Hybrid Protective Layer for Stable Sodium Metal Anodes at High Utilization

Na metal is a promising anode for Na batteries owing to its high theoretical capacity and low reduction potential. Nevertheless, an unstable and inhomogeneous solid electrolyte interphase originated from the instantaneous reactions between the Na metal anode and organic liquid electrolyte causes the intractable hurdles of dendrite growth and low Coulombic efficiency. Here, a sodium fluoride (NaF)-poly(vinylidene difluoride) (PVDF) inorganic-organic hybrid protective layer is constructed on a commercial Cu current collector via a simple blade-coating technique. A flexible PVDF matrix can endure volume change, maintaining the integrity of the anode/coating interface, while NaF particles provide improved Na+ diffusion conductivity and mechanical strength, suppressing the dendrite initiation and growth. Based on these synergetic effects, an excellent cycle life of more than ∼2100 h is realized at 1 mA cm-2 at 50% depth of discharge (DOD), which outperforms 10-fold lifetime of the Cu current collector (∼170 h). Moreover, the Cu current collector with a NaF-PVDF protective layer also delivers good cycling stability at 5 mA cm-2 and an ultrahigh DOD (80%). The rational design of the hybrid protective layer offers a new approach to realize stable Na metal batteries.