Chemical properties of various organic electrolytes for lithium rechargeable batteries: 1. Characterization of passivating layer formed on graphite in alkyl carbonate solutions

The characteristics and reaction mechanisms of the passivating film formed on the surface of graphite were investigated in ethylene carbonate-diethyl carbonate solutions containing LiClO4, LiPF6 and LiN(SO2CF3)2. The electron consumption resulting on the irreversible capacity of graphite was almost equivalent to that used in the one-electron reduction of Li+ found in the film. The electrochemical reactions in the first discharge process may be divided into the following steps: (i) ‘initial film formation step’ from 1.4 to 0.55 V; (ii) ‘main film formation step’ from 0.55 to 0.2 V, and (iii) ʹlithium intercalation step from 0.2 to 0.0 V. Most of the passivating film is formed together with the lithium intercalation reaction at step (ii). The passivating film formed at this step contained a significant amount of organic film such as EtOCO2Li, (CH2OCO2Li)2, etc. Through the consecutive formation of passivating film at steps (i) and (ii), lithium intercalation into graphite proceeds smoothly without further decomposition of the organic electrolyte.