Role of Solution Structure in Solid Electrolyte Interphase Formation on Graphite with LiPF₆ in Propylene Carbonate

Abstract

An investigation of the interrelationship of cycling performance, solution structure, and electrode surface film structure has been conducted for electrolytes composed of different concentrations of LiPF 6 in propylene carbonate (PC) with a binder-free (BF) graphite electrode. Varying the concentration of LiPF 6 changes the solution structure, altering the predominant mechanism of electrolyte reduction at the electrode interface. The change in mechanism results in a change in the structure of the solid electrolyte interface (SEI) and the reversible cycling of the cell. At low concentrations of LiPF 6 in PC (1.2 M), electrochemical cycling and cyclic voltammetry (CV) of BF graphite electrodes reveal continuous electrolyte reduction and no lithiation/delithiation of the graphite. The solution structure is dominated by solvent-separated ion pairs (Li + (PC) 4 //PF 6 – ), and the primary reduction product of the electrolyte is lithium propylene dicarbonate (LPDC). At high concentrations of LiPF 6 in PC (3.0–3.5 M), electrochemical cycling and CV reveal reversible lithiation/delithiation of the graphite electrode. The solution structure is dominated by contact ion pairs (Li + (PC) 3 PF 6 – ), and the primary reduction product of the electrolyte is LiF.