Systematic Transmission Electron Microscopy Study Investigating Lithium and Magnesium Intercalation in Vanadium Oxide Polymorphs A. Mukherjee, 1 R. F. Klie 1 , H.D. Yoo, 2 G. Nolis, 2 J. Cabana, 2 J. Andrews 3 and S. Banerjee 3 1. Department of Physics, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 2. Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street and Engineering South (MC 111), Chicago, IL 3. Department of Chemistry, Texas A&M University, Ross Street, College Station, TX Magnesium-ion based batteries promise a competitive alternative to conventional lithium-ion battery technology. Batteries combining Mg metal anode with a suitable intercalation-based cathode can offer much higher volumetric energy density, as well as significant cost and safety benefits over lithium ion batteries. Recent first-principles and experimental reports have established that orthorhombic α-V2O5 is a promising intercalation cathode for Mg ion batteries. However, several crucial aspects of the intercalation phenomenon, such as the specific intercalation sites for Mg within α-V2O5 or the formation of different phases upon Mg insertion into α-V2O5 remain unclear. Further systematic characterization of the Mg intercalation behaviour is therefore required. This contribution will focus on systematic investigation of Mg intercalation into α-V2O5 by combining aberration-corrected scanning transmission electron microscopy (STEM) imaging, electron diffraction, electron energy loss (EEL) and energy dispersive x-ray spectroscopy (XEDS). More specifically, we will present a comparison of Mg insertion sites in two different samples: i) electrochemically cycled α-V2O5 cathode in a prospective full cell vs Mg metal anode and ii) chemically synthesized MgV2O5 sample. In the case of electrochemically cycled α-V2O5, our results determine the Mg intercalation sites and it is concluded that this sample exhibits the local formation of the ε-Mg0.5V2O5 phase, as predicted by earlier first-principles density functional theory (DFT) calculations [1]. Figure 1a) and b) present atomic resolution high-angle annular dark-field (HAADF) and annular bright-field (ABF) images, respectively, for the electrochemically-cycled orthorhombic α-V2O5 cathode. Simulated HAADF and ABF images for the DFT predicted ε-Mg0.5V2O5 phase are overlaid on the experimental STEM images. The structural model for the ε-Mg0.5V2O5 phase is shown in Fig 1(c). We will also show that the chemically synthesized sample presents the δ-MgV2O5 phase [2]. Recent theoretical calculations have also predicted that the migration barrier for ionic intercalation can be decreased by exploiting different anion coordination environments in metastable vanadium oxide polymorphs, such as ζ-V2O5 [3]. Figure 2a) presents atomic-resolution HAADF image for ζ-V2O5 nanowires clearly showing the heavier V atoms and elucidating the tunnel structure for this novel polymorph; a structural model for the ζ-V2O5 phase is presented in Figure 2b). We have investigated the lithium intercalation in this tunnel-structured ζ-V2O5 polymorph, and will focus on showing that ζ-V2O5 nanowires show much better Li-cycling properties (i.e. reversibility) compared to orthorhombic α-V2O5 [4]. Moreover, Mg intercalation into ζ-V2O5 nanowires will be investigated in detail, comparing electrochemical performance at both low and high temperature cycling followed by systematic STEM characterization. The results obtained for this novel polymorph ζ-V2O5 will be directly compared with our previous work investigating Mg intercalation in α-V2O5 [2]. Other V2O5 polymorphs, such as ε-V2O5 will also be tested for their ability to intercalate Li or Mg [5]. 2012 doi:10.1017/S1431927617010728 Microsc. Microanal. 23 (Suppl 1), 2017 © Microscopy Society of America 2017 https://doi.org/10.1017/S1431927617010728 Published online by Cambridge University Press