Coral-Like Yolk–Shell-Structured Nickel Oxide/Carbon Composite Microspheres for High-Performance Li-Ion Storage Anodes
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Coral-like yolk–shell-structured nickel oxide/carbon composite microspheres were synthesized.
Phase separation and polystyrene nanobead decomposition affected the structure formation.
Coral-like yolk with interconnected mesopores provided excellent Li-ion storage properties.
KeywordsYolk–shell Nickel oxide Carbon composite Anode materials Spray pyrolysis Lithium-ion batteries
With the rapid increase in the energy demand, lithium-ion batteries (LIBs) have gained immense attention as next-generation energy storage devices and sources of vehicle energy [1, 2, 3, 4, 5, 6, 7]. Hence, in order to improve the performance of LIBs, it is imperative to develop innovative anode materials [8, 9, 10, 11].Transition metal oxides (TMOs) have been recognized as appropriate anode materials owing to their higher theoretical capacities as compared to that of graphite, high abundance, and chemical stability [12, 13, 14, 15, 16]. However, the drastic capacitance fading of TMOs owing to their large volume expansion during cycling has hindered their application as LIB anodes [17, 18, 19, 20, 21]. Therefore, various TMO nanostructures including nanoparticles, nanowalls, nanotubes, nanofibers, and nanoflakes have been extensively studied [22, 23, 24, 25, 26, 27, 28]. Recently, the yolk–shell structure materials have been used to improve the anode performance of LIBs [29, 30, 31, 32, 33, 34]. For example, Zhang et al. synthesized an iron oxide/carbon yolk–shell structure by carbonizing α-Fe2O3/SiO2/poly-dopamine composite nanoparticles followed by the removal of the SiO2 layer using NaOH. The Fe2O3/carbon yolk–shell structure exhibited a high reversible capacity of 810 mAh g−1 at 0.2 C rate and an excellent cycling stability while maintaining a capacity of 790 mAh g−1 after 100 cycles . Yu et al. also prepared yolk–shell Ni–Co mixed oxide nanoprisms through simple thermal annealing of Ni–Co precursor particles in air. These nanoprisms exhibited a reversible capacity of 1029 mAh g−1 after 30 cycles at 200 mA g−1 . Furthermore, Kim et al. integrated N-doped carbon in the hollow space between the yolk and the shell to achieve high capacity, accommodate volume change, improve electrical conductivity, and form a stable solid–electrolyte interphase (SEI) layer .
However, although yolk–shell structures with various compositions have been studied thus far, their long-term cycle properties are unsatisfactory for practical applications owing to their intrinsic low structural stability. An effective approach to overcome this limitation is to make TMO composites with carbonaceous materials. However, it is difficult to prepare yolk–shell-structured TMO/carbon hybrids using traditional synthesis methods. Therefore, the uniform composition of carbon and TMO and their even distribution in both the yolk and shell are quite challenging and have not been studied before.
In this study, we proposed a novel facile method for the synthesis of yolk–shell-structured TMO/carbon hybrid microspheres. The yolk had a coral-like structure with interconnected mesopores. Coral-like yolks shorten the Li+-ion diffusion path, facilitating the penetration of the electrolyte into the yolk during cycling. In addition, yolk–shell carbon composites can expand freely and hence show a highly stable SEI at the surface. Owing to the highly stable SEIs, such composites show excellent Li+-ion storage properties. Based on this concept, we synthesized coral-like yolk–shell-structured NiO/C composite microspheres via a one-pot spray pyrolysis process and a subsequent heat treatment. During the spray pyrolysis, polyvinylpyrrolidone (PVP) in the droplet partially phase-separated from the polystyrene (PS) colloidal solution and migrated outward, and interconnected mesopores were formed owing to the decomposition of PS. The subsequent thermal contraction of the inner part of the composite at high reaction temperatures during the spray pyrolysis process resulted in the formation of unique coral-like yolk–shell-structured NiO/C composite microspheres. The resulting NiO/C composite microspheres showed an ideal structure, and their long-term cycling and rate performances were superior to those of the other NiO-based nanomaterials with various morphologies reported till date.
