Theoretical Design and Structural Modulation of a Surface-Functionalized Ti3C2Tx MXene-Based Heterojunction Electrocatalyst for a Li-Oxygen Battery

Two-dimensional MXene with high conductivity has metastable Ti atoms and inert functional groups on the surface, greatly limiting application in surface-related electrocatalytic reactions. A surface-functionalized nitrogen-doped two-dimensional TiO₂/Ti₃C₂T_x heterojunction (N-TiO₂/Ti₃C₂T_x) was fabricated theoretically, with high conductivity and optimized electrocatalytic active sites. Based on the conductive substrate of Ti₃C₂T_x, the heterojunction remained metallic and efficiently accelerated the transfer of Li⁺ and electrons in the electrode. More importantly, the precise regulation of active sites in the N-TiO₂/Ti₃C₂T_x heterojunction optimized the adsorption for LiO₂ and Li₂O₂, facilitating the sluggish kinetics with a lowest theoretical overpotential in both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Employed as an electrocatalyst in a Li-oxygen battery (Li-O₂ battery), it demonstrated a high specific capacity of 15 298 mAh g^-1 and a superior cyclability with more than 200 cycles at 500 mA g^-1, as well as the swiftly reduced overpotential. Furthermore, combined with the in situ differential electrochemical mass spectrometry, ex situ Raman spectra, and SEM tests, the N-TiO₂/Ti₃C₂T_x heterojunction electrode presented a superior stability and reduced side reaction along with the high performance toward the ORR and OER. It provides an efficient insight for the design of high-performance electrocatalysts for metal-oxygen batteries.

A long-life lithium-oxygen battery via a molecular quenching/mediating mechanism

The advancement of lithium-oxygen (Li-O2) batteries has been hindered by challenges including low discharge capacity, poor energy efficiency, severe parasitic reactions, etc. We report an Li-O2 battery operated via a new quenching/mediating mechanism that relies on the direct chemical reactions between a versatile molecule and superoxide radical/Li2O2. The battery exhibits a 46-fold increase in discharge capacity, a low charge overpotential of 0.7 V, and an ultralong cycle life >1400 cycles. Featuring redox-active 2,2,6,6-tetramethyl-1-piperidinyloxy moieties bridged by a quenching-active perylene diimide backbone, the tailor-designed molecule acts as a redox mediator to catalyze discharge/charge reactions and serves as a reusable superoxide quencher to chemically react with superoxide species generated during battery operation. The all-in-one molecule can simultaneously tackle issues of parasitic reactions associated with superoxide radicals, singlet oxygen, high overpotentials, and lithium corrosion. The molecular design of multifunctional additives combining various capabilities opens a new avenue for developing high-performance Li-O2 batteries.

In-situ characterization of discharge products of lithium-oxygen battery using Flow Electrochemical Atomic Force Microscopy

The increasing interest in lithium-oxygen batteries (LOB), having the highest theoretical energy densities among the advanced lithium batteries, has triggered the search for in-situ characterization techniques, including Electrochemical Atomic Force Microscopy (EC-AFM). In this work we addressed the characterization of the formation and decomposition of lithium peroxide (Li₂O₂) on a carbon cathode using a modified AFM technique, called Flow Electrochemical Atomic Force Microscopy (FE-AFM), where an oxygen-saturated solution of the non-aqueous lithium electrolyte is circulated through a liquid AFM cell. This novel technique does not require keeping the AFM equipment inside a glove-box, and it allows performing a number of experiments using the same substrate with different electrolytes without disassembling the cell. We study the morphology of Li₂O₂ on graphite carbon using lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in dimethyl sulphoxide (DMSO) as electrolyte under different operational conditions, in order to compare our results with those reported using other electrolytes and in-situ and ex-situ EC-AFM.

