Stefan A Freunberger

• Ph.D. (Dr Sc ETH Zürich)
• Professor (Assistant) at Institute of Science and Technology Austria

Oxygen evolution from superoxide is a critical aspect of oxygen redox chemistry. However, the factors determining the formation of often harmful singlet oxygen are unclear. Here, we report that the release of triplet or singlet oxygen is governed by individual Marcus normal and inverted region behavior. Using a wide range of chemical oxidants, we f...

The short history of research on Li−O2 batteries has seen a remarkable number of mechanistic U‐turns over the years. From the initial use of carbonate electrolytes, that were then found to be entirely unsuitable, to the belief that (su)peroxide was solely responsible for degradation, before the more reactive singlet oxygen was found to form, to the...

Next-generation metal-air batteries are limited by factors such as high overpotential, and low energy efficiency. These factors lead to significant electrode and electrolyte decomposition, and consequently, limit the cell life. The primary cause of these parasitic reactions has been identified as the generation of singlet oxygen ( ¹ O 2 ) ¹ from th...

The necessity to move to renewable energy sources has increased over the last few years. However, to ensure a good transition and effective energy supply, efficient, cheap, and green technologies are required as grid-level storage. ¹ Redox-Active Organic Materials (ROMs) are a fast-growing research topic in different electrochemical storage devices...

Redox flow batteries (RFBs) are one potential solution to grid-level electrical energy storage (EES) benefiting from a decoupled power and capacity scaling. 1–3 High durability, long-calendar life, high efficiency EES with a low cost and fast response time is needed 1,4 for the transition from fossil fuels to renewable sources. ³ However, the low e...

For more than a century, electron transfer kinetics at electrode/electrolyte interface has been studied for dissolved redox active species at conductors (heterogeneous) or amongst dissolved redox species (homogeneous). However, a third major case is almost unexplored: dissolved redox species exchanging charge with redox active insulators. The elect...

The short history of research on Li‐O2 batteries has seen a remarkable number of mechanistic U‐turns over the years. From the initial use of carbonate electrolytes, that were then found to be entirely unsuitable, to the belief that (su)peroxide was solely responsible for degradation, before the more reactive singlet oxygen was found to form, to the...

Singlet oxygen (1O2) formation is now recognised as a key aspect of non-aqueous oxygen redox chemistry. For identifying 1O2, chemical trapping via 9,10-dimethylanthracene (DMA) to form the endoperoxide (DMA-O2) has become the main method due to its sensitivity, selectivity, and ease of use. While DMA has been shown to be selective for 1O2, rather t...

The inadequate understanding of the mechanisms that reversibly convert molecular sulfur (S) into lithium sulfide (Li2S) via soluble polysulfides (PSs) formation impedes the development of high-performance lithium-sulfur (Li-S) batteries with non-aqueous electrolyte solutions. Here, we use operando small and wide angle X-ray scattering and operando...

Formation of the redox active insulators such as Lithium peroxide (Li 2 O 2 ), Lithium sulfide (Li 2 S) are the salient features for next generation ‘beyond intercalation’ batteries like metal-air (O 2 ) and metal-sulfur (Li-S) batteries (1-3). The interest in these batteries arises from high theoretical energies, abundant elements, low cost, and e...

Capacity, rate performance, and cycle life of aprotic Li-O2 batteries critically depend on reversible electrodeposition of Li2O2. Current understanding states surface-adsorbed versus solvated LiO2 controls Li2O2 growth as surface film or as large particles. Herein, we show that Li2O2 forms across a wide range of electrolytes, carbons, and current d...

Capacity, rate performance, and cycle life of aprotic Li-O2 batteries critically depend on reversible electrodeposition of Li2O2. Current understanding states surface adsorbed versus solvated LiO2 to control Li2O2 growth as surface film or as large particles. Here we show that Li2O2 forms across a wide range of electrolytes, carbons, and current de...

