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

Abstract

Surface‐enhanced Raman spectroscopy (SERS), as a nondestructive and ultra‐sensitive 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.

Interfacial wettability and mass transfer characterizations for gas–liquid–solid triple‐phase catalysis

“…Theoretically, the FES measurement is suitable for other gas-consuming reactions (oxygen reduction, nitrogen fixation, etc.). [113][114][115] The only difference lies in selecting suitable fluorescent dyes for the specific reactant/product response. Experimentally detecting interfacial concentration and mass transfer coefficient is helpful to illustrate the interfacial molecular mass transfer property in triple-phase catalysis and could give strong support to the widely applied theoretical simulations introduced in the next section.…”

True Reaction Sites on Discharge in Li–O2 Batteries

“…The Li–air battery has a high specific energy of 3500 Wh kg –1 (based on cathode reaction of forming Li 2 O 2 ); however, its performance is restricted by the cathode reaction, i.e., low capacity, large overpotential, and poor cyclability. − Extensive effort has been devoted to the catalyst design for the cathode reactions, namely, reversible formation and decomposition of Li 2 O 2 . − A large number of catalysts, including homogeneous catalysts like soluble redox mediators and heterogeneous catalysts, e.g., metal alloy, metal oxides, MOFs, etc., have been extensively explored to decrease the overpotentials, increase the capacity, and improve the cyclability. − However, so far the true reaction sites, where the O 2 molecules are reduced to produce Li 2 O 2 , have not been unambiguously identified. The way catalysts work during the discharging and charging process is under debate, particularly when the catalyst surface is covered with a thin layer of insulating Li 2 O 2 . , Therefore, it brings confusion to researchers in the catalyst design for Li–air batteries.…”

Plasma Surface Engineering of NiCo2S4@rGO Electrocatalysts Enables High-Performance Li–O2 Batteries

“…Owing to the favorable theoretical energy density (3500 W h kg –1 , O 2 + 2Li + + 2e – ↔ Li 2 O 2 ) and cost-effectiveness, lithium–oxygen batteries (LOBs) have been recognized as a viable alternative to lithium-ion batteries. − Nevertheless, the practical viability of LOBs suffers from the retard conversion of insoluble and non-conductive intermediate-product during the oxygen reduction and evolution reactions (ORR and OER). , The Li 2 O 2 , produced during discharging, can passivate the electrode surface, as it tends to accumulate and block the pores of the cathode, thereby leading to the impeded charge transfer. − Meanwhile, the insulating Li 2 O 2 is difficult to decompose over charging, resulting in the inadequate dynamics, high over-potential, and poor reversibility. − Also, the slow air diffusion can restrain the reaction rate, causing the unsatisfied discharge capacity and rate performance. Herein, tremendous efforts have been devoted to optimizing the surface chemistry and electrocatalytic ability of the catalysts for LOBs, targeting the decreased overpotential and prolonged lifespan. − To this end, a suitable catalyst is expected to have sufficient active sites for ORR/OER, interoperable channels for the species transmission, and high void volume for the precipitation of non-soluble Li 2 O 2 .…”