Hydrothermal Synthesis of MnV 2 O 6 Nanorods as an Anode Material for Lithium-ion Batteries

. The MnV 2 O 6 nanorods anode materials was prepared by a simple hydrothermal method, MnCl 2 ·2H 2 O and NH 4 VO 3 as raw stuffs. The MnV 2 O 6 nanorods anode materials were tested by SEM, XRD, and galvanostatic charge/discharge profile measurement. Time-dependent experiments were designed to examine the morphology evolution of the MnV 2 O 6 nanorods anode materials. As an anode material, the MnV 2 O 6 nanorods showed the good discharge capacity (403 mAh g – 1 of 100th). The good electrochemical performance can be attributed to the synergistic effect with Mn and V elements, and fast lithium ion diffusion of the 1D nanorods structure.


Introduction
Lithium ion battery (LIBs) have been more and more widely used with pure electric vehicles, hybrid electric vehicles, mobile electronic products, et.al [1]. However, commercialization anode material (graphite: 372 mAh g -1 ) limit the further development and application of LIBs. So the development of low cost, non-toxic, long cycle life of electrode materials become the urgent demand of the LIBs [2]. In all TMOs (transition metal oxides), metal vanadates have be used incatalytic, optical, and electrode materials fields [3]. According to the literature, metal vanadates (such as Co 3 V 2 O 8 , CoV 2 O 6 , FeVO 4 , MoV 2 O 8 , Ni 3 V 2 O 8 , and Cu 3 V 2 O 8 ) also have good electrochemical performance, which can be used well in electrode materials (lithium-ion batteries, sodium-ion battery, zinc ion battery, et al.) [4][5][6].
MnV 2 O 6 have generated remarkable attention because of the facile preparation, safety, and high theoretical capacity. Some reports were related to research it as anode materials, the electrochemical property of pure MnV 2 O 6 is dissatisfactory, they have modified the MnV 2 O 6 by means of coating, compounding and constructing nanostructures to achieve the purpose of improving electrochemical performance. Kim and Ikuta synthesized a brannerite structure MnV 2 O 6 by a polymer gelation method, and the MnV 2 O 6 electrode material displayed a high initial discharge capacity [7]. In this work, the MnV 2 O 6 nanorods anode materials was prepared by a simple hydrothermal method. The morphology, crystal structure and electrochemical properties of the prepared composites were systematically investigated.
Time-dependent experiments were designed to examine the morphology evolution of the MnV 2 O 6 nanorods anode materials.

Experimental
Firstly, MnCl 2 ·2H 2 O and NH 4 VO 3 with stoichiometric amounts ratio of 1:2 were dissolved in 30 ml distilled water under magnetic stirring for 1 h. Then the mixed solution was transferred into a 50 mL teflon-lined stainless steel autoclave and annealed at 180 °C for 24 h. After naturally cooled down to room temperature, the resulting product was collected by filtration, and washed with deionized water and absolute alcohol for three times. It was further dried at 60 °C for 12 h.
The crystal structure and surface morphologies of the MnV 2 O 6 were characterized by X-ray diffraction (XRD) with Cu Kα radiation (Bruker AXS, D8 diffractometer) and scanning electron microscopy (SEM; JEOL JSM, 6510 V).The typical Electrochemical measurements process of MnV 2 O 6 material could be briefly shown in ref [8].

Results and discussion
The morphologies of the as-obtained MnV 2 O 6 sample was measured via SEM. The morphology of the MnV 2 O 6 sample was shown in Fig. 1a and b, which emerges severe aggregation of irregular nanorods structure, and the nanorods morphology of MnV 2 O 6 sample is composed of non-uniform nanorods (diameters of ~ 200 nm and length of 2~10 μm).  To confirm the formation mechanism of the MnV 2 O 6 nanorods, time-dependent experiments were designed to examine the morphology evolution of the sample under different reaction times in Fig. 2. The morphologies of the as-obtained five compounds were measured via SEM. The morphology of the MnV 2 O 6 (1 h) was shown in Fig. 2a, which emerges severe aggregation of irregular particles. After reaction time of 3 h, the morphology changed significantly, the morphology was made up nanoparticles and nanorods. As the reaction time continued to increase, the morphology continued to change. After reaction time of 12 h, the morphology was consisted of a small number of nanoparticles and a mass of nanorods. The 1D nanorods structure is beneficial to lithium ion diffusion [9].  The beginning discharge specific capacity of the MnV 2 O 6 electrode is 1123 mAh g -1 , which can be attributed to store 10.2 Li mole of MnV 2 O 6 . The extra specific capacity can be attributed to the SEI (solid electrolyte interphase) layer, which be formed by the decomposition of solvent in the electrolyte solution [10]. It can be seen that the MnV 2 O 6 electrode is 709.6 mAh g -1 of the 2 nd cycle, which means 36.8% of the first capacity loss. The samples respectively retain capacity of 403 mAh g -1 of 100 th cycle. Huang and Gao synthesized uniform MnV 2 O 6 nanobelts as anode materials by a hydrothermal method; the MnV 2 O 6 nanobelts displayed high cycling stability (1085 mAh g -1 at 100 mA g −1 ) and rate capability [11]. Some research work to improve rate performances by coating or composite method for its application in our working. For example, polymer coating of MnV 2 O 6 materials and MnV 2 O 6 /graphene nanocomposites are also well known to be for improving electrode performance because it is simple, low cost, and scalable. The MnV 2 O 6 nanorods electrode exhibits higher capacity. Outstanding electrochemical properties for the MnV 2 O 6 nanorods attributed to one dimensional nanorod structure can provided a larger surface area, shorter lithium ion diffusion path, maintain stable structure, guaranteed the good rate performance [4]. Fig. 3c shows the CV curves of MnV 2 O 6 electrode at 0.1 mV s −1 . During the first cathodic scans, the peaks of MnV 2 O 6 electrode at 1.89, 1.30, 0.63, 0.41 and 0.05 V were corresponding to the reduction of MnO and V 2 O 5 to form Mn 0 and Li x+y V 2 O 5 as well as formation of SEI. In the following scan, the reduction peaks moved to 0.74 V and 0.50 V. The anodic scans feature three oxidation peak at 0.19, 0.65, 0.85 and 2.52V, which could be associated with the oxidations of Mn to MnO, Li x+y V 2 O 5 to Li x V 2 O 5 and the decomposition of Li 2 O. Furthermore, the CVs of MnV 2 O 6 electrode remain almost the same of the follow cycle, which can also indicate that the good rate performance of MnV 2 O 6 electrode. The possible electrochemical reactions of the MnV 2 O 6 electrode was as follows [7,11]: LixV2O5 + yLi+ + ye-→Lix+yV2O5 (3)

Conclusions
In this work, the MnV 2 O 6 nanorods anode materials was prepared by a simple hydrothermal method, MnCl 2 ·2H 2 O and NH 4 VO 3 as raw stuffs. The MnV 2 O 6 nanorods anode materials were tested by SEM, XRD, and galvanostatic charge/discharge profile measurement. Time-dependent experiments were designed to examine the morphology evolution of the MnV 2 O 6 nanorods anode materials. As an anode material, the MnV 2 O 6 nanorods showed the good discharge capacity (403 mAh g -1 of 100th). The good electrochemical performance can be attributed to the synergistic effect with Mn and V elements, and one dimensional nanorod structure can provided a larger surface area, shorter lithium ion diffusion path, and maintain stable structure.