Highly Oxidation-Resistant Electrolyte for 4.7 V Sodium Metal Batteries Enabled by Anion/Cation Solvation Engineering

Tris(pentafluoro)phenylborane electrolyte additive regulates the highly stable and uniform CEI membrane components to improve the high-voltage behaviors of NCM811 lithium-ion batteries

Broadening the charging and discharging voltage window of high nickel cathode material NCM811 is the most expected method to improve the high specific energy density of batteries currently, yet the cathode-electrolyte interface (CEI) formed by the oxidized and decomposed products of carbonate-based electrolyte under high voltage are always so unsatisfied. Therefore, a voltage-stabilizer, TPFPB (Tris(pentafluoro)phenylborane), added into baseline electrolyte (1 M LiPF6 in EC:EMC:DMC=1:1:1 vol%) to promote the electrochemical performance of the battery at 4.5 V. The results interpret that the TPFPB-contained NCM811-Li half-cells exhibit high specific capacity (167.10 mAh/g), excellent capacity retention rate (CRR) (75.37 %), and high rate performance (173.3 mAh/g at 5C) during 4.5 V. Meanwhile, through the analysis of the physical characterization techniques. the B- and F-rich interfacial layer, named as CEI film, existing at the interface between the cathode and the electrolyte, produced under 4.5 V, is superior, resulting in impeding the structural collapse of the cathode material and the continued dissolution of transition metal ions (TMn+) from the cathode material, as well as, ameliorate the electrochemical polarization of the battery, ultimately, it can stabilize the electrochemical performance of the battery under high voltage. Therein, the present work elucidate a new and substantial approach to enhance the high-voltage performances of rich-Ni cathode materials.

Regulating Sodium Deposition Behavior by a Triple-Gradient Framework for High-Performance Sodium Metal Batteries

An efficient method for the synthesis of a self-supporting carbon framework (denoted Gra-GC-MoSe2) is proposed with a triple-gradient structure-in sodiophilic sites, pore volume, and electrical conductivity-which facilitates the highly efficient regulation of Na deposition. In situ and ex situ measurements, together with theoretical calculations, reveal that the gradient distribution of Se heteroatoms in MoSe2, and its derivatives tailor the sodiophilicity, while the gradient distribution of porous nanostructures homogenizes the Na+ diffusion. Therefore, Na deposition occurs from the bottom to the top of the Gra-GC-MoSe2 framework without dendrite formation. In addition, the gradient in electrical conductivity ensures the stripping process does not lead to dead Na. As a result, a Gra-GC-MoSe2 modified Na anode (Na@Gra-GC-MoSe2) shows impressive cycling stability with a high average Coulombic efficiency in an asymmetric cell. In symmetric cells, it also exhibits a long cycling life of 2000 h with a low polarization voltage and works stably even under a large capacity of 10 mAh cm-2. Moreover, a Na@Gra-GC-MoSe2|| Na3V2(PO4)3 full cell delivers a high energy density with an excellent cycling performance.

Solvation Effect: The Cornerstone of High-Performance Battery Design for Commercialization-Driven Sodium Batteries

Sodium batteries (SBs) emerge as a potential candidate for large-scale energy storage and have become a hot topic in the past few decades. In the previous researches on electrolyte, designing electrolytes with the solvation theory has been the most promising direction is to improve the electrochemical performance of batteries through solvation theory. In general, the four essential factors for the commercial application of SBs, which are cost, low temperature performance, fast charge performance and safety. The solvent structure has significant impact on commercial applications. But so far, the solvation design of electrolyte and the practical application of sodium batteries have not been comprehensively summarized. This review first clarifies the process of Na+ solvation and the strategies for adjusting Na+ solvation. It is worth noting that the relationship between solvation theory and interface theory is pointed out. The cost, low temperature, fast charging, and safety issues of solvation are systematically summarized. The importance of the de-solvation step in low temperature and fast charging application is emphasized to help select better electrolytes for specific applications. Finally, new insights and potential solutions for electrolytes solvation related to SBs are proposed to stimulate revolutionary electrolyte chemistry for next generation SBs.

