Challenges and Strategies for High-Energy Aqueous Electrolyte Rechargeable Batteries

Electrolyte Engineering Enables High Performance Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) feature high safety, low cost, environmental-friendliness, and promising electrochemical performance, and are therefore regarded as a potential technology to be applied in large-scale energy storage devices. However, ZIBs still face some critical challenges and bottlenecks. The electrolyte is an essential component of batteries and its properties affect the mass transport, energy storage mechanisms, reaction kinetics, and side reactions of ZIBs. The adjustment of electrolyte formulas usually has direct and obvious impacts on the overall output and performance. In this review, advanced electrolyte strategies are overviewed for optimizing the compatibility between cathode materials and electrolytes, inhibiting anode corrosion and dendrite growth, extending electrochemical stability windows, enabling wearable applications, and enhancing temperature tolerance. The underlying scientific mechanisms, electrolyte design principles, and recent progress are presented to provide a better understanding and inspiration to readers. In addition, a comprehensive perspective about electrolyte design and engineering for ZIBs is included.

Challenges and prospects of high-voltage aqueous electrolytes for energy storage applications

Aqueous electrolytes have attracted widespread attention as they are safe, environmentally benign and cost effective, holding great promise for future low-cost and sustainable energy storage devices. Nonetheless, the narrow electrochemical stability window caused by water electrolysis, as well as the trade-off between the stability window and other properties remain the bottleneck problem for the practical applications of aqueous electrolytes. Deep insights into the correlations between the microscopic physicochemical and electrochemical mechanisms and the macroscopic properties of aqueous electrolyte are essential for the envisaged applications, yet a systematic analysis of the recent progress in this area is still lacking. In this Perspective article, the basic mechanisms and influencing factors of water electrolysis including the hydrogen evolution and oxygen evolution reactions is critically examined. We systematically review the current state-of-the-art on high-voltage aqueous electrolytes focusing on the fundamental mechanisms of ion kinetics leading to dynamic electrolyte restructuring. Recent advances on the optimization of high-voltage aqueous electrolytes are also summarized. The existing challenges are identified and perspectives for exploring and developing future high-voltage aqueous electrolytes are provided.

Unlocking Layered Double Hydroxide as a High-Performance Cathode Material for Aqueous Zinc-Ion Batteries

Advanced cathode materials play an important role in promoting aqueous battery technology for safe energy storage. Transition metal double hydroxides are usually elusive as a stable cathode for aqueous zinc-ion batteries (AZIBs) due to their unstable crystal structure, sluggish ion transportation, and insufficient active sites for zinc-ion storage. Here, a trinary layered double hydroxide (LDH) with hydrogen vacancies (Ni₃ Mn_0.7 Fe_0.3 -LDH) is reported as a new cathode material for AZIBs. A reversible high capacity up to 328 mAh g^-1 can be obtained and cycle stably over 500 cycles with a capacity retention of 85%. Experimental and theoretical studies reveal that the hydrogen vacancies in LDH can expose lattice oxygen atoms as active sites for zinc-ion storage and accelerate ion diffusion by reducing the electrostatic interactions between zinc ions and the host structure. In addition, the synergy of the trinary transitional metal cations can suppress the Jahn-Teller distortion of manganese (III) oxide octahedron and enable long cycle stability. This work provides not only a series of high-performance cathode materials for AZIBs but also a novel materials design strategy that can be extended to other multi-valence metal-ion batteries.

Assessing the impact of valence asymmetry in ionic solutions and its consequences on the performance of supercapacitors

Molecular dynamics simulations were performed to describe the properties of hypothetical salt electrolytic solutions. The main focus of this work is the valence asymmetry, which in recent years has been considered an important aspect in the physical chemistry of aqueous electrolytes. In general, our results show that the structural, energetic, and dynamic properties respond differently to the asymmetry of ionic solutions, but in all cases, appreciable changes were observed. Graphene supercapacitors based on the investigated electrolytes were studied in light of their electrostatic properties. We observed that the electrode capacitances, positive and negative, were greatly influenced by the presence of cations in the electrical double layer of the negative electrode and by the absence of these cations, in the double layer of the positive electrode. In general, we assess that quantitative variations due to valence asymmetry may indeed be an important factor for the development of new and more efficient electrolytes.