Redox mediators could catalyse otherwise slow and energy-inefficient cycling of Li–S and Li–O2 batteries by shuttling electrons or holes between the electrode and the solid insulating storage materials. For mediators to work efficiently they need to oxidize the solid with fast kinetics but with the lowest possible overpotential. However, the depend...

Insufficient understanding of the mechanism that reversibly converts sulphur into lithium sulphide (Li2S) via soluble polysulphides (PS) hampers the realization of high performance lithium-sulphur cells. Typically Li2S formation is explained by direct electroreduction of a PS to Li2S; however, this is not consistent with the size of the insulating...

Insufficient understanding of the mechanism that reversibly converts sulfur into lithium sulfide (Li 2 S) via soluble polysulfides (PS) hampers the realization of high performance lithium-sulfur (Li-S) cells. Typically Li 2 S formation is explained by direct electroreduction of a PS to Li 2 S; however, this is not consistent with the size and shape...

Insufficient understanding of the mechanism that reversibly converts sulfur into lithium sulfide (Li 2 S) via soluble polysulfides (PS) hampers the realization of high performance lithium-sulfur (Li-S) cells. Typically Li 2 S formation is explained by direct electroreduction of a PS to Li 2 S; however, this is not consistent with the size and shape...

Redox mediators could catalyse otherwise slow and energy-inefficient cycling of Li-S and Li-O 2 batteries by shuttling electrons/holes between the electrode and the solid insulating storage materials. For mediators to work efficiently they need to oxidize the solid with fast kinetics yet the lowest possible overpotential. Here, we found that when t...

Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) based water-in-salt electrolytes (WiSEs) has recently emerged as a new promising class of electrolytes, primarily owing to their wide electrochemical stability windows (∼3–4 V), that by far exceed the thermodynamic stability window of water (1.23 V). Upon increasing the salt concentration towards s...

Aprotic alkali metal–O2 batteries face two major obstacles to their chemistry occurring efficiently, the insulating nature of the formed alkali superoxides/peroxides and parasitic reactions that are caused by the highly reactive singlet oxygen (¹O2). Redox mediators are recognized to be key for improving rechargeability. However, it is unclear how...

Significance Lithium–air batteries are promising next-generation energy storage devices and operate by electrodepositing insulating lithium peroxide (Li 2 O 2 ). Understanding how large amounts of Li 2 O 2 form, corresponding to large capacities, and what limits the amount is therefore paramount and requires following its structural evolution from...

Large overpotentials upon discharge and charge of Li-O2 cells have motivated extensive research into heterogeneous solid electrocatalysts or non-carbon electrodes with the aim to improve rate capability, round-trip efficiency, and cycle life. These features are equally governed by parasitic reactions, which are now recognized to be caused by format...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

We show the synthesis of a redox‐active quinone, 2‐methoxy‐1,4‐hydroquinone (MHQ), from a bio‐based feedstock and its suitability as electrolyte in aqueous redox flow batteries. We identified semiquinone intermediates at insufficiently low pH and quinoid radicals as responsible for decomposition of MHQ under electrochemical conditions. Both can be...

Aqueous iodine based electrochemical energy storage is considered a potential candidate to improve sustainability and performance of current battery and supercapacitor technology. It harnesses the redox activity of iodide, iodine, and polyiodide species in the confined geometry of nanoporous carbon electrodes. However, current descriptions of the e...

With the lithium-ion technology approaching its intrinsic limit with graphite-based anodes, Li metal is recently receiving renewed interest from the battery community as potential high capacity anode for next-generation rechargeable batteries. In this focus paper, we review the main advances in this field since the first attempts in the mid-1970s....

A vanillin‐based 2‐methoxyhydroquinone is proposed as the catholyte for redox‐flow batteries. We overcame the tendency of such quinones to form reactive radicals that trigger side‐reactions by carefully choosing the medium. This enabled 87.4 % capacity retention after 250 cycles. Abstract We show the synthesis of a redox‐active quinone, 2‐methoxy‐...

