Electron and Molecule Transport across Thin Li₂O₂ Layers: How Can Dense Layers Be Distinguished from Porous Layers?

Abstract

In Li–O2 batteries, charge and mass transport across the discharge product Li2O2 plays an important role for the kinetics. In general, it is distinguished between laterally homogeneous transport across dense Li2O2 layers and heterogeneous transport across porous layers. However, in many studies, the dense or porous nature was not verified. Here, we use a combination of scanning electron microscopy, atomic force microscopy-based scratching experiments, and electrochemical measurements on thin Li2O2 layers to demonstrate a simple method for verifying the dense nature of a layer. We show that dense layers with a fraction of the free electrode surface below 10–5 exhibit virtually the same charge-transfer resistance for oxygen reduction and for the redox reaction of Co­(Cp)2 +/Co­(Cp)2 redox probe molecules, indicating that both charge-transfer resistances are determined by electron transport across the dense layers. In contrast, if this fraction exceeds 10–5, the charge-transfer resistance of the Co­(Cp)2 +/Co­(Cp)2 redox reaction is much lower than that of the oxygen reduction. Our results lead to the conclusion that measuring the charge-transfer resistance of the oxygen reduction alone is not sufficient for characterizing charge-transport limitations, but additional information about the dense/porous nature of the Li2O2 layer is indispensable.

Unraveling the Control Mechanism of Carbon Nanotubes on the Oxygen Reduction Reaction and Product Growth Behavior in Lithium–Air Batteries

“…Based on the previously reported equilibrium potential (2 Li + + 2 e – + O 2 = Li 2 O 2 , U 0 = 2.96 V), the overpotentials (η = U 0 – U ) corresponding to the initial nucleation of Li 2 O 2 and Li 2 O can also be obtained. In addition, the threshold potential of Li 2 O 2 nucleation on the DWCNT surface is slightly larger than that of SWCNTs, and DWCNTs perform better in inhibiting the formation of Li 2 O, which is consistent with the research conclusion by Ma et al As reported in previous experiments, the generation and growth of Li 2 O and Li 2 O 2 are closely related to the potential. − Therefore, precise control of the discharge potential of LABs can achieve the purpose of regulating the proportion and morphology of products, thereby improving the cycle performance of batteries. Our results show that the electrode material can control the reaction type and growth path, so as to achieve the purpose of improving the cycle performance of the battery.…”

Computational Insights into LixOy Formation, Nucleation, and Adsorption on Carbon Nanotube Electrodes in Nonaqueous Li–O2 Batteries

“…Intensive attention has been paid to electrochemistry/chemistry reactions during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), − in terms of the extremely complex formation/decomposition mechanism of the main discharge product (Li 2 O 2 ). ,− The growth pathway of Li 2 O 2 is still not fully understood, which is believed to be a multistep reaction and is divided into three cases according to the amount of electron transfer: two-electron transfer (paths 1 and 2), direct two-electron transfer (path 3), and four-electron transfer (path 4). ,− …”