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Unique hollow NiO nanooctahedrons fabricated through the Kirkendall effect as anodes for enhanced lithium-ion storage
Accepted Manuscript Unique hollow NiO nanooctahedrons fabricated through the Kirkendall effect as anodes for enhanced lithium-ion storage Seung-Keun Park, Jae Hun Choi, Yun Chan Kang PII: DOI: Reference:
S1385-8947(18)31485-2 https://doi.org/10.1016/j.cej.2018.08.018 CEJ 19632
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
24 May 2018 26 July 2018 3 August 2018
Please cite this article as: S-K. Park, J.H. Choi, Y.C. Kang, Unique hollow NiO nanooctahedrons fabricated through the Kirkendall effect as anodes for enhanced lithium-ion storage, Chemical Engineering Journal (2018), doi: https:// doi.org/10.1016/j.cej.2018.08.018
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Unique hollow NiO nanooctahedrons fabricated through the Kirkendall effect as anodes for enhanced lithium-ion storage
Seung-Keun Park, Jae Hun Choi, and Yun Chan Kang*
Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea *Corresponding author
E-mail: [email protected]. Tel.: +82-2-928-3584. Fax: +82-2-3290-3268.
Abstract: The Kirkendall effect, which is a simple and novel phenomenon, has been widely employed for the fabrication of hollow metal oxide nanostructures with designed pore structures. For the first time, we demonstrate the application of the Kirkendall effect to nickel selenides (NiSe2) as precursors for the preparation of unique hollow NiO nanooctahedrons. The NiSe2 precursors prepared via a facile hydrothermal method underwent post-treatment in air. During the controlled oxidation process, the outward diffusion of Ni cations and the Se component in NiSe2 was quicker than the inward diffusion of O2 gas, resulting in the formation of NiO nanooctahedrons with hollow voids. As lithium-ion battery anode materials, these nanooctahedrons exhibited stable cycling performance (a specific discharge capacity of 1234 mA h g-1 after 150 cycles at 1 A g-1) and high rate capability (specific discharge capacities of 895, 887, 853, 808, 761, and 713 mA h g-1 at 0.5, 0.7, 1.0, 1.5, 2.0, and 3.0 A g-1, respectively). The excellent electrochemical properties of the unique hollow NiO nanooctahedrons can be ascribed to the substantial void space, which increases the contact area between the electrolyte and active materials and accommodates the volume expansion of NiO during cycling.
Keywords: Kirkendall effect; nickel selenide; hollow nanostructure; nickel oxide; lithium ion batteries; hydrothermal;
1. Introduction The Kirkendall effect, which originates from an unbalanced counter diffusion through an interface of two species, has been widely employed for the fabrication of hollow metal compounds [1-5]. This strategy is particularly attractive because the hollow voids in metal oxides are formed via a simple reaction without an additional template-removal process. In most previous studies, highly reactive transition metallic nanoparticles have been mainly employed as a precursor to form hollow metal oxide nanoparticles [2-5]. Unfortunately, the morphology of metallic nanoparticles is mostly limited to a spherical shape, which is thermodynamically stable. Therefore, it is a significant challenge to control the morphology of hollow metal oxide nanoparticles via the Kirkendall effect. As a strategy to overcome these limitations, some research groups have proposed the application of metal chalcogenides as precursors (e.g., metal sulfides and selenides) to fabricate unique structured hollow metal oxides via the Kirkendall effect. For example, Liu and Xue prepared hollow SnO2 nanocapsules through the oxidation of SnS nanobelts synthesized via the hydrothermal method [6]. Park et al. fabricated hierarchical-structured Fe2O3 rod clusters with numerous empty nanovoids by combining hydrothermal with one-step thermal oxidation [6]. During the oxidation process, FeSe2 precursors were sequentially transformed into FeSe2@FeOx–Se@Fe2O3, FeOx– Se@Fe2O, and porous Fe2O3 rod clusters. Nickel oxide (NiO) is a useful material in various applications including gas sensors, supercapacitors, catalysts, and bio sensors [7-17]. In particular, it is expected to be a promising anode for lithium-ion batteries (LIBs) owing to its natural abundance, low cost, and high theoretical capacity [18-26]. However, the practical application of NiO as an anode in LIBs is uncertain owing to the rapid capacity decay resulting from large volume expansion during cycling. Introducing the hollow voids into NiO nanoparticles is an effective strategy to solve this problem. Correspondingly, numerous research works have demonstrated hollow-structured NiO nanoparticles as anodes for LIBs, and they exhibited better electrochemical properties [27-32]. Nevertheless, studies on employing the Kirkendall effect to unique-structured nickel chalcogenides for the preparation of hollow nickel oxide have not been reported yet.
In this study, we develop for the first time a new approach to fabricate unique hollow NiO nanooctahedrons via the controlled Kirkendall effect. NiSe2 nanooctahedrons synthesized via a facile hydrothermal process were employed as a precursor. During a controlled oxidation process, the selenides were completely transformed into similar structured hollow NiO materials via the Kirkendall effect. From the morphological and structural changes depending on the temperature conditions, the detailed transformation mechanism of NiSe2 nanooctahedrons was also proposed. The as-obtained unique hollow NiO nanooctahedrons exhibited superior lithium-ion storage performance as anodes for LIBs. 2. Experimental section 2.1. Sample Preparation: Unique hollow NiO nanooctahedrons were synthesized by combining hydrothermal with a controlled thermal oxidation. To prepare the NiSe2 nanooctahedrons, nickel (ii) acetate tetrahydrate (1 mmol; Sigma) was dissolved in 12 mL of deionized (DI) water under vigorous stirring for 10 min. Subsequently, a red-colored solution containing 5 mmol of Se (99.5 %, Sigma) and 18 mL of hydrazine monohydrate (98%, Junsei) was added to the above solution drop by drop. After stirring for 30 min, the mixture was transferred to a Teflon-lined autoclave of capacity 100 mL and subsequently subjected to thermal treatment at 170 °C for 12 h. A dark grey product was obtained after filtration with DI water and ethanol. The obtained NiSe2 nanooctahedrons underwent the oxidation process at temperatures of 400 and 500 °C under air atmosphere for 3 h, and these samples were denoted as NiSe2-A400 and NiO-A500, respectively. 2.2. Characterization Techniques: X-ray diffraction (XRD, X’Pert PRO, at the Korea Basic Science Institute (Daegu)), X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha), thermogravimetric (TG) analyzer (Pyris 1, Perkin Elmer), and N2 adsorption–desorption isotherms were employed to determine the composition or structure of the samples. Scanning electron microscopy (FE-SEM, Tescan, VEGA3) and field-emission transmission electron microscopy (FETEM, JEOL, JEM-2100F) were used to investigate the morphologies and structures of the samples. 2.3. Electrochemical Measurements: The electrochemical properties of the NiSe2 and NiO
nanooctahedrons were evaluated by fabricating a standard 2032-type coin cell. The electrodes were fabricated by casting a mixture consisting of 70 wt% active materials, 20 wt% Super P, and 10 wt% sodium carboxymethyl cellulose in DI water onto a Cu foil. Li metal foil, porous polymer membrane, and 1 M LiPF6 dissolved in fluoroethylene carbonate/dimethyl carbonate at a volumetric ratio of 1:1 were employed as the counter electrode, separator, and electrolyte, respectively. Galvanostatic charge/discharge measurements were performed using a WBCS-3000s cycler (WonATech, Korea) in the potential range 0.001–3.0 V at various current densities. The mass loading of the electrode was 1.4 mg cm-2.