Mechanism for Preserving Volatile Nitrogen Dioxide and Sustainable Redox Mediation in the Nonaqueous Lithium-Oxygen Battery

Excessive overpotential during charging is a major hurdle in lithium-oxygen (Li-O₂) battery technology. NO₂^-/NO₂ redox mediation is an efficient way to substantially reduce the overpotential and to enhance oxygen efficiency and cycle life by suppressing parasitic reactions. Considering that nitrogen dioxide (NO₂) is a gas, it is quite surprising that NO₂^-/NO₂ redox reactions can be sustained for a long cycle life in Li-O₂ batteries with such an open structure. A detailed study with in situ differential electrochemical mass spectrometry (DEMS) elucidated that NO₂ could follow three reaction pathways during charging: (1) oxidation of Li₂O₂ to evolve oxygen, (2) vaporization, and (3) conversion into NO₃^-. Among the pathways, Li₂O₂ oxidation occurs exclusively in the presence of Li₂O₂, which suggests that NO₂ has high reactivity to Li₂O₂. At the end of the charging process, most of the volatile oxidized couple (NO₂) is stored by conversion to a stable third species (NO₃^-), which is then reused for producing the reduced couple (NO₂^-) in the next cycle. The dominant reaction of Li₂O₂ oxidation involves the temporary storage of NO₂ as a stable third species during charging, which is an innovative way for preserving the volatile redox couple, resulting in a sustainable redox mediation for a high-performance Li-O₂ battery.

Electrochemical stability of glyme-based electrolytes for Li-O2 batteries studied by in situ infrared spectroscopy

In situ subtractively normalized Fourier transform infrared spectroscopy (SNIFTIRS) experiments were performed simultaneously with electrochemical experiments relevant to Li-air battery operation on gold electrodes in two glyme-based electrolytes: diglyme (DG) and tetraglyme (TEGDME), tested under different operational conditions. The results show that TEGDME is intrinsically unstable and decomposes at potentials between 3.6 and 3.9 V vs. Li⁺/Li even in the absence of oxygen and lithium ions, while DG shows a better stability, and only decomposes at 4.0 V vs. Li⁺/Li in the presence of oxygen. The addition of water to the DG based electrolyte exacerbates its decomposition, probably due to the promotion of singlet oxygen formation.

LiF Protective Layer on a Li Anode: Toward Improving the Performance of Li-O2 Batteries with a Redox Mediator

The high charging overpotential and insulating/insoluble Li₂O₂ discharge products have seriously hindered the development of Li-O₂ batteries. Here, we report a highly concentrated 5.0 M lithium nitrate (LiNO₃) in N,N-dimethylacetamide (DMA) with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a redox mediator (RM) and with the LiF layer by adding N,N-dimethyltrifluoroacetamide (DMTFA) to the electrolyte, which reduces the charge voltage (3.6 V over 65 cycles) and allows stable cycling for 100 cycles without noticeable fading in capacity. The Li plating/stripping test and electrochemical impedance of the Li/Li symmetric cell results reveal that a Li/Li symmetric cell with DMTFA is stable due to the formation of a LiF protective layer on the Li metal, which suppresses the RM shuttle effect, improving the interface stability of Li and the electrolyte and also restraining the growth of dendrite during cell cycling. This work may provide a novel strategy for designing a protective layer for Li anodes in Li-O₂ batteries when using an RM.

Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future

The goal of limiting global warming to 1.5 °C requires a drastic reduction in CO₂ emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal-air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal-air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li-O₂ cells but include Na-O₂, K-O₂, and Mg-O₂ cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O₂ reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li-O₂ cells.

Implications of Testing a Zinc-Oxygen Battery with Zinc Foil Anode Revealed by Operando Gas Analysis

Zinc-oxygen batteries are seen as promising energy storage devices for future mobile and stationary applications. Introducing them as secondary battery is hindered by issues at both the anode and cathode. Research efforts were intensified during the past two decades, mainly focusing on catalyst materials for the cathode. Thereby, zinc foil was almost exclusively used as the anode in electrochemical testing in the lab-scale as it is easy to apply and shall yield reproducible results. However, it is well known that zinc metal reacts with water within the electrolyte to form hydrogen. It is not yet clear how the evolution of hydrogen is affecting the performance results obtained thereof. Herein, we extend the studies and the understanding about the evolution of hydrogen at zinc by analyzing the zinc-oxygen battery during operation. By means of electrochemical measurements, operando gas analysis, and anode surface analysis, we elucidate that the rate of the evolution of hydrogen scales with the current density applied, and that the roughness of the anode surface, that is, the pristine state of the zinc foil surface, affects the rate as well. In the end, we propose a link between the evolution of hydrogen and the unwanted impact on the actual electrochemical performance that might go unnoticed during testing. Thereof, we elucidate the consequences that arise for the working principle and the testing of materials for this battery type.

