Microwave-Assisted Synthesis of NiCo2O4 Double-Shelled Hollow Spheres for High-Performance Sodium Ion Batteries
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The ternary transitional metal oxide NiCo2O4 is a promising anode material for sodium ion batteries due to its high theoretical capacity and superior electrical conductivity. However, its sodium storage capability is severely limited by the sluggish sodiation/desodiation reaction kinetics. Herein, NiCo2O4 double-shelled hollow spheres were synthesized via a microwave-assisted, fast solvothermal synthetic procedure in a mixture of isopropanol and glycerol, followed by annealing. Isopropanol played a vital role in the precipitation of nickel and cobalt, and the shrinkage of the glycerol quasi-emulsion under heat treatment was responsible for the formation of the double-shelled nanostructure. The as-synthesized product was tested as an anode material in a sodium ion battery, was found to exhibit a high reversible specific capacity of 511 mAh g−1 at 100 mA g−1, and deliver high capacity retention after 100 cycles.
KeywordsNiCo2O4 Double-shelled hollow sphere Microwave Sodium ion battery
NiCo2O4 double-shelled hollow spheres were successfully synthesized via a rapid microwave-assisted solvothermal method in isopropanol with the aid of glycerol.
The roles of isopropanol, nitrate, glycerol, and the heating rate in the formation of the double shelled hollow spheres were systematically studied.
The as-synthesized NiCo2O4 double shelled hollow spheres showed good sodium storage performance with reversible specific capacity of 511 mAh g−1 at 100 mA g−1.
Presently, due to increasing energy consumption, there is an increasing demand for energy storage materials. Lithium ion batteries (LIBs) offer high energy storage density, long cycling life, and excellent safety properties, thus dominating the market for portable electronic device power sources . However, the depletion of lithium resources and the consequent high cost of lithium hinder the application of LIBs in several emerging areas, such as large-scale grid energy storage . Sodium, another Group I element, is much more abundant and has a much lower cost. As such, sodium ion batteries (SIBs), which have a charging/discharging mechanism similar to that of LIBs, are promising energy storage devices for the future and have received great research attention in the past few years . Nevertheless, the energy storage performance of SIBs is significantly limited by a lack of suitable electrode materials. For example, while graphite is used as the anode material in most commercial LIBs, it is nearly electrochemically inactive with sodium due to the large ionic radius of Na+ . Although many other carbonaceous materials have been intensively investigated as anode materials for SIBs, their sodium storage capabilities are too low to meet the demands of practical applications.
Transitional metal oxides have been widely investigated as substitutes for carbonaceous anode materials in LIBs . In particular, ternary transition metal oxides such as NiCo2O4 are extremely attractive, due to their high theoretical storage capacities (e.g., 890 mAh g−1 for NiCo2O4 compared to 372 mAh g−1 for graphite) and superior electrical conductivity (2 orders higher than that of single-component cobalt or nickel oxides) . Theoretically, NiCo2O4 has equivalent storage capacities for both sodium and lithium. Recently, some work has been reported on the successful application of NiCo2O4 as an anode material for SIBs [7, 8]. However, due to the sluggish sodiation/desodiation reaction kinetics, as well as the large volume change during the charging/discharging process induced by the large ionic radius of Na+, the reported NiCo2O4 materials exhibit greatly inferior capacities for sodium storage. In order to increase the practical sodium storage capacity of this material, a new strategy to engineer robust nanostructured NiCo2O4 is urgently needed. One attractive avenue amongst the various approaches is the use of hollow multi-shelled spheres, due to their unique structural features [9, 10, 11, 12, 13, 14, 15, 16, 17].
Recently, microwave-assisted nanotechnology has attracted a great deal of research interest, due to the interest in green chemistry in both academia and industry. Microwaves heat the reactants directly via dielectric loss, rather than by heat convection as in the conventional heating method. This unique heating mechanism allows the use of microwaves to greatly enhance the fabrication rate of nanomaterials, thus saving both time and energy. Additionally, nanomaterials synthesized via microwave heating have been widely reported to exhibit excellent performance due to the formation of better dispersed nanoparticles with more uniform size distributions .
