Redefining Separator Design and Water Activity for High-Energy Zinc Batteries Using Covalent Organic Framework.

While lithium-ion energy storage has found wide applications, the use of lithium ions as charge carrier has a number of issues, such as safety concerns and resource scarcity. In comparison with lithium, sodium is naturally abundant and cheaper. Therefore, recent years have seen a great deal of research interest in using sodium ions as charge carrier to develop sodium-ion energy storage technologies, such as sodium-ion batteries (NIBs) and sodium-ion capacitors (NICs). NICs have emerged as a promising technology for large-scale energy storage applications because this energy storage system combines the advantages of batteries and electrochemical capacitors (ECs). A NIC cell is configured with a battery electrode as the anode, an EC electrode as the cathode and an electrolyte containing sodium ions.However, finding a high-performance anode material has been one of the great challenges in developing this sustainable electrochemical energy storage technology. Transition-metal oxides (TMOs), such as TiO2, Nb2O5, NiCo2O4, and Fe3O4, have been demonstrated to be promising anode materials for sodium-ion storage. Nickel cobaltite (NiCo2O4) is of particular importance because of its low cost, abundance in nature, and high theoretical specific capacity (890 mAh g-1). However, this material suffers from critical problems, including sluggish sodium ion diffusion kinetics, low electrical conductivity, and large volume changes during charge/discharge, resulting in poor rate capability and cycling stability in NICs.This PhD thesis project aims to improve the electrochemical properties of NiCo2O4 (NCO) with regard to a number of physical and electrochemical aspects, including morphology, structure, electrical conductivity, ion diffusivity, and cycling stability. The main innovations and key findings in this thesis work include:• Spherical NCO particles have been synthesized by using a solvothermal method. It was found that subsequent thermal treatment temperature played an important role in determining the crystalline structure and particle size of the NCO. The NCO sample thermally treated at 350 oC showed an optimal and promise performance in NICs. Hollow NCO spheres with a chestnut shell morphology have also been synthesized using the solvothermal method.• Mechanism study revealed that the NCO phase was converted to metallic nickel, cobalt and sodium oxide phases upon pre-sodiation. The pre-sodiation of NCO was found to significantly improve its energy density. A NIC assembled with the pre-sodiated NCO as the anode and an activated carbon (AC) as the cathode exhibited an energy density of 60 Wh kg-1 at a power density of 10,000 W kg-1, much better performance than that reported in the literature. • The physical and electrochemical properties of the NCO particles were improved by using nitrogen-doped reduced graphene oxide (N-rGO). A porous N-rGO framework was used to encapsulate NCO particles to form a NCO@N-rGO composite. In addition to enhance electronic conductivity and alleviated the volume changes of NCO during charge/discharge, the N-rGO network also contributed to charge storage via a capacitive mechanism. A NIC assembled with NCO@N-rGO as the anode and an AC as the cathode delivered an energy density of 48.8 Wh kg-1 at a power density of 9750 W kg-1 with a stable cycle life.• A three-dimensional (3D) nitrogen-doped holey graphene (N-HG) was prepared and used to stabilize NCO particles, forming a NCO@N-HG composite. This composite exhibited good rate capabilities with a capacity of 403 mAh g-1 at a current density of 1 A g-1, which was higher than the 358 mAh g-1 of NCO@N-rGO in a sodium half-cell. A NIC assembled with NCO@N-HG as the anode and an AC as the cathode also delivered a higher energy density of 52 Wh kg-1 at a power density of 10,000 W kg-1. The physical and electrochemical properties of magnetite (Fe3O4) nanoparticles were also improved by using the 3D N-HG. The thin graphene sheets in the composites facilitate the electron transport and buffer the volume changes during charge/discharge, while the interconnected 3D macroporous network with a pore size in several micrometers range, combined with the nanopores in the N-HG provide pathways for rapid ion transport. The good electrochemical performance of the composites indicates that using N-HG to support TMOs particles is an effective and general approach towards developing high-performance anode materials for sodium-ion storage.In summary, this PhD thesis work has significantly improved the physical and electrochemical properties of nickel cobaltite with regard to electrical conductivity, structural stability, and sodium ion diffusivity in the bulk electrode. The good electrochemical performance indicates that NCO-based materials hold a great promise as anode for high-performance NICs with competing energy and power densities.

