Conducting Electrolyte Membranes Research Articles

Robust Solid-State Na-CO2 Battery with Na2.7Zr2Si2PO11.7F0.3-PVDF-HFP Composite Solid Electrolyte and Na15Sn4/Na Anode.

Solid-state Na-CO2 batteries are a kind of energy storage devices that can immobilize and convert CO2. They have the advantages of both solid-state batteries and metal-air batteries. High-performance solid electrolyte and electrode materials are important for improving the performance of solid-state Na-CO2 batteries. In this work, we investigate the influence of fluorine doping on the structure and ionic conductivity of Na3Zr2Si2PO12 (NZSP). An ionic conductive solid electrolyte membrane was prepared by compositing the inorganic solid electrolyte Na2.7Zr2Si2PO11.7F0.3 (NZSPF3) with poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP). It shows an ionic conductivity of up to 2.17 × 10-4 S cm-1 at room temperature, a high sodium ionic transfer number of ∼0.70, a broad electrochemical window of ∼5.18 V, and better mechanical strength. Furthermore, we studied the Na15Sn4/Na composite foil with the ability to inhibit dendrite as the anode for solid-state Na-CO2 batteries. Through density functional theory (DFT) calculations, the Na15Sn4 particle has been verified with a strong sodiophilic property, which reduces the nucleation barrier during the deposition process, leading to a lower overpotential. The symmetric cell assembled with the composite solid-state electrolyte NZSPF3-PVDF-HFP and Na15Sn4/Na composite anode can inhibit the growth of Na dendrites effectively and maintain the stability of the whole cell structure. Solid-state Na-CO2 batteries assembled with Ru-carbon nanotube (Ru-CNTs) as cathode catalysts exhibit a high discharge capacity of 6371.8 mAh g-1 at 200 mA g-1, excellent cycling stability for 1100 h, and good rate performance. This work provides a promising strategy for designing high-performance solid-state Na-CO2 batteries.

Highly conductive and nonflammable boron-based gel type single ion conducting electrolyte membranes toward high-safety and dendrite-free lithium metal batteries

Single-ion conducting polymer electrolytes (SIPEs), having a high Li-ion transference number (tLi+) close to unity, hold great promise for circumventing the lithium dendrite growth of lithium metal batteries (LMBs). However, SIPEs synthesized by a simple method that also simultaneously fulfill the requirements of high ionic conductivity, good thermal stability, and remarkable fire resistance are still quite rare. Herein, a novel single-ion conducting fibrous membrane (es-DDBSB-Li) based on borate anions of single-ion conductors (denoted as DDBSB-Li) and poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) as a binder was prepared via facile electrospinning technology. The introduction of borate anions promotes the homogeneous distribution of highly concentrated Li-ions and accelerates the ionic migration, leading to an enhanced ionic conductivity of 3.75 × 10−4 S cm−1 at 25 °C for DDBSB-Li saturated with organic solvents. Additionally, the thermally stable and nonflammable DDBSB-Li endows the resultant fibrous membranes to possess good high-temperature stability and fire-resistance properties. The Li||Li cell using this electrolyte displays a high tLi+ of 0.91 and effectively mitigates concentration polarization, thereby enabling a remarkably stable cyclic performance over 2000 h at 2.5 mA cm−2 at 25 °C. Also, this gel SIPE-based Li||LiFePO4 battery demonstrates superior rate performances up to 3C and delivers long-term cycling durability over 600 cycles under 1C at 25 °C. We believe this novel gel SIPE-based fibrous membrane has great potential for practical application in high-safety and dendrite-proof LMBs.

Flexible, high-temperature-resistant, highly conductive, and porous siloxane-based single-ion conducting electrolyte membranes for safe and dendrite-free lithium-metal batteries

