Quantitative Analysis of Solid Electrolyte Interphase and Its Correlation with The Electrochemical Performance of Lithium Ion Batteries Using Concentrated LiPF6/propylene Carbonate

Abstract

The charge and discharge performance of graphite negative and LiNi0.5Co0.2Mn0.3O2 (NCM523) positive electrodes in highly concentrated 4.45 mol kg−1 LiPF6/propylene carbonate electrolyte solution was investigated to clarify the chemical species in the surface film formed on both electrode surfaces, and compared with those obtained with conventional 1 mol l−1 LiPF6/ethylene carbonate + dimethyl carbonate electrolyte solution. For the graphite negative electrode, the total electrons consumed to form surface film was well correlated with irreversible capacity, and total mole number and chemical species of the surface film were also correlated with interfacial resistance. In the conventional electrolyte solution, the reductive decomposition of solvents progressed preferentially, while LiPF6 decomposed to form surface film in the concentrated electrolyte solution. While both organic- and inorganic-based surface films can achieve high coulombic efficiency and high capacity retention over charge/discharge cycles, the inorganic-based surface film resulted in a significant increase in interfacial resistance. As for the NCM523 positive electrode in the concentrated electrolyte solution, the formation of inorganic-based surface film and a remarkable increase in interfacial resistance were observed clearly, as with the graphite electrode. However, there was no direct correlation among mole number of chemical species in surface films formed, their chemical composition and interfacial resistance.