Study of Electrode Surface by IR Spectroscopy

BACKGROUND Advance Technology Development (ATD) Program is a new program put forth by Department of Energy to develop high power lithium ion batteries for hybrid electric vehicles and electric vehicles. It is a joint effort of five national labs and LBNL is the lead lab in investigating SEI (solid electrolyte interface) layer formation and dissolution. Our group is responsible for the characterization of SEI utilizing Infrared Spectroscopy. The basic electrochemistry of lithium ion battery involves only the transfer of Li ions between the two insertion electrodes (anode and cathode), therefore it is also called the rocking chair battery. The electrolyte commonly employed in lithium ion batteries is mixture of aprotic organic solvents and lithium salt such as LiPF6. The cells operate in a wide electrochemical potential window (0 to 5 volts versus Li/Li) which is beyond the thermodynamic limit of the organic electrolyte and therefore, the electrolyte is being reduced and oxidized at the anode and cathode respectively during the charging process. Fortunately, the decomposition product forms a protective film (SEI layer) on the electrode surface, which is electronic insulating but ionic conducting. It allows the transportation of the Li ions, but prevent further decomposition of the electrolyte. Therefore, SEI formation is crucial in the operation of lithium and lithium ion batteries, dissolution of which is believed to closely relate to the battery failure. Density functional theory and Ab initio calculations were performed by our group recently [1] to look at the reduction and oxidation mechanisms of common organic solvent used in lithium and lithium ion batteries, including diethyl carbonate (DEC (1)) and ethylene carbonate (EC (2)) which are currently employed in the ATD cells. Their reduction mechanisms are proposed below: