We characterized and studied a system of polymer-supported aqueous electrolytes containing a series of gels that exhibited double glass transitions and high conductivities. These electrolytes, composed of a 12 m aqueous solution of bis(trifluoromethanesulfonyl)imide and a monomer of poly(ethylene glycol) Diacrylate (PDA), were characterized, both before and after polymerization by ultraviolet irradiation, with thermoconductometry, rheometry, and glass transition analysis. For the gel compositions, the polymerization transformed the single liquid structure of the starting liquid into the double structure of the resulting gel constituting a polymer and a solution substructure, as evidenced by the associated transformations in the glass transition temperatures of these electrolytes. Individually, the polymer substructure in the gel gave it a storage modulus that rose rapidly with its PDA content, and the solution substructure, an unusually high conductivity. The generation of a polymer substructure in a gel also made the remaining solution substructure more conductive and less viscous, resulting in conductivity gains upon polymerization and in other unusual phenomena. The coexistence in the gel of two substructures, separately responsible for its mechanical and electrical properties, amenable to alterations in chemical composition and materials processing, holds great interest and promise for future electrolyte development.

With the increasing interest in highly concentrated electrolyte systems, correct determination of the cation transference number is important. Pulsed-field gradient NMR technique, which measures self-diffusion coefficients, is often applied on liquid electrolytes because of the wide accessibility and simple sample preparation. However, since the assumptions of this technique, that is, complete salt dissociation, all ions participating in motion, and all of them moving independently, no longer hold true in concentrated solutions, the transference numbers, thus obtained are often over-estimated. In the present work, impedance spectroscopy at a frequency range of 1 MHz to 0.1 mHz was used to examine the concentration effect on lithium-ion transference number under anion-blocking conditions TLi+abc for two electrolytes: lithium bis(fluorosulfonyl)imide (LiFSI) in sulfolane (SL) and lithium bis(trifluorosulfonyl)imide (LiTFSI) in tetraglyme (G4). The TLi+abc of the former was almost an order of magnitude higher than that of the latter. It also appeared to increase with increasing concentration while the latter followed an opposite trend. The faster Li+ transport in the SL system is attributed to the formation of a liquid structure consisting of extended chains/bridges of SL molecules and the anions, which facilitate a cation-hopping/ligand-exchanged-typed diffusion mechanism by partially decoupling the cations from the anions and solvent molecules. The G4 system, in contrast, is dominated by the formation of long-lived, stable [Li(G4)](+) solvation cages that results in a sluggish Li+ transport. The difference between the two transport mechanisms is discussed via comparison of the bulk ionic conductivity, viscosity, ion self-diffusion coefficients, and the Onsager transport coefficients.

Electrification of transportation and rising demand for grid energy storage continue to build momentum around batteries across the globe. However, the supply chain of Li-ion batteries is exposed to the increasing challenges of resourcing essential and scarce materials. Therefore, incentives to develop more sustainable battery chemistries are growing. Here we show an aqueous ZnCl2 electrolyte with introduced LiCl as supporting salt. Once the electrolyte is optimized to Li2ZnCl4⋅9H2O, the assembled Zn–air battery can sustain stable cycling over the course of 800 hours at a current density of 0.4 mA cm−2 between −60 °C and +80 °C, with 100% Coulombic efficiency for Zn stripping/plating. Even at −60 °C, >80% of room-temperature power density can be retained. Advanced characterization and theoretical calculations reveal a high-entropy solvation structure that is responsible for the excellent performance. The strong acidity allows ZnCl2 to accept donated Cl− ions to form ZnCl42− anions, while water molecules remain within the free solvent network at low salt concentration or coordinate with Li ions. Our work suggests an effective strategy for the rational design of electrolytes that could enable next-generation Zn batteries. Zinc batteries are receiving growing attention due to their sustainability merits not shared by lithium-ion technologies. Here the aqueous electrolyte design features unique solvation structures that render Zn–air pouch cell excellent cycling stability in a wide temperature range from −60 to 80 °C.

