Low-Concentration Redox-Electrolytes for High-Rate and Long-Life Zinc Metal Batteries.

The uncontrolled zinc electrodeposition and side reactions severely limit the power density and lifespan of Zn metal batteries. Herein, the multi-level interface adjustment effect is realized with low-concentration redox-electrolytes (0.2 m KI) additives. The iodide ions adsorbed on the zinc surface significantly suppress water-induced side reactions and by-product formation and enhance the kinetics of zinc deposition. The distribution of relaxation times results reveal that iodide ions can reduce the desolvation energy of hydrated zinc ions and guide the deposition of zinc ions due to their strong nucleophilicity. As a consequence, the Zn||Zn symmetric cell achieves superior cycling stability (>3000 h at 1 mA cm-2 , 1 mAh cm-2 ) accompanied by a uniform deposition and a fast reaction kinetics with a low voltage hysteresis (<30 mV). Additionally, coupled with an activated carbon (AC) cathode, the assembled Zn||AC cell delivers a high-capacity retention of 81.64% after 2000 cycles at 4 A g-1 . More importantly, the operando electrochemical UV-vis spectroscopies show that a small number of I3 - can spontaneously react with the dead zinc as well as basic zinc saltsand regenerate iodide ions and zinc ions; thus, the Coulombic efficiency of each charge-discharge process is close to 100%.

Redox Electrolytes-Assisting Aqueous Zn-Based Batteries by Pseudocapacitive Multiple Perovskite Fluorides Cathode and Charge Storage Mechanisms.

Aqueous Zn-based batteries (AZBs) have attracted intensive attention. However, to explore advanced cathode materials with in-depth elucidation of their charge storage mechanisms, improve energy storage capacity, and construct novel cell systems remain a great challenge. Herein, a new pseudocapacitive multiple perovskite fluorides (ABF3 ) cathode is designed, represented by KMF-(IV, V, and VI; M = NiCoMnZn/-Mg/-MgFe), and constructed Zn//KMF-(IV, V, and VI) AZBs and their flexible devices. Ex situ tests have revealed a typical bulk phase conversion mechanism of KMF-VI electrode for charge storage in alkaline media dominated by redox-active Ni/Co/Mn species, with transformation of ABF3 nanocrystals into amorphous metal oxide/(oxy)hydroxide nanosheets. By employing single or bipolar redox electrolyte strategies of 20 mm [Fe(CN)6 ]3- or/and 10 mm SO3 2- /Cu[(NH3 )4 ]2+ acting on KMF-(IV, V, and VI) cathode and Zn anode, the AZBs show an improved energy storage owing to additional capacity contribution of redox electrolytes. The as-designed Zn//polyvinyl alcohol (PVA)-KOH-K3 [Fe(CN)6 ]//KMF-(IV, V, and VI) redox gel electrolytes-assisting flexible AZBs (RGE-FAZBs) exhibit remarkable performance under different bending angles because of slight dissolution corrosion of zinc anode compared with liquid electrolytes. Overall, the work demonstrates the novel idea of conversion-type multiple ABF3 cathode for redox electrolytes-assisting AZBs (RE-AZBs) and their flexible systems, showing great significance on electrochemical energy storage.

Coupling W18 O49 /Ti3 C2 Tx MXene Pseudocapacitive Electrodes with Redox Electrolytes to Construct High-Performance Asymmetric Supercapacitors.

A pseudocapacitive electrode with a large surface area is critical for the construction of a high-performance supercapacitor. A 3D and interconnected network composed of W18 O49 nanoflowers and Ti3 C2 Tx MXene nanosheets is thus synthesized using an electrostatic attraction strategy. This composite effectively prevents the restacking of Ti3 C2 Tx MXene nanosheets and meanwhile sufficiently exposes electrochemically active sites of W18 O49 nanoflowers. Namely, this self-assembled composite owns abundant oxygen vacancies from W18 O49 nanoflowers and enough active sites from Ti3 C2 Tx MXene nanosheets. As a pseudocapacitive electrode, it shows a big specific capacitance, superior rate capability and good cycle stability. A quasi-solid-state asymmetric supercapacitor (ASC) is then fabricated using this pseudocapacitive anode and the cathode of activated carbon coupled with a redox electrolyte of FeBr3 . This ASC displays a cell voltage of 1.8 V, a capacitance of 101 F g-1 at a current density of 1 A g-1 , a maximum energy density of 45.4 Wh kg-1 at a power density of 900 W kg-1 , and a maximum power density of 18 000 W kg-1 at an energy density of 10.8 Wh kg-1 . The proposed strategies are promising to synthesize different pseudocapacitive electrodes as well as to fabricate high-performance supercapacitor devices.

Lignin-Derived Quinone Redox Moieties for Bio-Based Supercapacitors.

