Aging of Electrode/Electrolyte Interfaces in LiFePO₄/Graphite Cells Cycled with and without PMS Additive

LiFePO 4 /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, LiPF 6 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.