Unraveling the Magnesium-Ion Intercalation Mechanism in Vanadium Pentoxide in a Wet Organic Electrolyte by Structural Determination

Abstract

Magnesium batteries have received attention as a type of post-lithium-ion battery because of their potential advantages in cost and capacity. Among the host candidates for magnesium batteries, orthorhombic α-VO is one of the most studied materials, and it shows a reversible magnesium intercalation with a high capacity especially in a wet organic electrolyte. Studies by several groups during the last two decades have demonstrated that water plays some important roles in getting higher capacity. Very recently, proton intercalation was evidenced mainly using nuclear resonance spectroscopy. Nonetheless, the chemical species inserted into the host structure during the reduction reaction are still unclear (i.e., Mg(HO), Mg(solvent, HO), H, HO, HO, or any combination of these). To characterize the intercalated phase, the crystal structure of the magnesium-inserted phase of α-VO, electrochemically reduced in 0.5 M Mg(ClO) + 2.0 M HO in acetonitrile, was solved for the first time by the ab initio method using powder synchrotron X-ray diffraction data. The structure was tripled along the b-axis from that of the pristine VO structure. No appreciable densities of elements were observed other than vanadium and oxygen atoms in the electron density maps, suggesting that the inserted species have very low occupancies in the three large cavity sites of the structure. Examination of the interatomic distances around the cavity sites suggested that HO, HO, or solvated magnesium ions are too big for the cavities, leading us to confirm that the intercalated species are single Mg ions or protons. The general formula of magnesium-inserted VO is MgHVO, (0.66 ≤ x ≤ 1.16). Finally, density functional theory calculations were carried out to locate the most plausible atomic sites of the magnesium and protons, enabling us to complete the structure modeling. This work provides an explicit answer to the question about Mg intercalation into α-VO.

On the strange case of divalent ions intercalation in V2O5

“…Despite promising steps in this di rection [17,18], detailed structural characterization of the phases formed is only starting. Interesting results have been reported by Hong et al [43] in the course of the present work, reporting the formation of a Mg 0.17 H x V 2 O 5 (0.66 < x < 1.16) phase upon reduction of sub micrometric α V 2 O 5 sample in beaker type (flooded) cell at room temperature, using 0.5 M Mg(ClO 4 ) 2 + 2 M H 2 O in ACN (acetonitrile) electrolyte (Panel g of Fig. 2).…”

“…Novak and De Silvestro [42] and later other authors [17,36,43], speculated that the addition of water to the electrolyte results in en hanced electrochemical response of V 2 O 5 towards magnesium inter calation basing on the higher current observed in cyclic voltammetry experiments and the higher capacity recorded. This hypothesis is re lated to the ability of H 2 O molecules to strongly coordinate to magne sium ions and hence shield their high polarizing character, thus al lowing faster diffusion kinetics within the host material [16].…”

“…XRD peaks reported in the literature for the phases formed upon Mg 2+ (a-h) and Ca 2+ (i-j) electrochemical intercalation in α-V 2 O 5 at RT (Ref. [17,30,33,38,39,43,48]); (k-n) Characteristic XRD reflections of chemically hydrogenated V 2 O 5 phases (Ref. [44][45][46][47]).…”

“…Surprisingly, authors state that the formation of Mg 0.17 H x V 2 O 5 (together with an additional unknown phase) does also take place when the reduction is carried out in dry (2 ppm H 2 O), 0.5 M Mg(ClO 4 ) 2 ACN electrolyte, without the formation of any other phase upon further reduction. At this point the need for a careful eva luation of the actual water content in the electrolytes is worth recalling, since conventional drying methods may not be adequate for some of the compositions used [43,49]. Moreover, alternative side reactions invol ving other proton sources (e.g.…”

Preparation and electrochemical performance of VO2(A) hollow spheres as a cathode for aqueous zinc ion batteries

“…In the process of exploring alternative systems, rechargeable multivalent metal ion batteries, such as Al-ion battery, Ca-ion battery and Zn-ion battery, have shown potential application due to their abundant sources and environmental friendliness. [8][9][10][11][12][13] In particular, rechargeable Zn-ion batteries will play a role in energy storage devices owing to the following advantages: (a) the high theoretical Zn metal anode capacity (820 mA h g À1 ), and the low oxidation-reduction potential of Zn 2+ /Zn (À0.76 V vs. standard 2H + /H 2 ); (b) the lowcost (USD $2 kg À1 ) and abundant Zn sources, as well as the nontoxic and safe aqueous electrolyte; and (c) transfer of more charges during electrochemical reactions to provide more energy storage than the univalent batteries. [14][15][16][17][18][19][20][21] The rst aqueous ZIB was reported by Kang et al using MnO 2 as the Zn-storage cathode with zinc sulfate or nitrate aqueous electrolyte.…”