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

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(epsilon-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.

Automated summary

The chemical modification of solid polymer electrolytes in Li-metal batteries has been studied to enhance cell performance. This study investigates simultaneous chemical modification of the polymer matrix and lithium salt, examining ion coordination environments, ion transport mechanisms, and molecular speciation. By substituting F atoms with H atoms in the LiTFSI salt, lithium(difluoromethanesulfonyl) (trifluoromethanesulfonyl)imide (LiDFTFSI) and lithium bis(difluoromethanesulfonyl)imide (LiDFSI) salts are formed, promoting anion immobilization and increasing the lithium transference number. Additionally, replacing poly(ethylene oxide) (PEO) with poly(epsilon-caprolactone) (PCL) alters charge carrier speciation, affecting macroscopic properties such as Li+ conductivity and transference number.