Effect of Adsorption or Electro-Adsorption of Electrolyte Components at Interfaces on the Electrochemical Performances of Li-Ion Batteries

Abstract

In order to improve the stability of the electrode/electrolyte interface, film forming additives, such as vinylene carbonate (VC) or fluoroethylene carbonate (FEC), are usually added in small quantities to the electrolyte. When a stable SEI is formed, good cycling performances are obtained. These additives will compete with solvent molecules and electrolyte ions, for adsorption at the electrode surface. This work concerns the study of liquid/gas (L/G) and liquid/solid (S/L) interfaces for which the liquid is the electrolyte (a mixture of alkylcarbonates containing a lithium salt) and the solid a TiSnSb negative electrode. Surface free energy measurements lead to the conclusion that if lithium salts (LiPF 6 and LiTFSI) are preferentially adsorbed at the L/G interface, the fluorinated additives (FEC and difluoroethylene carbonate) are adsorbed at the S/L interface. A consequence is that these additives are readily reduced at the active material surface when the potential is driven to low values and that it is not surprising to find that the reduction potential of such additives varies strongly with the composition of the electrolyte and the nature of the electrode material.