High energy aqueous sodium-ion capacitor enabled by polyimide electrode and high-concentrated electrolyte

Carboxyl-Substituted Organic Molecule Assembled with MXene Nanosheets for Boosting Aqueous Na+ Storage

Aqueous alkali-ion batteries have enormous promise as a kind of safe, reliable, and sustainable energy technologies for power supplies. Although organic molecules with tunable and diverse configurations are potential electroactive materials, their inadequate redox activity and electron affinity hinder the practical application for aqueous alkali-ion storage. Herein, a novel electron-withdrawing carboxyl-substituted dipyridophenazine (CDPPZ) organic molecule is designed and synthesized for aqueous Na+ storage. Significantly, the introduction of carboxyl functional groups not only serves as additional redox-active sites for reversible Na+ coordination, but also causes the rearrangement of intramolecular electron cloud density to reduce the energy level, thereby ensuring the high redox activity and superior electron affinity of the CDPPZ molecule. For portable electronics, a self-supporting, adhesive-free, and flexible CDPPZ@MXene electrode is further constructed by incorporating highly redox-active CDPPZ molecule with MXene nanosheets, which delivers a fast, stable, and unrivaled aqueous Na+ storage capability with a high reversible capacity of 172.6 mAh cm-3 and excellent redox stability over 4000 cycles. In situ dynamic analysis combined with theoretical calculations illustrates the Na+ storage mechanism and corresponding coordinated pathway. Finally, a high-performance flexible aqueous Na-ion battery is fabricated with exceptional energy/power density and remarkable cycling lifespan, further confirming its promising application prospect.

High-Energy Aqueous Magnesium Ion Batteries with Capacity-Compensation Evolved from Dynamic Copper Ion Redox

The low specific capacity and low voltage plateau are significant challenges in the advancement of practical magnesium ion batteries (MIBs). Here, a superior aqueous electrolyte combining with a copper foam interlayer between anode and separator is proposed to address these drawbacks. Notably, with the dynamic redox of copper ions, the weakened solvation of Mg²⁺ cations in the electrolyte and the enhanced electronic conductivity of anode, which may offer effective capacity-compensation to the 3,4,9,10-perylenetetracarboxylic diimide (PTCDI)-Mg conversion reactions during the long-term cycles. As a result, the unique MIBs using expanded graphite cathode coupled with PTCDI anode demonstrate exceptional performance with an ultra-high capacity (205 mAh g^-1 , 243 Wh kg^-1 at 5 A g^-1 ) as well as excellent cycling stability after 600 cycles and rate capability (138 mAh g^-1 , 81 Wh kg^-1 at 10 A g^-1 ).

Exploring 2D Energy Storage Materials: Advances in Structure, Synthesis, Optimization Strategies, and Applications for Monovalent and Multivalent Metal-Ion Hybrid Capacitors

The design and development of advanced energy storage devices with good energy/power densities and remarkable cycle life has long been a research hotspot. Metal-ion hybrid capacitors (MHCs) are considered as emerging and highly prospective candidates deriving from the integrated merits of metal-ion batteries with high energy density and supercapacitors with excellent power output and cycling stability. The realization of high-performance MHCs needs to conquer the inevitable imbalance in reaction kinetics between anode and cathode with different energy storage mechanisms. Featured by large specific surface area, short ion diffusion distance, ameliorated in-plane charge transport kinetics, and tunable surface and/or interlayer structures, 2D nanomaterials provide a promising platform for manufacturing battery-type electrodes with improved rate capability and capacitor-type electrodes with high capacity. In this article, the fundamental science of 2D nanomaterials and MHCs is first presented in detail, and then the performance optimization strategies from electrodes and electrolytes of MHCs are summarized. Next, the most recent progress in the application of 2D nanomaterials in monovalent and multivalent MHCs is dealt with. Furthermore, the energy storage mechanism of 2D electrode materials is deeply explored by advanced characterization techniques. Finally, the opportunities and challenges of 2D nanomaterials-based MHCs are prospected.

