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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 devices, this leads to high-energy batteries with limited power and high-power supercapacitors with a well-known energy deficiency. For such devices the ideal system should endow the liquid state with a density of redox species close to the solid state. Here we report an approach based on biredox ionic liquids to achieve bulk-like redox density at liquid-like fast kinetics. The cation and anion of these biredox ionic liquids bear moieties that undergo very fast reversible redox reactions. As a first demonstration of their potential for high-capacity/high-rate charge storage, we used them in redox supercapacitors. These ionic liquids are able to decouple charge storage from an ion-accessible electrode surface, by storing significant charge in the pores of the electrodes, to minimize self-discharge and leakage current as a result of retaining the redox species in the pores, and to raise working voltage due to their wide electrochemical window. Electrochemical storage devices in the liquid state are promising but they suffer from low redox species density. An approach based on biredox ionic liquids now demonstrates bulk-like redox density compatible with supercapacitor applications.

The non-aqueous Li–air (O 2 ) battery is receiving intense interest because its theoretical specific energy exceeds that of Li-ion batteries. Recharging the Li–O 2 battery depends on oxidizing solid lithium peroxide (Li 2 O 2 ), which is formed on discharge within the porous cathode. However, transporting charge between Li 2 O 2 particles and the solid electrode surface is at best very difficult and leads to voltage polarization on charging, even at modest rates. This is a significant problem facing the non-aqueous Li–O 2 battery. Here we show that incorporation of a redox mediator, tetrathiafulvalene (TTF), enables recharging at rates that are impossible for the cell in the absence of the mediator. On charging, TTF is oxidized to TTF + at the cathode surface; TTF + in turn oxidizes the solid Li 2 O 2 , which results in the regeneration of TTF. The mediator acts as an electron–hole transfer agent that permits efficient oxidation of solid Li 2 O 2 . The cell with the mediator demonstrated 100 charge/discharge cycles. Recharging Li–O 2 batteries requires oxidation of the discharge product solid Li 2 O 2 . Now a redox-mediating molecule is shown to assist this process by transferring electron–holes between solid Li 2 O 2 and the positive electrode in a non-aqueous Li–O 2 cell. This allows the cell to be charged at rates that are otherwise impossible.

Rechargeable lithium–air (O 2 ) batteries are receiving intense interest because their high theoretical specific energy exceeds that of lithium-ion batteries. If the Li–O 2 battery is ever to succeed, highly reversible formation/decomposition of Li 2 O 2 must take place at the cathode on cycling. However, carbon, used ubiquitously as the basis of the cathode, decomposes during Li 2 O 2 oxidation on charge and actively promotes electrolyte decomposition on cycling. Replacing carbon with a nanoporous gold cathode, when in contact with a dimethyl sulphoxide-based electrolyte, does seem to demonstrate better stability. However, nanoporous gold is not a suitable cathode; its high mass destroys the key advantage of Li–O 2 over Li ion (specific energy), it is too expensive and too difficult to fabricate. Identifying a suitable cathode material for the Li–O 2 cell is one of the greatest challenges at present. Here we show that a TiC-based cathode reduces greatly side reactions (arising from the electrolyte and electrode degradation) compared with carbon and exhibits better reversible formation/decomposition of Li 2 O 2 even than nanoporous gold (>98% capacity retention after 100 cycles, compared with 95% for nanoporous gold); it is also four times lighter, of lower cost and easier to fabricate. The stability may originate from the presence of TiO 2 (along with some TiOC) on the surface of TiC. In contrast to carbon or nanoporous gold, TiC seems to represent a more viable, stable, cathode for aprotic Li–O 2 cells. Although rechargeable lithium–air batteries are receiving significant attention because of their high theoretical specific energy, carbon cathodes that are currently used decompose during oxidation and promote electrolyte decomposition on cycling. A titanium carbide-based cathode is now shown to reduce side-reactions, and exhibits enhanced reversible formation and decomposition of Li 2 O 2 .

The inadequate understanding of the mechanisms that reversibly convert molecular sulfur (S) into lithium sulfide (Li 2 S) 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 small angle neutron scattering (SANS) measurements to track the nucleation, growth and dissolution of solid deposits from atomic to sub-micron scales during real-time Li-S cell operation. In particular, stochastic modelling based on the SANS data allows quantifying the nanoscale phase evolution during battery cycling. We show that next to nano-crystalline Li 2 S the deposit comprises solid short-chain PSs particles. The analysis of the experimental data suggests that initially, Li 2 S 2 precipitates from the solution and then is partially converted via solid-state electroreduction to Li 2 S. We further demonstrate that mass transport, rather than electron transport through a thin passivating film, limits the discharge capacity and rate performance in Li-S cells. Understanding lithium sulfide’s electrodeposition and stripping is key to developing practical Li-S batteries. Here, the authors demonstrate that the discharge product in Li-S batteries consists of nano-size solid polysulfide particles and nano-crystalline lithium sulfide.

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 electrochemical reaction mechanism to interconvert these species are elusive. Here we show that electrochemical oxidation of iodide in nanoporous carbons forms persistent solid iodine deposits. Confinement slows down dissolution into triiodide and pentaiodide, responsible for otherwise significant self-discharge via shuttling. The main tools for these insights are in situ Raman spectroscopy and in situ small and wide-angle X-ray scattering (in situ SAXS/WAXS). In situ Raman confirms the reversible formation of triiodide and pentaiodide. In situ SAXS/WAXS indicates remarkable amounts of solid iodine deposited in the carbon nanopores. Combined with stochastic modeling, in situ SAXS allows quantifying the solid iodine volume fraction and visualizing the iodine structure on 3D lattice models at the sub-nanometer scale. Based on the derived mechanism, we demonstrate strategies for improved iodine pore filling capacity and prevention of self-discharge, applicable to hybrid supercapacitors and batteries. Iodide based energy storage is a potential candidate to improve performance of hybrid supercapacitors and batteries. Here, the authors revisit the previous understanding and show that electrochemical oxidation of iodide results in solid iodine deposits stabilized by the confinement of nanoporous carbons.

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 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 explain the side reactions. Here we show that singlet oxygen forms at the cathode of a lithium–oxygen cell during discharge and from the onset of charge, and accounts for the majority of parasitic reaction products. The amount increases during discharge, early stages of charge, and charging at higher voltages, and is enhanced by the presence of trace water. Superoxide and peroxide appear to be involved in singlet oxygen generation. Singlet oxygen traps and quenchers can reduce parasitic reactions effectively. Awareness of the highly reactive singlet oxygen in non-aqueous metal–oxygen batteries gives a rationale for future research towards achieving highly reversible cell operation. The application of Li–O 2 batteries is hindered by severe parasitic reactions in battery cycling. Here the authors show that the highly reactive singlet oxygen is the main cause for the electrolyte and carbon electrode degradation on discharge and charge.

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-O 2 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.

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

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.