such as silicene [9a] and ionic compounds [10a] were shown to speed up the Li 2 S lateral passivation rate, leading to lower capacities.Solvent has a strong impact on Li 2 S deposition. The kinetics and morphology of Li 2 S deposition in glyme-based polysulfide solutions was studied and a progressive nucleation and a 2D island growth model was proposed. [6a] In addition, the use of discharge mediator was reported to slow down the impingement of insulating Li 2 S islands on carbon and transform the 2D growth to a 3D growth. [11] This is in line with Cuisinier et al. [12] reporting a distinct Li 2 S deposition mechanism in electron pair donor electrolytes compared to glymes due to the partial solvation of Li 2 S and the additional chemical pathways provided by the increased stabilization of polysulfide radicals. Recently, Pan et al. [4] reported that solvents with medium donor number (DN) yield flower-like Li 2 S morphology, low-DN solvents make Li 2 S films, and high-DN solvents give rise small particles. These pioneering studies suggest that the solution mediation process plays a critical role in Li 2 S deposition and highlight the urgent need for developing quantitative and comprehensive correlation between solvent property and Li 2 S nucleation and growth.In this work, we establish structure-property relationship of solvent in controlling solid Li 2 S deposition and develop quantitative solvent-mediated Li 2 S growth models as guides to solvent selection. We investigate three solvent's properties and their roles on Li 2 S deposition: (1) the donicity which governs the stability of the polysulfide anions (i.e., the precursor of Li 2 S deposition) through (Li + ) sol -polysulfide interactions; [12,13] (2) the polarity (dielectric constant) which governs the solvation ability of the final product Li 2 S; [14] (3) the viscosity which strongly affects the diffusivity of polysulfide and dissolved Li 2 S. [15] We show that these solvent-controlled properties are essential factors pertaining to the sulfur utilization, electrode kinetics, and reversibility of electrochemical reduction of elemental sulfur. Finally, we demonstrate the effectiveness of the solvent selection criteria developed in this study in identifying new and more effective electrolytes for Li-S batteries. Results and DiscussionWe study Li 2 S deposition in electrolyte model systems of two major groups: (i) ether-based solvents: 1,2-dimethoxyethane (G1), diethylene glycol dimethyl ether (G2), triethylene glycol Controlling electrochemical deposition of lithium sulfide (Li 2 S) is a major challenge in lithium-sulfur batteries as premature Li 2 S passivation leads to low sulfur utilization and low rate capability. In this work, the solvent's roles in controlling solid Li 2 S deposition are revealed, and quantitative solventmediated Li 2 S growth models as guides to solvent selection are developed. It is shown that Li 2 S electrodeposition is controlled by electrode kinetics, Li 2 S solubility, and the diffusion of polysulfide/Li 2 S, which is dictated...

A new concept of exploiting bromide ions as a complexing agent to ‘free-up’ iodide ions for energy storage.

We report direct evidence of soluble LiO 2 generation upon Li 2 O 2 oxidation and reveal a strong solvent-controlled Li 2 O 2-oxidation reaction mechanism in Li-O 2 batteries. In high-donicity solvents, Li 2 O 2 oxidation follows a solution pathway by forming soluble LiO 2 intermediate. While in low-donicity solvent, Li 2 O 2 oxidation follows a solid-solution pathway by forming solid Li 2Àx O 2 intermediate. The preferential formation of soluble LiO 2 promotes the charging kinetics but leads to poor cycling stability. Our work shows that bypassing the generation of soluble LiO 2 will improve the stability of Li-O 2 batteries.

Reversible protonic ceramic electrochemical cells (R-PCECs) are a promising option for efficient and low-cost generation of electricity and hydrogen. Commercialization of R-PCECs, however, hinges on the development of highly active and robust air electrodes. Here, we report an air electrode consisting of PrBa 0.8 Ca 0.2 Co 2 O 5+δ and in situ exsolved BaCoO 3−δ nanoparticles (PBCC−BCO) that shows minimal polarization resistance (∼0.24 Ω cm 2 at 600 °C) and high stability when exposed to humidified air with 3−50% H 2 O. An R-PCEC utilizing PBCC-BCO demonstrates remarkable performances at 600 °C: achieving a peak power density of 1.06 W cm −2 in the fuel cell mode and a current density of 1.51 A cm −2 at 1.3 V in an electrolysis mode. More importantly, the R-PCECs demonstrate an exceptionally high durability over 1833 h of continuous operation in the electrolysis mode. This work offers an efficient approach to design of high-performance and durable electrodes for R-PCECs.

