Nanostructured V2O5 Cathode for Rechargeable Mg-Ion Batteries

Introduction Rechargeable battery systems with different transporting ions are an alternative choice to Li batteries that can expand and diversify the existing energy storage market. Magnesium batteries are the most promising technology to achieve substantially greater energy density than Li-ion batteries, due to their divalent nature (two electron redox couple). 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.

The current quest for new cathode materials for rechargeable Mg batteries is focused on intercalation compounds exhibiting high working potential and capacity¹. When transporting ions have much larger diameter or charge than the host metal ion, segregation of transporting and host metal ions in the crystalline structure is forseable.  In this case deintercalation will lead to the collapse of the crystalline structure and breaking of electric conductance. Therefore, our approach to achieving magnesium intercalation is to use nanoscale material that have adjustable d spacing and two-dimensional layered structure that can to control polarizability of Mg²⁺ions and accommodate large charge/volume changes.

Nanostructured electrode materials represent one of the most attractive strategies to dramatically enhance battery performance. Short diffusion length associated with the nanosized dimensions effectively reduces the distance that ions and electrons must travel during cycling. Their large surface area increases the contact area between electrode and electrolyte and hence the number of active sites for electrode reactions, which in turn reduces electrode polarization loss and improves power density, energy efficiency, and usable energy.

Nanostructured bilayered V₂O₅ is highly efficient 3 V cathode material for ambient temperature sodium ion batteries². It shows superb performance: theoretical reversible capacity for Na₂V₂O₅ stochiometry of 250 mAh/g, excellent rate capability and cycle life, as well as high energy and power densities. 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.

Results and Discussion As–prepared nanostructured bilayered V₂O₅ cathode is synthesized in the charged state. To provide source enrichment, prior to assembling into full Mg-ion cell, it was necessary to pre-condition V₂O₅ cathode (intercalate with Mg²⁺ ions). Half-cells were discharged galvanostatically at 20mA down to and 0.01 V versus Mg/ Mg²⁺, respectively, using an automated Maccor battery tester at ambient temperature. As shown in Figure 1., upon electrochemical pre-condition (Mg-ion half cell) the X-ray edge energy for vanadium is shifted toward the lower energy as a consequence of a vanadium reduction due to Mg intercalation. Pre-edge peak is proportional to the octahedral symmetry of the vanadium sites and its decrease upon transporting ion intercalation is due to modification of vanadium local environment into more ordered, symmetric structure. Based on the main-edge peak position and comparison with Na-ion half cell we estimate 1 Mg is incorporated in the V₂O₅ structure (V⁵⁺ is reduced to V⁴⁺). It was previously shown that the V⁵⁺ was reduced to V⁴⁺ when discharged to 1.5V versus Na/ Na⁺ (Na-ion half cell)². As a result of this electrochemical pre-condition we created Mg-rich V₂O₅ cathodes that were able to provide source of Mg-ions in full-cell. Furthermore, we employed HRTEM, X-Ray fluorescence (XRF) imaging and classical molecular dynamics simulations to understand the role of environment in the intercalation processes of Mg-ions into V₂O₅cathode.

Conclusions We have developed high-voltage nanostructured V₂O₅ cathode for Mg-ion rechargeable batteries that can operate in commercial electrolytes. Our results emphasize the importance of tailoring nanoarchitecture of electrode materials and open up new opportunities for rechargeable Mg batteries. In the near future, we plan using the combined experimental and theoretical approach to guide our efforts to increase the Mg enrichment to reach the ultimate goal of inserting 2 Mg-ions/V₂O₅.

Acknowledgement.  This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by DOE, Office of Science, Basic Energy Science (BES). 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)    On the Way to Rechargeable Mg Batteries: The Challenge of New Cathode Materials, E. Levi, Y. Gofer, and D. Aurbach, Chem. Mater. 2010, 22, 860–868.

(2)    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.