Remarkable improvement of cyclic stability in Li–O2 batteries using ruthenocene as a redox mediator

Effects of Central Metal Ion on Binuclear Metal Phthalocyanine-Based Redox Mediator for Lithium Carbonate Decomposition

Li2CO3 is the most tenacious parasitic solid-state product in lithium-air batteries (LABs). Developing suitable redox mediators (RMs) is an efficient way to address the Li2CO3 issue, but only a few RMs have been investigated to date, and their mechanism of action also remains elusive. Herein, we investigate the effects of the central metal ion in binuclear metal phthalocyanines on the catalysis of Li2CO3 decomposition, namely binuclear cobalt phthalocyanine (bi-CoPc) and binuclear cobalt manganese phthalocyanine (bi-CoMnPc). Density functional theory (DFT) calculations indicate that the key intermediate peroxydicarbonate (*C2O62-) is stabilized by bi-CoPc2+ and bi-CoMnPc3+, which is accountable for their excellent catalytic effects. With one central metal ion substituted by manganese for cobalt, the bi-CoMnPc's second active redox couple shifts from the second Co(II)/Co(III) couple in the central metal ion to the Pc(-2)/Pc(-1) couple in the phthalocyanine ring. In artificial dry air (N2-O2, 78:22, v/v), the LAB cell with bi-CoMnPc in electrolyte exhibited 261 cycles under a fixed capacity of 500 mAh g-1carbon and current density of 100 mA g-1carbon, significantly better than the RM-free cell (62 cycles) and the cell with bi-CoPc (193 cycles).

OUP accepted manuscript

Aprotic lithium–oxygen (Li–O₂) batteries are receiving intense research interest by virtue of their ultra-high theoretical specific energy. However, current Li–O₂ batteries are suffering from severe barriers, such as sluggish reaction kinetics and undesired parasitic reactions. Recently, molecular catalysts, i.e. redox mediators (RMs), have been explored to catalyse the oxygen electrochemistry in Li–O₂ batteries and are regarded as an advanced solution. To fully unlock the capability of Li–O₂ batteries, an in-depth understanding of the catalytic mechanisms of RMs is necessary. In this review, we summarize the working principles of RMs and their selection criteria, highlight the recent significant progress of RMs and discuss the critical scientific and technical challenges on the design of efficient RMs for next-generation Li–O₂ batteries.

Charge Rate‐Dependent Decomposition Mechanism of Toroidal Li2O2 in Li‐O2 Batteries

Using in situ atomic force microscopy, we visualized the oxidation dynamic process of toroidal lithium peroxide (Li2O2) in a working Li‐O2 battery and clarified the correlation between the decomposition behavior and the charging rate. It was found that at a low charge rate, the decomposition could occur at the Li2O2/electrolyte interface, providing strong evidence that a tiny current is allowed to pass through the bulk phase of toroidal Li2O2. Further, the evolution of the periphery thickness, diameter and center thickness of the toroid as a function of charge capacity was quantitatively analyzed. In the early stage of charging, the diameter of the toroid radially shrinks and the shrinking rate slows down in the later stage. Besides, the periphery thickness of the toroid decreases at a faster and uniform rate, while the center thickness decreases obviously in the later stage of charging. Increasing the charging rate promotes the decomposition occurring at the Li2O2/electrode interface, causing direct desorption of the Li2O2 from the electrode and irreversible capacity degradation.