Results and discussion Electrochemical characterisation Fig. The spectrometer energy scale was calibrated using Cu2p 3 /2 (932.7 eV), Ag3d 5 /2 (386.2 eV) and Au4f 7 /2 (84.0 eV) emissions of 3keV sputter-cleaned metal foils. XPS measurements using monochromatized Al Kα at 1486.6 eV were performed on a PHI 5500 system. All measurements were performed on unwashed electrodes to preserve any surface species formed during the cycling. The cells were then dismantled in the glove-box and small pieces of the electrodes cut out, mounted on a sample-holder, and transported to the XPS equipment, using a specially designed transport chamber to avoid contamination by air or moisture. Cycling was stopped either in the discharged or the charged state. Cells were precycled three times at a C/20 rate (current density: 4.7 mA g −1) between 2.0 and 3.7 V. Galvanostatic cycling was performed at 55 ☌ using a Digatron BTS 600 testing system.

The LiTFSI salt was dried under vacuum at 120 ☌ prior to electrolyte mixing the LiPF 6 salt was dried at 80 ☌. Each lithium salt was dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate ( DEC) in a 2 : 1 volumetric ratio. Two different electrolytes were used for the study: 1 M LiTFSI (lithium bis(trifluoromethylsulfonyl)imide) and 1 M LiPF 6 (lithium hexafluorophosphate). A better understanding of the surface chemistry of carbon-coated Li 2FeSiO 4 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: were assembled in an Ar-filled glove-box (<5 ppm H 2O and O 2). There are also indications that Li 2FeSiO 4 is degraded by the HF formed in the electrolyte by the hydrolysis of LiPF 6. Solvent reaction products together with lithium carbonate were also found.

A LiPF 6-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. However, solvent reaction products from EC were found together with lithium carbonate. The LiTFSI-based electrolyte exhibited high salt stability and no significant formation of LiF. 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiPF 6) based electrolytes were used with ethylene carbonate (EC) and diethyl carbonate ( DEC) as organic solvents. X-Ray photoelectron spectroscopy (XPS) has been used to characterise the surfaces of carbon-coated Li 2FeSiO 4 cathodes extracted from Li-ion batteries in both a charged and discharged state.

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