Rechargeable-battery chemistry based on lithium oxide growth through nitrate anion redox

Next-generation lithium-battery cathodes often involve the growth of lithium-rich phases, which enable specific capacities that are 2-3 times higher than insertion cathode materials, such as lithium cobalt oxide. Here, we investigated battery chemistry previously deemed irreversible in which lithium oxide, a lithium-rich phase, grows through the reduction of the nitrate anion in a lithium nitrate-based molten salt at 150 °C. Using a suite of independent characterization techniques, we demonstrated that a Ni nanoparticle catalyst enables the reversible growth and dissolution of micrometre-sized lithium oxide crystals through the effective catalysis of nitrate reduction and nitrite oxidation, which results in high cathode areal capacities (~12 mAh cm^-2). These results enable a rechargeable battery system that has a full-cell theoretical specific energy of 1,579 Wh kg^-1, in which a molten nitrate salt serves as both an active material and the electrolyte.

Oxygen Redox Reaction in Ionic Liquid and Ionic Liquid-like Based Electrolytes: A Scanning Electrochemical Microscopy Study

Improving the stability of the cathode interface is one of the critical issues for the development of high-performance Li/O₂ batteries. The most critical feature to address is the development of electrolytes that mitigate side reactions that bring about cathode passivation. It is well-known that the superoxide anion (O₂^•-) plays a critical role. Here, we propose scanning electrochemical microscopy (SECM) as an analytical tool to screen the electrolyte of Li/O₂ batteries. We demonstrate that by using SECM it is possible to evaluate the stability of O₂^•- and of the cathode to the passivation process occurring during the oxygen redox reaction. Specifically, we report a study carried out at a glassy carbon electrode in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and in tetraethylene glycol dimethyl ether with LiTFSI, the latter ranging from the salt-in-solvent to solvent-in-salt regions.

Understanding the Reaction Chemistry during Charging in Aprotic Lithium-Oxygen Batteries: Existing Problems and Solutions

The aprotic lithium-oxygen (Li-O₂ ) battery has excited huge interest due to it having the highest theoretical energy density among the different types of rechargeable battery. The facile achievement of a practical Li-O₂ battery has been proven unrealistic, however. The most significant barrier to progress is the limited understanding of the reaction processes occurring in the battery, especially during the charging process on the positive electrode. Thus, understanding the charging mechanism is of crucial importance to enhance the Li-O₂ battery performance and lifetime. Here, recent progress in understanding the electrochemistry and chemistry related to charging in Li-O₂ batteries is reviewed along with the strategies to address the issues that exist in the charging process at the present stage. The properties of Li₂ O₂ and the mechanisms of Li₂ O₂ oxidation to O₂ on charge are discussed comprehensively, as are the accompanied parasitic chemistries, which are considered as the underlying issues hindering the reversibility of Li-O₂ batteries. Based on the detailed discussion of the charging mechanism, innovative strategies for addressing the issues for the charging process are discussed in detail. This review has profound implications for both a better understanding of charging chemistry and the development of reliable rechargeable Li-O₂ batteries in the future.

Deactivation of redox mediators in lithium-oxygen batteries by singlet oxygen

Non-aqueous lithium-oxygen batteries cycle by forming lithium peroxide during discharge and oxidizing it during recharge. The significant problem of oxidizing the solid insulating lithium peroxide can greatly be facilitated by incorporating redox mediators that shuttle electron-holes between the porous substrate and lithium peroxide. Redox mediator stability is thus key for energy efficiency, reversibility, and cycle life. However, the gradual deactivation of redox mediators during repeated cycling has not conclusively been explained. Here, we show that organic redox mediators are predominantly decomposed by singlet oxygen that forms during cycling. Their reaction with superoxide, previously assumed to mainly trigger their degradation, peroxide, and dioxygen, is orders of magnitude slower in comparison. The reduced form of the mediator is markedly more reactive towards singlet oxygen than the oxidized form, from which we derive reaction mechanisms supported by density functional theory calculations. Redox mediators must thus be designed for stability against singlet oxygen.

Redox mediators can enhance redox reactions in Li-O₂ batteries; however, their gradual degradation remains unclear. Here the authors show that organic redox mediators are decomposed by singlet oxygen formed during cycling, indicating a strategy for the rational design of stable redox mediators.