Aqueous zinc-ion hybrid supercapacitors (Zn-HSCs) are promising devices for sustainable and efficient energy storage. However, they suffer from a limited energy density compared to lithium-ion batteries. This limitation can be overcome by developing novel electrode materials, with covalent organic frameworks (COFs) standing out as a particularly intriguing option. Herein, peri-xanthenoxanthene (PXX) has been integrated for the first time into a COF scaffold to take advantage of its straightforward synthesis, chemical stability, π-conjugated backbone, and heteroatom content endowing reversible redox reactions at low potentials. Two novel hexagonal COFs have been designed and synthesized by tethering of a PXX-diamine unit having a C2 symmetry with two distinct tris-aldehydes acting as C3-symmetric cornerstones, i.e., triformyl benzene (TFB) and triformylphloroglucinol (Tp), ultimately yielding COF PXX(PhNH2)2-TFB and COF PXX(PhNH2)2-Tp, respectively. As cathodes in Zn-HSCs, COF PXX(PhNH2)2-Tp exhibits a remarkable specific capacitance, energy, and power densities (237 F g-1, 106.6 Wh kg-1, and 3.0 kW kg-1, respectively), surpassing those of COF PXX(PhNH2)2-TFB (109 F g-1, 49.1 Wh kg-1, and 0.67 kW kg-1). Importantly, both COFs display outstanding long-term stability, over 5000 charge/discharge cycles, with capacitance retention >92%. These findings underscore the potential of PXX-based COFs as high-performance cathode materials for HSCs, thereby offering a promising new avenue for energy storage technologies.