3. Results and Discussion The unique hollow NiO nanooctahedrons were synthesized by applying the Kirkendall effect to NiSe2 nanooctahedrons as depicted in Scheme 1. First, NiSe2 nanooctahedrons with dense structures were prepared via a facile hydrothermal method (Scheme 1-a). During the process, the NiSe2 crystal nuclei rapidly grew and agglomerated to reduce their surface energy. At that time, the difference between the growth rates of the crystal faces resulted in the formation of NiSe2 nanooctahedrons [33]. As the NiSe2 crystals grow, the (110) crystal face gradually disappeared due to its highest surface energy, while the crystal face of (111) grew at the slowest rate, resulting in the formation of NiSe2 nanooctahedron. In the subsequent oxidation process at 500 °C, the NiSe2 nanooctahedrons transformed into the unique hollow NiO nanooctahedrons via the Kirkendall diffusion process (Scheme 1-b). According to previous studies, the different diffusion rates of metal ions, Se component, and O2 gas during the oxidation led to the formation of inner voids of metal oxides [34,35]. Similarly, the outward diffusion of Ni cations and Se component in NiSe2 crystals was quicker than the inward diffusion of O2 gas, resulting in the formation of various intermediate products. Finally, unique hollow NiO nanooctahedrons were obtained by evaporation of the Sebased materials in the intermediate products. The NiSe2 precursors synthesized via a hydrothermal method had an apparent octahedral shape
with a size of approximately 200 nm and a dense structure, as shown in the SEM and TEM images (Fig. 1a-c). The high-resolution (HR) TEM image (Fig. 1d) showed distinct 0.27 nm-spaced lattice fringes, corresponding to the (210) crystal plane of cubic-phase NiSe2 (#41-1495). The selected area electron diffraction (SAED) and XRD patterns of NiSe2 nanooctahedrons were also well matched with those of phase pure cubic NiSe2 (Figs. 1e and S1, respectively). The sharp peaks indicate the high crystallinity of the NiSe2 nanooctahedrons, and no impurity could be detected. The elemental mapping images displayed in Fig. 1f show uniform distributions of Ni and Se elements throughout the NiSe2 nanooctahedrons. To investigate the formation mechanism of the unique hollow NiO nanooctahedrons, the morphological and structural features of NiSe2 nanooctahedrons treated at the oxidation temperatures of 400 and 500 °C were evaluated using various analytical methods. As shown in Fig. 2a-c, the morphology and structure of NiSe2 nanooctahedrons treated at the oxidation temperature of 400 °C rarely changed compared with those of the precursor, but a thin oxidized layer was formed on their surface (Fig. 2c). These results indicated that the oxidation temperature of 400 °C was too low to transform NiSe2 to its corresponding oxide (Fig. 2a-c). From the HR-TEM image, SAED, and XRD patterns, the pure phase NiSe2 was also confirmed (Figs. 2d and e, and S1, respectively). The low content of oxygen element was detected from the elemental mapping images of the NiSe2 nanooctahedrons, as displayed in Fig. 2f. In contrast with NiSe2-A400, the morphology and crystalline structure of NiSe2 nanooctahedrons treated at the oxidation temperature of 500 °C significantly changed, as shown in Fig. 3. The overall morphology of the sample observed using SEM (Fig. 3a) was similar to that of pristine NiSe2 nanooctahedrons, but the TEM images showed different inner structures (Figs. 3b and c). The nanooctahedrons had distinct internal void spaces, as confirmed by the contrast between the edges and centers. They also comprise of numerous primary nanocrystals with different sizes. The HRTEM image (Fig. 3d) apparently showed 0.21 nm-wide crystal lattice fringes corresponding to the (210) plane of NiO. As shown in Figs. 3e and S1, the SAED and XRD patterns further verified the complete transformation of NiSe2 into phase-pure NiO (#44-1159). These results indicated the
formation of unique hollow NiO nanooctahedrons via the Kirkendall effect during the thermal oxidation. The elemental mapping images in Fig. 3f showed the existence of Ni, O, and Se elements, but Se element was detected in a small amount. This also confirmed the absence of Se element in the samples after the oxidation at 500 °C. To study the formation mechanism of hollow NiO nanooctahderons, we conducted the controlled experiment; NiSe2 precursors were treated in the time range of 15 ~ 30 min at 550 oC under air (Fig. S2). Notably, in the XRD pattern of sample that was heat treated for 20 min, the peaks which correspond to NiSeO3 newly appeared together with the peaks corresponding to crystalline NiSe2 and NiO. Further, the peaks correlated with NiSe2 completely disappeared after thermal treatment for 30 min. It means that NiSe2 nanooctahedrons transformed into NiO via intermediate NiSeO3 phase. Based on these results, we assumed the formation of intermediate phases including NiSe2@NiO, NiSex@NiSeO3-NiO and NiSeO3-NiO during the oxidation process. XPS analysis was performed to study the chemical information and electron states of NiSe2 nanooctahedrons before and after the oxidation process (Figs. 4 and S3). As shown in Fig. S3, the presence of Ni, Se, and O elements was confirmed in both XPS survey spectra. Compared with that of NiSe2 survey spectrum, the O 1s of NiO-A500 survey spectrum had an increased peak intensity owing to the existence of NiO nanocrystals after the oxidation step. As shown in Figs. 4a and b, the Ni 2p spectra of both samples were well fitted with two spin-orbit doublets, which were characteristic of Ni3+ and Ni2+, and two shake-up satellites (denoted as Sat.) [36,37]. The binding energies of Ni3+ and Ni2+ are 853.5 and 855.5 eV in Ni 2p3/2 spin-orbit level, respectively, and 870.7 and 873.8 eV in Ni 2p1/2 spin-orbit level, respectively. The existence of Ni3+ in both samples was attributed to the oxygen-rich nickel oxide phase on the surface of the samples. As confirmed in the XRD and SAED patterns, the hollow nanooctahedrons exhibited pure NiO crystal phase without Ni2O3 crystal phase. This result indicated that trace amount of Ni3+ exists on the surface of the sample, which rarely affect the electrochemical performances for LIBs. The Se 3d spectrum of NiSe2 showed two broad shoulders at 55.1 and 58.9 eV, which were deconvoluted into several peaks at 54.7, 55.5, 56.4, and 58.9 eV, representing Se 3d5/2, Se 3d3/2, Se-Se, and Se-O, respectively