Aqueous rechargeable Zn metal batteries (RZMBs) are promising candidates for coupling with intermittent renewable energy sources to realize a carbon-neutral energy transition. However, irreversible issues of Zn metal anodes and a poor understanding of the interphasial chemistry severely limit the viability of RZMBs. Here, we demonstrate that the addition of an asymmetric alkylammonium cation, trimethylethyl ammonium-bis(trifluoromethylsulfonyl)imide (Me3EtN-TFSI), as a supporting salt into a traditional aqueous electrolyte results in improved Zn anode reversibility. Performance improvements are attributed to the formation of interphasial chemistries including ZnF2, ZnCO3, and fluoro-polymeric species, especially when combined with CO2. By tailoring the Zn interphase, this electrolyte exhibited excellent stability in Na2V6O16 $ 1.63H(2)O (HNVO)/Zn full cells, with a high specific capacity sustained (> 100 mAh g(-1)) over 1,000 cycles at 300 mA g(-1). A combination of experiments and modeling showed the importance of tuning interphases to further improve Zn reversibility and RZMBs.

Extensive efforts have been made to seek new battery chemistries based on multivalent working ions, with the aim to replace the mature lithium-ion batteries. These efforts were initially driven by the pursuit of higher capacity/energy, better safety and lower cost, and more recently have significantly intensified with the increasing concerns over the climate change, the limited resources of Co and Ni, and the anxieties over geopolitical as well as ethical risks of the corresponding supply chain. But how far are we from a practical multivalent battery? This talk rigorously examines the achievements made in MV batteries as reported in the current literature, while attempting to explore a pathway through the fog-of-war ahead of us.

Rechargeable Zn metal batteries (RZMBs) may provide a more sustainable and lower-cost alternative to established battery technologies in meeting energy storage applications of the future. However, the most promising electrolytes for RZMBs are generally aqueous and require high concentrations of salt(s) to bring efficiencies toward commercially viable levels and mitigate water-originated parasitic reactions including hydrogen evolution and corrosion. Electrolytes based on nonaqueous solvents are promising for avoiding these issues, but full cell performance demonstrations with solvents other than water have been very limited. To address these challenges, we investigated MeOH as an alternative electrolyte solvent. These MeOH-based electrolytes exhibited exceptional Zn reversibility over a wide temperature range, with a Coulombic efficiency > 99.5% at 50% Zn utilization without cell short-circuit behavior for > 1,800 h. More important, this remarkable performance translates well to Zn || metal-free organic cathode full cells, supporting < 6% capacity decay after > 800 cycles at -40 °C.

Polymerization via ultraviolet irradiation of LiTFSI21mH2O + PDA liquid mixtures turned most of these liquids into gels or solids. Here, LiTFSI21mH2O denotes a 21 m aqueous solution of lithium bis(trifluoromethanesulfonyl)imide, and PDA stands for a monomer of poly(ethylene glycol) diacrylate of Mn 575. Systematic thermoconductometric measurement on these electrolytes, both before and after the polymerization, showed the gel electrolytes to be considerably more conductive than their precursor liquid mixtures, especially at lower temperatures. A parallel measurement of glass transition temperature, θ g, revealed in these gel electrolytes a unique double glass transition enveloping two sub-transitions each with its own θ g’s. These and a number of other related experimental observations can be consistently and clearly explained based on the existence of a solution and a polymer substructure in the polymerized electrolytes, and on these substructures becoming codominant in the gels. The exceptional conductivity in these gel electrolytes points to a promising direction to formulating a polymer-supported aqueous electrolyte with a set of desirable physical traits and an uncompromising conductivity.

A thermoconductometric method, with a uniquely designed sample cell and an in situ polymerization procedure, was applied to a polymer-supported aqueous electrolyte of w PDA + (1− w )LiTFSI 21m H 2 O, where w is in mass fraction, PDA stands for a monomer of poly(ethylene glycol) diacrylate of Mn 575, and LiTFSI 21m H 2 O denotes a 21 m aqueous solution of lithium bis(trifluoromethanesulfonyl)imide. The thermoconductometry curves of temperature differential, Δ θ , and conductivity, κ , were collected for these samples in the w range of (0, 0.5) and θ range of (−70, 60) °C. The Δ θ curves yielded a partial phase diagram revealing a high degree of compatibility between PDA and LiTFSI 21m H 2 O, and the κ curves formed for the unpolymerized and polymerized electrolytes a pair of data sets which were fitted to yield a pair of κ ( w , θ ) functions for the definition of a conductivity loss function upon polymerization. This function showed that a significant portion of the conductivity was retained upon polymerization in a large part of the ( w , θ ) space, where a safe and robust electrolyte with a decent conductivity could be formulated. Molecular and thermodynamic considerations were applied to correlate and discuss the results of the Δ θ and κ measurements.

