Confinement-enhanced Li+ ion dynamics in an ionic liquid-based electrolyte in porous material

Abstract

While Ionic Liquids (IL) are promising liquid electrolyte components for Li-ion batteries due to their high electrochemical stability and low volatility and flammability, unfavorable Lithium-anion clusters lead to poor Li⁺ transport properties such as low transference numbers. A confinement of ILs in nanoporous materials could overcome these problems, based on altered structural and dynamic properties of the confined ILs. We investigate the coordination and the Li⁺ dynamics in an IL/Li-salt mixture of 1-butyl-1-methylpyrrolidinium bis(trifluormethyl-sulfonyl)imide (Pyr₁₄TFSA) and LiTFSA and reveal in how far the confinement has positive or negative effects on ion clustering in the electrolyte. To this end, the electrolyte is confined in mesoporous silica SBA-15 (pore diameter 8 nm or 4 nm) or the metal–organic framework (MOF) ZIF-8 (pore diameter 1.16 nm). Raman spectra elucidate the Li-anion coordination and the interaction of the ions with the walls. Temperature-dependent ⁷Li spin relaxation rates, analyzed within the model of Bloembergen, Purcell and Pound (BPP), allow statements on the local Li⁺ environment, the local Li⁺ dynamics and its activation. In the SBA-15 materials the Li⁺ coordination is unchanged with persisting Li-TFSA clusters. Furthermore, the local dynamics of Li⁺ is reduced upon confinement, as expected due to geometrical restrictions. At the same time, however, both structural and dynamic parameters do not show a pronounced dependence on the pore size. Surprisingly, upon confinement in ZIF-8 Li⁺ displays faster local dynamics and a more asymmetric environment in comparison to the bulk electrolyte. The enhanced dynamics is accompanied by a reduced coordination to TFSA⁻, suggesting the breakup of Li-TFSA clusters. Differences between the porous materials are attributed to the nature of the wall surface, as Raman spectra suggest that in SBA-15 the TFSA⁻ ion is preferentially interacting with the pore walls, whereas in ZIF-8 the Pyr14⁺ ion is immobilized by the pore walls. These results demonstrate a strong influence of internal interfaces on IL structure and dynamics and bear potential for further tailoring ion dynamics.