Dense Li deposition enabled by weakly coordinated Li and fast Li transport in a single-ion conducting gel-polymer electrolyte

Abstract

Polymer electrolytes face fundamental challenges in simultaneously achieving rapid Li⁺ transport and weak Li⁺–anion/solvent bonding. To address this bottleneck, this study introduces molecular-level stepwise regulation of solvation structures in a gel-polymer electrolyte (GPE). By weakening the strong Li⁺–solvent coordination via NO₃⁻, immobilizing anions with PFPN, and utilizing the shielding effect of the NO₃⁻–TFSI⁻–FSI⁻ triplet anion in the solvated structure, the Li-coordination is sequentially reduced. The lithium-ion transport mechanism evolves from a vehicular transport mechanism of the entire primary solvation sheath (directional movement within the first solvation shell) to a Li⁺-hopping conduction mechanism (Li⁺ jumping between different coordination sites). Consequently, a single ion conducting GPE (SIC-GPE + PFPN + LiFSI) achieves a high lithium-ion transference number of 0.92 and high conductivity of 2.58 mS cm⁻¹. Due to the alleviation of the space-charge effect at the anode interface with higher t_Li⁺, the Li nucleation over-potential and deposition over-potential are significantly reduced, while the critical current density (CCD) reaches 8 mA cm⁻² for SIC-GPE + PFPN + LiFSI. Additionally, the exchange current density of SIC-GPE + PFPN + LiFSI is increased, which results in smooth and dense Li deposition morphology. With the PFPN derived cathode interphase interlayer (CEI) on the NCM622 cathode, the high-voltage lithium metal battery (LMB) operates stably for over 300 cycles, which is 30 times higher than that of the GPE without PFPN. This research unveils the details of the relationship between ultra-high lithium-ion transference number electrolytes and dense Li deposition and provides essential insights for the development of high-energy-density lithium metal batteries.