Dense Li deposition enabled by weakly coordinated Li and fast Li transport 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 a 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 (0.92) and high conductivity of 2.58 mS cm-1. Due to the alleviation of space-charge effect at anode interface with higher tLi+, 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 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 the GPE without PFPN. This research unveils the secrets of relationship between ultra-high lithium-ion transference number electrolytes with dense Li deposition and provides essential insights for the development of high-energy-density lithium metal batteries.