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Li–O2batteries can provide greater gravimetric energy than Li-ion batteries but suffer from poor efficiency and cycle life due to the instability of aprotic electrolytes. In this study, we show that the apparent four-electron oxygen reduction to form Li2O in Li–O2batteries with molten nitrate is facilitated by the electrochemical reduction of nitrate to nitrite, and subsequent chemical oxidation of nitrite to nitrate by molecular oxygen, instead of a four-electron oxygen reduction aided by disproportionation of Li2O2generated from two-electron reduction of molecular oxygen. By examining a series of transition metal catalysts using experiments and computation, optimizing the surface binding of nitrate to enhance the kinetics of the electrochemical reduction of nitrate to nitrite, as well as increasing the kinetics of nitrite oxidation by O2was shown to increase the discharge voltage and render the observed high-rate capability for NiO-based surfaces in Li–O2batteries.

Li–O2batteries can provide significantly higher gravimetric energy density than Li-ion batteries, but their practical use is limited by a number of fundamental issues associated with oxidizing discharge products such as Li2O2and LiOH during charging. Soluble inorganic redox mediators (RMs) like LiI and LiBr have been shown to enhance round-trip efficiency where different solvents can greatly shift the redox potential of the RMs, significantly altering the overpotential during charging, as well as their oxidizing power against the discharge product. Unfortunately, other design requirements like (electro)chemical stability with the electrode as well as reactive discharge products greatly constrain the selection of solvent, making it impractical to additionally design the solvent to provide optimal RM performance. In this work, we demonstrate that interhalide RMs based on LiI/LiBr and LiI/LiCl mixtures can enable tuning of the oxidizing power of the RM in a given solvent. I–Br interhalides I2Br–to IBr2–showed increasing chemical oxidizing power toward Li2O2and LiOH with increasing Br, and DEMS measurements during charging of Li–O2cells demonstrated that these I–Br interhalide RMs led to increased O2evolution with respect to LiI and reduced charging potential and CO2evolution with respect to LiBr.

Lithium–oxygen (Li–O2) batteries offer considerably higher gravimetric energy density than commercial Li-ion batteries (up to three times) but suffer from poor power, cycle life, and round-trip efficiency. Tuning the thermodynamics and pathway of the oxygen reduction reaction (ORR) in aprotic electrolytes can be used to enhance the Li–O2battery rate and discharge capacity. In this work, we present a systematic study on the role of the solvent and anion on the thermodynamics and kinetics of Li+-ORR, from which we propose a unified descriptor for its pathway and kinetics. First, by thoroughly characterizing the solvation environment of Li+ions using Raman spectroscopy, 7Li NMR, ionic conductivity, and viscosity measurements, we observe increasing Li+–anion interactions with increasing anion DN in low DN solvents such as 1,2-dimethoxyethane and acetonitrile but minimal Li+–anion interactions in the higher DN dimethyl sulfoxide. Next, by determining the electrolyte-dependent Li+/Li, TBA+,O2/TBA+–O2–, and Li+,O2/Li+–O2–redox potentials versus the solvent-invariant Me10Fc reference potential, we show that stronger combined solvation of Li+and O2–ions leads to weaker Li+–O2-coupling. Finally, using rotating ring disk electrode measurements, we show that weaker Li+–O2–coupling in electrolytes with strong combined solvation leads to an increased generation of soluble Li+–O2–-type species and faster overall kinetics during Li+-ORR.

Electrolytes with improved thermal and oxidative stability must be developed to achieve greater power and energy densities without compromising safety in modern energy storage devices. Because of their much-reduced solvent vapor pressure and expanded electrochemical windows, solvate ionic liquids (SILs) of lithium salts have recently attracted significant attention in this regard. The current palette of SILs is, however, limited to only a few suitable anions with limited chemical functionality. Guided by fundamental physical organic chemistry principles, we designed a new family of S-substituted fluorinated aryl sulfonamide-tagged anions that feature variable numbers of electronically neutral or withdrawing sulfide, sulfoxide, and sulfone substituents. Several salts of these electron deficient anions display very high electrochemical oxidative stability, good solubility, and a weakly coordinating nature that enables the synthesis of Li-based SILs with high thermal and electrochemical oxidative stability. This new family of functional, noncoordinating anions will potentially expand the scope of applications of SILs as safe electrolytes in battery devices.

