Heterogeneous Li coordination in solvent-in-salt electrolytes enables high Li transference numbers

The transport properties and the underlying coordination structure of a ternary electrolyte consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1,2-dimethoxyethane (DME), and 1,3-dioxolane (DOL) is studied over a wide concentration range, up to that of a Solvent-in-Salt (SiS) electrolyte. Among other advantages for next-generation battery applications, SiS electrolytes offer a high lithium transference number (t_Li) of 0.73. We analyze the transport mechanism by electrophoretic NMR (eNMR), providing the mobilities (μ) of all species. Intriguingly, in the SiS region, the mobility of the neutral species DME exceeds the cation mobility (μ_DME > μ_Li), suggesting a heterogeneous transport mechanism, where the Li⁺ mobility is averaged over different species. Based on Raman spectroscopy, NMR spectroscopy and MD simulations, we derive a model for a concentration-dependent Li⁺ coordination environment with a heterogeneous Li⁺ coordination in the SiS region, where the 1^st coordination shell either consists of TFSI^- and DOL only, or of DME, TFSI^-, and DOL. Lithium ions partially coordinated by DME migrate faster in an electric field, in contrast to lithium ions solely coordinated by anions and DOL molecules, explaining the peculiarity of the rapidly migrating neutral DME molecules. Further, DME is identified as an exclusively bidentate ligand, while TFSI^- and DOL act as bridging ligands coordinating different Li⁺ ions. Thus, Li⁺ coordination heterogeneity is the basis for Li⁺ transport heterogeneity and for achieving very high Li⁺ transference numbers. In addition, an effective dynamic decoupling of Li⁺ and anions occurs with an Onsager coefficient σ_+- ≈ 0. These results provide a deeper understanding of the very efficient lithium-ion transport in SiS electrolytes, with the potential to bring further improvements for battery applications.