Clarification of Solvent Effects on Discharge Products in Li–O₂ Batteries through a Titration Method

As a substitute for the current lithium-ion batteries, rechargeable lithium oxygen batteries have attracted much attention because of their theoretically high energy density, but many challenges continue to exist. For the development of these batteries, understanding and controlling the main discharge product Li₂O₂ (lithium peroxide) are of paramount importance. Here, we comparatively analyzed the amount of Li₂O₂ in the cathodes discharged at various discharge capacities and current densities in dimethyl sulfoxide (DMSO) and tetraethylene glycol dimethyl ether (TEGDME) solvents. The precise assessment entailed revisiting and revising the UV–vis titration analysis. The amount of Li₂O₂ electrochemically formed in DMSO was less than that formed in TEGDME at the same capacity and even at a much higher full discharge capacity in DMSO than in TEGDME. On the basis of our analytical experimental results, this unexpected result was ascribed to the presence of soluble LiO₂-like intermediates that remained in the DMSO solvent and the chemical transformation of Li₂O₂ to LiOH, both of which originated from the inherent properties of the DMSO solvent

Revealing the Reaction Mechanism of Na–O₂ Batteries using Environmental Transmission Electron Microscopy

Sodium–oxygen (Na–O₂) batteries are being extensively studied because of their higher energy efficiency compared to that of lithium oxygen (Li–O₂) batteries. The critical challenges in the development of Na–O₂ batteries include the elucidation of the reaction mechanism, reaction products, and the structural and chemical evolution of the reaction products and their correlation with battery performance. For the first time, in situ transmission electron microscopy was employed to probe the reaction mechanism and structural evolution of the discharge products in Na–O₂ batteries. The discharge product was featured by the formation of both cubic and conformal NaO₂. It was noticed that the impingement of the reaction product (NaO₂) led to particle coarsening through coalescence. We investigated the stability of the discharge product and observed that the reaction product NaO₂ was stable in the case of the solid electrolyte. The present work provides unprecedented insight into the development of Na–O₂ batteries

Revealing the Reaction Mechanism of Na–O₂ Batteries using Environmental Transmission Electron Microscopy

Sodium–oxygen (Na–O₂) batteries are being extensively studied because of their higher energy efficiency compared to that of lithium oxygen (Li–O₂) batteries. The critical challenges in the development of Na–O₂ batteries include the elucidation of the reaction mechanism, reaction products, and the structural and chemical evolution of the reaction products and their correlation with battery performance. For the first time, in situ transmission electron microscopy was employed to probe the reaction mechanism and structural evolution of the discharge products in Na–O₂ batteries. The discharge product was featured by the formation of both cubic and conformal NaO₂. It was noticed that the impingement of the reaction product (NaO₂) led to particle coarsening through coalescence. We investigated the stability of the discharge product and observed that the reaction product NaO₂ was stable in the case of the solid electrolyte. The present work provides unprecedented insight into the development of Na–O₂ batteries

Revealing the Reaction Mechanism of Na–O₂ Batteries using Environmental Transmission Electron Microscopy

Sodium–oxygen (Na–O₂) batteries are being extensively studied because of their higher energy efficiency compared to that of lithium oxygen (Li–O₂) batteries. The critical challenges in the development of Na–O₂ batteries include the elucidation of the reaction mechanism, reaction products, and the structural and chemical evolution of the reaction products and their correlation with battery performance. For the first time, in situ transmission electron microscopy was employed to probe the reaction mechanism and structural evolution of the discharge products in Na–O₂ batteries. The discharge product was featured by the formation of both cubic and conformal NaO₂. It was noticed that the impingement of the reaction product (NaO₂) led to particle coarsening through coalescence. We investigated the stability of the discharge product and observed that the reaction product NaO₂ was stable in the case of the solid electrolyte. The present work provides unprecedented insight into the development of Na–O₂ batteries

A Mo₂C/Carbon Nanotube Composite Cathode for Lithium–Oxygen Batteries with High Energy Efficiency and Long Cycle Life

Although lithium–oxygen batteries are attracting considerable attention because of the potential for an extremely high energy density, their practical use has been restricted owing to a low energy efficiency and poor cycle life compared to lithium-ion batteries. Here we present a nanostructured cathode based on molybdenum carbide nanoparticles (Mo₂C) dispersed on carbon nanotubes, which dramatically increase the electrical efficiency up to 88% with a cycle life of more than 100 cycles. We found that the Mo₂C nanoparticle catalysts contribute to the formation of well-dispersed lithium peroxide nanolayers (Li₂O₂) on the Mo₂C/carbon nanotubes with a large contact area during the oxygen reduction reaction (ORR). This Li₂O₂ structure can be decomposed at low potential upon the oxygen evolution reaction (OER) by avoiding the energy loss associated with the decomposition of the typical Li₂O₂ discharge products

Synergistic Integration of Soluble Catalysts with Carbon-Free Electrodes for Li–O₂ Batteries

The instabilities associated with solid catalysts and carbon electrode materials are one of the challenges that prevent Li–O₂ batteries from achieving a truly rechargeable high energy density. Here, we seek to achieve reversible Li–O₂ battery operations with high energies by tackling these instabilities. Specifically, we demonstrate synergistic integration of dual soluble catalysts (2,5-di-<i>tert</i>-butyl-1,4-benzoquinone (DBBQ) for discharging and (2,2,6,6-tetramethylpiperidin-1-yl)­oxyl (TEMPO) for charging) with antimony tin oxide (ATO) noncarbon electrodes with a porous inverse opal structure. The dual soluble catalysts showed a synergistic combination without any negative interference with each other, leading to higher capacity and rechargeability. Moreover, noncarbon porous antimony tin oxide (pATO) cathodes guaranteed improved stability against catalyst degradation, while KB carbon electrodes severely threatened stability of the soluble catalysts during cycling. We also found that the surface properties of the electrodes influenced the discharge mechanism, even in the presence of a solution-phase growth promoter such as DBBQ, which implies that further interface engineering may improve the performance. This study shows the great potential of the integration of soluble catalysts with electrode materials for further improvements in capacity, energy efficiency, and rechargeability for the practical development of Li–O₂ batteries

