Ammonium Vanadium Bronze, (NH₄)₂V₇O₁₆, as a New Lithium Intercalation Host Material

A new type of ammonium vanadium bronze, (NH₄)₂V₇O₁₆, was synthesized by the hydrothermal method. The triclinic crystal structure (P1¯) is successfully identified by the single-crystal X-ray diffraction method. The layered structure is similar to that of other vanadium bronzes but with an unprecedented stoichiometry and crystal structure. The structure is composed of a stack of V₇O₁₆ layers along the c axis, and two NH₄ ⁺ ions occupy the interlayer space per formula unit. Each ammonium ion is hydrogen-bonded to four lattice oxygen atoms, resulting in a stable structure with a large interlayer space, thus enabling the intercalation of various guest ions. Lithium ions are electrochemically intercalated into (NH₄)₂V₇O₁₆, with an initial discharge capacity of 232 mAh g^-1 and an average discharge voltage of 2 V (vs Li/Li⁺). Upon the first discharge, lithium ions are inserted, whereas ammonium ions are extracted. Upon charging, a reverse reaction takes place. However, only a fraction of the extracted ammonium ions are reaccommodated. Despite the small quantity, the reinsertion of ammonium ions contributes crucially to the structural stability, improving the electrochemical performance. These results could provide a general understanding of the intercalation mechanism of host materials containing ammonium ions. In addition, (NH₄)₂V₇O₁₆ intercalates Na⁺ ions reversibly, implying a potential capability as a host material for other guest ions.