Polymer gel electrolytes have been seeing a significant revival in their scientific interest and development because they have ionic conductivities similar to their liquid electrolyte counterparts, but lack of electrochemical degradation and flammability commonly observed in commercially used liquid electrolytes. Polymer gel electrolytes are formed when a liquid lithium ion electrolyte is mixed with a polymer at sufficiently large concentration to gel. The study of polymer gel electrolytes is not new, but the direct connection between their microscopic structure and their macroscopic properties remains elusive. For example, the question of whether the polymer in these gels systems acts as an inert, stabilizing scaffold for the electrolyte or whether interactions with the polymer actually alter the physical and chemical properties of the system is still unknown. More specifically, recent work called into question the possible interaction between the lithium cation and the polymer and its effect on the conductivity mechanism and other physical properties. In addition, it is also unknown what is the effect of the polymer concentration in structure of the system, since some studies have shown that the conductivity of the electrolyte can increase with the addition of polymer even though the viscosity also rises. Hence, we report here our study of the structure and dynamics of a gel polymer electrolyte comprised of a prototypical lithium-carbonate electrolyte and polyacrylonitrile. The microscopic properties of the gel electrolyte were studied using infrared spectroscopies (FTIR and 2DIR) and other experimental methods such as conductivity and viscosity. In particular, we focused our attention on the effect of the polymer concentration in both the macroscopic and microscopic structure of the electrolyte. Our results indicated that the polymer appears to interact with lithium ions even at very high concentrations (10% weight) when the system has a gel structure. Moreover, while the polymer structure and interaction with the liquid electrolyte are severely impacted by the concentration of the polymer, the structure and dynamics of the liquid electrolyte do not appear to be significantly altered. Our results highlight the critical role that the polymer concentration plays in determining the macroscopic properties of the system and its inability to modulate the dynamics of the liquid component in the gel electrolyte, which explains the conductivity of these systems.

Polymer gel electrolytes: A look into the structure and dynamics from electrolyte and polymer perspectives

ACS Spring 2022

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