Charge–discharge curves with a C/2 rate between 1.5 and 4.5 V in sodium half-cells starting with a discharge mode (a) and charge mode (b). The cells operate firstly at 20 °C (cycles from 1 to 100) and then at 40 °C (cycles from 101 to 200); cycling stability and Coulombic efficiency with a C/2 rate at 20 and 40 °C (c); (d) ex situ XRD patterns of electrodes cycled between 1.5 and 4.5 V at a C/2 rate: (a) pristine electrode; (b) discharged electrode cycled for 200 cycles (100 cycles at 20 °C and subsequent 100 cycles at 40 °C), the cell beginning with a charge mode; (c) charged electrode cycled for 200 cycles (100 cycles at 20 °C and subsequent 100 cycles at 40 °C), the cell beginning with a discharge mode; and (d) discharged electrode cycled for 100 cycles at 40 °C, the cell beginning with a charge mode