Molecular Crowding Electrolytes for Stable Proton Batteries

Advanced electrolytes for high-performance aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) have garnered significant attention in the realm of large-scale and sustainable energy storage, primarily owing to their high safety, low cost, and eco-friendliness. Aqueous electrolytes, serving as an indispensable constituent, exert a direct influence on the electrochemical performance and longevity of AZIBs. Nonetheless, conventional aqueous electrolytes often encounter formidable challenges in AZIB applications, such as the limited electrochemical stability window and the zinc dendrite growth. In response to these hurdles, a series of advanced aqueous electrolytes have been proposed, such as "water-in-salt" electrolytes, aqueous eutectic electrolytes, molecular crowding electrolytes, and hydrogel electrolytes. This comprehensive review commences by presenting an in-depth overview of the fundamental compositions, principles, and distinctive characteristics of various advanced aqueous electrolytes for AZIBs. Subsequently, we systematically scrutinizes the recent research progress achieved with these advanced aqueous electrolytes. Furthermore, we summarizes the challenges and bottlenecks associated with these advanced aqueous electrolytes, along with offering recommendations. Based on the optimization of advanced aqueous electrolytes, this review outlines future directions and potential strategies for the development of high-performance AZIBs. This review is anticipated to provide valuable insights into the development of advanced electrolyte systems for the next generation of stable and sustainable multi-valent secondary batteries.

An improved N-N dimethylacetamide-based non-flammable molecular crowding electrolyte using NaOTf for cobalt-based prussian blue//hard carbon aqueous solution battery

Aqueous sodium-ion batteries (ASIBs) are promising for large-scale electrical energy storage (LSEES) applications due to their cost and safety advantages. However, the low voltage stabilization window of water (∼1.23 V) and the lack of cathode with high specific capacity and long cycle life have limited their development. Cobalt-based Prussian blue analogues (NaCoPBAs) have the advantage of high theoretical specific capacity but short cycle life. Recently, the molecular crowding electrolyte (MCE) strategy has been proposed to improve the electrolyte voltage stability window (ESW) of electrolytes, in this work, we report an improved xMC (x: ratio, MC: molecular crowding agent) electrolyte that uses N-N dimethylacetamide (DMAC) as the molecular crowding agent and NaOTf as the advanced salt with an ESW of 2.65 V and excellent nonflammability. The side reactions of the NaCoPBA//Hard Carbon (HC) full-cell active material are improved with the aid of the electrolyte. Capacity retention of 75 % after 600 cycles with excellent cycling stability. These results demonstrate that this advanced MCE strategy can be utilized for practical applications designed for safety, high specific capacity and long cycle (ASIB).

Aqueous Electrolyte With Weak Hydrogen Bonds for Four-Electron Zinc-Iodine Battery Operates in a Wide Temperature Range

In the pursuit of high-performance energy storage systems, four-electron zinc-iodine aqueous batteries (4eZIBs) with successive I-/I2/I+ redox couples are appealing for their potential to deliver high energy density and resource abundance. However, susceptibility of positive valence I+ to hydrolysis and instability of Zn plating/stripping in conventional aqueous electrolyte pose significant challenges. In response, polyethylene glycol (PEG 200) is introduced as co-solvent in 2 m ZnCl2 aqueous solution to design a wide temperature electrolyte. Through a comprehensive investigation combining spectroscopic characterizations and theoretical simulations, it is elucidated that PEG disrupts the intrinsic strong H-bonds of water by global weak PEG-H2O interaction, which strengthens the O─H covalent bond of water and intensifies the coordination with Zn2+. This synergistic effect substantially reduces water activity to restrain the I+ hydrolysis, facilitating I-/I2/I+ redox kinetics, mitigating I3 - formation and smoothening Zn deposition. The 4eZIBs in the optimized hybrid electrolyte not only deliver superior cyclability with a low fading rate of 0.0009% per cycle over 20 000 cycles and a close-to-unit coulombic efficiency but also exhibit stable performance in a wide temperature range from 40 °C to -40 °C. This study offers valuable insights into the rational design of electrolytes for 4eZIBs.

