High reactivity of the nickel-rich LiNi1-x-yMnxCoyO2 layered materials surface towards H2O/CO2 atmosphere and LiPF6-based electrolyte

Abstract

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 O2, CO2 and H2O evolution profile in the 50–500 °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 19F 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.