Emerging Iontronic Sensing: Materials, Mechanisms, and Applications

Iontronic sensors represent a novel class of soft electronics which not only replicate the biomimetic structures and perception functions of human skin but also simulate the mechanical sensing mechanism. Relying on the similar mechanism with skin perception, the iontronic sensors can achieve ion migration/redistribution in response to external stimuli, promising iontronic sensing to establish more intelligent sensing interface for human-robotic interaction. Here, a comprehensive review on advanced technologies and diversified applications for the exploitation of iontronic sensors toward ionic skins and artificial intelligence is provided. By virtue of the excellent stretchability, high transparency, ultrahigh sensitivity, and mechanical conformality, numerous attempts have been made to explore various novel ionic materials to fabricate iontronic sensors with skin-like perceptive properties, such as self-healing and multimodal sensing. Moreover, to achieve multifunctional artificial skins and intelligent devices, various mechanisms based on iontronics have been investigated to satisfy multiple functions and human interactive experiences. Benefiting from the unique material property, diverse sensing mechanisms, and elaborate device structure, iontronic sensors have demonstrated a variety of applications toward ionic skins and artificial intelligence.

Synthesis and Electrochemical Performance of the Orthorhombic V2O5·nH2O Nanorods as Cathodes for Aqueous Zinc Batteries

Aqueous zinc-ion batteries offer the greatest promise as an alternative technology for low-cost and high-safety energy storage. However, the development of high-performance cathode materials and their compatibility with aqueous electrolytes are major obstacles to their practical applications. Herein, we report the synthesis of orthorhombic V₂O₅·nH₂O nanorods as cathodes for aqueous zinc batteries. As a result, the electrode delivers a reversible capacity as high as 320 mAh g⁻¹ at 1.0 A g⁻¹ and long-term cycling stability in a wide window of 0.2 to 1.8 V using a mild ZnSO₄ aqueous electrolyte. The superior performance can be attributed to the improved stability of materials, inhibited electrolyte decomposition and facilitated charge transfer kinetics of such materials for aqueous zinc storage. Furthermore, a full cell using microsized Zn powder as an anode within capacity-balancing design exhibits high capacity and stable cycling performance, proving the feasibility of these materials for practical application.

High-Efficiency and Stable Zn-Na3 V2 (PO4 )3 Aqueous Battery Enabled by Electrolyte-Induced Interphasial Engineering

Aqueous zinc batteries have been regarded as a promising energy storage technology due to their high energy, high material abundance, low toxicity, and intrinsic safety. NASICON-type materials have been proposed as efficient cathodes for rechargeable batteries, yet they suffer from fast degradation and low Coulombic efficiency in aqueous batteries. Here we demonstrate that a rationally designed aqueous electrolyte containing a supporting Na salt and polymer additive can efficiently suppress the water activity through hydrogen bonding and facilitate the anion involvement in interfacial reactions, thus enabling the stable operation of sodium superionic conductor (NASICON) cathodes in aqueous zinc batteries. As exemplified by a Na₃ V₂ (PO₄ )₃ cathode, the cell with zinc metal anode exhibits high cycling Coulombic efficiencies (around 99.9 % in average) with a steady output voltage and capacity retention for 300 cycles. This work addresses the potential issues with NASICON-type cathodes in aqueous zinc batteries and proposes an effective solution via fundamental interphasial chemistry to design efficient and sustainable aqueous electrolytes.

A Review of Energy Storage Mechanisms in Aqueous Aluminium Technology

This systematic review covers the developments in aqueous aluminium energy storage technology from 2012, including primary and secondary battery applications and supercapacitors. Aluminium is an abundant material with a high theoretical volumetric energy density of –8.04 Ah cm−3. Combined with aqueous electrolytes, which have twice the ionic storage potential as non-aqueous versions, this technology has the potential to serve many energy storage needs. The charge transfer mechanisms are discussed in detail with respect to aqueous aluminium-ion secondary batteries, where most research has focused in recent years. TiO2 nanopowders have shown to be promising negative electrodes, with the potential for pseudocapacitive energy storage in aluminuim-ion cells. This review summarises the advances in Al-ion systems using aqueous electrolytes, focusing on electrochemical performance.