A Critical Review on Room-Temperature Sodium-Sulfur Batteries: From Research Advances to Practical Perspectives

Room-temperature sodium-sulfur (RT-Na/S) batteries are promising alternatives for next-generation energy storage systems with high energy density and high power density. However, some notorious issues are hampering the practical application of RT-Na/S batteries. Besides, the working mechanism of RT-Na/S batteries under practical conditions such as high sulfur loading, lean electrolyte, and low capacity ratio between the negative and positive electrode (N/P ratio), is of essential importance for practical applications, yet the significance of these parameters has long been disregarded. Herein, it is comprehensively reviewed recent advances on Na metal anode, S cathode, electrolyte, and separator engineering for RT-Na/S batteries. The discrepancies between laboratory research and practical conditions are elaborately discussed, endeavors toward practical applications are highlighted, and suggestions for the practical values of the crucial parameters are rationally proposed. Furthermore, an empirical equation to estimate the actual energy density of RT-Na/S pouch cells under practical conditions is rationally proposed for the first time, making it possible to evaluate the gravimetric energy density of the cells under practical conditions. This review aims to reemphasize the vital importance of the crucial parameters for RT-Na/S batteries to bridge the gaps between laboratory research and practical applications.

Electrolytes for Sodium Ion Batteries: The Current Transition from Liquid to Solid and Hybrid systems

Sodium-ion batteries (NIBs) have recently garnered significant interest in being employed alongside conventional lithium-ion batteries, particularly in applications where cost and sustainability are particularly relevant. The rapid progress in NIBs will undoubtedly expedite the commercialization process. In this regard, tailoring and designing electrolyte formulation is a top priority, as they profoundly influence the overall electrochemical performance and thermal, mechanical, and dimensional stability. Moreover, electrolytes play a critical role in determining the system's safety level and overall lifespan. This review delves into recent electrolyte advancements from liquid (organic and ionic liquid) to solid and quasi-solid electrolyte (dry, hybrid, and single ion conducting electrolyte) for NIBs, encompassing comprehensive strategies for electrolyte design across various materials, systems, and their functional applications. The objective is to offer strategic direction for the systematic production of safe electrolytes and to investigate the potential applications of these designs in real-world scenarios while thoroughly assessing the current obstacles and forthcoming prospects within this rapidly evolving field.

Electrolyte Engineering with Tamed Electrode Interphases for High-Voltage Sodium-Ion Batteries

Sodium-ion batteries (SIBs) hold great promise for next-generation grid-scale energy storage. However, the highly instable electrolyte/electrode interphases threaten the long-term cycling of high-energy SIBs. In particular, the instable cathode electrolyte interphase (CEI) at high voltage causes persistent electrolyte decomposition, transition metal dissolution, and fast capacity fade. Here, this work proposes a balanced principle for the molecular design of SIB electrolytes that enables an ultra-thin, homogeneous, and robust CEI layer by coupling an intrinsically oxidation-stable succinonitrile solvent with moderately solvating carbonates. The proposed electrolyte not only shows limited anodic decomposition thus leading to a thin CEI, but also suppresses dissolution of CEI components at high voltage. Consequently, the tamed electrolyte/electrode interphases enable extremely stable cycling of Na3V2O2(PO4)2F (NVOPF) cathodes with outstanding capacity retention (>90%) over 3000 cycles (8 months) at 1 C with a high charging voltage of 4.3 V. Further, the NVOPF||hard carbon full cell shows stable cycling over 500 cycles at 1 C with a high average Coulombic efficiency (CE) of 99.6%. The electrolyte also endows high-voltage operation of SIBs with great temperature adaptability from -25 to 60 °C, shedding light on the essence of fundamental electrolyte design for SIBs operating under harsh conditions.

Recent advances in electrolyte molecular design for alkali metal batteries

In response to societal developments and the growing demand for high-energy-density battery systems, alkali metal batteries (AMBs) have emerged as promising candidates for next-generation energy storage. Despite their high theoretical specific capacity and output voltage, AMBs face critical challenges related to high reactivity with electrolytes and unstable interphases. This review, from the perspective of electrolytes, analyzes AMB failure mechanisms, including interfacial side reactions, active materials loss, and metal dendrite growth. It then reviews recent advances in innovative electrolyte molecular designs, such as ether, ester, sulfone, sulfonamide, phosphate, and salt, aimed at overcoming the above-mentioned challenges. Finally, we propose the current molecular design principles and future promising directions that can help future precise electrolyte molecular design.