Limiting Interfacial Free Water and Proton Concentration by Hydrogel Electrolytes for Stable MoO3 Anode in a Proton Battery

Aqueous rechargeable proton batteries are attractive due to the small ionic radius, light mass, and ultrafast diffusion kinetics of proton as charge carriers. However, the commonly used acidic electrolyte is usually very corrosive to the electrode material, which seriously affects the cycle life of the battery. Here, it is proposed that decreasing water activity and limiting proton concentration can effectively prevent side reactions of the MoO3 anode such as corrosion and hydrogen precipitation by using a lean-water hydrogel electrolyte. The as-prepared polyacrylamide (PAAM)-poly2-acrylamide-2-methylpropanesulfonic acid (PAMPS)/MnSO4 (PPM) hydrogel electrolyte not only has abundant hydrophilic groups that can form hydrogen bonds with free water and inhibit solvent-electrode interaction, but also has fixed anions that can maintain a certain interaction with protons. The assembled MoO3||MnO2 full battery can stably cycle over 500 times for ≈350 h with an unprecedented capacity retention of 100% even at a low current density of 0.5 A g-1. This work gives a hint that limiting free water as well as proton concentration is important for the design of electrolytes or interfaces in aqueous proton batteries.

Realization of Long-Life Proton Battery by Layer Intercalatable Electrolyte

Proton batteries have attracted increasing interests because of their potential for grid-scale energy storage with high safety and great low-temperature performances. However, their development is significantly retarded by electrolyte design due to free water corrosion. Herein, we report a layer intercalatable electrolyte (LIE) by introducing trimethyl phosphate (TMP) into traditional acidic electrolyte. Different from conventional role in batteries, the presence of TMP intriguingly achieves co-intercalation of solvent molecules into the interlayer of anode materials, enabling a new working mechanism for proton reactions. The electrode corrosion was also strongly retarded with expanded electrochemical stability window. The half-cell therefore showed an outstanding long-term cycling stability with 91.0 % capacity retention at 5 A g-1 after 5000 cycles. Furthermore, the assembled full batteries can even deliver an ultra-long lifetime with a capacity retention of 74.9 % for 2 months running at -20 °C. This work provides new opportunities for electrolyte design of aqueous batteries.

An Emerging Chemistry Revives Proton Batteries

Developing new energy techniques that simultaneously integrate the fast rate capabilities of supercapacitors and high capacities of batteries represents an ultimate goal in the field of electrochemical energy storage. A new possibility arises with an emerging battery chemistry that relies on proton-ions as the ion-charge-carrier and benefits from the fast transportation kinetics. Proton-based battery chemistry starts with the recent discoveries of materials for proton redox reactions and leads to a renaissance of proton batteries. In this article, the historical developments of proton batteries are outlined and key aspects of battery chemistry are reviewed. First, the fundamental knowledge of proton-ions and their transportation characteristics is introduced; second, Faradaic electrodes for proton storage are categorized and highlighted in detail; then, reported electrolytes and different designs of proton batteries are summarized; last, perspectives of developments for proton batteries are proposed. It is hoped that this review will provide guidance on the rational designs of proton batteries and benefit future developments.

Dual-Salt Mixed Electrolyte for High Performance Aqueous Aluminum Batteries

A dual-salt electrolyte with 5 M Al(OTF)3 and 0.5 M LiOTF is proposed for aqueous aluminum batteries, which can effectively prevent the corrosion caused by the hydrogen evolution reaction. With the addition of LiOTF in the electrolyte, the solvation phenomenon has changed with the coordination mode of Al3+ conversion from an all octahedral structure to a mixed octahedral and tetrahedral structure. This change can reduce the hydrogen bond between water molecules, which will minimize the occurrence of hydrogen evolution reactions. Moreover, the new electrolyte improves the cycle life of the battery. With MnO as the cathode, 2.1 V high charging platform and 1.5 V high discharge platform can be obtained. The electrochemical stability window (ESW) has been improved to 3.8 V. The first cycle capacity is up to 437 mAh g-1, which can be maintained at 103 mAh g-1 after 100 cycles. This work provides solutions for the future development of electrolyte for aqueous aluminum batteries.

