A Robust Solid–Solid Interface Using Sodium–Tin Alloy Modified Metallic Sodium Anode Paving Way for All‐Solid‐State Battery

Abstract All‐solid‐state alkaline metal batteries are perceived as the “holy‐grail” high energy density storage system. A robust physical contact between the anode and the solid‐state electrolyte is paramount for a stable cycling. However, the sluggish Na + diffusion kinetic in the metallic sodium results in loss of physical contact during desodiation and promotes rapid sodium penetration. Herein, instead of applying high stacking pressure, a composite anode consisting of Na and Na 15 Sn 4 is proposed to be the anode utilized with sodium superionic conductor solid‐state electrolyte. The addition of Na 15 Sn 4 in the Na matrix increases the Na + diffusivity in the anode layer that reduces the tendency to form pores at the interface. As a result, the symmetrical composite anode cell shows a high critical current density of 2.5 mA cm −2 and a stable galvanostatic cycling for more than 500 cycles at 0.5 mA cm −2 . According to the operando electrochemical impedance spectroscopy, an analytical diffusion model has been proposed to describe the diffusion mechanism in the anode during desodiation. This work shows that the electrode needs high Na + diffusion kinetics to integrate with the solid‐state electrolyte to achieve a robust physical interface.