Anion-Induced Uniform and Robust Cathode-Electrolyte Interphase for Layered Metal Oxide Cathodes of Sodium Ion Batteries

Layer metal oxides demonstrate great commercial application potential in sodium-ion batteries, while their commercialization is extremely hampered by the unsatisfactory cycling performance caused by the irreversible phase transition and interfacial side reaction. Herein, trimethoxymethylsilane (TMSI) is introduced into electrolytes to construct an advanced cathode/electrolyte interphase by tuning the solvation structure of anions. It is found that due to the stronger interaction between ClO4- and TMSI than that of ClO4- and PC/FEC, the ClO4--TMSI complexes tend to accumulate on the surface of the cathode during the charging process, leading to the formation of a stable cathode/electrolyte interface (CEI). In addition, the Si species with excellent electronic insulation ability are distributed in the TMSI-derived CEI film, which is conducive to inhibiting the continuous side reaction of solvents and the growth of the CEI film. As a result, under a current density of 250 mA g-1, the capacity retention of the NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode after 200 cycles in the TMSI-modified electrolyte is 74.4% in comparison to 51.5% of the bare electrolyte (1 M NaClO4/PC/5% FEC). Moreover, the NFM cathode shows better kinetics, with the specific discharge capacity increasing from 22 to 67 mAh g-1 at 300 mA g-1. It also demonstrates greatly improved rate capability, cycling stability, and Coulombic efficiency under various operating conditions, including high temperature (55 °C) and high cutoff voltage (2.0-4.3 V vs Na+/Na).

Non-aqueous Liquid Electrolyte Additives for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have been recognized as one of the most promising new energy storage devices for their rich sodium resources, low cost and high safety. The electrolyte, as a bridge connecting the cathode and anode electrodes, plays a vital role in determining the performance of SIBs, such as coulombic efficiency, energy density and cycle life. Therefore, the overall performance of SIBs could be significantly improved by adjusting the electrolyte composition or adding a small number of functional additives. In this review, the fundamentals of SIB electrolytes including electrode-electrolyte interface and solvation structure are introduced. Then, the mechanisms of electrolyte additive action on SIBs are discussed, with a focus on film-forming additives, flame-retardant additives and overcharge protection additives. Finally, the future research of electrolytes is prospected from the perspective of scientific concepts and practical applications.

Regulation of anion-Na+ coordination chemistry in electrolyte solvates for low-temperature sodium-ion batteries

High-performance sodium storage at low temperature is urgent with the increasingly stringent demand for energy storage systems. However, the aggravated capacity loss is induced by the sluggish interfacial kinetics, which originates from the interfacial Na+ desolvation. Herein, all-fluorinated anions with ultrahigh electron donicity, trifluoroacetate (TFA-), are introduced into the diglyme (G2)-based electrolyte for the anion-reinforced solvates in a wide temperature range. The unique solvation structure with TFA- anions and decreased G2 molecules occupying the inner sheath accelerates desolvation of Na+ to exhibit decreased desolvation energy from 4.16 to 3.49 kJ mol-1 and 24.74 to 16.55 kJ mol-1 beyond and below -20 °C, respectively, compared with that in 1.0 M NaPF6-G2. These enable the cell of Na||Na3V2(PO4)3 to deliver 60.2% of its room-temperature capacity and high capacity retention of 99.2% after 100 cycles at -40 °C. This work highlights regulation of solvation chemistry for highly stable sodium-ion batteries at low temperature.

Unraveling the Solvent Effect on Solid-Electrolyte Interphase Formation for Sodium Metal Batteries

Ether-based electrolytes are considered as an ideal electrolyte system for sodium metal batteries (SMBs) due to their superior compatibility with the sodium metal anode (SMA). However, the selection principle of ether solvents and the impact on solid electrolyte interphase formation are still unclear. Herein, we systematically compare the chain ether-based electrolyte and understand the relationship between the solvation structure and the interphasial properties. The linear ether solvent molecules with different terminal group lengths demonstrate remarkably distinct solvation effects, thus leading to different electrochemical performance as well as deposition morphologies for SMBs. Computational calculations and comprehensive characterizations indicate that the terminal group length significantly regulates the electrolyte solvation structure and consequently influences the interfacial reaction mechanism of electrolytes on SMA. Cryogenic electron microscopy clearly reveals the difference in solid electrolyte interphase in various ether-based electrolytes. As a result, the 1,2-diethoxyethane-based electrolyte enables a high Coulombic efficiency of 99.9 %, which also realizes the stable cycling of Na||Na3 V2 (PO4 )3 full cell with a mass loading of ≈9 mg cm-2 over 500 cycles.