(Fig. 4c) [38,39]. The existence of SeOx was attributed to the thin surface oxide layer formed under atmosphere. Notably, the Se 3d spectrum of NiO-A500 showed only two peaks corresponding to Se-Se and SeOx, indicating that NiSe2 nanocrystal transformed into NiO nanocrystal after the oxidation process at 500 °C, but trace amounts of SeOx remained in the crystals (Fig. 4d). The TG curve of NiSe2 was obtained under air atmosphere (Fig. S4). The weight loss of NiSe2 powders mainly occurred at the temperature range 500 to 650 °C, which was ascribed to the oxidation of NiSe2 and the subsequent volatilization of SeO2 formed from the remaining Se. These results emphasized that the oxidation of NiSe2 occurred at a temperature more than 500 °C. Hence, the crystal phase of NiSe2 was not changed even after the thermal oxidation at 400 °C. The specific pore structures of the samples were investigated using N2 adsorption–desorption measurement. The isotherm of NiO-A500 sample showed a type-IV graph with an apparent hysteresis loop, indicating the existence of mesopores within the nanooctahedrons (Fig. S5a) [40,41]. In contrast, a hysteresis loop in the isotherm of NiSe2 samples was not observed owing to the absence of pores. Thus, the surface area of NiO-A500 sample (13.7 m2 g-1) was higher than those of the NiSe2 (3.3 m2 g-1) sample. Similarly, the Barrrett–Joyner–Halenda (BJH) plot of the NiO-A500 sample shows a distinct peak located at 18 nm, which originated from the interstices between NiO nanocrystals closely packed each other (Fig. S4b). In contrast, the BJH plot of NiSe2 sample showed no apparent peaks. The cyclic voltammetric (CV) and charge–discharge measurements were conducted to evaluate the lithium-ion storage performances of the NiSe2 and NiO nanooctahedrons. The first, second, and fifth CV graphs of the NiSe2 and NiO-A500 samples obtained at 0.1 mV s–1 were shown in Fig. 5. Two reduction peaks at 1.6 and 1.4 V in the initial cathodic scan of the NiSe2 sample were observed, which were attributed to the multiple reduction stages for the conversion of NiSe2 into Ni and Li2Se (Fig. 5a) [42]. From the second cycle onward, a new cathodic peak at 1.7 V appeared, corresponding to the electrochemical formation of Li2Se from metalloid Se [43-45]. Thus, the anodic peaks at 2.0 and 2.2 V could be due to the formation of Se-deficient NiSe and Se, respectively. In the initial CV graph of NiO-A500 sample, a small cathodic peak at 1.8 V (indicated
by the arrow) was attributed to the electrochemical reaction between Li+ and the partially remaining Se (Fig. 5b). In the subsequent anodic sweep, however, the oxidation peak related to the formation of Se from Li2Se was not observed, indicating that the metalloid Se transformed into electrochemically inert phases such as SeO2 [34]. Thus, the metalloid Se in NiO-A500 nanooctahedrons did not contribute to the specific capacities of electrodes from the second cycle onward. A sharp cathodic peak and two broad anodic peaks in the initial cycle correspond to the redox reaction between NiO and Li+. Notably, the reduction peak shifted to a higher potential level of 1.0 V after the initial cycle owing to the formation of ultrafine nanocrystals during the first discharge and charge processes. An extra shoulder located at 1.5 V originated from the imperfection of the nanostructured NiO lattice [46]. Except for the initial cycle, all the peaks were nearly overlapped in the following cycles, suggesting that NiO-A500 sample exhibits excellent electrochemical reversibility (Fig. 5b). The initial discharge and charge profiles of the NiSe2 and NiO-A500 samples at a current density of 1 A g-1 were shown in Fig. 6a. NiO-A500 showed a distinct plateau at 0.6 V in the initial discharge profiles owing to the reduction of NiO to Ni and the formation of a solid electrolyte interphase (SEI) layer. Moreover, the initial discharge curve of the NiSe2 sample displayed a plateau at 1.4 V, which was higher than that of the NiO-A500 sample. These results were consistent with the CV results. As the electrochemical reaction between Li+ and NiSe2 occurred at a relatively high potential level, the NiSe2 sample was not suitable as an anode for LIBs. The initial discharge capacities of the NiSe2 and NiO-A500 samples were 704 and 1175 mA h g-1, respectively, and their corresponding Coulombic efficiencies were 80 and 73 %, respectively. The formation of SEI layers and the structural transformation of nanocrystals in the initial cycle decreased the Coulombic efficiencies of the two samples [47,48]. Owing to the high reversibility of LiSe2 formed during the initial discharge process, the NiSe2 sample has higher initial Coulombic efficiency than NiO-A500. The charge–discharge capacities vs. cycle number plots of the three samples at a current density of 1 A g-1 are shown in Fig. 6b. The NiO-A500 with unique hollow structures showed the highest capacities and stable cycling performance up to 150 cycles. A gradual increase in the capacities
