Limited Stability of Ether-Based Solvents in Lithium–Oxygen Batteries

Li–O₂ batteries offer the tantalizing promise of a specific energy much greater than current Li ion technologies; however, many challenges remain before the development of commercial energy storage applications based on the lithium–oxygen couple can be realized. One of the most apparent limitations is electrolyte stability. Without an electrolyte that is resistant to attack by reduced oxygen species, optimizing other aspects of the redox performance is challenging. Thus, identifying electrolyte decomposition processes that occur early in the redox process will accelerate the discovery process. In this study, ATR–FTIR was used to examine various reported Li–O₂ electrolytes taken directly from the cell separators of cycled electrochemical cells. Specifically, we examined, 1 M LiPF₆ in propylene carbonate (PC), 1 M LiCF₃SO₃ in tetraethyleneglycoldimethylether (TEGDME), and 1 M LiCF₃SO₃ in a siloxane ether (1NM3) and looked for soluble decomposition products. Each electrolyte was tested using a regular Li–O₂ cathode with no catalyst and either an O₂ atmosphere or an Ar atmosphere and a Li metal anode as well as in a Li–Li symmetric cell. The 1NM3 electrolyte was found to form soluble decomposition products under all cell conditions tested, and a decomposition pathway has been proposed. It was also found that 1NM3 and TEGDME were consumed as part of the charging process in a working Li–O₂ cell, even at moderate voltages in the absence of O<sub>2.</sub

Bulk-Sensitive Characterization of the Discharged Products in Li–O₂ Batteries by Nonresonant Inelastic X‑ray Scattering

Understanding the nature of discharged products is critical to identifying suitable electrolyte systems for Li–O₂ batteries. We have employed nonresonant inelastic X-ray scattering (NIXS), which is a hard X-ray photon-in photon-out technique to monitor low energy core–shell excitations and to obtain bulk sensitive information on the solid discharged products in Li–O₂ batteries using various electrolyte solvent/salt combinations. NIXS measurements were performed on cathodes after discharging the Li–O₂ cells using low discharge current (∼25 mA/g of carbon). NIXS results reveal that, even in cells containing current state-of-the-art electrolytes, the oxygen in the discharged products is bound predominantly to species other than a peroxide or lithia. This finding shows that electrolyte decomposition is a significant pathway during discharge of Li–O₂ batteries using ether and oligoether substituted silane based electrolytes

The Coupling between Stability and Ion Pair Formation in Magnesium Electrolytes from First-Principles Quantum Mechanics and Classical Molecular Dynamics

In this work we uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potentials, e.g., at the metal anode. Through comprehensive calculations using both first-principles as well as well-benchmarked classical molecular dynamics over a matrix of electrolytes, covering solvents and salt anions with a broad range in chemistry, we elucidate systematic correlations between molecular level interactions and composite electrolyte properties, such as electrochemical stability, solvation structure, and dynamics. We find that Mg electrolytes are highly prone to ion pair formation, even at modest concentrations, for a wide range of solvents with different dielectric constants, which have implications for dynamics as well as charge transfer. Specifically, we observe that, at Mg metal potentials, the ion pair undergoes partial reduction at the Mg cation center (Mg²⁺ → Mg⁺), which competes with the charge transfer mechanism and can activate the anion to render it susceptible to decomposition. Specifically, TFSI^– exhibits a significant bond weakening while paired with the transient, partially reduced Mg⁺. In contrast, BH₄^– and BF₄^– are shown to be chemically stable in a reduced ion pair configuration. Furthermore, we observe that higher order glymes as well as DMSO improve the solubility of Mg salts, but only the longer glyme chains reduce the dynamics of the ions in solution. This information provides critical design metrics for future electrolytes as it elucidates a close connection between bulk solvation and cathodic stability as well as the dynamics of the salt

Improved Hydrogen Release from Ammonia–Borane with ZIF-8

The promotion for hydrogen release from ammonia–borane (AB) was observed in the presence of ZIF-8. Even at concentrations of ZIF-8 as low as 0.25 mol %, a reduction of the onset temperature for dehydrogenation accompanies an increase in both the rate and amount of hydrogen released from AB

Role of Chloride for a Simple, Non-Grignard Mg Electrolyte in Ether-Based Solvents

