Probing Mg Intercalation in the Tetragonal Tungsten Bronze Framework V₄Nb₁₈O₅₅

Abstract

While commercial Li-ion batteries offer the highest energy densities of current rechargeable battery technologies, their energy storage limit has almost been achieved. Therefore, there is considerable interest in Mg batteries, which could offer increased energy densities compared to Li-ion batteries if a high-voltage electrode material, such as a transition metal oxide, can be developed. However, there are currently very few oxide materials which have demonstrated reversible and efficient Mg 2+ insertion and extraction at high voltages; this is thought to be due to poor Mg-ion diffusion kinetics within the oxide structural framework. Herein, the authors provide conclusive evidence of electrochemical insertion of Mg 2+ into the tetragonal tungsten bronze V4Nb18O55, with a maximum reversible electrochemical capacity of 75 mA h g −1 , which corresponds to a magnesiated composition of Mg4V4Nb18O55. Experimental electrochemical magnesiation/de-magnesiation revealed a large voltage hysteresis with charge/discharge (1.12 V vs Mg/Mg 2+ ); by limiting magnesiation to a composition of Mg2V4Nb18O55, this hysteresis can be reduced down to only 0.5 V. Hybrid-exchange Density Functional Theory (DFT) calculations suggest that a limited number of Mg sites are accessible via low-energy diffusion pathways, but that larger kinetic barriers need to be overcome to access the entire structure. The reversible Mg-ion intercalation involved concurrent V and Nb redox activity and changes in crystal structure, confirmed by an array of complimentary methods including powder X-ray diffraction, X-ray absorption spectroscopy, and energy-dispersive X-ray spectroscopy. Consequently, It can be concluded that the tetragonal tungsten bronzes show promise as intercalation electrode materials for Mg batteries. 30 the development of new energy storage technologies is par-31 amount. While Li-ion batteries have achieved great com-32 mercial success, 1 and are the most energy-dense recharge-33 able storage technology for portable applications, the tech-34 nology already heavily optimized, and future increases in35 energy density are predicted to be incremental. 2 Therefore,36 there is a need to develop new, low cost, and sustainable 37 battery chemistries for applications such as future electric 38 vehicle and grid storage. Mg metal anodes have a lower pro-39 pensity to form dendrites (that can cause cell failure) from 40 stripping and plating, which means that it could be used di-41 rectly as a metallic anode, 3-5 which would offer up to a five-42 fold increase in anodic volumetric energy density (Mg metal 43 = 3833 mA h cm −3 ) compared to Li-ion graphite anodes 44 (∼800 mA h cm −3 ), leading to an overall doubling in energy 45 density if a suitably high-energy density electrode material 46 can be found. 6 In comparison to Li + , the divalent ion Mg 2+ 47 transfers twice the charge (2 e − per Mg 2+ ion) when moving 48 between anode and cathode. This phenomenon decouples 49 the effect of electronic changes from the availability of va-50 cant sites in th...

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

“…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. a capacity of >200 mA h g −1 with a very large calculated voltage hysteresis of 1.68 V (Fig.…”

“…This irreversibility has been commonly observed in metal oxide electrodes with Mg electrolytes. 23,24 Fig. 4c and d also depicts integrated O K-edge X-ray absorption spectra acquired for pristine and electrochemically cycled V 2 O 5 -TiO 2 samples, compared with standard α-V 2 O 5 .…”

Fast lithium-ion conductivity in the ‘empty-perovskite’ n = 2 Ruddlesden–Popper-type oxysulphide Y2Ti2S2O5

“…64 Typically, mobility is higher within sulphides than oxides, and indeed there are only a handful of oxide materials that report reversible Mg 2+ intercalation. [65][66][67] Experimental results indicate that Mg 2+ ions can be introduced into Y 2 Ti 2 S 2 O 5 via a solid-state reaction route at 300 C, to a stoichiometry of Mg 0.32 Y 2 Ti 2 S 2 O 5 . Mg 2+ ions were found to occupy 12-coordinated sites within the type I cavities.…”

“…The 66 which is structurally-related to the tetragonal tungsten bronzes, and has units similar to type I and II W-R-type cavities. Activation barriers of similar magnitude were reported for selected pathways and Mg-ions we found to occupy distorted square-or rectangular-planar insertion sites.…”

Enhanced charge storage of nanometric ζ-V2O5 in Mg electrolytes

“…The use of CHFS enabled the production of nano-sized crystallites via highly supersaturating conditions and a short reaction time (residence time on the order of a few seconds), 25,26 and the process has previously produced a wide variety of high-performance nanosized electrode materials for Mg batteries, Liion batteries and supercapacitors. 17,[27][28][29][30][31] Experimental section…”