Voltage decay and redox asymmetry mitigation by reversible cation migration in lithiumrich layered oxide electrodes.

Calcium‐ion batteries (CIBs) are considered to be promising next‐generation energy storage systems because of the natural abundance of calcium and the multivalent calcium ions with low redox potential close to that of lithium. However, the practical realization of high‐energy and high‐power CIBs is elusive owing to the lack of suitable electrodes and the sluggish diffusion of calcium ions in most intercalation hosts. Herein, it is demonstrated that calcium‐ion intercalation can be remarkably fast and reversible in natural graphite, constituting the first step toward the realization of high‐power calcium electrodes. It is shown that a graphite electrode exhibits an exceptionally high rate capability up to 2 A g−1, delivering ≈75% of the specific capacity at 50 mA g−1 with full calcium intercalation in graphite corresponding to ≈97 mAh g−1. Moreover, the capacity stably maintains over 200 cycles without notable cycle degradation. It is found that the calcium ions are intercalated into graphite galleries with a staging process. The intercalation mechanisms of the “calciated” graphite are elucidated using a suite of techniques including synchrotron in situ X‐ray diffraction, nuclear magnetic resonance, and first‐principles calculations. The versatile intercalation chemistry of graphite observed here is expected to spur the development of high‐power CIBs.

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...

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.

For the realization of high-power lithium–oxygen batteries, a comparative study was conducted to investigate the kinetic properties of redox mediators.