Hydrothermal-Assisted Sintering Strategy Towards Porous- and Hollow-Structured LiNb3O8 Anode Material
© The Author(s). 2017
Received: 8 April 2017
Accepted: 16 July 2017
Published: 25 July 2017
Porous- and hollow-structured LiNb3O8 anode material was prepared by a hydrothermal-assisted sintering strategy for the first time. The phase evolution was studied, and the formation mechanism of the porous and hollow structure was proposed. The formation of the unique structure can be attributed to the local existence of liquid phase because of the volatilization of Li element. As the anode material, the initial discharge capacity is 285.1 mAhg−1 at 0.1 C, the largest discharge capacity reported so far for LiNb3O8. Even after 50 cycles, the reversible capacity can still maintain 77.6 mAhg−1 at 0.1 C, about 2.5 times of that of LiNb3O8 samples prepared by traditional solid-state methods. The significant improvement of Li storage capacity can be attributed to the special porous and hollow structure, which provides a high density of active sites and short parallel channels for fast intercalation of Li+ ions through the surface.
In recent years, much attention has been paid to hollow and porous structures due to their widespread applications in catalysis, energy, environmental engineering, drug delivery, and sensor systems [1–4]. Compared with other new energy batteries, lithium-ion batteries (LIBs) have gained commercial success as the predominant power source for portable electronics and show great potential in large-scale applications because of its high energy density, long lifespan, and environmental benignity . To obtain high electrochemical performance, the electrodes of LIBs always have open structures, which can provide a high density of active sites and parallel channels for faster intercalation of Li+ ions through the surface . However, it is challenging to synthesize the nanomaterials with open structures including porous and hollow architectures.
To improve LIBs performances, people have been seeking for high-performance electrode materials, including anode and cathode materials. LiFePO4 , LiCoO2 , LiMn2O4 , LiVPO4F , and various hybrid materials [11, 12] have been seriously considered as candidates for cathode materials. For anode materials, many different materials have been investigated as alternatives to graphite-based anode materials, such as transition metal oxides (TMOs) , molybdenum disulfide (MoS2), and graphene-based hybrids . In recent literature, niobium has been shown to have superior electrochemical performance ; some traditional compounds doped with Nb element and novel Nb-based compounds are well developed [16–19]. Nb-based oxides have been considered as promising anode materials for LIBs with improved safety. Compared with Li4Ti5O12 (with a theoretical capacity of 175 mAhg−1), Nb-based oxides have a relatively high theoretical capacity of 389 mAhg−1. Also, it is notable that the two Nb redox couples, Nb5+/Nb4+ and Nb4+/Nb3+, can suppress the formation of solid electrolyte interface (SEI) film during cycling . LiNb3O8, a well-known material, always appear in the preparation process of LiNbO3 as an impurity phase due to Li volatilization . Jian et al. firstly introduced LiNb3O8 material prepared by a solid-state reaction as an anode for LIBs. It is found that the as-prepared LiNb3O8 sample ball-milled with acetylene black (LiNb3O8-BM) largely improved the initial discharge/charge capacities (351 and 212 mAhg−1) than those of the as-prepared LiNb3O8 sample (250 and 170 mAhg−1) at 0.05 C; after 50 cycles, the capacity reached 150 mAhg−1 for LiNb3O8-BM at 0.1 C, only 30 mAhg−1 for LiNb3O8 sample . Porous LiNb3O8 nanofibers also exhibited improved capacity and cyclability in virtue of the high surface area, small nanocrystals, and porous structure with the initial discharge capacity of 241.1 mAhg−1 at 0.1 C . Due to the difficulty to obtain pure phase, as a novel anode material with high theoretical capacity, LiNb3O8 has rarely been studied.
In this paper, porous- and hollow-structured LiNb3O8 anode material was successfully prepared by a hydrothermal-assisted sintering process. The phase evolution was studied, and the formation mechanism of the porous and hollow structure was proposed. The morphological and electrochemical properties of LiNb3O8 as the anode material were also studied in detail.
