A comparative XPS surface study of Li₂FeSiO₄/C cycled with LiTFSI- and LiPF₆-based electrolytes

X-Ray photoelectron spectroscopy (XPS) has been used to characterise the surfaces of carbon-coated Li₂FeSiO₄ cathodes extracted from Li-ion batteries in both a charged and discharged state. 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiPF₆) based electrolytes were used with ethylene carbonate (EC) and diethyl carbonate (DEC) as organic solvents. The LiTFSI-based electrolyte exhibited high salt stability and no significant formation of LiF. However, solvent reaction products from EC were found together with lithium carbonate. A LiPF₆-based electrolyte, on the other hand, showed inferior salt stability with Li_xPF_y, Li_xPO_yF_z and LiF species formed on the surface. Solvent reaction products together with lithium carbonate were also found. There are also indications that Li₂FeSiO₄ is degraded by the HF formed in the electrolyte by the hydrolysis of LiPF₆. A better understanding of the surface chemistry of carbon-coated Li₂FeSiO₄ after the first cycles in a Li-ion battery has thus been achieved, thereby facilitating the optimisation of Li-ion batteries based on this potentially cheap and electrochemically most promising cathode material giving excellent capacity retention: <3% drop over 120 cycles.