Coupling structural evolution and oxygen-redox electrochemistry in layered transition metal oxides

Abstract

Lattice oxygen redox activity offers an unexplored way to access the latent superior electrochemical property of transition metal oxides for rechargeable batteries. However, the redox reaction of the lattice oxygen is often accompanied by unfavorable structural transformations and the corresponding degradation of electrochemical performances, precluding its practical application. Herein, we explore the close interplay between the local structural change during the dynamic intercalation process and the solid-state oxygen electrochemistry in the short-or long-term battery operation for layered transition metal oxides. By employing two model systems of the layered Na 0.6 (Li 0.2 Ti x Mn 0.8−x )O 2 with the oxygen redox capability, it is demonstrated that the substantially distinct evolutions in the oxygen redox activity and reversibility are caused by different cation migration mechanisms available in the system during the de/intercalation (i.e. out-of-plane and in-plane migrations of transition metals (TMs)). We show that the π stabilization upon the oxygen oxidation initially aids in the reversibility of the oxygen redox and is predominant in the absence of TM migrations, however, the π-interacting oxygens are gradually replaced by the σ-interacting oxygens that trigger the formation of O-O dimers and the structural destabilization over cycles. More importantly, it is revealed that the distinct TM migration paths available in the respective layered materials govern the conversion from π to σ interactions and its kinetics. It infers that regulating the dynamics of TMs in the layered materials can play a key role in delaying or inhibiting the deterioration of the oxygen redox reversibility. These ndings constitute a step forward in unraveling the correlation between the local structural evolution and the reversibility of solid-state oxygen electrochemistry, and provide a guidance for developing oxygen-redox layered electrode materials. Main TextThe use of reversible lattice oxygen redox has been a transformative strategy for accessing superior electrochemical activity of transition metal oxide-based materials such as in catalysts and battery electrodes. [1][2][3] In particular, with the growing demands for the next-generation battery technology, extensive efforts have been devoted to exploiting the lattice oxygen redox in developing novel electrode materials with higher energy densities. Lithium-rich layered oxides (Li 1 + x TM 1−x O 2 , TM: transition metal) are one of the examples, which could exhibit the remarkable oxygen redox activity. 4,5 The cumulative cationic and anionic redox activities from TM and oxygen, respectively, enable them to deliver energy and power densities that can surpass those of conventional lithium layered oxides (LiTMO 2 ). More recently, various transition metal oxides have been investigated as being capable of showing the anionic redox activity, which include not only lithium-rich layered compounds but also sodium layered oxides, disordered rocksalt phases, partially ordered spinels and...

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