High Chaos Induced Multiple-Anion-Rich Solvation Structure Enabling Ultrahigh Voltage and Wide Temperature Lithium-Metal Batteries

The optimal electrolyte for ultrahigh energy density (>400 Wh/kg) lithium-metal batteries with a LiNi0.8Co0.1Mn0.1O2 cathode is required to withstand high voltage (≥4.7 V) and be adaptable over a wide temperature range. However, the battery performance is degraded by aggressive electrode-electrolyte reactions at high temperature and high voltage, while excessive growth of lithium dendrites usually occurs due to poor kinetics at low temperature. Accordingly, the development of electrolytes has encountered challenges in that there is almost no electrolyte simultaneously meeting the above requirements. Herein, a high chaos electrolyte design strategy is proposed, which promotes the formation of weak solvation structures involving multiple anions. By tailoring a Li+-EMC-DMC-DFOB--PO2F2--PF6- multiple-anion-rich solvation sheath, a robust inorganic-rich interphase is obtained for the electrode-electrolyte interphase (EEI), which is resistant to the intense interfacial reactions at high voltage (4.7 V) and high temperature (45 °C). In addition, the Li+ solvation is weakened by the multiple-anion solvation structure, which is a benefit to Li+ desolventization at low temperature (-30 °C), greatly improving the charge transfer kinetics and inhibiting the lithium dendrite growth. This work provides an innovative strategy to manipulate the high chaos electrolyte to further optimize solvation chemistry for high voltage and wide temperature applications.

Carbon dots from alcohol molecules: principles and the reaction mechanism

Carbon dots (CDs) have attracted significant attention in the energy, environment, and biology fields due to their exceptional physicochemical properties. However, owing to the multifarious precursors and complex reaction mechanisms, the production of carbon dots from organic molecules is still a mysterious process. Inspired by the color change of sodium hydroxide ethanol solution after standing for some time, in this work, we thoroughly investigated the reaction mechanism from alcohol molecules to carbon dots through a lot of experiments and theoretical calculations, and it was found that the rate-controlling reaction is the formation of aldehydes, and it is also confirmed that there is a self-catalysis reaction, which can accelerate the conversion from alcohol to aldehyde, further facilitating the final formation of CDs. After the rate-controlling reaction of alcohol to aldehyde, under strongly alkaline conditions, an aldol reaction occurs to form unsaturated aldehydes, followed by further condensation and polymerization reactions to form long carbon chains, which are cross-linked and dehydrated to form carbon dots with a carbon core and surface functional groups. Additionally, it is found that the reaction can be largely accelerated with the assistance of electricity, which indicates the great prospect of industrial production. Furthermore, the obtained CDs with rich functional groups can be utilized as electrolyte additives to optimize the deposition behavior of Na metal, manifesting great potential towards safe and stable Na metal batteries.

Low-Cost Preparation of High-Performance Na-B-H-S Electrolyte for All-Solid-State Sodium-Ion Batteries

All-solid-state sodium-ion batteries have the potential to improve safety and mitigate the cost bottlenecks of the current lithium-ion battery system if a high-performance electrolyte with cost advantages can be easily synthesized. In this study, a one-step dehydrogenation-assisted strategy to synthesize the novel thio-borohydride (Na-B-H-S) electrolyte is proposed, in which both raw material cost and preparation temperature are significantly reduced. By using sodium borohydride (NaBH4 ) instead of B as a starting material, B atoms can be readily released from NaBH4 with much less energy and thus became more available to generate thio-borohydride. The synthesized Na-B-H-S (NaBH4 /Na-B-S) electrolyte exhibits excellent compatibility with current cathode materials, including FeF3 (1.0-4.5 V), Na3 V2 (PO4 )3 (2.0-4.0 V), and S (1.2-2.8 V). This novel Na-B-H-S electrolyte will take a place in mainstream electrolytes because of its advantages in preparation, cost, and compatibility with various cathode materials.

