Surface characterization and stability phenomena in Li2FeSiO4 studied by PES/XPS

Photoelectron spectroscopy (PES) has been used to characterise the surface of Li2FeSiO4 cathodes extracted from lithium-ion batteries. Pristine, uncycled, air-exposed electrodes were first analysed and found to carry significantly greater amounts of Li2CO3 on their surfaces than electrodes stored under inert atmosphere. The surface film formed on electrochemical cycling of Li2FeSiO4 electrodes at 60 degrees C using a LiN(SO2CF3)(2) salt based electrolyte revealed high salt stability and only small amounts of solvent reaction products. These were mainly of Li-carboxylate type; neither carbonates nor LiF were found. The excellent capacity retention (<3% over 120 cycles) and minimal irreversible capacity during the first cycle are probably a direct result of this very thin surface film. Li2FeSiO4 must therefore be seen as a most promising ( and potentially cheap) positive electrode material for future large-scale Li-ion battery applications.