High Discharge Capacity and Ultra-Fast-Charging Sodium Dual-Ion Battery Based on Insoluble Organic Polymer Anode and Concentrated Electrolyte

Sodium-based dual-ion batteries have shown great promise for large-scale energy storage applications due to their wide operating voltages, environmental friendliness, abundant sodium resources, and low cost, which are widely investigated by researchers. However, the development of high-performance anode materials is a key requirement for the realization of such electrochemical energy storage systems at the practical application level. Carbonaceous anode materials based on intercalation/deintercalation mechanisms typically exhibit low discharge capacities, while metal-based materials based on conversion or alloying reactions show unsatisfactory stability in performance. On the contrary, organic materials display high theoretical capacities due to their flexible molecular structure designability and stable cyclic performance with fast reaction kinetics based on the unique enolization reaction. Herein, we report an organic polymer anode material of polyimide (PNTO), combined with a high-concentration electrolyte; the sodium-based dual-ion battery system constructed exhibits outstanding electrochemical performance. The full battery shows an ultra-high specific discharge capacity of 293.2 mAh g-1 and can be cycled stably for 3200/5600/4100 cycles at ultra-high rates of 60/120/150 C without degradation. Furthermore, the dual-ion battery system demonstrates an extremely low self-discharge rate of 0.03% h-1 and superior fast-charging-slow-discharging performance. It is one of the best performances reported up to now for a dual-ion full battery based on an organic polymer anode. This novel battery system design strategy will facilitate the advancement of high-performance organic-based dual-ion batteries and is expected to be a promising candidate for large-scale energy storage applications.

New Insights on the Sodium Water-in-Salt Electrolyte and Carbon Electrode Interface from Electrochemistry and Operando Raman Studies

Comprehensive electrochemical and operando Raman studies are performed to investigate the electrochemical stability window (ESW) of supercapacitors filled with normal (salt-in-water) and highly concentrated (water-in-salt, WiSE) electrolytes. Impedance and chronoamperometric experiments are employed and combined with cyclic voltammetry to correctly define the ESW for a WiSE-based device. The total absence of water-splitting resulted in phase angles close to -90° in the impedance data. It is verified that a 17 m NaClO₄ electrolyte avoids the water-splitting up to 1.8 V. Furthermore, Raman studies under dynamic and static polarization conditions corroborate the existence of a solvent blocking interface (SBI), which inhibits the occurrence of water-splitting. Also, the reversible nature of the charge-storage process is assessed as a function of the applied voltage. At extreme polarization, the SBI structure is disrupted, thus allowing the occurrence of water-splitting and anionic (ClO₄^-) intercalation between the graphene sheets.

High-voltage liquid electrolytes for Li batteries: progress and perspectives

Since the advent of the Li ion batteries (LIBs), the energy density has been tripled, mainly attributed to the increase of the electrode capacities. Now, the capacity of transition metal oxide cathodes is approaching the limit due to the stability limitation of the electrolytes. To further promote the energy density of LIBs, the most promising strategies are to enhance the cut-off voltage of the prevailing cathodes or explore novel high-capacity and high-voltage cathode materials, and also replacing the graphite anode with Si/Si-C or Li metal. However, the commercial ethylene carbonate (EC)-based electrolytes with relatively low anodic stability of ∼4.3 V vs. Li⁺/Li cannot sustain high-voltage cathodes. The bottleneck restricting the electrochemical performance in Li batteries has veered towards new electrolyte compositions catering for aggressive next-generation cathodes and Si/Si-C or Li metal anodes, since the oxidation-resistance of the electrolytes and the in situ formed cathode electrolyte interphase (CEI) layers at the high-voltage cathodes and solid electrolyte interphase (SEI) layers on anodes critically control the electrochemical performance of these high-voltage Li batteries. In this review, we present a comprehensive and in-depth overview on the recent advances, fundamental mechanisms, scientific challenges, and design strategies for the novel high-voltage electrolyte systems, especially focused on stability issues of the electrolytes, the compatibility and interactions between the electrolytes and the electrodes, and reaction mechanisms. Finally, novel insights, promising directions and potential solutions for high voltage electrolytes associated with effective SEI/CEI layers are proposed to motivate revolutionary next-generation high-voltage Li battery chemistries.

