A study of the kinetics of lithium transport through the 'solid electrolyte interphase' (SEI) that forms on carbons used in lithium-ion batteries

One of the primary challenges facing the lithium-ion battery field is to increase the rates at which the cell can be charged and discharged. In particular, this thesis is concerned with the kinetics of lithium transport through the surface films that form on carbon in non-aqueous electrolyte. These surface films have often been described as a solid electrolyte interphase or “SEI”. To investigate the morphology and physical properties of the surface films, in situ electrochemical atomic force microscopy (ECAFM) was used. The ECAFM experiments showed that the surface films are thicker than previously believed. Cyclic voltammetry (CV), potentiostatic intermittent titration technique (PITT), constant current cycling, and electrochemical impedance spectroscopy (EIS) experiments were used to investigate the kinetics of charging and discharging of fresh and cycled cells. These experiments showed that the kinetics of a fresh cell are limited by lithium diffusion in the bulk carbon. The kinetics of a cycled cell, however, are slower than that of a fresh cell due to an increased impedance of the surface films. A model is proposed to explain how the surface films are slowing the kinetics of lithium transport. Consequences of a high impedance surface film are discussed.