Mechanism of self-discharge in graphite–lithium anode

In order to evaluate the anode contribution to the lithium-ion battery self-discharge, three electrode coin cells composed of metallic lithium as reference and counter electrode, organic liquid electrolyte and graphite composite working electrode were constructed as test cells. They were first cycled for a dozen cycles and then stored in the full lithiated state of graphite, at 70 °C for periods from 1 to 8 days. The capacity loss was determined during the first delithiation following storage. The latter was found composed of two terms, a reversible and an irreversible one, where the relative amounts are storage time dependants. Electrochemical impedance spectroscopy (EIS) was used to investigate the changes in the cell interfacial characteristics. A model involving the formation of an absorbed electron-ion–electrolyte complex on the graphite surface is proposed as the mechanism of the reversible and irreversible capacity losses. It is also suggested that precipitation/dissolution reactions are taking place at the solid electrolyte interphase (SEI). Precipitation occurs with insoluble inorganic species such as, LiF and Li2CO3, whereas dissolution may concern the organic and/or polymer part of the SEI. The continuous growth of the inorganic (and most resistive) part of the SEI with the subsequent electrode isolation is proposed as the major mechanism of the electrode end of life.