Preparation of Samples
LiNb3O8 powders were prepared by the hydrothermal-assisted sintering process. Lithium hydroxide monohydrate (LiOH·H2O, Aladdin, ACS, ≥98.0%) and niobium pentoxide (Nb2O5, Aladdin, AR, 99.9%) were purchased as raw materials without further purification. First, 3.5 mmol of Nb2O5 was dispersed into 35 ml of LiOH·H2O transparent aqueous solution (the mole ratio of Li:Nb = 8:1) with magnetic stirring for 1 h. Then, the suspension solution was put into a 50-ml Teflon-lined hydrothermal synthesis autoclave reactor. After that, the reactor was sealed and maintained at 260 °C for 24 h and then cooled down to room temperature naturally. Finally, the as-prepared products were centrifuged and rinsed with deionized water and ethanol. After drying in an oven at 60 °C for 12 h, the white Li-Nb-O powders were collected and calcined at various temperatures from 500 to 800 °C for 2 h with a ramp rate of 5 °C/min.
The thermal decomposition characteristic of Li-Nb-O powder was studied by a thermogravimetric and differential scanning calorimeter (TG/DSC, Netzsch STA 409 PC/PG) from room temperature to 1200 °C with a ramp rate of 10 °C/min under N2 atmosphere. The crystal structures of the calcined powders were analyzed using X-ray powder diffraction (XRD; Bruker D8 Discover) with Cu Kα radiation. The morphologies of the calcined powders were characterized by scanning electron microscopy (SEM; JSM-6700F). X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo-Fisher Escalab 250Xi instrument.
The LiNb3O8 electrodes were prepared by spreading slurry of LiNb3O8 powders, carbon black, and polyvinylidene fluoride (PVDF) with a weight ratio of 8:1:1 onto an aluminum foil. Afterward, the electrode was dried at 120 °C in a vacuum oven overnight. The anodes were punched into disks with a diameter of 16 mm. For the electrochemical measurements, CR2025 coin-type cells were assembled in an argon-filled glove box using lithium foil as the counter electrode and polypropylene microporous membrane (Celgard 2320) as a separator to isolate the two electrodes, and then, the 1.0-M LiPF6 electrolyte was dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 by volume). Galvanostatic charge-discharge tests of the cells were performed using a Land electric test system (Wuhan Land Electronics Co., Ltd., China) between 0 and 3 V (vs. Li/Li+) at different current densities of 0.1–1 C (1 C = 389 mAhg−1). Cyclic voltammetry (CV) curves were recorded on an electrochemical workstation (CHI604E, Shanghai Chenhua Instruments Co., Ltd., China) in the voltage range of 1–3 V.
Results and Discussion
At 700 °C, the monoclinic LiNb3O8 is the predominant phase with almost negligible impurity. The pure phase of LiNb3O8 is obtained at 800 °C with all the diffraction peaks indexed to the monoclinic phase (JCPDS card no. 36–0307), a space group of P21/a. Compared with the traditional solid-state method, the pure phase of LiNb3O8 is more easily obtained using the hydrothermal-assisted sintering process.
In summary, porous- and hollow-structured LiNb3O8 anode material was successfully prepared by the hydrothermal-assisted sintering strategy. The phase evolution was studied, and the formation mechanism of the porous and hollow structure was proposed. The formation of the unique structure can be attributed to the local existence of liquid phase because of the Li volatilization. As the anode material, the initial discharge capacity is 285.1 mAhg−1 at 0.1 C, the largest discharge capacity reported so far for LiNb3O8. After 50 cycles, the reversible capacity can still maintain 77.6 mAhg−1, about 2.5 times of that of the LiNb3O8 samples prepared by traditional solid-state methods. The significant improvement of Li storage capacity can be attributed to the special porous and hollow structure of LiNb3O8 powder, which provides a high density of active sites and short parallel channels for fast intercalation of Li+ ions through the surface.
This work is financially supported by the National Natural Science Foundation of China (No. 51202107) and Foundation of Henan Educational Committee (No. 16A140028).
HZ carried out the main part of the experimental work and XRD measurements. HLiu performed the XPS tests. HLi, LZ, and XZ participated in the preparation of the samples. CH performed the SEM images measurements. QL and JY carried out the electrochemical measurements and participated in the analysis of the data. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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