Development of safe aqueous batteries and supercapacitors critically relies on expanding the electrolyte electrochemical stability window. A novel mechanism responsible for widening the ...electrochemical stability window of water-in-salt electrolytes (WiSEs) compared to conventional salt-in-water electrolytes is suggested based on molecular dynamics (MD) simulations of the electrolyte–electrode interface. Water exclusion from the interfacial layer at the positive electrode provided additional kinetic protection that delayed the onset of the oxygen evolution reactions. The interfacial structure of a WiSE at negative electrodes near the potential of zero charge clarified why the recently discovered passivation layers formed in WiSEs are robust. The onset of water accumulation at potentials below 1.5 V vs Li/Li+ leads to formation of water-rich nanodomains at the negative electrode, limiting the robustness of the WiSE. Unexpectedly, the bis(trifluoromethanesulfonyl)imide anion adsorbed and trifluoromethanesulfonate desorbed with positive electrode polarization, demonstrating selective anion partitioning in the double layer.
A number of correlations between heat of vaporization (H vap), cation−anion binding energy (E ±), molar volume (V m), self-diffusion coefficient (D), and ionic conductivity for 29 ionic liquids have ...been investigated using molecular dynamics (MD) simulations that employed accurate and validated many-body polarizable force fields. A significant correlation between D and H vap has been found, while the best correlation was found for −log(DV m) vs H vap + 0.28E ±. A combination of enthalpy of vaporization and a fraction of the cation−anion binding energy was suggested as a measure of the effective cohesive energy for ionic liquids. A deviation of some ILs from the reported master curve is explained based upon ion packing and proposed diffusion pathways. No general correlations were found between the ion diffusion coefficient and molecular volume or the diffusion coefficient and cation/anion binding energy.
Many applications in chemistry, biology, and energy storage/conversion research rely on molecular simulations to provide fundamental insight into structural and transport properties of materials with ...high ionic concentrations. Whether the system is comprised entirely of ions, like ionic liquids, or is a mixture of a polar solvent with a salt, e.g., liquid electrolytes for battery applications, the presence of ions in these materials results in strong local electric fields polarizing solvent molecules and large ions. To predict properties of such systems from molecular simulations often requires either explicit or mean-field inclusion of the influence of polarization on electrostatic interactions. In this manuscript, we review the pros and cons of different treatments of polarization ranging from the mean-field approaches to the most popular explicit polarization models in molecular dynamics simulations of ionic materials. For each method, we discuss their advantages and disadvantages and emphasize key assumptions as well as their adjustable parameters. Strategies for the development of polarizable models are presented with a specific focus on extracting atomic polarizabilities. Finally, we compare simulations using polarizable and nonpolarizable models for several classes of ionic systems, discussing the underlying physics that each approach includes or ignores, implications for implementation and computational efficiency, and the accuracy of properties predicted by these methods compared to experiments.
Lithium-ion batteries raise safety, environmental, and cost concerns, which mostly arise from their nonaqueous electrolytes. The use of aqueous alternatives is limited by their narrow electrochemical ...stability window (1.23 volts), which sets an intrinsic limit on the practical voltage and energy output. We report a highly concentrated aqueous electrolyte whose window was expanded to ~3.0 volts with the formation of an electrode-electrolyte interphase. A full lithium-ion battery of 2.3 volts using such an aqueous electrolyte was demonstrated to cycle up to 1000 times, with nearly 100% coulombic efficiency at both low (0.15 coulomb) and high (4.5 coulombs) discharge and charge rates.
Metallic zinc (Zn) has been regarded as an ideal anode material for aqueous batteries because of its high theoretical capacity (820 mA h g
), low potential (-0.762 V versus the standard hydrogen ...electrode), high abundance, low toxicity and intrinsic safety. However, aqueous Zn chemistry persistently suffers from irreversibility issues, as exemplified by its low coulombic efficiency (CE) and dendrite growth during plating/ stripping, and sustained water consumption. In this work, we demonstrate that an aqueous electrolyte based on Zn and lithium salts at high concentrations is a very effective way to address these issues. This unique electrolyte not only enables dendrite-free Zn plating/stripping at nearly 100% CE, but also retains water in the open atmosphere, which makes hermetic cell configurations optional. These merits bring unprecedented flexibility and reversibility to Zn batteries using either LiMn
O
or O
cathodes-the former deliver 180 W h kg
while retaining 80% capacity for >4,000 cycles, and the latter deliver 300 W h kg
(1,000 W h kg
based on the cathode) for >200 cycles.
