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Lithium-metal batteries (LMBs) are promising alternatives to state-of-the-art Lithium-ion batteries (LIBs) to achieve higher energy densities. However, the poor cyclability of LMBs resulting from Li metal anode (Li0) irreversibility and concomitant electrolyte decompositions limits their practical applications. In this study, we reported a per-fluorinated salt, lithium tetrakis(perfluoro-tertbutyloxy)borate (abbreviated as Li-TFOB) as an electrolyte additive for Li-metal batteries, which contains 36 F atoms per molecule. This newly designed ionic additive tuned the chemical composition of the solid-electrolyte interphase (SEI) on Li0 by increasing the amount of LiF and Li-B-O inorganic species. DFT calculations and Molecular dynamics (MD) simulations indicated the preferential reduction of the TFOB anions at Li0, which occurs with a lower free energy change than PF6anions. The designed ionic additive enables the 4.6 V Li||LiNi0.6Mn0.2Co0.2O2 (NMC622) cell to achieve an average CE of 99.1 % and a high-capacity retention of >50 % after 500 cycles. This experiment-simulation joint study illustrated an attractive approach to accelerating the design of electrolytes and interphases for LMBs. 

Water soluble ferrocene (Fc) derivatives are promising cathode materials for aqueous organic redox flow batteries (AORFBs) towards scalable energy storage. However, their structure–performance relationship and degradation mechanism in aqueous electrolytes remain unclear. Herein, physicochemical and electrochemical properties, battery performance, and degradation mechanisms of three Fc catholytes, (ferrocenylmethyl)trimethylammonium chloride (C1-FcNCl), (2-ferrocenyl-ethyl)trimethylammonium chloride (C2-FcNCl), and (3-ferrocenyl-propyl)trimethylammonium chloride (C3-FcNCl) in pH neutral aqueous electrolytes were systemically investigated. UV-Vis and gas chromatography (GC) studies confirmed the thermal and photolytic C x -Cp − ligand dissociation decomposition pathways of both discharged and charged states of C1-FcNCl and C2-FcNCl catholytes. In contrast, in the case of the C3-FcNCl catholyte, the electron-donating 3-(trimethylammonium)propyl group strengthens the coordination between the C 3 -Cp − ligand and the Fe 3+ or Fe 2+ center and thus mitigates the ligand-dissociation degradation. Consistently, the Fc electrolytes displayed cycling stability in both half-cell and full-cell flow batteries in the order of C1-FcNCl < C2-FcNCl < C3-FcNCl. 

Aqueous redox flow batteries could provide viable grid-scale electrochemical energy storage for renewable energy because of their high-power performance, scalability, and safe operation ( 1 , 2 ). Redox-active organic molecules serve as the energy storage materials ( 2 , 3 ), but only very few organic molecules, such as viologen ( 4 , 5 ) and anthraquinone molecules ( 6 ), have demonstrated promising energy storage performance ( 2 ). Efforts continue to develop other families of organic molecules for flow battery applications that would have dense charge capacities and be chemically robust. On page 836 of this issue, Feng et al. ( 7 ) report a class of ingeniously designed 9-fluorenone (FL) molecules as high-performance, potentially low-cost organic anode electrolytes (anolytes) in aqueous organic redox flow batteries (see the figure, top). These FL anolytes not only display exceptional energy storage performance but also exhibit an unprecedented two-electron storage mechanism. 

Abstract Aqueous organic redox flow batteries (AORFBs) have received increasing attention as an emergent battery technology for grid‐scale renewable energy storage. However, physicochemical properties of redox‐active organic electrolytes remain fine refinement to maximize their performance in RFBs. Herein, we report a carboxylate functionalized viologen derivative, N,N′‐dibutyrate‐4,4′‐bipyridinium,(CBu)₂V, as a highly stable, high capacity anolyte material under near pH neutral conditions.(CBu)₂Vcan achieve solubility of 2.1 M and display a reversible, kinetically fast reduction at −0.43 V vs NHE at pH 9. DFT studies revealed that the high solubility of(CBu)₂Vis attributed to its high molecular polarity while its negative reduction potential is benefitted from electron‐donating carboxylate groups. A 0.89 V (CBu)₂V/(NH)₄Fe(CN)₆AORFB demonstrated exceptional energy storage performance, specifically, 100 % capacity retention with a discharge energy density of 9.5 Wh L⁻¹for 1000 cycles, power densities of up to 85 mW cm⁻², and an energy efficiency of 70 % at 60 mA cm⁻².(CBu)₂Vnot only represents the most capacity dense viologen with pendant ionic groups and also exhibits the longest (1200 hours or 50 days) and the most stable flow battery performance to date. 

Aqueous redox flow batteries using low-cost organic and inorganic active materials have received growing interest for sustainable energy storage. In this study, a low-cost, high redox potential (1.08 V vs. NHE) and high capacity ammonium bromide (NH 4 Br, 214.4 A h L −1 ) catholyte was coupled with an organic viologen anolyte to demonstrate 1.51 V high voltage (SPr) 2 V/Br − aqueous redox flow batteries under pH neutral conditions for the first time. Benefitting from the high water solubility of both the NH 4 Br catholyte and (SPr) 2 V anolyte, the newly designed (SPr) 2 V/Br − organic flow battery was operated at up to 1.5 M and an energy density of up to 30.4 W h L −1 . Using multiwall carbon nanotubes as an electrochemical additive for the Br 3 − /Br − redox couple, the highly energy dense (SPr) 2 V/Br − flow battery manifested outstanding current performance, up to 78% energy efficiency at 40 mA cm −2 current density and 227 mW cm −2 power density, the highest power density known for pH neutral organic flow batteries.