Molecule Isolation Protective Interface Formed by Planar Additive for Stable Sodium Metal Anodes

Sodium (Na) metal is an ideal anode for Na-based batteries because of its high specific capacity and low potential. However, interface issues such as side reactions with the electrolyte and uneven deposition severely hinder its practical application. Here, we report a zinc phthalocyanine (ZnPc) electrolyte additive with a planar molecular structure that can form a dense molecular layer when tightly adsorbed on the Na metal anode surface. Such a planar molecular layer can suppress side reactions between the anode and the electrolyte as well as homogenize Na+ flux to reduce dendrite growth. As a result, the molecular isolation interface formed by ZnPc adsorption on the surface of the Na metal anode enhances the interface stability and the cycling performance of the Na metal anode, with the average Coulombic efficiency of the half-cell of 99.95% after 350 stable cycles at 1 mA cm-2 for 1 mAh cm-2. Moreover, the assembled Na||Na3V2(PO4)3 full-cell with this additive delivers excellent stability over 120 cycles, proving the effectiveness of the ZnPc additive in practical application.

Increasing the Sodium Metal Electrode Compatibility with the Na3 PS4 Solid-State Electrolyte through Heteroatom Substitution

Rechargeable batteries are essential to the global shift towards renewable energy sources and their storage. At present, improvements in their safety and sustainability are of great importance as part of global sustainable development goals. A major contender in this shift are rechargeable solid-state sodium batteries, as a low-cost, safe, and sustainable alternative to conventional lithium-ion batteries. Recently, solid-state electrolytes with a high ionic conductivity and low flammability have been developed. However, these still face challenges with the highly reactive sodium metal electrode. The study of these electrolyte-electrode interfaces is challenging from a computational and experimental point of view, but recent advances in molecular dynamics neural-network potentials are finally enabling access to these environments compared to more computationally expensive conventional ab-initio techniques. In this study, heteroatom-substituted Na3 PS3 X1 analogues, where X is sulfur, oxygen, selenium, tellurium, nitrogen, chlorine, and fluorine, are investigated using total-trajectory analysis and neural-network molecular dynamics. It was found that inductive electron-withdrawing and electron-donating effects, alongside differences in heteroatom atomic radius, electronegativity, and valency, influenced the electrolyte reactivity. The Na3 PS3 O1 oxygen analogue was found to have superior chemical stability against the sodium metal electrode, paving the way towards high-performance, long lifetime and reliable rechargeable solid-state sodium batteries.

Electrolyte Salts for Sodium-Ion Batteries: NaPF6 or NaClO4?

NaClO4 and NaPF6, the most universally adopted electrolyte salts in commercial sodium-ion batteries (SIBs), have a decisive influence on the interfacial chemistry, which is closely related to electrochemical performance. The complicated and ambiguous interior mechanism of microscopic interfacial chemistry has prevented reaching a consensus regarding the most suitable sodium salt for high-performance SIB electrolytes. Herein, we reveal that the solvation structure induced by different sodium salt anions determines the Na+ desolvation kinetics and interfacial film evolution process. Specifically, the weak interaction between Na+ and PF6- promoted sodium desolvation and storage kinetics. The solvation structure involving PF6- induced the anion's preferential decomposition, generating a thin, inorganic compound-rich cathode-electrolyte interphase that ensured interface stability and inhibited solvent decomposition, thereby guaranteeing electrode stability and promoting the charge transfer kinetics. This study provides clear evidence that NaPF6 is not only more compatible with industrial processes but also more conducive to battery performance. Commercial electrolyte design employing NaPF6 will undoubtedly promote the industrialization of SIBs.

Molten salt-assisted synthesis of bismuth nanosheets with long-term cyclability at high rates for sodium-ion batteries

Bismuth is a promising anode material for sodium-ion batteries (SIBs) due to its high capacity and suitable working potential. However, the large volume change during alloying/dealloying would lead to poor cycling performance. Herein, we have constructed a 3D hierarchical structure assembled by bismuth nanosheets, addressing the challenges of fast kinetics, and providing efficient stress and strain relief room. The uniform bismuth nanosheets are prepared via a molten salt-assisted aluminum thermal reduction method. Compared with the commercial bismuth powder, the bismuth nanosheets present a larger specific surface area and interlayer spacing, which is beneficial for sodium ion insertion and release. As a result, the bismuth nanosheet anode presents excellent sodium storage properties with an ultralong cycle life of 6500 cycles at a high current density of 10 A g-1, and an excellent capacity retention of 87% at an ultrahigh current rate of 30 A g-1. Moreover, the full SIBs that paired with the Na3V2(PO4)3/rGO cathode exhibited excellent performance. This work not only presents a novel strategy for preparing bismuth nanosheets with significantly increased interlayer spacing but also offers a straightforward synthesis method utilizing low-cost precursors. Furthermore, the outstanding performance demonstrated by these nanosheets indicates their potential for various practical applications.

