Nature of Li2O2 and its relationship to the mechanisms of discharge/charge reactions of lithium-oxygen batteries.

Lithium-air batteries (LABs) are considered one of the most promising energy storage devices because of their large theoretical energy density. However, low cyclability caused by battery degradation prevents its practical use. Thus, to realize practical LABs, it is essential to improve cyclability significantly by understanding how the degradation processes proceed. Here, we used online mass spectrometry for real-time monitoring of gaseous products generated during charging of lithium-oxygen batteries (LOBs), which was operated with pure oxygen not air, with 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) tetraethylene glycol dimethyl ether (TEGDME) electrolyte solution. Linear voltage sweep (LVS) and voltage step modes were employed for charge instead of constant current charge so that the energetics of the product formation during the charge process can be understood more quantitatively. The presence of two distinctly different types of Li2O2, one being decomposed in a wide range of relatively low cell voltages (2.8-4.16 V) (l-Li2O2) and the other being decomposed at higher cell voltages than ca. 4.16 V (h-Li2O2), was confirmed by both LVS and step experiments. H2O generation started when the O2 generation rate reached a first maximum and CO2 generation took place accompanied by the decomposition of h-Li2O2. Based on the above results and the effects of discharge time and the use of isotope oxygen during discharge on product distribution during charge, the generation mechanism of O2, H2O, and CO2 during charging is discussed in relation to the reactions during discharge.

Evaluation of performance metrics for high energy density rechargeable lithium-oxygen batteries.

The demand for practical implementation of rechargeable lithium-oxygen batteries (LOBs) has grown owing to their extremely high theoretical energy density. However, the factors determining the performance of cell-level high energy density LOBs remain unclear. In this study, LOBs with a stacked-cell configuration were fabricated and their performance evaluated under different experimental conditions to clarify the unique degradation phenomenon under lean-electrolyte and high areal capacity conditions. First, the effect of the electrolyte amount against areal capacity ratio (E/C) on the battery performance was evaluated, revealing a complicated voltage profile for an LOB cell operated under high areal capacity conditions. Second, the impact of different kinds of gas-diffusion layer materials on the "sudden death" phenomenon during the charging process was investigated. The results obtained in the present study reveal the importance of these factors when evaluating the performance metrics of LOBs, including cycle life, and round-trip energy efficiency. We believe that adopting a suitable experimental setup with appropriate technological parameters is crucial for accurately interpreting the complicated phenomenon in LOBs with cell-level high energy density.

On-Surface Synthesis of Multiple Cu Atom-Bridged Organometallic Oligomers.

A metal-metal bond between coordination complexes has the nature of a covalent bond in hydrocarbons. While bimetallic and trimetallic compounds usually have three-dimensional structures in solution, the high directionality and robustness of the bond can be applied for on-surface syntheses. Here, we present a systematic formation of complex organometallic oligomers on Cu(111) through sequential ring opening of 11,11,12,12-tetraphenyl-1,4,5,8-tetraazaanthraquinodimethane and bonding of phenanthroline derivatives by multiple Cu atoms. A detailed characterization with a combination of scanning tunneling microscopy and density functional theory calculations revealed the role of the Cu adatoms in both enantiomers of the chiral oligomers. Furthermore, we found sufficient strength of the bonds against sliding friction by manipulating the oligomers up to a hexamer. This finding may help to increase the variety of organometallic nanostructures on surfaces.

Size dependent electrocatalytic activities of h-BN for oxygen reduction reaction to water.

Electrocatalytic activities for the oxygen reduction reaction (ORR) of Au electrodes modified by as prepared and size selected (0.45-1.0, 0.22-0.45, and 0.1-0.22 µm) h-BN nanosheet (BNNS), which is an insulator, were examined in O2 saturated 0.5M H2SO4 solution. The overpotential was reduced by all the BNNS modifications, and the smaller the size, the smaller the overpotential for ORR, i.e., the larger the ORR activity, in this size range. The overpotential was reduced by as much as ∼330 mV compared to a bare Au electrode by modifying the Au surface by the BNNS of the smallest size range (0.1-0.22 µm). The overpotential at this electrode was only 80 mV more than that at the Pt electrode. Both the rotation disk electrode experiments with Koutecky-Levich analysis and rotating ring disk electrode measurements showed that more than 80% of oxygen is reduced to water via the four-electron process at this electrode. These results strongly suggest and theoretical density functional theory calculations support that the ORR active sites are located at the edges of BNNS islands adsorbed on Au(111). The decrease in size of BNNS islands results in an effective increase in the number of the catalytically active sites and, hence, in the increase in the catalytic activity of the BNNS/Au(111) system for ORR.

