacity is 276 at 0.2 A g-1 and 43 mAh g-1 at 10 A g-1. The rate performance of NiSe2 is good as a SIBs anode, Figure 7d shows the cycling performance of the NiS and NiSe2 batteries at 1 A g-1. The NiSe2 electrode shows not only the higher reversible sodium storage capacity but also the better cycling stability, and delivers a specific charge capacity of 311 mAh g-1 during the 100th cycle at 1 A g-1. For NiS electrode, it can keep the capacity of 166 mAh g-1 after 100th cycle at the same current density. Since metal sulfides and selenides have been widely investigated as anodes for sodium battery, Table S1 (Supporting information) provides the comparison of the rate capability and cycle stability of the as-prepared NiSe2 battery and other reported chalcogenides for sodium ion batteries. It is to be noted that our desired NiSe2 nanoplate exhibits better rate performance or cycle stability than those of metal chalcogenides electrodes reported previously.

Figure 8. (a) Nyquist plots of fresh NiS and NiSe2 SIBs; (b) CV curves of NiSe2 SIB at various scan rates from 0.1 mV s-1 to 2.0 mV s-1. To further investigate the better SIB performance of NiSe2, the EIS spectra of the two electrodes were recorded, as shown in Figure 8a, it can be seen that the two electrodes both show a semicircle in the high-frequency region and a straight line in the low-frequency region, which are attributed to the charge-transfer resistance of the electrode (Rct) and the internal resistance (Rs), respectively. Rct is one of the limiting factors for the batteries performance, corresponding to the charge-transfer resistance at the electrode-electrolyte interface. From the diameter of the semicircle on the real axis (Rct), it is obvious that the Rct of the NiSe2 electrode has smaller resistance in comparison to NiS electrode. Figure S8 exhibits the Nyquist plots of NiS and NiSe2 sodium ion batteries after 100 galvanostatic charge-discharge cycles at a specific current of 1 A g-1. The results demonstrate that both the NiSe2 and NiS electrodes show increased of Rct upon the charge/discharge cycles, which revealed that the electrochemical reaction became much more difficult with the increase of the cycling number. Figure 8b show the CV curves of the NiSe2 anode at different scan rates ranging from 0.1 to 2 mV s-1. The intensities of both the cathodic and anodic peaks increase with the increasing scan rate, indicating a good rate capability behavior. The dependence of the cathodic and anodic peak currents on the square root of the scan rate (ν1/2) is presented in Figure S9 (Supporting information), which could also be analyzed by the equation 4-5. It is clear that the both cathodic and anodic peak show a linear relationship with the square root of scan rate, which suggests that the sodiation/desodiation reaction rate is diffusioncontrolled process. In this case, the peak current variation of CV



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curves for the NiSe2 battery is determined by the Na+ diffusion and scan rates. According to the fitting results, NiSe2 shows the high slope of 2.24 for positive peaks and 1.37 for negative peaks, which indicates high Na+ diffusion coefficients and leads to excellent Na+ storage capability. 4. Conclusion In summary, a facile and bottom-up approach has been demonstrated to prepare 2D Ni-MOFs, which are converted into NiS and NiSe2 nanoplates through thermally induced sulfurization and selenization. Carbon coating has been formed due to the decomposition of its organic parts. When evaluated as anodes of LIBs and SIBs, NiS and NiSe2 nanoplates show high specific capacities, excellent rate capabilities and stable cycling stability. As anodes of LIBs, NiS nanoplates exhibit excellent Li ion storage ability due to its high theoretical capacity and high diffusion coefficient of Li+ into NiS electrode, showing a high capacity of 972 mA h g1 at a current density of 0.1 A g-1 and a reversible capacity of 468 mA h g-1 after the 100 cycle at 1.0 A g-1. As anodes of NIBs, NiSe2 nanoplates deliver a stable discharge capacity of 318 mAh g-1 after 100 cycles at 1A g-1 and excellent rate capability, which can be attribute its low charge transfer resistance and high diffusion coefficients of Na ion into NISe2 electrode, leading a promising Na ion storage ability. By taking advantage of the unique reactivity of cyanide-bridged hybrid coordination polymer for the preparation of 2D NiS and NiSe2 structure in this work, largescale synthesis of unique metal sulfides or selenides with fascinating architectures and potential application toward the development of advanced anode materials for LIBs and NIBs could be expected.

ASSOCIATED CONTENT Supporting Information Experimental details, additional material characterization and discussion. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author Email: [email protected], [email protected].

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We gratefully acknowledge Singapore MOE AcRF Tier 1 grants RG2/13 and RG113/15.

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Nanosheets for Enhanced Sodium Storage. ACS Nano 2015, 9, 1137111381.



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8 ACS Paragon Plus Environment

Controllable Preparation of Square Nickel Chalcogenide (NiS and

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