Water‐in‐salt electrolytes based on highly concentrated bis(trifluoromethyl)sulfonimide (TFSI) promise aqueous electrolytes with stabilities nearing 3 V. However, especially with an electrode approaching the cathodic (reductive) stability, cycling stability is insufficient. While stability critically relies on a solid electrolyte interphase (SEI),...

Water‐in‐salt electrolytes have received particular attention for their electrochemical stability properties. It is now shown that water reduction is always present from the first measured currents, and that associated with this water reduction, LiTFSI precipitation occurs. Abstract Water‐in‐salt electrolytes based on highly concentrated bis(trifl...

Aqueous iodine based electrochemical energy storage is considered a potential candidate to improve sustainability and electrochemical performance of current battery and supercapacitor technology. It harnesses the redox activity of iodide, iodine and polyiodide species in the confined geometry of nanoporous carbon electrodes. However, current descri...

Large overpotentials upon discharge and charge of Li-O2 cells have motivated extensive research into heterogeneous solid electrocatalysts or non-carbon electrodes with the aim to improve rate capability, round-trip efficiency and cycle life. These features are equally governed by parasitic reactions, which are now recognized to be caused by the hig...

Rechargeable Li–O 2 batteries have gathered enormous attention in the research community for having amongst the highest theoretical energy storage. Realizing the promise, even in part, in practice could produce a device that stores significantly more energy than other rechargeable batteries. Fundamental understanding of the reaction mechanisms is n...

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...

Electrodepositing insulating and insoluble Li2O2 is the key process during discharge of aprotic Li-O2 batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and dissolved LiO2 governs whether Li2O2 grows as a conformal surface film or larger particles, leading to low or h...

Beyond-intercalation batteries promise a step-change in energy storage compared to intercalation based lithium- and sodium-ion batteries. However, only performance metrics that include all cell components and operation parameters can tell whether a true advance over intercalation batteries has been achieved.

In a recent issue of Joule, Dongmin Im and coworkers from Samsung in South Korea describe a prototype lithium-O2 battery that reaches 700 Wh kg-1 and 600 Wh L-1 on cell level. They cut all components to the minimum to reach this value. Difficulties to fill the pores with discharge product and inhomogeneous cell utilization turn out to limit the ach...

Interphases that form on the anode surface of lithium-ion batteries are critical for performance and lifetime, but are poorly understood. Now, a decade-old misconception regarding a main component of the interphase has been revealed, which could potentially lead to improved devices.

Aprotic alkali metal-oxygen batteries require reversible formation of metal superoxide or peroxide on cycling. Severe parasitic reactions cause poor rechargeability, efficiency, and cycle life and have been shown to be caused by singlet oxygen (¹O2) that forms at all stages of cycling. However, its formation mechanism remains unclear. We show that...

Singlet oxygen (¹O2) causes a major fraction of the parasitic chemistry during the cycling of non‐aqueous alkali metal‐O2 batteries and also contributes to interfacial reactivity of transition‐metal oxide intercalation compounds. We introduce DABCOnium, the mono alkylated form of 1,4‐diazabicyclo[2.2.2]octane (DABCO), as an efficient ¹O2 quencher w...

Potassium–air batteries, which suffer from oxygen cathode and potassium metal anode degradation, can be cycled thousands of times when an organic anode replaces the metal.

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...

Singlet oxygen (1O2) causes a major fraction of parasitic chemistry during cycling of non‐aqueous alkali metal‐O2 batteries and also contributes to interfacial reactivity of transition‐metal oxide intercalation compounds. We introduce DABCOnium, the mono alkylated form of 1,4‐diazabicyclo[2.2.2]octane (DABCO), as an efficient 1O2 quencher with an u...

In this issue of Joule, Dongmin Im and coworkers from Samsung in South Korea describe a prototype lithium-O 2 battery that reaches ∼700 Wh kg –1 and ∼600 Wh L –1 on the cell level. They cut all components to the minimum to reach this value. Difficulties filling the pores with discharge product and inhomogeneous cell utilization turn out to limit th...