FeOOH Nanocubes Anchored on Carbon Ribbons for Use in Li/O2 Batteries

A composite of FeOOH nanocubes anchored on carbon ribbons has been synthesized and used as a cathode material for Li/O₂ batteries. Fe²⁺ ion-exchanged resin serves as a precursor for both FeOOH nanocubes and carbon ribbons, which are formed simultaneously. The as-prepared FeOOH cubes are proposed to have a core-shell structure, with FeOOH as the shell and Prussian blue as the core, based on information from XPS, TEM, and EDS mapping. As a cathode material for Li/O₂ batteries, FeOOH delivers a specific capacity of 14816 mA h g^-1 _cathode with a cycling stability of 67 cycles over 400 h. The high performance is related to the low overpotential of the oxygen reduction/evolution reaction on FeOOH. The cube structure, the supporting carbon ribbons, and the -OOH moieties all contribute to the low overpotential. The discharge product Li₂ O₂ can be efficiently decomposed in the FeOOH cathode after a charging process, leading to higher cycling stability. Its high activity and stability make FeOOH a good candidate for use in non-aqueous Li/O₂ batteries.

Improving the cyclability and capacity of Li-O2 batteries via low rate pre-activation

Simple low rate pre-activation effectively prolonged the cycle life of Li-O2 batteries with MWNT cathodes in a 1 M LiClO4/DMSO electrolyte from 55 to 290 cycles, and the ultimate capacity and rate performance were also significantly enhanced, attributed to reconstructed homogeneous and compact SEI layers on the Li anodes by pre-activation.

Negative differential resistance as a critical indicator for the discharge capacity of lithium-oxygene batteries

In non-aqueous lithium-oxygen batteries, the one-electron reduction of oxygen and subsequent lithium oxide formation both occur during discharge. This lithium oxide can be converted to insulating lithium peroxide via two different pathways: a second reduction at the cathode surface or disproportionation in solution. The latter process is known to be advantageous with regard to increasing the discharge capacity and is promoted by a high donor number electrolyte because of the stability of lithium oxide in media of this type. Herein, we report that the cathodic oxygen reduction reaction during discharge typically exhibits negative differential resistance. Importantly, the magnitude of negative differential resistance, which varies with the system component, and the position of the cathode potential relative to the negative differential resistance determined the reaction pathway and the discharge capacity. This result implies that the stability of lithium oxide on the cathode also contributes to the determination of the reaction pathway.

A versatile functionalized ionic liquid to boost the solution-mediated performances of lithium-oxygen batteries

Due to the high theoretical specific energy, the lithium-oxygen battery has been heralded as a promising energy storage system for applications such as electric vehicles. However, its large over-potentials during discharge-charge cycling lead to the formation of side-products, and short cycle life. Herein, we report an ionic liquid bearing the redox active 2,2,6,6-tetramethyl-1-piperidinyloxy moiety, which serves multiple functions as redox mediator, oxygen shuttle, lithium anode protector, as well as electrolyte solvent. The additive contributes a 33-fold increase of the discharge capacity in comparison to a pure ether-based electrolyte and lowers the over-potential to an exceptionally low value of 0.9 V. Meanwhile, its molecule facilitates smooth lithium plating/stripping, and promotes the formation of a stable solid electrolyte interface to suppress side-reactions. Moreover, the proportion of ionic liquid in the electrolyte influences the reaction mechanism, and a high proportion leads to the formation of amorphous lithium peroxide and a long cycling life (> 200 cycles). In particular, it enables an outstanding electrochemical performance when operated in air.

Atomic-Thick TiO2(B) Nanosheets Decorated with Ultrafine Co3O4 Nanocrystals As a Highly Efficient Catalyst for Lithium-Oxygen Battery

Development of highly efficient catalysts based on transition metal oxides (TMOs) is desirable and remains a big challenge for lithium-oxygen (Li-O₂) batteries. In the present work, atomic-thick TiO₂(B) nanosheets decorated with ultrafine Co₃O₄ nanocrystals (Co₃O₄-TiO₂(B)) were synthesized and utilized as cathode catalyst in Li-O₂ batteries by designing a hybrid and inducing oxygen vacancies. The XPS characterization results suggested that the introduction of Co₃O₄ nanocrystals could induce numerous oxygen vacancies in the TiO₂(B) nanosheets through Co doping in the hybrid catalyst. The subsequent electrochemical experiments indicated that the Li-O₂ batteries with the prepared hybrid catalysts showed high specific capacity (11000 mAhg^-1), and good cycling stability (200 cycles at a limited capacity of 1000 mAhg^-1) with low polarization (above 2.7 V for discharge medium voltage and below 4.0 V for charge medium voltage within 80 cycles). Furthermore, a possible working mechanism was proposed for a better understanding of the high performance of Co₃O₄-TiO₂(B) catalysts for the Li-O₂ batteries. This work also provided new insights into designing efficient catalysts through interface engineering between 2D (two-dimensional) TMOs and 0D (zero-dimensional) TMOs for Li-O₂ batteries or other catalysis-related fields.

