Tuning the solution structure of electrolyte for optimal solid-electrolyte-interphase formation in high-voltage lithium metal batteries

The continuous reduction of electrolytes by Li metal leads to a poor lifespan of lithium metal batteries (LMBs). Low Coulombic efficiency (CE) and safety concern due to dendrite growth are the challenging issues for LMB electrolyte design. Novel electrolytes such as highly concentrated electrolytes (HCEs) have been proposed for improving interphase stability. However, this strategy is currently limited for high cost due to the use of a large amount of lithium salts as well as their high viscosity, reduced ion mobility, and poor wettability. In this work, we propose a new type of electrolyte having a moderate concentration. The electrolyte has the advantage of HCEs as the anion is preferentially reduced to form inorganic solid-electrolyte-interphase (SEI). Such optimization has been confirmed through combined spectroscopic and electrochemical characterizations and supported with the first-principle molecular dynamics simulation. We have shown the intrinsic connections between solution structure and their electrochemical stability. The 2.0 M LiDFOB/PC electrolyte, as predicted by our characterizations and simulations, allows stable charge–discharge of LNMO|Li cells at 5C for more than 1500 cycles. The 2.0 M electrolyte generates a dense layer of SEI containing fluoro-oxoborates, Li₃BO₃, LiF, Li₂CO₃, and some organic species effectively passivating the lithium metal, as confirmed by electron microscopy, X-ray photoelectron spectroscopy, and solid-state nuclear magnetic resonance.