NMR quantitative analysis of solid electrolyte interphase on aged Li-ion battery electrodes

• NMR analysis of the surface chemistry of cycled Li4Ti5O12 and LiFePO4 electrodes.
• The interphase composition is described thanks to a calibration of the spectra.
• Mechanisms of the formation of LiF are proposed at each electrode.
• The formation of LiF at LiFePO4 electrode can explain 15% of the capacity loss.

The surface chemistry of aged Li4Ti5O12 and LiFePO4 electrodes that have been cycled in a full cell configuration are examined using nuclear magnetic resonance. The failure mechanism of such cells has been previously identified to be an electrode capacity slippage process caused by the loss of charge carriers: electrons or lithium ions. The electrode-electrolyte interphase is quantitatively analyzed after cycling thanks to a calibration of NMR spectra. LiF is detected at both electrodes: about 1.2 μmol mg−1 at the Li4Ti5O12-based electrode and about 0.35 μmol mg−1 at the LiFePO4-based electrode, and it is the main component amongst lithiated species. By comparison with gravimetric studies, LiF cannot be the only component of the interphase. From a correlation between the amount of detected LiF and the electrode capacity slippage of the battery, different reaction paths are proposed at each electrode, involving either moisture-driven catalysis or reductive process that consumes a quantity of charge from the electrode.