Confined Lithium Deposition Triggered by an Integrated Gradient Scaffold for a Lithium-Metal Anode

Constructing a composite lithium anode with a rational structure has been considered as an effective approach to regulate and relieve the tough problems of a sparkling Li anode. However, the potential short circuits risk that Li deposition at the surface of the framework has not yet been resolved. Here, we present a simple regulating-deposition strategy to guide the preferentially bottom-up deposition/growth of Li. The triple-gradient structure of modified porous copper with electrical passivation (top) and chemical activation (bottom) shows significant improvements in the morphological stability and electrochemical performance. Meanwhile, the in situ generation of Li₂Se can as an advanced artificial SEI layer be devoted to homogeneous Li plating/stripping. As a result, the composite anode exhibits a long-term cycling over 250 cycles with a high average CE of 98.2% at 1 mA cm^-2. Furthermore, a capacity retention of 94.4% in full cells can be achieved when pairing with LiFePO₄ as the cathode. These results ensure a bright direction for developing high-performance Li metal anodes.

High-Energy Batteries: Beyond Lithium-Ion and Their Long Road to Commercialisation

Rechargeable batteries of high energy density and overall performance are becoming a critically important technology in the rapidly changing society of the twenty-first century. While lithium-ion batteries have so far been the dominant choice, numerous emerging applications call for higher capacity, better safety and lower costs while maintaining sufficient cyclability. The design space for potentially better alternatives is extremely large, with numerous new chemistries and architectures being simultaneously explored. These include other insertion ions (e.g. sodium and numerous multivalent ions), conversion electrode materials (e.g. silicon, metallic anodes, halides and chalcogens) and aqueous and solid electrolytes. However, each of these potential "beyond lithium-ion" alternatives faces numerous challenges that often lead to very poor cyclability, especially at the commercial cell level, while lithium-ion batteries continue to improve in performance and decrease in cost. This review examines fundamental principles to rationalise these numerous developments, and in each case, a brief overview is given on the advantages, advances, remaining challenges preventing cell-level implementation and the state-of-the-art of the solutions to these challenges. Finally, research and development results obtained in academia are compared to emerging commercial examples, as a commentary on the current and near-future viability of these "beyond lithium-ion" alternatives.

Eliminating Stubborn Insulated Deposition by Coordination Effect to Boost Zn Electrode Reversibility in Aqueous Electrolyte

Aqueous rechargeable zinc-ion batteries (ZIBs) have recently shined in energy storage and transmission, which are due to high safety and low cost. However, the extremely stubborn by-products in the Zn anode severely inhibited the Zn²⁺ adsorption/desorption and exacerbated the dendrite formation. Herein, we report a facile strategy to eliminate inert Zn₄(OH)₆SO₄·xH₂O for the improvement of ZIBs according to the coordination effect by employing ethylenediaminetetraacetic acid-diamine (EDTA-2Na) as a coordination additive in traditional electrolyte. Zn²⁺ is coordinated with the carboxyl group of the four acetyl carboxyl groups and the N in C–N bonds, forming a new chelating structure, and thus stubborn deposition will be dissolved in the electrolyte. As a result, the discharge capacity of 102 mAh g⁻¹ in the ZnSO₄/Li₂SO₄ with EDTA-2Na electrolyte at a current density of 4 C and a stable cycle life with a capacity of 90.3% after 150 cycles are achieved. It has been concluded that the coordination effect strategy provides a valuable idea for solving the defects of ZIBs.

Fast Ionic Storage in Aqueous Rechargeable Batteries: From Fundamentals to Applications

The highly dynamic nature of grid-scale energy systems necessitates fast kinetics in energy storage and conversion systems. Rechargeable aqueous batteries are a promising energy-storage solution for renewable-energy grids as the ionic diffusivity in aqueous electrolytes can be up to 1-2 orders of magnitude higher than in organic systems, in addition to being highly safe and low cost. Recent research in this regard has focussed on developing suitable electrode materials for fast ionic storage in aqueous electrolytes. In this review, breakthroughs in the field of fast ionic storage in aqueous battery materials, and 1D/2D/3D and over-3D-tunnel materials are summarized, and tunnels in over-3D materials are not oriented in any direction in particular. Various materials with different tunnel sizes are developed to be suitable for the different ionic radii of Li⁺ , Na⁺ , K⁺ , H⁺ , NH₄ ⁺ , and Zn²⁺ , which show significant differences in the reaction kinetics of ionic storage. New topochemical paths for ion insertion/extraction, which provide superfast ionic storage, are also discussed.