The lithium (Li) metal polymer battery (LMPB) is a promising candidate for solid-state batteries with high safety. However, high voltage stability of such a battery has been hindered by the use of polyethylene oxide (PEO), which oxidizes at a potential lower than 4 V versus Li. Herein, we adopt the polymer-in-salt electrolyte (PISE) strategy to circumvent the disadvantage of the PEO-lithium bis(fluorosulfonyl)imide (LiFSI) system with EO/Li ≤ 8 through a dry ball-milling process to avoid the contamination of the residual solvent. The obtained solid-state PISEs exhibit distinctly different morphologies and coordination structures which lead to significant improvement in oxidative stability. P(EO)1LiFSI has a low melting temperature, a high ionic conductivity at 60 °C, and an oxidative stability of ∼4.5 V versus Li/Li+. With an effective interphase rich in inorganic species and a good stability of the hybrid polymer electrolyte toward Li metal, the LMPB constructed with Li||LiNi1/3Co1/3Mn1/3O2 can retain 74.4% of capacity after 186 cycles at 60 °C under the cutoff charge voltage of 4.3 V. The findings offer a promising pathway toward high-voltage stable polymer electrolytes for high-energy-density and safe LMPBs.

As aqueous electrolytes have been gaining renewed interest in battery research, understanding how the properties of these electrolytes change at icing and other phase transitions becomes imperative for both monitoring such occurrences in extreme environments and identifying the service temperature limits for the battery devices containing such electrolytes. In this study, we devised a coupled thermoconductometric measurement system in which the impedance and temperature of an electrolyte sample, along with the temperature of a reference, were concurrently, continuously, and speedily measured while the sample was subjected to an external temperature ramp across its phase transitions. The synchronized curves of conductivity and temperature differential thus obtained were used effectively to study and understand the different processes in the phase transitions and their impact on the apparent conductivity of the electrolytes. The solutions of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in water and in ethylene carbonate (EC) were chosen as the representative aqueous and nonaqueous electrolytes, respectively. Results showed that while the differential temperature signal at a phase transition was always pronounced, a change in apparent conductivity could be deceptively unremarkable and easily escape notice, which may lead to erroneous estimation for the temperature limits of the electrolytes. It was also revealed that the nature of the solvent, the degree of supercooling, and the electrolyte composition were factors in determining the magnitude of the apparent conductivity drop at a phase transition. Furthermore, the apparent conductivity curves of a precipitated hypoeutectic LiTFSI + EC electrolyte were accurately calculated from the properties and proportion of its remaining liquid, using several functional relationships made available by an experimentally mapped phase diagram and a measurement-based conductivity function plus a form of mixing rule with an adjustable factor to account for the spatial configuration of the precipitates. Such in-depth understanding of ion-transport behavior at phase transition sets the foundation for quantitative prediction of electrolyte properties at extreme temperatures.

As an important alternative for Li-based battery technology, rechargeable zinc metal batteries promise attractive advantages including safety, high volumetric energy density and low cost; however, such benefits cannot be unlocked unless Zn reversibility meets stringent commercial viability1-2.