High-temperature-resistant polymer electrolyte membranes with satisfactory Li-ion transference number (tLi+) and ionic conductivity is desirable for the application in safe and dendrite-proof lithium metal batteries (LMBs). In this study, siloxane-based single-ion conducting polymer electrolyte (SIPE) membranes with high porosity are fabricated by in-situ sol-gel and non-solvent induced phase separation methods. Benefiting from the well-designed three-dimensional interpenetrating polymer network, the thermal and dimensional ability of the as-developed membranes are significantly enhanced. In addition, the porous structure enabling a high liquid electrolyte uptake (468.6%) combines with the intrinsic nature of SIPE for the membranes, giving rise to much higher ionic conductivity of 1.72 × 10−3 S cm−1 and tLi+ of 0.72 compared to commercial polypropylene (PP) separators (3.56 × 10−4 S cm−1 and 0.33) at 25 °C. Such excellent electrochemical properties are beneficial to inhibiting the Li dendrites and impeding short-circuiting of batteries. As expected, the Li/Li symmetrical cells show a stable galvanostatic Li plating/stripping cycling performance with a low overpotential of more than 400 h at 2 mA cm−2. Remarkably, the LiFePO4/Li batteries using the membranes exhibit impressive rate capacity and superior cycling ability compared to the batteries assembled with PP separators. This novel design and exciting results offer tremendous potential in the construction of dendrite-free LMBs with high safety.

Flexible, High-Temperature-Resistant, Highly Conductive, and Porous Siloxane-Based Single-Ion Conducting Electrolyte Membranes for Safe and Dendrite-Free Lithium-Metal Batteries

Flexible, High-Temperature-Resistant, Highly Conductive, and Porous Siloxane-Based Single-Ion Conducting Electrolyte Membranes for Safe and Dendrite-Free Lithium-Metal Batteries

Designing double comb copolymer as highly lithium ionic conductive solid-state electrolyte membranes

We present highly mechanical strength and ionic conductive solid-state electrolyte membranes based on amphiphilic double comb copolymer, i.e., poly(vinylidene chloride)-graft-poly(methyl methacrylate) (PVDC-g-PMMA) synthesized through atomic transfer radical polymerization (ATRP). Well-defined nanophase-separated amphiphilic double comb copolymers are complexed with two types of Li salts (LiTFSI and LiClO4) to form a solid-state electrolyte membrane. Compared to other types, the highest ionic conductivity of the solid-state electrolyte membranes is observed in an amphiphilic double comb copolymer with PVDC-g-PMMA/LiTFSI, due to the synergy of the dissociability of TFSI− and the hopping transport of lithium ions facilitated by PMMA chains. Moreover, the lowest activation energy (0.15 eV) and excellent ionic conductivity (1.3 × 10−3 S cm−1) are also observed in the same solid-state electrolyte membranes at room temperature.

Nanostructured Polymer Composite Electrolytes with Self-Assembled Polyoxometalate Networks for Proton Conduction

Nanostructured Polymer Composite Electrolytes with Self-Assembled Polyoxometalate Networks for Proton Conduction

Lithiated polyanion supported Li1.5Al0.5Ge1.5(PO4)3 composite membrane as single-ion conducting electrolyte for security and stability advancement in lithium metal batteries

Polymer electrolytes with single ion conducting behavior, high ion conductivity and fire-resistant play a vital role in suppressing lithium dendrite growth for high security, stability and high energy density of rechargeable lithium metal batteries (LMBs). Herein, we develop a gel single-ion composite electrolyte (SICE) by incorporating Li1.5Al0.5Ge1.5(PO4)3 nanoparticles into alternating diblock lithiated bis(benzene sulfonyl)imide polyanion with repeating ether groups. Li1.5Al0.5Ge1.5(PO4)3 nanoparticles can significantly endow SICE membrane more Li+ transportation channels, which offers elevated ionic conductivity (8.3 × 10−4 S cm−1) and Li+ transference number (0.93) to promote homogenously Li+ deposition in lithium foil for dendrite suppression. As a result, a highly reversible Li plating/stripping in a Li symmetric cell enables to run stably for at least 1600 h at 0.5 mA cm−2. Furthermore, Li1.5Al0.5Ge1.5(PO4)3 nanoparticles increases the electrochemical window of SICE up to 4.6 V (vs. Li+/Li), meanwhile enhances the thermal stability and fire-resistance, and promotes fast electrolyte wetting. Accordingly, a single-ion conducting LiFePO4/Li cell assembled with optimized SICE membrane endowed a high capacity retention of 92.9% after 800 cycles at 1 C, superior to the commercial LiPF6/PP system. This work paves a scalable and deployable way for design of advanced single-ion conducting electrolyte membranes to develop secure and stable LMBs.

