Abstract
Solid-state polymer electrolytes and high-concentration liquid electrolytes, such as water-in-salt electrolytes and ionic liquids, are emerging materials to replace the flammable organic electrolytes widely used in industrial lithium-ion batteries. Extensive efforts have been made to understand the ion transport mechanisms and optimize the ion transport properties. This perspective reviews the current understanding of the ion transport and polymer dynamics in liquid and polymer electrolytes, comparing the similarities and differences in the two types of electrolytes. Combining recent experimental and theoretical findings, we attempt to connect and explain ion transport mechanisms in different types of small-molecule and polymer electrolytes from a theoretical perspective, linking the macroscopic transport coefficients to the microscopic, molecular properties such as the solvation environment of the ions, salt concentration, solvent/polymer molecular weight, ion pairing, and correlated ion motion. We emphasize universal features in the ion transport and polymer dynamics by highlighting the relevant time and length scales. Several outstanding questions and anticipated developments for electrolyte design are discussed, including the negative transference number, control of ion transport through precision synthesis, and development of predictive multiscale modeling approaches.
Highlights
Lithium ion batteries have enabled transformative technologies such as wireless electronics, electric vehicles, and renewable energies, which was recognized by the 2019 Nobel Prize in Chemistry.[1]
Combining experimental and theoretical/computational findings, we show that a consistent explanation of transport properties for both liquid electrolytes and Polymer electrolytes (PEs) is possible when framed in terms of a set of microscopic, molecular properties such as ion pairing and ion correlation, salt concentration, solvent molecular weight, and the microstructure morphology
We have illustrated how the changes in the ion solvation environment and the relevant timescales can be used to systematically describe different ion transport mechanisms, to link macroscopic transport coefficients to microscopic timescales, and to understand many non-ideal solution behaviors observed in these electrolytes
Summary
Lithium ion batteries have enabled transformative technologies such as wireless electronics, electric vehicles, and renewable energies, which was recognized by the 2019 Nobel Prize in Chemistry.[1]. Increasing salt concentration in classical electrolytes above a threshold (typically >∼3M–5M depending on the salt–solvent combination) drastically changes the solvation environment of the ions and at sufficient concentration results in an ion-dominant solution known as “solvent-in-salt electrolytes” (SISEs).[3] In these SISEs, most of the solvent molecules are bound with the ions, which confers increased electrochemical stability, allowing wider ranges of operation temperature and voltage.[6,7] Room temperature ionic liquid electrolytes (ILs) can be considered as a limiting case of concentrated liquid electrolytes. Our understanding of ion conduction in each of these electrolytes has been greatly improved, but the wide range of composition and electrolytic properties of these materials has made it challenging to formulate a consistent and unified view of the diverse set of issues.[21] In this perspective, we attempt to connect and explain ion transport mechanisms for different types of classical electrolytes and PEs from a theoretical perspective. We conclude with a discussion of some unresolved questions and emerging approaches that are critical for improved mechanistic understanding and further development of the generation electrolyte materials
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