Water-in-salt electrolytes for high voltage aqueous electrochemical energy storage devices

If were not by their low electrochemical stability, aqueous electrolytes would be the preferred alternative to be used in electrochemical energy storage devices. Their abundance and nontoxicity are key factors for such application, especially in large scale. The development of highly concentrated aqueous electrolytes, so-called water-in-salt electrolytes, has expanded the electrochemical window of aqueous electrolyte up to 3.0 V (whereas salt-in-water electrolytes normally shows up to 1.6 V), showing that water can be an alternative after all. Many devices, ranging from metal-ion batteries to electrochemical capacitors, have been reported recently, making use of such wider electrochemical stability and enhancing devices energy density. Different salts have also been proposed not only to gain in costs but also to improve physicochemical properties.     

Understanding the electrochemistry of “water-in-salt” electrolytes: basal plane highly ordered pyrolytic graphite as a model system

A new approach to expand the accessible voltage window of electrochemical energy storage systems, based on so-called “water-in-salt” electrolytes, has been expounded recently. Although studies of transport in concentrated electrolytes date back over several decades, the recent demonstration that concentrated aqueous electrolyte systems can be used in the lithium ion battery context has rekindled interest in the electrochemical properties of highly concentrated aqueous electrolytes. The original aqueous lithium ion battery conception was based on the use of concentrated solutions of lithium bis(trifluoromethanesulfonyl)imide, although these electrolytes still possess some drawbacks including cost, toxicity, and safety. In this work we describe the electrochemical behavior of a simple 1 : 1 electrolyte based on highly concentrated aqueous solutions of potassium fluoride (KF). Highly ordered pyrolytic graphite (HOPG) is used as well-defined model carbon to study the electrochemical properties of the electrolyte, as well as its basal plane capacitance, from a microscopic perspective: the KF electrolyte exhibits an unusually wide potential window (up to 2.6 V). The faradaic response on HOPG is also reported using K₃Fe(CN)₆ as a model redox probe: the highly concentrated electrolyte provides good electrochemical reversibility and protects the HOPG surface from adsorption of contaminants. Moreover, this electrolyte was applied to symmetrical supercapacitors (using graphene and activated carbon as active materials) in order to quantify its performance in energy storage applications. It is found that the activated carbon and graphene supercapacitors demonstrate high gravimetric capacitance (221 F g⁻¹ for activated carbon, and 56 F g⁻¹ for graphene), a stable working voltage window of 2.0 V, which is significantly higher than the usual range of water-based capacitors, and excellent stability over 10 000 cycles. These results provide fundamental insight into the wider applicability of highly concentrated electrolytes, which should enable their application in future of energy storage technologies.

The stability of water-in-salt electrolyte systems is investigated using highly concentrated solutions of KF(aq) with graphite as a model system.     

Promoting operating voltage to 2.3 V by a superconcentrated aqueous electrolyte in carbon-based supercapacitor

Aqueous supercapacitors(SCs) have attracted more and more attention for their safety,fast charge/discharge capability and ultra-long life.However,the application of aqueous SCs is limited by the low working voltage due to the narrow electrochemical stability window(ESW) of wate r.Herein,we report a new "water in salt"(WIS) electrolyte by dissolving potassium bis(fluorosulfonyl) amide(KFSI) in water with an ultra-high mass molar concentration of 37 mol/kg.The highly concentrated electrolyte can achieve a wide ESW of 2.8 V.The WIS electrolyte enables a safe carbon-based symmetrical supercapacitor to operate stably at 2.3 V with an ultra-long cycle life and excellent rate performance.The energy density reaches 20.5 Wh/kg at 2300 W/kg,and the capacity retention is 83.5% after 50,000 cycles at a current density of 5 A/g.This new electrolyte will be a promising candidate for future high-voltage aqueous supercapacitors.     

Ionic liquid-mediated aqueous redox flow batteries for high voltage applications

Aqueous redox flow batteries with high cell voltages represent a promising approach for low-cost, high safety and high energy density applications. However, water breakdown is a major concern because it limits cell voltage. For the first time, we report the use of a highly concentrated aqueous ionic liquid electrolyte, 1-butyl-3-methylimidazolium chloride (BMImCl)-H₂O, in an aqueous flow battery operating with a broad electrochemical stability window of 3 V. The proof-of-concept was demonstrated using 2 V redox couples of metal acetylacetonates and a hybrid Zn/Ce flow battery.     