2 Experimental Section
2.1 Sample Preparation
Coral-like yolk–shell-structured metal oxide/C composite microspheres were prepared via one-pot spray pyrolysis. First, the NiO–Ni–C composite microspheres with the coral-like yolk–shell structure (denoted as CYS-Ni/NiO/C) were directly prepared by spray pyrolysis using a 0.2 M aqueous spray solution of nickel nitrate hexahydrate [Ni(NO3)2·6H2O, Daejung, 97%], 20 g L−1 of PVP [(C6H9NO), Mw 40,000, Daejung], and 20 g L−1 of PS nanobeads (40 nm). The size-controlled PS nanobeads (40 nm) were synthesized using an emulsifier-free emulsion polymerization method. The spray pyrolysis system used in this study is shown in Fig. S1. In the spray pyrolysis process, droplets were generated with the aid of a 1.7 MHz ultrasonic spray generator consisting of six vibrators. Subsequently, the droplets were transferred to a quartz reactor (length = 1200 nm and diameter = 50 nm) by N2 gas (carrier) at a flow rate of 10 L min−1. During the spray pyrolysis process, the reactor temperature was maintained at 700 °C. After the spray pyrolysis process, the as-prepared microspheres (CYS-Ni/NiO/C) were post-treated at 250 °C at a heating rate of 5 °C min−1 for 1 h under an air atmosphere in order to optimize the carbon content in the structure and transform the residual metallic Ni into the NiO phase. After the heat treatment, coral-like yolk–shell NiO/C composite microspheres (denoted as CYS-NiO/C) were obtained. For comparison, bare NiO microspheres with a hollow structure (denoted as hollow NiO) were also prepared via spray pyrolysis at 700 °C in an air atmosphere. The spray solution consisted of only nickel nitrate hexahydrate (without PVP and PS nanobeads).
2.2 Characterization Techniques
The morphology of the samples was examined using field-emission scanning electron microscopy (FE-SEM, ULTRA PLUS, ZEISS) and field-emission transmission electron microscopy (FE-TEM, JEOL, JEM-2100F). The phase analysis of the samples was carried out by X-ray diffraction (XRD, D8 Discover with GADDS, Bruker) using Cu Kα radiation (λ = 1.5418 Å). The chemical composition of the samples was investigated by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) using a focused monochromatic Al Kα radiation at 12 kV and 20 mA. Raman spectroscopy (Jobin–Yvon LabRam, HR800, excitation source = 514 nm He–Ne laser) was conducted to confirm the presence of a graphitic structure in the samples. The surface areas of the samples were estimated using the Brunauer–Emmett–Teller (BET) method where N2 was used as the adsorbate gas. Thermogravimetric analysis (TGA) was carried out using a Pyris 1 TG analyzer (Perkin Elmer) over the temperature range of 25–700 °C at a heating rate of 10 °C min−1 under an air atmosphere.
2.3 Electrochemical Measurements
The electrochemical performances of the samples as LIB anodes were evaluated using 2032-type coin cells. The prepared NiO samples were used as the working electrode composed of 70 wt% active material, 20 wt% carbon black (Super-P) as the conductive material, and 10 wt% sodium carboxymethyl cellulose as the binder on a copper foil. The Li metal and microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte used was 1 M LiPF6 in a mixture of fluoroethylene carbonate and dimethyl carbonate with a volume ratio of 1:1. The cells were assembled in a glove box under an Ar atmosphere. The electrochemical performances of the samples were evaluated using cyclic voltammetry (CV), charge–discharge testing, and electrochemical impedance spectroscopy (EIS). The mass loading of the samples for the test was 1.0 mg cm−2. The CV measurements of the samples were carried out at a scan rate of 0.1 mV s−1 over the potential range of 0.001–3.0 V. The charge–discharge testing of the samples was carried out at current densities of 0.5–10.0 A g−1 within the same potential window of 0.001–3.0 V. The EIS of the samples was carried out over the frequency range of 100 kHz–0.01 Hz using a perturbation of 10 mV.
3 Results and Discussion
3.1 Synthesis of Ni/NiO/C Microspheres
3.2 Synthesis of NiO/C Microspheres
3.3 Evaluation of Li-Ion Storage Performance
The initial discharge–charge profiles of the CYS-NiO/C, CYS-Ni/NiO/C, and hollow NiO microspheres at a high current density of 1.0 A g−1 are shown in Fig. 7b. The discharge–charge profiles of the samples were consistent with their CV results. The discharge curve of the hollow NiO microspheres featured a clear long plateau at approximately 0.59 V owing to the presence of highly crystalline NiO crystals in them [24, 51]. However, the CYS-NiO/C and CYS-Ni/NiO/C microspheres exhibited an unclear plateau because of the presence of C-surrounded NiO crystals (low crystallinity) in them [24, 53]. The initial discharge capacities of the CYS-NiO/C, CYS-Ni/NiO/C, and hollow NiO microspheres were 1124, 770, and 1148 mAh g−1, respectively, and their corresponding charge capacities were 778, 426, and 819 mAh g−1, respectively. The theoretical capacity of the CYS-NiO/C microspheres was about 588 mAh g−1, as calculated using the theoretical specific capacities of NiO (718 mAh g−1) and C (372 mAh g−1). The high capacity of the CYS-NiO/C microspheres can be attributed to the partial reversible formation and decomposition of the gel-like SEI film on the surface of the electrode and their pseudo-capacitance .The initial Coulombic efficiencies (CE) of the CYS-NiO/C, CYS-Ni/NiO/C, and hollow NiO microspheres were found to be 69%, 55%, and 71%, respectively. The CYS-Ni/NiO/C microspheres showed the lowest CE among the samples because of their high C content with a high initial irreversible capacity loss [24, 30]. Although the CYS-NiO/C microspheres also had a C content of 18 wt%, their CE was comparable to that of the C-free NiO hollow microspheres. The high structural stability of the CYS-NiO/C microspheres in the first discharge and charge cycles resulted in a high initial CE.