(Invited) HT-PEM Fuel Cell Development for Product Definition and Scaleup

“High Temperature” Proton conducting Electrolyte Membrane (HT-PEM) fuel cells offer an attractive method of converting renewable energy carriers (e.g. methanol) to electricity, especially remotely and in transportation. A particular advantage is their ability to digest reformates containing a few % CO, thus not requiring high purity hydrogen.Membranes of polybenzimidazole (PBI) allow for operation in the 150-200 °C range, as they do not require liquid water for continuous moisturizing as do fluorinated polymer membranes. This temperature range is advantageous for the typically used Pt catalyst, as the passivation by CO is limited.A central challenge is cost reduction, with particular focus on Pt loading of electrodes. The critical trade-off is against lifetime and electrode performance while maintaining adequate tolerance to CO and fuel trace impurities. These limitations are sensitive to system and customer parameters via selection of fuel, reformer characteristics and operation mode.Results will be presented and discussed on single cell lifetime in the 10.000 h range; objectives and results in electrode development and Pt loading, as well as results on methanol slip testing, start-stop testing and testing at elevated pressure. Lessons from early upscaling will be shared.

Ultrathin Aramid/COF Heterolayered Membrane for Solid-State Li-Metal Batteries.

Ultrathin, ultrastrong, and highly conductive solid-state polymer-based composite electrolytes have long been exploited for the next-generation lithium-based batteries. In particular, the lightweight membranes that are less than tens of microns are strongly desired, aiming to maximize the energy densities of solid-state batteries. However, building such ideal membranes are challenging when using traditional materials and fabrication technologies. Here we reported a 7.1 μm thick heterolayered Kevlar/covalent organic framework (COF) composite membrane fabricated via a bottom-up spin layer-by-layer assembly technology that allows for precise control over the structure and thickness of the obtained membrane. Much stronger chemical/mechanical interactions between cross-linked Kevlar and conductive 2D-COF building blocks were designed, resulting in a highly strong and Li+ conductive (1.62 × 10-4 S cm-1 at 30 °C and 4.6 × 10-4 S cm-1 at 70 °C) electrolyte membrane that can prevent solid-state batteries from short-circuiting after over 500 h of cycling. All-solid-state lithium batteries using this membrane enable a significantly improved energy density.

Synthesis, Phase Structure, and Ion Conductivity of Poly(p-phenylene) Functionalized with Lithium Trifluoromethanesulfonimide and Tetra(ethylene Oxide) Side Chains

Rigid-rod polymers tethered with delocalized anions and flexible ion conductive side chains present a synthetic pathway toward thin, single-ion conducting electrolyte membranes with low bulk resist...

A mediator-ion nitrobenzene - iodine nonaqueous redox flow battery with asymmetric solvents

Nonaqueous redox flow batteries (RFBs) are recently receiving growing attention as a promising candidate for grid energy storage applications. Primary benefits of nonaqueous RFBs over the aqueous RFBs are high cell voltage and ability to operate at low temperatures. However, due to the relatively low solubility of the redox couples in nonaqueous solvents and the crossover tendency of the redox species, the low energy density and limited cycle life of nonaqueous RFBs are two major concerns that hamper their applicability. We present here an asymmetric electrolyte approach that employs two different organic solvents in a single RFB to access high concentrations of both the anolyte (nitrobenzene (NB) in acetonitrile (ACN) solvent) and catholyte (iodine in tetraethylene glycol dimethyl ether (TEGDME) solvent). To avoid a crossover of the two redox-active solutions between the anode and cathode, a sodium-ion conductive solid electrolyte (Na3Zr2Si2PO12) membrane is employed as a separator. Sodium salts (NaClO4 or NaI) are added into the anolyte and catholyte as a supporting electrolyte. During the operation of the NB–I2 nonaqueous RFB, the electrochemical reactions of the redox couples are connected ionically by the shuttling of sodium ions through the Na3Zr2Si2PO12 membrane, i.e., Na+-ions are not involved in the electrode reactions but serve as mediator ions.

