Transition Metal Phthalocyanines as Redox Mediators in Li-O2 Batteries: A Combined Experimental and Theoretical Study of the Influence of 3d Electrons in Redox Mediation

Redox mediation is an innovative strategy for ensuring efficient energy harvesting from metal-oxygen systems. This work presents a systematic exploratory analysis of first-row transition-metal phthalocyanines as solution-state redox mediators for lithium-oxygen batteries. Our findings, based on experiment and theory, convincingly demonstrate that d5 (Mn), d7 (Co), and d8 (Ni) configurations function better compared to d6 (Fe) and d9 (Cu) in redox mediation of the discharge step. The d10 configuration (Zn) and non-d analogues (Mg) do not show any redox mediation because of the inability of binding with oxygen. The solution-state discharge product, transition-metal bound Li2O2, undergoes dissociation and oxidation in the charging step of the battery, thus confirming a bifunctional redox mediation. Apart from the reaction pathways predicted based on thermodynamic considerations, density functional theory calculations also reveal interesting effects of electrochemical perturbation on the redox mediation mechanisms and the role of the transition-metal center.

Theoretical Design and Structural Modulation of a Surface-Functionalized Ti3C2Tx MXene-Based Heterojunction Electrocatalyst for a Li-Oxygen Battery

Two-dimensional MXene with high conductivity has metastable Ti atoms and inert functional groups on the surface, greatly limiting application in surface-related electrocatalytic reactions. A surface-functionalized nitrogen-doped two-dimensional TiO₂/Ti₃C₂T_x heterojunction (N-TiO₂/Ti₃C₂T_x) was fabricated theoretically, with high conductivity and optimized electrocatalytic active sites. Based on the conductive substrate of Ti₃C₂T_x, the heterojunction remained metallic and efficiently accelerated the transfer of Li⁺ and electrons in the electrode. More importantly, the precise regulation of active sites in the N-TiO₂/Ti₃C₂T_x heterojunction optimized the adsorption for LiO₂ and Li₂O₂, facilitating the sluggish kinetics with a lowest theoretical overpotential in both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Employed as an electrocatalyst in a Li-oxygen battery (Li-O₂ battery), it demonstrated a high specific capacity of 15 298 mAh g^-1 and a superior cyclability with more than 200 cycles at 500 mA g^-1, as well as the swiftly reduced overpotential. Furthermore, combined with the in situ differential electrochemical mass spectrometry, ex situ Raman spectra, and SEM tests, the N-TiO₂/Ti₃C₂T_x heterojunction electrode presented a superior stability and reduced side reaction along with the high performance toward the ORR and OER. It provides an efficient insight for the design of high-performance electrocatalysts for metal-oxygen batteries.

Composite Cathode Architecture with Improved Oxidation Kinetics in Polymer-Based Li-O2 Batteries

The Li-O₂ battery based on the polymer electrolyte has been considered as the feasible solution to the safety issue derived from the liquid electrolyte. However, the practical application of the polymer electrolyte-based Li-O₂ battery is impeded by the poor cyclability and unsatisfactory energy efficiency caused by the structure of the porous cathode. Herein, an architecture of a composite cathode with improved oxidation kinetics of discharge products was designed by an in situ method through the polymerization of the electrolyte precursor for the polymer-based Li-O₂ battery. The composite cathode can provide sufficient gas diffusion channels, abundant reaction active sites, and continuous pathways for ion diffusion and electron transport. Furthermore, the oxidation kinetics of nanosized discharge products formed in the composite cathode can be improved by hexamethylphosphoramide during the recharge process. The polymer-based Li-O₂ batteries with the composite cathode demonstrate highly reversible capacity when fully charged and a long cycle lifetime under a fixed capacity with low overpotentials. Moreover, the interface contact between hexamethylphosphoramide and the Li metal can be stabilized simultaneously. Therefore, the composite cathode architecture designed in this work shows a promising application in high-performance polymer-based Li-O₂ batteries.