The oxidative stability and initial oxidation-induced decomposition reactions of common electrolyte solvents for batteries and electrical double layer capacitors were investigated using quantum ...chemistry (QC) calculations. The investigated electrolytes consisted of linear (DMC, EMC) and cyclic carbonate (EC, PC, VC), sulfone (TMS), sulfonate, and alkyl phosphate solvents paired with BF4 –, PF6 –, bis(fluorosulfonyl)imide (FSI–), difluoro-(oxalato)borate (DFOB–), dicyanotriazolate (DCTA–), and B(CN)4 – anions. Most QC calculations were performed using the M05-2X, LC-ωPBE density functional and compared with the G4MP2 results where feasible. The calculated oxidation potentials were compared with previous and new experimental data. The intrinsic oxidation potential of most solvent molecules was found to be higher than experimental values for electrolytes even after the solvation contribution was included in the QC calculations via a polarized continuum model. The presence of BF4 –, PF6 –, B(CN)4 –, and FSI– anions near the solvents was found to significantly decrease the oxidative stability of many solvents due to the spontaneous or low barrier (for FSI–) H- and F-abstraction reaction that followed the initial electron removal step. Such spontaneous H-abstraction reactions were not observed for the solvent complexes with DCTA– or DFOB– anions or for VC/anion, TMP/PF6 – complexes. Spontaneous H-transfer reactions were also found for dimers of the oxidized carbonates (EC, DMC), alkyl phosphates (TMP), while low barrier H-transfer was found for dimers of sulfones (TMS and EMS). These reactions resulted in a significant decrease of the dimer oxidation potential compared to the oxidation potential of the isolated solvent molecules. The presence of anions or an explicitly included solvent molecule next to the oxidized solvent molecules also reduced the barriers for the oxidation-induced decomposition reaction and often changed the decomposition products. When a Li+ cation polarized the solvent in the EC n /LiBF4 and EC n /LiPF6 complexes, the complex oxidation potential was 0.3–0.6 eV higher than the oxidation potential of EC n /BF4 – and EC n /PF6 –.
Metallic zinc is an ideal anode due to its high theoretical capacity (820 mAh g
), 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
-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
in a Zn||VOPO
full battery with 88.7% retention for >6,000 cycles, 325 Wh kg
in a Zn||O
full battery for >300 cycles and 218 Wh kg
in a Zn||MnO
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
VOPO
at 100% depth of discharge for 100 cycles, thus establishing aqueous zinc batteries as viable cell systems for practical applications.
As a legacy left behind by classical analytical electrochemistry in pursuit of ideal electrodics, and classical physical electrochemistry in pursuit of the most conductive ionics, the study of ...non-aqueous electrolytes has been historically confined within a narrow concentration regime around 1 molarity (M). This confinement was breached in recent years when unusual properties were found to arise from the excessive salt presence, which often bring benefits to electrochemical, thermal, transport, interfacial, and interphasial properties that are of significant interest to the electrochemical energy storage community. This article provides an overview on this newly discovered and under-explored realm, with emphasis placed on their applications in rechargeable batteries.
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The traditional efforts in electrolytes have been mostly evolving around the “1 M” region, where maximum ion conductivities occur in the majority of non-aqueous electrolyte systems. However, recent deviation from this “optimum” concentration has revealed to us that there is a new world. In the super-concentrated regions, the reversed salt/solvent ratio brought dramatic changes in bulk liquid structures, ion transport, and interfacial and interphasial properties. Some of these unusual properties have been found to introduce benefits to electrochemical, thermal, transport, interfacial, and interphasial properties that are of significant interest to the electrochemical energy storage community. This article provides a comprehensive overview on this newly discovered and under-explored realm.
Borodin et al. review advances in designing electrolytes for aqueous and non-aqueous batteries with a focus on going beyond the conventional “1 M” salt concentration to super-concentration that brings about multitudes of unexpected electrolyte properties from novel interfaces, interphases, ion transport mechanisms, and thermal stability.
The importance of the solid-electrolyte interphase (SEI) for reversible operation of Li-ion batteries has been well established, but the understanding of its chemistry remains incomplete. The current ...consensus on the identity of the major organic SEI component is that it consists of lithium ethylene di-carbonate (LEDC), which is thought to have high Li-ion conductivity, but low electronic conductivity (to protect the Li/C electrode). Here, we report on the synthesis and structural and spectroscopic characterizations of authentic LEDC and lithium ethylene mono-carbonate (LEMC). Direct comparisons of the SEI grown on graphite anodes suggest that LEMC, instead of LEDC, is likely to be the major SEI component. Single-crystal X-ray diffraction studies on LEMC and lithium methyl carbonate (LMC) reveal unusual layered structures and Li
coordination environments. LEMC has Li
conductivities of >1 × 10
S cm
, while LEDC is almost an ionic insulator. The complex interconversions and equilibria of LMC, LEMC and LEDC in dimethyl sulfoxide solutions are also investigated.
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