Mg battery operates with Chevrel phase (Mo₆S₈, ∼1.1 V vs Mg) cathodes that apply Grignard-based or derived electrolytes, which allow etching of the passivating oxide coating forms at the magnesium metal anode. Majority of Mg electrolytes studied to date are focused on developing new synthetic strategies to achieve a better reversible Mg deposition. While most of these electrolytes contain chloride as a component, and there is a lack of literature which investigates the fundamental role of chloride in Mg electrolytes. Further, ease of preparation and potential safety benefits have made simple design of magnesium electrolytes an attractive alternative to traditional air sensitive Grignard reagents-based electrolytes. Work presented here describes simple, non-Grignard magnesium electrolytes composed of magnesium bis­(trifluoromethane sulfonyl)­imide mixed with magnesium chloride (Mg­(TFSI)₂-MgCl₂) in tetrahydrofuran (THF) and diglyme (G2) that can reversibly plate and strip magnesium. Based on this discovery, the effect of chloride in the electrolyte complex was investigated. Electrochemical properties at different initial mixing ratios of Mg­(TFSI)₂ and MgCl₂ showed an increase of both current density and columbic efficiency for reversible Mg deposition as the fraction content of MgCl₂ increased. A decrease in overpotential was observed for rechargeable Mg batteries with electrolytes with increasing MgCl₂ concentration, evidenced by the coin cell performance. In this work, the fundamental understanding of the operation mechanisms of rechargeable Mg batteries with the role of chloride content from electrolyte could potentially bring rational design of simple Mg electrolytes for practical Mg battery

Origin of Electrochemical, Structural, and Transport Properties in Nonaqueous Zinc Electrolytes

Through coupled experimental analysis and computational techniques, we uncover the origin of anodic stability for a range of nonaqueous zinc electrolytes. By examination of electrochemical, structural, and transport properties of nonaqueous zinc electrolytes with varying concentrations, it is demonstrated that the acetonitrile–Zn­(TFSI)₂, acetonitrile–Zn­(CF₃SO₃)₂, and propylene carbonate–Zn­(TFSI)₂ electrolytes can not only support highly reversible Zn deposition behavior on a Zn metal anode (≥99% of Coulombic efficiency) but also provide high anodic stability (up to ∼3.8 V vs Zn/Zn²⁺). The predicted anodic stability from DFT calculations is well in accordance with experimental results, and elucidates that the solvents play an important role in anodic stability of most electrolytes. Molecular dynamics (MD) simulations were used to understand the solvation structure (e.g., ion solvation and ionic association) and its effect on dynamics and transport properties (e.g., diffusion coefficient and ionic conductivity) of the electrolytes. The combination of these techniques provides unprecedented insight into the origin of the electrochemical, structural, and transport properties in nonaqueous zinc electrolytes

Phase-Controlled Electrochemical Activity of Epitaxial Mg-Spinel Thin Films

We report an approach to control the reversible electrochemical activity (i.e., extraction/insertion) of Mg²⁺ in a cathode host through the use of phase-pure epitaxially stabilized thin film structures. The epitaxially stabilized MgMn₂O₄ (MMO) thin films in the distinct tetragonal and cubic phases are shown to exhibit dramatically different properties (in a nonaqueous electrolyte, Mg­(TFSI)₂ in propylene carbonate): tetragonal MMO shows negligible activity while the cubic MMO (normally found as polymorph at high temperature or high pressure) exhibits reversible Mg²⁺ activity with associated changes in film structure and Mn oxidation state. These results demonstrate a novel strategy for identifying the factors that control multivalent cation mobility in next-generation battery materials

Structural Evolution of Reversible Mg Insertion into a Bilayer Structure of V₂O₅·<i>n</i>H₂O Xerogel Material

Functional multivalent intercalation cathodes represent one of the largest hurdles in the development of Mg batteries. While there are many reports of Mg cathodes, many times the evidence of intercalation chemistry is only circumstantial. In this work, direct evidence of Mg intercalation into a bilayer structure of V₂O₅·<i>n</i>H₂O xerogel is confirmed, and the nature of the Mg intercalated species is reported. The interlayer spacing of V₂O₅·<i>n</i>H₂O contracts upon Mg intercalation and expands for Mg deintercalation due to the strong electrostatic interaction between the divalent cation and the cathode. A combination of NMR, pair distribution function (PDF) analysis, and X-ray absorption near edge spectroscopy (XANES) confirmed reversible Mg insertion into the V₂O₅·<i>n</i>H₂O material, and structural evolution of Mg intercalation leads to the formation of multiple new phases. Structures of V₂O₅·<i>n</i>H₂O with Mg intercalation were further supported by the first principle simulations. A solvent cointercalated Mg in V₂O₅·<i>n</i>H₂O is observed for the first time, and the ²⁵Mg magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to elucidate the structure obtained upon electrochemical cycling. Specifically, existence of a well-defined Mg–O environment is revealed for the Mg intercalated structures. Information reported here reveals the fundamental Mg ion intercalation mechanism in a bilayer structure of V₂O₅·<i>n</i>H₂O material and provides insightful design metrics for future Mg cathodes