Electrochemical Interface Engineering on a Silicon-Based Anode via Fluorinated-Additive-Assisted Interplay with the Electric Double Layer

Incorporating functional fluorinated additives into the electrolyte has demonstrated a promising strategy for improving the electrochemical and interfacial stability of silicon-based anode materials. In previous studies, these additives are claimed responsible for formation of a fluorinated solid electrolyte interphase (SEI) owing to matched orbital energy level with the other electrolyte components. The electric double layer (EDL) created via ionic-electronic coupling at a (sub)nanoscale shows potential influence on the initial SEI formation at the anode, yet the underlying relationship among electrolyte additive, EDL and SEI remains obscure. Here, it is shown that, introduction of 0.5 wt.% trimethylsilyl trifluoromethanesulfonate (TMSOTF) additive into a conventional LiPF₆-based electrolyte helps to refine the EDL configuration, allowing stronger participation of additive molecules and counter anions for building a fluoride-rich layers during initial SEI formation. This dynamic maintenance of an inorganic-rich matrix (LiF, Li_xPO_yF_z, and Li_xS_y) throughout the electrochemical process results in a SEI with optimized chemical composition, enhancing Li⁺ transport, mechanical strength, and structural integrity. Consequently, a SiO_x (x≈1) anode exhibits improved cycling and rate performance, and electrode conformality. This work helps to clarify the EDL-SEI interplay and provide guidelines for rational design of kinetically-stable SEI on a high-capacity anode with substantial volume variations.