Toward Unraveling the Origin of Lithium Fluoride in the Solid Electrolyte Interphase

The solid electrolyte interphase (SEI) is an integral part of Li-ion batteries and their performance, representing the key enabler for reversibility and also serving as a major source of capacity loss and dictating the cell kinetics. In the pervasive LiPF6-containing electrolytes, LiF is one of the SEI's major components; however, its formation mechanism remains unclear. Electrochemically, two separate reduction pathways could lead to LiF, either via direct anion reduction or electrocatalytic transformation of HF. This work aims to shed light on understanding the role played by these pathways. In a multimodal experimental and theoretical approach, we carried out operando structural characterization on an inert model single crystalline N-doped SiC working electrode during voltammetric scans in LiPF6 baseline electrolytes and complemented these with ex situ chemical characterization. These results were supplemented by cyclic voltammetry measurements using a variety of electrolyte formulations under different cycling rates as well as quantum chemical calculations and Born–Oppenheimer molecular dynamics simulations. Our results reveal that the reductive formation of LiF in these systems is likely a combined mechanism, which concomitantly involves both direct anion reduction and electrocatalytic transformation of HF. Specifically, LiF nucleates via the electrocatalytic transformation of HF followed by significant anion reduction.