Enhancing low-temperature durability and sodium-ion transport of anode-free sodium metal batteries through utilization of a solvent adsorption separator

A battery with high-energy density at low-temperature has been actively pursued in energy storage systems for decades. Anode-free sodium metal batteries (AFSMBs) have emerged as a promising battery configuration for enhanced energy densities by eliminating conventional anode materials. Nevertheless, their practical implementation in low-temperature environments remains constrained by two critical challenges: insufficient dynamics for sodium plating/stripping processes during cycling and instability of the solid electrolyte interphase. Herein, a strategy of multifunctional separator design by employing a solvent adsorption separator with Na supplementation (SAS-N) is proposed to enhance the low-temperature performance of AFSMBs. SAS-N acts as a supplemental sodium reservoir to mitigate irreversible sodium depletion and enhance interfacial compatibility through improved electrolyte wettability. Furthermore, SAS-N modulates more contact ion pair solvation structures, facilitating the formation of an inorganic-rich solid electrolyte interphase (SEI). This reconstructed interface simultaneously stabilizes electrochemical reactions at the electrode/electrolyte interface and accelerates sodium-ion transport kinetics at low temperature. SAS-based AFSMBs demonstrate ultralong-term cyclability, retaining 95.06% capacity over 600 cycles at 25 °C while sustaining 92.53% capacity retention through 1000 cycles under harsh −20 °C operation. This work provides a new approach of separator engineering to improve the low-temperature performance of AFSMBs.