Low-Polarization Lithium-Oxygen Battery Using [DEME][TFSI] Ionic Liquid Electrolyte

The room-temperature molten salt mixture of N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethanesulfonyl) imide ([DEME][TFSI]) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt is herein reported as electrolyte for application in Li-O₂ batteries. The [DEME][TFSI]-LiTFSI solution is studied in terms of ionic conductivity, viscosity, electrochemical stability, and compatibility with lithium metal at 30 °C, 40 °C, and 60 °C. The electrolyte shows suitable properties for application in Li-O₂ battery, allowing a reversible, low-polarization discharge-charge performance with a capacity of about 13 Ah g-1carbon in the positive electrode and coulombic efficiency approaching 100 %. The reversibility of the oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) is demonstrated by ex situ XRD and SEM studies. Furthermore, the study of the cycling behavior of the Li-O₂ cell using the [DEME][TFSI]-LiTFSI electrolyte at increasing temperatures (from 30 to 60 °C) evidences enhanced energy efficiency together with morphology changes of the deposited species at the working electrode. In addition, the use of carbon-coated Zn_0.9 Fe_0.1 O (TMO-C) lithium-conversion anode in an ionic-liquid-based Li-ion/oxygen configuration is preliminarily demonstrated.

Rotating-disk electrode analysis of the oxidation behavior of dissolved Li2O2 in Li–O2 batteries

The RDE method introduced in this study is a facile and informative technique to screen for high performance electrolytes for LOB.

Effect of halogen dopants on the properties of Li2O2: is chloride special?

DFT calculations reveal that halogens do not form metallic states or extra polarons that would increase bulk conductivity, and do not behave like donor dopants.

Composite Gel Polymer Electrolyte for Improved Cyclability in Lithium-Oxygen Batteries

Gel polymer electrolytes (GPE) and composite GPE (cGPE) using one-dimensional glass microfillers have been developed for their use in lithium-oxygen batteries. Using glass microfillers, tetraglyme solvent, UV-curable polymer, and lithium salt at various concentrations, the preparation of cGPE yielded free-standing films. These cGPEs, with 1 wt % of microfillers, demonstrated increased ionic conductivity and lithium transference number over GPEs at various concentrations of lithium salt. Improvements as high as 50% and 28% in lithium transference number were observed for 0.1 and 1.0 mol kg^-1 salt concentrations, respectively. Lithium-oxygen batteries containing cGPE similarly showed superior charge/discharge cycling for 500 mAh g^-1 cycle capacity with as high as 86% and 400% increase in cycles for cGPE with 1.0 and 0.1 mol kg^-1 over GPE. Results using electrochemical impedance spectroscopy, Raman spectroscopy, and scanning electron microscopy revealed that the source of the improvement was the reduction of the rate of lithium carbonates formation on the surface of the cathode. This reduction in formation rate afforded by cGPE-containing batteries was possible due to the reduction of the rate of electrolyte decomposition. The increase in solvated to paired Li⁺ ratio at the cathode, afforded by increased lithium transference number, helped reduce the probability of superoxide radicals reacting with the tetraglyme solvent. This stabilization during cycling helped prolong the cycling life of the batteries.

High-Performance Lithium-Oxygen Battery Electrolyte Derived from Optimum Combination of Solvent and Lithium Salt

To fabricate a sustainable lithium-oxygen (Li-O₂) battery, it is crucial to identify an optimum electrolyte. Herein, it is found that tetramethylene sulfone (TMS) and lithium nitrate (LiNO₃) form the optimum electrolyte, which greatly reduces the overpotential at charge, exhibits superior oxygen efficiency, and allows stable cycling for 100 cycles. Linear sweep voltammetry (LSV) and differential electrochemical mass spectrometry (DEMS) analyses reveal that neat TMS is stable to oxidative decomposition and exhibit good compatibility with a lithium metal. But, when TMS is combined with typical lithium salts, its performance is far from satisfactory. However, the TMS electrolyte containing LiNO₃ exhibits a very low overpotential, which minimizes the side reactions and shows high oxygen efficiency. LSV-DEMS study confirms that the TMS-LiNO₃ electrolyte efficiently produces NO₂^-, which initiates a redox shuttle reaction. Interestingly, this NO₂^-/NO₂ redox reaction derived from the LiNO₃ salt is not very effective in solvents other than TMS. Compared with other common Li-O₂ solvents, TMS seems optimum solvent for the efficient use of LiNO₃ salt. Good compatibility with lithium metal, high dielectric constant, and low donicity of TMS are considered to be highly favorable to an efficient NO₂^-/NO₂ redox reaction, which results in a high-performance Li-O₂ battery.

