Electronic and structural properties of V$_2$O$_5$ layered polymorphs

Abstract

V$_2$O$_5$ is a promising battery electrode material that can intercalate not only Li, but also more abundant alkaline metals such as Na and K, and even multivalent ions such as Al, Ca, Cu, Mg, and Zn. V$_2$O$_5$ exhibits several different polymorphs, and phase transitions between the polymorphs can occur depending on intercalant or external conditions. At least 8 different layered polymorphs have been observed. However, detailed information about the energetics and structural properties of each polymorph is still lacking. To obtain a reliable computational reference, we use hybrid density functional theory calculations to investigate the properties of layered V$_2$O$_5$ polymorphs. We benchmarked several methods to include van der Waals interactions in combination with hybrid functionals, and found that the Grimme D3 method is most accurate. We obtain detailed information on the electronic properties and structures of the various unintercalated polymorphs and show that the main electronic effect of intercalants is a filling of the lowest conduction bands, as the intercalant contributions are well above the conduction-band minimum. Despite the structural differences between the unintercalated polymorphs, we find that they have very similar band gaps and band structures, with the exception of the high temperature and pressure phase $β$.

Figures (24)

• Figure 1: The single-layer polymorphs investigated in this study: (a) $\alpha$-V$_2$O$_5$, (b) $\beta$-V$_2$O$_5$, (c) $\delta$-V$_2$O$_5$, and (d) $\gamma$-V$_2$O$_5$. Polyhedra are shown around the V atoms (large spheres) to indicate the bonding environment. Smaller spheres are O atoms.
• Figure 2: The double-layer polymorphs investigated in this study: (a) $\delta$-Ag$_{0.84}$-V$_2$O$_5$, (b) $\epsilon$-Cu$_{0.85}$-V$_2$O$_5$, (c) $\nu$-Ca$_{0.6}$-V$_2$O$_5$, and (d) $\rho$-K$_{0.5}$-V$_2$O$_5$. The black and purple indicators in (d) show that each alternating layer is identical and consists of D4 for the black layers and D4M for the purple layers.
• Figure 3: Deviation (in percent) of the calculated lattice constants as compared to the experimental lattice constants Enjalbert1986Balog2007, for different computational methods Grimme2006Grimme2010Grimme2011Tkatchenko2009Bucko2014 for the (a) $\alpha$ and (b) $\beta$ V$_2$O$_5$ polymorphs.
• Figure 4: The energy per formula unit (f.u.) as function of volume per formula unit for various layered V$_2$O$_5$ polymorphs. The energy per formula unit of the $\alpha$ polymorph is set to 0.
• Figure 5: The calculated band structures for the (a) single-layer $\alpha$-V$_2$O$_5$ polymorph and (b) the double-layer $\epsilon$-Cu$_{0.85}$ polymorph. The colors represent the band character, where red colors indicate V $d$ character and blue O $p$ character.