Synthesis of Mn3O4-Based Aerogels and Their Lithium-Storage Abilities
© Tang et al. 2015
Received: 17 March 2015
Accepted: 28 May 2015
Published: 10 June 2015
Mn3O4 aerogels and their graphene nanosheet (GN) composite aerogels were synthesized by a simple supercritical-ethanol process. In the process, supercritical ethanol acted as a reductant to reduce graphene oxide and MnO2 gels simultaneously. The synthesized aerogels consisted of 10–20 nm Mn3O4 nanocrystallites, with BET-specific surface areas around 60 m2/g. The performance of the aerogels as anode materials for lithium-ion batteries was also evaluated in this study. The results showed that Mn3O4 aerogels as anode materials exhibited a reversible capacity of 498.7 mAh/g after 60 charge/discharge cycles while the reversible capacity for Mn3O4/GN composite aerogels could further increase to 665 mAh/g. The mechanisms for the enhanced capacity retention could be attributed to their porous structures and improved electronic contact with GN addition. The process should also offer an effective and facile method to fabricate many other porous metal oxide/GN nanocomposites for low-cost, high-capacity, environmentally benign material for lithium-ion batteries.
Nanostructured materials such as nanowires, nanotubes, nanosheets, and porous nanomaterials have attracted great interest in the recent years because of the novel properties from their reduced dimensionality. They are becoming increasingly important for electrochemical energy-storage applications, especially for lithium-ion batteries [1–5]. The advantages of using nanostructured materials as electrodes for lithium-ion batteries include short path lengths for electron and Li-ion diffusion for high power, and accommodation of the strain due to Li intercalation/extraction for good cycle life, and so on [6–8]. Aerogels are well-known nanostructured materials with very high surface areas. They have a three-dimensional network of nanosized particles surrounded by macro-, meso-, and micropores. The diffusion distances for lithium ions and electrons in the nanosized aerogel particles are expected to be shorter compared with that of the solid electrode materials. Electrolytes can also easily infiltrate the entire aerogel particles. Porous structures may accommodate the strain induced by the volume expansion during the lithiation process. The above characteristics could enhance the performance (i.e., cyclability and rate capability) of corresponding materials such as the electrodes for lithium-ion batteries. For example, V2O5 aerogels were reported to have electroactive capacities greater than polycrystalline-solid V2O5 powders . Thereafter, other aerogels, including MnO2 and LixMnO2 as cathode materials for lithium-ion batteries, have also been investigated [10–12]. However, to our knowledge, the successful synthesis of Mn3O4 aerogels has not been reported and their lithium-storage abilities have not been studied so far. Mn3O4 is an attractive anode material for lithium-ion batteries due to high abundance of Mn element in natural resource, low cost, and environmental benignity. Its theoretical capacity can reach as high as ~936 mAh/g. However, previous studies reported that Mn3O4 showed poor performance as anode materials for lithium batteries. At a current density of 40 mA/g, the pure solid Mn3O4 powder showed a capacity lower than 300 mAh/g, which further decreased to ~200 mAh/g after ten cycles. Even for Co-doping Mn3O4, the first discharge capacity of ~900 mAh/g could be reached; however, it also further decreased to ~400 mAh/g after ten cycles . Mn3O4 nanoparticles even showed worse performance. After 10 cycles, only a capacity of ~115 mAh/g was retained. Only recently, have the capacity and rate capability of Mn3O4 been greatly improved by wiring up the Mn3O4 nanoparticles through a two-step solution-phase method .
In this paper, we have synthesized Mn3O4 aerogels and its graphene composite aerogels for the first time by a straightforward method, a supercritical-ethanol process. The results indicate that the supercritical-ethanol process can not only serve as a drying method to obtain the porous structure of aerogels but also reduce high-valence manganese oxide and graphene oxide (GO) simultaneously. The lithium-storage abilities of Mn3O4 aerogels were also investigated. The anode based on pure Mn3O4 aerogels exhibited an initial capacity of 1274.3 mAh/g. After 60 discharge/charge cycles, the capacity of 498.7 mAh/g was retained while the capacity increased to 665 mAh/g when graphene nanosheets (GNs) were incorporated into aerogel structure to improve their electronic contact.
Synthesis of GO
GO was prepared by a modified Hummers method [15, 16]. Briefly, 1 g-powdered flake graphite (500 mesh) and 0.75 g of NaNO3 were placed in a flask. 75 mL of H2SO4 (98 wt %) was then added with mechanical stirring in an ice-water bath. After 10 min, 4.5 g of KMnO4 was added gradually in the flask in 1 h. After the mixture was stirred vigorously for 5 days at room temperature, 3 mL of H2O2 (30 wt % aqueous solution) was added, and the mixture was stirred for 2 h at room temperature. The mixture was washed thoroughly with a mixed aqueous solution of 3 wt % H2SO4/0.5 wt % H2O2 to remove the excess manganate and the sulfate. Then, the solution was subjected to dialysis for 3–4 days to completely remove residual metal ions and acids. A typical AFM of GO is shown in Additional file 1: Figure S1.