The environmental-related issues arising from the fossil fuel assorted industrial revolution and worldwide development have prompted the quest for rechargeable batteries. In these predicaments, lithium-ion batteries (LIBs) took ownership to reshape our lives. However, the limited abundance, non-uniform geographical distribution and severe flammability of organic electrolytes, increase the uncertainty over their large-scale application. Recently, aqueous rechargeable sodium-ion batteries (ARSIBs) have gained considerable curiosity for large-scale energy storage due to their much-assured safety, environment friendliness, high-rate capacity, and low cost. However, the prospects of ARSIBs seeing commercial success remained remote due to the narrow water stability window (1.23 V), which translates into low cell voltage (< 1.6 V), low energy density (< 70 Wh Kg-1), and compromised cycling stability. The aforesaid dilemmas can be resolved by generating a protective layer known as a solid electrolyte interface (SEI) like in organic electrolytes. However, the SEI concept in aqueous electrolytes is relatively unexplored, as water dissociation leads to O2 and H2, and will enhance the parasitic reactions. The SEI formed in WiSE due to salt reduction is often inhomogeneous with a porous mosaic structure and is susceptible to mechanical cracking, and increase the overall cost. Hence, high-capacity electrodes and high voltage electrolytes capable of forming a stable SEI are urgently required to fulfill the dream of the large-scale application of ARSIBs.Recently low cost, highly abundant sulfur-based electrode material has attracted significant research attention due to its high theoretical capacity (1675 mA h g-1) and energy density. However, sluggish sulfur redox kinetics, acute polysulfide shutting, dendritic growth on the metal-based anode and low conductivity of sulfur and its discharge products proves to be a major roadblock for its commercialization. Utilizing abundant sulfur in an aqueous electrolyte along with abundant Na+ can resolve the kinetics and conductivity anxieties and leads to a new greener and safer Na-ion/S batteries chemistry. However lower order polysulfide dissolution in water is more feasible, leading to the rapid capacity decay and active sulfur loss due to H2S formation. Polysulfide dissolution is an interfacial mechanism occurring at the electrode-electrolyte interface and depends on both electrode and electrolyte merits. Therefore, an effective approach will be to couple efficient sulfur host with an electrolyte capable of generating a stable SEI on the electrode surface to prevent the direct attack of water on polysulfide.Herein we have explored urchin-like CoWO4 as a sulfur host coupled with Na-W-U-D electrolyte. The CoWO4 exhibits high conductivity, strong chemical interaction for sulfur and its discharge products, and urchin-like morphology having exposed edges facilitate the charge transport results in excellent polysulfide redox kinetics. The high voltage Na-W-U-D electrolyte was prepared by mixing NaClO4, urea, and N, N-dimethylformamide DMF in 1:2:1 ratio in water. Each component in the electrolyte plays an important role. Urea has very high water solubility and tends to form stable SEI, while DMF has a high dielectric constant, good solvation ability, and develop stabilize SEI. As a result, Na-W-U-D shows a stability window close to the 3.1 V regime due to reduced water activity resulting from complex ion solvent interaction and stable and uniform SEI formation, consisting of Na2CO3, polyurea, and reduction products of DMF. Despite the addition of DMF, non-flammable features of aqueous electrolytes remain well maintained. Herein for the first time, the SEI concept was successfully used for the aq. Na-ion/S battery. We discovered that the lower water activity of Na-W-U-D electrolyte hindered polysulfide dissolution and stable SEI prevent the direct attack of water on polysulfide and results in extended cycling stability. At the same time, the urchin-like CoWO4 host enhances the sluggish polysulfide redox kinetics and provides an abundant anchoring site for polysulfide adsorption. We investigate the effect of time, C-rate, depth of discharge, and dissolved oxygen on polysulfide dissolution and self-discharge of the negative electrode. The high electrode capacity combined with the safety and stable SEI of Na-W-U-D electrolyte translated into a record high initial capacity of 834 mA h g-1 w.r.t sulfur, with remarkable cycling stability up to 500 cycles @ 0.5 C. Post analysis by SEM and XPS, evident that the stable SEI consists of Na2CO3, polyurea, and reduced products of DMF (CO and NHMe2), which also prevent the negative electrode from self-discharge by mitigating the parasitic reaction of dissolved oxygen in the electrolyte. Moreover, a full cell assembled by integrating S@CoWO4 anode and Na0.44MnO2 cathode showed remarkable stability and a high energy density of 119 Wh kg-1, making it a promising candidate for a future energy storage system. Figure 1

Covalent organic frameworks (COFs) having a large surface area, porosity, and substantial amounts of heteroatom content are recognized as the ideal class of materials for energy storage and gas sorption applications. In this work, we have synthesized four different porous COF materials by the polycondensation of a heteroatom-rich flexible triazine-based trialdehyde linker, namely 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TPT-CHO), with four different triamine linkers. Triamine linkers were chosen based on differences in size, symmetry, planarity, and heteroatom content, leading to the synthesis of four different COF materials named IITR-COF-1, IITR-COF-2, IITR-COF-3, and IITR-COF-4. IITR-COF-1, synthesized within 24 h from the most planar and largest amine monomer, exhibited the largest Brunauer-Emmett-Teller (BET) surface area of 2830 m2 g-1, superior crystallinity, and remarkable reproducibility compared to the other COFs. All of the synthesized COFs were explored for energy and gas storage applications. It is shown that the surface area and redox-active triazene rings in the materials have a profound effect on energy and gas storage enhancement. In a three-electrode setup, IITR-COF-1 achieved an electrochemical stability potential window (ESPW) of 2.0 V, demonstrating a high specific capacitance of 182.6 F g-1 with energy and power densities of 101.5 Wh kg-1 and 298.3 W kg-1, respectively, at a current density of 0.3 A g-1 in 0.5 M K2SO4 (aq) with long-term durability. The symmetric supercapacitor of IITR-COF-1//IITR-COF-1 exhibited a notable specific capacitance of 30.5 F g-1 and an energy density of 17.0 Wh kg-1 at a current density of 0.12 A g-1. At the same time, it demonstrated 111.3% retention of its initial specific capacitance after 10k charge-discharge cycles. Moreover, it exhibited exceptional CO2 capture capacity of 25.90 and 10.10 wt % at 273 and 298 K, respectively, with 2.1 wt % of H2 storage capacity at 77 K and 1 bar.

