Synthesis of Sea Urchin-Like NiCo2O4 via Charge-Driven Self-Assembly Strategy for High-Performance Lithium-Ion Batteries
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In this study, hydrothermal synthesis of sea urchin-like NiCo2O4 was successfully demonstrated by a versatile charge-driven self-assembly strategy using positively charged poly(diallydimethylammonium chloride) (PDDA) molecules. Physical characterizations implied that sea urchin-like microspheres of ~ 2.5 μm in size were formed by self-assembly of numerous nanoneedles with a typical dimension of ~ 100 nm in diameter. Electrochemical performance study confirmed that sea urchin-like NiCo2O4 exhibited high reversible capacity of 663 mAh g−1 after 100 cycles at current density of 100 mA g−1. Rate capability indicated that average capacities of 1085, 1048, 926, 642, 261, and 86 mAh g−1 could be achieved at 100, 200, 500, 1000, 2000, and 3000 mA g−1, respectively. The excellent electrochemical performances were ascribed to the unique micro/nanostructure of sea urchin-like NiCo2O4, tailored by positively charged PDDA molecules. The proposed strategy has great potentials in the development of binary transition metal oxides with micro/nanostructures for electrochemical energy storage applications.
KeywordsHydrothermal synthesis NiCo2O4 Self-assembly Lithium-ion batteries
Differential scanning calorimetry
Electrochemical impedance spectra
Field emission scanning electron microscope
Transition metal oxides
X-ray photoelectron spectroscopy
Spinel nickel cobaltite (NiCo2O4) is one of the most important binary transition metal oxides (TMOs) with wide applications in electro-catalytic water splitting, supercapacitors and rechargeable battery materials, etc. [1, 2, 3, 4, 5, 6, 7]. Particularly, spinel NiCo2O4, having a theoretical specific capacity (890 mAh g−1), can be used as promising high-capacity anode materials for electrochemical lithium storage, owing to the higher electrical conductivity and electrochemical activities than monometallic oxides (Co3O4 and NiO) [8, 9]. However, lithium storage performance of NiCo2O4 was highly dependent on the distinct structure and morphology, which showed significant effects on cycling stability and rate capability.
In recent years, various NiCo2O4 with interesting morphologies, including nanowires , nanosheets , nanoflakes , nanobelts , sea urchin-like , and flower-like structures , have been synthesized by hydrothermal and solvothermal method. Previous studies suggested that micro/nanostructures manifested dual benefits from microscale and nanoscale dimensions for improved electron and ion transport, thereby leading to superior electrochemical performances [15, 16]. Generally, structure design of NiCo2O4 with micro/nanostructures was directed by choosing appropriate morphology controlling reagents. Zhang et al. employed polyvinylpyrrolidone (PVP) to synthesize NiCo2O4 for controlling morphology, based on coordination of metal ions with functional groups (e.g., -N and/or C=O) of pyrrolidone . However, limited effective structure directing reagents are feasible for synthesis of binary TMOs with unique morphology. Thus, it is highly desirable to explore versatile reagents for synthesizing NiCo2O4 with micro/nanostructures. Recently, we reported positively charged reagents, such as diallyldimethylammonium chloride (DDA) and its homopolymer, exhibited potentials in synthesizing Co3O4 for lithium-ion batteries (LIBs) [15, 16]. However, we are not aware of any binary TMOs (e.g., NiCo2O4) with micro/nanostructures synthesized by such charged molecules for electrochemical lithium storage applications.
Herein, we reported charge-driven self-assembly strategy for NiCo2O4 with sea urchin-like structure, followed by thermal treatment. The positively charged poly(diallydimethylammonium chloride) (PDDA) molecules were considered as a crucial structure directing reagent in hydrothermal synthesis. Sea urchin-like NiCo2O4 with micro/nanostructures also demonstrated superior lithium storage performance in repeated charge-discharge cycles. Obviously, it is the first work on charge-driven self-assembly synthesis of binary TMOs with assistance of charged organic molecules. This novel strategy is expected to pave a new way of synthesizing binary TMOs with novel micro/nanostructures for energy storage materials.
Synthesis of Sea Urchin-Like NiCo2O4
In a typical synthesis, 0.5 g of nickel acetate tetrahydrate (≥ 99%), 1.0 g of cobalt acetate tetrahydrate (≥ 98%), and 3.0 g of urea (99.5%) received from Acros Organics were dissolved in 55 mL deionized water, followed by adding 5 g PDDA solution (20 wt.% in H2O, Sigma-Aldrich). The mixed solution was carefully transferred into a sealed Teflon-lined stainless steel autoclave and placed in an electric oven maintained at 120 °C for 2 h. The resulting precipitation was collected by vacuum-assisted filtration and washed with deionized water for three times. Finally, the filtered sample was thermal treated in a muffle furnace at 450 °C for 2 h. The as-synthesized black samples were directly used in material characterizations and electrochemical performance evaluation.