Li-O2 Cell with LiI(3-hydroxypropionitrile)2 as a Redox Mediator: Insight into the Working Mechanism of I- during Charge in Anhydrous Systems

Redox mediators (RMs) have been widely applied to reduce the charge overpotential of nonaqueous lithium-oxygen (Li-O₂) batteries. Among the reported RMs, LiI is under hot debate with lots of controversial reports. However, there is a limited understanding of the charge mechanism of I^- in anhydrous Li-O₂ batteries. Here, we study the chemical reactivity between the oxidized state of I^- and Li₂O₂. We confirm that the Li₂O₂ particles could be chemically oxidized by I₂ rather than I₃^- species. Furthermore, our work demonstrates that the generated I^- from Li₂O₂ oxidation would combine with I₂ to give I₃^- species, hindering further oxidation of Li₂O₂ by I₂. To improve the working efficiency of I^- RMs, we introduce a compound LiI(3-hydroxypropionitrile)₂ (LiI(HPN)₂) with a high binding ability of I^-. Compared with LiI, the cell that contained LiI(HPN)₂ shows a significantly increased amount of I₂ species during charge and enhanced Li₂O₂ oxidation efficiency under the same working conditions.

Modified Tetrathiafulvalene as an Organic Conductor for Improving Performances of Li-O2 Batteries

Large over-potentials owing to the sluggish kinetics of battery reactions have always been the drawbacks of Li-O₂ batteries, which lead to short cycle life. Although redox mediators have been intensively investigated to overcome this issue, side-reactions are generally induced by the solvated nature of redox mediators. Herein, we report an alternative method to achieve more efficient utilization of tetrathiafulvalene (TTF) in Li-O₂ batteries. By coordinating TTF⁺ with LiCl during charging, an organic conductor TTF⁺ Cl_x^- precipitate covers Li₂ O₂ to provide an additional electron-transfer pathway on the surface, which can significantly reduce the charge over-potential, improve the energy efficiency of Li-O₂ batteries, and eliminate side-reactions between the lithium metal anode and TTF⁺ . When a porous graphene electrode is used, the Li-O₂ battery combined with TTF and LiCl shows an outstanding performance and prolonged cycle life.

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.

Poly(vinylidene fluoride) (PVDF) Binder Degradation in Li-O2 Batteries: A Consideration for the Characterization of Lithium Superoxide

We show that a common Li-O₂ battery cathode binder, poly(vinylidene fluoride) (PVDF), degrades in the presence of reduced oxygen species during Li-O₂ discharge when adventitious impurities are present. This degradation process forms products that exhibit Raman shifts (∼1133 and 1525 cm^-1) nearly identical to those reported to belong to lithium superoxide (LiO₂), complicating the identification of LiO₂ in Li-O₂ batteries. We show that these peaks are not observed when characterizing extracted discharged cathodes that employ poly(tetrafluoroethylene) (PTFE) as a binder, even when used to bind iridium-decorated reduced graphene oxide (Ir-rGO)-based cathodes similar to those that reportedly stabilize bulk LiO₂ formation. We confirm that for all extracted discharged cathodes on which the 1133 and 1525 cm^-1 Raman shifts are observed, only a 2.0 e^-/O₂ process is identified during the discharge, and lithium peroxide (Li₂O₂) is predominantly formed (along with typical parasitic side product formation). Our results strongly suggest that bulk, stable LiO₂ formation via the 1 e^-/O₂ process is not an active discharge reaction in Li-O₂ batteries.

