Association of paramagnetic species with formation of LiF at the surface of LiCoO2

To examine formation of LiF at the surface of LiCoO2, 7Li and 19F double-resonance NMR was applied to LiCoO2 electrode samples without any charge/discharge cycles. The two-dimensional (2D) 7Li–19F heteronuclear correlation (HETCOR) experiment using cross polarization (CP) shows a large cross peak at (7Li, 19F) = (ca. 3 ppm, ca. −145 ppm) for the sample without charge/discharge cycles. Since the magnetization build-up curve during 19F to 7Li CP shows a characteristic dipolar oscillation observed in pure LiF, the observed cross peak was assigned to LiF formed in the LiCoO2 electrode. For LiF in the LiCoO2 electrode, not only the apparent chemical shift differences from those of pure LiF (ca. −1 ppm, −203 ppm), but also the significant reduction of the spin-lattice relaxation times (T1 and T1ρ) were observed. These anomalies are ascribed to the paramagnetic effects. Creation of certain paramagnetic species in associate with the LiF formation on LiCoO2 or the LiF formation at paramagnetic sites at the surface of LiCoO2 was invoked.

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