Mn3O4 /CNTs composite as anode materials for lithium-ion batteries

Transition metal oxides especially manganese oxides are being intensively studied as candidate anode materials for next generation lithium ion batteries in high efficiency energy storage applications. In this paper, Mn3O4 /CNTs composite is prepared via a facile one-step solvothermal method. The results of XRD and SEM showed that Mn3O4 uniformly coated on the surface of CNTs. It could deliver a reversible charge capacity of 809.9 mA h g at the current density of 40 mA g, and the specific discharge capacity slightly increased from 644.2 mA h g to 796.1 mA h g after 50 cycles at a current density of 160 mA g demonstrating excellent cycling stability.


Introduction
Poizot et al. [1,2] first reported that lithium can be stored reversibly in nanostructured transition-mental oxides (M x O y , M=Fe, Co, Ni, Cu, etc.) through heterogeneous conversion reaction: M x O y + 2Li + + 2ye -→ xM 0 + yLi 2 O. These nanocompounds have attracted great attention as anode materials, due to their high theoretical capacity, natural abundance and environmental benignity. The theoretical specific capacity of Mn 3 O 4 is about 937 mA h g -1 , which is nearly three times higher than that of graphite. But Mn 3 O 4 exhibits extremely low electrical conductivity (~10 -7 -10 -8 S cm -1 ), limiting its capacity, cycling stability and rate capability as anode material for LIBs. Some strategies have been proposed to improve its electrochemical performance. Co doped Mn 3 O 4 have better cycle stability than undoped Mn 3 O 4 , with a capacity is only 400 mA h g -1 at a current density of 33-55 mA g -1 [3]; Gao et al. [4] have prepared spongelike nanosized Mn 3 O 4 by precipitation method with a stable reversible capacity of 800 mA h g -1 after 40 cycles at a current rate of 0.25 C. In this paper, we have employed a facile and fast one-step solvothermal method to prepare Mn 3 O 4 /CNTs composite and evaluated its electrochemical performance as anode for lithium-ion batteries. We also used X-ray diffraction and scanning electron microscopy to characterize the structure and morphology of Mn 3 O 4 /CNTs composite.

Experimental
CNTs was first functionalized by sonicating with a concentrated solution of H 2 SO 4 /HNO 3 (3/1, volume ratio) in a water bath for 12 h to remove the impurities and to improve their dispersion. The so-obtained CNTs was dissolved in ethanol, followed by sonicating for 2 h. Then KMnO 4 was added into the as-prepared CNTs dispersion under vigorous magnetic stirring for 1 h at room temperature. The mixture was transferred into a Teflonlined stainless steel autoclave and was sealed into an oven with a heat treatment of 180 °C for 12 h, and then cooled down to ambient temperature. The black products were filtered for several times and dried at 80 °C for 12 h under vacuum. The material was characterized by a X'Pert Pro PANalytical X-ray diffractometer, using filtered Cu Kα radiation (λ=1.5406 Å). The general morphology and particle size of the synthesized products were investigated by an FEI Quanta 200 FEG field emission scanning electron microscopy (FESEM). The electrochemical experiments were performed using 2016 coin-type cells assembled in an argon filled glove box. The working electrodes were fabricated by using mixed slurry of as-prepared samples, carbon black, and polyvinylidene fluorine (PVDF) in a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) solvent. The resultant slurry was uniformly pasted onto Cu foil and dried at 120 °C overnight under vacuum. The electrolyte was 1.0 mol L -1 LiPF 6 in 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Pure lithium foil was used as counter electrode. The batteries were discharge/charged at constant currents on a CT2001A Land Battery Testing System to evaluate their electrochemical performance in the galvanostatic mode between 0.01 V and 3.0 V. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits distribution, and reproduction in any medium, provided the original work is properly cited.

XQUHVWULFWHG XVH
Article available at http://www.matec-conferences.org or http://dx.doi.org/10.1051/matecconf/20153101005 weak peak at 26.3° can be assigned to the hexagonal carbon in CNTs (JCPDS No. 01-075-1621), and no other impurity phases was observed in the synthesized products, indicating high purity and crystallinity of the as prepared product.

Fig. 1 X-ray diffraction pattern of Mn 3 O 4 /CNTs composite
To investigate the morphology of the products, field emission scanning electron microscope (FESEM) images were collected fo the Mn 3 O 4 /CNTs composite, as shown in Fig. 2. Fig. 2(a) presents the overall FESEM images of the as prepared Mn 3 O 4 /CNTs composite sample. It exhibits homogeneous nanotube architecture. At high magnification ( Fig. 2(b)), it can be easily seen that Mn 3 O 4 are uniformly coated on the surface of CNTs. The average diameter of the nanotube is about 50 nm. So the coating thickness of Mn 3 O 4 is about 10 nm, and Mn 3 O 4 particle that coated on CNTs is very small. . The initial specific discharge capacity extends to 1528.7 mA h g -1 , slightly higher than the theoretical capacity of 937 mA h g -1 for the conversion reaction to Mn and Li 2 O. This should be ascribed to the decomposition of the electrolyte at low voltage to form a SEI layer and further lithium storage via interfacial charging at Mn/Li 2 O interface [6].The discharge voltage plateau of Mn 3 O 4 /CNTs electrode has shifted to 0.5 V from the second cycle, which is higher than the first discharge, indicating that the lithium insertion reaction has become easier [7]. The first reversible charge capacity is 809.9 mA h g -1 , which is much higher than the capacity of commercial graphitic carbon. There is a significant irreversible capacity loss for the first cycle, which is common to almost all systems based on conversion reactions. The cycling performance under a constant current density of 160 mA g -1 with a potential window from 0.01V to 3.0V of Mn 3 O 4 /CNTs composites electrode is shown in Fig.4 prevent the aggregation of Mn 3 O 4 nanoparticles and the cracking of the electrode material upon continuous cycling. Secondly, CNTs can improve the conductivity of the electrode materials and stabilize the electrode structure during the charge/discharge process because of the intimate interaction between CNTs and Mn 3 O 4 nanoparticles directly grown on them.

Conclusion
We synthesized a Mn 3 O 4 /CNTs composite via a one-step hydrothermal method. It displays a high specific discharge capacity of up to 796.1 mA h g -1 after 50 cycles at 160 mA g -1 , The excellent electrochemical performance of the Mn 3 O 4 /CNTs composite could be attributed to its unique architecture.