The redox chemistry of O 2 moieties has come into the focus of much of the forefront battery research such as metal-O 2 batteries and Li-rich layered oxides (1, 2). O 2 evolution is in either case a critical yet not fully understood phenomenon (1, 3). For example, operation of the rechargeable metal-O 2 batteries depends crucially on the reversible...

Hydrogelation, the self-assembly of molecules into soft, water-loaded networks, bridges the structural gap between single molecules and functional materials. The potential of hydrogels, such as those based on perylene bisimides, lies in their chemical, physical, optical, and electronic properties, which are all governed by the inter- and supramolec...

The solid electrolyte interphase (SEI) in Li and Na ion batteries forms when highly reducing or oxidizing electrode materials get in contact with liquid organic electrolyte. Its ability to form a mechanically robust, ion conducting and electron insulating layer critically determines performance, cycle life and safety. Li or Na alkyl carbonates (LAC...

Solid alkaline carbonates are universal passivation layer components of intercalation battery materials, common side products in metal-O₂ batteries, and are believed to form and decompose reversibly in metal-O₂/CO₂ cells. In all of these cathode chemistries, Li₂CO₃ decomposes to CO₂ when exposed to potentials >3.8 V vs. Li/Li+. Notably, though, O₂...

Solid alkaline carbonates are universal passivation layer components of intercalation battery materials, common side products in metal-O₂ batteries, and are believed to form and decompose reversibly in metal-O₂/CO₂ cells. In all of these cathode chemistries, Li₂CO₃ decomposes to CO₂ when exposed to potentials >3.8 V vs. Li/Li+. Notably, though, O₂...

Passivation layers on electrode materials are ubiquitous in nonaqueous battery chemistries and strongly govern performance and lifetime. They comprise break down products of the electrolyte including carbonate, alkyl carbonates, alkoxides, carboxylates and polymers. Parasitic chemistry in metal-O2 batteries forms similar products and is tied to the...

Kinetics of electrochemical reactions are several orders of magnitude slower in solids than in liquids as a result of the much lower ion diffusivity. Yet, the solid state maximizes the density of redox species, which is at least two orders of magnitude lower in liquids because of solubility limitations. With regard to electrochemical energy storage...

Non-aqueous metal-oxygen batteries depend critically on the reversible formation/decomposition of metal oxides on cycling. Irreversible parasitic reactions cause poor rechargeability, efficiency, and cycle life and have predominantly been ascribed to the reactivity of reduced oxygen species with cell components. These species, however, cannot fully...

Nebenreaktionen sind die größte Hürde für die Entwicklung von Na-O2-Batterien. In ihrer Zuschrift (DOI: 10.1002/ange.201709351) zeigen S. A. Freunberger et al., dass sich der hochreaktive Singulett-Sauerstoff in allen Phasen des Zellbetriebes bildet, besonders bei hohen Ladespannungen, und dass er die Hauptursache für die Nebenreaktionen ist. Wasse...

The biggest hurdle facing the development of Na–O2 batteries is parasitic chemistry. In their Communication (DOI: 10.1002/anie.201709351) S. A. Freunberger et al. show evidence that the highly reactive singlet oxygen forms at all stages of cycling, especially at high charging voltages, and that it is the main driver for parasitic reactions. However...

Aprotic sodium-O2 batteries require the reversible formation/dissolution of sodium superoxide (NaO2) on cycling. Poor cycle life has been associated with parasitic chemistry caused by the reactivity of electrolyte and electrode with NaO2, a strong nucleophile and base. Its reactivity can, however, not consistently explain the side reactions and irr...

Aprotic sodium-O2 batteries require the reversible formation/dissolution of sodium superoxide (NaO2) on cycling. Poor cycle life has been associated with parasitic chemistry caused by the reactivity of electrolyte and electrode with NaO2, a strong nucleophile and base. Its reactivity can, however, not consistently explain the side reactions and irr...