Material Characterizations and Electrochemical Performance Evaluation
Crystal phases, material morphologies, microstructures, and valence states of the as-prepared samples were characterized by powder X-ray diffractometer (XRD, Philips PW1830), field emission scanning electron microscope (FE-SEM, Hitachi S4800), transmission electron microscope (TEM, FEI Tecnai G2 20 scanning), and X-ray photoelectron spectroscopy (XPS, Model PHI5600), respectively. Thermal conversion study of precursors was conducted on thermogravimetric analysis (TGA, Mettler Toledo) and differential scanning calorimetry (DSC, Mettler Toledo) under oxygen atmosphere. In addition, specific surface area and pore size distributions of NiCo2O4 were performed on a surface area analyzer (Quantachrome Instruments) by N2 adsorption-desorption isotherms at 77 K. The specific surface area and pore size distribution were obtained by multi-point Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) method, respectively. Electrochemical lithium storage performance and rate capability were evaluated in CR2025 coin-type cell with NiCo2O4 as working electrode, lithium metal as counter electrode, microporous membrane (Celgard® 2400) as separator, and 1 M LiPF6 in 50 vol.% ethylene carbonate and 50 vol.% dimethyl carbonate as electrolyte. The working electrode was composed of 80% active NiCo2O4 materials, 10% PVdF binder, and 10% SuperP conductive carbon. Cyclic voltammetry (CV) analysis was measured in the voltage range of 0.005–3 V vs. Li+/Li and electrochemical impedance spectra (EIS) of sea urchin-like NiCo2O4 anodes were also recorded on electrochemical station (CorrTest® Instruments) in the frequency range of 100 kHz to 0.01 Hz with an amplitude of 5 mV. Galvanostatic charge-discharge test was conducted on a battery testing system (LAND CT2001A) at room temperature. The cycling performance was conducted at a current density of 100 mA g−1 for 100 cycles and rate capability test was performed with various current densities ranging from 100 mA g−1 to 3000 mA g−1.
Results and Discussion
Note that fluctuation of coulombic efficiency was also observed in the C-rate measurement, particularly at the changing points of current densities. For instance, when the current density was switched from 1000 to 2000 mA g−1, coulombic efficiency of the 40th cycle was suddenly declined from 100 to about 80%. In the following 9 cycles, coulombic efficiency was immediately stabilized at about 100%. The sudden drop of coulombic efficiency might be related to the partial loss of electrical connectivity between NiCo2O4 materials and conductive network by volume variation in the charging process, due to the applied high current density. Similar phenomena were also reported in previous C-rate studies on anode materials for rechargeable batteries [27, 28].
In this study, the improved performance of NiCo2O4 should be attributed to the micro/nanostructures of sea urchin-like morphology, compared to previous work on nanostructures (e.g., mesoporous nanowires). Basically, the lithium storage performance was associated with efficient transport of lithium ions and electrons in electrochemical charge-discharge cycles. The numerous nanoneedle, viewed as the building unit of sea urchin-like structure, could greatly improve solid-state Li+ diffusion behaviors, due to the shortened nanoscale length. In addition, the uniform microspheres, regarded as the secondary particles of sea urchin-like structure, could significantly enhance electron transport behaviors, owing to long-range electron transport network. The combined benefits of micro/nanostructures in sea urchin-like structure could result in better electrochemical performance than nanostructures. Overall, the superior electrochemical performance of NiCo2O4 was ascribed to the unique physical properties of sea urchin-like structure, which were tailored by PDDA-assisted charge-driven self-assembly strategy. This proposed strategy has potential in facile synthesis of energy storage materials for next generation LIBs.
In conclusion, sea urchin-like NiCo2O4 were successfully synthesized by charge-driven self-assembly strategy with positively charged PDDA, followed by thermal treatment. The charged molecules play a pivotal role in the formation of sea urchin-like structure, due to electrostatic adsorption and steric hindrance. Also, sea urchin-like NiCo2O4 demonstrated great potentials in electrochemical lithium storage. The superior performance was ascribed to the unique sea urchin-like structure of NiCo2O4 for enhanced electron and ion transport. Overall, charge-driven self-assembly strategy is an appealing route to synthesize energy storage materials for high-performance lithium-ion batteries.
This research was financially supported by Hong Kong Competitive Research Funding for Faculty Development Scheme (No.: UGC/FDS25/E07/16). Also, support from Hong Kong Environment and Conservation Fund (No.: 22/2016) and National Natural Science Foundation of China (No.: 51708358) were gratefully acknowledged.
Availability of Data and Materials
All data generated or analyzed during this study are included in this published article.
XYL conceived the idea and designed the experiments. BW, CWT, KHL, and XYL performed material synthesis, electrochemical performance evaluation, and analyzed the data. YYT and YPM supported in the material characterizations. BW, KHL, CWT, and XYL contributed in the manuscript preparation. XYL supervised this work and finalized the manuscript. All authors discussed the results and approved the final manuscript.
The authors declare that they have no competing interests.
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