Investigation of alkali and alkaline earth solvation structures in tetraglyme solvent

Origin of Li+ Solvation Ability of Electrolyte Solvent: Ring Strain

Developing new organic solvents to support the use of Li metal anodes in secondary batteries is an area of great interest. In particular, research is actively underway to improve battery performance by introducing fluorine to ether solvents, as these are highly compatible with Li metal anodes because fluorine imparts high oxidative stability and relatively low Li-ion solvation ability. However, theoretical analysis of the solvation ability of organic solvents mostly focuses on the electron-withdrawing capability of fluorine. Herein, we analyze the effect of the structural characteristics of solvents on their Li+ ion solvation ability from a computational chemistry perspective. We reveal that the structural constraints imposed on the oxygen binding sites in solvent molecules vary depending on the structural characteristics of the N-membered ring formed by the interaction between the organic solvent and Li+ ions and the internal ring containing the oxygen binding sites. We demonstrate that the structural strain of the organic solvents has a comparable effect on Li+ solvation ability seen for the electrical properties of fluorine elements. This work emphasizes the importance of understanding the structural characteristics and strain when attempting to understand the interactions between solvents and metal cations and effectively control the solvation ability of solvents.

AMOEBA Polarizable Force Field for Molecular Dynamics Simulations of Glyme Solvents

Classical molecular dynamics (MD) simulations of electrolyte systems are important to gain insight into the atom-scale properties that determine the battery-relevant performance. The recent Tinker-HP software release enables efficient and accurate MD simulations with the AMOEBA polarizable force field. In this work, we developed a procedure to construct a universal AMOEBA model for the solvent family of glymes (glycol methyl ethers), which involves a refinement scheme for valence parameters by fitting the AMOEBA-derived atomic forces to those computed at the DFT level. The refined AMOEBA model provides a good description of both local and nonlocal properties in terms of the spectroscopic response of glyme molecules, as well as the liquid glyme density and dielectric constant. In addition, the complexation energies of alkali and alkaline-earth metal cations with tetraglyme molecules obtained from AMOEBA calculations are in good agreement with DFT results, demonstrating the suitability of the developed AMOEBA model for an accurate simulation of glyme-based battery electrolytes. We also expect the procedure to be transferable to the development of AMOEBA models for other battery electrolyte systems.

Investigating the abnormal conductivity behaviour of divalent cations in low dielectric constant tetraglyme-based electrolytes

Solutions made of tetraglyme (G4) containing Ca(TFSI)₂ have been studied as models to understand the solvation structure and the conductivity properties of multivalent ions in low dielectric constant ethereal electrolytes. These solutions have been characterised using electrochemical impedance spectroscopy, rheological measurement, and Raman spectroscopy. The ionic conductivity of these electrolytes shows an intriguing non-monotonic behaviour with temperature which deviates from the semi-empirical Vogel-Tammann-Fulcher equation at a critical temperature. This behaviour is observed for both Mg(TFSI)₂ and Ca(TFSI)₂, but not LiTFSI, indicating a difference in the solvation structure and the thermodynamic properties of divalent ions compared to Li⁺. The origin of this peculiar behaviour is demystified using temperature-controlled Raman spectroscopy and first-principles calculations combined with a thermodynamic analysis of the chemical equilibrium of Ca²⁺ ion-pairing versus solvation. As long-range electrostatic interactions are critical in solutions based on low dielectric ethereal solvents, a periodic approach is here proposed to capture their impact on the solvation structure of the electrolyte at different salt concentrations. The obtained results reveal that the thermodynamic and transport properties of Ca(TFSI)₂/G4 solutions stem from a competition between enthalpic (ionic strength) and entropic factors that are directly controlled by the solution concentration and temperature, respectively. At high salt concentrations, the ionic strength of the solution favours the existence of free ions thanks to the strong solvation energy of the polydentate G4 solvent conjugated with the weak complexation ability of TFSI^-. At elevated temperatures, the configurational entropy associated with the release of a coordinated G4 favours the formation of contact ion-pairs due to its flat potential energy surface (weak strain energy), offering a large configuration space. Such a balance between ion-pair association and dissociation not only rationalises the ionic conductivity behaviour observed for Ca(TFSI)₂/G4 solutions, but also provides valuable information to extrapolate the ionic transport properties of other electrolytes with different M(TFSI)_n salts dissolved in longer-chain glymes or even poly(ethylene oxide). These findings are essential for the understanding of solvation structures and ionic transport in low-dielectric media, which can further be used to design new electrolytes for Li-ion and post Li-ion batteries as well as electrocatalysts.

Quantifying the ion coordination strength in polymer electrolytes

In the progress of implementing solid polymer electrolytes (SPEs) into batteries, fundamental understanding of the processes occurring within and in the vicinity of the SPE are required. An important but so far relatively unexplored parameter influencing the ion transport properties is the ion coordination strength. Our understanding of the coordination chemistry and its role for the ion transport is partly hampered by the scarcity of suitable methods to measure this phenomenon. Herein, two qualitative methods and one quantitative method to assess the ion coordination strength are presented, contrasted and discussed for TFSI-based salts of Li+, Na+ and Mg2+ in polyethylene oxide (PEO), poly(ε-caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC). For the qualitative methods, the coordination strength is probed by studying the equilibrium between cation coordination to polymer ligands or solvent molecules, whereas the quantitative method studies the ion dissociation equilibrium of salts in solvent-free polymers. All methods are in agreement that regardless of cation, the strongest coordination strength is observed for PEO, while PTMC exhibits the weakest coordination strength. Considering the cations, the weakest coordination is observed for Mg2+ in all polymers, indicative of the strong ion-ion interactions in Mg(TFSI)2, whilst the coordination strength for Li+ and Na+ seems to be more influenced by the interplay between the cation charge/radius and the polymer structure. The trends observed are in excellent agreement with previously observed transference numbers, confirming the importance and its connection to the ion transport in SPEs.