CNT-MXene ultralight membranes: fabrication, surface nano/microstructure, 2D-3D stacking architecture, ion-transport mechanism, and potential application as interlayers for Li-O2 batteries.

Multiwalled carbon nanotubes (MWCNTs) have shown effectiveness in improving the suitability of MXenes for energy-related applications. However, the ability of individually dispersed MWCNTs to control the structure of MXene-based macrostructures is unclear. Here, the correlation among composition, surface nano- and microstructure, MXenes' stacking order, structural swelling, and Li-ion transport mechanisms and properties in individually dispersed MWCNT-Ti3C2 films was investigated. The compact surface microstructure of MXene film, characterized by prominent wrinkles, is dramatically changed as MWCNTs occupy MXene/MXene edge interfaces. The 2D stacking order is preserved up to 30 wt% MWCNTs despite a significant swelling of ∼400%. Such alignment is completely disrupted at 40 wt%, and a more pronounced surface opening and internal expansion of ∼770% are realized. Both 30 wt% and 40 wt% membranes show stable cycling performance under a significantly higher current density due to faster transport channels. Notably, for the 3D membrane, the overpotential during repeated Li deposition/dissolution reactions is further reduced by ∼50%. Ion-transport mechanisms in the absence and presence of MWCNTs are discussed. Furthermore, ultralight yet continuous hybrid films comprising up to ∼0.027 mg cm-2 Ti3C2 can be prepared using aqueous colloidal dispersions and vacuum filtration for specific applications. The potential application of such ultralight membranes as interlayers for Li-O2 batteries is briefly examined.

Real time monitoring of generation and decomposition of degradation products in lithium oxygen batteries during discharge/charge cycles by an online cold trap pre-concentrator-gas chromatography/mass spectroscopy system.

Degradation products of lithium oxygen batteries with a tetraethylene glycol dimethyl ether (TEGDME) electrolyte solution during discharge/charge cycles were monitored by an online cold trap pre-concentrator-gas chromatography/mass spectroscopy system in real time. A total of 37 peaks were detected and 27 of them were assigned to specific molecules. Degradation compounds were generated and decomposed in very complex manners during discharge/charge cycles. Most molecules were generated during charge as a result of the degradation of TEGDME by active oxygen species and/or electrochemical oxidation. These molecules generated during charge were decomposed during discharge by active oxygen species.

Criteria for evaluating lithium-air batteries in academia to correctly predict their practical performance in industry.

Although the market share for Li-ion batteries (LiBs) has continuously expanded, the limited theoretical energy density of conventional LiBs will no longer meet the advanced energy storage requirements. Lithium-air batteries (LABs) are potential candidates for next-generation rechargeable batteries because of their extremely high theoretical energy density. However, the reported values for the actual energy density of LABs are much lower than those for LiBs, mainly due to the excess amount of electrolyte in the cell. In the present review article, the practical energy density is estimated for the representative LABs reported in academia, and the critical factors for improving the energy density of LABs are summarized. The criteria for evaluating LABs in laboratory-based experiments are also proposed for accurately predicting the performance of practical cells in industry.

Heterocyclic Ring-Opening of Nanographene on Au(111).

Cyclo-dehydrogenation is of importance to induce the planarization of molecules on noble surfaces upon annealing. In contrast to a number of successful syntheses of polycyclic aromatic hydrocarbons by forming carbon-carbon bonds, it is still rare to conduct conjugation and cleavage of carbon-nitrogen bonds in molecules. Here, we present a systematic transformation of the C-N bonds in11,11,12,12-tetraphenyl-1,4,5,8-tetraazaanthraquinodimethane as well as three other derivatives on Au(111). With bond-resolved scanning tunneling microscopy, we discovered novel the "heterocyclic segregation" reaction of one pyrazine ring with two nitrogen atoms to form two quinoline rings with one nitrogen each. Density functional theory calculations showed that the intramolecular ring-forming and -opening of N-heterocycles are strongly affected by the initial hydrogen-substrate interaction.

Probing Molecular Mechanisms during the Oscillatory Adsorption of Propyl Chain Functionalized Organosilane Films with Sum Frequency Generation Spectroscopy.