Replacing graphite with alloying Al negative electrodes would allow for the development of high energy density Li-ion batteries. However, large volume changes associated with the alloying/dealloying process often result in pulverization of the electrode and rapid capacity fade during cycling due to the continuous formation of solid electrolyte interphase (SEI) layers and loss of electronic contact. In this study, we report that increasing salt concentration in the electrolyte to > 5 mol dm[?]3 led to enhanced capacity retention during cycling of Li-Al half-cells, which was accompanied by nearly constant impedance for the Al electrode in lithium bis(fluorosulfonyl)imide (LiFSI)/dimethyl carbonate (DMC) 1:1.1 (mol/mol) superconcentrated electrolyte. X-ray photoelectron spectroscopy (XPS) revealed that a potential hold in the superconcentrated electrolyte formed an SEI layer with a greater LiF concentration than in standard 1 mol dm[?]3 solution. This was supported by Raman spectroscopy of LiFSI solutions in DMC, supplemented with density functional theory calculations, which showed an increased driving force for the reduction of FSI[?] anions to form LiF from Li+-coordinated DMC complexes with increasing salt concentration. Therefore, the enhanced capacity retention and stability can be attributed to the stability of LiF-rich SEI layers which limit carbonate reduction and charge transfer impedance growth.

Understanding the kinetics of electrochemical oxygen reduction reaction (ORR) and controlling the chemistry, morphology, and size of discharge products are critical to realize reversible operation of metal–air batteries. Here we show that increasing Na+activity and free DME (not coordinated to Na+) activity in the solution increases the solubility of NaO2and size of NaO2cubes in Na–O2cells. With increasing Na salt concentration, Raman spectroscopy revealed that Na+activity increased while free DME activity decreased. NaO2solubility and NaO2cube size were found to exhibit a maximum at a medium concentration of Na+, which was accompanied by the highest full discharge capacity. This trend was attributed to two competing effects that stabilize NaO2in solution; both higher Na+activity and higher free DME activity can enhance NaO2solubility. These results highlight immense opportunities in the design of discharge/charge characteristics such as reaction product sizes and discharge capacity through the manipulation of the chemical physics of electrolytes as well as the solvation of reaction intermediates in the electrolytes.

Fundamental understanding of the reactivity between electrode and electrolyte is key to design the safety and life of Li-ion batteries. Herein X-ray photoelectron spectroscopy was used to examine the electrode/electrolyte interface (EEI) on carbon-free, binder-free LiCoO2 powder and thin-film electrodes in LP57 electrolyte as function of potential. Upon charging of LiCoO2 a marked growth of oxygenated and carbonated species was observed on the surface, consistent with electrolyte oxidation at high potentials. We also demonstrated that LiCoO2 oxide surface was prone to decompose the salt starting at 4.1 VLi, as evidenced by the increase of LiF and LixPFyOz species upon charging. By DFT calculations we proposed a correlation between the interface composition and the thermodynamic tendency of the EC solvent for dissociative adsorption on the LixCoO2 surface, through the generation of reactive acidic OH groups on the oxide surface, which can have a role in the observed salt decomposition. This is consistent with the evidence of HF and PF2O2[?] species at 4.6 VLi observed by solution 19F-NMR measurements. Finally we compared EEI composition between composite and model electrodes and discussed the changes and mechanisms induced by the electrode composition or the use of electrolyte additives. We showed that the addition of diphenyl carbonate (DPC) in the electrolyte has a strong impact on the formation of solvent and salt decomposition products at the EEI layer.

Diels–Alder cycloaddition with furans as dienes and subsequent dehydrative aromatization are potentially valuable processes for sustainable conversion of biomass-derived furans to aromatics. We have performed electronic structure calculations to investigate the catalytic activity of HY and of alkali-exchanged Y zeolites in connection with the conversion of 2,5-dimethylfuran and ethylene to p-xylene. We have used two active site settings: an active site cluster model on which we have carried out density functional theory calculations and a mechanically embedded active site cluster model on which we have performed hybrid quantum mechanics/molecular mechanics calculations with the ONIOM scheme. Even though Lewis catalyzed Diels–Alder cycloaddition has received considerable attention over the years, we show that confinement and charge transfer in zeolite catalysts play a significant role in catalysis. Both HY and alkali-Y can catalyze the aromatization of the cycloadduct through dehydration but HY is found to be far more effective. Our analysis shows that the electron withdrawing ability of the cations and the catalytic activity of alkali-Y as Lewis acids are diminished by substrate binding-induced electron density shift from the framework oxygen atoms to the cations. On account of these inductive phenomena, we show that the DMF–ethylene cycloaddition follows a bidirectional instead of normal electron flow mechanism.