Stability of Glyme Solvate Ionic Liquid as an Electrolyte for Rechargeable Li-O2 Batteries

A solvate ionic liquid (SIL) was compared with a conventional organic solvent for the electrolyte of the Li-O₂ battery. An equimolar mixture of triglyme (G3) and lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), and a G3/Li[TFSA] mixture containing excess glyme were chosen as the SIL and the conventional electrolyte, respectively. Charge behavior and accompanying gas evolution of the two electrolytes was investigated by electrochemical mass spectrometry (ECMS). From the linear sweep voltammetry performed on an as-prepared cell, we demonstrate that the SIL has a higher oxidative stability than the conventional electrolyte and, furthermore, offers the advantage of lower volatility, which would benefit an open-type lithium-O₂ cell design. Moreover, CO₂ evolution during galvanostatic charge was less in the SIL, which implies less side reaction. However, O₂ evolution during charge did not reach the theoretical value in either of the two electrolytes. Several mass spectral fragments were generated during the charge process, which provided evidence for side reactions of glyme-based electrolytes. We further relate the difference in observed discharge product morphology for these electrolytes to the solubility of the superoxide intermediate, determined by rotating ring disk electrode (RRDE) measurements.

Electrolyte-controlled discharge product distribution of Na–O2 batteries: a combined computational and experimental study

Tuning the composition of discharge products is an important strategy to reduce charge potential, suppress side reactions, and improve the reversibility of metal–oxygen batteries.

Why Do Lithium-Oxygen Batteries Fail: Parasitic Chemical Reactions and Their Synergistic Effect

As an electrochemical energy-storage technology with the highest theoretical capacity, lithium-oxygen batteries face critical challenges in terms of poor stabilities and low charge/discharge round-trip efficiencies. It is generally recognized that these issues are connected to the parasitic chemical reactions at the anode, electrolyte, and cathode. While the detailed mechanisms of these reactions have been studied separately, the possible synergistic effects between these reactions remain poorly understood. To fill in the knowledge gap, this Minireview examines literature reports on the parasitic chemical reactions and finds the reactive oxygen species a key chemical mediator that participates in or facilitates nearly all parasitic chemical reactions. Given the ubiquitous presence of oxygen in all test cells, this finding is important. It offers new insights into how to stabilize various components of lithium-oxygen batteries for high-performance operations and how to eventually materialize the full potentials of this promising technology.

The effect of water on discharge product growth and chemistry in Li-O2 batteries

Understanding what controls Li-O2 battery discharge product chemistry and morphology is key to enabling its practical deployment as a low-cost, high-specific-energy energy conversion technology. Several studies have recently shown that the addition of substantial quantities (hundreds to thousands ppm) of water and weak acids to dimethoxyethane (DME)-based electrolytes can significantly increase Li-O2 battery discharge capacity, without substantially changing the discharge product chemistry, which remains Li2O2. The exact mechanisms behind these device-level improvements, however, are not yet understood. In this study, we show that the presence of water in a DME-based electrolyte decreases the rate of Li2O2 nucleation on the electrode surface during Li-O2 battery discharge, using potentiostatic electrochemical measurements, and direct, ex situ observations of Li2O2 particles. We also show that adding water to an acetonitrile (MeCN)-based electrolyte results in LiOH upon discharge, as opposed to only Li2O2. Using first principles calculations, we propose that this change in discharge product chemistry is attributable to increased proton availability, as shown by a lower pKa for water in MeCN than in DME. This study combines kinetic and morphological analyses with first principles calculations, and elucidates relationships among electrolyte composition, discharge product chemistry and growth mechanisms for the rational design of efficient metal-air batteries.

Sodium-Oxygen Battery: Steps Toward Reality

Rechargeable metal-oxygen batteries are receiving significant interest as a possible alternative to current state of the art lithium ion batteries due to their potential to provide higher gravimetric energies, giving significantly lighter or longer-lasting batteries. Recent advances suggest that the Na-O2 battery, in many ways analogous to Li-O2 yet based on the reversible formation of sodium superoxide (NaO2), has many advantages such as a low charge overpotential (∼100 mV) resulting in improved efficiency. In this Perspective, we discuss the current state of knowledge in Na-O2 battery technology, with an emphasis on the latest experimental studies, as well as theoretical models. We offer special focus on the principle outstanding challenges and issues and address the advantages/disadvantages of the technology when compared with Li-O2 batteries as well as other state-of-the-art battery technologies. We finish by detailing the direction required to make Na-O2 batteries both commercially and technologically viable.

A highly efficient Li2O2 oxidation system in Li–O2 batteries

We demonstrated a novel indirect charging system for Li–O₂ batteries, chemical regeneration, which reduces both charging time and capacity fade.