Enthalpic and Entropic Contributions to Fast Lithium Ion Conduction in Solid-State Aqueous Polymer Electrolytes

Abstract

Solid-state aqueous polymer electrolytes (SAPEs), a mixture of hydrophilic polymers and an appropriate amount of water, can produce high Li-ion conductivity while maintaining a solid state. Also, they can overcome the limitations of normal solid electrolytes. This study reports that the very high SAPE ionic conductivity (∼10 mS/cm at T = 298.15 K) originates from a low energy barrier (∼0.28 eV) closely correlated with water-filled ion passages in the medium. The low energy barrier is ascribed to a considerable reduction of the enthalpic barrier due to water addition despite a growth of the entropic barrier incurred by the negative nature of entropy change across water tubes. The extremely high ionic conductivity, coupled with an exceptionally low energy barrier, provides a unique advantage to SAPEs over conventional solid electrolytes.

Ionic Conduction and Dielectric Response of Nanoparticle-Coupled Hydrogel Network Polymer Electrolytes

“…In addition, after normalizing inverse temperature by the extrapolated σ relaxation temperature T σ , defined at ω σ ( T σ ) = 10 –2 rad/s (Figure d), all the conduction frequency ω σ data of these electrolytes merge into a single VFT curve (see the inset of Figure d). Therefore, this indicates that the lithium cations, bound to the carboxylate (COO – ) groups in PLiA, are released from the polymer chains by hydration to form conducting ions, migrate along the water channel formed by phase separation, and then are again trapped by other PLiA chains . These releasing and trapping processes are the main hopping mechanism of ion transport and are closely related to the ion-exchanging α 2 process.…”

“…Therefore, this indicates that the lithium cations, bound to the carboxylate (COO − ) groups in PLiA, are released from the polymer chains by hydration to form conducting ions, migrate along the water channel formed by phase separation, and then are again trapped by other PLiA chains. 40 These releasing and trapping processes are the main hopping mechanism of ion transport and are closely related to the ion-exchanging α 2 process.…”

Ion and Water Dynamics in the Transition from Dry to Wet Conditions in Salt-Doped PEG

“…While significant progress has been made in understanding ion transport mechanisms in the extremes of dry polymeric electrolytes and highly swollen polymer membranes, these fields have generally been developed independently, leading to knowledge gaps regarding transport in low- to moderately hydrated polymer systems. Recently, significant interest has arisen in such a regime for energy storage applications. − For example, aqueous solid polymer electrolytes (polymer and water-in-salt electrolytes) exhibit high ionic conductivities, preferential lithium transport, and improved safety compared to solid polymer electrolytes, as well as improved electrochemical stability compared to salt-in-water electrolytes . The ability to control ion permeation and ionic conductivity in low to moderately hydrated polymer systems hinges on developing fundamental insights into the interplay between ion–ion, ion–polymer, and ion–water interactions and the role of ion-polymer solvation on ion transport in the transition between “dry” and “hydrated” polymer systems.…”