Nonaqueous redox flow batteries (NRFBs) exhibit a wide potential window (>3.0 V) but have been limited by the low solubility of the active materials. Here, we propose and demonstrate a high-energy-density nonaqueous redox flow battery based on a lowmelting-point (37−40 °C) ferrocene derivative, 1,1-dimethylferrocene (DMFc), operated at its liquid state. The liquid redox-active DMFc not only contributes to high capacity but also acts as a solvating medium to the ion-conducting salts. Taking advantage of DMFc's high concentration (3 M), superior stability, and fast kinetics, the lithium/DMFc battery achieves a high volumetric density (∼68 Ah L −1 catholyte ) with a high Coulombic efficiency (>95%) and high cycling stability. Our work demonstrates that exploiting a low-meltingpoint redox-active species at its melting state is a promising direction for developing high-energy-density NRFBs for next-generation energy storage technologies.

One of the main bottlenecks that limit the performance of reversible protonic ceramic electrochemical cells (R‐PCECs) is the sluggish kinetics of the oxygen reduction and evolution reactions (ORR and OER). Here, the significantly enhanced ORR and OER kinetics and stability of a conventional La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) air electrode by an efficient catalyst coating of barium cobaltite (BCO) is reported. The polarization resistance of a BCO‐coated LSCF air electrode at 600 °C is 0.16 Ω cm2, about 30% of that of the bare LSCF air electrode under the same conditions. Further, an R‐PCEC with the BCO‐coated LSCF air electrode shows exceptional performance in both fuel cell (peak power density of 1.16 W cm−2 at 600 °C) and electrolysis (current density of 1.80 A cm−2 at 600 °C at 1.3 V) modes. The performance enhancement is attributed mainly to the facilitated rate of oxygen surface exchange.

The commercialization of reversible protonic ceramic electrochemical cells is hindered by the lack of highly active and durable air electrodes exposed to high concentration of steam under operating conditions. Here, findings that dramatically enhance the electrocatalytic activity and stability of a conventional (La0.6Sr0.4)0.95Co0.2Fe0.8O3−δ (LSCF) air electrode by a multiphase catalyst coating composed of a conformal Pr1−xBaxCoO3−δ thin film and exsolved BaCoO3−δ nanoparticles, are reported. At 600 °C, the catalyst coating decreases the polarization resistance of the LSCF air electrode by a factor of 25 (from 1.09 to 0.043 Ω cm2) in air and the degradation rate by two orders of magnitude (from 1.0 × 10−2 to 1.8 × 10−4 Ω cm2 h−1 in humidified air with 30 vol% H2O). Further, a single cell with the catalyst‐coated LSCF air electrode at 600 °C demonstrates a high peak power density of 1.04 W cm−2 in the fuel cell mode and a high current density of 1.82 A cm−2 at 1.3 V in the electrolysis mode. The significantly enhanced performance of the LSCF air electrode is attributed mainly to the high rate of surface oxygen exchange, fast surface proton diffusion, and the rapid H2O and O2 dissociation on the catalysts.

Reversible solid oxide cells based on ceramic proton conductors have potential to be the most efficient system for large‐scale energy storage. The performance and long‐term durability of these systems, however, are often limited by the ionic conductivity or stability of the proton‐conducting electrolyte. Here new family of solid oxide electrolytes, BaHfxCe0.8−xY0.1Yb0.1O3−δ (BHCYYb), which demonstrate a superior ionic conductivity to stability trade‐off than the state‐of‐the‐art proton conductors, BaZrxCe0.8−xY0.1Yb0.1O3−δ (BZCYYb), at similar Zr/Hf concentrations, as confirmed by thermogravimetric analysis, Raman, and X‐ray diffraction analysis of samples over 500 h of testing are reported. The increase in performance is revealed through thermodynamic arguments and first‐principle calculations. In addition, lab scale full cells are fabricated, demonstrating high peak power densities of 1.1, 1.4, and 1.6 W cm−2 at 600, 650, and 700 °C, respectively. Round‐trip efficiencies for steam electrolysis at 1 A cm−2 are 78%, 72%, and 62% at 700, 650, and 600 °C, respectively. Finally, CO2H2O electrolysis is carried out for over 700 h with no degradation.