Electrochemical Intercalation of Mg 2+ into Anhydrous and Hydrated Crystalline Tungsten Oxides

This is the authors' abstract. We don't add key points for this paper.

The reversible intercalation of multivalent cations, especially Mg 2+, into a solid-state electrode is an attractive mechanism for next-generation energy storage devices. These reactions typically exhibit poor kinetics due to a high activation energy for interfacial charge-transfer and slow solid-state diffusion. Interlayer water in V 2 O 5 and MnO 2 has been shown to improve Mg 2+ intercalation kinetics in nonaqueous electrolytes. Here, the effect of structural water on Mg 2+ intercalation in nonaqueous electrolytes is examined in crystalline WO 3 and the related hydrated and layered WO 3 · n H 2 O ( n = 1, 2). Using thin film electrodes, cyclic voltammetry, Raman spectroscopy, X-ray diffraction, and electron microscopy, the energy storage in these materials is determined to involve reversible Mg 2+ intercalation. It is found that the anhydrous WO 3 can intercalate up to ∼0.3 Mg 2+ (75 mAh g –1 ) and can maintain the monoclinic structure for at least 50 cycles at a cyclic voltammetry sweep rate of 0.1 mV s –1 . The kinetics of Mg 2+ storage in WO 3 are limited by solid-state diffusion, which is similar to its behavior in a Li + electrolyte. On the other hand, the maximum capacity for Mg 2+ storage in WO 3 · n H 2 O is approximately half that of WO 3 (35 mAh g –1 ). However, the kinetics of both Mg 2+ and Li + storage in WO 3 · n H 2 O are primarily limited by the interface and are thus pseudocapacitive. The stability of the structural water in WO 3 · n H 2 O varies: the interlayer water of WO 3 ·2H 2 O is removed upon exposure to a nonaqueous electrolyte, while the water directly coordinated to W is stable during electrochemical cycling. These results demonstrate that tungsten oxides are potential candidates for Mg 2+ cathodes, that in these materials structural water can lead to improved Mg 2+ kinetics at the expense of capacity, and that the type of structural water affects stability.