Solid-state NMR -Solid-state NMR measurements were performed on an Agilent 400 MHz spectrometer (B0=9.4 T, 155.5 MHz for Li) and an Agilent 300 MHz spectrometer (B0=7.1 T, 116.6 MHz for Li and 44 MHz for Li). Chemical shifts were referenced with respect to a 0.1 M LiCl solution. Variable temperature measurements were performed using a 5 mm static goniometer probe. T1 relaxation times were determined at various temperatures using a saturation recovery experiment. The T1s as well as the static line-shapes were fit using the ssNAKE program . For a detailed explanation of the theory behind the spectral density fitting as well as the CSA, see SI Text 1.