Fig. 2: Electrochemical and physicochemical characterizations of Na metal electrodes cycled in 1 M NaOTf-DEGDME at various temperatures.

a Galvanostatic cycling of Na||Na symmetric cells at current densities up to 1.0 mA cm⁻² with capacities up to 1.0 mAh cm⁻² at −40 °C. b CE as a function of cycle number for Na||stainless steel cells at 0.5 mA cm⁻² and 0.5 mAh cm⁻² at +20 °C and −40 °C. c Ex situ postmortem SEM measurements of Na metal electrodes after 50 cycles at 0.5 mA cm⁻² and 0.5 mAh cm⁻² at +20 °C, −20 °C and −40 °C. d Contents of elements (left column) and inorganic components (right column) determined by ex situ postmortem XPS depth profiling of the Na metal electrodes shown in Fig. 2c.