We report a family of Pt and Pd benzoporphyrin dyes with versatile photophysical properties and easy access from cheap and abundant chemicals. Attaching 4 or 8 alkylsulfone groups onto a meso-tetraphenyltetrabenzoporphyrin (TPTBP) macrocylcle renders the dyes highly soluble in organic solvents, photostable and electron-deficient with the redox pote...

Rechargeable Li-O2 batteries have amongst the highest formal energy and could store significantly more energy than other rechargeable batteries in practice if at least a large part part of their promise could be realized. Realization, however, still faces many challenges than can only be overcome by fundamental understanding of the processes taking...

Nowadays, commercial supercapacitors are based on purely capacitive storage and porous carbons are used as electrodes. Unfortunately their energy density is limited, so that many study investigated new materials that could improve the capacitance of the device. This new type of electrodes (e.g. RuO2, MnO2…) involves different mechanisms for energy...

Beyond-intercalation batteries promise a step-change in energy storage compared to intercalation-based lithium-ion and sodium-ion batteries. However, only performance metrics that include all cell components and operation parameters can tell whether a true advance over intercalation batteries has been achieved.

Non-aqueous lithium-oxygen batteries depend critically on the reversible formation and on the decomposition of lithium oxides on cycling [1-3] . The discharge reaction in the cathode of nonaqueous Li-O 2 batteries involves the reduction of O 2 and the formation of solid Li 2 O 2 . The process is reverse on charge. True reversibility of the cathode...

Na-ion batteries are amongst the most appealing alternatives to Li-ion batteries due to a similar intercalation chemistry and lower cost. Novel Na-ion chemistries are often assessed in half cells that contain Na-metal counter and reference electrodes (Figure 1). These electrodes are highly reactive and require a passivation layer to work, which is...

Non-aqueous metal–oxygen batteries depend critically on the reversible formation/decomposition of metal oxides on cycling. Irreversible parasitic reactions cause poor rechargeability, efficiency, and cycle life, and have predominantly been ascribed to the reactivity of reduced oxygen species with cell components. These species, however, cannot full...

Kinetics of electrochemical reactions are several orders of magnitude slower in solids than in liquids as a result of the much lower ion diffusivity. Yet, the solid state maximizes the density of redox species, which is at least two orders of magnitude lower in liquids because of solubility limitations. With regard to electrochemical energy storage...

Na battery chemistries show poor passivation behavior of low voltage Na storage compounds and Na metal with organic carbonate based electrolytes adopted from Li-ion batteries. A suitable electrolyte remains therefore a major challenge for establishing Na batteries. Here we report highly concentrated sodium bis(fluorosulfonyl)imide (NaFSI) in dimeth...

Mesoporous nanocrystalline TiO2 and TiO2–V2O5 microspheres were prepared by non-hydrolytic sol–gel from TiCl4, VOCl3, and iPr2O at 110 °C without any solvent or additives. The samples were characterized by elemental analysis, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, nitrogen physisorption, and impedance measurements. At...

If proton exchange membrane fuel cells (PEMFC) are ever to succeed in sustainable energy landscape as a potential zero emission technology, it is inevitable to reduce electricity production cost associated mainly with its MEAs, cell hardware and gas storage units. We demonstrate a diverse strategy for achieving this target with a concomitant amplif...

Redox mediators facilitate the oxidation of the highly insulating discharge product in metal-oxygen batteries during recharge and offer opportunities to achieve high reversible capacities. Now a design principle for selecting redox mediators that can recharge the batteries more efficiently is suggested.

Within the scope of developing a multi-physical model describing battery behavior during and after the mechanical load (accelerations, intrusions) of a vehicle’s high voltage battery, an internal short circuit model is of deep interest for a virtual hazard assessment. The internal short resistance and the size of the affected area must be known as...

Lithium-ion batteries are in widespread use in electric vehicles and hybrid vehicles. Besides features like energy density, cost, lifetime, and recyclability the safety of a battery system is of prime importance. The separator material impacts all these properties and requires therefore an informed selection. The interplay between the mechanical an...

