Discontinuous coordination boosting ion transport in solid polymer electrolytes

Abstract

Decoupling Li⁺ transport from polymer segmental dynamics is crucial for enhancing ionic conductivity (σ) and transference number (t₊) in solid polymer electrolytes (SPEs). Herein, by studying four ether-based SPEs with varying oxygen density, we identify a transition from polymer relaxation-limited ion transport in poly(ethylene oxide) (PEO) to ion hopping-dominant transport in poly(tetrahydrofuran) (PTHF), poly(1,3-dioxolane) (PDOL), and poly(trioxymethylene) (PTOM). Molecular dynamics simulations and solid-state ⁷Li nuclear magnetic resonance reveal origins of the transition. In PTHF, weak solvation with lithium bond characteristics contributes to a less-shielded Li⁺ environment, while in PDOL and PTOM, the discontinuous coordination (DC) structure and multi-chain binding are pivotal. The presence of DC structures is experimentally confirmed by in situ attenuated total reflection Fourier transform infrared spectroscopy and supported by quantum chemistry calculations. As a result, PDOL and PTOM exhibit t₊ values exceeding 0.5 and enhanced σ values of 4.3 × 10⁻³ and 8.5 × 10⁻³ S cm⁻¹ at 373 K, respectively. The Li/SPEs/LiFePO₄ cell with ex situ-prepared PDOL achieves a superior capacity retention of 90.8% after 50 cycles. This work underscores the significance of functional group spacing in tuning the transport mechanisms and demonstrates how the decoupling strategy can guide the bottom-up design of advanced SPEs.