A materials perspective to magnesium ion solid-state electrolytes
As economically viable alternatives to lithium-ion batteries, magnesium-ion based all-solid-state batteries are pursued to meet the criteria for an ideal energy storage device. With an energy-dense magnesium metal anode, these batteries can provide almost double the volumetric energy density at half the cost compared with state-of-the-art lithium-ion batteries. Although the development of solid-state magnesium batteries is hindered by various factors, the identification of an appropriate electrolyte remains the most challenging and limiting factor. In this review, we provide a survey of inorganic ceramic, metal-organic framework, glass and organic polymer solid electrolytes developed to date. We discuss the relationship between structure, composition and ionic conductivity of these Mg2+-ion inorganic solid-state electrolytes as well as the fundamental Mg-ion conduction mechanisms that govern magnesium transport in these solids with emphasis on Mg2+-ion conducting inorganic materials. Through a comparison of Mg2+ and Na+ ion conductors under the theoretical framework of multi-excitation entropy (or Meyer–Neldel rules), we highlight possible differences between these two systems, which lead to substantially different ion transport characteristics.