Various strategies to tune the ionic/electronic properties of electrode materials

Abstract

This Perspective highlights, through several snapshot examples, the importance of electrochemically-driven redox reactions in tuning the electronic/ionic as well as magnetic properties of 3d-metal-based inorganic compounds through a careful control of the metal oxidation state. Although such redox reactions usually imply the electron-ionic duality, they can be extended to insulating compounds (LiFePO₄) or semiconductors (CoO) as long as we can combine electrochemistry at the nanoscale to reduce diffusion and migration limitations, and provide the compounds with electrons through metallic coating techniques. A thorough investigation of the composition–structure–property relationships of the Li_xCoO₂ system, through the assembly of LiCoO₂/Li electrochemical cells has led to the identification of the CoO₂ phase, whose property and stability are discussed in terms of cationic–anionic redox competition, thus bearing some similarity with the high T_c cuprate superconductors. Such a d–sp redox competition could have structural and electronic consequences. Encouraged by the recently reported superconductivity in Na_xCoO₂;yH₂O phase, the room temperature Li_xCuO₂ phase diagram was reinvestigated through Li-driven electrochemical reactions. A solid solution domain was unravelled but superconductivity was not evident. With Cu-based materials such as Cu_2.33V₄O₁₁, we have shown the feasibility of a new reversible Li electrochemically-driven copper extrusion/insertion process, owing to the enhanced copper diffusion within the structure.