Feasibility of Full (Li-Ion)–O₂ Cells Comprised of Hard Carbon Anodes

Aprotic Li–O₂ battery is an exciting concept. The enormous theoretical energy density and cell assembly simplicity make this technology very appealing. Nevertheless, the instability of the cell components, such as cathode, anode, and electrolyte solution during cycling, does not allow this technology to be fully commercialized. One of the intrinsic challenges facing researchers is the use of lithium metal as an anode in Li–O₂ cells. The high activity toward chemical moieties and lack of control of the dissolution/deposition processes of lithium metal makes this anode material unreliable. The safety issues accompanied by these processes intimidate battery manufacturers. The need for a reliable anode is crucial. In this work we have examined the replacement of metallic lithium anode in Li–O₂ cells with lithiated hard carbon (HC) electrodes. HC anodes have many benefits that are suitable for oxygen reduction in the presence of solvated lithium cations. In contrast to lithium metal, the insertion of lithium cations into the carbon host is much more systematic and safe. In addition, with HC anodes we can use aprotic solvents such as glymes that are suitable for oxygen reduction applications. By contrast, lithium cations fail to intercalate reversibly into ordered carbon such as graphite and soft carbons using ethereal electrolyte solutions, due to detrimental co-intercalation of solvent molecules with Li ions into ordered carbon structures. The hard carbon electrodes were prelithiated prior to being used as anodes in the Li–O₂ rechargeable battery systems. Full cells containing diglyme based solutions and a monolithic carbon cathode were measured by various electrochemical methods. To identify the products and surface films that were formed during cells operation, both the cathodes and anodes were examined ex situ by XRD, FTIR, and electron microscopy. The HC anodes were found to be a suitable material for (Li-ion)–O₂ cell. Although there are still many challenges to tackle, this study offers a more practical direction for this promising battery technology and sets up a platform for further systematic optimization of its various components

Green Strategy to Single Crystalline Anatase TiO₂ Nanosheets with Dominant (001) Facets and Its Lithiation Study toward Sustainable Cobalt-Free Lithium Ion Full Battery

A green hydrothermal strategy starting from the Ti powders was developed to synthesis a new kind of well dispersed anatase TiO₂ nanosheets (TNSTs) with dominant (001) facets, successfully avoiding using the HF by choosing the safe substitutes of LiF powder. In contrast to traditional approaches targeting TiO₂ with dominant crystal facets, the strategy presented herein is more convenient, environment friendly and available for industrial production. As a unique structured anode applied in lithium ion battery, the TNSTs could exhibit an extremely high capacity around 215 mAh g^–1 at the current density of 100 mA g^–1 and preserved capacity over 140 mAh g^–1 enduring 200 cycles at 400 mA g^–1. As a further step toward commercialization, a model of lithiating TiO₂ was built for the first time and analyzed by the electrochemical characterizations, and full batteries employing lithiated TNSTs as carbon-free anode versus spinel LiNi_<i>x</i>Mn_2–<i>x</i>O₄ (x = 0, 0.5) cathode were configured. The full batteries of TNSTs/LiMn₂O₄ and TNSTs/LiNi_0.5Mn_1.5O₄ have the sustainable advantage of cost-effective and cobalt-free characteristics, and particularly they demonstrated high energy densities of 497 and 580 Wh kg_anode^–1 (i.e., 276 and 341 Wh kg_cathode^–1) with stable capacity retentions of 95% and 99% respectively over 100 cycles. Besides the intriguing performance in batteries, the versatile synthetic strategy and unique characteristics of TNSTs may promise other attracting applications in the fields of photoreaction, electro-catalyst, electrochemistry, interfacial adsorption photovoltaic devices etc

2,4-Dimethoxy-2,4-dimethylpentan-3-one: An Aprotic Solvent Designed for Stability in Li–O₂ Cells

In this study, we present a new aprotic solvent, 2,4-dimethoxy-2,4-dimethylpentan-3-one (DMDMP), which is designed to resist nucleophilic attack and hydrogen abstraction by reduced oxygen species. Li–O₂ cells using DMDMP solutions were successfully cycled. By various analytical measurements, we showed that even after prolonged cycling only a negligible amount of DMDMP was degraded. We suggest that the observed capacity fading of the Li–O₂ DMDMP-based cells was due to instability of the lithium anode during cycling. The stability toward oxygen species makes DMDMP an excellent solvent candidate for many kinds of electrochemical systems which involve oxygen reduction and assorted evaluation reactions

2,4-Dimethoxy-2,4-dimethylpentan-3-one: An Aprotic Solvent Designed for Stability in Li–O₂ Cells

In this study, we present a new aprotic solvent, 2,4-dimethoxy-2,4-dimethylpentan-3-one (DMDMP), which is designed to resist nucleophilic attack and hydrogen abstraction by reduced oxygen species. Li–O₂ cells using DMDMP solutions were successfully cycled. By various analytical measurements, we showed that even after prolonged cycling only a negligible amount of DMDMP was degraded. We suggest that the observed capacity fading of the Li–O₂ DMDMP-based cells was due to instability of the lithium anode during cycling. The stability toward oxygen species makes DMDMP an excellent solvent candidate for many kinds of electrochemical systems which involve oxygen reduction and assorted evaluation reactions