Probing Electrochemical Mg-Ion Activity in MgCr_2–xV_xO₄ Spinel Oxides

Abstract

Mg migration in oxide spinels is impeded by strong affinity between divalent Mg and oxygen, suggesting a necessity of exploring new chemistry of solid lattices for functional Mg-ion electrode materials. Cationic mobility with a suitable activation energy in Cr spinels is evidenced by theoretical and experimental results, while redox potentials of V are appropriate to operate with currently limited candidates of nonaqueous electrolytes. By controlling the structure, composition, and complexity, a largely solid-solution MgCrVO4 spinel was synthesized, which, unlike nanocomposites, can bring together the advantages of each transition metal in the lattice. The spinel was successfully synthesized by a simple solid-state reaction with minor inactive Cr- or V-rich components, which was confirmed via 25Mg MAS NMR and high-resolution X-ray diffraction analyses. A thermally and anodically stable Mg­(TPFA)2/triglyme electrolyte was utilized for high-temperature electrochemistry and lowering kinetic barriers at and across interfaces so as to observe intercalation behavior of Mg in the designed lattice. Multimodal characterization confirmed an apparent bulk demagnesiation from MgCrVO4 with partial reversibility by probing evolution of the local and long-range structure as well as vanadium and chromium electronic states within the lattice. Characterization experiments also provided a direct evidence for (de)­intercalation reactions that occurred without any major competitive conversion reactions or insertion of protons into the lattice, except for the formation of a surface rock salt phase upon charge. These findings in Mg-ion activity expand opportunities to design Mg spinel oxide materials while highlighting the need to identify the origins of reversibility challenges due to, but not limited to, phase stability, particularly for the charged states, barriers at the interface, electrolyte stabilities, and desolvation phenomena, collectively hindering practical use as cathode materials.

Control of crystal size tailors the electrochemical performance of α-V2O5 as a Mg2+ intercalation host

“…47 This observation suggests that the V 2 O 5 -TiO 2 composite not only possessed smaller crystallite sizes than the V 2 O 5 sample, but a greater degree of crystalline disorder. 28 Electrochemical analysis of the two materials under study was performed in conditions previously found to give high degrees of Mg insertion (110°C in 0.5 Mg(TFSI) 2 in PY 14 TFSI ionic liquid electrolyte). 23,24 For the first discharge/charge cycle of the V 2 O 5 sample, the behavior resembled that observed in bulk α-V 2 O 5 previously by Yoo et al, 23 i.e.…”

“…11,23 However, we note that other spinel compounds (AM 2 O 4 , where A = Mg or Zn, M = Cr, Co, Fe, Mn, V) have also consistently demonstrated Mg removal on charge (∼200 mA h g −1 ). 14,[26][27][28][29][30][31] Comparing between these materials, the report of α-V 2 O 5 is arguably the most striking with a high capacity of 300 mA h g −1 at 110°C, where this degree of Mg intercalation was confirmed and quantified by a combination of EDS, FTIR, XRD and XAS analysis. 23 The voltage hysteresis of the first cycle (1.35 V) was significantly higher than latter cycles (0.73 V), and inspection of the α-V 2 O 5 particles revealed significant delamination of the α-V 2 O 5 layers following the first discharge: it can therefore be hypothesized that the delamination of α-V 2 O 5 reduced the observed voltage hysteresis.…”

Enhanced charge storage of nanometric ζ-V2O5 in Mg electrolytes

“…This study revealed that diffusive processes impose a limit on the degree and reversibility of the observed Mg electrochemistry in ζ-V 2 O 5 , and that nanosizing can significantly improve electrode kinetics, revealing a key contribution to the voltage hysteresis that is pervasive to oxides used as Mg intercalation electrodes. 14,19,42,43 However, questions remain regarding the uniformity of magnesiation of cathode particles throughout the electrode. Therefore, it is suggested that future efforts should not only target ultrafine particles of candidate oxide electrode materials, but also probe their activity as a function of electrode depth, to elucidate kinetic limitations imposed by electrolyte permeation in the electrode and bulk electrode resistance.…”

“…[10][11][12][13] In contrast, oxide materials can offer much higher potentials vs. Mg/Mg 2+ , e.g. 2.5 V for V 2 O 5 , 14,15 2.9 V for MgMn 2 O 4 , 16 and 3.5 V for MgCr 2 O 4 , 14,15,[17][18][19] although these materials typically exhibit Mg insertion and removal kinetics that are moderateto-poor (e.g. overpotentials of >0.7 V for V 2 O 5 , corresponding to ∼70% energy efficiency).…”

Electron-beam-induced Spinel to Defect Rocksalt Phase Transition in MgCrMnO4

“…Several theoretical and experimental studies have revealed that transition-metal oxide spinels constitute a class of materials that provide possible host structures for favorable multivalent ion intercalation due to their three-dimensional pathways of migration, offering a diversity of favorable intercalation sites [3]. Initial studies have demonstrated low levels of Mg (de)intercalation into the spinel cathodes [4]. It was suggested that the sluggish intercalation chemistry during electrochemical cycling is due to structural changes and phase transitions hindering the Mg mobility and reversibility [5].…”