(Invited) HT-PEM Fuel Cell Development for Product Definition and Scaleup

“High Temperature” proton conducting electrolyte membrane (HT-PEM) fuel cells offer an attractive method of converting renewable energy carriers (e.g. methanol) to electricity, especially remotely and in transportation. A particular advantage is their ability to digest reformates containing a few % CO, thus not requiring high purity hydrogen.Membranes of polybenzimidazole (PBI) allow for operation in the 150-200 °C range, as they do not require liquid-water for continuous moisturizing as do classical fluorinated polymer membranes. This temperature range is advantageous for the typically used Pt catalyst, as passivation by CO is limited.A central challenge is cost reduction, with particular focus on Pt loading of electrodes. The critical trade-off is against lifetime and electrode performance while maintaining adequate tolerance to CO and fuel trace impurities. These limitations are sensitive to system and customer parameters via selection of fuel, reformer characteristics and operation mode.Results will be presented and discussed on single cell lifetime in the 10.000 h range; objectives and results in electrode development and Pt loading, as well as results on methanol slip testing, start-stop testing and testing at elevated pressure. Lessons from early upscaling will be shared.

Spray Pyrolysis Strategy for Preparation of Cathode-Supported Protonic Ceramic Fuel Cells

Since the discovery of proton conducting ceramic materials many research efforts have been focused into overcoming processing difficulties that hinder long-term stability and high performance of protonic ceramic fuel cells (PCFCs). Notably, high processing temperatures exceeding 1600 °C are often required to meet the microstructural demand of a dense proton conducting electrolyte membrane [1-2]. At this temperature range most potential electrode materials are chemically incompatible and therefore preclude the fabrication of electrode supported structures. In addition, for barium-based electrolytes (e.g. BaZr1-xYxO3-d) barium loss is anticipated resulting in lower conductivities and formation of structural defects. To overcome this major challenge thin film deposition techniques can be employed[3-4].Spray pyrolysis is a cost effective and simple deposition technique for preparation of ultra-thin films (i.e.<5μm) enabling the reduction of operating and processing temperatures of fuel cells [5]. Microstructures and electrochemical performance of multi-layered thin film PCFCs fabricated at exceptional low temperatures of 950 °C, are investigated. Large area 5x5cm2 cathode substrates of (La0.8Sr0.2)0.95MnO3-d prepared by tape casting were used for mechanical support of the cells. To modify and reduce the surface porosity of the substrate Ce0.9Gd0.1O3-d was coated to facilitate the deposition of the BaZr0.9Y0.1O3-d (BZY) thin electrolyte. Subsequently, the anode electrode was sprayed in two stages comprised by a nano-porous layer of BZY and infiltrated Ni particles. Characterization of the cathode-supported PCFCs was performed by electrical measurements operating up to 600°C and scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) techniques.[1] Shanwen Tao and John T.S. Irvine, Adv. Mater., 18, 1581-1584, (2006).[2] Peter Babilo and Sossina M. Haile, J. Am. Ceram. Soc., 88 2362-2368, (2005).[3] Kiho Bae et al., Adv. Energy Mater., 8, 1801315, (2018).[4] Georgios Tsimekas et al., ECS J. Solid State Sci. Technol.,6(8), P553-P560, (2017).[5] Georgios Tsimekas et al., ECS Trans., 70(1), 205-212, (2015).

NaClO4 added, corn and arrowroot starch based economical, high conducting electrolyte membranes for flexible energy devices

The increasing flexible and wearable electronic technology, demands cost effective and flexible energy devices which are safe to human body. Hence the benign biodegradable materials are becoming important class of materials for wearable energy devices. Starch is one such potential host renewable polymer which is abundant in nature and economical. Being a food ingredient, it is safe for human body. Its properties depends upon the amylose and amylopectin content in it and hence two different starches, corn (~ 27% amylose) and arrowroot (~ 15% amylose) are modified by sodium salt (NaClO4) and glutaraldehyde to develop flexible, transparent and free standing electrolyte membranes with high conductivity (> 10–3 S/cm). They have wide electrochemical stability window (> 2 V reaching upto 3.5 V) and low ESR. The relaxation time is of the order of µs and the cyclic voltammetry has indicated EDLC type of charge storage. At low frequency, the values of Cp/Cs are approaching to 1, indicating that all the available charges are polarizable and contributing to charge storage. The resonance frequency and frequency (f−45°) at which phase angle is −45°, are in kHz frequency range, i.e. the working frequency range is quite high. Electrolytes having corn starch (i.e. greater amount of amylose) have better performance on every electrochemical figure of merit.

