Molecular-Level Insight into Charge Carrier Transport and Speciation in Solid Polymer Electrolytes by Chemically Tuning Both Polymer and Lithium Salt

Abstract

The advent of Li-metal batteries has seen progress toward studies focused on the chemical modification of solid polymer electrolytes, involving tuning either polymer or Li salt properties to enhance the overall cell performance. This study encompasses chemically modifying simultaneously both polymer matrix and lithium salt by assessing ion coordination environments, ion transport mechanisms, and molecular speciation. First, commercially used lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt is taken as a reference, where F atoms become partially substituted by one or two H atoms in the -CF₃ moieties of LiTFSI. These substitutions lead to the formation of lithium(difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (LiDFTFSI) and lithium bis(difluoromethanesulfonyl)imide (LiDFSI) salts. Both lithium salts promote anion immobilization and increase the lithium transference number. Second, we show that exchanging archetypal poly(ethylene oxide) (PEO) with poly(ε-caprolactone) (PCL) significantly changes charge carrier speciation. Studying the ionic structures of these polymer/Li salt combinations (LiTFSI, LiDFTFSI or LiDFSI with PEO or PCL) by combining molecular dynamics simulations and a range of experimental techniques, we provide atomistic insights to understand the solvation structure and synergistic effects that impact macroscopic properties, such as Li⁺ conductivity and transference number.

Figure 1

DSC traces of (a) PEO- and (b) PCL-based SPEs. DSC scan recorded during the second heating at 10 K min^–1. Temperature dependence of the ionic conductivity for (c) PEO- and (d) PCL-based SPEs based on EIS. Neat polymers are indicated in purple, LiTFSI-based SPEs in black, LiDFTFSI-based SPEs in red, and LiDFSI-based SPEs in blue. The error bars for (c) and (d) are based on at least 3 samples per studied system.

Figure 2

Histogram illustrating the total (full color) and Li⁺ (striped) ionic conductivity with error bars at 70 °C of (a) PEO- and (b) PCL-based SPEs as a function of the studied lithium salt. LiTFSI-based SPEs are shown in black, LiDFTFSI-based SPEs in red, and LiDFSI-based SPEs in blue. The error bars are based on at least 3 samples per studied system.

Figure 3

Raman spectra of (a) neat lithium salts, (b) LiTFSI-, (c) LiDFTFSI-, and (d) LiDFSI-based SPEs; representing LiTFSI (black), LiDFTFSI (red), LiDFSI (blue), neat PEO (cyan), PEO-based SPEs (navy), neat PCL (magenta), and PCL-based SPEs (dark red). The vertical dotted lines indicate the studied bonds as well as their vibration and coordination type.

Figure 4

Molecular speciation analysis for Li⁺ coordination in (a) PEO- and (b) PCL-based SPEs. The vertical bars consist of different colors which represent different types of molecular speciation, as shown in the legends above the respective figures.

Figure 5

Molecular speciation analysis for anion coordination for (a) PEO- and (b) PCL-based SPEs. The vertical bars consist of different colors which represent different types of molecular speciation, as shown in the legends above the respective figures.

Figure 6

Molecular-level snapshots of MD simulations of (a) LiTFSI/PEO, (b) LiDFTFSI/PEO, (c) LiDFSI/PEO, (d) LiTFSI/PCL, (e) LiDFTFSI/PCL, and (f) LiDFSI/PCL at 70 °C. Li⁺ is represented by purple balls; TFSI^– (black), DFTFSI^– (red) and DFTFSI^– (blue) by stick models; and PEO (yellow) and PCL (green) by stick models within a coordination of 3 Å. The system name is labeled below each snapshot. Notice how in PEO-based SPEs (a–c) the lithium salts are more dissociated and mixed compared to PCL-based SPES (d–f).