Partial Desolvation Causes Lithium Structural Transport in Liquid and Gel Polymer Electrolytes

A molecular-level understanding of ion transport is critical for optimizing lithium diffusion in gel polymer electrolytes (GPEs). Using polarization-selective pump-probe and two-dimensional infrared (2D IR) spectroscopy, we quantified lithium solvation structures and desolvation dynamics across the transition from propylene carbonate (PC)/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) liquid electrolytes to GPEs containing up to 60% poly(propylene carbonate) (PPC). Lithium ions remain preferentially coordinated by PC across all compositions, but the Li⁺-PC coordination number (CN_Li-PC) decreases from four to two with increasing polymer, exhibiting a sharp transition near 30% PPC. Li⁺-solvent residence times, extracted independently from 2D IR chemical exchange and spectral diffusion measurements, slow from ∼500 ps in liquids to ∼2 ns in 60% GPEs. Strikingly, we find that the residence time, a metric commonly used to quantify structural transport, fails to capture ionic conductivity trends when the coordination environment changes; instead, we introduce a new descriptor, the structural step times (τ_ss), which is defined as residence times normalized by CN_Li-PC, capturing the total rate of parallel dissociation pathways available to a lithium cluster. τ_ss quantitatively predicts ionic conductivity across both liquid and gel electrolytes, even when salt concentration and polymer fraction are varied independently. These findings show that lithium structural diffusion is initiated by partial desolvation events, not through concerted "hopping" between solvation sites, underscoring the need for caution when invoking ion hopping as the structural transport mechanism in common nonaqueous electrolytes.