Multinuclear NMR Study of the Solid Electrolyte Interface on the Li-FeSn2 Negative Electrodes for Li-Ion Batteries

The composition of the solid electrolyte interface (SEI) layer on the Li-FeSn 2 negative electrode has been determined by a multinuclear solid state NMR study. The strong e-nuclear dipolar and contact interactions between the as-formed superparamagnetic Fe nanoparticles and the SEI layer allows a layer wise observation highly depending on the proximity. Li + from LiPF 6, which is traditionally considered as a postdeposited product from the electrolyte solution,has been observed as presenting different solvation states by 19 F → 7 Li cross polarization magic angle spinning (CPMAS) NMR. Direct contact between LiOH and Li x Sn or Fe nanoparticles is suggested by Fermi-Contact shifts of −11 and −2 ppm observed in 7 Li and 1 H NMR spectra, respectively. The Li x Sn alloy signal remains invisible to 7 Li NMR due to the close proximity to the Fe nanoparticles. Effects of different discharging rates and aging at the discharge state are discussed as well.