Redox-active covalent organic frameworks (COFs) are a new class of material with the potential to transform electrochemical energy storage due to the well-defined porosity and readily accessible redox-active sites of COFs. However, combining both high specific capacity and energy density in COF-based batteries remains a considerable challenge. Herein, we demonstrate the exceptional performance of Aza-COF in rechargeable sodium-ion batteries (SIBs). Aza-COF is a microporous 2D COF synthesized from hexaketocyclohexane and 1,2,4,5-benzenetetramine by a condensation reaction, which affords phenazine-decorated channels and a theoretical specific capacity of 603 mA h g-1. The Aza-COF-based electrode exhibits an exceptional average specific capacity (550 mA h g-1), energy density (492 W h kg-1) at 0.1 C, and power density (1182 W kg-1) at 40 C. The high capacity and energy density are attributed to swift surface-controlled redox processes and rapid sodium-ion diffusion inside the porous electrode. Rate capability studies showed that the battery also performs well at high current rates: 1 C (363 mA h g-1), 5 C (232 mA h g-1), 10 C (161 mA h g-1), and 20 C (103 mA h g-1). In addition, the long-term cycling stability test revealed very good capacity retention (87% at 5 C) and Coulombic efficiencies near unity over 500 cycles.

Next-Generation Batteries.

Covalent organic frameworks (COFs), thanks to their adjustable porous structure and abundant build-in functional motifs, have been recently regarded as promising electrode materials for a variety of batteries. There still remain grand opportunities to further utilizing their merits for developing advanced COFs-based batteries. In this paper, we propose a hybrid acid/alkali all-COFs battery by coupling pyrene-4,5,9,10-tetraone based COF cathode with anthraquinone based COF anode. In such a hybrid acid/alkali all-COFs battery, the cathodic COF favorably works in acid with a relatively positive potential, while the anodic COF preferably runs in alkali with a relatively negative potential. It thus can deliver a decently high discharge capacity of 92.97 mAh g-1 with a wide voltage window of 2.0 V, and exhibit high energy density of 74.2 Wh kg-1 along with a considerable cyclic stability over 300 cycles. The development of the proof-of-concept all-COFs battery may drive forward the improvement of newly cost-effective and performance-reliable energy storage devices.