after 50 cycles could be ascribed to the formation of a gel-like polymer film on the electrode material during cycling. The gel-like polymer film called SEI was formed by electrolyte decomposition during cycling in the voltage range of cell test, which consisted of lithium oxide and lithium carbonate [49-51]. As the cycling proceeded, more polymer films were formed owing to the access of lithium ion deep inside the electrodes. Thus, the specific capacity of NiO-A500 gradually increased. Moreover, the capacity of the NiSe2 sample gradually decreased during the early cycles, which could be attributed to the pulverization of the electrode materials. Consequently, the NiSe2 and NiO-A500 samples exhibited discharge capacities of 126 and 1234 mA h g-1 after 150 cycles, respectively. To further study the electrochemical behaviors of NiO-A500 sample, the rate capability at various current densities from 0.5 to 3.0 A g-1 was also measured (Fig. 6c). As the current density increased, NiO-A500 sample exhibited good capacity retention. The average discharge capacities at the current densities of 0.5, 0.7, 1.0, 1.5, 2.0, and 3.0 A g-1 were 895, 887, 853, 808, 761, and 713 mA h g-1, respectively. Notably, when the current density returned to 0.5 A g-1, a discharge capacity of 870 mA h g-1 was recovered, which demonstrated that the unique hollow NiO nanooctahedrons have a good potential as high-rate anodes for LIBs. The excellent electrochemical properties of the unique hollow NiO nanooctahedrons can be ascribed to the substantial void space, which increased the contact area between the electrolyte and active materials and accommodates the volume expansion of NiO during cycling. The electrochemical impedance spectroscopy results of NiO-A500 sample before and after 1, 30, and 100 cycles were shown in Fig. 7. The Nyquist plots were composed of semicircles in the highmedium-frequency range and an inclined straight line in the low-frequency region, which corresponded to the charge transfer resistance (Rct) and solid-state diffusion of Li+ in the electrode, respectively [52-54]. Compared with that of a fresh electrode, the Rct value of the electrode after the initial cycle significantly decreased, which could be ascribed to the formation of nano-sized domains from the hollow NiO nanooctahedrons [55]. As the cycles proceeded, the Rct value increased slightly, as shown in Fig. 7b, indicating that the structural integrity was well retained
during cycling. Correspondingly, the unique morphology of NiO nanooctahedrons was observed from the cycled electrode (Fig. S6). These results revealed that the hollow interiors of NiO nanooctahedrons effectively inhibit the pulverization resulting from the large volume expansion of electrode materials.
4. Conclusions In summary, unique structured NiO nanooctahedrons with well-defined hollow voids were successfully fabricated by applying the Kirkendall effect to NiSe2 precursors. During the oxidation process at 500 °C, the outward diffusion of nickel cations and selenium component in the NiSe2 was quicker than the inward diffusion of oxygen gas, which results in the formation of hollow NiO nanooctahedrons. When evaluated as LIB anodes, the unique hollow NiO nanooctahedrons exhibited exceptional electrochemical properties owing to their substantial inner voids, which increase the contact area between the electrolyte and active materials and accommodate the volume expansion of the nanocrystal during cycling. We believe that the strategy proposed in this work can be extended to the fabrication of various hollow metal oxides from their corresponding metal selenides via the Kirkendall effect.
Author information Corresponding Author * E-mail: [email protected]. Tel.: +82-2-928-3584. Fax: +82-2-3290-3268. (Yun Chan Kang)
Acknowledgements This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. 2017R1A2B2008592). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by
the Ministry of Science, ICT & Future Planning (NRF-2017R1A4A1014806). This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20153030091450).
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Figure captions
Scheme 1. Schematic illustration of formation mechanism of unique hollow NiO nanooctahedron. Fig. 1. Morphologies and crystal phase of NiSe2 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images. Fig. 2. Morphologies and crystal phase of NiSe2-A400 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images. Fig. 3. Morphologies and crystal phase of NiO-A500 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images. Fig. 4. High resolution XPS spectra of (a, b) Ni 2p, and (c, d) Se 3d of NiSe2 and NiO-A500 nanooctahedrons. Fig. 5. Cyclic voltammetry (CV) profiles of (a) NiSe2 and (b) NiO-A500 nanooctahedrons obtained at a scan speed of 0.1 mV s-1. Fig. 6. Electrochemical properties of NiSe2 and NiO-A500 nanooctahedrons: (a) initial galvanostatic charge-discharge profiles, (b) cycling performance and the Coulombic efficiencies ata current density of 1.0 A g-1, and (c) rate capability at various current densities. Fig. 7. Nyquist plots of NiO-A500 nanooctahedrons (a) before cycling and (b) after cycling.
Scheme 1. Schematic illustration of formation mechanism of unique hollow NiO nanooctahedron
Fig. 1. Morphologies and crystal phase of NiSe2 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images.
Fig. 2. Morphologies and crystal phase of NiSe2-A400 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images.
Fig. 3. Morphologies and crystal phase of NiO-A500 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images.
Fig. 4. High resolution XPS spectra of (a, c) Ni 2p, and (b, d) Se 3d of NiSe2 and NiO-A500 nanooctahedrons.
Fig. 5. Cyclic voltammetry (CV) profiles of (a) NiSe2 and (b) NiO-A500 nanooctahedrons obtained at a scan speed of 0.1 mV s-1.
Fig. 6. Electrochemical properties of NiSe2 and NiO-A500 nanooctahedrons: (a) initial galvanostatic charge-discharge profiles, (b) cycling performance and the Coulombic efficiencies ata current density of 1.0 A g-1, and (c) rate capability at various current densities.
Fig. 7. Nyquist plots of NiO-A500 nanooctahedrons (a) before cycling and (b) after cycling.
Graphitic abstract
Unique hollow NiO nanooctahedrons fabricated through the Kirkendall effect as anodes for enhanced lithium-ion storage Seung-Keun Park, Jae Hun Choi, and Yun Chan Kang*
Unique hollow NiO nanooctahedrons were successfully prepared via the controlled Kirkendall effect for the first time. NiSe2 nanooctahedrons synthesized via a facile hydrothermal process were employed as a precursor. During a controlled oxidation process, the selenides were completely transformed into similar structured hollow NiO materials via the Kirkendall effect. The as-obtained unique hollow NiO nanooctahedrons exhibited superior lithium-ion storage performance as anodes for LIBs.
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A new approach to fabricate unique hollow NiO nanooctahedrons via the controlled Kirkendall effect is introduced.
