Divalent cations like Mg 2+ , although similar in size to Li + , have higher charge densities, leading to stronger electrostatic interactions that may cause lattice strain and higher activation barriers for ion diffusion in the solid state . As a result, the mobility of these larger or higher charged ions is generally lower in the lattice impacting the durability and efficiency of the material in applications requiring rapid ion mobility and cycling stability. For this study, H2SO4 was selected for further device optimization. Additionally, the current response of h-WO3@Ti is tested in a more positive window up to 2 V vs. RE, and the CV shows that no significant current is observed, indicating high stability (Figure ).