Aviation and long-distance transport represent more than 6% of global CO2 emissions, but electrifying these fleets requires significant improvements in battery specific energy and power. Regional and narrow-body all-electric aircraft require 600-1,000 Wh kg-1 while mass-produced rechargeable batteries have energy densities below 300 Wh kg-1. Furthermore, batteries should be able to recharge instantly to near-continuously operate the machines they power and offset their high sunk costs, yet fully charging current batteries still takes 1-16 hours.Overcoming these technical bottlenecks requires entirely new approaches for powering electric vehicles that go beyond the pioneering intercalation materials that have dominated modern advances in batteries. For example, there are many energy-dense materials that can improve battery specific energy but do not work well in an intercalation battery architecture, especially small molecules such as organic compounds, organosulfurs, halides, and gases. Batteries with energy-dense small molecules suffer from active material loss by dissolution, irreversible reactions due to material crossover between electrodes, slow ion transport in the bulk material, and the use of excessive carbon to provide electronic conduction. These challenges cause current batteries to fall short of their high theoretical energy and power densities and suffer from poor reversibility and slow recharging.In this talk, we explore the potential for electrochemical fuels that are filled with energy-dense small molecules that can be electrically discharged in a battery cell, not a fuel cell, at the high rates needed for electric aircraft, and can be ejected from the vehicle and recharged in an external electrochemical cell so that new fuel can be rapidly filled into the vehicle for continuous operation.Our solution uses CH3S3CH3 small molecules stored in a supporting electrolyte as a catholyte paired with Li metal anodes, which we discharge in our extractor cell. The active material ratio in the catholyte is 65-80 wt.% to maximize cell specific energy. The discharge reaction occurs on an engineered porous current collector and the products are made soluble in the catholyte, which is critical to removing them for recycling and recharging. The liquid-to-liquid conversion enables fast kinetics, minimal polarization, promotes ion, electron, and mass transport and enables full conversion of the small molecules, which has been challenging in prior work.We demonstrate this concept in a toy robot dog and a commercial DJI quadrotor helicopter. A 10.4 Ah extractor cell provided up to 192 A kg-1 and achieved a specific energy of up to 850 Wh kg-1 (counting the full cell mass), which doubled the flight time of the Li-ion battery powered helicopter from 20 to 41 minutes. The fuel was mechanically recharged in 77 seconds, so that multiple rapid refills allowed a helicopter to operate for 23 hours in a 24-hour period. The fuel had a 3.2-year storage life and delivered 98% of its room temperature capacity at -20°C. Different from the fuels used in engines and fuel cells, the discharged liquid is rechargeable (99.97% chemical efficiency) with electrical input and the regenerated Li and catholyte can be used again in the stomach in what we refer to as a mechagenic cycle. This recycling makes the stomach more cost-efficient than batteries and gasoline after 41 flights in an electric aircraft. We also demonstrate fuels that have up to 1,447 Wh kg-1 or are completely aqueous for fire-safe applications. Overall, the use of abundant small molecules as electrochemical fuels could lead to a more sustainable and better performing energy cycle than current batteries. Figure caption: A sustainable energy cycle based on electrochemically rechargeable fuels. a, Envisioned mechagenic energy cycle for electric vehicles and robots that are rapidly refilled and return the discharged fuels to a recycler where it is converted back to a charged catholyte and metal using sustainable electricity. b, Photographs of a DJI Mini 2 helicopter powered by an extractor cell (a) and the recorded flight time compared to its factory-pack Li-ion battery (b). The fuel-powered helicopter had a 41-minute flight time, which is two times longer than the helicopter with the factory-pack Li-ion battery. c, The conversion of CH3S3CH3 monitored by 1H NMR. d, Voltage profile of the extractor cell operated at room temperature and -20°C. e, The voltage profile of the catholyte stored after 3.2 years. f, Rate performance of the extractor cell. With current densities of 32 to 192 A kg-1, the cell delivered at least 1 V output. g, A recycler separated by a LiSICON solid electrolyte plate. The two split chambers were filled with a CNT foam and a liquid electrolyte, respectively, to recycle the catholyte and Li metal at high efficiency. Figure 1

Supercapacitors have aroused considerable attention due to their high power capability, which enables charge storage/output in minutes or even seconds. However, to achieve a high energy density in a supercapacitor has been a long-standing challenge. Here, graphite is reported as a high-energy alternative to the frequently used activated carbon (AC) cathode for supercapacitor application due to its unique Faradaic pseudocapacitive anion intercalation behavior. The graphite cathode manifests both higher gravimetric and volumetric energy density (498 Wh kg-1 and 431.2 Wh l-1 ) than an AC cathode (234 Wh kg-1 and 83.5 Wh l-1 ) with peak power densities of 43.6 kW kg-1 and 37.75 kW l-1 . A new type of Li-ion pseudocapacitor (LIpC) is thus proposed and demonstrated with graphite as cathode and prelithiated graphite or Li4 Ti5 O12 (LTO) as anode. The resultant graphite-graphite LIpCs deliver high energy densities of 167-233 Wh kg-1 at power densities of 0.22-21.0 kW kg-1 (based on active mass in both electrodes), much higher than 20-146 Wh kg-1 of AC-derived Li-ion capacitors and 23-67 Wh kg-1 of state-of-the-art metal oxide pseudocapacitors. Excellent rate capability and cycling stability are further demonstrated for LTO-graphite LIpCs.

