Understanding formation of solid electrolyte interface film on LiMn2O4 electrode

We studied formation of solid electrolyte interface (SEI) film on the surface of spinel LiMn2O4 electrodes by evaluating irreversible capacity and monitoring impedance change in the first charge and discharge cycle of a Li/LiMn2O4 cell. Results show that during the first cycle, LiMn2O4 produces 10-15% irreversible capacity in both 1 m LiPF6 3:7 ethylene carbonate/ethyl methyl carbonate (EC/EMC) and 1 m LiPF6 1:1:3 propylene carbonate (PC)/EC/EMC electrolytes. Formation of the irreversible capacity mainly takes place in two voltage regions of similar to3.1 V, near open-circuit voltage of a fresh cell, and 3.7-4.2 V, in the voltage range of delithiation and lithiation of LiMn2O4. It is believed that the irreversible capacity is associated with the formation of SEI film on the surface of LiMn2O4 electrode. Impedance data indicate that electrolyte solvents greatly affect properties of the SEI film. The SEI film formed with PC/EC/EMC electrolyte is more resistive and more stable, while that formed with EC/EMC electrolyte is subjected to a reversible breakdown at voltages higher than 3.8 V. It is observed that after the cell is cycled, the SEI film becomes more conductive while the bulk resistance of electrolyte and electrode increases. (C) 2002 The Electrochemical Society.