Revealing the Intrinsic Atomic Structure and Chemistry of Amorphous LiO₂-Containing Products in Li–O₂ Batteries Using Cryogenic Electron Microscopy

Abstract

Aprotic lithium–oxygen batteries (LOBs) are promising energy storage systems characterized by ultrahigh theoretical energy density. Extensive research has been devoted to this battery technology, yet the detailed operational mechanisms involved, particularly unambiguous identification of various discharge products and their specific distributions, are still unknown or are subjects of controversy. This is partly because of the intrinsic complexity of the battery chemistry but also because of the lack of atomic-level insight into the oxygen electrodes acquired via reliable techniques. In the current study, it is demonstrated that electron beam irradiation could induce crystallization of amorphous discharge products. Cryogenic conditions and a low beam dosage have to be used for reliable transmission electron microscopy (TEM) characterization. High-resolution cryo-TEM and electron energy loss spectroscopy (EELS) analysis of toroidal discharge particles unambiguously identified the discharge products as a dominating amorphous LiO2 phase with only a small amount of nanocrystalline Li2O2 islands dispersed in it. In addition, uniform mixing of carbon-containing byproducts is identified in the discharge particles with cryo-EELS, which leads to a slightly higher charging potential. The discharge products can be reversibly cycled, with no visible residue after full recharge. We believe that the amorphous superoxide dominating discharge particles can lead researchers to reconsider the chemistry of LOBs and pay special attention to exclude beam-induced artifacts in traditional TEM characterizations.

Kinetically Accelerated Lithium Storage in High‐Entropy (LiMgCoNiCuZn)O Enabled By Oxygen Vacancies

“…For one thing, more oxygen vacancies increased the equilibrium electron density, thereby pushing the Fermi level upward. [ 36 ] For another, the surface adsorbed oxygen species changed after Li substitution (more O − and were adsorbed [ 37,38 ] ). Figure 3e evinces the DSC curves of MO and Li‐MO precursors.…”

Reversible Discharge Products in Li–Air Batteries

“…More leading‐edge in situ technologies are required to monitor the products’ dynamic evolution of electronic state, morphology, crystallinity, elemental composition, mass migration, and others. For example, in situ electrochemical TEM could observe the product growth in the liquid electrolyte environment, in situ X‐ray absorption spectroscopy could track the average information of products’ bulk particles, cryo‐TEM could study the LiO 2 growth pathway on different cathodes, [ 99 ] synchrotron‐based XRD could explore the transient changes in the LiOH crystal during battery cycling, operando pressure measurements could confirm discharge products via the number of electrons consumed per mole of gas, [ 140 ] EQCM could reveal the quantitative information of the deposited products, [ 141 ] X‐ray absorption near edge structure analysis could reveal the change in chemical oxidation state and chemical compositions of elements. [ 142 ] In addition, the combination of diverse in situ technologies to monitor the product evolution in one setup could simultaneously achieve more reliable results. Theoretical Calculations : The DFT‐based theoretical investigation has been widely used in the studies of discharge products in metal–air batteries and fetched much advancement.…”

“…Reproduced with permission. [ 99 ] Copyright 2022, American Chemical Society. h) Pd‐rGO catalyzes the amorphous LiO 2 products in Li–O 2 battery, dramatically reducing the voltage gap to 0.3 V at 200 mA g −1 .…”

“…used cryogenic‐TEM to study the toroidal discharge products in Li–O 2 batteries. [ 99 ] The results revealed that the toroidal products were primarily composed of amorphous LiO 2 with a tiny amount of crystalline Li 2 O 2 (Figure 6g). The amorphous LiO 2 was also obtained based on a 3D‐architectured Pd‐rGO cathode, and the resultant battery displayed an ultralow overpotential of 0.3 V (Figure 6h).…”

Morphology-Dictated Mechanism of Efficient Reaction Sites for Li2O2 Decomposition

“…However, although various in situ characterizations have been performed to study the charge process, violent controversies still exist about whether it is the electrode/Li 2 O 2 /electrolyte triple-phase boundary, the electrode/Li 2 O 2 interface, ,− or the Li 2 O 2 /electrolyte interface ,− that serves as the reaction site for Li 2 O 2 decomposition. Hou et al followed real-time decomposition of Li 2 O 2 particles by in situ transmission electron microscopy and found that the decomposition initially occurred at the electrolyte–RuO 2 –Li 2 O 2 triple-phase boundary, in which RuO 2 served as a solid catalyst for decomposition.…”