Nuclear magnetic resonance measurements. Solid-state NMR experiments were performed using a Bruker Avance II spectrometer operating at a magnetic field strength of 9.4 T, corresponding to a 1 H Larmor frequency of 400 MHz and Na Larmor frequency of 105.8 MHz. All experiments were carried out using a Bruker 4.0 mm H-X MAS NMR probe at a MAS frequency of 12.5 kHz unless otherwise stated. All 23 Na spectra were referenced using a 0.1M NaCl solution (aq) at room temperature.