The selectivity rules of sum frequency generation spectroscopy were exploited to determine propyl chain order during the time-dependent oscillatory adsorption of propyltrimethoxysilane (PTMS) and Langmuir-type growth of propyldimethylmethoxysilane (PDMMS). During the early stages of film growth, molecular packing density determines the extent of propyl chain defects within both films with high surface coverage resulting in a film with fewer defects. Following this, an ordered monolayer-like film stabilizes on the Al2O3 substrate for both silanes. Although this result is intuitive for the Langmuir-type growth of PDMMS, the stabilization of molecular ordering despite the continuing oscillation in PTMS surface coverage indicates the presence of a stable monolayer, while it is the oligomerized PTMS dendrimers which continue to desorb and readsorb to the substrate. We also reveal for the first time, the formation of a physisorbed bilayer during the self-assembly process of PTMS. The presence of this ordered, physisorbed bilayer on top of the covalently bound PTMS film plays a key role in the process of the molecular self-assembly mechanism and is proposed to enable further condensation of the covalently bound film.

Electrochemical Growth of Very Long (∼80 µm) Crystalline Li2O2 Nanowires on Single-Layer Graphene Covered Gold and Their Growth Mechanism.

For the development of lithium-air battery (LAB), which is one of the most promising next generation batteries, it is essential to understand the structure and properties of Li2O2, which is the discharged product at the positive electrode of a LAB, as well as the mechanism of Li2O2 growth because its deposition limits the discharge capacity and is the origin of the high charging overpotential of LAB. Characterization of the structure and properties of the Li2O2 formed in LABs is, however, difficult because it is usually in the form of poorly ordered small particles. In this study, we successfully grew well-aligned very long (∼80 µm) crystalline Li2O2 nanowires (NWs: average diameter of 22 nm) electrochemically at a gold electrode covered with single-layer graphene (SLG/Au). Preferential growth of the NWs along c-axis was confirmed by X-ray diffraction, transmission electron microscopy with electron diffraction, and Raman scattering. Raman imaging indicated that the sites of NW growth were the grain boundaries of single-layer graphene. The long, crystalline Li2O2 NWs provided the opportunity to investigate not only their structure and properties but also their growth mechanism during discharge. Raman measurements in the O-O stretching frequency region of the SLG/Au electrode at various depths of the discharge combined with exchange of oxygen in the solution from 18O2 to 16O2 during the discharge revealed that the growth took place at the bottom of the NWs, i.e., the Li2O2/electrode interface, not the top of the NWs, i.e., the solution/Li2O2 interface. This growth mechanism can explain why such long NWs can be grown despite the insulating nature of Li2O2.

Anomalously Slow Conformational Change Dynamics of Polar Groups Anchored to Hydrophobic Surfaces in Aqueous Media.

Water molecules within a thin hydration layer, spontaneously generated on hydrophobic protein surfaces, are reported to form a poorly dynamic network structure. However, how such a water network affects the conformational change dynamics of polar groups has never been explored, although such polar groups play a critical role in protein-protein and protein-ligand interactions. In the present work, we utilized as model protein surfaces a series of self-assembled monolayers (SAMs) appended with polar (Fmoc) or ionic (FITC) fluorescent head groups that were tethered via a 1.5-nm-long flexible oligoether chain to a hydrophobic silicon wafer surface, which was densely covered with paraffinic chains. We found that, not only in deionized water but also in aqueous buffer, these oligoether-appended head groups at ambient temperatures both displayed an anomalously slow conformational change, which required ∼10 h to reach a thermodynamically equilibrated state. We suppose that these behaviors reflect the poorly dynamic and low-permittivity natures of the thin hydration layer.

A quantum chemical study of substituent effects on CN bonds in aryl isocyanide molecules adsorbed on the Pt surface.

A periodicity implemented scheme of natural bond orbital (NBO) theory and normal mode analysis has been employed to investigate the tendency of the chemical bond strength of aryl isocyanide molecules with different para-substituted groups adsorbed on the Pt(111) surface. The NC bond order shows a clear correspondence with the NC stretching frequency; both of them exhibit a "volcano-like" profile as a function of the Hammett constant of the para-substituted groups for isolated molecules. When a molecule is adsorbed on the Pt(111) surface, the NC stretching frequency variations are determined by the resultant effect of σ donation and π back-donation between the molecule and the surface. The present comprehensive and systematic computations clarify the electron donating and withdrawing effects of the substituted groups on the interaction between the aryl isocyanide molecule and the transition metal substrate.

Effects of HF on the Lithiation Behavior of the Silicon Anode in LiPF6 Organic Electrolyte Solution.