Metallic zinc is an ideal anode due to its high theoretical capacity (820 mAh g −1 ), low redox potential (−0.762 V versus the standard hydrogen electrode), high abundance and low toxicity. When used in aqueous electrolyte, it also brings intrinsic safety, but suffers from severe irreversibility. This is best exemplified by low coulombic efficiency, dendrite growth and water consumption. This is thought to be due to severe hydrogen evolution during zinc plating and stripping, hitherto making the in-situ formation of a solid–electrolyte interphase (SEI) impossible. Here, we report an aqueous zinc battery in which a dilute and acidic aqueous electrolyte with an alkylammonium salt additive assists the formation of a robust, Zn 2+ -conducting and waterproof SEI. The presence of this SEI enables excellent performance: dendrite-free zinc plating/stripping at 99.9% coulombic efficiency in a Ti||Zn asymmetric cell for 1,000 cycles; steady charge–discharge in a Zn||Zn symmetric cell for 6,000 cycles (6,000 h); and high energy densities (136 Wh kg −1 in a Zn||VOPO 4 full battery with 88.7% retention for >6,000 cycles, 325 Wh kg −1 in a Zn||O 2 full battery for >300 cycles and 218 Wh kg −1 in a Zn||MnO 2 full battery with 88.5% retention for 1,000 cycles) using limited zinc. The SEI-forming electrolyte also allows the reversible operation of an anode-free pouch cell of Ti||Zn x VOPO 4 at 100% depth of discharge for 100 cycles, thus establishing aqueous zinc batteries as viable cell systems for practical applications.

Aqueous rechargeable zinc metal batteries promise attractive advantages including safety, high volumetric energy density, and low cost; however, such benefits cannot be unlocked unless Zn reversibility meets stringent commercial viability. Herein, we report remarkable improvements on Zn reversibility in aqueous electrolytes when phosphonium-based cations are used to reshape interfacial structures and interphasial chemistries, particularly when their ligands contain an ether linkage. This novel aqueous electrolyte supports unprecedented Zn reversibility by showing dendrite-free Zn plating/stripping for over 6400 h at 0.5 mA cm −2 , or over 280 h at 2.5 mA cm −2 , with coulombic efficiency above 99 % even with 20 % Zn utilization per cycle. Excellent full cell performance is demonstrated with Na 2 V 6 O 16 ⋅1.63 H 2 O cathode, which cycles for 2000 times at 300 mA g −1 . The microscopic characterization and modeling identify the mechanism of unique interphase chemistry from phosphonium and its functionalities as the key factors responsible for dictating reversible Zn chemistry

A water-in-salt electrolyte (WiSE) offers an electrochemical stability window much wider than typical aqueous electrolytes but still falls short in accommodating high-energy anode materials, mainly because of the enrichment of water molecules in the primary solvation sheath of Li+. Herein, we report a new strategy in which a non-Li cosalt was introduced to alter the Li+-solvation sheath structure. The presence of an asymmetric ammonium salt (Me3EtN center dot TFSI) in water increases the solubility of LiTFSI by two times, pushes the salt/water molar ratio from 0.37 in WiSE to an unprecedented value of 1.13, and significantly suppresses the water activity in both bulk electrolyte and the Li+-solvation sheath. This new 63 m (mol kg(solvent)(-1)) aqueous electrolyte (42 m LiTFSI + 21 m Me3EtN center dot TFSI) offers a wide potential window of 3.25 V and supports a 2.5 V aqueous Li-ion battery (LiMn2O4//Li4Ti5O12) to deliver a high energy density of 145 Wh kg(-1) stably over 150 cycles.

A system of electrolytes using water as a solvent was successfully used to support a typical lithium-ion battery chemistry that operates at 3.7V-4.2 V using standard ultraviolet-cured acrylic-based polymers as hydrophobic barriers. The aqueous electrolyte is contained in a system of poly(ethylene glycol) acrylate polymers crosslinked to produce an electrolyte gel that has electrochemical properties similar to that of the liquid phase component. The electrolyte gels have elastic moduli in the kPa range, making them soft enough to tolerant flexing, cutting, and blunt force impacts while keeping the electrodes covered and safe from shorting. While batteries based on water-in-salt electrolyte provides intrinsic safety that is otherwise unavailable from typical non-aqueous electrolytes, acrylate-based aqueous gel electrolytes offer the potential of large-scale manufacturing owing to the relatively low volatility of the electrolyte components and the low complexity of the proposed manufacturing process.