Modulating residual ammonium in MnO2 for high-rate aqueous zinc-ion batteries

Manganese dioxide (MnO₂), as a promising cathode candidate, has attracted great attention in aqueous zinc ion batteries (ZIBs). However, the undesirable dissolution of Mn²⁺ and the sluggish kinetic reaction are still two challenges to overcome before achieving good cycling stability and high-rate performance of ZIBs. Herein, β-MnO₂ with chemically residual NH₄⁺ (NMO) was successfully fabricated by controlling the washing condition and utilized as a cathode in a ZIB. Interestingly, NH₄⁺, as a layer pillar in the tunnel structure of NMO, could enhance its conductivity by changing the chemical structure, contributing to accelerating the kinetics of the charge carrier. Moreover, the residual NH₄⁺ in NMO could stabilize the chemical microstructure through the cationic electrostatic shielding effect and the formation of Mn-O⋯H bonds in NMO, promoting the reversible and successive insertion/extraction of H⁺/Zn²⁺ in the ZIB. As a result, the Zn/NMO battery exhibits excellent rate performance (up to 8.0 A g^-1) and cycling stability (10 000 cycles). This work will pave the way for the design of cathode materials with nonmetallic doping for high-performance ZIB systems.

Supramolecular-induced 2.40 V 130 °C working-temperature-range supercapacitor aqueous electrolyte of lithium bis(trifluoromethanesulfonyl) imide in dimethyl sulfoxide-water

Increasing the electrochemical stability window and working temperature range of supercapacitor aqueous electrolyte is the major task in order to advance aqueous electrolyte-based supercapacitors. Here, a supramolecular induced new electrolyte of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in dimethyl sulfoxide (DMSO) and water co-solvent system is proposed. Adjusting the coordination structure among LiTFSI, DMSO, and water in the electrolyte via supramolecular interactions results in its high ionic conductivity, low viscosity, wide electrochemical stability window, and large working temperature range. The new electrolyte-based supercapacitors can work in 2.40 V working potential and 130 °C working-temperature range from -40 to 90 °C. The devices exhibit good electrochemical performances, especially the energy density over 21 Wh kg^-1, which is much higher than that with traditional aqueous electrolytes (<10 Wh kg^-1). The work paves a way to develop high-performance aqueous electrolytes for supercapacitors.

Application of Advanced Vibrational Spectroscopy in Revealing Critical Chemical Processes and Phenomena of Electrochemical Energy Storage and Conversion

The future of the energy industry and green transportation critically relies on exploration of high-performance, reliable, low-cost, and environmentally friendly energy storage and conversion materials. Understanding the chemical processes and phenomena involved in electrochemical energy storage and conversion is the premise of a revolutionary materials discovery. In this article, we review the recent advancements of application of state-of-the-art vibrational spectroscopic techniques in unraveling the nature of electrochemical energy, including bulk energy storage, dynamics of liquid electrolytes, interfacial processes, etc. Technique-wise, the review covers a wide range of spectroscopic methods, including classic vibrational spectroscopy (direct infrared absorption and Raman scattering), external field enhanced spectroscopy (surface enhanced Raman and IR, tip enhanced Raman, and near-field IR), and two-photon techniques (2D infrared absorption, stimulated Raman, and vibrational sum frequency generation). Finally, we provide perspectives on future directions in refining vibrational spectroscopy to contribute to the research frontier of electrochemical energy storage and conversion.

