Metastable layered lithium-rich niobium and tantalum oxides via nearly instantaneous cation exchange†

Abstract

Lithium-rich early transition metal oxides are the source of excess removeable lithium that affords high energy density to lithium-rich battery cathodes. They are also candidates for solid electrolytes in all-solid-state batteries. These highly ionic compounds are sparse on phase diagrams of thermodynamically stable oxides, but soft chemical routes offer an alternative to explore new alkali-rich crystal chemistries. In this work, a new layered polymorph of Li₃NbO₄ with coplanar [Nb₄O₁₆]¹²⁻ clusters is discovered through ion exchange chemistry. A more detailed study of the ion exchange reaction reveals that it takes place almost instantaneously, changing the crystal volume by more than 22% within seconds. The transformation of coplanar [Nb₄O₁₆]¹²⁻ in L-Li₃NbO₄ into the supertetrahedral [Nb₄O₁₆]¹²⁻ clusters found in the stable cubic c-Li₃NbO₄ is also explored. Furthermore, this synthetic pathway is extended to access a new layered polymorph of Li₃TaO₄. NMR crystallography with ^6,7Li, ²³Na, and ⁹³Nb NMR, X-ray diffraction, neutron diffraction, and first-principles calculations is applied to A₃MO₄ (A = Li, Na; M = Nb, Ta) to identify local and long-range atomic structure, to monitor the unusually rapid reaction progression, and to track the phase transitions from the metastable layered phases to the known compounds found using high-temperature synthesis. A mechanism is proposed whereby some sodium is retained at short reaction times, which then undergoes proton exchange during water washing, forming a phase with hydrogen bonds bridging the coplanar [Nb₄O₁₆]¹²⁻ clusters. This study has implications for lithium-rich transition metal oxides and associated battery materials and for ion exchange chemistry in non-framework structures. The role of techniques that can detect light elements, local structure, and subtle structural changes in soft-chemical synthesis is emphasized.