High reactivity of the nickel-rich LiNi_(1-x-y)Mn_xCo_yO_2 layered materials surface towards H_2O/CO_2 atmosphere and LiPF_6-based electrolyte

摘要

Storage of nickel-rich LiNi1-x-yMnxCoyO2 positive active materials under improper environmental conditions brings about undesirable surface reactions causing poor Li-ion cell electrochemical performances and gas generation. In this paper, a detailed stepwise investigation of the increasing reactivity of 532, 622, 811 and 901 NMC materials towards a specific H2O/CO2 atmosphere is reported. Water-soluble salts as lithium carbonate, hydroxide and sulfates are accurately quantified thanks to acid-base titration and inductively coupled plasma atomic emission spectroscopy techniques. On the other hand, the presence of insoluble species as transition metal oxyhydroxides and oxides is unveiled through a thermal decomposition study using thermogravimetric mass spectrometer coupling technique. Their formation and thermal decomposition mechanisms are proposed after careful analysis of the characteristic mass loss and O-2 , CO2 and H2O evolution profile in the 50-500 degrees C region. Interestingly, the results also highlight an in situ lithiated layered oxide material reformation reaction. To assess their chemical reactivity towards LiPF6-based electrolyte, the latter was analyzed through F-19 nuclear magnetic resonance after storage with various reference salts and 811 NMC. LiPO2F2 is detected from storage tests with Li2CO3, Li2O and Li2SO4, and fluorophosphate-type molecules are detected from LiOH; this experiment can help discriminate between LiOH and Li2O as NMC surface species.