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The influence of a film-forming additive, propargyl methanesulfonate (PMS), on electrochemical performance and electrode/electrolyte interface composition of LiFePO 4 /graphite Li-ion batteries has been studied. Combined use of in-house X-ray photoelectron spectroscopy (XPS) and soft and hard X-ray photoelectron spectroscopy (PES) enabled nondestructive depth profiling at four different probing depths in the 2− 50 nm range. Cells cycled with PMS and LiPF 6 in ethylene carbonate/diethyl carbonate (EC/DEC) were compared to a reference sample cycled without PMS. In the first cycle, PMS cells showed a higher irreversible capacity, which is explained by formation of a thicker solid electrolyte interphase (SEI). After three cycles, the SEI thicknesses were determined to be 19 and 25 nm for the reference and PMS samples, respectively. After the initial cycling, irreversible losses shown by the PMS cells were lower than those of the reference cell. This could be attributed to a different SEI composition and lower differences in the amount of lithium between lithiated and delithiated electrodes for the PMS sample. It was suggested that PMS forms a triple-bonded radical on reduction, which further reacts with the electrolyte. The PMS additive was shown to influence the chemical composition at the positive electrode/electrolyte interface. Thicker interface layers with higher C−O and smaller LiF contributions were formed on LiFePO 4 cycled with PMS.

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LiFePO4/graphite cells are studied with hard X-ray photoelectron spectroscopy (HAXPES) in order to evaluate the influence of aging on the electrode/electrolyte interfaces. Cells cycled with the standard electrolyte, LiPF6 in ethylene carbonate/diethyl carbonate, are compared to cells cycled with addition of the film-forming additive propargyl methanesulfonate (PMS). Cycling is performed to accelerate aging at both 21 and 60 °C. The results show that PMS cells have better capacity retention at both temperatures. The thorough HAXPES analysis of the samples reveals that the PMS additive affects the interfaces on both negative and positive electrodes. The effects on the negative electrode show important variations in thicknesses and surface chemistry. Lower loss of cyclable lithium in the SEI and interface thicknesses that are stable during cycling are some reasons for the better capacity retention at 21 °C. The cycling fades quickly at 60 °C for both electrolytes, but the PMS electrolyte gives still a more stable cycling with thinner formed interface layers as probed by depth profiling characterization. PMS give a better long-term interface protection for Li-ion batteries, but capacity fading still cannot be fully prevented.