Lithium batteries: Improving solid-electrolyte interphases via underpotential solvent electropolymerization

Highlights: • Solid-electrolyte interfaces (SEI) are polymers derived from solvent reduction. • SEI properties depend on the degree of solvent polymerization. • Slow reduction rates yield electronically insulating, ion-conducting, solvent-impermeable SEI films. Understanding the mechanism of formation of solid-electrolyte interphases (SEI) is key to the prospects of lithium metal batteries (LMB). Here, we investigate via cyclic voltammetry, impedance spectroscopy and chronoamperometry the role of kinetics in controlling the properties of the SEI generated from the reduction of propylene carbonate (PC, a typical solvent in LMB). Our observations are consistent with the operation of a radical chain PC electropolymerization into polymer units whose complexity increases at lower initiation rates. As proof-of-concept, we show that slow initiation rates via one-electron PC reduction at underpotentials consistently yields compact, electronically insulating, Li⁺-conducting, PC-impermeable SEI films.