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NiSe2 nanooctahedrons synthesized via a facile hydrothermal process transform into NiO
nanooctahedrons. -
Hollow NiO nanooctahedrons exhibit excellent lithium-ion storage performances.
S1385-8947(18)31485-2 https://doi.org/10.1016/j.cej.2018.08.018 CEJ 19632
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
24 May 2018 26 July 2018 3 August 2018
Please cite this article as: S-K. Park, J.H. Choi, Y.C. Kang, Unique hollow NiO nanooctahedrons fabricated through the Kirkendall effect as anodes for enhanced lithium-ion storage, Chemical Engineering Journal (2018), doi: https:// doi.org/10.1016/j.cej.2018.08.018
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Unique hollow NiO nanooctahedrons fabricated through the Kirkendall effect as anodes for enhanced lithium-ion storage
Seung-Keun Park, Jae Hun Choi, and Yun Chan Kang*
Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea *Corresponding author
E-mail: [email protected]. Tel.: +82-2-928-3584. Fax: +82-2-3290-3268.
Abstract: The Kirkendall effect, which is a simple and novel phenomenon, has been widely employed for the fabrication of hollow metal oxide nanostructures with designed pore structures. For the first time, we demonstrate the application of the Kirkendall effect to nickel selenides (NiSe2) as precursors for the preparation of unique hollow NiO nanooctahedrons. The NiSe2 precursors prepared via a facile hydrothermal method underwent post-treatment in air. During the controlled oxidation process, the outward diffusion of Ni cations and the Se component in NiSe2 was quicker than the inward diffusion of O2 gas, resulting in the formation of NiO nanooctahedrons with hollow voids. As lithium-ion battery anode materials, these nanooctahedrons exhibited stable cycling performance (a specific discharge capacity of 1234 mA h g-1 after 150 cycles at 1 A g-1) and high rate capability (specific discharge capacities of 895, 887, 853, 808, 761, and 713 mA h g-1 at 0.5, 0.7, 1.0, 1.5, 2.0, and 3.0 A g-1, respectively). The excellent electrochemical properties of the unique hollow NiO nanooctahedrons can be ascribed to the substantial void space, which increases the contact area between the electrolyte and active materials and accommodates the volume expansion of NiO during cycling.
Keywords: Kirkendall effect; nickel selenide; hollow nanostructure; nickel oxide; lithium ion batteries; hydrothermal;
1. Introduction The Kirkendall effect, which originates from an unbalanced counter diffusion through an interface of two species, has been widely employed for the fabrication of hollow metal compounds [1-5]. This strategy is particularly attractive because the hollow voids in metal oxides are formed via a simple reaction without an additional template-removal process. In most previous studies, highly reactive transition metallic nanoparticles have been mainly employed as a precursor to form hollow metal oxide nanoparticles [2-5]. Unfortunately, the morphology of metallic nanoparticles is mostly limited to a spherical shape, which is thermodynamically stable. Therefore, it is a significant challenge to control the morphology of hollow metal oxide nanoparticles via the Kirkendall effect. As a strategy to overcome these limitations, some research groups have proposed the application of metal chalcogenides as precursors (e.g., metal sulfides and selenides) to fabricate unique structured hollow metal oxides via the Kirkendall effect. For example, Liu and Xue prepared hollow SnO2 nanocapsules through the oxidation of SnS nanobelts synthesized via the hydrothermal method [6]. Park et al. fabricated hierarchical-structured Fe2O3 rod clusters with numerous empty nanovoids by combining hydrothermal with one-step thermal oxidation [6]. During the oxidation process, FeSe2 precursors were sequentially transformed into FeSe2@FeOx–Se@Fe2O3, FeOx– Se@Fe2O, and porous Fe2O3 rod clusters. Nickel oxide (NiO) is a useful material in various applications including gas sensors, supercapacitors, catalysts, and bio sensors [7-17]. In particular, it is expected to be a promising anode for lithium-ion batteries (LIBs) owing to its natural abundance, low cost, and high theoretical capacity [18-26]. However, the practical application of NiO as an anode in LIBs is uncertain owing to the rapid capacity decay resulting from large volume expansion during cycling. Introducing the hollow voids into NiO nanoparticles is an effective strategy to solve this problem. Correspondingly, numerous research works have demonstrated hollow-structured NiO nanoparticles as anodes for LIBs, and they exhibited better electrochemical properties [27-32]. Nevertheless, studies on employing the Kirkendall effect to unique-structured nickel chalcogenides for the preparation of hollow nickel oxide have not been reported yet.
In this study, we develop for the first time a new approach to fabricate unique hollow NiO nanooctahedrons via the controlled Kirkendall effect. NiSe2 nanooctahedrons synthesized via a facile hydrothermal process were employed as a precursor. During a controlled oxidation process, the selenides were completely transformed into similar structured hollow NiO materials via the Kirkendall effect. From the morphological and structural changes depending on the temperature conditions, the detailed transformation mechanism of NiSe2 nanooctahedrons was also proposed. The as-obtained unique hollow NiO nanooctahedrons exhibited superior lithium-ion storage performance as anodes for LIBs. 2. Experimental section 2.1. Sample Preparation: Unique hollow NiO nanooctahedrons were synthesized by combining hydrothermal with a controlled thermal oxidation. To prepare the NiSe2 nanooctahedrons, nickel (ii) acetate tetrahydrate (1 mmol; Sigma) was dissolved in 12 mL of deionized (DI) water under vigorous stirring for 10 min. Subsequently, a red-colored solution containing 5 mmol of Se (99.5 %, Sigma) and 18 mL of hydrazine monohydrate (98%, Junsei) was added to the above solution drop by drop. After stirring for 30 min, the mixture was transferred to a Teflon-lined autoclave of capacity 100 mL and subsequently subjected to thermal treatment at 170 °C for 12 h. A dark grey product was obtained after filtration with DI water and ethanol. The obtained NiSe2 nanooctahedrons underwent the oxidation process at temperatures of 400 and 500 °C under air atmosphere for 3 h, and these samples were denoted as NiSe2-A400 and NiO-A500, respectively. 2.2. Characterization Techniques: X-ray diffraction (XRD, X’Pert PRO, at the Korea Basic Science Institute (Daegu)), X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha), thermogravimetric (TG) analyzer (Pyris 1, Perkin Elmer), and N2 adsorption–desorption isotherms were employed to determine the composition or structure of the samples. Scanning electron microscopy (FE-SEM, Tescan, VEGA3) and field-emission transmission electron microscopy (FETEM, JEOL, JEM-2100F) were used to investigate the morphologies and structures of the samples. 2.3. Electrochemical Measurements: The electrochemical properties of the NiSe2 and NiO
nanooctahedrons were evaluated by fabricating a standard 2032-type coin cell. The electrodes were fabricated by casting a mixture consisting of 70 wt% active materials, 20 wt% Super P, and 10 wt% sodium carboxymethyl cellulose in DI water onto a Cu foil. Li metal foil, porous polymer membrane, and 1 M LiPF6 dissolved in fluoroethylene carbonate/dimethyl carbonate at a volumetric ratio of 1:1 were employed as the counter electrode, separator, and electrolyte, respectively. Galvanostatic charge/discharge measurements were performed using a WBCS-3000s cycler (WonATech, Korea) in the potential range 0.001–3.0 V at various current densities. The mass loading of the electrode was 1.4 mg cm-2.