Electrolyte Technologies for High Performance Sodium-Ion Capacitors

Bridging the energy gap between batteries and capacitors, while in principle delivering a supercapacitor-like high power density and long lifespan, sodium-ion capacitors (SIC) have been considered promising energy storage devices that could be commercialized in the near future due to the natural abundance of sodium sources and the performance superiority of SIC devices. Therefore, in the past decade, substantial research efforts have been devoted to their structure and property improvements. With regard to the electrochemical performance of an ion capacitor, except for the electrode, the composition and structure of the electrolytes are also of great importance. Thus, in this mini review, we present a brief summary of the electrolytes developed recently for high performance SIC, including aqueous, organic, and ionic liquid based electrolytes. The influence factors such as ionic conductivities, electrolyte concentrations, electrochemical stable windows, as well as the cost and safety issues are discussed. Furthermore, the future perspectives and challenges in the science and engineering of new electrolytes are also considered. We hope that this review may be helpful to the energy storage community regarding the electrolytes of advanced SIC systems.

A Universal Approach to Aqueous Energy Storage via Ultralow-Cost Electrolyte with Super-Concentrated Sugar as Hydrogen-Bond-Regulated Solute

Aqueous energy-storage systems have attracted wide attention due to their advantages such as high security, low cost, and environmental friendliness. However, the specific chemical properties of water induce the problems of narrow electrochemical stability window, low stability of water-electrode interface reactions, and dissolution of electrode materials and intermediate products. Therefore, new low-cost aqueous electrolytes with different water chemistry are required. The nature of water depends largely on its hydroxyl-based hydrogen bonding structure. Therefore, the super-concentrated hydroxyl-rich sugar solutions are designed to change the original hydrogen bonding structure of water. The super-concentrated sugars can reduce the free water molecules and destroy the tetrahedral structure, thus lowering the binding degree of water molecules by breaking the hydrogen bonds. The ionic electrolytes based on super-concentrated sugars have the expanded electrochemical stability window (up to 2.812 V), wide temperature adaptability (-50 to 80 °C), and fair ionic conductivity (8.536 mS cm^-1 ). Aqueous lithium-, sodium-, potassium-ion batteries and supercapacitors using super-concentrated sugar-based electrolytes demonstrate an excellent electrochemical performance. The advantages of ultralow cost and high universality enable a great practical application potential of the super-concentrated sugar-based aqueous electrolytes, which can also provide great experimental and theoretical assistance for further research in water chemistry.

Advanced Materials for Sodium-Ion Capacitors with Superior Energy-Power Properties: Progress and Perspectives

Developing electrochemical energy storage devices with high energy-power densities, long cycling life, as well as low cost is of great significance. Sodium-ion capacitors (NICs), with Na⁺ as carriers, are composed of a high capacity battery-type electrode and a high rate capacitive electrode. However, unlike their lithium-ion analogues, the research on NICs is still in its infancy. Rational material designs still need to be developed to meet the increasing requirements for NICs with superior energy-power performance and low cost. In the past few years, various materials have been explored to develop NICs with the merits of superior electrochemical performance, low cost, good stability, and environmental friendliness. Here, the material design strategies for sodium-ion capacitors are summarized, with focus on cathode materials, anode materials, and electrolytes. The challenges and opportunities ahead for the future research on materials for NICs are also proposed.

Molecular Design Strategies for Electrochemical Behavior of Aromatic Carbonyl Compounds in Organic and Aqueous Electrolytes

To sustainably satisfy the growing demand for energy, organic carbonyl compounds (OCCs) are being widely studied as electrode active materials for batteries owing to their high capacity, flexible structure, low cost, environmental friendliness, renewability, and universal applicability. However, their high solubility in electrolytes, limited active sites, and low conductivity are obstacles in increasing their usage. Here, the nucleophilic addition reaction of aromatic carbonyl compounds (ACCs) is first used to explain the electrochemical behavior of carbonyl compounds during charge-discharge, and the relationship of the molecular structure and electrochemical properties of ACCs are discussed. Strategies for molecular structure modifications to improve the performance of ACCs, i.e., the capacity density, cycle life, rate performance, and voltage of the discharge platform, are also elaborated. ACCs, as electrode active materials in aqueous solutions, will become a future research hotspot. ACCs will inevitably become sustainable green materials for batteries with high capacity density and high power density.