Hydronium Intercalation Enables High Rate in Hexagonal Molybdate Single Crystals

Rapid proton transport in solid-hosts promotes a new chemistry in achieving high-rate Faradaic electrodes. Exploring the possibility of hydronium intercalation is essential for advancing proton-based charge storage. Nevertheless, this is yet to be revealed. Herein, a new host is reported of hexagonal molybdates, (A2 O)x ·MoO3 ·(H2 O)y (A = Na+ , NH4 + ), and hydronium (de)intercalation is demonstrated with experiments. Hexagonal molybdates show a battery-type initial reduction followed by intercalation pseudocapacitance. Fast rate of 200 C (40 A g-1 ) and long lifespan of 30 000 cycles are achieved in electrodes of monocrystals even over 200 µm. Solid-state nuclear magnetic resonance confirms hydronium intercalations, and operando measurements using electrochemical quartz crystal microbalance and synchrotron X-ray diffraction disclose distinct intercalation behaviours in different electrolyte concentrations. Remarkably, characterizations of the cycled electrodes show nearly identical structures and suggest equilibrium products are minimally influenced by the extent of proton solvation. These results offer new insights into proton electrochemistry and will advance correlated high-power batteries and beyond.

Aqueous Organic Batteries Using the Proton as a Charge Carrier

Benefiting from the merits of low cost, nonflammability, and high operational safety, aqueous rechargeable batteries have emerged as promising candidates for large-scale energy-storage applications. Among various metal-ion/non-metallic charge carriers, the proton (H+ ) as a charge carrier possesses numerous unique properties such as fast proton diffusion dynamics, a low molar mass, and a small hydrated ion radius, which endow aqueous proton batteries (APBs) with a salient rate capability, a long-term life span, and an excellent low-temperature electrochemical performance. In addition, redox-active organic molecules, with the advantages of structural diversity, rich proton-storage sites, and abundant resources, are considered attractive electrode materials for APBs. However, the charge-storage and transport mechanisms of organic electrodes in APBs are still in their infancy. Therefore, finding suitable electrode materials and uncovering the H+ -storage mechanisms are significant for the application of organic materials in APBs. Herein, the latest research progress on organic materials, such as small molecules and polymers for APBs, is reviewed. Furthermore, a comprehensive summary and evaluation of APBs employing organic electrodes as anode and/or cathode is provided, especially regarding their low-temperature and high-power performances, along with systematic discussions for guiding the rational design and the construction of APBs based on organic electrodes.

Effects of Molecular Crowding on the Structure, Stability, and Interaction with Ligands of G-quadruplexes

G-quadruplexes (G4s) are widely found in cells and have significant biological functions, which makes them a target for screening antitumor and antiviral drugs. Most of the previous research on G4s has been conducted mainly in diluted solutions. However, cells are filled with organelles and many biomolecules, resulting in a constant state of a crowded molecular environment. The conformation and stability of some G4s were found to change significantly in the molecularly crowded environment, and interactions with ligands were disturbed to some extent. The structure of the G4s and their biological functions are correlated, and the effect of the molecularly crowded environment on G4 conformational transitions and interactions with ligands should be considered in drug design targeting G4s. This review discusses the changes in the conformation and stability of G4s in a physiological environment. Moreover, the mechanism of action of the molecularly crowded environment affecting the G4 has been further reviewed based on previous studies. Furthermore, current challenges and future research directions are put forward. This review has implications for the design of drugs targeting G4s.