In this work, we developed a synthesis method for NiCo2O4 double-shelled hollow spheres, using a fast microwave-assisted solvothermal treatment followed by annealing. Double-shelled hollow nanostructures are beneficial in facilitating a high specific surface area to expose more active materials for reaction, as well as in buffering the volume change during the charging/discharging process. The macropores in the hollow structure can act as a Na+ transport system, shortening pathways for the diffusion of Na+, thus leading to faster reaction kinetics in hollow nanostructures. The as-synthesized product was further tested as an anode material in SIBs and showed excellent sodium storage capability.
All chemical materials were purchased from Aladdin Chemical Corporation and were of analytical grade. The materials were used without further purification.
3.2 Synthesis of NiCo2O4 Double-Shelled Hollow Spheres
In a typical procedure, 1 mmol of Co(NO3)2 and 0.5 mmol of Ni(NO3)2 were dissolved in 80 mL of isopropanol, to which 16 mL of glycerol was added. The mixture was stirred vigorously for 30 min. Subsequently, 60 mL of the mixture was pipetted into a 100-mL vessel and exposed to microwave solvothermal treatment in a microwave hydrothermal reactor (Xianghu, Beijing) at 180 °C for 30 min. To avoid any abnormal increases in pressure due to hot spots during the microwave heating, a ramping procedure was used to raise the temperature from room temperature to 180 °C over 20 min. Finally, the precipitate was collected, washed with ethanol and DI water, dried, and annealed in air at 350 °C for 2 h with a temperature ramping rate of 1 °C min−1.
3.3 Characterization of Materials
The crystal structure of the as-prepared samples was examined using X-ray diffraction (XRD, DX-2700, Cu Kα radiation, λ = 1.542 Å). The morphologies and the structural characterization of the products were observed using field emission scanning electron microscopy (FESEM, JEOL, JSM-7500F) and transmission electron microscopy (TEM, Zeiss, Libra200). N2 adsorption–desorption measurements were carried out using a NOVA1000e analyzer at 77 K. The pore size distributions of the samples were analyzed using the Barrett Joyner Halenda (BJH) method.
3.4 Electrochemical Measurements
The electrochemical performance of the material was evaluated using CR2032-type coin cells, which were assembled in a glove box filled with highly pure argon gas, with water and oxygen contents of less than 1 ppm. For the fabrication of the working electrode, the NiCo2O4 material, acetylene black, and the binder polyvinylidene fluoride (PVDF) were mixed in N-methyl-2-pyrrolidinone to form a homogeneous slurry with a weight ratio of 8:1:1, which was then coated on copper foil and finally dried for 12 h in a vacuum oven. The copper foil was cut into rounds with a diameter of 14 mm. The average loading mass of the active material was about 1 mg cm−2. Pure sodium foil was used as the counter electrode, and Whatman glass fiber (GF/C) was used as the separator. The electrolyte was 1 M NaClO4 in a mixture of ethylene carbonate and propylene carbonate (1:1, w/w). Galvanostatic charge/discharge tests were conducted using a Neware battery measurement system in the voltage range of 0.01–3.0 V (vs. Na+/Na). Cyclic voltammetry (CV) measurements were performed using a CHI 660E electrochemical workstation.
4 Results and Discussion
From the N2 adsorption/desorption isotherm curve (shown in Fig. S2), the surface area of the as-synthesized double-shelled hollow product was determined to be 30.7 m2 g−1, with the pore size distribution centered around 6.9 nm. This porous hollow structure is beneficial, both because the high surface area exposes more active materials for reaction during the charging/discharging process and because it could help to buffer the volume change during the sodiation/desodiation process. Thus, this morphology is vital to improve the storage capacity and cycling stability of the electrode.
5 Electrochemical Performance
In summary, a microwave-assisted fast solvothermal synthetic procedure for NiCo2O4 double-shelled hollow spheres in the presence of isopropanol and glycerol was developed. Both isopropanol and glycerol played a vital role in the synthetic procedure. The as-synthesized product exhibited excellent sodium storage performance when tested as an anode material in SIBs.
The work was financially supported by the Science Foundation of Sichuan Province (Grant No. 2016FZ0070) and the Natural Science Foundation of China (NSFC, 201476145). The authors also appreciate the technical support for Materials Characterization from The Analytical and Testing Center of Sichuan University.
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