3. Results and Discussion The unique hollow NiO nanooctahedrons were synthesized by applying the Kirkendall effect to NiSe2 nanooctahedrons as depicted in Scheme 1. First, NiSe2 nanooctahedrons with dense structures were prepared via a facile hydrothermal method (Scheme 1-a). During the process, the NiSe2 crystal nuclei rapidly grew and agglomerated to reduce their surface energy. At that time, the difference between the growth rates of the crystal faces resulted in the formation of NiSe2 nanooctahedrons [33]. As the NiSe2 crystals grow, the (110) crystal face gradually disappeared due to its highest surface energy, while the crystal face of (111) grew at the slowest rate, resulting in the formation of NiSe2 nanooctahedron. In the subsequent oxidation process at 500 °C, the NiSe2 nanooctahedrons transformed into the unique hollow NiO nanooctahedrons via the Kirkendall diffusion process (Scheme 1-b). According to previous studies, the different diffusion rates of metal ions, Se component, and O2 gas during the oxidation led to the formation of inner voids of metal oxides [34,35]. Similarly, the outward diffusion of Ni cations and Se component in NiSe2 crystals was quicker than the inward diffusion of O2 gas, resulting in the formation of various intermediate products. Finally, unique hollow NiO nanooctahedrons were obtained by evaporation of the Sebased materials in the intermediate products. The NiSe2 precursors synthesized via a hydrothermal method had an apparent octahedral shape
with a size of approximately 200 nm and a dense structure, as shown in the SEM and TEM images (Fig. 1a-c). The high-resolution (HR) TEM image (Fig. 1d) showed distinct 0.27 nm-spaced lattice fringes, corresponding to the (210) crystal plane of cubic-phase NiSe2 (#41-1495). The selected area electron diffraction (SAED) and XRD patterns of NiSe2 nanooctahedrons were also well matched with those of phase pure cubic NiSe2 (Figs. 1e and S1, respectively). The sharp peaks indicate the high crystallinity of the NiSe2 nanooctahedrons, and no impurity could be detected. The elemental mapping images displayed in Fig. 1f show uniform distributions of Ni and Se elements throughout the NiSe2 nanooctahedrons. To investigate the formation mechanism of the unique hollow NiO nanooctahedrons, the morphological and structural features of NiSe2 nanooctahedrons treated at the oxidation temperatures of 400 and 500 °C were evaluated using various analytical methods. As shown in Fig. 2a-c, the morphology and structure of NiSe2 nanooctahedrons treated at the oxidation temperature of 400 °C rarely changed compared with those of the precursor, but a thin oxidized layer was formed on their surface (Fig. 2c). These results indicated that the oxidation temperature of 400 °C was too low to transform NiSe2 to its corresponding oxide (Fig. 2a-c). From the HR-TEM image, SAED, and XRD patterns, the pure phase NiSe2 was also confirmed (Figs. 2d and e, and S1, respectively). The low content of oxygen element was detected from the elemental mapping images of the NiSe2 nanooctahedrons, as displayed in Fig. 2f. In contrast with NiSe2-A400, the morphology and crystalline structure of NiSe2 nanooctahedrons treated at the oxidation temperature of 500 °C significantly changed, as shown in Fig. 3. The overall morphology of the sample observed using SEM (Fig. 3a) was similar to that of pristine NiSe2 nanooctahedrons, but the TEM images showed different inner structures (Figs. 3b and c). The nanooctahedrons had distinct internal void spaces, as confirmed by the contrast between the edges and centers. They also comprise of numerous primary nanocrystals with different sizes. The HRTEM image (Fig. 3d) apparently showed 0.21 nm-wide crystal lattice fringes corresponding to the (210) plane of NiO. As shown in Figs. 3e and S1, the SAED and XRD patterns further verified the complete transformation of NiSe2 into phase-pure NiO (#44-1159). These results indicated the
formation of unique hollow NiO nanooctahedrons via the Kirkendall effect during the thermal oxidation. The elemental mapping images in Fig. 3f showed the existence of Ni, O, and Se elements, but Se element was detected in a small amount. This also confirmed the absence of Se element in the samples after the oxidation at 500 °C. To study the formation mechanism of hollow NiO nanooctahderons, we conducted the controlled experiment; NiSe2 precursors were treated in the time range of 15 ~ 30 min at 550 oC under air (Fig. S2). Notably, in the XRD pattern of sample that was heat treated for 20 min, the peaks which correspond to NiSeO3 newly appeared together with the peaks corresponding to crystalline NiSe2 and NiO. Further, the peaks correlated with NiSe2 completely disappeared after thermal treatment for 30 min. It means that NiSe2 nanooctahedrons transformed into NiO via intermediate NiSeO3 phase. Based on these results, we assumed the formation of intermediate phases including NiSe2@NiO, NiSex@NiSeO3-NiO and NiSeO3-NiO during the oxidation process. XPS analysis was performed to study the chemical information and electron states of NiSe2 nanooctahedrons before and after the oxidation process (Figs. 4 and S3). As shown in Fig. S3, the presence of Ni, Se, and O elements was confirmed in both XPS survey spectra. Compared with that of NiSe2 survey spectrum, the O 1s of NiO-A500 survey spectrum had an increased peak intensity owing to the existence of NiO nanocrystals after the oxidation step. As shown in Figs. 4a and b, the Ni 2p spectra of both samples were well fitted with two spin-orbit doublets, which were characteristic of Ni3+ and Ni2+, and two shake-up satellites (denoted as Sat.) [36,37]. The binding energies of Ni3+ and Ni2+ are 853.5 and 855.5 eV in Ni 2p3/2 spin-orbit level, respectively, and 870.7 and 873.8 eV in Ni 2p1/2 spin-orbit level, respectively. The existence of Ni3+ in both samples was attributed to the oxygen-rich nickel oxide phase on the surface of the samples. As confirmed in the XRD and SAED patterns, the hollow nanooctahedrons exhibited pure NiO crystal phase without Ni2O3 crystal phase. This result indicated that trace amount of Ni3+ exists on the surface of the sample, which rarely affect the electrochemical performances for LIBs. The Se 3d spectrum of NiSe2 showed two broad shoulders at 55.1 and 58.9 eV, which were deconvoluted into several peaks at 54.7, 55.5, 56.4, and 58.9 eV, representing Se 3d5/2, Se 3d3/2, Se-Se, and Se-O, respectively
