Recent development in the field of ceramics solid-state electrolytes: I—oxide ceramic solid-state electrolytes

Many elements in the periodic table form ionic compounds; the crystal lattices of such compounds contain cations and anions, which are arranged in the way that these cations and anions form two interpenetrated sub-lattices (cation and anion sub-lattices). Up to now, a number of ionic compounds are known, in which cations or anions are fairly mobile within the corresponding sub-lattice; these compounds are termed as "solid-state electrolytes". Many solid-state electrolytes with such moveable cations and moveable anions are known to date. Following the footsteps of the established Li-ion battery technology, an interest in the Li+-conducting solid-state electrolytes appears, and all-solid-state lithium battery has started its journey to accompany the reigning counterpart. The valence and ionic radius of ions, the crystal structure, and intrinsic defects of the material are the prime properties of the solid-state electrolytes, which determine the ion mobility in the crystal framework. There are a number of solid-state electrolyte structures that demonstrate high Li+-mobility and high Li+ conductivity (Li+ superconductors) in the range of 10−2 to 10−3 S/cm at room temperature, which is comparable to the ionic conductivity of 1 M LiPF6 (~ 10−2 S/cm), but the conductivity can dwindle highly by up to 5–6 orders of magnitude within the different materials that belonged to the same crystal structure family. Moreover, the surface or interface properties are also crucial factors in tailoring the ionic conductivity of practical polycrystalline solid electrolytes. The interfacial properties and compatibility with electrode materials have a high impact on the performance of electrochemical cells with solid electrolytes. Although the potential window of many solid electrolytes is high enough, there are solid electrolytes which are unstable at low operating potentials while others are not stable towards the cathodes; these features result in the appearance of non-conductive interface layers resulting in a low interfacial charge–transfer kinetics. In this review, we discuss the latest advancements in the field of Li-ion conducting electrolytes from the points of their fundamental properties. The latest achievements in the fields of cell design and improvements of (solid-state electrolytes)/(various anodes) and (solid-state electrolytes)/(various cathodes) compatibilities are considered as well.