Advanced Multifunctional Aqueous Rechargeable Batteries Design: From Materials and Devices to Systems

Multifunctional aqueous rechargeable batteries (MARBs) are regarded as safe, cost-effective, and scalable electrochemical energy storage devices, which offer additional functionalities that conventional batteries cannot achieve, which ideally leads to unprecedented applications. Although MARBs are among the most exciting and rapidly growing topics in scientific research and industrial development nowadays, a systematic summary of the evolution and advances in the field of MARBs is still not available. Therefore, the review presented comprehensively and systematically summarizes the design principles and the recent advances of MARBs by categories of smart ARBs and integrated systems, together with an analysis of their device design and configuration, electrochemical performance, and diverse smart functions. The two most promising strategies to construct novel MARBs may be A) the introduction of functional materials into ARB components, and B) integration of ARBs with other functional devices. The ongoing challenges and future perspectives in this research and development field are outlined to foster the future development of MARBs. Finally, the most important upcoming research directions in this rapidly developing field are highlighted that may be most promising to lead to the commercialization of MARBs and to a further broadening of their range of applications.

Recent Advances in Flexible Zn-Air Batteries: Materials for Electrodes and Electrolytes

Flexible Zn-air batteries (ZABs) draw much attention due to the merits of high energy density, stability, and safety, and show potential applications for wearable devices. However, the development of flexible ZABs with great energy density, high round-trip efficiency, and long cycle life for practical applications is highly restricted by the lack of highly active oxygen catalysts, high ion-conducting solid-state electrolytes, appropriate Zn anodes, and advanced battery configuration. Promising oxygen catalysts should possess both, superior oxygen reduction reaction and oxygen evolution reaction performance and can be directly used as self-supporting cathodes without loading catalysts on support materials such as carbon cloth. In addition, electrolytes play an important role in ZABs; a good electrolyte should be in all-solid state with high ion conductivity. Moreover, for an excellent Zn anode, it is required to stably contact the electrolyte interface during the bending process. Therefore, in this review, recent advances in ZABs are summarized, including: i) the powder and 3D self-supporting oxygen catalysts, ii) the species of solid-state electrolytes, and iii) the rational design of Zn anodes. Finally, the challenges and opportunities of this promising field are presented.

Development of advanced electrolytes in Na-ion batteries: application of the Red Moon method for molecular structure design of the SEI layer

This review aims to overview state-of-the-art progress in the collaborative work between theoretical and experimental scientists to develop advanced electrolytes for Na-ion batteries (NIBs). Recent investigations were summarized on NaPF6 salt and fluoroethylene carbonate (FEC) additives in propylene carbonate (PC)-based electrolyte solution, as one of the best electrolytes to effectively passivate the hard-carbon electrode with higher cycling performance for next-generation NIBs. The FEC additive showed high efficiency to significantly enhance the capacity and cyclability of NIBs, with an optimal performance that is sensitive at low concentration. Computationally, both microscopic effects, positive and negative, were revealed at low and high concentrations of FEC, respectively. In addition to the role of FEC decomposition to form a NaF-rich solid electrolyte interphase (SEI) film, intact FECs play a role in suppressing the dissolution to form a compact and stable SEI film. However, the increase in FEC concentration suppressed the organic dimer formation by reducing the collision frequency between the monomer products during the SEI film formation processes. In addition, this review introduces the Red Moon (RM) methodology, recent computational battery technology, which has shown a high efficiency to bridge the gap between the conventional theoretical results and experimental ones through a number of successful applications in NIBs.

Salt-in-water and water-in-salt electrolytes: the effects of the asymmetry in cation and anion valence on their properties

We investigated the structural, dynamic, energetic, and electrostatic properties of electrolytes based on the ion pairs LiCl and Li₂SO₄. Atomistic molecular dynamics simulations were used to simulate these aqueous electrolytic solutions at two different concentrations 2 M (normal) and 21 M (superconcentrated, WiSE). The effects of the valence asymmetry of the Li₂SO₄ electrolyte were also discussed for both salt concentrations. Our results differ in the physical aspect of pure electrolytes, showing the drastic effect of high concentration, in particular on the viscosity, which is dramatically increased in WiSE. This is a consequence of their reduced ionic mobility and has a direct effect on ionic conductivity. Also, our results for graphene-based supercapacitors, as indicated by some experimental work, do not indicate any better performance of WiSEs over normal electrolytes. In fact, the differences in the total capacitance, due to the concentration of ions, presented by both electrolytes are negligible. The valence asymmetry can be clearly observed in some properties but for most of them its effects could not be quantified or isolated.