高电压/宽温域水系碱金属离子电池的研究进展

水系储能器件具有固有的高安全性、环境友好性和成本低的优势，在未来智能电网、便携式/可穿戴电子产品等领域显示出巨大的应用潜力。然而水的热力学分解电压低、冰点高，导致水系电解液电化学稳定电压窗口窄以及凝固点高，极大地限制了水系储能器件的能量密度与宽温域应用。因此，设计耐高电压、抗冻的水系电解液，成为水系储能器件大规模、多场景应用的关键。本文系统综述了高电压/宽温域水系碱金属离子电池电解液设计的研究进展，从热力学和动力学角度出发，分别重点介绍提高电解液电压窗口和工作温度范围的各类策略以及相关作用机制。进一步提出宽温域、高压水系电解液的潜在设计思路，并对高性能水系碱金属离子电池的发展方向进行展望。     

Interfacial Speciation Determines Interfacial Chemistry: X-ray-Induced Lithium Fluoride Formation from Water-in-salt Electrolytes on Solid Surfaces

Super-concentrated “water-in-salt” electrolytes recently spurred resurgent interest for high energy density aqueous lithium-ion batteries. Thermodynamic stabilization at high concentrations and kinetic barriers towards interfacial water electrolysis significantly expand the electrochemical stability window, facilitating high voltage aqueous cells. Herein we investigated LiTFSI/H₂O electrolyte interfacial decomposition pathways in the “water-in-salt” and “salt-in-water” regimes using synchrotron X-rays, which produce electrons at the solid/electrolyte interface to mimic reductive environments, and simultaneously probe the structure of surface films using X-ray diffraction. We observed the surface-reduction of TFSI⁻ at super-concentration, leading to lithium fluoride interphase formation, while precipitation of the lithium hydroxide was not observed. The mechanism behind this photoelectron-induced reduction was revealed to be concentration-dependent interfacial chemistry that only occurs among closely contact ion-pairs, which constitutes the rationale behind the “water-in-salt” concept.     

“Water-in-salt”聚合物电解质制备及其电化学性能研究

为提高柔性锂离子电池安全性和循环稳定性能,本实验以自由基聚合结合冷冻干燥得到的聚丙烯酰胺膜为电解质载体,引入21 mol·kg^-1 LiTFSI 高浓度电解液,得到“water-in-salt”聚合物电解质。通过聚合物膜的形貌和孔道结构表征,红外光谱分析,离子电导率及电化学稳定窗口测试等对其基本物化特性进行了研究。冷冻干燥得到的聚丙烯酰胺膜内部具有大量微孔结构,有利于电解液的载入。将该吸附了电解液的聚合物电解质膜与锰酸锂(LiMn₂O₄)正极和磷酸钛锂(LiTi₂(PO₄)₃)负极组装全电池进行充放电性能测试。结果表明,制得的柔性聚合物电解质具有良好的拉伸性能,高离子电导率(20°C,4.34 mS·cm^-1)和宽电化学稳定窗口(3.12 V)。以“water-in-salt”聚合物电解质为隔膜组装的LiMn₂O₄||LiTi₂(PO₄)₃ 全电池表现出优异的倍率性能和长循环稳定性。     

Supercapattery: Merit merge of capacitive and Nernstian charge storage mechanisms

Supercapattery is the generic name for hybrids of supercapacitor and rechargeable battery. Batteries store charge via Faradaic processes, involving reversible transfer of localised or zone-delocalised valence electrons. The former is governed by the Nernst equation. The latter leads to pseudocapacitance (or Faradaic capacitance) which may be differentiated from electric double layer capacitance with spectroscopic assistance such as electron spin resonance. Because capacitive storage is the basis of supercapacitors, the combination of capacitive and Nernstian mechanisms has dominated supercapattery research since 2018, covering nanostructured and compounded metal oxides and sulphides, water-in-salt and redox active electrolytes and bipolar stacks of multicells. The technical achievements so far, such as specific energy of 270 Wh/kg in aqueous electrolyte, and charging–discharging for more than 5000 cycles, benchmark a challenging but promising future of supercapattery.     

