Electrochemical Intercalation of Mg²⁺ into Anhydrous and Hydrated Crystalline Tungsten Oxides

The reversible intercalation of multivalent cations, especially Mg²⁺, 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 V₂O₅ and MnO₂ has been shown to improve Mg²⁺ intercalation kinetics in nonaqueous electrolytes. Here, the effect of structural water on Mg²⁺ intercalation in nonaqueous electrolytes is examined in crystalline WO₃ and the related hydrated and layered WO₃·nH₂O (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 Mg²⁺ intercalation. It is found that the anhydrous WO₃ can intercalate up to ∼0.3 Mg²⁺ (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 Mg²⁺ storage in WO₃ are limited by solid-state diffusion, which is similar to its behavior in a Li⁺ electrolyte. On the other hand, the maximum capacity for Mg²⁺ storage in WO₃·nH₂O is approximately half that of WO₃ (35 mAh g^-1). However, the kinetics of both Mg²⁺ and Li⁺ storage in WO₃·nH₂O are primarily limited by the interface and are thus pseudocapacitive. The stability of the structural water in WO₃·nH₂O varies: the interlayer water of WO₃·2H₂O 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 Mg²⁺ cathodes, that in these materials structural water can lead to improved Mg²⁺ kinetics at the expense of capacity, and that the type of structural water affects stability.