(Invited) Electrochemical Nucleation and Growth of Lithium Fluoride at Lithium Battery Interfaces

Lithium fluoride (LiF) is a commonly observed phase in many battery interfaces including at the solid-electrolyte interphase (SEI) of graphite and Li metal, and the cathode electrolyte interphase (CEI) of many high-voltage cathode materials in Li-ion batteries, given the battery field’s reliance on fluorinated salt anions and increasingly, on fluorinated solvents. For Li electrodeposition, LiF formation has been proposed to correlate with high Coulombic efficiency (CE) of Li plating/stripping, which still remains below 99.9%. Analytical methodologies to study LiF within the SEI have historically been limited, relying on techniques such as X-ray photoelectron spectroscopy that can detect presence of LiF and quantify bulk-average amounts, but lack spatial or morphological information. Meanwhile, the processes by which LiF nucleates and grows at battery interfaces, the morphology and properties of LiF particles formed in relevant battery electrolytes, and the relation to their properties and functionality remain poorly understood. In this talk, I will present insights gained into the physicochemical properties and nucleation and growth behavior of electrochemically formed LiF under highly reducing conditions such as those found at Li anodes. We first studied a series of exemplar conversion reactions based on cathodic reduction of fluorine-rich carrier molecules (dissolved-gas or liquid solutions up to 5 M) that yield LiF upon reduction, with redox potentials ranging from 2–3 V vs. Li. The resulting particle size, morphology and crystallinity are highly sensitive to solvent donor number and discharge rate, evidencing a solution-mediated growth pathway involving supersaturation and precipitation of LiF crystallites onto a carbon or Cu substrate. The LiF growth modes are altered from cubic particles with large average particle sizes (> 1 µm) and sparse coverage in DMSO-based electrolytes to < 50 nm in carbonate- and ether-based electrolytes more relevant to battery applications. Next, we investigated how pre-nucleated LiF particles of varied morphologies and coverage influences Li plating on Cu current collectors. Nucleation overpotentials, Coulombic efficiencies as a function of plated/stripped capacity, and resulting Li deposition morphology are investigated to shed light on structural-electrochemical interplays relevant to those occurring at a Li metal interface.