Direct Evidence of LiF Formation at Electrode/Electrolyte Interface by 7Li and 19F Double-Resonance Solid-State NMR Spectroscopy

7Li and 19F double-resonance NMR was applied to examine LiF formed at the surface layer of a LiCoO2 electrode during charge-discharge cycles in an electrochemical cell with organic electrolytes containing LiPF6. Not only the chemical shifts observed by using cross polarization (CP) between 7Li and 19F, but also the magnetization-transfer curves during CP indicate the observed CPsignals are of LiF in the LiCoO2 electrode. Examination of the effect of 7Li-7Li spin diffusion between LiF and LiCoO2 on the 7Li spin-lattice relaxation time suggested that LiF forms macroscopic domains which is not directly adhere the LiCoO2 particles.