(Fig. 4c) [38,39]. The existence of SeOx was attributed to the thin surface oxide layer formed under atmosphere. Notably, the Se 3d spectrum of NiO-A500 showed only two peaks corresponding to Se-Se and SeOx, indicating that NiSe2 nanocrystal transformed into NiO nanocrystal after the oxidation process at 500 °C, but trace amounts of SeOx remained in the crystals (Fig. 4d). The TG curve of NiSe2 was obtained under air atmosphere (Fig. S4). The weight loss of NiSe2 powders mainly occurred at the temperature range 500 to 650 °C, which was ascribed to the oxidation of NiSe2 and the subsequent volatilization of SeO2 formed from the remaining Se. These results emphasized that the oxidation of NiSe2 occurred at a temperature more than 500 °C. Hence, the crystal phase of NiSe2 was not changed even after the thermal oxidation at 400 °C. The specific pore structures of the samples were investigated using N2 adsorption–desorption measurement. The isotherm of NiO-A500 sample showed a type-IV graph with an apparent hysteresis loop, indicating the existence of mesopores within the nanooctahedrons (Fig. S5a) [40,41]. In contrast, a hysteresis loop in the isotherm of NiSe2 samples was not observed owing to the absence of pores. Thus, the surface area of NiO-A500 sample (13.7 m2 g-1) was higher than those of the NiSe2 (3.3 m2 g-1) sample. Similarly, the Barrrett–Joyner–Halenda (BJH) plot of the NiO-A500 sample shows a distinct peak located at 18 nm, which originated from the interstices between NiO nanocrystals closely packed each other (Fig. S4b). In contrast, the BJH plot of NiSe2 sample showed no apparent peaks. The cyclic voltammetric (CV) and charge–discharge measurements were conducted to evaluate the lithium-ion storage performances of the NiSe2 and NiO nanooctahedrons. The first, second, and fifth CV graphs of the NiSe2 and NiO-A500 samples obtained at 0.1 mV s–1 were shown in Fig. 5. Two reduction peaks at 1.6 and 1.4 V in the initial cathodic scan of the NiSe2 sample were observed, which were attributed to the multiple reduction stages for the conversion of NiSe2 into Ni and Li2Se (Fig. 5a) [42]. From the second cycle onward, a new cathodic peak at 1.7 V appeared, corresponding to the electrochemical formation of Li2Se from metalloid Se [43-45]. Thus, the anodic peaks at 2.0 and 2.2 V could be due to the formation of Se-deficient NiSe and Se, respectively. In the initial CV graph of NiO-A500 sample, a small cathodic peak at 1.8 V (indicated
by the arrow) was attributed to the electrochemical reaction between Li+ and the partially remaining Se (Fig. 5b). In the subsequent anodic sweep, however, the oxidation peak related to the formation of Se from Li2Se was not observed, indicating that the metalloid Se transformed into electrochemically inert phases such as SeO2 [34]. Thus, the metalloid Se in NiO-A500 nanooctahedrons did not contribute to the specific capacities of electrodes from the second cycle onward. A sharp cathodic peak and two broad anodic peaks in the initial cycle correspond to the redox reaction between NiO and Li+. Notably, the reduction peak shifted to a higher potential level of 1.0 V after the initial cycle owing to the formation of ultrafine nanocrystals during the first discharge and charge processes. An extra shoulder located at 1.5 V originated from the imperfection of the nanostructured NiO lattice [46]. Except for the initial cycle, all the peaks were nearly overlapped in the following cycles, suggesting that NiO-A500 sample exhibits excellent electrochemical reversibility (Fig. 5b). The initial discharge and charge profiles of the NiSe2 and NiO-A500 samples at a current density of 1 A g-1 were shown in Fig. 6a. NiO-A500 showed a distinct plateau at 0.6 V in the initial discharge profiles owing to the reduction of NiO to Ni and the formation of a solid electrolyte interphase (SEI) layer. Moreover, the initial discharge curve of the NiSe2 sample displayed a plateau at 1.4 V, which was higher than that of the NiO-A500 sample. These results were consistent with the CV results. As the electrochemical reaction between Li+ and NiSe2 occurred at a relatively high potential level, the NiSe2 sample was not suitable as an anode for LIBs. The initial discharge capacities of the NiSe2 and NiO-A500 samples were 704 and 1175 mA h g-1, respectively, and their corresponding Coulombic efficiencies were 80 and 73 %, respectively. The formation of SEI layers and the structural transformation of nanocrystals in the initial cycle decreased the Coulombic efficiencies of the two samples [47,48]. Owing to the high reversibility of LiSe2 formed during the initial discharge process, the NiSe2 sample has higher initial Coulombic efficiency than NiO-A500. The charge–discharge capacities vs. cycle number plots of the three samples at a current density of 1 A g-1 are shown in Fig. 6b. The NiO-A500 with unique hollow structures showed the highest capacities and stable cycling performance up to 150 cycles. A gradual increase in the capacities
after 50 cycles could be ascribed to the formation of a gel-like polymer film on the electrode material during cycling. The gel-like polymer film called SEI was formed by electrolyte decomposition during cycling in the voltage range of cell test, which consisted of lithium oxide and lithium carbonate [49-51]. As the cycling proceeded, more polymer films were formed owing to the access of lithium ion deep inside the electrodes. Thus, the specific capacity of NiO-A500 gradually increased. Moreover, the capacity of the NiSe2 sample gradually decreased during the early cycles, which could be attributed to the pulverization of the electrode materials. Consequently, the NiSe2 and NiO-A500 samples exhibited discharge capacities of 126 and 1234 mA h g-1 after 150 cycles, respectively. To further study the electrochemical behaviors of NiO-A500 sample, the rate capability at various current densities from 0.5 to 3.0 A g-1 was also measured (Fig. 6c). As the current density increased, NiO-A500 sample exhibited good capacity retention. The average discharge capacities