New lithium-ion conducting compounds 3Li3N-MI (M = Li, Na, K, Rb) and their application to solid-state lithium-ion cells

In the quasi-binary systems Li3N-MI (M = Li, Na, K, Rb) new intermediate compounds 3Li3N-MI were synthesized by a solid-state reaction between Li3N and MI at 600°C. They were isomorphous and had a tetragonal unit cell. Their sintered bodies exhibited lithium-ion conductivity of 1.1 × 10−4–7.0 × 10−5 S cm−1 at room temperature. The grain boundary resistances were negligibly small because of their good sinterability. The decomposition voltages were approximately 2.5–2.8 V, much higher than that of Li3N. Even the compacts of the powdered compounds showed total conductivities more than 10−5 S cm−1. This allowed the cell construction of the solid-state lithium-ion cell, C/3Li3N-KI/LiTiS2, simply by pressing the powdered electrolyte and electrode materials. The cell was able to charge and to discharge at a constant current of 15 μA cm−2 at room temperature; however, severe polarization in the positive electrodes limited the charge/discharge performance.