1D Na MAS single-pulse (SP) NMR spectra were recorded after a selective 90° pulse of 10.5 µs (B1 field strength ~ 23.8 kHz) to excite only the central transition. The recycle delays were optimized for each sample to ensure quantitative NMR spectra. 2D Na MQMAS NMR spectra were recorded using a z-filtered 4-pulse sequence 39 with a split-t1 evolution at a magnetic field of 9.4 T (MAS rate 12.5 kHz, recycle delay optimized per sample, selective 90° pulse at B1-field of 24-27 kHz, z filter 20µs) to extract isotropic chemical shifts and average quadrupolar coupling constants. These were then used to deconvolute the 1D Na MAS NMR spectra using the software ssNake. 40 2D 23 Na- Na exchange (EXSY) correlation spectra were acquired using a Na B1 field strength of ~45 kHz and a mixing time of 3 ms. A rotor-synchronized t1 evolution (160µs) was employed between two Na pulses.