Ordered Oxygen Vacancies in the Lithium-Rich Oxide Li(4)CuSbO(5.5), a Triclinic Structure Type Derived from the Cubic Rocksalt Structure

[Image: see text] Li-rich rocksalt oxides are promising candidates as high-energy density cathode materials for next-generation Li-ion batteries because they present extremely diverse structures and compositions. Most reported materials in this family contain as many cations as anions, a characteristic of the ideal cubic closed-packed rocksalt composition. In this work, a new rocksalt-derived structure type is stabilized by selecting divalent Cu and pentavalent Sb cations to favor the formation of oxygen vacancies during synthesis. The structure and composition of the oxygen-deficient Li(4)CuSbO(5.5)□(0.5) phase is characterized by combining X-ray and neutron diffraction, ICP-OES, XAS, and magnetometry measurements. The ordering of cations and oxygen vacancies is discussed in comparison with the related Li(2)CuO(2)□(1) and Li(5)SbO(5)□(1) phases. The electrochemical properties of this material are presented, with only 0.55 Li(+) extracted upon oxidation, corresponding to a limited utilization of cationic and/or anionic redox, whereas more than 2 Li(+) ions can be reversibly inserted upon reduction to 1 V vs Li(+)/Li, a large capacity attributed to a conversion reaction and the reduction of Cu(2+) to Cu(0). Control of the formation of oxygen vacancies in Li-rich rocksalt oxides by selecting appropriate cations and synthesis conditions affords a new route for tuning the electrochemical properties of cathode materials for Li-ion batteries. Furthermore, the development of material models of the required level of detail to predict phase diagrams and electrochemical properties, including oxygen release in Li-rich rocksalt oxides, still relies on the accurate prediction of crystal structures. Experimental identification of new accessible structure types stabilized by oxygen vacancies represents a valuable step forward in the development of predictive models.