Unveiling the Reversibility and Stability Origin of the Aqueous V2 O5 -Zn Batteries with a ZnCl2 "Water-in-Salt" Electrolyte

Aqueous V2 O5 -Zn batteries, an alternative chemistry format that is inherently safer to operate than lithium-based batteries, illuminates the low-cost deployment of the stationary energy storage devices. However, the cathode structure collapse caused by H2 O co-insertion in aqueous solution dramatically deteriorates the electrochemical performance and hampers the operation reliability of V2 O5 -Zn batteries. The real-time phase tracking and the density functional theory (DFT) calculation prove the high energy barrier that inhibits the Zn2+ diffusion into the bulk V2 O5 , instead the ZnCl2 "water-in-salt electrolyte" (WiSE) can enable the dominant proton insertion with negligible lattice strain or particle fragment. Thus, ZnCl2 WiSE enables the enhanced reversibility and extended shelf life of the V2 O5 -Zn battery upon the high temperature storage. The improved electrochemical performance also benefits by the inhibition of vanadium cation dissolution, enlarged voltage window, as well as the suppression of the Zn dendrite protrusion. This study comprehensively elucidates the pivotal role of a concentrated ZnCl2 electrolyte to stabilize the aqueous batteries at both the static storage and dynamic operation scenarios.

Horizontally arranged zinc platelet electrodeposits modulated by fluorinated covalent organic framework film for high-rate and durable aqueous zinc ion batteries

Rechargeable aqueous zinc-ion batteries (RZIBs) provide a promising complementarity to the existing lithium-ion batteries due to their low cost, non-toxicity and intrinsic safety. However, Zn anodes suffer from zinc dendrite growth and electrolyte corrosion, resulting in poor reversibility. Here, we develop an ultrathin, fluorinated two-dimensional porous covalent organic framework (FCOF) film as a protective layer on the Zn surface. The strong interaction between fluorine (F) in FCOF and Zn reduces the surface energy of the Zn (002) crystal plane, enabling the preferred growth of (002) planes during the electrodeposition process. As a result, Zn deposits show horizontally arranged platelet morphology with (002) orientations preferred. Furthermore, F-containing nanochannels facilitate ion transport and prevent electrolyte penetration for improving corrosion resistance. The FCOF@Zn symmetric cells achieve stability for over 750 h at an ultrahigh current density of 40 mA cm^-2. The high-areal-capacity full cells demonstrate hundreds of cycles under high Zn utilization conditions.

An Overcrowded Water-Ion Solvation Structure for a Robust Anode Interphase in Aqueous Lithium-Ion Batteries

The water-in-salt electrolyte (WISE) features intimate interactions between a cation and anion, which induces the formation of an anion-derived solid electrolyte interphase (SEI) and expands the aqueous electrolyte voltage window to >3.0 V. Although further increasing the salt concentration (even to >60 molality (m)) can gradually improve water stability, issues about cost and practical feasibility are concerned. An alternative approach is to intensify ion-solvent interactions in the inner solvation structure by shielding off outward electrostatic attractions from nearby ions. Here, we design an "overcrowded" electrolyte using the non-polar, hydrogen-bonding 1,4-dioxane (DX) as an overcrowding agent, thereby achieving a robust LiF-enriched SEI and wide electrolyte operation window (3.7 V) with a low salt concentration (<2 m). As a result, the electrochemical performance of aqueous Li₄Ti₅O₁₂/LiMn₂O₄ full cells can be substantially improved (88.5% capacity retention after 200 cycles, at 0.57 C). This study points out a promising strategy to develop low-cost and stable high-voltage aqueous batteries.