Prospects and challenges of graphene based fuel cells

Abstract Novel characteristics of graphene have captured great attention of researchers for energy technology applications. Incorporation of graphene related hybrid and composite materials have demonstrated high performance and durability for fuel cell energy conversion devices. This article overviews graphene based materials for fuel cell technology applications such as electrodes additives, bipolar plates and proton conducting electrolyte membrane. The graphene dispersion over electrodes has revealed enhanced exposure of electrochemically active surface area for improved electro-catalytic activity towards fuel oxidation and oxidant reduction reactions. The issue of device stack durability and degraded performance due to corrosion of bipolar plates is discussed by incorporating graphene based materials. In proton exchange membrane devices, graphene as an electrolyte has shown an excellent performance towards high ionic conductivity and power density. The graphene incorporation in fuel cell devices has exhibited commendable performance and has bright future for commercial applications.

Redox Reaction of 2,2,6,6-Tetramethylpiperidine-1-Oxyl (TEMPO) in Lithium Bis(trifluoromethylsulfonyl)Amide-Tetraglyme Solvate Ionic Liquid

Electrode reactions of the redox couples dissolved in liquid electrolytes are able to be utilized for energy storage devices. Vanadium redox flow battery is one of the typical large-scaled energy storage devices using aqueous solutions for both anolye and catholyte.[1] The cell voltages of such aqueous redox flow batteries are often limited by the electrochemical potential window of water. In order to enlarge the cell voltage and increase energy density, it is necessary to use aprotic electrolytes having wider electrochemical potential windows. Typical aprotic electrolytes used for non-aqueous batteries like lithium-ion batteries are composed of organic solvents, which are volatile and flammable. However, such organic electrolytes are unfavorable especially for the large-scaled batteries requiring large amount of electrolytes because of safety in case of accident. Therefore, it is necessary to select less flammable electrolytes for non-aqueous redox flow batteries. The equimolar mixture of lithium bis(trifluoromethylsulfonly)amide (LiTFSA) and tetraglyme (G4) has been known to form the solvate ionic liquid, which is composed of a lithium-G4 complex, [Li(G4)]+, and TFSA–, and exhibits such properties as less volatility, less flammability, and wide electrochemical potential window, similar to those of ionic liquids.[2,3] The high concentration of lithium species in the solvate ionic liquid is also advantageous to the batteries using lithium ion as a charge carrier. Since the cathodic limit of the solvate ionic liquid is determined by deposition and dissolution of lithium, it is possible to construct the batteries combining a lithium anode with the redox reactions dissolved in the solvate ionic liquid using a lithium-ion conductive solid-state electrolyte as a separator between the anolyte and catholyte. We have already proposed the battery using the redox reaction of tris(2,2'-bipyridine)iron(III/II) complexes in the solvate ionic liquid with the cell voltage of about 4 V.[4] In the present study, the redox reaction of a stable free radical, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), was investigated in the solvate ionic liquid, [Li(G4)]TFSA, for application to the redox flow battery. Both TEMPO and its oxidized form, TEMPO+, were found to be soluble in the solvate ionic liquid at ambient temperature. The redox reaction of TEMPO+/TEMPO was observed in the cyclic voltammetry of a glassy carbon electrode in [Li(G4)]TFSA containing TEMPO. The diffusion of TEMPO was found to be faster than that of TEMPO+, probably due to coulombic interaction of TEMPO+ with the ions composing the solvate ionic liquid. The standard rate constant was determined by electrochemical impedance spectroscopy. The temperature dependence of the standard rate constant suggested the redox reaction of TEMPO+/TEMPO can be regarded as an outer-sphere electron transfer reaction with a small reorganization energy. The redox battery was constructed using a lithium anode and the catholyte of the solvate ionic liquid containing TEMPO with a thin lithium-ion conductive solid-state electrolyte membrane. The redox battery could be operated successfully with the average discharge voltage of 3.4 V.

