Synthesis and Structures of Novel Solid-State Electrolytes

Abstract

Two classes of new materials possessing ion conductivity have been developed: a lithium ion conductor and a hydride ion conductor. Conventional perovskite and ordered rock-salt structures were adopted as frameworks for lithium migration, and electrochemically stable elements such as Al, Ga, Ta, and Sc were used in the materials to facilitate their use as low-potential negative electrodes. New compositions of (Li 0.25 Sr 0.625 V (Li,Sr)0.125 )(Ga 0.25 Ta 0.75 )O 3, and Li 0.9 Sc 0.9 Zr 0.1 O 2 were found to be novel oxide-based lithium ion conductors. Oxyhydrides with K 2 NiF 4 -type structures were synthesized via a high-pressure synthesis method and their use in pure hydride ion conduction was demonstrated. The La 2-x-y Sr x+y LiH 1-x+y O 3-y oxyhydrides showed wide composition ranges of solid solution formation and the conductivity increased with anion vacancies or the introduction of interstitial hydride ions. The performance of an all-solid-state TiH 2 /o-La 2 LiHO 3 (x = y = 0, o: orthorhombic)/Ti cell provided conclusive evidence of pure H -conduction. [6,7]. In this section, we focus on Li + and H -as charge carriers and describe the structural characteristics of the corresponding newly developed materials. Keywords Lithium Ion ConductorsLithium ion conductors continue to attract much attention owing to their practical applications in all-solid-state lithium batteries [5,8]. A wide variety of such conductors exists (e.g., LISICON, perovskite, garnet, glass, glass ceramics, thio-LISICON, and LGPS), some of which were developed in the 1970s [4,[9][10][11][12][13][14]. For instance, LGPS-based materials (σ > 10 mS cm −1 at 25°C) enable high-power operation of solid-state lithium batteries; this is an intrinsic merit of solid-state systems, in addition to their safety and reliability. However, sulfide-based solids are sensitive to atmospheric moisture. As a result, most current research focuses on oxide-based materials, in order to satisfy the requirements of practical applications and engineering processes.Novel ion conductors are typically developed using three methods: (i) element substitution-based, (ii) structure-based, and (iii) composition-based material searches. Approach (i) relies on existing materials with ionic conductivity of the target charge carrier [15], which are amenable to tuning of their physical and electrochemical properties [16]. Therefore, although it is relatively easy to find new materials using this method, remarkable performance improvements are difficult to achieve. Approach (ii) is initiated by selecting a suitable crystal structure candidate for ion diffusion [6,11], which can be complicated by the fact that the diffusion of the target ion in the selected structure has usually not been demonstrated. Finally, approach (iii) is the most challenging, but also has the greatest potential to afford new materials with unique structures and properties. This approach starts with the selection of a suitable phase diagram [17]. Subsequently, materials corresponding to the chosen ...