Recent progress in organic redox flow batteries:Active materials,electrolytes and membranes

Redox flow batteries （RFBs） have great potentials in the future applications of both large scale energy storage and powering the electrical vehicle. Critical challenges including low volumetric energy density. high cost and maintenance greatly impede the wide application of conventional RFBs based on inorganic materials. Redox-active organic molecules have shown promising prospect in the application of RFBs, benefited from their low cost, vast abundance, and high tunability of both potential and solubility. In this review, we discuss the advantages of redo~ active organic materials over their inorganic compart and the recent progress of organic based aqueous and non-aqueous RFBs. Design considerations in active materi- als, choice of electrolytes and membrane selection in both aqueous and non-aqueous RFBs are discussed. Finally. we discuss remaining critical challenges and suggest future directions for improving organic based RFBs.

Carbon-Nitride-Based Materials for Advanced Lithium-Sulfur Batteries

Lithium-sulfur(Li-S)batteries are promising candidates for next-generation energy storage systems owing to their high energy density and low cost.However,critical challenges including severe shuttling of lithium polysulfides(LiPSs)and sluggish redox kinetics limit the practical application of Li-S batteries.Carbon nitrides(C_(x)N_(y)),represented by graphitic carbon nitride(g-C_(3)N_(4)),provide new opportunities for overcoming these challenges.With a graphene-like structure and high pyridinic-N content,g-C_(3)N_(4) can effectively immobilize LiPSs and enhance the redox kinetics of S species.In addition,its structure and properties including electronic conductivity and catalytic activity can be regulated by simple methods that facilitate its application in Li-S batteries.Here,the recent progress of applying C_(x)N_(y)-based materials including the optimized g-C_(3)N_(4),g-C_(3)N_(4)-based composites,and other novel C_(x)N_(y) materials is systematically reviewed in Li-S batteries,with a focus on the structure-activity relationship.The limitations of existing C_(x)N_(y)-based materials are identified,and the perspectives on the rational design of advanced C_(x)N_(y)-based materials are provided for high-performance Li-S batteries.

A Low-Crossover and Fast-Kinetics Thiolate Negolyte for Aqueous Redox Flow Batteries

Aqueous redox flow batteries(ARFBs)are a promising technology for large-scale energy storage.Developing high-capacity and long-cycle negolyte materials is one of major challenges for practical ARFBs.Inorganic polysulfide is promising for ARFBs owing to its low cost and high solubility.However,it suffers from severe crossover resulting in low coulombic efficiency and limited lifespan.Organosulfides are more resistant to crossover than polysulfides owing to their bulky structures,but they suffer from slow reaction kinetics.Herein,we report a thiolate negolyte prepared by an exchange reaction between a polysulfide and an organosulfide,preserving low crossover rate of the organosulfide and high reaction kinetics of the polysulfide.The thiolate denoted as 2-hydroxyethyl disulfide+potassium polysulfide(HEDS+K_(2)S_(2))shows reduced crossover rate than K_(2)S_(2),faster reaction kinetics than HEDS,and longer lifespan than both HEDS and K_(2)S_(2).The 1.5M HEDS+1.5M K_(2)S_(2)static cell demonstrated 96 Ah L^(-1)negolyte over 100 and 200 cycles with a high coulombic efficiency of 99.2%and 99.6%at 15 and 25mAcm^(-2),respectively.The 0.5M HEDS+0.5M K_(2)S_(2)flow cell delivered a stable and high capacity of 30.7 Ah L^(-1)negolyte over 400 cycles(691 h)at 20mAcm^(-2).This study presents an effective strategy to enable lowcrossover and fast-kinetics sulfur-based negolytes for advanced ARFBs.

External field–assisted batteries toward performance improvement

Rechargeable batteries are essential for the increased demand for energy storage technologies due to their ability to adapt intermittent renewable energies into electric devices,such as electric vehicles.To boost the battery performance,applying external fields to assist the electrochemical process has been developed and exhibits significant merits in energy efficiency and cycle stability enhancement.This perspective focuses on recent advances in the development of external field–assisted battery technologies,including photo-assisted,magnetic field–assisted,sound field–assisted,and multiple field–assisted.The workingmechanisms of external field–assisted batteries and their challenges and opportunities are highlighted.

Design strategies for low temperature aqueous electrolytes

Low temperature aqueous batteries(LT-ABs)have attracted extensive attention recent years.The LT-ABs suffer from electrolyte freezing,slow ionic diffusion and sluggish interfacial redox kinetics at low temperature.In this review,we discuss physicochemical properties of aqueous electrolytes in terms of phase diagram,ion diffusion and interfacial redox kinetics to guide the design of low temperature aqueous electrolytes(LT-AEs).Firstly,the characteristics of equilibrium and non equilibrium phase diagrams are introduced to analyze the antifreezing mechanisms and propose design strategies for LT-AEs.Then,the temperature/concentration/charge carrier dependence conductivity characteristics in aqueous electrolytes are reviewed to comprehend and regulate the ion diffusion kinetics.Moreover,we introduce interfacial studies in aqueous and non-aqueous batteries and propose potential improvement strategies for interfacial redox kinetics in LT-ABs.Finally,we summarize design strategies of LT-AEs for developing high performance LT-ABs.

The Potassium−Air Battery:Far from a Practical Reality?

CONSPECTUS:An energy storage system is the key bottleneck toward the widespread use of renewable energy and the development of electric vehicles(EVs).Alkali metal−oxygen batteries,which have higher gravimetric energy densities(3500−935 Wh kg−1)than conventional lithium−ion batteries(100−265 Wh kg−1),are considered to be one of the promising next-generation energy storage systems.Over the past decade,Li−O2 batteries have been the center of the research effort owing to their highest energy density.However,the poor reversibility,low round-trip efficiency,and limited cycle life originating from sluggish kinetic and serious parasitic chemistry induced by singlet oxygen hamper the development of Li−O2 batteries.Both the sluggish kinetics and severe parasitic reactions are closely related to the discharge product Li2O2.Unlike Li−O2 batteries,K−O2 batteries based on potassium superoxide offer an attractive theoretical energy density(935 Wh kg−1)with a significantly improved energy efficiency and lifetime compared to other alkali metal−O2 batteries.The fast and reversible O2/KO2 single−electron reaction exhibits higher redox kinetics compared to the Li−O2 redox chemistries and removes the needs of catalysts or redox mediators.In addition,the earth abundant K greatly alleviates the global shortage and uneven regional distribution of Li.These unique advantages of the K−O2 system make it a promising candidate for low-cost and large-scale energy storage.However,the development of a K−O2 battery is still in its early stages and its round-trip efficiency is still lower than that of lithium−ion batteries.Further improvement in energy efficiency and cycle life of the K−O2 batteries is crucial prior to practical applications.The present Account combines our efforts and other representative works on fundamental understandings and design strategies toward next-generation K−O2 batteries.Insights are offered on oxygen electrode reversibility and stability,anode stabilization and alternative anodes,and the closed system based on KO2−K2O2 conversion.Five physicochemical factors that affect the oxygen electrode reversibility and stability are discussed in light of recent findings,including electrolyte design,growth mechanism,operation environment,degradation mechanism,and electrode−electrolyte design.Furthermore,the alternative anode materials development to solve the long-standing potassium anode issue are discussed and the pros and cons of alternative anodes are compared.In addition,due to oxygen crossover to the anode and the electrolyte evaporation problem in open K−air battery systems,the feasibility and strategies to develop closed systems are briefly discussed.At the end of the Account,future directions in deepening understanding of K−O2 reaction and battery design to realize practical applications of K−O2 systems are highlighted.