High-Voltage Rechargeable Alkali–Acid Zn–PbO2 Hybrid Battery

Aqueous rechargeable batteries have attracted attention owning to their advantages of safety, low cost, and sustainability, while the limited electrochemical stability window (1.23 V) of water leads to their failure in competition with organic-based lithium-ion batteries. Herein, we report an alkali–acid Zn–PbO₂ hybrid aqueous battery obtained by coupling an alkaline Zn anode with an acidic PbO₂ cathode. It shows the capability to deliver an impressively high open-circuit voltage (V_oc) of 3.09 V and an operate voltage of 2.95 V at 5 mA cm⁻², thanks to the contribution of expanding the voltage window and the electrochemical neutralization energy from the alkali–acid asymmetric-electrolyte hybrid cell. The hybrid battery can potentially deliver a large area capacity over 2 mAh cm⁻² or a high energy density of 252.39 Wh kg⁻¹ and shows almost no fading in area capacity over 250 charge–discharge cycles.     

In situ Observation of Evolving H2 and Solid Electrolyte Interphase Development at Potassium Insertion Materials within Highly Concentrated Aqueous Electrolytes

The solid-electrolyte interphase (SEI) is key to stable, high voltage lithium-ion batteries (LIBs) as a protective barrier that prevents electrolyte decomposition. The SEI is thought to play a similar role in highly concentrated water-in-salt electrolytes (WISEs) for emerging aqueous batteries, but its properties remain unknown. In this work, we utilized advanced scanning electrochemical microscopy (SECM) and operando electrochemical mass spectrometry (OEMS) techniques to gain deeper insight into the SEI that occurs within highly concentrated WISEs. As a model, we focus on a 55 mol/kg K(FSA)_0.6(OTf)_0.4 electrolyte and a 3,4,9,10-perylenetetracarboxylic diimide negative electrode. For the first time, our work showed distinctly passivating structures with slow apparent electron transfer rates alike to the SEI found in LIBs. In situ analyses indicated stable passivating structures when PTCDI was stepped to low potentials (≈−1.3 V vs. Ag/AgCl). However, the observed SEI was discontinuous at the surface and H₂ evolution occurred as the electrode reached more extreme potentials. OEMS measurements further confirmed a shift in the evolution of detectable H₂ from −0.9 V to <−1.4 V vs. Ag/AgCl when changing from dilute to concentrated electrolytes. In all, our work shows a combined approach of traditional battery measurements with in situ analyses for improving characterization of other unknown SEI structures.     

Ion Conducting Polyethylene Oxide-Based Polymer Electrolyte Dopped with Ionic Liquid-Trifluoromethanesulfonic Chloride

In last few decades, polymer electrolyte is the most promising candidate for the fabrication of electrochemical devices. In current work, the influence of adding the room-temperature ionic liquid (trifluoromethanesulfonic chloride – CClF3O2S) in polyethylene oxide (PEO): ammonium iodide (NH4I) polymer electrolyte has been studied. The IL-doped polymer electrolyte films are synthesized by solution casting method with varying stoichiometric ratios. Several experimental techniques including optical polarizing microscope, impedance spectroscopy, X-ray diffraction, Linear sweep voltammetry, Ionic transference number thermal analysis, and electrical conductivity measurements at room temperature have been studied in detail. The complex material's maximum conductivity has been determined to be 3.3 × 10⁻⁵ S cm⁻¹ at room temperature. The POM images show the increase in amorphous region which further confirm the improvement in ionic conductivity. Ionic transference number 0.96 shows the system is purely ionic in nature. The ESW of the IL doped polymer electrolyte is also sawed to be 3.32 V which is suitable for the fabrication of electrochemical devices.     

Realizing high-voltage aqueous zinc-ion batteries with expanded electrolyte electrochemical stability window

Aqueous zinc-ion batteries(AZIBs) have aroused significant research interest around the world in the past decade. The use of low-cost aqueous electrolytes and a metallic Zn anode with a suitable redox potential and high energy density make AZIBs a potential alternative to commercial Li-ion batteries in the development of next-generation batteries. However, owing to the narrow electrochemical stability window(ESW) of aqueous electrolytes, the choice of cathode materials is limited, because of whi...     