Electrochemical Energy Storage with an Aqueous Quinone–Air Chemistry

Organic electrode materials such as quinones are drawing rising attention as promising redox-active materials for the development of rechargeable batteries. In aqueous solutions, the redox potential of quinones is dependent on the alkalinity and acidity of the medium. Under an alkaline condition, the oxidation potential of hydroquinone (existing as diphenolate) is ca. 0.8 V lower than that under an acidic condition. On the other hand, under an acidic condition, the reduction potential of oxygen is ca. 0.8 V higher than that under an alkaline condition. By taking these advantages, a Quinone-air cell with a rational voltage is strategically demonstrated with an alkaline anode electrolyte and an acidic cathode electrolyte, which are physically separated by a Na+-ion conductive solid-state electrolyte membrane. The Na+-ions shuttling through the solid-state membrane act as ionic mediators/messengers to sustain and link the redox reactions at the two electrodes.

Cover Feature: Nanofiber Single‐Ion Conducting Electrolytes: An Approach for High‐Performance Lithium Batteries at Ambient Temperature (ChemElectroChem 9/2017)

The Inside Back Cover picture illustrates a nanofiber single-ion conducting electrolyte membrane that is fabricated by using an electrospinning technique. By virtue of its nanofibrous morphology, it performs up to a 5 C charge/discharge rate under ambient temperature conditions. It delivers a discharge capacity 163 mAh g−1 at the 0.5 C rate, which is better than conventional liquid electrolytes. More information can be found in the Article by J.-T. Lee and co-workers on page 2178 in Issue 9, 2017 (DOI: 10.1002/celc.201700389).

Constructing Straight Polyionic Liquid Microchannels for Fast Anhydrous Proton Transport.

Polymeric ionic liquids (PILs) have triggered great interest as all solid-state flexible electrolytes because of safety and superior thermal, chemical, and electrochemical stability. It is of great importance to fabricate highly conductive electrolyte membranes capable to operate above 120 °C under anhydrous conditions and in the absence of mineral acids, without sacrificing the mechanical behavior. Herein, the diminished dimensional and mechanical stability of poly[1-(3H-imidazolium)ethylene]bis(trifluoromethanesulfonyl)imide has been improved thanks to its infiltration on a polybenzimidale (PBI) support with specific pore architecture. Our innovative solution is based on the synergic combination of an emerging class of materials and sustainable large-scale manufacturing techniques (UV polymerization and replication by microtransfer-molding). Following this approach, the PIL plays the proton conduction role, and the PBI microsieve (SPBI) mainly provides the mechanical reinforcement. Among the resulting electrolyte membranes, conductivity values above 50 mS·cm-1 at 200 °C and 10.0 MPa as tensile stress are shown by straight microchannels of poly[1-(3H-imidazolium)ethylene]bis(trifluoromethanesulfonyl)imide cross-linked with 1% of dyvinylbenzene embedded in a PBI microsieve with well-defined porosity (36%) and pore diameter (17 μm).

Investigation on lithium ion conductivity and structural stability of yttrium-substituted Li7La3Zr2O12

Advanced Li-air battery architecture demands a high Li+ conductive solid electrolyte membrane that is electrochemically stable against metallic lithium and aqueous electrolyte. In this work, an investigation has been carried out on the microstructure, Li+ conduction behaviour and structural stability of Li7La3-xYxZr2O12 (x = 0.125, 0.25 and 0.50) prepared by conventional solid-state reaction technique. The phase analysis of Li7La3-xYxZr2O12 (x = 0.125, 0.25 and 0.50) sintered at 1200 °C by powder X-ray diffraction (PXRD) and Raman confirms the formation of high Li+ conductive cubic phase (\( Ia\overline{3}d \)) lithium garnets. Among the investigated lithium garnets, Li7La2.75Y0.25Zr2O12 sintered at 1200 °C exhibits a maximized room temperature total (bulk + grain boundary) Li+ conductivity of 3.21 × 10−4 S cm−1 along with improved relative density of 96 %. The preliminary investigation on the structural stability of Li7La2.75Y0.25Zr2O12 in the solutions of 1 M LiCl, dist. H2O and 1 M LiOH at 30 °C/50 °C indicates that the Li7La2.75Y0.25Zr2O12 is relatively stable against 1 M LiCl and dist. H2O. Further electrochemical investigation is essential for practical application of Li7La2.75Y0.25Zr2O12 as protective solid electrolyte membrane in aqueous Li-air battery.