The chemistry of ternary and higher lithium nitrides

Lithium, as the lightest metallic element, forms a wide range of compounds of increasing importance as functional materials. This is especially true in an energy storage and conversion context, for example, where high energy density and high lithium ion mobility provide the drivers behind technologies such as rechargeable batteries and hydrogen storage. As a small, monovalent, mobile cation, Li⁺, is amenable structurally to a variety of coordination environments and its ability to readily occupy vacancies and interstitial positions lends it to a rich insertion and intercalation chemistry and the flexibility to form a myriad of structure types across a large stoichiometric range. This flexibility is as prevalent in nitrides as in oxides and other inorganic solids and lithium forms a larger number of ternary and higher compounds with nitrogen than any other single metal. Nevertheless, there are clear trends in the crystal chemistry of lithium nitrides and patterns to the bonding within these structures; key structure types and motifs dominate. Hence, not only does it become possible to anticipate composition-structure relationships in the synthesis of new nitrides, but also materials design and prescribed properties from magnetism through semiconducting and optical properties to superionic conductivity becomes a realistic prospect. This review presents a comprehensive account of the crystal chemistry of ternary and higher lithium nitrides across the periodic table and highlights the opportunities for materials design from the emerging understanding of structure-property relationships in these compounds.