Aging of the LiFePO4 positive electrode interface in electrolyte

The evolution of lithium-containing species on the surface of grains of 500 nm LiFePO4 and 100 nm carbon-coated LiFePO4 materials during the aging process in LiPF6 electrolyte has been followed using coupled 7Li MAS NMR, EIS (Electrochemical Impedance Spectroscopy) and XPS for materials synthesized with and without carbon coating.LiFePO4 undergoes surface reactivity upon immersion in classical LiPF6 electrolyte, although its open circuit voltage (∼3.2 V) lies in the thermodynamical stability voltage range. The evolution of the NMR signal shows that the reaction of formation of the interphase is very slow as no evidence of passivation could be found even after 1 month of contact with the electrolyte. 7Li MAS NMR combined with XPS indicates that carbon coating has a strong protective role towards formation of surface species on the material and hinders iron dissolution at elevated temperature. Coupled NMR, EIS and XPS experiments showed that the surface of the material grains is not covered by an homogenous layer. Increasing the storage temperature from 25 °C to 55 °C promotes the formation of organic species on the surface, most probably covering inorganic species such as LiF, LixPFy and LiPOyFz. No evidence of the formation of a resistive film is deduced from the evolution of EIS measurements. The interphase growth can accelerate the degradation of the electrochemical performance, leading to a loss of electrical contact within the electrode.