Polymer electrolytes (PEs) have attracted tremendous research interest for their potential to offer improved safety and energy capacity in next-generation battery technologies. Among the different classes of PEs, single-ion conductors (SICs) are particularly interesting due to their high transference numbers. Nevertheless, a detailed understanding of how molecular structure impacts the properties of SIC-PEs is absent, limiting the ability to design improved materials. Here, we present the synthesis and characterization of a new class (seven examples provided) of polyanions featuring fluorinated aryl sulfonimide tagged (FAST) anions as side chains. These “polyFAST” salts are shown to outperform the widely used poly[(4-styrenesulfonyl) (trifluoromethanesulfonyl)imide] due to their strongly electron-withdrawing side chains and enhanced distance between anionic sites, providing higher electronic conductivities at all salt concentrations and in some cases superior electrochemical oxidative stability. Moreover, they provide a platform for discovery of fundamental relationships between macromolecular composition, as programmed through monomer structure, and SIC-PE bulk properties. Finally, we leverage the electron-deficient nature of polyFAST salts to demonstrate a new poly(solvate ionic liquid) (polySIL) concept that offers a promising pathway toward high-performance PEO-free SIC-PEs.

Replacing graphite with alloying Al negative electrodes would allow for the development of high energy density Li-ion batteries. However, large volume changes associated with the alloying/dealloying process often result in pulverization of the electrode and rapid capacity fade during cycling due to the continuous formation of solid electrolyte interphase (SEI) layers and loss of electronic contact. In this study, we report that increasing salt concentration in the electrolyte to > 5 mol dm−3led to enhanced capacity retention during cycling of Li-Al half-cells, which was accompanied by nearly constant impedance for the Al electrode in lithium bis(fluorosulfonyl)imide (LiFSI)/dimethyl carbonate (DMC) 1:1.1 (mol/mol) superconcentrated electrolyte. X-ray photoelectron spectroscopy (XPS) revealed that a potential hold in the superconcentrated electrolyte formed an SEI layer with a greater LiF concentration than in standard 1 mol dm−3solution. This was supported by Raman spectroscopy of LiFSI solutions in DMC, supplemented with density functional theory calculations, which showed an increased driving force for the reduction of FSI−anions to form LiF from Li+-coordinated DMC complexes with increasing salt concentration. Therefore, the enhanced capacity retention and stability can be attributed to the stability of LiF-rich SEI layers which limit carbonate reduction and charge transfer impedance growth.

Fundamental understanding of the reactivity between electrode and electrolyte is key to design the safety and life of Li-ion batteries. Herein X-ray photoelectron spectroscopy was used to examine the electrode/electrolyte interface (EEI) on carbon-free, binder-free LiCoO2powder and thin-film electrodes in LP57 electrolyte as function of potential. Upon charging of LiCoO2a marked growth of oxygenated and carbonated species was observed on the surface, consistent with electrolyte oxidation at high potentials. We also demonstrated that LiCoO2oxide surface was prone to decompose the salt starting at 4.1 VLi, as evidenced by the increase of LiF and LixPFyOzspecies upon charging. By DFT calculations we proposed a correlation between the interface composition and the thermodynamic tendency of the EC solvent for dissociative adsorption on the LixCoO2surface, through the generation of reactive acidic OH groups on the oxide surface, which can have a role in the observed salt decomposition. This is consistent with the evidence of HF and PF2O2−species at 4.6 VLiobserved by solution 19F-NMR measurements. Finally we compared EEI composition between composite and model electrodes and discussed the changes and mechanisms induced by the electrode composition or the use of electrolyte additives. We showed that the addition of diphenyl carbonate (DPC) in the electrolyte has a strong impact on the formation of solvent and salt decomposition products at the EEI layer.