Nanostructured V2O5/Sn Mg-Ion Full Batteries
Authors: Tepavcevic Sanja , Zhou Dehua , Johnson Christopher S. , Rajh Tijana 
Published: February 2020

Introduction Rechargeable battery systems with different transporting ions that can provide an alternative choice to Li batteries would bring substantial relief and expansion of the existing energy storage market. To achieve substantially greater energy density than Li-ion magnesium batteries are the most promising technology. Furthermore, because of its “green” character Mg is environmentally friendly, with high natural abundance in the Earth’s crust (13.9% as compared to 0.0007 % for Li), and atmospheric stability. Due to the divalent nature of Mg ions and the use of a magnesium metal anode, magnesium batteries can obtain higher energy density than state-of-the-art lithium batteries. However, magnesium electrochemistry suffers from serious limitations due to anode/electrolyte incompatibility. In 2008, the Aurbach group at Bar-Ilan University made a key breakthrough in rechargeable magnesium battery technology demonstrating the battery with an electrolyte solution based on Mg organohaloaluminate salts, a magnesium metal anode, and an MgxMo3S4 cathode1. However, such a design exhibits relatively narrow electrochemical stability window (up to 2.2 V vs. Mg) limiting greatly the choice of cathodes and ultimately its potential specific energy. Alternative material families, as well as new design approaches, are highly desirable for ultimate industrialization of Mg secondary batteries.
Some researchers are searching for new electrolytes that work well with magnesium metal. Another potential solution is to use a different type of anode, one that works with familiar, conventional electrolytes. There have been recent and encouraging experimentations on Mg batteries with Sb, Sn, and Bi anodes by Singh et al.2

Exploration for new cathode materials for rechargeable Mg batteries is focused on intercalation compounds exhibiting a higher working potential and capacity. Nanostructured bilayered V2O5 is highly efficient 3 V cathode material for ambient temperature sodium ion batteries3. It shows superb performance: theoretical reversible capacity for Na2V2O5 stoichiometry of 250 mAh/g, excellent rate capability and cycle life, as well as high energy and power densities. Our preliminary results show that it is possible to extend the reversible ion-insertion chemistry of nanostructured layered V2O5 beyond lithium and larger in size Na+ ions to those with higher charge: Mg2+ions.
Experimental. In order to increase V2O5 loadings to be suitable for practical applications, we developed in situ growth of the V2O5 nanoribbons on commercially available, highly conductive, interconnected substrate: carbon nanofoam (CNF). Electrochemically discharged (Mg-rich) nanostructured V2O5 cathode was used as a source of Mg-ions. NanoSn/C composites used as anode were prepared by high-energy ball milling under an argon atmosphere. The SEM images showed the primary particle size was about 10-20 nanometers while the secondary particle aggregated to 1-3 micrometers. Coin cells (2032) were cycled in 1M Mg (ClO4) 2/acetonitrile and 1M Mg (ClO4) 2/propylene carbonate. The cell performance was evaluated galvanostatically at current densities of 0.04 mAcm-2 for both charge and discharge at room temperature.
Results and Discussion Sn appears to be promising anode material for advanced Mg-ion batteries because theoretical capacity of this electrode is much higher than of the carbon materials. In order to compare performance of CNF and Sn as anode in Mg-ion batteries we constructed two Mg-ion full cells: first one from nanostructured bilayered V2O5 cathode and CNF anode (V2O5/CNF) and second one from bilayered V2O5/CNF cathode and nanoSn anode (V2O5/Sn) in 1M Mg (ClO4)2/acetonitrile (Figure 1.). We believe that CNF as anode shows reversible capacity due to Mg-ion adsorption on the anode side, while higher Sn anode capacity is due to alloying of Mg-ions into NanoSn composite electrode. In our preliminary results these nanostructured electrode materials were able to retain the local electro-neutrality upon reversible insertion of Mg-ions in commercial electrolytes such as acetonitrile and propylene carbonate. In addition to pre-conditioning of the cathode size, we will discuss the process for Mg pre-treatment of Sn anode to be used as Mg-rich electrode and the future source of Mg ions in the full cell.

Figure 1. The first two cycles for a V2O5/CNF (red) and a V2O5/Sn (black) Mg-ion nanostructured full-cells in a conventional-type electrolyte.

Conclusions We have developed safe, Mg-ion battery system exclusively made by nanoelectrodes that operate in commercial electrolytes. These results emphasize the importance of tailoring nanoarchitecture of electrode materials and open up new opportunities for rechargeable Mg batteries.

Acknowledgement. This work was supported by the U. S. Department of Energy, US DOE-BES, under Contract No. DE-AC02-06CH11357. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
References
(1) Prototype systems for rechargeable magnesium batteries D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich & E. Levi, Nature, 2000, 407, 724-727.
(2) A high energy-density tin anode for rechargeable magnesium-ion batteries Nikhilendra Singh, Timothy S. Arthur, Chen Ling, Masaki Matsui and Fuminori Mizuno, Chem. Commun. 2013, 49, 149
(3) Vanadium Oxide Electrodes for Rechargeable Sodium-Ion Batteries, Sanja Tepavcevic, Hui Xiong, Vojislav R. Stamenkovic, Xiaobing Zuo, Mahalingam Balasubramanian, Vitali B. Prakapenka, Christopher S. Johnson, and Tijana Rajh, Nanostructured Bilayered ACS Nano, 2012, 6 (1), 530–538.