Because of their rapid charging and discharging, high power densities, and excellent cycling life stabilities, supercapacitors have great potential for use in electric vehicles, portable electronics, and for grid frequency modulation. The growing need for supercapacitors that are both efficient and ecologically friendly has generated curiosity in developing sustainable biomass-based electrode materials and electrolytes. Lignin, an aromatic polymer with remarkable electroactive redox characteristics and a large number of active functional groups, is one such candidate for use in renewable supercapacitors. Because its chemical structure features an abundance of quinone groups, lignin undergoes various surface redox processes, storing and releasing both electrons and protons. Accordingly, lignin and its derivatives have been tested as electroactive materials in supercapacitors. This review discusses recent examples of supercapacitors incorporating electrode materials and electrolytes derived from lignin, focusing on the pseudocapacitance provided by the quinone moieties, with the goal of encouraging the use of lignin as a raw material for high-value applications. Employing lignin and its derivatives as active materials in supercapacitor electrodes and as a redox additive in electrolytes has the potential to minimize environmental pollution and energy scarcity while also providing economic benefits.

Electrochemical Capacitors with Confined Redox Electrolytes and Porous Electrodes.

Electrochemical capacitors (ECs), including electrical-double-layer capacitors and pseudocapacitors, feature high power densities but low energy densities. To improve the energy densities of ECs, redox electrolyte-enhanced ECs (R-ECs) or supercapbatteries are designed through employing confined soluble redox electrolytes and porous electrodes. In R-ECs the energy storage is based on diffusion-controlled faradaic processes of confined redox electrolytes at the surface of a porous electrode, which thus take the merits of high power densities of ECs and high energy densities of batteries. In the past few years, there has been great progress in the development of this energy storage technology, particularly in the design and synthesis of novel redox electrolytes and porous electrodes, as well as the configurations of new devices. Herein, a full-screen picture of the fundamentals and the state-of-art progress of R-ECs are given together with a discussion and outlines about the challenges and future perspectives of R-ECs. The strategies to improve the performance of R-ECs are highlighted from the aspects of their capacitances and capacitance retention, power densities, and energy densities. The insight into the philosophies behind these strategies will be favorable to promote the R-EC technology toward practical applications of supercapacitors in different fields.

Understanding the Operating Mechanism of Aqueous Pentyl Viologen/Bromide Redox-Enhanced Electrochemical Capacitors with Ordered Mesoporous Carbon Electrodes.

Compared to traditional electric double-layer capacitors, redox-enhanced electrochemical capacitors (redox-ECs) show increased energy density and steadier power output thanks to the use of redox-active electrolytes. The aim of this study is to understand the electrochemical mechanisms of the aqueous pentyl viologen/bromide dual redox system at the interface of an ordered mesoporous carbon (CMK-8) and improve the device performance. Cells with CMK-8 carbon electrodes were investigated in several configurations using different charging rates and potential windows. The pentyl viologen electrochemistry shows a mixed behavior between solution-based diffusion and adsorption phenomena, with the reversible formation of an adsorbed layer. The extension of the voltage window allows for full reduction of the viologen molecules during charge and a consequent increase in the specific discharge energy delivered by the cell. Investigation of the mechanism indicates that a 1.5 V charging voltage with a 0.5 A g-1 charging rate and fast discharge rate produces the best overall performance.

A Chlorine-Based Redox Electrochemical Capacitor.

Electrochemical capacitors are under the spotlight due to their high power density, but they have a low energy density. Redox electrolytes have emerged as a promising approach to design high-energy electrochemical energy storage devices. Herein, a chlorine-based redox electrochemical capacitor is reported in an ionic liquid electrolyte. The commercial activated carbon is employed as the working electrode to render the reversible redox of chloride ions in an ionic liquid, by the restriction of micropores on neutral chlorine. The carbon material can simultaneously provide electrical double-layer capacitance. The effective integration of a chlorine redox reaction and electrical double layer allows for high-energy electrochemical capacitors. By this means, a rechargeable chlorine-based redox electrochemical capacitor with reversible capacity and good rate capability and cycling stability is obtained. This work offers a solution for a new type of high-energy electrochemical capacitors.

Solid-State Double-Network Hydrogel Redox Electrolytes for High-Performance Flexible Supercapacitors.

Flexible supercapacitors have great potential applications in wearable and portable electronics, but their practical applications were limited due to the low energy density and mechanical flexibility of solid-state electrolytes used for the construction of flexible supercapacitors. In this study, we first report the solid-state double-network (DN) hydrogel electrolytes (HEs) incorporated with Na2MoO4 redox additives. It is found that the solid-state DN HEs with Na2MoO4 redox additives exhibit high electrochemical performance, excellent mechanical properties, and fast self-recovery features. We then demonstrate novel symmetric supercapacitors (SSCs) incorporated with the solid-state Na2MoO4 DN HEs and the active carbon cloths as the electrodes. The SSCs exhibit a specific capacitance of 84 mF/cm2 at a current density of 1 mA/cm2 and an energy density of 70 µWh/cm2 at a power density of 3800 µWh/cm2. Moreover, the SSCs retain approximately 80% capacitance retention after 7000 charge/discharge cycles, which indicates that the SSCs possess excellent flexibility and stability. All of these results demonstrate that the SSCs incorporated with the solid-state Na2MoO4 DN HEs as energy-storage devices have great practical applications in wearable and portable electronics.