Suppression of Interphase Dissolution Via Solvent Molecule Tuning for Sodium Metal Batteries

Solvent molecule tuning is used to alter the redox potentials of solvents or ion-solvent binding energy for high-voltage or low-temperature electrolytes. Herein, an electrolyte design strategy that effectively suppresses solid electrolyte interphase (SEI) dissolution and passivates highly-reactive metallic Na anode via solvent molecule tuning is proposed. With rationally lengthened phosphate backbones with ─CH2 ─ units, the low-solvation tris(2-ethylhexyl) phosphate (TOP) molecule effectively weakens the solvation ability of carbonate-based electrolytes, reduces the free solvent ratio, and enables an anion-enriched primary Na+ ion solvation sheath. The decreased free solvent and compact lower-solubility interphase established in this electrolyte prevent electrodes from continuous SEI dissolution and parasitic reactions at both room temperature (RT) and high temperature (HT). As a result, the Na/Na3 V2 (PO4 )3 cell with the new electrolyte achieves impressive cycling stability of 95.7% capacity retention after 1800 cycles at 25 °C and 62.1% capacity retention after 700 cycles at 60 °C. Moreover, the TOP molecule not only maintains the nonflammable feature of phosphate but also attains higher thermal stability, which endows the electrolyte with high safety and thermal stability. This design concept for electrolytes offers a promising path to long-cycling and high-safety sodium metal batteries.

Construction of Inorganic-Rich Cathode Electrolyte Interphase on Co-Free Cathodes

Lithium-rich layered oxides (LRLOs), with the chemical formula of xLi₂MnO₃·(1 - x)LiMO₂, delivering higher specific discharge capacity, are potential cathode materials for lithium-ion batteries. However, the dissolution of transition metal ions and the instability of the cathode-electrolyte interphase (CEI) hinder the commercial application of LRLOs. Herein, a simple and affordable method is developed for the construction of a robust CEI layer by quenching a kind of cobalt-free LRLO, Li_1.2Ni_0.15Fe_0.1Mn_0.55O₂ (denoted as NFM), in 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether solvent. This robust CEI, with well-distributed LiF, TMF_x, and partial organic component CF_x, performs as a physical barrier to prevent NFM from direct contact with the electrolyte, suppresses the oxygen release, and ensures the CEI layer stability. The customized CEI with LiF and TMF_x-rich phase considerably enhances the NFM cycle stability and the initial coulomb efficiency and inhibits voltage fading. This work provides a valuable strategy for designing stable interface chemistry on the cathode of lithium-ion batteries.

A Mesoporous Tungsten Oxynitride Nanofibers/Graphite Felt Composite Electrode with High Catalytic Activity for the Cathode in Zn-Br Flow Battery

High electrochemical polarization during a redox reaction in the electrode of aqueous zinc-bromine flow batteries largely limits its practical implementation as an effective energy storage system. This study demonstrates a rationally-designed composite electrode that exhibits a lower electrochemical polarization by providing a higher number of catalytically-active sites for faster bromine reaction, compared to a conventional graphite felt cathode. The composite electrode is composed of electrically-conductive graphite felt (GF) and highly active mesoporous tungsten oxynitride nanofibers (mWONNFs) that are prepared by electrospinning and simple heat treatments. Addition of the 1D mWONNFs to porous GF produces a web-like structure that significantly facilitates the reaction kinetics and ion diffusion. The cell performance achieves in this study demonstrated high energy efficiencies of 89% and 80% at current densities of 20 and 80 mA cm^-2 , respectively. Furthermore, the cell can also be operated at a very high current density of 160 mA cm^-2 , demonstrating an energy efficiency of 62%. These results demonstrate the effectiveness of the mWONNF/GF composite as the electrode material in zinc-bromine flow batteries.