Low-concentration electrolyte with lithium nitrate as the sole salt for constructing a LiF/Li3N inorganic composite SEI and enabling stable lithium metal battery cycling

Abstract

Lithium nitrate (LiNO₃) exhibits exceptional solid electrolyte interphase-forming capabilities, cost efficiency, high thermal stability, and low environmental impact. However, its limited solubility in ester-based electrolytes means that it is frequently used only as an electrolyte additive. This study presents a low-concentration electrolyte (LCE) formulation comprising 1,2-dimethoxyethane (DME), fluoroethylene carbonate (FEC), and ethoxylated pentafluorocyclotriphosphazene (PFPN), with 0.5 M lithium nitrate (LiNO₃) serving as the sole lithium salt. In LiNO₃, strong Li⁺–NO₃⁻ interactions arise from the high binding affinity between NO₃⁻ and Li⁺, driving preferential incorporation of NO₃⁻ into the Li⁺ solvation shell to form a solvation structure dominated by contact ion pairs (CIPs). Furthermore, FEC and PFPN pull out part of DME from the Li⁺ solvation shell via intermolecular interactions, thereby reducing the proportion of DME solvent participation in the Li⁺ solvation shell and promoting the formation of nitrate-rich aggregates (AGG/AGG+). This design confers high voltage tolerance (4.4 V) and non-flammability characteristics to a 0.5 M low-salt-concentration ether-based electrolyte. It tackles the challenge inherent in LCEs, where solvent-dominated solvation architectures give rise to the formation of an organic-rich solid electrolyte interphase (SEI), culminating in suboptimal cycling stability. The approach markedly improves the cycling performance of NMC811 (9.2 mg cm⁻²)‖Li (50 μm) full cells, achieving 80% capacity retention after 150 cycles, while promoting the formation of a LiF/Li₃N inorganic composite solid electrolyte interphase (SEI). The key strategy of this work is to utilize LiNO₃ as the sole lithium salt, which paves a novel pathway for the rational design of advanced low-concentration electrolytes.