at the current densities of 0.5, 0.7, 1.0, 1.5, 2.0, and 3.0 A g-1 were 895, 887, 853, 808, 761, and 713 mA h g-1, respectively. Notably, when the current density returned to 0.5 A g-1, a discharge capacity of 870 mA h g-1 was recovered, which demonstrated that the unique hollow NiO nanooctahedrons have a good potential as high-rate anodes for LIBs. The excellent electrochemical properties of the unique hollow NiO nanooctahedrons can be ascribed to the substantial void space, which increased the contact area between the electrolyte and active materials and accommodates the volume expansion of NiO during cycling. The electrochemical impedance spectroscopy results of NiO-A500 sample before and after 1, 30, and 100 cycles were shown in Fig. 7. The Nyquist plots were composed of semicircles in the highmedium-frequency range and an inclined straight line in the low-frequency region, which corresponded to the charge transfer resistance (Rct) and solid-state diffusion of Li+ in the electrode, respectively [52-54]. Compared with that of a fresh electrode, the Rct value of the electrode after the initial cycle significantly decreased, which could be ascribed to the formation of nano-sized domains from the hollow NiO nanooctahedrons [55]. As the cycles proceeded, the Rct value increased slightly, as shown in Fig. 7b, indicating that the structural integrity was well retained
during cycling. Correspondingly, the unique morphology of NiO nanooctahedrons was observed from the cycled electrode (Fig. S6). These results revealed that the hollow interiors of NiO nanooctahedrons effectively inhibit the pulverization resulting from the large volume expansion of electrode materials.
4. Conclusions In summary, unique structured NiO nanooctahedrons with well-defined hollow voids were successfully fabricated by applying the Kirkendall effect to NiSe2 precursors. During the oxidation process at 500 °C, the outward diffusion of nickel cations and selenium component in the NiSe2 was quicker than the inward diffusion of oxygen gas, which results in the formation of hollow NiO nanooctahedrons. When evaluated as LIB anodes, the unique hollow NiO nanooctahedrons exhibited exceptional electrochemical properties owing to their substantial inner voids, which increase the contact area between the electrolyte and active materials and accommodate the volume expansion of the nanocrystal during cycling. We believe that the strategy proposed in this work can be extended to the fabrication of various hollow metal oxides from their corresponding metal selenides via the Kirkendall effect.
Author information Corresponding Author * E-mail: [email protected]. Tel.: +82-2-928-3584. Fax: +82-2-3290-3268. (Yun Chan Kang)
Acknowledgements This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. 2017R1A2B2008592). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by
the Ministry of Science, ICT & Future Planning (NRF-2017R1A4A1014806). This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20153030091450).
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Figure captions
Scheme 1. Schematic illustration of formation mechanism of unique hollow NiO nanooctahedron. Fig. 1. Morphologies and crystal phase of NiSe2 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images. Fig. 2. Morphologies and crystal phase of NiSe2-A400 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images. Fig. 3. Morphologies and crystal phase of NiO-A500 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images. Fig. 4. High resolution XPS spectra of (a, b) Ni 2p, and (c, d) Se 3d of NiSe2 and NiO-A500 nanooctahedrons. Fig. 5. Cyclic voltammetry (CV) profiles of (a) NiSe2 and (b) NiO-A500 nanooctahedrons obtained at a scan speed of 0.1 mV s-1. Fig. 6. Electrochemical properties of NiSe2 and NiO-A500 nanooctahedrons: (a) initial galvanostatic charge-discharge profiles, (b) cycling performance and the Coulombic efficiencies ata current density of 1.0 A g-1, and (c) rate capability at various current densities. Fig. 7. Nyquist plots of NiO-A500 nanooctahedrons (a) before cycling and (b) after cycling.
Scheme 1. Schematic illustration of formation mechanism of unique hollow NiO nanooctahedron
Fig. 1. Morphologies and crystal phase of NiSe2 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images.
Fig. 2. Morphologies and crystal phase of NiSe2-A400 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images.
Fig. 3. Morphologies and crystal phase of NiO-A500 nanooctahedrons: (a) SEM image, (b, c) TEM images, (d) HR-TEM image, (e) SAED pattern, and (f) elemental mapping images.
Fig. 4. High resolution XPS spectra of (a, c) Ni 2p, and (b, d) Se 3d of NiSe2 and NiO-A500 nanooctahedrons.
Fig. 5. Cyclic voltammetry (CV) profiles of (a) NiSe2 and (b) NiO-A500 nanooctahedrons obtained at a scan speed of 0.1 mV s-1.
Fig. 6. Electrochemical properties of NiSe2 and NiO-A500 nanooctahedrons: (a) initial galvanostatic charge-discharge profiles, (b) cycling performance and the Coulombic efficiencies ata current density of 1.0 A g-1, and (c) rate capability at various current densities.
Fig. 7. Nyquist plots of NiO-A500 nanooctahedrons (a) before cycling and (b) after cycling.
Graphitic abstract
Unique hollow NiO nanooctahedrons fabricated through the Kirkendall effect as anodes for enhanced lithium-ion storage Seung-Keun Park, Jae Hun Choi, and Yun Chan Kang*
Unique hollow NiO nanooctahedrons were successfully prepared via the controlled Kirkendall effect for the first time. NiSe2 nanooctahedrons synthesized via a facile hydrothermal process were employed as a precursor. During a controlled oxidation process, the selenides were completely transformed into similar structured hollow NiO materials via the Kirkendall effect. The as-obtained unique hollow NiO nanooctahedrons exhibited superior lithium-ion storage performance as anodes for LIBs.
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A new approach to fabricate unique hollow NiO nanooctahedrons via the controlled Kirkendall effect is introduced.
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NiSe2 nanooctahedrons synthesized via a facile hydrothermal process transform into NiO
nanooctahedrons. -
Hollow NiO nanooctahedrons exhibit excellent lithium-ion storage performances.