Li₃Mo₄P₅O₂₄: A Two-Electron Cathode for Lithium-Ion Batteries with Three-Dimensional Diffusion Pathways

The structure of the novel compound Li₃Mo₄P₅O₂₄ has been solved from single crystal X-ray diffraction data. The Mo cations in Li₃Mo₄P₅O₂₄ are present in four distinct types of MoO₆ octahedra, each of which has one open vertex at the corner participating in a Moî-?O double bond and whose other five corners are shared with PO₄ tetrahedra. On the basis of a bond valence sum difference map (BVS-DM) analysis, this framework is predicted to support the facile diffusion of Li⁺ ions, a hypothesis that is confirmed by electrochemical testing data, which show that Li₃Mo₄P₅O₂₄ can be utilized as a rechargeable battery cathode material. It is found that Li can both be removed from and inserted into Li₃Mo₄P₅O₂₄. The involvement of multiple redox processes occurring at the same Mo site is reflected in electrochemical plateaus around 3.8 V associated with the Mo⁶⁺/Mo⁵⁺ redox couple and 2.2 V associated with the Mo⁵⁺/Mo⁴⁺ redox couple. The two-electron redox properties of Mo cations in this structure lead to a theoretical capacity of 198 mAh/g. When cycled between 2.0 and 4.3 V versus Li⁺/Li, an initial capacity of 113 mAh/g is observed with 80% of this capacity retained over the first 20 cycles. This compound therefore represents a rare example of a solid state cathode able to support two-electron redox capacity and provides important general insights about pathways for designing next-generation cathodes with enhanced specific capacities.