A binary phase diagram of zinc bis(trifluoromethylsulfonyl)imide and water, Zn(TFSI)(2)-H2O, was mapped out from 0 to 0.375 mole fraction of salt, revealing the existence of di-, tetra-, hexa-, and octahydrates of Zn(TFSI)(2). Samples of this solution in molalities from 0.060 to 4.2 were also measured for their electrolytic conductivity in the temperature range of (-40, 80) degrees C. The conductivity data were further fitted with a Vogel-Fulcher-Tammann-based function and compared with those of LiTFSI-H2O obtained previously, showing Zn(TFSI)(2)-H2O to be almost as conductive as LiTFSI-H2O despite the divalence of the cation.

With high energy density and improved safety, rechargeable battery chemistries with a zinc (Zn) metal anode offer promising and sustainable alternatives to those based on lithium metal or lithium-ion intercalation/alloying anode materials; however, the poor electrochemical reversibility of Zn plating/stripping, induced by parasitic reactions with both aqueous and non-aqueous electrolytes, presently limits the practical appeal of these systems. Although recent efforts in rechargeable Zn metal batteries (RZMBs) have achieved certain advancements in Zn metal reversibility, as quantified by the Coulombic efficiency (CE), a standard protocol for CE has not been established, and results across chemistries and systems are often conflicting. More importantly, there is still an insufficient understanding regarding the critical factors dictating Zn reversibility. In this work, a rigorous, established protocol for determining CE of lithium metal anodes is transplanted to the Zn chemistry and is used for systematically examining how a series of factors including current collector chemistry, current density, temperature, and the upper voltage limit during stripping affect the measured reversibility of different Zn electrolytes. With support from density functional theory calculations, this standardized Zn CE protocol is then leveraged to identify an important correlation between electrolyte solvation strength toward Zn2+ and the measured Zn CE in the corresponding electrolyte, providing new guidance for future development and evaluation of Zn electrolytes.

Rechargeable zinc metal batteries (RZMBs) offer a compelling complement to existing lithium ion and emerging lithium metal batteries for meeting the increasing energy storage demands of the future. Multiple recent reports have suggested that optimized electrolytes resolve a century-old challenge for RZMBs by achieving extremely reversible zinc plating/stripping with Coulombic efficiencies (CEs) approaching 100%. However, the disparity among published testing methods and conditions severely convolutes electrolyte performance comparisons. The lack of rigorous and standardized protocols is rapidly becoming an impediment to ongoing research and commercialization thrusts. This Perspective examines recent efforts to improve the reversibility of the zinc metal anode in terms of key parameters, including CE protocols, plating morphology, dendrite formation and long-term stability. Then we suggest the most appropriate standard protocols for future CE determination. Finally, we envision future strategies to improve zinc/electrolyte stability so that research efforts can be better aligned towards realistic performance targets for RZMB commercialization. Zinc metal batteries (ZMBs) provide a promising alternative to lithium metal batteries but share the formidable challenges in reversibility. The authors discuss the key performance metrics of ZMBs and propose a protocol to assess the true reversibility of zinc metal anodes.

Highly concentrated solutions of lithium salts in water have made sweeping strides from the time in the early 2010s where aqueous electrolytes could operate a battery within an electrochemical window no more than 1.5V wide. In this presentation, we discuss the construction of a lithium ion battery using graphite as the anode and LiCoO2 as the cathode to make a cell with a 4.2V potential. The primary electrolyte is a water:trimethylphosphate hybrid with a water mole fraction of 0.44 and LiTFSI salt at a concentration of 9 molal. This aqueous hybrid electrolyte can be formed into a gel electrolyte by directly polymerizing acrylate-based monomers and crosslinkers dissolved in the electrolyte. We demonstrate that by protecting the graphite anode using an acrylate gel with a fluoroethylene carbonate-based liquid electrolyte, the battery cell can be cycled repeatedly between 3.0V and 4.2V just like a cell using organic carbonate electrolytes. The advantage of the aqueous hybrid electrolyte is that it is non-flammable, and a cell using aqueous gel electrolytes can withstand damage and even be cut open while operating with no risk of fire or explosion. The manufacturing and performance characteristics of the aqueous 4V battery will be discussed as well as the interfacial issues that come about with the use of aqueous gel electrolytes in a 4V-capable battery system.