Despite the unparalleled theoretical gravimetric energy, Li-O2 batteries are still under a research stage because of their insufficient cycle lives. While the reversibility in air-cathodes has been lately improved significantly by the deepened understanding on the electrode–electrolyte reaction and the integration of diverse catalysts, the stabilit...

In the long term high-volume applications such as electric vehicles and the storage of electricity from renewables require a step change in energy density, material sustainability and cost that stepwise improvement of current Li-ion technology based on intercalation electrodes cannot hope to deliver. Radical new approaches are required, motivating...

Li-ion and related battery technologies will be important for years to come. However, society needs energy storage that exceeds the capacity of Li-ion batteries. We must explore alternatives to Li-ion if we are to have any hope of meeting the long-term needs for energy storage. One such alternative is the Li-air (O 2 ) battery; its theoretical spec...

When lithium-oxygen batteries discharge, O2 is reduced at the cathode to form solid Li2O2. Understanding the fundamental mechanism of O2 reduction in aprotic solvents is therefore essential to realizing their technological potential. Two different models have been proposed for Li2O2 formation, involving either solution or electrode surface routes....

Photoinduced electron transfer (PET), which causes pH-dependent quenching of fluorescent dyes, is more effectively introduced by phenolic groups than by amino groups which have been much more commonly used so far. That is demonstrated by fluorescence measurements involving several classes of fluorophores. Electrochemical measurements show that PET...

Li-ion and related battery technologies will be important for years to come. However, society needs energy storage that exceeds the energy and power of common Li-ion batteries. 1,2 Many prospective applications of batteries require improving both energy density and rate capability. The simultaneous improvement of these two parameters is not suffici...

While O 2 reduction in aqueous environments has been studied extensively for many decades, O 2 reduction in aprotic solvents has received much less attention. One spin-off of the recent interest in rechargeable Li-O 2 batteries, Fig. 1, 1,2 based on aprotic electrolytes is that it has highlighted the importance of understanding the fundamental mech...

Understanding charge carrier transport in Li2O2, the storage material in the non-aqueous Li-O2 battery, is key to the development of this high-energy battery. Here, we studied ionic transport properties and Li self-diffusion in nanocrystalline Li2O2 by conductivity and temperature variable 7Li NMR spectroscopy. Nanostructured Li2O2, characterized b...

Li-ion and related battery technologies will be important for years to come. However, society needs energy storage that exceeds the capacity of Li-ion batteries. We must explore alternatives to Li-ion if we are to have any hope of meeting the long-term needs for energy storage. One such alternative is the Li-air(O 2 ) battery; its theoretical speci...

Lithium-air batteries have received extraordinary attention recently owing to their theoretical gravimetric energies being considerably higher than those of Li-ion batteries. There are, however, significant challenges to practical implementation, including low energy efficiency, cycle life, and power capability. These are due primarily to the lack...

The electrolyte in the non-aqueous (aprotic) lithium air battery has a profound influence on the reactions that occur at the anode and cathode, and hence its overall operation on discharge/charge. It must possess a wide range of attributes, exceeding the requirements of electrolytes for Lithium ion batteries by far. The most important additional is...

Several problems arise at the O2 (positive) electrode in the Li-air battery, including solvent/electrode decomposition and electrode passivation by insulating Li2O2. Progress partially depends on exploring the basic electrochemistry of O2 reduction. Here we describe the effect of complexing-cations on the electrochemical reduction of O2 in DMSO in...

Rechargeable lithium-air (O2) batteries are receiving intense interest because their high theoretical specific energy exceeds that of lithium-ion batteries. If the Li-O2 battery is ever to succeed, highly reversible formation/decomposition of Li2O2 must take place at the cathode on cycling. However, carbon, used ubiquitously as the basis of the cat...

The non-aqueous Li-air (O2) battery is receiving intense interest because its theoretical specific energy exceeds that of Li-ion batteries. Recharging the Li-O2 battery depends on oxidizing solid lithium peroxide (Li2O2), which is formed on discharge within the porous cathode. However, transporting charge between Li2O2 particles and the solid elect...