Our modern and technological society requests enhanced energy storage devices to tackle the current necessities. In addition, wearable electronic devices are being demanding because they offer many facilities to the person wearing it. In this manuscript, a historical review is made about the available energy storage devices focusing on super-capacitors and lithium-ion batteries, since they currently are the most present in the industry, and the possible polymeric materials suitable on wearable energy storage devices. Polymers are a suitable option because they not only possess remarkable mechanical resistance, flexibility, long life-times, easy manufacturing techniques and low cost in addition to they can be environmentally friendly, nontoxic, and even biodegradable too. Moreover, the electrical and electrochemical polymer properties can be tunning with suitable fillers giving to versatile conducting polymer composites with a good cost and properties' ratio. Although the advances are promising, there are still many drawbacks that need to be overcome. Future research should focus on improving both the performance of materials and their processability on an industrial scale, where additive manufacturing offers many possibilities. The sustainability of new energy storage devices should not

Although solid-state batteries (SSBs) are high potential in achieving better safety and higher energy density, current solid-state electrolytes (SSEs) cannot fully satisfy the complicated requirements of SSBs. Herein, a covalent organic framework (COF) with multi-cationic molecular chains (COF-MCMC) was developed as an efficient SSE. The MCMCs chemically anchored on COF channels were generated by nano-confined copolymerization of cationic ionic liquid monomers, which can function as Li+ selective gates. The coulombic interaction between MCMCs and anions leads to easier dissociation of Li+ from coordinated states, and thus Li+ transport is accelerated. While the movement of anions is restrained due to the charge interaction, resulting in a high Li+ conductivity of 4.9×10-4 S cm-1 and Li+ transference number of 0.71 at 30 °C. The SSBs with COF-MCMC demonstrate an excellent specific energy density of 403.4 Wh kg-1 with high cathode loading and limited Li metal source.

Supercapacitors (SCs) display intrinsic advantages such as high power density and high rate capability but low energy density. Thus, the development of advanced pseudocapacitive electrode materials is crucial for the advancement of supercapacitor technologies. These electrode materials significantly influence the performance of supercapacitors in electrical energy storage (EES) systems in terms of energy density and cycling stability. In this review, we first discuss EES technologies and their development and types of SCs, followed by an overview of the importance of organic electrode materials in pseudocapacitor (PSC) applications. Moreover, we present the principles of different redox-active organic molecule design strategies and their theoretical calculations to understand their electrochemical characteristics. Furthermore, we highlight the role of redox-active organic electrode materials in achieving a wider potential voltage window and, in turn, higher energy density, thus enhancing the electrochemical performance of PSCs. Subsequently, we discuss the role of molecular structures, the composition of electronic conducting materials and their structural and electrochemical performance relationship. Moreover, we highlight the advantages and disadvantages of organic materials compared with traditional transition-metal oxide inorganic materials for PSCs. Then, we present a brief discussion on the advances in small redox-active molecular architectures and their use in the fabrication of novel electrode materials, including polymers, covalent organic frameworks and metal organic frameworks. We provide an in-depth discussion on how material development from small redox-active molecules advances the charge-storage field and their application in illuminating light-emitting diodes. We hope that this review article will help provide a fundamental basis for the design and development of next-generation pseudocapacitive electrode materials from renewable sources for sustainable supercapacitor systems with higher charge-storage capability.

Materials Design and Engineering for High Energy-Density Rechargeable Zinc-air Batteries

