This progress report reviews the most recent discoveries regarding Li–O2chemistry during each discharge and charge process.

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A multi-redox phenazine molecule (5,10-dihydro-5,10-dimethyl phenazine or DMPZ) is used as a positive electrode material for redox flow batteries to boost energy density. Its redox mechanism and chemical stability are investigated. The marked color change during redox reaction can be utilized for the estimation of the state of charge.

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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...

To meet the ever-increasing energy demands and sustainability requirements, next-generation battery systems must provide superior energy densities while employing eco-friendly components. Transition metal oxide-based materials have served as important high-energy-density battery electrodes over the past few decades; however, their further development is challenging as we approach the theoretical limits arising from their crystal structures and constituting elements. Exploiting materials from biological systems, or bio-inspiration, offers an alternative strategy to overcome the conventional energy storage mechanism through the chemical diversity, highly efficient biochemistry, sustainability, and natural abundance provided by these materials. Here, we overview recent progress in biomimetic research focused on novel electrode material design for rechargeable batteries, exploiting redox-active molecules involved in the biometabolism and diverse bioderived materials with various morphologies. Successful demonstrations of energy storage using biomimetic materials that simultaneously exhibit outstanding performance and sustainability would provide insight toward the development of an eco-friendly and highefficiency energy storage system.

Governing the fundamental reaction in lithium–oxygen batteries is vital to realizing their potentially high energy density. Here, novel oxygen reduction reaction (ORR) catalysts capable of mediating the lithium and oxygen reaction within a solution‐driven discharge, which promotes the solution‐phase formation of lithium peroxide (Li2O2), are reported, thus enhancing the discharge capacity. The new catalysts are derived from mimicking the biological redox mediation in the electron transport chain in Escherichia coli, where vitamin K2 mediates the oxidation of flavin mononucleotide and the reduction of cytochrome b in the cell membrane. The redox potential of vitamin K2 is demonstrated to coincide with the suitable ORR potential range of lithium–oxygen batteries in aprotic solvent, thereby enabling its successful functioning as a redox mediator (RM) triggering the solution‐based discharge. The use of vitamin K2 prevents the growth of film‐like Li2O2 even in an ether‐based electrolyte, which has been reported to induce surface‐driven discharge and early passivation of the electrode, thus boosting the discharge capacity by ≈30 times. The similarity of the redox mediation in the biological cell and lithium–oxygen “cell” inspires the exploration of redox active bio‐organic compounds for potential high‐performance RMs toward achieving high specific energies for lithium–oxygen batteries.

Lithium dendrite growth in solid electrolytes is one of the major obstacles to the commercialization of solid-state batteries based on garnet-type solid electrolytes. Herein, we propose a strategy that can simultaneously resolve both the interface and electronic conductivity issues via a simple one-step procedure that provides multilayer protection at low temperature. We take advantage of the facile chemical conversion reaction, showing the wet-coated SnF 2 particles on the solid electrolyte effectively produces a multifunctional interface composed of LiF and Li−Sn alloy upon contact with lithium. We demonstrate the multifunctional interface enables the remarkably high critical current density up to 2.4 mA cm −2 at 25 °C and the stable galvanostatic cycling for over 1000 h at 0.5 mA cm −2 in the lithium symmetric cell. Moreover, the full cell delivers a robust cycle life of more than 600 cycles at 1.0 mA cm −2 , which is the highest performance at room temperature reported to date.