Lithium ion mobility in metal oxides: a materials chemistry perspective

Abstract

Metal oxides containing mobile lithium ions are technologically important materials in the context of design and development of electrolytes and electrodes for solid-state lithium batteries. Mobility of lithium in a solid manifests itself in the following measureable ways: ionic conductivity/diffusion, redox insertion/deinsertion and ion exchange. While ionic conductivity and redox insertion/deinsertion determine the practical use of a material as an electrolyte and electrodes, respectively, ion exchange involving lithium in aqueous/molten salt media under mild conditions not only provides a convenient probe for the investigation of lithium mobility in solids, but also enables synthesis of new metastable phases. In this article, we present a chemical (rather than electrochemical) perspective of lithium ion mobility in inorganic oxide materials, in an attempt to bring out the relationships between structure and properties associated with lithium ion mobility. The survey shows that considerable lithium ion mobility occurs both in close-packed (rocksalt and its relatives, spinel, LiNbO₃, rutile and perovskite) as well as open-framework (e.g. NASICON) oxide structures. LiCoO₂ (α-NaFeO₂), LiMn₂O₄ (spinel), LiNbO₃/LiTaO₃ (structure based on HCP array of anions), LiNbWO₆ (trirutile) and (Li,La)TiO₃ (perovskite) are some of the oxide materials (structure type indicated in parentheses) where high lithium mobility has been well established by various experimental studies. An investigation of the factors that control lithium ion conductivity in the (Li,La)TiO₃ perovskite has enabled us to design new perovskite oxides in the Li–Sr–B–B′–O (B = Ti, Zr; B′ = Nb, Ta) systems that exhibit high lithium ion mobility/conductivity. Among the framework materials, NASICON (e.g. Na₃Zr₂PSi₂O₁₂) turns out to be a versatile structure that supports high lithium mobility under ion-exchange, ionic conductivity and redox insertion/deinsertion conditions.