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

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 LiPF6 in propylene carbonate (PC) with a binder- free (BF) graphite electrode. Varying the concentration of LiPF6 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 LiPF6 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)//PF6-), and the primary reduction product of the electrolyte is lithium propylene dicarbonate (LPDC). At high concentrations of LiPF6 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)PF6-), and the primary reduction product of the electrolyte is LiF.