Understanding oxygen electrochemistry in aprotic Li-O_2 batteries

In the past decade, the aprotic lithium-oxygen(Li-O_2) battery has generated a great deal of interest because theoretically it can store more energy than today's lithium-ion batteries. Although considerable research efforts have been devoted to the R&D of this potentially disruptive technology, many scientific and engineering obstacles still remain to be addressed before a practical device could be realized. In this review, we summarize recent advances in the fundamental understanding of the O_2 electrochemistry in Li-O_2 batteries, including the O_2 reduction to Li_2O_2 on discharge and the reverse Li_2 O_2 oxidation on recharge and factors that exert strong influences on the redox of O_2/Li_2O_2. In addition,challenges and perspectives are also provided for the future study of Li—O_2 batteries.

Oxygen electrochemistry in Li-O2 batteries probed by in situ surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy(SERS),as a nondestructive and ultrasensitive single molecular level characterization technique,is a powerful tool to deeply understand the interfacial electrochemistry reaction mechanism involved in energy conversion and storage,especially for oxygen electrochemistry in Li-O2 batteries with unrivaled theoretical energy density.SERS can provide precise spectroscopic identification of the reactants,intermediates and products at the electrode|electrolyte interfaces,independent of their physical states(solid and/or liquid)and crystallinity level.Furthermore,SERS’s power to resolve different isotopes can be exploited to identify the mass transport limitation and reactive sites of the passivated interface.In this review,the application of in situ SERS in studying the oxygen electrochemistry,specifically in aprotic Li-O2 batteries,is summarized.The ideas and concepts covered in this review are also extended to the perspectives of the spectroelectrochemistry in general aprotic metal-gas batteries.

Metal phosphonate-derived cobalt/nickel phosphide@N-doped carbon hybrids as efficient bifunctional oxygen electrodes for Zn-air batteries

The exploration of efficient bifunctional electrocatalysts for oxygen reduction reaction and oxygen evolution reaction is pivotal for the development of rechargeable metal–air batteries.Transition metal phosphides are emerging as promising catalyst candidates because of their superb activity and low cost.Herein,a novel metal phosphonate-derived cobalt/nickel phosphide@N-doped carbon hybrid was developed by a carbothermal reduction of cobalt/nickel phosphonate hybrids with different Co/Ni molar ratios.The metal phosphonate derivation method achieved an intimately coupled interaction between metal phosphides and a heteroatom-doped carbon substrate.The resultant Co_(2)P/Ni_(3)P@NC-0.2 enables an impressive electrocatalytic oxygen reduction reaction activity,comparable with those of state-of-the-art Pt/C catalysts in terms of onset potential(0.88 V),4e‒selectivity,methanol tolerance,and long-term durability.Moreover,remarkable oxygen evolution reaction activity was also observed in alkaline conditions.The high activity is ascribed to the N-doping,abundant accessible catalytic active sites,and the synergistic effect among the components.This work not only describes a highefficiency electrocatalyst for both oxygen reduction reaction and oxygen evolution reaction,but also highlights the application of metal phosphonate hybrids in fabricating metal phosphides with tunable structures,which is of great significance in the energy conversion field.

Electronically coupled layered double hydroxide/MXene quantum dot metallic hybrids for high-performance flexible zinc–air batteries

Precise control of the local electronic structure and properties of electrocatalysts is important for enhancing the multifunctionality and durability of electrocatalysts and for correlating the structure/chemistry with the catalytic properties.Herein,we report electronically coupled metallic hybrids of NiFe layered double hydroxide nanosheet/Ti3C2 MXene quantum dots deposited on a nitrogen-doped graphene surface(LDH/MQD/NG)for high-performance flexible Zn-air batteries(ZABs).As verified from the Mott-Schottky and Nyquist plots,as well as spectroscopic,electrochemical,and computational analyses,the electronic and chemical coupling of LDH/MQD/NG modulates the local electronic and surface structure of the active LDH to provide metallic conductivity and abundant active sites,leading to significantly improved bifunctional activity and electrocatalytic kinetics.The rechargeable ZABs with LDH/MQD/NG hybrids are superior to the previous LDH-based ZABs,demonstrating a high power density(113.8 mW cm^(-2))and excellent cycle stability(150 h at 5 mA cm^(-2)).Moreover,the corresponding quasi solid-state ZABs are completely flexible and practical,affording a high power density of 57.6 mW cm^(-2)even in the bent state,and in real-life operation of tandem cells for powering various electronic devices.

The origin of potential rise during charging of Li-O2 batteries

When aprotic Li-O_2 batteries recharge, the solid Li_2O_2 in the positive electrode is oxidized, which often exhibits a continuous or step increase in the charging potential as a function of the charging capacity, and its origin remains incompletely understood.Here, we report a model study of electro-oxidation of a Li_2O_2 film on an Au electrode using voltammetry coupled with in situ Raman spectroscopy. It was found that the charging reaction initializes at the positive electrode|Li_2O_2 interface, instead of the previously presumed Li_2O_2 surface, and consists of two temporally and spatially separated Li_2O_2 oxidation processes, accounting for the potential rise during charging of Li-O_2 batteries. Moreover, the electrode surface-initialized oxidation can disintegrate the Li_2O_2 film resulting in a loss of Li_2O_2 into electrolyte solution, which drastically decreases the charging efficiency and highlights the importance of using soluble electro-catalyst for the complete charging of Li-O_2 batteries.