Spent Coffee Grounds as Eco-Friendly Additives for Aluminum-Air Batteries

A new approach to the recycling of spent coffee grounds is described in which lignin, a chemical component of spent coffee, is used as an electrolyte additive in aluminum-air batteries. The effect of lignin on the performance of aluminum-air batteries has been investigated by weight loss measurement, galvanostatic discharge test, and electrochemical impedance spectroscopy (EIS). The corrosion inhibition efficiency is improved up to 37.3% and fuel efficiency up to 21.7% at 500 ppm of lignin molecules. The chemisorption of lignin molecules on the aluminum surface improves battery performance. Adsorption of lignin molecules onto the aluminum surface is driven by the electrostatic interaction between the lignin's hydroxyl group and the aluminum surface. The mechanism for the performance improvement is explained by the chemisorption behavior of lignin molecules. The adsorption behavior has been investigated by scanning electronic microscopy with energy-dispersive spectroscopy (SEM-EDS), laser scanning microscopy (LSM), atomic force microscopy (AFM), Freundlich adsorption isotherm, Fourier-transform infrared (FT-IR) spectroscopy, and the computational calculation of adsorption energies based on the density functional theory (DFT).

Physicochemical and Electrochemical Properties of Water-in-Salt Electrolytes

Aqueous electrolytes are attractive for applications in electrochemical technologies due to features like being eco-friendly, cost effective, and non-flammable. Very recently, superconcentrated aqueous electrolytes, such as so-called water-in-salt, water-in-bisalt, and hydrate melt, have received a significant attention for electrochemical energy storage due to enhanced stability and much wider electrochemical stability window. This Review focuses on the physicochemical properties of the highly concentrated electrolytes that are derived from several analysis techniques and simulation. A summary of most common features such as ions-water interactions, structure of species present in the electrolyte, conductivity, and viscosity of the electrolytes found in the literature are presented as well. In addition, this Review explains how these characteristics affect the electrochemical behavior of the electrolyte such as double layer structure and electrode/electrolyte interface leading to enhanced electrochemical stability of aqueous electrolytes.

Direct Proof of the Reversible Dissolution/Deposition of Mn2+/Mn4+ for Mild-Acid Zn-MnO2 Batteries with Porous Carbon Interlayers

Mild-acid Zn-MnO2 batteries have been considered a promising alternative to Li-ion batteries for large scale energy storage systems because of their high safety. There have been remarkable improvements in the electrochemical performance of Zn-MnO2 batteries, although the reaction mechanism of the MnO2 cathode is not fully understood and still remains controversial. Herein, the reversible dissolution/deposition (Mn2+/Mn4+) mechanism of the MnO2 cathode through a 2e- reaction is directly evidenced using solution-based analyses, including electron spin resonance spectroscopy and the designed electrochemical experiments. Solid MnO2 (Mn4+) is reduced into Mn2+ (aq) dissolved in the electrolyte during discharge. Mn2+ ions are then deposited on the cathode surface in the form of the mixture of the poorly crystalline Zn-containing MnO2 compounds through two-step reactions during charge. Moreover, the failure mechanism of mild-acid Zn-MnO2 batteries is elucidated in terms of the loss of electrochemically active Mn2+. In this regard, a porous carbon interlayer is introduced to entrap the dissolved Mn2+ ions. The carbon interlayer suppresses the loss of Mn2+ during cycling, resulting in the excellent electrochemical performance of pouch-type Zn-MnO2 cells, such as negligible capacity fading over 100 cycles. These findings provide fundamental insights into strategies to improve the electrochemical performance of aqueous Zn-MnO2 batteries.

Evaluation of the Stability of Carbon Conductor in the Cathode of Aqueous Rechargeable Lithium Batteries against Overcharging

Three major components in a cathode of aqueous rechargeable lithium batteries are the active material, the polymer binder, and the carbon conductive additive. The stability of each component in the battery is the key to long service life. To evaluate the stability of the carbon component, we introduce here a quick and direct testing method. LiMn2O4 is chosen as a typical active material for the preparation of the cathode, with polyvinylidene fluoride (PVdF), and a commercial carbon, which is chosen among Acetylene black, superP, superP-Li, Ketjen black 1, Ketjen black 2, Graphite, KS-6, splintered glassy carbon, and splintered spherical carbon. This method reveals the correlation between the electrochemical stability of a carbon and its physical and structural properties. This helps researchers choose the right carbon component for a Li-ion cathode if they want the battery to be robust, especially at near full state of charge.