Directing selective solvent presentations at the electrochemical interfaces to enable initially anode-free sodium metal batteries

Initially, anode-free sodium metal batteries offer a high energy density
at lower costs than lithium-ion batteries, making them a promising
alternative for portable electronics, transportation, and power grids.
However, side reactions at the electrode/electrolyte interface hinder
their practical applications. Our study reveals that negative electrode
stability is primarily influenced by the solvents in the cation’s first
solvation shell, whereas positive electrode stability is dictated by
weakly bonded solvents. Based on this insight, we introduce an electrolyte
design strategy to selectively direct 2-methyltetrahydrofuran to the Na
metal electrode and tetrahydrofuran to the NaNi1/3Fe1/3Mn1/3O2 positive
electrode interface, optimizing stability for both electrodes. With this
tailored electrolyte, we achieve an average Coulombic efficiency of 99.91%
in Na-Cu cells for 400 cycles at 1 mA/cm2 with 1 mAh/cm2 and demonstrate
stable Na plating/stripping for 5000 hours at 2 mA/cm2 with 2mAh/cm2 in
Na-Na cells. Furthermore, an initially anode-free sodium metal battery
with a positive electrode active material loading of 15.88 mg cm^-2^
retains 90.9% of its capacity over 150 cycles at 110 mA g^-1^, even after
aging, underscoring its potential for practical applications.