Synthesis of Mn3O4-Based Aerogels
MnO2 wet gels were prepared by the previously reported method . Briefly, 0.948 g of KMnO4 was dissolved in 12.5-ml de-ion water. 0.232 fumaric acid was then added into the vigorously stirred KMnO4 solution. The resulting brown sol was then poured into polypropylene molds for gelation and aging for 24 h. The gel then was rinsed in 1-M sulfuric acid, followed by multiple rinses with water to remove impurity. MnO2/GO composite was prepared by adding GO into MnO2 sol first. The rest of the gelation and aging procedures were the same. Wet gels were then rinsed with ethanol for several times. The rinsed MnO2 gels and its graphene nanosheet (GN) hybrid were subjected to a supercritical-ethanol process at 260 °C at 10 MPa for 6 h to form Mn3O4 aerogels and Mn3O4/GN composite aerogels.
X-ray diffraction (XRD) patterns were obtained by an X-ray diffractometer (Rigaku D/Max-RB) with high intensity Cu Kα radiation (λ = 1.5418 Å, 40 kV, 100 mA).The morphology of samples was observed by a scanning electron microscopy (SEM, JSM6700F). Transmission electron microscopy (TEM) was conducted with a JEOL JEM-2010 electron microscope operating at 200 keV. The GN content in the Mn3O4/GN composite was determined by a thermogravimeter (TG, SDT Q600), and the measurements were carried out in air over a temperature range of 30–500 °C with a ramp rate of 10 °C /min. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a RBD-upgraded PHI-5000C ESCA system (Perkin Elmer) with Al Kα radiation (1486.6 eV). XPS Peak Version 4.1 software was used to perform curve fitting. The N2 absorption-desorption analysis was conducted on an Autosorb-1 instrument. The distribution of pore size was calculated from the desorption data using the Barret-Joyner-Halenda (BJH) method.
The powder of Mn3O4 aerogels and Mn3O4/GN composite aerogels as active materials, Super P carbon black and polyvinyldifluoride (weight ratio 80:10:10), were mixed in N-Methylpyrrolidone (NMP) solvent to produce an electrode slurry. The slurry was coated onto a copper foil using the doctor-blading method and then dried to form the working electrode. The electrochemical tests were performed using two-electrode coin-type cells with lithium as both the counter and reference electrode. 1 M of LiPF6 in a 1:1:1 (volume ratio) mixture of ethylene carbonate and diethyl carbonate and dimethyl carbonate was used as the electrolyte. Cell assembling was carried out in an argon-filled glove box. Galvanostatic charge-discharge cycling was conducted using a battery tester (Land, CT2100A) with a voltage window of 3–0.01 V at the current density of 50 mA/g.
Results and Discussion
Synthesis and Characterization of Mn3O4 and Mn3O4/GN Composite Aerogels
Summary of N2 adsorption/desorption data for Mn3O4 aerogels and Mn3O4/GN composite aerogels
Specific surface area
Average pore diameter
Electrochemical Properties of Anodes Based on Mn3O4 Aerogels and Mn3O4/GN Composite Aerogels
Mn3O4 aerogels and its GN composite aerogels were synthesized by the simple supercritical-ethanol process on MnO2 and MnO2/GO gels, respectively. Supercritical ethanol served as both drying and reducing agents of MnO2 and GO in the process. The samples were characterized by X-ray, SEM, XPS, and TEM. The electrochemical measurements demonstrated that Mn3O4 aerogels possessed better capacity retention compared with Mn3O4 nanoparticles. The enhanced performance of the synthesized Mn3O4 was ascribed to its porous aerogel structure. With GN addition to Mn3O4 aerogels, their electrochemical property was improved further. The higher capacity retention was attributed to the fact that GNs enhanced the electron transfer and reduced the resistance within Mn3O4. The process should also offer an effective and convenient technique to fabricate many other metal oxide/graphene porous nanocomposites for low-cost, high-capacity, environmentally benign material for lithium-ion batteries.
This work was supported by the Scientific Research Foundation for Returned Scholars, the Ministry of Education of China, Key Basic Research Projects of Science and Technology Commission of Shanghai (no. 11JC1412900), and the National Science Foundation of China program (no. 21271140, 51472182). Thank Dr. Suyang Yu for help on the experiment.
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