Synthesis of Mn3O4-Based Aerogels and Their Lithium-Storage Abilities

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. Electronic supplementary material The online version of this article (doi:10.1186/s11671-015-0960-x) contains supplementary material, which is available to authorized users.

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, V 2 O 5 aerogels were reported to have electroactive capacities greater than polycrystalline-solid V 2 O 5 powders [9]. Thereafter, other aerogels, including MnO 2 and Li x MnO 2 as cathode materials for lithium-ion batteries, have also been investigated [10][11][12]. However, to our knowledge, the successful synthesis of Mn 3 O 4 aerogels has not been reported and their lithium-storage abilities have not been studied so far. Mn 3 O 4 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 Mn 3 O 4 showed poor performance as anode materials for lithium batteries. At a current density of 40 mA/g, the pure solid Mn 3 O 4 powder showed a capacity lower than 300 mAh/g, which further decreased to~200 mAh/g after ten cycles. Even for Co-doping Mn 3 O 4 , the first discharge capacity of~900 mAh/g could be reached; however, it also further decreased to~400 mAh/g after ten cycles [13]. Mn 3 O 4 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 Mn 3 O 4 been greatly improved by wiring up the Mn 3 O 4 nanoparticles through a two-step solution-phase method [14].
In this paper, we have synthesized Mn 3 O 4 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 Mn 3 O 4 aerogels were also investigated. The anode based on pure Mn 3 O 4 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 NaNO 3 were placed in a flask. 75 mL of H 2 SO 4 (98 wt %) was then added with mechanical stirring in an icewater bath. After 10 min, 4.5 g of KMnO 4 was added gradually in the flask in 1 h. After the mixture was stirred vigorously for 5 days at room temperature, 3 mL of H 2 O 2 (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 % H 2 SO 4 /0.5 wt % H 2 O 2 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 Mn 3 O 4 -Based Aerogels
MnO 2 wet gels were prepared by the previously reported method [17]. Briefly, 0.948 g of KMnO 4 was dissolved in 12.5-ml de-ion water. 0.232 fumaric acid was then added into the vigorously stirred KMnO 4 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. MnO 2 /GO composite was prepared by adding GO into MnO 2 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 MnO 2 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 Mn 3

O 4 aerogels and Mn 3 O 4 /GN composite aerogels.
Characterization 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 Mn 3 O 4 /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 N 2 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.

Electrochemical Measurement
The powder of Mn 3 O 4 aerogels and Mn 3 O 4 /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 LiPF 6 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.

Synthesis and Characterization of Mn 3 O 4 and Mn 3 O 4 /GN Composite Aerogels
The process for preparing Mn 3 O 4 and Mn 3 O 4 /GN aerogels is illustrated schematically in Fig. 1. MnO 2 gels were prepared by adding fumaric acid to a stirred KMnO 4 solution, following the reaction below [18,19]: as synthesized monoliths of Mn 3 O 4 gels and Mn 3 O 4 /GN composite gels after the supercritical-ethanol process are brown and gray, respectively (Additional file 1: Figure S2). Figure 2a shows the XRD pattern of MnO 2 dried gel sample. The broad-peak profile indicates its amorphous nature. Broad peaks at 2θ = 36.6°, 37.5°and 65.8°can be indexed to α-MnO 2 (JCPDS No. 44-0141). After the supercritical-ethanol process (260°C, 10 MPa), plenty of sharp peaks appear (Fig. 2b and c)  At the same time, alcohols were reported as effective reductants to reduce GO into highly conductive GN [22,23]. Thus, supercritical ethanol under high temperature and high pressure was expected to have improved reducibility to reduce GO pre-mixed in MnO 2 gels into GN. Here, XPS (Fig. 3) was used to analyze GO powder and GO in composite after the supercritical-ethanol process. Curve fitting of the spectra was performed using Gaussian-Lorentzian peak shape after a Shirley background correction. For GO, three peaks located at 284.6, 286.8, and 288.0 eV could be assigned to the C-C/C = C, C-O, and C = O species, respectively [24,25]. The peak at 286.8 eV after supercritical-ethanol treatment almost disappeared, indicating that the C-O species were removed by the supercritical-ethanol process. Hydrogenating capacity of ethanol under the supercritical condition was responsible for the partial elimination of oxygen-containing functional groups on GO. The GN content in the Mn 3 O 4 /GN nanocomposite was quantitatively determined to be 7.3 wt % by thermogravimetric analysis (Additional file 1: Figure S3). Figure 4 shows SEM images of the Mn 3 O 4 aerogels and Mn 3 O 4 /GN composite aerogels. Samples   Table 1. Nitrogen adsorption/desorption isotherms (Additional file 1: Figure S4) of the Mn 3 O 4 aerogels and Mn 3 O 4 /GN composite aerogels are type IV isotherms with H1 hysteresis loops, which are characteristic of an interconnected mesoporous system with cylindrical pores. BET-specific surface areas are 69 and 67 m 2 /g, respectively. The BET-specific areas are not as high as the reported value (~200 m 2 /g) of MnO 2 aerogels [17], which might be due to crystallization and

Electrochemical Properties of Anodes Based on Mn 3 O 4 Aerogels and Mn 3 O 4 /GN Composite Aerogels
To evaluate the electrochemical performance of Mn 3 O 4based aerogels, the samples were used as the anodes for Li-ion battery. Figure 6a,b show the charge and discharge curves for anodes based on Mn 3 O 4 and Mn 3 O 4 / GN aerogels, respectively. In the first discharge curve, a sloping plateau in the range of~1.5-0.3 V is observed. It might be due to the formation of solid-electrolyte interface (SEI) film on the active materials. The welldefined voltage plateau around 0.35 V reflected the reduction reaction of Mn 3 O 4 as follows: Mn 3 O 4 + 8Li + + 8e − → 3Mn(0) + 4Li 2 O [26]. The charge curve shows a plateau at~1.2 V due to the reverse reaction [14]. mAh/g after ten cycles [14]. The improved electrochemical performance of Mn 3 O 4 aerogels could be attributed to the structure of aerogels. The high porosity from mesopores of aerogel structures facilitates the transportation of lithium ions in the aerogel particle, and the large surface area of aerogel enhances the utilization of active materials [27]. Further improvement of electrochemical properties of Mn 3 O 4 /GN can be attributed to the wiring effect of  Fig. 6d. Both Nyquist plots consist of one semicircle at mediumfrequency region, which could be related to charge-transfer resistance [28].

Additional file
Additional file 1: The AFM image of GO, the photo of Mn 3 O 4 aerogel monoliths, and the thermogravimetric analyses and Nitrogen adsorption/desorption isotherms of Mn 3 O 4 aerogel. Figure S1. AFM image of GO, Figure S2. Photos of monoliths of Mn 3 O 4 -based aerogels, Figure S3. Thermogravimetric analyses of Mn 3 O 4 -based aerogels, Figure S4.