(a) First charge–discharge curves between 2.0 and 3.25 V (red curves) and between 1.5 and 3.25 V (blue curves); first charge–discharge and second discharge curves between 1.5 and 4.2 V (black curves). The cells start with a discharge mode, and they are kept at discharged and charged states for 10 h. All curves are recorded at a C/20 rate at 20 °C. The intercalation of 1 Na per formula unit corresponds to a capacity of 64 mA h g⁻¹; (b) changes in the unit-cell volumes of cycled electrodes with respect to the pristine electrode depending on Na amount. The cells are stopped at different potentials (discharged and charged states are described in detail in Table S1, ESI†)