The cycling performances of the samples at the current density of 1.0 A g−1 are shown in Fig. 7c. The hollow NiO microspheres showed a gradual increase in the capacity up to 80 cycles. The initial increase in the capacity was due to their pulverization with large NiO crystals, which resulted in the generation of a fresh metal surface in every cathodic process and the formation of a continuous reversible SEI layer [46, 54, 55]. However, the microspheres showed a drastic decrease in the capacity to 312 mAh g−1 after 250 cycles because of the collapse of their structure by large volume changes during the repeated cycles. In contrast, both the CYS-NiO/C and CYS-Ni/NiO/C microspheres exhibited excellent cycling performances even at the high current density of 1.0 A g−1. The CYS-NiO/C microspheres showed a higher specific capacity than that of the CYS-Ni/NiO/C microspheres. This is because the CYS-Ni/NiO/C microspheres are composed of metallic Ni with inactivity for the LIB reaction and a relatively larger amount of C content with a low discharge capacity in the structure [52, 53, 56]. The CYS-NiO/C, CYS-Ni/NiO/C, and hollow NiO microspheres delivered reversible specific discharge capacities of 991, 430, and 191 mAh g−1 after 500 cycles, respectively. The CYS-NiO/C microspheres maintained a steady CE of more than 99.3%. Because of the C-surrounded NiO crystals, interconnected mesopores in the core, and hollow space between the yolk and the shell, the CYS-NiO/C microspheres effectively accommodated the volume expansion induced by the repeated lithiation/delithiation of Li+ ions and showed high cycling stability. Moreover, the fast Li+ ion and electron diffusion in these microspheres resulted in a superior rate performance, as shown in Fig. 7d. The final discharge capacities of the CYS-NiO/C microspheres at the current densities of 0.5, 1.5, 3.0, 5.0, 7.0, and 10.0 A g−1 were 753, 648, 560, 490, 440, and 389 mAh g−1, respectively. The coral-like yolk with numerous interconnected mesopores provided easy electrolyte accessibility to the electrode, thus providing a short diffusion length for Li+ ions, which resulted in an excellent rate performance. When the current density was reduced to 0.5 A g−1 again, the discharge capacity of the CYS-NiO/C microspheres recovered well to 737 mAh g−1, indicating that their Li+-ion storage performance was not degraded even at high current densities. Since the capacity of the hollow NiO microspheres increased gradually up to 80 cycles (Fig. S6), they showed a higher capacity than the CYS-NiO/C microspheres at the same current density. This can be attributed to the pulverization of the hollow NiO microspheres with large NiO crystals, which resulted in the generation of a fresh metal surface in every cathodic process and the formation of a reversible SEI layer continuously up to 80 cycles.
The long-term cycling performance and CE of the CYS-NiO/C microspheres at the high current density of 2.0 A g−1 are shown in Fig. 7e. The discharge capacities at the 2nd and 1000th cycles were 699 and 635 mAh g−1, respectively, and the capacity retention calculated from the second cycle was 91%. The CE of the CYS-NiO/C microspheres reached 99.1% after the 15th cycle and remained constant during the subsequent cycles. The CYS-NiO/C microspheres showed the best reversible capacities at high current densities and long-term cycling performance as compared to the other NiO materials and their carbon hybrids with various morphologies reported previously (Table S1). This can be attributed to the synergetic effect of the coral-like yolk–shell structure with well-defined interconnected mesopores and conductive carbon in these microspheres.
In this study, coral-like yolk–shell-structured metal oxide/carbon composite microspheres were prepared using spray pyrolysis for the first time. During the spray pyrolysis, PVP in the droplet partially phase-separated from the PS colloidal solution and migrated outward, and interconnected mesopores were formed by the decomposition of PS. The subsequent thermal contraction of the inner part of the composites at high reaction temperatures during the spray pyrolysis resulted in the formation of unique CYS-NiO/C microspheres. The CYS-NiO/C microspheres exhibited excellent electrochemical properties for Li+-ion storage because of their high structural stability, shortened Li+-ion diffusion paths, high electrical conductivity, and easy penetration of the electrolyte into the yolk during the repeated Li+ lithiation/delithiation processes. We believe that this novel strategy can be used for designing and synthesizing unique coral-like yolk–shell-structured metal oxide/carbon composites for a wide range of applications such as catalysis, gas sensors, and hydrogen evolution reactions, and energy storage.
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) (NRF-2018R1A4A1024691, NRF-2017M1A2A2087577, and NRF-2018R1D1A3B07042514).
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