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

Abstract

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 NaNi_1/3Fe_1/3Mn_1/3O₂ 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 h at 2 mA/cm² with 2 mAh/cm² in Na | |Na cells. Furthermore, an initially anode-free sodium metal battery with a positive electrode active material loading of 14.05 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.