The depletion of non-renewable energy resources has intensified the need for advanced energy storage materials, with metal sulfides emerging as particularly promising candidates for supercapacitor applications. This study presents a systematic investigation of zinc sulfide (ZnS)-based nanocomposites, including binary composites with nitrogen-doped reduced graphene oxide (ZnS/NrGO, denoted as ZnG) and polyaniline (ZnS/PANI, denoted as ZnP), as well as a ternary composite (ZnS/NrGO/PANI, denoted as ZnPG). These materials were synthesized via a combined hydrothermal and in situ polymerization approach to optimize their electrochemical properties for supercapacitor applications. Structural characterization confirmed the cubic phase purity of ZnS, while morphological and elemental analyses (SEM-EDX) verified successful composite formation. Spectroscopic techniques (FTIR, XPS) elucidated the chemical bonding and electronic interactions within the materials. The ternary composite demonstrated superior electrical conductivity (I-V measurements) and a mesoporous architecture with an enhanced surface area of 168 m2 g-1 (BET analysis). Remarkably, electrochemical evaluations revealed outstanding performance metrics: a specific capacitance (C s) of 2645.94 F g-1, energy density (E d) of 111.2 Wh kg-1, and exceptional cycling stability (95.3% retention after 10 000 cycles). In the asymmetric device configuration, the ZnPG electrode delivered 1901.51 F g-1 at 2 A g-1, with E d and power densities (P d) of 95.05 Wh kg-1 and 2201.21 W kg-1 respectively, demonstrating practical viability by powering an LED for 58 seconds. These results establish ZnPG as a highly efficient electrode material, showcasing the significant potential of metal sulfide-based nanocomposites for next-generation energy storage systems.

Magnesium (Mg) batteries have been considered one of the most promising “post-lithium-ion” energy storage technologies owing to their high theoretical energy density, earth abundance, and intrinsic safety with air and moisture. However, the cell performance of Mg batteries has been limited to cathode materials leading to sluggish kinetics, low reversible energy density, and poor cycling stability. The development of Mg intercalation cathode materials[1–3] has been impeded by slow Mg2+ solid diffusion due to high charge density, sluggish charge redistribution, and strong electrostatic interaction with host and anions.[4] Many of the Li+-intercalation cathodes are not suitable for Mg2+ insertion.[5] Organic materials with high tunability have attracted significant attention,[6] but it has been challenging to achieve organic Mg batteries with a satisfactory energy density as the commercial lithium-ion battery (100-170 Wh kg-1 [7]) with high power and long cycle life. Here, a new organic cathode for rechargeable Mg batteries is reported based on ethyl viologen (EV), which not only has a fast redox couple EV2+/EV0 but also is capable of coupling with redox-active anions, such as iodide (I-), achieving a total four-electron storage. The EV2+/EV0 redox couple demonstrates a stable cycle life (500 cycles) with a superior rate performance (10 C) owing to intrinsic fast electrode kinetics, and a high material utilization (>80%) was achieved at 1.0 C under a high areal loading of 5 mg cm−2. When coupling with iodide I-, a reversible four-electron storage is achieved with a high energy density (304.2 Wh kg-1) and a stable cycle life (>100 cycles). This study provides effective strategies for designing reversible multielectron storage for high-rate and high-energy rechargeable Mg batteries.[8] Reference s : [1] X. Sun, P. Bonnick, V. Duffort, M. Liu, Z. Rong, K. A. Persson, G. Ceder, L. F. Nazar, Energy Environ. Sci. 9 (2016) 2273-2277.[2] X. Sun, P. Bonnick, L. F. Nazar, ACS Energy Lett. 1 (2016) 297-301.[3] B. Liu, T. Luo, G. Mu, X. Wang, D. Chen, G. Shen, ACS nano 7 (2013) 8051-8058.[4] E. Levi, M. D. Levi, O. Chasid, D. Aurbach, J Electroceram. 22 (2009) 13-19.[5] M. Bouroushian, Electrochemistry of metal chalcogenides, Springer Science & Business Media, Heidelberg, Berlin, 2010.[6] M. Mao, T. Gao, S. Hou, C. Wang, Chem. Soc. Rev. 47 (2018) 8804-8841.[7] J.-M. Tarascon, M. Armand, in: V. Dusastre (Ed.), Materials for Sustainable Energy: A Collection of Peer-Reviewed Research and Review Articles from Nature Publishing Group, World Scientific, Singapore, 2011, pp. 171–179.[8] Y. Sun, Q. Zou, Y. Lu, Adv. Energy Mater. 9 (2019) 1903002. Acknowledgements : This work was supported by a grant from the Research Grants Council (RGC) of the Hong Kong Administrative Region, China, under an RGC project No. T23-601/17-R.