N,O co-doped porous carbon with rich pseudocapacitive groups exhibiting superior energy density in an acidic 2.4 V Li2SO4 electrolyte

Designing a carbon material with a unique composition and surface functional groups for offering high specific capacity in a wide voltage window is of great significance to improve the energy density for the supercapacitor in a cheap and eco-friendly aqueous electrolyte. Herein, we develop an efficient strategy to synthesize a N, O co-doped hierarchically porous carbon (NODPC-1.0) with moderate specific surface area and pore volume as well as rich heteroatoms using a deep eutectic solvent (DES) as an activator. It is found that NODPC-1.0 with a large proportion of pseudocapacitive functional groups (pyrrole-N, pyridine-N and carbonyl-quinone) can work stable in an acidic 2 mol/L Li₂SO₄ (pH 2.5) electrolyte, exhibiting specific capacities of 375 and 186 F/g at the current densities of 1.0 and 100 A/g, respectively. Also, the assembled symmetric capacitor using the NODPC-1.0 as the active material and 2 mol/L acidic Li₂SO₄ (pH 2.5) as the electrolyte shows an outstanding energy density of 74.4 Wh/kg at a high power density of 1.44 kW/kg under a broad voltage window (2.4 V). Relevant comparative experiments indicate that H⁺ of the acidic aqueous electrolyte plays a crucial part in enhancement the specific capacity, and the abundant pseudocapacitive functional groups on the surface of the NODPC-1.0 sample play the key role in the improvement of electrochemical cycle stability under a broad voltage window.     

Origins of Boosted Charge Storage on Heteroatom-Doped Carbons

Although tremendous efforts have been devoted to understanding the origin of boosted charge storage on heteroatom-doped carbons, none of the present studies has shown a whole landscape. Herein, by both experimental evidence and theoretical simulation, it is demonstrated that heteroatom doping not only results in a broadened operating voltage, but also successfully promotes the specific capacitance in aqueous supercapacitors. In particular, the electrolyte cations adsorbed on heteroatom-doped carbon can effectively inhibit hydrogen evolution reaction, a key step of water decomposition during the charging process, which broadens the voltage window of aqueous electrolytes even beyond the thermodynamic limit of water (1.23 V). Furthermore, the reduced adsorption energy of heteroatom-doped carbon consequently leads to more stored cations on the heteroatom-doped carbon surface, thus yielding a boosted charge storage performance.     

High-Current Capable and Non-Flammable Protic Organic Electrolyte for Rechargeable Zn Batteries

Rechargeable Zinc batteries (RZBs) are considered a potent competitor for next-generation electrochemical devices, due to their multiple advantages. Nevertheless, traditional aqueous electrolytes may cause serious hazards to long-term battery cycling through fast capacity fading and poor Coulombic efficiency (CE), which happens due to complex reaction kinetics in aqueous systems. Herein, we proposed the novel adoption of the protic amide solvent, N-methyl formamide (NMF) as a Zinc battery electrolyte, which possesses a high dielectric constant and high flash point to promote fast kinetics and battery safety simultaneously. Dendrite-free and granular Zn deposition in Zn-NMF electrolyte assures ultra-long lifespan of 2000 h at 2.0 mA cm⁻²/2.0 mAh cm⁻², high CE of 99.57 %, wide electrochemical window (≈3.43 V vs. Zn²⁺/Zn), and outstanding durability up to 10.0 mAh cm⁻². This work sheds light on the efficient performance of the protic non-aqueous electrolyte, which will open new opportunities to promote safe and energy-dense RZBs.     

