7Li NMR Chemical Shifts in Battery-Relevant Electrolytes: Ligand-Induced Shifts Cancel in Both Experiment and Calculation.

Abstract

⁷Li nuclear magnetic resonance (NMR) experiments in the literature have shown that lowering the concentration from 1 M to 0.1 M of LiPF₆ salt dissolved in a 50:50 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) has only a minor effect on the Li chemical shift (≈0.1 ppm). This is unexpected as the number of Li⁺-PF₆ ^- contact ion pairs is expected to be large in the 1 M solution but small in the more dilute solution. The aim of this study is to explore how the chemical shifts for the two solutions can differ so little despite the fact that the ion pair concentration differs markedly between the two solutions. To resolve this apparent paradox, molecular dynamics (MD) simulation results from the literature are combined with density functional theory-based chemical shift calculations. The calculated chemical shifts are in good agreement with the experimental values for both the 1 M and 0.1 M solutions, and so is the small chemical shift difference between them, which can be rationalized in terms of a cancellation of many contributions of opposite signs to the ⁷Li chemical shifts. The workflow considers effects of both short-time and long-time dynamical averaging.