Ionically Conductive Tunnels in h-WO₃ Enable High-Rate NH₄ ⁺ Storage

Compared to the commonly applied metallic ion charge carriers (e.g., Li⁺ and Na⁺ ), batteries using nonmetallic charge carriers (e.g., H⁺ and NH₄ ⁺ ) generally have much faster kinetics and high-rate capability thanks to the small hydrated ionic sizes and nondiffusion control topochemistry. However, the hosts for nonmetallic charge carriers are still limited. In this work, it is suggested that mixed ionic-electronic conductors can serve as a promising host for NH₄ ⁺ storage. Using hexagonal tungsten oxide (h-WO₃ ) as an example, it is shown that the existence of ionic conductive tunnels greatly promotes the high-rate NH₄ ⁺ storage. Specifically, a much higher capacity of 82 mAh g^-1 at 1 A g^-1 is achieved on h-WO₃ , in sharp contrast to 14 mAh g^-1 of monoclinic tungsten oxide (m-WO₃ ). In addition, unlike layered materials, the insertion and desertion of NH₄ ⁺ ions are confined within the tunnels of the h-WO₃ , which minimizes the damage to the crystal structure. This leads to outstanding stability of up to 200 000 cycles with 68% capacity retention at a high current of 20 A g^-1 .