Study on electrochemical properties of Pb(BF4)2 electrolyte for improvement of cycle lifetime and efficiency in soluble lead flow batteries

The electrochemical characteristics of a soluble lead flow battery are key factors for the improvement of its cyclic lifetime and efficiency. In this work, we suggested aqueous solution of Pb(BF₄)₂ and HBF₄ as the electrolytes of soluble lead flow battery and observed the kinetic behavior of the electrode by cyclic voltammetry of the cyclic lifetime, quantum efficiency and voltage efficiency of the SLFB with respect to concentration of the electrolyte. Both the density and viscosity of the electrolyte increase with an increasing [Pb(BF₄)₂]. The conductivity exhibits a peak at 1.5 mol/L Pb(BF₄)₂ when [HBF₄] is below 1.0 mol/L. According to the cyclic voltammetry, the electrode process is fast with a low overvoltage and no additional reactions for the aqueous solution with 1.5 mol/L Pb(BF₄)₂ and 0.25 mol/L HBF₄. Overall, for the aqueous solution of 1.5 mol/L Pb(BF₄)₂ and 0.25 mol/L HBF₄, the battery showed a best performance with the cyclic lifetime being 48 times, the average quantum efficiency 88%, the voltage efficiency 70% and the battery efficiency 62%. These results are promising for fresh parameters of the electrolyte that could play a major role for the improvement of SLFBs.     

Origins of Boosted Charge Storage on Heteroatom‐Doped Carbons

Although tremendous efforts have been devoted to understanding the origin of boosted charge storage on heteroatom‐doped carbons, none of the present studies has shown a whole landscape. Herein, by both experimental evidence and theoretical simulation, it is demonstrated that heteroatom doping not only results in a broadened operating voltage, but also successfully promotes the specific capacitance in aqueous supercapacitors. In particular, the electrolyte cations adsorbed on heteroatom‐doped carbon can effectively inhibit hydrogen evolution reaction, a key step of water decomposition during the charging process, which broadens the voltage window of aqueous electrolytes even beyond the thermodynamic limit of water (1.23 V). Furthermore, the reduced adsorption energy of heteroatom‐doped carbon consequently leads to more stored cations on the heteroatom‐doped carbon surface, thus yielding a boosted charge storage performance.     

The network gel polymer electrolyte based on poly(acrylate-co-imide) and its transport properties in lithium ion batteries

Poly (acrylate-co-imide)-based gel polymer electrolytes are synthesized by in situ free radical polymerization. Infrared spectroscopy confirms the complete polymerization of gel polymer electrolytes. The ionic conductivity of gel polymer electrolytes are measured as a function of different repeating EO units of polyacrylates. An optimal ionic conductivity of the poly (PEGMEMA₁₁₀₀-BMI) gel polymer electrolyte is determined to be 4.8 × 10^–3 S/cm at 25 °C. The lithium transference number is found to be 0.29. The cyclic voltammogram shows that the wide electrochemical stability window of the gel polymer electrolyte varies from −0.5 to 4.20 V (vs. Li/Li⁺). Furthermore, we found the transport properties of novel gel polymer electrolytes are dependent on the EO design and are also related to the rate capability and the cycling ability of lithium polymer batteries. The relationship between polymer electrolyte design, lithium transport properties and battery performance are investigated in this research.     

Solid asymmetric electrochemical capacitors using proton-conducting polymer electrolytes

Solid asymmetric electrochemical capacitors (EC) using polyvinyl alcohol (PVA)–heteropoly acid (HPA) electrolytes and RuO₂–graphite electrodes were developed. The devices were about 0.2 mm thick and had a working voltage window of 0–1.5 V, 50% wider than that of any proton-conducting symmetric EC. Pseudocapacitance from HPA contributes to the total capacitance of the asymmetric EC within a certain potential window. The PVA–HPA polymers have been proven to function both as electrolyte and as pseudocapacitive electrode material in EC cells.     

An aqueous rechargeable lithium battery based on doping and intercalation mechanisms

An aqueous rechargeable lithium battery (ARLB) using an electroactive polymer, polypyrrole (PPy), as a negative electrode; a lithium ion intercalation compound LiCoO₂ as a positive electrode; and Li₂SO₄ aqueous solution as an electrolyte and its working mechanism are described. The charge/discharge process is associated with the doping/un-doping of anions at the negative electrode and intercalation/deintercalation of lithium ions at the positive electrode. The average output voltage of the PPy//LiCoO₂ battery is about 0.85 V. This battery exhibits excellent cycling performance. This new technology solves the major problem of poor cycling life of ARLBs and will provide a new strategy to explore advanced energy storage and conversion systems.