Exploring the Ion Solvation Environments in Solid-State Polymer Electrolytes through Free-Energy Sampling

Abstract

Lithium-ion batteries are high energy density power sources that find uses in the automobile industry for electric cars and various portable devices like smartphones, laptops etc. There are strong motivations for developing solid-state electrolytes due to safety concerns associated with the combination of flammable liquid electrolytes and cell shorting due to lithium dendrite growth in current technology. A major issue with solid electrolytes is their low conductivity, which limits the power density (rate of charge/discharge), and possible loss of electrical contact due to anode shrinking. Lithium salts dissolved in poly(ether-acetals) are prime candidates for such electrolytes but the major challenge in improving the performance of these batteries is that the exact mechanism of the motion of the Li+ ions in such systems is not yet well understood. Our goal is to understand the underlying structural differences in the lithium coordination environments in these different systems and how this would affect the movement of the lithium ion from one coordination cage to another. This insight will allow us to suggest new compositions for electrolytes with improved performance for battery applications.Molecular dynamics simulations using newly developed, accurate interaction potentials have helped elucidate possible mechanisms for various transport properties in these systems. The presence of a second coordination shell around the Li ion, observed in some of these systems, seems to distort the first coordination shell, creating a more open cage structure that may help facilitate transport in these polymers. Furthermore, this distorted coordination may also assist in the formation of a lower coordination transition state while the Li ions jump from one cage to another during diffusion. This may be similar to the pathways followed in solid-state transport - either during topotactic intercalation, as ions move from interstitial to interstitial site; or within materials with solid-solid phase transitions related to thermally induced rotations of complex anions. Free energy calculations further elucidate the relative stability of these different Li-ether coordination environments and the possible pathways to move between them (See figure 1 for example). Furthermore, to understand the associated correlations of glass transition temperature in these systems with microscopic chemical details like the polymer backbone chemistry, ion clustering and coordination, we study the trends in these different systems with extensive statistics from long time molecular dynamics simulations at different temperatures and compositions. Figure 1