As one of the major impurities in the organic electrolyte, HF can react with the alkali components in the solid electrolyte interphase (SEI), such as lithium alkoxide and lithium carbonate, to form more LiF-rich SEI. Here, the effects of HF on the lithiation behavior of the single crystal Si(111) anode were studied using scanning electron microscopy, soft X-ray emission spectroscopy, and windowless energy-dispersive X-ray spectroscopy. When the Li-Si alloy is formed in 1.0 M LiPF6 in the propylene carbonate solvent, it has a layered structure that contained the first layer of crystalline Li15Si4 (c-Li15Si4) alloy pyramids, the second layer of amorphous Li13Si4 (a-Li13Si4) alloy, and a third layer of Li-diffused Li x Si alloy. When the more concentrated HF is in the electrolyte solution, less amount of the c-Li15Si4 alloy is formed in the first layer. It suggests that the Si lithiation can form only amorphous Li x Si alloy relative to the components in the electrolytes. The study also explains why only amorphous Li x Si alloy formation was observed in some previous studies.

Material balance in the O2 electrode of Li-O2 cells with a porous carbon electrode and TEGDME-based electrolytes.

This work figures out the material balance of the reactions occurring in the O2 electrode of a Li-O2 cell, where a Ketjenblack-based porous carbon electrode comes into contact with a tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte under more practical conditions of less electrolyte amount and high areal capacity. The ratio of electrolyte weight to cell capacity (E/C, g A h-1) is a good parameter to correlate with cycle life. Only 5 cycles were obtained at an areal capacity of 4 mA h cm-2 (E/C = 10) and a discharge/charge current density of 0.4 mA cm-2, which corresponds to the energy density of 170 W h kg-1 at a complete cell level. When the areal capacity was decreased to half (E/C = 20) by setting a current density at 0.2 mA cm-2, the cycle life was extended to 18 cycles. However, the total electric charge consumed for parasitic reactions was 35 and 59% at the first and the third cycle, respectively. This surprisingly large amount of parasitic reactions was suppressed by half using redox mediators at 0.4 mA cm-2 while keeping a similar cycle life. Based on by-product distribution, we will propose possible mechanisms of TEGDME decomposition and report a water breathing behavior, where H2O is produced during charge and consumed during discharge.

Quantum-to-Classical Transition of Proton Transfer in Potential-Induced Dioxygen Reduction.

We report an observation of a quantum tunneling effect in a proton-transfer (PT) during potential-induced transformation of dioxygen on a platinum electrode in a low overpotential (η) region at 298 K. However, this quantum process is converted to the classical PT scheme in the high η region. Therefore, there is a quantum-to-classical transition of the PT (QCT-PT) process as a function of the potential, which is confirmed by theoretical analysis. This observation indicates that the quantum tunneling governs the multistep electron-proton-driven transformation of dioxygen in the low η condition.

Electrical Matching at Metal/Molecule Contacts for Efficient Heterogeneous Charge Transfer.

In a metal/molecule hybrid system, unavoidable electrical mismatch exists between metal continuum states and frontier molecular orbitals. This causes energy loss in the electron conduction across the metal/molecule interface. For efficient use of energy in a metal/molecule hybrid system, it is necessary to control interfacial electronic structures. Here we demonstrate that electrical matching between a gold substrate and π-conjugated molecular wires can be obtained by using monatomic foreign metal interlayers, which can change the degree of d-π* back-donation at metal/anchor contacts. This interfacial control leads to energy level alignment between the Fermi level of the metal electrode and conduction molecular orbitals, resulting in resonant electron conduction in the metal/molecule hybrid system. When this method is applied to molecule-modified electrocatalysts, the heterogeneous electrochemical reaction rate is considerably improved with significant suppression of energy loss at the internal electron conduction.

Potassium Ions Promote Solution-Route Li2O2 Formation in the Positive Electrode Reaction of Li-O2 Batteries.

Lithium-oxygen system has attracted much attention as a battery with high energy density that could satisfy the demands for electric vehicles. However, because lithium peroxide (Li2O2) is formed as an insoluble and insulative discharge product at the positive electrode, Li-O2 batteries have poor energy capacities. Although Li2O2 deposition on the positive electrode can be avoided by inducing solution-route pathway using electrolytes composed of high donor number (DN) solvents, such systems generally have poor stability. Herein we report that potassium ions promote the solution-route formation of Li2O2. The present findings suggest that potassium or other monovalent ions have the potential to increase the volumetric energy density and life cycles of Li-O2 batteries.