Highly Reversible Mg Insertion in Nanostructured Bi for Mg Ion Batteries Yuyan Shao,* Meng Gu, Xiaolin Li, Zimin Nie, Pengjian Zuo, Guosheng Li, Tianbiao Liu, Jie Xiao, Yingwen Cheng, Chongmin Wang, Ji-Guang Zhang, and Jun Liu* Pacific Northwest National Laboratory, Richland, Washington 99352, United States * S Supporting Information ABSTRACT: Rechargeable magnesium batteries have at- tracted wide attention for energy storage. Currently, most studies focus on Mg metal as the anode, but this approach is still limited by the properties of the electrolyte and poor control of the Mg plating/stripping processes. This paper reports the synthesis and application of Bi nanotubes as a high- performance anode material for rechargeable Mg ion batteries. The nanostructured Bi anode delivers a high reversible specific capacity (350 mAh/g Bi or 3430 mAh/cm 3 Bi ), excellent stability, and high Coulombic efficiency (95% initial and very close to 100% afterward). The good performance is attributed to the unique properties of in situ formed, interconnected nano- porous bismuth. Such nanostructures can effectively accom- modate the large volume change without losing electric contact and significantly reduce diffusion length for Mg 2+ . Significantly, the nanostructured Bi anode can be used with conventional electrolytes which will open new opportunities to study Mg ion battery chemistry and further improve its properties. KEYWORDS: Energy storage, magnesium battery, anode, bismuth nanotube, insertion M agnesium-based batteries have attracted increasing interest in the past few years. 1-5 This technology uses earth-abundant Mg element with a bivalent charge carrier Mg 2+ which could potentially lead to low cost and high energy density. To date, most efforts in this field have been focused on Mg metal anode based rechargeable Mg battery 1,4 by developing high-performance electrolytes 5-9 and catho- des. 2,10-14 The use of Mg metal is still limited by the properties of the electrolyte and the poor control of the electrochemical reaction at the metal-electrolyte interfaces. 1,4 Conventional electrolytes made by mixing simple Mg salts (e.g., Mg(ClO 4 ) 2 ) and nonaqueous solvents (e.g., propylene carbonate), as widely used in lithium batteries, 15 do not produce reversible plating/ stripping of Mg. 16,17 This is likely due to a nonconductive layer formed on Mg surface in these conventional electrolytes. 18 Specially designed and synthesized electrolytes are needed for Mg metal anode. 4,19 So far, there are only a limited number of electrolytes that show reversible Mg plating/stripping. 5-7,20 The synthesis of these electrolytes usually requires strictly controlled chemical processes. 5,6,9 Low electrochemical stable window, 1,4,21 the corrosive nature of the electrolytes 21-24 and high volatility, usually involving solvents such as tetrahydrofur- an (THF), 4,5,7,9 are also significant limiting factors. A new approach that does not rely on the special electrolytes associated with Mg plating/stripping could lead to significant breakthroughs in this field. The revolution and large scale commercialization of Li-ion batteries were facilitated by the successful development of the intercalation and insertion materials for both cathode and anode. 25-31 Significant progresses have been made in the synthesis, understanding and utilization of such intercalation and insertion materials. 27,32-35 Extensive knowledge has been gained on the chemistry, properties and optimization of electrolytes for intercalation and insertion electrode materi- als. 15,36 If such mechanisms can be successfully used in Mg chemistry, the battery may not be limited by the Mg plating/ stripping reaction and the special electrolytes associated with such reactions. Instead, conventional electrolytes commonly used in Li-ion batteries could be used. The new approach will significantly widen the choices of the electrode and electrolyte materials, provide great opportunity to gain better under- standing of the reaction mechanisms involved Mg chemistry, and may lead to new cost-effective materials and technologies. Recently, a few groups studied bismuth and tin as the anode materials for rechargeable Mg-ion batteries. 37-39 These materials still have poor rate performance and fast capacity fading upon cycling. Taking tin as an example, only ∼20% of its theoretical capacity was obtained at the rate C/20 (discharge in 20 h), and the capacity fading is fast. 38 The Coulombic Received: October 16, 2013 Revised: November 20, 2013 Published: November 26, 2013 Letter pubs.acs.org/NanoLett © 2013 American Chemical Society 255 dx.doi.org/10.1021/nl403874y | Nano Lett. 2014, 14, 255-260