A‑Site Vacancy Engineering in KNbO₃ Perovskite for Enhanced Lithium Storage

Design of tailored materials using innovative approaches that allow faster charging/discharging processes could be the key for improvement of electric mobility. In this work, a strategy is developed to modify KNbO₃ perovskite structure by partially substituting K⁺ with La³⁺ at the A-site of the structure, creating two cation vacancies per substitution in the lattice. Materials with the general formula K_1–3xLa_x□_2xNbO₃ (with 0 ≤ x ≤ 0.15; □ is an A-site vacancy) have been synthesized by the sol–gel method. With La substitution and creation of artificial vacancies in the structure, KNbO₃ became activated for Li⁺ insertion. The highly substituted K_0.55La_0.15□_0.30NbO₃ (30% atomic A-site vacancies) exhibited 164 mAh g^–1 at 0.02 A g^–1 in the 0.05–3.0 V vs Li⁺/Li potential window. Ex situ ⁷Li and ⁹³Nb MAS NMR confirmed an increased Li⁺ insertion in relation to vacancies and corresponding changes in Nb⁵⁺ local environment, respectively. In situ X-ray diffraction (XRD) analysis revealed a solid-solution-type storage mechanism with a maximum volume change of only 1.3% upon Li⁺ insertion for highly substituted material. This accounts for the remarkable capacity retention obtained after 900 cycles at 0.1 A·g^–1. Diverged from the classical design of insertion materials, this study presents an alternative approach of creating vacancies without sacrificing the pristine phase, with a possibility to use the not so common class of ABO₃-type perovskites as the battery electrode.